Effects Of Organic Carbon Content On The Composition And Residue Of Polycyclic Aromatic Hydrocarbons In Soil Profile

In order to reveal the effect of total organic carbon(TOC)on the vertical migration and residues of polycyclic aromatic hydrocarbons(PAHs)in soil profiles,simulation leaching experiments of PAHs vertical migration in soil-column were carried out.Fourteen groups of experiments have been conducted according to the TOC of soil and the amount of leaching water.Soil samples were collected from Mentougou district,Yanqing district,Fangshan district,Tongzhou district and Haidian district,Beijing.Sixteen priority PAHs were chosen as simulating pollutants.Experiments were leached intermittently by deionized water for 10～120 days.Migration characteristics of PAHs in nature soil profiles and soil leaching stimulation experiments were discussed and the distribution of PAHs in leachate and soil particulate matter(SOM)were analyzed.It was especially focus on the effect of TOC and the amount of leaching water on the residues of PAHs in soil profiles.The results show that:(1)After leaching,PAHs in different columns all migrate vertically.The residue of PAHs mainly distributes in the surface soils.Both of total PAHs and monomer PAHs firstly significantly decrease and then become stable with the depth in the profiles.The migrational characteristics of different monomer PAHs are different,but PAHs with similar rings or isomers are of a similar migration trend.(2)The amount and compositions of PAHs residue in topsoil with different TOC varies each other,suggesting TOC and physicochemical property of PAHs all have important influences on the migration of PAHs in soil.(3)The content of PAHs in leachate maintain at a lower level at the early period,then gradually increase,and trend to stable at the middle-late period.The composition and content of PAHs in leachate from different profiles is different.The composition and relative content of PAHs with higher rings is low,indicating PAHs with higher rings is hard to dissolve in water and to migrate vertically.(4)PAHs are also detected in POM which migrated with leaching water,suggesting PAHs could be absorbed in POM and migrate vertically under leaching.In addition,there are the amounts of PAHs in POM are different in profiles with different TOC.The amounts of PAHs in POM from profiles with high TOC is higher than that from lower TOC.(5)The content of total PAHs and monomer PAHs residue in soil profile firstly decrease with increasing depth,then become stable,which is consistent with the distribution of TOC in soil profile.It indicates that the TOC is the main factor which affect the residue of PAHs in soil.The effect of TOC on residue of PAH with different rings in topsoil is different.PAHs with lower rings are more readily to migrate,while PAHs with higher rings combine with topsoil more closely,and hard to migrate under leaching.(6)The amount of PAHs residue in topsoil decrease gradually with increasing leaching water and the compositions of PAHs also varies,suggesting the amount of leaching water could affect the residue of PAHs in soil profile.It has more impact on PAHs with low-middle ring than PAHs with higher ring.(7)PAHs which migrates from topsoil to deep soil isn’t easy to be absorbed in deeper soil for their lower TOC,then could continue to migrate,thereby causing the pollution of shallow groundwater.