Catalytic Ozonation of Nitrite in Denitrification Wastewater Based on Mn/ZSM-5 Zeolites: Catalytic Performance and Mechanism

Abstract

In wet flue gas desulfurization and denitrification processes, nitrite accumulation inhibits denitrification efficiency and induces secondary pollution due to its acidic disproportionation. This study developed a Mn-modified ZSM-5 zeolite catalyst, achieving efficient resource conversion of nitrite in nitrogen-containing wastewater through an O₃ + Mn/ZSM-5 catalytic system. Mn/ZSM-5 catalysts with varying SiO₂/Al₂O₃ ratios (prepared by wet impregnation) were characterized by BET, XRD, and XPS. Experimental results demonstrated that Mn/ZSM-5 (SiO₂/Al₂O₃ = 400) exhibited a larger specific surface area, enhanced adsorption capacity, abundant surface Mn³⁺/Mn⁴⁺ species, hydroxyl oxygen species, and chemisorbed oxygen, leading to superior oxidation capability and catalytic activity. Under the optimized conditions of reaction temperature = 40 °C, initial pH = 4, Mn/ZSM-5 dosage = 1 g/L, and O₃ concentration = 100 ppm, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency reached 94.33%. Repeated tests confirmed that the Mn/ZSM-5 catalyst exhibited excellent stability and wide operational adaptability. The synergistic effect between Mn species and the zeolite support significantly improved ozone utilization efficiency. The O₃ + Mn/ZSM-5 system required less ozone while maintaining high oxidation efficiency, demonstrating better cost-effectiveness. Mechanism studies revealed that the conversion pathway of $\mathrm{N}{\mathrm{O}}_2^{-}$ followed a dual-path catalytic mechanism combining direct ozonation and free radical chain reactions. Practical spray tests confirmed that coupling the Mn/ZSM-5 system with ozone oxidation flue gas denitrification achieved over 95% removal of liquid-phase $\mathrm{N}{\mathrm{O}}_2^{-}$ byproducts without compromising the synergistic removal efficiency of NO_x/SO₂. This study provided an efficient catalytic solution for industrial wastewater treatment and the resource utilization of flue gas denitrification byproducts.

1. Introduction

Nitrogen oxides (NO_x), as one of the primary atmospheric pollutants, not only contribute to the formation of photochemical smog [1], haze [2], and acid rain [3] but also pose significant risks to the human respiratory system [4]. In recent years, low-temperature oxidation flue gas denitrification technology using ozone has been widely adopted in boilers, kilns, and non-power industries due to its high efficiency, broad applicability, and capability for synergistic multi-pollutant control [5,6]. This technology uses the strong oxidative capacity of ozone to convert poorly soluble NO in flue gas into higher-valence species (e.g., NO₂, NO₃, N₂O₅) [7,8], oxidize Hg⁰ to Hg²⁺ [9], and degrade VOCs into CO₂ and H₂O [10]. Coupled with a wet scrubbing process, it enables integrated removal of multiple pollutants [11]. In wet flue gas treatment systems, the primary absorption products of NO_x are nitrite ($\mathrm{N}{\mathrm{O}}_2^{-}$) and nitrate ($\mathrm{N}{\mathrm{O}}_3^{-}$) [12,13,14]. While nitrates are chemically stable and serve as key raw materials for construction, fertilizers, and phase-change materials [15,16,17], liquid-phase $\mathrm{N}{\mathrm{O}}_2^{-}$ is unstable and prone to disproportionation reactions under acidic conditions, releasing NO back into the atmosphere and causing secondary pollution [18]. Moreover, high concentrations of $\mathrm{N}{\mathrm{O}}_2^{-}$ in the absorbent slurry inhibit NO_x absorption, reducing denitrification efficiency. The accumulation of $\mathrm{N}{\mathrm{O}}_2^{-}$ also necessitates frequent replacement of industrial water, exacerbating water resource wastage. Additionally, $\mathrm{N}{\mathrm{O}}_2^{-}$ itself is highly toxic, and its excessive emission poses severe threats to ecosystems and human health [19,20]. Therefore, developing cost-effective methods for $\mathrm{N}{\mathrm{O}}_2^{-}$ removal or its resource recovery is critical for the large-scale application of ozone oxidation flue gas denitrification technology and wet process technology in industrial settings [11,21].

A variety of physical, chemical, and biological methods have been employed for pollutant removal from water [22,23]. Among these, physical adsorption has been preferentially adopted for nitrite elimination in industrial wastewater treatment, with typical adsorbents including activated carbon [24] and reverse osmosis membranes [25,26]. However, this technology suffers from inherent limitations such as insufficient adsorption selectivity, susceptibility to micropore clogging, and constraints in absorption tower modifications. These drawbacks directly lead to declining $\mathrm{N}{\mathrm{O}}_2^{-}$ removal efficiency and increased operational costs. Current wastewater treatment systems also encompass diversified approaches, including catalytic reduction [27,28], γ-ray irradiation [29], and ion exchange [30]. Nevertheless, the requirements for precious metal catalysts and complex equipment configurations make them poorly compatible with the engineering demands of wet scrubbing processes. Aerobic denitrification technology [31,32] has emerged as a novel biological nitrogen removal strategy, where denitrifying bacteria metabolically convert nitrite into N₂. However, this method typically requires prolonged treatment durations, and its temperature-sensitive nature presents significant compatibility challenges with the slurry environment in wet scrubbing processes, thereby limiting its practical application.

