Rapid Decolorization of Methyl Blue in Aqueous Solution by

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Rapid Decolorization of Methyl Blue in Aqueous Solution by Recyclable Microchannel-like La0.8K0.2FeO3 Hollow Microfibers Lianli Zou,† Qiuju Wang,† Zhou Wang,† Lina Jin,† Ruijiang Liu,†,‡ and Xiangqian Shen*,† †

Institute for Advanced Materials and ‡School of Pharmacy, Jiangsu University, Zhenjiang 212013, People’s Republic of China ABSTRACT: A novel catalyst of microchannel-like La0.8K0.2FeO3 hollow microfibers for methyl blue (MB) decolorization in aqueous solution via the catalytic wet air oxidation (CWAO) process was prepared by the citrate−gel process. The microchannellike La0.8K0.2FeO3 hollow microfibers were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nitrogen adsorption−desorption, and X-ray photoelectron spectroscopy (XPS) methods. The hollow microfibers have a diameter of about 2 μm, length of tens of micrometers, and hollow structure, and the ratio of hollow diameter to fiber diameter is around 0.5. There are at least two kinds of oxygen species on the hollow microfibers according to XPS analysis. The results show these microfibers have a high activity for MB decolorization and most MB can be decolorized in about 30 min under ambient conditions. The rapid decolorization of MB by La0.8K0.2FeO3 hollow microfibers is largely due to the unique microchannel-like structure of the microfibers and active oxygen on the microfiber surface. The MB decolorization data follow well with the second-order reaction model, and the calculated activation energy for MB decolorization process by the Arrhenius equation is 14.27 kJ/mol. The catalytic activity of the microfibers did not exhibited an obvious degradation after cycling ten times. rare earth element and B is a transition metal of the first row) have been widely used as catalysts for environmental protection due to their excellent thermostability, chemical stability, and high oxygen activity.15−17 In particular, substitution of potassium at the A site can result in more oxygen vacancies and increase the mobility of surface oxygen species on the catalyst during reactions.18,19 From the engineering application view, the hollow microfibers of porous perovskite-type compound oxides are potential microreactors, as they have a microchannel-like structure and are easy to separate and recover from wastewater. Therefore, the aim of this work was to prepare microchannel-like perovskite La0.8K0.2FeO3 hollow microfibers and investigate their catalytic performances to decolorize methyl blue (MB) in aqueous solution under ambient conditions.

1. INTRODUCTION Dye pollution in aqueous solution is an environmental problem, as most dyes are a potential source threatening human health. Recently, dye decolorization in aqueous solution has attracted much more attention.1,2 In order to solve the dye pollution problem, various technologies have been used, such as electrolytic oxidation,3 biodegradation,4 photodecomposition,5,6 adsorption,7 etc. However, these processes usually have drawbacks of high cost, long time period, and energy consumption. Wet air oxidation (WAO) was considered to be an efficient and economic method to decolorize dyes from wastewater, but the process commonly requires high temperature and high pressure. Catalytic wet air oxidation (CWAO) has been developed to relax the oxidation conditions.8−12 For example, CuO/γ-Al2O3 catalyst had excellent catalytic activity in treating azo dyes at 80 °C.9 Zhang et al.10 reported the polyoxometalate Zn1.5PMo12O40 catalyst with high activity and stability for pollutant dyes oxidization under ambient conditions. Liu and Sun11 prepared the Fe2O3−CeO2−TiO2/γ-Al2O3 catalyst, which exhibited high catalytic activity for methyl orange at room temperature. Safranin-T (ST) could be oxidized in wet air under ambient conditions by use of ZnO/MoO3 and Zn1.5PMo12O40 as catalyst.12 Although the CWAO process has been extensively investigated for a variety of dye decolorizations, the micro- and nanocatalysts used are very difficult to completely separate and recover from aqueous solutions, which will result in secondary pollution and high cost. Owing to regular pore structures and large surface area, nanoporous materials will have practical applications in many fields including chemistry and chemical engineering, electronics, medicine, and environmental and biotechnological engineering.13,14 The nanoporous perovskite-type compound oxides (usually with the general formula ABO3, where A is a © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of La0.8K0.2FeO3 Hollow Microfibers. Perovskite-type La0.8K0.2FeO3 hollow microfibers were synthesized by the citrate−gel process. The starting reagents La(NO3)3·6H2O, Fe(NO3)3·9H2O, K2CO3, and citric acid (C6H8O7·H2O) were all analytical-grade, and the process was described in detail in ref 20. The molar ratio of citric acid to total metal ions was 1.4:1. The required organic acid and metal salts were dissolved in deionized water to form aqueous solutions, with magnetic stirring for about 20 h at room temperature. This solution was transferred to a rotary evaporator and evaporated in a vacuum at 60 °C to remove surplus water until a viscous liquid was obtained. The gel Received: Revised: Accepted: Published: 658

