Removal of Disperse Yellow-42 Dye by Catalytic Ozonation Using Iron and Manganese-Loaded Zeolites

Abstract

In this research, the efficiency of the catalytic activity of iron and manganese-loaded (bimetallic) sodium zeolite was investigated for the ozonation-based removal of disperse yellow 42 dye. The impregnation method was used to deposit Fe and Mn on the surface of sodium zeolite. The morphological analysis of sodium zeolite before and after Fe and Mn deposition was conducted by SEM, EDX, and FTIR. It was found that several variables, including the ozone dose, contact time, pH, catalyst dose, and hydroxyl radical scavenger action, greatly influenced the efficiency of dye removal. The chemical oxygen demand (COD) removal by catalytic ozonation using Fe and Mn-loaded sodium zeolite from real dye textile wastewater was also investigated. After 30 min of treatment with catalytic ozonation at pH 6, the maximum 73% removal of disperse yellow 42 dye was achieved with a catalyst dose of 0.5 g and an ozone dose supply of 1.8 mg/min. In catalytic ozonation with the hydroxyl radical scavenger effect (HRSE), the decline in removal efficiency from 73% to 61% demonstrated that removal efficiency was highly dependent on hydroxyl radical production. The COD removal efficiency in the real textile wastewater was 59% with the ozonation process, which increased to 79% after catalytic ozonation.

1. Introduction

The textile sector is a big concern when it comes to releasing untreated wastewater into water streams causing severe water pollution [1,2,3]. The textile industry is the backbone of the economy of Pakistan, accounting for 65% of exports and 9% of gross national product (GNP). A large amount of water is used in various operations of the textile industry, which leads to a huge amount of wastewater discharge containing high loads of dyes, and the chemical oxygen demand (COD) is consequently found to be of a high concentration exceeding the regulatory National Environmental Quality Standards (NEQSs) [3]. Due to the lack of implementation strategies, resources, and technical facilities, effective effluent treatments are not being conducted to comply with these regulatory standards [3,4]. Dye is one of the most toxic pollutants and persistent organic compounds have been found in textile wastewater with great chemical stability and resistance to environmental factors such as light, heat, oxidation, and aerobic digestion [5]. Under normal conditions, they can persist in aquatic environments for up to 40 years, even at a concentration of <1 mg/L [6]. Dye residues in wastewater discharge substantially affect the environment, particularly aquatic ecosystems, because of their carcinogenic properties and relative biodegradability [5]. Dyes interfere with the physiological functions of aquatic plants by interfering with photosynthetic activity, resulting in reduced oxygen circulation and light absorption in the marine environment. As a result, dyes accumulate throughout the aquatic food chain, disrupting the physiological functioning of aquatic vegetation [7].

Disperse dyes are widely employed in the textile industry. The disperse yellow 42 dye belongs to class C with good sublimation fastness and dyeing capabilities. The molecular structure and properties of the dye are presented in

Table 1. Molecular structure and characteristics of disperse yellow 42 dye.

Dye	Disperse Yellow 42
Molecular structure
Chemical formula	C18H15N3O4S
Chemical name	4-anilino-3-nitro-N-phenyl benzene sulfonamide
Molecular weight (g/mol)	369.40
λmax (nm)	488

[8].

Various treatment techniques have been reported for the removal of dye, such as adsorption [5,9], biodegradation [10], coagulation [3], electrocoagulation [11], membrane filtration [12], and photolysis and photocatalysis [13]. Each technology has its own advantages and disadvantages. The major drawbacks include the expensive treatment, dye sludge, dye-sorbed adsorbent materials, and the disposal of the concentrated dye solution. In the past few years, advanced oxidation processes (AOPs) based on the production of hydroxyl radicals (OH•) have been studied for the abatement of various pollutants [14,15]. Ozonation has been applied successfully for the treatment of textile wastewater, either alone or in the presence of a catalyst [9,16]. Ozone is a strong oxidant with an electrochemical oxidative potential (E⁰) of 2.08 V and produces OH• radicals as efficient oxidants having E⁰ of 2.80 V [3]. The complex aromatic rings of textile dyes can be easily broken down by ozone because of its high oxidation potential, which degrades the dye compounds [15]. Moreover, nanocomposites can be synthesized and applied for the removal of dyes in water solutions [17]. Methylene blue and C. I. Acid Red 88 dye removal was investigated using CoFe₂O₄/mpg-C₃N₄ under ultrasonic irradiation conditions and the UV/H₂O₂ method [18], respectively.

