Apparatus for Measuring Coupled Membrane and Diffusion Behavior of Unsaturated Sodium Bentonite

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Core Ideas Coupled membrane and diffusion behavior was measured in unsaturated Na‐bentonite. A new testing apparatus advances the state of the art for coupled processes in clays. The membrane efficiency of Na‐bentonite increased as the degree of saturation decreased. Sodium bentonite (Na‐bentonite) has been shown to exhibit semipermeable membrane behavior—the ability to selectively restrict the migration of dissolved chemical species through the pores of the clay. However, experimental research to date has focused on the membrane behavior of Na‐bentonite almost exclusively under water‐saturated conditions (i.e., degree of saturation S = 1), even though membrane behavior under unsaturated conditions is expected to be more significant. Further, the limited number of studies that have been performed to evaluate membrane behavior in unsaturated soils have used only open systems to quantify membrane efficiency (ω), despite the testing advantages of using closed systems (e.g., more accurate measurement of ω, easier control of boundary conditions). Thus, a closed‐system testing apparatus capable of measuring coupled membrane and diffusion behavior in unsaturated Na‐bentonite was developed and then used to measure ω and salt diffusion in Na‐bentonite with S of 0.84 and 1.0. For a source solution (concentration difference) of 20 mM KCl, ω increased from 0.61 to 0.71 as S decreased from 1.0 to 0.84, which is consistent with the current conceptual understanding of membrane behavior and trends in the literature. In contrast, the effective diffusion coefficient for Cl− was essentially the same (i.e., ∼1.8 × 10−10 m2 s−1) for both specimens due to the small difference in S. The development of the testing apparatus advances the state of the art for laboratory measurement of coupled membrane and diffusion behavior in unsaturated clays commonly used as chemical containment barriers.