Improving chlorine resistance and separation performance of thin-film composite nanofiltration membranes with in-situ grafted melamine

Abstract

Nanofiltration (NF) membranes exhibit great performance deterioration when contacting with high concentration chlorine solutions, which is unbeneficial to their sustainable operation. Herein, a chlorine-stable melamine was surface-grafted to the polyamide membrane by a covalent modification method to improve the anti-chlorine property of NF membranes. The grafting of melamine significantly changed the surface features of NF membranes. The grafted membranes owned rougher morphology, narrower pore size, higher hydrophilicity, and greater positive charge compared to the unmodified membrane. Higher salt rejections (NaCl, MgCl2, MgSO4, and Na2SO4) with slightly declined water permeability were obtained for the grafted membranes. Importantly, the optimized membrane with melamine grafting could maintain a high separation performance when contacting with 2000 ppm NaClO solution for 10 h at a pH of 10.5. For the control membrane without modification, more defects appeared on the membrane surface when the pH of the chlorine solution decreased from 10.5 to 3.5. In contrast, the original surface characteristics and chemical structures of the grafted membranes were well preserved under the same conditions. The grafting layer endowed the grafted membrane outstanding chlorine resistance and high separation performance, making it attractive for the construction of chlorine tolerant desalination membranes for the environmental process.