Carbon and TiO2 synergistic effect on methylene blue adsorption

Carbon and TiO2 synergistic effect on methylene blue adsorption

Materials Chemistry and Physics xxx (2016) 1e9 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsev...

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Materials Chemistry and Physics xxx (2016) 1e9

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Carbon and TiO2 synergistic effect on methylene blue adsorption Evelyn Alves Nunes Simonetti*, Luciana de Simone Cividanes, Tiago Moreira Bastos Campos, Beatriz Rossi Canuto de Menezes, Felipe Sales Brito, Gilmar Patrocínio Thim ~o de Ci^ utica, Praça Marechal Eduardo Gomes, Sa ~o Jos gico de Aerona Departamento de Física, Divisa encias Fundamentais, Instituto Tecnolo e dos Campos, SP, Brazil

h i g h l i g h t s  This article deals with the adsorption of methylene blue onto TiO2-Carbon composite.  The sol-gel synthesis was efficient to insert TiO2 particles on carbon structure.  The TiO2 nanoparticles presented mesopore area around the carbon structure.  The wedge shapes pores which were the main responsible for methylene blue removal.  A new TiO2-Carbon materials can be used for removal of large pollutants molecules.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 February 2015 Received in revised form 6 April 2016 Accepted 8 April 2016 Available online xxx

Due to its high efficiency, low cost and a simple operation, the adsorption process is an important and widely used technique for industrial wastewater treatment. Recent studies on the removal of artificial dyes by adsorption include a large number of adsorbents, such as: activated carbon, silicates, carbon nanotube, graphene, fibers, titanates and doped titanates. The carbon insertion in the TiO2 structure promotes a synergistic effect on the adsorbent composite, improving the adsorption and the chargetransfer efficiency rates. However, there are few studies regarding the adsorption capacity of TiO2/Carbon composites with the carbon concentration. This study evaluates the effect of carbon (resorcinol/ formaldehyde) insertion on TiO2 structure through the adsorption process. Adsorbents were prepared by varying the carbon weight percentages using the sol-gel method. The physicochemical properties of the catalysts prepared, such as crystallinity, particle size, surface morphology, specific surface area and pore volume were investigated. The kinetic study, adsorption isotherm, pH effect and thermodynamic study were examined in batch experiments using methylene blue as organic molecule. In addition, the effect of carbon phase on the adsorption capacity of TiO2-carbon composite was deeply investigated. SEM micrographs showed that TiO2 phase grows along the carbon phase and FT-IR results showed the presence of TieOeC chemical bonding. The experiments indicate that the carbon phase acted as a nucleation agent for the growth of TiO2 during the sol-gel step, with a TiO2 structure suitable for blue methylene adsorption, resulting in a material with large surface area and slit-like or wedge-shaped pores. Further experiments will show the best carbon concentration for methylene blue adsorption using a TiO2 based material. © 2016 Elsevier B.V. All rights reserved.

Keywords: Composite materials Interfaces Nanostructures Surfaces Sol-gel growth Adsorption

1. Introduction The environmental regulatory agencies rigidly control the levels of contaminants in industrial wastes. This rigid control has brought

* Corresponding author. E-mail address: [email protected] (E.A.N. Simonetti).

about numerous effluent treatment techniques to be used prior to discarding the wastewater into the environment. The adsorption process is among the most important industrial wastewater treatment techniques due to its high efficiency and low cost combined with a simple operation process. The principle of this technique regards the development of highly porous materials which are able to remove different and complex molecules of pollutants from

http://dx.doi.org/10.1016/j.matchemphys.2016.04.035 0254-0584/© 2016 Elsevier B.V. All rights reserved.

