RADIOLYTIC OXIDATION OF PYRITE BY GAMMA RADIATION

Radiolysis of water as a consequence of natural radioactivity and cosmic rays can produce a mixture of strong oxidants, including hydrogen peroxide (H₂O₂) and hydroxyl radicals (HO^●), as well as molecular hydrogen (H₂). Subsequent radiation-induced chemical oxidation reactions are particularly significant in geologic environments where the fugacity of molecular oxygen is negligible. Oxidation by radiolytically generated reagents can be observed in modern groundwater associated with uranium ores and can be inferred for ancient groundwaters. Prior to the development of an O₂-rich atmosphere on Earth, radiolytically produced oxidants could have reacted with pyrite and provided local sources of partially to fully oxidized sulfur species available for microbial metabolism.

Radiolytic oxidation of pyrite was quantitatively studied using a series of sealed-tube experiments in which de-oxygenated water was mixed in varying proportions with freshly fractured pyrite and exposed to gamma radiation. Gaseous, liquid, and solid products were recovered, identified, and analyzed for sulfur isotopic composition. Molecular hydrogen was the principal gaseous species recovered. Yields of H₂ are linear over our experimental range and correspond closely to the proportion of water and pyrite. Aqueous sulfate was the only sulfur species identified in solution after gamma radiation. Using extraction with methylene chloride, we recovered elemental sulfur from gamma radiated pyrite-water experiments. Sulfur isotopic values remained unchanged for partially reacted pyrite, but showed distinct enrichment of ³⁴S in produced sulfate and depletion of ³⁴S in elemental sulfur.

Our experiments demonstrate that radiolysis is an effective mechanism for the production of oxidizing species in subsurface environments over geologic time. Recognizing geochemical signatures of radiolytic oxidation is particularly important for diagnosis and understanding of biotic and abiotic reaction pathways in environments where molecular oxygen is negligible, and for assessing potential sources of chemical energy for microbial metabolism in the deep subsurface of Earth and Mars.