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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976IN  INTERNATIONAL JOURNAL OF A...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –                            ...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 649...
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Physico chemical studies on the adsorption of atrazin on locally mined montmorillonites

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

  1. 1. 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) IJARETVolume 4, Issue 1, January- February (2013), pp. 79-95© IAEME: www.iaeme.com/ijaret.asp ©IAEMEJournal Impact Factor (2012): 2.7078 (Calculated by GISI)www.jifactor.com 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
  2. 2. 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), © IAEME1. INTRODUCTION The intensification of agriculture in response to the increase of the world populationin recent years has resulted in an equal increase in the use of fertilizers and pesticides toimprove crop yields. Despite their strength in the protection of crops and increase inagricultural yields, these chemicals have caused tremendous damage to the environment andhuman, especially when it is not appropriately used. Pollution of surface and undergroundwater by infiltration or leaching of pesticides was reported [1] ely into surface and groundwaters are examples that need not be demonstrated. The principal active components of thesechemical compounds are highly biologically active, toxic and represent a potential risk tohuman 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 killmany type of weeds on various crop fields. High concentration of atrazin has been detected insurface 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 surfaceand ground waters [6,7]. Atrazin is a compound classified as potentially carcinogenic tohumans [8]. The U.S. Environmental Protection Agency (USEPA) has also shown thatatrazin and its metabolites (Figure 1(a)) act as endocrine disruptors. The maximumadmissible 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 DDAFigure 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 canbind easily to soils. Hence, this compound travels long range, seap or leaches through the soiland 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]. 80
  3. 3. 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), © IAEMEOnce applied to the field, ATR can be carried with runoffs or storm water to surface water andpercolate to groundwater, or be retained in the soil column [9]. Degradation products are subject tothese same processes. [11] found that desethylatrazin (DEA) and hydroxyatrazin (HYA) are the mostprevalent degradation products in bulk soil, depending on the depth of the soil and the incubationperiod [12]. HYA is the least mobile degradation product [13]; DEA and deisopropylatrazin (DIA) areexpected to be more mobile than the other compounds [14].Considering the negative effects of atrazin and its metabolites on the environment, many studies havebeen 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 drinkingwater [16,17]. Ozonation [18,19] and membrane filtration [20, 21, 22] has been successful for atrazinremoval from water,however thes technique are too expensive. Adsorption of atrazin on activatedcarbons 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 twoco-workers [25] on the adsorption of atrazin and its metabolites (degradation products) by vermiculiteand montmorillonite modified by intercalation with iron (III) appeared to improve the adsorption ratebut 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 thatthe adsorption capacity of atrazin was the lowest (2.0 mmoL.g-1). [27] using modified clays foradsorbing 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 massof 20 gL-1 of montmorillonite solution in distilled water doped with atrazin , 38% removal wasobtained. However, these studies do not provide clear understanding of the mechanistic processesinvolved in the montmorillonite-atrazin interaction..Montmorillonite, are microscopic crystals of the 2:1 clay type, classified as phyllosilicate group ofminerals. They are known as a member of the smectite family [30]. Chemically, montmorillonite ishydrated 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) 81
