Photocatalytic Degradation of Organic Dyes from Clinical Laboratory Wastewater
Round 1
Reviewer 1 Report
The manuscript submitted by Ramírez Franco et al. reports about the usage of natural photocatalytic minerals for treatment of clinical wastewater containing laboratory dyes. The topic is interesting and fits with the scope of the chosen Journal Water. The manuscripts, however, lacks in insufficient description of methods used and poorly description of their results. The discussion is a mixture of results and discussion making it unfavorable to read. Independently, I believe that the results are worth to be published after an intensive revision and improvement of the whole manuscript. There are several issues to be revised before accepting the manuscript for publication. Please find my recommendations below.
Important issues to be changed or revised:
1. Overall, the introduction is well written. However, please adjust numbering of all equations in line. They look a bit chaotic (see line 86, 87, 103, 104, 105, 106).
2. Numbering of all chapters must be adjusted. I
3. in general, Figures show results of something and not a decolouration process for example. Please revise it in all results chapters.
4. In general, figures are presented and described in the result section. Direct discussion on figures in the discussion section is not favoured. Please consider this in your discussion section. Most part missing as description on your figures is unfortunately presented in the discussion section. This must be clearly separated in a scientific paper. Please revise before I restart reading through your discussion section.
5. Sunlight degradation might be not comparable to UVA treatments because the treatment conditions during UV experiments are very different from environmental conditions and high-power UV sources. The latter, especially, might be of crucial importance because the light spectrum emitted by UV lamps does not completely represent the natural sunlight spectrum, and the emitted UVA irradiation flux is much higher than for solar light. For this reason, the degradation pattern and kinetics might be different between artificial UV treatment and natural sunlight treatment! Therefore, a proper experimental set-up description is urgently required.
6. Did the authors consider the pH of they treated wastewater? Was it neutral or rather acidic? Since the table of characterization is missing it is difficult to understand if pH adjustment was required.
7. The authors might think about the influence either of iron and titanium separately. Since you have performed your treatment at acidic condition ferrous iron might have a more important influence on the degradation kinetic as Ti (line 367-374)? This might be also critically discussed.
8. The refence style must be adjusted to the guideline requirement.
9. Very often sentences are too long and a full stop is missing.
General issues to be revised:
Line 90-94: Please add a reference. Photocatalytic reactions are highly depending on the pH and therefore they cannot always be generalised. Commonly a pH screening is required to determine best condition for specific photocatalysers.
Line 119: Please delete the full stop behind the reference.
Line 139-141: I recommend removing the curve of the undiluted sample. It is well known that without proper sample dilution wrong measurements are generated. Instead, I recommend adding spectra of the corresponding dyes for preliminary sample compound identification. A compound identification only based on a spectrum is an assumption and would require proper identification based on MS techniques, for example.
Line 147: Replace (V/V) by (v/v)
Line 156: Table 1 is missing. Please add.
Line 161: Please use a full stop behind “mA” and start with a new sentence.
Line 136-197: Please add for all equipment used for your study the seller, city and country in brackets.
Line 171-173: Could you give more details on the UV reactor, please? A scheme would be favourable. What was the reactor volume? Was the system cooled? What was the incident photon flux and corresponding light intensity? What was the radiation range?
Line 172-177: Is information must be move behind line 185. Please describe first the reactor design and set-up and subsequent the sample measurement.
Line 187: What was the percentage of H2O2 and how much was added to the reaction? In other terms, what was the given concentration? This goes also for all tested photocatalysts.
Line 210: Table 2 is missing. Please add.
Line 221: The quality of Figure 2 (i.e., pixel) is very low. Please improve. This is true for all graphs in the manuscript. Please use and uniform layout and avoid frames. Be carefully with decimal number, i.e., for example 10.0 instead of 10,0. Please carefully check all graphs and revise if necessary (especially Fig. 12 &13).
Line 224: What is the particle size distribution observed finally? What is the major distribution?
Line 231: Is there nothing else to state about the result presented in Figure 4? To me the different loads seem to result pretty much in similar results. It is necessary to show all loads? Did you measure triplicates? There is no standard deviation.
Line 236: Please at the sample source used in the headline of the Figure.
Line 242: There is no note about Figure 7 in the main text. Please revise.
