Accepted Manuscript

Title: Synthesis of MgO nanoparticle loaded mesoporousAl2O3 and its defluoridation study

Author: Desagani Dayananda Venkateswara R. SarvaSivankutty V. Prasad Jayaraman Arunachalam PadmanabhanParameswaran Narendra N. Ghosh

PII: S0169-4332(14)02745-7DOI: http://dx.doi.org/doi:10.1016/j.apsusc.2014.12.057Reference: APSUSC 29292

To appear in: APSUSC

Received date: 29-9-2014Revised date: 6-12-2014Accepted date: 7-12-2014

Please cite this article as: D. Dayananda, V.R. Sarva, S.V. Prasad, J.Arunachalam, P. Parameswaran, N.N. Ghosh, Synthesis of MgO nanoparticle loadedmesoporous Al2O3 and its defluoridation study, Applied Surface Science (2014),http://dx.doi.org/10.1016/j.apsusc.2014.12.057

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Highlights

Simple and cost effective preparation of MgO nanoparticles loaded mesoporous Al2O3.

Adsorbents possess high surface area and mesoporous structure.

Higher fluoride removal capacity of MgO loaded Al2O3 than that of pure Al2O3.

Faster fluoride adsorption kinetics of MgO loaded Al2O3 from water.

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Synthesis of MgO nanoparticle loaded mesoporous Al2O3 and its

defluoridation study

Desagani Dayananda a, Venkateswara R. Sarva b, Sivankutty V. Prasad c, Jayaraman

Arunachalam b, Padmanabhan Parameswaran d, 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, India

b National Centre for Compositional Characterization of Materials (CCCM), Bhabha Atomic

Research Centre, ECIL Post, Hyderabad 500062, India

c Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science

and Technology (NIIST-CSIR), Thiruvananthapuram 695019, Kerala, India

d Materials Synthesis and Structural Characterisation Division, Physical Metallurgy Group,

Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603102 India

*Corresponding author (N. N. Ghosh): Tel: + 91 83 22580318; Fax: + 91 83 2557033. E-mail

address: naren70@yahoo.com

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Keywords: Mesoporous alumina, MgO, fluoride removal, adsorption isotherm, adsorption

kinetics.

Abstract

MgO nanoparticle loaded mesoporous alumina has been synthesized using a simple aqueous

solution based cost effective method for removal of fluoride from water. Wide angle powder X-

ray diffraction, nitrogen adsorption desorption analysis, transmission electron microscopy

techniques and energy dispersive X-ray spectroscopy were used to characterize the synthesized

adsorbents. Synthesized adsorbents possess high surface area with mesoporous structure. The

adsorbents have been thoroughly investigated for the adsorption of F- using batch adsorption

method. MgO nanoparticle loading on mesoporous Al2O3 enhances the F- adsorption capacity of

Al2O3 from 56% to 90% (initial F- concentration = 10 mg L-1). Kinetic study revealed that

adsorption kinetics follows the pseudo second order model, suggesting the chemisorption

mechanism. The F- adsorption isotherm data was explained by both Langmuir and Freundlich

model. The maximum adsorption capacity of 40MgO@Al2O3 was 37.35 mg g-1. It was also

observed that, when the solutions having F- concentration of 5 mg L-1 and 10 mg L-1 was treated

with 40MgO@Al2O3, the F- concentration in treated water became < 1 mg L-1, which is well

below the recommendation of WHO.

1. Introduction

Though, presence of trace amount of F- in drinking water is essential to prevent dental caries

but an excess intake of F- is detrimental to human health. Excess intake of F- can cause dental/

skeleton fluorosis [1,2]. It not only affects teeth and skeleton, but also accumulation of F- over a

long period of time can lead to cancer, osteosclerosis (brittle bones and calcified ligaments) and

sometimes neurological impairment [1]. According to World Health Organization (WHO)

recommendation, desirable limit and permissible limit of F- in drinking water are 1.0 mg L-1 and

1.5 mg L-1 respectively [3]. Many technologies are available for defluoridation of water such as

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membrane separation, ion-exchange, adsorption, coagulation, electrochemical techniques etc [2].

