Three-phase modeling of polycyclic aromatic hydrocarbon association with pore-water-dissolved organic carbon

Log–log plots of measured organic carbon-normalized sediment pore-water distribution coefficients (K′OCS) for several polycyclic aromatic hydrocarbons (PAHs) versus their octanol-water partition coefficients (K′owS) at two sites in the Elizabeth River, Virginia, show large deviations from linearity. Organic-carbon normalized distribution coefficients for these PAHs between sediments and pore waters decreased by more than two orders of magnitude with depth as well. To determine to what extent pore water dissolved and colloidal organic carbon (DOC) was responsible for the observed nonlinearity and decrease in K′OC, a three-phase model was used to estimate pore-water PAH-DOC binding coefficients (KDOC). Partitioning of PAHs to pore-water DOC (i.e., KDOC) enhances the observed “dissolved” phase PAH concentration, especially for high-Kow compounds, contributing to the nonlinearity in K′OC-Kow plots. However, our application of the three-phase partitioning model to these data indicate that, at most, pore-water PAH-DOC binding accounts for one order of magnitude of the observed decrease in K′OC with depth in the sediment bed. The results of this study are consistent with three-phase partitioning theory for hydrophobic organic compounds between sediment organic matter, pore-water DOC, and freely dissolved aqueous phases in natural systems.