Synergistic Adsorption-Catalytic Oxidation of o-xylene with Ozone over Manganese Oxide-Supported Bamboo Biochar

Abstract

Volatile Organic Compounds (VOCs) are significant pollutants emitted during industrial processes, with their release continuously increasing due to rapid industrialization, posing critical threats to environmental quality and human health. Catalytic oxidation is regarded as one of the most effective methods for VOC removal due to its low cost and high elimination efficiency. In pursuit of safety, energy savings, cost-effectiveness, and environmental friendliness, extensive efforts have been devoted to developing efficient catalysts that leverage the synergistic effects of catalysts and reactive oxygen species (ROSs) by incorporating ozone molecules, thereby reducing the temperature required for VOC catalytic oxidation. Based on research into large-scale treatment of industrial VOC waste gases, this study proposes an integrated adsorption-catalysis process system that balances cost and efficiency. Waste disposable bamboo chopsticks were recycled as precursors for carbon-based materials, and transition metal manganese oxides with different crystal phases were loaded onto the carbon substrate. The MnO_x-BC composite with the highest catalytic activity was selected for the catalytic oxidation of recalcitrant industrial o-xylene.

INTRODUCTION

Volatile Organic Compounds (VOCs), defined as organic compounds with boiling points below 260°C at room temperature, are major pollutants. VOCs act as precursors for tropospheric ozone formation, photochemical reactions, and secondary organic aerosols (SOA), significantly contributing to urban photochemical smog and global warming, thereby exacerbating atmospheric environmental degradation.¹ Most VOCs exhibit malodorous, irritating, toxic, and carcinogenic properties, posing substantial threats to human health and living conditions.^2,3

VOCs are primarily categorized into various species, including alkanes, alkenes, aromatic hydrocarbons, alcohols, aldehydes, and halogenated hydrocarbons.⁴ Among them, aromatic VOCs—emitted from widespread sources and exhibiting high toxicity—pose significant environmental and health hazards. Aromatic VOCs, collectively referred to as BTEX (benzene, toluene, ethylbenzene, and o-xylene), contain stable benzene ring structures with low energy barriers, rendering them resistant to degradation.⁵

Existing VOCs abatement technologies are divided into recovery and degradation methods.^6,7 Recovery techniques (e.g., adsorption, condensation, membrane separation) are suitable for low-concentration VOCs but require subsequent desorption and carry secondary pollution risks. Degradation methods (e.g., thermal incineration, biodegradation, catalytic oxidation) can completely decompose VOCs into harmless small molecules but often suffer from high energy consumption. Low-temperature catalytic oxidation stands out as one of the most effective methods due to its low cost and high efficiency,⁸ without requiring high temperatures, high pressure, discharge, or ultraviolet light.

Recent years have witnessed dedicated efforts to develop highly active, non-selective, and stable catalysts to lower the reaction temperature of VOC catalytic oxidation. Noble metal catalysts exhibit excellent low-temperature activity and stability,¹¹ such as Na-Pt/TiO₂,¹² Pt/MnOx-CeO₂,¹³ and Pd/TiO₂,¹⁴ but their high cost limits large-scale application. Non-noble metal oxides, especially transition metal-based catalysts, have emerged as cost-effective alternatives. FeOx-CeOx/SBA-15-350,¹⁵ Mn-Co/γ-Al₂O₃,¹⁶ and β-MnO₂/SBA-15,¹⁷ have also shown promising low-temperature activity. Mn-based catalysts, in particular, are widely studied for their high activity, large specific surface area, and low toxicity,^18-20 with factors like crystal phase, morphology, and tunnel structure significantly influencing performance.^21,22 Different MnO₂ crystal phases (α-, β-, γ-, δ-) show distinct catalytic activities, with δ-MnO₂ (layered structure, abundant oxygen vacancies, and high lattice oxygen mobility) demonstrating superior VOC oxidation performance.²¹

Biochar, produced via pyrolysis of agricultural waste(e.g., straw, rice husks, fruit shells), forestry residues (e.g., wood chips, sawdust), and municipal solid waste, aligns with the circular economy concept of “treating waste with waste”, reducing resource waste and environmental burden. Owing to its metal-free nature, abundance, resistance to acid-base corrosion, biocompatibility, and tunable physicochemical properties, biochar is widely used as an adsorbent and catalyst support.²³ However, pristine biochar exhibits unsatisfactory removal efficiency for industrial aromatic VOCs, necessitating modification to enhance cost-effectiveness and performance.