However, each method has its own limitations [2]. Among these methods, adsorption is the most

suitable process for drinking water treatment because of its cost effectiveness and simple

operation procedure [3]. Review articles describing several aspects of F- removal from water

using different adsorbents have been reported in the literature [2-5]. Among the available

adsorbents for F- removal, activated alumina has been explored extensively because of its good

F- removal capacity and relatively low operation cost [2]. F- adsorption capacity of activated

alumina mainly dependents on the crystalline form, surface properties, activation process,

solution pH and alkalinity. The disadvantage associated with activated alumina is that, maximum

fluoride removal occurs only when pH value of solution is below 6 and this factor limits the

practical applicability of activated alumina for this purpose. Moreover, it has also been reported

that alumina begins to leach below pH 6 and poses severe threat to human health [2]

High surface area aluminas with porous structure have gained importance due to their

potential applications as adsorptive material in the separation processes [6-12]. In recent years

mesoporous alumina has attracted considerable attention because of its capability of removal of

inorganic anions [6-9], heavy metals [10], organic dyes [11], rare earth elements [12] etc from

water. In the literature variety of synthetic methods have been reported for preparation of

mesoporous Al2O3 [13-17]. However, in most of the cases to prepare alumina, aluminum

alkoxides were used as starting material and alcohol as solvent. Aluminum alkoxides are not

only costly but also reactive to moisture. The use of alcohol as solvent and sometimes use of

autoclave as reactor make these synthesis processes difficult and expensive. In our previous

study, we have developed a simple but cost effective method to prepare mesoporous Al2O3

which can act as an efficient adsorbent to remove F- from water [18,19]. We have also observed

that F- removal capacity of mesoporous Al2O3 is better than that of commercially available Al2O3

[18].

It has already been demonstrated by several researchers that, F- adsorption capacity of

Al2O3 can be increased by chemical modification of its surfaces [3,14,19-23]. Impregnation of

positively charged cations (such as Ca2+, Mg2+, La4+, Zr4+, Fe3+, Ce4+ etc) onto the adsorbent

helps to create positive charges on the adsorbent surface which attracts F- and improves F-

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adsorption capacity [3,14,19-23]. The chemistry involve in this type of adsorption can be

presented as follows [3].

≡Me-OH2+ + F-251658240 ≡Me-F + H2O

(1)

≡Me-OH + F- 251658240 ≡Me-F + OH-

(2)

(≡Me represents the multivalent metallic cation surfaces)

Magnesium based adsorbents for removal of F- from water has been reported by several

researchers [23-28]. Devi et al have reported that, MgO nanoparticle has the capability to adsorb

F- with adsorption capacity of 14 mg g-1 [24]. The isomorphous substitution via ion exchange

mechanism between F- and -OH group, present on the surface of MgO, in aqueous media is

responsible for adsorption of F- onto MgO. This substitution is possible because F- and OH- are

similar size with comparable ionic radii as well as they are iso-electric in nature. The high

surface area of MgO nanoparticles provide a large number of base sites for ion-exchange with F-

at aqueous interface, which leads to the faster removal rate at the initial stage of adsorption

process [25,26]. The mechanism can be expressed as given below [25,26].

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Therefore, to prepare an adsorbent to remove F- from water with enhanced adsorption

capacity, we have adopted a strategy to impregnate MgO nanoparticle within/on the surface of

mesoporous Al2O3. In the present study, MgO nanoparticle loaded mesoporous Al2O3 adsorbents

were prepared by using a simple aqueous solution based cost effective method for removal of F-

from aqueous solution. F- sorption properties of the synthesized adsorbents have been

investigated using batch adsorption studies. The F- adsorption capacity of MgO nanoparticle

loaded mesoporous Al2O3 was compared with pure mesoporous Al2O3 [19].

2. Experimental Section

2.1. Materials

Aluminium nitrate nonahydrate, sodium hydroxide, sodium sulphate were procured from

Fisher Scientific, India; triethanol amine (TEA), sodium hydrogen carbonate, sodium chloride,

sodium nitrate and sodium fluoride were procured from Merck, India; magnesium nitrate

hexahydrate, 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 [18,29]. 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)3 9H2O 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)3

9H2O 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

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to obtain precursor powder. The dried precursor was then calcined at 550 °C for 4 h in air to

obtain porous Al2O3 powder.

2.3. Preparation of MgO loaded mesoporous Al2O3

MgO loaded mesoporous aluminas were synthesized using a wet impregnation technique.

MgO loaded mesoporous aluminas were synthesized with different loading percentages (5, 10,

20, 30, 40 and 50 wt.%) of MgO on mesoporous Al2O3. In a typical synthesis, in a beaker

calculated amount of aqueous solution of magnesium nitrate hexahydrate was mixed with desired

amount of mesoporous Al2O3 powder and stirred for 12 h. The mixture was dried at 150 °C for 5

h and then calcined at 550 °C for 4h in air atmosphere to obtain MgO loaded mesoporous Al2O3

adsorbent. The 5, 10, 20, 30, 40 and 50 wt.% MgO loaded mesoporous Al2O3 are now onwards

will be referred as 5MgO@Al2O3, 10MgO@Al2O3, 20MgO@Al2O3, 30MgO@Al2O3,

40MgO@Al2O3 and 50MgO@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 Kα radiation. The

diffractograms were recorded in the 2θ 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 Brunauer-

Emmett-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 10h.

