Study On Chemical Immobilization Of Heavy Metal In Soil Promoted By External Electric Field

The environmental risks of heavy metals reactivation and secondary pollution caused by excessive addition of stabilizers are still existing in remediation technology of chemical immobilization.The reasons are that low activation heavy metal compounds which abound in soils prevents the heavy metal from reacting with stabilizing agents to generate more stable compounds(residual fractions),and the contact reaction efficiency between heavy metal and stabilizing agents is not high in complex soil environment.In order to improve the long-term stability of heavy metals after chemical immobilization remediation and reduce the dosage of stabilizing agents,the joint technology of chemical immobilization combined with external electric field was selected to stabilize heavy metals in soils,and the mechanism of chemical immobilization of heavy metal in soil promoted by external electric field was explored in this paper.The main conclusions are as follows:(1)The existing form of heavy metals in soils and soil environment can affect the efficiency of chemical immobilization remediation;Complex agent with 2% triple superphosphate(TSP)+ 9% biochar can make the leaching concentration of heavy metals in soils meet the requirements of GB5085.3-2007;After being stabilized by the complex stabilizers,the speciation of Pb,Cd and Cd were dominated by stable forms such as oxidizable and residual fractions,and environmental risks of heavy metals were significantly reduced;As for the change degree about forms of three heavy metals,Pb was the most significant,followed by Cu and Cd;Complex stabilizer could maintain a high stability rate for heavy metals in a long period of time.(2)Both alternating current(AC)field and horizontal polar exchange electric field of direct current(DC)could effectively change the existing forms of heavy metals Pb、Cu and Cd in soils from stable fractions to unstable fractions,and DC electric field had more significant impact on the speciation of heavy metals than AC electric field;The impact of electric field on the leaching concentration of heavy metals is in order Cd>Cu>Pb.Therefore,the activity of heavy metals in soil can be significantly improved by electric field,which is beneficial for reaction between heavy metals and complex stabilizers to generate more stable compounds.(3)Compared with the single application of chemical immobilization technology,the dosage of stabilizers could be reduced by 22.7%~36.4% under the application of electric field;The joint technology was considered to be the optimal decision of soil remediation technology in this study,and its technical parameters as follows: Complex agent(2% TSP+5% biochar)combined with horizontal polar exchange electric field of direct current(DC);The joint technology could further reduce the content of reducible fractions of heavy metals in soil,while the percentage content of oxidizable and residual fractions increased by 7.61%~18%.(4)Both single application of chemical immobilization technology and joint technology could gradually change the main crystal structure of heavy metals from silicon-type to phosphorus-type in soil,and under application of the horizontal polar exchange electric field of DC,the contact reactions between heavy metals and stabilizers is more efficient,in addition,the Si atoms in the crystal structure of heavy metals in soil was further replaced by P atoms,and the crystal structure of mineral compositions formed by heavy metals and phosphate was more complex.It can be concluded that the application of external electric field can not only desorb heavy metals from the surface of soil particles to the soil solution,but also destroy the composition and structure of some organic matter and mineral crystal lattices in the soil to a certain extent which result in release and activation of heavy metals.Under the action of electric field,the contact reactions between heavy metals and stabilizers is more efficient which play an important role in the generation of stable fractions of heavy metals,in addition,the increase in content of stable metal fractions and the presence of higher efficiency chemical reactions can improve the long-term stability of heavy metals in soil and reduce the dosage of stabilizers.