High-performance nanofiltration membranes with a polyamide-polyester composite layer and a polydopamine surface layer for desalination and dye pollutant removal

Due to water shortages, people have to use wastewater, seawater and brackish groundwater. People prefer to use membrane-based technologies to produce high-quality water, because they are environmentally friendly and have higher energy efficiencies than traditional thermal methods. The desalination nanofiltration (NF) membranes developed demonstrate a trade-off between water permeabilities and selectivities. Researchers are devoted to develop desalination NF membranes with both high water permeabilities and high selectivities. Using novel strategies, we successfully constructed double-high-performance NF membranes with a polyamide-polyester composite layer and a surface layer via a facile additional two-step approach. We used both triethanolamine (TEOA) and piperazine (PIP) to react with trimesoyl chloride (TMC) to form a composite active layer and coated the self-assembled polydopamine (PDA) on the surface of the composite active layer. To our knowledge, their performances are among the topmost of developed NF membranes for desalination and dye pollutant removal. We used scanning electron microscope (SEM), atomic force microscope (AFM) and water contact angle gauge (WCA) to investigate the membrane surface characteristics. The membrane obtained via the two-step approach has a water permeability of up to 21.3 LMH/bar, while that of the control polyamide NF membrane is as low as 8.2 LMH/bar. The developed membrane owes apparently improved water permeability in addition to maintaining high selectivities. Furthermore, the coating of the self-assembled PDA can alleviate membrane fouling. We also performed molecular dynamics (MD) simulations to study why the use of TEOA in addition to PIP to react with TMC improves membrane performances and give guidances and inspirations to future researches.