Remarkable Ti-promotion in vanadium doped anatase titania for methylene blue adsorption in aqueous medium

Remarkable Ti-promotion in vanadium doped anatase titania for methylene blue adsorption in aqueous medium

Journal of Environmental Chemical Engineering 6 (2018) 5212–5220 Contents lists available at ScienceDirect Journal of Environmental Chemical Enginee...

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Journal of Environmental Chemical Engineering 6 (2018) 5212–5220

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Remarkable Ti-promotion in vanadium doped anatase titania for methylene blue adsorption in aqueous medium

T



Kamalesh Pala, , Kalyan Ghoraia, Sudiksha Aggrawalb, Tapas Kumar Mandalb, Paritosh Mohantyb, ⁎ Md Motin Seikhc, Arup Gayena, a

Department of Chemistry, Jadavpur University, Kolkata, 700032, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247667, India c Department of Chemistry, Visva-Bharati University, Santiniketan, West Bengal, 731235, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: V2O5 TiO2 V-doped TiO2 Surfactant assisted coprecipitation Methylene blue adsorption

We report here the synthesis of vanadium (5–20 at.%) doped titania and the pristine oxides via surfactant assisted coprecipitation method using cetyltrimethylammonium bromide as the surfactant and investigated their adsorption behavior towards hazardous methylene blue dye. The formation of anatase phase in all the doped materials is confirmed by the powder x-ray diffraction (P-XRD) analysis as corroborated by the Fourier transform-infrared (FT-IR) analysis, with particle sizes in the range 8–11 nm. Pure titania doesn’t show any adsorption property, but its presence enhances the adsorption property of vanadia considerably suggesting a promoting role of titania in the doped materials. The 10 at.% V-doped material, V0.1Ti0.9O2 is found to exhibit the best adsorption behaviour. The low resolution transmission electron microscopy studies reveal homogeneous nature, while ring type electron diffraction pattern suggests polycrystalline nature of the material. The high resolution transmission electron microscopy analysis suggests formation of single phase oxide. Kinetic and equilibrium studies have revealed the pseudo-second-order kinetics and Langmuir isotherm nature for this physisorption process. The pH dependency and recycling ability of this material have been discussed. Finally, X-ray photoelectron spectroscopy, FT-IR and P-XRD studies have been applied to understand the mechanism of regeneration of the material.

1. Introduction The presence of dyes in industrial wastewater is a major concern due to their adverse effects to many forms of life. Industries like textile, leather, paper, plastics, etc., use dyes in order to introduce colorful products. They consume a large amount of water during colorization technique and thus they also generate a considerable amount of colored wastewater [1]. Thus, dye wastewater needs to be treated for the protection of human health and environmental safety. The discharge of dyes in the environment is a dramatic source of aesthetic pollution, eutrophication and perturbation degradation [2]. It is well-known that public awareness regarding water quality is greatly influenced by the color because color is the first contaminant to be identified in wastewater. Even a very small amount of dyes in water (less than 1 mg L−1 for some dyes) is highly visible and undesirable [3,4]. Methylene blue (MB; C16H18N3SCl), a familiar basic dye is usually used for dying cotton, wood and silk. It can cause permanent injury to the eyes of human and animals. Several other injuries like burning



sensation of mouth, nausea, vomiting, profuse sweating, mental confusion and methemoglobinemia may also be caused by this dye [5–7]. MB remains very stable in the environment due to its synthetic origin and complex aromatic molecular structure. Therefore, the treatment of effluent containing such a dye is an important issue for researchers and environmentalists. Several treatment processes have been reported for the removal of these kinds of dyes from wastewater, such as: photocatalytic degradation [8,9], electrochemical degradation [20], sonochemical degradation [10], integrated chemical–biological degradation [11], photoassisted-biological treatment [12], Fenton-biological treatment scheme [13], cation exchange membranes [14] and adsorption processes [15–18]. Among the several separation techniques adsorption is a very effective process for the removal of such dyes because of its low initial cost, insensitivity to the toxic substances, flexibility in design and simplicity in operation [19,20]. Adsorption does not result in the formation of harmful substances and thus it is a well known equilibrium separation technique and an effective method for water

Corresponding authors. E-mail addresses: [email protected] (K. Pal), [email protected], [email protected] (A. Gayen).

https://doi.org/10.1016/j.jece.2018.08.015 Received 29 May 2018; Received in revised form 21 July 2018; Accepted 7 August 2018 Available online 08 August 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.

