Zeolites and Activated Carbons in Hydroponics: A Systematic Review of Mechanisms, Performance Metrics, Techno-Economic Analysis and Life-Cycle Assessment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the paper entitled “Zeolites and Activated Carbons in Hydroponics: Mechanisms, Performance Metrics, Techno-Economic Analysis and Life-Cycle Assessment,” the authors offer a review of topics related to title. It is not clear what the aim of this paper is, this must be stated at the end of the Introduction. Also, in the Introduction, the authors must compare this review with other in the literature. This section must be significantly expanded.

This is a strong, well-structured review manuscript with a clear focus on an important and timely topic in sustainable agriculture. The integration of techno-economic and life-cycle assessment perspectives is a significant strength.

I recommend improving the Abstract, the current abstract is a good summary but it can be more impactful. I think that it must start by more explicitly stating the core problem in hydroponics (e.g., "The accumulation of phytotoxic ammonium, heavy metals, and organic metabolites in recirculating nutrient solutions poses a major challenge to the sustainability and productivity of hydroponic systems."). Also, the key findings should be mentioned; this is that, instead of just listing material properties, authors must state the key comparative findings. For example: "This review reveals that while zeolites excel in ammonium and cation management, activated carbons are superior for organic contaminant removal. Their combination, especially when functionalized or composited with 2D/3D materials, offers a synergistic solution." The conclusions must mention broader implications, they could end with a stronger conclusion on the impact (e.g., "The judicious application of these advanced sorbents paves the way for more resilient, efficient, and circular hydroponic systems, reducing environmental footprint and enhancing food security.").

The content could be enhanced. This by clarifying the "Techno-Economic Analysis and Life-Cycle Assessment". In the title and aims it is mentioned TEA and LCA, but these are not explicitly addressed as dedicated sections in the body of the review. This is a significant gap. Also, it could be added a new section, where techno-economic and environmental perspectives must be discussed. The relative costs of natural vs. modified/advanced sorbents must be discussed. Compare the cost-benefit of regeneration vs. replacement. Also, the authors can mention the economic impact of extended nutrient solution life and reduced fertilizer use.

In this paper, the authors must discuss the environmental footprint of sorbent production (e.g., high-temperature activation for AC, chemical use for modifications). Compare this to the environmental benefits they provide (water savings, reduced eutrophication potential from discharged nutrients). Address end-of-life: are spent sorbents hazardous waste? Can they be repurposed (e.g., as soil amendments)?

The comparative analysis can be improved, this by creating a summary Table. Table 1 is a good start but is situational. Add a more comprehensive Table 2 that directly compares Zeolites, ACs, and their Composites across key metrics, namely: primary mechanism (ion exchange vs. adsorption vs. catalysis), key target pollutants (NH₄⁺, K⁺, heavy metals, DOC, pesticides), advantages (selectivity, surface area, regenerability), disadvantages/limitations (competition, cost, fouling, stability), relative cost (low, medium, high), regeneration potential (easy, difficult, not feasible).

The synergies must be discussed in dept. The text mentions combined use. Dedicate a paragraph to explaining why the combination is synergistic beyond "separating functions." For instance, zeolites removing NH₄⁺ might reduce biofouling on AC, or AC removing organics might prevent pore blockage in zeolites.

The conclusion touches on future research, but it can be more specific. This can be done by addressing gaps and future directions more concisely.

The English is generally good but can be polished for a more academic and fluid style, and redundancy must be avoided. The acronyms must be defined.

Figure 6 is Unclear: The schematic in Figure 6 is too simplistic and lacks explanatory power. What do "O," "C," "T-TC" represent? This figure should be redesigned to clearly show the synergistic interaction between the zeolite, activated carbon, and the 2D material, perhaps illustrating simultaneous NH₄⁺ exchange, organic adsorption, and photocatalytic degradation.

Ensure all figures (especially SEM images and graphs) are high-resolution (at least 300 DPI) for publication.