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contaminated effluents [1]. Some industrial sectors, such as textile, leather, paper and plastic are responsible for discarding synthetic dyes into the wastewater, contaminants which are harmful and toxic to human beings and microorganism [2]. These artificial dyes have aromatic amines (azo dyes) in their chemical structure, making them highly carcinogenic. Therefore, the presence of these dyes in the water, even in small amounts, represents a serious environmental problem. Recent studies related to the removal of artificial dyes by adsorption include a large number of adsorbents, such as: activated carbon [1e4], silicates [5], carbon nanotube [6], graphene [7], fibers [8], titanates [9], modified titanates surface [10], and doped titanates [11]. A new series of carbon based materials based on TiO2/Carbon composites has attracted great attention. The carbon insertion in the TiO2 structure promotes a synergetic effect on the adsorbent composite, improving the adsorption and the charge-transfer efficiency rate. However, few reports are related to the adsorption capacity of TiO2/Carbon composites with carbon concentration [12e14]. Nguyen-Phan et al. (2011) [15] prepared TiO2/graphene oxide composite using the colloidal blending method. The results showed that the amount of dye removed was proportional to the graphene oxide content. Zhang et al. (2011) [16] developed TiO2/activated carbon composites for humic acid removal and achieved 15% of removal capacity even using a dye with complex molecular structure. More recently Zhai et al. (2015) [17] developed a nanostructured TiO2 with sulfonated coal adsorbent for methylene blue removal. The large number of pores on its surface proved to be an efficient trap for the adsorption of the organic molecules. In this context, the objective of this study was to investigate the interaction between TiO2 and carbon structure in a TiO2-carbon adsorbent for methylene blue (MB) removal through an adsorption process. The carbon structure was synthesized by resorcinol-formaldehyde (ReF) sol-gel method. Similarly, TiO2 was synthesized by sol-gel method using titanium isopropoxide as titanium source. In addition, this study describes the kinetics, isotherm and thermodynamics of methylene blue adsorption from aqueous solutions onto TiO2-carbon composite. Moreover, a study related to the modifications promoted by the carbon structure on TiO2 properties was investigated.

where x represents the amount of carbon added. 2.2. Adsorption studies All adsorption studies were carried out in polyethylene flasks of 100 mL by retaining a given dose of adsorbent with 20 mL of methylene blue solution in a thermostated orbital shaker for different periods of shaking. The pH of methylene blue solution was adjusted using dilute HCl and NaOH solutions, and controlled using a pH meter. Next, the supernatant solution was separated from the adsorbent by filtration and the methylene blue concentration in the supernatant was determined spectrophotometrically, monitoring the absorbance at 665 nm. The adsorption studies were performed using the following conditions: 2.2.1. Kinetics study Adsorbent dose of 0.25 g mL1; initial methylene blue concentration 10.32 mg L1; pH ¼ 6; shaking time 0e360 min; T ¼ 25  C. 2.2.2. Dosage Adsorbent dose ranged from 0.25 g mL1 to 0.5 g mL1; initial methylene blue concentration 10.32 mg L1; pH ¼ 6; T ¼ 25  C. 2.2.3. Effect of pH Adsorbent dose of 0.25 g mL1; initial methylene blue concentration 10.32 mg L1; pH 1e14; predetermined shaking time value; T ¼ 25  C. 2.2.4. Adsorption isotherms Adsorbent dose of 0.75 g mL1; initial methylene blue concentration 5e50 mg L1; pH ¼ 6; predetermined shaking time value; temperature range 25e55  C. 2.2.5. Thermodynamics study Adsorbent dose of 0.75 g mL1; initial methylene blue concentration 5e50 mg L1; pH ¼ 6; predetermined shaking time value; temperature range 25e55  C. 2.3. Characterization

2. Materials and methods 2.1. Preparation of TiO2 and TiO2eC composites The carbon structure was synthesized by polycondensation of resorcinol (R) and formaldehyde (F) using an initial R:F molar ratio of 1:2. Deionized water (W) was used as diluent and the R:W molar ratio was 1:170. After the polycondensation reaction the mixture was kept at 353 K for 24 h for gel formation and then dried at 100  C for 48 h. Next, the dried samples were activated at 800  C in argon atmosphere for 2 h. This substance was identified as Sample C. TiO2 was obtained from titanium isopropoxide (IV) (Ti(OCH(CH3)2)4) (97%-Sigma Aldrich), acetic acid (99.7%-Labsynth) and ethylene glycol (99.0%-Labsynth). The mixture was kept at 100  C for 24 h until complete gelation, after that the obtained gel was dried at 110  C and then calcined at 400  C for 2 h. The molar ratio of alkoxide:acid:glycol was fixed at 1:20:21 e this adsorbent was then designated as TiO2eS. TiO2 (P25) obtained from Degussa was used as comparative adsorbent and methylene blue (Merck) as organic dye. Two amounts of Sample C (0.5% and 1.0% m/m) were added to mixtures of Ti(OCH(CH3)2)4, acid acetic and ethylene glycol. This mixture was stirred vigorously for 60 min followed by 60 min of sonication. The resulting colloidal solution was kept at 100  C for 24 h to produce the corresponding gel. The gel samples were dried at 110  C and then calcined at 400  C for 2 h at argon atmosphere. The composites doped with carbon were identified as TiO2-xC,

