Physico chemical studies on the adsorption of atrazin on locally mined montmorillonites

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976IN
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 1, January - February (2013), © IAEME
ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
IJARET
Volume 4, Issue 1, January- February (2013), pp. 79-95
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PHYSICO-CHEMICAL STUDIES ON THE ADSORPTION OF
ATRAZIN ON LOCALLY MINED MONTMORILLONITES

P.S. Thué1, J. M. Siéliéchi 2*, P.P. Ndibewu3, R. Kamga1
1
ENSAI, University of Ngaoundere, P.O. Box. 455 Ngaoundéré, Cameroon
pascalsilasthue@yahoo.fr, ; rickamga@yahoo.fr,
2
IUT, University of Ngaoundéré, P.O. Box. 455 Ngaoundéré, Cameroon
jsieliechi@yahoo.fr,
3
Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa
ndibewup@tut.ac.za,

ABSTRACT

Atrazin is an herbicide used intensively on large plantations for crop protection.
Unfortunately, this toxic usuallyin water intended for human consumption due to the well
known phenomena of leaching and infiltration. In the present work, the efficiency of local
montmorillonite for atrazin removal from aqueous solution is described. The adsorption
kinetics study showes that atrazin is quickly adsorbed on the surface of montmorillonite and
the adsorption equilibrium is attaind after 30 to 40 min. The adsorbed amount increases with
atrazin initial concentration and with the increased of the ionic strength. On the contrary,
there was a reduction of the amount adsorbed when the pH varied from 2 to 12 and when the
clay mass increased from 100 to 400 mg. The kinetics studies indicated that the adsorption
process was best described by the pseudo-first-order and intra-particle kinetics. The
Freundlich isotherm with a correlation coefficient of R2 = 0.99 and n = 1.76 was found to be
the model that best explain the adsorption of atrazin on the montmorillonite. It was also
shown that the affinity between the adsorbent and the adsorbate was strong for this type of
material. The application of the Temkin isotherm to the experimental data allowed to infer
that the adsorbate-adsorbent interaction energy was low (0.347 J.mol-1). This lead to the
conclusion that the mechanism of atrazin adsorption onto montmorillonite is probably a
physisorption process.

Keywords: Atrazin, adsorption, montmorillonite, kinetic, modelling

79

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –

1. INTRODUCTION

The intensification of agriculture in response to the increase of the world population
in recent years has resulted in an equal increase in the use of fertilizers and pesticides to
improve crop yields. Despite their strength in the protection of crops and increase in
agricultural yields, these chemicals have caused tremendous damage to the environment and
human, especially when it is not appropriately used. Pollution of surface and underground
water by infiltration or leaching of pesticides was reported [1] ely into surface and ground
waters are examples that need not be demonstrated. The principal active components of these
chemical compounds are highly biologically active, toxic and represent a potential risk to
human health, flora and fauna [1]. Atrazin (ATR) (6-chloro-N-ethyl-N'-(propan-2-yl)-1,3,5-
triazine-2,4-diamine) is one of the most widely used herbicides because of its ability to kill
many type of weeds on various crop fields. High concentration of atrazin has been detected in
surface and underground waters in Europe and North America [3,4] .
Despite the fact that it has been baned since November 2010 in the list of obsolete herbicides
[5], atrazin is still being used in many parts of Africa country including Cameroon.
In areas where the use is very intense, atrazin and its metabolites may contaminate surface
and ground waters [6,7]. Atrazin is a compound classified as potentially carcinogenic to
humans [8]. The U.S. Environmental Protection Agency (USEPA) has also shown that
atrazin and its metabolites (Figure 1(a)) act as endocrine disruptors. The maximum
admissible amount fof atrazin in drinking waters in the United State is 3ppb (3µg/L) [8].
Cl

CH3 N N

H3C NH N NH CH3

ATR

Cl OH

N N CH3 N N CH3

H2N N NH CH3 Cl H2N N NH CH3
DEA
N N HYA

H2N N NH CH3
DIA
Cl

N N

H2N N NH2

DDA

Figure 1(a) Chemical structures of atrazin (ATR) and its major degradation products,
desethylatrazin (DEA), deisopropylatrazin (DIA), didealkylatrazin (DDA),
and hydroxyatrazin (HYA) [9]

Once in the environment, ATR can remain chemically intact, or it can degrade. The physical-
chemical properties of ATR greatly enhance its mobility in both aqueous solution and it can
bind easily to soils. Hence, this compound travels long range, seap or leaches through the soil
and enters groundwater, especially in areas where table or groundwater is close to the surface.
This is true for areas where soils are loamy and well-drained (very permeable) [10].

