Chemosphere 150 (2016) 139e144
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Photocatalytic degradation and removal mechanism of ibuprofen via monoclinic BiVO4 under simulated solar light Fuhua Li, Yapu Kang, Min Chen, Guoguang Liu*, Wenying Lv, Kun Yao, Ping Chen, Haoping Huang School of Environmental Science and Engineering, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Panyu District, Guangzhou, 510006, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synthesis and characterization of monoclinic BiVO4. Photocatalytic activity for degradation of ibuprofen under simulated solar light. Effect of operational parameters on the photocatalytic degradation of ibuprofen. Removal mechanism of ibuprofen via monoclinic BiVO4 under simulated solar light.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2015 Received in revised form 7 February 2016 Accepted 9 February 2016 Available online xxx
Characterized as by X-ray diffraction, scanning electron microscopy and UVevis diffuse reflectance spectra techniques, BiVO4 photocatalyst was hydrothermally synthesized. The photocatalytic degradation mechanisms of ibuprofen (IBP) were evaluated in aqueous media via BiVO4. Results demonstrated that the prepared photocatalyst corresponded to phase-pure monoclinic scheelite BiVO4. The synthesized BiVO4 showed superior photocatalytic properties under the irradiation of visible-light. The photocatalytic degradation rate of IBP decreased with an increase in the initial IBP concentration. The degradation process followed first-order kinetics model. At an IBP concentration of 10 mg L1, while a BiVO4 concentration of 5.0 g L1 with pH value of 4.5, the rate of IBP degradation was obtained as 90% after 25 min. The photocatalytic degradation of IBP was primarily accomplished via the generation of superoxide radical (O 2 ) and hydroxyl radicals ( OH). During the process of degradation, part of the OH was conþ . The direct oxidation of holes (h ) made a minimal contribution to the degradation verted from the O 2 of IBP. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: Klaus Kümmerer Keywords: BiVO4 Photocatalytic degradation Ibuprofen Kinetics Mechanism
1. Introduction The compound 2-[3-(2-methylpropyl)phenyl] propanoic acid is commercially available as ibuprofen (IBP). Relative to other anti-
* Corresponding author. E-mail addresses: [email protected] (F. Li), [email protected] (G. Liu). http://dx.doi.org/10.1016/j.chemosphere.2016.02.045 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
inflammatory and antipyretic drugs like aspirin and acetaminophen, the IBP appears excellent properties of low toxicity, high efficacy and minimal side effects (Buser et al., 1999). Thus, IBP is more widely used at present. Due to its widespread application, IBP brought the formation of pseudo persistent phenomena. IBP inflicted damage on species of flora and fauna in aquatic ecosystems (Mendez-Arriaga et al., 2009; Ternes et al., 2004). Hence, IBP emerged as a new form of organic micropollutant.
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Photocatalytic technology is a potential way in the treatment of organic pollutants. As a regularly studied semiconductor photocatalyst, TiO2 is widely adopted in environmental modification and water treatment (Anandan and Ashokkumar, 2009; Hu et al., 2007; Im et al., 2012). However, TiO2 has a wide band gap of 3.20 eV. The band gap is selective for the absorption of ultraviolet light and it accounts for only ~5% of the ambient sunlight spectrum (Asahi et al., 2001; Wang et al., 2010; Yu et al., 2003). So it is difficult to develop TiO2 as commercially acceptable products for photocatalyst candidate. New visible-light-driven photocatalytic materials is indispensable to develop to efficiently utilize a greater portion of the solar spectrum. The materials must possess robust activity. Recently, BiVO4 come into the focus of researchers for its such unique properties as ferroelasticity (Nepochatenko and Dudnik, 2005), ionic conductivity (Rullens et al., 2006), photocatalytic activity for water splitting (Sayama et al., 2006; Su et al., 2010; Tokunaga et al., 2001) and capacity for the degradation of harmful pollutants (Jiang et al., 2008; Yin et al., 2010; Zhang et al., 2006). The photocatalytic activity of BiVO4 depends strongly on its crystalline form (Kohtani et al., 2003; Li et al., 2009; Zhang et al., 2007) and morphology (Liu et al., 2010; Wang et al., 2009). BiVO4 possesses such three primary crystalline forms, as tetragonal zircon, monoclinic scheelite and tetragonal scheelite structures. Monoclinic scheelite BiVO4 owns a moderate band gap (2.40 eV) and exhibits a best photocatalytic activity under visible-light irradiation (Tokunaga et al., 2001). The photocatalytic degradation of ibuprofen was not reported via monoclinic BiVO4 under solar light. Hence, the aim of this study was to investigate the photocatalytic activity of monoclinic BiVO4 in the degradation of IBP, and to illustrate the effects of operational parameters. The photocatalytic degradation mechanism was studied by quenching analysis as well.
