Mixed α-Fe2O3/Bi2WO6 oxides for photoassisted hetero-Fenton degradation of Methyl Orange and Phenol

Mixed α-Fe2O3/Bi2WO6oxides for Photoassisted Hetero-Fenton Degradation of Methyl Orange and Phenol C. Jaramillo-Páez1*, J.A. Navío1, M.C. Hidalgo1, Asmae Bouziani2,Mohammed EL AZZOUZI2 1Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Sevilla, Spain 2Department of Chemistry, Faculty of Sciences Rabat, University Mohammed V 4 Avenue IBN BATTOUTA B.P.1014 RPRabat, Morocco. Abstract Mixed oxides, α-Fe2O3/Bi2WO6, were prepared using a mechanical mixing procedure by adding to the Bi2WO6previously obtained by hydrothermal method the corresponding amount of a prepared α-Fe2O3, the latter obtainedby thermal decomposition of Fe(NO3).9H2O. The physicochemical surface, structural, morphological characteristics and optical properties of the samples, single and mixed, were determined by BET, XRD, FE-SEM, XPS and UV-Visible diffuse reflectance spectroscopy. UV–vis diffuse reflectance spectra showed that incorporating a 5%wt. of α-Fe2O3to the corresponding amount of Bi2WO6 sample broadened the visible light absorption of Bi2WO6as expected. The photocatalytic activity, of single and mixed catalysts, to degrade a selected dye such as Methyl Orange (MO) as well as the transparent substrate Phenol (Ph)were studied, in aqueous medium (pH ≈ 5.5)under UV and sun-like illumination conditions in the absence and presence of H2O2. In the present study the use ofaα-Fe2O3-Bi2WO6/H2O2systemdemonstrate much higher photocatalytic efficiency to degrade both MO and Phthan pristineBi2WO6or αFe2O3, single or mixed. Using the system α-Fe2O3-Bi2WO6/H2O2, around 85% ofMO was degraded in 60 min under sun-like illumination whereas 100% was degraded in 60 min under UV-illumination. However, just around 30% of Ph was degraded in 120 min in the α-Fe2O3-Bi2WO6/H2O2system under sun-like illumination whereas around a 95% was degraded in 90 min under UVillumination. Under UV-illumination, the generation of hydroxyl radicals is favorable; whereas under sun-like illumination, only the small fraction of the UV can produces the •OH.Under illumination, the H2O2could react with photoinduced electrons from the photocatalysts leading to the production of hydroxyl radicals (•OH). Keywords:Bismuth Tungstate; Iron oxide; Photocatalysis; Photo-Fenton; Phenol; Dyes; Hydroxyl radicals. * Corresponding author. Tel.: +34955420998 E-mail address: nav[email protected](J.A. Navío). 1. Introduction The increase of industrial activities has become a serious problem that leads to the augmentation ofpollution inair, water and soil. To facethis problem, the scientific community works to find new methods to undo thecontamination.During the past few decades, a variety of practical strategies have been implemented to develop viable wastewater treatment technologies[1–6]. Those technologies are very appealing alternatives for the degradation of organic pollutants because they permit a partial or complete mineralization of pollutants. It is based on the production of the very reactive and nonselective entities (particularly the hydroxylradicals•OH) having a higher oxidizing capacity than traditional oxidants (O2, Cl2, ClO2, H2O2, O3…)[7–9]. Photocatalytic degradation of organic pollutants to purity wastewater from industries and household has received extensive attention in recent years. In particular, heterogeneous photocatalysis shows promising potential in depthoxidation of pollutants to non-toxic inorganic molecules at ambient temperature[10]. Dyes are molecules commonly found in real effluents from textile and other industrial wastewaters[11,12]. Dyes are major organic pollutants, which can cause severe environmental disruption and health damages[13]. Similarly, phenol is one of the most abundant pollutants in industrial wastewaters and its toxicity, carcinogenicity and persistence, makesthiscompound dangerous for life at rather low concentrations. In this sense, heterogeneous photocatalysis, among a group of available technologies known as advanced oxidation processes (AOPs), is an important alternative to remove a wide range of organic compounds, including phenolsand dyes, in polluted streams. Other technologies as Fenton and Photo-Fenton have been