Ultra-Highly permeable loose nanofiltration membrane containing PG/PEI/Fe3+ ternary coating for efficient dye/salt separation

Abstract

Loose nanofiltration membrane (LNM) with high separation toward dye/salt and high permeability is a promising candidate for resource recovery of textile wastewater. In this study, to prepare a high permeable LNM, a ternary coating system containing a small polyphenol molecule named pyrogallol (PG), polyethylenimine (PEI) and Fe3+ ion was co-deposited on the hydrolyzed polyacrylonitrile substrate via regulating the competition reactions between coordination bond (COB) and covalent bond (CB). The membrane formation mechanism, morphology, surface properties, and separation performance of dye/salts were investigated by characterizations and filtration experiments. The characterization results indicated that the addition of Fe3+ limited the CB formation between PG and PEI via Michael addition and Schiff base reaction, due to its strong coordination ability toward PG and PEI via COB with –OH and –NH- groups, further reducing the selective layer thickness and improving the membrane permeance as a result. The content of PEI and Fe3+ in ternary coating solution obviously affected membrane morphology and hydrophilicity, thus influencing the permeance. More importantly, The LNM-5 exhibited an ultrahigh water permeability of 141.5 LHM·bar−1, high rejection of Congo Red (CR, 99.5%) and Alcian blue 8GX (ALBL, 99.7%) as well as high transmission of salts (98.8% for NaCl, 97.3% for Na2SO4, respectively.), which performed much better than many reported state-of-the-art LNMs prepared by different methods. Even under 60 g/L salt content, high rejection of CR (99.1%) and high pass of salts (98.8% for Na2SO4) could be still maintained. Furthermore, the LNM-5 had a superior antifouling performance and long-term stability during filtration (22 h) of dye/salt mixture. This work indicated that small polyphenol/PEI/Fe3+ trinity coating could be used as a promising alternative in practical applications, realizing excellent molecular separation towards sustainable textile wastewater remediation.