Double-charged self-assembled rGO/g-C 3 N 4 membrane prepared by "functional group substitution" for heavy metal ions rejection at low pressure.

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-C ₃ N ₄ -P membrane was prepared for removal of heavy metals at low pressure. Graphitic carbon nitride (g-C ₃ N ₄ ) 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-C ₃ N ₄ -P membrane was evaluated under low pressure (1-2 bar). The rejection rates of Cu ²⁺ , Cr ³⁺ , Mn ²⁺ , Cd ²⁺ , and Pb ²⁺ by membranes reduced by UV for 18 h (rGO/g-C ₃ N ₄ -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 NO ₃ ^- and SO ₄ ^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-C ₃ N ₄ -18-P membrane was as high as 37.14-50.16 L m ^-2 h ^-1 bar ^-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-C ₃ N ₄ . Obviously, flux and heavy metal rejection of rGO/g-C ₃ N ₄ -18-P membrane were simultaneously improved, breaking "trade-off" effect. Furthermore, rGO/g-C ₃ N ₄ -18-P membrane showed excellent antifouling ability and the potential for heavy metal wastewater filtration in comparison with other NF membranes reported in literature.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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