Insight into the visible light activation of sulfite by Fe/g-C₃N₄ with rich N vacancies for pollutant removal and sterilization: A novel approach for enhanced generation of oxysulfur radical

A novel Fe-modified g-C₃N₄ with rich N vacancies (NVs) under N₂ calcination was developed to active sulfite for the efficient removal of contaminants and bacterial inactivation under visible light (VIS). The degradation rates of VIS/sulfite/Fe/g-C₃N₄(N₂) for acetaminophen (ACT) were 10.94, 4.88, and 2.85 times higher than those of VIS/Fe/g-C₃N₄(N₂), VIS/sulfite/g-C₃N₄(N₂), and VIS/sulfite/Fe/g-C₃N₄(CO₂), respectively. Escherichia coli (E. coli) with an initial concentration of 2.5 × 10⁸ CFU/mL decreased by 6.1 and 8.4 log CFU/mL within 60 min and 80 min of inactivation, respectively. The activation of sulfite led to the generation of oxysulfur radicals, and the consumption of hole (h⁺) by sulfite inhibited its recombination with electron (e⁻), promoting the formation of superoxide radical (^•O₂^–). The synergistic effect of Fe, NVs, and g-C₃N₄ in VIS/sulfite/Fe/g-C₃N₄(N₂) system promoted the generation of oxysulfur radicals. Density functional theory (DFT) calculations indicated that NVs facilitated charge transfer from Fe to g-C₃N₄ substrate, accelerating the Fe³⁺/Fe²⁺ and oxysulfur radical cycles. Fe/g-C₃N₄ with rich NVs exhibited lower adsorption binding energy, faster charge transfer, and longer S-O bond length than pristine Fe/g-C₃N₄ during sulfite adsorption. This work unraveled the synergistic mechanism of Fe, NVs, and g-C₃N₄, which provides new insights into heterogeneous photocatalysts on sulfite activation for water treatment.