Geoscience Reference
In-Depth Information
Table 18.2 Percent desorption of paraquat (PQ) and diquat (DQ) (modified after Pateiro-Moure
et al.
2007
) Reprinted with permission from Pateiro-Moure et al. (
2007
). Copyright 2007
American Chemical Society
Desorption (%)
Soil sample
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
1
PQ
1.0
0.4
0.3
0.2
\0.08
\0.08
DQ
0.5
0.5
0.3
0.3
0.2
\0.01
EDTA-1
PQ
\0.08
\0.08
\0.08
\0.08
\0.08
\0.08
DQ
\0.01
\0.01
\0.01
\0.01
\0.01
\0.01
2
PQ
1.5
0.9
0.7
0.7
0.5
0.2
DQ
3.2
3.2
3.6
3.3
3.2
2.1
EDTA-2
PQ
0.3
0.2
0.4
0.4
0.3
0.2
DQ
1.9
1.5
0.5
1.4
1.1
0.5
Standard deviations: lower than 3 %
air-dried and moist samples of chlordimeform-montmorillonite complex were
practically equal (14.8 and 15.2 Å). Based on additional infrared spectra
(Fig.
18.16
), indicating that the adsorbed pesticide fixes the interlayer space,
Hermosin and Perez Rodriguez (
1981
) considered that (1) water molecules asso-
ciated inorganic exchangeable cations were displaced by chlordimeform, changing
the natural clay matrix and forming a coordination complex X(CfHCl)
m
n+
in the
interlayer spaces, or (2) inorganic exchangeable cations were displaced with their
hydration spheres from the interlamellar spaces by nonhydrated chlordimeform
molecules.
Glyphosate (N-phosphonomethylglycine)—a broad-spectrum herbicide—is a
small molecule comprising three polar functional groups (phosphonic acid, car-
boxylic acid, and secondary amines), which is sorbed strongly by soil minerals.
Glyphosate (GP) can only be sorbed onto variable-charge surfaces and not onto
permanent negative sites of layer silicates, because it has anionic properties in a
specific pH range (Borggaard and Gimsing
2008
, and references therein). The
most important soil and subsurface sorption sites for GP retention may be found on
aluminum and iron oxides, defectively ordered aluminum silicates, and edges of
layer silicates. Table
18.3
provides a view of ensemble GP sorption values on
various synthetic earth minerals, as a function of their surface area. It can be seen
that silicate clay (saturated with K
+
) has a limited capacity to sorb GP; the amount
appears to depend on the solution-saturating cation. Sorption of GP on montmo-
rillonite, illite, and kaolinite decreases according to the following order of cation
presence: Al
3+
[ Fe
3+
[ Cu
2+
[ Mg
2+
[ Zn
2+
[ Mn
2+
[ Ca
2+
[ Na
+
(Glass
1987
). The increased sorption of divalent and trivalent cations in multilayered
clays may be explained by the formation of complexes between interlayer cations
and glyphosate. Borggaard and Gimsing (
2008
) suggest that the discrepancy
between the two mineral groups (Table
18.3
) can be attributed to the number and
distribution of sorption sites. Oxides contain many adsorption sites; clay silicates
offer available adsorption sites only on octahedral layers exposed to mineral edges.
