Sulfate radical promotes aminyl radical coupling for selective ammonia conversion to harmless dinitrogen

The selective conversion of ammonia to environmentally benign dinitrogen (N₂) remains a major challenge in water treatment. This study systematically investigates the efficiency and selectivity of various advanced oxidation processes, revealing a fundamental divergence between sulfate radical (SO₄^•-)- and hydroxyl radical (HO^•)-mediated pathways. We demonstrate that SO₄^•--based systems (e.g., UV/PDS) achieve highly efficient ammonia removal (>96 %) with exceptional N₂ selectivity (>89 %) under environmentally relevant conditions (pH ∼8.5). In stark contrast, HO^•-based systems (e.g., UV/H₂O₂, O₃) favor over-oxidation to yield nitrate. By employing a combination of in-situ ¹⁵N nuclear magnetic resonance (NMR) and surface-enhanced Raman spectroscopy (SERS), we directly captured hydrazine-like intermediates (N₂H_x, 1≤ x ≤ 4), providing unambiguous evidence that SO₄^•- promotes the Gerischer-Mauerer pathway via aminyl radical (NH₂^•) coupling. This mechanism is initiated by the inherent preference of SO₄^•- for hydrogen atom abstraction from N-H bonds. Density functional theory calculations confirm the thermodynamic favorability of this pathway over sequential dehydrogenation. Importantly, the detection of similar N₂H_x intermediates during both chlorination and SO₄^•--mediated oxidation suggests that radical coupling is a convergent and critical step for N₂ generation. This work provides a mechanistic foundation for designing selective nitrogen-removal technologies based on hydrogen-abstraction-initiated radical coupling.