al-bentonit untuk adsorpsi hg

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RESEARCH ARTICLE

Modelling the adsorption of mercury onto natural

and aluminium pillared clays

Mabrouk Eloussaief &Ali Sdiri &Mourad Benzina

Received: 11 February 2012 /Accepted: 12 March 2012 /Published online: 25 April 2012# Springer-Verlag 2012

Abstract

Introduction The removal of heavy metals by natural adsor-bent has become one of the most attractive solutions forenvironmental remediation. Natural clay collected from theLate Cretaceous Aleg formation, Tunisia was used as a naturaladsorbent for the removal of Hg(II) in aqueous system.

Methods Physicochemical characterization of the adsorbentwas carried out with the aid of various techniques, includingchemical analysis, X-ray diffraction, Fourier transform in-frared and scanning electron micrograph. Batch sorptiontechnique was selected as an appropriate technique in thecurrent study. Method parameters, including pH, temperature,initial metal concentration and contact time, were varied in

order to quantitatively evaluate their effects on Hg(II) adsorp-tion onto the original and pillared clay samples. Adsorptionkinetic was studied by fitting the experimental results to the

pseudo-first-order and pseudo-second-order kinetic models.The adsorption data were also simulated with Langmuir,Freundlich and Temkin isotherms.

Results Results showed that the natural clay samples aremainly composed of silica, alumina, iron, calcium and mag-nesium oxides. The sorbents are mainly mesoporous materialswith specific surface area of


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to the amino acid cysteine in cases of mercury poisoning(Clarkson1993). While inorganic mercury is the most preva-lent form of mercury in aquatic systems, its biochemicalconversion to a more toxic methylmercury (i.e. organic form)

by microorganisms is feasible in water and soil (Stephen et al.2009). Even low doses of mercury accumulation in variousorgans (e.g. liver, kidneys, brain, spleen and bones) may cause

adverse effects like carcinogenic and mutagenic troubles,serious intestinal and urinary complications and even deathin more severe cases (Bhakta et al. 2009). Mercury usuallyenters food chain as methyl mercury through bacterial trans-formation of a wide variety of food, especially fish. Thethreshold limit for mercury (1 g L1) is the lowest amongall heavy metal ions (Bayramolu and Arica 2007). Therefore,the removal of Hg(II) from wastewater is an important issuethat was extensively investigated (Sreedhar et al. 1999;Mohan et al. 2001). Common techniques available for theremoval of mercury from solutions include chemical precipi-tation (Sampaio et al.2009), reverse osmosis (Patterson and

Fendorf1997) and ion exchange (Gode and Pehlivan2003).The main shortcomings of these methods are their costs,incomplete metal removal, high energy requirements andgeneration of toxic sludge. Adsorption is the most preferredmethod for removal of heavy metals from aqueous solutionsdue to its simplicity and its high effectiveness (Gupta et al.1998,2006a,b; Gupta and Ali2004; Shafaei et al.2007; Sdiriet al.2011,2012a,b). The applicability of adsorption in theremoval of heavy metals has been investigated by multipleresearchers (Gupta et al. 1997, 2001, 2007a, 2009, 2010;Gupta and Ali2008; Gupta and Rastogi2008a,b; Gupta and

Nayak2012). These studies confirmed the potential use of

adsorption as an appropriate technique for the removal ofvarious heavy metals. Moreover, the use of natural and cost-effective adsorbents (i.e. clay and limestones) to reduce heavymetal contamination is fundamentally important, especiallyfor developing countries (Mohan et al. 2001; Sdiri et al.2011,2012a). In this regard, this study has been undertakento develop a cheap adsorbent with large surface area and smalldiffusion resistance. Pillared clays have emerged as potentialadsorbents for the removal of heavy metal ions from aqueoussolutions due to their large surface areas. In addition, the

preparation of pillared adsorbents with high number of activesurface sites would show high adsorptive capacities. Among

