Effect of the Fe₂O₃/SBA-15 Surface on Inducing Ozone Decomposition and Mass Transfer in Water

Abstract

Catalytic ozonation with metal oxides is of interest for advanced water treatment technology. The amount of active oxygen-containing radicals produced is a primary objective of this process. Fe₂O₃ is a widely used catalyst because of its high performance. In this study, Fe₂O₃/SBA-15 was synthesized and characterized. The results revealed that Fe₂O₃/SBA-15 was a nano-/mesoporous material with high-order hexagonal array structures and exhibited greater catalytic performance than Fe₂O₃ in ozonation processes. To investigate the role of the Fe₂O₃/SBA-15 surface in O₃ decomposition, the kinetic constant was measured, and the interfacial reactions were discussed. Compared with Fe₂O₃, Fe₂O₃/SBA-15 significantly increased the formation of hydroxyl radicals (•OH) and the efficient utilization of O₃ in the catalytic O₃ decomposition process. The SBA-15 support decreased the O₃ self-decomposition rate during catalytic ozonation with Fe₂O₃/SBA-15, which resulted in increased formation of •OH via the reaction between O₃ and Fe₂O₃. From a practical point of view, Fe₂O₃/SBA-15 is an efficient green ozonation catalyst for water treatment.

1. Introduction

The increasing discharge of organic wastewater from industry and agriculture poses potential risks to human health and ecosystems. A variety of contaminants in wastewater require advanced treatment processes. Oxidation is an efficient water treatment technology for contaminant removal. Many contaminants (double bonds and molecules incorporating activated aromatic structures) can be removed easily by typical oxidants [1]. O₃ has received widespread attention in water treatment because of its oxidizing ability and potential as a disinfectant. However, its low stability and solubility result in the low efficiency of using O₃ alone in decomposing refractory contaminants. Advanced oxidation processes have been confirmed to be efficient for the removal of contaminants, especially in treating dissolved trace refractory contaminants [2]. Many advanced oxidation processes, such as heterogeneous catalytic ozonation (metal oxides, activated carbon and minerals), homogeneous catalytic ozonation (metal ions), O₃/UV, O₃/H₂O₂, and O₃/H₂O₂/UV, have been studied as potential water treatment methods to overcome the disadvantage of using O₃ alone [3].

Owing to its low negative effect and high oxidation capacity, heterogeneous catalytic ozonation has been the subject of increasing interest from the water treatment industry [4,5]. In this system, the active oxygen-containing radicals (•O²⁻, ¹O₂, and •OH) are important oxidant species resulting from O₃ decomposition in water [6,7]. The production of active oxygen-containing radicals predominantly depends on the decomposition behavior of dissolved O₃ on the catalyst surface.

Metal oxides, including MnO₂, Al₂O₃, TiO₂, CuO, and Fe₂O₃, are widely used as catalysts in catalytic ozonation [8,9,10,11]. However, the difficulty of reuse limits the use of these metal oxides. To improve the catalytic performance and ease of use, metal oxides on supports (Al₂O₃, SiO₂, and activated carbon) are often synthesized via wet impregnation methods [12]. The use of Al₂O₃ and activated carbon supports is limited by the specific surface area, mechanical strength, and easy oxidation. However, SiO₂ shows promise because of its high chemical stability and safety. SBA-15 is a pure SiO₂ molecular sieve with a large surface area, an ordered mesoporous structure, a narrow pore size distribution and thick pore walls [13]. It is often selected as a support for industrial catalysis [14]. In the past decade, SBA-15 has attracted much attention as a promising nanomaterial in environmental technology [15,16]. However, many previous studies have investigated gas adsorption on SBA-15 and modified SBA-15, and the catalytic ozonation of contaminants in water by SBA-15 and modified SBA-15 has rarely been investigated [17].

