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Pest Management Science
Pest Manag Sci 56:565±570 (2000)
Removal of paraquat and atrazine from water
by montmorillonite-(Ce or Zr) phosphate
cross-linked compounds
E González-Pradas,1* M Villafranca-Sánchez,1 F Del Rey-Bueno,2
MD Ureña-Amate1 and M Fernández-Pérez1
Department of Inorganic Chemistry, University of Almerı́a, La Cañada San Urbano s/n, 04120 Almerı́a, Spain
Department of Inorganic Chemistry, University of Granada, Campus Fuentenueva, Avda Severo Ochoa, s/n, 18071 Granada, Spain
Abstract: The adsorption of paraquat (1,1'-dimethyl-4,4'-bipyridilium dichloride) and atrazine (6chloro-N 2-ethyl-N 4-isopropyl-1,3,5-triazine-2,4-diamine) from aqueous solution onto two montmorillonite-(Ce or Zr) phosphate cross-linked compounds at different temperatures (288 K and 308 K)
has been studied using batch experiments. The adsorption isotherms obtained for paraquat on both
adsorbents may be classi®ed as H-type of the Giles classi®cation, which suggests that paraquat
molecules are strongly adsorbed on the samples. For the adsorption of atrazine, L-type isotherms were
obtained for both montmorillonite-(Ce or Zr) phosphate compounds, which suggests that these
compounds have a medium af®nity for this herbicide. The increase in temperature from 288 K to 308 K
did not have any clear effect on the adsorption process of paraquat on either adsorbent whereas
atrazine adsorption decreased slightly as temperature increased, possibly due to a mainly physical
process. Fourier transform infrared (FTIR) spectroscopic studies revealed that at the pH generated by
the adsorbents, the cationic herbicide interacted to a greater extent with the negatively charged surface
of the adsorbents than did atrazine. For both herbicides, the Ce-montmorillonite adsorbent showed a
higher adsorption capacity than the Zr-montmorillonite adsorbent.
# 2000 Society of Chemical Industry
Keywords: atrazine: paraquat; montmorillonite; phosphate; adsorption
The vast agricultural use of pesticides in Southern
Spain has important implications on the contamination of ground water systems which are used both for
human consumption and for crop irrigation. Areas
such as AlmerõÂa (southeastern AndalucõÂa) have low
rainfall and an intensive horticultural production
based on plastic greenhouses, so this contamination
of aquifer systems is an increasingly serious problem.
Water analyses have shown pesticide residues at
concentrations of 0.01±0.5 mg litreÿ1 in the AlmerõÂa
wells.1 One of the main reasons that removal and
disposal of these potentially hazardous waste chemicals is such a complex problem arises from the wide
range of chemical compounds which are used as
pesticides. This makes it extremely dif®cult to produce
a single method for pesticide disposal that applies
universally. Therefore, several speci®c methods for the
removal and disposal of these chemicals may be
required to solve the problem.