  4. 4. 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 Many studies has been carried out on atrazin adsorption from water on clay. Kaolinite typehave low adsorption capacity, natural montmorillonite have medium capacity and pillaredmontmorillonite have very high efficiency.This study reported on the influence of pH, ionic strength, the clay mass and concentration ofatrazin on the retention of atrazin in water by montmorillonite. Modelling of the kinetic andisotherms data are also reported.2. EXPERIMENTAL STUDY2.1. Study materials2.1.1. Montmorillonite sampling, fractionation and characterizationSoil aggregates were collected from Koussérie, a locality in the Far North region ofCameroon (Africa). Aggregates were collected by digging with a shovel to a depth of 50cmon 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 anddry bench top. They were then disaggregated by pounding in a wooden mortar and thenhomogenized for at least ten minutes using a roller mixer (Heindolph, Type Reax 2 fromGermany). 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 wasthen removed by drying at 105°C in an oven (Type P180 Jumo No. 84001 from USA). Thedried fraction was pulverized in an agate mortar and the powder obtained was stored in atightly closed glass jar for adsorption studies.The particle size distribution of the clay fraction was performed using a Mastersizer 2000particle size analyzer (Malvern Instrument Ltd, UK). For the <2µm fraction, it was found thatmore 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. AtrazinPure 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 itsmetabolites was performed using ACD/Structure Design Suite Version 12 [32].2.2. Adsorption Studies2.2.1. Preparation of synthetic solutionThe initial atrazin solution was prepared at 9.37 x 10-3M in methanol with 99.5% (obtainedfrom Aldrich, Germany) for spectroscopic analysis, by dissolving 0.02 g of atrazin in 10mLof methanol. This solution was stored at 6°C in the refrigerator in a brown glass bottle. Thesynthetic water solution was thus prepared by dissolving a given amount of this initialsolution 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 aknown mass of the prepared clay fraction and 1L of water containing atrazin as previouslydescribed. 82
  5. 5. 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), © IAEME2.2.3. Adsorption KineticsThe study of the adsorption of atrazin on montmorillonite was carried out in a stirred reactorusing 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 claysuspension of known quantity.The mixture was subjected to fast stirring (200 rpm) for 2 min followed by slow stirring at60rpm. The adsorption kinetics was carried out by varying the pH of the suspension, theconcentration 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-Visiblespectrophotometer (Metertech Spectrophotometer UV / Vis. SP8001, Taiwan) at 230 nm using aquartz cuvette. The absorption spectrum was obtained in the range of 200 to 500 nm. Thisspectrum has an absorption band at wavelengths between 215 and 230 nm with a maximumabsorption peak at 230 nm. The reference solution was the supernatant from centrifugation of theclay suspension prepared under the same conditions as the sample but without any atrazin. Thevalues of residual atrazin concentrations were established on the basis of triplicate adsorptiontests.2.2.4 Modelling of adsorption kineticsThe adsorption kinetics was modelled using the pseudo-first-order model and intra-particlediffusion model.The intra-particle diffusion kinetic equation is given by: qt = k int t 1 / 2 + CWhere qt is the relative amount of atrazin adsorbed at time t, kint is the intra-particle diffusionconstant 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 k1tand 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 isothermsThe adsorption of atrazin on montmorillonite was carried out in a stirred reactor in a 1L beakersprepared in batches. In these beakers were introduced a mass (m) of clay, of 200mg to which wasadded various concentrations of atrazin. These beakers were stirred in the Jar test (FisherBioblock, France) at 60rpm for a contact time of 40 min. Then, 3mL of suspension was removedfrom each beaker using a pipette followed by centrifugation at 3700rpm (DL 6000 B, USA). Theadsorption 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 initialconcentration of atrazin introduced into the solution and the residual concentration afteradsorption. The absorbed quantity expressed as per unit mass of clay is given by the relation: C − C0q= V mWhere 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. 83
