Environ Sci Pollut Res DOI 10.1007/s11356-015-5985-2

RESEARCH ARTICLE

Preparation of magnetic photocatalyst nanohybrid decorated by polyoxometalate for the degradation of a pharmaceutical pollutant under solar light Tahereh Rohani Bastami 1 & Ali Ahmadpour 1

Received: 5 June 2015 / Accepted: 14 December 2015 # Springer-Verlag Berlin Heidelberg 2016

Abstract Magnetic polyoxometalate nanohybrid was prepared by the surface modification of γ-Fe2O3/SrCO3 nanoparticles with PW12O340− polyoxometalate (POM) anions. The results of Fourier transform infrared (FTIR) and energydispersive X-ray (EDX) confirm the presence of POM on the surface of γ-Fe2O3/SrCO3 nanoparticles. TEM results revealed the ellipsoid-like structure of nanohybrid which was 23 nm in length and 6 nm in width. The activity of the photocatalyst was investigated by the photocatalytic degradation of ibuprofen (IBP) in an aqueous solution under solar light. It was found that in comparison with the γ-Fe2O3/ SrCO3, the degradation of IBP after 2-h exposure to the solar light irradiation was significantly higher for POM-γ-Fe2O3/ SrCO3 nanohybrids. The degradation of IBP was enhanced by the addition of H2O2 to the air saturated solution, while the addition of NaHCO3 and isopropanol restricted the degradation process. In the presence of H 2 O 2 , the Fenton photocatalyst degradation under solar light irradiation led to relatively complete degradation of IBP. Furthermore, the photocatalytic activity and magnetization properties of this magnetic photocatalyst nanohybrid provide a promising solution for the degradation of water pollutants and photocatalyst recovery.

Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-015-5985-2) contains supplementary material, which is available to authorized users. * Ali Ahmadpour [email protected]

1

Department of Chemical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

Keywords Magnetic nanohybrids . Photocatalysis . Surface modification . Polyoxometalate . Solar light degradation

Introduction In recent years, hybrid nanomaterials have been the subject of growing attention due to synergistic properties of initial compounds and their applications in functional materials (MasteriFarahani et al. 2012; Ayala et al. 2004; Jin et al. 2009). So far, two main types of photocatalysis have been studied: semiconductor metal oxide and polyoxometalates (POMs) (Shi et al. 2006; Herrmann 1999). POM-based photocatalyst is identical to the semiconductor in terms of properties, which is mainly due to analogous electric characteristic (band gap transition for semiconductors and HOMO-LUMO transition for POM) (Dolbecq et al. 2012; Hiskia et al. 2001). For a semiconductor photocatalyst, the photogeneration of hole (h+)–electron (e−) pair is pivotal to its photoactivity, while for polyoxometalate catalysts, the formation of the O → W (oxygen to tungsten) charge transfer plays an important role (Li et al. 2004). Polyoxometalates have been widely studied for their high activity under UV irradiation in homogeneous and heterogeneous processes (Dolbecq et al. 2012). Also, they possess both acidic and redox catalytic properties (Masteri-Farahani et al. 2012). However, there is room for further enhancements of their photocatalytic activities under visible light irradiation (λ > 400 nm). Despite their advantages, catalyst-based POMs are characterized by some drawbacks such as low surface area (1– 5 m2 g−1) and solubility in water and polar solvents, which hinder their separations (Masteri-Farahani et al. 2012; Karimi and Mahjoub 2011). The POM immobilization on the heterogeneous support represents a big step toward the expansion of catalytic applications of POM-based catalyst (Mizuno et al. 2006;

