Study On The Remediation Mechanism Of Cd And Pb In Rape Cultivated Soils By Biochar In Hanzhong Area

Soil heavy metal pollution has produced incalculable harm to human production and life,and the research of soil heavy metal pollution management has become one of the hot topics in current environmental pollution management.In recent years,biochar has become more and more well-known as a new type of soil heavy metal pollution remediation material and physicochemical property improver,but the original biochar has disadvantages such as unstable effect and short duration in the remediation of passivated soil heavy metal pollutants.In order to improve the remediation and treatment of soil heavy metals by biochar,many scholars have used various methods to optimize the performance of the original biochar,thus changing its physicochemical properties to achieve efficient remediation of soil heavy metal pollution.In this study,the following aspects were carried out:（1）The biochar was prepared from rice straw and corn straw,and then its physicochemical properties were optimized using alkali modification and metal modification methods,respectively.（2）To analyze the changes of physicochemical properties of biochar before and after modification by characterizing the properties of biochar（3）To conduct adsorption kinetics and isothermal adsorption tests of biochar on Cd²⁺and Pb²⁺in water bodies,and to investigate the adsorption mechanism by fitting the adsorption model.（4）Combining the adsorption test results with the physical and chemical properties of biochar,the best biochar for adsorption was selected as Fe-Mn-modified biochar,and the stability of Fe-Mn-modified biochar on soil Cd²⁺and Pb²⁺adsorption was investigated.（5）A potting simulation test was used to simulate the process of soil remediation by Fe-Mn biochar,and to investigate the effect of Fe-Mn modified biochar on soil physicochemical properties and its remediation mechanism on Cd and Pb contaminated soil.The conclusions of the study were as follows:（1）The specific surface area,pore size and types of surface functional groups of the modified biochar increased,and the characterization results of rice straw biochar were all better than those of corn straw biochar.The alkali modification and metal modification methods optimized the physicochemical properties of the original biochar to different degrees from different mechanisms of action:the metal modified biochar was loaded with more metal oxides on the surface,which increased the adsorption active sites of the biochar;the alkali modification increased the types of oxygen-containing functional groups on the surface of the biochar,which led to an increase in its specific surface area and pore size.（2）The results of kinetic adsorption experiments showed that the adsorption of Cd²⁺and Pb²⁺by the biochar before and after modification were in accordance with the quasi-secondary kinetic adsorption model and were chemisorption processes.The fitted curves of Weber-Morris model indicate that the adsorption process of Cd²⁺and Pb²⁺by biochar in water includes two stages:adsorption acceleration and adsorption equilibrium,and besides the influence of particle diffusion itself,there are Other factors influence the adsorption process.Through the analysis of the fitted parameters of the adsorption model of Cd²⁺and Pb²⁺in water by biochar,the adsorption effect of iron-manganese composite modified rice straw biochar on Cd²⁺and Pb²⁺in water was better than that of other modified biochar.（3）The adsorption-resolution test showed that Fe-Mn-modified biochar could effectively improve the adsorption immobilization of soil heavy metals Cd²⁺and Pb²⁺,but the excessive concentration of Cd²⁺and Pb²⁺in the soil would increase the resolution rate of Fe-Mn-modified biochar to Cd²⁺and Pb²⁺in the soil,resulting in its lower immobilization efficiency to Cd²⁺and Pb²⁺in the soil.（4）Pot simulations showed that Fe-Mn-modified biochar could enhance the activities of soil peroxidase and urease to some extent,increase the contents of effective phosphorus and organic matter in soil,and could effectively improve the physicochemical properties of Cd and Pb contaminated soil;meanwhile,Fe-Mn-modified biochar could effectively reduce the contents of DTPA-Cd and DTPA-Pb in soil,and by BCR morphological analysis,the contents of Cd and The content of weak acid extractable,reducible and oxidizable states of Pb were reduced to different degrees by BCR morphology analysis,and the content of residue state was increased,indicating that Fe-Mn modified biochar could effectively passivate Cd and Pb elements in the soil.The best passivation effect of Fe-Mn-modified biochar on Cd and Pb was observed at the treatment time of 60 d and 90 d.