Sphalerite oxidation pathways detected by oxygen and sulfur isotope studies

Sphalerite oxidation is a common process under acid-mine drainage (AMD) conditions and results in the release of SO42-, Zn and potentially toxic trace metals, which can pollute rivers and oceans. However, there are only a few studies on the mechanisms of aerobic sphalerite oxidation. Oxygen and S isotope investigations of the produced SO42- may contribute to the understanding of sphalerite oxidation mechanisms so helping to interpret field data from AMD sites. Therefore, batch oxidation experiments with an Fe-rich sphalerite were performed under aerobic abiotic conditions at different initial pH values (2 and 6) for different lengths of time (2-100 days). The O and S isotope composition of the produced SO42- indicated changing oxidation pathways during the experiments. During the first 20 days of the experiments at both initial pH values, molecular O-2 was the exclusive O source of SO42-. Furthermore, the lack of S isotope enrichment processes between SO42- and sphalerite indicated that O-2 was the electron acceptor from sphalerite S. As the oxidation proceeded, a sufficient amount of released Fe(II) was oxidized to Fe(III) by O2. Therefore, electrons could be transferred from sphalerite S sites to adsorbed hydrous Fe(III) and O from the hydration sphere of Fe was incorporated into the produced SO42- as indicated by decreasing delta O-18(SO4) values which became more similar to the delta O-18(H2O) values. The enrichment of S-32 in SO42- relative to the sphalerite may also result from sphalerite oxidation by Fe(III). The incorporation of O-2 into SO42- during the oxidation of sphalerite was associated with an O isotope enrichment factor epsilon(SO4-O2) of ca. -22 parts per thousand. The O isotope enrichment factor epsilon(SO4-H2O) was determined to be <= 4.1%. A comparison with O and S studies of other sulfides suggests that there is no general oxidation mechanism for acid-soluble sulfides. (C) 2011 Elsevier Ltd. All rights reserved.