Page 121 - Partition & Adsorption of Organic Contaminants in Environmental Systems

112 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

are treated in some detail later. Since there is a continuum of the organic
matter content in soil, the analysis and discussion of the sorption data in
aqueous systems is restricted to soils (or sediments) having more than 0.1 to
0.2% organic content so that the effect of SOM on contaminant uptake is
significant enough to be reliably quantified.

7.3 SORPTION FROM WATER SOLUTION

7.3.1 General Equilibrium Characteristics

We begin by looking at the sorption data for relatively nonpolar organic com-
pounds (solutes), and then look at the data for relatively polar organic com-
pounds, because of some characteristic differences in their behaviors. Whereas
the demarcation between polar and nonpolar compounds is not straight-
forward, polar compounds are considered to be those that possess significant
polar groups in their molecular structures, and nonpolar compounds those that
contain little or no polar groups. Some common strong polar groups are
—OH, —NH 2, —COOH, —CO—, and —NO 2, as illustrated in Table 5.3. Polar
groups enhance molecular interactions of the compounds by polar forces and
H-bonding with each other and with other polar compounds. In general, the
effect of a polar group in a molecule is more significant for small molecules
than for large molecules. Polar organic solutes generally exhibit low partition
from water to a water-immiscible (or partially miscible) organic phase relative
to nonpolar solutes because the former have a high affinity for water. For
instance, as shown in Table 5.3, the logK ow (octanol–water) values of highly
polar organic solutes are generally <2.
In water solution the sorption isotherms for relatively nonpolar organic com-
pounds on soils or sediments are usually virtually linear, as has been demon-
strated in a number of studies (see, e.g., Yaron and Saltzman, 1972; Chiou
et al., 1979, 1983; Karickhoff et al., 1979; Means et al., 1980; Schwarzenbach and
Westall, 1981; Kile et al., 1995). Similar isotherm linearity has been reported
for the soil uptake of volatile nonpolar pesticide vapors onto water-saturated
soils (Leistra, 1970; Spencer and Cliath, 1970). In some studies where slight
isotherm curvatures were shown (see, e.g., Mingelgrin and Gerstl, 1983), the
extent of isotherm nonlinearity (either concave upward or downward) appears
to be comparable with the normal range of data scatter and hence cannot be
distinguished from a linear isotherm. This is especially true in sorption studies
of low-organic-content soils when the solute uptake is computed by the
difference in solute concentrations in water before and after equilibration;here
the uncertainty in detecting small concentration changes is expected to be
relatively large. Since the curvatures in allegedly nonlinear isotherms for
some relatively nonpolar pesticides (e.g., ethylene dibromide and lindane)
(Mingelgrin and Gerstl, 1983) are quite small and show no consistent shape,
the results offer no clear evidence for strong solute adsorption over the con-
centration range studied.