successfully used in depollution of water[14–18]. To meet the requirement of future environmental applications, in the field of AOPs, it is still essentialto not only further improve the photocatalytic activity by synthesizing new photocatalysts but also to explore new combined processes. The development of new photocatalysts is attracting vast interest. Among them the Bismuth tungstate (Bi2WO6) is a typical n-type direct band gap semiconductor with a band gap of 2.8 eV and has prospective applications for the degradation of organic pollutants under visible light illumination due to their low valence band and high chemical stability[19]. In the same context, iron oxide (α-Fe2O3, hematite) with a narrow band gap at 2.2 eV, absorbing the light up to 600 nm and collecting about 40% of the solar spectrum energy, is also another of the promising materials for photocatalytic applications[20]. H2O2is a distinctive oxidative agent and has been frequently used in practical water treatment, because it’s a very common source of very active hydroxyl radicals (•OH) by its decomposition after being illuminated with ultraviolet (UV) light.The use of H2O2in photochemical processes (UV/H2O2) and UV (H2O2/ Fe3+ (Photo-Fenton)) has been investigated[1,21–24], however, long periods of UV-illumination are required,thuspoor degree of mineralization isobtained, makingall these processes not perspectives as potential methods for wastewater purification. From another perspective, as an electron capture agent, H2O2can also react with photogenerated electrons, from a photocatalytic process, to producehydroxyl radicals (•OH) as established in eq. (1-2) in which (SC) is a general semiconductor photocatalyst[25]. (SC) + h  → (SC)(e-CB + h+VB) (light absorption) (eq. 1) (H2O2) +e-CB → •OH+ OH-(eq.2) (H2O) + h+VB→ •OH+ H+(eq.3) The formation, the adsorption and the degradation of H2O2on different samples (TiO2and ZnO) have been investigated to better understand its participation in the photocatalytic reactions [26]. In a typical UV/H2O2/TiO2system, the active radical formation can arises not only from the direct UV-photolysiswhich takes place throughahomolytic process (H2O2+ hv → 2 •OH) but also from the photocatalytic ones (equation 1 to 3). Thus, if the photocatalyst only absorbs in the UV region, then the hydroxyl radicals photogeneration, during the combined photochemical process UV/(SC)/H2O2, can take place not only by the homogeneous photolytic decomposition of H2O2, but also according to the processes indicated in equations 1 and 2. But if the photocatalyst absorbs in the visible region, the generation of hydroxyl radicals (OH.)would be expected to occur, under visible illumination,by the photogenerated electron capture of H2O2, since the photolytic decomposition of H2O2requires shorter wavelengths[1]. The generation of H2O2and hydroxyl radicals on Bi2WO6for phenol degradation under visible light has been reported [27]. From this work, Authors concluded that the observed organic degradation over the irradiated Bi2WO6in aerated aqueous solution is due to the production of •OH and H2O2. Both photocatalysts, Bi2WO6and α-Fe2O3, display potential catalytic activity to many chemicals organic degradation under conditions of sun-like illumination. However, due tothe high recombination rate of photogenerated charge carriers, the ability isthereof limited. Various strategies, such as heterostructured constructing are being developed[28–30]. The aim of this work is not focused on developing a αFe2O3/Bi2WO6heterostructure, but to use a physical mixture of both materials and explore their photochemical activity in the presence or absence of H2O2in the degradation of two selected substrates. Therefore, when H2O2is co-present with single or physically mixed Bi2WO6and α-Fe2O3, different activities could be displayed because, parallel to the mechanism of homogeneous degradation by hydroxyl radicals photogenerated by H2O2,the effect generated by the intrinsic photocatalytic activity of single or mixed systemsisalso expected to occur. Methyl orange (MO) and phenol (Ph) are used to imitate non biodegradable, toxic organic compounds. The photocatalytic activity, under UV or sun-like Illumination, of α-Fe2O3, Bi2WO6and mixed α-Fe2O3/Bi2WO6samples,to Methyl Orange and Phenol degradation, in the absence and presence of H2O2is reported. 