Chemical Engineering Journal 248 (2014) 430–439

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Chemical Engineering Journal

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Preparation of CaO loaded mesoporous Al2O3: Efficient adsorbent forfluoride removal from water

http://dx.doi.org/10.1016/j.cej.2014.03.0641385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 83 22580318; fax: +91 83 2557033.E-mail address: naren70@yahoo.com (N.N. Ghosh).

Desagani Dayananda a, Venkateswara R. Sarva b, Sivankutty V. Prasad c, Jayaraman Arunachalam b,Narendra N. Ghosh a,⇑a Nano-Materials Lab, Department of Chemistry, Birla Institute of Technology and Science Pilani, K.K. Birla Goa Campus, Zuarinagar, Goa 403726, Indiab National Centre for Compositional Characterization of Materials (CCCM), Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500062, Indiac Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST-CSIR), Thiruvananthapuram 695019, Kerala, India

h i g h l i g h t s

� Simple preparation route tosynthesize CaO nanoparticles loadedmesoporous Al2O3.� Higher fluoride removal capacity of

CaO loaded Al2O3 than that of pureAl2O3.� Faster fluoride adsorption kinetics of

CaO loaded Al2O3 from water.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 February 2014Received in revised form 16 March 2014Accepted 18 March 2014Available online 27 March 2014

Keywords:Mesoporous aluminaCalcium oxideFluoride removalAdsorption isothermAdsorption kinetics

a b s t r a c t

In this paper, we report a simple chemical method for the preparation of CaO loaded mesoporous Al2O3

based adsorbents, which can be used for fluoride removal from water. The synthesized adsorbents werecharacterized by using powder X-ray diffractometer, N2 adsorption–desorption surface area and pore sizeanalyzer and high resolution transmission electron microscope. CaO loaded mesoporous aluminas exhib-ited poor crystalline mesoporous structure having c-Al2O3 phase. The fluoride removal capacities of thesynthesized adsorbents were evaluated using batch adsorption studies. Kinetic data revealed that, thefluoride sorption on 20 wt.% CaO loaded mesoporous Al2O3 was rapid, and �90% fluoride removal wasachieved within 15 min. CaO loaded mesoporous Al2O3 showed higher fluoride adsorption capacity(137 mg/g) and faster kinetics than mesoporous Al2O3.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

World Health Organization (WHO) has classified fluoride as oneof the major contaminants in drinking water [1]. Though, minuteamount of fluoride is essential to prevent dental caries, but excessintake of fluoride through drinking water leads to dental fluorosis(discolouration and pitting of teeth), skeletal fluorosis (pain and

stiffness in the back bone and joints) and nonskeletal fluorosis(rashes on skin, nervousness, muscle weakness, etc.) [2]. The fluo-ride concentration in drinking water in some states of India as wellas several other countries has been found as high as 30 mg L�1 [3].As per WHO recommendation, the desirable limit and the permis-sible limit of fluoride in drinking water are 1.0 mg L�1 and1.5 mg L�1 respectively. Hence, there is a need to develop suitableadsorbent which is capable of removing fluoride from water.

Since 1930s there has been a continuous interest in the researcharea of removal of fluoride from water using various methods.

D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439 431

Defluoridation processes are generally categorized into four maingroups: (i) adsorption method: in this method adsorbents (suchas activated alumina, bone charcoal, clay, and various metal oxi-des) are used in batch or column systems, (ii) ion exchange meth-od: here ion exchange resins are used to remove fluoride fromwater. However, this method is expensive, (iii) co-precipitationand contact precipitation method: in this method F� is precipitatedusing aluminum sulfate and lime (Nalgonda Technique) or withcalcium and phosphate compounds [4], (iv) membrane processes:reverse osmosis, nanofiltration and electrodialysis methods fall inthis category and are effective for fluoride removal. However, alongwith fluoride other ions, which are essential nutrients, are also re-moved from water in these processes [5]. Among the various meth-ods, adsorption is a widely used technique for this purpose due toits simple operation process and cost effectiveness [2]. Various as-pects of fluoride removal from water using different adsorbentshave been reported in the literature [6–8]. Activated Al2O3 is themost extensively used adsorbent for removal of fluoride fromdrinking water due to its high affinity and selectivity for fluoride.Crystal structure, morphology and surface properties of aluminasplay important roles on their fluoride removal capacity. For in-stance, c-Al2O3 is ten times more efficient than a-Al2O3 [9,10].Presence of hydroxyl groups on the surface play critical role indetermining the ion adsorption behavior of Al2O3. Protonationand deprotonation of these surface hydroxyl groups cause electri-cal charge promoting adsorption [11]. Two models namely (i) ionexchange model and (ii) ligand exchange model have been pro-posed by Ayoob et al. [8] to explain the fluoride adsorption ofAl2O3 in aqueous medium.

