Enhanced Decolorization and Mineralization of Acid Violet 19 Dye by Potassium Ferrate (VI)

No

Reviewers Comments

Response

1

Lines 62-63: The purity of AV19 and Potassium Ferrate (VI) must be stated.

We have added the purity information in the Materials and Synthesis sections. AV19 dye from Samchun Chemicals was used with ≥98% purity. The synthesized Potassium Ferrate (VI) used in this study had a purity of 93% at line 81.

2

Lines 70-76: These sentences are meant as a guide for authors to prepare their manuscripts correctly. Why thses sentences appeared here? There is no explanation of the synthesis of potassium ferrate (VI) in the text

We appreciate the reviewer’s observation. The unintended template sentences have been removed entirely. A complete synthesis procedure for Fe (VI) has now been added and clearly described based on the modified wet-chemical method at lines 87 – 103.

3

Line 88: How did the author determine the value of Molar absorptivity of Fe(VI)?

The molar absorptivity value (ε = 1070 M⁻¹·cm⁻¹ at 505 nm) was obtained from a calibration curve created using serial dilutions of freshly Fe (VI). This explanation and justification have now been added to the manuscript at lines 91 – 92.

4

Line 98: Where is the equation, as stated in this line?

Thank you for pointing this out. The equation used to calculate Fe (VI) concentration based on the Beer–Lambert Law is shown as Equation (1) at line 107, and the corresponding purity equation is reintroduced as Equation (2) at line 115.

5

The experimental procedure of AV19 decolorization is still unclear. The procedure must be explained in detail so that others can repeat the experiment.

From Section 2.3. Batch Reactor Preparation change to 2.3. Experimental Procedure. Then, the entire experimental procedure has been rewritten and expanded for full reproducibility. Details added include: AV19 initial concentration (0.34 mM), pH adjustment protocol, Fe(VI) dosing, reaction temperature control (15–65°C), sampling intervals (0.5–12 min), quenching method, λmax measurement at 547 nm, TOC filtration and analysis, and reactor schematic (Figure 2) at lines 116 – 140.

6

The equation for the plots in Fig. 3b is not found. Please add the equation in the text. What is the order of the decolorization reaction (first order or second order)?

We have now added the kinetic model equation used for analysis. The regression equations for each pH are now included in the text. Kinetic evaluation of the experimental data showed that the second-order kinetic model provided a better fit (R² = 0.9859) than the first-order model (R² = 0.9648). Therefore, the decolorization of AV19 by Fe (VI) follows second-order kinetics, consistent with previously reported Fe (VI) oxidation of organic contaminants at lines 200 – 207.

7

Line 196: The error bars, as stated in the text are not found in the figure.

Thank you for your comments. Please take a careful look at the figures. Error bars representing the standard deviation of triplicate measurements (n = 3) have been added to Figures 3, 4, and 5. In several points, the error bars are not visually distinguishable because the variability among replicates was minimal (standard deviation < marker size). As a result, error bars appear prominently only at data points with slightly higher variation. All error bars are included in the figures, even if they overlap with the markers.

8

The equation for the reaction between AV19 and Fe (VI) needs to be added in the discussion section.

The generalized oxidation reaction between Ferrate (VI) and organic dyes has been added in the Results section at lines 305 - 306, and provides a complete and detailed explanation about the degradation mechanism in the Discussion section at lines 517 – 567.

9

Line 259: How did the author determine the Ea value?

The activation energy (Eₐ = 47.78 kJ/mol) was determined using the Arrhenius equation from the slope of ln(kₐₚₚ) vs. 1/T, constructed using kinetic rate constants at 15–65°C on Figure 5b at line 235. The complete Arrhenius equation and calculation method have been added at lines 254 – 260.

10

The EDS spectra that show all the peaks present on the Fe (VI) should be attached.

The complete EDS spectra for AV19 before and after treatment with Fe (VI), showing elemental   peaks, have been added as Supplementary Figure S1 (Supplementary Materials).

11

The FTIR and XPS spectra for Fe(VI) (before and after reaction with AV19) should be presented. 

Thank you for this valuable suggestion. We fully agree that FTIR and XPS can provide additional structural information on Fe(VI) and its transformation products. However, in our laboratory, we do not currently have access to FTIR or XPS equipment, and due to a limited research budget, we are unable to outsource these analyses at this time. Although FTIR and XPS could not be performed, we have strengthened our characterization using the available analytical techniques in our laboratory:

1.      UV–Vis spectroscopy was used to confirm Fe (VI) purity and monitor its characteristic absorbance at 505 nm.

2.      FE-SEM and dual EDS mapping were added to show morphological changes and elemental redistribution before and after Fe (VI) treatment.

3.      LC–MS/MS analysis was used to identify degradation intermediates and confirm the structural transformation of AV19.