October 12, 2013 December 9, 2013 December 17, 2013 December 17, 2013 dx.doi.org/10.1021/ie403417s | Ind. Eng. Chem. Res. 2014, 53, 658−663

Industrial & Engineering Chemistry Research

Article

Figure 1. Characteristics of La0.8K0.2FeO3 hollow microfibers: (a) XRD pattern; (b) SEM morphology; (c) N2 adsorption−desorption isotherm.

solution was taken out, and the dye-loaded catalyst was separated by a centrifuge at 12 000 rpm for 5 min. The initial and final concentrations of MB solutions were measured on a UV spectrophotometer (UV-2550) at 600 nm.

microfibers were drawn from the viscous gels by a drawing device and dried in an oven at 80 °C for about 24 h to obtain the fiber precursor. Then the dried fibers were put in an alumina crucible and subsequently, at a heating rate of 3 °C/ min, calcined at 600 °C for 6 h to form perovskite-type La0.8K0.2FeO3 hollow microfibers. 2.2. Characterization. Microfiber morphology was studied by use of a field emission scanning electron microscope (FESEM, JSM-7001F). The crystalline structure was determined on an X-ray diffractometer (Rigaku D/Mmax2500PC) using Cu Kα radiation (λ = 1.54 Å) with voltage and current of 40 kV and 200 mA, respectively, and the scanning rate was 6°/ min in a 2θ range from 20° to 80°. The specific surface area was measured by the Brunauer−Emmett−Teller (BET) method on a Nova 2000e instrument, and the pore size distribution was calculated from adsorption data of the isotherm based on the Barrett−Joyner−Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) analysis was carried out with a Phi Quantera II system (Ulvac-Phi, Inc,). 2.3. Decolorization Measurement. The CWAO process was used to decolorize MB in aqueous solution. MB was selected as the model pollutant because it was widely used as a coloring agent and disinfector in rubbers and pharmaceuticals and has been found to be harmful to humans.21 For the CWAO process, a general procedure was carried out as follows: 0.1 g of La0.8K0.2FeO3 microfiber catalyst was suspended in 100 mL of solution containing MB in a 250 mL glass bottle, and then the air was inputted into the bottom of the glass bottle at a flow rate of 0.5 L/min. At given intervals, about 2 mL of suspension

3. RESULTS AND DISCUSSION 3.1. Characteristics of La0.8K0.2FeO3 Hollow Microfibers. Figure.1 shows the characteristics of as-prepared La0.8K0.2FeO3 hollow microfibers. After calcination at 600 °C for 6 h, the XRD pattern of the resultant product is shown in Figure 1a, and all peaks can be indexed to the perovskite structure of LaFeO3 (JCPDS 37-1493). The narrow peaks on the XRD pattern imply a well-crystallized structure of the sample, and the average grain size is about 21.29 nm, as calculated by Scherrer’s formula: D = kλ/β cos θ, where k is the Scherrer content of 0.9, λ is the wavelength of Cu Kα radiation, β is the full width at half-maximum of the diffraction peak, and θ is the angle of diffraction. The SEM morphology of the La0.8K0.2FeO3 hollow microfibers is shown in Figure 1b. It can be seen that the hollow microfibers have a diameter of about 2 μm, length of tens of micrometers, and a hollow structure, and the ratio of hollow diameter to fiber diameter is around 0.5. These hollow microfibers have a microchannel-like structure and can be used as microreactors. The nitrogen adsorption−desorption isotherm of La0.8K0.2FeO3 hollow microfibers is presented in Figure 1c. The isotherm possesses a hysteresis loop, implying the presence of mesopores with slit-shaped pores.5,22,23 The estimated 659

