Encapsulation of Vanadium Phosphorus Oxide into TiO2 Matrix for

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Encapsulation of Vanadium Phosphorus Oxide into TiO2 Matrix for Selective Adsorption of Methylene Blue from Aqueous Solution Zahra Akbari,† Mehran Ghiaci,*,† and Fahime Nezampour† †

Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran

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S Supporting Information *

ABSTRACT: Vanadium phosphorus oxide (VPO) has been embedded in a titania matrix. The adsorption capacity of the VPO(5−30 wt %)@TiO2 composites and VPO alone for the removal of harmful cationic and anionic dyes such as methylene blue (MB), rhodamine B (RhB), and methyl orange (MO) have been extensively investigated. The FT-IR, XRD, TGA, FE-SEM, EDX, and BET techniques were applied for characterization of the adsorbents. Batch-adsorption experiments demonstrated that the VPO(30 wt %)@TiO2 as the best composite selectively removes MB with more than 94% removal efficiency. The adsorption behavior of the MB on the VPO(5−30 wt %)@TiO2 composites was investigated with variations in the mass of sorbent, initial dye concentration, solution pH, temperature, and contact time. Kinetic experiments demonstrated that the pseudo-second order kinetic model showed the best fit with the adsorption data. The efficiency of the VPO(30 wt %)@TiO2 as the preferred composite remained unchanged after five adsorption cycles, and could be regenerated easily by simple washing with NaOH (0.1 mol/L) and water.

1. INTRODUCTION The fear of human beings of global warming and severe water contamination is understandable, and these phenomena require serious efforts for correction by NGOs and governments. Considering the strict regulations that governments have regarding water quality, removing pollutants such as colors, fertilizers and toxic substances from water prior to consumption by humans and animals is essential.1−4 In this respect, water contamination in particular has attracted the interest of scientific research communities.5−7 Among a number of industries such as textile, pharmaceutical, and paper processing, the textile industry is probably the major source of discharging colored materials to water resources.3,8−10 The colored materials could prevent sunlight penetration into water, and as a result disturb the photosynthetic reactions of plants and even more seriously, some synthetic dyes could cause severe damages to ecological environment and human beings.3,11,12 Moreover, certain dyes produce chemicals, which can be carcinogenic even at low concentrations, by degradation.8,13,14 Among common dyes, methylene blue (MB) and rhodamine B (RhB) are used as coloring agents.15 MB is an important cationic dye, which is very soluble in water and is widely used for dyeing, printing cotton and tanning, and dyeing leather.9,16−19 In the past, RhB was used as a food color. However, the application of RhB has been restricted due to a suspected carcinogenic nature.15 Therefore, removing RhB as a pollutant from water resources is essential. Several methods have been used for removing contaminants from wastewaters, such as adsorption, oxidation, photocatalytic © XXXX American Chemical Society

oxidation, ion exchange, coagulation, electro-oxidation, and membrane separation.4,6,7,15,20−22 One of the most effective and relatively inexpensive procedures for the removal of dyes is adsorption.6,7,16 The suitable adsorbents should have desirable adsorption capability, fast adsorption rate, good selectivity, and be easily prepared.21 In this respect, various types of adsorbents such as activated carbon,23 magnetic nanocomposite,24−28 hybrid hydrogel,29 activated carbon nanotubes,16 magnetic hydroxyapatite-immobilized oxidized multiwalled carbon nanotubes,30 and EDTA modified β-cyclodextrin/chitosan,31 have been applied for removing dyes from water. Although past efforts have been successful in utilizing TiO2 for its adsorption and photocatalytic properties,32−34 in this study TiO2 has been used as an adsorbent protector. The introduction of vanadium phosphorus oxide (VPO) as an absorbent for removing colored contaminants from water may be an irrelevant issue, but today VPO is used industrially as a catalyst for the synthesis of maleic anhydride. Many efforts have been made to regenerate the VPO catalysts during the past decades.35−40 In this work, the capability of VPO as a selective adsorbent for MB removal from aqueous solution has been investigated. However, VPO is moderately soluble in water (2 mg/mL) and forms a green-yellow color in water, which is definitely a disadvantage for an adsorbent. By encapsulating the VPO in a matrix of TiO2 (VPO@TiO2), it Received: June 29, 2018 Accepted: September 14, 2018

