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<br />McNEAL & BALISTRIERI
<br />were reported for cattle (Bos sp.), sheep (Ovis aries), or humans in China
<br />in 1295, Columbia in 1560, Mexico in 1764, South Dakota in 1857 and 1893,
<br />and Wyoming in 1907 and 1908 (Rosenfeld & Beath, 1964). Selenium toxici-
<br />ty diseases may appear in animals when dietary intake exceeds 4 mg/kg (Lakin
<br />& Davidson, 1973). The concentration of Se ingested by animals or humans
<br />and the exposure time determine the symptoms and the type of disease (Old-
<br />field, 1972). Later reports indicate that Se is also an essential element in animal
<br />nutrition (Schwarz & Foltz, 1957). Although several Se deficiency diseases
<br />have been identified (Oldfield, 1972; Wilber, 1983), the most common one
<br />is white muscle disease in cattle and sheep. Lakin and Davidson (1973) esti-
<br />mate that Se deficiency diseases appear when animal dietary intake falls below
<br />0.04 mg/kg.
<br />The narrow gap between necessary and toxic concentrations of Se makes
<br />it imperative to understand the processes controlling the distribution of this
<br />element in the environment. Beginning in the 1930s, a large research effort
<br />was begun to identify the sources and distribution of Se and the mechanisms
<br />by which it accumulates in plants and animals. Although much has been
<br />learned, many questions remain and research on Se continues.
<br />The biogeochemistry and occurrences of Se in the environment have been
<br />reviewed in detail by many researchers (e.g., Rosenfeld & Beath, 1964; Alla-
<br />way, 1968; Frost, 1972; Lakin, 1972, 1973; Vokal-Borek, 1979; Lo & Sandi,
<br />1980; Sarquis & Mickey, 1980; Reddy & Massaro, 1983; Wilber, 1983; May-
<br />land, 1985; Adriano, 1986). These authors agree that Se is ubiquitous in na-
<br />ture and its biogeochemistry is complex and not completely understood.
<br />This chapter will briefly summarize the geochemistry and occurrences
<br />of Se in the environment. The basic chemistry of Se and its distribution and
<br />movement through the environment will be discussed.
<br />SELENIUM CHEMISTRY
<br />Selenium was discovered in 1817 by Jon Jakob Berzelius. It has an atomic
<br />number of 34 and an electronic configuration of [Ar]3d10 4sZ 4p4. It is
<br />located in the oxygen group of the periodic table between nonmetallic sulfur
<br />(S) and metallic tellurium (Te). Selenium has several naturally occurring iso-
<br />topes [~4Se (0.87%), 76Se (9.0201o)m ~~Se (7.58010), 78Se (23.52%), 80Se
<br />(49.82°10), and gZSe (9.19°10)], resulting in a composite molecular weight of
<br />78.96. Natural variation in Se isotope ratios parallels those of S (Krouse &
<br />Thode, 1962).
<br />The chemistry of Se resembles that of S; however, the differences in
<br />the melting and boiling points and oxidation potentials allow for the separa-
<br />tion of those elements in the environment (Lakin, 1973). Selenium, like S,
<br />can exist in the 2 -, 0, 4+, and 6+ oxidation states. The concentration, speci-
<br />ation, and association of Se in a given environment depend on the pH and
<br />redox conditions, the solubility of its salts, the complexing ability of soluble
<br />and solid ligands, biological interactions, and reaction kinetics. The redox
<br />speciation of Se is shown in Fig. 1-1. The Eh-pH diagram was constructed
<br />
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