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Investigation of Properties Responsible for Phenol Removal via Titania Coated Activated Carbon


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INVESTIGATION OF PROPERTIES RESP ONSIBLE FOR PHENOL REMOVAL VIA TITANIA COATED ACTIVATED CARBON By VIVEK SHYAMASUNDAR A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

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Copyright 2005 By Vivek Shyamasundar

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iii ACKNOWLEDGMENTS I would sincerely like to th ank Dr. David Mazyck for giving me the chance to work on this challenging project. His guidance a nd constant encouragement enabled me to work through and succeed during some frustrating times. I have gained a lot of knowledge by working with him. I would like to thank my committee Dr. Paul Chadik, Dr. Joseph Delfino and Dr. Joseph Geunes who were always willing to help and provided me with a lot of suggestions throughout the duration of my research. I would like to thank all the students in Dr. Mazycks research group Ameena Khan, William Beau Kostedt, Morgana Bach, Jennifer Stokke and Heather Byrne. Their suggestions and help at all the stages of my work help ed me immensely. I consider myself very fortunate to have worked with such a great group. Thanks also to Aly Byrne and Gustavo Avila for their assistance on a num ber of experiments. I would also like to acknowledge Gautam Kini for his help. I thank Rick and Matt at Engineering Performance Solutions who were very effici ent and quick in analyzing my samples. I especially thank Sudeepti Southekal for be ing a great friend and a constant source of encouragement. I hope to help her in the same way as she pursues her PhD. Above all, I must thank my family for th eir love and support and for the principles and ethics that they have instilled in me.

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iv TABLE OF CONTENTS Page ACKNOWLEDGMENTS.................................................................................................iii TABLE OF CONTENTS...................................................................................................iv LIST OF TABLES.............................................................................................................vi LIST OF FIGURES..........................................................................................................vii ABSTRACT....................................................................................................................... ix CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW.............................................................................................4 2.1 Activated Carbon....................................................................................................4 2.1.1 Physical Properties of AC............................................................................5 2.1.2 Chemical Properties of AC...........................................................................7 2.1.3 Adsorption Mechanisms...............................................................................9 2.2 Photocatalysis.......................................................................................................12 2.2.1 Titanium Dioxide........................................................................................13 2.2.2 Mechanism of Photocatalysis.....................................................................14 2.2.3 Improvements in Photocatalysis.................................................................16 2.3 Fermi Energies/Levels, Electrical Conductivity and Schottky Barrier................18 2.4 Previous Synergy in AC-TiO2 Systems................................................................21 3 MATERIALS AND METHODS...............................................................................26 3.1. Target Pollutant...................................................................................................26 3.2 Carbon Materials..................................................................................................26 3.2.1 TiO2-AC Composites a nd Coating Procedures..........................................26 3.2.2 Mass Based Coating (m/m)........................................................................31 3.2.3 Molar Based Coating (mol/mol).................................................................31 3.2.4 Low Surface Area Carbon-TiO2 Composite...............................................33

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v 3.3 Experimental Setup...............................................................................................34 3.3.1 Batch Reactor Configuration......................................................................34 3.3.2 Analytical Equipment.................................................................................34 3.4 Experimental Procedure........................................................................................35 3.4.1 Concentration Measurements.....................................................................35 3.4.2 Batch Adsorption/Oxidation Tests with Activated Carbon........................36 3.4.3 Batch Adsorption/Oxidation Tests with Low Surface Area Carbons........37 4 RESULTS AND DISCUSSION.................................................................................39 4.1 Evaluation of Carbon Coating Strategies.............................................................39 4.2 Activated Carbon Studies.....................................................................................41 4.2.1 Ash Analyses of Activated Carbons...........................................................41 4.2.2 SEM Images of Titania Coated ACs..........................................................47 4.2.3 Activated Carbon Batch Studies.................................................................52 4.2.4. Determination of Existence of Phenol* (Phenol with Delocalized Electron) in the Presence of UV......................................................................62 4.3 Low Surface Area Carbon Studies.......................................................................66 4.3.1 Metals Content of Lo w Surface Area Carbons...........................................68 4.3.2 Batch Studies with Lo w Surface Area Carbons.........................................71 5 SUMMARY AND CONCLUSIONS.........................................................................78 5.1 Summary...............................................................................................................78 5.2 Conclusions...........................................................................................................81 6 CONTRIBUTIONS TO SCIENCE AND ENGINEERING......................................82 6.1 Contributions to Science.......................................................................................82 6.2 Contributions to Engineering................................................................................83 APPENDIX A VIRGIN AC STUDIES IN PRESENCE OF UV.......................................................84 B COLUMN STUDIES..................................................................................................85 C COLUMN STUDIES II..............................................................................................86 LIST OF REFERENCES...................................................................................................87 BIOGRAPHICAL SKETCH.............................................................................................91

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vi LIST OF TABLES Table Page 2-1. Photocatalysis reaction mechanisms..........................................................................16 3-1. Summary of AC properties.........................................................................................26 3-2. Carbon contents of virgin and titani a coated ((m/m) and (mol/mol)) activated carbons and their surface areas.................................................................................32 3-3. Surface areas of virgin and coated low surface area carbons.....................................33 3-4. Carbon contents of low surface area carbons.............................................................33 4-1. Summary of coating techniques.................................................................................40 4-2. Ash contents of ACs and mass % of titania deposited (m/m) on AC surface............42 4-3. Phenol removal as a function of AC coa ting strategy with and without irradiation..63 4-4. Electrical conductiv ity values for low surface area carbons......................................67 4-5. Elemental carbon and ash contents of low surface area carbons................................68 4-6. Average densities of 3% (m/m ) titanium dioxide coated carbons..............................74

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vii LIST OF FIGURES Figure Page 2-1. TiO2 structure. A) anatase, B) rutile...........................................................................14 2-2. Photocatalysis mechanism..........................................................................................15 2-3. Band gaps for (a) metals, (b) semiconducto rs and (c) insulators with Fermi level ( F) indicated............................................................................................................19 2-4. Alignment of Fermi levels a nd formation of Schottky barrier...................................20 2-5. Kinetics of phenol disappearance in th e presence and absence (photolysis) of various illuminated solids.........................................................................................22 3-1. Coating activated carbon with TiO2 via mechanical attachment (Theta Composer, Tokuju Corp., Japan)................................................................................................31 3-2. Concentration vs. Absorbance correla tion for phenol on UV spectrophotometer measured at 270nm wavelength...............................................................................35 4-1. EDS of A) ash residue of uncoated Bionuchar, B) Bionuchar coated with TiO2, C) ash residue of uncoated F400, D) F400 coated with TiO2, E) ash residue of uncoated HD4000, F) HD4000 coated with TiO2....................................................44 4-2. Titania coated Bionuchar with the TiO2 blocked pores circled..................................47 4-3. Bionuchar showing cellulose structure of the wood based AC..................................48 4-4. Titania coated on the surface of F400........................................................................49 4-5. TiO2 agglomerate (shown circled) near a macropore on the surface of HD4000......50 4-6. Surface of HD4000 showi ng uniform coating of TiO2 which tends to form agglomerates.............................................................................................................51 4-7. Batch adsorption studies of vi rgin ACs in the absence of UV...................................53 4-8. Batch adsorption studies of 3% (m/m) co ated ACs in the dark compared to TiO2 slurries......................................................................................................................5 4

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viii 4-9. Batch adsorption-photocatalysis studies of 3% (m/m) coated ACs in the presence of UV........................................................................................................................56 4-10. Batch adsorption-photocatal ysis studies of 3% (m/m) coated ACs loaded to the reactors on a volume basis in the presence of UV...................................................59 4-11. Batch adsorption studies: A) 3% (mo l/mol) coated ACs in the absence of UV loaded to the reactors by volume, B) Photocatalysis studies of 3% (mol/mol) coated ACs in the presence of UV loaded to the reactors on a volume basis..........60 4-12. Comparison of irradiated virgin ACs w ith virgin and coated ACs in the presence and absence of UV...................................................................................................62 4-13. EDS scan of ash residue of uncoated anthracite coal...............................................69 4-14. EDS scan of ash residue of uncoated bituminous coal.............................................69 4-15. EDS scan of ash residue of pitch coke.....................................................................70 4-16. Adsorption studies of coated low surface area carbons a nd titanium dioxide slurry......................................................................................................................... 71 4-17. Batch photocatalysis st udies of carbons and titaniu m dioxide slurry (m/m)............73 4-18. Photocatalysis studies of coated low surface area carbons in the presence of UV (m/v).........................................................................................................................7 5 4-19. Photocatalysis studies of low surface area carbons in the presence of UV (mol/v).76 A-1. Replicate data sets for virgin AC studies performed in the presence of UV demonstrating the phenol* phenomenon observed in Section 4.2.4........................84 B-1. Recirculation column studies perform ed with F400 using different flowrates (degrees of fluidization) in the presence and absence of UV...................................85 C-1. Adsorption runs of column tests pe rformed with HD4000 in the presence and absence of UV radiation measuring phe nol concentration in the effluent...............86 C-2. Concentration of phenol in effluent dur ing regeneration runs of column studies.....86

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ix Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science INVESTIGATION OF PROPERTIES RESP ONSIBLE FOR PHENOL REMOVAL VIA TITANIA COATED ACTIVATED CARBON By Vivek Shyamasundar December 2005 Chair: David Mazyck Major Department: Environmental Engineering Sciences Activated carbon has long been used to remove organic pollutants from water. However, a major disadvantage is that it n eeds to be regenerated regularly, which is primarily done offsite. The regeneration decreases the adsorption capacity of the activated carbon and hence increases costs. Titanium dioxide, thr ough the mechanism of photocatalysis, mineralizes the organic po llutants to carbon dioxide and water. A drawback of this technology is that when used in the form of a slurry, the separation of the titanium dioxide from the tr eated effluent incurs costs. The use of a titanium dioxide coated activated carbon would therefore resu lt in the elimination of the separation of titanium dioxide from water and would at the same time result in the in-situ regeneration of activated carbon. It has however been found in previous res earch that the behavior of the TiO2 coated activated carbons varies for different ACs. The objective of this research was to iden tify a simple yet effective procedure to coat the ACs with titanium di oxide and to identify propert ies of activated carbons that

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x influence a synergistic effect between the activated carbon an d the titanium dioxide. On the premise that conductivity du e to the presence of metals influences photocatalysis in the TiO2-AC composites, batch studies were pe rformed comparing the adsorption and photocatalysis of three activat ed carbons having different properties. Characterization studies were also performed on the ac tivated carbons comparing their surface morphologies and compositions of their ash. It was observed that ACs having less acidic groups on their surface resulted in better adsorp tion of the pollutant (i.e., phenol). It was also observed that the coal based ACs had a rougher surface resulting in a more uniform deposition of titanium dioxide on their surface. To further isolate and study the mechanis m of photocatalysis, low surface area carbons were used in separate studies. Thes e carbons differed in terms of ash composition and electrical conductivity. These carbons were coated with titanium dioxide and tested in batch for photocatalytic activity. The as h composition of these carbons was also compared. For activated carbons and low surface area carbons, results showed no noticeable influence of conductivity on the photocatalyt ic activity of the ACs or the carbons. Photocatalysis was observed as being a s econdary mechanism compared to adsorption which was clearly dominant.

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1 CHAPTER 1 INTRODUCTION Over the past 20 years, the planets dri nking water resources have entered into a very fragile state. A number of organic pollu tants, the sources of which are solvents, fertilizers (e.g., NPK), pesticides and ch lorophenols, among others, have resulted in widespread groundwater contamin ation (Hoffman et al., 1995). To cope with the growing pollution of the hydrosphere, a number of regulations and programs are being implemented. The primary strategy being applie d currently is the chemical treatment of polluted drinking water, surface water, groundwat er and wastewater. Pollutant removal in drinking waters may involve flocculation, sterilization, filtra tion and disinfection processes (Legrini et al., 1993). Technologies utilized may also include ultrafiltration, air stripping, carbon adsorption and oxidation (via ozonation or hydrogen peroxide) (Serpone, 1995). Recent advances in the field of water tr eatment have led to the development of oxidative degradation techniques for orga nic compounds dissolved in water. These processes are known as advanced oxidat ion processes (AOPs). Heterogeneous photocatalysis is one type of advanced oxidation process that can be used to counter the growing contamination of precious water resources due to organic compounds. This process entails the use of a photocatalyst th at is activated under the influence of UV radiation which ultimately result s in the oxidation of the or ganic contaminants to water and carbon dioxide, resulting in an acceptably mi nimal waste stream. This lack of toxic organic byproducts is a signi ficant advantage of this te chnology over the contemporary

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2 technologies mentioned earlier, which simply transfer the pol lutant from one phase to another. The technology can be very useful in applications for su stenance during longterm NASA space missions. Due to the limited re serves of water that may be carried on the space shuttle, a closed-loop photocatalytic system would theore tically be able to extend the lifespan of a given finite reserv e of water almost inde finitely. Additionally, due to the depleted resources of drinking wate r here on Earth, it is of great interest to adapt this technology for industrial purposes as well as for household water purification. The use of photocatalysis has been wide ly studied with th e photocatalyst (TiO2) in the form of a slurry. However, a slurry based sy stem used for industrial purposes or for household use may result in a waste stream consisting of the photocatalyst itself. Separation of TiO2 from the treated effluent coul d further complicate matters while increasing the cost of the system. Filters containing activated carbon and other treatment media are currently used in many households as secondary treatment system s. However, these filters need to be replaced from time to time due to the exha ustion of the activat ed carbon. Hence, combining the theoretically infinite sustaina bility of the photocatalysis phenomenon with the simplicity of an activated carbon filter, a compact treatment device is sought to be developed with the photocatalyst (titanium di oxide) coated on the activated carbon. The main challenges in the development of such a system as a viable application for industrial and household use are as follows: To identify a simple technique by which th e activated carbons may be coated with titanium dioxide. Preferably, the co ating technique would not incur any considerable additional costs. To develop an AC-TiO2 composite that demonstrates both high capacity as well as high photocatalytic efficiency.

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3 Numerous coating technique s were tested. The techni ques were evaluated on the basis of simplicity and effectiv eness. Making use of the coating technique that best fits the need; efforts were concentrated on optimizing an AC-TiO2 composite that shows the best performance with respect to photocatalysis of a model organic contaminant; in this case phenol (C6H5OH). Hence, it is imperative to is olate the intrinsic and physical properties of activated carbons which aid in photocatalysis, assuming photocatalysis is dependant on the intrinsic prope rties of the ACs. To do this experiments were performed on a variety of different ACs. The need for greater scie ntific understanding of the phenomenon of photocatalysis and the factors that affect it has provoked the author to concentrate part of the efforts on the science behind photocatalysis. Focus has es pecially been placed on whether a synergy exists between titanium dioxide and any particular activated carbon. This has been investigated thr ough the use of TiO2 coated carbonaceous materials with extremely low adsorption capacity. Electrical conductivity of the ACs due to the presence of conductive metals has been hypothesized to be a factor in fluencing the photocatal ysis seen with the AC-TiO2 composites. Identification of the factor s affecting photocatalysis will in the future allow us to further optimize a system. In summary, this study is an effort to de velop a system for in-situ regeneration of activated carbon for household as well as industrial purposes with a concentration on identifying a TiO2-AC composite that performs with the most synergy with respect to adsorption and photocatalysis.

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4 CHAPTER 2 LITERATURE REVIEW 2.1 Activated Carbon Activated carbons are adsorbents created from carbonaceous precursors that either experience thermal or chemical activation to increase the internal porosity of the raw material giving rise to high surface area carbonaceous materials (500-1500 m2/g). Suitable raw materials can include wood, coal (anthracite, bituminous, and lignite), coke, coconut shells, fly ash and even rice husk (Karthikeyan et al., 2005; Kinoshita, 1988). Depending on the precursor, the ash conten t (which can include iron, aluminum, and silica) of activated carbons can vary (e.g., 1-20% (w/w)). Thermal or steam activation entails the oxi dation of char, which is created during the first phase of the activation process via pyrolysis, with the help of oxidizing agents such as steam, air or carbon dioxide at temperatures between 800-1000 C. Chemical activation involves heating the carbonaceous precursor and a dehydrating agent (ex. phosphoric acid, carboxylic acid, sulfuric acid, nitric acid) to a temperature between 200650C. The dehydrating agent may then be leached out and reused (Kinoshita, 1988). Hence, chemically activated carbons typica lly have a low pH due to acidic groups on their surface. This has resulted in loose defi nitions such as L-type activated carbons for chemically activated carbons (due to l ow pH) and H-type activated carbons for physically activated carbons (due to hi gh pH). The difference in precursors and activation processes result in activated carbons having vary ing physical and chemical

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5 properties. As an aside, a physically activated carbon that is acid-washed to remove organics or for other purposes may exhibit L-type characteristics. Activated carbon is widely used in po table and wastewater treatment for the removal of organic and inorgani c pollutants. It may be used in its powdered form, known as powdered activated carbon (PAC) (particl es pass through a 325 mesh sieve), or in the granular form (GAC). 2.1.1 Physical Properties of AC Surface area. The defining physical characteristic of activated carbon (AC) is its welldefined pore structure which is one of the main reasons for its high internal surface area. It is this internal surface area that is available for the adsorption of pollutants. Even though properties such as pore size dist ribution, surface chemistry and adsorbateadsorbent interactions play a role, surface area is still considered the limiting factor for adsorption of the target pollutant by practitione rs. Hence, a greater su rface area of the AC will result in a greater potential for the adso rption of the pollutant. The typical surface areas of activated carbons are between 500 m2/g-1500 m2/g. Much of the carbon literature focuses on th e importance of surface area but surface chemistry is the dominant variable that controls adsorption in most cases. Therefore, the role of surface chemistry will be addressed th roughout this review and in the results and discussions of the data. The most common method for determining the surface area of an adsorbent is using the Brunauer, Emmett, and Teller (BET) theor y. The BET theory was initially developed for characterization of physical adsorption. Hence its adaptation to characterize surface area is not considered error fr ee as it cannot universally be ap plied to all types of physical adsorption (Dabrowski, 2001). The surface area analysis by the BET theory is performed

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6 with N2 which may result in an inaccurate pred iction of adsorption trends for other molecules (Dabrowski, 2001). Hence, the size of the molecule being adsorbed must be considered while characterizi ng the surface area of the adso rbent. For this reason, the pore size distribution (PSD) of the adsorbent is an important factor in characterizing the adsorption of activated carbon. The BET isotherm is given below. 1/ (W*((Po/P)-1)) = 1/ (Wm*C) + (C-1)/ (Wm*C)*(P/Po) W: Weight of gas adsorbed at P/Po P: Pressure of overlying gas Po: Saturation pressure of the gas Wm: Adsorbate weight in monolayer coverage C: Constant related to the energy of adsorption Pore size distribution. The pores of activated carbon c onsist of three size ranges according to IUPAC recommendations. Micropores: Less than 20 (2 nm) Mesopores: Between 20 (2 nm) and 500 (50 nm) Macropores: Greater than 500 (50 nm) Nowadays, the word nanopore is used in general to define micropores and mesopores (Dabrowski, 2001). The activation protoc ol to a large extent dictates the pore sizes in the AC. Micropores in activated carbon are usually co mparable to the sizes of the adsorbate molecules (e.g., size of phenol molecule is 6 (Mazyck, 2000)). Therefore, all the atoms of the adsorbent (activated carbon) can intera ct with the adsorbate species (Dabrowski, 2001) which is the main difference between adsorption mechanisms of micropores and that of mesopores and macropores. Micropore adsorption is hence a pore filling process in which the volume of the micropores is the controlling factor.

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7 The walls of macropores are formed by a great number of adsorbent atoms or molecules. The boundary of interfaces as well as the adsorbent surface area have a distinct physical meaning (Dabrowski 2001). Mono and multilayer adsorption successively takes place on the surface of mesopor es and their final fill is in accordance with the mechanism of capillary adsorbat e condensation (Dabrows ki, 2001). Mesopores participate in the transport of the pollutant molecules to the major adsorption sites in the micropores. They are characterized mainly by their specific surface area and pore size distribution. In the case of macropores, the action of adsorption forces does not occur throughout their void volume but at a short distance from their walls. Like mesopores, macropores are also diffusion por es in that they principa lly transport the pollutant molecules to smaller pores. During the adsorption process in activated carbon, four main steps occur: 1. Bulk diffusion or the diffusion of th e molecule through the bulk liquid. 2. Film diffusion or the diffusion of molecu les through the thin film (boundary layer) surrounding the activated carbon. 3. Pore diffusion or the diffusion of the mo lecules through the pores (or along the pore walls) of activated carbon. 4. Adsorption of the adsorbate. The rate determining step is step 3 whic h is the diffusion through the pores of the activated carbon, which is largely influenced by the pore size and the size of the diffusing molecule. 2.1.2 Chemical Properties of AC According to Coughlin and Ezra (1968) the structure of activated carbon is graphitic in nature with a la rge number of molecular layers The layers contain carbon

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8 atoms which are bound together by three bonds and one bond and result in sp2 hybridization. There is also the possibility of sp3 (tetrahedral) hybrid ization taking place resulting in the cross linking of the graph ite layers (Coughlin a nd Ezra, 1968; Hobbs, 2005) Van der Waals forces are responsible for holding together the carbon atoms within the graphite layers resulting in the microcry stalline structure. Other atoms that are bound in this structure may be present within the layers; known as basal planes, forming heterocyclic rings or at the edges of the carbon molecules, known as edge sites. These atoms form functional groups. Hence, as explained by Leon y Leon and Radovic (1994), the surface of activated carbon consists of two t ypes of sites: 1) basal planes and 2) edge sites. The edge sites are cons idered to be more reactive w ith oxygen groups than the basal planes due to the fact that edge sites c ontain a single unpaired el ectron (Radovic et al., 1997). Functional groups. Functional groups on the surface of activated carbons can be classified as acidic and basic. A number of functional groups have been found on the surface of activated carbon. These groups consist of carboxyl, lactonic, phenolic, carbonyl, quinones and pyrones am ong others (e.g., peroxide). Oxygen functional groups, despite being a small fraction of the overall carbon surface, are how ever very active and exhibit a large influence on adsorption capacity (Nevskaia et al., 1998; Nevskaia et al., 2004; Leon y Leon et al., 1994). The behavior of functional groups is governed by the variable electronegativ ity of the base atoms. Hence, atoms such as oxygen with higher electronegativity pull electrons towards its nucle us, resulting in a nega tive charge on that atom and a positive charge on the remaining at oms. This results in the groups becoming polar, hence affecting their in teractions with the adsorb ing molecules. Hence, bound

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9 oxygen functionality plays a crucia l role in adsorption behavior of the activated carbon towards the pollutant. The surface oxides may either be acidic or basic as explained earlier. Acidic surface oxides result in a positive surface charge due to protonation whereas basic surface oxides result in a negative surface charge due to deprotonation of the functional groups. The source of a positive surface is illu strated by the following equation: C + 2H2O CH3O+ + OHThe formation of acidic or basic surface oxides depends upon the temperature to which the carbon is exposed to during its activation process. Exposing the carbon to temperatures between 200-500C results in the formation of acidi c oxides (carboxylic, lactonic and phenolic). Basic surfaces on the ca rbon are formed when it is heated in an inert environment to re move initial oxides and cooled pr ior to exposing it to oxygen. It has also been found that the catalytic pr operties of ACs can be enhanced by the elimination of some of the acidic functiona l groups and introducti on of basic functional groups on the carbon surface (Salame et al., 2003; Tessmer et al., 1997). 2.1.3 Adsorption Mechanisms The topic of adsorption mechanisms seen in activated carbon has been one of much debate. There are two schools of thought the bonding theory proposed by Coughlin and Ezra (1968) and the electron donor acceptor theory (EDA) proposed by Mattson (1969). The two theories have been explained as follows: bonding theory. This theory proposes that the bond of the aromatic ring of carbon covalently bonds with the aromatic ring of the adsorbate or in other words that bonds occur between two p-orbitals. The pres ence of electron withdrawing groups such as oxygen on the carbon surface results in a reduction in the electron density in the

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10 system of the carbon basal plane. Hence, in theory the presence of electron withdrawing groups should weaken the dispersive ad sorption forces between the adsorbate electron and the carbon basal planes by creating pos itive holes in the conduction band of the electron system (Coughlin and Ez ra, 1968; Radovic et al., 2001). Adsorption at the basal planes is hence due to interactions and is typically weaker whereas adsorption at the edge sites is typically str onger (Coughlin and Ezra, 1968). In fact species bonded to the edge sites may cause the disturbances in the electron density of the basal plane, resulting in decreased interactions a nd reduced adsorption of organi c compounds. The presence of oxygen groups or electron withdrawing groups in general is hence thought to be one of the primary reasons for reduced adsorption of organics by activated carbon. It should be noted that the opposite is also true, where the removal of electr on withdrawing groups from the surface of ACs results in an increase in adsorption potential. Electron donor theory. The electron donor theory was proposed by Mattson et al. (1969) and refuted Coughlin and Ezra (1968) by suggesting that an ex change of electrons takes place during the adsorption process in which the carbonyl oxygen serves as the donor and the aromatic ring serves as the accep tor. Mattson et al. ( 1969) suggest that the carbonyl group serves as the major electron donor in the donor-acceptor complex and is hence responsible for an increase in phenol adsorption. The subse quent decrease in adsorption has been credited to carboxyl functional group formation (Mattson et al., 1969). Subsequent findings by numerous research groups has shown that an increase in oxygen functional groups results in the increased formation of both carboxyl as well as carbonyl groups, hence refuting the findings of Mattson et al (1969).

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11 Adsorption of phenol. Phenol adsorption is dependent mainly on the porosity and the surface chemistry of the act ivated carbon. Other factors in its adsorption include the surface area, ash content of the AC and the c oncentration of carbon atoms in the matrix. The adsorption of aromatic compounds from solution has been studied extensively (Radovic et al., 1997; Mattson et al., 1969; Coughlin and Ezra, 1968; Coughlin et al., 1968; Moreno-Castilla et al., 1995; Nevskaia et al., 2004). It has been found by Leng and Pinto (Leng and Pinto., 1997: as reported by Salame et al., 2003) that the uptake of phenol is due to a combination of the inte raction of phenol with the basal planes (physisorption) as well as by surface polymer ization. It has also been found that the phenol uptake increases as the carboxylic functional groups are removed from the carbon surface. The presence of carboxylic func tional groups (and hence oxygen groups) which are primarily formed during activation, resu lts in the AC surface becoming more polar resulting in water behaving as a competing speci es for the adsorption si tes. In addition to the decreased polarity of the AC surface, the removal of carboxylic functional groups also enhances interactions. Furthermore, activated carbons, under oxic conditions are prevented from adsorbing phenol in the pr esence of functional gr oups (Salame et al., 2003; Nevskaia et al., 1998, Nevskaia et al., 2004) This is due to the fact that the ability for the adsorption of phenol vi a oxidative coupling decreases as a result of the functional groups. Therefore, the catalytic properties of AC can be e nhanced by the introduction of basic functional groups and the elimination of some of its acidic groups. Hence, at low pH, the amount of phenol adsorbed increases slightly with an in crease in pH until a certain point after which a decrease in uptake is noticed.

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12 The pH of the phenol solution is not belie ved to play a signifi cant role in its adsorption onto activated carbon. Phenol is cons idered to be a weak acid. The loss of a hydrogen ion results in the formation of a phe nolate anion, the negati ve charge of which is delocalized around the ring. The negative charge gets spread out ove r the entire ion due to the overlapping of one of the lone pairs of electrons on the oxygen with the delocalized electrons on the benzene ring. Hence, the phenolate ion becomes relatively more stable. However, since oxygen is the most electronega tive element on the anion, the delocalized electrons will be drawn towards it which re sults in the hydrogen ion being attracted towards it again rendering solu tion pH a less important factor in the adsorption of phenol. 2.2 Photocatalysis The phenomenon of photocatalysis was di scovered by Fujishima and Honda in 1972 (Linsebigler et al., 1995). Sinc e then it has been extensively studied as an option for the treatment of contaminants in water. Phot ocatalysis results in the mineralization of organics to CO2, water and the corresponding mineral acids as the final products. Photocatalytic treatment of water requir es the use of the titanium dioxide (TiO2) photocatalyst in the form of a slurry or the immobilization of the titanium dioxide on a substrate. The use of a slurry is beneficial due to the large surface area of the catalyst which would be available for the photocatalys is reaction (Byrne et al., 1997). However, due to the size of particles used in the slurry, this method of treatment requires centrifugation or microfiltration techniques to re move the fine particles from the treated liquid, resulting in technical a nd economical problems (Legri ni et al., 1993). The use of titania as a slurry also results in an imp edence to the penetrati on of light through the solution due to increased opacity caused by the TiO2 particles. This however only takes place after a certain optimum titanium dioxide loading which is system dependent.

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13 Alternatively, the separa tion procedure problem can be overcome by immobilizing the TiO2 on a substrate (Dijkstra et al., 2001; Ollis et al ., 1991; Ray et al., 1997). A number of techniques have been used such as immobilization on beads, glass pearls (Trillas et al., 1996) inside tubes of glass or Teflon (D ijkstra et al., 2001) fiberglass or woven fibers, silica-titania composites and immobilization on to activated carbon (Dijkstra et al., 2001). This procedure has ha d varying results depending on the substrate onto which the TiO2 has been immobilized. Historic ally, it has been found that a suspended slurry is more efficient due to the absence of mass transfer limitations and larger specific surface area av ailable for photocatalysis (Dijk stra et al., 2001). However, it has also been shown that a TiO2-activated carbon composite particle has resulted in a synergy by which the degradation of a dye (methylene blue) was increased (Khan, 2003; Khan et al., 2006). 2.2.1 Titanium Dioxide Titanium dioxide is the most commonly used photocatalyst due to its properties of being relatively inert, corrosion resist ant and low cost as compared to other photocatalysts such as ZnS, WO3 and SrTiO3. Along with ZnO, it also shows the most photocatalytic activity as compared to othe r photocatalysts. It is found in three major forms: anatase (octahedral) (Figure 2-1 A), ru tile (tetragonal) (Figur e 2-1 B) and brookite (orthorhombic). Anatase and ru tile are the forms that show photocatalytic activity with anatase showing higher activity than rutile (O htani et al., 1993). The synthesis of anatase from rutile titania can usually be achieved by heat treatment at temperatures between 300C-600C. Pure grades of titanium dioxide are seldom found natura lly as it is often complexed with other minerals and metals. Recent studies have however shown that a combination of anatase (70%) and rutile (30%) are more activ e than pure anatase

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14 (Bacsa and Kiwi., 1998; Muggli et al. 2001). The grade of titanium dioxide used herein was commercially available, synthetically manufactured Degussa P25. Degussa P25 has a structure which is a combination of anatase (70%) and rutile (30%). It has repeatedly demonstrated good photocatalytic degradation. Figure 2-1. TiO2 structure. A) anatase, B) rutile 2.2.2 Mechanism of Photocatalysis When UV light energy is incident on TiO2, a valence band electron is excited to the conductance band; on the condition that the incident photons have a higher energy ( < 380 nm for anatase) than the band-gap ener gy. When this occurs, it results in the formation of a hole in the va lence band and hence an elec tron-hole (e-h) pair. A high quantum yield of the photocatalytic react ion can be achieved by the prevention or prolongation of the recombination of the elec tron-hole pair. This can be done with the help of an electron scavenger. An electron scavenger which may also be called a hole trap may consist of adso rbed hydroxide ions (OH-), oxygen or water molecules which react with the electron-hole to create a hydroxyl radical (OH*). An illustration of the mechanism of photocatalysis is shown in Fi gure 2-2 (adapted from Linsebigler et al., A B

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15 1995). The presence of water or hydroxide i ons is believed to be essential for the complete oxidation of organic molecules to CO2 and H2O since their absence has resulted in incomplete oxidation of organic mol ecules (Turchi and Ollis., 1990). Dissolved oxygen is believed to play an important role as an electron acceptor hence preventing the electron-hole pair from recombining. In the absence of an electron acceptor, recombination may take place in picoseconds, resulting in no photocatalytic degradation. Studies performed (Chhor et al., 2004; Dijkstra et al., 2001) have shown higher efficiencies in photocat alytic activity for phenol, salicyli c acid and formic acid with an increase in the presence of dissolv ed oxygen. Semiconductors such as TiO2 however have an advantage in that their recombination times are slower than those for conductors (metals) due to the absence of a continuum of interband st ates which would aid in the recombination of the electron-hole pair (Car p et al., 2004). This ensures that the electronhole pair has a sufficiently long time to diffuse to the surface of the ca talyst and initiate the redox reaction (Carp et al., 2004). Figure 2-2. Photocatalysis mechanism O2 OH.

