Sorption of Sr and Cs onto Czech natural bentonite: experiments and modelling

Within the development of deep geologic repository in the Czech Republic, the local Mg/Ca bentonite has been tested with the aim to assess its sorption qualities with regard to cations of interest for the performance assessment (PA) of the repository. The data of Sr and Cs sorption obtained by batch method were evaluated with the use of ion exchange and surface complexation models. The selectivity coefficients of exchangeable cations were determined, and the selectivity coefficient of Sr was set to be the same as for Ca. The selectivity coefficient of Cs was chosen in accordance with literature and the concentration and stability constant of the surface complex was fitted. One-site ion exchange model for Sr sorption was able to predict the observed K d dependence on solid-to-liquid ratio. Two-site ion exchange/surface complexation model for Cs sorption simulated the dual sorption mechanism of Cs adequately, thus the predictive ability was lower due to the fact that the protonation of edge sites or competition with Na ion was not taken into account.