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Influence of Inorganic Ions and pH on the Photodegradation of 1-Methylimidazole-2-thiol with TiO<sub>2</sub> Photocatalyst Based on Magnetic Multi-walled Carbon Nanotubes
Influence of Inorganic Ions and pH on the Photodegradation of 1-Methylimidazole-2-thiol with TiO2 Photocatalyst Based on Magnetic Multi-walled Carbon Nanotubes
Bulletin of the Korean Chemical Society. 2014. Jan, 35(1): 76-82
Copyright © 2014, Korea Chemical Society
  • Received : June 18, 2013
  • Accepted : October 08, 2013
  • Published : January 20, 2014
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About the Authors
Yinhua Jiang
Yingying Luo
Ziyang Lu
School of the Environment, Jiangsu University, Zhenjiang 212013, China
Pengwei Huo
Weinan Xing
Ming He
Jiqin Li
Yongsheng Yan

Abstract
1-Methylimidazole-2-thiol, as a kind of mercaptans, is a typical organic pollutant which has not been efficiently removed. In this study, titanium dioxide (TiO 2 ) photocatalyst based on magnetic multi-walled carbon nanotubes (MWCNTs) was synthesized via hydrothermal and sol-gel methods. The as-prepared photocatalyst was extensively characterized by X-ray diffraction (XRD), X-ray energy diffraction spectrum (EDS), transmission electron microscope (TEM), Fourier transform infrared (FT-IR) spectra, UV-Vis diffuse reflectance spectra (UV-vis DRS) and vibrating sample magnetometer (VSM). This photocatalyst of TiO 2 /Fe 3 O 4 /MWCNTs was proved to exhibit high photocatalytic efficiency and the photodegradation rate could reach nearly 82.7% for the degradation of 1-methylimidazole-2-thiol under ultraviolet irradiation. In addition, the results demonstrated that inorganic ions had a negative impact on photodegradation of 1-methylimidazole-2-thiol to varying degrees. Moreover, pH had a great and complex effect on photocatalytic degradation of 1-methylimidazole-2-thiol under ultraviolet irradiation.
Keywords
Introduction
Mercaptans come into our surrounding environment from various sources such as petrochemical, paper industry and biological processes. As a matter of fact, mercaptans are one of the most undesirable organic sulfur compounds which are volatile and the presence of a very little amount in air will make us uncomfortable. 1 In addition to toxicity, mercaptans can cause some problems due to their acidic properties such as corrosion on metallic surfaces. 1-methylimidazole-2-thiol, which is used for hyperthyroidism and non-cyanide silver plating, 2 is also well known as a widespread, toxic and poorly biodegradable pollutant of mercaptans. As a consequence, it seems important to obtain more effective methods to eliminate or convert mercaptans to milder compounds.
Currently, various techniques such as adsorption, biodegradation, hydrodesulfurization and electrochemical degradation have been suggested to treat mercaptans. 3 - 6 Among them, photocatalytic technology has played an important part in the field of environment for its non-toxicity, stability, quick effect and low energy consumption. 7 8 TiO 2 , as a widely used photocatalyst, which has the merit of good photocatalytic activity, highest stability, lowest cost and lowest toxicity, has been extensively employed in wastewater treatment. 9 However, the photocatalytic activity of pure titania and the recovery rate of TiO 2 is not high enough for practical use. 10 In order to solve these problems, the development of new materials is indispensable for the modification of TiO 2 in the field of photocatalysis.
Carbon nanotubes (CNTs), which have unique structure, high mechanical strength, remarkable electrical conductivity and thermal stability, are considered to enhance the photocatalytic activity in environmental cleaning. 11 On the other side, magnetic materials such as ferroferric oxide (Fe 3 O 4 ) nanoparticles with CNTs as the ideal carriers have been introduced as a promising material in environmental cleaning. 12 13 Owing to magnetism of Fe 3 O 4 nanoparticles, they can be easily separated by an external magnetic field and the synthetic magnetic photocatalyst can be reused.
