Study on Treatment of Basic Yellow 28 dye Wastewater by Micro-nano Bubble Ozone Catalytic Oxidation

Jie Wei; Hongwei Sha; Ruonan Wang

doi:10.4491/eer.2022.606

Environ Eng Res > Volume 28(5); 2023 > Article

Wei, Sha, and Wang: Study on Treatment of Basic Yellow 28 dye Wastewater by Micro-nano Bubble Ozone Catalytic Oxidation

Research

Environmental Engineering Research 2023; 28(5): 220606.

Published online: January 13, 2023

DOI: https://doi.org/10.4491/eer.2022.606

Study on Treatment of Basic Yellow 28 dye Wastewater by Micro-nano Bubble Ozone Catalytic Oxidation

Jie Wei¹, Hongwei Sha^1,^†

, Ruonan Wang^1,²

¹Department of Environmental Engineering, School of Municipal and Environmental Engineering, Shenyang Jianzhu University, Shenyang, Liaoning Province, 110168, China

²Liaoning COSCO Architectural Design Corporation, Shenyang, Liaoning Province, 110021, China

^†Corresponding author: E-mail: 13904052793@163.com, Tel: +86-13904052793, Fax:,

Received October 20, 2022 Revised December 12, 2022 Accepted January 12, 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

Mn loaded AC (Mn/AC), Cu loaded AC (Cu/AC), and Fe loaded AC (Fe/AC) catalysts were prepared by the impregnation-calcination method, which can be used to degrade Basic Yellow 28 dyes in simulated printing and dyeing wastewater for catalytic ozonation. The catalyst preparation and reaction conditions were optimized with the degradation efficiency of chemical oxygen demand (COD) and the absorbance of some organic compounds under 254 nm UV light (UV254) as indicators. The surface structure, specific surface area, and active component distribution of the catalysts were analyzed by scanning electron microscopy (SEM), nitrogen adsorption, and X-ray diffraction (XRD) characterization. Results indicated that Mn was the best active component, and a catalyst dosage of 3.5 g/L, an initial pH of 9.4, and an initial temperature of 31.7°C were the best reaction parameters to degrade Basic Yellow 28 dyes. After five recycling experiments, the catalysts could retain a relatively stable catalytic effect. Furthermore, this study can be informative for the sustainable development of future catalysts.

Keywords: Catalyst, Cationic dye wastewater, Micro-nano bubbles, Ozonation

Graphical Abstract

Keywords: Catalyst, Cationic dye wastewater, Micro-nano bubbles, Ozonation

1. Introduction

Ozone oxidation is a typical advanced oxidation method. The ozone-activated advanced oxidation process often generates ·OH, O⁻₂ and ¹O₂ [1, 2]. Although O₃ has a high reduction potential (E=2.07V) [1], oxidation by ozone alone is highly selective and it only attacks the nucleophilic sites of the aromatic ring due to ozone’s dipolar, electrophilic, and nucleophilic nature [1, 3]. In recent years, the heterogeneous catalytic ozonation has been proposed to overcome these problems. The use of catalysts in this technology can increase the generation of ·OH during the ozonation process [2]. This technology can minimize the loss of active components of the catalyst and ensure higher stability and no secondary pollution for the catalyst [4]. O₃ can produce ·OH with a redox potential of 2.80 V in the presence of catalysts. ·OH is an ion with a strong oxidizing property and is effective in degrading organic matters [5].

Activated carbon, molecular sieves, zeolites, and Al₂O₃ have been used as supporters for catalytic ozonation [4]. Activated carbon has become one of the most widely used supporters in catalytic ozonation due to its easy availability, large surface area, high stability, and good adsorption performance [6, 7]. Several metal oxides have been investigated as the active components for ozonation such as Mn, Fe and Cu [8–10]. Mn and Fe exist in multiple valence states, which offer excellent catalytic properties [11–13]. Cu is also a common and effective precursor in catalytic applications [14, 15]. During the catalytic ozonation process, catalysts can accelerate the generation of free active radicals by ozone and enhance the kinetics of pollutant conversion by ozone [16–18]. Many previous studies have indicated that compared with ozonation alone, metal oxides supported on activated carbon can enhance the pollutant removal efficiency [19–22].

