Research On The Preparation Of Biochar From Ramie And Its Application On The High-efficiency Adsorption Of Lead And Cadmium In Water

With the acceleration of urbanization and industrialization,a large number of heavy metals（HMs）are discharged into the water,which has caused serious damage to the water environment.In addition,lead(Pb²⁺)and cadmium(Cd²⁺)cause particularly serious water pollution problems due to their widely used,bio-accumulative and highly toxic characteristics.Among the methods of water pollution treatment,adsorption method is favored by researchers because of its high removal efficiency,low input cost and environmental friendliness.With the proposal of"Dual Carbon"strategy,it has great potential and possibility to use agricultural and forestry waste to produce biochar.The application of biochar as adsorptive material in water pollution control has become a research hotspot.In addition,Ramie as a raw material for textiles,it only uses 5%of the whole plant,and the rest is discarded or incinerated,resulting in a large amount of resource waste.Therefore,it is a very meaningful research topic to prepare biochar from waste Ramie and apply it to water pollution control.This study is based on the high-value utilization of hemp straw as a starting point,to test the biomass components and physical and chemical properties of Ramie,In addition,Mg O supported porous biochar（K-Mg O-RB）was prepared from Ramie by alkalization with potassium hydroxide to increase the specific surface area and then loading with magnesium oxide.Modern material characterization techniques such as scanning electron microscopy and X-ray energy spectroscopy elemental analysis（SEM-EDX）,specific surface area analyzer（BET）,X-ray diffraction（XRD）,Raman spectroscopy（Raman）,Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,Zeta potential analysis were used to analyze different biochar Physical and chemical properties.The best biochar（K-Mg O-RB）was selected by comparative experiments,and the adsorption behavior of K-Mg O-RB to remove Pb²⁺and Cd²⁺was studied by adsorption isothermal,kinetic and thermodynamic static adsorption experiments.In addition,the adsorption mechanism of K-Mg O-RB to remove Pb²⁺and Cd²⁺was studied by XRD,FT-IR and XPS,SEM.Finally,through the anti-cation interference ability experiment,the actual wastewater experiment,the circulation experiment and the fixed bed column experiment,the experiment shows that the biochar（K-Mg O-RB）has good practical application performance.The specific research results are as follows:（1）The content of cellulose,hemicellulose and lignin was 41.115%,20.935%and18.405%,respectively.The elemental analysis of hemp stalk shows that it contains very small amount of N and S,and it will not release acidic gas during pyrolysis（NOx and SO₂）,so it is an environmentally friendly biomass raw material.The results of thermogravimetric analysis showed that the burning process of flax culm was divided into three stages.（2）Elemental analysis showed that the O/C and（O+N）/C of biochar increased after alkalization,indicating that KOH activation could increase the hydrophilicity and polarity of biochar.SEM-EDX shows that K-Mg O-RB is a cluster structure with random openings,and a large number of tubular pore structures appear,and Mg elements are uniformly distributed on the carbon material.The BET analysis showed that the specific surface area and pore volume of K-Mg O-RB were 404.5m²/g and0.17cm³/g,respectively,which were mesoporous carbon materials.XRD analysis showed that the four biochars had typical locally ordered structures of carbon materials.Raman analysis showed that the graphitization degree of the four biochars decreased successively,RB>K-Mg O-RB>Mg O-RB>K-RB.FT-IR analysis showed that all four biochars were rich in oxygen functional groups such as-C=O and-OC=O.Zeta analysis shows that the zero charge point of K-Mg O-RB is p H=5.2.When p H>5.2,the surface of K-Mg O-RB is negatively charged,otherwise,it is positively charged.XPS analysis shows that Mg O is successfully loaded on K-Mg O-RB.（3）K-Mg O-RB has good adsorption performance for Pb²⁺and Cd²⁺through batch adsorption experiments.Compared with the original biochar（RB）,Mg O loaded biochar（Mg O-RB）and KOH activated biochar（K-RB）,K-Mg O-RB increased the adsorption capacity of Pb²⁺and Cd²⁺by 18 times and 98 times,6 times and 67 times,0.38 times and 0.32 times,respectively.The adsorption of Pb²⁺and Cd²⁺by K-Mg O-RB conforms to the quasi-second-order kinetic equation and Langmuir model,which indicates that the adsorption process is chemisorption of single molecular layer.In addition,the intra-particle diffusion model shows that the adsorption process is divided into two stages:surface adsorption and intra-particle diffusion.The maximum adsorption amounts of Pb²⁺and Cd²⁺by K-Mg O-RB calculated by Langmuir model were 1276.46 mg/g and 861.82 mg/g,respectively.The thermodynamic results show that the reaction of K-Mg O-RB to Pb²⁺and Cd²⁺is spontaneous endothermic.In addition,the removal of Pb²⁺and Cd²⁺by K-Mg O-RB was not significantly affected by the p H of the solution.Through the characterization before and after adsorption,the adsorption mechanism of K-Mg O-RB on Pb²⁺and Cd²⁺includes precipitation,ion exchange,complexation and electrostatic effect.（4）K-Mg O-RB has practical application in the removal of Pb²⁺and Cd²⁺.K-Mg O-RB can effectively remove Pb²⁺and Cd²⁺in both Pb²⁺and Cd²⁺solutions containing common coexisting cations and in actual water bodies,indicating that K-Mg O-RB has good anti-interference and practicality.The recycling experiments showed that K-Mg O-RB still had good Pb²⁺and Cd²⁺adsorption capacity after four cycles of regeneration,which revealed its excellent recycling.The effective treatment volumes of K-Mg O-RB at initial concentrations of 20 mg/L Pb²⁺and 2 mg/L Cd²⁺were 3480m L and 2520 m L,respectively,indicating that K-Mg O-RB had good dynamic adsorption performance.