Connecting molecular simulations and laboratory experiments for the study of time-resolved cation-exchange process in the interlayer of swelling clay minerals

Abstract In the context of element migration in clay-rich media, self-diffusion coefficients of interlayer cations in swelling clay minerals obtained from molecular simulations are rarely used by macroscopic models predicting cation-exchange processes. Based on experiments and simulations, this study aims at (i) making a connection between molecular and sample scale processes to predict the dynamics of cation-exchange reactions between the interlayer space of millimetre disks of vermiculite and aqueous reservoirs, and (ii) assessing the role played by both self-diffusion and selectivity coefficients on this process. Time-resolved cation exchange experiments were performed using Ca-saturated vermiculite disks immersed in aqueous reservoirs with different NaCl or SrCl2 salinities. The results were reproduced via a finite-volume model constrained by (i) cation self-diffusion coefficients calculated by molecular dynamics simulations and (ii) interlayer selectivity coefficients drawn from “batch” cation-exchange isotherms. Results showed that considering the averaged values for both the cation-exchange selectivity coefficients and self-diffusion coefficients of the slowest interlayer cation led to good agreement between the experiments and simulations, validating the modelling strategy for the connection between the molecular and laboratory time scales. A sensitivity test regarding the influence of the two input parameters on the overall results was then performed. This study underlined a constrained upscaling strategy to better assess the role played by different intrinsic parameters of the clay/water systems (molecular self-diffusion coefficients in the interlayer space vs. selectivity coefficient) on the diffusion of cations during cation-exchange reaction in clay-rich media.