all these adsorbents, aluminium pillared clays (Al-PILCs)have the required technical specifications and the potentialfor use in environmental applications due to their physical andchemical stability, large surface area and stable colloidalsuspension (Maira et al. 2001; Ni et al. 2007; Nabi et al.2009). In this study, we prepared a new adsorbent (Al-PILCs) for Hg(II) removal from aqueous solutions. Effectsof pH, temperature, contact time and initial concentrationof Hg(II) on the removal process were studied in detail.The obtained results were fitted to Freundlich, Langmuir

and Temkin isotherms. Thermodynamic parameters werealso determined for the Hg(II) adsorption on natural andaluminium pillared clays.

2 Experimental

2.1 Materials and methods

For the purpose of this study, a clay sample (RC) was collectedfrom the Late Cretaceous outcropping of the Aleg formation,Jebel Semmama (Kasserine, Tunisia). The adsorbent waswashed with distilled water to remove soluble salts, sieved andthe desired fraction of less than 2 m was collected for subse-quent analysis. Finally, the obtained clay was kept at 60C.

XRD patterns of the randomly oriented powders as well asthe separated clay fraction (2 m sized sub-sample) obtainedwith an X-ray diffractometer (Philips PW 1710, Germany),using CuKradiation (40 kV/40 mA) were recorded between

3245 with a counting time of 10 min. The powderedrock and three oriented glass slides (untreated, glycolated andheated to 520C) were used to identify clay mineral associa-tions. The clay fraction (2 m) was separated by sedimenta-tion and centrifugation (Sdiri et al.2010).

Chemical composition of the collected clay sample wasperformed by dissolving 1 g of dried clay in 50 mL of HNO3.The mixture was evaporated to dryness; distilled water wasthen added to remove the remaining traces of acid, and thewhole suspension was filtered with an ashless filter paper(Whatman, England). Finally, the filtrate was analysed forHg(II) content, at a wavelength of 253.7 nm, by atomic

absorption spectrophotometer (ZEEnit 700, Analytik Jena,Germany). The insoluble residue mainly composed of insol-uble silica (SiO2) was estimated by gravimetric method (Ben-zina1990) after calcination to 900C for 1 h. The specificsurface area was determined by the adsorptiondesorptionisotherms of nitrogen (BETSurface Area Analyzer 2010,ASAP, USA). The total pore volume was determined by

pycnometry. Infrared spectra were obtained using an FT-IR-420 spectrophotometer (JASCO, Shimadzu Corp., Japan).

2.2 Preparation of aluminium pillared clay

The 2-m-sized clay sub-sample was dispersed in distilledwater and saturated with 1 M NaCl solution for three timesunder continuous stirring. The obtained Na+ homoionic claysample was washed for several times to remove chloride.Pillaring procedure was performed according to Bergaya(1995). The pillaring solution was prepared by titrating aque-ous 0.5 M NaOH with aqueous 0.2 M AlCl36 H2O until theOH/Al ratio was equal to 2.4. At this hydrolysis ratio, Al13ismajor species in solution. The pillaring solution was main-tained for 3 h at 90C and then kept overnight at 30C. The

470 Environ Sci Pollut Res (2013) 20:469479


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obtained solution was mixed with an appropriate volume ofthe Na-exchanged clay suspension (1 % of clay) to prepare20 mmol of aluminium per gram of clay. The slurry was stirredfor 6 h at 30C, filtered, washed (to remove Cl anions) andcalcined to 250C for 2 h and subsequently at 450C for 3 h.Finally, the obtained product (i.e. Al-PILC clay) was pow-dered, sieved and dried at 60C. Both original (RC) and

pillared (Al-PILC) clay samples were used for the removalexperiments.

