The solid-water distribution ratios (K d values) of " native " PAHs, PCBs, and PCDDs in Boston and New York Harbor sediments were determined using small passive polyethylene samplers incubated for extended times in sediment-water suspensions. Observed solid-water distribution coefficients exceeded the corresponding f oc K oc products by 1-2 orders of magnitude. It was hypothesized that black carbon (f BC), measured in the Boston harbor sediment at about 0.6% and in the New York harbor sediment at about 0.3%, was responsible for the additional sorption. The overall partitioning was then attributed to absorption into the organic carbon and to adsorption onto the black carbon via K d) f oc K oc + f BC K BC C w n-1 with C w in µg/L. Predictions based on published K oc , K BC , and n values for phenanthrene and pyrene showed good agreement with observed K d,obs values. Thus, assuming this dual sorption model applied to the other native PAHs, PCBs, and PCDDs, black carbon-normalized adsorption coefficients, K BC s, were deduced for these contaminants. Log K BC values correlated with sorbate hydrophobicity for PAHs in Boston harbor (log K BC ≈ 0.83 log γ w sat -1.6; R 2) 0.99, N) 8). The inferred sorption to the sedimentary BC phase dominated the solid-water partitioning of these compound classes, and its inclusion in these sediments is necessary to make accurate estimates of the mobility and bioavailability of PAHs, PCBs, and PCDDs.
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