Based on the above comprehensive analysis, current nitrite treatment technologies are generally constrained by slow reaction kinetics, high operational costs, and stringent reaction requirements, making them incompatible with wet spray systems. In contrast, heterogeneous wet catalytic oxidation technology enables precise regulation of reaction pathways through the construction of efficient catalytic systems [33,34,35], while facilitating the conversion of nitrite to nitrate, thereby achieving resource utilization of pollutants. Regarding oxidants, ozone oxidation has remained a research focus in the advanced oxidation area due to its strong oxidative capacity [36,37]. Building upon this, the heterogeneous wet catalytic ozonation system has been successfully applied for deep mineralization of typical pollutants, including pharmaceuticals, pesticides, dyes, aromatic hydrocarbons, and organic acids, with commonly used catalysts such as activated carbon [38], TiO₂ [39], and Al₂O₃ [40]. However, radical-dominated advanced oxidation mechanisms generally suffer from a lack of reaction selectivity and are prone to competitive reaction interference, leading to significant degradation efficiency reduction for target pollutants [41]. Therefore, comprehensive consideration is required for catalyst selection. SiO₂-supported catalysts exhibit superior dispersion of active metal particles [42], but anomalous phenomena occurring during the reaction process, including carbon deposition, sintering of active metals, and structural loss of the SiO₂ support, can lead to the degradation of the active metal/SiO₂ architecture, thereby accelerating catalyst deactivation [43]. HZSM-5 zeolite has demonstrated potential in catalytic applications due to its high specific surface area, tunable acid sites, and well-defined pore structure [44,45,46]. Moreover, HZSM-5 exhibits properties such as easy handling, strong thermal stability, acid resistance, and recyclability, making it an ideal catalyst support [47]. However, its narrow channels hinder reactant/product diffusion [48,49,50], thereby constraining its selective oxidation capability toward $\mathrm{N}{\mathrm{O}}_2^{-}$. Meanwhile, Mn-based catalysts often demonstrate excellent catalytic oxidation performance through modulation of surface acidity and oxygen vacancy density [51,52]. During ozonation processes, Mn loading on catalysts can enhance ozone decomposition capacity and optimize reaction pathways by constructing redox-active centers [53,54,55]. Existing studies have also revealed that synergistic effects between Mn species and zeolite supports can form bifunctional catalytic interfaces, simultaneously promoting oxidant activation and target pollutant adsorption [56,57,58].

In response to the technical challenges in current nitrite removal processes, this study developed an innovative O₃ + Mn/ZSM-5 heterogeneous wet catalytic oxidation system. The tailored design of Mn/ZSM-5 catalysts with controlled SiO₂/Al₂O₃ ratios (fabricated via wet impregnation) demonstrated exceptional $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation performance, with the Mn/ZSM-5 structure–activity correlations and $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion pathways systematically unraveled through kinetic studies and multi-technique characterizations. The catalyst’s stability and potential for engineering applications were further validated. This work provided technical support for the resource utilization of nitrogen-containing wastewater and the green upgrading of wet flue gas desulfurization/denitrification processes.

2. Experiment and Methodology

2.1. Materials and Reagents

The HZSM-5 zeolites used in this work were supplied by Xfnano technology Co., Ltd. (Nanjing, China). Manganese nitrate (50% in H₂O, Mn(NO₃)₂), ethanol (≥99.5%, C₂H₆O), sodium nitrite (≥99.0%, NaNO₂), phosphoric acid (≥85.0%, H₃PO₄), and sodium hydroxide (≥96.0%, NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nitrogen gas (99.999%, N₂) and oxygen gas (99.999%, O₂) were supplied by Jingong Material Co., Ltd. (Hangzhou, China). All chemicals were procured from commercial suppliers and were used directly in this study without further purification.

2.2. Catalyst Preparation

Mn-modified ZSM-5 zeolite catalysts with different SiO₂/Al₂O₃ ratios were synthesized through a wet impregnation approach. The detailed preparation procedure was as follows: 2.0 g of HZSM-5 zeolite support was precisely weighed and transferred into a 100 mL beaker. Subsequently, 20 mL of anhydrous ethanol was added, followed by the addition of manganese nitrate corresponding to a 5 wt% Mn loading (10.589 g Mn(NO₃)₂ solution). The mixture was magnetically stirred in a water bath maintained at 40 °C until complete evaporation of the ethanol solvent. The resulting solid was dried at 105 °C for 12 h, followed by calcination in a muffle furnace at 400 °C for 4 h to obtain the final Mn/ZSM-5 catalysts with different SiO₂/Al₂O₃ ratios. The metal loading and SiO₂/Al₂O₃ ratios of the prepared catalysts were quantitatively analyzed by ICP-OES, as presented in

Table 1. The metal loading and SiO₂/Al₂O₃ ratios of the prepared catalysts.

Catalysts	Actual SiO2/Al2O3	Loaded Metal	Actual Mn Content
HZSM-5 (400)	400.1	-	0
Mn/ZSM-5 (21)	21.2	Mn	5.06 wt%
Mn/ZSM-5 (85)	85.3	Mn	5.11 wt%
Mn/ZSM-5 (200)	200.1	Mn	4.99 wt%
Mn/ZSM-5 (300)	300.0	Mn	5.04 wt%
Mn/ZSM-5 (400)	399.6	Mn	5.01 wt%

. All five catalysts exhibited consistent Mn loadings of approximately 5 wt%, with measured SiO₂/Al₂O₃ ratios of 21.2, 85.3, 200.1, 300.0, and 399.6, respectively. These values showed excellent agreement with the predetermined specifications, confirming the successful preparation of the target catalysts.

2.3. Catalytic Activity Tests

This study established an ozone-assisted heterogeneous wet catalytic oxidation system, as illustrated in Figure 1. The structure–performance relationship of the catalysts was precisely modulated by controlling two critical factors: the framework SiO₂/Al₂O₃ ratio of ZSM-5 zeolite and the loading of active metal species. Furthermore, the reaction kinetic characteristics were thoroughly analyzed by examining the coupling effects between ozone mass transfer efficiency and key thermodynamic parameters.

All gas streams in the system were precisely regulated using mass flow controllers (ALICAT, Tucson, AZ, USA). Ozone was generated by an ozone generator (AZCOZON-HTU500, Vancouver, BC, Canada) and subsequently divided into two pathways. The O₃/O₂ stream was directed to an ozone analyzer (BMT-964BT, Filderstadt, Germany) for real-time monitoring of ozone concentration. Based on calculation results, the O₃/O₂ stream was introduced as the reaction gas into a gas-mixing equipment. The mixed gas was introduced into the liquid phase via a fine bubble diffuser to facilitate the reaction. Sodium nitrite was employed as the source of $\mathrm{N}{\mathrm{O}}_2^{-}$ in the aqueous phase. The reaction solution, containing predetermined amounts of catalyst, was maintained in the reactor with continuous pH monitoring using a digital pH meter (OHAUS-ST3100, Parsippany, NJ, USA). Temperature control was achieved through a circulating thermostatic water bath (Aohua HH-S, Changzhou, China). The exhaust gas from the reaction system passed through a pretreatment unit before being analyzed by a Fourier-transform infrared gas analyzer (Gasmet Dx4000, Helsinki, Finland) for compositional quantification. Concentrations of $\mathrm{N}{\mathrm{O}}_2^{-}$ and $\mathrm{N}{\mathrm{O}}_3^{-}$ were determined by ion chromatography (Metrohm 930, Herisau, Switzerland).

