In this paper, we address the manner in which the continuumscale diffusive properties of smectite-rich porous media arise from their molecular- and pore-scale features. Our starting point is a successful model of the continuum-scale apparent diffusion coefficient for water tracers and cations, which decomposes it as a sum of pore-scale terms describing diffusion in macropore and interlayer "compartments." We then apply molecular dynamics (MD) simulations to determine molecularscale diffusion coefficients Dinterlayer teriayer of water tracers and representative cations(Na+, Cs+, Sr2+) in Na-smectite interlayers. We find that a remarkably simple expression relates D interiayer to the pore-scale parameter δnanopore ≤ 1, a constrictive factor that accounts for the lower mobility in interlayers as compared to macropores: δnanopore = D interlayer/D0, where D0, is the diffusion coefficient in bulk liquid water. Using this scaling expression, we can accurately predict the apparent diffusion coefficients of tracers H 2O, Na+, Sr2+1, and Cs+ in compacted Nasmectite-rich materials.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry