Preparation Of High Efficient Chemical Eluent And Its Application In Electrokinetic Remediation Of Heavy Metal Soil

In recent years,with the rapid development of China’s industrial production,automobile exhaust emissions and the abuse of pesticides and fertilizers,a large number of heavy metals are discharged into the soil,causing serious damage to the ecological environment,but also a greater threat to food safety and human health.Chemical leaching technology can effectively repair heavy metal soil,which has the advantages of simple operation,short elution cycle and high removal efficiency,and is widely used in the actual remediation of contaminated sites.The key of leaching technology is to select an efficient and economical environmental friendly eluent.Citric acid fermentation broth contains a lot of organic acids and other metabolites.It has strong acidity and complexation ability.It has good removal effect on heavy metals,and is easy to be degraded by soil microorganisms.The risk of secondary pollution is low.It has a good application prospect in chemical leaching.In this paper,Aspergillus niger was selected as the fermentation strain,and the citric acid fermentation broth of Aspergillus niger was used as the chemical eluent by shake flask fermentation method.The contaminated soil containing five typical heavy metals（Cu,Ni,Pb,Cd,Zn）was used as the remediation object.The leaching performance of citric acid fermentation broth on heavy metal contaminated soil was studied by shaking leaching test,and the best leaching conditions were determined.Based on the mild chemical properties of chloride and less influence on soil properties,citric acid fermentation broth was mixed with three kinds of chloride（sodium chloride,calcium chloride and ferric chloride）to explore the leaching efficiency and influencing factors of the complex eluent;the leaching technology was combined with electric remediation technology,and the citric acid fermentation broth of Aspergillus niger and the complex eluent were used as the electrolyte The effects of electrolyte,voltage and electrode exchange frequency on the migration of heavy metal ions in soil were analyzed.The enrichment of heavy metal ions in soil of different areas was analyzed.The effects of electrokinetic leaching remediation on soil pH,water content and other physical and chemical properties were discussed,as well as the mechanism of removing heavy metal ions in soil.The main achievements are as follows:（1）The citric acid fermentation broth of Aspergillus niger with 4.1%acidity could be obtained by using Aspergillus niger as fermentation strain,shaking flask medium in constant temperature shaker,and culturing at 32℃and 220 R/min for 25 days.In the study of leaching performance of citric acid fermentation broth,the solid-liquid ratio of citric acid fermentation broth of Aspergillus niger was 1:10,pH was 2,and the leaching time was 5 h.The leaching efficiency of Cu,Ni,Cd,Pb and Zn reached 75.8%,61.1%,62.2%,43.6%and 59.4%respectively.Citric acid fermentation broth of Aspergillus niger can effectively remove acid soluble,reducible and oxidizable heavy metals.It was found that the removal of Cu,Ni,Cd,Pb and Zn was a heterogeneous diffusion process including physical diffusion and chemical adsorption.（2）The results showed that the best leaching effect was obtained when the raw liquor of Aspergillus niger citric acid fermentation broth（acidity 4.1%）was mixed with 0.1 mol/L FeCl₃ at the volume ratio of 1:4,the solid-liquid ratio was 1:10,and the leaching time was 4 h.The removal rates of Cu,Ni,Cd,Pb and Zn were 86.4%,62.5%,89.6%,73.8%and 75.6%respectively,which were better than that of citric acid fermentation broth and FeCl3.The complex eluent can effectively remove the heavy metals in acid extractable state and reducible state,and the unstable forms of Cu,Ni,Cd,Pb and Zn decreased by84.7%,75.73%,88%,74.3%and 78%respectively.The analysis of spectral characteristics showed that the heavy metal leaching method with citric acid fermentation broth as eluent had little effect on the mineral composition of soil,and did not damage the structure of soil system.（3）When the voltage gradient is 1 V/cm,the repair time is 7 The electrokinetic remediation efficiencies of Cu,Ni,Cd,Pb and Zn were 79.7%,64.7%,73.5%,61.8%and 71.8%respectively when citric acid fermentation broth of Aspergillus niger was used as electrolyte,while the electrokinetic remediation efficiencies of Cu,Ni,Cd,Pb and Zn were 87.7%,69.4%,80.5%,61.8%and 75.9%respectively when citric acid fermentation broth FeCl₃complex eluent was used as electrolyte.The effect of electrokinetic repair increases with the increase of voltage gradient.The exchange electrode can effectively prevent the occurrence of focusing effect,re dissolve the heavy metals in the precipitation state,reduce the precipitation of heavy metals in the cathode,and improve the removal efficiency of heavy metals.The higher the electrode exchange frequency,the greater the impact on current,conductivity,soil pH and heavy metal distribution.（4）When the voltage gradient is 1V/cm,the repair time is 6 The electrokinetic remediation efficiency of Cu,Ni,Cd,Pb and Zn was 70.3%,59.7%,68.1%,61.6%and 66.4%respectively when citric acid fermentation broth was used as electrolyte,and 78.2%,72.4%,80.5%,68.2%and 74.9%respectively when citric acid fermentation broth FeCl₃ complex eluent was used as electrolyte.After the restoration,the content of heavy metals in the upper layer of the soil column treated with anode increased,while that in the lower layer decreased;when the cathode was on,the vertical electric field could accelerate the downward migration of heavy metal complex anions,and the heavy metals were mainly distributed in the lower layer of the soil column.No matter anode or cathode on the electrode treatment,the change of soil pH and conductivity was almost not affected.In addition,intermittent power on can effectively reduce the power on time,improve the removal rate of heavy metals,and reduce the power consumption in the remediation process.