Nitrobenzene is a typical man-made compound that is abundantly used in many fields (solvents, metal polishes, perfume, plastics, explosives, and pesticides) and is found in surface water and soil, causing great environmental concern because of its toxicity and difficult biological treatment, even at low concentrations [18,19]. Nitrobenzene rarely reacts with O₃ (0.09 M⁻¹s⁻¹) but reacts rapidly with •OH (2.2 × 10⁸ M⁻¹s⁻¹) [20]. Therefore, nitrobenzene, a special •OH indicator, was selected as a model contaminant in catalytic ozonation.

Compared with other metal oxides, Fe₂O₃ is ubiquitous in natural systems and poses a low risk to human health. In the present study, the Fe₂O₃/SBA-15 material was synthesized as a catalyst to investigate its catalytic performance, active oxygen-containing radicals and O₃ decomposition pathway on the catalyst surface.

2. Materials and Methods

2.1. Materials

The SBA-15 molecular sieve was obtained from XFNANO Co., Ltd. (Nanjing, China). FeCl₃·6H₂O was obtained from Hengxing Chemical Preparation Co., Ltd. (Tianjin, China). The nitrobenzene (NB) reagent was obtained from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). The 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) reagent was obtained from Sigma Aldrich, Inc. (Merck KGaA, Darmstadt, Germany). The tert-butanol reagent was obtained from Xilong Chemical Co., Ltd. (Beijing, China). The experimental water was obtained with a Milli-Q ultrapure system. The water pH was adjusted with HCl solution (Xiongda Chemical Co., Ltd., Maoming, China) and NaOH solution (Shuangshuang Chemical Co., Ltd., Yantai, China). All chemical reagents used in the tests were of analytical grade. The glass vessel was washed with H₂CrO₄ (Sinopharm Chemical Reagent Beijing Co., Ltd., Beijing, China) and then rinsed with ultrapure water.

2.2. Catalyst Preparation

Fe₂O₃/SBA-15 was synthesized via the wet impregnation method [21]. For example, 25 mg of FeCl₃·6H₂O was dissolved completely in 200 mL of ultrapure water in a conical glass flask, and 45 mg of SBA-15 was dispersed into this mixture. A stirrer was used to mix the suspension for 10 min to maintain a dispersion of Fe and SBA-15 in water. The pH of the reaction mixture was adjusted to 1.0 with HCl. After ultrasonication for 20 min, the suspension was slowly dried and finally calcined at 350 °C for 2 h (loading percentage of Fe₂O₃ wt.%, 10%). The solid was washed repeatedly with ultrapure water until the pH values of the filtrates did not vary. The samples were oven dried at 100 °C for 6 h. These dry samples were crushed and sieved into powder for the experiments. For comparison, Fe₂O₃ was synthesized through the same process without the introduction of SBA-15.

2.3. Experimental Procedure

The catalytic ozonation test was conducted in batch oxidation mode (Figure 1). The glass reactor was a modified round bottom flask (1 L), and gaseous O₃ was added once to the aqueous solution via an ozone generator (COM-AD-01, ANSEROS Company, Tübingen, Germany) from dried oxygen gas. The catalysts and nitrobenzene were then rapidly added to this reactor. The reaction temperature was maintained via a thermostatic bath (HH-2, Jintan Shuangjie Experimental Instrument Factory, Changzhou, China). A magnetic stirrer (84-1, Shanghai Meiyingpu Instrument Manufacturing Co., Ltd., Shanghai, China) was used to mix the reaction mixture continuously. The solution collected with a syringe at different times was quenched with 0.1 mol/L Na₂S₂O₃ (Beijing Omiget Pharmaceutical Technology Co., Ltd., Beijing, China). The water sample was filtered through a 0.45 µm glass membrane before analysis. The adsorption–decomposition of O₃ on the catalyst surface was investigated through the same process without nitrobenzene.

