Density Functional Theory Study on the Adsorption of Fe(OH)²⁺ on Kaolinite Surface in Water Environment

Round 1

Reviewer 1 Report

Comments for author File: Comments.docx

Author Response

#1 Article contribution

Please use a separate paragraph with the article's contribution. Now it is within the literature review, and it is vital to highlight the contribution of this paper.

Respond: Thank you for your suggestion. We have revised it according to your comments. In line 111-115, we use a separate paragraph to explain the contribution to the article.

 

#2 Extend introduction

Please extend the introduction with a summary of the article organization. I mean “section 2 concerns…, section 3 presents…, etc.” It is noted that previous studies reviewed are very few, and it is recommended to increase the previous studies to clarify the gap.

Respond: Thanks for your comments. According to your comments, we use a separate paragraph in the introduction to summarize the paper, and compare this question with previous studies. Relevant modifications are marked in red in line 101-110 of the paper.

 

#3 Results

The results in the article are well presented. This is a strong element of the article, but there is a lack of discussion.  Pay more attention to the discussion and it is important to make a comparison with previous relevant studies if any. I recommend that discussion section be separated from results.

Respond: Thank you for your opinion. In the section of results and discussion, we have appropriately added the discussion content of results, and compared the research content with the previous research. Due to the strong logic between the calculation results and the results discussion, it is the most appropriate to put the results together with the discussion in our paper. If it is described separately, the logical relationship of the whole article cannot be closely linked, so we put the result together with the discussion part to better explain.

 

#4 language

There are several grammatical and typographical errors present that impede the flow of reading. The English language needs to be improved for the whole manuscript.

Respond: We are very sorry for our language problems. We consulted native English speakers and made appropriate language adjustments. If necessary, we will also polish the paper through the polishing agency.

 

#5Conclusion

Please extend the conclusions part with future research direction

Respond: Thank you for your suggestion. According to your comments, we have expanded the future research direction in the conclusion part.

Reviewer 2 Report

 

Comments:

0. Major revision. 1. The novelty of this study should be inserted in the text clearly. 2. The advantages and disadvantages of the kaolinite as an adsorbent should be investigated. 3. The mechanism of adsorption should be studied in detail.   4. The stability of the adsorbent after the adsorption process should be presented. 5.  The regeneration of the adsorbent should be presented in detail. 6. The ion leaching from the adsorbent during the adsorption process should be studied. 7. Adsorption isotherm, kinetics and thermodynamics should be investigated. 8. The “introduction” and “results and discussion” sections of the manuscript can be strengthened and supported with some papers related to the literature and cited (optional for authors): Chemical Engineering Journal 146 (2009), 189-193; Materials Research Bulletin 42 (2007), 797-806; Colloids and Surfaces A: Physicochemical and Engineering Aspects 290 (2006), 125-131; Colloids and Surfaces A: Physicochemical and Engineering Aspects 355 (2010), 183-186; Biotechnology and Bioprocess Engineering 20 (2015), 109-116; Composites Part B: Engineering 154 (2018), 388-409; Fibers and Polymers 11 (2010), 234-240; Journal of Molecular Liquids 282 (2019), 115-130; Journal of Environmental Chemical Engineering 7 (2019), 103243; Journal of Applied Polymer Science 122 (2011), 1489-1499; Environmental monitoring and assessment 186 (2014), 5595-5604; J. Environ. Eng. 139 (2013), 1382-1390; Journal of Applied Polymer Science 109 (2008), 4043-4048; Journal of Chemical & Engineering Data 55 (11), 4660-4668.    

 

Author Response

Reviewer 2#

Q1. The novelty of this study should be inserted in the text clearly. 

Respond: Thank you for your comments. The novelty of this study was explained in lines 92-100 of the revised manuscript.

Q2. The advantages and disadvantages of the kaolinite as an adsorbent should be investigated.

