BIOGEOCHEMICAL BEHAVIOR OF SELENIUM IN ANOXIC SOILS AND SEDIMENTS - AN EQUILIBRIUM THERMODYNAMICS APPROACH

Equilibrium thermodynamic calculations were performed to evaluate mineral stability in anoxic soil and sediment porewater systems in which reducing conditions lead to microbially mediated transformations of Se oxyanions. pe-pH diagrams displaying mineral stability boundaries in the Fe-S-Se-H2O system are presented. Several diagrams were constructed assuming typical concentrations of dissolved species for anaerobic soil and sediment porewaters. Under reducing conditions, elemental Se and the formation of FeSe or FeSe2 control Se solubility. Native Se has a wide stability field, particularly under acid conditions. Under weakly acid-to-alkaline reducing conditions the formation of ironselenides is favored. If we assume that selenide can substitute for sulfide in a solid solution phase, precipitated FeS will contain an FeSe component. Because iron-selenides are thermodynamically unstable with respect to their sulfur counterparts both FeS and FeS2 are important in controlling the geochemical and environmental behavior of Se. Under conditions of FeS2 formation, FeSe and FeSe2 become unstable and elemental Se is produced.