α-MnO₂ Reactive Lattice Oxygen Promotes Peroxymonosulfate-Activated Sulfamethoxazole Degradation

Abstract

Activated peroxymonosulfate (PMS) processes have emerged as a highly effective advanced oxidation technique for the removal of emerging organic contaminants in water. This study successfully converted δ-MnO₂ into α-MnO₂ through a crystal phase transformation method via the application of a mild water bath heating process, enhancing its catalytic properties. α-MnO₂ (k = 0.092 ± 0.0059 min⁻¹) exhibited significantly higher activity than δ-MnO₂ (k = 0.027 ± 0.0075 min⁻¹) in the PMS-activated degradation of sulfamethoxazole (SMX). Importantly, ¹O₂ was identified as the primary reactive oxygen species in the α-MnO₂ + PMS system for SMX degradation. XPS and O₂-TPD characterizations demonstrated that α-MnO₂ possesses a higher concentration of active lattice oxygen a and lower concentration of Mn(III) than δ-MnO₂. Further analysis reveals that both surface Mn(III) and active lattice oxygen in α-MnO₂ are crucial for PMS activation. Notably, ¹O₂ is predominantly generated through the interaction between PMS and reactive lattice oxygen. Moreover, a heterogeneous PMS activation mechanism toward α-MnO₂ was proposed. This research underscores the critical role of active lattice oxygen in MnO₂ for PMS activation, providing valuable insight relevant to the design of catalysts aimed at pollutant elimination in environmental applications. To the best of our knowledge, our study is the first to report a pathway for MnO₂ crystal phase transition under relatively mild conditions.

1. Introduction

The increasing prevalence of emerging organic contaminants (EOCs) in global water systems has become a critical environmental and public health challenge [1,2,3]. Of particular concern within this contaminant class are antibiotic compounds, which constitute a dominant subset of EOCs, owing to their extensive application in human healthcare, livestock husbandry, and aquacultural practices. Notably, these substances retain biological activity at trace concentrations (ranging from ng/L to μg/L), while demonstrating environmental persistence and recalcitrance to natural degradation mechanisms. Consequently, the presence of antibiotics in aquatic ecosystems has prolonged ecological impacts, such as disruption of microbial community structures, inhibition of aquatic organism reproduction, and alterations in nutrient cycling processes. Even more concerning is that their widespread presence fosters the proliferation of antibiotic-resistant bacteria, contributing to a silent epidemic of antimicrobial resistance that may eventually render conventional antibiotics ineffective in clinical settings.

In light of the challenges posed by antibiotic compounds in water systems, it is paramount to effectively address their removal from aquatic environments. Conventional remediation strategies for antibiotic contaminants in aquatic environments (e.g., activated carbon adsorption, biological degradation) are confronted with critical challenges, including high operational costs and insufficient treatment efficiency. In contrast, advanced oxidation processes (AOPs) offer a more robust solution for the degradation of antibiotic pollutants and have emerged as a potentially effective technology for their removal from wastewater [4,5,6]. AOPs utilize powerful oxidants to generate highly reactive species, such as hydroxyl radicals, which can non-selectively oxidize and break down complex organic molecules into simpler, less harmful compounds. This approach is particularly advantageous for addressing the environmental persistence of antibiotics, as traditional wastewater treatment methods often fall short of fully eliminating these substances. Among various AOPs, peroxymonosulfate (PMS)-based AOPs are regarded as the preferred choice for the degradation of antibiotics, since PMS is more easily induced to generate reactive oxygen species (ROSs) than H₂O₂ and persulfate (PS) under catalytic activation [7,8].

Over the past several decades, transition metal oxides have emerged as critical catalytic activators in peroxymonosulfate (PMS)-mediated oxidation systems. Representative compounds in this category, including Co₃O₄ [9,10], MnO₂ [11,12,13], and Fe₂O₃ [14,15], have been systematically investigated through diverse synthesis approaches for PMS activation. Notably, MnO₂ possesses characteristics such as abundant natural reserves, low cost, environmental friendliness, and versatile valence states. The combination of these advantages has propelled MnO₂ to stand out in PMS activation research.

