The structure of U6+ sorption complexes on vermiculite and hydrobiotite

The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ion-exchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L-3-edge extended X-ray absorption fine structure (EXAFS). Polarized L-1- and L-3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ion-exchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.