Rapid adsorption of anionic dyes by ordered nanoporous alumina

Rapid adsorption of anionic dyes by ordered nanoporous alumina

Chemical Engineering Journal 209 (2012) 589–596 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: ww...

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Chemical Engineering Journal 209 (2012) 589–596

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Rapid adsorption of anionic dyes by ordered nanoporous alumina Bahareh Yahyaei, Saeid Azizian ⇑ Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran

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

" Dyes removal by OMA is much faster

than removal by activated carbons. " The adsorption capacity of anionic

dyes by OMA is comparable to activated carbon. " OMA is one of the best adsorbents for removal anionic dyes from aqueous solutions.

a r t i c l e

i n f o

Article history: Received 26 June 2012 Received in revised form 18 August 2012 Accepted 20 August 2012 Available online 25 August 2012 Keywords: Ordered mesoporous alumina Rapid adsorption Equilibrium isotherms Adsorption kinetics

a b s t r a c t Here we show that the ordered nanoporous alumina has high adsorption capacity for removal of dye pollutants from aqueous solution. The removal percentage of dyes by ordered nanoporous alumina is more than 30% only after 30 s and about 60% after 6 min which means that the dye removal is too fast in comparison to the activated carbon. An ordered nanoporous alumina has been synthesized by a fast and facile method by using aluminum iso-propoxide, F127 triblock copolymer and ethanol as a solvent. The TEM, XRD and N2 adsorption/desorption techniques have been used for characterization and the UV–vis spectroscopy has been used to follow the adsorption process. N2 sorption measurements reveal that the pore size of the obtained nanoporous alumina is about 5 nm and therefore can be categorized as mesoporous materials. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years the preparation, characterization and application of nanostructured mesoporous materials have been studied by many researchers due to their unique properties such as similar pore size, high surface area and tunable structures [1]. These properties make nanostructured mesoporous materials as ideal adsorbents and catalysts. Different nanoporous materials such as mesoporous ZnTiO3 [2], mesoporous carbons [3–6], mesoporous silica [7], and mesoporous alumina (MA) [8–11] have been synthesized by using different methods. There are many applications for these materials, for example the bio-application of mesoporous silica (MS) because of the well accordance between the MS pore size and proteins dimen⇑ Corresponding author. Tel.: +98 8118282807; fax: +98 8118380709. E-mail address: [email protected] (S. Azizian). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.08.055

sions [1], the optical and electronic applications of mesoporous WO3ATiO2 composites, can be mentioned [1] and the removal of pollutant from water by MA [12]. Although alumina is one of the adsorbent which has been used for removal of pollutants from aqueous solution [13] ordered mesoporous alumina (OMA) due to its specific structure gained a great attraction as an adsorbent, recently. For example, Yu et al. used MA to remove arsenate and orthophosphate anions from drinking water [12]. Bansiwal et al. used copper oxide incorporated MA for defluoridation of drinking water [8]. Li et al. used highly ordered MA to remove fluoride and arsenic from water [14]. Different methods have been used to synthesize MA. Lesaint et al. synthesized MA by double hydrolysis route. They used different surfactants as a pore directing agent, Al(NO)39H2O and NaAlO2 as metal precursors and deionized water as a solvent. The MA preparation has been followed by hydrothermal process and calcination to remove the surfactant [9]. Yuan et al. synthesized

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mesoporous c-alumina by sol–gel process. They used aluminum iso-propoxide as aluminum source, Pluronic P123 as a pore directing agent and ethanol as solvent [10]. Oveisi et al. used spin coating method to prepare the titania–alumina mixed oxide mesoporous film. They used titanium tetra-iso-propoxide, aluminum tetraiso-propoxide and triblock copolymer F127 as reactants [15]. Recently Grant et al. prepared OMA in the presence of Pluronic F127 triblock copolymer. Briefly the ethanolic solution of F127 and aluminum iso-propoxide was stirred at room temperature. The process was followed by evaporating solvent and calcination of prepared sample [16]. In the present work the OMA was synthesized by using Grant et al. method [16] and TEM images and XRD patterns were used for characterization. Synthesized OMA was used for removal of dye pollutants from aqueous solution; because the organic dyes are one of the major pollutants in water and their removal (special by adsorption method) is very important [17–20]. The adsorption process was followed by UV–vis spectroscopy. The adsorption equilibrium, adsorption kinetics and the effect of pH on the dye removal efficiency were investigated.

Fig. 1. XRD pattern of the prepared OMA.