2.3. Characterization of BiVO4 photocatalysts The samples were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and ultravioletevisible (UVeVis) diffuse reflectance spectroscopy. The XRD patterns were acquired through X-ray diffractometer (ULTIMA-III, Rigaku, Japan). The SEM images were obtained with a field emission scanning electron microscope (SXL-30ESM, Philips, Netherlands). The absorption spectrum was measured with a UVevis spectrophotometer (UV-2450, Shimadzu, Japan). 2.4. Ibuprofen solution The 1 g L1 IBP stock solution was prepared with acetonitrile and reserves were stored in a refrigerator at 4 C. Experimental solutions were prepared by the addition of an appropriate volume of stock solution into a brown volumetric flask. Nitrogen was supplied to evaporate the acetonitrile completely. Ultrapure water was then added. Next, agitation was introduced to promote the process. Dissolution was facilitated using sonication. The pH value was subsequently adjusted through the addition of 1% sulfuric acid or sodium hydroxide solution. Finally, the required volumes were achieved. 2.5. Photocatalytic degradation of ibuprofen
2. Materials and methods
All experiments were performed within an XPA-7 photochemical reactor. Pyrex test tubes were adopted to contain the solutions. The irradiation was provided by a 350 W long-arc Xe lamp (l > 290 nm). Prior to illumination, the suspension was magnetically stirred in the dark for 1 h. The aim was to ensure the adsorption/desorption equilibrium of IBP with the photocatalyst powders. The samples were removed at different time intervals for analysis. Three parallel experiments were conducted for each group. The results of each group were averaged in the analysis.
2.1. Reagents
2.6. Analytical method
Bi(NO3)3$5H2O and NH4VO3 were purchased from Aladdin Industrial Corporation (Shanghai). Ibuprofen was purchased from TCI Development Co. Ltd. (Shanghai). Sulfuric acid, nitric acid, sodium hydroxide, acetic acid, ethanol, benzoquinone, isopropanol were purchased from Chengdu Kelong Chemical Reagent Co. Ltd. All of the reagents above were of analytical grade and required no further purification. The methanol and acetonitrile purchased from Anaqua Chemicals Supply Co. Ltd. (USA) were of HPLC grade. Ultrapure water from a TKA Smart 2 Pure system (Germany) was used in the preparation of all aqueous solutions.
Ibuprofen concentrations were determined with a HPLC system (Shimadzu, Japan). The chromatographic conditions were as follows. Column: Zorbax Eclipse XDB-C18 (2.1 mm 150 mm, 5 mm); Temperature: 30 C; Mobile phase: acetonitrile/acetic acid buffer solution (containing 0.3% acetic acid, 50:50, v/v); Flow rate: 0.2 mL min1; Injection volume: 4 mL; Photodiode array detector: 263 nm wavelength. 3. Results and discussion 3.1. Morphology and structure
2.2. Synthesis of BiVO4 photocatalysts The BiVO4 photocatalyst was prepared by a hydrothermal process. In a typical procedure, Bi(NO3)3$5H2O (5 mM) and NH4VO3 (5 mM) were dissolved in 40 mL of HNO3 (4 M) and 40 mL of NaOH (4 M) respectively. To obtain a stable homogeneous mixture, these two solutions were combined under ultrasonication for 10 min. The pH value of the final mixture was adjusted to 7 with the addition of NaOH. After standing for 24 h, the suspension was transferred into a 100 mL Teflon autoclave. The autoclave was filled up to 80% of the total volume. The hydrothermal treatment was performed at 180 C for 3 h. Subsequently, the product was collected by filtration and rinsed several times with ultrapure water and ethanol. Finally, the products were dried at 80 C for 6 h in ambient air (Ge, 2008; Zhang et al., 2006).