Together, these techniques provide a comprehensive understanding of Fe (VI) behavior during oxidation and sufficiently support the mechanistic interpretation presented in the manuscript.

No

Reviewers Comments

Response

1

Figure 1 & Figure 2. The content in the figure is hard to read compared with main manuscript. The authors should consider rearrange the figure and content table to make sure it’s easy and clear to read.

Thank you for your comments. We have just made the Figure 1 captions and explanation in the above and below both figures at lines 87 – 106. Especially for Figure 2, we have rearranged the figure and content table to make sure it’s easy and clear to read at lines 116 – 165.

2

In the introduction, the author claimed that ‘the optimal conditions for AV19 oxidation are a pH of 7.0, a molar ratio of Fe (VI): AV19 of 1.7:1.’ But results section later uses 1:5, and figures report ratios as 1:1 → 1:5. The abstract says mineralization ≈31%, but earlier text  claims “molar ratio determined to be 5:1”. What is the actual optimized molar ratio? Why do three inconsistent values (1.7:1, 5:1, 1:5) appear across the manuscript?

We have corrected all inconsistencies and confirm that the optimized molar ratio is AV19:Fe (VI) = 1:5, which yielded the highest decolorization and mineralization performance. Earlier values (1.7:1 and 5:1) were removed as they originated from preliminary calculations or typographical errors.

3

In the degradation process, the author claimed three sequential stages: oxidative bond cleavage, ring opening, and mineralization. I have concern regarding the ring-opening process. The author mentioned they used LC/MS/MS to detect the product chemicals to prove the opening ring process. Do authors have quantify those chemicals concentration to further support the ring-opening process?

Thank you for raising this important point. In this study, LC–MS/MS was employed primarily for qualitative identification of degradation intermediates formed during AV19 oxidation, rather than for quantification. The chromatographic and mass spectral data enabled identification of multiple ring-opened products, including low–molecular–weight carboxylic acids, sulfonated aromatic fragments, and amine-containing species, confirming that oxidative bond cleavage and subsequent ring opening occurred during the reaction.

However, quantitative LC–MS/MS methods were not performed due to instrument limitations and the absence of calibration standards for all detected intermediates. We acknowledge that quantitative measurement of intermediate concentrations would provide additional evidence to support the proposed degradation pathway.

To address this, we have added a statement in the manuscript clarifying that LC–MS/MS analysis was qualitative, and that future work should include quantitative profiling of intermediates to validate kinetic relationships and mechanistic pathways more comprehensively.

4

In reference citation part, Reference 4&5 I think the format are different with others. Especially for website link. Please fix it.

Thank you for bringing this to our attention. We have revised References 4 and 5 to ensure complete consistency with MDPI reference formatting guidelines. The formatting of author names, publication year, article titles, journal information, and URLs has been standardized. The website links have been updated, properly formatted, and accompanied by access dates. The entire reference list has been reviewed to ensure accuracy, uniformity, and compliance with MDPI style requirements.

No

Reviewers Comments

Response

1

Supplement detailed characterization data of synthesized Potassium Ferrate (VI). Beyond purity (93%), provide information such as crystal structure (XRD), surface morphology (FE-SEM with higher magnification), and elemental composition (EDS) to confirm the oxidant’s structural integrity.

Thank you for this suggestion. Due to instrumental limitations and funding constraints, XRD analysis could not be performed in this study. To strengthen material characterization, we have added high-magnification FE-SEM images and EDS spectra of AV19 after treatment with Fe (VI), shown in Figures S1 and S2. However, for synthesized Fe (VI), we are only using SEM images in Supplementary Figure S3. FE-SEM Dual EDS spectra micrographs confirm the crystalline morphology. At the same time, EDS results verify the elemental composition (Fe, O, K), supporting the purity and structural stability of the synthesized Fe (VI). These results have been incorporated into the revised manuscript. We have also added a statement acknowledging the absence of crystallographic data and suggesting XRD analysis in future work.

2

Clarify the inconsistency in molar ratio descriptions. The abstract mentions "optimal molar ratio of Fe(VI):AV19 = 1.7:1" but later states "molar ratio of 5:1" for mineralization; specify the corresponding performance indicators (decolorization vs. mineralization) and explain the rationale for different optimal ratios.

We have corrected all inconsistencies and confirm that the optimized molar ratio is AV19:Fe (VI) = 1:5, which yielded the highest decolorization and mineralization performance. Earlier values (1.7:1 and 5:1) were removed as they originated from typographical errors.

3

Strengthen the degradation mechanism analysis. The proposed pathway mentions ·OH radicals, but no experimental evidence (e.g., radical quenching experiments) is provided to confirm their role. Supplement tests with scavengers (e.g., tert-butanol) to verify the contribution of ·OH and Fe(VI) species.

