Ion transport in anion exchange membranes in presence of multivalent anions like sulfate or phosphate

There is a demand for industrial processes for recycling salts, e.g. sodium sulfate, by splitting them electrochemically into their corresponding acids and bases. But, until now, the permselectivities of ion exchange membranes, especially of anion exchange membranes (AEMs), have been insufficient and have not proved suitable for cost-effective applications. In experiments, using the anion exchange membrane Neosepta AHA(R) (Tokuyama Corp.) for sodium sulfate and phosphate electrolysis, no effective H+ ion leakage, i.e. 100% permselectivity, has been observed for concentrations of up to about 7-8 wt.% acid. A transport model is presented that explains all of the results, and is based on two properties of multivalent anions. On the one hand, multiply charged anions can interact with the fixed ions of the membrane, reducing its ability to stop H+ ion leakage out of the anolyte. On the other hand, they can combine with H+ ions and then transport them in the desired direction into the anolyte. These opposing effects influence the overall permselectivity of anion exchange membranes. (C) 2002 Elsevier Science B.V. All rights reserved.