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The Performance of Silica-Titania Composites in a Packed-Bed Reactor for Photocatalytic Degradation of Gray Water

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THE PERFORMANCE OF SILICA-TITANIA COMPOSITES IN A PACKED-BED REACTOR FOR PHOTOCATALYTIC DEGRADATION OF GRAY WATER By CHRISTINA YVETTE LUDWIG 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 ENGINEERING UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by Christina Yvette Ludwig

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ACKNOWLEDGMENTS I would like to thank my advisor, Dr. David Mazyck, for the opportunity to perform this research and for his guidance. It has been a challenging and rewarding experience to learn from him. I would also like to thank the other members of my committee, Dr. Paul Chadik, Dr. Kevin Powers, and Dr. Chang-Yu Wu, for their constant supply of knowledge and suggestions throughout the duration of this research. I am grateful for the opportunity to work as part of an excellent research group. I thank Ameena Khan, Matt Tennant, Thomas Chestnutt, Jack Drwiega, Jennifer Hobbs, Jennifer Stokke, Morgana Bach, and Danielle Londeree for their ideas and suggestions as well as their general support and advice. Each group member contributed to this research as well as my education. I must also thank my family for all their love and support. Special thanks go to my husband, Matthew, for his constant encouragement and support throughout all of my education. iii

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TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES.............................................................................................................vi LIST OF FIGURES..........................................................................................................vii ABSTRACT.......................................................................................................................ix CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW.............................................................................................5 Photocatalysis...............................................................................................................5 Titanium Dioxide...................................................................................................6 Kinetics..................................................................................................................7 Catalyst Supports..........................................................................................................8 Activated Carbon........................................................................................................10 Silica...........................................................................................................................12 Regeneration........................................................................................................14 Synthesis by Sol-Gel Method..............................................................................14 Silica Gel Properties............................................................................................17 Silica-Carbon Composites..........................................................................................17 3 MATERIALS AND METHODS...............................................................................19 SiO2-TiO2 Composites................................................................................................19 Reactor System...........................................................................................................22 Simulated Wastewater................................................................................................24 Reactor Dynamics.......................................................................................................25 Experimental Procedures............................................................................................27 Adsorption and Destruction.................................................................................27 Adsorption of Intermediates................................................................................29 iv

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4 RESULTS...................................................................................................................30 System Optimization..................................................................................................30 Flow Rate.............................................................................................................30 Pore Size..............................................................................................................35 Titanium Dioxide Loading..................................................................................38 Activated Carbon Loading..................................................................................40 Kinetics.......................................................................................................................46 5 SUMMARY AND CONCLUSION...........................................................................49 APPENDIX SILICA-TITANIA COMPOSITE CHARACTERIZATION......................51 LIST OF REFERENCES...................................................................................................52 BIOGRAPHICAL SKETCH.............................................................................................56 v

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LIST OF TABLES Table page 1 Chemicals Used for Silica Gel Synthesis.................................................................19 2 Properties of Degussa P25 TiO2...............................................................................20 3 Reactor Specifications..............................................................................................23 4 Simulated Wastewater Composition........................................................................24 5 Rate Constants for TOC Mineralization to 500 ppb................................................48 A-1 Surface Area, Pore Volume, and Pore Size of Composites Tested..........................51 vi

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LIST OF FIGURES Figure page 1 Illustrations of TiO2 crystalline structures: rutile (left) and anatase (right)...............6 2 Particle size distribution of activated carbon used in AC-SiO2-TiO2 composites....20 3 Silica sols doped with titanium dioxide were mixed using magnetic stir plates and transferred to 96 well assay plates before forming a gel..........................................21 4 Reactor packed with SiO2-TiO2 composites with a UV light through the center.....22 5 Treatment system consisting of reactor packed with SiO2-TiO2 composites, UV lamp, pump, and reservoir of simulated wastewater.................................................23 6 Calibration of conductivity meter.............................................................................25 7 This figure compares the residence time distribution of the reactor as compared to a CSTR-in-series model........................................................................................27 8 This figure compares the residence time distribution of the reactor as compared to a CSTR-in-series model........................................................................................27 9 Diagram of the system used to test the adsorption of the intermediates formed from photocatalysis at different EBCTs...................................................................29 10 TOC removal by adsorption at flow rates of 5, 10, 20, and 30 mL/min, corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and hydraulic loading rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively.......................................31 11 TOC removal by destruction in a recirculating system at flow rates of 5, 10, 20, and 30 mL/min, corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and hydraulic loading rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively..........32 12 TOC removal by adsorption of wastewater effluents resulting from destruction at various EBCTs..........................................................................................................34 13 Effect of pellet size on adsorption of the wastewater...............................................36 14 Effect of pellet size on destruction of the wastewater..............................................36 15 Effect of pore size on adsorption of the wastewater.................................................37 vii

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16 Effect of pore size on the destruction of the wastewater..........................................37 17 Effect of TiO2 loading on adsorption of the wastewater...........................................39 18 Effect of TiO2 loading on destruction of the wastewater..........................................39 19 Effect of activated carbon addition to the adsorption properties of the pellets........42 20 Effect of activated carbon loading on the destruction performance of the pellets....42 21 Comparison of the first adsorption cycle (ads1) to the adsorption of the pellets after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118 angstroms, and 1.3wt% activated carbon loading.....................................................43 22 Comparison of the first adsorption cycle (ads1) to the adsorption fo the pellets after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118 angstroms, and 3.9wt% activated carbon loading.....................................................43 23 Effect of activated carbon loading on the adsorption properties of the pellets.........45 24 Effect of activated carbon loading on the destruction performance of the pellets....45 25 Comparison of the first adsorption cycle (ads1) to the adsorption of the pellets after the destruction cycle (ads2), using pellets of 12% TiO2 loading, 118 angstroms, and 9.6wt% activated carbon loading.....................................................46 viii

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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 Engineering THE PERFORMANCE OF SILICA-TITANIA COMPOSITES IN A PACKED-BED REACTOR FOR PHOTOCATALYTIC DEGRADATION OF GRAY WATER By Christina Yvette Ludwig August 2004 Chair: David Mazyck Major Department: Environmental Engineering Sciences Photocatalysis is used to mineralize organic water pollutants, providing water treatment without a waste stream. This water treatment method allows for a compact reactor design that is applicable in future NASA missions that will require water recovery. The objective of this research was to optimize a photocatalytic reactor that will reduce the total organic carbon (TOC) concentration of a gray water influent to meet NASAs drinking water quality standards. The system utilizes a reactor packed with titanium dioxide (TiO2) supported by silica gel (SiO2) and was optimized with respect to empty bed contact time (EBCT), pore size of the SiO2-TiO2 composite, and TiO2 loading of the SiO2-TiO2 composite. The addition of activated carbon to the SiO2-TiO2 composite was also investigated. An optimum EBCT of seven minutes was found based on the fastest destruction rate to reach below the 500 ppb TOC maximum limit established by NASA. Also based on destruction rate, a SiO2-TiO2 composite having a 12% TiO2 loading (weight/volume ix

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basis) and 118 angstrom pore size was shown to have the best performance. Activated carbon was found to improve the overall performance of the system through its increase to the composites adsorption capabilities. Greater adsorption capabilities can allow for lower energy requirements, enabling the UV light to be turned off during a period of contaminant adsorption and turned on for mineralization of the contaminants already surrounding the photocatalyst. Adsorption capabilities of the support can lead to more efficient, complete destruction of the contaminants by surrounding the photocatalyst with a high concentration of contaminants; therefore, it was predicted that increasing the ability of the photocatalyst composite to adsorb the organic contaminants would increase the rate of photocatalytic destruction. Adsorption was shown to be an important system parameter, but increasing adsorption did not always lead to a better destruction rate due to the contribution of other parameters controlling the system, such as UV light exposure and hydraulic flow through the reactor. x

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CHAPTER 1 INTRODUCTION Contamination of our water, air and soil by various agricultural and industrial processes has traditionally been controlled through ultrafiltration, extraction, air stripping, carbon adsorption, incineration, and oxidation via ozonation or via hydrogen peroxide (Serpone, 1995). Problems with the successful and efficient removal of toxic pollutants from our environment have led to the search for more advanced methods (Hoffmann et al., 1995). Groundwater contamination is expected to be a primary source of human contact with a majority of the toxic pollutants in our environment, many of which are organic compounds, such as solvents, pesticides, chlorophenols, and volatile organics (Hoffman et al., 1995). Photocatalysis offers an advanced technology for the removal of toxic organics from our water. Many of the previously mentioned technologies simply transfer the pollutant out of the water, requiring additional treatment and/or disposal of the compound. In photocatalysis, the organic contaminants are oxidized, ultimately to carbon dioxide and water, leaving no waste to dispose. Utilizing UV irradiation, photocatalysis has the added advantage of being able to simultaneously disinfect and destroy organics (Serpone, 1995). Because of its promise as a more advanced environmental technology, photocatalysis is being developed for the destruction of toxic organic compounds in water. Given its ability to eliminate organic compounds in one process, this technology has a variety of applications. Its potential to be a compact, low maintenance system makes it particularly useful for small-scale water recovery. Water recovery is a vital component of NASA space 1

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2 missions. The reclamation of water from gray sources, which include shower waste, hand wash waste, oral hygiene waste, urine, urine flush waste, and spacecraft humidity condensate, enables long-term presence in space without the penalty of transporting large masses of clean water to the crew members. It is also a necessity for missions involving long flight distances, such as a mission to Mars. It is important that the water recovery system be compact and reliable with as little energy requirements as possible, allowing crew members to direct time and resources toward scientific investigations. A photocatalytic system that meets NASAs water recovery system requirements is being developed to be used as a post processor for the destruction of organic compounds that may not be removed in preceding water subsystems. These subsystems may include biological reactors, ion exchange, and reverse osmosis. The objective of the post processor is to ensure that the total organic carbon (TOC) concentration remains below the maximum limit required by NASA water quality standards, which is 500 ppb (Lange and Lin, 1998). The post-processor must also be able to treat the wastewater at a rate of 11.5 L/person/day, which is the quantity of wastewater produced by each person per day (Campbell et al., 2003). A system that destroys organic contaminants would also be applicable in a home water treatment system. Some homeowners, particularly those with well water as their water supply source, choose to use additional treatment processes, such as an activated carbon filter, before consumption. Depending on the water quality, it is suggested that 10 to 20 L/day of water for drinking or cooking could be processed using a 15 W to 40 W photocatalytic unit at a cost comparable to other treatment processes (Matthews, 1993).

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3 The ability of photocatalysis to mineralize organic compounds has already been proven (Matthews, 1987). More efficient, complete destruction of organic compounds in water can be achieved through the use of an adsorbent as a support for the photocatalyst. Because silica is transparent, it does not interfere with the absorbance of energy on the titania surface but does provide a means of immobilizing the titania to avoid the issue of separating it from the water after treatment. The porous structure of silica also allows for the adsorption of contaminants, providing a pathway to the photocatalyst. Adsorption capabilities can also allow for lower energy requirements, enabling the UV lamp to be turned off during a period of contaminant adsorption and turned on for mineralization of the contaminants already surrounding the photocatalyst. The overall objective of this research is to optimize a photocatalytic reactor system that will reduce the TOC concentration of a gray water influent to meet NASAs drinking water quality standards. The system will utilize a reactor packed with titanium dioxide (TiO2) supported by silica gel (SiO2) and will be optimized with respect to empty bed contact time (EBCT), pore size of the SiO2-TiO2 composite, and TiO2 loading of the SiO2-TiO2 composite. The addition of activated carbon to the SiO2-TiO2 composite will also be investigated. Increasing the ability of the photocatalyst composite to adsorb the organic contaminants is expected to increase the rate of photocatalytic destruction; therefore, it is predicted that the composite characteristics resulting in the highest adsorption will also provide the best photocatalytic degradation of the contaminants. Because activated carbon is known as an adsorbent of organic compounds, it is hypothesized that the addition of activated carbon to the SiO2-TiO2 composite will increase the photocatalytic destruction rate of the system. Because activated carbon is

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4 opaque, it may prevent some of the UV light from reaching the photocatalyst. There may be a need to balance the adsorption and the opacity, resulting in an optimum activated carbon loading.

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CHAPTER 2 LITERATURE REVIEW Photocatalysis Heterogeneous photocatalysis involves more than one phase. In this discussion, the media involved in the reaction are a solid phase catalyst and a liquid solution containing the organic contaminants. The photocatalytic process begins with the excitation of an electron (Eq. 1). TiO2 + hv e+ h+ (Eq. 1) When light of the appropriate wavelength is absorbed by the catalyst (e.g., TiO2), an electron (e-) is transferred from the valence band to the conduction band, leaving a positively charged hole behind (h+). The wavelength of light necessary to provide the energy to move the electron into the conduction band varies with the specific photocatalyst; for titanium dioxide (TiO2), UV light of less than 388 nm is required. The electron can then recombine with the electron hole, or the hole can react with other species. Oxygen plays an important role as an electron acceptor, thus preventing the electrons from recombining and keeping the electron holes open for reaction. Water and hydroxide ions react with the electron holes to form hydroxyl radicals, proven the primary oxidant in the photocatalytic oxidation of organics (Turchi and Ollis, 1990). Repeated hydroxyl radical attack can eventually lead to complete oxidation of the contaminant. 5

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6 Titanium Dioxide Titanium dioxide is the photocatalyst most often used due to its low cost and better activation over other photoactive metal oxides. It has also shown stability in that the TiO2 itself does not change over time, unlike other photocatalysts. Zinc oxide has shown good photocatalytic abilities, but has been found to be unstable due to photodecomposition (Peral et al., 1988). TiO2 can be synthesized in different crystalline forms. The two most applicable structures are anatase and rutile. Both consist of titanium atoms surrounded by six oxygen atoms in different octahedron formations (Figure 1). Figure 1. Illustrations of TiO2 crystalline structures: rutile (left) and anatase (right). Rutile Anatase Diebold, 2003 The anatase structure has been found to be more photocatalytically active (Ohtani and Nishimoto, 1993; Tanaka et al., 1993). Degussa P-25 is a commercially available TiO2 with a structure that is 70% anatase and 30% rutile. This particular TiO2 is

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7 commonly used as it has repeatedly demonstrated successful photocatalytic degradation of organics. Kinetics Langmuir-Hinshelwood (LH) kinetics has been shown to successfully describe the photocatalytic degradation of organic contaminants (Al-Ekabi and Serpone, 1988; Turchi and Ollis, 1989). The LH model assumes the reactions take place at the surface of the catalyst, and the reaction rate is proportional to the fraction of the surface covered by the reactant. KCkKCkdtdCr1 (Eq. 2) Where r is the rate of the reaction, C is the concentration of the contaminant, t is time, k is the rate constant, is the fraction of the surface covered by the reactant, and K is the adsorption equilibrium constant. Eq. 2 can be simplified when the concentration is either very high (KC >> 1) or very low (KC << 1). Under conditions of low concentration, KC becomes negligible compared to 1 and Eq. 2 reduces to a first order reaction: kKCdtdC (Eq. 3) Similarly, under conditions of high concentration, 1 becomes negligible compared to KC. Eq. 2 reduces to a zero order reaction, where the reaction rate is equal to the rate constant, k. This simplification is illustrated in the photocatalytic degredation of 4-chlorophenol, where no additional increase in rate was observed with an increase in initial concentration above 0.2 millimoles (Al-Ekabi and Serpone, 1989). It was reasoned that at a high enough contaminant concentration, the surface sites of the catalyst are

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8 completely saturated, so further increase in concentration cannot further increase the rate of reaction, according to LH kinetics. Conversely, below the specified concentration, the catalyst surface sites are not completely saturated; therefore, the fraction of the surface covered by the contaminant will vary with concentration, thereby varying the reaction rate. The applicability of the LH model can be validated by inverting Eq. 2 and plotting 1/r vs. 1/C. The slope of a linear plot provides 1/kK and the 1/k value is given by the y-intercept. kCkKdtdC111/1 (Eq. 4) This equation has been found to be accurate for the reaction rate in a single component system, with no competition for reactive sites, but must be expanded to accurately describe a multi-component system (Turchi and Ollis, 1989). Catalyst Supports The utilization of a photocatalyst as a slurry can be effective, but creates difficulties in recovering the catalyst due to its small particle size. For this reason, the use of a variety of supports, such as glass beads, carbon, sand, clay, and silica gel, have been investigated (Matthews, 1993). There are additional advantages to using a support that is capable of adsorbing the organic contaminants. The rate of destruction is generally controlled by the concentration of the contaminant; therefore, the rate of mineralization will decrease as the contaminant concentration decreases. This makes reducing the concentration of organics to low levels a slow process. If an adsorbent is used as a support, a high concentration of the contaminant is created around the photocatalyst, which increases the rate of mineralization (Yoneyama and Torimoto, 2000). The

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9 adsorbent support can also retain intermediates that are formed during the destruction process. The possibility of creating toxic intermediates during photocatalysis is a concern, but if the intermediates are held near the catalyst they are more likely to be completely destroyed. Silica gel and activated carbon are two porous media that are being investigated as catalyst supports. Activated carbon is commonly used in water treatment to adsorb undesired organic components. Its proven adsorption capabilities make it a good candidate for a photocatalyst support. An obvious adverse affect of the addition of activated carbon is its opaque nature. The activated carbon could prevent photons from reaching the photocatalyst, decreasing activation and subsequently reducing mineralization. Silica gel generally has a lower surface area than carbon, but has the advantage of being transparent, allowing the light to penetrate and activate the titanium dioxide. There is some disagreement in the literature as to the level of adsorption desired in the catalyst support. A study of the photocatalytic destruction of propionaldehyde in the air phase revealed that a catalyst support with medium level adsorption abilities compared to the other adsorbents used in the study (carbon having high adsorption and zeolum having low adsorption) showed the fastest degradation (Yoneyama and Torimoto, 2000). The reasoning was based on the longer time needed for diffusion from composites involving strong adsorption. Similarly, it was found that the use of activated carbon as a catalyst support was not advantageous in the destruction of dichloromethane in water (Torimoto et al., 1997). The reason for this result was attributed to slower diffusion of dichlormethane from the adsorbent compared to the bulk solution. However, an

