Construction of an ultrathin, compacted clay membrane for use in reverse osmosis

The purpose of this project was to test a combination sedimentation/vacuum filtering method for forming ultrathin (0.041 to 0.064 mm) bentonite membranes and to see if ultrathin clay membranes can be effectively compacted and used in reverse osmosis purification of water. We used a flat-leaf reverse osmosis (RO) cell specifically constructed for use with clay membranes. This cell differs from commercially available cells in that it has an integral piston so that the clay membrane can be compacted in a hydraulic press. Our results showed compaction of such ultrathin clay membranes to be problematic. The thickness of the membranes was exceeded by the dimensional variation in the machined experimental cell, and this is believed to have resulted in local bypassing of the membrane with a resultant decrease in solute rejection efficiency. In two of the experiments, permeant flow was varied as a percentage of the total flow to investigate the results of changing permeant flow on solute rejection. In one experiment, the permeant flow was varied between 2.4% and 10.3% of the total flow with no change in solute rejection. In another experiment, the permeant flow was varied between 24.6% and 52.5% of the total flow. In this experiment, the solute rejection rate decreased as the permeant occupied greater fractions of the total flow. Solute rejection was found to decrease with increasing salt concentration and ranged between 62.9% for 0.092 M and 19.7% for 0.989 M chloride and between 61.5% and 16.8% for sodium. Due to problems with the compaction procedure and potential membrane bypassing, these rejection rates are probably not the upper limit for NaCl rejection by bentonite membranes. (C) 2003 Elsevier B.V. All rights reserved.