This work focuses on recycling waste disposable bamboo chopsticks to produce bamboo charcoal through thermal treatment and acid washing. Manganese oxides were grown in situ on the bamboo charcoal via hydrothermal-redox methods. The adsorption-catalytic oxidation performance of MnO_x-BC materials with different crystal phases was systematically evaluated for o-xylene removal, and the reaction mechanisms were explored. This study provides a theoretical basis for further optimizing material properties and promoting the application of adsorption-catalysis technology in VOCs abatement.

EXPERIMENTAL

Materials

Potassium permanganate (KMnO₄), manganese sulfate monohydrate (MnSO₄·H₂O), ammonium persulfate ((NH₄)₂S₂O₈), concentrated nitric acid (HNO₃), and o-xylene were utilized as primary reagents.

Catalysts Preparation

BC: Disposable bamboo chopsticks were recycled and pretreated. Initially, they were thoroughly washed with deionized water, cut into segments of 5–8 mm length, and sealed in a ceramic crucible. The crucible was placed in a muffle furnace and heated at a ramp rate of 10°C min^-1 to 700°C, followed by a 2 h isothermal hold. The resulting carbonized bamboo charcoal was ground into powder, and 75 g of the powder was immersed in 500 mL of a 2 mol/L dilute nitric acid solution for 6 h of acid washing. The acid-treated powder was subsequently rinsed three times with deionized water and dried at 80°C. The material was then blended with bentonite and sodium bicarbonate at a mass ratio of 80:20:5, granulated in a pelletizer with an appropriate amount of water to form spheres of 3–6 mm diameter, and finally dried to achieve structural stability. This processed bamboo charcoal was designated as BC.

MnO₂-BC: Various MnO₂ phases were synthesized under distinct conditions:

α-MnO₂-BC: 1.13 g KMnO₄, 0.47 g MnSO₄·H₂O, and 7.5 g BC.

β-MnO₂-BC: 1.52 g MnSO₄·H₂O, 2.05 g (NH₄)₂S₂O₈, and 7.5 g BC.

γ-MnO₂-BC: 3.04 g MnSO₄·H₂O, 4.10 g (NH₄)₂S₂O₈, and 7.5 g BC.

δ-MnO₂-BC: 0.275 g MnSO₄·H₂O, 1.5 g KMnO₄, and 7.5 g BC. Hydrothermal synthesis was conducted under specific conditions: 160°C for 12 h (α-MnO₂ and β-MnO₂), 90°C for 24 h (γ-MnO₂), and 255°C for 24 h (δ-MnO₂). After cooling to room temperature, the products were filtered under vacuum, dried overnight at 80°C, and collected. Subsequently, the samples were calcined in static air at 300°C for 3 h with a heating rate of 5°C/min. The resulting catalysts were labeled as α-MnO₂-BC, β-MnO₂-BC, γ-MnO₂-BC, and δ-MnO₂-BC, respectively.

MnO₂-BC Synthesized at Different Hydrothermal Temperatures: Using identical reagent conditions as δ-MnO₂-BC but varying hydrothermal temperatures (180°C, 205°C, 230°C, and 255°C), the samples were designated as MnO₂-BC180, MnO₂-BC205, MnO₂-BC230, and MnO₂-BC255.

Characterization

The synthesized catalysts were comprehensively characterized using a suite of analytical techniques. Morphology and microstructure were examined by field emission scanning electron microscopy (FE-SEM; Quanta 400F, FEI, USA). Crystalline phase and composition were determined via X-ray diffraction (XRD; Ultima IV, Rigaku, Japan). Functional groups and active sites were analyzed using Fourier transform infrared spectroscopy (FT-IR; Thermo Fisher Nicolet iS10, Thermo Fisher, USA). Nitrogen adsorption-desorption isotherms were measured at -196°C, with Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods employed to calculate specific surface area and pore size distribution. Prior to analysis, samples were degassed under vacuum at 150°C for 8 h. Metal element content was quantified by inductively coupled plasma optical emission spectrometry (ICP-OES; PerkinElmer). Chemical composition, molecular structure, and surface elemental valence states were characterized by X-ray photoelectron spectroscopy (XPS; ESCALAB 250, Thermo Fisher Scientific, USA), with spectral fitting performed using CasaXPS software. Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMXplus) was utilized to detect reactive oxygen species (e.g., hydroxyl radicals •OH or superoxide radicals •O₂^-), using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP) as spin-trapping agents for •OH and singlet oxygen (¹O₂), respectively.

