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reduced graphene oxide under visible-light irradiation
Journal of Environmental Management 232 (2019) 713–721
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Research article
Photolysis and photocatalysis of tetracycline by sonochemically heterojunctioned BiVO4/reduced graphene oxide under visible-light irradiation
T
Tayyebeh Soltani, Ahmad Tayyebi, Byeong-Kyu Lee∗ Department of Civil and Environmental Engineering, University of Ulsan, Nam-gu, Daehak-ro 93, Ulsan 44610, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Photolysis Tetracycline BVO/rGO nanocomposite Visible light Ultrasound
The widespread use of antibiotics in pharmaceutical therapies and agricultural practice has led to severe environmental pollution. In this study, the simultaneous photolysis and photocatalysis behaviors of tetracycline (TC), one of the most frequently prescribed groups of antibiotics, were investigated using BiVO4 (BVO) supported on reduced graphene oxide (rGO). The resulting BVO/rGO nanocomposite (NC) showed prominent adsorption performance and photocatalytic ability under wide initial pH conditions (from acidic to alkaline: pH 2.5, 6.7, 9.2 and 10.5). This study analyzed the kinetics and proposed a mechanism for the photolytic and photocatalytic degradation of TC under visible light irradiation with BVO and BVO/rGO. The photolysis and photocatalytic degradation efficiency of TC was largely influenced by the solution pH and increased with increasing initial pH. The TC was stable without significant photolysis at pH 2.5, while TC photolysis increased up to 17% at pH 9.2. With further increase in the solution pH from 9.2 to 10.5, the light absorption of TC at 356 nm showed a red shift to 372 nm and new absorption peaks at around 533 nm were formed due to the formation of new colored intermediates. The photocatalytic degradation activities of TC by BVO/rGO under visible light irradiation reached 55, 67, 92 and 99% at initial pH 2.5, 6.7, 9.2 and 10.5, respectively. However, when using BVO only, the photocatalytic degradation of TC was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. The great improvement of photocatalytic activity of BVO/rGO is attributed to the reduced particle size, increased adsorption ability of rGO, extended photo responding range of BVO, and efficient separation of photogenerated charge carriers, which are derived from the ultimate coverage of the BVO by the rGO.
1. Introduction Tetracycline (TC), one of the most frequently prescribed groups of antibiotics, has received considerable attention due to its application in veterinary medicines and aquaculture. After medication, due to its poor absorption by organisms, more than 70% of consumed TC is excreted in active form into the environment, causing growing concerns about its potential environmental impacts. Therefore, the development of new and efficient methods to decrease the discharge of TC antibiotics into the environment is required (Liu et al., 2018). Photolytic and photocatalytic processes has been extensively investigated for ameliorating environmental pollution problems using visible light or solar energy (Soltani and Lee, 2016b). Various photocatalysts, such as TiO2, WO3, ZnO, CdS, TiO2 and γ-Fe2O3 embedded in polyvinylalcohol–alginate beads, and Ag3PO4, have received considerable attention for the photocatalytic degradation of pollutants and the
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production of hydrogen from water splitting (Majidnia and Fulazzaky, 2017). However, many of these developed photocatalysts have a problem with photocorrosion, wide bandgap and high recombination rate of induced electron−hole pairs. Bismuth vanadate (BiVO4:BVO), is one of the promising n-type visible-light driven photocatalysts that demonstrates unique properties, such as bright color, non-toxicity, high stability against photocorrosion and high ferroelasticity. Among three different kinds of BVO crystal structure, the monoclinic structure can provide more effective utilization of visible light (Tayyebi et al., 2019) However, some limitations, such as low absorption, low conductivity, and the high recombination rate of charge carriers in BVO, make it more difficult to use in photocatalytic fields (Tayyebi et al., 2018). Furthermore, due to its very low solubility in water, BVO nanoparticles (NPs) are accumulated rapidly after mixing the precursors, which greatly hinders the fabrication of BVO crystals with uniform morphology and small particle size by
Corresponding author. E-mail address: [email protected] (B.-K. Lee).
https://doi.org/10.1016/j.jenvman.2018.11.133 Received 21 September 2017; Received in revised form 3 November 2018; Accepted 27 November 2018 0301-4797/ © 2018 Published by Elsevier Ltd.
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2.2.1. Preparation of graphene oxide (GO) GO nanosheets were synthesized from graphite powder according to the modified ultrasonic Hummers method that we have previously reported (Soltani and Lee, 2017).
traditional solution-phase methods (Xi and Ye, 2010). In order to solve these problems, researchers have focused on combining BVO with other materials to form nanocomposites (NCs) such as CuO/BVO (Jiang et al., 2009a), WO3/BVO (Chatchai et al., 2009), V2O5/BVO (Jiang et al., 2009b), Bi2O3/BVO (Li and Yan, 2009), BiFeO3/rGO (Soltani and Lee, 2016c) and graphene/BVO (Soltani et al., 2018). Reduced graphene oxide (rGO), having two-dimensional conjugated structure, has extremely high theoretical specific surface area (∼2600 m2 g−1) and excellent electrical conductivity because of its high abundance of delocalized electrons in its π - conjugated electronic structure. The superior carrier mobility (200000 cm2 V−1) and extended π-electron conjugation make graphene an excellent and costeffective redox mediator to improve photogenerated charge carrier transport between two or more semiconductors. (Li et al., 2011). Furthermore, the various and complex adsorptive sites of graphene surfaces contain polarized electron-rich and electron-depleted sites, surface defects, wrinkles, and functional groups attached to the surfaces and edges of the graphene nanosheets. In particular, graphene and chemically modified graphene (rGO) hold great promise for the adsorption of various organic and inorganic pollutants, due to their hydrophobic properties, strong π – π interactions of their aromatic 2D structure and also large surface area. Thus, a reliable way to improve the photocatalytic activity of pure BVO is coupling with graphene-related materials (e.g., rGO). Wang et al. reported that a considerable enhancement of the photoactivity of BVO has been demonstrated through the degradation of methylene blue upon the covering of rGO due to the increased surface adsorption effect of rGO (Wang et al., 2014). Xiong et al. reported that, after being incorporated with rGO, the leaf-like BVO displayed exhibited excellent performance in adsorption and photocatalytic degradation of RhB in aqueous solution due to the enhanced light harvesting efficiency, and the reduced charge recombination rate (Xiong et al., 2017). However, these reported studies have required complicated processes for BVO/rGO composite preparation. This necessitates the development of an easy and mild synthetic method for the synthesis of BVO/rGO NC. In this paper, we present a simple ultrasonic approach for the preparation of BVO/rGO NC with quick ultrasonic treatment to evaluate whether the use of BVO NPs embedded in rGO can have good efficiency for the adsorption and photocatalytic ability of TC from aqueous solution, compared to BVO. Also, the effects of rGO on the phase structure, vibrational mode, particle size, surface area, optical behavior and morphology of BVO are examined in detail. Photodegradation as well as a parallel experiment are performed for the photolysis of TC. The effect of solution pH on the photolytic and photocatalytic degradation is investigated to elucidate the different mechanisms at diverse pH values under visible light irradiation.