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16 The following four cases of hydroxyl attack are typically considered which are also illustrated in Table 2-1 (adapted from Turchi and Ollis, 1990). Reaction occurs when the radical and the pollutant are adsorbed. The radical is not bound and it reacts w ith the organic pollutant molecule. The adsorbed radical reacts with a free organic molecule moving past the surface. Both the radical as well as the organi c molecule are unbound and they react with each other in the fluid phase. Table 2-1. Photocatalysis reaction mechanisms Excitation TiO2 + hv e+ h+ Recombination e+ h+ heat Adsorption O2 + TiIV OHTiIV + H2O TiIV H2O Trapping TiIV OH+ h+ TiIV OH* TiIV H2O + h+ TiIV OH* + H+ Hydroxyl attack Case1 TiIV OH* + R1,ads TiIV + R2,ads Case 2 OH* + R1,ads R2,ads Case 3 TiIV OH* + R1 TiIV + R2 Case 4 OH* + R1 R2 2.2.3 Improvements in Photocatalysis A number of techniques ha ve been attempted to enhance the efficiency of photocatalytic systems. One of thes e techniques is the doping of TiO2 particles with metals. Doping of the semiconductor with variou s transition metal ions has in the past led to an enhanced efficiency of photocatalytic systems. TiO2 particles can be substituted or interstitially doped with diffe rent cations or can form mixed oxides. However, this technique also poses a complex problem. The net induced alteration of the photocatalytic activity is made up from the sum of the changes which occurs due to the following factors.

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17 The light absorption capability of the TiO2 photocatalyst Adsorption capacity of the s ubstrate (activated carbon) molecules at the catalyst surface Interfacial charge transfer rate As the concentration of the dopant in creases, the space charge region becomes narrower resulting in the efficient separation of the electron-hole pair s within the region by the dielectric field before recombina tion. An increase in dopant concentration however, results in the narrowing of the sp ace charge region. When this occurs, the penetration of light into the TiO2 greatly exceeds the space charge layer. Due to the absence of a sufficient driving force ( dopant), to separate them, electron-hole recombinations increase due to the absence of a driving force to separate them. Hence, an optimum concentration of dopant ions is requ ired which would make the thickness of the space charge layer similar in magnitude to th e light penetration dept h. (Carp et al., 2004; Hoffman et al., 1995; Linsebigler et al., 1995). The recombination of photogenerated electrons and holes are often times influenced by doping ions. In most cases th e recombination times are enhanced by the doping ions, hence impeding the progress of th e reaction. For example, in both p-type doping (cations having valencie s lower than that of Ti+4 such as Al+3, Cr+3 and Ga+3) and n-type doping (cations havi ng valencies higher than Ti+4 such as Nb+5, Ta+5 and Sb+5) an inhibition effect which is ascribed to the reco mbination of the electrons and holes is seen. P-type dopants act as acceptor centers for photoelectrons wh ereas n-type dopants act as donor centers of photoelectrons (Carp et al., 2004; Hoffman et al., 1995; Linsebigler et al., 1995; Matos et al., 1998). Metallic ions such as Fe3+ and Ru3+ however behave differently than P-type and N-type dopants. This is due to their half-filled electronic

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18 configuration which results in them being more stable. This electronic configuration is destroyed when metallic ions are trapped, leading to a decrease in their stability. Moreover, a transfer of the trapped electrons to oxygen adsorbed to the surface of the catalyst may take place with the metallic ions returning to the original stable half filled electron structure resulti ng in the transfer of charge a nd efficient separation of the electrons and holes by trapped electrons. Hen ce, co-doping may be a viable technique to improve charge separation, thereby improving phot ocatalytic efficiency. In fact, studies by Leng et al. (2002) have shown a synergis tic effect resultin g in the increased photocatalytic degradation of chloroform in so lution to as much as 5 times the original rate when TiO2 has been doped with Fe+3 and Eu+3. In this case, Fe+3 serves as a hole trap and Eu+3 serves as an electron trap (Carp et al., 2004) It can therefore be summarized that the process of photocatalytic oxidation through a semiconductor such as titanium dioxide can be modified and controlled to a certain extent depending on the type of metals, the extent to which they are doped on the titania and the combinations of metals used in the doping process. 2.3 Fermi Energies/Levels, Electrical Conductivity and Schottky Barrier Fermi level is a term used to describe th e top of the sea of electrons (electron energy levels) at a temperature of absolute ze ro. At absolute zero, electrons pack into the lowest available energy states to form this co llection of energy states. Hence, at absolute zero, no electron will have enough energy to ri se above the surface of the Fermi sea. Ordinary electrical processes involve energies in the range of a fraction of an electron volt. However, the Fermi energies in the case of metals are on the order of electron volts, implying that most of these electrons ca nnot receive sufficient energy from those processes.

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19 The Fermi energy is also important in char acterizing the electric al conductivity of metals. The velocities of the electrons participating in c onduction can be calculated from the Fermi energy hence providing us with the dr ift velocities of electrons encountered in the metals. Also crucial to the conduction pro cess is the Band theory of solids which is useful in visualizing the difference between conductors, semiconductors and insulators. The primary requirement for the conduction process is the presen ce of electrons in the conduction band. In insulators, electrons in the valence band are separated from the conduction band by a very large gap whereas in conductors, the valence band and the conduction band overlap. The case of semiconduc tors (such as titanium dioxide) is unique in that the intrinsic Fermi level is half way between the bottom of the conduction band and the top of the valence band. Theref ore, even a small amount of doping of the semiconductor can result in a large increase in conductivity. Hence, as the Fermi level for semiconductors approaches the conduction band, it denotes an n-type whereas when the Fermi level is closer to the valence band, it denotes a p-type semiconductor. Figure 2-3 and Figure 2-4 illustrate the concept of Fe rmi levels of metals, semi-conductors and insulators along with the associated band gap. Figure 2-3. Band gaps for (a) metals, (b) semi conductors and (c) insulators with Fermi level ( F) indicated

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20 The effect of metals added to semiconductors such as TiO2 is as follows. The dopant metals change the photocatalytic prope rties of the semiconductor by changing the distribution of electrons. Hence, when a metal and a semiconductor are electrically connected, electron migration takes place from the semiconductor to the metal until the two Fermi levels are aligned as shown in Figure 2-4 (Linsebigler et al., 1995; Carp et al., 2004; Hoffman et al., 1995) (figure adapted from Linsebigler et al., 1995). This phenomenon forms what is known as the Scho ttky barrier. It must be noted that the particular case mentioned above ta kes place when the work function ( ) of the metal is higher than that of the semiconductor. The wo rk function is the least amount of energy required to remove an electron from the surf ace of a conducting material to a point just outside the metal. Figure 2-4. Alignment of Fermi levels and formation of Schottky barrier The Schottky barrier produced at the in terface of the metal and the semiconductor acts as an electron trap, he nce hindering electron-hole recombination which results in

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21 increased photocatalysis (Lin sebigler et al., 1995; Carp et al., 2004). This scenario is what will be referred to as a favorable differe nce in Fermi energies during the progress of this paper. 2.4 Previous Synergy in AC-TiO2 Systems Activated carbon is a well known adsorbent especially for systems dealing with organics. It is also well known that TiO2 is capable of oxidizing organics to water and carbon dioxide. Hence, it can be hypothesized that a combination of these two materials would result in a combination of adsorption a nd degradation which in theory would result in a synergy. This would effectively mean that the pollutant is adsorbed onto the activated carbon and is then degraded due to the TiO2 photocatalyst in the presence of UV radiation resulting in the theore tically infinite life of AC. However, a number of variables are invol ved in the process which may limit the effect of photocatalysis on the adsorbed pollu tant. Properties of the AC such as pore size distribution, surface area, surface functional groups and the acid ic-basic charac teristics of the AC may make this theory extremely system specific when applied Considerable research has been conducted in the field of TiO2-AC composites (Matos et al., 1998; Matos et al., 2001; Khan., 2003; Khan et al., 2006). Matos et al. (1998) and Khan et al. (2006) both found that titania seems to show a synergistic effect with activat ed carbon. To elaborate, the presence of activated carbon alone in a system for the removal of a pollutant has been shown to be less effective than a combination of the activated carbon and titania in the pr esence of UV. Similarly, the presence of the titania and AC (in the pr esence of UV) in systems has shown better performance than a system consisting solely of titania.

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22 A system tested by Matos et al. ( 1998) consisted of 50 mg of TiO2 combined with 10 mg of AC in the form of a slurry with the target polluta nt being phenol. This system was compared to other systems consisting solely of either titania or activated carbon. The experiments were conducted such that the systems were not exposed to UV radiation until equilibrium had been established with respect to adsorption. Once equilibrium was established, the systems were exposed to UV radiation (340 nm). The following figure (Figure 2-5, adapted fr om Matos et al., 1998) illustrates the results of the kinetics of phenol disappearance under the various systems tested. Figure 2-5. Kinetics of phenol di sappearance in the presence and absence (photolysis) of various illuminated solids

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23 The results of Matos et al. (1998) show that the appare nt rate constant for the system consisting solely of titania is 5.6 x 10-3/min whereas the apparent rate constant for the TiO2 AC system is 1.39 x 10-2/min. Hence it has been concluded that the photocatalytic efficiency seen in the case of the titania-AC sl urry is 2.5 times that seen by the system with titania alone. Th e authors ascribe this result to the adsorption of phenol to the AC which is followed by a mass transfer to the photoactive tita nia. In effect, the essence of the synergy in the case of the titania-AC system is the common interface between the AC and TiO2 which has been postulated to cons ist of almost half of the total surface of exposed titania. However, the rate constants would be cal culated from a point at which adsorption of phenol has taken place and the systems have reached equilibrium. In the case that the rate constants of the two syst ems are calculated solely from the time of irradiation, it may be noticed that the AC-titania system actua lly has a slower rate than the system containing only titania. Hence, although a benefi cial effect is seen to be taking place with the addition of AC, it does not agree with the definition of synergy as proposed by this author. From the arguments made above, it is cl ear that a synergy was not prevalent in the titania-AC system tested by Matos et al. (1998). A subsequent paper by the same authors (M atos et al., 2001) inve stigated the role of the type of AC on the photo catalytic degradation of organi c pollutants. Similar tests to the work described previously have been performed although emphasis has been given to the surface properties of the AC and their in fluence on photocatalysis. It was concluded by Matos et al. (2001) that H-type ACs (High pH/ High temperature activated) when added to a titania slurry show a beneficial trend in photocatalysis while the opposite is

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24 true for L-type ACs (Low pH/Low temper ature activated). Alt hough the terms H-type and L-type carbons can only be used loos ely, the veracity and extent to which the conclusion by Matos et al. (2001) may apply has been investigated by this author. Another case of the synergistic effect of titania with AC has been investigated by Khan (2003). Their study was performed by coating TiO2 (Degussa P25) on various commercially available ACs. The coated AC s were then used in the adsorption/ degradation of dyes (i.e., met hylene blue, reactive red). The results seen by Khan (2003) showed that the activated carbons F400 (Calgon), HD4000 (NORIT) and a wood-based activated carbon synthesized in their laboratory showed an increased removal of the dye in the presence of UV as opposed to in the absence of it. On the other hand, Bionuchar (Westvaco) showed the opposite tr end. It was concluded that th e presence of ionic metals such as iron (Fe++) in the coal based carbons (F400, HD4000) resulted in the synergy. This was based on the hypothesis that the ionic metals presen t in the coal based carbons act as sinks for electrons, thereby hinder ing electron hole recombination and hence enhancing photocatalysis. Iron in the form of Fe++ is commonly used in the doping of TiO2 to enhance photocatalysis. Iron is also a good conductor and would hence contribute to the electrical conductivity of the material in which it is pr esent. Hence, from the theory of Fermi levels and Schottky barrier explained earlier, it is believed that this hypothesis by Khan (2003) could hold true in the case of dye s. It is a topic of interest to see whether the synergy noticed in the case of dyes also holds true in the case of organic compounds such as phenol.

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25 The work by Khan (2003) is the basis fo r the hypothesis formed by this author which attributes the co nductivity of activated carbons (due primarily to metals) as an influence over the photocatalys is observed with the TiO2-AC composite.

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26 CHAPTER 3 MATERIALS AND METHODS 3.1. Target Pollutant The target pollutant, phenol (C6H6O), was selected because it is representative of pollutants experienced in water treatment. It was obtained in the li quid form (90 % w/w) from Fisher Scientific. The stock solu tion was diluted to obtain the required concentration (average concentration ~ 55 mg /L 2 mg/L). The molecular weight of phenol is 94.11. The pH of phenol solution is 5-6. 3.2 Carbon Materials 3.2.1 TiO2-AC Composites and Coating Procedures Each AC differed in terms of physical and chemical properties due to differences in precursor and activation technique. A summary of the properties of the ACs is provided in Table 3-1. Table 3-1. Summary of AC properties Activated carbon Precursor Activation type BET surface area (m2/gm) Micro pores (%) Meso pores (%) Bionuchar Wood Chemical 1509 (+191, 270) 32 58 F400 Bituminous coal Thermal 1000 (+134, -200) 60 32 HD4000 Lignite coal Thermal 539 (+80, 114) 25 66 The Bionuchar which was chemically activat ed is an AC supersaturated with carboxylic and phosphoric groups and hence has a very acidic surface. The F400 is a more basic AC whereas the HD4000, although it has been thermally activated has an acidic surface due to an acid wash after its activation procedure.

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27 The activated carbons were standardized to a 20 x 35 mesh using American Standard Sieves (Fisher Scient ific) and a Ro-Tap sieve shaker (Fisher Scientific). They were then subjected to a number of coati ng procedures which were assessed based on simplicity and efficiency. Particle synthesis. In order to synthesize the TiO2-carbon particles, four coating techniques were considered. Some of these coating techniques were only applicable to activated carbons, versus the low surface area ra w materials, due to its porous nature. The coating techniques that were considered are as follows: Boil deposition Pore Volume Impregnation (PVI) Chemical Attachment (Modified Sol-Gel technique) Mechanical Attachment (Theta Composer) It was found during the course of experime nts that a 3-5% (m/m) coating provided efficient and optimum activity for photocatal ysis. The techniques were assessed based on simplicity and efficiency as the ultimate engineering goal is the development of a portable RAC filter. Boil deposition. The boil deposition procedure is the most convenient method of coating a substrate with TiO2. It is based on the mechanis m of agitation of a mixture during boiling. The theory implies that the TiO2 would be deposited on the carbon surface due to the agitation of water, TiO2 and carbon while boiling, resulting in the impaction and hence deposition of TiO2 on the carbon/AC surface in th e presence of heat. Although this procedure is not very consistent, it result s in convenient coating of the particles and is efficient in terms of performance of carbons or activated carbons. The procedure was adapted from the t echnique used by Khan (2003). In this procedure, the carbon/activated carbon sample to be coated was first washed with

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28 deionized water. The sample was added to a 250 ml Erlenmeyer fl ask and a mass of TiO2 was also added to the flask corresponding to the required extent of coating. Deionized water (200-220 ml) was added to the mixture. The flask was then placed on a hot plate and heated to 120 C and allowed to boil. On ce all the water had evaporated, the sample was once again rinsed with DI water and was le ft to dry in a furnace maintained at 120C to remove any residual moisture. Pore Volume Impregnation (PVI). Impregnation is a commonly used method in supported catalyst preparation. The impregna tion method involves three steps: 1) Contacting the support with the impregnating so lution for a certain period of time, 2) Drying the support to remove the impregnate d/absorbed liquid and 3) Activating the catalyst by calcinations or reduction (Tao, 2003). The principle of impregnation involves using a volume of the precursor solution such th at it is equivalent to the pore volume of the support. Hence, it is evident that this technique can only be successful for activated carbons or substrates with a well established pore structure. A weighed quantity of AC was taken in a 40 ml vial. The total pore volume of that quantity of AC was determined. The precurs or (titanium isopropoxide (TTIP), ACROS) and isopropanol (Fisher Scientif ic) were then mixed with the AC sample such that the volume of the solution was equal to the total pore volume of the AC sample. Hence, for a 3% mass loading of titanium dioxide on the AC, the solution of TTIP and isopropanol would be prepared such that the TTIP was 3% of the solution (v/v). The solution of TTIP and isopropanol was introduced into the 40 ml vial containi ng the AC. The contents of the vial were agitated for a few minutes. Th e vial was then left open under the fume hood for 24 hours to allow the TTIP-isopropanol solu tion to be impregnated in the AC. After

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29 24 hours of drying, the activated carbon wa s heat treated in a furnace at 300 C to convert the titanium dioxide to the anatase phase. The performance of this coating procedure was questioned due to the fact that the titanium dioxide is impregnated predominantly in the pores of the AC. This was believed to hinde r the incidence of UV light on the titanium dioxide given the random structure of the pores as well as the opacity of activated carbon. Also, as stated by Matos et al (1998), the presence of TiO2 in the pores as opposed to at the surface of AC would mean the absence of a concentration gradient forming, resulting in hindered photocatalytic activ ity. The results from the tests performed with ACs coated with TiO2 by the PVI technique seemed to agree with this argument. Chemical attachment (mod ified sol-gel technique). The chemical attachment coating procedure makes use of a sol gel to coat the activated carbon surface. This technique was adapted from the technique em ployed to coat barium ferrite particles. (Drwiega, 2004). Four gr ams of AC (HD4000, 20 x 35) and some volume of polyethyleneimine (PEI (Alfa Aesar)) and 80 mL of nanopure were combined and mixed for 10 minutes until the entire PEI was disso lved. 250 mL of nanopure was added to a 3mouth flask. The carbon particles were then introduced into the 3-mouth flask. The glassware consisting of th e reflux tube, 3-mouth flask, gas adapter, funnel and thermometer were assembled. The nitrogen and water flow were then turned on for a few minutes. Five milliliters of isopropanol was a dded to the closed funnel after which 100 l of titanium isopropoxide (TTIP) was added to the closed funnel. The particles were slowly stirred with a magnetic stirrer. The so lution in the funnel was slowly introduced into the water. The mixing was continued for 10 minutes and the solution with the particles was heated to 95 C. This was continued for 20-24 hours. The particles were

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30 then removed and rinsed thoroughly with deio nized water. The AC particles were then heated in an oven at 110 C for 1 hour to dr y the particles. The pa rticles then underwent heat treatment at 350-400 C in presence of nitrogen fl ow for 3 hours to convert the titanium dioxide to the anatase phase. This coating technique perf ormed comparably well to the boil deposition. However, the main di sadvantage of this technique was that it required a complex setup of equipment. Since one of the ultimate goals of the project is the development of a household filter, it was no t selected as the coating technique for the experiments performed. Mechanical attachment. Mechanical attachment consists of forcing the carbon and TiO2 through a 1 mm gap at the same instant, hence implanting the TiO2 onto the carbon surface due to the high forces on the particles. The techni que, also known as mechanofusion consists of equipment known as a Theta-Composer (Tokuju Inc, Japan). The Theta-Composer basically consists of a ro tor and a vessel set in a way such that they are concentric and rotate in opposite directions (rotor speed: 2500 rpm; vessel speed: 77 rpm). This is shown in Figure 3-1 (Adapted from Khan, 2003). The AC particles (Sieved to 20 x 35 mesh) and 3% TiO2 (m/m) was added to the vessel and the theta-composer was allowed to run for 15 minutes. The disadvantage of this techni que was the separation of TiO2 from the carbon particles when introduced in a liquid medium as well as the powdering of the AC due to the natural mechanical motion and abrasion of the thetacomposer. After selection of the boil deposition procedure as the preferred coating technique, the carbons were coated on two bases: 3% TiO2 on the basis of mass of carbon. 3% TiO2 on the basis of moles of elemental carbon in the AC.

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31 Figure 3-1. Coating activated carbon with TiO2 via mechanical attachment (Theta Composer, Tokuju Corp., Japan) 3.2.2 Mass Based Coating (m/m) The activated carbons we re coated with TiO2 in the ratio of 3:100 (weight of TiO2/weight of AC). The coating percentage was decided upon using evidence from previous experiments which showed an op timal coating percentage range of 3%-5% (mass/mass). For coating the ACs on a mass ba sis, a certain mass of AC (1-2 gms) was mixed with TiO2 (3% of that mass). The boil depositi on procedure was then used to coat the TiO2 onto the ACs. After coating, the carbons were loaded to their reactors on the basis of mass and in other experiments on th e basis of volume. They may be denoted in the future as mass-mass and mass-volume respectively. 3.2.3 Molar Based Coating (mol/mol) For coating the carbons with TiO2 on a molar basis, the amount of elemental carbon (C) in each sample was found by performi ng an ash analysis in which the ACs were heated to approximately 575 C. The difference between the initial masses of the ACs and their final masses (of the ash) provi ded the amount of elemental carbon in each sample. The following table (Table 3-2) pr ovides a summary of the elemental carbon

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32 contents of the ACs followed by a sample calculation of the TiO2-elemental Carbon ratios used in the composites. Table 3-2. Carbon contents of virgin and ti tania coated ((m/m) and (mol/mol)) activated carbons and their surface areas Activated carbon Carbon content (%) Surface area (m/m) (m2/gm) Surface area (mol/mol) (m2/gmm3) Bionuchar 92 98 1483 1380 F400 86 94 1034 1009 HD4000 82 88 602 624 A sample calculation is provided. Activated carbon A contains X mg Carbon/ gm AC For 3% (mol/mol) loading: MolesTiO2 / MolesCarbon = 3% (m/M)TiO2 / (m/M)C = 3/100 (m/79.86)TiO2 / (X/12)C = 0.03 mTiO2 / XCarbon = 0.19965 mTiO2 = Mass of TiO2 to be added mC = Mass of Carbon (Elemental Carbon) MTiO2 = Molecular weight of TiO2 = 79.86 MC = Molecular weight of Carbon = 12 The TiO2 and AC mixture then underwent boil deposition, standardizing the Carbon-TiO2 ratio in all the carbons. The carbons after coating were loaded to their reactors on the basis of volume. They may be denoted in the future as mole-volume. It must be noted that the tests performed with the coated ACs were performed using TiO2-AC composites prepared in different ba tches. Hence the replicate sets of experiments are indicative of the performan ces of different batches of composite as opposed to composites prepared in the same ba tch. In this regard, the consistency of the batches with respect to performance of th e composites will also be observed. The TiO2low surface area carbon composites (explained in section 3.2.4) were also prepared and tested similarly.

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33 3.2.4 Low Surface Area Carbon-TiO2 Composite The low surface area carbons used herein c onsisted of Pitch Coke (Asbury Graphite Mills, Asbury, NJ), Graphite (Asbury Graphite Mills, Asbury, NJ), Anthracite Coal (Reading Anthracite Coal Company, Maha ntongo St., Pottsville, PA), and Bituminous Coal (International Industri es, Logan, WV). The titanium dioxide was commercially available Degussa P25 (70% Anatase, 30% Rutile). The virgin carbons used in these studies had extremely low surface areas as s hown in Table 3-3. In fact, the surface areas of some of the samples were below the mini mum value that was detectable. The values are still provided although they may contain some error. Table 3-3. Surface areas of virgin an d coated low surface area carbons Surface area (m2/gm) Carbon Virgin carbons 3% TiO2 (mass/mass coating) 3% TiO2 (mole/mole of C) Pitch coke 0.62 4.21 8.15 Anthracite 0.92 4.1 7.51 Graphite 1.35 3.63 7.99 Bituminous 0.5 1.00 3.34 Although in both cases (i.e., (m/m) and (mol/mol)) there was a considerable percentage increase in the surface areas of the coated carbons as compared to the virgin carbons, it was still extremely low for any si gnificant sorption phenomena to occur. We also analyzed the amount of elemental carbon in each sample (as for the ACs) in order to coat the car bons on the basis of mole/mole which is provided in Table 3-4. The coating technique was exactly the same as that used to coat the ACs. Table 3-4. Carbon contents of low surface area carbons Low Surface Area CarbonCarbon Content (%) Pitch Coke 93 Graphite 99 Anthracite 90 Bituminous 65-72

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34 3.3 Experimental Setup 3.3.1 Batch Reactor Configuration Batch testing was performed initially to investigate how the carbons with and without TiO2 performed for phenol removal. The se tup for the batch tests consisted of a rotator (Analytical Research Systems, Ga inesville, FL) onto which were mounted 4 reactors. The reactors consis ted of 100 mL glass syringes (SGE; Obtained from Fischer Scientific). The syringes were fitted with gastight plunge rs, the ends of which were made of Teflon. The syringes were also mount ed horizontally on the rotator such that they were parallel to the axis of the rotator. The entire system rotate d at a rate of 3 rpm. Three UV lamps (365 nm wavelength) were placed such that they were 120 apart and at a distance of 0.5 cm from the outer wall of th e syringes at the nearest point. The entire assembly was enclosed in a wooden box, the wa lls of which were covered with reflective aluminum foil so that any stra y radiation could be reflected back towards the reactors. 3.3.2 Analytical Equipment The concentration of phenol solution was determined on a UV-VIS spectrophotometer (HACH DR/4000U). The BET surface area analyses for the activated and low surface area carbons as well as their composites were performed using a Quantachrome NOVA 2200e Gas Sorption Anal yzer (Boynton Beach, Florida). Samples were outgassed for 24 hours prior to testing. Be fore outgassing, the samples were dried at 110 C. The SEM images and the EDS scans were performed using a scanning electron microscope (SEM JEOL JSM 6400). The images and scans were performed at the major analytical instrumentation cente r (MAIC, Department of Materials Science, University of Florida, Gainesville). Multiple images and scans of the same sample as well as different particles of the same sample were take n. Density measurements of the Carbon-TiO2 and

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35 the AC-TiO2 composites were performed on an Ultrapychnometer (Quantachrome). Replicate sets of measurements were taken. Mu ltiple samples from within batches as well as across different batched were taken. The valu e provided is the average of all the values with the maximum and minimum values also provided. 3.4 Experimental Procedure 3.4.1 Concentration Measurements The concentration of the phenol solution was measured at the peak absorbance wavelength = 270nm for phenol which was determ ined by performing an absorbance scan of the phenol solution. The concentr ation vs. absorbance profile for a phenol solution measured at 270 nm on the UVVIS spectrophotometer is shown in Figure 3-1 and obeys Beers law. The R2 for the profile was found to be 0.97. y = 62.445x R2 = 0.970 10 20 30 40 50 60 70 00.20.40.60.81AbsorbanceConcentration (ppm) Concentration vs Absorbance Profile for Phenol Linear (Concentration vs Absorbance Profile for Figure 3-2. Concentration vs. Abso rbance correlation for phenol on UV spectrophotometer measured at 270nm wavelength

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36 3.4.2 Batch Adsorption/Oxidation Te sts with Activated Carbon The synergistic phenomena observed in certain TiO2-AC composites were attempted to be determined for phenol by performing batch tests with the various composites. The three activat ed carbons selected Bionuchar, F400 and HD4000 were tested initially in a batch system. The test s were performed usi ng granular activated carbon (GAC) for the convenience of separation and to simulate a real world system. Experiments were performed with ACs coated with TiO2 on the basis of mass as well as on a molar basis, which has been explained earlier. The ACs coated by mass were loaded into the reactors on two bases: 1) mass basis and 2) volume basis. The loading of reactors on the basis of mass was done such that 50 mg of AC was loaded for 80 mL of phenol solution whose concentration wa s 55 mg/L. The volume loadi ng of the composites in the reactors was done such that 0.025 mL of AC was used for 80 mL of phenol solution whose concentration was 55 mg/L. The ACs coat ed on a molar basis were loaded in the reactors only on the basis of volume. The volume based loading of reactors with these composites was done such that 0.025 mL of AC was used for 80 mL of phenol solution with a concentration of 55 mg/L. The proce dure of coating the AC s by molar ratios and loading the composites on a volume basis was done to normalize the TiO2-carbon ratio for each sample since the ACs were of varyi ng densities, resulting in different quantities of AC (and hence TiO2) in each reactor. The parameters such as initial concentr ation of phenol solu tion, volume of phenol solution used in each reactor and the time for each run remained constant for the adsorption-oxidation studies for both types of AC loading. Both experiments were carried out for a period of 24 hours. For adsorption studies, experiments were performed in the absence of UV radiation. At th e end of 24 hours, the GAC part icles were separated from

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37 the phenol solution by filtration with 0.45 m filters (Fisher Scientific) using vacuum filtration. The final concentration of the phenol solution was measured using a UV-VIS spectrophotometer. The difference between the initial concentration of the stock solution and the final concentrations were attributed to adsorption. For the de gradation studies, the experiments took place in the presence of UV light. Again, the experiments were carried out for 24 hrs at the end of which the GAC particles were separated from the phenol solution by vacuumfiltration. The final concen tration of the separated phenol solution was measured using the spectrophotomete r. The difference between the initial concentration of the stock solution and the fi nal concentration of the phenol sample in this case was attributed to a combination of adsorption and photocat alytic degradation. In addition to adsorption and oxidation tests, tests were also pe rformed with virgin ACs in phenol solution in the presence of UV. Replicate sets were run for each sample and the error between the samples has been denoted as the maximum and minimum values that were recorded. The actual value plotted is the average of all the values obtained for that result. 3.4.3 Batch Adsorption/Oxidation Test s with Low Surface Area Carbons As explained earlier, the conf iguration for the batch tests involving the low surface area carbons was identical to that employed for the AC-TiO2 tests. The TiO2-carbon composites (35 x 200 sieve) were coated on the basis of mass as well as on the basis of mole. For the carbons coated on a mass basis, the composites were loaded into the reactors on two bases: by mass and by vol ume. For the mass loading, 80 mg of the carbons were loaded into the reactors wh ich contained 80 mL of 55 mg/L phenol solution. For the volume loading, 0.05 mL of the carbons were loaded into the reactors

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38 containing 80 mL of 55 mg/L solution. Th e volume loading was done in order to normalize to some extent the amount of car bon (and hence titania) in each reactor. For the carbons coated on a molar basis, the composites were loaded into the reactors only on a volume basis. Here again, 0. 05 mL of sample were loaded into reactors containing 80 mL of 55 mg/L phenol solu tion. This would in theory completely normalize the ratio of titania to carbon (elemental) in each reactor. The testing procedure for both cases was id entical to the one employed for the AC TiO2 composites as far as runtime and sample analysis procedures are concerned. Replicate sets were run for each sample and the error between the samples has been denoted as the maximum and minimum values that were recorded. The actual value plotted is the average of all the values obtained for that result.