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Once applied to the field, ATR can be carried with runoffs or storm water to surface water and
percolate to groundwater, or be retained in the soil column [9]. Degradation products are subject to
these same processes. [11] found that desethylatrazin (DEA) and hydroxyatrazin (HYA) are the most
prevalent degradation products in bulk soil, depending on the depth of the soil and the incubation
period [12]. HYA is the least mobile degradation product [13]; DEA and deisopropylatrazin (DIA) are
expected to be more mobile than the other compounds [14].
Considering the negative effects of atrazin and its metabolites on the environment, many studies have
been carried out aimed at their elimination from water intended for human consumption.
Conventional [6,7] water treatment process are ineffective for the removal of atrazin from drinking
water [16,17]. Ozonation [18,19] and membrane filtration [20, 21, 22] has been successful for atrazin
removal from water,however thes technique are too expensive. Adsorption of atrazin on activated
carbons also successful [23,24], but production and regeneration of activated carbon is costly.
Other studies have focused on the elimination of pesticides by clay. For example, investigation by two
co-workers [25] on the adsorption of atrazin and its metabolites (degradation products) by vermiculite
and montmorillonite modified by intercalation with iron (III) appeared to improve the adsorption rate
but with too long an adsorption reaction time of more than 24h. Also, from research conducted by
[26] on the removal of atrazin, lindane and diazinon from water using organo-zeolites, it appeared that
the adsorption capacity of atrazin was the lowest (2.0 mmoL.g-1). [27] using modified clays for
adsorbing atrazin in water showed that the adsorption coefficient was, hitherto, low. Furthermore,
work carried out by [28,29] on the adsorption of atrazin on montmorillonite showed that with a mass
of 20 gL-1 of montmorillonite solution in distilled water doped with atrazin , 38% removal was
obtained. However, these studies do not provide clear understanding of the mechanistic processes
involved in the montmorillonite-atrazin interaction..
Montmorillonite, are microscopic crystals of the 2:1 clay type, classified as phyllosilicate group of
minerals. They are known as a member of the smectite family [30]. Chemically, montmorillonite is
hydrated sodium, calcium, aluminum and magnesium silicate hydroxide (Figure (b)).

Figure 1(b) Structure of montmorillonite (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2.nH2O)

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Many studies has been carried out on atrazin adsorption from water on clay. Kaolinite type
have low adsorption capacity, natural montmorillonite have medium capacity and pillared
montmorillonite have very high efficiency.
This study reported on the influence of pH, ionic strength, the clay mass and concentration of
atrazin on the retention of atrazin in water by montmorillonite. Modelling of the kinetic and
isotherms data are also reported.

2. EXPERIMENTAL STUDY

2.1. Study materials

2.1.1. Montmorillonite sampling, fractionation and characterization
Soil aggregates were collected from Koussérie, a locality in the Far North region of
Cameroon (Africa). Aggregates were collected by digging with a shovel to a depth of 50cm
on average. These aggregates were placed in nylon bags and transported to the laboratory.
The aggregates were dried in the laboratory by spreading them on the surface of a clean and
dry bench top. They were then disaggregated by pounding in a wooden mortar and then
homogenized for at least ten minutes using a roller mixer (Heindolph, Type Reax 2 from
Germany). 1500 g of this sample were soaked in 3L of distilled water for 24 hours. The
<2µm fraction was obtained by gravity separation after 8 hours of decantation. The water was
then removed by drying at 105°C in an oven (Type P180 Jumo No. 84001 from USA). The
dried fraction was pulverized in an agate mortar and the powder obtained was stored in a
tightly closed glass jar for adsorption studies.
The particle size distribution of the clay fraction was performed using a Mastersizer 2000
particle size analyzer (Malvern Instrument Ltd, UK). For the <2µm fraction, it was found that
more than 50% of the particles had a size of about 1.952 microns. Earlier investigation by
[31] on this clay fraction had confirmed that it is actually a type 2:1 clay.

2.1.2. Atrazin
Pure atrazin molecule (99%) was obtained from Riedel de Haen (Germany).

2.1.3 Physicochemical and associated chemical properties of atrazin and its metabolites
(degradation products)
Evaluation of the physicochemical and their associated chemical properties of atrazin and its
metabolites was performed using ACD/Structure Design Suite Version 12 [32].

