Surface modification of nanofiltration membranes with zwitterions to enhance antifouling properties during brackish water treatment: A new concept of a “buffer layer”

Nanofiltration (NF)-based brackish water desalination has been labeled an emerging avenue for alleviating water scarcity, however, membrane fouling still restricts its broad application. Herein, zwitterion-modified membranes were prepared with polydopamine (PDA) and sulfobetaine methacrylate (SBMA) monomers via surface-initiated atom-transfer radical-polymerization (SI-ATRP) and named TFC-PDA-PSBMA. Compared with the control membrane, the TFC-PDA-PSBMA possessed enhanced hydrophilicity, lower negative charge and smaller pore size. Superior salt retention of Na2SO4 and NaCl of up to 99.1% and 80.7% was obtained for TFC-PDA-PSBMA with only a 10% decrease in permeance. Furthermore, TFC-PDA-PSBMA exhibited an outstanding flux recovery of 92.1% during humic acid (HA) filtration. The mechanism driving the improved antifouling properties was analyzed from the aspects of the modified layer, free interfacial energy and hydration layer. TFC-PDA-PSBMA also exhibited superior antibiofouling with reducing 86.1% of E. coli colonies adhered on membrane surface. The SBMA layer and hydration layer acted together as a buffer layer with robust repulsion of foulants. In addition, the TFC-PDA-PSBMA membrane was applied in authentic brackish water desalination and exhibited superior selectivity, desalination and antifouling performance, which matched the simulated solution results well. A new concept of a “buffer layer” was proposed in this work, which may provide insight into antifouling membrane design during brackish water treatment.