Novel S-scheme heterojunction of red mud-based Fe2O3/Co-Al-LDH for the photo-Fenton degradation of gatifloxacin under visible light

Abstract

Gatifloxacin (GAT), as one of the fourth-generation fluoroquinolone antibiotics, has been widely used in the medical industry to treat bacterial infections, while also was increasingly detected in aquatic environments. Red Mud (RM) was one kind of industrial solid waste residue discharged from the extraction process of alumina by Bayer technology. In this study, a novel RM-based heterojunction catalyst composed of Fe₂O₃ and Co-Al layered double hydroxide (Fe/Co-Al-LDH/RM) was fabricated using RM as raw material through a simple Mechanochemical Synthesis (MCS) method, and Fe/Co-Al-LDH/RM was then used as catalyst for photo-Fenton system to degrade GAT in aquatic environments under visible light (Vis + Fe/Co-Al-LDH/RM + H₂O₂ system). After reacting for 120 min, 94.0 % of GAT (20 mg/L, 300 mL) could be removed with the optimum reaction conditions as follows: [Fe/Co-Al-LDH/RM] = 0.03 g/L, [H₂O₂] = 90 mmol/L and initial pH = 6.5. The constructed reaction system was proven to be effective in degrading GAT in various natural water sources. The characterizations revealed that the action of MCS could significantly improve the physicochemical and photoelectrochemical properties of Fe/Co-Al-LDH/RM, and density functional theory (DFT) calculation proved that Fe/Co-Al-LDH/RM exhibited an unique S-type charge transfer pathway under visible light. The quenching experiments showed that Co(IV) and Fe(IV) were the predominant active species for GAT degradation in the weak acidic system of Vis + Fe/Co-Al-LDH/RM + H₂O₂, while h⁺ and ·OH were the predominant active species for GAT degradation in the strong acidic system of Vis + Fe/Co-Al-LDH/RM + H₂O₂. This study not only uses industrial waste RM as raw material to successfully synthesize a catalyst with S-scheme heterojunctions though a simple MCS method, but also advances the development of GAT degradation technology in photo-Fenton system, achieving the goal of “using waste to treat waste”.