Two mechanisms of H+/OH− ion generation in anion-exchange membrane systems with polybasic acid salt solutions.

Electrodialysis are increasingly used in hybrid membrane technologies for the recovery and separation of weak polybasic acids from municipal and industrial wastewater and other solutions. It is known that the rate of generation of H+ and OH− ions at ion-exchange membrane boundaries during the treatment of such solutions is greater than that in the case of strong electrolytes, such as NaCl. One of the mechanisms of this generation, water splitting involving catalytic participation of functional groups, is relatively well understood. However, the second possible mechanism, where H+ ions are generated during acid dissociation at the depleted solution/anion-exchange membrane interface, is not sufficiently clear. Current-voltage characteristics (CVCs) of a Neosepta AMX anion-exchange membrane in 0.02 M NaH 2 PO 4 , NaH 2 Cit, KHT, and NaCl solutions are recorded. The effective transport number of H+ ion in the depleted boundary solution is measured to quantify the rate of the H+ ion generation. While the "water splitting" mechanism takes place in any kind of electrolytes, the "acid dissociation" mechanism is characteristic only for weak polybasic acids and other ampholytes, the electric charge of which depends on the pH. With that, the first mechanism occurs only at overlimiting currents, and the second occurs both in the underlimiting and overlimiting current modes. The dissociation of acid species at an AEM interface is caused by Donnan exclusion of protons as co-ions from the membrane. The rate of this reaction increases in the order KHT < NaH 2 PO 4 <NaH 2 Cit. This order is determined by the values of equilibrium acid dissociation constants pK ai for the first and second ionization steps as well as by the dissociation rate constants for acid anions. The contribution of each of the H+/OH− ion generation mechanisms depends on the current density, more precisely, on the interval between the 1st, 2nd or 3rd limiting currents, recorded on the I–V characteristic. At relatively low currents, the contribution of "water splitting" in the studied weakly acidic systems is insignificant. At i > 2.6 i lim Lev, there is a competition between both mechanisms of H+ ion generation. The obtained results give new insight into the understanding of the kinetics of H+/OH− ion generation and the impact of this process on the efficiency of electrodialysis of solutions containing polybasic acid species. [Display omitted] • Two mechanisms of H+/OH− ion generation occur in electrodialysis of weak acid salts. • "Water splitting" mechanism (1) is similar to that for the case of strong electrolytes. • "Acid dissociation" mechanism (2) occurs only in the case of polybasic acid salts. • Mechanism (2) is due to the Donnan exclusion of protons as co-ions from AEM. • Mechanism (1) requires a threshold potential difference, mechanism (2) is thresholdless; it dominates up to 3 i lim (the case of NaH2Cit). [ABSTRACT FROM AUTHOR]