There is a growing interest in the application and
study of clays as precursors of cross-linked compounds, especially since with these one can now obtain
solids of controllable porosity for possible use as
adsorbents.2 Phosphates of tetravalent metals are
another type of inorganic layered compound that has
been widely studied because of their catalytic and ionexchange properties. These compounds have been
compared with some smectites due to their structural
similarities.3 So the use of cross-linked compounds
obtained using montmorillonite (a layered silicate
belonging to the group of smectites) and phosphates
of tetravalent metals has gained widespread acceptance as a technique to eliminate contaminants in
natural waters, as a result of the improvement of their
surface properties (speci®c surface areas and porosity),
with regard to those exhibited by each compound
independently. This allows the new compounds to be
used as better adsorbents in aqueous media. The
association between montmorillonite and phosphates
of tetravalent metals yields a material which does not
disperse in water and undergoes only a very low degree
of hydrolysis of its phosphate groups.4 The use of these
compounds in preference to others such as anionic
exchangers and some types of clay (natural or
modi®ed), can be justi®ed by the superior results
* Correspondence to: E González-Pradas, Department of Inorganic Chemistry, University of Almerı́a, La Cañada San Urbano s/n, 04120
Almerı́a, Spain
E-mail: [email protected]
(Received 26 January 1999; revised version received 15 October 1999; accepted 14 February 2000)
# 2000 Society of Chemical Industry. Pest Manag Sci 1526±498X/2000/$17.50
E GonzaÂlez-Pradas et al
obtained in the removal of contaminants from
As adsorption on solid substrates is one of the
methods which has been used for removing pesticides
from water,7 we considered it useful to study the
sorption processes of two herbicides, atrazine and
paraquat, as a function of temperature on two
montmorillonite-(Ce or Zr) phosphate cross-linked
Atrazine (6-chloro-N 2-ethyl-N 4-isopropyl-1,3,5triazine-2,4-diamine) is a systemic herbicide which
inhibits photosynthesis and is applied for general weed
control.8 Paraquat (1,1'-dimethyl-4,4'-bipyridinium
ion), normally applied in the form of the dichloride
salt, is an extremely effective, non-selective herbicide
which also interferes with the redox reactions related
to photosynthesis.9 Its persistence and polar character
mean that this compound may be present as residues
in surface water.10,11 Both herbicides are extensively
used in the AlmerõÂa region.
Taking into account the above, this study was
initiated to determine the effectiveness of the new
cross-linked compounds in removing atrazine and the
cationic pesticide paraquat from aqueous solutions.
This evaluation was carried out by studying the
sorption of the two pesticides in batch experiments,
in order to obtain the corresponding sorption isotherms and sorption capacities, as well as to study the
effect of temperature on the adsorption process.
The materials used as adsorbents in this study were
two samples of montmorillonite-Ce(IV) phosphate
and montmorillonite-Zr(IV) phosphate cross-linked
compounds (labelled as 5C and 5Z, respectively),
which have been characterized by the present authors
in a previous paper.4 Tetravalent metal phosphate-clay
compounds were obtained by using a method adapted
from the one proposed by Sterte,12 to obtain titanium
oxide±montmorillonite compounds. The starting material was a bentonite from `Los Trancos' deposit,
Minas de Gador SA, Cabo de Gata, Almeria, Spain.
Samples of montmorillonite, the < 2-mm fraction,
(5 g), were dispersed in 50 ml of the corresponding
tetravalent cation (Ce(IV) or Zr(IV)) solution at an
appropriate concentration that exceeded by ®ve times
the exchange capacity of the clay (90 meq 100 gÿ1).
The suspensions obtained were continuously stirred at
room temperature for 9 h and then allowed to stand for
12 h. Next, they were re¯uxed under constant stirring
with a volume of 50 ml of phosphoric acid at a
concentration double that of the tetravalent cation
used to saturate the clay. The mixture was re¯uxed for
a further 7 h and allowed to stand at room temperature
for 12 h. The samples obtained were then washed with
distilled water until sulphate (Ce sample) or chloride
(Zr sample) was completely removed in the washing
water, followed by drying at room temperature and
then at 383 K to constant weight.
Analytical grade atrazine (99%) and paraquat
(99%) were purchased from Riedel-De HaeÈn, (the
latter obtained as the dichloride salt) and used without
further treatment or puri®cation.
Atrazine and paraquat adsorption experiments were
performed by using the batch-equilibration method.
Duplicate samples of each adsorbent (0.1 g) were
equilibrated in 100 ml conical ¯asks with 50 ml of
aqueous solution of the herbicides with varying initial
concentrations, ranging from 1 10ÿ4 to 7.5 10ÿ2 cM
for the 5C sample and from 1 10ÿ4 to 6.7 10ÿ2 cM
for the 5Z sample. The experiments were carried out
in a thermostatic shaker bath at 288 K and 308 K.
