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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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© 2013 Amy M. Borello

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To my parents for their unparalleled support

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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. I wouldn’t be where I am today without his support and

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” but created memories I’ll never forget. 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 Anna Casasus for their support and guidance.

This process was even more rewarding having my fiancé, Alec Gruss, there for

every step of my journey – even through all the dropped Skype calls. I couldn’t have

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. I’m grateful they’ve 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 when their background isn’t even slightly related to mine. I cannot put into words

how grateful I am to my parents, Dan and Mary Borello, and how they have supported

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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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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 of Mercury Pollution .................................................................. 20

Industries ................................................................................................... 20 Geography ................................................................................................. 21 Atmospheric Transport ............................................................................... 21

Regulations ............................................................................................................. 22 Health Impacts of 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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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

254 nm ............................................................................................................. 50 365 nm ............................................................................................................. 51

Oxygen Purge ......................................................................................................... 52 High Hg Concentrations ................................................................................... 52 Low Hg Concentrations .................................................................................... 52

5 EFFECT OF DISSOLVED ORGANIC MATTER CONCENTRATIONS ON PHOTOCHEMICAL AQUEOUS MERCURY REMOVAL ........................................ 57

Background ............................................................................................................. 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 Future 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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Effect of Wavelength & pH ......................................................................... 85 Effects of UV Intensity ................................................................................ 85

Extended Run Times .............................................................................................. 86

LIST OF REFERENCES ............................................................................................... 91

BIOGRAPHICAL SKETCH .......................................................................................... 103

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LIST OF TABLES

Table page 2-1 Mercury removal efficiency (%) for some technologies 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 kinetics 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 nitrogen 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 Suwannee 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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LIST OF FIGURES

Figure page 2-1 Proportions of global anthropogenic sources 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 anthropogenic emissions of total mercury to the atmosphere 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 Mercury Eh-pH diagram for systems containing Hg, O, H, S, and Cl ................. 37

2-8 Distribution of Hg(II) at various pH ...................................................................... 38

2-9 Speciation of 100 ppb Hg in deionized water as prepared from Hg(NO3)2 ......... 39

3-1 Batch reactor set-up schematic .......................................................................... 45

3-2 Spectrum of 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 N2 and 254 nm UV .......................................................................... 53

4-2 Aqueous Hg removal of various Hg concentrations versus time in the presence of N2 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 O2 and 254 nm UV .......................................................................... 55

4-6 Aqueous Hg removal of various Hg concentrations versus time in the presence of O2 and 365nm UV ........................................................................... 55

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5-1 Comparison of Hg C/C0 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/C0 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/C0 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/C0 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/C0 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/C0 versus time of FA and HA in the presence of a nitrogen purge .................................................................................................... 76

6-4 Comparison of Hg C/C0 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/C0 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; SRFA = 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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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

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 oxygen saturated aqueous solutions, mercury

removal reached 90% after 60 minutes. Since dissolved organic matter, such as humic

and fulvic acids, 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;

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however, the concentration ratio of Hg to humic or fulvic acids did not affect the overall

mercury removal.

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CHAPTER 1 INTRODUCTION

Problem Statement

Man’s fascination with mercury (Hg) is seen throughout history, from the works of

Lewis Carroll 1 to Sir Isaac Newton’s obsession with alchemy 2. The First Emperor of

China, Qin Shinhuang, was especially obsessed with this metal. Thought to make him

immortal, 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.

It wasn’t until the 1950’s with the Minamata tragedy in Japan that its disastrous

effects were discovered. Industrial waste released into Minamata Bay contained Hg and

was naturally converted into the highly toxic form of methylmercury (MeHg).

Bioaccumulation of MeHg occurred in the bay’s fish population, the main source of

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, after consumption 5, also known as Minamata Disease 6-8. Recent research

shows Hg contamination is a rising international problem. Currently, 10% of childbearing

age women in the U.S. have Hg concentrations above the level considered by the

United States Environmental Protection Agency (EPA) to be safe to fetuses and young

children 9.

