Biogeochemistry of Environmentally Important Trace Elements

Chapter 1

Biogeochemistry of Environmentally Important Trace Elements: Overview Yong Cai

Downloaded by UNIV OF CALGARY on May 4, 2013 | http://pubs.acs.org Publication Date: October 30, 2002 | doi: 10.1021/bk-2003-0835.ch001

Department of Chemistry and Southeast Environmental Research Center, Florida International University, Miami, FL 33199

The papers included in this book cover three main themes: speciation, transport between phases, and transformation and chemical reactions involved in the biogeochemistry of arsenic, mercury, and selenium. These main themes are supplemented by some detailed case studies. A few papers are also included to represent the current status on the study of trace element cycling in some localized geographical regions.

Trace elements that are of environmental importance include many that are listed in the periodic table as it is these elements that constitute the earth itself. It is, of course, not our intention, for the symposium or this book, to include all these elements. Rather we focus on several elements, arsenic, mercury, and selenium, that have drawn researchers' great attention in the past decade. All three elements have been listed as metals of major interest to the U.S. EPA (1). Arsenic is an element of great concern in the terrestrial as well as aquatic environments because of the high toxicity of certain arsenic species (2-4). The natural occurrence of arsenic in the environment is usually associated with sedimentary rocks of marine origin, weathered volcanic rocks, geothermal areas, and fossil fuels. Most of the arsenic derived from anthropogenic sources is released as a by-product of mining, metal refining processes, the burning of fossil fuels, and agricultural use (4-6). Recent research suggests that arsenic in

© 2003 American Chemical Society In Biogeochemistry of Environmentally Important Trace Elements; Cai, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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drinking water may be more dangerous than previously believed (7). A recent report by the National Academy of Sciences concluded that the previous arsenic standard of 50 μg/L in drinking water does not achieve the U.S.EPA's goal of protecting public health and should be lowered (2,5). EPA has recently decreased its drinking water standard from 50 to 10 μg/L, effective January 1, 2003, to adequately protect public health. The environmental impacts of arsenic contamination and the implementation of new regulations reducing arsenic in drinking water have resulted in a need for detailed studies of the biogeochemical cycling of arsenic and the development of arsenic decontamination technologies. Mercury is one of the most prevalent and toxic contaminants in the environment (8-10). Mercury is emitted into the environment from a number of natural as well as anthropogenic sources. It has been suggested that anthropogenic emissions are leading to a general increase in mercury on local, regional and global scales (9). Among the metals and metalloids of concern for their potentially harmful effects in the environment, mercury is unique for a number of reasons (10). Mercury and some of its compounds are volatile and can be transported over a long distance, making target populations exposure to mercury potentially serious even in remote areas. Mercury can be efficiently transformed into its most toxic form, methylmercury, under environmental conditions. Mercury is perhaps the only metal which indisputably bioaccumulates and biomagnifies through the food chain, causing harmful effects to animals and humans. These facts have motivated intensive research on mercury as a pollutant of global concern (8,9) Selenium is one of the most intensively studied inorganic components of diet. Ever since it was recognized in the 1950s that the element, which had until then been known only for its toxic effects, was also an essential nutrient, it has attracted growing interest in both human health and environmental fields of science (11,12). Selenium deficiency diseases and excesses resulting in toxicity in animals and human beings have been reported frequently (11-14, chapter 22 of this volume). There is a rather narrow range between selenium's action as toxicant and as a nutrient to human and animals. Selenium is also a toxicant and possibly a nutrient to plants. Selenium levels exceeding 2 mg kg-1 are found in many seleniferous soils throughout the western U.S., Ireland, Australia, Israel, and other countries (chapter 22 of this volume). These soils are derived from marine parent materials containing high levels of selenium. Anthropogenic sources of selenium in the environment include oil refining, mining, and fossil fuel combustion. A number of books and reviews have been written on the biogeochemistry of arsenic, mercury, and selenium. To name only a few of them published in the last decade, Biogeochemistry of Trace Metals by Adriano, published in 1992 (15), covered a broad aspects on the occurrence, fate, and transport of trace metals in the environment. Stoeppler (16) was editor of a book which dealt with