MnO₂ exhibits various crystal forms [16,17,18], including tetragonal, rhombohedral, and monoclinic structures, each possessing distinct properties. To identify the crystal phase that optimally activates PMS, researchers have synthesized various crystalline types of MnO₂ and conducted extensive studies of their performance. The current literature indicates that α-MnO₂ demonstrates superior catalytic activity compared to β-MnO₂ and δ-MnO₂ in the activation of PMS [19], and the relationship between the surface chemical state and catalytic activity has been explored. For instance, Shen et al. established a correlation between the presence of surface Mn(III) species and enhanced catalyst activity [20]. Similarly, Wang et al. highlighted the essential role of Mn(IV) species in a KBr-assisted β-MnO₂ + PMS system [21]. In addition, many researchers argue that oxygen vacancies are crucial in facilitating PMS activation. Therefore, methods to create oxygen vacancies, including element doping, mechanical grinding, and other means, have been carried out and reported recently [22,23,24,25]. Despite advancements in elucidating the structure–activity relationships of MnO₂ catalysts in the activation of PMS, there is a range of perspectives among researchers. Continued research is necessary to deepen our understanding and refine these relationships, paving the way for more effective applications in pollutant degradation and environmental remediation.

In the present study, δ-MnO₂ and α-MnO₂ were prepared and utilized to activate PMS for the degradation of SMX. The ability of δ-MnO₂ and α-MnO₂ to catalytically activate PMS was evaluated by the degradation of SMX. Furthermore, the role of ROSs and their contribution to the elimination of SMX was assessed through radical scavenger experiments and EPR analysis. Finally, a key role for active lattice oxygen in ROS generation and PMS activation by α-MnO₂ was proposed.

2. Results and Discussion

2.1. Characterization of As-Prepared Materials

The XRD patterns of the synthesized materials are illustrated in Figure 1. δ-MnO₂ exhibited weak crystallization, characterized by four prominent peaks at 12.5°, 25.2°, 37.1°, and 65.8°, which correspond to birnessite-type MnO₂ (JCPDS 80-1098). For treated MnO₂, the peaks at 12.7°, 18.0°, 28.7°, 37.6°, 42.0°, 49.9°, 56.2°, 60.2°, and 65.5° correspond to the (110), (200), (310), (121), (301), (411), (600), (521), and (002) diffraction planes, respectively. These peaks are in good agreement with the standard tetragonal phase of α-MnO₂ (JCPDS 72-1982). This indicates that δ-MnO₂ can be effectively converted into α-MnO₂ through heating in a Mn²⁺-rich aqueous environment.

SEM images have been obtained to observe the morphologies of the prepared samples. As shown in Figure 2, the as-prepared δ-MnO₂ displayed an irregular particle morphology, whereas α-MnO₂ exhibited a distinct nanorod morphology with a significantly larger length. This indicates that crystal transformation of δ-MnO₂ in a Mn²⁺-rich aqueous environment is accompanied by crystal growth. During this process, the existing structure of δ-MnO₂ is altered, allowing for incorporation of Mn²⁺ ions from the surrounding solution. As the Mn²⁺ ions interact with the δ-MnO₂ structure, they facilitate the formation of new bonds and reconfiguration of the crystal lattice. The specific surface area, calculated from the nitrogen adsorption–desorption isotherm (Figure S1), is presented in

Table 1. Physicochemical parameters of as-prepared samples.

Sample	BET (m2/g)	AOS of Mn	Mn 2p3/2			OII/(Ototal) by XPS
			Mn2+	Mn3+	Mn4+
δ-MnO2	135	3.33	11.2%	42.7%	46.1%	25.2%
α-MnO2	97	3.53	9.4%	37.4%	53.2%	37.8%

. The BET specific surface areas were determined to be 135 m²/g and 97 m²/g for δ-MnO₂ and α-MnO₂, respectively.

