Study On The Effect Of Different Leaching Agents On Leaching And Remediation Of Cd-contaminated Soil

At present,the remediation technologies of Cd contaminated soil mainly include engineering remediation technology,physicochemical remediation technology,bioremediation technology and chemical remediation technology.Among them,the soil leaching method of physicochemical remediation technology has the advantages of high leaching efficiency,good leaching effect and thorough treatment,which is suitable for the treatment of high concentration and small area contaminated soil.The effect of soil leaching method depends on the performance of the eluent.Among the commonly used eluents,biosurfactant and natural organic acid have the advantages of high efficiency,fast,little damage to soil,high efficiency and high efficiency With the advantages of low secondary pollution and environmental friendliness,it has become a hot spot in the research of soil eluent and promoted the application of soil eluent in the remediation of contaminated soil.In this study,five kinds of eluents,citric acid,malic acid,rhamnolipid,tea saponin and tannic acid,were selected from Yunnan red soil with pH6.85 from the perspective of high efficiency,little damage to soil,low secondary pollution and environmental friendliness,The best eluent was screened out,the mechanism of eluent remediation was explored,and the applicability and potential risk of eluent in actual environment were proved（1）The leaching equilibrium time of rhamnolipid on lightly,moderately and heavily polluted soil was 30 min,tannic acid 20 min and tea saponin 15 min,which indicated that the leaching process of biosurfactant was a rapid reaction process.The leaching equilibrium time of citric acid for slightly,moderately and severely polluted soil was 60 min,that of malic acid for slightly polluted soil was 30 min,that for moderately polluted soil was 60 min,and that for heavily polluted soil was 120 min.（2）Among the three biosurfactants,rhamnolipid had the best leaching effect.The best leaching conditions were pH=7,mass fraction 2%,and the leaching rates of light,moderate and severe soil were 22.91%,24.49%and30.20%,respectively.Among the two natural organic acids,citric acid had the best leaching effect.The best leaching conditions were pH=6,mass concentration 0.15 mol·L-1,and the leaching rates of light,moderate and severe soil were 22.91%,24.49%and 30.20%,respectively The leaching rates of moderate and severe soils were 74.92%,77.09%and 79.79%respectively.In conclusion,citric acid and rhamnolipid were the best leaching agents from the perspective of high efficiency and environmental friendliness.（3）Among the five combinations,the combination of rhamnolipid and citric acid showed synergistic effect,and the leaching rate was the highest.The leaching rates of the combination of citric acid and tannic acid,rhamnolipid and malic acid were improved to a certain extent,but the combination of citric acid and tea saponin showed antagonistic effect.Therefore,2%rhamnolipid and 0.15 mol·L^-1 citric acid were the best combination.The leaching rates of light,moderate and severe polluted soil were 94.88%,96.57%and 98.54%respectively.（4）The results showed that the optimum conditions of the complex eluent composed of 0.15 mol·L^-1 citric acid and 2%rhamnolipid were as follows:pH=7,volume ratio of 1:1,leaching time of 30 min.the leaching rates of mild,moderate and severe polluted soil were 91.37%,92.49%and96.79%respectively The concentrations of Cd in moderate and severe soils were less than the requirements of"soil environmental quality standard for soil pollution risk control of agricultural land（Trial）"（GB 15618-2018）.（5）In the soil column leaching,the leaching effect of each eluent was complex eluent>citric acid>rhamnolipid>UPW（pure water）,among which the complex eluent of citric acid and rhamnolipid had the best leaching effect,and the leaching rates of Cd were 90.03%,91.93%and93.25%for the soils with mild,moderate and severe pollution levels,respectively,which were much higher than those of citric acid and rhamnolipid alone,The results show that the complex eluent can be used for the remediation of contaminated soil.（6）The removal rates of acid extractable,oxidizable,reducible and residual were 96.63%,97.82%and 98.43%,96.46%,98.63%and 96.42%,9.43%,28.44%and 25%,9.43%,18.96%and 23.1%,respectively,The results show that the complex eluent can remove acid extractable and oxidizable Cd ions in soil,so as to achieve the purpose of remediation of contaminated soil.（7）The recovery kinetics of the complex eluent can be described by quasi second order kinetic equation,which indicates that the complex eluent mainly removes Cd ions through chemical action.The results showed that the complex eluent of citric acid and rhamnolipid had high efficiency,rapid and environment-friendly remediation effect on Yunnan red soil with pH=6.85,and could be directly used for in situ remediation of Cd contaminated soil with small area,heavy pollution and concentrated pollution.The complex eluent can effectively remove acid extractable and oxidizable Cd ions from Cd contaminated soil to achieve the purpose of remediation of contaminated soil.The remediation kinetics can be described by quasi second order kinetic equation,which provides a theoretical basis for the treatment and remediation of Cd contaminated soil.