Preliminary experiments were conducted for various
time intervals to determine when sorption equilibrium
was reached. The time required for atrazine was 48 h
and 168 h for paraquat. Following the equilibration
period, the adsorption systems were centrifuged at
19 000 rev minÿ1 for 10 min and the concentration of
the herbicide in the supernatant solutions (Ce)
determined by high performance liquid chromatography (HPLC) using a Beckman liquid chromatographic
system equipped with diode-array detector and data
The HPLC operating conditions were as follows:
separation by isocratic elution was performed on a
150 3.9 mm Nova-Pack LC-18 bonded-phase column (Waters, Millipore Corporation); sample volume, 20 ml; ¯ow rate, 1.0 ml minÿ1; and the mobile
phase, HPLC grade acetonitrile (Riedel-De HaÈen) ‡
demineralized water (milli-Q quality, Millipore Corp)
(60‡40 by volume) for atrazine measurements. For
paraquat measurements, the mobile phase was prepared as follows: a solution containing the speci®c
equivalent of 7.5 mM sodium heptanesulphonate
(Sigma Chemical Co) and 0.10 M orthophosphoric
acid (85%, Panreac) was made up in 0.45 mm ®ltered
doubly distilled water. The pH was adjusted to 3.00
with triethylamine (99.6%, Merck), and the organic
modi®er, acetonitrile, was added to yield a 10% (v/v)
proportion. Atrazine was analysed at 222 nm and
paraquat at 257 nm, their wavelengths of maximum
absorption. The amount of pesticide adsorbed on the
5C and 5Z samples, (X), was calculated from the
difference between the initial (C0) and equilibrium
herbicide solution concentrations (Ce). The pH of the
adsorbent solutions was also measured before and
after the adsorption experiments to determine if the
pH was stable during the experiment. Blanks containing no pesticide were used for each series of experiments. No degradation products of any herbicide were
found in the supernatant.
The FTIR spectra were recorded on an ATI
Mattson spectrometer. An aqueous pesticide solution
(0.05 litre) with a concentration corresponding to the
maximum used in the adsorption experiments was
added under continuous stirring to 0.25 g of each
adsorbent in 100 ml conical ¯asks. These ¯asks were
shaken for the equilibration time at 298 K. Following
the equilibration period, the samples were centrifuged
Pest Manag Sci 56:565±570 (2000)
Adsorption of paraquat and atrazine by metal phosphate montmorillonite
and the supernatant concentration measured. Another
0.05-litre portion of the same pesticide solution was
added and the process was repeated until the supernatant concentration was constant. The remaining
solids were then air dried without washing and pressed
into potassium bromide pellets.
The adsorption isotherms of paraquat and atrazine on
the 5C and 5Z samples at 288 K and 308 K are shown
in Figs 1 and 2. According to the slope of the initial
portion of the curves, the isotherms corresponding to
the adsorption of paraquat on 5C and 5Z samples (Fig
1) may be classi®ed as H-type of the Giles classi®cation.13 This suggests that both adsorbents have a high
af®nity for this pesticide and that there is no competition from solvent for adsorption sites. The curves
tend to de®ne a plateau, suggesting the formation of a
complete monolayer of paraquat molecules covering
the adsorbent surface (isotherms belonging to subgroup II of the Giles classi®cation). However, the
isotherms corresponding to the adsorption of atrazine
(Fig 2) show a decreased slope in the initial portion of
the curves and may be classi®ed as L-type isotherms.
This indicates that both adsorbents (5C and 5Z) have
a medium af®nity for atrazine and that no strong
competition from the solvent for adsorption sites
occurs. The curves in this case do not tend to de®ne
a plateau, the isotherms belonging to subgroup III of
the Giles classi®cation. Therefore, it seems reasonable
to suppose that in this case a complete monolayer of
atrazine molecules covering the adsorbents surface is
not formed.
