Ion-exchange membranes (IEMs), which are highly charged polymeric membranes, are widely used in water and energy applications, such as electrified water treatment, energy harvesting from salinity gradient, and electrolytic energy storage. Concentration-induced tradeoff relationships between conductivity and permselectivity constrain the performance of IEM-based processes. Though classic Donnan-equilibrium-based theory offers a good qualitative or semi-quantitative explanation on the origin of the phenomena, the tradeoff behaviors of IEMs in different electrolyte solutions and the influence of electrolyte on the tradeoff relationships have not been fully investigated. The knowledge gap hinders the development of IEM separation for selective pollutant removal or material recovery, when IEMs are applied to treat mixed electrolyte solutions. In this study, we investigated the impacts of electrolyte type on the concentration-induced conductivity-permselectivity tradeoff of commercial cation and anion exchange membranes. The effective ionic conductivity and apparent permselectivity of IEMs were characterized in different electrolyte solutions and concentration levels. The increasing external concentration enhanced the effective conductivity, which was mainly determined by the counterion identity and was insensitive to the co-ion identity. The apparent permselectivity declined with external concentration, with higher valency counterions and lower valency co-ions yielding lower permselectivity. By combining the conductivity and permselectivity data, different tradeoff behaviors caused by external concentration were observed in different electrolyte solutions. Lastly, conductivity and permselectivity models were developed to facilitate the understanding of the primary factors that affect the tradeoff relationships. Qualitative consistency between experimental and models was observed, indicating that the theoretical framework underpinning the model can explain the transport phenomena. The study advances the understanding of the IEM conductivity-permselectivity relationship in different electrolytes and provides guidance for improving IEM performance in complex water chemistries.

Influence of electrolyte on concentration-induced conductivity-permselectivity tradeoff of ion-exchange membranes

ACS Fall 2021

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