Preparation Of Walnut Green Peel-based Porous Carbon And Its Adsorption Performance For Metal Ions In Wastewater

New energy and electric vehicles are becoming more and more popular,but the process of recycling waste NCM811 batteries will produce a large amount of high and low concentrations of Ni²⁺,Co²⁺and Mn²⁺wastewater.Electroplating wastewater contains a large amount of Zn²⁺.These heavy metal ions can cause serious biological poisoning.Biomass is an excellent carbon source material.A suitable carbon source should be chosen to prepare carbon materials with high adsorption capacity and high removal rate for Zn²⁺,Ni²⁺and Co²⁺by a series of synthesis methods,which provides reliable support for removal of heavy metal ion pollution in industry.In this paper,walnut green peel was used as raw material to prepare walnut green peel-based biomass charcoal by high-temperature pyrolysis.SEM,FTIR,BET,XPS,XRD,VSM and ZETA potential were used to characterize carbon materials before and after adsorption of metal ions,and their specific surface area and pore size distribution,micro morphology,surface charge properties and surface organic functional groups were analyzed.By studying the adsorption isotherms of carbon materials for Zn²⁺,Ni²⁺and Co²⁺,adsorption kinetics,adsorption thermodynamics,ion competition reactions,fixed bed adsorption and carbon regeneration,the adsorption mechanism of carbon materials and the feasibility of industrial applications are explored.The main results are as follows:（1）The walnut green peel-based hierarchical biomass charcoal（HBCx）was prepared from walnut green peel with hydrothermal pretreatment by high temperature pyrolysis（700℃,750℃,800℃）.The morphology of HBCx is an irregular block structure with a large number of pores distributed on the outer surface.The pore volume and specific surface area of HBC₈₀₀are 0.0710 cm³/g and 94.39 m²/g,respectively.It has a hierarchical porous structure;HBCx is rich in containing N,O surface functional groups,in which the surface-OH group participates in the ion exchange process;adsorption isotherms and kinetics show that the adsorption process can be reflected by the Langmuir isotherm and the pseudo-secondary kinetic model better,respectively.The maximum adsorption capacity for metal ions on HBC₈₀₀is in accordance with the following sequence:Zn²⁺（249.38 mg/g）>Ni²⁺（128.21 mg/g）>Co²⁺（115.74 mg/g）.The adsorption process of metal ions on HBC₈₀₀are mainly controlled by chemical adsorption.The adsorption thermodynamics result shows that the adsorption process is spontaneous.（2）The hierarchical S,N-doped biomass carbon charcoal（HSBCx）was prepared from walnut green peel as the carbon source and thiourea as nitrogen and sulfur source with hydrothermal pretreatment by high temperature pyrolysis（700℃,750℃,800℃）.The morphology of HSBCx is an irregular block porous structure.The pore volume and specific surface area of HSBC₈₀₀are 0.20 cm³/g and 194 m²/g,respectively;HSBCx is rich in surface functional groups containing N,O and S,in which the surface-OH group participates in the process of ion exchange,and the N and S functional groups can promote the adsorption of metal ions;the pH_PZCof HSBC₈₀₀is 3.12;the adsorption isotherm and kinetics show that the adsorption process can be reflected by Langmuir isotherm and the pseudo-secondary kinetic model better,respectively.The maximum adsorption capacity for metal ions on HSBC₈₀₀is in accordance with the following sequence:Zn²⁺（292.40 mg/g）>Ni²⁺（170.36 mg/g）>Co²⁺（152.91 mg/g）.The adsorption process of metal ions on HSBC₈₀₀are mainly controlled by chemical adsorption.（3）The magnetic biomass carbon（MBCx）was prepared from walnut green peel with hydrothermal pretreatment mixed by ferric nitrate uniformly in the ratio of 15:7,15:9,15:11 by high temperature pyrolysis（800℃）.The surface of MBCx has a large amount of aggregates which may be a mixture of Fe⁰or Fe₃O₄.The pore volume and specific surface area of MBC₉are 0.28 cm³/g and 249 m²/g,respectively.It has hierarchical porous structure;MBCx is rich in the surface functional groups containing N and O,in which the surface-OH group participates in the ion exchange process.Oxide-N group shows high adsorption selectivity for Zn²⁺and Ni²⁺.pH_PZCof MBCx is 8.90;XRD results show that the crystal phase structure of Fe in MBCx is dominated by Fe⁰.The saturation magnetization of MBC₁₁are as high as 37.76 emu/g.The adsorption process can be reflected by the Langmuir isotherm and the pseudo-second-order kinetic model better,respectively.The maximum adsorption capacity for metal ions on MBC₉is in accordance with the following sequence:Zn²⁺（263.16 mg/g）>Ni²⁺（137.36 mg/g）>Co²⁺（130.38 mg/g）.The adsorption process of metal ions on MBC₉are mainly controlled by chemical adsorption.The adsorption thermodynamics result shows that the adsorption process is spontaneous.The adsorption thermodynamics,fixed-bed,competition adsorption for metal ions and carbon regeneration were also carried out for the as-made materials.The results show that the adsorption process is a spontaneous.The results show that HBCx,HSBCx and MBCx can be used to treat the wastewater containing Zn²⁺,Ni²⁺and Co²⁺as high-efficiency and recyclable adsorbents.MBCx can be separated from water quickly through the action of a magnetic field.The synergistic effect of ion exchange,co-precipitation,surface complexation,pore filling effect and electrostatic adsorption may be the adsorption mechanism of Zn²⁺,Ni²⁺and Co²⁺on HBCx,HSBCx and MBCx.