2.2. Adsorption Studies
2.2.1. Preparation of synthetic solution
The initial atrazin solution was prepared at 9.37 x 10-3M in methanol with 99.5% (obtained
from Aldrich, Germany) for spectroscopic analysis, by dissolving 0.02 g of atrazin in 10mL
of methanol. This solution was stored at 6°C in the refrigerator in a brown glass bottle. The
synthetic water solution was thus prepared by dissolving a given amount of this initial
solution of atrazin in a 1L of distilled water as per the desired concentration.
The preparation of the clay slurry was carried out in a batch of 1L beakers containing a
known mass of the prepared clay fraction and 1L of water containing atrazin as previously
described.

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2.2.3. Adsorption Kinetics
The study of the adsorption of atrazin on montmorillonite was carried out in a stirred reactor
using a Jar test device (Fisher Bioblock, France). The adsorption kinetics was determined at 25 ±
1 °C in a dispersed medium by putting into contact the atrazin in aqueous solution with the clay
suspension of known quantity.
The mixture was subjected to fast stirring (200 rpm) for 2 min followed by slow stirring at
60rpm. The adsorption kinetics was carried out by varying the pH of the suspension, the
concentration of atrazin, the mass of montmorillonite and ionic strength.
The atrazin concentration in solution was determined at regular time intervals of 10 min. for 2 h.
For this purpose, 3mL of the mixture were taken using a pipette bulb, centrifuged at 3700rpm
(DL 6000 B, USA) for 20 min. The supernatant was collected and analyzed using a UV-Visible
spectrophotometer (Metertech Spectrophotometer UV / Vis. SP8001, Taiwan) at 230 nm using a
quartz cuvette. The absorption spectrum was obtained in the range of 200 to 500 nm. This
spectrum has an absorption band at wavelengths between 215 and 230 nm with a maximum
absorption peak at 230 nm. The reference solution was the supernatant from centrifugation of the
clay suspension prepared under the same conditions as the sample but without any atrazin. The
values of residual atrazin concentrations were established on the basis of triplicate adsorption
tests.
2.2.4 Modelling of adsorption kinetics
The adsorption kinetics was modelled using the pseudo-first-order model and intra-particle
diffusion model.
The intra-particle diffusion kinetic equation is given by:
qt = k int t 1 / 2 + C
Where qt is the relative amount of atrazin adsorbed at time t, kint is the intra-particle diffusion
constant and C is a constant.
The kinetic equation for the pseudo-first order model is given by the relation:

Where qt is the amountt ) = Lnqe −after a stirring time t, qe the amount adsorbed at equilibrium,
Ln(qe − q adsorbed k1t

and k1 is the rate constant. Representing the function Ln(qe-qt) = f(t), we obtain a line with slope-
k1 and the intercept ln(qe).
2.2.4. Adsorption isotherms
The adsorption of atrazin on montmorillonite was carried out in a stirred reactor in a 1L beakers
prepared in batches. In these beakers were introduced a mass (m) of clay, of 200mg to which was
added various concentrations of atrazin. These beakers were stirred in the Jar test (Fisher
Bioblock, France) at 60rpm for a contact time of 40 min. Then, 3mL of suspension was removed
from each beaker using a pipette followed by centrifugation at 3700rpm (DL 6000 B, USA). The
adsorption was carried out at a temperature of 25°C, pH of 6.5 and an ionic strength of 5.10-3 M.
The adsorbed amount of atrazin (q) was determined by the difference between the initial
concentration of atrazin introduced into the solution and the residual concentration after
adsorption. The absorbed quantity expressed as per unit mass of clay is given by the relation:
C − C0
q= V
m
Where Co is the initial concentration of atrazin introduced in µg.L-1;
C: Concentration of atrazin in solution at time t;
m: mass of adsorbent used in g,
V: The volume of the solution in L.