2.3 Batch adsorption

A batch sorption method was used to study the adsorption ofmercury (Hg(II)) metals onto the Late Cretaceous clays ofAleg formation (Ali and Gupta2007). A stock solution of1,000 mg L1 Hg(II) was prepared by dissolving appropriateamount of HgCl2 (Aldrich Corp., Germany) in distilledwater. This solution was diluted to 550 mg L1 for adsorp-tion experiments. Fifty milligrams of clays was added to

50 mL of Hg(II) solution at initial pH 3.2. The suspensionwas mixed on a thermostated shaker bath operating at 25Cand 200 rpm during 240 and 360 min for Al-PILC and RCsamples, respectively. It is noteworthy that the removal ofmercury by the pillared and the original clay sample attainedtheir equilibrium within 240 and 360 min, respectively.Therefore, different equilibrations times (240 and 360 min)were adopted. After the reaction, the suspension was centri-fuged at 2,500 rpm for 20 min; the supernatant was with-drawn and stored at 4C until analysis for Hg(II) by flameatomic absorption spectroscopy at a wavelength of253.7 nm. The removed amount was determined from the

difference between the initial and final concentrations. Allexperiments were performed in triplicate.

Effects of pH, temperature, initial metal concentration andcontact time were also investigated. To study the effect ofcontact time, a set of samples were prepared as describedabove but then shaken for 30, 60, 120, 180, 240, 300, 360,420, 480 and 540 min. The initial metal concentration,20mgL1, wasthe same in all of the contact time experiments,as were the temperature (25C), clay amount (1 gL1) andinitial metal solution pH (3.2). To investigate the effect of pH,the initial pH of the solution was adjusted to 39. The contacttime in the pH experiments was fixed at 240 and 360 min, and

the shaking rate was 200 rpm; metal concentrations were heldat 20 mg L1. Temperature effects were evaluated by conduct-ing this method at 25, 35 and 45C, while maintaining thesame initial pH, contact time and metal concentration.

Adsorption process was quantified by calculating theadsorption amount (milligrams per gram) as defined by thefollowing:

qe Ci Cf V

M 1

where qe is the amount of Hg2+ ions adsorbed on the clay

(milligrams per gram), Ci the initial Hg2+ ionsconcentration in

solution (milligrams per litre), Cfthe final Hg2+ ions concen-

tration in solution (milligrams per litre),Vthe volume of themedium (litres) and M the amount of the clay used in thereaction mixture (grams).

3 Results and discussions

3.1 Characterization of materials

Mineralogical analysis by X-ray diffraction indicated thatthe raw clay was mainly composed of smectite (montmoril-lonite) associated with kaolinite, illite, quartz, calcite andfeldspar. In the aluminium pillared sample, the untreatedoriented clay (2 m) showed a remarkable peak near18.4indicating an increase in d-spacing when comparedto the original clay (Fig. 1). This was expected due to the

expansion of the interlayer spacing in 2:1 clays after alu-minium treatment (i.e. pillaring). The loss on ignition, de-termined by calcination at 1,000C for 2 h, was attributed tothe evaporation of physically bound water near 100C, thedehydroxylation of clay minerals near 500C and the de-composition of calcite around 750C. The specific surfacearea of the original sample, estimated to be 109 m2 g1,suggested that the studied smectitic clay may exhibit anenhanced removal of heavy metals. Table 1 summarizesthe main physicochemical and mineralogical properties ofthe studied clay materials. Our results indicated that thestudied clay samples were mainly composed of silica, alu-

minium and iron oxides. The collected clay samplecontained about 52.5 % of silica, 18.2 % alumina, but only3 % iron oxides (Table 1). The original clay sample (RC)contained higher amounts of iron and calcite than the alu-minium pillared clay sample (Al-PILC). This was expectedsince the treatment of the raw clay sample may cause thedissolution of various constituents. The Al-PILC materialshowed better textural proprieties when compared to the

Fig. 1 X-ray diffraction patterns of the untreated oriented clay fraction(2m) of the Al-PILC and the RC (Al-PILCaluminium pillared clay,

