The Experimental Study On The Migration Of Heavy Metal Ions In Different Environments Under Freeze-Thaw Cycles

China has a vast territory and a wide distribution of frozen soil.However,in recent years,with the exploitation of mineral resources and the construction of oil pipelines,highways and railways in the permafrost area,the content of heavy metals in the soil in the permafrost area has exceeded the standard in different degrees.Heavy metal elements in soil will not only enter the hydrosphere and atmosphere and affect the fragile permafrost ecological environment,but also may threaten human health through the food chain.Therefore,it is particularly important to study the migration characteristics of heavy metal pollutants in the permafrost,and it is of great significance to protect the soil environment in the permafrost area.Based on the artificial preparation of soil heavy metal pollution as the research object,indoor soil column freeze-thaw cycling test,explore in E1(groundwater),E2(rain),E3(deep pollution)and E4(quartz sand soil)four kinds of environment conditions,Cu,Zn,Cd,Cr,Ni,Pb after seven times freeze-thaw cycle of six kinds of heavy metal ions migration patterns.The transport of six heavy metals under different freeze-thaw cycles and rainfall intensity was simulated and predicted by using the HYDRUS-1D software.The preliminary study showed that:(1)During the freeze-thaw cycle,the temperature and unfrozen water content in the soil column have roughly the same changing trend with time.The temperature and unfrozen water content in the soil column change rapidly with time,then slowly change,and finally tend to be stable.The closer the temperature and unfrozen water content are to the top of the soil column,the more obvious the variation trend is.(2)After seven freeze-thaw cycles,stratified sampling results show that the content of heavy metal ions in the soil has a certain effect on the p H value of the soil,that is,the p H value of the soil decreases with the increase of the concentration of heavy metals.(3)After seven freeze-thaw cycles,the heavy metal contents in the four environmental conditions all changed.In general,the contents of the six heavy metals vary greatly between 0-150 mm and 150-450 mm of soil column,while the changes are relatively stable between 150-450 mm of soil column.In other words,with the increase of soil column depth,the contents of the six heavy metals in each layer gradually decrease.Because in the process of frozen soil column,the side to low side of the water by high temperature migration,heavy metal ions and migration along with the water,and soil column melts,and some heavy metal ions migrate down,so overall,heavy metal ions is mainly concentrated in the part of the soil column,and with the increase of soil column depth,migration quantity reduce gradually.Under the action of groundwater,the contents of Cu and Zn migrated to the bottom of the soil column were about 30mg/kg and 40mg/kg,respectively.The concentrations of Cr and Pb in the bottom of the soil column reached 80mg/kg and 70mg/kg,respectively.However,the contents of Cd in the 150-450 mm of the soil column were almost zero,indicating that Cd migration ability was poor under the action of groundwater.Ni has migrated,but not significantly.Rainfall can promote the migration of heavy metals,and the migration of heavy metals under this condition is more obvious than that under the action of groundwater.Under this condition,the contents of Cu and Zn at the bottom are higher than those under the action of groundwater,and the contents are33mg/kg and 58mg/kg,respectively.The content of Cd measured in the range of150-450 mm in the soil column is about 0.5mg/kg,indicating that Cd migrates downward under the condition of rainfall.The contents of Cr,Pb and Ni reached69mg/kg,112mg/kg and 33mg/kg,respectively.In the deeply polluted soil,the content of heavy metal ions is generally higher in the range of 0-300 mm,so the content of the six heavy metal ions is higher in the lower part of the soil column under the condition of deep pollution.The contents of Cu and Zn at the bottom are 40mg/kg and 90mg/kg,the contents of Cd at the bottom are 0.73mg/kg,the contents of Cr,Pb and Ni are413mg/kg,171mg/kg and 91mg/kg,respectively.In the quartz sandy soil,the contents of Cu and Zn are 31mg/kg and 81mg/kg,respectively.The downward migration of Cd in the quartz sandy soil is less than that in the rainfall condition,while the migration of Cr is more obvious than that in the rainfall condition.The contents of Pb and Ni were 79mg/kg and 11mg/kg,respectively.It can be concluded that the migration effect of different heavy metal ions under the action of quartz sand and rainfall is different.Through comparison,it is found that the four environmental conditions tested in this paper have the following effects on the migration of heavy metal ions in the freeze-thaw cycle: E3(deep pollution)>E2(rain)>E4(quartz sand soil)>E1(groundwater).(4)On the basis of experiment,through the HYDRUS-1D model was used to simulate the migration of heavy metals by different number of freeze-thaw and rainfall intensity,the influence of the simulation results show that the heavy metal migration quantity after freezing and thawing 30 times more than the freezing and thawing 20 times after the migration amount,the increase of number of freezing and thawing to promote the transfer of heavy metal ions,at the same time under the different rainfall intensity,heavy metal migration quantity is not the same,migrated to40 mm of rainfall amount is higher than for 20 mm rainfall amount of migration,reveal that the increase of rainfall intensity on the part of the soil heavy metal ions in the soil column under part,makes the increasing heavy metal concentrations of deep soil column.