2. Experimental details 2.1 Preparation of α-Fe2O3, Bi2WO6and mixed α-Fe2O3/Bi2WO6samples All the reagents used in this procedure were analytical grade without further purification. The detailed synthesis procedure for single α-Fe2O3, Bi2WO6and mixedα-Fe2O3/Bi2WO6sample was as follow: The iron oxide was prepared by drying iron(III) nitrate nonahydrate Fe(NO3).9H2O at 120°C for 2h then submitting the samples to a further calcinations treatment at 300°C for 2h. The Bi2WO6was prepared according to the method previously described [31]by dissolving 4.85 g of Bi (NO3)3.5H2O in 10 mL of glacial acetic acid, and 1.7 g of Na2WO4.2H2O in 90 mL of distilled water, then those two solutions were mixed forming a white suspension (pH≈2), which was kept under stirring for 1h. The white suspension was transferred into a Teflon recipient insideastainless steel autoclave.The hydrothermal treatment was done at 140°C for 20h, and then the precipitate was filtered, washed and dried overnight at 120°C. Finally the sample was submitted to a calcination treatment at 300°C for 4h. The α-Fe2O3/Bi2WO6mixed samples were obtained with a mechanical mixing in agate mortar, by adding the prepared α-Fe2O3to the corresponding amount of Bi2WO6for a 5wt. % of iron oxide in the mixture. This sample will hereafter be named as BW-Fe(5)-2 indicating a 5% of iron oxide and that Bi2WO6was prepared at pH=2. 2.2. Characterization of the photocatalysts BET surface areas (SBET) of all samples were evaluated by N2adsorption measurement with a Micromeritics ASAP 2010 instrument. Degasification of the samples was performed at 150 ºC for 30 min in He flow. Crystalline phase composition of the samples was estimated by X-ray diffraction (XRD). XRD patterns were obtained on a Siemens D-501 diffractometer with Ni filter and graphite monochromator using Cu Kα radiation. The morphology for all the samples was analyzed by field Scanning electron microscopy (FE-SEM) using a Hitachi S 4800 microscope. Light absorption properties of the samples were studied by UV–Vis spectroscopy. The Diffuse Reflectance UV–Vis Spectra (UV–Vis DRS) were recorded on a Varian spectrometer model Cary 100 equipped with an integrating sphere and using BaSO4as reference. Band-gaps values were calculated from the corresponding Kubelka–Munk functions, F(R∞), which are proportional to the absorption of radiation, by plotting (F(R∞)×hν)1/2 against hν. X-ray photoelectron spectroscopy (XPS) studies were carried out on a LeyboldHeraeus LHS-10 spectrometer, working with constant pass energy of 50 eV. The spectrometer main chamber, working at a pressure <2×10−9Torr, is equipped with an EA-200 MCD hemispherical electron analyzer with a dual Xray source working with Al Kα (hν=1486.6 eV) at 120 W and 30 mA. C1s signal (284.6 eV) was used as internal energy reference in all the experiments. Samples were outgassed in the pre-chamber of the instrument at 150 ºC up to a pressure <2×10−8Torr to remove chemisorbed water.All photoelectron spectra were analyzed using Casa-XPS software. 2.3. Photodegradation tests The photocatalytic activity of the catalysts prepared was tested in the photodiscoloration of a selected dye, Methyl Orange (MO), as well as on the photodegradation of Phenol (Ph) as a selected transparent, toxic substrate.Methyl Orange and Phenol (ReagentPlus >99%) were supplied by Sigma-Aldrich.Photocatalytic tests were carried out using a discontinuous batch system, this includes a 250 mL Pyrex reactor enveloped by an aluminum foil, filled with an aqueous suspension (100 mL) containing either the single substrates (concentrations: 20 ppm of MO or 50 ppm of phenol) or a mixture of both (10 ppm of MO/25 ppm phenol) and the photocatalyst (1g/L). On the experiments in which H2O2is co-present, either with substrates or with substrates and catalysts, before illumination, a certain amount (~ 3 mM) of H2O2 (wt. 30%) was added in the medium. The mixed solution was magnetically stirred in the dark for 20 min. Systems were illuminated through a UVtransparent Plexiglas® top window (threshold absorption at 250 nm) by an Osram Ultra-Vitalux lamp (300 W) with sun-like radiation