Heterogeneous catalytic ozonation for highly efficient mineralization of phenol with La-modified Ce/γ-Al<sub>2</sub>O<sub>3</sub>

Qijie Liu; Erhao Gao; Jing Li; Jiali Zhu; Sheng Feng; Zuliang Wu; Shuiliang Yao

doi:10.4491/eer.2022.743

Environ Eng Res > Volume 28(6); 2023 > Article

Liu, Gao, Li, Zhu, Feng, Wu, and Yao: Heterogeneous catalytic ozonation for highly efficient mineralization of phenol with La-modified Ce/γ-Al2O3

Research

Environmental Engineering Research 2023; 28(6): 220743.

Published online: February 16, 2023

DOI: https://doi.org/10.4491/eer.2022.743

Heterogeneous catalytic ozonation for highly efficient mineralization of phenol with La-modified Ce/γ-Al₂O₃

Qijie Liu^1,², Erhao Gao^1,², Jing Li^1,², Jiali Zhu^1,², Sheng Feng¹, Zuliang Wu^1,^2,^†

, Shuiliang Yao^1,^2,^†

¹School of Environmental Science and Engineering, Changzhou University, Changzhou 213164, China

²Key Laboratory of Advanced Plasma Catalysis Engineering for China Petrochemical Industry, Jiangsu 213164, China

^†Corresponding author: E-mail: (Z.L.W.) wuzuliang@cczu.edu.cn, (S.L.Y.) yaos@cczu.edu.cn, Tel: (Z.L.W.) +0086-0519-86330086, (S.L.Y.) +0086-0519-86330086

Received December 9, 2022 Revised February 3, 2023 Accepted February 15, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, La-Ce/γ-Al₂O₃ materials for the catalytic ozonation of phenol were prepared by impregnation. In order to analyze the physicochemical properties, BET, SEM, XRD, XPS, and FTIR were used to characterize the catalysts. The removal performance of phenol under different reactive conditions (catalyst dosage, initial pH value, initial phenol concentration, and ozone concentration) was investigated, and the possible catalytic oxidation mechanism and conversion pathway of phenol were proposed through the analysis of ROS and intermediates in solution. The results indicated that the conversion rate of 2.5% La-Ce/γ-Al₂O₃ increased by 20.53% within 20 min compared with ozonation alone. Moreover, the CO₂ mineralization rate was more than twice that of ozonation alone within 250 min, reaching 92.53%, and superior CO₂ mineralization was maintained after repeated use. The CO₂ mineralization rate of phenol after adding 3 g/L 2.5% La-Ce/γ-Al₂O₃ catalyst reached almost 100%, and could be maintained in a solution with a wide pH range of 3~9.5. From the characterization results, it could be seen that the 2.5% La-Ce/γ-Al₂O₃ catalyst obtained higher adsorbed oxygen and hydroxyl species content than γ-Al₂O₃ and Ce/γ-Al₂O₃. In the process of phenol catalyzed ozonation, the •O₂⁻ species in the solution were deduced to be the key ROS.

Keywords: Catalytic ozonation, La-Ce, γ-Al₂O₃, Phenol wastewater, Removal mechanism

Graphical Abstract

Keywords: Catalytic ozonation, La-Ce, γ-Al₂O₃, Phenol wastewater, Removal mechanism

1. Introduction

Important intermediates and raw ingredients in many industrial manufacturing processes are phenolic compounds, mainly used in coal carbonization, oil refining, plastics production, dyes synthesis, pharmaceutical production, etc. [1, 2]. As a result, phenolic compounds are common in industrial wastewater. Due to its properties of high toxicity and being difficult for biodegradation, it has been listed as one of the key hazardous wastewater for water pollution prevention in the world [3]. The U.S. Environmental Protection Administration also lists it as one of 129 priority pollutants for control [4]. In China, the maximum discharge concentration of phenolic pollutants in wastewater must be controlled below 1 mg/L [5]. Therefore, the removal of phenolic compounds from wastewater is overwhelming imperative.

