Novel high-flux positively charged aliphatic polyamide nanofiltration membrane for selective removal of heavy metals

The toxic heavy metals produced by the discharge of industrial wastewater pose a serious threat to the ecological environment and human health. Nanofiltration (NF) membrane separation technology is widely used in fields such as water softening, heavy metal removal and dye separation due to its environmental friendliness and low cost. Herein, a novel positively charged aliphatic polyamide NF membrane (PEI-BTC) has been developed by using 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC) monomer bearing a stereoscopic structure which undergoes classic interfacial polymerization (IP) with polyethyleneimine (PEI) on the Polyether sulfone (PES) support membrane. The physicochemical properties revealed that the PEI-BTC membrane had a larger mean effective pore size (0.285 nm), a thinner separation layer (40 nm) and a stronger positively charged membrane surface (IEP = 7.25) than the traditional PEI-TMC membrane. Compared with previously reported PEI-based and commercial NF membranes, the optimized PEI-BTC membrane exhibits a higher MgCl2 (2000 ppm) rejection of 97.53% and pure water flux of 156.85 kg·m−2·h−1 at 1.0 MPa. Moreover, the prepared PEI-BTC NF membrane shows excellent toxic heavy metal (1000 ppm) removal efficiency in the order of Mn (98.78%) > Zn (98.32%) > Ni (97.74%) > Cu (95.67%) > Cd (90.49%). The results demonstrate that the prepared positively charged aliphatic polyamide NF membrane (PEI-BTC) has a unique industrial production potential for water softening and heavy metal removal.