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 O2. However, the incorporation of O2 into SO42- has been shown to decrease continuously during pyrite oxidation experiments. Hence, it remains uncertain if (and how) O2 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, 18O-enriched O2 was injected into the headspace of the reaction vessels. Increasing δ18OSO4δ18OSO4 values with increasing injection volume of 18O-enriched O2 indicated the permanent incorporation of about 9% O2 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 O2 consumed during the experiments was incorporated into SO42-. Slightly increased δ18OH2Oδ18OH2O 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 εSO4-O2εSO4-O2 value indicated that the oxidation of dissolved Fe2+ by O2 did not play an important role. Furthermore, the lack of 32S enrichment in SO42- 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.

Research highlights
► A small amount of molecular oxygen is permanently incorporated into sulfate during pyrite oxidation.
► Molecular oxygen may be incorporated into sulfate by sulfite oxidation and tetrathionate oxidation.
► Molecular oxygen is incorporated into water molecules during its cathodic reduction.
► Molecular oxygen mainly accepts electrons from pyrite and minor from dissolved and adsorbed ferrous iron.