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Photochemical Reaction Mechanisms of Aqueous Mercury for Application of Removal Technologies

Permanent Link: http://ufdc.ufl.edu/UFE0045228/00001

Material Information

Title: Photochemical Reaction Mechanisms of Aqueous Mercury for Application of Removal Technologies
Physical Description: 1 online resource (103 p.)
Language: english
Creator: Borello, Amy M
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

Subjects

Subjects / Keywords: aqueous -- fulvic -- humic -- mercury -- photochemistry -- purge -- reduction -- removal -- volitilization
Environmental Engineering Sciences -- Dissertations, Academic -- UF
Genre: Environmental Engineering Sciences thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Mercury pollution is an issue of global concern primarily due to its ability to be naturally converted to methylmercury, a highly toxic compound that bioaccumulates in the food chain. It’s mobility and cycling in the natural environment is significantly dependent upon speciation. This research focuses on the photochemical reactions that affect speciation in order to better understand natural cycling as well as aid in the development of future water treatment technologies. Photochemical transformations of mercury were studied in synthetic waters to better understand the intricate species changes of mercury. Solutions of aqueous mercuric nitrate, at varying concentrations, and deionized water were exposed to ultraviolet light to promote mercury reduction while a purge gas is used as a driving force to release the volatile mercury from solution. With a nitrogen purge in the presence of 254 nm irradiation, mercury removal reached over 99 % removal. Even in solutions abundant with oxygen, mercury removal reached 90%after 60 minutes.  Since dissolved organic matter, such as humic and fulvic acid, is ubiquitous in natural waters,it is important to understand how it affects the photochemical mechanisms of mercury. With the addition of humic acid, the overall removal of Hg decreased due to increased complexation with the humic acid; however, the ratio of Hg concentration to humic acid did not affect overall removal.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Amy M Borello.
Thesis: Thesis (Ph.D.)--University of Florida, 2013.
Local: Adviser: Mazyck, David W.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045228:00001

Permanent Link: http://ufdc.ufl.edu/UFE0045228/00001

Material Information

Title: Photochemical Reaction Mechanisms of Aqueous Mercury for Application of Removal Technologies
Physical Description: 1 online resource (103 p.)
Language: english
Creator: Borello, Amy M
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2013

Subjects

Subjects / Keywords: aqueous -- fulvic -- humic -- mercury -- photochemistry -- purge -- reduction -- removal -- volitilization
Environmental Engineering Sciences -- Dissertations, Academic -- UF
Genre: Environmental Engineering Sciences thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Mercury pollution is an issue of global concern primarily due to its ability to be naturally converted to methylmercury, a highly toxic compound that bioaccumulates in the food chain. It’s mobility and cycling in the natural environment is significantly dependent upon speciation. This research focuses on the photochemical reactions that affect speciation in order to better understand natural cycling as well as aid in the development of future water treatment technologies. Photochemical transformations of mercury were studied in synthetic waters to better understand the intricate species changes of mercury. Solutions of aqueous mercuric nitrate, at varying concentrations, and deionized water were exposed to ultraviolet light to promote mercury reduction while a purge gas is used as a driving force to release the volatile mercury from solution. With a nitrogen purge in the presence of 254 nm irradiation, mercury removal reached over 99 % removal. Even in solutions abundant with oxygen, mercury removal reached 90%after 60 minutes.  Since dissolved organic matter, such as humic and fulvic acid, is ubiquitous in natural waters,it is important to understand how it affects the photochemical mechanisms of mercury. With the addition of humic acid, the overall removal of Hg decreased due to increased complexation with the humic acid; however, the ratio of Hg concentration to humic acid did not affect overall removal.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Amy M Borello.
Thesis: Thesis (Ph.D.)--University of Florida, 2013.
Local: Adviser: Mazyck, David W.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2013
System ID: UFE0045228:00001


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1 PHOTOCHEMICAL REACTION MECHANISMS OF AQUEOUS MERCURY FOR APPLICATION OF REMOVAL TECHNOLOGIES By AMY M BORELLO A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013

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2 2013 Amy M Borello

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3 To my parents for their unparalleled support

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4 ACKNOWLEDGMENTS I would like to thank my advisor, Dr David Mazyck for giving me this magnificent opportunity to follow my passion. I have not only grown as a researcher under his guidance but also as a person. faith in me. I am also thankful for my advisory committee Dr. Paul Chadik, Dr. Jean Claude Bonzongo, and Dr. James Jawitz, and the guidance they provided me I am not only honored to have them on my committee for their intellect but for who they are as individuals I am grateful to have a wonderful research group and co workers who have helped me along the way in a laboratory setting as well as saving my sanity. The board game breaks were not only educational I especially would like to thank Christine Valcarce, Erica Borges Wallace Gonzaga Sanaa Jaman, Christina Griggs, Natalia Hoogesteijn, Alec Gruss, Taccara Williams, Emily Faulconer, Heather Byrne, Dave Baun, and An n a C asasus for their support and guidance This process was even more rewarding having my fianc, Alec Gruss, there for every step of my journey even through all the dropped Skype calls done it without his advice and support and am so grateful I always had someone to be there for me. Lastly, I am especially thankful to my family put up with my analytical equipment sob stories for the past few years, because there were plenty of them. I am so lucky to have a support group that will review my poster presentations even ot put into words how grateful I am to my parents, Dan and Mary Borello, and how they have supported

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5 me throughout the years. I strive to be like them and am fortunate to have them as my family. I would like to acknowledge the National Science foundation for their support of this dissertation. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 0802270

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 14 Problem Statement ................................ ................................ ................................ 14 Hypothesis ................................ ................................ ................................ .............. 15 Objectives ................................ ................................ ................................ ............... 16 2 LITERATURE REVIEW ................................ ................................ .......................... 18 Mercury Pollution ................................ ................................ ................................ .... 18 Types of Mercury Emissions ................................ ................................ ............ 18 Natural ................................ ................................ ................................ ....... 18 Anthropogenic ................................ ................................ ............................ 19 Demographics o f Mercury Pollution ................................ ................................ .. 20 Industries ................................ ................................ ................................ ... 20 Geography ................................ ................................ ................................ 21 Atmospheric Transport ................................ ................................ ............... 21 Regulations ................................ ................................ ................................ ............. 22 Health Impacts o f Mercury ................................ ................................ ................ 22 Entry ................................ ................................ ................................ .......... 22 Exposure ................................ ................................ ................................ .... 23 Effects ................................ ................................ ................................ ........ 23 Rulings and Acts ................................ ................................ .............................. 26 General Chemistry ................................ ................................ ................................ .. 27 Photochemistry of Aqueous Mercury ................................ ................................ ...... 28 Aqueous Mercury Photochemistry in the Presence of Dissolved Organic Matter ... 29 3 EXPERIMENTAL ................................ ................................ ................................ .... 41 Chemicals ................................ ................................ ................................ ............... 41 Batch Experiments ................................ ................................ ................................ .. 42 UV Irradiation ................................ ................................ ................................ ... 42 Gas Purge ................................ ................................ ................................ ........ 43 Mercury Collection and Analysis ................................ ................................ ............. 43 Humic Acid Analysis ................................ ................................ ............................... 44

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7 4 PHOTOCHEMICAL MECHANISMS OF AQUEOUS MERCURY REMOVAL IN SYNTHETIC SOLUTIONS ................................ ................................ ...................... 47 Background ................................ ................................ ................................ ............. 47 Control Studies ................................ ................................ ................................ ....... 48 Nitrogen Purge ................................ ................................ ................................ ........ 49 254 nm ................................ ................................ ................................ ............. 49 365 nm ................................ ................................ ................................ ............. 49 Air Purge ................................ ................................ ................................ ................. 50 25 4 nm ................................ ................................ ................................ ............. 50 365 nm ................................ ................................ ................................ ............. 51 Oxygen Purge ................................ ................................ ................................ ......... 52 High Hg Concentr ations ................................ ................................ ................... 52 Low Hg Concentrations ................................ ................................ .................... 52 5 EFFECT OF DISSOLVED ORGANIC MATTER CONCENTRATIONS ON PHOTOCHEMICAL AQUEOUS MERCURY REMOVAL ................................ ........ 57 Ba ckground ................................ ................................ ................................ ............. 57 Control Studies ................................ ................................ ................................ ....... 58 Mercury Volatilization as a Function of Nitrogen Purge Gas and Increasing Humic Acid ................................ ................................ ................................ .......... 59 Impact of Hg:DOM Ratio on Hg Volatilization Induced by Nitrogen Purge .............. 61 Effect of Purge Gas on Hg and HA Removal ................................ .......................... 62 6 EFFECT OF VARIOUS TYPES OF ORGANIC MATTER ON PHOTOCHEMICAL AQUEOUS MERCURY REMOVAL ................................ ........ 69 Background ................................ ................................ ................................ ............. 69 Mercury Volatilization as a Function of Organic Matter Concentrations Induced by Nitrogen Purge ................................ ................................ ................................ 70 The Effect Fulvic Acid Reduction on Dissolved Gaseous Mercury Removal .......... 72 The Effect of Purge Gas Type ................................ ................................ .......... 72 The Effect of UV Wavelength ................................ ................................ ........... 73 7 CONCLUSIONS ................................ ................................ ................................ ..... 79 Contributions to Science ................................ ................................ ......................... 79 Futu re Research Avenues ................................ ................................ ...................... 80 APPENDIX: SUPPLEMENTAL INFORMATION ................................ ........................... 82 UV Calculations ................................ ................................ ................................ ...... 82 Photon Energy ................................ ................................ ................................ .. 82 Intensity ................................ ................................ ................................ ............ 82 Comparison to Sunlight ................................ ................................ .................... 83 Effect of Dissolved Organic Matter ................................ ................................ ... 84 Absorption ................................ ................................ ................................ .. 84

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8 Effect of Wavelength & pH ................................ ................................ ......... 85 Ef fects of UV Intensity ................................ ................................ ................ 85 Extended Run Times ................................ ................................ .............................. 86 LIST OF REFERENCES ................................ ................................ ............................... 91 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 103

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9 LIST OF TABLES Table page 2 1 Mercury removal efficiency (%) for some techno logies and different categories ................................ ................................ ................................ ........... 40 4 1 Control studies for Hg concentrations of 50 ppb with run times of 60 minutes ... 56 4 2 Reaction rate kine tics for various Hg concentrations with nitrogen purge .......... 56 5 1 Control studies for Hg Concentrations of 100 ppb with run times of 60 minutes ................................ ................................ ................................ ............... 67 5 2 P values generated from factorial ANOVA modeling for various concentrations under 254 nm irradiation and a nitro gen purge .......................... 67 5 3 Reduction of HA after 60 minutes under various experimental conditions as measured by TOC removal ................................ ................................ ................. 68 6 1 Elemental analysis in percent of Suwann ee River Humic and Fulvic acids ........ 78 A 1 Variables used to calculate final intensity for various humic acid concentrations irradiated with a 254 nm lamp ................................ .................... 90 A 2 Variables used to calculate final intensity for various humic acid concentrations irradiated with a 365 nm lamp ................................ .................... 90