A

DOI: 10.1021/acs.jced.8b00549 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Scheme 1. Structure of MB, MO, and RhB Dyes

green. Stirring was continued at 130 °C for 3 h. Finally, VOHPO4·0.5H2O was formed as a blue precipitate. The suspension was separated, centrifuged, and then washed with n-butanol/acetone solution. Next, the sample was dried in an oven at 100 °C for 24 h, followed by calcination at 400 °C for 20 h to yield the deep greenish VPO. 2.2.2. Synthesis of VPO@TiO2. For the preparation of VPO(30 wt %)@TiO2, in the first step, VOHPO4·0.5 H2O (134 mg, 0.78 mmol) was added to 10 mL of water containing 5 wt % PVP and the resulting mixture was sonicated for 1 h. Titanium tetra-n-butoxide as a precursor of TiO2, was then added to resulting suspension followed by continuous sonication for 1 h. For a better dispersion, the suspension was stirred vigorously overnight at room temperature. Water was then removed upon heating. The resulting solid powder was dried and calcined at 450 °C for 22 h. The above procedure was used for the preparation of other composites of VPO (5, 10, 20, and 40 wt %) into TiO2 matrix. 2.2.3. Adsorption Study. The removal efficiency of TiO2, and VPO(5−30 wt %)@TiO2 was calculated for MB, RhB, and MO according to the following equation: (eq 1):24

becomes insoluble and can selectively adsorb MB from aqueous solutions. The kinetic and equilibrium parameters of the adsorption of MB, RhB, and methyl orange (MO) were then studied to understand the adsorption. The effect of pH, adsorption time, and concentration parameters on the adsorption kinetics of MB, RhB, and MO on TiO2 and VPO(5−30 wt %)@TiO2 were extensively investigated. The experimental results showed that VPO(30 wt %)@TiO2 could be a highly efficient adsorbent for the removal of MB from aqueous solutions containing MO and RhB (Scheme 1).

2. EXPERIMENTAL SECTION 2.1. Materials and Analysis. All reagents were commercially available. V2O5, H3PO4 (85%) and titanium tetra-nbutoxide Ti[O(CH2)3CH3]4 (97%) were purchased from Sigma-Aldrich Chemical Co. Benzyl alcohol, n-butanol, MB, RhB, and MO were purchased from Merck Chemical Co. Fourier transform infrared (FTIR) spectra of the prepared intermediates and the adsorbent were recorded using a FTIR spectrophotometer (Jasco 680-plus) using KBr pellets.The Xray diffraction (XRD) pattern of the adsorbent was obtained using a Philips X’pert diffractometer (Netherland) with Cu anode. Thermogravimetric analysis of the VPO and VPO(30 wt %)@TiO2was performed in the N2 atmosphere at a heating rate of 10 °C min−1 using a BAHR-STA/TGA-503 instrument. Adsorption−desorption isotherms of nitrogen at 77.3 K were prepared using NOVAWin2, version 2.2 (Quantachrome instruments) and the specific surface area of the adsorbents was determined by Brunauer−Emmett−Teller (BET) technique. Field emission scanning electron microscopy images were obtained using a Mira 3-XMU FESEM (Germany) EDX spectrometer. The electronic absorption spectra of the solutions were recorded in the UV−vis region using a Jasco, V-570 spectrophotometer (Japan). 2.2. Preparation of the Adsorbents. 2.2.1. Synthesis of VPO. On the basis of the previously reported procedure,37 V2O5 (1 g, 5.5 mmol) was added to a mixture of n-butanol and benzyl alcohol (15 mL) at room temperature, and the mixture obtained was continuously stirred under reflux (130 °C) for 5 h. The brown color of the mixture turned to greenish and finally deep green. H3PO4 (1.27 mL) was then added slowly which changed the color of the reaction mixture to bluish

%removal =

(Co − Ce) × 100 Co

(1)

The equilibrium adsorption capacity (qe) (mg g−1) of MB, RhB, and MO was calculated by the following expression (eq 2):16 qe =

