Unveiling the Potential of Novel Struvite–Humic Acid Composite Extracted from Anaerobic Digestate for Adsorption and Reduction of Chromium
Round 1
Reviewer 1 Report
The manuscript presented for review concerns the issue of environmental pollution in the form of chromium compounds. Indeed, it is one of the most important environmental problems related to mining and various industries today. For this reason, work is important as it fits in with the current trend. The characteristics of the obtained composite do not raise any particular concerns, as opposed to the compliance of the title with the content of the article, hence my comments:
I am asking for a more detailed explanation of the kinetics and an indication of which stage is decisive, whether changes take place and whether the phenomenon under study is chemistry or physical adsorption.
The resulting composite, as the title suggests, is to be used for adsorption, while the manuscript does not contain any research customarily carried out in developing the conditions for carrying out this process. Therefore, please complete the research with isotherms determined on the basis of two- and three-parameter models with an appropriate discussion of the results obtained.
Author Response
Manuscript Number: catalysts-1773480
Answers to Editors and Reviewers
The manuscript is revised carefully for publication in Catalysts. We are obliged to the comments from the reviewers of this manuscript, which contributed to improving its quality. This point-by-point response explains how the authors have dealt with the specific referees’ comments. The changes are also marked throughout the manuscript text.
Reviewer #1
Your work is very good and suitable for publication due to novel material and process, however there are some comments that could be taken into account to further improve your work:
Response: Bundle of thanks for your valuable comments. Point by point response to comments is given below.
[1]. Over using the deterministic coefficient (R2) as the only criterion in this case and because the values are very close, it is suggested that another additional criterion be used to reinforce this conclusion, for example, the normalized standard deviation, Δq%
Response: Thanks for the valuable suggestion. The normalized standard deviation is calculated and incorporated in the revised manuscript as per recommendation [Page#6, Line 179-187].
[2] Why were only two models used? There is a wide range of kinetic models (Elovich, Avrami, etc.) which may have valuable information on the Cr (VI) removal process.
Response: We appreciate the comment. The current study was mainly conducted to extract novel struvite-humic acid composite and its detailed characterization employing different analytical techniques. Also, the potential of the derived composite for adsorption and reduction of Cr (VI) using the modeling and analytical approach. We agree that there are a variety of kinetic models to explore the adsorption mechanism of contaminants employing various adsorbents. Our preliminary study has only used the most commonly employed models herein. However, your suggestion is constructive we will consider it in our future studies.
[3]. Why with the pseudo first and pseudo second order models, can it be inferred that only physical forces intervene in the adsorption mechanism? These models give us information about the active sites involved in metal removal and the Elovich model, to mention one, cannot indicate the nature of the surface (chemical or physical).
Response: Thanks for pointing it out. We agreed that with the implication of pseudo first order and pseudo second order, we could not precisely confirm the adsorption mechanism. Generally, the best fitting of pseudo first order implies that the adsorption is physisorption since the rate limiting step in this kind of mechanism is diffusion and is not dependent on the concertation of both reactants (physical exchange) (Wang et al., 2015; Ajmal et al., 2018). Similarly, the pseudo-second order model fitting suggested that the adsorption’s mechanism is chemically rate controlling, called chemisorption (Hubbe et al., 2019). As the data herein in this study was more fitted to pseudo first order, it was inferred that physical forces might be involved in the adsorption. However, the mechanism can be cross verified by the thermodynamic fit into Langmuir or Freundlich’s isotherms, which determine chemisorption and physisorption. Likewise, enthalpy of adsorption can also be used to confirm either the adsorption in chemisorption or physisorption. Similarly, the Elovich model used to verify the nature of adsorption to be physical or chemical. There are different ways to confirm the nature of adsorption. We are sorry for the ambiguity arising from using only pseudo first and second-order models. This study is a part of a project being a preliminary study; only two models were used; however, we have confirmed the adsorption mechanism through analytical techniques also. In our future study, the adsorption mechanism will be explored by employing various models.
References
Wang, J., Liu, G., Li, T and Zhou, C. (2015). Physicochemical studies toward the removal of Zn(II) and Pb(II) ions through adsorption on montmorillonite-supported zero-valent iron nanoparticles. RSC Advances. 5(38), 1-38.
Hubbe, M. A., Azizian, S., and Douven, S. (2019). "Implications of apparent pseudo-second-order adsorption kinetics onto cellulosic materials: A review," BioRes. 14(3), 7582-7626.