dx.doi.org/10.1021/ie403417s | Ind. Eng. Chem. Res. 2014, 53, 658−663


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Figure 2. XPS spectra of La0.8K0.2FeO3 hollow microfibers: (a) La 3d; (b) Fe 2p; (c) O 1s.

specific surface area, pore volume, and average pore diameter are 17.56 m2/g, 0.069 cm3/g, and 11.69 nm, respectively. As displayed in the inset of Figure 1c, the dominant pore size ranges from about 10 to 20 nm, which confirms that the microfibers have a mesoporous structure and very high pore volume. Theoretically, the microfibers can be microreactors for oxidation and decolorization of dyes, and the larger pore size of the catalyst is easier for transport of dye molecules into pores. Thus, the La0.8K0.2FeO3 hollow microfiber catalyst could closely contact the dyes and has a high catalytic performance. Furthermore, XPS analysis was conducted to understand the surface chemical state of the La0.8K0.2FeO3 hollow microfiber catalyst. La 3d5/2, Fe 2p, and O 1s spectra of this catalyst are shown in Figure 2. The binding energy at around 834 and 838 eV (Figure 2a) can be attributed to La 3d5/2.24 Two strong peaks, as shown in Figure 2b, are observed at 710 and 724 eV, which can be assigned to Fe 2p3/2 and Fe 2p1/2. This suggests the oxidation state of Fe ions in the catalyst is +3, but some of the Fe3+ ions are transformed into Fe4+ as the La3+ ions are partially replaced by K+ ions.5,18,19 There are two peaks in the XPS spectrum of O 1s, shown in Figure 2c, indicating that at least two kinds of oxygen species are present on La0.8K0.2FeO3 hollow microfiber surfaces. The dominant peak at about 530.0 eV is characteristic of metallic oxides, which belongs to the lattice oxygen. The peak at 531.8 eV can be associated with adsorbed surface oxygen (O22‑, O2−) species, which may exist in oxygen vacancies.5,19 In the decolorizaion process of MB, the adsorbed oxygen species will play an essential role due to its high activity. 3.2. Catalytic Activity for MB Decolorization. MB decolorization by the CWAO process is mainly influenced by catalyst dosage, catalyst composition, temperature, and dye concentration in aqueous solution. As shown in Figure 3, the MB decolorization efficiency is increased from about 70% to 98% with the La0.8K0.2FeO3 catalyst dosage increasing from 0.5 to 3.0 mg/mL. This is due to a higher catalyst dosage, with larger catalyst surface area and reacting sites between the dye molecules and catalyst. It is attractive that, even with a low catalyst dosage of 1.0 mg/mL, the decolorization efficiency can reach above 85%, meaning a promising cost-effective performance for this La0.8K0.2FeO3 hollow microfiber catalyst. In order to compare the decolorization of MB for LaFeO3 and La0.8K0.2FeO3, as shown in Figure 3, the decolorization efficiency is improved about 25% when some of the La sites are substituted by K, possibly due to a higher mobility of surface oxygen species and more oxygen vacancies on the catalyst resulting from this substitution. 18,19 For the La0.8K0.2FeO3 hollow microfiber catalyst, without air, the decolorization efficiency is still relatively high, from about

Figure 3. Effect of catalyst dosage on decolorization efficiency (at room temperature, with initial MB concentration of 0.5 mg/mL and reaction time of 60 min, with and without air).

