Understanding the Efficiency of Catalytic Ozonation for the Degradation of Synthetic Dyes in Water: A Study of Degradation Mechanism and Pathways

Abstract

Dyes in wastewater are an environmental issue due to the persistent nature of these compounds. This comparative study examined the efficiency of ozonation and catalytic ozonation using Fe³⁺/O₃ for the degradation of two selected dyes, Methylene Blue (MB) and Methyl Orange (MO). For MB, ozonation alone achieved 65% degradation within the maximum reaction time of 15 min, whereas 100% degradation was obtained with the Fe³⁺/O₃ method in the same time. On the other hand, for MO, ozonation alone resulted in 85% degradation within 15 min, while the Fe³⁺/O₃ method achieved 100% degradation in 10 min. The effect of Fe³⁺ dose was also investigated, and 3 ppm was found to be the most efficient. The scavenger effect highlighted that ^•OH radicals were the dominant species for degradation. For MB, the highest degradation rate was observed at pH 9, which is attributed to catalyzed ozone decomposition, thereby enhancing the generation of ^•OH radicals to a higher concentration. For MO, the degradation rate was highest at pH 5. LC-MS analysis was performed to explore MB degradation products formed during Fe³⁺/O₃ treatment. Five main degradation products were observed, with the main pathway involving the generation of P1, P2, and P3. Based on the results, the Fe³⁺/O₃ method is considered efficient for wastewater treatment. This study highlights the Fe³⁺/O₃ method as a sustainable solution for the degradation of dyes from textile wastewater.

1. Introduction

Dyes are used in the textile industry and pose several environmental impacts [1]. The industries use water, which discharges pollutants into surface water [2]. Worldwide, the textile industry releases dyes, which are hazardous to human health and the environment [3]. Synthetic dyes also have uses in industries, including food production, pharmaceuticals, and plastics [4]. Dyes have unique properties, such as resistance to heat, oxidation and aerobic digestion. Synthetic dyes interact with fiber macromolecules, and heating dyed fabrics results in the appearance of different colors [5]. Traditional wastewater treatment methods are insufficient to degrade dyes due to their stability [6]. Owing to this fact, advanced oxidation processes (AOPs), such as UV/Cl, ozonation alone, and peroxymonosulfate (PMS), have been explored to degrade the persistent compounds in water [7,8]. The sulfate radical-based AOPs are also reported as efficient methods [9]. Recently, Fenton technology has also gained attention due to its efficiency and low cost [10]. In addition, semiconductor photocatalytic technology is also found to be a sustainable way to degrade compounds into non-toxic byproducts [11]. This technology uses solar energy to initiate the redox reactions [12]. Biological treatments are also used to degrade the dyes, though there are many challenges to applying them on a large scale [13]. The methods mentioned were found to be efficient but were not cost-effective.

Ozonation is an AOP which is widely considered for its efficiency in wastewater treatment [14,15]. Ozonated degradation involves the generation of ozone to degrade organic pollutants in water. Ozone reacts with pollutants either directly or indirectly through the ^•OH radicals in the presence of water [16,17]. ^•OH radicals are non-selective and highly reactive, enabling the degradation of pollutants into less harmful byproducts [18]. Ozonation is effective in degrading pollutants which are resistant to traditional wastewater treatment methods [12,17]. Ozonation limitations include the cost of application and the generation of carcinogenic byproducts [19,20]. On the other hand, catalytic ozonation has emerged as an efficient method, which involves the use of catalysts to enhance the efficiency of ozone reactions, thereby reducing cost and minimizing harmful byproducts [21,22], as displayed in Table S1. Catalytic ozonation catalysts include the transition metal ions and metal oxides [6,20]. Heterogeneous catalysts have more benefits due to their stability [23]. For instance, materials such as zeolites, activated carbon, and nanostructured materials have been widely investigated for their catalytic properties in ozonation treatment. These catalysts increase the generation of reactive oxygen species (ROS), which facilitate the degradation of pollutants [15].