spectrum and a main line in the UVA range at 365 nm. The intensity of the incident UV-Visible light on the solution was measured with a Delta OHM photo-radiometer HD2102.1, being ca. 110 W/m2whereas the intensity of the incident UV light on the solution was of ca. 90 W/m2. In order to favor the adsorption–desorption equilibrium between the catalysts and substrates, prior to irradiation the suspension was magnetically stirred for 20 min in thedark. Magnetic stirring and a constant oxygen flow of 20 L/h, as an oxidant, were used to produce a homogeneous suspension of the photocatalyst in the solution. A tank bubbler was used as a source of natural oxygen. All photocatalytic tests started at pH ca. 5.5 and the total reaction time was 120 min. During the Methyl Orange photoreaction, samples were collected at different times and in order to evaluate the dye discoloration rate, the concentration of Methyl Orange during the photodegradation reaction was analyzed by UV– Visible spectroscopy, considering the main peak of this dye in the visible range, located at 465 nm. For this analysis a UV–vis spectrometry with a Cary 100 (Varian) spectrometerwas used. Phenol concentrations were followed by HPLC technique (Agilent, 1200 Series) using anElipse XDB-C18 column (4.6 x 150 mm i.d., 5 μm; Agilent) at 40ºC. Mobile phase was water/methanol (65:35) at a flow rate of 0.8 ml/min. Samples of about 2 mL were removed periodically during the experiment and filtered (Millipore Millex 25 0.45 mm membrane filter) previous to HPLC measurements. Photolysis tests of substrates under illumination and in absence of catalyst were carried out. Under the experimental conditions used in this work, substrate photolysis was negligible. Reproducibility of the measurements was ensured by double testing of selected samples. Total organic carbon was followed also by means of a TOC analyzer (Shimadzu 5000). Mineralization degrees (%) were evaluated by the TOC values upon 2 h of illumination,for all the photo-assisted processes studied. 3. Results and discussion 3.1. Characterization Figure 1 shows X-ray diffraction patterns (XRD) of the prepared materials.The XRD of the as-prepared iron oxide sample showed the diffraction peaks corresponding to the standard α-Fe2O3(JCPDS no. 33-0664). The main peaks at 24.1, 33.1, 35.6, 49.5 and 54.1º was observed, which correspond to (012), (104), (110), (024) and (116) diffraction planes of hematite respectively. For the Bi2WO6sample all the diffraction peaks are in good consistent with the standard data of the pure russelite orthorhombic Bi2WO6phase (JCPDS no. 39-0256). catalysts, which obviously condition the capacity of surface adsorption for substrates of different chemical natures. In fact, according to the results reported in the literature [26], the adsorption capacity of H2O2seems to be linked to the number of OH groups present on the surface of solids. Regardless of this, it is evident that the Bi2WO6-H2O2system increases the degrees of mineralization of both substrates, with respect to those obtained when only the catalyst or H2O2are used. A proposed mechanism to explain the combined process of Bi2WO6and H2O2, would be the generation of hydroxyl radicals, either by capturing electrons by H2O2and the simultaneous oxidation of H2O by holes, as stated in the equations 1 to 3 in which the semiconductor (SC) would be Bi2WO3, as well as by the direct photolytic processes (H2O2+ hv → 2 OH.). 3.2.3 α-Fe2O3andα-Fe2O3-H2O2 Figure 6A shows the processes in the dark and under illumination, which occur for MO substrate in the presence of α-Fe2O3. As can be seen in the dark, a large adsorption of MO occurs at the surface of α-Fe2O3. A tentative explanation of the adsorption process in the dark would be the establishment of a weak Lewis acid-base interaction between the electron density of the chromophore group (-N=N-) and d-orbitals of Fe3+.After equilibration, the backlight, both in the UV and sun-like, generates a photo-desorption process of the MO.These photodesorption processes are accompanied by photocatalytic degradation processes of MO, since after 120 min under lighting conditions TOC values obtained indicatethat there has been a percentage of 27.4% of mineralization(Table 1).However, as