2.2.2. Preparation of BVO First, 0.008 mol of Bi (NO3)3·5H2O and 0.008 mol of NH4VO3 were dissolved in 40 mL of EG and 40 mL of hot water, respectively. After stirring both individually for 15 min, the NH4VO3 solution was slowly added to the Bi (NO3)3·5H2O solution under vigorous stirring for 5 min and then irradiated in an ultrasonic bath at 35 °C for 30 min. The yellowish resultants obtained from the reaction were precipitated naturally and washed with pure ethanol and distilled water several times, and finally dried overnight at 80 °C to obtain the sample of BVO particles. 2.2.3. Preparation of BVO/GO A specific amount (weight fractions of 2%) of GO prepared from natural graphite powder by using our previously reported, modified ultrasonic hummer method was dispersed into 10 mL of ethanol and maintained in a sonication bath for 30 min to obtain a well-dispersed suspension. Then, prepared GO solution was added dropwise to the mixture of Bi (NO3)3·5H2O and NH4VO3 in EG/H2O solution and ultrasonically treated in the bath for another 30 min. The precipitation product was washed with ethanol and water and dried at 80 °C for 12 h to obtain BVO/GO. 2.2.4. Preparation of BVO/GO BVO/rGO nanocomposites were synthesized via simple visible-lightassisted photocatalytic reduction of graphene oxide (GO) by using BVO for the preparation of BVO/rGO. Typically, the as-prepared GO with weight fractions of 2% was dispersed in a 25 mL ethanol solution by ultrasonication, and then 0.2 g BVO was added into the solution. Following further sonication for 30 min in an ultrasonic bath at 30 °C, a homogeneous suspension was obtained. The suspensions were then illuminated using a 250 W Xe lamp (Oriel) with a 420 nm cut-off filter for another 60 min under sealed conditions. When the BVO/GO suspension was irradiated with visible light, its color changed from yellow brown to dark green because the π network was successfully restored within the carbon structure due to the BVO-induced reduction of the GO sheets (Soltani and Lee, 2016c). After centrifugation, the dark green precipitate product was washed with ethanol and DI water three times and dried in an oven at 70 °C for 14 h to obtain the BVO/rGO. 2.3. Characterization and equipment The crystal structure of the obtained nanocrystalline powders of nanomaterials were determined using X-ray diffraction (XRD; Bruker D8Advance) with monochromatic Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 10–80° at room temperature (RT). The Raman vibrational frequency was analyzed with the aid of a labRam HR microRaman spectrophotometer (Bruker, model: Senteraa 2009, Germany) with the 785 nm laser line as excitation source at RT. The morphology of the BVO and BVO/rGO was examined using a scanning electron microscope (SEM), Model Quanta 250 FEG (Field Emission Gun). The particle size, shape, and surface of the nanomaterials were examined using high resolution transmission electron microscopy (HR-TEM) with JEOL TEM Model 2100. The chemical states of the elements in the nanomaterials was provided by the X-ray photoelectron spectroscopy (XPS) data recorded on a Thermo Scientific Sigma Probe spectrometer with a monochromatic AlKα source (photon energy 1486.6 eV), spot size 400 μm, pass energy of 200 eV and energy step size of 1.0 eV. The absorbance of the TC at λmax of 365 nm was obtained with a Genesis 10S UV–Vis spectrophotometer. High-performance liquid chromatography (HPLC; Thermo Scientific Dionex Model UltiMate®3000) was used to monitor the formation of reaction intermediates. A C18 column
2. Materials and methods 2.1. Chemicals and materials Graphite powder (purity 99%, mesh 325), potassium permanganate (KMnO4), sulfuric acid (H2SO4), phosphoric acid (H3PO4), hydrochloric acid (HCl), hydrogen peroxide (H2O2), ethylene glycol (EG), ammonium metavanadate (NH4VO3) and TC were obtained from Merck and used without further purification. Hydrogen peroxide (30%, H2O2) and Bismuth nitrate (Bi (NO3)3·5H2O) purchased from Fluka were used without further purification. Isopropanol (IP) and ethylenediaminetetraacetic acid disodium (EDTA-2Na) of analytical reagent grade were used without further purification. Ethanol without further purification and deionized (DI) water were used for the sample preparation. DI was used throughout the entire GO synthesis procedure. 2.2. Preparation of nanoparticles (NPs) Typical synthesis process for BVO/GO NC is described below. 714
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(25 cm × 4.6 mm I.D., 5 m, Germany) with a UV detector at wavelengths of 275 and 356 nm was utilized under stable conditions (temperature: 25 °C, injection volume: 100 μL). The HPLC mobile phase was 0.01 M oxalic acid and acetonitrile at a volume ratio of 1:1 and flow rate of 1.0 mL/min.
2.4. Activity test This study focused on the application of BVO/rGO as an efficient photocatalyst for quick degradation of TC from aqueous solution. Aqueous suspensions (50 mL) containing 45 mg of BVO or BVO/rGO and 25 mg L−1 of TC in a 250 mL Erlenmeyer flasks were prepared. Before the visible light irradiation, TC was allowed to adsorb on the surface of the catalyst in dark for 40 min to establish adsorption-desorption equilibrium. Then, the amount of adsorption was calculated by using the following formula:
qe = (C0 − Ce ) m / v
Fig. 1. XRD patterns of GO, pure BVO, and BVO/rGO. The inset of Fig. 1 shows photographs of the pure BVO and BVO/rGO NCs obtained.
(1)
where qe (mg g−1) represents the amount of adsorption at equilibrium, C0 and Ce (mg L−1) represent the initial concentration and the equilibrium concentration of TC in the solution, respectively, V (L) represents the volume of the solution, and m (g) represents the dosage of the adsorbent. The glass reactor was then illuminated directly exposed to a 55 W fluorescent lamp with an emission peak at 550 nm under continuous magnetic shaking. The distance between the solution and the lamps was approximately 30 cm. The solution was placed in a water bath to maintain a temperature range of 28–30 °C. At each designated time interval for photolysis experiment, about 5 mL of the reaction solution was sampled and the liquid phase was easily separated from the photocatalyst. However, the photocatalytic experiment was performed separately (without intermediate samplings). The UV–detector with a maximum absorption wavelength of 365 nm was adjusted for TC analysis. After repeating the experiment three times, the absorption of the TC solution was converted to the TC concentration using a standard curve. In order to investigate the effect of pH on TC photodegradation, the photolysis and photodegradation experiments were conducted at different pHs (pH 2.5, 6.7, 9.2, and 10.5) by adding HCl or NaOH into the TC solution. Fig. 2. (a) UV–visible diffuse reflectance spectra and the inset shows the estimated band gaps of BVO and BVO/rGO.
3. Results and discussion 3.1.2. UV–vis absorption spectra The UV–vis absorption spectra of GO, BVO and the BVO/rGO heterostructure are shown in Fig. 2. GO presents a peak located at 225 nm, related to the π-π transitions of the aromatic C]C band, and a weak shoulder at 305 nm, ascribed to the n - π transitions of the C]O band in GO (Paredes et al., 2008). Pure BVO exhibits an absorption edge at around 600 nm, showing good visible light response. In comparison to BVO, BVO/rGO exhibits slightly stronger visible-light absorption and a red shift of absorption edge due to the absorption of rGO (Yin et al., 2011). Due to the incorporation of rGO, BVO/rGO possessed much stronger visible-light absorption than BVO in the range of 600–800 nm. The band gaps of bare BVO and BVO/rGO based on Tauc's equation (Butler, 1977) are estimated to be 2.30 and 2.21 eV, respectively. The imperceptible band gap narrowing could be related to the formation of BieC bonds in BVO/rGO through chemical bonding between BVO and rGO (Soltani et al., 2018).
3.1. Characterization of BVO/rGO nanocomposite (NC) 3.1.1. XRD results Fig. 1 displays the XRD patterns of the as-prepared BVO and 2% BVO/rGO NC. Typical peaks of GO (001) at around 2θ = 9.35° are observed in the GO curve. The sharp XRD peaks of the as-prepared BVO and BVO/rGO with high crystallinity and without any secondary phase are in good agreement with pure monoclinic phase BVO (JCPDS 140688). Three peaks at around 19.0°, 35.2°, and 46.0°, which can be indexed to the Scheelite structure of BVO with active monoclinic phase (Tokunaga et al., 2001), can be clearly observed in the patterns of the BVO particles and BVO/rGO NCs. The monoclinic phase is the most active phase among the three phases of BVO for O2 evolution under visible light irradiation (Tokunaga et al., 2001). The absence of any impure peaks for BVO and BVO/rGO NC suggested the high purity and crystallinity of the samples. Furthermore, the structural change of the (001) plane of GO to the (002) plane of rGO in BVO/rGO implied that GO had been reduced to rGO through the ultrasonic reduction process (Krishnamoorthy et al., 2013). Thus, the proposed ultrasonic reduction process is a highly efficient, simple and green method for preparing the composites. The inset of Fig. 1 shows photographs of the pure BVO and BVO/rGO NCs obtained.
3.1.3. FT-IR analysis The presence of the characteristic peaks of GO, BVO and BVO/rGO was confirmed by FT-IR and the results are shown in Fig. S1.
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Fig. 3. XPS of the core-level C 1s electrons in (a) GO and (b) BVO/rGO.
3.1.4. XPS analysis The surface chemical states of various elements, including C, O, Bi, and V, in the BVO/rGO NCs were confirmed by XPS analysis and the results are shown in Fig. S2. Fig. 3 shows the C 1s, Bi 4f, V 2p and O1s XPS spectra with Gaussian fitting for BVO/rGO NCs. The appearance of carbon peaks confirms the existence of rGO in the BVO/rGO nanostructures. As shown in Fig. 3a, the C1s peak is deconvoluted into three peaks, corresponding to carbons in the form of sp2 bonds at 284.6 eV, and oxygen-containing functional groups at 286.6 eV (CeO, epoxy, and hydroxyl) and 288.9 eV (C]O, carboxyl), respectively. The fairly low peak intensity for oxygenated functional groups in BVO/rGO represented that most of the oxygenous functional groups in GO were removed by ultrasonic reduction method. In the case of Bi, the deconvoluted peaks centered at 159.0 eV and 165 correspond to the elemental composition of Bi 4f7/2 and Bi 4f7/2, respectively (Fig. 3b). Besides, the split peaks for V 2p were identified at binding energies 524.2 eV for V2p1/2 and 516.7 eV for V 2p3/2, as shown in Fig. 3c. The identically located binding energy O 1s in BVO/rGO at 529.9 eV could be assigned to the lattice oxygen of BVO (Fig. 3c). However, another peak at 531.6 eV, which is ascribed to the surface hydroxyl groups, does not exist in the BVO/rGO system.
Fig. 4. Raman spectra of the as-synthesized GO, BVO and BVO/rGO.
2007), and the diminished D/G ratio of BVO/rGO indicates that the reduction of GO to rGO increased the average size of the graphene domains.
