Modeling the electric transport of HCl and H3PO4 mixture through anion-exchange membranes

The electric transport of the mixture of hydrochloric and phosphoric acids through strong base (Neosepta ACM) and weak base (Selemion AAV) anion-exchange membranes was investigated. The instantaneous efficiency of HCl removal from the cathode solution, CECl, with and without H3PO4 was determined. It was found that CECl was 0.8-0.9 if the number of moles of elementary charge passed through the system, nF, did not exceed ca. 80% of the initial number of HCl moles in the cathode solution, nCl,ca,0. The retention efficiency of H3PO4 in that range was close to one. The transport of acid mixtures was satisfactorily described by a model based on the extended Nernst-Planck and Donnan equations for nF not exceeding nCl,ca,0. Among the tested model parameters, most important were: concentration of fixed charges, the porosity-tortuosity coefficient, and the partition coefficient of an undissociated form of H3PO4. For the both membranes, the obtained optimal values of fixed charge concentration, , were up to 40% lower than the literature values of obtained from the equilibrium measurements. Regarding the H3PO4 equilibria, it was sufficient to consider H3PO4 as a monoprotic acid.