Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Selenium
Selenium (CAS Registry No. 7782-49-2) is a metalloid with both metallic and non-metallic properties. It is present in the environment in both inorganic and organic forms. Inorganic forms of selenium include selenide (oxidation state −2, H2Se), elemental selenium (oxidation state 0) and the species selenite (oxidation state +4, SeO32−, HSeO3−) or selenate (oxidation state +6, SeO42−, HSeO4−). Organic forms of selenium include selenomethionine and selenocysteine, which can be found in plants (ATSDR, 2003; Johnson et al., 2010; Dennert et al., 2011; Ferguson et al., 2012).
Selenium is found naturally throughout the environment. The elemental form of selenium in the environment is rare; selenium is mostly found in combination with other elements (ATSDR, 2003). In soil, the most common forms of selenium are selenate and selenite. Selenide, selenium sulphide and elemental selenium are essentially insoluble and tend to be immobile in soils (U.S. EPA, 1990a; ATSDR, 2003). The elemental form of selenium has appreciable volatility and hence will enter the atmospheric environmental compartment, as will selenium dioxide (e.g., in emissions from smelting operations and coal burning) and volatile organoselenium compounds produced by plants (IARC, 1975). Selenium compounds may be methylated by soil microorganisms, such as bacteria and fungi, and by plants and animals. The methylated species volatilize to the atmosphere (Shamberger, 1981). During volcanic activity, selenium present in the lava volatilizes, explaining the low concentration found in magmatic rocks (Fordyce et al., 2000). Rock erosion results in selenium entering oceans and inland waters (IARC, 1975); thus, selenate and selenite can be found in fresh water and seawater (U.S. EPA, 1990a; CCME, 2009). Selenate and selenite are soluble and mobile in soil. The form of selenium in the soil and its bioavailability depend on pH, texture, mineralogy, the presence of competing ions (PO43− and SO42−) and the organic matter content of the soil. An increase in pH, oxidizing conditions and a low organic matter content increase the formation of soluble and mobile species. An alkaline soil environment favours the formation of selenate (CCME, 2009; Johnson et al., 2010), which is thus more readily available than selenite for plant uptake and utilization (Fairweather-Tait et al., 2011). In acidic soil, selenite combines with ferric oxide and clays to form less soluble complexes.
Anthropogenic release is considered the main factor in selenium migration (Johnson et al., 2010). Coal ash from coal-fired power plants and mining and refining of copper and other metals are the main human-caused sources of selenium in water (Casey and Siwik, 2000; ATSDR, 2003). When coal is burned, selenium is released to the atmosphere. Selenium can enter water bodies through coal ash pond effluent and the ash and dusts that settle from the atmosphere on water (Fishbein, 1983).
Selenium compounds are used in the laboratory for the synthesis of organic chemicals and as reducing agents. They are used by the glass, paint, plastics and ceramic industries to produce tints and colours, and selenium is present at high concentrations in gun bluing agents, which are lubricants to polish metals (ATSDR, 2003). Selenium's semiconductor properties are utilized in electronics and photoelectric cells. Selenium is also used as a replacement for lead in brass alloy plumbing fittings, in nutritional supplements, as an agricultural fertilizer and in metallurgical applications (ATSDR, 2003; CCME, 2009).
Selenium is not directly mined, but rather is a by-product of the production of other metals (Johnson et al., 2010). Selenium production in Canada is divided among three provinces. In 2010, 18, 58 and 22 tonnes were mined in Quebec, Ontario and Manitoba, respectively (Natural Resources Canada, 2010); in 2011, 19 and 16 tonnes were mined in Quebec and Ontario, respectively (Natural Resources Canada, 2011).
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