In Situ Surface Modification of Thin-Film Composite Polyamide Membrane with Zwitterions for Enhanced Chlorine Resistance and Transport Properties

High-performance chlorine-resistant thin-film composite (TFC) membranes with zwitterions were fabricated by in situ surface modification of polyamide with 2,6-diaminopyridine and the subsequential quaternization with 3-bromopropionic. The successful modification of the TFC polyamide surface with zwitterions was confirmed by various characterizations including surface chemistry, surface hydrophilicity, and surface charge. The transport performance of the membrane was measured in both of the cross-flow reverse osmosis (RO) and forward osmosis processes, and the results showed that the modified TFC membrane improved both of its water permeability and perm-selectivity with the increased A and A/B ratios upon modification with zwitterions. The chlorination challenging experiments were performed to demonstrate that the modified membrane enhanced its chlorine resistance without affecting its salt rejection upon 16 000 ppm·h chlorination exposure. A chlorination mechanism study illustrated that the modified membrane with zwitterions could prevent the Orton rearrangement of the benzene ring of the polyamide layer. Importantly and excitingly, the optimal chlorinated TFC membrane with zwitterions achieved a very high water flux of 72.15 ± 2.55 LMH with 99.67 ± 0.09% of salt rejection in the cross-flow RO process under 15 bar.