Page 203 - Partition & Adsorption of Organic Contaminants in Environmental Systems

194 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

While the uptake from hexane by dry and nearly dry soils is remarkably higher
than that from water, such uptake is strongly suppressed by humidity and
approaches zero when the soils become water saturated (to be in contrast with
the observation that parathion exhibits a definitive uptake on the same soils
from water). The saturation water content ranges from about 2wt% for the
sandy Mivtahim soil (6% clay; f om = 0.003) to 17% for the clay-rich Har Barqan
soil (56% clay; f om = 0.019). These observations led the authors to suggest that
different mechanisms govern the parathion “adsorption” in aqueous soil
suspensions and in hydrated soil–organic solvent systems. They assumed that
in soil–water systems the solvent (water) is preferentially adsorbed but that
small amounts of parathion diffuse through water films and get adsorbed as
they approach the colloid surfaces.
It is evident from the parathion sorption data of Yaron and Saltzman (1972)
that the dominant mechanism of the soil sorption in organic-solvent systems
is different from that in aqueous systems. Results found in aqueous systems
are reconcilable with the assumed solute partition into SOM. The high
parathion uptake from hexane on dry soils is attributable to adsorption on soil
minerals (mainly clays), on which the specific interactions of parathion’s polar
groups reduce the adsorptive competition of nonpolar hexane (while the par-
tition of parathion into the organic matter is minimized by the good solvency
of hexane). The parathion uptake from hexane would therefore be depressed
by humidity because of the strong adsorptive competition of water for min-
erals (in this case,water is considered as a competing solute),which leads even-
tually to a nearly complete suppression of parathion uptake when the soils
become fully water saturated. By comparison, parathion exhibits a definitive
uptake on soil from aqueous solution because the poor solvency of water (in
addition to its suppression of mineral adsorption) makes the partition of
parathion into the SOM a favorable process. The failure of the soils to sorb
parathion from polar organic solvents is due apparently to the fact that such
solvents minimize solute (parathion) adsorption on minerals because of their
polarity and reduce the solute partition into SOM by their good solvency.
Therefore, polar solvents are much more effective than nonpolar solvents to
recover nonionic organic contaminants from soil.
Since the presumed adsorption of organic solutes from an organic solvent
onto a soil occurs by a competition of solute and solvent for relatively polar
soil minerals, the amounts of adsorption for different solutes on a given soil
will be closely related to the solute polarity. In light that solute adsorption
would be most effective from a nonpolar solvent, as illustrated above, a polar
solute should exhibit much higher adsorption than a nonpolar or weakly
polar solute from a nonpolar solvent onto a soil. The sorption isotherms of
parathion, lindane, 2,4¢-PCB, and 1,2-dichlorobenzene from hexane onto
Woodburn soil (f om = 0.019), as shown in Figure 7.37, are in agreement with
this expectation. Here the uptake of parathion is greatly enhanced over
those of other relatively nonpolar solutes (lindane, 2,4¢-PCB, and 1,2-
dichlorobenzene) because parathion has a much higher polar-group content.