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

Area of Science:

• Environmental Chemistry
• Water Treatment Technologies
• Advanced Oxidation Processes

Background:

• Selective conversion of ammonia to dinitrogen (N₂) is critical for water treatment but remains a significant challenge.
• Advanced oxidation processes (AOPs) are explored for ammonia removal, but pathway selectivity is often poor.
• Divergent reaction pathways between sulfate radical (SO₄•⁻) and hydroxyl radical (HO•) mediated AOPs require systematic investigation.

Purpose of the Study:

• To systematically investigate the efficiency and selectivity of SO₄•⁻ and HO• based AOPs for ammonia conversion.
• To elucidate the underlying reaction mechanisms for selective N₂ generation versus nitrate formation.
• To provide a mechanistic foundation for designing advanced nitrogen removal technologies.

Main Methods:

• Comparative study of UV/persulfate (PDS) for SO₄•⁻ and UV/H₂O₂, O₃ for HO• mediated ammonia oxidation.
• In-situ ¹⁵N nuclear magnetic resonance (NMR) and surface-enhanced Raman spectroscopy (SERS) for intermediate detection.
• Density functional theory (DFT) calculations to assess thermodynamic favorability of proposed reaction pathways.

Main Results:

• SO₄•⁻-based systems (UV/PDS) achieved >96% ammonia removal with >89% N₂ selectivity at pH ~8.5.
• HO•-based systems (UV/H₂O₂, O₃) predominantly led to nitrate formation via over-oxidation.
• Hydrazine-like intermediates (N₂Hₓ) were directly detected, confirming SO₄•⁻ promotes aminyl radical coupling (Gerischer-Mauerer pathway) via hydrogen atom abstraction.

Conclusions:

• Sulfate radicals exhibit superior selectivity for ammonia conversion to N₂ compared to hydroxyl radicals.
• Radical coupling, initiated by hydrogen abstraction, is a key convergent step for N₂ generation.
• The findings provide a mechanistic basis for developing targeted nitrogen removal technologies using SO₄•⁻.