Carbonate modified polyester nanofiltration membrane: Separation performance and mechanism investigation

The development of loose nanofiltration (LNF) membranes with high water flux and excellent separation performance is of great significance for wastewater treatment and resource recovery. As a green polyphenol with high molecular weight, tannic acid (TA) is an attractive aqueous monomer to prepare LNF membranes via interfacial polymerization (IP) with trimesoyl chloride (TMC). To the best of our knowledge, this work first reports a facile and delicate method to regulate reaction sites of the aqueous monomer by oxidizing a certain proportion of hydroxyl groups under weak alkaline condition caused by carbonate to tailor the morphology of polyester layer. The resulted nanobubbles generated by the reaction of carbonate with H+ produced during the IP reaction (which also promoted the IP process) rendered polyester layer nanovoids structure. Compared with the pristine membrane, the tailored morphology of active layer decreased 93.24% membrane resistance and significantly improved water flux up to 13.55 times, while maintained effective dye/salt separation. Furthermore, the polyester NF membrane possessed high electronegativity and abundant oxygen-containing functional groups, which could effectively remove heavy metal ions Pb2+, Cu2+ and radionuclide uranium (U(VI)) from water through Donnan effect and adsorption, with the rejection rates of above 95.39%, 94.71% and 99.35%, respectively. This work provides a noteworthy modification idea for the preparation of high-performance multifunctional LNF membranes derived from green polyphenols to deal with some challenges such as dye separation, heavy metal ions and radionuclide removal.