Adsorption and Catalytic Oxidation Procedures

The experimental setup is illustrated schematically. O-xylene, as the target pollutant, was mixed with reactive oxygen species (ROSs, herein ozone) and introduced into a catalytic reactor for degradation. The inlet concentrations were set at 500 ppm for o-xylene and 100 mg/L for ozone, whilecatalyst loading is 6.5 g. Triplicate experiments were conducted, each with a reaction duration of 80 min. O-xylene concentration was monitored using a gas chromatograph (GC; FL9790, Zhejiang Fuli) equipped with a flame ionization detector (FID). Pollutant concentration was calculated using Equation (1):

\(\begin{align}\eta=\frac{C_{0}-C_{t}}{C_{0}}\end{align}\)       (1)

where C₀ and C_t represent the o-xylene concentrations at the inlet and the outlet under steady-state conditions, and η represents the removal efficiency of o-xylene.

The O-xylene Gas Blending System comprised rotameters, buffer bottles, bubbling bottles, and mixing bottles. Compressed air from a cylinder was split into two streams controlled by rotameters. A minor stream passed through a bubbling bottle containing o-xylene reagent, while the major stre am mixed with the vaporized o-xylene in a mixing bottle to ensure homogeneous distribution.

O-xylene Concentration Detection: A GC-FID system (FL9790, Zhejiang Fuli) was employed to measure o-xylene concentrations at the inlet and outlet. Optimal analytical conditions were established based on literature review and empirical trials. A calibration curve was constructed using standard o-xylene gas, relating peak area to concentration via linear regression.

Figure 1. Experimental setup.

Ozone conversion rate was calculated by measuring the ozone concentrations at the reactor inlet (C_in) and outlet (C_out) using an convenient Ozone Analyzer, with the calculation formula shown in Equation (2)

\(\begin{align}\eta_{O_3}=\frac{C_{in}-C_{out}}{C_{in}}\end{align}\)       (2)

where η_O3 represents the ozone conversion rate (%), C_in is the inlet ozone concentration (100 mg/L, set in the experiment), and C_out is the outlet ozone concentration under steady-state conditions.

Intermediate Product Analysis: Reaction intermediates were identified using gas chromatography-mass spectrometry (GC-MS; ISQ7000, Thermo Fisher, USA). Key instrument specifications included: MS sensitivity: EI scan S/N > 400 (rms) for 1 pg OFN at m/z 272; PCI scan S/N > 500 (rms) for 100 pg BZP at m/z 183; NCI scan S/N > 600 (rms) for 200 fg OFN at m/z 272. FID detection limit: <1.8 pg carbon/sec for propane. ECD detection limit: <0.008 pg/sec for hexachlorobenzene.

RESULTS AND DISCUSSION

Structural Characterization

X-ray diffraction (XRD) characterization was employed to analyze the crystalline structure of the synthesized samples. Figure 2 presents the XRD patterns of the prepared catalysts. The diffraction peaks of the four samples correspond well with the standard lattice parameters of α-MnO₂ (JCPDS 44-0141), β-MnO₂ (JCPDS 24-0735), γ-MnO₂ (JCPDS 14-0644), and δ-MnO₂ (JCPDS 80-1098),21 confirming the successful loading of MnO₂ with four distinct crystal phases. Simultaneously, the biomass bamboo charcoal (BC) exhibits a broad diffraction peak at 23°, attributed to the (002) crystal plane of carbon. Among the four crystal structures, δ-MnO₂ typically possesses a disordered structure along certain crystallographic directions, resulting in broader and weaker XRD peaks compared to α-MnO₂ and β-MnO₂.

