Freely-dissolved concentration of persistent, bioaccumulating and toxic compounds (PBTs) in the environment controls numerous diffusive processes, such as volatilization, sorption and bioaccumulation. Thus, the freely-dissolved concentration has been established as the bioavailable fraction of PBTs. One of the main sources of PBTs into aquatic systems is sediments. Measurements as well as prediction of the freely-dissolved pore water concentration in sediment are essential. However, both measurements and predictions are complex. For example, low water solubility of PBTs complicates the measurements, and sorption into non-linear behavior sorbents in the sediment, makes prediction tough. Here, polychlorinated biphenyls (PCBs) freely-dissolved sediment pore water concentrations were measured and predicted from sediment from Indiana Harbor and Ship Canal. PCBs in these sediments are among the highest in the Great Lakes region (up to 100 000 ng g-1 dw), with high values of black carbon (~30% of total organic carbon). PCB freely-dissolved sediment pore water concentrations were measured using SPME PDMS-fiber and ranged from 6 to 1400 ng L-1 (Sum of 39 congeners). Equilibrium partitioning into organic carbon over-predicted the freely-dissolved sediment pore water concentration in 7-fold. Because of these outcomes, black carbon sorption was included into the prediction model. Thus, sediment was chemically oxidized to remove the organic carbon and leave the black carbon and PCBs. Using the oxidized sediment, a set of experiments was performed to determine the black carbon-normalized partition coefficients (KPCBi BC) for 22 PCB congeners. A strong log-log correlation was found between the measured KPCBi BC and the octanol-water partition coefficient (log KPCBi BC = 1.05 x log KPCBi OW + 0.55, R2 = 0.81, p < 0.001). Partitioning into organic carbon together with adsorption onto black carbon reduced the over-prediction of freely-dissolved sediment pore water concentration to only 1.5-fold.

Measurements and prediction of freely-dissolved PCB sediment pore water concentrations in enriched black carbon sediment

246th National Meeting (2013)

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