Despite awareness of mercury’s adverse health and environmental impacts, its

appeal endures. Mercury is still used today in industrial settings for fluorescent light bulb

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production, gold production, chlor-alkali facilities, as well as others10. 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 providing a

driving force (gas purge) to release the volatile mercury from solution and subsequently

remove it via gas-phase adsorption.

This study focuses on photochemical removal of Hg from synthetic solutions in

order to better understand the reduction mechanisms that are occurring. This method is

preferable because direct adsorption 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(NO3)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 reactions 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(NO3)2, the DGM production is affected by

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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 due 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 photochemical 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 increasing concentrations of DOM affect the DGM production and subsequent removal.

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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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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 imperative 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 color16. While cinnabar is has low

solubility, the presence of iron and humic substances enhance cinnabar solubility 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 anthropogenic Hg 19-

21. These natural pathways account for one-third of Hg emissions to the atmosphere22.

Typically, elemental mercury (Hg0) is found in the atmosphere, but oxidized forms either

in the gas phase or attached to particles are also present 23. Hg salts, HgCl2, HgS, and

HgO, are also naturally occurring. While they can cause gastrointestinal tract damage

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and kidney failure, they are not common forms of naturally occurring Hg24 and only

HgCl2 is volatile enough to exist as an atmospheric gas 22.

Anthropogenic

Burning of fossil fuels, primarily coal, is the largest source of anthropogenic

emissions accounting for 45% of total anthropogenic emissions 22. Other anthropogenic

mercury sources include metal and chemical manufacturing, caustic soda production,

incineration 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

technologies 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.

Using chlorine electrolytic cells, chlor-alkali facilities 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 Act31. 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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FGD wet scrubbers are widely used by coal-fired power plants in the U.S. to

remove sulfur dioxide (SO2) 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

published on Hg concentrations of FGD 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

Industries

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,

batteries, 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 contains 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 and deposition subsequently

occurs (Figure 2-2).

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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 Europe combined. Figure 2-3 shows the total

amount of industrial Hg emissions separated by various regions 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 discrepancy is likely do to the large increase in

mercury emission in Africa 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 annually, 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, respectively22. The two

industries that produce the most Hg emissions in Asia are fossil fuel combustion for

energy and artisanal gold production22. Due to Hg’s ability to cycle globally, international

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 anthropogenic 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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receiving some deposited Hg from Eastern Asia22. 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 that Hg poses to human health, it is important to regulate this toxic

metal. The 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 because of its ability to be naturally converted biotically to

methylmercury in the water, the most toxic form of Hg that bioaccumulates in the food

chain 44. Thus, a modest increase in Hg pollution can result in larger Hg concentrations

in fish and other vertebrates, sometimes thousands of levels higher than the

surrounding water22. 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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admitted into the atmosphere, it will not remain in the gaseous phase it’s entire lifetime

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 animals45, which means the most toxic form of Hg is

consumed.

It is also possible to be exposed to Hg in the gaseous form. Typically, 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 through the blood stream and enters the brain

by mimicking the amino acid methionine4. 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 reference does (RfD) of 0.1 μg Hg/kg body weight/day,

which is equivalent to 5.8 μg/L (ppb) Hg46. Below this level, adverse health effects of

mercury are not expected.

These 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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preservative and antifungal agent in the form of thiomersal for vaccines, cleaning

solution for contact lenses, and makeup 47. Currently the federal government allows 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 stricter

standards than the rest of the U.S.49.

An organometallic cation of Hg, methylmercury, causes the most damage 50-52. As

stated previously, exposure to methylmercury most commonly occurs from eating fish.

Since the amount of mercury varies from fish to fish, and eating fish is known to be

good for one’s health due to the omega-3 fatty acids, eating fish has been compared to

“mercury Russian roulette.”53 When methylmercury was first synthesized in the 1860s,

two of the laboratory technicians died from mercury poisoning and production of

methylmercury was put on hold. It wasn’t until the twentieth century, when it’s antifungal

property was discovered, that Hg was applied to seeds for cereal crops in the farming

industry 54. While Hg aided in the green revolution, 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

consume high quantities of fish containing Hg can pass this toxic metal to their children

59-61. Studies also show that increased methylmercury exposure correlates to decreased

performance of intelligent quotient (IQ) tests 57.

