Degradation and Mineralization of Bisphenol A by Mesoporous

Aug 12, 2010 - College of Environmental Science and Engineering, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Key Labora...
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Environ. Sci. Technol. 2010, 44, 6843–6848

Degradation and Mineralization of Bisphenol A by Mesoporous Bi2WO6 under Simulated Solar Light Irradiation CHUNYING WANG, HAO ZHANG, FANG LI, AND LINGYAN ZHU* College of Environmental Science and Engineering, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Key Laboratory of Pollution Processes and Environmental Criteria, Ministry of Education, Nankai University, Tianjin, P.R. China 300071

Received June 2, 2010. Revised manuscript received July 25, 2010. Accepted July 30, 2010.

Bismuth tungstate (Bi2WO6) catalysts of different morphology were synthesized with a hydrothermal method by controlling the pH of the reaction solution. The properties of the synthesized catalysts were characterized and all catalysts presented high photoabsorption capacity in the range of UV light to visible light around 450 nm. The surface area of the catalysts decreased but the crystallinity increased with the pH of the hydrothermal reaction solution in the range of 4-11. It was found that the crystallinity of the catalysts played an important role on their degradation capacity to Bisphenol A (BPA). Bi2WO6 catalyst prepared at pH 11 displayed a mesoporous structure and it showed the highest photocatalytic activity to degrade BPA under simulated solar light irradiation. Nearly 100% of BPA with original concentration at 20 ppm was removed after 30 min irradiation in a solution with pH 10 and Bi2WO6 amount of 1.0 g L-1. Furthermore, 86.6 and 99.1% of the total organic carbon was eliminated after 60 and 120 min irradiation, respectively. Only one intermediate at m/z 133 was observed by LC/MS and a simple pathway of BPA degradation by Bi2WO6 was proposed.

Introduction Bisphenol A [2,2-bis(4-hydroxyphenyl)propane, BPA] has been widely used as raw materials for epoxy and polycarbonate resins, such as baby bottles, lining of food cans, and dental sealants (1). The total global production capacity of BPA was about 3.2 million metric tons in 2008 (2). Due to the wide usage in household and commercial products, a large amount of BPA has been released into the environment and it is widely present in aquatic environment with concentration in river water at level of ng L-1- µg L-1 (3-5). Toxicity studies have found that BPA may cause various adverse effects on aquatic organisms even at low exposure levels (6-9). BPA was first described as a synthetic estrogenic agent in 1936 (10). It was characterized as one of the representative endocrine disrupting chemicals (EDCs) by Ministry of the environment government of Japan in 1998 (11). Several methods have been developed to remove BPA from water, including physical (12, 13), biological (14), * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-22-23500791. Fax: +86-2223503722. 10.1021/es101890w

 2010 American Chemical Society

Published on Web 08/12/2010

ultrasonic (15, 16) and chemical (17, 18) techniques. Carbonaceous absorbents can quickly adsorb BPA from water (13), but BPA molecules remain intact and present potential threat to the environment. Biodegradation method usually takes a long time and it depends on many environmental factors such as bacterial counts, salinity, temperature and so on (19, 20). Among these methods, photocatalytic degradation is promising due to its high degradation and mineralization efficiency (21). Since the discovery of the photocatalytic splitting of H2O on the TiO2 electrodes by Fujishima and Honda in 1972 (22), TiO2 has found a wide application in degradation of organic chemicals in water. BPA could be completely mineralized to CO2 by nano-TiO2 under UV irradiation (23). However, the main shortcoming of TiO2 is that it only absorbs ultraviolet light no longer than 387.5 nm, which only accounts for about 4% of sunlight (24-26). High energy is necessary to keep its degradation efficiency when TiO2 is used to treat organic chemicals in water. Therefore, it is of great interest to develop visible-light-driven photocatalysts. Bismuth tungstate (Bi2WO6) was reported to have photocatalytic activity under visible light due to its Bi2O2 layered structure with perovskite-like slab of WO6 (27). Bi2WO6 could be synthesized by solid-state reaction (28) and hydrothermal method (29). Hydrothermal synthesis produces Bi2WO6 with smaller crystal size and higher surface area. Hydrothermal conditions, such as reaction temperature and reaction time, play important roles in simultaneously controlling the size, morphology, and dispersivity of the nanocrystals (30). The photocatalytic activity of Bi2WO6 was demonstrated by photodegrading azo dyes such as Methylene Blue and Rhodamine B (31, 32). But its ability to degrade EDCs has not been extensively studied. The impact of pH of hydrothermal reaction solution on the physicochemical properties of Bi2WO6 catalysts and their photocatalytic activity are not well understood. The objectives of this study were: 1, to synthesize Bi2WO6 catalysts of different morphology using hydrothermal method by varying the pH values of the reaction solutions; 2, to characterize the physicochemical properties of the prepared catalysts and investigate their degradation capacity to BPA; 3, to study the impacts of different reaction factors on the degradation efficiency of the prepared catalyst; 4, to investigate the degradation mechanism and pathway of BPA using the prepared catalyst.

