Effects of ageing and soil properties on the oral bioavailability of benzo[a]pyrene using a swine model

Oral bioavailability of benzo[a]pyrene (B[a]P) was studied in a swine model using eight spiked soil samples after incubation for 50 and/or 90 days. Silica sand was used as a reference material and the relative bioavailability (RB) of B[a]P in soils was calculated as the quotient of the area under the plasma B[a]P curve (AUC) for soil and AUC for the silica sand. Significantly reduced RB was observed in all study soils after 90 days ageing, ranging from 22.1 ± 0.4% to 62.7 ± 10.1%, except for one very sandy soil (sand content 87.6%) where RB was unchanged (108.1 ± 8.0%). Apart from this, bioavailability decreased during ageing with the decrease (from day 50 to day 90) being only significant for a clayey soil containing expandable clay minerals. Statistical analyses of B[a]P RB at day 90 (eight soils) and soil properties showed no direct correlation between RB and specific soil properties such as total organic carbon (TOC) and clay content which were commonly linked to organic contaminant sequestration. However, strongly significant relationships (p < 0.001) were found between RB and the fine particle associated carbon (FPAC) defined as (Silt + Clay)/TOC, and between RB and the soil mesopore (< 6 nm; p < 0.001) fraction, after two samples with high pH and high EC being excluded from the analyses. The bioaccessibility estimated by four in vitro extraction methods: dichloromethane/acetone sonication (DCM/Ace), butanol vortex (BuOH), hydroxypropyl-β-cyclodextrin extraction (HPCD) and Milli Q water leaching methods at different sampling time (1 day, 50 days and 90 days after spiking) also showed a decreasing trend. Significant correlations were found between B[a]P RB and DCM/Ace (R2 = 0.67, p < 0.05) extractable fraction and BuOH (R2 = 0.75, p < 0.01) extractable fraction.