For the past few years, advanced oxidation processes have been reported to be widely used in wastewater treatment [6, 7], such as Fenton oxidation [8], Photocatalytic oxidation [9], Electrochemical oxidation [10], Wet air oxidation [11], and Ozone oxidation [12–15]. Among them, ozone oxidation has attracted many attentions because ozone has strong oxidizing properties, and it can decompose various organic compounds through direct or indirect reactions [16]. Although ozone oxidation is considered as an effective phenolic pollutants removal method, it hardly reacts with some intermediates (such as aldehydes and carboxylic acids) produced during phenolic oxidation, resulting in a low mineralization rate [17–19]. Mu et al. [20] evaluated the effect of ozonation alone on bisphenol A (BPA) removal. The BPA conversion efficiency reached 99.9% within 20 minutes, but the mineralization rate of BPA was only 27.2%. Fang et al. [21] found that after ozonation alone for 60 min, the removal rate of phenol reached 70%, but only 10.0% of TOC. Therefore, in order to improve the mineralization rate of phenolic pollutants, various catalysts were employed to absorb ozone and promote its decomposition to generate more active free radicals for reactions, which has been proven to be an effective method [22–26].

Due to the advantages of low cost, good thermal stability and environmental friendliness of rare earth metals and transition metals, they have become hot spots in the research of catalyst development in recent years [27–30]. Among them, CeO₂ has a special oxygen defect structure and two valence states of Ce³⁺/Ce⁴⁺, which make it have a high oxygen storage capacity [31]. Qu et al. [32] prepared Ce/SEP catalysts, and found that high oxygen vacancy accelerated the Ce⁴⁺/Ce³⁺ cycle, which significantly improved the TOC removal of p-nitrophenol (PNP) compared with SEP alone. Similarly, a nano-CeO₂-doped ZSM-5 catalyst was prepared for catalytic ozonation to sulfamethoxazole (SMX), and its optimal TOC removal rate was twice that of O₃ alone [33]. Besides, it was found that the compounding of other metal oxides with CeO₂ could increase the active sites and generate active radicals, further improving the catalytic performance [34–37]. Ma et al. [38] concluded that COD removal rate of Mn_xCe_1−xO₂/γ-Al₂O₃/O₃ reached 70.2%, while that of CeO₂/γ-Al₂O₃/O₃ was only 65.8%. Additionally, Mohebali et al. [39] prepared Fe₃O₄@Ce-UiO-66 composites and evaluated their degradation and mineralization ability for acetaminophen removal. They found that 54.9% of TOC could be removed within 10 min, 17.8% higher than that of Ce-UiO-66. It also has been shown that La-doped Ce can increase the generation of surface oxygen defect structure of Ce-based catalyst, which further promote Ce⁴⁺/Ce³⁺ cycling and improve oxygen mobility [40–43]. Li et al. [44] found that La-doped Ce microspheres had a better catalytic performance for toluene oxidation, and T₉₀ (the reaction temperature when conversion reaches 90%), T₅₀ and T₁₀ were 52 °C, 32 °C and 14 °C lower than that of CeO₂, respectively. Similarly, a high specific surface area La-doped nano-CeO₂ catalyst had high oxygen vacancy content and indicated that the conversion rate of CO reaches 50% at 170 °C, thus proving that the catalyst had excellent catalytic activity [45]. According to previous studies, the combination of La and Ce is mainly used in the catalysis oxidation of gaseous organic pollutants, but there is almost no report on liquid organic pollutants. Considering the obvious difference between the catalytic interface reaction in the gas phase and liquid phase, it is necessary to study the catalytic effect of La and Ce in wastewater containing organic pollutants.

In this study, different amounts of La-modified Ce/γ-Al₂O₃ catalysts were prepared by equal volume impregnation. The catalysts were characterized by using N₂ adsorption-desorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) to analyze the physicochemical properties. Taking phenol as the target pollutant, the catalytic performance of different catalysts via catalytic ozonation was evaluated by the conversion and the mineralization. The effects of catalyst dosage, initial pH value, initial phenol concentration, and ozone concentration on phenol removal performance were investigated, and possible mechanisms of phenol degradation were proposed.