Furthermore, the commercial application of ozone has been limited due to its small specific surface area, low dissolved ozone concentration and low partial pressure [23]. A gas-liquid mixing pump can be used to disperse ozone into numerous micro-nano bubbles (MNBs) to enhance the mass transfer rate of ozone [23, 24]. Unlike the conventional fine-bubble aeration, the micro-nano bubble-aerated systems (MNBAs) possess more advantages such as higher O₃ stability, higher gas-liquid mass transfer efficiency, and larger specific surface areas [25]. MNBs can increase the mass transfer efficiency of ozone significantly and thus enhance the treatment efficiency [26, 27]. In addition, the bubbles generated by MNBAs could remain stable in water over a period of days or months owing to the high zeta potential [28, 29], and then promote the proliferation of reactive oxygen species (ROS), such as hydroxyl radicals (·OH), and superoxide radical anion (

O_{2}^{-} \cdot

), through ozone decomposition [30, 31]. Studies have shown that the O₃/MBAs process removes organic matters at a higher efficiency than ozonation alone, which can be ascribed to accelerated decomposition of ozone and enhanced generation of hydroxyl radicals during the collapse of ozone microbubbles [32, 33]. Therefore, the MNBA technology can be widely used in catalytic ozonation. However, very few studies have focused on the combination of the MNBA technology and heterogeneous catalytic ozonation along with the use of metal-loaded activated carbon as catalysts [34].

In this work, we combined O₃/MNBA technology and catalysts to degrade Basic Yellow 28 dye which are typical cationic dyes with benzene ring and unsaturated structure and are widely used in industry [35]. Then we optimized the reaction conditions in order to provide a reference for the efficient application of the catalysts. We also investigated the stability of the catalysts during the reaction process.

2. Experimental

2.1. Materials

The Mn(NO₃)₂, Cu(NO₃)₂, Fe(NO₃)₂, and activated carbon used for the preparation of catalysts were purchased from Shenyang Shenglongfu Experimental Equipment Co., Ltd. and Shenyang Dongxing Reagent Factory; the Basic Yellow 28 dyes used for the preparation of experimentally simulated wastewater were obtained from Hangzhou Advance Technology Co. Ltd. Additional information on other chemicals and reagents is shown in Table S1. All chemicals and reagents were analytically pure. The water used throughout the whole work was deionized.

2.2. Preparation of Catalysts

The catalysts were prepared by the impregnation-calcination method. First, 6–8 mesh granular activated carbon was washed with pure water 2–3 times. Next, a certain amount of pretreated granular activated carbon was dipped into the impregnation solution. After being stirred for 6 h at room temperature, the impregnated activated carbon was first steamed dried in a water bath at 80 °C and then dried in an oven at 120 °C. Finally, the dried activated carbon was roasted in a muffle furnace at a specific temperature for a while to obtain the catalysts.

2.3. Characterization of Catalysts

The surface crystal structure of the catalysts was analyzed by an X-ray diffractometer (XRD-7000, Shimadzu, Japan) using Cu K α radiation. Scanning electron microscopy (SEM) images of the catalysts were obtained through a high-energy electron beam using a cold field emission scanning electron microscope (S-4800, HITACHI, Japan). The pore structure and specific surface area of the catalysts were measured by a Brunauer-Emmett-Teller (BET) physical adsorption instrument (Autosorb-IQ, Conta, USA). Nitrogen was used as the test gas for adsorption. The samples were dried and degassed under vacuum at 300 °C before testing.

