Municipal Solid Waste Fly Ash-Derived Zeolites as Adsorbents for the Recovery of Nutrients and Heavy Metals—A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper submitted to the Water journal discusses the utilization of municipal solid waste fly ash (MSW-FA) as a feedstock for the production of zeolites, which can then be applied to recover nutrients and heavy metals from wastes. The authors thoroughly analyze constraints associated with using MSW-FA, including its low and variable Si and Al content as well as high contamination levels. Methods for extracting and purifying zeolite precursors are considered, covering acid and alkali leaching. Structural properties allowing zeolites' sorption activity for cations and anions are then discussed. Factors impacting selectivity and sorption rates are analyzed. Data on natural and synthetic zeolite properties are provided. In conclusion, the authors emphasize the need for further optimizing methods to obtain pure zeolite precursors from MSW-FA and selecting optimal zeolite frameworks. Thus, the article systematically consolidates knowledge on utilizing MSW ash for zeolite production, multifariously considering opportunities and limitations of this approach.

The review's value for publication in the journal Water; however, significant revision addressing the comments below could markedly strengthen the article by more fully developing important elements:

1. The abstract currently appears as an unstructured text lacking specific analyzed knowledge and proposals. It must be supplemented with the aims and objectives of developing the literature review by specifying details with real figures and conclusions (what was analyzed and what approaches do the authors propose?).

2. The concluding part of the Introduction could be better written by adding clarification and properly stating the research aims—the specific problems addressed by the authors are lacking. The main research objective is clearly traceable throughout the article—a critical assessment of using zeolites synthesized from components contained in municipal solid waste fly ash (MSW-FA) as sorbents for extracting nutrients (nitrates and phosphates) and heavy metals. A line of specific tasks leading the review to achieve this aim can also be discerned: 1) consider the shortcomings of using MSW-FA as a source for zeolite synthesis; 2) review various precursor extraction and purification methods from MSW-FA; 3) analyze how zeolite properties and application conditions influence cation and heavy metal adsorption; 4) consider modified zeolites for nitrate and phosphate extraction; 5) analyze approaches for substance recovery and regeneration after adsorption; 6) examine regenerating zeolite adsorption capacity. As a recommendation to the authors: embed (summarize) the narrative sequence in the concluding paragraphs of the Introduction. This would significantly improve readability and ease comprehension for readers—who are often less specialized experts in the field.

3. The authors did not sufficiently detail the methodology for extracting Si and Al from MSW ash. While briefly outlining acid and alkali leaching approaches, specific experimental protocols were not presented. Including such methodological details would strengthen the review by allowing evaluation and replication of the approaches discussed.

4. The manuscript would be strengthened by providing a more robust examination of zeolite contamination resulting from toxic components in MSW fly ash. Inclusion of compositional data from both raw ash and synthesized zeolites investigating hazardous impurities, coupled with an evaluation of removal efficacy via documented methods, would enhance the discussion.

5. The manuscript would benefit from expanded coverage of waste management considerations relating to zeolite production and potential environmental impact. Assessing this element through a cited analysis, alongside recommended approaches for preferential waste handling, could strengthen the evaluation of this important aspect.

6. The review would be enhanced through inclusion of a safety evaluation pertaining to the handling of zeolites derived from MSW fly ash. Consideration of documented risk factors and recommended safety precautions through an analyzed discussion would helpfully address this relevant topic.

7. The manuscript could be further strengthened by providing a more in-depth treatment of methods used to characterize the sorption properties of the synthesized zeolites and evaluate their potential applications. Supplementary findings from the authors' own research or pertinent literature concerning the optimization of sorption conditions for various ions and selectivity determinations may aid comprehension and assessment of this important topic.

8. The review would benefit from inclusion of a section addressing magnetic separation and magnetic zeolite composites, an important topic not presently covered. Discussion of magnetic zeolites, with citation of relevant works such as the given reference demonstrating their advantages for simplified recovery of spent material, could represent a valuable addition to the analysis (10.1016/j.net.2021.12.010).

9. The review would benefit from a more robust examination of structure-property relationships. Clarification of how parameters such as XRD structure impact zeolite performance, along with comparison of amorphous versus crystalline and natural versus synthetic varieties via referenced evidence, could strengthen comprehension and analysis. Expanding upon this critical topic through additional scholarly discussion would usefully enhance this section.

10. The article discusses sorption of heavy metals but does not address the problem of decontamination from radioactive metals. Without this information, the review seems incomplete. It is necessary to supplement this section with additional context and references (10.1016/j.net.2022.04.005, 10.1016/j.jeurceramsoc.2022.02.007, 10.1016/j.ceramint.2021.10.164).

11. Little attention is given in the article to issues of durability and regeneration of zeolites. It would be beneficial to supplement with data on the stability of sorption properties over time and the possibility of their renewal.

