Model Test Study On The Remediation Of Heavy Metal Contaminated Clay By Freeze-thaw And Washing

Based on the National Natural Science Foundation of China project "Research on the migration and purification technology of specific heavy metal pollutants under freeze-thaw action",this paper intends to solve the problems of long single repair time and low removal efficiency of freeze-thaw washing method.In this paper,a clay contaminated by Pb and Cd was taken as the research object.The continuous freeze-thaw washing test of double layer water supplement,gradient cooling,EDTA and clear water alternately was used to improve the single heavy metal removal rate.On the basis of the experimental results,the freeze-thaw scheme was adjusted,that is,by adding a heating layer in the middle,the repeated segregation into ice was realized,the mass transfer efficiency of the eluent and the contaminated soil was improved,and the removal rate of heavy metals was further improved.The following conclusions were reached:Freeze-thaw washing test:(1)In the continuous freeze-thaw washing test,the thawing time of the soil column is always shorter than the freezing time,the water replenishment amount of the lower soil body is smaller than that of the upper soil body,and the conductivity of the washing liquid gradually increases with freezing.and decreases rapidly with melting.(2)The continuous freeze-thaw washing test can effectively remove Pb and Cd in the soil,and the heavy metal removal effect of the soil near the middle water layer is better than that of the soil at both ends.After four experiments,the removal rates of Pb and Cd reached 60.01% and 70.91%,respectively.The bioavailable states of Pb and Cd decreased by 66.01% and 74.18%,respectively.(3)Continuous freeze-thaw washing increases the fragmentation of soil particles and reflects the removal effect of heavy metal pollutants in the soil in this region.The soil near the middle of the soil column at 30-40 cm has the best removal effect.(4)The continuous freeze-thaw washing causes significant changes in the particle size distribution of the soil column as a whole.Among them,the percentages of clay particles,fine powder particles and coarse powder particles gradually increased with the increase of the number of tests,while the percentage contents of sand particles gradually decreased.Repeated coagulation test:(1)By setting the heating layer,the formation of segregation ice in soil was effectively promoted,and the migration of washing solution was increased.The water absorption was 8.36 kg,which was 9.3% higher than that of 89.87 kg during freezing.(2)Compared with the first freeze-thaw washing test,the concentration and removal rate of heavy metal pollutants in the washing solution were significantly improved by repeated fractionation and coagulation washing test,and the mass concentrations of Pb and Cd were increased by 122.62 % and 211.11 %,respectively.The removal rate increased by 251.6% and 264.7% respectively.This paper has 53 figures,10 tables and 76 references.