Extractability of polycyclic aromatic hydrocarbons in sediments: A matter of association?

Bioavailability and accumulation of sediment-bound polycyclic aromatic hydrocarbons (PAHs) by benthic biota are closely related to their extractability by water or mild aqueous solvents. Nevertheless, PAH accumulation by benthic organisms is sometimes considerably different from predictions based on an equilibrium partition coefficient K-OC between water and bulk sedimentary organic carbon (OC). We present evidence that PAH extractability is strongly affected by the type of OC acting as a sorbent. We compared extractability of spiked [C-14] fluoranthene from a variety of natural and man-made OC matrices, including bulk sediment organic carbon, peat moss, power plant fly ash, diesel soot, petroleum/natural gas soot, coal dust, and carbon black. Artificial sediments were prepared from glass beads amended with equal weight percentages (2%) of nine different types of OC. Amended sediments were spiked with [C-14] fluoranthene and batch-extracted with seawater and 0.5% sodium dodecyl sulfate (SDS) after 65 and 12 h of equilibration, respectively. Fluoranthene extractability by seawater ranged between 0.03% and 0.9%, corresponding to a 50-fold variation of apparent K-OC, and 0.03-18% for SDS. Correlation between seawater and SDS extraction efficiencies was weak, suggesting differences in the mechanism of solubilization. These results demonstrate that use of a single value of K-OC to predict bioavailability of fluoranthene should be avoided, and that attempts to extrapolate PAH extractability from water-only extraction experiments to aqueous solutions containing surface-active dissolved organic carbon, such as the gut fluids of deposit feeding macrofauna, is very likely to lead to erroneous predictions.