Reversibility and modeling of adsorption behavior of cesium ions on colloidal montmorillonite particles

In the geological disposal system with stable colloidal particles, the influence of the colloidal particles on the migration of radionuclides has to be evaluated. Not only the distribution coefficient, which is a measure for adsorption of radionuclides on colloidal particles, but also the reversibility of adsorption should be determined. In this study, adsorption and desorption of Cs+ ions on colloidal montmorillonite particles were investigated in the wide range of the initial Cs+ concentrations. A rapid, almost linear and reversible adsorption of Cs+ on montmorillonite was observed at basic condition, indicating that the adsorption of Cs+ on montmorillonite was mostly dominated by ion exchange. The size of the montmorillonite particles slightly affected the distribution coefficients at lower Cs+ concentrations, suggesting the contribution of surface complexation. The ion exchange and surface complexation model reproduced the adsorption and desorption behavior of Cs+. To explain the fixation of Cs+, montmorillonite was conditioned with the solutions of various Cs+ concentrations and the basal spacing was measured by X-ray diffraction (XRD). Conditioning montmorillonite at higher Cs+ concentrations > 5 × 10− 3 M decreased the basal spacing which may result in the fixation of Cs+ in the interlayer space. Since the adsorption experiments were carried out at lower Cs+ concentration < 10− 4 M, Cs+ adsorption was reversible.