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 N-2, and the production of toxic photocatalytic oxidation by-products of NO and NO 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 the effective photocatalytic removal of aqueous NH4+-NH3 in a weak alkaline solution was achieved for the first time. (C) 2014 Elsevier B.V. All rights reserved.