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

Chemical Engineering Journal 248 (2014) 430–439

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Preparation of CaO loaded mesoporous Al2O3: Efficient adsorbent for fluoride removal from water Desagani Dayananda a, Venkateswara R. Sarva b, Sivankutty V. Prasad c, Jayaraman Arunachalam b, Narendra N. Ghosh a,⇑ a b c

Nano-Materials Lab, Department of Chemistry, Birla Institute of Technology and Science Pilani, K.K. Birla Goa Campus, Zuarinagar, Goa 403726, India National Centre for Compositional Characterization of Materials (CCCM), Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500062, India 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

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

 Simple preparation route to

synthesize CaO nanoparticles loaded mesoporous Al2O3.  Higher fluoride removal capacity of CaO loaded Al2O3 than that of pure Al2O3.  Faster fluoride adsorption kinetics of CaO loaded Al2O3 from water.

a r t i c l e

i n f o

Article history: Received 2 February 2014 Received in revised form 16 March 2014 Accepted 18 March 2014 Available online 27 March 2014 Keywords: Mesoporous alumina Calcium oxide Fluoride removal Adsorption isotherm Adsorption 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 were characterized by using powder X-ray diffractometer, N2 adsorption–desorption surface area and pore size analyzer and high resolution transmission electron microscope. CaO loaded mesoporous aluminas exhibited poor crystalline mesoporous structure having c-Al2O3 phase. The fluoride removal capacities of the synthesized adsorbents were evaluated using batch adsorption studies. Kinetic data revealed that, the fluoride sorption on 20 wt.% CaO loaded mesoporous Al2O3 was rapid, and 90% fluoride removal was achieved within 15 min. CaO loaded mesoporous Al2O3 showed higher fluoride adsorption capacity (137 mg/g) and faster kinetics than mesoporous Al2O3. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction World Health Organization (WHO) has classified fluoride as one of the major contaminants in drinking water [1]. Though, minute amount of fluoride is essential to prevent dental caries, but excess intake of fluoride through drinking water leads to dental fluorosis (discolouration and pitting of teeth), skeletal fluorosis (pain and ⇑ Corresponding author. Tel.: +91 83 22580318; fax: +91 83 2557033. E-mail address: [email protected] (N.N. Ghosh). http://dx.doi.org/10.1016/j.cej.2014.03.064 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

stiffness in the back bone and joints) and nonskeletal fluorosis (rashes on skin, nervousness, muscle weakness, etc.) [2]. The fluoride concentration in drinking water in some states of India as well as several other countries has been found as high as 30 mg L1 [3]. As per WHO recommendation, the desirable limit and the permissible limit of fluoride in drinking water are 1.0 mg L1 and 1.5 mg L1 respectively. Hence, there is a need to develop suitable adsorbent which is capable of removing fluoride from water. Since 1930s there has been a continuous interest in the research area of removal of fluoride from water using various methods.

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

Defluoridation processes are generally categorized into four main groups: (i) adsorption method: in this method adsorbents (such as activated alumina, bone charcoal, clay, and various metal oxides) are used in batch or column systems, (ii) ion exchange method: here ion exchange resins are used to remove fluoride from water. However, this method is expensive, (iii) co-precipitation and contact precipitation method: in this method F is precipitated using aluminum sulfate and lime (Nalgonda Technique) or with calcium and phosphate compounds [4], (iv) membrane processes: reverse osmosis, nanofiltration and electrodialysis methods fall in this category and are effective for fluoride removal. However, along with fluoride other ions, which are essential nutrients, are also removed from water in these processes [5]. Among the various methods, adsorption is a widely used technique for this purpose due to its simple operation process and cost effectiveness [2]. Various aspects of fluoride removal from water using different adsorbents have been reported in the literature [6–8]. Activated Al2O3 is the most extensively used adsorbent for removal of fluoride from drinking water due to its high affinity and selectivity for fluoride. Crystal structure, morphology and surface properties of aluminas play important roles on their fluoride removal capacity. For instance, c-Al2O3 is ten times more efficient than a-Al2O3 [9,10]. Presence of hydroxyl groups on the surface play critical role in determining the ion adsorption behavior of Al2O3. Protonation and deprotonation of these surface hydroxyl groups cause electrical charge promoting adsorption [11]. Two models namely (i) ion exchange model and (ii) ligand exchange model have been proposed by Ayoob et al. [8] to explain the fluoride adsorption of Al2O3 in aqueous medium. It has been reported by various researchers that the fluoride adsorption capacity of Al2O3 can be increased by chemical modification of its surfaces. Due to high electro negativity and small ionic size fluoride ion is classified as hard base. So, it has a strong affinity towards 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 helps to create positive charges on the adsorbent surface which attracts F (Eqs. (1) and (2)) [6] and improves fluoride adsorption capacity. These metallic cations act as a bridge in between adsorbed fluoride and Al2O3 surface. The chemistry involve in this type of adsorption can be presented as follows.