RCraw clay)

Environ Sci Pollut Res (2013) 20:469479 471


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original sample (i.e. higher porosity and specific surfacearea). This indicates that the exchange of the interlayercations (i.e. Ca2+and Mg2+) with Na+ cations during thesaturation process enhanced the number of active sites dueto the difference in the atomic radii between the exchangedcations. Figure2shows the FT-IR spectra of the RC and theAl-PILC indicating the characteristic bands of surface hy-droxyl group (OH) vibrating near 3,625 cm1. The addition-al band near 1,100 cm1 was attributed to the presence ofsilica, while the stretching of C0O and MgOH groups

occurred at 720 and 500 cm1

, respectively. After the alu-minium pillaring, the spectrum of the Al-PILC changedgreatly. The broad band at 3,428 shifted to 3,625 cm1. This

band corresponded to the stretching vibration of the OHgroups of natural clay; the change indicates the introductionof the OH groups onto the Al-PILC. The new band identi-fied near 1,490 cm1 was attributed to the stretching

vibration of the Al-OH groups (Fig. 2 b). These structuralmodifications were attributed to the pillaring effects of claymaterials on their textural characteristics.

3.2 Effect of pH

The initial solution pH was adjusted to the interval 39 with

appropriate volume of solutions; all other parameters werekept constant. The results are shown in Fig. 3. Adsorptionstudies indicated that the solution pH was an important ad-sorption parameter that strongly affects heavy metal removal

because of the decrease of positive charges on the adsorbentsurface with increasing the solution pH (Eloussaief et al.2009). The adsorbed amounts of Hg(II) onto the pillared clay(i.e. Al-PILC) decreased from 17 to 2 mg g1 when pH of thesolution was increased from 3 to 9. The possible explanationof the decreased removal capacity could be assigned to themercury species present at higher pH, as discussed later.Positively charged species (Hg2+ ions) was dominant in thesolution at pH5. For higher pH (>8.0), the adsorption of Hg(II) becameslightly lower because of the formation of hydroxyl com-

plexes like [Hg(OH)]+ or Hg(OH)2, as described by Eq. 2(Ngah and Fatinathan 2010). The optimum of pH was found at3.2 values; therefore, subsequent adsorption experimentswerecarried out at pH 3.2.

Hg2 2OH ,Hg OH 2 2

3.3 Effect of contact time

Fifty milligrams of adsorbent was placed in a polypropylenetube containing 50 mL of metal solution and then shaken for30, 60, 120, 180, 240, 300, 360, 420, 480 and 540 min at theoptimum pH and temperature. The results show that adsorp-tion process is clearly time dependent (Fig. 4). From thisfigure, it was observed that more than 95 % of the total

Table 1 Chemical and physical characteristics of the raw and pillaredclays (% by weight)

Oxides (%) RC Al-PILC

SiO2 52.501 57.301

Al2O3 18.202 29.003

Fe2O3 3.001 0.503

CaO 2.810 1.002

MgO 2.450 0.740

K2O 1.503 1.002

Na2O 1.780 0.060

LOI 16.001 9.003

Vp (cm3 g1) 0.183 0.421

Pore size (nm) 2.233 5.222

(%) 32.011 75.003

SBET (m2 g1) 109.020 251.004

LOIloss on ignition, Vpvolume of pore,porosity,SBETBET specificsurface area

14903625

Fig. 2 FT-IR spectra of RC (a)and Al-PILC (b)