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3. RESULTS AND DISCUSSION

3.1 Physicochemical and associated chemical properties of atrazin and its
metabolites
ACD/Structure Design Suite (SDS) is a valuable computational tool that uses proven
predictive algorithms and models to help optimize lead compounds towards producing
analogues with improved physicochemical characteristics in pharmacokinetic studies [33, 34]
. This tool was used in this study to provide advanced knowledge for the understanding of
structure-property relationships and improved physicochemical properties of atrazin and its
degradation products.
Table 1a compares the physicochemical and associated chemical properties of atrazin
(ATR) and two most prevalent metabolites, hydroxyatrazin (HYA) and desethylatrazin
(DEA) in soil. This is, probably, due the similarities in parameters such as molar refractivity,
molar volume, parachor, index of refraction, surface tension and density between them.
Although these physicochemical parameters are distinctively different in deisopropylatrazin
(DIA) when compared with DEA, these two degradation compounds of atrazin are the most
mobile [10].

The difference between ATR and DDA (didealkylatrazin) (the most infrequent metabolite of
atrazin) is illustrated in Table 1b. The surface tension of this molecule is very high and this
may explain its almost complete immobility, hence, not often detected in both soil and
ground water.

Table 1a Comparison of the physicochemical and associated chemical properties of
atrazin (ATR) and two most prevalent metabolites (DEA & HYA).
ATR DEA HYA
Molecular formula C8H14ClN5 C6H10ClN5 C6H11N5O
215.68326 187.6301 169.18444
Formula weight Atrazin* DEA** HYA***
58.49 ± 0.3 cm 3
48.49 ± 0.3 cm3 45.47 ± 0.3 cm3
Composition 169.8 ± 3.0 cm3 136.1 ± 3.0 cm3 122.6 ± 3.0 cm3
Molar refractivity 460.1 ± 4.0 cm3 387.2 ± 4.0 cm3 366.4 ± 4.0 cm3
1.604 ± 0.02 1.630 ± 0.02 1.663 ± 0.02
Molar volume 53.8 ± 3.0 dyne/cm 65.3 ± 0.05 dyne/cm 79.6 ± 3.0 dyne/cm
3
Parachor 1.269 ± 0.06 g/cm3 1.377 ± 0.06 g/cm3 1.379 ± 0.06 g/cm
-24 3
Index of refraction 23.19 ± 0.5 10-24cm3 19.22 ± 0.5 10-24cm3 18.02 ± 0.5 10 cm
215.093773 Da 187.062473 Da 169.09636 Da
Surface tension 215.6833 Da 187.6301 Da 169.1844 Da

Density
Polarizability
Monoisotopic mass

Average mass
ATR*:C(44.55%)H(6.54%)Cl(16.44%)N(32.47%) ; DEA**: C(38.41%) H(5.37%)
Cl(18.90%) N(37.33%); HYA***: C(42.60%) H(6.55%) N(41.39%) O(9.46%)

84


Table 1b Comparison of the physicochemical and associated chemical properties of atrazin
(ATR) with the most mobile (DIA) and the most infrequent (DDA) metabolites.
ATR DIA DDA
Molecular formula C8H14ClN5 C11H19ClN10O C3H4ClN5
215.68326 342.78796 145.55036
Formula weight Atrazin* DIA**** DDA*****
58.49 ± 0.3 cm3 NA† 33.89 ± 0.3 cm3
Composition 169.8 ± 3.0 cm3 NA† 85.6 ± 3.0 cm3
Molar refractivity 460.1 ± 4.0 cm3 NA† 277.2 ± 4.0 cm3
1.604 ± 0.02 NA† 1.722 ± 0.02
Molar volume 53.8 ± 3.0 dyne/cm NA† 109.9 ± 3.0 dyne/cm
Parachor 1.269 ± 0.06 g/cm3 NA† 1.700 ± 0.06 g/cm3
Index of refraction 23.19 ± 0.5 10-24cm3 NA† 13.43 ± 0.5 10-24cm3
215.093773 Da 342.143183 Da 145.015523 Da
Surface tension 215.6833 Da 342.788 Da 145.5504 Da
Density
Polarizability
Monoisotopic mass

Average mass
ATR*:C(44.55%)H(6.54%)Cl(16.44%)N(32.47%); DIA****: C(38.54%) H(5.59%)
Cl(10.34%) N(40.86%) O(4.67%); DDE*****:(24.76%) H(2.77%) Cl(24.36%) N(48.12%);
NA†: Not available
3.2 Atrazin adsorption on montmorillonite
3.2.1 Adsorption kinetics of atrazin
Figure 2 (a and b) show changes in the quantity of atrazin adsorbed as a function of contact time
for atrazin concentrations ranging from 100 to 400 µg.L-1 and pH of 3 and 10.
The adsorption kinetics has two phases: a rapid growth phase which indicates that atrazin is
rapidly adsorbed whatever the pH or the concentration of atrazin in solution and the second
phase, which is in the form of a plateau wherein the adsorption of the solute is at the maximum.