  6. 6. 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), © IAEME3. RESULTS AND DISCUSSION3.1 Physicochemical and associated chemical properties of atrazin and its metabolitesACD/Structure Design Suite (SDS) is a valuable computational tool that uses provenpredictive algorithms and models to help optimize lead compounds towards producinganalogues with improved physicochemical characteristics in pharmacokinetic studies [33, 34]. This tool was used in this study to provide advanced knowledge for the understanding ofstructure-property relationships and improved physicochemical properties of atrazin and itsdegradation 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 mostmobile [10].The difference between ATR and DDA (didealkylatrazin) (the most infrequent metabolite ofatrazin) is illustrated in Table 1b. The surface tension of this molecule is very high and thismay explain its almost complete immobility, hence, not often detected in both soil andground water. Table 1a Comparison of the physicochemical and associated chemical properties of atrazin (ATR) and two most prevalent metabolites (DEA & HYA). ATR DEA HYAMolecular formula C8H14ClN5 C6H10ClN5 C6H11N5O 215.68326 187.6301 169.18444Formula weight Atrazin* DEA** HYA*** 58.49 ± 0.3 cm 3 48.49 ± 0.3 cm3 45.47 ± 0.3 cm3Composition 169.8 ± 3.0 cm3 136.1 ± 3.0 cm3 122.6 ± 3.0 cm3Molar 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 ± 0.02Molar volume 53.8 ± 3.0 dyne/cm 65.3 ± 0.05 dyne/cm 79.6 ± 3.0 dyne/cm 3Parachor 1.269 ± 0.06 g/cm3 1.377 ± 0.06 g/cm3 1.379 ± 0.06 g/cm -24 3Index 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 DaSurface tension 215.6833 Da 187.6301 Da 169.1844 DaDensityPolarizabilityMonoisotopic massAverage massATR*: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
  7. 7. 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 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 Atrazin adsorbed (mg.g ) -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
  8. 8. 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), © IAEMEThe first phase corresponds to the adsorption of the pesticide on the most accessible siteslocated 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 aplateau on the kinetic curves in the second step indicate that the adsorption equilibrium hasbeen attained. The equilibrium time is almost identical and varied between 30 and 40 min.3.3 Influence of parameters on the adsorption kinetics of atrazin3.3.1 Influence of the concentration of atrazinFigure 3 shows the changes in the amounts of atrazin adsorbed on montmorillonite as afunction of the contact time for different initial concentrations of 100µg.L-1, 200 µg.L-1; 300µg.L-1 and 400 µg.L-1 at pH = 6.5, temperature of 25 °C and 200 mg of clay. It can be seenfrom 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 MThe concentration adsorbed after 20 min. ranges from 0,2 to 1.0 mg.g-1for initial load concentrationsfrom 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-ordermodel and intra-particulate diffusion model. From this data (Table 2), it appears that the pseudo-firstorder model and intra-particle diffusion model best describes the phenomenon studied in view of thecorrelation coefficient. The speed constant of the pseudo-first order model increases with increasingatrazin concentration. Similarly, the speed constants of the intra-particle diffusion model increaseswith increasing atrazin concentration.It can thus be deduced that an increase in the concentration of atrazin have a positive influence on itsretention rate. 86
  9. 9. 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), © IAEMETable 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 0,901This could be explained by the fact that at low concentrations of atrazin, the diffusion of themolecule to the adsorption sites on the surface of the clay is much lower than at highconcentrations. [35] observed similar results in the adsorption of endrin on montmorillonite.3.3.2 Influence of the clay mass introducedFigure 4 shows changes in the quantity of atrazin adsorbed on montmorillonite as a functionof 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 87