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Kholdeeva 2008). Polyoxometalates were immobilized onto various substrates to prepare supported POM photocatalysts such as mesoporous silica (Karimi and Mahjoub 2011; Kozhevnikov et al. 1995)), titanium dioxide (Li et al. 2004), activated carbon (Izumi and Urabe 1981), and zeolite (Ozer and Ferry 2002). However, the separation difficulty inherent in the photocatalyst is unavoidable in these approaches. In this regard, magnetic photocatalyst nanomaterials offer promising materials as they permit convenient recovery by the external magnetic field with high degradation efficiency (Shi et al. 2006). In this regard, some studies have focused their attention on the synthesis and catalytic application of POM-based magnetic photocatalysts (Masteri-Farahani et al. 2012; Shi et al. 2006; Karimi and Mahjoub 2011). An ideal magnetic photocatalyst should have the following properties: high photocatalytic activity and high magnetization for convenient magnetic separation (Shi et al. 2006). In this paper, for the first time, POM-γ-Fe2O3/SrCO3 nanohybrids were prepared and used for the degradation of pharmaceutical ibuprofen (IBP) in an aqueous solution under solar light. Ibuprofen (IBP) (2-(3-(2-methylpropyl) phenyl] propanoic acid) belongs to nonsteroidal anti-inflammatory drugs that are widely used for the treatment of musculature pain, inflammatory disorders, fever, migraine, and toothaches (Madhavan et al. 2010). It has been reported that several kilotons of IBP are produced annually in the world, part of them rejected to the effluents and excreted by patients (Zheng et al. 2011; Buser et al. 1999). The concentration of IBP in the environment is estimated to be between 10 ng L−1 and 169 μg L−1 (Santos et al. 2007). IBP is not easily biodegradable in the municipal wastewater treatment plants. Thus, the removal or degradation of this pollutant by different techniques has been the subject of many studies (Esplugas et al. 2007; Gonzalez et al. 2007). The degradation techniques include advanced oxidation processes such as gamma irradiation (Zheng et al. 2011), pulse radiolysis (Illés et al. 2013), ultrasonic irradiation (Me´ndez-Arriagad et al. 2008)), solar photo-Fenton (Me´ndez-Arriag et al. 2010), photocatalysis and sonophotocatalysis (Madhavan et al. 2010; Choina et al. 2013), UV/H2O2 processes (Kwong et al. 2015), sono-enzymatic degradation (Chakma and Moholkar 2015), ozonation (Quero-Pastor et al. 2014), and photocatalytic ozonation (Rey et al. 2014). To the best of the author’s knowledge, there are few studies on the degradation of IBP by POM-based nanomagnetic photocatalyst under solar light. The goal of this work is to investigate the photocatalytic activity of this novel nanohybrid as well as the role of surface modification with POM in the presence of solar photocatalyst degradation of IBP as a model of nonsteroidal anti-inflammatory drug (NSAID) in water. Further, the influence of reaction conditions like contact time, pH of the solution, photo-Fenton catalytic activity, and different additives was examined.

Materials and methods Chemicals and materials All chemicals were of analytical grade and applied without further purification. Iron (III) chloride hexahydrate (FeCl3· 6H2O), strontium nitrate (Sr (NO3)2), sodium acetate anhydrous (NaAC), ethylene glycol anhydrous (EG), 1,6diaminohexan, hydrogen peroxide (30 %), isopropanol, sodium hydrogen carbonate (NaHCO3), and tungstophosphoric acid hydrate (H3PW12O40; Keggin type of POM) were purchased from Merck Company. Ibuprofen (IBP), C13H18O2, was obtained from Shandong Xinhua Pharmaceutical Company in China. Milli-Q water with a resistivity of at least 18.2 MΩ·cm−1 was used. Preparation of γ-Fe2O3/SrCO3 The γ-Fe2O3/SrCO3 nanoparticles were prepared as follows: 1.35 g (5 mmol) of FeCl3·6H2O and 1.06 g (5 mmol) of (Sr (NO3)2) were dissolved in the ethylene glycol (EG) (40 mL) to obtain a yellow clear solution. Then, 3.6 g sodium acetate was added under vigorous stirring for 10 min at a temperature of 80 °C. In the next step, 4 mL of 1,6-diaminohexan was added to the mixture. Finally, the mixture was transferred to a 60 mL Teflon-lined autoclave and maintained at 180 °C for 12 h before being cooled down to the room temperature. The products were magnetically separated and washed several times with deionized water and then dried at 50 °C overnight. Preparation of POM-γ-Fe2O3/SrCO3 nanohybrid First, γ-Fe2O3/SrCO3 nanoparticles (0.3 g) were dispersed in 50 mL of deionized water in a 100-mL Erlenmeyer flask. Then, 0.6 g of H3PW12O40 was added to the solution and stirred for 24 h to obtain POM-γ-Fe2O3/SrCO3 nanohybrid. The products, magnetically separated, were washed with two deionized water twice and then dried at 50 °C overnight. Characterization The composition and crystal phase of the samples were characterized by X-ray diffraction (XRD) on a X’Pert Pro MPD X-ray diffractometer equipped with Cu irradiation (λ = 1.5406 Å). The size and morphology of the samples were characterized using TEM method (CM30 Philips, Netherlands, 150 kV). The sample was dispersed in ethanol and dropped on the copper grid before loading to the instrument. To determine the composition of γ-Fe2O3/SrCO3 nanoparticles, the inductivity coupled plasma (ICP) was used (OPTIMA 7300DV). The Fourier transform infrared (FTIR) of the samples were recorded using Thermo Nicolet, Avatar 370 FTIR in the range of 400– 4000 cm−1. Energy-dispersive X-ray (EDX) spectra were