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per step and analyzed by the database of ICDD (International Centre for Diffraction Data) for phase identification. Average particle sizes were calculated from the line-width broadening of the peaks using Scherrer’s equation. Fourier transform-infrared (FT-IR) spectra were recorded on the powdered materials in transmission emission mode in a Perkin Elmer RX-1 FTIR spectrometer in the range of 4000–400 cm−1. The high resolution transmission electron microscopy (HRTEM) experiment was performed on a JEOL (2000EX II) microscope operated at 200 kV. The energy-dispersive X-ray spectroscopy (EDX) elemental mapping analysis were performed on a JEM ARM200 F cold FEG double aberration corrected electron microscope equipped with a large solidangle CENTURIO EDX detector and Quantum GIF. Materials for TEM were ground powder under ethanol, and the resulting dispersion was transferred to a holey carbon film deposited on Ni supported grid.

decontamination applications [21–23]. TiO2 plays a very important role in the photodegradation of such dyes due to the narrow band-gap value of 3.2 eV for the anatase form [24]. Besides photoactivity, TiO2 also shows adsorption behavior due to its tunable surface properties, i.e., acidity/ basicity or open coordination sites, of the resultant materials, which is of importance to the adsorption of molecules [24]. Many researchers have also reported the beneficial effect of vanadium on TiO2 by enhancing its photocatalytic activity [2,25,26] and enable it in the field of gas sensing ability [27]. With this background, we have prepared pure oxides of titanium and vanadium, and a number of their mixed oxides by surfactant assisted co-precipitation (SAC) method. We have chosen the cationic dye MB as the adsorbate because of its known strong adsorption onto solids. Based on material screening, Ti0.9V0.1O2 is reported to show the best adsorption property. The equilibrium and adsorption kinetics, materials reusability and pH dependence of the adsorption property are discussed.

2.2.2. Surface area analysis The surface area and porosity analysis of the materials were analyzed from the N2 sorption isotherms using Autosorb iQ (Quantachrome Inc.) gas sorption instrument. Prior to N2-sorption experiments all the materials were degassed at 90 °C for 1 h.

2. Experimental 2.1. Material preparation

2.2.3. XPS analysis The XPS analysis was carried out by PHI-5000 VersaProbe III ULVAC-PHI Inc., XPS spectrophotometer. The analysis was performed by mounting the specimens on the carbon tape and keeping it overnight in the sample introduction chamber under vacuum. The measurement was performed using monochromatic AlKα radiation (1486.6 eV) under ultra-high vacuum condition. The binding energy for individual elements are scaled with reference to C1 s at 284.8 eV.

Pure and doped materials of titanium and vanadium with nominal composition VxTi1-xO2 (x = 0, 0.05, 0.1, 0.15, 0.2 and 1) were prepared by a surfactant assisted coprecipitated technique using cetyltrimethylammonium bromide (CTAB). In a typical synthetic process, 3.64 g (0.1 M in the resulting solution) of CTAB (Spectrochem India, 98%) was dissolved in 50 mL of millipore water in a beaker and in another beaker calculated amount of ammonium metavanadate (VN; Sigma Aldrich, 97%) was dissolved in 50 mL of millipore water. The solutions were left for stirring until clear solution appeared. The contents of the beakers were then mixed and stirred again for 15 min. Titanium tetraisopropoxide (TTIP; Sigma Aldrich, 97%) was then added drop wise in this mixture under vigorous stirring. After complete addition of TTIP, the solution was left for ageing for 1 h. The solution was then alkalinized with NH4OH (∼pH 10) and the mixture was left for ageing for overnight under constant stirring at 400 rpm. The precipitate obtained was separated by centrifuging at 10,000 rpm (REMI PR-24). The crude obtained was washed 3 times with millipore water followed by drying in an oven at 110 °C for ∼4 h. The materials of this study are the ones obtained on calcination of the crushed crude materials at 400 °C for 3 h at a heating rate 10 °C min−1. Table 1 lists all the materials studied here with their name and nominal composition.