In this paper, Fe 3 O 4 nanoparticles were introduced to the surface of multi-walled carbon nanotubes (MWCNTs) through hydrothermal synthesis method and then the photocatalyst was synthesized by coating Fe 3 O 4 /MWCNTs with TiO 2 via sol-gel method. Afterwards, the photocatalyst was characterized by XRD, EDS, TEM, UV-vis spectra, FT-IR spectra and VSM. 1-methylimidazole-2-thiol, as a kind of mercaptan compounds, was degraded under ultraviolet light by the synthetic photocatalyst of TiO 2 /Fe 3 O 4 /MWCNTs. It has been demonstrated that the photocatalytic reactions strongly depends on the photogenerated electron-hole recombination rate, light intensity, catalyst concentration, temperature and pH. 14 Given that natural water and industrial wastewater usually contains a vast variety of inorganic ions (such as Na + , K + , SO 4 2− and Cl ) which are very likely to affect the photodegradation activity of the catalyst, 15 the influence of various parameters such as cations, anions and pH were investigated during the photocatalytic process under the ultraviolet irradiation.
Experimental
Materials and Characterization. MWCNTs provided by Beijing DK nano technology Co. Ltd. with special surface area of more than 40 m 2 /g and about 50 nm and 10-20 μm in diameter and length, respectively. 1-methylimidazole-2-thiol (C 4 H 6 N 2 S) was purchased by Aladdin Chemistry Co. Ltd. Ethylene glycol (C 2 H 6 O 2 , AR), iron (III) nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O), polyethylene glycol 1500 (PEG 1500), polyvinylpyrrolidone k-30 (PVP), tetrabutyl titanate (TBOT, CP), ethanol (C 2 H 5 OH, AR), sodium chloride (NaCl), sodium acetate trihydrate (NaAc·3H 2 O, AR) and magnesium chloride hexahydrate (MgCl 2 ·6H 2 O) were all purchased from Sinopharm Chemical Reagent Co. Ltd. Sulfuric acid (H 2 SO 4 , 95%-98%), nitric acid (HNO 3 , 65%-68%), hydrochloric acid (HCl, 36%-38%), calcium chloride (CaCl 2 ), sodium nitrate (NaNO 3 ), sodium sulfate decahydrate (Na 2 SO 4 ·10H 2 O), sodium bicarbonate (NaHCO 3 ) and potassium chloride (KCl, AR) were used as received. Deionized and doubly distilled water was used throughout this work. The pH value of solution was adjusted by using buffer solution with the PHS-3C precision pH meter. X-ray diffraction (XRD) patterns were obtained with a D/max-RA X-ray diffractometer (Rigaku, Japan) equipped with Ni-filtrated Cu Kα radiation (40 kV, 200 mA) to characterize the crystal structure. The 2θ scanning angle range was 20-80° with a step of 0.02°/0.2 s. The specific surface area (BET) was estimated from the N2 adsorption/desorption isotherms, measured by a Quantachrome NOVA2000 surface area apparatus. The X-ray energy diffraction spectrum (EDS) was examined with S-4800 scanning electron microscopy (HITACHI, Japan) and the transmission electron microscope (TEM) images were examined with JEM-2100 transmission electron microscopy (JEOL, Japan). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 FT-IR (America thermoelectricity Company) with 2 cm −1 resolution in the range 400- 4000 cm −1 , using KBr pellets. Ultraviolet-visible spectrophotometer (UV-2450, Shimadzu, Japan) was used for spectrophotometric determination of mercaptans. UV-vis diffuse reflectance spectra (UV-vis DRS) of photocatalyst powder were obtained for the dry-pressed disk samples using it equipped with the integrating sphere accessory for diffuse reflectance spectra, using BaSO 4 as the reflectance sample.
Preparation of TiO2/Fe3O4/MWCNTs photocatalyst. The purification of the MWCNTs was treated as follows: 0.5 g MWCNTs was added in a mixture of nitric acid (15 mL) and sulfuric acid (45 mL) and then stirred for 6 h at 60 ℃. Afterwards, the sample was washed with deionized water to neutral and dried under vacuum over night at 50 ℃ for further use. These treatments helped remove impurities such as amorphous carbon and metallic catalyst in the MWCNTs. 16
The Fe 3 O 4 /MWCNTs were prepared via a one-step hydrothermal synthesis by Zhang with some modifications. 17 30 mg purified MWCNTs was dispersed in 25 mL ethylene glycol by sonication for 20 min. Afterwards, 1.2 g Fe(NO 3 ) 3 ·9H 2 O was added into the above solution and sonicated for another 20 min, 1.5 g NaAc·3H 2 O, 0.4 g PEG (1500) and 10 mg PVP were added into the solution and sonicated for another 20 min. Finally, the mixture was transferred into a 50 mL teflon-lined stainless steel autoclave and maintained at 200 ℃ for 12 h, afterwards, the mixture was cooled to room temperature. The black products were collected by a magnet and rinsed with deionized water and ethanol for several times until there were no nitrate ions in the solution, then sealed and dried in a vacuum oven at 50 ℃ over night.
