Diffusion and Restrictive Tortuosity for a Semipermeable Sand–Bentonite Mixture

Le résumé fourni par la source

Compacted sand–bentonite mixtures can be used as engineered, low-permeability (≤1.0×10−9 m/s) hydraulic and chemical containment barriers for waste containment when a suitable source of natural clay is not available. Under this low-permeability condition, solute diffusion becomes a necessary consideration. This study reports the results of the measurement of the effective diffusion coefficients, D*, for salt (KCl) diffusion through a compacted specimen of a sand–bentonite mixture known to exhibit semipermeable membrane behavior due to solute restriction. The measured D* values for chloride (Cl−) are used to determine the restrictive tortuosity factor, τr, that reflects the increased tortuosity due to solute restriction (0≤τr≤1), and the τr values are correlated with the membrane efficiency coefficient, ω, representing the degree of solute restriction (0≤ω≤1). The resulting τr-versus-ω relationship is nonlinear, which is inconsistent with a linear relationship, τr=1−ω, based on previously developed theory and supported extensively by results from the literature for other bentonite-based barriers. The difference in behavior is attributed primarily to the suppression of Cl− diffusion due to competition with bromide (Br−) in satisfying electroneutrality. The presence of Br− resulted from the addition of biocide to prevent bioclogging as a mechanism for solute restriction. The results illustrate that the linear relationship, τr=1−ω, is not applicable when there is significant competition between two or more anionic chemical species in the specimen pore water due to the requirement for electroneutrality. This finding has important implications for practical applications.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.