Bioaccessibility of PAHs and PAH derivatives in a fuel soot assessed by an in vitro digestive model with absorptive sink: Effects of aging the soot in a soil-water mixture

- Aging significantly reduces the bioaccessibility of PAHs and PAH derivatives.
- One cause is dissolution of pore-blocking substances, enhancing sorption.
- Another is competitive sorption by the soil during digestion.
- Soot-to-soil redistribution and blockage by organic matter play no roles.

Aging soot in soil under neutral aqueous condition for 30 days significantly (p < 0.05) reduced the apparent gastrointestinal bioaccessibility (Bapp) of polycyclic aromatic hydrocarbons (PAHs) and PAH derivatives (d-PAHs) natively present in a composite fuel soot sample. Bapp was determined under fasting conditions by a previously developed in vitro digestive model that includes silicone sheet as a third phase absorptive sink in the small intestinal stage. Redistribution of contaminants from soot to soil, determined in independent experiments, was too small to affect Bapp. Prior uptake by soot of a commercial humic acid representing dissolved soil organic matter had no impact on Bapp. We identified two causes for the reduction in Bapp by soil and found they were approximately additive. One is an aging time-independent “matrix effect” attributable to competitive sorption by the soil of labile contaminant that is desorbed from the soot during the digestion test. The other is the dissolution of soluble substances from the soot during the aging process that increases soot surface area and nanoporosity. The increased surface area and nanoporosity drive contaminants from labile to nonlabile states in the soot and decrease the desorption into the digestive fluid, the former contributing most to the reduction in Bapp. The present study shows that mixing of raw soot with soil has important effects, both aging and non-aging, on the bioaccessibility of soot-borne contaminants.

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