The release of carcinogenic PAHs, such as benzo(a)pyrene (BaP), from soils contaminated with coal tar is crucial to bioavailability/bioaccessibility-based risk assessment of former manufactured gas plants (MGP). Due to the carbonaceous nature of these materials, they may act as both sink and source of PAHs. This study focuses on the release kinetics of BaP from two Australian soils with different textures and organic carbon contents (OC) as well as a silica sand artificially contaminated with BaP solution, coal tar pitch, and liquid coal tar, at BaP concentration of 50 mg/kg. The desorption kinetics of BaP were determined by sequential Tenax extraction and a three-compartment (rapid, slow, and very slow) desorption model was characterised after 1, 24, and 48 days of ageing with dry-wet cycles. The results show different desorption patterns of BaP from soils containing coal tar pitch and liquid coal tar compared to soils with BaP in solution. The desorption of BaP from silica sand and the clayey soil with higher OC (MTA) was inhibited by coal tar pitch and liquid coal tar compared to parallel treatments of BaP solution after 1, 24 and 48 days of ageing. For the sandy soil with low OC (GIA), a similar trend was identified at 1 day, but desorption of BaP was largely enhanced in GIA with liquid coal tar after 24 days of ageing. Overall, MTA yielded the lowest rapidly desorbing fractions (Frap) for BaP (&lt; 2.2%), followed by GIA (&lt; 28%), and silica sand (&lt; 33%). The strongest ageing effects were identified in GIA treated with BaP solution and coal tar pitch (27% and 2% decreased Frap after 48 days, respectively). The ageing effects were less prominent in soils treated with pitched and liquid coal tar compared to BaP solution spiked soils. For MTA, declines in Frap in these two treatments over 48 days were less than 0.1%. In GIA and sand with liquid coal tar Frap increased after 24 days and decreased after 48 days of ageing. These findings show that coal tar as a source material will affect the release of the PAH in different ways, based on soil properties and its status (solid or liquid), during ageing process. This is important in the illustration of the re-mobilisation mechanisms of non-extractable PAH residues in MGP sites. The results of this study also help in the understanding of the changes in PAH bioaccessibility in aged MGP soils as the correlations between rapidly desorbing PAHs and their bioaccessibility have been reported in previous studies.

Effects of coal tar as source material on the desorption kinetics of benzo(a)pyrene from contaminated soils

256th National Meeting (2018)

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