Respond: The research content of this paper is closely related to the actual rare earth extraction process. In ion-type rare earth minerals, kaolinite is the main clay mineral adsorbed by rare earth ions (about 90% of ion-type rare earth minerals are kaolinite), so kaolinite is selected as the adsorbent. In the process of exchange rare earth ions from the surface of kaolinite, impurity ions such as Fe are inevitably exchanged, which seriously affects the quality of the rare earth leaching solution. Therefore, we focus on the adsorption mechanism of iron ions on the surface of kaolinite to provide a reference for the study of more selective leaching agents. The advantages and disadvantages of kaolinite as adsorbent are not the focus of this paper, so it is not studied.

Q3. The mechanism of adsorption should be studied in detail.

 Respond: In this paper, the quantum chemical calculation method has been mainly used to calculate the hydrogen bond and chemical bond formed by adsorption of adsorbents and kaolinite surface through the calculation and comparison of adsorption energy. Mulliken population and density of state analysis were used to investigate the adsorption structure, adsorption principle and adsorption properties of Fe hydroxyl hydrate on the ideal kaolinite surface in detail

Q4. The stability of the adsorbent after the adsorption process should be presented. 

Respond: The stability of the adsorption structure of Fe hydrated on kaolinite surface depends on the value of adsorption energy, the number and length of hydrogen bonds and chemical bonds, etc. By comparing these data, the adsorption stability of Fe hydrated on kaolinite (001) surface was obtained. In the state density analysis module, we briefly described the stable state of adsorbent before and after outer/inner layer adsorption on the kaolinite surface, and the results showed that the stability of kaolinite surface was enhanced after adsorption.

Q5. The regeneration of the adsorbent should be presented in detail. 

Respond: The purpose of our study is to investigate the occurrence of Fe hydroxyl hydrate on the surface of kaolinite. In ionic rare earth ore, rare earth ions are mainly adsorbed on the surface of kaolin. Leaching agent is used to exchange rare earth ions from the surface of kaolin as our rare earth leaching solution, which is the goal of rare earth extraction. After rare earth ions are desorbed from the surface of kaolinite, the mission of kaolinite is completed, so the regeneration of adsorbent is not considered in our study.

Q6. The ion leaching from the adsorbent during the adsorption process should be studied. 

Respond: Thank you for your opinion. In this paper, the occurrence state of ions on the surface of kaolinite is mainly studied by means of quantum chemical calculation method, which is also the goal of this paper. Our team plans to study the dynamic analytical process of ions by using molecular dynamics next.

Q7. Adsorption isotherm, kinetics and thermodynamics should be investigated. 

Respond: The purpose of our study is to simulate the adsorption state of impurity Fe on the surface of ion-type rare earth minerals through quantum chemical calculation, and reveal the microscopic adsorption coordination mechanism of hydroxy-hydrated Fe on kaolinite (001) surface. Because the adsorption isotherm and kinetics study is difficult to accurately obtain these information, so no research has been carried out in this field. And the adsorption energy we calculated has reflected the adsorption thermodynamics.

Q8. The “introduction” and “results and discussion” sections of the manuscript can be strengthened and supported with some papers related to the literature and cited (optional for authors):  Chemical Engineering Journal 146 (2009), 189-193; Materials Research Bulletin 42 (2007), 797-806; Colloids and Surfaces A: Physicochemical and Engineering Aspects 290 (2006), 125-131; Colloids and Surfaces A: Physicochemical and Engineering Aspects 355 (2010), 183-186; Biotechnology and Bioprocess Engineering 20 (2015), 109-116; Composites Part B: Engineering 154 (2018), 388-409; Fibers and Polymers 11 (2010), 234-240; Journal of Molecular Liquids 282 (2019), 115-130; Journal of Environmental Chemical Engineering 7 (2019), 103243; Journal of Applied Polymer Science 122 (2011), 1489-1499; Environmental monitoring and assessment 186 (2014), 5595-5604; J. Environ. Eng. 139 (2013), 1382-1390; Journal of Applied Polymer Science 109 (2008), 4043-4048; Journal of Chemical & Engineering Data 55 (11), 4660-4668.  

 Respond:  In view of your opinion, after our discussion, we find out the articles related to this paper from the references you recommend and cited them in line 53-58 to strengthen support for our research.

Round 2

Reviewer 1 Report

No comments 

Reviewer 2 Report

Accept