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10 investigation of a single compound, methylene blue, using various activated carbon types as catalyst supports for TiO2 was completed (Khan, 2003). Here it was found that the stronger adsorbents led to better destruction. Further analysis as to the cause of varied destruction levels resulting from different activated carbon supports revealed that higher metal content led to enhanced photocatalysis. Khan explains that the metals contained in the AC could act as electron acceptors, reducing the recombination of electron/hole pairs and leading to increased photocatalysis. Activated Carbon Activated carbon (AC) can be made from a variety of carbonaceous materials, including coal, peat, peanut shells, and wood, which are charred and subsequently activated through a chemical or physical process, such as the application of steam or CO2 at temperatures typically around 900C and higher. The final product is a material that has a high surface area (generally 500-1500 m2/g) and high energy for removing pollutants. AC is a well-known technology for the removal of organic compounds from water through adsorption. The drawback of AC is that once its adsorption capacity is reached, it must either be replaced or regenerated, a process that usually involves burning off the adsorbed pollutants and a corresponding loss of carbon mass. This drawback could be overcome through a combination of AC and TiO2. The AC adsorbs the contaminants, leading to more efficient photocatalysis, and the photocatalysis destroys the pollutants, leading to regeneration of the adsorbent. This concept has been investigated by generating composites of AC coated with titanium dioxide (Lu et al., 1999; Torimoto et al., 1996; Torimoto et al., 1997; Yoneyama and Torimoto, 2000; Khan, 2003). The TiO2 can be coated on the AC through various methods, such as impregnation or boiling

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11 deposition. Different methods will lead to AC-TiO2 composites with different properties. The disappearance of contaminants as well as the disappearance of total organic carbon has been monitored (Lu et al., 1999; Torimoto et al., 1996) and found that although TiO2 alone decreased contaminant concentrations at a faster rate, the overall decomposition of the contaminants and intermediates was faster with the AC-TiO2 composite. This supports the idea that the AC will adsorb the intermediates, holding them close to the photocatalyst until completely mineralized. Several studies claim enhanced photocatalysis through the addition of AC in an aqueous suspension of TiO2 (Matos et al., 1998; Herrmann et al., 1999; Matos et al., 2001; Gomes da Silva and Faria, 2003). The location of the degradation process in aqueous suspensions has been studied (Minero et al., 1992) and shows that the oxidation reaction takes place close to the catalyst surface; therefore, the use of the two materials in an aqueous suspension relies on the transfer of the pollutants from the adsorbent to the photocatalyst. This creates an added step compared to the AC-TiO2 composites; the contaminants must be somehow transferred, possibly during a collision, from the AC to the TiO2. Two studies directly compared the use of a mixture versus a composite using the same quantity of carbon and TiO2 in each experiment with the same organic compound (Torimoto et al., 1997; Nagaoka et al., 2002). These authors agreed that enhanced removal occurred with the addition of carbon compared to TiO2 alone, but found that physically combining the two materials was more advantageous. Torimoto reasons that the photodecomposition of the adsorbed species must take place during collisions, and this occurrence is simply not high enough to compete with the AC-TiO2 composite. In addition, the aqueous suspension defeats the original purpose of

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12 supporting TiO2 in the first place, which is to enable the TiO2 particles to be easily separated from the solution after photocatalysis. The regeneration of AC-TiO2 has been studied and shown to be difficult. Regeneration after adsorption of an organic dye, methylene blue, of two composites that differed in the type of AC was studied (Khan, 2003). The first attempt at regeneration of one AC-TiO2 composite showed a loss of its capacity with each 24-hour regeneration cycle. Khan reasoned that some of the carbon may not be exposed to UV light, preventing photocatalysis; therefore, a second regeneration attempt involved simultaneous fluidization with UV irradiation. This succeeded in regenerating the AC-TiO2 back to its initial uptake, but the AC-TiO2 reached exhaustion faster than in the first cycle. The other AC-TiO2 composite, made with a different AC, was regenerated only through intermittent 12 hour irradiation, but was not restored to its original level. Another study that focused on the regeneration of AC-TiO2 also showed long regeneration times (Crittenden et al., 1997). In order to increase the regeneration rate, Crittenden tried heating the AC-TiO2 during regeneration with the idea that this would increase the desorption rate, which was thought to be a limiting step in the regeneration rate. The regeneration of AC-TiO2 was found to be only 10% more efficient than the regeneration of the control, which was AC without TiO2. This indicated that the efficiency was based on simple desorption rather than oxidation of the organic compounds. Silica Silica (SiO2) is a transparent material that can be synthesized through various chemical methods, providing several options for the creation of SiO2-TiO2 composites. Synthesis can occur through preparation of the two oxides simultaneously, as one mixed

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13 material, or through deposition of TiO2 on SiO2. Sol-gel hydrolysis, coprecipitation, and flame hydrolysis are the most popular mixed oxide methods, while impregnation, chemical vapor deposition, and precipitation are the methods used for supported oxides, though the supported oxide methods have not received as much attention (Gao and Wachs, 1999). Each method will lead to composites with different behaviors based on differences in surface area, pore size, dispersion of TiO2 on or in the silica, and bonding between the silica and photocatalyst. SiO2-TiO2 composites have been proven successful in the photocatalytic degradation of a variety of organic compounds (Matthews, 1988; Anderson and Bard, 1995; Anderson and Bard, 1997; Xu et al., 1999; Vohra and Tanaka, 2003). In some studies, SiO2-TiO2 has been compared directly to bare TiO2 and shown to be more advantageous for specified compounds (Xu et al., 1999; Anderson and Bard, 1995). Xu and co-workers synthesized particles from a titania sol and silica powder and tested for photocatalytic destruction of acetophenone. SiO2-TiO2 was found to be at least 10% more effective than bare TiO2. This increased destruction was related to higher adsorption of acetophenone and better dispersion of TiO2 due to the presence of silica. A similar comparison was performed using SiO2-TiO2 and bare TiO2 particles prepared by a sol-gel technique and tested for the removal of rhodamine-6G (Anderson and Bard, 1995). Again, SiO2-TiO2 was found to have a faster degradation rate, which was related to the increased adsorption of the contaminant. These studies demonstrate the idea that increasing adsorption will increase the concentration of the contaminant around the TiO2 and lead to faster destruction of the contaminant.

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14 Regeneration Multiple studies have been performed that prove the ability of SiO2-TiO2 to be used over and over again. Phenol was repeatedly degraded by greater than 99% in a reactor packed with SiO2-TiO2 particles (Matthews, 1988). Cycles of adsorption and destruction of phenol were performed four times without loss of capacity. A series of adsorption and destruction tests were also performed using SiO2-TiO2 pellets for the photocatalytic degradation of volatile organics (Holmes, 2003). At least ten runs were performed using the same pellets and achieving 80% or more removal of each of the contaminants. No decrease in the systems original photocatalytic ability was observed in the data. SiO2-TiO2 pellets are used in four adsorption and destruction cycles for the removal of an organic dye, crystal violet (Londeree, 2002). No decrease in the pellets performance is shown; in fact, the adsorption ability of the pellets actually increased with each cycle. Synthesis by Sol-Gel Method Compared to other synthesis processes, the sol-gel technique is the most widely used based on its potential for control over the textural and surface properties of the final material (Gao and Wachs, 1999). In addition, because this preparation method begins with high purity chemical precursors, it produces high purity, homogeneous materials (Kirk and Othmer, 1991). Starting from a liquid also has the advantage of shaping the material in a mold, making it easier to produce particles of the size and shape needed for a specific application. A sol is a suspension of colloidal particles dispersed in a liquid. Under the proper conditions, the sol will form a gel, which is a solid network that holds the liquid in its pores. A sol-gel can be formed through a series of hydrolysis and condensation reactions

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15 of alkoxide precursors. Once the gel is formed, it may undergo aging and drying steps to allow the gel to mature to the desired structure and expel the liquid held in its pores. Commonly used silica precursors are tetra methyl ortho silicate (TMOS) and tetra ethyl ortho silicate (TEOS). The chemical formulas for these silica precursors are Si(OCH3)4 and Si(OC2H5)4 for TMOS and TEOS, respectively. These can be represented by Si(OR)4 in equations 5 and 6. The alkoxide precursor reacts with water in a hydrolysis reaction (Eq. 5) and the silica network begins to form through condensation reactions (Eq. 6). The network continues to grow through polycondensation. Because the alkoxide precursor is not readily soluble in water, an alcohol is generally used as a solvent in the mixing of these reactants. Depending on the precursor, methanol or ethanol is formed as a byproduct of the hydrolysis reaction and water is produced in the condensation reactions. Si(OR)4 + 4H2O Si(OH)4 + 4ROH (Eq. 5) Si(OH)4 + Si(OH)4 (OH)3Si O Si(OH)3 + H2O (Eq. 6) The rates of the hydrolysis and condensation reactions that begin formation of the gel have a strong impact on the structure of the final material. These reaction rates will vary with temperature, nature of the solvent, type of alkoxide precursor, and nature and concentration of the acid or base (Hench and West, 1990). The acid or base concentration has been shown to be the dominant factor (Orcel, 1987). In general, increasing the concentration of H+ or H3O+ in acidic conditions or increasing the concentration of OHin basic conditions will increase the rate of hydrolysis. The ratio of water to the alkoxide precursor, the R ratio, also affects the rate of hydrolysis. Water can be used in excess to promote hydrolysis. The rate of the hydrolysis and condensation

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16 reactions will ultimately affect the gel time and the structure of the gel. Hydrofluoric acid has been used to decrease the gel time and increase the pore size of the resulting gel (Powers, 1998). After gelation, at which point the three dimensional silica network has formed and the sol ceases to move as a liquid, the polycondensation reactions continue as the silica network further develops. Syneresis and Ostwald ripening also contribute to this aging process. Syneresis involves the contraction of the silica gel network as a result of the condensation reactions, which create additional silica network links and cause more liquid to be expelled. Ostwald ripening describes the formation of the silica gel network in the areas of negative curvature, filling in these crevices on the surface of the silica gel. This process strengthens the gel, which helps to protect the gel against cracking during drying. The time and temperature under which the aging process is allowed to occur will impact the surface area and pore size of the gel. Alcohol and water produced in the aforementioned reactions remain in the pores of the gel. The gel must be dried, typically at temperatures between 100C and 180C, to remove the liquid from the pores. Capillary forces are created by the evaporation of the liquid, causing some of the pores to collapse and the gel to shrink. Silica gels dried in this manner are termed xerogels and will generally be reduced in volume by 40-60% (El Shafei, 2000). Steps can be taken to reduce the amount of shrinking and create a silica gel of higher surface area and pore volume. One method involves replacing the water with a liquid of lower surface tension, such as an alcohol, before drying. A gel can also be supercritically dried to completely eliminate the destructive forces of surface tension, minimizing the amount that the gel shrinks.

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17 Silica Gel Properties The silica network consists of four oxygen atoms bonded to each silicon atom, forming a tetrahedron, and each oxygen atom is shared by two silicon atoms. These Si-O-Si, or siloxane, bonds make up the bulk silica structure. The surface of the silica is hydroxylated in the presence of water, forming Si-OH, or silanol, groups. It is these silanol groups that make the silica surface hydrophilic and determine the reactivity of the silica (El Shafei, 2000). The silanol groups can be defined as single, geminal, or vincinal. Single silanol groups are isolated and formed with a silicon atom that is bonded to three other oxygen atoms in siloxane bonds. When at a close enough distance, the hydroxyl groups can interact with each other through hydrogen bonding, creating vincinal silanols. Geminal silanols exist when two hydroxyl groups are linked to the same silicon atom. The silica surface can be dehydroxylated through heating above temperatures of 200C. The silanol groups will condense to form siloxane bonds. Dehydroxylation will occur at a greater degree as the temperature is increased. In the presence of water, the silanol groups become ionized and the hydrogen atom will associate or dissociate depending on the pH of the solution (Icenhower and Dove, 2000). The point of zero charge (PZC) of silica is between a pH of 2 and 3; therefore, at pH values above this PZC, the hydrogen atoms will begin to dissociate. This deprotonation will leave a negative charge on the surface of the silica (Eq. 7). Si-OH Si-O+ H+ (Eq. 7) Silica-Carbon Composites Combining silica and carbon can potentially combine the advantages of each material to create a composite with improved adsorption capabilities. These composites may possess a heterogeneous surface through combination of the properties of the

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18 nonpolar, hydrophobic carbon with the polar, hydrophilic silica to create a material that can adsorb both inorganic and organic contaminants (Skubiszewska-Zieba, 2002). Carbon-silica composites have been successfully created through several methods, including pyrolysis of methylene chloride on the surface of silica gel (Villieras et al., 1998), pyrolyzed elutrilithe in a sol-gel (Deng et al., 1998), and coating of silica gel on the carbon surface through hydrolysis of a silica precursor (Cheong et al., 2003). Deng showed that combining the carbon and silica created a composite with different adsorption capacities than the individual materials. For certain adsorbates, namely water, cyclohexane, acetone, and pyridine, carbon-silica composites had superior uptake when compared to silica or carbon alone (Deng et al., 1998). Composites consisting of carbon, silica and titanium dioxide have also been generated for the epoxidation of cyclohexene (Cheong, 2003). The titanium dioxide supported by both carbon and silica was found to provide greater conversion of cyclohexene compared to titanium dioxide support by silica or carbon alone. The advantages of each material were successfully combined: the carbon provided higher selectivity for the epoxide due to its hydrophobic nature, and the silica provided a better environment for catalytic activity.

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CHAPTER 3 MATERIALS AND METHODS SiO2-TiO2 Composites The SiO2-TiO2 composites were created using a sol-gel method (Londeree, 2002). The silica precursor was tetra-ethyl-ortho-silicate (TEOS) (Fisher Scientific, reagent grade). It was mixed with nanopure water using a water to TEOS mole ratio of 16:1. Ethanol (Aaper Alcohol, 200 proof) was used as the solvent to facilitate the miscibility between the TEOS and water. Two acid catalysts were used: a 1 M nitric acid solution, made from 15.8 M nitric acid (Fisher Scientific, certified A.C.S.) and nanopure water, and a 3% solution of hydroflouric acid, formulated from 49% hydrofluoric acid (Fisher Scientific, reagent A.C.S.) and nanopure water. Table 1 summarizes the quantity of each chemical used to make one batch, approximately 10 g, of silica gel. The volume of hydrofluoric acid was varied to control the pore size of the gel. Volumes of 2, 3, and 4 mL correspond to approximate pore diameters of 70, 118, and 234 angstroms, respectively. Table 1. Chemicals Used for Silica Gel Synthesis Chemical Volume Nanopure water 25 mL Ethanol 50 mL TEOS 35 mL 1 M Nitric Acid 4 mL 3% Hydrofluoric Acid 2-4 mL The chemicals in Table 1 were mixed in polystyrene containers in the order listed using a magnetic stir plate. While mixing, the liquid was doped with the desired quantity 19

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20 of Degussa P25 TiO2. According to Degussa, the TiO2 possesses the properties listed in Table 2. Table 2. Properties of Degussa P25 TiO2 Property Value Specific surface area (BET) 50 +/15 m2/g Average primary particle size 30 nm Tapped density 130 g/L pH value in 4% dispersion 3.5-4.5 For the composites that included activated carbon, the mixture was doped with activated carbon after the addition of TiO2. The activated carbon used in the composites was ground to an average particle diameter between 1 and 2 microns as determined by the LS2 13 320 Laser Diffraction Particle Size Analyzer. The distribution of the particle size is shown in Figure 2. Figure 2. Particle size distribution of activated carbon used in AC-SiO2-TiO2 composites. Analysis A and Analysis B are duplicates of the particle size distribution analysis. 01234567891002468101214161820Particle Diameter (micron)Number (%) Analysis A Analysis B

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21 The TiO2 and activated carbon loadings stated throughout this document are referred to as percentages. The TiO2 loadings are reported on a basis of TiO2 weight per TEOS volume. For example, the 12% TiO2 loading is determined using 4.2 g of TiO2 per 35 mL of TEOS as a percentage. The activated carbon loadings are reported by a weight percentage using the activated carbon mass per total mass of the pellets. The sol was allowed to mix between 0.5 and 7 hours, depending on the amount of 3% HF used. For the amounts of 2, 3, and 4 mL, approximate mixing times of 7, 2, and 0.5 hours were used, respectively. The liquid was then transferred to 96-well assay plates and allowed to gel (Figure 3). The assay plates were Fisherbrand, polystyrene plates that contained 0.45 mL in each well. Figure 3. Silica sols doped with titanium dioxide were mixed using magnetic stir plates and transferred to 96 well assay plates before forming a gel. Upon reaching the gel point, at which point the sol no longer moved as a liquid, the trays were capped and wrapped in aluminum foil to prevent any of the contents from volatilizing during the aging process. The gels were aged for 48 hours at room temperature and 48 hours at 65C in an Oakton Stable Temperature oven. After aging, the pellets were removed from the assay plates and transferred into Teflon screw cap jars.

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22 Each lid had a small hole to allow the liquid expelled from the gels pores to slowly escape as vapor during the drying process. The drying process involved heating at 103C for 18 hours and 180C for 6 hours in a Yamato DVS 400 Drying Oven. The BET (Brunauer, Emmett, and Teller equation) surface areas and pore volumes of the gels were analyzed using a Quantachrome NOVA 1200 Gas Sorption Analyzer. Reactor System The pellets were packed into a cylindrical reactor (Figure 4). The reactor was designed with a hollow center and thin annulus to allow the UV lamp to be placed in the center, providing maximum UV light exposure to the pellets. The inner wall of the annulus was a quartz tube that could be completely removed, making it simple to remove the pellets after testing. The lamp in the center of the quartz tube provided UV light at a wavelength of 365 nm. As measured at the center of the lamp, the intensity was 7.4 mW/cm2 at the inner diameter of the annulus and decreased to 5 mW/cm2 at the outer diameter. Specifications of the reactor are listed in Table 3. The reactor was enclosed in a box to provide control over its exposure to ambient light. Figure 4. Reactor packed with SiO2-TiO2 composites with a UV light through the center.

