CH3COO(BiO) heterostructured nanocomposite

Science of the Total Environment 647 (2019) 245–254

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Highly efficient photocatalytic removal of multiple refractory organic pollutants by BiVO4/CH3COO(BiO) heterostructured nanocomposite Xia Zhang, Yuhao Ma, Lulu Xi, Guifen Zhu, Xiang Li, Dongyang Shi, Jing Fan ⁎ School of Environment, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huai River Water Environment and Pollution Control, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, 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

• One-pot facile synthesis of BiVO4/ CH3COO(BiO) heterojunction was presented. • The material showed high degradation and mineralization rate for multiple pollutants. • The degradation mechanism of 4-AAP and IBP was proposed. • Good activity of the heterojunction was maintained in real wastewater treatment.

a r t i c l e

i n f o

Article history: Received 15 June 2018 Received in revised form 27 July 2018 Accepted 30 July 2018 Available online 31 July 2018 Editor: Paola Verlicchi Keywords: BiVO4/CH3COO(BiO) Heterojunction Photocatalysis Degradation Organic pollutant Mechanism

a b s t r a c t Highly efficient photocatalytic degradation of refractory organic contaminants in wastewater remains a great challenge due to low quantum efficiency and poor solar energy utilization of the currently employed photocatalysts. Herein, a novel BiVO4/CH3COO(BiO) heterojunction photocatalyst is designed and prepared by a simple one-pot solvothermal method, and characterized by various techniques. By using this photocatalyst, degradation efficiency of four kinds of emerging refractory organic pollutants (sulfamethoxazole, bisphenol A, 4 aminoantipyrine and ibuprofen) in water is investigated under simulated solar irradiation. Then, total organic carbon is measured to determine the mineralization degree, and degradation intermediates of the pollutants are identified to propose their degradation pathway. It is found that under the given conditions, complete degradation of the pollutants is observed within the irradiation of 5–24 h, and 81–96% mineralization degree is achieved in 24 h. Furthermore, it is shown that the degradation kinetics can be described by pseudo-first order model. Based on the detected intermediates during the degradation process of 4 aminoantipyrine and ibuprofen, the degradation pathways of these two pollutants are suggested to involve cleavage of side chain, heterocyclic ring opening and hydroxylation of aromatic ring. In addition, the application of the BiVO4/CH3COO(BiO) heterojunction photocatalyst in the purification of the spiked real wastewater is also investigated. © 2018 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (J. Fan).

https://doi.org/10.1016/j.scitotenv.2018.07.450 0048-9697/© 2018 Elsevier B.V. All rights reserved.

Refractory organic contaminations are characterized by easy bioaccumulation and resistance to chemical oxidation and biological transformation due to their lipophilic and stable chemical structures. In

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recent decades, an increasing occurrence of emerging organic contaminants with potential hazards, such as pharmaceuticals, metabolite, and plasticizer, has been frequently reported in a wide variety of water (Giraldo et al., 2010). These compounds have become a worldwide concern due to their extensive use, release and widespread distribution in environment (Rosal et al., 2010). In particular, low concentration (ng/L or μg/L) of such contaminants can lead to the problems of chronic toxicity, endocrine disruption, and the development of pathogen resistance (Hernando et al., 2006; Luo et al., 2014; Murgolo et al., 2015). Common methods, such as biodegradation, adsorption and chemical oxidative technique, are often used to remove such toxic organic substances from environmental matrices (Alvarino et al., 2015; Elmolla et al., 2010; Li and Zhang, 2010). Unfortunately, the complexity, low efficiency, essential post-treatment and possible secondary pollution make these methods less efficient for practical use (Cui et al., 2016; Yan et al., 2016). Comparatively speaking, “green” and “sustainable” heterogeneous photocatalysis becomes a promising alternative approach at present and is expected to play an important role in the decomposition of such pollutants because of its comprehensive merits of simplicity, eco-friendly, low-cost, gentle operation, and capability of simultaneously removal of multiple pollutants in complex water matrix (Chala et al., 2014; Zhang et al., 2017). Recently, considerable attention has been drawn to the Bismuthbased organic acid salt semiconductor photocatalysts owing to their “green” elements composition and powerful oxidization properties resulted from deep valence band level dominated by O2p (Duan et al., 2010; Xiao et al., 2015; Yang et al., 2015). Bismuth oxide acetate (CH3COO(BiO)) is one of the most attractive photocatalysts and has been reported to exhibit excellent photocatalytic performance for the removal of organic dyes (Zhang et al., 2016). However, this photocatalyst can solely absorb UV light due to its wide bandgap, which greatly limits its applications. On the other hand, the photocatalytic efficiency of CH3COO(BiO) is also limited by the recombination of photogenerated e−/h+ pairs. Thus, efforts to reduce the recombination of the carriers and to improve its visible light responsive ability are extremely necessary. Contrast to individual photocatalyst, constructing composite photocatalyst has been recognized as effective strategy to simultaneously overcome the defect of the single photocatalyst (Grigioni et al., 2015). Particularly, the type-II semiconductor heterojunction with matchable band structures displays the best electron-hole separation, thus the selection of components for assembling unit composite system is crucial (Jia et al., 2017). Yellow Bismuth vanadate (BiVO 4 ) has been recognized as a promising candidate photocatalyst capable of utilizing visible light, and extensively explored for decomposition of organic contaminants and water splitting (García Pérez et al., 2011; Kudo and Miseki, 2009; Tan et al., 2016), but its poor photo-activity resulted from high recombination of the photogenerated e−/h+ pairs limits its extensive application. As a result, BiVO 4 is often employed to combine with other semiconductor photocatalyst to form heterojunction, leading to simultaneous improvement of visible light capture and separation efficiency of carriers, and thus to the enhancement of activity (Chatchai et al., 2013; Madhusudan et al., 2011). In this context, types of heterojunction, matching degree of band structure and interface contact of the two materials greatly affect the extent of catalytic activity enhancement. Therefore, further researches on the design, preparation and properties of novel BiVO4 based composite photocatalysts are of great significance for the practical application of photocatalysis technology. In this work, we have designed and synthesized new BiVO4/CH3COO (BiO) heterojunction by a facile simple one-pot solvothermal approach for the first time. Under simulated solar light irradiation, four representative emerging refractory organic compounds, including sulfamethoxazole, bisphenol A, 4 aminoantipyrine and ibuprofen, have been effectively degraded and mineralized by the BiVO4/CH3COO(BiO) heterojunction in water.

2. Materials and methods 2.1. Chemical reagents Bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O), ammonium metavadate (NH4VO3), nitric acid (HNO3, 65%), ammonia (NH3·H2O, 25%) and glacial acetic acid (CH3COOH) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Sulfamethoxazole (SMX), bisphenol A (BPA), 4 aminoantipyrine (4 AAP) and ibuprofen (IBP) were acquired from Sigma-Aldrich (Steinheim, Germany). All these chemicals were analytical grade and used as received. Deionized water was used in solution preparations. Here, sulfamethoxazole, bisphenol A, 4 aminoantipyrine and ibuprofen were selected as model compounds of pharmaceuticals, drug intermediates and endocrine disrupting chemicals, which are typical refractory organic contaminations. The selection of such organic compounds was based on their widespread occurrence in domestic sewage and diverse physicochemical properties (Gong and Chu, 2016; Kim et al., 2014; Loos et al., 2013). The chemical structure and main properties of these contaminants were summarized in Table 1.

