Preparation Of Graphene-based Membrane Materials And Their Separation Performance

Membrane separation has received wide attention due to its advantages such as low cost,low energy consumption,high efficiency and easy operation.Recently,the research boom of graphene has promoted the rapid development of graphene-based membrane materials,and graphene oxide(GO)has become one of the research hotspots attracting much attention.The abundance of oxygen-containing groups endows GO good dispersion and excellent filmforming properties,leading to the formation of a stacked separation membrane,which exhibits high permeability and efficiency.However,the inevitable swelling and the single separation process of GO membrane seriously affect the stability in aqueous solutions and the separation effectiveness.Therefore,to improve the permeability and separation performance,we designed and prepared different functionalized graphene-based membranes for the research of metal ion separation,the main contents and innovations are summarized as follows:(1)Preparation of cyclodextrin-modified graphene-based membrane and its separation performance for copper ions(Cu2+).Cyclodextrin-modified reduced graphene oxide membrane(rGO-β-CD)were obtained by vacuum/pressure-assisted filtration.Due to the functionalization of β-CD effectively inhibits the swelling,the composite membrane exhibited high stability in polar solvent.In addition,as the host molecules,β-CD can form complexes with organic molecules or metal ions via the host-guest interaction,demonstrating the enhanced permeability and selectivity.As a result,the membrane retained organic molecules such as rhodamine B,methyl orange,and Evans blue at a rate of over 99%.Importantly,the membrane exhibited higher adsorption selectivity for Cu2+ in mixed ionic solutions,with selective permeability ratio of 2.6,2.8 and 4.6 for Co2+/Cu2+,Ni2+/Cu2+and Zn2+/Cu2+,respectively.(2)Preparation of graphene-based composite membrane loaded with manganese dioxide(MnO2)and their separation properties for lithium ions(Li+).Based on the previous work,we synthesized MnO2 nanotubes with a one-dimensional hollow structure,and prepared graphenebased composite membrane(GO-β-CD/MnO2)using MnO2 as an inorganic filler.Since Li+can easily enter the defective sites in MnO2,the adsorption separation performance of the composite membrane is enhanced by using MnO2 as an ideal Li+adsorbent.The permeation and adsorption experiments showed that the selective permeability ratio for K+/Li+ and Na+/Li+ in a mixed ionic solution are 2.5 and 3.2,respectively.In addition,the membrane shows a strong adsorption capacity for Li+,with a maximum adsorption of 37.5 mg g-1.Importantly,the membrane can enrich Li+concentration to 1.2 mg L-1 after 100 adsorption-desorption cycles.(3)Preparation of MXene/GO composite membrane and its voltage-gated ion transport performance.An electro-responsive MXene/GO membrane(MGOm)was prepared by introducing MXene nanosheets between GO sheet layers for achieving voltage-gated ion transport behavior.Since MXene has a high electrical conductivity,the composite of MXene and GO nanosheets imparted good electrical conductivity to the membrane;meanwhile the membrane also shows excellent stability and resistance to acids and bases in different solutions.When a voltage of+0.6 V is applied to the membrane,the interlayer spacing expanded,thereby increasing the ion permeability.Conversely,the permeability is decreased when a voltage of-0.6 V is applied.In addition,the effect of KCl-treated membrane on the ion permeation was investigated.When K+enters the GO layers,the force between the interlayer is enhanced by cation-π interaction.Hence,the KCl-treated membrane shows lower permeation rates,which are lower than 2 mmol m-2 h-1.(4)Preparation of porous self-supported graphene-based composite membrane and its adsorption and separation performance on Li+.Inspired by the electro-responsive membrane,porous self-supported graphene-based membrane(GCS)was prepared to investigate the effect of applied electric field on the selective adsorption separation of Li+.The porous structure constructed with polystyrene microspheres can provide a large solid-liquid contact area for the membranes.In addition,the incorporation of single-walled carbon nanotubes endows the membranes good electrical conductivity and voltage adjustable adsorption behavior.Meanwhile,the crown ether modified on the membrane achieves specific recognition of Li+via host-guest interaction.In ionic mixed solution,the selective separation factors for Li+/Na+,Li+/K+,Li+/Mg2+and Li+/Ca2+are 18.97,26.19,16.67 and 19.64,respectively.Based on density functional theory,Li+can preferentially enter the cavities of the crown ether,leading to the selective adsorption separation of the membrane.In summary,we have prepared a series of graphene-based composite membranes with multilayer stacked structures by vacuum/pressure assisted filtration method.Due to the functional modification,the membranes exhibit multiple coupling mechanisms in separation,improving the effect of ion separation.In addition,we also focused on exploring the effect of voltage on the ion permeability and selectivity.Thus,this doctoral thesis provides feasible ideas for graphene-based membrane materials in the field of separation,especially in the ion selective and controllable separation.