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

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.