Effect of anion exchange membrane capacity loss on pH and electric conductivity of saline solution during neutralization dialysis

Abstract Neutralization dialysis (ND) is a relatively simple and easy-to-use method of water desalination, which does not require large investments, like reverse osmosis or electrodialysis. One of the problems encountered when using this method is a relatively rapid deterioration of the effective exchange capacity of the anion-exchange membrane caused by its contact with the alkali compartment. We report the results of experimental study of this method using a fresh Neosepta® AMX membrane (Astom, Japan) and the same membrane after its ND operation for about 400 h. As well, we describe a new non-steady state mathematical model of the ND process in batch mode. The model is based on the Nernst-Planck equations and considers a cell containing an acid, a saline and an alkali compartments separated by a cation- and an anion-exchange membrane. The incomplete dissociation degree of secondary and tertiary amino groups present in the used AMX membrane is taken into account as a function of the local pH value. It is accepted that the pristine AMX contains only the quaternary amino groups, positively charged at all pH. When assuming that the fraction of the quaternary groups in the used AMX is 35% of the initial value, we find a very good agreement between calculated and experimental time-dependences of the conductivity and pH of the feed saline solution. Simulations show that the fraction of weakly basic functional groups in anion-exchange membrane has a strong effect on the course of ND process. Deprotonation of weakly basic functional groups in the AMX causes a decrease in the counterion flux through this membrane and an increase in the co-ion leakage, which reduces the rate of desalination and causes acidification of the saline solution. The time needed to decrease the conductivity of the initial saline solution by 70% in the case of the used AMX membrane is twice as long as that of the fresh membrane. The pH of the product solution is 7 in the case of the fresh AMX and only 3.5 in the case of the used one.