Table of Contents
Title Page
Copyright
Preface
Acknowledgment
Contributors
Chapter 1: Overview of Mercury in the Environment
1.1 Introduction
1.2 Toxicity and Health Risks of Mercury Exposure
1.3 Sources of Mercury
1.4 Overview of Mercury Biogeochemical Cycling
1.5 Structure of the Book
1.6 Concluding Remarks
References
Part I: Analytical Developments
Chapter 2: Advances in Speciation Analysis of Mercury in the Environment
2.1 Introduction
2.2 Sample Preparation for Hg Speciation in Environmental Samples
2.3 Application of GC Technique in Hg Speciation Analysis
2.4 Application of HPLC Technique in Hg Speciation Analysis
2.5 Application of Capillary Electrophoresis Techniques in Hg Speciation Analysis
2.6 Application of X-Ray Absorption Spectroscopy in Probing Chemical Microenvironment of Hg
2.7 Application of Stable Isotope Dilution Technique in Mercury Speciation Analysis
2.8 Summary
References
Chapter 3: Measuring Gas Phase Mercury Emissions from Industrial Effluents
3.1 Introduction
3.2 Standardized Methods for Measuring Mercury
3.3 Mercury Continuous Emission Monitors (CEMs)
3.4 Future Outlook
References
Part II: Speciation and Transformation
Chapter 4: Atmospheric Chemistry of Mercury
4.1 Introduction
4.2 The Overall Picture
4.3 Chemical Transformations in the Gas Phase
4.4 Chemical Transformations in the Aqueous Phase
4.5 Redox Chemistry at the Interface between the Atmosphere and Earth's Surfaces
4.6 Atmospheric Implications of the Identified Redox Pathways
4.7 Future Research Needs
References
Chapter 5: Microbial Transformations in the Mercury Cycle
5.1 Introduction
5.2 Mercury Methylation
5.3 Methylmercury Degradation
5.4 Redox Cycling of Inorganic Hg
5.5 Conclusions
5.6 Acknowledgments
References
Chapter 6: Photoreactions of Mercury in Aquatic Systems
6.1 Significance of Mercury Photoreactions
6.2 Concepts in Mercury Photoreactions
6.3 Current Methods in Mercury Photochemistry
6.4 Summary
References
Chapter 7: Chemical Speciation of Mercury in Soil and Sediment
7.1 Introduction
7.2 Physicochemical Properties, Oxidation States, Chemical Forms, Structures, and Concentrations of Mercury in the Environment
7.3 Aqueous Phase: Major Ligands and Their Affinities for Mercury(II)
7.4 Liquid and Solid Phases of Mercury in Soils and Sediments
7.5 Reactions of Mercury(II) with Soil and Sediment Particle Surfaces
7.6 Stabilization of Nanoparticulate Mercury(II) Sulfides by Natural Organic Matter
7.7 Solubility and Chemical Speciation of Mercury(II) in Soils and Sediments
7.8 Methods for Studying the Chemistry of Mercury(II) In Soils and Sediments
7.9 Future Research Needs
7.10 Appendix
References
Chapter 8: The Effects of Dissolved Organic Matter on Mercury Biogeochemistry
8.1 Introduction
8.2 Dissolved Organic Matter
8.3 Field Observations
8.4 Effects of DOM on Mercury Distributions Between Solution and Particles
8.5 Mercury Binding Strength
8.6 Mercury Binding Environment
8.7 Methylmercury Binding Strength and Environment
8.8 DOM and Mercury Mineral Dissolution
8.9 DOM and Mercury Mineral Precipitation
8.10 Acknowledgements
References
Chapter 9: Tracking Geochemical Transformations and Transport of Mercury through Isotope Fractionation
9.1 Introduction
9.2 Fractionation of Mercury Isotopes in Environmental Processes
9.3 Hg Isotope Variations in Nature
9.4 Summary
References
Part III: Transport and Fate
Chapter 10: Atmospheric Transport of Mercury
10.1 Introduction
10.2 General Concepts of Mercury Cycling in the Atmosphere
10.3 Methods for Studying Atmospheric Mercury Transport
10.4 Assessments of Airborne Mercury Pollution
10.5 Knowledge Gaps
References
Chapter 11: Adsorption of Mercury on Solids in the Aquatic Environment
11.1 Introduction
11.2 Adsorption of Mercury on Solids
11.3 Role of Colloids in Mercury Adsorption
11.4 Concluding Remarks
References
Chapter 12: Exchange of Elemental Mercury between the Oceans and the Atmosphere
12.1 Introduction
12.2 Models of Gas Exchange of Elemental Mercury at the Air–Sea Interface
12.3 Field Studies of Ocean-To-Air Fluxes of Mercury
