Ultrasound-Enhanced Catalytic Ozonation Oxidation of Ammonia in Aqueous Solution

Abstract

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.

1. 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 (O₃) 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 are easily separated and maintain their catalytic activity for a long time are employed for decomposing ozone into hydroxyl radicals (·OH) [9,11]. ·OH is more oxidative than O₃, because their oxidation potentials are 2.80 V and 2.07 V, respectively [12]. In addition, the solid catalyst in ozonation can directly convert ammonia into harmless substances (such as N₂) under certain conditions [13,14]. Therefore, the suitable solid catalysts are very important for the ammonia conversion by heterogeneous catalytic ozonation.

Recently, the focus has been on alkaline earth metal oxides due to their lower solubility in the reaction process and the basic oxygen-containing groups (hydroxyl groups) on their surface [15]. The order of catalytic activity of alkaline earth metal oxides per unit surface area is SrO > CaO > MgO [16]. Furthermore, surface hydroxyl groups on the catalyst react easily with O₃ to form ·OH [12]. However, the alkaline earth metal oxides are powdery, without mechanical strength, and are not easily separated from the effluent [17,18]. To overcome these limitations, attaching the alkaline earth metal oxide to some supports has been considered. Activated alumina, with acid-base active sites on their surface, high mechanical strength and good thermal stability, can be used as a suitable support [18]. Cotman et al. [19] prepared ruthenium metal supported on alumina catalyst (Ru-Al₂O₃) to catalyze ozonation of bisphenol A, and the catalyst Ru-Al₂O₃ showed excellent stability and reusability. Wang et al. [20] studied the manganese metal loading of 4% on Al₂O₃ (Mn-Al₂O₃) exhibited highest catalytic activity.

In the process of heterogeneous catalytic ozonation, catalyst deactivation is also an important issue affecting the catalytic efficiency due to the accumulation of by-products on the catalyst surface [21]. Currently, ultrasound (US) is combined with the heterogeneous catalytic ozonation process to treat organic pollutants in water [22,23,24]. When the ultrasound catalytic reaction is carried out, transient cavitation enhances the turbulent of the solution, thereby accelerating the mass transfer process of the reactants and by-products between the solution and the catalyst surfaces [25]. At the same time, the collapse of acoustic cavitation bubbles generates shock waves, thus creating a continuous cleaning effect on the surface of the catalyst [25,26,27]. To date, ultrasound-enhanced catalytic ozonation of ammonia in aqueous solution has rarely been studied.

Accordingly, (1) to enhance the ozone oxidation ability and its utilization rate, (2) to power the mechanical strength of alkaline earth metal oxides and easily separate from the solution, and (3) to delay the deactivation of the catalyst, this study combines an ultrasonic, supported alkaline earth metal oxide with ozone (SrO-Al₂O₃/US/O₃) to oxidize the ammonia in aqueous solution. The effects of initial solution pH, reaction time, dosage of catalyst, ozone flow, ultrasonic power and duty cycle on ammonia conversion were investigated in the SrO-Al₂O₃/US/O₃ system. For comparison, the effects of the same operating parameters on ammonia conversion were studied in SrO-Al₂O₃/O₃ system to understand the role of US in the catalytic ozonation reaction. A set of experiments were also designed for assessing the effect of tert-butanol and inorganic ions (Na⁺; K⁺; Ca²⁺; Mg²⁺; SO₄²⁻; CO₃²⁻; HCO₃⁻) on the conversion of ammonia in the SrO-Al₂O₃/US/O₃ system. In addition, the mechanism for ultrasound-enhanced catalytic ozonation of ammonia in aqueous solution is discussed.

2. Experimental

2.1. Materials

Ammonium chloride (NH₄Cl, analytical grade) was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China), which was used to prepare simulated water containing NH₄⁺. Both activated-alumina (Al₂O₃, analytical grade) and barium salt (Sr(NO₃)₂, analytical grade) were purchased from Shanghai Zhanyun Chemical Co. Ltd. (Shanghai, China).

