CHEMICAL EQUILIBRIA OF SELENIUM IN SOILS

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Abstract
Thermodynamic data were used to develope equilibrium reactions and constants for 83 selenium (Se) minerals and solution species that relate to soils. The solubilities of the metal-selenate minerals are very high in well-aerated soils and are not expected to persist in these systems. The solubilities of 17 metal-selenite minerals studied suggest that MnSeO3(c) is the only selenite mineral that might persist in strongly acid soils. Other selenite minerals appear to be too soluble to form in soils particularly in the alkaline pH range. Metal-selenide minerals, in general, are extremely insoluble in soils under highly reducing conditions. Of the 17 metal selenides studied, Cu2Se(c) is the most stable mineral in acid soils, and both PbSe(c) and SnSe(c) are more stable under neutral and alkaline conditions. Formulation of these metal-selenides may prevent the precipitation of elemental Se in soils. The redox (pe+pH) of soils controls Se speciation in solution. Selenate (SeO42-) is the major species in solution at high redox (pe+pH > 15.0). In the medium redox range (pe+pH 7.5-15.0) either SeO32- or HSeO3-species is predominant, depending upon pH. At low redox (pe+pH < 7.5), HSe- is the major species in soil solution. Only in strongly acid soils do H2Se0 species contribute significantly to Se in solution. None of the 27 metal solution complexes studied was found to contribute significantly to total soluble Se in normal cultivated soils. Except for MnSeO3(c), none of the other known selenite or selenate minerals is expected to persist in soils. In moderately to highly oxidized conditions it is possible that an adsorption mechanism may control the distribution of Se between solid surfaces and the solution phase. However, at low redox, either elemental Se or a metal-selenide mineral, e.g., Cu2Se(c), PbSe(c) may control Se solubility in soils.