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23 Table 3. Reactor Specifications Specification Value Length of Reactor 19 cm Inner Diameter 2.5 cm Outer Diameter 4.2 cm Empty Bed Volume of Pellets 138.6 mL The system consisted of the reactor, 6 mm PTFE tubing, a Masterflex L/S Digital Standard Drive peristaltic pump, and a reservoir of simulated wastewater (Figure 5). The system was used in two different configurations. One system configuration allowed the wastewater to pass through the reactor only once, and the other formed a closed loop system in which the wastewater was recirculated through the reactor. A cover (not shown in figure 5) was placed over the front of the reactor to completely enclose it during operation of the system. The reservoir was a one liter flask that was covered with aluminum foil to prevent any outside UV light exposure and capped with parafilm to create a closed system. Figure 5. Treatment system consisting of reactor packed with SiO2-TiO2 composites, UV lamp, pump, and reservoir of simulated wastewater.

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24 Simulated Wastewater The wastewater feed was created using a formula for typical wastewater provided by Johnson Space Center (JSC). The chemical composition includes the shower waste, hand wash waste, oral hygiene waste, urine, and urine flush waste that would be expected from a crew of four people. Table 4 lists the amounts of each constituent present in the simulated wastewater. Table 4. Simulated Wastewater Composition Wastewater Component Quantity Pert Plus for Kids 1.2 g Deionized water 999.4 ml Ammonium bicarbonate, NH4HCO3 2726 mg Sodium chloride, NaCl 850 mg Potassium bicarbonate, KHCO3 378 mg Creatinine, C4H7N3O 248 mg Hippuric acid, C9H9NO3 174 mg Potassium dihydrogen phosphate, KH2PO4 173 mg Potassium bisulfate, KHSO4 111 mg Citric acid monohydrate, C6H8O7H2O 92 mg Tyrosine, C9H11NO3 66 mg Glucuronic acid, C6H10O7 60 mg 1.48N Ammonium hydroxide, NH4OH 10 ml This formula represents the raw wastewater that would be the influent for the beginning of the water treatment system; therefore, it would be treated by several processes before actually becoming the feed for the post-processor. To account for pretreatment, the simulated wastewater is diluted using 9 mL of the concentrated solution with 991 mL of deionized water to make each liter of wastewater feed. This creates a solution of approximately 3 ppm of total organic carbon (TOC), which is within the range of the TOC concentration expected of the post processor influent (Cambel et al., 2003). It was found that the wastewater was not stable over long periods of time, particularly at room temperature. The liquid would change in color, from a cloudy, white mixture to

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25 yellow, and the TOC concentration decreased over time. Therefore, a new solution of the concentrated wastewater was created each week and stored in an amber bottle at 4C. A small fraction of the wastewater was found to be volatile, so the wastewater feed was mixed for approximately 1 hour before use to allow the volatiles to be removed and the TOC concentration to stabilize. Reactor Dynamics A tracer analysis was conducted to determine the behavior of the reactor. Sodium chloride was chosen as a tracer because it was not expected to be adsorbed by the media and had been used successfully as a tracer in a previous study (Holmes, 2002). The sodium chloride concentration was measured using conductivity measurements read by a Fisher Scientific conductivity probe. A linear correlation between conductivity and sodium chloride concentration was observed and used to translate conductivity measurements to a sodium chloride concentration (Figure 6). y = 2.589x + 0.7543R2 = 0.999702040608010012014002040NaCl concentration (mg/L)Conductivity (umho) 60 Figure 6. Calibration of conductivity meter

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26 DI water was pumped through the reactor until a stable conductivity was achieved. For the tracer analysis, this conductivity was subtracted from effluent readings to determine the sodium chloride concentration exiting the reactor. The tracer analysis was performed two ways to provide a duplicate study. First, a solution of 50 mg/L sodium chloride was pumped through the reactor and the conductivity of the effluent was monitored until the concentration of the effluent reached 50 mg/L. A second experiment was conducted in which DI water was pumped through the reactor and the effluent conductivity was monitored until it indicated that all the sodium chloride had been washed out of the reactor. The flow rate used for both tests was 10 mL/min. The data collected from the tracer analysis were used to calculate a mean residence time and to model the reactor behavior. The mean residence time was determined to be 12.5 minutes. A fractional age distribution (E-curve) of the sodium chloride in the reactor was created for both reactor tests. This distribution was then compared to that of the model of continuously stirred tank reactors (CSTRs) in series to determine the number of tanks in series the reactor behavior was approximately equivalent to (Figures 7 and 8). The E-curve for the CSTR-in-series model was created using Eq. 8. 1)!1(ntnttntntneE Eq. 8 Where n is the number of CSTRs in series, t is the time in the reactor, and tbar is the mean residence time. A comparison of the data with the CSTR-in-series model reveals that the reactor behaves as five CSTRs in series.

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27 Figure 7. This figure compares the residence time distribution of the reactor as compared to a CSTR-in-series model. Test A was performed by pumping 50 mg/L of NaCl through the reactor until the effluent reached the same concentration. Residence Time Distribution, Test A 0.000.020.040.060.080.100.120.14010203040Time (min)dF/dt Actual Model, n =5 Figure 8. This figure compares the residence time distribution of the reactor as compared to a CSTR-in-series model. Test B was performed by pumping DI water through the reactor until the NaCl concentration decreased from 50 mg/L. Residence Time Distribution, Test B0.000.020.040.060.080.100.120.1401020304Time (min)dF/dt 0 Actual Model, n = 5 Experimental Procedures Adsorption and Destruction In order to optimize the system, experiments were performed to test its sensitivity to flow rate, pore size of the silica gel support, titanium dioxide loading, and addition of activated carbon to the catalyst support. With each change to the system, removal of TOC achieved by adsorption and destruction was quantified. All samples were analyzed using a Tekmar Dohrmann Apollo 9000 combustion analyzer.

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28 The amount of pellets packed in the reactor was measured by volume as 115 mL in a 100 mL graduated cylinder and held constant for each experiment. The regeneration ability and effects of the experiments on the pellets was unknown; therefore, the reactor was packed with new pellets for each experiment. Because of the acidic properties of the silica gel, all pellets were pre-washed with a sodium hydroxide solution (pH = 8.5) to reduce the pH changed caused by the pellets. The pellets were washed until a pH of 4 was reached in the effluent stream. To study TOC removal by adsorption, wastewater was first pumped through the reactor in a single-pass configuration, in the dark, until the pellets were exhausted, meaning that the influent TOC concentration equaled the effluent TOC concentration. After the adsorption capacity had been reached, the system, including the reactor and tubing, remained full of 264 mL of wastewater and the reservoir was filled with 1 liter of wastewater. The wastewater was recirculated and the UV lamp was turned on. Because the pellets were previously exhausted with respect to TOC adsorption, TOC removal during this experiment was assumed to be due to destruction. Samples of 25 mL were taken from the reservoir every few hours until the TOC concentration reached below 500 ppb. Because the pellets were still acidic, even after a period of washing, the pellets changed the pH of the wastewater as it flowed through the reactor. For the single pass adsorption experiments, the pH was decreased from 8 at the influent to 6 at the effluent. Recirculation of the wastewater for the destruction tests decreased the initial pH of 8 to a final pH of 4. The pH of the influent and effluent was monitored in all experiments using

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29 a Fisher Scientific pH meter and these pH changes remained consistent throughout all the tests. Adsorption of Intermediates Experiments were performed to test the adsorption of the intermediates formed after UV exposure in the reactor at different empty bed contact times (EBCTs), which is defined as the time needed for one bed volume of water to pass through the reactor (calculated using the empty bed volume divided by the flow rate). The pre-washed pellets in the reactor were first exhausted in the dark. The UV light was then turned on and the effluent was collected in a reservoir. This experiment was performed at flow rates of 5, 10, 20, and 30 mL/min. A 1 inch glass tube packed with 65 mL of pre-washed pellets was used to test the adsorption of the effluent. In each case, the adsorption experiment was performed at a flow rate of 10 mL/min with a new set of pellets and conducted immediately following collection of the effluent. A diagram of the apparatus is shown in Figure 9. Reactor 1(constant UV)Destruction at various EBCTs Reactor 2(no UV)Adsorption at same EBCT WastewaterWastewater and Intermediates Reactor 1(constant UV)Destruction at various EBCTs Reactor 2(no UV)Adsorption at same EBCT WastewaterWastewater and Intermediates Figure 9. Diagram of the system used to test the adsorption of the intermediates formed from photocatalysis at different EBCTs.

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CHAPTER 4 RESULTS System Optimization The photocatalytic system was optimized with respect to flow rate, pore size of the silica support, titanium dioxide loading, and activated carbon loading. For each test of the system, one parameter was varied while the others were held constant. The adsorption and destruction abilities of the system were tested at each set of conditions. The optimum system parameters were chosen based on the ability to reduce the TOC concentration to 500 ppb in the shortest amount of time. The wastewater concentration is represented in all graphs as a normalized TOC concentration, showing the fraction of the effluent TOC concentration (Ce) over the influent TOC concentration (C0, 3 ppm). The pellet pore sizes reported in this results section are approximate; the BET surface area, pore volume, and calculated pore size of each pellet composition that was tested can be found in the Appendix. Flow Rate Flow rate was the first parameter to be varied. Experiments were performed using flow rates of 5, 10, 20, and 30 mL/min, which are of the appropriate order of magnitude to treat the wastewater produced by 1-4 crew members per day. A flow rate of 16 mL/min would provide treatment for wastewater produced by 2 people per day, assuming a production of 11.5 L of wastewater per crew member (Campbell et al., 2003). The SiO2-TiO2 pellets used to test each flow rate had a 12% TiO2 loading and pore size of 118 angstroms. The system was first operated in a single pass configuration with no UV light 30

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31 to test the adsorption behavior of the pellets. The results are displayed in Figure 10. The adsorption curves at each flow rate are compared directly using bed volumes as the independent variable. Figure 10 shows that lower flow rates lead to lower effluent concentrations in the first few bed volumes. The lower flow rates create longer EBCTs, allowing more time for diffusion of the wastewater through the pores of the silica and then subsequent adsorption. The 5, 10, and 20 mL/min flow rates cause exhaustion of the pellets after approximately 6 bed volumes, while the pellets tested at the 30 mL/min flow rate reach exhaustion sooner, at 3 bed volumes. 0.00.10.20.30.40.50.60.70.80.91.00.02.04.06.08.010.0Bed Volumes Ce/Co 5 mL/min 10 mL/min 20 mL/min 30 mL/min Figure 10. TOC removal by adsorption at flow rates of 5, 10, 20, and 30 mL/min, corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and hydraulic loading rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively. After the pellets were exhausted, or close to exhaustion, the system was reconfigured from a single pass to a recirculating mode, with constant UV light exposure in the reactor. Because the pellets were previously exhausted, it was assumed that TOC removal during this testing period was due to complete mineralization. Figure 11 shows

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32 that the rate of destruction depends on the flow rate, with the 20 mL/min flow rate reaching below the desired 500 ppb in the shortest amount of time. Despite the fact that the pellets tested at 5 mL/min were not entirely exhausted, meaning that some of the removal during the destruction period could be due to adsorption, the destruction test conducted at 5 mL/min required the longest time to reach 500 ppb. This is surprising, since the slower flow rates are shown to allow more time for the contaminants to adsorb, and increased adsorption was expected to lead to increased destruction. 0.00.10.20.30.40.50.60.70.80.91.00510152025Time (hours)Ce/Co 5 mL/min 10 mL/min 20 mL/min 30 mL/min 500 ppb Figure 11. TOC removal by destruction in a recirculating system at flow rates of 5, 10, 20, and 30 mL/min, corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and hydraulic loading rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively. It is important to realize that the recirculation means that the total time the wastewater is in the reactor is approximately equal, regardless of the flow rate. For example, at a flow rate of 20 mL/min, the wastewater might pass through the reactor 4 times as fast compared to a 5 mL/min flow rate, but it would also experience 4 times as

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33 many passes. Therefore, one of the differences between the flow rates is that the wastewater spends intermittent time in the reactor at the higher flow rates compared to longer, continuous passes through the reactor at slower flow rates. It is possible that different intermediates, or quantity of intermediates, are created at different EBCTs, which could change the adsorption of the wastewater after the first pass through the reactor. In order to test the adsorption of the intermediate wastewater (i.e., wastewater and intermediates created after contact with the irradiated SiO2-TiO2 composites), the wastewater was first passed through the reactor with constant UV light at a certain EBCT. Then, the effluent from the first reactor was passed through a second reactor, which was in the dark, to test for adsorption of these intermediates. The EBCT of the first reactor was varied to create effluents equivalent to that after a single pass through the reactor at the flow rates used during the recirculating destruction test. The EBCT in the second reactor was held constant so that the adsorption of the various wastewater effluents could be directly compared. Figure 12 presents the adsorption curves for the intermediate wastewater created from the single pass through the second reactor. The data show no difference in the adsorption of the wastewater after passing through the reactor at different EBCTs. Therefore, the better performance at 20 mL/min shown in Figure 11 did not result from the formation of intermediates that were more adsorbable. A second hypothesis based on diffusion of the wastewater was formulated to explain the variation in the destruction rates as the flow rate was changed. The rate of destruction depends on the ability of the contaminants to reach the titanium dioxide in the porous surface of the pellets and the ability of the products resulting from the oxidation

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34 reactions to travel out of the pores. It is possible that the higher flow rates increase the diffusion of the molecules in and out of the pellets. The transport of the wastewater contaminants and the photocatalysis products in and out of the pellets pores depends on several steps. These steps include transport through the bulk solution, transport through the film layer formed by stationary water surrounding the pellet, and travel through the pores. Higher flow rates decrease the film layer surrounding the pellet; thus, decreasing the distance the molecules must diffuse to approach the pellet surface. While higher flow rates were shown to have a negative impact on adsorption behavior in a single pass system, the impact of higher flow rates on the adsorption behavior of intermediate products as well as the transport of final products out of the pores and into the bulk solution is unknown. 0.00.20.40.60.81.01.2020406080100Time (min)Ce/Co 5.25 min EBCT 28 min EBCT 14 min EBCT 7 min EBCT Figure 12. TOC removal by adsorption of wastewater effluents resulting from destruction at various EBCTs.

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35 In order to test the hypothesis of diffusion being a limiting factor in the system, pellets of the same composition but approximately half the size (smaller diameter, same length) were tested using a flow rate of 20 mL/min. If diffusion were a limiting factor, the expectation was that the smaller pellets would increase the bulk surface area and decrease the distance a contaminant must travel inside the pellet, leading to a faster destruction rate. The results presented in Figures 13 and 14 show that the smaller pellets performed worse than the original size pellets. These data indicate that diffusion is not the limiting factor controlling the destruction rate. It is possible that changing the size of the pellets also changed another system parameter, such as the light penetration or the hydraulic flow through the reactor, which decreased the destruction rate of the system. Pore Size Using the optimum flow rate of 20 mL/min and the same 12% TiO2 loading, the pore size was varied to determine its effect on the adsorption and destruction capabilities of the system. Figure 15 shows the adsorption curves for three different pore sizes: 70, 118, and 234 angstroms. The surface area of the gels decreased with increasing pore size. The 234 angstrom gel, with the lowest surface area, exhibited the worst adsorption, exhausting at 4 bed volumes. All the curves show exhaustion of the pellets by 8 bed volumes, indicating that only a small amount of adsorption is occurring with any of the three pore sizes. This is not unexpected since the surface of the silica gel is hydrophilic, and the contaminants being adsorbed are hydrophobic. In addition, the adsorption is occurring at a pH of 6, which is well above silicas isoelectric point of 2-3; therefore, the surface of the silica is likely to be negatively charged. The main ingredient in the wastewater is soap, which is an anionic surfactant, and some of the other organics are carboxylic acids, which will also

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36 form anions when dissociated. Charge repulsion could also be a reason for the low adsorption. 0.00.10.20.30.40.50.60.70.80.91.00246810Bed VolumesCe/Co 3mm x 6mm Pellets 4mm x 6mm Pellets Figure 13. Effect of pellet size on adsorption of the wastewater. 0.00.10.20.30.40.50.60.70.80.91.00510152025Time (hours)Ce/Co 3mm x 6mm Pellets 4mm x 6mm Pellets Figure 14. Effect of pellet size on destruction of the wastewater.

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37 0.00.10.20.30.40.50.60.70.80.91.00246810Bed VolumesCe/Co 70 angstroms 118 angstroms 234 angstroms Figure 15. Effect of pore size on adsorption of the wastewater. 0.00.10.20.30.40.50.60.70.80.91.00510152025Time (hours)Ce/Co 70 angstroms 118 angstroms 234 angstroms 500 ppb Figure 16. Effect of pore size on the destruction of the wastewater.

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38 The results of the destruction test are displayed in Figure 16. The 118 angstrom composite performs better than the composite with a 234 angstrom pore size, which could be due to the higher adsorption of the 118 angstrom composite as shown in Figure 15. Although the 70 angstrom composite shows adsorption behavior similar to the 118 angstrom composite, it displays the slowest destruction rate of all three pore sizes. The 70 angstrom pore size could be too small to allow adequate diffusion of the molecules in and out of the pores. The middle pore size of 118 angstroms shows the fastest destruction of the three pore sizes tested, possibly due to the best combination of adsorption and diffusion, and was chosen as the optimum pore size for future experiments. Titanium Dioxide Loading The TiO2 loading was varied while keeping the chosen flow rate and pore size constant. TiO2 loadings of 0, 3, 12, 24, and 32% (% by weight of TiO2/volume of TEOS) were tested and compared (Figures 17 and 18). A clear trend is not observed in the adsorption curves of pellets with various TiO2 loadings. The pellets show similar adsorption, with only the 24% TiO2 loading exhibiting a slight decrease in adsorption, exhausting about 15 minutes before the other pellets. The destruction curves presented in Figure 18 show the best performance with a TiO2 loading of 12%. The test of the composite with no TiO2, only SiO2, serves as a control, showing the destruction that occurs in the system through photolysis alone. The curve shows that approximately 20-30% of the TOC is able to be removed without the addition of TiO2 to the system. The 12% TiO2 is significantly better than the 3% loading, showing that more catalyst is needed in the system for faster destruction; however, further increase past 12% TiO2 does not greatly improve the destruction rate of the system. The 24% loading

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39 0.00.10.20.30.40.50.60.70.80.91.0010203040506070Time (minutes)Ce/Co No TiO2 3% TiO2 12% TiO2 24% TiO2 32% TiO2 Figure 17. Effect of TiO2 loading on adsorption of the wastewater. 0.00.10.20.30.40.50.60.70.80.91.00510152025Time (hours)Ce/Co No TiO2 3% TiO2 12% TiO2 24% TiO2 32% TiO2 500 ppb Figure 18. Effect of TiO2 loading on destruction of the wastewater.

