Crosslinking Approaches for Polyethylene Imine (PEI) and Its Uses in Adsorption of Heavy Metals, Dyes, and Carbon Dioxide

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This review, entitled “Crosslinking approaches of polyethylene imine (PEI) and their uses in adsorption of heavy metals, dyes and carbon dioxide” different strategies for crosslinking PEI and reviews some of the most promising applications in terms of adsorption of metallic and organic compounds.

 

Overall, it is a well-written and well-conducted review with a clear and comprehensive presentation of the different approaches to crosslinking, as well as an exposition of the most significant studies in terms of adsorption. However, there are some small details that I think could be improved:before being accepted for publication.

Below, I detail what I consider to be the most important:

• On reading the abstract I get the feeling that it is only a very brief introduction to the subject, and nothing is said about the objective of carrying out this work, or what gap it is intended to fill. In my opinion, a few lines with this idea should be included and generalities eliminated. Furthermore, I don't quite see the meaning and/or relationship of the phrase: “copper ion is stored in a thin film of crosslinked PEI…” with the idea being developed. Could the authors link to the general idea?
• Similarly, I feel that the introduction lacks the objective of this work or the gap that it aims to fill with respect to the rest of the reviews that have been published to date.
• Line 219: a bracket is missing in the formula Na(BH(OAC)₃)
• Units should be unified. in most cases they appear, for example, as mg g^-1, and in others as mg/g (line 608, 609…) or as “mg (blank space) L^-1”, and in others mAcm^-2 (lines 774-780)

Comments for author File: Comments.pdf

Author Response

Reviewer 1

We thank the reviewer for reading our review article and for giving us important comments. We have revised our article based on the comments received.

This review, entitled “Crosslinking approaches of polyethylene imine (PEI) and their uses in adsorption of heavy metals, dyes and carbon dioxide” different strategies for crosslinking PEI and reviews some of the most promising applications in terms of adsorption of metallic and organic compounds.

Overall, it is a well-written and well-conducted review with a clear and comprehensive presentation of the different approaches to crosslinking, as well as an exposition of the most significant studies in terms of adsorption. However, there are some small details that I think could be improved:before being accepted for publication.

Below, I detail what I consider to be the most important:

• On reading the abstract I get the feeling that it is only a very brief introduction to the subject, and nothing is said about the objective of carrying out this work, or what gap it is intended to fill. In my opinion, a few lines with this idea should be included and generalities eliminated. Furthermore, I don't quite see the meaning and/or relationship of the phrase: “copper ion is stored in a thin film of crosslinked PEI…” with the idea being developed. Could the authors link to the general idea?

Response: We have now added a short paragraph at the end of the abstract about the main objective of the review

Response: The preparation of crosslinked PEI film on a substrate is discussed in section 3.2 of the review article. It can also be referred to the original article (Porous PEI coating for copper ion storage and its controlled electrochemical release. Adv. Sustain. Syst. 2020, 4 (3), 1900123). The study was about adsorption of copper on a thin film of crosslinked PEI and its controlled release by electrochemical method with a sought application in fouling prevention. We have now re-phrased the sentences.

• Line 219: a bracket is missing in the formula Na(BH(OAC)₃)

Response: This is edited

• Units should be unified. in most cases they appear, for example, as mg g^-1, and in others as mg/g (line 608, 609…) or as “mg (blank space) L^-1”, and in others mAcm^-2 (lines 774-780)

Response: the units are edited in the revised review article.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The evaluation report is given in the following for the manuscript (Ref No: 3552303) entitled "Crosslinking approaches of polyethylene imine (PEI) and their uses in adsorption of heavy metals, dyes and carbon dioxide". This is a review paper comprising 101 references, and the approached subject is appropriate for the scope of the Journal. The emerged comments during the manuscript evaluation are given below.

 

1) In the introduction part, the authors should also discuss about the use of free (non-crosslinked) PEI for the removal of heavy metal ions from aqueous solution in a hybrid adsorption-ultrafiltration process, also known as polymer enhanced ultrafiltration (PEUF) or polymer assisted ultrafiltration (PAUF), see for example:

https://doi.org/10.5004/dwt.2023.30174

https://doi.org/10.1016/j.jhazmat.2009.03.145

What would be the advantages of using crosslinked PEI material compared to using free PEI in PEUF?  This aspect should be discussed in this paper. 

 

2) A suggestive scheme should be illustrated showing the possible mechanisms of binding of heavy metal ions and dye molecules by the PEI macromolecule.

