A Novel Magnetic Nano-Adsorbent Functionalized with Green Tea Extract and Magnesium Oxide to Remove Methylene Blue from Aqueous Solutions: Synthesis, Characterization, and Adsorption Behavior

Round 1

Reviewer 1 Report

The following comments need to be addressed before the work can be accepted for the publication.

1. The authors mentioned that the synthesized magnetite particles are possessing the superparamagnetic behaviour with the observation of S-shaped hysteresis loop. However, they need to provide the magnetization values at the zero magnetic field (0 Oe) with a zoom image as an inset. This exactly provided by the coercivity and magnetic resonance values and confirms the superparamagnetic behaviour. Therefore, the authors need to provide this information.

2.  When the adsorption studies were conducted as batch experiments, it is necessary to analyse the adsorption data with statistics. It will be good if the authors can provide the statistically significant and highly significant values.

3. Since the iron oxide nanoparticles can switch their isotopic form (oxidation state) from magnetite to maghemite or others as per the time and environmental conditions, what is the proof that the authors used form is magnetite only. Any other analysis technique like XPS for the investigation of oxidation state of iron oxide will be needed.

4. The rationale behind the selection of Green Tree extracts, Mg onto Fe3O4 as an adsorbent in this work is missing.

5. Why the authors have selected the methylene blue adsorbate when several other dye molecules like Rhodamine B, methyl orange, Fast green etc are available.

6. The adsorption mechanism needs to be provided with a proper visualization/schematic representation.

7. There have been several works available on the usage of iron oxide nanoparticles with several modifications and used as adsorbents. The authors need to compare their results with that of literature studies and confirm the usefulness of their prepared ones.

8. The Introduction section needs to provide the literature on Green tree extracts and iron oxide nanoparticles for methylene blue extraction and other studies.

9. The iron oxide nanoparticles are very much prone to agglomeration, I am wondering whether the Green tree extracts are strong enough to protect the particles from clumping and prevent the inter molecular surface attractions. The authors need to explain.

10. The authors need to provide a section linking the functional groups of green tree extracts that can be suitable for the effective bonding of methylene blue functional groups. In this way, one can see the effectiveness of the adsorption process.

Author Response

We thank the reviewers for their time and effort in evaluating the manuscript. We also thank the reviewers for their favorable opinions on our work and their constructive suggestions that have improved the manuscript. We have taken into account their suggestions, and revised accordingly the text to improve the overall quality of paper while remaining faithful to the main purpose of the work.

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript includes the adsorbent preparation by magnetite and tea composite. Manuscript is well written, but followings are comments,

1) please mention the amount ratio of introduced tea and MB based on mole.

2) from diffusion phenomena, please mention the macro and micro structure of the adsorbent.

3) please mention the effect of treatment of HCl and acetic acid during cycling.

Author Response

We thank the reviewers for their time and effort in evaluating the manuscript. We also thank the reviewers for their favorable opinions on our work and their constructive suggestions that have improved the manuscript. We have taken into account their suggestions, and revised accordingly the text to improve the overall quality of paper while remaining faithful to the main purpose of the work.

Author Response File: Author Response.pdf

Reviewer 3 Report

Title: A Novel Green Tea/Mg-Functionalized Magnetic Nano-Adsorbent for Methylene Blue Removal from Aqueous Solutions: Synthesis, Characterization, and Adsorption Behavior

Overall Comments: The paper requires careful revision as its current state leads to considerable confusion regarding the chemistry, results, and presentation. I recommend against publication.

Comments on the Title, Abstract, and References:

1. Authors may consider revising the title to: "Magnetic nano-adsorbent functionalized with green tea extract and magnesium oxide to remove methylene blue from aqueous solutions."

2. Authors should revise the abstract using "green tea extract" instead of "green tea" and "MgO" instead of "Mg."

3. Line 18-19: "Batch experiments revealed that the adsorption behavior was influenced by various operating parameters, such as pH and ionic strength."  Please inlude other parameters.

4. Line 21-22: "The experimental adsorption capacity of GT/Mg@Fe3O4 for MB surpassed 174.93 mg g−1, markedly superior to the performance of numerous other adsorbents." Authors have not provided any comparison with reported literature but stated it as markedly superior. Please include a comparison in a table.

5. Authors stated that adsorption of MB by the nano composite occurred by chemisorption. However, they also stated that it was successfully regenerated by acids (HCl, AcOH, and EtOH), creating confusion. Moreover, magnetic nanoparticles are difficult to separate from the solution without using a good magnet. They must stay suspended in water. This supports a misleading message for the reader.

6. Please check all references to see whether they are properly cited or not. E.g. [24].  

Comments on Introduction:

7. The introduction is incomplete and needs careful revision.

8. Authors should include different processes for removing MB from water, highlighting the advantages of the adsorption process. Then, they should include various commercial and reported adsorbents, including polymer-based, bio-polymer-based, and biopolymer-based nanocomposite adsorbents. Also, highlight some biomass-based adsorbents that have been reported. What is the role of green tea extract in this study? Then, include your research hypothesis and the novelty of this study.

9. What do authors mean by "Mg species"? Please correct this in the manuscript.

10. Line 81-82: "Finally, desorption-adsorption cycle experiments are also performed to assess the potential for using GT/Mg@Fe3O4 as an efficient and stable adsorbent in practical wastewater treatment applications." Have the authors used industrial effluents? Without using any industrial effluents, how could you make this statement?