I conclude that this manuscript must be improved prior to publication.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The topic of the paper is interesting and topical, however, I would like to make some comments and suggestions.

In my opinion, this does not look like a review work and I do not understand why there are so many authors at work when the work is short with few results.

Technically arrange work, pictures, tables.
Also, your font is not uniform everywhere.
Your references must be recent.
You lack more physical and chemical analyzes in your work.
You lack more comparisons with the literature in the form of some tables. You should compare the results of other authors and then include it all.
Your entire work is very general. You should be concise and specific in some parts of the work.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The current review paper is well written and presents a comprehensive picture of the use of selected materials (zeolites, activated carbons, and 2D materials) in hydroponics. While the review seems to cover the topic well, there are some weaker aspects that should be corrected before the acceptance of the article.

Please correct Figure 3 panel a - it does not represent the correct model of the zeolite composition. Please note that all Si and Al atoms are directly connected to oxygen atoms and so the current picture may be misleading for a non-expert Reader. The same, the arrow pointing to Al sites as the negative charges, is a simplification going too far.

Lines 452-458: authors refer to zeolite-based systems which can exert their catalytic function. I have some doubts regarding this application here - these catalytic processes requires high temperatures and thus these reactions would not operate under room temperatures and mild conditions of hydroponics.

I have some comments on the use of the references cited: reference 87 refers to the catalytic activity of the Pd-zeolite/Al₂O₃ whereas it is meant to support the properties of the activated carbon - transition metal systems for plant growth. The same holds true for references 84 - 86 (the articles refer to zeolites!). Earlier, when one checks placement for references 80 and 81 a very similar patters is observed. The text refers to zeolites, while references refer to photocatalytic properties of other systems. All references should be carefully checked and placed where they are supposed to be.

On  the editorial side: there is an outlying "(" in Table 1.

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper has been improved, I believe that it is ready for publication.

Author Response

Comments 1:The paper has been improved, I believe that it is ready for publication.

Response 1: We sincerely thank you for your positive evaluation of our revised manuscript. We greatly appreciate your time, effort, and constructive feedback throughout the review process. Your comments were valuable in improving the clarity and quality of our work. We are pleased to hear that you now consider the manuscript ready for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Please revise the cited references once more. For instance ref 89 refers to the work on Mn-oxide system and not Mn-zeolite (this serves as the reference for paragraph in lines 572-576: "Particular attention has been given to Mn- and Co-modified zeolites, which display catalytic activity in the oxidation of phenolic compounds and pesticides that may accumulate in recirculating nutrient solutions. Such materials combine ion-exchange and catalytic filtration functions, reducing the load on carbon-based sorbents and enhancing the stability of hydroponic systems [89]." 

No mention of the word "zeolite" in the whole reference 89! Please find the correct references.

Author Response

Comments 1: Please revise the cited references once more. For instance ref 89 refers to the work on Mn-oxide system and not Mn-zeolite (this serves as the reference for paragraph in lines 572-576: "Particular attention has been given to Mn- and Co-modified zeolites, which display catalytic activity in the oxidation of phenolic compounds and pesticides that may accumulate in recirculating nutrient solutions. Such materials combine ion-exchange and catalytic filtration functions, reducing the load on carbon-based sorbents and enhancing the stability of hydroponic systems [89]." No mention of the word "zeolite" in the whole reference 89! Please find the correct references.

Response 1: 

Thank you for your comment. We have corrected the citation and revised the corresponding text. The paragraph now accurately reflects the content of reference 89 and reads:

“Particular attention has been given to Mn-modified zeolites, which exhibit catalytic activity due to various forms of MnOx in the structure or on the surface. Their multivalent Mn states enable efficient oxidation of organic compounds, improving adsorption and catalytic purification in hydroponic systems [89].” (page 18)

Thank you for helping us improve the accuracy of the manuscript.