BMeAOHþ2 þ F ! BMeAF þ H2 O

ð1Þ

BMeAOH þ F ! BMeAF þ 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 efficient method because of its easy operation and low maintenance cost [4]. However, its efficiency is limited for low or medium fluoride concentrations. Camacho et al. [13] have reported the preparation of CaO loaded activated alumina bead by using wet impregnation technique. Though, in the literature variety of synthetic methods have been reported for preparation of mesoporous Al2O3 [19–21] but most of the methods are associated with some limitations. In most of the sol–gel based synthesis, aluminum alkoxides are used as starting materials which are not only costly but also very reactive and controlling their hydrolysis rate is difficult. So, sometimes mixtures of organic solvents or addition of chelating agents are used to control the hydrolysis of aluminum alkoxides. The use of organic solvents and requirement of autoclave (for hydrothermal synthesis) make these processes difficult for large scale production of Al2O3 and expensive. To address these issues, we have developed

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a simple aqueous solution based method for preparation of mesoporous Al2O3, where cheap aluminum nitrate was used as starting material and water was used as solvent. This process does not require any autoclave or complicated reaction setup [22]. In this paper, we are reporting a facile cost effective method for preparation of CaO loaded mesoporous Al2O3 adsorbent, which exhibited high fluoride removal capacity. The effects of various parameters (such as adsorbent dose, initial fluoride concentration, pH, contact time, competing ions) on the fluoride removal capacity of the synthesized materials have also reported here. The fluoride removal efficiency of CaO loaded Al2O3 was compared with pure mesoporous Al2O3. 2. Experimental section 2.1. Materials Aluminum nitrate nonahydrate, sodium hydroxide, sodium sulfate were procured from Fisher Scientific, India; triethanol amine (TEA), sodium hydrogen carbonate, sodium chloride, sodium nitrate and sodium fluoride were procured from Merck, India; calcium nitrate tetrahydrate, stearic acid, and hydrochloric acid from s.d fine-chem. limited, India. All these chemicals were used as received. 2.2. Synthesis of mesoporous Al2O3 Mesoporous Al2O3 was synthesized by using an aqueous solution based method developed by us [22]. In a typical synthesis, 3.41 g of stearic acid was warmed at 80 °C. An aqueous solution of aluminum nitrate was prepared by dissolving 14.72 g of Al(NO3)39H2O in 20 mL water and warmed at 80 °C. A solution of TEA was prepared by mixing 16 mL of TEA with 30 mL of water. Aqueous solution of TEA was mixed with stearic acid and stirred continuously to get a clear solution. This mixture was added to the aqueous solution of Al(NO3)39H2O with constant stirring and the temperature of the reaction mixture was maintained 80 °C. White precipitate was formed. This reaction mixture was then stirred for 12 h at room temperature. Then it was transferred in a Teflon bottle, closed it tightly and aged for 24 h at 90 °C. The white gel thus formed was then filtered, washed with distilled water and dried at 90 °C to obtain precursor powder. Precursor powder was then 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 wet impregnation technique. CaO loaded mesoporous aluminas were synthesized with different loading percentages (5, 10, 15, 20 and 30 wt.%) of CaO on mesoporous Al2O3. In a typical synthesis, in a beaker calculated amount of aqueous solution of calcium nitrate tetrahydrate was mixed with desired amount of mesoporous Al2O3 powder and stirred for 12 h. The mixture was then dried on a hot plate at 90 °C. The dried material was calcined at 550 °C for 4 h in air atmosphere to obtain CaO loaded mesoporous Al2O3 adsorbent. The 5, 10, 15, 20, 25 and 30 wt.% CaO loaded mesoporous Al2O3 are now onwards will be referred as CaO5@Al2O3, CaO10@Al2O3, CaO15@Al2O3, CaO20@Al2O3, CaO25@Al2O3 and CaO30@Al2O3 respectively. 2.4. Characterization of materials Powder X-ray diffraction (XRD) patterns of the samples were recorded using a Rigaku powder X-ray diffractometer (Mini FlexII, Rigaku, Japan) using Cu Ka radiation. The diffractograms were

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recorded in the 2h ranges 10–80° with a scanning speed of 3°/min. Nitrogen adsorption–desorption isotherms of the synthesized materials were obtained by using a surface area and porosity analyzer (Micromeritics Tristar 3000, USA) to determine BrunauerEmmett-Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size. Prior to the adsorption measurements all samples were out gassed using nitrogen flow at 200 °C for 10 h. High Resolution Transmission Electron micrographs (HRTEM) of the synthesized samples were obtained using HRTEM (FEI, Tecnai G2 30 S-Twin, USA) operated at 300 kV.

fluoride concentrations varying from 5 mg L1 to 1000 mg L1. The effect 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 fluoride concentration 30 mg L1. The effects of co-existing anions (such as chloride, nitrate, sulfate and bicarbonate) on fluoride adsorption were investigated by performing fluoride adsorption experiments using a solution mixture having fluoride concentration of 10 mg L1 and other ions (10 mg L1 and 100 mg L1). Reproducibility of measurements was determined in triplicate and average values are reported here.

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

2.7. Fluoride analysis

pHPZC plays an important role in the surface characterization of metal oxides/hydroxides. In the adsorption process, pHPZC determines how easily a substrate can adsorb ions present in solution. pHPZC of the adsorbents can be determined by using the salt addition method [23,24]. For determination of pHPZC of the synthesized materials, a solution of 0.01 M NaCl was prepared, and its pH was adjusted in between 3 and 12 by using calculated amount of 0.01 M HCl and 0.01 M NaOH solutions and pH of the mixture was measured using a pH meter (EUTECH instruments pH 700). 10 mL of 0.01 M NaCl solutions having different pH, were taken in 15 mL centrifuge tubes and 30 mg of adsorbent was added in each of these solutions. These tubes were then kept on a mechanical shaker (Niolab instruments, Mumbai, India) at (30 ± 2) °C for 24 h and the equilibrium (final) pH of the solutions were recorded. DpH (the difference between initial and final pH) values were plotted against their initial pH values. The pHinitial at which DpH was zero was considered as pHPZC.