2.4. Analytical Methods

X-ray diffraction (XRD) patterns were obtained with a D8 ADVANCE diffractometer (Bruker, Ettlingen, Germany) to analyze crystallinity and phases of the catalysts. Fourier transform infrared spectroscopy (FT-IR) was performed with a Spectrum One instrument (PerkinElmer, Shelton, CT, USA) to analyze the structure and functional groups. Scanning electron microscopy (SEM) was performed with a Quanta 200 instrument (FEI, Hillsboro, OR, USA) to analyze the morphology of the catalyst. The Brunauer–Emmet–Teller (BET) method was performed with an ASAP 2020 (Micromeritics, Norcross, GA, USA) analyzer to study the structure of the catalyst. The concentration of nitrobenzene was analyzed via an HPLC-1200 system (Agilent, Santa Clara, CA, USA) with a Venusil Mp C18 column via UV detection at 262 nm. The mobile phase was a mixture of 4/5 CH₃OH and 1/5 ultrapure water introduced at a 0.8 mL/min flow rate. An EMX-8/2.7 spectrometer (Bruker, Germany) was used to analyze the amount of active radicals formed. A T6 spectrophotometer (Persee, Taoyuan City, Taiwan) was used to analyze dissolved O₃ at a wavelength of 258 nm. An Optima 5300DV spectrometer (PerkinElmer, Shelton, CT, USA) was used to analyze the concentrations of ions leached from the catalyst surface.

3. Results and Discussion

3.1. Characterization

Fe₂O₃/SBA-15 was characterized to determine the relationship between its ozonation activity and its structure.

The crystal structure of Fe₂O₃/SBA-15 was analyzed via X-ray diffraction. Figure 2A shows wide-angle XRD patterns of Fe₂O₃/SBA-15. Analysis of the patterns revealed that the major phase of SBA-15 was amorphous SiO₂ (broad peaks in the 2θ range from 20° to 30°). The XRD patterns of Fe₂O₃/SBA-15 differ from those of SBA-15. Seven intense peaks appeared in the wide-angle patterns of Fe₂O₃/SBA-15, which were assigned to hematite (No. 33-0664). In the low-angle range, Fe₂O₃/SBA-15 showed a strong peak near 1°, indicating that the hexagonal structure belonged to the {100} lattice family (Figure 2B). The two weaker peaks were attributed to the {110} and {200} lattice planes [22]. Therefore, Fe₂O₃/SBA-15 has a highly ordered hexagonal array structure.

The physisorption isotherms and pore structures of Fe₂O₃/SBA-15 were measured via the BET method. The nitrogen adsorption–desorption isotherms of the samples are shown in Figure 2C. A type IV H1 isotherm with a hysteresis loop from 0.6 to 0.8 relative pressure was observed for Fe₂O₃/SBA-15 after N₂ adsorption–desorption, and the data suggested the presence of a mesoporous structure. S_BET decreased from 721.5 m²/g (SBA-15) to 382.3 m²/g (Fe₂O₃/SBA-15). Moreover, the pore diameter distribution of Fe₂O₃/SBA-15 is shown in Figure 2D. Fe₂O₃/SBA-15 is a mesoporous material with pore sizes ranging from 6.2 to 11 nm.

The surface functional groups of Fe₂O₃/SBA-15 were characterized via FT-IR spectroscopy. The FT-IR spectrum of Fe₂O₃/SBA-15 is shown in Figure 2E. The –O–H peaks at 3450 cm⁻¹ and 1634 cm⁻¹ suggested the presence of surface –OH and water on Fe₂O₃/SBA-15 [23]. The peaks at 1052 cm⁻¹ and 685 cm⁻¹ represent characteristic bands of Si–O–Si bonds [24]. The peaks at 532 cm⁻¹ and 451 cm⁻¹ represent the Fe–O bonds of Fe₂O₃ [25].

