Al2O3 nanoparticles synthesized using various oxidizing agents: Defluoridation performance

Accepted Manuscript Al2O3 nanoparticles synthesized using various oxidising agents: Defluoridation performance M. Changmai, J.P. Priyesh, M.K. Purkait PII:

S2468-2179(17)30072-2

DOI:

10.1016/j.jsamd.2017.09.001

Reference:

JSAMD 121

To appear in:

Journal of Science: Advanced Materials and Devices

Received Date: 24 May 2017 Revised Date:

16 August 2017

Accepted Date: 6 September 2017

Please cite this article as: M. Changmai, J.P. Priyesh, M.K. Purkait, Al2O3 nanoparticles synthesized using various oxidising agents: Defluoridation performance, Journal of Science: Advanced Materials and Devices (2017), doi: 10.1016/j.jsamd.2017.09.001. 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 proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Al2O3 nanoparticles synthesized using various oxidising agents: Defluoridation performance

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M. Changmai*, J.P. Priyesh and M. K. Purkait

Department of Chemical Engineering, Indian Institute of Technology Guwahati,

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Guwahati-781039, India.

* Corresponding author

Tel: + 91 - 361 - 2582262 Fax: +91 - 361 - 2582291 E. Mail: [email protected]

ACCEPTED MANUSCRIPT Abstract This study concerns the removal of fluoride using aluminium oxide nanoparticles synthesized in the presence of oxidising agents H2SO4, KMnO4 and K2Cr2O7 which had a significant effect on the size and shape of the nanostructure. The obtained nanoparticles were characterized using TGA, FESEM, EDX and XRD analysis. From the TGA data it was

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observed that the weight loss was almost constant from 400-650 oC, hence the experimental temperature range was set to be 400-650 oC. XRD analysis with and without oxidising agents indicated the crystalline behaviour to have increased with increasing temperature. Al2O3 nanoparticles thus

prepared had a considerable potential for fluoride adsorption from

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aqueous medium in the concentration range of 2-8 mg/l. Around 92 % fluoride was adsorbed at pH=4.7 for the range of fluoride concentration considered herein. The equilibrium data

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were well fitted with freundlich adsorption isotherm whereas the adsorption kinetic data followed the pseudo second order model.

Keywords: Al2O3; nanoparticles; characterization; fluoride; adsorption; regeneration 1. Introduction

Fluoride is one of the major water polluting components and occurs due to both natural and

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man-made reasons. It is highly reactive and is found naturally as CaF2. Fluoride ions infiltrate surface water and ground water mainly from soil leeching, precipitation, weathering of fluoride bearing rocks and human emissions. Although, minute quantity of fluoride is required for the formation of dental enamel and normal bone mineralization in the human

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body but excessive intake leads to slow, progressive crippling scourge known as fluorosis. Studies have estimated that fluorosis is prevalent in 17 states of India [1]. The safe limit of

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fluoride in drinking water is 1 mg/l [2]. India is one of 21 nations with serious health problems due to consumption of fluoride-contaminated drinking water. In India, the fluoride concentrations in drinking water generally vary from 1.5 to 39 ppm [3] and is estimated that more than 200 million people worldwide depend on drinking water with the fluoride content exceeding WHO guideline. The requirement of fluoride content changes from place to place and it depends on the geographical conditions and the age of human being. The fluoride content should be less than 1 mg/l in water according to the Bureau of Indian Standards (BIS). According to US standard, the fluoride content should be between 0.6 and 0.9 ppm [4]. The concentration and the duration of continuous intake determines whether the impact of fluoride in drinking water can be beneficial or detrimental to mankind. Fluoride generally 1

ACCEPTED MANUSCRIPT gets deposited in the joints of pelvic, knee, neck and shoulder bones and makes it difficult to move or walk .It may even lead to a rare bone cancer, spondylitis or arthritis osteo-sarcoma and finally spine, major joints, muscles and nervous system may get damaged [5]. Hence, it has become a necessity to reduce the fluoride content to the safe limit. Various techniques have been introduced to reduce the fluorine content i.e. chemical and physical methods.

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Chemical methods include electro-coagulation processes and coagulation precipitation whereas the physical methods include membrane separation and adsorption technique, mainly nano-filtration and reverse osmosis. Adsorption using a nano-adsorbent such as schwertmannite with adsorption capacity of 17.24 mg/g is one such technique which can be

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used for defluoridation [6,7]. Recent work on defluoridation using an iron oxide hydroxide suggested a considerable potential for fluoride removal of 11.3 mg/g from aqueous medium [8]. Water remediation using efficient nano-adsorbent is getting more importance since

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recently. A wide variety of nano-adsorbents have been produced till date for the removal of fluoride from water [9-11]. Alumina supported metal oxide nanoparticles are the most extensively used nano-adsorbent for the defluoridation process [12-14]. Metal oxide nanoparticles are quite promising in the fields of adsorption for their large surface area and porous structure along with short diffusion route [15]. Because of comparatively large surface

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areas, it is likely that nanosized adsorbents with strong affinity towards fluoride can be a useful tool in enhancing the adsorption capacity in drinking water treatment. However, due to their small particle size, isolation of nanosized adsorbents from matrices is difficult for practical application. Al2O3 nanoparticles were found to have a high affinity towards fluoride

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ions moreover, because of its cost-efficiency for large scale defluoridation process they have a preferred advantage over other adsorbents [16-18].

