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

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, the author investigated the efficiency of ozonation and catalytic ozonation for the degradation of MB and MO. Degradation tests demonstrated that ozonation alone led to a degradation of dyes and the introduction of Fe⁺³ significantly improved the degradation rate, which is attributed to the ability of Fe⁺³ to generate additional reactive oxygen species, such as •OH through catalytic ozone decomposition. In addition, the effect of pH on the degradation rate and possible degradation pathways are also comprehensively investigated. However, there existed some problems in this paper, these problems must be considered and addressed before acceptance:

1. The purity of the chemicals should be given in the experimental section.
2. The uploaded Supplementary Materials are incorrect.
3. The annotation of superscripts and subscripts are not standardized (e.g. Line 156). Please modify it according to the journal's formatting requirement.
4. In the Line 255, the k value is missing.
5. In the Line 163-188, the text description does not match the Figure 3.1.

Author Response

"Please see the attachment".

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

After reading the manuscript, the reviewer suggests the following issues should be addressed to improve the quality of the manuscript.

(1) "Fe+3" is suggested to be revised as "Fe3+".

(2) In Fig. 3.2b, the y-axis "Ln(C)" could be "Ln(C/C0)"? Please check and confirm it is correct.

(3) The authors are suggested to provide the first-order and second-order kinetic equations so that readers can clearly understand the degradation kinetic behaviors of the dyes.

(4) As the authors emphasize the •OH free radicals are the main radicals causing the dye degradation, experimental measurement of the •OH free radicals is necessary.

(5) The authors are suggested to investigate the effect of Fe3+/O3 ratio on the degradation efficiency of the dyes.

(6) The authors are suggested to draw a diagram elucidating the synergistic effect between Fe3+ and O3 in the dye degradation.

(7) As there are many methods that can be used to eliminate the environmental pollutants such as various advanced oxidation technologies (e.g., Appl. Surf. Sci. 679 (2025) 161275, J. Environ. Chem. Eng. 12 (2024) 114405), the authors are suggested to summarize these methods based on the papers and point out the advantage or uniqueness of the adsorption method developed in their work.

 

 

Author Response

"Please see the attachment."

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript addresses the degradation of synthetic dyes, Methylene Blue (MB) and Methyl Orange (MO), from wastewater using ozonation and Fe³⁺ catalyzed ozonation processes. The study investigates the effect of pH, degradation kinetics, and the MB degradation. The manuscript deals with an important topic in environmental remediation and presents promising results, however, I have several comments and suggestions for improving the manuscript:

1. The keywords does not adequately reflect the main focus of the study. Terms such as “Oxidants” “Degradation” and “Textile” are too broad. Please consider more specific keywords like: Catalytic Ozonation, Fe³⁺ Catalyst, Methylene Blue, Methyl Orange, Synthetic Dyes, etc.
2. Consider revising the phrase “materials zeolites” (Line 58) to “zeolites”, “materials such as zeolites”, or similar.
3. There is a typographical error in the dye name (Figure 1): “Methyle orange” should be corrected to “Methyl orange”.
4. When referring to ferric ions, the oxidation state should be consistently written as Fe³⁺ throughout the manuscript (not Fe⁺³).
5. Lines 91 and 110 refer to supplementary material, but it has not been provided with the manuscript.
6. The manuscript states: “For the ferric catalyzed ozonation (Fe⁺³/O₃) the desired concentration of Fe₂(SO₄)₃) which ranged from 1- 5 ppm, was also added to observe the effect on the reaction.”, yet only results for 3 ppm are presented. If other concentrations were not tested, please revise the sentence.
7. There is a typographical error in Line 97: the chemical formula “Fe₂(SO₄)₃)” should be corrected to “Fe₂(SO₄)₃”.
8. In Section 2.4, both LC-MS and LCMS are used. Please unify the terminology throughout the manuscript.
9. Figures 2, 3.3, 3.4, 3.5, and 3.6 show experiments over varying time intervals, up to 15 minutes. Therefore, it is not accurate to refer to the reaction time as “15 min”, this is the maximum time, not a fixed value. Please correct this in the Figure captions.
10. Subsection 3.4: The manuscript reports a first-order rate constant as –0.195 s⁻¹. Rate constants are typically positive. Please explain how a negative value was obtained or correct the text.
11. Line 253 contains the incomplete sentence: “The observed k value was s”. Please revise.
12. Figure 3.7 caption: The figure shows the data for MB only. Please explicitly state this in the figure caption.
13. Why was the degradation pathway only investigated for MB and not for MO as well?
14. Conclusions should be rewritten to support the presented results. The sentence “In addition, MO was 100% degraded in just 10 min with the Fe³⁺/O₃ method, highlighting the efficiency of this catalytic method for the degradation of MB.” should be rewritten.