2.3. Preparation of dye stock solution The dye stock solution was prepared by dissolving calculated amount of MB (Merck India Ltd. with water solubility of 50 g L–1 at 20 °C) in millipore water to the concentration of 100 mg L−1. The experimental solutions of different concentrations were obtained by diluting the dye stock solution in accurate proportions. 2.4. Equilibrium studies For equilibrium studies, different initial concentrations of MB (C0 = 20, 30, 40 and 50 mg L−1) contained in a beaker were treated with 20 mg of V10TO material (based on screening tests). The pH of the mixtures was adjusted to 7.0 using buffer solution. The mixtures were then magnetically stirred (∼350 rpm) to reach the equilibrium. After 3 h, the suspensions were taken, centrifuged at ∼14,000 rpm for 2 min and the residual MB concentrations were analyzed by a Jasco spectrophotometer (Model no. V-630, Japan) at 662 nm using a of 1 cm path length. The amount of MB adsorbed (qe mg g−1) by the material in each case was calculated using the mass balance equation:

2.2. Material characterization 2.2.1. Structural and microstructural analysis The powder XRD data were collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. The XRD patterns were recorded in the 2θ range between 10° and 100° using Lynxeye detector (1D mode) and a scan time of 2 s

qe = (C0 − Ce ) ∗V / M

Table 1 List of synthesized materials with name, BET and porosity data and MB adsorption property for the screening of materials. Composition

Name

Scherrer size (nm)

Surface area (m2 g−1)

Half pore width (Å)

Pore volume (cm3 g−1)

MB adsorption, q (mg g−1) at 3h

TiO2 V0.05Ti0.95O2 V0.1Ti0.9O2 V0.15Ti0.85O2 V0.2Ti0.8O2 V2O5

TO V5TO V10TO V15TO V20TO VO

11 9.6 8.4 8.8 8.8 39

174 166 170 168 168 17

18 18.84 18.97 18.97 18.97 8.44

0.09 0.15 0.17 0.23 0.33 0.018

0 37 56 52 53 35

And the MB removal (%) was calculated using the following equation:

MB removal(%) = (C0 − Ce ) ∗100/C0 where, C0 and Ce (mg L−1) are the initial and equilibrium concentrations of MB. V is the volume of the solution (mL) and M is the mass of catalyst (mg). To estimate the equilibrium parameters, four models were used: Langmuir, Freundlich, Temkin and Dubinin-Radushkevich [22]. 2.5. Kinetic studies All the kinetic studies were performed using the same initial 5213

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Table 2 Review of some adsorption and kinetic models. Sl. No.

Equations

1

Adsorption isotherm Langmuir adsorption isotherm Ce qe

=

RL =

2

1 q0 KL

Parameters and their significance

+

1 1 + KL Ce

Freundlich adsorption isotherm 1 n

ln qe = ln KF + 3

ln Ce

Temkin adsorption isotherm RT b

qe = 4

q0 (mg g−1)= Maximum adsorption capacity for monolayer KL= Langmuir adsorption constant RL= Feasibility of the adsorption process 1/n = Intensity of adsorption KF (mg g−1(L mg−1)1/n)=Adsorption coefficient b = Heat of adsorption A = Binding energy

Ce q0

ln A +

RT b

ln Ce

lnqe = lnqD − βε 2 1 ) Ce (−2β )−0.5

ε = RT (1 +

E=

1 2

Kinetic model Pseudo first order kinetic model ln (qe − qt ) = ln qe − k1 t Pseudo second order kinetic model t qt

=

1 k 2 qe2

+

Fig. 1. P-XRD patterns of pure oxides of titanium and vanadium and the various vanadium doped titania obtained at 400 °C.

qD = Theoretical adsorption capacity β (kJ−2 mol2)= A constant related to the mean free energy of adsorption per mole of MB ε = Polanyi potential E = Free energy

Dubinin-Radushkevich isotherm

crystallite size calculated from full width at half maxima (FWHM) of (101) diffraction lines lie in the range 8–11 nm for all titania related materials.

k1 (min−1)= Pseudo-firstorder rate constant

3.2. BET studies

k2 (g mg−1 min−1)= Pseudosecond-order rate constant

t qe

The BET specific surface areas (SAs) of the pure and doped titania materials are found to be high and lay in the range of 166–174 m2 g−1 (see Table 1). Although the SA is considerably low (∼17 m2 g-1) for pure vanadium oxide, its doing in titania doesn’t have such a lowering effect. Among the doped materials, 10 at.% vanadium doped material that is shown in our studies to have the best adsorption activity possesses the highest surface area.