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In the present work, nano-alumina structure is prepared by the chemical treatment on the surface of aluminium foil in the range of 10 nm to 150 nm. The prepared nanoparticles had a considerable prospective to be utilized in fluoride removal. To improve the oxidation efficiency, oxidation agents such as H2SO4, KMnO4 and K2Cr2O7 in mild concentration (10 mM) were treated on the surface before annealing. It was found that each oxidizing agent resulted in the nanoparticles with different morphologies. With H2SO4 as oxidizing agent, flower like structure with nanochains were observed, K2Cr2O7 formed plate like structures and KMnO4 formed uniform nanoparticles on the foil surface without any chain formation. Fluoride adsorption efficiency was determined in batch mode. The adsorption studies were

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ACCEPTED MANUSCRIPT carried out considering various parameters such as contact time, initial fluoride concentration, adsorbent mass, pH, stirring speed and the effect of other ions. 2. Materials and methods 2.1. Materials

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Aluminium foil of 2 mm thickness was obtained from Hindalco Ltd., Sodium fluoride (NaF) from Titan biotech Ltd., India, HCl and NaOH from Merck, India. Other chemicals such as acetone, ethanol, H2SO4, KMnO4 (99% purity) and K2Cr2O7 (99.9% purity) were obtained from Merck, India. All the chemicals were of analytical grade and used without further

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purification. 2.2. Synthesis and characterization of Al2O3 nanostructure

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Aluminium foil of 2.5 cm × 2.5 cm were washed with acetone and ethanol to remove the organic impurities from the surface. The foils were then treated with 1 M HCl solution to eliminate the surface oxidation layer formed already on the surface by the reaction with air and washed with deionized water to remove HCl from the surface. The foil was annealed in presence of air for 3 h to produce Al2O3 nanostructures (Fig. 1). The experiment was repeated by changing annealing temperature as 400 oC, 450 oC, 500 oC and 550 oC. To improve the

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oxidation efficiency, oxidation agents such as H2SO4, KMnO4 and K2Cr2O7 in mild concentration were treated on the surface before annealing [19-22]. Figure 1 here

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2.3. Adsorption experiment

Batch adsorption was carried out to remove excess fluoride from the water using Al2O3 nano

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structures. Al2O3 nano structures prepared without oxidising agent at 450 oC and with oxidising agents H2SO4, K2Cr2O7 and KMnO4 at 550 oC were used as adsorbent. The initial concentrations of fluoride used are 2 mg/l, 4 mg/l and 8 mg/l with an adsorbent dose of 4 g/l. The removal efficiency R was calculated by R = (Co – Ce)/Co× 100%

(1)

2.4. Characterization techniques Thermo gravimetric analysis (TGA) (Make: Mettler Toledo) analysis with N2 gas flow rate of 40 ml/min and purge gas flow rate of 20 ml/min was carried out to determine the oxidation properties of the Aluminium foil. Field Emission scanning electron microscope (FESEM by 3

ACCEPTED MANUSCRIPT LEO 1430 vp at 3.00-5.00 KV) was used to examine the morphological structure and to measure the average particle size. The FESEM analysis of aluminium foil treated without any oxidising agents showed the formation of nanostructures. A wide angle X- Ray diffractometer (Bruker D8) was used to study the crystallite structure of the nanoparticles. Energy-dispersive X-ray spectroscopy (EDX) technique was used for the elemental analysis

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or chemical characterization of the sample and confirmed the formation of Al2O3 nanoparticle and only while using H2SO4 sulphur remained in the nanoparticle. 3. Results and discussion

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3.1. Characterization of Al2O3 nanostructures

To determine the oxidising characteristics of Al foil Thermo gravimetric analyser (TGA) was carried out. Oxidation of pure aluminium foil at elevated temperature in presence of air was

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done in temperature range of 28-650 oC. Highest temperature was restricted to 650 oC, because the melting point of Al foil is 665 oC. The heating rate was set at 20 oC/min. The % weight loss from 28-120 oC was due to moisture removal with a weight loss of 8-9 % which then linearly decreased to 400 oC with weight loss of 16 %. From 400-650 oC the weight loss was very less (Fig. 2). For the preparation of nanostructures optimum conditions are required.

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If the oxidation rate are very high, foil will oxidise completely and form bulk material, on the other hand if the oxidation rates are very low, only surface oxidation will occur and an oxidation layer will form on the surface. From the data, it was observed that at temperature of 400-650 oC, the weight loss almost remained constant confirming the attainment of thermal

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stability. So the experimental temperature range was set to be 400-650 oC. Figure 2 here

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The morphology and size of Al2O3 nanostructures after annealing at 400 oC, (Fig. 3a), were examined by FESEM which showed only surface oxidation without the formation of nanostructure. The surface was totally covered by oxidation layer which was an onset of the initialisation of nanoparticle formation with the symmetry of the oxidation layer being round structures joined together. Al foil treated without oxidising agent at 450 oC, (Fig. 3b) clearly showed the formation of nanostructure. The nanoparticle size range was about 10-80 nm and average chain length was 1200 nm. At 500 oC (Fig. 3c) and 550 oC (Fig. 3d) not only the surface but the entire aluminium foil was oxidised. At 500 oC along with bulk structures nano-chain like structures were also visible with average particle size of about 100 nm. Figure 3 here