Thank you for this insightful comment. We agree that radical quenching experiments could further elucidate the contribution of ·OH radicals. However, scavenger experiments were not conducted in this study due to time and resource limitations. In the revised manuscript, we have removed speculative statements regarding ·OH and focused the mechanistic discussion on direct Fe (VI) oxidation pathways and the identification of transformation reaction products, and known Fe (VI) reaction mechanisms reported in the literature. A recommendation for future work involving radical scavenging assays has been added.

4

Improve the comparative analysis with existing technologies. Expand Table 1 to include key parameters of Fe (VI)-based AV19 degradation from other studies (e.g., catalyst dosage, reaction time, decolorization/mineralization efficiency) to highlight the advantages of this work.

Thank you for the suggestion. Table 1 has been expanded to include a comparative summary of key operational parameters reported in recent Fe (VI)-based studies (oxidant dosage, reaction time, pH, decolorization). This revision highlights the higher decolorization efficiency achieved in the present work, with lower oxidant dosage and shorter reaction time. Relevant references have been added to support the comparison at line 454.

5

Supplement stability and reusability evaluations of Potassium Ferrate (VI). Investigate the oxidant’s performance over multiple cycles and characterize its structural changes after reaction (e.g., XRD, FTIR) to assess its practical application potential.

Thank you for this insightful comment. We agree that stability, reusability, and structural evolution of Fe (VI) are critical aspects for practical application. Due to limited sample availability and instrumental constraints, multi-cycle performance tests and crystallographic analyses (XRD and FTIR) were not conducted in this study.

To partially address this point, we have included dual EDS mapping and high-magnification FE-SEM micrographs in Supplementary Figures S1 and S2, showing elemental and morphological changes in AV19 before and after Fe(VI) treatment, and FE-SEM images of synthesized Fe(VI) in Supplementary Figure S3.

The EDS spectra (Fig. S1) reveal a shift in elemental composition following oxidation, with increased Fe and O signals and the disappearance/reduction of S-containing peaks, suggesting oxidative transformation of sulfonated dye species. FE-SEM images (Fig. S2) show a transition from large, plate-like AV19 aggregates to fragmented, porous microstructures, consistent with the breakdown of the aromatic chromophore. Additionally, the FE-SEM images of Fe (VI) (Fig. S3) illustrate a rough, granular morphology typical of high-surface-area oxidants and support the structural characteristics of the synthesized material.

A recommendation for future work on reusability testing, structural tracking using XRD/FTIR, and long-term performance evaluation has been added to the revised manuscript at lines 591 – 594.

6

Clarify the mineralization efficiency data. The abstract reports 31% mineralization, while the results section mentions ~32%—standardize the data and explain why mineralization is significantly lower than decolorization (e.g., aromatic ring stability, intermediate accumulation).

Thank you for pointing this out. The value has been standardized to 31% mineralization across the manuscript. The lower mineralization relative to decolorization is attributed to the persistence of aromatic intermediates and to partial oxidation of aromatic rings. The completed explanation has been added to lines 466–473.

7

Optimize the toxicity assessment. The TEST software results show conflicting developmental toxicity for IP1 and IP2 (above threshold) and others (below threshold); supplement experimental toxicity data (e.g., aquatic organism tests) to validate the computational predictions.

We appreciate this thoughtful suggestion. Due to instrumental and time constraints, experimental toxicity assays (e.g., aquatic acute toxicity tests) were not conducted in this study. The toxicity assessment relies on computational predictions using EPA TEST, which provides toxicity estimates based on QSAR algorithms. The observed variability among intermediates has been acknowledged and interpreted as an indication that some degradation products may still pose ecological risks, reinforcing the need for further degradation steps. A limitation statement and recommendation for future experimental validation have been added.

8

Some papers should be cited (Carbon Neutralization, 2024; 3: 737-767; EcoEnergy, 2024; 2(1): 83-113; EcoEnergy, 2024; 2(3): 489-502; Tungsten, 2024;6(2):447-453; Tungsten, 2024;6(2):447-453.)

Thank you for the recommendation. The cited papers have been reviewed and integrated into the revised manuscript where relevant at Introduction lines 56 – 58 and Discussion at lines 484 – 487. But we couldn’t access the latest literature (Tungsten,2024;6(2):447-453). Unfortunately, our institution does not currently subscribe to these journals, and we were unable to obtain full access to the cited articles through our library services. As a result, we could not extract sufficient detail to incorporate or contextualize the findings in our manuscript accurately.

No

Reviewers Comments

Response

1

Please use only one abbreviation for Potassium Ferrate (VI) throughout the text.

Thank you for pointing out this inconsistency. We have revised the entire manuscript to use a single consistent abbreviation, Fe (VI), for Potassium Ferrate (VI). All previous variations have been standardized.

2

The introduction should clearly articulate the innovation, and to highlight the uniqueness of this research, including its potential impact on this field of research.

