Relevance of Chemico-Osmotic and Electro-Osmotic Phenomena in Bentonite-Based Barriers

Osmosis is known to play a key role in reducing the transport rate of contaminants through the natural and engineered clay barriers that are used for a number of geoenvironmental applications, such as the lining of landfills and the deep geological disposal of radioactive wastes. Although a significant body of experimental research has focused on the quantification of osmotic phenomena in smectite clays permeated with single-electrolyte solutions, no evidence has been provided about the membrane behaviour of clays in solute mixtures and, specifically, about the so-called osmotic anomalies (i.e. membrane efficiency coefficient outside the 0 to 1 range) that have been documented in the biological and chemical literature for fine-porous charged diaphragms in the presence of two or more electrolytes. In view of the similarities between such fine-porous media and smectite clays, the aim of the paper is to discuss the conditions under which bentonite-based barriers are expected to exhibit the aforementioned osmotic anomalies, which are shown to be caused by the different diffusivities and electrochemical valences of the migrating cations.