Fluoride concentrations in the solutions (before and after treatment with adsorbent) were measured using UV–Vis spectrophotometer (V-570, Jasco, Japan) at 550 nm with a zirconium-xylenol orange complex reagent [25]. Xylenol orange dye, sodium salt of 3,30 -bis[N,N-di(carboxymethyl)-aminomethyl]-o-cresolsulphonphthalein, forms an orange colored complex with Zr4+. Zr-xylenol orange was prepared by mixing the dye with depolymerized zirconium solution in HCl. This complex decolorizes when it reacts with fluoride ions. During the reaction, fluoride ions dissociate the zirconyl-xylenol orange complex and forms colorless zirconium fluoride. This reaction was used for the spectrophotometric determination of fluoride [26,27]. At the time of analysis, 1 mL of reagent solution was added to 4 mL of fluoride solution (1:4 volume ratio) [25].

3.1. Characterization

2.6. Fluoride adsorption experiments In order to determine the effect of different controlling parameters (such as adsorbent dose, contact time, initial fluoride concentration, pH and co-existing anions) on fluoride adsorption capacity of the synthesized adsorbents, batch adsorption experiments were performed. All adsorption experiments were carried out at (30 ± 2) °C. A stock solution containing 1000 mg L1 fluoride was prepared by dissolving 2.23 g of sodium fluoride in 1000 mL of reverse osmosis (RO) water. Working solutions were prepared by diluting this stock solution. In a typical experiment, 30 mg of adsorbent was mixed with 10 mL of fluoride solution in a 15 mL capped centrifuge tube. The mixture was placed in a mechanical shaker and agitation with a speed of 120 rpm. When equilibrium was reached, the adsorbent was separated from the mixture by centrifuging at 4000 rpm for 10 min. The concentration of fluoride present in the solution after treatment with adsorbent was determined using UV–Vis spectrophotometer. The amount of fluoride adsorbed by the adsorbent, qe (mg g1), was calculated using the following equation:

qe ¼ ðC 0  C e Þ ðV=mÞ

3. Results and discussion

Wide angle powder XRD analysis was carried out for the synthesized materials to identify their crystalline phase and shown in Fig. 1. Though the XRD patterns of the materials showed poor crystalline structure with broad diffraction peaks, characteristic peaks of c-Al2O3 were identified (ICDD card No. 10-0425). The peak intensities corresponding to c-Al2O3 phase were found to be decreased with increasing CaO loading in the sample. For the samples having higher loading of CaO (e.g. CaO20@Al2O3, CaO30@Al2O3) a broad peak in the region of 2h = 26–35° was observed which can be assigned for amorphous CaO (ICDD card No. 28-0775). This fact indicates the amorphous nature of CaO in CaO loaded mesoporous Al2O3 samples.

ð3Þ

where C0 and Ce are the initial and equilibrium concentrations of fluoride in solution (mg L1) respectively, V is the volume of solution (L) and m is mass of the adsorbent (g). The effect of adsorbent dose on fluoride adsorption was investigated by varying adsorbent dose from 0.1 g L1 to 8 g L1. The effect of contact time was examined with initial fluoride concentration of 30 mg L1 and adsorbent dose of 3 g L1. Adsorption kinetic experiments were carried out by using 3 g L1 of the adsorbent with different fluoride concentrations ranging from 5 mg L1 to 30 mg L1. At a designed interval time, the samples were centrifuged and residual fluoride concentrations were determined. The effect of initial fluoride concentration and the adsorption isotherms were studied using solutions having various

Fig. 1. XRD pattern of (a) Al2O3, (b) CaO5@Al2O3, (c) CaO10@Al2O3, (d) CaO15@Al2O3, (e) CaO20@Al2O3 and (f) CaO30@Al2O3.

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N2 adsorption–desorption isotherm analysis were conducted to evaluate the surface area and pore structure of the synthesized Al2O3 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 of pores with narrow necks with wide bodies (often referred to as ink bottle pores) [28]. Large hysteresis loop for pure mesoporous Al2O3 can be attributed to its large pores [29]. However, loading of CaO on mesoporous Al2O3 does not change the shape of the isotherms. The pore size distribution of all the synthesized samples also confirmed the mesoporosity of the materials (Fig. 2(ii)). Surface area and pore size parameters of the synthesized adsorbents are summarized in Table 1. It was observed that BET surface area and pore volume of CaO loaded aluminas decreased with increasing CaO loading on mesoporous Al2O3. Decrease of pore volume indicated that CaO particles are incorporated into the pores of Al2O3. In order to study the surface morphology of the synthesized materials HRTEM images of the samples were recorded and shown in Fig. 3. The morphological aspects of the samples show wormhole-like, highly connected porous structure of the materials. However, the pores are disordered in nature. In Fig. 3(b) it was observed that presence of CaO nanoparticles (5–8 nm) on mesoporous Al2O3 matrix in the CaO loaded sample was observed. In aqueous solution, point of zero charge of an adsorbent plays an important role in the adsorption process. The pHPZC indicates the pH at which the net surface charge on adsorbent is zero. When pH of the solution is pHPZC of adsorbent, the net surface charge of the adsorbent becomes negative due to desorption of H+. Now, adsorption of anions on the negatively charged surface of adsorbent competes with coulombic repulsion [23,24]. The pHPZC values were determined 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 were found to be 8.2 and 11.91 respectively (Fig. 4). This indicates that CaO loaded Al2O3 should have better F- adsorption capacity than that of pure Al2O3. 3.2. Fluoride adsorption studies 3.2.1. Optimization of composition of adsorbent Fluoride adsorption studies of various amount of CaO loaded aluminas were performed using a solution having initial fluoride