The diffraction pattern (Fig. 1) shows that all of the peaks could be indexed as the standard card (JCPDS Card No. 14-0688) of monoclinic BiVO4 (Yu and Kudo, 2005; Zhou et al., 2006) and no other peaks from possible impurities were detected. The XRD analysis results indicated that the prepared photocatalyst corresponded to phase-pure monoclinic scheelite BiVO4. The SEM micrographs (Fig. 2) of the BiVO4 sample indicates that the prepared BiVO4 was in the form of microspheres. The surfaces of the sample were formed by many small particle agglomerations. These surfaces were uneven, and part of which were comprised of hollow structures. The UVeVis DRS of the BiVO4 sample is illustrated in this work (Fig. 3). Based on the equation of Eg ¼ 1240/lg, the Eg value of the BiVO4 sample was obtained as 2.32 eV. The Eg value is slightly lower than, or similar to, those of the BiVO4 materials reported in
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visible-light absorption wavelength is 535 nm. Consequently, the synthesized monoclinic BiVO4 photocatalyst was expected to show superior photocatalytic properties under visible-light irradiation. 3.2. Photocatalytic activity The photocatalytic performance of the BiVO4 is revealed in this section (Fig. 4). It can be seen that the photodegradation and dark adsorption could not efficiently remove IBP. However, the removal efficiency of IBP attained by the photocatalytic process was up to 90%. This result indicates that the catalyst showed excellent photocatalytic activity under simulated sunlight. 3.3. Effect of pH The effect of pH on the degradation rate of IBP was determined after irradiation for 40 min (Fig. 5a). It can be seen that catalysis was more likely to occur under acidic conditions than under neutral or alkaline conditions. The removal rate of IBP was maximized at the pH value of 4.5. This is because that IBP is a weak acid (4.52 pKa 4.9) (Madhavan et al., 2010; Mendez-Arriaga et al., 2008; Rafols et al., 1997), while the point of zero charge of BiVO4 is
Fig. 1. XRD pattern of the BiVO4 sample.
Fig. 2. SEM images of the BiVO4.
Fig. 3. UVevis diffuse reflectance spectra of the BiVO4.
the literature (Jiang et al., 2012; Zhu et al., 2013). Its maximum
Fig. 4. Dark adsorption, photodegradation and photocatalytic degradation of IBP under simulated solar irradiation. [IBP]0 ¼ 10 mg L1, [BiVO4] ¼ 2.0 g L1, pH ¼ 4.5. Error bars indicate one standard deviation.
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Fig. 5. Influence of pH value (a) and catalyst concentration (b) on the degradation of IBP. a: [IBP]0 ¼ 10 mg L1, [BiVO4] ¼ 2.0 g L1 b: [IBP]0 ¼ 10 mg L1, pH ¼ 4.5. Error bars indicate one standard deviation.
widely reported to be pH ¼ 2.7 (Xie et al., 2006). In weak acidic medium (2.7 pH 4.9), IBP is electrically neutral and exists in molecular form, while BiVO4 is deprotonated and electronegative. This facilitated the adsorption of IBP to the surface of the catalyst. The contact probability of IBP with reactive oxygen species (ROS) and hþ were increased then. However, in strong acidic media (pH < 2.7), BiVO4 is electropositive and the concentration of Hþ is excessive. This condition will lead to the reductions of OH. Hence, the removal rate of IBP was decreased. In neutral and alkaline media (pH > 4.9), IBP is electronegative due to ionization. Therefore, it can repel electronegative BiVO4 and OH. Several studies have shown that high concentrations of OH enabled CO2 to enter 2 into the solution. CO2 3 was produced consequently. The CO3 is a radical scavenger (Kashif and Ou-Yang, 2009). 3.4. Effect of catalyst concentration The effect of BiVO4 concentration on the degradation rate was investigated after irradiation for 25 min (Fig. 5b). The degradation rate of IBP increased as the BiVO4 concentration was increased. When the latter was increased to 5.0 g L1, the degradation rate was maximized, at rates reaching 91%. It has been reported that TiO2/ UV-A photocatalytic systems is able to degrade 92.6% of the original IBP after a treatment time of 120 min (Silva et al., 2014). To achieve the same degradation rate, BiVO4 need less time than TiO2. On the other hand, BiVO4 photocatalytic systems improves the costeffectiveness through photoactivation with solar irradiation. Therefore, the synthesized BiVO4 showed superior photocatalytic properties under the irradiation of visible-light. As BiVO4 concentration was further increased, the degradation rate was shown to decrease. This was primarily due to ROS. The concentration of ROS played a critical role in the photocatalytic oxidation reactions. More ROS were produced with higher BiVO4 concentration. However, when the BiVO4 concentration reached a certain level, the shielding and reflection of light by the catalyst resulted in a decrease of light utilization. The production of ROS was decreased as a result. In addition, the overuse of the BiVO4 would result in the aggregation of catalyst. The aggregation phenomenon consequently reduced the contact area between BiVO4, light and IBP. 3.5. Effect of substrate concentration The effect of IBP concentration on degradation rate was studied after irradiation for 40 min (Fig. 6). It can be seen that the
Fig. 6. Influence of substrate concentration on the degradation [BiVO4] ¼ 2.0 g L1, pH ¼ 4.5. Error bars indicate one standard deviation.
of
IBP.
degradation process of IBP could be fitted by the first-order kinetics model. The degradation rate of IBP decreased as the substrate concentration was increased. This result can be explained by considering the competition for absorption of the limited quantity of active sites by the IBP. Because the initial concentration of IBP increased but the number of active sites did not change, the number of active sites available per molecule of IBP decreased, so the degradation rate of IBP decreased.
