Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is quite a unique review in the field of geopolymers as adsorbents for wastewater treatment. There is indeed a major research gap in the field and this review tries to address it. One of the major issue with this review is that ideally it should be critical review with critique based on authors expertise rather than literature summary. The major concerns are listed below:

The title emphasizes “multifunctional adsorbents,” but the concept of multifunctionality is not consistently developed. The review should clearly define multifunctionality and distinguish between multi contaminant adsorption, combined adsorption/catalysis, and shaped adsorbent functionality.

As I mention above, there are many literature examples but critical comparison is missing. The authors should compare other state of the art adsorbents such as MOFs, GOs, COFs for example 10.66173/jenmas.2026.1 and many others are perform better than conventional adsorbents. These emerging adsorbents and their comparison with geopolymers will give the reader an idea about true standing of the geopolymers as adsorbents.

The discussion in table 1 should clearly separate adsorption of dyes, metal ions, anions, and emerging pollutants. These pollutant classes follow different mechanisms and should not be treated as one broad category.

How does the different mechanicsms of adsorption such as Ion exchange, electrostatic attraction, surface complexation, precipitation, pore filling, and π–π are linked to the specific geopolymer compositions and functional groups.

How about the regeneration and reuse of geopolymers. This is a critical issues when it comes to the commercialization and scale up of the adsorbents. This should be included with a special section near the end dedicated to it.

The section on carbon-based functionalization again falls short of providing critique on such adsorbents. Its currently more of a summary. As i mentioned above the comparison with state of the art with critique is important.

For such a review, the conclusion should be more forward looking. What should be future research priorities such as pilot testing, techno-economic analysis, lifecycle assessment, regeneration protocols, and real wastewater validation.

Author Response

We sincerely thank the Editor and the Reviewers for their careful evaluation of our manuscript and for the constructive suggestions. The comments helped us improve the clarity, critical discussion, mechanistic interpretation, and application-oriented perspective of the review. In the revised manuscript, changes were introduced in the Introduction, Sections 4 and 5, the Conclusions, figures, tables, and references. The point-by-point responses are reported below.

Reviewer 1

We sincerely thank the Reviewer for the positive evaluation of the topic and for recognizing the uniqueness and relevance of this review in the field of geopolymer adsorbents for wastewater treatment. We also appreciate the critical comments, which helped us improve the manuscript by moving from a mainly descriptive literature overview toward a more critical and application-oriented discussion.

Comment 1

The title emphasizes “multifunctional adsorbents,” but the concept of multifunctionality is not consistently developed. The review should clearly define multifunctionality and distinguish between multi contaminant adsorption, combined adsorption/catalysis, and shaped adsorbent functionality.

Response: We thank the Reviewer for this important comment. We agree that the concept of multifunctionality needed to be clarified more explicitly. Accordingly, the Introduction has been revised to define multifunctionality at different but complementary levels. In the revised manuscript, multifunctionality is now described as: i) multi-contaminant adsorption, namely the ability of the same material to interact with pollutants of different chemical nature, such as metal ions, dyes, anions, and emerging organic contaminants; ii) the coexistence of different adsorption pathways within the same material, including physical adsorption, ion exchange, electrostatic attraction, surface complexation, precipitation, pore filling, hydrogen bonding, and π–π stacking; and iii) the practical/engineering functionality associated with the possibility of shaping geopolymers into monoliths, foams, membranes, spheres, or 3D-printed structures suitable for continuous-flow or easily recoverable treatment systems. This revision better connects the title of the manuscript with the scope and structure of the review.

Comment 2

As I mention above, there are many literature examples but critical comparison is missing. The authors should compare other state of the art adsorbents such as MOFs, GOs, COFs for example 10.66173/jenmas.2026.1 and many others are perform better than conventional adsorbents. These emerging adsorbents and their comparison with geopolymers will give the reader an idea about true standing of the geopolymers as adsorbents.