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concentration 30 mg L1 and adsorbent dose of 3 g L1. It was observed that pure Al2O3 adsorbed 28% F whereas CaO20@Al2O3 adsorbed 90% F. However, it was also observed that, due to 5 wt.% CaO loading BET surface area of Al2O3 has decreased from 284 m2/g to 228 m2/g and percentage of F- adsorption has also decreased from 28% to 13% in comparison with pure Al2O3 (Fig 5). But, further increase of CaO loading (up to 20 wt.%) resulted in increase of percentage of F adsorption (up to 90%) though the surface area of the adsorbents was decreased with increasing CaO loading (Table 1). This is because of the fact that, surface area of the adsorbent is not the only factor which dictates the F adsorption capacity of CaO loaded Al2O3 adsorbents. Presence of CaO also plays an important role. According to Patel et al. [18], in presence of water CaO forms Calcium hydroxide and adsorption of F occurs by surface chemical reaction, in which AOH groups of Ca(OH)2 are replaced by F resulting in formation of CaF2. This process can be represented by the following equations:

CaO þ H2 O ! CaðOHÞ2 þ Hþ

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 mesoporous Al2O3 matrix which caused the reduction of the surface area of the adsorbent but these CaO particles might not got chance to come in contact with F ions. So, the fluoride adsorption on CaO5@Al2O3 was found to be lower than that of Al2O3. But when CaO loading was further increased, some CaO particles were also deposited on the surface of Al2O3 matrix. These CaO particles reacted with F and formed CaF2. Due to this reason, adsorbents having higher CaO loading exhibited their higher F adsorption capacities than that of pure Al2O3. 20 wt.% CaO loaded Al2O3 sample exhibited its capacity to absorb 90% fluoride from solution with a very fast rate (detailed discussion is in Section 3.2.4), further increase of CaO loading did not show much enhancement in F adsorption capacity of the adsorbents. So, further studies were performed by using CaO20@Al2O3. 3.2.2. Determination of optimum adsorbent dose The effect of adsorption dose on removal of fluoride using pure Al2O3 and CaO20@Al2O3 is shown in Fig. 6, in which percent of fluoride removal is plotted against adsorption dose. It was observed that, initially the percent of fluoride removal increased rapidly with the increase of adsorbent dose and maximum F removal occurred when adsorbent was 3 g L1. The percentage of fluoride removal increased from 20% to 90% with increase in CaO20@Al2O3 dose from 0.25 to 7.5 g L1 at C0 10 mg L1. However, when the

Fig. 2. (i) N2-adsorption desorption isotherms and (ii) pore size distributions of (a) Al2O3, (b) CaO5@Al2O3, (c) CaO10@Al2O3, (d) CaO15@Al2O3, (e) CaO20@Al2O3 and (f) CaO30@Al2O3.

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Table 1 Surface area and pore size parameters of the synthesized adsorbents obtained by means of N2 adsorption–desorption study. Sample description

BET surface area (m2/g)

BJH average pore size (nm)

BJH pore volume (cm3/g)

Al2O3 CaO5@Al2O3 CaO10@Al2O3 CaO15@Al2O3 CaO20@Al2O3 CaO30@Al2O3

284 228 175 135 93 86

7.4 7.5 8.2 8.6 10.1 10.9

0.70 0.55 0.45 0.36 0.29 0.23

adsorbent dose was more than 3 g L1, not much increase in F removal was observed with increasing adsorbent dose. Hence, 3 g L1 of adsorbent dose of Al2O3 and CaO20@Al2O3 was considered for further studies. It was observed that 3 g L1 of pure Al2O3 removed 56% fluoride from a solution having fluoride concentration of 10 mg L1 whereas, CaO20@Al2O3 removed 90% fluoride from the same solution. 3.2.3. Determination of time of equilibrium The fluoride sorption on Al2O3 and CaO20@Al2O3 was investigated as a function of time using a solution having initial fluoride concentration 30 mg L1 and adsorbent dose was 3 g L1. It was observed that, fluoride adsorption by Al2O3 and CaO20@Al2O3 was increased with time (Fig. 7). Initially, fluoride adsorption on adsorbents occurred fast, followed by slower adsorption till the equilibrium was reached. In case of Al2O3, 22% fluoride sorption occurred within 1 h contact time and equilibrium reached in 5 h and 29% fluoride was adsorbed. In case of CaO20@Al2O3, 82% fluoride was adsorbed within 15 min and the equilibrium was reached in 30 min with 92% fluoride adsorption. 3.2.4. Adsorption kinetics Various 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 or transport of fluoride ions from solution phase to exterior surface of the adsorbent particles, adsorption on the particle surfaces, nature of pore structure of adsorbents, attachment of fluoride ions on the surface of adsorbents via complexation or intraparticle precipitation etc. The rate of sorption depends on structural properties of the adsorbent, initial concentration of fluoride solution, interaction between fluoride ions, active sites of adsorbents etc. [30]. To understand the fluoride adsorption kinetics on the synthesized adsorbents, we have investigated their change of fluoride adsorption capacity with time for the solutions having different fluoride concentrations. The adsorption kinetics of fluoride onto

