Visible-light activation of H₂O₂ by red-mud/biochar: ¹O₂ pathway for efficient tetracycline degradation

Abstract

The implementation of the nonradical (¹O₂) pathway in conventional Fenton reactions is a promising approach for degrading organic pollutants in water. In this study, a one-step co-pyrolysis method of red mud (RM) and spent coffee grounds (SCG) was employed to prepare an iron-rich catalyst (RMSCG70) for activating H₂O₂ in a photo-Fenton system to degrade tetracycline hydrochloride (TCH). RMSCG70 exhibited excellent TCH degradation performance under visible-light irradiation. At a catalyst dosage of 0.1 g/L, H₂O₂ concentration of 3 mmol/L, and TCH concentration of 20 mg/L, the TCH removal rate reached 99.3% within 60 min. Quenching experiments conducted under the light and dark conditions, together with Electron Paramagnetic Resonance (EPR) analyses, indicated that ¹O₂ is the main reactive oxygen species (ROS). The {\text{O}}_2^{\text{●}-} generated by O₂ reduction reacted with the {}^{\text{●}}{\text{OH}}_{\mathrm{a}\mathrm{d}\mathrm{s}} on the catalyst surface to form ¹O₂, and the FeAl₂O₄ component in RMSCG70 in the photo-Fenton system promoted the generation of {\text{O}}_2^{\text{●}-} and {}^{\text{●}}{\text{OH}}_{\mathrm{a}\mathrm{d}\mathrm{s}}. Cyclic experiments showed that the TCH degradation efficiency remained around 90% after five cycles. Further investigation of its application potential in a continuous-flow reactor revealed that when the hydraulic retention time (t_hr) was 30 min, the TCH removal rate reached approximately 80%. This study revealed the mechanism of organic pollutants under visible-light irradiation using RM and SCG as photocatalysts, while enabling the resource utilization of solid waste.

Graphical abstract

Keywords

Photo-Fenton / Singlet oxygen / Adsorbed hydroxyl radical / Red mud

Highlight

	● A photocatalyst material was prepared in one-step using two types of waste materials.
	● In the photo-Fenton system, 1O2 is the dominant active substance.
	● The {\text{O}}_2^{\text{●–}} produced by O2 reduction reacts with the {}^{\text{●–}}OHads generated on the surface of the catalyst to convert into 1O2.
	● Efficient TCH degradation can be achieved under different hydraulic residence times in continuous flow reactors.

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