Preparation Of Reduced Graphene Oxide-MXene Membranes And Their Electro-Enhanced Performance For Molecular Rejection

Benefits from its sp² hybrid structure,reduced graphene oxide（RGO）nanosheets can be used to constructing separation membranes with ultrafast water transport channels for efficient wastewater treatment.However,weak hydrophilicity and narrow interlayer spacing of RGO membranes increase the energy barrier for water molecules to enter the RGO membrane channel,resulting in a low actual water flux.In addition,the fewer oxygen-containing functional groups on the surface of the RGO membranes weak the electrostatic interaction between the membrane and charged species,which is unfavorable for rejecting charged species by the membrane.To solve these problems,conductive RGO-MXene membranes with wettability channels were prepared in this work;the water fluxes of the membranes were improved by enhancing the affinity between water and the entrance of the membrane channels and reducing the resistance of water molecules entering the membrane channels;the rejections of charged dyes by the membranes were increased by enhanced electrostatic interactions under electro-assistance.The main research contents and conclusions are as follows:（1）RGO-MXene membranes were prepared by vacuum filtration on polyvinylidene fluoride membrane substrates,and the effects of MXene mass percentage on the surface morphology,molecular structure,interlayer spacing,hydrophilicity and conductivity of the RGO-MXene membrane were investigated.The intercalating of MXene reduces the surface wrinkle structure and internal defects of the membrane.The interlayer spacing increases from0.38 nm to 0.65 nm.The water contact angle of the membrane decreases from 80.1°to 42.3°and the zeta potential decreases from-20.3 to-36.2 m V,indicating the hydrophilicity of the membrane is increased.The conductivity of membrane maintains～1100 S m^-1.（2）The water permeability of RGO-MXene membrane was tested by cross-flow filtration method.The mechanism of MXene intercalation to improve membrane water flux was investigated by analysis of membrane properties and molecular dynamics simulation.The results show that as the mass percentage of MXene increases from 0 to 70 wt.%,the pure water flux of the membrane increases from 3.7 to 62.3 L m^-2 h^-1 bar^-1.The surface free energy of membrane increases from 49.3 to 66.5 m J m^-2,indicating that the intercalation of MXene increases the wettability of the membrane surface and the affinity between water and the entrance of the membrane channels,thereby the resistance of water entering the membrane channel is reduced.Molecular dynamics simulation result shows that the rate of water molecules entering the RGO-MXene membrane channel is 4 times high than that for the RGO membrane channels.In addition,the increased interlayer spacing could also contribute to more water molecules to enter the membrane channels.Based on the above analysis,intercalating MXene into the RGO membrane accelerates the entrance of more water molecules into the membrane channels and improves the water flux of the membrane.（3）The rejection performance of the RGO-MXene membrane was investigated,and the effects of MXene mass percentage and applied voltage on the rejection performance of the membrane were analyzed under the condition of electro-assistance.The results show that the as the mass percentage of MXene increases from 0 to 70 wt.%,the permeate flux of the membrane increases from 1.8 to 29.9 L m^-2 h^-1 bar^-1,while the rejection for orange G（OG）decreases from 88.8%to 27.8%.Electro-assistance can effectively improve the rejection of ionic dyes by the membrane.Especially for OG,the rejection of it by the membrane increases from 55.9%at 0 V to 91.4%at 2.0 V.In addition,the rejection rate of methyl blue（MB）reaches nearly 100%.Mechanism analysis shows that enhanced electrostatic repulsion is the main reason for the improvement of the rejection performance of the RGO-MXene membrane under electro-assistance.