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 L1, adsorbent dose = 3 g L1, contact time = 8 h, pH = 6.8 ± 0.2).

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

Fig. 3. HRTEM images of (a) Al2O3 and (b) CaO20@Al2O3. CaO nanoparticles are shown within the circle.

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and slope of the plot (Fig. 9). The plots log(qe  qt) vs t according to pseudo-first order kinetics are shown in supplementary document Fig. S1. Parameters obtained after fitting the experimental data in the pseudo-first order and pseudo-second order kinetic models are shown in Table 2. As indicated in Table 2, the R2 values of pseudo second order kinetic model (>0.99) were much higher than those of pseudo first order kinetic model (<0.94) and the adsorption capacity values (qe(cal)) calculated from pseudo second order kinetic model were much closer to the experimental values (qe(exp)). These facts indicate the applicability of the pseudo second order kinetic model for fluoride adsorption on Al2O3 and CaO20@Al2O3. Similar trend was also reported by Camacho et al. [13] and Jagtap et al. [33]. Pseudo-second order fitting suggests that chemisorption might be responsible for the fluoride adsorption on Al2O3 and CaO20@Al2O3. Aluminum fluoride complexes and CaF2 are the main components forming on the surface of the adsorbents during fluoride adsorption process [13,34].

Fig. 6. Effect of adsorbent dose on fluoride adsorption capacity of adsorbents (C0 = 10 mg L1, contact time = 8 h, pH = 6.8 ± 0.2).

Fig. 7. Effect of contact time on fluoride adsorption capacity of adsorbents (C0 = 30 mg L1, adsorbent dose = 3 g L1, pH = 6.8 ± 0.2).

slow increase was observed until equilibrium was reached. In case of CaO20@Al2O3, within 15 min 90% fluoride adsorption was observed 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 identify the dynamics of the fluoride adsorption process. The mathematical representations of these models are as follows: Pseudo first order adsorption kinetic model:

logðqe  qt Þ ¼ log qe  ðk1 =2:303Þt

ð6Þ

Pseudo second order adsorption kinetic model:

t=qt ¼ 1=ðk2 q2e Þ þ ð1=qe Þt

ð7Þ

where qe is the amount of fluoride adsorbed on adsorbent (mg g1) at equilibrium, qt is the amount of fluoride adsorbed on adsorbent (mg g1) at time t (min), k1 is the rate constant (min1) for pseudo first order kinetics and k2 is the rate constant (g mg1 min1) for pseudo-second order kinetics. For pseudo second order kinetics model, the kinetics data were plotted t/qt vs t and k2 values were calculated from the intercept

3.2.5. Effect of initial fluoride concentration The effect of initial fluoride concentration on fluoride adsorption capacity by Al2O3 and CaO20@Al2O3 was studied by keeping all other parameters constant (adsorbent dose 3 g L1, contact time = 8 h, temperature 30 ± 2 °C) as shown in Fig. 10. It was observed that, with increase in fluoride concentration (C0), fluoride adsorption capacity (qe) of the adsorbent increases and then reaches a plateau. This should be due to more availability of fluoride ions at higher fluoride concentration (up to C0 = 750 mg L1) for adsorption and the plateau forms due to the saturation of active sites of the adsorbent surfaces. The adsorption capacity of CaO20@Al2O3 is higher than that of Al2O3. The presence of CaO in CaO20@Al2O3 enhances the fluoride removal capacity of the adsorbents by forming insoluble CaF2. Moreover, the higher pHPZC of CaO20@Al2O3 may also contribute towards its higher adsorption capacity. A closer look of these results infers that the increase in qe is very significant as 135 mg g1 of qe could be achieved at higher fluoride concentration. 3.2.6. Adsorption Isotherms Adsorption isotherms provide important information about the adsorption processes. Equilibrium studies are useful to obtain the adsorption capacity of the adsorbents. To obtain the equilibrium data, initial concentrations of fluoride solutions were varied keeping the adsorbent dose constant. The time of equilibrium was chosen considering the results of kinetic studies of fluoride removal by adsorbents. In the present study two well known isotherm models, viz, Freundlich isotherm [35] and Langmuir isotherms [36] were used. Freundlich model indicates the heterogeneity of the adsorbent surface and considers multilayer adsorption. The Freundlich model is represented as follows:

log qe ¼ log K f þ ð1=nÞ log C e

ð8Þ

where Kf and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity (heterogeneity) factor respectively. The values of Kf and 1/n were obtained from the slope and intercept of the linear Freundlich plot of log qe vs log Ce shown in Fig. 11(i). The Langmuir adsorption isotherm model is based on mono layered adsorption on uniform homogeneous surface with sites of identical nature. The Langmuir model is represented in linear form as follows:

C e =qe ¼ 1=ðQ 0 bÞ þ C e =Q 0

ð9Þ 1

where Ce is equilibrium concentration (mg L ), qe is amount of fluoride adsorbed at equilibrium (mg g1), Q0 (mg g1) is maximum

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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 L1, adsorbent dose = 3 g L1, 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 L1, adsorbent dose = 3 g L1, pH = 6.8 ± 0.2).