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ACCEPTED MANUSCRIPT The oxidation rate was adjusted by adding oxidizing agents H2SO4, KMnO4 and K2Cr2O7. FESEM results showed the formation of various types of nanoparticles when treated with different oxidizing agents (H2SO4, KMnO4 and K2Cr2O7) and different annealing temperature (450 and 550 oC). Based on standard potential values of 0.45 V, 1.33 V and 1.51 V for H2SO4, K2Cr2O7 and KMnO4 respectively, KMnO4 is the best oxidizing agent. Hence stronger

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the oxidizing agents smaller the nanoparticles formed. For Aluminium foil annealed at 450 oC (Fig. 4a) with H2SO4 the structures formed were large and the arrangement was similar to flower like structure along with nano-chain formation. The nanoparticle size ranged from 50150 nm and the average chain length was about 1800 nm. Nano-rods were also formed on the surface with nano-chain which were spread all over the surface. The width of the nano-chain

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was about 20 nm and length about 200 nm. At 550 oC, (Fig. 4b), the Al foil was oxidised with traces of nanoparticle and chains. For Al foil treated with K2Cr2O7 at 450 oC, (Fig. 4c),

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the nanoparticle size ranged from 10-120 nm with average length of 600 nm. At 550 oC, (Fig. 4d), the average size of the nanoparticle ranged from 15-180 nm. The chains had plate like structures with average chain length of 2000 nm. Al foil were also treated with KMnO4 at 450oC and 550 oC. At 450 oC (Fig. 4e), the size range of nanoparticle was from 25-130 nm and instead of chains the nanoparticles were formed uniformly throughout the surface as a

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layer. At 550 oC, (Fig. 4f), KMnO4 being a stronger oxidizing agent than K2Cr2O7 resulted in the formation of smaller nanoparticles with a size range of 10-110 nm and an average chain length of 600 nm. In this case also nanoparticles were formed throughout the surface and the structure was almost round everywhere.

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Figure 4 here

Energy-dispersive X-ray spectroscopy (EDX) was done to confirm the Al2O3 formation on

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the surface and to check if there were any impurities present in the particle from the oxidising agent for the sample at 550 oC. As seen from Table 1 EDX confirmed the presence of the elements Al and O. It could also be observed that in the presence of oxidizing agents the weight % of Al decreased and that of O increased. Table 1 here X-Ray diffraction analysis (XRD) was done for the samples prepared at 450 oC and 500 oC with and without oxidising agents. It was found that either in the presence or in the absence of oxidizing agents, the crystalline behaviour increased with increasing temperature due to the development of large crystallite and a higher degree of crystallinity [38]. Peaks

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ACCEPTED MANUSCRIPT corresponding to Al foil were observed as (111), (200), (220) and (311). Similarly peaks corresponding to Al2O3 were observed as (311), (222), (400), (511) and (444). The intensity of the peaks increased in the presence of oxidizing agents and increased reaction temperature. The reason being an increase in crystalline phase with increasing temperature. Three phases α, γ and θ were found to be present in the prepared sample when compared with literature

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[14,20-21].

3.2. Adsorption Studies

3.2.1. Effect of Contact Time and Initial Fluoride Concentration

Al2O3 nano structures prepared without oxidising agent at 450 oC and with oxidising agents

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H2SO4, K2Cr2O7 and KMnO4 at 550 oC were used as adsorbent. The initial concentrations of fluoride used are 2 mg/l, 4 mg/l and 8 mg/l with an adsorbent dose of 4 g/l. The rapid

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adsorption of the fluoride took place within 20 min after which adsorption became slow and almost reached equilibrium within 90 min. Further increase in contact time for 24 h increased the fluoride removal only by less than 1 %. Nanostructures without oxidising agent at 450 oC with initial fluoride concentration of 2 mg/l offered more than 90 % removal and 74.87 % removal obtained for 8 mg/l. Percentage fluoride removal by adsorption of nanoparticle

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prepared by oxidising agents H2SO4, K2Cr2O7 and KMnO4 at 550 oC showed fluoride removal for the initial concentration of 2 g/l as 88.5 %, 91 % and 92 % respectively for the adsorbent (Fig. 5). And for the initial concentration 8 g/l, the % fluoride removal were 82.75 %, 87.87 % and 88.12 % respectively. It was observed that the adsorption was fast for lower

Figure 5 here

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initial fluoride concentration.

3.2.1. Effect of Adsorbent Mass With increase in the Al2O3 nanoparticle dose, at 500 oC without any oxidizing agent, from 1 g/l to 4 g/l, the residual fluoride concentration was decreased and permissible limit (1 mg/l) was achieved. 4 g/l of alumina was required to maintain the permissible limit for 2 mg/l, 4 mg/l and almost for 8 mg/l initial fluoride concentration (Fig. 6). Up to an adsorbent dose of 1 g/l, the residual fluoride concentration decreased sharply beyond which it was almost constant for 2 mg/l and 4 mg/l. This sharp decrease in fluoride concentration was due to the greater surface area and availability of more adsorption sites. The number of active sites and the bulk fluoride concentration were decreased and reached in equilibrium with increase in

