Kinetics and Mechanisms of Cr(VI) Removal by nZVI: Influencing Parameters and Modification

Round 1

Reviewer 1 Report

The authors developed a thorough study of the synthesis, characterization and use of bare and surface-modified nZVI in the removal of Cr(VI) and explored the effect of different operational parameters in the removal efficiency. 

The work is very well designed, and each question is tackled from multiple sides which builds a work with well analyzed results and solid conclusions. The work results easy to read, the information is concisely presented and all the removal experiments were performed by duplicate with proper interval error bars for each data point, which denotes a great volume of work.

However, I consider that the work lacks of scientific interest for the community dedicated to the study of use of nZVI for environmental remediation, particularly in the case of Cr(VI). On the one hand, the nZVI were synthesized by traditional pathway (reduction with borohidryde) and modified with agents reported usually used. On the other hand, the removal of Cr(VI) was tested in conditions already thoroughly studied in the specific literature as in the works of Alidokht et al. 2011; Gheju 2011; Nahuel Montesinos et al. 2014; Zhang et al. 2018 and referencies therein, but also in the literature cited in the manuscript. 

Finally, the mechanism proposed is just an outline of the mechanism largely proposed in the specific literature and the same techniques were also used in the past to lead to same general conclusions. It is always good to find a work that replicates data already obtained by other authors, but I think this manuscript does not show novel results nor new information about the system of study.

Author Response

Responses to Reviewer #1

Overall comments

The authors developed a thorough study of the synthesis, characterization and use of bare and surface-modified nZVI in the removal of Cr(VI) and explored the effect of different operational parameters in the removal efficiency. 

The work is very well designed, and each question is tackled from multiple sides which builds a work with well analyzed results and solid conclusions. The work results easy to read, the information is concisely presented and all the removal experiments were performed by duplicate with proper interval error bars for each data point, which denotes a great volume of work.

We appreciate your favorable and helpful comments and suggestions to improve this manuscript.

Minor comments

Reviewer comment 1:

However, I consider that the work lacks of scientific interest for the community dedicated to the study of use of nZVI for environmental remediation, particularly in the case of Cr(VI). On the one hand, the nZVI were synthesized by traditional pathway (reduction with borohidryde) and modified with agents reported usually used. On the other hand, the removal of Cr(VI) was tested in conditions already thoroughly studied in the specific literature as in the works of Alidokht et al. 2011; Gheju 2011; Nahuel Montesinos et al. 2014; Zhang et al. 2018 and references therein, but also in the literature cited in the manuscript.

Finally, the mechanism proposed is just an outline of the mechanism largely proposed in the specific literature and the same techniques were also used in the past to lead to same general conclusions. It is always good to find a work that replicates data already obtained by other authors, but I think this manuscript does not show novel results nor new information about the system of  study.

The authors thank the reviewer for this comment.

We fully agree with the reviewer’s comment about the abundant research work performed with Cr(VI), which is why we chose this target pollutant based on the serious harm of Cr(VI) to the environment and humans. It is true that some relevant literatures have focused on elucidating the mechanism of Cr(VI) removal in water by nZVI, and we still believe that this is a research direction that needs to be well explored.

Comprehensive comparison of the existing nZVI synthesis routes, whether it is top-down (Lithography grinding and Precision milling method), or bottom-up (Borohydride chemical reduction, Carbothermal reduction, Ultrasonic method, Electrochemical method, Green synthesis, etc.), all have certain advantages and disadvantages. Among these methods, the chemical reduction method has the advantages of being simple and easy to use, can be used in any laboratory, and can stably synthesize nZVI particles with smaller particle size and larger specific surface area. It has the potential to be widely used in practical wastewater, which is also the original intention of our choice of this method for scientific research.

