Bio-inspired co-deposition of dopamine and N-oxide zwitterionic polyethyleneimine to fabricate anti-fouling loose nanofiltration membranes for dye desalination

Developing anti-fouling loose nanofiltration membranes (LNMs) that can achieve dye desalination of saline textile wastewater through dye retention and salt permeation for resource utilization remains an urgent challenge that needs to be addressed. Inspired by the biomimetic adhesion of mussels and the excellent fouling resistance of saltwater fish surfaces, novel LNMs were prepared through co-deposition strategy using dopamine (DA) and N-oxide zwitterionic polyethyleneimine (ZPEI) as biomimetic functional materials. There are multiple interactions between them, including Schiff base reaction, Michael addition reaction, cation-π interaction and covalent polymerization. Compared with PEI-LNM prepared under the similar method, the optimized ZPEI-LNM displays more uniform surface, smaller average roughness (6 nm vs 22 nm), and denser pore size (0.69 nm vs 0.79 nm), indicating the zwitterionic N-oxide moieties in ZPEI regulate the deposition process and the cross-linking structure of the selective layer. ZPEI-LNM exhibits a high pure water flux of 203.0 L m−2h−1 under 0.6 MPa. Importantly, ZPEI-LNM shows high retention towards different dyes, including Congo red (CR, negative, 696.8 g/mol) of 100.0 %, Acid fuchsin (AF, negative, 585.5 g/mol) of 99.5 %, Crystalline violet (CV, positive, 408.0 g/mol) of 98.2 %, and Indigo carmine (negative, 466.4 g/mol) of 96.0 %, while the rejection of NaCl (8.7 %) and Na2SO4 (12.7 %) are both lower. As expected, for the mixture solution of AF/NaCl, the dyes are efficiently rejected, while salts can pass through, indicating that ZPEI-LNM displays an excellent dye desalination capability. Furthermore, multiple-cycle antifouling tests for bovine serum albumin (BSA) and sodium alginate (SA) reveal that the flux recovery ratios are higher than those of PEI-LNM (BSA: 90.4 % vs 81.9 %; SA: 86.6 % vs 80.5 %), indicating ZPEI-LNM exhibits better anti-fouling performance towards different model foulants owing to the outstanding hydration layer caused by zwitterionic N-oxide moieties and lower surface roughness. Consequently, the work offers a new viewpoint for the fabrication of anti-fouling LNMs via biomimetic strategy for dye desalination.