Comparison of coupled solute flux through sodium- and enhanced-bentonite barriers leveraging two decades of experimental data

Enhanced bentonites (EBs) developed for improved hydraulic compatibility (i.e., low hydraulic conductivity, k, to chemical solutions) relative to unamended sodium bentonite (NaB) are increasingly being used in geosynthetic clay liners (GCLs). Both NaB and EBs have been shown to exhibit semipermeable membrane behavior. Thus, predictions of barrier performance for different bentonites that focus only on advection and ignore the influence of membrane behavior and diffusion on solute flux are inaccurate. In this paper, data from solute transport studies conducted over the last 20 years were used to compare expected solute flux through NaB and EB barriers with and without accounting for membrane behavior. Coupled effective diffusion coefficients (D*), membrane efficiency coefficients (w), and k values for NaBs and EBs from the literature were used with an analytical solution for coupled solute transport to predict dimensionless transient and steady-state solute fluxes exiting the barrier (J*j and J*ss) under typical GCL boundary conditions. Values of J*j and J*ss for a range of salt solutions were compared for NaB GCLs and five different EBs. For a given source concentration (Co) of a given salt, EBs generally exhibit lower k, higher w, and similar D* relative to NaB GCLs. As a result, J*ss values for EBs were 10–60% lower relative to NaB for monovalent (KCl, NaCl) solutions and up to 95% lower relative to NaB for divalent (CaCl2) solutions. The bentonites with the highest w, but not necessarily the lowest k, correlated to the lowest J*ss for all solutions.