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

Pseudo second order

C0 (mg L1)

qe(exp) (mg g1)

qe(cal) (mg g1)

k1 (min1)

R2

qe(cal) (mg g1)

k2 (g mg1 min1)

R2

Al2O3

5 10 20 30

1.38 1.80 2.24 2.89

0.49 1.12 1.43 1.31

0.0055 0.0114 0.0094 0.0107

0.8740 0.7113 0.8488 0.7336

1.40 1.89 2.35 3.00

0.0315 0.0191 0.0131 0.0169

0.9976 0.9943 0.9919 0.9985

CaO20@Al2O3

5 10 20 30

1.48 3.20 6.56 9.87

0.29 0.54 1.04 0.44

0.0074 0.0107 0.0031 0.0154

0.9392 0.7797 0.6980 0.7781

1.49 3.22 6.55 9.88

0.0753 0.0649 0.0151 0.1315

0.9994 0.9994 0.9974 1

adsorption capacity for Langmuir isotherm and b (L mg1) is the Langmuir constant related to sorption energy. The values of Langmuir parameters Q0 and b were calculated from the slope and intercept of the linear Langmuir plot of Ce/qe vs Ce (Fig. 11(ii)). Parameters obtained from Freundlich and Langmuir isotherms are listed in Table 3. Freundlich adsorption isotherm model fitted well for pure Al2O3 (R2 = 0.9690) (Fig. 11(i)) whereas, Langmuir adsorption isotherm model fitted well for CaO20@Al2O3 (R2 = 0.9942) (Fig. 11(ii)). The maximum fluoride adsorption capacities of Al2O3 and CaO20@Al2O3 obtained from Langmuir isotherm were 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). But Q0 value of CaO20@Al2O3 obtained from experiments is 136.99 mg/g, which is 88% of the theoretical value. This is because of the fact that, certain amount of CaO nanoparticles were deposited within the pores of mesoporous Al2O3 and these CaO particles might not get a chance to come in contact with F ions and so did not participate in F adsorption process. To predict the adsorption efficiency, the dimensionless quantity ‘r’ was calculated using the following equation [37]:

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Table 3 Langmuir and Freundlich isotherm parameters for fluoride adsorption on Al2O3 and CaO20@Al2O3 at pH of 6.8 ± 0.2 and temperature = (30 ± 2) °C. Langmuir isotherm

Al2O3 CaO20@Al2O3

Freundlich isotherm

Q0 (mg g1)

b (L mg1)

R2

Kf

1/n

R2

24.45 136.99

0.004 0.084

0.7391 0.9942

0.56 13.59

0.5115 0.4182

0.9690 0.5779

Fig. 10. Effect of initial fluoride concentration on fluoride adsorption capacity of Al2O3 and CaO20@Al2O3 (adsorbent dose = 3 g L1, contact time = 8 h, pH = 6.8 ± 0.2).

r ¼ 1=ð1 þ bC 0 Þ

ð10Þ

where C0 and b are the initial fluoride concentration and Langmuir isotherm constant respectively. If the value of r is <1, it signifies the favorable adsorption whereas, r > 1 indicates the unfavorable adsorption [33]. Since, in the present case ‘r’ values were found to be <1 (varies from 0.98 to 0.01 with the variation of C0 from 5 to 1000 mg L1), it was assumed that favorable adsorption of fluoride occurred on Al2O3 and CaO20@Al2O3.

Fig. 12. Effect of initial pH on fluoride adsorption capacity of Al2O3 and CaO20@Al2O3 (adsorbent dose = 3 g L1, C0 = 30 mg L1, contact time = 8 h).

3.2.7. Effect of initial pH The effect of initial pH of the solution on fluoride removal by Al2O3 and CaO20@Al2O3was investigated at different pH ranging from 4 to10, with a constant adsorbent dose of 3 g L1, initial fluoride concentration 30 mg L1, contact time 8 h, temperature (30 ± 2 °C) and shown in Fig. 12. The adsorption of fluoride on Al2O3 does not change much within the pH range of solution 4– 9. However, when initial pH was 10, the fluoride adsorption capacity of pure Al2O3 was decreased. This might be due to the fact that pHZPC value of Al2O3 8.2 which is <10. The fluoride adsorption capacity of CaO20@Al2O3 does not affect much within the pH range of 4–10.

