Hydroxypropyl-β-cyclodextrin (HPCD) extraction of urban soils: mass balance, bioaccessibility, and cancer risk assessment of 71 polycyclic aromatic hydrocarbons (PAH) from petrogenic and pyrogenic sources

Bioaccessibility of polycyclic aromatic hydrocarbons (PAH) in soils is often determined by extraction with hydroxypropyl-β-cyclodextrin (HPCD), yet there is no standardized approach for determining bioaccessible concentrations (c_bio), which are either calculated or directly measured. Additionally, most studies analyze solely the 16 EPA PAH, neglecting more carcinogenic PAH with higher relative potency factors (RPFs) and often disregarding PAH sources. This study examines optimal c_bio determination by providing mass balances of HPCD extraction. Moreover, this study investigates the bioaccessibility of 71 PAH in five urban soils, considering their petrogenic to pyrogenic sources and their carcinogenic potential. Checking HPCD mass balances revealed that mechanical stress and residual HPCD can artificially increase the residual PAH content (c_non-bio), making c_bio calculation using c_non-bio unfeasible. We therefore recommend re-extraction of HPCD extracts to measure c_bio. The measured c_bio of PAH depended on PAH sources, with lower c_bio observed in petrogenic coal-containing soils. Within c_bio, 7H-benzo[c]fluorene contributes most to toxicity equivalents (TEQs), while the most carcinogenic EPA PAH, benzo[a]pyrene, contributes little. Dibenzopyrenes, considered highly carcinogenic, were not bioaccessible. TEQs could only be calculated for 24 PAH with available RPFs. However, additional PAH with evidence for carcinogenicity were also bioaccessible, with 2-methylfluoranthene and benzo[c]phenanthrene reaching c_bio of >1 mg/kg. Notably, benzo[c]phenanthrene is classified by the IARC as a group 2B carcinogen. These findings suggest that particularly 7H-benzo[c]fluorene and benzo[c]phenanthrene should be included in routine risk assessments due to their carcinogenic categorization and bioaccessibility. Future studies should focus on expanding carcinogenicity data and RPF values for additional PAH.