3.1.5. Raman analysis Raman spectroscopy analysis was used to investigate the true state of the carbon-based materials. Fig. 4 shows the Raman spectra of GO, BVO and BVO/rGO. Raman peaks at 1593 and 1352 cm−1 for GO correspond to the G and D-bands, respectively. Raman bands of around 820, 707, 366, 323, and 210 cm−1 for bare BVO are in agreement with previous reports (Tayyebi et al., 2018). As for the BVO/rGO NCs, besides the distinctive peaks assigned to BVO, the D and G bands are slightly blue- and red-shifted to 1346 and 1602 cm−1, respectively, which is likely due to the change in surface strain resulting from the contact between rGO and BVO. In general, the D/G ratio is inversely proportional to the average size of the sp2 domains (Stankovich et al.,
3.1.6. SEM analysis Fig. 5 shows the SEM images of GO, BVO and BVO/rGO NC. BVO consisted of dumbbell aggregated BVO particles with an average size of 1–2 μm. A wrapped dumbbell-like morphology was observed for BVO/ rGO, which is significantly smaller than that of bare BVO. When GO is introduced to BVO, this combination affects the nucleation and growth of BVO on the rGO surface, thereby improving the dispersion and adhesion of BVO particles on the GO sheets, decreasing the agglomeration of BVO NPs, and restraining the restacking of rGO (Novoselov et al., 2005). Thus, the dumbbell-like BVO NPs were densely covered with 716
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Fig. 5. SEM images of (a) GO, (b) BVO, and (c) BVO/rGO.
order model. It can be seen that the pseudo-second-order kinetics model, having a higher coefficient of determination (R2 > 0.995), was more appropriate for describing the adsorption kinetics than the pseudo-first-order kinetics model was. Fan et al. (2016) also reported that the pseudo-second-order kinetics model better explained the adsorption of TC onto hazelnut shell derived activated carbons.
rGO nanosheets, suggesting the possibility of sufficient contact and effective interfacial interaction between BVO and rGO. This intimate interaction enabled the electron transfer from BVO to rGO to facilitate the efficient charge separation within the NC during the TC photodegradation process. 3.2. . Adsorption of TC
3.2.2. Proposed adsorption mechanism The adsorption mechanisms of TC by graphene materials at various pHs are strongly dependent on the molecular structure of TC and the functional groups present on the surface of rGO, which can be interpreted by the π–π interaction, hydrophobic effect, hydrogen bonding and electrostatic interaction (Soltani and Lee, 2016a). The charges on TC and rGO are both dependent on pH. Numerous studies have reported the negatively charged graphene-based materials through all measured pHs due to the presence of carboxylic, hydroxyl, aldehyde and ketone groups (Soltani and Lee, 2016a). TC has several polar/ionizable groups containing amino, carboxyl, phenol, alcohol, and ketone. TC has three acid dissociation constants (pKa = 3.3, 7.7, and 9.7) and exists as a cationic (TCH3+), zwitterionic (TCH2), and anionic (TCH−, TC2−) species under acidic, moderately acidic to neutral, and alkaline conditions, respectively (Gu et al., 2007). Our previous study indicated that carboxyl and carbonyl groups on the edges of rGO are highly electrophilic, which can improve the interaction by hydrogen bonding with polar molecules in acidic and neutral pH (Soltani and Lee, 2016a). Thus, the adsorption of TC by BVO/rGO under acidic (pH 2.5) and neutral (pH 6.7) conditions can be attributed to hydrogen bonding. However, the lower oxygen functional groups and increased π-π electron delocalization through the π-bond structure in rGO reduced the TC adsorption by rGO in acidic and neutral media. In basic media (pH 9.2), GO is negatively charged and TC is also mainly negatively charged (TCH−, TC2−). Thus, the previously proposed mechanisms operating through hydrogen bonding cannot be responsible for the strong adsorption of TC on BVO/rGO in the basic medium due to the resulting repulsion of similar charges in the graphene surfaces and TC. TC can be adsorbed onto the surface of BVO/rGO through π-π electron donoraccepter interaction (Riskin et al., 2008) between the TC molecules
3.2.1. Effect of contact time and kinetic studies The adsorption characteristics of TC by BVO and BVO/rGO at initial solution pHs of 2.5, 6.7, 9.2 and 10.5 were investigated with varying adsorption time between 0 and 50 min, and results are described in the supplementary information in Fig. S3. Fig. S3 (a) shows that the adsorbed amount of TC increased with increasing contact time, and then reached equilibrium within 20 min for BVO and 40 min for BVO/rGO. The adsorption capacity increased from 0.51 to 2.42 mg g−1 for BVO and from 3.62 to 8.71 mg g−1 for BVO/rGO as the solution pH increased from 2.5 to 10.5, respectively. This result represents that the TC adsorption capacity for BVO/rGO across the tested pH range (2.5–10.5) is higher than that for BVO. The adsorption kinetics was also investigated to describe the adsorption rate and mechanism with a determination of the controlling factor for TC adsorption. The kinetic data were estimated using the linearized pseudo-first-order and pseudo-second-models (Zhou et al., 2016) as expressed by Eqs (2) and (3), respectively:
log(qe − qt ) = logqe − K1 2.303 t t
1 t qt = K2 qe2 + qe
(2) (3)
where qe and qt are the maximum adsorption capacity (mg g−1) and equilibrium capacity (mg g−1) at the time t, respectively. K1 (min−1) and K2 (g mg−1 min−1) are the pseudo-first-order and pseudo-secondorder rate constants, respectively. Figs. S3b and S3c show the curve fitting for linear pseudo-first-order and pseudo-second-order kinetics for the experimental data of TC adsorption. The calculated values of kinetic parameters from the models are shown in Table S1 for the pseudo- first- order kinetics and in Table S2 for the pseudo-second– 717
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molecule was significantly inhibited in TC+ form but progressed well in TC− form. A completely different behavior occurred for the initial absorbance spectrum of TC after the pH was further increased from 9.2 to 10.5. The initial absorbance spectrum of TC was red shifted from 356 nm to 379 nm with higher absorbance (Fig. 6b). This behavior is related to the transfer of the TC molecule from TCo to the TC− anions associated with the transfer of the π-π * state (HOMO−1 to LUMO) of the TC chromophore. This behavior for transferring the TC molecule from TCo to TC− anions involves transferring the π-π * state of the TC chromophore (Santos et al., 2000). In addition, TC photolysis results at pH 10.5 revealed a photolysis efficiency of 62% and also showed the formation of a new and relatively small absorption peak in the visible region (533 nm) due to the formation of 4a,12a-anhydro-4-oxo-4-dedimethylaminotetracycline that originated from TC photolysis (Addamo et al., 2005). Furthermore, as shown in the inset of Fig. 6b, the color of the TC solution was changed from light yellow to pink, indicating the formation of new intermediates at pH 10.5. The formation of these colorful intermediates represented that the more polar intermediate compounds were formed in the solution during the photolysis processes due to the loss of some functional groups in TC (Wang et al., 2011a). 3.3.2. Photocatalytic degradation 3.3.2.1. Photodegradation comparison between BVO and BVO/rGO. The effect of pH on the TC photocatalytic degradation by BVO and BVO/ rGO was investigated at pH 2.5, 6.7, 9.2 and 10.5. As shown in Fig. 7, before the visible light irradiation, TC was allowed to adsorb on the surface of the catalyst in dark for 40 min to establish adsorptiondesorption equilibrium. Only 2–5% of the TC was adsorbed by BVO in the different pHs; however, the adsorption capacity of BVO/rGO increased from 12% to 30% as the solution pH was increased from 2.5 to 10.5. As Compared to BVO, BVO/rGO provided more adsorption sites for TC due to the relatively larger surface area of BVO/rGO. In a TC removal comparison between dark and visible light conditions, the removal efficiency of BVO and BVO/rGO NCs differed significantly. The photocatalytic activity of NPs increased as the solution pH was increased from 2.5 to 10.5. By using BVO, the TC photodegradation after 85 min visible light irradiation was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. Compared to BVO, BVO/ Fig. 6. Photolysis of TC at (a) pH of 2.5, 6.7 and 9.2, and (b) pH of 10.5. The inset shows the color change of the initial TC solution from light yellow to pink. This experiment was performed with sampling.