In addition to oxidation reactions, the disproportionation reaction of $\mathrm{N}{\mathrm{O}}_2^{-}$ under acidic conditions must also be considered (Equation (1)):

$$3\mathrm{N}{\mathrm{O}}_2^{-}+2{\mathrm{H}}^{+}=2\mathrm{N}\mathrm{O}\uparrow +\mathrm{N}{\mathrm{O}}_3^{-}+{\mathrm{H}}_2\mathrm{O}$$

(1)

The conversion efficiency of $\mathrm{N}{\mathrm{O}}_2^{-}$ considering both oxidation and disproportionation reactions can be calculated using the following formula:

$${\left[\mathrm{N}{\mathrm{O}}_2^{-}\right]}_{con.} = {\displaystyle \frac{c_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right) - c\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)}{c_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)}} \times 100\%$$

(2)

$${\left[\mathrm{N}{\mathrm{O}}_2^{-}\right]}_{con.O}={\displaystyle \frac{69\times \left(c\left(\mathrm{N}{\mathrm{O}}_3^{-}\right) {-c}_{initial}\left(\mathrm{N}{\mathrm{O}}_3^{-}\right)\right)-31\times \left(c_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)-c\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)\right)}{62\times c_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)}}\times 100\mathrm{\%}$$

(3)

$${\left[\mathrm{N}{\mathrm{O}}_2^{-}\right]}_{con.D}={\displaystyle \frac{93\times \left(c_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)-c\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)\right)-69\times \left(c\left(\mathrm{N}{\mathrm{O}}_3^{-}\right) {-c}_{initial}\left(\mathrm{N}{\mathrm{O}}_3^{-}\right)\right)}{46\times { c}_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)}}\times 100\mathrm{\%}$$

(4)

where ${\left[\mathrm{N}{\mathrm{O}}_2^{-}\right]}_{con.}$, ${\left[\mathrm{N}{\mathrm{O}}_2^{-}\right]}_{con.O}$, ${\left[\mathrm{N}{\mathrm{O}}_2^{-}\right]}_{con.D}$ are the total conversion efficiency of $\mathrm{N}{\mathrm{O}}_2^{-}$, conversion efficiency via oxidation reaction, and conversion efficiency via disproportionation reaction, respectively. $c_{initial}\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)$, $c_{initial}\left(\mathrm{N}{\mathrm{O}}_3^{-}\right)$ are the initial concentration of $\mathrm{N}{\mathrm{O}}_2^{-}$ and $\mathrm{N}{\mathrm{O}}_3^{-}$ in the liquid phase, unit mg/L, respectively. $c\left(\mathrm{N}{\mathrm{O}}_2^{-}\right)$, $c\left(\mathrm{N}{\mathrm{O}}_3^{-}\right)$ are final concentration of $\mathrm{N}{\mathrm{O}}_2^{-}$ and $\mathrm{N}{\mathrm{O}}_3^{-}$ after reaction, unit mg/L, respectively.

The ozone oxidation flue gas pollutant coupling wet spray system is shown in Figure 2. A lab-scale spray tower was designed following industrial specifications. The impact of the O₃ + Mn/ZSM-5 catalytic system on NO_x/SO₂ removal efficiency in the flue gas and the scrubbing stability of the original system were investigated. The ozone generation and flue gas analysis procedures were consistent with those described for Figure 1. Simulated flue gas premixed with O₃ was introduced into a quartz cylindrical reactor (650 mm length × 50 mm inner diameter) housed in a tubular furnace (Yifeng YFK60×600/10QK-G, Shanghai, China) for oxidation. The scrubbing system comprised a spray tower (inner diameter: 76 mm), a gear pump, an absorbent slurry circulation system, and a slurry sampling and analysis unit. The system operated under controlled conditions: slurry temperature: 50 ± 0.5 °C (maintained by an electronically controlled system with triple-blade impeller and mixed-flow pump agitation) and pH: 5.5 ± 0.1 (monitored in real-time using a calibrated pH meter). The gear pump (Levu CT3001S, Baoding, China) delivered the absorbent slurry to a fine-atomization solid-cone nozzle within the tower. The absorbent slurry comprised 3 L phosphate buffer solution (Na₂HPO₄-NaH₂PO₄) and a predetermined dosage of Mn/ZSM-5 zeolite catalyst. The concentration dynamics of nitrite byproducts in the liquid phase were quantitatively monitored through the integrated sampling and analysis system.

Similarly, the removal efficiency of $\mathrm{N}{\mathrm{O}}_2^{-}$ in the liquid phase can be calculated using Equations (2)–(4). The removal efficiency of NOₓ ($\eta \left(N{O}_x\right)$) and SO₂ ($\eta \left(S{O}_2\right)$) in the flue gas are determined by the following formula:

$$\eta \left(\mathrm{N}{\mathrm{O}}_x\right)={\displaystyle \frac{c_{inlet}\left(\mathrm{N}{\mathrm{O}}_x\right)-c_{outlet}\left(\mathrm{N}{\mathrm{O}}_x\right)}{c_{inlet}\left(N{O}_x\right)}}\times 100\mathrm{\%}$$

(5)

$$\eta \left(\mathrm{S}{\mathrm{O}}_2\right)={\displaystyle \frac{c_{inlet}\left(\mathrm{S}{\mathrm{O}}_2\right)-c_{outlet}\left(\mathrm{S}{\mathrm{O}}_2\right)}{c_{inlet}\left(\mathrm{S}{\mathrm{O}}_2\right)}}\times 100\mathrm{\%}$$

(6)

where $c_{inlet}\left(N{O}_x\right)$, $c_{outlet}\left(N{O}_x\right)$ are the concentrations of NO_x at the system inlet and after spray absorption, unit mg/Nm³, respectively. $c_{inlet}\left(S{O}_2\right)$, $c_{outlet}\left(S{O}_2\right)$ are the concentrations of SO₂ at the system inlet and after spray absorption, unit mg/Nm³, respectively.

2.4. Catalyst Characterization

After catalysts were pretreated at 200 °C in vacuum, the BET-specific surface area was measured by N₂ adsorption–desorption isotherms using an automatic surface area and porosity analyzer (Micromeritics ASAP 2460, Norcross, GA, USA) at 77 K. And the BJH method was used to calculate the pore volume and average pore diameter of catalysts. Elemental analysis (ICP-OES) was conducted on an inductively coupled plasma-optical emission spectrometer (Agilent 5110, Santa Clara, CA, USA). It was used for quantitative analysis of manganese loading and SiO₂/Al₂O₃ ratio in Mn/ZSM-5 catalysts. The X-ray diffraction (XRD) patterns of the catalysts were recorded by a Rigaku D/max 2550PC diffractometer (Rigaku, Tokyo, Japan) using monochromatized Cu Kα radiation (λ = 0.15406 nm), over a diffraction angle range of 10° to 80° at a scanning speed of 1°/min. The surface atomic chemical states were determined on an X-ray photoelectron spectrometer (Waltham, MA, USA) using a standard Al Kα source (1486.6 eV), and all the binding energies were referenced to the C 1s peak at 284.8 eV.

