Study On Passivation Efficiency And Mechanism Of Iron-sulfur Codoped Biochar For Arsenic And Lead

With the rapid development of smelting,mining,electroplating and other industries and the excessive use of pesticides and fertilizers in agricultural production activities,various heavy metals,which are difficult to be degraded naturally,have been released to the environment.Among them,the pollution of arsenic（As）and lead（Pb）is particularly prominent.As and Pb pollution in soil can not only affect the soil fertility,but also amplify its biological toxicity by the food chain,thereby posing an enormous risk to human health.In the As-Pb co-polluted soil,the differences in the properties and pollutant behavior of As and Pb lead to more complicated soil pollution and make it more difficult to remediate.Therefore,it is necessary to find a material that can improve the remediating performance effectively.In this thesis,rice straw was used as the material to prepare iron-sulfur codoped biochar（Fe/S-BC）by stirring and impregnation method.The adsorption characteristics and mechanism of Fe/S-BC in As-Pb co-polluted water was investigated.In addition,Fe/S-BC was applied to the passivation of As-Pb co-polluted agricultural soil,and its passivation efficiency and mechanism were explored,providing technical reference for the remediation of compound heavy metal-contaminated soil.The mainly conclusions were as follows:（1）Fe/S-BC was successfully prepared by impregnation calcination method.Scanning electron microscopy（SEM）,the Brunauer-Emmett-Teller（BET）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）were used to characterize the morphology,structure and elemental composition.As the result,compared with BC,Fe/S-BC had larger specific surface area and more functional groups.（2）The adsorption mechanism of Fe/S-BC for As（III）and Pb（II）in composite solution was investigated by adsorption kinetics,adsorption isotherms and thermodynamics calculations.The results show that the adsorption was mainly controlled by chemisorption and was a spontaneous endothermic process.The Freundlich modle fitted the removal procedure of As（III）on Fe/S-BC better,while the Langmuir model could better clarified the removal procedure of Pb（II）on Fe/S-BC.In combined pollution system,the optimal removal capacities of Fe/S-BC for As（III）and Pb（II）were 91.2 mg/g and 631.7 mg/g.Competitive and synergistic effects existed simultaneously in the co-contamination system.The mechanism of As（III）removal could be illustrated by surface complexation,oxidation and precipitation.In addition to precipitation and complexation,the elimination mechanism of Pb（II）also contained ion exchange and electrostatic interactions.（3）The remediation effect of Fe/S-BC on As-Pb co-contaminated farmland soil was investigated by soil culture in laboratory.In order to simulate operations such as roasting and flooding in the process of planting crops in actual farmland,aerobic and anaerobic comparative cultivation was carried out.It was showed that the addition of Fe/S-BC to the soil would affect the physicochemical properties such as p H,Eh,SOM,CEC,and nitrogen（N）,phosphorus（P）and potassium（K）in the soil.The soil p H value with the addition of Fe/S-BC increased at the beginning and then stabilized over time.Fe/S-BC could steadily increase the Eh in aerobic environment while make it decrease in anaerobic conditions.The content of SOM,CEC,N,P and K of soil could be affected by the addition of Fe/S-BC in both aerobic and anaerobic treatments,improving soil fertility effectively.（4）The addition of Fe/S-BC transformed As and Pb in soil from active state to more stable form.The passivation efficiency of Fe/S-BC for As and Pb,which was higher in aerobic environment,increased over time.In aerobic environment,Fe/S-BC could better promote the conversion exchangeable stage and specifically sorbed stage to amorphous Fe oxides bound stage for As,meanwhile,convert exchangeable stage,carbonate stage to Fe-Mn oxides stage and residual stage for Pb.In anaerobic conditions,Fe/S-BC could better promote the conversion exchangeable stage to amorphous Fe oxides bound stage and crystalline Fe oxides bound stage for As,meanwhile,convert exchangeable stage,carbonate stage to Fe-Mn oxides stage for Pb.The passivation efficiency of As and Pb in soil was effectively improved by the mechanism mentioned above.