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 degrees 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.,) were recovered from all experiments. Ferric oxide (hematite) was detected only in high temperature experiments. Reactants were selected with large differences in initial delta O-18 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. 6180 of water did not show detectable change during any experiment. delta O-18 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 center dot). These two competing mechanisms for oxygen incorporation into sulfate express comparable influences at 25 degrees C. With increasing reaction temperatures from 4 to 100 degrees 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 degrees 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 H202 to water and molecular oxygen (02) occurs within minutes in mineral-blank experiments and suggests little possibility for direct oxidation of pyrite by H2O2 above 100 degrees C. We hypothesize that a Fe-O-2 mechanism is responsible for oxygenating pyrite to sulfate usingO(2) from the preceding thermal decomposition of H2O2 (c) 2007 Elsevier Ltd. All rights reserved.