Page 175 - Partition & Adsorption of Organic Contaminants in Environmental Systems

166 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

solids is at least one of the primary causes of the nonlinear effect of nonpolar
compounds at low C e/S w values. Although the available information does not
exclude the possibility of other nonlinear sources for nonpolar compounds
(e.g., other materials in natural solids with carbon-like adsorptive properties),
the HSACM model for nonpolar solutes is currently supported by more exper-
imental evidence. For the nonlinear effect of polar solutes, such as that shown
for DCP on HA, where the nonlinear capacity greatly surpasses the amount
attributable to surface adsorption, the solute–SOM specific interaction, as sug-
gested by Spurlock and Biggar (1994), seems to be a reasonable explanation
of the result, although the mode of specific interaction remains to be
substantiated. At this point, greater effort is needed to explore all possible
nonlinear sources and causes for nonionic compounds with soils and natural
solids.
We now shift our attention to another potential source of sorption non-
linearity for polar solutes, which has to do with their adsorption on certain
minerals. We recall from our deliberation in Chapter 6 that a montmorillonite
+
(SAz-1) exchanged with K exhibits a lower water uptake than benzene uptake
at low P/P° values, as a result of the weak cation hydration and the low affin-
ity of water for siloxane surfaces. This suggests that very polar solutes may
compete effectively against water for adsorption on such clay minerals in soil.
Although it would be very unusual for ordinary soils to have a high content
+
of montmorillonite with predominantly weak hydrating cations (e.g., K and
+
Cs ), one cannot rule out this potential adsorptive effect for very polar con-
taminants at low C e/S w values with such soils or sediments that exhibit un-
usually high montmorillonite contents.
Laird et al. (1992) studied atrazine uptake from water solution by 13 indi-
vidual Ca-saturated smectitic clays, where the cation exchange capacity (CEC)
varies from 79 to 134cmol/kg. They found that the net atrazine uptake ranged
widely from 0 to nearly 100%. The extent of atrazine uptake decreased gen-
erally with increasing clay surface charge density or largely with increasing
CEC of the clay, since the siloxane plane areas of all smectites are relatively
comparable. Haderlein and Schwarzenbach (1993) and Haderlein et al. (1996)
further investigated the clay uptake of polar contaminants from water. The
latter study compared the sorption data of a series of nitroaromatic com-
pounds (NACs) (e.g., nitrobenzenes, nitrotoluences, nitroanilines, and nitro-
phenols) on relatively pure kaolinite (Cornwall, UK), illite (Tokay, Hungary),
and montmorillonite (SAz-1) in different cationic forms. On a given clay type,
+
the uptake of NACs was relatively high when the clay was exchanged with K ,
+
+
Cs , or NH 4 ion, and the isotherms are notably nonlinear, characteristic of
adsorption; the uptake became negligible when the clay was exchanged with
3+
2+
2+
+
strongly hydrating Na ,Ca ,Mg , and Al ions. The discrepant results were
attributed to the powerful hydration of the latter set of cations on clay’s
siloxane surfaces, which reduces their water-unoccupied surfaces accessible to
polar solutes (Weissmahr et al.,1997). On this basis,the high uptake of atrazine
by low-CEC Ca-saturated smectites, as observed by Laird et al. (1992), may