Page 151 - Partition & Adsorption of Organic Contaminants in Environmental Systems

142 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

with increasing molecular weight or K ow. From these results, PAHs should
exhibit much greater partition with the SOM’s aromatic components (espe-
cially, those without polar substituents) than with its other components or
molecular segments.
To relate the K oc data to solute partition with the SOM’s components, it is
necessary to know the aromatic and other group contents in SOM. Based on
13
C-NMR data for whole soils, the organic carbon in SOM is about 20 ± 5%
aromatic, 25 ± 6% alkyl, 40 ± 10% O-alkyl (e.g., carbohydrate), and 15 ± 5%
carboxyl + amide + ester (Baldock et al., 1992). The aromatic content includes
hydrogen- and carbon-substituted aromatics, oxygenated aromatics (e.g., those
with —OH and —COOH), and unsaturated carbons (Baldock et al., 1992);
the separation of these components is technically difficult, however. Using the
above-estimated carbon fractions in SOM, if one assumes that (1) the PAHs
partition to SOM’s aromatic components as effectively as they do to a pure
aromatic solvent (as in benzene); (2) the partition to SOM’s aliphatic compo-
nents is about the same as to n-hexane; and (3) the partition to the remaining
SOM components is not significant, the K oc values calculated for NAP, PHN,
and PYR by the assumed solubilities in SOM and in water should be about
the same as the values measured. However, the K oc values calculated, even
discounting the PAH partition to SOM’s aliphatic fraction, are significantly
higher than the values measured, to be explained shortly; the partition to
aliphatic fraction alone is not sufficient to account for the measured K oc values.
If an equivalent “nonpolar aromatic carbon” fraction of about 0.10 in SOM is
assumed, the calculated logK oc values for NAP (3.12), PHN (4.50), and PYR
(5.02) are then comparable with their values measured with sediments in Table
7.8 (2.88, 4.42, and 5.18, respectively) and with other sediment logK oc values
reported by Karickhoff et al. (1979) in Table 7.6 (3.11, 4.36, and 4.92, respec-
tively). The similarly calculated logK oc for anthracene (4.55) using its solubil-
-2
ities in benzene (8.90 ¥ 10 mol/L) (Acree and Rytting, 1983) and in water
-7
(2.51 ¥ 10 mol/L) (Chiou et al., 1982b) also agrees well with the logK oc value
of 4.41 measured with sediments (Karickhoff et al., 1979).
Whereas a quantitative account of the K oc values requires well-defined
SOM molecular structures that are presently unavailable, the results above
support the assumed influence of the SOM’s aromatic components on PAH
partition. The observation that the calculated K oc values based on the total
aromatic content in SOM are much higher than the actual values suggests that
most aromatic structures in SOM are substituted with some polar and non-
polar groups, which reduce their overall compatibilities with PAHs. Since
sediments exhibit slightly higher K oc values than soils, the sediment organic
matter should have either a lower polar-group content or a higher aromatic
content. The approximate aromatic fractions in SOM for some of the present
13
soils and sediments will be estimated later using the solid-state C-NMR
spectra of the whole samples.
The enhanced affinity of PAHs over other nonpolar solutes for natural
organic matter has been attributed to the PAH’s planar structures that facili-