Author Response

"Please see the attachment"

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The work is interesting and useful from a technical point of view. However, I would like to suggest important revisions that should be made before acceptance.

1. The abstract and conclusion are too general. Please expand them with clearer details on key results, highlight the main findings, and emphasize the novelty and practical significance of the study.
2. The novelty and significance of this study should be clearly highlighted in the text.
3. LC–MS pathways are proposed, but no chromatograms or spectra are shown. Please provide LC–MS/MS graphs to support intermediate identification.
4. All figures are low resolution, improve figure quality, clarity, and labeling for better readability.
5. The discussion in this article still needs to be deepened. Here are some recent articles closely related to catalytic activity that might enrich your content. (1) Yu et al., 2025 (doi.org/10.1016/j.jece.2025.116048) demonstrates pollutant degradation via advanced oxidation mechanisms, similar to catalytic ozonation. (2) Chen et al., 2023 (doi.org/10.1016/j.seppur.2023.124252) highlights combined reductive and oxidative pathways in AOPs, aligning with your Fe³⁺/ozone system. 
6. Comparison in the tabulated should be provided. Besides, the advantages of this work over previous works should be addressed.
7. There are many careless mistakes, the authors should check the manuscript word by word and the paper must be properly organized.

Author Response

"Please see the attachment."

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

Recommendations to improve the manuscript.

Page 1, Line 36

This statement is inaccurate and misleading.

Typically, synthetic dyes do not form chemical bonds with fiber macromolecules, only reactive dyes do so, via covalent bond formation. Additionally, mordant dyes (metal-complex dyes) interact through coordination with metal ions bound to the fiber.

It is recommended to revise the sentence for accuracy.

Also, "fiber molecules" should be corrected to "fiber macromolecules", as fibers are polymeric in nature.

 

Page 3, Line 90

The supplementary material (containing Figures S1, S2, and S4) has not been uploaded or included in the submitted file. Without access to these figures, it is not possible to judge the results.

Please ensure all supplementary files are provided.

 

Page 4, Line 140

The term "fenton" should be capitalised as "Fenton", since it refers to the Fenton reaction, named after Fenton who discovered it over a century ago.

All instances of "fenton" in the manuscript must be corrected to "Fenton".

 

Page 5, Figure 3.1

The legend labels for the two dyes, “MB(O3)” and “MO(O3)”,  are identical in style and color, making it difficult to distinguish between them without close inspection.

To improve clarity, consider using different line styles, markers, or colors for MB and MO, and include a clearly labeled legend.

Page 6, Line 199 (also applies to Figure 3.2)

A negative rate constant is physically impossible in kinetic analysis. Rate constants for first-order reactions must be positive.

This suggests an error in data fitting or sign convention during linear regression

Please analyse the kinetic data and correct the reported value. The negative sign should be removed unless it results from incorrect plotting.

 

Page 6, Lines 201–206

The claim of second-order kinetics is not well-supported by the methodology. The authors base this conclusion solely on the fit of dye concentration decay, without measuring the catalyst concentration over time. In catalytic reactions, apparent "second-order" behavior may arise from pseudo-first-order conditions, especially when catalyst concentration is constant.