2. Experimental 2.1. Materials Aluminum iso-propoxide (for synthesis), ethanol (99.8%), alumina (90 active, neutral) and nitric acid (65%) were purchased from Merck Co. and Pluronic F127 (MW = 12,600) from Sigma–Aldrich. Methyl orange (MO) and bromothymol blue (BTB) purchased from Fluka, and BDH companies, respectively. 2.2. Instruments The adsorption process was followed using UV–vis spectra obtained by a PG UV–vis spectrophotometer. A Phillips apparatus ‘‘PW 1710’’ with Cu Ka radiation was used to obtain the X-ray diffraction patterns of ordered mesoporous alumina. Transmission electron images were obtained by transmission electron microscope Philips CM120. N2 adsorption–desorption isotherms were obtained with a Quantachrome Nova Station A at 77.3 K. The samples were outgassed at 300 °C for 3 h, before nitrogen adsorption. The pore size distributions were calculated based on the DFT method. 2.3. Preparation and characterization of OMA OMA was synthesized by Grant et al. method [16]. Briefly 0.75 g F127 was dissolved in 40 ml absolute ethanol and allowed to stir at room temperature for 4 h. Then 1.0213 g of aluminium iso-propoxide and 0.85 ml HNO3 65% was added to above solution and the obtained mixture was stirred at room temperature for another 5 h. To evaporate the solvent, the mixture was moved to oven at 60 °C for 48 h. After that, the obtained specimen was calcined in a quartz tube furnace by heating at 1 °C/min to 400 °C and keeping at 400 °C for 4 h [16]. XRD patterns and TEM images have been used to characterize the microscopic features of OMA. Fig. 1 shows the XRD pattern of the obtained OMA. Broad peaks at the ranges of 10–40° and 50– 80° prove the formation of OMA with amorphous wall [10,14]. The strong peak around 1.0°, which is shown in the inset of Fig. 1, is another evidence for the formation of OMA. This peak identifies the p6mm hexagonal symmetry [9] which has been approved by the TEM images too. The TEM images of the obtained OMA, which are shown in Fig. 2, show the highly ordered hexagonal pore openings along

Fig. 2. The TEM images of the obtained OMA along (a) [1 1 0] and (b) [0 0 1] direction. The scale-bar represents 50 nm.

[0 0 1] direction and the cylindrical pores along [1 1 0] direction [10,14]. Fig. 2b confirms the p6mm hexagonal symmetry for the synthesized OMA.

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B. Yahyaei, S. Azizian / Chemical Engineering Journal 209 (2012) 589–596 Table 1 Textual properties of obtained OMA and different adsorbents. Adsorbent

Surface area (m2/g)

Pore volume (cm3/g)

Pore size (nm)

Reference

OMA

242.8

0.420

4.97

Commercial Al2O3 MA-0.17Eta MPCb MA-nc MCd CaCl2 AC-51e

123

0.193

4.15

268 757 338 147 1.287 1607

0.300 0.550 0.450 0.280 0.005 1.420

7.80 4.50 9.60 7.62 8.42 –

Present study Present study [16] [21] [22] [23] [24] [25]

a

Mesoporous alumina with 0.17 mol of ethanol. Ordered mesoporous carbon. Mesoporous alumina synthesized from aluminum nitrate nonahydrate as aluminum precursor. d Mesoporous carbon. e AC-51 is an activated carbon prepared from grapevine rhytidome. b

c

and 25 °C) for 24 h. The equilibrium concentration of MO and BTB, Ce, was measured at 464 nm and 430 nm, respectively, and the amount of adsorbed MO and BTB per unit mass of OMA at equilibrium, qe, was calculated by following equation:

qe ¼

ðC 0  C e Þ V W

ð1Þ

where C0 is the initial concentration of dye, V the solution volume and W is the mass of adsorbent. Kinetic experiments were performed at concentrations of 10, 15 and 20 mg/l for both MO and BTB. In each concentration a series of 5 ml of MO and BTB solutions with 3 mg of OMA were placed in a shaker (150 rpm and 25 °C) and at the proper time intervals the residual MO and BTB concentrations, Ct, was determined at 464 nm and 430 nm, respectively. The amount of adsorbed MO and adsorbed BTB per unit mass of OMA at time t was calculated by following equation:

qt ¼

ðC 0  C t Þ V W

ð2Þ

The calculated error of qe and qt is about 3.4%. Fig. 3. (a) N2 adsorption–desorption isotherm (b) the DFT pore-size distribution curve of the obtained OMA and commercial Al2O3 and (c) N2 adsorption–desorption isotherm of commercial Al2O3.