This study focused on comprehensively elucidating the kinetics, influencing factors and mechanisms of Cr(VI) removal from water by nZVI. What's more important is to compare and explore the reasons for the promotion or inhibition of the experimental results by different modification methods. On the one hand, the previous literature is the basis of this study, and on the other hand, this study further verifies the reliability and stability of this material, which provides an essential theoretical basis for the further improvement and wide application of nZVI in the future.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript described the influencing parameters, mechanism and kinetics behind removal of hexavalent chromium from aqueous solution using nanoscale zero-valent iron nanoparticles. After a careful review of the manuscript, I do not find it fit for publication in the current form. My reviews as follow:

 

The introduction does not illustrate the background correctly with particular reference to novelty of this work as compared to other studies available in literature. In fact, there are many examples in the synthesis zero-valent iron nanoparticles using chemical reduction method and application for removal of hexavalent chromium. Here, I am listing only few of them:  

https://www.sciencedirect.com/science/article/pii/S1474706510000355;

https://www.sciencedirect.com/science/article/pii/S2213343721020546; https://www.sciencedirect.com/science/article/pii/S0147651320314093;

https://www.sciencedirect.com/science/article/pii/S1385894718314190;

https://www.sciencedirect.com/science/article/pii/S1383586616318391;

https://www.sciencedirect.com/science/article/pii/S1385894720306306

https://www.sciencedirect.com/science/article/pii/S0927775722014522

 

In this work, authors just stated common trends in synthesis, application of  nZVI and evaluation of performance. There are no noticeable differences, key features and novelty in this work as compared recent studies in the subject. The discussions of the mechanism are too simplistic. Authors are looking so careless, just trying increase number of publication without novelty.

Author Response

Responses to Reviewer #2

Recommendation:

The manuscript described the influencing parameters, mechanism and kinetics behind removal of hexavalent chromium from aqueous solution using nanoscale zero-valent iron nanoparticles. After a careful review of the manuscript, I do not find it fit for publication in the current form. My reviews as follow:

The authors thank the reviewer for providing valuable comments.

Specific comments

Reviewer comment 1:

The introduction does not illustrate the background correctly with particular reference to novelty of this work as compared to other studies available in literature. In fact, there are many examples in the synthesis zero-valent iron nanoparticles using chemical reduction method and application for removal of hexavalent chromium. Here, I am listing only few of them:  

https://www.sciencedirect.com/science/article/pii/S1474706510000355;

https://www.sciencedirect.com/science/article/pii/S2213343721020546; https://www.sciencedirect.com/science/article/pii/S0147651320314093;

https://www.sciencedirect.com/science/article/pii/S1385894718314190;

https://www.sciencedirect.com/science/article/pii/S1383586616318391;

https://www.sciencedirect.com/science/article/pii/S1385894720306306

https://www.sciencedirect.com/science/article/pii/S0927775722014522

In this work, authors just stated common trends in synthesis, application of  nZVI and evaluation of performance. There are no noticeable differences, key features and novelty in this work as compared recent studies in the subject. The discussions of the mechanism are too simplistic. Authors are looking so careless, just trying increase number of publication without novelty.

Thank you for this valuable feedback.

We fully agree with the reviewer's comments on the extensive research work on Cr(VI) removal by nZVI, which is why we chose this target pollutant based on the serious harm of Cr(VI) to the environment and humans. It is true that some relevant literatures have focused on elucidating the mechanism of Cr(VI) removal in water by nZVI, and we still believe that this is a research direction that needs to be well explored.

To further highlight the significance and value of this study, we have adjusted the relevant presentations in the Abstract and Conclusions and added new content in the Introduction.

Lines 73-80; Text was add to: “Comprehensive comparison of existing nZVI synthesis methods, whether top-down (lithographic grinding and precision milling) or bottom-up (borohydride chemical reduction, carbothermic reduction, ultrasonic, electrochemical , green synthesis, etc.), all have certain advantages and limitations[1]. Among these methods, the chemical reduction method is simple and feasible, suitable for use in any laboratory; meanwhile the product obtained is characterized by a homogeneous structure that displays a high reactivity[2], and has the potential to be widely used in practical application.”

Lines 104-107; Text was altered to: “This study also devoted to the development of easily accessible, economical, and environmentally friendly materials as possible substitutes for modified nZVI, providing as many theoretical foundations as possible for more kinds of modified nZVI.”

1. Stefaniuk, M.; Oleszczuk, P.; Ok, Y. S., Review on nano zerovalent iron (nZVI): From synthesis to environmental applications. Chemical Engineering Journal 2016, 287, 618-632.
2. Jamei, M. R.; Khosravi, M. R.; Anvaripour, B., Investigation of ultrasonic effect on synthesis of nano zero valent iron particles and comparison with conventional method. Asia-Pacific Journal of Chemical Engineering 2013, 8 (5), 767-774.