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39 CHAPTER 4 RESULTS AND DISCUSSION 4.1 Evaluation of Carbon Coating Strategies Titanium dioxide was coated on the activ ated carbons using 4 different coating techniques: boil deposition, por e volume impregnation (PVI), chemical attachment, and mechanical attachment. Boil deposition was by far the easiest and mo st convenient coating technique as it required using a commercially available TiO2 (Degussa P25), water and heat. It was nevertheless a very inconsistent and unpredic table of the coating methods as the TiO2 deposited not only on the AC but also on the beaker in which it was being carried out. Therefore, although a known mass of titania was used in the coating process, the actual amount of TiO2 deposited on the carbon was likely le ss than the targeted value. The average phenol removal observed for HD4000 coated with titania was 78%. Pore volume impregnation resulted in the immobilization of the TiO2 within the pores of the AC. The titania depositing in the pores of the ACs was considered a disadvantage for this coating technique, part icularly due to the opacity of activated carbon and the random orientation of its pores It was hypothesized th at due to titania deposition within the pores it would result in a low probabil ity of the UV light reaching the titanium dioxide, resulting in a system with reduced photocatalytic activity. The results obtained seemed to agree with this hypothesis. The photocatalytic tests which were performed with HD4000 showed a removal of only 42%.

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40 Chemical attachment of the TiO2 to the carbon was also attempted to manufacture a more rigid composite. Although th is coating technique would provide a more consistent coating on the surface of the ACs, the perfor mance of the ACs coated by this technique did not show any marked improvement over the boil deposition technique. The sol-gel coated HD4000 resulted in 81% removal vi a adsorption and photocatalysis, which was comparable to the removal seen with the boil deposition method. Table 4-1. Summary of coating techniques Mechanical attachment was achieved th rough impaction using the Theta Composer instrument (Tokuju Inc., Japan) which resulted in the attrition of the granular activated carbon (GAC) particles, which orig inated as approximately 1 mm in size, to particles less than 45 m (Powdered Activated Carbon or PAC). The technique also resulted in separation of the TiO2 particles from the substrate upon introduction in water. The Coating technique PerformanceAdvantages Disadvantages Boil deposition 78% Simple procedure Good performance 1) Inconsistent coating 2) Poor control of TiO2 loading Pore volume impregnation (PVI) 62% Good coating control 1) Requires additional hydrolysis and calcination steps 2) Titania deposition in macropores (Tao, 2005) 3) Poor performance Chemical attachment 81% Good coating control Good performance 1) Requires additional hydrolysis step 2) Experimental setup is cumbersome Mechanical attachment --1) Attrition of AC particles 2) TiO2 separation from substrate

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41 advantages and the disadvantages of each tec hnique are summarized in Table 4-1. It must be noted that multiple batches of the TiO2-AC composites were tested. Based on these results, the boil deposition tec hnique was selected for coati ng the carbons w ith titania. 4.2 Activated Carbon Studies Khan et al. (2003) showed that F400 a nd HD4000 showed a synergy when coated with TiO2, whereas Bionuchar did not. The author s concluded that metals, naturally occurring in activated carbon may have been responsible for the observed synergy (i.e., removal of target pollutant increased when titania was coated on AC as compared to activated carbon alone). Nevertheless, they did not investigate this phenomenon in further detail. It was hence a viable exercise to investigate whether metals, in particular electrically conductive metals such as iron and aluminum, could be a plausible mechanism for creating th e described synergy. 4.2.1 Ash Analyses of Activated Carbons To determine the metallic content of Bionuchar, F400 and HD4000, the ACs used by Khan (2003), the uncoated (i.e., virgin) carbons were combusted in air at 575C until only an ash residue remained. The ash resi due of each AC was then analyzed by a scanning electron microscope (SEM) to pe rform an energy-dispersive spectroscopy (EDS) study. EDS was also used to verify the presence of titani a on the surface of the coated ACs. Note that the ACs of which the ash residues were examined were not coated with titania prior to ashing. Even though the EDS data canno t be used as a complete quantitative comparison, it is still useful as a semi-quantitative as well as a qualitative tool for characterizing elemental composition. Th e percentage of ash (by weight) in each sample is provided in Table 4-2. Also provide d in the table is the amount of titania (as a percentage) by weight actually coated on the AC samples. This was quantified simply as

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42 the difference between the weight of the ash re sidue of a virgin AC and that of a coated AC sample. The theoretical amount of titania which was to be coated on the surface of each of the ACs was 3% (i.e., weight of titania added equals 3% of the weight of the AC sample). Table 4-2 shows the inconsistency of the boil deposition procedure in coating the ACs. On an average it is seen from th e data provided in Tabl e 4-2 that Bionuchar has the lowest TiO2 deposited on its surface. This is at tributed to morphological differences between the wood based Bionuchar and the coal based F400 and HD4000. Table 4-2. Ash contents of ACs and mass % of titania deposited (m/m) on AC surface Activated carbon Precursor Carbon% (virgin ACs) Ash% (virgin ACs) TiO2 % Bionuchar Wood 92-98% 2-8% 0.5-1.3% F400 Bituminous86-94% 6-14% 1.6-2.2% HD4000 Lignite 82-88% 12-18% 1.4-2.3% The ash analyses of the ACs provided an estimate as to the magnitude of the composition of elements seen in the EDS scan s. Figures 4-1(A-F) show the EDS scans of the ash residues of the virgin ACs and the scans of the coated ACs. The EDS scan of the Bionuchar ash residue (F igure 4-1A) shows that it consists of primarily phosphorous, silicon and sodium. A ve ry faint trace of alum inum as well as a peak corresponding to iron is also observ ed. The phosphorous peak is likely remnant from the chemical activation technique using phosphoric acid. Figure 4-1B represents an ED S image of a Bionuchar sample coated with titania. It can be seen from the scan that smaller peaks corresponding to phosphorous, silicon and sodium are again observed. A titanium peak is also seen in addition to the peaks seen in the ash residue of the same AC. It is importa nt to note than the to tal ash content of the Bionuchar samples is a maximum of only 10% of the weight of the sample which is inclusive of the titania coated on the surface. Hence, coupled with the results from the

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43 EDS scan, it may be concluded that a very small amount of each of the elements is present in Bionuchar. Hence the low presence of conductive metals such as aluminum in Bionuchar (also observed by Khan (2003)) may resu lt in very few conductive sites on the surface of Bionuchar. The residual ash sample of F400 (Figure 4-1C) consisted of ve ry large peaks of aluminum and silicon. Smaller peaks pertai ning to iron, sodium, ma gnesium and calcium were also noticed. In comparison to the ED S scan performed on the ash of the Bionuchar sample which showed an aluminum peak of about 200 counts (in replic ate sets of scans), the F400 ash residue showed aluminum peaks having magnitudes between 3800 and 4000 counts in the replicate sets of scans prefor med. The presence of iron should also be noted. The activated carbon sample of TiO2-F400 (Figure 4-1D) sh ows the presence of aluminum, silicon, iron and titanium. What is interesting to note is the extremely high titania peak of about 5000 counts. It may be recalled that the corresponding peak in the Bionuchar sample did not have a magnitude of even 2000 counts. This suggests that the morphological difference between the wood based Bionuchar and the coal based F400 resulted in a difference in TiO2 coating. For instance, variations in roughness may cause the titania to adhere to AC surfaces differen tly. This would then affect the adsorption of phenol as well as photocatalytic mechanisms Another observation to be made is the higher ash content of F400 (6 -14%) as compared to Bionuchar. This indicates the presence of larger quantities of the elemen ts seen in the EDS scans of F400 relative to Bionuchar.

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44 Figure 4-1. EDS of A) ash residue of uncoa ted Bionuchar, B) Bionuchar coated with TiO2, C) ash residue of uncoated F400, D) F400 coated with TiO2, E) ash residue of uncoated HD4000, F) HD4000 coated with TiO2 A B C

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45 Figure 4-1. Continued The differences in the ash compositions as well as the compositions of the ACs (Bionuchar and F400) are noticed, with th e prominent peak of aluminum and the presence of iron being the most pertinent to this study. The relatively high conductivity of aluminum and iron may play a role in the distinction between th e performances of D E F

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46 Bionuchar and F400 that were observed in th e work of Khan (2003). This may be a possible cause for F400 showi ng a synergistic phenomena with respect to dyes. The results of the EDS scan of F400 also agree with Khan (2003) about the presence of iron in F400. F400 would have different metals contents because it is a coal based activated carbon that originated as bituminous coal. Th erefore, metals that are present in the subterranean environment would be inherent in the coal. However, Bionuchar is a wood based activated carbon that not only would have different metals than coal based activated carbons, but since it is chemically activated, during the chemical impregnation step at low pH, some of the metals would become soluble and leach out of the wood. In the case of HD4000 the EDS image (Figur e 4-1E) of the ash residue prominently consisted of silicon and aluminum. In additi on, magnesium and sodium were also fairly prominent. However, iron that was hypothesize d to be important for the observed synergy in the work of Khan (2003), was absent in the EDS scan. The presence of aluminum and silicon were also detected in the TiO2 coated HD4000 sample (Figure 4-1F). The EDS image also showed a very high titanium peak (about 5000 counts). In replicate sets which provided reproducible re sults, counts as high as 8000 were found for titanium. The EDS scans for the HD4000 samples also showed a relatively low carbon peak indica ting that a titanium dioxide agglomerate may have been focused on. The fact that this was the case in replicate sets between different TiO2 coated HD4000 samples may also indicate to some ex tent a uniform coating on the surface of this AC. Noteworthy is the fact that HD4000, like F400 is also a coal based AC. This strengthens the argument that morphology may pl ay a role in the deposition of titania on

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47 the AC. This can also be attributed to the e nhanced performances seen in the coal based carbons with respect to adsorption and photooxidation of dyes (Khan, 2003). The fact that HD4000 also contains highly conductive me tals such as aluminum strengthens the argument that conductivity may play a role in enhanced photocatalysis. Also, the fact that the ash content of HD4000, a lignite based AC is relatively high (12-18%) increases the probability of the coated titanium dioxide being in the vicinity of a site on the AC which possesses a higher conductivity (due to the presence of conductive metal ions). 4.2.2 SEM Images of Titania Coated ACs SEM images were used to further charac terize the coating of titania on the ACs. The images showed a variation in the depos ition of titania on the activated carbons as well as the degree to which each AC had been coated. Figure 4-2 shows an image of titania coated Bionuchar. Figure 4-2. Titania coated Bionuchar with the TiO2 blocked pores circled

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48 It can be noticed from the image that the surface of Bionuchar seems very smooth. The smooth nature of the AC surface may cause a lack of adherence of the titania to the AC surface which may result in decreased photo catalytic activity. Another notable aspect of this image is the fact that a number of por es (shown circled) appear to be partially or completely blocked by the titanium dioxide This may in fact be due to the poor adherence of the TiO2 surface resulting in the titania be ing deposited inside the pores of the AC. Deposition in the pores would of course reduce the diffusion of phenol to adsorption sites, thereby decreasing the adsorp tion of phenol (as discussed in subsequent sections). The wood based nature of this AC is evident in Figure 4-3. Figure 4-3. Bionuchar showing cellulose structure of the wood based AC. The Bionuchar AC particle seen in this im age shows what seems to be a celluloselike structure due to the wood precursor. It ma y also be noticed that the particle appears to have a smooth honeycomb structure which as mentioned earlier may not allow TiO2 to

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49 adhere well to its surface. The author c onsiders the morphology of the Bionuchar surface to be one of the reasons why a relatively small titanium peak is detected in the EDS of Bionuchar and its ash residue. Indeed, Table 42 showed that the pe rcentage of titania on the Bionuchar samples was less than the other two samples. In contrast to the morphology of Bionucha r, the titania seems to be uniformly coated on the surface of F400 (Figure 4-4). Figure 4-4. Titania coated on the surface of F400 The uniformity in coating is thought to be an explanation for the well defined peaks of titania seen in the EDS scans of F400 and c ould also be an explanation for the synergy seen in the study by Khan (2003). A uniform coating would increase the probability of titania contacting metals present on the AC surface which could hypothetically improve photocatalysis.

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50 An SEM image of HD4000 coated with tita nia is shown in Figure 4-5. It may be noticed that the TiO2 agglomerates on the surface of HD4000; here the TiO2 agglomerate is near the opening of a macropore (s ee circled region). The size of the TiO2 agglomerates (about 15 m) is observed to be much larger than the individual TiO2 particles (i.e., 27 nm) (Li et al., 2004). Figure 4-5. TiO2 agglomerate (shown circled) near a macropore on the surface of HD4000 However, it may also be noticed that si milar to the case of F400, the titania is coated more uniformly on the surface th an in the case of Bionuchar; a phenomenon believed to be dictated by the morphol ogy of HD4000. Figure 4-6 shows the uniform coating and the agglomeration of titania particles along with the rough surface of HD4000. As in the case of F400, the uniformity in coating may be an explanation for the large titania presence seen in the EDS s cans for HD4000. This may also be a possible

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51 explanation for the synergy noticed by Khan (2003) for the same reasons as mentioned for F400. Figure 4-6. Surface of HD4000 showing uniform coating of TiO2 which tends to form agglomerates Hence, it may be summarized that th e wood based carbon and the coal based carbons differ morphologically. Titania coated on Bionuchar seems to concentrate around the pores. It is also believed that due to the smoothness of the Bionuchar surface, there is less adherence of the TiO2 particles to the Bionuchar surf ace. However in the case of both F400 and HD4000, a more uniform coating of tita nia is observed with the titania forming agglomerates on the surface of HD4000. The ash analysis, EDS scans and the SEM im ages together provide us with some possible explanations for the syne rgistic phenomenon of certain TiO2-AC particles observed by Khan (2003). Both F400 and HD 4000 contain relativel y high ash contents as compared to Bionuchar. Their EDS scans also show a very high titanium presence on

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52 their surface in addition to the higher presence of electr ically conductive metals (i.e., aluminum, iron). It may be concluded that there is a great er possibility of titania being in contact with a conductive site in the case of F400 and HD4000 relative to Bionuchar. 4.2.3 Activated Carbon Batch Studies The results of the batch tests conducted on the activated carbons which compared the performance of the ACs (or the TiO2-AC composites) as a func tion of the removal of phenol are presented. It must be noted that differences in phenol removal of less than 45% were not considered significant. Virgin activated carbons. In the work of Khan (2003), the focus was on the removal of dyes. Herein, the focus was on a model aromatic adsorbate (phenol) and the intent was to learn if the c onclusions made by Khan (2003) and as previously discussed, were applicable to the removal of phenol. To determine the adsorption capacity of the ACs, batch tests were performed in the dark for a period of 24 hours. The studies were first performed with virgin ACs. Three activated carbons were used: Bionuchar, F400 and HD4000. The results of the tests performed with the virgin ACs are presented in Figure 4-7. It can be seen from the results that th e virgin F400 showed the most adsorption capacity with 87% removal whereas Bionuchar s howed the least adsorption capacity with only 52% removal. HD4000 performed relativel y well with about 76% removal. The high adsorption capacity of F400 may be explained by the fact that its pores are predominantly (60%) microporous. The molecula r size of phenol is about 6 (0.6 nm). Therefore, it is expected to adsorb mainly in pores havi ng a microporous size range. The low adsorption capacity observed for Bionuchar is attributed to the acidity of its surf ace. The surface of

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53 Bionuchar is highly acidic. It is well documente d in the literature th at L-type carbons or acidic activated carbons show reduced adsorp tion of organics such as phenol (Matos et al., 1998; Salame et al., 2003; Coughlin et al., 1968, Mattson et al., 1969). 0 10 20 30 40 50 60SamplePhenol Concentration (mg/L) Initial Concentration Virgin Bionuchar Virgin F400 Virgin HD4000 Figure 4-7. Batch adsorption studies of virgin ACs in the absence of UV It is thought that carboxyl and other such electron w ithdrawing functional groups result in a lowering of phenol ad sorption capacity by removal of the -electron from the AC aromatic ring. This decrease in the de nsity of the electron cloud in the carbon basal planes causes a decrease in the strength of interactions betwee n the benzene ring of phenol and the basal planes of AC. Phenol also reacts with carboxylic groups on the carbon surface forming ester bonds which contri butes to this phenomenon (Salame et al., 2003; Nevskaia et al., 1998). As a result, th ere is decreased adso rption of phenol on the basal planes of carbon. The case of HD4000 however is peculiar at first glance. HD4000 is acidic due to its activation procedure, but th is acidity is a function of residual acid used for acid-washing versus the presence of oxyge nated functional groups located on the edge

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54 sites. Therefore, although it displays characteristics of an L-type carbon, the manner in which this is manifested is di fferent than traditional L-t ype carbons. At the nano-scale, HD4000 behaves more like an H-type carbon, which suggests that bonding is the dominant adsorption mechanism. Adsorption capacity for titania-coated activated carbons. Similar trends were seen when batch adsorption studies were perf ormed in the dark with 3% (m/m) (i.e., mass of titania per mass of activated carbon) titaniacoated AC samples (Figure 4-8). Note that the targeted titania loading wa s 3%, but as was shown earlier the actual amount of titania on the surface of each AC was less than 3 %. 0 10 20 30 40 50 60SamplePhenol Concentration (mg/L) Initial Concentration 50 mg 3% Bionuchar 50 mg 3% F400 50 mg 3% HD4000 1.5 mg TiO2 Slurry 2.4 mg TiO2 Slurry Figure 4-8. Batch adsorption studies of 3% (m /m) coated ACs in the dark compared to TiO2 slurries A slight decrease in adsorption capacity as compared to the virgin ACs was observed in all the ACs. This was attributed to the partial blockage of pores and hence reduced surface area and active sites available for adsorption. In addition to the ACs, Figure 8 presents the adsorption capacities of two titania slurries (no AC). These masses

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55 of titania were chosen because they are very near the masses of titania that was present on the ACs. Clearly the titania slurries show very low adsorption capacities (approximately 8% removal). In fact, it is noticed that even when a higher amount of titania is loaded to the reactor, the decrease in phenol concentration does not change. Adsorption and photocatalysis for ti tania-coated activated carbons. The next step was to determine the performance of th e coated ACs in the presence of UV light. As in the earlier experiment, performances of tw o titania slurries were also included. It was expected that UV would initiate photocatalys is and therefore cons iderably enhance the removal of phenol through the combined f unctions of adsorption and photocatalysis. The tests with the titania coated ACs were conducted in a sequence which would eventually normalize the amount of carbon and titanium dioxide for each sample. The initial batch tests consisted of ACs coated with titania on the basi s of mass. The ACs were loaded to their respective reactors such that each reactor contained 50 mg of the titania-coated AC. These samples have been de noted in places as mass-mass or m/m. The amounts of titanium dioxide for the slurry systems were 1.5 (3% of 50 mg) and 2.4 mg (3% of 80 mg as a comparison) respectivel y. It must be noted that the ACs do not have the same density which resulted in each reactor having different volumes of ACs. The results of the tests are shown in Figure 4-9. The irradiated Bionuchar composite with only 39% removal performed poorly compared to HD4000 (78% phenol removal) and F400 (88% phenol removal). The performance of both titania-HD4000 and titani a-F400 composites slightly improved in the presence of UV; a fact th at is attributed to photocat alysis by UV. However, this refutes Matos et al. (1998, 2001) who claim that an L-type AC inhi bits photocatalysis

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56 whereas an H-type AC enhances photocatal ysis. The surface of HD 4000 is acidic and can therefore be categorized as an L-type carbon, but as was previously mentioned, the manner in which HD4000 exhibits a low pH is mechanistically different than traditional L-type carbons. Therefore, further investiga tion into this phenomenon is warranted. 0 10 20 30 40 50 60SamplePhenol Concentration (mg/L) Initial Concentration 50 mg 3% Bionuchar 50 mg 3% F400 50 mg 3% HD 4000 1.5 mg TiO2 Slurry 2.4 mg TiO2 Slurry Figure 4-9. Batch adsorption-photocatalysis studie s of 3% (m/m) coated ACs in the presence of UV Matos et al. (1998, 2001) clearly erred in their oversight that L-type carbons exhibit less adsorption than H-type carbons. Therefore, decreased adsorption was actually responsible for their results as opposed to decreased photocatalysis. As was previously discussed in detail, a possible explanation as to why the coalbased carbons behaved synergistically is b ecause both contained conductive metals. The enhanced performance of the ACs may point to a favorable difference in Fermi energies between the titania coated on their surface and a conductive metal in their vicinity, resulting in longer recombin ation times and improved phot ocatalysis. As explained

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57 earlier, due to the relatively large ash cont ent and metals composition, there is a greater probability of the titania on the surface of AC being in the vicinity of a conductive metal. Contrary to the performance of the coal based carbons, the performance of the irradiated Bionuchar-titania composite, which is acidic, was considerably worse than the performance of the virgin Bionuchar (13% difference). Whereas this phenomenon may be attributed to pore blockages in the coated Bionuchar, the obs ervation that the irradiated Bionuchar composite performed worse than th e Bionuchar composite (6% difference) in the absence of UV makes for an inte resting discussion (Figure 4-9). A trend similar to the one seen in Fi gures 4-8 and 4-9 was noticed in the experiments performed by Khan (2003). It wa s found that irradiated dyes (denoted as Dye*) showed tendencies to adsorb differently than dyes which had not been irradiated (denoted as Dye). According to Khan (2003) an d Khan et al. (2006), the irradiation of the dye caused the excitation of the delocalized electrons in the aromatic structure which would affect the adsorp tion of the compound (Dye*). These delocalized electrons in the aromatic structure would then interact with the carbon basal planes of Bionuchar which due to acidic functional groups would cont ain a less dense electron cloud resulting in decreased uptake (due to decreased interactions). The decreased uptake would also result in decreased photocatalysis. Hence, ACs which preferentially adsorbed the unadulterated dye compounds and had reduced uptake for Dye* would have a reduced overall performance. This was due to the fact that their affinity for the unadulterated dye compound resulted in the overall hindrance to adsorption of the dye compound. Hence, in the case of dyes, the two theories cumulativ ely may explain the reduced performance of titania-Bionuchar composites in the presence of UV. Continuing in the same vein, the

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58 synergy observed with F400 and HD4000 in the presence of UV was attributed to their relative affinity for Dye*. In the case of ACs that show ed good removal, a situation of augmented adsorption was theorized wherein activated carbons having an affinity to adsorb both Dye and Dye* at comparable levels showed increased decolorization of the dye solution (Dye + Dye*). Whether the same theory may apply to phenol (hence forming phenol*) was thought to be an idea worth invest igating and is discussed later. Regarding the performance of the titania coated ACs and titania alone, each of the ACs showed more removal of phe nol than the titania slurries. In fact, the titania slurry containing a lower concentration of titania slig htly outperformed the slurry with a higher concentration. Although this may be attribut ed to decreased penetration of UV with increasing TiO2 concentration, the fact still remain s that the removal was considerably lower than the AC-TiO2 composites. Therefore, it must be noted that although titanium dioxide may result in a decrease in adsorp tion capacity when coated on ACs, the ACs coated with titania provide for better overall removal of phenol in the presence of UV as compared to the titania slurry. Volumetric loading of the reactors and mol/mol coating. Due to the variation in the densities of the ACs, it was deemed n ecessary to normalize the differences in the volumes of ACs loaded to th e reactors. In other words, since the wood-based carbon was less dense than the coal based carbons, th e mass of carbon added for each experiment was less. Since the mass of carbon was less, then mass of titania would be less. Therefore, the system could be normalized by adding the carbons to the r eactor based on their volumes versus mass. Hence, 0.025 mL of AC was loaded to each of the respective reactors.

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59 The results of the experiment carried out under these conditions are shown in Figure 4-10 0 10 20 30 40 50 60SamplePhenol Concentration (mg/L) Initial Concentration 0.025ml 3% Bionuchar 0.025ml 3% F400 0.025ml 3% HD4000 Figure 4-10. Batch adsorption-photocatalysis st udies of 3% (m/m) coated ACs loaded to the reactors on a volume basi s in the presence of UV The tests did not result in a ny significant difference in performance of the ACs. A trend similar to the one seen in Figure 4-9 was seen for this experiment as well. With this said, the change in basis of loading from mass to volume only succeeded in normalizing the amount of AC in each reactor. It was al so of interest to learn if normalizing the amount of titania on each AC based on titania to carbon would impact the results. This ratio would be different for each carbon becau se of the different ash contents. To normalize the titania to carbon loading, the ACs were coated such that 3 moles of titania were present for every 100 moles of elem ental carbon. In doing so, the titania to elemental carbon ratio in each AC remained constant. In addition to the change in coating basis, the titania-coated ACs were continued to be loaded by volume into their

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60 respective reactors. Using these bases for coa ting the titania on the ACs and loading them to the reactors, tests were performed in the abse nce as well as in the presence of UV light. 0 10 20 30 40 50 60SamplesPhenol Concentration (mg/L) Initial Concentration 3% Bionuchar (mol/mol) loaded by volume 3% F400 (mol/mol) loaded by volume 3% HD4000 (mol/mol) loaded by volume 0 10 20 30 40 50 60SamplesPhenol Concentration (mg/L) Initial Concentration 3% Bionuchar (mol/mol) loaded by volume 3% F400 (mol/mol) loaded by volume 3% HD4000 (mol/mol) loaded by volume Figure 4-11. Batch adsorption studies: A) 3% (mol/mol) coated ACs in the absence of UV loaded to the reactors by volume, B) Photocatalysis studies of 3% (mol/mol) coated ACs in the presence of UV loaded to the reactors on a volume basis B A

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61 Tests in the absence of UV were performed to assess as to how much the change in the basis of coating affected adsorption of phe nol since in each case the amount of coated titania increased. Similarly, tests in the pr esence of UV were perf ormed to assess the effect that the increased titania coating on the ACs ha d on simultaneous adsorptionphotocatalysis. Figures 4-11a and 4-11b show the results of the experiments in the absence and presence of UV light respectively. As in the earlier tests, an analysis of th e data revealed no significant differences in the trends of their performances. Although th ere are signs of photo catalysis having some effect on phenol removal, the differences not iced in the trends are not of sufficient magnitude to arrive at any definite conclu sion with respect to how photocatalysis was influenced by the variations. The results seemed to show that adsorp tion dominated over photocatalysis as a mechanism. Hence, the extent to which phot ocatalysis plays a role in the removal of phenol in a co-adsorbent with high surface area was questioned Unlike results shown by Matos et al. (1998) in the case of organi c compounds and Khan (2003) in the case of dyes, the results seen herein failed to show any clear evidence of a synergy of any kind existing between ACs and TiO2 with respect to performan ce. The premise that a coadsorbent with high surface area would result in an increase in photocatalytic efficiency of titania (Matos et al., 1998; Takeda et al ., 1995) can hence be questioned. The theory by Matos et al. (2001) that L-t ype ACs inhibit photocatalysis whereas H-type ACs enhance photocatalysis has also gone unfounded based on the data that has been presented. The failure by Matos et al. (2001) to perform adso rption control studies s eems to be the cause of the unverifiable hypothesis.

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62 4.2.4. Determination of Existence of Phenol* (Phenol with Delocalized Electron) in the Presence of UV The trends observed in the titania-AC co mposites, particularly when compared to the performances of the virgin ACs resulted in the author investigating the possibility of a phenol* phenomenon, similar to the dye* phenomenon noticed by Khan et al (2006). The virgin ACs (50 mg) were exposed to UV fo r 24 hours without the presence of any photocatalyst. As a control study, phenol solu tion (no AC, no titania) was also exposed to UV light for 24 hours. The resulting da ta are provided in Figure 4-12. 0 10 20 30 40 50 60 Initial Concentratio nBionucharF400HD4000AC SamplesPhenol Concentration (mg/L) Virgin AC (NO UV) Coated AC (No UV) Coated AC (With UV) Virgin AC (With UV) Figure 4-12. Comparison of irra diated virgin ACs with virg in and coated ACs in the presence and absence of UV The results of the batch tests performed in section 4.2.3 are provided in Table 4-3. The table quantifies to an extent the do mination of the adsorption mechanism over photocatalysis.

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63 Table 4-3. Phenol removal as a function of AC coating strategy with and without irradiation Virgin AC performance (% removal) 3% (m/m) AC performance (% removal) 3% (m/v) AC performance (% removal) 3% (mol/v) AC performance (% removal) Activated carbons Precursor No UV With UV No UV With UV With UV No UV With UV Bionuchar Wood 51.5 36.9 44.7 38.7 37.7 37.1 37.9 F400 Bituminous 87.1 84.7 82.3 88.6 91.3 91.3 92.5 HD4000 Lignite 75.9 61.3 73.4 78.2 74.2 74.5 75.5 Comparison of Bionuchar systems. In the case of Bionuchar, the virgin AC in the presence of UV (36.9% removal) performed poo rly compared to its performance in the dark (51.5% removal). This can be accounted for by the theory put forth by Khan (2006) whereby the excitation of delocalized electrons in the aromatic structure of phenol results in reduced uptake by Bionuchar whose surface is supersaturated with acidic functional groups. The decrease in the electron density in the basal planes of Bionuchar due to the acidic functional groups coupled with the exc itation of delocalized electrons in phenol (resulting in phenol*) contributes to decreased interactions and hence decreased adsorption. Bionuchars low affinity for phenol* results in the overa ll low adsorption of phenol. The coated Bionuchar exposed to UV may show a similarly low performance due to a combined effect of pore bl ockage and the effect of phenol* on its adsorption. Comparison of F400 systems. F400, unlike Bionuchar and HD4000 does not show much variation regardless of whether the ca rbon was coated with ti tania and exposed to UV or if the virgin carbon was exposed to UV. The virgin F400 in the absence of UV (87% removal) is observed to perform as we ll as the coated F400 which was irradiated (88% removal).

