1-s2.0-S0021979714010376-main

Highly active magnetic bismuth tungstate/magnetite composite under visible light irradiation in the presence of hydrogenperoxideGuoqiang Shan,Yu Fu,Xiaolong Chu,Chun Chang,Lingyan Zhu ⇑Key Laboratory of Pollution Processes and Environmental Criteria,Ministry of Education,Tianjin Key Laboratory of Environmental Remediation and Pollution Control,Collegeof Environmental Science a r t i c l e i n f o Article history:Received 18October 2014Accepted 20December 2014Available online 31December 2014Keywords:Bi 2WO 6Fe 3O 4Visible light Magnetic Rhodamine BHydroxyl radicalsa b s t r a c tBi 2WO 6/Fe 3O 4composites were synthesized using hydrothermal method and their photocatalytic activity to degrade rhodamine B (RhB)under visible light irradiation assisted with H 2O 2and underlying mecha-nisms were investigated.The composites were 3D flower-like microspheres constructed by 2D Bi 2WO 6nanosheets loaded with spherical Fe 3O 4nanoparticles.The composited photocatalysts could be easily harvested from the reaction solution by an external magnetic field.In the presence of H 2O 2,a large amount of hydroxyl radicals (ÅOHs)were produced by H 2O 2reacting with photogenerated electrons.Fe 3O 4not only promoted the separation of hole–electron pairs but also acted as a Fenton-like reagent,expediting the production of ÅOH.Thus,the composites in the presence of H 2O 2displayed much higher photocatalytic efficiency to degrade RhB than pure Bi 2WO 6.98%of RhB (initial concentration 10mg/L)was degraded in 2h visible irradiation and 60%of total organic carbon (TOC)was removed in 3h by 0.5g/L Bi 2WO 6/Fe 3O 4(5:1).The composite displayed high photodegradation efficiency at pH 3–9.Our study suggests that a visible light driven and highly active magnetic photocatalyst-Fenton coupling oxi-dation system may have potential application in water treatment and environmental cleaning.Ó2015Elsevier Inc.All rights reserved.1.IntroductionIn recent years,semiconductor photocatalytic process has found its wide application in water treatment as a low-cost,environmen-tal friendly and sustainable treatment technology [1].TiO 2is one of the most widely applied photocatalysts under irradiation of ultraviolet light.However,TiO 2can only absorb ultraviolet wave-length,which constitutes only $3–5%of solar light [2].In order to make better use of sunlight in nature,efforts have been done to develop catalysts which are active to visible light.Bismuth phot-ocatalysts,including Bi 2WO 6,are active to visible light and display potential photocatalytic efficiency to many contaminants and azo dyes under visible light irradiation [3–7].As a visible-light-driven photocatalyst,Bi 2WO 6displays potential catalytic capacity to many organic chemicals under irradiation of visible light and it is primarily synthesized by hydrothermal or solvothermal method [8].In order to improve catalytic efficiency,Bi 2WO 6catalysts with/10.1016/j.jcis.2014.12.0680021-9797/Ó2015Elsevier Inc.All rights reserved.⇑Corresponding author at:The College of Environmental Science and Engineer-ing,NankaiUniversity,Weijin Road 94,Tianjin 300071,PR China.Fax:+862223508807.E-mail address:zhuly@ (L.Zhu).different morphology and crystal structures were synthesized [8,9].Another effective technique is to dope Bi2WO6with metal, or metal oxide,which might trap the photogenerated electrons and restrain the recombination of hole–electron pair[10–13].However,one of the shortcomings which limit the real applica-tion of these catalysts in water treatment is that it is very difficult to separate the catalysts from the reaction solution at the end of reaction without the assistance of centrifugation orfiltration. Recently,many studies have been conducted to prepare photocat-alysts with magnetic materials,which could be separated and recy-cled conveniently by applying an external magneticfield.Many magnetic iron materials,such as CoFe2O4[14],NiFe2O4[15], Fe3O4[16],were coupled with different photocatalysts to prepare composites with magnetic property.In many cases,doping with