1,6

1,4
2,0
Atrazin adsorbed (mg.g )
-1

1,2
-1

-1
100 µg.L 1,0
1,5 200 µg.L
-1
-1
100 µg.L
-1
300 µg.L 0,8 200 µg.L -1
-1
400 µg.L 300 µg.L
-1

-1
0,6 400 µg.L
1,0

0,4

0,5
(a) 0,2 (b)
0,0
0 20 40 60 80 100 120
Contact time (min)
0,0
0 20 40 60 80 100 120

Contact time (min)

Figure 2 Kinetics of adsorption of atrazin at pH = 3 (a) and pH = 10 (b) FI = 2M,100mg

85


The first phase corresponds to the adsorption of the pesticide on the most accessible sites
located on the outer surface of the particles as well as on the interlayer spaces of the clay.
At the end of this phase the retained amount of atrazin stops evolving and the presence of a
plateau on the kinetic curves in the second step indicate that the adsorption equilibrium has
been attained. The equilibrium time is almost identical and varied between 30 and 40 min.

3.3 Influence of parameters on the adsorption kinetics of atrazin

3.3.1 Influence of the concentration of atrazin

Figure 3 shows the changes in the amounts of atrazin adsorbed on montmorillonite as a
function of the contact time for different initial concentrations of 100µg.L-1, 200 µg.L-1; 300
µg.L-1 and 400 µg.L-1 at pH = 6.5, temperature of 25 °C and 200 mg of clay. It can be seen
from figure 3 that the maximum amount of atrazin has been adsorbed after 20 min.
Quantity adsorbed (mg.g-1)

1,2

1,0

0,8 100 µg.L-1
200 µg.L-1
300 µg.L-1
0,6 400 µg.L-1

0,4

0,2

0,0
0 20 40 60 80 100 120
Contact time (min)

Figure 3 Influence of the concentration of atrazin adsorption kinetics: 200 mg, pH = 6.5
and FI = 5.10-3 M

The concentration adsorbed after 20 min. ranges from 0,2 to 1.0 mg.g-1for initial load concentrations
from 100 to 400 µg.L-1. The adsorption efficiency increases with an increase in the initial adsorbent.
Table 2 shows the influence of the concentration of atrazin on the parameters of the pseudo-first-order
model and intra-particulate diffusion model. From this data (Table 2), it appears that the pseudo-first
order model and intra-particle diffusion model best describes the phenomenon studied in view of the
correlation coefficient. The speed constant of the pseudo-first order model increases with increasing
atrazin concentration. Similarly, the speed constants of the intra-particle diffusion model increases
with increasing atrazin concentration.
It can thus be deduced that an increase in the concentration of atrazin have a positive influence on its
retention rate.

86


Table 2 Influence of atrazin concentration on the kinetic parameters of the pseudo-
first-order and intra-particulate diffusion models.

C Pseudo-first order model Intra-particle diffusion model
(µg/
L-1)
K1 qe cal R2 Kint1 C1 R12 Kint2 C2 R22
(min-1) (g/mg. (g/mg.
min) min)
100 0,0868 0,347 1 0,077 - 0,047 0,995 0,017 0,215 0,985

200 0,147 0,516 0,991 0,183 - 0,077 0,995 0,019 0,495 0,814
300 0,185 0,706 0,990 0,289 - 0,052 0,976 0,022 0,738 0,848
400 0,244 0,934 0,997 0,325 - 0,124 0,992 0,064 0,678 0,901

This could be explained by the fact that at low concentrations of atrazin, the diffusion of the
molecule to the adsorption sites on the surface of the clay is much lower than at high
concentrations. [35] observed similar results in the adsorption of endrin on montmorillonite.

3.3.2 Influence of the clay mass introduced
Figure 4 shows changes in the quantity of atrazin adsorbed on montmorillonite as a function
of the contact time for various masses of clay introduced which ranged from 100, 200, 300,
400 mg, at pH of 6.5, temperature of 25 °C and 250 µg.L-1.

