ABSTRACT
Because the major oxyanions of selenium (Se) in the environment, selenate (Se042-) and selenite (S e0 32-)— and their associated acids in solution-have negligible vapor pressure at normal biospheric temperatures, the volatile chemical species of Se are the reduced and methylated forms. Five different volatile forms of reduced Se have been detected in laboratory experiments, and three of these have been determined in the environment. They are hydrogen selenide (H2Se), methaneselenol (CH3SeH), dimethyl selenide (CH3SeCH3), dimethyl selenenyl sulfide (CH3SeSCH3), and dimethyl diselenide (CH3SeSeCH3). The relatively high vapor pressures of these organometalloidal compounds, in theory, make them significant in the biogeochemical cycling of Se, where they can play a part in the global processes of Se distribution from sediments to aqueous and vapor phases and back; however, the rapid oxidation of the first two in this list and the lower vapor pressure-at normal temperatures-of the last two probably leave dimethyl selenide as the most significant contributor to environmental Se mobility. The available physicochemical data for these five compounds are listed in Table 1, ordered by boiling point. One more compound is included, dimethylselenone, CH3S e 0 2CH3. With a melting point of 153°C, this compound is not a significant vapor phase Se-containing compound; however, its historical importance in the discussion will become clear below. A few other mixed sulfur-selenium alkylated species have recently been detected in laboratory experiments with Se-rich plants such as garlic; however, these experiments were carried out at temperatures substantially above room temperature, and except for high temperature environ ments like hot springs or ocean vents, the presence of these compounds in the environment probably is unimportant.
