MODIFICATION OF POLYAMIDE MEMBRANES BY SELECTED MOLECULES FOR IMPROVED BORON REJECTION

Membrane filtration is one of the main technologies used in water treatment and in industrial separation processes. Most of the commercial RO/NF membranes today are composed of polyamide which is cost effective and shows very high salt rejection with relatively high flux. Although effective in terms of flux and salt removal, these membranes have a few drawbacks. Our focus is on poor rejection of boron (B) present in sea water at ca. 5 ppm as undissociated boric acid that needs to be removed to the level of 0.5 ppm, unachievable today in a one-pass process. This complicates the process and available solutions increase the cost of desalinated water by 15-20%.

We explore feasibility of a simple and inexpensive in-situ modification process for commercial RO membranes that can significantly increase their boron rejection. This is achieved by reaction of the membrane with certain modifying molecules that reduce the affinity of the active layer to boric acid and tighten the polyamide structure and thus increase its selectivity. The results show that the proposed treatment may indeed reduce the boron passage by a factor of 2 to 4, while simultaneously maintaining or improving the salt rejection of the membrane. The selectivity improvement comes at the expense of some flux reduction, but the flux-selectivity tradeoff is superior to commercial polyamide membranes.