Study On Migration Characteristics Of Heavy Metals In Surface Soil

At present,heavy metal pollution in China's soil is relatively serious,especially in polluted sites.There are many types of heavy metal pollutants,high pollution concentration and complicated pollution forms.Heavy metal pollutants enter the soil and undergo migration and transformation,which in turn endangers human health.In the process of site soil treatment and restoration,the determination of the scope of pollution and risk assessment are particularly important.There are two main types of heavy metal migration in soil.One is migration through soil solution,and the main role of migration is convection.The other is migration caused by concentration difference,and its migration is dominated by dispersion.At present,the migration of heavy metals is mainly studied in the laboratory through the soil column leaching test.Only the migration of heavy metals dominated by convection is analyzed,and the migration of heavy metals under the dominant diffusion is not considered.The migration of heavy metals in contaminated sites is mainly caused by the concentration difference.At this time,heavy metal migration mainly includes molecular diffusion and mechanical dispersion.This experiment mainly studies the migration of heavy metal concentration difference,that is,adding contaminated soil on one side of the test device,adding test soil on the other side,sampling and measuring the concentration of heavy metals in the soil for a certain period of time,and further determining the migration amount of heavy metals.This experiment takes brown soil as the research object,and its basic physical and chemical properties are:moisture 19.55%±0.5%,bulk density 1.47g/cm~3,pH 8.16±0.12,organic matter 16.57g/kg,cation exchange capacity 16.22cmol/kg;soil The texture is sandy clay loam,the sand content is 51.26%,the powder content is 22.63%,and the clay content is 26.11%.The background values of five heavy metals such as Pb,Cd,Cr,Hg and As in soil were 27.49 mg/kg,0.243 mg/kg,60.61 mg/kg,0.152 mg/kg and 12.77mg/kg,respectively.The results show that the migration of five heavy metals such as Pb,Cd,Cr,Hg and As has a great relationship with the properties of heavy metals.Molecular diffusion and mechanical dispersion,adsorption and desorption and chemical reactions promote or inhibit the migration of heavy metals..The migration distance of heavy metal Pb at60d was:horizontal migration of 3 cm and vertical migration of 4 cm.The migration distance of heavy metal Cd at 60d was:horizontal migration 2cm,vertical migration4cm.The migration distances of heavy metal Cr at 1d,3d,5d,7d,14d,30d,60d are:horizontal migration 1cm,2cm,2cm,2cm,3cm,5cm,10cm;vertical migration 1cm,2cm,2cm,2cm,4cm,7cm,12cm.The heavy metal Cr has obvious migration in both horizontal and vertical directions,and the vertical migration is relatively fast.The migration distances of heavy metal Hg at 1d,3d,5d,7d,14d,30d,60d are:horizontal migration 2cm,3cm,6cm,7cm,7cm,10cm,12cm;vertical migration 1cm,1cm,2cm,2cm,5cm,9cm,10cm.The horizontal migration of heavy metal Hg is faster than the vertical migration,and the concentration of mercury migrated horizontally at the same distance at the same time is higher.During the test period,there was no significant migration of As horizontal migration and vertical migration.As the migration time increases,the migration of heavy metals at the same distance becomes smaller and smaller,and the migration rate becomes smaller and smaller.At the same time,as the migration distance increases,the amount of migration decreases.The migration of Pb and Cd in the soil is not obvious,and As has no migration during the test period,and adsorption-precipitation is the main influencing factor.At 60 days of migration,the migration of Pb at a horizontal migration of 1 cm was 0.288 g/(m~2·a),the migration of 3 cm was 0.188 g/(m~2·a),and the migration at a vertical migration of1 cm was 0.138 g/(m~2·a).The migration amount at 4 cm was 0.092 g/(m~2·a).The migration amount of Cd horizontal migration at 1cm is 0.0047g/(m~2·a),the migration at 2cm is 0.0045g/(m~2·a),and the migration of Cd at 1cm vertical migration is0.010g/(m~2·a).The migration amount of Cd at 4 cm was 0.0062 g/(m~2·a).Cr and Hg have obvious migration in the soil,which is less affected by adsorption-precipitation.After heavy metals enter the soil,they can migrate by molecular diffusion-mechanical dispersion.The vertical migration of heavy metal Cr is relatively faster than horizontal migration.The migration of Cr at a horizontal migration of 1 cm was 9.72g/(m~2·a),the migration at 10 cm was 0.23 g/(m~2·a),and the migration at a vertical migration of 1 cm was 11.01 g/(m~2·a),at 12 cm.The migration amount was 0.30g/(m~2·a).The horizontal migration of heavy metal Hg is faster than vertical migration.The migration amount of Hg horizontal migration at 1cm is 0.425g/(m~2·a),the migration at 12cm is 0.0013g/(m~2·a);the migration at 1cm vertical migration is0.220g/(m~2·a),at 10cm The migration amount was 0.0029 g/(m~2·a).The heavy metals of Pb,Cd,Cr,Hg and As have different migration abilities at different times.With the passage of time,the migration distance increases and the migration of heavy metals decreases at the same distance.In the horizontal migration process,the migration rate is Cr>Hg>Pb>Cd>As;during the vertical migration process,the migration rate is Cr>Hg>Pb>Cd>As.The migration of five heavy metals such as Pb,Cd,Cr,Hg and As in soil can be used as the basis for determining the range of heavy metal pollution in soil.