Research workers, working with commercial bentonite-based membranes, reported different diffusivity for co- and counterions through those membranes. Many of these researchers called this difference in diffusivity ultrafiltration. Some of these workers also noted that commercial bentonite membranes often contain divalent metal carbonates as impurities. Recently it has been suggested that the ultrafiltration is associated with the co-diffusion of divalent metal ions. Previous workers did not recognize this co-diffusion, probably due to an absence of imposed concentration gradient for the divalent metal ions. The present investigation suggests that the required concentration gradient is self-generated by increased solubility of the divalent metal carbonates with increasing concentration of alkali halide in solution. The presence of dissolved divalent metal ions alters the interfacial potentials at the outside solution and bentonite membrane junctions and reduces the electrostatic potential gradient across the membrane. A reduction in the electrostatic potential gradient across the membrane reduces the coupling between the diffusing co- and counterions and alters their diffusivity. (c) 2006 Elsevier Inc. All rights reserved.