12.4 Rate Constants for Reduction and Oxidation of Mercury Species in Ocean Waters
12.5 Modeling Studies Estimating Oceanic Air–Sea Exchange
12.6 CONCLUSIONS AND FUTURE DIRECTIONS
References
Chapter 13: Exchange of Mercury Between the Atmosphere and Terrestrial Ecosystems
13.1 General Overview
13.2 Methods and Tools Applied for Measurement and Understanding of Air–Terrestrial Surface Exchange
13.3 Measured Fluxes
13.4 Conclusions
13.5 Acknowledgments
References
Part IV: Bioaccumulation, Toxicity, and Metallomics
Chapter 14: Bioaccumulation and Biomagnification of Mercury Through Food Webs
14.1 Introduction
14.2 Mercury in Aquatic and Terrestrial Organisms
14.3 Mercury within Organisms
14.4 Factors Affecting Mercury in Biota
14.5 Biomagnification of Mercury Through Food Webs
14.6 Mercury Stable Isotopes in Bioaccumulation Studies
14.7 Case Study—Kejimkujik National Park and Historic Site, Nova Scotia, Canada
14.8 Conclusions
References
Chapter 15: A Review of Mercury Toxicity with Special Reference to Methylmercury
15.1 Introduction
15.2 Global Mercury Emission into the Atmosphere
15.3 Metabolism and Toxicity of Chemical Forms of Mercury
15.4 Risk Assessment of Prenatal Exposure to Methylmercury
15.5 Risks and Benefits of Fish Consumption for Brain Development
15.6 Exceptional Methylmercury Exposure Through Rice
15.7 Summary
References
Chapter 16: Metallomics of Mercury: Role of Thiol- and Selenol-Containing Biomolecules
16.1 Introduction
16.2 Metallomics of Mercury
16.3 Mercury and Methylmercury Complexes with Thiol-Containing Biomolecules
16.4 Mercury and Methylmercury Binding to Selenol-Containing Biomolecules
16.5 Lability of Mercury or Methylmercury Complexes with Thiols or Selenols
16.6 Thiol-Containing Biomolecules in the Uptake and Metabolism of Mercury
16.7 Selenium Aided Biomineralization of Mercury and Methylmercury
16.8 Analytical and Modeling Approaches
16.9 Conclusion
16.10 Acknowledgment
References
Chapter 17: Human Health Significance of Dietary Exposures to Methylmercury
17.1 Introduction
17.2 Methylmercury Exposure
17.3 Nutrients in Fish and Seafood
17.4 Major Prospective Cohort Studies
17.5 Health Effects
17.6 Cardiovascular Outcomes
17.7 Nutrient and Methylmercury Exposure as Predictors of Developmental Outcomes
17.8 Confounding Variables
17.9 Risk Assessment and Exposure Imprecision
17.10 Conclusions
References
Index
Contributors
GEORGE R. AIKEN, US Geological Survey, 3215 Marine St., Suite E127, Boulder, CO
MARC AMYOT, GRIL, Département de sciences biologiques. Université de Montréal, Montréal, Quebec, Canada
TAMAR BARKAY, Department of Biochemistry and Microbiology, Rutgers University, New Brunswick, NJ and National Environmental Research Institute (NERI), Aahus University, Roskilde, Denmark
SURESH K. BHARGAVA, Advanced Materials and Industrial Chemistry Group, School of Applied Sciences, RMIT University, Melbourne, Victoria, Australia
YONG CAI, Department of Chemistry and Biochemistry and Southeast Environmental Research Center, Florida International University, Miami, FL
ANNA L. CHOI, Department of Environmental Health, Harvard School of Public Health, Boston, MA
MEREDITH CLAYDEN, Canadian Rivers Institute and Biology Department, University of New Brunswick, Saint John, New Brunswick, Canada
XINBIN FENG, State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
CHASE A. GERBIG, Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, CO
PHILIPPE GRANDJEAN, Department of Environmental Health, Harvard School of Public Health, Boston, MA Department of Environmental Medicine, University of Southern Denmark, Odense, Denmark
MAE SEXAUER GUSTIN, Department of Natural Resources and Environmental Sciences, University of Nevada-Reno, Reno, NV
HOLGER HINTELMANN, Department of Chemistry, Trent University, Peterborough, Ontario, Canada
KONRAD HUNGERBUHLER, Safety and Environmental Technology Group, Swiss Federal Institute of Technology (ETH ZUrich), ZUrich, Switzerland
SAMUEL J. IPPOLITO, Advanced Materials and Industrial Chemistry Group, School of Applied Sciences, RMIT University, Melbourne, Victoria, Australia