Furthermore, the linear regression in Figure 3.3 shows a noticeable scatter of data points, and the standard deviations of the slope and intercept are likely high. While it is true that standard errors of regression coefficients are often omitted in chemical literature, fundamental statistical principles should not be ignored, especially when drawing conclusions about mechanism of reaction.

The first-order model for ozonation alone appears reasonable, as it reflects the dependence on a single reactant (dye) under excess ozone. However, the assignment of true second-order kinetics requires more rigorous justification, including control experiments and error analysis.

 

Page 7, Line 225

While methylene blue (MB) carries a permanent positive charge on its heterocyclic nitrogen (making it a dye of cationic nature), it is more accurate to classify it as a basic dye, not simply a "cationic" one.

So, names “cationic” and “basic” is a part of dye classification according to Colour Index in spite of their similar charge.

 

Page 9, Figure 3.5

It is recommended to report the regression coefficient and rate constant with at least two or three significant figures.

 

Page 9, Lines 262–269

This statement is scientifically incorrect.

Methyl Orange (MO) undergoes a well-known pH-dependent structural transformation.

At pH 5, MO exists entirely in its deprotonated (anionic) form, not protonated. Therefore, attributing higher reactivity to a "protonated form" at pH 5 is chemically invalid.

Instead, the enhanced degradation at pH 5 is likely due to:

Electrostatic interaction between anionic MO and partially hydrolysed iron species, which act as active catalytic sites.

Formation of a complex that promotes localised OH radical generation.

Protonation of aromatic amines typically reduces electron density, making them less reactive toward electrophilic radicals like OH-radicals. Hence, protonation generally decreases, not increases, reactivity in AOPs.

 

Page 9, Lines 270–275

While the decrease in degradation efficiency at pH 9 is plausible, the explanation requires refinement:

At pH 9, MO remains in its reactive anionic form, so reduced reactivity cannot be attributed to deprotonation.

Instead, iron ions undergo extensive hydrolysis, forming iron hydroxide, which may precipitate and reduce available catalyst.

The authors do not report any observation of iron precipitation, which would be expected at pH > 8. This omission raises questions about catalyst stability.

For MB, the lower degradation at acidic pH is better explained by:

Suppressed OH-radicals generation, due to lack of hydroxyl-ions initiated ozone decomposition.

Slower initiation of radical chain reactions in acidic media, a well-established phenomenon in advanced oxidation processes (AOPs).

Comments on the Quality of English Language

Minor editorial comments

Line 11. ...study investigated the efficiency of ozonation...
Recommended “we investigated”, or “authors investigated”, or “this study examined”

Line 67, 114 – similar

Line 14, led to a degradation
Recommended “led to a degradation”

Line 16, …improved the degradation rate…
Recommended “…enhanced BM degradation…”

Line 18, …ions were found to increase the production of hydroxyl radicals…
Recommended “…catalyse ozone decomposition, enhancing the generation of hydroxyl radicals…"

Line 23, For LC-MS analysis, MB degradation by the Fe+3/O3 method was investigated.
Sounds hard. Recommended “LC-MS analysis was performed to investigate MB degradation products formed during Fe3+/O3 treatment.”

Line 196, 3 g/h/L
Please clarify the unit here and everywhere in the text

Line 353, The Fe+3/O₃ method is highly recommended for wastewater treatment.
This conclusion is overstated. No data are provided on cost comparison and catalyst recovery.

 

 

Author Response

"Please see the attachment"

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript could been accepted for publication. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed the issues raised by the reviewer, and the manuscript could be accepted for publication in Sustainability. 

Reviewer 3 Report

Comments and Suggestions for Authors

I asppreciate the acceptance of my suggestions and the thorough responses to all the questions raised. The effort invested in addressing the suggestions and further improving the quality of the paper is acknowledged.

Reviewer 4 Report

Comments and Suggestions for Authors

I have carefully reviewed the revised manuscript and am satisfied with the improvements. The authors have addressed the comments effectively, and the manuscript has been significantly enhanced. Therefore, I recommend its acceptance for publication.

Reviewer 5 Report

Comments and Suggestions for Authors

I am satisfied by revisions of the text in the corrected manuscript