Figure 2. XRD pattern of α-, β-, γ-, and δ-MnO₂-BC samples.

The specific morphology of the materials was further examined by scanning electron microscopy (SEM). SEM images of BC, α-, β-, γ-, and δ-MnO₂-BC samples are shown in Figure 3. The biomass bamboo charcoal, after high-temperature calcination and acid washing, predominantly exhibits a flake-like structure. α-MnO₂ loaded on BC displays a dendritic nanostructure (Figure 3b), composed of uniform nanorods with dimensions of approximately 2.5 μm inlength and 30 nm in width. β-MnO₂ shows a similar dendritic nanostructure (Figure 3c), consisting of tetragonal prismatic nanorods with lengths of 2 μm and widths of 50–100 nm. Studies indicate that γ-MnO₂ adopts a spherical nanostructure formed by sharp-tipped MnO₂ nanoneedles. δ-MnO₂ also exhibits a spherical morphology with micrometerscale diameters, comprising interwoven nanosheets assembled from numerous fine nanowires.

Figure 3. SEM images of (a) BC, (b) α-MnO₂-BC, (c) β-MnO₂-BC, (d) γ-MnO₂-BC, (e) δ-MnO₂-BC and HRTEM image of (f) δ-MnO₂-BC.

In Figure 3e, distinct spherical particles loaded on biomass bamboo charcoal are observable; however, the interwoven nanowire structure is less evident, potentially due to the specific hydrothermal reaction temperature and duration.The HRTEM images in Figure 3f presented typical layered oxide structure and easily recognizable lattice fringes of 0.70nm spacing, which is consistent with the (001) facet of δ-MnO₂.

Figures 4 and 5 illustrate the nitrogen adsorption-desorption isotherms and pore size distribution curves of the prepared materials. According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the α-MnO₂-BC, β-MnO₂-BC, γ-MnO₂-BC, and δ-MnO₂-BC samples all exhibit Type IV isotherms with H4-type hysteresis loops, indicating mesoporous structures characterized by slit-like pores formed by the stacking of layered sheets, consistent with SEM observations. Pore size distribution curves reveal that the pore diameters of these samples primarily range from 2 to 8 nm. As analyzed in Table 1, δ-MnO₂-BC demonstrates a higher specific surface area compared to BC, whereas loading with α-MnO₂, β-MnO₂, and γ-MnO₂ reduces the specific surface area, suggesting that these MnO₂ phases grow on the biochar and partially block the existing pores. In contrast, δ-MnO₂, with its layered structure, facilitates the formation of slit-like pores through sheet stacking, thereby enhancing specific surface area and pore volume. This provides δ-MnO₂-BC with more reactive sites, and the fluffy stacking of nanosheets is identified as the primary reason for large pore formation and increased surface area. ICP-OES results (Table 1) show that the Mn loading rates for all catalysts are approximately 2.3%, aligning well with theoretical calculations and reinforcing the comparability among different MnO₂-phase-loaded biochar materials.

Figure 4. N₂ absorption-desorption isotherm.

Figure 5. Corresponding pore size distribution curves.

Table 1. Textural properties and Mn loading of the different samples

Fourier transform infrared (FT-IR) spectroscopy was utilized to analyze the functional groups and chemical bonding of the materials. The FT-IR spectra of the prepared samples (Figure 6) exhibit identical characteristic peaks at 3420 cm^-1, 1610 cm^-1, 1100 cm^-1, and 800 cm^-1, corresponding to the asymmetric stretching vibrations of O–H, C=O, C–O, and C–H groups, respectively.²⁵ Previous studies indicate that the bending vibration of Mn–O–C bonds typically appears near 1400 cm^-1; weak absorption peaks observed in the spectra of these four materials suggest that MnO₂ is primarily loaded onto the biochar in a supported form, with a low probability of direct bonding with carbon.²⁶

Figure 6. FT-IR spectra of α-MnO₂-BC, β-MnO₂-BC, γ-MnO₂-BC, and δ-MnO₂-BC.

