Page 201 - Partition & Adsorption of Organic Contaminants in Environmental Systems

192 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

environments and of the plausibility of certain surfactants or microemulsions
for remediating soils and natural solids contaminated by specific organic
compounds.

7.4 SORPTION FROM ORGANIC SOLVENTS

7.4.1 Effect of Solvent Polarity
There are relatively few studies reported on the soil sorption of organic com-
pounds and pesticides from organic-solvent solutions. Results from such
studies are useful for illustrating soil sorptive behavior in general and for
establishing the theoretical basis for selection of suitable solvents to recover
contaminants from soil, either for laboratory analysis or for remediation.
Obviously, the best solvents to recover contaminants are those that effectively
suppress the contaminant sorption to soil (or sediment). Based on the
behaviors of soil minerals and organic matter as described earlier, one would
anticipate that the partitioning of nonionic organic compounds from organic
solvents into soil organic matter (SOM) should be weak because of the good
solvency of the solution phase (Chiou et al., 1981; Chiou and Shoup, 1985).
Thus the extent of soil uptake would be determined mainly by the efficiency
of the compounds to compete with the solvent for adsorption on soil miner-
als. Because of the inherent polarity of inorganic minerals, the adsorptivity
of a compound (solute or solvent) would depend strongly on its ability to
engage in polar interactions with minerals.
Hance (1965) reported a set of experimental data which highlight the dif-
ference in the sorption of diuron from aqueous and petroleum solutions by an
oxidized soil (f om £ 0.015) and an organic soil (f om = 0.76). The sorption of
diuron to the oxidized soil was remarkably greater from petroleum solution
than from water solution, whereas the sorption to the organic soil was remark-
ably greater from water solution than from petroleum solution. The diuron
sorption from water solution to both soils was largely linear, whereas the
sorption from petroleum solution was noticeably nonlinear. The sorption
isotherms from petroleum solution are presented in Figure 7.36. The solubil-
ity of diuron in petroleum is about 30mg/L and in water about 36mg/L at room
temperature. From the data observed, Hance concluded that there was a com-
petition between diuron and water for adsorption sites in soil under aqueous
slurry conditions and that diuron competed more effectively for SOM than for
soil mineral surfaces. The high uptake of diuron from the petroleum solution
by the oxidized soil along with the noted isotherm curvature is illustrative
of the strong (competitive) adsorption of the polar solute (diuron) on soil
minerals and the relatively weak competition of a nonpolar solvent (petro-
leum spirit) for adsorption on minerals. The isotherm curvature with the
organic soil in Figure 7.36 is not evident, as the overall uptake is very weak;
this is because the soil has a low mineral content. On the other hand, the high