Mechanistic insight into developing a novel loose nanofiltration membrane for effective dye/salt separation by epigallocatechin gallate

Loose nanofiltration (LNF) membrane prepared by hydroxyl-based materials with excellent separation and antifouling properties has attracted great attention for treating textile wastewater. However, the construction and formation mechanism of the selective layer by using large polyphenol-based monomers was not well studied. Herein, a novel LNF membrane was fabricated by interfacial polymerization, in which a polyphenol named epigallocatechin gallate (EGCG) was employed as aqueous monomer to design the selective layer. The results illustrated that the formed polyester (PE) selective layer possessed a loose structure with smooth and hydrophilic surface. Notably, the EGCG with large molecular volume and polyphenol groups contributed to the larger molecular weight cut-off (MWCO) and hydrophilic PE layer with high permeability, whereas the small molecular trimesoyl chloride (TMC) accounted for the pore size reduction with low permeance. More importantly, the optimized EGCG-based LNF-4 membrane displayed a high water permeability of 41.7 LMH·bar−1, high rejection of dyes (98.5 % for Congo Red (CR), 93.4 % for Methyl Blue (MB), and 94.8 % for Evans Blue (EB), respectively) and low rejection of salt (7.4 % for NaCl). Besides, under 60 g/L salt content, a high retention of CR (93.0 %) and high permeation of NaCl (95.1 %) could be still maintained. Furthermore, the LNF-4 membrane had a superior fouling resistance towards CR and EB with flux recovery rate above 96.4 %. The excellent antibacterial property also made LNF-4 more competitive in practical applications. This work can provide a new strategy for fabrication of outstanding multifunctional LNF membranes by large molecular polyphenol monomers.