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Adsorption of methylene blue and zinc ions on raw and acid-activated bentonitefrom Morocco

M. Hajjaji ⁎, H. El ArfaouiLaboratoire de Physico-chimie des Matériaux et Environnement, Département de Chimie, Avenue Prince My Abdellah, Faculté des Sciences Semlalia, Université Cadi Ayyad, B.P. 2390,

Marrakech, Morocco

a b s t r a c ta r t i c l e i n f o

 Article history:

Received 13 May 2009Received in revised form 18 September 2009Accepted 24 September 2009Available online 6 October 2009

Keywords:

BentoniteAdsorptionMethylene blueZincAcid-activated bentonite

Adsorption of methyleneblue (BM) andzinc ions on rawand acid-activated Moroccan bentonitecomposedof montmorillonite(88 mass%),a mixture of quartz and K-feldspar (9 mass%), calcite (3 mass%) and insignificantamounts of organic matter was evaluated at 25 °C using UV –visible and atomic absorption spectrometry. Theadsorption capacity of MB andZn ions by rawbentonite were about 2.2and 1.1 mmol/(g of bentonite) andthebest-fit isotherm models were those of Harkins– Jura and Langmuir. Acid-activation of the bentonite reducedthe maximum uptake of MB and Zn ions by 30 and 95% and the best- fit models of the isotherms wereFreundlich and Dubinin–Radushkevich for MB and Zn ions respectively. The reduced adsorption wasassociated with partial collapse of the montmorillonite particles and the formation of amorphous silica.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Methylene blue (MB), a cationic dye, is found in wastewaters of dyeing industries. It is stable to sunlight and natural oxidizing agents,and resists to biodegradation. Zinc is quantitatively encountered inef fluents of mining and refining plants and it is often present in fun-gicide treated fields.

Removalofsuchpollutantsfromaqueoussolutionscanbeachievedby adsorption, precipitation, ion exchange and membrane separation.Adsorption on activated carbon and synthetic resins is the most po-pular method for removing cationic pollutants. Unfortunately, theseadsorbents are high cost materials.

The considered pollutants are taken as examples for comparing theorganic and inorganic cations sorption on a local bentonite. In fact,because of theirlamellarstructure,fine particle size andcationexchangecapacity, as well as their low cost and local availability, clay minerals,particularly smectites, prove to be ef ficient for removing cationicpollutants from aqueous solutions.

Adsorption mechanisms of MB on smectites, especially montmo-rillonite, were investigated (Margulies et al., 1988; Rytwo et al., 1995;Schoonheydt and Johnston, 2006; Hajjaji and Alami, 2009). Most of the studies were conducted on montmorillonite from Wyoming.

Zn, as a soil pollutant, has been the subject of studies mainlyrelated to the fixation by clay fractions which are the most active partof top soils (e.g., Kiekens, 1995). Few studies were devoted to Znfixation on individual clay minerals, particularly montmorillonite(Mellah and Chegrouche, 1997; Kaya and Ören, 2005).

To enhance their retention ability, clay minerals have been thesubject of different chemical modifications (e.g., Bergaya and Lagaly,2001). Acid-activation and cation exchange are the most importantmodification reactions (Komadel, 2003; Komadel and Madejová, 2006).

The aim of this study is to evaluate the equilibrium adsorption of MB and Zn ions on raw and acid-activated bentonite, consisting es-sentially of montmorillonite.

2. Materials and experimental procedures

The used bentonite was from a Miopliocene deposit located atTassaout (High Atlas — Morocco) (Driouich, 1993). It exhibited asoapy feature and consisted of calcium montmorillonite (88 mass%),K-feldspar and quartz (9 mass%), calcite (3 mass%) and minoramounts of organic matter (0.2 mass%) (El Arfaoui, 2002).

The specific surface area and cation exchange capacity of thebentonite, determined by MB adsorption (Hang and Brindley, 1970)were 608 m2/g and 1.15 meq/g, respectively.

Applied Clay Science 46 (2009) 418–421

 Abbreviations: A, constant in the H– J adsorption model [(mmol/g)2]; B, constant in theD–R adsorption model [(mol/J)2]; B′, constant in the H– J adsorption model; B1, constant inthe Temkin adsorption model (mmol/g); C e, equilibrium concentration of ions in solution(mo/l); E , mean free energy(J/mol); K F, Freundlich constant; K L , Langmuirconstant (l/mol);K T, Temkin constant (l/mol); qe, adsorbed amount at equilibrium (mmol/g);qm, adsorptioncapacity (mmol/g); 1/n, constant in the Freundlich adsorption model.⁎ Corresponding author. Fax: +212 254 43 74 08.

E-mail address: hajjaji@ucam.ac.ma (M. Hajjaji).

0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.clay.2009.09.010

Contents lists available at ScienceDirect

Applied Clay Science

 j o u r n a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / c l a y

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MB (C16H18N3S+, Cl−) and zinc ions, taken as ZnCl2, were used intheir dry states. They were high purity products.

