Research On Washing Remediation Of Heavy Metals Contaminated Soil By Chelating Agents

Soil heavy metal pollution has seriously threatened ecosystems and human health.Soil washing technology is an efficient and convenient treatment method,and the choice of eluent is the key.Chelating agents have been favored by people because of their low cost and strong elution ability.In view of the problem that traditional eluents such as ethylenediaminetetraacetic acid(EDTA)are difficult to degrade in the environment.This study using the degradable chelating agents named imidodisuccinic acid(IDS)and N,N-bis(carboxylmethyl)-L-glutamic acid(GLDA)as the eluents to remediate heavy metal contaminated soils by soil washing.The main results and progress are as follows:(1)The removal efficiency of heavy metals in soil increased with the increase of washing time.The optimal shaking washing time for three chelating agents was 8 h.(2)The effect of EDTA on washing removal rate of Cu was not significant in the range of pH 4 to 10;when pH=4,the removal rate of Cd reached the highest,72.59%.When the system pH was 4,the removal rates of Cu by IDS and GLDA reached 31.58% and 29.34%,respectively,which were higher than that of EDTA under the same conditions.The removal rate of Cd by GLDA increased with the increase of pH,but was lower than the removal rate of Cd by EDTA under the same conditions.(3)The removal efficiency of heavy metal Cu and Cd by three chelating agents increased with the increase of the amount of eluent.When the molar ratio of heavy metal and chelating agent is 1:1,the removal rate of Cu by IDS exceeds that of EDTA by 27.69%;while the ratio of heavy metal and chelating agent is 1:10,the removal rate of Cd by GLDA was 6.37% lower than the same amount of EDTA.When three kinds of chelating agents were used to remove heavy metals in the soil,the best solid-liquid ratio was 1:10.(4)Compared with EDTA,IDS has a higher removal rate of residual Cu,which accounts for 10.6% of the total removal.The removal rate of the exchangeable,reducible and oxidizable forms of Cd by GLDA accounted for 99.5% of the total removal,which is similar to that of EDTA for these three forms.The chelating agent has no significant effect on the removal of residual Cu and Cd.(5)The IDS and GLDA compounding system has comparable elution efficiency to the equivalent amount of EDTA in Cu and Cd in contaminated soil.It can be concluded that when the compounding ratio of IDS:GLDA=4:6,the comprehensive reduction index L is the largest,337.89,and the rinsing effect is the best.The removal rates of Cu and Cd at this time were 33.61% and 71.75%,respectively.(6)Different washing times have a greater influence on the removal of heavy metals.Among them,the removal efficiency of Cu and Cd in contaminated soils reached the maximum by two leaching methods,which were 41.36% and 82.71%,respectively.(7)Through the study of the elution efficiency of heavy metals on aggregates of different sizes,it can be seen that the elution efficiency of the heavy metal Cu and Cd on the powdered aggregates(<0.053mm)reaches the maximum,which is 39.53% and 98.68%,respectively.(8)The dose-effect relationship of the three chelating agents on the relative luminescence of luminescence bacteria and the comprehensive toxicity of the soil after washing were studied by means of the luminescence toxicity assay(LBT).The results showed that all three chelating agents have inhibitory effects on luminescent bacteria,and the toxicity of the three chelating agents is GLDA>EDTA>IDS;the comprehensive toxicity of the soil after leaching is reduced.After the soil was washed by chelating agent,the comprehensive toxicity was: IDS treatment> GLDA treatment > EDTA treatment.