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1989; Jacks and Bhattacharya, 1998; Juillot et al., 1999; Matschullat, 2000; Pacyna and Pacyna, 2001). Elevated concentrations of As in soils occur only locally, but in areas of former industrial areas it may cause environmental concern (Nriagu, 1994; Smith et al., 1998; Kabata-Pendias and Pendias, 2001). Although many minerals contain As compounds, the anthropogenic contribution to the environment in the past accounted for 82,000 metric tons/year worldwide (Nriagu and Pacyna, 1988). Inorganic As compounds such as calcium arsenate, lead arsenate, sodium arsenate and many others were used by farmers as insecticides pesticides for debarking trees, in cattle and sheep dips to control ticks, fleas, lice and also in aquatic weed control. Water soluble preparatives, such as chromated copper arsenate (CCA) and other As-based chemicals used as wood preservatives during the past have lead to widespread metal contamination in soils around the wood preservation facilities (Bhattacharya et al., 2002c). However, the use of inorganic As compounds in agriculture has gradually disappeared since the 1960s due to greater understanding of As toxicity and awareness regarding food safety and environmental contamination (Vaughan, 1993; Sanok et al., 1995; Smith et al., 1998). In addition, during manufacturing of As-containing pesticides and herbicides, release of waste and As-laden liquids near the manufacturing areas may contaminate soil and water bodies (Mahimairaja et al., 2005). There are several “hot spots” around the world where soils have very high concentrations of As caused by natural geochemical enrichment and long-lasting ore mining and processing. For example, in Poland, mine spoils, slag dumps and tailings, that remained in the areas of As manufacturing and industrial processes, also contain extremely high concentrations of As (Karczewskam et al., 2004, 2005). There is a widespread concern regarding bioavailability of As in the terrestrial environment in industrialized regions of the world. The majority of incidences of soil As pollution could be traced back to a period prior to extensive statutory controls over As emissions (Meharg et al., 1994). For example, England was one of the cradles of the industrial revolution in the 19th century that has left behind an extensive legacy of As-contaminated sites. As part of the Land Ocean Interaction Study (LOIS) the As concentrations in the rivers of northeastern England reveal As enrichment within the urban and industrially affected rivers (Neal and Robson, 2000; Neal and Davies, 2003). The study revealed that the concentration of dissolved As in the rural areas averaged between 0.6 and 0.9 mg/L, while for the rivers influenced by industrial discharges the average between 3.2 and 5.6 mg/L, while suspended particulate As is much lower (average 0.1 to 0.2 mg/L for the rural and 0.2 to 0.8 mg/L for the
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industrial rivers). However, for the industrialized rivers dissolved As concentrations can be as high as 25.6 mg/L.
The possible mobilization of As in the soils, and subsequent leaching into ground or surface water or entry into the human food chain, should always be considered as a serious hazard. Detailed investigations are therefore necessary to estimate the total concentrations of As in soils in such areas, its chemical fractionation, and potential solubility to evaluate the potential risks from As mobilization.
1.6. Microbial transformations of arsenic
Mobilization of As in natural ecosystems is predominantly driven by microbially mediated biogeochemical interactions. Microbial reduction of As(V) to the more toxic and mobile As(III) species occurs via detoxification (Cervantes et al., 1994) or respiration processes (Ahmann et al., 1994). The genes that encode the proteins involved in As resistance are either plasmid or chromosomally borne, and have been best studied in Escherichia coli. Plasmid R773 comprises of five genes arsRDABC organized in an operon (Chen et al., 1986). The arsC gene encodes the As(V)-reductase; arsA and arsB act as the As(III) efflux pumps; arsR and arsD regulate the ars operon. Only a handful of microorganisms capable of respiring As(V) have been isolated (Oremland and Stolz, 2003). The As(V)- reductase genes (arrA and arrB) involved in As(V) reduction have been identified in a number of bacteria, and they share high sequence identities (Santini and Stolz, 2004). The As(V)-respiring microorganisms can use different electron donors (e.g. acetate, hydrogen), and range from mesophiles to extremophiles (Oremland and Stolz, 2003). These laboratory studies indicate that microbial processes involved in As(V) reduction and mobilization is many times faster than inorganic chemical transformations (Ahmann et al., 1997; Jones et al., 2000). This has led researchers to conclude that these microorganisms play an important role in As cycling in the sub-surface (Ahmann et al., 1997; Jones et al., 2000; Islam et al., 2004).