 

Author Response

We thank the reviewer for reading our review article and for giving us important comments. We have revised our article based on the comments received.

The evaluation report is given in the following for the manuscript (Ref No: 3552303) entitled "Crosslinking approaches of polyethylene imine (PEI) and their uses in adsorption of heavy metals, dyes and carbon dioxide". This is a review paper comprising 101 references, and the approached subject is appropriate for the scope of the Journal. The emerged comments during the manuscript evaluation are given below.

 

1) In the introduction part, the authors should also discuss about the use of free (non-crosslinked) PEI for the removal of heavy metal ions from aqueous solution in a hybrid adsorption-ultrafiltration process, also known as polymer enhanced ultrafiltration (PEUF) or polymer assisted ultrafiltration (PAUF), see for example:

https://doi.org/10.5004/dwt.2023.30174

https://doi.org/10.1016/j.jhazmat.2009.03.145

Response: we have now added a short paragraph in the introduction section about the use of non-crosslinked PEI for heavy metal ion adsorption and relevant articles are cited.

What would be the advantages of using crosslinked PEI material compared to using free PEI in PEUF?  This aspect should be discussed in this paper. 

 Response: We understand that both ways can have their own advantages and drawbacks. However, the advantages of crosslinked PEI is briefly presented in the revised manuscript.

2) A suggestive scheme should be illustrated showing the possible mechanisms of binding of heavy metal ions and dye molecules by the PEI macromolecule.

Response: We have included some chelation mechanisms (schematic and word description) under section 3.1 of the revised review article. The interaction mechanisms of ions with the chelating material will be different based on the coordination number of the metal ions and the functional groups found in the adsorbent.

Reviewer 3 Report

Comments and Suggestions for Authors

The present review discusses the various crosslinking reactions of PEIs and possible reaction mechanisms reported in literature. In addition to presenting the reactions, the review discusses the resulting crosslinked material properties. Finally, the application of cross-linked PEIs in real life is introduced. However, there are still the following problems in the review, and it is recommended that it be accepted after some revisions.

1. When different crosslinkers are listed, it is suggested that the crosslinkers should be classified, such as inductive classification according to different crosslinking mechanisms. And the types of crosslinkers should be comprehensive.
2. In Scheme 1, the PEI structural formula should be written as a polymer, or the annotation should be changed to PEI monomer.
3. The article does not explain how water exerts the effectiveness of the adhesives.
4. The experimental steps in the article are too detailed, and it is recommended to focus on the details related to the reaction mechanisms.
5. It is recommended to give a schematic diagram of crosslinked polyethyleneimines in its application, and to draw a structural diagram of crosslinked PEIs complexing and chelating with transition metals.

Additional comments:

Although the article cites a lot of literature, it does not cover enough preface research in the past three years. For example, the application of new dynamic covalent crosslinkers (e.g., dynamic imine bonds, disulfide bonds) in PEI crosslinking is not mentioned, and there are important advances in the field of controlled adsorption-desorption. It is suggested that the latest literature be supplemented to enhance the forefront of the review.

A variety of crosslinkers (such as glutaraldehyde, epichlorohydrin, etc.) are listed in this paper, but there is a lack of horizontal comparison of different crosslinkers in terms of mechanical strength, porosity, adsorption selectivity, etc. It is suggested to add a table or subsection to systematically summarize the correlation between the reaction conditions, crosslinking density and adsorption performance of various crosslinkers

The description of the adsorption mechanism of heavy metals/dyes is mainly electrostatic and coordination, but there is no theoretical support such as molecular dynamics simulation or density functional theory (DFT) calculation. It is advisable to cite relevant computational studies to illustrate the differences in the selectivity of amino groups (primary, secondary, terary) for specific metal ions.

Some adsorption parameters are vaguely described (e.g., pH is not specified for "acidic conditions", and specific temperature is not indicated for "room temperature"). It is recommended to supplement key experimental conditions (e.g., pH, ionic strength, etc.) to enhance the scientificity.

The article emphasizes the laboratory performance of PEI materials, but does not discuss the challenges faced by its commercial application (e.g., crosslinker cost, regeneration energy consumption, long-term stability). It is suggested that a paragraph should be added to analyze the economy, adsorbent life and secondary pollution risk of large-scale production, and propose targeted solutions (such as modular design and green crosslinker development).

The units in the "Removal Efficiency" column in Tables 1 and 2 are not uniform (e.g., "mg g⁻¹" is mixed with percentages), and some charts are of low resolution (e.g., Scheme 12). A schematic diagram of the structure of the key reactions should be added so that the reader can better understand it.

