Sulfite activation by ultralow-dose Mn(III) porphyrin for efficient degradation of sulfadiazine via superoxide radical and singlet oxygen

Sulfite (SO 3 2− ) has emerged as an economical and eco-friendly precursor for reactive species generation in advanced oxidation processes (AOPs). However, conventional activation strategies typically require high sulfite inputs and generate non-selective radicals, limiting their efficiency and practicality. Here, we demonstrate that a trace amount of water‑soluble manganese porphyrin complex (Mn(III)‑TCPP, as low as 4 μM), can efficiently activate Na 2 SO 3 at ultralow concentrations (10–100  μM) to degrade sulfadiazine. The Mn(III)-TCPP/Na 2 SO 3 system exhibited an initial rate 41 times faster than the Mn 2+ /Na 2 SO 3 system and 4.2 times greater than the reported -Mn(III)-NTA/Na 2 SO 3 system. A linear correlation between the degradation rate constant and the Hammett constant (σ + ) of porphyrin substituents confirms that ligand electronic structure modulates Mn redox cycling during SO 3 2− oxidation by dissolved O 2 . Mechanistic studies, including time‑resolved spectroscopy, electrochemical analysis, and reactive species trapping, reveal that the reaction proceeds primarily via superoxide radical and singlet oxygen, representing a distinct departure from conventional sulfate radical or high-valent Mn(V) that has been reported in other metal–ligand AOP systems. This work establishes a new, highly efficient sulfite activation driven by an ultralow-dose metalloporphyrin catalyst and provides fundamental insight into non‑radical oxidation routes for sulfite-based AOPs.