The TiO 2 /Fe 3 O 4 /MWCNTs were synthesized through a sol-gel method. 36 mL ethanol was mixed with 10 mL TBOT with a vigorous stirring for 15 min at 40 ℃ in a sealed condition. Afterwards, a mixture of 36 mL ethanol, 3 mL deionized water and 0.2 mL hydrochloric acid was gradually added into the above solution. The mixed solution was stirred for 10 min at 40 ℃ under an enclosed condition. After the formation of sol was observed, 1.0 g Fe 3 O 4 / MWCNTs was added into this sol to obtain gel. Finally, the gel was dried under vacuum at 50 ℃ and calcined for 4 h at 500 ℃ under a nitrogen atmosphere.
Photocatalytic Activity Experiment. The photocatalytic activity was evaluated by the degradation of 1-methylimidazole- 2-thiol under UV irradiation. 50 mL 10 mg/L 1-methylimidazole-2-thiol was mixed with 0.1 g TiO 2 /Fe 3 O 4 / MWCNTs or additional inorganic ions and kept in the chamber of GHX-2 photocatalytic reactor under UV light irradiation. After the desired adsorption time, the sampling for photo-treatment was taken with an injector in 10 min interval in an hour. The concentration of treated solution was decanted with a magnet and measured by UV-vis spectrophotometer in the wavelength of 251 nm. The degradation rate (Dr) of catalyst is calculated by the following formula:
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Where C 0 is the initial concentration of 1-methylimidazole-2-thiol (mg/L), C represents the residual concentration (mg/ L) at time t (min). The ratio of reduced concentration ( C 0 C ) to initial concentration ( C 0 ) of 1-methylimidazole-2-thiol calculated by ( C 0 C )/ C 0 is plotted against irradiation time in order to observe the photocatalytic degradation rate.
In order to find out the factors of photodegradation, the influence of cations, anions, molar of sodium chloride and pH was investigated in ultraviolet photodegradation process. After the experiment, recycled sample was collected by an external magnet to ascertain the reproducibility of TiO 2 / Fe 3 O 4 /MWCNTs photocatalyst.
Results and Discussion
XRD Analysis. As shown in Figure 1(a) , the XRD pattern of purified MWCNTs exhibited a characteristic peak at 2θ = 26.1°, corresponding to the PDF#03-0401 data file. The 2θ peaks of Figure 1(b) at 30.2°, 35.4°, 43.1°, 57.2° and 62.8° corresponding well with PDF#02-1035 indicated that the Fe 3 O 4 nanoparticles had a cubic spinel structure. 18 Compared to Figure 1(a) , complete disappearance of the strong peak in Figure 1(b) was a result of MWCNTs decorated by Fe 3 O 4 nanoparticles. The peaks of Figure 1(c) appeared at 2θ values of 25.3°, 38.0°, 48.1°, 54.0° and 75.1° which can be indexed to the anatase phase of TiO 2 , 19 corresponding to PDF#01- 0562. Average crystallite size of TiO 2 was estimated according to Scherrer’s equation, d = kλ/(βcosθ), where d is the average crystallite size (nm), λ is the wavelength of the Cu Kα applied (λ = 0.15406 nm), β is the peak width at halfmaximum and k is the constant usually applied as 0.89. The average crystallite size of TiO 2 was calculated to be 26 nm. The specific surface area of TiO 2 /Fe 3 O 4 /MWCNTs photocatalyst was calculated as 168.32 m 2 /g using BET method, and with average pore sizes of 4.41 nm. At the same time, the diffraction peaks of Fe 3 O 4 were still existed in the TiO 2 / Fe 3 O 4 /MWCNTs photocatalysts.