Experimental Section Materials and Reagents. BPA (purity 99.5%), was purchased from Dr. Ehrenstorfer GmbH, Augsburg, Germany. Stock solution of BPA was prepared by dissolving a certain amount of BPA in purified water. Other materials and reagents used are given in the Supporting Information. Preparation and Characterization of Bi2WO6. Bi2WO6 samples were prepared using hydrothermal method. Twenty mL of 0.05 mol L-1 Na2WO4•2H2O solution dissolved in 1.0 mol L-1 NaOH was added slowly into the same volume of 0.1 mol L-1 Bi (NO3)3•5H2O solution which was dissolved in 1.0 mol L-1 HNO3. The mixed solution was vigorously stirred at room temperature for 10 min and then ultrasonicated for 30 min. The pH of the reaction solution was adjusted with diluted HNO3 or NaOH solution to 4, 7, 9, 11, and 13, respectively. The mixed solution was transferred to a 50 mL Teflon-lined autoclave. The autoclave was sealed and heated to 140 °C for 20 h in an oven. After cooling down to room temperature, the reaction solution was centrifuged at 3000 rpm. The precipitate was collected and washed with distilled water and ethanol for several times to remove any possible VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. XRD patterns of the prepared catalysts. ionic residuals, and finally dried at 120 °C for 4 h. The characterization methods for the prepared samples are described in the Supporting Information. Degradation Experiments. The photocatalytic degradation experiments were carried out in a photochemical reactor (Figure S1). Simulated sunlight irradiation was provided by an 800 W xenon lamp (Institute of Electric Light Source, Beijing), which was positioned in the cylindrical quartz cold trap. The system was cooled by circulating water and maintained at room temperature. Before the irradiation, the suspension was magnetically stirred in the dark for 30 min to ensure adsorption equilibrium of BPA on the catalysts. Approximately 0.5 mL of reaction solution was taken at given time intervals and filtered through 13 mm × 0.45 µm membrane for BPA analysis. The reuse of the catalyst was evaluated by collecting the catalyst, which was used for another degradation experiment. The details about the reuse experiment are described in the Supporting Information. Analysis of BPA and Its Intermediates. BPA in the reaction solution was analyzed by high performance liquid chromatograph and the analysis of the intermediates of BPA degradation was performed on a liquid chromatograph coupled with mass spectrometer system (LC-MS). Further details about the instrumental conditions are available in the Supporting Information. Total organic carbon (TOC) was measured with a Shimadzu TOC-V CPH analyzer. The removal ratio (R) of BPA (or TOC) was determined as follows: R ) (1 - C/C0) × 100%

(1)

Where C0 is the initial concentration of BPA (or TOC) and C is the concentration at reaction time t (min).

Results and discussion Catalyst Characterization. Figure 1 shows the XRD patterns of the catalysts prepared at different pH conditions. The diffraction peaks of those catalysts prepared at pH 4, 7, 9, 11 are consistent with those of russellite Bi2WO6 [JCPDS No. 39-0256] (its standard XRD pattern is shown at the bottom of Figure 1). However, as pH increased to 13, the peaks shifted significantly and matched exactly those of Bi3.84W0.16O6.24 [JCPDS No. 43-0447] (its standard XRD pattern is shown on the top of Figure 1). These suggest that the catalyst prepared at pH 13 has different chemical compositions from the catalysts prepared at other pH values. Therefore, the catalyst prepared at pH 13 will not be discussed further in this study. Apparently, pH played an important role in the formation of bismuth tungsten oxide. pH in the precursor suspensions may affect the solubility of WO42- and [Bi2O2]2+ and finally lead to the formation of different phases of bismuth tungsten oxide (33). The peaks of Bi2WO6 became narrower and their 6844

