Study On Washing Effect And Mechanism Of Organic Acid Compound Washing Agent On The Cadmium And Lead Composite Contaminated Soil

As an important part of the ecosystem,the presence of soil pollution is related to the ecosystem and human health closely.Soil washing technology is considered to be an efficient and quick soil remediation technology,but its remediation efficiency to a large extent depends on the choice of the washing agent.Focus on the Cd-Pb composite polluted soil problem,this subject researched the remediation effect,potential mechanism and ecological risk assessment on the Cd-Pb composite contaminated soil with glycolic acid single washing,glycolic acid（GA）and sodium tripolyphosphate（ST）compound washing.In order to further reduce the dosage of washing agent and improve the soil remediation efficiency,this subject also combined washing with electrokinetic remediation technology to study the remediation of Cd-Pb composite contaminated soil.The main research contents of this article are as follows:（1）As a soil washing agent for single washing,natural and biodegradable glycolic acid（GA）was used to remediate the Cd-Pb composite contaminated soil.And the experiment results showed that under the washing conditions(GA concentration:3 mol·L^-1,washing time:3 h,washing liquid-to-solid ratio:1/5),the Cd and Pb removal efficiency reached80.19%and 48.54%,respectively.In which,the content of exchangeable,reducible and oxidable Cd are reduced markedly,and the exchangeable and reducible Pb content reduced simultaneously,so that the potential ecological risk value in composite contaminated soil is significantly decreased after soil washing remediation.However,after glycolic acid single washing,there are still small part of Cd and Pb residued in the soil with a decreased p H value which will easily results in soil acidification,and the ecological risk value is still at a high risk level.So it need a further optimization on the washing technological process to promote the Cd and Pb removal efficiencies and reduce its impact on the washed soil properties.（2）Mixed sodium tripolyphosphate with glycolic acid to be a compound washing agent for the Cd-Pb composite contaminated soil remediation,experimental results demonstrated that the the compound washing effect is much better than glycolic acid single washing.Furthermore,the response surface methodology was used for the compound washing conditions optimization and obtained the best compound washing conditions(GA concentration:1.0 mol·L^-1+ST concentration:0.095 mol·L^-1,washing time:3 h,washing liquid-to-solid ratio:1/10),the removal efficiency of Cd and Pb reached 89%and 68%,respectively.And the residual Cd concentration in the soil is lower than the control value in the"Soil Pollution Risk Management Quality Standard"(Cd:0.91 mg kg^-1<1.5 mg kg^-1),the Pb concentration is reduced and close to a screening value(Pb:81.7 mg kg^-1>80 mg kg^-1),the exchangeable and reducible Cd and Pb content reduced obviously,and no apparent residue of compound washing agent in washed soil.Additionally,the soil p H changed slightly,soil fertility element content had a certain increase,and the ecological risk level reduced from the extremely high risk level to a low risk level,indicating that glycolic acid and sodium tripolyphosphate compound washing agent is a green,efficient and environmentally-friendly organic acid compound washing agent.（3）The results of GA washing combined with elctrokinetic remediation,GA and ST mixed solution wasing combined with electrokinetic remediation,and ultrapure water washing（purely elctrokinetic remediation）control experiments showed that GA washing combined elctrokinetic remediation with the highest remobal efficiency of Cd and Pb.More than 95%Cd and 28%Pb were removed by the joint remediation mode of GA washing and electrokinetic remediation.It indicated that organic acid washing combined with electrokinetic remediation can improve the remediation efficiency of the Cd-Pb composite contaminated soil,especially enhanced the Cd remvoal,and reduced the dosage of washing agent used in the remediation effectively,which can provide a theoretical reference to the in-site remediation in the actual pollution site.In summary,glycolic acid single washing,organic acid compound washing agent（glycolic acid+sodium tripolyphosphate）washing and washing combined with electrokinetic remediation technology all shows good removal efficiency on the Cd-Pb composite contaminated soil,and realize an efficient removal of Cd and Pb simultaneously.Specially,the organic acid compound washing agent（glycolic acid+sodium tripolyphosphate）has the best effect on the Cd-Pb composite contaminated soil remediation with few impact on the soil physicochemical properties,and the remediated soil ecological risk level decreased significantly to be a low ecological risk level,which is good for the remediated soil reuse.