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