The existing conclusions are more generalised, and the unique contribution of this article compared with similar reviews is not clear. It is recommended to refine the innovation points of PEI crosslinking technology.

There were a few grammatical errors (e.g., "pro-environmental PEI gels" should be "environmentally friendly PEI gels") and inconsistent citation formats (page numbers were missing in some literatures).

Author Response

Reviewer 3

We thank the reviewer for reading our review article and for giving us important comments. We have revised our article based on the comments received.

The present review discusses the various crosslinking reactions of PEIs and possible reaction mechanisms reported in literature. In addition to presenting the reactions, the review discusses the resulting crosslinked material properties. Finally, the application of cross-linked PEIs in real life is introduced. However, there are still the following problems in the review, and it is recommended that it be accepted after some revisions.

1. When different crosslinkers are listed, it is suggested that the crosslinkers should be classified, such as inductive classification according to different crosslinking mechanisms. And the types of crosslinkers should be comprehensive.

Response: Our discussion in the review articles is based on the type of crosslinkers and reaction mechanisms. The review article is structured in the following order of crosslinkers:  aldehydic functional group, epoxide, α, β-unsaturated compounds (Michael addition), nucleophilic substitution reactions. We have attempted to discuss as many crosslinker as possible.  

2. In Scheme 1, the PEI structural formula should be written as a polymer, or the annotation should be changed to PEI monomer.

Response: PEI is a polymer itself and the R groups added on the structure is to show that the structure grows in all directions to form the polymer. We believe that this a short way to represent PEI instead of trying to draw differently. We have also drawn the longer form of PEI in the introduction section (Figure 1) for readers to refer.

3. The article does not explain how water exerts the effectiveness of the adhesives.

Response: The adsorbent is mostly used to remove/ concentrate metal cations from water samples. The presence and absence of water molecule in the adsorbent, however, can potentially affect the CO₂ adsorption efficiency of the crosslinked material from the environment. We found one article about the impact of water in CO2 adsorption (Carbon capture with polyethylenimine hydrogel beads (PEI HBs). J. Mater. Chem. A 2018, 6 (43), 21468-21474). However, this is something that needs to be studied further in the future and is briefly stated in “future direction” section of our review article.

4. The experimental steps in the article are too detailed, and it is recommended to focus on the details related to the reaction mechanisms.

Response: The reason why we discussed the experimental steps briefly for each reaction is to give the reader a better idea about how the materials have been prepared which will have a consequence on the material property.   

5. It is recommended to give a schematic diagram of crosslinked polyethyleneimines in its application, and to draw a structural diagram of crosslinked PEIs complexing and chelating with transition metals.

Response: We have included some chelation mechanisms (schematic and word description) under section 3.1 of the revised review article. The interaction mechanisms of ions with the chelating material will be different based on the coordination number of the metal ions and the functional groups found in the adsorbent.

Additional comments:

Although the article cites a lot of literature, it does not cover enough preface research in the past three years. For example, the application of new dynamic covalent crosslinkers (e.g., dynamic imine bonds, disulfide bonds) in PEI crosslinking is not mentioned, and there are important advances in the field of controlled adsorption-desorption. It is suggested that the latest literature be supplemented to enhance the

Response: We have now mentioned dynamic covalent crosslinkers at the end of section 2 even though there are not many publications for PEI. We hope future studies will look into this.

A variety of crosslinkers (such as glutaraldehyde, epichlorohydrin, etc.) are listed in this paper, but there is a lack of horizontal comparison of different crosslinkers in terms of mechanical strength, porosity, adsorption selectivity, etc. It is suggested to add a table or subsection to systematically summarize the correlation between the reaction conditions, crosslinking density and adsorption performance of various crosslinkers

Response: Table 1 and Table 2 shows the type of crosslinkers and the adsorption efficiencies. We have also added a short paragraph just above Table 1 that compares crosslinkers horizontally. 

The description of the adsorption mechanism of heavy metals/dyes is mainly electrostatic and coordination, but there is no theoretical support such as molecular dynamics simulation or density functional theory (DFT) calculation. It is advisable to cite relevant computational studies to illustrate the differences in the selectivity of amino groups (primary, secondary, terary) for specific metal ions.

Response: Future DFT work is recommended in the last section of our article.