Adsorption-Catalytic Oxidation Performance for o-xylene Removal

To evaluate the adsorption capacity of the different samples, o-xylene adsorption tests were performed, with each repeated test to ensure reliability. Figure 7 shows the adsorption curves of the various samples, where error bars represent the standard deviation from the average of each measurement. The error bars generally show a variation of 3-5% of the average values, indicating that the overall trends in adsorption performance remain consistent despite minor measurement variations. The adsorption experiment was conducted at ambient temperature by introducing a defined mixture of o-xylene and nitrogen into the reactor. In the absence of oxygen and ROSs, catalytic oxidation is deemed not to occur. The saturation adsorption capacities were determined by measuring the amount of o-xylene adsorbed until the outlet concentration equated to the inlet concentration. The order of adsorption capacities of the different samples is as follows: δ-MnO₂-BC>α-MnO₂-BC>β-MnO₂-BC>BC>γ-MnO₂-BC. Compared to other adsorbents used for o-xylene adsorption, δ-MnO₂-BC exhibits superior performance, which can be attributed to large specific surface area and total pore volume, which are favorable for adsorption processes by providing sufficient adsorption sites. Conversely, γ-MnO₂-BC exhibited the lowest adsorption capacity, likely due to its smaller surface area and total pore volume. In addition, the loading of manganese oxides with distinct crystal phases introduces supplementary active sites, which exert a notable influence on the adsorption capacity of the catalyst. These sites encompass a diverse array of structural defects and active functional groups, ultimately modulating the adsorption capacity through enhanced surface reactivity and tailored pore architecture, which explains that α-MnO₂-BC and β-MnO₂-BC exhibits a significantly greater adsorption capacity compared to BC.

Figure 7. Adsorption tests of o-xylene. Reaction conditions: 500 ppm o-xylene, N₂ balance, 25°C.

The adsorption-catalytic treatment efficiency of the prepared catalysts for VOCs was evaluated by monitoring the o-xylene removal rates over different MnO₂-BC phases in Figure 8. Experimental conditions were set as follows: o-xylene concentration of 500 ppm, air flow rate of 3 L/min, ozone injection rate of 100 mg/L, catalytic packing mass of 6.5 g, and a continuous reaction duration of 80 min. As shown in Figure 8a, all catalysts exhibit a gradual decline in performance over the 80-min period, attributed to reduced adsorption capacity as pollutant molecules accumulate on the catalyst surface, leading to decreased catalytic efficiency. Among the various catalysts, δ-MnO₂-BC demonstrates superior adsorption-catalytic performance, achieving a maximum o-xylene removal efficiency exceeding 80% and maintaining 65% removal after 80 min, which can be attributed to the material’s enhanced adsorption capability coupled with a high density of catalytic active centers. The removal performance of different MnO₂-BC phases is proportional to the ozone conversion rate in the system. δ-MnO₂-BC exhibits an ozone conversion rate of up to 92%, indicating that a portion of ozone directly participates in o-xylene oxidation, while another portion generates additional reactive species under catalytic action for oxidative degradation.

Figure 8. Degradation curves of o-xylene and Ozone Conversion.

The initial pollutant concentration is a critical factor influencing removal efficiency. Industrial VOC emissions typically range from 100 to 1000 ppm, classified as low to medium concentration waste gases. To explore the large-scale application of the adsorption-catalytic system, the performance in treating varying concentrations was investigated. As illustrated in Figure 9a, the δ-MnO₂-BC system maintains removal rates above 60% for o-xylene initial concentrations between 500 and 1000 ppm over 80 min; however, performance is poorer at lower concentrations. Thus, the system is more suitable for medium-concentration VOC treatment, with optimal efficiency at 500 ppm.

Figure 9. Degradation curves of o-xylene under (a) different concentration and (b) hydrothermal temperature.

Studies show that δ-MnO₂ can be synthesized via hydrothermal methods within a temperature range of 180–260℃ using fixed precursor ratios. Selecting an appropriate hydrothermal temperature is crucial for preserving catalytic activity and stability, thereby enhancing practical application effectiveness. Under controlled conditions (o-xylene concentration: 500 ppm; air flow: 3 L/min; ozone injection: 100 mg/L; catalytic packing: 6.5 g; reaction time: 80 min), the influence of hydrothermal temperature (180℃, 205℃, 230℃, 255℃) on o-xylene removal was investigated (Figure 9b). Results indicate that o-xylene removal efficiency increases with rising temperature. However, excessively high temperatures may cause the layered structure of δ-MnO₂ to collapse, emphasizing the importance of precise temperature control to enhance active sites, optimize pore structure, improve stability, and increase surface area—key factors determining overall catalytic efficacy.