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. Energy

Dispersive X-ray Spectroscopy (EDS) was carried out using EDAX EDS system attached to

HRTEM.

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2.5. Determination of pHPZC (Point of Zero Charge) of the adsorbents

pHPZC of the adsorbents was determined by using the salt addition method [19,30,31]. For

determination of pHPZC of the synthesized adsorbents, a solution of 0.01 M NaCl was prepared

and its pH was adjusted in between 3 and 12 by using 0.01 M HCl and 0.01 M NaOH solutions.

pH of the mixtures was measured using a pH meter (EUTECH instruments pH 700). 10 mL of

0.01M NaCl solutions, having different pH were taken in 15 mL centrifuge tubes and 30 mg

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 then the samples were

centrifuged with 4000 rpm for 10 min. The equilibrium (final) pH of the solutions was recorded.

∆pH (the difference between initial and final pH) values were plotted against their initial pH

(pHinitial) values. The pHinitial at which ∆pH was zero was considered as pHPZC.

2.6. Fluoride adsorption experiments

Batch method was used to determine the F- sorption properties of MgO loaded

mesoporous Al2O3 adsorbents. A stock solution (1000 mg L-1) was prepared by dissolving 0.221

g anhydrous NaF to 1 L of RO (reverse osmosis) water. This solution was further diluted to

desired concentrations for experimental use. Certain amount of adsorbent was added to 10 mL of

F- solution (with known F- concentration) in a 15 mL capped centrifuge tube. The mixture was

then placed in a mechanical shaker at 30 °C temperature. After continuous shaking in the

mechanical shaker for a fixed time interval, the samples were centrifuged with 4000 rpm for 10

min. F- concentration in the residual solution was analyzed by using UV-Vis spectrophotometer

[32]. The amount of F- adsorbed by the adsorbent, qe (mg g-1), was calculated using the following

equation:

qe = (C0-Ce) (V/m) (5)

where C0 and Ce are the initial and equilibrium concentrations of F- in solution (mg L-1)

respectively, V is the volume of solution (L) and m is mass of the adsorbent (g).

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The effect of adsorbent dose on F- adsorption was investigated by varying adsorbent dose

from 0.1 g L-1 to 8 g L-1. The effect of contact time was examined using solutions with initial F-

concentration of 30 mg L-1 and adsorbent dose of 3 g L-1. Adsorption kinetic experiments were

carried out using solutions having different F- concentrations ranging from 5 mg L-1 to 30 mg L-1

and keeping the adsorbent dose constant (3g L-1). At a designed time interval, the samples were

centrifuged and adsorbent was separated. F- concentrations of solutions after treatment with

adsorbent were determined. The effect of initial F- concentration and the adsorption isotherms

were investigated using solutions having various F- concentrations varying from 5 mg L-1 to 1000

mg L-1. Effect of initial pH of the solution on F- removal was determined by varying pH (pH was

adjusted using 0.01 N HCl and 0.01 N NaOH) from 4 to 10 with a solution having F-

concentration 30 mg L-1 and adsorbent dose 3 g L-1. The effect of co-existing anions (such as

chloride, nitrate, sulphate and bicarbonate) on F- adsorption were investigated by performing F-

adsorption experiments using 10 mg L-1 of F- concentration and 10 mg L-1 and 100 mg L-1 of co-

existing anions. Reproducibility of measurements was checked in triplicate and average values

are reported here.

2.7. Fluoride analysis

F- concentration in the solutions (before and after treatment with adsorbent) was

measured using UV-Visible spectrophotometer (V-570, Jasco, Japan) at 550 nm with a

zirconium-xylenol orange complex reagent [18,19,32]. Xylenol orange dye, sodium salt of 3,3′-

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 F- ions. During the

reaction, F- ions dissociate the zirconyl-xylenol orange complex and forms colorless zirconium

fluoride. This reaction was used for the spectrophotometric determination of F- [33]. At the time

of analysis, 1 mL of zirconium-xylenol orange complex reagent was added to 4 mL of F- solution

(1:4 volume ratio) [32]. Absorbance of different F- solutions with known F- concentration

(ranging from 0 - 1.2 mg L-1) was measured at λmax = 550 nm. Calibration curve was constructed

by plotting absorbance vs. F- concentration (R2 > 0.997). Concentrations of F-, present in the

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solution, before and after treatment with the synthesized adsorbents were determined from the

calibration curve.

3. Results and Discussion

3.1. Characterization of the synthesized materials

The X-ray powder diffraction pattern of MgO loaded Al2O3 samples are shown in Fig.1.

