Synthesis of Manganese Ferrite/Graphene Oxide Magnetic Nanocomposite for Pollutants Removal from Water

Round 1

Reviewer 1 Report

Katubi et al., have done an interesting research work with a novel material for environmental application.

It can be accepted after Major revision.

The comments are as follows:

1. ABSTRACT should be presented with more numerical, analysis, and fitting results.
2. The level of English throughout the manuscript is needed to be improved.
3. The introduction is needed to be improved with literature. It should be improved the following references:

• Ultrasonics Sonochemistry, Volume 49, 2018, 1-12.
• Ultrasonics Sonochemistry, Volume 40, Part A, 2018, 748-762.
• Desalination and Water Treatment, Volume 57, Issue 44, 2016, 21091-21104.
• Journal of Industrial and Engineering Chemistry, 65, 2018, 349-362.
• Journal of the Taiwan Institute of Chemical Engineers, Volume 91, 2018, 186-199.
• Journal of Saudi Chemical Society, 21, Supplement 1, 2017, S219-S230.

4. Please add the models to the related tables and delete them from the text.
5. The units should be added to the Figures.
6. Use “Fig.” instead of Figure.
7. The resolution of TEM and SEM is low. No information is clear. Please replace with higher resolutions. Please use another color instead of Red to mention the names in Fig. 1d.
8. Please add the q_m of the adsorbents used in the articles mentioned in comment 3 for comparison.
9. Conclusion could be presented with more information.

I look forward to receiving the revised version of this manuscript for reconsideration.

Author Response

We appreciate your valuable comments which have been addressed. Please see the attachment.

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Reviewer 2 Report

This work "Synthesis of Manganese Ferrite/Graphene Oxide Magnetic 2
Nanocomposite for Pollutants Removal from Water" looks promising as the magnetic nanocomposite is used for pollutant removal from water. But few comments should be addressed in revised version.

Please find below comments:

1) Adsorption capacity results should be removed from Abstract as they are already mentioned in Conclusions section.

2) Line # 62 should be corrected. "Uses advantages" should be rephrased.

3) Figure 6 a and b should be provided symmetrically.

4) If possible removal efficiency in different aqueous media (1XPBS, seawater, NaOH, HCl)s should be provided.

Author Response

We appreciate your valuable comments which have been addressed. Please see the attachment.

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Reviewer 3 Report

The manuscript titled “Synthesis of Manganese Ferrite/Graphene Oxide Magnetic Nanocomposite for Pollutants Removal from Water” focus on the synthesis, characterization and application as lead and red dye remover of a graphene oxide-based composite. The authors adopted different characterization techniques and tested the composite under different experimental condition including contact time, pH, initial concentration, and adsorbent dose. Unfortunately, some points compromise the overall quality of the manuscript.

First of all, I have found a paper dated back to 2014 (Kumar et al. ACS Appl. Mater. Interfaces 2014, 6, 17426−17436) dealing with single-layer graphene oxide with manganese ferrite nanoparticles used for removal of Pb(II), As(III), and As(V) from contaminated water. The authors must take into account this reference and all others (for instance also J. Mater. Chem., 2012, 22, 1033–1039 dealing with magnetite-graphene oxide composite for the removal of neutral red; or FlatChem 26 (2021) 100233) have already proposed such kind of materials for the same application. I wonder why the authors have not included these papers in their references list. The novelty of their work has to be stated unambiguously in comparison with the available literature on the theme.

Secondly, the authors stated that the composite is made of graphene oxide and manganese ferrite but the XRD pattern does not evidence the formation of a spinel ferrite. Indeed, the authors ascribed the XRD reflections to manganese ferrite although those reflections do not match with the expected angular positions (for some peaks more than 5 degrees of shift are detected in the attribution the authors made). The authors justified it as due to “the formation of oxygen-containing groups among graphene oxide sheets.” However, I am not convinced: to the best of my knowledge, XRD is not a spectroscopy and a shift in the XRD reflections can be ascribed to a shrinkage or an enlarging of the cell unit of the crystalline lattice. I do not expect that a surface bond of the manganese ferrite on the graphene oxide may produce such a shift. In addition, the band associated with the graphene oxide in the pattern appeared to be changed after the synthesis of the magnetic phase, with a shift in the band position and in the width. Something seems to happen to graphene oxide. Therefore, I think that the interpretation of the XRD data presented in the manuscript does not sound and a revision is necessary.