Conventional biochars (e.g., wood charcoal, straw carbon, coconut shell carbon, bamboo charcoal) typically exhibit desorption temperatures below 200℃. To evaluate the actual contribution of catalytic oxidation in the adsorption-catalytic system, the reaction system was heated using a hot plate at temperatures above 200℃, where adsorption is negligible or minimal, allowing o-xylene removal rates to be regarded as conversion rates.²⁷ Four temperature gradients (200℃, 250℃, 300℃, 400℃) were tested (Figure 10). A positive correlation between reaction temperature and conversion rate is observed. At 400℃, o-xylene conversion rates exceed 80% within 80 min, demonstrating that ozone decomposes more readily at high temperatures, generating increased reactive oxygen species for degradation. However, high-temperature operations entail higher costs and contradict principles of cost efficiency, and since removal rates at 400℃ are comparable to those of the room-temperature adsorption-catalytic system, the latter is more practical for real-world applications.

Figure 10. High-Temperature o-xylene conversion over δ-MnO₂-BC.

Cycle experiments were conducted to assess catalyst stability and regenerability (Figure 11). After each use, the catalyst was placed in a 200℃ oven for 10 min to desorb o-xylene, followed by subsequent reactions under identical conditions (o-xylene: 500 ppm; air flow: 3 L/min; ozone: 100 mg/L; δ-MnO₂-BC packing: 6.5 g; duration: 80 min). Results show that catalytic activity remains unaffected for the first 240 min, with peak efficiency sustained around 80%. After the fourth cycle, efficiency declines significantly, but the material still achieves over 50% o-xylene removal after five cycles (400 min), confirming excellent stability and regenerability.

Figure 11. Cycling Test of δ-MnO₂-BC.

Industrial VOC emissions typically consist of multicomponent mixtures and are often accompanied by coexisting gases such as water vapor, NO_x, and SO₂. These factors significantly influence the ozone-assisted catalytic performance through competitive adsorption, active site poisoning, and intermediate accumulation. Preliminary experiments indicate that in a binary mixture with acetone, the removal efficiency of o-xylene over δ-MnO₂-BC decreases from 80% to 68%, while the acetone conversion remains at only 35%. Nevertheless, its large specific surface area and layered structure confer superior tolerance compared to other crystal phases. Under moderate humidity (RH = 50%), the catalyst maintains a 75% removal efficiency; however, exposure to high humidity (RH = 80%) leads to a significant decline in efficiency to 50%. The presence of 100 ppm NO reduces the removal efficiency to 70%, whereas 50 ppm SO₂ causes irreversible deactivation (efficiency drops to 35%), a trend consistent with literature reports.²⁸ The stable performance of δ-MnO₂-BC in mixed VOC systems and under moderate humidity supports its potential for industrial applications. Further optimization through surface modification represents a promising direction for future development.

Reaction Mechanism Analysis

Electron spin resonance (ESR) spectroscopy was first performed to gain insights into oxygen vacancies within the samples (Figure 12). The signal at g = 2.003 is ascribed to oxygen vacancies, and the surface oxygen vacancy density correlates well with catalytic activity. Previous studies suggest that surface oxygen vacancy clusters are more reactive in toluene elimination compared to other vacancy types.²⁹ Further investigation into oxidative active species in the electrocatalytic system revealed that using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent, the δ-MnO₂-BC system exhibits a characteristic 1:2:2:1 quartet signal in ESR spectra (Figure 13a), indicating the formation of ·OH radicals. The signal intensity diminishes after five reaction cycles, confirming reduced •OH production. Similarly, with 2,2,6,6-tetramethylpiperidine (TEMP) as a trapping agent, the δ-MnO₂-BC system shows a 1:1:1 triplet signal (Figure 13b), corresponding to the formation of ¹O₂ species, which also decreases after five cycles. To identify the primary ROSs involved in catalytic ozonation, scavenger experiments were conducted. Specific scavengers—carotene for ¹O₂, ascorbic acid for ·O₂^-, and isopropanol for •OH—were used. Figure 13 demonstrates that o-xylene removal efficiency by δ-MnO₂-BC is significantly reduced upon addition of D-mannitol (·OH scavenger) and carotene (¹O₂ scavenger), whereas only a slight reduction occurs with ascorbic acid (·O₂ scavenger), indicating that ·OH and ¹O₂ are the key ROSs in toluene degradation.