TIM JARDINE, Australian Rivers Institute, Griffith University, Brisbane, Queensland, Australia
GUIBIN JIANG, State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
AKIYOSHI KAKITA, Department of Pathological Neuroscience, Resource Branch for Brain Disease Research CBBR, Brain Research Institute, Niigata University, Niigata, Japan
MOHAMMAD A. K. KHAN, Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada
KAREN KIDD, Canadian Rivers Institute and Biology Department, University of New Brunswick, Saint John, New Brunswick, Canada
MARCOS LEMES, Department of Environment and Geography and Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada
YANBIN LI, Department of Chemistry and Biochemistry and Southeast Environmental Research Center, Florida International University, Miami, FL
CHE-JEN LIN, Department of Civil Engineering, Lamar University, Beaumont, TX
CHU-CHING LIN, Department of Biochemistry and Microbiology, Rutgers University, New Brunswick, NJ
GUANGLIANG LIU, Department of Chemistry and Biochemistry and Southeast Environmental Research Center, Florida International University, Miami, FL
MATTHEW MACLEOD, Department of Applied Environmental Science, Stockholm University, Stockholm, Sweden
KATSUYUKI MURATA, Department of Environmental Health Sciences, Akita University School of Medicine, Akita, Japan
NELSON J. O'DRISCOLL, Department of Earth and Environmental Sciences, K.C. Irving Environmental Science Center, Acadia University, Wolfville, Nova Scotia, Canada
SIMO O. PEHKONEN, Department of Chemical Engineering, Masdar Institute, Abu Dhabi, United Arab Emirates
ASIF QURESHI, Safety and Environmental Technology Group, Swiss Federal Institute of Technology (ETH ZUrich), ZUrich, Switzerland
JOSEPH N. RYAN, Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, CO
YLIAS M. SABRI, Advanced Materials and Industrial Chemistry Group, School of Applied Sciences, RMIT University, Melbourne, Victoria, Australia
MINESHI SAKAMOTO, Department of International Affairs and Environmental Sciences, National Institute for Minamata Disease, Minamata, Japan
MASANORI SASAKI, Department of Basic Medical Science, National Institute for Minamata Disease, Minamata, Japan
PATTARAPORN SINGHASUK, Department of Industrial Engineering, Lamar University, Beaumont, TX
ULF SKYLLBERG, Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
ELSIE SUNDERLAND, School of Public Health, Harvard University, Boston, MA
OLEG TRAVNIKOV, Meteorological Synthesizing Centre-East, EMEP, Moscow, Russia
EMMA E. VOST, Department of Earth and Environmental Science, K. C. Irving Environmental Science Center, Acadia University, Wolfville, Nova Scotia, Canada
FEIYUE WANG, Department of Environment and Geography and Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada
NATHAN YEE, Department of Environmental Sciences, Rutgers University, New Brunswick, NJ
YONGGUANG YIN, State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
WANG ZHENG, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN
Chapter 1
Overview of Mercury in the Environment
Guangliang Liu, Yong Cai, Nelson O'Driscoll, Xinbin Feng, and Guibin Jiang
1.1 Introduction
Mercury (Hg) is a naturally occurring element that is present throughout the environment. Mercury is recognized as a global contaminant because it can undergo long-range transport in the atmosphere, be persistent in the environment, be accumulated in the food web, and pose severe adverse effects on the human and ecosystem health (Nriagu, 1979; Fitzgerald et al., 2007b). The environmental contamination of land, air, water, and wildlife in various ecosystems with mercury around the world due to the natural release and extensive anthropogenic use of Hg has been a global concern for decades (Lindberg and Turner, 1977; Ebinghaus et al., 1999; Fitzgerald et al., 2005; Mason et al., 2009). This being the first chapter of the book, it will briefly discuss the health risks associated with mercury exposure and the natural and anthropogenic sources of mercury emissions, and then provide a very brief overview of the biogeochemical cycling of mercury.
