Degradation and Mineralization ofBisphenol A by Mesoporous Bi2WO6under Simulated Solar LightIrradiationC H U N Y I N G W A N G , H A O Z H A N G ,F A N G L I , A N D L I N G Y A N Z H U *

College of Environmental Science and Engineering, TianjinKey Laboratory of Environmental Remediation and PollutionControl, Key Laboratory of Pollution Processes andEnvironmental Criteria, Ministry of Education, NankaiUniversity, Tianjin, P.R. China 300071

Received June 2, 2010. Revised manuscript received July25, 2010. Accepted July 30, 2010.

Bismuth tungstate (Bi2WO6) catalysts of different morphologywere synthesized with a hydrothermal method by controlling thepH of the reaction solution. The properties of the synthesizedcatalysts were characterized and all catalysts presented highphotoabsorption capacity in the range of UV light to visiblelight around 450 nm. The surface area of the catalysts decreasedbut the crystallinity increased with the pH of the hydrothermalreaction solution in the range of 4-11. It was found thatthe crystallinity of the catalysts played an important role ontheir degradation capacity to Bisphenol A (BPA). Bi2WO6 catalystprepared at pH 11 displayed a mesoporous structure and itshowed the highest photocatalytic activity to degrade BPA undersimulated solar light irradiation. Nearly 100% of BPA withoriginal concentration at 20 ppm was removed after 30 minirradiation in a solution with pH 10 and Bi2WO6 amount of 1.0 gL-1. Furthermore, 86.6 and 99.1% of the total organic carbonwas eliminated after 60 and 120 min irradiation, respectively. Onlyone intermediate at m/z 133 was observed by LC/MS and asimple pathway of BPA degradation by Bi2WO6 was proposed.

Introduction

Bisphenol A [2,2-bis(4-hydroxyphenyl)propane, BPA] hasbeen widely used as raw materials for epoxy and polycar-bonate resins, such as baby bottles, lining of food cans, anddental sealants (1). The total global production capacity ofBPA was about 3.2 million metric tons in 2008 (2). Due to thewide usage in household and commercial products, a largeamount of BPA has been released into the environment andit is widely present in aquatic environment with concentrationin river water at level of ng L-1- µg L-1 (3-5). Toxicity studieshave found that BPA may cause various adverse effects onaquatic organisms even at low exposure levels (6-9). BPAwas first described as a synthetic estrogenic agent in 1936(10). It was characterized as one of the representativeendocrine disrupting chemicals (EDCs) by Ministry of theenvironment government of Japan in 1998 (11).

Several methods have been developed to remove BPAfrom water, including physical (12, 13), biological (14),

ultrasonic (15, 16) and chemical (17, 18) techniques. Car-bonaceous absorbents can quickly adsorb BPA from water(13), but BPA molecules remain intact and present potentialthreat to the environment. Biodegradation method usuallytakes a long time and it depends on many environmentalfactors such as bacterial counts, salinity, temperature and soon (19, 20). Among these methods, photocatalytic degradationis promising due to its high degradation and mineralizationefficiency (21). Since the discovery of the photocatalyticsplitting of H2O on the TiO2 electrodes by Fujishima andHonda in 1972 (22), TiO2 has found a wide application indegradation of organic chemicals in water. BPA could becompletely mineralized to CO2 by nano-TiO2 under UVirradiation (23). However, the main shortcoming of TiO2 isthat it only absorbs ultraviolet light no longer than 387.5 nm,which only accounts for about 4% of sunlight (24-26). Highenergy is necessary to keep its degradation efficiency whenTiO2 is used to treat organic chemicals in water. Therefore,it is of great interest to develop visible-light-driven photo-catalysts. Bismuth tungstate (Bi2WO6) was reported to havephotocatalytic 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 hy-drothermal method (29). Hydrothermal synthesis producesBi2WO6 with smaller crystal size and higher surface area.Hydrothermal conditions, such as reaction temperature andreaction time, play important roles in simultaneously con-trolling the size, morphology, and dispersivity of the nano-crystals (30). The photocatalytic activity of Bi2WO6 wasdemonstrated by photodegrading azo dyes such as MethyleneBlue and Rhodamine B (31, 32). But its ability to degradeEDCs has not been extensively studied. The impact of pH ofhydrothermal reaction solution on the physicochemicalproperties of Bi2WO6 catalysts and their photocatalytic activityare not well understood.