(Co − Ce)V m

(2)

where the initial and equilibrium concentrations of the adsorbate in the solution are Co and Ce (mg/L), respectively, V (L) is the volume of solution, and m (g) is the adsorbent weight. The stock solutions of MB, MO and RhB (1000 ppm) were prepared by dissolving the dyes in deionized water. Aqueous solutions with different concentrations of each dye (5−100 ppm) were prepared by successive dilution of the stock solution with water. The dye concentrations were determined using UV−vis spectroscopy at wavelengths of 664, 554, and 464 nm for MB, RhB, and MO, respectively (Figure S1). The B



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chosen as the preferred adsorbent and was thoroughly investigated. 3.1. Characterization of the VPO(30 wt %)@TiO2. The FT-IR spectra of VOHPO4·0.5H2O, bulk VPO, TiO2, and VPO(30 wt %)@TiO2 are shown in Figure S2. The FT-IR spectrum of VOHPO4·0.5H2O shows the stretching and bending vibrations of the coordinated and adsorbed water at 3370 and 1640 cm−1, respectively. The bands in the region of 1000−1200 cm−1, including those at1195, 1103, and1050 cm−1are due to the asymmetric stretching of PO3(P−O stretching). The stretching vibration of the VO groups was observed at 975 cm−1. Moreover, the peaks at 928, 640, and 417 cm−1 were attributed to the δoop (P−OH) and δoop of -OPO vibration. The formation of (VO)2P2O7 (named VPO) phase after calcination was confirmed by the appearance of the absorption bands at 1237 and 1141 cm−1 which was attributed to the phosphate group. In addition, the two bands observed at 968 and 630 cm−1are due to VO and δ-OPO vibrations, respectively.38,39,41 In the spectrum of TiO2, OH stretching and bending vibrations are observed at 3434 and 1638 cm−1, belonging to the surface OH groups and adsorbed water molecules. The broad peak appearing at 470−550 cm−1 was attributed to the stretching vibration of the Ti−O−Ti bonds.42−44 In the spectrum of VPO(30 wt %)@TiO2, the broad peak in the range of 450−600 cm−1 is assigned to the stretching vibrational mode of the Ti−O−Ti bonds at TiO2 matrix in VPO(30 wt %)@TiO2.45 The X-ray diffraction patterns of VPO, VPO(30 wt %)@ TiO2, and used VPO(30 wt %)@TiO2 after five runs were studied (Figure S3). VPO as a reference shows a diffraction pattern at 2θ = 18.48°, 22.88°, 28.28°, 29.93°, 43.16°, and 58.35° which is characteristic of VPO phase (JCPDS: 0410698) (Figure S3a).41,46 The characteristic peaks of TiO2 appear at 25.28°, 37.93°, 48.37°, 53.88°, 55.29°, 62.72°, 68.99°, 70.17° and 75.37° (JCPDS: 01-0562) (Figure S3b). It is worth mentioning that because of good dispersion of the VPO particles into the TiO2 matrix, the corresponding diffraction peaks cannot be observed. The thermogravimetric analysis (TGA) data for VPO and VPO(30 wt %)@TiO2 are shown in Figure S4. There are three weight loss stages for VPO. The first stage up to 200 °C with about 8 wt % corresponds to the desorption of physically adsorbed water and the other two in the ranges of 200−600 °C and 650−700 °C with less than 4 wt % weight loss are attributed to the removal of phosphate groups.47,48 When VPO is encapsulated into TiO2 matrix, only 5% weight loss occurs for physically adsorbed water. This demonstrates that VPO(30 wt %)@TiO2 has good thermal stability at high temperatures. 3.1.1. BET Surface Area and Pore Volume. The BET specific surface area of VOP(30 wt %)@TiO2 was measured to be 264 m2·g−1. According to the data of BJH analysis, the prepared adsorbent has a pore size of 2.5−2.6 nm and a total pore volume of 0.54 cm3·g−1. From the shape of the isotherm and hysteresis loop at around p/p0 = 0.8−0.9, the VOP(30 wt %)@TiO2 adsorbent is concluded to have a mesoporous network (Figure S5). 3.1.2. FESEM and EDX Analysis. The morphological SEM images of VPO and the VPO(30 wt %)@TiO2 adsorbent are shown at various magnifications (1 μm, 500 and 200 nm) in Figure 1A. As it can be observed, VPO shows plate-like crystals very similar to rosebud clusters (Figure 1a,b), and when encapsulated into a TiO2 matrix, the solid shows a new morphology and the composite becomes a floppy agglomerate

calibration curve was obtained from the spectra of the standard solutions at a specific pH (usually 7). An exact amount of the adsorbents (10 mg) was added to the aqueous dye solutions (10 mL) with dye concentrations from 5 to 100 ppm. The dye solution containing the adsorbent was stirred overnight at room temperature followed by decantation, and the dye concentration was calculated after dilution (if necessary).