the TOC measurement is performed on the liquid phase, it is also possible that MO is not completely desorbed and this apparent decrease on TOC could not be related to a mineralization. This fact is important since iron is the second most abundant metal on Earth, and the mineral hematite is most often formed in natural water. Thus, a natural photocatalytic degradation process with particles of α-Fe2O3suspended in water or sediments could have a beneficial impact to the water ecosystem contaminated by MO. In the degradation test of MO with α-Fe2O3, an initial adsorption and later desorption under illumination is observed (Figure 6A).This behavior observed for the MO in the presence of α-Fe2O3, is not observed when Phenol is used. Figure 6B shows the plot of the conversion percentages of Phenol, using the prepared catalysts α-Fe2O3.As with the use of Bi2WO6, the use of α-Fe2O3leads to conversion percentages which are negligible for both substrates, both under UV and sun-like illumination. These results indicate that the as prepared iron oxide sample presents, not only a low adsorption capability for Phenol but also a low photoactivity, although optical absorption results, by DRS (Fig.2), indicate a wide optical absorption both in the ultraviolet and in the visible, being in accordance with the narrow band gap at 2.2 eV. The poor photoactivity, tested for the prepared α-Fe2O3, similarly to that observed for Bi2WO6, could be associated with the high recombination of charge carriers in the prepared samples.It is interesting to note that although nosignificant degrees of conversion for phenol were observed, however TOC values indicate that there has been a certain degree of mineralization (25-35%) for phenol(Table 1). It is interesting to note that the simultaneous presence ofthe catalyst (α-Fe2O3) and H2O2, leads toa significant increase in conversion percentages for both substrates, both in the UV and the sun-like (Figures 6C and 6D) and that they are different from those obtained for the twosubstrates both withonly the catalyst (Figures 6A and 6B) and with H2O2alone (Figures 4A and 4B).These results are not surprising if they are explained in the context of a photoassisted hetero-Fenton process implemented by the α-Fe2O3/H2O2 system.Thus,regardless of whether the H2O2-photolytic process generating •OH radicals (equation 7), the α-Fe2O3/H2O2systemwould also be acting by generating more hydroxyl radicals via a simultaneous heterogeneous photoFenton mechanism involving Fe3+/Fe2+ pairs, as the following: H2O2+ hv → 2•OH(eq. 7) ˃Fe 3+ + e-→ ˃Fe2+ [O2] → ˃Fe 3+(eq. 8) ˃Fe2+ + H2O2+ H+→ ˃Fe3+ +•OH + H2O (eq. 9) ˃Fe2+ +•OH→ ˃Fe 3+ + OH-(eq. 10) ˃OH-+ h+→ •OH(eq. 11) In the presence of oxygen, re-oxidation of ˃Fe 2+ occurs at a high rate before detaching from the surface (photocorrosion).For this reason, the photocorrosion of iron oxide in photocatalytic reaction (under oxygen) is generally very low with this process being hindered[35–37]. 3.2.4 α-Fe2O3/Bi2WO6andα-Fe2O3/Bi2WO6-H2O2 In Fig. 7, we show the MO and Phenol conversion plots using the mixed αFe2O3/Bi2WO6photocatalysts both in the absence (Figures 7A and 7B) or in the presence of hydrogen peroxide (Figures 7C and 7D), under UV and sun-like irradiation. It is possible to observe that using the mixedα-Fe2O3/Bi2WO6 photocatalysts,MOremains almost unaffected in solutions under illumination. With BW-Fe(5)-2 under these conditions, there is a slight improvement of Phenol conversion, after 120 min of illumination (Fig.7B), compared with that obtained for MO (Figure 7A). However, the small conversion values observed are higher under conditions of UV-irradiation than that observed under sun-like illumination, being comparatively similar to the results shown in Figures 4A and 4B, as is expected, since the catalyst used, BW-Fe(5)-2, is a physical mixture of 95% Bi2WO6. Thus, we can conclude that the mechanical mixture of Bi2WO6 and α-Fe2O3, does not affect the intrinsic photocatalytic behavior observed for single catalysts under the same experimental conditions. Figures 7C an 7D shows the variation of the MO (Figure 7C) and Phenol (Figure 7D) conversion with time when BW-Fe(5)-2 is usedas aphotocatalystin the co-presence of H2O2, under UV