Preparation Of Functionalized Graphene Oxide Composite Material And Study On Its Adsorption Mechanism For Dyes And Rare Earth Elements

Industrial production and human daily life lead to many pollutants discharged into the water environment,thus endangering human health.At present,the focus is mainly on organic and inorganic environmental pollutants.Among them,organic dyes in textile industry have attracted much attention in the field of water treatment due to their difficulty in degrading and carcinogenicity.Azo dye ethyl red and cationic dye methylene blue are typical pollutants of organic dyes.Rare earth elements,as an inorganic pollutant,will disturb the aquatic system and endanger human health through food chain.Therefore,the treatment and removal of pollutants such as organic dyes and inorganic rare earth elements in water environment has become the focus of research at domestic and foreign.At present,the main methods of treatment and removal include membrane separation,adsorption,photocatalysis,biodegradation,etc.Among them,the adsorption method is considered to be one of the effective methods to remove pollutants in water due to the advantages of simple operation,economic efficiency and good adsorption effect.In this method,the selection of adsorption materials is the key,because the structural characteristics of materials directly affect the adsorption performance.As a new material,graphene oxide（GO）has a great potential in the field of water treatment functional materials because of its rich active groups and large specific surface area.However,the practical application in the treatment of water pollutants is limitedbecause of the easy agglomeration between the layers,high hydrophilicity anddifficulty in solid-liquid separation.Functional modification of GO,expansion of the application of GO materials in the treatment of pollutants in the water environment,and the development and preparation of new composite materials with strong adsorption capacity and easy separation are current hot issues.At the same time,it is of great significance to introduce molecular docking,molecular dynamics simulation and quantum chemical calculation to understand and evaluate the performance,removal effect and adsorption mechanism atthe molecular/atomic level.In this paper,we focused on the preparation of adsorption materials with good removal effect of organic dyes and rare earth elements pollutants in water environment,and carried out the research on the adsorption performance,removal effect,adsorption model and adsorption mechanism of cyclodextrin derivatives and GO based materials as adsorbents.In particular,the application of molecular dynamics simulation and quantum chemistry calculation deeply analyzed the micro mechanism of GO matrix composites in the removal of four rare earth elements,which is an important contribution of this paper.The main research contents and results are as follows,（1）Study on the inclusion mechanism of cyclodextrin derivatives with the azo dye ethyl red（ER）.The main purpose of this chapter was to explore the inclusion mechanism of two kinds of cyclodextrin derivatives with azo dye ER by taking the advantages of good water solubility of cyclodextrin derivatives,and to provide preliminary basic research for their application in the removal of insoluble dye pollutants.Twoβ-cyclodextrinderivatives2,6-dimethyl-β-cyclodextrin（DM-β-CD）and hydroxypropyl-β-cyclodextrin（HP-β-CD）were used as host molecules,and ER as guest molecule.The prediction results of molecular docking and kinetic simulation（MD）showed that ER spontaneously formed stable inclusion complexes with DM-β-CD and HP-β-CD at a stoichiometric ratio of 1:1,and the main driving forces were hydrogen bonding and hydrophobic interaction.Molecular dynamics trajectory analysis showed that the dominant configuration was that the ethyl end embedded in the cavity,and the structure of DM-β-CD was less disturbed by ER.The results of phase solubility and Job plot experiments confirmed the predicted results of MD simulation with a stoichiometric ratio of 1:1.The binding constant indicated that DM-β-CD had stronger binding ability to ER than HP-β-CD.Thermodynamic analysis showed that the inclusion process of DM-β-CD and HP-β-CD to ER was spontaneously driven by entropy.The results of ¹H NMR and 2D ¹H-¹H ROESY showed that the ethyl end of ER molecule entered the cavity of DM-β-CD and HP-β-CD,which confirmed the rationality of MD simulation results.The results of various solid-state characterization methods showed that the thermal stability of ER was enhanced,and the apparent morphology changed significantly.The formation of inclusion complex significantly increased the water solubility and decreased the accumulation of ER in water environment,which was conducive to the removal of ER.Therefore,the study on the inclusion process and mechanism of two cyclodextrin derivatives with azo dye ER provided an important reference for the application of cyclodextrin materials in the removal of dye pollutants.（2）Carboxymethyl functionalizedβ-cyclodextrin modified graphene oxide（CM-β-CD-GO）for the removal of methylene blue（MB）.Using GO as the carrier matrix,β-CD was grafted to overcome the agglomeration of GO,and then carboxymethylation was used to increase its active sites.The adsorption materials with high performance were prepared to achieve the efficient adsorption and removal of cationic dye MB,and the adsorption mechanism of CM-β-CD-GO was explored from the perspective of experiment and quantum chemistry.The results showed that the carboxyl functional groups provided more active adsorption sites,which makes CM-β-CD-GO have high removal efficiency for MB in a wide range of p H.CM-β-CD-GO could rapidly adsorb MB,and the removal efficiency could reach 90%within 20 min,which accorded with the pseudo-second-order model.The adsorption process was related to chemical adsorption,and the adsorption rate was