3.6. Mechanism of ibuprofen photocatalytic degradation 3.6.1. Impact of benzoquinone on photocatalytic degradation of IBP Quenching experiments were employed to study removal mechanism of IBP (Fig. 7a). The O 2 was quenched with the addition of benzoquinone (Chen et al., 2008; Zhang et al., 2011). Then the degradation rate constant declined from 0.0622 min1 to 0.0094 min1. The phenomenon indicated that the contribution rate of O 2 was 84.9%. It was accordingly inferred that the photocatalytic degradation of IBP primarily via the generation of O 2 , as indicated in the following Eqs. (1)e(3).
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Fig. 7. Effect of quenching agents (a) and dissolve oxygen (b) on the degradation of IBP. [IBP]0 ¼ 10 mg L1, [BiVO4] ¼ 2.0 g L1, pH ¼ 4.5. Error bars indicate one standard deviation.
BiVO4 þ hv/BiVO4 þ e þ hþ
(1)
O2 þ e /O$ 2
(2)
* IBP þ O$ 2 /IBPproduct
(3)
3.6.2. Impact of isopropanol on photocatalytic degradation of IBP The OH was quenched with the addition of isopropanol(Buxton et al., 1988; Vione et al., 2001). Then the degradation rate constant decreased from 0.0622 min1 to 0.0309 min1. It could be determined that the contribution rate of OH was 50.3%. In addition to the reaction of holes (hþ) and H2O, the OH was also further transformed from O 2 . IBP degradation was achieved through the reaction with the OH radicals, as shown in Eqs. (4)e(8).
H2 O þ hþ /$OH þ Hþ
(4)
þ O$ 2 þ H /$HO2
(5)
2$HO2 /H2 O2 þ O2 $
(6)
O$ 2 þ H2 O2 /$OH
(7)
* IBP þ $OH/IBPproduct
(8)
3.6.3. Impact of methanol on photocatalytic degradation of IBP When methanol was introduced, both hþ and OH were quenched (Hu et al., 2007; Im et al., 2012; Santato et al., 2001). Compared with the addition of isopropanol, the degradation rate constant (0.0326 min1) was slightly increased. An inference was obtained in this work that the direct oxidation by the hþ was less significant in the IBP photocatalytic oxidation process. The tiny difference between the rate constants of methanol and isopropanol is probably due to that the quenching of hþ and OH reduced the recombination of electronehole pair. Afterward, the concentration of electrons was increased. Then the formation of ROS was facilited.
3.6.4. Impact of dissolved oxygen on photocatalytic degradation of IBP Dissolved oxygen concentration is an important parameter. Experiments were conducted to study the effect of dissolved oxygen in the solution, using nitrogen or oxygen as purging gas. Nitrogen and oxygen was bubbled through the solution to regulate the dissolved oxygen concentration. The gas was bubbled through the solution for at least 30 min before each experiment. As can be seen from Fig. 7b, the degradation rate of IBP subsequent to the introduction of O2 was much higher than that with a supply of N2. The degradation rate constant increased from 0.0360 min1 to 0.0639 min1. The supply of O2 served to enhance the formation of O 2 .This phenomenon further indicated that the degradation of IBP was due to the generation of O 2 . 4. Conclusions Phase-pure monoclinic scheelite BiVO4 was synthesized through a hydrothermal process. The prepared BiVO4 showed good performance in the degradation of IBP under simulated solar irradiation. The degradation process could be depicted by first-order kinetics model. In the condition of constant catalyst volume, the degradation rate of IBP decreased with an increasing initial concentration of substrate. The optimal conditions for IBP degradation were that the concentration of BiVO4 was 5.0 g L1 and the pH value was 4.5. The photocatalytic degradation of IBP primarily via the generation of superoxide radical (O 2 ) and hydroxyl radicals ( OH). During the process of degradation, part of the OH was converted þ from the O 2 . The direct oxidation of holes (h ) made a minimal contribution to the degradation of IBP. This work showed that photocatalytic degradation of ibuprofen via monoclinic BiVO4 under solar light is feasible. Monoclinic BiVO4 has a broad prospects in the commercial application. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21377031). References Anandan, S., Ashokkumar, M., 2009. Sonochemical synthesis of AueTiO2 nanoparticles for the sonophotocatalytic degradation of organic pollutants in aqueous environment. Ultrason. Sonochemistry 16, 316e320. Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Visible-light