It has been reported by various researchers that the fluorideadsorption capacity of Al2O3 can be increased by chemical modifi-cation of its surfaces. Due to high electro negativity and small ionicsize fluoride ion is classified as hard base. So, it has a strong affinitytowards electropositive multivalent metal ions like Fe3+, Ca2+, Zr4+,La3+, Ce4+ [12–14] etc. Impregnation of positively charged cations(such as Ca2+, La4+, Zr4+, Fe3+, and Ce4+) onto the adsorbent helpsto create positive charges on the adsorbent surface which attractsF� (Eqs. (1) and (2)) [6] and improves fluoride adsorption capacity.These metallic cations act as a bridge in between adsorbed fluorideand Al2O3 surface. The chemistry involve in this type of adsorptioncan be presented as follows.

BMeAOHþ2 þ F� ! BMeAFþH2O ð1Þ

BMeAOHþ F� ! BMeAFþ OH� ð2Þ

(�Me represents the multivalent metallic cation surfaces).Calcium compounds (such as Ca(OH)2, CaCl2, CaSO4) have been

used in many fluoride affected areas to remove fluoride from water[15–18]. The Nalgonda technique is recognized as a very efficientmethod because of its easy operation and low maintenance cost[4]. However, its efficiency is limited for low or medium fluorideconcentrations. Camacho et al. [13] have reported the preparationof CaO loaded activated alumina bead by using wet impregnationtechnique.

Though, in the literature variety of synthetic methods havebeen reported for preparation of mesoporous Al2O3 [19–21] butmost of the methods are associated with some limitations. In mostof the sol–gel based synthesis, aluminum alkoxides are used asstarting materials which are not only costly but also very reactiveand controlling their hydrolysis rate is difficult. So, sometimesmixtures of organic solvents or addition of chelating agents areused to control the hydrolysis of aluminum alkoxides. The use oforganic solvents and requirement of autoclave (for hydrothermalsynthesis) make these processes difficult for large scale productionof Al2O3 and expensive. To address these issues, we have developed

a simple aqueous solution based method for preparation of meso-porous Al2O3, where cheap aluminum nitrate was used as startingmaterial and water was used as solvent. This process does not re-quire any autoclave or complicated reaction setup [22].

In this paper, we are reporting a facile cost effective method forpreparation of CaO loaded mesoporous Al2O3 adsorbent, whichexhibited high fluoride removal capacity. The effects of variousparameters (such as adsorbent dose, initial fluoride concentration,pH, contact time, competing ions) on the fluoride removal capacityof the synthesized materials have also reported here. The fluorideremoval efficiency of CaO loaded Al2O3 was compared with puremesoporous Al2O3.

2. Experimental section

2.1. Materials

Aluminum nitrate nonahydrate, sodium hydroxide, sodium sul-fate were procured from Fisher Scientific, India; triethanol amine(TEA), sodium hydrogen carbonate, sodium chloride, sodium ni-trate and sodium fluoride were procured from Merck, India; cal-cium nitrate tetrahydrate, stearic acid, and hydrochloric acidfrom s.d fine-chem. limited, India. All these chemicals were usedas received.

2.2. Synthesis of mesoporous Al2O3

Mesoporous Al2O3 was synthesized by using an aqueous solu-tion based method developed by us [22]. In a typical synthesis,3.41 g of stearic acid was warmed at 80 �C. An aqueous solutionof aluminum nitrate was prepared by dissolving 14.72 g ofAl(NO3)3�9H2O in 20 mL water and warmed at 80 �C. A solutionof TEA was prepared by mixing 16 mL of TEA with 30 mL of water.Aqueous solution of TEA was mixed with stearic acid and stirredcontinuously to get a clear solution. This mixture was added tothe aqueous solution of Al(NO3)3�9H2O with constant stirring andthe temperature of the reaction mixture was maintained 80 �C.White precipitate was formed. This reaction mixture was thenstirred for 12 h at room temperature. Then it was transferred in aTeflon bottle, closed it tightly and aged for 24 h at 90 �C. The whitegel thus formed was then filtered, washed with distilled water anddried at 90 �C to obtain precursor powder. Precursor powder wasthen calcined at 550 �C for 4 h in air to obtain mesoporous Al2O3

powder.

2.3. Preparation of CaO loaded mesoporous Al2O3

CaO loaded mesoporous aluminas were synthesized using a wetimpregnation technique. CaO loaded mesoporous aluminas weresynthesized with different loading percentages (5, 10, 15, 20 and30 wt.%) of CaO on mesoporous Al2O3. In a typical synthesis, in abeaker calculated amount of aqueous solution of calcium nitratetetrahydrate was mixed with desired amount of mesoporousAl2O3 powder and stirred for 12 h. The mixture was then driedon a hot plate at 90 �C. The dried material was calcined at 550 �Cfor 4 h in air atmosphere to obtain CaO loaded mesoporous Al2O3

adsorbent. The 5, 10, 15, 20, 25 and 30 wt.% CaO loaded mesopor-ous Al2O3 are now onwards will be referred as CaO5@Al2O3,CaO10@Al2O3, CaO15@Al2O3, CaO20@Al2O3, CaO25@Al2O3 andCaO30@Al2O3 respectively.