(electron acceptors) and graphene materials (electron donors). 3.3. Decomposition of tetracycline (TC) The photocatalytic activities of pure BVO and BVO/rGO NC were evaluated by checking the degradation of TC, at different initial pHs under visible-light irradiation. 3.3.1. Photolysis of TC The photolysis of TC is affected by the pH of the reaction solution. Fig. 6 compares the photolysis of TC with UV–Vis spectra in the range from 200 to 500 nm with initial pH varying from acidic to basic medium. (2.5, 6.7 (neutral), 9.2 and 10.5). As shown in Fig. 6a, TC photolysis at a neutral pH (6.7) is higher than that at pH 2.5, which indicates that TC is more stable at pH 2.5. The further increase in pH from 6.7 to pH 9.2 increased the photolysis up to 17%. The solution pH dependence of TC photolysis is linked to the protonation states and was monitored by analyzing the absorbance spectra of organic pollutants (Boreen et al., 2004). TC molecule is amphoteric with pKa values of 3.3, 7.7, 9.7 and 12 (Figueroa et al., 2004) and it exists in neutral and positively charged (TC+) forms at pH 4.0 and negatively charged (TC−) form at pH of 9.0. This information indicates that the photolysis of TC
Fig. 7. Photocatalytic degradation rates of TC using BVO/rGO and pure BVO photocatalysts at different initial pHs. This experiment was performed without any sampling. 718
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rGO NCs exhibited better photocatalytic performance at all tested pHs. Under the same conditions with BVO, 55, 67, 92% of TC was photodegraded from aqueous solution at pH 2.5, 6.7, 9.2 with BVO/rGO, respectively. However, the complete photocatalytic degradation of TC was reduced to 50 min when the pH was further increased from 9.2 to 10.5. TC has four possible molecular species that can be involved in protonation or deprotonation depending on the solution pH (Parolo et al., 2008). As the solution pH increases, the degree of deprotonation and consequently the degree of degradation increase accordingly. In addition, due to the high electrical density on the ring system, TC molecules with a negative charge tend to absorb reactive species such as •OH, thereby facilitating TC photolysis and photodegradation in base media (Jiao et al., 2008). Generally, the photocatalytic degradation of organic compounds with a photocatalyst depends on several parameters, including bandgap energy, crystal structure, specific surface area and crystalline size. The presence of rGO in the BVO/rGO NC played a vital role in capturing and shuttling electrons as an electron reservoir, which is also beneficial for electron transmission from BVO to rGO (Wang et al., 2013). Also, the decrease in the band gap energy from 2.30 for BVO to 2.21 eV for BVO/rGO could be another reason for the higher photocatalytic activity of BVO/rGO as compared to BVO. In addition, the introduction of rGO into BVO effectively increased the surface area for holding more TC and increasing the reaction interspace. As previously reported (Dong et al., 2016), the lower surface area of BVO is one of the drawbacks for improving photocatalytic activity. The nitrogen adsorption–desorption technique was applied to measure the specific surface area and pore structure of the BVO and BVO/rGO samples. As can be seen in Fig. S4, the isotherm of BVO is of type IV with hysteresis loops in the relative range of 0.68–1.0 for BVO and 0.46–1.0 for BVO/rGO. Furthermore, the BET specific surface area of BVO/rGO (1.4 m2/g) was much larger than that of BVO (0.80 m2 g−1). The BET data clearly demonstrated that after the introduction of only 2% GO into the BVO structure, BVO/rGO showed a 1.75-fold larger surface area than BVO, which is beneficial for overcoming the lower photocatalytic activity of BVO.
Fig. 8. Temporal evolution of the spectra during the photodegradation of TC mediated by BVO/rGO under visible light. The inset shows the change of initial pH during photodegradation at different irradiation times and also the solution color change from light yellow (initial TC) to colorless after 50 min of photocatalytic degradation
information. In the photolysis process, some new intermediate products with high intensity and shorter retention time were identified after 85 min visible light irradiation. After 50 min visible light irradiation in the photodegradation process, the initial TC peak had almost disappeared and only one negligible peak remained, which indicated the complete mineralization of the TC during the photocatalytic process. In general, complete mineralization of organic contaminants decreases their toxicity to organisms. In contrast, the presence of photolysis products may increase the toxicity (Soderquist and Crosby, 1975). However (Jiao et al., 2008), recently evaluated the toxicity effect of TC photolysis using luminescent bacteria. They found that the TC photolysis products increased the high adversity risk on bacteria. In the current study, the photolysis products were determined using HPLC data analysis, as shown in Fig. S5b. The spectra show the newly emerged peaks at a retention time of 2.4 min, which might be associated with some photolysis intermediates generated from the visible irradiation at high pHs (> pH 9.7). These generated intermediates could be associated with certain toxic substances to bacteria according to the previous report of new peaks identified at retention times of 1.9 and 2.2 min (Jiao et al., 2008). The photolysis efficiency of 62% at an initial pH of 10.5 is indicative of the low stability of TC at this pH. However, in these conditions, TC is mainly transformed to new products rather than being completely mineralized. This necessitates further investigation of the photocatalytic degradation of TC in aqueous solution using BVO or BVO/rGO as a photocatalyst under visible irradiation to its mineralization to small molecules such as CO2, H2O, and NH3 (Thi and Lee, 2017).
3.3.2.2. UV–vis spectra. The UV–vis spectrum of TC in the range of 290–550 nm as a function of the time of visible light irradiation (λ > 420 nm) is shown in Fig. 8. The UV spectrum of TC consists of three specific absorption bands at 357 nm, 275 nm and 250 nm corresponding mainly to π-π * transitions (Wang et al., 2011b). As shown in Fig. 8, the peak intensities of all absorption bands decreased with increasing irradiation time. However, the decay of the absorbance at 357 nm band was faster than that at 275 nm. This means that the ring containing the N-groups, which is responsible for the absorbance at 275 nm, is barely opened, in contrast to the other rings (Dalmázio et al., 2007). The visible band may have disappeared with increasing reaction time due to the fragmentation of phenolic groups connected to aromatic rings by radical species produced in the photocatalytic medium. When the radiation time was increased to 50 min, all TC absorption peaks disappear completely, implying that BVO/rGO, as a promising photocatalyst material, can completely photodegrade TC in aqueous solution within 50 min. The solution color changed from light yellow (initial TC) to colorless after 50 min of photocatalytic degradation, as shown in the inset of Fig. 8, which also shows a fast decrease from an initial pH 6.7 to pH 4.5 after 40 min of light irradiation, followed by a slight decrease to pH 4.1 as the irradiation time was increased to 60 min. The pH decrease indicates that some intermediates may have been produced in the form of organic acids during the TC degradation (Zhu et al., 2013).
3.4. Possible photodegradation mechanism Based on the empirical equation, the valence band potential (VVB) of BVO NPs is about 2.67 V, which is lower than the standard redox potential of OH, H+/H2O (2.7 V versus NHE) (Shi et al., 2017) (Scheme 1). Therefore, the photogenerated hole cannot oxidize the H2O to form •OH. Even though the photoinduced electrons in the conduction band
3.3.3. Identification of intermediates HPLC-UV chromatograms for the initial TC peak, as well as the peaks from the photolysis and photocatalytic degradation of TC after 85 min of visible light irradiation, are shown in Fig. S5 in supporting 719
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Soltani, T., Lee, B.-K., 2017. Low intensity-ultrasonic irradiation for highly efficient, ecofriendly and fast synthesis of graphene oxide. Ultrason. Sonochem. 38, 693–703. Soltani, T., Tayyebi, A., Lee, B.-K., 2018. Efficient promotion of charge separation with
Scheme 1. Schematic illustration of the proposed mechanism for photogenerated charge carrier transfer in the BVO/rGO NCs under visible light irradiation for TC degradation.
(CB) of BVO could not reduce O2 to produce •O2−, due to the edge potential of BVO (+0.46 eV) that is more positive than that of O2/•O2−(−0.33 eV vs. NHE), these electrons in the CB of BVO easily transferred to the surface of rGO where they were utilized for reducing O2 to •O2−(Chen et al., 2017). Based on these results, we therefore tentatively concluded that hv+ and •O2− are the main active species in TC photodegradation, whereas •OH has little contribution to the TC photodegradation, as shown in Scheme 1. Furthermore, complete coverage of rGO sheets on the BVO surfaces accelerates the transfer of photogenerated electrons from BVO to rGO to achieve efficient charge separation (Tan et al., 2012). 4. Conclusions This study investigated the photolysis and photocatalysis behaviors of TC after fabricating BVO/rGO NCs to improve the photocatalytic activities of bare BVO. The photolysis and photocatalytic degradation of TC by BVO and BVO/rGO were highly dependent on the initial pH and greatly improved as the initial pH was increased from 2.5 to 10.5. TC is completely stable against photolysis at pH 2.5. However, the TC photolysis slightly increased to 17% as the initial pH was increased from 2.5 to 9.2. With further pH increase up to 10.5, the light absorption of TC at 356 nm was red-shifted to 372 nm with an intensity reduction of 62%. In addition, new absorption peaks at around 533 nm were observed due to the formation of new colored intermediates. Using only BVO, the photocatalytic degradation of TC was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. BVO/rGO exhibited greatly improved photocatalytic activity of TC, as compared with BVO, reaching 55, 67, 92 and 99% at the corresponding pHs, respectively. This great improvement is due to the effective incorporation of rGO into BVO in BVO/rGO, which inhibited the aggregation of BVO, improved the adsorption capacity of TC, and reduced the recombination rate of photogenerated electrons and holes. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Education) (No. 2016R1D1A1B03931215). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2018.11.133. 720
