The Study On Stabilization And Mechanism Of Biochar-based Composites On Lead And Cadmium Co-contaminated Soil

Soil pollution has become increasingly prominent,and the remediation of contaminated soils has attracted great attention of the state and society.Baotou City is a typical heavy industrial city and the soil quality has declined seriously with the rapid industry development.As a new type material for contaminated soil remediation,biochars has attracted extensive attention of environmental workers.However,the study of biochars on stabilization remediation for soil heavy metal pollution pays more attention to its application effect in acidic soils,less attention has been paid to the effect and mechanism of stabilization in alkaline soils.The biochar-based composites which prepared by introducing metal oxides and other functional materials has been proved to be able to improve the adsorption performance of raw biochar to heavy metals such as lead and cadmium.However,the study on biochar-based composites were focused on the removal of heavy metals from solution,and the application effect and mechanism in soil was relatively few.Based on these,two new biochar-based composites were prepared derived corn straw.On the basis of discussing their adsorption properties and mechanism for Pb and Cd in solution,two new biochar-based composites were applied to the remediation of weak alkaline Pb-Cd co-contaminated chestnut soil in Baotou City.The stabilization effects on Pb and Cd were investigated and the mechanism were discussed.This study is expected to achieve the dual objectives of contaminated soil remediation and waste recycling,and provide technical reference for the soil remediation contaminated by heavy metal in Baotou.This paper includes the following study:1.The heavy metals pollution in soil of Baotou area and two typical non-ferrous metals smelting enterprises was investigated and evaluated.The results showed that heavy metals contaminated soil of Baotou City was in a“medium”level of potential ecological crisis,in which the pollution of industrial zone was more serious,and lead and cadmium were the main elements.The content of Pb and Cd in the soil around two typical non-ferrous metals smelting enterprises exceeded the standard at the sampling points in this study,reaching the level of moderate or severe pollution.2.A novel composite material of amorphous Fe-Mn binary oxides-loaded biochar?BFM?was successfully fabricated by the route of impregnation/pyrolysis.The structure and morphology of BFM were characterized by using SEM-EDS,BET,XRD,FTIR and XPS.Batch adsorption experiments were carried out to investigate the effect of BFM dosages and solution pH on the adsorption of Pb²⁺and Cd²⁺by BFM.the adsorption characteristics of Pb²⁺and Cd²⁺were studied by kinetics,isotherms and thermodynamics,and the adsorption mechanism was discussed.The results showed that BFM had strong adsorption ability to Pb²⁺and Cd²⁺,and the maximum adsorption capacities is 113.715and 109.031 mg/g at 25?,respectively.The adsorption of Pb²⁺and Cd²⁺onto BFM followed the pseudo-second-order kinetics equation and Langmuir isotherm,indicating that the adsorption process was a mono-molecular chemical adsorption.?G<0 and?S>0,indicating a spontaneous and endothermic adsorption.Competitive adsorption occurs when Pb²⁺and Cd²⁺coexist,and the adsorption of Pb²⁺by BFM takes precedence over that of Cd²⁺.The main removal mechanisms of BFM for Pb²⁺and Cd²⁺are the complexation of oxygen-containing functional groups?-OH,-COOH?and Fe-O,Mn-O with Pb and Cd,as well as ion exchange.The BFM is a potential adsorption material for the efficient removal for Pb²⁺and Cd²⁺.3.A novel biochar-magnetic MnFe₂O₄ nanocomposite?BC/FM?was prepared by sol-gel/pyrolyzing route.Following that,its composition,morphology and structure was characterized by various techniques including SEM-EDS,BET,XRD,FTIR and XPS.Batch adsorption experiments were carried out to investigate the influencing factors and adsorption characteristics of BC/FM for Pb²⁺and Cd²⁺.The results demonstrated that biochar could effectively support MnFe₂O₄ which displayed high dispersion on the surface of biochar.The adsorption process was described well by the pseudo-second-order kinetic model and Sips?Freundlich-Langmuir?model.Thermodynamic studies suggested that the process was spontaneous and endothermic.The maximum adsorption capacity of BC/FM was 154.94 and 127.83 mg/g for Pb???and Cd???,respectively,which was higher than that of some other adsorbents of biochar or biochar-based composite.In bi-solute system,the preferential adsorption order of BC/FM for two metals was Pb???prior to Cd?II?.The main mechanism of Pb???and Cd???removal by BC/FM was formation of Pb/Cd-O or complexation of carboxyl and hydroxyl and ion exchange.Therefore,the prepared magnetic BC/FM composite is an excellent adsorbent for Pb???and Cd???removal.4.In order to study the effects of BC,BFM and BC/FM on the remediation of weak alkaline Pb-Cd co-contaminated chestnut soil,the effects of BC,BFM and BC/FM on soil pH,cation exchange capacity?CEC?,organic matter?SOM?and soil electric conductivity?EC?,and the fraction distribution change of Pb and Cd and TCLP leaching toxicity were studied by soil culture experiment.The results showed that the application of BC,BFM and BC/FM significantly increased soil pH,CEC and SOM,and effectively promoted the transformation of Pb and Cd from bioavailable to stable state.The contents of available Pb and Cd were negatively correlated with soil pH,SOM,CEC and EC.Three materials can reduce the TCLP-extracted content of Pb and Cd in soil effectively.The immobilization ability showed BC/FM>BFM>BC,and the immobilization effect of Pb is better than that of Cd.The stabilization mechanism of BC,BFM and BC/FM for Pb and Cd in the weak alkaline Pb-Cd co-contaminated chestnut soil is mainly complexation,at the same time,ion exchange and precipitation is existed.The application of BC,BFM and BC/FM realizes the remediation for the weak alkaline Pb-Cd co-contaminated chestnut soil.5.In order to further analyze the effects of BC,BFM and BC/FM on the availability of Pb and Cd in soil,and study the behavior of Pb and Cd in plant-soil system,the effects of BC,BFM and BC/FM on the growth and heavy metal uptake of Maize under Pb-Cd stress were studied by pot experiment.The results showed that BC,BFM and BC/FM effectively promoted the growth of maize and reduced the absorption of Pb and Cd.The overall effect was BC/FM>BC>BFM.The three materials are expected to be applied to stabilization remediation of Pb-Cd contaminated soils.