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FIGURE 2. Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curve calculated from adsorption branch of the nitrogen isotherm (inside) of Bi2WO6 prepared at pH 11. intensities increased as pH increased from 4 to 11, indicating the increase of crystallinity and formation of bigger Bi2WO6 crystallites. The intensity ratio of the (200) peak to the (131) peak of Bi2WO6 is more than 0.50, obviously larger than the standard value of 0.20. This suggests that these crystals have special anisotropic growth along the (200) or (020) direction. These results could be attributed to their unique sheet-shaped morphologies. Morphologies of the samples were characterized by FESEM (Figure S2) and TEM (Figure S3). Their morphologies were strongly dependent on the pH of the hydrothermal reaction solution. When the pH was in the range of 4-7, Bi2WO6 was present as a mixture of sphere- and sheet-shaped crystal forms. The crystallite size was about 50 nm. As pH increased to 9-11, irregular sheet-shaped morphology predominated and the crystal size increased to 100-200 nm, which is in agreement with the XRD results. The FESEM images show a highly rough surface of the catalysts. Selected area electron diffraction (SAED) (Figure S3-e) demonstrates that the catalyst prepared at pH 11 was single crystal. BET gas sorptometry measurements were conducted to examine the porous nature of the catalyst prepared at pH 11 (the catalyst prepared at pH 11 displayed the highest photocatalytic activity, which will be discussed later). Figure 2 shows the N2 adsorption/desorption isotherm and the poresize distribution (inset) of the catalyst. The isotherm is identified as type IV, which is characteristic of mesoporous materials (34, 35). The pore-size distribution obtained from the isotherms indicates a number of pores less than 10 nm in the sample. These pores presumably arise from the spaces between the sheets of the product. The sharp peak at around 2 nm suggests that the nanopores are distributed very evenly. The BET specific surface area (SBET) of the sample was calculated from N2 isotherms at -196.68 °C, and was found to be as much as 16.82 m2 g-1. The single-point total volume of pores at P/P0) 0.9673 was 0.0329 cm-3 g-1. The relatively large surface area and total pore volume support the fact that the catalyst has a nanoporous structure. The SBETs of other catalysts prepared at pH 4, 7, 9 were 37.45, 34.84, 18.68 m2 g-1 respectively. The surface area decreased with the pH increasing from 4 to 11, which agrees the result that the particle size increased with pH in the range of 4-11. It is well-known that the electronic structure feature of a semiconductor affects its optical absorption and migration of the light induced electrons (e-1) and holes (h+), and then determines its photocatalytic activity. Figure S4 shows the UV-vis DRS of the catalysts prepared at different pH values. All of them demonstrated high photoabsorption capacity in

FIGURE 3. Photocatalytic degradation of BPA by Bi2WO6 under simulated solar irradiation: (a) Bi2WO6 prepared at pH 4, 7, 9, 11; (b) The removal efficiency of BPA by Bi2WO6 as compared to the results by TiO2, only with irradiation and Bi2WO6 without irradiation; (c) Effect of catalyst/substrate molar ratio on the photodegradation of BPA by Bi2WO6 (pH 11); (d) Effect of pH values on the photodegradation BPA by Bi2WO6 (pH 11). For (a, b and d), the initial BPA concentration was 20 ppm and the catalyst amount was 1.0 g L-1. the range of UV light to visible light around 450 nm, suggesting their potential photocatalytic activity under visible light. The color of the catalysts was yellow, which is in accordance with the absorption band edge at 450 nm. An intensive absorption band with a steep edge was observed, indicating that the visible light absorption was induced by the intrinsic band gap transition instead of the transition from impurity levels (36). The XPS spectrum of Bi2WO6 prepared at pH 11 is shown in Figure S5. Its Bi/W ratio is close to 2:1. Characteristic binding energy values of 159.1 eV and 164.3 for Bi 4f7/2 and Bi 4f5/2 reveal a trivalent oxidation state for bismuth. The binding energy at 37.5 and 35.4 eV for W 4f5/2 and W 4f7/2 can be assigned to a W6+ oxidation state (37). The O element may be fitted into two kinds of chemical states: crystal lattice oxygen and adsorbed oxygen (38). Degradation Efficiency of the Catalysts Prepared at Different pH. The degradation efficiency of BPA by Bi2WO6 prepared at different pH is illustrated in Figure 3(a). The photodegradation efficiency increased as the pH of the hydrothermal reaction solution increased from 4 to 11. The catalyst prepared at pH 11 displayed the highest degradation efficiency. This suggests that Bi2WO6 prepared at basic conditions displayed higher photocatalytic activity than those prepared at acidic or neutral conditions. Many factors such as surface area and crystallinity may affect the activity of photocatalysts. A large surface area favors the sorption of substrates to the catalyst surface and leads to faster reaction. However, the SBETs of the Bi2WO6 catalysts decreased with pH of the hydrothermal reaction solutions in the range of 4-11. On the other hand, the crystallinity increased a lot from pH 4 to 11. It is reported that a high degree of crystallinity (or few surface and bulk defects) helps to reduce e--h+ recombination (39, 40) and stimulates the photocatalytic reaction. So the crystallinity may be the main factor in controlling the BPA degradation by Bi2WO6. Herein, Bi2WO6 prepared at pH 11 was selected as the optimum catalyst and it will be discussed in detail in the following sections. Previous studies show that a suitable conformation of pores allows light waves to penetrate deep inside the