The surface area measurements of the catalysts were carried out using BET equipment, MicrotracBEL - model Belsorp II, using nitrogen adsorption at 196  C and relative pressures (P/P0) ranging from 0.05 to 0.35. The specific area was determined by the BET method. Fourier transform infrared spectroscopy (FTIR) analysis was carried out in a Perkin Elmer Spectrum One spectrophotometer with KBr pellets in the range of 4000e500 cm1. The X-ray diffraction (XRD) analysis was performed using the powder method on SEISERT equipment, model 1001, radiation CuKa (l ¼ 1.54178 Å) and a nickel filter. Crystalline phases were identified using JCPDS (Joint Committee of Powder Diffraction Standards). The surface morphology of the adsorbents was investigated by scanning electron microscopy (SEM) and field-emission gun scanning electron microscope (FEG-SEM). 3. Results and discussion 3.1. Kinetics study Fig. 1(A) shows the kinetics curves obtained for the methylene blue removal from aqueous solutions of TiO2eS, P25, C, TiO2-0.5C and TiO2-1.0C. The adsorption of MB by P25, TiO2eS and sample C was very fast, however the removal capacity was too small when compared to the composite adsorbents. In 15 min samples TiO2eS, P25 and C reached the adsorption equilibrium with approximately

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3

(B)

(A) 1.2

15

TiO2-1.0C

P25 TiO2-S

TiO2-0.5C

0.8

Linear Fit

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TiO2-1.0C TiO2-0.5C

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2000

TiO2-1.0C

-1

t.qt (min.g.mg )

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Fig. 1. Effect of contact time on methylene blue adsorption (A). The pseudo first-order (B), pseudo second-order (C), and intra-particle diffusion models (D) for batch adsorption of methylene blue (T ¼ 25  C, adsorbent dose: 0.25 g mL1, C0 ¼ 10.32 mg L1).

0.13 mg g1, 0.2 mg g1 and 1.1 mg g1, respectively. However the equilibrium time for samples TiO2-0.5C and TiO2-1.0C was of 120 min and 180 min, respectively. Despite the longer time, the adsorption capacity of the composites is quite higher, reaching about 9 mg g1 and 15 mg g1 at equilibrium for TiO2-0.5C and TiO2-1.0C, respectively. The electrostatic interaction is the primary binding strength between the oxygen containing groups of TiO2 on samples TiO2eS and P25 during the adsorption process. In addition, the adsorption process of the carbon sample (C) is related to p-p stacking interaction between the methylene blue molecules and the aromatic rings of the carbon structure. For TiO2-0.5C and TiO2-1.0C both electrostatic and p-p stacking interaction take place [8,18,19]. Several kinetic models such as the pseudo first- and secondorder equations and intra-particle diffusion equations have been used to examine the controlling mechanism of the adsorption process [8,18,19]. The linear pseudo first-order equation is given as follows [15]:

logðqe  qt Þ ¼ log qe 

k1 t 2:303

(1)

where qt and qe are the amounts of adsorbed methylene blue at equilibrium (mg g1), respectively, and k1 is the rate constant of the

pseudo first-order adsorption process (min1). The slopes and intercepts of the log plots (qe-qt) versus t (Fig. 1(B)) were used to determine the first-order rate constant (k1) and the amount adsorbed when the equilibrium (qe) was reached. The obtained results using Equation (1) are shown in Table 1. The correlation coefficients obtained for all samples using the first-order kinetic model were low and qe values deviated considerably from the experimental ones. These two observations indicated that the pseudo first-order equation might not be adequate to describe the mechanism for methylene blue removal using all samples as adsorbents.

Table 1 Kinetic constants for methylene blue adsorption by P25, TiO2eS, C, TiO2-0.5C, TiO21.0C. Qe ¼ experimental amount adsorbed at equilibrium. Sample

P25 TiO2eS C TiO2-0.5C TiO2-1.0C

First-order-model

Second-order-model

Qe

k1

qe1

R

k2

qe2

R

0.02 0.02 0.03 0.02 0.02

0.38 0.35 0.97 2.25 2.87

0.93 0.90 0.85 0.93 0.87

0.84 0.87 0.10 0.01 0.005

0.22 0.13 1.24 8.96 15.37

0.997 0.995 0.997 0.998 0.998

0.22 0.12 1.10 8.63 15.24

k1 (min1); k2 (g mg1 min1); qe1 and qe2 (mg g1).

