Monte Carlo analysis of montmorillonite particle structures and modeling of dissolution rate reduction

The equilibrium structure of montmorillonite clay particles was calculated using Monte Carlo simulation to evaluate the long-term behavior of alteration for bentonite buffer materials in geological repositories of radioactive waste. It has been experimentally observed that the dissolution rate of montmorillonite strongly decreases as the density increases. One of the primary causes of this reduction in dissolution rate is the reduction in reactive surface area when the edges of particles are covered with other particles (physical masking). In this study, we proposed a geometrical model in which the masking area (the edge surface masked by other particles) is calculated from the edge-to-face distance of particles. The effective edge surface area (ESA) from the equilibrium structure obtained from Monte Carlo simulation was computed using the masking model. The numerical results of the effective ESA are in good agreement with the experimental measurements of the dissolution rate by atomic force microscopy in a wide range of density conditions. Moreover, we propose a prediction expression of the effective ESA of montmorillonite particles over a wide range of density conditions, which produces physically reasonable predictions.