Soil Immobilization Technology For Heavy Metal Lead Pollution

With the advancement of China’s modernization in recent years,the agriculture in China has made great achievements.However,soil pollution,especially heavy metal pollution,will greatly reduce the grain output of the country,seriously threaten the food safety and harm people’s health.According to statistics,about 5 million tons of lead（Pb）are emitted into the environment globally every year,and about one fifth of arable land in China is contaminated with heavy metal lead.In-situ immobilization,due to its simple operation,low cost,and short remediation period,will become the most promising remediation method in the future.Among them,the most promising method for soil remediation is to treat lead pollution in the soil environment by adding phosphate to react with Pb in the soil to generate lead phosphate minerals through precipitation reactions.Among lead phosphate minerals,pyromorphite has the minimum solubility and is the most stable mineral.Therefore,the purpose of this paper is to study the application of phosphate-based immobilizing agents to prepare pyromorphite for the remediation of lead pollution in soil.To achieve this purpose,two curing agents,potassium dihydrogen phosphate（soluble phosphate）and hydroxyapatite（insoluble phosphate）,were firstly selected in this paper to investigate the feasibility of repairing remediation contamination by forming pyromorphite,and then to explore the effect of each influencing factor（time,p H,amount of added P and Cl,temperature）on the effect of lead cure,and to screen out the best curing agents.Then,the optimal process parameters for immobilizing and removing lead were determined using response surface analysis.Finally,based on the optimal process,the immobilization characteristics and mechanism of lead pollution in soil were studied,including extraction toxicity,content of available heavy metals,analysis of chemical forms of heavy metals,remediation effect,etc.The stability of synthesized pyromorphite was also analyzed.The main research contents and conclusions are as follows:（1）The two phosphate-based curing agents selected in this paper,potassium dihydrogen phosphate and hydroxyapatite,could both generate stable pyromorphite in the presence of Cl^-to achieve immobilized remediation of lead contamination.The results showed that the presence of OH^-hindered the curing of Pb²⁺in solution.With the addition of P,the curing rate of both curing agents for lead gradually increased.With the addition of Cl,the curing rate of potassium dihydrogen phosphate for lead gradually increased,while the curing of hydroxyapatite for lead was first increased and then decreased.The increase in temperature promoted the curing effect of hydroxyapatite on solution lead,while potassium dihydrogen phosphate had less effect on the curing of solution lead.The optimal reaction times for curing Pb2+in solution were respectively 30 min and 60 min for potassium dihydrogen phosphate and hydroxyapatite,with ideal p H values of 6 and 3,and the maximum curing rates of 96.43%and 73.25%respectively.?（2）The effects of time,p H,P/Pb and Cl/Pb on the curing efficiency of Pb were optimized by response surface analysis.The results showed that the significant factors affecting the curing efficiency of Pb were in the order of P/Pb>Cl/Pb>p H>time.At the same time,the optimal parameters determined by the model were:time of 40 min,p H of 6.0,P/Pb of 1.0 and Cl/Pb of 0.6.The predicted curing efficiency of Pb by the model was 99.18%,which was close to the experimental result of 99.32%,indicating that the process parameters optimized by the model were credible.（3）It was found that the lead-polluted soil treated with potassium dihydrogen phosphate and potassium chloride could be remediated by generating stable pyromorphite.Toxicity characteristics leachate procedure（TCLP）and Ca Cl2 extraction experiment showed that adding potassium dihydrogen phosphate and potassium chloride can effectively reduce the leaching concentration of Pb in soil,with immobilization efficiencies ranging from 80.3%to93%and 87%to 96.6%,respectively,and meeting the limit standards of the“Identification Standards for Hazardous Wastes-Identification for Extraction Toxicity”（GB 5058.3-2007）.The BCR sequential extraction method was used to study the morphological changes of lead in soil.It was found that with the increase of potassium dihydrogen phosphate and potassium chloride,the content of weak acid extractable,reducible,and oxidizable lead gradually decreased,which were transformed into residual lead.（4）The effects of different p H values and organic acids（oxalic acid,citric acid,malic acid,acetic acid and mixed acid）on the stability of pyromorphite were studied.The results showed that different p H values and organic acids did not change the phase composition of pyromorphite,but lead oxalate was found after high-concentration oxalic acid is used to react with pyromorphite.H⁺in acid environment accelerated the dissolution of pyromorphite,and the content of lead and phosphorus decreased with the increase of p H value.However,when p H was 4（close to the minimum p H of real soil）,the dissolution rate of pyromorphite was pretty low,which could help to realize its long-term stability in soil.The increase in content of organic acid promoted the dissolution and release of lead and phosphorus,and the specific degree of effects was in the order of oxalic acid>mixed acid>citric acid>malic acid>acetic acid.To sum up,this paper successfully obtained pyromorphite crystals through artificial simulation of lead-containing wastewater,and realized the removal of lead pollution in wastewater.At the same time,through experimental analysis and response surface analysis,the optimal immobilization process parameters of lead were established,the in-situ immobilization of lead contaminated soil was realized,and the stability of pyromorphite was analyzed.This paper lays a foundation for the low-cost and high-efficiency treatment of heavy metal lead pollution.