Influence of Surface Compositions on the Reactivity of Pyrite toward Aqueous U(VI)

Abstract

Pyrite plays a significant role in governing the mobility of toxic uranium in anaerobic envi-ronment via an oxidation-reduction process occurring at the mineral-water interface, but the factors influencing the reaction kinetics remain poorly understood. In this study, natural pyri-tes with different impurities (Pb, As and Si) and different surface pretreatments were used to react with aqueous U(VI) from pH ~3.0 to ~9.5. Both aqueous and solid results indicated that freshly crushed pyrites, which do have more surface Fe2+/Fe3+ and S2- sites that were generat-ed from breakage of Fe(S)-S bonds during ball-milling, exhibited a much stronger reactivity than those treated with acid-washing. Besides, U(VI) reduction which involves the possible intermediate U(V) and the formation of hyperstoichiometric UO2+x(s) was found to preferen-tially occur at Pb- and As-rich spots on the pyrite surface, suggesting that the incorporated impurities could act as reactive sites due to the generation of lattice defects and galena- and arsenopyrite-like local configurations. These reactive surface sites can be removed by acid-washing, leaving a pyrite surface nearly inert towards aqueous U(VI). Thus, reactivity of py-rite towards U(VI) is largely governed by its surface compositions, which provides an insight on the chemical behavior of both pyrite and uranium in various environments.