2.2. Preparation of the photocatalysts For the preparation of BiVO4/CH3COO(BiO) composite, one-step solvothermal method was developed. In a typical procedure, 5.0 mmol of Bi (NO3)3·5H2O was dissolved in 30 mL of mixed solvent of glacial acetic acid and deionized water (v/v = 2:1) to form solution A. Meanwhile, another solution was prepared by dissolving certain amount of NH4VO3 in 20 mL of diluted ammonia water (v/v = 1:1). Then, the ammonia solution containing NH4VO3 was added dropwise into solution A under vigorous stirring. Then, pH of the final suspension was adjusted to desired value (3.0, 5.5, 6.0, 7.0, 9.0, or 10.0) using ammonia. After stirring for 30 min, the resulting suspension was transferred into a 100 mL of Teflon-lined autoclave and heated at 160 °C for 20 h. Afterward cooling down naturally, the brilliant yellow precipitate was collected by centrifugation, washed by deionized water and absolute ethanol for several times, and then dried in an oven at 60 °C overnight. Meanwhile, changing the added amount of NH4VO3 (0.5, 1.0, 1.25, 1.5 or 2.0 mmol), BiVO4/CH3COO(BiO) composites were synthesized and denoted as BCB-1, BCB-2, BCB-3, BCB-4, and BCB-5, respectively. For the sake of comparison, neat BiVO4 and CH3COO(BiO) photocatalysts were also prepared by the same procedures but without the addition of CH3COOH and NH4VO3.

2.3. Characterization of the photocatalysts The crystal structure and phase composition of the as-prepared samples were analyzed by powder X-ray diffraction (XRD, D8 Focus, Bruker, German) with Cu Kα as a radiation source. The size and morphology of the as-prepared samples were observed with scanning electron microcopy (SEM, JSM-6390LV, Japan). Further microstructure characterization was performed on transmission electron microscopy (TEM) and high-resolution transmission spectroscopy (HRTEM, JEM-2100, Japan). Fourier transform infrared (FT-IR) spectra were recorded using a Spectrum 400 analyzer (Perkin-Elmer, USA), and KBr was used as reference. Surface properties of the samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, USA) with Al Kα Xray irradiation. Binding energy values were calibrated to C 1s peak of 284.6 eV. The UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded over the range of 200–800 nm on an UV–vis spectrophotometer (Lambd 950, PerkinElmer, USA) using BaSO4 as a reference. The room temperature photoluminescence (PL) emission spectra were measured using a fluorescence spectrophotometer (FP-6500, Japan) equipped with a Xenon lamp at an excitation wavelength of 300 nm.

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Table 1 Chemical information of the target contaminants investigated in the present work. Molecular formula

MW (g/mol)

logKow

pKa

CAS no.

Sulfamethoxazole

Compound

Chemical structure

C10H11N3O3S

253.28

0.89a

5.7a

723-46-6

Bisphenol A

C15H16O2

228.29

3.30b

9.6–10.2b

80-05-7

4 Aminoantipyrine

C11H13N3O

203.24

Nf

4.3c

83-07-8

Ibuprofen

C13H18O2

206.28

3.5d

4.9a

15687-27-1

Nf: not found. a (Kosma et al., 2014). b (Yoon et al., 2003). c (Rosal et al., 2010). d (Stuer-Lauridsen et al., 2000).

2.4. Water matrixes Two different water matrixes were investigated: (i) Deionized water, which was used to investigate the influencing factors and photocatalytic activity of the photocatalysts in the absence of any organic and inorganic compounds that may interfere with the process; (ii) Wastewater effluent from the municipal wastewater treatment plant located in Xinxiang (Henan province, China), which was selected to study the application of the photocatalysts developed in this work in the treatment of real water matrixes. Wastewater was used as received within the next 3 days. 2.5. Photocatalytic degradation and mineralization experiments All experiments were conducted in a photochemical batch reactor made of borosilicate glass. In a typical run, 250 mL of aqueous solution with a certain concentration of target pollutant was loaded in the reaction vessel along with an appropriate amount of catalyst. It was worth noting that the concentration of pollutant was set at ppm level (1.5–15 ppm), which was higher than the actual value (0.01–4.24 μg/L) of pollutants in sewage in China (Liu et al., 2017). The reason for choosing ppm level of pollutant was for the convenience of determination, assurance of the accuracy and repeatability of the treatment process, as well as the advantage of investigating chemical reaction kinetics (Mecha et al., 2016). Prior to light irradiation, the suspension was magnetically stirred in the dark for 40 min to establish the adsorption/desorption equilibrium. A 300 W Xenon lamp was used as simulated solar light source. Then turned on the lamp and began the photocatalytic reaction. Samples were drawn at frequent time intervals and were filtered by 0.22 μm polyethersulfone membrane filters (ANPEL, China) to remove the catalyst particles for the analysis of the pollutants. 2.6. Analytical methods 2.6.1. HPLC analysis The concentrations of sulfamethoxazole, bisphenol A, 4 aminoantipyrine and ibuprofen were determined using a high performance liquid chromatography (HPLC, Wasters 2696 series, USA) equipped with a Symmetry C18 column (150 × 4.6 mm, 5 μm) and an ultraviolet-visible detector. The column temperature was maintained at 30 °C. Samples were analyzed at a flow rate of 1.0 mL/min, and the injection volume was 20 μL. The mobile phase used was methanol-water mixture (50,50 v/v). The detection wavelength was 256 nm for sulfamethoxazole, 276 nm for bisphenol A, 243 nm for 4 aminoantipyrine,

and 264 nm for ibuprofen, respectively. The removal efficiency (η) of the target pollutant was calculated by Eq. (1): ηð%Þ ¼ ð1−C t =C 0 Þ  100%

ð1Þ

where C0 and Ct are the concentrations of target pollutant before reaction and at reaction time t, respectively. 2.6.2. GC–MS analysis Analysis of intermediate products after 5 h light irradiation was performed by gas chromatography–mass spectrometry (GC–MS, Agilent 7890B/7000C series, USA) with the electron ionization mode. A HP-5 column (30 m × 0.25 mm × 0.25 μm) was used for analysis. Prior to GC–MS measurements, the suspension in solution was filtered through a glass microfiber filter (Whatman GF/C, UK), and then the filtrate was extracted with n hexane dichloromethane (1:1 v/v) mixed solvent. After phase separation, the organic phase was dried over anhydrous Na2SO4 and concentrated to 0.5 mL by rotary evaporation. The initial temperature of the column oven was hold at 80 °C for 1 min and then increased to 280 °C with a heating rate of 10 °C/min. Mass spectrometric detection was operated with 70 eV electron impact (EI) mode at an ionization current of 50 μA and an ion source temperature of 250 °C. The mass spectra were recorded in full scan mode (m/z 50–500) for qualitative analysis. The structural identification was based on the interpretation of the fragmentation pathways and the NIST2014 library for the mass spectra. 2.6.3. LC-TOF-MS analysis Due to the polarity of certain degradation products, liquid-liquid extraction is not applicable for sample preparation. Therefore, direct injection of the samples in LC-TOF-MS system was also performed for the analysis of intermediate products. After filtration to remove catalyst particles, samples were injected into a liquid chromatography (LC, Agilent series 1100, USA) equipped with a Zorbax C18 analytical column (4.6 × 50 mm, 2.7 μm). Column temperature was maintained at 25 °C. The mobile phase used was methanol-water (90:10 v/v). The flow rate was 0.5 mL/min, and the injection volume was 20 μL. Detection was performed by using a time of flight mass spectrometer (Bruker microTOF-II, USA) equipped with an ESI interface. The setting parameters for the ESI interface were as follows: 180 °C drying gas temperature, 4.0 L/min ESI drying gas flow and 1.0 bar ESI nebulizer gas pressure. The mass spectrometric detection was performed in positive detection mode with a capillary voltage of 4500 V and over the range 50 b m/z b 1200.

