Ultra-thin double Janus nanofiltration membrane for separation of Li+ and Mg2+: “Drag” effect from carboxyl-containing negative interlayer

To explore nanofiltration (NF) membrane with high permeability and good separation for Li+ from salt-lake, a triple-layered ultra-thin double Janus NF membrane was prepared with polyethersulfone (PES) substrate membrane, a carboxylated cellulose nanocrystal (CNC-COOH) interlayer and an ultra-thin polyamide (PA) layer via interfacial polymerization of polyethylenimine and trimesoyl chloride. The interlayer contain a large number of carboxyl groups, which could improve not only the negative potential of interlayer, but also the positive potential of the PA layer because of the increase of unreacted NH2. In addition, the hydrophilic of interlayer was stronger than the PA layer. Therefore, the interlayer and the ultra-thin PA layer could construct a double Janus NF membrane with hydrophobic/hydrophilic, positive/negative charge. It was found that the content of carboxyl groups and unreacted amino groups on the surface of Janus PES/CNC-COOH/PA NF membrane was increased obviously. The thickness of Janus NF membrane PA layer was 60.29 nm, 55.56% thinner than that in traditional PES/PA NF membrane and 25.89% thinner than that in weak Janus PES/cellulose nanocrystal/PA NF membrane. The Janus NF membrane had a minimum average surface roughness of 6.21 nm, an effective pore diameter of 1.38 nm, and exhibited a positive surface charge with a highly hydrophilic nature. Under the Mg2+/Li+ ratio of 30 and 60, the Janus NF membrane exhibited the rejection difference between Mg2+ and Li+ reached to 74.35% and 83.96%, the water flux was 4.17 and 3.4 L/(m2·h·bar), respectively, and the separation factor SLi,Mg was 12.15 and 5.84, respectively.