Biogeochemical behavior of selenium in anoxic soils and sediments: An equilibrium thermodynamics approach

Abstract

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 ironselenides are thermodynamically unstable with respect to their sulfur counterparts both FeS and FeS₂ are important in controlling the geo‐chemical and environmental behavior of Se. Under conditions of FeS₂ formation, FeSe and FeSe₂ become unstable and elemental Se is produced.