affected by multiple stages.The Langmuir model was more suitable to describe the adsorption process,which indicated that MB was deposited on the surface of CM-β-CD-GO in the form of monolayer adsorption,and it is an endothermic spontaneous process.Co-existing dyes interference experiments showed that CM-β-CD-GO could adsorb cationic dyes,and it still maintained good adsorption performance after recycling for five times.The composition and morphology of the composites were characterized by X-ray photoelectron spectroscopy（XPS）,atomic force microscopy（AFM）and laser particle size analyzer.Oxygen-containing functional groups（carboxyl and hydroxyl）were active adsorption sites.Comprehensive experimental results,the adsorption mechanism was proposed as follows:MB molecules were adsorbed to the surface of CM-β-CD-GO under electrostatic attraction,and then the electron sharing or transfer occured between the oxygen-containing functional groups and MB,completing the adsorption of MB from aqueous solution to the surface of solid materials.Density functional theory（DFT）calculation results explained the improvement of adsorption capacity of carboxyl from the perspective of mechanism.In addition,the interaction between graphenesheets and MB confirmed the contribution ofπ-πconjugation to the adsorption.Differential density and density of states results indicated that there was electron transfer or sharing between CM-β-CD-GO and MB.In this study,DFT calculation was used to explore the micro adsorption mechanism of CM-β-CD-GO adsorption of organic pollutants,which enriches the theoretical analysis of the adsorption of GO substrates.（3）Preparation of graphene oxide mesoporous silica composite material（GO-NH₂Si O₂）and study on the adsorption of rare earth elements.In view of the fact that inorganic rare earth elements are mostly in ionic state in water,the main research content of this chapter was to prepare the graphene oxide composite GO-NH₂Si O₂on the basis of not destroying the active sites,and realize the rapid adsorption of rare earth elements Nd,Ce,La and Y.Expanding the application of GO base materials in the removal of inorganic elements.Langmuir isothermal model and SEM-EDS results showed that all four rare earth elements formed a uniform monolayer adsorption on the surface of GO-NH₂Si O₂.The adsorption rate of Nd（Ⅲ）in 8minutes reached more than 90%,and the four ions reached equilibrium after 10 min,which showed the kinetic properties of the rapid adsorption of GO-NH₂Si O₂.The pseudo-second-order model was more in line with the experimental results,indicating that the main rate-limiting step of adsorption was chemical adsorption,and the adsorption process was divided into fast surface adsorption,slow pore diffusion and adsorption equilibrium.The results of binary and quaternary adsorption of four rare earth elements showed that there was competition among the elements,and their adsorption sites were the same.When the competitive alkali,alkaline soil and transition metal ions were low concentration,GO-NH₂Si O₂ had good selectivity.The results of adsorption-desorption cycle experimentsshowed that the recovery of rare earth elements was higher than 95%after the GO-NH₂Si O₂material was recycled for five times.Based on the morphological and composition characterization of GO-NH₂Si O₂ by SEM-EDS,XPS and other modern analytical methods,combined with the results of adsorption experiments,it is predicted that the possible adsorption mechanism was,electrostatic attraction induced rare earth elements to adsorb on the surface of GO-NH₂Si O₂,or has electron sharing or ion exchange with carboxyl and hydroxyl groups,or complexation with secondary amine groups,so as to achieve rapid and efficient adsorption.In this study,the GO matrix composite with three-dimensional sandwich structure was prepared,and the rapid adsorption of four inorganic rare earth elements was achieved.The research results not only expand the application of GO as adsorbent in inorganic rare earth elements,but also provide a new idea for the removal of Nd,Ce,La,Y in water environment.（4）Theoretical study on the mechanism of GO-NH₂Si O₂ adsorption of four rare earth elements.The three-dimensional sandwich structure model of GO-NH₂Si O₂ was constructed.The dynamic adsorption behavior of Nd,Ce,La,Y on GO-NH₂Si O₂ was simulated by molecular dynamics（MD）simulation.Based on density functional theory（DFT）calculation,the adsorption energy and charge transfer of different functional groups and four rare earth elements were compared and analyzed.The micro mechanism of adsorption was revealed from molecular and atomic level.MD simulation results showed that the diffusion coefficient of Y was the largest among the four rare earth elements Nd,Ce,La,and Y.The adsorption distances of the four ions were r<2.6?,indicating the chemisorption,and carboxyl and hydroxyl groups were the main active sites.According to the average potential PMF,the adsorption energy of GO-NH₂Si O₂for four rare earth ions was Nd>Ce>La>Y,which explained the adsorption capacity from binding mechanism.The energy analysis showed that the electrostatic action drives the adsorption reaction.According to DFT theoretical calculation,the adsorption strength of the three functional groups to the rare earth ions was carboxy>hydroxy>epoxy group.The adsorption energy of four rare earth ions on active sites is consistent with MD result,which was Nd>Ce>La>Y.The bond length,atomic charge and differential electric density analysis of each functional group confirmed the electron transfer and sharing proposed by the experiments.In short,the results of MD simulation and DFT calculation provided theoretical support for the adsorption mechanism proposed by adsorption kinetics,thermodynamics and XPS characterization experiments.The results of this study have important guidance for interpreting the mechanism of GO materials on rare earth elements from the micro level.