3. RESULTS AND DISCUSSION As shown in Scheme 2, and stated in the introduction, VPO is dissolved in water (2 mg/mL), giving rise to a yellowish green Scheme 2. Solubility Test of VPO and VPO(30 wt %)@ TiO2

solution. However, by encapsulating VPO in a matrix of TiO2 the composite prevents the VPO from escaping, probably though bonding between the VPO and TiO2. By using this method, the VPO wastes can be used in the maleic anhydride units. In the next step, to consider the economic aspect of the procedure, various weight percents of VPO were loaded into the TiO2 matrix, and the qe values of VPO(x wt %)@TiO2 composites were measured (Table 1). As observed, by increasing the load of the VPO in the composite, the adsorption capacity of the composite increased. Although the VPO(40 wt %)@TiO2 adsorbent had a higher adsorption tendency, VPO leached from the TiO2 matrix at that weight percent; therefore, the composite with 30 wt % of VPO was Table 1. Effect of Different Amount of VPO Loading on the TiO2 on MB Adsorptiona entry

VPO(x wt %)@TiO2

qe (20 ppm)

1 2 3 4 5

5% 10% 20% 30% 40%

2.95 6.9 17.54 19.05 19.67

a

Adsorption conditions: C0 = 20 mg/L, adsorbent dosage = 10 mg, pH = 7.0, adsorption time = 24 h at room temperature. C



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of particles with dimensions of about 10 nm, Figure 1c,d. Images of used VPO(30 wt %)@TiO2 adsorbent after five runs are seen in Figure 1e,f). EDX spectra of the two samples (Figure 1B) show the presence of O, P, and V atoms in the two sorbents, while the Ti atom only exists in the VPO(30 wt %)@ TiO2 sample. Other magnifications of SEM images of VPO and the VPO(30 wt %)@TiO2 are illustrated in Figure S6. 3.2. Adsorption of MB, RhB, and MO. 3.2.1. Adsorption Isotherms. Various isotherms employed for adsorption of MB, MO, and RhB on VPO(5−30 wt %)@TiO2. To analyze the experimental data, the Langmuir, Freundlich, and Redlich− Peterson isotherm models were used. The Langmuir isotherm shows that the adsorption is monolayer, and there is not any interaction between the sorbent molecules on the surface of the adsorbent. The Langmuir isotherm has the following form (eq 3):9 1 1 1 = + qe Qo KLQ oCe

(3)

where Qo (mg/g) is the Langmuir maximum adsorbed dye per unit mass of adsorbent and KL (L/mg) is the Langmuir constant related to the affinity of binding sites and the rate of adsorption. RL is a dimensionless equilibrium parameter and indicates the suitability of the isotherm. If RL > 1, the isotherm is unfavorable, RL < 1, favorable, RL = 1, linear, or RL = 0, irreversible. RL is calculated using eq 4:9

Figure 1. (A) FESEM images of (a,b) VPO, (c,d) VPO(30 wt %)@ TiO2, and (e,f) used VPO(30 wt %)@TiO2 adsorbent after five runs, (B) EDX analysis of (a) VPO and (b) VPO(30 wt %)@TiO2.

RL =

1 (1 + KLCo)

(4)

Figure 2. Adsorption isotherm of MB on (a) TiO2 and (b−e) VPO(5, 10, 20, and 30 wt %)@TiO2 measured at different concentrations (5−100 mg/L). Adsorption conditions: adsorbent dosage = 10 mg, pH = 7.0, adsorption time = 24 h at room temperature. D



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wt %)@TiO2, while for the VPO(20 wt %)@TiO2 the Langmuir equation with the R2 value of 0.99 was calculated to be the best fit model. In this work, the adsorptions of MO and RhB on TiO2 and VPO(30 wt %)@TiO2 have also been analyzed by the three isotherms, namely Langmuir, Freundlich, and Redlich− Peterson. The concentration of the dyes were measured by UV−vis spectrophotometry. On the basis of the obtained results, none of these sorbents that is, TiO2, and VPO(30 wt %)@TiO2 were suitable adsorbents for the removal of MO and RhB (Table 3). MB and RhB are cationic dyes but MO is