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collected on a Leo-1450-VP scanning electron microscope (SEM). The magnetization curve of synthesized magnetic photocatalyst nanohybrid was investigated using vibrating sample magnetometer (VSM, Leckeshore model). The optical properties of the magnetic photocatalyst nanohybrid were determined by UV-vis spectroscopy (Cecil 8000 series). Point of zero charge measurement The pHpzc of the POM-γ-Fe2O3/SrCO3 nanohybrid was determined by salt addition method (Noh and Schwarz 1989; Rohani Bastami and Entezari 2012). Also, pHpzc was measured using 0.01 M NaCl aqueous solutions at pH 2–10. The pH values were fixed with HCl and NaOH aqueous solutions. Each prepared solution (50 mL) was contacted with 0.05 g of the sample and stirred for 48 h. The supernatant was decanted and its pH was measured. The pHpzc value was determined by plotting the pH of the filtered solution (pHf) as a function of initial pH value (pHi) solutions. Photocatalytic activity test Photocatalytic activities of POM-γ-Fe2O3/SrCO3 nanohybrid were evaluated based on the degradation of ibuprofen in an aqueous solution (chemical properties shown in Table 1) under solar light, and the results were compared to the dark condition. In each experiment, 50 mg POM-γ-Fe 2 O 3 /SrCO 3 nanohybrid was dispersed into 50 mL of IBP solution (10 mg L−1) and then exposed to solar light (clear sky) with constant stirring at a temperature between 27 and 32 °C (GPS coordinates, N = 36° 18′ 41.6″, E = 59° 31′ 54.2″; Scheme 1). Some other experiments were also conducted under identical conditions, with a 500-W tungsten lamp and a LED blue lamp as visible irradiators (distance from liquid surface = 15 cm), and in the dark condition to allow comparison of the results. Then, the samples were separated by an external magnetic field and centrifuged at 12,000 rpm for 10 min to complete the Table 1

Desorption study To determine the amount of IBP adsorbed onto the surface of POM-γ-Fe2O3/SrCO3 nanohybrids, the desorption experiment was carried out. In this experiment, 0.05 g nanohybrids, which were pre-adsorbed with 50 mL of 10 mg L−1 IBP at different pHs during 120 min of photocatalytic treatment under solar light and dark conditions, were dispersed in ethanol and stirred continuously for 2 h. The concentration of desorbed IBP was analyzed using a UV-vis spectrophotometer at λ = 224 nm.

Results and discussion Characterization of POM-γ-Fe2O3/SrCO3 nanohybrids The process of the functionalization of γ-Fe2O3/SrCO3 nanoparticles with H3PW12O40 was as follows: In the first step, the γ-Fe2O3/SrCO3 was prepared with amine groups. Then, the reaction of amine groups with H3PW12O40 polyoxometalate yielded the POM-γ-Fe2O3/SrCO3 nanohybrids. In the next step, the protonation of amine groups gave positively charged ammonium cations which bounded electrostatically to the heteropolyanions (Masteri-Farahani et al. 2012). Figure 1 depicts the XRD patterns of the prepared γ-Fe2O3/ SrCO3 nanoparticles. The XRD peaks could be identified by orthorhombic SrCO3 (ICDD-01-074-1491) and orthorhombic

Chemical properties of IBP

Pharmaceutical name Ibuprofen

separation of photocatalyst nanohybrids from aqueous solution. The concentration of IBP solutions was analyzed using Cecil 8000 series UV-vis spectrophotometer at λ = 224 nm. In the pH study, the initial pH was adjusted by HCl and NaOH solutions. The pH was measured using a pH-meter (HANNA instruments, Inc.). Further, an experiment was carried out with γ-Fe2O3/SrCO3 under solar light with the same conditions to determine the influence of POM surface modification on the degradation of IBP.

