Prediction of Intrinsic Cesium Desorption from Na-Smectite in Mixed Cation Solutions

Quantitative understanding of the stability of sorbed radionuclides in smectite is necessary to assess the performance of engineering barriers used for nuclear waste disposal. Our previous study demonstrated that the spatial organization of the smectite platelets triggered by the divalent cations led to the apparent fixation of intrinsic Cs in smectite, because some Cs is retained inside the formed tactoids. Natural water is usually a mixture of Na⁺ and divalent cations (Ca²⁺ and Mg²⁺). This study therefore investigated the desorption behavior of intrinsic Cs in Na-smecite in mixed Na⁺-divalent cation solutions under widely various cation concentrations using batch experiments, grain size measurements, and cation exchange modeling (CEM). Results show that increased Na⁺ concentrations facilitate Cs desorption because Na⁺ serves as the dispersion agent. A linear relation was obtained between the logarithm of the Na⁺ fraction and the accessible Cs fraction in smectite. That relation enables the prediction of accessible Cs fraction as a function of solution cationic compositions. The corrected CEM considering the effects of the spatial organization suggests that the stability of intrinsic Cs in the smectite is governed by the Na⁺ concentration, and suggests that it is almost independent of the concentrations of divalent cations in natural water