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Off of Japan’s Kyushu Island, the inhabitants of Minamata experienced these

devastating effects of methylmercury poisoning 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 affecting 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 methylmercury 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 ventilation24. This is common in developing countries with

small-scale gold production. If elemental Hg is ingested it is absorbed slowly 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 effects,

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 weakness66.

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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 regulated (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 hazardous 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 would 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.

Currently, 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 in benefits – not including the assumed $9.6 billion yearly in compliance

costs71. Compliance is going to be based on maximum achievable control technology

(MACT) per requirements in the Clean Air Act with a total of 20 tons of Hg emissions cut

27

by 2016. With wet scrubbers as a MACT, increased wastewater containing Hg is

expected in the near future 14. Compliance is required in the next three years and

therefore developing control technologies is vital.

Due to the fact that Hg pollution is a global human health 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 well as reducing it’s use in many

products and processes by 2020 73. The treaty still needs to be ratified and in October of

this year another meeting will be held and is expected to be put into effect in 3-4

years73.

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, Hg0, Hg+, and Hg2+, and is found in many forms in the environment. In

environmental aqueous systems 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 aqueous 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:

Hg2+ + H2O Hg(OH)+ + H+ (2-1)

28

Hg(OH)+ + H2O Hg(OH)2 + H+ (2-2)

The speciation of Hg used in these experiments was Hg(NO3)2, which is known to

ionize in solution to form Hg2+ and NO3- 74. Figure 2-9 shows the species of Hg in

prepared solutions of Hg(NO3)2 and deionized water. Indeed, Hg(OH)2 is the dominant

species.

Photochemistry of Aqueous Mercury

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 Hg0 and Hg2+ 75. Hg0 is volatile and therefore not as soluble in water as Hg2+.

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 photoreduction 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 Hg0 + OH (2-3)

Hg(OH)aq + H+ + e- Hg0 + H2O (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

29

oxidation reactions, can also occur without the presence of chlorine. These equations

are as follows 84,87:

Hg0 + hv Hg2+ + 2e- (2-6)

Hg0 + 2OH Hg2+ + 2OH- (2-7)

Hg0 + 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 the 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 decomposing organic matter 88.

According to the hard and soft Lewis acids and bases concept, Hg is a “soft” acid that

binds strongly with “soft” bases, such as thiol, as opposed to “hard” bases that contain

oxygen 89. The “soft” functional groups on humic substances that Hg binds the strongest

to are reduced 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 humic 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 seen that high concentrations of Hg first bind with reduced sulfur and

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 known 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 enhanced by the presence of DOM. Other

studies also found that the presence of DOM enhances the production rate of DGM 95,96.

Control studies performed in the dark by Ravichandran 97 with 100 ppt Hg in the

presence of DOM produced no measurable amounts of DGM although 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 production 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 al. 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 O’Driscoll et al. 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

31

UV-irradiation or from increasing complexation of Hg-DOM. These reports also suggest

direct photolysis of DOM:

DOM + H2O + hv OH + products (2-10)

where 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 that the photolysis of DOM is capable of Hg

reduction and may 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 hydroxide or hydrogen ions:

DOM + hv DOM* (2-12)

DOM* + O2 DOM+ + O2− (2-13)

O2− + Hg(II) Hg(0) + O2 (2-14)

O2− + O2

− + H+ H2O2 + O2 (2-15)

H2O2 + 2OH− + Hg(II) O2 + 2H2O + Hg(0) (2-16)

H2O2 + 2H+ + Hg(0) 2H2O + 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 DOM. 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.

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 affects the binding constants of Hg-DOM. The binding

constants of Hg-DOM in the literature span as low as 104.7 to as high as 1032.2 88. Only

recently has the role of the ratio of Hg:DOM concentration been recently studied. Using

XANES technology, it can be seen that the ratio of Hg-S bonds to Hg-O bonds

increases with decreasing concentrations of Hg 102. Haitzer et al. 103 measured a wide

range of Hg:DOM ratios and measured the binding strength using an equilibrium

dialysis ligand exchange method. At Hg:DOM ratios less than 1 μg/mg (i.e.: 10 ppb Hg

with 10 ppm DOM), Hg is able to primarily bind to the reduced sulfur groups while ratios

greater than 10 μg/mg (i.e.: 100 ppb Hg with 10 ppm DOM) resulted 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.