The type IV shape of nitrogen adsorption/desorption isotherm of the obtained OMA which is shown in Fig. 3a, proves the formation of OMA with uniform mesopores [21]. The DFT pore-size distribution curve which is shown in Fig. 3b, reveals that most of the pores of the obtained OMA have a uniform pore size distribution, centered at 4.97 nm. The multipoint BET surface area and total pore volume are 242.8 m2/g and 0.42 cm3/g, respectively. N2 adsorption–desorption isotherm of commercial Al2O3 has also been presented in Fig. 3c. The textural properties of obtained OMA, commercial alumina and other mesoporous materials and adsorbents have been compared in Table 1.

2.4. Kinetic and equilibrium experiments For the equilibrium experiments several solutions of dyes with different initial concentrations (10–150 mg/l for MO and 10– 120 mg/l for BTB) were prepared and then a set of 5 ml of each solution with 3 mg of OMA were placed in a shaker (150 rpm

2.5. Effect of pH To study the effect of solution pH on the removal percentage of MO and BTB by OMA, a series of dye solutions (10 mg/l) in the pH range 3.4 6 pH 6 9.6 with 3 mg of OMA were prepared and were shaken for 4 h (150 rpm and 25 °C). The removal percentage of dye was calculated by following equation:

% Re ¼

ðA0  AÞ  100 A

ð3Þ

where A0 and A are the initial and equilibrium absorbance at corresponding wavelength for each dye. 3. Results and discussions 3.1. Kinetic studies An efficient adsorbent should have several characters. The high rate of adsorption is one of the most important characters of each adsorbent. Fig. 4 shows the UV–vis spectra of the dye solution (15 mg/l) in contact with 3 mg of OMA at different times. The removal percentage of MO by OMA is more than 30% only after 30 s and about 60% after 6 min (Fig. 4a). A similar result was

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Fig. 6. Experimental kinetic data for the adsorption of MO and BTB by OMA and commercial Al2O3 at concentration of 20 mg/l for both dyes.

Fig. 4. UV–vis spectra of the dye solution (15 mg/l) in contact with 3 mg of OMA at different time (a) MO, (b) BTB.

13%, respectively, after 4800 s [25] while this percentage by OMA was obtained after about 20 s for both dyes. Thus OMA can remove MO and BTB much faster than the commercial and experimentally prepared activated carbons [25]. This high rate of adsorption is very important from practical point of view, especially for flow systems. Fig. 5 shows the time dependency of the adsorption of MO and BTB by OMA at different initial concentrations (10, 15 and 20 mg/l). It is clear that the adsorption rate and the amount of adsorbed species increase by increasing the initial concentration. In both cases the dye removal has been performed within few minutes, which is too fast in comparison to the activated carbons. For comparison of dye removal efficiency of OMA and commercial Al2O3, kinetic experiments have been performed at concentration of 20 mg/l for both MO and BTB. The results of these experiments have been shown in Fig. 6. It is clear that the adsorption of MO and BTB onto the surface of OMA is much faster than the adsorption onto the surface of Al2O3. Also the amount of ad-species (both dyes) by OMA is higher than commercial Al2O3. Although the hysteresis loops in the N2 adsorption/desorption isotherms of both OMA and commercial Al2O3 (Fig. 3a and c) confirm the presence of mesopores in both materials but the amount of adsorbed N2 on the surface of commercial Al2O3 is much lower than the amount of adsorbed N2 on the surface of OMA, suggesting that commercial Al2O3 has fewer surface area and pore volume. Pseudo first order and pseudo second order rate equations [26] were used to analyze the kinetic data. Integrated form of the pseudo first order (PFO) model can be expressed as:

qt ¼ qe ð1  expðk1 tÞÞ

ð4Þ

where qt and qe are the amounts of the dye adsorbed at time, t, and at equilibrium, respectively, and k1 is the pseudo first order rate coefficient. The pseudo second order (PSO) model can be declared as [26]:

qt ¼

k2 q2e t 1 þ k2 qe t

ð5Þ

where k2 is the pseudo second order rate coefficient. According to the obtained correlation coefficients, the experimental kinetic data were best fitted with pseudo second order equation for both MO and BTB. The results of fitting are listed in Table 2. The predicted values of qt based on PSO model as a function of time are presented as solid lines in Fig. 5. Fig. 5. Experimental kinetic data for the adsorption of (a) MO and (b) BTB by OMA at different initial concentrations.

observed in the case of BTB (Fig. 4b). So the adsorption process of MO and BTB by OMA is too fast. The removal percentage of MO and BTB by different granular activated carbons with the same concentration is about 25% and

