Membrane behavior of compacted sand–bentonite mixture

Abstract

Semi-permeable membrane behavior contributes to the containment function of engineered barriers used for waste containment by restricting the migration of dissolved chemical species (solutes) such as aqueous-phase contaminants. The existence of membrane behavior has been demonstrated extensively for virtually all categories of bentonite-based containment barriers except compacted sand–bentonite mixtures. Accordingly, membrane tests were conducted on two specimens of a compacted sand–bentonite mixture comprising 15% bentonite (dry weight) with sufficiently low hydraulic conductivity (i.e., ≤1.0 × 10−9 m/s) to be suitable for use as a waste containment barrier. Despite the imposition of relatively complex chemical conditions, including the use of tap water versus de-ionized water as a circulating liquid and incomplete flushing (leaching) of soluble salts from the specimens prior to membrane testing, the results were in good agreement with those previously reported for other bentonite-based engineered barriers when exposed to similar or the same types of salts and salt concentrations. Also, both compacted soil–bentonite specimens exhibited similar magnitudes of membrane behavior, with measured membrane efficiencies ranging from 0.395 ± 0.053 to 0.063 ± 0.012 when exposed to source KCl solutions with concentrations ranging from 5 to 80 mmol/L, respectively. Thus, compacted sand–bentonite mixtures suitable for use as engineered containment barriers can exhibit semi-permeable membrane behavior, thereby potentially enhancing the containment function of the barriers.