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adsorptive capacity occurred within 240 min for the Al-PILC and 360 min for the RC sample, after what the remov-al further increased but to a much lower extent. Fromtheoretical point of view, the adsorption process requires

long equilibration time while the practical approach needsa short contact time (Sdiri et al. 2011). Based on the kineticresults, an equilibration time of 240 and 360 min wasselected as compromises between theoretical and practicalapproach. It was also observed that pillared clay sampledemonstrated a faster adsorption due to its enhanced specificsurface area (>250 m2 g1) which provides more active sitesfor Hg(II) adsorption, therefore allows easier diffusion ofmetal cations. Several studies reported much lower equili-

bration time. For instance, Gupta and Rastogi (2008a) men-tioned that raw biomass (i.e.Oedogoniumsp.) needed muchshorter time to remove Cd(II) from aqueous solutions. The

difference in equilibration time could be attributed to thefact that the presence of amino, carboxyl, hydroxyl andcarbonyl groups in algal biomasses enhanced their adsorp-tion capacity despite the low specific surface area. Further-more, the affinity of natural adsorbent to heavy metalsseemed to be lower than that of algal biomasses.

3.4 Effect of temperature

The effect of temperature on adsorption of Hg(II) ions ontoAl-PILC and RC was investigated by conducting this methodat 298, 308 and 318 K. The experimental results (Fig. 5)showed that the Al-PILC adsorption capacity (qe) decreasedfrom 17 to 8.5 mg g1 as temperature increased from 298 to

318 K at 20 mg L1. This slight change could be related to thehigher collision frequencies at higher temperature and there-fore lower adsorption capacity. With increased temperature,the adsorption of mercury ions decreased (Eloussaief et al.2009; Gupta and Rastogi2008b) confirming that the processwas exothermic. The observed change may be related to thesaturation of surface sites by the Hg(II) ions (nl and Ersoz2006).

3.5 Effect of initial concentration

The effect of initial concentration of Hg(II) solution was

varied from 5 to 50 mg L1. The equilibration times were setto 240 and 360 min for Al-PILC and RC, respectively. Theadsorbent concentration was fixed at 1 gL1, and a metalsolution of pH 3.2 was used. The amount of mercury adsorbedfor different initial concentrations onto the samples is shownin Fig. 5. Our results showed that the removal of mercuryincreased with the increase of the initial Hg(II) concentrationdue to the free reactive sites available for the adsorption of Hg(II) ions until saturation. However, the adsorbed amount (qe)was higher at high concentrations (Fig. 5) due to the largerdriving forces for mass transfer at higher concentrations.These forces increased the loading capacityof the studied clay

samples (Mahvi et al.2004). For instance, the initial concen-tration, 20 mg L1, decreased substantially after treatment toreach 3.29 and 15.15 mg L1 for Al-PILC and RC samples,respectively. Furthermore, the saturation of the surface adsorp-tion sites was favoured due to the higher concentration ofHg(II) ions (Herrero et al.2005).

0

5

10

15

20

0 2 4 6 8 10

Adsorbed

amounts(mgg-1)

pH

Al-PILC

RC

Fig. 3 Effect of pH on the Hg(II) removal on Al-PILC and RC

0

5

10

15

20

0 100 200 300 400 500

Adsorbed

amount

(mgg-1)

Time (min)

Al-PILC

RC

Fig. 4 Effect of agitation time on the adsorption efficiency of Hg(II)on Al-PILC and RC

0

5

10

15

20

0 10 20 30 40 50 60

Adsorbed

amounts(mgg-1)

Initial concentration (mg L-1)

25 C

35 C

45 C

25 C

35 C

45 C

Al-PILC

RC

Fig. 5 Effect of temperature on the adsorption efficiency of Hg(II) onAl-PILC and RC



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3.6 Adsorption isotherms

An isotherm describes the equilibrium relationship be-tween the amount of the adsorbed metal ions and theremaining concentration in the liquid phase. It gives im-

portant information about the main mechanisms involvedin the removal of heavy metal (i.e. Hg(II)) by natural and

aluminium pillared clay samples (Eloussaief et al. 2011;Erdem et al. 2004; Naiya et al. 2009). In this study, theisotherm data were analysed using the Langmuir, Freund-lich and Temkin equations. Isotherm constants were cal-culated by using Software CurveExpert 1.3.

