The isotopic composition of sulfate from anaerobic and low oxygen pyrite oxidation experiments with ferric iron — New insights into oxidation mechanisms

Although pyrite oxidation has been extensively studied by oxygen and sulfur isotope investigations (as well as by further methods), details of oxidation mechanisms are still under discussion. In this study, the chemical oxidation of pyrite was studied under anaerobic and low oxygen conditions with different amounts of ferric iron in order to investigate oxidation mechanisms and to determine an exact value of the oxygen isotope enrichment factor εSO4–H2O.Results showed that pyrite was predominantly oxidized by ferric iron in anaerobic experiments with high ferric iron/pyrite surface ratios. The obtained oxygen isotope enrichment factor εSO4–H2O = 2.3‰ was in the range of recently determined values and indicated the preferential incorporation of 18O from water molecules into sulfate during pyrite oxidation.δ18OSO4 values from anaerobic and low oxygen experiments with low ferric iron/surface ratios indicated a significant proportion of molecular oxygen in sulfate which should be unintentionally adsorbed onto the pyrite surface during placing the pyrite into the experimental flasks. In the presence of water, adsorbed molecular oxygen acted as pyrite oxidant and was incorporated into the produced sulfate.A sulfur isotope enrichment factor εSO4–FeS2 = − 0.8‰ was observed from pyrite oxidation by ferric iron and may indicate the formation of elemental sulfur. A lack of sulfur isotope enrichment in sulfate relative to pyrite was observed in most of the experiments where pyrite was oxidized by molecular oxygen. However, a sulfur isotope enrichment in sulfate relative to pyrite may also occur during aerobic pyrite oxidation.

Research Highlights► The oxygen isotope enrichment factor εSO4-H2O is 2.3‰ for pyrite oxidation. ► Molecular oxygen is adsorbed onto pyrite surfaces and incorporated into the produced sulfate. ► The sulfur isotope enrichment factor εSO4-FeS2 is -0.8‰ for abiotic pyrite oxidation by ferric iron.