Some adsorption parameters are vaguely described (e.g., pH is not specified for "acidic conditions", and specific temperature is not indicated for "room temperature"). It is recommended to supplement key experimental conditions (e.g., pH, ionic strength, etc.) to enhance the scientificity.

Response: Where possible, the word “acidic conditions” are replaced with specific concentration of the acid used like 2% HNO₃ in the revised article. If pH values are not available in the original article, we still used the same word.

Response: The word “room temperature” were used in the original articles without specifying the actual room temperature. So used the same word.

The article emphasizes the laboratory performance of PEI materials, but does not discuss the challenges faced by its commercial application (e.g., crosslinker cost, regeneration energy consumption, long-term stability). It is suggested that a paragraph should be added to analyze the economy, adsorbent life and secondary pollution risk of large-scale production, and propose targeted solutions (such as modular design and green crosslinker development).

Response: The cost and stability of crosslinked PEI is an important aspects and fundamental studies are needed which are mentioned in the revised articles. In some cases, stability of the materials and desorption efficiencies are already discussed.

The units in the "Removal Efficiency" column in Tables 1 and 2 are not uniform (e.g., "mg g⁻¹" is mixed with percentages), and some charts are of low resolution (e.g., Scheme 12). A schematic diagram of the structure of the key reactions should be added so that the reader can better understand it.

Response: Scheme 12 is edited and the units are changed for consistency.

The existing conclusions are more generalised, and the unique contribution of this article compared with similar reviews is not clear. It is recommended to refine the innovation points of PEI crosslinking technol

Response: We have revised the conclusions and the main components of our review paper are captured in the latest version of our article.

There were a few grammatical errors (e.g., "pro-environmental PEI gels" should be "environmentally friendly PEI gels") and inconsistent citation formats (page numbers were missing in some literatures).

Response: The phrase is edited as suggested and the references are formatted.

Reviewer 4 Report

Comments and Suggestions for Authors

This review paper discusses crosslinked polyethyleneimine (PEI) and its applications as an adsorbent for heavy metals, dyes, and carbon dioxide. While the use of PEI as an adsorbent is already well known, this paper has the potential to contribute to addressing global environmental challenges. However, I have several concerns that should be addressed before publication. My comments are as follows:

1. Table 1: The units for recovery efficiency vary across the references. The authors should standardize these values, using either mg g⁻¹ or %, by recalculating the efficiencies based on the data provided in the original references. Additionally, data on non-crosslinked PEI should be included for comparison.
2. The authors should discuss how the chemical structure or physical properties of the crosslinkers influence recovery efficiency.
3. Table 2: As with Table 1, the units should be unified to allow for meaningful comparison.
4. As the authors noted, the regeneration of the adsorbent is just as important as its adsorption efficiency. I strongly recommend that the authors compare the regeneration efficiencies of different crosslinkers.

 

Author Response

Reviewer 4

We thank the reviewer for reading our review article and for giving us important comments. We have revised our article based on the comments received.

This review paper discusses crosslinked polyethyleneimine (PEI) and its applications as an adsorbent for heavy metals, dyes, and carbon dioxide. While the use of PEI as an adsorbent is already well known, this paper has the potential to contribute to addressing global environmental challenges. However, I have several concerns that should be addressed before publication. My comments are as follows:

1. Table 1: The units for recovery efficiency vary across the references. The authors should standardize these values, using either mg g⁻¹ or %, by recalculating the efficiencies based on the data provided in the original references. Additionally, data on non-crosslinked PEI should be included for comparison.

Response: Our paper is limited to discussing crosslinking reactions, mechanisms, material properties and uses. However, we briefly discussed the use of non-crosslinked materials and a brief comparison with crosslinked analogous are added in the introduction section.

-The units are unified.

  2. The authors should discuss how the chemical structure or physical properties of the crosslinkers influence recovery efficiency.

Response: Table 1 and Table 2 shows the type of crosslinkers and the adsorption efficiencies. We have also added a short paragraph just above Table 1 that compares crosslinkers horizontally. The influence of crosslinkers on recovery efficiency are already mentioned under each type of crosslinked material. 

3. Table 2: As with Table 1, the units should be unified to allow for meaningful comparison.

Response: Yes, this is edited.

4. As the authors noted, the regeneration of the adsorbent is just as important as its adsorption efficiency. I strongly recommend that the authors compare the regeneration efficiencies of different crosslinkers.

Response: the regeneration of the adsorbent is already briefly mentioned under each type of crosslinked material. We have also some more details in the revised article.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

My questions have been well addressed, and it can be accepted for publication.