12. The work does not present an analysis of the economic feasibility of applying various approaches. It would be advisable to supplement the article with cost calculations and possible savings compared to traditional zeolite production methods. The article contains no conclusions on the commercialization opportunities of the technologies. It would be advisable to provide an assessment of the industrial implementation prospects of the described methods for obtaining and applying zeolites from municipal solid waste.

13. The work does not contain conclusions about the prospects for further research in this area. It would be advisable to formulate recommendations on the most priority directions for deepening the topic.

14. The review consists of a very large set of literature sources (a total of 183). Please increase the number of citations from the last 3-5 years. In particular, authors can draw attention to articles published in journals from the MDPI publisher.

Author Response

Thanks you very much for very valuable suggestions to improve our manuscript. Please find below our replies to each of your comments, and please also find the detailed adjustments and additions in the attached revised manuscript with track changes (the new references in the list of references are just marked in red).  

1. The abstract currently appears as an unstructured text lacking specific analyzed knowledge and proposals. It must be supplemented with the aims and objectives of developing the literature review by specifying details with real figures and conclusions (what was analyzed and what approaches do the authors propose?).	The abstract has been revised taking the reviewer’s comments into account
2. The concluding part of the Introduction could be better written by adding clarification and properly stating the research aims—the specific problems addressed by the authors are lacking. The main research objective is clearly traceable throughout the article—a critical assessment of using zeolites synthesized from components contained in municipal solid waste fly ash (MSW-FA) as sorbents for extracting nutrients (nitrates and phosphates) and heavy metals. A line of specific tasks leading the review to achieve this aim can also be discerned: 1) consider the shortcomings of using MSW-FA as a source for zeolite synthesis; 2) review various precursor extraction and purification methods from MSW-FA; 3) analyze how zeolite properties and application conditions influence cation and heavy metal adsorption; 4) consider modified zeolites for nitrate and phosphate extraction; 5) analyze approaches for substance recovery and regeneration after adsorption; 6) examine regenerating zeolite adsorption capacity. As a recommendation to the authors: embed (summarize) the narrative sequence in the concluding paragraphs of the Introduction. This would significantly improve readability and ease comprehension for readers—who are often less specialized experts in the field.	The final part of the introduction has been revised taking the reviewer’s comments into account
3. The authors did not sufficiently detail the methodology for extracting Si and Al from MSW ash. While briefly outlining acid and alkali leaching approaches, specific experimental protocols were not presented. Including such methodological details would strengthen the review by allowing evaluation and replication of the approaches discussed.	This section (2.5.2) has been extended significantly, and a section (2.5.3) concerning hydrothermal treatment of MSW-FA has been included.
4. The manuscript would be strengthened by providing a more robust examination of zeolite contamination resulting from toxic components in MSW fly ash. Inclusion of compositional data from both raw ash and synthesized zeolites investigating hazardous impurities, coupled with an evaluation of removal efficacy via documented methods, would enhance the discussion.	New sections on heavy metals (2.5.4) and dioxins/furans (2.5.5)
5. The manuscript would benefit from expanded coverage of waste management considerations relating to zeolite production and potential environmental impact. Assessing this element through a cited analysis, alongside recommended approaches for preferential waste handling, could strengthen the evaluation of this important aspect.	Included in a new section (2.5.6)
6. The review would be enhanced through inclusion of a safety evaluation pertaining to the handling of zeolites derived from MSW fly ash. Consideration of documented risk factors and recommended safety precautions through an analyzed discussion would helpfully address this relevant topic.	This is partially included in the waste management section, though indicating that the main hazards are in the off gas from the hydrothermal treatment and in the contaminated wastewater. However, we believe the main risk here is related to managing the initial MSW-FA.
7. The manuscript could be further strengthened by providing a more in-depth treatment of methods used to characterize the sorption properties of the synthesized zeolites and evaluate their potential applications. Supplementary findings from the authors' own research or pertinent literature concerning the optimization of sorption conditions for various ions and selectivity determinations may aid comprehension and assessment of this important topic.	We have included some new text related to this but have decided to omit an extended description of any method related to this.
8. The review would benefit from inclusion of a section addressing magnetic separation and magnetic zeolite composites, an important topic not presently covered. Discussion of magnetic zeolites, with citation of relevant works such as the given reference demonstrating their advantages for simplified recovery of spent material, could represent a valuable addition to the analysis (10.1016/j.net.2021.12.010).	We included a new (short) chapter that focus on practical application of zeolites for adsorption. Section 5.2 deals with powdered zeolites, and magnetic zeolites have been included as an important topic there.
9. The review would benefit from a more robust examination of structure-property relationships. Clarification of how parameters such as XRD structure impact zeolite performance, along with comparison of amorphous versus crystalline and natural versus synthetic varieties via referenced evidence, could strengthen comprehension and analysis. Expanding upon this critical topic through additional scholarly discussion would usefully enhance this section.	We have added more related to this topic in Section 2.1 (methodologies; just mentioned), Section 2.4 (amorphous and crystalline structures in MSW-FA), Section 2.5 (producing zeolite-like material from MSW-FA).
10. The article discusses sorption of heavy metals but does not address the problem of decontamination from radioactive metals. Without this information, the review seems incomplete. It is necessary to supplement this section with additional context and references (10.1016/j.net.2022.04.005, 10.1016/j.jeurceramsoc.2022.02.007, 10.1016/j.ceramint.2021.10.164).	The review focus on resource recovery of abundant resources, hence we have chosen to leave radioactive metals out.
11. Little attention is given in the article to issues of durability and regeneration of zeolites. It would be beneficial to supplement with data on the stability of sorption properties over time and the possibility of their renewal.	We have extended the text on desorption (6.2.1) and on regeneration (6.2.3).
12. The work does not present an analysis of the economic feasibility of applying various approaches. It would be advisable to supplement the article with cost calculations and possible savings compared to traditional zeolite production methods. The article contains no conclusions on the commercialization opportunities of the technologies. It would be advisable to provide an assessment of the industrial implementation prospects of the described methods for obtaining and applying zeolites from municipal solid waste.	We don’t believe it is possible/advisable to do this given the current state of development. A quick search on the topic has also been fruitless. Furthermore, none of the authors are economists, hence any trial to make a good guestimate would be more or less useless.
13. The work does not contain conclusions about the prospects for further research in this area. It would be advisable to formulate recommendations on the most priority directions for deepening the topic.	The conclusion is now more or less rewritten to include more prospects for further research, and to include new aspects that have come up during the revision.
14. The review consists of a very large set of literature sources (a total of 183). Please increase the number of citations from the last 3-5 years. In particular, authors can draw attention to articles published in journals from the MDPI publisher.	We had 40 refs from the last 5 years (22%) and 22 refs from last 3 years (12%). Of the new we have added, 32 refs from last 5 years (39%) and 16 refs from last 3 years (19%).