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10 LIST OF FIGURES Figure page 2 1 Proportions of global anthropogenic sour ces of Hg based on industry ............... 34 2 2 Schematic of mercury cycle in the environment ................................ ................. 34 2 3 Breakdown of industrial Hg emissions by region ................................ ................ 35 2 4 Change of global a nthropogenic emissions of total mercury to the atmo sphere from 1990 2000 ................................ ................................ .............. 35 2 5 Model of atmospheric transport of anthropogenic Hg emissions from East Asia ................................ ................................ ................................ .................... 36 2 6 Timeline of mercury use and subsequent regulations in the U.S. ....................... 36 2 7 Merc ury Eh pH diagram for systems containing Hg, O, H, S, and Cl ................. 37 2 8 Distr ibution of Hg(II) at various pH ................................ ................................ ...... 38 2 9 Speciation of 100 ppb Hg in deionized water as prepared from Hg(NO 3 ) 2 ......... 39 3 1 Batch reactor set up schematic ................................ ................................ .......... 45 3 2 Spectrum o f 254nm lamp ................................ ................................ ................... 45 3 3 Spectrum of 365 nm lamp ................................ ................................ .................. 46 4 1 Aqueous Hg removal of various Hg concentrations versus time in the presence of N 2 and 254 nm UV ................................ ................................ .......... 53 4 2 Aqueous Hg removal of various Hg concentrations versus time in the presence of N 2 and 365 nm UV ................................ ................................ .......... 53 4 3 Aqueous Hg removal of various Hg concentrations versus time in the presence of air and 254 nm UV ................................ ................................ .......... 54 4 4 Aqueous Hg removal of various Hg concentrations versus time in the presence of air and 365 nm UV ................................ ................................ .......... 54 4 5 Aqueous Hg removal of various Hg concentrations versus time in the presence of O 2 and 254 nm UV ................................ ................................ .......... 55 4 6 Aqueous Hg removal of various Hg concentrations versus time in the presence of O 2 and 365nm UV ................................ ................................ ........... 55

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11 5 1 Comparison of Hg C/C 0 verses time in the presence of various concentrations of HA and 100 ppb Hg with 254 nm UV and nitrogen purge. ...... 65 5 2 Comparison of Hg C/C 0 verses time in the presence of nitrogen purge and 254 nm UV with varying concentrations of Hg and HA for comparison of 100:1 and 10:1 Hg:DOM ratios ................................ ................................ ........... 65 5 3 Comparison of Hg C/C 0 versus time in the presence of Air and oxygen purges and 254 nm UV with Hg concentrations of 100 ppb and varying HA ...... 66 6 1 Comparison of Hg C/C 0 verses time in the presence of nitrogen purge and 254 nm UV with varying concentrations of Hg and FA for comparison of 100:1 and 10:1 Hg:DOM ratios ................................ ................................ ........... 75 6 2 Comparison of Hg C/C 0 versus time in the presence of a nitrogen purge and 365 nm UV with varying concentrations of Hg and FA for comparison of 100:1 and 10:1 Hg:DOM ratios ................................ ................................ ........... 75 6 3 Comparison of Hg C/C 0 versus time of FA and HA in the presence of a nitrogen purge ................................ ................................ ................................ .... 76 6 4 Comparison of Hg C/C 0 versus time in the presence of oxygen or air purge and 365 nm UV with 10 ppb Hg and 1 ppm FA ................................ .................. 77 6 5 Comparison of Hg C/C 0 versus time of 254 nm and 365 nm in the presence of an air purge with 10 ppb Hg, 1 ppm FA ................................ .......................... 77 A 1 Solar spectrum as a function of wavelength ................................ ....................... 87 A 2 Molar absorptivity as a function of wavelength for various humic substances (SRHA = Suwannee River humic acid; SRF A = Suwannee River fulvic acid) .... 87 A 3 UV intensity as a function of Suwannee River humic acid absorbance when irradiated by a 254 nm bulb ................................ ................................ ................ 88 A 4 UV intensity as a function of Suwannee River humic acid absorbance when irradiated by a 365 nm bulb ................................ ................................ ................ 88 A 5 Aqueous Hg removal of 1000 ppb Hg versus time in the presence of a nitrogen purge and 254 nm UV ................................ ................................ ........... 89 A 6 Aqueous Hg removal of 1000 ppb Hg versus time in the presence of a nitrogen purge and 254 nm UV ................................ ................................ ........... 89

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12 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PHOTOCHEMICAL REACTION MECHANISMS OF AQUEOUS MERCURY FOR APPLICATION OF REMOVAL TECHNOLOGIES By Amy M Borello May 2013 Chair: David Mazyck Major: Environmental Engineering Sciences Mercury pollution is an issue of global concern primarily due to its ability to be naturally converted to methylmercury, a highly toxic compound that bioaccumulates in dependent upon speciation. This research focuses on the photochemical reactions that affect spe ciation in order to better understand natural cycling as well as aid in the development of future water treatment technologies. Photochemical transformations of mercury were studied in synthetic waters to better understand the intricate species changes of mercury. Solutions of aqueous mercuric nitrate, at varying concentrations, and deionized water were exposed to ultraviolet light to promote mercury reduction while a purge gas is used as a driving force to release the volatile mercury from solution. With a nitrogen purge in the presence of 254 nm irradiation, mercury removal reached over 99% removal. Even in oxygen saturated aqueous solutions, mercury removal reached 90% after 60 minutes. Since dissolved organic matter, such as humic and fulvic acid s is u biquitous in natural waters, it is important to understand how it affects the photochemical mechanisms of mercury. With the addition of humic acid, the overall removal of Hg decreased due to increased complexation with the humic acid;

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13 however, the concentr ation ratio of Hg to humic or fulvic acid s did not affect the overall mercury removal.

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14 CHAPTER 1 INTRODUCTION Problem Statement Lewis Carroll 1 2 The First Emperor of China, Qin Shinhuang, was especially obsessed with this metal. Thought to make him immor tal, his daily drink of Hg eventually cost him his life. Unknown to him the cause of his death, he wished to surround himself for all eternity with this mysterious metal by creating shimmering rivers, streams, and oceans of liquid mercury that represented the rivers and seas of China. Over 2000 years later in 1987, geologists measured soil samples above the site that contained Hg concentrations as high as 1 .5 ppm 3 effects were discovered. Industrial waste released into Minamata Bay contained Hg and was naturally converted into the highly toxic form of methylmercury (MeHg). Bioaccumulat protein for the villagers. MeHg enters the brain by mimicking the amino acid methionine 4 preventing protein synthesis and causing widespread neurological damage, as well as death, afte r consumption 5 also known as Minamata Disease 6 8 Recent research shows Hg contamination is a rising international problem. Currentl y 10% of childbearing age women in the U.S. have Hg concentrations above the level considered by the United States Envir onmental Protection Agency (EPA) to be safe to fetuses and young children 9 appeal endures. Mercury is still used today in industrial settings for fl u orescent light bulb

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15 product ion, gold production, chlor alkali facilities, as well as others 10 It is critical that Hg waste streams be reduced to protect human health and the environment. Currently, laboratory studies in prepared aqueous mercury solutions have not been well explored and are typically performed to clarify natural Hg cycling 11 However, this principle can be applied to treatment technologies for Hg contaminated waters. This research uses ultraviolet (UV) light to promote mercury reduction in aqueous solution while pr oviding a driving force (gas purge) to release the volatile mercury from solution and subsequently remove it via gas phase adsorption. This study focuses o n photochemical removal of Hg from synthetic solutions in order to better understand the reduction m echanisms that are occurring. This method is preferable because direct ad sorption of Hg in the aqueous phase is difficult because of competing contaminants. Hypothesis Dissolved gaseous mercury (DGM) is a highly volatile form of Hg that can be removed from aqueous solutions via gas exchanged. It is believed that DGM production is a function of UV wavelength, initial Hg concentration, purge gas type, and contact time when spiking deionized water with Hg(NO 3 ) 2 However, it is unclear as to how these variables affect redox reactions and radical chemistry. Therefore, it is hypothesized that DGM production is a function of competing oxidation and reduction reactions and that Hg is reduced instantaneously via UV photons, but can be re oxidized by secondary reactio ns that are a function of radical chemistry associated with superoxides and nitrate radicals. It is also hypothesized that when dissolved organic matter (DOM) is introduced into a solution of deionized water and Hg(NO 3 ) 2 the DGM production is affected by

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16 complexation of Hg to the DOM and decreasing the overall Hg removal. It is also believed that purge gas type, UV wavelength, and ratio of DOM and Hg concentrations will affect these complex solutions as well. Lastly, it is hypothesized that various types of organic matter will affect the DGM production and, thusly, overall removal of Hg. It is expected that organic matter high in sulfur sites will bind with Hg more readily and cause less Hg removal than DOM containing low amounts of reduced sulfur sites du e to strong binding constants of Hg and sulfur Objectives This research aims to simplify the aqueous Hg matrix by preparing controlled, synthetic solutions of typical concentrations of Hg in order to understand the core concepts of photoche mical mechanisms of aqueous Hg volatilization and subsequent removal. Comprehension of Hg photoreduction mechanisms will provide a scientific basis for the development of water treatment technologies for compliance with projected worldwide regulations as well as insight to Hg cycling in the natural environment This research will clarify key mechanisms and provide insight in order to optimize Hg removal from the aqueous phase and advance scientific knowledge. Major objectives to be investigated include: Studying the impact that UV wavelength, dissolved oxygen concentration, and Hg concentration have on total DGM production. Conduct experiments under various contact times to determine removal rates of Hg. Identify key mechanisms of photochemical DGM production from aqueous Hg. How in creasing concentrations of DOM a ffect the DGM production and subsequent removal.

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17 Studying the total organic carbon (TOC) concentration in DOM experiments with UV illumination The effect that humic acid and fulvic acid sulfur sites have on Hg removal and its correlation to DGM production.

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18 CHAPTER 2 LITERATURE REVIEW Mercury Pollution It is vital to study mercury (Hg) removal and treatment since laws regulating mercury are on the rise The United Nations is currently negotiating a global Hg treaty requiring reduced Hg production 12,13 Furthermore, the Energy Information Administration estimates world coal consumption to increase by 50% to 2035 14 Coal is cheap, readily available, and contains Hg ; therefore, this research is imper ative to understand how to treat the wastewaters generated from coal fired power plants in order to reduce Hg cycling in our environment Types of Mercury Emissions Natural The most common naturally occurring Hg mineral is cinnabar (HgS) 15 Cinnabar is mined to produce Hg for industrial applications such as electrical equipment, fungicides, and has even been used as tattoo ink due to its red color 16 While cinnabar is has low solubility the presence of iron and humic substances enhance cinnabar solubil ity and affect mobilization and bioavailability 17 Natural sources of Hg emissions include weathering of rocks, volcanoes, and geothermal activity to name a few 18 A proportion of mercury emissions also may occur from forest fires that re emit anthropoge nic Hg 19 21 These natural pathways account for one third of Hg emissions to the atmosphere 22 Typically, elemental mercury (Hg 0 ) is found in the atmosphere, but oxidized forms either in the gas phase or attached to particles are also present 23 Hg salts HgCl 2 HgS, and HgO are also natur ally occurring. While they can cause gastrointestinal tract damage

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19 and kidney failure they are not common forms of nat urally occurring Hg 24 and only HgCl 2 is volatile enough to exist as an atmospheric gas 22 Anthropog enic Burning of f ossil fuels, primarily coal, is the largest so urce of anthropogenic emissions accounting for 45% of total anthropogenic emissions 22 Othe r anthropogenic mercury sources include metal and chemical manufacturing, caustic soda production, in cineration of municipal and medical waste, and cement manufacturing 25 It is common to treat Hg emissions by using best available control technologies (BACT) instead of enforcing final Hg emission concentrations. A list of the best available control techn ologies for treating Hg emissions from these industries can be seen in Table 2 1. This research focuses on treating industrial wastewaters from anthropogenic sources Therefore, it is vital to look at various types of contaminated wastewaters and their average mercury concentration. Common types of Hg laden wastewaters include chlor alkali wastewater and flue gas desulfurization (FGD) wastewater. Usin g chlorine electr olytic cells chlor alkali facil ities use mercury as a cathode to produce chlorine and caustic soda 26 29 von Canstein and collaborators 30 sampled chlor alkali facilities in Europe and measured their wastewater for Hg concentration. Typically, total Hg concentration was around 1.6 ppm with some reaching as high as 7.6 ppm. In the US, chlor alkali plants are required to treat effluent wastewater discharge using the best available technology economically achievable (BAT) in accordance to the Clean Water Ac t 31 This consists of chemical precipitation with sulfide compounds followed by filtration. Typically, this results in final effluent concentrations of 10 50 ppb Hg 32

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20 FGD wet scrubbers are widely used by coal fired power plants in the U.S. to remove sul fur dioxide (SO 2 ) from exhaust gases 33 However, these wet scrubbers have been shown to remove Hg as well 34 Although not currently considered a hazard, it is likely that future regulations will require additional Hg removal. Although little is publ ishe d on Hg concentrations of F G D wastewater since it is not regulated, one report shows that it can be highly concentrated and range from 500 ppb 800 ppb Hg if using water that is recycled in the removal system 35 Demographics of Mercury Pollution Industri es Various industries contribute to global Hg pollution. It is important to examine each of these industries in order to understand how Hg should be regulated. Manufacturing processes that use mercury include chlorine and caustic soda production, cement, b atteries, lamps and bulbs, and gold mining 36 Other industries that emit significant amounts of Hg are power plants waste incinerators, and hospitals, mostly from dental care 22,37 39 Currently, fossil fuel combustion contributes to roughly 45% of all anthropogenic emissions of Hg 22 This occurs when coal which naturally contain s Hg, is burned for electricity and subsequently releases Hg into the air. Figure 2 1 shows that small scale gold production, metal production, and cement production follow fossil fuel combustion in total Hg emissions, respectively. While these sources emit mostly gaseous Hg, the atmospheric lifetime of the toxic element is 1 2 years 40 a nd deposition subsequently occurs ( Figure 2 2 ).