2.4. Catalytic Ozonation Experiment

Pressurized dissolved gas was used to produce MNBs. The experimental setup of catalytic ozonation is schematically illustrated in Fig. S1. An O₃ generator (Shijiazhuang Ouneng General Technology Co., Ltd.) was used to generate O₃. A gas-liquid mixing pump (model 20QY-1, power 0.55 kW–7.5kW, gas-liquid volume ratio: 11%) mixed the sucked wastewater and O₃, pressed the mixture into a dissolved gas tank through a connecting pipe and then into a reactor through an aeration disk. This process generated MNBs 20–30 μm in diameter [36]. The experimental reactor was a plastic tank with an effective working volume of 20 L. The catalysts were fixed in the middle of the reactor. At 15-min intervals, a water sample was taken from the sampling port at the bottom of the reactor. During the experiment, the residual O₃ was quenched by 10 % KI.

2.5. Analytical Methods

The pH of the wastewater was determined by a composite glass electrode (SX711, San-Xin Instrumentation Factory, Shanghai). The COD of the wastewater was measured using the rapid closed catalytic digestion method. The concentration of O₃ in the solution was measured using the sodium indigo disulfonate (IDS) spectrophotometry method. The point of zero charge (pH_pzc) of the catalysts was determined by the salt titration method [37]. The content of the metal in the solution was measured by ICP-MS. A UV-visible spectrophotometer (UV-5500, Yuan Analysis Instrument Co., Ltd., Shanghai) was used to determine the absorbance of water samples at 440 nm, which is the maximum absorption wavelength of the Basic Yellow 28 dyes. UV₂₅₄ is a parameter representing the amount of carbon source in wastewater, which directly reflects the ability of ozone to mineralize organic pollutants.

3. Results and Discussion

3.1. Comparation of Different Aeration Systems

The experiment was intended to discuss the performance of O₃-Micro-nano Bubbles (O₃-MNBs) and O₃-Ordinary Bubbles (O₃-OBs) concretely. The ozone inlet flowrate and concentration were set at 3 L/min and 10.3 mg/L, respectively, to serve as the reaction conditions in the subsequent experiments.

The effects of O₃-MNBs and O₃-OBs on the liquid phase ozone concentration and ozone mass transfer coefficient were investigated. We assumed that the dissolution of ozone in water can be described by a mass transfer coefficient equation of gas in solution (based on Eqs (1)–(2)) [38]:

(1)

\frac{d C}{d t} = K_{la} (C_{s} - C)

(2)

- K_{la} t = ln (C_{s} - C)

where K_la, C_s, and C_t are the ozone mass transfer coefficient, ozone saturated concentration, and bulk concentration in water, respectively. The results are shown in Fig. 1a and b. Clearly, the O₃-MNBs exhibited a higher liquid phase ozone concentration and a higher ozone mass transfer efficiency than the O₃-OBs. According to Young–Laplace equation, MNBs have a higher internal pressure than ordinary bubbles, leading to the dissolution of more ozone in water [28, 39]. As described in a previous study, more ozone can be dissolved into water through the bubble interface because of the larger total surface area of the MNBs [40]. The ozone mass transfer coefficient of the O₃-MNBs was 1.77 times higher than that of the O₃-OBs, which agreed with the conclusion in the literature [41, 42]. Studies have shown that the gas-liquid transfer efficiency of ozone was effectively improved due to the self-pressurization effect of MNBs [40, 43]. It can be concluded from Fig. 1c that the efficiency of COD removal by O₃/MNBAs was approximately 3.1 and 1.7 times higher than that by MNBAs alone and by ozonation alone, respectively. This could be attributed to the fact that O₃/MNBAs produced a higher concentration of ·OH in solution, which corresponded to a higher COD removal rate. Nam et al. demonstrated that microbubble ozonation performed particularly well in benzo pyrene degradation due to the higher ·OH concentration [44]. Therefore, the O₃/MNBA technology was applied in this work.

3.2. Comparation of Different Catalyst Systems

Different roasting temperatures (400 °C, 500 °C, and 600 °C), roasting times (2 h, 3 h, and 4 h), and impregnation solution concentrations (0.5 mol/L, 1.0 mol/L, and 2.0 mol/L) were set to optimize the preparation conditions of the catalysts. To degrade 20 L of simulated dye wastewater with a concentration of 100 mg/L, 2 L/min ozone dosage, 10.47 mg/L ozone concentration, and 1 g/L catalyst dosage or activated carbon were used as the reaction conditions.