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is a relevant review that already has a high quality, please find below comments to further improve this work:

-line 30-31, stay with the same sig-fig, ie. 3.4 billion tons… - why is it increasing so much?

-line 74-76 – peak phosphorous is actively discussed in the scientific literature. I think with this work, you do not want to get into this debate…

-“In this review we are focusing on the feasibility of extracting Al and Si from MSW- 89 FA and use these as precursors in the synthesis of zeolites to be used as sorbents for the 90 recovery of nutrients and heavy metals from sources rich in these adsorbates.”- I think you should also mention in the title of the paper that you focus on Al and Si, and not heavy metals…

-“involves some sort of stabilization or solidification of the FA 227 prior to landfilling or other confined placement to minimize potential leakage of salts, 228 heavy metals and organic micropollutants” please specify

-Since this is a review I suggest including a chapter on the methodology you used for your literature review, and also a figure that contains the number of papers, etc. that were reviewed for this. (Not only the cited literature, but all of it).

Author Response

Thanks you very much for your suggestions to improve our manuscript. Please find below our replies to each of your comments, and please also find the more detailed adjustments and additions in the attached revised manuscript with track changes (the new references in the list of references are just marked in red). 

-line 30-31, stay with the same sig-fig, ie. 3.4 billion tons… - why is it increasing so much?	The reason is the increased interest in incineration due to the large weight/volume loss you get with concomitant recovery of energy/heat and destruction of organic pollutants and pathogens (stated right above).
-line 74-76 – peak phosphorous is actively discussed in the scientific literature. I think with this work, you do not want to get into this debate…	We have rephrased this: “The need to drastically improve the recovery of phosphorous from available sources due to reduced availability of non-renewable phosphate rock is well known.”
-“In this review we are focusing on the feasibility of extracting Al and Si from MSW- 89 FA and use these as precursors in the synthesis of zeolites to be used as sorbents for the 90 recovery of nutrients and heavy metals from sources rich in these adsorbates.”- I think you should also mention in the title of the paper that you focus on Al and Si, and not heavy metals…	The main aim of the review paper is stated the title; to assess if MSW-FA can be used to synthesize zeolites that can be used to adsorb and recover nutrients and heavy metals – both.
-“involves some sort of stabilization or solidification of the FA 227 prior to landfilling or other confined placement to minimize potential leakage of salts, 228 heavy metals and organic micropollutants” please specify	We have now mentioned some typical processes.
-Since this is a review I suggest including a chapter on the methodology you used for your literature review, and also a figure that contains the number of papers, etc. that were reviewed for this. (Not only the cited literature, but all of it).	This has not been a scoping review, so we have not tried to cover all relevant literature

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thanks to the authors for a job well done with the comments. I believe that this publication deserves to be published in the journal and presented to the scientific community. I wish the authors success in their future work!

Reviewer 2 Report

Comments and Suggestions for Authors

Comments have been addressed well - this can be published now.