Double-charged self-assembled rGO/g-C₃N₄ membrane prepared by "functional group substitution" for heavy metal ions...

Heavy metals are still the critical pollutants in industrial wastewater and there is an urgent need for efficient and environmentally friendly treatment technologies. Reduced graphene oxide (rGO) is widely used for preparations of nanofiltration (NF) membranes but suffers from poor hydrophilicity and electronegativity. In this work, a double-charged rGO/g-C₃N₄-P membrane was prepared for removal of heavy metals at low pressure. Graphitic carbon nitride (g-C₃N₄) assisted reduction of GO membranes under ultraviolet (UV) irradiation, and the modification of functional groups with high polarity improved the hydrophilicity of membrane surface. The filtration performance for heavy metals of rGO/g-C₃N₄-P membrane was evaluated under low pressure (1-2 bar). The rejection rates of Cu²⁺, Cr³⁺, Mn²⁺, Cd²⁺, and Pb²⁺ by membranes reduced by UV for 18 h (rGO/g-C₃N₄-18-P membrane) reached 94.72 %, 98.05 %, 82.32 %, 88.2 % and 77.15 %, respectively. In the experiment of mixed simulated wastewater, the rejection rates of NO₃^- and SO₄^2- both reached >95 %. Outstanding rejection rates were attributed to the interaction and the synergy effect of double-charged layers as well as steric effects. Meanwhile, the water flux of rGO/g-C₃N₄-18-P membrane was as high as 37.14-50.16 L m^-2h^-1bar^-1 (1-2 bar). The high flux was due to the reduced degree of oxidation so that water molecules transported between GO nanochannels more smoothly and the transport path was shortened through the nanopores of g-C₃N₄. Obviously, flux and heavy metal rejection of rGO/g-C₃N₄-18-P membrane were simultaneously improved, breaking "trade-off" effect. Furthermore, rGO/g-C₃N₄-18-P membrane showed excellent antifouling ability and the potential for heavy metal wastewater filtration in comparison with other NF membranes reported in literature.