Remediation Of Multiple Heavy Metal-contaminated Soil Through The Combination Of Soil Washing And In Situ Immobilization

With the fast development of the society and economy,the industrialization in China has been significantly enhanced.However,environmental problems also appeared and have already become the focus of attention,especially the problem of heavy metal pollution in farmland.Soil heavy metal pollutions do great harm to the soil environment:it can influence the physical and chemical property of the soil,damage the soil microorganisms,threat the whole soil ecosystem and cause the degradation of the soil.Furthermore,soil heavy metal pollutions can threaten human health through accumulation in food chain.Earlier studies have mainly focused on only soil washing or in situ immobilization to amend contaminated soils,which ignored their respective disadvantages.The combination of soil washing and in situ immobilization is beneficial as it exploits their advantages and compensates for their respective deficiencies.In this study,we investigated the combined effect of soil washing?with FeCl₃?and in situ immobilization?with lime,biochar and carbon black?to remediate the soil contaminated with multi metals.In addition,we analysed the changes in heavy metal?total and bio-available metals?contents,soil properties and soil enzymes activities after the combined effect.A total of three interrelated research works were designed in this study and the obtained conclusions are as follow:?1?Soil washing with FeCl₃ presented excellent removal efficiency to remove heavy metals from the soil.The results showed that the removal rate of Cd,Pb,Zn,and Cu was 62.9%,52.1%,30.0%,and 16.7%,respectively.FeCl₃ washing can remove most of the acid-soluble and reducible heavy metals,however,it has bad effect on the removal of the oxidizable and residual metal s.The physicochemical property of the soil has changed greatly after FeCl ₃ washing:soil pH decreased 1.55,soil cation exchange capacity?CEC?was 26.4%lower,dissolved organic matter increased by 88.9%.Therefore,FeCl₃ washing can promote the loss of soil organic matter and may cause soil degradation after washing.?2?FeCl₃ washing combined with lime immobilization can alleviate the adverse effects on soil caused by FeCl₃ washing.After the combined immobilization,soil pH increased from 3.69 to 7.52,soil CEC content increased from 15.83 cmol/kg to 27.66cmol/kg,growing by 75%.The activities of soil enzymes?urease,sucrase and catalase?increased significantly compared with the soil treated with FeCl ₃ washing alone,the growth rates were 52%,64%and 45%,respectively.Correlation analysis demonstrated that the increase in soil pH and the addition of nutrition can effectively remediate the washed soil.Though washing combined with biochar or carbon black immobilization is less effective on the remediation compared with lime immobilization,it still can increase the activity of the soil enzymes?urease,sucrase,and catalase?to some extend.?3?FeCl₃ washing combined with lime immobilization exhibited good effects on stabilizing the metals.The percentage of acid-soluble Cd,Pb,Cu and Zn decreased by36.5%,70.91%,53.3%and 73.6%,respectively.Meanwhile,the percentage of reducible,oxidizable and residual metals had the corresponding increase.Soil washing combined with biochar immobilization tends to i ncrease the activity of Cd,Pb and Zn with the influence of the acid soil pH,whereas the activity of Cu decreases.Furthermore,soil washing combined with carbon black immobilization can increase the migration of the heavy metals?Cd,Pb,Zn and Cu?.In conclusion,FeCl₃ washing combined with lime immobilization is an effective method to amend the soils contaminated with multiple heavy metals.The combined remediation can increase the activity of soil enzymes?urease,sucrase and catalase?compared soil washing alone.Therefore,the combination of soil washing and in situ immobilization can be considered while amending soil contaminated with multiple heavy metals.