Chemical structure

MW (g mol−1)

pKa

206.29

4.9

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Scheme 1 Experimental setup for the photocatalytic degradation of IBP on POM-γ-Fe2O3/SrCO3 under solar light and dark condition (volume of container = 500 mL)

γ-Fe2O3 phases (ICDD-00-052-1449). Some diffraction peaks of γ-Fe2O3/SrCO3 nanoparticles assigned to the SrCO3 at 2θ values of 25.4°, 44.2°, and 47.3° correspond to the crystal planes of (111), (203), and (213), respectively. Moreover, peaks assigned to the γ-Fe2O3 at 2θ values of 36.3°, 50°, and 63.1° correspond to the (131), (115), and (243), respectively. It is assumed that the SrCO3 phase is the result of interaction of strontium species with the carbon dioxide generated in the oxidation process of EG as carbon sources (Ji et al. 2012). However, the XRD peaks of γ-Fe2O3 and Fe3O4 are close to each other. The subsequent characterization confirms the presence of γ-Fe2O3 phase (see chemical analysis of γ-Fe2O3/ SrCO3). As shown in Fig. 1, the background in the XRD pattern is due to poor crystalinity, fine size, and large number of broken bonds at the surface. The above result confirms nanosized and distorted surface morphology (Layek et al. 2010).

Fig. 1 XRD pattern of γ-Fe2O3/SrCO3 nanoparticles (square and asterisk indicate the phase of γ-Fe2O3 and SrCO3, respectively)

According to the chemical analysis of γ-Fe2O3/SrCO3, the amounts of Fe and Sr in the sample are 37.126 and 23.453 %, respectively. The Fe/Sr ratio is approximately 1.6:1, which confirms the proposed structure as XRD result. To confirm the surface modification of γ-Fe2O3/SrCO3 nanoparticle with POM, the FTIR spectra of the prepared γFe2O3/SrCO3, POM-γ-Fe2O3/SrCO3, and Keggin-type of heteropolyanions (POM) were obtained, as shown in Fig. 2. The presence of magnetic nanoparticles is indicated by the peaks around 540 and 555 cm−1 for the POM-γ-Fe2O3/ SrCO3 and γ-Fe2O3/SrCO3 samples, respectively. These peaks are related to the M–O band. According to the results, the red shifts (increased wavelength) for the POM-γ-Fe2O3/ SrCO3 samples confirm the interaction of nanoparticle surface with POM. The four peaks in the wave number of 800– 1100 cm −1 are due to Keggin-type heteropolyanions (Masteri-Farahani et al. 2012). As shown in the FTIR spectrum of POM, the peak around 980 cm −1 is due to the stretching vibration of W=O, which is shifted to 952 cm−1 in the presence of POM-γ-Fe2O3/SrCO3. Furthermore, the peaks at 893 and 800 cm−1 in the case of POM are due to corner and edge sharing of W–O–W vibration, which shifted to 858 and 814 cm−1 for POM-γ-Fe2O3/SrCO3. The interaction between the surface of nanoparticle and POM is assumed to be the source of red shift in the POM peaks. The EDX spectra of POM-γ-Fe2O3/SrCO3 nanohybrids from two different areas of sample are shown in Fig. 3. Here, the tungsten (W) peaks suggest the existence of POM on the surface of nanohybrids. Furthermore, strong Fe peak and weak Sr peak reveals that Fe ratio is greater than Sr, as confirmed by the chemical analysis data. As shown in Fig. 3, a weak peak of C is consistent with SrCO3 as XRD results. The images of the -resolution TEM analysis of POM-γFe2O3/SrCO3 are shown in Fig. 4a, b. It can be seen that

