Nanofiltration membranes with dually charged composite layer exhibiting super-high multivalent-salt rejection

• Nanofiltration membrane with dually charged composite layer was designed.
• The membrane was fabricated via A-IP on PVC substrate.
• The membrane exhibit super-high salts rejection and a reasonably high flux.
• Synergistic effect of dually charged composite layer upgrade separation efficiency.

State-of-the-art nanofiltration (NF) membranes generally positively or negatively charged could not remove both multivalent cations and anions once and for all according to the Donnan Exclusion. Designing NF membranes with facile approach for effective removal of multivalent-salt without compromising the permeation is still a tough challenge. In this work, we demonstrate a sort of novel NF membrane consists of a positively charged interlayer sandwiched between substrate and negatively charged outermost surface. The restructured NF membranes with dually charged composite layer were fabricated via amination-interfacial polymerization (A-IP) on poly (vinyl chloride) (PVC) substrate. The resultant NF membranes exhibited super high salts rejection of up to 99.0% MgCl2, 99.0% MgSO4, 98.5% CaCl2, 95.7% Pb(NO3)2, and 96.0% Na2SO4. Additionally, reasonably high flux (8.7 L m−2 h−1 bar−1, (pure water permeability (PWP))) approaching those of commercial NF membranes was obtained due to the relatively loose structure of the composite layer. Furthermore, the transport and separation mechanism of the restructured NF membranes reveals that the significant upgraded separation performance could be attributed to the synergistic effect of the dually charged layer. Moreover, the dually charged composite layer shown none unfavorable interaction and the resultant NF membranes possessed good interfacial stability.