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ACCEPTED MANUSCRIPT time. Therefore, the amount of residual fluoride concentration will be almost same with further increase in adsorbent dose. Figure 6 here 3.2.3. Effect of pH The equilibrium sorption of fluoride with initial fluoride concentration of 8 mg/l was

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investigated over a pH range of 2.67 to 11.28 and operating temperature of 25 oC to determine the optimum pH for maximum removal of fluoride (Fig 7 (inset)). From the figure it was clear that the fluoride adsorption on alumina was strongly dependent on pH. The maximum fluoride removal of 88.12 % took place at pH=4.7. The fluoride adsorption

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increased with pH, reached maximum at pH=4.7 and then decreased slowly up to pH=9.31. Beyond pH=9.31, the percentage removal decreased sharply as seen in (Fig. 7). As pH goes

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on increasing the amount of OH- ions in the solution would also go on increasing. Since our ion concerned for removal is F-, hence as pH would go on increasing the repulsion between Fand OH- ions would increase resulting in a decrease in fluoride removal. Al2O3 remains stable at pH=11 whereas it starts to dissolve at a pH lower than 3 [10,11-14].

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Figure 7 here

3.2.4. Effect of Stirring Speed

Stirring is an important parameter in adsorption phenomena, which provides the proper

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contact between the adsorbent and the solution. It helps to distribute the solute in bulk solution and also aids the formation of an external boundary film. We utilized the shaking incubator from Labtech to study the effect of stirring speed in our samples. Stirring speed of

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100, 200 and 300 rpm were used with contact time of 90 min. KMnO4 treated nanoparticle was used with an adsorbent dosage of 4 g/l at 25 oC. With increase in stirring speed from 100 to 300 rpm the percentage fluoride removal changed from 85.87-87 %, respectively. The increase in the fluoride removal with increase stirring speed was explained by the fact that increase in stirring speed reduced the film boundary layer surrounding the adsorbent, thus increasing the external film transfer coefficient and hence better fluoride adsorption. 3.2.5. Effect of Other Ions Natural fluoride contaminated water contains several other ions such as nitrate (KNO3), chloride (NaCl), sulphate (Na2SO4), carbonate (Na2CO3) and bicarbonate (NaHCO3) which 7

ACCEPTED MANUSCRIPT can affect the fluoride ion adsorption process. To study the effects of Na ions, 300 mg/l of NaCl solution for each ion were added separately to the fluoride solution. The adsorption experiments were performed in 8 mg/l fluoride solution at 25 oC with 4 g/l of adsorbent prepared with KMnO4 as oxidizing agent. It was observed that the effect of chloride, nitrate and sulphate ions were negligible and had the equilibrium fluoride removal were 87.62, 86

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and 85.32 % respectively. This decrease in fluoride removal is due to the competition of these anions for the adsorption with fluoride. The percentage fluoride removal decreased to 32.5 and 26.12 % by the effect of bicarbonate and carbonate ions respectively, and it was due to the significant increase in pH of the solution. With increasing pH, OH- ions would increase

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resulting in repulsion of F- ions thereby reducing the % fluoride removal. 3.3. Thermodynamic Study

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To study the effect of temperature on the percentage fluoride removal, experiments were conducted with the operating temperatures 25 oC, 35 oC, 45 oC and 55 oC. The experiment was conducted maintaining an initial fluoride concentration 8 mg/l and an adsorbent dose of 4 g/l of Al2O3 prepared with KMnO4. It was observed that, with increase in temperature from 25-55 oC, the percentage removal increased from 88.12-89.2 % which indicated the endothermic behaviour of the adsorption.

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The thermodynamic parameters such as change in standard free energy, enthalpy, and entropy were calculated to study the spontaneous nature and the thermodynamic feasibility of the process

[23,24].

These

parameters

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calculated

by

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following

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equations ∆G 0 = ∆H 0 − T∆S 0 .The enthalpy of adsorption (∆H ) and entropy ( ∆S 0 ) can be calculated from the slope and intercept of

) versus

plot where, m is the

adsorbent dose (g/l

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),qe is the amount of arsenic adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration (mg/L), T is the temperature in Kelvin, qe/Ce is called the adsorption affinity (Fig. 8).

The thermodynamic parameters at initial fluoride concentration of 8 mg/l is summarized in Table 2. The positive enthalpy of adsorption ( ∆H 0 ) and the negative Gibbs free energy change ( ∆G 0 ), indicated the process to be endothermic and spontaneous in nature. The low value of ∆S 0 implies that no remarkable change in entropy occurred during the fluoride adsorption process. The positive value of ∆S 0 reflects an increase in randomness at the solidsolution interface during the adsorption. The low value of enthalpy of adsorption ( ∆H 0 ) 8

ACCEPTED MANUSCRIPT indicated that the physical adsorption process dominated the fluoride adsorption process. In order to further confirm the assertion that the physical adsorption is the predominant mechanism in the fluoride adsorption process, the values of activation energy (Ea) and sticking coefficient (S*) were determined from the experimental data by using a modified Arrhenius type equation related to surface coverage (θ) as follows

.The

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Sticking coefficient S * is a function of the adsorbate-adsorbent system under investigation and its value lies in the range 0< S * <1 and is completely dependent on temperature of the system. As the name specifies, the parameter S * indicates the potentiality of an adsorbate to remain on the adsorbent system. The θ can be calculated by using following equation