  10. 10. 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), © IAEMEThe shapes of the kinetic curves are the same as previously described whatever the mass of clayintroduced. We however noticed that the amount of atrazin adsorbed decreases as the clay massintroduced into the medium increases. This amount reduces by almost four-folds with masses of clayintroduced from 400 to 100 mg.Table 3 shows the influence of the mass of clay on the model parameters of pseudo-first-order andintra-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 modelC 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 0,951M200 0,2851 0,808 0,998 0,263 - 0,062 0,982 0,027 0,617 0,977M300 0,197 0,406 0,999 0,115 - 0,060 0,998 0,027 0,249 0,948M400 0,141 0,326 0,997 0,094 - 0,037 0,993 0,011 0,24 0,911From Table 3, it was found that the pseudo-first order model and intra-particle diffusion best describethe 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 inmass of clay has a negative influence on the retention rate of atrazin. This result could be explained bythe fact that the increase in mass would reduce the mobility of atrazin in solution. Indeed, atrazin is aweak base which is strongly hindered by the presence of a triazine cycle, two amino groups and twoalkyl groups in position 4 and 6. This molecular structure does not only give atrazin anelectronegative 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 mixtureFigure 5 shows the change in amounts of atrazin adsorbed on montmorillonite as a function of thecontact time at different pH values (from 2 – 12), 250µg.L-1, 200mg and at 25°C. 1 ,0 Atrazin adsorbed (mg.g ) -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 88
  11. 11. 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 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 intra-particle diffusion.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.m min) in)pH=2 0,252 0,784 0,999 0,269 - 0,107 0,995 0,056 0,526 0,926pH=6 0,218 0,626 0,999 0,22 - 0,086 0,995 0,055 0,392 0,962pH=7 0,201 0,609 0,999 0,21 - 0,064 0,989 0,036 0,438 0,947pH=8 0,217 0,504 0,997 0,190 - 0,064 0,991 0,030 0,403 0,924pH=9 0,197 0,451 1,000 0,116 - 0,062 0,995 0,020 0,318 0,930pH=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
  12. 12. 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), © IAEME3.3.4 Influence of ionic strength Figure 7 shows the variation of the amount of atrazin adsorbed on montmorillonite as afunction 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 Atrazin adsorbed (mg.g ) -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 atrazinIt is clear from figure 7 that increasing the ionic strength of the mixture results in a netincrease in the amount of atrazin adsorbed. Table 5 shows the influence of ionic strength onthe 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 andintra-particle diffusion. C Pseudo-first order model Intra-particle diffusion model -1 (µg.L ) K1 qe cal R2 Kint1 C1 R12 Kint2 C2 R22 -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 0,963From Table 5, it is found that the pseudo-first order model and intra-particle diffusion betterdescribe the phenomenon studied with correlation coefficient greater than 0.8. The velocityconstant of the pseudo-first order model decreases with increasing ionic strength. Similarly,the velocity constants of intra-particle diffusion model decreases with increasing ionicstrength. This confirms that a decrease in ionic strength has a negative influence on the 90
  13. 13. 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), © IAEMEretention of atrazin. This result could be explained by the fact that the ionic solution has aninfluence on the diffused double layer of the clay [37]. Indeed, the addition of salt in themedium would result in compression of the diffused double layer and promote interactionbetween atrazin and the surface of montmorillonite. [35] also observed similar results.3.3.5. Adsorption isotherm of atrazinThe 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 equilibriumtime which was determined in the adsorption kinetics study (40 min.). This is shown inFigure 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 ofisotherm, indeed, indicates that the available adsorption sites decreases gradually as theconcentration of solute in solution increases. This implies that the solid has a greater affinityfor 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 (10-400µg.L-1).A significant difference in the amounts of atrazin adsorbed was nevertheless observed. Forthe 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 theinfluence of the concentration of atrazin introduced but also by the pH. These results are inagreement with the observations made during the study of the influence of atrazinconcentration and pH on the adsorption kinetics of atrazin [36].3.4 Modelling the adsorption isotherm of atrazinThe Langmuir, Freundlich and Temkin models which are widely used for modellingadsorption isotherms were used in this work to describe those of atrazin. The Langmuirmodel parameters (qmax and KL), Freundlich (Kf and n) and Temkin (A and B) were obtainedby linearization of the model equations and are presented in Table 6.A comparison of the regression coefficients (R2) shows that the adsorption isotherm ofatrazin can be described by the three models. However, the Freundlich model was found to be 91
