Molecular simulation on Cs, Rb retention in Na/K-montmorillonite interlayer coupling clay swelling/collapse

Abstract

A key issue about radioactive waste disposal and nuclear accident contamination control is the retention of radionuclides in clay minerals. The cation (Cs⁺, Rb⁺, Na⁺, K⁺) selectivity in montmorillonite (Mt) interlayers have not been quantitatively studied. This work employs classical molecular dynamics (CMD) to systematically investigate the interlayer structure, swelling properties, diffusion dynamics, and cation exchange processes. The selectivity of alkali ions within the interlayer space coupled with clay swelling/collapse under different water activity (a_w) and cation activity, has been quantified. Both Cs-, Rb–Mt demonstrate a monolayer hydrate configuration as the most stable state. The mobility of intercalated species, as indicated by self-diffusion coefficients, exhibits a stepwise trend with increasing water content. The cationic selectivity within the interlayer follows the order Cs⁺ > Rb⁺ > K⁺ > Na ⁺ at a_w = 1.0. The logarithm values of the selectivity coefficients for Cs/Rb relative to Na/K are as follows: logK_c(Cs/K) = 0.73, logK_c(Cs/Na) = 1.62, logK_c(Rb/K) = 0.62, and logK_c(Rb/Na) = 1.57 at a_w = 1.0. A model correlating selectivity coefficients with water activity has been proposed. It is noted that Cs⁺ and Rb⁺ ions tend to accumulate within the interlayer as water activity decreases, and interlayer Rb⁺ competes with Cs⁺ for exchange positions at low water activity. These results can be used to quantify cationic partitioning during the remediation of radiocesium contamination in soil and weathering processes of sediments.