Surface complexation of U(VI) on goethite (-FeOOH)

Sherman, D.M., Peacock, C.L. and Hubbard, C.G. (2008) Surface complexation of U(VI) on goethite (-FeOOH). Geochimica et Cosmochimica Acta, 72 (2), 298-310. (doi:10.1016/j.gca.2007.10.023).

Abstract

It has previously been established that U(VI), as the uranyl cation UO2+2, sorbs to Fe oxide hydroxide phases by forming an inner-sphere complex with the surface hydroxyls. Here, we argue that on goethite (a-FeOOH) there are two inner-sphere surface complexes of UO2+2: the E2 complex resulting from bidentate edge-sharing with free FeO6 polyhedra on the {210} and {010} surfaces (space group setting Pnma) and a C2 complex resulting from bidentate corner-sharing with two adjacent FeO6 polyhedra on (predominantly) the {101} surface. Ab initio (density functional theory) predictions of the surface-complex geometries and stabilities suggest that the C2 complex is 0.12 eV (12 kJ/mol) less stable than the E2 complex. However, since the {101} sites are ~10(2) more abundant than the {210} and {010} sites, sorption on the {101} sites via the C2 complexes is favoured by configurational entropy. We argue that the C2 complex is dominant at high surface loadings and only this complex can account for the sorption capacity of goethite for U. We show that EXAFS spectra are consistent with this hypothesis. Inclusion of multiple scattering paths in the UO2(OH)+ and UO2CO3 surface complexes can account for nearly all of the signal previously attributed to the edge-sharing (E2) complex. We develop a new surface complexation model for UO2+2 on goethite using the 1pK model for surface protonation and the Basic Stern model for surface electrostatics. The model is able to account for sorption and desorption of UO2+2 in both ambient and reduced CO2 environments at surface loadings of 0.02-2.0 wt. % U.

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