3.6.1 Langmuir isotherm

Langmuir isotherm model is based on the assumption ofmonolayer adsorption assuming that all surface sites areenergetically identical and surface itself is homogeneous

(Ngah et al.2004). It also assumes that intermolecular forcesdecrease rapidly with the distance from the adsorption sur-face. Langmuir isotherm is expressed as:

qe KLqmCe

1KLCe3

The above equation can be rearranged to its linear form:

Ce

qe

1

KL

Ce

qm4

both qm and KL could be determined from the slope andintercept of the linear plotCe/qeagainstCe, respectively.Ceis the equilibrium concentration of Hg(II) ion (milligrams

per litre), qe is the adsorbed amount of Hg(II) (milligramsper gram), qm (milligrams per gram) is the maximum ad-sorption capacity andKLis the Langmuir constant related tothe adsorption energy, especially the adsorption enthalpy(Jiang et al. 2009). Our experimental data were fitted tothe Langmuir isotherm to calculate the maximum adsorptioncapacity of the studied clay samples. The results indicatedthat higher removal efficiency was achieved by Al-PILC

sample (Fig. 6). The maximum adsorption capacity (qm)was 49.75 mg g1 and 9.70 mg g1 for Al-PILC and RC,respectively. This may indicate that the aluminium treatmentof the collected clay sample enhanced its textural propertiesthat would contribute to the higher adsorptive capacities.Furthermore, the high coefficients of determinations (R200.982 and 0.992) further confirmed that our experimentaldata better fit to the Langmuir model. This is indicative ofthe homogenous clay surface as supported by KLvalues (KLwas 0.029 and 0.012 Lmg1 for Al-PILC and RC,

respectively). The Langmuir isotherm can be expressed interms of the dimensionless constant (RL) defined as (Yong-

Mei et al.2010):

RL 1

1KLC05

whereC0is the initial metal concentration. The value ofRLindicates whether the adsorption process is favourable asfollows (Eloussaief and Benzina2010):

RL>1 unfavourable adsorptionRL01 linear 0


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3.6.2 Freundlich isotherm

Freundlich isotherm describes multilayer adsorption on ener-getically heterogeneous surfaces. It is an empirical equationsuitable for high and middle range of solute concentration, butnot for low concentrations. Freundlich isotherm is usuallydescribed by the following equation (Hu et al.2005):

qe KFC1ne 6

This equation can be arranged in its linear form:

log qe logKF1

n log Ce 7

whereKFandnare Freundlich constants related to the adsorp-tion capacity and intensityof adsorption, respectively.KF and nwere determined from the linear plot of logqeversus log Ce.Our data showed a good fitting to the Freundlich model(Fig.6). The high correlation coefficient (R20.97) may con-firm this hypothesis. In addition, the calculatednvalues were1.28 and 1.68 for Al-PILC and RC, respectively. This indicatesfavourable adsorption since the calculated adsorption intensityranged between 1 and 10 (1< n


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3.8 Model validation tests

To evaluate the fit of the above-mentioned theoretical mod-els to the experimental data, linear coefficients of determi-nation and non-linear chi-square test were used. The linearcoefficient of determination,R2, represents the percentage ofvariability in the dependent variable that has been explained

by the regression line.The chi-square test statistic was also used to calculate the

sum of the squares of the differences between the experimen-tal data and those calculated from the model. The equivalentmathematical statement could be described as following:

c2

X qeqe;m 2

qe;m13

where qe and qe,m are the experimental and calculated theequilibrium capacity (milligrams per gram), respectively. Ifdata from model are similar to the experimental data,2 will

be a small number; if not,2

will be a bigger number.The fitness of the sorption data were further analysed bycalculating the sum of squared error (SSE) as given by thefollowing equation:

SSEX qt;e qt;m

2

q2t;m14

where qt,e and qt,m are the experimental and calculated ad-sorption capacity (milligrams per gram) a timet, respectively.