Thermodynamic parameters 1

ΔG 0 = − RT ln KD ln KD =

KD =

qe Ce

ΔS 0 R



ΔH 0 RT

ΔG° = Gibbs free energy change ΔH° = Enthalpy change ΔS° = Entropy change KD= Distribution coefficient R= Gas constant (8.314 J mol−1 K−1) T = Temperature (K)

3.3. FTIR analysis concentrations of MB and mass of V10TO material keeping other conditions identical to the equilibrium studies. The mixtures were magnetically stirred at ∼350 rpm. At appropriate time intervals, the suspensions were taken, centrifuged and analyzed by the same UV–vis spectrophotometer. The amounts of MB adsorbed and MB removal (%) were calculated similarly. The corresponding kinetic parameters were calculated using two different models namely, pseudo-first-order and pseudo-second-order kinetic models [16].

Fig. 2 shows the FTIR spectra of pure and doped titania materials. For all the materials broad bands observed around 3400 and 1634 cm−1 are assigned to the stretching and bending modes of physisorbed water on titania [28]. The vibrational mode of the Ti-O-Ti stretching is observed at 427 cm−1 [5]. The symmetric stretching mode of V]O of the terminal oxygen and the asymmetric and symmetric stretching modes of VeOeV of bridged oxygen are observed at 1007, 819 and 583 cm−1, respectively [25].

2.6. Thermodynamic studies To understand the nature of adsorption and the effect of temperature on the adsorption of MB onto V10TO, the changes in free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were determined [17]. The required equations for the calculation of these thermodynamic parameters are listed in Table 2. 3. Results and discussion 3.1. X-ray diffraction analysis The powder XRD (P-XRD) patterns of pure and doped materials of titanium and vanadium obtained on calcination at 400 °C are shown in Fig. 1. The major diffraction peaks for VO belongs to the Pmmn space group with orthorhombic structure (ICDD PDF # 89-0612). The major diffraction peaks of the other materials are indexed with a tetragonal structure belonging to I41/amd space group (ICDD PDF # 89-4921) and confirms the formation of pure anatase phase of TiO2. The average

Fig. 2. FTIR spectra of pure and doped oxides of titanium and vanadium. 5214

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3.6.1. Applicability of the different isotherm models It is obvious from the fitted data of Table 3 that the adsorption of MB can appropriately be described by the monolayer Langmuir isotherm with coefficient of determination, R2 value > 0.99. The maximum monolayer MB adsorption capacity for V10TO material was calculated to be 57.47 mg g−1, which agrees quite well with the experimental value of 56.64 mg g−1. The low value of KL indicates that the van der Waals type of adsorption is predominant in the MB adsorption process [16]. In this regard the feasibility of the adsorption process (RL) was found to be in the range 0 < RL < 1, confirming monolayer adsorption as the favorable process. Specifically, the RL values decrease from ∼0.396 to ∼0.039 with the increase in initial concentration of MB solution from 20 ppm to 50 ppm indicating the more favorable adsorption at lower MB concentration. The lower value of R2 for Freundlich isotherm (see Table 3) suggests that multilayer adsorption was not favored. The applicability of Temkin model is associated with a decrease of adsorption subsequent to covering of the solid surface, which gave lower values for both the binding energy (0.72 kJ mol−1) and R2 (0.75). The adsorption energy (E) was calculated to be 2.15 kJ mol−1 for the Dubinin–Radushkevich isotherm suggesting physisorption nature of the MB adsorption onto V10TO.

Fig. 3. Adsorption behavior of pure titania and its V-doped oxides.

3.4. Screening of materials For screening stage we used 50 mL of 10 mg L−1 MB dye solution with 5 mg of each material and the data are plotted in Fig. 3. Pure titania does not show any adsorption property, while pure vanadium oxide shows an adsorption of ∼10% (∼8 mg g-1) initially that is increased to ∼33% (∼30 mg g-1) after 3 h. All the vanadium doped titania oxides show an increase in adsorption behavior suggesting promoting effect of titanium in the doped oxide (see Fig. 3). The 5 at.% vanadium doped material shows a sharp rise in the activity at the initial stage (up to 30 min) and the adsorption activity lies always above that of the pure oxide of vanadium. The adsorption study clearly reveals that the 10% vanadium doped titania shows the maximum adsorption activity. Further loading of vanadium results in a decrease of adsorption behavior of the material (both V15TO and V20TO shows nearly similar adsorption behavior). Therefore V10TO is chosen as the best material for rest of the studies.

