Beyond the Nernst-Planck model: Ion transport and species-specific interactions in ion-exchange membranes in high-salinity environments

Abstract

Ionic transport in ion-exchange membranes (IEMs) is governed by the interaction forces among the various components of the solution-equilibrated membrane, including mobile ions, water molecules and the fixed charges of the polymeric matrix. In this work, we investigated ionic transport in IEMs equilibrated with highly concentrated saline solutions within the framework of the Maxwell–Stefan transport theory to quantitatively evaluate the importance of species-specific frictional interactions. In this work, a comprehensive and thorough physical-mathematical model is proposed to quantify fundamental ion-membrane specific interactions from an independent set of experiments conducted on a commercial CEM equilibrated with NaCl and KCl solutions at concentrations up to 5 mol L−1. The retrieved parameters were subsequently employed to analyse the specific contribution of the binary interactions on the mass and charge transport resistances. The results of this work clearly show that ion-membrane and ion-water interactions largely determine the salt permeability and ionic conductivity of IEMs, with ion-membrane interactions being progressively weakened at increasing salt concen- tration possibly due to electrostatic screening. Ultimately, the proposed theoretical framework provides a fundamental insight into the physics of ionic transport in membrane and paves the way for the development of physically grounded models able to accurately describe the local interactions among the mobile species and the membrane structure. Within the framework of the proposed model, such understanding will allow prediction of the behaviour of IEMs in terms of macroscopic parameters, such as ion selectivity and ionic conductivity, in highly concentrated systems.