Kinetic Modelling of Aromaticity and Colour Changes during the Degradation of Sulfamethoxazole Using Photo-Fenton Technology

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this work, the degradation kinetics and mechanisms of sulfamethoxazole were systematically studied through the photo-Fenton technology. According to the experimental analysis, it was evident that the radical attack played the significant contributions for their superior degradation in relation to the turbidity, color, and aromatic intermediates. This is an alternative strategy for developing the effective and rapid treatment of antibiotic in natural water. However, there are few questions or concerns must be fully addressed in the revised version. My comments are listed below: 

1. In this work, you had to highlight the the unique advantages of photo-Fenton than typical Fenton or other advance oxidation method (i.e., PMS) in introduction and experimental part. 

2. The time nodes in Figure 1 (a) and (b) are not obvious, please collect the point to compare the rate constant in the first 20 minutes.

3. According to your results involved the cleavage of bonds and rings, it is necessary to identify the intermediates using LC-MS to propose the possible degradation pathways of SMX.

4. For a clear comparison, it was found that the concentrations of H₂O₂ has a significant effect on the degradation. How about the effect of initial pH, natural organic matter and water matrix?

5. Having gone through the paper, I just considered that the radical determination is not clear yet. It is better to provide the additional experiments for uncovering the detailed reactive mechanism and selection. 

6. From the consideration of practical application, how about the continuous reusability? 

7. Both the water colour and turbidity are discussed in article, please reconsider your title.

8. The uniform type of reference is required.

Comments on the Quality of English Language

No

Author Response

Author's Reply to the Review Report (Reviewer 1)

 

Comments and Suggestions for Authors

 

In this work, the degradation kinetics and mechanisms of sulfamethoxazole were systematically studied through the photo-Fenton technology. According to the experimental analysis, it was evident that the radical attack played the significant contributions for their superior degradation in relation to the turbidity, color, and aromatic intermediates. This is an alternative strategy for developing the effective and rapid treatment of antibiotic in natural water. However, there are few questions or concerns must be fully addressed in the revised version. My comments are listed below:

 

Coments 1. In this work, you had to highlight the the unique advantages of photo-Fenton than typical Fenton or other advance oxidation method (i.e., PMS) in introduction and experimental part.

 

Response 1.

 

We have added in the Introduction Section:

 

Various AOPs have been developed and applied for wastewater treatment, but the Fenton and photo-Fenton processes stand out as the most powerful, efficient, and cost-effective options. These methods are particularly effective in treating persistent pollutants, whether used alone or in combination with conventional or biological treatments [29]. The photo-Fenton process offers several key advantages over the conventional Fenton method. By utilizing light energy, it generates more hydroxyl radicals, leading to faster and more efficient pollutant degradation. It also enhances the regeneration of ferrous iron, reducing the need for additional chemicals like hydrogen peroxide and iron. Unlike traditional Fenton, which works best in acidic conditions, photo-Fenton can operate effectively at a wider pH range, including near-neutral conditions, making it more versatile and environmentally friendly. Based on this, studies reported in the literature show that the photo-Fenton process is an effective treatment both in the degradation of micropollutants and in bacterial inactivation [30], verifying that in the case of the elimination of SMX both the degradation performance as the speed at which it is eliminated, is much greater using photo-Fenton technology than using a conventional Fenton process [31].

 

 

29. Ameta, R.; Chohadia, A.K.; Jain, A.; Punjabi, P.B. Fenton and Photo-Fenton Processes. In Advanced Oxidation Processes for Waste Water Treatment; Ameta, S.C., Ameta, R., Eds.; Academic Press, 2018; pp. 49-87. https://doi.org/10.1016/B978-0-12-810499-6.00003-6.

 

30. Soriano-Molina, P.; De la Obra, I.; Miralles-Cuevas, S.; Gualda-Alonso, E.; Casas López, J.L.; Sánchez Pérez, J.A. Assessment of Different Iron Sources for Continuous Flow Solar Photo-Fenton at Neutral pH for Sulfamethoxazole Removal in Actual MWWTP Effluents. J. Water Process Eng. 2021, 42, 102109. https://doi.org/10.1016/j.jwpe.2021.102109.

 

31. Martínez-Costa, J.I.; Rivera-Utrilla, J.; Leyva-Ramos, R.; Sánchez-Polo, M.; Velo-Gala, I.; Mota, A.J. Individual and Simultaneous Degradation of the Antibiotics Sulfamethoxazole and Trimethoprim in Aqueous Solutions by Fenton, Fenton-like and Photo-Fenton Processes Using Solar and UV Radiations. J. Photochem. Photobiol. A Chem. 2018, 360, 95-108. https://doi.org/10.1016/j.jphotochem.2018.04.014.

