Study On Remediation Of Heavy Metal Contamination In Clay Soil By Composite Freeze-thaw And Soil Washing

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,the bench-scale shaking test and freezing temperature test were conducted to selecte suitable eluent and determine washing parameters.And then,through the freeze-thaw leaching test,the effects of freeze-thaw cycles,eluent concentration and times of supplying the eluent were studied,and the best leaching scheme was explored.Finally,the morphological changes of heavy metals in the contaminated soil and the changes of pH and electrical conductivity were analyzed to improve the freeze-thaw leaching mechanism.The following conclusions are drawn:(1)Freeze-thaw leaching method can effectively remove the heavy metal pollutants in clay soil.Because freezing-thawing cycle can reduce cohesive force of cohesive soil and destroy its original particle structure,particle rearrangement contributes to full contact and reaction between eluent and contaminants,movement and migration of mixture,which improve the elution efficiency.(2)The removal effect of cohesive soil with Pb and Cd is influenced by the leaching agent,ratio of solution to soil and freeze-thaw mechanism.Therefore the removal efficiency can be improved by selecting the appropriate eluent concentration,freezing and thawing mechanism at the low ratio of solution to soil conditions.On the countrary the excessive increase of the concentration of the eluent and the number of freeze-thaw can not only improve the leaching efficiency,but also cause the waste of resources.Compared with ordinary leaching,freeze-thaw leaching can save the dosage of leaching agent,improve the efficiency of leaching and reduce the workload of recycle,which contributes to field remediation.(3)Freeze thaw leaching method can remove the residue content of heavy metals in soil,and at the same time it has less effect on the physicochemical properties of the soil due to the smaller amount of the eluent.