1,2
Atrazin adsorbed (mg.g-1)

1,0

0,8

0,6
m=100 mg
m=200 mg
m=300 mg
0,4 m=400 mg

0,2

0,0
0 20 40 60 80 100 120
Contact time (min)

Figure 4 Influence of the clay mass introduced on the adsorption kinetics

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The shapes of the kinetic curves are the same as previously described whatever the mass of clay
introduced. We however noticed that the amount of atrazin adsorbed decreases as the clay mass
introduced into the medium increases. This amount reduces by almost four-folds with masses of clay
introduced from 400 to 100 mg.
Table 3 shows the influence of the mass of clay on the model parameters of pseudo-first-order and
intra-particle diffusion models.
Table 3 Influence of the mass of clay on the kinetic parameters of the model pseudo-first-order and
intra-particle diffusion.
Pseudo-first order model Intra-particle diffusion model
C K1 qe cal R2 Kint1 C1 R12 Kint2 C2 R22
(µg/ -1
(min ) (g/mg.
L-1) min)
M100 0,235 0,909 0,997 0,354 - 0,119 0,992 0,063 0,693 0,951
M200 0,2851 0,808 0,998 0,263 - 0,062 0,982 0,027 0,617 0,977
M300 0,197 0,406 0,999 0,115 - 0,060 0,998 0,027 0,249 0,948
M400 0,141 0,326 0,997 0,094 - 0,037 0,993 0,011 0,24 0,911
From Table 3, it was found that the pseudo-first order model and intra-particle diffusion best describe
the phenomenon studied with correlation coefficient greater than 0.9. The rate constant of the pseudo-
first order model decreases with an increase in the mass of clay. Similarly, the rate constants of intra-
particle diffusion model decreases with increasing mass of clay. This confirms that the increase in
mass of clay has a negative influence on the retention rate of atrazin. This result could be explained by
the fact that the increase in mass would reduce the mobility of atrazin in solution. Indeed, atrazin is a
weak base which is strongly hindered by the presence of a triazine cycle, two amino groups and two
alkyl groups in position 4 and 6. This molecular structure does not only give atrazin an
electronegative character, that is to say the same charge as that on the surface of the clay in solution,
but also reduces the possibility of attaching another molecule to neighbouring adsorption sites.

3.3.3 Influence of the pH of the mixture

Figure 5 shows the change in amounts of atrazin adsorbed on montmorillonite as a function of the
contact time at different pH values (from 2 – 12), 250µg.L-1, 200mg and at 25°C.

1 ,0
-1

0 ,8

pH =2
0 ,6 pH =6
pH =7
pH =8
pH =9
0 ,4
p H = 12

0 ,2

0 ,0
0 20 40 60 80 100 120 140
C o n t a c t t im e ( m in )

Figure 5 Effect of pH on the adsorption kinetics of atrazin

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It can be noted from figure 5 that the amount of atrazin adsorbed increases significantly when
the pH of the mixture decreases and tends towards to be more acidic.
Table 4 shows the influence of pH on the kinetic parameters of the pseudo-first-order and
intra-particle diffusion models.
Apparently, the pseudo-first order model and intra-particle diffusion model best describe the
phenomenon studied with a correlation coefficient greater than 0.9 (Table 4). The velocity
constant of the pseudo-first order model decreases with increasing pH. Similarly, the velocity
constants of the intra-particle diffusion model decreases with increasing pH. This confirms
that pH increase has a negative influence on the retention of atrazin.

Table 4 Effect of pH on the kinetic parameters of the model pseudo-first-order and
C pseudo-first order model Intra-particle diffusion model
(µg.L-1)
(min-1) (g/mg. (g/mg.m
min) in)
pH=2 0,252 0,784 0,999 0,269 - 0,107 0,995 0,056 0,526 0,926
pH=6 0,218 0,626 0,999 0,22 - 0,086 0,995 0,055 0,392 0,962
pH=7 0,201 0,609 0,999 0,21 - 0,064 0,989 0,036 0,438 0,947
pH=8 0,217 0,504 0,997 0,190 - 0,064 0,991 0,030 0,403 0,924
pH=9 0,197 0,451 1,000 0,116 - 0,062 0,995 0,020 0,318 0,930
pH=12 0,117 0,208 0,999 0,066 - 0,023 0,992 0,0126 0,139 0,909

This could be explained by the fact that a decrease in the pH of the solution leads to an
increase in the cationic fraction of atrazin, which would therefore favour its retention by the
negatively charged clay at this pH. Indeed, at low pH, atrazin through its amine function fixes
the proton H+ (protonation) and forms cations that are easily removed (Figure 6).