3. Results and Discussion

3.1. Relationship Between Catalytic Performance and Catalyst Properties

3.1.1. Effect of Zeolite SiO₂/Al₂O₃ on Catalytic Ozonation of $\mathrm{N}{\mathrm{O}}_2^{-}$

As shown in Figure 3, the catalytic oxidation performance of $\mathrm{N}{\mathrm{O}}_2^{-}$ was significantly influenced by the SiO₂/Al₂O₃ ratios of the Mn/ZSM-5 zeolites. The physicochemical properties of ZSM-5 exhibit a distinct structure–activity relationship with its framework SiO₂/Al₂O₃ ratio, while the incorporation of transition metal Mn further modulates the distribution of active sites. To elucidate the intrinsic material characteristics governing $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion pathways in the liquid phase, the catalytic performance of Mn/ZSM-5 with varying SiO₂/Al₂O₃ ratios was systematically investigated. For clarity in subsequent discussion, the following terms were defined: NC is the total conversion efficiency of $\mathrm{N}{\mathrm{O}}_2^{-}$, NCO is the $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion efficiency attributed to the oxidation reaction, and NCD is the $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion efficiency resulting from the disproportionation reaction. Experimental results revealed that higher SiO₂/Al₂O₃ ratios enhanced oxidative reaction selectivity. At SiO₂/Al₂O₃ = 400, the system achieved 97.3% total $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion efficiency with 96.95% contribution from the oxidative pathway.

3.1.2. Textual Properties

N₂ adsorption–desorption isotherms and the pore size distribution curves for Mn/ZSM-5 zeolites are shown in Figure S1, and Table S1 presents the textual properties of the catalysts. All the catalysts exhibited type IV isotherms with a H4 hysteresis loop [59], indicating the plentiful micropores and small mesopores. The pore size distributions of the five Mn/ZSM-5 catalysts were similar. The intensity of the curve was higher below 3 nm, and there was a small peak at 3.4–4.4 nm. This also indicated that the pores on the catalyst were mainly composed of abundant micropores and small mesopores, which was in line with the typical characteristics of zeolite catalysts. The BET surface areas for different catalysts ranked as follows: Mn/ZSM-5 (400) > Mn/ZSM-5 (300) > Mn/ZSM-5 (200) > Mn/ZSM-5 (85) > Mn/ZSM-5 (21). Results found that the specific surface area of zeolites increased significantly with the increase in the SiO₂/Al₂O₃ ratio. A higher BET surface area would produce more active sites for adsorption, activation, and oxidation, which would eventually contribute to the excellent catalytic performance of catalysts [60]. Among all the catalysts, Mn/ZSM-5 (400) had the largest surface area (398.9 m²·g⁻¹) and pore volume (0.103 cm³⋅g⁻¹), in accordance with its highest catalytic activity.

3.1.3. Crystalline Structures

The XRD patterns of various Mn/ZSM-5 zeolites are shown in Figure S2. All of the catalysts exhibited typical characteristic peaks at 13.8°, 14.8°, 23.1°, 23.3°, 23.9°, and 29.8°, corresponding to the (102), (112), (501), (051), (303), and (630) plane of ZSM-5 (JCPDS#42-0023) and HZSM-5 (JCPDS#49-0657) [61]. Results found that the strongest characteristic diffraction peaks of Mn/ZSM (21), Mn/ZSM (85), Mn/ZSM (200), Mn/ZSM (300), and Mn/ZSM (400) samples were at 23.09°, 23.06°, 23.02°, 23.03°, and 23.05°, respectively, which corresponded well with the (501) crystal plane in ZSM-5 (23.12°) and shifted slightly to the left. The strongest characteristic diffraction peak of HZSM-5 was at 22.99°. It showed that the strongest characteristic diffraction peaks of the Mn/ZSM-5 samples were all between those of ZSM-5 and HZSM-5. The reason was that the introduction of Mn caused the crystal plane spacing to become smaller, which also confirmed the loading and dispersion of Mn on the catalyst. In the five catalysts, no characteristic diffraction peaks corresponding to Mn species were found, indicating that Mn has excellent dispersion and weak crystallinity on the zeolite carrier. The dispersed Mn species did not damage the skeleton structure, but instead filled or modified the zeolite support.

3.1.4. Surface Properties

The XPS analysis was used to characterize the oxidation states and distribution of surface species, and the results are shown in Figure S3. Furthermore, the quantitative ratios were calculated by deconvolution of XPS curves and presented in Table S2. The Mn 2p_3/2 spectrum (shown in Figure S3a) exhibited three main peaks at ∼643.3 eV, 641.9 eV, and 640.9 eV, which were assigned to Mn⁴⁺, Mn³⁺, and Mn²⁺, respectively [62,63]. It could be seen that there were abundant species of Mn⁴⁺, Mn³⁺, and Mn²⁺ on all five Mn/ZSM-5 catalysts. It is noteworthy that the order of Mn³⁺ proportion was as follows: Mn/ZSM-5 (400) > Mn/ZSM-5 (300) > Mn/ZSM-5 (200) > Mn/ZSM-5 (85) > Mn/ZSM-5 (21), which was similar to the catalytic activity order. Mn³⁺ is generally believed to be closely related to the generation of oxygen vacancies, which can promote the decomposition of ozone and act as active sites for the ozonation [64,65,66].