Zeolites were reported as excellent nanocatalysts because of their microporous crystalline structure with exchangeable cations and negatively charged surface [19,20]. A catalyst in ozonation accelerates ozone decomposition through the generation of OH• radicals [3,21]. Iron (Fe)-based catalysts in ozonation showed high catalytic activity due to their numerous benefits, including their abundance in nature, ease of synthesis, non-toxicity, and some other unique properties: the magnetic properties of Fe₃O₄ and the high density of hydroxyl groups in FeOOH [22,23]. Manganese oxides were reported as highly efficient for usage in practical applications due to their low cost, natural availability, and environmental friendliness. Manganese (Mn) has multiple oxidation states, that can help in electron transmission, and manganese dioxide possesses a negatively charged surface in a pH ranging from 5 to 11, which is of great importance for water sources and water treatment activities. Ozone’s electrophilic properties cause it to react quickly with the negatively charged surface [24].

The current investigation aims to investigate the effectiveness of multi-metal-loaded sodium zeolite as a catalyst in the ozonation process for the removal of dyes in wastewater. This study was focused on the application of zeolites for the removal of dyes and the modification of sodium zeolite by loading Fe and Mn onto it. Operational working conditions, such as the pH, ozonation flow, catalyst dose, and the contact time of ozonation, were studied and optimized to make the treatment process effective and economical. The hydroxyl radical scavengers’ effect (HRSE) was studied in single ozonation and catalytic ozonation to explore the possible treatment mechanism. Moreover, the COD experiments were conducted for real textile wastewater to study the reduction in the organic pollution load by implying multi-metal-loaded sodium zeolite as a catalyst in the ozonation process. This study may provide sustainable treatment solutions for the treatment of dye wastewater to achieve sustainable development goals 3 and 6.

2. Materials and Methods

2.1. Materials

Sodium zeolite, FeSO₄, MnSO₄, and disperse yellow 42 dye were purchased from Sigma Aldrich, UK. All the reagents and chemicals, including KI, HNO₃, NaCl, HCl, sodium thiosulfate, and the scavenger agents, were of analytical grade and did not require any treatment. Deionized water was utilized in standard solutions for preparation and washing purposes. The pH of samples was determined using the Hanna HI 9811, USA.

2.2. Sampling of Real Textile Dye Wastewater

The real textile dye wastewater sample was collected from a textile and dyeing factory situated in Lahore, Pakistan. The sample was collected in a pre-washed container and kept in the laboratory for further experimentation at 4 °C without adding any preservative.

2.3. Preparation of Fe and Mn-Loaded Sodium Zeolite Catalyst

Before Fe and Mn deposition, the sodium zeolite was washed with deionized water and dried at 103 °C in an oven (EYELA-NDO-450ND) for 6 h. The impregnation method was used to deposit Fe and Mn on the sodium zeolite [25]. The impregnation of 30 g of sodium zeolite was carried out using 9 g of FeSO₄ and 9 g of MnSO₄ dissolved in 100 mL of deionized water. The mixture was agitated using a magnetic stirrer at a speed of 120 rpm on a hot plate at a temperature of 103 °C for 4–5 h. After complete evaporation, the sample was dried in the oven at 103 °C. After drying, the sample was dipped in a 1 M HNO₃ solution overnight to remove unreacted FeSO₄ and MNSO₄. The Fe- and Mn-loaded sodium zeolite catalyst was washed with distilled water until a constant pH was attained and then dried at a temperature of 103 °C for 1–2 h. The prepared catalyst was calcined in a furnace (TELEX 54524-CARBOLITE SHFLD) at 550 °C for 6 h. The prepared sample was stored according to standard practice for further use. Figure 1 shows the schematic diagram for catalyst preparation.