Cl
Cl

CH3 N N
CH3 N N
+
+ H
H3C NH N NH CH3
H3C NH N NH CH3 +
H

Figure 6 Reaction of protonation of atrazin in aqueous medium

Similarly, at low pH, the adsorbent capacity of clay is increased due to the replacement of
exchangeable cations (Ca2+, Na+, Mg2+ and K+) by H+ ions. This greatly increases their
negative charge. This result was also observed by [29] who showed that the adsorption of
atrazin on modified montmorillonite was best in acidic than alkaline conditions. A negative
adsorption correlation with pH was also reported in the case of the adsorption of basic
pesticides such as prochloraz [39], atrazin, terbuthylazine or the fluoroxypyr (family of
triazines) [37].

89


3.3.4 Influence of ionic strength
Figure 7 shows the variation of the amount of atrazin adsorbed on montmorillonite as a
function of the contact time at different values of ionic strength at 250µg.L-1, with pH = 6.5,
temperature of 25 ° C and 200 mg clay.

0,7
-1

0,6

0,5

0,4

0,3

0,2
FI=10-3M
0,1 FI=10-2M
FI=10-1M
0,0
0 20 40 60 80 100 120
Contact time (min)

Figure 7 Effect of ionic strength on the adsorption kinetics of atrazin

It is clear from figure 7 that increasing the ionic strength of the mixture results in a net
increase in the amount of atrazin adsorbed. Table 5 shows the influence of ionic strength on
the kinetic parameters of the pseudo-first-order and intra-particle diffusion models.

Table 5 Effect of ionic strength on the kinetic parameters of the model pseudo-first-order and
C Pseudo-first order model Intra-particle diffusion model
-1
(µg.L )
-1
(min ) (g/mg. (g/mg.
min) min)
-1
FI=10 0,256 0,611 0,998 0,200 - 0,061 0,988 0,027 0,442 0,899
FI=10-2 0,247 0,522 0,991 0,149 - 0,048 0,990 0,019 0,351 0,832
FI=10-3 0,122 0,253 0,999 0,065 - 0,035 0,995 0,021 0,130 0,963

From Table 5, it is found that the pseudo-first order model and intra-particle diffusion better
describe the phenomenon studied with correlation coefficient greater than 0.8. The velocity
constant of the pseudo-first order model decreases with increasing ionic strength. Similarly,
the velocity constants of intra-particle diffusion model decreases with increasing ionic
strength. This confirms that a decrease in ionic strength has a negative influence on the

90


retention of atrazin. This result could be explained by the fact that the ionic solution has an
influence on the diffused double layer of the clay [37]. Indeed, the addition of salt in the
medium would result in compression of the diffused double layer and promote interaction
between atrazin and the surface of montmorillonite. [35] also observed similar results.

3.3.5. Adsorption isotherm of atrazin
The adsorption isotherm of atrazin was obtained for concentrations ranging from 10, 100,
200, 300, and 400µg.L-1. The contact time between the clay and atrazin was the equilibrium
time which was determined in the adsorption kinetics study (40 min.). This is shown in
Figure 8.

1,2
Qe (mg.g-1)

1,0

0,8

0,6

0,4

0,2

0,0
0 50 100 150 200 250
Ce ( g.L-1)
Figure 8 Adsorption isotherm of atrazin at pH = 6.5, m = 200 mg; FI = 5.10-3M, T =
25°C)

The analysis of the isotherm shows a resemblance with the type L isotherm. Such type of
isotherm, indeed, indicates that the available adsorption sites decreases gradually as the
concentration of solute in solution increases. This implies that the solid has a greater affinity
for the solute in solution. We note, however, that the isotherm does not present a plateau,
indicating that the adsorption sites are not saturated in the concentration range used (10-400
µg.L-1).
A significant difference in the amounts of atrazin adsorbed was nevertheless observed. For
the same mass introduced, significant amounts of atrazin was adsorbed on the clay at pH =
6.5 (0.045 mg.g-1 ≤ 1 mg.g qe-1). This was not is thought not only to be facilitated by the
influence of the concentration of atrazin introduced but also by the pH. These results are in
agreement with the observations made during the study of the influence of atrazin
concentration and pH on the adsorption kinetics of atrazin [36].