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The linear pseudo second-order equation is given as follows [1]:

t 1 t ¼ þ qt k2 q2e qe

(2)

where k2 is the pseudo second-order rate constant (min1 g mg1). The slopes of the plots t.qt1 versus t give the value of qe and the intercepts give k2. The plot of t.qt1 versus t (Fig. 1(C)) yields very good straight lines using all samples as adsorbents. Table 1 also lists the computed results obtained from the second-order equation. The correlation coefficients for the second-order kinetics equation were near 1.0 for all samples and the calculated qe values showed good agreement with the experimental data. These two observations indicate that the adsorption processes here studied belong to a second order kinetic model. Adsorption kinetics was controlled by the adsorption mechanism and rate-limiting step of the process. The adsorption of MB on adsorbent can occur through three consecutive steps: (1) the external diffusion stage depicting macro-pore or inter-particle diffusion; (2) the gradual adsorption until equilibrium is reached, where the adsorption is controlled by intra-particle (micro-pore) diffusion, and (3) adsorption of methylene blue on the interior surface of the adsorbent. The third step is a fast and non-limiting step in the adsorption process. Therefore, the adsorption rate can be limited by the first (inter-particle diffusion) or the second steps (intra-particle diffusion). The intra-particle diffusion model was used to identify the diffusion mechanism during the adsorption process. It can be expressed by the following equation [20]:

qt ¼ kdif t 0:5 þ Ci

(3)

where qt (mg g1) is the amount of MB adsorbed at time t (min), kdif is the intra-particle diffusion rate constant (mg g1.h0.5) and Ci is the intercept. Fig. 1(D) shows a linearized form plot of intra-particle diffusion model for all adsorbents. It can be seen that the plots of the samples P25, TiO2eS and C for the pronounced portion correspond to the instantaneous adsorption or external surface adsorption. Another indication that intra-particle diffusion is not the rate limiting process is that the plots did not pass through the origin [4]. The deviation from the origin is because of the difference in the mass transfer rate in the initial and final stages of adsorption. However, the plots of samples TiO2-0.5C and TiO2-1.0C are linear with several linear coefficients, implying that more than one process affects the methylene blue removal. Each plot can be divided into three stages and only one of them passes through the origin. It can then be concluded that intra-particle diffusion is not the sole rate limiting step during the adsorption process. Samples TiO2-0.5C and TiO2-1.0C showed close values of kdif for the first and second stages, suggesting that inter-particle and intra-particle diffusions may control the adsorption kinetics (Table 2). One can conclude that the pseudo second-order kinetic model provides the best correlation for all adsorption processes. In addition, the sample TiO2-1.0C was chosen as the ideal adsorbent for the next Table 2 Intra-particle diffusion constants for methylene blue adsorption by P25, TiO2eS, C, TiO2-0.5C, TiO2-1.0C. Sample

