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, CO32−, HCO3−, and SO42− 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.


Introduction
Ammonia is a common contaminant. In particular, a large amount of ammonia wastewater is produced in the process of mining extraction and separation of rare-earth ore [1]. Once the discharge of ammonia exceeds the environmental capacity of the receiving waters, it causes several problems, including eutrophication, poisoning aquatic life, reducing water quality, and even threatening human health [2,3]. Hence, it is necessary to treat the ammonia in wastewater.
As an oxidant, ozone (O 3 ) has been used in wastewater treatment and deep purification of drinking water [4,5]. Ozonation alone has low oxidation efficiency because of its unstable chemical properties (the half-life is approximately 15 min under neutral conditions at 298 K), which limits its oxidation ability [6,7]. To improve the oxidation efficiency of ozone, the synergy of catalyst has been investigated, namely catalytic ozonation, which can be classified into homogeneous and heterogeneous depending on the type of catalyst [8,9]. In homogeneous catalytic ozonation, liquid catalysts (mostly transition metal ions) are used to decompose the ozone, but it is difficult to separate from the effluent, thus causing secondary pollution in water bodies [9,10]. In heterogeneous catalytic ozonation, solid catalysts that Instrument Factory, Shanghai, China) to obtain pretreated activated-Al2O3 used as a support for the catalyst.
An impregnation method was used to prepare the SrO-Al2O3 catalyst. Briefly, the pretreated activated-Al2O3 and 0.1 mol/L Sr(NO3)2 solution were mixed in a solid-liquid ratio of 1 g/20 mL and shaken in a 60 °C water bath thermostat (SHA-C, Jintan Ronghua Instrument Manufacturing Co. Ltd., Jintan, China) for 20 h. The mixture was filtrated, dried at 80 °C for 3 h and heated to 110 °C for 2 h in an oven and then calcined at 700 °C for 4 h in a muffle furnace (SHF.M25/10, Shanghai Lingyi Industrial Co. Ltd., Shanghai, China).

Procedure
The experimental setup of the ultrasound-enhanced catalytic ozonation of ammonia process (SrO-Al2O3/US/O3) is shown in Figure 1. To begin with, 100 mL NH4Cl solution was prepared at an initial concentration of 50 mg/L and acidified or alkalized to a set pH value (4.5~11.5) with 1 mol/L HCl or NaOH before each experimental run. Then the resulting solution was placed in a 200 mL reactor and a predetermined amount of SrO-Al2O3 (0.5~3.0 g/L) was added. Subsequently the instruments were connected and run according to Figure 1. O3 was generated by an ozone generator (FL-815ET, FeiLi, Shengzhen, China) with an oxygen source and a control device for ozone flow. The ozone generator has a maximum ozone flow of 15 g/h and the ozone flow (0.4~3.8 g/h) was set by the control device. The ozone was continuously bubbled into NH4Cl solution and the remaining ozone was absorbed by KI solution.
Ultrasonic treatment was performed using an ultrasonic generator (XO-SM50N, Nanjing Xianou Instrument Manufacturing Co. Ltd., Nanjing, China) equipped with a 6 mm diameter titanium probe. The probe was inserted to a depth of about 10 mm in the solution and the ultrasound was performed in a pulse mode of 1 s (or 2 s) on and 1 s (or 2 s) off at a given ultrasonic power (90~450 W) and an ultrasonic frequency of 25 Hz. The temperature of the whole reaction system was maintained at 25 °C by an intelligent thermostat.
During the catalytic ozonation of ammonia experiment (SrO-Al2O3/O3), the ultrasonic generator was turned off while other conditions are consistent with SrO-Al2O3/US/O3.

