PONE-D-24-08931Optimization of chromium (VI) reduction in aqueous solution using magnetic Fe3O4 sludge resulting from electrocoagulation processPLOS ONE

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Additional Editor Comments:

I have raised few concerns that might be useful for author’s to improve their manuscript at current stage, after successful implementation the improved manuscript could be considered for further publication.

Editor Comments to the author

Author has prepared Optimization of chromium (VI) reduction in aqueous solution using magnetic Fe3O4 sludge resulting from electrocoagulation process

The experimental studies need to be more systematic and scientifically must be strong enough to support in order to attract this article reader which are lacking in the current form the manuscript, especially

1. In the section 3.3, and Table 2, author mentioned that, Cr(VI) could be reduced to Cr(III), how did you author confirmed their reduction, have you performed high resolution XPS spectra, Electron paramagnetic resonance to confirm their reduction, I suggest the following article might be useful for authors for further understanding to read and refer them Self-Assembly of Poly(ethyleneimine)-Modified g-C3N4 Nanosheets with Lysozyme Fibrils for Chromium Detoxification, Langmuir 2021, 37, 7147−7155,

2. In the section 3 Results and Discussion, Please include FT-IR spectra and also please provide before and after reduction of Cr(VI) based on FT-IR spectra studies this spectra would be helpful to identify their functional groups are Cr-O stretching and Cr=O stretching after sorption of Cr(VI) along with adsorbent for further information please carefully read and refer [Enhanced adsorption of hexavalent chromium arising out of an admirable interaction between a synthetic polymer and an ionic liquid, Chemical Engineering Journal 222 (2013) 454–463 an additionally Ultrasound-assisted preparation and characterization of crystalline cellulose–ionic liquid blend polymeric material: A prelude to the study of its application toward the effective adsorption of chromium, Journal of Colloid and Interface Science 367 (2012) 398–408.

3. In Figure 2 & 3, please discuss SEM images coupled with Energy dispersive spectra analysis (EDS) along with elemental compositions for before and after reduction of Chromium (VI) subsequently discussion need to be incorporate for further, an additional clarification I ask author to go through this research article

An enhanced adsorption methodology for the detoxification of chromium using n-octylamine impregnated Amberlite XAD-4 polymeric sorbent, Journal of Environmental Science and Health, Part A 2011, 46, 1598-1610.

4. In the introduction section, what are the new aspects and originality of this manuscript? authors have to discuss a detailed previously published Chromium (VI) removal and reduction-based literature studies for examples please include

Heavy metal and organic dye removal via a hybrid porous hexagonal boron nitride-based magnetic aerogel, npj Clean Water, 2022, 5, 24. Chemical Engineering Journal 222 (2013) 454–463. Journal of Colloid and Interface Science 367 (2012) 398–408. Langmuir 2021, 37, 7147−7155, rather than discussing (introduction part) about dyes removal and reduction and also how do you correlate from previous studies and what are the additional advantages from existing scientific published articles, a concise literature studies should be added into the introduction part that should be relevant to the prosed adsorbent materials?

5. In the XRD spectra, Figure 4, I didn’t find the existence Fe3+ and Fe2+ ions in the prominent magnetic nanoparticle (Fe3O4) for further information please carefully read this article will be helpful npj Clean Water, 2022, 5, 24.

6. Please provide magnetic hysterias curve for the as synthesised magnetic Fe3O4 sludge nano-adsorbent, provide digital images for suspension solution subsequently showcase after keeping the magnet how this suspension could be resulted as an transparent? For further clarification I ask author to go through this research article for further information’s Heavy metal and organic dye removal via a hybrid porous hexagonal boron nitride-based magnetic aerogel, npj Clean Water, 2022, 5, 24.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: No

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: No

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This manuscript reported the “Optimization of chromium (VI) reduction in aqueous solution using magnetic Fe3O4 sludge resulting from electrocoagulation process”. The manuscript needs some alterations before one can take a final decision. This review manuscript is still far from Major Revision in its current status because of following reasons:

1. Check the page for typo errors (e.g., space, superscript, subscript, spelling problems) on individual lines, such as 214, 240, 252, and 332.