2. Experimental Methods

2.1. Catalyst Preparation

La-Ce/γ-Al₂O₃ catalysts were prepared by equal volume impregnation. First, γ-Al₂O₃ powder (0.2 μm) was washed with C₂H₅OH and deionized water. Subsequently, the washed γ-Al₂O₃ powder was dried at 110 °C for 6 h. Finally, it was calcined at 500 °C for 3 h and cooled for standby. A certain amount of La(NO₃)₃·6H₂O and Ce(NO₃)₃·6H₂O were dissolved simultaneously in a suitable amount of deionized water. Then evenly loaded on the corresponding quality of the γ-Al₂O₃ powder prepared previously by equal volume impregnation method. The impregnation time was 12 h. Subsequently, the drying and calcination conditions were consistent with the above mentioned γ-Al₂O₃. Finally, the La-Ce/γ-Al₂O₃ catalyst was cooled to obtain. In this study, La-modified Ce/γ-Al₂O₃ catalysts with different loading capacities were prepared, namely x% La-Ce/γ-Al₂O₃ (1% La-Ce/γ-Al₂O₃, 2% La-Ce/γ-Al₂O₃, 2.5% La-Ce/γ-Al₂O₃, and 3% La-Ce/γ-Al₂O₃). Ce content remained unchanged at 2% based on the previous works (Fig. S2).

2.2. Experimental Setup and Methods

Fig. S1 is used to investigate the catalytic ozonation of phenol in this experiment. The experimental setup is mainly divided into ozone generation system, catalytic reaction system and analysis system. Ozone (O₃) is supplied by O₂ DBD (Dielectric barrier discharge) using a pulsed power supply. The catalytic ozonation reactor is a 100 mL bubbling bottle with phenol solution and catalyst, which are stirred with a magnetic stirrer to facilitate effective mixture.

Except of influencing parameter experiment, the reaction conditions are: 2 g/L of catalyst, 100 mL of phenol solution, 200 mg/L of initial concentration, 7 of initial pH and 24 mg/L of O₃ concentration. Before experiment, add the catalyst to the phenol solution and stir to achieve an adsorption-desorption equilibrium for 30 min. Every 5 minutes, the gas sample and liquid sample were analyzed, respectively. During the experiment, each sample was tested three times, and its average value was taken.

The ozone analyzer measures the mass concentration analysis of ozone from the DBD reactor. The tail gas of catalytic ozonation reactor was injected into gas chromatography (GC-2014, Shimadzu, Japan; 2 m Porpark N column). Every 5 minutes, we took samples and recorded the peak area of CO₂. During the experiment, 0.2 mL solution was removed from the catalytic ozonation reactor and then the change of phenol concentration in the reaction solution was analyzed by HPLC (Shimadzu, Japan; Eclipse Plus C18 column). The liquid sample was filtered with a filter (0.22 μm) before analyzed. The intermediates of phenol removal solution were qualitatively analyzed by LC-MS (Agilent, USA; Waters BEH C18 column).

The phenol conversion rate is calculated by:

(1)

Phenol conversion (%) = \frac{C_{0} - C_{t}}{C_{0}} \times 100 %

C₀ refers to the initial phenol concentration; C₁ refers to the residual phenol concentration at a certain time t (mg/L).

The CO₂ mineralization rate is calculated by:

(2)

m (g) = \sum_{i = 5}^{250} (\frac{C_{t_{i}} * υ * (t_{i} - t_{i - 5}) * M}{22.4 * 6}) Δ t

(3)

{CO}_{2} mineralization (%) = \frac{m (g)}{Phenol content (g)} \times 100 %

m refers to the cumulative amount of phenol conversion into CO₂ (g); C_{t_i} refers to the amount of CO₂ (ppm) produced at a specific time t_i; υ refers to O₃ traffic (mL/min); M refers to the relative molecular mass of phenol; 22.4 refers to the molar volume (L) of the gas in the standard state; 6 refers to the number of C atoms of phenol.

3. Results and Discussion

3.1. Evaluation of Phenol Removal Activity of Different Catalysts

Fig. 1 shows the effect of La-Ce/γ-Al₂O₃ catalysts on the catalytic ozonation performance for phenol removal. Fig. 1(a) shows that the phenol conversion under different catalysts was very fast, and the conversion rate reached more than 99% after 60 min. However, the addition of catalysts improved the phenol conversion, and 2.5% La-Ce/γ-Al₂O₃ obtained the highest conversion rate of phenol at the same time. Specifically, for the x% La-Ce/γ-Al₂O₃ (x = 1, 2, 2.5, and 3) catalysts, the conversion rate of phenol reached 77.50%, 82.91%, 83.92% and 80.60% within 20 min, respectively. By contrast, the conversion rate of phenol was 78.89% over the Ce/γ-Al₂O₃ catalyst, and only 66.04% for ozone alone. However, the conversion rate of phenol during the catalytic ozonation only reflects the preliminary oxidation capacity of catalytic ozonation, and the deep oxidation requires further study of the CO₂ mineralization rate.