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XRD patterns of purified MWCNTs (a), Fe3O4/MWCNTs (b) and TiO2/Fe3O4/MWCNTs photocatalysts (c).
TEM and EDS Analysis. TEM images of purified MWCNTs, Fe 3 O 4 /MWCNTs and TiO 2 /Fe 3 O 4 /MWCNTs photocatalysts were shown in Figure 2(a) , 2(b) and 2(c) , respectively. The MWCNTs was approximately 50 nm in diameter after being purified, without any changes to pristine MWCNTs. Obviously, unlike purified MWCNTs, Figure 2(b) showed a lot of additional tiny nanoparticles decorated on the surface of the MWCNTs owing to Fe 3 O 4 /MWCNTs nanocomposites. As shown in inset of Figure 2(b) and Figure 2(c) , the existence of Fe 3 O 4 and TiO 2 was confirmed by characterization of high resolution TEM. The mean particle size of TiO 2 was estimated to be 25.26 nm through the particle size histogram from TEM images (shown in Fig. S1, Supporting Information), which was similar with the result of XRD. The EDS analysis (shown in Fig. S2) illustrated that the major constituents for the photocatalysts were Ti, O, Fe and C. The peaks of Ti were mainly generated by TiO 2 and the peaks of Fe were generated by magnetic materials of iron oxide compounds. This result was consistent with the XRD data and further confirmed the existence of TiO 2 .
FT-IR Spectra Analysis. FT-IR spectra of pristine MWCNTs, Fe 3 O 4 /MWCNTs and TiO 2 /Fe 3 O 4 /MWCNTs photocatalysts were investigated (shown in Fig. S3).The absorption peaks near 3400-3500 cm −1 were belonged to -OH and the 1670 cm −1 band might be due to H-O-H bending of the physically adsorbed water. In the low wavenumber ranges, the absorptive peaks of MWCNTs and Fe 3 O 4 /MWCNTs were significantly different. This was attributed to the bond of Fe-O (582 cm −1 ) according to the characterization before. It could be observed from TiO 2 /Fe 3 O 4 /MWCNTs that the absorption peaks of Fe-O became wakened owing to TiO 2 layers. The absorption peaks in the range 700-400 cm −1 was corresponded to the Ti-O bond vibration. Moreover, the absorption peaks at 1389 cm −1 was ascribed to the O-H bond vibration of the surface-adsorbed species of particles (TiOH). The result indicated that the magnetic photocatalysts were well modified.
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TEM images of purified MWCNTs (a), Fe3O4/MWCNTs. (b) and TiO2/Fe3O4/MWCNTs photocatalysts. (c) Inset images were the corresponding high resolution TEM images of Fe3O4 and TiO2.
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The UV-vis diffuse reflectance spectra of purified MWCNTs (a), Fe3O4/MWCNTs (b), TiO2/Fe3O4/MWCNTs (c) and P25 (d) Inset was the plot for TiO2/Fe3O4/MWCNTs.
UV-vis DRS Analysis. The UV-vis diffuse reflectance spectra of purified MWCNTs, Fe 3 O 4 /MWCNTs, TiO 2 /Fe 3 O 4 /MWCNTs and P25 were shown in Figure 3 . It could be obviously observed that the light absorption intensity of composite photocatalyst was enhanced by MWCNTs. Assuming TiO 2 /Fe 3 O 4 /MWCNTs to be an indirect semiconductor, plots of (Ahν) 1/2 versus the energy of absorbed light afforded the band gaps of TiO 2 /Fe 3 O 4 /MWCNTs as shown in the inset of Figure 3 . The band gap was estimated to be 1.2 eV, which was much lower than that of TiO 2 (3.2 eV). This could be attributed to the possible electronic transition of π→π* of MWCNTs and n→π* between the n-orbit of the oxygen species of TiO 2 and π* of MWCNTs. 20 As a consequence, it could be effectively excitated and generated more electron-hole pairs under UV light, which could improve photocatalytic activity. The possible photodegradation mechanism of 1-methylimidazole-2-thiol with TiO 2 /Fe 3 O 4 /MWCNTs under ultraviolet light could be described as Scheme 1 . When the TiO 2 /Fe 3 O 4 /MWCNTs were under UV irradiation, the electrons of TiO 2 could be excitated and migrate to electron conductor of MWCNTs to generate •OH. At the same time, the electrons could be injected to the conduction band. The electrons in valence band of TiO 2 could recombine with holes, while holes generated in the valence band of TiO 2 . Thus, an increasing number of photogenerated electrons and holes formed in TiO 2 nanoparticles. The photo-generated electrons were so active that they could react with O 2 to generate superoxide radical anion •O 2 . Then the photo-generated holes reacted with OH- or H 2 O to generate more •OH which resulted in the enhanced photocatalytic activity. 21 22 As a result, 1-methylimidazole-2-thiol could be oxidized to organic radicals or other intermediates.
