The Development And Application Of Remediation Of Heavy Metal Contaminated Soils By Chemical Leaching Combined Plant And Microorganism

With the continuous development of industry and rapid urbanization,soil pollution has become increasingly serious,especially heavy metal pollution.The general situation of soil environment in China is not optimistic and the heavy metal contamination of soil is very serious in some area,especially the urgent cultivated soil quality according to the national soil pollution survey bulletin released in 2014.Point exceeding standard rate of cultivated soil is highest which reached 19.4%and moderate pollution points accounted for 1.8%.These moderate pollution points are mainly concentrated around industrial and mining enterprises.They bring pollutants into the soil by atmospheric dustfall,sewage irrigation and solid waste stacking,etc.As a result,soil ecological function and crop quality are affected,human health are endangered.In May 2016,the soil pollution prevention plan of action was introduced and fully implemented by state council.Starting the detailed survey of nationwide soil pollution and Carrying out orderly management and restoration.Compared with the single repair technology,the combination of chemical leaching with plant and microorganism has high repair efficiency and good application prospect.It is widely concerned by domestic and overseas scholars.In this article,we take two kinds of heavy metal contaminated paddy soils of Guangdong shaoguan and Nanjing qixia as the research subjects,combinating chemical leaching with plant and microorganism to repair them and systenatically stuyding the treatment methods of leaching wastewater.The results are as follows,(1)In this paper,we sampled two types of soils polluted by Pb,Cd,Zn and Cu of Guangdong shaoguan and Nanjing qixia.According to the previous research results of our laboratory,20 mmol·L-1 FeCl3,100 mmol·L-1 citric acid and their combined solution were chosen as leaching agents.The leaching time was set to 0.5,1,2,4,6,8,10,12 and 15 h.Its aim is to study the leaching mechanism of Pb,Cd,Zn and Cu in soil.It was found that combined leaching was better than that of single leaching.It reached equilibrium state at 15 h,the leaching rate of Pb,Cd,Zn and Cu in Shaoguan soil reached 24.4%,30.2%,10.1%and 20.8%and that of Qixia soil reached 13.5%,39.6%,30.8%,and 22.7%respectively by FeC13+CTA.The leaching rate of Pb,Cd,Zn and Cu increased rapidly in the beginning,then gradually slowed down and eventually stabilized.The leaching tOemperature was set to 15?,25?C and 35?.It was found that the leaching rate of Pb,Cd,Zn,and Cu gradually increased with the temperature,and the lower leaching rate of heavy metal increased significantly.The equation fitting of heavy metal leaching process showed that Elovich equation and Double constant equation are the optimal equation to describe the dynamic leaching process of heavy metal in the soil,which is a non-homogeneous diffusion process.BCR fractionation method results showed that Cd in the soil of Shaoguan mainly exist in acid-soluble fraction and Fe-Mn oxides bound fraction,accounting for 70.6%.And the main form of Pb,Zn and Cu is residual fraction,accounting for 57.5%,67.8%and 61.9%.Pb and Cd in the soil of Qixia mainly exist in acid-soluble fraction and Fe-Mn oxides bound fraction,accounting for 79.3%and 73.1%.The main form of Zn and Cu is residual fraction,accounting for 40.6%and 45.0%.The content of acid-soluble fraction and Fe-Mn oxides bound fraction of Pb,Cd,Zn,and Cu significantly reduced which decreased by 14.2 to 69.9%.The content of sulfide-organic bound fraction and residual fraction remained unchanged.Acid-soluble fraction and Fe-Mn oxides bound fraction are the main fraction to remove heavy metal in soil by leaching agents.The changes of organic matter,available nitrogen,phosphorus and potassium of two soil samples during the leaching process were determined.We found that organic matter and available nitrogen slowly reduced,available phosphorus and potassium significantly reduced.And it reached more than 50%of the maximum amount of washing within 0.5 hour,indicating that the leaching of available phosphorus and potassium in the soil by the leaching agent happened in the beginning.(2)After washing,the soil was fertilized and planted with hyperaccumulator Sedum plumbizincicola.The rhizobium was applied to promote the absorption of heavy metal by Sedum plumbizincicola and achieve the goal of complete restoration.It was found that the removal rate of heavy metal by phytoremediation was 9.2%to 21.1%.Compared with the control group,the biomass of Sedum plumbizincicola increased by 6.9%to 38.5%,the removal rate of heavy metal in soil increased by 1.7%to 8.7%,and the concentration of heavy metal in Sedum plumbizincicola increased by 3.4%to 53.7%.Rhizobia can promote growth of Sedum plumbizincicola and absorption of heavy metal.Sedum plumbizincicola has a certain bioconcentration of Pb,Cd,Zn and Cu.Pb,Cd and Cu were mainly concentrated in the root of Sedum plumbizincicola,and their bioconcentration factors were all greater than 0.5.Except concentrated in the root of Sedum plumbizincicola,Zn was also abundant in the shoot of Sedum plumbizincicola,and its translocation factor was greater than 1.It showed that the ability to transfer Zn of Sedum plumbizincicola is high.(3)For the leaching waste water,a purification method using CaO precipitation combined n-HAP adsorption.Adjusting pH of the leaching waste water to 5.0.Then 0.8 g·L-1 CaO were used to precipitate wastewater for 30 minutes under the condition of 25?.After precipitation,adjusting pH of the solution to 6.0 and 1.0 g·L-1 n-HAP was added to adsorb heavy metal for 180 minutes.The concentration of Pb,Cd,Zn and Cu in leaching wastewater fell below the standard of environmental control which can be discharged safely after processing.