Spectroscopic identification of ternary carbonate complexes upon U(VI) sorption onto ferrihydrite

The sorption processes of uranium(VI) onto ferrihydrite (Fh) were investigated by in situ Attenuated Total Reflection Fourier-transform Infrared (ATR FT-IR) spectroscopy. This technique provides structural information of the molecular complexes occurring during the sorption processes of actinide ions on mineral phases in aqueous solution [1].
The influence of the presence of atmospheric carbon dioxide during the sorption processes of the actinide ions was studied by sorption experiments which were carried out under inert gas conditions and in an ambient atmosphere.
The spectra demonstrate that the uranyl ion recovers in a similar molecular environment irrespective to the prevailing atmospheric conditions. However, the carbonate ions sorbed to the mineral phase in ambient atmosphere undergo significant structural changes upon the sorption of the actinide ions. While carbonate binds monodentately to the pristine Fh-surface, it forms bidendate surface complexes upon the sorption of U(VI). Moreover, the formation of ternary inner-sphere complexes can be derived from spectra recorded during the sorption processes of atmospheric carbonate onto a Fh-film which was preloaded with U(VI) under inert gas conditions. With respect to the absorption frequency of the uranyl ion sorbed onto Fh under different conditions, we suggest the formation of [Fh•••UO2•••O2CO] complexes. Surface complexes with carbonate serving as a bridging ligand between Fh and UO22+ ions were not found in our experiments. These findings are in good agreement with recent EXAFS results [2].
Furthermore, we investigated the sorption processes at acidic (5.5) and at ambient (7.8) pH values. No significant differences were observed in the spectra. However, the sorption processes obviously occur with reduced velocity at higher pH values which might be due to the dominating aqueous U(VI) species at different pH values [3].

[1] Lefèvre (2004) Adv. Colloid Interface Sci. 107, 109-123. [2] Rossberg et al. (2009) Environ. Sci. Technol. (in press). [3] Müller et al. (2008) Inorg. Chem. 47, 10127-10134.