Line 246: What is visible white light? Please provide the corresponding wavelength.
Line 246/247: “Experiments were carried out using visible white light, sunlight and ultraviolet light type A” I have serious problems to find a proper experimental description of these experiments in your Material and Methods section. I recommend to carefully revise in the “M&M” section the method description as I proposes for 171-173. Furthermore, I would like to know how you performed your sunlight experiments? Sunlight degradation might be not comparable to UVA treatments because the treatment conditions during UV experiments are very different from environmental conditions and high-power UV sources. The latter, especially, might be of crucial importance because the light spectrum emitted by UV lamps does not completely represent the natural sunlight spectrum, and the emitted UVA irradiation flux is much higher than for solar light. For this reason, the degradation pattern and kinetics might be different between artificial UV treatment and natural sunlight treatment! Therefore, a proper experimental set-up description is urgently required.
Line 258: From your experiments you should determine the degradation rate constants at least. You could also determine the half-life of your different treatment conditions.
Line 267: Why is the x-axes starting at 45%? It remains unclear whether 45% is the highest particles size distribution. The graph shows only “size 1” and “size 3”. What’s about “size 2”?
Line 269-273: More detailed description regarding the results presented in Figure 12 and 13 is required to better understand and interpreted them. Please revise. For example, what do the different abbreviations in the legends of both figures mean?
Line 286 to 296: This part might be better moved up to the results section “Ilmenite characterization”. These information are missing there and are more descriptive rather than a discussive.
Line 297-462: Please see my comment above in “important issues”.
Author Response
Respected reviewer, attached I am sending the responses to your comments.Author Response File: 
Author Response.docx
Reviewer 2 Report
The present manuscript deals with the photocatalytic degradation of clinical wastewater samples containing organic dyes. I consider that the findings can result very interesting to the journal readership but there is a long pathway of improvement to be considered before acceptance. Before recommending the acceptance of this manuscript, authors need to consider the following major comments:
1. Abstract: Please include relevant data and findings such as degradation yields. Abstract should comprise the following aspects: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: briefly describe the main methods or treatments applied; (3) Results: summarize the article’s main findings; (4) Conclusions
2. Abstract needs an introductory phrase within the problematic dealt in the article and a conclusion phrase. What are the principal achievements of this work?
3. Introduction lacks of updated references. There are several phrases that are not referenced. For example, lines 31-38, 39-47, 48-50.
4. Some references that could be included:
https://doi.org/10.1007/s10311-020-01045-2
https://doi.org/10.1016/j.jiec.2021.02.017
https://doi.org/10.3390/nano13020270
https://doi.org/10.3390/nano12193290
https://doi.org/10.3390/catal12080899
https://doi.org/10.1016/j.scitotenv.2021.152434
5. In general, I consider that the manuscript is not well sustained wiht updated references. The discussion lacks of comparison with other works performed on the same subject.
6. Authors should include in the introduction what have they done in previous research to state the real breakthrough done in the present work. I see that authors have performed some work on the degradation of Orange dye II based on dark Fenton (https://doi.org/10.1007/s11356-016-7263-3). Then, it could be interesting to say that in previous work you have achieved those specific results and that in here you go further in the analysis of the degradation of real wastewater samples considering a photocatalytic process.
7. Line 133-135: Where did you get the wastewater samples?
8. Figure 1: The reliable absorbance range for the majority of spectrometers and colorimeters is between 0.1 and 1. Values of absorbance more than or equal to 1.0 are excessive. Your solution is overly concentrated if the absorbance readings you are getting are 1.0 or higher.
9. I don’t understand why authors have selected 525 nm wavelength. Even though is the highest absorbance peak, the one at around 660 nm seems more reliable as it is not distorted when the sample is concentrated. Authors must include the calibration curve concentration vs. absorbance.
10. Where are Table 1 and Table 2?
11. Where does the ilmenite samples were obtained?
12. Adsorption is not the same as color degradation. The removal of color is a consequence of adsorption which causes the adhesion of atoms, ions or molecules to a surface. On the other hand, when the free radicals are formed they can degrade the organic molecules through first discoloring the solution and later degrading the sample. In the first stage, discoloration occurs due to the breakage of azo bonds or aromatic rings and the mineralization is when the smaller molecules (byproducts) are transformed into CO2 and H2O. In fact, the sample is not fully degraded until it reaches 100 % mineralization which is not directly related to the discoloration. Mineralization of samples can be estimated by analysing the total organic carbon on the samples (TOC): Mineralization (%) = 100 – 100(TOCt/TOC0).
