Page 117 - Partition & Adsorption of Organic Contaminants in Environmental Systems

108 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

sorption data. In earlier studies of pesticide–soil interactions, the soil was gen-
erally assumed to be a single adsorbent, or at best a mixed adsorbent of some
kind, analogous to other well-defined conventional adsorbents. Although this
view reconciled to a large extent the sorptive behavior of organic pesticides
on relatively dry soils and minerals, it ran into serious technical difficulties in
explaining the sorption data with water-saturated soils. We shall see later that
the different sorptive characteristics with relatively dry and water-saturated
soils are directly responsible for the change in chemical activity, bioavailabil-
ity, and toxicity of contaminants sorbed to the soil.
The adsorptive character of soils and minerals has been illustrated
unequivocally in earlier studies on the vapor uptake of chloropicrin (Stark,
1948), ethylene dibromide (Hanson and Nex, 1953; Wade, 1954), and methyl
bromide (Chisholm and Koblitsky, 1943) by water-unsaturated soils and
minerals, in which the vapor uptake is suppressed by soil moisture. The uptake
of parathion and lindane by soils from hexane solution exhibits a similar sup-
pression by soil moisture (Yaron and Saltzman, 1972; Chiou et al., 1985). These
observations indicate that organic compounds and water compete for adsorp-
tion on initially water-unsaturated soil minerals, which comprise most of the
available surface area of the soil solid. Moreover, the isotherms measured
for the uptake of pesticides from either the vapor phase or from a nonpolar
solvent (e.g., hexane) on water-unsaturated soils and minerals are commonly
nonlinear, characteristic of an adsorption process.
In keen contrast to the findings above, the uptake of the same nonionic
compounds from water by soils, or from vapor phase by water-saturated soils,
displays uniquely different features. Most notably, the extent of soil uptake for
given organic compounds shows a strong dependence on the SOM content
(Kenaga and Goring, 1980; Means et al., 1980; Kile et al., 1995). The uptake of
organic vapors by wet soils displays a similar effect (Wade, 1954; Leistra, 1970).
The predominant effect of SOM content in this case is demonstrated by the
relative invariance of the sorption coefficients of given organic compounds
among soils, or size fractions of soil, when the coefficients are normalized to
the SOM content (Karickhoff et al., 1979; Kenaga and Eoring, 1980; Kile
et al., 1995). The sorption isotherms of nonionic compounds on water-satu-
rated soils are all relatively linear (Chiou et al., 1979; Karickhoff et al., 1979;
Schwarzenbach and Westall, 1981; Sun and Boyd, 1991; Rutherford et al., 1992)
and are not strongly temperature dependent, exhibiting only small exother-
mic heats of sorption (Mills and Biggar, 1969; Spencer and Cliath, 1970;Yaron
and Saltzman, 1972; Pierce et al., 1974; Chiou et al., 1979). Moreover, the soil
uptake of binary nonpolar solutes from water occurs without a significant com-
petition between the solutes (Schwarzenbach and Westall, 1981; Chiou et al.,
1983, 1985) in contrast to the strong competitive effects found in sorption
by dry soil from the vapor phase and from nonpolar organic solvents
(Chisholm and Koblitsky, 1943; Wade, 1954; Spencer et al., 1969; Mills and
Biggar, 1969; Yaron and Saltzman, 1972; Chiou and Shoup, 1985; Chiou et al.,
1985; Pennell et al., 1992; Thibaud et al., 1993).