2.4. Characterization of materials

Powder X-ray diffraction (XRD) patterns of the samples were re-corded using a Rigaku powder X-ray diffractometer (Mini FlexII,Rigaku, Japan) using Cu Ka radiation. The diffractograms were

Fig. 1. XRD pattern of (a) Al2O3, (b) CaO5@Al2O3, (c) CaO10@Al2O3, (d) CaO15@Al2-

O3, (e) CaO20@Al2O3 and (f) CaO30@Al2O3.

432 D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439

recorded in the 2h ranges 10–80� with a scanning speed of 3�/min.Nitrogen adsorption–desorption isotherms of the synthesizedmaterials were obtained by using a surface area and porosity ana-lyzer (Micromeritics Tristar 3000, USA) to determine Brunauer-Emmett-Teller (BET) surface area and Barrett–Joyner–Halenda(BJH) pore size. Prior to the adsorption measurements all sampleswere out gassed using nitrogen flow at 200 �C for 10 h. HighResolution Transmission Electron micrographs (HRTEM) of thesynthesized samples were obtained using HRTEM (FEI, Tecnai G230 S-Twin, USA) operated at 300 kV.

2.5. Determination of pHPZC (point of zero charge) of the adsorbents

pHPZC plays an important role in the surface characterization ofmetal oxides/hydroxides. In the adsorption process, pHPZC deter-mines how easily a substrate can adsorb ions present in solution.pHPZC of the adsorbents can be determined by using the salt addi-tion method [23,24]. For determination of pHPZC of the synthesizedmaterials, a solution of 0.01 M NaCl was prepared, and its pH wasadjusted in between 3 and 12 by using calculated amount of0.01 M HCl and 0.01 M NaOH solutions and pH of the mixturewas measured using a pH meter (EUTECH instruments pH 700).10 mL of 0.01 M NaCl solutions having different pH, were takenin 15 mL centrifuge tubes and 30 mg of adsorbent was added ineach of these solutions. These tubes were then kept on a mechan-ical shaker (Niolab instruments, Mumbai, India) at (30 ± 2) �C for24 h and the equilibrium (final) pH of the solutions were recorded.DpH (the difference between initial and final pH) values were plot-ted against their initial pH values. The pHinitial at which DpH waszero was considered as pHPZC.

2.6. Fluoride adsorption experiments

In order to determine the effect of different controlling param-eters (such as adsorbent dose, contact time, initial fluoride concen-tration, pH and co-existing anions) on fluoride adsorption capacityof the synthesized adsorbents, batch adsorption experiments wereperformed. All adsorption experiments were carried out at(30 ± 2) �C. A stock solution containing 1000 mg L�1 fluoride wasprepared by dissolving 2.23 g of sodium fluoride in 1000 mL of re-verse osmosis (RO) water. Working solutions were prepared bydiluting this stock solution. In a typical experiment, 30 mg ofadsorbent was mixed with 10 mL of fluoride solution in a 15 mLcapped centrifuge tube. The mixture was placed in a mechanicalshaker and agitation with a speed of 120 rpm. When equilibriumwas reached, the adsorbent was separated from the mixture bycentrifuging at 4000 rpm for 10 min. The concentration of fluoridepresent in the solution after treatment with adsorbent was deter-mined using UV–Vis spectrophotometer. The amount of fluorideadsorbed by the adsorbent, qe (mg g�1), was calculated using thefollowing equation:

qe ¼ ðC0 � CeÞ ðV=mÞ ð3Þ

where C0 and Ce are the initial and equilibrium concentrations offluoride in solution (mg L�1) respectively, V is the volume of solu-tion (L) and m is mass of the adsorbent (g).

The effect of adsorbent dose on fluoride adsorption was investi-gated by varying adsorbent dose from 0.1 g L�1 to 8 g L�1. The ef-fect of contact time was examined with initial fluorideconcentration of 30 mg L�1 and adsorbent dose of 3 g L�1. Adsorp-tion kinetic experiments were carried out by using 3 g L�1 of theadsorbent with different fluoride concentrations ranging from5 mg L�1 to 30 mg L�1. At a designed interval time, the sampleswere centrifuged and residual fluoride concentrations weredetermined. The effect of initial fluoride concentration and theadsorption isotherms were studied using solutions having various

fluoride concentrations varying from 5 mg L�1 to 1000 mg L�1. Theeffect of pH was investigated by adjusting solution pH from 4 to 10,using 0.01 N HCl and 0.01 N NaOH for a solution having initial fluo-ride concentration 30 mg L�1. The effects of co-existing anions(such as chloride, nitrate, sulfate and bicarbonate) on fluorideadsorption were investigated by performing fluoride adsorptionexperiments using a solution mixture having fluoride concentra-tion of 10 mg L�1 and other ions (10 mg L�1 and 100 mg L�1).Reproducibility of measurements was determined in triplicateand average values are reported here.

