Spectroscopic characterization of heavy metal species in aqueous solutions and at mineral surfaces
For a reliable assessment of the migration of heavy metal contaminants, a detailed knowledge of the molecular events occurring in the environment is mandatory. This includes the chemical reactions of the contaminants in aqueous solution and as well at the mineral-water interface. Comprehensive molecular information can be obtained by a multi method approach where complementary information is obtained from each spectroscopic technique.
The aqueous speciation of heavy metal ions is generally a function of pH, temperature, redox reactions and ionic strength. It is typically derived from potentiometric experiments at relatively high metal concentrations (millimolar range). For lower metal concentrations, that is the micromolar range and below, it is mostly extrapolated and needs verification by non-invasive spectroscopic techniques with a sufficiently low detection limit, such as luminescence (TRLFS, →) and vibrational (IR, →) spectroscopy.
Fig. 1: TRLFS spectra of the aqueous U(VI)-citrate system recorded at low temperature (T = −120 °C). From such series of spectra, the formation constants (log K values) of aqueous complexes can be derived. 
Structural information of the sorption species formed on mineral surfaces can be obtained from various spectroscopic techniques, such as luminescence (TRLFS, →), vibrational (IR, →) and X-Ray absorption (EXAFS, →) spectroscopy providing complementary molecular information. From the data obtained, information about inner- or outer sphere complexation and the formation of ternary surface complexes can be derived.
Fig. 2: U(VI) sorption on TiO2 investigated by time-resolved IR spectroscopy ([U(VI)]init = 20 µM, pH 5). Spectra allow to monitor on line the subsequent formation of different surface species. 
Due to the radioactive decay of the actinides and their fission products, elevated temperatures have to be considered in the near field of a deep waste repository. For reliable safety assessment studies, the validation of thermodynamic at elevated temperatures is lacking. Similar challenges are imposed by the very low redox potentials and high salinity developing in such environments. Hence, the investigation of complex species in aqueous solutions and at mineral surfaces, as a function of pH, temperature, redox reactions and ionic strength becomes essential for the safety assessment in the near and far field of nuclear repositories.
Fig. 3: FT filtered peak from EXAFS spectra of the U(VI)-gibbsite sorption system fitted with single an multiple scattering paths. The formation of a dimeric U(VI) carbonato surface species on the mineral surface is derived. 
Our research addresses these topics by constantly updating and upgrading experimental methods, like spectroscopic tools (XAS, IR, TRLFS, NMR). The influence of salinity and/or temperature on the spectroscopic properties can be investigated by UV-vis, TRLFS, and ATR FT IR spectroscopy up to an ionic strength of 5 M and 70 °C.
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