Combining Chemical Leaching And Electrokinetics To Remediate Heavy Metals Contaminated Soils

With the rapid development of the economy,the environmental pollution problems have seriously affected people's living environment.The heavy metals contaminations of soils are widespread,hiden and irreversible,and detrimental to environment.Excessive heavy metals in the soils will lead to the excessive absorption of heavy metals by food crops,may indirectly cause serious health hazards to human.At present,there are various technologies for remediating heavy metals contaminated soils.Chemical leaching is a simple,fast and efficient method for soils clean-up,and attracts attention of scientists.At the same time,electrokinetic(EK)remediation can remove heavy metals from contaminated soil without adding chemical,has also been favored by researchers.However,a single electrokinetics can not achieve satisfactory results.Facing to these problems,this paper combines chemical leaching and electrokinetic technology to remediate soils contaminated by heavy metals,by conducting field tests and laboratory experiments,to compare the rice straw fermentation liquid,potassium fertilizer and commonly used chemical washing agents to leach out heavy metals from soil.Furthermore,combining electrokinetic technology with chemical leaching is studied to further increase the removal rates of heavy metals from contaminated soils.The main outcomes of this paper are as follows:(1)The Shaotian field experiment selected anaerobic fermentation of straw straw,aerobic fermentation of straw straw,KCl and HAc for the chemical leaching.After 120days of treatment,the p H value of soil under different treatments was not obvious;KCl had the best removal effect on heavy metals(Cd,Pb,Cu and Zn)in soil.Among them,the removal rate of Cd was 19.20%,the removal rate of Pb was 6.84%,the removal rate of Cu was 7.56%and the removal rate of Zn was 15.94%.(2)The Guangzhou field experiment selected selected FeCl₃,KCl and HAc for the chemical leaching.After 120 days of treatment,the p H of the soil treated with FeCl₃decreased significantly.After 40 days,the p H dropped to below 3,and as the rinsing time prolonged,it rose back to 3.2 in the later stage;acetic acid and KCl also caused a slight decrease in soil p H and a rebound in the later stage.FeCl₃ had the best removal effect on heavy metals(Cd,Pb,Cu and Zn)in soil.Among them,the removal rate of Cd was 39.08%,the removal rate of Pb was 12.41%,the removal rate of Cu was 28.08%and the removal rate of Zn was 20.91%.At the same time,groundwater monitoring was carried out during the leaching process,the monitoring results showed that the eluent did not pollute the groundwater.(3)Vertical leaching experiments with different amounts of KCl(0.2%,0.5%and 1%)showed that KCl is a strong ion exchange salt,which can remove Cd from acidic soil,when the amount of KCl was increased from 0.2%to 1%of the soil weight,the removal rate of Cd increased from 38.6%to 84.1%.When the amount of KCl was increased from 0.2%to 1%of the soil weight,the removal rate of Cd increased from38.6%to 84.1%.The vertical EK treatment alone did not effectively transfer heavy metals from the upper surface soil(0-20 cm)to the deeper layers,but significantly increases the p H of the soil near the cathode,which can passivate heavy metals,thereby preventing the absorption of heavy metals by rice or vegetables.KCl leaching combined with EK improved the removal efficiency of heavy metals in topsoil(0-20 cm)soil,especially 1%KCl+EK treatment.The removal efficiency of Cd was 94.8%,which reached the national agricultural soil standard.It was worth further stuied in the field experiment.(4)Batch experiments of soil washing by FeCl₃,Fe(NO₃)₃,KCl,KNO₃ and HCl showed that,FeCl₃ and Fe(NO₃)₃ had good removal effects on heavy metals in soil,and the neutral salts KCl and KNO₃ were generally removed.By comparing the removal effects of Fe³⁺,H⁺,K⁺and Cl^-on heavy metals Cd,Pb,Cu and Zn,the role of different ions in the removal of Cd followed order:H⁺>Fe³⁺>K⁺>Cl^-,For Pb,Fe³⁺>H⁺>K⁺>Cl^-;For Cu,Fe³⁺>H⁺>K⁺>Cl^-;For Zn,H⁺>K⁺>Cl^->Fe³⁺.(5)Electrokinetic remediation combined with soil washing with different chemicals showed that Cd,Pb,Cu and Zn migrated out-off the topsoil(0-10 cm),downward to the layer of 10-20 cm and accumulated there,this was unfavorable to clean-up agricultural soils,and the previously proposed vertical leaching and electrokinetic remediation was necessary.