2.2. Preparation of Supported Catalyst

Actived-Al₂O₃ (48–212 μm) was washed three times with deionized water, followed by ultrasonic cleaning for 15 min in an ultrasonic cleaner (KQ-100TDE, Kunshan Ultrasonic Instrument Co. Ltd., Kunshan, China) and dried at 110 °C for 5 h in an oven (101A-3, Shanghai Experimental 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).

2.3. 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.

2.4. 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⁺ ($C_{N{H}_4^{+}}$) and the spectrophotometry method [29] for determining the concentration of NO₂⁻ ($C_{N{O}_2^{-}}$) 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 ($C_{N{O}_3^{-}}$) 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 NH₄⁺ were gaseous nitrogen in addition to residual NH₄⁺, NO₂⁻, and NO₃⁻ in liquid phase. Gaseous nitrogen may include N₂, N₂O, NO, NO₂, etc. [13]. Percentages of NH₄⁺ ($P_{N{H}_4^{+}}$), NO₃⁻ ($P_{N{O}_3^{-}}$), NO₂⁻ ($P_{N{O}_2^{-}}$) and gaseous nitrogen ($P_{\mathrm{gaseous} \mathrm{nitrogen}}$) were calculated using Equations (1)–(4):

$$P_{N{H}_4^{+}}=\frac{C_1}{C_0}\times 100\%$$

(1)

$$P_{N{O}_3^{-}}=\frac{C_2}{C_0}\times 100\%$$

(2)

$$P_{N{O}_2^{-}}=\frac{C_3}{C_0}\times 100\%$$

(3)

$$P_{gaseous nitrogen}=100\%-P_{N{H}_4^{+}}-P_{N{O}_3^{-}}-P_{N{O}_2^{-}}$$

(4)

$$N{H}_4^{+}conversion=100\%-P_{N{H}_4^{+}}$$

(5)

where $C_1$, $C_2$, $C_3$ are the concentration, of residual NH₄⁺, formed NO₃⁻ and NO₂⁻ after the reaction, respectively, and $C_0$ is the initial ammonia concentration (mg/L).

3. Results and Discussions

3.1. Performance Comparison of Different Treatment Systems

Tests were carried out in the absence and presence of SrO-Al₂O₃ to evaluate the adsorption capacity of SrO-Al₂O₃ on ammonia, the results is presented in Figure 2a. As shown in Figure 2a, when aqueous solution pH 8.5, 3.5% of NH₄⁺ was converted to NH₃ when no catalyst was present. In the SrO-Al₂O₃ alone system, the conversion rate of NH₄⁺ was 10.3% and no nitrate and nitrite nitrogen were produced, since the yield of NH₃ was 3.5%, indicating that 6.8% of NH₄⁺ was adsorbed by the SrO-Al₂O₃ catalyst.

To evaluate the ammonia conversion rate in various systems, experiments were carried out with US, O₃, US/O₃, SrO-Al₂O₃/US, SrO-Al₂O₃/O₃ and SrO-Al₂O₃/US/O₃ systems, respectively, and the results are given in Figure 2b. Figure 2b shows that the US alone and O₃ 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 NO₃⁻ were 4.4% and 5.8%, respectively.

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 transfer of ozone [31]. On the other hand, the decomposition of ozone and H₂O 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].

$${\mathrm{O}}_3+)))\to \cdot \mathrm{O}+{\mathrm{O}}_2$$

(6)

$$\cdot {\mathrm{O}+\mathrm{H}}_2\mathrm{O}\to 2\cdot \mathrm{OH}$$

(7)

$${\mathrm{H}}_2\mathrm{O}+\left)\right))\to \cdot \mathrm{OH}+\cdot \mathrm{H}$$

(8)

When US was introduced into the SrO-Al₂O₃ system, both the ammonia conversion and gaseous nitrogen yield were low (11.2% and 3.6%, respectively). In the SrO-Al₂O₃/O₃ 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-Al₂O₃ can stimulate the decomposition of ozone to produce ·OH. Compared to the SrO-Al₂O₃/US system, the oxidation effect of ammonia is obviously enhanced, which can be explained by co-oxidation of O₃ and ·OH in the SrO-Al₂O₃/O₃ system.