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Activation energy ( Ea ) and sticking coefficient ( S * ) were estimated from the plot of ln(1 − θ ) versus 1/T and were shown in Table 2. The lower value of ( Ea ) & ( S * )

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confirmed the physisorption behaviour of the present adsorption system [27,34-35]. Figure 8 here Table 2 here

3.4. Kinetic Study

The kinetics of fluoride adsorption on the surface of the Al2O3 nanostructures were studied

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using different models such as pseudo-first-order and pseudo-second-order [23,24]. Four different Al2O3 nanoadsorbents were prepared without oxidising agent at 450 oC, with oxidising agents H2SO4, K2Cr2O7 and KMnO4 at 550 oC. The pseudo-first order model

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assumes that the rate of change of solute uptake with time is directly proportional to the amount of solute adsorbed with time and the difference in equilibrium concentration. The rate constant of adsorption is expressed as a first-order rate expression given as

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where qt and qe are the amount of fluoride adsorbed (mg/g) at

contact time t (min) and at equilibrium, k1 is the pseudo-first-order rate constant (g/mg min). The adsorption rate constant k1 and equilibrium adsorption capacity qe can calculated from the plot of ln(qe − qt ) versus t. From these calculated values it is clear that the kinetics of fluoride adsorption on the Al2O3 nanoparticles was not following the pseudo first order kinetics and hence not diffusion controlled phenomena. The sorption kinetics may be represented by pseudo-second-order model as

,where, k 2 is the

equilibrium rate constant for pseudo-second order sorption (g/mg min).The values of qe and k2 were calculated from the slope and intercept of t/qt versus t plot (Fig. 9) shows the plots of 9

ACCEPTED MANUSCRIPT t/qt versus t plots for different adsorbents. The calculated values of k2, qe and R2 were showed in Table 3. The values of the regression coefficient R2 was nearly unity (0.99) for all the adsorbents at all the initial fluoride concentrations. These values confirmed that the kinetics of fluoride adsorption followed pseudo-second order process. Hence, it was concluded that the adsorption process was favoured by the pseudo-second order kinetic model.

Table 3 here

3.5 Equilibrium Study

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Figure 9 here

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It is essential to establish the most appropriate correlation for the equilibrium curve to optimize the design of an adsorption system. The commonly used and most useful equilibrium isotherms are the langmuir and the freundlich isotherms [23,24]. Langmuir

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isotherm is based on the assumption that there are a finite number of active sites which are homogenously distributed on the adsorbent surface. These binding sites on the surface of the adsorbent have the same affinity for the adsorption of a single molecular layer, and there is no interaction between adsorbed molecules. The equation of Langmuir isotherm is , where qe the amount is adsorbed at

represented as follows

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equilibrium (mg/g), Ce is the equilibrium concentration of the arsenic (mg/l), constant b is related to the energy of adsorption (l/mg), and qm is the Langmuir monolayer adsorption capacity (mg/g). The parameters can be determined from the plot of 1 / qe versus 1/ Ce (Fig.

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10). To check feasibility of the isotherm, the dimensionless equilibrium parameter RL was determined by the following equation

, where b (l/mg), is the Langmuir

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constant and Co (mg/l) is the initial concentration in the liquid phase. The value of

RL indicates the shape of the isotherm to be either unfavourable ( RL >1), linear ( RL =1), favourable (0< RL < 1) or irreversible ( RL =0). For the present study RL values obtained are in the range of 0.36-0.10 for the initial fluoride concentration of (2-10 mg/l). The RL value indicated that the fluoride adsorption was more favourable for higher initial fluoride concentrations than the lower ones. The freundlich isotherm model is based on the multilayer adsorption of an adsorbate onto the heterogeneous surface of an adsorbent. The expression , (Fig. 10) where K F is the freundlich

for freundlich isotherm is given as

constant is related to the bonding energy, and 1 / n is a measure of intensity of the adsorption. Higher the 1 / n value, more favourable is the adsorption. From the Table 4, it is clear that 10

ACCEPTED MANUSCRIPT both the models showed high regression correlation coefficient (R2= 0.99 & 0.99), so both Langmuir model and freundlich models can be used for describing adsorption equilibrium of fluoride. From the langmuir and freundlich adsorption isotherm data, it is clear that both isotherms are comparable with experimental values [27,29-31,34,35].

Table 4 here Table 5 here 4. Regeneration Study

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Figure 10 here

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The regeneration study has been done to develop an efficient adsorbent that can be reused thereby making it cost effective. KMnO4 treated nanoparticle was used as the adsorbent. 8 mg/l fluoride was adsorbed on 4 g/l of adsorbent, the adsorbent was transferred to 100 ml

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water and pH was adjusted to acidic pH=4.7. In the acidic pH range hardly any fluoride was leached, as the pH increases above 4.7, more than 75 % of fluoride was desorbed in about 90 min. The desorbed adsorbent thus obtained was washed with 0.1 M HCl to make it acidic in nature and to activate it for further reuse in adsorption. 5. Conclusion

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Al2O3 nanostructures were synthesized on the surface of aluminium foil in the range of 10-80 nm. In presence of oxidizing agents and an increasing temperature, the size of the nanoparticles increased. H2SO4, KMnO4 and K2Cr2O7 gave particle sizes of around 150, 130 and 150 nm respectively. EDX analysis confirmed that Al2O3 nanostructures were formed on

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the surface of aluminium foil. By the temperature analysis it was clear that the suitable temperature was 450 oC for nanostructures prepared without any oxidising agent and with

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H2SO4, whereas with the oxidising agents K2Cr2O7 and KMnO4, the suitable temperature obtained was 550 oC. Moreover, nanoparticle density was high in presence of KMnO4 as the oxidizing agent at 550 oC. The current study highlighted that the Al2O3 nanoparticles which were acidic in nature had a considerable potential for fluoride adsorption from aqueous medium with a maximum of 92 % fluoride being adsorbed by Al2O3 nanoparticles at pH=4.7. The regeneration study were carried out to study the reusability of the used activated carbon adsorbent and results suggested desorption of fluoride as high as 75 %.

