Abstract
sol was prepared for the degradation of methylene blue (MB) solution under ultraviolet (UV) irradiation. The absorption spectra of MB indicated that the maximum wavelength, 663 nm, almost kept the same. The performance of 92.3% for color removal was reached after 160 min. The particle size of sol was about 22.5 nm. X-ray diffraction showed that consisted of a single anatase phase. The small size and anatase phase probably resulted in high photocatalytic activity of sol. The degradation ratio decreased as the initial concentration of MB increased. The photodegradation efficiency decreased in the order of . Regarding catalyst load, the degradation increased with the mass of catalyst up to an amount of 1.5 then decreased as the mass continued to increase. The addition of to sol resulted in an increase on the degradation ratio.
1. Introduction
The textile industry produced dye pollutants that were becoming a major source of environmental contamination. It was estimated that 10 to 15 percent of dyestuff without treatment was released in water during dyeing processes [1]. A variety of physical, chemical, and biological methods, such as adsorption, coagulation, membrane process, and oxidation-ozonation were presently available for treatment of dye wastewater [2–4]. The conventional processes were insufficient to purify the wastewaters. They just transferred the compounds from aqueous to another phase, thus causing secondary pollution problem [5, 6]. Semiconductor photocatalysts TiO2 was acting much interest for their potential applicability in degradation of dye pollutants [7–10]. The TiO2 catalyst could transform organic pollutants into biodegradable compounds of low molecular weight.
Many papers [11–15] reported that TiO2 was prepared by the sol-gel progress for it was nonselective, room-temperature and inexpensive manufacturing cost. Huang et al. [16] prepared TiO2 sol by a sol-gel method. The microstructure and morphology of sol sample were characterized. Its performance for photodegradation of formaldehyde was investigated. However, the operational effects, such as catalyst concentration, initial formaldehyde concentration, and pH, in the degradation process were not considered. The TiO2 sol usually contained organic solvent as a major constituent [17–20]. Silva and Faria [21] first mixed tetrabutyl titanate, absolute alcohol, and polyethyleneglycol. Few quantity of deionized water was added and the volume fraction of organic solvent reached almost 98.2%. The degradation of TiO2 prepared using this method was only around 60%. The organic solvent slowed down the hydrolysis rate of tetrabutyl titanate and then decreased the photocatalytic activity of TiO2 [22]. It also resulted in more serious environmental pollution [23, 24]. To overcome the disadvantages, the use of aqueous solution instead of organic solvent was desired.
In this paper, aqueous base titanium sol was synthetized via a sol-gel method. Its photocatalytic activity was evaluated on the degradation of MB. The effects of key experimental parameters were investigated.
2. Experimental
2.1. Materials
The absolute ethanol (EtOH, AR), Hydrocholoric acid (HCl, AR), Hydrogen Peroxide (H2O2, AR), and methylene blue (MB) were purchased from Sinopharm Chemical Company (China). Tetrabutyl titanate (Ti(OC4H9)4, CP) was chosen as a precursor. The chemical structure of MB was shown in Figure 1. The water used in the experiments was double distilled and deionized.
2.2. Preparation of TiO2 Sol
8 mL Ti(OC4H9)4 was dissolved in 4 mL EtOH. After stirring vigorously by a dynamoelectric stirrer for 10 min at room temperature, it was added dropwise to the mixed solution of H2O and HCl (1 molL−1) with a burette under stirring. An aqueous solution of H2O2 was then added to the mixture, and clear sol was obtained after 7 days. The mole ratio of Ti(OBu)4 : H2O : C2H5OH : HCl was about 1.3 : 320.6 : 3.9 : 1. The water quality content of the sol was about 90%.
2.3. Characterization
The Zeta potential of reaction solution and particle size of TiO2 sol were measured with the Zeta potential and particle size analysis apparatus (Nano-ZS90, Malvern Co., UK), respectively.
The crystalline phase of TiO2 was studied by X-ray diffractometer (Rigaku Corporation, Japan) with Cu K radiation. A 2 range of to was scanned with a step of using an integration time of 3 seconds.
UV-vis spectroscopy of the TiO2 sol was recorded on a UV-vis spectrophotometer (Puxi Tongyong Apparatus Ltd. of Beijing TU-1901) with wavelength range of 500–700 nm.