The morphology of Fe₂O₃/SBA-15 was characterized via SEM. The patterns shown in Figure 2F clearly indicate that the presence of Fe₂O₃ in the Fe₂O₃/SBA-15 structure remarkably changed the morphology of SBA-15. The inset of Figure 2F shows that the morphologies of SBA-15 formed wheat ears. However, introducing Fe₂O₃ affected the morphologies, and the morphologies of Fe₂O₃/SBA-15 formed numerous dispersed irregular short rods.

3.2. Catalytic Performance and O₃ Utilization Efficiency

The catalytic performance of Fe₂O₃/SBA-15 was studied through nitrobenzene removal in different systems, involving adsorption on the catalyst, O₃ alone, and catalytic ozonation. In this experiment, Fe₂O₃ was selected as a reference.

The results shown in Figure 3 indicate that the removal rate of nitrobenzene was only 38.5% after 10 min in O₃ alone. Nitrobenzene removal during this process was difficult to achieve in a short period of time. The introduction of Fe₂O₃ and Fe₂O₃/SBA-15 led to a synergistic reaction with O₃ in the removal of nitrobenzene. Compared with that of O₃ alone, the amount of nitrobenzene removed was greater with Fe₂O₃ and Fe₂O₃/SBA-15 during catalytic ozonation, and the amount of nitrobenzene removed was greater with Fe₂O₃/SBA-15. The nitrobenzene removal rate achieved with Fe₂O₃/SBA-15 after 10 min was 61.5%, which was 1.16 times greater than that achieved with Fe₂O₃. SBA-15 inhibited nitrobenzene removal in the ozonation system. In addition, nitrobenzene showed weak adsorption onto Fe₂O₃ and Fe₂O₃/SBA-15 at less than 3% under these test conditions. Adsorption had a weak effect on removal via catalytic ozonation with Fe₂O₃ and Fe₂O₃/SBA-15. Therefore, the removal of nitrobenzene mainly depends on the behavior of O₃ decomposition on the catalyst surface.

In this work, SBA-15 was used to support Fe₂O₃. After the synthesis of modified SBA-15, the chemical composition and physical structure of SBA-15 were altered, leading to improved catalytic performance. Catalytic ozonation is a complex reaction process that includes the following three routes [26]: (1) O₃ adsorption onto the catalyst results in the production of active oxygen-containing radicals that decompose contaminants, (2) contaminants adsorb onto the catalyst and react with O₃, and (3) O₃ and contaminants adsorb onto the catalyst surface before interaction.

Tert-butanol (TBA) is a stronger hydroxyl radical (•OH) scavenger, as indicated by its high reaction rate constants with •OH (5 × 10⁸ M⁻¹s⁻¹) and with O₃ (3 × 10⁻³ M⁻¹s⁻¹) [27]. TBA reacts with •OH in aqueous solution, generating inert intermediates, which induce the end of the radical chain reaction. As shown in Figure 3, the presence of TBA had a very strong negative effect on nitrobenzene removal during both the O₃-alone and catalytic ozonation processes, causing a primary reduction in the removal rate, even at a low concentration of TBA (2.0 mg/L). The removal rate of nitrobenzene via catalytic ozonation with Fe₂O₃ decreased from 53.2% to 21.2%, and the removal rate via Fe₂O₃/SBA-15-catalyzed ozonation decreased from 61.5% to 22.8%. The introduction of TBA effectively inhibited •OH formation in the solution. The above results indicated that TBA competitively traps and quickly consumes •OH in the reaction system. Moreover, the results also suggested that in both the O₃-alone and catalytic ozonation processes, nitrobenzene is removed mainly by •OH in the ozonation process.

The O₃ utilization efficiency was investigated during the O₃-alone and catalytic ozonation processes (Figure 4). The O₃ utilization efficiency was used as an indicator of catalyst performance. The O₃ utilization efficiency was expressed as follows:

$$R_U \left(\%\right)={\displaystyle \frac{\left[\mathrm{NB}{]}_0-\right[\mathrm{NB}{]}_t}{\left[{\mathrm{O}}_3{]}_0-\right[{\mathrm{O}}_3{]}_t}}\times 100\%$$

(1)

where R_U represents the O₃ utilization efficiency (%), the subscript (0) represents the initial concentration, and the subscript (t) represents the terminal concentration.

