Page 152 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 143

tate their gaining a closer approach to the aromatic components in natural
organic matter (Chin et al., 1997) or their intermolecular attractions through
p–p electron interactions (Gauthier et al., 1987). From the partition stand-
point, the favored interaction between aromatic components seems more
likely to be related to similarity in their cohesive energy densities (CEDs) (or
their solubility parameters, d). As shown in Table 2.2, for example, the d values,
3 1/2
in (cal/cm ) , for benzene (9.2), NAP (9.9), PHN (9.8), and anthracene (9.9)
are consistently high compared with those of other nonpolar solutes. The d
value estimated for PYR from its solubilities in hydrocarbons is about 10.6
(Acree, 1998). Thus, on going from benzene to PYR, the d values for PAHs
stay relatively constant with increasing molecular size. The d values for most
short-chain halogenated compounds are in the range 8.5 to 10 (e.g., d= 8.6 for
CT) and the values for aliphatic hydrocarbons (e.g., n-hexane) are much lower,
mainly in the range 7 to 8. The d values for low-molecular-weight alkyl-
substituted aromatic compounds (e.g., toluene and xylenes) are about 8.6 to
9.0 (Hildebrand et al., 1970). The d data for halogenated aromatic compounds
(e.g., chlorinated benzenes and PCBs) are essentially unavailable. The d for
SOM is reported to range from 10.3 with relatively nonpolar solutes to 17.2
with highly polar and H-bonding solutes (Chiou and Kile, 1994). As noted,
since the aromatic compounds have similar structures and presumably com-
parable CEDs, the two mechanisms considered to favor the partition of PAHs
with SOM are interrelated.
On the basis that the SOM is a natural heterogeneous polymer, where the
various components (or molecular segments) may be present in sizable
domains, it is possible that a given class of solutes would interact more favor-
ably with certain SOM components of comparable CEDs, such as for PAHs
with SOM’s aromatic regions. This consideration is consistent with the vari-
able d value of SOM, as stated earlier. The solubility of a compound in a
polymer medium may thus depend more on the polymer’s functional-group
content than on its overall polarity, the latter being more important for low-
molecular-weight organic substances. Since the d values of PAHs are fairly
constant, their solubilities (as supercooled liquids) in SOM should be approx-
imately the same and largely independent of the molecular weight; this is con-
trary to the significant solubility decrease of other nonpolar solutes in SOM
with increasing molecular weight. This dissimilarity provides an a priori
account of the increased deviation between the K oc values of PAHs and other
nonpolar solutes with an increase in K ow .
The relative amounts of different structural carbons in SOM, as revealed
13
by the solid-state C-NMR spectra of whole soils and sediments, provide
added insights into the effect of SOM composition on solute K oc . However,
such assignment of organic carbon types is only semiquantitative because of
the overlapping chemical shift of certain functional-group carbons and of the
imprecise quantification of some functional groups (Baldock et al., 1992). The
13
C-NMR spectra of Marlette soil, Helena Mississippi River sediment,
Spectacle Island sediment, and Fort Point Channel sediment are selected for