P25 TiO2eS C TiO2-0.5C TiO2-1.0C

(First stage) intra-particle diffusion model (second stage) C1

kdif1

R1

C2

kdif2

R2

0.19 0.08 0.84 0.05 0.23

0.001 0.003 0.017 1.265 1.716

0.643 0.787 0.876 0.995 0.994

e e e 5.09 11.40

e e e 0.31 0.24

e e e 0.97 0.98

experiments due to its higher adsorption capacity. The adsorption difference between samples TiO2 and TiO2/carbon is a strong indication that the phases TiO2 and C are not only mechanically mixed, since the carbon concentration is too small. There is a coupled effect when the action of two phases in conjunction is much more intense than when conducted separately. The coupled effect must be the result of a chemical interaction between the carbonaceous phase and TiO2. Fig. 2 shows the FTIR spectra of samples TiO2eS, C, TiO2-0.5C, TiO2-1.0C and TiO2e10C. Samples C, TiO2-0.5C and TiO2-1.0C showed a strong and broad peak around 3430 cm1, which corresponds to the stretching mode of the OeH group. Sample C showed bands around 2932, 2863, 1629, 1446 and 1026 cm1, which were attributed to asymmetric (aCH2), symmetric (sCH2) stretching of CeH, C]C stretching mode and CeO stretching vibrations, respectively. Samples TiO2eS, TiO20.5C and TiO2-1.0C showed a main band at 511 cm1 that corresponds to OeTieO bond vibrations of anatase and rutile phases. TiO2-1.0C also shows a band at 1059 cm1, related to CeO stretching vibrations, which indicates an interaction with the carbon structure. The FTIR spectrum of sample TiO2e10C showed the same peaks of samples TiO2eS, TiO2-0.5C and TiO2-1.0C, however, its peaks at 1057 cm1 is much stronger than TiO2-1.0C sample, which might be associated to TieOeC chemical bonding due to the increase in the carbon content [21,22]. Fig. 3 shows the X-ray diffraction patterns of P25, TiO2eS, C, TiO2-0.5C and TiO2-1.0C. The TiO2eS material prepared has a single and crystalline phase (anatase (JCPDS 21e1272)). Sample C showed broads peaks at 2q ¼ 25 and 2q ¼ 44 corresponding to the micrographitic phase, which is a typical characteristic of carbon materials [4,22,23]. TiO2-0.5C also presented an anatase phase and no peak was observed in the carbon phase. In addition, sample TiO2-1.0C showed anatase and rutile crystalline phases (JCPDS 211276). This can be observed by the intense peak at 2q ¼ 27.5 relative to (110) rutile plane. The rutile phase should be related to the growth mechanism of TiO2 particles on the carbon surface, since sample TiO2-1.0C have more carbon and the influence of this phase on the growth of TiO2 is more pronounced. The formation of rutile normally occurs at 660  C, and the calcination temperature was 400  C, promoting the formation of anatase. The growth of rutile particles on the carbon surface might be associated to carbon bed hot spot, since the minimum temperature required for this phenomenon is between 400 and 500  C. The hot spot occurs due to the formation of localized heated areas in carbon beds, after the exothermic oxidation reaction between carbon and oxygen-bearing species. These hot spot zones stimulate the formation of rutile, and, as TiO2-1.0C sample has a higher carbon content this development phase is accentuated [24]. Calculated through the Scherrer equation, the crystal size of TiO2eS exhibits an average size of 11.41 nm for the (101) plane of the anatase phase (Table 3). With the addition of carbon in the TiO2 matrix the anatase crystal size is reduced to 7.55 nm and 9.18 nm for TiO2-0.5C and TiO2-1.0C, respectively. The TiO2-1.0C also exhibits an average crystal size of 33.50 nm for the (110) plane of the rutile phase. Table 3 also lists the pore size, pore volume and surface area of all adsorbents. With the increasing carbon content of TiO2, the mesopore surface area increases from 30.32 to 67.22 m2 g1, and the total pore volume increases from 0.041 to 0.078 cm3 g1 [16]. Sample C exhibits micropore surface area of 651.83 m2 g1 and pore diameter of 0.74 nm. Despite the high surface area, the presence of micropores is unfavorable for the removal of methylene blue. Fig. 4 shows micrographs of samples TiO2eS (A,E), C (B) and TiO2-1.0C (C,D,F). The micrographs show that the surface of sample TiO2eS (micrograph 4-A) is much less porous than sample TiO21.0C (micrographs 4-D), since they are shown with the same

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5

-OH -C-H C=C C-O

O-Ti-O

TiO2-10C

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Sample C

3500

P25

TiO2-0.5C 4000

TiO2-S

3500

Fig. 2. FT-IR spectra of TiO2e10C, TiO2eS, TiO2-1.0C, C, TiO2-0.5C, and P25 samples.

magnification. In addition, they also show that the carbonaceous

A(101)

R(110)

TiO2-1.0C

A

R AR

A

R A A R

A

Intensity (a.u.)

TiO2-0.5C

Sample C

C C

TiO2-S

P25

10

20

30

40



50

60

70

80

Fig. 3. XRD profiles of TiO2-1.0C; TiO2-0.5C; C; TiO2eS and P25 samples. A ¼ anatase, R ¼ rutile.

