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Polyamide TFC membranes are widely applied in membrane-based water treatment but generally suffer various fouling problems. In this work, the layer-by-layer assembly of phytic acid (PA) and metal ions (M) is constructed on the surface TFC membrane for the first time, to improve the bio/organic fouling resistances and separation performance of TFC membranes simultaneously. The PA molecule with six phosphonic acid groups of strong chelation ability acts as the organic ligand, and the metal ion acts as the inorganic cross-linker, inducing the assembly of hydrophilic and antibacterial PA-M (Ag or Cu) complexes on the TFC membrane surface. Various characterizations including FTIR, XPS, SEM, AFM, and EDX are employed to confirm the successful and uniform modification of PA-M. FO performance of the PA-M modified TFC membranes, i.e., TFC_PA-Ag and TFC_PA-Cu, is optimized by varying PA concentration and assembly cycles, where the water flux can be improved by 57% and 68%, respectively, without compromising the membrane selectivity. Additionally, the PA-M modification improves the biofouling and organic fouling resistances of the TFC membrane remarkably, owing to the enhanced antibacterial ability and hydrophilicity. The modified TFC membranes are also proven to show the excellent stability by the quantitative release test.

Biofouling can be overcome with zwitterion grafts and antimicrobial, metallic nanoparticles. However, the mechanism underpinning this effective approach remains unclear. To elucidate the role of each component in this system while maximizing membrane antifouling and antimicrobial properties, here we performed a comparative study to investigate the impact of zwitterion type and their interactions with Ag of various states. Two different zwitterions (SO 3 − -based and COO − -based) were employed to modify polyamide (PA) thin film composite (TFC) membranes, and the metallized and mineralized membranes were developed via in situ formation of silver (Ag) nanoparticles and deposition of silver chloride (AgCl) particles on the zwitterion-modified TFC membranes. The presence of zwitterions was key to enhance Ag content, resulting in significantly improved antimicrobial and antifouling properties without compromising the nanofiltration separation performance. COO −based zwitterions were found more favorable toward Ag metallization and mineralization compared to SO 3 − -based zwitterions. The underlying mechanisms underpinning this discovery were further revealed using density functional theory (DFT) to reveal Gibbs free energy of the binding between zwitterions and Ag + ions. This fundamental knowledge is crucial for designing next-generation antibiofouling strategies.

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