As shown in Fig. 1(b), when ozone oxidation alone was performed, the CO₂ mineralization rate was only 35.76% after 250 min of reaction. By contrast, the CO₂ mineralization rate of the Ce/γ-Al₂O₃ catalyst increased to 70.54%. As the x% La-Ce/γ-Al₂O₃ (x = 1, 2, 2.5, and 3) catalyst was dosed, the CO₂ mineralization rate furtherly increased to 78.77%, 87.13%, 92.53% and 82.02%, respectively. The CO₂ mineralization over 2.5% La-Ce/γ-Al₂O₃ was more than twice that of ozonation alone, and the deep oxidation effect of phenol was greatly improved. The results proved that the addition of La in the catalytic ozonation of phenol played a leading role, and 2.5% La-Ce/γ-Al₂O₃ catalyst in this experiment was the best.

3.2. Catalyst Characterization

3.2.1. BET and SEM analysis of the samples

N₂ adsorption-desorption isotherm was used to measure the pore structure and data of the samples. It could be seen from Fig. 2 that these three catalysts exhibited a typical type IV isotherm with H2-hysteresis loop in the range of 0.6–1.0 relative pressure (P/P₀), indicating that the catalysts still maintained the mesoporous structure of the γ-Al₂O₃ vector after loading Ce and La. The specific pore structure data for different catalysts are compiled in Table S1. When La was introduced, the pore data of the catalyst decreased, probably because some metal oxides were embedded in the pores of the γ-Al₂O₃ support, leading to pore blockage. However, the catalytic activity was greatly improved as demonstrated (Fig. 1), indicating that the pore structure may not be the key factor.

In order to detect the surface morphology of the catalysts, SEM is used to characterize them. Fig. 3 shows that the three catalysts exhibited similar morphology. In combination with the results of N₂ adsorption-desorption isotherms, suggesting that although the metal loading caused some specific surface area loss, they did not greatly change the basic structure of the γ-Al₂O₃ support. Besides, 2.5% La-Ce/γ-Al₂O₃ catalyst had less surface aggregation and more evenly grain distribution than Ce/γ-Al₂O₃ catalyst.

3.2.2. XRD, FTIR and XPS analysis of the samples

Fig. 4(a) shows the XRD patterns of the different catalysts. All these three samples indicated diffraction peaks of 2θ at 66.8°, 60.5°, 45.8°, 39.5°, 37.6°, 31.9°, and, 19.6°, corresponding to γ-Al₂O₃ (PDF#29-0063) [46], indicating that the γ-Al₂O₃ structure remained stable. No diffraction peaks of CeO₂ were observed of Ce/γ-Al₂O₃, demonstrating that CeO₂ species were highly dispersed on γ-Al₂O₃ [47]. However, after the further introduction of La on Ce/γ-Al₂O₃, obvious diffraction peaks appeared near 29.1° and 55.9°, corresponding to La₂O₃ (PDF#74-2430). Therefore, we thought that these two peaks could be diffraction peaks of La₂O₃.

The chemical groups on the catalyst were identified by infrared spectroscopy (Fig. 4(b)). The bands at 1630 cm⁻¹ and 3435 cm⁻¹ which attributed to the bending vibration (δ) and stretching vibration (ν) of the H₂O adsorption and surface hydroxyl groups, respectively, were clearly observed [48]. The peak intensities followed the sequence of 2.5% La-Ce/γ-Al₂O₃ > Ce/γ-Al₂O₃ > γ-Al₂O₃, indicating that the presence of cerium oxides and lanthanum oxides brought about more hydroxyl groups to be produced on the surface of the catalyst. The highest surface hydroxyl content of the 2.5% La-Ce/γ-Al₂O₃ should be related to its superior catalytic activity [47].