or sun-like illumination. A different trend is observed for both substrates. Thus, for MO the simultaneous presence of the mixed catalysts BW-Fe(5)-2 and H2O2has a marked influence on the photoassisted discoloration process.This effect is more marked under conditions of UV-illumination for which almost 100%discolorationis achievedin30 min, while high conversion values are obtained (ca. 85% at 60 min) under sun-like irradiation.These conversion values turn out to be higher than those obtained for the same substrate (MO) in homogeneous phase with light-H2O2(Figure 4A), in the photocatalytic processes using single photocatalysts (Figures5A and 6A) and when the single photocatalysts are used with H2O2(Figures 5C and 6C).These results indicate a synergistic effect in the mixture of the photocatalystsBW-Fe(5)-2 when co-existing with H2O2under illumination, at least for the substrate used (MO), since this synergistic effect is not observed for Phenol (Figure 7D).The most striking result is that the synergistic effect occurs even in the visible for MO. Using this system BW-Fe(5)-2 + H2O2, apart from the relatively high values obtained for photo-bleaching of MO, under both UV and under sun-like illumination, relatively high values of mineralization degrees of MO are also obtained(see Table 1). This fact could be explained by assuming a contribution of photosensitization of a dye moleculesuchas MO, asit has alsobeen observed when using Rhodamine B [31]. As mentioned in the introduction, due to high recombination rate of charge carriers in the single oxides, α-Fe2O3and Bi2WO6, have developed strategies that lead to the separation of charge carriers, such as the development of heterostructural constructing of both catalysts [28–30]. In these systems, it has been achieved an improvement in the photocatalytic activity compared with that obtained with the singles catalysts. The α-Fe2O3acts as a hole-accepting semiconductor and photogenerated electrons are injected with high efficiency from the conduction band of α-Fe2O3to the conduction band of Bi2WO6. In our work we used a α-Fe2O3/ Bi2WO6composite by a mechanical mixing procedure and in order to achieve high performance, the extra H2O2was required. However, the goal is the same in both cases, that is, achieve efficient separation of the charge carriers. In our case, H2O2acts as an electron acceptor, thus generating •OH radicals while the H2O can act as holes acceptor, generating more •OH radicals. The advantage, of our method, can be found in the amount of hydroxyl radicals generated in the process, thereby increasing photo-assisted degradation of the substrates. Regardless of this, in our work we have assessed the photocatalytic activity with two different substrates, the MO and Phenol and not with the Rhodamine B, since we have evidence that the evaluation of photocatalytic activity with Rhodamine B generates results that are more spectacular, than when a transparent substrate, such as Phenol is used. There are other works, that achieve improved photo-Fenton mechanism by incorporating the α-Fe2O3to a graphene oxide (GO) [38] or to Kaolin [39]. In both cases, however, the improvement obtained could be attributed to the synergetic effects of the adsorptive power of GO or Kaolin and the hydroxyl radicals produced by heterogeneous photo-Fenton reactions. In any case, in these works, the evaluation of the activity is done also by using RhB. Among the mixed catalyst BW-Fe(5)-2,in the presence of H2O2, higher conversion values for MOwere displayed, both under UV and sun-like illumination. XPS analyses have been conducted on the prepared mixed oxides system BW-Fe(5)-2, just before and after the photo-assisted Methyl Orange degradation under visible illumination in the presence of hydrogen peroxide, by recovering the catalyst powder from the reaction system, after a prolonged time of illumination (Experiment reported in Figure 7C). Figure 8 shows the results of XPS analysis of the original sampleofαFe2O3/Bi2WO6, in which only O(1s), W(4f), Bi(4f) and Fe(2p) peaks were detected but no peaks for residual sodium weredetected. The overview spectrums of the mixed system demonstrate that Bi, W, O and Fe exist, further confirming that the sample was composed