Influence of carbonate on U(VI) aqueous speciation and adsorption to ferrihydrite investigated by advanced EXAFS data analysis methods

At alkaline pH conditions, formation of soluble uranyl carbonato complexes commonly prevents U(VI) sorption to mineral surfaces, thereby promoting uranium migration in the liquid phase. Microbial activity may increase the partial CO2 pressure by several orders of magnitude in relation to the atmosphere, hence negatively charged carbonato complexes may play a significant role even at acidic pH values, where sorption to positively charged surfaces may compete with formation of binary inner-sphere sorption complexes. Therefore, we studied by EXAFS the sorption of UO22+ on ferrihydrite in aqueous solution as a function of pH and at elevated carbonate concentrations. We used iterative target transformation factor analysis (ITFA) [1] to derive the spectra of the pure species from the EXAFS spectral mixtures. Then we applied Monte Carlo simulations (MCTFA) [2] to determine the three-dimensional structure of the pure species and the radial pair distribution functions of the single and multiple scattering paths. The spectra of the mixtures can be described by a linear combination of two species. Species 1 is an inner-sphere binary U(VI)-ferrihydrite complex, with U(VI) coordinating to the edge of an Fe octahedron. In species 2, U(VI) is coordinated to two or three carbonato ligands, suggesting formation of [(UO2)(CO2)2]2- or [(UO2)(CO2)3]4- complexes associated with the ferrihydrite solid.

[1] Rossberg, A., Reich, T. & Bernhard, G. Complexation of uranium(VI) with protocatechuic acid - application of iterative transformation factor analysis to EXAFS spectroscopy. Analytical and Bioanalytical Chemistry 376, 631-638 (2003).
[2] Rossberg, A. & Scheinost, A. C. Three-dimensional modeling of EXAFS spectral mixtures by combining Monte Carlo Simulations and Target Transformation Factor Analysis. Analytical and Bioanalytical Chemistry 383(1), 56-66 (2005).