Figure 12. ESR spectroscopy of δ-MnO₂-BC.

Figure 13. ESR spectroscopy of (a) DMPO-·OH, (b)TEMP-¹O₂ and (c-d) Catalytic ozonation of o-xylene over δ-MnO₂-BC in the presence of various scavenger molecules.

The generation of hydroxyl radicals (·OH) during the reaction was qualitatively assessed using coumarin as a probe molecule via photoluminescence (PL) spectroscopy (Figure 14). The catalytic oxidation reaction was performed in a sealed container under the following conditions: 500 ppm xylene, 100 ppm ozone, and 6.5 g of catalyst. The corresponding fluorescence intensities at these time points provide a direct indicator of the dynamic ·OH concentration profile throughout the reaction process. The concentration of hydroxyl radicals (·OH) in the catalytic oxidation system exhibited a continuous increase with prolonged reaction time, demonstrating the effective activation and dissociation of ozone into ·OH species. After 80 minutes of reaction, the δ-MnO₂-BC catalyst yielded the highest ·OH concentration among all tested crystalline phases, which corresponds directly to its superior ozone conversion rate as observed in prior analyses.

Figure 14. The photoluminescence spectroscopy of (a) α-MnO₂-BC, (b) β-MnO₂-BC, (c) γ-MnO₂-BC and (d) δ-MnO₂-BC.

The XPS C1s spectra in Figure 15 were deconvoluted into four peaks corresponding to C-C (284.8±0.3 eV), C-O (286.5±0.2 eV), C=O (288.0±0.5 eV), and O-C=O (289.0±0.5 eV). The C-O groups improve gaseous VOCs adsorption via dipole-dipole interactions and hydrogen bonding, while simultaneously serving as electron donors to promote ozone decomposition into ·OH radicals. Meanwhile, the C=O groups directly activate ozone to generate singlet oxygen (¹O₂) that attacks unsaturated bonds in VOCs, and synergize with manganese to form charge-transfer channels reducing reaction energy barriers. Furthermore, the O-C=O groups anchor metal active centers forming MnO-C interfacial sites that modulate surface charge distribution, thereby enhancing ozone adsorption and decomposition efficiency.^30,31 This cooperative action of functional groups creates an integrated adsorption-oxidation system that optimizes the degradation pathway of o-xylene.

Figure 15. High-resolution XPS spectra of C1s.

XPS analysis of the O1s orbital was performed to quantify oxygen vacancy concentrations (Figure 16). The O1s spectra were deconvoluted into three peaks corresponding to lattice oxygen (529.0–530.5 eV), oxygen vacancies (530.5–531.5 eV), and adsorbed oxygen (531.5–533.5 eV). The relative oxygen vacancy content was calculated as the ratio of the oxygen vacancy peak area to the total O1s area. Results show the following order: γ-MnO₂-BC < β-MnO₂-BC < α-MnO2-BC < δ-MnO₂-BC. This positive correlation between oxygen vacancy concentration and o-xylene removal efficiency confirms the involvement of oxygen vacancies in catalytic oxidation. Additionally, the surface chemical states of Mn in δ-MnO₂-BC before and after reaction were studied via XPS (Figure 17). Curve fitting revealed that the Mn³⁺/Mn⁴⁺ ratio decreased from 1.76 (fresh catalyst) to 1.42 (after five cycles). Since the presence of Mn³⁺ induces oxygen vacancies due to electrostatic balance, a higher Mn³⁺/Mn⁴⁺ ratio corresponds to greater oxygen vacancy content, which serves as critical active sites in oxidation reactions.

Figure 16. High-resolution XPS spectra of O1s.

Figure 17. High-resolution XPS spectra of Mn2p for δ-MnO₂-BC before and after cycling tests.

