Page 214 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM VAPOR PHASE 205

and K om = 380 (Chiou et al., 1985). Thus, for the amounts of dieldrin and
lindane applied to water-saturated Gila soil, the pesticide vapor concentra-
tions will remain unchanged at saturation with increasing soil-water content.
This effect should persist until the water content becomes high enough to
dilute the excess pesticides to below saturation.
The analysis above accounts further for the observation that the lindane
sorption capacity on the hydrated Gila soil is largely the same as that on the
same soil with 3.9% or 10% water (Spencer and Cliath, 1970). At 3.9% water
on Gila soil, which corresponds to about the point of water saturation, the heat
of lindane vapor sorption calculated from the temperature dependence of the
isotherms was less exothermic than the reverse heat of lindane vaporization
(-DH v ), due presumably to the dominance of vapor partition in SOM of a
water-saturated soil.
To substantiate the effect of humidity on the mechanism and capacity
of soil sorption of organic compounds, Chiou and Shoup (1985) determined
the vapor sorption of benzene, chlorobenzene, m-dichlorobenzene, p-
dicholorobenzene, 1,2,4-trichlorobenzene, and water on dry Woodburn soil,
and of benzene, m-dichlorobenzene, and 1,2,4-trichlorobenzene as functions
of relative humidity (RH). Isotherms for all compounds as single vapors on
dry soil in a normalized plot of Q versus P/P° (where P and P° are equilib-
rium and saturation vapor pressures, respectively) were distinctly nonlinear,
with water showing the greatest capacity (Figure 7.43). The data observed
closely fit the BET adsorption model. In addition to the isotherm nonlinear-
ity, the sorption capacities of organic vapors on dry soil were about two orders
of magnitude greater than those of the same compounds from water on the
same soil shown in Figure 7.2 (when the data therein are expressed as Q versus
C e /S w ). The considerably greater sorption on dry soil is attributed to strong
adsorption on soil minerals, which predominates over the simultaneous
uptake by partition into the SOM. The reason for this strong suppression by
water of adsorption of organic compounds on soil minerals is elucidated in
Chapter 6.
The sorption of benzene, m-dichlorobenzene, and 1,2,4-trichlorobenzene by
initially dry Woodburn soil was depressed progressively by increasing RH
(Chiou and Shoup, 1985). The results for m-dichlorobenzene and 1,2,4-
trichlorobenzene are given in Figures 7.44 and 7.45, respectively. In addition
to the reduced uptake in the presence of water vapor, the isotherms for these
relatively nonpolar vapors at RH ≥ 50% assume a practically linear shape at
P/P° £ 0.5. At high RH before water saturation, the vapor uptake observed
should consist largely of adsorption on adsorbed-water surfaces, as these non-
polar vapors cannot effectively displace the adsorbed water, and of a con-
comitant partition into the SOM, producing relatively linear isotherms up to
moderate P/P°. At high P/P° values of the vapor, the vapor isotherm may
exhibit nonlinearity, as shown by Call (1957) for EDB, due to multilayer vapor
adsorption on water surfaces, if a significant portion of the water-associated
mineral surface area remains available (which should decrease with the