Membrane behavior of compacted sand-bentonite mixture

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 x 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.