Reverse osmosis properties of bentonite/glass bead mixtures at low compaction pressures

It is well-known that clays and shales can be membrane-functioning. When a hydraulic head difference exists across a membrane-functioning barrier, a portion of the solute is rejected from solution passing through the membrane. Many engineering structures such as landfill liners, mixed soil augered barriers, and retention pond liners consist of soil-clay mixes, which may have clay contents as low as 12%. Some shallow geologic environments may also contain similar clay/soil mixes. No previous testing has been performed to investigate hydraulic-head-induced solute rejection in such mixed soils. Therefore, we performed six experiments using five different mixes of Na-bentonite and glass beads (100%, 50%, 25%, 12%, and 0% clay) to determine if any of these mixes exhibited membrane properties and to determine what effect clay content had upon the membrane properties. All the mixtures were compacted to 690 kPa and the compacted soil samples were 0.56-1.40 mm thick. An approximately 35 ppm Cl− solution and a hydraulic head of approximately 90 kPa were used for all experiments. Experimental data shows that these simulated clay-sand mixtures do exhibit measurable membrane properties with as low as 12% clay by weight under these conditions. Calculated reflection coefficient values ranged from a low of 0.07 for 12% Na-bentonite to a high of 0.26 for 100% clay. Solute rejection ranged from 12.6% for 12% clay to a high of more than 40% for the 50% and 100% clay samples. The glass beads represent fine-grained sand and were shown to have no membrane properties.