phase (micrograph 4-B) grows in a spherical form. A spherical form can also be observed in micrograph 4-C (yellow arrows) with different roughness when compared to micrograph 4-B. The carbon spheres, in this case, are enveloped by TiO2 particles. Micrograph 4C also shows that some spheres are completely filled by TiO2 particles (red arrows). In Fig. 4-E and F the red arrows shows the crystallites that are present in TiO2eS and TiO2-1.0C samples, respectively. On the other hand, the yellow arrows in Fig. 4-E and F shows the pores also present in both samples, TiO2eS and TiO21.0C. It can be observed that the pore and crystallite size are in the same order of magnitude as the BET (pore diameter size) and XRD (crystal size) results. The coupled effect on the capacity adsorption is probably the result of the interaction between the carbonaceous phase and TiO2. It is plausible that this new growth mechanism alters the morphologic properties of the TiO2 particles. TiO2 was synthesized in a medium which already had a solid carbonaceous phase. This condition could grow a TiO2 phase with structural properties that are different from the TiO2eS phase. These “new” structural characteristics could be responsible for the increase in the adsorption capacity of samples TiO2-0.5C and TiO2-1.0C. Therefore, each structural property alone is not solely responsible for the different adsorption capacities. Accordingly, Table 3 shows that samples TiO2eS (30.32 m2/g; 5.45 nm) and TiO2-1.0C (67.22 m2/g; 4.65 nm) have similar mesopore surface areas and pore diameter. The difference between them does not justify the large difference in adsorption capacity (TiO2eS ¼ 0.13 mg g1; TiO21.0C ¼ 15.37 mg g1). These values are not able to explain why sample TiO2eS showed such a low adsorption capacity value, since its surface area is half that of sample TiO2-1.0C and the pore diameter is larger. Fig. 5 shows the nitrogen adsorption/desorption isotherms of samples TiO2eS and TiO2-1.0C. The isotherm of sample

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Table 3 Physicochemical properties of TiO2eS, C, TiO2-0.5C and TiO2-1.0C samples. A ¼ anatase, R ¼ rutile. Sample

Crystal phase

Crystal sizea (nm)

Surface areab (m2 g1)

Pore volume (cm3 g1)

Average pore diameter (nm)

TiO2eS C TiO2-0.5C TiO2-1.0C

A e A A/R

11.41 (A) e 7.55 (A) 9.18 (A)/33.50 (R)

30.32 651.83 54.24 67.22

0.041 0.227 0.049 0.078

5.45 0.74 3.62 4.65

a b

Obtained from XRD, using Scherrer equation: D ¼ 0.9ll/B x cosq, where l ¼ 0.154 nm and B ¼ Full Width at Half Maximum (FWHM) of the highest peak. Surface area calculated by BET and t-plot methods.

Fig. 4. FEG-SEM micrographs of samples TiO2eS (A,E), C (B) and TiO2-1.0C (C,D,F).

TiO2-0.5 is similar to that of TiO2-1.0C and was omitted. The hysteresis of sample TiO2eS is a H2 type (bottle type) and that of sample TiO2-1.0C is a H3 (slit, wedge or layers). Therefore, although both samples show similar mesopore surface area, pore volume and pore size, the shapes of the pores are completely different. Methylene blue molecules cannot enter inside the pores of sample TiO2eS because its size should be larger than the pore opening, justifying the very low adsorption capacity value. In addition, sample TiO2-1.0C was chosen as the ideal adsorbent for the next experiments because of its highest adsorption capacity, thus a complete adsorption characterization was performed. The dosage study was conducted using methylene blue concentration of 10.32 mg L1 and several amounts of Sample TiO2-

1.0C. Fig. 6(A) shows the effect of the adsorbent dose on the removal of methylene blue, showing that the amount of methylene blue removed increased sharply with the increasing adsorbent dose, from 0.005 g to 0.015 g. After that the amount removed slowly increased when the adsorbent dose was changed from 0.015 g to 0.045 g. Consequently, the amount of 0.015 g was chosen as ideal adsorbent dose (maximum removal quantity with minimum adsorbent quantity) of methylene blue and low quantities of adsorbents. This amount of sample TiO2-1.0C was chosen as default mass of the adsorbent in the following experiments. Fig. 6(B) shows the pH effect on the adsorption of methylene blue in the TiO2-1.0C as a function of pH values. At low pH values (<7) the adsorption rate of methylene blue on TiO2-1.0C was small, probably due to the

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30

TiO2-S - adsorption

25

TiO2-1.0C - adsorption

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TiO2-S - desorption

N2 adsorbed (cm /g STP)

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Fig. 5. N2 adsorptionedesorption isotherms of TiO2eS and TiO2-1.0C samples.