2.4. Photocatalytic Activity Tests
The photocatalytic activity of TiO2 sol was evaluated on the degradation of MB in an aqueous solution under illumination of UV light (mercury vapor lamp, 40 W, 40 cm long, predominantly wavelength 253.7 nm) in a photoreactor system. The UV lamp was fixed down below the top of the reactor and vertically hung. Four test tubes contained with reaction solution were equably around the lamp. The depth of the solution was 8 cm and the internal diameter of test tube was 1.6 cm. The distance from test tubes to lamp was 10 cm. In a typical experiment, the TiO2 sol was added into aqueous solution of MB, and the initial MB concentration and the amount of TiO2 were set at 0.2 gL−1 and 1.5 gL−1, respectively. The reaction time ranged from 0 to 160 min. The degradation of MB was calculated by formula: Degradation = , where and were the absorbance of the primal and remaining MB, respectively. The absorbance was measured with UV/vis spectrophotometer (UV-2100, China).
3. Results and Discussion
3.1. Absorption Spectra of MB Degradation
Figure 2 displayed maximum UV-vis absorption at wavelength of 663 nm. The maximum absorption almost kept the same during the photodegradation progress. This indicated that it was a sign of MB concentration level after degradation. So it was possible to measure the absorbance at 663 nm each time, and the resulting data of degradation ratio were valid. The absorption peak of the spectra rapidly decreased with increased time and almost disappeared for 160 min light irradiation as seen from Figure 2. The chromophores responsible for characteristic color of the MB were broken down and MB had been degraded. The maximum absorption value and calculated degradation ratio were shown in Table 1 COD values of MB were presented in Table 2. The performance of 92.3% for color removal and 71.4% for COD value removal were reached, which indicated that TiO2 sol prepared in the experiment had high photocatalytic activity.
3.2. Particle Size and Its Distribution of TiO2 Sol
The nanometer particle size was probably one of the reasons for the high photocatalytic activity of the catalyst. Figure 3 showed the particle size of TiO2 sol. It distributed between 10 nm and 80 nm and the mean particle size was 22.5 nm. The small crystal size of the TiO2 sol caused blue shifts, which was consistent with findings reported earlier [25, 26]. The blue shift probably resulted in an increment of reduction potential for the electron and oxidizing potential for the hole. The electrons and holes with high reduction and oxidation power increased the photodegradation ratio. Particle size also had an influence on the surface area of the reactive species. The decrease of the particle size caused an increase of specific surface area, which was beneficial to accelerate the photocatalytic reaction.
3.3. The Crystallinity of TiO2 Sol
The crystalline phase affected the activity of TiO2. It was observed from Figure 4 that a broad peak at (corresponding to (101)) was observed. The rutile diffraction peak at of TiO2 film (corresponding to (110)) was not found. These indicated that TiO2 thin film without high-temperature treatment was mainly based on the existence of anatase phase. Between the two crystalline phases, anatase was proved to have better optoelectronic properties [27]. Simultaneously, the full width at half maximum of XRD peak corresponded to the crystal size of TiO2 catalyst. Scherrer formula was used for the calculation of crystallite size [28]. The size was about 23.8 nm. Both the anatase phase and small size improved the photocatalytic activity of TiO2 sol.
3.4. Photocatalytic Activity of TiO2 Sol
3.4.1. Effect of Initial Concentration of Dye
The initial concentration of MB influenced the UV light absorption of TiO2 catalyst. It could be seen from Figure 5 that MB was degraded slowly at a high concentration. As the MB concentration increased, some of the UV light photons were absorbed by the substantial amount of MB molecules. The quantity of effective photons which was absorbed by the surface of catalyst was reduced. The quantity of excited TiO2 electrons produced by effective photons decreased, making the generating holes lessened. Simultaneously, a large amount of impurities and other organic substances were probably adsorbed on the surface of TiO2, making it more difficult for holes to enter the solution. So fewer and fewer holes entered the MB solution and reacted with adsorbed hydroxide ions to produce hydroxyl radicals, the main oxidizing species responsible for the photooxidation of organic compounds [29]. It was noted that the degradation of MB solution decreased with the increasing concentration. The almost complete degradation took place for dye concentrations at 0.20 gL−1.
3.4.2. Effect of pH Value of the Solution
Solution pH was an important variable in the evaluation of aqueous phase mediated photocatalytic degradation reactions. It influenced the surface electric charge of TiO2 catalyst. Figure 6 revealed that the photodegradation efficiency decreased in the order of pH 2 pH 9 > pH 7. Rideh et al. [30] concluded that the interaction of TiO2 with cationic electron donors and electron acceptors would be favored for photocatalytic activity at high pH greater than the zero point charge (ZPC) of TiO2, while anionic electron donors and electron acceptors would be favored at low pH less than ZPC. ZPC of TiO2 was found to be 6.25.
