Efficient photocatalytic removal of aqueous NH4+–NH3 by palladium-modified nitrogen-doped titanium oxide nanoparticles under visible light illumination, even in weak alkaline solutions

• Photocatalytic removal of aqueous NH4+–NH3 under visible light illumination.
• High aqueous NH4+–NH3 removal ratio of over 90%.
• Minimized production of toxic by-products of NO3− and NO2−.
• Effective photocatalytic removal of aqueous NH4+–NH3 in a weak alkaline solution.
• An environmentally friendly and cost-effective approach to treat aqueous NH4+–NH3.

Aqueous ammonium–ammonia (NH4+–NH3) is a commonly found pollutant in industrial and domestic wastewater, which poses a great threat to both the environment and the human health. Photocatalytic treatment had been proved to be an efficient approach to oxidize aqueous NH4+–NH3 for its removal. However, most current researches still suffer from the requirements of UV illumination and strong alkaline environment, low removal rates, and high ratios of toxic oxidation by-products of NO3− and NO2−. In this study, palladium-modified nitrogen-doped titanium oxide (TiO2−xNx/PdO) nanoparticles were synthesized by a simple sol–gel process, which demonstrated an effective photocatalytic removal of aqueous NH4+–NH3 under visible light illumination. By optimizing the reaction conditions, a high removal rate of over 90% could be achieved by TiO2−xNx/PdO nanoparticles under only visible light illumination with the major product of N2, and the production of toxic photocatalytic oxidation by-products of NO3− and NO2− was minimized. The real time solution pH was found to be critical for the photocatalytic oxidation of aqueous NH4+–NH3. By keeping the solution pH constantly at ∼8, the effective photocatalytic removal of aqueous NH4+–NH3 in a weak alkaline solution was achieved for the first time.

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