Page 211 - Partition & Adsorption of Organic Contaminants in Environmental Systems

202 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

TABLE 7.21. Limiting Partition Capacities (Q° om ), Volume Fraction Solubilities (f°)
of Liquids, and Solubility Parameters (d) of Liquids in Peat Organic Matter
3 0.5
Liquid d (cal/cm ) Q° om (mg/g) f°
n-Hexane 7.3 28.2 0.053
Carbon tetrachloride 8.6 65.9 0.051
Benzene 9.2 38.9 0.054
Trichloroethylene 9.2 80.0 0.067
1,4-Dioxane 10.0 80.2 0.092
EGME 10.5 190 0.21
Acetone 9.9 171 0.22
Nitroethane 11.1 272 0.25
Acetonitrile 11.9 344 0.36
1-Propanol 11.9 313 0.34
Ethanol 12.7 396 0.40
Methanol 14.5 620 0.51
Water 23.4 a 370 0.33
Source: Data from Chiou and Kile (1994).
a Value uncertain.

(CT), trichloroethylene (TCE), n-hexane, 1,4-dioxane, EGME, nitroethane,
acetone, acetonitrile, methanol, ethanol, 1-propanol, and water. Except for
water vapor, the isotherms for the organic vapors are largely linear, reflecting
the predominance of the partition effect. Water exhibits a large uptake and a
unique isotherm, with a profound concave-downward curvature at low RH but
a good linearity at moderate to high RH. It appears that water engages ini-
tially in a hydrate formation with SOM and then partitions subsequently into
the hydrated SOM net work (Chiou and Kile, 1994). For the essentially linear
uptakes of organic vapors, their limiting partition capacities (i.e., solubilities),
Q° om, with the SOM may be obtained by extrapolating the respective isotherms
to P/P° = 1 and normalizing the capacities to f om. The Q° om values calculated in
mg/g and the corresponding volume-fraction solubilities, f°, of the liquids with
dry SOM are given in Table 7.21.
We recall that SOM is a relatively polar phase. As a consequence, the Q° om
values of nonpolar liquids (e.g., hexane, CT, and benzene) are about an order
of magnitude smaller than those of highly polar liquids (e.g., methanol and
ethanol), in keeping with the solubility criterion. As seen, the Q° om values of
nonpolar liquids with dry SOM of the peat in Table 7.21 are comparable with
the corresponding S° om values of the liquids with water-saturated SOM in Table
7.2. In Table 7.21, one also notes that the nonpolar liquids display very con-
sistent f° values, because the solubilities of solutes in a polymer or a macro-
molecular substance are usually better accounted for by their volume fractions
(Flory, 1941). The much higher Q° om values for polar liquids reflect their
enhanced solubilities in relatively polar SOM, as assisted by more powerful
polar and H-bonding forces. Since the solubility parameter (d) of a liquid is a