Page 133 - Partition & Adsorption of Organic Contaminants in Environmental Systems

124 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

raise their relative concentrations). Later in this chapter, examples of non-
linear sorption for both polar and nonpolar solutes on soils and the potential
sources of such nonlinearity will be brought into a better perspective. As
a logical sequence of our presentation, we shall first consider the sorptive
behavior of relatively nonpolar solutes.

7.3.2 Effect of Soil Organic Matter versus Sediment Organic Matter

In the study of solute sorption coefficients (K om or K oc), a subject of practical
interest is how much the SOM medium property varies between soils and
between sediments to affect the sorption coefficient (K om or K oc) of an organic
contaminant. This information is critical to whether soils or sediments from
dispersed geographic locations need to be studied individually (if K om or K oc
values vary widely) or can be treated rather indiscriminately (if K om or K oc
values are relatively invariant). Before the more extensive investigation by
Kile et al. (1995), the literature data on this subject were limited to only a few
selected solutes and to a relatively small set of soil samples examined by dif-
ferent analytical techniques (Bailey and White, 1964; Goring, 1967). Based on
K oc data from different reports for a few selected solutes, Kenaga and Goring
(1980) observed that the K oc variation between soils is generally less than a
factor of 3 to 4. Mingelgrin and Gerstl (1983) indicated that the K oc could vary
by as large as a factor of 10 or greater, based on selected K oc values of some
pesticides. Utilizing a correlation of K oc with (O + N)/C weight ratio of natural
organic matter, Rutherford et al. (1992) estimated the K oc variation of non-
polar solutes between soils to be less than a factor of 3, based on the range of
(O + N)/C values for common soil organic matter.
Since the different analytical procedures employed led inevitably to K om or
K oc variation, especially for soil or sediment samples with very low organic
contents, a more accurate account of the K om or K oc variation between soils or
sediments could only be achieved through the use of a large set of soils and
sediments with significant SOM contents to be analyzed by consistent and
rigorous analytical methods. With this consideration, Kile et al. (1995) meas-
ured the K oc values of two relatively nonpolar solutes, carbon tetrachloride
(CT) and 1,2-dichlorobenzene (DCB), on 32 “normal” soils and 36 “normal”
bed sediments collected from diverse geographic locations in both the United
States and China. For all samples, solute concentrations in both water and
soil/sediment were solvent-extracted and analyzed by gas chromatography.
Partition data of low-polarity solutes with the SOM of soils and sediments
should best detect differences in SOM polarity and composition, if any,
because the solubility of nonpolar solutes is sensitive to the organic medium
polarity (see Chapter 5) and because the adsorption of such solutes on
soil/sediment minerals should be most effectively suppressed by water.
The sources of soils and bed sediments, the sample BET-N 2 surface areas
(SA), the organic carbon contents (% OC), and the measured K oc values for
CT and DCB from Kile et al. (1995) are shown in Table 7.3. Sorption isotherms