Membrane Behavior of Unsaturated Sodium Bentonite

Abstract

Chemical containment barriers comprising sodium bentonite (Na-bentonite) have been shown to exhibit semipermeable membrane behavior under saturated conditions. Since membrane behavior results in restricted migration of aqueous-phase chemicals (solutes), the existence of membrane behavior in bentonite-based barriers can significantly improve the containment function of the barriers. However, some bentonite-based barriers exist under unsaturated conditions, and the extent to which the degree of saturation, S, of the barrier affects the existence and magnitude of membrane behavior has not been evaluated heretofore. Accordingly, the membrane efficiency coefficient, ω, of Na-bentonite was measured in the laboratory at constant S in response to applied differences in potassium chloride (KCl) concentrations under closed-system conditions. The results indicated that, for a given S, ω decreased as the source concentration of KCl (Cot) increased, which is consistent with previous studies based on S=1.0. However, for a given Cot, ω increased with decreasing S, as expected on the basis that a reduction in S corresponds to a reduction in the water-filled pore space accessible for solute migration. Overall, ω ranged from 0.31 at S=1.0 with Cot=50 mM KCl to 0.75 at S=0.79 for Cot=20 mM KCl. Although the range in S that was evaluated was limited by the testing conditions, which resulted in test durations ranging from 232 to 335 days, the results of this study provide the first quantitative evidence illustrating the effect of S on ω for Na-bentonite.