Geoscience Reference
In-Depth Information
Table 8.9 Adsorption from aqueous solutions of s-triazine compounds by Na
+
-montmorillonite
(Hayes and Mingelgrin
1991
)
Compound
pK
a
Water solubility (ppm)
Adsorption (lmol/g)
pH 2
pH 3
pH 4
pH 5
Atrazine
1.68
33
275
200
115
70
Atratone
4.20
1,654
410
450
475
400
Ametryne
3.12
193
520
610
650
560
Propazine
1.85
9
150
110
20
20
Hydroxypropazine
5.20
310
150
220
240
245
Prometone
4.30
750
290
380
400
350
Prometryn
3.08
48
460
490
490
415
Sposito et al. (
1996
) analyzed a broad set of data and suggested two general
complexation mechanisms to characterize the retention of s-triazines on humic
acids: Proton transfer is favored for humic acids of high acidic functional groups
and for s-triazine with low basicity; electron transfer mechanisms are favored for
humic acids of low acidic functional group content and for s-triazines of high
basicity.
The effect of initial pH (3-11) on paraquat intake rate by activated clay
adsorption also is reported by Tsai et al. (
2003
). As the pH increased, the amount
of adsorbed cationic paraquat increased in response to the increasing number of
negatively charged sites, which are available due to the loss of H
+
from the
surfaces. The surface of activated clay at pH [ 3.0 exhibited negative charges due
mainly to the variable charge from pH-dependent surface hydroxyl sites. With the
increase in pH, the negative charges increased, which in turn increased paraquat
sorption. The distribution coefficient, K
d
, decreased as pH decreased, for example,
from 3,521 (L/kg) at pH 11.0 to 981 (L/kg) at pH 3.0. In fact, the rates of
adsorption for all of the studied pH values decreased with time, until they grad-
ually approached a plateau.
Barriuso et al. (
1992
) analyzed the relationship between soil properties and
adsorption behavior of cationic, anionic, and neutral herbicides. Atrazine, terbut-
ryn, and 2,4-D from the triazine herbicides family were considered. The study
included 58 soils, covering a wide range of pH values, OC contents, and miner-
alogical compositions. For this range of soils and pH, atrazine was in a neutral
form, 2,4-D in an anionic form, and terbutryn in either neutral or cationic form.
Based on the adsorption measurements, the authors concluded that (1) K
d
for
atrazine is strongly related to the OC content but not correlated to the soil pH; (2)
K
d
for terbutryn is less correlated to the OC content and is correlated to the soil
pH; and (3) K
d
for 2,4-D is not correlated to the OC content, but it is inversely
correlated to the soil pH.
Because pesticides in the subsurface can reach layers with different properties,
as they are redistributed with depth, it is interesting to examine their adsorption on
earth materials during vertical transport. Dror et al. (
1999
) report adsorption of two
s-triazines (atrazine and terbuthylazine), as they are redistributed in a 120-cm-deep
