PYRITE OXIDATION USING MILLIMOLAR H2O2 AS AN ANALOG FOR RADIATION-INDUCED CHEMICAL REACTIONS IN GROUNDWATER

Radiolytic dissociation of water produces oxidizing (e.g., H₂O₂, OH radicals, and O₂) and reducing (e.g., H atoms and H₂) species.  In groundwaters associated with uranium deposits, radiation-induced chemical reactions between radiolytically produced radicals and aqueous or solid media can accelerate water-rock interaction.  These processes are particularly significant in geologic environments where molecular oxygen derived from the atmosphere is a negligible input.  If radiolytically generated oxidants react with sulfide minerals or elemental sulfur then partially to fully oxidized sulfur species are potentially available to support microbial metabolism.  In order to investigate processes and mechanisms of radiolytic oxidation we have performed a series of sealed-tube experiments using pyrite and at 0.2 to 0.02 mM hydrogen peroxide, from 4 to 150 C.   Water used in these experiments was de-oxygenated to minimize competing reactions with O₂.

The key result of this work is that radiolytically produced oxidants, such as H₂O₂ and O₂, could efficiently oxidize pyrite in an otherwise oxygen-limited environment.  Our results show that the rate of pyrite oxidation by H₂O₂ increases with increasing H₂O₂ concentration, pyrite surface area, and temperature.  The combination of reactive oxygen species from peroxide decomposition products (e.g., O₂, OH radicals) and from pyrite dissolution (e.g., Fe³⁺) aggressively oxidizes the residual pyrite surface.  This observation suggests that the overall mechanism of pyrite oxidation by peroxide is more complex than previously recognized.  Competing oxidants with different oxidation efficiencies allows for multiple mechanism of reactions at different temperatures and surface conditions.

In natural groundwater environments where radiolysis occurs, coupled abiotic and biotic reactions will complicate the products of radiation-induced chemical reactions.  Radiolytic pyrite oxidation can buffer oxygen production, maintain the anaerobic conditions, and supply partially to fully oxidized sulfur species for microbial reduction or disproportionation.