Average oxidation state (AOS) and element speciation analyses of Mn were performed for both δ-MnO₂ and α-MnO₂. Figure 3a presents the Mn 3s spectra, and the AOS of Mn was determined from the binding energy difference (∆E) of the Mn 3s splitting peaks, utilizing the formula AOS = 8.956 − 1.126 × ∆E. As presented in Table 1, the AOS of Mn in α-MnO₂ is slightly higher than that in δ-MnO₂. Then, an analysis of Mn speciation was conducted for both δ-MnO₂ and α-MnO₂ (Figure 3b), and the proportion of Mn atoms in different valence states is listed in Table 1. δ-MnO₂ exhibited the presence of Mn⁴⁺, Mn³⁺, and Mn²⁺ in proportions of 46.1%, 42.7%, and 11.2%, respectively. Following the crystal transformation process, the proportions of Mn⁴⁺, Mn³⁺, and Mn²⁺ in α-MnO₂ were found to be 53.2%, 37.4%, and 9.4%, respectively. This suggests that α-MnO₂ has a higher proportion of high-valent Mn species than δ-MnO₂. In addition, the O 1s peaks were deconvoluted into three distinct components (Figure 3c), with binding energies at 529.5 eV, 531.1 eV, and 533.2 eV, corresponding to bulk lattice oxygen (O_I), surface active oxygen species (O_II), and adsorbed water (O_III) [26], respectively. Obviously, the crystal transformation process described in this study significantly enhances the content of active lattice oxygen in MnO₂.

To further explore the nature and reactivity of the lattice oxygen species present in the prepared materials, both H₂-TPR and O₂-TPD experiments were carried out. These techniques are widely employed to assess the reducibility and oxygen storage capacity of metal oxides, which are crucial factors influencing their catalytic performance. The first reduction peak in the H₂-TPR profile typically indicates the mobility of lattice oxygen. As shown in the H₂-TPR profiles (Figure 4a), δ-MnO₂ exhibited a single prominent peak at 282 °C. However, after its transformation into α-MnO₂, a new reduction peak emerged at a higher temperature, resulting in two reduction peaks at 288 °C and 331 °C, which are typically associated with the reduction of surface and/or more mobile lattice oxygen species. This finding suggests that the crystal transformation process did not enhance the mobility of lattice oxygen in MnO₂; rather, it introduced a new type of active lattice oxygen species. Subsequently, O₂-TPD tests were conducted to evaluated the oxygen storage capacity of the prepared samples. As illustrated in Figure 4b, the O₂-TPD spectra can be categorized into three characteristic temperature ranges. The low-temperature desorption region, below 250 °C, is generally attributed to the adsorbed oxygen species that are easily released. The intermediate temperature range, spanning from 250 to 600 °C, reflects the desorption of more strongly bound active lattice oxygen. Finally, the high-temperature region beyond 600 °C is associated with the desorption of bulk lattice oxygen, which is less mobile and tightly integrated into the crystal structure [27]. In comparison with the δ-MnO₂ sample, the α-MnO₂ variant exhibited a significantly increased amount of active lattice oxygen and a corresponding decrease in bulk lattice oxygen. This implies that the structural transformation process effectively promoted the conversion of bulk oxygen into a more reactive lattice oxygen form.

2.2. Catalytic Activity of Prepared MnO₂

The catalytic activity of the synthesized MnO₂ was evaluated by catalytic degradation of SMX in the presence of PMS at pH 7.0 ± 0.2 under ambient conditions. As illustrated in Figure 5, SMX exhibited the highest removal efficiency in the α-MnO₂ + PMS system. δ-MnO₂ facilitated the degradation of 34% of SMX within 15 min in the presence of PMS. In comparison, α-MnO₂ demonstrated a significantly higher removal rate, achieving 77% degradation of SMX under the same conditions and within the same time frame. Moreover, the degradation of SMX followed pseudo first-order kinetics (Figure S2), with reaction rate constants (k_obs) of 0.027 ± 0.0075 min⁻¹ for the δ-MnO₂ + PMS system and 0.092 ± 0.0059 min⁻¹ for the α-MnO₂ + PMS system (Figure 5). As the concentration of SMX remained relatively constant, the contributions of SMX absorption and degradation in the PMS only, δ-MnO₂ only, and α-MnO₂ only systems can be considered negligible within the reaction time. Additionally, the concentration of Mn²⁺ in the leaching solution following reaction in the α-MnO₂ + PMS system was measured at 0.976 µg/L, which is extremely low and indicates minimal risk of secondary pollution. In the homogeneous control tests, negligible SMX degradation was observed in the presence of 1.0 and 10.0 µg/L Mn²⁺, as well as in the leaching solution control group (Figure S3). Consequently, the degradation of SMX was predominantly attributed to the heterogeneous activation of PMS by α-MnO₂. In addition, to evaluate the stability and reusability of α-MnO₂, degradation experiments were conducted over six cycles. As illustrated in Figure 6, α-MnO₂ exhibited remarkable long-term stability, because the SMX removal efficiency was comparable in each of the six cycles.