To further analyze the mechanism underlying the catalytic degradation of o-xylene for sample δ-MnO₂-BC, gas chromatography-mass spectrometry (GC/MS) was employed to detect intermediate products formed during the reaction process. By monitoring intermediates under varying reaction durations and conditions, the pathway for catalytic oxidation of o-xylene within this system was analytically deduced. As delineated in Table 2, identified reaction intermediates included benzyl alcohol, benzoic acid, o-hydroxybenzoic acid (salicylic acid), maleic anhydride, oxalic acid, and acetic acid, among others. Notably, benzoic acid, benzyl alcohol, and benzaldehyde were identified as the predominant intermediates in the catalytic oxidation of o-xylene. The ring-opening reaction of o-xylene molecules constitutes a critical step in the decomposition of aromatic compounds. Direct ring cleavage of toluene involves a high energy barrier, whereas the bond energy linking the benzene ring to the methyl groups in o-xylene is substantially lower than that of the conjugated π-bonds within the aromatic ring. Thus, o-xylene decomposition typically initiates via attack on the methyl substituents. As shown in Figure 18, the proposed reaction pathway for o-xylene proceeds as follows:^32,33 In the initial step, stepwise oxidation of the methyl groups yields benzyl alcohol, which undergoes rapid oxidation to benzaldehyde, followed by further oxidation to benzoic acid. In the subsequent step, the benzene ring of benzoic acid is subjected to attack by ·OH and ¹O₂, resulting in the formation of hydroxylated intermediates such as salicylic acid. Ultimately, the ring undergoes further hydroxylation and cleavage, generating intermediates including maleic anhydride, oxalic acid, and acetic acid. A fraction of these intermediates is completely mineralized to carbon dioxide and water.

Table 2. Some intermediates in the degradation of o-xylene detected by GC/MS

Figure 18. The evolution pathway of the o-xylene over δ-MnO₂-BC.

Thus, the reaction mechanism for adsorption-catalytic oxidation of xylene over manganese oxide-loaded bamboo biochar is proposed as follows: Initially, physical adsorption and enrichment occur. The microporous structure (pore size: 1–10 nm) and high specific surface area (>400 m²/g) of biomass bamboo charcoal (BC) preferentially adsorb gaseous O₃ molecules, enhancing the local concentration. Subsequently, oxygen vacancies induce ozone decomposition into highly reactive oxygen atoms and molecular oxygen. The generation of ROSs—specifically hydroxyl radicals (·OH) and singlet oxygen (¹O₂)—for xylene attack proceeds via two primary pathways: (i) highly reactive oxygen atoms react with surface O–H groups on the biochar, releasing ·OH as the dominant oxidative species; (ii) ozone interacts with surface quinone groups (C=O) to form ¹O₂. Furthermore, the Mn³⁺/Mn⁴⁺ redox cycle facilitates electron transfer, accelerating ozone activation and utilization, thereby improving xylene treatment efficiency

CONCLUSION

Volatile organic compounds (VOCs), particularly o-xylene as a representative pollutant, pose significant threats to ecological systems and human health. Driven by the principle of cost-effectiveness and efficiency enhancement, the development of green and efficient VOC treatment technologies has been a globally researched subject. This study establishes an ozone-assisted adsorption-catalysis system utilizing manganese oxide-biochar composites to evaluate the treatment efficiency of carbon-based catalysts loaded with different manganese oxide crystalline phases.

Results demonstrate that the δ-MnO₂-BC catalyst exhibits the largest specific surface area and pore volume, predominantly mesoporous in structure, yielding excellent adsorption-catalytic performance. It achieves over 80% o-xylene removal efficiency while maintaining remarkable stability and regenerability, retaining 50% removal efficiency after five adsorption-desorption cycles. Within this system, ozone molecules are catalytically decomposed at active sites on the δ-MnO₂-BC surface into hydroxyl radicals and singlet oxygen, which are subsequently adsorbed into oxygen vacancies to facilitate o-xylene oxidation, ultimately converting it into intermediates including benzoic acid, maleic anhydride, and oxalic acid. The developed δ-MnO₂-BC system demonstrates advantages of environmental compatibility, high efficiency, stability, and low operational costs, providing a promising strategy for scalable treatment of industrial o-xylene emissions.