3.2. Equilibrium studies Langmuir, Freundlich and Langmuir–Freundlich isotherms are the common isotherms used in equilibrium studies. The Langmuir isotherm is [27]:

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B. Yahyaei, S. Azizian / Chemical Engineering Journal 209 (2012) 589–596 Table 2 The obtained constants of PFO and PSO kinetic models for adsorption of MO and BTB onto the OMA at different initial concentrations. PFO

qm (mg/g)

KL (l/ mg)

KF (mg/g) (l/ mg)1/n

Ks (l/ mg)1/m

n

r2

MO Langmuir Freundlich Langmuir– Freundlich

235.6 – 193.0

0.51 – –

– 87.92 –

– – 0.52

– 2.8 0.45

0.920 0.744 0.962

BTB Langmuir Freundlich Langmuir– Freundlich

166.3 – 140.5

0.25 – –

– 44.00 –

– – 0.12

– 2.7 0.45

0.929 0.833 0.981

Isotherm

PSO qe (mg/g)

r2

k2 (l mg1 min1)

qe (mg/g)

r2

[MO] (mg/l) 10 0.035 15 0.020 20 0.042

4.4 9.1 15.1

0.877 0.900 0.962

0.054 0.013 0.011

12.3 19.1 28.1

0.999 0.989 0.995

[BTB] (mg/l) 10 0.080 15 0.057 20 0.078

5.3 6.1 11.0

0.980 0.900 0.844

0.0514 0.0471 0.0349

9.5 15.6 25.9

0.999 0.998 0.999

k1 (min1)

Table 3 Obtained isotherms constants for adsorption of MO onto the OMA.

Table 4 The maximum adsorption capacity, qm (mg/g), of OMA and different activated carbons.

OMA GACa AC-51 MWCNTs PAACb a b

MO (mg/g)

BTB (mg/g)

Reference

193.0 172.5 138.3 51.74 217.39

140.5 133.7 136.4 – –

Present study [25] [25] [30] [31]

Granular activated carbon. Phragmites australis activated carbon.

The results of fitting with adsorption isotherm models were listed in Table 3. According to the obtained correlation coefficients the experimental equilibrium data were fitted to the Langmuir– Freundlich isotherm for both MO and BTB. The predicted values of qe based on Langmuir–Freundlich equation are presented in Fig. 7 as solid lines. High deviation of the n values from unity indicates that OMA provides a heterogeneous surface for adsorption of both dyes. The obtained adsorption capacity, qm, for OMA is comparable with different activated carbons [25,30,31] (Table 4). This indicates that OMA is a suitable adsorbent for dye removal from aqueous solution. 3.3. Effect of pH

Fig. 7. Adsorption isotherm of (a) MO and (b) BTB onto the OMA at 25 °C.

qe ¼

qm K L C e 1 þ K LCe

ð6Þ

where qm (mg/g) and KL (l/mg) are the maximum adsorption capacity and Langmuir constant, respectively. The Freundlich isotherm is used for multisite adsorption process [28]:

qe ¼ K F C 1=n e

ð7Þ

where KF is the Freundlich constant and n is the parameter describes the system heterogeneity. The Langmuir–Freundlich (Sips) isotherm which is used for heterogeneous surfaces is a combination of Langmuir and Freundlich isotherms [29]:

qe ¼

qm K s C e1=n 1 þ K s C e1=n

ð8Þ

where KS is the adsorption constant. Fig. 7 shows the equilibrium experimental data for the adsorption of MO and BTB by OMA.

The pH of solution can affect on the removal efficiency of dye by different adsorbents. To investigate whether the pH affects on removal efficiency of dyes by OMA or not, we have investigated the removal percentage of MO and BTB by OMA at different pH. Fig. 8 shows the removal percentage of MO and BTB by OMA at different pH. In acidic pH the removal percentage of MO has increased slightly (about 10%), reached to its maximum value at pH = 6.2, and then decreased slightly (about 10%) in basic pH. In the case of BTB, maximum removal percentages have been obtained in acidic pH and the in basic pH the removal percentage has decreased dramatically. The pH of point of zero charge (pHPZC) for OMA was determined by the method proposed by Faria et al. [32]. Briefly a series of NaCl (0.01 M) solutions with pH range between 2.8 and 11.1 were prepared using HCl (0.01 M) and NaOH (0.01 M). Then 10 ml of each solution with 30 mg of OMA were placed in a shaker (150 rpm and 25 °C). After 24 h, the equilibrium pH of each solution was measured using a pH-meter. pHPZC has determined by plotting final pH (pHf), versus initial pH (pHi) of solutions (Fig. 9) [32]. According to this plot the pHPZC for OMA is found to be as 7.8. At solution pH higher than pHPZC, the surface of the adsorbent has negative charge while at the pH lower than pHPZC, the surface charge is positive [33]. The concentrations of dyes in the molecular

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Fig. 8. Effect of pH on the removal percentage of (a) MO and (b) BTB by OMA. Fig. 10. Calculated molecular and ionized concentrations of (a) MO and (b) BTB at different pH (right axes). The dyes removal percentages are presented on left axes too.