Author Response File: Author Response.pdf

Reviewer 3 Report

Reviewer’s comments

Manuscript Number: catalysts-1859738

Title: Kinetics and mechanisms of Cr(VI) Removal by nZVI: Influencing parameters and modification

Journal: Catalysts

The manuscript reports the removal of Cr)VI) using nanoscale Zero Valent Iron. The manuscript is well written and organized; however, revision is needed according to the following comments.

1.     The novelty of the work needs to be highlighted where the are many related published papers, such as:

https://doi.org/10.1007/s00128-011-0425-6

https://ieeexplore.ieee.org/document/5515574

https://doi.org/10.1038/s41598-020-58639-7

2.    Since the specific surface area is mentioned, the N₂ adsorption/desorption isotherm should be provided.

3.    All equations listed in text S1 need to be supported with relevant reference.

4.    Table of comparison should be provided to compare the removal efficiency of nZVI with other related materials.

5.    A deep discussion is needed for many parts.

6.     The correlation between structural/morphological findings and catalytic performance should be discussed. The following literature may help: 

https://doi.org/10.1016/j.ecoenv.2020.111552

https://doi.org/10.1680/jenes.21.00002

Author Response

Responses to Reviewer #3

General comments:

The manuscript reports the removal of Cr)VI) using nanoscale Zero Valent Iron. The manuscript is well written and organized; however, revision is needed according to the following comments.

We are grateful for your positive comments and helpful suggestions. We revised the manuscript according to your suggestions.

Specific comments:

Reviewer comment 1:

The novelty of the work needs to be highlighted where the are many related published papers, such as:

https://doi.org/10.1007/s00128-011-0425-6

https://ieeexplore.ieee.org/document/5515574

https://doi.org/10.1038/s41598-020-58639-7

The authors thank the reviewer for this comment.

To further highlight the significance and value of this study, we have adjusted the relevant presentations in the Abstract and Conclusions and added new content in the Introduction.

Lines 73-80; Text was add to: “Comprehensive comparison of existing nZVI synthesis methods, whether top-down (lithographic grinding and precision milling) or bottom-up (borohydride chemical reduction, carbothermic reduction, ultrasonic, electrochemical , green synthesis, etc.), all have certain advantages and limitations[1]. Among these methods, the chemical reduction method is simple and feasible, suitable for use in any laboratory; meanwhile, the product obtained is characterized by a homogeneous structure that displays a high reactivity[2], and has the potential to be widely used in practical application.”

Lines 104-107; Text was altered to: “This study also devoted to the development of easily accessible, economical, and environmentally friendly materials as possible substitutes for modified nZVI, providing as many theoretical foundations as possible for more kinds of modified nZVI.”

1. Stefaniuk, M.; Oleszczuk, P.; Ok, Y. S., Review on nano zerovalent iron (nZVI): From synthesis to environmental applications. Chemical Engineering Journal 2016, 287, 618-632.
2. Jamei, M. R.; Khosravi, M. R.; Anvaripour, B., Investigation of ultrasonic effect on synthesis of nano zero valent iron particles and comparison with conventional method. Asia-Pacific Journal of Chemical Engineering 2013, 8 (5), 767-774.

Reviewer comment 2:

Since the specific surface area is mentioned, the N₂ adsorption/desorption isotherm should be provided.

Thanks for pointing out this accidental omission. Regrettably, although we have searched as much data as possible, with the graduation of the original experimenter, we were unable to find data of the N₂ adsorption and desorption isotherms.

Reviewer comment 3:

All equations listed in text S1 need to be supported with relevant reference.

The authors thank the reviewer for this comment, and the relevant references were added to support the equations listed in Text S1:

Text S1.

1. The reduction rates of nZVI for Cr(VI) reduction were obtained by following formula[3]:

Cr removal efficiency (%)= ×100%

C₀: is the initial Cr(VI) concentration in the solution;

C_t: the concentration of Cr(VI)in the solution at min.

2. The kinetic equation for the removal of Cr (VI) by nZVI was as follows:

first-order[4]: LnC = k₁t + C₀

 second-order[5]: 1/C = k₂t + C₁

pseudo-first-order[6]: Ln(q_e– q_t) = k₃t + C₂

pseudo-second-order[7]: t/q_t = k₄t + C₃

C: the concentration of Cr(VI)in the solution at min.

