Page 132 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 123

tion applies only to polar solutes and is considered to be most significant at
low C e/S w, due to “site” availability. At high C e/S w, with the specific sites nearly
saturated, the polar solutes then exhibit a linear partition into the entire SOM.
Although the presumed polar solute–SOM specific interaction as a potential
source of sorption nonlinearity seems sensible, it is debatable that it could be
portrayed as a partition (solubility) process, since it operates only over a short
range of solute concentrations. Much more work is needed to substantiate the
assumed specific interaction between solute and SOM and related active SOM
sites and active solute groups.
By considering the dual mechanistic functions of the soil, the unsuppressed
adsorption of polar solutes on certain minerals could also lead to significant
nonlinear sorption by soils at low C e/S w (Chiou, 1995). On some special soil
minerals (e.g., certain montmorillonite clays as discussed in Chapter 6), water
may not sufficiently suppress the adsorption of polar organic solutes as it does
the low-polarity solutes. In contrast, the residual mineral adsorption of low-
polarity solutes should be relatively weak and linear over a large concentra-
tion range, as noted for phenanthrene (Huang et al., 1996). The consequence
of this unsuppressed adsorption with soil minerals or of the specific interac-
tion with SOM to the overall solute sorption to a mineral soil having a sig-
nificant SOM content is illustrated schematically in Figure 7.1b. Since the
presumed unsuppressed mineral adsorption or the solute–SOM specific inter-
action for polar solutes is significant largely at low C e/S w, the effect may over-
whelm the linear partition with SOM at low C e/S w, while the partition
contribution usually prevails at moderate to high C e/S w. As a result, the
isotherm would be nonlinear at low C e/S w but virtually linear at moderate to
high C e/S w. The solute concentration at which the linear partition and the non-
linear contribution cross is expected to be a function of the SOM content, the
magnitude of K om or K oc, and the mineral type and content, which determine
the relative magnitudes of partition and adsorption.
Because soils and sediments from different geographic locations differ con-
siderably in their compositions, there is no simple a priori way to predict the
magnitude of sorption nonlinearity and the specific cause for its occurrence.
The possibilities of all mechanisms contributing to the concentration depend-
ence of the polar-solute sorption coefficient have to be settled by experiments
together with relevant soil and solute physical and chemical properties. The
potential nonlinear sorption of polar solutes to soils and sediments at low con-
centrations is important because the concentrations of many polar solutes in
natural water (usually at low mg/L to low mg/L) fall into a low C e /S w range,
owing to their high water solubilities. However, one should keep in mind that
the magnitude of nonlinear sorption that one observes in single-solute systems
is usually attenuated in multisolute systems because of the solute competition,
as shown later. Thus the data derived from single-solute studies may not prop-
erly reflect the actual solute behavior in natural systems. The nonlinear effect
for low-polarity solutes should be far less serious in consideration of their
wider isotherm linearity and their generally lower water solubilities (which