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40 actually exhibits slower degradation, and the 32% loading decreased the TOC concentration to 500 ppb about an hour sooner than the 12% loading. It is possible that there is a limit to the amount of TiO2 that can be effectively dispersed near the surface of the silica gel, and at higher loadings the TiO2 will agglomerate and some of the TiO2 will be blocked from the UV light. The 32% TiO2 pellet shows that the addition of more TiO2 can lead to better performance; however, 2.5 times the amount of TiO2 in the 12% TiO2 composite was required to decrease the destruction time by approximately 20%. Because this level of improvement in the destruction rate does not justify such a large increase in TiO2 loading, the 12% TiO2 loading was chosen as the optimum TiO2 loading. Activated Carbon Loading Activated carbon was added to the SiO2-TiO2 pellets to explore its effect on the performance of the system. It was expected that the adsorption of the organic contaminants would increase, but it was difficult to predict the effects on the systems destruction abilities. The increased adsorption could enhance the destruction rate, but the activated carbon could also have a negative impact on the system by blocking the UV light needed to activate the catalyst. In order to find the optimum activated carbon loading that increased adsorption without blocking too much UV light, several different activated carbon loadings were tested at two different TiO2 loadings of 12% and 24%. The increase in adsorption with the addition of activated carbon to pellets of 24% TiO2 is illustrated in Figure 19. The time needed to exhaust the pellets doubles with even the lowest activated carbon loading, 1.3wt%. The destruction performance of the pellets containing activated carbon is compared to pellets without any carbon in Figure 20. It is interesting to find that in the two lowest carbon loadings, 1.3wt% and 2.6wt%, the carbon addition had no impact on the time

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41 needed to reach the 500 ppb TOC limit. At the higher loading of 3.9wt%, the carbon begins to negatively impact the destruction rate, most likely due to blocking of UV light needed to activate the catalyst. Because practical operation of the system would involve exhaustion of the carbon in the dark, destruction of the organics under constant UV, and then repeating, the cycle, it was important to test the ability of the activated carbon to be used again. For this reason, an additional adsorption test was conducted after the usual adsorption and destruction cycle. The second adsorption cycle showed complete regeneration for the 1.3% and 3.9% activated carbon loadings (Figures 21 and 22). Regeneration was not tested for the 2.6% carbon loading. The effect of activated carbon addition was also tested with a 12% TiO2 loading. A higher carbon loading of 9.6wt% was explored, and the results are shown in Figures 23 and 24. It is clear that the 9.6wt% greatly increases the adsorption of the organic contaminants. After more than 6 hours, the system was not yet exhausted. The destruction test shows that the carbon does inhibit the destruction rate, though the concentration still reaches the necessary 500 ppb level. Because the carbon was exhausted only to a Ce/C0 ratio of 0.6, it is possible that the some of the removal shown in Figure 24 was due to adsorption. A second adsorption experiment was conducted after the destruction cycle to determine if the carbon had been regenerated. Figure 25 shows complete regeneration of the carbon. The regeneration displayed in Figure 25 indicates that destruction was also occurring, and enough of the organic contaminants were destroyed to clean the surface of the activated carbon.

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42 0.00.10.20.30.40.50.60.70.80.91.0050100150200Time (min)Ce/Co 24% TiO2, No C 24% TiO2, 1.3wt% AC 24% TiO2, 2.6wt% AC 24% TiO2, 3.9wt% AC Figure 19. Effect of activated carbon addition to the adsorption properties of the pellets. 0.00.10.20.30.40.50.60.70.80.91.0051015202530Time (hours)Ce/Co 24% TiO2, No C 24% TiO2, 1.3wt% AC 24% TiO2, 2.6wt% AC 24% TiO2, 3.9wt% AC 500 ppb Figure 20. Effect of activated carbon loading on the destruction performance of the pellets.

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43 0.00.20.40.60.81.01.2050100150200Time (min)Ce/Co Ads1 Ads2 Figure 21. Comparison of the first adsorption cycle (ads1) to the adsorption of the pellets after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118 angstroms, and 1.3wt% activated carbon loading. 0.00.10.20.30.40.50.60.70.80.9050100150200Time (min)Ce/Co Ads1 Ads2 Figure 22. Comparison of the first adsorption cycle (ads1) to the adsorption fo the pellets after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118 angstroms, and 3.9wt% activated carbon loading.

PAGE 54

44 Similar to the trend in the 3.9wt% carbon loading in Figure 20, the destruction curve for the 9.6wt% carbon shows an increase in the TOC concentration to a Ce/C0 ratio of 0.3 just before decreasing to 500 ppb. It is possible that this phenomenon seen in the destruction tests of the highest activated carbon loadings could be due to desorption of the organics from the carbon. Because the destruction cycle is performed following exhaustion of the pellets, the pellets with higher activated carbon loadings have more organic contaminants contained in the pores at the start of the destruction test. It is possible that as the TOC concentration of the contaminants in solution decreases to a certain level, some of the contaminants held within the pores desorb into solution before becoming completely mineralized. The advantage of the activated carbon addition is the increase in the adsorption cycle with the ability to regenerate the carbon surface, meaning that the system could be operated in repeated cycles of adsorption and destruction. The increased adsorption properties allow longer periods in the absence of UV light, which decreases the energy requirements of the system. When considering this type of system operation, the 2.6wt% exhibits the most potential compared to the other activated carbon loadings that were tested. Although the 3.9% and 9.6% loadings were shown to require the longest times to reach exhaustion, creating longer dark periods, the destruction periods were lengthened by more then double, increasing the time requiring the presence of UV light by about 15 hours. The destruction period was the same for the composites containing zero, 1.3%, and 2.6% activated carbon loading. Of these three composite compositions, the composite containing 2.6% carbon is the best choice because it exhibited the longest period of adsorption.

PAGE 55

45 0.00.10.20.30.40.50.60.70.80.91.00100200300Time (min)Ce/Co 12% TiO2, No AC 12% TiO2, 9.6wt% AC Figure 23. Effect of activated carbon loading on the adsorption properties of the pellets. 0.00.10.20.30.40.50.60.70.80.91.0051015202530Time (hours)Ce/Co 12% TiO2, No AC 12% TiO2, 9.6wt% AC 500 ppb Figure 24. Effect of activated carbon loading on the destruction performance of the pellets.

PAGE 56

46 0.00.10.20.30.40.50.60.70.80.91.00100200300400Time (min)Ce/Co Ads1 Ads2 Figure 25. Comparison of the first adsorption cycle (ads1) to the adsorption of the pellets after the destruction cycle (ads2), using pellets of 12% TiO2 loading, 118 angstroms, and 9.6wt% activated carbon loading. Kinetics The rate constants for the destruction performance at each set of system conditions were calculated using LH kinetics. It was assumed that the concentration was low enough to reduce the LH equation to a first order reaction, although no experiments were performed to verify first order behavior. Using the number of CSTRs in series, n, determined through the tracer analysis to be 5, and assuming a first order reaction, the equation for the effluent concentration at any particular time, t, is described by the following equation: nenktCC10 Eq. 9 where k is the rate constant. In this discussion, the k values that are calculated describe the destruction behavior of the photocatalytic system and represent the product

PAGE 57

47 of the rate constant and the equilibrium adsorption constant, or the kK term shown in the reduced LH equation (Eq. 3). The time each plug of wastewater spent in the reactor was estimated for use in Eq. 9. This time was calculated from the time at which the destruction curve crossed the 500 ppb threshold. The k constants resulting from these calculations are shown in Table 5. The rate constants quantify the optimum choice for each system property: 20 ml/min flow rate, 118 angstrom pore size, and 12% TiO2 loading. For the TiO2 loading, the 32% loading actually provides the highest destruction rate, but it is only slightly higher than the 12% loading. Because the 32% loading requires 2.5 times the weight of TiO2 in the system to reach the higher destruction rate, the 12% TiO2 loading is considered optimum. The rate constants for the composites containing activated carbon are also displayed in Table 5. Because the composites containing activated carbon have a higher adsorption capacity than composites without activated carbon, more organic carbon will be held in the pores of the composite at the of the adsorption period. This increase in the amount of organic carbon contained within the composite signifies a higher quantity of TOC in the system at the start of the destruction period. It is important to consider the additional organic carbon that was photocatalytically degraded as well as the longer dark adsorption periods when comparing the kinetics of the composites containing activated carbon. The 2.6% activated carbon composite shows the same rate constant as the composite with a lower activated carbon loading, but it would allow for a longer dark adsorption period; therefore, the 2.6% activated carbon loading is considered the best choice out of the activated carbon loadings that were tested.

PAGE 58

48 Table 5. Rate Constants for TOC Mineralization to 500 ppb Flow Rate (mL/min) Avg. Time in Reactor (min) k (min-1) 5 128.8 0.017 20 56.4 0.038 30 143.4 0.015 Pore Size (angstroms) 60 133.5 0.016 140 56.4 0.038 320 59.3 0.036 Titanium Dioxide Loading (%) 3 >148.3 <0.014 12 56.4 0.038 24 62.3 0.034 32 47.5 0.045 Activated Carbon Loading (%) 1.3% (24% TiO2) 62.3 0.034 2.6% (24% TiO2) 83.1 0.034 3.9% (24% TiO2) 148.3 0.014 9.6% (12% TiO2) 148.3 0.014

PAGE 59

CHAPTER 5 SUMMARY AND CONCLUSION SiO2-TiO2 composites were tested for their ability to photocatalytically degrade the organic contaminants in gray water. The composites were tested in a packed-bed reactor, and the system was optimized with respect to flow rate, TiO2 loading, and pore size of the SiO2-TiO2 composite. Activated carbon was also added to the composites and the effect on the performance of the system was investigated. The SiO2-TiO2 composites were successful in photocatalytically degrading the gray water influent in a recirculating system to a TOC concentration below the 500 ppb maximum limit. The optimum pore size and TiO2 loading were found to be 118 angstroms and 12% (wt/vol), respectively. The system performed best when operated at a flow rate of 20 mL/min. Activated carbon was found to be capable of improving the overall system performance through increased adsorption, which decreases the energy requirements of the system. It was hypothesized that increasing the adsorption of the SiO2-TiO2 composites would increase the destruction rate of the system. Adsorption was shown to be an important system parameter, but increasing adsorption did not always lead to a better destruction rate. The performances of the 234 angstrom composite and the 30 mL/min flow rate condition illustrate situations where adsorption became a limiting factor and the decreased adsorption led to slower destruction. However, adsorption was not always the limiting factor. Adsorption was increased by lowering the flow rate to 5 mL/min and by adding activated carbon. In each of these cases, other factors, such as UV light exposure, 49

PAGE 60

50 were affected and the destruction rate did not improve. Adsorption is one of several factors that must be considered in order to optimize the system performance.

PAGE 61

APPENDIX SILICA-TITANIA COMPOSITE CHARACTERIZATION The BET surface area and pore volume were measured for each composite that was tested. The average pore size was calculated from the following equation (Powers, 1998): R = 2Vp/SA where r is the pore radius, Vp is the specific pore volume and SA is the specific surface area. The measurements for each composite are listed in Table A-1. Table A-1. Surface Area, Pore Volume, and Pore Size of Composites Tested Test TiO2 Loading Carbon Loading BET SSA (m2/g) Pore Volume (cm3/g) Planned Pore Size (angstroms) Calculated Pore Size (angstroms) Flow Rate 5 mL/min 12% None 274.5 0.854 140 124 20 mL/min, Test A 12% None 292.0 0.714 140 98 20 mL/min, Test B 12% None 259.9 0.871 140 134 Small Pellets, Test A 12% None 274.0 0.828 140 121 Small Pellets, Test B 12% None 309.9 0.871 140 112 Average 282.1 0.828 118 Max 309.9 0.871 134 Min 259.9 0.714 98 Pore Size 60 ang 12% None 393.6 0.690 60 70 320 ang 12% None 136.7 0.800 320 234 TiO2 Loading 24% 24% None 263.7 0.770 140 117 32% 32% None 275.1 0.640 140 93 Activated Carbon Loading 1.3% 24% 1.3% 256.9 0.682 140 106 2.6% 24% 2.6% 285.0 0.865 140 121 3.9% 24% 3.9% 241.6 0.649 140 107 9.6% 12% 9.6% 317.0 0.809 140 102 51

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LIST OF REFERENCES Al-Ekabi H, Serpone N. Kinetic studies in heterogeneous photocatalysis. 1. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over TiO2 supported on a glass matrix. Journal of Physical Chemistry, 1988, 92, 5726-5731. Al-Ekabi H, Serpone N. Kinetic studies in heterogeneous photocatalysis. 2. TiO2-Mediated degradation of 4-chlorophenol alone and in a three-component mixture of 4-chlorophenol, 2, 4-dichlorophenol, and 2,4,5-trichlorophenol in air-equilibrated aqueous media. Langmuir, 1989, 5, 250-255. Anderson C, Bard A. An improved photocatalyst of TiO2/SiO2 prepared by a sol-gel synthesis. Journal of Physical Chemistry, 1995, 99, 9882-9885. Anderson C, Bard A. Improved photocatalytic activity and characterization of mixed TiO2/SiO2 and TiO2/AlO3 materials. Journal of Physical Chemistry B, 1997, 101, 2611-2616. Campbell M, Finger B, Verostko C, Wines K, Pariani G, Pickering K. Integrated Water Recovery System Test. International Conference on Environmental Systems, July, 2003. Cheong H, Kang K, Rhee H. Preparation of titanium-containing carbon silica composite catalysts and their liquid-phase epoxidation activity. Catalysis Letters, 2003, 86, 1-3, 145-149. Crittenden J, Suri R, Perram D, Hand D. Decontamination of water using adsorption and photocatalysis. Water Research, 1997, 31, 3, 411-418. Deng X, Yue Y, Gao Z. New carbon-silica composite adsorbents from elutrilithe. Journal of Colloid and Interface Science, 1998, 206, 52-57. Diebold U. The surface science of titanium dioxide. Surface Science Reports, 2003, 48, 53-229. Ding Z, Hu X, Yue P, Lu G, Greenfield P. Synthesis of anatase TiO2 supported on porous solids by chemical vapor deposition. Catalysis Today, 2001, 68, 173-182. El Shafei G. Silica surface chemical properties. Adsorption on Silica Surfaces. E. Papirer, Ed. Surfactant Science Series, 2000, 90, 35-61. 52

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53 Gao X, Wachs I. Titania-silica as catalysts: molecular structural characteristics and physico-chemical properties. Catalysis Today, 1999, 51, 233-254. Gomes da Silva C, Faria JL. Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation. Journal of Photochemistry and Photobiology A: Chemistry, 2003, 155, 133-143. Hench L, West J. The sol-gel process. Chemical Review, 1990, 90, 33-72. Herrmann JM, Matos J, Disdier J, Guillard C, Laine J, Malato S, Blanco J. Solar photocatalytic degradation of 4-chlorophenol using the synergistic effect between titania and activated carbon in aqueous suspension. Catalysis Today, 1999, 54, 255-265. Hoffmann M, Martin S, Choi W, Bahnemann D. Environmental applications of semiconductor photocatalysis. Chemical Review, 1995, 69-96. Holmes F. The performance of a reactor using photocatalysis to degrade a mixture of organic contaminants in aqueous solution. Masters Thesis, University of Florida, 2003. Icenhower J, Dove P. Water behavior at silica surfaces. Adsorption on Silica Surfaces. E. Papirer, Ed. Surfactant Science Series, 2000, 90, 280. Khan A. Titanium dioxide coated activated carbon: A regenerative technology for water recovery. Masters Thesis, University of Florida, 2003. Kirk R, Othmer D. Sol-gel technology. Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed. J. Kroschwitz, M Howe-Grant, Eds. New York: Wiley, 1991, 22, 497. Lange KE, Lin CH. Advanced Life Support Program: Requirements Definition and Design considerations. Document #CTSD-ADV-245 (REV A) presented for the Crew and Thermal Systems Division in Houston, TX. Liu T, Cheng T. Effects of SiO2 on the catalytic properties of TiO2 for the incineration of chloroform. Catalysis Today, 1995, 26, 71-77. Londeree D. Silica-titania composites for water treatment. Masters Thesis, University of Florida, 2002. Lu M, Chen J, Chang K. Effect of adsorbents coated with titanium dioxide on the photocatalytic degradation of propoxur. Chemosphere, 1999, 38, 3, 617-627. Matos J, Laine J, Herrmann JM. Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon. Applied Catalysis B: Environmental 1998, 18, 281-291.

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54 Matos J, Laine J, Herrmann JM. Effect of the type of activated carbons on the photocatalytic degradation of aqueous organic pollutants by UV-irradiated titania. Journal of Catalysis, 2001, 200, 10-20. Matthews R. Photooxidation of organic impurities in water using thin films of titanium dioxide. Journal of Physical Chemistry, 1987, 91, 3328-3333. Matthews R. An adsorption water purifier with in situ photocatalytic regeneration. Journal of Catalysis, 1988, 113, 549-555. Matthews R. Photocatalysis in water purification: Possibilities, problems and prospects. Photocatalytic Purification and Treatment of Water and Air, 1993, 121. Minero C, Catozzo F, Pelizzetti E. Role of adsorption in photocatalyzed reactions of organic molecules in aqueous TiO2 suspensions. Langmuir, 1992, 8, 481-486. Nagaoka S, Hamasaki Y, Ishihara S, Nagata M, Iio K, Nagasawa C, Ihara H. Preparation of carbon/TiO2 microsphere composites from cellulose/TiO2 microsphere composites and their evaluation. Journal of Molecular Catalysis A: Chemical, 2002, 177, 255-263. Ohtani B, Nishimoto S. Effect of surface adsorption of aliphatic alcohols and silver ion on the photocatalytic activity of TiO2 suspended in aqueous solutions. Journal of Physical Chemistry, 1993, 97, 920-926. Orcel G. The chemistry of silica sol-gel. Ph.D. Dissertation. University of Florida, Gainesville, Fl, 1987. Peral J, Casado J, Domenech J. Light-induced oxidation of phenol over ZnO powder. Journal of Photochemistry and Photobiology A: Chemistry, 1988, 44, 209-217. Powers K. The development and characterization of sol gel substrates for chemical and optical applications. Ph.D. Dissertation. University of Florida, Gainesville, FL, 1998. Serpone N. Brief introductory remarks on heterogeneous photocatalysis. Solar Energy Material and Solar Cells, 1995, 38, 369-379. Skubiszewska-Zieba J, Charmas B, Leboda R, Staszczuk P, Kowalczyk P, Oleszczuk P. Effect of hydrothermal modification on the porous structure and thermal properties of carbon-silica adsorbents (carbosils). Materials Chemistry and Physics, 2002, 78, 486-494. Tanaka K, Hisanaga T, Rivera P. Effect of crystal form of TiO2 on the photocatalytic degradation of pollutants. Photocatalytic purification and treatment of water and air, 1993, 169.