In addition, the sum of squared error (SSE) test was alsodone to support the best-fit adsorption model (Table3). SSE

values were lower for pseudo-second-order model than thepseudo-first-order model. Based on R2 and SSE values, itwas confirmed that pseudo-second-order model best fits theexperimental data.

By comparing the results, the values of2 and the cor-relation coefficients (R2) (Table 3), it was found that theLangmuir isotherm best represented the equilibrium adsorp-tion of Hg(II) onto natural and aluminium pillared clays. It isnecessary to analyse the data set using the chi-square test toconfirm the best-fit isotherm for the adsorption system(Meenakshi and Viswanathan 2007). If the data from themodel are similar to the experimental data, 2 will be a

small number, while if they differ, 2 will be a bigger

number. The 2 values of the isotherms are comparableand hence the adsorption of Hg(II) follows in the order as:Langmuir>Freundlich>Temkin isotherms.

4 Thermodynamic study

To evaluate the nature of adsorption of mercury onto naturaland aluminium pillared clays, the removal processwas analysedin term of thermodynamic behaviour. To achieve this goal, threethermodynamic parameters, including free energy (G), en-thalpy (H) and entropy change (S), were determined by thefollowing equations (zcan et al.2006; Akcay2004).

Kd qeCe

15

G RTlnKd 16

lnKd S

R

H

RT 17

whereKd is the distribution coefficient for the adsorption;S,H and G are the changes of entropy, enthalpy and theGibbs energy; T(kelvin) is the temperature and R (8.314 Jmol1 K1) is the gas constant. The values ofH and Swere determined from the slopes and intercepts of the plots ofln Kd versus 1/T. The calculated thermodynamic parametersindicated thatH values were 16.31 and 30.77 kJ mol1

for Al-PILC and RC, respectively(Table 4). The high values ofH indicated strong interactions between Hg(II) ions and the

studied clay samples. Similar finding was mentioned byEloussaief and Benzina (2010) when studying the efficiencyof natural and acid activated clay in the removal of Pb(II) fromaqueous solution by Tunisian clays. The negative valuesobtained forG indicated the spontaneous nature of adsorp-tion(Table 4). Moreover, theincrease ofG with temperatureindicated that adsorption was unfavourable at higher temper-atures (Eloussaief and Benzina2010). Exothermic adsorptionof Hg2+ ion was also observed on china clay and lemna minor

powder (Shun-Xing et al.2011). To follow the evolution ofH versus amount adsorbed of Hg(II) adsorption on studiedsamples, the lnCeversus 1/Tat different amounts (m08, 6, 4

and 2 mg g1) adsorbed was plotted for Al-PILC (Fig.7a) and

Table 4 Kinetic models and (SSE) parameters for the Hg(II) adsorption onto natural and aluminium pillared clays

Measuredqe (mg/g) Pseudo-first order Pseudo-second order

k1 (1/min) qe(mg/g) R2 SSE k2 (g/mg min) qe (mg/g) R

2 SSE

RC 4.851 0.005 0.997 0.8911 7.394 0.003 5.185 0.9179 4.497

Al-PILC 16.520 0.008 0.985 0.7768 7.242 0.002 17.906 0.9854 0.110



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(m02.5, 2, 1.5 and 1 mg g1) for RC (Fig.7b). Then, Hversus amount adsorbed was illustrated in Fig.7c, and it wasnoted that the amount adsorbed increased with the increasingof the released energy orH. During the adsorption of Hg(II)onto the raw clay, the released energy was higher because ofthe existence of more impurities that would prevent the diffu-sion of the positively charged ion (i.e. Hg(II)). The adsorbedamount by the original RC clay was much lower than thatrecorded for Al-PILC clay.