Following these studies, filtered natural waters (0.5 μm PAC briquette with lead

reduction cartridge) were spiked with varying Hg:DOM ratios to measure the varying

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

saturated reduced sulfur sites. Because these bonds are weak, it is assumed it’s easier

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

production.

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

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 understand

mercury cycling with the additional potential of improved treatment methods.

34

Figure 2-1. Proportions of global anthropogenic sources of Hg based on industry22

Figure 2-2. Schematic of mercury cycle in the environment

35

Figure 2-3. Breakdown of industrial Hg emissions by region22

Figure 2-4. Change of global anthropogenic emissions of total mercury to the

atmosphere from 1990-2000 (in tons) 105

36

Figure 2-5. Model of atmospheric transport of anthropogenic Hg emissions from East

Asia22

Figure 2-6. Timeline of mercury use and subsequent regulations in the U.S.

37

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

38

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

39

Figure 2-9. Speciation of 100 ppb Hg in deionized water as prepared from Hg(NO3)2

109

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

41

CHAPTER 3 EXPERIMENTAL

Chemicals

Mercury solutions of 10, 50, 100, 500, and 1000 μg/L were synthesized through

dilution of a 1000 mg/L mercury nitrate, Hg(NO3)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 Scientific) 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

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 before 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 and 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 spectrum of the 254 nm and 365 nm

lamps, respectively. Figure 3-2 illustrates that there is a very narrow spectrum range for

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 ¼ 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 connected

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 solution 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 through a 0.45 μm mixed cellulose membrane filter

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

SnCl2 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 HNO3 and 1 mL H2SO4 was added to acidify

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% KMnO4, 1.6 mL of 5% K2S2O8 and

1.2 of 12% NaCl-hydroxylamine sulfate solution (Fisher Scientific). Hg removal is

reported as C/Co, the final concentration over initial concentration. Error bars illustrate

the range of duplicate reactor runs.

Humic Acid Analysis

In order to determine the transformation of humic and fulvic acids, 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 for 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 oC) furnace. CO2 is analyzed with an NDIR (non-

dispersive infrared gas analyzer) detector.

45

Figure 3-1. Batch reactor set-up schematic

Figure 3-2. Spectrum of 254nm lamp 113

46

Figure 3-3. Spectrum of 365 nm lamp 114

47

CHAPTER 4 PHOTOCHEMICAL MECHANISMS OF AQUEOUS 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 anthropogenic 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 eliminated 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-μg/L levels. In the Great

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 techniques 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

and 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 air

phase. This method of transferring Hg from the water phase to the air phase allows for

less adsorption competition in the diverse water matrix.

48

Control Studies

To ensure that aqueous Hg loss is a result of photochemical reactions, control

experiments were performed. When studying controls in the dark, the UV lamp was not

illuminated but did remain in solution. Experiments run in the dark, ones with purging

alone, and others with no purge or UV irradiation were performed. The lowest Hg

concentration used 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 performed with both 254 nm and 365 nm UV. The

purge gas utilized for the control experiments was N2 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 presented 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 N2 purge, a C/C0

value of 0.97 is obtained after 60 minutes. When 254 nm and 365 nm UV lamps lit and

no purge is present, C/C0 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/C0 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 N2 purge

gas was run concurrently with 254 nm or 365 nm UV, low C/C0 values of 0.02 and 0.01

were measured, respectively. These results suggest that photochemical reactions are

responsible for the transformation of soluble ionic Hg to volatile elemental Hg where it is

then transferred to the purge gas and subsequently removed from the system. In

49

addition, there is no significant difference between filtered and unfiltered samples,

implying that species of Hg greater than 0.45 μm were not formed.

