English

• 0.   Introduction

  Pharmaceutical antibiotics and relative derivatives are emerging pollutants of public concerns originated from pharmaceuticals and personal care products (PPCPs)^[1-2]. Since most pharmaceutical anti-biotics are poorly metabolized and absorbable, their exposures can cause potential adverse effects on aquatic ecology as well as on human health^[3-4]. To solve the environmental issues caused by pharmaceutical antibiotics^[5], the development of photocatalysts for antibiotics removal has been considered as one of the most viable advanced oxidation technologies^[6-7], in view of the usage of clean and sustainable solar energy^[8].

  Recently, semiconductor photocatalysts have attracted considerable research focuses due to their excellent capacities in the degradation of organic contaminants^[9-11]. Metal oxides and sulfides, such as titanium oxides^[12-13], zinc oxides and sulfides^[14-16], and cadmium sulfides^[17-18], have been widely explored for the photocatalytic degradation of organic compounds. More recently, bismuth tungstates have been demonstrated to show superior performances in the visible-light-driven photocatalysis, thanks to their appropriate band energy, for applications such as water splitting and environmental remediation^[19-21]. In this context, various synthetic methods^[22-24] have been developed for the fabrication of crystalline Bi₂WO₆. However, surface morphology and phase structure of bismuth tungstates are somehow not easily manipulated due to the general application of high temperature reactions. Further to the precise determination on photocatalytic performances, Bi₂WO₆ materials prepared at viable and comparable synthetic conditions, with well-defined physical structures and properties, are extremely demanded.

  The hydrothermal process has been known as a robust and flexible pathway in the synthesis of advanced materials with high purity, uniform particle size, and excellent dispersion^[25]. In this work, hierar-chically structured flower-like bismuth tungstate (f-Bi₂WO₆) were fabricated through a template-free hydrothermal synthesis. The as-prepared f-Bi₂WO₆ was applied for the degradation of tetracycline antibiotics (tetracycline (TC) and oxytetracycline (OTC)) under visible irradiation. Specifically, the f-Bi₂WO₆ exhibited photocatalytic degradation efficiency of ca. 89.7% (for TC) and ca. 75.8% (for OTC), respectively. Further-more, the f-Bi₂WO₆ photocatalyst showed excellent stability at various pH values and could be recycled for reuse, which was promising for the potential application in wastewater treatment.

  1.   Experimental

  1.1   Materials

  Bi(NO₃)₃·5H₂O, Na₂WO₄·2H₂O, ethanol, ethylene glycol, tetracycline, oxytetracycline, tert-butyl alcohol, ammonium oxalate and p-benzoquinone were commercially purchased and used without further purification. Reaction solutions and stock solutions were prepared using deionized water supplied with a UPT-I-5T ultrapure water system.

  1.2   Methods

  Scanning electron microscopy (SEM) images were photographed by using a JSM6700-F with a working voltage of 10 kV. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were recorded by using an FEIT 20 working at 200 kV. Powder X-ray diffraction (PXRD) was carried out on a Rigaku Miniflex 600 diffractometer with Cu Kα radiation (λ=0.154 nm, U=40 kV, I=40 mA, 2θ=10°~80°). N₂ adsorption-desorption isotherms were obtained on a Micromeritics ASAP 2460 instrument and used for Brunauer-Emmett-Teller (BET) surface area and pore size distribution (PSD) calculations. UV-Vis and diffuse reflectance spectra (DRS) were recorded on a Shimadzu UV-Vis spectrophotometer (UV-2550).

  1.3   Synthesis

  Bi(NO₃)₃·5H₂O (2.0 mmol) and Na₂WO₄·2H₂O (1.0 mmol) were dissolved in 30 mL mixed ethylene glycol and ethanol (1:2, V/V), followed by stirring for 30 min at 500 r·min^-1 and sealed in a solvothermal autoclave and kept in an oven (160 ℃) for 12 h. The f-Bi₂WO₆ was isolated as white powder product and washed with ultrapure water and absolute ethanol, respectively, and collected by using a centrifuge and dried overnight in an oven (50 ℃).

  1.4   Photocatalytic reaction

  In a typical procedure, the f-Bi₂WO₆ photo-catalyst (20 mg) and tetracycline solution (100 mL, 20 mg·L^-1) were added in a Pyrex glass vessel (200 mL) with simultaneous shaking. The above mixture was left for 30 min in dark then exposed to visible light. Photocatalytic experiments were monitored by UV-Vis measurements of the characteristic absorbency of tetracycline molecules after certain time intervals.