photocatalyst and leads to high mobility of charges (41, 42). It is speculated that the abundant mesopores in the catalyst favor the penetration of light waves and bring about the BPA molecules in solution deep inside the photocatalyst, which results in promoted photocatalytic activity (17). Figure S6 shows that the photocatalytic degradation of BPA by Bi2WO6 (pH 11) followed a pseudo first-order kinetic model (43): -ln

( )

C ) kappt C0

(2)

Where kapp represents the apparent degradation rate constant, which was determined by plotting ln(C/C0) versus reaction time t. Figure 3(b) illustrates the photocatalytic degradation effect of Bi2WO6 (pH 11) as compared with P25 TiO2 (SBET ≈ 50 m2 g-1). The adsorption of BPA by Bi2WO6 was negligible in 60 min. BPA could be degraded without any catalyst under simulated solar light irradiation, but the reaction was very slow and only 19.6% of BPA could be removed in 60 min irradiation. The removal efficiency was enhanced to 28.3% when 1.0 g L-1 P25 TiO2 was added. However, it was enhanced to 95.6% when the same amount of Bi2WO6 (pH 11) was added as catalyst. Bi2WO6 (pH 11) displayed much higher activity than P25 TiO2 under simulated solar light irradiation. The potential to reuse Bi2WO6 was investigated and the result is shown in Figure S7. It can be seen that the catalyst still kept high photocatalytic activity after reuse for 2 times. Due to the mass loss during the filtering and transferring processes, the recovered amount of Bi2WO6 decreased from initial 0.2 g in the first run to 0.1806 g in the second run and 0.1618 g in the third run. As a result, the removal efficiency decreased slightly. Effect of Catalyst/Substrate Molar Ratio on the Degradation. Influence of the catalyst/substrate molar ratio was studied with BPA concentration set at 10, 20, and 40 ppm (Figure 3(c)), and the corresponding catalyst amount set at 0.25-2.0, 0.5-4.0, and 1.0-8.0 g L-1, respectively. At all three levels of BPA, kapp of the reaction increased with the catalyst/ substrate molar ratio in the range of 8.16-49. When the molar VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ratio further increased, the kapp still increased at BPA level of 10 ppm; but leveled off at 20 ppm and decreased slightly at 40 ppm. Given the amount of BPA in the reaction solution was constant, more catalyst would provide more e--h+. As a result, the catalytic activity increased. However, when the amount of catalyst increased continuously, the generation of e--h+ might be affected due to the increase of light scattering and decrease of light penetration (44). Effect of Initial pH of BPA Solution. In heterogeneous media, the active sites on the surface of most semiconductors are dependent on the concentration of hydrogen ion (H+) or hydroxide ion (OH-) in aqueous solution. As a result, pH of the reaction solution may affect the adsorption property and catalytic activity of the photocatalyst. The effect of the initial pH on the photodegradation efficiency of BPA in water is shown in Figure 3(d). Obviously, the degradation rate constant kapp increased as the pH increased from 3 to 10, while it shows a decreasing trend in the range of 10-12. It was reported that Bi2WO6 is unstable in acidic solution, and it could be transformed to H2WO4 and Bi2O3 (45), leading to poor photocatalytic activity of Bi2WO6 at low pH. The redox potential of TiO2 was reported to vary in different pH conditions (46). The redox potential of Bi2WO6 could also be affected by the pH of solution. The slight alkaline conditions (pH 8-10) could help the generation of e--h+ in the catalyst, but further research is needed. When pH increased to 10-12, bisphenolate anion may be formed since the pKa value of BPA is 9.59-10.2 (47). Therefore, the degradation rate decreased due to electrostatic repulsion between the produced bisphenolate anions and the negatively charged surface of Bi2WO6. TOC Removal and Photocatalytic Degradation Mechanism of BPA by Bi2WO6. Complete mineralization of organic compounds is of great significance in the treatment of organic pollutants in water. To access the advantage of Bi2WO6 to completely destruct organic molecules in water solution, TOC was monitored during the entire reaction process. 86.6% of TOC was removed from the reaction system (the initial concentration of BPA was 20 ppm, pH was 10) after 60 min reaction with Bi2WO6 under simulated solar irradiation while 99.1% was removed in 120 min (Figure 4(a)). However, as long as 20 h was necessary when TiO2 was used to photocatalytically degrade BPA (initial concentration of 40 ppm) to CO2 under UV irradiation (23). Only 79% TOC was eliminated in ultrasound/UV/Fe(II) system for 120 min (48). Photocatalytic degradation of BPA by Bi2WO6 displayed an outstanding advantage in mineralization BPA to CO2 and H2O. The intermediates formed in the photocatalytic degradation process were monitored using LC-MS analysis. Except for the peak of BPA at 227, only one byproduct at m/z 133 was observed. The intermediate at m/z 133 eluted earlier than BPA, indicating that it is more polar. The species at m/z 133 was indentified as 4-isopropenylphenol in previous studies (17, 18, 49). The peak areas of m/z 133 and m/z 227 varied with the irradiation time and the result is shown in Figure 4(b). The photocatalytic degradation of BPA by TiO2 was investigated in many previous studies. Ohko et al detected intermediates at m/z 133, 135, 173, and 207 (23). Guo et al detected 5 main intermediates at m/z 133, 185, 199, 245, and 259. The detection of different number of intermediates could be due to the different LC-MS operation conditions. But we used exactly the same operation conditions of LC-MS as Guo et al (17). This suggests that other intermediates were not formed or their concentrations were very low during the photocatalytic degradation by Bi2WO6 catalyst under simulated solar light irradiation. Guo et al (17) investigated the degradation mechanisms of BPA by mesoporous TiO2 nanoparticles under UV irradiation. They suggested that both hydroxyl radicals (HO · ) and 6846