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leachate using ion exchange, Water Environ. Res., 71 (1999), pp. 36–42. [18] S. Saha, Treatment of aqueous effluent for fluoride removal, Water Resour., 27 (1993), pp. 1347–1350.

[19] D. Clifford, J. Matson, R. Kennedy, Activated alumina: Rediscovered adsorbent for fluoride, humic acids and silica, Ind. Water Eng., 15 (1978), pp. 6–12.

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[20] V. S. Giri, R. Sarathi, S.R. Chakravarthy, C. Venkataseshaiah, Studies on production and characterization of nano-Al2O3 powder using wire explosion technique, Mater. Lett., 58 (2004), pp. 1047– 1050.

[21] J. Li, Y. Pana, C. Xianga, Q. Gea, J. Guoa, Low temperature synthesis of ultrafine α-

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Al2O3 powder by a simple aqueous sol–gel process, Ceram. Int., 32 (2006), pp. 587–591. [22] S. Anandan, X. Wen, S. Yang, Room temperature growth of CuO nanorod arrays on

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copper and their application as a cathode in dye-sensitized solar cells, Mater. Chem. Phys., 93 (2005), pp. 35–40.

[23] C.C. Tang, S.S. Fan, P. Li, M. Lamy, H.Y. Dang, In situ catalytic growth of Al2O3 and Si nanowires, J. Cryst. Growth, 224 (2001), pp. 117–121. [24] X. Zhichuan, B. Zhuanfang, S. Chengmin, A facile fabrication of Cu2O nanowire arrays on Cu substrates, Cent. Eur. J. Eng., 2 (3) (2012), pp. 364-368. [25] M.G. Sujana, H.K. Pradhan, S. Anand, Studies on sorption of some geomaterials for fluoride removal from aqueous solutions, J. Hazard. Mater., 161 (2009), pp. 120–125. [26] M.G. Sujana, S. Anand, Fluoride removal studies from contaminated ground water by using bauxite, Desalination, 267 (2011), pp. 222–227.

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ACCEPTED MANUSCRIPT [27] V. Gopal, K.P. Elango, Equilibrium, kinetic and thermodynamic studies of adsorption of fluoride onto plaster of Paris, J. Hazard. Mater., 141(1) (2007), pp. 98-105. [28] A. Tor, N. Danaoglu, G. Arslan, Y. Cengeloglu, Removal of fluoride from water by using granular red mud: Batch and column studies, J. Hazard. Mater., 164 (1) (2009), pp. 271–278

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[29] A. Goswami, M.K. Purkait, Kinetic and equilibrium study for the fluoride adsorption using pyrophyllite, Separ. Sci. Technol., 46 (2011), pp. 1797-1807.

[30] S. Kagne, S. Jagtap, P. Dhawade, S.P. Kamble, S. Devotta, S.S. Rayalu, Hydrated cement: A promising adsorbent for the removal of fluoride from aqueous solution, J.

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Hazard. Mater., 154 (2008), pp. 88–95.

[31] S. Ghorai, K.K. Pant, Equilibrium, kinetics and breakthrough studies for adsorption of fluoride on activated alumina, Sep. Purif. Technol., 42 (2005), pp. 265–271

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[32] S.M. Maliyekkal , A.K. Sharma, L. Philip, Manganese-oxide-coated alumina: A promising sorbent for defluoridation of water, Water Res., 40 (2006), pp. 3497–3506. [33] A. Elhalil, S. Qourzal, F.Z. Mahjoubi, R. Elmoubarki , M. Farnane, H. Tounsadi, M. Sadiq, M. Abdennouri , N. Barka, Defluoridation of groundwater by calcined Mg/Al layered double hydroxide, Emerging Contaminants, 2 (2016), pp. 42–48.

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[34] P. K. Raul, R. R. Devi, I. M. Umlong, S. Banerjee, L. Singh, M. Purkait, Removal of fluoride from water using iron oxide-hydroxide nanoparticles, J. Nanosci. Nanotechnol., 12 (2012) ,pp. 3922.

[35] M. Changmai, M.K. Purkait, Kinetics, equilibrium and thermodynamic study of

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phenol adsorption using NiFe2O4 nanoparticles aggregated on PAC, J. Water Proc. Engg. 16 (2017), pp. 90–97.

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[36] S. Karaca, A. Gurses, M. Ejder, M. Acikyildiz, Adsorptive removal of phosphate from aqueous solutions using raw and calcinated dolomite, J. Hazard. Mater. 128 (2006), pp. 273-279.

