Soil Washing Remediation Of Heavy Meatals From Contained Soil

Chemical washing remediation technology is widely used in heavy metal contaminated soil remediation engineering in China.The selection of eluent is the key in this technology.Single eluent often has certain limitations on the removal of heavy metals,and the combination of eluents with different mechanisms can effectively improve the removal efficiency of heavy metals.In this study,a laboratory study on washing remediation was carried out on the soil of a lead-contaminated plot in Qingpu District of Shanghai and an arsenic-contaminated soil in Jiading District.Different chemical eluents were selected for lead-contaminated soil and arsenic-contaminated soil,respectively.The types of eluents and main wahing parameters were optimized by single factor experiments,and then the response surface analysis method was used to carry out the small-scale test of compound eluents,and the optimal washing parameters were obtained.The reuse effect of compound eluent and the changes of occurrence forms of heavy metals in soil were explored in order to provide experimental basis for the remediation of two heavy metal contaminated plots.The main contents and results of this study are as follows:（1）In the pilot test of single factor washing of lead contaminated soil in Qingpu,the removal rates of lead by the four eluents were as follows:Fe Cl₃>EDTA>HCl>CA,the preferred eluents are Fe Cl₃ and EDTA,the concentration is 0.10 mol·L^-1,the liquid-solid ratio is 10:1,and the washing time is 4 h,the removal rates are 79.69%and72.75%,respectively.In the Jiading arsenic washing experiment,the arsenic removal efficiency of the four eluents is OA>EDTA>Na OH>KH₂PO₄,the preferred eluents are OA and EDTA,the concentration is 0.10 mol·L^-1,the liquid-solid ratio is 10:1,and the washing time is 4 h,the removal rates are 75.95%and 67.83%,respectively.（2）On the basis of the above single factor experiments,aiming at Qingpu lead contaminated soil and Jiading arsenic contaminated soil,two kinds of four-factor and three-level small-scale test schemes were designed by using Box-Behnken method in Design-Expert 12 software to optimize the eluent concentration,washing time and liquid-solid ratio.Taking the heavy metal removal rate as the response value,the response surface model（RSM）was used to optimize the test results,and the optimal washing parameters were obtained.The optimal compound washing parameters for lead contaminated soil are EDTA concentration 0.03 mol·L^-1,the Fe Cl₃ concentration 0.20mol·L^-1,the washing time was 12 h,and the liquid-solid ratio was 15.The model predicted optimal value of lead removal rate under this parameter is 98.87%.The actual lead removal rate of the verification test based on this parameter is 99.63%,which is basically consistent with the optimal value predicted by the model,and much higher than the lead removal rate of a single eluent.The optimal washing parameters for arsenic contaminated soil are the concentration of EDTA was 0.02 mol·L^-1,the concentration of OA was 0.20 mol·L^-1,the elution time was 12 h,and the liquid-solid ratio was 13,and the removal rate was 83.24%.The model predicted optimal value of arsenic washing removal rate under this parameter is 84.13%.According to this parameter,the actual arsenic removal rate is 83.24%,which is similar to the optimal value predicted by the model,and is significantly higher than the arsenic removal rate of a single eluent.The results provide an important experimental basis for the formulation of washing remediation schemes for two heavy metal contaminated plots.（3）After use,the compound eluent still has certain heavy metal removal ability and can be reused,but with the increase of reuse times,the washing ability decreases gradually.When the optimized lead compound eluent(0.03 mol·L^-1EDTA+0.20 mol·L^-1Fe Cl₃)was reused for the third time to treat lead-contaminated soil,the lead removal rate was still 65.86%;when the optimized arsenic compound eluent(0.02 mol·L^-1EDTA+0.20 mol·L^-1Fe Cl₃)was reused for the third time,the arsenic removal rate was52.27%.The results provide a basis for the reuse of compound eluents in the actual washing repair project.was reused for the third time,the arsenic removal rate was52.27%.The results provide a basis for the reuse of compound eluents in the actual washing repair project.（4）Lead and arsenic compound eluents have good removal rates of different forms of lead or arsenic in the soils treated by them.The removal rates of various forms of lead in Qingpu lead contaminated soil by lead compound eluent were in the following order:oxidizable state（86.19%）>reducible state（85.05%）>acid soluble state（71.5%）>residue state（67.34%）.The removal rates of various forms of arsenic in Jiading arsenic contaminated soil by arsenic compound eluent were as follows:iron type arsenic（95.92%）>calcium type arsenic（85.60%）>residual arsenic（62.84%）>exchangeable arsenic（62.83%）>aluminum type arsenic（50.70%）.After washing,the total content of lead and arsenic in the soil not only decreased greatly,but also the proportion of residual state increased,the stability of heavy metals increased,the bioavailability decreased,and the risk of heavy metal pollution was further reduced.