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Photocatalytic degradation and removal mechanism of ibuprofen via monoclinic BiVO4 under simulated solar light Fuhua Li, Yapu Kang, Min Chen, Guoguang Liu*, Wenying Lv, Kun Yao, Ping Chen, Haoping Huang School of Environmental Science and Engineering, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Panyu District, Guangzhou, 510006, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synthesis and characterization of monoclinic BiVO4. Photocatalytic activity for degradation of ibuprofen under simulated solar light. Effect of operational parameters on the photocatalytic degradation of ibuprofen. Removal mechanism of ibuprofen via monoclinic BiVO4 under simulated solar light.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2015 Received in revised form 7 February 2016 Accepted 9 February 2016 Available online xxx
Characterized as by X-ray diffraction, scanning electron microscopy and UVevis diffuse reflectance spectra techniques, BiVO4 photocatalyst was hydrothermally synthesized. The photocatalytic degradation mechanisms of ibuprofen (IBP) were evaluated in aqueous media via BiVO4. Results demonstrated that the prepared photocatalyst corresponded to phase-pure monoclinic scheelite BiVO4. The synthesized BiVO4 showed superior photocatalytic properties under the irradiation of visible-light. The photocatalytic degradation rate of IBP decreased with an increase in the initial IBP concentration. The degradation process followed first-order kinetics model. At an IBP concentration of 10 mg L1, while a BiVO4 concentration of 5.0 g L1 with pH value of 4.5, the rate of IBP degradation was obtained as 90% after 25 min. The photocatalytic degradation of IBP was primarily accomplished via the generation of superoxide radical (O 2 ) and hydroxyl radicals ( OH). During the process of degradation, part of the OH was conþ . The direct oxidation of holes (h ) made a minimal contribution to the degradation verted from the O 2 of IBP. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: Klaus Kümmerer Keywords: BiVO4 Photocatalytic degradation Ibuprofen Kinetics Mechanism
1. Introduction The compound 2-[3-(2-methylpropyl)phenyl] propanoic acid is commercially available as ibuprofen (IBP). Relative to other anti-
* Corresponding author. E-mail addresses: [email protected] (F. Li), [email protected] (G. Liu). http://dx.doi.org/10.1016/j.chemosphere.2016.02.045 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
inflammatory and antipyretic drugs like aspirin and acetaminophen, the IBP appears excellent properties of low toxicity, high efficacy and minimal side effects (Buser et al., 1999). Thus, IBP is more widely used at present. Due to its widespread application, IBP brought the formation of pseudo persistent phenomena. IBP inflicted damage on species of flora and fauna in aquatic ecosystems (Mendez-Arriaga et al., 2009; Ternes et al., 2004). Hence, IBP emerged as a new form of organic micropollutant.
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Photocatalytic technology is a potential way in the treatment of organic pollutants. As a regularly studied semiconductor photocatalyst, TiO2 is widely adopted in environmental modification and water treatment (Anandan and Ashokkumar, 2009; Hu et al., 2007; Im et al., 2012). However, TiO2 has a wide band gap of 3.20 eV. The band gap is selective for the absorption of ultraviolet light and it accounts for only ~5% of the ambient sunlight spectrum (Asahi et al., 2001; Wang et al., 2010; Yu et al., 2003). So it is difficult to develop TiO2 as commercially acceptable products for photocatalyst candidate. New visible-light-driven photocatalytic materials is indispensable to develop to efficiently utilize a greater portion of the solar spectrum. The materials must possess robust activity. Recently, BiVO4 come into the focus of researchers for its such unique properties as ferroelasticity (Nepochatenko and Dudnik, 2005), ionic conductivity (Rullens et al., 2006), photocatalytic activity for water splitting (Sayama et al., 2006; Su et al., 2010; Tokunaga et al., 2001) and capacity for the degradation of harmful pollutants (Jiang et al., 2008; Yin et al., 2010; Zhang et al., 2006). The photocatalytic activity of BiVO4 depends strongly on its crystalline form (Kohtani et al., 2003; Li et al., 2009; Zhang et al., 2007) and morphology (Liu et al., 2010; Wang et al., 2009). BiVO4 possesses such three primary crystalline forms, as tetragonal zircon, monoclinic scheelite and tetragonal scheelite structures. Monoclinic scheelite BiVO4 owns a moderate band gap (2.40 eV) and exhibits a best photocatalytic activity under visible-light irradiation (Tokunaga et al., 2001). The photocatalytic degradation of ibuprofen was not reported via monoclinic BiVO4 under solar light. Hence, the aim of this study was to investigate the photocatalytic activity of monoclinic BiVO4 in the degradation of IBP, and to illustrate the effects of operational parameters. The photocatalytic degradation mechanism was studied by quenching analysis as well.
