Experimental Study On Remediation Of Cd And Pb Contaminated Clays By Freeze-thaw Synergistic Washing Method

This paper is based on the Natural Science Foundation of China"Study of transport and remediation of specific heavy metal pollutants based on freezing and thawing".In consideration of the property that water would migrate from unfrozen region to freezing front and soil permeability will also enhance by freeze-thaw action under the certain temperature gradient and water conditions,the test was carried out by using artificial freezing combined with chemical washing to remediate heavy metal polluted clay soils for the purposes of providing new ideas and methods.Firstly,taking the local farmland clay contaminated by Cd and Pb as the research object,the best leaching agent was obtained by comparing the removal efficiency of EDTA,oxalic acid,FeCl₃,citric acid and tartaric acid as leaching agents alone or in combination with through the indoor shaking leaching test.Secondly,the freeze-thawing leaching soil column was tested by selecting EDTA,citric acid and EDTA combined with oxalic acid as the leaching agent according to the removal effect of indoor shaking leaching.By studying the effects of soil loading density,leaching agent combination and concentration,freeze-thaw cycle times,and leaching agent supply mode on the freeze-thaw leaching effect,the optimal leaching scheme was explored.The following conclusions are drawn:?1?In the laboratory shaking leaching test,the leaching effect of EDTA combined with oxalic acid was the best,which was better than the removal rate of the two alone,and also better than the leaching combination of FeCl₃ with citric acid and tartaric acid.The optimal concentration of EDTA and oxalic acid was 0.005 mol/L EDTA+0.01 mol/L oxalic acid.?2?Leaching agents have an impact on soil frost heave.Different types and concentrations of leaching agents have different effects on frost heave water absorption.EDTA has a greater impact on frost heave and water absorption than citric acid.The combination of EDTA and oxalic acid has less effect on frost heave and water absorption,and the influence of each leaching agent on frost heave is greater than that of deionized water.?3?Freeze-thaw leaching can effectively remove the heavy metals Cd and Pb from the clay.Because the freeze-thaw cycle can reduce the cohesion of clay and destroy its original particle structure,the rearrangement of particles is conducive to the full contact between the leaching liquid and pollutants,thus improving the leaching efficiency.When the compound solution of EDTA and oxalic acid was used as the eluent,the leaching efficiency is significantly improved compared to single eluent,and the removal rates of Cd and Pb reached 47.71%and 54.70%after 9 times of freeze-thaw in group FTW9-4 respectively.?4?The freeze-thawing and leaching method has obvious leaching effect,and the liquid soil is relatively small,which provides reference for large-scale remediation of heavy metal contaminated soil in seasonal frozen areas in the future by using freeze-thawing alternation phenomenon.