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64 The similarities in the performances of th e two systems could be a result of either of two processes. The coated F400 compensates for the bloc kage of pores through photocatalytic degradation of phenol or The coated F400 in the presence of UV behaves the same as the virgin F400. Hence pore blockage is not a factor. However, this would also put to question the presence of any photocatalytic effect. It is also noticed that virgin F400 in the presence of UV performs slightly worse (ca. 85%) than the F400 experiment in the dark (ca. 87%). This difference is however not considered to be significant. Comparison of HD4000 systems. Finally, for HD4000, the irradiated titania coated sample displays maximum performan ce (78% removal) which is only slightly greater than the performances of the virgin HD4000 (no UV) with 76% removal and the coated HD4000 (no UV). It is however interestin g to note that the irradiated samples of virgin HD4000 perform considerab ly worse (61% removal) than the other samples. This may point to phenol* having a large influen ce over the adsorpti on capacity of HD4000. Noticing that the coated HD4000 in the pr esence of UV performs well, it may be theorized that the performa nce of HD4000 observed in ir radiated samples may be attributed more to enhanced photocatalysis. It may be this enhanced photocatalysis which compensates for the poor adsorption of phenol* attributed to its surface acidity. The performances of the ACs in the rema ining systems do not show any significant deviation. The slight changes in performa nce are attributed to a combination of adsorption and photocatalysis. Hence, in summary, from the explanation provi ded above, it is clear that in terms of photocatalytic activity, HD4000 seems to be the most active AC. The ac idic nature of its

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65 surface is thought to slightly inhibit adsorption. However, as seen from the SEM and EDS images, it seems to have the most unifo rm coating and the most adhesive surface towards TiO2. This could possibly be an explanat ion to its superior photocatalytic activity. The poor performance of Bionucha r has been extensively discussed and attributed to its characteristic acidic surf ace. A point has also been made regarding the morphology of the Bionuchar resulting in deposition of TiO2 in its pores, hence resulting in decreased adsorption. Th e poor adherence of TiO2 to the Bionuchar surface is the cause of the low photocatalytic degrada tion of phenol observed with the TiO2-Bionuchar composite. F400 largely displayed a consistent performance over each of its systems. Its more basic surface, morphology resul ting in good affinity towards TiO2 and high adsorption capacity seem to be important factors. The effect of conductivity, if any, seems to be overwhelmed by other factors such as adsorption and morphology of the surface. In fact, in the case of activated carbons, the phenomenon of irradiated phenol resulting in a change in adsorption characteristics proves that a good photocatalytic system would require adsorption as a primary mechanism. Although it has not been investigated, the author also proposes the possibility of UV irradiation having an effect on the AC surface. Hence, it is possible that the UV irradiation affects the AC such that the carbon system changes. This would also result in noticeable changes in adsorption of phenol by each of the ACs. For instance, excitation of the electrons in the carbon basal planes may result in a phenomenon similar to the one proposed with the phenol*. The author ha s however not investigated the possibility of this phenomenon in any further detail.

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66 The replicate sets for the experiments invol ving the virgin ACs in the absence of UV radiation is provided in appendix A. 4.3 Low Surface Area Carbon Studies The multitude of variables observed in the performance of ACs with respect to adsorption and photocatalysis re sulted in the investigation of carbon substrates having low surface areas. To further test whether conductivity and metals play a role in enhancing photocatalysis, low surface area carbons (< 8 m2/g) with varying conductivities were used in further tests. By selecting a variety of carbons such that they differed in their properties that were conducive or inhibitive to photo catalysis, it would be possible to relate specific properties of the carbons to the observed synergy. As hypothesized by Khan (2003), the reason for the superior performance of certain ACs is the presence of ionic metals. These metals act as electron sinks, cau sing the hindrance of electron-hole recombinations, thus enhancing ph otocatalysis. As explai ned earlier, this is interpreted by this author as enhanced photocatalytic activ ity being a function of higher electrical conductivity primarily due to specific sites on the carbons where metals are present. In theory, the presence of titania in the vicinity of a c onductive site (metal) will result in a favorable difference in Fermi energies between the TiO2 and the metal. The resulting effect is the change in the distribution of elect rons of the semiconductor (TiO2). The formation of the Schottky barrier will cause the migration of electrons from the titania to the metal increasing the electr on-hole recombination times and therefore enhancing photocatalysis. It should be noted that in general, the addition of conductive materials to semiconductors or insulators has a positiv e impact on their conductivity. This property may be approximated as a summation of th e conductivities of the materials involved.

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67 However, at the same time it is important to note that metals content is by no means the sole factor influencing electrical conductivit y. In the case of graphi te, a major influence of its electrical conductivity is its crystalline structure. Crys talline graphite consists of parallel sheets of carbon atoms, each sheet co ntaining hexagonal arrays of carbon atoms. Each carbon atom exhibits sp2 hybridization and forms sigma bonds with its three nearest neighbors. There is also di stributed pi bonding between the carbon atoms in the sheet. This delocalized pi system is responsible fo r the electrical conductiv ity of graphite. The disruption of this graphitic structure is the cause for a loss in electrical conductivity in carbonaceous materials. With this in mind, carbon materials having varying electrical conductivites were selected. The carbons selected for the studies were graphite pitch coke, bituminous coal and anthracite coal. The gradient in electrica l conductivity between the selected carbons, graphite being the highest and bituminous the lowest is shown in Table 4-4. These conductivities were measured as resistivitie s by compacting the carbons in a column at pressures between 125-150 psi and measuring th eir resistivity to ch arge (Data provided by and experiments performed by Asbury Carb ons, NJ). Hence the resistance of the compacted powdered carbons was measured and obtained which was then easily translated to electrical conductivity by the recipr ocal of the resistivity. Table 4-4. Electrical conductivity va lues for low surface area carbons Carbon Conductivity (m -1cm-1) (mho) Graphite 20-50 Pitch Coke ~ 10 Anthracite 0.001-0.01 Bituminous< 0.001

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68 4.3.1 Metals Content of Low Surface Area Carbons Before discussing how the titania coated low surface area carbons performed for phenol adsorption, it was of intere st to also investigate the as h contents of these samples. To determine if conductive metals were ind eed present in any of the carbons, an EDS study similar to the one performe d on the ACs was performed. Prio r to that, as in the case of ACs, the quantity of ash for each carbon was determined. This would enable us to analyze the EDS scans with mo re perspective. The ash cont ents of the low surface area carbons are provided in Table 4-5. Table 4-5. Elemental carbon and ash c ontents of low surface area carbons Carbon Elemental carbon (%)Ash (%) Graphite 99% 1% Pitch coke 93% 7% Anthracite 90% 10% Bituminous65-72% 28-35% EDS scans of all the carbons were perform ed with the exceptio n of graphite. The electrical conductivity of graphite is extremely high and its ash content is negligible. Hence, the amount of impurities found in gr aphite (metals) would be negligible and would not make any significant difference. Howe ver, it is interesting to see whether the conductivity due to its structure pl ays a role in p hotocatalysis. The EDS scan of the ash residue of the an thracite coal is shown in Figure 4-13. It shows prominent peaks corres ponding to aluminum and iron. These metals have very high electrical conductivities (376/mohm-cm and 102 /mohm-cm respectively) as compared to titanium (23 /mohm-cm). A smalle r peak of sodium is also seen. Another peak present is that of silicon (electri cal conductivity of si licon = 0.012 /mohm-cm). A number of such EDS scans were taken, all of which showed similar results.

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69 Figure 4-13. EDS scan of ash resi due of uncoated anthracite coal Again, it must be noted that EDS is by no means a quantitative analytical tool and is at best semi-quantitative. However, the images do show a greater prominence of some elements over others. Figure 4-14 shows an EDS scan of the ash re sidue of bituminous coal. The image shows a prominent aluminum peak. In addition to this, smaller peaks of iron, calcium and magnesium and sodium are also seen. Again, as in the case of anthracite, a silicon peak is noticed. Figure 4-14. EDS scan of ash residue of uncoated bituminous coal

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70 Although anthracite and bituminous are low conductivity carbons, the presence of metals such as aluminum and iron means that there is a high probability of a number of conductive sites on the surface of these carbons The amount of ash contained in these carbons (10% and 28-35% respectively) also poi nts towards there being a relatively large amount of these metals in these carbons. A similar scan was performed on the ash of pitch coke (Figure 4-15). Small peaks of aluminum and silicon in addition to small peaks of sodium and sulfur are seen. Figure 4-15. EDS scan of ash residue of pitch coke From the EDS scans of the low surface area carbons, it is evident that aluminum is commonly found in each of the carbons. A very prominent iron peak is seen in anthracite coal with the bituminous sample also show ing some presence of iron. Recalling the study by Khan (2003) which hypothesized metals (s pecifically iron) to be the cause of increased photocatalysis, it may be expected that anthracite and bituminous should show the greatest removal when the carbons ar e subjected to batch adsorption-oxidation studies.

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71 4.3.2 Batch Studies with Low Surface Area Carbons Batch studies were performed with the lo w surface area carbons. These tests were performed in a procedure identical to that used in the activated carbon studies. The carbons used were sized to 35 x 200 sieve in order to normalize as far as possible the external surface area of the carbons. Titanium dioxide was coated on the ACs via boil deposition on a mass basis. A mass of 80 mg of each sample was used with 80 mL of 55 ppm phenol solution. Adsorption studies performed on coated low surface area carbons in the dark. In order to verify the neglig ible adsorption capacity of the carbons, they were put through adsorption studies in the absence of UV li ght. In addition to the carbons, a titanium dioxide (Degussa P25) slurry was also tested as a comparison to the coated substrates. The titanium dioxide was weighed such that its mass was equal to the theoretical mass of the titania on the surface of the carbons (3% of 80 mg, i.e., 2.4mg). It was expected that all the samples including the titanium dioxide slurry would show negligible adsorption. Figure 4-16 shows the results of the adsorption studies. 0 10 20 30 40 50 60 70 SamplesPhenol Concentration (mg/L) Initial Concentration 3% Pitch Coke 3% Graphite 3% Anthracite 3% Bituminous 2.4 mg TiO2 Slurry Figure 4-16. Adsorption studies of coated lo w surface area carbons and titanium dioxide slurry

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72 It can be seen that the adsorption seen in each of the carbon samples including the slurry is negligible. In fact the maximum removal seen is only 4.6%. This emphatically proves that the carbon composites are indeed of low surface area, incapable of any significant adsorption. It was thought unnecessary to perform adsorption studies for the virgin carbons due to th eir low surface areas. Photocatalysis studies performed with coated low surface area carbons. The carbons and the titania slurry were then made to undergo photocatalysis studies. Using an identical configuration, the samples were e xposed to 365 nm UV light. It was expected that carbons containing a larger concentration of metals in their matrices would perform better than those with a lo wer content of metals (Kha n, 2003). Although all the carbons with the exception of graphite contained a considerable amount of metals, due to the presence of metals and the relatively hi gh natural conductivity of pitch coke, it was expected to perform better than the other ca rbon composites. Due to better mass transfer, the titanium dioxide was also expected to perform relatively well. The results of the experiment are shown in Figure 4-17. Although there was little di stinction in the performa nce of the carbons, it was noticed that the titania coated bituminous coal showed the great est removal of 20%. Anthracite coal with 18% removal also perf ormed relatively well. Pitch coke which was expected to perform well only removed 6% of the phenol. The graphite which is the most electrically conductive, although lacking in ash content showed a removal of 13%. However, a surprising result was that of th e titania slurry which only accounted for 11% removal. This was attributed to a reduction of the surface area of TiO2 available for irradiation. It was thought that the amount of titania loaded was too high.

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73 0 10 20 30 40 50 60 70SamplesPhenol Concentration (mg/L) Initial Concentration 3% Pitch Coke 3% Graphite 3% Anthracite 3% Bituminous 2.4 mg TiO2 Slurry Figure 4-17. Batch photocatalysis studies of carbons and tita nium dioxide slurry (m/m) Hence for the specific configuration of the reactor, the titania concentration in the slurry was greater than the optimal load ing, causing low penetration of UV through the slurry. It has been previously observed in Fi gure 4-9 that a slurry w ith a lower loading of titania has performed better. Comparing the oxidation studies to the adsorption studies in the absence of UV, it is evident that there is photocatalysis taking pl ace. A comparison between the carbon-titania particles shows no trend with respect to co nductivity or metals content in their ash residues. However, a trend was noticed in the difference in performances being similar to the carbons differences in densities. Due to this, the volume of carbons and hence the amount of titania in each reac tor would be different. Highe r density would result in a smaller volume of that particular carbon resulting in a lower TiO2 dose in the respective reactor. Table 4-6 summarizes the densities of the coated carbons.

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74 Table 4-6. Average densities of 3% (m/m) titanium dioxide coated carbons Carbon Density g/m3 Bituminous1.37 Anthracite 1.93 Pitch coke 2.25 Graphite 2.58 From the table it can be seen that bitumi nous coal and anthracite coal are the two carbons with the least density. This difference in density is reflect ed in the difference seen between the performances of the carbons Since the adsorption capacities for all the carbons were negligible, the coated carbons we re loaded on the basis of volume in order to normalize the volumes and titanium dioxide lo aded to each reactor. This was expected to magnify differences in phenol decom position between the carbons. The following graph shows the results of the batch studies in which the carbons were coated with TiO2 on the basis of mass and loaded to the reactors such that their volumes equaled 0.05 mL. This set, shown in Figure 4-18, is refe rred to as mass-volume or m/v. The results reiterated the fact that there wa s no real difference in performance in the carbons with respect to adsorption or photoc atalysis. The lack of distinction in photocatalytic performance as well as ad sorption between the carbons prompted the author to discard the theory regarding phenol* having an influence on photocatalysis when adsorption was absent. It was postulate d that the excitation of the electrons in phenol only has an effect on its adsorpti on characteristics and does not affect photocatalysis. The influence of metals on photocatalysis was also questioned as clearly, there was no distinction seen in photo catalysis between the carbons ev en though they differed in the content and composition of metals.

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75 0 10 20 30 40 50 60 70SamplesPhenol Concentration (mg/L) Initial Concentration 3% Pitch Coke 3% Graphite 3% Anthracite 3% Bituminous Figure 4-18. Photocatalysis studies of coated low su rface area carbons in the presence of UV (m/v) To further verify these results and norm alize the amount of carbon and titania in each reactor, tests were performed using a mo lar basis for coating the carbons with TiO2. This basis, explained in detail in the materi als and methods section c onsists of coating the carbons such that the mole ratio of titanium dioxide to carbon is 3%. The carbons were then loaded to their reactors on the basis of volume as in the earlier case (0.05 mL). This set of samples has been referred to as molevolume or mol/v. The results from these tests which were otherwise performed in an identical manner to the previous batch tests are shown in Figure 4-19. For the mass-volume and the mole-volume sets, it was deemed unnecessary to perform contro l studies in the absence of UV light as the carbons contained no internal pore structure. Hence, a higher concentration of TiO2 coated on the carbon surface would not make any noticeable difference to adsorption. The results from these tests showed a s light increase (less than 2%) in the photocatalytic degradation of

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76 phenol. However, this was seen in all the samples, resulting ag ain in no significant distinction between the samples. 0 10 20 30 40 50 60 70SamplePhenol Concentration (mg/l) Initial Concentration 3% Pitch Coke 3% Graphite 3% Anthracite 3% Bituminous Figure 4-19. Photocatalysis studies of low surface area carbons in the presence of UV (mol/v) It may be noticed that the performances of the ACs with the exception of the molevolume samples which showed a slight incr ease in performance, remains relatively constant. The only considerable distinction ca n be made in the case of the mass-mass carbons. However, this has been explained by th e differences in thei r densities. It is evident from these studies that when titanium dioxide is coated on a substrate such as carbon, adsorption plays a very important role More importantly, what is shown by the low surface area carbon tests is that as prev iously hypothesized by this author (Section 2.4), conductivity may not be playing any si gnificant role in the enhancement of photocatalysis. Even though carbons were sel ected whose conductivities varied by orders of magnitude, there was no significant effect or even any trend that was noticed in the performance of the titania coated carbons. Hence, the conclusion by Khan (2003) about metals such as iron enhancing photocatalysis due to their role as electron traps is

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77 uncorroborated. At the same time, the theo ry by Matos et al. (2001) regarding the synergy seen in the case of titania systems containing H-type ACs is also unsubstantiated. Furthermore, the experiments conducted with the AC composites and the low surface area carbonaceous composites show that a slight synergy may be noticed in TiO2AC composites, primarily driven by adso rption. The difference in the electrical conductivities of the low surface area substrates did not show any considerable synergy.

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78 CHAPTER 5 SUMMARY AND CONCLUSIONS 5.1 Summary In this study, activated carbons which di ffered in physical and chemical properties were coated with titanium dioxide and subj ected to a variety of experiments. The AC composites were primarily tested with respec t to the impact of el ectrical conduc tivity of the substrate on photocatalysis in addition to photocatalysis as a function of morphology and surface chemistry. The results did not definitively suppor t my initial hypothesis that electrical conductivity of the ACs infl uences the photocatalytic properties of the AC-TiO2 composite. A slight improvement in photocat alysis was observed in coal based ACs which were believed to possess higher conduc tivities. The improvement in performance did not significantly prove that a synergy was present between TiO2 and the conductive ACs (F400 and HD4000); however, the definite tre nd seen in the tests leads the author to believe that conductive sites on the surface of AC may result in a localized enhancement of photocatalysis. Moreover, this localized enhanced photocatalysis mechanism is insignificant in the perspective of the adsorp tion mechanism at the sc ale at which it was carried out. EDS scans of the ACs showed the absence of key metals (i.e., absence of iron in HD4000) which had been attributed to the s ynergy observed in the tests performed by Khan (2003). The premise that trace metals in the AC serve as electron traps, prolonging electron-hole recombination times leadi ng to enhanced photocatalysis was thus

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79 questioned. In effect, the presence of metals and hence the influe nce of conductivity of ACs on enhanced photocatalysis is a minor f actor for photocatalysis due to their low distribution and the low probability that a TiO2 particle would be deposited in their vicinity. The argument was further strengthene d after reviewing the performances of the low surface area carbons which contained varying quantities of metal composition but showed no significant differences in performances. There is evidence that the morphology of th e material (AC) to an extent governs the amount of photocatalyst that adheres to it. The smoothness of surfaces such as in the case of the wood based Bionuchar resulted in decreased amounts of titania being deposited. In fact, it was noticed that of th e titania that was deposited, a large quantity was driven into the pores, pointing towards adsorption of titania in the pores during the coating process as opposed to its deposition on the exterior surface of Bionuchar. The pore blockages as a result of this are one of the reasons for Bionuchars lower phenol adsorption. In contrast, rough surfaces su ch as the coal based F400 and HD4000 displayed greater quantities of TiO2 deposited on their surf aces. SEM images of the coated ACs also showed a more uniform coati ng of titania. It was noticed that the same ACs also displayed greater adsorption capacity Presence of surface functional groups on the AC is also believed to be a major influence on adsorption and hence indirectly on photocatalysis. Surface functional groups are well known to have an influence on th e adsorption of organic compounds by activated carbons. This fact was related to another phenomenon in explaining the variation in trends seen in the ACs when they were subjected to UV. The phenomenon involved a similar observation to the one by Khan (2003) in the case of dyes. Irradiated virgin ACs

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80 showed a decrease in performance when comp ared to the adsorption demonstrated by virgin ACs in the dark. Phenol* which is the excited state of phenol in which delocalized electrons in phenol are excite d is the suggested theory. Th e adulteration of the initial compound is believed to be a reaction whic h occurs in the picosecond time scale, explaining why irradiated phenol solution do es not show any change in spectroscopic absorbance. Another possibility which has not been inve stigated by this author is that the UV radiation affects the activated carbon itself. He nce, it is possible that there is a change in the system of the electrons in the carbon basal planes of the activated carbon resulting in a decrease in the dispersive adsorption forces. Due to the decreased interactions; in this case due to the carbon electrons, a decrease in adsorption of phenol by the irradiated ACs may be noticed. Another possibi lity is that both the theories may take place simultaneously causing the observed results. The superior performance of F400 across all the systems is attributed to a combination of factors. The basic surface of F400 does not inhibit adsorption of the organic contaminant as its interactions are strong. Th e pore blockages of F400 are overcome by the superior photocatalysis that it shows. Although the presence of metals such as aluminum might point towards a Fe rmi energy difference influencing the results, it is highly unlikely given the large do minance of the adsorption mechanism. On the basis of photocatalysis alone HD4000 performed the best. The poor adsorption capacity of HD4000 for irradiated phenol was compensated by photocatalysis. The process of photocatalysis, especially when combined with the mechanism of adsorption is one that is extremely comple x and not very well understood. When the

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81 mechanism of photocatalysis is combined w ith an adsorption mechanism, it is often difficult to distinguish between a nd isolate the two mechanisms. I propose that a synergy in activated carbon-TiO2 composites may indeed show no significant improvements. However, even small improvements in performances may actually be a synergistic phenomenon in wh ich the AC is being photocatalytically regenerated during its adsorption cycle. When the ACs are compared, it is evident that there is a difference in their overall perf ormance with the use of the titania-carbon composite. 5.2 Conclusions In the case of Bionuchar, the decrease in th e electron density in its basal planes due to its acidic surface functional groups coupl ed with the excitation of delocalized electrons in phenol explains the decrease in performance of its irradiated coated composite as compared to its non-irradiated composite. It is due to a combination of these factors that Bionuchar demonstrates poor performance. The synergy demonstrated by F400 is attributed to a combination of high adsorption capacity of phenol as well as a balanced affinity for phenol*, which would provide more pollutant to be photo catalytically degraded on the surface of F400 which is uniformly coated with TiO2. Even though HD4000 may be considered an L-type AC, it actually shows a synergistic effect with resp ect to adsorption and photocat alysis which refutes Matos et al. (2001). Although electrical conductivity of the low surface area carbons varied by orders of magnitude, they demonstrated the same performance. The absence of any adsorption in fact decreased the distinction between carbons. Hence, adsorption is clearly the dominant mechanism over photoc atalysis. The key factors of adsorption are the morphology of the AC, surface area and pore size of the AC and the surface functional groups on the AC.

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82 CHAPTER 6 CONTRIBUTIONS TO SCIENCE AND ENGINEERING 6.1 Contributions to Science The experiments carried out have yielded a very interesting se t of results. An analysis of the results has shown that co mbining two mechanisms (i.e., adsorption and photocatalysis) creates a complex system wh ich needs to be further understood. This complex system consists of various mechanis ms which may be dependent on each other. The systems behave in such a manner that it is extremely difficult to distinguish between the two mechanisms. In lieu of this, the contri butions of this thesis to science in the opinion of the author are as follows: It has been adequately demonstrated that surface functional groups primarily affect adsorption and hence only indirectly a ffect photocatalysis. This is not in accordance to what Matos et al. (1998, 2001) proposed. It is only due to the fact that adsorption is a dominant mechanism that photocatalysis is observed to be influenced by the surface functiona lity of the activated carbons. The phenomenon of the excitation of the de localized electron in dyes (dye*) which was observed by Khan et al. (2006) is also observed in the case of phenol (i.e., phenol*). However, whereas in the case of dyes it was observed to enhance or inhibit adsorption depending upon the type of AC, in the case of phenol it was primarily noticed to inhibit adsorption of both L-type and H-type ACs. It is also possible that the UV radiati on has an affect on the carbon electron system, thereby affecting adsorption. Finally, it has been shown that individual material properties do not have much of an influence on the adsorption-photocatalysis system. It is more likely that a synergy exists between the general mech anisms of photocatalysis and adsorption with adsorption being the dom inant mechanism. It is proposed that an enhancement due to synergy may be noticed in systems in which the AC needs to be regenerated in-situ. Prior to research being perfor med on individual material properties and their effects on photocatalysis, further res earch must be performed to distinguish between photocatalysis and adsorption and how they behave synergistically.

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83 6.2 Contributions to Engineering As mentioned in section 6.1, the research presented by the author demonstrates a possibility to apply adsorption-photocatalysis systems to regenerate activated carbons insitu. Activated carbon in its contemporary use re quires to be regenerated (usually off-site) after its adsorption capacity has been reached. By the use of activated carbon coated with titanium dioxide, it is possible to regenera te the ACs in-situ by exposing them to UV radiation during their adsorption process. Such a system would require a certain amount of fluidization to allow the radiation to be incident on all of the TiO2-AC particles. Preliminary column studies were performed by this author in which HD4000 and F400 were used in a column configuration. Phenol was allowed to flow through the column and the particles were slightly fluidized. The entire column was exposed to UV radiation. Comparing the irradiated system to a syst em not exposed to UV, a small difference was seen in the two sets of data with the i rradiated system performing better. The data is provided in Appendix B and Appendix C. It was found that although the two systems were regenerated by flowing DI water to th e same extent, the effluent showed lower concentrations of phenol in the irradiat ed sample pointing towards photocatalytic degradation of the phenol. Hence, these bench scale column studies show the engineer ing application of titanium dioxide coated activated carbon in th e in-situ regeneration of activated carbon.

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84 APPENDIX A VIRGIN AC STUDIES IN PRESENCE OF UV 0 10 20 30 40 50 60SampleConcentration (mg/L) Initial Bionuchar F400 HD4000 Figure A-1. Replicate data sets for virgin AC studies performed in the presence of UV demonstrating the phenol* phenomenon observed in Section 4.2.4.

PAGE 95

85 APPENDIX B COLUMN STUDIES 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 0500100015002000 Time (min)Concentration (mg/L) With UV 1500 mL/hr Without UV 1500 mL/hr With UV Without UV With UV Packed Bed Without UV Packed Bed Figure B-1. Recirculation column studies perf ormed with F400 using different flowrates (degrees of fluidization) in the presence and absence of UV.

PAGE 96

86 APPENDIX C COLUMN STUDIES II 0.00 0.20 0.40 0.60 0.80 1.00 1.20 0100200300400500600700Time (minutes)C/C o 1st Adsorption Run (Averaged) 2nd Adsorption Run (Only DI) 2nd Adsorption Run (UV + DI) Figure C-1. Adsorption runs of column tests performed with HD4000 in the presence and absence of UV radiation measuring phe nol concentration in the effluent 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 0100200300400500600700800Time (minutes)Concentration (mg/L) DI + UV Regeneration Only DI Regeneration Figure C-2. Concentration of phenol in effl uent during regenerati on runs of column studies

PAGE 97

87 LIST OF REFERENCES Bacsa RR and Kiwi J. Effect of rutile phase on the photocatalytic properties of nanocrystalline titania during the degradation of p-coumaric acid. Applied Catalysis B: Environmental 1998; Vol.16; Issue 1: 19-29. Byrne JA, Eggins BR, Brown NMD, McKinne y B, Rouse M. Immobilization of TiO2 powder for the treatment of polluted water. Applied Catalysis B: Environmental 1998; 17: 25-36. Carp O, Huisman CL, Reller A. Photoi nduced reactivity of titanium dioxide. Progress in Solid State Chemistry 2004; 32: 33-177. Castilla CM, Rivera-Utrilla J, Lopez-Ram on MV, Carrasco-Marin F. Adsorption of some substituted phenols on activated carbons from a bituminous coal. Carbon. 1995; 33: 845-851. Chhor K, Bocquet JF, Colbeau-Justin C. Comp aritive studies of phenol and salicylic acid photocatalytic degradation: Influence of adsorbed oxygen. Materials Chemistry and Physics. 2004; 86: 123-131. Coughlin RW and Ezra FS. Role of surface acidity in the ad sorption of organic pollutants on the surface of carbon. Environmental Science and Technology 1968; vol.2; No. 4: 291-297. Coughlin RW, Ezra FS, Tan RN. Influence of chemisorbed oxygen in adsorption onto carbon from aqueous solution. Journal of Colloid and Interface Science. 1968; Vol. 28; No. 3: 386-396. Dabrowski A. Adsorption from theory to practice. Advances in Colloid and Interface Science. 2001; 93: 135-224. Dijkstra MFJ, Buwalda H, De Jong AWF, Michorius A, Winkelman JGM, Beenackers AACM. Experimental comparison of thr ee reactors designs for photocatalytic water purification. Chemical Engineering Science 2001; 56: 547-555. Dijkstra MFJ, Michorius A, Buwalda H, Panneman HJ, Winkelman JGM, Beenackers AACM. Comparison of the efficiency of immobilized and suspended systems in photocatalytic degradation. Catalysis Today 2001; 66: 487-494.

PAGE 98

88 Hobbs J. Adsorption of substituted arom atic compounds by activated carbon: A mechanistic approach to quantitative st ructure activity relationships. 2005; Masters Thesis The University of Florida, Gainesville, FL-32611. Hoffman MR, Martin ST, Choi W, Bahnema nn DW. Environmental applications of semiconductor photocatalysis. Chemical Review 1995; 95: 69-96. Karthikeyan T, Raj gopal S, Miranda LR. Journal of Hazardous Materials 2005; B124: 192-199. Khan AY and Mazyck DW. The effect of UV irradiation on adsorption by activated carbon/TiO2 composites. Letters to the Editor/Carbon 2006; 44: 158-193. Khan AY. Titanium dioxide coated activated carbon: A regenerative technology for water recovery. 2003. Masters Thesis The University of Florida, Gainesville, FL-32611. Kinoshita K. Carbon. Electrochemical and Physicochemical Properties. Wiley 1988. Legrini O, Oliveros E, Braun AM. Phot ochemical processed for water treatment. Chemical Review 1993; 93: 671-698. Leng CC and Pinto NG. Effects of surface prope rties of activated carbons on adsorption behavior of selected aromatics. Carbon 1997; Vol. 35; No.9: 1375-1385. Leon y Leon C and Radovic L. Interfacial Ch emistry and Electrochemistry of Carbon Surfaces. In: Thrower P. editor: Chemistry and Physics or Carbon. Volume 25, New York: Dekker, 1994. Li W, Ni C, Lin H, Huang CP, Shah IS. Size dependence of thermal stability of TiO2 nanoparticles. Journal of Applied Physics 2004; Vol.96; No.11. Linsebigler AL, Guanquan L, Yate s JT Jr. Photocatalysis on TiO2 surfaces: Principles, mechanisms and selected results. Chemical Review 1995; 95: 735-758. Matos J, Laine J, Herrmann JM. Effect of the type of activated carbons on the photocatalytic degradation of aqueous or ganic pollutants by UV-irradiated titania. Journal of Catalysis 2001; 200: 10-20. Matos J, Laine J, Herrmann JM. Synergy eff ect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon. Applied Catalysis B: Environmental. 1998; 18: 281-291. Mattson JS, Harry MB Jr., Malbin MD, Weber WJ Jr., Crittenden JC. Surface chemistry of active carbon: specific adsorption of phenols. Journal of Colloid and Interfacial Science 1969; Vol. 31; No.1; 116-130. Mattson JS, Mark HB. Active carbon surface chemistry and adsorption from solution. Dekker New York; 1971.

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89 Mazyck DW. Steam-curing plus ramped temperature N2 treatment: A novel technology for thermal reactivation of granul ar activated carbons (GAC). 2000; PhD dissertation The Pennsylvania State Univer sity, University Park, PA-16802. Moreno-Castilla C, Carrasco-Marin F, Mal donado-Hodar F, Rivera-Utrilla J. Effect of non-oxidant acid treatments on the surface properties of activat ed carbons with very low ash content. Carbon 1998; 36 (1-2): 145-151. Nevskaia DM, Castillejos-Lopez E, Munoz V, Guererro-Ruiz A. Adsorption of aromatic compounds from water by treated carbon materials. Environmental Science and Technology. 2004; 38: 5786-5796. Nevskaia DM, Santianes A, Munoz V, Guerrero -Ruiz A. Interaction of aqueous solutions of phenol with commercial activated carbons: an adsorption and kinetic study. Carbon 1999; 37: 1065-1074. Ohtani B, Kakimoto M, Nishimoto S, Kagiya T. Photocatalytic reaction of neat alcohols by metals loaded titanium (IV) oxide particles. Journal of Photochemistry and Photobiology A: Chemistry. 1993; Vol. 70; 70;3:265-272. Ollis DF, Pelizzetti E, Serpone N. Photocat alyzed destruction of water contaminants. Environmental Science and Technology. 1991; Vol. 25; 9: 1522-1529. Radovic L, Sivla I, Ume J, Mendes J, Leon y Leon C, Scaroni A. An experimental and theoretical study of the adsorption of aromatics possessing electron withdrawing and electron donating functiona l groups by chemically m odified activated carbons. Carbon. 1997; 35(9): 1339-1348. Ray AK and Beenackers AACM. Novel swirl flow reactor for kinetic studies of semiconductor photocatalysis. A.I.Ch.E. Journal 1997; 43(10): 2571-2578. Salame II and Bandosz TJ. Role of surface chem istry in adsorption of phenol on activated carbons. Journal of Colloid and Interfacial Sciences 2003; 264: 307-312. Serpone N. Brief introductory rema rks on heterogeneous photocatalysis. Solar Energy Materials and Solar Cells 1995; 38: 369-379. Tao Y, Schwartz S, Wu CY, Mazyck DW. Development of a TiO2/AC composite photocatalyst by dry impregnation for th e treatment of methanol in humid airstreams. Industrial and Engineeri ng Chemistry Research. Revision submitted: 2005 Tessmer CH, Vidic RD, Uranowski L. Imp act of oxygen containing surface functional groups on activated carbon. Environmental Science and Technology 1997; 31: 1872-1878.