2.3. Characterization of BiVO4 photocatalysts The samples were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and ultravioletevisible (UVeVis) diffuse reflectance spectroscopy. The XRD patterns were acquired through X-ray diffractometer (ULTIMA-III, Rigaku, Japan). The SEM images were obtained with a field emission scanning electron microscope (SXL-30ESM, Philips, Netherlands). The absorption spectrum was measured with a UVevis spectrophotometer (UV-2450, Shimadzu, Japan). 2.4. Ibuprofen solution The 1 g L1 IBP stock solution was prepared with acetonitrile and reserves were stored in a refrigerator at 4 C. Experimental solutions were prepared by the addition of an appropriate volume of stock solution into a brown volumetric flask. Nitrogen was supplied to evaporate the acetonitrile completely. Ultrapure water was then added. Next, agitation was introduced to promote the process. Dissolution was facilitated using sonication. The pH value was subsequently adjusted through the addition of 1% sulfuric acid or sodium hydroxide solution. Finally, the required volumes were achieved. 2.5. Photocatalytic degradation of ibuprofen
2. Materials and methods
All experiments were performed within an XPA-7 photochemical reactor. Pyrex test tubes were adopted to contain the solutions. The irradiation was provided by a 350 W long-arc Xe lamp (l > 290 nm). Prior to illumination, the suspension was magnetically stirred in the dark for 1 h. The aim was to ensure the adsorption/desorption equilibrium of IBP with the photocatalyst powders. The samples were removed at different time intervals for analysis. Three parallel experiments were conducted for each group. The results of each group were averaged in the analysis.
2.1. Reagents
2.6. Analytical method
Bi(NO3)3$5H2O and NH4VO3 were purchased from Aladdin Industrial Corporation (Shanghai). Ibuprofen was purchased from TCI Development Co. Ltd. (Shanghai). Sulfuric acid, nitric acid, sodium hydroxide, acetic acid, ethanol, benzoquinone, isopropanol were purchased from Chengdu Kelong Chemical Reagent Co. Ltd. All of the reagents above were of analytical grade and required no further purification. The methanol and acetonitrile purchased from Anaqua Chemicals Supply Co. Ltd. (USA) were of HPLC grade. Ultrapure water from a TKA Smart 2 Pure system (Germany) was used in the preparation of all aqueous solutions.
Ibuprofen concentrations were determined with a HPLC system (Shimadzu, Japan). The chromatographic conditions were as follows. Column: Zorbax Eclipse XDB-C18 (2.1 mm 150 mm, 5 mm); Temperature: 30 C; Mobile phase: acetonitrile/acetic acid buffer solution (containing 0.3% acetic acid, 50:50, v/v); Flow rate: 0.2 mL min1; Injection volume: 4 mL; Photodiode array detector: 263 nm wavelength. 3. Results and discussion 3.1. Morphology and structure
2.2. Synthesis of BiVO4 photocatalysts The BiVO4 photocatalyst was prepared by a hydrothermal process. In a typical procedure, Bi(NO3)3$5H2O (5 mM) and NH4VO3 (5 mM) were dissolved in 40 mL of HNO3 (4 M) and 40 mL of NaOH (4 M) respectively. To obtain a stable homogeneous mixture, these two solutions were combined under ultrasonication for 10 min. The pH value of the final mixture was adjusted to 7 with the addition of NaOH. After standing for 24 h, the suspension was transferred into a 100 mL Teflon autoclave. The autoclave was filled up to 80% of the total volume. The hydrothermal treatment was performed at 180 C for 3 h. Subsequently, the product was collected by filtration and rinsed several times with ultrapure water and ethanol. Finally, the products were dried at 80 C for 6 h in ambient air (Ge, 2008; Zhang et al., 2006).