In Biogeochemistry of Environmentally Important Trace Elements; Cai, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


3 the sampling, sample treatment, chemical speciation, and environmental mobility of heavy metals, including mercury, arsenic, and selenium, in sediment and soils. In 1999, Ebinghaus et al. (9) edited a book which dealt with the characterization, risk assessment and remediation issues in mercury contaminated sites. Arsenic in the Environment edited by Nriagu, published in two volumes in 1994, describes in considerable detail the cycling, characterization, and health effects of arsenic (5,17). The Society of Environmental Chemistry and Health has been organizing, biennially, an international conference on arsenic exposure and health effects. Books based on these meetings have been published (18,19). Frankenberger and Benson (12) were editors of a book entitled Selenium in the Environment which addresses a number of important issues regarding the fate and transport of selenium in the environment. In 1996 Reilly (11) wrote a detailed monograph on selenium in food and health. In addition to the books, a number of monograph chapters and journal review articles have described certain phases of the biogeochemistry of these trace elements. One of them is the review paper published by Cullen and Reimer on Arsenic Speciation in the Environment (4), which is still a classic and highly cited in its field. Furthermore, reports and documents from various agencies are also important resources for the research in these areas (e.g. 2,3,8). There have been great advances in the past decade in the understanding of the biogeochemical cycling of arsenic, mercury, and selenium in environmental systems. This book is the result of an ACS symposium on trace element biogeochemistry at the 221 ACS National Meeting, which was held in San Diego, April 1-5, 2001. The symposium and the present volume make no attempt to cover every aspect of the biogeochemistry of these elements. Instead we have attempted to emphasize the areas of current interest in the scientific community because of their key roles in elucidating the biogeochemistry of these elements. Three main themes are covered in these chapters: Speciation, transport between phases, and transformation and chemical reactions involved in the fate and transport of these trace elements. These main themes are supplemented by some detailed case studies. A few papers are also included to represent the current status of the study of trace element cycling in some localized geographical regions. st

Speciation Speciation analysis of an element has been defined by Florence (20) as the determination of the concentrations of the individual physico-chemical forms of the element in a sample that together, constitute its total concentration. Speciation analysis involves a complex scheme of operations. Figure 1 illustrates an approach to differentiate speciation analysis into several categories.



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s

HgClf;

Others differentiation based on binding strength, hydrophobicity

Speciation Λ Analysis^

Figure 1 Scheme of an approach showing speciation of trace elements

e.g. HgiOH)»

Complex Forces or Ionic Bonds

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g.Hg +,Hg°, As0 ,As0 -, SeO/', SeO/'

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Oxidation States

e.g. particulate, colloidal and dissolved phases

Speciation Based on Particle Size



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There has been increasing interest over the past decade in speciation information about elements present in environmental and biological samples, since the toxicological and biogeochemical importance of many metals and metalloids greatly depends upon their chemical forms (21,22). The determination of the total amount of an element is important, but is insufficient to assess its toxicity and overall biogeochemical cycling. Information about concentrations of the individual species of an element, including its organic derivatives, is particularly crucial. Frequently the lack of the speciation information is the major limitation to our understanding of the biogeochemical cycling of the element. The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Not only should the separation steps (which are required either because of the insufficient sensitivity of the detection technique and/or the problems of interferences caused by concomitant elements) be considered closely in order to maintain the integrity of the species, but the detection techniques should allow both species detection and characterization. Mercury, arsenic, and selenium are three of these elements whose speciation is of particular interest. Arsenic occurs in a variety of chemical forms in water as a result of many chemical and biological transformations in the aquatic environment. In a review paper (Chapter 2), Watt and Le detail the arsenic species detected in groundwater, surface freshwater, estuarine/coastal, and seawater and the variables that affect the speciation. In Chapter 3, Lee and Nriagu present their work performed to characterize the nature of the interactions between carbonate ions and As(III). Their results from this study, along with data reported previously by the same group (25), seem to support the formation of arsenic carbonate complexes. Arsenic speciation in soils and sediments is much more complicated compared to the speciation in water. Loepper et al. (Chapter 4) describe the assessment of quantity and speciation of arsenic in soils by chemical extraction. Many different methods have been developed to accurately determine the concentrations of mercury species in various environmental matrices. Only recently has inductively coupled plasma mass spectrometry (ICP/MS) emerged as a competitive technique for mercury species analysis. Hintelmann and Ogrinc (Chapter 21) give a comprehensive description of the various methods in use in their laboratory for mercury species determinations in a variety of matrices, focusing especially on water column measurements. They describe the concept of speciated isotope dilution and stable tracer techniques introducing a novel approach to accurately calculate individual mercury isotope concentrations in multiple isotope addition experiments. In nature, selenium can be found in a variety of chemical forms at different oxidation states; for example, Se(VI) is usually present as the oxyanion selenate


