Quantification of the Dilute Sedimentary Soot Phase:  Implications for PAH Speciation and Bioavailability

Existing field data indicate that soot may significantly affect the environmental speciation of polycyclic aromatic hydrocarbons (PAHs). To expand hydrophobic partition models to include soot partitioning, we need to quantify fsc, the soot fraction of the solid matrix, and Ksc, the soot-carbon-normalized partition coefficient. To this end, we have developed a method that allows quantification of soot carbon in dilute and complex sedimentary matrices. Non-soot organic carbon is removed by thermal oxidation, and inorganic carbonates are removed by acidification, fol lowed by CHN elemental analysis of the residual soot carbon. The selectivity of the soot carbon method was confirmed in tests with matrices of known compostion. The soot quantification technique was applied to two sets of natural sediments, both previously analyzed for PAHs. The input histories of PAHs and soot recorded in a lacustrine sediment core followed the same general trends, and we thus infer a coupling between the two. Our measures of fsc and calculations of Ksc, approximated from studies of PAH sorption onto activated carbon, were applied to rationalize previously generated in situ Kocvalues. Intriguingly, we find that the elevated PAH Kd values of two marine sediment−porewater systems are now quantitatively explain able through the extended, soot-partioning inclusive, distribution model. The importance of the soot phase for PAHs in the environment has implications for how we perceive (and should test) in situ bioavailability and, consequently, also for the development of sediment quality criteria.