2.3. Identification of Active Species

Previous studies have shown that •OH, •SO₄⁻, •O₂⁻, and ¹O₂ are the primary ROSs active in EOC degradation in catalyst + PMS systems. To investigate the corresponding ROSs generated in the α-MnO₂ + PMS system, several typical scavengers were employed for quenching experiments. EtOH was commonly applied as a scavenger of •SO₄⁻ (k_EtOH/•SO4⁻ = 4.3 × 10⁷ M⁻¹·s⁻¹) and •OH (k_EtOH/•OH = 2.2 × 10⁹ M⁻¹·s⁻¹), TBA was employed as a scavenger of •OH (k_TBA/•OH = 6.0 × 10⁸ M⁻¹·s⁻¹), and BQ was commonly used to determine the existence of •O₂⁻. As shown in Figure 7, the degradation rate of SMX was inhibited to different degrees in the presence of TBA, EtOH, and BQ, indicating that •OH, •SO₄⁻, and •O₂⁻ contributed to some extent to the degradation of SMX. In contrast, the efficiency of SMX degradation was significantly reduced upon the addition of L-H (a scavenger for ¹O₂), suggesting the existence of ¹O₂ and its crucial role in the reaction system. The identified active species that we found to be active in SMX degradation were also reported in literature [23].

To identify the ROSs involved in SMX degradation during the activation of PMS, EPR experiments were performed, applying DMPO and TEMP as trapping agents. As shown in Figure S4, no peaks were observed in the control groups containing α-MnO₂, indicating that no radicals were produced in the absence of PMS. Upon the addition of PMS (see Figure 8), a strong characteristic 1:1:1 triplet signal was detected in the presence of TEMP, confirming the existence of ¹O₂ following PMS activation. Additionally, in the presence of DMPO, a set of weak peaks was obtained, which could be assigned to the DMPO−•OH and DMPO•−•SO₄⁻ adducts. Moreover, MeOH was employed as a solvent to eliminate the effect of other radicals during evaluation of the EPR signal for •O₂⁻, and a characteristic peak related to the DMPO-•O₂⁻ adduct was detected. All the EPR signals were consistent with the findings from the quenching experiments. The above findings indicate that the α-MnO₂ + PMS system can generate reactive species, with ¹O₂ identified as the dominant reactive species, while •OH, •SO₄⁻, and •O₂⁻ contribute to the degradation of SMX to a lesser extent.

2.4. PMS Activation Mechanism

The decomposition of PMS over δ-MnO₂ and α-MnO₂ was investigated and is presented in Figure S5. Compared to δ-MnO₂, α-MnO₂ demonstrated significantly higher efficiency with regard to PMS decomposition. The results from the quenching experiments and EPR characterization confirmed the generation of •OH, •SO₄⁻, •O₂⁻, and ¹O₂ in the α-MnO₂ + PMS system, with ¹O₂ being identified as the primary ROS responsible for the degradation of SMX.

Mn(III) is commonly employed as the active site for persulfate, playing a pivotal role in the Mn(III)/Mn(IV) cycle within Mn-based catalysts [4,28,29]. The surface Mn(III) sites effectively donate electrons, facilitated by the electrostatic attraction between persulfate and α-MnO₂, which in turn promotes the generation of •OH and •SO₄⁻ radicals (Equations (1)–(3)). Subsequently, •O₂⁻ and ¹O₂ can be generated through a series of reactions (Equations (4)–(10)). The XPS results presented above indicate that the proportion of Mn(III) in α-MnO₂ is 37.4%, which is significantly lower than the 42.7% proportion of Mn(III) in δ-MnO₂. This suggests that, in the present study, Mn(III) may not be the primary active site for the activation of PMS, and that ¹O₂ may be generated through alternative pathways.