Comments on Materials and Methods:

11. "Fried green tea leaves": Are they fried or dried?

12. "All the other chemicals and reagents were of analytical purity and used as received." Please add the chemical names used in this research in detail.

13. "A sealed beaker." What is it?

14. "The obtained GT was stored in the refrigerator (277.15 K) under dark conditions." How could the author set up dark conditions in the refrigerator? Since green tea leaves were used to prepare the green tea extract, I suggest writing GTE instead of GT.

15. Line 101: "Commercial Fe3O4 nanoparticles." Please include the nanoparticle size and its magnetic values.

16. Why did the authors use MgSO4 and sodium acetate instead of magnesium acetate? Please include synthesis and characterization of separately prepared MgO using Mg(OAc) and green tea extract.

17. How do the MgO nanoparticles form in this mixture? Please include the chemistry and color of the materials obtained.

18. Please write "GTE-MgO-Fe3O4 composites" instead of "GT/Mg@Fe3O4." You may write "nanocomposites" if you know the size of MgO and Fe3O4.

19. Since the tea extract contains lots of chemicals, it needs to be calcined to remove any organic impurities in the adsorbents.

20. Comments on Batch Adsorption Experiments:

21. What was the amount of MB?

Comments on Results:

22. Please include the SEM micrographs in the main text instead of the supplementary file.

23. "The particle size of GT/Mg@Fe3O4 particles was approximately 160 nm." Please include the particle size distribution (Histogram) graph with standard deviation.

24. Please include the XRD analysis and BET in Section 2.3 Characterization.

25. Please correct the mistakes in the Figure 1 caption.

26. Line 217: "As presented in Figure 4b." I hope it will be Fig. 2b. Please change the figure to lines-symbols instead of column graphs.

27. I request the authors to carefully check the text and the presentation from Section 3.2.2 to 3.5. It confuses me to proceed further.

28. Revise the conclusions with limitations and future scope.

29. "In summary, the GT/Mg@Fe3O4 adsorbent exhibited strong practical application potential for the efficient elimination of MB from wastewater." What do you mean?

30. Please go through the iThenticate report and modify the English to minimize the percentage to 15-20%.

Please go through the iThenticate report and modify the English to minimize the percentage to 15-20%.

Author Response

We thank the reviewers for their time and effort in evaluating the manuscript. We also thank the reviewers for their favorable opinions on our work and their constructive suggestions that have improved the manuscript. We have taken into account their suggestions, and revised accordingly the text to improve the overall quality of paper while remaining faithful to the main purpose of the work.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have incorporated all the necessary corrections and have provided satisfactory answers to my comments. Therefore, I recommend this article for the final publication in its present form.

Author Response

Thank your for your favorable opinions on our work.

Reviewer 3 Report

Dear Authors,

Thank you for considering the suggestions for improving the manuscript. However, some issues still require clarification before this article can be accepted for publication.

1. Comment 8: Please include information regarding using biopolymer-based nanocomposites as adsorbents for water treatment.

2. Please address comment 16. If you utilized Mg (OAc) 2 directly instead of MgSO4 and sodium acetate, kindly explain the chemistry behind MB's limited adsorption capacity.

3. How many times did the authors conduct adsorption experiments? While standard error bars were used.

4. As some error bars are not visible, it is recommended that Figure 3 be converted into a line symbol, making the data trends more apparent and easier to interpret (Refer to comment 25).

5. Regarding comment 24: "FI-IR," please correct it to "FT-IR."

6. The conclusion section should not contain citations or references. Therefore, Table A4 should be included before the conclusion section ( make a new section to "comparison with the literature"). The table should only present MB dye removal, excluding metal ions. Additionally, it is recommended that some previously reported magnetic nanocomposites capable of removing 98% of MB from water be included in the comparison table. Discusses the findings.

7. Finally, the limitations of this study should be addressed in the conclusion section (Refer to comment 27).

Author Response

We appreciate your positive overall comments and instructive suggestions. We have carefully considered your suggestions and have made substantial improvements to the paper accordingly. Here is our point-by-point response to your comments.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Dear Authors,

Thank you for revising and improving the manuscript. After the minor revision, I can recommend your paper for publication.  

1. Regarding comment 2, I understood that the experiment was not conducted using Mg(OAC)₂. 
2. Could you please provide the names of the salts used to study the influence of commonly co-existing ions, specifically Na⁺, K⁺, Ca²⁺, Cl−, NO³⁻, and SO4²⁻?
3. I noticed a slight decrease in adsorption at pH 5.0 compared to pH 6.0. Could you please explain the reason for this difference?
4. Is there a specific reason for not introducing a new "Comparison with the Literature" section before the conclusion? You can consider this point to better distribute your paper within this journal. Additionally, please include the pH values in Table A4.
5. Please incorporate a figure depicting the relationship between adsorbent dosage removal efficiency(%) and adsorption capacity (mg/g) into the manuscript.
6. Line 450 has a typographical error: "(for example, the amounts of added GTE, MgSO₄ and NaOAc)." The correct spelling is "and."
7. Please check the accuracy of the thermodynamic parameter calculations. I have reviewed them and found the Enthalpy to be approximately -123.62 kJ/mol.....please check it.

Author Response

Thank you for your suggestions. The modifications coresponding to your comments have been done in the revised manuscript. Please kindly check them out.

Author Response File: Author Response.pdf