In vitro gastrointestinal mobilization and oral bioaccessibility of PAHs in contrasting soils and associated cancer risks: Focus on PAH nonextractable residues

The gastrointestinal mobilization and oral bioaccessibility of polycyclic aromatic hydrocarbon (PAH) nonextractable residues (NERs) from soils remain unexplored, including associated incremental lifetime cancer risks. This study investigated the gastrointestinal mobilization of PAHs and their NERs from contrasting soils, using a physiologically based extraction test that incorporates a silicone-rod (Si-Org-PBET) as PAH sink. Associated cancer risks following soil ingestion were also evaluated. Four solvent-spiked and aged soils, and four long-term contaminated manufactured gas plant (MGP) soils, were utilized. Total-extractable PAH concentrations were measured after exhaustive solvent extractions of soils. We evaluated the PAH sorption efficiency of the silicone rods and associated sorption kinetics, using PAH-spiked silica sand as the contaminated matrix. We then assessed gastrointestinal mobilization of benzo[a]pyrene and benzo[a]pyrene NERs from the solvent-spiked soils, and mobilization of six PAHs and their NERs from the MGP soils. PAH oral bioaccessibility was determined. The incremental lifetime cancer risks (ILCRs), using Si-Org-PBET- and total-extractable PAH concentrations from the MGP soils, were calculated. Sorption kinetics modelling showed that 95% of mobilized PAHs sorbed to the silicone rods within 2-19 h, depending on PAH physico-chemical properties. Total-extractable and Si-Org-PBET extractable PAH concentrations exceeded health investigation levels (3 mg/kg based on benzo[a]pyrene toxic equivalent quotients) in soils. PAH oral bioaccessibility approached 100% for solvent-spiked soils, but only 24-36% for the MGP soils. Associated ILCRs exceeded target levels (10^-5) for one MGP soil, particularly for 2-3 year olds, despite oral bioaccessibility considerations. In contrast, mobilized PAH NERs did not exceed health investigation and ILCR levels, as the NERs were highly sequestered, especially in the MGP soils. PAH nonextractable residues in long-term contaminated soils are unlikely to be mobilized in concentrations that pose cancer risks to humans following soil ingestion, and do not need to be considered in risk assessments.