In situ identification of the U(VI) surface speciation on iron oxide phases by ATR FT-IR spectroscopy

The identification of the molecular interactions occurring at solid-liquid interfaces is of great signifi-cance to the assessment of the migration behavior of heavy metal ions in the environment. In particu-lar, the dissemination of radioactive metals, such as uranium (U), in soils and aquifers is determined by sorption and desorption processes at mineral surfaces.
Information of the molecular structures of the sorption complexes can be obtained by vibrational spec-troscopy. The application of the Attenuated Total Reflection (ATR) technique in combination with a flow cell experiment potentially provides insights into the dynamic processes occurring during com-plex formation at the solid-liquid interface [1]. This technique allows an on line monitoring of the sorption processes with a time resolution in the sub minute range and under selective conditions ap-proaching near environmental relevant conditions [2,3]. In particular, the variation of selective ex-perimental parameter, e.g. pH, c(UO22+) or pCO2, and the selection of modified solid phases are ex-pected to generate selective spectral changes which potentially facilitate the identification of molecular features.
In this work, we provide vibrational spectroscopic data from binary and ternary U(VI) surface species on iron oxide mineral phases in the absence and presence of atmospherically derived CO2¸ respec-tively. In a comparative study of two iron oxide phases, namely ferrihydrite and maghemite, the dif-ferent character of sorption complexes can be spectroscopically identified. From the frequency of the ν3(UO2) mode, the formation of different types of surface species, that is inner- and outer-sphere com-plexes, can be derived. This is corroborated by time-resolved spectra of the sorption step and of the subsequently induced desorption process. From the time courses of these reactions, a first assignment to the different types of surface species predominantly formed at the different mineral surfaces can be given.
Results from the ternary sorption systems (U(VI)/atm. CO2/iron oxide phase) demonstrate signifi-cantly different affinities of the carbonate ions to the different mineral phases. While atm. CO2 forms binary and ternary sorption species on ferrihydrite in the absence and presence of U(VI), respectively, only ternary uranyl carbonato species were observed at the maghemite-water interface.

[1] Voegelin, A. et al. (2003) Environ. Sci. Technol. 37, 972-978.
[2] Müller, K. et al. (2012) Geochim. Cosmochim. Acta 76, 191-205.
[3] Foerstendorf, H. et al. (2012) Journal of Colloid and Interface Science 377, 299-306.