5 Comparison to other studies

Based on previous relevant studies, the amount of heavy metalsremoved by various clay materials is highly variable (Table5).In the current study, pillared clay sample (Al-PILC) demon-strated an enhanced removal rate when compared with theoriginal form (RC). Sdiri et al. (2012a) mentioned that the

removal efficiency was dependent upon the physicochemicalcharacteristicsof the clay andthe metal removed, which was thecase of the present clay samples. The calculated Langmuircapacity jumped from 9.70 mg/g for mercury removal by theoriginal RC sample to 49.75 mg/g, confirming the tight rela-tionship between the physicochemical properties of the adsor-

bent and its removal capacity. These results indicate muchhigher removal efficiency for the present clay samples thanwas shown by Bhattacharyya and Gupta (2006), who reviewedthe removal of metals like lead, cadmium andcopperby variouskindsof clay (Table 5). However, the removal capacity occurredto a much lower extent in comparison to the study of Mohan et

al. (2001). We found that aluminium pillared clay from westernTunisia exhibited greater removal efficiency than those reportedin literature. Therefore, it is plausible to confirm that the LateCretaceous clays outcroppings of Jebel Semmama, Tunisia aresuitable for the removal Hg(II) from aqueous solutions.

Fig. 7 aPlot of lnCeversus 1/Tfor Hg(II) adsorption on Al-PILC indifferent amount adsorbed (metres); bplot of lnCeversus 1/Tfor Hg(II)adsorption on RC in different amount adsorbed (metres) and c Hversus amount adsorbed for Hg(II) adsorption on Al-PILC and RC

Table 5 Comparison of adsorption capacity with those of previousremoval studies with natural clays

Metal Sorbent qmax(mg/g)

kL(L/mg) Literature

Mercury RC 9.701 0.029 Present studyAl-PILC 49.750 0.012

Activated

carbon

724.000 208.936 Mohan et al.

(2001)

Fe2O4nanoparticles

50.001 5.000 Gupta and

Nayak (2012)

Magnetic

nano-

adsorbent

71.430 4.670

Orange peel

powder

35.710 5.600

Lead Smectitic clay 50.761 0.552 Sdiri et al. (2011)

Illitic clay 25.441 0.041 Eloussaief and

Benzina (2010)

Kaolinite 11.520 20.701 Bhattacharyya andGupta (2006)

Cadmium Bentonite 9.272 22.702 Ulmanu et al.

(2003)

Kaolinite 6.781 32.300 Gupta and

Bhattacharyya

(2006)

Copper Illitic clay 17.983 0.212 Eloussaief et al.

(2009)

Kaolinite 4.303 19.901 Bhattacharyya and

Gupta (2008)



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6 Conclusions

Clay deposits of the Late Cretaceous Aleg formation ofwestern Tunisia were found to be potential adsorbents forHg(II) adsorption from aqueous solutions. In this study, rawand aluminium pillared clay samples achieved high removalof Hg2+ ions from aqueous solutions. Our study showed that

good adsorption could be achieved under the operatingconditions of pH 3.2, contact time (240 and 360 min forAl-PILC and RC, respectively) and 1 gL1 of clay powderunder the controlled temperature of 298 K. The experimen-tal data demonstrated a high degree of fitness to Langmuirmodel, indicating a monolayer adsorption and homogenoussurface. From thermodynamic point of view, it was conclud-ed that the adsorption process was spontaneous and exother-mic in nature. These results indicated that natural clays ofwestern Tunisia are promising adsorbents for Hg(II) remov-al in aqueous systems. Further study is needed to evaluatethe adsorption behaviour of raw and pillared clays under

various conditions using other toxic heavy metals.

Acknowledgments We would like to thank Mr. Nidhal Baccar,Technician in the Biotechnology Research Center of Sfax, for his helpin the analysis of our samples by atomic absorption spectrometer andhis assistance in the laboratory work. The authors would like to extendtheir thanks to Professor Vinod Kumar Gupta and his team for their

prompt reviews and the constructive comments and suggestions.

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