magnetic materials would reduce photocatalytic activities of the original catalysts due to decreasing activity sites available for reac-tion.Strategies are needed to prepare magnetic composites which retain the good photoactivity of the original catalyst and obtain good magnetic property.Magnetite(Fe3O4)is an ideal magnetic material and widely used in catalysis because of its low cost and easy preparation [17].Besides its ultrahigh-density magnetic property,Fe3O4has been used as heterogeneous Fenton-like catalyst for catalytic oxi-dation of organic compounds[18,19].H2O2is a typical oxidative agent and has been widely used in practical water treatment [20].As an electron capture agent,H2O2can react with photogen-erated electrons to produce hydroxyl radicals(ÅOHs)[4,21].H2O2þeÀCB!ÅOHþOHÀð1ÞThus,when H2O2is co-present with Bi2WO6/Fe3O4composite,it might interact with the composite and affect the photocatalytic capacity.It was reported that Bi2WO6and its composites such as Co3O4/Bi2WO6,C/Fe–Bi2WO6,Bi2WO6@carbon/Fe3O4exhibited higher photodegradation efficiency with the assistance of H2O2 [21–24].Xu et al.and Liu et al.synthesized Bi2WO6/Fe3O4compos-ites and achieved high degradation efficiency for RhB[25,26].How-ever,the mechanisms involved in the complex system were not fully investigated.The current study aimed to dope Bi2WO6with Fe3O4to prepare magnetic Bi2WO6/Fe3O4composites and investigate the combined degradation mechanisms of Bi2WO6/Fe3O4in the presence of H2O2. Rhodamine B(RhB)was used to imitate nonbiodegradable,toxic organic compounds with multiple benzene rings.The photocata-lytic activity of the magnetic composites to RhB under visible light (k>400nm)in the presence of H2O2was evaluated.The respective roles of Bi2WO6,Fe3O4and H2O2and the reaction mechanism were investigated extensively.The impacts of H2O2concentration and solution pH on the photocatalytic performance were studied.The degradation pathway was also investigated.The proposed strategy was useful for designing a visible light driven and magnetic photo-catalyst-Fenton coupling oxidation system for wastewater treatment.2.Materials and experiment2.1.Materials and reagentsRhodamine B(RhB)(98%)was purchased from J&K Scientific Ltd.(Beijing,China).FeCl3Á6H2O(analytical reagent,AR)was bought from Guangfu Technology Co.Ltd.(Tianjiin,China).Sodiumacetate(AR),NaOH(AR)and HNO3(65%)were purchased from Jiangtian Chemical Technology Co.Ltd.(Tianjin,China).Bi(NO3)3-Á5H2O,Na2WO4Á4H2O(AR)and KI(Guaranteed reagent)was pur-chased from Yingda Chemical Technology Co.Ltd.(Tianjin, China).Isopropanol(AR)was provided by Fengchuan Chemical Technology Co.Ltd.(Tianjin,China).AgNO3(AR)was obtained from Tairuier Chemical Co.Ltd.(Shanghai,China).Glycol and absolute ethyl alcohol(AR)were purchased from Concord Technology Co. Ltd.(Tianjin,China).Terephthalic acid(TPA)(99%)was purchased from ACROS(New Jersey,USA).124G.Shan et al./Journal of Colloid and Interface Science444(2015)123–1312.2.Preparation of nanosphere Fe3O4by solvothermal method1.35g of FeCl3was dissolved in75mL glycol,and then3.6g sodium acetate was added in the solution.After sodium acetate was dissolved completely,the solution was ultrasonicated for 30min.The mixture was transferred to a50mL autoclave,heated at200°C for16h.Then the black solid powder was separated from the solution by centrifugation,washed with deionized water and absolute ethanol consecutively and dried under120°C for5h[27].2.3.Preparation of Bi2WO6/Fe3O4and Bi2WO6A certain amount of Fe3O4was dispersed in20mL of glycol by magnetic stirring for10min and then ultrasonicated for30min. Ten mL of Bi(NO3)3solution at0.2mol/L(dissolved in1mol/L HNO3aqueous solution)was added to the dispersed Fe3O4-glycol solution.After magnetic stirring for20min,10mL of Na2WO4solu-tion at0.1mol/L(dissolved in1mol/L NaOH aqueous solution)was added dropwisely into the mixed solution.It was then stirred vig-orously for10min and ultrasonicated for30min.Xu et al.pre-pared Bi2WO6and Bi2WO6/Fe3O4(pH=1,3,5,7)and found that those prepared at pH1did not show