  14. 14. 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), © IAEMEthe 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 TemkinKL (L.mg-1) qm(mg.g )-1 R 2 Kf (L.mg-1) n R2 A B R20,011 1,375 0,989 0,051 1,760 0,994 0,089 0,347 0,978Indeed, the Freundlich model assumes that the adsorption of molecules at the solid-solutioninterface occurs on heterogeneous surfaces having different types of adsorption sites, whilethe Langmuir model describes adsorption taking place on homogeneous sites. The Temkinmodel assumes that the adsorption energy of any molecule decreases linearly with thecovering of the surface of the adsorbent by the adsorbed species.It is clear from this table that the maximum adsorption capacity, qm , predicted by theLangmuir model is greater than the amount adsorbed which correspond to a concentration ofatrazin of 400 µg.L-1 given by the plot of the isotherm. Thus, the determined value of qmsuggest 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 1indicates a good affinity between atrazin and clay. This confirms the idea that it is anisotherm 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 thesurface 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 theatrazin molecule and clay, is very low (B = 0.347 J.mol-1), thus we can deduce that theadsorption is physical. The main bonds implicated are therefore weak links of low adsorptionenergies 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 onmontmorillonite is described by the maximum growth exponential model which consists oftwo steps: the first which is the growth phase, corresponding to adsorption of molecule on themost accessible sites located on the external surface of the clay and the second, constantphase, corresponding to the equilibrium adsorption. The equilibrium time obtained variesbetween 30 and 40 min.From the study of the influence of parameters on the adsorption kinetics, it was found that theadsorbed amounts were better when working at pH of 3 or 38 mg of clay or at high ionicstrength, 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 insolution.The experimental results on the adsorption of atrazin were compared with theoretical modelsof Langmuir, Freundlich and Temkin. Although the three models showed correlation factorsR2>0.95, the best correlation was obtained with the Freundlich model. This strong correlationindicates the heterogeneity of the surface of the clay (montmorillonite) used. Moreover, the 92
  15. 15. 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), © IAEMEcoefficient n>1, indicates that there is a great affinity between atrazin and clay. Maybe, themost important interpretation of montmorillonite excellent sorptive ability as proven by thephysical and chemical parameters measured in this work would be their relationship with itscolloidal size and crystalline structure in layers, resulting in a high specific surface area andoptimum 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 ismuch appreciated. The authors acknowledge the Department of Chemistry & EnvironmentalEngineering (ENSAI-IUT) of the University of Ngaoundéré, Cameroon, for their help insample collection and experimental work. Finally, the Advanced Chemistry Development,Inc. (ACD/Labs), 8 King St. E., Ste. 107, Toronto, Ontario M5C 1B5, Canada, isacknowledged as physical chemical data of atrazin and its metabolites were generated usingtheir ACD/Structure Design Suite (SDS) and advanced in-silico chemistry tools.REFERENCES1. Kordel. W., et. al. (1997), ‘‘The importance of natural organic material for environmental processes in water and soils’’, Pure Appl. Chem., Vol.69, pp.1571-1600.2. Kiely. T., et. al. (2004), ‘‘Pesticide Industry Sales and Usage: 2000 and 2001 Market Estimates’’. U.S. Environmental Protection Agency, Washington, DC.3. Graziano. N., et. al. (2006), ‘‘2004 National Atrazine Occurrence Monitoring Program using the Abraxis ELISA method’’, Environ. Sci.Technol., Vol. 40, pp.1163–1171.4. U.S. Geological Survey. (2003), ‘‘Pesticides in Streams and Ground Water’’, http://ca.water.usgs.gov/pnsp/pestsw/Pest-SW_2001_Text.html [Accessed, 1/12/2012].5. MINAGRI (2003), ‘‘Liste des produits homologues pour dix ans’’,Yaounde, Cameroon.6. Jiang. H., et. al. (2005), ‘‘Determination of chloro-s-triazines including didealkylatrazine using solid phase extraction coupled with gas chromatography/mass spectrometry’’, J. Chromatogr. A, Vol.1064, No.2, pp. 219–226.7. Jiang. H., et. Al.