Fig. 4(c) shows the results of XPS O 1s. The O 1s was deconvoluted into adsorbed oxygen (O_ads), lattice oxygen (O_latt) and hydroxyl (OH), corresponding binding energies of 530.2 eV, 531.5 eV and 532.9 eV, respectively [49]. Hydroxyl groups were important surface reactive oxygen species, which acted as an active site to improve the activity of catalytic [50–52]. The relative ratios of various oxygen species were calculated (Table S2). 2.5% La-Ce/γ-Al₂O₃ catalyst compared with Ce/γ-Al₂O₃ catalyst, the results of XPS O 1s showed that the content of O_latt decreased from 50.3% to 37.7%, the content of O_ads increased from 32.5% to 43.8%, and the content of OH also increased from 17.2% to 18.5%. A higher ratio of hydroxyl groups represented more surface active sites of the catalyst, and helped to improve the phenol catalyzed ozonation [53].

3.3. Influence of Experimental Parameters

Furthermore, the effects of 2.5% La-Ce/γ-Al₂O₃ catalyst on the conversion and mineralization of phenol under different catalyst dosage, initial pH value, initial phenol concentration, and ozone concentration were studied by catalytic ozonation, so as to optimize the reaction process for phenol removal.

3.3.1. Catalyst dosage

The effects of different 2.5% La-Ce/γ-Al₂O₃ catalyst dosages (1, 2, 3, and 4 g/L) on the removal of phenol were studied. Fig. S3(a) shows that the conversion rate improves from 92% to 95.52% at 30 min with the increase of catalyst dosage from 1g/L to 3g/L. But the catalyst dosage further increased to 4 g/L, and the phenol conversion rate was hardly unchanged at 30 min. The catalyst dosage had almost no impact on the preliminary oxidation of phenol. In Fig. S3(b), after catalytic ozonation for 250 min, the CO₂ mineralization was 83.16% (1 g/L), 92.53% (2 g/L), 99.69% (3 g/L) and 91.8% (4 g/L), respectively. The catalyst dosage had a vital effect on the CO₂ mineralization rate, and the best phenol mineralization was reached at a 3 g/L dosage of 2.5% La-Ce/γ-Al₂O₃ catalyst. All in all, with the catalyst dosage increased, the conversion rate and CO₂ mineralization rate of phenol raised. Probably because the increase of catalyst dosage could provide more active sites, which promotes O₃ decomposition, being conducive to produce more active free radicals. Nevertheless, the CO₂ mineralization rate decreased when the dosage was increased to 4 g/L. This was probably because the excessive catalyst leading to the active sites close to saturation, and even aggregating with each other catalyst, thereby reducing the efficient contact between the catalyst surface with O₃ and the pollutants [28, 54, 55]. As a result, ozone conversion and mineralization were inhibited, and the catalytic activity was greatly suppressed. The results indicated that the catalyst dosage distinctly affected the phenol removal, and the catalyst dosage in subsequent experiments was recognized to be 3 g/L.

3.3.2. Initial pH

During the process of water treatment, the initial pH is an important condition. Herein, in order to adjust the initial pH of the solution, HCl (0.1 mol/L) and NaOH (0.1 mol/L) were added to the solution. The catalytic ozonation effect of phenol under different pH conditions was studied. As shown in Fig. S3(c) and (d), the change in pH had little effect on the phenol conversion when the initial pH was between 3 and 9.5. The conversion rates of phenol could all reach more than 99.5% under different initial pH after 60 min of reaction. After 250 min of reaction, the CO₂ mineralization rate also reached more than 99%. Therefore, the 2.5% La-Ce/γ-Al₂O₃ catalyst is capable of catalytic ozonation of phenol in a wide range of pH, which reduces the pH adjustment process of water treatment.

3.3.3. Initial phenol concentration

After 30 min of reaction, Fig. S3(e) shows that the conversion rates of phenol were 99.09%, 97.58%, 95.52%, 88.56%, and 82.13%, when the initial phenol concentration was 100, 150, 200, 250 and 300 mg/L. With the increase of the initial phenol concentration, the results indicated that the conversion rate gradually decreased. In addition, the CO₂ mineralization rates after 85 min of reaction were 99.76%, 76.59%, 54.7%, 46.47%, and 38.65% (Fig. S3(f)), respectively, showing that the different initial phenol concentrations significantly influenced the CO₂ mineralization rate. Both the conversion rate and CO₂ mineralization rate of phenol decreased significantly with the increase of the initial phenol concentration. The higher the phenol concentration implied the more phenol molecules in the solution, while the production number of active free radicals was almost the same as the O₃ addition unchanged. This suggests that more active free radicals were involved in the preliminary oxidized phenol molecules with the increasing phenol concentration, and the deep oxidation of phenol conversion was restrained [53, 54].