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 using Henry’s Law at standard temperature and pressure. Experimental

results show that at an initial concentration of 50 ppb with 60 minutes of exposure time,

the final Hg concentration reached 0.4 ppb (C/C0 = 0.008). Even at higher

concentrations of 1000 ppb, almost 80% removal of Hg is achieved after 60 minutes.

365 nm

When solutions of the same initial Hg concentration as listed above were exposed

to nitrogen purge while being irradiated by 365 nm UV, the decrease in photon energy

caused less reduction of 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 total 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/C0 = 0.02), comparable to results seen from 254 nm UV experimental runs

(Figure 4-1).

50

The reaction kinetics for Hg removal 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 compared

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 UV.

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. This shows a faster initial conversion rate of divalent

Hg2+ to Hg0 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 option for treatment technologies than nitrogen due to cost and

availability. Since the addition of air would cause a higher dissolved oxygen

concentration within solution (calculated as 8 mg/L using Henry’s Law) and would

51

therefore increase the number of possible 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

When air purged experiments were performed 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 removal when exposed to 365 nm than with 254 nm UV. Only 30% Hg

removal was achieved with 254 nm irradiation (C/C0 = 0.71) where 85% removal

achieved with 365 nm UV (C/C0 = 0.15).

It is believed that photolysis of nitrate might be the cause of the disparity of Hg

removal between the two wavelengths. At wavelengths ≤ 302 nm, photolysis of nitrate

occurs by the following pathways 115,116:

NO3− + hv NO2 + O−

(4-1)

O− + H2O OH− + OH (4-2)

OH + Hg0 OH− + Hg2+ (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.

52

Oxygen Purge

High Hg Concentrations

The same initial Hg concentrations as discussed previously were then exposed to

an oxygen purge 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 and 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(NO3)2.

Increasing concentrations of nitrate consume more photons and lead to Hg oxidation,

especially under highly oxidizing conditions created from an oxygen purge. It is also

likely that the large gap in Hg concentrations causes a large gap in Hg removal.

53

Figure 4-1. Aqueous Hg removal of various Hg concentrations versus time in the

presence of N2 and 254 nm UV

Figure 4-2. Aqueous Hg removal of various Hg concentrations versus time in the

presence of N2 and 365 nm UV

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 versus time in the

presence of air and 365 nm UV

55

Figure 4-5. Aqueous Hg removal of various Hg concentrations versus time in the

presence of O2 and 254 nm UV

Figure 4-6. Aqueous Hg removal of various Hg concentrations versus time in the

presence of O2 and 365nm UV

56

Table 4-1. Control studies for Hg concentrations of 50 ppb with run times of 60 minutes

Condition C/Co Range* Sample Treatment

No UV, no purge 1.0 ±0.03 Unfiltered No UV, N2 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, N2 purge 254 nm UV, N2 purge 365 nm UV, N2 purge 365 nm UV, N2 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 (R2)

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 Ms-1 0.994 1000 Zero 5 × 10-8 Ms-1 0.948

57

CHAPTER 5 EFFECT OF DISSOLVED ORGANIC MATTER CONCENTRATIONS ON

PHOTOCHEMICAL AQUEOUS 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 production 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 humic 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 (Sred) sites in DOM 91. Therefore, the

amount of Sred 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.

58

Since DOM is ubiquitous in natural waters and numerous waste and industrial

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

understand 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 its correlation

to DGM production, and (3) the effect of increased oxygen 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 illuminated but did

remain in solution. Hg concentrations of 100 μg/L were used for all control experiments.