2.4. Characterization

The sodium zeolite before and after Fe and Mn loading was characterized using various instrumental techniques. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were performed using an electron microscope (Inspect S50 using FEI) to determine the surface morphology and elemental properties. The point of zero charges (pHpzc) of multi-metal-loaded sodium zeolite was measured using the mass transfer method. Functional groups on the sodium zeolite and multi-metal-loaded sodium zeolite were studied using Fourier transform infrared spectroscopy (FTIR) analysis (FTIR spectrometer, Bruker ALPHA, Germany). A Brunauer Emmett Teller (BET) analyzer (NOVA 2200E BET) was used to determine the average pore size and pore volume and the specific surface area of sodium zeolite with and without multi-metal loading.

2.5. Preparation of the Disperse Yellow 42 Dye Solution

A 100 ppm standard solution of disperse yellow 42 dye was prepared. From this stock solution, dilutions of 5, 10, 15, 20, 25, and 30 ppm were prepared and examined using a UV/VIS spectrophotometer (PerkinElmer LAMBDA 365).

2.6. Experimental Setup

Figure 2 presents a schematic diagram of the experimental setup used for the ozonation/catalytic ozonation processes. A semi-batch reactor was used to perform the experimental analysis. The experiment was conducted using a Pyrex glass reactor connected to an ozone reactor, along with two conical flasks for the KI traps. Each time an experiment was conducted, 500 mL of the sample was poured into the reactor. Ozone was added to the reactor using sparges from the ozone generator (DA-12025-B12, Sky ozone, Karachi, Pakistan). The analysis was carried out for samples after the required treatment at regular intervals. The tools used to carry out the ozonation and catalytic ozonation experiments included flasks, a magnetic stirrer, a beaker, and a suction filtration assembly.

2.7. Experimental Analysis

The percentage removal of dye by catalytic ozonation was compared with the ozonation process to determine the efficacy of Fe- and Mn-loaded sodium zeolite for the removal of disperse yellow 42 dye.

For the ozonation and catalytic ozonation of the dye solution, 500 mL of the synthetic solution of disperse yellow 42 dye was prepared and treated in a reactor tank for about 30 min at room temperature with an ozone dose of 1.8 mg/min. A weighed quantity of Fe- and Mn-loaded zeolite was added as a catalyst to the reactor during catalytic ozonation. Every 5 min, the sample was taken out in a sample bottle, to which 2–3 drops of 0.025 N sodium thiosulfate solution were added. The samples were then analyzed using a UV-Vis spectrophotometer at a maximum wavelength of 495 nm and the corresponding absorbance was calculated. The relative absorbance and dye concentration calibration curves were created.

The experiment was carried out with varying experimental conditions, such as the catalyst dose, ozone dose, hydroxyl radical scavenger effect, pH, and reaction time. The basic and acidic pH was attained by the addition of NaOH and HCl, respectively. Sodium bicarbonate was added to the dye solution to examine the formation of radicals during the ozonation and catalytic ozonation processes. The following calculation formula was used to determine the dye removal percentage.

$$\mathrm{D}\mathrm{y}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}\left(\%\right)=\frac{(C_i-C_f)}{C_i}\times 100$$

where C_i is the initial concentration of dye and C_f is the final concentration of dye.