2.7. Fluoride analysis

Fluoride concentrations in the solutions (before and after treat-ment with adsorbent) were measured using UV–Vis spectropho-tometer (V-570, Jasco, Japan) at 550 nm with a zirconium-xylenolorange complex reagent [25]. Xylenol orange dye, sodium salt of3,30-bis[N,N-di(carboxymethyl)-aminomethyl]-o-cresolsulphonph-thalein, forms an orange colored complex with Zr4+. Zr-xylenolorange was prepared by mixing the dye with depolymerizedzirconium solution in HCl. This complex decolorizes when it reactswith fluoride ions. During the reaction, fluoride ions dissociate thezirconyl-xylenol orange complex and forms colorless zirconiumfluoride. This reaction was used for the spectrophotometricdetermination of fluoride [26,27]. At the time of analysis, 1 mL ofreagent solution was added to 4 mL of fluoride solution (1:4volume ratio) [25].

3. Results and discussion

3.1. Characterization

Wide angle powder XRD analysis was carried out for the syn-thesized materials to identify their crystalline phase and shownin Fig. 1. Though the XRD patterns of the materials showed poorcrystalline structure with broad diffraction peaks, characteristicpeaks of c-Al2O3 were identified (ICDD card No. 10-0425). The peakintensities corresponding to c-Al2O3 phase were found to be de-creased with increasing CaO loading in the sample. For the sampleshaving higher loading of CaO (e.g. CaO20@Al2O3, CaO30@Al2O3) abroad peak in the region of 2h = 26–35� was observed which canbe assigned for amorphous CaO (ICDD card No. 28-0775). This factindicates the amorphous nature of CaO in CaO loaded mesoporousAl2O3 samples.

D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439 433

N2 adsorption–desorption isotherm analysis were conducted toevaluate the surface area and pore structure of the synthesizedAl2O3 and CaO loaded aluminas. Isotherms for all the samples(Fig. 2(i)) are typical type IV isotherms with H2 hysteresis loop,which indicates the mesoporous nature of synthesized materials.H2 hysteresis loops for these isotherms suggest the presence ofpores with narrow necks with wide bodies (often referred to asink bottle pores) [28]. Large hysteresis loop for pure mesoporousAl2O3 can be attributed to its large pores [29]. However, loadingof CaO on mesoporous Al2O3 does not change the shape of the iso-therms. The pore size distribution of all the synthesized samplesalso confirmed the mesoporosity of the materials (Fig. 2(ii)). Surfacearea and pore size parameters of the synthesized adsorbents aresummarized in Table 1. It was observed that BET surface area andpore volume of CaO loaded aluminas decreased with increasingCaO loading on mesoporous Al2O3. Decrease of pore volume indi-cated that CaO particles are incorporated into the pores of Al2O3.

In order to study the surface morphology of the synthesizedmaterials HRTEM images of the samples were recorded and shownin Fig. 3. The morphological aspects of the samples showwormhole-like, highly connected porous structure of the materials.However, the pores are disordered in nature. In Fig. 3(b) it wasobserved that presence of CaO nanoparticles (5–8 nm) on meso-porous Al2O3 matrix in the CaO loaded sample was observed.

In aqueous solution, point of zero charge of an adsorbent playsan important role in the adsorption process. The pHPZC indicatesthe pH at which the net surface charge on adsorbent is zero. WhenpH of the solution is <pHPZC of adsorbent, the net surface charge onsolid surface of adsorbent is positive because of the adsorption ofexcess H+. This situation favors the adsorption of anions on the sur-face of adsorbent due to coulombic attraction. When pH of thesolution is >pHPZC of adsorbent, the net surface charge of the adsor-bent becomes negative due to desorption of H+. Now, adsorption ofanions on the negatively charged surface of adsorbent competeswith coulombic repulsion [23,24]. The pHPZC values were deter-mined from DpH (the difference between initial and final pH)versus pHinitial plots, as pH at which DpH is zero, that is, pHinitial =pHfinal. The pHPZC values for pure Al2O3 and CaO20@Al2O3 werefound to be 8.2 and 11.91 respectively (Fig. 4). This indicates thatCaO loaded Al2O3 should have better F- adsorption capacity thanthat of pure Al2O3.

3.2. Fluoride adsorption studies

3.2.1. Optimization of composition of adsorbentFluoride adsorption studies of various amount of CaO loaded

aluminas were performed using a solution having initial fluoride

Fig. 2. (i) N2-adsorption desorption isotherms and (ii) pore size distributions of (a) Al2

CaO30@Al2O3.