13. I suggest authors to delve into the degradation mechanisms and to analyse TOC in the wastewater samples. Is important to indicate that what you are showing is just the discoloration of the samples.
14. Figure 2: add the legend of the symbols (star, circle and triangle).
15. There are too many figures along the manuscript. Figure 4 – 7 can be condensed in one and be labelled as Figure X. a) – d). The same with Figures 8 – 10 and 11 - 14.
16. Figures like 7 and 14 are unnecessary if you are showing already the discoloration percentage, those figures can be sent to the supporting information.
17. Include error bars in all figures.
18. Figure 7. Why is the treated sample showing much absorbance than the diluted sample? I believe there is a mistake in the legend.
19. In Figures’ captions: the subscripts numbers of H2O2 need to be corrected.
20. Figure 11: There is a mistake in the legend. It should be Size 1 and Size 2, right? Authors must name the samples in the same way in all Figures. In Figure 12, 13 and 14 they are called ILM-1 and 2.
21. In some Figures the decimal point is a comma and in others is a dot.
22. Figure 12 and 15. I don’t understand why the pH is varied with time. The pH of the sample should not vary through time because it is a very important factor to consider when performing a degradation experiment. pH must be fixated during the entire reaction. The use of a buffer solution is necessary to avoid this problem.
23. Why is it important to analyse the effect of different parameters on pH? Is that really necessary? The sole effect of the pH on the degradation process is more important. For this, authors can fixate degradation time and ilmenite load while varying the pH.
24. Where is the N2 adsorption/desorption isotherm? Please include it on the supplementary file.
25. Line 324, COD is not an indicator of the mineralization. Although it can be related to the TOC, but it is not quite reliable.
26. Line 331: according to Fan should be written as according to Fan et al.
27. Authors should estimate the pollutant concentration with a calibration curve concentration vs. absorbance. In this case, chemical oxygen demand can be an indicator of the pollutants concentration and therefore discoloration. Here is a reference that can be used as a guide where the authors analyse discoloration by the COD: https://doi.org/10.1016/j.jclepro.2021.127385. It is well-known that COD is a colorimetric assay. In the same work you can see that even though COD is almost fully decreased, samples are not completely mineralized.
28. Please sustain the discussion with more references.
29. Is better to call ilmenite reusability rather than stability.
30. Discussion needs an extensive English and grammar editing.
Author Response
Respected reviewer, attached I am sending the responses to your comments.Author Response File: Author Response.docx
Reviewer 3 Report
Comments to the author
1. The introduction section need to be more precisely, discuss more details about ilmenite related materials, advantages and disadvantages, why author chosen this materials as a photo catalyst, how the author improved from the previous studies to till date, Please support with catalytic efficiencies with previously reported materials and discuss your current studies?
2. Please author need to be more careful, during the advanced Oxidation Processes (AOPs) it’s generate hydroxyl radicals OH• How do you confirm your proposed materials ilmenite that was produced hydroxyl radicals OH•?? Have you performed any supporting evidence are trapping theses radicals by electron paramagnetic resonance (EPR)??
3. The whole introduction part need to be re write, please discuss your proposed materials of ilmenite, rather than discussing about TiO2 most of them are well understood about TiO2 photo catalytic mechanism, please delete from the whole part start from page 2 line 53 to114.
4. In page 4, Figure 1, author showcasing the clinical laboratory wastewater which contains, crystal violet how do you justify?, I suggest you to showcase your samples in digital photo images and comparing with commercially available crystal violet in diluted various concentration, subsequently the mass values must be determined by mass spectrometry (m/z) obtained from clinical laboratory wastewater and compare with commercially available crystal violet.
5. Please provide double beam UV-Visible spectrophotometer for before initiation of clinical laboratory wastewater degradation, subsequently for each time interval how this spectrum were decreased in following concentration were decreased during the degradation as well as in different time of intervals (kinetics), so that this article reader can clearly understand of this studies and compare your kinetics data with your proposed material against previously published literature data’s.
