Remediation Of Heavy Metals Contaminated Soil By Combined Chemical Leaching And Biotechnology

In this study,chemical leaching and biological technology were combined to repair heavy metal contaminated soil around a lead-zinc mine in Baoji Feng County,Shaanxi Province.Firstly,this paper analyzed the content and morphology of Pb,Cd,Cu and Zn in contaminated soil,then,this paper used EDTA-Na2,citric acid,Ca Cl2,Fe Cl3 and HCl to repair the polluted soil with the method of ectopic chemical leaching and compared the physicochemical properties of soil and the changes of the heavy metal form before and after leaching;after that,used chemical method(lime),biological charcoal(500 ? bagasse)and microbial method(phosphate bacteria OPW2-6)to passivate the residual heavy metals in the soil and selects combined restoration techniques with best passivation effect and nutrient improvement effect by comparing the three kinds of methods;finally,used soil column leaching method to repair contaminated soil in situ(simulated),the chemical method(lime),biological carbon method(500 ? bagasse)and microbial method(sulfate reducing bacteria 2-2-5a)were used for passivation in the position of 30cm-40 cm,and selected the in-situ joint remediation technique with minimum underwater pollution after rinsing from top to bottom.The main achievements of this paper were as follows:(1)Compared with the element background values in shaanxi province,the content of Pb,Cd,Cu and Zn in polluted soil is 123.61,51.02,2.88 and 28.01 times higher,which is heavy pollution;Cd and Zn mainly exist in exchangeable and carbonate binding states,which indicated that their bioavailability is high,while Pb and Zn mainly exist in residue state.(2)After the experiment of ectopic chemical leaching,this paper concluded that Fe Cl3 had the best removal effect on Cu and Cd and the maximum removal rate is 69.34%,65.68% respectively;while the EDTA-Na2 had the best removal effect on Pb and Zn,and the maximum removal rate was 52.62% and 25.76% respectively;because Fe Cl3 leaching had less damage to soil physical and chemical properties and could remove most exchangeable states and carbonate binding states of Pb,Cd,Cu and Zn,this paper selected Fe Cl3 as a high removal rate and environmental friendly washing agent.(3)After optimizing the conditions of FeCl3,leaching time and washing times,this paper concluded that in ectopic chemical leaching,the best concentration of leaching process for Fe Cl3 was 0.3 mol/L,the solid-liquid ratio was 1:7,the leaching time was 4h,and the washing times were 2 times.The removal rate of Pb,Cd,Cu and Zn could reach 55.12%,50.63%,63.58% and 16.6% respectively.(4)After comparing the passivation effect of Biological Carbon method(500 ? bagasse),chemical method(lime)and microbial method(OPW2-6),this paper concluded that the OPW2-6 had the best passivation effect on Pb,Cd,Cu and Zn in soil,the exchangeable states and the carbonate binding states of Pb,Cd,Cu,Zn were decreased by 50.1%,48.72%,57.9% and 37.64% respectively.According to the monitoring of soil nutrients,after adding OPW2-6 to phosphate bacteria,the enzyme activity of soil increased significantly and the p H of soil,content of available potassium and organic matter also increased obviously,which indicated that the strain had better effect on soil acidity,nutrients and enzyme activity,as a result,the effect of different chemical leaching and microbial method on the remediation of soil contaminated by heavy metals was best.(5)Through in-situ simulated elution experiment this paper concluded that,considering the problem of environmental protection and cost,the optimal leaching time was 42h;comparing the passivation effect of adding lime bio-carbon and sulfate-reducing bacterium 2-2-5a in the position of 20-30 cm in the soil,the results showed combined repair of in-situ chemical leaching and deep adding sulfate-reducing bacteria 2-2-5a passivation had the best effect,the content of Pb,Cd,Cu,Zn was decreased by 24.83%,10.52%,5.26% and 8.91% at the position of 70 cm in the soil.