Journal of Radioanalytical and Nuclear Chemistry, Vol. 249, No. 2 (2001) 385–390
Competitive sorption between glyphosate and inorganic phosphate
on clay minerals and low organic matter soils
H. M. Dion,1,3* J. B. Harsh,2,3** H. H. Hill Jr.1,3+
1
Department of Chemistry, Washington State University, USA
Department of Crop and Soil Sciences, Washington State University, USA
3
Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164, USA
2
(Received December 13, 2000)
Inorganic phosphate may influence the adsorption of glyphosate to soil surface sites. It has been postulated that glyphosate sorption is dominated
by the phosphoric acid moiety, therefore, inorganic phosphate could compete with glyphosate for surface sorption sites. We examine sorption of
glyphosate in low organic carbon systems where clay minerals dominate the available adsorption sites using 32P-labeled phosphate and 14C-labeled
glyphosate to track sorption. We found glyphosate sorption strongly dependent on phosphate additions. Isotherms were generally of the L type,
which is consistent with a limited number of surface sites. Most sorption on whole soils could be accounted for by sorption observed on model
clays of the same mineral type as found in the soils.
Introduction
Glyphosate [N-(phosphonomethyl)glycine] is a nonselective, broad-spectrum, post-emergent herbicide used
in a variety of agricultural and domestic settings. It is
already one of the most commonly applied herbicides
and its use is likely to increase in the coming years due
to the increased demand for Round-up Ready crops.3
It has been determined that the phosphate moiety of
glyphosate is responsible for its strong adsorption to
soils and that soil phosphate capacity is related directly
to glyphosate adsorption.4,9,10 Competitive effects of
phosphate on glyphosate adsorption have yet to be
quantified which is of increasing importance due to the
inception of Round-up Ready crops. It is estimated
that 57% of all soybeans grown in the United States are
Round-up Ready (USDA 1999).
Knowledge of the fate and transport of pesticides
applied to soils has become increasingly important as we
seek to protect groundwater from toxic chemicals and
crops from persistence. The residence time for a specific
pesticide depends upon soil and analyte characteristics,
such as, texture, cation exchange capacity, pH, organic
carbon content, degradation or transformation rate, and
analyte chemical properties. One of the factors that is
particularly important with regard to more polar
compounds, such as weak acids and bases, is the clay
content of the soil.
The objective of this study was to quantify the
competitive chemical adsorption phenomenon between
glyphosate and ortho-phosphate using dual-tracer batch
equilibrium experiments. Studies were conducted on
several whole soils (kaolinitic, illitic, smectitic) before
and after removing organic matter and pure clay
minerals. Pure minerals were examined in order to
develop a comparison between sorption on high clay
subsoils and clay minerals.
Experimental
Chemicals
Unlabeled and 14C-labeled glyphosate, 98.7% pure,
were obtained from Monsanto (St. Louis, MO).
Potassium phosphate dibasic was used in addition to
diammonium hydrogen phosphate to achieve desired
phosphate concentrations (96% pure, J. T. Baker). All
compounds were used as delivered.
Neutron activation
The 32P was prepared at Washington State
University’s Nuclear Radiation Center by neutron
activation using the 31P(n,γ)32P reaction. Diammonium
hydrogen phosphate [(NH4)2HPO4] was purchased from
J. T. Baker (99.6% pure) and used as received. A known
amount of dry ammonium phosphate was triple sealed in
polyethylene vials and irradiated in the WSU Triga III
fueled reactor for 6 hours, which equals a thermal
neutron fluence of ~1016 cm–2. A cool-down time of 12
hours was used to allow for short-lived activation
products to decay. After 12 hours the sample was
counted on a germanium detector, this revealed trace
quantities of 24Na (T1/2 = 14.5 h); however, this impurity
does not interfere with competitive sorption because the
24Na was completely decayed before the experiments
were begun. Ammonium phosphate was quantitatively
dissolved in ultra-pure water. The solution pH was 8.02.