6. Please provide the band gap details of your proposed materials of ilmenite, and discuss about how this band gap are helping to your photo-catalytic studies?!!!.
7. Please move all the Figures 4,5,6,7,8,9,10,11,12,13,14,15 to supporting information, I suggest author to digest your data and give precisely and most significant information to scientific research publication.
8. The title need to be revise, I suggest author to cite more relevant studies which are recently published in within 5-7 years. Author has indexed several references which are not related to current study.
Comments for author File: Comments.docx
Author Response
Respected reviewer, attached I am sending the responses to your comments.Author Response File: Author Response.docx
Reviewer 4 Report
· The introduction section of the paper requires a thorough revision. There is a lot of introductory information which, in many cases, has not been properly cited. In its current form, it reads more like an assignment than a scientific paper. Many of the cited papers are outdated and the references need to be updated to reflect recent field findings.
· It is unclear how the authors identified the UV-Vis peaks of a certain molecule without using any separation method. HPLC-UV analysis is necessary for this. The section from lines 136 to 155 should be a part of the results and discussion section. It is also unclear what "undiluted and diluted" refers to.
· The section from lines 144 to 155 does not make much sense from an analytical chemistry perspective. The sentences need to be made clearer and it is unclear what the authors are trying to achieve.
· Table 1 is missing.
· The section from lines 170 to 197 requires much more detail. What was the initial concentration of the pollutant? What was the volume? How much catalysis was used? What was the distance of the light source to the sample? What was the volume of the sample holder? How was the sample temperature controlled? What was the volume of H2O2 used for the blank? It is very confusing to understand how the experiments were conducted.
· The information from lines 200 to 211 cannot be evaluated as Table 2 is missing.
· BET analysis is missing.
· The XRD analysis is presented in poor-quality graphs and the discussion is inadequate. More analysis of blank materials is necessary.
· Other graphs are also presented in poor quality and the paper, in its current form, SHOULD BE REJECTED. The manuscript requires extensive revision before any submission to any journal in order to meet the minimal standards of a scientific paper.
Author Response
Respected reviewer, attached I am sending the responses to your comments.Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The resubmitted manuscript submitted by Ramírez Franco tries to report results of a photocatalytic batch treatment of a real clinical wastewater predominantly containing the dyes crystal violet and methylene blue from gram staining. The topic is very interesting since the authors try to present results for potential wastewater treatment. However, I cannot see significant improvement according to my recommendations. Further, I cannot see a clear improvement regarding the English. I have serious doubts that with additional recommendation the authors will be able to finally understand and improve the manuscript as it will be required for publication in the Journal Waters. Please find my response below:
1. Why didn’t the authors use the template form of waters?
2. Line numbering is missing and make it very difficult to report to the authors where to improve mistakes in the manuscript!
3. Explanation of abbreviation is missing in the abstract (BET) and in the introduction (TOC), for example.
4. Use of capital letters in sentences without proper justification. For example, why is Advanced Oxidation Process (AOP) written with capital letters?
5. Senseless use of semicolon instead to usage of full stop in the complete manuscript.
6. The authors show the Vis-spectrum of the untreated wastewater. There are cleary two absorbance maxima maybe corresponding to the investigated dyes. Only from studying the literature, it is well known that the absorbance maximum of crystal violet is in the range of 580 to 585 nm and of methylene blue between 660 to 665 nm. The authors have decided to measure the effect of photocatalytic degradation of their dyes at 525 nm. There is no clear justification for their decision. Certainly, the wastewater contains a lot of organic carbon and therefore different wavelengths might be investigated if this is the basic parameter in their investigation. However, choosing 525 mn without any statement occurs a serious mistake in the methodology. Why was only the Vis spectra chosen? The water was characterised by COD, for example. In my understanding both parameter, ie., Vis spectra and COD or for example TOC must be investigated for sufficient result interpretation. The authors want to demonstrate decolourisation of the wastewater. However, only demonstrating decolourisation of the wastewater does not automatically allow to conclude that the treatment was successful. What’s about potential formation of harmful transformation products? There is no information concerning this problem.