In the environment and in biological systems, mercury can exist in three oxidation states, namely, Hg(0) (metallic), Hg(II) (mercuric), and Hg(I) (mercurous), with the monovalent form being rare owing to its instability (Ullrich et al., 2001; Fitzgerald et al., 2007a,b). In general, the dominant form of mercury in water, soil, and sediment is the inorganic Hg(II) form while methylmercury (MeHg) is dominant in biota, and in the atmosphere Hg(0) is the primary species (USEPA, 1997; Ullrich et al., 2001).
1.2 Toxicity and Health Risks of Mercury Exposure
All forms of mercury are toxic, but particularly problematic are the organic forms such as MeHg, which is a neurotoxin (Committee on the Toxicological Effects of Methylmercury, 2000; Clarkson and Magos, 2006). Acute mercury exposure can produce permanent damage to the nervous system, resulting in a variety of symptoms such as paresthesia, ataxia, sensory disturbances, tremors, blurred vision, slurred speech, hearing difficulties, blindness, deafness, and death (USEPA, 1997; Committee on the Toxicological Effects of Methylmercury, 2000; Clarkson and Magos, 2006). In addition to neurotoxicity, mercury, in inorganic and/or organic forms, can affect other systems and sequentially cause adverse effects including renal toxicity, myocardial infarction, immune malfunction, and irregular blood pressure (USEPA, 1997; Committee on the Toxicological Effects of Methylmercury, 2000).
Human exposure to Hg can pose a variety of health risks, with the severity depending largely on the magnitude of the dose. Historically, there were two notorious poisoning episodes associated with the extremely high MeHg exposures, that is, in Minamata where individuals were poisoned by MeHg through consumption of contaminated fish and in Iraq where the consumption of MeHg-treated (as a fungicide) grain led to poisoning (Committee on the Toxicological Effects of Methylmercury, 2000). Nowadays, acute poisoning incidents from high Hg exposure are rare and the health risks mercury poses to human population are mainly from chronic MeHg exposure through consumption of contaminated fish and other aquatic organisms, particularly large predatory fish species (USEPA, 1997). A major concern related to the health risks of chronic MeHg exposure is the possibility of developmental toxicity in the fetal brain, since MeHg can readily cross the placenta and the blood–brain barrier (Clarkson and Magos, 2006). Prenatal Hg exposure interferes with the growth and migration of neurons and has the potential to cause irreversible damage to the developing central nervous system (Committee on the Toxicological Effects of Methylmercury, 2000). For instance, because of prenatal MeHg exposure from maternal fish consumption, infants might display deficits in subtle neurological endpoints such as IQ deficits, abnormal muscle tone, and decrements in motor function (Committee on the Toxicological Effects of Methylmercury, 2000).
1.3 Sources of Mercury
Both naturally occurring and anthropogenic processes can release mercury into air, water, and soil, and emission into the atmosphere is usually the primary pathway for mercury entering the environment (Camargo, 1993; Berg et al., 2006; Jiang et al., 2006; Bone et al., 2007; Bookman et al., 2008; Streets et al., 2009; Cheng and Hu, 2010). It is estimated that the total annual global input to the atmosphere from all sources (i.e., from natural and anthropogenic emissions) is around 5000–6000 t (Mason et al., 1994; Lamborg et al., 2002; Gray and Hines, 2006). The relative importance of natural versus anthropogenic sources of mercury has not been accurately determined, with the ratio of natural to anthropogenic mercury emissions being reported to be within a wide range (e.g., from 0.8 to 1.8) (Nriagu and Pacyna, 1988; Nriagu, 1989, 1994; Bergan et al., 1999; Gustin et al., 2000; Lin and Tao, 2003; Nriagu and Becker, 2003; Seigneur et al., 2003, 2004; Gbor et al., 2007; Shetty et al., 2008).