Fig. 1(a) shows almost amorphous nature of γ- Al2O3 with broad X-ray diffraction peaks at 2θ =

19.41, 45.45 and 67.14 corresponding to (111), (400) and (440) diffraction planes (ICDD card

no. 10-0425). The peak intensities corresponding to γ- Al2O3 phase were found to be decreased

with increasing MgO loading in the sample. The samples containing MgO loading upto 20 wt%

did not show any characteristic peaks of MgO. However, further increase of MgO loading on

Al2O3, characteristic peaks of cubic MgO at 2θ = 37.01, 42.86, 62.44, 74.86 and 78.82

corresponding to (111), (200), (220), (311) and (222) planes were observed [ICDD card no: 65-

0476].

<Figure 1>

N2 adsorption- desorption isotherms and pore size distributions of MgO nanoparticle loaded

mesoporous Al2O3 are shown in Figure 2. N2 adsorption- desorption isotherms for pure Al2O3

and MgO nanoparticle loaded adsorbents (upto 30wt%, i.e. 5MgO@Al2O3, 10MgO@Al2O3,

20MgO@Al2O3 and 30MgO@Al2O3) are typical type IV isotherms with H2 hysteresis loop. This

indicates that these adsorbents possess mesoporous structure with pores having narrow necks and

with wide bodies (often referred to as ink bottle pores) [34]. However, when more than 30wt%

MgO was impregnated within mesoporous Al2O3 matrix (i.e. 40MgO@Al2O3 and

50MgO@Al2O3), the nature of the isotherms has changed to H3 type hysteresis loop, indicating

the formation of aggregated slit-shaped porous structure. The change in pore size of the MgO

loaded Al2O3 adsorbents (Fig. 2(ii)) was found to be minor (from 6.5 to 7.0 nm) when MgO

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nanoparticle loading was increased from 0 to 30%. Then, further increase of MgO loading led to

dramatic decrease in pore size (from 6.5 to 3.8 nm). This might be due to the fact that pores have

collapsed to some extent and formed aggregates of nanoparticles [34,35]. Moreover, when MgO

loading is high then MgO may be deposited on the walls of pore and caused reduction of pore

size. The BJH pore volumes of the samples were found to be decreased with increasing MgO

loading. The physical parameters obtained by means of N2 adsorption- desorption study are

summarized in Table 1. The BET surface area and pore volume of the adsorbents also found to

be decreased with increasing MgO nanoparticle loading.

<Figure 2>

<Table 1>

HRTEM micrographs of the synthesized adsorbents Al2O3 and 40MgO@Al2O3 are

shown in Fig 3(a) and (b). Morphological aspects of the samples showed warmhole-like highly

connected porous structure of mesoporous Al2O3 matrix and MgO nanoparticles (size < ~8 nm)

are within the porous structure (Fig. 3b). Fig. 3c reveals that, after F- adsorption needle like

particles were formed on the adsorbent surface. EDS analysis of this sample shows the presence

of F- in the adsorbent (Fig. 3d). This might be due to the formation of Mg(OH)2-xFx [25,26].

From EDS analysis it was also found that, the weight percentage ratios of Al: Mg: O: F content

in the adsorbent after F- adsorption are 24.6: 28: 45: 2.4 respectively.

<Figure 3>

One of the most important characteristics of adsorbent surface is the point of zero charge.

This point corresponds to the pH of the solution when the net electrical charge of the surface of

adsorbent is zero. In the adsorption process, pHPZC determines how easily a substrate can adsorb

ions present in solution. When pH of the solution is < pHPZC of adsorbent, the net surface charge

on solid surface of adsorbent becomes positive because of the adsorption of excess H+. This

situation favours the adsorption of anions on the surface of adsorbent due to coulombic

attraction. When pH of the solution is > pHPZC of adsorbent, the net surface charge of the

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adsorbent becomes negative due to desorption of H+. In this situation, adsorption of anions on

the negatively charged surface of adsorbent competes with coulombic repulsion [30, 31].

Therefore, at a pH, which is lower than the point of zero charge value of the adsorbent, F- ions

may be attracted on the adsorbents surface and when pH of the solution is higher, F- ions may be

repelled. The pHPZC values were determined from pH (the difference between initial and final

pH) versus pHinitial plots, as pH at which pH is zero that is pHinitial= pHfinal. The pHPZC values for

pure mesoporous Al2O3 and 40MgO@Al2O3 were found to be 8.2 and 11.2 respectively (Fig.4).

This indicates that 40MgO@Al2O3 should have better F- adsorption capacity than that of pure

Al2O3.

