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-Al(2)O(3)), ultrasonic ozonation (US/O(3)), ultrasound-enhanced SrO-Al(2)O(3) (SrO-Al(2)O(3)/US), SrO-Al(2)O(3) ozonation (SrO-Al(2)O(3)/O(3)) and ultrasound-enhanced SrO-Al(2)O(3) ozonation (SrO-Al(2)O(3)/US/O(3)) under the same experimental conditions. The results indicated that the combined SrO-Al(2)O(3)/US/O(3) process achieved the highest NH(4)(+) conversion rate due to the synergistic effect between US, SrO-Al(2)O(3) and O(3). Additionally, the effect of different operational parameters on ammonia oxidation in SrO-Al(2)O(3)/O(3) and SrO-Al(2)O(3)/US/O(3) systems was evaluated. It was found that the ammonia conversion increased with the increase of pH value in both systems. The NH(3)(aq) is oxidized by both O(3) and ·OH at high pH, whereas the NH(4)(+) oxidation is only carried out through ·OH at low pH. Compared with the SrO-Al(2)O(3)/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-Al(2)O(3)/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, CO(3)(2−), HCO(3)(−), and SO(4)(2−) inhibited the ammonia oxidation in the SrO-Al(2)O(3)/US/O(3) system. During ammonia conversion, SrO-Al(2)O(3) catalyst not only has a certain adsorption effect on NH(4)(+) but accelerates the O(3) decomposition to ·OH.