Adsorption And Separation Properties Of Graphene-based Macrostructures For Water Treatment And Interaction Mechanisms

Graphene-based macrostructures(GBMs)are porous macrostructures formed by the crosslinking and assembly of graphene-based nanosheets(GBNs).Compared with GBNs,GBMs assembled by GBNs have excellent adsorption and separation capability and recycling performance for pollution control,especially in the removal and purification of water pollutants.Three-dimensional graphene-based macrostructures and graphene-based membranes formed by the liquid-phase assembly,functional modification or layer by layer stacking are two main GBMs.The surface property regulation,micro/nano structure construction and functional modification of GBMs are crucial to their structural stability,surface wettability,adsorption and separation performance and catalytic conversion performance,which is a hot topic in the field of environment.In this study,different types of GBMs were constructed by sol-gel,surface modification and filtration assembly methods.The structure of GBMs was regulated from three aspects:structural stability,wetting property and separation performance.The adsorption and separation properties of GBMs were investigated,and their surface morphology,chemical composition and wettability were characterized.The relationship between the microstructure and the macroscopic properties of GBMs and their interaction mechanism was elucidated,so as to provide theoretical reference for the design and preparation of GBMs and their application in water pollution control.The main conclusions and innovations are shown as follows:(1)Because of the covalent cross-linking of cysteamine,the interaction between graphene oxide(GO)layers was effectively regulated,the structural stability of graphene aerogel was enhanced,and the porosity and mechanical properties were improved simultaneously.As a crosslinking agent,cysteamine can react with epoxy groups on GO surface through nucleophilic ring-opening reaction,thus improving the cross-linking degree between GO nanosheets and forming more micropores and mesopores.The specific surface area and porosity of aerogel increased obviously.The reduction ability of cysteamine improves the hydrophobicity of the composite aerogel and forming a bread-like structure with a smooth external surface and rough porous interior.The composite aerogel can fully maintain its original morphology under strong acid and strong alkali conditions,and their structural stability and mechanical strength were significantly improved with good oil(148-226 g/g)and dye adsorption capacities.(2)By using GBNs as the interface micro/nano structure modification materials,the wettability of different porous substrates,like melamine foam and copper mesh,was effectively regulated,the oil/water selectivity and cyclic performance were significantly improved.The two-dimensional flexible wrinkle structure of rGO/GO makes it easy to modify the surface of porous substrate and form a variety of micro-nano scale rough structures.The hydrophobic reduced graphene oxide(rGO)can also shelter the hydrophilic functional groups on the porous melamine foam(MF)substrate,and the surface energy was further reduced by the modification of octadecylamine(ODA).The obtained ODA-rGO@MF exhibited superhydrophobic/superoleophilic properties with a water contact angle of 153.5°.It also shows excellent oil-water selectivity,good oil absorption capacity(44–111 g/g)and cycle performance.The hydrophilicity of GO and the cross-linking effect of Cu²⁺endowed the modified copper mesh with stable superhydrophilic/underwater superoleophobic.The obtained GO@Cu O mesh demonstrated excellent anti-oil-fouling property and its oil-water separation efficiency is up to 99.5%.(3)A graphene-based membrane with excellent photocatalytic self-cleaning performance was successfully constructed.The coupling of photocatalytic degradation and membrane separation process effectively alleviates the inevitable membrane fouling problem in the practical application of graphene-based separation membrane.As an intercalation material,Ag nanoparticles modified graphitic carbon nitride(Ag@g-C₃N₄)can significantly improve the water permeability of the composite membrane while maintaining its high rejection performance.The composite membrane exhibit excellent photocatalytic self-cleaning performance.In the dye molecules separation and rejection process,the decreased water flux can recover to 98.1%of its initial flux after visible light irradiation.The composite membrane also has good stability and universality,which suggested that this membrane is also an ideal candidate for the effective separation of other organic pollutants with good photoregeneration ability.The oxidation and degradation of organic pollutants by·OH and·O₂-radicals under light irradiation is the basis of its photocatalytic self-cleaning ability.(4)By coupling the external electric field with the membrane separation process and using carbon dots(CDs)as intercalation material,the interlayer nanochannel and electrochemical performance of rGO membrane were regulated,and the flux and retention performance was improved simultaneously with the mediation of external electric field.CDs can effectively regulate the roughness,hydrophilicity and electrochemical properties of the membrane.The salt rejection performance significantly improved with the mediation of an external electric field,and the highest Na Cl rejection rate was 89.3%,which was 62.1%higher than that without voltage.The salt rejection performance of the membrane under the electric field is closely related to its specific capacitance.Membrane with higher specific capacitance shows better salt rejection performance.The ion adsorption and electric field force of the membrane have no significant effect on salt rejection.The enhancement of electrostatic interaction between membrane and ions is the main reason for the improvement of rejection performance.