1.3.1 Natural Sources of Mercury
There are a number of natural processes that can emit Hg into the atmosphere. These processes may include geologic activities (in particular volcanic and geothermal emissions), volatilization of Hg in marine environments, and emission of Hg from terrestrial environments (including substrates with elevated Hg concentrations and background soils) (Nriagu, 1989, 1993, 1994; Gustin et al., 2000, 2008; Gustin, 2003; Nriagu and Becker, 2003; Gray and Hines, 2006). Owing to the lack of data and the complexity of geological processes (e.g., vast variability spatially and temporally) (Gustin et al., 2000, 2008), it is rather difficult to accurately estimate natural Hg emissions, resulting in high degrees of uncertainties being associated with the reported Hg emissions from natural sources. The annual global Hg emissions from natural sources are estimated to range from 800 to 5800 t, with a middle range from 1800 to 3000 t (Lindberg and Turner, 1977; Nriagu, 1989; Lindberg et al., 1998; Bergan et al., 1999; Pirrone et al., 2001; Seigneur et al., 2001, 2004; Lamborg et al., 2002; Mason and Sheu, 2002; Pacyna and Pacyna, 2002; Pirrone and Mahaffey, 2005; Pacyna et al., 2006; Shetty et al., 2008). Among different natural processes, the global volcanic, geothermal, oceanic, and terrestrial Hg emissions are estimated to be 1–700, ∼60, 800–2600, and 1000–3200 t per year, respectively (Nriagu, 1989; Lindberg et al., 1998, 1999; Bergan et al., 1999; Ferrara et al., 2000; Lamborg et al., 2002; Mason and Sheu, 2002; Nriagu and Becker, 2003; Pyle and Mather, 2003; Seigneur et al., 2004; Fitzgerald et al., 2007b). Gaseous elemental mercury (GEM) is the predominant form (>99%) of Hg from natural emissions, which is different than anthropogenic emissions that may also contain reactive gaseous mercury (RGM) and particulate Hg (PHg) (Stein et al., 1996; Streets et al., 2005; Pacyna et al., 2006). It should be noted that some processes of natural Hg emissions include reemission of Hg previously deposited from the atmosphere by wet and dry processes derived from both anthropogenic and natural sources. For instance, emission from low Hg-containing substrates and background soils is assumed to be predominantly reemission of Hg previously deposited (Gustin et al., 2000; Seigneur et al., 2004; Gustin et al., 2008; Shetty et al., 2008).
1.3.2 Anthropogenic Sources of Mercury
Extensive anthropogenic emission and use of Hg have caused worldwide mercury contamination in many aquatic and terrestrial ecosystems (Lee et al., 2001; Streets et al., 2005, 2009; Hope, 2006; Wu et al., 2006; Zhang and Wong, 2007; Sunderland et al., 2009). Comparisons of contemporary (within the past 20–30 years) measurements and historical records indicate that the total global atmospheric mercury burden has increased by a factor of between 2 and 5 since the beginning of the industrialized period (USEPA, 1997). Although anthropogenic emission of Hg has been reduced in the past three decades, anthropogenic processes are still responsible for a significant proportion of global Hg input to the environment. It has been suggested that, among the 5000–6000 t of Hg that is estimated to be released into the atmosphere each year, about 50% may be from anthropogenic sources (Mason et al., 1994; Lamborg et al., 2002; Gray and Hines, 2006), which agrees with some other studies where the annual global anthropogenic emissions of mercury are estimated to be in the range of 2000–2600 t (Pacyna et al., 2001, 2006; Pirrone et al., 2001; Pacyna and Pacyna, 2002; Pirrone and Mahaffey, 2005). Unlike natural sources, anthropogenic sources can emit different species of Hg including GEM, RGM, and PHg with a distribution of about 50–60% GEM, 30% RGM, and 10% PHg (Streets et al., 2005; Pacyna et al., 2006).