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21 Geography When studying Hg emissions by continents, Asia, North America, and Europe are the three largest emitters 41 Hg emissions from Asia are four times higher than the emissions from North American and E urope combined. Figure 2 3 shows the total amount of industrial Hg emissions separated by various region s of the world. Other reports show that the top three countries are Asia, Africa, and Europe, with North America following a close fourth 36 This discr epancy is likely do to the large increase in mercury emission in Af rica in recent years (Figure 2 4 ). According to the latest United Nations Environmental Programme report on mercury, China is by far the largest producer of Hg emitting over 800 tons annual ly, while India follows second with an annual emission of slightly less than 200 tons 22,25 The United States, Russia, and Indonesia round off the top five Hg emitters in the world, respectively 22 The two industries that produce the most Hg emissions in Asia are f ossil fuel combustion for energy and artisanal gold production 22 laws regulating mercury are needed to protect global human health. Atmospheric Transport Hg is commonly found in the atmosphere as elemental Hg, possibly due to oxidation from bromine, ozone, or hydroxyl radicals 42 After a span of 1 2 years, Hg leaves the atmosphere and is subsequently deposited into the water or soils. Hg has the ability to be re emitted and travel farther and longer than the usual atmospheric lifetime allows. This can result in accumulation in the Polar Regions where re emission is not favorable 22 Using only East Asian anthrop ogenic emission data, Figure 2 5 shows a model of long range transport of Hg. These data show the western part of North America

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22 receiving some deposited Hg from Eastern Asia 22 Therefore, to prevent Hg pollution in the US, action will not only be required in the US, but China as well. Regulations Health Impacts of Mercury With the risk t hat Hg pos es to human health, it is important to regulate this toxic metal. T he following is a risk analysis model that focuses on (a) entry, (b) exposure, and (c) effects 43 of Hg in order to understand how and why it should be regulated Entry Hg enters the environment naturally through volcanoes, weathering of rocks, and geothermal activity that release mineral Hg from the earth. It can also be released through anthropogenic sources from various industries such as coal fired power plants, chlor alkali facilities, and metal manufacturers, among others. Both these and natural sources of Hg emissions are discussed in greater detail above. The reason all types of Hg entry into the environment are hazardous to human health and our environment is beca use of its ab ility to be naturally converted biotically to methylmercury in the water the most toxic form of Hg that bioaccumulate s in the food chain 44 Thus, a modest increase in Hg pollution can result in larger Hg concentrations in fish and other vert ebrates, sometimes thousands of levels higher than the surrounding water 22 With global migration patterns of fish and long range atmospheric transfer (Figure 2 1) this is a rising international problem. The ability of Hg to cycle and be re emitted causes it to be a very dangerous pollutant to the environment and human health. It has the ability to be in the air phase, aqueous phase, and can also be found in sediments. Therefore, although Hg might be

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23 admitted into the atmosphere, it will not remain in the and instead enters into a continuous cycle through various phases Exposure There are various routes of exposure to Hg. It can be inhaled, absorbed through the skin, or ingested. As discussed above, the most toxic form, methylmercury is the most common form of Hg exposure to humans primarily from its dominance in fish, shellfish, and other marine animals 45 which means the most toxic form of Hg is consumed. It is also possible to be exposed to Hg in the gaseous form. Ty pically, this is less toxic than other forms of Hg exposure and is more commonly consumed in low concentrations since it is diluted upon entering the atmosphere. Gaseous Hg is more of an issue in artisanal/small scale gold mining where gold is collected by amalgamation with Hg 22 The amalgam is heated to remove the Hg, which is then released into the atmosphere or the indoor air and inhaled. This is more common in developing countries as seen in Figure 2 3 below. Effects Once Hg enters the body, it moves t hrough the blood stream and enters the brain by mimicking the amino acid methionine 4 This prevents protein synthesis and can lead to widespread neurological damage, as well as death 5 In order to prevent this, the EPA has established an exposure referenc 46 Below this level, adverse health effects of mercury are not expected The se health effects of Hg are dependent on the type of Hg exposure. Organic forms of mercury are an especially toxic version of Hg. It is typically found as a

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24 preservative and antifungal agent in the form of thiomersal for vaccines, cleaning solution for contact lenses, and makeup 47 Currently the federal governm ent allow s eye products to contain 65 ppm Hg 48 Although even at this high concentration, it is believed to only cause sensitivity and allergic reactions if any side effects at all. However, in 2007 Minnesota banned the use of mercury in cosmetics, giving it stric ter stan dards than the rest of the U.S. 49 An organometallic cation of Hg methylmercury, causes the most damage 50 52 As stated previously, e xposure to methylmercury most commonly occurs from eating fish. Since t he amount of mercur y varies from fish to fish, and eating fish is known to be 3 fatty acids, eating fish has been compared to 53 When methylme rcury was first synthesized in the 1 860s, two of the laboratory technicians died from mercury poisoning and production of methylmercury was put on hold until the twentieth century property was discovered that Hg was applied to seeds for cereal crops in the farming industry 54 While Hg aided in the green r evolution, in 1952 the Swedish realized it also caused neurological damage in the birds and small mammals that lived off of the treated grains 54 Methylmercury is especially dangerous to children, infants, and fetuses since its primary effect is impaired neurological development 55 58 Mothers who breast feed and c onsume high quantities of fish contain ing Hg can pass this toxic metal to their childr en 59 61 Studies also show that increased methylmercury exposure correlates to decreased performance of intelligent quotient (IQ) tests 57

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25 experienced t hese devastating effects of methylmercury p oisoning en masse In 1955 many residents experienced neurological defects and in 1959 it was finally linked to Hg exposure 62 Wastewater discharge from the Chisso factory that manufactured acetaldehyde for fertilizers was identified as the culprit affect ing over 2,000 victims of Hg poisoning, labeled Minamata Disease and over 10,000 more received compensation 63 They contracted the disease through the Hg bioaccumulating in fish, a major source of protein in the area. Intake of such high quantities of me thylmercury leads to cerebella atrophy causing vision, language, balance, hearing, and motor function disorders with sever cases (over 1,700 people) leading to death 64,65 Elemental Hg is a less toxic form than organic Hg When inhaled and absorbed in the lungs, it may cause tremors, gingivitis, and headaches if inhaled over a long period of time and in places with little ventilation 24 This is common in developing countries with small scale gold production. If elemental Hg is ingested it is absorbed sl owly and may pass through the digestive system without causing damage 24 Higher exposure levels have more destructive effects such as kidney damage, respiratory failure, and even death 66 Inorganic Hg compounds are also known to cause adverse health effe cts, although it is less common. The major sources of inorganic Hg exposure are dental amalgams occupational exposure, and accidental contact from consumer products such as fluorescent lamps, Hg switches in thermostats, and pilot sensors 67 High exposure to inorganic Hg may result in skin rashes, emotional changes, memory loss, and muscle weakness 66

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26 Rulings and Acts Due to the fact that the disastrous effects of Hg were only just realized in the last 50 years Hg is still in the early stages of being reg ulated (Figure 2 6) In 1974, the Safe Drinking Water Act was passed to protect human health from contaminants in drinking water. For Hg, the maximum contaminant level has been set to 2 ppb because it is believed that this concentration does not adversely affect human health 68 The Environmental Protection Agency is also in charge of regulating Hg under the Clean Water Act, Resource Conservation and Recovery Act, and the Clean Air Act. The Clean Air Act passed in 1990 suggested the regulation of hazardou s air pollutants, including Hg. It did not, however, include coal fired power plants in the list of industries to control Hg emissions. Ten years, the EPA deemed mercury from power facilities worthy of being regulated. In 2005 the EPA under the Bush administration issued the Clean Air Mercury Rule the first time Hg was regulated from coal fired power plants 69 Three years later the D.C. Circuit vacated the new rule. The Clean Air Mercury Rule proposed a cap and trade program that wo uld reduce Hg emissions from 48 tons per year to 38 tons per year and eventually 15 tons; however, the EPA did not list power plants as toxic sources and environmental groups such as the Sierra Club said the regulations were not stringent enough 70 C urren tly, the EPA is working on Mercury and Air Toxics Standards (MATS ) the first national emission standards for power plants. The EPA states that these new standards will prevent 11,000 premature deaths and 130,000 asthma attacks, providing $37 90 billion i n benefits not including the assumed $9.6 billion yearly in compliance costs 71 Compliance is going to be ba sed on maximum achievable control t echnology (MACT) per requirements in the Clean Air Act with a total of 20 tons of Hg emissions cut

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27 by 2016 Wit h wet scrubbers as a MACT, increased wastewater containing Hg is expected in the near future 14 C ompliance is required in the next thr ee years and therefore developing control technologies is vital. Due to the fact that Hg pollution is a global human heal th issue, the United Nations Environment Programme (UNEP) began negotiations for a mercury treaty in 2009 72 After five rounds of negotiations taking four years, over 140 countries signed the first global mercury treaty to limit atmospheric emissions as w many products and processes by 2020 73 The treaty still needs to be ratified and in Oc t ober of this year another meeting will be held and is expected to be put into effect in 3 4 years 73 General Chemistry In order to comply with new regulations, it is important to understand the chemistry of mercury to better understand how to remove it Hg is a complicated element with the term mercurial not only describing it, but deriving from it. Hg has three oxidation states, H g 0 Hg + and Hg 2+ and is found in many forms in the environment. In environmental aqueous system s such as the ocean, chloride greatly influences the speciation of Hg. Hg is also influenced by sulfur, an abundant chemical in the environment due to strong likelihood of complexation Figure 2 7 shows these common Hg species in a Pourbaix diagram illustrating possible states of Hg in aqueous solutions. When Hg is not in the presence of additional constituents Hg speciation is dependent upon the pH in aqueou s solutions. At pH values of 4 and higher, Hg(OH) 2 is the most common speciation (Figure 2 8) Hydrolysis of Hg can be seen in Equations 2 1 and 2 2 below: Hg 2+ + H 2 O Hg(OH) + + H + (2 1)

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28 Hg(OH) + + H 2 O Hg (OH) 2 + H + (2 2 ) The speciation of Hg used in these experiments was Hg(NO 3 ) 2 which is known to ionize in solution to form Hg 2+ and NO 3 74 Figure 2 9 shows the species of Hg in prepared solutions of Hg(NO 3 ) 2 and deionized water. Indeed, Hg(OH) 2 is the dominant species. Photochemistry of Aqueous Merc ury Photochemical reactions have been shown to influence Hg speciation within aqueous solutions and thus alter its bioavailability. Typically, free Hg exists in natural waters as Hg 0 and Hg 2+ 75 Hg 0 is volatile and therefore not as soluble in water as Hg 2 + Field studies have shown that an introduction of photons from solar radiation lead to Hg reduction by the production of volatile dissolved gaseous Hg ( DGM ) 76 78 It has also been shown in natural studies that DGM production increases with a correlated increase in solar irradiation energy 76,79 Photons are therefore believed to cause direct and indirect photoreactions where Hg is reduced. The volatilized aqueous Hg can then be released into the air and captured by commercially available sorbent removal. Because sorbents used for aqueous Hg removal continuously battle competitive adsorption with other constituents within the water, air phase sorbents are a much more viable option for Hg removal 80 82 Common photor eduction reactions reported in literature are listed below 83 : Hg(OH) 2 + hv Hg(OH) 2 Hg(OH) aq + OH (2 2 ) Hg(OH) aq + hv Hg 0 + OH (2 3 ) Hg(OH) aq + H + + e Hg 0 + H 2 O (2 5 ) Oxidation of Hg has also been observed 42,84 86 In natural waters, when chloride ions are added the photooxidation rate increases 85 Photooxidation, as well as other