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absorbance, respectively. In this formula, n is determined from the type of optical transition of semiconductor (n = 1 for direct transition and n = 4 for indirect transition) (Cao et al. 2012). A direct band gap of 1.8 eV was determined from a plot of (αhν)0.5 versus band gap energy (hν) for both samples (Supplementary Fig. S1b). The reported direct band gap of pure γ-Fe2O3 was Eg = 2.43 eV (Chakrabarti et al. 2004). The lower band gap in our case is assumed to be due to the presence of SrCO 3 in the structure of nanoparticles. Furthermore, the nonsteep shape of the spectrum indicates that the visible light absorption band is the result of a transition from impurity levels (Guo et al. 2012). The pHpzc is defined as the pH where the net surface charge is zero. The surface charge is positive when pH of the solution is lower than pHpzc and vice versa. The pHpzc of POM-γFe2O3/SrCO3 nanohybrid was about 7.0. Photocatalytic degradation of IBP The photocatalytic degradation of IBP under solar light and dark condition by POM-γ-Fe2O3/SrCO3 nanohybrid was followed by a UV-vis spectroscopy. The main absorption band of IBP is located at 224 nm, which belongs to the benzene ring. Fig. 2 FTIR spectra of POM, γ-Fe2O3/SrCO3, and POM-γ-Fe2O3/ SrCO3 nanohybrids

nanohybrids are ellipsoid in shape and structure, resembling a structure of 23 nm in length and 6 nm in width. Furthermore, the structure is not completely crystalline and consistent with the significant background in XRD patterns. The SAED image of the sample is shown in Fig.4c. The SAED pattern is characterized by spotty diffraction rings with the sample assuming to be a polycrystalline. The magnetization curve of the prepared POM-γ-Fe2O3/ SrCO3 at room temperature is shown in Fig. 5. As can be seen, the saturation magnetization and remanence are 11 and 0.9 emu/g, respectively. Given that the remanence is close to zero and no hysteresis is observed, it can be easily separated in a suspended system. Furthermore, the nanohybrid can be redispersed after separation by an external magnetic field as a result of its superparamagnetic behavior. Supplementary Fig. S1a shows the absorption spectra of γFe2O3/SrCO3 and POM-γ-Fe2O3/SrCO3. According to the results, the absorption edges of both samples were in the visible region (>400 nm). The band gaps of samples were calculated by the following formula (Cao et al. 2012):
n2 ahv ¼ A HV −Eg ð1Þ where α, h, ν, Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap energy, and measured

Influence of contact time To identify the effect of contact time, the pH values were adjusted to 4.7 and 9.5. Since the pKa of the IBP is 4.9, the concentration of protonated and deprotonated forms of IBP at pH 4.7 will be equal (Illés et al. 2013). At pH 9.5, the ionic and deprotonated form of IBP is predominant. Supplementary Fig. S2 shows the UV-vis spectra of 10 mg L−1 IBP main solution at different pHs. Figure 6a, b shows the UV-vis spectra of IBP degradation at pH 4.7 under solar light and dark conditions, respectively. The starting solution of IBP exhibits an absorption band of 224 nm which belongs to the benzene ring. After irradiation under solar light, the intensity of this band rises, indicating the formation of changed aromatic molecules (Illés et al. 2013). The hydroxylation is assumed to have occurred at the beginning of the transformation. This hydroxylation may take place both in the side chains and in the ring (Illés et al. 2013). It was observed that the simultaneous side group oxidation and benzene ring oxidation of IBP were associated with the appearance of a new red shift band at a higher wavelength (~250 nm) and a broad band in the range of 240–300 nm. This observation confirms the production of intermediates and the change in aromatic molecules (Illés et al. 2013; Choina et al. 2013). In our case, the aromatic molecule hydroxylation was observed at the beginning. It may occur both

Environ Sci Pollut Res Fig. 3 EDX spectra of POM-γFe2O3/SrCO3 nanohybrids

in the side chains and in the ring. After 60 min, a new peak appeared at ~250 nm under solar light, but in the dark condition, only a simple hydroxylation emerged with a broad band observed at a higher wavelength. It has been reported that the substitution in the side chain does not change the absorption spectrum at higher wavelengths (Illés et al. 2013; Choina et al. 2013). Supplementary Fig. S3 shows the influence of contact time on the appearance of intermediates during the photocatalytic treatment of IBP based on UV-vis absorbance at ~250 nm. Intermediates emerge immediately after the photocatalytic treatment of IBP. The formation rate of intermediates is highest under solar light, but it is significantly reduced in the dark condition. This is due to photocatalytic activity of nanohybrid under solar light. In a study, the use of γ-Fe2O3/SrCO3 under solar light for 120 min revealed a simple hydroxylation without any significant broad band at higher wavelengths. These results confirm the effect of surface modification by POM on the photodegradation of IBP. The enhanced activity in the case of POM-γ-Fe2O3/SrCO3 can be attributed to more efficient separation of photo-induced electron–hole pairs in the