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90 Trillas M, Peral J, Domenech X. P hotocatalyzed degradation of phenol, 2,4Dichlorophenol, phenoxyacetic acid a nd 2,4-dichlorophenoxyacetic acid over supported TiO2 in a flow system. Journal of chemical technology and biotechnology 1996; 67: 237-242. Turchi CS and Ollis DF. Photocatalytic degradation of organic water contaminants. Mechanisms involving hydroxyl radical attack. Journal of Catalysis 1990; 122: 178-192. Yang P, Lu C, Hua N, Du Y. Titanium dioxide nanoparticles co-doped with Fe3+ and Eu3+ ions for photocatalysis. Materials Letters. 2002; 57: 794.

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91 BIOGRAPHICAL SKETCH Vivek Shyamasundar was born in India wher e he spent a major portion of his life living in Bombay. After graduating from the University of Bombay in 2003 with a degree in chemical engineering, he decided to pursue his graduate degree at the University of Florida in environmental engine ering. During this time, he al so took courses in industrial engineering. Following his graduation in December 2005, Vivek hopes to apply his knowledge of chemical engineering and environmental engineering as he pursues a career in the industry.


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Title: Investigation of Properties Responsible for Phenol Removal via Titania Coated Activated Carbon
Physical Description: Mixed Material
Copyright Date: 2008

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INVESTIGATION OF PROPERTIES RESPONSIBLE FOR PHENOL REMOVAL VIA
TITANIA COATED ACTIVATED CARBON
















By

VIVEK SHYAMASUNDAR


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005


































Copyright 2005

By

Vivek Shyamasundar















ACKNOWLEDGMENTS

I would sincerely like to thank Dr. David Mazyck for giving me the chance to work

on this challenging project. His guidance and constant encouragement enabled me to

work through and succeed during some frustrating times. I have gained a lot of

knowledge by working with him. I would like to thank my committee Dr. Paul Chadik,

Dr. Joseph Delfino and Dr. Joseph Geunes who were always willing to help and

provided me with a lot of suggestions throughout the duration of my research.

I would like to thank all the students in Dr. Mazyck's research group Ameena

Khan, William 'Beau' Kostedt, Morgana Bach, Jennifer Stokke and Heather Byrne. Their

suggestions and help at all the stages of my work helped me immensely. I consider

myself very fortunate to have worked with such a great group. Thanks also to Aly Byrne

and Gustavo Avila for their assistance on a number of experiments. I would also like to

acknowledge Gautam Kini for his help. I thank Rick and Matt at Engineering

Performance Solutions who were very efficient and quick in analyzing my samples.

I especially thank Sudeepti Southekal for being a great friend and a constant source

of encouragement. I hope to help her in the same way as she pursues her PhD.

Above all, I must thank my family for their love and support and for the principles

and ethics that they have instilled in me.
















TABLE OF CONTENTS


Page

A C K N O W L E D G M E N T S ................................................................................................. iii

TA BLE OF CON TEN TS................................................................ .......................... iv

LIST OF TABLES .............. ................. ........... ................... ........ vi

L IST O F FIG U R E S .... ...... ...................... ........................ .. ....... .............. vii

ABSTRACT ........ .............. ............. ...... ...................... ix

CHAPTER

1 INTRODUCTION ............... ................. ........... ................. ... .... 1

2 LITERA TURE REVIEW .......................................................... ..............4

2.1 Activated Carbon ........................ .. .......... ........ .........................
2.1.1 Physical Properties of A C .................................. ............... ...................5
2.1.2 Chem ical Properties of A C ................................................ ...... ......... 7
2.1.3 A dsorption M echanism s.................................................................... .... .9
2 .2 P h oto cataly sis ................................................................................... 12
2 .2 .1 T itanium D ioxide........... ...... ...................................... .......... ... ......... 13
2.2.2 Mechanism of Photocatalysis ..................... ............... 14
2.2.3 Improvements in Photocatalysis................................ ...............16
2.3 Fermi Energies/Levels, Electrical Conductivity and Schottky Barrier ................18
2.4 Previous Synergy in AC-TiO2 Systems..... .......... ...................................... 21

3 M ATERIALS AND M ETHODS ........................................ ......................... 26

3.1. T target P ollutant ........ ............... ... ......... .... ....... .. ... .......... .. ......... .... 26
3.2 Carbon M materials ................. .............. ........ .. .. .... ...... .. ...............26
3.2.1 TiO2-AC Composites and Coating Procedures....................................26
3.2.2 Mass Based Coating (m/m) ................................................. 31
3.2.3 M olar Based Coating (m ol/m ol)...................................... ............... 31
3.2.4 Low Surface Area Carbon-TiO2 Composite.............................................33









3 .3 E xperim ental Setup ...................................................................... ...................34
3.3.1 Batch Reactor Configuration........................................................ ........ 34
3.3.2 A nalytical E quipm ent....................................................................... ... ...34
3.4 Experimental Procedure....................................................... .................. 35
3.4.1 Concentration Measurements ....................................... ............... 35
3.4.2 Batch Adsorption/Oxidation Tests with Activated Carbon......................36
3.4.3 Batch Adsorption/Oxidation Tests with Low Surface Area Carbons ........37

4 RESULTS AND DISCU SSION ........................................... .......................... 39

4.1 Evaluation of Carbon Coating Strategies .................................. ............... 39
4.2 A activated C arbon Studies ................................................. ........................ 41
4.2.1 Ash Analyses of Activated Carbons........... .......................41
4.2.2 SEM Images of Titania Coated ACs .................................. ............... 47
4.2.3 Activated Carbon Batch Studies.......................... ......................52
4.2.4. Determination of Existence of Phenol* (Phenol with Delocalized
Electron) in the Presence of UV ... ................. ..... ...............62
4.3 Low Surface Area Carbon Studies .................................................. 66
4.3.1 Metals Content of Low Surface Area Carbons................ .... ........... 68
4.3.2 Batch Studies with Low Surface Area Carbons ......................................71

5 SUMMARY AND CONCLUSIONS ............................................. ...............78

5 .1 S u m m a ry ......................................................................................................... 7 8
5 .2 C o n c lu sio n s ..................................................................................................... 8 1

6 CONTRIBUTIONS TO SCIENCE AND ENGINEERING .....................................82

6 .1 C contribution s to Science ............................................................ .....................82
6.2 Contributions to Engineering........................................... .......... ............... 83

APPENDIX

A VIRGIN AC STUDIES IN PRESENCE OF UV ....................................................84

B C O L U M N ST U D IE S ....................................................................... .....................85

C C O L U M N STU D IE S II .............................................................................. ............86

LIST OF REFEREN CES ............................................................ .................... 87

B IO G R A PH IC A L SK E T C H ..................................................................... ..................91
















LIST OF TABLES


Table Page

2-1. Photocatalysis reaction m echanism s ............................................... ............... 16

3-1. Sum m ary of A C properties............................................... .............................. 26

3-2. Carbon contents of virgin and titania coated ((m/m) and (mol/mol)) activated
carbons and their surface areas........................................... .......................... 32

3-3. Surface areas of virgin and coated low surface area carbons.................................33

3-4. Carbon contents of low surface area carbons ......................................................33

4-1. Sum m ary of coating techniques ........................................... .......................... 40

4-2. Ash contents of ACs and mass % of titania deposited (m/m) on AC surface ............42

4-3. Phenol removal as a function of AC coating strategy with and without irradiation ..63

4-4. Electrical conductivity values for low surface area carbons ....................................67

4-5. Elemental carbon and ash contents of low surface area carbons.............................68

4-6. Average densities of 3% (m/m) titanium dioxide coated carbons.............................74
















LIST OF FIGURES


Figure Page

2-1. TiO 2 structure. A ) anatase, B ) rutile.................................. ....................... .. .......... 14

2-2. Photocatalysis m echanism .......................................................... ............... 15

2-3. Band gaps for (a) metals, (b) semiconductors and (c) insulators with Fermi level
(SF) in dictated .........................................................................19

2-4. Alignment of Fermi levels and formation of Schottky barrier ...............................20

2-5. Kinetics of phenol disappearance in the presence and absence (photolysis) of
various illum inated solids............................................... .............................. 22

3-1. Coating activated carbon with TiO2 via mechanical attachment (Theta Composer,
Tokuju Corp., Japan) .................. ............................. ........ .. ........ .... 31

3-2. Concentration vs. Absorbance correlation for phenol on UV spectrophotometer
m measured at 270nm w avelength ........................................ .......................... 35

4-1. EDS of A) ash residue of uncoated Bionuchar, B) Bionuchar coated with TiO2, C)
ash residue of uncoated F400, D) F400 coated with TiO2, E) ash residue of
uncoated HD4000, F) HD4000 coated with TiO2 ................... .... .......... 44

4-2. Titania coated Bionuchar with the TiO2 blocked pores circled..............................47

4-3. Bionuchar showing cellulose structure of the wood based AC...............................48

4-4. Titania coated on the surface of F400 ............................................. ............... 49

4-5. TiO2 agglomerate (shown circled) near a macropore on the surface of HD4000 ......50

4-6. Surface of HD4000 showing uniform coating of TiO2 which tends to form
agglom erates..................................... ............................... ........... 51

4-7. Batch adsorption studies of virgin ACs in the absence of UV..............................53

4-8. Batch adsorption studies of 3% (m/m) coated ACs in the dark compared to TiO2
slu rrie s ............................................................................... 5 4









4-9. Batch adsorption-photocatalysis studies of 3% (m/m) coated ACs in the presence
o f U V ..............................................................................................5 6

4-10. Batch adsorption-photocatalysis studies of 3% (m/m) coated ACs loaded to the
reactors on a volume basis in the presence of UV ................................................59

4-11. Batch adsorption studies: A) 3% (mol/mol) coated ACs in the absence of UV
loaded to the reactors by volume, B) Photocatalysis studies of 3% (mol/mol)
coated ACs in the presence of UV loaded to the reactors on a volume basis ..........60

4-12. Comparison of irradiated virgin ACs with virgin and coated ACs in the presence
and absence of U V ............................... .. .. .......... ...... .... ...... ...... 62

4-13. EDS scan of ash residue of uncoated anthracite coal ................... ....... .........69

4-14. EDS scan of ash residue of uncoated bituminous coal...........................................69

4-15. EDS scan of ash residue of pitch coke .... ............................................... 70

4-16. Adsorption studies of coated low surface area carbons and titanium dioxide
slurry .................................. ..................................71

4-17. Batch photocatalysis studies of carbons and titanium dioxide slurry (m/m)............73

4-18. Photocatalysis studies of coated low surface area carbons in the presence of UV
(m /v ) ...................................... .................................................... 7 5

4-19. Photocatalysis studies of low surface area carbons in the presence of UV (mol/v).76

A-1. Replicate data sets for virgin AC studies performed in the presence of UV
demonstrating the phenol* phenomenon observed in Section 4.2.4......................84

B-1. Recirculation column studies performed with F400 using different flowrates
(degrees of fluidization) in the presence and absence of UV..............................85

C-1. Adsorption runs of column tests performed with HD4000 in the presence and
absence of UV radiation measuring phenol concentration in the effluent ..............86

C-2. Concentration of phenol in effluent during regeneration runs of column studies .....86















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

INVESTIGATION OF PROPERTIES RESPONSIBLE FOR PHENOL REMOVAL VIA
TITANIA COATED ACTIVATED CARBON

By

Vivek Shyamasundar

December 2005

Chair: David Mazyck
Major Department: Environmental Engineering Sciences

Activated carbon has long been used to remove organic pollutants from water.

However, a major disadvantage is that it needs to be regenerated regularly, which is

primarily done offsite. The regeneration decreases the adsorption capacity of the

activated carbon and hence increases costs. Titanium dioxide, through the mechanism of

photocatalysis, mineralizes the organic pollutants to carbon dioxide and water. A

drawback of this technology is that when used in the form of a slurry, the separation of

the titanium dioxide from the treated effluent incurs costs. The use of a titanium dioxide

coated activated carbon would therefore result in the elimination of the separation of

titanium dioxide from water and would at the same time result in the in-situ regeneration

of activated carbon. It has however been found in previous research that the behavior of

the TiO2 coated activated carbons varies for different ACs.

The objective of this research was to identify a simple yet effective procedure to

coat the ACs with titanium dioxide and to identify properties of activated carbons that









influence a synergistic effect between the activated carbon and the titanium dioxide. On

the premise that conductivity due to the presence of metals influences photocatalysis in

the TiO2-AC composites, batch studies were performed comparing the adsorption and

photocatalysis of three activated carbons having different properties. Characterization

studies were also performed on the activated carbons comparing their surface

morphologies and compositions of their ash. It was observed that ACs having less acidic

groups on their surface resulted in better adsorption of the pollutant (i.e., phenol). It was

also observed that the coal based ACs had a rougher surface resulting in a more uniform

deposition of titanium dioxide on their surface.

To further isolate and study the mechanism of photocatalysis, low surface area

carbons were used in separate studies. These carbons differed in terms of ash composition

and electrical conductivity. These carbons were coated with titanium dioxide and tested

in batch for photocatalytic activity. The ash composition of these carbons was also

compared.

For activated carbons and low surface area carbons, results showed no noticeable

influence of conductivity on the photocatalytic activity of the ACs or the carbons.

Photocatalysis was observed as being a secondary mechanism compared to adsorption

which was clearly dominant.














CHAPTER 1
INTRODUCTION

Over the past 20 years, the planet's drinking water resources have entered into a

very fragile state. A number of organic pollutants, the sources of which are solvents,

fertilizers (e.g., NPK), pesticides and chlorophenols, among others, have resulted in

widespread groundwater contamination (Hoffman et al., 1995). To cope with the growing

pollution of the hydrosphere, a number of regulations and programs are being

implemented. The primary strategy being applied currently is the chemical treatment of

polluted drinking water, surface water, groundwater and wastewater. Pollutant removal in

drinking waters may involve flocculation, sterilization, filtration and disinfection

processes (Legrini et al., 1993). Technologies utilized may also include ultrafiltration, air

stripping, carbon adsorption and oxidation (via ozonation or hydrogen peroxide)

(Serpone, 1995).

Recent advances in the field of water treatment have led to the development of

oxidative degradation techniques for organic compounds dissolved in water. These

processes are known as advanced oxidation processes (AOPs). Heterogeneous

photocatalysis is one type of advanced oxidation process that can be used to counter the

growing contamination of precious water resources due to organic compounds. This

process entails the use of a photocatalyst that is activated under the influence of UV

radiation which ultimately results in the oxidation of the organic contaminants to water

and carbon dioxide, resulting in an acceptably minimal waste stream. This lack of toxic

organic byproducts is a significant advantage of this technology over the contemporary









technologies mentioned earlier, which simply transfer the pollutant from one phase to

another. The technology can be very useful in applications for sustenance during long-

term NASA space missions. Due to the limited reserves of water that may be carried on

the space shuttle, a closed-loop photocatalytic system would theoretically be able to

extend the lifespan of a given finite reserve of water almost indefinitely. Additionally,

due to the depleted resources of drinking water here on Earth, it is of great interest to

adapt this technology for industrial purposes as well as for household water purification.

The use of photocatalysis has been widely studied with the photocatalyst (TiO2) in the

form of a slurry. However, a slurry based system used for industrial purposes or for

household use may result in a waste stream consisting of the photocatalyst itself.

Separation of TiO2 from the treated effluent could further complicate matters while

increasing the cost of the system.

Filters containing activated carbon and other treatment media are currently used in

many households as secondary treatment systems. However, these filters need to be

replaced from time to time due to the exhaustion of the activated carbon. Hence,

combining the theoretically infinite sustainability of the photocatalysis phenomenon with

the simplicity of an activated carbon filter, a compact treatment device is sought to be

developed with the photocatalyst (titanium dioxide) coated on the activated carbon. The

main challenges in the development of such a system as a viable application for industrial

and household use are as follows:

* To identify a simple technique by which the activated carbons may be coated with
titanium dioxide. Preferably, the coating technique would not incur any
considerable additional costs.

* To develop an AC-TiO2 composite that demonstrates both high capacity as well as
high photocatalytic efficiency.









Numerous coating techniques were tested. The techniques were evaluated on the

basis of simplicity and effectiveness. Making use of the coating technique that best fits

the need; efforts were concentrated on optimizing an AC-TiO2 composite that shows the

best performance with respect to photocatalysis of a model organic contaminant; in this

case phenol (C6H50H). Hence, it is imperative to isolate the intrinsic and physical

properties of activated carbons which aid in photocatalysis, assuming photocatalysis is

dependant on the intrinsic properties of the ACs. To do this, experiments were performed

on a variety of different ACs.

The need for greater scientific understanding of the phenomenon of photocatalysis

and the factors that affect it has provoked the author to concentrate part of the efforts on

the science behind photocatalysis. Focus has especially been placed on whether a synergy

exists between titanium dioxide and any particular activated carbon. This has been

investigated through the use of TiO2 coated carbonaceous materials with extremely low

adsorption capacity. Electrical conductivity of the ACs due to the presence of conductive

metals has been hypothesized to be a factor influencing the photocatalysis seen with the

AC-TiO2 composites. Identification of the factors affecting photocatalysis will in the

future allow us to further optimize a system.

In summary, this study is an effort to develop a system for in-situ regeneration of

activated carbon for household as well as industrial purposes with a concentration on

identifying a TiO2-AC composite that performs with the most synergy with respect to

adsorption and photocatalysis.














CHAPTER 2
LITERATURE REVIEW

2.1 Activated Carbon

Activated carbons are adsorbents created from carbonaceous precursors that either

experience thermal or chemical activation to increase the internal porosity of the raw

material giving rise to high surface area carbonaceous materials (500-1500 m2/g).

Suitable raw materials can include wood, coal (anthracite, bituminous, and lignite), coke,

coconut shells, fly ash and even rice husk (Karthikeyan et al., 2005; Kinoshita, 1988).

Depending on the precursor, the ash content (which can include iron, aluminum, and

silica) of activated carbons can vary (e.g., 1-20% (w/w)).

Thermal or steam activation entails the oxidation of char, which is created during

the first phase of the activation process via pyrolysis, with the help of oxidizing agents

such as steam, air or carbon dioxide at temperatures between 800-1000 C. Chemical

activation involves heating the carbonaceous precursor and a dehydrating agent (ex.

phosphoric acid, carboxylic acid, sulfuric acid, nitric acid) to a temperature between 200-

6500C. The dehydrating agent may then be leached out and reused (Kinoshita, 1988).

Hence, chemically activated carbons typically have a low pH due to acidic groups on

their surface. This has resulted in loose definitions such as L-type activated carbons for

chemically activated carbons (due to 'low' pH) and H-type activated carbons for

physically activated carbons (due to 'high' pH). The difference in precursors and

activation processes result in activated carbons having varying physical and chemical









properties. As an aside, a physically activated carbon that is acid-washed to remove

organic or for other purposes may exhibit L-type characteristics.

Activated carbon is widely used in potable and wastewater treatment for the

removal of organic and inorganic pollutants. It may be used in its powdered form, known

as 'powdered activated carbon' (PAC) (particles pass through a 325 mesh sieve), or in the

granular form (GAC).

2.1.1 Physical Properties of AC

Surface area. The defining physical characteristic of activated carbon (AC) is its

well-defined pore structure which is one of the main reasons for its high internal surface

area. It is this internal surface area that is available for the adsorption of pollutants. Even

though properties such as pore size distribution, surface chemistry and adsorbate-

adsorbent interactions play a role, surface area is still considered the limiting factor for

adsorption of the target pollutant by practitioners. Hence, a greater surface area of the AC

will result in a greater potential for the adsorption of the pollutant. The typical surface

areas of activated carbons are between 500 m2/g-1500 m2/g.

Much of the carbon literature focuses on the importance of surface area but surface

chemistry is the dominant variable that controls adsorption in most cases. Therefore, the

role of surface chemistry will be addressed throughout this review and in the results and

discussions of the data.

The most common method for determining the surface area of an adsorbent is using

the Brunauer, Emmett, and Teller (BET) theory. The BET theory was initially developed

for characterization of physical adsorption. Hence its adaptation to characterize surface

area is not considered error free as it cannot universally be applied to all types of physical

adsorption (Dabrowski, 2001). The surface area analysis by the BET theory is performed









with N2 which may result in an inaccurate prediction of adsorption trends for other

molecules (Dabrowski, 2001). Hence, the size of the molecule being adsorbed must be

considered while characterizing the surface area of the adsorbent. For this reason, the

pore size distribution (PSD) of the adsorbent is an important factor in characterizing the

adsorption of activated carbon. The BET isotherm is given below.

1/ (W*((Po/P)-)) = 1/ (Wm*C) + (C-l)/ (Wm*C)*(P/Po)

W: Weight of gas adsorbed at P/Po
P: Pressure of overlying gas
Po: Saturation pressure of the gas
Wm: Adsorbate weight in monolayer coverage
C: Constant related to the energy of adsorption

Pore size distribution. The pores of activated carbon consist of three size ranges

according to IUPAC recommendations.

* Micropores: Less than 20 A (2 nm)
* Mesopores: Between 20 A (2 nm) and 500 A (50 nm)
* Macropores: Greater than 500 A (50 nm)

Nowadays, the word nanopore is used in general to define micropores and

mesopores (Dabrowski, 2001). The activation protocol to a large extent dictates the pore

sizes in the AC.

Micropores in activated carbon are usually comparable to the sizes of the adsorbate

molecules (e.g., size of phenol molecule is 6 A (Mazyck, 2000)). Therefore, all the atoms

of the adsorbent (activated carbon) can interact with the adsorbate species (Dabrowski,

2001) which is the main difference between adsorption mechanisms of micropores and

that of mesopores and macropores. Micropore adsorption is hence a pore filling process

in which the volume of the micropores is the controlling factor.









The walls of macropores are formed by a great number of adsorbent atoms or

molecules. The boundary of interfaces as well as the adsorbent surface area have a

distinct physical meaning (Dabrowski, 2001). Mono and multilayer adsorption

successively takes place on the surface of mesopores and their final fill is in accordance

with the mechanism of capillary adsorbate condensation (Dabrowski, 2001). Mesopores

participate in the transport of the pollutant molecules to the major adsorption sites in the

micropores. They are characterized mainly by their specific surface area and pore size

distribution.

In the case of macropores, the action of adsorption forces does not occur

throughout their void volume but at a short distance from their walls. Like mesopores,

macropores are also diffusion pores in that they principally transport the pollutant

molecules to smaller pores.

During the adsorption process in activated carbon, four main steps occur:

1. Bulk diffusion or the diffusion of the molecule through the bulk liquid.

2. Film diffusion or the diffusion of molecules through the thin film (boundary layer)
surrounding the activated carbon.

3. Pore diffusion or the diffusion of the molecules through the pores (or along the
pore walls) of activated carbon.

4. Adsorption of the adsorbate.

The rate determining step is step 3 which is the diffusion through the pores of the

activated carbon, which is largely influenced by the pore size and the size of the diffusing

molecule.

2.1.2 Chemical Properties of AC

According to Coughlin and Ezra (1968) the structure of activated carbon is

graphitic in nature with a large number of molecular layers. The layers contain carbon









atoms which are bound together by three a bonds and one n bond and result in sp2

hybridization. There is also the possibility of sp3 (tetrahedral) hybridization taking place

resulting in the cross linking of the graphite layers (Coughlin and Ezra, 1968; Hobbs,

2005). Van der Waals forces are responsible for holding together the carbon atoms within

the graphite layers resulting in the microcrystalline structure. Other atoms that are bound

in this structure may be present within the layers; known as basal planes, forming

heterocyclic rings or at the edges of the carbon molecules, known as edge sites. These

atoms form functional groups. Hence, as explained by Leon y Leon and Radovic (1994),

the surface of activated carbon consists of two types of sites: 1) basal planes and 2) edge

sites. The edge sites are considered to be more reactive with oxygen groups than the basal

planes due to the fact that edge sites contain a single unpaired electron (Radovic et al.,

1997).

Functional groups. Functional groups on the surface of activated carbons can be

classified as acidic and basic. A number of functional groups have been found on the

surface of activated carbon. These groups consist of carboxyl, lactonic, phenolic,

carbonyl, quinones and pyrones among others (e.g., peroxide). Oxygen functional groups,

despite being a small fraction of the overall carbon surface, are however very active and

exhibit a large influence on adsorption capacity (Nevskaia et al., 1998; Nevskaia et al.,

2004; Leon y Leon et al., 1994). The behavior of functional groups is governed by the

variable electronegativity of the base atoms. Hence, atoms such as oxygen with higher

electronegativity pull electrons towards its nucleus, resulting in a negative charge on that

atom and a positive charge on the remaining atoms. This results in the groups becoming

polar, hence affecting their interactions with the adsorbing molecules. Hence, bound









oxygen functionality plays a crucial role in adsorption behavior of the activated carbon

towards the pollutant.

The surface oxides may either be acidic or basic as explained earlier. Acidic surface

oxides result in a positive surface charge due to protonation whereas basic surface oxides

result in a negative surface charge due to deprotonation of the functional groups. The

source of a positive surface is illustrated by the following equation:

C, + 2H20 CH30+ + OH-

The formation of acidic or basic surface oxides depends upon the temperature to

which the carbon is exposed to during its activation process. Exposing the carbon to

temperatures between 200-5000C results in the formation of acidic oxides carboxylicc,

lactonic and phenolic). Basic surfaces on the carbon are formed when it is heated in an

inert environment to remove initial oxides and cooled prior to exposing it to oxygen. It

has also been found that the catalytic properties of ACs can be enhanced by the

elimination of some of the acidic functional groups and introduction of basic functional

groups on the carbon surface (Salame et al., 2003; Tessmer et al., 1997).

2.1.3 Adsorption Mechanisms

The topic of adsorption mechanisms seen in activated carbon has been one of much

debate. There are two schools of thought the 7t-7t bonding theory proposed by Coughlin

and Ezra (1968) and the electron donor acceptor theory (EDA) proposed by Mattson

(1969). The two theories have been explained as follows:

7r-7r bonding theory. This theory proposes that the 7t bond of the aromatic ring of

carbon covalently bonds with the aromatic ring of the adsorbate or in other words that 7t

bonds occur between two p-orbitals. The presence of electron withdrawing groups such

as oxygen on the carbon surface results in a reduction in the electron density in the 7t









system of the carbon basal plane. Hence, in theory the presence of electron withdrawing

groups should weaken the dispersive adsorption forces between the adsorbate 7t electron

and the carbon basal planes by creating positive holes in the conduction band of the 7n

electron system (Coughlin and Ezra, 1968; Radovic et al., 2001). Adsorption at the basal

planes is hence due to 7t-7t interactions and is typically weaker whereas adsorption at the

edge sites is typically stronger (Coughlin and Ezra, 1968). In fact species bonded to the

edge sites may cause the disturbances in the electron density of the basal plane, resulting

in decreased interactions and reduced adsorption of organic compounds. The presence of

oxygen groups or electron withdrawing groups in general is hence thought to be one of

the primary reasons for reduced adsorption of organic by activated carbon. It should be

noted that the opposite is also true, where the removal of electron withdrawing groups

from the surface of ACs results in an increase in adsorption potential.

Electron donor theory. The electron donor theory was proposed by Mattson et al.

(1969) and refuted Coughlin and Ezra (1968) by suggesting that an exchange of electrons

takes place during the adsorption process in which the carbonyl oxygen serves as the

donor and the aromatic ring serves as the acceptor. Mattson et al. (1969) suggest that the

carbonyl group serves as the major electron donor in the donor-acceptor complex and is

hence responsible for an increase in phenol adsorption. The subsequent decrease in

adsorption has been credited to carboxyl functional group formation (Mattson et al.,

1969). Subsequent findings by numerous research groups has shown that an increase in

oxygen functional groups results in the increased formation of both carboxyl as well as

carbonyl groups, hence refuting the findings of Mattson et al (1969).









Adsorption of phenol. Phenol adsorption is dependent mainly on the porosity and

the surface chemistry of the activated carbon. Other factors in its adsorption include the

surface area, ash content of the AC and the concentration of carbon atoms in the matrix.

The adsorption of aromatic compounds from solution has been studied extensively

(Radovic et al., 1997; Mattson et al., 1969; Coughlin and Ezra, 1968; Coughlin et al.,

1968; Moreno-Castilla et al., 1995; Nevskaia et al., 2004). It has been found by Leng and

Pinto (Leng and Pinto., 1997: as reported by Salame et al., 2003) that the uptake of

phenol is due to a combination of the interaction of phenol with the basal planes

(physisorption) as well as by surface polymerization. It has also been found that the

phenol uptake increases as the carboxylic functional groups are removed from the carbon

surface. The presence of carboxylic functional groups (and hence oxygen groups) which

are primarily formed during activation, results in the AC surface becoming more polar

resulting in water behaving as a competing species for the adsorption sites. In addition to

the decreased polarity of the AC surface, the removal of carboxylic functional groups

also enhances 7t-7t interactions. Furthermore, activated carbons, under oxic conditions are

prevented from adsorbing phenol in the presence of functional groups (Salame et al.,

2003; Nevskaia et al., 1998, Nevskaia et al., 2004). This is due to the fact that the ability

for the adsorption of phenol via oxidative coupling decreases as a result of the functional

groups. Therefore, the catalytic properties of AC can be enhanced by the introduction of

basic functional groups and the elimination of some of its acidic groups. Hence, at low

pH, the amount of phenol adsorbed increases slightly with an increase in pH until a

certain point after which a decrease in uptake is noticed.









The pH of the phenol solution is not believed to play a significant role in its

adsorption onto activated carbon. Phenol is considered to be a weak acid. The loss of a

hydrogen ion results in the formation of a phenolate anion, the negative charge of which

is delocalized around the ring. The negative charge gets spread out over the entire ion due

to the overlapping of one of the lone pairs of electrons on the oxygen with the delocalized

electrons on the benzene ring. Hence, the phenolate ion becomes relatively more stable.

However, since oxygen is the most electronegative element on the anion, the delocalized

electrons will be drawn towards it which results in the hydrogen ion being attracted

towards it again rendering solution pH a less important factor in the adsorption of phenol.

2.2 Photocatalysis

The phenomenon of photocatalysis was discovered by Fujishima and Honda in

1972 (Linsebigler et al., 1995). Since then it has been extensively studied as an option for

the treatment of contaminants in water. Photocatalysis results in the mineralization of

organic to CO2, water and the corresponding mineral acids as the final products.