3.7. Adsorption kinetics MB adsorption kinetics of V-doped titania is studied using two well known models, namely, (a) pseudo-first order and (b) pseudo-second order kinetics and the experimental as well as the fitted data are shown in Fig. 6. It is observed that there is maximum deviation of the experimental points from the fitted line in pseudo-first order kinetics (see Fig. 6). The deviation between the experimental points and the fitted line is minimum when a pseudo-second order kinetics is being considered (see Fig. 6). Various parameters like the rate constant (from the slope) and mass of adsorbed MB at equilibrium (from the intercept value), calculated from the kinetic analysis are presented in Table 4. The rate constant values suggest a decrease with an increase in the MB concentration.

3.5. Microstructural analysis 3.7.1. Applicability of pseudo-second-order kinetic model According to this model, adsorption rate depends on both the adsorption sites of adsorbent and adsorbate/solute molecules, and related to the numbers of occupied sites and the vacant available sites for the adsorbent at equilibrium. The formation of a chemical bond between an adsorption site and a solute molecule is the rate-limiting step. The calculated parameters of the model confirms much better fitting with R2 value more than 0.999 (≈1.00) when compared with the pseudo-firstorder kinetic model (see Table 4). Specifically, the close agreement between the experimental qe value of 56.64 mg g−1 and the calculated value of 54.95 mg g-1 (for the maximum concentrated solution) unequivocally suggests the applicable model to be pseudo-second-order for the adsorption process.

The transmission electron microscopy (TEM) image of V10TO indicates that the material is purely homogeneous and the particle size is fairly small (see Fig. 4(a)). The average particle size is ∼10 nm, which is in good agreement with the XRD results. The lattice fringe analysis (d = 0.35 nm) obtained from the high resolution transmission electron microscopy (HRTEM) clearly displays the (101) plane of anatase titania (see Fig. 4(b)). The selected area electron diffraction (SAED) also exhibits diffraction rings corresponding to the (101), (103), (200), (105) and (213) lattice planes (inside out) signifying the polycrystalline nature of the anatase titania (see Fig. 4(c)). The EDX spectra with the material composition (see Fig. 4(d) & (e)) suggest that Ti : V is very close to the nominal composition.

3.8. Thermodynamic parameters 3.6. Adsorption isotherms The calculated thermodynamic parameters at various temperatures are presented in Table 5. The negative ΔG° values confirm the feasibility and spontaneity of the adsorption process. Also, the ΔG° values increase with the increase in temperature suggesting better performance of the V10TO at lower temperature as expected for a physisorption process. The ΔH° and ΔS° values for the investigated temperature range were calculated respectively from the slope and intercept of ln KD versus 1/T plot shown in Fig. 7. The ΔH° value for the MB adsorption onto V10TO was found to be −18.3 kJ mol−1, which confirms physisorption and exothermic nature of the adsorption process. The ΔS° value was also found to be negative that could be due to the restricted movement of the free MB molecules at the material interface compared to that in the

The nature of MB adsorption isotherms of the V-doped titania is considered using four well-known adsorption isotherms, namely, (a) Langmuir, (b) Freundlich (c) Temkin and (d) Dubinin-Radushkevich models. The fitted data are shown in Fig. 5, while the values of the corresponding parameters are listed in Table 3. It is noted that there is minimum deviation in the experimental points from the fitted line when the Langmuir model is considered (see Fig. 5(a)). Whereas, the deviation is maximum when the experimental data are fitted following a Dubinin-Radushkevich adsorption isotherm (see Fig. 5(d)). Deviations are also clearly evident from the fittings based on Freundlich and Temkin models (see Fig. 5(b) and (c)). 5215

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Fig. 4. (a) Low resolution TEM, (b) HRTEM, (c) SAED, (d) EDX spectra and (e) EDX elemental composition of V10TO.

solution phase.

amount in the washing process.

3.9. pH dependent behavior

3.11. Material properties subsequent to adsorption and regeneration mechanism

Adsorption activity of MB solution at different pH was analyzed by the Britton–Robinson buffer (BR buffer) when other conditions for the adsorption process were kept unchanged (see Fig. 8). It is observed that only 1% adsorption takes place at a lower pH of 4 essentially due to the acidic nature of MB. So the adsorption behavior is inhibited in the acidic medium, whereas the catalyst shows much improved adsorption behavior at higher pH values (neutral to alkaline) as expected.