The diffraction pattern (Fig. 1) shows that all of the peaks could be indexed as the standard card (JCPDS Card No. 14-0688) of monoclinic BiVO4 (Yu and Kudo, 2005; Zhou et al., 2006) and no other peaks from possible impurities were detected. The XRD analysis results indicated that the prepared photocatalyst corresponded to phase-pure monoclinic scheelite BiVO4. The SEM micrographs (Fig. 2) of the BiVO4 sample indicates that the prepared BiVO4 was in the form of microspheres. The surfaces of the sample were formed by many small particle agglomerations. These surfaces were uneven, and part of which were comprised of hollow structures. The UVeVis DRS of the BiVO4 sample is illustrated in this work (Fig. 3). Based on the equation of Eg ¼ 1240/lg, the Eg value of the BiVO4 sample was obtained as 2.32 eV. The Eg value is slightly lower than, or similar to, those of the BiVO4 materials reported in
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visible-light absorption wavelength is 535 nm. Consequently, the synthesized monoclinic BiVO4 photocatalyst was expected to show superior photocatalytic properties under visible-light irradiation. 3.2. Photocatalytic activity The photocatalytic performance of the BiVO4 is revealed in this section (Fig. 4). It can be seen that the photodegradation and dark adsorption could not efficiently remove IBP. However, the removal efficiency of IBP attained by the photocatalytic process was up to 90%. This result indicates that the catalyst showed excellent photocatalytic activity under simulated sunlight. 3.3. Effect of pH The effect of pH on the degradation rate of IBP was determined after irradiation for 40 min (Fig. 5a). It can be seen that catalysis was more likely to occur under acidic conditions than under neutral or alkaline conditions. The removal rate of IBP was maximized at the pH value of 4.5. This is because that IBP is a weak acid (4.52 pKa 4.9) (Madhavan et al., 2010; Mendez-Arriaga et al., 2008; Rafols et al., 1997), while the point of zero charge of BiVO4 is
Fig. 1. XRD pattern of the BiVO4 sample.
Fig. 2. SEM images of the BiVO4.
Fig. 3. UVevis diffuse reflectance spectra of the BiVO4.
the literature (Jiang et al., 2012; Zhu et al., 2013). Its maximum
Fig. 4. Dark adsorption, photodegradation and photocatalytic degradation of IBP under simulated solar irradiation. [IBP]0 ¼ 10 mg L1, [BiVO4] ¼ 2.0 g L1, pH ¼ 4.5. Error bars indicate one standard deviation.
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Fig. 5. Influence of pH value (a) and catalyst concentration (b) on the degradation of IBP. a: [IBP]0 ¼ 10 mg L1, [BiVO4] ¼ 2.0 g L1 b: [IBP]0 ¼ 10 mg L1, pH ¼ 4.5. Error bars indicate one standard deviation.
widely reported to be pH ¼ 2.7 (Xie et al., 2006). In weak acidic medium (2.7 pH 4.9), IBP is electrically neutral and exists in molecular form, while BiVO4 is deprotonated and electronegative. This facilitated the adsorption of IBP to the surface of the catalyst. The contact probability of IBP with reactive oxygen species (ROS) and hþ were increased then. However, in strong acidic media (pH < 2.7), BiVO4 is electropositive and the concentration of Hþ is excessive. This condition will lead to the reductions of OH. Hence, the removal rate of IBP was decreased. In neutral and alkaline media (pH > 4.9), IBP is electronegative due to ionization. Therefore, it can repel electronegative BiVO4 and OH. Several studies have shown that high concentrations of OH enabled CO2 to enter 2 into the solution. CO2 3 was produced consequently. The CO3 is a radical scavenger (Kashif and Ou-Yang, 2009). 3.4. Effect of catalyst concentration The effect of BiVO4 concentration on the degradation rate was investigated after irradiation for 25 min (Fig. 5b). The degradation rate of IBP increased as the BiVO4 concentration was increased. When the latter was increased to 5.0 g L1, the degradation rate was maximized, at rates reaching 91%. It has been reported that TiO2/ UV-A photocatalytic systems is able to degrade 92.6% of the original IBP after a treatment time of 120 min (Silva et al., 2014). To achieve the same degradation rate, BiVO4 need less time than TiO2. On the other hand, BiVO4 photocatalytic systems improves the costeffectiveness through photoactivation with solar irradiation. Therefore, the synthesized BiVO4 showed superior photocatalytic properties under the irradiation of visible-light. As BiVO4 concentration was further increased, the degradation rate was shown to decrease. This was primarily due to ROS. The concentration of ROS played a critical role in the photocatalytic oxidation reactions. More ROS were produced with higher BiVO4 concentration. However, when the BiVO4 concentration reached a certain level, the shielding and reflection of light by the catalyst resulted in a decrease of light utilization. The production of ROS was decreased as a result. In addition, the overuse of the BiVO4 would result in the aggregation of catalyst. The aggregation phenomenon consequently reduced the contact area between BiVO4, light and IBP. 3.5. Effect of substrate concentration The effect of IBP concentration on degradation rate was studied after irradiation for 40 min (Fig. 6). It can be seen that the
Fig. 6. Influence of substrate concentration on the degradation [BiVO4] ¼ 2.0 g L1, pH ¼ 4.5. Error bars indicate one standard deviation.
of
IBP.
degradation process of IBP could be fitted by the first-order kinetics model. The degradation rate of IBP decreased as the substrate concentration was increased. This result can be explained by considering the competition for absorption of the limited quantity of active sites by the IBP. Because the initial concentration of IBP increased but the number of active sites did not change, the number of active sites available per molecule of IBP decreased, so the degradation rate of IBP decreased.
