Page 129 - Partition & Adsorption of Organic Contaminants in Environmental Systems

120 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

where S om is the solute solubility in SOM and S w the solute solubility in water.
5
For solid DDT with S w = 5.5mg/L (Weil et al., 1974) and K om 1.5 ¥ 10 at
25°C, one therefore gets S om 830mg/kg, or 0.83g/kg. By comparison, the
solubility of DDT in pure octanol is about 42g/L (Chiou et al., 1982b), which
is some 50 times its solubility in SOM. The low estimated solubility of DDT
in SOM is much expected for a relatively nonpolar solid compound in a polar
macromolecular amorphous material. It is evident from these data that the
very high sorption coefficient of DDT results primarily from its extremely low
water solubility, which gives rise to a high partition coefficient.
On the premise of solute partition, one expects organic compounds with
high water solubility to also exhibit high S om values because these compounds
are usually also more compatible with organic solvents. For example, as shown
in Table 7.2, benzene, with S w = 1780mg/L and K om 18, gives S om 32g/kg
according to Eq. (7.10), which is about 40 times greater than the S om value of
solid DDT (due partly to the fact that benzene is a liquid and DDT is a solid).
This is consistent with the fact that benzene is completely miscible with octanol
and most organic solvents. As shown in Table 7.2, the S om values for solid
compounds are smaller because of the melting-point effect. Thus, although
the S om values for a given solute vary somewhat among soils or sediments
due to compositional differences in their organic matters, the magnitudes
of the S om values fall largely into the range to be expected for low-
polarity organic compounds in relatively polar organic polymers. To explain
differences in soil uptake of organic compounds from water, Mingelgrin and
Gerstl (1983) suggested that the less polar the compound, the more it will
tend to adsorb on a hydrophobic surface (SOM) from a polar solvent (water),
while removing solvent molecules from that surface. This hydropho-
bic adsorption concept is not consistent with the fact that the limiting uptake

TABLE 7.2. Estimated Solubilities of Some Organic Liquids and Solids in Soil
Organic Matter by Use of Eq. (7.10) and the Sorption Data on Woodburn Soil
Compound K om S w (mg/L) S om (mg/g)
Liquids
Benzene 18.2 1,780 32.4
Chlorobenzene 47.9 491 23.5
o-Dichlorobenzene 186 148 27.5
m-Dichlorobenzene 170 134 22.8
1,2,4-Trichlorobenzene 501 48.8 24.5
Solids
p-Dichlorobenzene 159 72.0 11.5
2-PCB 1,700 3.76 6.4
2,2¢-PCB 4,790 0.717 3.4
2,4¢-PCB 7,760 0.635 4.9
2,4,4¢-PCB 24,000 0.115 2.8
Lindane 360 7.8 2.8
Source: Data from Chiou et al. (1983).