Photocatalytic treatment of water requires the use of the titanium dioxide (TiO2)

photocatalyst in the form of a slurry or the immobilization of the titanium dioxide on a

substrate. The use of a slurry is beneficial due to the large surface area of the catalyst

which would be available for the photocatalysis reaction (Byrne et al., 1997). However,

due to the size of particles used in the slurry, this method of treatment requires

centrifugation or microfiltration techniques to remove the fine particles from the treated

liquid, resulting in technical and economical problems (Legrini et al., 1993). The use of

titania as a slurry also results in an impedence to the penetration of light through the

solution due to increased opacity caused by the TiO2 particles. This however only takes

place after a certain optimum titanium dioxide loading which is system dependent.









Alternatively, the separation procedure problem can be overcome by immobilizing

the TiO2 on a substrate (Dijkstra et al., 2001; Ollis et al., 1991; Ray et al., 1997). A

number of techniques have been used such as immobilization on beads, glass pearls

(Trillas et al., 1996) inside tubes of glass or Teflon (Dijkstra et al., 2001) fiberglass or

woven fibers, silica-titania composites and immobilization onto activated carbon

(Dijkstra et al., 2001). This procedure has had varying results depending on the substrate

onto which the TiO2 has been immobilized. Historically, it has been found that a

suspended slurry is more efficient due to the absence of mass transfer limitations and

larger specific surface area available for photocatalysis (Dijkstra et al., 2001). However,

it has also been shown that a TiO2-activated carbon composite particle has resulted in a

synergy by which the degradation of a dye (methylene blue) was increased (Khan, 2003;

Khan et al., 2006).

2.2.1 Titanium Dioxide

Titanium dioxide is the most commonly used photocatalyst due to its properties of

being relatively inert, corrosion resistant and low cost as compared to other

photocatalysts such as ZnS, WO3 and SrTiO3. Along with ZnO, it also shows the most

photocatalytic activity as compared to other photocatalysts. It is found in three major

forms: anatase (octahedral) (Figure 2-1 A), rutile (tetragonal) (Figure 2-1 B) and brookite

(orthorhombic). Anatase and rutile are the forms that show photocatalytic activity with

anatase showing higher activity than rutile (Ohtani et al., 1993). The synthesis of anatase

from rutile titania can usually be achieved by heat treatment at temperatures between

3000C-6000C. Pure grades of titanium dioxide are seldom found naturally as it is often

completed with other minerals and metals. Recent studies have however shown that a

combination of anatase (70-75%) and rutile (30-35%) are more active than pure anatase









(Bacsa and Kiwi., 1998; Muggli et al. 2001). The grade of titanium dioxide used herein

was commercially available, synthetically manufactured Degussa P25. Degussa P25 has a

structure which is a combination of anatase (70%) and rutile (30%). It has repeatedly

demonstrated good photocatalytic degradation.














A B

Figure 2-1. TiO2 structure. A) anatase, B) rutile

2.2.2 Mechanism of Photocatalysis

When UV light energy is incident on TiO2, a valence band electron is excited to the

conductance band; on the condition that the incident photons have a higher energy (X <

380 nm for anatase) than the band-gap energy. When this occurs, it results in the

formation of a hole in the valence band and hence an electron-hole (e-h) pair. A high

quantum yield of the photocatalytic reaction can be achieved by the prevention or

prolongation of the recombination of the electron-hole pair. This can be done with the

help of an electron scavenger. An electron scavenger which may also be called a 'hole

trap' may consist of adsorbed hydroxide ions (OH-), oxygen or water molecules which

react with the electron-hole to create a hydroxyl radical (OH*). An illustration of the

mechanism of photocatalysis is shown in Figure 2-2 (adapted from Linsebigler et al.,









1995). The presence of water or hydroxide ions is believed to be essential for the

complete oxidation of organic molecules to CO2 and H20 since their absence has resulted

in incomplete oxidation of organic molecules (Turchi and Ollis., 1990). Dissolved

oxygen is believed to play an important role as an electron acceptor, hence preventing the

electron-hole pair from recombining. In the absence of an electron acceptor,

recombination may take place in picoseconds, resulting in no photocatalytic degradation.

Studies performed (Chhor et al., 2004; Dijkstra et al., 2001) have shown higher

efficiencies in photocatalytic activity for phenol, salicylic acid and formic acid with an

increase in the presence of dissolved oxygen. Semiconductors such as TiO2 however have

an advantage in that their recombination times are slower than those for conductors

(metals) due to the absence of a continuum of interband states which would aid in the

recombination of the electron-hole pair (Carp et al., 2004). This ensures that the electron-

hole pair has a sufficiently long time to diffuse to the surface of the catalyst and initiate

the redox reaction (Carp et al., 2004).


A'

Figure 2-2. Photocatalysis mechanism









The following four cases of hydroxyl attack are typically considered which are also

illustrated in Table 2-1 (adapted from Turchi and Ollis, 1990).

* Reaction occurs when the radical and the pollutant are adsorbed.

* The radical is not bound and it reacts with the organic pollutant molecule.

* The adsorbed radical reacts with a free organic molecule moving past the surface.

* Both the radical as well as the organic molecule are unbound and they react with
each other in the fluid phase.

Table 2-1. Photocatalysis reaction mechanisms
Excitation TiO2 + hv e- + h+
Recombination e- + h+ heat
Adsorption 02 + TiV OH-
TiIv + H20 Tiv H20
Trapping Tiv OH- + h+ Ti" OH*
TiV H20 + h+ TiV OH* + H
Hydroxyl attack
Casel Ti" OH* + R1,ads TiIv + R2,ads
Case 2 OH* + R1,ads R2,ads
Case 3 Ti" OH* + R1 Ti" + R2
Case 4 OH* + R1 R2


2.2.3 Improvements in Photocatalysis

A number of techniques have been attempted to enhance the efficiency of

photocatalytic systems. One of these techniques is the doping of TiO2 particles with

metals. Doping of the semiconductor with various transition metal ions has in the past led

to an enhanced efficiency of photocatalytic systems. TiO2 particles can be substituted or

interstitially doped with different cations or can form mixed oxides. However, this

technique also poses a complex problem. The net induced alteration of the photocatalytic

activity is made up from the sum of the changes which occurs due to the following

factors.









* The light absorption capability of the TiO2 photocatalyst

* Adsorption capacity of the substrate (activated carbon) molecules at the catalyst
surface

* Interfacial charge transfer rate

As the concentration of the dopant increases, the space charge region becomes

narrower resulting in the efficient separation of the electron-hole pairs within the region

by the dielectric field before recombination. An increase in dopant concentration

however, results in the narrowing of the space charge region. When this occurs, the

penetration of light into the TiO2 greatly exceeds the space charge layer. Due to the

absence of a sufficient driving force dopantt), to separate them, electron-hole

recombinations increase due to the absence of a driving force to separate them. Hence, an

optimum concentration of dopant ions is required which would make the thickness of the

space charge layer similar in magnitude to the light penetration depth. (Carp et al., 2004;

Hoffman et al., 1995; Linsebigler et al., 1995).

The recombination of photogenerated electrons and holes are often times

influenced by doping ions. In most cases the recombination times are enhanced by the

doping ions, hence impeding the progress of the reaction. For example, in both p-type

doping cationss having valencies lower than that of Ti+4 such as Al+3, Cr+3 and Ga+3) and

n-type doping cationss having valencies higher than Ti+4 such as Nb+5, Ta+5 and Sb+5) an

inhibition effect which is ascribed to the recombination of the electrons and holes is seen.

P-type dopants act as acceptor centers for photoelectrons whereas n-type dopants act as

donor centers of photoelectrons (Carp et al., 2004; Hoffman et al., 1995; Linsebigler et

al., 1995; Matos et al., 1998). Metallic ions such as Fe3+ and Ru3+ however behave

differently than P-type and N-type dopants. This is due to their half-filled electronic









configuration which results in them being more stable. This electronic configuration is

destroyed when metallic ions are trapped, leading to a decrease in their stability.

Moreover, a transfer of the trapped electrons to oxygen adsorbed to the surface of the

catalyst may take place with the metallic ions returning to the original stable half filled

electron structure resulting in the transfer of charge and efficient separation of the

electrons and holes by trapped electrons. Hence, co-doping may be a viable technique to

improve charge separation, thereby improving photocatalytic efficiency. In fact, studies

by Leng et al. (2002) have shown a synergistic effect resulting in the increased

photocatalytic degradation of chloroform in solution to as much as 5 times the original

rate when TiO2 has been doped with Fe+3 and Eu+3. In this case, Fe+ serves as a hole trap

and Eu+3 serves as an electron trap (Carp et al., 2004). It can therefore be summarized

that the process of photocatalytic oxidation through a semiconductor such as titanium

dioxide can be modified and controlled to a certain extent depending on the type of

metals, the extent to which they are doped on the titania and the combinations of metals

used in the doping process.

2.3 Fermi Energies/Levels, Electrical Conductivity and Schottky Barrier

Fermi level is a term used to describe the top of the 'sea' of electrons (electron

energy levels) at a temperature of absolute zero. At absolute zero, electrons pack into the

lowest available energy states to form this collection of energy states. Hence, at absolute

zero, no electron will have enough energy to rise above the surface of the Fermi 'sea'.

Ordinary electrical processes involve energies in the range of a fraction of an electron

volt. However, the Fermi energies in the case of metals are on the order of electron volts,

implying that most of these electrons cannot receive sufficient energy from those

processes.








The Fermi energy is also important in characterizing the electrical conductivity of

metals. The velocities of the electrons participating in conduction can be calculated from

the Fermi energy hence providing us with the drift velocities of electrons encountered in

the metals. Also crucial to the conduction process is the 'Band theory of solids' which is

useful in visualizing the difference between conductors, semiconductors and insulators.

The primary requirement for the conduction process is the presence of electrons in

the conduction band. In insulators, electrons in the valence band are separated from the

conduction band by a very large gap whereas in conductors, the valence band and the

conduction band overlap. The case of semiconductors (such as titanium dioxide) is

unique in that the intrinsic Fermi level is half way between the bottom of the conduction

band and the top of the valence band. Therefore, even a small amount of doping of the

semiconductor can result in a large increase in conductivity. Hence, as the Fermi level for

semiconductors approaches the conduction band, it denotes an n-type whereas when the

Fermi level is closer to the valence band, it denotes a p-type semiconductor. Figure 2-3

and Figure 2-4 illustrate the concept of Fermi levels of metals, semi-conductors and

insulators along with the associated band gap.



(a(b) (c)



- - - - -




Figure 2-3. Band gaps for (a) metals, (b) semiconductors and (c) insulators with Fermi
level (SF) indicated









The effect of metals added to semiconductors such as TiO2 is as follows. The

dopant metals change the photocatalytic properties of the semiconductor by changing the

distribution of electrons. Hence, when a metal and a semiconductor are electrically

connected, electron migration takes place from the semiconductor to the metal until the

two Fermi levels are aligned as shown in Figure 2-4 (Linsebigler et al., 1995; Carp et al.,

2004; Hoffman et al., 1995) (figure adapted from Linsebigler et al., 1995). This

phenomenon forms what is known as the Schottky barrier. It must be noted that the

particular case mentioned above takes place when the work function (
higher than that of the semiconductor. The work function is the least amount of energy

required to remove an electron from the surface of a conducting material to a point just

outside the metal.



Ex



E 7-b ---CB





VB


Metal Semiconductor (n-type)

Figure 2-4. Alignment of Fermi levels and formation of Schottky barrier

The Schottky barrier produced at the interface of the metal and the semiconductor

acts as an electron trap, hence hindering electron-hole recombination which results in









increased photocatalysis (Linsebigler et al., 1995; Carp et al., 2004). This scenario is

what will be referred to as a favorable difference in Fermi energies during the progress of

this paper.

2.4 Previous Synergy in AC-TiO2 Systems

Activated carbon is a well known adsorbent especially for systems dealing with

organic. It is also well known that TiO2 is capable of oxidizing organic to water and

carbon dioxide. Hence, it can be hypothesized that a combination of these two materials

would result in a combination of adsorption and degradation which in theory would result

in a synergy. This would effectively mean that the pollutant is adsorbed onto the

activated carbon and is then degraded due to the TiO2 photocatalyst in the presence of UV

radiation resulting in the theoretically infinite life of AC.

However, a number of variables are involved in the process which may limit the

effect of photocatalysis on the adsorbed pollutant. Properties of the AC such as pore size

distribution, surface area, surface functional groups and the acidic-basic characteristics of

the AC may make this theory extremely system specific when applied. Considerable

research has been conducted in the field of TiO2-AC composites (Matos et al., 1998;

Matos et al., 2001; Khan., 2003; Khan et al., 2006).

Matos et al. (1998) and Khan et al. (2006) both found that titania seems to show a

synergistic effect with activated carbon. To elaborate, the presence of activated carbon

alone in a system for the removal of a pollutant has been shown to be less effective than a

combination of the activated carbon and titania in the presence of UV. Similarly, the

presence of the titania and AC (in the presence of UV) in systems has shown better

performance than a system consisting solely of titania.










A system tested by Matos et al. (1998) consisted of 50 mg of TiO2 combined with

10 mg of AC in the form of a slurry with the target pollutant being phenol. This system

was compared to other systems consisting solely of either titania or activated carbon. The

experiments were conducted such that the systems were not exposed to UV radiation

until equilibrium had been established with respect to adsorption. Once equilibrium was

established, the systems were exposed to UV radiation (340 nm). The following figure

(Figure 2-5, adapted from Matos et al., 1998) illustrates the results of the kinetics of

phenol disappearance under the various systems tested.




20 m



oo
%0 0 2
a 0 TiO2
0 AC
a A

| o o Photolyti



0 o
S TO D 1 D T02 -A




0 0
0


0

9- ?
-60 0 60 126 180 240 300 360 420 4s0

Time (min)

Figure 2-5. Kinetics of phenol disappearance in the presence and absence (photolysis) of
various illuminated solids









The results of Matos et al. (1998) show that the apparent rate constant for the

system consisting solely of titania is 5.6 x 10-3/min whereas the apparent rate constant for

the TiO2 AC system is 1.39 x 10-2/min. Hence it has been concluded that the

photocatalytic efficiency seen in the case of the titania-AC slurry is 2.5 times that seen by

the system with titania alone. The authors ascribe this result to the adsorption of phenol to

the AC which is followed by a mass transfer to the photoactive titania. In effect, the

essence of the synergy in the case of the titania-AC system is the common interface

between the AC and TiO2 which has been postulated to consist of almost half of the total

surface of exposed titania.

However, the rate constants would be calculated from a point at which adsorption

of phenol has taken place and the systems have reached equilibrium. In the case that the

rate constants of the two systems are calculated solely from the time of irradiation, it may

be noticed that the AC-titania system actually has a slower rate than the system

containing only titania. Hence, although a beneficial effect is seen to be taking place with

the addition of AC, it does not agree with the definition of synergy as proposed by this

author. From the arguments made above, it is clear that a synergy was not prevalent in the

titania-AC system tested by Matos et al. (1998).

A subsequent paper by the same authors (Matos et al., 2001) investigated the role

of the type of AC on the photocatalytic degradation of organic pollutants. Similar tests to

the work described previously have been performed although emphasis has been given to

the surface properties of the AC and their influence on photocatalysis. It was concluded

by Matos et al. (2001) that H-type ACs (High pH/ High temperature activated) when

added to a titania slurry show a beneficial trend in photocatalysis while the opposite is









true for L-type ACs (Low pH/Low temperature activated). Although the terms H-type

and L-type carbons can only be used loosely, the veracity and extent to which the

conclusion by Matos et al. (2001) may apply has been investigated by this author.

Another case of the synergistic effect of titania with AC has been investigated by

Khan (2003). Their study was performed by coating TiO2 (Degussa P25) on various

commercially available ACs. The coated ACs were then used in the adsorption/

degradation of dyes (i.e., methylene blue, reactive red). The results seen by Khan (2003)

showed that the activated carbons F400 (Calgon), HD4000 (NORIT) and a wood-based

activated carbon synthesized in their laboratory showed an increased removal of the dye

in the presence of UV as opposed to in the absence of it. On the other hand, Bionuchar

(Westvaco) showed the opposite trend. It was concluded that the presence of ionic metals

such as iron (Fe+) in the coal based carbons (F400, HD4000) resulted in the synergy.

This was based on the hypothesis that the ionic metals present in the coal based carbons

act as sinks for electrons, thereby hindering electron hole recombination and hence

enhancing photocatalysis. Iron in the form of Fe+ is commonly used in the doping of

TiO2 to enhance photocatalysis. Iron is also a good conductor and would hence contribute

to the electrical conductivity of the material in which it is present. Hence, from the theory

of Fermi levels and Schottky barrier explained earlier, it is believed that this hypothesis

by Khan (2003) could hold true in the case of dyes. It is a topic of interest to see whether

the synergy noticed in the case of dyes also holds true in the case of organic compounds

such as phenol.






25


The work by Khan (2003) is the basis for the hypothesis formed by this author

which attributes the conductivity of activated carbons (due primarily to metals) as an

influence over the photocatalysis observed with the TiO2-AC composite.














CHAPTER 3
MATERIALS AND METHODS

3.1. Target Pollutant

The target pollutant, phenol (C6H60), was selected because it is representative of

pollutants experienced in water treatment. It was obtained in the liquid form (90 % w/w)

from Fisher Scientific. The stock solution was diluted to obtain the required

concentration (average concentration 55 mg/L 2 mg/L). The molecular weight of

phenol is 94.11. The pH of phenol solution is 5-6.

3.2 Carbon Materials

3.2.1 TiO2-AC Composites and Coating Procedures

Each AC differed in terms of physical and chemical properties due to differences in

precursor and activation technique. A summary of the properties of the AC's is provided

in Table 3-1.

Table 3-1. Summary of AC properties
Activated Precursor Activation BET surface area Micro Meso
carbon type (m2/gm) pores pores
___ __(%) (%)
Bionuchar Wood Chemical 1509 (+191, 270) 32 58
F400 Bituminous Thermal 1000 (+134, -200) 60 32
coal
HD4000 Lignite coal Thermal 539 (+80, 114) 25 66

The Bionuchar which was chemically activated is an AC supersaturated with

carboxylic and phosphoric groups and hence has a very acidic surface. The F400 is a

more basic AC whereas the HD4000, although it has been thermally activated has an

acidic surface due to an acid wash after its activation procedure.









The activated carbons were standardized to a 20 x 35 mesh using American

Standard Sieves (Fisher Scientific) and a Ro-Tap sieve shaker (Fisher Scientific). They

were then subjected to a number of coating procedures which were assessed based on

simplicity and efficiency.

Particle synthesis. In order to synthesize the TiO2-carbon particles, four coating

techniques were considered. Some of these coating techniques were only applicable to

activated carbons, versus the low surface area raw materials, due to its porous nature. The

coating techniques that were considered are as follows:

* Boil deposition
* Pore Volume Impregnation (PVI)
* Chemical Attachment (Modified Sol-Gel technique)
* Mechanical Attachment (Theta Composer)

It was found during the course of experiments that a 3-5% (m/m) coating provided

efficient and optimum activity for photocatalysis. The techniques were assessed based on

simplicity and efficiency as the ultimate engineering goal is the development of a

portable RAC filter.

Boil deposition. The boil deposition procedure is the most convenient method of

coating a substrate with TiO2. It is based on the mechanism of agitation of a mixture

during boiling. The theory implies that the TiO2 would be deposited on the carbon surface

due to the agitation of water, TiO2 and carbon while boiling, resulting in the impaction

and hence deposition of TiO2 on the carbon/AC surface in the presence of heat. Although

this procedure is not very consistent, it results in convenient coating of the particles and is

efficient in terms of performance of carbons or activated carbons.

The procedure was adapted from the technique used by Khan (2003). In this

procedure, the carbon/activated carbon sample to be coated was first washed with









deionized water. The sample was added to a 250 ml Erlenmeyer flask and a mass of TiO2

was also added to the flask corresponding to the required extent of coating. Deionized

water (200-220 ml) was added to the mixture. The flask was then placed on a hot plate

and heated to 1200 C and allowed to boil. Once all the water had evaporated, the sample

was once again rinsed with DI water and was left to dry in a furnace maintained at 1200C

to remove any residual moisture.

Pore Volume Impregnation (PVI). Impregnation is a commonly used method in

supported catalyst preparation. The impregnation method involves three steps: 1)

Contacting the support with the impregnating solution for a certain period of time, 2)

Drying the support to remove the impregnated/absorbed liquid and 3) Activating the

catalyst by calcinations or reduction (Tao, 2003). The principle of impregnation involves

using a volume of the precursor solution such that it is equivalent to the pore volume of

the support. Hence, it is evident that this technique can only be successful for activated

carbons or substrates with a well established pore structure.

A weighed quantity of AC was taken in a 40 ml vial. The total pore volume of that

quantity of AC was determined. The precursor (titanium isopropoxide (TTIP), ACROS)

and isopropanol (Fisher Scientific) were then mixed with the AC sample such that the

volume of the solution was equal to the total pore volume of the AC sample. Hence, for a

3% mass loading of titanium dioxide on the AC, the solution of TTIP and isopropanol

would be prepared such that the TTIP was 3% of the solution (v/v). The solution of TTIP

and isopropanol was introduced into the 40 ml vial containing the AC. The contents of

the vial were agitated for a few minutes. The vial was then left open under the fume hood

for 24 hours to allow the TTIP-isopropanol solution to be impregnated in the AC. After









24 hours of drying, the activated carbon was heat treated in a furnace at 3000C to convert

the titanium dioxide to the anatase phase. The performance of this coating procedure was

questioned due to the fact that the titanium dioxide is impregnated predominantly in the

pores of the AC. This was believed to hinder the incidence of UV light on the titanium

dioxide given the random structure of the pores as well as the opacity of activated carbon.

Also, as stated by Matos et al. (1998), the presence of TiO2 in the pores as opposed to at

the surface of AC would mean the absence of a concentration gradient forming, resulting

in hindered photocatalytic activity. The results from the tests performed with AC's coated

with TiO2 by the PVI technique seemed to agree with this argument.

Chemical attachment (modified sol-gel technique). The chemical attachment

coating procedure makes use of a sol gel to coat the activated carbon surface. This

technique was adapted from the technique employed to coat barium ferrite particles.

(Drwiega, 2004). Four grams of AC (HD4000, 20 x 35) and some volume of

polyethyleneimine (PEI (Alfa Aesar)) and 80 mL of nanopure were combined and mixed

for 10 minutes until the entire PEI was dissolved. 250 mL of nanopure was added to a 3-

mouth flask. The carbon particles were then introduced into the 3-mouth flask. The

glassware consisting of the reflux tube, 3-mouth flask, gas adapter, funnel and

thermometer were assembled. The nitrogen and water flow were then turned on for a few

minutes. Five milliliters of isopropanol was added to the closed funnel after which 100 [tl

of titanium isopropoxide (TTIP) was added to the closed funnel. The particles were

slowly stirred with a magnetic stirrer. The solution in the funnel was slowly introduced

into the water. The mixing was continued for 10 minutes and the solution with the

particles was heated to 950C. This was continued for 20-24 hours. The particles were









then removed and rinsed thoroughly with deionized water. The AC particles were then

heated in an oven at 1100 C for 1 hour to dry the particles. The particles then underwent

heat treatment at 350-4000C in presence of nitrogen flow for 3 hours to convert the

titanium dioxide to the anatase phase. This coating technique performed comparably well

to the boil deposition. However, the main disadvantage of this technique was that it

required a complex setup of equipment. Since one of the ultimate goals of the project is

the development of a household filter, it was not selected as the coating technique for the

experiments performed.

Mechanical attachment. Mechanical attachment consists of forcing the carbon

and TiO2 through a 1 mm gap at the same instant, hence implanting the TiO2 onto the

carbon surface due to the high forces on the particles. The technique, also known as

mechanofusion consists of equipment known as a Theta-Composer (Tokuju Inc, Japan).

The Theta-Composer basically consists of a rotor and a vessel set in a way such that they

are concentric and rotate in opposite directions (rotor speed: 2500 rpm; vessel speed: 77

rpm). This is shown in Figure 3-1 (Adapted from Khan, 2003). The AC particles (Sieved

to 20 x 35 mesh) and 3% TiO2 (m/m) was added to the vessel and the theta-composer was

allowed to run for 15 minutes.

The disadvantage of this technique was the separation of TiO2 from the carbon

particles when introduced in a liquid medium as well as the powdering of the AC due to

the natural mechanical motion and abrasion of the theta-composer.

After selection of the boil deposition procedure as the preferred coating technique,

the carbons were coated on two bases:

* 3% TiO2 on the basis of mass of carbon.
* 3% TiO2 on the basis of moles of elemental carbon in the AC.










(a) (b) (c)
Vessel Rotor





9.6



Clearance

Nanosized TiO2 & micrometer-sized
magnetic substrate particles

Figure 3-1. Coating activated carbon with TiO2 via mechanical attachment (Theta
Composer, Tokuju Corp., Japan)

3.2.2 Mass Based Coating (m/m)

The activated carbons were coated with TiO2 in the ratio of 3:100 (weight of

TiO2/weight of AC). The coating percentage was decided upon using evidence from

previous experiments which showed an optimal coating percentage range of 3%-5%

(mass/mass). For coating the ACs on a mass basis, a certain mass of AC (1-2 gms) was

mixed with TiO2 (3% of that mass). The boil deposition procedure was then used to coat

the TiO2 onto the ACs. After coating, the carbons were loaded to their reactors on the

basis of mass and in other experiments on the basis of volume. They may be denoted in

the future as 'mass-mass' and 'mass-volume' respectively.

3.2.3 Molar Based Coating (mol/mol)

For coating the carbons with TiO2 on a molar basis, the amount of elemental

carbon (C) in each sample was found by performing an ash analysis in which the AC's

were heated to approximately 5750 C. The difference between the initial masses of the

ACs and their final masses (of the ash) provided the amount of elemental carbon in each

sample. The following table (Table 3-2) provides a summary of the elemental carbon









contents of the ACs followed by a sample calculation of the TiO2-elemental Carbon

ratio's used in the composites.

Table 3-2. Carbon contents of virgin and titania coated ((m/m) and (mol/mol)) activated
carbons and their surface areas
Activated Carbon content Surface area (m/m) Surface area (mol/mol)
carbon(%) (m2/gm) (m2/gmm3)
Bionuchar 92 98 1483 1380
F400 86 94 1034 1009
HD4000 82 88 602 624

A sample calculation is provided.

Activated carbon A contains X mg Carbon/ gm AC
For 3% (mol/mol) loading:
MolesTiO2 / MoleScarbon = 3%
> (m/M)Tio2 / (m/M)c = 3/100
S (m/79.86)Tio2 / (X/12)c = 0.03
S mTi2 / Xcarbon = 0.19965

mTiO2 = Mass of TiO2 to be added
mc = Mass of Carbon (Elemental Carbon)
MTiO2 = Molecular weight of TiO2 = 79.86
Mc = Molecular weight of Carbon = 12

The TiO2 and AC mixture then underwent boil deposition, standardizing the

Carbon-TiO2 ratio in all the carbons. The carbons after coating were loaded to their

reactors on the basis of volume. They may be denoted in the future as 'mole-volume'.

It must be noted that the tests performed with the coated ACs were performed using

TiO2-AC composites prepared in different batches. Hence the replicate sets of

experiments are indicative of the performances of different batches of composite as

opposed to composites prepared in the same batch. In this regard, the consistency of the

batches with respect to performance of the composites will also be observed. The TiO2-

low surface area carbon composites (explained in section 3.2.4) were also prepared and

tested similarly.









3.2.4 Low Surface Area Carbon-TiO2 Composite

The low surface area carbons used herein consisted of Pitch Coke (Asbury Graphite

Mills, Asbury, NJ), Graphite (Asbury Graphite Mills, Asbury, NJ), Anthracite Coal

(Reading Anthracite Coal Company, Mahantongo St., Pottsville, PA), and Bituminous

Coal (International Industries, Logan, WV). The titanium dioxide was commercially

available Degussa P25 (70% Anatase, 30% Rutile). The virgin carbons used in these

studies had extremely low surface areas as shown in Table 3-3. In fact, the surface areas

of some of the samples were below the minimum value that was detectable. The values

are still provided although they may contain some error.

Table 3-3. Surface areas of virgin and coated low surface area carbons
Carbon Surface area (m /gm)
Virgin 3% TiO2 3% TiO2
carbons (mass/mass coating) (mole/mole of C)
Pitch coke 0.62 4.21 8.15
Anthracite 0.92 4.1 7.51
Graphite 1.35 3.63 7.99
Bituminous 0.5 1.00 3.34

Although in both cases (i.e., (m/m) and (mol/mol)) there was a considerable

percentage increase in the surface areas of the coated carbons as compared to the virgin

carbons, it was still extremely low for any significant sorption phenomena to occur.

We also analyzed the amount of elemental carbon in each sample (as for the AC's) in

order to coat the carbons on the basis of mole/mole which is provided in Table 3-4. The

coating technique was exactly the same as that used to coat the ACs.

Table 3-4. Carbon contents of low surface area carbons
Low Surface Area Carbon Carbon Content (%)
Pitch Coke 93
Graphite 99
Anthracite 90
Bituminous 65-72









3.3 Experimental Setup

3.3.1 Batch Reactor Configuration

Batch testing was performed initially to investigate how the carbons with and

without TiO2 performed for phenol removal. The setup for the batch tests consisted of a

rotator (Analytical Research Systems, Gainesville, FL) onto which were mounted 4

reactors. The reactors consisted of 100 mL glass syringes (SGE; Obtained from Fischer

Scientific). The syringes were fitted with gas-tight plungers, the ends of which were

made of Teflon. The syringes were also mounted horizontally on the rotator such that

they were parallel to the axis of the rotator. The entire system rotated at a rate of 3 rpm.

Three UV lamps (365 nm wavelength) were placed such that they were 1200 apart and at

a distance of 0.5 cm from the outer wall of the syringes at the nearest point. The entire

assembly was enclosed in a wooden box, the walls of which were covered with reflective

aluminum foil so that any stray radiation could be reflected back towards the reactors.

3.3.2 Analytical Equipment

The concentration of phenol solution was determined on a UV-VIS

spectrophotometer (HACH DR/4000U). The BET surface area analyses for the activated

and low surface area carbons as well as their composites were performed using a

Quantachrome NOVA 2200e Gas Sorption Analyzer (Boynton Beach, Florida). Samples

were outgassed for 24 hours prior to testing. Before outgassing, the samples were dried at

110 C. The SEM images and the EDS scans were performed using a scanning electron

microscope (SEM JEOL JSM 6400). The images and scans were performed at the major

analytical instrumentation center (MAIC, Department of Materials Science, University of

Florida, Gainesville). Multiple images and scans of the same sample as well as different

particles of the same sample were taken. Density measurements of the Carbon-TiO2 and











the AC-TiO2 composites were performed on an Ultrapychnometer (Quantachrome).

Replicate sets of measurements were taken. Multiple samples from within batches as well


as across different batched were taken. The value provided is the average of all the values


with the maximum and minimum values also provided.


3.4 Experimental Procedure

3.4.1 Concentration Measurements

The concentration of the phenol solution was measured at the peak absorbance


wavelength X = 270nm for phenol which was determined by performing an absorbance


scan of the phenol solution. The concentration vs. absorbance profile for a phenol


solution measured at 270 nm on the UV-VIS spectrophotometer is shown in Figure 3-1


and obeys Beer's law. The R2 for the profile was found to be 0.97.



