Fabrication of an electrically conductive separation layer with stable nanonetworks for efficient removal of heavy metals under an external voltage

Positively charged nanofiltration membrane shows great potentials in removing heavy metal ions from wastewater, but excellent separation and antifouling properties remain a challenge. Herein, a novel nanocomposite membrane with a dual-charged electrically conductive separation layer was fabricated by combining phase inversion, vacuum filtration and chemical crosslinking. The microstructure of PSF/sodium styrene-maleic anhydride copolymer (SMANa) composite membranes was systematically manipulated by adjusting the intermolecular binding state of SMA. Subsequently, various deposition layers were generated on the membrane surface by tuning the CNT/rGO ratios, and polyethylenimine (PEI) was also introduced by crosslinking with tannic acid (TA). The optimized membrane possessed a dual-charged separation layer with excellent conductivity and stable three-dimensional (3D) nanonetworks, and acted as cathode to achieve outstanding rejections of Cu2+(92.9%), Ni2+ (99.0%), Pb2+ (99.9%) and Cd2+ (99.5%) at a -3V voltage, with the permeate fluxes of 14.4～18.6 L/m2 h bar. This is ascribed to the synergistic effect among the electroreduction of metal ions, the chemical reaction with OH− to form hydroxides, the chelation with –OH and –NH2 groups, and the adsorption-induced electrostatic repulsion. Importantly, it also displayed superior stability over multiple runs and outstanding antifouling performances owing to the superposition of electrochemistry effect and stable hydration layers.