Preparation Of Modified Biochar Adsorbents And The Removal Performance And Mechanisms Of Cr(Ⅵ)

With the rapid development of social economy,various human activities such as the unstoppable industrialization process and modern agricultural practices have discharged toxic heavy metals into the environment,resulting in environmental pollution.Hexavalent chromium（Cr（Ⅵ））pollution is the most serious environmental pollution problem of various heavy metals,because Cr（Ⅵ）cannot be naturally decomposed and also has migration ability,which can threaten the survival of animals and plants,and human health.Therefore,the problem of Cr（Ⅵ）pollution urgently needs to be resolved.Among various methods and materials used to reduce Cr（Ⅵ）pollution,biochar is a promising biomass-derived functional material that can be used as a low-cost environmentally friendly adsorbent on Cr（Ⅵ）removal.The important aspect of the ability of Cr（Ⅵ）effectively adsorption by biochar is owing to its tunable surface functional groups and structures.In this study,biochar was modified by iron doping,enrichment of oxygen-containing functional groups and introduction of a large number of amino groups to improve its surface properties.The behavioral mechanism and performance of modified biochar on Cr（Ⅵ）removal from aqueous solutions were investigated.The surface properties of modified biochar were studied using characterization methods such as SEM,FTIR,XRD,and XPS.The main research contents and results are as follows:（1）Tea straw biochar loaded with nano-zero-valent iron（800n ZVI/BC）was prepared by carbon thermal method,the maximum Cr（Ⅵ）adsorption capacity on 800n ZVI/BC was 48.45 mg/g.800n ZVI/BC had a specific surface area of 305.26 m²/g and rich pore structures,which could provide more active sites for pollutant adsorption.The Cr（Ⅵ）removal efficiency could reach 99.84%within 30 min.In addition,Its removal efficiency of Cr（Ⅵ）could still reach 87.99%and66.99%after aging in air and simulated groundwater environment for 21 days.The fitting results of kinetics and thermodynamics indicated that the adsorption process involved chemical reactions,which was monolayer adsorption.After material characterization and adsorption performance study,the process of its removal of Cr（Ⅵ）was obtained,including:Electrostatic attraction,reduction and complexation of Cr（Ⅵ）by Fe⁰ and Fe O,provision of electrons by C-O functional groups,and complexation precipitation of Cr（III）with Fe（III）.In this study,we have found that tea stem biochar was not only a stabilizer and dispersant of nano zero valent iron,but also an electron donor.（2）Tannic acid modified chestnut shell based porous carbon（T-PBC）was prepared at room temperature,and its maximum adsorption capacity for Cr（Ⅵ）was 218.71 mg/g.The complex pore structure of T-PBC also meat that it had more contact sites with pollutants.The removal rate of Cr（Ⅵ）from the stock solution of electroplating wastewater by T-PBC was to 83.04%.The kinetic and thermodynamic fitting results indicated that chemical adsorption dominated the adsorption process.Electrostatic attraction,the phenolic hydroxyl group on T-PBC having coordination with HCr O₄^-and providing electrons for its reduction were the main mechanisms of Cr（Ⅵ）removal on T-PBC.（3）Amino functionalized porous carbon（P₁-PBC）was prepared by cross-linking with polyethyleneimine,and its maximum adsorption capacity for Cr（Ⅵ）was 193.05 mg/g.When the p H of the solution changes from acidic to neutral,the adsorption capacity of P₁-PBC for Cr（Ⅵ）decreased,but the rate of decrease was relatively slow.By contrast,It was noteworthy that the dependence of P₁-PBC on solution p H was reduced.The removal rate of Cr（Ⅵ）from the stock solution of electroplating wastewater by P₁-PBC was 85.48%.The kinetic and thermodynamic fitting results showed that the adsorption process was chemical adsorption.The main removal mechanisms of Cr（Ⅵ）removal of P₁-PBC were that the protonated amino group increased the electrostatic attraction between P₁-PBC and HCr O₄^-,and the reduction of Cr（Ⅵ）mainly provided electrons through the amino group.From the adsorption results,there were different degrees of improvement in surface properties and in the adsorption capacity of Cr（Ⅵ）compared with that before heteroatom doping.（4）In general,800n ZVI/BC has the fastest reaction rate,T-PBC has the largest adsorption capacity,and P₁-PBC has the smallest dependence on p H.From the adsorption results,the surface performance and Cr（Ⅵ）adsorption capacity of the three adsorbents have been improved to varying degrees compared to before the addition of exogenous substances.This study provided modification ideas in surface optimization of biochar modified adsorbents for Cr（Ⅵ）and adsorption performance enhancement,and provided some reference for the application of biochar modified materials through the material characterization and adsorption result analysis.