Persistence of semipermeable membrane behavior for a geosynthetic Clay Liner

Abstract

Geosynthetic clay liners (GCLs) consist of sodium bentonite (Na-bentonite) sandwiched between two geotextiles. Na-bentonite is preferred for GCLs due to the high swell and low hydraulic conductivity of the bentonite. Bentonites also may exhibit semipermeable membrane behavior, or the ability to restrict the migration of solutes through the clay. In previous studies, membrane behavior of GCLs has been evaluated primarily on the basis of exposure to relatively low salt concentrations (= 54 mM) of simple monovalent salts, such as potassium chloride (KCl), with increasing salt concentrations corresponding to decreasing membrane behavior. The objective of this study was to evaluate the persistence of membrane behavior for a GCL specimen exposed to salt solutions containing increasingly higher concentrations of KCl until any observed membrane behavior was completely destroyed. The results indicate that membrane behavior persisted until the specimen was exposed to a KCl concentration of 400 mM, which is well beyond the salt concentrations used in any previous study evaluating the membrane behavior of GCLs. In addition, the results clearly show that the relationship between the membrane efficiency coefficient and the logarithm of the salt concentration becomes nonlinear with increasing salt concentration, a trend that previously has been hypothesized to exist on the basis of limited experimental data. Thus, the results of this study represent the first evaluation of the persistence of membrane behavior of a GCL exposed to simple monovalent salt solutions.