The role of colloidal organic matter in the marine geochemistry of PCB’s

Abstract

Polychlorinated biphenyls (PCBs) were used as model hydrophobic organic compounds (HOC) to study physical~chemical processes which affect the speciation and fate of HOC in coastal environments. The focus of this study is on the sorption of PCBs with colloidal organic matter in seawater, and the influence of this process on the distribution of PCBs in coastal sediments. Laboratory and field experiments were used to make quantitative estimates of PCB-organic matter sorption, and to test predictions of three-phase equilibrium models. Astatic headspace partitioning method was developed to measure the sorption of several individual chlorobiphenyls with colloidal organic matter enriched from coastal seawater by hollow fiber ultrafiltration. This technique directly measured the dissolved phase fugacity of PCBs in experimental bottles, and avoided uncertain separation techniques often used in isolating various chemical phases. Colloidal organic carbon normalized partition coefficients (Koc) were determined from linear sorption isotherms, and increased from 1.9 x 104 (L/Kg) for 2,4'-dichlorobiphenyl to 3.5 x 105 for 2,2 1 ,3,4,5'-pentachlorobiphenyl. Sorption tended to increase with increasing octanol-water partition coefficients (Kow ) of the sorbate, and values of Koc were within a range of those reported in other experimental sorption studies with sediments and dissolved humic substances. Experimental partitioning results support the hypothesis that HOC-organic colloid sorption is similar to HOC sorption by sediment organic matter. PCBs were measured in the interstitial waters and sediments of three box cores obtained from New Bedford Harbor and Buzzards Bay, Massachusetts. The three sites studied had a wide range of sediment PCB concentrations, and reducing conditions provided environments containing high concentrations of colloidal organic matter. Dissolved organic carbon (DOC) and total sediment organic carbon (TOC) were measured in two cores to test the role of organic matter on the observed partitioning of PCBs. PCB concentrations, particularly those of less soluble chlorobiphenyls, were highly elevated in interstitial waters compared to water column concentrations at all three sites. The measured apparent distribution coefficients (K'd) of individual chlorobiphenyls did not increase with hydrophobicity (Kow) and indicate that a large fraction of PCBs in interstitial waters must be sorbed to organic colloids. A three-phase equilibrium sorption model, in which dissolved PCBs are in a dynamic equilibrium with colloidal and sediment organic matter, accounts for many aspects of the field data. There is good agreement of observed partitioning at New Bedford Station 67 and Buzzards Bay Station M with model calculations based on predictions derived from laboratory experiments of HOC sorption with sediments and organic colloids, but model calculations underpredict K'd at Station 84 in New Bedford Harbor. The sediment-interstitial water results were contrasted with a study of PCB partitioning in the water column at two stations in New Bedford Harbor. Measured K'ds of PCBs in the water column increased with Kow and approached predictions based on two-phase, water-suspended particulate partitioning when the effect of organic colloids on high Kow-PCBs was considered. The effect of a mobile colloid-sorbed PCB phase on a general sediment transport model was evaluated. Enhanced migration of highly sorbed PCBs can be included in an apparent diffusivity term when steady-state conditions exist. Model calculations showed that diffusive migration of PCBs in typical coastal sediments is minor over 20 yr timescales. Comparisons with the 210Pb profile provided additional evidence that the solid phase depth profiles of PCBs at Station M were controlled by biological mixing of the sediments and not pore water migration. Predictions based on results from Station M indicate that sediment diffusion and bioturbation of colloid-sorbed PCBs in surface sediments could provide a potentially important flux of PCBs across the sediment-water interface to the water column in Buzzards Bay.