  Degradation efficiency was estimated by the following equation: D=C/C₀×100%, where D is the degradation efficiency, and C₀ and C is the initial and tested characteristic absorbencies of tetracycline, respectively. The rate constant (k) was estimated by the following equation: -ln(C/C₀)=kt.

  1.5   Photocatalyst stability test

  Stability of the f-Bi₂WO₆ photocatalyst was studied by repeated cycles of photocatalytic reactions under set conditions. Between two consecutive runs of a cycling reaction, the photocatalyst was recycled by using a centrifuge and washed with ultrapure water and ethanol for several times and dried at 50 ℃ in an oven. The f-Bi₂WO₆ was then separated at the end of a cycling reaction, washed, and dried as stated above and used for further PXRD measurements.

  2.   Results and discussion

  2.1   Physical characterization

  Surface morphology of the f-Bi₂WO₆ was revealed by using SEM and TEM techniques. As shown in Fig. 1a and 1b, the f-Bi₂WO₆ displayed hierarchically flower-like structure (around 1.0 micron in size and ca. 20 nm in thickness) assembled from plates of bismuth tungstates. TEM images also clearly showed the flower-like structure (Fig. 1c) and lattice fringes with d-spacing of ca. 0.312 nm (Fig. 1d), which corresponded to the (113) crystal plane of f-Bi₂WO₆. PXRD was applied to identify the phase purity of f-Bi₂WO₆ (Fig. 2a). The characteristics at 28.31°, 32.93°, 47.28°, 55.83°, 58.56°, 68.75°, 75.96 and 78.03° were well indexed as (113), (020), (028), (313), (226), (400), (139) and (145) crystal planes of orthorhombic Bi₂WO₆ (Fig. 2a; PDF No.73-1126)^[26]. N₂ adsorption/desorption isotherms were recorded to evaluate the porosity of the f-Bi₂WO₆. As shown in Fig. 2b, the f-Bi₂WO₆ showed typical adsorption/desorption isotherms for nanomaterials with clear hysteresis loop, possessing dominant mesopores. The BET surface area was estimated to be ca. 78 m²·g^-1 which was fairly reasonable for hierarchically structured flower-like Bi₂WO₆. UV-Vis diffuse reflectance spectra (DRS) of the f-Bi₂WO₆ are shown in Fig. 2c. The absorption bands in the visible region indicated considerable light harvesting capacity of the f-Bi₂WO₆, which generally favored the visible-light-driven photocatalytic activity for target reactions^[27]. The main band structure of the f-Bi₂WO₆ in the visible region was calculated by the converted Kubelka-Munk (K-M) equation αhν=A(hν-Eg)^1/2, where α is the adsorption coefficient, hν is the photon energy, E_g is the direct band gap (eV), and A is a constant^[28-29]. The calculated E_g for the f-Bi₂WO₆ was 2.66 eV (Fig. 2d), which was suitable for excitation under visible irradiation.

  Figure 1

  

  Figure 1.  SEM (a, b) and TEM (c, d) images of the f-Bi₂WO₆

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  Figure 2

  

  Figure 2.  (a) PXRD patterns, (b) N₂ adsorption-desorption isotherms, (c) DRS spectrum, and (d) K-M plot of the f-Bi₂WO₆

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  2.2   Photocatalytic activity

  Photocatalytic activity of the f-Bi₂WO₆ was studied towards the degradation of tetracycline antibiotics (TC and OTC) under visible irradiation. In a typical procedure, photocatalyst was first immersed in TC/OTC solutions and reacted in dark for 30 min to reach the adsorption/desorption equilibrium. Upon visible irradiation, the UV-Vis absorption characteris-tics of TC/OTC decreased gradually with time, indica-ting continuous photocatalytic TC/OTC degradation. Of note, the f-Bi₂WO₆ was able to degrade ca. 75.8% of OTC and ca. 89.7% of TC within 1 h (Fig. 3a), which was comparable to some of the best performing photocatalysts for the degradation of tetracycline antibiotics documented in the literatures^[30]. Quantita-tively, the apparent rate constant k of 0.041 9 min^-1 for TC degradation was calculated by the pseudo-first-order reaction kinetics equation (Fig. 3a, inset), which was approximately 2.1 times of that for the cubic Bi₂MoO₆ and 4.4 times of that for the WO₃/g-C₃N₄ Z-scheme photocatalysts showing similar TC degradation efficiencies^[31-32]. The excellent photocatalytic capacity of the f-Bi₂WO₆ was likely derived from the enhanced separation efficiency of photogenerated charge carriers at host-guest interfaces.