The Freundlich model is a multilayer adsorption. In other words, there is a heterogeneous surface without uniform distribution of adsorption sites over the surface of the adsorbent. The Freundlich isotherm is applied in the following form (eq 5):24 qe = KFC1/ nF

(5)

KF and nF are the Freundlich constants, which correspond to the capacity and intensity of the adsorption, respectively. A 1/n value in the range of 0.1−1.0 indicates a highly favorable adsorption, while 1/n > 2 demonstrates an unfavorable adsorption. A 1/n value of 0 < 1/n < 1, is a measure of surface heterogeneity or adsorption intensity. Therefore, as the value tends to approach zero, the surface becomes more heterogeneous. The Redlich−Peterson isotherm is applied according to the following eq (eq 6).49

Table 3. Comparison of the Adsorption Capacities and Removal Efficiency of the Dyes onto Various Adsorbents (TiO2 and VPO(30 wt %)@TiO2) dye

ji C zy lnjjjjKR e − 1zzzz = g ln Ce + ln aR j q z (6) e k { The Redlich−Peterson isotherm constants are K, and aR. The g parameter is the exponent, which lies between 1 and 0. Briefly, if g goes toward zero, it approaches the Freundlich isotherm model, and if the g value is approximately equal to 1, it satisfies the ideal Langmuir condition. The adsorption isotherms of MB on TiO2 and the VPO@ TiO2 composites with different weight percents of VPO (5, 10, 20, and 30 wt %) are shown in Figure 2, at pH = 7.0 and room temperature. As shown in Table 1, and in the adsorption isotherms the VPO (30 wt %)@TiO2 composite was chosen as the best adsorbent. The three isotherm parameters for the adsorption of MB onto the VPO@TiO2 composites and TiO2 are reported in Table 2 and Table S1. On the basis of the R2 values for the

MB MO RhB

Langmuir

Freundlich

Redlich−Peterson

parameter

VPO(30 wt %)@TiO2

KL RL Qo R2 KF 1/nF R2 KR aR g R2

0.4256 0.0555 61.350 0.9988 22.88 0.3014 0.9676 0.004 7.2901 × 10−5 0.6505 0.9575

VPO(30 wt %)@TiO2 TiO2 VPO(30 wt %)@TiO2 TiO2 VPO(30 wt %)@TiO2

qe

R (%)b

4.3 18.71

21.89 94.41

0.69 1.22

3.64 7.21

a

Commercial TiO2. bRemoval efficiency. Adsorption conditions: C0 = 20 mg/L, adsorbent dosage = 10 mg, pH = 7.0, adsorption time = 24 h at room temperature.

anionic. If VPO(30 wt %)@TiO2 is belived to appreciably adsorb MB as a cationic dye, but not the other cationic dye (RhB), the steric hindrance was probably the main factor in this regard. The mechanism of this physisorption phenomenon might show that in the VPO as the main adsorbent, oxygen atoms have a high density on the surface,47 which causes strong electrostatic interaction with the cationic dye, and such interaction could not be appropriate for methyl orange as an anionic dye. As previously indicated, although VPO is not a suitable adsorbent for removing MB due to its solubility in water, not only the solubility was controlled by encapsulating VPO in a matrix of TiO2, but also the VPO(30 wt %)@TiO2 composite showed a very good efficiency for adsorbing MB. 3.2.2. Effect of pH. One of the most important factors for studying the adsorption property of an adsorbent is the pH of the aqueous solution. The pH of the solution influences the degree of ionization of the dye molecules and surface charge of the adsorbent. The effect of pH on adsorption was studied using concentrations of 20 and 40 mg/L of dye at room temperature for 24 h. Figure 3a shows the effect of the initial pH on the adsorption of MB onto VPO(30 wt %)@TiO2. It can be observed that a negligible amount of MB was adsorbed on the surface of VPO(30 wt %)@TiO2 in acidic pH. For MB cationic dye, the removal capacity dramatically increased, by increasing the initial pH, and raised the removal efficiency of MB to 90%. The influence of pH for removing 20 mg/L of RhB by VPO(30 wt %)@TiO2 was also studied. Figure 3b indicates that the removal of RhB was less sensitive to the initial pH variation of the dye solution, and the steric hindrance was thought to be very important in this respect. 3.2.3. Adsorption Kinetics. The effect of contact time on the removal efficiency of MB by VPO(30 wt %)@TiO2 at pH = 6.0 is shown in Figure 4. As observed, adsorption takes place in two steps for 20 mg/L solution of MB, in which the highest adsorption was perceived in the first 3 min, and the removal efficiency reached 85%. After the first 3 min, the rate of

