Improving permselectivity of the polyamide reverse osmosis membrane by a controlled-release sulfate radical modification

The sulfate radical (SO4.-) is an effective reactive oxidative species that can improve the permselectivity of polyamide membranes. However, the coexistence of hydroxyl radicals, which have been previously reported in a thermal-activated persulfate (PDS) system, have detrimental effects on membrane integrity and salt rejection. In this study, we adopted a facile citric acid chelated Fe(II)/PDS modification method to generate a controlledrelease SO4.- oxidation condition at room temperature, which enhanced membrane performance. The rapid oxidation by Fe(II)/PDS significantly increased membrane water permeability. However, ferric oxyhydroxides were formed and deposited on the membrane surface, reducing surface hydrophilicity and negative charge. Citric acid was then introduced into the modification as a chelating agent to control the release of Fe(II) and improve the modification efficiency. The modified membrane showed further increased water permeability (from 26.0 to 53.0 L m(-2)h(-1)) and maintained a salt rejection of 98.4%. The optimized membrane enhanced surface hydrophilicity and negative charge, slightly improving anti-fouling ability. The membrane chemical composition characterization using ATR-FTIR and XPS indicated the formation of excess oxygenous groups. Furthermore, SO4. oxidized the chlorine-sensitive sites, endowing the modified membrane with significantly improved chlorine resistance. The results from this study provide a facile SO4.--based modification method for comprehensively improving permselectivity, surface properties, and anti-fouling/anti-chlorine resistances of the polyamide membranes.