Integrated Ozonation Ni-NiO/Carbon/g-C3N4 Nanocomposite-Mediated Catalytic Decomposition of Organic Contaminants in Wastewater under Visible Light

Abstract

Developing a hybrid process for wastewater purification is of utmost importance to make conventional methods more efficient and faster. Herein, an effective visible light-active nickel-nickel oxide/carbon/graphitic carbon nitride (Ni-NiO/C/g-C₃N₄)-based nanocatalyst was developed. A hybrid process based on ozonation and Ni-NiO/C/g-C₃N₄ visible light photocatalysis was applied to decolourize the Congo red (CR), Alizarin Red S (ARS), and real dairy industry wastewater. The synthesized catalyst was characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Χ-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-Vis diffuse reflectance spectrophotometry (UV-Vis DRS). The factors affecting the catalytic process were evaluated, including contact time, solution pH, initial dye concentration, etc. The degradation rate of CR and ARS was compared between the photocatalysis, ozonation, and integrated photocatalytic ozonation (PC/O₃) methods. The results showed 100% degradation of CR and ARS within 5 min and 40 min, respectively, by integrated PC/O₃. The reusability of the modified catalyst was evaluated, and four successive regenerations were achieved. The modified Ni-NiO/C/g-C₃N₄ composite could be considered an effective, fast, and reusable catalyst in an integrated PC/O₃ process for the complete decolourization of wastewater.

Keywords: hybrid catalytic process; ozonation; photocatalysis; wastewater treatment.

Figure 1

XRD pattern of the NiO, g-C₃N₄, and Ni-NiO/C/g-C₃N₄ hybrid nanocomposite.

Figure 2

SEM images of the Ni-NiO/C/g-C₃N₄ composite at different magnifications: (a) 10k, (b) 30k.

Figure 3

XPS of Ni-NiO/C/g-C₃N₄ composite catalyst: (a) wide scan survey, (b) C1s, (c) N1s, (d) O1s, (e) Ni2p.

Figure 4

(a) UV-visible absorption spectrum. (b) Tauc plots of NiO and Ni-NiO/C/g-C₃N₄ nanohybrid.

Figure 5

Comparison of the photocatalytic performance of NiO, g-C₃N₄, and Ni-NiO/C/g-C₃N₄ composite and the ozonation catalytic degradation of CR and ARS (concentration 20 mg/L, pH 5.0 for CR and 9.0 for ARS, contact time 180 min, Vol. 50 mL, catalyst mass 0.05 g).

Figure 6

Effect of pH on photocatalysis, ozonation, and integrated PC/O₃: (a) CR degradation, (b) ARS degradation (concentration 20 mg/L; contact time 180 min; vol. 50 mL; catalyst Ni-NiO/C/g-C₃N₄; catalyst mass 0.05 g; ozone flow rate 20 g/h).

Figure 7

Effect of contact time on photocatalysis, ozonation and integrated photocatalytic ozonation: (a) CR degradation, (b) ARS degradation (concentration 20 mg/L for both dyes, pH 5.0 for CR and 9.0 for ARS, Vol. 50 mL, catalyst: Ni-NiO/C/g-C₃N₄, catalyst mass 0.05 g, ozone flow rate 20 g/h).

Figure 8

Effect of initial concentration of photocatalysis, ozonation, and integrated photocatalytic ozonation: (a) CR degradation, (b) ARS degradation (pH 5.0 for CR and 9.0 for ARS, contact time 180 min, Vol. 50 mL, catalyst: Ni-NiO/C/g-C₃N₄, catalyst mass 0.05 g, ozone flow rate 20 g/h).

Figure 9

COD removal by photocatalysis, ozonation, and integrated photocatalytic ozonation (initial COD concentration 4200 mg/L, pH 4.0, Vol. 50 mL, contact time 180 min, catalyst: Ni-NiO/C/g-C₃N₄, catalyst mass 0.05 g, ozone flow rate 20 g/h).

Figure 10

Regeneration study of (a) CR and (b) ARS degradation using the Ni-NiO/C/g-C₃N₄ composite (concentration 20 mg/L, pH 5.0 for CR and 9.0 for ARS, contact time 180 min, Vol. 50 mL, catalyst: Ni-NiO/C/g-C₃N_4, catalyst mass 0.05 g).

Figure 11

A schematic diagram showing the production of active radical species under (a) photocatalysis and (b) integrated photocatalysis–ozonation.