3.6. Mechanism of ibuprofen photocatalytic degradation 3.6.1. Impact of benzoquinone on photocatalytic degradation of IBP Quenching experiments were employed to study removal mechanism of IBP (Fig. 7a). The O 2 was quenched with the addition of benzoquinone (Chen et al., 2008; Zhang et al., 2011). Then the degradation rate constant declined from 0.0622 min1 to 0.0094 min1. The phenomenon indicated that the contribution rate of O 2 was 84.9%. It was accordingly inferred that the photocatalytic degradation of IBP primarily via the generation of O 2 , as indicated in the following Eqs. (1)e(3).
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Fig. 7. Effect of quenching agents (a) and dissolve oxygen (b) on the degradation of IBP. [IBP]0 ¼ 10 mg L1, [BiVO4] ¼ 2.0 g L1, pH ¼ 4.5. Error bars indicate one standard deviation.
BiVO4 þ hv/BiVO4 þ e þ hþ
(1)
O2 þ e /O$ 2
(2)
* IBP þ O$ 2 /IBPproduct
(3)
3.6.2. Impact of isopropanol on photocatalytic degradation of IBP The OH was quenched with the addition of isopropanol(Buxton et al., 1988; Vione et al., 2001). Then the degradation rate constant decreased from 0.0622 min1 to 0.0309 min1. It could be determined that the contribution rate of OH was 50.3%. In addition to the reaction of holes (hþ) and H2O, the OH was also further transformed from O 2 . IBP degradation was achieved through the reaction with the OH radicals, as shown in Eqs. (4)e(8).
H2 O þ hþ /$OH þ Hþ
(4)
þ O$ 2 þ H /$HO2
(5)
2$HO2 /H2 O2 þ O2 $
(6)
O$ 2 þ H2 O2 /$OH
(7)
* IBP þ $OH/IBPproduct
(8)
3.6.3. Impact of methanol on photocatalytic degradation of IBP When methanol was introduced, both hþ and OH were quenched (Hu et al., 2007; Im et al., 2012; Santato et al., 2001). Compared with the addition of isopropanol, the degradation rate constant (0.0326 min1) was slightly increased. An inference was obtained in this work that the direct oxidation by the hþ was less significant in the IBP photocatalytic oxidation process. The tiny difference between the rate constants of methanol and isopropanol is probably due to that the quenching of hþ and OH reduced the recombination of electronehole pair. Afterward, the concentration of electrons was increased. Then the formation of ROS was facilited.
3.6.4. Impact of dissolved oxygen on photocatalytic degradation of IBP Dissolved oxygen concentration is an important parameter. Experiments were conducted to study the effect of dissolved oxygen in the solution, using nitrogen or oxygen as purging gas. Nitrogen and oxygen was bubbled through the solution to regulate the dissolved oxygen concentration. The gas was bubbled through the solution for at least 30 min before each experiment. As can be seen from Fig. 7b, the degradation rate of IBP subsequent to the introduction of O2 was much higher than that with a supply of N2. The degradation rate constant increased from 0.0360 min1 to 0.0639 min1. The supply of O2 served to enhance the formation of O 2 .This phenomenon further indicated that the degradation of IBP was due to the generation of O 2 . 4. Conclusions Phase-pure monoclinic scheelite BiVO4 was synthesized through a hydrothermal process. The prepared BiVO4 showed good performance in the degradation of IBP under simulated solar irradiation. The degradation process could be depicted by first-order kinetics model. In the condition of constant catalyst volume, the degradation rate of IBP decreased with an increasing initial concentration of substrate. The optimal conditions for IBP degradation were that the concentration of BiVO4 was 5.0 g L1 and the pH value was 4.5. The photocatalytic degradation of IBP primarily via the generation of superoxide radical (O 2 ) and hydroxyl radicals ( OH). During the process of degradation, part of the OH was converted þ from the O 2 . The direct oxidation of holes (h ) made a minimal contribution to the degradation of IBP. This work showed that photocatalytic degradation of ibuprofen via monoclinic BiVO4 under solar light is feasible. Monoclinic BiVO4 has a broad prospects in the commercial application. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21377031). References Anandan, S., Ashokkumar, M., 2009. Sonochemical synthesis of AueTiO2 nanoparticles for the sonophotocatalytic degradation of organic pollutants in aqueous environment. Ultrason. Sonochemistry 16, 316e320. Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Visible-light
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