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McNEAL & BALISTRIERI
<br />position, SeO3 - concentration, and the concentration of competing anions
<br />such as phosphate (PO4-) (Brown & Carter, 1969; Cary &Gissel-Nielsen,
<br />1973; Balistrieri & Chao, 1987; Ryden et al., 1987).
<br />Selenate (SeOq-)
<br />The 6+ oxidation state of Se (SeO4-) is stable in well oxidized environ-
<br />ments (Fig. 1-1). Selenic acid, H2SeO4, is a strong acid and, therefore, not
<br />protonated over the pH range of natural waters (Fig. 1-1 and 1-2).
<br />Selenate salts are very soluble (Fig. 2-2; Elrashidi et al., 1987), Se04-
<br />is not as strongly adsorbed as SeO3- by soil constituents (Merrill et al.,
<br />1986; Balistrieri & Chao, 1987, 1989), and the conversion of SeO4- to less
<br />mobile forms of Se (SeO3 - or elemental Se) is a slow process (Sarquis &
<br />Mickey, 1980). Selenate is the form of Se most readily taken up by plants
<br />(Gissel-Nielsen & Bisbjerg, 1979; Eisler, 1985).
<br />DISTRIBUTION AND BEHAVIOR OF SELENIUM
<br />IN THE ENVIRONMENT
<br />Selenate is the most mobile form of Se in aqueous systems. Selenium
<br />can become immobile or biologically unavailable by reduction to elemental
<br />Se, by formation of metal selenides or Se-sulfides, or by SeO3 - adsorption.
<br />Therefore, the biogeochemistry of Se and environmental parameters, such
<br />as pH and redox conditions, strongly influence the concentration, mobility,
<br />and distribution of Se in the environment.
<br />Because of its complex chemistry, it is not surprising that Se is found
<br />in virtually all materials on earth. The total concentration of Se has been
<br />determined in rocks, soils, fossil fuels, volcanic gas, natural waters, and plant
<br />and animal tissue (Eisler, 1985) (Table 1-l). Only limited data is available
<br />on the chemical speciation and modes of occurrence of Se in natural sub-
<br />stances. Measures and Burton (1980) determined the proportion of Se04-
<br />and SeO3- in seawater, whereas Cutter and Bruland (1984) and Takayana-
<br />gi and Cossa (1985) determined SeO4-, SeO3-, and organic See- in sea-
<br />water. Various organic Se compounds have been identified in plants (Shrift,
<br />1973) and the metal content of various extractable fractions of soils and oxide
<br />mixtures have been examined (Pickering, 1981; Chao, 1984; Rapin et al.,
<br />1986). Much more work is required in this key area.
<br />A schematic diagram summarizing the behavior of Se in the environ-
<br />ment is shown in Fig. 1-3, showing that Se is involved in a variety of physi-
<br />cal, chemical, and biological processes. Selenium concentrations will be
<br />discussed in the content of the process that distribute it throughout the en-
<br />vironment. These processes include volcanic activity, combustion of fossil
<br />fuels, weathering of rocks and soils, soil leaching, groundwater transport,
<br />plant and animal uptake and release, adsorption and desorption, chemical
<br />and biological redox reactions, and mineral formation. The importance of
<br />a given process is largely determined by the particular speciation of Se.
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