 

 

Coments 2. The time nodes in Figure 1 (a) and (b) are not obvious, please collect the point to compare the rate constant in the first 20 minutes.

 

Response 2:

 

We have added graduation marks to the graph axes to improve readability.

 

 

Coments 3. According to your results involved the cleavage of bonds and rings, it is necessary to identify the intermediates using LC-MS to propose the possible degradation pathways of SMX.

 

Response 3

 

In this work we have focused on the kinetic study of colour and turbidity during the degradation of diclofenac using photo-Fenton technology. The main objective was to demonstrate that colour does not evolve randomly, and that it responds to a global measure of the formation of intermediates that generate colour in water. Precisely the novelty of this work is based on studying organoleptic parameters such as colour and turbidity, which are restricted by environmental legislation. Based on the kinetic studies carried out, a kinetic model has been proposed for the colour changes and the loss of aromaticity of the water depending on the operating conditions under which the treatment has been carried out, which allows estimating the minimum amount of reagents to obtain quality water suitable for discharge into natural channels. We take into consideration your proposal to carry out the study of reaction intermediates in future work using specific analytical equipment that allows us to determine the degradation routes followed in these processes.

 

 

Coments 4. For a clear comparison, it was found that the concentrations of H₂O₂ has a significant effect on the degradation. How about the effect of initial pH, natural organic matter and water matrix?

 

Response 4

 

We have carried out previous studies on the effect of initial pH on the degradation of sulfamethoxazole (Villota et al. 2024).

 

Villota, N.; Jankelevitch, S.; Lomas, J.M. (2024). Kinetic Modelling of Colour and Turbidity Formation in Aqueous Solutions of Sulphamethoxazole Degraded by UV/H₂O₂. Environ. Technol., 45, 349-359. https://doi.org/10.1080/09593330.2022.2109997.

 

When studying the degradation of sulfamethoxazole using the photo-Fenton process, in addition to the effect of H₂O₂ concentration, the initial pH, natural organic matter, and the water matrix can also significantly influence the degradation efficiency. The study of these parameters is included in the Basic and/or Applied Research projects (PIBA) project: “Degradation of emerging contaminants (drugs) that cause high turbidity and color in water using ultraviolet light and ozone”, PIBA 2023 1 0032 University of the Basque Country. Department of Education of the Basque Government, which we are currently developing in the Doctoral thesis carried out by the doctoral student Unai Duoandicoechea.

 

 

Coments 5. Having gone through the paper, I just considered that the radical determination is not clear yet. It is better to provide the additional experiments for uncovering the detailed reactive mechanism and selection.

 

Response 5

 

In the kinetic modeling that we propose in the article for the degradation of aromaticity and colour of waters, the radical factor that you mention does not intervene, therefore, we do not find the need to complete the experimental part to meet this objective. We are going to consider it in future work that we are going to carry out within the line of work that you suggest in Comment 3.

 

 

Coments 6. From the consideration of practical application, how about the continuous reusability?

 

Response 6

 

To apply it at an industrial level in a continuous and efficient manner, several aspects would need to be addressed to allow the reuse of the process. Some key strategies would be recirculation of reagents, optimization of UV light or radiation sources, design of continuous reactors, improving phase separation (filtration) and considering economic viability. Once the reaction is complete, it is important to separate the solid products and the catalyst. Sedimentation, filtration or centrifugation techniques can help recover any iron or solids generated, so they do not interfere with the continuous process. Minimizing the use of reagents, such as H₂O₂ and iron salts, can help reduce operating costs. Furthermore, the reuse of these materials can help make the process more profitable in the long term.

 

 

Coments 7. Both the water colour and turbidity are discussed in article, please reconsider your title.

 

Response 7

 

We have changed the title:

 

Kinetic modelling of aromaticity and colour changes during the degradation of sulfamethoxazole using photo Fenton technology

 

 

Coments 8. The uniform type of reference is required.

