Nanoscale zero-valent iron loaded vermiform expanded graphite for the removal of Cr (VI) from aqueous solution

Cr (VI) is indispensable in industrial manufacturing, and its extensive use leads to severe heavy-metal pollution in the water environment around people, posing a great danger to physical health and living environment of multitudinous organisms. Expanded graphite (EG) is considered as a typical material for adsorption, while nanoscale zero-valent iron (nZVI) can be applied to degrade and sedimentate various organic or inorganic pollutants. In this study, a simultaneous collaboration of EG and nZVI is carried out, with the investigation on the influence of different test conditions for adsorption performances. These findings demonstrate that nZVI@EG manifests favourable adsorptive performance on the removal of hexavalent chromium efficiently. nZVI, acting as an electron donor, is supposed to reduce Cr (VI) to Cr (III), turning itself into iron oxide or hydroxide. The whole process is an exothermic reaction, accompanying chemical reduction and physical adsorption. And Cr (III) is fastened on the appearance by deposition of chromium hydroxide or ferrochromium complex precipitation, which greatly reduces the total chromium content in the aqueous solution. Herein, as a new composite adsorbent, nZVI@EG shows promising prospects of practical applications in water contamination and environmental remediation.

2 In the portion on the preparation of nZVI@EG, PEG and MF dispersant were added into the system. What is the purpose of these reagents? 3 The axis values and titles of the inserting figures in Figure 3a should be bold to achieve a clear expression. 4 What is the specific chemical reaction on the redox process between nZIV@EG and Cr(VI)? And Chemical equations may reflect the whole process obviously.
Decision letter (RSOS-210801.R0) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Jiao: Title: Nanoscale Zero-valent Iron Loaded Vermiform Expanded Graphite for the Removal of Chromium (VI) from Aqueous Solution Manuscript ID: RSOS-210801 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) In this manuscript, the authors synthesized nZVI@EG and characterized the composites in detail. The prepared composites were applied for the sorption and reduction of Cr(VI) to Cr(III) and the results showed that the composited could efficiently reduce Cr(VI) to Cr(III). After reading the manuscript, I think it can be accepted for publication after revision. Special comments: 1. In the main text, I suggest the authors to revise "chromium (VI)" to Cr(VI). 2. In the Introduction section, two critical reviews should be added in the revised form such as: The Innovation, 2021, 2(1), 100076; Biochar. 2020, 2, 47-64. 3. It is better to add the effect of pH and ionic strength on Cr(VI) removal as this is helpful to understand the interaction mechanism from batch sorption results. 4. How about the stability of the composites in the removal of Cr(VI)? 5. The interaction mechanism should be discussed more detail in the revised form. Several important papers are helpful to improve the discussion of the results such as: Environmental Research，2021, 196，110349; Chemosphere，2021, 262，127901; Environmental Science & Technology. 2019, 53, <a href="tel:6454-6461">6454-6461</a>. 6. How about the reusability of the composites in the removal of Cr(VI) from solutions?
Reviewer: 2 Comments to the Author(s) The authors have described a simply method to prepare a new type of absorbent materials, combining nanoscale zero-valent iron nanoparticles with expanded graphite with an enhancement on the performance properties to degrade different pollutants in aqueous solution. Characterization results and systematic condition experiments were presented to demonstrate the great removal effect, revealing the redox process as the adsorption mechanism with detailed simulation calculation. Thus, I would like to recommend the publication of the manuscript after a minor revision. 1 For the expanded graphite, the expanded volume is a significant measurable indicator. After heating, the acid treated graphite can be expanded to generate a vermiform and fluffy form. And what is the expanded volume? This should be added into the manuscript. 2 In the portion on the preparation of nZVI@EG, PEG and MF dispersant were added into the system. What is the purpose of these reagents? 3 The axis values and titles of the inserting figures in Figure 3a should be bold to achieve a clear expression. 4 What is the specific chemical reaction on the redox process between nZIV@EG and Cr(VI)? And Chemical equations may reflect the whole process obviously.
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Dear Dr Jiao: Title: Nanoscale Zero-valent Iron Loaded Vermiform Expanded Graphite for the Removal of Cr (VI) from Aqueous Solution Manuscript ID: RSOS-210801.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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"In the main text, I suggest the authors to revise "chromium (VI)" to Cr(VI)"
R1. All the expressions of "chromium (VI)" have been revised to "Cr (VI)" in the main text.