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reduced graphene oxide (rGO) in BiVO4/rGO photoanode for greatly enhanced photoelectrochemical water splitting. Sol. Energy Mater. Sol. Cells 185, 325–332. Stankovich, S., Dikin, D.A., Piner, R.D., Kohlhaas, K.A., Kleinhammes, A., Jia, Y., Wu, Y., Nguyen, S.T., Ruoff, R.S., 2007. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 45, 1558–1565. Tan, L.-L., Chai, S.-P., Mohamed, A.R., 2012. Synthesis and applications of graphenebased TiO2 photocatalysts. ChemSusChem 5, 1868–1882. Tayyebi, A., Soltani, T., Hong, H., Lee, B.-K., 2018. Improved photocatalytic and photoelectrochemical performance of monoclinic bismuth vanadate by surface defect states (Bi1-xVO4). J. Colloid Interface Sci. 514, 565–575. Tayyebi, A., Soltani, T., Lee, B.-K., 2019. Effect of pH on photocatalytic and photoelectrochemical (PEC) properties of monoclinic bismuth vanadate. J. Colloid Interface Sci. 37–46. Thi, V.H.T., Lee, B.-K., 2017. Great improvement on tetracycline removal using ZnO rodactivated carbon fiber composite prepared with a facile microwave method. J. Hazard Mater. 324, 329–339. Tokunaga, S., Kato, H., Kudo, A., 2001. Selective preparation of monoclinic and tetragonal BiVO4 with scheelite structure and their photocatalytic properties. Chem. Mater. 13, 4624–4628. Wang, J., Wang, P., Cao, Y., Chen, J., Li, W., Shao, Y., Zheng, Y., Li, D., 2013. A high efficient photocatalyst Ag3VO4/TiO2/graphene nanocomposite with wide spectral response. Appl. Catal. B Environ. 136–137, 94–102. Wang, P., Yap, P.-S., Lim, T.-T., 2011a. C–N–S tridoped TiO 2 for photocatalytic
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Research article
Photolysis and photocatalysis of tetracycline by sonochemically heterojunctioned BiVO4/reduced graphene oxide under visible-light irradiation
T
Tayyebeh Soltani, Ahmad Tayyebi, Byeong-Kyu Lee∗ Department of Civil and Environmental Engineering, University of Ulsan, Nam-gu, Daehak-ro 93, Ulsan 44610, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Photolysis Tetracycline BVO/rGO nanocomposite Visible light Ultrasound
The widespread use of antibiotics in pharmaceutical therapies and agricultural practice has led to severe environmental pollution. In this study, the simultaneous photolysis and photocatalysis behaviors of tetracycline (TC), one of the most frequently prescribed groups of antibiotics, were investigated using BiVO4 (BVO) supported on reduced graphene oxide (rGO). The resulting BVO/rGO nanocomposite (NC) showed prominent adsorption performance and photocatalytic ability under wide initial pH conditions (from acidic to alkaline: pH 2.5, 6.7, 9.2 and 10.5). This study analyzed the kinetics and proposed a mechanism for the photolytic and photocatalytic degradation of TC under visible light irradiation with BVO and BVO/rGO. The photolysis and photocatalytic degradation efficiency of TC was largely influenced by the solution pH and increased with increasing initial pH. The TC was stable without significant photolysis at pH 2.5, while TC photolysis increased up to 17% at pH 9.2. With further increase in the solution pH from 9.2 to 10.5, the light absorption of TC at 356 nm showed a red shift to 372 nm and new absorption peaks at around 533 nm were formed due to the formation of new colored intermediates. The photocatalytic degradation activities of TC by BVO/rGO under visible light irradiation reached 55, 67, 92 and 99% at initial pH 2.5, 6.7, 9.2 and 10.5, respectively. However, when using BVO only, the photocatalytic degradation of TC was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. The great improvement of photocatalytic activity of BVO/rGO is attributed to the reduced particle size, increased adsorption ability of rGO, extended photo responding range of BVO, and efficient separation of photogenerated charge carriers, which are derived from the ultimate coverage of the BVO by the rGO.
1. Introduction Tetracycline (TC), one of the most frequently prescribed groups of antibiotics, has received considerable attention due to its application in veterinary medicines and aquaculture. After medication, due to its poor absorption by organisms, more than 70% of consumed TC is excreted in active form into the environment, causing growing concerns about its potential environmental impacts. Therefore, the development of new and efficient methods to decrease the discharge of TC antibiotics into the environment is required (Liu et al., 2018). Photolytic and photocatalytic processes has been extensively investigated for ameliorating environmental pollution problems using visible light or solar energy (Soltani and Lee, 2016b). Various photocatalysts, such as TiO2, WO3, ZnO, CdS, TiO2 and γ-Fe2O3 embedded in polyvinylalcohol–alginate beads, and Ag3PO4, have received considerable attention for the photocatalytic degradation of pollutants and the
∗
production of hydrogen from water splitting (Majidnia and Fulazzaky, 2017). However, many of these developed photocatalysts have a problem with photocorrosion, wide bandgap and high recombination rate of induced electron−hole pairs. Bismuth vanadate (BiVO4:BVO), is one of the promising n-type visible-light driven photocatalysts that demonstrates unique properties, such as bright color, non-toxicity, high stability against photocorrosion and high ferroelasticity. Among three different kinds of BVO crystal structure, the monoclinic structure can provide more effective utilization of visible light (Tayyebi et al., 2019) However, some limitations, such as low absorption, low conductivity, and the high recombination rate of charge carriers in BVO, make it more difficult to use in photocatalytic fields (Tayyebi et al., 2018). Furthermore, due to its very low solubility in water, BVO nanoparticles (NPs) are accumulated rapidly after mixing the precursors, which greatly hinders the fabrication of BVO crystals with uniform morphology and small particle size by
Corresponding author. E-mail address: [email protected] (B.-K. Lee).
https://doi.org/10.1016/j.jenvman.2018.11.133 Received 21 September 2017; Received in revised form 3 November 2018; Accepted 27 November 2018 0301-4797/ © 2018 Published by Elsevier Ltd.
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2.2.1. Preparation of graphene oxide (GO) GO nanosheets were synthesized from graphite powder according to the modified ultrasonic Hummers method that we have previously reported (Soltani and Lee, 2017).
traditional solution-phase methods (Xi and Ye, 2010). In order to solve these problems, researchers have focused on combining BVO with other materials to form nanocomposites (NCs) such as CuO/BVO (Jiang et al., 2009a), WO3/BVO (Chatchai et al., 2009), V2O5/BVO (Jiang et al., 2009b), Bi2O3/BVO (Li and Yan, 2009), BiFeO3/rGO (Soltani and Lee, 2016c) and graphene/BVO (Soltani et al., 2018). Reduced graphene oxide (rGO), having two-dimensional conjugated structure, has extremely high theoretical specific surface area (∼2600 m2 g−1) and excellent electrical conductivity because of its high abundance of delocalized electrons in its π - conjugated electronic structure. The superior carrier mobility (200000 cm2 V−1) and extended π-electron conjugation make graphene an excellent and costeffective redox mediator to improve photogenerated charge carrier transport between two or more semiconductors. (Li et al., 2011). Furthermore, the various and complex adsorptive sites of graphene surfaces contain polarized electron-rich and electron-depleted sites, surface defects, wrinkles, and functional groups attached to the surfaces and edges of the graphene nanosheets. In particular, graphene and chemically modified graphene (rGO) hold great promise for the adsorption of various organic and inorganic pollutants, due to their hydrophobic properties, strong π – π interactions of their aromatic 2D structure and also large surface area. Thus, a reliable way to improve the photocatalytic activity of pure BVO is coupling with graphene-related materials (e.g., rGO). Wang et al. reported that a considerable enhancement of the photoactivity of BVO has been demonstrated through the degradation of methylene blue upon the covering of rGO due to the increased surface adsorption effect of rGO (Wang et al., 2014). Xiong et al. reported that, after being incorporated with rGO, the leaf-like BVO displayed exhibited excellent performance in adsorption and photocatalytic degradation of RhB in aqueous solution due to the enhanced light harvesting efficiency, and the reduced charge recombination rate (Xiong et al., 2017). However, these reported studies have required complicated processes for BVO/rGO composite preparation. This necessitates the development of an easy and mild synthetic method for the synthesis of BVO/rGO NC. In this paper, we present a simple ultrasonic approach for the preparation of BVO/rGO NC with quick ultrasonic treatment to evaluate whether the use of BVO NPs embedded in rGO can have good efficiency for the adsorption and photocatalytic ability of TC from aqueous solution, compared to BVO. Also, the effects of rGO on the phase structure, vibrational mode, particle size, surface area, optical behavior and morphology of BVO are examined in detail. Photodegradation as well as a parallel experiment are performed for the photolysis of TC. The effect of solution pH on the photolytic and photocatalytic degradation is investigated to elucidate the different mechanisms at diverse pH values under visible light irradiation.