3.2.8. Effect of co-existing anions Beside fluoride ions the natural ground water always contains various other ions, which may compete with fluoride during adsorption and affect the efficiency of the adsorbent. To study the effect of co-existing anions on fluoride adsorption on synthesized adsorbents, 10 mg L1 and 100 mg L1 initial concentrations  2 of Cl, NO 3 , SO4 and HCO3 were used while keeping the initial fluoride concentration as 10 mg L1. The effect of co-ions on the fluoride removal efficiency of Al2O3 and CaO20@Al2O3 is shown in Fig. 13. It was observed that the presence of Cl did not affect the fluoride removal capacity of the adsorbents. Presence of NO 3 1 (10 and 100 mg L1) and SO2 ) ions slightly decreased 4 (10 mg L

Fig. 11. (i) Freundlich adsorption isotherm for adsorption of fluoride on Al2O3 and (ii) Langmuir adsorption isotherm for adsorption of fluoride on CaO20@Al2O3. (C0 = 5 mg L1 to 1000 mg L1, adsorbent dose = 3 g L1, contact time = 8 h, pH = 6.8 ± 0.2).

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Fig. 13. Effect of co-existing anions on fluoride adsorption capacity of Al2O3 and CaO20@Al2O3 (adsorbent dose = 3 g L1, C0 = 10 mg L1, contact time = 8 h).

(2–3%) the fluoride removal capacity of the adsorbent, whereas 8–9% decrease was observed when HCO 3 was present. This is  due to the competition between HCO 3 and F for the adsorption on active sites of the adsorbent.

4. Conclusion Here, the preparation of CaO loaded mesoporous Al2O3 based adsorbents was reported for removal of F from water. It has been demonstrated that loading of 20 wt.% CaO significantly enhances the F removal capacity of mesoporous Al2O3. CaO20@Al2O3 is capable of removing 90% F within 15 min using a moderate adsorbent dose of 3 g L1where as, pure Al2O3 removes only 22% after 1 h. Generally, the fluoride concentration in contaminated ground water is 5–10 mg L1. It was observed that, when the solutions having F concentration of 5 mg L1 and 10 mg L1 was treated with CaO20@Al2O3 with adsorbent dose of 3 g L1, the F concentration of treated water became <1 mg L1, which is well below the recommendation of WHO. The maximum fluoride adsorption capacities of Al2O3 and CaO20@Al2O3 were found to be 24.45 mg/g and 136.99 mg/g respectively. The CaO loaded mesoporous Al2O3 exhibited higher fluoride removal capacity than that of pure Al2O3 and commercial Al2O3 (ca 7 mg g1) [10], because of Ca may react with fluoride ions to form CaF2 precipitates and high value of pHPZC of CaO20@Al2O3. CaO20@Al2O3 exhibited good fluoride removal efficiency over a wide range of pH. The high adsorption capacity and fast rate of adsorption makes CaO20@Al2O3 a potential candidate as an adsorbent in fluoride removal devices. Acknowledgment Authors gratefully acknowledges financial support from Board of 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, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.03.064. References [1] J. Fawell, K. Bailey, J. Chilton, E. Dahi, L. Fewtrell, Y. Magara, Fluoirde in Drinking-Water, IWA Publishing, London, 2006. [2] S. Jagtap, M.K. Yenkie, N. Labhsetwar, S. Rayalu, Fluoride in drinking water and defluoridation of water, Chem. Rev. 112 (2012) 2454–2466.