nanohybrid through the transference of excited electrons of γ-Fe2O3/SrCO3 to POM. The photocatalytic treatment of IBP at pH 9.5 displays the simple oxidation in both dark and solar light conditions (Supplementary Fig. S4a, b). Influence of pH of solution Figure 7 shows the UV-vis spectra of IBP degradation at different initial pHs of the solution under solar light irradiation and dark conditions for 2-h periods. According to the results, the degradation of IBP was found to be higher at lower pH values (pH 2.5). However, the production of intermediates was significantly increased at pH 4.7 (red-shifted peak at ~250 nm). To determine the amount of adsorbed species during the photocatalytic treatment under both solar light and dark conditions, desorption study was performed (Fig. 8). As shown in the desorption UV-vis spectrum of the sample with the catalytic treatment under dark condition, three peaks at 208, 230, and 260 nm are related to the adsorption of IBP and hydroxylated

Environ Sci Pollut Res Fig. 4 a, b HRTEM and c SAED pattern of POM-γ-Fe2O3/SrCO3 nanohybrid

intermediates. According to the results, the highest desorption was obtained in pH 4.7 with one peak at 208 nm. The desorption spectrum of the sample with photocatalytic activity under solar light shows a small peak at 208 nm, indicating the residual amount of IBP on the surface of catalyst. It is assumed that solar light irradiation is responsible for the degradation of IBP on the surface of nanohybrid and surface cleaning. The initial pH of the solution plays an important role in the photocatalytic degradation of organic pollutants. The surface charge of the catalyst can be changed relative to the point of zero charge (PZC) and chemical nature of the catalyst (Choina et al. 2013). At pH values lower than pHpzc, the surface of catalyst is protonated and thus positively charged. On the contrary, at pH higher than pHpzc, the surface is deporotonated and thereby negatively charged (Choina et al. 2013; Rohani Bastami and Entezari 2012. The point of zero charge for POM-γ-Fe2O3/SrCO3 nanohybrids is close to 7.0. Since IBP is a weak acid with a pKa of about 4.9, low pH will lead to the protonation of catalyst surface and the carboxyl group of IBP. On the other hand, high pH will increase the negative charge of the catalyst surface as a result of the deprotonation of its surface and IBP. Under this condition, the predominance of electrostatic repulsion leads to the reduction of reaction rate. In acidic media, the surface of catalyst and IBP are protonated. It seems that the main adsorption mechanism between the surface of catalyst and IBP in acidic media is hydrogen bonding

between protonated IBP and oxygen functional group of catalyst surface. Furthermore, at low pH, the carboxylic group on the IBP molecule is shifted to the acidic form. The state of equilibrium leaves some carboxylic anions in the solution, which can be adsorbed on the surface. Also, the higher degradation of IBP under solar light in acidic media might be due to the following reactions (Dolbecq et al. 2012): ðPOM −S Þðhν Þ→ðPOM −S Þ*

ð2Þ

Fig. 5 Magnetization versus applied magnetic field for POM-γ-Fe2O3/ SrCO3

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According to Eq. (2), the photoexcitation of POM-γFe2O3/SrCO3 (POM-S) leads to (POM-S)*. Then, the charge transfer and photoreduction of POM occurs: ðPOM −S Þ* →POM red þ S þ

ð3Þ

The re-oxidation of POM after exposure to O2 is POM red þ O2 →POM þ O•− 2

ð4Þ

In acidic media, HO•2 is formed according to the following reaction: þ • O•− 2 þ H →HO2