* E-mail: [email protected]
** E-mail: [email protected]
+
E-mail: [email protected]
0236–5731/2001/USD 17.00
© 2001 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht
H. M. DION et al.: COMPETITIVE SORPTION BETWEEN GLYPHOSATE AND INORGANIC PHOSPHATE
Table 1. Soil physical and chemical characteristics
Soil
Illitic
Kaolinitic
Smectitic
Organic carbon, %
0.557 ± 0.001
0.0368 ± 0.0003
1.739 ± 0.0012
pH
6.78
7.77
7.50
Soils
Three soils were collected and air-dried for the
experiments. X-ray diffraction analysis was conducted
on the three soils to determine the underlying mineralogy
(Philips X-ray diffractometer). Oriented samples of the
three test soils were used and treatments included airdried, heated to 300 and 550 °C saturated with both
potassium and magnesium, and glycerol solvated. The
three soils used were an illitic Palouse silt loam; an
unmapped weathered kaolinite soil obtained from a
roadcut in Latah County, Idaho; and a Sharpsburg silty
clay loam dominated by smectites obtained from Union
County, Iowa.
Each of the three soils was characterized for texture
(pipet method, modified by WSU Pedology and
Quaternary Studies Laboratory), organic carbon
content,7 and pH (soil saturation extract). Results of the
soil characterization are contained in Table 1.
Clay minerals
Pure clay minerals were selected for comparison to
the clay fraction in the selected soils. Illite and kaolinite
clay minerals were obtained from Washington State
University’s mineral collection and were pulverized
prior to obtain a mean particle size between 25 and
2 µm. Beidellite clay (Bid-1) was purchased from the
Clay Mineral Repository and pulverized prior to use
(University of Missouri, St. Louis, MO).
Competitive sorption experiments
The procedure for the sorption experiments was
conducted using a soil:solution ratio of 1:10 m/m. One
gram of soil or clay mineral was placed in a sterilized
50 ml polyethylene centrifuge tube and 10 ml of solution
was added. The 32P was added as a spike to a matrix
solution of potassium phosphate dibasic to reach the
desired initial concentration, 0.005, 0.05, and 0.1 mol/l.
A mixture of unlabeled and carbon 14-labeled
glyphosate was added at 7.75E-3 (3.82E+3 dpm), 7.78E1 (2.785E+3 dpm), 1.48 (4.35E+3 dpm), and
2.96 mmol/l (8.68E+3 dpm) with initial activity shown
in brackets. All glyphosate and phosphate spike
additions were less than 0.250 ml in total added volume.
Blanks carried out through the same sample treatments
386
Sand, %
Silt, %
Clay, %
4.037 ± 0.012
4.288 ± 0.002
14.28 ± 0.03
52.03 ± 5.99
49.40 ± 16.06
43.85 ± 1.07
43.94 ± 2.01
45.05 ± 2.55
42.39 ± 1.70
showed that degradation due to microbiological or
chemical pathways or loss to equipment was negligible.
Sampling time was determined elsewhere by
conducting timed experiments during which samples
were taken at 1, 5, 12, 24, 48, 72 h, 1, 2, 4, and 6 weeks.
