Gamma-irradiation-induced reduction of aqueous Se(VI) by natural pyrite

Abstract

Although the reduction of Se(VI) to Se0 and FeSe2 by pyrite is thermodynamically favorable, well-crystallized natural pyrite remains chemically inert to aqueous Se(VI) under conventional conditions. In this study, the interaction of aqueous Se(VI) with natural arsenic-rich and arsenic-free pyrites was studied under gamma-irradiation at pH ∼4.0 to ∼9.0, mimicking the conditions near a nuclear waste repository. The results showed that gamma-irradiation significantly boosts the reductive precipitation of Se(VI) by arsenic-rich pyrite at pH ∼4.0 to ∼9.0. For arsenic-free pyrite, this enhancement was observed within a narrower pH range of ∼5.0 to ∼6.0, and a distinct red precipitate, indicative of Se0, formed during the reaction at pH ∼6.0 under an absorbed dose of 100 kGy at a dose rate of 100 Gy/min. The reduction process involving arsenic-rich pyrite is more sensitive to the absorbed dose rate, with a decrease in Se(VI) removal efficiency at higher dose rates. Conversely, both the absorbed dose and the dose rate have significant effects on the reactions involving the arsenic-free pyrite. Transmission electron microscopy and X-ray absorption spectroscopy analyses verified the occurrence of Se0 nanoparticles on pyrite surfaces via the intermediate product of Se(IV). It is suggested that the ·OH radical generated by water radiolysis can be scavenged by pyrite and/or its non-oxidative dissolved solutes such as Fe2+ and H3AsO3/AsO33− impurities, allowing the concurrent eaq− and ·H, primarily eaq−, to reduce Se(VI), thereby altering the reaction pathway between pyrite and aqueous Se(VI) under gamma-irradiation. This work offers important insights into the geochemical behavior of 79Se in the near-field of a nuclear waste repository.