PPY-PEI Engineered Surface With Quaternized Crosslinking for Enhanced K+/Mg2+ Selectivity in Cation Exchange Membranes

The monovalent cation exchange membrane (MCEM) plays a crucial role in strategic resource recovery fields such as lithium/magnesium separation and seawater softening. This study proposes a two-stage modification strategy of “nanoparticle deposition – quaternization cross-linking”: first, deposit the Polypyrrole -Polyethyleneimine (PPY-PEI) composite nanoparticles in situ on the commercial Cation Exchange Membrane (CEM) surface, and then undergo quaternization cross-linking to prepare a high-performance monovalent cation exchange membrane (MCEM). The rigid structure of Polypyrrole -Polyethyleneimine (PPY-PEI) nanoparticles completely avoids the traditional problem of monomer penetration, and the rate of membrane pore blockage is significantly reduced. Electrified nanoparticles form gradient nanochannels to achieve dual mechanism synergy: The surface positive charge layer repels and blocks Mg 2+ through the Donnan effect; 2–5 nm mesoporous channels facilitate K + transport through size sieving. Subsequent quaternization crosslinking strengthened the modified layer and increased the positive membrane charge density, leading to further improvement in perm-selectivity. The results show that the modified membrane achieves a breakthrough performance in the separation of K + /Mg 2+ : the selectivity reaches 6.9; the K + flux remains at 0.14/(m 2 ·s). Molecular dynamic simulation of the ion diffusion behavior confirmed that the presence of Polypyrrole -Polyethyleneimine (PPY-PEI) can accelerate the diffusion rate of K + while limiting the transport of Mg 2+ , providing a theoretical basis for the performance prediction of selective ion exchange membranes.