Study On Remediation Effect Of Passivator On Heavy Metal Contaminated Soil And Stability Of Citric Acid To Passivation Products

Immobilization soil remediation technologies have been widely used in the remediation of heavy metal contaminated soil,the remediation of heavy metal contamination in farmland soils should pay attention to the following aspects.On the one hand,the selection of passivators are crucial,the passivators should not only ensure the quality of the soil,but also control the concentration of heavy metals in soil within the risk range.On the other hand,low-molecular-weight organic acids(LMWOAs),secreting from the rhizosphere of crops,play an important role in the process of adsorption-desorption,dissolution and release of heavy metals in soil,thus enhancing their bioavailability.Therefore,it is necessary to select suitable passivators and analyze their effect on soil physical and chemical properties and passivation,and assess the risk of heavy metals under rhizosphere environment.In order to provide suitable passivators for heavy metals contaminated farmland,and evaluate the stability of heavy metals immobilization when environmental conditions change.In this study,we selecte saprolite,biochar and potassium dihydrogen phosphate as passivators from passivation remediation effect,mechanism and soil quality control,which added to heavy metal contaminated soils for soil culture experiments.The effects of three passivators on remediation of Cd,Pb and Zn contaminated soils were comprehensively analyzed from the content of available heavy metals,toxic leaching concentration and the speciation was determined by BCR.The stability of heavy metals passivation products were analyzed by adding different concentrations of citric acid to simulate the changes of rhizosphere environmental conditions.Adopted DTPA/TEA to evaluated the effects of citric acid on bioavailability of heavy metals,and TCLP to evaluate the concentration of heavy metals in soil was under safe level after dissolution release and evaluate their risk to ecological environment,and to analyze the changes of heavy metal form in soil to judge the risk of heavy metal dissolution release.In the study of the remediation effects on heavy metals,the results showed that the p H value,the contents of CEC and SOM in soil increased with the increase of biochar,but there was no significantly change of SOM content in the soil with sepiolite.The content of available heavy metals in soil decreased with the increase of the proportion of biochar and sepiolite.When the proportion of biochar and sepiolite is higher than5%,the concentration of Cd and Zn extracted by TCLP method are lower than international standard,but the concentration of TCLP-Pb is higher than international standard.This may be related to the lower percentage of the exchange state of Pb in the soil.BCR analysis showed that heavy metals transformed into more stable forms.All these show that biochar and sepiolite reduce the bioavailability and mobility of heavy metals in soil.With the increase of the proportion of biochar,the evaluation of the ecological environment risk by TCLP method is below the safe level,which indicates that the immobilization of heavy metals by biochar and sepiolite are effective and feasible.It was found that sepiolite and biochar would preferentially adsorb Cd because of the strong migration of Cd in soil.With the increase of passivation time,Zn and Pb would compete with Cd to adsorb the adsorptive sites on soil,thus desorbing part of Cd.The addition of potassium dihydrogen phosphate reduces the p H value in soil,therefore,in the initial stage of passivation,some Cd and Zn will dissolve and release.With the increase of passivation time,dissolved Cd and Zn reacted with phosphate to form precipitation,which reduces the bioavailability of Cd and Zn.Pb in soil,which showed a decrease trend with the increase of passivation time.The passivation effect of potassium dihydrogen phosphate is the best on Pb immobilization when the ratio of potassium dihydrogen phosphate is 1%.In the study of the stabilization of the passivation products of heavy metals in soil after adding citric acid,when the proportion of sepiolite and biochar was constant,2mmol/kg citric acid decreased the bioavailability of Cd in soil,but increased the mobility of Zn and Pb.10 and 20 mmol/kg citric acid would enhance the desorption and dissolution of heavy metals in soil,thus enhancing the bioavailability and migration of heavy metals.When the concentration of citric acid is constant,the bioavailability and migration of heavy metals show a downward trend with the increase proportion of passivators,which indicated that the addition of passivators can inhibit the desorption and dissolution of heavy metals in soil in a certain extent,and reduced the ecological risk of heavy metals.When the proportion of sepiolite and biochar was more than 5%,adopted TCLP method to analyze the risk to the ecological environment,it was found that TCLP-Cd and TCLP-Zn in soil were still below the safe level.Through BCR speciation analysis,it was found that citric acid would dissolve and release the heavy metals which combined with ferromanganese oxides and organic matter,increasing the content of exchangeable heavy metals,and thus enhance the toxic effect of heavy metals.In summary,it was found that biochar and sepiolite are effective in the remediation of Cd,Pb and Zn contaminated soils.However,with the change of environmental conditions,the dissolution and release of passivated heavy metals will increase the risk to the ecological environment.However,when the proportion of passivators is high,the dissolution and release of heavy metals in soils would be inhibited to a certain extent and the environmental risk will be reduced.Potassium dihydrogen phosphate has a good passivation effect on Pb,and the passivation products are stable.