Towards Bottom-up Mesoscale Computer Simulation of Diffusion in Polydisperse Sodium Bentonite

Abstract

Understanding the transport of solutes through hydrated sodium montmorillonite (Na-MMT) is of significant interest to better model its ability to control the transport of ions. Porosity and tortuosity are often used as scaling factors connecting diffusion coefficients in the clay material to free diffusion coefficients of solute in water. In this work, the porosity and tortuosity of Na- MMT were calculated using a bottom-up approach using a new coarse-grained representation capable of accounting for polydispersity. We considered both monodispersed and polydisperse Na- MMT models. Accessible porosity of these models was estimated using PorosityPlus. The pore network information was used to create a 3D image of hydrated Na-MMT, where random walkers were employed to evaluate its tortuosity. Finally, the porosity and tortuosity values estimated using the new CG representation were compared to macroscopic experimental values describing tritium and iodide diffusion in Na-MMT dominant bentonite. The result of the new representation better matches the experimental data than the previous representation. The new representation be used to study the diffusion properties of polydisperse systems containing platelets with diameters ranging from 100-500 Å. The effect of direction of compaction on the measured diffusion will also be explored using the new model.