3. The Langmuir and Freundlich isotherm models can be represented as follows:

Langmuir[8]: q = kc^1/n

Freundlich[9]: q = qe*bc/(1+bc)

q: adsorption capacity at different time(mg·g^-1)

4. Intraparticle[10]: q_t=k*t ^0.5 + C
5. Formula for calculating adsorption capacity[11]:

q: Adsorption capacity at adsorption equilibrium, mg/g;

V: Solution volume, L;

C₀:Initial concentration of solution, mg/L;

C: The concentration of solution at adsorption equilibrium at min, mg/L;

Qt: adsorption capacity of solution at adsorption equilibrium at min , mg/g;

Qe: maximum adsorption capacity, mg/g.

 

1. Stefaniuk, M.; Oleszczuk, P.; Ok, Y. S., Review on nano zerovalent iron (nZVI): From synthesis to environmental applications. Chemical Engineering Journal 2016, 287, 618-632.
2. Jamei, M. R.; Khosravi, M. R.; Anvaripour, B., Investigation of ultrasonic effect on synthesis of nano zero valent iron particles and comparison with conventional method. Asia-Pacific Journal of Chemical Engineering 2013, 8 (5), 767-774.
3. Xu, Y.; Chen, J.;  Chen, R.;  Yu, P.;  Guo, S.; Wang, X., Adsorption and reduction of chromium(VI) from aqueous solution using polypyrrole/calcium rectorite composite adsorbent. Water Research 2019, 160, 148-157.
4. Sadef, Y.; Poulsen, T. G.; Bester, K., Modeling organic micro pollutant degradation kinetics during sewage sludge composting. Waste Management 2014, 34 (11), 2007-2013.
5. Ho, Y.-S., Review of second-order models for adsorption systems. Journal of Hazardous Materials 2006, 136 (3), 681-689.
6. Feng, J.; Su, L.;  Ma, Y.;  Ren, C.;  Guo, Q.; Chen, X., CuFe2O4 magnetic nanoparticles: A simple and efficient catalyst for the reduction of nitrophenol. Chemical Engineering Journal 2013, 221, 16-24.
7. Ho, Y. S.; Ng, J. C. Y.; McKay, G., Kinetics of pollutant sorption by biosorbents: Review. Separation and Purification Methods 2000, 29 (2), 189-232.
8. Foo, K. Y.; Hameed, B. H., Insights into the modeling of adsorption isotherm systems. Chemical Engineering Journal 2010, 156 (1), 2-10.
9. Al-Ghouti, M. A.; Da'ana, D. A., Guidelines for the use and interpretation of adsorption isotherm models: A review. Journal of Hazardous Materials 2020, 393.
10. Kegl, T.; Kosak, A.;  Lobnik, A.;  Novak, Z.;  Kralj, A. K.; Ban, I., Adsorption of rare earth metals from wastewater by nanomaterials: A review. Journal of Hazardous Materials 2020, 386.
11. Zhao, J.; Boada, R.;  Cibin, G.; Palet, C., Enhancement of selective adsorption of Cr species via modification of pine biomass. Science of the Total Environment 2021, 756.
12. Komarek, M.; Koretsky, C. M.;  Stephen, K. J.;  Alessi, D. S.; Chrastny, V., Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto Nanomaghemite: A Spectroscopic and Modeling Approach. Environmental Science & Technology 2015, 49 (21), 12851-12859.
13. Wang, T.; Zhang, L.;  Li, C.;  Yang, W.;  Song, T.;  Tang, C.;  Meng, Y.;  Dai, S.;  Wang, H.;  Chai, L.; Luo, J., Synthesis of Core-Shell Magnetic Fe3O4@poly(m-Phenylenediamine) Particles for Chromium Reduction and Adsorption. Environmental Science & Technology 2015, 49 (9), 5654-5662.
14. Ben Tahar, L.; Oueslati, M. H.; Abualreish, M. J. A., Synthesis of magnetite derivatives nanoparticles and their application for the removal of chromium (VI) from aqueous solutions. Journal of Colloid and Interface Science 2018, 512, 115-126.
15. Hu, Y.; Peng, X.;  Ai, Z.;  Jia, F.; Zhang, L., Liquid Nitrogen Activation of Zero-Valent Iron and Its Enhanced Cr(VI) Removal Performance. Environmental Science & Technology 2019, 53 (14), 8333-8341.
16. Mahmoud, M. E.; Mohamed, A. K.; Salam, M. A., Self-decoration of N-doped graphene oxide 3-D hydrogel onto magnetic shrimp shell biochar for enhanced removal of hexavalent chromium. Journal of Hazardous Materials 2021, 408.
17. Tang, J.; Zhao, B.;  Lyu, H.; Li, D., Development of a novel pyrite/biochar composite (BM-FeS2@BC) by ball milling for aqueous Cr(VI) removal and its mechanisms. Journal of Hazardous Materials 2021, 413.
18. Yang, C.; Ge, C.;  Li, X.;  Li, L.;  Wang, B.;  Lin, A.; Yang, W., Does soluble starch improve the removal of Cr(VI) by nZVI loaded on biochar? Ecotoxicology and Environmental Safety 2021, 208.

