Nanostructured MnO x /g-C3N4 for photodegradation of sulfamethoxazole under visible light irradiation

Abstract

The effectiveness of g-C₃N₄ as photocatalyst is hindered by the rapid recombination of photo-generated electron/hole pairs. To improve its photocatalytic performance, the incorporation of g-C₃N₄ with co-catalysts can promote charge separation efficiency and enhance redox capabilities. In our study, a two-step approach involving calcination and solvothermal method was utilized to fabricate a proficient MnO _x /g-C₃N₄ heterojunction photocatalyst with high photocatalytic activity. MnO _x is effective at capturing holes to impede the recombination of electron/hole pairs. The MnO _x /g-C₃N₄ composite shows a notable improvement in photocatalytic degradation of SMX, obtaining an 85% degradation rate, surpassing that of pure g-C₃N₄. Furthermore, the MnO _x /g-C₃N₄ composite exhibits remarkable and enduring catalytic degradation capabilities for sulfamethoxazole (SMX), even after four consecutive reuse cycles. The intermediates produced in the MnO _x /g-C₃N₄ system are found to be less hazardous to common aquatic creatures such as fish, daphnids, and green algae when compared to SMX. With its high tolerance, exceptional degradation ability, and minimal ecological risk, the MnO _x /g-C₃N₄ composite emerges as a promising candidate for eliminating antibiotics from wastewater resources.

Fig. 1. (A) XRD pattern, (B) FT-IR spectra of CN, 1MnCN, 3MnCN, and 5MnCN; (C) N₂ adsorption–desorption isotherms, and (D) pore width of CN and 3MnCN.

Fig. 2. SEM images of CN (A), 1MnCN (B), 3MnCN (C), and 5MnCN (D); TEM images of CN (E) and 3MnCN (F); EDS mapping analysis of 3MnCN (G). SEM image of the selected-area EDS (G1), and the elemental distribution images of N (G2), O (G3), C (G4), and Mn (G5).

Fig. 3. XPS spectra of CN and 3MnCN. High resolution C 1s (A), O 1s (B), N 1s (C), and Mn 2p (D).

Fig. 4. UV-vis diffuse reflectance spectra (A) and PL spectra (B) of as prepared CN, 1MnCN, 3MnCN, and 5MnCN; the electrochemical impedance spectra (EIS) (C), transient photocurrent curves (D), Mott–Schottky plots (E), and band structures (F) of CN and 3MnCN.

Fig. 5. Effect of different photocatalyst on (A) photodegradation of SMX, and first-order kinetics plots (B) for CN, 1MnCN, 3MnCN, and 5MnCN; UV-Vis spectra for photodegradation of SMX (λ = 267 nm) on 3MnCN (C) and CN (D). Conditions: [SMX]₀ = 15 mg L⁻¹, [CN]₀ (or [xMnCN]₀) = 200 mg L⁻¹, V = 100 mL, light source: 40 W white LED lamp. The source data is presented in Table S3.

Fig. 6. Effect of initial pH on (A) SMX photodegradation, and first-order kinetics (B) plots; effect of various photocatalyst weight on (C) degradation of SMX and first-order kinetics (D) plots; conditions: [SMX]₀ = 15 mg L⁻¹, 3MnCN as a photocatalyst, V = 100 mL, light source: 40 W white LED lamp. The source data is presented in Tables S4 and S5.

Fig. 7. Scavengers (A) of K₂Cr₂O₇/e⁻ (a), Na₂C₂O₄/h⁺ (b), TBA/˙OH (c), and BQ/˙O₂⁻ (d), and no scavenger (e); photodegradation mechanism of SMX over 3MnCN under visible light irradiation (B). Conditions: [SMX]₀ = 15 mg L⁻¹, [catalyst]₀ = 200 mg L⁻¹, V = 100 mL, light source: 40 W white LED lamp. The source data is presented in Table S6.

Fig. 8. The proposed SMX photodegradation pathway by 3MnCN (A); change in toxicity during the photodegradation of SMX by 3MnCN via ECOSAR (B, C).

Fig. 9. Recycling of photodegradation (A) of SMX upon the 3MnCN photocatalyst. XRD pattern (B) of 3MnCN of fresh (a) and reuse (b) and TEM image (C) of 3MnCN reuse.