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 (N2, Air, O2) 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/C0 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 solutions without HA, there seems to be

slightly less Hg removal than there is with humic acid present (C/C0 of 0.22). Therefore,

due to the presence of HA, some Hg is transformed to volatile elemental Hg and

59

subsequently leaves the system via purge gas. It is possible 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 concurrently as opposed to separately in

solutions containing only Hg, final Hg concentrations are lowered (C/Co 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 volatilization 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 ppm 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, when 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 after 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 was 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 case. The decrease of Hg removal as the concentration of humic acid

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 increasing 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 due to DOM limiting UV-irradiation from reaching the mercury

99,100. Other reports involving natural waters suggest direct photolysis of DOM 87:

DOM + H2O + hvOH + products (5-1)

where the hydroxyl radical then initiates oxidation reactions that lead to the subsequent

oxidation of Hg:

OH + Hg0 Hg2+ + OH− (5-2)

However, it has also been proposed that the photolysis of DOM is also capable of

Hg reduction 83:

DOM + hv DOM* (5-3)

DOM* + O2DOM+ + O2

− (5-4)

O2−+ Hg2+

Hg0 + O2 (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, the superoxide anion could also lead to

reduction of Hg by a different mechanism 83,100:

61

O2−+ O2

− + H+ H2O2 + O2 (5-6)

H2O2 + 2OH− + Hg2+ O2 + 2H2O + Hg0 (5-7)

or could subsequently lead to re-oxidation in the presence of hydrogen ions when

reacting with newly formed hydrogen peroxide 120:

H2O2 + 2H+ + Hg0 2H2O + Hg2+ (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-molecular 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 functional 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/C0 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 performed, but with the ratio of Hg and HA

concentrations as 10:1. Similarly, various concentrations of Hg and HA showed a similar

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), they 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 that at high ratios of Hg:DOM, such as 100:1, Hg primarily bonds to

the more abundant carboxyl groups as opposed to forming stronger bonds with reduced

sulfur groups 103. Therefore, a higher ratio should have a faster rate of removal than a

smaller ratio does. 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

between 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,

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 in 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/C0 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

reduction 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 could 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

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. Additionally, the total

photochemical destruction 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 which aid in the reduction of Hg, as

seen in Equations 5-3 through 5-7 above. Thus, since the O2 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 O2 purge, as seen in Figure 5-3.

65

Figure 5-1. Comparison of Hg C/C0 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/C0 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

66

Figure 5-3. Comparison of Hg C/C0 versus time in the presence of Air and oxygen

purges and 254 nm UV with Hg concentrations of 100 ppb and varying HA

67

Table 5-1. Control studies for Hg Concentrations of 100 ppb with run times of 60 minutes

Condition Average Hg, C/C0 Range

100 ppb Hg, 10 ppm HA

No purge, 254 nm 1.0 ±0.01

N2 purge, no UV 0.89 ±0.01

Air purge, no UV 0.90 ±0.02

O2 purge, no UV 0.93 ±0.01

N2 purge, 254 nm 0.22 ±0.02

100 ppb Hg, No HA

No purge, 254 nm 1.0 ±0.05

N2 purge, no UV 0.97 ±0.06

N2 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

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

N2 purge, no UV 0.1% ±0.34

Air purge, no UV 1.8% ±0.31

O2 purge, no UV 0.2% ±0.19

Concurrent Experiments

N2 purge, 254 nm 1.7% ±0.03

Air purge, 254 nm 24.1% ±0.38

O2 purge, 254 nm 43.2% ±0.07

69

CHAPTER 6 EFFECT OF VARIOUS TYPES OF ORGANIC MATTER ON PHOTOCHEMICAL

AQUEOUS MERCURY REMOVAL

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 Hg2+ captured in the wastewater have been known to

convert back to elemental Hg (Hg0) 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 mechanisms 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 composition of organic matter

affects Hg removal and reemission. To elucidate the complex mechanisms 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) the ratio of

70

Hg:DOM and its correlation to DGM production, and (3) how destruction of FA by UV

affects overall Hg removal.

Mercury Volatilization as a Function of Organic Matter Concentrations Induced by Nitrogen Purge

Total mercury removal was measured when 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

exposure 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 was also seen in Chapter 5. However, the phenomena in the lack of variation

between 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 looked 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. This 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.

71

It could also be argued that no matter the Hg concentration, if the aqueous

solution has the same concentration of organic matter, then it might result in the same

Hg removal due to the saturated amount 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/C0 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 comparing 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 of Hg:DOM, they all showed roughly the same Hg

removal. This demonstrates that the total amount of Hg bonding to Sred sites on organic

matter is not the sole determining factor for overall Hg removal.

