Optimisation strategies for the preparation of bipolar membranes with reduced salt ion leakage in acid-base electrodialysis

The salt ion fluxes across commercial bipolar membranes (BPMs) result in th e salt contamination of the produced acids or bases especially at increased product concentrations. Often, bipolar membrane electrodialysis can only b e applied when these fluxes are reduced. Here, a model is presented to pred ict the salt impurities using the limiting current density measured for a s ingle bipolar membrane. The model is extended to relate the limiting curren t density to the experimentally determined properties of the separate membr ane layers. A direct dependence has been found for the salt ion fluxes acro ss the bipolar membrane on the square of the solution concentration and the effective salt diffusion coefficient. Further, the salt ion transport is i nversely dependent on the fixed charge density and the thickness of the lay ers. The latter is not trivial - the thickness in general does not play a r ole in the selectivity of separate anion or cation exchange membranes. The dependence of the salt ion transport on the membrane layer properties has b een verified experimentally by characterising membranes prepared from comme rcially available anion exchange membranes and tailor-made cation-permeable layers. The presented mode I has proven to be both, simple and accurate en ough to guide bipolar membrane development towards increased selectivity. ( C) 2001 Elsevier Science B.V. All rights reserved.