For the sorption experiments, bentonite (0.5 g/l) was dispersed indistilled water. pH was to 8.6. Solutions of zinc chloride or meth-ylene blue with different concentrations and constant temperature(25 °C) were added to samples of 40 ml bentonite dispersions. Theinitial concentration of MB and Zn was varied up to 3.5×10−3 and1.5×10−3mol/l.The dispersions were kept at 25 °C, stirred for6 h andcentrifuged at 4000 rpm. The adopted contact time largely exceededthe required time for equilibrium (around 40 min).

Theamount of MB inthe supernatant wasdetermined by a Genesys20 spectrophotometer operating at 656 nm, while the equilibrium Zn

concentration was evaluatedby atomic absorption spectrometry using

a Unicam 925 apparatus. The adsorbed amount was derived from theconcentration changes.

For acid-activation, samples of 40 g of the oven-dried (110 °C)bentonite were reacted with HCl solution (6.7 N) for 3 h at boilingtemperature,using a conventional reflux apparatus(Vengris etal., 2001).

The changes due to acid-activation were investigated by InductiveCoupled Plasma-Atomic Energy Spectroscopy (ICP-AES), X-ray dif-fraction (XRD) and infrared spectroscopy (IR). The XRD examinations

were carried out by a Siemens D5000 diffractometer. For the IR analyses, a Nicolet 205 spectrometer was used.

3. Results and discussion

 3.1. Adsorption by raw bentonite

Typical adsorption isotherms relevant to MB and Zn ions arereported in Fig. 1. The isotherms followed the Langmuir equation(Langmuir, 1918) (Table 1). Thus, MB species were adsorbed on similaractive sites and only a monolayer was built up at saturation. The max-imum adsorption of MB, 2.22 mmol/g bentonite, exceeded the CEC of the bentonite of 1.15 meq/g. To be consistent with the Langmuir modelassumptions, MB would be essentially adsorbed as dimers (MB+)2. Themaximum adsorption of Zn ions (Table 1) was close to the CEC.

Since the Langmuir constant is proportional to the binding energyand because of  K L (Zn)>K L (MB) (Table 1), zinc ions were morestrongly bound to the bentonite particles. This may result from theirhigher ionic potential (2.7 Å−1 for Zn2+ and only 0.2 Å−1 for MB+).

The use of the Freundlich model (Freundlich, 1907) indicated aless suitable fit of the experimental data (Table 1) and revealed thepreference of MB over Zn ions in contrast to the Langmuir model.

A marked discrepancy was observed between the MB experimen-tal and the Temkin (Temkin and Pyzhev, 1940) isotherms. Also, theDubinin–Radushkevich (D–R) model (Dubinin and Radushkevich,1966) (Table 1) only fitted the amounts of Zn ions adsorbed. Takinginto consideration the values of E =(2B)−1/2 (the mean free energy of adsorption per mole of adsorbate when it is transferred to the surfaceof solid from infinity in the solution) (Table 1), the MB adsorption

appeared somewhat preferred.The Harkins– Jura (H– J) equation (Harkins and Jura, 1943), which

accounts for multilayer adsorption and the existence of a heteroge-neous pore distribution in the absorbent, fitted well the MB data, butless Zn2+ adsorption (Table 1).

To determine the best-fit isotherm model, the HYBRID fractionalerror function (HYBRID) was applied (Gunay, 2007; Oubagaranadinet al., 2007)

HYBRID =100n− p

∑n

1

ðqe;exp−qe;calÞ2

qe;exp

" #

where n and p are the number of data points and the model para-meters respectively. qe,exp and qe,cal are the experimental and cal-

culated equilibrium adsorbed amounts.

Fig. 1. Isotherms for the adsorption of MB and Zn ions on raw bentonite.

 Table 1

Adsorption isotherm models and fitting parameters for the adsorption of MB and Zn ions on raw bentonite.

Model Equation Linearized form Values of the model parametersa Fitting parameters

MB Zn MB Zn

Langmuir qe=K L C eqm/(1+K L C e) C e/qe=1/qmK L +C e/qm K L =7120.9; qm=2.22 K L =22038.7; qm=1.10 0.9888 0.9996Freundlich qe=K F(C e)

1/ n log qe=log K F+(1/n)logC e K F=3.64; 1/n=0.103 K F=1.99; 1/n=0.09 0.9322 0.9392Temkin qe=(RT/b)Ln(K TC e) qe=B1LnK T+B1LnC e K T=3043.2×105; B1=0.145 K T=1404×105; B1= 0.090 0.8832 0.9413

RT/b =B1

D–R  qe=qmexp(−Be2) Ln qe=l n qm−Be2 E =24967.9; qm=2.27 E =22707.7; qm=1.42 0.9048 0.9426e =RTLn(1+1/C e)

E =1 / (2B)1/2

H– J qe=(B′/ A−(1/ A)logC e)−1/2 1/qe

2=B′/ A−(1/ A)logC e A =3.60; B′=−1.777 A =2.41; B′=−0.922 0.9903 0.9351

a

Dimensions, see list of abbreviations.