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Mechanism of TiO2/Fe3O4/MWCNTs under ultraviolet irradiation.
VSM Analysis. Magnetic measurement was carried out by using a vibrating sample magnetometer (VSM) with the applied field sweeping from 10 to 10 kOe (shown in Fig. S4). Symmetric hysteresis showed good magnetic properties and the saturation magnetization (Ms) was 7.25 emu/g. It was obviously lower than that of the bulk Fe 3 O 4 (90 emu/g). 23 This phenomenon could be attributed to the surface spin effect on Fe 3 O 4 nanoparticles caused by modification of TiO 2 . The coercivity and remanence of TiO 2 /Fe 3 O 4 /MWCNTs were 207.68 Oe and 1.59 emu/g, respectively. To further clarify the magnetic properties, a simple experiment was conducted. When an external magnetic field was applied, the particles were attracted to the wall of beaker and the dispersion became clear and transparent.
Effect of Cations on Photocatalytic Activity. Effect of Cations on Photocatalytic Activity. There are dissenting views about the effect of metal cations on photocatalytic degradation of organic compounds in literature. 24 In order to highlight the effect of different cations on the photodegradation of 1-methylimidazole-2-thiol, the effects of Na + , K + , Ca 2+ and Mg 2+ were studied by adding 0.1 mmol NaCl, KCl, CaCl 2 and MgCl 2 to the solution of 1-methylimidazole-2-thiol, respectively. Different exogenous cations on the photodegradation of 1-methylimidazole-2-thiol were shown in Figure 4 .
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Effect of different cations on the photocatalytic activity.
The TiO 2 /Fe 3 O 4 /MWCNTs exhibited the highest photocatalytic activity which could reach nearly 82.7% in an hour. However, the photocatalytic efficiency with exogenous cations was inhibited and varied in the following order.
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On the one hand, Mg 2+ and Ca 2+ were provided with high valency and total ion concentration in comparison with Na + and K + . The addition of cations led to the increase of the ionic strength, hindered the electrostatic repulsive effects by a compression of the electric double layer as argued by Guillard et al . and Hoogeven et al . 25 26 As cations with a strong electrostatic field (multi-charged) were the most efficient at compressing the electric double layer (pzc of TiO 2 /Fe 3 O 4 /MWCNTs with Mg 2+ , Ca 2+ , Na + and K + was 2.35, 2.37, 2.46 and 2.55, respectively.), high ionic strength of Mg 2+ and Ca 2+ showed better efficiency to improve degradation than Na + and K + . On the other hand, chloride ion had been shown to be chemically inert in photocatalytic treatment. 27 As a consequence, the addition of exogenous cations slightly led to the inhibition of photodegradation.
Effect of Anions on Photocatalytic Activity. 0.1 mmol of different exogenous anions on the photodegradation of 1- methylimidazole-2-thiol were shown in Figure 5 . In this work, the effects of Cl , NO 3 , HCO 3 and SO 4 2− were studied by adding 0.1 mmol NaCl, NaNO 3 , NaHCO 3 and Na 2 SO 4 to the solution of 1-methylimidazole-2-thiol, respectively. The photocatalytic efficiency was varied in the following order.
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The addition of exogenous anions was observed to have a negative effect which inhibited the photocatalytic activity. The existed research showed that inorganic anions could capture ·OH and form corresponding inorganic free radicals. 28 As SO 4 2− had a weak capturing affect and competition combination to ·OH, it would take place with the following reaction in the water. 29
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Effect of different anions on the photocatalytic activity.
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Though SO 4 2− showed the highest activity for the additional ions, the process of photodegradation was ultimately inhibited.