Iodometric analysis was used to calculate the ozone dose [3,26]. The ozone produced by the O₃ generator was circulated for 30 min through flasks containing a 2% KI solution (200 mL), to which 1 N of nitric acid was added for titration against 0.005 N of sodium thiosulfate using starch as an indicator. The following formula was used to calculate the ozone dose.

$$\mathrm{Ozone}\mathrm{dose}\left(\frac{\mathrm{m}\mathrm{g}}{\mathrm{m}\mathrm{i}\mathrm{n}}\right)=\frac{\mathrm{N}\times \mathrm{V}\times 24}{\mathrm{T}}$$

where N denotes the sodium thiosulfate normality, V denotes the titrant volume, and T denotes the ozonation time.

To study the adsorption effect, 500 mL of the dye solution with a weighed amount of catalyst was placed on a magnetic stirrer for 30 min at room temperature and with a neutral pH.

The COD of the untreated and treated samples was measured using the open reflux method. A blank reagent was run through a similar procedure to control the amount of organic material in the sample.

3. Results and Discussion

3.1. Characterization of Catalysts

The FTIR of sodium zeolite before and after Fe and Mn loading are shown in Figure 3. The FTIR spectra of sodium zeolite (Figure 3a) were composed of vibrational peaks of the framework and a sharp peak of Si–O or Al–O at 1172 cm⁻¹, which drifted to 1008 cm⁻¹ for the Fe- and Mn-loaded sodium zeolite (Figure 3b) [27]. The Si–O(Si) and Si–O(Al) vibrations were observed within a range of 1200–400 cm⁻¹. The bands for zeolite water ranged between 1600 and 3700 cm⁻¹. Whereas the bands in the range of 500–700 cm⁻¹ were a result of the pseudo lattice vibrations of structural units. A range below 400 cm⁻¹ was suggested for the lattice vibrations [28].

The SEM images in Figure 4 were taken at a magnification of 100× with an ETH of 5.00 KV to examine the surface morphological characteristics of sodium zeolite before (Figure 4a) and after Fe and Mn loading (Figure 4b). The SEM images may indicate the surface coating with Fe and Mn. The bimetallic coating on the surface of the sodium zeolite was composed of tiny particles that were placed on top of a micron-scale layer that was more condensed [16].

Figure 5 presents the EDX analysis of sodium zeolite (Figure 5a) and the bimetal (Fe and Mn)-loaded sodium zeolite (Figure 5b). The additional peaks of Fe and Mn were observed in Figure 5b, which confirmed the loading of Fe and Mn on the surface of sodium zeolite.

The BET analysis results are presented in

Table 2. Catalyst characterization.

Material	Surface Area (m2/g)	Average Pore Size (nm)	Pore Volume (cc/g)	Fe (%)	Mn (%)	PHpzc
Sodium zeolite	9.4	5.45	7.19	-	-	6.2 ± 0.2
Multi-metal-loaded sodium zeolite	39.6	5.96	12.45	0.66	0.45	6.1 ± 0.2

. The specific surface area of sodium zeolite (9.4 m²/g) was significantly increased with multi-metal loading on the sodium zeolite (39.6 m²/g). Other properties, such as the average pore size and the pHpzc of sodium zeolite before and after multi-metal loading were 5.45, 5.96 nm, and 6.2 ± 0.2 and 6.1 ± 0.2, respectively.

The pH of the solution plays an important role in defining the charge on the material. If the pH of the water is higher than the pHpzc of the material, this material surface will be positively charged, and when the pH is than the pHpzc of the material, then the material surface will be negatively charged, affecting the charge of the material for the adsorption of the pollutants on its surface. The dye studied has both positive and negative ends to it. Therefore, at the studied pH, sodium zeolite would be negatively charged, so it can attract the positive end of the dye, which leads to the adsorption process and results in a surface reaction for both molecular ozone and hydroxyl radicals.