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29 oxidation reactions, can also occur without the presence of chlorine. These equations are as follows 84,87 : Hg 0 + hv Hg 2+ + 2e (2 6 ) Hg 0 + 2 OH Hg 2+ + 2OH (2 7 ) Hg 0 + OH HgOH (2 8 ) HgOH + OH Hg(OH) 2 (2 9 ) Due to the co occurrence of oxidation and reduction reactions, the production of DGM occurs when Hg reduction reaction rates exceed oxidation reaction rates. Because of t he complexity of this system, mechanisms of DGM production are poorly understood. Aqueous Mercury Photochemistry in the Presence of Dissolved Organic Matter Dissolved organic matter (DOM) is known to form strong complexes with Hg affecting its mobility, speciation, and, thusly, its bioaccumulation 83 Roughly 80% of DOM is composed of humic substances formed from decomposi ng organic matter 88 According to the oxygen 89 humic substances that Hg binds the strongest to are r educed sulfur sites such as sulfide, thiol, and thiophene Xia et al. 90 identified sulfur functional groups of aquatic and soil humic substances using X ray absorption near edge structure (XANES) spectroscopy. The XANES results showed soil and aquatic hum ic substances have 10% and 50%, respectively, of total sulfur in its reduced state, which indicates that aquatic humic substances play a key role in Hg complexation. More recent reports use XANES to identify the binding sites between Hg and DOM 91 It is s een that high concentrations of Hg first bind with reduced sulfur and

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30 subsequently with oxygen containing ligands (ie: phenol and carboxyl groups) due to the low density of reduced sulfur sites available for bonding. Aside from complexing Hg, DOM is also k nown to reduce Hg abiotically to DGM which can subsequently be removed via gas exchange 92 95 Allard & Arsenie 94 spiked synthetic waters with 400 ppb, with and without DOM. While irradiating the samples, they found the production rate of DGM was enhance d by the presence of DOM. Other studies also found that the presence of DOM enhances the production rate of DGM 95,96 Control studies p e r form ed in the dark by Ravichandran 97 with 100 ppt Hg in the presence of DOM produced no measurable a mounts of DGM alt hough data is not shown. When additional inhibitors were added (i.e.: Cl Eu) to solution, there was a significant decrease in rate 94 Because Eu binds with DOM and reduces the number of complexing sites available to Hg it is believed that DGM productio n requires intra molecular electron transfer and, therefore, Hg must first be attached to the DOM before it is reduced. Conflicting results have mostly occurred in complex natural waters 77,79,87,98,99 Amyot et a l. 79 found natural water spiked with 1 8 mg/L humic substances did not have any significant difference in DGM production. A different experiment performed by Amyot and colleagues 100 observed higher DGM production in temperate lakes with low levels of DOM (2.6 ppm ) as compared to those with high levels of DOM (5 5.6 ppm ). Matthiessen 101 and 99 found similar results where the DGM production was limited by the amount of Hg available rather than the DOM concentration. It is possible that high levels of DOM could inhibit Hg photoreduction due to DOM limiting

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31 UV irradiation or from increasing comple xation of Hg DOM. These reports also suggest direct photolysis of DOM: DOM + H 2 O + hv OH + products ( 2 10 ) w here the hydroxyl radical initiates oxidation reactions that lead to the subsequent oxidation of Hg: OH + Hg(0) Hg(II) + OH ( 2 11 ) However, it has also been proposed th at the photolysis of DOM is capable of Hg reduction and ma y depend on the pH. Hydrogen peroxide may originate from UV induced transformation of DOM that can lead to the oxidation or reduction of Hg depending on the number of h ydroxide or hydrogen ions: DOM + hv DOM ( 2 12 ) DOM + O 2 DOM + + O 2 ( 2 13 ) O 2 + Hg(II) Hg(0) + O 2 ( 2 14 ) O 2 + O 2 + H + H 2 O 2 + O 2 ( 2 15 ) H 2 O 2 + 2OH + Hg(II) O 2 + 2H 2 O + Hg(0) ( 2 16 ) H 2 O 2 + 2H + + Hg(0) 2H 2 O + Hg(II) ( 2 17 ) It is important to note that the pH dependence may not be as straight forward as the abundance of hydroxide and hydrogen ions but may also affect the chemistry of the functional groups on the DO M. For example, Alberts et al. 92 measured a high DGM production at lower pH levels rather than high pH levels which conflicts with the equations listed above

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32 Other possibilities for such varying results could be the different types of DOM studied and the ratio of Hg: DOM concentrations. As discussed above, the amount of reduced sulfur sites greatly affect s the binding constants of Hg DOM. The binding constants of Hg DOM in the literature span as low as 10 4.7 to as high as 10 32.2 88 Only recently has the role of the ratio of Hg: DOM conce ntration been recently studied. Using XANES technology, it can be seen that the ratio of Hg S bonds to Hg O bonds increases with d ecreasing concentrations of Hg 102 Haitzer et al. 103 measured a wide range of Hg: DOM ratios and measured the binding strengt h using an equilibrium dialysi s ligand exchange method. At Hg: with 10 ppm DOM), Hg is able to primarily bind to the reduced sulfur groups while ratios ppm DOM) re sulted in smaller binding constants due to there being more Hg than reduced sulfur sites and subsequent binding to weak functional groups containing oxygen, such as carboxyl groups. h lead reduction cartrid ge) were spiked with varying Hg: DOM ratios to measure the v arying rates of DGM production 104 A faster rate was seen when observing high Hg: DOM ratios because the Hg is forced to bind with weak oxygen functional groups due to the s aturated reduced sulfur sites. Because these bo for DGM to form. This rate eventually slowed when only the stronger Hg DOM bonds were left Therefore, the Hg: DOM ratio must be a consideration when observing DGM produ ction. DOM significantly affects the speciation, mobility, and fate of Hg. DOM high in reduced sulfur functional groups are very likely to form strong bonds that can either

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33 reduce DGM production due to its strong complexation with Hg or increase DGM production if intra molecular electron transfer is required. Because of the complex structure of DOM, it is seen to play a significant part in the photoreduction of Hg as well as possible reoxidation. Further studies need to be preformed to better understa nd mercury cycling with the additional potential of improved treatment methods.

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34 Figure 2 1. Proportions of global anthropogenic sources of Hg based on industry 22 Figure 2 2 Schematic of mercury cycle in the environment

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35 Figure 2 3. Breakdown of industrial Hg emissions by region 22 Figure 2 4 Change of global anthropogenic emissions of total mercury to the atmosphere from 1990 2000 (in tons) 105

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36 Figure 2 5 Model of atmospheric transport of anthropogenic Hg emissions from East Asia 22 Figure 2 6. Timeline of mercury use and subsequent regulations in the U.S.

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37 Figure 2 7. Mercury Eh pH diagram for systems containing Hg, O, H, S, and Cl 106,107

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38 Figure 2 8 Distribution of Hg(II) at various pH 108

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39 Figure 2 9. Speciation of 100 ppb Hg in deionized water as prepared from Hg(NO 3 ) 2 109

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40 Table 2 1. Mercury removal efficiency (%) for some technologies and different categories 25 Technology Coal Burning Cement Waste Caustic Soda Production Battery Production Electrostatic precipitators 32 35 Fabric filter 42 50 75 Flue gas desulfurization 18 97 Activated carbon 50 95 90 Gas stream cooling 90 Mist eliminators 90

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41 CHAPTER 3 EXPERIMENTAL Chemicals dilution of a 1000 mg/L mercury nitrate, Hg(NO 3 ) 2 standard solution (Fisher Scientific) with deionized water (18.2 megohm cm). All containers storing Hg were made of glass and capped with Teflon lids. The pH of prepared solutions measured 4 0.5 and was not manually adjusted for any of the experiments The pH was calibrated before use and measured via specific electrode on a Fisher Scientific Accumet AR 20 benchtop multiparameter meter. Ultra high purity nitrogen, ultra high purity oxygen, and breathing grade air used in this study were acquired from Airgas, Inc. and were selected as purge gases to introduce variable concentrations of oxygen into the solutions at atmospheric pressure. This pressure was the same for all experiments. All glassware was washed and soaked in 25% nitric acid (Fisher Scienti fic) overnight. Containers were then rinsed with deionized water to remove acid before use. Humic acid (HA) used for experimentation was Suwannee River Humic Acid Standard II and Suwannee River Fulvic Acid Standard II obtained from the International Humic Substance Society, St. Paul, Minnesota. These standards were chosen because the concentration of functional groups on the organic matters is widely reported within the literature. Organic matter stock solutions of 100 mg/L were prepared by dissolving the measured amount of freeze dried HA or FA in deionized water (18.2 megohm cm). Concentrations of humic acid included 1 mg/L, 5 mg/L, and 10 mg/L. Solutions containing humic acid and Hg are continuously magnetically stirred for one hour before use to ensure proper mixing and shielded from ambient light. When samples were

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42 mixed for 3 hours there was no statistical difference between the final experimental results; therefore, mixing reached equilibrium after one hour of mixing. Initial samples were collected be fore and after all experimentation in order to assure no changes were occurring within the original solution. Batch Experiments Batch experiments were conducted in a custom designed cylindrical reactor (Figure 3 1), which contained 100 mL of Hg solution an d was continuously mixed by magnetic stirring. UV Irradiation All experiments were performed in the presence of a single ended PL S Twin Tube Short Compact Fluorescent Lamp (Bulbs.com, Worcester, MA) of 254 nm or 365 nm and an in situ gas purge. This lamp was inserted through the Teflon lid and immersed with the aqueous solution. During experimentation, the reactor was shielded from ambient light. Wavelengths of 254 nm and 365 nm were chosen because they were known to cause photochemical reactions, based on extensive literature review 42,92,94,96,110 In addition, 185 nm wavelengths are known to cause ozone generation and therefore a 185 nm bulb was not chosen to avoid this interference with Hg mechanisms. Although 365 nm is within the solar spectrum, it is not an accurate representation of sunlight since it has a much higher intensity than sunlight would on the sample (see Appendix A.). The manufacturer of the UV lamps in the batch reactors was contacted and Figures 3 2 and 3 3 illustrate the wavelength s pectrum of the 254 nm and 365 nm lamps, respectively. Figure 3 2 illustrates that there is a very narrow spectrum range for

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43 the 254 nm lamp. The 365 nm lamp, however, has a little bit of a wider spectrum that ranges from 340 400 nm with a dominant peak at 365 nm and two very narrow and shorter peaks around 400 nm and 435 nm (Figure 3 3). Gas Purge The reactor lid is equipped with a inch glass purge tube submerged in the liquid to enable an in situ gas purge directly into the reactor solution and exits the reactor though a glass ventilation outlet. Gas flow was regulated at 2 L/min and was connecte d to the purge tube at ambient pressure. The flow rate was selected because it is the maximum flow that fits within the confines of the reactor 111 Various combinations of UV conditions, type of purge gas, Hg concentration, DOM type, and DOM concentration were observed for 5, 15, 30, 45, and 60 minutes to determine rate constants. This factorial design totals over 1000 combinations of experiments not including duplicates. Mercury Collection and Analysis After the desired run time elapsed, 20 mL of the s olution was collected and stored in a glass vial and spiked with 0.5 mL of concentrated nitric acid as a preservative until further analysis, which is the first step of the EPA method 245.1 protocol. The remaining solution was collected and filtered throug using vacuum filtration. A sample of 20 mL of the filtrate was stored in a glass vial and preserved in the same manner as the unfiltered sample. These collected samples were analyzed within 14 days from the time of collection. Resulting aqueous Hg concentrations after experimentation were determined by SnCl 2 reduction technique and detection by atomic absorption spectrometry (Teledyne Leeman Labs) per EPA standard method 245.1 112 In compliance to EPA method 245 .1, within 12 hours of collection 0.5 mL HNO 3 and 1 mL H 2 SO 4 was added to acidify

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44 and preserve each sample. Within 24 hours, completion of EPA method 245.1 was performed by digesting the samples with 3 mL of 5% KMnO 4 1.6 mL of 5% K 2 S 2 O 8 and 1.2 of 12% NaC l hydroxylamine sulfate solution (Fisher Scientific). Hg removal is reported as C/C o the final concentration over initial concentration. Error bars illustrate the range of duplicate reactor runs. Humic Acid Analysis In order to determine the transformatio n of humic and fulvic acid s total organic carbon (TOC) was analyzed on a Tekmar Dohrmann Apollo 9000HS autosampler. After the desired run time elapsed, 30 mL of solution was collected in glass vials for TOC analysis. Combustion techniques were utilized f or TOC analysis where, after samples are acidified with 10% phosphoric acid (Fisher Scientific) and stripped of inorganic carbon by zero grade air (Airgas, Inc.), samples are oxidized on a platinum/alumina catalyst in a high temperature (680 o C) furnace. C O 2 is analyzed with an NDIR (non dispersive infrared gas analyzer) detector.