2.2.2. Preparation of BVO First, 0.008 mol of Bi (NO3)3·5H2O and 0.008 mol of NH4VO3 were dissolved in 40 mL of EG and 40 mL of hot water, respectively. After stirring both individually for 15 min, the NH4VO3 solution was slowly added to the Bi (NO3)3·5H2O solution under vigorous stirring for 5 min and then irradiated in an ultrasonic bath at 35 °C for 30 min. The yellowish resultants obtained from the reaction were precipitated naturally and washed with pure ethanol and distilled water several times, and finally dried overnight at 80 °C to obtain the sample of BVO particles. 2.2.3. Preparation of BVO/GO A specific amount (weight fractions of 2%) of GO prepared from natural graphite powder by using our previously reported, modified ultrasonic hummer method was dispersed into 10 mL of ethanol and maintained in a sonication bath for 30 min to obtain a well-dispersed suspension. Then, prepared GO solution was added dropwise to the mixture of Bi (NO3)3·5H2O and NH4VO3 in EG/H2O solution and ultrasonically treated in the bath for another 30 min. The precipitation product was washed with ethanol and water and dried at 80 °C for 12 h to obtain BVO/GO. 2.2.4. Preparation of BVO/GO BVO/rGO nanocomposites were synthesized via simple visible-lightassisted photocatalytic reduction of graphene oxide (GO) by using BVO for the preparation of BVO/rGO. Typically, the as-prepared GO with weight fractions of 2% was dispersed in a 25 mL ethanol solution by ultrasonication, and then 0.2 g BVO was added into the solution. Following further sonication for 30 min in an ultrasonic bath at 30 °C, a homogeneous suspension was obtained. The suspensions were then illuminated using a 250 W Xe lamp (Oriel) with a 420 nm cut-off filter for another 60 min under sealed conditions. When the BVO/GO suspension was irradiated with visible light, its color changed from yellow brown to dark green because the π network was successfully restored within the carbon structure due to the BVO-induced reduction of the GO sheets (Soltani and Lee, 2016c). After centrifugation, the dark green precipitate product was washed with ethanol and DI water three times and dried in an oven at 70 °C for 14 h to obtain the BVO/rGO. 2.3. Characterization and equipment The crystal structure of the obtained nanocrystalline powders of nanomaterials were determined using X-ray diffraction (XRD; Bruker D8Advance) with monochromatic Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 10–80° at room temperature (RT). The Raman vibrational frequency was analyzed with the aid of a labRam HR microRaman spectrophotometer (Bruker, model: Senteraa 2009, Germany) with the 785 nm laser line as excitation source at RT. The morphology of the BVO and BVO/rGO was examined using a scanning electron microscope (SEM), Model Quanta 250 FEG (Field Emission Gun). The particle size, shape, and surface of the nanomaterials were examined using high resolution transmission electron microscopy (HR-TEM) with JEOL TEM Model 2100. The chemical states of the elements in the nanomaterials was provided by the X-ray photoelectron spectroscopy (XPS) data recorded on a Thermo Scientific Sigma Probe spectrometer with a monochromatic AlKα source (photon energy 1486.6 eV), spot size 400 μm, pass energy of 200 eV and energy step size of 1.0 eV. The absorbance of the TC at λmax of 365 nm was obtained with a Genesis 10S UV–Vis spectrophotometer. High-performance liquid chromatography (HPLC; Thermo Scientific Dionex Model UltiMate®3000) was used to monitor the formation of reaction intermediates. A C18 column
2. Materials and methods 2.1. Chemicals and materials Graphite powder (purity 99%, mesh 325), potassium permanganate (KMnO4), sulfuric acid (H2SO4), phosphoric acid (H3PO4), hydrochloric acid (HCl), hydrogen peroxide (H2O2), ethylene glycol (EG), ammonium metavanadate (NH4VO3) and TC were obtained from Merck and used without further purification. Hydrogen peroxide (30%, H2O2) and Bismuth nitrate (Bi (NO3)3·5H2O) purchased from Fluka were used without further purification. Isopropanol (IP) and ethylenediaminetetraacetic acid disodium (EDTA-2Na) of analytical reagent grade were used without further purification. Ethanol without further purification and deionized (DI) water were used for the sample preparation. DI was used throughout the entire GO synthesis procedure. 2.2. Preparation of nanoparticles (NPs) Typical synthesis process for BVO/GO NC is described below. 714
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(25 cm × 4.6 mm I.D., 5 m, Germany) with a UV detector at wavelengths of 275 and 356 nm was utilized under stable conditions (temperature: 25 °C, injection volume: 100 μL). The HPLC mobile phase was 0.01 M oxalic acid and acetonitrile at a volume ratio of 1:1 and flow rate of 1.0 mL/min.
2.4. Activity test This study focused on the application of BVO/rGO as an efficient photocatalyst for quick degradation of TC from aqueous solution. Aqueous suspensions (50 mL) containing 45 mg of BVO or BVO/rGO and 25 mg L−1 of TC in a 250 mL Erlenmeyer flasks were prepared. Before the visible light irradiation, TC was allowed to adsorb on the surface of the catalyst in dark for 40 min to establish adsorption-desorption equilibrium. Then, the amount of adsorption was calculated by using the following formula:
qe = (C0 − Ce ) m / v
Fig. 1. XRD patterns of GO, pure BVO, and BVO/rGO. The inset of Fig. 1 shows photographs of the pure BVO and BVO/rGO NCs obtained.
(1)
where qe (mg g−1) represents the amount of adsorption at equilibrium, C0 and Ce (mg L−1) represent the initial concentration and the equilibrium concentration of TC in the solution, respectively, V (L) represents the volume of the solution, and m (g) represents the dosage of the adsorbent. The glass reactor was then illuminated directly exposed to a 55 W fluorescent lamp with an emission peak at 550 nm under continuous magnetic shaking. The distance between the solution and the lamps was approximately 30 cm. The solution was placed in a water bath to maintain a temperature range of 28–30 °C. At each designated time interval for photolysis experiment, about 5 mL of the reaction solution was sampled and the liquid phase was easily separated from the photocatalyst. However, the photocatalytic experiment was performed separately (without intermediate samplings). The UV–detector with a maximum absorption wavelength of 365 nm was adjusted for TC analysis. After repeating the experiment three times, the absorption of the TC solution was converted to the TC concentration using a standard curve. In order to investigate the effect of pH on TC photodegradation, the photolysis and photodegradation experiments were conducted at different pHs (pH 2.5, 6.7, 9.2, and 10.5) by adding HCl or NaOH into the TC solution. Fig. 2. (a) UV–visible diffuse reflectance spectra and the inset shows the estimated band gaps of BVO and BVO/rGO.
3. Results and discussion 3.1.2. UV–vis absorption spectra The UV–vis absorption spectra of GO, BVO and the BVO/rGO heterostructure are shown in Fig. 2. GO presents a peak located at 225 nm, related to the π-π transitions of the aromatic C]C band, and a weak shoulder at 305 nm, ascribed to the n - π transitions of the C]O band in GO (Paredes et al., 2008). Pure BVO exhibits an absorption edge at around 600 nm, showing good visible light response. In comparison to BVO, BVO/rGO exhibits slightly stronger visible-light absorption and a red shift of absorption edge due to the absorption of rGO (Yin et al., 2011). Due to the incorporation of rGO, BVO/rGO possessed much stronger visible-light absorption than BVO in the range of 600–800 nm. The band gaps of bare BVO and BVO/rGO based on Tauc's equation (Butler, 1977) are estimated to be 2.30 and 2.21 eV, respectively. The imperceptible band gap narrowing could be related to the formation of BieC bonds in BVO/rGO through chemical bonding between BVO and rGO (Soltani et al., 2018).
3.1. Characterization of BVO/rGO nanocomposite (NC) 3.1.1. XRD results Fig. 1 displays the XRD patterns of the as-prepared BVO and 2% BVO/rGO NC. Typical peaks of GO (001) at around 2θ = 9.35° are observed in the GO curve. The sharp XRD peaks of the as-prepared BVO and BVO/rGO with high crystallinity and without any secondary phase are in good agreement with pure monoclinic phase BVO (JCPDS 140688). Three peaks at around 19.0°, 35.2°, and 46.0°, which can be indexed to the Scheelite structure of BVO with active monoclinic phase (Tokunaga et al., 2001), can be clearly observed in the patterns of the BVO particles and BVO/rGO NCs. The monoclinic phase is the most active phase among the three phases of BVO for O2 evolution under visible light irradiation (Tokunaga et al., 2001). The absence of any impure peaks for BVO and BVO/rGO NC suggested the high purity and crystallinity of the samples. Furthermore, the structural change of the (001) plane of GO to the (002) plane of rGO in BVO/rGO implied that GO had been reduced to rGO through the ultrasonic reduction process (Krishnamoorthy et al., 2013). Thus, the proposed ultrasonic reduction process is a highly efficient, simple and green method for preparing the composites. The inset of Fig. 1 shows photographs of the pure BVO and BVO/rGO NCs obtained.
3.1.3. FT-IR analysis The presence of the characteristic peaks of GO, BVO and BVO/rGO was confirmed by FT-IR and the results are shown in Fig. S1.
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Fig. 3. XPS of the core-level C 1s electrons in (a) GO and (b) BVO/rGO.
3.1.4. XPS analysis The surface chemical states of various elements, including C, O, Bi, and V, in the BVO/rGO NCs were confirmed by XPS analysis and the results are shown in Fig. S2. Fig. 3 shows the C 1s, Bi 4f, V 2p and O1s XPS spectra with Gaussian fitting for BVO/rGO NCs. The appearance of carbon peaks confirms the existence of rGO in the BVO/rGO nanostructures. As shown in Fig. 3a, the C1s peak is deconvoluted into three peaks, corresponding to carbons in the form of sp2 bonds at 284.6 eV, and oxygen-containing functional groups at 286.6 eV (CeO, epoxy, and hydroxyl) and 288.9 eV (C]O, carboxyl), respectively. The fairly low peak intensity for oxygenated functional groups in BVO/rGO represented that most of the oxygenous functional groups in GO were removed by ultrasonic reduction method. In the case of Bi, the deconvoluted peaks centered at 159.0 eV and 165 correspond to the elemental composition of Bi 4f7/2 and Bi 4f7/2, respectively (Fig. 3b). Besides, the split peaks for V 2p were identified at binding energies 524.2 eV for V2p1/2 and 516.7 eV for V 2p3/2, as shown in Fig. 3c. The identically located binding energy O 1s in BVO/rGO at 529.9 eV could be assigned to the lattice oxygen of BVO (Fig. 3c). However, another peak at 531.6 eV, which is ascribed to the surface hydroxyl groups, does not exist in the BVO/rGO system.
