Reconfiguring reaction pathways for rapid low-temperature catalytic ammonia removal from wastewater

Ammonia removal is an essential part of wastewater treatment, and complex biological processes are currently the mainstream, converting ammonia to N₂ with a large footprint and lengthy period. High-temperature (150°C or higher) and high-pressure catalytic oxidation were applied to ammonia conversion, which suffered high investment and operational costs. This study developed a low-temperature catalytic air oxidation process with a fabricated nanoscale Ru/Al catalyst. Through continuous reaction with pH adjustment, it achieved an ammonia conversion rate of 92.0 % at 90°C, with 72.8 % converted to N₂. During the treatment, ammonia was converted to the key intermediate product N₂H₂, accompanied by a small amount of N₂H₄, and they were eventually turned into N₂. In contrast, at 150°C, ammonia was converted to NHO and then to N₂, and the selectivity was only 43.9 % (most became nitrate). Ammonia form was the critical factor influencing activity energy and conversion efficiency, and maintaining ammonia molecules by pH adjustment could reduce the energy barrier. The formation of the proposed intermediates, N₂H₂ and N₂H₄, was corroborated by FTIR, Raman spectroscopy, and DFT calculations, which confirmed their role in the low-temperature pathway. These findings revealed a new pathway from ammonia to N₂ and provided a unique technical option to clean ammonia-containing wastewater.