desirable photodegradation capacity for RhB[26].Thus,to investigate the enhancement effect of H2O2,the pH of the precursor suspension was adjusted to1 using HCl.The suspension was then transferred to a50mL of Tef-80°.FT-IR spectrum was obtained by MAGNA-560Nicolet.The morphology of the photocatalyst was characterized by a Hitachi-S3500N scanning electron microscope(SEM).Brunauer–Emmett–Teller(BET)surface area measurements were performed on a Quantachrome AUTOSORB sorption analyzer.The nitrogen adsorp-tion and desorption isotherms were measured at393K after degassing the samples.Diffuse reflection spectra(DRS)were mea-sured with a Hitachi U-3010spectrometer at the range of300–700nm.Their magnetic properties(M–H curve)were measured at room temperature on a VSM9600magnetometer.Fluorescence spectra for solid and liquid samples were measured by Hitachi F-4500fluorescence spectrophotometer.2.5.Photocatalytic experimentsPhotocatalytic activity of the catalysts was evaluated by photo-catalytic degradation of RhB(10mg/L).A350W Xe lamp with 400nm lightfilter was used as light source.The loading amount of the catalysts was0.5g/L.Before adding the catalysts,the solu-tion of RhB was adjusted to pH7.0using HCl or NaOH.Before illu-mination,a certain amount of H2O2(wt.30%)was added in the suspension of the catalyst and RhB.The mixed solution was mag-netically stirred in dark for1h to ensure the establishment of an adsorption–desorption equilibrium between the catalyst and RhB.The dispersion was then exposed to visible light irradiationimages of the prepared catalysts.(A)pure Bi2WO6;(B)Fe3O4;(C)the front of Bi2WO6/Fe3O4(5:1);(D)the side of BiG.Shan et al./Journal of Colloid and Interface Science444(2015)123–13112540mL of mixed solution of NaOH(1mol/L)and absolute ethanol (v:v=1:1)was added in the vial to extract RhB remaining on the solids.The mixture was stirred for4h.The RhB concentration in the supernatant was measured by Cary UV–Vis spectrophotometer at553nm wavelength.The removal efficiency was calculated as follows:Removal rate¼ðC dþC aÞ=C iÂ100%ð2ÞC d,C a and C i are the concentrations of RhB degraded,RhB residue adsorbed on the catalysts and the initial RhB in solution, respectively.2.7.Determination ofÅOH in the reaction systemThe amount ofÅOH in the reaction system was determined by fluorescence spectrophotometer analysis method with tere-phthalic acid(TPA)capture[28,29].Four mL of TPA solution (0.2mol/L,dissolved in0.5mol/L NaOH solution)as trapping agent was added in the reaction solution.The producedÅOHs during irra-diation may react with TPA following Eq.(3)to form2-hydroxy terephthalic acid(HTPA),which was detected byfluorescence spectrophotometer(excitation wavelength at315nm,emission wavelength at about425nm).ð3Þ3.Results and discussion3.1.Materials characterizationFig.1A shows the XRD spectra of Fe3O4and Bi2WO6/Fe3O4com-posites.For Fe3O4,the diffraction peaks correspond to the standard Fe3O4(JCPDS No.65-3107),which is cubic with cell parameters as a=b=c=8.391Å,a=b=c=90°.Diffraction peaks at about31.2°, 35.5°,43.2°,57.0°and62.6°correspond to the(220)(311)(400) (511)(440)crystal planes of Fe3O4respectively[30].In the com-posites with Bi2WO6,the diffraction peaks at26.5°,28.2°,47.2°and 55.8°are attributed to the(131)(200)(202)(133)planes of Bi2-WO6(JCPDS No.39-0256)respectively[31].As the content of Fe3O4 increases,the peak intensities at35.5°,43.8°,62.5°increase consis-tently,indicating that Fe3O4was doped in Bi2WO6successfully.The FT-IR spectra of Fe3O4,Bi2WO6and Bi2WO6/Fe3O4are shown in Fig.1B.For Fe3O4,the peak at584cmÀ1corresponds to the stretching vibration of Fe–O–Fe in Fe3O4[32].In the spectrum of Bi2WO6/Fe3O4composite,the peaks at730and628cmÀ1attri-bute to Bi–O,W–O stretching and W–O–W bridge stretching modes;the peak at572cmÀ1belongs to Fe–O–Fe.As compared with pure Fe3O4,the peaks shift slightly,implying that doping with Bi2WO6may affect the Fe–O–Fe bond.Other peaks at3427and 1620cmÀ1are ascribed to the stretching and bending vibrations of free or bonded hydroxyl groups,which may be due to the