<Figure 4>

3.2. Optimization of composition of MgO loaded Al2O3 adsorbents

F- adsorption analysis of different amounts of MgO loaded mesoporous aluminas are

shown in Fig 5. The analysis was performed using a solution having initial F- concentration of 30

mg L-1 and adsorbent dose of 3 g L-1. It was clearly observed that, loading of MgO nanoparticles

on high surface area mesoporous Al2O3 enhanced its F- adsorption capacity. For instance, pure

mesoporous Al2O3 adsorbed ~28% F-, whereas adsorbent having 40 wt% MgO adsorbed ~83%

F-. However, when MgO loading exceeded 40 wt%, much enhancement in F- adsorption capacity

of adsorbents was not observed. So, further studies were conducted using 40MgO@Al2O3.

<Figure 5>

3.3. Determination of optimum adsorption dose

The effect of adsorbent dose on F- removal at initial F- concentration of 10 mg L-1 is

shown in Fig. 6. It is evident that, the percent of F- removal was increased with increasing

adsorbent dose. This is due to the fact that, a greater amount of adsorbent provides greater

number of available binding sites. It was also observed that, the percent of F- adsorbed increased

drastically with increasing adsorbent dose from 0.25 g L-1 to 3g L-1. However, when the

adsorbent dose was more than 3g L-1, not much increase in F- removal was observed with

increasing adsorbent dose. Hence, 3 g L-1 of adsorbent dose of mesoporous Al2O3 and

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40MgO@Al2O3 was considered for further studies. Pure Al2O3 removed ~ 56% F- from a

solution having F- concentration of 10 mg L-1 whereas, 40MgO@Al2O3 exhibited its capacity of

removing ~90% F- from the same solution with 3 g L-1 adsorbent dose.

<Figure 6>

3.4. Determination of equilibrium time

The fluoride sorption on mesoporous Al2O3 and 40MgO@Al2O3 was investigated as a

function of time using a solution having initial F- concentration 30 mg L-1 and adsorbent dose

was 3 g L-1. F- adsorption of 40MgO@Al2O3 was found to be increased from 12% to 76% as the

contact time increased from 15 min 480 min and equilibrium was achieved at 480 min (Fig. 7).

Pure mesoporous Al2O3 adsorbed ~ 23% F- within 90 min contact time and equilibrium reached

in 300 min and ~ 29 % F- was adsorbed. Hence, equilibrium time of 480 min was applied for

further studies. F- adsorption was initially fast upto 120 min and then it became low. The initial

rapid adsorption was presumably due to exchange of F- ions with surface hydroxyl ions of the

adsorbent. Slow adsorption in the later stage caused by the gradual uptake of F- at the inner

surface of adsorbent [36,37].

<Figure 7>

3.5. Adsorption Kinetics

The adsorption kinetics of F- adsorption on mesoporous Al2O3 and 40MgO@Al2O3 were

studied using initial F- concentration (C0) of 5, 10, 20, 30 mg L-1 and adsorbent dose of 3 g L-1 at

30 °C. The adsorption kinetics of F- on 40MgO@Al2O3 are shown in Figure 8. The kinetic

curves show that, the sorption rate was rapid at the beginning of the processes and then became

slow as equilibrium was approached towards 480 min.

<Figure 8>

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The rate of sorption depends on structural properties of the sorbent, initial F-

concentration in solution, interaction between F- ions and active sites of adsorbents [38]. To

understand the kinetics of F- adsorption onto the surface of the adsorbent, Lagergren’s pseudo-

first order kinetic model [39] and Ho’s pseudo- second order kinetic model [40] equations were

used. In this study, the kinetic data obtained for Al2O3 and 40MgO@Al2O3 using various initial

F- concentrations (C0 = 5, 10, 20 and 30 mg L-1) with constant adsorbent dose (3g L-1) were fitted

to kinetic models.

The mathematical equation representing Lagergren’s pseudo-first order kinetic model is

as follows:

log(qe- qt) = log qe- (k1/2.303)t (6)

where qe and qt are the amount of F- adsorbed per unit mass of adsorbent at equilibrium and at a

given time t, respectively. k1 is the rate constant (min-1). Values of k1 and qe were calculated from

the intercept and slope of log(qe- qt) vs. t plot.

The mathematical equation representing pseudo-second order kinetic model is as follows:

t/qt = 1/(k2qe2) + (1/qe)t (7)

where k2 is the rate constant (g mg-1min-1) for the pseudo-second order reaction. qe and qt are the

amount of F- adsorbed at equilibrium and at a given time t, respectively. Values of qe and k2 were

obtained from the slope and intercept of t/qt vs. t plot.

