Speciation of U(VI) in natural systems by advanced EXAFS analysis II. Application of MCTFA to solve the spatial structure of U(VI) sorption complexes on ferrihydrite

The motivation of this study was to investigate the binding structure of uranium attached to mine water colloids and sediments gained from an abandoned uranium mine currently being flooded. U LIII-edge EXAFS spectroscopy was applied to identify the oxidation state and the atomic coordination sphere of bound uranium. First results from shell fitting showed that U(VI) was coordinated by a mononuclear, inner-sphere complex to the edge of an Fe(O,OH)6 octahedron originating from 2-line ferrihydrite (Fh) which predominated the bulk solid phase. This is in accordance with [1].
However, a small spectral contribution at R+Δ ~2.4 Å in the Fourier transform (FT) of the EXAFS could not be explained. Therefore, synthetic U(VI)-Fh coprecipitates were prepared under varied chemical conditions in order to step by step rule out possible contributions of other constituents in the samples such as sulfur, silicon, and carbon. Interestingly, the FT peak was independent of the preparation conditions, i.e. it also appeared under CO2-free atmosphere and in absence of other ligands. Hence Monte Carlo Target Transformation Factor Analysis (MCTFA) [2] was employed to find a structural model consistent with both spectroscopic and experimental data. It was found that the FT peak originates from a third O-atom of the Fe octahedron at a radial distance of 2.84 Å which consistently explains the other atomic distances of the sorption complex calculated from the experimental data. This 3D topology is presented and discussed in comparison with the complex structure of U(VI) sorbed onto aluminum hydroxide gel.
By another set of sorption experiments the influence of carbonate was examined at different pH conditions under ambient air (pCO2 = 35.5 Pa) and increased CO2 atmosphere (pCO2 = 1014 Pa). It was shown by iterative factor analysis that two eigenvectors are sufficient to fully reproduce the measured spectra. One is given by the aforementioned binary complex structure, the other is represented by the UO2(CO3)34- aqueous complex suggesting accessory outer-sphere complexation of uranylcarbonate ions on ferrihydrite at elevated carbonate concentrations. Further research is needed to understand the complex topology in detail and to explain the role of steric influences by neighboring Fe octahedra of the adsorbent.

References
[1] Waite, T.D. et al. (1994) Geochim. Cosmochim. Acta 58, 5465-5478.
[2] Rossberg, A. et al. (2005) Anal. and Bioanal. Chem. 383, 56-66.