Importance of black carbon to sorption of native PAHs, PCBs, and PCDDs in Boston and New York, Harbor sediments

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 mug/L. Predictions based on published K-oc, K-BC, and n values for phenanthrene and pyrene showed good agreement with observed K-d,K-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 approximate to 0.83 log gamma(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.