Ultrasound-Enhanced Catalytic Ozonation Oxidation of Ammonia in Aqueous Solution

Excessive ammonia is a common pollutant in the wastewater, which can cause eutrophication, poison aquatic life, reduce water quality and even threaten human health. Ammonia in aqueous solution was converted using various systems, i.e., ozonation (O₃), ultrasound (US), catalyst (SrO-Al₂O₃), ultrasonic ozonation (US/O₃), ultrasound-enhanced SrO-Al₂O₃ (SrO-Al₂O₃/US), SrO-Al₂O₃ ozonation (SrO-Al₂O₃/O₃) and ultrasound-enhanced SrO-Al₂O₃ ozonation (SrO-Al₂O₃/US/O₃) under the same experimental conditions. The results indicated that the combined SrO-Al₂O₃/US/O₃ process achieved the highest NH₄⁺ conversion rate due to the synergistic effect between US, SrO-Al₂O₃ and O₃. Additionally, the effect of different operational parameters on ammonia oxidation in SrO-Al₂O₃/O₃ and SrO-Al₂O₃/US/O₃ systems was evaluated. It was found that the ammonia conversion increased with the increase of pH value in both systems. The NH₃(aq) is oxidized by both O₃ and ·OH at high pH, whereas the NH₄⁺ oxidation is only carried out through ·OH at low pH. Compared with the SrO-Al₂O₃/O₃ system, the ammonia conversion was significantly increased, the reaction time was shortened, and the consumption of catalyst dosage and ozone were reduced in the SrO-Al₂O₃/US/O₃ system. Moreover, reasonable control of ultrasonic power and duty cycle can further improve the ammonia conversion rate. Under the optimal conditions, the ammonia conversion and gaseous nitrogen yield reached 83.2% and 51.8%, respectively. The presence of tert-butanol, CO₃^2-, HCO₃^-, and SO₄^2- inhibited the ammonia oxidation in the SrO-Al₂O₃/US/O₃ system. During ammonia conversion, SrO-Al₂O₃ catalyst not only has a certain adsorption effect on NH₄⁺ but accelerates the O₃ decomposition to ·OH.