Sorption of uranium(VI) on ferrihydrite – Influence of atmospheric carbonate on surface complex formation investigated by ATR-FT-IR spectroscopy

The migration behaviour of uranium in the geosphere is generally influenced by sorption processes in aqueous media. The solubility, mobility and bioavailability is determined by the molecular interactions between the dissolved actinide species and mineral surfaces in groundwater aquifers. In its hexavalent form, uranium usually exists in the environment as the dioxouranyl cation (UO22+) which can form highly soluble complexes with a variety of anionic species commonly increasing the solubility of uranium. However, since there is a high affinity of the UO22+ ion to hydrous iron oxides the mobility of U(VI) can be considerably reduced in the environment. The affinity of U(VI) to hydrous ferric oxides is strongest in the pH range from 5 to 8. But it is also dependent on the composition of the liquid phase. Strong complexing ligands such as carbonate potentially inhibit adsorption of U(VI) onto the mineral phase (1). Therefore, the structures of the uranyl surface complexes at iron hydroxide mineral phases such as hematite and ferrihydrite (Fh) in the presence of CO2 are still in the focus of recent spectroscopic investigations (2,3).
In this work we present results from sorption experiments of U(VI) on Fh using Attenuated Total Reflection Fourier-transform Infrared (ATR-FT-IR) spectroscopy which provide in situ spectral information of the sorption processes in aqueous solution. The sorption of the UO22+ ion was investigated at mildly acidic conditions in atmospheric equilibrium and under an inert gas atmosphere in order to study the influence of dissolved CO2 on the sorption process. The infrared spectra show a similar surface complex of the uranyl ion at the Fh-phase irrespective of the presence of atmospheric CO2. However, the binding of the carbonate ligand to the mineral phase changes considerably upon sorption of the actinide ion and a change from a monodentate to a bidentate binding upon sorption of UO22+ is concluded. Furthermore, we investigated the sorption and desorption behavior of carbonate ions on pristine Fh and after sorption of UO22+ onto the iron oxide phase.

References
(1) Hsi, C. D.; Langmuir, D. Geochim. Cosmochim. Acta 1985, 49, 1931-1941.
(2) Bargar, J. R.; Reitmeyer, R.; Davis, J. A. Environ. Sci. Technol. 1999, 33, 2481-2484.
(3) Ulrich, K. U.; Rossberg, A.; Foerstendorf, H.; Zänker, H.; Scheinost, A. C. Geochim. Cosmochim. Acta 2006, 70, 5469-5487.