As shown in Table 1, the optimal preparation conditions for Mn/GAC, Cu/GAC, and Fe/GAC were 500 °C, 500 °C, and 400 °C roasting times, 3 h, 4 h, and 4 h roasting times, and 0.5 mol/L, 0.5 mol/L, and 1.0 mol/L impregnation solution concentrations, respectively. According to the COD removal results, the Mn/GAC catalyst showed a better treatment performance than the other two catalysts.

3.3. Characterization of Catalysts

The three catalysts with the best removal performance were characterized and analyzed to observe the surface morphology and crystal structure. The SEM images of the unmodified activated carbon and the catalysts are provided in Fig. S2a, b, c, and d. Unlike the unmodified activated carbon (Fig. S2a), the surface of the Mn-nitrate-modified activated carbon (Fig. S2b) was covered by lamellar particles, which may be the typical morphology of MnO₂ as reported in the literature [45, 46]. From Table S2, it can be known that the activated carbon showed a decrease in its specific surface area, pore volume, and pore size after being loaded with three metals. In addition, the pore structure may change and recombine after calcination [47]. This helps to ensure the successful loading of metal oxides on the surface and in the pore channels of the activated carbon [48]. It is worth noting that the activated carbon modified by iron nitrate showed a sharp decrease in its specific surface area, pore volume, and pore size. This could be attributed to the fact that the iron oxide particles were mainly distributed on the surface of the supporters in the form of metal clusters (Fig. S2d) [47, 49]. After modification by manganese nitrate and copper nitrate, the activated carbon did not show apparent change in its specific surface area, but its COD removal rate changed significantly.

Fig. 2 shows the effects of different reaction systems on the COD removal efficiency. When activated carbon was used as an adsorbent, the COD removal efficiency was only 2.4%. This suggested that activated carbon offered very limited adsorption for the removal of contaminants in the system. Compared with ozonation alone, adding activated carbon to the ozonation system could increase the efficiency of COD removal by 16.11 %, implying that activated carbon could promote the transformation of aqueous ozone into more[50]. The figure also provides a clear illustration of the performance of Mn/AC, Cu/AC, and Fe/AC catalysts in the removal of COD from dye wastewater. During the reaction process, all these catalysts showed good performance in COD treatment, with Mn/AC performing the best of all. Mn is a particular transition metal oxide with valence states from +2 to +7, compared to Cu or Fe with a maximum formal oxidation state of +4 or +3. This advantage promotes the participation of manganese in the electron transfer reaction with O₃ during the catalytic cycle [51]. After data analysis and catalyst characterization, Mn/AC was finally selected as the optimal catalyst.

The XRD patterns of activated carbon and Mn/AC are presented in Fig. S3. The unmodified activated carbon exhibited characteristic diffraction peaks of SiO₂ and CaCO₃, which indicated that the activated carbon contained a small amount of SiO₂ and CaCO₃ impurities [52]. After modification by manganese nitrate, characteristic diffraction peaks at 12.80°, 18.10°, and 32.88° were observed, which might be the crystalline phases of α-MnO₂ [53]. The intensity of the diffraction peaks of the activated carbon became weaker, indicating that the diffraction peaks of the activated carbon decreased after loading of manganese onto the activated carbon. It has also been demonstrated that MnO₂ [54] has the best catalytic treatment performance among the various valence states of manganese oxides.

3.4. Optimization of Process Parameters

Given its good catalytic performance, Mn/AC was used in this part of the experiment, and the operating conditions were optimized by investigating the basic parameters (catalyst dosage, pH, and reaction temperature).