Parameters obtained after fitting the F- adsorption kinetic data of mesoporous Al2O3 and

40MgO@Al2O3 in the pseudo-first order and pseudo-second order kinetic models are listed in

Table 2. Figure 9 (i) and (ii) show the representative plots of pseudo-first order and pseudo-

second order kinetic models when experimentally obtained kinetic data for 40MgO@Al2O3 were

fitted with the kinetic model equations (equation 4 and 5). As indicated in Table 2, R2 values of

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pseudo second order kinetic model were much higher than those of pseudo first order kinetic

model 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 that F-

adsorption on Al2O3 and 40MgO@Al2O3 follows pseudo second order kinetics. Pseudo second

order kinetic model suggests that chemisorption might be responsible for the F- adsorption on

mesoporous Al2O3 and 40MgO@Al2O3 [14,41].

<Figure 9>

<Table 2>

3.6. Effect of initial fluoride concentration

The effect of initial F- concentration on F- adsorption capacity of mesoporous Al2O3 and

40MgO@Al2O3 was studied by keeping all other parameters constant (adsorbent dose 3g L-1,

contact time = 8h, temperature 30 ± 2 °C) (Fig. 10). It was observed that, with increasing initial

F- concentration (C0), F- adsorption capacity (qe) of the adsorbent increased and then reached a

plateau. This might be due to more availability of F- ions at higher F- concentration (upto C0 =

500 mg L-1) for adsorption and the plateau forms due to the saturation of active sites of the

adsorbent surfaces at higher F- concentrations. The adsorption capacity of 40MgO@Al2O3 was

found to be higher than that of mesoporous Al2O3. The presence of MgO in 40MgO@Al2O3

enhanced the F- removal capacity of the adsorbents (for example, when initial F- concentration

was 1000 mg L-1, the qe values of mesoporous Al2O3 and 40MgO@Al2O3 were 23 mg g-1 and 37

mg g-1 respectively). Moreover, higher pHPZC of 40MgO@Al2O3 (pHPZC= 11.2) in comparison

with pure mesoporous Al2O3 (pHPZC= 8.2) may also contribute towards its higher adsorption

capacity.

<Figure 10>

3.7. Adsorption Isotherms

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After conducting equilibrium studies to determine the optimum conditions for maximum

F- removal of mesoporous Al2O3 and 40MgO@Al2O3, the obtained equilibrium data were

analyzed by fitting them in linear isotherm model equations, viz. Langmuir (eq 8) [42],

Freundlich (eq 9) [43]. Langmuir model indicates the monolayer adsorption on uniform

homogeneous surface having identical sites, whereas, Freundlich model indicates the

heterogeneity of the adsorbent surface and considers multilayer adsorption.

The linear form of the Freundlich equation is:

log qe = log Kf + (1/n) log Ce (8)

where Ce is equilibrium concentration (mg L-1), qe is amount of F- adsorbed at equilibrium (mg g-

1). 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 plot of logqe vs. logCe.

The linear form of the Langmuir model is:

Ce/qe = 1/(Q0b) + Ce/Q0 (9)

where Ce is equilibrium concentration (mg L-1), qe is amount of F- adsorbed at equilibrium (mg g-

1), Q0 is adsorption capacity for Langmuir isotherm and b is the Langmuir constant related to

sorption energy. The values of Q0 and b were calculated from the slope and intercept of the linear

Langmuir plot of Ce/qe vs. Ce.

The fitted parameters obtained from these models are summarized in Table 3. After

fitting the F- adsorption data to the Freundlich and Langmuir isotherm models following

important points were noted: (i) in case of mesoporous Al2O3, adsorption data for F- ion

concentration range 5 to 1000 mg L-1, fitted well with Freundlich model (R2(Freundlich) = 0.9690

and R2(Langmuir) = 0.7391), whereas data for 40MgO@Al2O3 fitted well with both Freundlich

(R2=0.9639) as well as Langmuir (R2 = 0.9752) model (Fig. 11). When the adsorption data for

low F- concentration range (5-100 mg L-1) were considered, the data for mesoporous Al2O3 fitted

well with Freundlich model (R2(Freundlich) = 0.9609 whereas R2

(Langmuir) = 0.7902) and for

40MgO@Al2O3, F- adsorption data fitted well with both Freundlich (R2 = 0.9627) as well as

Langmuir (R2 = 0.9756) model. So, F- adsorption on 40MgO@Al2O3 can be considered to be

favorable and single layered [44-46]. (ii) Kf value of 40MgO@Al2O3 was greater than that of

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Al2O3. This indicates adsorption capacity of 40MgO@Al2O3 is higher than that of mesoporous

Al2O3. (iii) 1/n value of 40MgO@Al2O3 is less than unity as well as less than the 1/n value of

pure Al2O3. This fact also indicates that, favorable adsorption of F- on 40MgO@Al2O3. (iv)

within the experimental conditions, maximum adsorption capacity of 40MgO@Al2O3 is 37.45

mg g-1 which is higher than that of pure mesoporous Al2O3 (23.35 mg g-1).