Another way to look at the effect of Sred 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 than FA since Hg will form stronger

bonds and not be available for volatilization. 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 Hg removal in the presence of HA. In fact, a review of the literature shows

that only 1.6-2.0% of Sred sites are involved in strong interactions with Hg 103,128.

72

Therefore, it’s also important to look at other factors that may affect Hg removal such as

the breakdown of DOM versus only considering Sred bonding.

The Effect Fulvic Acid Reduction on Dissolved Gaseous Mercury Removal

The Effect of Purge Gas Type

Since so few sulfur functional groups bind strongly with Hg, it is important to look

at other mechanisms for Hg 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 O2_ 129-131. These free radicals can lead to

Hg oxidation in the following reactions:

DOM + hv DOM* (6-1)

DOM* + O2 DOM+ + O2− (6-2)

O2− + O2

− + H+ H2O2 + O2 (6-3)

H2O2 + 2H+ + Hg(0) 2H2O + Hg(II) (6-4)

DOM* + H2O 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.

O2 and air purges produced the same rate of decrease in the molecular weight of FA,

while a N2 purge caused very little change to the FA. It order to determine if

73

photochemical changes of organic matter are a factor for Hg removal, C/C0 was

measured under varying purge gases with 365 nm irradiation.

Figure 6-4 compares an O2 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 O2 and air

purge cause different DO concentrations, they showed the same trend of Hg removal

indicating that the breakdown 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 N2 purge under the same conditions. It

is known that very little organic matter breakdown occurs when in the presence of a N2

purge 132,133. Therefore, there should be even more Hg removal (than the O2 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 (N2 purge with 365 nm UV), the purge that produces

the most Hg removal is indeed the N2 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 way to observe how the reduction of FA affects Hg removal is to look at

varying UV wavelengths. 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,

74

more Hg removal occurs 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 removal 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.

75

Figure 6-1. Comparison of Hg C/C0 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/C0 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

76

Figure 6-3. Comparison of Hg C/C0 versus time of FA and HA in the presence of a nitrogen purge and (a) 254 nm and (b)

365 nm wavelength

77

Figure 6-4. Comparison of Hg C/C0 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/C0 versus time of 254 nm and 365 nm in the presence

of an air purge with 10 ppb Hg, 1 ppm FA

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

79

CHAPTER 7 CONCLUSIONS

Contributions to Science

The impact that UV wavelength, purge gas 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 studied), 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 Hg2+ and NO3- in water. Therefore, it is

important to understand how nitrate behaves. At wavelengths less than ≤ 302 nm,

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 when FA was added

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 with

80

Hg and decreased DGM production. However, when FA 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 Research 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 in 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 simplified 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.

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 the removal rate of Hg, as is the presence of

additional redox reactions not seen in simplified waters.

82

APPENDIX A SUPPLEMENTAL INFORMATION

UV Calculations

Photon Energy

Variations in wavelength were studied as well. When irradiated with 254 nm

wavelengths, 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 equation below represents the relationship between photon energy and

wavelength137:

E = h*c/λ (A-1)

Where E is photon energy, h is the Plank constant (6.63×10-34 J*s), c is the speed

of light in a vacuum (3.0×108 m/s), and λ is the wavelength. With this equation it can be

seen that as wavelength increases, the photon energy decreases. Therefore, 254 nm

has higher photon energy than 365 nm (due to it’s shorter wavelength) and thus causes

more reduction reactions in the same period of time.

Intensity

The UV output of the 254 nm UV lamp (according to manufacturer’s website:

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 cm2. With a UV output of 2.4 watts (2400 mW),

this equals an intensity of 87.62 mW/cm2.

83

The UV output of the 365 nm UV lamp was not available on the manufacture’s

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

another manufacture’s website (maestrogen.com – T5UV lamp 8W 365nm). This UV

output was equivalent to 1.6 watts. Therefore, using the same area as above, the

intensity is equal to 58.42 mW/cm2.

