Gastrointestinal Bioaccessibility of PAHs and PAH Derivatives Natively Present in Soot Particles Using an in Vitro Model: Effects of Soot Mixing with Soil.

Soot or black carbon distributes widely in soil and sediments and on environmental surfaces, especially in impacted areas, and typically contains high levels of PAHs. Gastrointestinal uptake of environmental particles via hand-to-mouth activities is a significant route of exposure to PAHs in humans, especially young children. The level of exposure depends on the bioavailability of PAHs in the soot within the digestion tract. We investigated the apparent bioaccessibility (B_app) of 11 EPA reference PAHs, one nitro-PAH and four oxo-PAH derivatives natively present in a fuel soot using an in vitro three-stage GI model. The model includes a silicone polymer sheet during the small intestinal stage as an absorptive sink to mimic uptake by the epithelium, which steepens the concentration gradient across the soot-digestive fluid interface and promotes desorption. Our previous studies confirmed an increase in B_eff in the presence of the sheet, and showed that B_eff increased under feeding conditions, especially when lipids were added. Aging of soot in a soot-soil-water mixture for up to 30 d caused a general decrease in B_eff. Pre-coating the soot with humic acid had little effect on B_eff, and mass transfer from soot to soil was too small to explain the aging effect. Part of the decrease in B_eff was caused by gradual dissolution of mineral salts present in the soot, which exposed hydophobic surface area and decreased desorption. Another part of the decrease in B_eff was attributed to a “matrix effect”, in which the soil competes with the soot and the silicone sheet for PAHs desorbed into the digestive fluid. The matrix effect is exhibited even in non-aged mixtures.The results have implications for the risk of PAHs through ingestion of soil and environmental particles.