70

60 -
Concentration vs
Absorbance Profile for
50 Phenol
Linear (Concentration vs
Absorbance Profile for
S40

30 -

20 -

10 y= 62.445x
R2 = 0.97
0
0 0.2 0.4 0.6 0.8 1
Absorbance



Figure 3-2. Concentration vs. Absorbance correlation for phenol on UV
spectrophotometer measured at 270nm wavelength









3.4.2 Batch Adsorption/Oxidation Tests with Activated Carbon

The synergistic phenomena observed in certain TiO2-AC composites were

attempted to be determined for phenol by performing batch tests with the various

composites. The three activated carbons selected Bionuchar, F400 and HD4000 were

tested initially in a batch system. The tests were performed using granular activated

carbon (GAC) for the convenience of separation and to simulate a real world system.

Experiments were performed with ACs coated with TiO2 on the basis of mass as well as

on a molar basis, which has been explained earlier. The ACs coated by mass were loaded

into the reactors on two bases: 1) mass basis and 2) volume basis. The loading of reactors

on the basis of mass was done such that 50 mg of AC was loaded for 80 mL of phenol

solution whose concentration was 55 mg/L. The volume loading of the composites in the

reactors was done such that 0.025 mL of AC was used for 80 mL of phenol solution

whose concentration was 55 mg/L. The ACs coated on a molar basis were loaded in the

reactors only on the basis of volume. The volume based loading of reactors with these

composites was done such that 0.025 mL of AC was used for 80 mL of phenol solution

with a concentration of 55 mg/L. The procedure of coating the ACs by molar ratio's and

loading the composites on a volume basis was done to normalize the TiO2-carbon ratio

for each sample since the AC's were of varying densities, resulting in different quantities

of AC (and hence TiO2) in each reactor.

The parameters such as initial concentration of phenol solution, volume of phenol

solution used in each reactor and the time for each run remained constant for the

adsorption-oxidation studies for both types of AC loading. Both experiments were carried

out for a period of 24 hours. For adsorption studies, experiments were performed in the

absence of UV radiation. At the end of 24 hours, the GAC particles were separated from









the phenol solution by filtration with 0.45 [tm filters (Fisher Scientific) using vacuum

filtration. The final concentration of the phenol solution was measured using a UV-VIS

spectrophotometer. The difference between the initial concentration of the stock solution

and the final concentrations were attributed to adsorption. For the degradation studies, the

experiments took place in the presence of UV light. Again, the experiments were carried

out for 24 hrs at the end of which the GAC particles were separated from the phenol

solution by vacuum-filtration. The final concentration of the separated phenol solution

was measured using the spectrophotometer. The difference between the initial

concentration of the stock solution and the final concentration of the phenol sample in

this case was attributed to a combination of adsorption and photocatalytic degradation.

In addition to adsorption and oxidation tests, tests were also performed with virgin

ACs in phenol solution in the presence of UV.

Replicate sets were run for each sample and the error between the samples has been

denoted as the maximum and minimum values that were recorded. The actual value

plotted is the average of all the values obtained for that result.

3.4.3 Batch Adsorption/Oxidation Tests with Low Surface Area Carbons

As explained earlier, the configuration for the batch tests involving the low surface

area carbons was identical to that employed for the AC-TiO2 tests. The TiO2-carbon

composites (35 x 200 sieve) were coated on the basis of mass as well as on the basis of

mole. For the carbons coated on a mass basis, the composites were loaded into the

reactors on two bases: by mass and by volume. For the mass loading, 80 mg of the

carbons were loaded into the reactors which contained 80 mL of 55 mg/L phenol

solution. For the volume loading, 0.05 mL of the carbons were loaded into the reactors









containing 80 mL of 55 mg/L solution. The volume loading was done in order to

normalize to some extent the amount of carbon (and hence titania) in each reactor.

For the carbons coated on a molar basis, the composites were loaded into the

reactors only on a volume basis. Here again, 0.05 mL of sample were loaded into reactors

containing 80 mL of 55 mg/L phenol solution. This would in theory completely

normalize the ratio of titania to carbon (elemental) in each reactor.

The testing procedure for both cases was identical to the one employed for the AC

TiO2 composites as far as runtime and sample analysis procedures are concerned.

Replicate sets were run for each sample and the error between the samples has been

denoted as the maximum and minimum values that were recorded. The actual value

plotted is the average of all the values obtained for that result.














CHAPTER 4
RESULTS AND DISCUSSION

4.1 Evaluation of Carbon Coating Strategies

Titanium dioxide was coated on the activated carbons using 4 different coating

techniques: boil deposition, pore volume impregnation (PVI), chemical attachment, and

mechanical attachment.

Boil deposition was by far the easiest and most convenient coating technique as it

required using a commercially available TiO2 (Degussa P25), water and heat. It was

nevertheless a very inconsistent and unpredictable of the coating methods as the TiO2

deposited not only on the AC but also on the beaker in which it was being carried out.

Therefore, although a known mass of titania was used in the coating process, the actual

amount of TiO2 deposited on the carbon was likely less than the targeted value. The

average phenol removal observed for HD4000 coated with titania was 78%.

Pore volume impregnation resulted in the immobilization of the TiO2 within the

pores of the AC. The titania depositing in the pores of the ACs was considered a

disadvantage for this coating technique, particularly due to the opacity of activated

carbon and the random orientation of its pores. It was hypothesized that due to titania

deposition within the pores it would result in a low probability of the UV light reaching

the titanium dioxide, resulting in a system with reduced photocatalytic activity. The

results obtained seemed to agree with this hypothesis. The photocatalytic tests which

were performed with HD4000 showed a removal of only 42%.









Chemical attachment of the TiO2 to the carbon was also attempted to manufacture a

more rigid composite. Although this coating technique would provide a more consistent

coating on the surface of the ACs, the performance of the ACs coated by this technique

did not show any marked improvement over the boil deposition technique. The sol-gel

coated HD4000 resulted in 81% removal via adsorption and photocatalysis, which was

comparable to the removal seen with the boil deposition method.

Table 4-1. Summary of coating techniques

Coating Performance Advantages Disadvantages
technique

Simple procedure 1) Inconsistent coating
Boil deposition 78% 2) Poor control of TiO2
Good performance loading
1) Requires additional
Pore volume hydrolysis and calcination
Pore volume
impregnation 62% Good coating control steps
( ) 2) Titania deposition in
macropores (Tao, 2005)
3) Poor performance

d c g c l 1) Requires additional
Chemical 81% hydrolysis step
attachmentGood performance 2) Experimental setup is
Good performance
cumbersome

1) Attrition of AC
Mechanical particles
attachment 2) TiO2 separation from
substrate

Mechanical attachment was achieved through impaction using the Theta Composer

instrument (Tokuju Inc., Japan) which resulted in the attrition of the granular activated

carbon (GAC) particles, which originated as approximately 1 mm in size, to particles less

than 45 [tm (Powdered Activated Carbon or PAC). The technique also resulted in

separation of the TiO2 particles from the substrate upon introduction in water. The









advantages and the disadvantages of each technique are summarized in Table 4-1. It must

be noted that multiple batches of the TiO2-AC composites were tested. Based on these

results, the boil deposition technique was selected for coating the carbons with titania.

4.2 Activated Carbon Studies

Khan et al. (2003) showed that F400 and HD4000 showed a synergy when coated

with TiO2, whereas Bionuchar did not. The authors concluded that metals, naturally

occurring in activated carbon may have been responsible for the observed synergy (i.e.,

removal of target pollutant increased when titania was coated on AC as compared to

activated carbon alone). Nevertheless, they did not investigate this phenomenon in

further detail. It was hence a viable exercise to investigate whether metals, in particular

electrically conductive metals such as iron and aluminum, could be a plausible

mechanism for creating the described synergy.

4.2.1 Ash Analyses of Activated Carbons

To determine the metallic content of Bionuchar, F400 and HD4000, the ACs used

by Khan (2003), the uncoated (i.e., virgin) carbons were combusted in air at 5750C until

only an ash residue remained. The ash residue of each AC was then analyzed by a

scanning electron microscope (SEM) to perform an energy-dispersive spectroscopy

(EDS) study. EDS was also used to verify the presence of titania on the surface of the

coated ACs. Note that the ACs of which the ash residues were examined were not coated

with titania prior to ashing. Even though the EDS data cannot be used as a complete

quantitative comparison, it is still useful as a semi-quantitative as well as a qualitative

tool for characterizing elemental composition. The percentage of ash (by weight) in each

sample is provided in Table 4-2. Also provided in the table is the amount of titania (as a

percentage) by weight actually coated on the AC samples. This was quantified simply as









the difference between the weight of the ash residue of a virgin AC and that of a coated

AC sample. The theoretical amount of titania which was to be coated on the surface of

each of the ACs was 3% (i.e., weight of titania added equals 3% of the weight of the AC

sample). Table 4-2 shows the inconsistency of the boil deposition procedure in coating

the ACs. On an average it is seen from the data provided in Table 4-2 that Bionuchar has

the lowest TiO2 deposited on its surface. This is attributed to morphological differences

between the wood based Bionuchar and the coal based F400 and HD4000.

Table 4-2. Ash contents of ACs and mass % of titania deposited (m/m) on AC surface
Carbon%
Activated carbon Precursor Carbon Ash% (virgin ACs) TiO2 %
(virgin ACs)
Bionuchar Wood 92-98% 2-8% 0.5-1.3%
F400 Bituminous 86-94% 6-14% 1.6-2.2%
HD4000 Lignite 82-88% 12-18% 1.4-2.3%

The ash analyses of the ACs provided an estimate as to the magnitude of the

composition of elements seen in the EDS scans. Figures 4-1(A-F) show the EDS scans of

the ash residues of the virgin ACs and the scans of the coated ACs.

The EDS scan of the Bionuchar ash residue (Figure 4-1A) shows that it consists of

primarily phosphorous, silicon and sodium. A very faint trace of aluminum as well as a

peak corresponding to iron is also observed. The phosphorous peak is likely remnant

from the chemical activation technique using phosphoric acid.

Figure 4-1B represents an EDS image of a Bionuchar sample coated with titania. It

can be seen from the scan that smaller peaks corresponding to phosphorous, silicon and

sodium are again observed. A titanium peak is also seen in addition to the peaks seen in

the ash residue of the same AC. It is important to note than the total ash content of the

Bionuchar samples is a maximum of only 10% of the weight of the sample which is

inclusive of the titania coated on the surface. Hence, coupled with the results from the









EDS scan, it may be concluded that a very small amount of each of the elements is

present in Bionuchar. Hence the low presence of conductive metals such as aluminum in

Bionuchar (also observed by Khan (2003)) may result in very few conductive sites on the

surface of Bionuchar.

The residual ash sample of F400 (Figure 4-1C) consisted of very large peaks of

aluminum and silicon. Smaller peaks pertaining to iron, sodium, magnesium and calcium

were also noticed. In comparison to the EDS scan performed on the ash of the Bionuchar

sample which showed an aluminum peak of about 200 counts (in replicate sets of scans),

the F400 ash residue showed aluminum peaks having magnitudes between 3800 and 4000

counts in the replicate sets of scans preformed. The presence of iron should also be noted.

The activated carbon sample of TiO2-F400 (Figure 4-1D) shows the presence of

aluminum, silicon, iron and titanium. What is interesting to note is the extremely high

titania peak of about 5000 counts. It may be recalled that the corresponding peak in the

Bionuchar sample did not have a magnitude of even 2000 counts. This suggests that the

morphological difference between the wood based Bionuchar and the coal based F400

resulted in a difference in TiO2 coating. For instance, variations in roughness may cause

the titania to adhere to AC surfaces differently. This would then affect the adsorption of

phenol as well as photocatalytic mechanisms. Another observation to be made is the

higher ash content of F400 (6-14%) as compared to Bionuchar. This indicates the

presence of larger quantities of the elements seen in the EDS scans of F400 relative to

Bionuchar.








44



Counts

8000-
A

6000-
0

4000-


2000 Na Si
Na

C AAl S K Fe
0-
0 2 4 8 8
Enerav fkeVi
Counts

8000-
B

8000-


4000-



2000 p Ti


Na Si Ti

0 2 4 0 8
Energy (keV)

Counts

8000-
C

8000-

0 Si
Al
4000-


2000-
Mg
Na S Ca
C e P K Ca Ti Ti Fe
0 1 1 I . I . .-. ,. . I . ,
0 2 4 8 8
Energy (keV)

Figure 4-1. EDS of A) ash residue of uncoated Bionuchar, B) Bionuchar coated with

TiO2, C) ash residue of uncoated F400, D) F400 coated with TiO2, E) ash
residue of uncoated HD4000, F) HD4000 coated with TiO2













Counts

8000--

D

6000-

Ti

4000- 0
0

2000-
Si Ti T
Al Ti
l S Ca Fe
0-:2
0 2 4 6 8 10
Energy(keV)

Counts

8000- E
E

Si
6000-



4000-


Al
2000-
Mg
C Na Ca Ti
0- 4

0 2 4 6 8
Energy (keV)
Counts

8000-
F

6000-



4000--



2000-
0
C Si Ti Ti
0 Al S
0 2 4 6 8 10
Energy (keV)



Figure 4-1. Continued


The differences in the ash compositions as well as the compositions of the ACs


(Bionuchar and F400) are noticed, with the prominent peak of aluminum and the


presence of iron being the most pertinent to this study. The relatively high conductivity of


aluminum and iron may play a role in the distinction between the performances of









Bionuchar and F400 that were observed in the work of Khan (2003). This may be a

possible cause for F400 showing a synergistic phenomena with respect to dyes. The

results of the EDS scan of F400 also agree with Khan (2003) about the presence of iron

in F400.

F400 would have different metals contents because it is a coal based activated

carbon that originated as bituminous coal. Therefore, metals that are present in the

subterranean environment would be inherent in the coal. However, Bionuchar is a wood

based activated carbon that not only would have different metals than coal based

activated carbons, but since it is chemically activated, during the chemical impregnation

step at low pH, some of the metals would become soluble and leach out of the wood.

In the case of HD4000 the EDS image (Figure 4-1E) of the ash residue prominently

consisted of silicon and aluminum. In addition, magnesium and sodium were also fairly

prominent. However, iron that was hypothesized to be important for the observed synergy

in the work of Khan (2003), was absent in the EDS scan.

The presence of aluminum and silicon were also detected in the TiO2 coated

HD4000 sample (Figure 4-1F). The EDS image also showed a very high titanium peak

(about 5000 counts). In replicate sets which provided reproducible results, counts as high

as 8000 were found for titanium. The EDS scans for the HD4000 samples also showed a

relatively low carbon peak indicating that a titanium dioxide agglomerate may have been

focused on. The fact that this was the case in replicate sets between different TiO2 coated

HD4000 samples may also indicate to some extent a uniform coating on the surface of

this AC. Noteworthy is the fact that HD4000, like F400 is also a coal based AC. This

strengthens the argument that morphology may play a role in the deposition of titania on









the AC. This can also be attributed to the enhanced performances seen in the coal based

carbons with respect to adsorption and photo-oxidation of dyes (Khan, 2003). The fact

that HD4000 also contains highly conductive metals such as aluminum strengthens the

argument that conductivity may play a role in enhanced photocatalysis. Also, the fact that

the ash content of HD4000, a lignite based AC, is relatively high (12-18%) increases the

probability of the coated titanium dioxide being in the vicinity of a site on the AC which

possesses a higher conductivity (due to the presence of conductive metal ions).

4.2.2 SEM Images of Titania Coated ACs

SEM images were used to further characterize the coating of titania on the ACs.

The images showed a variation in the deposition of titania on the activated carbons as

well as the degree to which each AC had been coated. Figure 4-2 shows an image of

titania coated Bionuchar.

'- U "


Figure 4-2. Titania coated Bionuchar with the TiO2 blocked pores circled









It can be noticed from the image that the surface of Bionuchar seems very smooth.

The smooth nature of the AC surface may cause a lack of adherence of the titania to the

AC surface which may result in decreased photocatalytic activity. Another notable aspect

of this image is the fact that a number of pores (shown circled) appear to be partially or

completely blocked by the titanium dioxide. This may in fact be due to the poor

adherence of the TiO2 surface resulting in the titania being deposited inside the pores of

the AC. Deposition in the pores would of course reduce the diffusion of phenol to

adsorption sites, thereby decreasing the adsorption of phenol (as discussed in subsequent

sections). The wood based nature of this AC is evident in Figure 4-3.
























Figure 4-3. Bionuchar showing cellulose structure of the wood based AC.

The Bionuchar AC particle seen in this image shows what seems to be a cellulose-

like structure due to the wood precursor. It may also be noticed that the particle appears

to have a smooth honeycomb structure which as mentioned earlier may not allow TiO2 to









adhere well to its surface. The author considers the morphology of the Bionuchar surface

to be one of the reasons why a relatively small titanium peak is detected in the EDS of

Bionuchar and its ash residue. Indeed, Table 4-2 showed that the percentage of titania on

the Bionuchar samples was less than the other two samples.

In contrast to the morphology of Bionuchar, the titania seems to be uniformly

coated on the surface of F400 (Figure 4-4).


Figure 4-4. Titania coated on the surface of F400

The uniformity in coating is thought to be an explanation for the well defined peaks

of titania seen in the EDS scans of F400 and could also be an explanation for the synergy

seen in the study by Khan (2003). A uniform coating would increase the probability of

titania contacting metals present on the AC surface which could hypothetically improve

photocatalysis.









An SEM image of HD4000 coated with titania is shown in Figure 4-5. It may be

noticed that the TiO2 agglomerates on the surface of HD4000; here the TiO2 agglomerate

is near the opening of a macropore (see circled region). The size of the TiO2

agglomerates (about 15 [tm) is observed to be much larger than the individual TiO2

particles (i.e., 27 nm) (Li et al., 2004).
























Figure 4-5. TiO2 agglomerate (shown circled) near a macropore on the surface of
HD4000

However, it may also be noticed that similar to the case of F400, the titania is

coated more uniformly on the surface than in the case of Bionuchar; a phenomenon

believed to be dictated by the morphology of HD4000. Figure 4-6 shows the uniform

coating and the agglomeration of titania particles along with the rough surface of

HD4000. As in the case of F400, the uniformity in coating may be an explanation for the

large titania presence seen in the EDS scans for HD4000. This may also be a possible









explanation for the synergy noticed by Khan (2003) for the same reasons as mentioned

for F400.


Figure 4-6. Surface of HD4000 showing uniform coating of TiO2 which tends to form
agglomerates

Hence, it may be summarized that the wood based carbon and the coal based

carbons differ morphologically. Titania coated on Bionuchar seems to concentrate around

the pores. It is also believed that due to the smoothness of the Bionuchar surface, there is

less adherence of the TiO2 particles to the Bionuchar surface. However in the case of both

F400 and HD4000, a more uniform coating of titania is observed with the titania forming

agglomerates on the surface of HD4000.

The ash analysis, EDS scans and the SEM images together provide us with some

possible explanations for the synergistic phenomenon of certain TiO2-AC particles

observed by Khan (2003). Both F400 and HD4000 contain relatively high ash contents

as compared to Bionuchar. Their EDS scans also show a very high titanium presence on









their surface in addition to the higher presence of electrically conductive metals (i.e.,

aluminum, iron).

It may be concluded that there is a greater possibility of titania being in contact

with a conductive site in the case of F400 and HD4000 relative to Bionuchar.

4.2.3 Activated Carbon Batch Studies

The results of the batch tests conducted on the activated carbons which compared

the performance of the ACs (or the TiO2-AC composites) as a function of the removal of

phenol are presented. It must be noted that differences in phenol removal of less than 4-

5% were not considered significant.

Virgin activated carbons. In the work of Khan (2003), the focus was on the

removal of dyes. Herein, the focus was on a model aromatic adsorbate (phenol) and the

intent was to learn if the conclusions made by Khan (2003) and as previously discussed,

were applicable to the removal of phenol.

To determine the adsorption capacity of the ACs, batch tests were performed in the

dark for a period of 24 hours. The studies were first performed with virgin ACs. Three

activated carbons were used: Bionuchar, F400 and HD4000. The results of the tests

performed with the virgin ACs are presented in Figure 4-7.

It can be seen from the results that the virgin F400 showed the most adsorption

capacity with 87% removal whereas Bionuchar showed the least adsorption capacity with

only 52% removal. HD4000 performed relatively well with about 76% removal. The high

adsorption capacity of F400 may be explained by the fact that its pores are predominantly

(60%) microporous. The molecular size of phenol is about 6 A (0.6 nm). Therefore, it is

expected to adsorb mainly in pores having a microporous size range. The low adsorption

capacity observed for Bionuchar is attributed to the acidity of its surface. The surface of










Bionuchar is highly acidic. It is well documented in the literature that 'L-type carbons' or

'acidic' activated carbons show reduced adsorption of organic such as phenol (Matos et

al., 1998; Salame et al., 2003; Coughlin et al., 1968, Mattson et al., 1969).


60


50
a
40 m Initial Concentration
40-
re Virgin Bionuchar
0 E0 Virgin F400
o Virgin HD4000
= 30

20
C-
20


10


0
Sample


Figure 4-7. Batch adsorption studies of virgin ACs in the absence of UV

It is thought that carboxyl and other such electron withdrawing functional groups

result in a lowering of phenol adsorption capacity by removal of the 7t-electron from the

AC aromatic ring. This decrease in the density of the electron cloud in the carbon basal

planes causes a decrease in the strength of interactions between the benzene ring of

phenol and the basal planes of AC. Phenol also reacts with carboxylic groups on the

carbon surface forming ester bonds which contributes to this phenomenon (Salame et al.,

2003; Nevskaia et al., 1998). As a result, there is decreased adsorption of phenol on the

basal planes of carbon. The case of HD4000 however is peculiar at first glance. HD4000

is acidic due to its activation procedure, but this acidity is a function of residual acid used

for acid-washing versus the presence of oxygenated functional groups located on the edge










sites. Therefore, although it displays characteristics of an L-type carbon, the manner in

which this is manifested is different than "traditional" L-type carbons. At the nano-scale,

HD4000 behaves more like an H-type carbon, which suggests that 7t-7t bonding is the

dominant adsorption mechanism.

Adsorption capacity for titania-coated activated carbons. Similar trends were

seen when batch adsorption studies were performed in the dark with 3% (m/m) (i.e., mass

of titania per mass of activated carbon) titania-coated AC samples (Figure 4-8). Note that

the targeted titania loading was 3%, but as was shown earlier the actual amount of titania

on the surface of each AC was less than 3 %.

60

Initial Concentration
50 50 mg 3% Bionuchar
2" 50 mg 3% F400
I[3 50 mg 3% HD4000
40 1.5 mg 1T02 Slurry
0 2.4 mg 1T02 Slurry

E 30


20
C-)
o20

10-
10



Sample


Figure 4-8. Batch adsorption studies of 3% (m/m) coated ACs in the dark compared to
Ti02 slurries

A slight decrease in adsorption capacity as compared to the virgin ACs was

observed in all the ACs. This was attributed to the partial blockage of pores and hence

reduced surface area and active sites available for adsorption. In addition to the ACs,

Figure 8 presents the adsorption capacities of two titania slurries (no AC). These masses









of titania were chosen because they are very near the masses of titania that was present on

the ACs. Clearly the titania slurries show very low adsorption capacities (approximately

8% removal). In fact, it is noticed that even when a higher amount of titania is loaded to

the reactor, the decrease in phenol concentration does not change.

Adsorption and photocatalysis for titania-coated activated carbons. The next

step was to determine the performance of the coated ACs in the presence of UV light. As

in the earlier experiment, performances of two titania slurries were also included. It was

expected that UV would initiate photocatalysis and therefore considerably enhance the

removal of phenol through the combined functions of adsorption and photocatalysis.

The tests with the titania coated ACs were conducted in a sequence which would

eventually normalize the amount of carbon and titanium dioxide for each sample. The

initial batch tests consisted of ACs coated with titania on the basis of mass. The AC's

were loaded to their respective reactors such that each reactor contained 50 mg of the

titania-coated AC. These samples have been denoted in places as 'mass-mass' or 'm/m'.

The amounts of titanium dioxide for the slurry systems were 1.5 (3% of 50 mg) and 2.4

mg (3% of 80 mg as a comparison) respectively. It must be noted that the ACs do not

have the same density which resulted in each reactor having different volumes of ACs.

The results of the tests are shown in Figure 4-9.

The irradiated Bionuchar composite with only 39% removal performed poorly

compared to HD4000 (78% phenol removal) and F400 (88% phenol removal). The

performance of both titania-HD4000 and titania-F400 composites slightly improved in

the presence of UV; a fact that is attributed to photocatalysis by UV. However, this

refutes Matos et al. (1998, 2001) who claim that an L-type AC inhibits photocatalysis










whereas an H-type AC enhances photocatalysis. The surface of HD4000 is acidic and can

therefore be categorized as an L-type carbon, but as was previously mentioned, the

manner in which HD4000 exhibits a low pH is mechanistically different than

"traditional" L-type carbons. Therefore, further investigation into this phenomenon is

warranted.


60

Initial Concentration
,50 -9 50 mg 3% Bionuchar
[ 50 mg 3% F400
E [0 50 mg 3% HD 4000
40 1.5 ng Ti02 Slurry
0 2.4 mng Ti02 Slurry

I 30
0
0
20


( 10


0
Sample


Figure 4-9. Batch adsorption-photocatalysis studies of 3% (m/m) coated ACs in the
presence of UV

Matos et al. (1998, 2001) clearly erred in their oversight that L-type carbons

exhibit less adsorption than H-type carbons. Therefore, decreased adsorption was

actually responsible for their results as opposed to decreased photocatalysis.

As was previously discussed in detail, a possible explanation as to why the coal-

based carbons behaved synergistically is because both contained conductive metals. The

enhanced performance of the ACs may point to a favorable difference in Fermi energies

between the titania coated on their surface and a conductive metal in their vicinity,

resulting in longer recombination times and improved photocatalysis. As explained









earlier, due to the relatively large ash content and metals composition, there is a greater

probability of the titania on the surface of AC being in the vicinity of a conductive metal.

Contrary to the performance of the coal based carbons, the performance of the

irradiated Bionuchar-titania composite, which is acidic, was considerably worse than the

performance of the virgin Bionuchar (13% difference). Whereas this phenomenon may be

attributed to pore blockages in the coated Bionuchar, the observation that the irradiated

Bionuchar composite performed worse than the Bionuchar composite (6% difference) in

the absence of UV makes for an interesting discussion (Figure 4-9).

A trend similar to the one seen in Figures 4-8 and 4-9 was noticed in the

experiments performed by Khan (2003). It was found that irradiated dyes (denoted as

Dye*) showed tendencies to adsorb differently than dyes which had not been irradiated

(denoted as Dye). According to Khan (2003) and Khan et al. (2006), the irradiation of the

dye caused the excitation of the delocalized electrons in the aromatic structure which

would affect the adsorption of the compound (Dye*). These delocalized electrons in the

aromatic structure would then interact with the carbon basal planes of Bionuchar which

due to acidic functional groups would contain a less dense electron cloud resulting in

decreased uptake (due to decreased 7t-7t interactions). The decreased uptake would also

result in decreased photocatalysis. Hence, ACs which preferentially adsorbed the

unadulterated dye compounds and had reduced uptake for 'Dye*' would have a reduced

overall performance. This was due to the fact that their affinity for the unadulterated dye

compound resulted in the overall hindrance to adsorption of the dye compound. Hence, in

the case of dyes, the two theories cumulatively may explain the reduced performance of

titania-Bionuchar composites in the presence of UV. Continuing in the same vein, the









synergy observed with F400 and HD4000 in the presence of UV was attributed to their

relative affinity for 'Dye*'. In the case of ACs that showed good removal, a situation of

augmented adsorption was theorized wherein activated carbons having an affinity to

adsorb both 'Dye' and 'Dye*' at comparable levels showed increased decolorization of

the dye solution (Dye + Dye*). Whether the same theory may apply to phenol (hence

forming phenol*) was thought to be an idea worth investigating and is discussed later.

Regarding the performance of the titania coated ACs and titania alone, each of the

ACs showed more removal of phenol than the titania slurries. In fact, the titania slurry

containing a lower concentration of titania slightly outperformed the slurry with a higher

concentration. Although this may be attributed to decreased penetration of UV with

increasing TiO2 concentration, the fact still remains that the removal was considerably

lower than the AC-TiO2 composites. Therefore, it must be noted that although titanium

dioxide may result in a decrease in adsorption capacity when coated on ACs, the ACs

coated with titania provide for better overall removal of phenol in the presence of UV as

compared to the titania slurry.

Volumetric loading of the reactors and mol/mol coating. Due to the variation in

the densities of the ACs, it was deemed necessary to normalize the differences in the

volumes of ACs loaded to the reactors. In other words, since the wood-based carbon was

less dense than the coal based carbons, the mass of carbon added for each experiment was

less. Since the mass of carbon was less, then mass of titania would be less. Therefore,

the system could be normalized by adding the carbons to the reactor based on their

volumes versus mass. Hence, 0.025 mL of AC was loaded to each of the respective

reactors.







59


The results of the experiment carried out under these conditions are shown in

Figure 4-10.


60


50

=E U Initial Concentration
S40 0 0.025ml 3% Bionuchar
0 0.025ml 3% F400
IM c3 0.025ml 3% HD4000
r- J
S30


20
C-,


( 10


0
Sample


Figure 4-10. Batch adsorption-photocatalysis studies of 3% (m/m) coated ACs loaded to
the reactors on a volume basis in the presence of UV

The tests did not result in any significant difference in performance of the ACs. A

trend similar to the one seen in Figure 4-9 was seen for this experiment as well. With this

said, the change in basis of loading from mass to volume only succeeded in normalizing

the amount of AC in each reactor. It was also of interest to learn if normalizing the

amount of titania on each AC based on titania to carbon would impact the results. This

ratio would be different for each carbon because of the different ash contents. To

normalize the titania to carbon loading, the ACs were coated such that 3 moles of titania

were present for every 100 moles of elemental carbon. In doing so, the titania to

elemental carbon ratio in each AC remained constant. In addition to the change in

coating basis, the titania-coated ACs were continued to be loaded by volume into their







60



respective reactors. Using these bases for coating the titania on the ACs and loading them


to the reactors, tests were performed in the absence as well as in the presence of UV light.



60


* Initial Concentration
* 3% Bionuchar (mol/mol) loaded by volume
O 3% F400 (mol/mol) loaded by volume
O 3% HD4000 (mol/mol) loaded by volume

A


Samples


* Initial Concentration
m 3% Bionuchar (mol/mol) loaded by volume
O 3% F400 (mol/mol) loaded by volume
O 3% HD4000 (mol/mol) loaded by volume


B


I ------


Samples

Figure 4-11. Batch adsorption studies: A) 3% (mol/mol) coated ACs in the absence of
UV loaded to the reactors by volume, B) Photocatalysis studies of 3%
(mol/mol) coated ACs in the presence of UV loaded to the reactors on a
volume basis


- 50
-_
05)
E
40 -
o

" 30
i-

o
--

0 20 -
o
10
0- 10


0 1


50
-J

E

o

" 30
0

20
o
'-
I'-



( 10



0









Tests in the absence of UV were performed to assess as to how much the change in

the basis of coating affected adsorption of phenol since in each case the amount of coated

titania increased. Similarly, tests in the presence of UV were performed to assess the

effect that the increased titania coating on the ACs had on simultaneous adsorption-

photocatalysis. Figures 4-1 la and 4-1 lb show the results of the experiments in the

absence and presence of UV light respectively.