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2.6.4. TOC analysis Total organic carbon (TOC) of the target pollutants before and after light irradiation was measured on a TC/TN analyzer (Multi N/C 2100, Analytikiena, Germany). The filtered samples were acidified by hydrochloric acid, and the injection volume was 300 μL. Oxygen was used as the carrying gas with a flow rate of 160 mL/min. The temperature of the oven was 800 °C. Calibration of the analyzer was achieved with potassium hydrogen phthalate standards. The mineralization degree (T) of the target pollutants was calculated using Eq. (2): T ð%Þ ¼ ð1−TOC t =TOC 0 Þ  100%

ð2Þ

where TOC0 and TOCt are the TOC values of the target pollutants in solution before irradiation and at the irradiation time t, respectively. 3. Results and discussion 3.1. Structure, composition, morphology and optical property of the samples It can be seen from Fig. 1(a, b) that the diffraction peaks of the control samples are well indexed to monoclinic phase BiVO4 (JCPDS No. 140688) and CH3COO(BiO) (JCPDS No. 14-0800), respectively. Fig. 1(c–h) displays the XRD patterns of the composite samples prepared at different pH values. It is clear that all the diffraction peaks in the composite samples prepared at pH 5.5 and 6.0 (Fig. 1d, e) agree well with those of monoclinic BiVO4 and CH3COO(BiO). Besides, the widening of diffraction peaks and the decrease of intensity of the composites synthesized at higher pH values of precursor solution (pH = 7.0, 9.0, 10.0) mean that the size of these samples is smaller or the crystallinity is poor. The morphology and microstructure of the as-synthesized CH3COO (BiO), BiVO4 and representative composite of BiVO4/CH3COO(BiO) (BCB-4) have been investigated by SEM, TEM and HRTEM. Fig. 2(a–b) is the typical SEM images of the composite photocatalyst prepared at pH 3.0 and 5.5, respectively. It can be seen that BiVO4 and CH3COO (BiO) in the composite sample prepared at pH 3.0 display the morphology of dumbbell with length of 2 μm and of bulk plate with length reaching tens of micrometers, respectively. However, their morphology changes into grain-like and ultrathin sheet when the pH of precursor solution increases to 5.5, and the BiVO4 bundles are embed into CH3COO (BiO) sheets to form uniform composites. TEM images of BiVO4/CH3COO(BiO) prepared at pH 5.5 (Fig. 2c) show that the spiral chain BiVO4 is tightly attached onto the CH3COO (BiO) thin sheet. Higher magnification image (inset in Fig. 2c) indicates that BiVO4 in the composites is composed of numerous nano olives-like

particles with length less than 10 nm, and the particles connect together to form hierarchical structure, which endows the as-prepared samples with superior performance, such as anti-aggregation property, higher surface-to-volume ratio for more active adsorption and reaction sites. All of these are considered to be favorable for the enhancement of photocatalytic performance (Guo et al., 2015). HRTEM image of BiVO4/ CH3COO(BiO) (Fig. 2d) suggests that the interplanar lattice spaces are 0.27 and 0.32 nm, corresponding to the lattice spacing of (204) and (111) planes for BiVO4 and CH3COO(BiO), respectively. In addition, all the separate morphology of BiVO4 and CH3COO(BiO) in the composites is consistent with that reported in literatures (Yu and Kudo, 2006; Zhang et al., 2016), which confirms phase composition of the samples. XPS analysis is further carried out to analyze the surface composition and oxidative state of elements in the as-synthesized sample (Fig. 3). Two strong peaks at 164.1 eV and 158.8 eV in high-resolution XPS spectrum are assigned to Bi 4f5/2 and Bi 4f7/2, which confirms that the bismuth species in the as-prepared composite photocatalyst (Fig. 3a) are Bi3+ cations (Sun et al., 2014). The peaks at binding energy of 523.6 and 516.3 eV are the splitting signals of V2p (Fig. 3b), indicating that the vanadium cations exist in pentavalence (Chen et al., 2012). The O 1s peaks at around 529.4, 531.1 and 533.0 eV (Fig. 3c) are characteristics of Bi\\O binding energy, acetate species, surface hydroxyl groups and oxygen adsorbed on the surface from the atmosphere, respectively (Rauf et al., 2015; Yang et al., 2015). Additionally, the three peaks of C1s at 284.6, 285.4 and 288.2 eV (Fig. 3d) are, respectively, assigned to C\\C, C\\O and C_O bonding in the CH3COO(BiO) (Zhang et al., 2016). Infrared spectrum analysis is a useful tool to verify chemical bonds and functional groups in materials. The characteristic band at 3441, 1420, 1521 and 532 cm−1 shown in Fig. S1a corresponds to O\\H stretch, symmetric and asymmetric stretching vibration of COO− group and Bi\\O stretch, respectively. Moreover, the band at 738 and 833 cm−1 for both the samples of BiVO4 and BiVO4/CH3COO(BiO) (Fig. S1b and c) is separately ascribed to symmetric and asymmetric stretching vibrations of V\\O bond (Gotić et al., 2005; Zhang et al., 2006). It is worth noting that shift of the peaks from 1521 and 1420 nm−1 to 1503 and 1388 nm−1, respectively, indicates that there is a strong interaction between CH3COO(BiO) and BiVO4 in the composite. This interaction can facilitate the flow of charge carriers across the contact interface and thus effectively reduce their recombination in the single material phase. The optical property of the photocatalyst is evaluated by UV–vis DRS analysis. The bandgap energy (Eg) is estimated using the equation (Chen et al., 2014): αhν = A(hν-Eg)n/2, where α, h, ν, Eg and A is absorption coefficient, Planck constant, light frequency, bandgap energy, and proportionality constant, respectively. Plotting (αhν)0.5 vs. (hν), the intercept of the tangent line to x-axis provides an approximation of Eg value. As shown in Fig. S2a, the adsorption edges of different BiVO4/CH3COO (BiO) samples red-shift gradually with the increase of BiVO4 content due to the strong visible light response of BiVO4. In addition, the Eg value of CH3COO(BiO), BiVO4, BCB-1, BCB-2, BCB-3, BCB-4 and BCB-5 is calculated to be 3.21 eV, 2.24 eV, 2.78 eV, 2.76 eV, 2.56 eV, 2.34 eV and 2.36 eV, respectively. Thus, the result indicates that the BCB-4 nanocomposite photocatalyst can respond to a wider range of visible light. Photoluminescence (PL) spectrum is a common and effective method to evaluate the recombination of charge carriers. As shown in Fig. S2b, both CH3COO(BiO) and BiVO4 display broad emitting peaks resulted from the recombination of e−/h+ pairs, while an obvious decrease in PL emission intensity is observed in the composite photocatalysts, which suggests that the recombination of e−/h+ pairs is significantly inhibited by coupling CH3COO(BiO) with BiVO4. 3.2. Photocatalytic degradation of the organic pollutants by BiVO4/CH3COO (BiO)

Fig. 1. XRD patterns of the samples: (a) BiVO4, (b) CH3COO(BiO), (c–h) composite samples prepared at different pH values (3.0, 5.5, 6.0, 7.0, 9.0, 10.0).

