Mineralogic and sulfur isotopic effects accompanying oxidation of pyrite in millimolar solutions of hydrogen peroxide at temperatures from 4 to 150 °C

Oxidation of pyrite by hydrogen peroxide (H₂O₂) at millimolar levels has been studied from 4 to 150 °C in order to evaluate isotopic effects potentially associated with radiolytic oxidation of pyrite. Gaseous, aqueous, and solid phases were collected and measured following sealed-tube experiments that lasted from 1 to 14 days. The dominant gaseous product was molecular oxygen. No volatile sulfur species were recovered from any experiment. Sulfate was the only aqueous sulfur species detected in solution, with sulfite and thiosulfate below the detection limits. X-ray diffraction patterns and images from scanning electron microscopy reveal solid residues composed primarily of hydrated ferric iron sulfates and sporadic ferric-ferrous iron sulfates. Hematite was detected only in solid residue produced during high temperature experiments. Elemental sulfur and/or polysulfides are inferred to be form on reacting pyrite surface based on extraction with organic solvents. Pyrite oxidation by H₂O₂ increases in rate with increasing H₂O₂concentration, pyrite surface area, and temperature. Rates measured in sealed-tube experiments at 25°C, for H₂O₂ concentration of 2 × 10⁻³ M are 8.8 × 10⁻⁹ M/m²/sec, which are higher than previous estimates. A combination of reactive oxygen species from H₂O₂ decomposition products and reactive iron species from pyrite dissolution is inferred to aggressively oxidize the receding pyrite surface. Competing oxidants with temperature-dependent oxidation efficiencies results in multiple reaction mechanisms for different temperatures and surface conditions. Sulfur isotope values of remaining pyrite were unchanged during the experiments, but showed distinct enrichment of ³⁴S in produced sulfate and depletion in elemental sulfur. The Δ_{sulfate-pyrite} and Δ_{elemental sulfur-pyrite} was +0.5 to +1.5‰ and was −0.2 to −1‰, respectively. Isotope data from high-temperature experiments indicate an additional ³⁴S-depleted sulfur fraction, with up to 4‰ depletion of ³⁴S, in the hematite. Sulfur isotope trends were not influenced by H₂O₂ concentration, temperature, or reaction time. Results of this study indicate that radiolytically produced oxidants, such as hydrogen peroxide and hydroxyl radicals, could efficiently oxidize pyrite in an otherwise oxygen-limited environment. Although H₂O₂ is generally regarded as being of minor geochemical significance on Earth, the H₂O₂ molecule plays a pivotal role in Martian atmospheric and soil chemistry. Additional experimental and field studies are needed to characterize sulfur and oxygen isotope systematics during radiolytical oxidation of metallic sulfides and elemental sulfur.