Page 155 - Partition & Adsorption of Organic Contaminants in Environmental Systems

146 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

Similarly, the correlation for PAHs on soils may be achieved by assuming the
soil K oc value to be one-half the sediment K oc value, which can in turn be
obtained by adjusting Eq. (7.17) to give

logK oc = 1.00logK ow - 0.51 (7.19)

The merits of Eqs. (7.16) through (7.19) have been largely substantiated by
Xia (1998) and Allen-King et al. (2002) in a statistical analysis of a large set
of nonpolar compounds on a wide range of soils and sediments if the SOM
content is more than about 0.1%. A more extensive testing of the generality
of these equations is warranted.
For soils and sediments having a SOM content less than about 0.1%, the
measured K oc values for given nonpolar solutes tend to be greater than pre-
dicted (i.e., higher than the values with high-SOM soils and sediments). A
logical explanation is that the occurrence of a weak and linear solute uptake
by soil minerals, due to solute concentration near (rather than condensation
on) mineral surfaces, as discussed earlier, may become quite significant for
low-SOM soils and sediments relative to the concurrent linear solute partition
to SOM. In this case, since the overall solute sorption would be virtually linear,
normalizing the sorption coefficient with SOM leads to a higher K oc than pre-
dicted. With a given low-SOM soil or sediment, the magnitude of this devia-
tion should be greater for more water-soluble solutes because of their smaller
partition coefficients with SOM, making the mineral effect more important.
Finally, one would not expect the soil versus sediment and PAH versus
nonPAH effects, as dealt with here for nonpolar solutes, to be equally evident
and important for relatively polar compounds because their partition to SOM
would be mediated strongly by more powerful polar interactions.

7.3.6 Sorption to Previously Contaminated Soils

We have so far considered primarily the sorptive behavior of contaminants
from water to relatively clean soils or sediments. In many organic-
contaminated sites, the sorption of a contaminant may be greatly influenced
by the preexisting contaminants in soil or sediment. If a local environment
contains an exotic organic plume (i.e., an excess organic phase), there is little
doubt that it will exert a large effect on contaminant uptake, as it will act effec-
tively as an excess partition phase to sequester the contaminant. However, if
the contaminated soil or sediment contains no clearly discernible separate
phase, the result would depend on the level of soil contamination, or more
exactly on whether the quantity in the soil exceeds the saturation limit in
SOM, because an excess phase will not appear before this limit is exceeded.
Often, the excess phase is not readily visible because it may be microscopic in
size. On the other hand, if the amount of contaminants in SOM is below sat-
uration, the effect on contaminant sorption should be insignificant (Sun and
Boyd, 1991). This is because the substance partitioned into SOM cannot func-
tion as a partition phase. For a substance to maintain a stable separate phase