Sorption of U(VI) onto TiO2. A vibrational spectroscopic analysis of the sorption processes

A detailed knowledge of the sorption processes of uranium(VI) onto titanium(IV) dioxide (TiO2) can serve as a model for the elucidation of molecular processes on more complex mineral surfaces. Because of the high stability, low solubility over a wide pH range and the well-known structure of TiO2, sorption studies with U(VI) can be carried out throughout a large range of experimental parameters [1-3].

In this work, we used attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy in an in situ investigation of the sorption processes of U(VI) onto TiO2. A set of highly purified and well characterized TiO2 phases from different origins differing in the ratio of the most stable polymorphs, i.e. anatase and rutile, in specific surface area and in particle size distribution was investigated. Irrespective of the composition of the mineral phase, it was shown that U(VI) mainly forms a similar inner sphere complex, showing an absorption maximum shifted of about 20 cm−1 to lower wavenumbers compared to the aqueous species.

The influence of the aqueous speciation on the sorption process was studied using different concentrations of the actinide ions and at different pH values. The high sensitivity of the in situ ATR FT-IR technique allows the study of U(VI) concentrations down to the lower micromolar range. From these results, it is concluded that only one significant surface U(VI) complex forms in the pH range between 4 and 7, and across a range of aqueous uranyl concentrations. The surface speciation of uranyl therefore involves fewer species than the aqueous speciation in these experimental systems, as is consistent with the findings of a previous study [4].

Furthermore, the capability of a fast scanning of IR spectra makes it feasible to carry out time-resolved experiments of the sorption processes with a time resolution in the sub-minute time range. It is shown that in the early steps of the U(VI) sorption a transient species is formed on the mineral phase showing a clearly red-shifted absorption maximum. After prolonged sorption the IR spectra indicate the formation of a stable surface species which is commonly found on TiO2 phases. These findings give new insights into the surface complex formation of U(VI) on titanium dioxide at a molecular level.

[1] Den Auwer, C., Drot, R., Simoni, E., Conradson, S. D., Gailhanou, M. and de Leon, J. M. (2003) "Grazing incidence XAFS spectroscopy of uranyl sorbed onto TiO2 rutile surfaces." New J. Chem. 27, 648-655.
[2] Wazne, M., Meng, X. G., Korfiatis, G. P. and Christodoulatos, C. (2006) "Carbonate effects on hexavalent uranium removal from water by nanocrystalline titanium dioxide." J. Hazard. Mater. 136, 47-52.
[3] Vandenborre, J., Drot, R. and Simoni, E. (2007) "Interaction mechanisms between uranium(VI) and rutile titanium dioxide: From single crystal to powder." Inorg. Chem. 46, 1291-1296.
[4] Lefèvre, G., Kneppers, J. and Fédoroff, M. (2008) "Sorption of uranyl ions on titanium oxide studied by ATR-IR spectroscopy." J. Colloid Interface Sci. 327, 15-20.