Page 141 - Partition & Adsorption of Organic Contaminants in Environmental Systems

132 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

The difference between soil and bed-sediment K oc values as detected by rel-
atively nonpolar solutes lends a basis for identifying the source of suspended
solids in rivers. In the study of Kile et al. (1995), the suspended solids from
the Mississippi River, Missouri River, and Yellow River were collected during
high river flows, and the sample from the Illinois River was collected during
a low-to-normal river flow. Here the K oc values of CT and DCB are typical of
those of soils for the former but are more representative of bed sediments for
the latter (Table 7.3). One may infer from these data that the suspended solids
during high water flows in these three rivers consist mainly of newly eroded
soils and the suspended solids from the Illinois River under low-to-normal
water flow consist largely of resuspended bed sediment. The Yellow River
suspended solid, which shows its origin as an eroded soil, is in keeping with
the river’s high carrying load of eroded soils during the high-flow season. In
contrast, bed sediment collected from the Yellow River (sediment 22) gives
K oc values typical of those for other bed sediments. Thus the sorption data
serves as a simple indicator of the source and time history of the suspended
solids.
The relatively low K oc values of CT and DCB, about one order of magni-
tude lower than their respective logK ow values, suggest that the SOM of soils
(or sediments) must be fairly polar in nature to limit the partition (solubility)
of these nonpolar organic solutes. To investigate the effect of SOM composi-
tion and polarity on solute partition, Rutherford et al. (1992) measured the
partition coefficients of two relatively nonpolar solutes, benzene and CT, in
relation to the elemental compositions of relatively ash-free natural organic
matters: cellulose, muck, peat, and treated peat (peat washed by 0.1N NaOH
to lower the oxygen content). The weight ratio of [oxygen + nitrogen] to
carbon of the natural organic matter [i.e., the (O + N)/C value] was used as
an approximate polarity index of the sample, which gives the relative polarity
order: cellulose > muck > peat > treated peat. An inverse relation is evident
between the K oc (or K om) of both CT and DCB and the (O + N)/C of the
organic matter sample, as shown in Figure 7.12.
The results for CT in Figure 7.12 illustrate several points of interest. First,
the partition of a nonpolar solute to a natural organic matter is sensitive to
the organic matter polarity (or composition). The small K oc value of CT with
cellulose compared to that with humified materials (such as muck or peat)
results from a poor match in polarity between a nonpolar solute and a highly
polar organic phase, which makes cellulosic materials a poor partition phase
for nonpolar contaminants. Second, the (O + N)/C values for normal soils
should fall into a relatively narrow range, in view of relatively constant K oc
values of CT on soils from diverse sources (mean K oc = 60; SD =±7), which
would place them somewhere between the (O + N)/C values of Houghton
muck (0.777) and Florida peat (0.657). Third, the low (O + N)/C value of the
treated peat (0.488) and the observed K oc value of 115 for CT with this sample,
which exceeds the K oc values with normal soils but resembles the K oc values
with bed sediments, substantiate the contention that the sediment OM has a