Analysis
After the reaction, liquid samples were withdrawn from the reactor at pre-determined intervals and then the concentrations of ammonium (NH4 + ), nitrite (NO2 -), and nitrate (NO3 -) in solution were measured. Nessler's reagent spectrophotometry method [28] was used for determining the concentration of NH4 + (

Analysis
After the reaction, liquid samples were withdrawn from the reactor at pre-determined intervals and then the concentrations of ammonium (NH 4 + ), nitrite (NO 2 − ), and nitrate (NO 3 − ) in solution were measured. Nessler's reagent spectrophotometry method [28] was used for determining the concentration of NH 4 ) and the spectrophotometry method [29] for determining the ) in the liquid samples by a visible spectrophotometer (SP-756PC, Shanghai Spectrum Instrument Co. Ltd., Shanghai, China). Ultraviolet spectrophotometry method [30] was used for determining the nitrate (C NO − 3 ) concentration using an ultraviolet spectrophotometer (722 N, Shanghai Spectrum Instrument Co. Ltd., Shanghai, China). In this study, according to the calculation of nitrogen balance, the products of oxidation of NH 4 + were gaseous nitrogen in addition to residual NH 4 + , NO 2 − , and NO 3 − in liquid phase. Gaseous nitrogen may include N 2 , N 2 O, NO, NO 2 , etc. [13]. Percentages of NH 4 ) and gaseous nitrogen (P gaseous nitrogen ) were calculated using Equations (1)-(4): where C 1 , C 2 , C 3 are the concentration, of residual NH 4 + , formed NO 3 − and NO 2 − after the reaction, respectively, and C 0 is the initial ammonia concentration (mg/L).

Performance Comparison of Different Treatment Systems
Tests were carried out in the absence and presence of SrO-Al 2 O 3 to evaluate the adsorption capacity of SrO-Al 2 O 3 on ammonia, the results is presented in Figure 2a. As shown in Figure 2a, when aqueous solution pH 8.5, 3.5% of NH 4 + was converted to NH 3 when no catalyst was present.
In the SrO-Al 2 O 3 alone system, the conversion rate of NH 4 + was 10.3% and no nitrate and nitrite nitrogen were produced, since the yield of NH 3 was 3.5%, indicating that 6.8% of NH 4 + was adsorbed by the SrO-Al 2 O 3 catalyst. In the US/O3 system, less than 13% of NH4 + was oxidized and the NO3 − yield was 11.2%, which were higher than those of O3 alone and US alone. When US was introduced into the O3 system, the propagation of ultrasonic waves in solution produced transient cavitation, causing turbulence and reducing the liquid film thickness of the ozone-containing gas bubbles, which facilitates the mass   Figure 2b. Figure 2b shows that the US alone and O 3 alone system have a small effect on the oxidation of ammonia in aqueous solution, with ammonia conversion and gaseous nitrogen yield of less than 7.4% and 3.0%, respectively. Simultaneously, the yields of NO 3 − were 4.4% and 5.8%, respectively. In the US/O 3 system, less than 13% of NH 4 + was oxidized and the NO 3 − yield was 11.2%, which were higher than those of O 3 alone and US alone. When US was introduced into the O 3 system, the propagation of ultrasonic waves in solution produced transient cavitation, causing turbulence and reducing the liquid film thickness of the ozone-containing gas bubbles, which facilitates the mass transfer of ozone [31]. On the other hand, the decomposition of ozone and H 2 O would produce ·OH in the cavitation bubbles (Equations (6)-(8)) [32]. Where the ")))" indicate ultrasound. Subsequently, the cavitation bubbles are ruptured, and the free radicals formed diffuse from the internal into the solution to oxidize the ammonia in aqueous solution [32,33].
H 2 O+))) → ·OH+ · H When US was introduced into the SrO-Al 2 O 3 system, both the ammonia conversion and gaseous nitrogen yield were low (11.2% and 3.6%, respectively). In the SrO-Al 2 O 3 /O 3 system, the ammonia conversion and production of gaseous nitrogen were 63.2% and 35.1%, respectively. One possible reason for this is that the active sites of SrO-Al 2 O 3 can stimulate the decomposition of ozone to produce ·OH. Compared to the SrO-Al 2 O 3 /US system, the oxidation effect of ammonia is obviously enhanced, which can be explained by co-oxidation of O 3  Comparison with the production of gaseous nitrogen, being 3.5% for the catalyst-free system and 6.8% NH 4 + adsorbed by the SrO-Al 2 O 3 catalyst in the catalyst alone system in Figure Figure 3 depicts the results showing that the pH of the initial solution is positively correlated with ammonia conversion and gaseous nitrogen selectivity. With the initial pH increasing from 4.5 to 11.5, the percentage of NH 4 + decreased, and the production of NO 3 − and gaseous nitrogen increased in the SrO-Al 2 O 3 /O 3 and SrO-Al 2 O 3 /US/O 3 system. On the one hand, the form of ammonia in aqueous solution depends on pH (Equation (9)), NH 4 + is predominant at pH lower than pK a , and free ammonia (NH 3 (aq)) increases at pH higher than pK a [34].