Previous research shows that PMS self-dissociation can produce a minor fraction of non-radical ¹O₂ (Equation (11)) [30,31]. Another significant pathway for the generation of ¹O₂ is the reaction between PMS and the lattice oxygen of MnO₂ (Equations (12) and (13)) [24,32,33]. The results from the XPS, H₂-TPR, and O₂-TPD analyses indicate that, during the transformation of δ-MnO₂ into α-MnO₂, a significant amount of bulk lattice oxygen is converted into active lattice oxygen. Consequently, α-MnO₂ contains a greater amount of active lattice oxygen compared to δ-MnO₂, which facilitates the release of more lattice oxygen from its surface. A previous study employed isotope-tracing techniques to demonstrate that lattice oxygen in MnO₂ can be mobilized and involved in the generation of ¹O₂ [24]. Consistent with our research, this highlights that oxygen vacancies can significantly enhance the mobility and reactivity of lattice oxygen, thereby promoting PMS activation. In short, the released lattice oxygen of MnO₂ subsequently reacts with PMS to generate ¹O₂, resulting in enhanced catalytic activity (Figure 9).

≡ Mn^IV + HSO₅⁻ → ≡ Mn^III + •SO₅⁻ + H⁺

(1)

≡ Mn^III + HSO₅⁻ → ≡ Mn^IV + •SO₄⁻ + OH⁻

(2)

≡ Mn^III + HSO₅⁻ → ≡ Mn^IV + SO₄²⁻ + •OH

(3)

2 •SO₅⁻ → 2 •SO₄⁻ + O₂

(4)

•SO₄⁻ + H₂O → •OH + SO₄²⁻ + H⁺

(5)

•OH + •OH → H₂O₂

(6)

•OH + H₂O₂ → HO₂• + H₂O

(7)

HO₂• → H⁺ + O₂⁻•

(8)

O₂⁻• + •OH → ¹O₂ + OH⁻

(9)

O₂⁻• + O₂⁻• + 2 H⁺ → ¹O₂ + H₂O₂

(10)

SO₅²⁻ + HSO₅⁻ → HSO₄⁻ + SO₄²⁻ + ¹O₂

(11)

O_lat → O^⁎

(12)

O^⁎ + HSO₅⁻ → HSO₄⁻ + ¹O₂

(13)

3. Experimental

3.1. Chemicals

Potassium permanganate (KMnO₄, 99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Manganese chloride (MnCl₂, 99%) and PMS (98%) were obtained from Damas-beta (Shanghai, China). Sulfamethoxazole (SMX, 99%), 3-morpholinopropanesulfonic acid (MOPS, 98%), L-ascorbic acid (99%), benzoquinone (BQ), and L-histidine (L-H) were purchased from HEOWNS (Tianjin, China). Ethanol (EtOH, chromatographic grade) and methanol were purchased from Sigma-Aldrich (Shanghai, China). Tert-butyl alcohol (TBA, 99.5%) was purchased from Alfa Assar (Shanghai, China). All other chemicals and reagents used were of analytical grade or higher. High-purity deionized (DI) water was generated using a Millipore Milli-Q water purification system (Billerica, MA, USA).

3.2. Catalyst Preparation

The catalyst was synthesized in 125 mL of deionized water (DI). Firstly, 7.5 g KMnO₄ was added to the system and dissolved, applying ultrasonic and magnetic agitation. Subsequently, 2.5 mL ethanol was introduced into the dissolved solution, and the mixture was stirred for 3 h at 60 °C. The resulting solid precipitate was then filtered, washed, and dried overnight at 105 °C. The resultant catalyst was labeled as δ-MnO₂.

In order to further modify the prepared δ-MnO₂, we carried out the following operations. A total of 4.34 g of MnCl₂ was dissolved in 100 mL DI water. Subsequently, 3 g δ-MnO₂ powder was introduced into the dissolved solution, and the mixture was maintained at 60 °C while stirring for 4 h. The resultant solid precipitate was then filtered, washed, and dried overnight at 105 °C. The resultant catalyst was labeled as treated MnO₂.

3.3. Characterization

X-ray diffraction (XRD) patterns were obtained using a Bruker X-ray diffractometer (D8-Advance, Karlsruhe, Germany) equipped with a Cu-Kα X-ray source. SEM images were recorded using a Merlin SEM microscope (Carl Zeiss, Oberkochen, Germany). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Fisher, Waltham, MA, USA), and the binding energy was calibrated with the C 1s peak at 284.8 eV. The specific surface area was determined using a Micromeriticis ASAP 2460 analyzer (iQ3 Quantachrome, Boynton Beach, FL, USA) and nitrogen adsorption data at 77 K.