We appreciate this suggestion. The Introduction has been revised to emphasize the novelty of combining performance evaluation, LC–MS/MS mechanistic profiling, FE-SEM/EDS structural characterization, and computational toxicity assessment of degradation products under varying operational conditions. Additionally, the environmental relevance and potential impact of Fe (VI) oxidation for rapid treatment at lines 52 – 73.

3

Suggestion is to remove lines 70-76 from the 2. Synthesis Potassium Ferrate (VI) part.

Thank you for the suggestion. These lines, which contained instructional guidance unrelated to the experimental work, have been removed. Any errors about it, and we have already made changes with the exact procedure for Synthesis Fe (VI) clearly at lines 87 – 107.

4

Please add which TOC analyzer was used in this research.

The requested information has been added. The manuscript now states that total organic carbon was measured using a TOC-L analyzer (Shimadzu, Japan) at line 143.

5

How the required pH of the solution was established at the beginning of each degradation/decolorization experiment of AV19? The pH of the solution often changes during degradation. What was the pH of the solutions after dye decolorization was complete?

We thank the reviewer for this important question. The initial pH was adjusted using 1.0 M NaOH and HCl solutions, and the reaction was monitored continuously using a calibrated pH meter. The pH experiment section has three variations of conditions, such as pH 5 (acidic), pH 7 (neutral), and pH 10 (basic). For example, in the pH 7 experiment, the initial pH of AV19 is 6.4–6.6. After that, we add the Fe (VI) around 3 – 4 seconds, beginning the pH immediately increasing to pH  10.1 – 10.5. Then, to neutralize the pH again, we immediately add 1 drop of HCl, and the pH will remain stable for about 12 minutes at 7.0–7.4.

6

What about temperature stability of AV19? In this manuscript, focus is on the temperature stability of Fe (VI), but the fact is that AV19 stability also decreases with temperature increase. Considering that the decolorization experiments were done at 45°C, please explain the stability of the color at 45°C.

Thank you for this insightful comment. While Fe (VI) degradation kinetics were strongly temperature-dependent, AV19 stability also decreases at elevated temperature, with partial thermal fading reported above 40°C in the absence of oxidants. However, preliminary control experiments showed that thermal degradation of AV19 at 45°C without Fe(VI) was negligible (<3% absorbance loss over 12 min), confirming that decolorization at this temperature was predominantly oxidation-driven.

7

Units need to be uniformly used, in accordance with the guidelines for this journal. For example, in the lines 88 and 92 M-1cm-1 have to be corrected in M^-1cm^-1.

Thank you. We have carefully checked and standardized all units throughout the manuscript to comply with journal guidelines. All instances of M-1cm-1 were corrected to M⁻¹ cm⁻¹, along with similar unit formatting adjustments at line 108.

8

In line 105 please add concentration of AV19 solution.

The AV19 dye solution has now been added. The manuscript specifies that AV19 solutions were prepared at 0.34 mM at line 122.

9

Please improve the quality of Figure 10, to show the results more visible. The existing one do not show the elemental composition of the analyzed samples.

Thank you for this observation. Figure 10 has been replaced with a higher-resolution version. The revised figure is now more readable and informative at line 358.

10

In the Conclusion part, please add, in one paragraph, the future research directions based on the findings, including industrial applications of this approach.

We appreciate this suggestion. A new paragraph has been added to the Conclusion outlining future work, including structural characterization, multi-cycle evaluation, hybrid AOP development, toxicity verification, and potential industrial applications such as wastewater treatment in the textile and printing industries at lines 591 – 594.

11

Please check that all references are cited in the same format.

Thank you. The entire reference list has been reviewed and reformatted in accordance with MDPI guidelines.

No

Reviewers Comments

Response

1

Please ensure the consistent symbols of AV19 and Fe(VI) are used in the manuscript, not AV 19 or Fe (VI) (no spacing)

Thank you for this valuable comment. We have carefully revised the entire manuscript to ensure consistent notation of the dye and oxidant throughout the text, figures, and tables. Specifically, all instances of “AV 19” were corrected to “AV19”, and all instances of “Fe (VI)” were corrected to “Fe(VI)” (i.e., no spacing).

2

Line 431. The correct symbol is Ea, not Ea

Thank you for pointing this out. We have corrected the symbol for activation energy in the manuscript. The notation was revised from Ea to Eₐ (subscript “a”) at Line 431 and checked for consistency throughout the manuscript.

No

Reviewers Comments

Response

1

The current version of the manuscript is acceptable for publication.

We sincerely thank Reviewer 3 for the careful evaluation of our revised manuscript and for the positive feedback. We greatly appreciate the reviewer’s confirmation that the current version is acceptable for publication. This encouraging comment supports the quality and completeness of the revisions made.