Enhanced visible light photocatalytic performance with metal-doped Bi₂WO₆ for typical fluoroquinolones degradation: Efficiencies, pathways and mechanisms

To clarify the photocatalytic mechanisms of metal-doped Bi₂WO₆ for fluoroquinolones (FQs) degradation, the effects of the chemical characteristics of four metals, molar ratios of the doped metals, morphology of the catalysts, and electrostatic interactions on the degradation of norfloxacin (NOR) and ciprofloxacin (CIP) were evaluated under visible light irradiation. Experimental results implied that the doping of Mg²⁺, Fe³⁺, Zn²⁺ and Cu²⁺ dramatically improved the photodegradation of Bi₂WO₆ for NOR and CIP removal, in which 1% Mg/Bi₂WO₆ exhibited the highest degradation rate of 89.44% for NOR and 99.11% for CIP. Photodegradation of NOR fitted to the pseudo-first-order model (k₁ value of 0.02576 min^-1), whereas that of CIP be better described by pseudo-second-order model. Moreover, the two FQs photodegradation pathways and the possible intermediates were summarized. The mechanisms of the metal dopants for the enhancement of photocatalytic activity were attributed to its enhanced specific surface area, electrostatic absorption, as well as the significant photogeneration of ·O₂^- and h⁺. Also, the photocatalyst exhibited a high stability with 78.5% photocatalytic performance after four cycles of repeated usage.