Page 185 - Partition & Adsorption of Organic Contaminants in Environmental Systems

176 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

ether and alcohol (Shin et al., 1970) might be more reasonably explained by
the reduction of soluble or suspended soil organic matter rather than by the
postulate that the applied solvents remove lipids from organic matter and
hence enhance the accessibility of DDT to soil organic matter.
By Eqs. (7.23) to (7.26), the effect of DOM on solute solubility, and hence
on the solute sorption coefficient, would be less significant for solutes with
higher water solubility because their K dom or K doc values are smaller. Con-
sequently, more water-soluble compounds, such as lindane and TCB, show no
detectable solubility increases even at relatively high DOM concentrations
(Caron et al., 1985; Chiou et al., 1986). This consequence is predicated by Eq.
(7.24). For example, for a solute–DOM pair to achieve S* = 2S w, it is required
w
that X = 1/K dom. Since K dom increases with decreasing (supercooled-liquid) S w,
a higher X is needed for a more water-soluble solute than for a less-soluble
solute to produce the same result. Thus, with soil humic acid as DOM, DDT
4
-5
with K dom = 6.6 ¥ 10 would give S* = 2S w at X = 1.5 ¥ 10 (or 15mg/L), while
w
2
the more soluble TCB with K dom = 6.3 ¥ 10 can achieve this result only at
X = 1.6 ¥ 10 -3 (or 1600mg/L). Other relatively soluble solutes (most polar
solutes) would likewise show no discernible solubility enhancement at X <
100mg/L.
At S* = 2S w, if one writes X = S w /(S wK dom), with S wK dom in units of g solute/g
w
DOM (i.e., dimensionless), a more explicit relation between S w and required
X can be obtained for water solubility enhancement. For nonpolar solutes, the
term S wK dom is small because the corresponding S wK om term, which expresses
the solute S om [Eq. (7.10)] is small (see Table 7.2), and because K om ≥ K dom for
soil-derived DOM. The dimensionless S om values vary to some extent among
nonpolar liquids and supercooled liquids (i.e., about 0.03 for benzene and
<0.01 for supercooled DDT). Hence, for a solute to achieve S* = 2S w with soil
w
humic acid as DOM (K dom 0.5K om), it is required that X ≥ 60S w, where S w is
for liquids or supercooled liquids [see Eqs. (3.7) and (3.9)]. This factor should
increase with increasing polarity of DOM (as with SRHA and SRFA) because
of their reduced K dom. For most nonpolar solutes, except possibly for PAHs, as
discussed below, an empirical rule is that the level of the truly dissolved natural
organic matter must be at least about two orders of magnitude greater than
the solute’s supercooled-liquid S w in order to produce an enhancement factor
of 2. Since the natural DOM level seldom exceeds 100mg/L, only those con-
taminants with S w < 1mg/L as supercooled liquids could possibly exhibit a sig-
nificant increase in their concentrations by DOM in natural water. However,
as discussed later, some human-made DOM, such as surfactant micelles, is able
to produce a much greater effect than natural DOM at low concentrations.
We now attend specifically to the potential impact of DOM on the behav-
ior of PAHs. As we recall, PAHs are a unique class of nonpolar solutes in
respect to their enhanced partition to SOM, which leads to higher K om or K oc
values, due to their aromatic moieties, as discussed earlier. Since the DOM
derived from a natural source also contains aromatic moieties, although its
content varies with the source (soil, river, lake, etc.), the DOM’s aromatic