Figure S3b presents the XPS spectra of O 1s, and the curves were deconvoluted into three peaks located at ∼532.8 eV, 531.6 eV, and 529.9 eV, corresponding to hydroxyl oxygen species (O_OH), surface-adsorbed oxygen species (O_ad), and lattice oxygen species (O_la), respectively [10,67]. The detailed species ratios are listed in Table S2. Results found that there were abundant hydroxyl oxygen species and surface adsorbed oxygen species on the surface of the Mn/ZSM-5 catalyst, which was related to the high specific surface area and H₂O adsorption of the zeolite carrier. It was beneficial for the adsorption and conversion of reactants on oxygen vacancies [68,69]. The abundant surface hydroxyl species (O_OH) can timely convert the adsorbed $\mathrm{N}{\mathrm{O}}_2^{-}$ on the catalyst into $\mathrm{N}{\mathrm{O}}_3^{-}$, improve the conversion rate and oxidation activity of $\mathrm{N}{\mathrm{O}}_2^{-}$, and reduce the selectivity of the disproportionation reaction. The density of O_OH on the heterogeneous catalytic surface was believed to be crucial for the initiation of ·OH from the decomposition of ozone [70]. In addition, surface adsorbed oxygen species (O_ad) were usually considered to be conducive to the formation of oxygen vacancies, which contributed to better catalytic performance [71,72]. As shown in Table S2, Mn/ZSM-5 (400) exhibited a significantly higher O_OH proportion compared to the other four samples, and it also had a rich O_ad proportion, which corresponds to its outstanding catalytic oxidation activity.

3.1.5. Adsorption Properties in Liquid Phase

The superior performance of high SiO₂/Al₂O₃ ratio Mn/ZSM-5 can be attributed to its key adsorption properties. Firstly, the higher zero charge point of Mn/ZSM-5 with an elevated SiO₂/Al₂O₃ ratio resulted in greater surface positive charges under acidic conditions, thereby enhancing $\mathrm{N}{\mathrm{O}}_2^{-}$ adsorption capacity [73]. Secondly, there was a competitive adsorption relationship between ozone and water molecules on the surface of zeolites. Zeolites with a high SiO₂/Al₂O₃ ratio had strong hydrophobicity due to their high silicon content, which was more conducive to the adsorption of ozone on the surface [74,75], thereby promoting the contact between $\mathrm{N}{\mathrm{O}}_2^{-}$ and active molecules.

3.2. Performance of O₃ + Mn/ZSM-5 System for Catalytic Oxidation of $N{O}_2^{-}$

As illustrated in Figure 4, significant differences were observed in the conversion pathways and efficiency of $\mathrm{N}{\mathrm{O}}_2^{-}$ among different oxidation systems. In the 20% O₂ direct oxidation system, the total $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion efficiency reached only 49.9%. Pathway analysis revealed that 95.91% of this conversion originated from the self-disproportionation of nitrite, which is considered an undesirable side reaction. This finding confirms that oxygen alone is insufficient to drive the directional oxidation of liquid-phase $\mathrm{N}{\mathrm{O}}_2^{-}$ effectively. To enhance oxidation efficiency, an ozone-assisted catalytic system was developed. Experimental results demonstrated that O₃ participation increased the total $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion efficiency to 69.1%. However, the oxidative pathway contribution rate remained limited to 39.77%, indicating the need for further optimization to improve the selectivity toward the desired oxidation pathway.

To address this limitation, HZSM-5 zeolite was introduced to construct a heterogeneous catalytic system. When SiO₂/Al₂O₃ = 400, the O₂ + HZSM-5 system achieved a remarkable increase in total $\mathrm{N}{\mathrm{O}}_2^{-}$ conversion efficiency to 84.5%, with the oxidation efficiency reaching 50.93% (approximately 24-fold higher than that of the pure O₂ system). This demonstrated that the HZSM-5 zeolite significantly enhanced the selectivity of the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation reaction while suppressing the disproportionation reaction. The improvement can be attributed to the HZSM-5 zeolite’s adsorption capability for both free $\mathrm{N}{\mathrm{O}}_2^{-}$ and O₂ in the liquid phase, thereby increasing their collision probability and promoting the oxidation of $\mathrm{N}{\mathrm{O}}_2^{-}$ to $\mathrm{N}{\mathrm{O}}_3^{-}$. Further enhancement was observed upon introducing O₃, which elevated the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency from 50.93% (O₂ + HZSM-5) to 73.79% (O₃ + HZSM-5). As shown in Figure 4, the Mn/ZSM-5 catalyst exhibited superior performance in optimizing oxidative pathway selectivity while minimizing disproportionation. Under optimized conditions (T = 40 °C, $c_{initial}\left(N{O}_2^{-}\right)$ = 100 mg/L, Mn/ZSM-5 dosage = 1 g/L, SiO₂/Al₂O₃ = 400, O₃ = 100 ppm, O₂ = 20%, and initial pH = 4), O₃ + Mn/ZSM-5 system achieved 94.33% NO₂⁻ oxidation efficiency (NCO) and only 2.97% disproportionation conversion (NCD). This represents near-complete selectivity for the oxidative pathway, effectively eliminating secondary pollution concerns associated with byproduct utilization.

The cycling stability and structural durability of catalysts are critical metrics for evaluating their industrial applicability, as these properties directly determine process economics and environmental benefits. The recycling performance of the Mn/ZSM-5 zeolite catalyst is presented in Figure 5. Results found that the Mn/ZSM-5 catalyst demonstrated exceptional and stable catalytic activity across five consecutive experimental cycles. During the 15-h continuous operation, $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency consistently exceeded 93%. Disproportionation reaction was effectively suppressed at a minimal level (<4%), and no detectable NO re-emission was observed. These results confirmed the outstanding reusability of Mn/ZSM-5, maintaining robust catalytic oxidation stability even under prolonged operation in acidic environments. The synergistic structure–performance stability endows Mn/ZSM-5 with significant industrial potential for high-concentration nitrite wastewater treatment.

3.3. Effects of Different System Conditions on $N{O}_2^{-}$ Oxidation Efficiency

3.3.1. Effect of Reaction Temperature

Temperature represents one of the most critical parameters in catalytic systems. Figure 6 illustrates the effect of reaction temperature on $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency and conversion pathways in the ozone-assisted wet catalytic oxidation system. The results demonstrated that elevated temperatures adversely affected oxidation efficiency. In the O₃ + HZSM-5 system, $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency decreased from 76.95% at 30 °C to 42.21% at 70 °C, while undesirable disproportionation increased from 7.15% to 56.89% over the same temperature range. Although the O₃ + Mn/ZSM-5 system showed improved overall performance, $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency still declined from 92.81% (30 °C) to 54.84% (70 °C), while disproportionation rose from 1.89% to 44.86%. Comparative analysis of the two systems revealed that the Mn/ZSM-5 catalyst significantly reduced the temperature sensitivity of the reaction system. When the temperature increased to 60 °C, the O₃ + Mn/ZSM-5 system maintained $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency above 75%. This demonstrated that the Mn/ZSM-5 catalyst can further enhance the selectivity of $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation pathway while effectively suppressing the disproportionation reaction. The superior catalytic performance and stability of Mn/ZSM-5 effectively broaden the optimal temperature operating window for this process.

