Page 174 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 165

peat (Figure 7.25). This indicates that the reduced HM polarity has less impact
on the partition uptake of a moderately polar solute (DUN) than that of a
nonpolar solute (EDB).
The sorption of DCP on peat, HA, and HM exhibits trends similar to those
observed for the DUN sorption. For relatively polar DCP, the nonlinear effect
probably results predominantly from its specific interaction with active SOM
groups, as mentioned before. Here the nonlinear capacity of DCP is about
25mg/g with the peat, 16mg/g with HA, and 45mg/g with HM (Table 7.13).
The disparate nonlinear capacities of DCP on these sorbents appear to reflect
some changes in either the affinity or the abundance of the active sites in HA
and HM materials when prepared from the peat soil. Nonetheless, the non-
linear capacities for DCP on all sorbents are far too high to be reconciled with
the measured BET-N 2 surface areas alone. Thus the solute–SOM specific inter-
actions appear to predominate over the much weaker solute adsorption on a
small amount of HSACM for the nonlinear sorption of relatively polar solutes.
The presumed different sources for the nonlinear effects of polar and non-
polar solutes is further illustrated by their different nonlinear-sorption ranges
[i.e., the observed (C e/S w) ans values are considerably greater for polar DCP and
DUN than for nonpolar EDB] (Table 7.13). As with the DUN sorption, the
slope of the upper DCP–HM isotherm is comparable with that of DCP on the
peat (Figure 7.26), suggesting that the contents of relatively polar sorbents do
not significantly affect the partition uptake of a relatively polar solute.
The nonlinear characteristics of EDB, DUN, and DCP on HA, HM, and
peat samples are inherently consistent with the expectations of the HSACM-
SI model, in which the nonlinear capacities observed for nonpolar EDB are
well related to the BET-N 2 surface areas (or to the presumed amounts of
HSACM) of the sorbents, whereas those for polar DUN and DCP call for
additional specific interactions with SOM. By contrast, the diversity of the
data cannot readily be reconciled with the glassy–rubbery SOM model nor
with the internal-hole model without much additional ad hoc hypothesis. The
glassy–rubbery SOM model does not consider specifically the disparate non-
linear effects for polar and nonpolar solutes. The results with density-
fractionated HA and base-insoluble HM would force this model to further
hypothesize that the impact of the glassy component in SOM on sorption non-
linearity also depends on the solute polarity; that is, the glassy component (or
its effect) exists only in HM but not in HA for nonpolar solutes, whereas it
occurs in both HA and HM for polar solutes. Similarly,the internal-hole model
would have to further assume that the compound-specific internal holes acces-
sible to nonpolar solutes are located only in HM but those accessible to polar
solutes exist in both HA and HM. It is difficult, however, to rationalize the
inconsistency on the origin and effect of the glassy component or internal
holes in SOM, since both polar and nonpolar solutes should have equal access
to the presumed glassy SOM or internal holes.
In light of the mutual consistency of the sorption, surface area, and petro-
graphic data, the existence of small amounts of HSACM in soils or natural