"In the Introduction section, two critical reviews should be added in the revised
form such as: The Innovation, 2021, 2(1), 100076;Biochar. 2020, 2, 47-64." R2. As the reviewer mentioned, the two critical reviews on typical absorbent materials for pollutants removal from aqueous solutions are meaningful to demonstrate the significance of heavy-metal ion removal and adsorption method, which have been added into the Introduction section as No.14 and 17 of references.

"It is better to add the effect of pH and ionic strength on Cr(VI) removal as this is
helpful to understand the interaction mechanism from batch sorption results." R3. The effect of pH on the removal efficiency of Cr (VI) in aqueous solution has been presented in Figure 3a, showing great adsorption effect at the condition of low pH. In order to investigate the ionic strength on Cr (VI) removal, 0.1 mol/L of NaCl, Na2SO4, NH4Cl and Na2CO3 have been added into the solution, respectively, with the result showing as below. From Figure R1, the removal efficiency at the presence of NH4Cl achieved a high level over 90%, promoted by the acidic hydrolysis of ammonium ions.
Inversely, the removal efficiency at the presence of Na2CO3 was low, due to the alkaline hydrolysis of carbonates, in accord with the pH testing experiments. Moreover, strong electrolyte ions such as Na + , Cland SO4 2-, showed weak effect on the adsorption process. Figure R1 The effect of various ions on the removal efficiency of Cr (VI) in aqueous solution.

"How about the stability of the composites in the removal of Cr(VI)."
R4. Before and after the adsorption of Cr (VI) in aqueous solutions, the mass change has been recorded to reflect the mechanical stability. And the results presented obvious weight increment, indicating few nanoparticles shedding and massive formation of sediments. In addition, the experiments were carried out at different pH, indicating good chemical stability in various aqueous environments. To verify the mechanical stability deeply, nZVI@EG were added into deionized water with powerful stirring. And the SEM images of the treated composite were presented in Figure R2, showing complete structure with abundant nanoparticles on the surface and no visible damage.
Furthermore, the SEM images of the composite after different adsorption time were shown in Figure 5a and b, exhibiting great adsorption effect on Cr (VI) with the formation of numerous floccules. Figure R2 SEM images of nZVI@EG after strong stirring in deionized water.
5. The interaction mechanism should be discussed more detail in the revised form.
Several important papers are helpful to improve the discussion of the results such as: Environmental Research， 2021, 196， 110349;Chemosphere， 2021, 262， 127901;Environmental Science & Technology. 2019, 53, 6454-6461. R5. The interaction mechanism has been discussed in detail, and the literatures were helpful to describe the reaction process. And the session of adsorption mechanism has been revised as below.

"The axis values and titles of the inserting figures in
R3. Figure 3a has been revised with bolder frame and fonts in the inserting figure as below. Figure R4 The effects of environmental pH on the removal efficiency of Cr (VI) in aqueous solution, inserting the form distribution of Cr (VI) at different pH.

"What is the specific chemical reaction on the redox process between nZIV@EG
and Cr(VI)? And Chemical equations may reflect the whole process obviously." R4. The whole adsorption process can be described that the Cr (VI) ions contacted the surface and multi-grade pores of the expanded graphite composite under the driving force of diffusion with physical adsorption. And then Fe 0 nanoparticles reduced Cr (VI) to Cr (III), with the reaction equation as follows.