3.2. Optimization of the Factors Affecting the Removal Efficiency

3.2.1. Ozone Contact Time

An ozonation process was performed for 30 min with samples taken out at intervals of 5, 10, 15, 20, 25, and 30 min. The ozone contact time with the sample is very important to determine the optimum duration required to achieve the highest reaction efficiency. Figure 6 demonstrates the dye removal efficiency for time duration. According to the results, a sharp increase in dye removal was attained with increasing ozonation duration. For example, ozonation for 3 min resulted in a removal efficiency of 9%, which increased to 12%, 16%, 18%, and 29% after ozonation for 10, 15, 20, and 25 min. A further increase in the ozonation duration to 30 min did not result in a significant increase in dye removal efficiency (30% dye removal efficiency after 30 min of ozonation). Therefore, a 30 min duration for ozonation was considered for all subsequent experiments.

3.2.2. Ozone Dose

Figure 7 depicts the influence of the ozone dose on the removal efficiency trends for disperse yellow 42 dye. Three ozone doses, i.e., 0.6, 1.2, and 1.8 mg/min, were applied for dye removal. According to the results, an increasing removal efficiency trend was observed for all three ozone doses. The lowest removal efficiency of 15% was attained for 0.6 mg/min of ozone dose supply after 30 min of ozonation. Whereas the removal efficiency after 30 min of ozonation increased to 22% and 30% with an increase in ozone dose to 1.2 mg/min and 1.8 mg/min, respectively. Hence the highest removal efficiency at a fixed ozonation contact time of 30 min was attained for an ozone dose of 1.8 mg/min. This fact was attributed to the increase in radical-based reactions occurring on the catalyst’s surface with an acceleration in the ozone dose, which substantially stimulated the dye removal efficiency [16].

3.2.3. Catalyst Dose

To determine the influence of the catalyst dose in ozonation for the removal efficiency of disperse yellow 42 dye, three different catalyst doses of 0.1 g, 0.5 g, and 1 g were used (Figure 8). The lowest catalyst dose of 0.1 g resulted in the lowest removal efficiency of 53%, which increased to 73% by increasing the catalyst dose to 0.5 g. The increase in catalyst amount resulted in an increased surface area, providing more catalytic sites for radical production and pollutant degradation. Therefore, the generation of hydroxyl radicals increased with an increase in the catalyst dose, facilitating the removal of the dye [29]. Whereas a further increase in the catalyst dose to 1 g did not significantly enhance the catalytic activity, giving a removal efficiency of 75%. Therefore, 0.5 g was selected as the optimal catalyst dose for subsequent experiments to make the process more resource- and cost-efficient.

3.2.4. pH Effect

The removal efficiency of the dye significantly depends on the pH of the solution. Figure 9 demonstrates the pH effect on the catalytic ozonation of disperse yellow 42 dye. The impact of the dye solution’s pH on disperse yellow 42 removal was examined using catalytic ozonation at three pH conditions of 3, 6, and 9. The highly acidic solution with a pH of 3 resulted in the lowest removal efficiency of 63%. The removal efficiency of the dye increased to 73% at a pH of 6. Whereas the highest dye removal efficiency was attained at a basic pH of 9. By increasing the pH to basic conditions, the removal of dye was increased, which resulted in the stimulation of hydroxyl radical production. Therefore, the higher hydroxyl radical concentrations produced at higher or basic pH levels enhanced the dye removal efficiency [29].

3.2.5. Hydroxyl Radical Scavenger Effect

Figure 10 presents the effect of the hydroxyl radical scavenger on the ozonation and catalytic ozonation process for the removal of disperse yellow 42 dye. According to the results, the application of hydroxyl radical scavengers decreased the dye removal efficiency. When the ozonation process was carried out in the presence of a hydroxyl radical scavenger, the dye removal efficiency declined from 30% to 23%. Likewise, the catalytic ozonation process also showed a reduction in removal efficiency from 71% to 61%. The addition of carbonates and bi-carbonate ions was considered a plausible reason for a decline in dye removal efficiency. Carbonates and bi-carbonate ions scavenge the hydroxyl radicals by the inhibition of active sites. Therefore, in the presence of hydroxyl radical scavenging, the efficiency of the ozonation and catalytic ozonation processes was negatively impacted by the presence of bicarbonate ions. The following equations (Equations (1) and (2)) present the scavenging effect of carbonates.

