Polarization Dependent Grazing Incidence GI XAFS Measurements of Uranyl Cation Sorption onto Mineral Surfaces

Measurement of the polarization dependence of surface sensitive grazing incidence (GI) x-ray absorption fine structure (XAFS) spectra at the U L3 edge is applied to identify reactive U(VI) sorption sites on various mineral surfaces. We use the known polarization dependence of the dioxouranium(VI) or 'uranyl' cation, [O=U=O] 2+ , to experimentally determine its orientation when sorbed onto single crystal mineral surfaces (α-Al 2 O 3 , CaCO 3 ). The GIXAFS signal measured at different orientations of a U(VI) sorbed α-Al 2 O 3 (11-20) wafer relative to the polarization vector of the incident synchrotron radiation, ε, indicates that the uranyl surface species exhibit a preferred surface orientation. Analysis of the EXAFS substantiates our previous findings, identifying the reactive sorption sites as AlO 6 octahedral edges on the surface. Initial results of uranyl sorbed onto freshly cleaved (10-14) surface of natural calcite indicate that uranyl cations sorb onto terrace step edges and not onto the surface plane or with an orientation of the [O=U=O] 2+ unit parallel to the (10-14) surface.