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(Se0 ~), Se(IV) is present as selenite (HSe0 \ Se0 ~), and Se(0) is present as a solid, including both red monoclinic and gray hexagonal forms. Se(-II) is found in a variety of organic compounds, including selenoamino acids and volatile Se forms such as dimethylselenide (DMSe) and dimethyldiselenide (DMDSe). Inorganic selenides, such as hydrogen selenide and metal selenides, are also possible. Fox et al. (Chapter 22) review the recent findings on selenium speciation in soils and plants, with a particular emphasis on reactions and processes important in the field. 4

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Transport between phases Transport of elements between different phases (water/air; sediment/water; soil to plant, etc.) is clearly one of the key factors that determine the final fate and cycling of these elements in the environment. Several review papers and case studies address this issue in this book. Compared to the intensive studies on the transport of arsenic crossing the water/sediment boundary, arsenic transport from soils to plants has gained little attention. Phytoremediation, an emerging, plant-based technology for the removal of toxic contaminants from soil and water is a potentially attractive approach (24-26). This technique has received much attention lately as a cost-effective alternative to the more established treatment methods used at hazardous waste sites. The recent discovery of arsenic hyperaccumulators (27,2S) will stimulate the research on arsenic uptake by plants. Cai and Ma (Chapter 8) review the mechanisms on the metal tolerance, accumulation, and detoxification in plants with particular emphasis on arsenic in terrestrial plants. In Chapter 9, research results on uptake of arsenic by plants in Southeast England are described by Farago et al. Many elements are involved in the transfer of gaseous species between earth surface and the atmosphere (Chapters 10-12). While there have been abundant studies and evidence involving the lighter elements (e.g. C, N , S etc), the atmospheric transfer of the heavier elements is much less recognized. The importance of volatile chemical forms of selected trace elements in the environment, including arsenic, mercury, and selenium, have been discussed by several authors. Feldmann (Chapter 10) describes the volatilization of metals from a landfill site in Delta (British Columbia). In Chapter 11, Hirner discusses the chemical and toxicological aspects of the volatile metal and metalloid species associated, with waste materials. Tessier et al. (Chapter 12) present their recent results on the biogenic volatilization of trace elements from European estuaries. Despite progress made in the last decade in the research of mercury emission from soils, considerable knowledge gaps still exist. In a comprehensive review, Zhang et al. (Chapter 18) discuss these uncertainties and identify some future priorities for the research of soil mercury emission. This would promote a more


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extensive and critical assessment of our understanding on soil mercury emission and global mercury cycling. The importance of microbial processes in the regulation of dissolved gaseous mercury (DGM) in deep freshwater has not been previously investigated. Siciliano et al. (Chapter 17) evaluate microbial mercury reductase and oxidase activities in a depth profile from Jack's lake, Canada, and determine the DGM depth profiles for four sampling stations on Lake Ontario, Canada. Their results indicate that microbial processes are an important factor regulating DGM in the hypolimnion. They also discuss the importance of DGM to atmospheric flux rates.

