Page 150 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 141

dard deviations of the mean K oc values for the soils and for the sediments. The
difference in PAH partition observed between the soil and sediment (i.e., by
about a factor of 1.6) is comparable in magnitude to that (by about a factor of
1.7) found for CT and DCB on a large set of soils and freshwater sediments
described earlier. The result suggests that the unequal solute partition effi-
ciencies with soil versus sediment OM accounts roughly for a factor of 2 for
the discrepancy in K oc as exhibited by Eq. (7.16) for soils and by Eq. (7.17) for
sediments. The remaining gap stems presumably from the unequal solubilities
of PAHs and substituted aromatic compounds in the SOM of soils or sediments.
The disparate partition effects between PAHs and substituted aromatic
solutes on SOM may be illustrated by comparing the mean soil K oc values of
PHN (logK oc = 4.21; logK ow = 4.57) and PYR (logK oc = 4.98; logK ow = 5.18) in
Table 7.8 with the reported soil logK oc values of 2-PCB (logK oc = 3.50; logK ow
= 4.51) and 2,4¢-PCB (logK oc = 4.16; logK ow = 5.10) in Table 7.4, where
the PAH and PCB solutes have comparable logK ow values. As seen, there is a
sharp decline in K oc for substituted aromatic solutes (PCBs) relative to that
for the PAHs with increasing molecular weight (or K ow); the K oc values for
PHN and PYR are 5.1 and 6.6 times, respectively, those of 2-PCB and
2,4¢-PCB. These differences are comparable in magnitude to those by Eqs.
(7.16) and (7.17), when a correction is made for the soil versus sediment
effect. In addition, the enhanced reduction in K oc with increasing K ow for chlo-
rinated solutes (PCBs) relative to PAHs indicates that the partition effect of
the former with SOM decreases more rapidly with increasing molecular
weight.
It is of interest to explore reasons for the enhanced partition of PAHs in
SOM over that of other nonpolar solutes. Since the SOM contains various
polar and nonpolar moieties (Baldock et al., 1992), including aromatic and
aliphatic groups, it is pertinent to compare the solubility behaviors of PAHs
in apolar aromatic (e.g., benzene) and aliphatic (e.g., n-hexane) solvents and
in a moderately polar solvent (e.g., octanol). The solubilities of NAP, PHN, and
PYR in benzene at room temperature are 3.26mol/L (Acree and Rytting,
1983), 2.27mol/L (Hildebrand et al., 1917), and 0.697mol/L (Judy et al., 1987),
respectively; they are about 2.5, 4.9, and 7.4 times higher than their respective
solubilities in n-hexane. The mole fraction solubilities of these PAHs in
benzene are in fact very close to their ideal solubilities by Raoult’s law [Eq.
(5.12)], with PYR showing the greatest deviation from its ideal solubility by
only about a factor of 2 (i.e., the PAH molecular size has no strong bearing
on the solubility in benzene relative to the ideal solubility). The solubilities of
relatively nonpolar PAHs in n-hexane should be comparable with those in
octanol (see related partition data in Table 5.2). The increased deviation of
PAH solubilities in benzene and n-hexane (or octanol) with increasing molec-
ular size suggests that there is an increasing incompatibility of PAHs with an
aliphatic phase (or a moderately polar phase) as the PAH molecular size
increases. This result is analogous to the increased disparity in K oc between
PAHs and other low-polarity solutes (e.g., chlorinated benzenes and PCBs)