The role of pyrite in controlling metal ion activities in highly reduced soils

This study examines the solubility relationships of metal sulfides in highly reduced soils. Soils obtained from a mountain bog previously contaminated with Fe, Zn, Cd, and Pb sulfide ores were subjected to oxidation followed by gradual but severe reduction. The sequence of metal sulfide precipitations that occur as pe + pH is lowered was predicted from thermodynamic calculations. These predictions were compared to the results obtained from two studies in which soil suspensions were equilibrated under controlled redox conditions. Oxidation of the surface horizon of these soils resulted in partial dissolution of existing metal sulfides as pH decreased. Subsequent reduction increased pH and formed reaction products that controlled the activities of Fe2+, Zn2+, Cd2+ and Pb2+. When decomposing organic matter was used as the electron source to reduce these soils. the solubility of these metals approached equilibrium with Fe3O4(amorp), ZnCO3(smithsonite), CdCO3(octavite), and PbCO3(cerrusite), respectively, as pH increased above 5.5 and pe + pH decreased to 7.7 at 65 d. The pe + pH did not go lower with organic matter as the sole reductant. A reducing gas mixture containing 1% H-2(g), 5% CO2(g), and 94% Ar was passed through identical soil suspensions to which organic matter had also been added to lower the pe + pH so metal sulfides could precipitate. The calculated pe + pH values at which the metal sulfides formed were: (1) CdS(greenockite) (4.85), (2) ZnS(sphalerite) (4.70), (3) PbS(galena) (4.40), and (4) FeS2(pyrite) (4.35). Since Fe is more abundant in most soils than SO42-, Cd, Zn, and Pb, FeS2(pyrite) is expected to control sulfide activity, and in turn, the activities of Cd2+, Zn2+, and Pb2+. Only under conditions in which there is an abundance of SO42- compared to Fe would the control by pyrite be negated.