Grazing incidence XAFS spectroscopy of uranyl sorbed onto TiO2 rutile surfaces

The surface complex formed by uranyl oxocations sorbed onto rutile titanium oxide has been probed by X-ray Absorption Spectroscopy. These measurements are part of a work that aims to model the interaction between heavy metal ions and mineral surfaces in aqueous conditions. In order to de ne the orientation and structure of the oxocation complex on the surface, both polycrystalline and monocrystalline ( 110) and (001) planes of rutile TiO2 have been investigated. Polarized XANES measurements show that the uranyl rod sorbs nearly parallel to the rutile surface, although this ideal configuration must be modulated by the surface topographic defects. Site by site simulation compared to the EXAFS data suggests that two surface oxygen sites are involved: shared edge and shared summit oxygen atoms of the TiO6 octahedron. Both grazing incidence EXAFS on the ( 110) plane and isotropic EXAFS on polycrystalline TiO2 reveal comparable sorption behavior: on average, the uranyl oxocation bonds to the surface as a bidentate complex with two short oxygen distances at 2.32 Angstrom and three larger distances at 2.47 Angstrom. Grazing incidence EXAFS on the ( 001) plane shows an unexpected low signal to noise ratio due to the lower uranium uptake. Data analysis suggests the formation of an outer sphere uranium complex on this plane. The discrepancy between both plane reactivities is still not understood.