Gradient crosslinking optimization for the selective layer to prepare polyvinyl alcohol (PVA) nanofiltration (NF) membrane: The enhanced filtration performance and potential rejection for EDCs

Polyvinyl alcohol (PVA) has been employed and crosslinked as the selective layer of NF membranes with enhanced hydrophilicity. However, the influences of different crosslinking methods and agents on the reaction mechanism and membrane performance still need to be comprehensively studied. In this study, NF membranes were prepared by coating crosslinking (CC-membrane) and gradient crosslinking (GC-membrane) methods. Compared with CC-membrane, the GC-membrane possessed a 65% higher flux with Na2SO4 rejection around 90%, under the optimal preparation conditions with PVA content of 0.10%, curing at 80 °C for 10 min. Crosslinking agents including 1,3,5-benzenetricarboxylic acid (TMA), 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and pyromellitic dianhydride (PMDA) with different spatial structure were used to optimize the GC-membrane. As confirmed by SEM, XPS, FTIR and contact angle tests, the 6FDA-GC membrane showed a thinner, smoother and more hydrophilic selective layer, and kept an excellent balance between the water flux (88.6 L/m2h) and Na2SO4 rejection (88.7%). This is because the 6FDA with a large molecule and flexible spatial structure could form a more integrated crosslinking structure compared to PMDA. Meanwhile, the 6FDA-GC membrane showed rejections of over 80% towards the endocrine-disrupting compound 17α-Ethinylestradiol and exhibited stable antifouling ability and durability. Overall, this work studied systematically the influences of various crosslinking agents on the GC-membrane, and verified the potential of hydrophilic PVA membranes for nanofiltration.