Polyamide layer sulfonation of a nanofiltration membrane to enhance perm‐selectivity via regulation of pore size and surface charge

Abstract In this study, sulfonated nanofiltration membranes were synthesized via interfacial polymerization (IP) of aqueous‐phase piperazine (PIP), 3‐amino‐1‐propanesulfonic acid (APA), and organic‐phase trimesoyl chloride (TMC) monomers on porous substrates. Membranes with different APA contents were thoroughly characterized to determine how APA affected the physicochemical characteristics and perm‐selectivity of the membranes. Membrane characterization confirmed the incorporation of APA into the generated polyamide (PA) layer. The optimal membrane performance was obtained at a 0.8 w/v% aqueous‐phase APA content, where the enhanced membrane molecular weight cutoff (MWCO) and surface hydrophilicity resulted in a pure water permeability of 11.9 L m −2 h −1 bar −1 , approximately 13.1% higher than that of the blank membrane. Enhanced electrostatic repulsion of the APA‐modified membrane on Na 2 SO 4 resulted in up to a 96.2% Na 2 SO 4 rejection rate. The 24.3% NaCl rejection rate resulted in NaCl/Na 2 SO 4 selectivity for the mixed‐salt solution of the optimized membrane of 19.9, which was a significant increase over that of the blank. The embedding of APA did not affect the long‐term stability of the NF membrane. The combined effects of the hydrophilicity, charge, and roughness of the membrane surface slightly enhanced the antifouling ability of the APA‐modified membrane over that of the blank. This study provides a facile strategy for the fabrication of NF membranes with high perm‐selectivity.