Selenium, element 34, is a metalloid in row VIB of the periodic table. The main oxidation states are selenide (−2), elemental selenium (0), selenite (+4), and selenate (+6). Selenate and selenite are found in the oxic zone, while Se(0) is more abundant in the anoxic zone. Although hydrogen selenide may be produced by microorganisms, it is highly toxic. Se(-II) is more often found as organoselenium in the form of selenoproteins. The common methylated species (e.g., dimethylselenide, dimethyldiselenide) are volatile. Selenium can be a substitute for sulfur and is often found as metal selenide (e.g., Clausthalite, Crookesite). Selenium occurs in eight stable isotopes, the most common of which are Se80 (∼50%) and Se78 (∼24%), and nine radioactive isotopes.
The microbial oxidation of selenium is a slow process, whereas the dissimilatory reduction of selenate via selenite to Se(0) is rapid (Dowdle and Oremland, 1998). Selenate and selenite may also methylated, and Se(0) can be...
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