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45 Figure 3 1. Batch reactor set up schematic Figure 3 2 Spectrum of 254nm lamp 113

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46 Figure 3 3. Spectrum of 365 nm lamp 114

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47 CHAPTER 4 PHOTOCHEMICAL MECHANISMS OF AQUEOU S MERCURY REMOVAL IN SYNTHETIC SOLUTIONS Background Mercury (Hg) is a global concern causing many forms of neurological damage that are especially harmful to children. The majority of Hg is introduced to the environment through anthropo genic sources, including coal fired power plants and chlor alkali plants. The concern is primarily due to the ability of methylmercury compounds to bioaccumulate in the food chain. This bioaccumulation of methylmercury in fish tissue is not easily elimin ated and is in greater concentrations at the time of human consumption. Currently, the EPA recommends the implementation of fish tissue based water quality criteria to meet the Clean Water Act requirements. Water column concentrations will vary based on location, but most likely would require sub Lakes area, for example, discharge Hg concentrations required to be below 1.3 ng/L as part of the Great Lakes Initiative. These levels call for the development of new remediation techniq ues that can treat trace level aqueous Hg waters to even lower concentrations. This research aims to focus on the photoreduction of Hg in order to decrease its solubility in aqueous solutions. With this reduction, the Hg will volatilize via a purge gas an d thereby be removed from the solution. The air borne Hg can then be captured using silica titania composites, a proven technology for capturing Hg from the air phase. Other sorbents such as activated carbon may work equally well in capturing Hg from the a ir phase This method of transferring Hg from the water phase to the air phase allows for less adsorption competition in the diverse water matrix.

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48 Control Studies To ensure that aqueous Hg loss is a resul t of photochemical re actions, control experiments w ere pe r formed. When studying controls in the dark, the UV lamp was not illuminated but did remain in solution. Experiments r u n in the dark, ones with purging alone, and others with no purge or UV irradiation were performed. The lowest Hg concentration use d in these experiments (50 ppb) and the longest reaction time (60 minutes) were studied based on experimental data that showed these conditions removed the most Hg These were p e r formed with both 254 nm and 365 nm UV. The purge gas utilized for the control experiments was N 2 purge since this purge gas is inert and resulted in the most Hg removal. These highly reducing conditions were used so that when ran in the dark, or without a purge, the difference in Hg removal could be measured accurately. Results p resented in Table 4 1 show control runs with a purge or UV alone did not remove substantial quantities of Hg. With no UV illuminated and a N 2 purge, a C / C 0 value of 0.97 is obtained after 60 minutes. When 254 nm and 365 nm UV lamps lit and no purge is present, C/ C 0 values of 1.0 and 0.97, respectively, were measured. These values are comparable to the blank with neither purge nor UV with a C/C 0 of 1.0. However, when both UV and purge gas occur concurrently there is a noticeable amount of Hg removal from the aqueous phase (Table 4 1). When the condition of a N 2 purge gas was run concurrently with 254 nm or 365 nm UV, low C/C 0 values of 0.02 and 0.01 were measured, respectively. The se results suggest that photochemical reactions are responsible for the tra nsformation of soluble ionic Hg to volatile elemental Hg where it is then transferred to the purge gas and subsequently removed from the system. In

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49 addition, there is no significant difference between filtered and unfiltered samples, implying that species Nitrogen Purge 254 nm Hg solutions with initial concentrations of 50, 100, 500, and 1000 ppb were purged with nitrogen while being irradiated with 254 nm UV (Figure 4 1). A nitrogen purge was selected due to its inertness, create a reducing environment, and, thusly, to serve as a driving force for volatile Hg removal from the aqueous phase. Nitrogen purge caused dissolved oxygen (DO) concentrations within the solution to be near 0 mg/L, according to calculations results show that at a n initial concentration of 50 ppb with 60 minutes of exposure time, the final Hg concentration reached 0.4 ppb (C/C 0 = 0.008). Even at higher concentrations of 1000 ppb, almost 80% removal of Hg is achieved after 60 minutes. 365 nm W hen solutions of the same initial Hg concentration as listed above we re exposed to nitrogen purge while being irradiated by 365 nm UV the decrease in photon energy caused less reduction o f Hg and removal from the solution (Figure 4 2). These results show that an increase in Hg concentration resulted in a decrease in overall Hg removal from the solution. However, even at such high concentrations of 1000 ppb, results show there is still a t otal of 60% removal after an exposure time of 60 minutes. At lower concentrations of 50 ppb with the longest exposure time of 60 minutes, the final Hg concentration reached 1 ppb (C/C 0 = 0.02), comparable to results seen from 254 nm UV experimental runs (F igure 4 1).

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50 The r eaction kinetics for Hg remova l were calculated and can be seen in Table 4 2. At concentrations of 50 ppb and 100 ppb for both 254 nm and 365 nm UV, Hg removal follows first order reaction kinetics where reaction rate depends upon initial Hg concentration in solution. Concentrations tested above 100 ppb for 365 nm better followed zero order reaction kinetics where reaction rates do not depend on initial concentrations. This means that there is such an abundance of Hg and nitrate in solution that the specific amount does not matter since the matrix is saturated. This is most likely due to the fact that there are a limited number of reduction reactions that can occur when irradiated with 365 nm UV due to less photon energy available as compare d to 254 nm. With 365 nm UV, increased concentrations of Hg and nitrate caused the reaction rate to be independent of divalent Hg concentrations after a certain point because the reactions were not occurring as fast as they were when irradiated by 254 nm U V. Reaction rates at 254 nm UV demonstrate first order kinetics for all concentrations. At an initial Hg concentration of 50 ppb, the rate constants are 0.18 s 1 and 0.11 s 1 for 254 nm and 365 nm, respectively. Th is shows a faster initial conversion rate of divalent Hg 2+ to Hg 0 with increased photon energy. Air Purge 254 nm Hg solutions with the same initial concentrations as listed above were then exposed to an air purge with 254 nm UV (Figure 4 3). Air purge was selected since it is a more feasible opti on for treatment technologies than nitrogen due to cost and availability. Since the addition of air would cause a higher dissolved oxygen

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51 therefore increase the number of poss ible oxidation reactions, it was hypothesized that the addition of an air purge would decrease the total amount of Hg removal. Indeed, this can be seen in Figure 4 3. 365 nm Whe n air purged experiments were p e r formed with 365 nm UV (Figure 4 4), unexpected results occurred. It was hypothesized that when the solution was exposed to a lower photon energy containing wavelength (365 nm), there would be less Hg removal than with a wavelength having higher photon energy (254 nm). However, there was greater Hg rem oval when exposed to 365 nm than with 254 nm UV. Only 30% Hg removal was achieved with 254 nm irradiation (C/C 0 = 0.71) where 85% removal achieved with 365 nm UV (C/C 0 = 0.15). It is believed that photolysis of nitrate might be the cause of the disparity o f Hg occurs by the following pathways 115,116 : NO 3 + hv NO 2 + O (4 1) O + H 2 O O H + O H (4 2 ) OH + Hg 0 OH + Hg 2+ (4 3) It can be seen in equations 4 1 through 4 3 that the abundance of hydroxyl radicals formed during irradiation of 254 nm UV likely lead to additional Hg reoxidation reactions that caused Hg to remain within the solution. Therefore, more removal of Hg occurred at 365 nm wavelengths because the photolysis of nitrate (which leads to oxidation reactions) does not occur at wavelengths greater than about 302 nm 115

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52 Oxygen Purge High Hg Concentrations The same initial Hg concentrations as discussed previously were then exposed to an oxygen purg e with 254 nm and 365 nm UV exposure (Figures 4 5 and 4 6, respectively) As expected with increased oxygen concentrations in solution (complete saturation), initial concentrations of 500 ppb and 1000 ppb Hg were reduced by less than 20% for both 254 nm a nd 365 nm UV. Low Hg Concentrations However, at lower concentrations of 50 ppb and 100 ppb Hg, removal reached roughly 80% with 365 nm UV and up to 90 % removal with 254 nm UV. Such a large distinction between these concentrations instead of a more gradual variation is likely due to increased concentrations of nitrate from the mercury standard Hg(NO 3 ) 2 Increasing concentrations of nitrate consume more photons and lead to Hg oxidation, especially under highly oxidizing conditions created from an oxygen purg e. It is also likely that the large gap in Hg concentrations causes a large gap in Hg removal.

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53 Figure 4 1. Aqueous Hg removal of various Hg concentrations versus time in the presence of N 2 and 254 nm UV Figure 4 2. Aqueous Hg removal of various Hg concentrations versus time in the presence of N 2 and 365 nm UV

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54 Figure 4 3. Aqueous Hg removal of various Hg concentrations versus time in the presence of air and 254 nm UV Figure 4 4. Aqueous Hg removal of various Hg concentrations v ersus time i n the presence of ai r and 365 nm UV

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55 Figure 4 5. Aqueous Hg removal of various Hg concentrations versus time in the presence of O 2 and 254 nm UV Figure 4 6. Aqueous Hg removal of various Hg concentrations versus time in the presence of O 2 and 365nm UV

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56 Table 4 1. Control studies for Hg concentrations of 50 ppb with run times of 60 minutes Condition C/C o Range Sample Treatment No UV, no purge 1.0 0.03 Unfiltered No UV, N 2 purge 0.97 0.06 Unfiltered 254 nm UV, no purge 1.0 0.05 Unfiltered 365 nm UV, no purge 0.97 0.02 Unfiltered Concurrent Experiments 254 nm UV, N 2 purge 254 nm UV, N 2 purge 365 nm UV, N 2 purge 365 nm UV, N 2 purge 0.02 0.01 Unfiltered 0.02 0.00 Filtered 0.01 0.00 Unfiltered 0.01 0.01 Filtered *Based on two replicates Table 4 2. Reaction rate kinetics for various Hg concentrations with nitrogen purge Initial Hg ( ppb) Rate Order Rate Constant Fit (R 2 ) 254 nm UV 50 First 0.182 s 1 0.994 100 First 0.118 s 1 0.962 500 First 0.031 s 1 0.986 1000 First 0.053 s 1 0.994 365 nm UV 50 First 0.111 s 1 0.996 100 First 0.029 s 1 0.989 500 Zero 4 10 8 M s 1 0.994 1000 Zero 5 10 8 M s 1 0.948