To evaluate the adsorption capacities of the 5C and
5Z samples, the experimental data points were ®tted to
the Langmuir equation:14
…b Xm † Xm
Figure 1. Adsorption isotherms at 288K and 308K for paraquat on the 5C
and 5Z samples (error bars represent the standard deviation of two
Pest Manag Sci 56:565±570 (2000)
Figure 2. Adsorption isotherms at 288K and 308 K for atrazine on the 5C
and 5Z samples (error bars represent the standard deviation of two
where X = pesticide adsorbed per kg of adsorbent,
(cmol kgÿ1)
Ce = equilibrium solution concentration, (cM)
Xm = maximum amount of pesticide that can be
adsorbed in a monolayer (adsorption capacity),
(cmol kgÿ1)
b = constant relating to the energy of adsorption,
(litre cmolÿ1)
Plotting Ce/X vs Ce for both compounds on both 5C
and 5Z samples (Figs 3 and 4) shows that the
Langmuir isotherm ®ts well (Table 1).
The most prominent feature of these results is the
different adsorption capacity (Xm) of the paraquat and
the atrazine herbicides on both 5C and 5Z samples.
The Xm values obtained for paraquat are in the order
of 10 times higher than those obtained for atrazine.
This may be explained by the different physicochemical properties of the two pesticides studied. Paraquat
is a cationic herbicide, so the main adsorption mechanism is a strong electrostatic interaction with the
negative charged surface of the adsorbents.15±17
However, atrazine is a weak basic herbicide with a
pKa value of 1.68, so non-protonated molecules
Figure 3. Application of the Langmuir equation to the adsorption data of
paraquat on the 5C and 5Z samples.
E GonzaÂlez-Pradas et al
Figure 4. Application of the Langmuir equation to the adsorption data of
atrazine on the 5C and 5Z samples.
If we compare the adsorption of the two herbicides
at the two considered temperatures on both adsorbents, it can be seen that, in general, the 5C sample
shows a higher adsorption capacity than the 5Z. One
possible explanation for these results is that the cerium
and zirconium phosphates form a type of lattice on the
clay particles. In this process, the ®brous cerium
phosphate is wrapped around the clay particles
whereas the microcrystalline zirconium phosphate is
less cross-linked.23 Overall, the study of pore structure
characteristics of Ce(IV)- or Zr(IV)-montmorillonite
phosphate cross-linked compounds indicates that the
5C sample is more porous than 5Z, also showing a
larger contribution of macropores (8.43 m2 gÿ1 vs
2.38 m2 gÿ1).4 All these factors would affect the
accessibility of the herbicide molecules to the adsorbent surface.
dominate over the protonated species at the pH
generated by the samples (3.7 and 3.9 for 5C and
5Z, respectively). Thus, retention of the solids to the
surface only takes place through weaker forces such as
hydrogen bonds or Van der Waals forces.18
The Xm values given in Table 1 are greater in
general than those reported by other authors for the
same pesticides but using different adsorbents such as
anionic exchangers19 (0.11 cmol kgÿ1 for atrazine
sorption), illite16 (0.30 cmol kgÿ1 for paraquat sorption), kaolinite20 (2.79 10ÿ7 cmol kgÿ1 for atrazine
sorption), peat (0.32 and 0.03 cmol kgÿ1 for paraquat
and atrazine sorption, respectively), humic acids21,22
(9.25 and 0.07 cmol kgÿ1 for paraquat and atrazine
sorption, respectively) or natural bentonite (21.50 and
2.54 10ÿ4 cmol kgÿ1 for paraquat and atrazine sorption, respectively) and acid-treated bentonites
(1.07 cmol kgÿ1 for atrazine sorption).7
Varying the temperature from 288 K to 308 K does
not have any clear effect on the adsorption process of
paraquat for either 5C or 5Z. In contrast, this increase
in temperature results in a slight decrease in the
amount of atrazine adsorbed on either adsorbent. In
this case, the decrease of the Xm values with the
increase in temperature might be explained if we
assume that weak physical forces between the adsorbate and adsorbent exist.18 This trend has also been
reported previously by other authors for the sorption of
atrazine on kaolinitic and montmorillonitic clays.20
Infrared spectra
Figure 5 shows as an example the FTIR spectra of the
pure adsorbent 5C together with the spectra for
paraquat and atrazine sorbed on 5C.