(B) 90

Adsorption capacity Removal

12

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6

50

4

40

0.00

0.01

0.02

0.03

Adsorbent dose (g)

0.04

TiO2-1.0C

-1

TiO2-1.0C

14

35

Adsorption capacity (mg.g )

-1

100

Removal of mehtyene blue (%)

Adsorption capacity (mg.g )

(A) 16

30

25

20

15

10

0.05

2

4

6

pH

8

10

12

14

Fig. 6. Effect of adsorbent dosage on adsorption capacity of methylene blue (A). Effect of different values of pH on the adsorption of methylene blue (B) (T ¼ 25  C, adsorbent dose: 0.25 g mL1, C0 ¼ 10.32 mg L1).

increasing methylene blue concentration the active sites for adsorption are completely filled, and then the removal capacity tends to saturation. The Freundlich adsorption equation model is expressed by Refs. [7,18]:

positive charge of the catalyst surface, which repels the cationic dye form of the methylene blue molecule. When the pH of the solution reaches the range of 9.5e14 the interface of the sample is negatively charged, adsorbing the positively charged form of the methylene blue molecule. The equilibrium adsorption isotherm is fundamental to describe the interactive behavior between adsorbates and adsorbents and to define adsorption systems. Fig. 7(A) shows the adsorption capacity of TiO2-1.0C with different dye concentrations (5e50 mg L1). The adsorption capacity increases with the increasing methylene blue concentration and then reached a saturation plateau. With the

1

qe ¼ KF Cen

where KF and 1/n are the Freundlich constants, and KF represents the relative adsorption capacity of the adsorbent and n represents

(A) 25

(B)

TiO2-1.0C

0.00

20

TiO2-1.0C Linear Fit Curve

-0.05

qe (mg.g )

15

-0.10

ln(Kc)

-1

(4)

328 K 313 K 298 K Fit Freundlich isotherm

10 5

-0.15

-0.20

-0.25

0 0

5

10

15

20 -1

Ce (mg.L )

25

30

35

0.00300

0.00305

0.00310

0.00315

0.00320

-1

0.00325

0.00330

0.00335

0.00340

-1

T (K ) 1

Fig. 7. Adsorption isotherms of methylene blue on TiO2-1.0C (A). Plot of lnKc vs. T

for methylene blue adsorption on TiO2-1.0C (B).

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E.A.N. Simonetti et al. / Materials Chemistry and Physics xxx (2016) 1e9

the adsorption dependence on equilibrium concentration of methylene blue. Langmuir isotherm model can be expressed as [18]:

qe ¼

Q0 bCe 1 þ bCe

(5)

where Q0 is the maximum adsorption capacity per adsorbent mass (mg g1) and b is a constant related to the adsorption energy (L mg1). The slopes of the linear form of Freundlich and Langmuir plots were used to determine the adsorption constants tabulated in Table 4. The regression coefficients show that the adsorption data has a better fit to Freundlich than to the Langmuir isotherm. The Freundlich model describes a reversible adsorption on heterogeneous surfaces of the adsorbent and it is consistent with a multilayer adsorption of methylene blue on the TiO2-1.0C structure. The results also revealed that the adsorption capacity increased from 20 to 25 mg g1 when the temperature was increased from 25 to 55  C. The thermodynamic parameters such as change in standard free energy (DG ), enthalpy (DH ) and entropy (DS ) were determined by using the following equations [10]:

ln Kc ¼

D S0 R



DH 0

(6)

RT

DG0 ¼ DH0  T DS0

(7)

where R (8.314 J mol1.K1) is the gas constant, T (K) the absolute temperature and Kc (g L1) is the distribution coefficient defined by   qe.C1 e . DH and DS can be calculated from the slope and intercept of Van't Hoff plots of lnKc versus T1 as shown in Fig. 7(B). The  positive values (Table 5) of DH suggest that the adsorption of methylene blue on TiO2-1.0C is an endothermic process and the  increase in DS values indicates an increase in the randomness at solid/solution interface during the dye removal. With the  increasing temperature the values of DG decrease, indicating more spontaneous processes at higher temperature [21]. Fig. 8 shows the FTIR spectra of TiO2-1.0C before and after methylene blue adsorption. No significant change was observed in intensity and wavenumber in the peak related to OeTieO bonding by the adsorbed molecules. However, the peak related to CeH, C]C and CeO stretching vibrations obtained after adsorption are more intense. Therefore, one can conclude that the adsorption process takes place preferentially by the p-p stacking interaction between the methylene blue molecules and the aromatic rings of the carbon structure [12]. 4. Conclusion TiO2/carbon composites were synthesized using sol-gel and resorcinol-formaldehyde method. Sample TiO2eC is an efficient adsorbent for the removal of MB. The kinetics was better fitted to pseudo second-order and the equilibrium data were best described

