Cd-Pb Contaminated Soil Chelation Enhanced Phytoremediation And Heavy Metal Leakage Risk Research

Chelant-enhanced phytoextraction is a kind of soil heavy metal remediation technology with good application prospect.It is of great significance to study the risk of heavy metal leakage migration caused by rainfall in the repair process.In this paper,with the background of Cd-Pb contaminated soil,the dynamic activation experiment was used to study the change of the active ability of six chelating agents over time,and select the suitable chelating agent for further plant cultivation-chelating enhancement-rainfall leaching experiment.The effects of chelating Enhancement and its Action time on extraction and remediation of typical plants（Zea mays and Brassica juncea）and leaching of heavy Metals were study.Finally,the adsorption and dispersion parameters of chelated heavy metals in the lower aerated soil zone were studied by chelated heavy metal adsorption experiments and soil column migration experiments.The（CDE）mathematical model of the convection-dispersion model for the transport of heavy metals in the lower vadose zone was further constructed,and the migration of chelated heavy metals in the lower aeration zone was simulated,and the risk of groundwater pollution was predicted and evaluated.The following major findings have been obtained:（1）The dynamic activation of the common chelating agent show that:According to the dynamic characteristics of water soluble heavy metals in Cd-Pb contaminated soil,the six chelating agents can be divided into three types:strong activation ability-slow decay type（EDTA）,moderate activation ability-attenuation significant type（GLDA,DTPA,NTA）,low activation energy type（EDDS,CA）;After adding GLDA and DTPA,the content of water soluble Cd was 0.56 and 0.58 times of EDTA,the half-lives of water-soluble Cd were 12 days and 25 days;the content of water soluble Pb was 0.62 and 0.73 times of EDTA and the half-lives of water-soluble Pb were 15 and 12 days.Considering that GLDA and DTPA has a good activation ability and a certain decay change（half-life）can be used as the suitable chelating agent for the activation of Cd-Pb contaminated soil.（2）Plant cultivation-chelating enhancement experiment showed that Zea mays showed tolerance to two chelating agents（GLDA,DTPA）and heavy metals（Cd,Pb）.Brassica juncea was poisoned on the 16th day after GLDA activation and biomass was 0.41 times that of the control group.On the 3rd day of chelating activation,the extraction ability of heavy metals in plants reached a high level.With the increase of chelating planting time,either the biomass increased but the heavy metal content decreased,or the chelating agent toxicity showed a significant decrease in biomass.In the end,the amount of heavy metal extraction increased little（DTPA-Zea mays,Brassica juncea）,and even decreased significantly（GLDA-Brassica juncea）;the effect of DTPA and GLDA on the extraction of Cd was weak,mainly in promoting Pb extraction.Under the condition of GLDA-Brassica juncea,the extraction ability of Pb was the strongest,and the extraction amount was 17.07 times of the control group.（3）Plant cultivation-chelation enhancement-filtration test showed that the total amount of GLDA,DTPA leaching from the two plants increased significantly under the conditions of chelating activation on the 3rd day and 16th day compared with the control group,in which the total amount of Cd,Pb leaching was significantly increased.On the 3rd day of chelation activation,the amount of Cd and Pb of leaching showed Brassica juncea>Zea mays;the leaching amount of Cd was the largest under the condition of DTPA-Brassica juncea,which was 13.6%of the total soil;the total leaching of Pb under GLDA-Brassica juncea conditions,the largest amount was 11.2%of the total soil;the total amount of leaching on the 16th day of chelation activation was significantly lower than that of the third day,and the percentage of Cd,Pb leaching from GLDA chelating enhanced Zea mays on the 16th day decreased more than that of DTPA,which was 1.93 times and 1.09 times as much as that of DTPA,respectively;the migration of heavy metals caused by the rainfall is more obvious.Under the two plant conditions,GLDA and DTPA chelate activation,the maximum leaching concentration points appear at the first rainfall collection point;the concentration of leaching filtrate Cd and Pb With the increase of rainfall,the index decreased or decreased linearly,and the goodness of fit R² was greater than 0.81.（4）The mathematical model construction and migration prediction results of DTPA chelated heavy metal undergass migration indicate that:the adsorption isotherms of chelated Cd and Pb in soil all follow the linear adsorption model.The dispersion coefficients of the DTPA-Cd（Pb as background）and DTPA-Pb（Cd as background）model are 0.009415 and0.00659 m²?d^-1,respectively.Under the condition of remediation of Zea mays and Brassica juncea,on the 3rd and 16th day of chelation activation,the migration distance of Cd and Pb in the first remediation cycle were 2.4-2.6m and 2.5-2.8m,respectively.There was no risk of groundwater pollution.In the third remediation cycle,there was a risk of contaminated groundwater,with a Cd migration distance of 5.4-5.8m and a Pb migration distance of6.0m-6.2m（local groundwater depth of 5 m）.The risk of groundwater pollution was:Brassica juncea>Zea mays;the third day of chelating activation>the 16th day of chelating activation.