Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The identification of the molecular interactions occurring at solid-liquid interfaces is of great significance to the assessment of the migration behavior of heavy metal ions in the environment. In particular, the dissemination of radioactive metals, such as uranium (U), in soils and groundwater aquifers is determined by sorption and desorption processes at mineral surfaces.
The molecular structures of the sorption complexes are mainly obtained by means of spectroscopic investigations of batch samples. These experiments provide no molecular information about the dynamic processes occurring during complex formation at the solid-liquid interface. However, such information, in particular about the early sorption steps, is expected to improve the understanding of the sorption processes. Therefore, we applied attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy for in situ studies of the molecular processes at solid-liquid interfaces in real time with time resolution in the sub minute range and under selective conditions approaching near environmental relevant conditions [1].
In this work, we provide vibrational spectroscopic data from binary and ternary sorption systems, namely U(VI)/TiO2 and U(VI)/atm. CO2/ferrihydrite(Fh), respectively. From the binary U(VI)/TiO2-system the subsequent formation of two different surface species was observed [2]. These species were identified as inner and outer sphere uranyl complexes substantiating basic principles of surface complexation modeling which are based on thermodynamic approaches.
The spectral data obtained from in situ sorption experiments of U(VI) onto Fh demonstrate the formation of a unique U(VI) surface species irrespective of the absence or presence of atmospherically derived CO2. In contrast, the surface speciation of the carbonate anions significantly changes upon U(VI) sorption strongly suggesting the formation of ternary surface species. Moreover, the online monitoring of the sorption and desorption reactions allows the analysis of the sorption kinetics. Because of the different reaction rates found for carbonate sorption and desorption reactions on pristine Fh and Fh pretreated with U(VI), it is shown that carbonate sorption is a faster reaction than the sorption of U(VI). From the structural information of the ternary sorption complex derived from the spectroscopic results, molecular structures of the surface species are proposed [3].

References:

[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, submitted.