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The sorption behaviour of Cs(I) on Czech Ca–Mg bentonite BCV_2017 was investigated under conditions relevant to the engineered barrier systems in deep geological repositories. Batch sorption experiments were performed in synthetic groundwater SGW2 of Ca-HCO$_3$ type using natural bentonite and bentonite samples altered at 95 °C in 1 mol L$^{-1}$ NaCl, KCl, MgCl$_2$, or SGW2. The effects of contact time, solid-to-liquid ratio, initial radionuclide concentration, and bentonite alteration were evaluated.
Kinetic experiments showed that Cs(I) uptake reached apparent equilibrium rapidly, between sample preparation and the first sampling point after 0.9 days. Based on these results, a conservative contact time of 72 h was selected for equilibrium experiments.
Cs(I) sorption showed pronounced nonlinearity, indicating heterogeneous sorption sites and gradual site saturation. The experimental data were interpreted using distribution coefficients and fitted by linear, Freundlich, Langmuir, and two-site Langmuir models. The two-site Langmuir model provided the best fit according to the lowest WSOS/df values. The effect of solid-to-liquid ratio was statistically significant, although its practical impact was limited. Alteration of bentonite affected Cs(I) uptake mainly in the low-concentration range, whereas at higher concentrations, the isotherms of individual bentonite samples tended to overlap. Distribution coefficient values for Cs(I) were generally in the range of 10$^2$–10$^4$ L kg$^{-1}$, in agreement with previously published data.
The results confirm the strong retention of Cs(I) by BCV_2017 bentonite and show that thermal and chemical alteration can influence radionuclide uptake, particularly at low aqueous concentrations. These findings contribute to the assessment of Czech bentonite as a radionuclide-retaining material in engineered barrier systems.