Removal Of Heavy Metals From Water By Graphene Oxide/Layered Double Hydroxide Composites

Layered double hydroxides（LDHs）and graphene oxide（GO）are nanomaterials having excellent properties.Their higher adsorption properties and great potential have attracted much attention.LDHs,also known as hydrotalcite,is a natural clay mineral,which is composed of metal cationic laminates and exchangeable interlayer anions,so it has a certain adsorption capacity for both anions and cations.GO is an ideal adsorbent for heavy metal ions because of its large specific surface area and abundant functional groups which can provide more adsorption sites for pollutants.In this dissertation,the magnetic graphene oxide/LDH and core-shell structure of ferroferric oxide@graphene oxide/LDH composites were synthesized and characterized.The adsorption performance for Cu²⁺,Pb²⁺,Cd²⁺in water was studied by batch adsorption experiment.The results were as follows:（1）MGL were successfully prepared by one-step solvothermal method.The results of X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,energy dispersive spectrum（EDS）,X-ray photoelectron spectroscopy（XPS）,vibrating sample magnetometer（VSM）and N₂ adsorption-desorption show that the prepared magnetic materials had complete crystal structure,with a specific surface area of 78.07 m²/g.Through batch adsorption experiment,it was found that MGL had good adsorption efficient for Cu²⁺,Pb²⁺,and Cd²⁺in water.The optimum adsorption conditions were as follows:dosage of 0.07 g;adsorption equilibrium time of 240 min;and without adjusting solution pH(Pb²⁺:5.37,Cu²⁺:5.50,Cd²⁺:5.85).The adsorption kinetics data accorded with the pseudo-second-order kinetics equation,and the adsorption isotherms data all fitted with the Langmuir model.The maximum adsorption amounts of MGL for Cu²⁺,Pb²⁺,and Cd²⁺were 23.04 mg/g,192.3 mg/g,and 45.05 mg/g,respectively,which indicated that the adsorption ability of the material for heavy metals was Pb²⁺>Cd²⁺>Cu²⁺.The main adsorption mechanisms of MGL on Cu²⁺,Pb²⁺and Cd²⁺were as follows:（i）complexation with oxygen-containing functional groups on the surface of materials;（ii）precipitation of heavy metal hydroxides or carbon oxides on the surface of materials;（iii）isomorphism of Mg²⁺by Cu²⁺,Pb²⁺,and Cd²⁺.（2）Fe₃O₄@GL were successfully synthesized by co-precipitation method and characterized by XRD,FTIR,SEM,TEM and XPS.By comparing the adsorptivity of LDH,graphene oxide/LDH（GL）and Fe₃O₄@GL on Cu²⁺,Pb²⁺,and Cd²⁺,the results show that both GL and Fe₃O₄@GL had good adsorption efficient.The adsorption capacity GL was slightly higher than that of Fe₃O₄@GL,but Fe₃O₄@GL can be rapidly separated from the mixture by using external magnet,increasing the practicability of the adsorptive material.The optimal amount of adsorbents was 0.05 g and the pH value did not need to be adjusted.The adsorption equilibrium was achieved when the reaction time was 240 min.The adsorption process of the two materials was in accordance with the pseudo-second-order kinetics equation.The adsorption isotherms data of GL and Fe₃O₄@GL for Cu²⁺and Cd²⁺were more consistent with the Freundlich model,however the adsorption process for Pb²⁺accorded with the Langmuir model.The maximum adsorption capacities of GL and Fe₃O₄@GL for Cu²⁺,Pb²⁺and Cd²⁺were as follows:89.26 mg/g,227.0 mg/g,76.67 mg/g and 80.72 mg/g,213.9 mg/g,70.26 mg/g.The adsorption mechanisms were as follows:（i）surface complexation of hydroxyl groups with heavy metal ions;（ii）formation of PbCO₃ and CdCO₃ precipitation between heavy metal ions and carbonate ions.