Modified heterogeneous anion-exchange membranes for desalination of brackish and recycled water

In the desalination of recycled water, the triple demand for a high recovery ratio, low energy consumption, and fouling resistance can be met by electrodialysis (ED) under certain conditions: high recovery is an inherent property of the process, while low energy consumption and fouling resistance can be obtained by the use of modified heterogeneous membranes. To this end, heterogeneous polyethylene-based anionexchange membranes were modified by a hydrophilic anion-exchange coating, having an ion-exchange capacity of 2.2 to 2.9 mEq/g (dry weight) and a specific conductance of 70 to 110 mS/cm. The coating was found to be stable in continuous ED operation (66 days). The modification suppressed water splitting and brought the performance of the coated heterogeneous membranes into line with that of homogeneous commercial anion-exchange membranes. Direct measurement of the polarization at the coated membranes indicated that this phenomenon decreased as a result of the coating. Mathematical modeling showed that the strong antipolarizing effect of the thin charged coating could be explained in terms of the change of ion distribution caused by the coating. A fixed charge density much smaller than that within the membrane bulk sufficed to increase the limiting current to the value obtained with homogeneous membranes, and electroneutrality kept a finite concentration of mobile ions near the entrance to the membrane even at very high current density. Current-voltage curves predicted from the model showed a decrease of polarization for the coated membranes, as was indeed obtained experimentally. Fouling was studied by the addition of dodecylbenzyl sulfonate (DBS) as a model foulant to the feed solution. The modified membranes showed a smaller fouling tendency than commercial anion-exchange membranes.