Response: We agree with the Reviewer that the position of geopolymers should be better contextualized with respect to other state-of-the-art adsorbents. A new discussion has therefore been added in Section 5, where geopolymer-based adsorbents are compared with graphene oxide/reduced graphene oxide materials, activated carbons, biomass-derived carbons, MOFs, COFs, and hybrid membrane systems. In this section, we acknowledge that several of these advanced adsorbents can show higher adsorption capacities or selectivity under ideal batch conditions, mainly due to their high surface area, ordered porosity, and tailored surface chemistry. However, we also clarify that geopolymers have a different technological strength: they combine moderate-to-high adsorption performance with low-temperature processing, possible use of abundant or waste-derived aluminosilicate precursors, good chemical/mechanical stability, and direct shaping into monoliths, foams, spheres, pellets, membranes, or 3D-printed architectures. Therefore, the revised discussion evaluates geopolymers not only as competitors of advanced adsorbents, but also as processable inorganic matrices able to host or stabilize high-performance phases for practical wastewater treatment configurations.

Comment 3

The discussion in table 1 should clearly separate adsorption of dyes, metal ions, anions, and emerging pollutants. These pollutant classes follow different mechanisms and should not be treated as one broad category.

Response: We thank the Reviewer for this useful suggestion. We agree that different pollutant classes should not be treated as a single broad category, because dyes, metal ions, anions, and emerging pollutants may follow different adsorption pathways. To address this point while preserving the compact structure of the manuscript, Table 1 has been revised by adding a “Pollutant class” column. The entries are now organized to distinguish anionic dyes, cationic dyes, metal ions, and multicomponent metal systems. We did not divide the table into several smaller tables in order to maintain readability and avoid fragmentation of the discussion. Moreover, we avoided adding a simplified mechanism column because the adsorption mechanisms are discussed more rigorously in Section 4 and schematically summarized in the new Figure 6. In the case of pristine geopolymers, the representative studies collected in Table 1 mainly concern dyes and metal ions, whereas emerging pollutants are discussed in the subsequent sections dealing with functionalized geopolymer adsorbents.

Comment 4

How does the different mechanicsms of adsorption such as Ion exchange, electrostatic attraction, surface complexation, precipitation, pore filling, and π–π are linked to the specific geopolymer compositions and functional groups.

Response: We agree that the link between adsorption mechanisms, geopolymer composition, and functional groups needed to be made clearer. For this reason, Section 4 has been substantially revised and a new mechanistic discussion has been introduced. The revised text now explicitly explains that adsorption is both pollutant-dependent and composition-dependent. Low-Si/Al aluminosilicate frameworks and zeolitic domains mainly enhance cation exchange and electrostatic interactions; silanol and aluminol groups favour polar interactions and surface complexation; iron-, calcium-, or aluminium-rich phases may contribute to precipitation or ligand-exchange phenomena; while carbonaceous domains introduce graphitic surfaces, oxygen-containing groups, and microporous regions that favour π–π stacking, hydrogen bonding, van der Waals interactions, and pore filling. A new Figure 6 has also been added to visually summarize these mechanisms in pristine, zeolite-functionalized, and carbon-functionalized geopolymer adsorbents.

Comment 5

How about the regeneration and reuse of geopolymers. This is a critical issues when it comes to the commercialization and scale up of the adsorbents. This should be included with a special section near the end dedicated to it.

Response: We fully agree with the Reviewer that regeneration and reuse are critical aspects for practical implementation and scale-up. A dedicated paragraph has been added near the end of Section 5. In this new discussion, we clarify that the literature on geopolymer adsorbent regeneration is still fragmented, with many studies reporting single-cycle adsorption data and fewer works evaluating adsorption–desorption cycles, capacity retention, mechanical integrity after reuse, or leaching after regeneration. We also discuss that the regeneration strategy depends on both pollutant class and dominant adsorption mechanism. For example, metal-ion adsorption driven by ion exchange or surface complexation may require acid, salt, or chelating solutions, whereas dye-loaded carbon/geopolymer composites may require pH adjustment, solvent washing, oxidative treatment, or thermal regeneration. The revised manuscript also emphasizes that regeneration should not be evaluated only as residual adsorption capacity, but together with mass loss, leaching behaviour, structural stability, and feasibility under flow conditions.