concentration 30 mg L�1 and adsorbent dose of 3 g L�1. It was ob-served that pure Al2O3 adsorbed �28% F� whereas CaO20@Al2O3

adsorbed �90% F�. However, it was also observed that, due to5 wt.% CaO loading BET surface area of Al2O3 has decreased from284 m2/g to 228 m2/g and percentage of F- adsorption has also de-creased from �28% to �13% in comparison with pure Al2O3 (Fig 5).But, further increase of CaO loading (up to 20 wt.%) resulted in in-crease of percentage of F� adsorption (up to 90%) though the sur-face area of the adsorbents was decreased with increasing CaOloading (Table 1). This is because of the fact that, surface area ofthe adsorbent is not the only factor which dictates the F� adsorp-tion capacity of CaO loaded Al2O3 adsorbents. Presence of CaO alsoplays an important role. According to Patel et al. [18], in presenceof water CaO forms Calcium hydroxide and adsorption of F� occursby surface chemical reaction, in which AOH groups of Ca(OH)2 arereplaced by F� resulting in formation of CaF2. This process can berepresented by the following equations:

CaOþH2O! CaðOHÞ2 þHþ ð4Þ

CaðOHÞ2 þ 2F� ! CaF2 ðsÞ þ 2OH� ð5Þ

When less amount of CaO (5 wt.%) was present in the adsorbent,CaO nanoparticles were deposited within some pores of mesopor-ous Al2O3 matrix which caused the reduction of the surface area ofthe adsorbent but these CaO particles might not got chance tocome in contact with F� ions. So, the fluoride adsorption onCaO5@Al2O3 was found to be lower than that of Al2O3. But whenCaO loading was further increased, some CaO particles were alsodeposited on the surface of Al2O3 matrix. These CaO particles re-acted with F� and formed CaF2. Due to this reason, adsorbents hav-ing higher CaO loading exhibited their higher F� adsorptioncapacities than that of pure Al2O3. 20 wt.% CaO loaded Al2O3 sam-ple exhibited its capacity to absorb �90% fluoride from solutionwith a very fast rate (detailed discussion is in Section 3.2.4), furtherincrease of CaO loading did not show much enhancement in F�

adsorption capacity of the adsorbents. So, further studies wereperformed by using CaO20@Al2O3.

3.2.2. Determination of optimum adsorbent doseThe effect of adsorption dose on removal of fluoride using pure

Al2O3 and CaO20@Al2O3 is shown in Fig. 6, in which percent offluoride removal is plotted against adsorption dose. It was ob-served that, initially the percent of fluoride removal increased rap-idly with the increase of adsorbent dose and maximum F� removaloccurred when adsorbent was 3 g L�1. The percentage of fluorideremoval increased from 20% to 90% with increase in CaO20@Al2O3

dose from 0.25 to 7.5 g L�1 at C0 10 mg L�1. However, when the

O3, (b) CaO5@Al2O3, (c) CaO10@Al2O3, (d) CaO15@Al2O3, (e) CaO20@Al2O3 and (f)

Table 1Surface area and pore size parameters of the synthesized adsorbents obtained bymeans of N2 adsorption–desorption study.

Sampledescription

BET surface area(m2/g)

BJH average poresize (nm)

BJH pore volume(cm3/g)

Al2O3 284 7.4 0.70CaO5@Al2O3 228 7.5 0.55CaO10@Al2O3 175 8.2 0.45CaO15@Al2O3 135 8.6 0.36CaO20@Al2O3 93 10.1 0.29CaO30@Al2O3 86 10.9 0.23

Fig. 4. Plot for determination of pHPZC of Al2O3 and CaO20@Al2O3 adsorbent.

Fig. 5. Effect of CaO loading on mesoporous Al2O3 for removal of fluoride(C0 = 30 mg L�1, adsorbent dose = 3 g L�1, contact time = 8 h, pH = 6.8 ± 0.2).

434 D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439

adsorbent dose was more than 3 g L�1, not much increase in F� re-moval was observed with increasing adsorbent dose. Hence, 3 g L�1

of adsorbent dose of Al2O3 and CaO20@Al2O3 was considered forfurther studies. It was observed that 3 g L�1 of pure Al2O3 removed�56% fluoride from a solution having fluoride concentration of10 mg L�1 whereas, CaO20@Al2O3 removed �90% fluoride fromthe same solution.

3.2.3. Determination of time of equilibriumThe fluoride sorption on Al2O3 and CaO20@Al2O3 was investi-

gated as a function of time using a solution having initial fluorideconcentration 30 mg L�1 and adsorbent dose was 3 g L�1. It wasobserved that, fluoride adsorption by Al2O3 and CaO20@Al2O3

was increased with time (Fig. 7). Initially, fluoride adsorption onadsorbents occurred fast, followed by slower adsorption till theequilibrium was reached. In case of Al2O3, �22% fluoride sorptionoccurred within 1 h contact time and equilibrium reached in 5 hand �29% fluoride was adsorbed. In case of CaO20@Al2O3, �82%fluoride was adsorbed within 15 min and the equilibrium wasreached in 30 min with �92% fluoride adsorption.