3.3.4. Ozone concentration

The addition of ozone provided a continuous supply of reactive radicals for the entire catalytic system and promoted the oxidation of phenol, so the effects of ozone concentration were studied on the conversion and mineralization of phenol. The ozone concentration was set to 8, 16, 24 and 32 mg/L, respectively. As shown in Fig. S3(g), indicating that with the ozone concentration increased from 8 to 24 mg/L, the conversion rate of phenol raised from 60.94% to 97.01% within 35 min. However, the ozone concentration was further increased to 32 mg/L, and the conversion rate only raised by 0.49%. At this time, ozone was more involved in the deep oxidation of phenol, which further improved the CO₂ mineralization. As shown in Fig. S3(h), as the ozone concentration increased from 24 to 32 mg/L, the CO₂ mineralization rates of phenol within 100 min were raised from 61.64% to 81.4%. Obviously, the increase in ozone concentration could significantly improve the CO₂ mineralization of phenol. This was mainly because the active radicals decomposed from ozone increase when the ozone concentration increased, which promoted phenol oxidation.

3.4. Catalyst Stability

The stability of 2.5% La-Ce/γ-Al₂O₃ catalysts was verified by repeated experiments. The used catalyst was washed with C₂H₅OH and deionized water for several times, and then dried at 60 °C for 6 h for reuse. Fig. 5 shows that the catalytic activity of the 2.5% La-Ce/γ-Al₂O₃ catalyst hardly reduced after four cycles. After 40 min, the conversion rates of phenol were 98.51%, 97.91%, 97.41%, and 96.86%, respectively. After 250 min, the CO₂ mineralization rate still remained at 94.61% after four cycles, indicating that the 2.5% La-Ce/γ-Al₂O₃ catalyst was quite stable during the phenol removal process.

3.5. Possible Removal Mechanisms of Phenol

3.5.1. Identification of ROS (Radical Oxygen Species) in solution

Since the phenol in the solution was almost not adsorbed on the catalyst, its oxidation reaction mainly occurred in the solution. In order to illustrate the oxidation mechanism of phenol in the catalytic ozonation process of 2.5% La-Ce/γ-Al₂O₃, p-BQ and TBA were added as probes for the detection of superoxide radicals (•O₂⁻) and hydroxyl radicals (•OH), respectively [56, 57]. As shown in Fig. 6, the phenol conversion rate was 91.37% after 30 min of reaction in the presence of TBA, which decreased by 4.15% compared with the absence of TBA. After 60 min of reaction, there was no obvious difference in conversion rate. This indicates that •OH species in solution are not the main ROS produced in the catalytic ozonation process. The conversion rate of phenol greatly decreased from 99.5% to 50.5% after 60 min after the addition of p-BQ, which showed that the •O₂⁻ in the solution may be the active oxygen group that played a dominant role in catalyzing ozone oxidation.

3.5.2. Possible mechanisms of •O₂⁻ formation

Based on the above experimental results and previous studies, the possible catalytic mechanism of ROS generation in the La-Ce/γ-Al₂O₃/O₃ system was proposed and shown in Fig. 7. As O₃ is introduced, the hydroxyl groups on the catalyst surface (M-OH) react with O₃ to generate •HO₂ and •O₂⁻ (Eq. (4)) [48], and the consumed surface hydroxyl group would be recovered through the decomposition of an adsorbed water molecule on the surface of the catalyst [38]. The •HO₂ species are unstable and would easily decompose into •O₂⁻ and H⁺ (Eq. (5)) [58]. In addition, •HO₂ also reacts with dissolved O₃ in solution to form •OH radical (Eq. (6)) [47], which is less important in the catalytic ozonation of phenol based on the results of section 3.5.1. Meanwhile, the Ce³⁺/Ce⁴⁺ cycle plays an important role. Ce³⁺ species are oxidized by O₃ to form Ce⁴⁺ and •O₂⁻ species (Eq. (7)), and the oxygen vacancy on the surface of La-Ce/γ-Al₂O₃ promotes the reduction of Ce⁴⁺ to Ce³⁺ (Eq. (8)) [33]. During the Ce³⁺ and Ce⁴⁺ cycle, the released electrons would combine with O₂ to form the reactive •O₂⁻ species (Eq. (9)) [59].