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55 Torimoto T, Ito S, Kuwabata S, Yoneyama H. Effects of adsorbents used as supports for titanium dioxide loading on photocatalytic degradation of propyzamide. Environmental Science and Technology, 1996, 30, 1275-1281. Torimoto T, Okawa Y, Takeda N, Yoneyama H. Effect of activated carbon content in TiO2-loaded activated carbon on photodegradation behaviors of dichloromethane. Journal of Photochemistry and Photobiology A: Chemistry, 1997, 103, 153-157. Turchi C, Ollis D. Mixed reactant photocatalysis: Intermediates and mutual rate inhibition. Journal of Catalysis, 1989, 119, 483-496. Turchi C, Ollis D. Photocatalytic degradation of organic water contaminant mechanisms involving hydroxyl radical attack. Journal of Catalysis, 1990, 122, 178-192. Villieras F, Leboda R, Charmas B, Bardot F, Gerard G, Rudzinski W. High resolution argon and nitrogen adsorption assessment of the surface heterogeneity of carbosils. Carbon, 1998, 36, 10, 1501-1510. Vohra M, Tanaka K. Photocatalytic degradation of aqueous pollutants using silica-modified TiO2. Water Research, 2003, 37, 3992-3996. Xu Y, Zheng W, Liu W. Enhanced photocatalystic activity of supported TiO2 dispersing effect of SiO2. Journal of Photochemistry and Photobiology A: Chemistry, 1999, 122, 57-60. Yoneyama H, Torimoto T. Titanium dioxide/adsorbent hybrid photocatalysts for photodestruction of organic substances of dilute concentrations. Catalysis Today, 2000, 58, 133-140.

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BIOGRAPHICAL SKETCH Christina Yvette TerMaath was born in Lafayette, Indiana, on January 7, 1977. Her family spent a few years in Lafayette and Panama City, Florida, before relocating to Springfield, Virginia, where Christina graduated from West Springfield High School in 1995. She pursued a Bachelor of Science degree in Chemical Engineering from the Pennsylvania State University with a focus on energy and fuels. Her interest in environmental engineering was realized as she studied the pollution and control of emissions from the use of fossil fuels. Upon graduation in December 1999, Christina began an eight month internship with the US Environmental Protection Agency in Washington, DC, where her interest in environmental engineering grew. After two years at an energy consulting company in Washington, DC, where she learned about hydrogen fuel technologies through her work with the Department of Energys Hydrogen Program, Christina decided to pursue graduate school in the Environmental Engineering Sciences Department at the University of Florida. 56


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Copyright Date: 2008

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THE PERFORMANCE OF SILICA-TITANIA COMPOSITES
IN A PACKED-BED REACTOR FOR PHOTOCATALYTIC DEGRADATION
OF GRAY WATER
















By

CHRISTINA YVETTE LUDWIG


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

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Christina Yvette Ludwig















ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. David Mazyck, for the opportunity to perform

this research and for his guidance. It has been a challenging and rewarding experience to

learn from him. I would also like to thank the other members of my committee, Dr. Paul

Chadik, Dr. Kevin Powers, and Dr. Chang-Yu Wu, for their constant supply of

knowledge and suggestions throughout the duration of this research.

I am grateful for the opportunity to work as part of an excellent research group. I

thank Ameena Khan, Matt Tennant, Thomas Chestnutt, Jack Drwiega, Jennifer Hobbs,

Jennifer Stokke, Morgana Bach, and Danielle Londeree for their ideas and suggestions as

well as their general support and advice. Each group member contributed to this research

as well as my education.

I must also thank my family for all their love and support. Special thanks go to my

husband, Matthew, for his constant encouragement and support throughout all of my

education.
















TABLE OF CONTENTS

page

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

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

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

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

CHAPTER

1 IN TRODU CTION ................................................. ...... .................

2 LITER A TU R E R EV IEW .............................................................. ...................... 5

P hotocataly sis ............. ....... .............................. ...... .......... .......... . .
T titanium D ioxide............ .......................................................... ........... ...
K inetics................................................... 7
Catalyst Supports ................................. ................................ ..............
A activated C arbon .................. ................................ ......... .. ............ 10
S ilic a ................... ............................................................ ................1 2
R eg en eratio n ..................................................................................................14
Synthesis by Sol-G el M ethod .................................................... .. ... .......... 14
Silica G el Properties ........................................... .. ...... ................. 17
Silica-Carbon Com posites .......................................................... ............... 17

3 M ATERIALS AND M ETHOD S ........................................ ......................... 19

Si02-TiO2 Composites .................. ............................ .. .. .................... 19
R e a cto r S y stem ..................................................................................................... 2 2
Sim ulated W astew after ........................................................................ .................. 24
R actor D ynam ics ........................................... ............... .......... ....25
Experim ental Procedures ............................................................. ............... .27
A dsorption and D estruction........................................... .......................... 27
A dsorption of Interm ediates ...................................................... ..... .......... 29










4 R E SU L T S ....................................................... 30

Sy stem O ptim ization .......................................................................... ................... 30
F lo w R a te ........................................................................................................ 3 0
P o re S iz e ................................................................................ 3 5
Titanium Dioxide Loading ...........................................................................38
Activated Carbon Loading ............................................................................40
K in e tic s .................................................................................................................. 4 6

5 SUMMARY AND CONCLUSION ...........................................................................49

APPENDIX SILICA-TITANIA COMPOSITE CHARACTERIZATION....................51

L IST O F R E F E R E N C E S ............................ ................................................................52

B IO G R A PH IC A L SK E T C H ...................................................................... ..................56







































v
















LIST OF TABLES

Table pge

1 Chemicals Used for Silica Gel Synthesis...................................... ............... 19

2 Properties of D egussa P25 TiO 2................................................................... ...... 20

3 R actor Specifications...................... .. .. ......... .. .......................... ............... 23

4 Simulated W astewater Composition ............................................. ............... 24

5 Rate Constants for TOC Mineralization to 500 ppb .............................................48

A-i Surface Area, Pore Volume, and Pore Size of Composites Tested........................51















LIST OF FIGURES


Figure pge

1 Illustrations of TiO2 crystalline structures: rutile (left) and anatase (right) ...............6

2 Particle size distribution of activated carbon used in AC-Si02-TiO2 composites ....20

3 Silica sols doped with titanium dioxide were mixed using magnetic stir plates and
transferred to 96 well assay plates before forming a gel. .......................................21

4 Reactor packed with Si02-TiO2 composites with a UV light through the center.....22

5 Treatment system consisting of reactor packed with SiO2-TiO2 composites, UV
lamp, pump, and reservoir of simulated wastewater .............................................23

6 Calibration of conductivity m eter ..................................... ......................... ......... 25

7 This figure compares the residence time distribution of the reactor as compared
to a C STR -in-series m odel ......................................................... ............... 27

8 This figure compares the residence time distribution of the reactor as compared
to a C STR -in-series m odel ......................................................... ............... 27

9 Diagram of the system used to test the adsorption of the intermediates formed
from photocatalysis at different EBCTs. ...................................... ............... 29

10 TOC removal by adsorption at flow rates of 5, 10, 20, and 30 mL/min,
corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and hydraulic loading
rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively. ......................................31

11 TOC removal by destruction in a recirculating system at flow rates of 5, 10, 20,
and 30 mL/min, corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and
hydraulic loading rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively. .........32

12 TOC removal by adsorption of wastewater effluents resulting from destruction at
various EB C T s. .......................... ...... ..................... .... ...... ...........34

13 Effect of pellet size on adsorption of the wastewater. .............................................36

14 Effect of pellet size on destruction of the wastewater. ..........................................36

15 Effect of pore size on adsorption of the wastewater....................... ................37









16 Effect of pore size on the destruction of the wastewater. .......................................37

17 Effect of TiO2 loading on adsorption of the wastewater................ .................. 39

18 Effect of TiO2 loading on destruction of the wastewater ..................................... 39

19 Effect of activated carbon addition to the adsorption properties of the pellets. .......42

20 Effect of activated carbon loading on the destruction performance of the pellets....42

21 Comparison of the first adsorption cycle (adsl) to the adsorption of the pellets
after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118
angstroms, and 1.3wt% activated carbon loading .................................................43

22 Comparison of the first adsorption cycle (adsl) to the adsorption fo the pellets
after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118
angstroms, and 3.9wt% activated carbon loading.........................................43

23 Effect of activated carbon loading on the adsorption properties of the pellets.........45

24 Effect of activated carbon loading on the destruction performance of the pellets....45

25 Comparison of the first adsorption cycle (adsl) to the adsorption of the pellets
after the destruction cycle (ads2), using pellets of 12% TiO2 loading, 118
angstroms, and 9.6wt% activated carbon loading.................... ..................46















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 Engineering

THE PERFORMANCE OF SILICA-TITANIA COMPOSITES IN A PACKED-BED
REACTOR FOR PHOTOCATALYTIC DEGRADATION OF GRAY WATER

By

Christina Yvette Ludwig

August 2004

Chair: David Mazyck
Major Department: Environmental Engineering Sciences

Photocatalysis is used to mineralize organic water pollutants, providing water

treatment without a waste stream. This water treatment method allows for a compact

reactor design that is applicable in future NASA missions that will require water

recovery. The objective of this research was to optimize a photocatalytic reactor that will

reduce the total organic carbon (TOC) concentration of a gray water influent to meet

NASA's drinking water quality standards. The system utilizes a reactor packed with

titanium dioxide (TiO2) supported by silica gel (Si02) and was optimized with respect to

empty bed contact time (EBCT), pore size of the SiO2-TiO2 composite, and TiO2 loading

of the Si02-TiO2 composite. The addition of activated carbon to the Si02-TiO2

composite was also investigated.

An optimum EBCT of seven minutes was found based on the fastest destruction

rate to reach below the 500 ppb TOC maximum limit established by NASA. Also based

on destruction rate, a Si02-TiO2 composite having a 12% TiO2 loading (weight/volume









basis) and 118 angstrom pore size was shown to have the best performance. Activated

carbon was found to improve the overall performance of the system through its increase

to the composite's adsorption capabilities. Greater adsorption capabilities can allow for

lower energy requirements, enabling the UV light to be turned off during a period of

contaminant adsorption and turned on for mineralization of the contaminants already

surrounding the photocatalyst.

Adsorption capabilities of the support can lead to more efficient, complete

destruction of the contaminants by surrounding the photocatalyst with a high

concentration of contaminants; therefore, it was predicted that increasing the ability of

the photocatalyst composite to adsorb the organic contaminants would increase the rate of

photocatalytic destruction. Adsorption was shown to be an important system parameter,

but increasing adsorption did not always lead to a better destruction rate due to the

contribution of other parameters controlling the system, such as UV light exposure and

hydraulic flow through the reactor.














CHAPTER 1
INTRODUCTION

Contamination of our water, air and soil by various agricultural and industrial

processes has traditionally been controlled through ultrafiltration, extraction, air

stripping, carbon adsorption, incineration, and oxidation via ozonation or via hydrogen

peroxide (Serpone, 1995). Problems with the successful and efficient removal of toxic

pollutants from our environment have led to the search for more advanced methods

(Hoffmann et al., 1995). Groundwater contamination is expected to be a primary source

of human contact with a majority of the toxic pollutants in our environment, many of

which are organic compounds, such as solvents, pesticides, chlorophenols, and volatile

organic (Hoffman et al., 1995). Photocatalysis offers an advanced technology for the

removal of toxic organic from our water. Many of the previously mentioned

technologies simply transfer the pollutant out of the water, requiring additional treatment

and/or disposal of the compound. In photocatalysis, the organic contaminants are

oxidized, ultimately to carbon dioxide and water, leaving no waste to dispose. Utilizing

UV irradiation, photocatalysis has the added advantage of being able to simultaneously

disinfect and destroy organic (Serpone, 1995). Because of its promise as a more

advanced environmental technology, photocatalysis is being developed for the

destruction of toxic organic compounds in water. Given its ability to eliminate organic

compounds in one process, this technology has a variety of applications.

Its potential to be a compact, low maintenance system makes it particularly useful

for small-scale water recovery. Water recovery is a vital component of NASA space









missions. The reclamation of water from gray sources, which include shower waste,

hand wash waste, oral hygiene waste, urine, urine flush waste, and spacecraft humidity

condensate, enables long-term presence in space without the penalty of transporting large

masses of clean water to the crew members. It is also a necessity for missions involving

long flight distances, such as a mission to Mars. It is important that the water recovery

system be compact and reliable with as little energy requirements as possible, allowing

crew members to direct time and resources toward scientific investigations. A

photocatalytic system that meets NASA's water recovery system requirements is being

developed to be used as a post processor for the destruction of organic compounds that

may not be removed in preceding water subsystems. These subsystems may include

biological reactors, ion exchange, and reverse osmosis. The objective of the post

processor is to ensure that the total organic carbon (TOC) concentration remains below

the maximum limit required by NASA water quality standards, which is 500 ppb (Lange

and Lin, 1998). The post-processor must also be able to treat the wastewater at a rate of

11.5 L/person/day, which is the quantity of wastewater produced by each person per day

(Campbell et al., 2003).

A system that destroys organic contaminants would also be applicable in a home

water treatment system. Some homeowners, particularly those with well water as their

water supply source, choose to use additional treatment processes, such as an activated

carbon filter, before consumption. Depending on the water quality, it is suggested that 10

to 20 L/day of water for drinking or cooking could be processed using a 15 W to 40 W

photocatalytic unit at a cost comparable to other treatment processes (Matthews, 1993).









The ability of photocatalysis to mineralize organic compounds has already been

proven (Matthews, 1987). More efficient, complete destruction of organic compounds in

water can be achieved through the use of an adsorbent as a support for the photocatalyst.

Because silica is transparent, it does not interfere with the absorbance of energy on the

titania surface but does provide a means of immobilizing the titania to avoid the issue of

separating it from the water after treatment. The porous structure of silica also allows for

the adsorption of contaminants, providing a pathway to the photocatalyst. Adsorption

capabilities can also allow for lower energy requirements, enabling the UV lamp to be

turned off during a period of contaminant adsorption and turned on for mineralization of

the contaminants already surrounding the photocatalyst.

The overall objective of this research is to optimize a photocatalytic reactor system

that will reduce the TOC concentration of a gray water influent to meet NASA's drinking

water quality standards. The system will utilize a reactor packed with titanium dioxide

(TiO2) supported by silica gel (SiO2) and will be optimized with respect to empty bed

contact time (EBCT), pore size of the SiO2-TiO2 composite, and TiO2 loading of the

SiO2-TiO2 composite. The addition of activated carbon to the SiO2-TiO2 composite will

also be investigated. Increasing the ability of the photocatalyst composite to adsorb the

organic contaminants is expected to increase the rate of photocatalytic destruction;

therefore, it is predicted that the composite characteristics resulting in the highest

adsorption will also provide the best photocatalytic degradation of the contaminants.

Because activated carbon is known as an adsorbent of organic compounds, it is

hypothesized that the addition of activated carbon to the SiO2-TiO2 composite will

increase the photocatalytic destruction rate of the system. Because activated carbon is






4


opaque, it may prevent some of the UV light from reaching the photocatalyst. There may

be a need to balance the adsorption and the opacity, resulting in an optimum activated

carbon loading.














CHAPTER 2
LITERATURE REVIEW

Photocatalysis

Heterogeneous photocatalysis involves more than one phase. In this discussion, the

media involved in the reaction are a solid phase catalyst and a liquid solution containing

the organic contaminants. The photocatalytic process begins with the excitation of an

electron (Eq. 1).

TiO2 + hv e- + h+ (Eq. 1)

When light of the appropriate wavelength is absorbed by the catalyst (e.g., TiO2),

an electron (e-) is transferred from the valence band to the conduction band, leaving a

positively charged hole behind (h+). The wavelength of light necessary to provide the

energy to move the electron into the conduction band varies with the specific

photocatalyst; for titanium dioxide (TiO2), UV light of less than 388 nm is required. The

electron can then recombine with the electron hole, or the hole can react with other

species. Oxygen plays an important role as an electron acceptor, thus preventing the

electrons from recombining and keeping the electron holes open for reaction. Water and

hydroxide ions react with the electron holes to form hydroxyl radicals, proven the

primary oxidant in the photocatalytic oxidation of organic (Turchi and Ollis, 1990).

Repeated hydroxyl radical attack can eventually lead to complete oxidation of the

contaminant.









Titanium Dioxide

Titanium dioxide is the photocatalyst most often used due to its low cost and better

activation over other photoactive metal oxides. It has also shown stability in that the

TiO2 itself does not change over time, unlike other photocatalysts. Zinc oxide has shown

good photocatalytic abilities, but has been found to be unstable due to

photodecomposition (Peral et al., 1988).

TiO2 can be synthesized in different crystalline forms. The two most applicable

structures are anatase and rutile. Both consist of titanium atoms surrounded by six

oxygen atoms in different octahedron formations (Figure 1).


[0101




o^4-o
[001] -

1.946 A titanium


S) oxygen '

-010] -
[100] 1.983A [001] -- 100]

Rutile Anatase
Diebold, 2003
Figure 1. Illustrations of TiO2 crystalline structures: rutile (left) and anatase (right).

The anatase structure has been found to be more photocatalytically active (Ohtani

and Nishimoto, 1993; Tanaka et al., 1993). Degussa P-25 is a commercially available

TiO2 with a structure that is 70% anatase and 30% rutile. This particular TiO2 is









commonly used as it has repeatedly demonstrated successful photocatalytic degradation

of organic.

