Amine-Modified ZIF Composite Membranes: Regulated Nanochannel Interactions for Enhanced Cation Transport and Precise Separation

Electromembrane water treatment technologies are attracting attention for their energy efficiency and precise separation of counterions. However, ion-exchange membranes exhibit low ionic conductance and selectivity for ions with similar charges. In this study, we developed a novel ZIF-8 composite membrane with amine-modified nanochannels through an in situ PEI-assisted seeding and secondary growth method. An integral and uniform selective layer was formed, and the amine-modified nanochannels induced differential transport of Li⁺, Na⁺, K⁺, and Mg²⁺ via the dehydration-hydration process. The composite membrane possessed a lower energy barrier for Na⁺ transport (E_a = 13 kJ mol^-1) compared to Mg²⁺ (E_a = 17 kJ mol^-1), showing a Na⁺ flux of 3.7 × 10^-8 mol·cm^-2·s^-1 and a Na⁺/Mg²⁺ permselectivity of 52 (∼60 times higher than the commercial membrane). The physicochemical and electrochemical properties of the composite membranes were systematically characterized, revealing the significant role of the Mg²⁺ layer in increasing Mg²⁺ repulsion and facilitating Na⁺ diffusion. Besides, DFT simulation and interaction energy calculation elucidated that a moderate binding energy and compensation effect between ions and nanochannels, which can be precisely regulated by PEI incorporation, are crucial for the favorable passage of Na⁺ while maintaining high Mg²⁺ rejection. The membrane also demonstrated performance stability during a 5-day test and maintained high selectivity across varying salinity and pH conditions. This work advances the development of efficient cation separation membranes for sustainable desalination and resource recovery.