Photocatalytic degradation of sulfamethoxazole is first performed and used to evaluate the activity of the photocatalysts prepared at different acidity of precursor (pH = 3.0, 5.5, 6.0, 7.0, 9.0 and 10.0) and

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Fig. 2. SEM and TEM images of the as-prepared samples: (a) BiVO4/CH3COO(BiO) prepared at pH 3.0, (b) BiVO4/CH3COO(BiO) prepared at pH 5.5, (c, d) TEM and HRTEM images of BiVO4/ CH3COO(BiO) prepared at pH 5.5. Inset in (c): high-magnification TEM image circled in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

with addition of different amounts of NH4VO3 under the optimum pH value (marked as BCB-1, BCB-2, BCB-3, BCB-4 and BCB-5). The adsorption measurements show that the difference in the adsorption of sulfamethoxazole on the surface of these photocatalysts is small (6.9–8.2%), indicating that the difference in the removal efficiency should be attributed to the difference in the photocatalytic performance of the material rather than the adsorption. Fig. S3a shows that pH 5.5 is the best pH value for the preparation of the composite photocatalysts. Under this pH condition, the obtained sample (BCB-4) displays the best

photocatalytic activity when the dosage of NH4VO3 is 1.5 mmol, and 98% of sulfamethoxazole can be degraded in 8 h (Fig. S3b). Nevertheless, the degradation efficiency of sulfamethoxazole only reaches 21% and 74% by neat BiVO4 and CH3COO(BiO), respectively. In addition, the gradual decrease and final disappearance of the peak at 256 nm in UV–vis absorption spectra of sulfamethoxazole indicate that the aromatic structure is efficiently destroyed by BCB-4 (Fig. S4). Furthermore, in order to evaluate the possible leaching of bismuth in the water solution, bismuth concentration in the sample drawn at different irradiation time

Fig. 3. XPS patterns of BiVO4/CH3COO(BiO): (a) Bi 4f, (b) V2p, (c) O1s, and (d) C1s.

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Fig. 4. (a) Degradation of different target pollutants by BCB-4 photocatalyst under solar irradiation, (b) the degradation dynamics of pollutants. Reaction conditions: catalyst loading = 1.0 g/L, [SMX]0 = [BPA]0 = [4-AAP]0 = [IBP]0 = 10 mg/L, pH = 6.0, 25 °C.

intervals (0.5 h, 1.0 h, 7.0 h, 24 h) over optimized BCB-4 photocatalyst is analyzed using inductively coupled plasma mass spectrometry (ICP-MS, ELANDRC-e, USA), and the results show that no bismuth is detected in each sample, which confirms the stability of the BCB-4 material. Therefore, BCB-4 composite photocatalyst is used for the next studies. The degradation conditions of sulfamethoxazole have been optimized in photocatalyst dosage and initial SMX concentration. In addition, considering the fact that the solvent used for wastewater simulation is deionized water, its pH value is 6.0 and close to neutral, therefore the pH value is not adjusted in this work. The results are shown in Figs. S5 and S6. It is clear from Fig. S5 that the degradation efficiency increases with the photocatalyst dosage up to 1.0 g/L due to an increase of absorption and reaction sites on the surface of the photocatalyst. However, a further increase of the photocatalyst loading (1.5 g/L) leads to light scatting and screening effects, and thus reducing the photocatalytic activity (Hu et al., 2014). Fig. S6 shows the degradation of SMX with an initial concentration of 1.5–15 mg/L in the presence of 1 g/L BCB-4 phogocatalyst. It is clear that SMX with concentration lower than 2.5 mg/L can be completely removed within 1–2 h. When the initial concentration range increases to 2.5–10 mg/L, more than 98% of SMX can be efficiently removed within 8 h. Further increasing the concentration from 10 to 15 mg/L, the degradation efficiency remarkably decreases because when the initial sulfamethoxazole concentration is too high, a larger number of intermediate products are produced and compete with the sulfamethoxazole for the adsorption and reaction sites on catalyst surface. Therefore, photocatalyst loading of 1.0 g/L, initial pollutant concentration of 10 mg/L, and solution pH of 6.0 have been used in the subsequent experiments. It is also found that the degradation of sulfamethoxazole by the BiVO4/CH3COO(BiO) composite photocatalyst can be described by: − ln ðC t =C 0 Þ ¼ kt

ð3Þ

where C0 is the concentration of sulfamethoxazole before degradation, Ct is the concentration of sulfamethoxazole at the irradiation time t, and k is the apparent rate constant of the degradation reaction. This result indicates that degradation of sulfamethoxazole follows the pseudofirst-order reaction kinetics. In addition, it is shown that the BCB-4 displays the fastest reaction rate (0.4040 h−1) at the given conditions,

which is about 2.5 and 14.8 times higher than that of individual CH3COO(BiO) (0.1616 h−1) and BiVO4 (0.0273 h−1), respectively. To further confirm the photocatalytic activity of the BCB-4 photocatalyst, the degradation of other three typical organic pollutants, bisphenol A (BPA, plasticizer), ibuprofen (IBP, non-steroidal antiinflammatory drug), and 4-aminoantiyrine (4-AAP, drug intermediate) have been also examined under the mentioned conditions, and the results are shown in Fig. 4. For the sake of comparison, the degradation result of sulfamethoxazole is also included in this figure. It is found that degradation efficiency of the pollutants by BCB-4 photocatalyst within 5 h decreases in the order: bisphenol A (99%) N sulfamethoxazole (85%) N ibuprofen (65%) N 4 aminoantiyrine (46%), and their rate constant is 0.7290, 0.4040, 0.2282 and 0.1272 h−1, respectively. The removal efficiency of organic compounds is closely related to their chemical structure. The electron-deficient radicals tend to attack structure with high electron density. Here, the donation of electrons by the hydroxyl and methyl functional groups to the benzene ring can be responsible for the high removal of bisphenol A. The enhancement of the reactivity with active substances due to the aliphatic functional and hydroxyl groups on the aromatic ring was also reported by Nakada et al. (2007). In addition, the high removal efficiency of sulfamethoxazole is attributed to the presence of sulfonylaniline moiety, and the aniline moiety can be activated via deprotonation of the sulfonamide nitrogen when the pH value is greater than pKa 5.7 (Dodd et al., 2006). Nevertheless, the poor removal of ibuprofen and 4 aminoantiyrine can be explained by the electron-withdrawing nature of the carboxyl and amide groups in the structure (Nakada et al., 2007). 3.3. Mineralization of the target pollutants and mechanism for photocatalysis In the process of photocatalytic degradation, fully mineralization of organic pollutants is the ultimate goal, because in this case the pollutants would be degraded into water and CO2 completely, and no any harmful products can be produced. In order to evaluate the mineralization degree of the organic pollutants investigated in this work, total organic carbon (TOC) values of the contaminants have been measured before and after 24 h simulated solar light irradiation and the results are given in Table 2, together with the degradation efficiency

Table 2 Photocatalytic removal and mineralization efficiency of the target contaminants (24 h). Pollutant

Sulfamethoxazole Bisphenol A 4 Aminoantipyrine Ibuprofen

Concentration (mg/L) Original

After degradation

10 10 10 10

– – – –

Degradation efficiency

TOC (mg/L)

Mineralization rate

Original

After degradation

100% 100% 100% 100%

4.54 10.5 6.11 14.83

0.16 1.05 1.15 1.94

96% 90% 81% 87%

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Fig. 5. Effect of scavengers on the photodegradation of sulfamethoxazole in water by BCB4 photocatalyst under solar light irradiation. Reaction condition: catalyst loading = 1.0 g/L, [SMX]0 = 10 mg/L, pH = 6.0, 25 °C.

determined under the same conditions. It is clearly noted that all the investigated target contaminants can be completely degraded. However, the TOC removal efficiency of the contaminants is lower than the degradation efficiency except for sulfamethoxazole which is nearly completely mineralized. The partial reason may ascribe to the formation of some stable intermediate products such as organic carboxylic acid in the mineralization process. Thus, the mineralization degree more than 81% means that the harm of these pollutants to the environment and human being can be significantly reduced. Furthermore, the lower the concentration of the target pollutant is, the shorter the irradiation time is taken to degrade. The purpose of using simulated solar light with Xenon lamp as the driving light source of photocatalyst is to make full use of solar energy. In fact, the performance of photocatalyst under actual solar light irradiation is remarkably higher than Xenon lamp (Hu et al., 2014), thus the irradiation time for complete removal of the target pollutants selected here should be significantly shortened if solar light is used. Thus, the practical application using solar light irradiation as driving energy of BiVO4/CH3COO(BiO) photocatalyst is potential for wastewater treatment. The band structure is a vital factor for photocatalytic performance. Here, the band edge position can be calculated by the empirical equation (Xu and Schoonen, 2000): EVB = X − Ee + 0.5Eg and ECB = EVB − Eg, where EVB and ECB are the valence band and conduction band edge position, respectively. X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. Ee is the energy of free electrons on the hydrogen scale (~4.5 eV), and Eg is the bandgap energy of the semiconductor. Herein, the electronegativity of BiVO4 and CH3COO(BiO) is calculated to be 6.04 eV and