To understand the recycling behavior and to look into the mechanistic aspect of MB adsorption, we have carried out (i) XPS surface analysis of the as-prepared (V10TO_AP) material, material collected after an adsorption cycle (V10TO_Aged) and the regenerated material (V10TO_Regn); (ii) bulk phase analysis of these three samples by along with the material collected after an adsorption cycle followed by the washing steps (V10TO_Washed) by P-XRD and (iii) FTIR analysis of these four samples in comparison to that of MB. Fig. 10(a) shows the Ti 2p XP spectra of V10TO in their as-prepared, aged and regenerated forms. The Ti2p3/2,1/2 spin–orbit doublets centered at binding energies (BEs) around 458.6 and 464.2 eV for AP and Regn materials can be attributed to Ti4+ species (see Fig. 10(a)). There is a shift of about 0.3 eV in the BE position on ageing suggesting minor reduction of the material during MB adsorption possibly due to formation of TiOx kind of species. Fig. 10(b) shows the V2p and O1 s core level region of the three materials. Accordingly, the V 2p3/2,1/2 spin–orbit doublets centered at BE around 516.4 and 524.0 eV for AP and Aged materials correspond to V4+ species. There occurs an increase in the BE by about 0.5 eV for the Regn material suggesting little more increase of oxidizing environment of vanadium on regeneration. Broad nature of the O 1 s region possibly suggests presence of a different kind of oxide species on the surface. The powder XRD patterns of the four forms of the materials are shown in Fig. 11(a). No signature of change in phase composition is

3.10. Recycling ability The stability of the synthesized catalyst was evaluated for four consecutive cycles. For the first run, 20 mg of V10TO was taken in 45 mL of millipore water and 5 mL of MB stock solution was added to it with constant stirring at ∼350 rpm in a dark place. After each run, the material was recovered with centrifugation, followed by washing, first with 1 N HCl solution then with millipore water. The crude was then dried at 110 °C for 2 h. The dried crude material was subsequently calcined at 400 °C for 3 h to regenerate the material that was used as the catalyst for the next run. The results are displayed in Fig. 9 from which we may conclude that the present vanadium doped titania catalyst shows nearly similar activity in each cycle of adsorption and experiences only a minor loss. Specifically, the adsorption reached around 86% in just 5 min and 93% in 1 h for the 1st cycle. This is followed by a little (∼2%) decrease in adsorption in the subsequent cycles. This loss of activity may be ascribed to be due to the certain loss of the catalyst 5216

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Fig. 5. MB adsorption isotherms using V10TO as catalyst and considering (a) Langmuir, (b) Freundlich, (c) Temkin and (d) Dubinin-Radushkevich models.

3.12. Adsorption mechanism

Table 3 Isotherm parameters for the adsorption of MB onto V10TO.

Three types of interactions play main role in adsorption reaction, namely electrostatic attraction, physical adsorption and chemical adsorption. Based on the above results, we may confirm the physical nature of MB adsorption onto V10TO, as reflected by the various parameters of different isotherm models. The monolayer adsorption type with bond energy below 80 kJ mol−1 highly recommend the physical nature. The study of kinetics of the process shows that the correlation coefficients of pseudo-second-order model is about unity, which also supports the physisorption phenomenon. It is to be highlighted that the BET specific surface area of V10TO is ∼ 174 m2 g−1, which provides sufficiently large area for this physical adsorption process. Both Kupfer et al. [15] and Siddiqui et al. [22] have confirmed the physisorption nature of MB on these kinds of oxide materials. Siddiqui et al. have derived two models for MB adsorption process, namely intra-particle diffusion model and liquid film diffusion model and they have advocated for the second model that gave well fitted data for the adsorption process. Whereas, Kupfer et al. have suggested the main role to be played by the intra-particle diffusion model. Our present study shows that the MB adsorption onto V10TO is a physical adsorption and follows a pseudo-second-order kinetics in parity with the earlier reports of MB adsorption as discussed above. A comparative study on the MB adsorption capacity is also done (see Table 6), from which one can assess the beneficial role of this kind of simple and well-known doped oxide material for use in the removal of hazardous dye like methylene blue.