mois-ture introduced in the process of mixing with KBr.The UV–vis DRS spectra are shown in Fig.1C.Pure Bi2WO6was pale yellow powder and displayed absorbing capacity in visible light region with an absorption edge at about450nm[25,33].After composited with Fe3O4,the light absorption of the Bi2WO6/Fe3O4 composites increased and extended in a wider range of200–700nm.This could be due to the dark red color of the composites after doping with Fe3O4[25].The SEM images of Bi2WO6,Fe3O4and5:1Bi2WO6/Fe3O4are shown in Fig.2.The nano-sheets of Bi2WO6with the crystallite size of about4–10nm in thickness and200–300nm in length are assembled to form particles withflower-like shape in a2l m diam-eter.The Fe3O4nanosphere particles(diameter at around200nm) were formed under the solvothermal conditions using ethylene glycol as solvent.The Fe3O4nano-particles were aggregated together due to their magnetic property.The3D morphologies of Bi2WO6/Fe3O4(5:1)composite in front and by the side indicate that nano-flakes of Bi2WO6were clustered asflower microspheres with diameter about2l m.It is clear that Fe3O4nanoparticles evenly distributed on the surface or inserted among the nanoflakes of Bi2WO6[26].The Brunauer–Emmett–Teller(BET)specific surface areas and porous volumes of Bi2WO6,Fe3O4and Bi2WO6/Fe3O4composites were determined using nitrogen adsorption and desorption iso-therms.The N2adsorption/desorption isotherms and porous struc-tures are shown in Fig.3A.The S BET of pure Bi2WO6was about 38.43cm2/g and that of Fe3O4was about27.94cm2/g.Doping Fe3O4in Bi2WO6significantly enhanced the S BET of the composites, which was53.80,58.30,47.89and55.02cm2/g for10:1,5:1,3:1 and2:1composites,respectively.The pore size of pure Bi2WO620004000distribution(inlet)of purecurves of(a)Fe3O4,(b)Bi2WOseparation of Bi2WO6/Fe3O4(5:1)of an external magnet.126G.Shan et al./Journal of Colloid and Interface Science444(2015)123–131and Bi 2WO 6/Fe 3O 4(5:1,2:1)was around 2nm and their single point total volume was 0.1018,0.1288,0.1162cm 3/g,respectively.Thus,the Bi 2WO 6/Fe 3O 4(5:1)composite had a slightly higher total volume.The magnetic behavior of pure Fe 3O 4and Bi 2WO 6/Fe 3O 4(2:1,5:1)was investigated and the results are shown in Fig.3B.The M –H hysteresis loop indicates that Fe 3O 4and Bi 2WO 6/Fe 3O 4composites possessed permanent magnetic properties.The satura-tion magnetization value (Ms )of them was 70.50,21.79,8.614emu/g respectively,which decreased significantly with decrease in the amount of Fe 3O 4in the composites.This was desir-able since the magnetic property is contributed by Fe 3O 4.Bi 2WO 6/Fe 3O 4(5:1)particles could be easily separated from the colloidal suspension by applying an external magnetic field in 5min,as shown in the lower right panel of Fig.3B.3.2.Photocatalytic performanceThe photocatalytic efficiency of the catalysts and Fe 3O 4in the absence of H 2O 2to degrade RhB is shown in Fig.4A.H 2O 2alone (10mM)displayed negligible degradation efficiency (only 4%)for RhB under visible light.Pure Fe 3O 4neither showed degradation efficiency to RhB.Pure Bi 2WO 6and Bi 2WO 6/Fe 3O 4composites,dis-played relatively high removal efficiency of 66–83%.However,most RhB removed from the solution was not degraded but strongly adsorbed on surface of catalysts.The adsorbed dye mole-attraction between RhB molecules and the catalyst surface (this will be discussed in details later).However,once the same amount of H 2O 2(10mM)was added to the reaction system,the maximum absorbance peak disappeared after 90min and no other peaks were observed in the range of 250–800nm (Fig.4B),suggesting the aromatic and azo groups of RhB were destroyed and decomposed into small molecules.This indicates that H 2O 2promoted the photocatalytic efficiency of both Bi 2WO 6and Bi 2WO 6/Fe 3O 4(Fig.4C).More than 95%RhB (10mg/L)was degraded under visible light irradiation within 2h irradiation.Less than 1–5%of RhB remained on surface of the composites (0.5g/L)at the end of irradiation.Xu et al.reported that only 67%of RhB (4.79mg/L)was removed by Bi 2WO 6/Fe 3O 