Initial Solution pH
C NH 3 The fraction of NH 3 (aq) at a given pH can be calculated according to Equation (10), where K b is the ionization constant, and its value is 1.774 × 10 −5 at 298 K [3]. Thus, NH 4 + is gradually turned into NH 3 (aq) with the increase of initial solution pH. For instance, the fraction of NH 3 (aq) at pH = 4.5 is only 1.8 × 10 −5 , which increases to 0.99 at pH = 11.5.
with ammonia conversion and gaseous nitrogen selectivity. With the initial pH increasing from 4.5 to 11.5, the percentage of NH4 + decreased, and the production of NO3 − and gaseous nitrogen increased in the SrO-Al2O3/O3 and SrO-Al2O3/US/O3 system. On the one hand, the form of ammonia in aqueous solution depends on pH (Equation (9)), NH4 + is predominant at pH lower than pKa, and free ammonia (NH3(aq)) increases at pH higher than pKa [34].
The fraction of NH3 (aq) at a given pH can be calculated according to Equation (10), where Kb is the ionization constant, and its value is 1.774 × 10 −5 at 298 K [3]. Thus, NH4 + is gradually turned into NH3 (aq) with the increase of initial solution pH. For instance, the fraction of NH3 (aq) at pH = 4.5 is only 1.8 × 10 −5 , which increases to 0.99 at pH = 11.5. On the other hand, as shown in Equations (11) and (12), ·OH would be generated from the ozone dissociation at alkaline conditions which contributed to the oxidation NH3(aq) [33]. Hence, according to Figure 3, the oxidation of NH3(aq) occurs by both O3 and ·OH at high pH. As more NH3(aq) is continuously oxidized, the reaction equilibrium (9) shifts to the formation of NH3(aq), resulting in an increase of ammonia conversion rate. However, since O3 cannot oxidize NH4 + directly [3,32,34], the oxidation of NH4 + can be carried out through ·OH at low pH, which was derived from ultrasound-enhanced decomposition of O3 and H2O (Equations (6)- (8)). On the other hand, as shown in Equations (11) and (12), ·OH would be generated from the ozone dissociation at alkaline conditions which contributed to the oxidation NH 3 (aq) [33]. Hence, according to Figure 3, the oxidation of NH 3 (aq) occurs by both O 3 and ·OH at high pH. As more NH 3 (aq) is continuously oxidized, the reaction equilibrium (9) shifts to the formation of NH 3 (aq), resulting in an increase of ammonia conversion rate. However, since O 3 cannot oxidize NH 4 + directly [3,32,34], the oxidation of NH 4 + can be carried out through ·OH at low pH, which was derived from ultrasound-enhanced decomposition of O 3 and H 2 O (Equations (6)- (8)).
Comparing the two system SrO-Al 2 O 3 /O 3 and SrO-Al 2 O 3 /US/O 3 , when the initial pH was 9.5, the introduction of ultrasound increased the ammonia conversion from 68.4% to 83.2%, and the proportion of gaseous nitrogen increased from 38.2% to 51.2%. It is confirmed that the combination of US, SrO-Al 2 O 3 and O 3 can synergistically enhance the ammonia conversion. In SrO-Al 2 O 3 /O 3 system, the reaction time for reaching the maximum ammonia conversion rate and gaseous nitrogen yield was twice that of the SrO-Al 2 O 3 /US/O 3 system, indicating that the introduction of US can greatly shorten the reaction time. Figure 4 shows that the effect of reaction time on ammonia conversion in SrO-Al2O3/O3 and SrO-Al2O3/US/O3 system. According to Figure 4, the ammonia conversion and the gaseous nitrogen yield increased with the prolongation of the reaction time in the two systems. The reaction time was 120 min in the SrO-Al2O3/O3 system, the ammonia conversion and gaseous nitrogen yield reached 81.6% and 49.6%, respectively, while the reaction time was 60 min in the SrO-Al2O3/US/O3 system, the ammonia conversion and gaseous nitrogen yield reached 83.4% and 51.4%, respectively. In SrO-Al2O3/O3 system, the reaction time for reaching the maximum ammonia conversion rate and gaseous nitrogen yield was twice that of the SrO-Al2O3/US/O3 system, indicating that the introduction of US can greatly shorten the reaction time.