Despite several advantages of catalytic ozonation for dye degradation, many challenges need to be addressed for real-time application [20,21]. One of the challenges is the cost of ozone generation, which limits its application for wastewater treatment [24]. The development of cost-effective ozone-based degradation methods is needed. Moreover, in catalytic ozonated degradation, the separation and recovery of the catalyst is also challenging.

The presented study examined the effect of ferric (Fe³⁺) ions on the efficiency of ozonation for the degradation of selected synthetic dyes, Methylene Blue (MB) and Methyl Orange (MO). The study highlighted the effect of key ozonation parameters (such as dye dosage, pH, contact time, effect of Fe ³⁺ ion as a catalyst) on the degradation efficiency of selected synthetic dyes. The structures of MB and MO are shown below in Figure 1.

The Fe³⁺/O₃ method offers a novel catalytic approach to enhance ozone decomposition, thereby accelerating the generation of ROS and improving degradation performance. Compared with conventional ozonation alone, the Fe³⁺/O₃ process provides faster degradation kinetics and complete removal of dyes within shorter reaction times. This study highlights the novelty of employing Fe³⁺ as a cost-effective and environmentally benign catalyst to enhance ozonation efficiency, demonstrating its potential for practical application in wastewater treatment.

2. Materials and Methods

2.1. Chemicals

Distilled water, with its high purity, was utilized for the preparation and dilution of solutions. MB (≥99% purity, analytical grade) and MO (≥99% purity, analytical grade) were used for investigation. Stock solutions of 1000 ppm were prepared. 0.01 M sodium thiosulfate (Na₂S₂O₃, ≥98% purity, analytical grade) concentration was used as a scavenger of oxidizing species like O₃, and ^•OH free radicals. To study the effect of ions (anions and cations), salts at concentrations of 0.01–0.05 M were used. For sample purification, the solid-phase extraction (SPE) was used. The pH of the solution was adjusted using a few drops of 0.01 M H₂SO₄ and NaOH (≥98% purity, analytical grade). An ozone generator with a production of 3 g h⁻¹ L⁻¹ was used. Iron sulfate (Fe₂(SO₄)₃, 98% purity, analytical grade), with a concentration of 1 to 3 mg/L, was used to generate Fe³⁺ ions.

2.2. Experimental Procedure

Preparation of Sample Solution

Samples were prepared from a stock solution of 1000 mg/L. The sample concentration was maintained at 20 ppm in the reaction solution. For the calibration curves, the λ_max used for MB was 662 nm, while for MO was 472 nm. A linear relationship between absorbance and concentration was obtained. These results are presented in Figure S1 and Figure S2, respectively.

2.3. Reaction Setup

2.3.1. Ozonation Experiment

A reaction solution of 250 mL was prepared with a concentration of selected dyes of 20 ppm. The reaction solution comprised either MB or MO and distilled water for ozonation alone (O₃). For the Fe³⁺/O₃ experiment, the effect of Fe³⁺ ion concentration (1–3 ppm) was also investigated, and findings are displayed in Figure S3. To maintain and control the pH at the desired level, H₂SO₄ (0.01 M) and NaOH (0.01 M) were used to change the pH. The effects of pH 5, 7 and 9 were investigated on the reaction system. The mentioned pH range was selected because synthetic dyes exist in different forms, which affect the reaction system.

2.3.2. Sample Collection

Samples were collected at specific time intervals of 0 min, 0.5 min, 1 min, 2 min, 3 min, 5 min, 7 min, 10 min, 12 min, and 15 min during the reaction to monitor degradation. Each collected sample was immediately quenched with 50 µL of 0.01 M Na₂S₂O₃ to stop the reaction and preserve the sample’s state at the respective time point. The changes in dye concentration over time were investigated using a double-beam spectrophotometer to explore degradation kinetics and provide insights into the reaction’s progression. The experimental setup and procedure for ozonation are shown in Figure S4.