Kinetics

Langmuir-Hinshelwood (LH) kinetics has been shown to successfully describe the

photocatalytic degradation of organic contaminants (Al-Ekabi and Serpone, 1988; Turchi

and Ollis, 1989). The LH model assumes the reactions take place at the surface of the

catalyst, and the reaction rate is proportional to the fraction of the surface covered by the

reactant.

dC kKC
r kO = (Eq. 2)
dt 1+KC

Where r is the rate of the reaction, C is the concentration of the contaminant, t is

time, k is the rate constant, 8 is the fraction of the surface covered by the reactant, and K

is the adsorption equilibrium constant.

Eq. 2 can be simplified when the concentration is either very high (KC >> 1) or

very low (KC << 1). Under conditions of low concentration, KC becomes negligible

compared to 1 and Eq. 2 reduces to a first order reaction:

dC
= kKC (Eq. 3)
dt

Similarly, under conditions of high concentration, 1 becomes negligible compared

to KC. Eq. 2 reduces to a zero order reaction, where the reaction rate is equal to the rate

constant, k. This simplification is illustrated in the photocatalytic degredation of 4-

chlorophenol, where no additional increase in rate was observed with an increase in

initial concentration above 0.2 millimoles (Al-Ekabi and Serpone, 1989). It was reasoned

that at a high enough contaminant concentration, the surface sites of the catalyst are









completely saturated, so further increase in concentration cannot further increase the rate

of reaction, according to LH kinetics. Conversely, below the specified concentration, the

catalyst surface sites are not completely saturated; therefore, the fraction of the surface

covered by the contaminant will vary with concentration, thereby varying the reaction

rate.

The applicability of the LH model can be validated by inverting Eq. 2 and plotting

1/r vs. 1/C. The slope of a linear plot provides 1/kK and the 1/k value is given by the y-

intercept.


+ I (Eq. 4)
dC/dt kK) C k

This equation has been found to be accurate for the reaction rate in a single

component system, with no competition for reactive sites, but must be expanded to

accurately describe a multi-component system (Turchi and Ollis, 1989).

Catalyst Supports

The utilization of a photocatalyst as a slurry can be effective, but creates difficulties

in recovering the catalyst due to its small particle size. For this reason, the use of a

variety of supports, such as glass beads, carbon, sand, clay, and silica gel, have been

investigated (Matthews, 1993). There are additional advantages to using a support that is

capable of adsorbing the organic contaminants. The rate of destruction is generally

controlled by the concentration of the contaminant; therefore, the rate of mineralization

will decrease as the contaminant concentration decreases. This makes reducing the

concentration of organic to low levels a slow process. If an adsorbent is used as a

support, a high concentration of the contaminant is created around the photocatalyst,

which increases the rate of mineralization (Yoneyama and Torimoto, 2000). The









adsorbent support can also retain intermediates that are formed during the destruction

process. The possibility of creating toxic intermediates during photocatalysis is a

concern, but if the intermediates are held near the catalyst they are more likely to be

completely destroyed.

Silica gel and activated carbon are two porous media that are being investigated as

catalyst supports. Activated carbon is commonly used in water treatment to adsorb

undesired organic components. Its proven adsorption capabilities make it a good

candidate for a photocatalyst support. An obvious adverse affect of the addition of

activated carbon is its opaque nature. The activated carbon could prevent photons from

reaching the photocatalyst, decreasing activation and subsequently reducing

mineralization. Silica gel generally has a lower surface area than carbon, but has the

advantage of being transparent, allowing the light to penetrate and activate the titanium

dioxide.

There is some disagreement in the literature as to the level of adsorption desired in

the catalyst support. A study of the photocatalytic destruction of propionaldehyde in the

air phase revealed that a catalyst support with medium level adsorption abilities

compared to the other adsorbents used in the study (carbon having high adsorption and

zeolum having low adsorption) showed the fastest degradation (Yoneyama and Torimoto,

2000). The reasoning was based on the longer time needed for diffusion from composites

involving strong adsorption. Similarly, it was found that the use of activated carbon as a

catalyst support was not advantageous in the destruction of dichloromethane in water

(Torimoto et al., 1997). The reason for this result was attributed to slower diffusion of

dichlormethane from the adsorbent compared to the bulk solution. However, an









investigation of a single compound, methylene blue, using various activated carbon types

as catalyst supports for TiO2 was completed (Khan, 2003). Here it was found that the

stronger adsorbents led to better destruction. Further analysis as to the cause of varied

destruction levels resulting from different activated carbon supports revealed that higher

metal content led to enhanced photocatalysis. Khan explains that the metals contained in

the AC could act as electron acceptors, reducing the recombination of electron/hole pairs

and leading to increased photocatalysis.

Activated Carbon

Activated carbon (AC) can be made from a variety of carbonaceous materials,

including coal, peat, peanut shells, and wood, which are charred and subsequently

activated through a chemical or physical process, such as the application of steam or CO2

at temperatures typically around 900C and higher. The final product is a material that

has a high surface area (generally 500-1500 m2/g) and high energy for removing

pollutants.

AC is a well-known technology for the removal of organic compounds from water

through adsorption. The drawback of AC is that once its adsorption capacity is reached,

it must either be replaced or regenerated, a process that usually involves burning off the

adsorbed pollutants and a corresponding loss of carbon mass. This drawback could be

overcome through a combination of AC and TiO2. The AC adsorbs the contaminants,

leading to more efficient photocatalysis, and the photocatalysis destroys the pollutants,

leading to regeneration of the adsorbent. This concept has been investigated by

generating composites of AC coated with titanium dioxide (Lu et al., 1999; Torimoto et

al., 1996; Torimoto et al., 1997; Yoneyama and Torimoto, 2000; Khan, 2003). The TiO2

can be coated on the AC through various methods, such as impregnation or boiling









deposition. Different methods will lead to AC-TiO2 composites with different properties.

The disappearance of contaminants as well as the disappearance of total organic carbon

has been monitored (Lu et al., 1999; Torimoto et al., 1996) and found that although TiO2

alone decreased contaminant concentrations at a faster rate, the overall decomposition of

the contaminants and intermediates was faster with the AC-TiO2 composite. This

supports the idea that the AC will adsorb the intermediates, holding them close to the

photocatalyst until completely mineralized.

Several studies claim enhanced photocatalysis through the addition of AC in an

aqueous suspension of TiO2 (Matos et al., 1998; Herrmann et al., 1999; Matos et al.,

2001; Gomes da Silva and Faria, 2003). The location of the degradation process in

aqueous suspensions has been studied (Minero et al., 1992) and shows that the oxidation

reaction takes place close to the catalyst surface; therefore, the use of the two materials in

an aqueous suspension relies on the transfer of the pollutants from the adsorbent to the

photocatalyst. This creates an added step compared to the AC-TiO2 composites; the

contaminants must be somehow transferred, possibly during a collision, from the AC to

the TiO2. Two studies directly compared the use of a mixture versus a composite using

the same quantity of carbon and TiO2 in each experiment with the same organic

compound (Torimoto et al., 1997; Nagaoka et al., 2002). These authors agreed that

enhanced removal occurred with the addition of carbon compared to TiO2 alone, but

found that physically combining the two materials was more advantageous. Torimoto

reasons that the photodecomposition of the adsorbed species must take place during

collisions, and this occurrence is simply not high enough to compete with the AC-TiO2

composite. In addition, the aqueous suspension defeats the original purpose of









supporting TiO2 in the first place, which is to enable the TiO2 particles to be easily

separated from the solution after photocatalysis.

The regeneration of AC-TiO2 has been studied and shown to be difficult.

Regeneration after adsorption of an organic dye, methylene blue, of two composites that

differed in the type of AC was studied (Khan, 2003). The first attempt at regeneration of

one AC-TiO2 composite showed a loss of its capacity with each 24-hour regeneration

cycle. Khan reasoned that some of the carbon may not be exposed to UV light,

preventing photocatalysis; therefore, a second regeneration attempt involved

simultaneous fluidization with UV irradiation. This succeeded in regenerating the AC-

TiO2 back to its initial uptake, but the AC-TiO2 reached exhaustion faster than in the first

cycle. The other AC-TiO2 composite, made with a different AC, was regenerated only

through intermittent 12 hour irradiation, but was not restored to its original level.

Another study that focused on the regeneration of AC-TiO2 also showed long

regeneration times (Crittenden et al., 1997). In order to increase the regeneration rate,

Crittenden tried heating the AC-TiO2 during regeneration with the idea that this would

increase the desorption rate, which was thought to be a limiting step in the regeneration

rate. The regeneration of AC-TiO2 was found to be only 10% more efficient than the

regeneration of the control, which was AC without TiO2. This indicated that the

efficiency was based on simple desorption rather than oxidation of the organic

compounds.

Silica

Silica (Si02) is a transparent material that can be synthesized through various

chemical methods, providing several options for the creation of Si02-TiO2 composites.

Synthesis can occur through preparation of the two oxides simultaneously, as one mixed









material, or through deposition of TiO2 on SiO2. Sol-gel hydrolysis, coprecipitation, and

flame hydrolysis are the most popular mixed oxide methods, while impregnation,

chemical vapor deposition, and precipitation are the methods used for supported oxides,

though the supported oxide methods have not received as much attention (Gao and

Wachs, 1999). Each method will lead to composites with different behaviors based on

differences in surface area, pore size, dispersion of TiO2 on or in the silica, and bonding

between the silica and photocatalyst.

SiO2-TiO2 composites have been proven successful in the photocatalytic

degradation of a variety of organic compounds (Matthews, 1988; Anderson and Bard,

1995; Anderson and Bard, 1997; Xu et al., 1999; Vohra and Tanaka, 2003). In some

studies, SiO2-TiO2 has been compared directly to bare TiO2 and shown to be more

advantageous for specified compounds (Xu et al., 1999; Anderson and Bard, 1995). Xu

and co-workers synthesized particles from a titania sol and silica powder and tested for

photocatalytic destruction of acetophenone. SiO2-TiO2 was found to be at least 10%

more effective than bare TiO2. This increased destruction was related to higher

adsorption of acetophenone and better dispersion of TiO2 due to the presence of silica. A

similar comparison was performed using SiO2-TiO2 and bare TiO2 particles prepared by a

sol-gel technique and tested for the removal of rhodamine-6G (Anderson and Bard,

1995). Again, SiO2-TiO2 was found to have a faster degradation rate, which was related

to the increased adsorption of the contaminant. These studies demonstrate the idea that

increasing adsorption will increase the concentration of the contaminant around the TiO2

and lead to faster destruction of the contaminant.









Regeneration

Multiple studies have been performed that prove the ability of SiO2-TiO2 to be used

over and over again. Phenol was repeatedly degraded by greater than 99% in a reactor

packed with SiO2-TiO2 particles (Matthews, 1988). Cycles of adsorption and destruction

of phenol were performed four times without loss of capacity. A series of adsorption and

destruction tests were also performed using SiO2-TiO2 pellets for the photocatalytic

degradation of volatile organic (Holmes, 2003). At least ten runs were performed using

the same pellets and achieving 80% or more removal of each of the contaminants. No

decrease in the system's original photocatalytic ability was observed in the data. SiO2-

TiO2 pellets are used in four adsorption and destruction cycles for the removal of an

organic dye, crystal violet (Londeree, 2002). No decrease in the pellets' performance is

shown; in fact, the adsorption ability of the pellets actually increased with each cycle.

Synthesis by Sol-Gel Method

Compared to other synthesis processes, the sol-gel technique is the most widely

used based on its potential for control over the textural and surface properties of the final

material (Gao and Wachs, 1999). In addition, because this preparation method begins

with high purity chemical precursors, it produces high purity, homogeneous materials

(Kirk and Othmer, 1991). Starting from a liquid also has the advantage of shaping the

material in a mold, making it easier to produce particles of the size and shape needed for

a specific application.

A sol is a suspension of colloidal particles dispersed in a liquid. Under the proper

conditions, the sol will form a gel, which is a solid network that holds the liquid in its

pores. A sol-gel can be formed through a series of hydrolysis and condensation reactions









of alkoxide precursors. Once the gel is formed, it may undergo aging and drying steps to

allow the gel to mature to the desired structure and expel the liquid held in its pores.

Commonly used silica precursors are tetra methyl ortho silicate (TMOS) and tetra

ethyl ortho silicate (TEOS). The chemical formulas for these silica precursors are

Si(OCH3)4 and Si(OC2H5)4 for TMOS and TEOS, respectively. These can be represented

by Si(OR)4 in equations 5 and 6. The alkoxide precursor reacts with water in a

hydrolysis reaction (Eq. 5) and the silica network begins to form through condensation

reactions (Eq. 6). The network continues to grow through polycondensation. Because

the alkoxide precursor is not readily soluble in water, an alcohol is generally used as a

solvent in the mixing of these reactants. Depending on the precursor, methanol or

ethanol is formed as a byproduct of the hydrolysis reaction and water is produced in the

condensation reactions.

Si(OR)4 + 4H20 "- Si(OH)4 + 4ROH (Eq. 5)

Si(OH)4 + Si(OH)4 4 (OH)3Si O Si(OH)3 + H20 (Eq. 6)

The rates of the hydrolysis and condensation reactions that begin formation of the

gel have a strong impact on the structure of the final material. These reaction rates will

vary with temperature, nature of the solvent, type of alkoxide precursor, and nature and

concentration of the acid or base (Hench and West, 1990). The acid or base

concentration has been shown to be the dominant factor (Orcel, 1987). In general,

increasing the concentration of H+ or H30+ in acidic conditions or increasing the

concentration of OH- in basic conditions will increase the rate of hydrolysis. The ratio of

water to the alkoxide precursor, the R ratio, also affects the rate of hydrolysis. Water can

be used in excess to promote hydrolysis. The rate of the hydrolysis and condensation









reactions will ultimately affect the gel time and the structure of the gel. Hydrofluoric

acid has been used to decrease the gel time and increase the pore size of the resulting gel

(Powers, 1998).

After gelation, at which point the three dimensional silica network has formed and

the sol ceases to move as a liquid, the polycondensation reactions continue as the silica

network further develops. Syneresis and Ostwald ripening also contribute to this aging

process. Syneresis involves the contraction of the silica gel network as a result of the

condensation reactions, which create additional silica network links and cause more

liquid to be expelled. Ostwald ripening describes the formation of the silica gel network

in the areas of negative curvature, filling in these crevices on the surface of the silica gel.

This process strengthens the gel, which helps to protect the gel against cracking during

drying. The time and temperature under which the aging process is allowed to occur will

impact the surface area and pore size of the gel.

Alcohol and water produced in the aforementioned reactions remain in the pores of

the gel. The gel must be dried, typically at temperatures between 100C and 180C, to

remove the liquid from the pores. Capillary forces are created by the evaporation of the

liquid, causing some of the pores to collapse and the gel to shrink. Silica gels dried in

this manner are termed xerogels and will generally be reduced in volume by 40-60% (El

Shafei, 2000). Steps can be taken to reduce the amount of shrinking and create a silica

gel of higher surface area and pore volume. One method involves replacing the water

with a liquid of lower surface tension, such as an alcohol, before drying. A gel can also

be supercritically dried to completely eliminate the destructive forces of surface tension,

minimizing the amount that the gel shrinks.









Silica Gel Properties

The silica network consists of four oxygen atoms bonded to each silicon atom,

forming a tetrahedron, and each oxygen atom is shared by two silicon atoms. These Si-

O-Si, or siloxane, bonds make up the bulk silica structure. The surface of the silica is

hydroxylated in the presence of water, forming Si-OH, or silanol, groups. It is these

silanol groups that make the silica surface hydrophilic and determine the reactivity of the

silica (El Shafei, 2000). The silanol groups can be defined as single, geminal, or vincinal.

Single silanol groups are isolated and formed with a silicon atom that is bonded to three

other oxygen atoms in siloxane bonds. When at a close enough distance, the hydroxyl

groups can interact with each other through hydrogen bonding, creating vincinal silanols.

Geminal silanols exist when two hydroxyl groups are linked to the same silicon atom.

The silica surface can be dehydroxylated through heating above temperatures of 2000C.

The silanol groups will condense to form siloxane bonds. Dehydroxylation will occur at

a greater degree as the temperature is increased.

In the presence of water, the silanol groups become ionized and the hydrogen atom

will associate or dissociate depending on the pH of the solution (Icenhower and Dove,

2000). The point of zero charge (PZC) of silica is between a pH of 2 and 3; therefore, at

pH values above this PZC, the hydrogen atoms will begin to dissociate. This

deprotonation will leave a negative charge on the surface of the silica (Eq. 7).

Si-OH <-4 Si-O- + H+ (Eq. 7)

Silica-Carbon Composites

Combining silica and carbon can potentially combine the advantages of each

material to create a composite with improved adsorption capabilities. These composites

may possess a heterogeneous surface through combination of the properties of the









nonpolar, hydrophobic carbon with the polar, hydrophilic silica to create a material that

can adsorb both inorganic and organic contaminants (Skubiszewska-Zieba, 2002).

Carbon-silica composites have been successfully created through several methods,

including pyrolysis of methylene chloride on the surface of silica gel (Villieras et al.,

1998), pyrolyzed elutrilithe in a sol-gel (Deng et al., 1998), and coating of silica gel on

the carbon surface through hydrolysis of a silica precursor (Cheong et al., 2003). Deng

showed that combining the carbon and silica created a composite with different

adsorption capacities than the individual materials. For certain adsorbates, namely water,

cyclohexane, acetone, and pyridine, carbon-silica composites had superior uptake when

compared to silica or carbon alone (Deng et al., 1998). Composites consisting of carbon,

silica and titanium dioxide have also been generated for the epoxidation of cyclohexene

(Cheong, 2003). The titanium dioxide supported by both carbon and silica was found to

provide greater conversion of cyclohexene compared to titanium dioxide support by silica

or carbon alone. The advantages of each material were successfully combined: the

carbon provided higher selectivity for the epoxide due to its hydrophobic nature, and the

silica provided a better environment for catalytic activity.














CHAPTER 3
MATERIALS AND METHODS

SiO2-TiO2 Composites

The Si02-TiO2 composites were created using a sol-gel method (Londeree, 2002).

The silica precursor was tetra-ethyl-ortho-silicate (TEOS) (Fisher Scientific, reagent

grade). It was mixed with nanopure water using a water to TEOS mole ratio of 16:1.