Another essential feature of the Langmuir model can be given in terms of dimensionless

separation factor ‘r’, which was defined by Weber and Chakravorti [47] as

r = 1/(1+bC0) (10)

where, C0 and b are the initial F- concentration and a Langmuir constant, respectively. If

the value of r is < 1, it signifies the favorable adsorption whereas, r > 1 indicates the unfavorable

adsorption [48]. Since, in the present case ‘r’ values were found to be < 1 (varies from 0.89 to

0.04 with the variation of C0 from 5 to 1000 mg L-1), it was assumed that favorable adsorption of

F- occurred on mesoporous Al2O3 and 40MgO@Al2O3.

<Figure 11>

<Table 3>

3.8. Effect of initial pH

The effect of initial pH of the solution on F- removal by mesoporous Al2O3 and

40MgO@Al2O3 was investigated at different pH ranging from 4 to10, with a constant adsorbent

dose of 3 g L-1, initial F- concentration 30 mg L-1, contact time 8 h, temperature (30 ± 2 ° C) and

shown in Fig. 12. It was observed that, in case of mesoporous Al2O3, F- adsorption did not affect

much within the pH range from 4-9. But at pH 10, its F- adsorption capacity was decreased. This

might be due to pHZPC value of mesoporous Al2O3 is 8.2 which is < 10. F- adsorption capacity of

40MgO@Al2O3 did not change much within the pH range of 4-10.

<Figure 12>

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3.9. Effect of co-existing anions

Beside F- ions the natural ground water always contains various other ions, which may

compete with F- during adsorption and affect the efficiency of the adsorbent. To study the effect

of co-existing anions on F- adsorption of synthesized adsorbents, 10 mg L-1 and 100 mg L-1 initial

concentrations of Cl-, NO3-, SO4

2- and HCO3- were used while keeping the initial F-

concentration as 10 mg L-1 and adsorbent dose 3 g L-1. The effect of competing anions (such as

Cl-, NO3-, SO4

2-, and HCO3-) on F- adsorption of mesoporous Al2O3 and 40MgO@Al2O3

adsorbents is shown in Figure 13. It was observed that, within the experimental condition

presence of Cl-, NO3-, SO4

2-, and HCO3- ions did not affect much (< 7 %) the F- removal capacity

of 40MgO@Al2O3 as well as Al2O3.

<Figure 13>

4. Conclusion

Here, preparation of MgO nanoparticle loaded mesoporous Al2O3 based adsorbents by

using a simple aqueous solution based method was reported. It was observed that loading of 40

wt% MgO nanoparticle within the porous matrix of mesoporous Al2O3 significantly enhanced

the F- removal capacity of the adsorbent (40MgO@Al2O3) in comparison with pure mesoporous

Al2O3 [19], commercial Al2O3 [18] as well as pure MgO nanoparticle [24]. 40MgO@Al2O3

exhibited its capability to remove ~90% F- from an aqueous F- solution having initial F-

concentration 10 mg L-1. Generally, F- concentration in contaminated ground water is ~ 5- 10 mg

L-1. It was observed that, when the solutions having F- concentration of 5 mg L-1 and 10 mg L-1

was treated with 40MgO@Al2O3, the F- concentration in treated water became < 1 mg L-1, which

is well below the recommendation of WHO. The novelty of mesoporous 40MgO@Al2O3

adsorbent lies in its (i) simple and cost effective preparation method, (ii) its high F- adsorption

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capacity and (iii) fast rate of adsorption. These features make mesoporous 40MgO@Al2O3 a

potential candidate as an adsorbent in F- removal devices.

Acknowledgment

Authors gratefully acknowledges financial support from Board of Research in Nuclear

Science (BRNS), India (Sanc no: 2010/37C/2/ BRNS/ 827).

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Figure Captions:

Fig. 1. XRD pattern of (a) Al2O3, (b) 10MgO@Al2O3, (c) 20MgO@Al2O3, (d) 30MgO @Al2O3,

(e) 40MgO@Al2O3, (f) 50MgO@Al2O3.

Fig. 2. (i) N2-adsorption desorption isotherms and (ii) Pore size distributions of pure alumina and

MgO loaded aluminas. (a) Al2O3, (b) 10MgO@Al2O3, (c) 20MgO@Al2O3, (d) 30MgO

@Al2O3, (e) 40MgO@Al2O3, (f) 50MgO@Al2O3.