[3] S.M. Maliyekkal, A.K. Sharma, L. Philip, Manganese-oxide-coated alumina: a promising sorbent for defluoridation of water, Water Res. 40 (2006) 3497– 3506. [4] W.G. Nawlakhe, D.N. Kulkarni, B.N. Pathak, K.R. Bulusu, Defluoridation of water by Nalgonda technique, Indian J. Environ. Health 17 (1975) 26–65. [5] H. Tamura, N. Katayama, R. Furuichi, Modeling of ion-exchange reactions on metal oxides with the frumkin isotherm: 1. Acid–base and charge characteristics of MnO2, TiO2, Fe3O4, and Al2O3 surfaces and adsorption affinity of alkali metal ion, Environ. Sci. Technol. 30 (1996) 1198–1204. [6] P. Loganathana, S. Vigneswarana, J. Kandasamya, R. Naidu, Defluoridation of drinking water using adsorption processes, J. Hazard. Mater. 248–249 (2013) 1–19. [7] A. Bhatnagar, E. Kumar, M. Sillanpaa, Fluoride removal from water by adsorption - a review, Chem. Eng. J. 171 (2011) 811–840. [8] S. Ayoob, A.K. Gupta, T.B. Venugopal, A conceptual overview on sustainable technologies for the defluoridation of drinking water, Crit. Rev. Environ. Sci. Technol. 38 (2008) 401–470. [9] Y.H. Li, S. Wang, A. Cao, D. Zhao, X. Zhang, C. Xu, Z. Luan, D. Ruan, J. Liang, D. Wu, B. Wei, Adsorption of fluoride from water by amorphous alumina supported on carbon nanotubes, Chem. Phys. Lett. 350 (2001) 412–416. [10] Y. Ku, Hwei-Mei Chiou, The adsorption of fluoride ion from aqueous solution by activated alumina, Water Air Soil Pollut. 133 (2002) 349–360. [11] M.G. Sujana, G. Soma, N. Vasumathi, S. Anand, Studies on fluoride adsorption capacities of amorphous Fe/Al mixed hydroxides from aqueous solutions, J. Fluorine Chem. 130 (2009) 749–754. [12] F. Luo, K. Inoue, The removal of fluoride ion by using metal (III)-loaded amberlite resins, Solvent Extr. Ion Exch. 22 (2004) 305–322. [13] L.M. Camacho, A. Torres, D. Saha, S. Deng, Adsorption equilibrium and kinetics of fluoride on sol–gel-derived activated alumina adsorbents, J. Colloid Interface Sci. 349 (2010) 307–313. [14] J. Wang, W. Xu, L. Chen, Y. Jia, L. Wang, X.-J. Huang, J. Liu, Excellent fluoride removal performance by CeO2–ZrO2 nanocages in water environment, Chem. Eng. J. 231 (2013) 1980–2205. [15] B.D. Turner, P.J. Binning, S.L.S. Stipp, Fluoride removal by calcite: evidence for fluorite precipitation and surface adsorption, Environ. Sci. Technol. 39 (2005) 9561–9568. [16] B.D. Turner, P.J. Binning, S.W. Sloan, A calcite permeable reactive barrier for the remediation of Fluoride from spent potliner (SPL) contaminated groundwater, J. Contam. Hydrol. 95 (2008) 110–120. [17] M. Yang, T. Hashimoto, N. Hoshi, H. Myog, Fluoride removal in a fixed bed packed with granular calcite, Water Res. 33 (1999) 3395–3402. [18] G. Patel, U. Pal, S. Menon, Removal of fluoride from aqueous solution by CaO nanoparticles, Sep. Sci. Technol. 44 (2009) 2806–2826. [19] Q. Yuan, A.-X. Yin, C. Luo, L.-D. Sun, Y.-W. Zhang, W.-T. Duan, H.-C. Liu, C.-H. Yan, Facile synthesis for ordered mesoporous c-aluminas with high thermal stability, J. Am. Chem. Soc. 130 (2008) 3465–3472. [20] W. Cai, J. Yu, C. Anand, A. Vinu, M. Jaroniec, Facile synthesis of ordered mesoporous alumina and alumina-supported metal oxides with tailored adsorption and framework properties, Chem. Mater. 23 (2011) 1147–1157. [21] B. Naik, N.N. Ghosh, A review on chemical methodologies for preparation of mesoporous silica and alumina based materials, Recent Pat. Nanotechnol. 3 (2009) 213–224. [22] B. Naik, V.S. Prasad, N.N. Ghosh, Development of a simple aqueous solution based chemical method for synthesis of mesoporous c-alumina powders with disordered pore structure, J. Porous Mater. 17 (2010) 115–121. [23] V. Srivastava, C.H. Weng, V.K. Singh, Y.C. Sharma, Adsorption of nickel ions from aqueous solutions by nano alumina: kinetic, mass transfer, and equilibrium studies, J. Chem. Eng. Data 56 (2011) 1414–1422. [24] M. Farooq, A. Ramli, D. Subbarao, Physiochemical properties of c-Al2O3-MgO and c-Al2O3–CeO2 composite oxides, J. Chem. Eng. Data 57 (2012) 26–32. [25] S.V. Rao, R. Singh, S.C. Chaurasia, Bhabha Atomic Research Centre (BARC) News letter, Mumbai, India 219 (2002) 6.

D. Dayananda et al. / Chemical Engineering Journal 248 (2014) 430–439 [26] J.A. Ruzicka, H. Jakschova, L. Mrklas, Determination of fluorine in bones and teeth with xylenol orange, Talanta 13 (1966) 1341–1344. [27] M.L. Cabello-Tomas, T.S. West, Kinetochromic spectrophotometry-I: determination of fluoride by catalysis of the zirconium-xylenol orange reaction, Talanta 16 (1969) 781–788. [28] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A.J. Pierotti, J. Rouquerol, R.T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (1985) 603–619. [29] J. Cejka, N. Zilkova, J. Rathousky, A. Zukal, Nitrogen adsorption study of organised mesoporous alumina, Phys. Chem. Chem. Phys. 3 (2001) 5076– 5081. [30] E.A. Oliveira, S.F. Montanher, A.D. Andrade, J.A. Nobrega, M.C. Rollemberg, Equilubrium studies for the sorption of chromium and nickel from aqueous solutions using raw rice bran, Process Biochem. 40 (2005) 3485–3490.

439

[31] S. Lagergren, About the theory of so-called adsorption of soluble substances, K. Sven. Vetenskapsakad Handl. 24 (1898) 1–39. [32] Y.S. Ho, G. McKay, Pseudo-second-order model for sorption processes, Process Biochem. 34 (1999) 451–465. [33] S. Jagtap, M.K.N. Yenkie, N. Labhsetwar, S. Rayalu, Defluoridation of drinking water using chitosan based mesoporous alumina, Micropor. Mesopor. Mater. 142 (2011) 454–463. [34] C.S. Sundaram, N. Viswanathan, S. Meenakshi, Defluoridation of water using magnesia/chitosan composite, J. Hazard. Mater. 163 (2009) 618–624. [35] H.M.F. Freundlich, Uber die adsorption in losungen, Z. Phys. Chem. 57A (1906) 385–470. [36] I. Langmuir, The constitution and fundamental properties of solids and liquids, J. Am. Chem. Soc. 38 (1916) 2221–2295. [37] T.W. Weber, R.K. Chakravorti, Pore and solid diffusion models for fixed bed adsorbents, J. Am. Inst. Chem. Eng. 20 (1974) 228–238.