Equilibrium Sorption of Monovalent and Divalent Organic Ions in Anion Exchange Membranes

Ion transport in ion exchange membranes (IEMs) involving weak electrolytes such as organic acid species can exhibit considerably different behavior compared to the transport of inorganic species, partly due to their pH-dependent dissociation behavior. In this work, the concentration-dependence of the equilibrium sorption of sodium co-ions and organic counterions in a strongly basic anion exchange membrane (AEM) is systematically studied for organic acids with one and two carboxylic acid groups, namely, lactic acid and tartaric acid, with the AEMs being equilibrated in organic salt solutions containing predominantly the highest valency of the organic ions. It is discovered that up to ∼28% of total lactic acid within the membrane phase exists as neutral lactic acid molecules and up to ∼30% of total tartaric acid exists as monovalent bitartrate ions, over the range of external salt concentration investigated (0.1–1 equiv/L). This indicates a shift in the acid dissociation equilibria from the external solution to the membrane phase, which appears to be more significant at higher salt concentrations. Our quantification of organic ion speciation in AEMs will contribute to future fundamental studies of organic ion transport in IEMs, enabling a rational design of polymer chemistry and development of electromembrane processes to maximize separation performance.