of Bi2WO6and Fe2O3. As shown in Figure 8A, the characteristic peak of O 1s around 530.1 eV could come from the overlapping contributions of several components, being the identification of the submerged peaks performed by Gaussian deconvolution and curve fitting: these peaks were located at 529.32 eV, 529.65 eV, 530.32 eV, 530.98 eV and 531.26 eV which corresponds to Fe-O, Bi-O, W-O lattice oxygen, chemisorbed water and •OHhydroxyl groups respectively [40,41]. The peaks located at 36.02 eV and 33.91 eV with a spin-orbital separation of 2.11 eV, as shown in Figure 8B, could be assigned to the +6 oxidation state of tungsten for the W 4f5/2 and W7/2respectively [40].The XPS spectrum of the Bi 4f region displayed in Figure 8C consisting oftwo characteristic peaks with binding energies of 159.04 eV and 164.31 eV correspond to the signals from doublets of Bi 4f7/2 and Bi 4f5/2 in the trivalent oxidation state, respectively for pure Bi2WO6. Figure 8D provides XPS peaks of Fe element, exhibiting two individual peaksinthe Fe 2p regionlocated at 710.42 eV and 723.75 eV, which can be assigned to Fe 2p3/2 and Fe 2p1/2 peaks in α-Fe2O3phase, respectively [28] confirming the existence of Fe2O3phase on the mixed system which was not detected by XRD technique. Besides two satellite peaks of Fe 2p located at 718.8 eV and 732.9 eV are clearly distinguishable. The satellite peaks were the result of the charge transfer screening attributed to the presence of Fe in 3+ oxidation state [42,43]. However, the splitted peaks of Bi 4f7/2 and 4f5/2attributed to Fe3+–O–Bi3+ linkage at lower energy values of 157.5 eV and 162.8 eV, respectively[44],donot appear, indicating that no interaction between α-Fe2O3and Bi2WO6existsin the mixed system, as expected by the preparation procedure. The used α-Fe2O3/Bi2WO6sample after photo-assisted discoloration process of Methyl Orange under visible illumination by using H2O2, was recovered and, once dried, was subjected to a XPS analysiswith theresultsbeingreported in Figure 9. It is interesting to note that, in the region of the O(1s)peak, two distinct peaks appear centered around 526.8 eV and 530.0 eV respectively. By a deconvolution analysis and fitting, several submerged peaks canbedistinguished. A clear peak centered at 526.58 eV can be attributed to a peroxide species stabilized in the system whichcannot be associated to Na2O or Na2O2[45]since sodium was not detected by XPS. No changes were observed in the oxidation states of W and Bi respectively. However, for this sample, Fe 2p photoelectron peaks appeared around 710.6 eV and 724.0 eV with satellite peaks. The peaks of Fe 2p1/2 and Fe 2p3/2 levels at 724.0 eV and 710.6 eV, respectively, separated 13.4 eV, verified the presence of Fe in 3+ oxidation state on the recovered α-Fe2O3/Bi2WO6catalysts. However, as shown in Figure 9D, a shoulder around 706.0 eV, not present on the original spectra (Figure 8D) is observed. This could be ascribed to the transformation of Fe(3+) to Fe(2+) after heterogeneous photo-Fenton reaction [46]. This finding, together with the presence of O(1s) peak associated to peroxide species, leadus to postulate the presence of iron(II) peroxide,Fe(O2),stabilized in the mixed system. The interaction of H2O2with iron oxide hasnotbeen extensivelystudied, and there are studies that suggest the formation of Fe(O2) by computer calculations[47,48]; a recent study [49]concluded that the dark Fenton process involving Fe(II) + H2O2consists of two regimes, a fast ferrous one that is triggered by the reaction of Fe2+ + H2O2and a slow ferric one that is dominated by the reduction of Fe(III).However, stabilization of peroxide species as η2Fe(II) -O22seems unlikely, due to the instability of the peroxide species. Other proposals could be made, based on results published in the literature [50].Pignatello et al. [51], evidenced the formation of an additional oxidant in the photoassisted Fenton reaction. The results suggest the participation of a highvalentoxoiron complex (ferryl) in addition to •OHin organic compound oxidations.They evidenced that hydrogen peroxide forms a complex with iron, (Fe3+-OOH)2+[K= 1.15 x10-2], that absorbs in the visible region and could be the precursor of the proposed ferryl complex[51]. If