Page 154 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 145

soil correlates more closely with the sediment’s lower polar-group content.
Thus, while the aromatic content in SOM appears to enhance the K oc values
for PAHs, the difference between soil and sediment K oc values is related, at
least in part, to their different polar-group contents.
We now examine how the logK oc data of PAHs with contaminated coastal
marine sediments compare to those with “clean” river sediments. The Fort
Point Channel sediment from Boston Harbor is known to be severely con-
taminated by highly aromatic organic wastes that resemble a coal tar
(McGroddy and Farrington, 1995), whereas Spectacle Island sediment and
Peddocks Island sediment are suspected of being contaminated to lesser
extents by similar organic wastes. For these samples, one notes that the K oc
values of the PAHs on Spectacle Island sediment are virtually the same as
those of “clean” freshwater sediments and Massachusetts Bay marine sedi-
ment. This suggests that the level of contamination in this sediment has not
significantly affected the overall organic composition of the sample. The
carbon fractions for alkyl (29%), aromatic (32%) and O-alkyl and carboxyl-
amide-ester components (39%) of Spectacle Island sediment are about the
same as those of Helena Mississippi River sediment (Figure 7.15). The mod-
erately high K oc data with Peddocks Island sediment indicate that the sedi-
ment is contaminated by a significant level of organic wastes (McGroddy and
Farrington, 1995), which either reduces the polar-group content or increases
the aromatic content of the sediment (NMR spectra not available). The
severely contaminated Fort Point Channel sediment exhibits a high aromatic
carbon content (46%); the combined carbon content of O-alkyl and carboxyl-
amide-ester components is low (21%), excluding the oxygenated aromatic
carbons (e.g., the phenolic carbons at 140 to 160ppm). The alkyl carbon
content (33%) is the same as that of Helena Mississippi River sediment. The
spectral data for this sediment correlate semiquantitatively with the much
higher than average K oc values of the PAHs. A comparison of the NMR and
K oc data between Helena Mississippi River sediment and Fort Point Channel
sediment further illustrates the point that the polar-group content in SOM is
a significant factor affecting the K oc values of PAHs and other solutes.

7.3.5 Estimation of Sorption Coefficients for Nonpolar Contaminants
The foregoing analysis of the SOM compositional effect on the K oc values of
nonpolar solutes with soils and sediments and of the enhanced partition of
PAHs versus other nonpolar solutes with SOM offers an improved algorithm
for estimating the K oc values of various nonpolar contaminants with soils and
sediments by applying appropriate logK oc –logK ow correlations. Starting with
Eq. (7.16) for estimating the K oc values of nonpolar non-PAH compounds on
soils, the corresponding K oc values for the compounds on sediments can be
obtained by assuming that the sediment K oc is twice the soil K oc , that is,

logK oc = 0.904logK ow - 0.211 (7.18)