3.2.4. Adsorption kineticsVarious models have been proposed to throw light on the

mechanisms of adsorption kinetics of anions onto solid particles.These mechanisms depend on several factors such as diffusion ortransport of fluoride ions from solution phase to exterior surfaceof the adsorbent particles, adsorption on the particle surfaces, nat-ure of pore structure of adsorbents, attachment of fluoride ions onthe surface of adsorbents via complexation or intraparticle precip-itation etc. The rate of sorption depends on structural properties ofthe adsorbent, initial concentration of fluoride solution, interactionbetween fluoride ions, active sites of adsorbents etc. [30].

To understand the fluoride adsorption kinetics on the synthe-sized adsorbents, we have investigated their change of fluorideadsorption capacity with time for the solutions having differentfluoride concentrations. The adsorption kinetics of fluoride onto

Fig. 3. HRTEM images of (a) Al2O3 and (b) CaO20@Al2O

Al2O3 and CaO20@Al2O3 are shown in Fig. 8(i) and (ii). Two stagesof adsorption kinetics were obtained for Al2O3: (i) the adsorptioncapacity increased quickly during the first 60 min and (ii) after that

3. CaO nanoparticles are shown within the circle.

Fig. 6. Effect of adsorbent dose on fluoride adsorption capacity of adsorbents(C0 = 10 mg L�1, contact time = 8 h, pH = 6.8 ± 0.2).

Fig. 7. Effect of contact time on fluoride adsorption capacity of adsorbents(C0 = 30 mg L�1, adsorbent dose = 3 g L�1, pH = 6.8 ± 0.2).

D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439 435

slow increase was observed until equilibrium was reached. In caseof CaO20@Al2O3, within 15 min 90% fluoride adsorption was ob-served and equilibrium was reached in 30 min time.

The kinetics of fluoride adsorption on Al2O3 and CaO20@Al2O3

were analyzed using Lagergren’s pseudo-first order kinetic model[31] and Ho’s pseudo-second order kinetic model [32] to identifythe dynamics of the fluoride adsorption process.

The mathematical representations of these models are asfollows:

Pseudo first order adsorption kinetic model:

logðqe � qtÞ ¼ log qe � ðk1=2:303Þt ð6Þ

Pseudo second order adsorption kinetic model:

t=qt ¼ 1=ðk2q2e Þ þ ð1=qeÞt ð7Þ

where qe is the amount of fluoride adsorbed on adsorbent (mg g�1)at equilibrium, qt is the amount of fluoride adsorbed on adsorbent(mg g�1) at time t (min), k1 is the rate constant (min�1) for pseudofirst order kinetics and k2 is the rate constant (g mg�1 min�1) forpseudo-second order kinetics.

For pseudo second order kinetics model, the kinetics data wereplotted t/qt vs t and k2 values were calculated from the intercept

and slope of the plot (Fig. 9). The plots log(qe � qt) vs t accordingto pseudo-first order kinetics are shown in supplementary docu-ment Fig. S1. Parameters obtained after fitting the experimentaldata in the pseudo-first order and pseudo-second order kineticmodels are shown in Table 2. As indicated in Table 2, the R2 valuesof pseudo second order kinetic model (>0.99) were much higherthan those of pseudo first order kinetic model (<0.94) and theadsorption capacity values (qe(cal)) calculated from pseudo secondorder kinetic model were much closer to the experimental values(qe(exp)). These facts indicate the applicability of the pseudo secondorder kinetic model for fluoride adsorption on Al2O3 andCaO20@Al2O3. Similar trend was also reported by Camacho et al.[13] and Jagtap et al. [33]. Pseudo-second order fitting suggeststhat chemisorption might be responsible for the fluoride adsorp-tion on Al2O3 and CaO20@Al2O3. Aluminum fluoride complexesand CaF2 are the main components forming on the surface of theadsorbents during fluoride adsorption process [13,34].

3.2.5. Effect of initial fluoride concentrationThe effect of initial fluoride concentration on fluoride adsorp-

tion capacity by Al2O3 and CaO20@Al2O3 was studied by keepingall other parameters constant (adsorbent dose 3 g L�1, contacttime = 8 h, temperature 30 ± 2 �C) as shown in Fig. 10. It was ob-served that, with increase in fluoride concentration (C0), fluorideadsorption capacity (qe) of the adsorbent increases and thenreaches a plateau. This should be due to more availability of fluo-ride ions at higher fluoride concentration (up to C0 = 750 mg L�1)for adsorption and the plateau forms due to the saturation of activesites of the adsorbent surfaces. The adsorption capacity ofCaO20@Al2O3 is higher than that of Al2O3. The presence of CaO inCaO20@Al2O3 enhances the fluoride removal capacity of the adsor-bents by forming insoluble CaF2. Moreover, the higher pHPZC ofCaO20@Al2O3 may also contribute towards its higher adsorptioncapacity. A closer look of these results infers that the increase inqe is very significant as 135 mg g�1 of qe could be achieved at high-er fluoride concentration.

3.2.6. Adsorption IsothermsAdsorption isotherms provide important information about the

adsorption processes. Equilibrium studies are useful to obtain theadsorption capacity of the adsorbents. To obtain the equilibriumdata, initial concentrations of fluoride solutions were varied keep-ing the adsorbent dose constant. The time of equilibrium was cho-sen considering the results of kinetic studies of fluoride removal byadsorbents. In the present study two well known isotherm models,viz, Freundlich isotherm [35] and Langmuir isotherms [36] wereused.