The reaction of surface charge of TiO2 catalyst performed in alkaline and acid media as follows:
Equation (1) showed that at pH higher than pHzpc, TiO− appeared to be the predominant form. The zeta potential of the sample was 25.49 mV as shown in Figure 7 (a), which indicated that TiO2 was negatively charged. The catalyst absorbed MB molecules with electropositive charge based on the law of electrostatic attraction, which made the degradation increased. The final degradation ratio was 33.8% at pH 9, much higher than 18.0% at pH 7 as shown in Figure 6. Regarding pH values of 2, the average zeta potential of TiO2 was approximately 19.04 mV in Figure (b). Because TiO2 was in the protonated form () as shown in (2), TiO2 catalyst had an effect of electrostatic repulsion against the MB molecules.
However, the effect on MB degradation was only in a low degree. In acidic media, electrons tended to move into the surface of catalyst for the electrostatic attraction between the positive charged TiO2 and negative charged electrons. They reacted with the oxygen molecules absorbed around the TiO2 surface to form oxidizing species such as and OOH. The reaction formulas were as follows [31]:
These oxidizing radicals made the degradation accelerate, which played a more important role than the electrostatic effect discussed above in the reaction progress. Thus, the catalytic activity of TiO2 sol improved in alkaline and acid media, especially in acid media.
3.4.3. Effect of Catalyst Load
In order to avoid an ineffective excess of catalyst and to ensure a total absorption of efficient photons, the fitting mass of catalyst needed to be found. The concentration was varied from 0.5 to 2.5 gL−1 in this study. In Table 3, the degradation ratio increased with the mass of catalyst up to an amount of 1.5 gL−1 for an increment of the active sites available for MB adsorption and degradation. However, an increase on the catalyst loading to 2.0 and 2.5 gL−1 resulted in a decrease in the degradation ratio. This could be attributed to deactivation of activated molecules by collision with ground state molecules of TiO2. Following these observations, it was decided to keep the amount of TiO2 at the value of 1.5 gL−1 in subsequent photocatalytic degradation experiments.
3.4.4. Effect of Added H2O2
Hydrogen peroxide played a dual role in photocatalytic reaction acting as electron acceptor and could decompose to produce OH radicals. As shown in Figure 8, the addition of H2O2 (0.03 molL−1) to TiO2 sol resulted in a significant increase on the degradation ratio of MB. At the end of 160 min of irradiation total removal, the degradation ratio of TiO2 with H2O2 reached 95.8%, more than 92.3%. The decolorization of MB by H2O2 itself was in a low degree. The degradation ratio was only 9.6%.
Photocatalytic reaction major energy wasting step consisted in the recombination of photo-generated electrons and holes leading to the low quantum yield of the process. Electron-hole recombination could be prevented by adding a proper electron acceptor to the system. So the addition of H2O2 to TiO2 sol was considered. H2O2 showed a positive effect on the degradation by avoiding electron/hole recombination, as it acted as electron acceptors.
Adding H2O2 was also a possible way to increase the concentration of OH radicals to increase the degradation as shown in the following [32]:
Hydroxyl radicals could as well be produced by reaction of H2O2 with superoxide radical anion as follows:
4. Conclusion
TiO2 sol synthesized via a sol-gel method was used in the photocatalytic degradation of MB under UV light. The maximum wavelength of MB in the absorption spectra was 663 nm and almost kept the same. The absorption peak decreased during the degradation progress. The particle size of TiO2 sol was 22.5 nm, which was one cause of the high photocatalytic activity of the sol. The high initial concentration of MB interfered with the penetration of UV light transmission. Fewer holes reacted and resulted in a decrease in the degradation ratio. The activity of TiO2 sol improved in alkaline media due to the electrostatic attractions between oppositely charged MB molecules and negatively charged TiO2 catalyst. In acidic media, more electrons reacted to form oxidizing species to increase the degradation. Regarding catalyst load, the degradation increased with the mass of catalyst up to an amount of 1.5 gL−1. It decreased as the mass continued to increase attributed to the deactivation of activated molecules. The addition of H2O2 to TiO2 sol resulted in an increase in the degradation ratio. The H2O2 acted as electron acceptors to make electron/hole recombination avoided and increased the concentration of OH radicals.
Acknowledgments
The authors are grateful to the financial support of the National Natural Science Foundation of China (no. 20674031), the Program of Key Laboratory of Eco-Textile (Jiangnan University) and the Business Doctoral Innovation Project of Jiangsu Province in China (BK2009672).