$$\mathrm{·}\mathrm{O}\mathrm{H}+\mathrm{C}{\mathrm{O}}_3^{2-}=\mathrm{C}{\mathrm{O}}_3^{\mathrm{·}-}+\mathrm{O}{\mathrm{H}}^{-}$$

(1)

$$\mathrm{C}{\mathrm{O}}_3^{-}+{\mathrm{O}}_3^{\mathrm{·}-}=\mathrm{C}{\mathrm{O}}_3^{2-}+{\mathrm{O}}_3$$

(2)

Carbonate stabilizes the ozone by acting as an inhibitor of ozone breakdown and reacts with the oxidant radicals produced during the process. As the dye removal depends on the radicals produced during the process, the presence of carbonates reduces the process efficiency. Therefore, the presence of carbonates in an aqueous solution severely limits the use of ozone in dye removal [30,31].

3.3. Comparison of Various Treatment Processes

The decolorization efficiency of different treatment processes—including adsorption, ozonation, and catalytic ozonation—was compared to determine the most efficient process for dye removal. Figure 11 demonstrates the results of decolorization efficiencies attained by various methods. According to the results, the lowest decolorization efficiency of 15% was attained by the adsorption process. The ozonation process demonstrated a relatively higher efficiency of 30%. The highest decolorization of 73% was attained by the catalytic ozonation process.

The results indicate that apart from for the ozonation process, adding a catalyst enhances the process efficiency. The addition of a catalyst to ozonation increases the rate of OH• radicals’ generation, which leads to the efficient removal of pollutants. Ozonation in the presence of a heterogeneous catalyst (O₃/heterogeneous catalysis) has a very complex mechanism with many influencing variables. The catalyst plays a variety of functions during the catalytic ozonation process. Three possible steps can be proposed in carrying out the O₃/heterogeneous catalytic process. In the first step, the catalyst may act as an adsorbent, attracting ozone or polar organic matter to its surface due to the surface charge and/or different functional groups present on the surface (e.g., hydroxyl groups). As a result of this step, ozone and organic matter react on the catalyst’s surface. Secondly, catalysts encourage OH• formation and ozone decomposition, which improves the removal of organic matter. Thirdly, catalysts and the adsorbed materials interact to form a surface complex that would, consequently, be oxidized by ozone or OH• [32]. Because of the above-stated steps involved in catalytic ozonation, pollutant/organic matter removal efficiency increased.

3.4. COD Removal of Real Textile Dye Wastewater

The initial characteristics of real textile wastewater were pH: 9.2 ± 0.6; turbidity: 214 ± 15 NTU; chlorides: 360 ± 40 mg/L; total solids (TS): 2647 ± 250 mg/L; total dissolved solids (TDS): 2406 ± 250 mg/L; total suspended solids (TSS): 221 ± 40 mg/L; COD: 680 ± 100 mg/L; sulfates: 288 ± 35 mg/L; biological oxygen demand (BOD₅): 198 ± 45 mg/L; total hardness 230 ± 15 mg/L as CaCO₃; and color: 3365 ± 150 Pt-Co. The major portion of TS contained more TDS than TSS and sulfates and chlorides complied with NEQS; however, the COD exceeded the NEQS. The COD removal from real textile dye wastewater was investigated after ozonation and catalytic ozonation methods to evaluate the effectiveness of the treatment processes (Figure 12). The real textile wastewater had an initial COD of 680 mg/L before treatment. The COD analysis with the reflux method demonstrated a 59% COD removal after the ozonation process, whereas the catalytic ozonation process using Fe- and Mn-loaded sodium zeolite resulted in an increased COD removal of 79%. Hence the application of Fe- and Mn-loaded sodium zeolite as a catalyst enhanced COD removal efficiency for the real textile dye wastewater.