3.4.1. Effect of catalyst dosage

Both Brønsted and Lewis acid sites are considered to be the active centers for catalytic ozonation [55, 56]. Water molecules can be adsorbed on manganese oxides to combine with Lewis acid sites, and then dissociate into protonated surface hydroxyl groups. Lots of research has shown that neutral hydroxyl groups and protonated hydroxyl groups are the active sites for catalytic ozonation [57]. Hence, the catalyst dosage affects the number of active sites indirectly. The effect of catalyst dosage in the range of 1 to 5 g/L on the COD and UV₂₅₄ removal rates of the Mn/AC system was investigated. The ozone dosage, pH, reaction temperature, and continuous aeration time were set at 2 L/min, 7.1, 25 °C, and 120 min, respectively. As shown in Fig. 3, the COD and UV₂₅₄ removal rates increased significantly when the catalyst dosage increased from 1.0 g/L to 3.0 g/L. This suggested that the addition of the catalyst enhanced the degradation efficiency of Basic Yellow 28 dyes. This may be explained as follows. As the catalyst dosage increased, the total specific surface area and the number of active sites of the catalyst were increased as well. This could facilitate the reaction with O₃ to promote the generation of ·OH [58, 59], thus improving the COD and UV₂₅₄ removal rates. From the stabilized COD and UV₂₅₄ removal rates, it can be deduced that excessive catalyst dosage hindered the generation of ·OH, which converted into without catalytic ability. Besides, as the number of O₃ molecules per unit volume of the solution was fixed, increasing the catalyst dosage failed to make the O₃ in the bubbles come into sufficient contact with the active sites on the catalyst surface [41, 60]. It can also be seen that the COD and UV₂₅₄ removal rates were not linear related with the increase of catalyst dosage, but there existed a critical value [60]. Therefore, from the perspectives of both cost efficiency and treatment performance, 3.0 g/L was used as the optimal catalyst dosage in the subsequent experiments.

3.4.2. Effect of initial pH of solution

The performance of heterogeneous catalytic oxidation is greatly affected by environmental factors, and as reported in the literature, it is particularly affected by the pH of the solution. On the one hand, the pH of the solution affects the oxidation mode and the decomposition rate of O₃ [57]. For instance, based on Eqs.(3)–(8) [61], OH⁻ in the solution will decompose O₃. Nevertheless, excessive OH⁻ will inhibit the generation of ·OH.

(3)

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

(4)

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

(5)

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

(6)

O_{2}^{-} \cdot + O_{3} \to O_{3}^{-} + O_{2}

(7)

O_{3}^{-} \to O_{2} + O^{-} \cdot

(8)

O^{-} \cdot + H_{2} O \to {OH}^{-} + \cdot OH

On the other hand, the pH of the solution affects the charged state of the catalyst surface, which can be expressed by the metal oxide proton balance equations [62] (Eqs.(9)–(10)).

(9)

MeOH + H^{+} \to {MeOH}_{2}^{+} (pH < {pH}_{pzc})

(10)

MeOH + {OH}^{-} \to {MeO}^{-} + H_{2} O (pH > {pH}_{pzc})