Anthropogenic emissions of mercury can be from point (e.g., incinerators and coal-fired power plants) as well as diffuse (e.g., landfills, sewage sludge amended fields, and mine waste) sources (Nriagu, 1989; Sigel and Sigel, 2005; Malm, 1998; Schroeder and Munthe, 1998; Quemerais et al., 1999; Lee et al., 2001; Horvat, 2002; Gustin, 2003; Nelson, 2007; Feng et al., 2010; Pacyna et al., 2010). Point sources, including combustion, manufacturing, and miscellaneous sources (e.g., dental amalgam), are thought to be the main anthropogenic sources of mercury, accounting for approximately more than 95% of anthropogenic mercury emissions (USEPA, 1997). Combustion sources include burning of fossil fuels (e.g., coal and oil), medical waste incinerators, municipal waste combustors, and sewage sludge incinerators. Fossil fuel combustion can be associated with power generation, industrial and residential heating, and various industrial processes. Combustion processes emit divalent mercury and elemental mercury, in gaseous as well as particulate form, depending on the fuels and materials burned (e.g., coal, oil, municipal waste) and fuel gas cleaning and operating temperature, into the atmosphere (USEPA, 1997; UNEP Chemicals Branch, 2008). Manufacturing sources refer to extensive use (especially in the past and in some undeveloped areas) of mercury compounds in many industrial processes such as gold mining, chlor-alkali production, and paper and pulp manufacturing. Unlike combustion sources, manufacturing processes can release mercurial compounds directly into aquatic and terrestrial environments, in addition to the atmosphere (Lindberg and Turner, 1977; Nriagu et al., 1992; Nriagu, 1994; USEPA, 1997; AMAP/UNEP, 2008; UNEP Chemicals Branch, 2008).
Of the three anthropogenic point sources, combustion generally contributes more than 80% of anthropogenic mercury emissions, although varying from region to region (USEPA, 1997; UNEP Chemicals Branch, 2008). Figure 1.1 illustrates the global inventory of mercury emissions from major anthropogenic sources, as estimated by the United Nations Environmental Programme (UNEP) (AMAP/UNEP, 2008; UNEP Chemicals Branch, 2008). Fossil fuel combustion for power generation and industrial and residential heating contributes about 45% of total global emission (880 t out of 1930 t) (Fig. 1.1). Owing to the enormous amount of coal that is burned, coal burning is the largest single source of anthropogenic emissions of Hg to the atmosphere (AMAP/UNEP, 2008). Waste incineration contributes another significant proportion (about 120 t) of mercury emission, but with a wide range between 50 and 470 t due to lack of reliable estimation data, in particular in countries outside Europe and North America. In addition, fuel combustion in industrial processes, including cement and metal production, can release mercury into the atmosphere. Meanwhile, these industrial processes, in particular, the production of iron and nonferrous metals, can release mercury as it can be present as impurity in ores (AMAP/UNEP, 2008). The data illustrated in Fig. 1.1 for these industrial processes include mercury from fuel combustion and from impurities in ores.
Manufacturing sources mainly include gold mining and chlor-alkali industry. Globally, gold mining and production, primarily artisanal and small-scale gold mining using mercury to extract gold, contribute about 20% of anthropogenic mercury emission, while the fraction for chlor-alkali production is about 3% (Fig. 1.1) (AMAP/UNEP, 2008; UNEP Chemicals Branch, 2008). Although industrial use of mercury has been largely reduced in developed countries, it may still contribute to a significant portion of Hg emission in developing countries (e.g., in Asia and South America). As seen from Fig. 1.2, there are significant geological disparities in anthropogenic mercury emissions, with Asia alone accounting for about 65% of total global emission (1280 t out of 1930 t). It should be borne in mind that the data in Fig. 1.2 refer merely to the current emission inventory by region estimated by UNEP, with historical contributions being unaccounted for. Moreover, the relative contributions of different sources to total anthropogenic mercury emission vary with geological region (Fig. 1.3). The most striking characteristic in geological variability of anthropogenic mercury emissions is the dominant contribution of gold mining to overall anthropogenic mercury emission in South America. On the global scale, fossil fuel combustion for power and heating is the primary source of mercury emission, but in South America, gold mining contributes over 60% of total anthropogenic mercury emission (Fig. 1.3).
1.4 Overview of Mercury Biogeochemical Cycling
After entering the environment, mercury undergoes a series of complicated transport and transformation processes during its biogeochemical cycling. The biogeochemical cycling of mercury is closely associated with the chemical forms of mercury present in different phases of the environment.