4(1g/L)after 2h irradiation [26].Liu et al.reported that the removal rate of RhB (4.79mg/L)in 2h irradiation was 80%by Bi 2WO 6/Fe 3O 4com-posite (1g/L)[25].In the present study,around 39.1%,60.0%of total organic carbon (TOC)was removed by pure Bi 2WO 6and Bi 2-WO 6/Fe 3O 4(5:1)composite after 3h irradiation in the presence of H 2O 2(Fig.4D).Fu et al.reported that the complete degradation of RhB (4.79mg/L)required 20h and only 5%of TOC was removed after 3h irradiation by pure Bi 2WO 6(0.5g/L)[3].It is obvious that addition of H 2O 2greatly enhanced the photocatalytic capacity for RhB.Among the catalysts,the 5:1Bi 2WO 6/Fe 3O 4composite dis-played the highest catalytic efficiency.Thus,the 5:1composite was selected for further research.G.Shan et al./Journal of Colloid and Interface Science 444(2015)123–131127However,its underlying mechanisms remain unclear.It is well known that H2O2can accept photogenerated electron to produce ÅOH,which is a strong oxidative species to degrade organic chem-icals[21].To ascertain the impacts of H2O2on the photocatalytic capacity of Bi2WO6and Bi2WO6/Fe3O4,the amount ofÅOH in the reaction solutions was determined.Fig.5shows thefluorescence intensities of HTPA in the reaction solutions with H2O2alone, Fe3O4/H2O2,Bi2WO6/H2O2and Bi2WO6/Fe3O4/H2O2during irradia-tion.For H2O2alone,the very weakfluorescence signal suggest that negligibleÅOHs were produced under visible light irradiation (Fig.5A).This might explain the very low degradation efficiency of H2O2under simulated solar light irradiation.In the system with Fe3O4/H2O2(Fig.5B),the strong signal indicates that a large amount ofÅOHs were produced,through Fenton-like reaction as shown in Eq.(4).Surface Fe3O4(…Fe II)could decompose H2O2to produceÅOHs[18]:B Fe IIþH2O2!B Fe IIIþÅOHþOHÀð4ÞHowever,only10%of RhB was degraded in the system of Fe3O4/ H2O2(Fig.4C).This might be owing to the low adsorption capacity of Fe3O4for RhB molecules.In the system of Fe3O4/H2O2,the solu-tion pH was7.5,at which the surface of Fe3O4(p H pzc=6.8)and RhB molecules(p K a=3.0)were negatively charged.Thus,there is a strong repulsive interaction between them,leading to very weak adsorption of RhB on the surface of Fe3O4.It is well recognized that photocatalytic degradation happens on or very near to the surface of the catalysts[35].As a result,RhB could not be degraded effi-ciently in the Fe3O4/H2O2system.As illustrated in Fig.5C and D,thefluorescence signal intensities in the Bi2WO6/H2O2and Bi2WO6/Fe3O4/H2O2systems increased rapidly during irradiation of120min.In the presence of H2O2, the photogenerated electrons of Bi2WO6might react with H2O2 quickly to generateÅOHs as Eq.(1).The photodegradation of RhB by Bi2WO6/H2O2was much higher than pure Bi2WO6,implying thatÅOHs played an important role in the photodegradation under visible light irradiation.The amount ofÅOH produced in the Bi2WO6/Fe3O4/H2O2system was twice of that in the Bi2WO6/H2O2system during120min irra-diation.This might be attributed to the heterogeneous Fenton-like reaction of surface Fe3O4with H2O2just as Eq.(4)and the coupling effect occurring at the interfaces of Fe3O4and Bi2WO6,which led to effective separation of electron–holes pairs of Bi2WO6.To ascertain this latter assumption,the photoluminescence(PL)spectroscopy of Bi2WO6/Fe3O4and Bi2WO6at room temperature was measured and the results are illustrated in Fig.S1.Upon photoexicitation at 325nm,pure Bi2WO6exhibited PL band at475–525nm.However, the Bi2WO6/Fe3O4(5:1)composite exhibited much weaker PL sig-nal.The lower PL intensity of the composite suggests that doping with Fe3O4led to more separation of photo-induced hole–electron pairs[36].As a result,the production ofÅOHs by the Bi2WO6/Fe3O4 composites was enhanced compared to equal amount of pure Bi2-WO6in the presence of H2O2.This also explained the relatively higher photocatalytic degradation efficiency of the magnetic composite.To further understand the reaction mechanism of the catalysts in the presence of H2O2,isopropanol(IPA)(v:v=1:20)asÅOH scav-enger,KI(5mmol/L)as a hole scavenger