 

Reviewer comment 4:

Table of comparison should be provided to compare the removal efficiency of nZVI with other related materials.

Thanks for your suggestion.

A detailed comparison of Cr(VI) removal efficiency by nZVI and those of other related materials is tabulated in Table A as below:

Table A. Comparison of Cr(VI) removal efficiency by nZVI and other related materials

Sorbent	pH	Adsorption rate (mg/g/h)	Adsorption capacity (mg/g)
nano-magnetite[12]	6.0	3.7	1.5
Fe3O4@poly(m-phenylenediamine) particle[13]	2.0	49.2	246.0
Magnetite[14]	2.0	7.5	12.5
liquid nitrogen treated zero-valent iron[15]	6.3	0.4	0.4
graphene oxide hydrogel with shrimp shell magnetic biochar[16]	1.0	28.6	85.9
ball milling synthesized FeS2@biochar composite[17]	3.0	57.5	134.0
nZVI H2A-nZVI Starch-nZVI Fe-Cu	3.0 3.0 3.0 3.0	25.0 66.7 200.0 100.0	50.0 50.0 50.0 50.0

Table A has been provided in the Supplementary Information as Table S5.

 

12. Komarek, M.; Koretsky, C. M.;  Stephen, K. J.;  Alessi, D. S.; Chrastny, V., Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto Nanomaghemite: A Spectroscopic and Modeling Approach. Environmental Science & Technology 2015, 49 (21), 12851-12859.
13. Wang, T.; Zhang, L.;  Li, C.;  Yang, W.;  Song, T.;  Tang, C.;  Meng, Y.;  Dai, S.;  Wang, H.;  Chai, L.; Luo, J., Synthesis of Core-Shell Magnetic Fe3O4@poly(m-Phenylenediamine) Particles for Chromium Reduction and Adsorption. Environmental Science & Technology 2015, 49 (9), 5654-5662.
14. Ben Tahar, L.; Oueslati, M. H.; Abualreish, M. J. A., Synthesis of magnetite derivatives nanoparticles and their application for the removal of chromium (VI) from aqueous solutions. Journal of Colloid and Interface Science 2018, 512, 115-126.
15. Hu, Y.; Peng, X.;  Ai, Z.;  Jia, F.; Zhang, L., Liquid Nitrogen Activation of Zero-Valent Iron and Its Enhanced Cr(VI) Removal Performance. Environmental Science & Technology 2019, 53 (14), 8333-8341.
16. Mahmoud, M. E.; Mohamed, A. K.; Salam, M. A., Self-decoration of N-doped graphene oxide 3-D hydrogel onto magnetic shrimp shell biochar for enhanced removal of hexavalent chromium. Journal of Hazardous Materials 2021, 408.
17. Tang, J.; Zhao, B.;  Lyu, H.; Li, D., Development of a novel pyrite/biochar composite (BM-FeS2@BC) by ball milling for aqueous Cr(VI) removal and its mechanisms. Journal of Hazardous Materials 2021, 413.

 

Reviewer comment 5:

A deep discussion is needed for many parts.

We are grateful for this suggestion. More detailed discussion has been provided Conclusion section in the revised manuscript.

Reviewer comment 6:

The correlation between structural/morphological findings and catalytic performance should be discussed. The following literature may help: 

https://doi.org/10.1016/j.ecoenv.2020.111552

https://doi.org/10.1680/jenes.21.00002

This comment is highly appreciated.