 

Response 8

 

We have corrected the references:

 

6. Dong, H.; Guo, X.; Yang, C.; Ouyang, Z. Synthesis of g-C₃N₄ by different precursors under burning explosion effect and its photocatalytic degradation for tylosin. Appl. Catal. B: Environ. 2018, 230, 65–76. https://doi.org/10.1016/j.apcatb.2018.02.044

 

11. Mertah, O.; Gómez-Avilés, A.; Kherbeche, A.; Belver, C.; Bedia, J. Peroxymonosulfate enhanced photodegradation of sulfamethoxazole with TiO₂@CuCo₂O₄ catalysts under simulated solar light. J. Environ. Chem. Eng. 2022, 10, 108438. https://doi.org/10.1016/j.jece.2022.108438

 

22. Poza-Nogueiras, V.; Gomis-Berenguer, A.; Pazos, M.; Sanroman, A.; Ania, C.O. Exploring the use of carbon materials as cathodes in electrochemical advanced oxidation processes for the degradation of antibiotics. J. Environ. Chem. Eng. 2022, 10, 107506. https://doi.org/10.1016/j.jece.2022.107506

 

34. Ji, Y.; Fan, Y.; Liu, K.; Kong, D.; Lu, J. Thermo activated persulfate oxidation of antibiotic sulfamethoxazole and structurally related compounds. Water Res. 2015, 87, 1–9. https://doi.org/10.1016/j.watres.2015.09.005

 

38. Villota, N.; Cruz-Alcalde, A.; Ferreiro, C.; Lombrana, J.I.; Esplugas, S. Changes in solution turbidity and color during paracetamol removal in laboratory and pilot-scale semicontinuous ozonation reactors. Sci. Total. Environ. 2023, 854, 158682. https://doi.org/10.1016/j.scitotenv.2022.158682

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The present manuscript “Water colour changes during the degradation of sulfamethoxazole using photo Fenton technology”. In general, the manuscript is well written and organized. However, it would be beneficial to integrate the corresponding discussion in the results section, include references, and compare results with the literature. The following comments must be addressed by the authors in order to be further considered accepted for publication in Catalysts after minor revision.

1.       Pag. 2 Line 69 homogenizes nomenclature with name or symbols iron or Fe.

2.       What is the factor that influences the color of the solution and how does it relate to SMX?

3.       Pag 2 Line 93 check syntaxis, spelling error.

4.       Pag 2 Line 67-93 homogenizes nomenclature photo-Fenton or Photo-Fenton.

5.       Pag 3 Line 99 which intermediaries? add references

6.       Pag 3 Line 98-102 improve the wording is not clear

7.       Pag 3 Line 3 Are intermediates or sub-products? What is the difference? Should the wording

8.       Pag 3 Line 112-113 which causes resistance in aromatic sub-products for degradation into hydroxyl radicals? Please add references.

9.       Pag 3 Line 108-121 Add references.

10.    Line 119 is degree or degradation percentage. ? What is the difference?

11.   Pag 3 Line 132-134 Should the wording, the wording should be in infinitive time.

12.   Pag 4 Line 163-169 as a suggestion, a diagram of the reaction mechanism occurring could be added.

13.    Pag 5 Line 177, check syntaxis.

14.   Pag 5 Line 170-192 add references.

15.   Why use 50.0 mg/L SMX as a standard?

16.   Is it possible to use different distinguishers in graph 4? since it is lost or enlarge or make an insert where the graphs are piled up?

17.   Line 310-322 Should be improved  writing

 

18.   A comparative analysis with the literature should be added where they use the photo penton process and highlight the importance of the work, or the originality of this.

Comments on the Quality of English Language

Should be checked for grammatical errors.

Author Response

Author's Reply to the Review Report (Reviewer 2)

 

Comments and Suggestions for Authors

 

The present manuscript “Water colour changes during the degradation of sulfamethoxazole using photo Fenton technology”. In general, the manuscript is well written and organized. However, it would be beneficial to integrate the corresponding discussion in the results section, include references, and compare results with the literature. The following comments must be addressed by the authors in order to be further considered accepted for publication in Catalysts after minor revision.

 

Coment 1.       Pag. 2 Line 69 homogenizes nomenclature with name or symbols iron or Fe.

 

Response 1.

 

We have changed iron for Fe²⁺

 

 

Coment 2.       What is the factor that influences the color of the solution and how does it relate to SMX?

 

Response 2

 

The factor that influences the colour of the SMX solution oxidized by the photo-Fenton process is the formation of intermediate compounds and the structural changes of SMX during the oxidation process. These intermediates are typically by-products of the breakdown of SMX under the influence of hydroxyl radicals, which are generated in the photo-Fenton process.