2.4. LC-MS Sample Preparation

LC-MS (liquid chromatography–mass spectrometry) was used to investigate degradation products. Since the ozonation alone method has already been extensively studied, the presented study investigated the degradation products generated during the Fe³⁺/O₃ treatment method. The detailed procedure for sample preparation for LC-MS analysis to investigate MB degradation products is as follows. SPE using silica cartridges was selected for the study based on the chemical properties of MB. Before use, the cartridges were washed with methanol and rinsed with distilled water. For the loading step, methanol-quenched samples were gradually passed through the cartridge, using a vacuum manifold to maximize interaction and adsorption of MB onto the sorbent material. The cartridge was rinsed with water to remove impurities. In the elution step, dye was removed from the cartridge using CH₃OH. For the analysis, LC-MS was used with positive ion mode.

2.5. Instruments and Data Analysis

High-Performance Liquid Chromatography (HPLC) was used, an Agilent (1260) Infinity II system (Agilent Technologies, Santa Clara, CA, USA). For LC-MS analysis, a TripleTOF 5600+ LC-MS/MS (AB Sciex, Marlborough, MA, USA) was used. PeakView Software version 2.2 (AB Sciex, Marlborough, MA, USA) was used to explore the peak intensities. For graphical representation and statistics, OriginPro 2023 (OriginLab, Northampton, MA, USA) were used.

3. Results

Ozonation is considered an efficient method for wastewater treatment [25]. Ozone attacks the chromophore of dyes by a process known as ozonolysis [21,26]. Two ozonolytic degradation pathways for pollutant degradation are the radical generation and the direct attack by ozone on the pollutants [25,26]. The high oxidation potential of ozone (2.07 V) makes it so powerful at destroying organic pollutants [27], while its counterpart (^•OH) possesses even more oxidation potential (2.8 V) [16]. Emerging contaminants, including pharmaceuticals, pesticides, hormones, UV filters, surfactants, and dyes, have presented a significant challenge for environmental researchers [28,29]. Initially proposed for the purification of drinking water in 1980, AOPs have since been employed in various wastewater treatment applications [30]. AOPs, or hybrid processes such as ultrasound-assisted Fenton, sono-photocatalytic, O₃/H₂O₂, have been developed to improve efficiency and minimize the limitations and drawbacks of AOPs in treating pollutants [31]. It has been found that conventional biotic treatments are not only ineffective but also problematic in degrading persistent antibiotics [32]. Various techniques, such as coagulation, membrane separation, sorption and biodegradation, have been discussed to degrade a range of drugs found in drinking and discarded water [33]. Since the early 21st century, ozone has been widely used to treat drinking water and industrial wastewater [34]. Ozone (bluish gas) is an immensely reactive and unstable allotrope of O₂ with a pungent odour. The oxidation potential of ^•OH radicals and molecular ozone is 2.80 V and 2.07 V, respectively [16,35]. Substantial research is needed on the integration of ozone with different water treatment methods to increase overall efficiency and to address the challenges associated with environmental applications and cost effectiveness [21]. In the presented study, Fe³⁺/O₃ is integrated for the degradation of selected synthetic dyes MB and MO. The findings are discussed in detail below.

3.1. Degradation of MB and MO by O₃ and Fe³⁺/O₃

This study compared the degradation efficiency of MB and MO using O₃ and Fe³⁺/O₃ under the same reaction conditions.

3.2. Degradation of MB

O₃ achieved 65% degradation of MB within 15 min, demonstrating ozone’s high oxidative potential but also indicating that MB’s structural stability limits complete degradation under these conditions. However, when Fe³⁺ ions were introduced as a catalyst in the Fe³⁺/O₃ method, MB degradation was enhanced to 100%, achieving complete removal within the same reaction time. Iron-based catalysts can be considered, as this metal is abundant on earth [36]. Iron catalysts have been used in the O₃ method to degrade different pollutants in water [37].

The result highlights the catalytic role of Fe³⁺ in accelerating the ozone decomposition process and generating additional reactive species, such as OH radicals, that effectively break down MB molecules. The findings are shown in Figure 2.

3.3. Degradation of MO

Compared to MB, MO displayed a higher degradation rate in both treatments, as depicted in Figure 2. With O₃, MO was 85% degraded within 15 min, indicating its higher susceptibility to O₃ oxidation. In the Fe³⁺/O₃ system, MO achieved 100% degradation in 15 min, underscoring the efficacy of the mentioned method. The results suggest that the presence of Fe³⁺ further facilitates the breakdown of MO by enhancing the generation of reactive species. The natural susceptibility of MO to ozone resulted in a rapid degradation rate even in the absence of a catalyst.

