Surface modification of an anion exchange membrane to improve the selectivity for monovalent anions in electrodialysis - experimental verification of theoretical predictions

Electrodialysis (ED) is a useful method to make drinking water from brackish water. In ED, high selectivity for monovalent anions is required to effectively remove harmful ions like NO3- and F-, and to prevent the precipitation of CaSO4 scale. In our previous paper, we theoretically discussed the effect of several parameters of the system (including the anion exchange membrane (AEM)) on the selectivity for monovalent anions to identify and understand the parameters that affect the selectivity for monovalent anions. In this paper, we modified an REM with polydopamine to make a negatively charged layer on the REM surface. The permselectivities for NO3-, F-, and SO42- (relative to Cl-) were measured to experimentally verify the theoretical predictions. The selectivity for monovalent (relative to divalent) anions was improved by the surface modification of the REM with a negatively charged layer, as theoretically expected. The permselectivity between monovalent anions was not affected by the surface modification. In addition, we analyzed the experimental results with our model and obtained the charge density of the negatively charged layer. The theory explains the experimental results well and the validity of our theoretical model was confirmed. (C) 2015 Elsevier B.V. All rights reserved.