Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 334

with the various kinds of functional groups. The dissolution of n -octanol in water is
roughly eight octanol molecules to 100,000 water molecules in an aqueous phase. This rep-
resents a ratio of about one to twelve thousand (Schwarzenbach et al., 1993). Since water-
saturated n -octanol has a molar volume of 0.121 L/mol as compared with 0.16 L/mol for
pure n -octanol, the close similarity permits one to ignore the effect of the water volume
on the molar volume of the organic phase in experiments conducted to determine the
octanol-water equilibrium partition coeficient. The octanol-water partition coeficient k ow
has been found to be suficiently correlated not only to water solubility, but also to soil
sorption coeficients. In the experimental measurements reported, the octanol is consid-
ered to be the surrogate for SOM.
Organic chemicals with k ow values less than 10 are considered to be relatively hydrophilic—
with high water solubilities and small soil adsorption coeficients. Organic chemicals with
k ow values greater than 10 4 are considered to be very hydrophobic and are not very water-
soluble. Chiou et al. (1982) has provided a relationship between k ow and water solubility S
as follows:
log k ow = 4.5 − 0.75 log S (ppm).
Aqueous concentrations of hydrophobic organics such as polyaromatic hydrocarbons
(PAH) in natural soil-water systems are highly dependent on adsorption/desorption equi-
librium with sorbents present in the systems. Studies of compounds that included normal
PAHs, nitrogen and sulfur heterocyclic PAHs, and some substituted aromatic compounds
suggest that the sorption of hydrophobic molecules (benzidine excepted) is governed by
the organic content of the substrate. The dominant mechanism of organic adsorption is the
hydrophobic bond established between a chemical and natural organic matter in the soil.
The extent of sorption can be reasonably estimated if the organic carbon content of the soil
is known (Karickhoff, 1984) using the expression: k p = k oc f oc , where f oc is the organic carbon
content of the SOM, k oc is the organic content coeficient, and k p is the linear Freundlich
isotherm obtained for the target organic chemical. This approach works reasonably well
in the case of high organic contents (e.g., f oc > 0.001). Relationships reported in the literature
relating k ow to k oc show that these can be grouped into certain types of organic chemicals.
For PAHs, the relationship given by Karickhoff et al. (1979) is
log k oc = log k ow − 0.21.
For pesticides, Rao and Davidson (1980) report that
log k oc = 1.029 log k ow − 0.18.
For chlorinated and methylated benzenes, the relationship given by Schwarzenbach and
Westall (1981) is
log k oc = 0.72 log k ow + 0.49.
The graphical relationship shown in Figure 9.17 uses some representative values reported
in the various handbooks (e.g., Verscheuren, 1983; Montgomery and Welkom, 1991) for log
k ow and log k oc . The values used for log k ow are essentially midrange results reported in
the handbooks and in many studies. Not all log k oc values are obtained as measured val-
ues. Many of these have been obtained through application of the various log k oc -log k ow