Table 2. Langmuir, Freundlich, and Redlich−Peterson Isotherm Parameters for MB Adsorption on VPO(30 wt %) @TiO2 isotherm

adsorbent TiO2a

KF((mg/g)(L/mg)1/n); KL (L/mg); Qo(mg/g); KR (L/g);aR(mg−1). Adsorption conditions: C0 = 20 mg/L, adsorbent dosage = 10 mg, pH = 7.0; adsorption time = 24 h at room temperature.

a

sorption of MB on the VPO (30 wt %)@TiO2, the Langmuir equation with an RL value of 0.0555 might provide a better fit in comparison with the other two isotherms. Furthermore, the g parameter in the Redlich−Peterson isotherm is 0.6505, demonstrating the adherence to the Langmuir isotherm. Moreover, the results show that Freundlich was the best fit model for the adsorption of MB on TiO2 and VPO(5 and 10 E



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Figure 3. Removal efficiency for (a) MB dye (C0 = 20 and 40 mg/L) and (b) RhB dye (C0 = 20 mg/L) at various pH values. Adsorption conditions: C0 = 20 and 40 mg/L, adsorbent dosage = 10 mg, adsorption time = 24 h at room temperature.

To understand the factors affecting the adsorption process, the mentioned kinetic models were used to fit the experimental data. The kinetic parameters are given in Table 4. The R2 Table 4. Kinetic Parameters of Pseudo-first-order and Pseudo-second-order Adsorption Kinetic Models for MB on VPO(30 wt %)@TiO2a pseudo-first-order

pseudo-second-order

initial concn

qe,1

k1

qe,2

k2

(mg/L)

(mg/g)

(min)−1

R2

(mg/g)

(min)−1

R2

20 40

3.88 8.0116

0.2183 0.0982

0.9621 0.9263

18.9393 37.31

0.0528 0.0355

1 0.9987

a

Adsorption conditions: C0 = 20 and 40 mg/L, adsorbent dosage = 10 mg, pH = 6.0 at room temperature.

values of the pseudo-second-order kinetic model are higher than those of pseudo-first-order model, and the calculated qe values (qe,2) of the PS model are close to the experimental data (Qo). Therefore, the pseudo-second order kinetic model was more suitable for describing the adsorption of MB onto VPO(30 wt %)@TiO2. 3.2.4. Effect of Adsorbent Dosage. Figure 5 demonstrates the effect of the amount of VPO(30 wt %)@TiO2 between 5 and 30 mg on adsorption of MB dye under the conditions of pH = 6.0, C0 = 20 and 40 mg/L, and contact time of 24 h. The results indicate that the adsorption capacity of MB greatly enhanced with increasing the adsorbent dosage to 20 mg due to the increased adsorption sites and surface area. By increasing the adsorbent dosage to 30 mg, the adsorption capacities almost did not change. 3.2.5. Adsorption Thermodynamics. The effects of temperature and thermodynamic behavior of MB adsorption onto VPO(30 wt %)@TiO2 were studied by measuring the changes in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS). By using the following eqs (eqs 9 and 10) the thermodynamic parameters were calculated.8,50

Figure 4. Effect of contact time on MB adsorption by VPO(30 wt %) @TiO2. Adsorption conditions: C0 = 20 and 40 mg/L, adsorbent dosage = 10 mg, pH = 6.0 at room temperature.

adsorption decreased and it almost became constant after 1 h. Adsorption kinetics for 40 mg/L solution of MB was a little different from that of 20 mg/L solution; the maximum removal obtained in the first 12 min was 89%, and then the adsorption rate decreased, constantly. Clearly, the high adsorption at the initial stage related to the presence of uncovered surfaces, and the availability of the active sites on the adsorbent. The adsorbed amount of MB at time t, qt (mg/g), was obtained by eq 2.16 Pseudo-first-order (PF), and pseudosecond-order (PS) models analyzed the experimental kinetic data for MB. These models are expressed as eqs7 and 8:

