Selective oxidation of ammonium to dinitrogen by a novel catalytic ozonation system: Regulating the N2 selectivity by sulfite

The selective oxidation of NH₄⁺-N into dinitrogen (N₂) is still a challenge. Currently, traditional advanced oxidation processes often involve in the chlorine free radicals to increase the selectivity of NH₄⁺-N oxidation products towards N₂ but is usually accompanied by the production of many toxic disinfection by-product. Herein, we reported a novel catalytic ozonation system (UV/O₃/MgO/Na₂SO₃) for selective NH₄⁺-N oxidation based on the reducing capability and photochemical properties of Na₂SO₃. In the UV/O₃/MgO/Na₂SO₃/NH₄⁺-N system, Na₂SO₃ could not only reduce the intermediate of NO₂⁻ or NO₃⁻ to N₂ by inducing the generation of hydrated electrons under UV irradiation, but also reduce the gaseous intermediate of NO_x to N₂, thus achieving a high N₂ selectivity (>85 %). Based on the analyses of each component roles, the determination of reactive oxygen species and the evolution of NH₄⁺-N oxidation intermediates, the possible mechanisms of NH₄⁺-N selective oxidation by UV/O₃/MgO/Na₂SO₃ system were revealed. This system exhibits a great potential for the NH₄⁺-N removal from water/wastewater. This work provides a new strategy for NH₄⁺-N oxidation into N₂ by advanced oxidation processes independent of the action of chlorine free radicals.