The objectives of this study were: 1, to synthesize Bi2WO6

catalysts of different morphology using hydrothermal methodby varying the pH values of the reaction solutions; 2, tocharacterize the physicochemical properties of the preparedcatalysts and investigate their degradation capacity to BPA;3, to study the impacts of different reaction factors on thedegradation efficiency of the prepared catalyst; 4, to inves-tigate the degradation mechanism and pathway of BPA usingthe prepared catalyst.

Experimental SectionMaterials and Reagents. BPA (purity 99.5%), was purchasedfrom Dr. Ehrenstorfer GmbH, Augsburg, Germany. Stocksolution of BPA was prepared by dissolving a certain amountof BPA in purified water. Other materials and reagents usedare given in the Supporting Information.

Preparation and Characterization of Bi2WO6. Bi2WO6

samples were prepared using hydrothermal method. TwentymL of 0.05 mol L-1 Na2WO4•2H2O solution dissolved in 1.0mol L-1 NaOH was added slowly into the same volume of0.1 mol L-1 Bi (NO3)3•5H2O solution which was dissolved in1.0 mol L-1 HNO3. The mixed solution was vigorously stirredat room temperature for 10 min and then ultrasonicated for30 min. The pH of the reaction solution was adjusted withdiluted HNO3 or NaOH solution to 4, 7, 9, 11, and 13,respectively. The mixed solution was transferred to a 50 mLTeflon-lined autoclave. The autoclave was sealed and heatedto 140 °C for 20 h in an oven. After cooling down to roomtemperature, the reaction solution was centrifuged at 3000rpm. The precipitate was collected and washed with distilledwater and ethanol for several times to remove any possible

* To whom correspondence should be addressed. E-mail:zhuly@nankai.edu.cn. Phone: +86-22-23500791. Fax: +86-22-23503722.

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Published on Web 08/12/2010

ionic residuals, and finally dried at 120 °C for 4 h. Thecharacterization methods for the prepared samples aredescribed in the Supporting Information.

Degradation Experiments. The photocatalytic degrada-tion experiments were carried out in a photochemical reactor(Figure S1). Simulated sunlight irradiation was provided byan 800 W xenon lamp (Institute of Electric Light Source,Beijing), which was positioned in the cylindrical quartz coldtrap. The system was cooled by circulating water andmaintained at room temperature. Before the irradiation, thesuspension was magnetically stirred in the dark for 30 minto ensure adsorption equilibrium of BPA on the catalysts.Approximately 0.5 mL of reaction solution was taken at giventime intervals and filtered through 13 mm × 0.45 µmmembrane for BPA analysis. The reuse of the catalyst wasevaluated by collecting the catalyst, which was used foranother degradation experiment. The details about the reuseexperiment are described in the Supporting Information.

Analysis of BPA and Its Intermediates. BPA in the reactionsolution was analyzed by high performance liquid chro-matograph and the analysis of the intermediates of BPAdegradation was performed on a liquid chromatographcoupled with mass spectrometer system (LC-MS). Furtherdetails about the instrumental conditions are available inthe Supporting Information. Total organic carbon (TOC) wasmeasured with a Shimadzu TOC-V CPH analyzer. Theremoval ratio (R) of BPA (or TOC) was determined as follows:

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

Results and discussionCatalyst Characterization. Figure 1 shows the XRD patternsof the catalysts prepared at different pH conditions. Thediffraction peaks of those catalysts prepared at pH 4, 7, 9, 11are consistent with those of russellite Bi2WO6 [JCPDS No.39-0256] (its standard XRD pattern is shown at the bottomof Figure 1). However, as pH increased to 13, the peaks shiftedsignificantly and matched exactly those of Bi3.84W0.16O6.24

[JCPDS No. 43-0447] (its standard XRD pattern is shown onthe top of Figure 1). These suggest that the catalyst preparedat pH 13 has different chemical compositions from thecatalysts prepared at other pH values. Therefore, the catalystprepared at pH 13 will not be discussed further in this study.Apparently, pH played an important role in the formationof bismuth tungsten oxide. pH in the precursor suspensionsmay affect the solubility of WO4

2- and [Bi2O2]2+ and finallylead to the formation of different phases of bismuth tungstenoxide (33). The peaks of Bi2WO6 became narrower and their

intensities increased as pH increased from 4 to 11, indicatingthe 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 thestandard value of 0.20. This suggests that these crystals havespecial anisotropic growth along the (200) or (020) direction.These results could be attributed to their unique sheet-shapedmorphologies.

