Study On Leaching Characteristics Of MICP Solidified Lead-contaminated Soil Under Acid Rain Leaching Condition

Due to the rapid development of China’s social economy,the demand for construction and land is large,heavy metal pollution problem is increasingly serious.At present,compared with other technologies,the Microbially Induced Carbonate Precipitation has lower cost,better effect and less pollution in the treatment of polluted soil.Acid rain is a serious problem in China.Acid rain infiltration not only promotes the release of toxic heavy metals in soil,but also dissolves calcium carbonate.Therefore,it is necessary to study the leaching characteristics of Microbial Induced Calcite Precipitation（MICP）for remediation of heavy metal contaminated soils under the action of acid rain infiltration.In this article,based on the method of Microbially Induced Carbonate Precipitation（MICP）,we adopted Sporosarcina pasteurii to repair the lead-polluted soil,and design the pH value of 3.5,4.5,5.6,3 kinds of typical conditions of acid rain,using deionized water as control of pH value of 7.5,through soil column leaching experiment method,the analysis discusses the lead-contaminated soil with microbial induced carbonate precipitation method in the soil column leaching characteristics of the unconfined compressive strength before and after,the changing rule of the leaching characteristics and microstructure.The main conclusions are as follows:（1）The unconfined compressive strength of MICP solidified lead-contaminated soil decreases with the decrease of pH value of leachate,and increases first and then decreases with the increase of initial Pb²⁺concentration.（2）After the MICP solidified lead-contaminated soil was leached by simulated acid rain solution with a pH value of 3.5,4.5 and 5.6,the pH value of the leached solution decreased with the decrease of the pH value of the leaching solution.When the leached solution’s pH value was 5.6,the leached solution’s pH value was close to that of the deionized water leached sample.When the pH value of the leacheate was 4.5,the pH value of the leacheate of the solidified samples with the initial Pb²⁺concentration of1000mg/kg remained stable with the leacheate.The pH value of the leacheate of the solidified samples with the initial Pb²⁺concentration of 500mg/kg decreased somewhat at the later stage of leacheate,while the pH value of the leacheate of the solidified samples with the initial Pb²⁺concentration of 100mg/kg and 5000mg/kg began to decline at the middle stage of leacheate.The concentration of Pb²⁺and Ca²⁺in leached solution was consistent.With the decrease of pH value of leached solution,the leached amount of Pb²⁺and Ca²⁺gradually increased.At the initial stage of leaching,a large amount of Pb²⁺and Ca²⁺was leached out,but only a small amount of Pb²⁺and Ca²⁺was leached out,and when the initial Pb²⁺concentration of the solidified samples was100mg/kg and 5000mg/kg,the leached amount of Pb²⁺and Ca²⁺was greater than that of the solidified samples with the initial Pb²⁺concentration of 500mg/kg and 1000mg/kg.（3）With the decrease of pH value of leachate,the morphology change of lead ion in MICP solidified samples shows that the exchangeable state increases gradually,while the carbonate binding state,ferric and manganese oxidation state,organic binding state and residue state decrease gradually.（4）By X-ray diffraction（XRD）,scanning electron microscopy and energy spectrum analysis under the action of the acid rain infiltration where solidification sample to study the microscopic structure,found where technology can effectively cure the Pb²⁺,under the effect of acid rain infiltration,with the loss of the simulated acid rain solution pH,calcium carbonate,carbonate lead content of the diffraction peak intensity is reduced,and with the increase of initial concentration of Pb²⁺,the diffraction peak intensity first increases then decreases.When the pH value of leachate is 3.5,4.5,it is difficult to observe the complete crystal of mineralized product.The results are mutually confirmed with the macroscopic property changes.