Freundlich model indicates the heterogeneity of the adsorbentsurface and considers multilayer adsorption. The Freundlich modelis represented as follows:

log qe ¼ log Kf þ ð1=nÞ log Ce ð8Þ

where Kf and 1/n are Freundlich constants related to adsorptioncapacity and adsorption intensity (heterogeneity) factor respec-tively. The values of Kf and 1/n were obtained from the slope andintercept of the linear Freundlich plot of logqe vs logCe shown inFig. 11(i).

The Langmuir adsorption isotherm model is based on mono lay-ered adsorption on uniform homogeneous surface with sites ofidentical nature. The Langmuir model is represented in linear formas follows:

Ce=qe ¼ 1=ðQ0bÞ þ Ce=Q0 ð9Þ

where Ce is equilibrium concentration (mg L�1), qe is amount offluoride adsorbed at equilibrium (mg g�1), Q0 (mg g�1) is maximum

Fig. 8. Adsorption kinetic curves of fluoride adsorption on (i) Al2O3 and (ii) CaO20@Al2O3 at different initial fluoride concentrations (C0 = 5, 10, 20 and 30 mg L�1, adsorbentdose = 3 g L�1, pH = 6.8 ± 0.2).

Fig. 9. Pseudo second order adsorption kinetic model for fluoride adsorption on (i) Al2O3 and (ii) CaO20@Al2O3 (C0 = 5, 10, 20 and 30 mg L�1, adsorbent dose = 3 g L�1,pH = 6.8 ± 0.2).

Table 2Comparison of pseudo-first order and pseudo-second order kinetic models parameters, and calculated qe(cal) and experimental qe(exp) values for different initial fluorideconcentrations of Al2O3 and CaO20@Al2O3.

Pseudo first order Pseudo second order

C0 (mg L�1) qe(exp) (mg g�1) qe(cal) (mg g�1) k1 (min�1) R2 qe(cal) (mg g�1) k2 (g mg�1 min�1) R2

Al2O3 5 1.38 0.49 0.0055 0.8740 1.40 0.0315 0.997610 1.80 1.12 0.0114 0.7113 1.89 0.0191 0.994320 2.24 1.43 0.0094 0.8488 2.35 0.0131 0.991930 2.89 1.31 0.0107 0.7336 3.00 0.0169 0.9985

CaO20@Al2O3 5 1.48 0.29 0.0074 0.9392 1.49 0.0753 0.999410 3.20 0.54 0.0107 0.7797 3.22 0.0649 0.999420 6.56 1.04 0.0031 0.6980 6.55 0.0151 0.997430 9.87 0.44 0.0154 0.7781 9.88 0.1315 1

436 D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439

adsorption capacity for Langmuir isotherm and b (L mg�1) is theLangmuir constant related to sorption energy. The values ofLangmuir parameters Q0 and b were calculated from the slopeand intercept of the linear Langmuir plot of Ce/qe vs Ce (Fig. 11(ii)).

Parameters obtained from Freundlich and Langmuir isothermsare listed in Table 3. Freundlich adsorption isotherm model fittedwell for pure Al2O3 (R2 = 0.9690) (Fig. 11(i)) whereas, Langmuiradsorption isotherm model fitted well for CaO20@Al2O3

(R2 = 0.9942) (Fig. 11(ii)). The maximum fluoride adsorption capac-ities of Al2O3 and CaO20@Al2O3 obtained from Langmuir isothermwere found to be 24.45 mg/g and 136.99 mg/g respectively.

Theoretically calculated Q0 value of CaO20@Al2O3 is 155 mg/g(considering Q0 of pure Al2O3 24.45 mg/g and assuming all CaO,present in CaO20@Al2O3, reacted with F� and formed CaF2). ButQ0 value of CaO20@Al2O3 obtained from experiments is136.99 mg/g, which is 88% of the theoretical value. This is becauseof the fact that, certain amount of CaO nanoparticles were depos-ited within the pores of mesoporous Al2O3 and these CaO particlesmight not get a chance to come in contact with F� ions and so didnot participate in F� adsorption process.

To predict the adsorption efficiency, the dimensionless quantity‘r’ was calculated using the following equation [37]:

Fig. 10. Effect of initial fluoride concentration on fluoride adsorption capacity ofAl2O3 and CaO20@Al2O3 (adsorbent dose = 3 g L�1, contact time = 8 h,pH = 6.8 ± 0.2).

Table 3Langmuir and Freundlich isotherm parameters for fluoride adsorption on Al2O3 andCaO20@Al2O3 at pH of 6.8 ± 0.2 and temperature = (30 ± 2) �C.