The experimental results indicated that the minimal O₃ utilization efficiency was 6.2% when O₃ alone was used (Figure 4). The O₃ utilization efficiency for Fe₂O₃-catalyzed ozonation was less than that for Fe₂O₃/SBA-15. The utilization efficiency of O₃ in catalytic ozonation with Fe₂O₃/SBA-15 after 10 min was 10.6%, which was 1.2 times greater than that in catalytic ozonation with Fe₂O₃. The O₃ in the solution can be consumed via the following two routes: (1) direct oxidation of O₃ with nitrobenzene or (2) O₃ decomposition into active free radicals. The oxidation rate constants of nitrobenzene with •OH and O₃ are 2.2 × 10⁸ M⁻¹s⁻¹ and 0.09 M⁻¹s⁻¹, respectively, indicating that nitrobenzene hardly reacts with O₃ [20]. On the basis of the above results in Figure 4, the improvement in O₃ utilization efficiency in catalytic ozonation was actually caused by the substantial formation of •OH, especially in catalytic ozonation with Fe₂O₃/SBA-15. Therefore, in subsequent experiments, we investigate the influence of variables and on O₃ decomposition and mass transfer and the interface behaviors.

3.3. Water Temperature and Catalyst Dose

Water temperature was a key factor influencing the decomposition behavior of O₃ in the ozonation reaction system. The influence of water temperature on nitrobenzene removal was investigated at different water temperatures during catalytic ozonation with Fe₂O₃/SBA-15.

As shown in Figure 5, the removal rates of nitrobenzene at 15 °C, 25 °C, 35 °C, and 45 °C were significantly different from those of catalytic ozonation with Fe₂O₃/SBA-15. The adsorption removal rates at different water temperatures did not significantly change. The removal rate of nitrobenzene increased from 52.5% to 61.5%, 63.2% and 55.7% as the water temperature increased from 15 °C to 45 °C. The removal of nitrobenzene via catalytic ozonation with Fe₂O₃/SBA-15 was obviously affected by the water temperature. Within a certain temperature range from 15 °C to 35 °C, increasing the water temperature can positively promote the activation of O₃, increasing the decomposition of O₃ into •OH. Moreover, O₃ showed a lower solubility with increasing water temperature. When the water temperature was further increased from 35 °C to 45 °C, the removal rate of nitrobenzene decreased from 63.2% to 55.7% after 10 min, which had a negative effect on the amount of •OH formed.

Heterogeneous catalytic ozonation is an oxidation process of the following three phases: gas–liquid–solid. Therefore, it was necessary to investigate the influence of the catalyst content on the removal of nitrobenzene in water.

The removal of nitrobenzene was studied by catalytic ozonation in the presence of different catalyst contents. The results shown in Figure 6 indicate that the removal rate increased gradually by 51.1% and 65.2%, respectively, with increasing catalyst content (25–150 mg/L). At a low content of 25 mg/L, the removal rate of nitrobenzene obviously improved compared with that of O₃ alone (38.5%). Only 4.2% of nitrobenzene can be removed by adsorption onto the Fe₂O₃/SBA-15 surface (150 mg/L); therefore, surface adsorption weakly contributes to the removal rate in the catalytic ozonation process. The increase in the catalyst content may also increase the solid catalytic surface area, resulting in the promotion of •OH formation, which increases the removal rate in the solution.

3.4. Ozone Decay

O₃ gas–liquid mass transfer from water onto the catalyst surface plays an important role in catalytic ozonation. The dissolved O₃ is unstable in water because of its resonance structure. The O₃ half-life ranges from a few seconds to a few minutes and relies on several environmental conditions. Therefore, the decay behavior of O₃ in solution was evaluated to determine the effect of the Fe₂O₃/SBA-15 surface in this process.

