Enhance U(VI) reduction on natural pyrite surfaces by gamma irradiation

Pyrite with acid-washed surfaces is chemically inert toward aqueous U(VI) at routine conditions. Given that there is a strong radiation field in the nuclear waste repository, the impact of gamma irradiation on the interaction between aqueous U(VI) and natural pyrite was investigated. Three different natural pyrites, namely Si-, Pb-, and As-pyrites, were used for reactions with uranyl nitrate and uranyl acetate, and the focus was set on Si-pyrite. With an initial pH of ∼ 4.5 and ∼ 6.5, results showed that the solution pH increased mainly due to the reduction of nitrate or the decomposition of acetate caused by the gamma irradiation, while it decreased for the reaction with an initial pH of ∼ 9.0 after gamma irradiation due to the oxidation of aqueous Fe2+. Meanwhile, significant amounts of U(IV)/U(V) were formed on pyrite surfaces at these three pH conditions, demonstrating that gamma irradiation can greatly enhance the U(VI) reduction by pyrite. We proposed that the oxidizing ·OH radical generated by water radiolysis could react with pyrite, leaving the concomitant eaq− and ·H radicals to be effective reductants for aqueous U(VI) or the U(VI)-precipitate formed on pyrite surfaces. Conversely, the impact of gamma irradiation on the U(VI) reduction is insignificant at pH ∼ 3.0, mainly due to the quenching effect of eaq− by H+. Besides, carbonate has an inhibitive effect, because it is a quencher of eaq− and can form anionic complexes with U(VI). The presence of pyrite can also protect UO2 from γ-irradiation-induced oxidation.