Page 140 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 131

bed sediments and the predominance of solute partitioning in SOM over
mineral adsorption; if mineral adsorption were important, the sorption coeffi-
cient would be affected by the sorbent’s SA, which is not observed. The range
of variation for the soils is much smaller than that reported in other studies,
with a smaller set of combined data analyzed by different analytical methods
(Kenaga and Goring, 1980; Mingelgrin and Gerstl, 1983). The extreme K oc
values for soils (or sediments) differ by less than a factor of 2. The relative
invariance in K oc suggests that the properties of the humified SOM that
mediate nonpolar solute solubility are quite similar for a wide variety of
uncontaminated soils, and also likely for relatively pristine bed sediments.
There does not appear to be a large variability in SOM polarity and compo-
sition between well-weathered soils from diverse geographic locations. In view
of the relative invariance in K oc between soils or between sediments as
illustrated, the use of average soil (or sediment) K oc values for assessing the
sorption of low-polarity contaminants to different soils (or sediments) would
seem sufficient in most environmental applications. However, an account of
the difference in sorption to soil and sediment of low-polarity contaminants
would seem warranted.
The fact that most soil K oc values are distinct from bed-sediment K oc
values implies that the process that turns eroded soils into bed sediments
brings about a noticeable change in the property of the organic constituent.
A possible cause for this change is that the sedimentation process fractionates
soil organic constituents such that the more polar and water-soluble
organic components in SOM (e.g., fulvic and humic acid fractions), or that
the soil particles with more polar organic components, are separated out
to form dissolved organic matter and colloids in water, with the less-polar soil
organic constituents preserved in the bed sediment. The time scale to
bring about a complete soil-to-sediment conversion should be a function of
hydrodynamics. Another possible cause for this change would be the biologi-
cal influence. However, this effect would seem small since the difference
between bed-sediment and soil K oc values, although statistically significant,
is not large.
Part of the variation in K oc within bed sediments reflects the extent of con-
version of the eroded soils to bed sediments. Consider, for example, the rela-
tively low K oc values of CT and DCB with sediments 1, 6, and 7 in Table 7.3.
Sediment 1 is a U.S. EPA sample taken from the mouth of Isaacs Creek at the
junction with the Ohio River near Ripley, Ohio. The fact that the K oc values
on sediment 1 are significantly lower than the rest but are very similar to soil
K oc values suggests that this sample could be a recently eroded soil which
retains most of its soil-organic-matter composition. The somewhat lower K oc
values with sediments 6 and 7 relative to the average sediment K oc may again
be a result of incomplete conversion of eroded soils to bed sediments. The K oc
data suggest that bed sediments from most large rivers and lakes are relatively
comparable in their SOM polarities and compositions, probably because they
are more aged and contain less recently eroded soils.