Mechanistic insights into monovalent cation selectivity in modified ion selective membranes: A model-based and experimental study

Abstract

Surface modification techniques that introduce oppositely charged thin layers onto ion exchange membranes have proven highly effective for separating monovalent and divalent ions in chemical and biochemical processes. Although monovalent cation selective membranes (MCSM) have been widely used in electrodialysis, the underlying separation mechanisms and ion transport behavior under varying modification layer charge densities and applied voltages remain insufficiently understood. This study establishes a nanochannel-based MCSM model grounded in the physical nature of porous media, providing clear insights into ion transport behavior and the underlying mechanism of ion selectivity. The model is further validated through electrodialysis experiments. The results show that the nanochannel model captures the influence of membrane pore structure on electric field distribution and ion transport behavior, and reasonably reproduces the experimentally observed trends in selectivity. Near the limiting current density (LC regime), ion transport is primarily governed by diffusion within the modification layer, where the electrostatic barrier more effectively suppresses the migration of divalent Mg²⁺, leading to enhanced Li/Mg selectivity. More importantly, unlike the uniform model, which predicts a continuous linear increase in selectivity with increasing charge concentration in the modification layer, the nanochannel model reveals a two-stage trend characterized by an initial rise followed by saturation. This nonlinear behavior arises from electrostatic potential saturation near the channel walls caused by counterion accumulation, which restricts further potential propagation and ultimately limits ion selectivity enhancement. These results significantly contribute to our understanding of the mechanisms underpinning the selective transport of monovalent cations and offer valuable guidance for their practical application in selective electrodialysis.