Coupling Effects during Steady-State Solute Diffusion through a Semipermeable Clay Membrane

Two separate coupling effects are evaluated with respect to steady-state potassium chloride (KCl) diffusion through a bentonite-based geosynthetic clay liner (GCL) that behaves as a semipermeable membrane. Both of the coupling effects are correlated with measured chemico-osmotic efficiency coefficients, ω, that range from 0.14 to 0.63 for the GCL. The first coupling effect is an explicit (theoretical) salt-sieving effect expressed as a coupled effective salt diffusion coefficient, D_ω*, that is lower than the true (uncoupled) effective salt diffusion coefficient, D_s*, because of the observed membrane behavior. However, the maximum difference between D_ω* and D_s* based on measured chloride concentrations is relatively small (i.e., = 10%), and the difference decreases with decreasing ω (i.e., D_ω* → D_s* as ω → 0). The second coupling effect is implicit (empirical) and is characterized by the measurement of concentration-dependent effective salt diffusion coefficients that results in an ∼300% decrease in D_s* as ω increases from 0.14 to 0.63. The decrease in D_s* resulting from implicit coupling is attributed to solute exclusion described in terms of a restrictive tortuosity factor.