3.5. Proposed Mechanism

The mechanism of catalytic ozonation using Fe- and Mn-loaded sodium zeolite as a catalyst is complicated and may be influenced by the surface properties of the catalyst, molecular ozone decomposition, and the nature of the targeted dye. Figure 13 shows that during catalytic ozonation, OH• are produced due to the reactions of aqueous ozone with hydroxyl ions present in bulk solutions. Surface hydroxyl groups and active sites of synthesized catalysts may also be involved in OH• generation. The hydroxyl radical scavengers effect was investigated previously [3,33] and also performed in the current study (Figure 10), as discussed above, to confirm the catalytic effect and OH• production in catalytic ozonation [27]. It was found that the reduction in removal efficiency of the studied dye in the presence of a hydroxyl radical scavenger during the single ozonation and catalytic ozonation process may lead to a dominant treatment mechanism based on the hydroxyl radicals. Therefore, it may be hypothesized that the interaction of ozone with the Fe- and Mn-loaded sodium zeolite produces higher generations of OH• radicals. Furthermore, ozone decomposition and OH• radicals-based reactions were demonstrated during the catalytic ozonation process (Figure 13). Besides, some inert intermediates were produced due to a reaction between the hydroxyl radical scavengers and OH• radicals that can hinder the ozone degradation rate. Briefly, previous studies reported that an OH•-based mechanism was dominant for heterogeneous catalytic ozonation [5,29,30,34,35], so the proposed mechanism agrees with past studies.

3.6. Discussion

The efficacy of catalytic ozonation using multi-metal-loaded sodium zeolite was compared with single ozonation and simple adsorption processes. The results obtained show that the catalytic ozonation process has a high removal efficiency of dye compared to the ozonation and adsorption processes [5,27,31]. Moreover, the authors investigated the hydroxyl radical scavenger effect, which clearly suggested that the process involved radical-based reactions [5,27,31]. Therefore, it may be hypothesized that the dominant mechanism for the degradation of disperse yellow 42 dye is based on OH• radicals. So, the proposed mechanism is in agreement with previous studies [5,29,30,34,35] reporting a radicals-based mechanism for catalytic ozonation. Besides, researchers have previously studied [16,27,30,31,33] the metal leaching and reuse performance of metal-loaded zeolites as a catalyst, and the study of another research group [21] also reported that a minute quantity of metal was leached out, as well as the good stability and reusability of zeolites in catalytic ozonation for the treatment of dyes and other pollutants in water and wastewater. Additionally, this study also explored the COD experiments that clearly suggested a significant reduction in the organic pollution of real textile wastewater with the addition of multi-metal-loaded sodium zeolite as catalyst in the ozonation process [27].

4. Conclusions

This study compared the efficiencies of the ozonation and catalytic ozonation processes for the removal of disperse yellow 42 dye from an aqueous solution. Using Fe- and Mn-loaded (bimetallic) sodium zeolite, the catalytic ozonation process proved to be an efficient method for dye removal. The highest catalytic removal was achieved by catalytic ozonation (73%), followed by ozonation (30%), and adsorption (15%). A catalyst dose of 0.5 g at a pH of 6 and with a maximum ozone supply rate of 1.8 mg/min was considered as the optimized conditions to attain the highest removal efficiency by catalytic ozonation within a time duration of 30 min. The ozonation and catalytic ozonation processes highly depended on the presence of scavengers. In the catalytic ozonation process, zeolite activity was decreased by the hydroxyl radical scavenger. The addition of carbonates and bicarbonate ions scavenged the hydroxyl radical and reduced the active sites. The significant decline in catalytic efficiency indicated that the dye removal was supported by radical production. In real wastewater, the catalytic ozonation process is relatively more efficient for COD removal, with 79% removal efficiency, compared to the ozonation process, with a removal efficiency of 59%.