H₂ temperature programmed reduction (H₂-TPR) and O₂ temperature programmed desorption (O₂-TPD) were performed on an AutoChem II 2920 instrument (Micromeritics, Norcross, GA, USA) equipped with a thermal conductivity detector (TCD). Prior to testing, 50 mg samples (40–60 meshes) were pretreated at 105 °C for 30 min in the helium flow. For H₂-TPR measurement, the sample was programmed to rise to 800 °C at a ramp rate of 5 °C/min in a 5% H₂/Ar atmosphere. For the O₂-TPD analysis, the sample was purged with 5% O₂/He for 30 min and then with helium for another 30 min. Subsequently, it was heated from 40 °C to 800 °C at a ramp rate of 5 °C/min in the helium flow.

3.4. Experimental Setup

SMX degradation experiments were performed in 30 mL beakers at a temperature of 25 ± 2 °C while being stirred magnetically. In a typical test, 5 mg of the catalysts was introduced into an aqueous solution (20 mL) containing 10 µM SMX and buffered at 7.0 ± 0.2 with 10 mM MOPs. Unless otherwise specified, 10 mM PMS was incorporated into the above solution to initiate the reaction. The reaction time for all the experiments was within 15 min. Samples were then periodically collected, filtered through a 0.22 µm membrane, quenched using L-ascorbic acid, and subjected to appropriate analytical methods for evaluation. The control experiments were conducted with catalysts alone without oxidants and PMS alone under the same conditions as those described above. To investigate the corresponding reactive species generated in the MnO₂ + PMS system, TBA (scavenger of •OH, 10 mM), EtOH (scavenger of •SO₄⁻ and •OH, 17 mM), BQ (scavenger of •O₂⁻, 0.1 mM), and L-H (scavenger for ¹O₂, 0.1 M) were employed for the quenching experiments. In order to evaluate the stability of the catalysts, δ-MnO₂ was recycled after each run, washed with DI water multiple times, dried and activated in an oven at 80 °C overnight for reuse [26]. All measurements except for those in the recycling experiment were repeated in triplicate and reported as the average values.

3.5. Analytical Methods

The concentration of SMX was determined using a SHIMADZU HPLC-DAD (LC-2030C) system, which featured a SHIMADZU C18 column (4.6 × 150 mm, 5 μm). Detection was performed at a wavelength of 254 nm. Gradient elution was used for SMX, with 0.1% phosphoric acid and methanol. The PMS solution concentrations were quantified through a spectrophotometric approach, which was modified based on a iodometric titration method [34]. Briefly, 0.5 mL sample was added to 4.0 mL aqueous solution containing 0.05 g NaHCO₃ and 0.4 g KI. The mixture was shaken manually and allowed to react for 15 min, after which the absorbance was measured using UV–vis spectrophotometry (SHIMADZU, Tokyo, Japan) at a wavelength of 352 nm. The adducts formed from ROS with DMPO or TEMP, utilized as spin trapping agents, were analyzed using a Bruker-A300 EPR spectrometer (Bruker (Beijing) Technology Co., Ltd., Beijing, China). Additionally, the Mn ions leached into the degradation solution were quantified via inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7900, Santa Clara, CA, USA).

4. Conclusions

In summary, α-MnO₂ enriched with active lattice oxygen was synthesized through a phase transformation approach to enhance activation of PMS for the degradation of SMX. Compared to δ-MnO₂, α-MnO₂ demonstrated a significantly higher efficacy in the PMS-activated degradation of SMX at pH 7.0. The reaction rate constant of the α-MnO₂ + PMS system was 0.092 ± 0.0059 min⁻¹, which was more than three times higher than that of δ-MnO₂+ PMS (k = 0.027 ± 0.0075 min⁻¹). The quenching experiments and EPR characterization indicated that ¹O₂ is the primary ROS responsible for SMX degradation. The analysis results show that both surface Mn(III) and active lattice oxygen in α-MnO₂ play important roles in the activation of PMS. ¹O₂ is primarily generated through the interaction between PMS and reactive lattice oxygen, rather than solely relying on the cycling of Mn(III). This study effectively demonstrates the catalytic activation potential of α-MnO₂, especially its enhanced lattice oxygen activity for SMX degradation, providing insights into catalyst design and the elimination of pollutants for environmental applications.