The FTIR spectra of the paraquat±adsorbent
complex for 5C and also for 5Z exhibit bands
characteristic of this herbicide centred at 3067 and
3135 cmÿ1, which are assigned to the C-H tension
mode of the methyl groups on the aromatic ring in the
paraquat molecule.24 A characteristic set of bands can
also be seen between 1200 and 1600 cmÿ1 that may be
assigned to the C-C tension mode and the C-H
deformation mode in the aromatic ring plane.24,25
Since paraquat readily adsorbs on the adsorbent
surfaces, these ®ndings can be related to the mechanism of the adsorption of this compound. In the cited
mechanism, as above indicated, coulombic forces play
an important role, as demonstrated in the IR
In the FTIR spectra of the samples treated with
atrazine (the herbicide which is retained to a lesser
extent), modi®cations in the spectra are less pronounced than with paraquat. However, an intense
band appears at 1384 cmÿ1, which may be assigned to
the symmetric deformation vibration mode of the
methyl groups present in the herbicide (dCH3).27,28
These results demonstrate that the af®nity of atrazine
for adsorption to the montmorillonite-phosphate
cross-linked compounds is much weaker than that
observed for paraquat. Thus, atrazine is retained on
Table 1. Parameters of the Langmuir equation for adsorption of paraquat and atrazine on the 5C and 5Z samples
T (K) Pesticide
Xm a (cmol kgÿ1)
b 10ÿ4 a (litre cmolÿ1)
29.7327 ( 0.0003)
3.883 ( 0.014)
30.3186 ( 0.0003)
3.601 ( 0.018)
1.461 ( 0.001)
0.047 ( 0.049)
1.486 ( 0.001)
0.017 ( 0.086)
Xm a (cmol kgÿ1)
0.999*** 27.2776 ( 0.0005)
2.348 ( 0.010)
0.999*** 27.9509 ( 0.0003)
2.000 ( 0.012)
b 10ÿ3 a (litre cmolÿ1)
5.617 ( 0.002)
0.440 ( 0.050)
10.049 ( 0.001)
0.235 ( 0.066)
95 CL.
***Signi®cant at P = 0.001.
Pest Manag Sci 56:565±570 (2000)
Adsorption of paraquat and atrazine by metal phosphate montmorillonite
Figure 5. IR spectra of 5C, paraquat-5C complex and atrazine-5C complex.
the surface of the 5C and 5Z samples through
relatively weak interactions such as van der Waals
forces or H bonding.29
Pest Manag Sci 56:565±570 (2000)
Details of the interactions of paraquat and atrazine
with the two cross-linked compounds (5C, 5Z) have
E GonzaÂlez-Pradas et al
been elucidated by a combination of adsorption and
FTIR studies.
The FTIR results show a large number of feature
bands, especially for paraquat, probably due to the
electrostatic interaction mechanism with the surface of
the adsorbents. The weaker interaction and adsorption
of atrazine shows no relevant changes in the IR
spectrum, only, one additional band in the spectrum
appearing in this case.
The variation of temperature from 288 K to 308 K
mainly affects the adsorption process of atrazine,
decreasing the amount of pesticide adsorbed on either
adsorbent. The results obtained from the adsorption
isotherms, together with those above from the FTIR
studies, demonstrate that the af®nity of paraquat for
adsorption to both adsorbents is much stronger than
that observed for atrazine. The 5C adsorbent showed
in all cases the higher adsorption capacity, so this
sample is the most effective at removing atrazine and
to a large extent, paraquat molecules from aqueous
solution, using the experimental conditions reported
in this paper.
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