251

6.67 eV, and the bandgap energy of BiVO4 and CH3COO(BiO) is 2.24 eV and 3.21 eV, respectively. Then, according to the above equations, the EVB value of BiVO4 and CH3COO(BiO) is calculated to be 2.66 eV and 3.77 eV; and the ECB is 0.42 eV and 0.56 eV for BiVO4 and CH3COO(BiO), respectively. It is found that the EVB and ECB values of CH3COO(BiO) are higher than those of BiVO4, and thus a type II heterojunction is suggested to be formed after their contact. Then, the injection of photogenerated electrons from BiVO4 to CH3COO(BiO) is favored thermodynamically, and thus efficiently restrains the recombination of e−/h+ pairs. To further understand the dominant active species in the photocatalytic degradation of organic pollutants over BiVO 4/CH 3COO (BiO) heterojunction, specific trapping agents, tert-butyl alcohol (tBuOH) and ethylenediaminetetraacetic acid disodium salt (EDTA2Na), are employed as scavengers of •OH radicals and valence band holes (h+ VB), respectively. In addition, in order to investigate the influence of O 2 •− on the degradation of sulfamethoxazole, which is generated from the reduction reaction of dissolved oxygen by photoinduced electrons on conduction band, the solution is continuously bubbled with high pure N2 gas throughout the whole experimental process (Chen et al., 2016b). The degradation dynamics of sulfamethoxazole mediated by different scavengers under solar light irradiation are displayed in Fig. 5. It is evident from Fig. 5 that both the addition of 1 mM t-BuOH and bubbling of N2 impose minor effects on the degradation of sulfamethoxazole compared with the case without any scavengers. However, 1 mM EDTA-2Na has a great influence on the degradation of sulfamethoxazole, suggesting that holes may be the main active species responsible for the degradation of sulfamethoxazole. This result is consistent with most of the photocatalytic mechanism of bismuth based photocatalyst (Chen et al., 2017a; Chen et al., 2016a; Xiao et al., 2013), and band structure properties of the bismuth based semiconductor and the effective inhibition of charge carrier recombination by constructing heterojunction are responsible for the generation of holes. The reasons for such a low level of •OH radicals can be understood as follows. First, the quantity of •OH radicals generated from oxidation of OH− by holes is small due to the low concentration of OH− at neutral condition. Second, since the amount of adsorbed materials on photocatalyst surface has an important effect on the reactivity of species with holes, the low adsorption of the species such as H2O or OH− on the surface of the photocatalyst may be one of the reasons; Third, it is generally difficult to derive •OH radicals from the oxidation of H2O or OH− by holes because of the smaller redox potential of BiV/BiIII (+1.59 eV) than that of •OH/H2O (+2.77 eV) and •OH/OH− (+1.99 eV) (Xiao et al., 2012). Moreover, the more positive potential of electrons accumulated on the CB of CH3COO(BiO) (0.56 eV) than the standard redox potential of O2/ O2•− (−0.33 eV vs. NHE) indicates that it is difficult for electrons to reduce the absorbed O2 on the surface to produce O2•− radical (Chen et al., 2017b), which can be responsible for minor role of O2•− radical in the photocatalytic process.

Table 3 Intermediate products of 4-AAP and IBP obtained from GC–MS analysis within 5 h of reaction. Compound

Intermediate

Retention time (min)

Main fragment (m/z)

4-AAP

P1

9.07

163, 120, 93, 77, 65, 51

2 oxo N phenylpropanamide

P2

11.01

164, 121, 92, 65, 51

N′ methyl N phenylacetohydrazide

P1

8.57

162, 120, 91

4 Isobutyl benzaldehyde

P2

9.51

178, 163, 135,91, 57

1 (4 Isobutylphenyl) ethanol

P3

10.04

176, 161, 134, 105, 91

1 (4 Isobutylphenyl) ethanone

IBP

Molecular structure

Name

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Table 4 Intermediate products of 4-AAP and IBP obtained from LC-TOF-MS in positive ion mode within 5 h of reaction. Compound

Intermediate

Formula

Experimental mass (m/z)

Calculated mass (m/z)

Err (ppm)

4-AAP

C1 C2 C1a

C10H13N3NaO3 C7H9N2O C12H17O C12H16NaO C10H17O5

258.0850 137.0709 177.1268 199.1092 217.1079

258.0849 137.0709 177.1274 199.1093 217.1071

−0.4 0 3.4 0.5 −3.7

IBP

C2 a

Detected also by GC–MS.

3.4. Product identification and degradation pathway of 4 aminoantipyrine and ibuprofen Here, 4 aminoantipyrine and ibuprofen with relatively low mineralization degree are taken to understand the degradation mechanism of the target pollutants. According to the experimental procedures described in the experimental section 2.6, degradation intermediates of 4 aminoantipyrine and ibuprofen have been analyzed. Blank sample without any target pollutant is simultaneously measured, and the results are shown in Fig. S7. Chemical structures of the intermediates identified by GC–MS are given in Table 3, and the total ion chromatogram and MS spectra for these intermediates from GC–MS are given in Figs. S8–S11. The identification of the byproducts was also performed on LC-TOFMS because of its high sensitivity on monitoring the m/z values. Structure assignation is based on the accurate mass measures provided by the TOF analyzer which allows to obtain the elemental composition of the protonated molecule with a high grade of accuracy (b5 ppm). The accurate mass spectra of the intermediate products are obtained and the information provided is summarized in Table 4, which shows the measured and calculated mass of the protonated ions [M + H]+/[M + Na]+. Total ion chromatogram and MS spectra for the intermediates of 4-aminoantipyrine and ibuprofen from LC-TOF-MS are given in Figs. S12–S15. The reporting of identification confidence follows the four-level system proposed by Schymanski et al. (2014). Based on the intermediates detected by GC–MS and LC-TOF-MS, degradation mechanism is proposed for 4 aminoantipyrine and ibuprofen. Two steps are suggested for the degradation of 4 aminoantipyrine as shown in Scheme 1. The amino group at the 12th position is first oxidized into nitro group to generate C1 intermediate followed by: (i) the removal of methyl group at the 13th position and nitro group at the 12th position, and the cleavage of C2-C3 bond, so that the pyrazolone ring is opened and intermediate P1 is generated. Next, further demethylation of the P1 form the product C2; (ii) the cleavage of N4-N5 and N4C3 bonds, the removal of methyl and nitro groups at the 13th and 12th positions, and the further oxidation of H atom at the 2nd position eventually lead to the formation of P2 intermediate. The proposed degradation pathway agrees with the conclusion that the penta-heterocycle is

