Elucidation of the adsorption mechanism of cesium ions into montmorillonite interlayers by classical molecular dynamics simulations

Abstract

In the geological disposal of high-level radioactive waste, bentonite plays a crucial role in retarding radionuclide migration by adsorbing cationic species. Cesium ions (Cs⁺), in particular, are strongly adsorbed by montmorillonite, the main component of bentonite. However, the detailed adsorption states and mechanisms remain unclear. We employed classical molecular dynamics simulations to investigate the adsorption behavior of Cs⁺ and sodium ions (Na⁺) from bulk water into the interlayer of Na-montmorillonite, by evaluating free energy profiles. Our results revealed that Cs⁺ adsorbs more strongly into the interlayer than Na⁺ due to the formation of the inner- and outer-sphere complexes of Cs⁺ and Na⁺, respectively. In the presentation, we will also discuss the fundamentals of classical molecular dynamics, the physical origin of the free energy profiles, detailed adsorption mechanisms, and comparisons with experimental data.