From the perspectives of reaction thermodynamics and kinetics, conventional oxidation reactions typically intensify with increasing temperature, primarily due to the temperature-dependent mechanism of chemical reaction rates. However, in our system, the oxidation efficiency of $\mathrm{N}{\mathrm{O}}_2^{-}$ decreased with rising temperature rather than improved. This observation suggested that although intensified ionic movement of $\mathrm{N}{\mathrm{O}}_2^{-}$ occurred in the liquid phase, effective collisions with oxidants were limited. Simultaneously, increased temperature enhanced collision probabilities between $\mathrm{N}{\mathrm{O}}_2^{-}$ and H⁺, which subsequently promoted an undesirable disproportionation reaction. The competitive relationship between oxidation and disproportionation pathways in the liquid phase ultimately suppressed $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency.

3.3.2. Effect of Initial Liquid pH Value

The effect of initial liquid pH on $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency and conversion pathways is illustrated in Figure 7. Experimental data demonstrated that the initial pH played a pivotal regulatory role in both the oxidation efficiency and reaction pathways of $\mathrm{N}{\mathrm{O}}_2^{-}$. Under acidic conditions (initial pH = 4), the O₃ + HZSM-5 system achieved 73.79% $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency. However, when the pH increased to six, the efficiency sharply decreased to 32.09%. This phenomenon correlated with the catalyst’s zero point of charge (ZPC) [76,77]. When pH < ZPC, positively charged sites form on the catalyst surface, enabling $\mathrm{N}{\mathrm{O}}_2^{-}$ enrichment at the interface. Conversely, when pH > ZPC, electrostatic repulsion occurred between the negatively charged Mn/ZSM-5 surface and $\mathrm{N}{\mathrm{O}}_2^{-}$ anions, significantly inhibiting surface adsorption and consequently reducing $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency. Further analysis revealed that pH also critically affected ozone stability. Molecular ozone concentration markedly decreased under alkaline conditions, while remaining nearly undecomposed in acidic media [78,79]. These combined factors accounted for the observed decline in $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency in the O₃ + HZSM-5 system. According to heterogeneous catalytic ozonation theory, solution pH served as a primary determinant of ozonation efficiency by altering reaction kinetics and pathways. At low pH, direct ozonation dominated and demonstrated selectivity, whereas alkaline conditions favored indirect pathways involving radical chain reactions [70,80]. Therefore, $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation in the O₃ + HZSM-5 system predominantly proceeded through direct ozonation mechanisms.

As shown in Figure 7, the O₃ + Mn/ZSM-5 system achieved 94.33% $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency at pH = 4. When the pH was increased to six, the Mn/ZSM-5 catalyst still maintained 75.73% oxidation efficiency. Remarkably, even under alkaline conditions, the Mn/ZSM-5 catalyst demonstrated considerable oxidative performance. These results indicated that the superior $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation in the O₃ + Mn/ZSM-5 system arose from the synergistic effect between direct ozonation and radical chain reactions. This phenomenon demonstrated that the Mn/ZSM-5 catalyst possessed superior pH adaptability, where the incorporation of active Mn species effectively broadened the operational pH window of the catalytic system. The doping of Mn species not only enhanced the acid stability of the zeolite support but also created Mn³⁺/Mn⁴⁺ active sites that facilitated the directional adsorption and activation of ozone molecules on the catalyst surface. These active sites lowered the activation energy for ozone decomposition into hydroxyl radicals, thereby maintaining efficient electron transfer between ozone and $\mathrm{N}{\mathrm{O}}_2^{-}$ through coordination activation mechanisms even when elevated pH reduces surface protonation. From the perspective of interfacial reaction kinetics, these findings substantiated the exceptional structural stability and catalytic durability of Mn/ZSM-5 zeolite catalyst under varying pH conditions.

3.3.3. Effect of Oxidant Concentration

The effect of ozone concentration on the heterogeneous catalytic oxidation of $\mathrm{N}{\mathrm{O}}_2^{-}$ was investigated in Figure 8. Kinetic experimental data revealed that in the O₃ + Mn/ZSM-5 catalytic system, $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency exhibited a significant transition from 79.45% to 94.33% as ozone concentration increased from 50 ppm to 100 ppm, following pseudo-first order reaction kinetics. When the ozone concentration was further elevated to 150 ppm, the oxidation efficiency demonstrated marginal effect characteristics with only a 0.5% enhancement, and upon exceeding 150 ppm, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency entered a stable plateau region at 95%, displaying distinct mass transfer limitation features. Synchronous monitoring indicated that the disproportionation side reaction rate stabilized at 2.5% when ozone concentration reached 150 ppm, showing significant correlation with the oxidation efficiency stagnation observed in the high ozone concentration range.

Comparative analysis of different catalytic systems revealed that when the ozone concentration decreased from 100 ppm to 50 ppm, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency in the O₃ + HZSM-5 system significantly dropped from 73.79% to 50.71%. This demonstrated that ozone concentration served as a critical parameter governing the catalytic oxidation performance. Remarkably, under identical ozone concentration reduction, the O₃ + Mn/ZSM-5 system maintained ~ 80% $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency, indicating that the synergistic effect between Mn species and the zeolite support substantially enhances ozone utilization efficiency. The Mn/ZSM-5 system achieved comparable oxidation performance with lower ozone consumption, demonstrating better economic feasibility. The observed limitation in $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency at elevated ozone concentrations can be mechanistically explained by interfacial regulation phenomena in the heterogeneous catalytic system: Mn active sites reached adsorption–desorption equilibrium under high ozone concentrations, rendering excess ozone ineffective for surface catalytic cycles, while simultaneous mass transfer limitations created diffusion barriers for $\mathrm{N}{\mathrm{O}}_2^{-}$ migration from the liquid phase to catalytic interfaces. The unconverted $\mathrm{N}{\mathrm{O}}_2^{-}$ remaining in the liquid phase consequently underwent competitive disproportionation reactions with H⁺, collectively constraining the overall oxidation efficiency through these parallel pathways.

