Study Of Chemical Leaching Remediation Mechanism Of Heavy Metal Contaminated Farmland Soils And The Purification Treatment Of Leaching Wastewater

In recent years,the regional farmland soil pollution,especially heavy metal pollution,has become increasingly serious in China,has already affected the ecological environment safety such as soil quality,agricultural products quality and human health.Remediation of heavy metal contaminted soil has already become a guarantee not only for regional ecological security,agricultural product quality and safety,but also for the national sustainable development.What's more,it is the inevitable requirement to protect people's health and promote their livelihood.In this paper,we sampled 3 types of soils near mines polluted by heavy metals in Shaoguan,Qixia and Chang'an.FeCl3,citric acid(CTA)and FeCl3+CTA were used several times to remove heavy metals(Pb,Cd,Zn and Cu)from these polluted soils by the batch extraction method,the extraction rate and the change of soil basic properties after extraction were measured and compared.The micro-aggregates of different particles were extracted by wet sieve method and freeze-drying method.The leaching of soil micro-aggregates was carried out by using FeCl3,citric acid and FeCl3+CTA to investigate extraction rate and morphological changes of Pb,Cd,Zn and Cu in different particle groups.Alkali precipitation and biochars adsorption were used to deal with the leaching wasterwater.The results are as follows,(1)Batch extraction experiments showed that,for the first time,leaching removal rate of Pb,Cd,Zn,and Cu was higher,the leaching removal rate of Pb,Cd,Zn and Cu gradually reduced on the second time and third time,compared to single FeCl3 or citric acid(CTA),FeCl3+CTA increased the heavy metal removal efficiency of Pb and Cu dramaticlly,has increased 70.4%?113.5%compared to citric acid(CTA),increased 70.4%?113.5%compared to FeCl3.After leaching solution,soil pH,TOC,CEC,OM,exchangeable calcium,magnesium and available nitrogen,available phosphorus and available potassium have dropped dramaticlly,the change of soil properties for the first time was the largest.And the value of the EC in the soil leaching decreased dramaticlly for the first time,increased slightly after the second leaching.(2)BCR extraction method results showed that Cd and Zn in the soil of Qixia mainly exist in acid-soluble forms and reduction forms,Cu and Pb mainly exists in residual forms,Cd and Zn was more easily to remove.Cd in the soil of shaoguan mainly exists in acid-soluble forms and reduction forms,Cu,Zn and Pb mainly exists in residual forms,and Cd was more easily to remove.The Cd is most easily to remove with different leaching agents.Cu mainly exists in residual forms.The removal rate of Pb,Cd,Zn and Cu are mainly as the acid-soluble and reduction forms.(3)CEC,TOC,OM,available nitrogen,phosphorus,potassium,and the content of Pb,Cd,Zn and Cu in the different size of the soil aggregate was clay>sand>powder sand>coarse silt.Heavy metals and the soil nutrient level in clay has enriched obviously.After the leaching the soil aggregate of qixia and shaoguan,the removal rate of Pb,Cd,Zn and Cu of the soil aggregate was coarse silt>sand>powder sand>clay.The heavy metal is hard to remove in the clay level.(4)Using FeCl3,citric acid(CTA)and FeCl3+CTA as leaching agents,besides the heavy metals in the leaching water,elements such as a Ca,Al,Mg and Mn also dissolution heavily.When using FeCl3,the content of Ca in soil is the largest,when using citric acid,Al,Mn and Mg content in the leaching water is far higher than that of FeCl3,when using FeCl3+CTA as leaching agents,the content of constant element increased dramaticlly,and for the first time,the content of constant element was the largest.(5)Adopted the method of alkaline precipitation combined with adsorbent dealt with the leaching wastewater purification treatment,the remove rate was good.To sum up,using FeCl3,citric acid(CTA)and FeCl3+CTA,removal rate of heavy metals were mainly as the acid-soluble and reduction forms.The remove rate of heavy metals was clay>powder sand>sand>coarse silt.The heavy metals in coarse silt forms are more easily to remove,while clay forms obvious fixed effect and nutrients to heavy metals in the soils.The leaching wastewater treatment by alkaline precipitation combined with adsorbent adsorption,can reduce the pH and the cost,set the precipitating agent(Ca(OH)2)and adsorbent(biochars)of solid-liquid ratio as 14:1 and 4:1 was good.