Sorption of U(VI) at the TiO₂ – water interface: An in situ vibrational spectroscopic study.

Molecular-scale knowledge of U(VI) sorption reactions at the water-mineral interface is important for predicting U(VI) transport processes in the environment. In this work, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy was used in a comprehensive investigation of the sorption processes of U(VI) onto TiO₂. The high sensitivity of the in situ ATR FT-IR technique allows the study of U(VI) concentrations down to the low micromolar range, which is relevant to most environmental scenarios.
A set of highly purified and well characterized TiO₂ phases differing in their origin, the ratio of the most stable polymorphs (anatase and rutile), in specific surface area, isoelectric points and in particle size distribution was investigated. Irrespective of the composition of the mineral phase, it was shown that U(VI) forms similar surface complexes, which was derived from the antisymmetric stretching mode υ₃(UO₂) showing a characteristic shift to lower wavenumbers compared to the respective aqueous species at a similar low concentration level.
The availability of a fast scanning IR device makes it feasible to perform time-resolved experiments of the sorption processes with a time resolution in the sub-minute range. It is shown that during the early stages of the U(VI) uptake a surface species on the mineral phase is formed characterized by a significantly red-shifted absorption maximum which is interpreted as a bidendate inner-sphere complex. After prolonged sorption, the IR spectra indicate the formation of a second surface species showing a smaller shift compared to the aqueous species. These findings were verified by a series of spectroscopic experiments performed on a U(VI)-saturated surfaces.
Further ATR FT-IR spectroscopic experiments focused on the impact of the U(VI) concentration, the pH value and the exclusion of atmospheric carbonate on the sorption process. The results confirm the presence of two surface species, occurring sequentially as a function of U(VI) surface loading.
This work provides new insights into the sorption processes of U(VI) on titanium dioxide on a molecular level. Basic thermodynamic ideas of surface complexation are substantiated by in situ infrared spectroscopy.