The results clearly demonstrate the enhanced efficiency of the Fe³⁺/O₃ method compared to O₃ alone method. These findings emphasize the significance of the Fe³⁺/O₃ method in not only accelerating dye degradation but also achieving complete removal under relatively short contact times. This enhanced performance demonstrates the novelty of the Fe³⁺/O₃ method and its practical potential for industrial wastewater treatment, where efficient degradation of contaminants is required [36].

3.4. Reaction Kinetics of MB Degradation

The degradation of MB using ozonation followed a first-order kinetic model, where the reaction rate was directly proportional to the ozone concentration. O₃, a strong oxidant, rapidly degrades organic pollutants through direct oxidation mechanisms [38], resulting in a first-order dependence on its concentration. The first-order equation is presented as follows:

$$\mathit{l}\mathit{n}\left({\displaystyle \raisebox{1ex}{${\mathit{C}}_{\mathit{t}}$}\!\left/ \!\raisebox{-1ex}{${\mathit{C}}_0$}\right.}\right)=-\mathit{k}\mathit{t}$$

(1)

Due to the high oxidative potential of O₃, it is possible to achieve 85% degradation of MB in only 15 min, demonstrating its efficiency as an oxidant.

Previous research has demonstrated similar first-order kinetics in ozone-based degradation processes [14,39], particularly for aromatic organic compounds. The first-order rate constant k for the ozonation process was determined to be 0.19 s⁻¹ (Figure 3).

3.5. Ferric Catalyzed Ozonation of MB Degradation

The Fe³⁺/O₃ method exhibited pseudo-first-order kinetics. The pseudo-first-order equation can be presented as follows:

$$\mathit{l}\mathit{n}\left({\displaystyle \raisebox{1ex}{${\mathit{C}}_{\mathit{t}}$}\!\left/ \!\raisebox{-1ex}{${\mathit{C}}_0$}\right.}\right)={\mathit{k}}_{\mathit{obs}}\mathit{t}$$

(2)

The interactions between Fe³⁺ ions and MB molecules generate reactive radicals, enhancing the degradation process through both direct and indirect oxidation pathways. The addition of an excessive concentration of Fe³⁺ promotes radical formation and creates a dual oxidation pathway [40] resulting in a pseudo-first-order rate dependence. Studies support the increased degradation efficiency and pseudo-first-order reaction in metal-catalyzed ozonation processes, particularly for complex organic molecules [41]. The rate constant k for this system was calculated to be 0.24 s⁻¹ (Figure 4).

3.6. Effect of pH on the Degradation of MB

The effect of pH on the degradation of MB via Fe³⁺/O₃ was evaluated to understand how pH influences the efficiency of this process. pH is a critical parameter in ozonation because it directly affects both the chemical form of MB and the generation of ROS, particularly ^•OH radicals, which are potent oxidizing agents [42]. For this study, pH values of 5, 7, and 9 were selected, representing the typical pH range of industrial wastewater [43]. The findings indicated that the degradation efficiency of MB increased significantly as the pH increased from acidic to alkaline conditions. As shown in Figure 5, at lower pH (pH 5), the degradation efficiency was relatively low compared to neutral and alkaline conditions. This can be attributed to the limited generation of ^•OH radicals [44,45], which are more prevalent in alkaline conditions [46]. To explore the role of ^•OH free radicals in the degradation of dyes, the scavenger effect was investigated. It was observed that radical scavenger Na₂S₂O₃ retarded the degradation rate at pH 9, which confirms the dominant role of ^.OH free radicals as displayed in Figure S5. In acidic environments, O₃ is the primary oxidant [45], but it is less effective in degrading MB due to the slower reaction kinetics with basic dyes. Additionally, MB exists predominantly in its protonated form at lower pH, which may reduce its reactivity with ozone.

