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

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 1, January - February (2013), © IAEME

79

PHYSICO-CHEMICAL STUDIES ON THE ADSORPTION OF

ATRAZIN ON LOCALLY MINED MONTMORILLONITES

P.S. Thué1, J. M. Siéliéchi 2*, P.P. Ndibewu3, R. Kamga1

1ENSAI, University of Ngaoundere, P.O. Box. 455 Ngaoundéré, Cameroon

[email protected], ; [email protected], 2 IUT, University of Ngaoundéré, P.O. Box. 455 Ngaoundéré, Cameroon

[email protected], 3

Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa

[email protected],

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

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN

ENGINEERING AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print)

ISSN 0976 - 6499 (Online)

Volume 4, Issue 1, January- February (2013), pp. 79-95

© IAEME: www.iaeme.com/ijaret.asp

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80

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].

CH3

CH3

Cl

NHNH2

N

N

N

N

N N

NH NH

Cl

CH3

CH3 CH3

ATR

DEA

CH3

CH3

OH

NHNH2

N

N

N

HYA

N

N N

NH2

NH

Cl

CH3

DIA

N

N N

NH2

NH2

Cl

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].


81

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)


82

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 10-3

M 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.


83

Ctkqt +=2/1

int

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:

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 amount adsorbed after a stirring time t, qe the amount adsorbed at equilibrium,

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:

Vm

CCq 0−

=

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.

tkLnqqqLn ete 1)( −=−


84

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

Formula weight

Composition

Molar refractivity

Molar volume

Parachor

Index of refraction

Surface tension

Density

Polarizability

Monoisotopic mass

Average mass

C8H14ClN5

215.68326

Atrazin*

58.49 ± 0.3 cm3

169.8 ± 3.0 cm3

460.1 ± 4.0 cm3

1.604 ± 0.02

53.8 ± 3.0 dyne/cm

1.269 ± 0.06 g/cm3

23.19 ± 0.5 10-24

cm3

215.093773 Da

215.6833 Da

C6H10ClN5

187.6301

DEA**

48.49 ± 0.3 cm3

136.1 ± 3.0 cm3

387.2 ± 4.0 cm3

1.630 ± 0.02

65.3 ± 0.05 dyne/cm

1.377 ± 0.06 g/cm3

19.22 ± 0.5 10-24

cm3

187.062473 Da

187.6301 Da

C6H11N5O

169.18444

HYA***

45.47 ± 0.3 cm3

122.6 ± 3.0 cm3

366.4 ± 4.0 cm3

1.663 ± 0.02

79.6 ± 3.0 dyne/cm

1.379 ± 0.06 g/cm3

18.02 ± 0.5 10-24

cm3

169.09636 Da

169.1844 Da

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%)


85

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

Formula weight

Composition

Molar refractivity

Molar volume

Parachor

Index of refraction

Surface tension

Density

Polarizability

Monoisotopic mass

Average mass

C8H14ClN5

215.68326

Atrazin*

58.49 ± 0.3 cm3

169.8 ± 3.0 cm3

460.1 ± 4.0 cm3

1.604 ± 0.02

53.8 ± 3.0 dyne/cm

1.269 ± 0.06 g/cm3

23.19 ± 0.5 10-24cm3

215.093773 Da

215.6833 Da

C11H19ClN10O

342.78796

DIA****

NA†

NA†

NA†

NA†

NA†

NA†

NA†

342.143183 Da

342.788 Da

C3H4ClN5

145.55036

DDA*****

33.89 ± 0.3 cm3

85.6 ± 3.0 cm3

277.2 ± 4.0 cm3

1.722 ± 0.02

109.9 ± 3.0 dyne/cm

1.700 ± 0.06 g/cm3

13.43 ± 0.5 10-24cm3

145.015523 Da

145.5504 Da

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.

(a) (b)

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

Contact time (min)

0 20 40 60 80 100 120

Atrazin adsorbed (mg.g-1)

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

100 µg.L-1

200 µg.L -1

300 µg.L -1

400 µg.L -1

Contact time (min)

0 20 40 60 80 100 120


0,0

0,5

1,0

1,5

2,0

100 µg.L-1

200 µg.L -1

300 µg.L -1

400 µg.L -1


86

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.

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.

Contact time (min)

0 20 40 60 80 100 120

Quantity adsorbed (mg.g-1)

0,0

0,2

0,4

0,6

0,8

1,0

1,2

100 µg.L-1

200 µg.L-1

300 µg.L-1

400 µg.L-1


87

Table 2 Influence of atrazin concentration on the kinetic parameters of the pseudo-

first-order and intra-particulate diffusion models.

C

(µg/

L-1

)

Pseudo-first order model Intra-particle diffusion model

K1

(min-1

)

qe cal

R2

Kint1

(g/mg.

min)

C1 R12 Kint2

(g/mg.

min)

C2 R22

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

.

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

Contact time (min)

0 20 40 60 80 100 120


0,0

0,2

0,4

0,6

0,8

1,0

1,2

m=100 mg

m=200 mg

m=300 mg

m=400 mg


88

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.

C

(µg/

L-1

)


K1

(min-1

)

qe cal R2 Kint1

(g/mg.

min)

C1 R12 Kint2 C2 R2

2

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.

Figure 5 Effect of pH on the adsorption kinetics of atrazin

C o n ta c t t im e (m in )

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0


0 ,0

0 ,2

0 ,4

0 ,6

0 ,8

1 ,0

p H = 2

p H = 6

p H = 7

p H = 8

p H = 9

p H = 12


89

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

(µg.L-1

)

pseudo-first order model Intra-particle diffusion model

K1

(min-1

)

qe cal

R2

Kint1

(g/mg.

min)

C1 R12 Kint2

(g/mg.m

in)

C2 R22

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).

N

N N

NH NH

Cl

CH3

CH3

CH3

+ H+

N

N N

NH NH

Cl

CH3

CH3

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+, Mg

2+ 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].


90

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.

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

(µg.L-1

)


K1

(min-1

)

qe cal

R2

Kint1

(g/mg.

min)

C1 R12 Kint2

(g/mg.

min)

C2 R22

FI=10-1

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

Contact time (min)

0 20 40 60 80 100 120


0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

FI=10-3M

FI=10-2M

FI=10-1M


91

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.

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

Ce (µg.L-1)

0 50 100 150 200 250

Qe (mg.g-1)

0,0

0,2

0,4

0,6

0,8

1,0

1,2


92

the one that best describes the adsorption relative to the Langmuir and Temkin models

(R2

Freundlich> R2

Langmuir> R2

Temkin).

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

) R2

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


93

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