Page 205 - Partition & Adsorption of Organic Contaminants in Environmental Systems

196 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

mic heat,the solute adsorption in this case must be either insignificant or weak.
For example, as noted earlier, adsorption of phenanthrene from water onto
various pure minerals (Huang and Weber, 1997) exhibits weak and linear
uptake with small exothermic heats (i.e., less exothermic than the reverse heat
of solute solution in water) because the solvent (water) is preferentially
adsorbed. However, in certain binary-solute systems, the adsorptive competi-
tion of one solute against the other for adsorbent surfaces may give rise to an
anomalous temperature effect, if their heats of adsorption per unit area as
single solutes are relatively comparable and if the displacement of one solute
by the other produces a large gain in entropy of the system.
In the parathion uptake from hexane on partially hydrated soils as reported
by Yaron and Saltzman (1972), the amounts of water in soils and in hexane
solution are below saturation. Since parathion contains many polar groups,
it should have a relatively high heat of adsorption per unit area with dry
soil minerals. In addition, parathion is considerably bigger in molecular size
than water. Adsorption of 1 mole of parathion by displacing water from
mineral surfaces of a partially hydrated soil (where there is enough water to
cover all mineral sites) would release many moles of water into hexane
solution. Although this displacement may be moderately endothermic for
parathion, the gain in system entropy by releasing more water from surface
sites to unsaturated hexane solution may be high enough to offset the
enthalpic deficiency and thus to make the process favorable (Chiou, 1998).
This entropic driving force diminishes as the hexane solution becomes more
saturated with water. When the water content in soil exceeds saturation, there
would be no more entropic driving force for parathion adsorption from
hexane, making it difficult to detect the temperature effect. On the other hand,
if the amount of water in soil is way below the monolayer capacity, the uptake
of parathion from hexane should be exothermic because of its adsorption on
abundant water-free mineral sites. Thus, although it is possible for the adsorp-
tion of a solute to be either exothermic or endothermic, as suggested by
Mingelgrin and Gerstl (1983), the endothermic adsorption occurs only for
special binary-solute systems in which the increase in total entropy is sufficient
to compensate for the unfavorable enthalpic balance.
Chiou et al. (1985) further explored the sorption of parathion and lindane
from aqueous and hexane solutions to substantiate the roles of soil minerals
and organic matter in uptake by soil, using Woodburn soil (a mineral soil) and
Lake Labish peat soil (an organic soil). The composition of Woodburn soil
consists of 1.9% organic matter (f om = 0.019), 68% silt, and 21% clay and that
of Lake Labish peat soil of 51% organic matter (f om = 0.51), 36% silt, and 3.5%
clay. In aqueous systems, both parathion and lindane show linear isotherms,
and there is no apparent sorptive competition between the two solutes;
the results on Woodburn soil are illustrated in Figure 7.7. In oven-dried
Woodburn soil–hexane systems, the sorption of parathion (and lindane) is
nonlinear and much greater than the corresponding uptake from water. On
air-dried Woodburn soil (with about 2.5% water), the uptake of parathion