Modulation Of Physical And Chemical Properties Of Graphene Oxide Membrane Materials And Their Application In Water Treatment

Graphene oxide（GO）,an important derivative of graphene,has attracted a lot of attention from researchers in the fields of materials and water treatment because of its unique microstructure and excellent ion-molecular sieving properties.The surface and edges of GO nanosheet layers contain massive oxygen-containing functional groups,such as epoxy,hydroxyl,carbonyl and carboxyl groups,and the presence of these functional groups causes serious swelling problems in GO membranes in aqueous solutions,i.e.,swelling leads to the widening of the layer spacing and reducing the separation performance of the membranes.Therefore,the development of GO nanofiltration（NF）membranes with high rejection of metal ions,high water flux and high structural stability is still a great challenge for the field of water treatment.Based on this,in this thesis,the research related to the rejection performance of heavy metal ions,sodium ions in seawater,etc.is carried out through structural modulation of GO NF membranes in terms of oxygen content,layer spacing,and membrane flatness,and the specific research contents and results are as follows.（1）The robust K⁺-modified reduced GO（r GO–K⁺）membranes are developed for treating wastewater containing multivalent heavy metal ions.The r GO–K⁺membranes demonstrate the water permeance of 86.1 L m^-2h^-1bar^-1 and a rejection rate of 99.8%for Fe Cl₃,which exceed the corresponding values of state-of-the-art NF membranes for multivalent metal ion rejection.In addition,the NF membranes maintain stable and efficient rejection characteristics over long periods of time up to 72 h,at pressures up to 9 bar,and under strong acid and alkaline conditions.The r GO-K⁺membrane water permeance improvement was attributed to the cation-πinteraction between K⁺and r GO sheets,which fixed and expanded the interlayer spacing while increasing the surface hydrophilicity,thus weakening the water transport resistance.In addition,K⁺connects adjacent sheets by cation-πinteractions,thus also enhancing the stability of the separation membrane.Interestingly,in the work on hydrated K⁺-modified reduced GO,we also found that r GO-K⁺membrane materials have better performance in photocatalytic degradation of organic dye molecules and have potential applications in optical applications such as photodegradation and photocatalytic treatment.（2）GO membrane suffers poor stability in aqueous solution and low desalination efficiency of small-sized metal sodium ions.The nano-sized cross-linker-thiourea（TU）molecules were introduced into GO membrane to form TU cross-linked GO membrane（TU-GO）.which improves the stability of GO membrane in aqueous solution.Then,KOH treated TU-GO（TU-GO-KOH）NF membranes with were prepared on the basis of TU-GO NF membranes by immersion in KOH solution.KOH treatment further enhanced the cross-linking degree of TU-GO NF membrane through K⁺-πinteraction and realized enhanced stability of GO membrane,thus effectively removing small-sized sodium ions from seawater.Compared with TU-GO separation membranes,KOH solution treatment is more favorable to the ion selectivity of GO NF membranes,i.e.,better water molecule permeability while maintaining a high ion rejection rate.The resulting TU-GO-KOH NF membrane exhibited high Na Cl rejection（～91.4%）with a water flux of 4.3 L h^-1m^-2,which exceeded the performance of most GO NF membranes reported in the literature.In addition,the separation membrane still exhibited good separation performance and structural stability at high pressures（up to 9 bar）.The cross-linking of TU molecules and the cation-πinteraction between K⁺and GO nanosheets combined to improve the desalination efficiency and stability of the membrane.This TU-GO-KOH NF membrane shows great potential for its application in seawater desalination and provides new ideas for the design and development of high-performance GO NF membranes.