[37] A. Goswami, M.K Purkait, The defluoridation of water by acidic alumina, Chem. Eng. Res. Design 90 (2012), pp. 2316–2324. [38] M. Changmai, M.K. Purkait, Interaction of fatty acid chain length with NiFe2O4 nanoparticles, Surf. Interfaces 8 (2017), pp. 45–50

14

ACCEPTED MANUSCRIPT Table 1: Atomic and weight percentage values from EDX data

Element

Weight %

Atomic %

Al foil after annealing at 550oC without any oxidizing agent

Oxygen Aluminium

33.60 66.40

Al foil after annealing at 550oC with H2SO4

Oxygen Aluminium Sulphur

41.94 27.40 30.36

Al foil after annealing at 550oC with K2Cr2O7

Oxygen Aluminium

42.61 57.39

55.60 44.40

Al foil after annealing at 550oC with KMnO4

Oxygen Aluminium

52.68 47.32

65.25 34.75

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Sample

46.05 53.95

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SC

57.05 22.34 20.61

15

ACCEPTED MANUSCRIPT Table 2: Thermodynamic parameters for adsorption of fluorine

∆Ho

∆SO

S*

Ea

- ∆Go

(oC)

(KJ/mol)

(KJ/mol)

(KJ/mol)

(KJ/mol)

(KJ/mol)

25

3.053

26.94

0.039

2.707

8.025

35

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45

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Temperature

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55

16

8.294 8.563 8.833

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Table 3: Kinetic model parameters with adsorbent treated without oxidising agent and with oxidising agents H2SO4, K2Cr2O7 and KMnO4 respectively

Psuedo first order

ln(qe-qt)=lnqe-k1t

qe,expt (mg/g) qe,cal (mg/g) k1 (min-1) R2

None

Psuedo second order

t/qt=1/k2qe+t/qe

qe,cal (mg/g) k2 (min-1) R2

0.41 2.79 0.99

0.80 1.02 0.99

1.52 0.37 0.99

ln(qe-qt)=lnqe-k1t

qe,expt (mg/g) qe,cal (mg/g) k1 (min-1) R2

0.44 0.11 0.06 0.99

0.86 0.26 0.07 0.99

1.65 0.73 0.07 0.99

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t/qt=1/k2qe+t/qe

qe,cal (mg/g) k2 (min-1) R2

0.44 3.84 0.99

0.86 1.90 0.99

1.66 0.59 0.99

K2Cr2O7

Psuedo first order

ln(qe-qt)=lnqe-k1t

qe,expt (mg/g) qe,cal (mg/g) k1 (min-1) R2

0.45 0.10 0.07 0.79

0.89 0.19 0.06 0.75

1.75 0.67 0.07 0.89

K2Cr2O7

Psuedo second order

qe,cal (mg/g) k2 (min-1) R2

0.45 5.12 0.99

0.89 2.70 0.99

1.76 0.64 0.99

KMnO4

Psuedo first order

ln(qe-qt)=lnqe-k1t

qe,expt (mg/g) qe,cal (mg/g) k1 (min-1) R2

0.46 0.13 0.07 0.82

0.90 0.30 0.08 0.88

1.76 0.65 0.05 0.83

t/qt=1/k2qe+t/qe

qe,cal (mg/g) k2 (min-1) R2

0.46 3.69 0.99

0.90 1.85 0.99

1.77 0.46 0.99

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Psuedo second order

KMnO4

t/qt=1/k2qe+t/qe

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H2SO4

Psuedo first order

Parameters

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None

Initial fluoride concentration (mg/l) 2 4 8 0.41 0.79 1.49 0.15 0.34 0.81 0.07 0.06 0.06 0.90 0.94 0.89

H2SO4

Equation

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Oxidizing Kinetics agent

Psuedo second order

17

ACCEPTED MANUSCRIPT Table 4: Langmuir and freundlich isotherm constants for fluoride adsorption Isotherm Value Langmuir

Equation

Plot A plot of

Parameters (mg/g)

versus

3.82

indicated a straight line with a

0.87

slope of

and intercept

of

Freundlich

The values of

1.75

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0.99

(L/mg )

and

(mg/g)

were obtained from the slope n (L/mg)

0.71

versus

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of

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and intercept of the linear plot

0.99

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ACCEPTED MANUSCRIPT Table 5: Comparison of fluoride adsorption capacity of acidic alumina with a few reported adsorbents Adsorbent capacity (mg/l)

Reference

Plaster of paris

0.37

Gopal et al., 2007 [27]

Granular red mud

0.85

Tor et al., 2009 [28]

Pyrophyllite

2.2

Goswami et al., 2011 [29]

Hydrated cement

2.68

Kagne et al., 2008 [30]

Activated alumina

2.41

Ghorai et al., 2005 [31]

Manganese oxide coated alumina

2.85

Maliyekkal et al., 2006 [32]

Mg/Al layered double hydroxide

0.99

Iron oxide-hydroxide nanoparticle

1.66

Alumina (KMnO4 oxidized at

3.82

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Elhalil et al., 2016 [33]

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Raul et al., 2012 [34] Present study

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550oC)

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Adsorbent

19

ACCEPTED MANUSCRIPT Washed with acetone and ethanol to remove organic impurities

Aluminium Foil (2.5×2.5×10-4 m2)