(2006), ‘‘Occurrence and removal of chloro-s-triazines in water treatment plants’’, Environ. Sci. Technol., Vol.40, No.11, pp. 3609-3616.8. USEPA (2000), ‘‘Office of Pesticide Programs’’, Health Effects Division, Washington DC http://www.epa.gov/pesticides/cumulative/triazines/newdocket.htm [Accessed, 18/01/2013].9. Panshin. S.Y., et. al. (2000), ‘‘Analysis of atrazine and four degradation products in the pore water of the Vadose Zone, Central Indiana’’, Environ. Sci. Technol., Vol.34, pp. 2131-2137.10. Kruger. E.L., et. al. (1996), ‘‘Relative mobilities of atrazine, five atrazine degradates, metolachlor, and simazine in soils of Iowa’’, Environ. Toxicol. Chem., Vol.15, pp. 691- 695.11. Koskinen. W. C.,et. al. (1996), ‘‘In Herbicide Metabolites in Surface Water and Groundwater’’; Meyer, M. T., Thurman, E. M., Eds.; ACS Symposium Series 630; American Chemical Society: Washington, DC, U.S.A.125-139.12. Kruger. E.L., et. al. (1997), ‘‘Comparative fates of atrazine and deethylatrazine in sterile and nonsterile soils’’, J. Environ. Qual., Vol.26, pp.95-101. 93
  16. 16. 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), © IAEME13. Lerch. R.N., et. al. (1998), ‘‘Contribution of hydrolyated atrazine degradation products to the total atrazine load in Midwestern streams’’, Environmental Science and Technology, Vol.32, pp. 40–48.14. Thurman. E.M. and Fallon. J.D (1996), ‘‘The deethylatrazine to atrazine ratio as an indicator of the onset of the spring flush of herbicides into surface water of the Midwestern United States’’, International Journal of Environmental Analytical Chemistry, Vol. 65, pp. 203–214.15. Yue. Z., et. al. (2006), ‘‘Chemically activated carbon on a fiberglass substrate for removal of trace atrazine from water’’, J. Mater. Chem., Vol.16, pp.3375–3380.16. Miltner. R.J., et. al. (1989), ‘‘Treatment of seasonal pesticides in surface waters’’, J. Am. Water Works Assoc., Vol.81, pp. 43–52.17. Pham. T., et. al. (2008), ‘‘To what extent are pesticides removed from surface water during coagulation-flocculation’’, Water and Environment Journal, pp. 1747-6585.18. Acero, J.L. et. al. (2000), ‘‘Degradation kinetics of atrazin and its degradation products with ozone and OH radicals: a predictive tool for drinking water treatment’’, Environ. Sci. Technol., Vol.34, pp.591-597.19. Adams. C.D. and Randtke. S.J (1992), ‘‘Ozonation byproducts of atrazin in synthetic and natural waters’’, Environ. Sci. Technol., Vol. 28, No.11, pp. 2218–2227.20. Devitt. E., et. al. (1998), ‘‘Effects of natural organic matter and the raw water matrix on the rejection of atrazine by pressure driven membranes’’, Water Res., Vol. 32, pp. 2563– 2568.21. Verstraeten. I.M., et. al. (2002), ‘‘Changes in concentrations of triazine and acetamide herbicides by bank filtration, ozonation, and chlorination in a public water supply’’, J. Hydrol., Vol. 266, pp.190–208.22. Tan. L (2007), ‘‘Nanofiltration treatment for pesticides removal: a case study for atrazine and dimethoate’’, Thesis submitted in fulfillment of the requirements for the degree of Master of Science, 137 pages.23. Adams. C.D. and Watson. T.L (1996), ‘‘Treatability of s-triazine herbicide metabolites using powered activated carbon’’, J. Environ. Eng., Vol.122, No.4, pp. 327–330.24. Guillon. M. and Font. R (2001), ‘‘Dynamic pesticide removal with carbon fibers’’, Wat. Res., Vol. 35, pp.516-520.25. Abate. G and Masini. J. C (2005), ‘‘Adsorption of atrazin, hydroxyatrazin, deethylatrazin, and deisopropylatrazin onto Fe(III) polyhydroxy cations intercalated vermiculite and montmorillonite’’, J. Agric. Food Chem., Vol. 53, pp.1612–1619.26. Lemic. J et. al. (2006), ‘‘Removal of atrazine, lindane and diazinone from water by organo-zeolites’’, Water Res., Vol.40, pp.1079–1085.27. Martin. G. and Font. R (2001), ‘‘Dynamic pesticide removal with carbon fibers’’, Wat. Res., Vol. 35, pp. 516-520.28. Polati. S., et. al. (2005), ‘‘Sorption of Pesticides on Kaolinite and Montmorillonite as a Function of Hydrophilicity’’, Journal of Environmental Science and Health, Part B, Vol. 41, No.4, pp.333 - 344.29. Zadaka. D, et. al. (2008), ‘‘Atrazine removal from water by polycation–clay composites: Effect of dissolved organic matter and comparison to activated carbon’’, Water Research, Vol.43, pp.677-683.30. Hill. C. And Forti. P (1997), ‘‘Deposition and Stability of Silicate Minerals’’, Cave Minerals of the World (Second ed.), National Speleological Society. pp. 177. 94
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