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57 CHAPTER 5 EFFECT OF DISSOLVED ORGANIC MATTER CONCE NTRATIONS ON PHOTOCHEMICAL AQUEOU S MERCURY REMOVAL Background Dissolved gaseous Hg (DGM) is a highly volatile form of Hg that can be removed from aqueous solutions via gas exchange. As a result of this volatilization, there is a reduction of Hg available in aqueous solutions for the produc tion of MeHg. The production of DGM can occur biotically or abiotically, though abiotic processes are of more importance in surface waters where sunlight induced reduction can occur 75,77 79 Direct photoreduction of Hg to DGM has also been observed in laboratory studies 109,110,117 In natural waters, however, direct photoreduction is complicated by complexation with organic and inorganic ligands. Dissolved organic matter (DOM), especially h umic substances, is well known to interact with Hg 83,92,93,95,99,118 Allard & Arsenie 94 spiked synthetic waters with Hg and found an increase of DGM production when irradiated in the presence of DOM versus when Hg was irradiated alone Conversely, Amyot et al. 79 found UV induced DGM production rate was less in high DOM lakes than it was in low DOM lakes. Other reports have found no correlation between the amount of DOM and rate of DGM production 97,100 These paradoxical results may be a result of the varying structures of DOM, the presence of competing ions reducing the availability of complexing sites, and the ratio of Hg DOM concentrations. A study involving X ray absorption spectroscopic method shows the complexation of Hg on reduced sulfur (S red ) s ites in DOM 91 Therefore, the amount of S red sites on the DOM as well as the Hg:DOM ratio can greatly affect the complexation of Hg and subsequently the DGM production 88,103,104

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58 Since DOM is ubiquitous in natural waters and numerous waste and industria l waters that may require treatment prior to discharge, it is important to understand how it affects the photochemical mechanisms of aqueous Hg and thus maximize DGM production as a treatment technology. This study focuses on synthetic waters to better und erstand the intricate photo redox mechanisms between Hg and DOM with emphasis on (1) how increasing the concentration of DOM affects DGM production and thusly the total removal of Hg from the aqueous phase, (2) the ratio of Hg:DOM and it s correlation to DG M production, and (3) the effect of increased oxyge n concentration from a purge gas. Control Studies Control studies were performed to explore aqueous Hg loss in the presence of humic acid. When studying controls in the dark, the UV lamp was not illuminate d but did The reaction time of 60 minutes was used in control runs since it was the longest time tested and caused substantial Hg removal. All purge gases (N 2 Air, O 2 ) were used individually for control runs. Results presented in Table 5 1 show the difference in Hg removal between UV and purge gases run separately and concurrently. In solutions containing 10 ppm humic acid (HA) with just the UV lamp illuminated and no purge, any volatile mercury formed has no way to leave the system thus showing no removal (C/C 0 of 1.0). However, some removal does occur when just the purge gas is running without the UV lamp illuminated. When these same parameters are applied to solutio ns without HA, there seems to be slightly less Hg removal than there is with humic acid present (C/C 0 of 0.22). Therefore, due to the presence of HA, some Hg is transformed to volatile elemental Hg and

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59 subsequently leaves the system via purge gas. It is po ssible that the HA provides an electron source for Hg thus increasing its ability to form volatile elemental Hg without the presence of UV and can subsequently be removed from the solution via purge gas. When both a purge and UV irradiation are run concur rently as opposed to separately in solutions containing only Hg, final Hg concentrations are lowered (C/C o of 0.02 versus 1.0 and 0.97, respectively). This is due to the reduction of soluble ionic Hg to elemental Hg via UV irradiation and subsequent volati lization from solution via purge gas 94 Mercury Volatilization as a Function of Nitrogen Purge Gas and Increasing Humic Acid When solutions of 100 ppb Hg and 0 pp m HA were exposed to 254 nm UV and a nitrogen purge (Figure 5 1), the removal is best modeled by first order reaction kinetics with a rate constant of 0.182 s 1 indicating that the reaction rate is dependent upon the Hg concentration in solution. As seen in Figure 5 1, w hen 1 ppm of HA was introduced into the system, it altered the rate of removal, which increased in the presence of HA. It is likely that the reaction rate increased from the increased electron source that the humic acid provided, causing more reduction reactions. Furthermore, the final amount of Hg remaining in solution afte r 60 minutes was higher in the presence of humic acid, indicating less overall removal. When the HA concentration was then increased to 10 ppm, as seen in Figure 5 1 the effect of sulfur complexation was more pronounced. During these experiments, there w as less Hg removal when compared to 1 ppm HA throughout the entire duration of the experiment. It may seem that with more organic matter in the system, the presence of more free electrons would increase Hg removal; the results show that this was not the ca se. The decrease of Hg removal as the concentration of humic acid

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60 increased from 1 ppm to 10 ppm was likely due to increased complexation of Hg to the more abundant sulfur sites on the humic acid 103,104,119 With a greater amount of HA in solution, the Hg more readily bonds to sulfur functional groups due to the strong binding constant 103 When there is less HA in solution, the Hg is forced to bond with functional groups containing oxygen, such as carboxylic functional groups, forming weaker bonds and inc reasing the likelihood of subsequent separation and reduction of Hg. Other studies involving natural waters have shown a decrease in overall DGM concentrations when DOM is increased. It is possible that high levels of DOM could inhibit Hg photoreduction d ue to DOM limiting UV irradiation from reaching the mercury 99,100 Other reports involving natural waters suggest direct photolysis of DOM 87 : DOM + H 2 O + hv OH + products (5 1) w here the hydroxyl radical then initiates oxidation reactions that lead to t he subsequent oxidation of Hg: OH + Hg 0 Hg 2+ + OH (5 2) However, it has also been proposed that the photolysis of DOM i s also capable of Hg reduction 83 : DOM + hv DOM (5 3) DOM + O 2 DOM + + O 2 (5 4) O 2 + Hg 2+ Hg 0 + O 2 (5 5) Hydrogen peroxide may originate from UV induced transformation of DOM that can lead to the oxidation or reduction of Hg depending on the number of hydroxide or hydrogen ions. In the presence of abundant hydroxide, t he superoxide anion could also lead to reduction of Hg by a di fferent mechanism 83,100 :

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61 O 2 + O 2 + H + H 2 O 2 + O 2 (5 6) H 2 O 2 + 2OH + Hg 2+ O 2 + 2H 2 O + Hg 0 (5 7) o r could subsequently lead to re oxidation in the presence of hydrogen ions when reacting with newly formed hydrogen peroxide 120 : H 2 O 2 + 2H + + Hg 0 2H 2 O + Hg 2+ (5 8) DOM high in reduced sulfur functional groups, such as the HA used in this work, are very likely to form strong bonds that can either reduce DGM production due to its strong complexation with Hg 79 or increase DGM production if intra molecula r electron transfer is required 95 Because of these conflicting results in the literature, it is important to observe the ratio of Hg to DOM. Impact of Hg:DOM Ratio on Hg Volatilization Induced by Nitrogen Purge If the ratio of Hg to reduced sulfur functi onal groups is a factor in overall Hg removal, it is plausible that keeping the ratio the same when varying the concentrations of both Hg and HA will produce the same C/C 0 results. A 100:1 ratio was observed using concentrations of 1000 and 100 ppb Hg with 10 and 1 ppm HA, respectively. Indeed, Figure 5 2 shows a similar trend between the various concentrations that were measured. A factorial analysis of variance (ANOVA) was conducted to determine if there was a statistical difference between the different concentrations. When comparing a ratio of 100:1 as seen in Table 5 2, the p value (0.97) is very close to one, indicating that there is not enough evidence to say that the data sets are statistically different from one another. A similar experiment was pe rformed, but with the ratio of Hg and HA concentrations as 10:1. Similarly, various concentrations of Hg and HA showed a similar

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62 trend in overall removal (Figure 5 2). Although the P value of the 10:1 ratio was smaller (0.66) than the 100:1 ratio (0.97), t hey still fall well above the critical limit of 0.05 and thus there is not enough evidence to say that they are statistically different from one another even though the amount of available sulfur sites between them deferrers (Table 5 2). It is believed th at at high ratios of Hg:DOM, such as 1 00:1, Hg primarily bonds to the more abundant carboxyl groups as opposed to forming stronger bonds with reduced sulfur gr oups 103 Therefore, a higher ratio should have a faster rate of removal than a smaller ratio doe s. Indeed, the experiments involving a 100:1 ratio had a faster rate of removal, if only slightly, and a smaller amount of mercury remaining in solution after 60 minutes. However, according to the statistical analysis, there is no significant difference be tween the 100:1 and 10:1 ratio. It is still important to note that the P value is 0.39, lower than the other observed P values and the closest to the critical limit of 0.05. Although statistical analysis were not completed, Zheng & Hintelmann 104 observed a faster rate of Hg removal at higher Hg:DOM ratios, yet their experiments involved simulated solar irradiation as opposed to 254 nm UV lamps. Therefore, the shorter wavelength could allow for faster breakdown or transformations of the HA or more free electrons present within the solution, allowing for a faster and more similar rate for all ratios. Since DOM is known to be a source and a sink for hydroxyl radicals 121,122 it is also possible that hydroxyl radicals that would normally be reducing Hg are instead reacting with the more abundant HA, thus limiting Hg removal in high HA waters. Effect of Purge Gas on Hg and HA Removal It is well known that the presence of oxygen leads to oxidation of Hg 85 which subsequently decreases the amount of DGM within the system, limiting removal. Yet,

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63 experiments involving the removal of Hg in the presence of DOM have not observed the impact of varying the oxygen content in the purge gas. In order to better understand the effect of dissolved oxygen (DO) on Hg i n the presence of DOM, experiments were performed using air and oxygen purge gases. Henry's Law was used to calculate a theoretical dissolved oxygen concentration of 8 mg/L and 40 mg/L for air and oxygen, respectively. Figure 5 3 illustrates the impact of purge gas on 100 ppb Hg irradiated by 254 nm UV. When compared to the presence of a nitrogen purge without any HA where removal after 60 min reached a C/C 0 of 0.02 (Figure 5 1), there is very little Hg volatilized in the presence of air or oxygen (Figure 5 3). This concurs with other experiments involving lower concentrations of Hg that show decreased Hg removal when DO levels increased 109 When 1 ppm HA is added to solution, as seen in Figure 5 3, the overall removal of Hg increased for both air and oxygen purges. Similarly to the nitrogen purge, it is possible that the HA provided an electron source, which would increase the number of reduc tion reactions and lead to an increased removal of Hg and without HA present. However, there was less overall removal of Hg during the air purge as opposed to the oxygen purge, indicating that the amount of DO in the system is not the only factor affecting Hg removal. Similar results have been observed in solutions without any HA present and it is believed that the carbon dioxide present in the air purge co u ld impact the photochemical reactions, decreasing removal 109 Studies measuring HA were conducted to better understand how DO concentrations affect Hg removal (Table 5 3). When a purge gas was running without

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64 the UV lamp illuminated, very little decrease in TOC occurred, which indicated UV was the driving force for the breaking apart of organic matter A dditionally, the total photochemical d estruction of HA positively correlated to the total concentration of DO in solution. It is likely that the breaking apart of HA created singlet oxygen, hydrogen peroxide, free electrons, or hydroxyl radicals, all of wh ich aid in the reduction of Hg, as seen in Equations 5 3 th r ough 5 7 above. Thus, since the O 2 purge caused more HA to break apart than the air purge, there are more constituents in the water that reduce Hg. This led to a greater overall Hg removal for the O 2 purge, as seen in Figure 5 3.