more vulnerable to be attacked by the •OH radicals than the aromatic rings (Belver et al., 2017). In Scheme 2, the degradation pathway of ibuprofen in the presence of BiVO4/CH3COO(BiO) composite photocatalyst has been demonstrated to occur mainly through decarboxylation at the 13th position and subsequent hydroxylation at the 11th position of ibuprofen. And then, further oxidation of the hydroxyl group in P2 to give keto intermediate P3 followed by demethylation to generate P1. Next, the polyhydroxylation of benzene ring and side chain leads to the formation of intermediate C2. It is worth noting that P3 intermediate is also detected by LC-TOF-MS (labeled as C1). The proposed mechanism is in agreement with the strong attracting electron effect of carboxyl group in ibuprofen, which is more easily attacked by •OH radicals. 3.5. Degradation of target pollutants in real wastewater matrices In order to verify the matrix effect on the photocatalytic degradation, real samples of wastewater effluents were collected and used. Typical characteristics of the effluent are displayed in Table S1. The actual wastewater sample containing target pollutant was individually prepared by spiking with small volume of a concentrated stock solution of sulfamethoxazole, bisphenol A, 4 aminoantipyrine or ibuprofen to obtain a desired concentration, and then the same photocatalytic experiments were performed as before. Comparison of photocatalytic degradation of the pollutants in deionized water and wastewater effluent is displayed in Fig. S16. It is shown that the efficiency of four pollutants removed from the actual wastewater by the BCB-4 photocatalyst is slightly lower than the efficiency from deionized water. The same phenomena were reported by other researchers (Ioannidou et al., 2017; Wang et al., 2017), in which the degradation of ibuprofen and sulfamethoxazole by g-C3N4/Bi2WO6 and WO3/TiO2 in real water matrix was delayed compared with the case in deionized water. Carbonate and bicarbonate are common substances in natural water and wastewater, which may react with •OH radical to produce carbonate radical (CO3•−). Compared with •OH radical, the high selectivity and low oxidation potential of CO3•− is one of the reasons for the lower removal efficiency of sulfamethoxazole in wastewater (Abargues et al., 2018). In addition, dissolved organic matters (DOM) such as humic acid and fulvic acid are reported to be the dominant organic species in wastewater effluent. One hand, they absorb light to generate triplet states of DOM (3DOM⁎) and then produce •OH radicals (He et al., 2016). On the other hand, DOM also competes with the target for the absorption and reactive sites on the catalyst surface (Brame et al., − 2015). In addition, the photolysis of NO− 2 and NO3 in wastewater can also generate •OH radical by absorbing UV light and accelerate the decomposition of pollutants, but the yield of •OH radicals is usually quite low (Mack and Bolton, 1999). A series of related reactions are shown in Eqs. (4)–(9). CO3 2− þ • OH→CO3 •

−

þ OH −

Scheme 1. Proposed degradation pathway for 4 aminoantipyrine in deionized water by BCB-4 photocatalyst under solar light irradiation.

ð4Þ

X. Zhang et al. / Science of the Total Environment 647 (2019) 245–254

253

Scheme 2. Proposed degradation pathway for ibuprofen in deionized water by BCB-4 photocatalyst under solar light irradiation.

Acknowledgments HCO3 − þ • OH→CO3 •− þ H2 O

ð5Þ This work was supported by the National Natural Science Foundation of China (Grant No. 21777038) and the Henan program for basic and frontier technology research project (Grant No. 132300410288).

 hv  DOM → 3 DOM →O• H

ð6Þ

DOM þ • OH→products

ð7Þ

Appendix A. Supplementary data

ð8Þ

Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.07.450.

ð9Þ

References

hv

H2 O

NO2 − → ½NO2 −  →• NO þ O•− → • NO þ • OH þ OH − hv

H2 O

NO3 − → ½NO3 −  →• NO2 þ O•− → • NO2 þ • OH þ OH −

Compared with the degradation in deionized water, the slightly lower removal efficiency of the target pollutants in wastewater effluent may be attributed to the above mentioned factors. Nevertheless, the BiVO4/CH3COO(BiO) composite photocatalyst can still remove most of the target pollutants in wastewater effluents with the prolonged irradiation time. Reusability tests indicate that BiVO4/CH3COO(BiO) has good recycling performance, which suggests its promising practicality for water purification. 4. Conclusions In this study, a novel BiVO4/CH3COO(BiO) heterojunction is prepared by a simple one-step solvothermal method using glacial acetic acid aqueous solution as solvent. It is found that the heterojunction (BCB-4) exhibits much higher photocatalytic activity for sulfamethoxazole, bisphenol A, ibuprofen and 4 aminoantiyrine than individual CH 3COO(BiO) and BiVO4 in water under simulated solar irradiation. Mechanism study shows that the enhanced activity is mainly attributed to the improvement of optical absorption and charge separation resulted from II type heterojunction. 99% bisphenol A, 85% sulfamethoxazole, 65% ibuprofen and 46% 4aminoantiyrine can be removed within 5 h of irradiation under given conditions. When the irradiation time is prolonged to 24 h, all the pollutants are completely degraded. Their high mineralization degree (81–96%) indicates that the potential hazards of these pollutants to environment and human beings are significantly reduced after degradation. Detection of degradation products confirms that side chain cleavage, heterocyclic ring opening and hydroxylation of aromatic ring are main degradation pathways. The one-pot synthetic strategy employed here is facile, convenient, environmentally friendly, and scalable. Especially, good reusability and less inhibition of photocatalyst activity in practical wastewater applications suggest that BiVO4/CH3COO(BiO) photocatalyst has the potential prospect in the disposal of refractory organic pollutant wastewater.

Abargues, M.R., Giménez, J.B., Ferrer, J., Bouzas, A., Seco, A., 2018. Endocrine disrupter compounds removal in wastewater using microalgae: degradation kinetics assessment. Chem. Eng. J. 334, 313–321. https://doi.org/10.1016/j.cej.2017.09.187. Alvarino, T., Suarez, S., Katsou, E., Vazquez-Padin, J., Lema, J.M., Omil, F., 2015. Removal of PPCPs from the sludge supernatant in a one stage nitritation/anammox process. Water Res. 68, 701–709. https://doi.org/10.1016/j.watres.2014.10.055. Belver, C., Hinojosa, M., Bedia, J., Tobajas, M., Alvarez, M.A., Rodriguez-Gonzalez, V., Rodriguez, J.J., 2017. Ag-coated heterostructures of ZnO-TiO2/delaminated montmorillonite as solar photocatalysts. Materials 10. https://doi.org/10.3390/ma10080960. Brame, J., Long, M., Li, Q., Alvarez, P., 2015. Inhibitory effect of natural organic matter or other background constituents on photocatalytic advanced oxidation processes: mechanistic model development and validation. Water Res. 84, 362–371. https:// doi.org/10.1016/j.watres.2015.07.044. Chala, S., Wetchakun, K., Phanichphant, S., Inceesungvorn, B., Wetchakun, N., 2014. Enhanced visible-light-response photocatalytic degradation of methylene blue on Feloaded BiVO4 photocatalyst. J. Alloys Compd. 597, 129–135. https://doi.org/10.1016/ j.jallcom.2014.01.130. Chatchai, P., Nosaka, A.Y., Nosaka, Y., 2013. Photoelectrocatalytic performance of WO3/ BiVO4 toward the dye degradation. Electrochim. Acta 94, 314–319. https://doi.org/ 10.1016/j.electacta.2013.01.152. Chen, L., Zhang, Q., Huang, R., Yin, S.-F., Luo, S.-L., Au, C.-T., 2012. Porous peanut-like Bi2O3BiVO4 composites with heterojunctions: one-step synthesis and their photocatalytic properties. Dalton Trans. 41, 9513–9518. https://doi.org/10.1039/c2dt30543g. Chen, J., Guan, M., Cai, W., Guo, J., Xiao, C., Zhang, G., 2014. The dominant {001} facetdependent enhanced visible-light photoactivity of ultrathin BiOBr nanosheets. Phys. Chem. Chem. Phys. 16, 20909–20914. https://doi.org/10.1039/c4cp02972k. Chen, F., Yang, Q., Sun, J., Yao, F., Wang, S., Wang, Y., Wang, X., Li, X., Niu, C., Wang, D., Zeng, G., 2016a. Enhanced photocatalytic degradation of tetracycline by AgI/BiVO4 heterojunction under visible-light irradiation: Mineralization efficiency and mechanism. ACS Appl. Mater. Interfaces 8, 32887–32900. https://doi.org/10.1021/ acsami.6b12278. Chen, F., Yang, Q., Zhong, Y., An, H.X., Zhao, J.W., Xie, T., Xu, Q.X., Li, X.M., Wang, D.B., Zeng, G.M., 2016b. Photo-reduction of bromate in drinking water by metallic Ag and reduced graphene oxide (RGO) jointly modified BiVO4 under visible light irradiation. Water Res. 101, 555–563. https://doi.org/10.1016/j.watres.2016.06.006. Chen, F., Yang, Q., Li, X.M., Zeng, G.M., Wang, D.B., Niu, C.G., Zhao, J.W., An, H.X., Xie, T., Deng, Y.C., 2017a. Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: An efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation. Appl. Catal. B Environ. 200, 330–342. https://doi.org/ 10.1016/j.apcatb.2016.07.021. Chen, F., Yang, Q., Wang, S., Yao, F., Sun, J., Wang, Y., Zhang, C., Li, X., Niu, C., Wang, D., Zeng, G., 2017b. Graphene oxide and carbon nitride nanosheets co-modified silver chromate nanoparticles with enhanced visible-light photoactivity and antiphotocorrosion properties towards multiple refractory pollutants degradation. Appl. Catal. B Environ. 209, 493–505. https://doi.org/10.1016/j.apcatb.2017.03.026.

