Anionic covalent organic framework as an interlayer to fabricate negatively charged polyamide composite nanofiltration membrane featuring ions sieving

Ions sieving is significant but challengeable for seawater desalination, industrial water treatment and salts recovery. Traditional nanofiltration (NF) membranes suffer from undesirable ion selectivity and permeance due to uncontrolled aromatic polyamide (PA) nanofilm. Herein, we first reported an anionic covalent organic framework (TpPa-SO3Na) with abundant sulfonic acid groups as a mediating interlayer to fabricate PA composite membrane. The uniform and negatively charged TpPa-SO3Na spherical nanoparticles were in situ soldering on nylon substrate by liquid–liquid interfacial Schiff reaction. The TpPa-SO3Na interlayer assisted in creating a defect-free PA nanofilm through mediating the diffusion–reaction process of amine monomers. According to enhanced Donnan effect, the composite polyamide nanofiltration membrane showed an outstanding salt rejection ratio (Na2SO4, 99.6%), high single salt selectivity (NaCl/Na2SO4, 178.5) and mono/divalent anion selectivity (Cl−/SO42−, 312.6), superior to most reported NF membranes. This work shed light on strategies to synthesize emerging covalent organic framework materials and to effectively construct NF composite membranes with excellent solute retention and ions sieving capabilities.