Chapter 22
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Selenium Speciation in Soils and Plants Patricia M. Fox, Danika L. LeDuc, Hussein Hussein, Zhi-qing Lin, and Norman Terry* Plant and Microbial Biology, University of California, Berkeley, CA 94720
With respect to living organisms, selenium has a dual character. At low concentrations, it may be an essential nutrient. When present at higher concentrations in the environment, however, it may be toxic. Whether selenium is a nutrient or a toxicant depends not only on its concentration but also on its chemical form. This fact has spurred great interest in its biogeochemistry and environmental cycling. Selenium may exist as both organic and inorganic species with different bioavailabilities and toxicities. Selenium speciation is strongly affected by the oxidation state and pH of the soil (or water) environment. It is also dramatically affected by transformations mediated by living organisms, especially plants and microbes. Recent technological advances such as the use of X-ray absorption spectroscopy and HPLC-ICP-MS have been particularly useful in speeding the study of selenium speciation. Here we review the research contributions from many fields including soil science, ecology, plant physiology and biochemistry, and microbiology, to provide a more complete picture of the speciation and transformation of selenium in soils and plants.
© 2003 American Chemical Society Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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340 In recent years, much attention has been paid to the biogeochemistry and environmental cycling of the trace element selenium (Se). There is a narrow range between Se's action as a toxicant and as a nutrient to humans and animals. Selenium is also a toxicant and possibly a nutrient to plants. Selenium levels exceeding 2 mg kg" are found in many seleniferous soils throughout the western U.S., Ireland, Australia, Israel, and other countries (1). These soils are derived from marine parent materials containing high levels of Se. Selenium contamination may also result from human activities such as oil refining, mining, and fossil fuel combustion. The toxicity of Se is dependent not only on overall levels of Se, but also on the chemical form, or speciation. Thus, much research has been performed in the past few years focusing on the speciation of Se in the environment. The purpose of this review is to summarize the recentfindingson Se speciation, with a particular emphasis on reactions and processes important in the field. In nature, selenium can be found in oxidation states of -II, 0, IV, and VI. A variety of chemical forms exist at these oxidation states; for instance, Se(VI) is usually present as the oxyanion selenate (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 (2,3). 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.
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Selenium Speciation in Soil
Techniques for Se Speciation in Soil In soils, the primary Se forms include selenate, selenite, elemental Se, and organic Se species. Traditionally, the speciation of Se in soils has been determined using a sequence of chemical extractants, which target specific pools or phases of an element in soil (4,5). Such pools include soluble, specifically adsorbed, organic matter associated, and oxide bound, among others. More recently, researchers have combined sequential extraction schemes with Se speciation by hydride-generation atomic absorption spectrometry (HGAAS) to achieve a greater understanding of Se speciation rather than just association (6,7). Because HGAAS only detects selenite in solution, selective oxidation and reduction of water samples or soil extracts allows one to distinguish between selenate, selenite, and Se(-II) compounds at levels down to 1 μg L" . These procedures are relatively easy to perform using commonly available equipment. 1
Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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However, due to the indirect nature of the methods, pools of Se identified may be operationally defined, particularly because chemical extraction may alter the speciation of Se in the soil. A recently exploited alternative to conventional extraction procedures is xray absorption spectroscopy (XAS) (2,3,8,9). X-ray absorption near-edge structure (XANES), with edge-fitting from model compounds, can accurately and quantitatively determine the oxidation states of Se present in soils (2). With this technique, the sample of interest is examined directly, without any extractions or treatments that may alter its composition or speciation. However, XAS facilities are not widely available, require specialized knowledge, and require higher concentrations (10-100 mg kg" ) for accurate analysis, so traditional extraction techniques are still quite useful. 1
Comparative Solubility and Sorption of Different Se Species Selenium may be incorporated into the solid phase of soil through either precipitation, co-precipitation, adsorption onto soil surfaces, or absorption into minerals or organic matter. Elrashidi et al. (10) reviewed the solubilities of a wide variety of Se mineral and precipitate forms. Because of their high solubility, metal-selenates and metal-selenites are unlikely to persist in soils, particularly at alkaline pH (10). By contrast, metal-selenides and elemental Se are quite insoluble in soils under reducing conditions (10). Due to the high solubility of selenate and selenite precipitates and minerals, the solubility of Se in aerobic environments is more likely to be controlled by adsorption and complexation processes. While selenite is strongly adsorbed to soil surfaces, selenate is much more weakly adsorbed (11-13). Studies on Se adsorption onto soils have indicated that the most important soil components are Fe- and Al-oxides and oxyhydroxides, Mn-oxides, clays, organic matter, and carbonates (11,13-15). The levels of Se adsorption by Fe- and Mn-oxides are similar, with Al-oxides adsorbing slightly lower levels (16-18). Kaolinite adsorbs greater levels of selenite than montmorillonite below pH 6, even when the minerals have similar surface areas (13,19). Selenite adsorption is sometimes correlated with organic carbon in soils (14,15). However, negatively charged organic substances may also block adsorption sites, preventing Se adsorption (20,21). The adsorption of both selenite and selenate depends on a number of soil characteristics. Adsorption is highly pH dependent, with greater sorption occurring at lower pH on soils (11-13) and soil minerals such as Fe-oxides (17,22-24), Al-oxides (25-27), Mn-oxides (17,18,28), and clays (13,19). Unlike these soil minerals, selenite sorption to calcite and hydroxyapatite is low in the acidic region and peaks at pH 8 (13,29). Studies on Mn- and Fe-oxides have
Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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342 demonstrated that the magnitude of selenite adsorption increases with increasing mineral surface area (18,22,28). In addition to pH and mineral characteristics, Se adsorption is affected by the presence of other anions in soil solution which may compete for adsorption sites. Competition of many anions with selenate is a function of ionic strength (23,25,27). Phosphate, molybdate, silicate, and citrate most effectively compete with selenite for adsorption sites, whereas other anions decrease adsorption to a much lesser extent (17,24,30,31). By contrast, certain anions such as carbonate, formate, and acetate promote selenate adsorption onto aluminum oxide, and CaCl increases selenite adsorption to soil (25,31). 2
Distribution of Se Species in Soils and Sediments In both aerobic and anaerobic soils, Se is present mostly as Se(0) (26-66 %) and Se(-II) (27-60 %), with only low amounts present as selenate (1-11 %) and selenite (1-16 %) (7,32,33). However, in some soils selenite may be more important; Martens and Suarez (6) reported that selenite comprised 68% of the total Se in a soil from Sumner Peck Ranch in Fresno, CA. In anaerobic environments such as sediments, Se may be present in much larger quantities (e.g., 83 mg kg" ) than in aerobic soils (e.g., 1.0 mg kg" ) (32). The most important factors affecting the speciation of Se in soil are oxidation-reduction status, pH, and microbial communities. Redox potential has long been known to control the speciation of Se in soils and solution. Under oxidizing conditions (>300 mV) selenate is the most dominant species, whereas under moderately reducing conditions (0-200 mV) selenite becomes the dominant species and under strongly reducing conditions (
