Contribution of the transmembrane electric potential to the set voltage in a single-anion exchange membrane electrodialysis-cell and the role of solution conditions

Abstract

The transmembrane electric potential (TMEP) drives ion transport via electromigration across ion exchange membranes (IEMs) during electrodialysis (ED). For ED operation, a voltage is either measured, or set to remain constant, between two electrodes on either side of a single IEM (simplified ED-cell) or a stack of IEMs (bench-scale ED system). The set/measured voltage has been used in the literature to approximate the TMEP in simplified ED-cells assuming that other elements between the electrodes (e.g., solutions, boundary layers, concentration gradient) contribute negligibly to the measured/set voltage; however, there is no experimental evidence in the literature comprehensively evaluating the accuracy of this assumption. Accordingly, our objectives were to (i) determine the contribution of the TMEP to the set voltage in a simplified ED-cell under operationally relevant solution conditions, and (ii) understand the role of solution conditions on the potential drop contributions from each element between reference electrodes. We studied sodium salts of eight anions (inorganic and organic) and three desalination levels at a set voltage of 0.4 V. Results showed that the set voltage was not a good approximation of the TMEP for any solution condition which was primarily attributed to the substantial potential drop from the solutions. The TMEP also varied substantially depending on solute identity and concentration. Additionally, the TMEP decreased substantially as the desalination level increased from 0% to 75%, which was attributed to the increase in potential drops due to the boundary layers and open circuit voltage. The reported findings provide important insights into the effective driving force of ion transport via electromigration when operating ED at a set voltage.