The results shown in Figure 7A indicate that the concentration of O₃ continuously decreased in the solution due to O₃ self-decomposition during the O₃-alone process. Compared with using O₃ alone, the use of Fe₂O₃ significantly improved O₃ decay. After 15 min of reaction, O₃ completely decomposed into Fe₂O₃. However, the presence of Fe₂O₃/SBA-15 strongly inhibited O₃ decay relative to that obtained from O₃ alone. The concentration of O₃ slowly decreased with fluctuations due to the presence of Fe₂O₃/SBA-15.

O₃ decomposition occurs via heterogeneous reactions or via homogeneous reactions [17]. The decomposition rate of O₃ in a heterogeneous system can be written as follows:

$$-{\displaystyle \frac{\mathrm{d}\left[{\mathrm{O}}_3\right]}{\mathrm{d}t}}=k_1\left[{\mathrm{O}}_3\right]+k_2\left[\mathrm{S}\right]\left[{\mathrm{O}}_3\right]$$

(2)

where k₁ and k₂ are the apparent rate constants of O₃ decomposition because of homogeneous and heterogeneous processes. [S] represents the number of active catalyst sites. The concentration of O₃ in the solution changed over a narrow range, and [S] is the pseudoconcentration and is indicated the concentration of surface reactive centers on the catalyst. By defining k_app = k₁ + k₂ [S], Equation (2) may be expressed as Equation (3).

$$-{\displaystyle \frac{\mathrm{d}\left[{\mathrm{O}}_3\right]}{\mathrm{d}t}}=k_{app}\left[{\mathrm{O}}_3\right]$$

(3)

O₃ decomposition followed first-order kinetics according to Equation (2), and kapp is the apparent rate constant.

The pseudo-first-order rate constant of O₃ increased remarkably in the presence of Fe₂O₃, with the apparent rate constant increasing 2.312-fold (Figure 7B). This result was consistent with that of previous studies, and Fe₂O₃ was the active material for O₃ decomposition in water [12]. Moreover, O₃ decomposition did not follow first-order kinetics in the presence of Fe₂O₃/SBA-15. This phenomenon represented the dynamic equilibrium between adsorption and desorption.

The solution acidity and alkalinity significantly affected the decomposition of O₃ in water because of O₃ reactions with hydroxyl ions and were also important factors for evaluating the activity of the catalyst. Therefore, the effect of the initial pH of the solution on the O₃ decay process was studied via catalytic ozonation with Fe₂O₃/SBA-15.

Figure 8A shows that the O₃ concentration rapidly decreased at pH 9 during catalytic ozonation with Fe₂O₃/SBA-15. In contrast, the O₃ concentration slowly decreased with increasing and decreasing fluctuations at pH 3 in the same process. O₃ decay followed a pseudo-first-order kinetics model (pH 9). O₃ decay exhibited a decay rate constant at pH 9, resulting in a rate constant of 0.308 min⁻¹ after 30 min. At pH 3, O₃ decay did not follow first-order kinetics at this reaction time. These results may be due to the following factors. First, the hydroxyl ions competitively decompose O₃ under alkaline conditions. Second, the active species were difficult to produce from O₃ decomposition under acidic conditions.

3.5. Active Species

The oxidation of contaminants in water by dissolved O₃ involves mainly O₃ or active oxygen-containing radicals. O₃ decomposition often produces many active oxygen-containing radicals, such as HO₄•, •O, •OH, HO₂•, and O²•, in catalytic ozonation. Among the above free radicals, •OH is the principal active radical due to its high oxidative activity and unselective reaction with contaminants [28]. Thus, many studies have focused on accelerating the production of •OH in this process.