After 48 hours, glyphosate sorption from solution was
negligible, for all soils tested, as determined by linear
regression analysis on the data points from 48 to 72
hours.1 Suspensions were placed on an end-over-end
mixer until sampling. At 48 hours, the suspensions were
centrifuged at 10,000 rpm for 15 minutes with the
supernatant solution retained for analysis. In addition to
the competitive sorption experiments, glyphosate
sorption was examined without added phosphate.1
Liquid scintillation counting
Two ml of supernatant solution was added to 12 ml
of scintillation cocktail (ScintiVerse II, Fisher Chemical,
Fair Lawn, NJ) and counted by LSC (Model 1900TR
LSC, Packard/Canberra, Meriden, CT). Because 32P is a
high-energy beta-emitter, crossover calculations were
necessary to deconvolute the 14C spectra from the 32P
spectra. Energy window A was set between 0 and
156 keV and window B was set between 156 and
2,000 keV; this allowed for minimal 14C contributions to
the activity in window B. Although, there was a
significant contribution to the activity in region A from
the 32P, this overlap was corrected by determining
contribution to the counts in region A due to 32P. In
addition, it was found that a certain ratio of 32P counts
were being detected in both region A and region B; this
was corrected by calculating a cross talk value which
allowed determinations to be made based on separate
samples containing only the initial activities of 14C and
32P respectively:
Ratio of counts in region A versus
cpmb − blank cpma − blank corrected cpmb Cross talk calculation = ratio AB counts in region B =
cpm 32P = countsB – blank
cpm
14C
= (countsA – blank)–crosstalk
(1)
(2)
(3)
(4)
H. M. DION et al.: COMPETITIVE SORPTION BETWEEN GLYPHOSATE AND INORGANIC PHOSPHATE
Sorbed glyphosate and phosphate were determined
through back calculations using the initial aqueous
concentration and the final aqueous concentration.
accurate prediction of total phosphate capacity it would
be necessary to determine sorbed concentrations much
higher than those studied in this experiment:
y=
Results
Sorption experiments
Adsorption isotherms from the soils and clay
minerals were plotted using the 48-hour equilibrium
concentrations. Previous studies have shown sorption
past 48 hours is inconsequential to the total amount
sorbed.1 Initial phosphate concentrations were 5.3, 50.5,
and 101.1 mmol/l in order to represent concentrations
normally added to agricultural soils and glyphosate
concentrations ranged from 7.75E-3 to 2.96 mmol/l.
Isotherms depicting glyphosate sorption with varying
levels of phosphate are shown in Fig. 1.
As the initial level of phosphate increases, sorption of
glyphosate decreases. For all systems, addition of as
little as 5.3 mmol/l of phosphate causes a significant
decrease in sorption of glyphosate. Also, as initial levels
of glyphosate increased, the effect of the phosphate
tended to have less of an impact on glyphosate sorption.
Isotherms were characterized as being of the
Langmuir type, indicating a decrease in the slope of the
sorption isotherm, consistent with sorption on material
with a limited number of sorption sites. It is also
consistent with the hypothesis that glyphosate sorption
will decrease in the presence of ortho-phosphate
assuming that they compete for the same surface sites.
Table 2 contains the Langmuir curves and regression
analysis for the phosphate and glyphosate sorption
isotherms. Langmuir curves are used to estimate the total
sorption due to surface site saturation [Eq. (5)]. Results
indicate that glyphosate sorption should be much greater
than phosphate sorption; however, only three points were
used to generate the phosphate curve and
underestimation is possible. In order to make a more
ax (1 + bx) (5)
where y – glyphosate
or
phosphate
sorbed
(mmol.kg–1),
x – glyphosate or phosphate concentration
(mmol.l–1),
a – empirical fitting parameter,
b – empirical fitting parameter.
Nevertheless, the Langmuir a parameter, which is
related to the strength of interaction of the sorbate with
the sorbent, indicates that glyphosate is more strongly
sorbed than phosphate to both smectitic and illitic
minerals and soils. The results for kaolinite and the
weathered soil containing kaolinite indicate that
phosphate and glyphosate are on a more equal footing in
competition for sorption sites. The Langmuir parameters
do not differ significantly for phosphate sorption among
the minerals and soils; therefore, the low a parameter for
glyphosate on kaolinite may indicate a fundamental
difference in sorption mechanism on this mineral.
Figure 2 qualitatively shows the relationship between
glyphosate sorption as a function of phosphate
concentration. This relationship was investigated further
by plotting the equilibrium sorbed glyphosate
concentration (initial concentration 2.96 mmol/l) versus
the equilibrium sorbed phosphate concentration (mmol/l)
at each phosphate concentration. This plot was examined
using linear regression analysis (Table 3):
y = mx + b
(6)
where y – glyphosate sorbed (mmol.kg–1),
x – phosphate sorbed (mmol.kg–1),
m – slope of regression line,
b – y-intercept.
