Page 167 - Partition & Adsorption of Organic Contaminants in Environmental Systems

158 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

see Figure 6.6). This behavior suggests that a small amount of high-affinity
adsorption sites is present, as pictured by the HSACM postulate. The HSACM
concept is thus in accord with the widespread natural occurrence of charcoal-
like materials that are commonly produced by biomass burning. The ubiquity
of low levels of charcoal-like materials in sediments has been well documented
(Smith et al., 1973; Griffin and Goldberg, 1983; Masiello and Druffel, 1998).
Many soils may thus be naturally blended with small amounts of charcoal-like
substances. Although the HSACM postulate does not rule out the possibility
of a small quantity of high-affinity adsorption sites in a specific fraction of the
SOM, it seems unlikely that the SOM contains such unique sites. In this
respect, the HSACM is better viewed as an extraneous substance in soil rather
than as a portion of SOM, even though it is often counted as part of the soil
organic carbon in SOM analysis by high-temperature combustion methods.
More work is needed to establish clearly whether a direct relation exists
between nonlinear capacity and SOM content.
We now consider the sorption data of polar solutes on peat soil and
Woodburn soil. For polar solutes, the greater nonlinear capacity requires an
additional nonlinear model. For DCP on peat, for instance, the nonlinear
capacity observed (25mg/g) greatly exceeds the adsorption capacity account-
able by the small surface area of the soil. The nonlinear capacities for MON
and DUN on peat are smaller but still higher than can be reconciled with the
soil surface area. These findings imply that the relatively large nonlinear sorp-
tion of polar solutes at low C e/S w values is strongly related to solute polarity
and occurs within the interior network of SOM. The data are compatible with
the specific-interaction (SI) model of Spurlock and Biggar (1994), which
captures the nonlinear features of polar pesticides at low (relative) concen-
trations. The model postulates that the specific interaction of polar solutes with
highly active SOM sites approaches saturation at a much lower concentration
than does the concurrent partition to SOM, and therefore the isotherm is non-
linear at low (relative) concentrations.
Since the assumed specific-interaction (SI) model involves the polar groups
of solute and SOM, it makes sense that the magnitude of nonlinear sorption
and the solute competitive power depend on the solute polarity, as manifested
by the experimental data. In this respect, a nonpolar co-solute is unable to
suppress the large nonlinear uptake of a polar solute. The small reduction of
DUN uptake on peat by co-solute EDB (with C e /S w = 0.30), as shown in Figure
7.18, and that of atrazine uptake at low concentrations on soil by co-solute
TCE (Xing et al., 1996) may be attributed to adsorptive competition on a small
amount of HSACM in soil. The finding that the nonlinear sorption capacities
of a polar solute (e.g., DCP) on peat and Woodburn soil (see Table 7.11)
correlate largely with respective SOM contents is in keeping with assumed
specific interactions of polar solutes with the interior active sites of SOM.
Chiou and Kile (1998) also found that the nonlinear sorption of DCP on peat
does not disappear by lowering the solution pH to 2.0, suggesting that the
active sites in SOM are not confined to ionizable groups. Overall, the combi-