Page 210 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM VAPOR PHASE 201

excess, onto the adsorbed water surface. We shall see later that the vapor
uptake on wet soils is better explained in terms of the vapor partition into
SOM.
Leistra (1970) presented results on the vapor uptake of cis- and trans-1,3-
dichloropropene on three types of soils: humus sand (f om = 0.055), peat sand
(f om = 0.18), and peat (f om = 0.95), with moisture contents of 17%, 41%, and
120% of the dry soil weight, respectively. The isotherms for all three soils were
highly linear, with the soil-to-vapor distribution coefficients being propor-
tional to the respective SOM contents; the SOM-normalized sorption co-
efficients (K om) were largely independent of the soils for each vapor. The
K om values of both compounds exhibited a small temperature dependence,
with DH being < 4kJ/mol exothermic and nearly constant (Hamaker and
Thompson, 1972). The data suggest that at these moisture contents the soils
were fully water saturated. Note that the amount of water needed to saturate
the soil in sorption is obviously much lower than the field water saturation
capacity; the former seems to be close to the water content at the soil wilting
point. Analogously, the moisture content in some conventional unsaturated
zones (i.e., the vadose zones) may be well above the water-sorption saturation
level. It is important that the water saturation level in soil sorption not be
confused with the field water-saturation capacity.
From the vapor sorption data analyzed so far, it is evident that dry and
slightly hydrated soil minerals (especially, clays) act as powerful adsorbents
for organic vapors and that the contribution by clay adsorption greatly exceeds
the concurrent vapor partition into the organic matter on most dry mineral
soils. Apparently, at saturation-water contents, the adsorptive power of soil
minerals for organic compounds is largely lost because of strong competitive
adsorption of water (Chiou and Shoup, 1985), leaving the partition with SOM
as the dominant mechanism. The fact that the organic-vapor uptake by dry
soils is closely related to clay content rather than to SOM content suggests
that dry clay is more powerful per unit weight in adsorption of organic com-
pounds than is SOM per unit weight in uptake by partition; the reverse is true
for the hydrated soils. The suppression of vapor uptake observed on soils by
moisture is essentially the same as noted in the suppression by water of
parathion and lindane uptake from a nonpolar solvent (hexane). The only dif-
ference is that the organic solvent also minimizes the solute partition in SOM,
making the total solute uptake approach zero at full water saturation.
Although the water content also affects the vapor partition in SOM, to be illus-
trated later, the partition uptake with water-saturated SOM remains substan-
tial. More data on vapor sorption in relation to RH or soil-water content are
presented later.
Relative to soil mineral adsorption, there have been few studies on the par-
tition uptakes of different vapors by relatively dry SOM. Using the Florida
peat (f om = 0.864) as a model for SOM, Rutherford and Chiou (1992) and
Chiou and Kile (1994) measured the vapor partition to dry SOM at room tem-
perature of some nonpolar and polar liquids: benzene, carbon tetrachloride