Red mud-based biochar activated peroxydisulfate under visible light for tetracycline removal: Synergistic effect of FeAl2O4 photocatalysis

Heterogeneous photocatalytic activation of persulfate (PDS) attracted much attention due to its positive environmental impact and efficacy in wastewater treatment. In this paper, a number of photocatalysts (RHRMX) were successfully synthesized by co-pyrolysis with rice husk (RH) using red mud (RM) as a precursor of Fe. Among them, RHRM50 exhibited excellent catalytic performance, with a high degradation rate of 99.5% for 20 mg/L tetracycline hydrochloride (TCH) in 60 min and its mineralization rate of 85.1% under the synthesized photocatalytic coupled PDS activation. In addition, the degradation efficiency under light conditioned remained above 95% after 5 cycles. The combination of the photoelectric properties and the Mott-Schottky spectrum of RHRM50 suggested that the photocatalysis was mainly attributable to FeAl2O4, with the energy band gap was determined to be 1.782 eV. Electron spin resonance (ESR) and burst experiments showed that under light, the FeAl2O4 generated holes mainly reacted with PDS to form 1O2, which was the main factor leading to the increase in the degradation rate. Based on density functional theory (DFT) calculations and identification of intermediates by liquid chromatography-mass spectrometry (LC-MS), hypothetical four potential pathways for TCH degradation were proposed. In addition, the potential ecotoxicity of TCH and its intermediates was evaluated using the ecological conformational relationship (ECOSAR). In conclusion, the green strategy of using RM to prepare RHRMX had a broad application prospect in the field of wastewater treatment and environmental protection.