  Figure 3

  

  Figure 3.  (a) Photocatalytic TC and OTC degradation efficiency of the f-Bi₂WO₆; (b) Photocatalytic TC degradation at various pH values; (c) Cyclic TC degradation reactions; (d) PXRD of the f-Bi₂WO₆ before and after cyclic reactions; (e) Photocatalytic efficiency of the f-Bi₂WO₆ with exposure to various scavengers

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  To further investigate the mechanism for TC degradation, scavenger experiments were applied to detect photogenerated active species^[33-34]. Considerable decreases on the photocatalytic efficiency of the f-Bi₂WO₆ were observed upon addition of ammonium oxalate (AO), tert-butyl alcohol (TBA) and p-benzoq-uione (PBQ), which were used as scavengers for h⁺, ·OH and ·O₂^- radicals, respectively^[35]. Specifically, PBQ exhibited better quenching effects towards the photo-catalytic reactions, which was rather superior to AO and TBA. Specifically, ·O₂^- accounted for ca. 33.2% of the total degradation efficiency, whereas h⁺ (ca. 20.4%) and ·OH (ca. 16.2%) contributed relatively less to the overall photodegradation of tetracycline molecules (Fig. 3e). These results suggested ·O₂^- radicals were likely dominant active species, whereas h⁺ and ·OH radicals played minor roles in this current system.

  We thus propose the visible-light-driven photo-catalysis might be initiated by the fast generation and separation of photogenerated carriers (h⁺, e^-), followed by electron transfer from the valence bond (VB) to conduction band (CB) of Bi₂WO₆^[36]. The highly reduc-tive e^- takes part in the formation of a series of active species including ·OH and ·O₂^- radicals, of which ·OH is initiated via a complex process involving the reduction of H₂O₂ generated by ·O₂^- species. The oxidative h⁺, ·OH and ·O₂^- attack and degrade organic substrates (TC and OTC) into secondary and final products (H₂O and CO₂).

  2.3   Stability

  We further studied photocatalytic TC degradation at various pH values. As shown in Fig. 3b, the f-Bi₂WO₆ displayed faster and more efficient TC degradation rate at higher pH values (9.0 and 11.0), than the TC degradation rate at lower pH value of 3.0, 5.0 and 7.0. These results indicated photocatalytic TC degradation were favored in alkaline solutions which might be reasonable to carry out TC removal before acidification or appropriate pH adjustment would be necessary during wastewater treatment. Moreover, recycling reactions of TC degradation were performed to verify the photostability and reusability of the f-Bi₂WO₆. It was demonstrated that the f-Bi₂WO₆ could preserve more than 95% of its initial catalytic ability at the end of four consecutive runs, as indicated in Fig. 3c. PXRD patterns recorded on the f-Bi₂WO₆ before and after recycling reactions matched well with each other (Fig. 3d), suggesting significant structural and crystalline stability of the photocatalyst. These results suggested that the as-prepared f-Bi₂WO₆ was promising for the potential application in wastewater treatment.

  3.   Conclusions

  In summary, we proposed a facile hydrothermal synthetic pathway to prepare hierarchically structured flower-like bismuth tungstate. The as-prepared f-Bi₂WO₆ photocatalysts exhibited considerably superior photocatalytic capacity towards tetracycline degradation at various pH, as well as cyclic photocatalytic reactions. Active species such as h⁺, ·OH and ·O₂^- were believed to be keys that were responsible for the high photocatalytic efficiency of the materials. Further study to fabricate Bi₂WO₆ photocatalysts with different morphology in similar reaction systems is currently underway, which may help to determine the specific role of structural morphology and possible effects of structure control on the optimization of photocatalytic activity of bismuth tungstate.

  
  Acknowledgements: We thank the International Science and Technology Cooperation and Exchange Project of Fujian Agriculture and Forestry University (Grant No.KXGH17010), the State Key Laboratory of Structural Chemistry (Grant No.20170032) and the New Century Excellent Talents in Fujian Province University for funding.
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• 

  Figure 1  SEM (a, b) and TEM (c, d) images of the f-Bi₂WO₆

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  Figure 2  (a) PXRD patterns, (b) N₂ adsorption-desorption isotherms, (c) DRS spectrum, and (d) K-M plot of the f-Bi₂WO₆

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  Figure 3  (a) Photocatalytic TC and OTC degradation efficiency of the f-Bi₂WO₆; (b) Photocatalytic TC degradation at various pH values; (c) Cyclic TC degradation reactions; (d) PXRD of the f-Bi₂WO₆ before and after cyclic reactions; (e) Photocatalytic efficiency of the f-Bi₂WO₆ with exposure to various scavengers

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