Oxygen isotope partitioning during oxidation of pyrite by H2O2 and its dependence on temperature

A detailed experimental study was conducted to investigate mechanisms of pyrite oxidation by determining product yields and oxygen isotopic fractionation during reactions between powdered pyrite (FeS₂) with aqueous hydrogen peroxide (H₂O₂). Sealed silica-tube experiments utilized aliquots of pyrite that were reacted with 0.2 M H₂O₂ for 7 to 14 days at 4 to 150 °C. No volatile sulfur species were detected in any experiment. The only gaseous product recovered was elemental oxygen inferred to result from decomposition of H₂O₂. Aqueous sulfate (S_aq) was the only sulfur product recovered from solution. Solid hydrated ferric iron sulfates (i.e., water-soluble sulfate fraction, S_ws) were recovered from all experiments. Ferric oxide (hematite) was detected only in high temperature experiments.Reactants were selected with large differences in initial δ¹⁸O values. The oxygen isotopic compositions of oxygen-bearing reactants and products were analyzed for each experiment. Subsequent isotopic mass-balances were used to identify sources of oxygen for reaction products and to implicate specific chemical reaction mechanisms. δ¹⁸O of water did not show detectable change during any experiment. δ¹⁸O of sulfate was similar for S_aq and S_ws and indicated that both H₂O and H₂O₂ were sources of oxygen in sulfate. Low-temperature experiments suggest that H₂O-derived oxygen was incorporated into sulfate via Fe³⁺ oxidation, whereas H₂O₂-derived oxygen was incorporated into sulfate via oxidation by hydroxyl radicals (HO). These two competing mechanisms for oxygen incorporation into sulfate express comparable influences at 25 °C. With increasing reaction temperatures from 4 to 100 °C, it appears that accelerated thermal decomposition and diminished residence time of H₂O₂ limit the oxygen transfer from H₂O₂ into sulfate and enhance the relative importance of H₂O-derived oxygen for incorporation into sulfate. Notably, at temperatures between 100 and 150 °C there is a reversal in the lower temperature trend resulting in dominance of H₂O₂-derived oxygen over H₂O-derived oxygen. At such high temperatures, complete thermal decomposition of H₂O₂ to water and molecular oxygen (O₂) occurs within minutes in mineral-blank experiments and suggests little possibility for direct oxidation of pyrite by H₂O₂ above 100 °C. We hypothesize that a Fe-O₂ mechanism is responsible for oxygenating pyrite to sulfate using O₂ from the preceding thermal decomposition of H₂O₂.