254

X. Zhang et al. / Science of the Total Environment 647 (2019) 245–254

Cui, K., He, Y., Jin, S., 2016. Enhanced UV–visible response of bismuth subcarbonate nanowires for degradation of xanthate and photocatalytic reaction mechanism. Chemosphere 149, 245–253. https://doi.org/10.1016/j.chemosphere.2016.01.111. Dodd, M., Buffle, M.-O., Von Gunten, U., 2006. Oxidation of antibacterial molecules by aqueous ozone: moiety-specific reaction kinetics and application to ozone-based wastewater treatment. Environ. Sci. Technol. 40, 1969–1977. Duan, F., Zheng, Y., Liu, L., Chen, M., Xie, Y., 2010. Synthesis and photocatalytic behaviour of 3D flowerlike bismuth oxide formate architectures. Mater. Lett. 64, 1566–1569. https://doi.org/10.1016/j.matlet.2010.04.046. Elmolla, E.S., Chaudhuri, M., Eltoukhy, M.M., 2010. The use of artificial neural network (ANN) for modeling of COD removal from antibiotic aqueous solution by the Fenton process. J. Hazard. Mater. 179, 127–134. https://doi.org/10.1016/j.jhazmat.2010.02.068. García Pérez, U.M., Sepúlveda-Guzmán, S., Martínez-de la Cruz, A., Ortiz, Méndez U., 2011. Photocatalytic activity of BiVO4 nanospheres obtained by solution combustion synthesis using sodium carboxymethylcellulose. J. Mol. Catal. A Chem. 335, 169–175. https://doi.org/10.1016/j.molcata.2010.11.030. Giraldo, A.L., Peñuela, G.A., Torres-Palma, R.A., Pino, N.J., Palominos, R.A., Mansilla, H.D., 2010. Degradation of the antibiotic oxolinic acid by photocatalysis with TiO2 in suspension. Water Res. 44, 5158–5167. https://doi.org/10.1016/j.watres.2010.05.011. Gong, H., Chu, W., 2016. Determination and toxicity evaluation of the generated products in sulfamethoxazole degradation by UV/CoFe2O4/TiO2. J. Hazard. Mater. 314, 197–203. https://doi.org/10.1016/j.jhazmat.2016.04.052. Gotić, M., Musić, S., Ivanda, M., Šoufek, M., Popović, S., 2005. Synthesis and characterisation of bismuth(III) vanadate. J. Mol. Struct. 744-747, 535–540. https://doi.org/ 10.1016/j.molstruc.2004.10.075. Grigioni, I., Stamplecoskie, K.G., Selli, E., Kamat, P.V., 2015. Dynamics of photogenerated charge carriers in WO3/BiVO4 heterojunction photoanodes. J. Phys. Chem. C 119, 20792–20800. https://doi.org/10.1021/acs.jpcc.5b05128. Guo, X., Zhen, M., Liu, H., Liu, L., 2015. BiOBr–BiOI microsphere assembled with atomthick ultrathin nanosheets and its high photocatalytic activity. RSC Adv. 5, 24777–24782. https://doi.org/10.1039/c5ra01086a. He, Y., Sutton, N.B., Rijnaarts, H.H.H., Langenhoff, A.A.M., 2016. Degradation of pharmaceuticals in wastewater using immobilized TiO2 photocatalysis under simulated solar irradiation. Appl. Catal. B Environ. 182, 132–141. https://doi.org/10.1016/j. apcatb.2015.09.015. Hernando, M., Mezcua, M., Fernandezalba, A., Barcelo, D., 2006. Environmental risk assessment of pharmaceutical residues in wastewater effluents, surface waters and sediments. Talanta 69, 334–342. https://doi.org/10.1016/j.talanta.2005.09.037. Hu, X., Fan, J., Zhang, K., Yu, N., Wang, J., 2014. Pharmaceuticals removal by novel nanoscale photocatalyst Bi4VO8Cl: influencing factors, kinetics, and mechanism. Ind. Eng. Chem. Res. 53, 14623–14632. https://doi.org/10.1021/ie501855r. Ioannidou, E., Frontistis, Z., Antonopoulou, M., Venieri, D., Konstantinou, I., Kondarides, D.I., Mantzavinos, D., 2017. Solar photocatalytic degradation of sulfamethoxazole over tungsten-modified TiO2. Chem. Eng. J. 318, 143–152. https://doi.org/10.1016/j. cej.2016.06.012. Jia, X., Cao, J., Lin, H., Zhang, M., Guo, X., Chen, S., 2017. Transforming type-I to type-II heterostructure photocatalyst via energy band engineering: A case study of I-BiOCl/ I-BiOBr. Appl. Catal. B Environ. 204, 505–514. https://doi.org/10.1016/j. apcatb.2016.11.061. Kim, E.S., Kang, H.J., Magesh, G., Kim, J.Y., Jang, J.-W., Lee, J.S., 2014. Improved photoelectrochemical activity of CaFe2O4/BiVO4 heterojunction photoanode by reduced surface recombination in solar water oxidation. ACS Appl. Mater. Interfaces 6, 17762–17769. https://doi.org/10.1021/am504283t. Kosma, C.I., Lambropoulou, D.A., Albanis, T.A., 2014. Investigation of PPCPs in wastewater treatment plants in Greece: occurrence, removal and environmental risk assessment. Sci. Total Environ. 466-467, 421–438. https://doi.org/10.1016/j.scitotenv.2013.07.044. Kudo, A., Miseki, Y., 2009. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–278. https://doi.org/10.1039/b800489g. Li, B., Zhang, T., 2010. Biodegradation and adsorption of antibiotics in the activated sludge process. Environ. Sci. Technol. 44, 3468–3473. https://doi.org/10.1021/es903490h. Liu, H.-Q., Lam, J.C.W., Li, W.-W., Yu, H.-Q., Lam, P.K.S., 2017. Spatial distribution and removal performance of pharmaceuticals in municipal wastewater treatment plants in China. Sci. Total Environ. 586, 1162–1169. https://doi.org/10.1016/j. scitotenv.2017.02.107. Loos, R., Carvalho, R., Antonio, D.C., Comero, S., Locoro, G., Tavazzi, S., Paracchini, B., Ghiani, M., Lettieri, T., Blaha, L., Jarosova, B., Voorspoels, S., Servaes, K., Haglund, P., Fick, J., Lindberg, R.H., Schwesig, D., Gawlik, B.M., 2013. EU-wide monitoring survey on emerging polar organic contaminants in wastewater treatment plant effluents. Water Res. 47, 6475–6487. https://doi.org/10.1016/j.watres.2013.08.024. Luo, Y., Guo, W., Ngo, H.H., Nghiem, L.D., Hai, F.I., Zhang, J., Liang, S., Wang, X.C., 2014. A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment. Sci. Total Environ. 473-474, 619–641. https://doi.org/10.1016/j.scitotenv.2013.12.065. Mack, J., Bolton, J.R., 1999. Photochemistry of nitrite and nitrate in aqueous solution-a review. J. Photochem. Photobiol. A Chem. 128, 1–13. https://doi.org/10.1016/S10106030(99)00155-0. Madhusudan, P., Ran, J.R., Zhang, J., Yu, J.G., Liu, G., 2011. Novel urea assisted hydrothermal synthesis of hierarchical BiVO4/Bi2O2CO3 nanocomposites with enhanced visible-light photocatalytic activity. Appl. Catal. B Environ. 110, 286–295. https:// doi.org/10.1016/j.apcatb.2011.09.014.