Ethanol (Aaper Alcohol, 200 proof) was used as the solvent to facilitate the miscibility

between the TEOS and water. Two acid catalysts were used: a 1 M nitric acid solution,

made from 15.8 M nitric acid (Fisher Scientific, certified A.C.S.) and nanopure water,

and a 3% solution of hydroflouric acid, formulated from 49% hydrofluoric acid (Fisher

Scientific, reagent A.C.S.) and nanopure water. Table 1 summarizes the quantity of each

chemical used to make one batch, approximately 10 g, of silica gel. The volume of

hydrofluoric acid was varied to control the pore size of the gel. Volumes of 2, 3, and 4

mL correspond to approximate pore diameters of 70, 118, and 234 angstroms,

respectively.

Table 1. Chemicals Used for Silica Gel Synthesis
Chemical Volume
Nanopure water 25 mL
Ethanol 50 mL
TEOS 35 mL
1 M Nitric Acid 4 mL
3% Hydrofluoric Acid 2-4 mL


The chemicals in Table 1 were mixed in polystyrene containers in the order listed

using a magnetic stir plate. While mixing, the liquid was doped with the desired quantity









of Degussa P25 TiO2. According to Degussa, the TiO2 possesses the properties listed in

Table 2.

Table 2. Properties of Degussa P25 TiO2
Property Value
Specific surface area (BET) 50 +/- 15 m2/g
Average primary particle size 30 nm
Tapped density 130 g/L
pH value in 4% dispersion 3.5-4.5


For the composites that included activated carbon, the mixture was doped with

activated carbon after the addition of TiO2. The activated carbon used in the composites

was ground to an average particle diameter between 1 and 2 microns as determined by

the LS2 13 320 Laser Diffraction Particle Size Analyzer. The distribution of the particle

size is shown in Figure 2.


Figure 2. Particle size distribution of activated carbon used in AC-Si02-TiO2 composites.
Analysis A and Analysis B are duplicates of the particle size distribution
analysis.


10
9
8 Analysis A
7 \-- Analysis B
S6
5
E 4
Z 3
2
1
0
0 2 4 6 8 10 12 14 16 18 20

Particle Diameter (micron)









The TiO2 and activated carbon loadings stated throughout this document are

referred to as percentages. The TiO2 loadings are reported on a basis of TiO2 weight per

TEOS volume. For example, the 12% TiO2 loading is determined using 4.2 g of TiO2 per

35 mL of TEOS as a percentage. The activated carbon loadings are reported by a weight

percentage using the activated carbon mass per total mass of the pellets.

The sol was allowed to mix between 0.5 and 7 hours, depending on the amount of

3% HF used. For the amounts of 2, 3, and 4 mL, approximate mixing times of 7, 2, and

0.5 hours were used, respectively. The liquid was then transferred to 96-well assay plates

and allowed to gel (Figure 3). The assay plates were Fisherbrand, polystyrene plates that

contained 0.45 mL in each well.
















Figure 3. Silica sols doped with titanium dioxide were mixed using magnetic stir plates
and transferred to 96 well assay plates before forming a gel.

Upon reaching the gel point, at which point the sol no longer moved as a liquid, the

trays were capped and wrapped in aluminum foil to prevent any of the contents from

volatilizing during the aging process. The gels were aged for 48 hours at room

temperature and 48 hours at 650C in an Oakton Stable Temperature oven. After aging,

the pellets were removed from the assay plates and transferred into Teflon screw cap jars.









Each lid had a small hole to allow the liquid expelled from the gel's pores to slowly

escape as vapor during the drying process. The drying process involved heating at 103C

for 18 hours and 180C for 6 hours in a Yamato DVS 400 Drying Oven. The BET

(Brunauer, Emmett, and Teller equation) surface areas and pore volumes of the gels were

analyzed using a Quantachrome NOVA 1200 Gas Sorption Analyzer.

Reactor System

The pellets were packed into a cylindrical reactor (Figure 4). The reactor was

designed with a hollow center and thin annulus to allow the UV lamp to be placed in the

center, providing maximum UV light exposure to the pellets. The inner wall of the

annulus was a quartz tube that could be completely removed, making it simple to remove

the pellets after testing. The lamp in the center of the quartz tube provided UV light at a

wavelength of 365 nm. As measured at the center of the lamp, the intensity was 7.4

mW/cm2 at the inner diameter of the annulus and decreased to 5 mW/cm2 at the outer

diameter. Specifications of the reactor are listed in Table 3. The reactor was enclosed in

a box to provide control over its exposure to ambient light.


Figure 4. Reactor packed with Si02-TiO2 composites with a UV light through the center.









Table 3. Reactor Specifications
Specification Value
Length of Reactor 19 cm
Inner Diameter 2.5 cm
Outer Diameter 4.2 cm
Empty Bed Volume of Pellets 138.6 mL


The system consisted of the reactor, 6 mm PTFE tubing, a Masterflex L/S Digital

Standard Drive peristaltic pump, and a reservoir of simulated wastewater (Figure 5). The

system was used in two different configurations. One system configuration allowed the

wastewater to pass through the reactor only once, and the other formed a closed loop

system in which the wastewater was recirculated through the reactor. A cover (not

shown in figure 5) was placed over the front of the reactor to completely enclose it during

operation of the system. The reservoir was a one liter flask that was covered with

aluminum foil to prevent any outside UV light exposure and capped with parafilm to

create a closed system.


















Figure 5. Treatment system consisting of reactor packed with Si02-TiO2 composites, UV
lamp, pump, and reservoir of simulated wastewater.









Simulated Wastewater

The wastewater feed was created using a formula for typical wastewater provided

by Johnson Space Center (JSC). The chemical composition includes the shower waste,

hand wash waste, oral hygiene waste, urine, and urine flush waste that would be expected

from a crew of four people. Table 4 lists the amounts of each constituent present in the

simulated wastewater.

Table 4. Simulated Wastewater Composition
Wastewater Component Quantity
Pert Plus for Kids 1.2 g
Deionized water 999.4 ml
Ammonium bicarbonate, NH4HCO3 2726 mg
Sodium chloride, NaCl 850 mg
Potassium bicarbonate, KHCO3 378 mg
Creatinine, C4H7N30 248 mg
Hippuric acid, C9H9N03 174 mg
Potassium dihydrogen phosphate, KH2PO4 173 mg
Potassium bisulfate, KHSO4 111 mg
Citric acid monohydrate, C6Hs07-H20 92 mg
Tyrosine, C9HllN03 66 mg
Glucuronic acid, C6H1007 60 mg
1.48N Ammonium hydroxide, NH40H 10 ml


This formula represents the raw wastewater that would be the influent for the

beginning of the water treatment system; therefore, it would be treated by several

processes before actually becoming the feed for the post-processor. To account for

pretreatment, the simulated wastewater is diluted using 9 mL of the concentrated solution

with 991 mL of deionized water to make each liter of wastewater feed. This creates a

solution of approximately 3 ppm of total organic carbon (TOC), which is within the range

of the TOC concentration expected of the post processor influent (Cambel et al., 2003).

It was found that the wastewater was not stable over long periods of time, particularly at

room temperature. The liquid would change in color, from a cloudy, white mixture to









yellow, and the TOC concentration decreased over time. Therefore, a new solution of the

concentrated wastewater was created each week and stored in an amber bottle at 40C. A

small fraction of the wastewater was found to be volatile, so the wastewater feed was

mixed for approximately 1 hour before use to allow the volatiles to be removed and the

TOC concentration to stabilize.

Reactor Dynamics

A tracer analysis was conducted to determine the behavior of the reactor. Sodium

chloride was chosen as a tracer because it was not expected to be adsorbed by the media

and had been used successfully as a tracer in a previous study (Holmes, 2002). The

sodium chloride concentration was measured using conductivity measurements read by a

Fisher Scientific conductivity probe. A linear correlation between conductivity and

sodium chloride concentration was observed and used to translate conductivity

measurements to a sodium chloride concentration (Figure 6).



140
S120 -
0 y=2.589x + 0.7543
E 100
SR2 = 0.9997
S80
S60
I 40
0
20 2

0
0 ----------------

0 20 40 60
NaCI concentration (mg/L)
Figure 6. Calibration of conductivity meter









DI water was pumped through the reactor until a stable conductivity was achieved.

For the tracer analysis, this conductivity was subtracted from effluent readings to

determine the sodium chloride concentration exiting the reactor. The tracer analysis was

performed two ways to provide a duplicate study. First, a solution of 50 mg/L sodium

chloride was pumped through the reactor and the conductivity of the effluent was

monitored until the concentration of the effluent reached 50 mg/L. A second experiment

was conducted in which DI water was pumped through the reactor and the effluent

conductivity was monitored until it indicated that all the sodium chloride had been

washed out of the reactor. The flow rate used for both tests was 10 mL/min.

The data collected from the tracer analysis were used to calculate a mean residence

time and to model the reactor behavior. The mean residence time was determined to be

12.5 minutes. A fractional age distribution (E-curve) of the sodium chloride in the

reactor was created for both reactor tests. This distribution was then compared to that of

the model of continuously stirred tank reactors (CSTRs) in series to determine the

number of tanks in series the reactor behavior was approximately equivalent to (Figures 7

and 8). The E-curve for the CSTR-in-series model was created using Eq. 8.

nt
E--
E= net nt Eq. 8
t(n 1)! )

Where n is the number of CSTRs in series, t is the time in the reactor, and tbar is the

mean residence time.

A comparison of the data with the CSTR-in-series model reveals that the reactor

behaves as five CSTRs in series.











Residence Time Distribution, Test A

0.14 -* Actual
0.12 Model, n =5
0.10 -
S0.08 *
L-
S0.06
0.04 *
0.02
0.00 *-*
0 10 20 30 40
Time (min)

Figure 7. This figure compares the residence time distribution of the reactor as compared
to a CSTR-in-series model. Test A was performed by pumping 50 mg/L of
NaC1 through the reactor until the effluent reached the same concentration.



Residence Time Distribution, Test B

0.14
0.12 Actual
0.10 Model, n 5
2 0.08
L-
0.06
0.04 *
0.02
0.00 __ I t
0 10 20 30 40
Time (min)
Figure 8. This figure compares the residence time distribution of the reactor as compared
to a CSTR-in-series model. Test B was performed by pumping DI water
through the reactor until the NaCl concentration decreased from 50 mg/L.

Experimental Procedures

Adsorption and Destruction

In order to optimize the system, experiments were performed to test its sensitivity

to flow rate, pore size of the silica gel support, titanium dioxide loading, and addition of

activated carbon to the catalyst support. With each change to the system, removal of

TOC achieved by adsorption and destruction was quantified. All samples were analyzed

using a Tekmar Dohrmann Apollo 9000 combustion analyzer.









The amount of pellets packed in the reactor was measured by volume as 115 mL in

a 100 mL graduated cylinder and held constant for each experiment. The regeneration

ability and effects of the experiments on the pellets was unknown; therefore, the reactor

was packed with new pellets for each experiment. Because of the acidic properties of the

silica gel, all pellets were pre-washed with a sodium hydroxide solution (pH = 8.5) to

reduce the pH changed caused by the pellets. The pellets were washed until a pH of 4

was reached in the effluent stream.

To study TOC removal by adsorption, wastewater was first pumped through the

reactor in a single-pass configuration, in the dark, until the pellets were exhausted,

meaning that the influent TOC concentration equaled the effluent TOC concentration.

After the adsorption capacity had been reached, the system, including the reactor and

tubing, remained full of 264 mL of wastewater and the reservoir was filled with 1 liter of

wastewater. The wastewater was recirculated and the UV lamp was turned on. Because

the pellets were previously exhausted with respect to TOC adsorption, TOC removal

during this experiment was assumed to be due to destruction. Samples of 25 mL were

taken from the reservoir every few hours until the TOC concentration reached below 500

ppb.

Because the pellets were still acidic, even after a period of washing, the pellets

changed the pH of the wastewater as it flowed through the reactor. For the single pass

adsorption experiments, the pH was decreased from 8 at the influent to 6 at the effluent.

Recirculation of the wastewater for the destruction tests decreased the initial pH of 8 to a

final pH of 4. The pH of the influent and effluent was monitored in all experiments using









a Fisher Scientific pH meter and these pH changes remained consistent throughout all the

tests.

Adsorption of Intermediates

Experiments were performed to test the adsorption of the intermediates formed

after UV exposure in the reactor at different empty bed contact times (EBCTs), which is

defined as the time needed for one bed volume of water to pass through the reactor

(calculated using the empty bed volume divided by the flow rate). The pre-washed

pellets in the reactor were first exhausted in the dark. The UV light was then turned on

and the effluent was collected in a reservoir. This experiment was performed at flow

rates of 5, 10, 20, and 30 mL/min. A 1 inch glass tube packed with 65 mL of pre-washed

pellets was used to test the adsorption of the effluent. In each case, the adsorption

experiment was performed at a flow rate of 10 mL/min with a new set of pellets and

conducted immediately following collection of the effluent. A diagram of the apparatus

is shown in Figure 9.

Reactor 1 Reactor 2
(constant UV) (no UV)
Destruction at Adsorption at
various EBCTs same EBCT











Wastewater Wastewater and
Intermediates
Figure 9. Diagram of the system used to test the adsorption of the intermediates formed
from photocatalysis at different EBCTs.














CHAPTER 4
RESULTS

System Optimization

The photocatalytic system was optimized with respect to flow rate, pore size of the

silica support, titanium dioxide loading, and activated carbon loading. For each test of

the system, one parameter was varied while the others were held constant. The

adsorption and destruction abilities of the system were tested at each set of conditions.

The optimum system parameters were chosen based on the ability to reduce the TOC

concentration to 500 ppb in the shortest amount of time. The wastewater concentration is

represented in all graphs as a normalized TOC concentration, showing the fraction of the

effluent TOC concentration (Ce) over the influent TOC concentration (Co, 3 ppm). The

pellet pore sizes reported in this results section are approximate; the BET surface area,

pore volume, and calculated pore size of each pellet composition that was tested can be

found in the Appendix.

Flow Rate

Flow rate was the first parameter to be varied. Experiments were performed using

flow rates of 5, 10, 20, and 30 mL/min, which are of the appropriate order of magnitude

to treat the wastewater produced by 1-4 crew members per day. A flow rate of 16

mL/min would provide treatment for wastewater produced by 2 people per day, assuming

a production of 11.5 L of wastewater per crew member (Campbell et al., 2003). The

SiO2-TiO2 pellets used to test each flow rate had a 12% TiO2 loading and pore size of 118

angstroms. The system was first operated in a single pass configuration with no UV light









to test the adsorption behavior of the pellets. The results are displayed in Figure 10. The

adsorption curves at each flow rate are compared directly using bed volumes as the

independent variable. Figure 10 shows that lower flow rates lead to lower effluent

concentrations in the first few bed volumes. The lower flow rates create longer EBCTs,

allowing more time for diffusion of the wastewater through the pores of the silica and

then subsequent adsorption. The 5, 10, and 20 mL/min flow rates cause exhaustion of the

pellets after approximately 6 bed volumes, while the pellets tested at the 30 mL/min flow

rate reach exhaustion sooner, at 3 bed volumes.


1.0
0.9 /
0.8 ,
0.7
o 0.6
C 0.5
o 0.4
0.3 A- 5 mUmin
10 mUmin
0.2 / 20 mUmin
0.1 / 30 mUmin
0.0
0.0 2.0 4.0 6.0 8.0 10.0
Bed Volumes
Figure 10. TOC removal by adsorption at flow rates of 5, 10, 20, and 30 mL/min,
corresponding to EBCTs of 28, 14, 7, and 5.25 minutes and hydraulic loading
rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min, respectively.

After the pellets were exhausted, or close to exhaustion, the system was

reconfigured from a single pass to a recirculating mode, with constant UV light exposure

in the reactor. Because the pellets were previously exhausted, it was assumed that TOC

removal during this testing period was due to complete mineralization. Figure 11 shows










that the rate of destruction depends on the flow rate, with the 20 mL/min flow rate

reaching below the desired 500 ppb in the shortest amount of time. Despite the fact that

the pellets tested at 5 mL/min were not entirely exhausted, meaning that some of the

removal during the destruction period could be due to adsorption, the destruction test

conducted at 5 mL/min required the longest time to reach 500 ppb. This is surprising,

since the slower flow rates are shown to allow more time for the contaminants to adsorb,

and increased adsorption was expected to lead to increased destruction.


1.0
0.9
0.8
0.7
S0.6
S0.5
O 0.4
0.3
n


0.0 -------------------------------------
0.1
0.0
0 5 10 15 20 25
Time (hours)

Figure 11. TOC removal by destruction in a recirculating system at flow rates of 5, 10,
20, and 30 mL/min, corresponding to EBCTs of 28, 14, 7, and 5.25 minutes
and hydraulic loading rates of 0.55, 1.1, 2.2, and 3.3 cm3/cm2-min,
respectively.

It is important to realize that the recirculation means that the total time the

wastewater is in the reactor is approximately equal, regardless of the flow rate. For

example, at a flow rate of 20 mL/min, the wastewater might pass through the reactor 4

times as fast compared to a 5 mL/min flow rate, but it would also experience 4 times as


--A-5 mL/min
10 mL/min
S- 20 mL/min
30 mL/min
S----- 500 ppb









many passes. Therefore, one of the differences between the flow rates is that the

wastewater spends intermittent time in the reactor at the higher flow rates compared to

longer, continuous passes through the reactor at slower flow rates. It is possible that

different intermediates, or quantity of intermediates, are created at different EBCTs,

which could change the adsorption of the wastewater after the first pass through the

reactor.

In order to test the adsorption of the intermediate wastewater (i.e., wastewater and

intermediates created after contact with the irradiated SiO2-TiO2 composites), the

wastewater was first passed through the reactor with constant UV light at a certain

EBCT. Then, the effluent from the first reactor was passed through a second reactor,

which was in the dark, to test for adsorption of these intermediates. The EBCT of the

first reactor was varied to create effluents equivalent to that after a single pass through

the reactor at the flow rates used during the recirculating destruction test. The EBCT in

the second reactor was held constant so that the adsorption of the various wastewater

effluents could be directly compared. Figure 12 presents the adsorption curves for the

intermediate wastewater created from the single pass through the second reactor. The

data show no difference in the adsorption of the wastewater after passing through the

reactor at different EBCTs. Therefore, the better performance at 20 mL/min shown in

Figure 11 did not result from the formation of intermediates that were more adsorbable.