2. The author should add a clear figure caption for Figure 5 (A, B, C, D) in the revised manuscript.

3. In section 3.6, the author said that Cr (VI) was reduced to Cr(III) in aqueous solution using a magnetic Fe3O4 sludge catalyst, however the author provided no data or results for the Cr(III) formation. UV-Vis spectra, a solution picture, and XPS data for Cr(III) reduction must all be included in the revised manuscript.

4. In Section 3.9, the author describes how to remove and reuse Cr(VI) from absorbed Fe3O4 sludge.

5. It is possible to test an industrial sample for Cr(VI) reduction using this Fe3O4 catalyst.

6. Line 132: The author said that "zeta potential analyses were performed for the characterization of electro-coagulated sludge." However, no zeta potential data is included in the manuscript. The revised manuscript must include zeta potential data.

7. Additionally, a hysteresis loop spectrum could possibly be provided for magnetic property confirmation.

Reviewer #2: 1. There are many errors in typo, grammatic and sentence formation throughout the manuscript. Authors can revisit the entire manuscript and correct them.

Examples: line 31- Cr(VI) reduction optimization adsorption experimental sets were designed using response surface design (sentence can be reframed).

Line 35- Freundlich, etc.

Line 98- Abbreviate the MH and MOH for better understanding.

Line 138- Abbreviate BBD for better understanding.

Line 214- Fe3O4 change to Fe3O4 , EDX to EDS (throughout the manuscript)

2. Line 69- Sharma et al., 2021 reference is not given in reference section.

3. Line 141- It is better to explain the synthesis procedure of adsorbent for better understanding.

4. Authors used “adsorption”, “reduction”, “removal” of Cr(VI) throughout the manuscript including the title. This may confuse the readers understanding that, what exactly the authors did? Is it reduction of Cr(VI) to Cr(III) using adsorbent? Or removal of Cr(VI) by adsorbent? If the work is focused on reduction of Cr(VI) to Cr(III), it should be mentioned clearly in introduction part.

5. Why authors calculated both adsorption capacity and removal efficiency? How do they differ? They both represent the same fact. Authors can use either adsorption capacity (qe) or removal efficiency (%) throughout the manuscript including figures.

6. Line 172- References can be given for the kinetics models.

7. Line 176- Gibbs free energy ( G), total enthalpy changes ( H), and entropy changes ( S) � is missing.

8. Line 246- Authors can explain more about how increasing iron ions plays a role in Cr(VI) removal.

9. Line 248- “At the same time, H+ in acidic conditions can increase the adsorption of Cr(VI) in the form of Cr2O7 2− or HCrO4 − by neutralizing the negative charge on the surface of electro-coagulated Fe3O4 sludge (Niu et al., 2019).”

How electro-coagulated Fe3O4 sludge surface is negatively charged at acidic pH?

10. Line 272- What are the functional groups present on the surface of electro-coagulated Fe3O4 sludge? Authors can mention them for better understanding.

11. How is adsorption capacity decreasing while increasing the dosage? At the same time removal efficiency increasing with increasing the dosage. Author can explain more about this behaviour.

12. Line 274- How stability of chromium ions affects the adsorption capacity.

13. Line 340- Regeneration method is unclear. How desorption performed? What reagents were used?

14. The deviation of the results were not mentioned for any analysis. Authors should comment on the reproducibility of the results or include error bars.

15. Author mentioned “chemisorption” in line 36 and “physical adsorption” in line 310.

16. Authors should emphasis the novelty of the adsorbent and how this electro-coagulated sludge Fe3O4 is better compared to already reported adsorbents.

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Reviewer #1: No

Reviewer #2: No

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Optimization of chromium (VI) reduction in aqueous solution using magnetic Fe3O4 sludge resulting from electrocoagulation process

PONE-D-24-08931R1

Dear Dr. Muhammad Akram

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Kind regards,

Santhana Krishna Kumar Alagarsamy

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Academic Editor comments

The Academic Editor thanks to the authors for this revised version that address most of the comments made in its previous report. The manuscript can be published as it stands.