Cu2+/alginate nanofiltration membranes fabricated at the aqueous contra-diffusion "interface" for salt/dye rejection

Thin-film composite (TFC) membranes can find broad uses in desalination and wastewater treatment. However, industrialized interfacial polymerization involves many organic solvents and toxic monomers. An aqueous fabrication process can meet the standards of green and sustainable production, but fabricating defect-free thin films in a homogenous phase remains challenging. Herein, we fabricate a series of TFC nanofiltration membranes through a contra-diffusion process based on the cross-linking of metal ions and alginate. The film formation is confined at the ultrafiltration substrate surface due to the competition between reaction and restricted diffusion. Among a series of metal ion/alginate composites, the Cu2+/alginate composite membrane displays superior nanofiltration performance because of the strong affinity between Cu2+ and alginate. The highly negatively charged membrane performs a high rejection to divalent anions and anionic dyes based on the Donnan effect. The effects of alginate concentration and diffusion time on membrane performance were investigated to achieve optimal fabrication conditions. The as-prepared membrane shows a rejection of 96.2% to Na2SO4 with permeation flux around 50.2 L.m(-2).h(-1) under 0.6 MPa, and 99.5% to several dye molecules. Moreover, the membrane performs outstanding anti-fouling and anti-bacterial properties because of its excellent hydrophilicity and the existence of Cu2+.