posted on 2023-11-15, 12:34authored bySin Sieng
Gianna Sim, Qiuyue Wang, Sandra E. Kentish, George Q. Chen
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.