ΔGo = ΔH o − T ΔS o

(9)

(8)

i ΔS o ΔH o yz zz ln Kd = jjj − RT { (10) k R For an adsorption process, Kd is defined as follows (eq 11): q Kd = e Ce (11)

where qe and qt (mg/g) are the uptakes of MB at equilibrium and at time t (min), respectively; k1 (1/min) is the adsorption rate constant and k2 (g/mg·min) is the rate constant of the second-order equation.

where qe is the amount of the dye adsorbed per unit mass of VPO(30 wt %)@TiO2. The adsorption of MB on VPO(5, 10, 20, and 30 wt %)@ TiO2 composites was investigated at three different temper-

Pseudo-first order model:

9

ln(qe,1 − qt ) = ln(qe,1) − k1t

Pseudo-second order model: t 1 t = + 2 qt qe,2 k 2qe,2

(7)

9

F



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Figure 5. Effect of the adsorbent dosage on MB adsorption by VPO(30 wt %)@TiO2. Adsorption conditions: C0 = 20 and 40 mg/L, pH = 6.0, adsorption time = 24 h at room temperature.

atures (313, 323, and 333 K). As shown in Figure S7, by increasing the temperature, the adsorption of MB on the adsorbents decreased. The thermodynamic parameters for VPO(5−30 wt %)@TiO2 are listed in Table 5. Table 5. Thermodynamic Data for Adsorption of MB on VPO(5-30 wt %)@TiO2a entry

VPO(x%)@ TiO2

temp (K)

ΔGo (kJ/mol)

ΔSo (kJ/mol K)

ΔHo (kJ/mol)

1

5%

−66.075

10%

0.007

−17.583

3

20%

0.042

−9.201

4

30%

−17.894 −16.354 −14.816 −20.051 −20.130 −20.209 −22.641 −23.071 −23.500 −25.335 −25.148 −24.962

−0.153

2

313 323 333 313 323 333 313 323 333 313 323 333

−0.018

−31.175

Figure 6. UV−vis absorption spectra for selectivity of MB-MO and MB-RhB. Adsorption conditions: C0 = 20 mg/L, adsorbent dosage = 10 mg, pH = 6.0, adsorption time = 24 h at room temperature.

a

Adsorption conditions: C0 = 20 mg/L, adsorbent dosage = 10 mg, pH = 6.0, adsorption time = 24 h.

and RhB might then be adsorbed through van der Waals interactions between MB and RhB (Table 6).

The negative value of ΔG, as shown in Table 5, indicates the thermodynamically spontaneous nature of the adsorption under experimental conditions.9 On the basis of the measured ΔG, one could judge that physisorption is the dominating mechanism. Negative ΔH is an indication of the exothermic nature of the adsorption, which could be either physical or chemical.15 A negative value of the change in entropy (ΔS) shows that at the solid−solution interface the molecular motions decrease during the adsorption of the dye on the VPO(5−30 wt %)@TiO2.2 3.3. Selectivity and Reusability. VPO(30 wt %)@TiO2 was separately added to a solution containing MB-MO or MBRhB with initially equal concentrations. After 24 h, the color of MB-MO turned to orange, while a purple color was obtained for the mixture of MB-RhB (Figure 6). The separated adsorbents were blue, suggesting that VPO(30 wt %)@TiO2 selectively adsorbed MB from the solutions. UV−vis spectra were used to check the concentration of the remaining dyes in solutions. The concentration of MB decreased evidently, by the increase in the contact time for both mixtures, while the concentration of MO did not show any change. Surprisingly, in the presence of MB, RhB was adsorbed much better (78%) than RhB as the only adsorbate (7.2%). As pointed out in section 3.2.1, RhB did not adsorb because of steric hindrance. However, when MB is also present in the solution, it adsorbed through electrostatic interaction with VPO in the first stage

Table 6. Data of Selectivity of Dyes Adsorption onto VPO(30 wt %)@TiO2 dye mix MB-MO mix MB-RhB

MB MO MB RhB

qe,exp

R (%)a

17.40

93

18.16 13.39

90 78

a

Removal efficiency. Adsorption conditions: C0 = 20 mg/L, adsorbent dosage = 10 mg, pH = 6.0, adsorption time = 24 h at room temperature.