 

As SMX is oxidized, its molecular structure degrades, leading to the formation of oxidized intermediates, which often differ in chemical structure from the parent compound. These intermediates often contain aromatic or nitrogenous compounds, which can absorb light in the visible spectrum, giving the solution a distinct colour.

 

Some of the intermediates formed during oxidation may contain chromophores, which are molecular structures that absorb visible light. The colour observed in the solution is due to these chromophores. These intermediates can absorb different wavelengths of light compared to the original SMX molecule, leading to a change in colour. SMX contains an aromatic ring and a sulfonamide group, both of which can undergo oxidative degradation in the photo-Fenton process. As the aromatic ring is broken down and transformed into different intermediates, the colour of the solution may shift due to changes in light absorption properties. The intensity and specific colour observed depend on the types of intermediates present and their concentrations.

 

 

Coment 3.       Pag 2 Line 93 check syntaxis, spelling error.

 

Response 3

 

We have changed for “Figure 1 shows the kinetics of several of the parameters that indicate water quality when aqueous solutions of SMX are oxidized using photo-Fenton technology.”

 

 

Coment 4.       Pag 2 Line 67-93 homogenizes nomenclature photo-Fenton or Photo-Fenton.

 

Response 4

 

We have changed for photo-Fenton

 

 

Coment 5.       Pag 3 Line 99 which intermediaries? add references

 

Response 5

 

The references are indicated at the end of the paragraph, in which the explanation of the intermediates generated in the first stages of SMX oxidation is developed. [32,38]

 

 

Coment 6.       Pag 3 Line 98-102 improve the wording is not clear

 

Response 6

 

We have rewrite the sentence:

 

This is because in the early stages of the photo-Fenton reaction, hydroxyl radicals attack the aromatic ring of SMX, generating partially oxidized intermediates. In these early stages, the hydroxylation of SMX occurs mainly at the benzene ring and the amino group, producing several oxidized mono and dihydroxylated intermediates, such as 3-hydroxysulfamethoxazole, 5-hydroxysulfamethoxazole and the oxidized derivatives of isoxazole [32,38 ].

 

 

Coment 7.       Pag 3 Line 3 Are intermediates or sub-products? What is the difference? Should the wording

 

Response 7

 

Intermediates are compounds or products formed in the middle of a chemical reaction and by-products are secondary products formed unintentionally or as a side reaction.

 

We have eliminated by-products.

 

 

Coment 8.       Pag 3 Line 112-113 which causes resistance in aromatic sub-products for degradation into hydroxyl radicals? Please add references.

 

Response 8

 

We have added:

 

The presence of certain substituent groups on aromatic molecules such as nitro, sulfonate or carboxylate groups can affect the reactivity of hydroxyl radicals because they may induce inductive or resonance effects that further stabilize the aromatic structure, making radical attack less likely. These groups can also increase the electron density at certain positions on the ring, protecting the compound from oxidative attack [42]. Furthermore, during the oxidation process, oxidized intermediates are generated that have a very stable structure, such as quinones or aromatic acids that can be especially resistant [43].

 

42. Ren, M.; Sun, S.; Wu, Y.; Shi, Y.; Wang, Z.; Cao, H.; Xie, Y. The structure-activity relationship of aromatic compounds in advanced oxidation processes: A review. Chemosphere 2022, 296, 134071. https://doi.org/10.1016/j.chemosphere.2022.134071

 

43. Villota, N.; Jankelevitch, S.; Lomas, J.M. Kinetic Modelling of Colour and Turbidity Formation in Aqueous Solutions of Sulphamethoxazole Degraded by UV/H₂O₂. Environ. Technol. 2024, 45, 349-359. https://doi.org/10.1080/09593330.2022.2109997

 

 

Coment 9.       Pag 3 Line 108-121 Add references.

 

Response 9

 

We have added:

 

42. Ren, M.; Sun, S.; Wu, Y.; Shi, Y.; Wang, Z.; Cao, H.; Xie, Y. The structure-activity relationship of aromatic compounds in advanced oxidation processes: A review. Chemosphere 2022, 296, 134071. https://doi.org/10.1016/j.chemosphere.2022.134071

 

43. Villota, N.; Jankelevitch, S.; Lomas, J.M. Kinetic Modelling of Colour and Turbidity Formation in Aqueous Solutions of Sulphamethoxazole Degraded by UV/H₂O₂. Environ. Technol. 2024, 45, 349-359. https://doi.org/10.1080/09593330.2022.2109997

 

 

Coment 10.    Line 119 is degree or degradation percentage. ? What is the difference?

