Limiting membrane and diffusion behavior of a geosynthetic clay liner

Bentonite based geosynthetic clay liners (GCLs) are commonly used as chemical containment barriers to minimize liquid flow and contaminant transport. The preference for GCLs relative to other materials for chemical containment include economy of use and superior engineering properties, including low hydraulic conductivity (k) and the ability to exhibit semipermeable membrane behavior that restricts the migration of solutes through the bentonite. In the case of low k (e.g., ≤10−10 m/s), diffusion is likely to be a significant, if not dominant, mechanism of chemical transport. The objective of this study was to evaluate the membrane and diffusion behavior of a GCL in the limit as the salt (KCl) concentration increased to the extent that any observed membrane behavior was destroyed. The results indicated that the observed membrane behavior for the GCL was essentially destroyed when the average KCl concentration across the GCL specimen, Cave, reached 200 mM KCl, which was greater than the value of Cave that would have been predicted on the basis of previous results. The results also indicated that the value of the effective diffusion coefficient, D*, for Cl− at this limiting value for Cave of 200 mM KCl was greater than the value of D* that would have been predicted on the basis of the use of lower values of Cave. However, the differences between the measured and predicted values of D* were relatively minor (≤11.3%), such that the differences likely would be insignificant for most practical applications.