Fig. 4. Raman spectra of the as-synthesized GO, BVO and BVO/rGO.
2007), and the diminished D/G ratio of BVO/rGO indicates that the reduction of GO to rGO increased the average size of the graphene domains.
3.1.5. Raman analysis Raman spectroscopy analysis was used to investigate the true state of the carbon-based materials. Fig. 4 shows the Raman spectra of GO, BVO and BVO/rGO. Raman peaks at 1593 and 1352 cm−1 for GO correspond to the G and D-bands, respectively. Raman bands of around 820, 707, 366, 323, and 210 cm−1 for bare BVO are in agreement with previous reports (Tayyebi et al., 2018). As for the BVO/rGO NCs, besides the distinctive peaks assigned to BVO, the D and G bands are slightly blue- and red-shifted to 1346 and 1602 cm−1, respectively, which is likely due to the change in surface strain resulting from the contact between rGO and BVO. In general, the D/G ratio is inversely proportional to the average size of the sp2 domains (Stankovich et al.,
3.1.6. SEM analysis Fig. 5 shows the SEM images of GO, BVO and BVO/rGO NC. BVO consisted of dumbbell aggregated BVO particles with an average size of 1–2 μm. A wrapped dumbbell-like morphology was observed for BVO/ rGO, which is significantly smaller than that of bare BVO. When GO is introduced to BVO, this combination affects the nucleation and growth of BVO on the rGO surface, thereby improving the dispersion and adhesion of BVO particles on the GO sheets, decreasing the agglomeration of BVO NPs, and restraining the restacking of rGO (Novoselov et al., 2005). Thus, the dumbbell-like BVO NPs were densely covered with 716
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Fig. 5. SEM images of (a) GO, (b) BVO, and (c) BVO/rGO.
order model. It can be seen that the pseudo-second-order kinetics model, having a higher coefficient of determination (R2 > 0.995), was more appropriate for describing the adsorption kinetics than the pseudo-first-order kinetics model was. Fan et al. (2016) also reported that the pseudo-second-order kinetics model better explained the adsorption of TC onto hazelnut shell derived activated carbons.
rGO nanosheets, suggesting the possibility of sufficient contact and effective interfacial interaction between BVO and rGO. This intimate interaction enabled the electron transfer from BVO to rGO to facilitate the efficient charge separation within the NC during the TC photodegradation process. 3.2. . Adsorption of TC
3.2.2. Proposed adsorption mechanism The adsorption mechanisms of TC by graphene materials at various pHs are strongly dependent on the molecular structure of TC and the functional groups present on the surface of rGO, which can be interpreted by the π–π interaction, hydrophobic effect, hydrogen bonding and electrostatic interaction (Soltani and Lee, 2016a). The charges on TC and rGO are both dependent on pH. Numerous studies have reported the negatively charged graphene-based materials through all measured pHs due to the presence of carboxylic, hydroxyl, aldehyde and ketone groups (Soltani and Lee, 2016a). TC has several polar/ionizable groups containing amino, carboxyl, phenol, alcohol, and ketone. TC has three acid dissociation constants (pKa = 3.3, 7.7, and 9.7) and exists as a cationic (TCH3+), zwitterionic (TCH2), and anionic (TCH−, TC2−) species under acidic, moderately acidic to neutral, and alkaline conditions, respectively (Gu et al., 2007). Our previous study indicated that carboxyl and carbonyl groups on the edges of rGO are highly electrophilic, which can improve the interaction by hydrogen bonding with polar molecules in acidic and neutral pH (Soltani and Lee, 2016a). Thus, the adsorption of TC by BVO/rGO under acidic (pH 2.5) and neutral (pH 6.7) conditions can be attributed to hydrogen bonding. However, the lower oxygen functional groups and increased π-π electron delocalization through the π-bond structure in rGO reduced the TC adsorption by rGO in acidic and neutral media. In basic media (pH 9.2), GO is negatively charged and TC is also mainly negatively charged (TCH−, TC2−). Thus, the previously proposed mechanisms operating through hydrogen bonding cannot be responsible for the strong adsorption of TC on BVO/rGO in the basic medium due to the resulting repulsion of similar charges in the graphene surfaces and TC. TC can be adsorbed onto the surface of BVO/rGO through π-π electron donoraccepter interaction (Riskin et al., 2008) between the TC molecules
3.2.1. Effect of contact time and kinetic studies The adsorption characteristics of TC by BVO and BVO/rGO at initial solution pHs of 2.5, 6.7, 9.2 and 10.5 were investigated with varying adsorption time between 0 and 50 min, and results are described in the supplementary information in Fig. S3. Fig. S3 (a) shows that the adsorbed amount of TC increased with increasing contact time, and then reached equilibrium within 20 min for BVO and 40 min for BVO/rGO. The adsorption capacity increased from 0.51 to 2.42 mg g−1 for BVO and from 3.62 to 8.71 mg g−1 for BVO/rGO as the solution pH increased from 2.5 to 10.5, respectively. This result represents that the TC adsorption capacity for BVO/rGO across the tested pH range (2.5–10.5) is higher than that for BVO. The adsorption kinetics was also investigated to describe the adsorption rate and mechanism with a determination of the controlling factor for TC adsorption. The kinetic data were estimated using the linearized pseudo-first-order and pseudo-second-models (Zhou et al., 2016) as expressed by Eqs (2) and (3), respectively:
log(qe − qt ) = logqe − K1 2.303 t t
1 t qt = K2 qe2 + qe
(2) (3)
where qe and qt are the maximum adsorption capacity (mg g−1) and equilibrium capacity (mg g−1) at the time t, respectively. K1 (min−1) and K2 (g mg−1 min−1) are the pseudo-first-order and pseudo-secondorder rate constants, respectively. Figs. S3b and S3c show the curve fitting for linear pseudo-first-order and pseudo-second-order kinetics for the experimental data of TC adsorption. The calculated values of kinetic parameters from the models are shown in Table S1 for the pseudo- first- order kinetics and in Table S2 for the pseudo-second– 717
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molecule was significantly inhibited in TC+ form but progressed well in TC− form. A completely different behavior occurred for the initial absorbance spectrum of TC after the pH was further increased from 9.2 to 10.5. The initial absorbance spectrum of TC was red shifted from 356 nm to 379 nm with higher absorbance (Fig. 6b). This behavior is related to the transfer of the TC molecule from TCo to the TC− anions associated with the transfer of the π-π * state (HOMO−1 to LUMO) of the TC chromophore. This behavior for transferring the TC molecule from TCo to TC− anions involves transferring the π-π * state of the TC chromophore (Santos et al., 2000). In addition, TC photolysis results at pH 10.5 revealed a photolysis efficiency of 62% and also showed the formation of a new and relatively small absorption peak in the visible region (533 nm) due to the formation of 4a,12a-anhydro-4-oxo-4-dedimethylaminotetracycline that originated from TC photolysis (Addamo et al., 2005). Furthermore, as shown in the inset of Fig. 6b, the color of the TC solution was changed from light yellow to pink, indicating the formation of new intermediates at pH 10.5. The formation of these colorful intermediates represented that the more polar intermediate compounds were formed in the solution during the photolysis processes due to the loss of some functional groups in TC (Wang et al., 2011a). 3.3.2. Photocatalytic degradation 3.3.2.1. Photodegradation comparison between BVO and BVO/rGO. The effect of pH on the TC photocatalytic degradation by BVO and BVO/ rGO was investigated at pH 2.5, 6.7, 9.2 and 10.5. As shown in Fig. 7, before the visible light irradiation, TC was allowed to adsorb on the surface of the catalyst in dark for 40 min to establish adsorptiondesorption equilibrium. Only 2–5% of the TC was adsorbed by BVO in the different pHs; however, the adsorption capacity of BVO/rGO increased from 12% to 30% as the solution pH was increased from 2.5 to 10.5. As Compared to BVO, BVO/rGO provided more adsorption sites for TC due to the relatively larger surface area of BVO/rGO. In a TC removal comparison between dark and visible light conditions, the removal efficiency of BVO and BVO/rGO NCs differed significantly. The photocatalytic activity of NPs increased as the solution pH was increased from 2.5 to 10.5. By using BVO, the TC photodegradation after 85 min visible light irradiation was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. Compared to BVO, BVO/ Fig. 6. Photolysis of TC at (a) pH of 2.5, 6.7 and 9.2, and (b) pH of 10.5. The inset shows the color change of the initial TC solution from light yellow to pink. This experiment was performed with sampling.
