Page 158 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 149

K hom (or K hoc) of the model solute is known or can be estimated with fair accu-
om
racy, the value of f hom (or f hoc) can then readily be estimated. The K* /K om
values for 2-PCB with four PCB-oil contaminated soils (Sun and Boyd, 1991)
are shown in the last column of Table 7.9. Similarly, using Eq. (7.22) with CT
and DCB as model nonpolar solutes, very large K* /K oc values are observed
oc
for soils and sediments in which a petroleum hydrocarbon phase exists (Kile
et al., 1995). In Table 7.8 we also find that the measured K oc values of PAHs
for some highly contaminated sediments are appreciably higher than those of
relatively clean sediments, due to the presence of a separate hydrocarbon
phase.

7.3.7 Deviations from Linear Sorption Isotherms
Some recent studies on the sorption of single contaminants (solutes) from
water on some soils and sediments indicate that the measured sorption at low
relative concentrations (C e/S w) may often be nonlinear with enhanced sorp-
tion coefficients compared to the upper linear sorption range. Young and
Weber (1995) found that the sorption of a nonpolar solute (phenanthrene) on
some soils and shales exhibits a significant nonlinearity with a concave-
downward shape at low concentrations. Spurlock and Biggar (1994) observed
nonlinear sorption of relatively polar substituted ureas (herbicides) on soils at
low concentrations, with the nonlinear sorption coefficient increasing with
decreasing solute concentration. Xing et al. (1996) also found deviations from
linear sorption at low concentrations for some polar pesticides (triazines) and,
to a lesser extent, for relatively nonpolar trichloroethylene (TCE) on selected
soil and organic-matter samples.
It is of practical interest to deliberate on the cause of such nonlinear sorp-
tion for organic solutes at low C e/S w, since a wide variety of relatively soluble
organic contaminants may fall into this range in natural systems. Although
the unsuppressed adsorption of polar solutes on certain clay fractions of
low-organic-content soils (Laird et al., 1992; Haderlein and Schwarzenbach,
1993; Weissmahr et al., 1997) could result in nonlinear sorption at low C e/S w,
the effect as noted for polar and nonpolar solutes on soils with relatively high
SOM contents points instead to the occurrence of a strong nonpartition effect
(e.g., adsorption or specific interaction) of solutes with either a small amount
of active SOM groups or with a small amount of nonmineral soil fraction.
A number of conceptual models have been postulated to account for the
nonlinear solute sorption on soils of significant SOM contents: (1) the differ-
ent equilibrium rates of the solute with the assumed two structural entities of
the SOM, one in a rubbery state and the other in a glassy state (Young and
Weber, 1995; Weber and Huang, 1996) where the solute sorption to rubbery
SOM is linear in reflection of partition and that to glassy SOM is nonlinear
in reflection of a surface adsorption; (2) the presence of a small amount of
high-surface-area carbonaceous material (HSACM) (such as charcoal or soot)
that exhibits a greater nonlinear adsorption at low relative concentrations than