Distribution Patterns, Toxicity and Environmental Risks of Polycyclic Aromatic Hydrocarbons in Different Density Fractions of Long-Term Field-Contaminated Soils

Soil organic matter (SOM) in different density fractions has different characteristics and turnover rates. The environmental fates and risks of polycyclic aromatic hydrocarbons (PAHs) associated with different SOM fractions are inevitably different during the transformation of SOM. Four long-term field contaminated soils were physically separated into free light fractions (F-LF), occluded light fractions (O-LF) and heavy fractions (HF) by using density liquids (1.60 g cm -3 ) combing with sonication, and the distribution, toxicity and environmental risks of PAHs in the three fractions were studied and assessed. The contents of total PAHs in F-LF and O-LF were in the range of 95.13-361.71 mg kg -1 and 226.32-1159.90 mg kg -1 , respectively, both of which were far greater than those of in HF ranging from 4.80 mg kg -1 to 8.94 mg kg -1 . Although the F-LF and O-LF accounted for only 0.57%-11.96% and 0.22%-1.07% of the soil, they were associated with 9.52%-47.88% and 11.43%-27.47% of soil total PAHs. The contents of total PAHs in different soil fractions have no significant correlation with their organic carbon contents, but have significant correlation with black carbon contents ( ** P < 0.01). The calculated average values of total TEQ BaP (Toxicity equivalent of benzo[a]pyrene) in soil fractions were in the order of O-LF > F-LF ≫ HF. Combing with the characteristics and turnover rates of OM in different soil fractions, we proposed a model of PAHs flow linking with F-LF, O-LF and HF in soil and the environmental risks of PAHs were considered to follow the sequence of F-LF > HF > O-LF