Abnormal electron-donation-dependent sulfite activation in the Mn(II)-nitrilotriacetic acid system: QSAR and mechanistic insights

In advanced oxidation processes (AOPs), electron-rich contaminants are generally susceptible to oxidation by reactive species. Herein, an unusual substrate-selective degradation was observed in sulfite (Na₂SO₃) based AOPs with the Mn(II)-nitrilotriacetic acid (NTA) complex. The strong electron-rich phenolics were hardly degraded, whereas electron-deficient phenols were efficiently removed. An asymmetric volcano-shaped correlation between the degradation rates and Hammett constants σ⁺ was established via quantitative structure-activity relationship analysis and can be expanded to antibiotics and dyes with anodic peak potential as a descriptor. Mechanistic investigations demonstrate that electron-rich phenolics complex with Mn(II)-NTA, suppressing O₂-driven Mn(III)-NTA formation and thus limiting Mn(V)-NTA accumulation, with a slow radical chain reaction for Na₂SO₃ oxidation. In contrast, electron-deficient substrates promote Mn(III)-NTA generation, fast Na₂SO₃ oxidation and transformation to peroxymonosulfate (PMS), and subsequent Mn(V)-NTA formation. Consequently, radical pathways participate in the degradation of electron-rich pollutants, whereas Mn(V)-NTA governs the oxidation of electron-deficient ones. The low-cost Na₂SO₃ system shows comparable or superior performance to PMS-based systems for electron-deficient pollutants, highlighting its economic viability. These findings underscore the importance of pollutant-catalyst interactions in AOPs and provide mechanistic insights for targeted contaminant remediation.