Fabrication of antifouling thin-film composite nanofiltration membrane via surface grafting of polyethyleneimine followed by zwitterionic modification

In this work, antifouling thin-film composite (TFC) nanofiltration (NF) membrane having zwitterionic PEI moieties was fabricated through the layer-by-layer interfacial polymerization (LbL-IP) via grafting hyperbranched polyethyleneimine (PEI) onto the polyamide surface, followed by in-situ N-methylation of PEI and then quaternization using 3-bromopropionic acid to form zwitterionic PEI. The successful surface modifications of TFC NF membrane with PEI and zwitterionic PEI were confirmed by XPS and ATR-FTIR results. Benefitting from the strong hydrophilicity and hyperbranched steric structure, the zwitterionic PEI-grafted TFC membrane (NF-ZPEI) exhibited a stronger hydration capacity and a thicker hydration layer than the PEI-grafted TFC membrane (NF-PEI) and the control sample (NF-PA). Meantime, the water permeability of NF-ZPEI membrane was enhanced by 11.6% higher than the control sample. The NF-ZPEI membrane also exhibited the improved ion selectivity of having a decreased rejection towards NaCl and maintaining a high rejection towards divalent ions. Dynamic fouling experiments were performed to demonstrate the impact of surface hydrophilicity and surface charge on the membrane fouling performances. Results illustrate that the zwitterionic PEI grafted on the TFC NF membrane mitigated the electrostatic attraction between the membrane surface and protein foulants, and exhibited the improved antifouling properties of having a low flux decline and a high flux recovery. Our work provides some insights on the zwitterionic surface modification of the TFC NF membrane with both the enhanced water permeability and the antifouling properties towards charged foulants.