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-characteriz ed samples of vermiculite and hydrobiotite were exposed to aqueous uranyl u nder conditions designed to promote surface sorption either at fixed charge ion-exchange sites or at amphoteric surface hydroxyl sites. The local stru cture of uranium in the sorption samples was directly measured using uraniu m 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 wer e used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to a ssess the effects of ion-exchange and dehydration upon the mineral structur e. The most significant findings are: (1) Under conditions which greatly fa vor ion-exchange sorption mechanisms, uranyl retains a symmetric local stru cture suggestive of an outer-sphere complex, with a preferred orientation o f the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ion-exchange complexes adopt a less symmetric structure, consistent with a n inner-sphere complex, with less pronounced orientation of the uranyl axis ; and (3) For conditions which favor sorption at surface hydroxyl sites, ur anyl has a highly distorted equatorial shell, indicative of stronger equato rial ligation, and the detection of a neighboring U atom suggests the forma tion of surface precipitates and/or oligomeric complexes.