Page 148 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 139

As noted, Eq. (7.17) predicts higher K oc than does Eq. (7.16) at the same K ow
value; the difference between the K oc values estimated by the two equations
increases significantly with increasing K ow value. In the range of logK ow = 3 to
6, the ratio of K oc from Eq. (7.17) for PAHs on sediment to that from Eq. (7.16)
for chlorinated benzenes and PCBs on soil increases from 4 to 8.
In view of these two distinct logK oc–logK ow correlations, it appears either
that PAHs on sediments exhibit appreciably higher K oc values than do substi-
tuted aromatic solutes on soils or else that PAHs exhibit considerably smaller
K ow values than do substituted aromatic solutes. The latter effect can be ruled
out on the basis that plots of logK ow versus logS w (supercooled liquid solubil-
ity) for different nonpolar solutes, including substituted aromatic solutes and
PAHs, fall virtually onto a single line (Kile et al., 1995). Similarly, since there
are no strong polar groups in PAHs, the much higher K oc values of PAHs
cannot be attributed to specific interactions with SOM or with soil minerals
in water solution (Haderlein and Schwarzenbach, 1993; Chiou, 1995). There-
fore, the higher K oc values of PAHs according to Eq. (7.16) result supposedly
from two effects: (1) higher partition with sediments than with soils for a wide
variety of nonpolar solutes, including PAHs; and (2) PAHs exhibiting higher
K oc values with SOM than other nonpolar solutes. The first effect was noted
earlier for the sorption data of CT and DCB on a wide range of soils and sed-
iments and presumably is also pertinent to PAHs. The contributions of these
two potential effects were investigated by Chiou et al. (1998) by analyzing the
sorption of three PAHs: naphthalene (NAP), phenanthrene (PHN), and
pyrene (PYR), on a series of “clean” soils as well as freshwater and coastal
sediments to elucidate their partition effects with SOM. Similar experiments
were performed on some “contaminated” coastal marine sediments. The
13
solid-state C-NMR data of some soils and sediments were also obtained to
substantiate the effect of SOM composition on the partition behavior of the
PAHs.
Molecular properties of NAP, PHN, and PYR are given in Table 7.7.
The sorption isotherms of these compounds on selected soils, river sediments,
and coastal sediments in Massachusetts Bay and in Boston Harbor (at
Spectacle Island, Peddocks Island, and Fort Point Channel) in Massachusetts
are essentially linear over a significant range of relative concentrations
(C e/S w). The experimental logK oc values of the PAHs are compiled in
Table 7.8.
To evaluate the impact of the original SOM polarity between soil and sed-
iment on K oc , the logK oc values for the three PAHs on five soils and the cor-
responding values on four sediments (the three freshwater sediments and the
coastal Massachusetts Bay marine sediment) are compared. The data for other
coastal sediments of Boston Harbor are excluded from this evaluation because
they are either known or suspected of being contaminated by organic wastes
(McGroddy and Farrington, 1995). As shown in Table 7.8, the K oc values
between soils or between sediments are relatively invariant, as previously
noted for CT and DCB on a large set of soils and freshwater sediments. The