Parameters Langmuir Freundlich

Temkin DubininRadushkevich

q0 (mg g−1) 57.47 KF (mg g−1(L mg−1)1/n) 43.51 A (L g−1) 3.31 qD (mg g−1)

KL (L mg−1) 0.90 n

R2 0.992 R2

14.75 b (kJ mol−1) 0.72 β (kJ−2 mol2)

52.76

0.11

0.787 R2 0.753 E (kJ mol−1) 2.15

R2 0.413

noted for all the forms, except the occurrence of an undefined impurity peak (at ∼18°) in the aged and washed material. Calcination leads to complete regeneration of the material. The FTIR analysis (see Fig. 11(b)) of the various forms of the catalyst together with that of MB clearly suggests its adsorption (V10TO_Aged) and almost removal after washing (V10TO_Washed) that is completed after the regeneration treatment (V10TO_Regn). Washing in acid and alkali seems not sufficient enough, suggesting possible presence of some chemisorbed species. Therefore the doped material can only be regenerated completely through introduction of an intermediate oxidation treatment and hence can be used continuously (repeated four times) for the removal of MB, which is really attractive. More importantly, such regenerated phase exhibits the similar activity as the pristine phase. Thus one can consider such periodic behavior as on-off-on modes of performance towards MB adsorption activity and can MB solution

be presented as “VTO ⥫==============⥬ (VTO ) MB”. washing followed by oxidation

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Fig. 6. MB adsorption kinetics evaluated on the basis of (a) pseudo-first order and (b) pseudo-second order models for V10TO material at 298 K.

Table 4 The values of parameters and correlation coefficients of kinetic models at 298 K. MB concentration (mg L−1)

Parameters

C = 20 Experimental value 45.77 qe (mg g−1) Pseudo-first order model 0.025 k1 (min−1) qe (mg g−1) 9.37 R2 0.90 Pseudo-second order model k2 (g mg−1 min−1) 0.015 qe (mg g−1) 45.66 R2 1.00

C = 30

C = 40

C = 50

49.10

52.23

56.64

0.020 13.16 0.95

0.019 13.43 0.94

0.017 15.47 0.98

0.010 48.31 1.00

0.010 51.02 1.00

0.008 54.95 1.00

Table 5 Thermodynamic parameters for methylene blue adsorption on V10TO. T (K)

KD

ΔG° (kJ mol−1 K−1)

ΔH° (kJ mol−1 K−1)

ΔS° (J mol−1 K−1)

293 298 303 313

3.05 2.73 2.42 1.89

−2.72 −2.49 −2.23 −1.66

−18.3

−53.1

Fig. 8. MB adsorption activity of V10TO at different pH.

Fig. 9. Recycling ability of V10TO material in MB adsorption.

4. Conclusions This study shows that vanadium doped titania exhibits great activity towards MB dye adsorption. It was found that 10 at.% vanadium doped titania shows best performance on MB adsorption. The adsorption kinetics could be quite successfully fitted by a pseudo-second order kinetic equation and the equilibrium adsorption can be explained on the basis of Langmuir isotherm model. The physisorption of MB onto V10TO is predominantly driven by weak van der Waals attractive force. The feasibility of the process is observed by noting the RL value, which

Fig. 7. Plot of ln KD vs. 1/T for methylene blue adsorption.

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Fig. 10. XPS core level spectra of (a) Ti 2p and (b) V 2p regions of AP (black line), Aged (red line) and Regn (blue line) of V10TO.

Fig. 11. (a) Powder XRD patterns and (b) FT-IR spectra (along with that of MB) for V10TO material in AP, Aged, Washed and Regn forms.

polluting world.

Table 6 Comparative study of methylene blue removal capacity of different adsorbents. Adsorbent

MB removal capacity (mg g−1)

Ref.

Cortaderia selloana flower spikes Phragmites australis ZnS:Ni-NP-AC Cu(OH)2-NP-AC Raw Algerian kaolin Spent yerba mate Ilex paraguariensis Kaolin Zeolite 4A Ball Clay V10TO

40.00 22.70 21.79 32.90 52.76 52.00

[29] [30] [31] [32] [33] [34]

45.00 22.00 25.10 56.64

[35] [35] [36] Present study

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decreases with the increase of MB concentration. The spontaneity of the adsorption is reflected by the negative value of ΔG°, which increases at higher temperature. The pH of the reaction mixture also plays a vital role towards adsorption. The reusability of the doped material for the removal of MB can be achieved through involvement of a regeneration step. Keeping in mind the role of MB in various injuries, this improved MB adsorption activity of anatase TiO2 in the presence of vanadium ions, the vanadia doped titania oxide, may play a crucial role in the field of adsorption catalysis by changing various operational parameters and expected to be useful for applications such as the waste water treatment in hazardous chemical industry making a relatively less 5219

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