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 O2 was the exclusive O source of SO42-. Furthermore, the lack of S isotope enrichment processes between SO42- and sphalerite indicated that O2 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 δ18OSO4 values which became more similar to the δ18OH2O values. The enrichment of 32S in SO42- relative to the sphalerite may also result from sphalerite oxidation by Fe(III).The incorporation of O2 into SO42- during the oxidation of sphalerite was associated with an O isotope enrichment factor εSO4–O2 of ca. −22‰. The O isotope enrichment factor εSO4–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.

► Oxygen and sulfur isotopes of sulfate indicate changing oxidation pathways during aerobic sphalerite oxidation.
► Initially, sphalerite was oxidized by molecular oxygen.
► Later on, sphalerite was oxidized by ferric iron generated from released ferrous iron.
► The oxygen isotope enrichment factors εSO4-O2 and εSO4–H2O are −22‰ and ⩽4.1‰, respectively.
► There is no general oxidation mechanism for acid-soluble sulfides.