At pH 9, the degradation rate of MB was the highest. The efficient degradation of MB is due to the presence of the deprotonated form of MB and the presence of ^•OH free radicals. Deprotonated form of MB, which is more prone to attack by both O₃ and ^•OH radicals. Moreover, OH⁻ ions react with O₃ to generate more ^•OH radicals. ^•OH radicals are more reactive than O₃ and have played a major role in degrading MB.

These results highlighted the role of pH in the efficient degradation of MB by O₃. The lower degradation efficiency of MB at acidic pH is attributed to the suppressed generation of ^•OH radicals, due to the limited availability of hydroxyl ions, which are required to decompose O₃. Moreover, the initiation of radical chain reactions is slower under acidic conditions.

It is concluded from the findings that pH plays a dominant role in the degradation of MB. pH affects both the chemical behavior of the MB and the generation of ROS.

3.7. Reaction Kinetics of MO Degradation

3.7.1. Degradation via Ozonation and Ferric Catalyzed Ozonation

The degradation of MO, an anionic azo dye, using ozonation and Fe³⁺/O₃ systems provides insight into the kinetics and mechanistic aspects of AOPs. As shown in Figure 6A, ozonation alone achieved complete MO degradation in 12 min. At the same time, the Fe³⁺/O₃ method reduced this time to 10 min, highlighting the catalytic efficiency of Fe³⁺ ions in accelerating the reaction. This enhancement is attributed to the ability of Fe⁺³ to generate additional reactive oxygen species, such as ^•OH through catalytic ozone decomposition.

As displayed in Figure 6B, the kinetic analysis revealed that the Fe³⁺/O₃ system follows second-order reaction kinetics, with the rate of degradation dependent on the concentrations of both Fe³⁺ and MO. The observed k value was 0.29 s⁻¹.

This research highlighted the importance of tailoring AOP conditions to the specific chemical properties of target pollutants for optimized degradation efficiency.

3.7.2. Effect of pH on the Degradation of MO

The findings presented in Figure 7 highlighted the effect of pH 5, 7 and 9 on the degradation rate of MO. The pH affected the degradation significantly. The highest degradation was observed at pH 5, where 100% degradation of MO was found within 5 min. At pH 5, MO exists in its deprotonated (anionic) form. The enhanced degradation at this pH is likely due to electrostatic interactions between the anionic dye and partially hydrolysed iron species acting as active catalytic sites. In addition, the possible formation of surface complexes may facilitate localized ^•OH radical generation, further accelerating the degradation process.

On the other hand, at pH 9, the degradation rate retarded, and 100% degradation was not achieved. At pH 9, the reduced degradation efficiency is attributed to extensive hydrolysis of iron ions, leading to the formation of ferric hydroxide precipitates that limit the availability of active catalytic sites. The formation of ferric hydroxide was also evident from the observed change in solution color, indicating catalyst instability under alkaline conditions.

These findings highlighted the crucial role of pH in MO degradation by the Fe³⁺/O₃ method.

These findings highlighted the influence of pH in the wastewater treatment processes for dyes like MO.

3.8. Product Identification

In this study, five products (P1, P2, P3, P4, and P5) were identified and characterized through MS/MS analysis performed on a liquid chromatography-mass spectrometry (LC-MS) system. These products were proposed based on the error calculation between the detected and calculated mass-to-charge ratios (m/z), as detailed in

Table 1. Product exploration through MS/MS analysis in positive mode.

Mode	Products	Retention Time (min)	Molecular Formula	Detected m/z	Calculated m/z	Error (ppm)
Positive	P1	9	C16H18ClN3OS	335.05	335.051	0.44
	P2	10.93	C9H14NO	152.143	152.144	−0.5
	P3	4.47	C8H14N	124.111	124.11	−2.56
	P4	6.01	C15H16ClN3OS	355.073	321.074	−0.5813
	P5	2.8	C14H12ClN3O2S	321.033	321.034	−0.531

.

The identification process relied on a combination of high peak intensity observed in LC-MS chromatograms and subsequent confirmation through MS/MS fragmentation analysis. Monohydroxylated product P1, which led to degradation to form P2 and P3, was proposed as the main product based on MS/MS spectra analysis, as displayed in Figures S6 and S7.