We investigated how the initial pH values 5.0, 7.1, 9.4, and 10.8 affect the COD and UV₂₅₄ removal rates of the Mn/AC/O₃ system. The ozone dosage, catalyst dosage, reaction temperature, and continuous aeration time were set at 2 L/min, 3.0 g/L, 25 °C, and 120 min, respectively. As shown in Fig. 4, when the pH of the solution increased in the range of 5.0 to 9.4, it could increase the rate of COD removal from dye wastewater. Therefore, it could be concluded that the increase of the pH in this range could increase the concentration of in the solution, which promoted the generation of ·OH. After 120 min of continuous aeration, when the initial pH was 9.4, the removal efficiencies of COD and UV₂₅₄ reached 73.6% and 79.3%, respectively, compared with the initial pH of 5.0, COD and UV₂₅₄ removal rates soared about 12.9% and 9.8%, respectively. First of all, the pH at the potential of zero-point charge (pH_pzc) of the catalyst was found to be approximately 6.54 lower than the pH of 9.4 in the solution. In this case, the catalyst surface was negatively charged and showed good catalytic performance [63]. This was conducive to the adsorption of the Basic Yellow 28 dyes with a positive charge. As reported in the literature, MnOx/SBA-15 can adsorb organic matters to combine with the hydroxyl groups on the catalyst surface to facilitate further oxidization and decomposition by ·OH in the solution [64]. It can be seen from Fig. 5, the catalysts were more stable under alkaline conditions and retained more manganese. The manganese caused the formation of oxygen vacancies, which adsorbed more pollutants to accelerate the ozonation process [65, 66]. Apart from this, ·OH could also facilitate the decomposition of O₃ to produce more ·OH. However, it is worth noting that when the pH exceeded 9.4, the COD and UV₂₅₄ removal rates declined sharply. This could be attributed to that excessive OH⁻ consumed the ·OH (Eq(8)) and competed with the catalyst to adsorb O₃, which inhibited the activity of the catalyst and thus affected the COD and UV₂₅₄ removal rates. The above-mentioned results demonstrated that the pH of the solution had a significant effect on the removal performance. It could affect both the decomposition rate of O₃ and the charged state of the catalyst surface. Moreover, it could also affect the surface properties of metal oxides covered by surface hydroxyl groups [67, 68]. The COD and UV₂₅₄ degradation efficiencies obtained under alkaline conditions were much higher than under acidic conditions. Given these findings, the initial pH of 9.4 was used in the subsequent experiments.

3.4.3. Effect of initial temperature of solution

We investigated how the initial solution temperatures 12.5°C, 23. 2°C, and 31.7°C affect the COD and UV₂₅₄ removal rates of the Mn/AC/O₃ system. The subsequent experiment was carried out under the optimum reaction conditions. From Fig. 6, it can be observed that COD and UV₂₅₄ degradation efficiencies increased slowly with the increase of reaction temperature. The reaction temperature affected the catalytic ozonation process from two perspectives: the activation energy of the reaction and the solubility of O₃ in the solution. The gradually increased reaction temperature could cause the catalysts to reduce the activation energy of the reaction. This could accelerate the chemical reaction between the catalysts, O₃, and pollutants. This could also promote the decomposition of O₃ molecules in the dye wastewater, which in turn accelerated the generation of ·OH to degrade organic matters. However, with the increase in the initial temperature of the dye wastewater, the solubility of O₃ in the wastewater would decrease [69–71]. This could hinder the conversion of O₃ from the gas phase to the liquid phase and affect the concentration of O₃ and ·OH, thus failing to facilitate the COD and UV₂₅₄ degradation efficiencies. It was also found that the removal rates varied little among the three solution temperatures, indicating that the initial temperature of the solution had little effect on the ozone-catalyzed oxidation system. However, the experimental results suggested that the initial temperature of the solution had a positive effect on the catalytic ozonation for the degradation of Basic Yellow 28 dyes. In other words, the increase in reaction temperature played a greater role in the increase in reaction rate than in the decrease in the solubility of O₃.

3.5. The Stability of Catalysts

To evaluate the stability of the Mn/AC catalyst, it was reused five times under the same conditions. As shown in Fig. 7, as the catalyst was used for an increased number of times, the COD and UV₂₅₄ removal efficiency decreased, but to a small extent. After the catalyst was used for the fifth time, the leaching percentage of Mn was only 1.4%, which could have little effect on the activity of the catalyst. These results demonstrated that the Mn/AC catalyst remained stable and effective during the reaction process.

