Geoscience Reference
In-Depth Information
Table 8.10 Percent of adsorption from aqueous solution of organophosphorus pesticides by
clays (Yaron
1978
)
Insecticides
Solution
equilibrium
concentration
(ppm)
Clays
Ca
2+
-
montmorillonite
(%)
Ca
2+
-
kaolinite
(%)
Ca
2+
-
attapulgite
(%)
Parathion (C
10
H
14
NO
5
PS)
6.5
73
14
87
Pirimiphos-methyl
(C
11
H
20
N
3
O
3
PS)
16.0
94
30
Pirimiphos-methyl
(C
13
H
24
N
3
O
3
PS)
16.0
92
75
Menazon (C
6
H
12
N
5
O
2
PS
2
)
16.0
16
5
esters are stable at neutral or acidic pH but susceptible to hydrolysis in the pres-
ence of alkali compounds, where the P-O-X ester bond breaks down. The rate of
hydrolysis is related to the nature of the constituent X, the presence of catalytic
agents, pH, and temperature. Because the solubilities of organophosphorus
compounds in water are low, their adsorption from water solutions also is low.
Table
8.10
shows the percentage of some organophosphorus compounds adsorbed
from aqueous solution onto clay surfaces. The maximum adsorption capacity of
clays for organophosphorus pesticides is achieved only when the chemical reaction
occurs in an appropriate organic solvent. The maximum adsorption capacity of
parathion, for example, on a monoionic clay surface from a hexane solution is
10 % of the initial concentration on montmorillonite, 8 % on attapulgite, and less
than 0.5 % on kaolinite.
The amount of adsorbed chemical is controlled by both properties of the chemical
and of the clay material. The clay-saturating cation is a major factor affecting the
adsorption of the organophosphorus pesticide. The adsorption isotherm of parathion
from an aqueous solution onto montmorillonite saturated with various cations
(Fig.
8.33
) shows that the sorption sequence (Al
3+
[ Na
+
[ Ca
2+
) is not in
agreement with any of the ionic series based on ionic properties. This shows that, in
parathion-montmorillonite interactions in aqueous suspension, such factors as clay
dispersion, steric effects, and hydration shells are dominant in the sorption process.
In general, organophosphorus adsorption on clays is described by the Freundlich
equation, and the K
d
values for parathion sorption are 3 for Ca
2+
-kaolinite, 125 for
Ca
2+
-montmorillonite, and 145 for Ca
2+
-attapulgite.
The hydration status of the clay or earth material may affect the adsorption
capacity of nonpolar (or slightly polar) toxic chemicals. Continuing with parathion
as a case study, Fig.
8.34
shows the increase adsorbed parathion on attapulgite
from a hexane solution, as the adsorbed water on the clay surface decreases. This
behavior may be explained by the competition for adsorption sites between the
polar water and the slightly polar parathion. Possibly, however, the reduction in
adsorption due to the presence of water is caused by the increased time required
for parathion molecules to diffuse through the water film to the adsorption sites.
