Rationalisation of the relationship between proton leakage and water flux through anion exchange membranes

Electromembrane concentration of nitric acid using anion exchange membranes has been studied in a laboratory pilot rig. The main factor limiting acid concentration has previously been reported to be proton leakage but in the present work water flux is also shown to be significant. Differential current efficiencies and mean water transport numbers are determined for each system via methematical manipulation of batch data. Water flux appears to be closely linked to proton leakage as an improvement in one of these leads to a deterioration of the other. Previous proton leakage theories have been developed further and a proton leakage and water flux model, based on sorbed species and their interactions within the membrane, has been postulated. It is suggested that the absolute amount of sorbed acid is more significant than the sorbed acid:water ratio. The model is validated by numerous cited publications, in addition to the data presented herein, to explain the performance variation of a given membrane when subjected to different operating conditions. Future work should consider water flux along with proton leakage as figures of merit for process evaluation, and investigate the applicability of the model to the comparative performance of different membranes when subjected to identical operating conditions.