Ion-exchange membranes (IEMs) are charged polymeric thin-films widely applied in electrically-driven environmental technologies, such as electrodialysis and membrane capacitive deionization. Traditionally, the charged membranes are used to select the permeation of oppositely charged counterions and block like-charged co-ions. Recently, separation beyond charge polarity, specifically, ion-selective separation, has attracted increasing research interest. The counterion-specific selectivity can unlock new applications, e.g., efficient hazard removal and resource recovery in water and wastewater treatment. However, gaps in the current understanding of the chemistry-structure-property relationships in membrane mobility exist, frustrating efforts on informed development of ion-specific selectivity. In this study, we integrate experimental and theoretical approaches to investigate the migration mobility of different counterions in IEMs. The proposed transport model establishes a quantitative structure-property relationship, based on two governing effects: the spatial effect of swollen water and the electrostatic interaction between mobile ions and fixed charges. The counterion diffusivities of commercial IEMs in different electrolytes were experimentally characterized and show good agreement (high regression R<sup>2</sup>) with the proposed model. The impact of counterion-dependent water swelling is assessed for ion-specific separation. The frictional effect of electrostatic interaction is quantitatively linked to the fixed charged density and effective dielectric constant. The analysis also reveals a weaker electrostatic effect in anion exchange membranes compared with cation exchange membranes, due to the molecular features. This study offers new insights on the underlying mechanisms governing ion-specific selectivity and will inform the rational application of IEMs for high-value ion-selective separation.

Elucidating counterion mobility in ion-exchange membranes for electrically-driven ion-selective separations: Spatial effect and valency-dependent electrostatic interaction

ACS Fall 2021

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