Photocatalytic efficacy of supported tetrazine on MgZnO nanoparticles for the heterogeneous photodegradation of methylene blue and ciprofloxacin

Abstract

MgZnO@SiO₂-tetrazine nanoparticles were synthesized and their photocatalytic efficiency was demonstrated in the decomposition of ciprofloxacin and methylene blue (MB). This new heterogeneous nanocatalyst was characterized by FT-IR, XRD, UV-vis, DRS, FE-SEM, ICP, and CHN. Distinctive variables including photocatalyst dose, pH, and degradation time were investigated. Up to 95% photodegradation was gained under the optimum conditions (20 mg photocatalyst, 3.5 ppm MB, pH 9) by using MgZnO@SiO₂-tetrazine nanoparticles after 20 min. An elementary kinetic study was carried out, and a pseudo-first-order kinetic with a reasonably high rate-constant (0.068 min^-1) was derived for the MB decay. Photoluminescence (PL) studies confirmed that the photocatalytic activity of MgZnO@SiO₂-tetrazine was almost consistent with the Taugh plots. Thus, it can be envisaged that the photocatalytic activity is closely related to the optical absorption. Furthermore, a photoreduction mechanism was suggested for the degradation process. Addition of scavengers and some mechanistic studies also revealed that O₂˙^- is the original radical accounting for the degradation of MB, considering this latter compound as a model type pollutant. Finally, efficacy of the present photocatalytic process was assessed in the degradation of ciprofloxacin as a model drug under the optimum reaction conditions.

Fig. 1. FT-IR spectra of (a) MgZnO, (b) MgZnO@SiO₂–Cl, (c) MgZnO@SiO₂-tetrazine, and (d) tetrazine.

Fig. 2. The FE-SEM images of (a) MgZnO and (b) MgZnO@SiO₂-tetrazine nanoparticles.

Fig. 3. The EDX image of MgZnO@SiO₂ nanoparticles.

Fig. 4. Wide-angle XRD patterns of (a) MgZnO and (b) MgZnO@SiO₂-tetrazine nanoparticles.

Fig. 5. Photoluminescence spectra of tetrazine (a), MgZnO (b), and MgZnO@SiO₂-tetrazine (c).

Fig. 6. DRS spectrum of MgZnO@SiO₂-tetrazine (a); UV-vis spectra of MgZnO@SiO₂-tetrazine, MgZnO, and tetrazine (b).

Fig. 7. TGA profile of MgZnO@SiO₂-tetrazine nanoparticles.

Fig. 8. Effects of light and photocatalyst type on the degradation of an aqueous solution of MB ([MB]: 4 ppm, degradation time: 40 min, pH 7).

Fig. 9. Effect of MB concentration on the discoloration efficiency (40 mL of MB solution, 5 mg of MgZnO@SiO₂-tetrazine, degradation time 20 min, and pH 7).

Fig. 10. Effect of photocatalyst dosage on the decolorization efficiency of 40 mL solution of dye, [MB]: 3.5 ppm, degradation time: 20 min, and pH 7.

Fig. 11. Effect of pH on the photodecolorization efficiency (40 mL of MB solution, MgZnO@SiO₂-tetrazine: 20 mg, [MB]: 3.5 ppm, degradation time: 20 min).

Fig. 12. Reusability of MgZnO@SiO₂-tetrazine in the photodegradation of MB. 40 mL solution of 3.5 ppm MB, MgZnO@SiO₂-tetrazine: 0.02 g, degradation time: 20 min, and pH 9.

Fig. 13. Degradation of ciprofloxacin with time in the presence of MgZnO@SiO₂-tetrazine.

Fig. 14. Kinetics of MB degradation catalyzed by tetrazine, MgZnO, and MgZnO@SiO₂-tetrazine (40 mL of 3.5 ppm MB solution, photocatalyst: 20 mg, degradation time: 20 min, and pH 9).

Fig. 15. Reaction kinetic rate constants for MB photodegradation over tetrazine, MgZnO, and MgZnO@SiO₂-tetrazine.

Fig. 16. Effects of some hole scavengers on the photodegradation of MB (40 mL of 8 ppm MB solution, 0.005 mmol of hole scavenger, pH 7, and MgZnO@SiO₂-tetrazine: 5 mg).

Fig. 17. Plots of (αhυ)²vs. photon energy of tetrazine, MgZnO, and MgZnO@SiO₂-tetrazine.

Scheme 1. Proposed photocatalysis mechanism for MgZnO@SiO₂-tetrazine.

Fig. 18. A schematic representation for the preparation of MgZnO@SiO₂–Cl.

Fig. 19. Synthetic scheme for the preparation of “tetrazine”.

Fig. 20. A schematic for the preparation of MgZnO@SiO₂-tetrazine.