In the atmosphere, elemental mercury (Hg(0)) constitutes the majority of Hg (>90%) and is the predominant form in the gaseous phase, which facilitates the long-range transport of Hg at a global scale (USEPA, 1997; Ebinghaus et al., 1999; Pirrone and Mahaffey, 2005). On the other hand, Hg(II) species present in atmospheric waters, either dissolved or adsorbed onto particles in droplets, has a tendency to readily deposit on the earth's surface through wet and dry deposition, which is important to the local and regional cycle of Hg (Nriagu, 1979; Schroeder and Munthe, 1998).
In water, sediment, and soil environments, mercury is present primarily as various Hg(II) compounds, including inorganic (e.g., mercuric hydroxide) and organic (e.g., MeHg) mercuric compounds, and secondarily as Hg(0), which plays an important role in the exchange of mercury between the atmosphere and aquatic and terrestrial surfaces (Stein et al., 1996; Ullrich et al., 2001; Fitzgerald et al., 2007a,b). These Hg(II) compounds (including inorganic and organic) are present in a variety of physical and chemical forms through complexing with various inorganic (e.g., chloride and sulfide) and organic (e.g., organic matter) ligands (Ullrich et al., 2001). Although in aquatic and soil environments MeHg may constitute a minor fraction of total mercury present (typically less than 10% and 3% in water and soil/sediment, respectively), the formation of MeHg is an important step in mercury cycling (USEPA, 1997; Ullrich et al., 2001). This is because MeHg can be bioaccumulated along the food web and reach high concentrations in organisms, in particular, in aquatic environments. In fishes and wildlife that prey on fish, MeHg can be the dominant form of mercury species owing to bioaccumulation and biomagnification (Stein et al., 1996; Fitzgerald et al., 2007a).
Associated with transformation between different mercury species and transport of mercury between different environmental phases, there are a number of processes that are important in the biogeochemical cycling of mercury. These processes include oxidation of Hg(0) and reduction of Hg(II) (including photochemical and microbial processes), methylation of inorganic mercury (primarily mediated by microbes), distribution of mercury between water and sediment, deposition of mercury from the atmosphere, long-range transport of mercury in the atmosphere, exchange of mercury between the earth surface (oceans and terrestrial ecosystems) and the atmosphere, and bioaccumulation of mercury through food webs (Nriagu, 1979; Ebinghaus et al., 1999; Pirrone and Mahaffey, 2005; Fitzgerald et al., 2007b).
1.5 Structure of the Book
The biogeochemical cycling of mercury is rather complicated, involving various transport and transformation processes that determine the fate of mercury and the health risks on ecosystem and humans. A comprehensive summary of the various aspects regarding transformation and transport of mercury is essential for better assessing the risks of mercury contamination. In the past years, a great deal of research has been done to advance the understanding of important aspects of mercury biogeochemical cycling and has produced a wealth of material. This book is aimed to develop a comprehensive review of the state of environmental mercury research by summarizing all the key aspects of the mercury cycle.
Following this opening chapter, environmental analytical chemistry of mercury species and measurement of industrial gas phase mercury emissions are discussed. The main body of the book is devoted to address the important transformation and transport processes of mercury in the environment (as mentioned in Section 1.4), which includes the interaction of mercury with organic matter and the isotope fractionation of mercury. In addition, the toxicity, metallomics, and health risks associated with mercury (in particular MeHg) exposure are discussed in Part IV of the book.
1.6 Concluding Remarks
Both naturally occurring and anthropogenic processes can release mercury into the environment, and the latter has led to a current total global atmospheric mercury burden two- to fivefold higher than before the industrialized period. There are significant geological disparities not only in the amounts of anthropogenic mercury emissions but also in the relative importance of different anthropogenic sources for each region.
After entering the environment, mercury undergoes a series of complicated transport and transformation processes during its biogeochemical cycling. Through formation of MeHg in the (particularly aquatic) environment and bioaccumulation of MeHg (particularly in fishes) through food webs, human populations can be exposed to mercury (especially MeHg) through consumption of mercury-contaminated fishes. Human exposure to mercury can pose a variety of health risks, mainly as neurological damages, especially in the fetal brain. This book covers environmental analysis of mercury, important transformation and transport processes of mercury in the environment, and toxicological aspects of mercury.
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