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1 Geochemistry and Occurrence of <br />Selenium: An Overview <br />James M. McNeal <br />U. S. Geological Survey <br />Reston, Virginia <br />Laurie S. Balistrieri <br />U. S. Geological Survey <br />Seattle, Washington <br />ABSTRACT <br />Selenium (Se) is both beneficial and toxic to animals, plants, and humans. Con- <br />sequently, it is imperative to know its concentration in the environment and to <br />understand the processes controlling its distribution. Determinations of Se concen- <br />trations in a variety of materials indicate that Se is widely distributed throughout <br />the environment. The processes responsible for its distribution include volcanic ac- <br />tivity, the burning of fossil fuels, the weathering of rocks and soils, groundwater <br />transport, precipitation of minerals, adsorption, chemical or bacterial reduction and <br />oxidation, and metabolic uptake and release by plants and animals. The importance <br />of a particular process in controlling the distribution of Se is intimately linked to <br />the speciation of Se, which is controlled by the pH and redox conditions of the en- <br />vironment. Selenium can exist as selenide (SeZ-), elemental Se (Se°), selenite <br />(Se03-), and selenate (SeO;-). Each oxidation state exhibits different chemical be- <br />havior. selenide and elemental Se occur in acidic, reducing, and organic-rich environ- <br />ments. Metal selenides, Se-sulfides, and elemental Se are insoluble, and therefore <br />biologically unavailable. For the pH and redox conditions of most soil and aquatic <br />environments, Se03- and Se04- should be the dominant forms of Se. selenite is <br />immobilized by adsorption onto particles, particularly Fe oxyhydroxides. selenate <br />is highly mobile and biologically available because of the solubility of its salts and <br />its weak adsorption by particles. Microbial action can change the speciation of Se <br />through oxidation or reduction, or through the formation of organic Se compounds. <br />Depending on its concentration, Se can either be beneficial (in some instances, <br />essential) or toxic to humans and certain plants and animals (Lakin, 1972). <br />Most early reports dealing with the health aspects of Se were concerned with <br />toxicity problems. Symptoms of what later was identified as Se poisoning <br />Copyright ©1989 Soil Science Society of America and American Society of Agronomy, 677 <br />S. Segoe Rd., Madison, WI 53711, USA. Selenium in Agriculture and the Environment, SSSA <br />Special Publication no. 23. <br />