Morphologies of the samples were characterized byFESEM (Figure S2) and TEM (Figure S3). Their morphologieswere strongly dependent on the pH of the hydrothermalreaction solution. When the pH was in the range of 4-7,Bi2WO6 was present as a mixture of sphere- and sheet-shapedcrystal forms. The crystallite size was about 50 nm. As pHincreased to 9-11, irregular sheet-shaped morphologypredominated and the crystal size increased to 100-200 nm,which is in agreement with the XRD results. The FESEMimages show a highly rough surface of the catalysts. Selectedarea electron diffraction (SAED) (Figure S3-e) demonstratesthat the catalyst prepared at pH 11 was single crystal.

BET gas sorptometry measurements were conducted toexamine the porous nature of the catalyst prepared at pH 11(the catalyst prepared at pH 11 displayed the highestphotocatalytic activity, which will be discussed later). Figure2 shows the N2 adsorption/desorption isotherm and the pore-size distribution (inset) of the catalyst. The isotherm isidentified as type IV, which is characteristic of mesoporousmaterials (34, 35). The pore-size distribution obtained fromthe isotherms indicates a number of pores less than 10 nmin the sample. These pores presumably arise from the spacesbetween the sheets of the product. The sharp peak at around2 nm suggests that the nanopores are distributed very evenly.The BET specific surface area (SBET) of the sample wascalculated from N2 isotherms at -196.68 °C, and was foundto be as much as 16.82 m2 g-1. The single-point total volumeof pores at P/P0) 0.9673 was 0.0329 cm-3 g-1. The relativelylarge surface area and total pore volume support the factthat the catalyst has a nanoporous structure. The SBETs ofother catalysts prepared at pH 4, 7, 9 were 37.45, 34.84, 18.68m2 g-1 respectively. The surface area decreased with the pHincreasing from 4 to 11, which agrees the result that theparticle size increased with pH in the range of 4-11.

It is well-known that the electronic structure feature ofa semiconductor affects its optical absorption and migrationof the light induced electrons (e-1) and holes (h+), and thendetermines its photocatalytic activity. Figure S4 shows theUV-vis DRS of the catalysts prepared at different pH values.All of them demonstrated high photoabsorption capacity in

FIGURE 1. XRD patterns of the prepared catalysts.

R ) (1 - C/C0) × 100% (1)

FIGURE 2. Nitrogen adsorption-desorption isotherms and thecorresponding pore size distribution curve calculated fromadsorption branch of the nitrogen isotherm (inside) of Bi2WO6prepared at pH 11.

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the range of UV light to visible light around 450 nm, suggestingtheir potential photocatalytic activity under visible light. Thecolor of the catalysts was yellow, which is in accordance withthe absorption band edge at 450 nm. An intensive absorptionband with a steep edge was observed, indicating that thevisible light absorption was induced by the intrinsic bandgap transition instead of the transition from impurity levels(36).

The XPS spectrum of Bi2WO6 prepared at pH 11 is shownin Figure S5. Its Bi/W ratio is close to 2:1. Characteristicbinding energy values of 159.1 eV and 164.3 for Bi 4f7/2 andBi 4f5/2 reveal a trivalent oxidation state for bismuth. Thebinding energy at 37.5 and 35.4 eV for W 4f5/2 and W 4f7/2 canbe assigned to a W6+ oxidation state (37). The O element maybe fitted into two kinds of chemical states: crystal latticeoxygen and adsorbed oxygen (38).

Degradation Efficiency of the Catalysts Prepared atDifferent pH. The degradation efficiency of BPA by Bi2WO6

prepared at different pH is illustrated in Figure 3(a). Thephotodegradation efficiency increased as the pH of thehydrothermal reaction solution increased from 4 to 11.The catalyst prepared at pH 11 displayed the highestdegradation efficiency. This suggests that Bi2WO6 preparedat basic conditions displayed higher photocatalytic activitythan those prepared at acidic or neutral conditions. Manyfactors such as surface area and crystallinity may affect theactivity of photocatalysts. A large surface area favors thesorption of substrates to the catalyst surface and leads tofaster reaction. However, the SBETs of the Bi2WO6 catalystsdecreased with pH of the hydrothermal reaction solutionsin the range of 4-11. On the other hand, the crystallinityincreased a lot from pH 4 to 11. It is reported that a highdegree of crystallinity (or few surface and bulk defects) helpsto reduce e--h+ recombination (39, 40) and stimulates thephotocatalytic reaction. So the crystallinity may be the mainfactor in controlling the BPA degradation by Bi2WO6. Herein,Bi2WO6 prepared at pH 11 was selected as the optimumcatalyst and it will be discussed in detail in the followingsections.