Catalyst Dosage
In general, increased catalyst dosages provide more surface-active sites, thus facilitating O3 decomposition into ·OH [35]. As seen in Figure 5, the ammonia conversion and gaseous nitrogen production improved with an increase of SrO-Al2O3 dosage. The dosage of SrO-Al2O3 was 2.5 g/L in the SrO-Al2O3/O3 system, the ammonia conversion and gaseous nitrogen yield reached 81.0% and 50.0%, respectively, while the dosage was 2.0 g/L in the SrO-Al2O3/US/O3 system, the ammonia conversion and gaseous nitrogen yield reached 83.2% and 51.2%, respectively. It is illustrated that the introduction of US cannot only enhance the ammonia conversion but reduce the catalyst dosage.

Catalyst Dosage
In general, increased catalyst dosages provide more surface-active sites, thus facilitating O 3 decomposition into ·OH [35]. As seen in Figure 5 Figure 6 shows the effect of the ozone flow on ammonia conversion. The results indicated that when ozone flow increased from 0.4 mg/min to 3.0 mg/min in SrO-Al2O3/O3 system, the ammonia conversion and gaseous nitrogen yield increased from 58.6% to 79.2% and 31.8% to 47.8%, respectively. While in the SrO-Al2O3/US/O3 system, ozone flow from 0.4 mg/min to 0.8 mg/min, the ammonia conversion and gaseous nitrogen yield increased from 75.8% to 83.1% and 46.2% to 51.7%,  Figure 6 shows the effect of the ozone flow on ammonia conversion. The results indicated that when ozone flow increased from 0.4 mg/min to 3.0 mg/min in SrO-Al 2 O 3 /O 3 system, the ammonia conversion and gaseous nitrogen yield increased from 58.6% to 79.2% and 31.8% to 47.8%, respectively. While in the SrO-Al 2 O 3 /US/O 3 system, ozone flow from 0.4 mg/min to 0.8 mg/min, the ammonia conversion and gaseous nitrogen yield increased from 75.8% to 83.1% and 46.2% to 51.7%, respectively. The increase in ozone flow means an increase in ozone concentration, which activates the ·OH generation and ultimately enhances the ammonia oxidation in the reaction system [36]. While continuing to increase the ozone flow, it was found that the ammonia conversion, gaseous nitrogen yield and NO 3 − production in the solution have a little bit increase. The reason for this is that with a certain amount of SrO-Al 2 O 3 catalyst, the number of active sites is fixed, and cannot completely decompose excessive ozone to ·OH.  Figure 7 shows the effect of ultrasonic power on ammonia conversion in SrO-Al2O3/US/O3 system. Presence of US improved the ammonia oxidation. With the increase of ultrasonic power from 0 to 450 W, residual NH4 + concentration and gaseous nitrogen yield after the reaction decrease first and then rises. Increasing ultrasonic power from 0 to 270 W can enhance the ultrasonic cavitation, but exceeding 270 W, acoustic shielding appears to reduce the ultrasonic cavitation and the utilization of acoustic energy [31,37]. When the ultrasonic power is 270 W, the ammonia conversion and gaseous nitrogen yield both maximize 83.2% and 51.8%. Increasing ultrasonic power from 0 to 270 W can enhance the ultrasonic cavitation, but exceeding 270 W, acoustic shielding appears to reduce the ultrasonic cavitation and the utilization of acoustic energy [31,37]. When the ultrasonic power is 270 W, the ammonia conversion and gaseous nitrogen yield both maximize 83.2% and 51.8%. Figure 7 shows the effect of ultrasonic power on ammonia conversion in SrO-Al2O3/US/O3 system. Presence of US improved the ammonia oxidation. With the increase of ultrasonic power from 0 to 450 W, residual NH4 + concentration and gaseous nitrogen yield after the reaction decrease first and then rises. Increasing ultrasonic power from 0 to 270 W can enhance the ultrasonic cavitation, but exceeding 270 W, acoustic shielding appears to reduce the ultrasonic cavitation and the utilization of acoustic energy [31,37]. When the ultrasonic power is 270 W, the ammonia conversion and gaseous nitrogen yield both maximize 83.2% and 51.8%.