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65 Figure 5 1. Comparison of Hg C/C 0 verses time in the presence of various concentrations of HA and 100 ppb Hg with 254 nm UV and nitrogen purge. Figure 5 2. Comparison of Hg C/C 0 verses time in the presence of nitrogen purge and 254 nm UV with varying concentrations of Hg and HA for comparison of 100:1 and 10:1 Hg:DOM ratios

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66 Figure 5 3. Comparison of Hg C/C 0 versus time in the presence of Air and oxygen purges and 254 nm UV wit h Hg concentrations of 100 ppb and varying HA

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67 Table 5 1. Control studies for Hg Concentrations of 100 ppb with run times of 60 minutes Condition Average Hg, C/C 0 Range 100 ppb Hg, 10 ppm HA No purge, 254 nm 1.0 0.01 N 2 purge, no UV 0.89 0.01 Air purge, no UV 0.90 0.02 O 2 purge, no UV 0.93 0.01 N 2 purge, 254 nm 0.22 0.02 100 ppb Hg, No HA No purge, 254 nm 1.0 0.05 N 2 purge, no UV 0.97 0.06 N 2 purge, 254 nm 0.02 0.01 Table 5 2. P values generated from factorial ANOVA modeling for various concentrations under 254 nm irradiation and a nitrogen purge Condition P value Std Error 100 ppb Hg, 1 ppm DOM vs. 1000 ppb Hg, 10 ppm DOM 0.97 0.09 vs 100 ppb Hg, 10 ppm DOM 0.39 0.08 100 ppb Hg, 10 ppm DOM vs. 10 ppb Hg, 1 ppm DOM 0.66 0.07

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68 Table 5 3. Reduction of HA after 60 minutes under various experimental conditions as measured by TOC removal Condition TOC Reduction Range Irradiated Experiments 254 nm, no purge 33.8% 0.45 Purge Gas Experiments N 2 purge, no UV 0.1% 0.34 Air purge, no UV 1.8% 0.31 O 2 purge, no UV 0.2% 0.19 Concurrent Experiments N 2 purge, 254 nm 1.7% 0.03 Air purge, 254 nm 24.1% 0.38 O 2 purge, 254 nm 43.2% 0.07

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69 CHAPTER 6 EFFECT OF VARIOUS TYPE S OF ORGANIC MATTER ON PHOTOCHEMICAL AQUEOUS MERCURY REMO VAL Background At the end of 2011, the EPA announced new standards to limit Hg removal from coal fired power plants 71 Currently, Hg is captured with wet FGD scrubbers, which are designed to prevent sulfur and other air pollutants from entering the atmosphere. However portions of the soluble Hg 2+ captured in the wastewater have been known to convert back to elemental Hg (Hg 0 ) and be re emitted into the stack 123 125 With new national standards being put into place, and global Hg regulations on the horizon 73 it is imperative to develop safe and reliable techniques for capturing Hg. Therefore, it is imperative to understand how Hg reemission occurs in order to develop removal technologies that prevent this from happening. Previous studies have shown that volatilization of Hg increases in the presence of organic matter (Chapter 5). However, when the concentration of humic acid (HA) is increased, less aqueous Hg removal is observed. Therefore, intricate mechanisms are at work that likely involves functional groups varying on different humic substances or the destruction of organic matter. It is important to not only observe how organic matter affects the photochemical mec hanisms of mercury, but to study different types of organic matter in order to better understand the mechanisms involved. This study focuses on synthetic waters containing a different type of humic substance, fulvic acid (FA), in order to compare how compo sition of organic matter affects Hg removal and reemission. T o elucidate the complex mechan isms involved between Hg and FA, this research focuses on (1) how increasing FA concentration affects Hg removal when the initial Hg concentration is constant, (2) t he ratio of

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70 Hg:DOM and it s correlation to DGM production, and (3) how destruction of FA by UV affects overall Hg removal. Mercury Volatilization as a Function of Organic Ma tter Concentrations Induced by N itrogen P urge Total mercury removal was measured w hen various concentrations of Hg and FA were exposed to 254 nm UV and a nitrogen purge. As seen in Figure 6 1, no matter the concentration of Hg or FA, Hg removal followed the same trend. After 60 minutes of expos ure the total Hg removal amounted to 92 95% for vastly differing Hg concentrations ranging from 10 1000 ppb Hg and FA concentrations of 1 10 ppm. This same trend of differing Hg and organic matter concentrations producing the same results w as also seen in Chapter 5. However, t he phenomena in the la ck of variation betw een different ratios of Hg:DOM was unexpected since much of the literature states that the Hg:DOM ratio of concentrations affects the overall DGM production and subsequent Hg removal 126,127 These studies, however, commonly look ed at sunlight and wavelengths that mimic sunlight. Therefore, it is also important to look at various types of UV exposure and how that affects the overall Hg removal in the presence of FA. When different Hg;DOM ratios were exposed to wavelengths of 365 nm and a nitrogen purge a similar trend was seen as with wavelengths of 254 nm. Figure 6 2 shows the same Hg removal trend no matter the ratio T his is possibly why the literature shows a correlation between varying concentrations of Hg and DOM with the same ratio to each other because all concentrations have the same correlation and removal no matter the ratio or initial concentration

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71 It could also be argued that no mat ter the Hg concentration, if the aqueous solution has the same concentration of o rganic matter, then it might result in the same Hg removal due to the saturated am ou n t of organic matter in the system. Nevertheless, even when the total FA concentration increases (from 1 ppm to 5 ppm FA) the same amount of DGM production and subsequent Hg removal occurs (Figure 6 2). Therefore, under the same UV and purge conditions, the same C/C 0 is seen for all Hg concentrations in the presence of FA. Upon closer inspection of the literature results actually show a similar trend as seen here. When com paring various graphs published by Zheng and Hintelmann 104 the same amount of Hg removal was seen for aqueous solutions containing 2 ppb Hg and 57.8 ppm DOC, 2 ppb Hg and 12 ppm DOC, and 10 ppb Hg and 12 ppm DOC. Although they were all different ratios o f Hg:DOM, they all show ed roughly the same Hg removal. This demonstrates that the total amount of Hg bonding to S red sites on organic matter is not the sole determining factor for overall Hg removal. Another way to look at the effect of S red sites is to compare FA and HA since they have varying concentrations of sulfur (Table 6 1). Because HA has more sulfur functional groups than FA (and Hg more readily forms stronger bonds with sulfur sites), less Hg removal should occur in the presence of HA tha n FA since Hg will form stronger bonds and not be available for vol atilization. Figure 6 3(a) showed this trend of less Hg removal in the presence of HA. However, when exposing the same conditions (nitrogen purge, 100 ppb Hg, and 1 ppm FA or HA) to 365 nm UV rather than 254 nm UV, there is initially more H g removal in the presence of HA. In fact, a review of the literature shows that only 1.6 2 .0 % of S red sites are involv ed in strong interactions with Hg 103,128

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72 Therefore o ther factors that may affect Hg removal such as the breakdown of DOM versus only considering S red bonding. The Effect Fulvic Acid Reduction on Dissolved Gaseous Mercury Removal The Effect of Purge Gas Type Since so few sulfur functional groups bind strongl y with Hg, it is important t o look at other mechanisms for H g removal such as the breaking apart of organic matter and TOC removal Photodegradation of organic matter is known to produce free radicals such as OH and superoxide anion radical O 2 129 131 Thes e free radicals can lead to Hg oxidation in the following reaction s: DOM + hv DOM ( 6 1 ) DOM + O 2 DOM + + O 2 ( 6 2 ) O 2 + O 2 + H + H 2 O 2 + O 2 ( 6 3 ) H 2 O 2 + 2H + + Hg(0) 2H 2 O + Hg(II) ( 6 4 ) DOM + H 2 O DOM + + OH ( 6 5 ) OH + Hg(0) Hg(II) + OH ( 6 6 ) Therefore, it seems that increased photodegradation of FA would lead to decreased Hg removal based on the equations above. Lou and Xie 132 completed in depth analysis on photochemical molecular weight reduction on various forms of dissolved organic matter The breaking apart of Suwannee River FA (used in these experiments) under irradiation that contains a peak wavelength of 365 nm under various purge gases was measured using TOC analysis O 2 and air purges produced the same rate of decrease in the molecular weight of FA, while a N 2 purge caused very little change to the FA. It order to determine if

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73 photochemical changes of organic matter are a factor for Hg removal, C/C 0 was measured under varying purge gases with 365 nm irradiation Figure 6 4 compares an O 2 and an air purge while keeping the UV and concentration of Hg an FA constant. Even though purge gases are known to affect the overall Hg removal (Chapter 4), when organic matter is added to solution it also has a great influence on Hg reduction. If, in fact, the destruction of high molecular weight fractions of FA has a dominant role in Hg removal, then Figure 6 4 should show the same trend for each line. Indeed, this is seen in Figure 6 4 Even though the O 2 and air purge cause di fferent DO concentrations, they show ed the same trend of Hg re moval indicating that the break down of organic matter greatly influences how DGM is produced and Hg is removed from the aqueous phase. To further prove this point, we can look at a N 2 purge u nder the same conditions. It is known t hat very little organic matter breakdown occurs when in the presence of a N 2 purge 132,133 Therefore, there should be even more Hg removal (than the O 2 and air purges) since there is less photodegradation of FA and, subsequently, less free radicals in solution to cause oxidation. When comparing Figure 6 4 to the 10 ppb Hg and 1 ppm FA conditions seen in Figure 6 2 (N 2 purge with 365 nm UV), the purge tha t produces the most Hg removal is indeed the N 2 purge. Therefore, less molecular weight alterations of FA under these conditions equate to more Hg removal likely as a result of less free radicals present in solution The Effect of UV Wavelength Another wa y to observe how the reduction of FA affects Hg removal is to look at varying UV wavelength s Figure 6 6 compared 254 nm and 365 nm wavelengths in the presence of an air purge with 10 ppb Hg and 1 ppm FA. When comparing the results,

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74 more Hg removal occur s when exposed to 365 nm UV as opposed to 254 nm. Justification for this observation may be found in how FA degrades at various wavelengths. When exposed to an air purge, 254 nm UV causes reduction of FA faster than 365 nm wavelengths 134 Therefore, it is possible that photoreduction of FA produced more free radicals and caused less Hg re moval under 254 nm wavelengths. While it is not mentioned in the literature, these studies have shown that Hg removal is not solely dependent upon bonding to functional groups, but instead that photodegradation of organic matter should be taken into account.

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75 Figure 6 1. Comparison of Hg C/C 0 verses time in the presence of nitrogen purge and 254 nm UV with varying concentrations of Hg and FA for comparison of 100:1 and 10:1 Hg:DOM ratios Figure 6 2. Comparison of Hg C/C 0 versus time in the presence of a nitrogen purge and 365 nm UV with v arying concentrations of Hg and FA for comparison of 100:1 and 10:1 Hg:DOM ratios

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76 Figure 6 3. Comparison of Hg C/C 0 versus time of FA and HA in the presence of a nitrogen purge and (a) 25 4 nm and (b) 36 5 nm wavelength

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77 Figure 6 4 Comparison of Hg C/C 0 versus time in the presence of oxygen or air purge and 365 nm UV with 10 ppb Hg and 1 ppm FA Figure 6 5 Comparison of Hg C/C 0 versus time of 254 nm and 365 nm in the presence of an a ir purge with 1 0 ppb Hg, 1 ppm FA

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78 Table 6 1. Elemental analysis in percent of Suwannee River Humic and Fulvic acids 135,136 Type C H N O P S ash other FA 54.65 3.71 0.47 39.28 0.2 0.5 0.95 0.24 HA 57.24 3.94 1.08 29.13 0.2 0.82 0.56 7.03

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79 CHAPTER 7 CONCLUSIONS Contributions to Science The impact that UV wavelength, purge ga s type, and Hg concentration have on DGM production and subsequent Hg removal were studied. Removal rates were also calculated. With a nitrogen purge, at higher concentrations, 500 and 1000 ppb Hg, under low photon energy conditions, 365 nm UV, Hg removal followed zero order kinetics and showed less overall removal. Results showed that optimal Hg removal conditions included a low Hg initial concentration (50 ppb was the lowest studi ed), a nitrogen purge, and 254 nm UV irradiation. Under these conditions, 99% removal of Hg occurred after 30 minutes. Key mechanisms involved with DGM production were identified. Mercuric nitrate was used for all solutions and ionizes into Hg 2+ and NO 3 in water. Therefore, it is photolysis of nitrate produces hydroxyl radicals, which in turn causes the oxidation of Hg. DOM was added to solutions in order to determine how DGM production and Hg removal is affected. When 1 ppm HA was added to a solution of 100 ppb Hg, 254 nm UV, and nitrogen purge, more Hg is removed from the system after 5, 15, and 30 minutes than without any HA present. A similar trend was seen w hen FA was adde d instead of HA. Increasing concentrations of DOM were investigated as well. When 10 ppb HA was added to a solution of 100 ppb Hg, 254 nm UV, and nitrogen purge, the overall Hg removal decreased when just 1 ppm HA was added. This indicates complexation wit h