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FIGURE 4. (a) Temporal change in BPA and TOC removal in the presence of Bi2WO6. The initial BPA concentration was 20 ppm, the catalyst amount was 1.0 g L-1, and the pH of reaction solution was 10; (b) Time course of the peak areas of BPA and the intermediate at m/z 133.

FIGURE 5. Suggested degradation pathway of BPA by Bi2WO6. photogenerated holes were responsible for the heterogeneous BPA degradation. The hydroxyl radicals may attack the different carbon atoms of BPA molecules and resulted in different intermediates. They proposed BPA was mainly degraded through demethylation and hydroxylation, producing hydroxylated and multihydroxylated intermediates, which were also observed by other researchers (23). They also suggested a minor pathway: BPA was oxidized directly by holes or HO · to produce 4-isopropenylphenol (m/z 133). In present study, no hydroxylated intermediate was detected, suggesting no hydroxylation occurred during the reaction. Many reports on the photocatalytic degradation of organic compounds in aqueous solutions have suggested the important role of HO · (17, 50, 51). For Bi2WO6, the standard oxidation

potential (1.59 eV) (52) of photogenerated hole is lower than the redox potential of HO · /OH- (1.99 eV) (53), implying that the photogenerated hole on the surface of Bi2WO6 could not react with OH-/H2O to form HO · . This may explain that no hydroxylated products were observed in the reaction solution. The observation of only m/z 133 indicates that the degradation of BPA by Bi2WO6 could be mainly due to the direct oxidation by the photogenerated holes. Based on the results above, the degradation of BPA by Bi2WO6 experienced the most direct and simple pathway (Figure 5). First of all, Bi2WO6 catalyst received solar simulated irradiation and generated e--h+. These e--h+ pairs reacted with BPA absorbed on the catalyst surface or in the nanopores. Photogenerated holes could oxidize BPA directly to form the immediate at m/z 133. Then, it was degraded to simple organic acids, which were further transferred to CO2 and H2O.

Acknowledgments The authors gratefully acknowledge the financial support of Ministry of Education (Grant 708020); Tianjin Municipal Science and Technology Commission (08ZCGHHZ01000, 07JCZDJC01900), Ministry of Science and Technology (2008ZX08526-003, 2009DFA91910), New Century Talent program, and China-US Center for Environmental Remediation and Sustainable Development.

Supporting Information Available Detailed description of the materials and experimental methods; the FESEM images and TEM images; UV-vis DRS of the catalysts; the XPS spectrum of Bi2WO6 prepared at pH 11; the photocatalytic degradation kinetics of BPA by Bi2WO6 prepared at pH 11; the result of reuse evaluation of the catalyst prepared at pH 11. This information is available free of charge via the Internet at http://pubs.acs.org/.

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