In situ enhanced chlorine oxide radical generation on a novel space-confined photoanode and its application for ammonia oxidation: Structure, performance, and mechanism

Oxidizing ammonia to nitrogen is a challenge in wastewater treatment. For this challenge ammonia oxidation via chlorine oxide radicals has been proved to be a potential method. In this study, a space-confined photoanode was designed and prepared by loading RuO 2 nanoparticles on TiO 2 /WO 3 nanowires for in situ enhancing the generation of chlorine oxide radical. Scanning electron microscopy characterization confirmed that RuO 2 nanoparticles were uniformly deposited on TiO 2 /WO 3 nanowires. The generation of chlorine oxide radical was markedly boosted, and its steady-state concentration reached 2.76 × 10 −12 M. This value was 3 times that of the traditional bifacial electrode (a type of electrode that has been widely used for chlorine oxide radical-related research). As a result, ammonia oxidation was greatly enhanced. In 90 min, 31.9 mg/L ammonia nitrogen could be degraded, yet only 0.94 mg/L nitrate was accumulated. In addition, the space-confined photoanode exhibited good performance for real wastewater treatment, and 41.0 mg/L ammonia nitrogen could be oxidized in 135 min. During photoelectrocatalysis, a synergistic index (47.7 %) was obtained. Besides the synergistic effect between photocatalysis and electrocatalysis, the unique morphology and structural design (TiO 2 /WO 3 and RuO 2 were mainly responsible to hydroxyl radical generation and free chlorine production, respectively) also synergistically promoted the generation of chlorine oxide radical, thereby improving ammonia oxidation. The mechanism for ammonia oxidation via chlorine oxide radicals over the space-confined photoanode was proposed based on experimental results and theoretical calculations.