Langmuir isotherm Freundlich isotherm

Q0

(mg g�1)b(L mg�1)

R2 Kf 1/n R2

Al2O3 24.45 0.004 0.7391 0.56 0.5115 0.9690CaO20@Al2O3 136.99 0.084 0.9942 13.59 0.4182 0.5779

Fig. 12. Effect of initial pH on fluoride adsorption capacity of Al2O3 andCaO20@Al2O3 (adsorbent dose = 3 g L�1, C0 = 30 mg L�1, contact time = 8 h).

D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439 437

r ¼ 1=ð1þ bC0Þ ð10Þ

where C0 and b are the initial fluoride concentration and Langmuirisotherm constant respectively. If the value of r is <1, it signifies thefavorable adsorption whereas, r > 1 indicates the unfavorableadsorption [33]. Since, in the present case ‘r’ values were found tobe <1 (varies from 0.98 to 0.01 with the variation of C0 from 5 to1000 mg L�1), it was assumed that favorable adsorption of fluorideoccurred on Al2O3 and CaO20@Al2O3.

3.2.7. Effect of initial pHThe effect of initial pH of the solution on fluoride removal by

Al2O3 and CaO20@Al2O3was investigated at different pH rangingfrom 4 to10, with a constant adsorbent dose of 3 g L�1, initial fluo-ride concentration 30 mg L�1, contact time 8 h, temperature(30 ± 2 �C) and shown in Fig. 12. The adsorption of fluoride onAl2O3 does not change much within the pH range of solution 4–9. However, when initial pH was 10, the fluoride adsorption capac-ity of pure Al2O3 was decreased. This might be due to the fact thatpHZPC value of Al2O3 8.2 which is <10. The fluoride adsorptioncapacity of CaO20@Al2O3 does not affect much within the pH rangeof 4–10.

Fig. 11. (i) Freundlich adsorption isotherm for adsorption of fluoride on Al2O3 and(C0 = 5 mg L�1 to 1000 mg L�1, adsorbent dose = 3 g L�1, contact time = 8 h, pH = 6.8 ± 0.2

3.2.8. Effect of co-existing anionsBeside fluoride ions the natural ground water always contains

various other ions, which may compete with fluoride duringadsorption and affect the efficiency of the adsorbent. To studythe effect of co-existing anions on fluoride adsorption on synthe-sized adsorbents, 10 mg L�1 and 100 mg L�1 initial concentrationsof Cl�, NO3

�, SO42� and HCO3

� were used while keeping the initialfluoride concentration as 10 mg L�1. The effect of co-ions on thefluoride removal efficiency of Al2O3 and CaO20@Al2O3 is shownin Fig. 13. It was observed that the presence of Cl� did not affectthe fluoride removal capacity of the adsorbents. Presence of NO3

�

(10 and 100 mg L�1) and SO42� (10 mg L�1) ions slightly decreased

(ii) Langmuir adsorption isotherm for adsorption of fluoride on CaO20@Al2O3.).

Fig. 13. Effect of co-existing anions on fluoride adsorption capacity of Al2O3 and CaO20@Al2O3 (adsorbent dose = 3 g L�1, C0 = 10 mg L�1, contact time = 8 h).

438 D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439

(�2–3%) the fluoride removal capacity of the adsorbent, whereas�8–9% decrease was observed when HCO3

� was present. This isdue to the competition between HCO3

� and F� for the adsorptionon active sites of the adsorbent.

4. Conclusion

Here, the preparation of CaO loaded mesoporous Al2O3 basedadsorbents was reported for removal of F� from water. It has beendemonstrated that loading of 20 wt.% CaO significantly enhancesthe F� removal capacity of mesoporous Al2O3. CaO20@Al2O3 iscapable of removing �90% F� within 15 min using a moderateadsorbent dose of 3 g L�1where as, pure Al2O3 removes only�22% after 1 h. Generally, the fluoride concentration in contami-nated ground water is �5–10 mg L�1. It was observed that, whenthe solutions having F� concentration of 5 mg L�1 and 10 mg L�1

was treated with CaO20@Al2O3 with adsorbent dose of 3 g L�1,the F� concentration of treated water became <1 mg L�1, which iswell below the recommendation of WHO. The maximum fluorideadsorption capacities of Al2O3 and CaO20@Al2O3 were found tobe 24.45 mg/g and 136.99 mg/g respectively. The CaO loaded mes-oporous Al2O3 exhibited higher fluoride removal capacity than thatof pure Al2O3 and commercial Al2O3 (ca 7 mg g�1) [10], because ofCa may react with fluoride ions to form CaF2 precipitates and highvalue of pHPZC of CaO20@Al2O3. CaO20@Al2O3 exhibited good fluo-ride removal efficiency over a wide range of pH. The high adsorp-tion capacity and fast rate of adsorption makes CaO20@Al2O3 apotential candidate as an adsorbent in fluoride removal devices.

Acknowledgment

Authors gratefully acknowledges financial support from Boardof Research in Nuclear Science (BRNS), India (Sanc no: 2010/37C/2/BRNS/827).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2014.03.064.

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