Following the reviewer's suggestion, lines 455-461(revised manuscript lines 396-406) have been revised to：“It can be seen from the SEM images in Fig.9 Fig.11, the particles exhibited an obviously single spherical shape with addition of starch and H₂A, which significantly improved the dispersity of the modified nZVI. Yang et al. [18] showed that soluble starch can act as a protecting and dispersing agent to prevent nZVI from agglomerating. The while the surface of the particles after adding Cu was relatively rough, which can also be considered to increase the effective sites for Cr(VI) adsorption by increasing the specific surface area. The remaining three kinds of modifiers (i.e., CMC, Zn, and Mn) with obvious inhibitory effect were agglomerated in different degrees. Therefore, it can be concluded that the aggregation degree of particles was closely related to the removal rate of Cr(VI) by nZVI. Improved dispersity of nZVI particles could facilitate the removal of target contaminants. A higher degree of agglomeration of nZVI particles causes a lower removal efficiency of target contaminant.”

18. Yang, C.; Ge, C.;  Li, X.;  Li, L.;  Wang, B.;  Lin, A.; Yang, W., Does soluble starch improve the removal of Cr(VI) by nZVI loaded on biochar? Ecotoxicology and Environmental Safety 2021, 208.

Reviewer 4 Report

Main question addressed by the research: The work addresses the influencing parameters of kinetics and mechanisms of CR(VI) removal by nZVI.

Originality and relevance of the topic: The topic is relevant to the field and it considers a suitableresearch gap.
Added value of the paper:  The manuscript takes into account the synthesis and characterization of  nZVI and the  corresponding CR(VI) removal. The paper should include what aspects are critical for these assessments and clearly explain why they are analysing those and why they are needed at the end of the Introduction.

Quality of figures: Very good and clear.
Specific improvements for the paper to be considered:

1. Abstract is too short and general. It should summarize the main findings and applications of the paper. More specific findings should be included in the abstract.
2. Numbering of the sections should be amended.
3. When talking to reactions is not clear in the main text the reaction discussed. It should be referred to with the scheme numbering.
4. Lines 312-315. If the Cr is totally removed, how can the reaction still be affected? Which reaction? 
5. The conclusions are poor and they would need more elaboration so they clearly match the results.

Author Response

Responses to Reviewer #4

 General comments:

Main question addressed by the research: The work addresses the influencing parameters of kinetics and mechanisms of CR(VI) removal by nZVI.

Originality and relevance of the topic: The topic is relevant to the field and it considers a suitable research gap.

Added value of the paper:  The manuscript takes into account the synthesis and characterization of  nZVI and the  corresponding CR(VI) removal. The paper should include what aspects are critical for these assessments and clearly explain why they are analysing those and why they are needed at the end of the Introduction.

Quality of figures: Very good and clear.

Specific improvements for the paper to be considered:

The authors thank the reviewer for providing valuable comments and suggestions, which are of great help to improve the clarity and quality of the manuscript. We gave the response to your specific comments point-to-point in the text according to your suggestions.

Specific comments:

Reviewer comment 1:

Abstract is too short and general. It should summarize the main findings and applications of the paper. More specific findings should be included in the abstract.

This comment is highly appreciated.

According to the requirements indicated by the journal, the abstract should be a single paragraph of about 200 words maximum. Therefore, we have combined the reviewer's suggestions and journal requirements to rewrite and replace the abstract in the manuscript, as shown in the following paragraph:

