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SELENIUM IN SELENIFEROUS ENVIRONMENTS 3I SELENIUM VOLATILITY Volative Se compounds are naturally released into the atmosphere as a result of biological activity in aquatic (Chau et al., 1976) and terrestrial ecosystems (Abu-Erreish et al., 1968; Doran & Alexander, 1977). The path- ways for Se volatilization in higher plants have been reviewed by Lewis (1976). The volatile compounds include dimethyl selenide (DMSe), dimethyl di- selenide, dimethylselenone, methane selenol, and hydrogen selenide (Zieve & Peterson, 1981, 1984c). Dimethyl selenide is the principle compound released by microbial and plant respiration (Lewis et al., 1966, 1974; Francis et al., 1974). Volatile Se was first collected from accumulator and nonaccumulator plants by Lewis et al. (1966, 1974), and the volatile Se respired by the accumulator A. racemo- sus was identified as DMSe (Lewis et al., 1974). Although DMSe was not found in the volatiles from a nonaccumulator Astragalus (Evans et al., 1968), failure to detect the DMSe in this latter case may possibly relate to detection levels or to differences in metabolism. Seven cruciferous species gave off volatile Se during growth on nutrient media containing selenite or selenate. The rate of Se respiration varied from <0.05% of leaf-Se per hour to 0.40% per hour (Lewis, 1976). The DMSe was identified as the single volatile Se compound produced by fresh cabbage leaves. Zieve and Peterson (1985) observed that ~SSeDMSe was also sorbed to soils. The amount sorbed was greater for wet soils than dry and was greater when the atmospheric concentration of DMSe was high than when it was low. Sorption of DMSe by individual soil constituents was greatest for or- ganic matter and clays, less for Mn and Fe oxides, and least for acid-washed sands. Experiments with sterilized soils indicated that microorganisms were not significantly involved in the sorption process but may have been respon- sible for the conversion of DMSe to other Se forms after sorption occurred. They concluded that the soil was an important natural sink for atmospheric DMSe. Under certain conditions, significant quantities of Se as DMSe are respired by animals as a volatile compound having agarlic-like odor (Mar- tin, 1973; Sternberg et al., 1968). This compound is formed in animals by methylating selenite. The amount exhaled increases with increasing Se in- take and is greater for orally administered selenite than selenate, selenomethio- nine, or seleniferous wheat (Martin, 1973). The formation of DMSe can be inhibited by As, Cd, and other heavy metals. This method of Se excretion is an important mechanism by which the animal removes Se from the body, especially when the diet is relatively high in Se. Measurable concentrations of alkylselenides (principally DMSe) may now be detected in the atmosphere in some areas. Values of 1 ng/m3 have been noted by Zieve and Peterson (1984a, 1986} around swamps and landfills. Zieve and Peterson (1984a) have shown that DMSe in the atmosphere can be ab- sorbed through the leaves of plants and transported to roots. This process occurred even when plants were grown in hydroponics with selenite-Se added to the nutrient culture.