The reusability of VPO(30 wt %)@TiO2 in dye adsorption was investigated by running five cycles of the adsorption. After each cycle, the adsorbent was washed with water and NaOH solution (0.1 mol/L). The resulting solid was then dried and utilized for the next run. The results indicated that the removal efficiency was maintained at over 90% for MB. The selective adsorption efficiency of VPO(30 wt %)@TiO2 composite for MB was compared with those reported for other adsorbents, and the results are demonstrated in Table 7. When comparing the adsorption capacity of the VPO(30 wt %)@ TiO2 for MB as a toxic pollutant it was concluded that G



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Table 7. Comparison of the Adsorption Capacities of MB onto Various Adsorbents adsorbents red mud biopolymeric membrane fruit peel [email protected]/2-Gel Br/Mo heterostructures TMFb Sporopollenin Ficus carica bast VPO(30 wt %)@TiO2

adsorption capacity 5.23×10 (mol/g) 20.83 (mg/g) 0.0068 (mmol/g) 6.47×10−4 (mmol/g)a 55.36 (mg/g) 50.8 (mg/g)a 1.70 (μmol/g) 47.62 (mg/g) 61.35 (mg/g)

a

*E-Mail: [email protected]. Tel./Fax: +98-031-33913254 (2350).

15 3 51 29 8 52 53 54 this work

ORCID

Mehran Ghiaci: 0000-0002-0686-7778 Funding

Authors acknowledge the Research Council of Isfahan University of Technology for supporting this work. Notes

The authors declare no competing financial interest.

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b

q is calculated from second-order kinetic model. Sodium titanate with uniform flower-like morphologies.

REFERENCES

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VPO(30 wt %)@TiO2 has a great potential for MB removal from aqueous solutions. In this work, the efficiency of the VPO(30 wt %)@TiO2 adsorbent was examined on real sewage which was donated by a textile industry (Golnesar Woolen Co.). The sewage had a mixture of dyes including blue color. A 10 mg portion of the adsorbent in the optimized conditions could adsorb more than 50% of the colors during 24 h at pH 7.0 (Figure S8).

4. CONCLUSION VPO(30 wt %)@TiO2 has been successfully fabricated via a facile encapsulation process. It seems that the applied methodology might be an efficient way to prevent leaching of the main reagent in a heterogeneous catalyst. The experimental results indicated an excellent performance for adsorption of MB onto VPO(30 wt %)@TiO2. The effects of different factors such as pH, adsorbent dosage, and contact time on adsorption was investigated. Equilibrium adsorption isotherms were implemented to evaluate the adsorption capacities of VPO(30 wt %)@TiO2. The pseudo-second order model was the best fitting model used for kinetic studies. The detailed investigation of the adsorption behavior of VPO(30 wt %)@TiO2 indicates that the adsorption was well fitted with the Langmuir model. MB adsorption onto the adsorbent was thermodynamically favorable and the VPO(30 wt %)@TiO2 selectively adsorbed MB.

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AUTHOR INFORMATION

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information file contains . The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00549. Figures S1−S8: UV−vis absorption spectra for MB, MO, and RhB; FTIR spectra of precursors, TiO2 and adsorbent; the XRD patterns of VPO, VPO(30 wt %) @TiO2, and used VPO(30 wt %)@TiO2 adsorbent after five runs; TGA curves of VPO and VPO(30 wt %)@ TiO2; N2 adsorption−desorption isotherms and pore size distribution of VPO(30 wt %)@TiO2; FESEM images of VPO and VPO(30 wt %)@TiO2; the effect of temperature on the adsorption of MB by VPO(5 wt %) @TiO2, VPO(10 wt %)@TiO2, VPO(20 wt %)@TiO2, and VPO(30 wt %)@TiO 2; and finally UV−vis absorption spectrum of a real water sample. Table S1: Langmuir, Freundlich, and Redlich−Peterson isotherm parameters for MB adsorption on TiO2 and VPO (5, 10, and 20 wt %)@TiO2 composites (PDF) H



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Encapsulation of Vanadium Phosphorus Oxide into TiO2 Matrix for

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