Abstract：In this study, single-spherical nanoscale Zero Valent Iron (nZVI) with large specific surface area were successfully synthesized was prepared by a simple and rapid chemical reduction method and the adsorptive and removal efficiency of Cr(VI) from aqueous solution by nZVI was explored. XRD spectra and SEM-EDS images showed that the synthesized nZVI particles had excellent crystal structure, but oxidation products such as γ-Fe₂O₃ and Fe₃O₄ were formed on the surface of the particles. with good crystalline spherical structure have been successfully synthesized. The nZVI exhibited a specific surface area of 15.19 m²/g by BET analysis. The effect of different factors on the removal of Cr(VI) by nZVI was studied and the optimum experimental conditions were found. Experimental results showed that approximately 97% of 25 mg/L Cr(VI) was removed by 0.5 g/L nZVI at pH 3 and 20℃ within 1 h. The process performance was significantly inhibited by the increase in the initial solution pH and Cr(VI) concentration or the decrease in the adsorbent dosage. The order of the inhibition of coexisting ions on Cr(VI) removal from strong to weak was HCO₃^–˃NO₃^–˃SO₄^2–˃ Cl^–. The presence of humic acid also had a strong inhibition on the process efficiency, while the effect of cations (i.e., Mg²⁺ and Ca²⁺) was insignificant. Kinetic and thermodynamic equations at different temperatures showed Kinetics studies demonstrated that the removal of Cr(VI) by nZVI was a single-layer chemical adsorption conforming removal fitted better to the pseudo-second-order kinetics. model and the isotherm data was well described by Langmuir model. By applying the Experimental results demonstrated that both the intraparticle diffusion model, the adsorption process was composed of three stages of a rapid diffusion, chemical reduction, and internal saturation.  and chemical reactions are important rate-limiting factors in controlling the adsorption process. FTIR, XPS, and XRD were used to characterize the nZVI before and after the reaction with Cr(VI). Mechanism analysis demonstrated that the removal of Accordingly, a plausible mechanism for Cr(VI) removal by nZVI involved was proposed, including adsorption, reduction, precipitation and co-precipitation. Meanwhile, Cr(VI) was reduced to Cr(III) by nZVI, while FeCr₂O₄, Cr_xFe_1‒xOOH, and Cr_xFe_1‒x(OH)₃ were formed as end products. Besides, various materials were examined for the modification of nZVI to further enhance the performance. In addition, the study It was found that ascorbic acid, starch, and Cu modified nZVI can promote the removal efficiency of Cr(VI) in varying degrees due to the enhanced mobility of the particles. These results can provide new insights into the removal mechanisms of Cr(VI) by nZVI.

Reviewer comment 2:

Numbering of the sections should be amended.

Thanks for pointing out that the number should be amended. Corrected.

Reviewer comment 3:

When talking to reactions is not clear in the main text the reaction discussed. It should be referred to with the scheme numbering.

Thanks for pointing out the omission. Corrected.

Reviewer comment 4:

Lines 312-315. If the Cr is totally removed, how can the reaction still be affected? Which reaction?

According to the original manuscript, lines 312-315 are “Moreover, HCO₃^– could react with Fe²⁺ to form FeCO₃ via Eq. 5 or iron (oxy) hydroxyl carbonate precipitate attaching on nZVI surface, consequently blocking the reactive sites and resulting in a decreased performance.”

As one of the important anions in water, HCO₃^- is often selected as one of the parameters of water. In this study, the addition of HCO₃^- directly affects the pH of the aqueous solution and the number of surface available sites of nZVI. On the one hand, it can be seen from the built-in diagram of Fig.6- HCO₃^- that the addition of HCO₃^- causes the pH of the aqueous solution to rise rapidly from 3 to about 6, and remains relatively stable. This phenomenon promotes the formation of iron hydroxide, which adheres to the surface of nZVI, further reducing its adsorption capacity for Cr(VI). On the other hand, HCO₃^- may react with the generated Fe²⁺ to form siderite (FeCO₃) or Fe²⁺/Fe³⁺ (oxy) hydroxycarbonate, attaching to the nZVI surface thereby consuming iron ions and blocking the reaction site.

Therefore, in the point made in lines 312-315, the specific reaction is shown in Equation 4 (in the revised manuscript), regardless of whether Cr(VI) is completely removed, the effect of HCO₃^- on nZVI will be as described above.

Fe2+ + 2HCO3–→Fe(HCO3)2→FeCO3 +CO2 + H2O

(4)

 

Reviewer comment 5:

The conclusions are poor and they would need more elaboration so they clearly match the results.