 

Response 10

 

We have changed degree for degradation level

 

 

Coment 11.   Pag 3 Line 132-134 Should the wording, the wording should be in infinitive time.

 

Response 11

 

We consider that the verbal tense used in the writing of the sentence that you indicate is appropriate in the sense used:

 

This occurs because, when SMX is attacked by hydroxyl radicals, its C-C and C-H bonds are broken, fragmenting the molecule into simpler compounds such as organic acids, aldehydes, alcohols, and other intermediate species.

 

 

Coment 12.   Pag 4 Line 163-169 as a suggestion, a diagram of the reaction mechanism occurring could be added.

 

Response 12

 

The study carried out is not focused on proposing a degradation mechanism for SMX, since the work is not focused on the identification and analysis of the degradation intermediates.

 

 

Coment 13.    Pag 5 Line 177, check syntaxis.

 

Response 13

 

We have corrected:

As these intermediates are mineralized into CO₂ and H₂O, the colour of the water gradually decreases.

 

 

Coment 14.   Pag 5 Line 170-192 add references.

 

Response 14

 

We have added:

 

43. Villota, N.; Jankelevitch, S.; Lomas, J.M. Kinetic Modelling of Colour and Turbidity Formation in Aqueous Solutions of Sulphamethoxazole Degraded by UV/H₂O₂. Environ. Technol. 2024, 45, 349-359. https://doi.org/10.1080/09593330.2022.2109997

 

 

Coment 15.   Why use 50.0 mg/L SMX as a standard?

 

Response 15

 

In all the works we have published on the degradation of emerging contaminants using AOPs we have worked with 50.0 mg/L because it is a concentration that allows us to be within the detection limits of the analytical equipment that we have used in the experimental part. We have also relied on the publications of other authors on the same topic to be able to compare the results we obtain.

 

 

Coment 16.   Is it possible to use different distinguishers in graph 4? since it is lost or enlarge or make an insert where the graphs are piled up?

 

Response 16

 

The SMX degradation kinetics are so fast that it is not possible to distinguish more precisely the experimental points.

 

 

Coment 17.   Line 310-322 Should be improved  writing

 

Response 17

 

We have changed:

 

The photo-Fenton treatment is capable of completely degrading the SMX load contained in the water. During oxidation, the aromaticity of water experiences a slight initial increase because partially oxidized intermediates are generated that contain greater aromaticity or that can even generate more aromatic structures. In addition, it is observed that a brown color is being generated in the water. The color is caused by some of the aromatic intermediates, such as quinones or polycyclic compounds, which are chromophoric in nature. It can also be caused by fragments of organic molecules reacting with each other forming condensation products. On the other hand, it must be considered that SMX contains a heterocyclic ring (containing sulfur and nitrogen) that can generate intermediate products with chromophore properties. Simultaneously with the color, turbidity is also generated in the water, which can be caused by the precipitation of iron hydroxides, the formation of colloidal particles and insoluble organic intermediates, which remain suspended in the water. As the photo-Fenton reaction proceeds, hydroxyl radicals continue to attack the aromatic intermediates, causing the opening of the benzene rings or the complete oxidation of the aromatic compounds to simpler products.

 

 

Coment 18.   A comparative analysis with the literature should be added where they use the photo penton process and highlight the importance of the work, or the originality of this.

 

Response 18

 

We have added in the introduction section:

 

Various AOPs have been developed and applied for wastewater treatment, but the Fenton and photo-Fenton processes stand out as the most powerful, efficient, and cost-effective options. These methods are particularly effective in treating persistent pollutants, whether used alone or in combination with conventional or biological treatments [29]. The photo-Fenton process offers several key advantages over the conventional Fenton method. By utilizing light energy, it generates more hydroxyl radicals, leading to faster and more efficient pollutant degradation. It also enhances the regeneration of ferrous iron, reducing the need for additional chemicals like hydrogen peroxide and iron. Unlike traditional Fenton, which works best in acidic conditions, photo-Fenton can operate effectively at a wider pH range, including near-neutral conditions, making it more versatile and environmentally friendly. Based on this, studies reported in the literature show that the photo-Fenton process is an effective treatment both in the degradation of micropollutants and in bacterial inactivation [30], verifying that in the case of the elimination of SMX both the degradation performance as the speed at which it is eliminated, is much greater using photo-Fenton technology than using a conventional Fenton process [31].

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

All concerned questions were resolved. The paper can be published in the journal.