Geoscience Reference
In-Depth Information
Fig. 14.1 Conversion of
3-aminotriazole to the imino
form (Russel et al.
1968
)
Many nonionic organic contaminants require extreme acid conditions to accept
H
+
ions. In clays, the extent of protonation is related to the electronegativity and
polarizing power of structural metal cations, in the order: H
+
[ Al
3+
[ Fe
3+
[
Mg
2+
[ Ca
2+
[ Na
+
[ K
+
(McBride
1994
). Mineral surface acidity also catalyzes
hydrolysis of organic contaminants in the subsurface. This transformation pathway
depends both on the type of clay mineral forming the solid phase and on the clay-
saturating cation. For example, Mg
2+
-montmorillonite exhibits much weaker cata-
lytic capacity than Ca
2+
-montmorillonite; and Ca
2+
-beidellite and Ca
2+
-nontronite
exhibit a lower catalytic capacity than Ca
2+
-montmorillonite (Mortland and Raman
1967
). As clay surfaces become drier, the protons become concentrated in a smaller
volume of water, and the surface acidity increases to an extreme value. Under these
conditions, even a very weak base can be protonated. Chaussidon and Calvet (
1965
)
showed that amines adsorbed on montmorillonite undergo transformation on
dehydration of the clay. Catalytic hydroxylation of an organic molecule (e.g.,
atrazine) on H
+
-montmorillonite involves the substitution of a chlorine atom by a
hydroxyl ion; the degradation product apparently remains adsorbed on the clay as
the keto form of the protonated hydroxy analog (Russel et al.
1968
). These authors
also showed that Mg
2+
-montmorillonite converts the amino form of 3-aminotriazole
to the imino form, as shown in Fig.
14.1
.
Surface-catalyzed degradation of pesticides has been examined in the context of
research on contaminant-clay interactions. Such interactions were observed ini-
tially when clay minerals were used as carriers and diluents in the crop protection
industry (Fowker et al.
1960
). Later specific studies on the persistence of potential
organic contaminants in the subsurface defined the mechanism of clay-induced
transformation of organophosphate insecticides (Saltzman et al.
1974
; Mingelgrin
and Saltzman
1977
) and s-triazine herbicides (Brown and White
1969
). In both
cases, contaminant degradation was attributed to the surface acidity of clay min-
erals, controlled by the hydration status of the system.
Rearrangement reactions catalyzed by the clay surface were observed for
parathion (an organophosphate pesticide) when it was adsorbed on montmoril-
lonite or kaolinite in the absence of a liquid phase. The rate of rearrangement
reactions increased with the polarization of the hydration water of the exchange-
able cation (Mingelgrin and Saltzman
1977
). Table
14.1
summarizes a series of
reactions catalyzed by clay surfaces, as reported in the literature.
Dissolved metals and metal-containing surfaces play an important role in the
transformation of organic contaminants in the subsurface environment. Metal ions
can catalyze hydrolysis in a way similar to acid catalysis. Organic hydrolyzable
compounds susceptible to metal ion catalysis include carboxylic acids, esters,