Previous studies show that a suitable conformation ofpores 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 catalystfavor the penetration of light waves and bring about the BPAmolecules in solution deep inside the photocatalyst, whichresults in promoted photocatalytic activity (17).

Figure S6 shows that the photocatalytic degradation ofBPA by Bi2WO6 (pH 11) followed a pseudo first-order kineticmodel (43):

Where kapp represents the apparent degradation rateconstant, which was determined by plotting ln(C/C0) versusreaction time t.

Figure 3(b) illustrates the photocatalytic degradation effectof Bi2WO6 (pH 11) as compared with P25 TiO2 (SBET ≈ 50 m2

g-1). The adsorption of BPA by Bi2WO6 was negligible in 60min. BPA could be degraded without any catalyst undersimulated solar light irradiation, but the reaction was veryslow and only 19.6% of BPA could be removed in 60 minirradiation. The removal efficiency was enhanced to 28.3%when 1.0 g L-1 P25 TiO2 was added. However, it was enhancedto 95.6% when the same amount of Bi2WO6 (pH 11) wasadded as catalyst. Bi2WO6 (pH 11) displayed much higheractivity than P25 TiO2 under simulated solar light irradiation.The potential to reuse Bi2WO6 was investigated and the resultis shown in Figure S7. It can be seen that the catalyst stillkept high photocatalytic activity after reuse for 2 times. Dueto the mass loss during the filtering and transferringprocesses, the recovered amount of Bi2WO6 decreased frominitial 0.2 g in the first run to 0.1806 g in the second run and0.1618 g in the third run. As a result, the removal efficiencydecreased slightly.

Effect of Catalyst/Substrate Molar Ratio on the Degra-dation. Influence of the catalyst/substrate molar ratio wasstudied with BPA concentration set at 10, 20, and 40 ppm(Figure 3(c)), and the corresponding catalyst amount set at0.25-2.0, 0.5-4.0, and 1.0-8.0 g L-1, respectively. At all threelevels of BPA, kapp of the reaction increased with the catalyst/substrate molar ratio in the range of 8.16-49. When the molar

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 thephotodegradation BPA by Bi2WO6 (pH 11). For (a, b and d), the initial BPA concentration was 20 ppm and the catalyst amount was1.0 g L-1.

-ln( CC0

) ) kappt (2)

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ratio further increased, the kapp still increased at BPA levelof 10 ppm; but leveled off at 20 ppm and decreased slightlyat 40 ppm. Given the amount of BPA in the reaction solutionwas constant, more catalyst would provide more e--h+. Asa result, the catalytic activity increased. However, when theamount of catalyst increased continuously, the generationof e--h+ might be affected due to the increase of lightscattering and decrease of light penetration (44).

Effect of Initial pH of BPA Solution. In heterogeneousmedia, the active sites on the surface of most semiconductorsare dependent on the concentration of hydrogen ion (H+) orhydroxide ion (OH-) in aqueous solution. As a result, pH ofthe reaction solution may affect the adsorption property andcatalytic activity of the photocatalyst. The effect of the initialpH on the photodegradation efficiency of BPA in water isshown in Figure 3(d). Obviously, the degradation rateconstant kapp increased as the pH increased from 3 to 10,while it shows a decreasing trend in the range of 10-12. Itwas reported that Bi2WO6 is unstable in acidic solution, andit could be transformed to H2WO4 and Bi2O3 (45), leading topoor photocatalytic activity of Bi2WO6 at low pH. The redoxpotential of TiO2 was reported to vary in different pHconditions (46). The redox potential of Bi2WO6 could also beaffected 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 ofBPA is 9.59-10.2 (47). Therefore, the degradation ratedecreased due to electrostatic repulsion between the pro-duced bisphenolate anions and the negatively charged surfaceof Bi2WO6.

