Membrane behavior and diffusion in bentonite for different anions

Abstract

Bentonite clay is widely used in engineered barriers, such as geosynthetic clay liners (GCLs), to safely contain wastes and chemicals. In addition to low hydraulic conductivity and diffusion coefficients, bentonite has been shown to exhibit membrane behavior (anion restriction) which can enhance long term containment performance. However, prior experimental studies on GCLs and other clay barriers have primarily focused on membrane behavior for chloride-based solutions. This study investigated diffusion and membrane behavior for two different anions (sulfate and chloride) through GCLs for a range of solution concentrations. Significant membrane behavior was observed for both KCl and K 2 SO 4 solutions, with measured membrane efficiency coefficients (ω) as high as 0.75. These results provide the first direct evidence of the significance of membrane behavior in GCLs for sulfate. Further, ω values were higher for sulfate than for chloride, aligning with existing conceptual models of ion restriction in clay membranes. The findings suggest that membrane effects in bentonite barriers containing solutions with anions with greater hydrated radii than chloride may be more significant than anticipated based on existing experimental literature. The results have important implications for understanding anion transport through bentonite, and support advancements in coupled transport modeling and performance-based design of containment systems.