Geoscience Reference
In-Depth Information
Fig. 16.14 Kinetics of
parathion degradation on
kaolinite (Saltzman et al.
1974
)
contact with the dissociated hydration water molecules. In the second stage,
parathion molecules that might have been initially bound to the clay surface by
different mechanisms are very slowly hydrolyzed, as they reach active sites with a
proper orientation.
Parathion hydrolysis on clay surfaces also is affected by environmental factors,
such as temperature and water content. A rise in temperature generally enhances
parathion hydrolysis on kaolinite, but the effect is greater in Na
+
-kaolinite than
Ca
2+
-kaolinite. These differences are due to the different hydrolysis pathways in
the presence of Na
+
- and Ca
2+
-saturating cations. In the limit of sorbed water, the
addition of water to kaolinite affects degradation kinetics and degradation rates.
A slight increase in the moisture content, above that corresponding to sorbed
water, results in a steep decrease in the degradation rate. To emphasize the rela-
tionship between degradation rate and moisture content, Saltzman et al. (
1974
)
plotted (Fig.
16.15
) the parathion remaining in Na
+
-, Ca
2+
-, and Al
3+
-kaolinites
after a 15-day incubation period against water content. The functional dependence
of degradation on moisture content on the various homoionic clays is similar. The
presence of free water, at about 11 % moisture content, almost completely hinders
the catalytic effect of the clay surface. Over a wide range of moisture contents, the
rate of parathion degradation is similar for Na
+
- and Ca
2+
-kaolinites, showing that,
in some cases, the effect of water may be stronger than that of the cation. For Al
3+
-
kaolinite, the opposite is true: The cation seems to be the determining factor of the
degradation rate.
In addition, the type of clay can affect the catalysis of organophosphate con-
taminants. As shown in the case of parathion transformation on kaolinite, the main
feature of the degradation process is the direct hydrolysis of the phosphate ester
bond, the catalysis being strongly moisture and cation dependent. The rear-
rangement of organophosphorus on kaolinite is a secondary process with a much