1.7. Remediation
Several technologies are currently available for As removal, ranging from simple and effective coagulation– flocculation, to sophisticated technologies such as ion exchange and reverse osmosis (Naidu and Bhattacharya, 2006). In addition, low-cost remediation methods, such as auto-attenuation and the use of geological material as natural sorbents for As (e.g. laterite, bauxsols, natural red earth or Fe-rich oxisols) have emerged as possible alternatives for the
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removal of As from groundwater in the developing world (Gen?Fuhrman et al., 2004, 2005; Naidu and Bhattacharya, 2006; Vithanage et al., 2006), but there is a pressing need to develop these methods further and in a cost-effective way. The concept of phytoremediation of As-contaminated sites was proposed over twenty years ago (Chaney, 1983). Phytoremediation has an advantage over conventional remediation of As-contaminated soils that include burial and chemical stabilization, which may pose long-term health threats due to leakage or chemical instability (Allen, 2001; Fostner and Haase, 1998). Thus phytoremediation has the potential to become an environmentally friendly and low-cost alternative remediation technique. It is well documented that some tropical and sub-tropical plant species can tolerate and uptake various inorganic and organic forms of As (Meharg and Hartley-Whitaker, 2002). Mesquite is am plant that grows well in humid and desert environments that has been shown to absorb Cr(VI) and other metals such as Pb (Aldrich et al., 2004). X-ray absorption spectroscopic (XAS) studies revealed that mesquite can bioreduce Cr(VI) to the less toxic Cr(III) (Aldrich et al., 2003). However, a significant gap of information exists on the ability of desert plant species to uptake As or other toxic elements.
1.8. Current research
Research on As is currently very active and includes assessment of interactions at scales ranging from molecular bonding to sub-continental, As speciation in inorganic and organic materials through a wide variety of chemical and spectroscopic approaches, and an emerging understanding of the role of microbes and other biota in As cycling. A recent review on health impacts of As resulted in drinking water standards of 10 μg/L or even lower in some countries (Kapaj et al., 2006). These lowered standards are projected to greatly increase water supply costs in many regions. The increased pressure on society to protect human health and the ecosystem has stimulated research using a wide multitude of approaches and techniques (Naidu et al., 2006; Bhattacharya et al., 2007). Considering the seriousness of this global As problem, a two-day symposium was organized to facilitate a thorough discussion on a broad range of inter-disciplinary issues that are related to the research on As in the environment. These include understanding the natural and anthropogenic processes which accelerate or control human exposure to As and different aspects of remediation. The outline of the symposium and the subsequent publications are described below.
2. Theme of the Special Symposium
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The Special Symposium (SYP-4) “Arsenic in the Environment: Biology and Chemistry” was organized as part of the 8th International Conference on Biogeochemistry of Trace Elements (ICOBTE) in Adelaide, Australia during April 2005. This Special Symposium attracted a wide range of contributions from a large number of multidisciplinary As researchers, that covered major themes, such as: 1) sources and characterization of As in groundwater environment; 2) processes that control mobility and speciation of As in soil, water and biota; 3) prediction of the fate of As in natural environments in response to geochemical, hydrologic, and biologic changes; 4) analytical techniques and speciation studies; 5) remediation and management of As-contaminated soils and groundwater; and 6) impact of As on agriculture and water supply management. The articles included in this special issue address many of these issues and pave the way through recent findings on the environmental behaviour of As in terms of its occurrence, sources, health impacts, and remediation. Besides understanding the fundamental processes of As mobilization, the articles discuss a wide variety of chemical and spectroscopic approaches, and increased understanding of the importance of microbes and other biota in As cycling. Although much has been learned about As in the environment the ability to predict the impact of intentional and unintentional changes to hydrologic and geochemical regimes often remains elusive.
Key research contributions from several international teams of scientists working on As in the environment, groundwater in the Bengal Delta Plain and elsewhere in the world were presented and discussed during the symposium and are amalgamated in this Special Issue of The Science of the Total Environment.
3. Layout and summary of the articles
This special issue comprises 14 articles and 1 short communication, grouped into four sections. 1) Arsenic in the groundwater environment; 2) arsenic in agricultural soils and mining environment; 3) biogeochemistry of As and toxicity, and 4) remediation of Ascontaminated soils and sediments.
3.1. Arsenic in the groundwater environment
This section has five articles. The first two contributions deal with the specific issues related to the occurrence of geogenic As in the alluvial aquifers of Bangladesh. The first paper (von Br?mssen et al., 2007-this volume) targets low-arsenic aquifers in areas with high concentrations of geogenic As in groundwater with a case study from Matlab Upazila in Southeastern Bangladesh. The local drillers are constructing deeper tubewells than in the recent past (60 m
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