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The lowest HYBRID values for MB and Zn ions corresponded to themodels of H– J and Langmuir respectively (Table 3).

 3.2. Adsorption by acid-activated bentonite

The adsorption isotherms of MB and Zn2+ (Fig. 2) correlated wellwiththe Langmuirand Freundlich equations(Table2).Ascomparedwiththe raw bentonite, K L and 1/n were increased indicating that the bindingenergy raised and cations af finity for bentonite particles declined. ThemaximumadsorptionforMBandZn2+wasreducedbyabout30and95%.This marked reduction was also indicated by the Temkin model.

The use of the D–R model showed a slight improvement for thecorrelationcoef ficient forboth adsorptionisotherms,butH– Jmodelwas

not usable (Tables 1 and 2). Considering the D–R parameters, the freeenergy of adsorption was reduced by about 50% for MB and Zn ions.

In view of the calculated HYBRID values (Table 3), the Freundlich

adsorption isotherm best fitted the MB isotherm. Therefore, cationadsorption seemed to occur on heterogeneous surfaces, and theinfluence of theadsorbent porosity on theMB adsorption wasno moresignificant. In contrast,it seemedthat porosity playeda role in thecaseof Zn adsorption, since the D–R model showed the best fit (Table 3).

As well known (Komadel and Madejová, 2006), acid-activationresulted in the change of montmorillonite. The amount of silicon inraw and activated bentonite was almost constant, whereas thequantity of the octahedral cations (Mg2+, Fe3+, Al3+) was stronglyreduced (Table 4). The leaching of Mg and Al was also evidenced by

Fig. 2. Isotherms for the adsorption of MB and zinc ions on the acid-activated bentonite.

 Table 2

Adsorption isotherm models and fitting parameters for the adsorption of MB and Znions on acid-activated bentonite.

Model Values of the model parametersa Fittingparameters

MB Zn MB Zn

Langmuir K L =63656.3; qm=1.56 K L =1053161.2;qm=0.06

0.9708 0.9882

Freundlich K F=133.85; 1/n=0.48 K F=55.16; 1/n =0.55 0.9818 0.9948Temkin K T=11.06×105;

B1=0.275K T=118.08×105;B1=0.013

0.9557 0.9861

D–R  E =12199.8; qm=8.64 E= 12621.4; qm=1.13 0.9824 0.9968H– J A =0.07; B′=−4.447 A= 1.24×10−4;

B′=−5.7040.8357 0.8931

a

Dimensions, see list of abbreviations.

 Table 3

HYBRID error function for the two-parameter isotherm models.

Model HYBRID

Raw bentonite Acid-activated bentonite

MB Zn2+ MB Zn2+

Langmuir 0.1009 0.0047 0.0344 0.0177Freundlich 0.0346 0.0067 0.0105 0.0116Temkin 0.0462 0.0077 0.0347 0.0316

D–R 0.0502 0.0064 0.0110 0.0079H–  J 0.0031 0.0072 0.0564 0.1667

 Table 4

Chemical compositions (mass%) of the raw (RB) and acid-activated bentonite (AB).

SiO2 Al2O3 Fe2O3 CaO MgO K2O TiO2aL.I.

RB 53.1 17.6 3.4 1.4 3.4 0.4 0.6 20AB 56.7 9.9 2.0 <0.1 1.3 0.4 0.9 bn.d

a Loss on ignition.b Not determined.

Fig. 3. Infrared spectra of the raw bentonite (a), <2 µm clay-sized fraction (b) and acid-

activated bentonite (c).

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the disappearance of the Al–Mg–OH infrared band and the intensitydecrease of Al–Al–OH bands (Fig. 3). The basal X-ray reflection of montmorillonite (Fig. 4) disappeared and the intensity of the otherreflections was reduced. Amorphous silica, which is characterized bythe prominent IR band located in the range of 1250–1100 cm−1

(Fig. 3) formed during acid-activation may provide additional activeadsorption sites. This may be the reason the adsorption isotherm forMB was well fitted by the Freundlich equation. Amorphous silicaparticles occupying micropores of montmorillonite particles mayimpede the access of MB to some internal sites. Apparently, this wasnot the case for distinctly smaller Zn ions since the adsorption iso-therm was well described by the D–R equation.

4. Conclusions

The best fit for the adsorption on raw and acid-activated bentonitewas obtained with the H– J and Langmuir models. Apparently, the

porosity of the particles of montmorillonite, which is the maincomponent of bentonite, influenced the adsorption of the MB ions.

The acid attack of the bentonite had a detrimental effect on theuptake capacity because of the marked destruction of montmorillon-ite and the formation of amorphous silica. In this condition, theFreundlich model became the best-fit isotherm model for MB. Theadsorption process of Zn, as compared to that of MB, seemed tohappen much more by pore volume filling since the D–R model was

the best-fi

t adsorption isotherm model.

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