Preparation And Separation Properties Of Graphene/Carbon Nanotubes Composite Membrane

Graphene shows a unique application value in many areas due to its superior physical and chemical properties.It is a one-atom-thick sheet of graphite comprising SP2-hybridized carbon atoms arranged in the hexagonal honeycomb lattices.Perfect structure graphene composed of continuous carbon atoms hexatomic ring.It is demonstrated that the perfect pristine graphene is impermeable to any gas or liquid.It can be used as anti-corrosion materials.However,The graphene sheet featuring nanoporous or microporous can be formed by selectively artifical etching.It has been proposed as a promising high-efficient separation membrane.Owing to the one-atom thickness of graphene,the transport rates of molecules through the graphene membranes are supposed to be extremely high.But the ideal monolayer graphene membrane preparation process was not mature.The actual application of separation membrane with larger area can't preparation.At the present,the study of the monolayer graphene is basically remained at the theoretical stage,and it is often used as air separation membrane.Graphene oxide(GO)with oxygen-containing functional groups,an oxidized derivative of graphene,maintains most of the extraordinary properties of graphene.It is easy to fabricate in large-scale and can be assemble into membrane simply by simple methods.In this work,we prepared high temperature thermal reduction graphene(GR),the oxide graphene/carbon nanotubes composite separation membranes(GO/CNT separation membranes)and graphene/carbon nanotubes composite separation membranes(rGO/CNT separation membranes),And They were used for the separation of heavy metal Pb2+ ions.and Co2+ ions,their separation properties and separation mechanism were studied.The main contents can be summarized below:(1)Graphite oxide was prepared from natural flake graphite via improved Hummers methods,After washed by HCl solution,The NaOH solution was added instead of water until the pH approximately equal to 7 for neutral solutions,Then graphite oxide was obtained by the vacuum freeze-drying method,which Greatly shortens the graphite oxide washing time.Graphene oxide(GO)were got by ultrasonic exfoliation of graphite oxide.Microstructures and properties of graphene oxide washed by the above-mentioned two methods were contrasted.The results of SEM showed that the microstructures of oxide graphene washed by NaOH solutions present thin layers with wrinkle.Furthermore,GO sheet with the thickness of 0.76 nm could be observed by atomic force microscope(AFM),The results of X-ray diffraction(XRD)and Fourier transform infrared spectroscope(FTIR)showed that the graphite oxide was lightly reduced by the alkali(NaOH),Thus the lightly reduced GO may improve performance of the GO/CNT separation membranes.The middletemperature reaction time on the influence of the structures and properties of the GO is studied.Properties are better more than 4h reaction time.The original CNT were preoxidation treatment,and the impurities in the CNT was removed.The result of SEM shows that dispersing performance is better than before treatment.(2)Graphene(GR)was obtained by reduced graphene oxide(GO)via thermal reduction quickly.and it was employed as adsorbent to study the adsorption property of heavy metal Pb2+ ions.The results of SEM,TEM and AFM showed that the the microstructures of Graphene(GR)presents thin layers with wrinkle,GR sheet with the thickness of 0.37 nm could be observed by atomic force microscope(AFM).The solution conditions such as pH,adsorption time and initial concentration of Pb2+ ions on the influence of the adsorption properties were investigated.The results showed that the pH value played an important role in the adsorption on graphene.When pH value of the solution was higher than 7,The adsorption capacity was increased very remarkably.and the equilibrium was reached within five minutes.The regeneration of the GR was investigated,the adsorption capacity decreased only after the 7th cycle to approximately 8.7%.Experimental data was fitted by Langmuir and Freundlich adsorption isotherms model,and the adsorption mechanism was studied.The results shows that the adsorption isotherms for Pb2+ was well fitted by the Langmuir model and the adsorption processes followed the pseudo-second-order kinetic model,It demonstrated that the adsorption reaction is a fast single molecularlayer chemical process.High temperature reduction graphene avoids the use of toxic chemical reductant.The maximum adsorption capacities of GR on Pb2+ was approximately 86.5mg/g.(3)GO/CNT separation membranes was prepared by vacuum filtration methods.The effect of GO/CNT composite membranes on separation properties of heavy metal Pb2+ and Co2+ions was tested.Scanning electron microscope(SEM)was employed to characterize the cross section microstructure.The results showed that GO nanosheets were interlocked with each other in a parallel manner forming the lamellar structures during the film formation process and CNTs are intercalated in the stack GO layers establishing the corrugations and nanochannels for fast water delivery.The impact of GO content of separation membranes on separation properties of Pb2+ ions was analyzed.With the increase of the content of GO in the separation membrane,the rejection rate was relatively stable,the water flux was decreased obviously.The GO/CNT composite membranes with GO/CNT mass ratio of 1 : 1 exhibited best separation properties.The factors influencing their rejection rate and water flux,such as the thickness of membrane,pressure,pressure mode and their regenerability,were investigated.The separation properties of GO/CNT separation membranes is best when the volume of GO/CNT dispersion is 5ml and the thickness of membrane is 2?m.the separation properties is best when pressure is 0.4 MPa and can be improved through interval pressing.When the pressure is 0.1 MPa,the water flux of GO/CNT separation membranes can reach up to 518Lm-2h-1bar-1 and the good rejection rate of PbCl2,Co(NO3)2,Pb(NO3)2 and CoCl2(over 98%).which exhibits very good performance of regeneration.(4)In order to further increase the water flux of GO/CNT separation membranes,GO is reduced by green non-toxic VC(ascorbic acid).Then rGO/CNT separation membranes was prepared by vacuum filtration methods.the impact of rGO content of separation membranes on separation properties of Pb2+ ions was analyzed.With the increase of the content of rGO in the separation membrane,the rejection rate was lightly improved,but the water flux was decreased obviously.Synthetically,the rGO/CNT composite membranes with GO/CNT mass ratio of 4 : 1 exhibited best separation properties.The water flux of rGO/CNT separation membrane significantly higher than the GO/CNT separation membrane.The water flux of rGO/CNT separation membranes can reach up to 945Lm-2h-1bar-1 and the good rejection rate of PbCl2,Co(NO3)2,Pb(NO3)2 and CoCl2(over 95%).which exhibits very good performance of regeneration.The separation mechanism of the rGO/CNT separation membranes is the combination of both the size sieving effect and adsorption.For the first time,rGO/CNT membrane and GO/CNT membrane are prepared and possessed the good rejection rate of Pb2+ and Co2+.The rGO/CNT membrane and GO/CNT membrane are facile to fabricate and exhibit high separation performance.It is believed that the graphene separation membrane have great attractive application in water treatment field.