(electron acceptors) and graphene materials (electron donors). 3.3. Decomposition of tetracycline (TC) The photocatalytic activities of pure BVO and BVO/rGO NC were evaluated by checking the degradation of TC, at different initial pHs under visible-light irradiation. 3.3.1. Photolysis of TC The photolysis of TC is affected by the pH of the reaction solution. Fig. 6 compares the photolysis of TC with UV–Vis spectra in the range from 200 to 500 nm with initial pH varying from acidic to basic medium. (2.5, 6.7 (neutral), 9.2 and 10.5). As shown in Fig. 6a, TC photolysis at a neutral pH (6.7) is higher than that at pH 2.5, which indicates that TC is more stable at pH 2.5. The further increase in pH from 6.7 to pH 9.2 increased the photolysis up to 17%. The solution pH dependence of TC photolysis is linked to the protonation states and was monitored by analyzing the absorbance spectra of organic pollutants (Boreen et al., 2004). TC molecule is amphoteric with pKa values of 3.3, 7.7, 9.7 and 12 (Figueroa et al., 2004) and it exists in neutral and positively charged (TC+) forms at pH 4.0 and negatively charged (TC−) form at pH of 9.0. This information indicates that the photolysis of TC
Fig. 7. Photocatalytic degradation rates of TC using BVO/rGO and pure BVO photocatalysts at different initial pHs. This experiment was performed without any sampling. 718
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rGO NCs exhibited better photocatalytic performance at all tested pHs. Under the same conditions with BVO, 55, 67, 92% of TC was photodegraded from aqueous solution at pH 2.5, 6.7, 9.2 with BVO/rGO, respectively. However, the complete photocatalytic degradation of TC was reduced to 50 min when the pH was further increased from 9.2 to 10.5. TC has four possible molecular species that can be involved in protonation or deprotonation depending on the solution pH (Parolo et al., 2008). As the solution pH increases, the degree of deprotonation and consequently the degree of degradation increase accordingly. In addition, due to the high electrical density on the ring system, TC molecules with a negative charge tend to absorb reactive species such as •OH, thereby facilitating TC photolysis and photodegradation in base media (Jiao et al., 2008). Generally, the photocatalytic degradation of organic compounds with a photocatalyst depends on several parameters, including bandgap energy, crystal structure, specific surface area and crystalline size. The presence of rGO in the BVO/rGO NC played a vital role in capturing and shuttling electrons as an electron reservoir, which is also beneficial for electron transmission from BVO to rGO (Wang et al., 2013). Also, the decrease in the band gap energy from 2.30 for BVO to 2.21 eV for BVO/rGO could be another reason for the higher photocatalytic activity of BVO/rGO as compared to BVO. In addition, the introduction of rGO into BVO effectively increased the surface area for holding more TC and increasing the reaction interspace. As previously reported (Dong et al., 2016), the lower surface area of BVO is one of the drawbacks for improving photocatalytic activity. The nitrogen adsorption–desorption technique was applied to measure the specific surface area and pore structure of the BVO and BVO/rGO samples. As can be seen in Fig. S4, the isotherm of BVO is of type IV with hysteresis loops in the relative range of 0.68–1.0 for BVO and 0.46–1.0 for BVO/rGO. Furthermore, the BET specific surface area of BVO/rGO (1.4 m2/g) was much larger than that of BVO (0.80 m2 g−1). The BET data clearly demonstrated that after the introduction of only 2% GO into the BVO structure, BVO/rGO showed a 1.75-fold larger surface area than BVO, which is beneficial for overcoming the lower photocatalytic activity of BVO.
Fig. 8. Temporal evolution of the spectra during the photodegradation of TC mediated by BVO/rGO under visible light. The inset shows the change of initial pH during photodegradation at different irradiation times and also the solution color change from light yellow (initial TC) to colorless after 50 min of photocatalytic degradation
information. In the photolysis process, some new intermediate products with high intensity and shorter retention time were identified after 85 min visible light irradiation. After 50 min visible light irradiation in the photodegradation process, the initial TC peak had almost disappeared and only one negligible peak remained, which indicated the complete mineralization of the TC during the photocatalytic process. In general, complete mineralization of organic contaminants decreases their toxicity to organisms. In contrast, the presence of photolysis products may increase the toxicity (Soderquist and Crosby, 1975). However (Jiao et al., 2008), recently evaluated the toxicity effect of TC photolysis using luminescent bacteria. They found that the TC photolysis products increased the high adversity risk on bacteria. In the current study, the photolysis products were determined using HPLC data analysis, as shown in Fig. S5b. The spectra show the newly emerged peaks at a retention time of 2.4 min, which might be associated with some photolysis intermediates generated from the visible irradiation at high pHs (> pH 9.7). These generated intermediates could be associated with certain toxic substances to bacteria according to the previous report of new peaks identified at retention times of 1.9 and 2.2 min (Jiao et al., 2008). The photolysis efficiency of 62% at an initial pH of 10.5 is indicative of the low stability of TC at this pH. However, in these conditions, TC is mainly transformed to new products rather than being completely mineralized. This necessitates further investigation of the photocatalytic degradation of TC in aqueous solution using BVO or BVO/rGO as a photocatalyst under visible irradiation to its mineralization to small molecules such as CO2, H2O, and NH3 (Thi and Lee, 2017).
3.3.2.2. UV–vis spectra. The UV–vis spectrum of TC in the range of 290–550 nm as a function of the time of visible light irradiation (λ > 420 nm) is shown in Fig. 8. The UV spectrum of TC consists of three specific absorption bands at 357 nm, 275 nm and 250 nm corresponding mainly to π-π * transitions (Wang et al., 2011b). As shown in Fig. 8, the peak intensities of all absorption bands decreased with increasing irradiation time. However, the decay of the absorbance at 357 nm band was faster than that at 275 nm. This means that the ring containing the N-groups, which is responsible for the absorbance at 275 nm, is barely opened, in contrast to the other rings (Dalmázio et al., 2007). The visible band may have disappeared with increasing reaction time due to the fragmentation of phenolic groups connected to aromatic rings by radical species produced in the photocatalytic medium. When the radiation time was increased to 50 min, all TC absorption peaks disappear completely, implying that BVO/rGO, as a promising photocatalyst material, can completely photodegrade TC in aqueous solution within 50 min. The solution color changed from light yellow (initial TC) to colorless after 50 min of photocatalytic degradation, as shown in the inset of Fig. 8, which also shows a fast decrease from an initial pH 6.7 to pH 4.5 after 40 min of light irradiation, followed by a slight decrease to pH 4.1 as the irradiation time was increased to 60 min. The pH decrease indicates that some intermediates may have been produced in the form of organic acids during the TC degradation (Zhu et al., 2013).
3.4. Possible photodegradation mechanism Based on the empirical equation, the valence band potential (VVB) of BVO NPs is about 2.67 V, which is lower than the standard redox potential of OH, H+/H2O (2.7 V versus NHE) (Shi et al., 2017) (Scheme 1). Therefore, the photogenerated hole cannot oxidize the H2O to form •OH. Even though the photoinduced electrons in the conduction band
3.3.3. Identification of intermediates HPLC-UV chromatograms for the initial TC peak, as well as the peaks from the photolysis and photocatalytic degradation of TC after 85 min of visible light irradiation, are shown in Fig. S5 in supporting 719
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Soltani, T., Lee, B.-K., 2017. Low intensity-ultrasonic irradiation for highly efficient, ecofriendly and fast synthesis of graphene oxide. Ultrason. Sonochem. 38, 693–703. Soltani, T., Tayyebi, A., Lee, B.-K., 2018. Efficient promotion of charge separation with
Scheme 1. Schematic illustration of the proposed mechanism for photogenerated charge carrier transfer in the BVO/rGO NCs under visible light irradiation for TC degradation.
(CB) of BVO could not reduce O2 to produce •O2−, due to the edge potential of BVO (+0.46 eV) that is more positive than that of O2/•O2−(−0.33 eV vs. NHE), these electrons in the CB of BVO easily transferred to the surface of rGO where they were utilized for reducing O2 to •O2−(Chen et al., 2017). Based on these results, we therefore tentatively concluded that hv+ and •O2− are the main active species in TC photodegradation, whereas •OH has little contribution to the TC photodegradation, as shown in Scheme 1. Furthermore, complete coverage of rGO sheets on the BVO surfaces accelerates the transfer of photogenerated electrons from BVO to rGO to achieve efficient charge separation (Tan et al., 2012). 4. Conclusions This study investigated the photolysis and photocatalysis behaviors of TC after fabricating BVO/rGO NCs to improve the photocatalytic activities of bare BVO. The photolysis and photocatalytic degradation of TC by BVO and BVO/rGO were highly dependent on the initial pH and greatly improved as the initial pH was increased from 2.5 to 10.5. TC is completely stable against photolysis at pH 2.5. However, the TC photolysis slightly increased to 17% as the initial pH was increased from 2.5 to 9.2. With further pH increase up to 10.5, the light absorption of TC at 356 nm was red-shifted to 372 nm with an intensity reduction of 62%. In addition, new absorption peaks at around 533 nm were observed due to the formation of new colored intermediates. Using only BVO, the photocatalytic degradation of TC was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. BVO/rGO exhibited greatly improved photocatalytic activity of TC, as compared with BVO, reaching 55, 67, 92 and 99% at the corresponding pHs, respectively. This great improvement is due to the effective incorporation of rGO into BVO in BVO/rGO, which inhibited the aggregation of BVO, improved the adsorption capacity of TC, and reduced the recombination rate of photogenerated electrons and holes. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Education) (No. 2016R1D1A1B03931215). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2018.11.133. 720
Journal of Environmental Management 232 (2019) 713–721
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