4. Conclusion

In this study, three catalysts were prepared by the impregnation-calcination method under optimized conditions. The Mn/AC catalyst obtained under the optimal conditions (500 °C roasting temperature, 3h roasting time, and 0.5 mol/L impregnation solution concentration) achieved the best COD removal efficiency. The characterization of the catalysts revealed that the Mn/AC catalyst had better surface characteristics and pore structure than the other two. Our investigation showed that different reaction conditions, i.e., catalyst dosage, initial pH of the solution, and initial temperature of solution, had different effects on the treatment performance of Basic Yellow 28 dye wastewater. During the reaction process, it was demonstrated that the increasing catalyst dosage could enhance the active sites in the system. For a good balance between the removal rates of COD and UV₂₅₄, 3.0 g/L was determined as the optimal catalyst dosage. An appropriate alkalinity could not only promote the decomposition of O₃ but also improve the surface characteristics of the catalysts. Accordingly, the initial pH 9.4 was beneficial to promoting catalytic ozonation. Besides, the rising reaction temperature played a positive role in catalytic ozonation. A possible area of future research would be to investigate methods to improve the COD and UV₂₅₄ degradation efficiencies by loading multiple active components onto the supporter. Furthermore, it is also important to go beyond laboratory models to explore the practical functionality and applicability of the catalysts.

Supplementary Information

eer-2022-606-Supplementary.pdf

Acknowledgements

We acknowledge the financial support provided by the Major Science and Technology Project on Water Pollution Control and Treatment (2018ZX0760100401) and the Liaoning Provincial Department of Education Project (lnjc202011).

Notes

Conflict-of-Interest Statement

The authors declare no competing interests.

Author Contributions

W.R.N. (student) conducted the experiments. S.H.W. (student) conducted the experiments, wrote and revised the manuscript. W.J. (associate professor) revised the manuscript.

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Fig. 1

Effect of different aeration processes (ozone dosage at 2 L/min, pH at 7.1, reaction temperature at 25 °C, and continuous aeration time at 10 min).

Fig. 2

COD removal rates of different catalysts

Fig. 3

Effect of different catalyst dosages on (a) COD removal rate and (b) UV₂₅₄ removal rate (ozone dosage at 2 L/min, pH at 7.1, reaction temperature at 25 °C, and continuous aeration time at 120 min).

Fig. 4

Effect of different initial pH values of the solution on (a) COD removal rate and (b) UV₂₅₄ removal rate (ozone dosage at 2 L/min, catalyst dosage at 3.0 g/L, reaction temperature at 25 °C, and continuous aeration time at 120 min).

Fig. 5

Effect of different initial pH values of the solution (ozone dosage at 2 L/min, catalyst dosage at 3.0 g/L, reaction temperature at 25 °C, and continuous aeration time at 120 min).

Fig. 6

Effect of different initial temperatures of the solution on (a) COD removal rate and (b) UV₂₅₄ removal rate (ozone dosage at 2 L/min, catalyst dosage at 3.0 g/L, pH at 7.1, and continuous aeration time at 120 min).

Fig. 7

The stability of catalysts (ozone dosage at 2 L/min, catalyst dosage at 3.0 g/L, pH at 9.4, and continuous aeration time at 120 min).

Table 1

COD removal rates of catalysts prepared under different preparation conditions.

Mn/GAC				Cu/GAC				Fe/GAC

Roasting temperature (°C)	Roasting time (h)	Impregnation solution concentration (mol/L)	COD removal rate (%)	Roasting temperature (°C)	Roasting time (h)	Impregnation solution concentration (mol/L)	COD removal rate (%)	Roasting temperature (°C)	Roasting time (h)	Impregnation solution concentration (mol/L)	COD removal rate (%)
400	2	0.5	50.90%	400	2	0.5	51.20%	400	2	0.5	51.50%
500	2	0.5	58.90%	500	2	0.5	53.70%	500	2	0.5	49%
600	2	0.5	48.70%	600	2	0.5	49.60%	600	2	0.5	48.00%
500	2	0.5	50.90%	500	2	0.5	53.70%	400	2	0.5	51.50%
500	3	0.5	63.20%	500	3	0.5	54.60%	400	3	0.5	52.30%
500	4	0.5	63.90%	500	4	0.5	57.30%	400	4	0.5	54.60%
500	3	1	64.30%	500	4	1	53.40%	400	4	1	56.20%
500	3	2	57.80%	500	4	2	51.80%	400	4	2	53.50%