39% to 61% with the catalyst dosage ranging from 0.5 to 3.0 mg/mL. This can be attributed to the adsorption of these porous microfibers and active surface oxygen species. Figure 4 shows the relationship of decolorization efficiency and reaction temperature. From Figure 4, it can be observed that the decolorization efficiency of MB increases from 84.4%

Figure 4. Effect of temperature on decolorization efficiency (at initial MB concentration of 0.5 mg/mL, catalyst dosage of 1.0 mg/mL). 660



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to 91.5% when the temperature is increased from 20 to 50 °C. The effect of temperature on decolorization of MB can be explained by higher activities of both dye molecules and oxygen species on La0.8K0.2FeO3 hollow microfiber catalyst at a higher temperature. At the measurement temperature range of 20−50 °C, the decolorization process for all the samples is very rapid: after about 5 min, more than 70% of MB is decolorized, and around 80% of MB is decolorized in 30 min. The time requirement comparison of MB removal for some recently reported catalysts and absorbents is presented in Table 1. It can

1 = kt + c Ct

(1)

where Ct is the concentration of MB at any time, k represents the apparent kinetic rate constant, t is the reaction time, and c is the kinetic constant related to the initial dye concentration. According to the measurements in Figures 3−5, the fitting results are shown in Figure 6. Accordingly, the rate constant

Table 1. Time Requirement Comparison for 80% MB Removal by Use of Catalysts and Absorbents catalyst La0.8K0.2FeO3 hollow microfibers graphene graphite/ZnO composites chitosan/graphite

process

C0 (MB mg/mL)

time

ref.

catalytic oxidation adsorption photocatalyst

0.5

90 min

23

be seen that the decolorization time requirement of MB for La0.8K0.2FeO3 hollow microfiber catalyst in this work is much shorter than those of about 60 min for graphene adsorbent,21 90 min for magnetic chitosan and graphene oxide adsorbent,25 and 120 min for graphite/ZnO catalyst.26 The effect of initial MB concentration at various reaction times on the decolorization efficiency is plotted in Figure 5. At

Figure 6. Second-order kinetic plots for decolorization of MB at different initial dye concentrations (at room temperature, with catalyst dosage of 1.0 mg/mL).

estimated for different initial MB concentrations is represented in Table 2. It can be seen that the second-order reaction model is suitable to describe the decolorization process, as the determination coefficient (R2) for various initial MB concentrations is larger than 0.98. As the MB initial concentration increases from 0.1 to 0.5 mg/mL, the rate constant, k2, decreases from 7.20 × 10−4 to 1.12 × 10−4L·mg/min. Similar results were obtained by Khorramfar et al.27 Usually, the reaction activation energy (Ea) can be calculated from28

Ea + ln C (2) RT where k is the kinetic rate constant, R is the universal gas constant (8.314 J/mol·K), and T is the temperature (kelvins). The estimated value of k at different temperatures is presented in Table 3, and the relationship of ln k versus 1/T is plotted in Figure 7. According to the slope of this plot, the calculated value of Ea is 14.27 kJ/mol. From the results shown above, the microchannel-like La0.8K0.2FeO3 hollow microfibers show a very high catalytic property for MB decolorization. As reported in ref 29, redox couples of the ions could account for the activity of ABO3 perovskite oxides in the CWAO process. According to the Fe 2p XPS spectrum, there are Fe4+ and Fe3+ ions present in the La0.8K0.2FeO3 hollow microfiber catalyst, and the Fe4+/Fe3+ redox couple could be active for MB oxidation reaction. Because the existence of adsorbed oxygen species (such as O22− and O2−) has been confirmed by the O 1s XPS spectrum, the catalyst possesses good oxygen storage capacity, which is beneficial to the oxidation of MB. The enhanced MB decolorization can be ascribed to the unique structure of La0.8K0.2FeO3 hollow microfibers, which ln k = −

Figure 5. Effect of initial MB concentration on decolorization efficiency (at room temperature, catalyst dosage of 1.0 mg/mL).

the initial MB concentration range from 0.1 to 0.5 mg/mL, the decolorization efficiency exhibits a slight decreasing tendency with various reaction times at room temperature. After 1 h reaction, the decolorization for each concentration (0.1−0.5 mg/mL) of MB achieves about 93%, 92%, 89%, 88%, and 86%, respectively. 3.3. Decolorization Kinetics. It is important to understand the decolorization of MB in the CWAO process by investigation of the reaction kinetics. The common equation for a second-order reaction model can be written as27 661