Thirdly, if the manganese ferrite is present, we should expect also some bands in the Raman Spectra, at least that at about 600-620 cm-1 associated with the A_1g band. The vibrational modes ascribed to Fe-O and Mn-O stretching are not unambiguously ascribable. XPS is used only to state that iron, manganese, oxygen and carbon are present in the material, but other information can be extracted, for instance in terms of oxidation state of the metal cations or in terms of bonds between the adsorbent and the adsorbate after adsorption test. Moreover, the saturation magnetization of the sample appears too low for manganese ferrite, and the authors stated “According to Figure 3b, the saturation magnetism (Ms) of MnFe2O4/GO was 28.8 emu/g that was smaller than that of manganese ferrite alone (67 emu/g) [35], indicating the reduced size of nanocomposite after the loading of manganese ferrite over graphene oxide. The nanocomposite’s excellent magnetic properties are attributed to the large load of magnetic ferrite over the graphene oxide.” Ref. 35 only talks about graphene oxide; the smaller size (11-26 nm) is declared to be responsible for the low Ms, which is then stated to be excellent (excellent magnetic properties) and ascribable to the large load of magnetic phase over the support, which is not clearly specified in the text.

Finally, concerning the adsorption tests, I was wondering if you have compared the adsorption capacity of the composite with those of the separate phases, i.e. graphene oxide and manganese ferrite. Indeed, as far as I know they can be used as removers for lead (ACS Appl. Mater. Interfaces 2014, 6, 17426−17436; Environ. Sci. Technol. 2011, 45, 10454–10462; Dalton Trans., 2011, 40, 10945, where C_smax = 842 mg g-1) and dyes (J. Chem. Eng. Data 2019, 64, 833−867). The comparison with the literature results in terms of performances in the lead removal do not take into account those phases as titanium oxide, hematite nanoparticles, Fe@MgO nanocomposite, etc., which are considered the best candidates for lead removal and other papers on manganese ferrite/ graphene oxide composite (Bathia et al. Int. J. Environ. Sci. Technol. (2017) 14:1135–1154; Barthi et al. Journal of Superconductivity and Novel Magnetism volume 33, pages3651–3665(2020)).

To sum up, something is missing or at least not fully clarified in the novelty, data interpretation and correlation between the different characterization techniques. A deeper investigation on the material is needed together with a comparison of its removal performances with reference samples to better evaluate the importance of proposing a composite instead of single phases, and a deeper literature search.

Other issues:

• The introduction does not describe which kinds of materials have been previously tested in the literature for lead and neutral red dye in order to highlight the possible novelties of the authors’ proposal.
• From the XRD pattern of the composite you might extract other information such as the crystallite size, besides the identification of the crystalline phases.
• The TEM images and SEM images are of poor quality. Can you provide further and better images? In addition, HRTEM together with TEM-EDX and SEM-EDX measurements might have helped the authors in identifying the manganese-bearing crystalline phase. A particle size distribution might help in understanding the magnetic behaviour.
• From the magnetization curve at 300 K it is possible to calculate the magnetic domain size, that can be then compared with the crystallite size obtained by XRD and particle size from TEM.
• If you would like to refer to lead ions with a charge 2+, please substitute in all the manuscript Pb+2 with Pb2+. Conventionally, the charge of an ion is written with the number first followed by the sign, on the contrary the oxidation state is written with the sign by first.

Author Response

We highly appreciate your valuable comments which have been addressed. Please see the attachment.

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Round 2

Reviewer 1 Report

The comments have been performed carefully.

It can be accepted for publication in Processes.

Author Response

Please see the attachment.

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Reviewer 3 Report

The discussion of the data in this manuscript can be improved. Generally speaking, more carefulness on the characterization and data presentation would be desirable, but overall the discussion of the data is lacking and it should focus on the comparison of the results obtained by the authors and those reported in the existing literature. The authors have not described the literature in detail, neither in the introduction or the paragraph "Comparative study ". The authors have just included in the bibliography the suggested references. However, differences between their work and, for instance, that from Kumar et al. ACS Appl. Mater. Interfaces 2014, 6, 17426−17436 should be clearly stated in the introduction section in order to provide to the readers the evidence of previous works on the same subject and highlighting the novelty of this manuscript with respect to the literature. With the same aim, the paragraph “Comparative study” should contain a discussion of the results obtained in the manuscript in the light of the literature listed in Table 2. For instance, why has better lead removal been achieved in references [68,69] for similar systems? What features can be responsible for these differences? All these kinds of comments must be included in the manuscript. It is not just the matter of adding references in the manuscript but instead discussing the obtained results in the light of the references by also better explaining the idea behind the proposed manuscript.

Concerning the XRD analysis, the corrections make coherent the text with the data, that now show the obtainment of a manganese ferrite phase. Nevertheless, the manganese ferrite reflections are prominent with respect to the graphene oxide XRD band, therefore I wonder if the authors can indicate the weight percentage of the two phases in the composite, which is also fundamental to normalize the magnetic measurement. Moreover, the authors have not considered the suggestions on the Raman, XPS, TEM and SEM analysis.

Author Response

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