Interaction between aqueous uranium (VI) and sulfide minerals: Spectroscopic evidence for sorption and reduction

The interaction of aqueous U(VI) with galena and pyrite surfaces under anoxic conditions has been studied by solution analysis and by spectroscopic methods. The solution data indicate that uranyl uptake is strongly dependent on pH; maximum uptake (>98%) occurs above a pH range of between 4.8 and 5.5, depending on experimental conditions. Increasing the sorbate/sorbent ratio results in a relative decrease in uptake of uranyl and in slower sorption kinetics. Auger electron spectroscopy (AES) analysis indicates an inhomogeneous distribution of sorbed uranium at the surface. In the case of galena, formation of small precipitates (~40 nm wide needles) of a uranium oxide compound are found. Pyrite shows a patchy distribution of uranium, mainly associated with oxidized surface species of sulfur and iron. X-ray photoelectron spectroscopy (XPS) yields insight into possible redox processes indicating, for both sulfides, the concomitant formation of polysulfides and a uranium oxide compound with a mixed oxidation state at a U(VI)/U(IV) ratio of ~2. Furthermore, in the case of pyrite, at pH above 6 increased oxidation of sulfur and iron and higher relative amounts of unreduced surface-uranyl are observed. Fourier Transformed Infrared (FTIR) analysis of surface-bound uranyl shows a significant shift of the asymmetric stretching frequency to lower wavenumbers which is consistent with the formation of a U3O8-type compound and thus, independently, confirms the partial reduction of uranyl at the sulfide surface. The combination of AES, XPS, and FTIR provides a powerful approach for identifying mechanisms that govern the interaction of redox sensitive compounds in aqueous systems. Our overall results indicate that sulfide minerals are efficient scavengers of soluble uranyl. Comparing our results with recent field observations, we suggest that thermodynamically metastable U3O8 controls uranium concentrations in many anoxic groundwaters.