Thin film composite nanofiltration membrane with tannic acid self-assembled coating as an interlayer achieves high selectivity by enhancing size exclusion effect

Traditional thin film composite polyamide nanofiltration (NF) membranes are difficult to simultaneously achieve high rejection for salts that are divalent anionic and cationic, and their separation selectivity for mono-/di-valent salt separation is usually poor. Herein, novel polyesteramide NF membranes were developed by interfacial polymerization on modified poly(vinylidene fluoride) (PVDF) substrates containing tannic acid (TA) self-assembled coating (TC@PVDF). TC@PVDF substrates were firstly fabricated via non-solvent induced phase separation technology using amphiphilic hyperbranched poly(methyl methacrylate-co-N-methylacrylamide) (HPMH) copolymers as in situ modifiers and TA aqueous solution as coagulation bath. The self-assembly behavior between TA and HPMH results in the increase of substrate surface hydrophilicity, electronegativity and pore size, which would facilitate the storage and uniform distribution of aqueous monomer piperazine on the substrate surface. For these reasons, TA coating interlayered NF (TCi-NF) membrane exhibits a thick polyesteramide layer with crisscrossing ridge surface morphology, a low molecular weight and a small pore size. The TCi-NF membrane not only demonstrates high rejection toward Na2SO4 (99.3 %) and MgCl2 (95.7 %), but also displays exceptional separation selectivity for both mono-/di-valent anions [α(Cl−/SO42−) = 49] and mono-/di-valent cations [α(Na+/Mg2+) = 52]. Since the surface charge property of TCi-NF membrane is close to weak negativity, electrostatic effect between charged ions and membrane surface would be weakened, so the separation mechanism mainly depends on the size exclusion effect. Specifically, the TCi-NF membrane displays stable long-term separation performance and high salt concentration resistance. Therefore, this research provides a new preparation strategy for improving the precise desalination performance of thin film composite NF membranes.