Ajmal, Z. Muhmood, A. Usman, M. Kizito, S. Lu, J. Dong, R. Wu, S. (2018). Phosphate removal from aqueous solution using iron oxides: Adsorption, desorption and regeneration characteristics. J Colloid Interface Sci. 528:145-155.
[4] With the FTIR results, it was found that there are functional groups on the surface. Are these not involved in the Chromium adsorption process?
Response: We appreciate the comment. Yes, the functional groups on the surface of struvite-humic composite confirmed through FTIR analysis are involved in chromium adsorption. These findings are described in section 2.4. Mechanisms of Cr (VI) sorption and reduction of the manuscript.
[5] There is information on the isoelectric point of the samples and the pH of the solution to know if the electrostatic forces are significantly present in the adsorption of the metal.
Response: Thanks for the comments. We agreed that the isoelectric point of the samples and the pH of the solution are important parameters to confirm the involvement of electrostatic forces in the adsorption of the metal. We are sorry that this aspect is not explored in the current study. Your suggestion is very nice we will consider it in future studies.
[6] There is a lack of discussion about the maximum adsorption capacity of the materials, comparing them with other materials that have been used for this process to better understand how good a candidate this material is for the elimination of the contaminant.
Response: Bundle of thanks for the valuable suggestion. The discussion regarding the comparison of adsorption capacity of the material used in the current study with already reported adsorbent is incorporated in the revised version of the manuscript [Page#6, Line#172-179].
Reviewer 2 Report
Dear authors
Your work is very good and suitable for publication due to novel material and process, however there are some comments that could be taken into account to further improve your work:
1. Over using the deterministic coefficient (R2) as the only criterion in this case and because the values ​​are very close, it is suggested that another additional criterion be used to reinforce this conclusion, for example, the normalized standard deviation, Δq%
2. Why were only two models used? There is a wide range of kinetic models (Elovich, Avrami, etc.) which may have valuable information on the Cr (VI) removal process.
3. Why with the pseudofirst and pseudosecond order models, can it be inferred that only physical forces intervene in the adsorption mechanism? These models give us information about the active sites involved in metal removal and the Elovich model, to mention one, cannot indicate the nature of the surface (chemical or physical).
4. With the FTIR results, it was found that there are functional groups on the surface. Are these not involved in the Chromium adsorption process?
5. There is information on the isoelectric point of the samples and the pH of the solution to know if the electrostatic forces are significantly present in the adsorption of the metal.
6. There is a lack of discussion about the maximum adsorption capacity of the materials, comparing them with other materials that have been used for this process to better understand how good a candidate this material is for the elimination of the contaminant.
Author Response
Manuscript Number: catalysts-1773480
Answers to Editors and Reviewers
The manuscript is revised carefully for publication in Catalysts. We are obliged to the comments from the reviewers of this manuscript, which contributed to improving its quality. This point-by-point response explains how the authors have dealt with the specific referees’ comments. The changes are also marked throughout the manuscript text.
Reviewer #2
The manuscript presented for review concerns the issue of environmental pollution in the form of chromium compounds. Indeed, it is one of the most important environmental problems related to mining and various industries today. For this reason, work is important as it fits in with the current trend. The characteristics of the obtained composite do not raise any particular concerns, as opposed to the compliance of the title with the content of the article, hence my comments:
I am asking for a more detailed explanation of the kinetics and an indication of which stage is decisive, whether changes take place and whether the phenomenon under study is chemistry or physical adsorption.
Response: Bundle of thanks for your valuable comments. The current study was mainly conducted to extract novel struvite-humic acid composite and its detailed characterization employing different analytical techniques. Also, the potential of the derived composite for adsorption and reduction of Cr (VI) using the modeling and analytical approach. Employing FTIR results and XPS analysis, we explored the involvement of different functional groups in chromium adsorption. Moreover, pseudo first order and pseudo second order kinetic models were employed to investigate the Cr adsorption mechanism. It was perceived that physical, chemical, and redox processes are involved in adsorption. Based on the analytical and modeling approaches' outcomes, we proposed the mechanism of Cr adsorption depicted in Figure 6. There are many models to verify the nature of adsorption, for example, further thermodynamic fitting of Langmuir and Freundlich model, enthalpy of adsorption, Elovich model etc. Being our preliminary study, we have employed the most commonly used models. Hopefully, in our future studies, we will explore the nature of chromium adsorption onto the struvite-humic acid composite by employing various models.
Round 2
Reviewer 1 Report
I am very sorry, but without carrying out standard adsorption studies, and only on the basis of incomplete kinetics, we cannot speak of the usefulness of the material.