Treated with HCl to remove surface oxidation layer

Treated with oxidizing agents to improve oxidation

Annealed at different temperature

Flow chart for the preparation of Al2O3 nanoparticles

105

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Weight loss due to moisture removal +burning of volatile impurities

100

Reduced weight loss due to surface oxidation

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Mass (%)

95 90 85

Weight loss is constant

80

0

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75 70

100

200

300

400

500 o

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Temperature ( C)

Figure. 2

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Figure. 1

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Thermo gravimetric analysis (TGA) plot of Al2O3 foil

600

700

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ACCEPTED MANUSCRIPT

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Figure. 3

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Mag=64.76KX

FESEM image of Al foil after annealing without any oxidising agent

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a) at 400 oC b) at 450 oC c) at 500 oC d) at 550 oC

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Figure. 4

FESEM images of Al foil after annealing using various oxidising agents at 450 oC and 500 oC for H2SO4 (a) and (b), K2Cr2O7 (c) and (d), KMnO4 (e) and (f)

ACCEPTED MANUSCRIPT

100

100

(b)

40

Initial fluoride concentration (mg/l)

20 0

2

4

40

60

60

40

20

Initial fluoride concentration (mg/l)

20

8 0

0

80

Contact time (min)

100

0

100

100

Initial fluoride concentration (mg/l)

20

Figure. 5

40

4

8

40

60

80

Contact time (min)

100

60

40

Initial fluoride concentration (mg/l)

20

8

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0

4

60 Contact time (min)

80

EP

0

Fluoride removal (%)

40

2

80

2

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Fluoride removal (%)

60

20

20

(d)

(c)

80

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60

80

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Fluoride removal (%)

80

Fluoride removal (%)

(a)

100

2 0

0

20

40

4 60

8 80

Contact time (min)

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Effect of contact time and initial fluoride concentration using adsorbent dose: 4 g/l (a) nanoparticles without oxidizing agent b) nanoparticles with H2SO4 as oxidizing agent c) nanoparticles with K2Cr2O7 as oxidizing agent d) nanoparticles with KMnO4 as oxidising agents.

100

9

Temperature = 25 oC, Stirring speed = 200 rpm Adsorbent dose (g/l) 2 4 8

8 7 6 5

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4 3 2 1 0

0

1

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Residual fluoride concentration (mg/l)

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2

3

4

5

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Adsorbed dose (g/l) Figure. 6

Variation of residual fluoride concentration with Al2O3 adsorbent without oxidizing agent

100

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80

14

60

12

20

EP

40

Final pH

10

0

8 6 4

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Fluoride removal (%)

Initial fluoride concentration = 8 mg/l, Ads. dose = 4 g/l Temperature = 25 oC

0

2

2 2

4

6

8

10

12

14

Initial pH

4

6

8

10

Initial pH of fluoride solution

Figure. 7 Effect of pH on fluoride adsorption, Inset: pHzpc of Al2O3 adsorbent

12

ACCEPTED MANUSCRIPT 0.93

y =-159.45x + 1.407 R2 = 0.988

0.91 0.90 0.89 0.88 0.87 0.0032

1/T (1/oC)

0.0033

0.0034

SC

0.0031

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log ((qe/Ce) × m)

0.92

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Figure. 8

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Arrhenius plot for the adsorption of fluoride by Al2O3 adsorbent prepared with KMnO4 as oxidizing agent

ACCEPTED MANUSCRIPT 250

250 (b)

Fluoride concentration (mg/l) 2 4 8

Fluoride concentration (mg/l) 2 4 8

200

t/qt (g min mg-1)

t/qt (gminmg-1)

200

(a)

150

150

100

50

0

20

40

60

80

0

100

Time (min) 240

t/qt (g min mg-1)

20

40

60

80

100

80

100

Time (min)

200

(c)

(d)

Fluoride concentration (mg/l) 2 4 8

200

0

Fluoride concentration (mg/l) 2 4 8

160

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t/qt (g min mg-1)

160

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0

50

RI PT

100

120

120 80 40 0

0

20

40

60

80

Figure. 9

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Time (min)

100

80

40

0

0

20

40

Pseudo second order kinetic model fitting

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(a) Adsorbent without oxidizing agent

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(c) Adsorbent with K2Cr2O7

60

Time (min)

(b) Adsorbent with H2SO4 (d) Adsorbent with KMNO4

ACCEPTED MANUSCRIPT

2.5

RI PT

1.5

1.0

SC

qe (mg/g)

2.0

Adsorbent dose = 4 g/l Experimental Langmuir Freundlich

0.0 0.0

0.2

0.4

0.6

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0.5

0.8

1.0

1.2

1.4

1.6

Ce (mg/l) Figure. 10

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Adsorption isotherm plot for the adsorption of fluoride by Al2O3 adsorbent prepared with KMnO4 as oxidizing agent

ACCEPTED MANUSCRIPT

Highlights: Al2O3 nanostructures were successfully synthesised on the surface of aluminium foil in the range of 10 nm to 150 nm.

•

The length of the nanochains could be increased by changing the oxidizing agents and other experimental parameters like temperature.

•

Al2O3 nanoparticles thus prepared were acidic in nature and had a considerable potential for fluoride adsorption from aqueous medium with a maximum of 92 % fluoride being adsorbed by Al2O3 nanoparticles at pH 4.7

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•