Grazing incidence (GI) XAFS measurements of Hf(IV) and U(VI) sorption onto mineral surfaces

Surface sensitive grazing incidence XAFS measurements of mineral sorbed metal cations are used to characterize the metal sorbed at reactive surface sites. Two different systems are studied: (1) Hf(IV) sorbed onto the (001) basal plane of freshly cleaved mica and a surface oxidized silicon wafer and (2) U(VI) sorbed onto the (110) surface of alpha-Al2O3 at two different concentrations and two different orientations of the crystal surface plane to the polarization vector of the incident synchrotron radiation. Similar to findings reported for Hf(IV) sorbed onto amorphous silica, the EXAFS metrical parameters for Hf(IV) on both the mica (001) basal plane and on the oxidized silicon surface show that a mononuclear surface species forms. The cations are bound to silanol/aluminol groups present as defects on the mica surface in a fashion equivalent to binding to clay (hk0) edge sites. The polarization dependency of the XANES/EXAFS spectra of the alpha-Al2O3 surface sorbed U( VI) uranyl moiety shows that the linear uranyl cations exhibit a preferred orientation on the (110) surface. Comparison of the XANES spectral trends with data previously recorded for UO2(CH3CO2)(2).2H(2)O single crystals and fits to the EXAFS data allows identification of the reactive sites as the AlO6 octahedral edges on the alpha-Al2O3 (110) surface.