and AgNO3(5mmol/L)as a photo-electron scavenger was added in the reaction systems, respectively.As shown in Fig.6A,the degradation of RhB was inhibited by the three reagents under visible light irradiation,sug-gesting thatÅOH,photoelectron and hole are related to the photo-degradation in the complex system with H2O2.It was reported that photogenerated hole oxidation might take part in the photodegra-dation process with Bi2WO6[3,37,38].However,Fig.4A shows RhB was hardly decomposed by Bi2WO6or Bi2WO6/Fe3O4without the assistance of H2O2.It is expected that the impact of hole oxidation128G.Shan et al./Journal of Colloid and Interface Science444(2015)123–131may not be notable due to the strong adsorption of RhB on the sur-face of the catalysts.As shown in Fig.6B,theÅOH generated in the system of Bi2WO6without H2O2under visible irradiation could be negligible while a large amount ofÅOHs were generated as H2O2 was added(Fig.6B).Thus,theÅOH generation owing to the addition of H2O2might be the principal pathway,particularly in the initial step of the formation of the observed N-deethylated intermediates of RhB(Scheme S1)[39].Once the adsorbed RhB was degraded by ÅOHs,the hole oxidation might then take part in the degradation of RhB.It was found that Bi2WO6/Fe3O4/H2O2displayed higher photo-catalytic efficiency to degrade RhB than Bi2WO6/H2O2.It also dis-played higher TOC removal(60%)than Bi2WO6/H2O2(39%).These suggest that Bi2WO6/Fe3O4(5:1)composite in the presence of H2O2displays stronger oxidative capacity.It may be explained by the moreÅOHs generated in the Bi2WO6/Fe3O4/H2O2system com-pared to that in the Bi2WO6/H2O2system during120min ually,the photocatalytic efficiency of the photocatalyts doped with magnetic materials decreases as compared to the ori-ginal catalysts with the same mass.For example,the degradation efficiencies of TiO2/CoFe2O4[40]and TiO2/ZnFe2O6[41]decreased significantly compared to pure TiO2due to reduced active sites available for photoactivity.However,in the present study,the pho-tocatalytic activity of the Bi2WO6/Fe3O4composites was not reduced but enhanced as compared with equal amount of pure Bi2-WO6.This suggests that doping with Fe3O4not only introduces magnetic property to the composites but also assists to enhance the photocatalytic activity.3.4.Effects of H2O2dosage and initial pHFig.7A shows the effect of H2O2with a dosage of0.1–40mM on photocatalysis of RhB by Bi2WO6/Fe3O4(5:1)composite.The deg-radation efficiency increased with increasing in the concentration of H2O2(0.1–20mM).As more H2O2was added in the solution, moreÅOHs were produced,and higher photocatalytic efficiency was resulted.However,when the concentration of H2O2was higher than20mM,the degradation efficiency decreased.This might be due to that the excess H2O2reacted withÅOHs in the solu-tion as following Eqs.(5)and(6)[42].Thus,the amount ofÅOH decreased,leading to lower photocatalytic efficiency.ÅOHþH2O2þOHÀ!H2OþHOÅ2ð5ÞÅOHþHOÅ2!H2OþO2ð6ÞSolution pH might be one of the most important factors affect-ing the adsorption and photocatalytic degradation of RhB [38,43,44].The impact of solution pH on the adsorption of Bi2WO6/ Fe3O4(5:1)is shown in Fig.7B.The adsorption capacity of the cat-alyst decreased as the initial pH increased in the range3–11.The p H pzc of Bi2WO6and Bi2WO6/Fe3O4(5:1)was determined to be 4.3(Fig.7C).RhB(p K a=3[45])molecules are negatively charged in the studied pH range.The surface of the catalyst is positively charged when the pH is lower than4.3while is negatively charged as the pH is higher than4.3.At pH3,there is a strong electrostatic attraction between RhB molecules and the catalyst surface,leading to higher adsorption affinity.With the increase of the solution pH, the interaction becomes gradually from attraction to repulsive interaction.As a result,the adsorption decreased gradually.More than50%of RhB was adsorbed on the catalyst surface at pH3–9. While only20%of RhB was adsorbed at pH11.During2h irradia-tion,RhB was degraded completely under pH3–9,as shown in Fig.7D.However,only13%of RhB was degraded at pH11.Accord-ing to Eq.