The role of colloidal organic matter in the marine geochemistry of PCB’s
The role of colloidal organic matter in the marine geochemistry of PCB’s
Date
1986-04
Authors
Brownawell, Bruce J.
Linked Authors
Person
Alternative Title
Citable URI
As Published
Date Created
Location
New Bedford Harbor
Buzzards Bay, MA
Buzzards Bay, MA
DOI
10.1575/1912/3932
Related Materials
Replaces
Replaced By
Keywords
Polychlorinated biphenyls
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.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1986
Embargo Date
Citation
Brownawell, B. J. (1986). The role of colloidal organic matter in the marine geochemistry of PCB's [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/3932