3.9. Hydroxylated Product (P1)

As displayed in Figure 8, P1, the primary hydroxylated product, was identified as the most prominent degradation product in the Fe³⁺/O₃ system.

A high-intensity peak in the LC-MS spectrum suggests that hydroxylation is the initial and dominant degradation pathway. Previous studies have also reported the generation of hydroxylated products during the degradation of pollutants by ozonation [47]. The detected m/z value of P1 showed minimal error when compared to the calculated m/z, confirming its identity with high accuracy. The MS/MS fragmentation pattern revealed characteristic fragments that support the addition of hydroxyl groups to the parent compound.

The degradation of P1 was proposed through MS/MS analysis, which identified the formation of P2 and P3. The m/z values of P2 and P3 had a small error, which confirms their identification. The reaction pathway for these degradation products explains the role of ^•OH radicals in the degradation of MB. The nitroso product (P4) was also suggested based on both MS/MS analysis. P4 suggests that nitrosation occurs as a pathway II under studied conditions. Nitroso products are reported to be generated during treatment with AOPs and are considered harmful byproducts [48]. In the presented study, the nitroso product is proposed as a minor product, highlighting the potential of the Fe³⁺/O₃ method to generate nitroso degradation products. A nitrated product (P5) was also identified, proposed by the addition of a nitro group to the MB. The nitrated products are also reported as harmful to the environment [47,48]. The LC-MS analysis suggested P5 as a minor product, with an m/z value with a small error. The MS/MS analysis further confirmed the identification of P5 as a nitrated product [49,50].

3.10. Proposed Degradation Pathways

The proposed degradation products suggest a main degradation pathway by hydroxylation, followed by cleavage into P2 and P3. The reaction pathway highlighted the oxidative potential of the Fe³⁺/O₃ method, driven by ^•OH free radicals. Nitroso (P4) and nitrated (P5) products are suggested as minor products based on their peak intensities.

These findings highlighted the efficiency of the Fe³⁺/O₃ method, where both hydroxylated and nitrated products play a crucial role in the degradation of MB. The small error between the detected and calculated m/z values for suggested degradation products is an indication of the reliability of the degradation pathways. The exploration of P2 and P3 from P1 highlights the capability of the studied method to degrade MB. The findings suggested that the Fe³⁺/O₃ method is efficient in degrading pollutants through hydroxylation, with subsequent degradation.

4. Conclusions

The findings of this study highlighted that the Fe³⁺/O₃ method is an efficient method for dye degradation. For MB, ozonation alone achieved 65% degradation within the maximum reaction time of 15 min, whereas 100% degradation was obtained with the Fe³⁺/O₃ method at the same time. On the other hand, for MO, ozonation alone resulted in 85% degradation within 15 min, while the Fe³⁺/O₃ method achieved 100% degradation in 10 min. Fe³⁺ ions have promoted the generation of ROS, which played a vital role in the degradation of MB. In addition, MO was 100% degraded in just 10 min with the Fe³⁺/O₃ method, highlighting the efficiency of this catalytic method for dye degradation.

The presented study also investigated the effect of pH on the degradation rate of both selected dyes. For MB, the lowest degradation rate was observed at pH 5, with the highest degradation occurring at pH 9. At pH 9, the higher concentration of ^•OH radicals facilitated the degradation of the MB. On the other hand, MO has the highest degradation rate at pH 5, where the dye exists in the deprotonated form, which likely enhances its degradation rate. At pH 9, the lowest degradation rate was observed.

MS/MS analysis confirmed the five degradation products. P1 (monohydroxylated product) degraded to form P2 and P3, while minor products, P4 and P5, were suggested as nitrated products. These findings highlighted the reaction mechanism and pathways involved in the ozonation and catalytic ozonation of synthetic dyes, contributing to a better understanding of the degradation process.

The 100% degradation of selected dyes within the reaction time highlights the potential of the studied method for large-scale applications. Moreover, information on the influencing factors can guide future research in optimizing reaction conditions. Future studies should focus on assessing the cost efficiency of the Fe³⁺/O₃ process in comparison with conventional treatment methods. Moreover, catalyst recovery and reusability are essential to ensure the practical applicability of this approach for large-scale wastewater treatment.