(1),alkaline condition does not favor the generation of ÅOHs,which may explain the low degradation efficiency of the composite at pH11[43].The solution pH was determined before and after adding the catalysts in the system.Due to the low p H pzc (4.3)of Bi2WO6/Fe3O4(5:1),the solution pH decreased to7and10 after adding the catalyst in the reaction solution with initial pH9 and11respectively.The amount ofÅOH produced in the reaction solutions at initial pH9and11was determined.As shown in Fig.S2,the amount ofÅOH in the solution at pH9increased signif-icantly during the irradiation,whileÅOH was not detected in the solution at initial pH11,supporting that basic conditions do not favor the formation ofÅOH and the degradation efficiency decreases significantly.This suitability in a wide pH range(3–9)is vital in the practical wastewater treatment.In traditional Fenton process,the pH of wastewater inflow requires an acidic condition(about pH 3)for an ideal efficiency[46,47].The highly effective Bi2WO6/ Fe3O4/H2O2system can overcome this shortcoming and be suitable for neutral or weak alkaline conditions of wastewater.3.5.Photodegradation pathwayTo further identify the major intermediates,the reaction solu-tion was analyzed by LC/MS and GC/MS(the analytical methods are shown in SI).Intermediates at m/z443,415,387,359and 331were observed in LC/MS spectrum and they were identified as RhB,N,N,N0,-triethylated Rh,N,N0,-diethylated Rh or N,N,-diethy-lated Rh,N-ethylated Rh and Rh respectively[48,49].The reactionG.Shan et al./Journal of Colloid and Interface Science444(2015)123–131129kinetics of these species is shown in Fig.S3.As the concentration of RhB in the solution decreased with the irradiation time,the species of N,N,N0,-triethylated Rh,N,N0,-diethylated Rh or N,N,-diethylated Rh,N-ethylated Rh and Rh were produced gradually due to the consecutive N-de-ethylation.In the GC–MS results,some small organic molecules were identified in the reaction solution at irradi-ation of60and120min,and they are listed in Tables S1and S2.At irradiation of60min,some aromatic organic molecules were iden-tified,while smaller molecules without benzene ring were identi-fied in the solution after irradiation for120min.The results suggest that RhB and the N-de-ethylated intermediates were degraded mainly by ring opening in thefirst60min irradiation. Then,the produced aromatic intermediates were further degraded to small organic acids,amines and alcohols.The proposed photo-degradation pathway is shown in Scheme S1.4.ConclusionsDoping Bi2WO6with Fe3O4not only introduced strongly mag-netic property to the composites,but also enhanced the photocat-alytic capability to degrade RhB in the presence of H2O2.The generation ofÅOH due to the addition of H2O2is vital for RhB deg-radation.Fe3O4not only promoted the separation of hole–electron pairs but also acted as a Fenton-like reagent,expediting the pro-duction ofÅOHs.As a result,the composites displayed highly effi-cient photodegradation and mineralization capacities to RhB in the presence of H2O2under simulated solar light irradiation.At the end of reaction,the photocatalysts could be harvested from reaction solution easily by applying an external magneticfield.The Bi2WO6/Fe3O4in the presence of a moderate H2O2can be suit-able for wide pH conditions of wastewater.AcknowledgmentsThe authors gratefully acknowledge thefinancial support of Ministry of Science and Technology(2014CB932001, 2012ZX07529-003),Tianjin Municipal Science and Technology Commission(13JCZDJC35900),and the Ministry of Education inno-vation team(IRT13024).Appendix A.Supplementary materialFor the photoluminescence spectra of the catalysts;thefluores-cence spectra of HTPA in Bi2WO6/Fe3O4/H2O2(pH9and11);the identification of the degradation intermediates of RhB by GC/MS; the proposed photodegradation pathway of RhB,see SI.Supple-mentary data associated with this article can be found,in the online version,at /10.1016/j.jcis.2014.12.068. 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