The redox potential of Cl 2 /Cl was 1.36 V which was lower than redox potential of TiO 2 valence band hole. The oxidation capacity of ·OH was only inferior to Cl and ·OH was the primary oxidant in the process of catalytic oxidation. Therefore the inhibition of pollutants activity of Cl may be caused by competing for the oxygen in the photodegradation processes. In addition, Cl could struggle with target pollutants through the capture of ·OH which would change highly active ·OH to other substances to inhibit the photocatalytic activity of TiO 2 . The reaction mechanism was demonstrated as follows: 30
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HCO 3 formed a strong combination on the surface of catalyst and trapped ·OH to produce the less reactive anion radical·CO 3 as listed below.
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As a result, HCO 3 played a negative part in the photocatalytic process.
Given that nitrate could form ·OH when irradiated, it might be able to increase the photodegradation rate of the catalytic processes.
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However, nitrate could absorb light in the UV range and acted as an inner filter for the UV light. 31 It could be seen from the experimental data that nitrate worked mainly as a UV light filter rather than as a generator of ·OH under the circumstances. As a consequence, the photocatalytic efficiency of the pollutants in the UV process decreased.
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Effect of different dosages of NaCl on the photocatalytic activity (a) and the last photodegradation rates (b).
Effect of Dosages of NaCl on Photocatalytic Activity. Different dosages of NaCl on the photodegradation of 1-methylimidazole-2-thiol were shown in Figure 6 . The catalytic efficiency varied in the following order.
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With the chloride concentration increased, the photodegradation rate exhibited from 77.4% to 70.2% which was gradually decreased. Just as described before, chloride ion was an effective ·OH scavenger and the existence of chloride ion inhabited the photodegradation process.
Effect of pH on Photocatalytic Activity. The effect of pH with 0.1 mmol NaCl on the photodegradation of 1-methylimidazole-2-thiol was shown in Figure 7 . The photocatalytic ability was varied in the following order.
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When the pH was 9, the degradation efficiency of the photocatalyst reached the best. The impact of Cl in the process of photodegradation was significantly linked to pH value.
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Acid conditions would release more H + which would let the solute play a role as acid catalyst. 32 On the other side, neutral or alkaline condition was expected to obtain higher photocatalytic efficiency in the presence of Cl . 33 In view of sensitivity analysis, it was interesting to point out that ·OH concentration was much more sensitive to changes than chloride concentrations due to pH variation.
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Effect of pH with 0.1 mmol NaCl on the photocatalytic activity (a) and last photodegradation rates (b).
Reproducibility of TiO2/Fe3O4/MWCNTs. The stability of TiO 2 /Fe 3 O 4 /MWCNTs was very important to the photocatalytic photocatalytic reaction. Fig. S5 showed the removal efficiency in different cycles. It could be clearly seen that TiO 2 /Fe 3 O 4 /MWCNTs could be used at least four cycles with a little loss of photocatalytic efficiency for degradation of 1-methylimidazole-2-thiol, indicating that TiO 2 /Fe 3 O 4 /MWCNTs had excellent reproducibility.
Conclusion
In this paper, a new magnetic photocatalyst of TiO 2 /Fe 3 O 4 /MWCNTs was synthesized through combination of hydrothermal and sol-gel methods. Afterwards, the as-prepared photocatalyst was used for the degradation of 1-methylimidazole-2-thiol under ultraviolet irradiation. The photocatalyst was proved to exhibit a high catalytic efficiency which could nearly reach 82.7% in an hour. The effects of various parameters such as cations, anions and pH were investigated during the photodegradation process. The results showed that the photodegradation rate slightly decreased in the presence of cations. The presence of anions was also observed to have a negative effect which inhibited the photocatalytic activity. However, the pH had a great and complex effect on photocatalytic degradation of 1-methylimidazole-2-thiol. In addition, the as prepared photocatalyst was expected to be used for recycling and utilization.
Acknowledgements
This work was financially supported by the Natural Science Foundation of China (No. 21306068), the Natural Science Foundation of Jiangsu Province (No. SBK201342486), the China Postdoctoral Science Foundation (No. 2012M521015) and the Innovation Programs Foundation of Jiangsu Province (Nos. CXZZ13_0693 and CXZZ13_0665). And the publication cost of this paper was supported by the Korean Chemical Society.
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