Paper No. 7
Presentation Time: 3:00 PM
URANYL SORPTION PROCESSES ON FE(II)-BEARING MINERALS
SINGER, David M., Department of Geology, Kent State University, 228 McGilvrey Hall, Kent, OH 44242, BANFIELD, Jill F., Earth and Planetary Science, University of California, Berkeley, 369 McCone Hall, Berkeley, 94720, WAYCHUNAS, Glenn A., Earth Sciences Division, Lawrence Berkeley National Lab, MS 90R1116, 1 Cyclotron Road, Berkeley, CA 94720, ZACHARA, John M., Fundamental Sciences Directorate, Northwest National Laboratory, Richland, WA 99354, MAHER, Kate, Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305 and BROWN, Gordon E., Geological & Environmental Sciences, Stanford University, Bldg. 320, 450 Serra Mall, Stanford, CA 94305-2115, dsinger4@kent.edu
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.
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