ð5Þ

As a result, the concentration of radicals reacting with IBP increases. Influence of additional oxygen supply and H2O2 From a practical point of view, the investigation of IBP degradation under air-saturated solution is improved. The oxygen content of the IBP aqueous solution is enhanced by bubbling synthetic air into the solution during the photocatalytic process. As Figs. 6a and 9b show, the enhancement of additional oxygen augmented the photocatalytic degradation of IBP. It is suggested that the increased oxygen content leads to formation of super peroxide radicals O•2− (Shivaraju 2011). According to Eq. (5), increasing O•2− in acidic media can improve the radical concentration and IBP degradation. It is well known that H2O2 is OH• radical promoter, which can accelerate the degradation of pollutants in an aqueous solution (Basfar et al. 2005). Figure 9a, b also shows the influence of different quantities of H2O2 on IBP degradation under solar light irradiation and in a ci di c m e d ia . T he re su lt s sh ow ed t ha t 1 0 μ L (0.01 mL, 0.0025 M) of H2O2 accelerated the degradation of IBP, leading to the production of intermediates by the appearance of new peaks at 260 nm. The increase of concentration of H2O2 to 80 μL (0.08 mL, 0.02 M) leads to nearly complete degradation of IBP. In this case, the solar-Fenton catalytic degradation of IBP takes place. Hydrogen peroxide contacts with Fe (III) sites on the surface of POM-γ-Fe 2 O 3 /SrCO 3 photocatalysts, initiating a chain reaction that leads to the formation of OH• radical. Under acidic condition, the mechanism of H2O2 activation by γ-Fe2O3/SrCO3 photocatalysts would be according to Eqs. (6)–(9) (Vikesland and Valentine 2002 First, Fe (III)-H2O2 complex is generated on the surface of photocatalyst (Eq. (6)). Then, the generated species is converted to Fe (II) and HO•2 according to Eq.7. In addition, Fe (II)

Fig. 6 UV-vis spectra of photocatalytic degradation of IBP on POM-γFe2O3/SrCO3 at pH = 4.7 under a solar light and b dark condition

species can be generated on the surface of photocatalyst according to Eq.8. All forms of Fe (II) species produced on the surface of photocatalyst can react with H2O2 and produce reactive radicals (HO•2 and HO•). FeðIII Þ þ H 2 O2 →FeðIII ÞH 2 O2

ð6Þ

FeðIII ÞH 2 O2 → FeðII Þ þ HO•2 þ H þ

ð7Þ

FeðIII Þ þ HO•2 → FeðII Þ þ O2 þ H þ

ð8Þ

FeðII Þ þ H 2 O2 þ H þ → FeðIII Þ þ OH • þ H 2 O

ð9Þ

According to Fig. 9a, the concentration of intermediate is higher in the presence of 160 μL (0.16 mL, 0.04 M) in comparison to 80 μL (0.08 mL, 0.02 M) which was concluded by

Environ Sci Pollut Res Fig. 7 UV-vis spectra of photocatalytic degradation of IBP on POM-γ-Fe2O3/SrCO3 at different pH after 2 h

higher amount of absorbance at higher wavelength (λ = 270 nm). Thus, It is assumed that with an increase in the concentration of H2O2 to 160 μL (0.16 mL, 0.04 M), the degradation rate of IBP is marginally decreased as a result of the competition of H2O2 with IBP for OH• (Eq. (10)) (Zheng et al. 2011). H 2 O2 þ OH • →H 2 O þ HO•2 Fig. 8 UV-vis spectra of desorption species of IBP and other intermediates (photocatalytic treatment for 2 h and desorption time of 2 h)

ð10Þ

Stability and reusability of photocatalyst From a practical point of view, the estimation of reusability and stability is of significant importance. For this reason, the photodegradation of IBP was repeated three times. After each run, POM-γ-Fe2O3/SrCO3 nanohybrids were separated from the solution by an external magnetic field. The collected sample was repeatedly used in the next cycle. In each cycle, POM-γ-Fe2O3/

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Fig. 9 Influence of additional oxygen supply and hydrogen peroxide on the a UV-vis spectrum of IBP degradation, and b the concentration of IBP as a function of catalytic reactions using POM-γ-Fe2O3/SrCO3 under sunlight (1) initial conc., (2) POM-γ-Fe2O3/SrCO3, (3) POM-γ-Fe2O3/ SrCO3 + air, (4) POM-γ-Fe2O3/SrCO3 + H2O2 (0.01 mL), (5) POM-γFe2O3/SrCO3 + H2O2 (0.08 mL), and (6) POM-γ-Fe2O3/SrCO3 + H2O2 (0.16 mL) during 2-h exposure to the solar light at pH = 2.5