Table 4 Langmuir, Freundlich isotherm constants for adsorption of methylene blue on TiO21.0C. Temperature

Langmuir constants

( C)

Qo(mg g1)

b (L mg1)

R

Freundlich constants KF

1/n

R

25 40 55

20.48 21.94 25.73

3.57 2.59 1.28

0.83 0.88 0.91

16.25 16.12 17.51

3.65 4.54 5.81

0.98 0.99 0.98

Table 5 Thermodynamic parameters for methylene blue adsorption on TiO2-1.0C. T (K)

DG (J mol1)

DH (kJ mol1)

DS (J mol1 K1)

298 313 328

568.81 296.48 24.16

5.979 e e

18.155 e e

4

0

Transmittance (%)

8

-4

-C-H

-12

-16

C-O

C=C

-8

-OH

O-Ti-O

TiO2-1.0C Before adsorption TiO2-1.0C After Adsorption

-20 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 8. FT-IR spectra of TiO2-1.0C sample before and after methylene blue adsorption (C).

by the Freundlich model and also the process is endothermic. The combination of TiO2 and carbon allows the formation of anatase and rutile phases at 400  C. The carbon structure was covered by rutile particles. This allows the formation of high surface area, mesoporous surfaces and wedge pores, which favor the methylene blue adsorption. It is believed that the interaction between TiO2 and carbon, as well as the carbon amount, is fundamental for preparing nanocomposites with appropriate characteristics for the adsorption process, a topic which deserves to be further investigated. References [1] M. Ghaedi, A.G. Nasab, S. Khodadoust, M. Rajabi, S. Azizian, Application of activated carbon as adsorbents for efficient removal of methylene blue: kinetics and equilibrium study, J. Ind. Eng. Chem. 20 (2014) 2317e2324. http:// dx.doi.org/10.1016/j.jiec.2013.10.007. [2] M. Ghaedi, M.D. Ghazanfarkhani, S. Khodadoust, N. Sohrabi, M. Oftade, Acceleration of methylene blue adsorption onto activated carbon prepared from dross licorice by ultrasonic: equilibrium, kinetics and thermodynamic studies, J. Ind. Eng. Chem. 20 (2014) 2548e2560. http://dx.doi.org/10.1016/j.jiec.2013. 10.039. [3] M. Roosta, M. Ghaedi, a Daneshfar, R. Sahraei, a Asghari, Optimization of the ultrasonic assisted removal of methylene blue by gold nanoparticles loaded on activated carbon using experimental design methodology, Ultrason. Sonochem. 21 (2014) 242e252. http://dx.doi.org/10.1016/j.ultsonch.2013.05.014. [4] H. Cherifi, B. Fatiha, H. Salah, Kinetics studies on the adsorption of methylene blue onto vegetal fiber activated carbons, Appl. Surf. Sci. 282 (2013) 52e59. http://dx.doi.org/10.1016/j.apsusc.2013.05.031. [5] J. Zhang, D. Cai, G. Zhang, C. Cai, C. Zhang, G. Qiu, K. Zheng, Z. Wu, Adsorption of methylene blue from aqueous solution onto multiporous palygorskite modified by ion beam bombardment: effect of contact time, temperature, pH and ionic strength, Appl. Clay Sci. 83 (2013) 137e143. http://dx.doi.org/10. 1016/j.clay.2013.08.033. [6] P. Wang, M. Cao, C. Wang, Y. Ao, J. Hou, J. Qian, Kinetics and thermodynamics of adsorption of methylene blue by a magnetic graphene-carbon nanotube composite, Appl. Surf. Sci. 290 (2014) 116e124. http://dx.doi.org/10.1016/j. apsusc.2013.11.010. [7] J.-G. Yu, L.-Y. Yu, H. Yang, Q. Liu, X.-H. Chen, X.-Y. Jiang, X.-Q. Chen, F.-P. Jiao, Graphene nanosheets as novel adsorbents in adsorption, preconcentration and removal of gases, organic compounds and metal ions, Sci. Total Environ. 502 (2015) 70e79. http://dx.doi.org/10.1016/j.scitotenv.2014.08.077. [8] Y. Wang, X. Zhang, X. He, W. Zhang, X. Zhang, C. Lu, In situ synthesis of MnO2

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E.A.N. Simonetti et al. / Materials Chemistry and Physics xxx (2016) 1e9

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