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Table 2. Estimated Kinetic Constant of Second-Order Reaction Model for Different Initial MB Concentrations C0(mg/mL) 0.1 −1

−1

k (L·mg min ) R2

7.20 × 10 0.9874

0.2 −4

2.27 × 10 0.9815

0.3 −4

0.4 −4

0.5 −4

1.95 × 10 0.9840

1.58 × 10 0.9801

1.12 × 10−4 0.9858

Table 3. Decolorization Efficiency and Estimated Second-Order Kinetic Constant at Different Temperatures for C0 = 0.5 mg/ mL T (K) decolorization efficiency (%) k (L·mg−1min−1)

293

303

313

323

84.4 1.29 × 10−4

87.5 1.56 × 10−4

89.3 1.83 × 10−4

91.5 2.23 × 10−4

Figure 7. Second-order kinetic plots for decolorization of MB at different temperatures and (inset) relationship of ln k vs 1/T (with initial MB concentration of 0.5 mg/mL and catalyst dosage of 1.0 mg/ mL).

Figure 8. Catalyst cycle times and decolorization of MB (at room temperature, with initial MB concentration of 0.5 mg/mL and catalyst dosage of 1.0 mg/mL).

have a microchannel-like structure with a very high ratio of surface to volume and act as a microreactor to decolorize MB in aqueous solution. Due to advantages of high efficiency and precise process control, microchannel-like microreactors have been widely designed and applied in chemistry and chemical engineering,30,31 medicine and biomaterials fields,32 etc. As the dye solution passes through the internal channel of hollow microfibers, the rough channel surface having active oxygen species captures the dye molecules and oxidizes them rapidly. Therefore, in the CWAO process, the active oxygen species on the microchannel-like La0.8K0.2FeO3 hollow microfibers will largely act as the rapidly oxidizing agent to decolorize MB in aqueous solution, leading to rapid MB decolorization with high efficiency. 3.4. Cycling Ability of Catalyst. The cycling ability of a catalyst is fateful for its practical applications. In this work, the cycled La0.8K0.2FeO3 hollow microfiber catalyst was regenerated by separation from the residual solution, washed with deionized water, and heat-treated at 200 °C for about 5 h. Figure 8 shows the MB decolorization efficiency of the microchannel-like La0.8K0.2FeO3 hollow microfiber catalyst for 10 cycles. It is found that the decolorization efficiency remains more than 78% even after recycling 10 times. Therefore, the microchannel-like La0.8K0.2FeO3 hollow microfiber catalyst is chemically stable in aqueous solution and has potential application in dye-polluted wastewater engineering.

4. CONCLUSIONS (1) Microchannel-like La0.8K0.2FeO3 hollow microfibers have been prepared by the citrate−gel process at calcination temperature of 600 °C for 6 h, with fiber diameter about 2 μm, length of tens of micrometers, pore size about 10− 20 nm, and clear hollow structure with a hollow diameter/fiber diameter ratio of approximately 0.5. Two kinds of oxygen species are detected on the microfiber surface, and these hollow microfibers with a microchannel-like structure can be used as microreactors with high oxygen storage capacity and surface oxygen activity. (2) Microchannel-like La0.8K0.2FeO3 hollow microfibers exhibit high decolorization ability for MB in aqueous solution under ambient conditions, and the rapid decolorization process is largely due to the unique microchannel-like structure and active oxygen on the microfiber surface in the CWAO process. In the temperature range 20−50 °C, the decolorization process is very rapid: after about 5 min, more than 70% of MB is decolorized, and around 80% of MB is decolorized in 30 min. (3) The decolorization process can be described well by a second-order reaction model, and the calculated activation energy is 14.27 kJ/mol. 662



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51274106), the Science and Technology Support Program of Jiangsu Province (Grant BE2012143, BE2013071), the Natural Science Research Program of Jiangsu Province Higher Education (Grant 12KJA430001), the Jiangsu Provincial Postgraduate Cultivation and Innovation Project (Grant CXZZ13_0662), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Rapid Decolorization of Methyl Blue in Aqueous Solution by

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