High-performance and acid-resistant nanofiltration membranes prepared by solvent activation on polyamide reverse osmosis membranes

We present a facile method for fabricating polyamide (PA) nanofiltration (NF) membranes exhibiting remarkable separation performance and high acid stability via solvent activation on PA reverse osmosis (RO) membranes with strong polar aprotic solvents (dimethyl sulfoxide (DMSO), dimethylformamide and N-methyl-2-pyrrolidone). The solvents with strong solvency power for PA greatly swelled and deformed the dense RO PA layer, making the PA network more permeable and looser, which significantly improved the water permeance of the RO membrane while maintaining its high rejection to divalent salts. Consequently, the solvent-activated RO membranes exhibited remarkable NF-grade separation performance, exceeding that of the commercial NF membrane (NF270, Dow Filmtec.). Particularly, the DMSO-activated membrane showed ∼30% higher water permeance, higher salt rejection and ∼6.8 times higher monovalent/divalent ion selectivity than NF270. This was attributed to the strongest solvency power of DMSO among the solvents used. Moreover, the solvent-activated membrane exhibited the superior acid stability to NF270 owing to the higher acid resistance of its fully-aromatic PA chemistry than that of the semi-aromatic PA. Our proposed method is a simple, effective and commercially viable strategy for fabricating high-performance and acid-resistant NF membranes that can expand the application spectrum of NF technology.