Distribution Patterns, Toxic Equivalence, 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 soil fractions are inevitably different during the transformation of SOM. Understanding the distribution and toxic equivalence of PAHs in soil density fractions helps in elucidating the SOM characteristics and position roles in environmental risks of PAHs in contaminated soils. 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) after sonication, and the distribution and toxic equivalence of PAHs in the three fractions were studied and assessed. Furthermore, the roles of organic carbon (OC) content, black carbon (BC) content, and physical morphological characteristics of different soil fractions in PAHs distribution and environmental risks were discussed. 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, and both of them were far higher than those of in HF, which ranged from 4.80 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 studied soils, 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 exhibited an insignificant correlation with their OC content but had a significant correlation with BC content (**P < 0.01). The calculated average values of total TEQBaP (toxic equivalence of benzo[a]pyrene) in soil fractions followed the order of O-LF > F-LF ≫ HF. It is noteworthy that the enrichment capacity and high toxicity of PAHs in O-LF related to high BC content. We suggested that organic matter losing or soil physical structure destruction could lead to the exposure of PAHs in O-LF and the increase of environmental risks of PAHs in soil.