g–C₃N₄ induced acceleration of Fe³⁺/Fe²⁺ cycles for enhancing metronidazole degradation in Fe³⁺/peroxymonosulfate process under visible light

In this study, a Fe³⁺/g-C₃N₄ hybrid catalyst system was proposed to activate peroxymonosulfate (PMS) for the metronidazole (MNZ) photocatalytic degradation. The two catalysts, Fe³⁺ and g-C₃N₄, exhibited an obvious synergistic effect in the photocatalytic degradation process. When 1 mM PMS, 0.04 mM Fe³⁺ and 0.05 g L⁻¹ g-C₃N₄ were applied, the rate constant of the Fe³⁺/g-C₃N₄/PMS/LED process at 0.07288 min⁻¹ is around 3.6 to 6.8 times faster than that of Fe³⁺/PMS/LED and g-C₃N₄/PMS/LED processes at 0.0198 and 0.01076 min⁻¹, respectively. Under visible light, electron transfer from photo-activated g-C₃N₄ to Fe³⁺, resulting in the continuous regeneration of Fe²⁺ in the system, which ensures non-stopping production of radicals for MNZ degradation. UV–visible spectra were used to confirm the regeneration of Fe²⁺. In addition, EPR tests were used to identify the reactive oxygen species involved in the reaction system. Typically, the effects of various operation parameters, including the catalyst dosage, PMS dosage, initial concentration of MNZ and initial pH were examined. This work provided a new idea of promoting pollutant degradation by accelerating Fe³⁺/Fe²⁺ redox through semiconductor, which could help to use the catalyst more effectively for wastewater treatment and/or chemical industries.