In addition, the total flux for these specific types of UV lamps and batch reactors

has been measured and recorded using ferrioxalate actinometry 111. For the 254 nm

and 365 nm lamps, the flux in the surrounding solution was found to be 2.46×105

einsteins/min and 1.25×105 einsteins/min, respectively. This shows more photon energy

for the 254 nm bulb than the 365 nm bulb.

Comparison to Sunlight

The 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. While 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/cm2 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/cm2 with a photon energy

from 3.12-4.14 eV 138.

The intensities of the lamps used in the batch reactor experiments were previously

calculated based on UV output in the range of 50-80 mW/cm2. These intensities are

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

Absorption

Beer-Lambert Law is defined as:

A = ε * b * c (A-2)

Where ε is the absorptivity coefficient, b is the path length, c is the concentration in

the sample, and A is absorbance.

Molar absorptivity, ε, for Suwannee River humic acid was found to be 1100

L/mol/cm 139 at a wavelength of 254nm. With a MW of 1851.5 g/mol-C, this ε is

equivalent to 0.06 L/m/mg 139.

Using a path length of 1 cm (this is the half-way point between the wall of the

batch reactor and the inserted UV lamp, i.e., the average distance, and the highest

humic acid concentration used (10 mg/L or 5 mg-C/L), the absorbance equal to 0.003.

At wavelengths of 365 nm, ε for Suwannee River humic acid was found to be 350

L/mol/cm 139 or 0.019 L/m/mg. Using the same method as above 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 SUVA is in units of L/mg-m, UVA (UV absorption at 254 nm) is in units of

cm-1, and DOC (dissolved organic carbon) is in units of mg/L. Using a DOC value of 5

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

point between the wall of the batch reactor and the inserted UV lamp, 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

absorbance142. Beer-Lambert’s Law describes a linear relationship between absorbance

and concentration; however, if there is ionization, this may cause nonlinearity. In the

batch reactor experiments studied pH is not altered and thus the humic acid absorbance

coefficient is not thought to change.

Effects of UV Intensity

Humic acid may affect the intensity of the UV lamps. Intensity is affected by

absorbance by the following equation 142:

A = -log(I/Io) (A-4)

where I0 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 wavelengths, can be seen in Table A-1 and Table A-2.

86

As seen in Figure A-3 and Figure A-4, as humic 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 with 254 nm UV irradiation, the

intensity decreases by only 0.6 mW/cm2. With an initial intensity of roughly 88 mW/cm2,

0.6 mW/cm2 is a negligible decline.

Extended Run Times

Extended contact time runs were studied in order to determine if there is an 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 solution 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 volatile 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

87

Figure A-1. Solar spectrum as a function of wavelength138

Figure A-2. Molar absorptivity as a function of wavelength for various humic substances

(SRHA = Suwannee River humic acid; SRFA = Suwannee River fulvic acid)139

88

Figure A-3. UV intensity as a function of Suwannee River humic acid absorbance when

irradiated 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 (m

W/

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 (m

W/

cm^

2)

Absorbance

89

Figure A-5. Aqueous Hg removal of 1000 ppb Hg versus time in the presence of a nitrogen purge and 254 nm 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)

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/cm2 87.62 mW/cm2

1 mg/L 0.0003 87.62 mW/cm2 87.56 mW/cm2

10 mg/L 0.003 87.62 mW/cm2 87.02 mW/cm2

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/cm2 58.42 mW/cm2

1 mg/L 9.5*10-5 58.42 mW/cm2 58.41 mW/cm2

10 mg/L 9.5*10-4 58.42 mW/cm2 58.29 mW/cm2

91

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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

engineering.

Ms. Borello received the Alumni Graduate Fellowship and the National Science

Foundation Graduate Fellowship, which fully funded her graduate studies. She was the

Vice President of the University of Florida’s chapter of the American Water Works

Association (AWWA), committee member for the university’s student union board, and

currently serves on the Graduate Student Advisory Board for 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

Outreach Program, AWWA’s Model Water Tower Competition, and Science

Engineering Communication Mathematics Enhancement (SECME).

Ms. Borello is completing her doctoral research in photochemical transformations

of aqueous mercury under the guidance of Dr. David Mazyck. She will graduate with her

Ph.D. in May 2013.