3.4 Modelling the adsorption isotherm of atrazin

The Langmuir, Freundlich and Temkin models which are widely used for modelling
adsorption isotherms were used in this work to describe those of atrazin. The Langmuir
model parameters (qmax and KL), Freundlich (Kf and n) and Temkin (A and B) were obtained
by linearization of the model equations and are presented in Table 6.

A comparison of the regression coefficients (R2) shows that the adsorption isotherm of
atrazin can be described by the three models. However, the Freundlich model was found to be

91


the one that best describes the adsorption relative to the Langmuir and Temkin models
(R2Freundlich> R2Langmuir> R2Temkin).

Table 6 Parameters of Langmuir, Freundlich and Temkin obtained by linearization of
the adsorption isotherm of atrazin.

Langmuir Freundlich Temkin
KL (L.mg-1) qm(mg.g )-1
R 2
Kf (L.mg-1) n R2
A B R2
0,011 1,375 0,989 0,051 1,760 0,994 0,089 0,347 0,978

Indeed, the Freundlich model assumes that the adsorption of molecules at the solid-solution
interface occurs on heterogeneous surfaces having different types of adsorption sites, while
the Langmuir model describes adsorption taking place on homogeneous sites. The Temkin
model assumes that the adsorption energy of any molecule decreases linearly with the
covering of the surface of the adsorbent by the adsorbed species.
It is clear from this table that the maximum adsorption capacity, qm , predicted by the
Langmuir model is greater than the amount adsorbed which correspond to a concentration of
atrazin of 400 µg.L-1 given by the plot of the isotherm. Thus, the determined value of qm
suggest that this model may well be used to describe the adsorption of atrazin in solution.
In addition, the value of the constant, n, of the Freundlich model which is greater than 1
indicates a good affinity between atrazin and clay. This confirms the idea that it is an
isotherm of type L. However, the correlation coefficient given by the Freundlich model (R2 =
0.994) allows us to deduce that there do not only exist sites of same adsorption energy on the
surface of the clay, but there also exists sites of variable energy in smaller proportion.
The B constant of the Temkin model, which translate the interaction energy between the
atrazin molecule and clay, is very low (B = 0.347 J.mol-1), thus we can deduce that the
adsorption is physical. The main bonds implicated are therefore weak links of low adsorption
energies such as hydrogen bonds and Van der Waals bonds.

4. CONCLUSION

It is clear from this work that the kinetic data for the adsorption of atrazin on
montmorillonite is described by the maximum growth exponential model which consists of
two steps: the first which is the growth phase, corresponding to adsorption of molecule on the
most accessible sites located on the external surface of the clay and the second, constant
phase, corresponding to the equilibrium adsorption. The equilibrium time obtained varies
between 30 and 40 min.
From the study of the influence of parameters on the adsorption kinetics, it was found that the
adsorbed amounts were better when working at pH of 3 or 38 mg of clay or at high ionic
strength, or at high atrazin concentrations (491µg.L-1). The pseudo-first-order and intra-
particle diffusion kinetic models can be used to describe the adsorption kinetics of atrazin in
solution.
The experimental results on the adsorption of atrazin were compared with theoretical models
of Langmuir, Freundlich and Temkin. Although the three models showed correlation factors
R2>0.95, the best correlation was obtained with the Freundlich model. This strong correlation
indicates the heterogeneity of the surface of the clay (montmorillonite) used. Moreover, the

92


coefficient n>1, indicates that there is a great affinity between atrazin and clay. Maybe, the
most important interpretation of montmorillonite excellent sorptive ability as proven by the
physical and chemical parameters measured in this work would be their relationship with its
colloidal size and crystalline structure in layers, resulting in a high specific surface area and
optimum rheological characteristics.

ACKNOWLEDGEMENT

This work was supported in part by the Tshwane University of Technology (TUT),
Arcadia campus, Pretoria – South Africa. Their assistance in data analysis and presentation is
much appreciated. The authors acknowledge the Department of Chemistry & Environmental
Engineering (ENSAI-IUT) of the University of Ngaoundéré, Cameroon, for their help in
sample collection and experimental work. Finally, the Advanced Chemistry Development,
Inc. (ACD/Labs), 8 King St. E., Ste. 107, Toronto, Ontario M5C 1B5, Canada, is
acknowledged as physical chemical data of atrazin and its metabolites were generated using
their ACD/Structure Design Suite (SDS) and advanced in-silico chemistry tools.

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