Effect of Tert-Butanol
Tert-butanol, a common ·OH scavengers' agent, is difficult to directly react with O 3 (reaction rate constant is K = 3.0 × 10 −2 M −1 S −1 ) but reacts quickly with ·OH (K = 6.0 × 10 8 M −1 S −1 ) [38]. In addition, tert-butanol would react with ·OH in bulk solution to form inert substances, which inhibited the chain reaction of ·OH [39,40]. The effect of tert-butanol on ammonia conversion in the SrO-Al 2 O 3 /US/O 3 system is shown in Figure 9. With tert-butanol concentration increasing from 0 to 18.0 mg/L, the residual NH 4 + increased from 16.8% to 49.2%, and the percentage of NO 3 − and gaseous nitrogen decreased from 31.3% to 20.5%, and 51.8% to 30.2%, respectively. That is to say, the addition of tert-butanol largely inhibits the ammonia oxidation in the SrO-Al

Actual Ammonium-Containing Wastewater Oxidized in the SrO-Al 2 O 3 /US/O 3 System
The actual wastewater was collected from a rare earth metallurgical plant located in Longnan County, Jiangxi province, China and transferred into clean plastic bottles, and immediately transported to the laboratory for proper storage for future use. The characteristics of the actual wastewater used in this study are shown in Table 1. Actual wastewater with an initial NH 4 + concentration of 50 mg/L was obtained by diluting the actual wastewater with an initial NH 4 + concentration of 302 mg/L with deionized water. As shown in Figure 11, there were some slight differences in the treatment between simulated water and actual wastewater with an initial NH 4 + concentration of 50 mg/L. Among them, the ammonia conversion was 83.2% and 79.6%, respectively, and the gaseous nitrogen yield was 51.8% and 53.9%, respectively. However, when the actual wastewater with an initial NH 4 + concentration of 302 mg/L was treated in SrO-Al 2 O 3 /US/O 3 system, the ammonia conversion and gaseous nitrogen yield were 58.1% and 40.7%, respectively. This indicates that the treatment efficiency of the SrO-Al 2 O 3 /US/O 3 process is limited and is currently only suitable for treating low concentrations of ammonia-containing wastewater.
ammonia conversion was 83.2% and 79.6%, respectively, and the gaseous nitrogen yield was 51.8% and 53.9%, respectively. However, when the actual wastewater with an initial NH4 + concentration of 302 mg/L was treated in SrO-Al2O3/US/O3 system, the ammonia conversion and gaseous nitrogen yield were 58.1% and 40.7%, respectively. This indicates that the treatment efficiency of the SrO-Al2O3/US/O3 process is limited and is currently only suitable for treating low concentrations of ammonia-containing wastewater.

Morphology Analysis of Catalyst
Surface topographies of Al2O3 and SrO-Al2O3 were observed by scanning electron microscopy (JSM-6360LV, Japanese electronics company, Japan) and the results are shown in Figure 12. Figure  12a shows that the surface of Al2O3 is rough and has obvious channels, which will facilitate the loading of SrO. Figure 12b shows that a large amount of irregular small particles is evenly distributed on the surface of SrO-Al2O3 and these small particles are closely joined.

Morphology Analysis of Catalyst
Surface topographies of Al 2 O 3 and SrO-Al 2 O 3 were observed by scanning electron microscopy (JSM-6360LV, Japanese electronics company, Japan) and the results are shown in Figure 12. Figure 12a shows that the surface of Al 2 O 3 is rough and has obvious channels, which will facilitate the loading of SrO. Figure 12b shows that a large amount of irregular small particles is evenly distributed on the surface of SrO-Al 2 O 3 and these small particles are closely joined.

