New insights into U(VI) sorption onto montmorillonite from batch sorption and spectroscopic studies at increased ionic strength.

The influence of ionic strength up to 3 mol kg (background electrolytes NaCl or CaCl) on U(VI) sorption onto montmorillonite was investigated as function of pH in absence and presence of CO. A multi-method approach combined batch sorption experiments with spectroscopic methods (time-resolved laser-induced fluorescence spectroscopy (TRLFS) and in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR)). In the absence of atmospheric carbonate, U(VI) sorption was nearly 99% above pH 6 in both NaCl and CaCl and no significant effect of ionic strength was found. At lower pH, cation exchange was strongly reduced with increasing ionic strength. In the presence of carbonate, U(VI) sorption was reduced above pH 7.5 in NaCl and pH 6 in CaCl system due to formation of aqueous UO(CO) and CaUO(CO) complexes, respectively, as verified by TRLFS. A significant ionic strength effect was observed due to the formation of CaUO(CO), which strongly decreases U(VI) sorption with increasing ionic strength. The joint analysis of determined sorption data together with literature data (giving a total of 213 experimental data points) allowed to derive a consistent set of surface complexation reactions and constants based on the 2SPNE SC/CE approach, yielding log K° = 2.42 ± 0.04, log K° = -4.49 ± 0.7, and log K° = -20.5 ± 0.4. Ternary uranyl carbonate surface complexes were not required to describe the data. With this reduced set of surface complexes, an improved robust sorption model was obtained covering a broad variety of geochemical settings over wide ranges of ionic strengths and groundwater compositions, which subsequently was validated by an independent original dataset. This model improves the understanding of U(VI) retention by clay minerals and enables now predictive modeling of U(VI) sorption processes in complex clay rich natural environments.