Fig.3. HRTEM images of (a) Al2O3, (b) 40MgO@Al2O3 (MgO nanoparticles are shown within

the circle), (c) 40MgO@Al2O3 after F- adsorption and (d) EDS spectra of 40MgO@Al2O3 after

F- adsorption.

Fig. 4. Plot for determination of pHPZC of Al2O3 and 40MgO@Al2O3.

Fig. 5. Effect of MgO loading on mesoporous Al2O3 for removal of fluoride (C0 = 30 mg L-1,

adsorbent dose = 3g L-1, contact time = 8h, pH = 6.8 ± 0.2).

Fig. 6. Effect of adsorbent dose on fluoride adsorption capacity of adsorbents (C0 = 10 mg L-1,

contact time = 8h, pH = 6.8 ± 0.2).

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Fig. 7. Effect of contact time on fluoride adsorption capacity of adsorbents (C0 = 30 mg L-1,

adsorbent dose = 3g L-1, pH = 6.8 ± 0.2).

Fig. 8. Adsorption kinetic curves of fluoride adsorption on 40MgO@Al2O3 at different initial

fluoride concentrations (C0 = 5, 10, 20 and 30 mg L-1, adsorbent dose = 3g L-1, pH = 6.8 ±

0.2).

Fig. 9. (i) Pseudo first order and (ii) Pseudo second order adsorption kinetic model for fluoride

adsorption on 40MgO@Al2O3 (C0 = 5, 10, 20 and 30 mg L-1, adsorbent dose = 3g L-1, pH = 6.8 ±

0.2).

Fig. 10. Effect of initial fluoride concentration on fluoride adsorption capacity of Al2O3 and

40MgO@Al2O3 (adsorbent dose = 3g L-1, contact time = 8h, pH = 6.8 ± 0.2).

Fig. 11. (i) Freundlich adsorption isotherm models and (ii) Langmuir adsorption isotherm models

for adsorption of fluoride on Al2O3 and 40MgO@Al2O3. (C0 = 5 mg L-1 to 1000 mg L-1,

adsorbent dose = 3g L-1, contact time = 8h, pH = 6.8 ± 0.2) (The inset is isotherm models

at low F- concentration ranging from 5 mg L-1 to 100 mg L-1).

Fig. 12. Effect of initial pH on fluoride adsorption capacity of Al2O3 and 40MgO@Al2O3

(adsorbent dose = 3g L-1, C0 = 30 mg L-1, contact time = 8h).

Fig. 13. Effect of co-existing anions on fluoride adsorption capacity of Al2O3 and

40MgO@Al2O3 (adsorbent dose = 3g L-1, C0 = 10 mg L-1, contact time = 8h).

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Table Captions:

Table 1 Surface area and pore size parameters of the synthesized adsorbents obtained by means

of N2 adsorption-desorption study.

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 40MgO@Al2O3.

Table 3 Langmuir and Freundlich isotherm parameters for fluoride adsorption on Al2O3 and

40MgO@Al2O3 at pH of 6.8 ± 0.2 and temperature = (30 ± 2) °C.

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Table 1.

SampleBET Surface area

(m2/g)

pore size (nm)

BJH pore volume(cm3/g)

Al2O

3 264 6.5 0.62

5MgO@Al2O3 216 6.6 0.55

10MgO@Al2O3 198 6.7 0.51

20MgO@Al2O3 152 6.6 0.41

30MgO@Al2O3 134 6.9 0.26

40MgO@Al2O3 105 3.8 0.23

50MgO@Al2O3 80 3.7 0.16

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Table 2.

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

5 1.38 0.49 0.0055 0.8740 1.40 0.0315 0.9976

10 1.80 1.12 0.0114 0.7113 1.89 0.0191 0.9943

20 2.24 1.43 0.0094 0.8488 2.35 0.0131 0.9919Al2O3

30 2.89 1.31 0.0107 0.7336 3.00 0.0169 0.9985

5 1.43 1.73 0.0101 0.9258 1.56 0.0044 0.9970

10 2.9 2.4 0.0067 0.9567 3.12 0.0036 0.9939

20 5.45 4.85 0.0076 0.9573 5.72 0.0018 0.995140MgO@Al2O3

30 7.35 6.4 0.0067 0.9812 7.83 0.0010 0.9962

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Table 3

Langmuir isotherm Freundlich isothermQ0

(mg g-1 )b

(l mg-1)R2 Kf 1/n R2

Al2O3 24.45 0.004 0.7391 0.56 0.51 0.9690

40MgO@Al2O3 37.35 0.0227 0.9752 3.33 0.38 0.9639

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Figure 13