As for the SrO-Al₂O₃/US/O₃ system, both ammonia conversion and gaseous nitrogen yield were higher than that of SrO-Al₂O₃/O₃ processes. The reason may be that the active sites of SrO-Al₂O₃ were gradually occupied by the NH₄⁺ and NO₃⁻ during the catalytic ozonation process in the SrO-Al₂O₃/O₃ system. The active sites of SrO-Al₂O₃ were opened by transient cavitation when US was introduced in the SrO-Al₂O₃/US/O₃ system. The experimental results indicated that the ammonia conversion is dominated by the oxidation of O₃ and ·OH, which comes from the synergistic effect of US, SrO-Al₂O₃ and O₃ in the SrO-Al₂O₃/US/O₃ system.

Comparison with the production of gaseous nitrogen, being 3.5% for the catalyst-free system and 6.8% NH₄⁺ adsorbed by the SrO-Al₂O₃ catalyst in the catalyst alone system in Figure 2a, the NO₃⁻ can be considered to come from the simultaneous oxidation of NH₄⁺ and NH₃ in the O₃, US, US/O₃, SrO-Al₂O₃/US, SrO-Al₂O₃/O₃ and SrO-Al₂O₃/US/O₃ systems.

3.2. Effect of Operating Parameters on Ammonia Conversion

3.2.1. Initial Solution pH

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 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 pK_a, and free ammonia (NH₃(aq)) increases at pH higher than pK_a [34].

$${\mathrm{NH}}_3(\mathrm{a}\mathrm{q})+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{N}{\mathrm{H}}_4^{+}+\mathrm{O}{\mathrm{H}}^{-}; {\mathrm{pK}}_{\mathrm{a}}=9.246$$

(9)

$$\frac{{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_3}}{{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_3}+{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_4^{+}}}=\frac{{10}^{\mathrm{p}\mathrm{H}\text{-}14}}{{\mathrm{K}}_{\mathrm{b}}+{10}^{\mathrm{p}\mathrm{H}\text{-}14}}$$

(10)

The fraction of NH₃ (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, 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)).

$${\mathrm{O}}_3{+\mathrm{OH}}^{-}\to {\mathrm{HO}}_2^{-}{+\mathrm{O}}_2$$

(11)

$${\mathrm{O}}_3{+\mathrm{HO}}_2^{-}\to \cdot \mathrm{OH}+\cdot {\mathrm{O}}_2^{-}{+\mathrm{O}}_2$$

(12)

Comparing the two system SrO-Al₂O₃/O₃ and SrO-Al₂O₃/US/O₃, 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-Al₂O₃ and O₃ can synergistically enhance the ammonia conversion.

3.2.2. 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.

3.2.3. 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.

3.2.4. Ozone Flow

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%, 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 NO₃⁻ production in the solution have a little bit increase. The reason for this is that with a certain amount of SrO-Al₂O₃ catalyst, the number of active sites is fixed, and cannot completely decompose excessive ozone to ·OH. Comparing the two system of SrO-Al₂O₃/O₃ and SrO-Al₂O₃/US/O₃, it can be concluded that the introduction of US can reduce the consumption of ozone to a large extent.

3.2.5. Ultrasonic Power

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%.

3.2.6. Duty Cycle

The US can be introduced either continuously or intermittently in the experiments. Since the continuous addition consumes a large amount energy, the intermittent method is adopted. To explore the effect of intermittent ultrasonic operation mode on ammonia conversion in the SrO-Al₂O₃/US/O₃ process, five intermittent operation modes were set up, which were (a) no ultrasound, (b) ultrasonic operation 1 s and interval 1 s (duty cycle 1:2), (c) ultrasonic operation 1 s and interval 2 s (duty cycle 1:3), (d) ultrasonic operation 2 s and interval 1 s (duty cycle 2:3), and (e) ultrasonic operation 2 s and interval 2 s (duty cycle 2:4), as observed in Figure 8. When the duty cycle was 1:3, the ammonia conversion was 83.2%, gaseous nitrogen reached the maximum of 51.8% and the production of NO₃⁻ 31.28%. When the duty cycle was 2:3, the ammonia conversion reached a maximum of 86.4%, gaseous nitrogen 41.3% and NO₃⁻ production 45.0%.