The effect of oxygen concentration on $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency was also considered, as shown in Figure 9. It was noteworthy that the single-ozone system maintained only ~60% oxidation efficiency while exhibiting a disproportionation rate of 20%. It demonstrated that the sole ozone oxidation pathway was constrained by mass transfer limitations arising from low oxidant concentration, and ozone alone cannot effectively achieve high-efficiency $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation. When ozone and oxygen coexist, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency significantly improved with concomitant suppression of the disproportionation reaction. This phenomenon can be attributed to synergistic effects in the ozone-oxygen dual-oxidant system: (1) molecular oxygen contributes partially to $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation; (2) competitive adsorption of oxygen inhibits ineffective ozone decomposition on the catalyst surface; and (3) the strong oxidative potential of ozone enhances interfacial electron transfer efficiency, thereby shifting the reaction pathway selectivity from disproportionation (Equation (1)) toward complete oxidation ($\mathrm{N}{\mathrm{O}}_2^{-}$ → $\mathrm{N}{\mathrm{O}}_3^{-}$).

3.4. Mechanisms of Catalytic Ozonation on Mn/ZSM-5 Zeolite Catalyst

Existing literature studies indicate that three possible scenarios may occur in heterogeneous catalytic ozonation systems [81,82]:

(1)

O₃ adsorbs on the catalyst surface and decomposes to form radicals, which subsequently react with $\mathrm{N}{\mathrm{O}}_2^{-}$ in the solution.

(2)

$\mathrm{N}{\mathrm{O}}_2^{-}$ adsorbs on the catalyst and is then attacked by O₃ molecules or other reactive species.

(3)

Both O₃ and $\mathrm{N}{\mathrm{O}}_2^{-}$ adsorb on the catalyst surface and undergo direct reaction.

Results found that the reaction mechanisms of heterogeneous wet catalytic ozonation could be primarily categorized into two pathways: the direct molecular ozone oxidation pathway and the radical chain reaction pathway. Surface adsorption played a crucial role in heterogeneous catalytic ozonation processes. In the O₃ + Mn/ZSM-5 catalytic system, $\mathrm{N}{\mathrm{O}}_2^{-}$ can react simultaneously with both ozone molecules and their derived radical species. It is worth emphasizing that under the acidic conditions employed in our experiments, ozone molecules exhibited high stability with a low tendency for self-decomposition into hydroxyl radicals. Particularly when ZSM-5 zeolite served as the catalyst, its surface acid sites demonstrate a negligible promotion effect on ozone decomposition for radical generation [41]. The introduction of Mn active species had been reported to significantly improve this situation [83]. Kinetic analysis of the experimental system revealed that $\mathrm{N}{\mathrm{O}}_2^{-}$ underwent two competitive transformation pathways during the reaction: oxidation and disproportionation. The disproportionation reaction predominantly occurred in the bulk liquid phase, whereas the oxidation process was highly dependent on surface adsorption and active sites on the catalyst.

To elucidate the dominant mechanism of the O₃ + Mn/ZSM-5 catalytic system in the oxidation reaction, a radical identification experimental strategy was employed to analyze the contribution of reactive oxygen species. As shown in Figure 10, by selectively adding hydroxyl radical scavenger tert-butanol (TBA) and superoxide radical scavenger benzoquinone (BQ), the variations in reaction kinetics under different radical inhibition conditions were systematically compared. This approach enabled effective discrimination between the direct ozone oxidation pathway and the radical chain reaction pathway in the heterogeneous wet catalytic oxidation system. In the O₃ + HZSM-5 zeolite catalytic system, the introduction of TBA and BQ did not induce significant changes in either the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency or the reaction pathway. This observation confirmed that the oxidation reaction in the O₃ + HZSM-5 system followed a non-radical-mediated mechanism, where the direct interaction between $\mathrm{N}{\mathrm{O}}_2^{-}$ and molecular ozone constituted the fundamental reaction pathway. The process was essentially characterized by direct electron transfer between $\mathrm{N}{\mathrm{O}}_2^{-}$ and molecular ozone occurring on the catalyst surface.

Notably, in the O₃ + Mn/ZSM-5 system, the addition of TBA resulted in a decrease in $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency from 94.33% to 73.38%, accompanied by an increase in the disproportionation ratio from 2.96% to 13.71%. These catalytic performance parameters approached those of the O₃ + HZSM-5 system. This demonstrated that the introduction of Mn active components promoted the generation of oxidative hydroxyl radicals (OH) in the reaction system, and ·OH played a crucial role in the deep ozonation of $\mathrm{N}{\mathrm{O}}_2^{-}$. When BQ was introduced, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency remained at 93.74%, showing no significant difference from the system without radical scavengers. The addition of BQ caused no observable changes in either $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency or reaction pathway. This indicated that the Mn active components did not facilitate the generation of superoxide radicals (${\mathrm{O}}_2^{-}$) in the reaction system. Therefore, the oxidation reaction in the O₃ + Mn/ZSM-5 heterogeneous wet catalytic oxidation system proceeded through a dual mechanism involving both the direct ozonation, as well as the reaction between $\mathrm{N}{\mathrm{O}}_2^{-}$ and hydroxyl radicals.

Based on comprehensive characterization and kinetic studies, this work proposed a mechanism for the heterogeneous wet catalytic oxidation in the O₃ + Mn/ZSM-5 system, as shown in Figure 11: The efficient catalytic oxidation of $\mathrm{N}{\mathrm{O}}_2^{-}$ proceeded through a synergistic combination of direct ozonation and radical chain reactions. Under acidic conditions, $\mathrm{N}{\mathrm{O}}_2^{-}$ preferentially adsorbed onto the acidic sites of Mn/ZSM-5 zeolite via electrostatic interactions, forming stable surface-adsorbed species. Molecular ozone underwent chemical adsorption on the catalyst surface, enabling direct oxidation of partially adsorbed $\mathrm{N}{\mathrm{O}}_2^{-}$ species. Concurrently, ozone activation occurs at Mn³⁺/Mn⁴⁺ active sites, generating highly reactive hydroxyl radicals (OH). These radical species subsequently participated in oxidation reactions with $\mathrm{N}{\mathrm{O}}_2^{-}$ through interfacial diffusion, thereby enhancing the overall oxidation performance. Furthermore, the strong adsorption of $\mathrm{N}{\mathrm{O}}_2^{-}$ by the Mn/ZSM-5 zeolite effectively suppressed its migration to the bulk liquid phase. Combined with the competitive mechanism between oxidation and disproportionation pathways, these factors collectively inhibited $\mathrm{N}{\mathrm{O}}_2^{-}$ disproportionation in the liquid phase. This dual-path synergistic mechanism elucidated the intrinsic regulation principle of Mn/ZSM-5 catalysts in achieving reaction pathway optimization through controlled surface adsorption–oxidation processes.