We appreciate your helpful suggestions to improve this manuscript. And the conclusions were rewrite and replace in the manuscript, as shown in the following paragraph:

“4. Conclusion

Single-spherical Nanoscale Zero Valent Iron was prepared by chemical reduction method and the adsorptive and removal efficiency of Cr(VI) from aqueous solution by nZVI was explored. XRD spectra and SEM-EDS images showed that the The synthesized nZVI particles had excellent crystal spherical structure, however, oxidation products such as γ-Fe₂O₃ and Fe₃O₄ were formed on the surface of the particles. BET analysis indicated a specific surface area of nZVI of 15.19 m²/g. Experimental results showed that at pH 3 and 20℃, the removal rate of Cr(VI) can reach 100% after 90 min of the reaction, which the adsorption capacity was 50 mg/g, and the initial pH，Cr(VI) concentration and nZVI dosage all affect the removal efficiency of Cr(VI). The effect of different factors on the removal of Cr(VI) by nZVI were studied and the optimum experimental conditions were found. The process of Cr(VI) removal by nZVI was in accordance with the Kinetic and thermodynamic equations at different temperatures showed that the removal of Cr(VI) by nZVI was a single-layer chemical adsorption conforming to pseudo-second-order kinetic model (R²˃ 0.99), the Langmuir adsorption isotherm model (R²˃ 0.97) and the three-stage diffusion process (adsorption stage, redox stage and adsorption-analytic equilibrium stage).kinetics. The order of the inhibition of coexisting ions on Cr(VI) removal from strong to weak was HCO₃^–˃NO₃^–˃SO₄^2–˃ Cl^–. The presence of humic acid also had a strong inhibition on the process efficiency, while the effect of cations (i.e., Mg²⁺ and Ca²⁺) was insignificant. FTIR, XPS, and XRD were used to characterize the nZVI before and after the reaction with Cr(VI).By applying the intraparticle diffusion model, the adsorption process was composed of three stages of a rapid diffusion, chemical reduction, and internal saturation. Accordingly, a plausible mechanism for Cr(VI) removal by nZVI was proposed, including Mechanism analysis demonstrated that the removal of Cr(VI) by nZVI involved adsorption, reduction, precipitation and co-precipitation, in which the reducing products were FeCr₂O₄, Cr_xFe_1‒xOOH, and Cr_xFe_1‒x(OH)₃ were formed as end products. In addition, the modification of nZVI by various materials has been investigated to further enhance the performance. It was found that Fe-starch, Fe-ascorbic acid and Fe-Cu had better removal rates of Cr(VI) than pure phase nZVI, due to the presence of starch and ascorbic acid could effectively reduce the agglomeration between nZVI particles. Some cost-effective materials were evaluated as alternative modifier for further improving the process performance, which provided a research idea for the research direction of nZVI modifier. Our study suggest that nZVI is an effective and green technology for Cr(VI) removal and has a promising application prospects.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you very much to the authors for their reply to my comments. I agree that the understanding of Cr(VI) mechanism removal by nZVI can be improved and that is in fact of high scientific interest but I do not find in the manuscript further improvement to that field of knowledge. Some of the recommended papers already set the fundamental reactions involved in Cr(VI) removal by nZVI and the main products, as well as the influence of the key factors explored in this manuscript.

On the other hand, as mentioned, the synthesis through chemical reduction by NaBH4 is the traditional laboratory-based procedure to obtain nZVI. In your reply you mention the comprehensive comparisson of different synthesis method, which I agree that it has to be done from many points of view, but it has not be studied in this work.

In my opinion the work lacks of scientific originality but I encourage the authors to persist in their good work practices that led them to explore the system from many points of view.

Reviewer 2 Report

Authors improved manuscript according to reviewer's suggestions. Thus present form of manuscript can be accepted for publications.

Reviewer 3 Report

Reviewer’s comments

Manuscript Number: catalysts-1859738

Title: Kinetics and mechanisms of Cr(VI) Removal by nZVI: Influencing parameters and modification

Journal: Catalysts

The authors addressed some of the comments in the revised version. However, further revision is needed according to the following comments.

1.     Since the specific surface area is mentioned, the N₂ adsorption/desorption isotherm should be provided.

Author reply: Thanks for pointing out this accidental omission. Regrettably, although we have searched as much data as possible, with the graduation of the original experimenter, we were unable to find data of the N₂ adsorption and desorption isotherms.

 Since the data are unavailable, it is recommended to omit the discussion on surface area or repeat the experiment to get new data.

 2.     The data listed in Table S5 should be arranged in ascending or descending.

 

3.      The mechanism of adsorption (how Cr ions bind with nZVI) and the correlation between structural/morphological findings and removal performance should be discussed. The following literature may help ChemistrySelect 5(1), 124-146. RSC Adv. 11(58), 36528-36553, J. Mol. Liq. 277, 175-180.