TOC Removal and Photocatalytic Degradation Mech-anism of BPA by Bi2WO6. Complete mineralization of organiccompounds is of great significance in the treatment of organicpollutants in water. To access the advantage of Bi2WO6 tocompletely destruct organic molecules in water solution, TOCwas monitored during the entire reaction process. 86.6% ofTOC was removed from the reaction system (the initialconcentration of BPA was 20 ppm, pH was 10) after 60 minreaction with Bi2WO6 under simulated solar irradiation while99.1% was removed in 120 min (Figure 4(a)). However, aslong as 20 h was necessary when TiO2 was used to photo-catalytically degrade BPA (initial concentration of 40 ppm)to CO2 under UV irradiation (23). Only 79% TOC waseliminated in ultrasound/UV/Fe(II) system for 120 min (48).Photocatalytic degradation of BPA by Bi2WO6 displayed anoutstanding advantage in mineralization BPA to CO2 andH2O.

The intermediates formed in the photocatalytic degrada-tion process were monitored using LC-MS analysis. Exceptfor the peak of BPA at 227, only one byproduct at m/z 133was observed. The intermediate at m/z 133 eluted earlierthan BPA, indicating that it is more polar. The species at m/z133 was indentified as 4-isopropenylphenol in previousstudies (17, 18, 49). The peak areas of m/z 133 and m/z 227varied with the irradiation time and the result is shown inFigure 4(b). The photocatalytic degradation of BPA by TiO2

was investigated in many previous studies. Ohko et al detectedintermediates at m/z 133, 135, 173, and 207 (23). Guo et aldetected 5 main intermediates at m/z 133, 185, 199, 245, and259. The detection of different number of intermediates couldbe due to the different LC-MS operation conditions. But weused exactly the same operation conditions of LC-MS as Guoet al (17). This suggests that other intermediates were notformed or their concentrations were very low during thephotocatalytic degradation by Bi2WO6 catalyst under simu-lated solar light irradiation.

Guo et al (17) investigated the degradation mechanismsof BPA by mesoporous TiO2 nanoparticles under UV irradia-tion. They suggested that both hydroxyl radicals (HO · ) and

photogenerated holes were responsible for the heterogeneousBPA degradation. The hydroxyl radicals may attack thedifferent carbon atoms of BPA molecules and resulted indifferent intermediates. They proposed BPA was mainlydegraded through demethylation and hydroxylation, pro-ducing hydroxylated and multihydroxylated intermediates,which were also observed by other researchers (23). Theyalso suggested a minor pathway: BPA was oxidized directlyby 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. Manyreports on the photocatalytic degradation of organic com-pounds in aqueous solutions have suggested the importantrole of HO · (17, 50, 51). For Bi2WO6, the standard oxidation

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

FIGURE 5. Suggested degradation pathway of BPA by Bi2WO6.

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potential (1.59 eV) (52) of photogenerated hole is lower thanthe redox potential of HO ·/OH- (1.99 eV) (53), implying thatthe photogenerated hole on the surface of Bi2WO6 could notreact with OH-/H2O to form HO · . This may explain that nohydroxylated products were observed in the reaction solution.The observation of only m/z 133 indicates that the degradationof BPA by Bi2WO6 could be mainly due to the direct oxidationby the photogenerated holes.

Based on the results above, the degradation of BPA byBi2WO6 experienced the most direct and simple pathway(Figure 5). First of all, Bi2WO6 catalyst received solar simulatedirradiation and generated e--h+. These e--h+ pairs reactedwith BPA absorbed on the catalyst surface or in the nanopores.Photogenerated holes could oxidize BPA directly to form theimmediate at m/z 133. Then, it was degraded to simpleorganic acids, which were further transferred to CO2 andH2O.

AcknowledgmentsThe authors gratefully acknowledge the financial support ofMinistry of Education (Grant 708020); Tianjin MunicipalScience and Technology Commission (08ZCGHHZ01000,07JCZDJC01900), Ministry of Science and Technology(2008ZX08526-003, 2009DFA91910), New Century Talentprogram, and China-US Center for Environmental Reme-diation and Sustainable Development.

Supporting Information AvailableDetailed description of the materials and experimentalmethods; the FESEM images and TEM images; UV-vis DRSof the catalysts; the XPS spectrum of Bi2WO6 prepared at pH11; the photocatalytic degradation kinetics of BPA by Bi2WO6

prepared at pH 11; the result of reuse evaluation of the catalystprepared at pH 11. This information is available free of chargevia the Internet at http://pubs.acs.org/.

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