Preparation Of Modified Metal Organic Framework Derived Carbon Materials And The Reduction And Adsorption Properties For Cr (Ⅵ)

With the rapid development of society,compounds containing Cr（Ⅵ）have been widely used in industrial production.Cr（Ⅵ）compounds containing strong toxicity are exposed to the environment and enter natural water bodies through rainfall and waste water discharge,causing great harm to the environment.Many methods have been developed to remove Cr（Ⅵ）from wastewater,including adsorption,chemical reduction,biological processes,photocatalysis,etc.Each method has its advantages and disadvantages,but most of the removal of Cr（Ⅵ）is limited to a single removal or reduction,and there are relatively few technologies for the reduction and removal of Cr（Ⅵ）together.Metal organic framework（MOF）is widely used as a precursor for its high specific surface area,orderly structure,high porosity and controllable pore size.MOF-based materials have great application prospects in the field of adsorption of pollutants.In this paper,MOF（MIL-53（Fe））was selected as the precursor,different amounts of Cu were added to MIL-53（Fe）,and a series of bimetallic@porous carbon materials were successfully prepared by in-situ transformation at different temperatures under nitrogen atmosphere.The introduction of Cu,the effects of different Cu/Fe and calcination temperatures on reduction and adsorption of Cr（Ⅵ）were investigated.MIL-53（Fe）was selected as the precursor,and divalent copper ions of different proportions were added to MIL-53（Fe）by dipping method.XCu@FexC-800 was obtained by pyrolysis in nitrogen at 800℃.After calcining,Cu⁰ and Fe⁰ coexisted in the material,and different Cu/Fe almost had no effect on the morphology of each element in the material.With the increase of the amount of Cu added,the material showed that the collapse gradually occurred,the porosity and specific surface area decreased,the Cu agglomeration was serious,and the proportion of O content gradually decreased,but the overall morphology remained.The addition of copper increased the adsorption site of the material and enhanced the adsorption effect of Cr（Ⅵ）.However,as the Cu continued to increase,the surface area and pore volume of the material decreased,resulting in the adsorption amount of Cr（Ⅵ）no longer increased.The redox reaction between Cu and Fe accelerates the electron cycle and improves the redox capacity.Cr（Ⅵ）can be completely reduced within 5min.The experiment shows that 1/2Cu@FexC-800 has the best reduction and adsorption effect,so 1/2Cu@MIL-53（Fe）is used for the next experiment.In order to explore the influence of calcining temperature on materials,1/2Cu@MIL-53（Fe）was calcined in nitrogen at different temperatures from 700℃to 900℃.With the increase of calcining temperature,the purer the Cu⁰ and Fe⁰ obtained,the better the reduction effect of Cr（Ⅵ）.With the increase of temperature,the content of oxygen vacancy（the defective structure of porous carbon）and surface functional groups increased,the C-C fracture in Sp²-C resulted in the content decreased.High temperature calcining reduced the coordination carboxyl group of Fe³⁺ions,the symmetrical（C-O）bond of the carboxyl group and the intermediate dicarboxylate ligand,but the calcining temperature had no effect on the functional group morphology of the material.The high temperature leads to the slight collapse of the frame structure and the agglomeration of metal ions.The experiment shows that 1/2Cu@FexC-800has the best adsorption effect on Cr（Ⅵ）,and 1/2Cu@FexC-900 can completely reduce Cr（Ⅵ）at the moment when the material contacts the target solution,and the reduction effect on Cr（Ⅵ）is the fastest.However,materials obtained at different calcination temperatures can eventually completely reduce Cr（Ⅵ）.Considering comprehensively,the next experiment was carried out by using 1/2Cu@FexC-800.The reduction and adsorption of Cr（Ⅵ）by 1/2Cu@FexC-800 are greatly affected by pH,and pH=3 is the most suitable reduction and adsorption condition.Acid conditions can provide H⁺for reduction,and the rapid corrosion of Fe⁰ and iron oxide leads to the formation of Fe²⁺on the surface of NZⅥ,while providing a large number of electrons for reduction reaction.At this time,Cr is in the most easily adsorbed form of HCrO₄,but too low pH（pH=1）leads to material digestion and structure destruction.The removal capacity of Cr（Ⅵ）by 1/2Cu@FexC-800 increased with the increase of the initial concentration of the solution.Adsorption isotherm and adsorption kinetics of reaction model analysis,the results show that 1/2Cu@FexC-800 to remove Cr（Ⅵ）the reaction,the Langmuir model（R²=0.977）can better describe the adsorption results concentration research scope,to show that the reaction to ignore the interaction between adsorption layer and the interaction between metal ions and its adsorption characteristics of monolayer coverage of maximum adsorption reached 725 mg g^-1.The adsorption kinetics of Cr（Ⅵ）by materials follows the pseudo-second-order model（R²=0.99）.The adsorption kinetics of Cr（Ⅵ）by adsorbents are affected by the diffusion in the membrane and the binding of metal by the surface adsorption sites.The adsorption kinetics of copper-doped materials after calcination is faster,and the main reaction resistance may be related to the adsorption kinetics.The adsorption reaction is controlled by three successive steps,namely the instantaneous adsorption phase,the mesoporous diffusion phase and the microporous diffusion phase.