Heterocyclic compound-anchored nanofiltration membrane with engineered diffusion energy barrier for Cl−/SO42− separation

Polyamide thin-film composite (PA TFC) membranes synthesized via conventional interfacial polymerization (IP) often underperform in Cl⁻/SO₄²⁻ separation due to their subpar spatial architectures. This study presents a novel strategy to fabricate advanced anion-selective nanofiltration (NF) membranes by functionalizing PA selective layer pore walls with nitrogen/oxygen-containing heterocyclic compounds (crown ether and cyclen) to tailor intrapore chemical microenvironments. Both experimental validations and molecular simulation results demonstrate that precise regulation of chemical interactions between the membrane matrix and permeation species (NaCl/Na₂SO₄) enables controlled tuning of diffusion energy barriers for anions traversing the membrane. Beyond size exclusion and charge repulsion mechanisms, we employ transition state theory to elucidate the fundamental Cl⁻/SO₄²⁻ separation mechanism. The distinct roles of these compounds in modulating IP reaction kinetics and their impacts on membrane microstructure are systematically investigated. Integration of heterocycles into the PA network yields more hydrophilic, less densely packed membranes. Resultant cyclen-modified membranes achieve exceptional separation performance (water flux of 26.65 L m⁻² h⁻¹ bar⁻¹, Cl⁻/SO₄²⁻ selectivity of 223), while 15-crown-5-modified membranes exhibit remarkable Cl⁻/SO₄²⁻ selectivity (690, Na₂SO₄ rejection of 99.71 %) with maintained water flux (17.50 L m⁻² h⁻¹ bar⁻¹). These modified membranes also exhibit robust long-term stability and excellent fouling resistance. This work establishes a novel pathway for developing high-performance Cl⁻/SO₄²⁻ separation membranes through engineered control of ion diffusion energy barrier.