3.5. Effect of O₃ + Mn/ZSM-5 System on Ozone Oxidation Flue Gas Denitrification System

In this section, the synergistic treatment of flue gas pollutants and liquid-phase absorption byproduct $\mathrm{N}{\mathrm{O}}_2^{-}$ was investigated by coupling the O₃ + Mn/ZSM-5 heterogeneous wet catalytic oxidation system with an ozone oxidation denitrification technology spray absorption system. The experiments were conducted using a designed wet scrubbing tower, with the following Group I–IV. The specific operating parameters are detailed in

Table 2. The specific operating parameters of the spray absorption system.

Group	Total Gas Volume	O3/NO	Catalyst	Catalyst Dosage
I	5 L/min	1.6	-	0 g/L
II	5 L/min	1.6	Mn/ZSM-5 (400)	1 g/L
III	5 L/min	1.6	Mn/ZSM-5 (400)	2 g/L
IV	5 L/min	1.6	Mn/ZSM-5 (400)	3 g/L

.

This study focused on investigating the impact of Mn/ZSM-5 zeolite catalyst introduction on both the flue gas pollutant removal efficiency and the operational stability of the spray absorption system. As shown in Figure 12, under an O₃/NO molar ratio of 1.6, the ozone oxidation denitrification system demonstrated excellent synergistic removal performance for NO_x/SO₂. Experimental data revealed that SO₂ was completely removed after oxidation–absorption treatment, while the overall NO_x removal efficiency remained consistently above 93%. Notably, comparative analysis between the blank control (Group I) and catalytic test groups (Groups II–IV) showed NO_x removal efficiency of 93.94%, 94.06%, 94.17%, and 94.49%, respectively, with no significant variation. This confirmed that the introduction of the Mn/ZSM-5 catalyst did not adversely affect the existing flue gas pollutant removal system. Further analysis indicated that, in the complex O₃-H₂O-NO_x-SO₂ multiphase system, the Mn/ZSM-5 catalyst neither altered the mass transfer and reaction kinetics of flue gas pollutants nor induced unfavorable phase transitions in the absorption liquid. This characteristic ensured the long-term stable operation of the spray absorption system.

As shown in Figure 12, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency remained merely 29.48% in the blank control group (Group I) without catalyst addition, while the catalytic systems exhibited remarkable $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation enhancement effects. In Group II with 1 g/L Mn/ZSM-5 dosage, the total $\mathrm{N}{\mathrm{O}}_2^{-}$ removal efficiency exceeded 95%, comprising 91.33% oxidation efficiency and 3.93% disproportionation rate. Groups III and IV showed similar performance to Group II. These results confirmed that the dual-path synergistic catalytic mechanism established by O₃ + Mn/ZSM-5 significantly enhanced $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency in the absorption slurry. This process successfully reduced the environmental risk of denitrification byproduct nitrite by 1–2 orders of magnitude while maintaining >90% synergistic NO_x/SO₂ removal efficiency. This validated the catalytic performance of Mn/ZSM-5 and provided critical technical parameters for the green upgrading of wet flue gas desulfurization and denitrification processes.

4. Conclusions

This study developed a Mn-modified ZSM-5 zeolite catalyst. By establishing an O₃ + Mn/ZSM-5 heterogeneous wet catalytic oxidation system, efficient resource utilization of nitrite in denitrification wastewater could be achieved. The main conclusions are as follows:

(1)

A significant structure–activity relationship existed between the SiO₂/Al₂O₃ ratio of Mn/ZSM-5 zeolite and its catalytic oxidation capability. The Mn/ZSM-5 (SiO₂/Al₂O₃ = 400) catalyst achieved an exceptional $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency of 94.33%, which could be attributed to its optimal physicochemical properties, including the largest specific surface area, enhanced adsorption capacity, abundant surface Mn³⁺/Mn⁴⁺ species, hydroxyl oxygen groups, and chemisorbed oxygen, which collectively contributed to remarkable improvements in both $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation selectivity and catalytic activity. Repeated testing confirmed the outstanding structural stability and catalytic durability of Mn/ZSM-5, maintaining over 93% oxidation efficiency after multiple prolonged reaction cycles.

(2)

The oxidation efficiency and conversion pathways of $\mathrm{N}{\mathrm{O}}_2^{-}$ were governed by several key operational parameters, including reaction temperature, initial pH, catalyst dosage, and oxidant concentration. Compared with HZSM-5, the Mn/ZSM-5 catalyst demonstrated superior environmental adaptability and catalytic stability, effectively broadening the optimal operational windows for both temperature and pH in the catalytic oxidation process. The synergistic effect between Mn species and the zeolite support significantly enhanced ozone utilization efficiency, enabling the O₃ + Mn/ZSM-5 system to achieve comparable oxidation performance with substantially reduced ozone consumption, thereby improving overall economics. Furthermore, the combined use of oxygen and ozone proved more effective in promoting $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation than ozone alone.

(3)

In the O₃ + Mn/ZSM-5 heterogeneous wet catalytic oxidation system, the highly efficient catalytic oxidation of $\mathrm{N}{\mathrm{O}}_2^{-}$ followed a dual-path reaction mechanism involving the synergistic combination of direct ozonation and radical chain reactions. Under acidic conditions, $\mathrm{N}{\mathrm{O}}_2^{-}$ preferentially adsorbed onto the acidic sites of Mn/ZSM-5 zeolite via electrostatic interactions, forming stable surface-adsorbed species. Molecular ozone underwent chemical adsorption on the catalyst surface, enabling direct oxidation of partially adsorbed $\mathrm{N}{\mathrm{O}}_2^{-}$ species. Concurrently, ozone activation occurred at the Mn³⁺/Mn⁴⁺ active sites, generating highly reactive hydroxyl radicals (OH). These radical species subsequently participated in oxidation reactions with $\mathrm{N}{\mathrm{O}}_2^{-}$ through interfacial diffusion, thereby enhancing the overall oxidation performance.

(4)

The O₃ + Mn/ZSM-5 system demonstrated excellent compatibility with existing spray absorption systems for multi-pollutant removal, requiring no installation of additional liquid-phase byproduct treatment facilities. The introduction of the Mn/ZSM-5 catalyst would not affect the long-term operational stability of both the flue gas purification system and spray absorption unit. Under conditions ensuring effective removal of flue gas pollutants, the O₃ + Mn/ZSM-5 system achieved 91.33% $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency, providing critical technical parameters for the green upgrading of wet flue gas desulfurization and denitrification processes.