Table 2. Langmuir fit for phosphate and glyphosate sorption isotherms
Soil
Illite soil
Illite mineral
Kaolinite soil
Kaolinite mineral
Smectite soil
K
14.463
15.72
14.75
11.83
12.69
Phosphate
b
r2
4.147 E-5
6.690 E-5
3.303 E-5
2.89 E-4
8.826 E-5
1
1
1
1
1
K
175.9
354.7
7.7
19.5
87.7
Glyphosate
b
r2
7.695 E-2
1.216 E-2
2.732 E-3
3.373 E-2
2.925 E-2
0.976
0.955
0.987
0.940
0.991
387
H. M. DION et al.: COMPETITIVE SORPTION BETWEEN GLYPHOSATE AND INORGANIC PHOSPHATE
Fig. 1. Glyphosate sorption on selected soils and clay minerals; a – illitic soil, b– illite mineral, c – kaolinitic soil
388
H. M. DION et al.: COMPETITIVE SORPTION BETWEEN GLYPHOSATE AND INORGANIC PHOSPHATE
Fig. 1. Glyphosate sorption on selected soils and clay minerals; d – kaolinite, e – smectitic soil
Table 3. Linear regression analysis of glyphosate sorption
(initial concentration of 2.96 mmol/l) versus phosphate sorption
(initial concentrations of 5.3, 50.5, and 101.1 mmol/l)
Soil
Illite soil
Illite mineral
Kaolinite soil
Kaolinite mineral
Smectite soil
m
b
r2
–19.66
–23.07
–11.29
–7.64
–12.19
10.51
13.81
5.81
3.04
11.05
0.934
0.936
0.806
0.733
0.894
If sorption was preferential for either of the analytes
then the slope of the regression line would be either less
than one indicating preference for glyphosate or greater
than one indicating phosphate preference. In all of the
systems, there appears to be preferential adsorption of
glyphosate to the surface sites because the slope of each
of the systems studied was less than one. The greatest
preference was on the illitic soil and illite mineral with
slopes of –23.07 and –19.66, respectively, with the other
surfaces in the order smectitic soil > kaolinitic soil >
kaolinite mineral. This preference for glyphosate on the
soils and minerals studied may be due to glyphosate’s
multiple reactive functional groups, which could form
bidentate or tridentate complexes with the mineral
surface. These results agree with the Langmuir K values
which show glyphosate sorbed far more strongly than
phosphate.
Sorption of glyphosate and phosphate on the soils are
similar to sorption on the clay minerals chosen as
analogs. This supports the idea that the predominant clay
minerals in a soil system may be the most important
factor for predicting glyphosate/phosphate sorption in
low organic carbon systems. Competition between
phosphate and glyphosate reinforces other studies that
implicate the phosphate moiety in glyphosate sorption to
soils. A quantitative analysis leads to the conclusion that
glyphosate is preferentially adsorbed over phosphate
with the exception of the kaolinitic soil.
389
H. M. DION et al.: COMPETITIVE SORPTION BETWEEN GLYPHOSATE AND INORGANIC PHOSPHATE
*
H. D. was partially supported by the National Science
Foundation’s Integrative Graduate Education and Research Training
Grant to Washington State University under Grant #9972817. In
addition, we would like to acknowledge Pat FUERST for loaning of
radio labeled 14C glyphosate and WSU’s Nuclear Radiation Center for
reactor time in preparation of the 32P used in these experiments.
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Fig. 2. Sorption of glyphosate on selected soils and clay minerals
with and without the presence of phosphate; a – illitic soil and illite
mineral, b – kaolinitic soil and kaolinite mineral, c – smectitic soil and
beidellite mineral
390