(4)

M-OH + O_{3} \to M + \cdot {HO}_{2} + \cdot O_{2}^{-}

(5)

\cdot {HO}_{2} \leftrightarrow \cdot O_{2}^{-} + H^{+}

(6)

O_{3} + \cdot {HO}_{2} \to \cdot OH + O_{2}

(7)

{Ce}^{3 +} + O_{3} \to \cdot O_{2}^{-} + {Ce}^{4 +} {+ e}^{-}

(8)

{Ce}^{4 +} + e^{-} \to {Ce}^{3 +}

(9)

O_{2} H + e^{-} \to \cdot O_{2}^{-}

3.5.3. Possible conversion pathways

The main intermediate products of phenol conversion were detected and characterized by LC-MS (30 min), as shown in Fig. S4. Based on the previous studies and experimental results, the main intermediate products and possible pathways of phenol conversion are speculated and shown in Fig. 8.

Pathway I: Phenol is attacked by •OH to generate hydroquinone/resorcinol/catechol (m/z = 110). Pathway II: Phenol is attacked by •O₂⁻ to form p-benzoquinone/o-benzoquinone (m/z = 108). Meanwhile, p-benzoquinone/o-benzoquinone (m/z = 108) and hydroquinone/resorcinol/catechol (m/z = 110) are interconvertible. The aromatic ring of the above intermediates is further attacked by •O₂⁻, resulting in the opening of the ring and maleic acid (m/z = 116) is produced. Maleic acid is a macromolecular organic acid, which further degrades into short-chain acids, such as oxalic acid (m/z = 90) and malonic acid (m/z = 104). Short-chain acids further degrade into formic acid (m/z = 46). Finally, formic acid is oxidized into CO₂ and H₂O [60].

4. Conclusions

The conversion rate of phenol could be significantly increased in the catalyst presence compared with ozonation alone, and the conversion rate of 2.5% La-Ce/γ-Al₂O₃ increased by 20.53% within 20 min. Moreover, the CO₂ mineralization rate of phenol over 2.5% La-Ce/γ-Al₂O₃ catalyst was more than twice that of ozonation alone within 250 min, reaching 92.53%, and superior CO₂ mineralization was maintained after repeated use. The CO₂ mineralization rate of phenol after adding 3 g/L 2.5% La-Ce/γ-Al₂O₃ catalyst reached almost 100%, and could be remained in a solution with a wide pH range (pH = 3~9.5). The increase of the initial phenol concentration was not conducive to its oxidation, while the increase of ozone concentration significantly improved the conversion and mineralization of phenol.

The best 2.5% La-Ce/γ-Al₂O₃ catalyst obtained a higher amount of adsorbed oxygen and hydroxyl species compared with Ce/γ-Al₂O₃ and γ-Al₂O₃, which were essential for the CO₂ mineralization of phenol. The •O₂⁻ species in the solution were deduced to be the key ROS during the catalytic ozonation of phenol, and they were continuously generated between the reactions of M-OH and O₃, and the Ce³⁺/Ce⁴⁺ cycle. The ring-opening and continuous oxidation of phenolic substances were mainly achieved by •O₂⁻ species. According to the high removal efficiency, catalytic activity and stability of La-Ce/γ-Al₂O₃ catalyst, the catalytic system provides a theoretical basis for the advanced treatment of phenol containing wastewater by ozone oxidation and has a good practical application prospect and value.

Supplementary Information

eer-2022-743-Supplementary.pdf

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No.22075032).

Notes

Conflict-of-interest

The authors declare no conflict of interest.

Author Contributions

Q.J.L. (Master student) took charge of investigation, scientific experiments, data curation, formal analysis and writing; E.H.G. (Lecturer) revised the manuscript; J.L. (Associate Professor) and J.L.Z. (Lecturer) did a formal analysis; S.F. (Professor) and S.L.Y. (Professor) took charge of investigation; Z.L.W. (Professor) provided supervision, theoretical foundation, and experimental guidance.

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