number of main reactive species (ROSs) like h+, H2O2, O•2−, and •OH (Cao et al. 2012; Zhou et al. 2010). Thus, the effects of two scavengers were investigated to obtain the reaction mechanism. In this study, the isopropanol was added to the reaction system as a •OH scavenger (Zhou et al. 2012; Cui et al. 2013) and NaHCO3 was introduced as •OH and holes scavenger (Zhou et al. 2012). Figure 10 shows that in the presence of isopropanol, the photodegradation of IBP was inhibited significantly compared to the absence of scavenger under similar conditions, indicating the main role of •OH in the degradation of IBP. The addition of NaHCO3 had a less significant effect on PCO progress of IBP, assuming that h+ had a comparatively minor effect on IBP degradation. In photocatalytic reactions, the activity of photocatalyst affected by the recombination of photoinduced electrons and holes will reduce the photodegradation efficiency (Cheng et al. 2010). According to scheme 2, a synergistic effect is generated between POM and γ-Fe2O3/SrCO3. It is suggested that the role of POM in POM-γ-Fe2O3/SrCO3 photocatalyst is critical in improving photoefficiency by retarding the fast recombination of a charge-pair (h+-e−) on the nanohybrid and producing a strong oxidant O•2− ( Dolbecq et al. 2012). In this system, γ-Fe2O3/SrCO3 can be excited by the solar light irradiation to deliver the electron from its conduction band into the LUMO of the POM unit coated on the surface. Then, adsorbed O2 and H2O2 can easily trap an electron in the LUMO of the POM anion to yield the oxidizing species •OH, which is followed by the attack of radicals to the organic molecules. The following reaction could occur in the degradation of IBP: POM −γ−Fe2 O3 =SrCO3 þ hν ðsolarÞ→POM −γ−Fe2 O3 =SrCO3 ðhþ þ e− Þ

ð11Þ

POM −γ−Fe2 O3 =SrCO3 ðhþ þ e− Þ→POM ðe− Þ þ γ−Fe2 O3 =SrCO3 ðhþ Þ

ð12Þ

POM ðe− Þ þ O2 →O•− 2

ð13Þ

• − O•− 2 þ H 2 O→HO2 þ OH

ð14Þ

þ • O•− 2 þ H →HO2

ð15Þ

HO•2 þ HO•2 →H 2 O2 þ O

ð16Þ

Mechanism of visible light photocatalytic degradation

H 2 O2 →2OH •

ð17Þ

The organic pollutant can be photodegraded via photocatalytic oxidation (PCO) process. A PCO process includes a large

OH • þ IBP→IBPox

ð18Þ

SrCO3 nanohybrids (0.05 g) were added to 50 mL of 10 mgL−1 IBP solution at pH 2.5 for 120 min. As shown in Supplementary Fig. S5, IBP was decomposed in each cycle without any significant loss of activity after two cycles with only a slight loss of activity observed in the third cycle. It is also confirmed that the POM-γ-Fe2O3/SrCO3 nanohybrids were stable during photocatalytic oxidation of IBP. Furthermore, there was no significant difference between the FTIR spectra of POM-γ-Fe2O3/SrCO3 nanohybrids and the samples after degradation at pH 2.5 and 4.7 (Supplementary Fig. S6).

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photocatalytic activity and magnetization properties of the novel magnetic photocatalyst nanohybrids provided a promising solution for the degradation of water pollutants and photocatalyst recovery. Acknowledgments The authors acknowledge the assistance of Mr. Bakhtiari from Chemical Engineering Research Lab. of Ferdowsi University of Mashhad. This project has been financially supported by the Iranian National Science Foundation (INSF) (No.92004798).

References

Fig. 10 Influence of different scavengers on the UV-vis spectrum of IBP degradation during 2-h exposure to the solar light at pH = 2.5

Conclusion The highly solar light photoactive POM-γ-Fe2O3/SrCO3 nanohybrids were fabricated by grafting the phosphotungstic acid anions to γ-Fe2O3/SrCO3 nanoparticles, which are used for the degradation of ibuprofen (IBP) in the aqueous solution under solar light. FTIR and EDX analyses demonstrated that phosphotungstic acid anions were anchored on the surface of γ-Fe2O3/SrCO3 nanoparticles. The IBP aqueous solution showed higher degradation in the presence of nanohybrids compared to that of γ-Fe2O3/SrCO3 in the absence of surface modification. The influence of different parameters like contact time, pH of solution, and different light sources were also examined. The results exhibited that the addition of hydrogen peroxide increased degradation, but the addition of NaHCO3 and isopropanol restrained the degradation process. The solarFenton degradation of IBP was occurred on the surface of photocatalyst in the presence of H2O2. Furthermore, the

Scheme 2 Schematic diagram of the photocatalytic reaction of IBP on POM-γ-Fe2O3/SrCO3 nanohybrid

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