Page 177 - Partition & Adsorption of Organic Contaminants in Environmental Systems

168 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

tions. Whereas the single-solute sorption data apply to most laboratory studies,
the behavior of a contaminant in natural systems at low C e/S w will depend
strongly on system conditions. The principal deciding factors for nonpolar
contaminants are the amount of HSACM relative to SOM, the contaminant
solubility, and the number and amount of other coexisting contaminants. If the
HSACM content is small, the sorption coefficients of most nonpolar solutes
should be relatively constant over the typical environmental concentration
range (i.e., at subparts per million or higher), as the linear partition to SOM
would outweigh the nonlinear adsorption on HSACM. This is expected to be
especially the case for all nonpolar contaminants with very low S w values (e.g.,
chlorinated hydrocarbons, PCBs, and PAHs) because at environmentally sig-
nificant concentrations they will be placed at relatively high C e/S w values (i.e.,
they will easily fall into the partition-dominated range). In the evaluation of
the sorption coefficient (K om or K oc) of a contaminant at low C e/S w in natural
systems, it is also imperative to keep in mind the effect of mutual solute com-
petition on the sorption behavior of individual solutes. The possible outcome
of this effect is outlined as follows.
If the system contains multiple solutes and the HSACM content is low, the
nonlinear sorption would be most significant when there are only few co-
existing solute species and/or if the solutes are at extremely low C e/S w values.
However, if the system contains a dominant component (of high C e/S w value),
the sorption of all nonpolar solutes, irrespective of the number of solute
species, at low C e/S w values should become relatively linear because of adsorp-
tive suppression by the dominant species on HSACM. In this case, the sorp-
tion coefficients of all contaminants, including the dominant species, should
conform largely to their respective linear partition coefficients with SOM. The
same should apply for nonpolar solutes if the system contains a large number
of solute species, each at about the same C e/S w value. For polar solutes in
multiple-solute systems, where the major cause for sorption nonlinearity is
the SOM content, or the specific clay content on rare occasions, the sorption
coefficients could be subject to greater variation with solute concentration,
since suppression of the nonlinear sorption in this case requires higher co-
solute C e/S w values and more specific co-solute polarity. Nonetheless, if a pow-
erful polar contaminant (e.g., phenol) dominates, the nonlinear sorption
effects of the less-polar contaminants should again be greatly diminished. The
observed suppression of the nonlinear sorption of a given polar solute by polar
co-solutes from other classes reduces the complexity of the multiple-solute
system.

7.3.8 Influence of Dissolved and Suspended Natural Organic Matter
For many highly water-insoluble compounds, the term log (g w /g w *) often has a
more significant impact on the logK om value in soil–water systems [Eq. (7.11)]
than, say, on logK ow values in octanol–water systems [Eq. (5.1)]. In soil–water
mixtures, the term g w /g w * expresses the enhancement of solute concentration