7. The set-up describes how the UV and sunlight experiments simply fails. For readers who want to reproduce the experiment important information are still missing. The authors state that they have used for UV-C treatment monochromatic light with the wavelengths of 254 nm. There is still the photon flux missing and intensity missing. Similar problems occur for the sunlight and visible light experiments which seems to be carried out by artificial illumination. How the authors performed these experiments and under which condition (again photo flux and intensity) is not sufficiently described. Actinometric experiments must be carried out before and the systems must be properly characterised. This information is not given in the manuscript and therefore potentially wrong result interpretation can be result.
8. The discussion on comparing sunlight experiments clearly show that the authors did not understand my concerns. As I understood they tried to simulate sunlight treatment. I have serious doubts if such an experiment is representing real sunlight conditions.
9. In some treatments the addition of hydrogen peroxide was reported. Still, I have got no answer to my question of how much H2O2 was added? I have no information found about the reactor volume and the volume of H2O2added. And what was the percentage of the H2O2 stock solution? Was it 100 % (I have doubts about), or was it 50 % or 30%. There is no clear indication in the M&M section. Without such information the manuscript fails in my opinion.
10. In the result section I cannot find significant improvement. The quality of figures is very low, and layout of table is not adequate. I recommended already to improve figure; however, this is not any improvement.
11. I also asked the authors to move text from the discussion section to the result section since several passages are result descriptive in the discussion. This text and information are urgently missing in the result section. Presenting only figures without more detailed descriptive information is inappropriate.
12. I recommended to have a look at potentially determine kinetic parameters. For sure, this is a real wastewater, and several influences can occur. If the authors had performed only their photocatalytic treatment under standard conditions meaning only with crystal violet and/or methylene blue their can also determine the degradation constant rate. In a second step they could also evaluate the obtained results from their treatment of the real wastewater and compare. The argumentation that they cannot further evaluate their results presented in this study fails because determine simply the slope of an obvious exponential degradation curve demonstrate that the author do not understand the relationship and laws of photocatalytic treatments.
13. The discussion is in my opinion obsolete with intensive revision of the M&M section and result section.
Author Response
We deeply appreciated all the commentaries and suggestions given by the reviewers, without a doubt they have been essential to improve the submitted article. Regarding the proper English language use, the text was review twice using English grammar correction software and a final revision by a native English writing consultant, all the suggestions of these software and the consultant were incorporated in the document final versión.
Author Response File: Author Response.docx
Reviewer 2 Report
Authors have adressed the majority of my comments and clearly explained the reason on why not addressing other comments. I consider that the manuscript still needs an extensive editing of english language, but I now find it more suitable for publication. Some comments:
The following paragraph I consider better to use it for comparison purposes in the discussion: Several researchers have assessed ilmenite as photocatalyst for organic pollutant degradation; Pataquiva-Mateus et. al. [11] found that degradation of Orange II was obtained by dark Fenton during the first 7 h of reaction reaching 90%of COD removal; similarly, GarciaMunoz et. al. [12] have found that a Catalytic Wet Peroxide Oxidation – Photo assisted process using ilmenite as low-cost Catalyst successfully accomplished sulfonamides degradation and between 35% to 85% mineralization
Did authors followed a standarized protocol to collect the wastewater obtained from the Central de procesamiento de Laboratorio Clínico? For example: ISO 5667–3:2019
Methods need to be extended. Explain in a more detailed way how these techniques were used: the chemical oxygen demand, biochemical oxygen demand, phenol content, total suspended solids, methylene blue active substances and settleable solids.
Author Response
We deeply appreciated all the commentaries and suggestions given by the reviewers, without a doubt they have been essential to improve the submitted article.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
based on my evaluation the current form of the manuscript does not suitable for publication.
Author Response
We deeply appreciated all the commentaries and suggestions given by the reviewers, without a doubt they have been essential to improve the submitted article.
Author Response File: Author Response.docx
Reviewer 4 Report
Authors have modified the manuscript and improved the quality and it seems more acceptable now.
Author Response
We deeply appreciated all the commentaries and suggestions given by the reviewers, without a doubt they have been essential to improve the submitted article.
Author Response File: Author Response.docx
Round 3
Reviewer 1 Report
Accepted in present form.