The Pathway for Ammonia in Water Oxidized in the SrO-Al2O3/US/O3 System
According to the above experimental results, the pathway for ammonia conversion in the SrO-Al2O3/US/O3 system can be inferred, as shown in Figure 13. Under alkaline conditions, ammonia mainly exists in a free state (NH3(aq)). The NH3(aq) oxidation occurs by both O3 and ·OH, whereas NH4 + oxidation is only carried out through ·OH. During ammonia conversion, the SrO-Al2O3 catalyst not only has a certain adsorption effect on NH4 + , but accelerates the O3 decomposition to ·OH. US enhances the turbulence degree of liquid phase to promote the ammonia conversion to NH3(aq), while the cavitation effect of US causes H2O to crack to ·OH. In addition, US improves the mass transfer rate of O3, strengthens the O3 decomposition and produces more ·OH. Moreover, the ultrasonic wave cleans the surface of catalyst and slows down the deactivation of the catalyst, thus prolonging the service life of the catalyst. Therefore, the ammonia conversion has been greatly improved due to the synergistic effect of US, SrO-Al2O3 and O3 in the SrO-Al2O3/US/O3 system.
In this study, the products of oxidation of ammonia in aqueous solution are gaseous nitrogen in the gas phase and the residual NH4 + , NO2 -, and NO3 -in the liquid phase. Among them, gaseous According to the above experimental results, the pathway for ammonia conversion in the SrO-Al 2 O 3 /US/O 3 system can be inferred, as shown in Figure 13. Under alkaline conditions, ammonia mainly exists in a free state (NH 3 (aq)). The NH 3 [13,43,44]. The production of NO 2 − and NO 3 − was derived from the oxidation of NH 4 + and NH 3 (aq).
mainly exists in a free state (NH3(aq)). The NH3(aq) oxidation occurs by both O3 and ·OH, whereas NH4 + oxidation is only carried out through ·OH. During ammonia conversion, the SrO-Al2O3 catalyst not only has a certain adsorption effect on NH4 + , but accelerates the O3 decomposition to ·OH. US enhances the turbulence degree of liquid phase to promote the ammonia conversion to NH3(aq), while the cavitation effect of US causes H2O to crack to ·OH. In addition, US improves the mass transfer rate of O3, strengthens the O3 decomposition and produces more ·OH. Moreover, the ultrasonic wave cleans the surface of catalyst and slows down the deactivation of the catalyst, thus prolonging the service life of the catalyst. Therefore, the ammonia conversion has been greatly improved due to the synergistic effect of US, SrO-Al2O3 and O3 in the SrO-Al2O3/US/O3 system. In this study, the products of oxidation of ammonia in aqueous solution are gaseous nitrogen in the gas phase and the residual NH4 + , NO2 -, and NO3 -in the liquid phase. Among them, gaseous nitrogen may include N2, N2O, NO, NO2, NH3 [13,43,44]. The production of NO2 -and NO3 − was derived from the oxidation of NH4 + and NH3 (aq).

Conclusions
The experimental results indicated that, compared with US, O 3 , US/O 3 , SrO-Al 2 O 3 /US and SrO-Al 2 O 3 /O 3 systems, the highest ammonia conversion rate was obtained by the SrO-Al 2 O 3 /US/O 3 system. The free ammonia (NH 3 (aq)) was oxidized by both O 3 and ·OH at high pH, while the NH 4 + was oxidized through ·OH generated by the decomposition of O 3 and H 2 O at low pH. Compared to the SrO-Al 2 O 3 /O 3 system, the reaction time required in the SrO-Al 2 O 3 /US/O 3 system is shortened, and the consumption of catalyst dosage and ozone was reduced. Moreover, reasonable control of ultrasonic power and duty cycle can further improve the ammonia conversion and gaseous nitrogen. Under the conditions of initial NH 4 + concentration 50 mg/L, initial pH 9.5, ozone flow 0.8 g/h, catalytic dosage 2.0 g/L, temperature 25 • C, reaction time 60 min, ultrasonic frequency 25 kHz, ultrasonic power 270 W, and duty cycle 1:3, the ammonia conversion and gaseous nitrogen yield were 83.2% and 51.8%, respectively. The oxidation of ammonia in the SrO-Al 2 O 3 /US/O 3 system was affected by tert-butanol, CO 3 2− , HCO 3 − , SO 4 2− , indicating that ·OH is an important oxidizer involved in the ammonia ozonation.
To better clarify the mechanism of oxidizing ammonia, it is necessary to further determine the components of gaseous nitrogen qualitatively and quantitatively.