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80 Hg and decreased DGM production. However, when F A was added in similar solutions in place of HA, the same trend of Hg removal was seen between the experiments with varying concentrations of FA in solution (1 ppm vs 10 ppm FA) This shows that the type of DOM is a determining factor of Hg removal. The reduction of DOM was studied by measuring the TOC before and after experimentation. The total amount of HA remaining in the solution correlated to the amount of DO in solution, provided by the purge gas. The oxygen purge caused more HA destruction than the air purge, which produced more free electrons and other constituents that affect Hg mechanisms FA was also studied and showed that the type of wavelength as well as the type of purge gas affected the total molecular weight of DOM decreased in the solutions When large amounts of FA photodegradation occurred, its correlation to less Hg removal was witnessed. Future R esearch Avenues While UV is more commonly studied from a treatment perspective, it is costly; therefore, it is important to look at the effect of ambient light. The addition of photocatalysts is also recommended for ambient light i n order to aid in the overall Hg removal since visible light does not have UV. It is also important to understand the mechanisms for Hg removal in the presence of anions. The addition of anions would clarify mechanisms involved with Hg removal in aqueous solutions that mimic FGD water. A simplifi ed aqueous Hg matrix of deionized water and anions would aid in understanding how Hg can be photochemically removed from FGD wastewater. Example anions could include chloride and sulfate, both of which are dominant in FGD wastewater.

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81 While the experiments presented in this dissertation showed up to 99% of Hg removal in concentrations similar to FGD wastewaters, it has not been tested on non simulated FGD wastewater. Competition between additional contaminants for photons and electrons is expected to alter t he removal rate of Hg, as is the presence of additional redox reactions not seen in simplified waters.

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82 APPENDIX A SUPPLEMENTAL INFORMA TION UV Calculations Photon Energy Variations in wavelength were studied as well. When irradiated with 254 nm wave lengths, there was more Hg removal than when irradiated by wavelengths of 365 nm. This is likely because there are more reduction reactions occurring at 254 nm than at 365 nm. The cause may be from 254 nm having higher photon energy than 365 nm. The equat ion below represents the relationship betwe en photon energy and wavelength 137 : (A 1) Where E is photon energy, h is the Plank constant (6.6310 34 J*s), c is the speed of light in a vacuum (3.010 8 on it can be seen that as wavelength increases, the photon energy decreases. Therefore, 254 nm more reduction reactions in the same period of time. Intensity The UV out 1000bulbs.com PL S 9W/TUV G23 Base) is 2.4 watts. The height the water level reaches within the batch reactor is 8.3 cm and the average width (between the center of the reactor and the wall of the reactor) is 3.3 cm. Therefore, the average area the lamp is irradiating is 3.3 cm 8.3 cm = 27.39 cm 2 With a UV output of 2.4 watts (2400 mW), this equals an intensity of 87.62 mW/cm 2

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83 The UV output of the 365 nm UV lamp was not available on the m website ( bulbs.com) or through contacting the manufacturer. However, a similar 8 W bulb (versus the 9 W UV bulb used in experiments) specifications were found on T5UV lamp 8W 365nm). This UV outp ut was equivalent to 1.6 watts. Therefore, using the same area as above, the intensity is equal to 58.42 mW/cm 2 In addition, the total flux for these specific types of UV lamps and batch reactors has been measured and recorded using ferrioxalate actinom et ry 111 For the 254 nm and 365 nm lamps, the flux in the surrounding solution was found to be 2.4610 5 einsteins/min and 1.2510 5 einsteins/min, respectively. This shows more photon energy for the 254 nm bulb than the 365 nm bulb. Comparison to Sunlight Th e sun gives off various types of light and does not have a specific wavelength. The wavelength and intensity spectrum for the sun can be seen in Figure 8. From the graph it can be seen that the highest irradiance is in the 400 500 nm wavelength ranges. Whi le there is some 254 nm wavelengths emitted from the sun, it is only a small portion. The 365 nm lamp is a better representation of a wavelength that is given off by the sun. The intensity of wavelengths between 200 300 nm has been recorded as 0.37 mW/cm 2 with a photon energy ranging between 4.15 6.22 eV 138 The solar intensity of wavelengths between 300 400 nm is recorded as 7.91 mW/cm 2 with a photon energy from 3.12 4.14 eV 138 The intensities of the lamps used in the batch reactor experiments were pr eviously calculated based on UV output in the range of 50 80 mW/cm 2 These intensities are

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84 much higher than the intensities given off by the sun. Therefore, the batch reactor experiments are better applied to water treatment applications with insertable UV lamps rather than natural water systems. Effect of Dissolved Organic Matter Ab sorption Beer Lambert Law is defined as: (A 2) the sample, and A is absorbance. L/mol/cm 139 at a wavelength of 254nm. With a MW of 1851.5 g/mol C equivalent to 0. 06 L/ m/ m g 139 Using a path length of 1 cm (this is the half way point betw een the wall of the batch reactor and the inserted UV l amp, i.e., the average distance, and the highest humic acid concentration used (10 mg/L or 5 mg C/L ), the absorb ance equal to 0. 00 3. to be 350 L/mol/cm 139 or 0.019 L/m/mg. Using the same method as ab ove to solve for the absorbance of Suwannee River humic acid, absorbance is equivalent to 9 5 *10 4 Another way to calculate absorption is to assume a SUVA (specific ultraviolet absorption) value. A SUVA value of 7.5 can be found in the literature for HA 140 The absorption can then be calculated from the following equation 141 : SUVA = UVA/DOC*100 cm/m (A 3) Where S UVA is in units of L/mg m, UVA (UV absorption at 254 nm) i s in units of cm 1 and DOC (dissolved organic carbon) is in units of mg/L. Using a DOC value of 5

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85 mg C/L to represent 10 mg/L of HA since 50% of HA is composed of carbon, the given values equal a UVA of 0.375 cm 1 Using a path length of 1 cm since this is the halfway poin t between the wall of the batch reactor and the inserted UV l amp, the absorbance can be calculated as 0.38. Effect of Wavelength & pH The Beer Lambert absorption coefficient for humic acid changes as a function of wavelength. As seen in Figure A 2 as the wavelength increases, the molar absorptivity decreases. The solid dark line represents the organic matter that was used in the batch reactor experiments. Changes in pH may cause shifts in ionization, which can change the absorbance 142 Beer escribes a linear relationship between absorbance and concentration; however, if there is ionization, this may cause nonlinearity. I n the batch reactor experiments studied pH is not altered and thus the humic acid absorbance coefficient is not thought to c hange. Effects of UV Intensity Humic acid may affect the intensity of the UV lamps. Intensity is affected by absorb ance by the following equation 142 : A = log(I/I o ) (A 4 ) where I 0 is initial intensity, I is final intensity, and A is absorbance. Using this equation and absorbance and intensity calculated above, Figure A 3 and Figure A 4 results based on 254 nm and 365 nm wavelengths, respectively. The calculations used to derive the graph, including dissolved organic matter concentrations and wavel engths, can be seen in Table A 1 and Table A 2

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86 As seen in Figure A 3 and Figure A 4 as hum ic acid concentration increases the average UV intensity decreases because of the increased absorbance. However, even at a maximum concentration of 10 mg/L of humic acid wi th 254 nm UV irradiation, the intensity decreases by only 0.6 mW/cm 2 With an initial intensity of roughly 88 mW/cm 2 0.6 mW/cm 2 is a negligible decline. Extended Run Times Extended contact time runs were studied in order to determine if there is a n agent within the solution that is causing reduction of Hg, or if the photons energy was exciting the Hg atoms directly. As seen in Figure A 5, the removal of Hg levels off after a certain period of time, indicating that there is a limited agent within the soluti on that is driving Hg reduction. Figure A 6 shows extended run times for 1000 ppb Hg, 254 nm UV, and an oxygen purge. Even after 3 hours, there is limited Hg removal. Therefore, it is unlikely that the photons are directly exciting the mercury into the vo latile form and then removed immediately. Instead, the driving mechanism likely involves the conversion of nitrate into oxidizing conditions since the DO is saturated under an oxygen purge

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87 Figure A 1. Solar spectrum as a function of wavelength 138 Figure A 2. Molar absorptivity as a function of wavelength for various humic substances (SRHA = Suwannee River humic acid; SRF A = Suwannee River fulvic acid) 139

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88 Figure A 3. UV intensity as a function of Suwannee River humic acid absorbance when irra diated by a 254 nm bulb Figure A 4. UV intensity as a function of Suwannee River humic acid absorbance when irradiated by a 365 nm bulb 86.9 87 87.1 87.2 87.3 87.4 87.5 87.6 87.7 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 I (mW/cm^2) Absorbance 58.28 58.3 58.32 58.34 58.36 58.38 58.4 58.42 58.44 0 0.0002 0.0004 0.0006 0.0008 0.001 I (mW/cm^2) Absorbance

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89 Figure A 5. Aqueous Hg removal of 1000 ppb Hg versus time in the prese nce of a nitrogen purge and 254 n m UV Figure A 6. Aqueous Hg removal of 1000 ppb Hg versus time in the presence of a nitrogen purge and 254 nm UV 0 0.2 0.4 0.6 0.8 1 1.2 0 25 50 75 100 125 150 175 200 C/Co Time (minutes) 0 0.2 0.4 0.6 0.8 1 1.2 0 25 50 75 100 125 150 175 200 C/Co Time (minutes)

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90 Table A 1. Variables used to calculate final intensity for various humic acid concentrations irradiated with a 254 nm lamp Humic Acid Concentration Absorbance Initial Intensity Final Intensity 0 mg/L 0 87.62 mW/cm 2 87.62 mW/cm 2 1 mg/L 0.00 0 3 87.62 mW/cm 2 87.56 mW/cm 2 10 mg/L 0.003 87.62 mW/cm 2 87.02 mW/cm 2 Table A 2. Variables used to calculate final intensity for various humic acid concentrations irradiated with a 365 nm lamp Humic Acid Concentration Absorbance Initial Intensity Final Intensity 0 mg/L 0 58.42 mW/cm 2 58.42 mW/cm 2 1 mg/L 9. 5*10 5 58.42 mW/cm 2 58.41 mW/cm 2 10 mg/L 9. 5*10 4 58.42 mW/cm 2 58.29 mW/cm 2

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103 BIOGRAPHICAL SKETCH Amy Marie Borello was born in Hollywood, FL and grew up in Vero Beach, FL. After Graduating from Vero Beach High School in 2005, she received the Florida Academic Scholars Award and attended the University of Florida. As an undergraduate student Ms. Borello interned at the Smithsonian Marine Station, MBV Engineering, and Sol Gel Solutions. In 2009, she completed her honors research on mercury removal from the water and graduated Magna Cum Laude with a B.S. in environmental e ngineering. Ms. Borello received the Alumni Graduate Fellowship and the National Science Foundation Graduate Fellowship, which fully funded her graduate studies. She was the ater Works Association (AWWA), committee m currently serves on the Graduate Student Advisory Board f or the Engineering School of Sustainable Infrastructure & Environment. She has volunteered with various programs to raise awareness about science and engineering to students such as Science Quest, Science Partners in Inquiry based Collaborative Education ( SPICE), Gator Engineering Engineering Communication Mathematics Enhancement (SECME). Ms. Borello is completing her doctoral research in photochemical transformations of aqueous mercury un der the guidance of Dr. David Mazyck. She will graduate with her Ph.D. in May 2013.