Mecha, A.C., Onyango, M.S., Ochieng, A., Fourie, C.J.S., Momba, M.N.B., 2016. Synergistic effect of UV–vis and solar photocatalytic ozonation on the degradation of phenol in municipal wastewater: A comparative study. J. Catal. 341, 116–125. https://doi.org/ 10.1016/j.jcat.2016.06.015. Murgolo, S., Petronella, F., Ciannarella, R., Comparelli, R., Agostiano, A., Curri, M.L., Mascolo, G., 2015. UV and solar-based photocatalytic degradation of organic pollutants by nano-sized TiO2 grown on carbon nanotubes. Catal. Today 240, 114–124. https://doi.org/10.1016/j.cattod.2014.04.021. Nakada, N., Shinohara, H., Murata, A., Kiri, K., Managaki, S., Sato, N., Takada, H., 2007. Removal of selected pharmaceuticals and personal care products (PPCPs) and endocrine-disrupting chemicals (EDCs) during sand filtration and ozonation at a municipal sewage treatment plant. Water Res. 41, 4373–4382. https://doi.org/10.1016/j. watres.2007.06.038. Rauf, A., Sher Shah, M.S.A., Choi, G.H., Humayoun, U.B., Yoon, D.H., Bae, J.W., Park, J., Kim, W.-J., Yoo, P.J., 2015. Facile synthesis of hierarchically structured Bi2S3/Bi2WO6 photocatalysts for highly efficient reduction of Cr(VI). ACS Sustain. Chem. Eng. 3, 2847–2855. https://doi.org/10.1021/acssuschemeng.5b00783. Rosal, R., Rodríguez, A., Perdigón-Melón, J.A., Petre, A., García-Calvo, E., Gómez, M.J., Agüera, A., Fernández-Alba, A.R., 2010. Occurrence of emerging pollutants in urban wastewater and their removal through biological treatment followed by ozonation. Water Res. 44, 578–588. https://doi.org/10.1016/j.watres.2009.07.004. Schymanski, E.L., Jeon, J., Gulde, R., Fenner, K., Ruff, M., Singer, H.P., Hollender, J., 2014. Identifying small molecules via high resolution mass spectrometry: communicating confidence. Environ. Sci. Technol. 48, 2097–2098. https://doi.org/10.1021/es5002105. Stuer-Lauridsen, F., Birkved, M., Hansena, L.P., Holten Lützhøft, H.-C., Halling-Sørensen, B., 2000. Environmental risk assessment of human pharmaceuticals in Denmark after normal therapeutic use. Chemosphere 40, 783–793. https://doi.org/10.1016/S00456535(99)00453-1. Sun, W., Zhang, H., Lin, J., 2014. Surface modification of Bi2O3 with Fe(III) clusters toward efficient photocatalysis in a broader visible light region: implications of the interfacial charge transfer. J. Phys. Chem. C 118, 17626–17632. https://doi.org/10.1021/ jp503845g. Tan, H.L., Amal, R., Ng, Y.H., 2016. Exploring the different roles of particle size in photoelectrochemical and photocatalytic water oxidation on BiVO4. ACS Appl. Mater. Interfaces 8, 28607–28614. https://doi.org/10.1021/acsami.6b09076. Wang, J., Tang, L., Zeng, G., Deng, Y., Liu, Y., Wang, L., Zhou, Y., Guo, Z., Wang, J., Zhang, C., 2017. Atomic scale g-C3N4/Bi2WO6 2D/2D heterojunction with enhanced photocatalytic degradation of ibuprofen under visible light irradiation. Appl. Catal. B Environ. 209, 285–294. https://doi.org/10.1016/j.apcatb.2017.03.019. Xiao, X., Liu, C., Hu, R., Zuo, X., Nan, J., Li, L., Wang, L., 2012. Oxygen-rich bismuth oxyhalides: generalized one-pot synthesis, band structures and visible-light photocatalytic properties. J. Mater. Chem. 22, 22840–22843. https://doi.org/10.1039/ c2jm33556e. Xiao, X., Jiang, J., Zhang, L., 2013. Selective oxidation of benzyl alcohol into benzaldehyde over semiconductors under visible light: The case of Bi12O17Cl2 nanobelts. Appl. Catal. B Environ. 142-143, 487–493. https://doi.org/10.1016/j.apcatb.2013.05.047. Xiao, K., Tian, N., Guo, Y., Huang, H., Li, X., Zhang, Y., 2015. Facile synthesis, electronic structure and photocatalytic activity of a novel Bi-based hydroxyl oxalate Bi(C2O4) OH. Inorg. Chem. Commun. 52, 5–8. https://doi.org/10.1016/j.inoche.2014.12.005. Xu, R., Schoonen, M.A.A., 2000. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am. Mineral. 85, 543–556. https://doi. org/10.2138/am-2000-0416. Yan, M., Wu, Y., Yan, Y., Zhu, F., Hua, Y., Shi, W., 2016. Synthesis and characterization of novel BiVO4/Ag3VO4 heterojunction with enhanced visible-light-driven photocatalytic degradation of dyes. ACS Sustain. Chem. Eng. 4, 757–766. https://doi.org/ 10.1021/acssuschemeng.5b00690. Yang, L., Han, Q., Wang, X., Zhu, J., 2015. Highly efficient removal of aqueous chromate and organic dyes by ultralong HCOOBiO nanowires. Chem. Eng. J. 262, 169–178. https://doi.org/10.1016/j.cej.2014.09.078. Yoon, Y., Westerhoff, P., Snyder, S.A., Esparza, M., 2003. HPLC-fluorescence detection and adsorption of bisphenol A, 17ß-estradiol, and 17α-ethynyl estradiol on powdered activated carbon. Water Res. 37, 3530–3537. https://doi.org/10.1016/s0043-1354(03) 00239-2. Yu, J., Kudo, A., 2006. Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv. Funct. Mater. 16, 2163–2169. https://doi. org/10.1002/adfm.200500799. Zhang, L., Chen, D., Jiao, X., 2006. Monoclinic structured BiVO4 nanosheets: hydrothermal preparation, formation mechanism, and coloristic and photocatalytic properties. J. Phys. Chem. B 110, 2668–2673. https://doi.org/10.1021/jp056367d. Zhang, J., Han, Q., Wang, X., Zhu, J., Duan, G., 2016. Synthesis of δ-Bi2O3 microflowers and nanosheets using CH3COO(BiO) self-sacrifice precursor. Mater. Lett. 162, 218–221. https://doi.org/10.1016/j.matlet.2015.10.024. Zhang, X., Shi, D., Fan, J., 2017. One stone two birds: novel carbon nanotube/Bi4VO8Cl photocatalyst for simultaneous organic pollutants degradation and Cr(VI) reduction. Environ. Sci. Pollut. Res. 24, 23309–23320. https://doi.org/10.1007/s11356-0179969-2.