How do proton-transfer reactions affect current-voltage characteristics of anion-exchange membranes in salt solutions of a polybasic acid? Modeling and experiment

Insufficient understanding of transport mechanism of polybasic acid species (phosphoric, citric, malic, etc.) through anion-exchange membranes (AEMs) hinders the widespread use of electrodialysis for processing salt solutions of such acids, e.g. the recovery of phosphates from mixed solutions. In this paper, we present experimental current-voltage characteristics (CVC) and partial fluxes of H2PO4− and HPO42− across an AEM. These data are simulated using a mathematical model based on the Nernst-Planck-Poisson equations coupled with the kinetic equations for chemical reactions. The model describes the nonstationary transport of phosphate acid species through an AEM and adjacent solution diffusion boundary layers. It is shown that the proton-transfer reactions determine the occurrence of two limiting currents. The electric current sweep rate and the values of dissociation rate constants of acid anions largely affect the values of these limiting currents and generally the shape of the CVC. It is found that due to chemical reactions involving polybasic anions, the formation of their concentration profiles in the membrane occurs much slower than in the case of monobasic salts. Numerical simulation shows that a quasi-stationary CVC is possible only when the current sweep rate is such that measurements are carried out for at least 10 h.