1. Occurrence and Significance
Selenium (Se) is the third element in Group VIA in the periodic table. It has an atomic number of 34, an atomic weight of 78.96, and valences of 2, 4, or 6. The average abundance of Se in the earth’s crust is 0.2 ppm; in soils it is 0.27 to 0.74 ppm; in streams it is 0.2 μg/L, and in groundwaters it is <0.1 mg/L. Selenium is used in electronics, ceramics, and shampoos.
The inorganic fraction of dissolved selenium consists predominantly of selenium as the selenate ion (SeO42−), designated here as Se(VI), and selenium as the selenite ion (SeO32−), Se(IV). Other common aqueous species include Se2−, HSe−, and Se0. Selenium is considered a nonessential trace element for most plants, but is an essential trace nutrient for most animals, and selenium deficiency diseases are well known in veterinary medicine. Above trace levels, ingested selenium is toxic to animals and may be toxic to humans. Although the selenium concentration of most natural waters is low, the pore water in seleniferous soils in semiarid areas may contain up to hundreds or thousands of micrograms of dissolved selenium per liter. Certain plants that grow in such areas accumulate large concentrations of selenium and may poison livestock that graze on them. Water drained from such soil may cause severe environmental pollution and wildlife toxicity. Selenopolysulfide ions (SSe2−) may occur in the presence of hydrogen sulfide in waterlogged, anoxic soils. Selenium derived from microbial degradation of seleniferous organic matter includes selenite, selenate, and the volatile organic compounds dimethyl selenide and dimethyl diselenide. Nonvolatile organic selenium compounds may be released to water by microbial processes. Soluble selenium may be leached from coal ash and fly ash at electric power plants that burn seleniferous coal.
The United Nations Food and Agriculture Organization recommended maximum level for selenium in irrigation waters is 20 μg/L. The U.S. EPA primary drinking water standard MCL is 50 μg/L.