3.3. Effect of Tert-Butanol and Inorganic Ions on Ammonia Conversion

3.3.1. Effect of Tert-Butanol

Tert-butanol, a common ·OH scavengers’ agent, is difficult to directly react with O₃ (reaction rate constant is K = 3.0 × 10⁻² M⁻¹ S⁻¹) but reacts quickly with ·OH (K = 6.0 × 10⁸ 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-Al₂O₃/US/O₃ system is shown in Figure 9. With tert-butanol concentration increasing from 0 to 18.0 mg/L, the residual NH₄⁺ increased from 16.8% to 49.2%, and the percentage of NO₃⁻ 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-Al₂O₃/US/O₃ system, indicating that the intermediate product (·OH) plays an important role.

3.3.2. Effect of Inorganic Ions

Large amounts of cations and anions often appear in the actual water body, the influence of main cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (CO₃²⁻, HCO₃⁻, SO₄²⁻) should be evaluated on ammonia conversion in SrO-Al₂O₃/US/O₃ system. As observed in Figure 10, the addition of Na⁺, K⁺, Ca²⁺ and Mg²⁺ had no significant effect on ammonia conversion, while CO₃²⁻, HCO₃⁻ and SO₄²⁻ suppressed the ammonia conversion. The anions’ radical-scavenging properties, as shown in Equations (13)–(15) [40,41,42], reduced the amount of ·OH produced, thereby causing a decrease in ammonia conversion.

$${\mathrm{CO}}_3^{2-}+\cdot \mathrm{OH}\to {\mathrm{CO}}_3^{-}\cdot {+\mathrm{OH}}^{-}$$

(13)

$${\mathrm{HCO}}_3^{-}+\cdot \mathrm{OH}\to {\mathrm{CO}}_3^{-}\cdot {+\mathrm{H}}_2\mathrm{O}$$

(14)

$${\mathrm{SO}}_4^{2-}+\cdot \mathrm{OH}\to {\mathrm{SO}}_4^{-}\cdot {+\mathrm{OH}}^{-}$$

(15)

3.4. Actual Ammonium-Containing Wastewater Oxidized in the SrO-Al₂O₃/US/O₃ 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. Characteristics of actual wastewater.

Parameter	pH	NH4+	NO3−	NO2−	Mg	Si	Ca	Mn	Rb	Na	Sr	Y
Concentration (mg/L)	8.45	302	70.2	0.57	1.04	1.26	27.8	3.10	1.15	0.69	0.69	0.51

. Actual wastewater with an initial NH₄⁺ concentration of 50 mg/L was obtained by diluting the actual wastewater with an initial NH₄⁺ 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 NH₄⁺ 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 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.

3.5. 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.

3.6. 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 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).

4. Conclusions

The experimental results indicated that, compared with US, O₃, US/O₃, SrO-Al₂O₃/US and SrO-Al₂O₃/O₃ systems, the highest ammonia conversion rate was obtained by the SrO-Al₂O₃/US/O₃ system. The free ammonia (NH₃ (aq)) was oxidized by both O₃ and ·OH at high pH, while the NH₄⁺ was oxidized through ·OH generated by the decomposition of O₃ and H₂O at low pH. Compared to the SrO-Al₂O₃/O₃ system, the reaction time required in the SrO-Al₂O₃/US/O₃ 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 NH₄⁺ 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-Al₂O₃/US/O₃ system was affected by tert-butanol, CO₃²⁻, HCO₃⁻, SO₄²⁻, 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.