Fig. 9. pH variation in terms of initial pH of solutions.

(C 0e ) and ionized (C ie ) forms as a function of pH have been obtained by following equations [34].

Ce

C 0e

¼

C ie

 ¼ Ce 1 

Fig. 11. MO structure at (a) acidic and (b) basic mediums.

ð9Þ

1 þ 10ðpHpK a Þ 1 1 þ 10ðpHpK a Þ

 ð10Þ

where C e is the MO or BTB concentration and K a is the acidity constant of MO (pKa = 3.4) or BTB (pKa = 7.1). The results of calculated molecular and ionized concentrations of MO and BTB at different pH are shown in Fig. 10. The MO structures in the acidic and basic mediums have been shown in Fig. 11. In the basic form of methyl orange, a hydrogen ion is removed from the ANNA bridge between the rings and the positive charge on the terminal nitrogen was neutralized. As shown in Fig. 10a, the removal percentage of MO by OMA, almost

remains unchanged in acidic and basic pH and also most of MO molecules exist in the ionic form (Fig. 11b). This suggests that MO molecules are not adsorbed through ionic ASO 3 group. It seems that the adsorption of MO is because of the interaction between the central nitrogens or benzene rings with the surface of OMA. Understanding the adsorption mechanisms of MO on the surface of OMA needs more spectroscopic study. Fig. 12 shows the BTB structure at different pH mediums. In the acidic medium the BTB molecules exist in the molecular form as shown in Fig. 10b while in basic medium the AOH functional groups convert to AO and @O and also an anion sulfate group appears, therefore BTB gain two negative charges. So in acidic solution the interaction between the positive surface of OMA and AO

B. Yahyaei, S. Azizian / Chemical Engineering Journal 209 (2012) 589–596

595

Fig. 12. The BTB structure at different pH.

Freundlich isotherm best-fit the equilibrium data for adsorption of both dyes. Pseudo-first order and pseudo second order rate equations were tested to investigate the adsorption kinetic. The experimental kinetic data of both dyes fit very well with the pseudo second order kinetic model. The study on the effect of temperature on the adsorption of MO and BTB by OMA indicated that the adsorption of both dyes is an endothermic process. The high BET surface area of OMA and also high adsorption capacity and very high rate of adsorption of both dyes pollutant by nanostructured mesoporous alumina is very important and interesting for practical applications. Thus the ordered mesoporous alumina is one of the best and efficient adsorbents for removal of MO and BTB from aqueous solutions. Acknowledgment

Fig. 13. Effect of temperature on the removal percentage of MO and BTB by OMA.

functional groups of BTB leads to high removal percentage of BTB by OMA and in the basic solution the strong repulsive interaction between the anionic form of BTB and the negative surface of OMA leads to dramatic decrease of removal percentage. 3.4. Effect of temperature The effects of temperature on MO and BTB adsorption onto OMA were tested at three temperatures (25 °C, 35 °C and 45 °C). The results of these experiments which have been presented in Fig. 13 shows that the removal percentage of both dyes by OMA has been increased with increasing temperature. It means that the adsorption of MO and BTB by OMA is an endothermic process. The number of dye molecules which obtain enough energy to get an interaction with active sites at the surface increase with increasing temperature [35] therefore the removal percentage of dyes is increased too. 4. Conclusion Although activated carbon is the common adsorbent for dye removal from aqueous solutions but nanostructured mesoporous alumina has some advantages compare to activated carbon for MO and BTB removal. The preparation of OMA is easier than the preparation of activated carbons. The adsorption of MO and BTB onto the surface of OMA is much faster than the adsorption onto the surface of activated carbons and commercial alumina. Also the adsorption capacity of MO and BTB by OMA is higher than the adsorption capacity of commercial Al2O3 and it is comparable to activated carbon. The BET surface area of OMA is also higher than commercial Al2O3. The difference between the adsorption capacity of MO and BTB (about 26%) by OMA maybe is due to the difference between the molecular sizes of two dyes. Langmuir–

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