The current test detected •OH production via spin trapping/EPR technology, which may capture •OH according to the intensity of the DMPO-OH signal. The signal was composed of quartet lines (1:2:2:1 peak height ratio). Figure 9 indicates that the •OH signal exists in three processes. The intensity of the peak was minimal for O₃ alone. The formation of •OH in O₃ alone was difficult to achieve in a short time. The test data also indicated that the introduction of a catalyst promoted O₃ decomposition. Relative to that observed with O₃ alone, the adduct signal increased during catalytic ozonation with Fe₂O₃ and Fe₂O₃/SBA-15, and the strongest signal was observed during catalytic ozonation with Fe₂O₃/SBA-15. The reason for this finding was the presence of SBA-15, which can significantly affect O₃ mass transfer at the solid/water interface for •OH generation.

3.6. Ion Effects

According to the XRD analysis of Fe₂O₃/SBA-15, there was some opportunity for the release of ions from Fe₂O₃/SBA-15. The release of ions could lead to two possible effects: (1) the leaching of ions, which could have a negative impact on human health and the environment, and (2) an effect on the performance of Fe₂O₃/SBA-15. Moreover, metal ions are often used as homogeneous catalysts for catalytic ozonation, which could further contribute to removal during the reaction process. Therefore, the ions released from Fe₂O₃/SBA-15 in this study are of particular concern.

After 30 min, ion leaching was detected in the treated water (Figure 10). The results indicated the presence of Fe and Si ions after the reaction. The concentration of Fe ions was minimal in the two processes, especially during catalytic ozonation with Fe₂O₃/SBA-15. This phenomenon indicated that the SBA-15 support can inhibit Fe ion leaching in catalytic ozonation with Fe₂O₃/SBA-15. In this experiment, the influence of the homogeneous reaction with Fe ions was insignificant compared with that of the heterogeneous reaction. Moreover, the residual Fe and Si ions are not harmful to human health because their concentrations are lower than the drinking water standards in China. On the basis of the above results, Fe₂O₃/SBA-15 has the potential to be used safely in catalytic ozonation.

3.7. Catalyst Reuse

Deactivation is a major issue with catalysts in water treatment technology. For catalytic ozonation reactions, it is essential to study the catalytic stability of spent/reused catalysts. The catalytic stability of Fe₂O₃/SBA-15 was evaluated by reusing the same Fe₂O₃/SBA-15 sample in three successive iterations of decomposing O₃ into •OH. At the end of the tests, the solid samples were recovered and then rinsed with water. The samples were dried at 100 °C for subsequent use.

As shown in Figure 11, the variation in peak intensity of the DMPO-OH adduct signal was minimal after three reuses in catalytic ozonation with Fe₂O₃/SBA-15. •OH formation was not affected by the continuous use of Fe₂O₃/SBA-15 over a long reaction time. These results indicated that the performance of Fe₂O₃/SBA-15 was relatively stable during catalytic ozonation. Therefore, the application of ozonation catalyzed with Fe₂O₃/SBA-15 for water treatment was feasible.

4. Conclusions

Fe₂O₃/SBA-15 is a nano-/mesoporous material with high-order hexagonal array structures. Compared with Fe₂O₃, Fe₂O₃/SBA-15, a useful ozonation catalyst, obviously enhances the removal of nitrobenzene, O₃ utilization efficiency, and O₃ decomposition into •OH. The introduction of the SBA-15 support can affect the pathway of O₃ decomposition in the ozonation system. Compared with O₃/Fe₂O₃, Fe₂O₃/SBA-15 significantly increases the amount of •OH generated. This phenomenon can be illustrated by the difference in the surface properties of the catalysts. The experimental results indicate that the SBA-15 support decreased the O₃ self-decomposition rate during catalytic ozonation with Fe₂O₃/SBA-15, which resulted in increased •OH production via the reaction between O₃ and Fe₂O₃. Fe₂O₃/SBA-15 is an efficient catalyst because of its high activity, stability and safety in the catalytic ozonation process. Further studies should investigate whether metal oxides support ozone-loaded materials and further illuminate the enhancement mechanism, including the reaction behavior and key structure.