Assessing the leakage rate through semipermeable bentonite-based barriers permeated with multi-electrolyte solutions

Most of the experimental research conducted to date on the semipermeable membrane behaviour of bentonites, which are used as part of pollutant containment systems, has involved permeant (electrolyte) solutions containing a single salt, thus not accounting for the simultaneous presence of two or more electrolytes. In view of the significance of the latter issue when bentonites are permeated with real contaminated liquids, the results of a multi-stage membrane test carried out on a powdered sodium bentonite in equilibrium with aqueous mixtures of sodium chloride (NaCl) and potassium chloride (KCl) are interpreted via a theoretical model, whereby the overall osmotic counter-flow of water in multi-electrolyte systems is decomposed into a chemico-osmotic effect, which arises from the partial exclusion of anions from the bentonite pores and also occurs in single-electrolyte systems, and an electro-osmotic effect, which stems from the condition of null electric current density in the presence of cationic species with different diffusivities and causes the measured reflection coefficient, w, to assume values both larger than unity (w = 1.064) and lower than zero (w = - 1.168). While positive anomalous osmosis (w > 1) correlates with an enhanced osmotic counter-advection of contaminants and, hence, with an improved containment performance of bentonite-based barriers, the occurrence of negative anomalous osmosis (w < 0) causes such barriers to behave worse than non-semipermeable clay liners.