Limiting membrane behavior of compacted sand–bentonite mixture

Compacted sand–bentonite mixtures are used as waste containment barriers (e.g. landfill liners and bentonite buffers for nuclear waste) to restrict contaminant transport. The potential for enhanced chemical containment of compacted sand–bentonite mixtures due to semipermeable membrane behavior has also been demonstrated. However, the extent to which membrane behavior persists in the presence of highly concentrated chemical solutions, which have been shown to degrade membrane behavior in bentonite-based barriers, remains largely unknown. Moreover, the limiting (threshold) salt concentrations at which membrane behavior of compacted sand–bentonite mixtures is effectively destroyed have not been evaluated. Accordingly, this study quantified the limiting membrane behavior of two duplicated specimens of compacted sand–bentonite mixture comprising 15% sodium bentonite (by dry mass) by determining the limiting salt concentrations at which measurable membrane behavior was eliminated. The specimens were exposed to increasingly higher source concentrations, C ot , of boundary monovalent salt solutions (KCl and NaCl) until measured values of the membrane efficiency coefficient, ω , were effectively zero. Overall, ω decreased from an average of 0.032 to zero as C ot increased from 160 mmol/L KCl to 3.27 mol/L NaCl, resulting in limiting threshold salt concentrations for the two tests between 1.63 mol/L and 3.27 mol/L NaCl, which are significantly higher than those at which measurable membrane behavior has previously been demonstrated.