Transformation and Chemical Reactions As indicated in Figure 1, a variety of forms of these three elements exist in environmental systems. These trace metal species are formed via biotransformation and chemical reactions modulated with redox and pH conditions. A better understanding of these reactions is the major objective to allow a critical assessment of the biogeochemistry of these elements. For example, the complexation and oxidation of arsenite are important pathways in the overall environmental cycling of arsenic. In order to identify soil components responsible for As(III) oxidation and the surface sites which bind the As(V) product, Manning and co-workers (Chapter 5) investigate the complexation and oxidation reactions of As(III) in three soils using standard batch reactions and Xray absorption spectrometry. The chemistry of arsenic in aquatic environments is complex because of its multiple oxidation states and its association with a variety of minerals through adsorption and precipitation. In Chapter 6, Meng and co workers provide a detailed review on the redox transformations of inorganic arsenic with the aim of understanding its mobility in aquatic environments. Hydroxyl and superoxide anion radicals are formed naturally and may contribute significantly to redox processes of arsenic in the environment. Studies in this area are scarce. In Chapter 7, Motamedi et al. present the results of their study on the reactions of ultrasonically generated hydroxyl radical with arsenic species in the presence of oxygen and argon over a range of solution pH and arsenic concentrations. This fundamental research on the oxidation of As(III) provides us useful information about the transformation between As(III) and As(V). While inorganic mercury is the major source of mercury to most aquatic systems, it is methylmercury (CH Hg) that bioconcentrates in aquatic food webs and is the source of health advisories worldwide that caution against the consumption of fish containing elevated CH Hg (8). Although extensive research has been conducted over the last four decades, further studies are definitely needed for a better understanding of the processes and factors affecting the methylation and bioconcentration of mercury through food webs. A 3

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comprehensive review by Benoit and co-workers (Chapter 19) address the geochemical and biological controls over CH Hg production and degradation in aquatic ecosystems. Despite the intensive research efforts on selenium cycling, little is known about the resulting speciation of selenium with mineralization of organic selenium compounds and no information is available for determining the importance of the different decomposition pathways for organic sulfur and selenium present in terrestrial soils. Martens and Suarez (Chapter 23) determine the decomposition of sulfur and selenium pathway intermediates in soil and evolution of volatile sulfur and selenium species indicative of the metabolizing pathways. Speciation of nonvolatilized selenium, following soil incubation with the selenium pathways intermediates, was also determined because the activity of methylation or demethylation pathways in soil may influence the accumulation of organic selenium compounds.


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Other Case Studies Several case studies which represent the current research in the general area of fate and transport of trace elements are included to illustrate actual locations where trace metals have posed significant problems or threats. Yellowknife, Canada has an extensive soil arsenic contaminant problem as a result of 60 years of gold mining activity. In Chapter 13, Reimer and co-workers describe an approach for characterizing arsenic sources and risk at the contaminated sites. Instead of measuring total concentration of arsenic, sequential selective extraction (SSE) and a stimulated gastric fluid extraction (GFE) were used to assess environmentally available and bioavailablefractionsin soils. It was found that these techniques can be used to identify actual risks and develop effective remediation strategies. Carbonell-Barrachina et al. (Chapter 14) present a paper on arsenic biogeochemistry in acidified pyrite mine waste from the Aznalcollar (Southwestern Spain) environmental disaster. Montezuma Well is part of Montezuma Castle National Monument, located in north-central Arizona. It contains elevated concentrations of geogenic arsenic. Compton-O'Brien et al. (Chapter 15) describe the study of occurrence and transport of arsenic in different matrices in this highly arsenic-enriched area. In Chapter 16, Wai and co-workers discuss Blackfoot disease and other related health problems associated with arsenic contamination of groundwater. In Chapter 20, Cossa and co-workers provide a synthesis of current knowledge about mercury dynamics and the state of contamination of the Seine estuary, France. After a study of the sources and level of mercury contamination in the Seine basin and an evaluation of internal inputs into the estuary, this paper considers the distribution of the metal between dissolved and particulate phases, the chemical reactions



9 governing the speciation and bioavailability of mercury, the state of contamination in the biota, and temporal changes in contamination within the last decade. Nguyen and Manning (Chapter 25) describe the spectroscopic and modeling study of lead adsorption and precipitation reactions on a mineral soil. The results from EXAFS analysis revealed a possible lead inner-sphere adsorption mechanism that was used in conjunction with a surface complexation model to quantitatively describe lead binding to soil. Dai and co-workers (Chapter 24) studied the sorption behavior of butyltin compounds in estuarine environments of the Haihe River, China. It is concluded that at acidic condition (pH

Biogeochemistry of Environmentally Important Trace Elements

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