the formation of (Fe3+-OOH)2+species is assumedthen an increasedphotoconversion process is likely to occur, in both the UV and visible, as seen in the results presented in Figures 7C and 7D. 3.2.5 Mixtures of Methyl Orange and Phenol Finally, we have studied the simultaneous degradation of MO and Phenol both ina mixed solution, using the mixed oxide photocatalyst BW-Fe(5)-2, in the presence of H2O2undersun-like illumination conditions. Figure 10 reports the variation in the concentration of MO and Phenol in the mixture MO/Phenol with time under sun-like illumination, using the BW-Fe(5)-2 catalyst in the presence of H2O2. As noted in Fig.10, it is possible to observe that in mixed solutions of both substrates, there is an increase in Phenol degradation influenced by the simultaneous presence of MO, while the profile of the conversion plot of MO is practically the same as that obtained for the single substrate under the same experimental conditions (Fig.7C). These results indicate that regardless of the synergistic effect observed in the physical mixture of the two materials studied, BW-Fe(5)-2 in the co-presence of H2O2, a marked influence of the simultaneous presence of MO over the Phenol degradation, is also observed. It is observed that this mixed system, BW-Fe(5)-2, is capable of completely makingMOdisappearin the mixture, after 120 min under sun-like illumination, leading toa residual amount of TOC at the final period, indicating a percentage of 50% of mineralization. From one point of view, having a photosensitizing molecule, suchas MO, has proved to have some effect in the photodegradation of a non-photosensitizing one, like Phenol, since by using these conditions, for single phenol a conversion value of ca. 30% was observed whereas a value of ca. 60% is reached in the co-presence of MO. Conclusions Mixed oxides, α-Fe2O3/Bi2WO6, were prepared using a mechanical mixing procedure by addingthe corresponding amount of a prepared α-Fe2O3to the Bi2WO6previously obtained by hydrothermal method,the former obtainedby thermal decomposition of Fe(NO3).9H2O. Despite exhibiting potential optical absorption capacity in the UV-vis region, however, the twosingle catalysts showed poor photocatalytic activity, both in the UV and in the visible, possibly due to high recombination rate of carrier’s photogenerated charges. Theprepared oxide α-Fe2O3shows a remarkable dark adsorption capability to MO, however under illumination conditions it displayed a photodesorption process which is accompanied by a simultaneous photo- Figure 1. X-ray diffraction patterns (XRD) of pristine α-Fe2O3, Bi2WO6and mixed BWFe(5)-2 samples Figure 2.Diffused reflectance spectra (DRS) of pristine α-Fe2O3, Bi2WO6and mixed BW-Fe(5)-2 samples. Figure 3.SEM images of pristine Bi2WO6(A and B), pristine α-Fe2O3(C), and mixed BW-Fe(5)-2 samples (D). Figure 4. Conversion plots for photochemical discoloration of Methyl Orange (A) and photochemical disappearance of Phenol (B), with only the presence of H2O2under UV or sun-like illumination. A B Figure 5. Conversion plots for photochemical discoloration of Methyl Orange and Phenol disappearance, under UV or sun-like illumination: with only the presence of Bi2WO6photocatalyst (A and B) or with the co-presence of both, Bi2WO6and H2O2(C and D). A B C D Figure 6. Conversion plots for photochemical discoloration of Methyl Orange and Phenol disappearance, under UV or sun-like illumination: with only the presence of αFe2O3photocatalyst (A and B) or with the co-presence of both, α-Fe2O3and H2O2(C and D). A B C D Figure 7.Conversion plots for photochemical discoloration of Methyl Orange and Phenol disappearance, under UV or sun-like illumination: with only the presence of mixed BW-Fe(5)-2 photocatalysts (A and B) or with the co-presence of BW-Fe(5)-2 photocatalysts and H2O2(C and D). A B C D Figure 8. XPS surface spectra of the as-prepared mixed BW-Fe(5)-2 photocatalysts. Figure 9. XPS surface spectra of the recoveredBW-Fe(5)-2 sample after being used on the photo-assisted discoloration process of Methyl Orange under visible illumination by using H2O2. Figure 10. Methyl Orange and Phenol conversion plots in mixed solution with BW-Fe(5)-2 under sun-like illumination, in the presence of H2O2