The role of dissolved molecular oxygen in abiotic pyrite oxidation under acid pH conditions - Experiments with 18O-enriched molecular oxygen

Several O isotope studies have shown that SO42- produced from aqueous pyrite oxidation mainly contains water-derived O and minor atmospherically-derived O-2. However, the incorporation of O-2 into SO42- has been shown to decrease continuously during pyrite oxidation experiments. Hence, it remains uncertain if (and how) O-2 is permanently incorporated into SO42- during pyrite oxidation. Abiotic aerobic batch pyrite oxidation experiments in aqueous solutions were performed under acid pH conditions. After 151 days. O-18-enriched O-2 was injected into the headspace of the reaction vessels. Increasing delta O-18(SO4) values with increasing injection volume of O-18-enriched O-2 indicated the permanent incorporation of about 9% O-2 into the produced SO42- during pyrite oxidation from 151 to 201 days. Molecular oxygen may be incorporated into SO42- by oxidation of the S intermediate species sulfite (and maybe tetrathionate) into SO42-. However, only 4% of the O-2 consumed during the experiments was incorporated into SO42-. Slightly increased delta O-18(H2O) values from experiments with the largest injection of O-18-enriched O-2 indicated the incorporation of O-2 into water molecules which may proceed during the cathodic reduction of O-2. Thus, O-2 was an important electron acceptor under aerobic acid conditions. The observed epsilon(SO4-O2) value indicated that the oxidation of dissolved Fe2+ by O-2 did not play an important role. Furthermore, the lack of S-32 enrichment in SO42- compared to pyrite indicated that the oxidation of adsorbed Fe2+ by O-2 should not be a dominant mechanism, although it may be catalyzed onto the pyrite surface. Hence, O-2 should accept electrons predominantly from pyrite. (C) 2010 Elsevier Ltd. All rights reserved.