As in the earlier tests, an analysis of the data revealed no significant differences in

the trends of their performances. Although there are signs of photocatalysis having some

effect on phenol removal, the differences noticed in the trends are not of sufficient

magnitude to arrive at any definite conclusion with respect to how photocatalysis was

influenced by the variations.

The results seemed to show that adsorption dominated over photocatalysis as a

mechanism. Hence, the extent to which photocatalysis plays a role in the removal of

phenol in a co-adsorbent with high surface area was questioned. Unlike results shown by

Matos et al. (1998) in the case of organic compounds and Khan (2003) in the case of

dyes, the results seen herein failed to show any clear evidence of a synergy of any kind

existing between ACs and TiO2 with respect to performance. The premise that a co-

adsorbent with high surface area would result in an increase in photocatalytic efficiency

of titania (Matos et al., 1998; Takeda et al., 1995) can hence be questioned. The theory by

Matos et al. (2001) that L-type ACs inhibit photocatalysis whereas H-type ACs enhance

photocatalysis has also gone unfounded based on the data that has been presented. The

failure by Matos et al. (2001) to perform adsorption control studies seems to be the cause

of the unverifiable hypothesis.










4.2.4. Determination of Existence of Phenol* (Phenol with Delocalized Electron) in
the Presence of UV

The trends observed in the titania-AC composites, particularly when compared to

the performances of the virgin ACs resulted in the author investigating the possibility of a

phenol* phenomenon, similar to the dye* phenomenon noticed by Khan et al (2006). The

virgin ACs (50 mg) were exposed to UV for 24 hours without the presence of any

photocatalyst. As a control study, phenol solution (no AC, no titania) was also exposed to

UV light for 24 hours. The resulting data are provided in Figure 4-12.


60


50
:J
o) 1 Virgin AC (NO UV)
-40 Coated AC (No UV)
o O Coated AC (Wth UV)
SVirgin AC (Wcth UV)
'c30

0
o
0
20


10
-C




0
Initial Concentration Bionuchar F400 HD4000
AC Samples

Figure 4-12. Comparison of irradiated virgin ACs with virgin and coated ACs in the
presence and absence of UV

The results of the batch tests performed in section 4.2.3 are provided in Table 4-3.

The table quantifies to an extent the domination of the adsorption mechanism over

photocatalysis.









Table 4-3. Phenol removal as a function of AC coating strategy with and without
irradiation
Activated Precursor Virgin AC 3% (m/m) AC 3% (m/v) 3% (mol/v)
carbons performance performance AC AC
(% removal) (% removal) performance performance
(% removal) (% removal)
No With No With With UV No With
UV UV UV UV UV UV
Bionuchar Wood 51.5 36.9 44.7 38.7 37.7 37.1 37.9
F400 Bituminous 87.1 84.7 82.3 88.6 91.3 91.3 92.5
HD4000 Lignite 75.9 61.3 73.4 78.2 74.2 74.5 75.5

Comparison of Bionuchar systems. In the case of Bionuchar, the virgin AC in the

presence of UV (36.9% removal) performed poorly compared to its performance in the

dark (51.5% removal). This can be accounted for by the theory put forth by Khan (2006)

whereby the excitation of delocalized electrons in the aromatic structure of phenol results

in reduced uptake by Bionuchar whose surface is supersaturated with acidic functional

groups. The decrease in the electron density in the basal planes of Bionuchar due to the

acidic functional groups coupled with the excitation of delocalized electrons in phenol

(resulting in phenol*) contributes to decreased xt-7t interactions and hence decreased

adsorption. Bionuchar's low affinity for phenol* results in the overall low adsorption of

phenol. The coated Bionuchar exposed to UV may show a similarly low performance due

to a combined effect of pore blockage and the effect of phenol* on its adsorption.

Comparison of F400 systems. F400, unlike Bionuchar and HD4000 does not show

much variation regardless of whether the carbon was coated with titania and exposed to

UV or if the virgin carbon was exposed to UV. The virgin F400 in the absence of UV

(87% removal) is observed to perform as well as the coated F400 which was irradiated

(88% removal).









The similarities in the performances of the two systems could be a result of either

of two processes.

* The coated F400 compensates for the blockage of pores through photocatalytic
degradation of phenol or

* The coated F400 in the presence of UV behaves the same as the virgin F400. Hence
pore blockage is not a factor. However, this would also put to question the presence
of any photocatalytic effect.

It is also noticed that virgin F400 in the presence of UV performs slightly worse

(ca. 85%) than the F400 experiment in the dark (ca. 87%). This difference is however not

considered to be significant.

Comparison of HD4000 systems. Finally, for HD4000, the irradiated titania

coated sample displays maximum performance (78% removal) which is only slightly

greater than the performances of the virgin HD4000 (no UV) with 76% removal and the

coated HD4000 (no UV). It is however interesting to note that the irradiated samples of

virgin HD4000 perform considerably worse (61% removal) than the other samples. This

may point to phenol* having a large influence over the adsorption capacity of HD4000.

Noticing that the coated HD4000 in the presence of UV performs well, it may be

theorized that the performance of HD4000 observed in irradiated samples may be

attributed more to enhanced photocatalysis. It may be this enhanced photocatalysis which

compensates for the poor adsorption of phenol* attributed to its surface acidity.

The performances of the ACs in the remaining systems do not show any significant

deviation. The slight changes in performance are attributed to a combination of

adsorption and photocatalysis.

Hence, in summary, from the explanation provided above, it is clear that in terms of

photocatalytic activity, HD4000 seems to be the most active AC. The acidic nature of its









surface is thought to slightly inhibit adsorption. However, as seen from the SEM and

EDS images, it seems to have the most uniform coating and the most adhesive surface

towards TiO2. This could possibly be an explanation to its superior photocatalytic

activity. The poor performance of Bionuchar has been extensively discussed and

attributed to its characteristic acidic surface. A point has also been made regarding the

morphology of the Bionuchar resulting in deposition of TiO2 in its pores, hence resulting

in decreased adsorption. The poor adherence of TiO2 to the Bionuchar surface is the

cause of the low photocatalytic degradation of phenol observed with the TiO2-Bionuchar

composite. F400 largely displayed a consistent performance over each of its systems. Its

more basic surface, morphology resulting in good affinity towards TiO2 and high

adsorption capacity seem to be important factors.

The effect of conductivity, if any, seems to be overwhelmed by other factors such

as adsorption and morphology of the surface. In fact, in the case of activated carbons, the

phenomenon of irradiated phenol resulting in a change in adsorption characteristics

proves that a good photocatalytic system would require adsorption as a primary

mechanism.

Although it has not been investigated, the author also proposes the possibility of

UV irradiation having an effect on the AC surface. Hence, it is possible that the UV

irradiation affects the AC such that the carbon 7n system changes. This would also result

in noticeable changes in adsorption of phenol by each of the ACs. For instance, excitation

of the 7t electrons in the carbon basal planes may result in a phenomenon similar to the

one proposed with the phenol*. The author has however not investigated the possibility

of this phenomenon in any further detail.









The replicate sets for the experiments involving the virgin ACs in the absence of

UV radiation is provided in appendix A.

4.3 Low Surface Area Carbon Studies

The multitude of variables observed in the performance of ACs with respect to

adsorption and photocatalysis resulted in the investigation of carbon substrates having

low surface areas. To further test whether conductivity and metals play a role in

enhancing photocatalysis, low surface area carbons (< 8 m2/g) with varying

conductivities were used in further tests. By selecting a variety of carbons such that they

differed in their properties that were conducive or inhibitive to photocatalysis, it would be

possible to relate specific properties of the carbons to the observed synergy. As

hypothesized by Khan (2003), the reason for the superior performance of certain ACs is

the presence of ionic metals. These metals act as electron sinks, causing the hindrance of

electron-hole recombinations, thus enhancing photocatalysis. As explained earlier, this is

interpreted by this author as enhanced photocatalytic activity being a function of higher

electrical conductivity primarily due to specific sites on the carbons where metals are

present. In theory, the presence of titania in the vicinity of a conductive site (metal) will

result in a favorable difference in Fermi energies between the TiO2 and the metal. The

resulting effect is the change in the distribution of electrons of the semiconductor (TiO2).

The formation of the Schottky barrier will cause the migration of electrons from the

titania to the metal increasing the electron-hole recombination times and therefore

enhancing photocatalysis.

It should be noted that in general, the addition of conductive materials to

semiconductors or insulators has a positive impact on their conductivity. This property

may be approximated as a summation of the conductivities of the materials involved.









However, at the same time it is important to note that metals content is by no means the

sole factor influencing electrical conductivity. In the case of graphite, a major influence

of its electrical conductivity is its crystalline structure. Crystalline graphite consists of

parallel sheets of carbon atoms, each sheet containing hexagonal arrays of carbon atoms.

Each carbon atom exhibits sp2 hybridization and forms sigma bonds with its three nearest

neighbors. There is also distributed pi bonding between the carbon atoms in the sheet.

This delocalized pi system is responsible for the electrical conductivity of graphite. The

disruption of this graphitic structure is the cause for a loss in electrical conductivity in

carbonaceous materials.

With this in mind, carbon materials having varying electrical conductivites were

selected. The carbons selected for the studies were graphite, pitch coke, bituminous coal

and anthracite coal. The gradient in electrical conductivity between the selected carbons,

graphite being the highest and bituminous the lowest is shown in Table 4-4. These

conductivities were measured as resistivities by compacting the carbons in a column at

pressures between 125-150 psi and measuring their resistivity to charge (Data provided

by and experiments performed by Asbury Carbons, NJ). Hence the resistance of the

compacted powdered carbons was measured and obtained which was then easily

translated to electrical conductivity by the reciprocal of the resistivity.

Table 4-4. Electrical conductivity values for low surface area carbons
Carbon Conductivity (mQ' cm' ) (mho)
Graphite 20-50
Pitch Coke 10
Anthracite 0.001-0.01
Bituminous < 0.001









4.3.1 Metals Content of Low Surface Area Carbons

Before discussing how the titania coated low surface area carbons performed for

phenol adsorption, it was of interest to also investigate the ash contents of these samples.

To determine if conductive metals were indeed present in any of the carbons, an EDS

study similar to the one performed on the ACs was performed. Prior to that, as in the case

of ACs, the quantity of ash for each carbon was determined. This would enable us to

analyze the EDS scans with more perspective. The ash contents of the low surface area

carbons are provided in Table 4-5.

Table 4-5. Elemental carbon and ash contents of low surface area carbons
Carbon Elemental carbon (%) Ash (%)
Graphite 99% 1%
Pitch coke 93% 7%
Anthracite 90% 10%
Bituminous 65-72% 28-35%

EDS scans of all the carbons were performed with the exception of graphite. The

electrical conductivity of graphite is extremely high and its ash content is negligible.

Hence, the amount of impurities found in graphite (metals) would be negligible and

would not make any significant difference. However, it is interesting to see whether the

conductivity due to its structure plays a role in photocatalysis.

The EDS scan of the ash residue of the anthracite coal is shown in Figure 4-13. It

shows prominent peaks corresponding to aluminum and iron. These metals have very

high electrical conductivities (376/mohm-cm and 102 /mohm-cm respectively) as

compared to titanium (23 /mohm-cm). A smaller peak of sodium is also seen. Another

peak present is that of silicon (electrical conductivity of silicon = 0.012 /mohm-cm). A

number of such EDS scans were taken, all of which showed similar results.











Counts

8000-


6000- Si

Al
4000-
Fe

2000-
Na
Ti
K eTi Fe
0 . .. . I.-' . ..
0 2 4 6 8
Energy (leV)


Figure 4-13. EDS scan of ash residue of uncoated anthracite coal


Again, it must be noted that EDS is by no means a quantitative analytical tool and


is at best semi-quantitative. However, the images do show a greater prominence of some


elements over others.


Figure 4-14 shows an EDS scan of the ash residue of bituminous coal. The image shows


a prominent aluminum peak. In addition to this, smaller peaks of iron, calcium and


magnesium and sodium are also seen. Again, as in the case of anthracite, a silicon peak is


noticed.


Counts


0 2 4


8
Energy (keV)


Figure 4-14. EDS scan of ash residue of uncoated bituminous coal







70


Although anthracite and bituminous are low conductivity carbons, the presence of

metals such as aluminum and iron means that there is a high probability of a number of

conductive sites on the surface of these carbons. The amount of ash contained in these

carbons (10% and 28-35% respectively) also points towards there being a relatively large

amount of these metals in these carbons.

A similar scan was performed on the ash of pitch coke (Figure 4-15). Small peaks

of aluminum and silicon in addition to small peaks of sodium and sulfur are seen.

Counts

8000-


6000-


4000-


2000-
O Si
Na Al S
0 I A . . i ,-,-,--
0 2 4 0 8
Energy (keV)

Figure 4-15. EDS scan of ash residue of pitch coke

From the EDS scans of the low surface area carbons, it is evident that aluminum is

commonly found in each of the carbons. A very prominent iron peak is seen in anthracite

coal with the bituminous sample also showing some presence of iron. Recalling the study

by Khan (2003) which hypothesized metals (specifically iron) to be the cause of

increased photocatalysis, it may be expected that anthracite and bituminous should show

the greatest removal when the carbons are subjected to batch adsorption-oxidation

studies.











4.3.2 Batch Studies with Low Surface Area Carbons

Batch studies were performed with the low surface area carbons. These tests were

performed in a procedure identical to that used in the activated carbon studies. The

carbons used were sized to 35 x 200 sieve in order to normalize as far as possible the

external surface area of the carbons. Titanium dioxide was coated on the ACs via boil

deposition on a mass basis. A mass of 80 mg of each sample was used with 80 mL of 55


ppm phenol solution.

Adsorption studies performed on coated low surface area carbons in the dark.

In order to verify the negligible adsorption capacity of the carbons, they were put through

adsorption studies in the absence of UV light. In addition to the carbons, a titanium

dioxide (Degussa P25) slurry was also tested as a comparison to the coated substrates.

The titanium dioxide was weighed such that its mass was equal to the theoretical mass of

the titania on the surface of the carbons (3% of 80 mg, i.e., 2.4mg). It was expected that

all the samples including the titanium dioxide slurry would show negligible adsorption.

Figure 4-16 shows the results of the adsorption studies.

70
Initial Concentration
3% Pitch Coke
o 3% Graphite
60 0 3% Anthracite
0 3% Bituminous
0 2.4 mg TiO2 Slurry
2 50

S40

30

20

10

0-
Sam pies


Figure 4-16. Adsorption studies of coated low surface area carbons and titanium dioxide
slurry









It can be seen that the adsorption seen in each of the carbon samples including the

slurry is negligible. In fact the maximum removal seen is only 4.6%. This emphatically

proves that the carbon composites are indeed of low surface area, incapable of any

significant adsorption. It was thought unnecessary to perform adsorption studies for the

virgin carbons due to their low surface areas.

Photocatalysis studies performed with coated low surface area carbons. The

carbons and the titania slurry were then made to undergo photocatalysis studies. Using an

identical configuration, the samples were exposed to 365 nm UV light. It was expected

that carbons containing a larger concentration of metals in their matrices would perform

better than those with a lower content of metals (Khan, 2003). Although all the carbons

with the exception of graphite contained a considerable amount of metals, due to the

presence of metals and the relatively high natural conductivity of pitch coke, it was

expected to perform better than the other carbon composites. Due to better mass transfer,

the titanium dioxide was also expected to perform relatively well. The results of the

experiment are shown in Figure 4-17.

Although there was little distinction in the performance of the carbons, it was

noticed that the titania coated bituminous coal showed the greatest removal of 20%.

Anthracite coal with 18% removal also performed relatively well. Pitch coke which was

expected to perform well only removed 6% of the phenol. The graphite which is the most

electrically conductive, although lacking in ash content showed a removal of 13%.

However, a surprising result was that of the titania slurry which only accounted for 11%

removal. This was attributed to a reduction of the surface area of TiO2 available for

irradiation. It was thought that the amount of titania loaded was too high.










70
Initial Concentration
3% Ptch Coke
60 3% Graphite
O[ 3% Anthracite
su- w 3% Bituminous
S50 2.4 mg Ti02 Slurry
E
0


30





10 -
0


Samples



Figure 4-17. Batch photocatalysis studies of carbons and titanium dioxide slurry (m/m)

Hence for the specific configuration of the reactor, the titania concentration in the

slurry was greater than the optimal loading, causing low penetration of UV through the

slurry. It has been previously observed in Figure 4-9 that a slurry with a lower loading of

titania has performed better.

Comparing the oxidation studies to the adsorption studies in the absence of UV, it is

evident that there is photocatalysis taking place. A comparison between the carbon-titania

particles shows no trend with respect to conductivity or metals content in their ash

residues. However, a trend was noticed in the difference in performances being similar to

the carbons' differences in densities. Due to this, the volume of carbons and hence the

amount of titania in each reactor would be different. Higher density would result in a

smaller volume of that particular carbon resulting in a lower TiO2 dose in the respective

reactor. Table 4-6 summarizes the densities of the coated carbons.









Table 4-6. Average densities of 3% (m/m) titanium dioxide coated carbons
Carbon Density
g/m3
Bituminous 1.37
Anthracite 1.93
Pitch coke 2.25
Graphite 2.58

From the table it can be seen that bituminous coal and anthracite coal are the two

carbons with the least density. This difference in density is reflected in the difference

seen between the performances of the carbons Since the adsorption capacities for all the

carbons were negligible, the coated carbons were loaded on the basis of volume in order

to normalize the volumes and titanium dioxide loaded to each reactor. This was expected

to magnify differences in phenol decomposition between the carbons. The following

graph shows the results of the batch studies in which the carbons were coated with TiO2

on the basis of mass and loaded to the reactors such that their volumes equaled 0.05 mL.

This set, shown in Figure 4-18, is referred to as 'mass-volume' or 'm/v'.

The results reiterated the fact that there was no real difference in performance in the

carbons with respect to adsorption or photocatalysis. The lack of distinction in

photocatalytic performance as well as adsorption between the carbons prompted the

author to discard the theory regarding phenol* having an influence on photocatalysis

when adsorption was absent. It was postulated that the excitation of the electrons in

phenol only has an effect on its adsorption characteristics and does not affect

photocatalysis.

The influence of metals on photocatalysis was also questioned as clearly, there was

no distinction seen in photocatalysis between the carbons even though they differed in the

content and composition of metals.













70
Initial Concentration
3% Ptch Coke
60 0 3% Graphite
Fi 4 O 3% Anthracite
s a 3% Bituminous
E 50

40

= 30
o

020

10

0
Samples


Figure 4-18. Photocatalysis studies of coated low surface area carbons in the presence of
UV (m/v)

To further verify these results and normalize the amount of carbon and titania in

each reactor, tests were performed using a molar basis for coating the carbons with TiO2.

This basis, explained in detail in the materials and methods section consists of coating the

carbons such that the mole ratio of titanium dioxide to carbon is 3%. The carbons were

then loaded to their reactors on the basis of volume as in the earlier case (0.05 mL). This

set of samples has been referred to as 'mole-volume' or 'mol/v'. The results from these

tests which were otherwise performed in an identical manner to the previous batch tests

are shown in Figure 4-19. For the mass-volume and the mole-volume sets, it was deemed

unnecessary to perform control studies in the absence of UV light as the carbons

contained no internal pore structure. Hence, a higher concentration of TiO2 coated on the

carbon surface would not make any noticeable difference to adsorption. The results from

these tests showed a slight increase (less than 2%) in the photocatalytic degradation of







76


phenol. However, this was seen in all the samples, resulting again in no significant

distinction between the samples.

70
m Initial Concentration
3% Pitch Coke
60 O 3% Graphite
O 3% Anthracite
50 3% Bituminous
E o50

40


30
o
-,
C 20

10


Sample


Figure 4-19. Photocatalysis studies of low surface area carbons in the presence of UV
(mol/v)

It may be noticed that the performances of the ACs with the exception of the 'mole-

volume' samples which showed a slight increase in performance, remains relatively

constant. The only considerable distinction can be made in the case of the 'mass-mass'

carbons. However, this has been explained by the differences in their densities. It is

evident from these studies that when titanium dioxide is coated on a substrate such as

carbon, adsorption plays a very important role. More importantly, what is shown by the

low surface area carbon tests is that as previously hypothesized by this author (Section

2.4), conductivity may not be playing any significant role in the enhancement of

photocatalysis. Even though carbons were selected whose conductivities varied by orders

of magnitude, there was no significant effect or even any trend that was noticed in the

performance of the titania coated carbons. Hence, the conclusion by Khan (2003) about

metals such as iron enhancing photocatalysis due to their role as electron traps is






77


uncorroborated. At the same time, the theory by Matos et al. (2001) regarding the

synergy seen in the case of titania systems containing H-type ACs is also unsubstantiated.

Furthermore, the experiments conducted with the AC composites and the low

surface area carbonaceous composites show that a slight synergy may be noticed in TiO2-

AC composites, primarily driven by adsorption. The difference in the electrical

conductivities of the low surface area substrates did not show any considerable synergy.














CHAPTER 5
SUMMARY AND CONCLUSIONS

5.1 Summary

In this study, activated carbons which differed in physical and chemical properties

were coated with titanium dioxide and subjected to a variety of experiments. The AC

composites were primarily tested with respect to the impact of electrical conductivity of

the substrate on photocatalysis in addition to photocatalysis as a function of morphology

and surface chemistry.

The results did not definitively support my initial hypothesis that electrical

conductivity of the ACs influences the photocatalytic properties of the AC-TiO2

composite. A slight improvement in photocatalysis was observed in coal based ACs

which were believed to possess higher conductivities. The improvement in performance

did not significantly prove that a synergy was present between TiO2 and the conductive

ACs (F400 and HD4000); however, the definite trend seen in the tests leads the author to

believe that conductive sites on the surface of AC may result in a localized enhancement

of photocatalysis. Moreover, this localized enhanced photocatalysis mechanism is

insignificant in the perspective of the adsorption mechanism at the scale at which it was

carried out.

EDS scans of the ACs showed the absence of key metals (i.e., absence of iron in

HD4000) which had been attributed to the synergy observed in the tests performed by

Khan (2003). The premise that trace metals in the AC serve as electron traps, prolonging

electron-hole recombination times leading to enhanced photocatalysis was thus









questioned. In effect, the presence of metals and hence the influence of conductivity of

ACs on enhanced photocatalysis is a minor factor for photocatalysis due to their low

distribution and the low probability that a TiO2 particle would be deposited in their

vicinity. The argument was further strengthened after reviewing the performances of the

low surface area carbons which contained varying quantities of metal composition but

showed no significant differences in performances.

There is evidence that the morphology of the material (AC) to an extent governs

the amount of photocatalyst that adheres to it. The smoothness of surfaces such as in the

case of the wood based Bionuchar resulted in decreased amounts of titania being

deposited. In fact, it was noticed that of the titania that was deposited, a large quantity

was driven into the pores, pointing towards adsorption of titania in the pores during the

coating process as opposed to its deposition on the exterior surface of Bionuchar. The

pore blockages as a result of this are one of the reasons for Bionuchar's lower phenol

adsorption. In contrast, rough surfaces such as the coal based F400 and HD4000

displayed greater quantities of TiO2 deposited on their surfaces. SEM images of the

coated ACs also showed a more uniform coating of titania. It was noticed that the same

ACs also displayed greater adsorption capacity

Presence of surface functional groups on the AC is also believed to be a major

influence on adsorption and hence indirectly on photocatalysis. Surface functional groups

are well known to have an influence on the adsorption of organic compounds by activated

carbons. This fact was related to another phenomenon in explaining the variation in

trends seen in the ACs when they were subjected to UV. The phenomenon involved a

similar observation to the one by Khan (2003) in the case of dyes. Irradiated virgin ACs









showed a decrease in performance when compared to the adsorption demonstrated by

virgin ACs in the dark. Phenol* which is the excited state of phenol in which delocalized

electrons in phenol are excited is the suggested theory. The adulteration of the initial

compound is believed to be a reaction which occurs in the picosecond time scale,

explaining why irradiated phenol solution does not show any change in spectroscopic

absorbance.

Another possibility which has not been investigated by this author is that the UV

radiation affects the activated carbon itself. Hence, it is possible that there is a change in

the system of the 7n electrons in the carbon basal planes of the activated carbon resulting

in a decrease in the dispersive adsorption forces. Due to the decreased 7n-7n interactions; in

this case due to the carbon 7n electrons, a decrease in adsorption of phenol by the

irradiated ACs may be noticed. Another possibility is that both the theories may take

place simultaneously causing the observed results.

The superior performance of F400 across all the systems is attributed to a

combination of factors. The basic surface of F400 does not inhibit adsorption of the

organic contaminant as its 7n-7n interactions are strong. The pore blockages of F400 are

overcome by the superior photocatalysis that it shows. Although the presence of metals

such as aluminum might point towards a Fermi energy difference influencing the results,

it is highly unlikely given the large dominance of the adsorption mechanism.

On the basis of photocatalysis alone, HD4000 performed the best. The poor

adsorption capacity of HD4000 for irradiated phenol was compensated by photocatalysis.

The process of photocatalysis, especially when combined with the mechanism of

adsorption is one that is extremely complex and not very well understood. When the









mechanism of photocatalysis is combined with an adsorption mechanism, it is often

difficult to distinguish between and isolate the two mechanisms.

I propose that a synergy in activated carbon-TiO2 composites may indeed show no

significant improvements. However, even small improvements in performances may

actually be a synergistic phenomenon in which the AC is being photocatalytically

regenerated during its adsorption cycle. When the ACs are compared, it is evident that

there is a difference in their overall performance with the use of the titania-carbon

composite.

5.2 Conclusions

* In the case of Bionuchar, the decrease in the electron density in its basal planes due
to its acidic surface functional groups coupled with the excitation of delocalized
electrons in phenol explains the decrease in performance of its irradiated coated
composite as compared to its non-irradiated composite. It is due to a combination
of these factors that Bionuchar demonstrates poor performance.

* The synergy demonstrated by F400 is attributed to a combination of high
adsorption capacity of phenol as well as a balanced affinity for phenol which
would provide more pollutant to be photocatalytically degraded on the surface of
F400 which is uniformly coated with TiO2.

* Even though HD4000 may be considered an L-type AC, it actually shows a
synergistic effect with respect to adsorption and photocatalysis which refutes Matos
et al. (2001).

* Although electrical conductivity of the low surface area carbons varied by orders of
magnitude, they demonstrated the same performance. The absence of any
adsorption in fact decreased the distinction between carbons. Hence, adsorption is
clearly the dominant mechanism over photocatalysis. The key factors of adsorption
are the morphology of the AC, surface area and pore size of the AC and the surface
functional groups on the AC.














CHAPTER 6
CONTRIBUTIONS TO SCIENCE AND ENGINEERING

6.1 Contributions to Science

The experiments carried out have yielded a very interesting set of results. An

analysis of the results has shown that combining two mechanisms (i.e., adsorption and

photocatalysis) creates a complex system which needs to be further understood. This

complex system consists of various mechanisms which may be dependent on each other.

The systems behave in such a manner that it is extremely difficult to distinguish between

the two mechanisms. In lieu of this, the contributions of this thesis to science in the

opinion of the author are as follows:

* It has been adequately demonstrated that surface functional groups primarily affect
adsorption and hence only indirectly affect photocatalysis. This is not in
accordance to what Matos et al. (1998, 2001) proposed. It is only due to the fact
that adsorption is a dominant mechanism that photocatalysis is observed to be
influenced by the surface functionality of the activated carbons.

* The phenomenon of the excitation of the delocalized electron in dyes (dye*) which
was observed by Khan et al. (2006) is also observed in the case of phenol (i.e.,
phenol*). However, whereas in the case of dyes it was observed to enhance or
inhibit adsorption depending upon the type of AC, in the case of phenol it was
primarily noticed to inhibit adsorption of both L-type and H-type ACs. It is also
possible that the UV radiation has an affect on the carbon 7t electron system,
thereby affecting adsorption.

* Finally, it has been shown that individual material properties do not have much of
an influence on the adsorption-photocatalysis system. It is more likely that a
synergy exists between the general mechanisms of photocatalysis and adsorption
with adsorption being the dominant mechanism. It is proposed that an enhancement
due to synergy may be noticed in systems in which the AC needs to be regenerated
in-situ. Prior to research being performed on individual material properties and
their effects on photocatalysis, further research must be performed to distinguish
between photocatalysis and adsorption and how they behave synergistically.









6.2 Contributions to Engineering

As mentioned in section 6.1, the research presented by the author demonstrates a

possibility to apply adsorption-photocatalysis systems to regenerate activated carbons in-

situ. Activated carbon in its contemporary use requires to be regenerated (usually off-site)

after its adsorption capacity has been reached. By the use of activated carbon coated with

titanium dioxide, it is possible to regenerate the ACs in-situ by exposing them to UV

radiation during their adsorption process.

Such a system would require a certain amount of fluidization to allow the radiation

to be incident on all of the TiO2-AC particles. Preliminary column studies were

performed by this author in which HD4000 and F400 were used in a column

configuration. Phenol was allowed to flow through the column and the particles were

slightly fluidized. The entire column was exposed to UV radiation.

Comparing the irradiated system to a system not exposed to UV, a small difference

was seen in the two sets of data with the irradiated system performing better. The data is

provided in Appendix B and Appendix C. It was found that although the two systems

were regenerated by flowing DI water to the same extent, the effluent showed lower

concentrations of phenol in the irradiated sample pointing towards photocatalytic

degradation of the phenol.

Hence, these bench scale column studies show the engineering application of

titanium dioxide coated activated carbon in the in-situ regeneration of activated carbon.



















APPENDIX A
VIRGIN AC STUDIES IN PRESENCE OF UV


~4-
S40



30
E

o
0
1 20



10


* Initial
* Bionuchar
O F400
0 HD4000


0 1


I


Sample


Figure A-i. Replicate data sets for virgin AC studies performed in the presence of UV
demonstrating the phenol* phenomenon observed in Section 4.2.4.


















APPENDIX B
COLUMN STUDIES


1 100


* With UV 1500 mL/hr
* Without UV 1500 mL/hr
Wth UV
Without UV
x With UV Packed Bed
* Without UV Packed Bed


*)KX
S


.~x


X
i
$ *


i i" n


1.000

0.900

0.800

0.700

0.600

0.500

0.400

0.300

0.200

0.100

0.000 -


2000


1000


Time (min)


Figure B-1. Recirculation column studies performed with F400 using different flowrates
(degrees of fluidization) in the presence and absence of UV.


1500


I






















APPENDIX C
COLUMN STUDIES II


1.20
1st Adsorption Run (Averaged)
S2nd Adsorption Run (Only DI)
1.00 2nd Adsorption Run (UV + DI)


0.80


0.60 .


0.40 .


0.20


0.00
0 100 200 300 400 500 600 700
Time (minutes)


Figure C-1. Adsorption runs of column tests performed with HD4000 in the presence and
absence of UV radiation measuring phenol concentration in the effluent


1.80
DI + UV Regeneration
1.60 m Only DI Regeneration

1.40

1.20

S1.00

0.80

0.60

0.40

0.20 *

0.00
0 100 200 300 400 500 600 700 800

Time (minutes)


Figure C-2. Concentration of phenol
studies


in effluent during regeneration runs of column















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