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 (O-3), ultrasound (US), catalyst (SrO-Al2O3), ultrasonic ozonation (US/O-3), ultrasound-enhanced SrO-Al2O3 (SrO-Al2O3/US), SrO-Al2O3 ozonation (SrO-Al2O3/O-3) and ultrasound-enhanced SrO-Al2O3 ozonation (SrO-Al2O3/US/O-3) under the same experimental conditions. The results indicated that the combined SrO-Al2O3/US/O-3 process achieved the highest NH4+ conversion rate due to the synergistic effect between US, SrO-Al2O3 and O-3. Additionally, the effect of different operational parameters on ammonia oxidation in SrO-Al2O3/O-3 and SrO-Al2O3/US/O-3 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 O-3 and center dotOH at high pH, whereas the NH4+ oxidation is only carried out through center dotOH at low pH. Compared with the SrO-Al2O3/O-3 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/O-3 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, CO32-, HCO3-, and SO42- inhibited the ammonia oxidation in the SrO-Al2O3/US/O-3 system. During ammonia conversion, SrO-Al2O3 catalyst not only has a certain adsorption effect on NH4+ but accelerates the O-3 decomposition to center dotOH.