Immobilization Remediation Of Cd And Pb In Contaminated Soil From Mining Areas With Biochar And Its Modified Materials

The heavy metal pollution has attracted more attention recently,especially in soil from mining areas with the process of mining,stacking tailings and smelting metals.Its remediation has become a topical aspect of environmental protection.Immobilization remediation is highly regarded because of its numerous advantages,such as convenient operation,obvious effect,low cost,and so on.The adsorption experiments and soil culture experiments were carried out with the biochar and its modified material as immobilization addition.The aim in this study was to enhance the immobilization effects in soil contaminated by heavy metals.The characteristics were studied about adsorption of biochar and its modified materials,including potassium phosphate modified biochar（BCKP）,magnesium chloride modified biochar（BCMg）and fly ash（FA）,on Cd²⁺and Pb²⁺in solution.The immobilization effects of the materials on Cd and Pb in soil were analyzed under several typical conditions.Finally,the the action mechanism and the responses of two enzymes concered with these effects were analyzed.The main results of the research are as follows:（1）Modification with potassium phosphate and magnesium chloride enhanced the adsorption of biochar on Cd²⁺and Pb²⁺.The adsorption capacities of BCKP on Cd²⁺and Pb²⁺were the highest in the coexistence solution of Cd²⁺and Pb²⁺,which were9.82 mg/g and 118.60 mg/g,respectively.The modification of phosphoric acid and ferric chloride reduced the adsorption capacity of biochar to Cd²⁺and Pb²⁺.The adsorption of Cd²⁺and Pb²⁺by materials conformed to the pseudo-second-order kinetic model.（2）In terms of the mechanism of action between biochar and heavy metals,the specific surface area of BCKP increased by 29.4%compared with BC,and the increase of phosphate groups promoted the adsorption and fixation of heavy metals.BC mainly immobilize Cd²⁺and Pb²⁺in the form of sulfate precipitation,while BCKP mainly precipitated Cd²⁺and Pb²⁺in the form of phosphate.（3）The application of BC,BCKP,BCMg and FA could promote the adsorption and retention of soil on Cd²⁺and Pb²⁺.The adsorption capacity of the soil treated with BCKP on Cd²⁺was 0.69 mg/g,and the one of the soil treated with FA on Pb²⁺was 4.14mg/g.The presence of Pb²⁺significantly inhibited adsorption of soil on Cd²⁺.Under acidic conditions,increasing p H was beneficial to the improvement of adsorption of soil treated with BCKP and FA on Cd²⁺and Pb²⁺,and was always better than BC and BCMg treatments.（4）The passivation effect of materials on soil Cd were FA,BCKP,BCMg and BC in descending order,and the passivation effect on Pb were BCKP,BC and BCMg in descending order.the addition,FA increased the risk of leaching toxicity of Pb in soil.The mixed addition of biochar and fly ash made the TCLP extract content of soil Cd reach a relative stability 30 in advance,compared with the addition of each material alone.（5）The residual content of Cd and Pb in the soil with BCKP was increased by72.2%and 35.7%,respectively.BCKP promoted the transformation of Cd and Pb from weakly acid-soluble,reducible and oxidizable states to residual states.Biochar and its modified materials both promoted the activities of soil urease and alkaline phosphatase,but the addition of FA had no significant effect on them.（6）For Cd and Pb composite polluted soil,when Pb is the passivation target,the application dosage is 3%BCKP with a particle size of 60 mesh is the best;when Cd is the passivation target,the application dosage is 1%BCKP with a particle size of 20mesh is the best.When Cd and Pb are regarded as passivation targets at the same time,the optimal application formula is consistent with that of Cd.