A second hypothesis based on diffusion of the wastewater was formulated to

explain the variation in the destruction rates as the flow rate was changed. The rate of

destruction depends on the ability of the contaminants to reach the titanium dioxide in the

porous surface of the pellets and the ability of the products resulting from the oxidation









reactions to travel out of the pores. It is possible that the higher flow rates increase the

diffusion of the molecules in and out of the pellets. The transport of the wastewater

contaminants and the photocatalysis products in and out of the pellets' pores depends on

several steps. These steps include transport through the bulk solution, transport through

the film layer formed by stationary water surrounding the pellet, and travel through the

pores. Higher flow rates decrease the film layer surrounding the pellet; thus, decreasing

the distance the molecules must diffuse to approach the pellet surface. While higher flow

rates were shown to have a negative impact on adsorption behavior in a single pass

system, the impact of higher flow rates on the adsorption behavior of intermediate

products as well as the transport of final products out of the pores and into the bulk

solution is unknown.



1.2

1.0

0.8

0.6

0.4 ---5.25 min EBCT
S-A28 min EBCT
0.2 -u14 min EBCT
.-*-7 min EBCT
0.0 1
0 20 40 60 80 100
Time (min)

Figure 12. TOC removal by adsorption of wastewater effluents resulting from
destruction at various EBCTs.









In order to test the hypothesis of diffusion being a limiting factor in the system,

pellets of the same composition but approximately half the size (smaller diameter, same

length) were tested using a flow rate of 20 mL/min. If diffusion were a limiting factor,

the expectation was that the smaller pellets would increase the bulk surface area and

decrease the distance a contaminant must travel inside the pellet, leading to a faster

destruction rate. The results presented in Figures 13 and 14 show that the smaller pellets

performed worse than the original size pellets. These data indicate that diffusion is not

the limiting factor controlling the destruction rate. It is possible that changing the size of

the pellets also changed another system parameter, such as the light penetration or the

hydraulic flow through the reactor, which decreased the destruction rate of the system.

Pore Size

Using the optimum flow rate of 20 mL/min and the same 12% TiO2 loading, the

pore size was varied to determine its effect on the adsorption and destruction capabilities

of the system. Figure 15 shows the adsorption curves for three different pore sizes: 70,

118, and 234 angstroms.

The surface area of the gels decreased with increasing pore size. The 234 angstrom

gel, with the lowest surface area, exhibited the worst adsorption, exhausting at 4 bed

volumes. All the curves show exhaustion of the pellets by 8 bed volumes, indicating that

only a small amount of adsorption is occurring with any of the three pore sizes. This is

not unexpected since the surface of the silica gel is hydrophilic, and the contaminants

being adsorbed are hydrophobic. In addition, the adsorption is occurring at a pH of 6,

which is well above silica's isoelectric point of 2-3; therefore, the surface of the silica is

likely to be negatively charged. The main ingredient in the wastewater is soap, which is

an anionic surfactant, and some of the other organic are carboxylic acids, which will also









form anions when dissociated. Charge repulsion could also be a reason for the low

adsorption.


p


--- 3mm x 6mm Pellets -
--4mm x 6mm Pellets -


Bed Volumes
Figure 13. Effect of pellet size on adsorption of the wastewater.


Time (hours)


Figure 14. Effect of pellet size on destruction of the wastewater.


-u-3mm x 6mm Pellets
---4mm x 6mm Pellets





U































Bed Volumes

Figure 15. Effect of pore size on adsorption of the wastewater.


Time (hours)


Figure 16. Effect of pore size on the destruction of the wastewater.


--70 angstroms
--118 angstroms
--234 angstroms
I


-- 70 angstroms
-- -118 angstroms
-.-- 234 angstroms
----- 500ppb









The results of the destruction test are displayed in Figure 16. The 118 angstrom

composite performs better than the composite with a 234 angstrom pore size, which could

be due to the higher adsorption of the 118 angstrom composite as shown in Figure 15.

Although the 70 angstrom composite shows adsorption behavior similar to the 118

angstrom composite, it displays the slowest destruction rate of all three pore sizes. The

70 angstrom pore size could be too small to allow adequate diffusion of the molecules in

and out of the pores. The middle pore size of 118 angstroms shows the fastest

destruction of the three pore sizes tested, possibly due to the best combination of

adsorption and diffusion, and was chosen as the optimum pore size for future

experiments.

Titanium Dioxide Loading

The TiO2 loading was varied while keeping the chosen flow rate and pore size

constant. TiO2 loadings of 0, 3, 12, 24, and 32% (% by weight of TiO2/volume of TEOS)

were tested and compared (Figures 17 and 18). A clear trend is not observed in the

adsorption curves of pellets with various TiO2 loadings. The pellets show similar

adsorption, with only the 24% TiO2 loading exhibiting a slight decrease in adsorption,

exhausting about 15 minutes before the other pellets. The destruction curves presented in

Figure 18 show the best performance with a TiO2 loading of 12%. The test of the

composite with no TiO2, only SiO2, serves as a control, showing the destruction that

occurs in the system through photolysis alone. The curve shows that approximately 20-

30% of the TOC is able to be removed without the addition of TiO2 to the system.

The 12% TiO2 is significantly better than the 3% loading, showing that more

catalyst is needed in the system for faster destruction; however, further increase past 12%

TiO2 does not greatly improve the destruction rate of the system. The 24% loading




























0 10 20 30 40 50
Time (minutes)
Figure 17. Effect of TiO2 loading on adsorption of the wastewater.


60 70


0 5 10 15 20 25
Time (hours)
Figure 18. Effect of TiO2 loading on destruction of the wastewater.









actually exhibits slower degradation, and the 32% loading decreased the TOC

concentration to 500 ppb about an hour sooner than the 12% loading. It is possible that

there is a limit to the amount of TiO2 that can be effectively dispersed near the surface of

the silica gel, and at higher loadings the TiO2 will agglomerate and some of the TiO2 will

be blocked from the UV light. The 32% TiO2 pellet shows that the addition of more TiO2

can lead to better performance; however, 2.5 times the amount of TiO2 in the 12% TiO2

composite was required to decrease the destruction time by approximately 20%. Because

this level of improvement in the destruction rate does not justify such a large increase in

TiO2 loading, the 12% TiO2 loading was chosen as the optimum TiO2 loading.

Activated Carbon Loading

Activated carbon was added to the SiO2-TiO2 pellets to explore its effect on the

performance of the system. It was expected that the adsorption of the organic

contaminants would increase, but it was difficult to predict the effects on the system's

destruction abilities. The increased adsorption could enhance the destruction rate, but the

activated carbon could also have a negative impact on the system by blocking the UV

light needed to activate the catalyst. In order to find the optimum activated carbon

loading that increased adsorption without blocking too much UV light, several different

activated carbon loadings were tested at two different TiO2 loadings of 12% and 24%.

The increase in adsorption with the addition of activated carbon to pellets of 24% TiO2 is

illustrated in Figure 19. The time needed to exhaust the pellets doubles with even the

lowest activated carbon loading, 1.3wt%.

The destruction performance of the pellets containing activated carbon is compared

to pellets without any carbon in Figure 20. It is interesting to find that in the two lowest

carbon loadings, 1.3wt% and 2.6wt%, the carbon addition had no impact on the time









needed to reach the 500 ppb TOC limit. At the higher loading of 3.9wt%, the carbon

begins to negatively impact the destruction rate, most likely due to blocking of UV light

needed to activate the catalyst.

Because practical operation of the system would involve exhaustion of the carbon

in the dark, destruction of the organic under constant UV, and then repeating, the cycle,

it was important to test the ability of the activated carbon to be used again. For this

reason, an additional adsorption test was conducted after the usual adsorption and

destruction cycle. The second adsorption cycle showed complete regeneration for the

1.3% and 3.9% activated carbon loadings (Figures 21 and 22). Regeneration was not

tested for the 2.6% carbon loading.

The effect of activated carbon addition was also tested with a 12% TiO2 loading. A

higher carbon loading of 9.6wt% was explored, and the results are shown in Figures 23

and 24. It is clear that the 9.6wt% greatly increases the adsorption of the organic

contaminants. After more than 6 hours, the system was not yet exhausted. The

destruction test shows that the carbon does inhibit the destruction rate, though the

concentration still reaches the necessary 500 ppb level.

Because the carbon was exhausted only to a Ce/Co ratio of 0.6, it is possible that the

some of the removal shown in Figure 24 was due to adsorption. A second adsorption

experiment was conducted after the destruction cycle to determine if the carbon had been

regenerated. Figure 25 shows complete regeneration of the carbon. The regeneration

displayed in Figure 25 indicates that destruction was also occurring, and enough of the

organic contaminants were destroyed to clean the surface of the activated carbon.































200


Time (min)


Figure 19. Effect of activated carbon addition to the adsorption properties of the pellets.


10 15 20 25 30


Time (hours)

Figure 20. Effect of activated carbon loading on the destruction performance of the
pellets.


- 24% Ti02, No C
- 24% Ti02, 1.3wt% AC
-*-24% T102, 2.6wt% AC
-x-24% T02, 3.9wt% AC


--A 24% Ti02, No C
-*-24% Ti02, 1.3wt% AC
-24% Ti02, 2.6wt% AC
x-24% Ti02, 3.9wt% AC
500 ppb





















0.4 Ads
----- Ads 1

0.2 -- Ads2
0.2

0.0
0 50 100 150 200
Time (min)


Figure 21. Comparison of the first adsorption cycle (adsl) to the adsorption of the pellets
after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118
angstroms, and 1.3wt% activated carbon loading.


50 100 150 200
Time (min)


Figure 22. Comparison of the first adsorption cycle (adsl) to the adsorption fo the pellets
after the destruction cycle (ads2), using pellets of 24% TiO2 loading, 118
angstroms, and 3.9wt% activated carbon loading.









Similar to the trend in the 3.9wtO carbon loading in Figure 20, the destruction

curve for the 9.6wt% carbon shows an increase in the TOC concentration to a Ce/Co ratio

of 0.3 just before decreasing to 500 ppb. It is possible that this phenomenon seen in the

destruction tests of the highest activated carbon loadings could be due to desorption of

the organic from the carbon. Because the destruction cycle is performed following

exhaustion of the pellets, the pellets with higher activated carbon loadings have more

organic contaminants contained in the pores at the start of the destruction test. It is

possible that as the TOC concentration of the contaminants in solution decreases to a

certain level, some of the contaminants held within the pores desorb into solution before

becoming completely mineralized.

The advantage of the activated carbon addition is the increase in the adsorption

cycle with the ability to regenerate the carbon surface, meaning that the system could be

operated in repeated cycles of adsorption and destruction. The increased adsorption

properties allow longer periods in the absence of UV light, which decreases the energy

requirements of the system. When considering this type of system operation, the 2.6wt%

exhibits the most potential compared to the other activated carbon loadings that were

tested. Although the 3.9% and 9.6% loadings were shown to require the longest times to

reach exhaustion, creating longer dark periods, the destruction periods were lengthened

by more then double, increasing the time requiring the presence of UV light by about 15

hours. The destruction period was the same for the composites containing zero, 1.3%,

and 2.6% activated carbon loading. Of these three composite compositions, the

composite containing 2.6% carbon is the best choice because it exhibited the longest

period of adsorption.



























0 100 200 300


Time (min)

Figure 23. Effect of activated carbon loading on the adsorption properties of the pellets.


S--12% 1102, No AC
-u-12% 1102, 9.6wt% AC
500 ppb










0 5 10 15 20 25 30


Time (hours)

Figure 24. Effect of activated carbon loading on the destruction performance of the
pellets.













1.0
0.9 --Ads 1
0.8 Ads2
0.7
0 0.6 -
S0.5
0.4
0.3
0.2
0.1
0.0
0 100 200 300 400
Time (min)


Figure 25. Comparison of the first adsorption cycle (adsl) to the adsorption of the pellets
after the destruction cycle (ads2), using pellets of 12% TiO2 loading, 118
angstroms, and 9.6wt% activated carbon loading.

Kinetics

The rate constants for the destruction performance at each set of system conditions

were calculated using LH kinetics. It was assumed that the concentration was low

enough to reduce the LH equation to a first order reaction, although no experiments were

performed to verify first order behavior. Using the number of CSTRs in series, n,

determined through the tracer analysis to be 5, and assuming a first order reaction, the

equation for the effluent concentration at any particular time, t, is described by the

following equation:


Ce = Co 1 + Eq. 9
n

where k is the rate constant. In this discussion, the k values that are calculated

describe the destruction behavior of the photocatalytic system and represent the product









of the rate constant and the equilibrium adsorption constant, or the kK term shown in the

reduced LH equation (Eq. 3).

The time each plug of wastewater spent in the reactor was estimated for use in Eq.

9. This time was calculated from the time at which the destruction curve crossed the 500

ppb threshold. The k constants resulting from these calculations are shown in Table 5.

The rate constants quantify the optimum choice for each system property: 20 ml/min flow

rate, 118 angstrom pore size, and 12% TiO2 loading. For the TiO2 loading, the 32%

loading actually provides the highest destruction rate, but it is only slightly higher than

the 12% loading. Because the 32% loading requires 2.5 times the weight of TiO2 in the

system to reach the higher destruction rate, the 12% TiO2 loading is considered optimum.

The rate constants for the composites containing activated carbon are also

displayed in Table 5. Because the composites containing activated carbon have a higher

adsorption capacity than composites without activated carbon, more organic carbon will

be held in the pores of the composite at the of the adsorption period. This increase in the

amount of organic carbon contained within the composite signifies a higher quantity of

TOC in the system at the start of the destruction period. It is important to consider the

additional organic carbon that was photocatalytically degraded as well as the longer dark

adsorption periods when comparing the kinetics of the composites containing activated

carbon. The 2.6% activated carbon composite shows the same rate constant as the

composite with a lower activated carbon loading, but it would allow for a longer dark

adsorption period; therefore, the 2.6% activated carbon loading is considered the best

choice out of the activated carbon loadings that were tested.









Table 5. Rate Constants for TOC Mineralization to 500 ppb
Flow Rate (mL/min) Avg. Time in Reactor k (min-1)
(min)
5 128.8 0.017
20 56.4 0.038
30 143.4 0.015
Pore Size angstromss)
60 133.5 0.016
140 56.4 0.038
320 59.3 0.036
Titanium Dioxide Loading (%)
3 >148.3 <0.014
12 56.4 0.038
24 62.3 0.034
32 47.5 0.045
Activated Carbon Loading (%)
1.3% (24% TiO2) 62.3 0.034
2.6% (24% TiO2) 83.1 0.034
3.9% (24% TiO2) 148.3 0.014
9.6% (12% TiO2) 148.3 0.014














CHAPTER 5
SUMMARY AND CONCLUSION

Si02-TiO2 composites were tested for their ability to photocatalytically degrade the

organic contaminants in gray water. The composites were tested in a packed-bed reactor,

and the system was optimized with respect to flow rate, TiO2 loading, and pore size of the

Si02-TiO2 composite. Activated carbon was also added to the composites and the effect

on the performance of the system was investigated.

The Si02-TiO2 composites were successful in photocatalytically degrading the gray

water influent in a recirculating system to a TOC concentration below the 500 ppb

maximum limit. The optimum pore size and TiO2 loading were found to be 118

angstroms and 12% (wt/vol), respectively. The system performed best when operated at

a flow rate of 20 mL/min. Activated carbon was found to be capable of improving the

overall system performance through increased adsorption, which decreases the energy

requirements of the system.

It was hypothesized that increasing the adsorption of the SiO2-TiO2 composites

would increase the destruction rate of the system. Adsorption was shown to be an

important system parameter, but increasing adsorption did not always lead to a better

destruction rate. The performances of the 234 angstrom composite and the 30 mL/min

flow rate condition illustrate situations where adsorption became a limiting factor and the

decreased adsorption led to slower destruction. However, adsorption was not always the

limiting factor. Adsorption was increased by lowering the flow rate to 5 mL/min and by

adding activated carbon. In each of these cases, other factors, such as UV light exposure,






50


were affected and the destruction rate did not improve. Adsorption is one of several

factors that must be considered in order to optimize the system performance.
















APPENDIX
SILICA-TITANIA COMPOSITE CHARACTERIZATION

The BET surface area and pore volume were measured for each composite that was

tested. The average pore size was calculated from the following equation (Powers,

1998):


R = 2Vp/SA


where r is the pore radius, Vp is the specific pore volume and SA is the specific surface

area. The measurements for each composite are listed in Table A-1.

Table A-i. Surface Area, Pore Volume, and Pore Size of Composites Tested
BET Pore Planned Pore Calculated
TiO2 Carbon SSA Volume Size Pore Size
Test Loading Loading (m2/g) (cm3/g) angstromss) angstromss)
Flow Rate
5 mL/min 12% None 274.5 0.854 140 124
20 mL/min, Test A 12% None 292.0 0.714 140 98
20 mL/min, Test B 12% None 259.9 0.871 140 134
Small Pellets, Test A 12% None 274.0 0.828 140 121
Small Pellets, Test B 12% None 309.9 0.871 140 112

Average 282.1 0.828 118
Max 309.9 0.871 134
Min 259.9 0.714 98
Pore Size
60 ang 12% None 393.6 0.690 60 70
320 ang 12% None 136.7 0.800 320 234
Ti02 Loading
24% 24% None 263.7 0.770 140 117
32% 32% None 275.1 0.640 140 93
Activated Carbon
Loading
1.3% 24% 1.3% 256.9 0.682 140 106
2.6% 24% 2.6% 285.0 0.865 140 121
3.9% 24% 3.9% 241.6 0.649 140 107
9.6% 12% 9.6% 317.0 0.809 140 102















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BIOGRAPHICAL SKETCH

Christina Yvette TerMaath was born in Lafayette, Indiana, on January 7, 1977. Her

family spent a few years in Lafayette and Panama City, Florida, before relocating to

Springfield, Virginia, where Christina graduated from West Springfield High School in

1995. She pursued a Bachelor of Science degree in Chemical Engineering from the

Pennsylvania State University with a focus on energy and fuels. Her interest in

environmental engineering was realized as she studied the pollution and control of

emissions from the use of fossil fuels. Upon graduation in December 1999, Christina

began an eight month internship with the US Environmental Protection Agency in

Washington, DC, where her interest in environmental engineering grew. After two years

at an energy consulting company in Washington, DC, where she learned about hydrogen

fuel technologies through her work with the Department of Energy's Hydrogen Program,

Christina decided to pursue graduate school in the Environmental Engineering Sciences

Department at the University of Florida.