Facile fabrication of a positively charged nanofiltration membrane for heavy metal and dye removal

Positively charged membranes are essential for the removal of heavy metals from wastewater by nanofiltration (NF). A simple fabrication procedure for synthesizing a positively charged membrane with high permeance and high selectivity is needed to make this realistic. This work prepared a positively charged P84 NF membrane by chemical cross-linking of the asymmetric plain P84 membrane using hyperbranched polyethyleneimine (PEI). Instead of using organic solvent and thermal annealing, the cross-linking reaction occurred in mild conditions with a cross-linker concentration of 15 g L^-1 and a cross-linking time of 20 h at ambient temperature in water. The modified membrane had increased surface hydrophilicity, surface roughness, and a positively charged surface. The resulting membrane had an excellent water flux of 43.2 L m^-2h⁻¹ at 4 bar. The cross-linked P84 NF membrane showed high rejections of divalent heavy metals PbCl₂, Ni(NO₃)₂, ZnCl₂, Cu(NO₃)₂ with rejections of 99.0%, 96.7%, 96.2%, 99.8%, respectively, which outperforms the commercial NF270 membrane tested in the same conditions. Due to the high cross-linking of the active surface layer, the membrane was found to reject positively charged organic dyes almost completely: Victory blue B (VBB, 506.08 g mo1^-1), methylene blue (MB, 319.85 g mo1^-1), and negatively charged Congo red (CR, 696.66 g mo1^-1). The convenient fabrication process could accelerate the progress of the positively charged NF membranes in potential applications of removing heavy metals and positively charged dyes from effluents in industries.