Synthesis and application of Bi2WO6 for the photocatalytic degradation of two typical fluoroquinolones under visible light irradiation

Abstract

Bismuth tungstate (Bi₂WO₆) was successfully synthesized by a method combining ultrasonic solvothermal treatment and high-temperature calcination. The products were affirmed by X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The characterization results indicated that calcination could improve the crystallinity and visible light utilization capacity of Bi₂WO₆. The photodegradation experiments showed that Bi₂WO₆ calcined at 450 °C for 3 h exhibited better photocatalytic activity for the degradation of norfloxacin and enrofloxacin under visible light irradiation than the catalyst prepared without calcination or calcined at other temperatures. Meanwhile, the effects of the amount of 450-Bi₂WO₆, the initial concentration of targets, and the pH of the solutions on the degradation were studied. Under the optimal conditions, the removal ratios reached to 92.95% (for norfloxacin) and 94.58% (for enrofloxacin) within 75 min. Furthermore, h⁺ and ·O₂ ^- were identified to affect the photodegradation process significantly, and the possible photocatalytic mechanism was proposed. The as-prepared sample was verified to possess good stability and reusability, suggesting its potential application prospect in the treatment of fluoroquinolone antibiotics.

Fig. 1. XRD patterns of the as-prepared Bi₂WO₆ samples.

Fig. 2. SEM images of (a) bulk-Bi₂WO₆, (b) 350-Bi₂WO₆, (c) 450-Bi₂WO₆, (d) 550-Bi₂WO₆.

Fig. 3. (a) UV-vis DRS spectra and (b) forbidden bandwidths of Bi₂WO₆ samples.

Fig. 4. High-resolution XPS of 450-Bi₂WO₆ (a) total survey, (b) Bi 4f, (c) W 4f, (d) O 1 s.

Fig. 5. FT-IR spectra of Bi₂WO₆ samples.

Fig. 6. Photocatalytic degradation of (a) NOR and (b) ENR with different photocatalysts (10 mg L⁻¹ drug solution, 0.5 g L⁻¹ photocatalyst, initial pH of 10.2 for NOR and of 10.7 for ENR).

Fig. 7. Effects of the operation parameters on the degradation of NOR or ENR (different amounts of 450-Bi₂WO₆ for the solutions with the initial concentration of 10 mg L⁻¹ at original pH; solutions with different initial concentrations at 450-Bi₂WO₆ dosage of 0.5 g L⁻¹ and original pH; solutions with the initial concentration of 10 mg L⁻¹ at 450-Bi₂WO₆ dosage of 0.5 g L⁻¹ and different initial pH).

Fig. 8. Effect of different scavengers on the degradation efficiency of NOR with 450-Bi₂WO₆.

Fig. 9. Mott–Schottky plots of 450-Bi₂WO₆.

Scheme 1. Possible photocatalytic mechanism of the 450-Bi₂WO₆ sample.

Fig. 10. (a) Results of recycling tests and (b) XRD of 450-Bi₂WO₆ after photocatalytic degradation of NOR for four cycles.