T64 Frontiers in Mineral Sciences: Mineral/Melt Energetics, Mineral Surface Chemistry, Mineral Nanoscience, and High-Pressure Mineralogy II

Sunday, 18 October 2009
1:30 PM - 5:30 PM
E145

URANYL SORPTION PROCESSES ON FE(II)-BEARING MINERALS

Paper number: T64-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

Uranyl uptake by Fe(II)-bearing minerals is likely dominated by three interactions: (1) formation of surface complexes at [Fe(II)(O,OH)6] octahedral sites; (2) formation of surface complexes and/or coprecipitation with ferrihydrite formed by the release of Fe(II) from Fe(II)-bearing minerals and subsequent oxidation; and (3) heterogeneous reduction of U(VI) to U(IV) by Fe(II) at the Fe(II)-bearing mineral surface. Examples of all these processes are found in the contaminated sediments at the Hanford 300 Area in Washington State. X-ray microprobe analyses revealed that U(VI) was associated with ferrihydrite-coated magnetite and Fe(II)-bearing clays (dominated by chlorite). Uranyl uptake by chlorite was examined by batch sorption/desorption experiments combined with X-ray absorption spectroscopy, SEM and TEM, and surface complexation modeling. Uranium(VI) sorption is independent of ionic strength, suggesting dominantly inner-sphere sorption, which was supported by selective chemical extraction results. Fits of the EXAFS spectra of the short-term sorption samples indicate that UO22+ forms inner-sphere sorption complexes with carbonate (when present) at [Fe(O,OH)6] octahedral sites in a bidentate, edge-sharing manner. EXAFS-derived structural parameters were used to constrain the type of U(VI)-bearing surface species and were combined with observed batch sorption trends as input for a diffuse double-layer surface complexation model (SCM), which successfully predicts U(VI) sorption over a range of U(VI) concentrations, pH values, and solution compositions. After long-term exposure of chlorite to U(VI) under anaerobic conditions at 90oC, uranium reduction occurred in the CO3-Ca-free system and CO3-bearing system samples, whereas no U(IV) was detected in the CO3-Ca-bearing system sample. Preliminary investigations of U(VI) sorption on magnetite by grazing incidence X-ray absorption spectroscopy, grazing incidence small angle X-ray scattering, and atomic force microscopy aim to determine the composition, structure, and size of surface precipitates from the initial exposure of a surface to aqueous U solution species, and the transformations that occur during nucleation and particle growth of U(IV)- and/or U(VI)-bearing phases on the magnetite surface.