Comment 6

The section on carbon-based functionalization again falls short of providing critique on such adsorbents. Its currently more of a summary. As i mentioned above the comparison with state of the art with critique is important.

Response: We thank the Reviewer for this observation. The section on carbon-based functionalization has been revised to include a more critical discussion. In particular, we now emphasize that the incorporation of carbonaceous phases does not automatically lead to improved adsorption performance, since poor dispersion, aggregation, or partial encapsulation within the geopolymer matrix can reduce the accessibility of the carbon surface. We also discuss that the increase in adsorption capacity should be balanced against the additional processing burden associated with carbon activation, graphene oxide synthesis/reduction, CNT dispersion, or biochar preparation. Furthermore, we highlight that many carbon-rich systems are still investigated as powders or crushed materials, while their actual advantage for wastewater treatment should be demonstrated in shaped bodies suitable for fixed-bed or continuous-flow configurations. Finally, the revised text identifies the stability of the carbon/geopolymer interface, possible release of nanosized carbon phases, and regeneration behaviour as still insufficiently investigated aspects.

Comment 7

For such a review, the conclusion should be more forward looking. What should be future research priorities such as pilot testing, techno-economic analysis, lifecycle assessment, regeneration protocols, and real wastewater validation.

Response: We agree with the Reviewer. The Conclusions have been revised to provide a more forward-looking perspective. The revised final section now highlights the main priorities for future research, including validation in real wastewater matrices, development of standardized regeneration and reuse protocols, evaluation of shaped adsorbents under continuous-flow and pilot-scale conditions, control of ion leaching and pH drift, preservation of mechanical integrity, and integration of techno-economic and life-cycle assessment indicators. We also emphasize that future studies should move beyond the search for the highest q_max under ideal batch conditions and should instead focus on engineered adsorbent bodies able to maintain stable performance under realistic operating conditions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Major Comments:

1. Lack of Intuitive Visual Representation for Core Adsorption Mechanisms

When discussing different modification strategies, the manuscript repeatedly mentions interfacial interaction mechanisms, such as enhanced ion exchange from zeolite modification, as well as π-π interactions and hydrogen bonding introduced by carbon-based materials. However, the main text relies entirely on textual descriptions and lacks intuitive mechanism diagrams. 

Suggestion: It is highly recommended that you should draw and include high-quality mechanism diagrams and a graphical abstract. Specifically, professional plotting software could be used to construct micro-scale models that visually demonstrate the specific interaction processes—such as complexation, electrostatic attraction, and π-π stacking—of multi-component pollutants (e.g., Pb²⁺, Cu²⁺, or organic dyes) on the surface of modified geopolymers.

2. Insufficient Discussion on Anti-Interference Mechanisms in Real Engineering Water Matrices

Although you correctly point out in the conclusion that most existing studies are limited to ideal single-pollutant batch experiments, the main text does not delve into the “anti-interference mechanisms” of modified geopolymers in complex water environments. In actual industrial wastewater detoxification or municipal tailwater advanced purification projects, high concentrations of co-existing ions and dissolved organic matter will intensely compete with the adsorbents. 

Suggestion: It is suggested to add a dedicated subsection in Section 4 to specifically discuss competitive adsorption behaviors in complex matrices (such as tailwater or industrial wastewater). Additional literature should be supplemented to demonstrate the targeted selectivity under different modification strategies (e.g., the anti-interference capabilities of MOF/COF-derived carbon materials or specific zeolite phases in complex water quality scenarios) to enhance the article's reference value for real-world engineering applications.

3. Absence of Regeneration and Life Cycle Assessment

As a paper submitted to Sustainability, the “sustainability” of the materials cannot merely stop at the use of solid waste precursors like fly ash or red mud. The service life, desorption/regeneration efficiency, and safe disposal of the adsorbents after saturation are decisive factors in evaluating their engineering feasibility, yet the article barely touches upon these aspects.

Suggestion: You should add a systematic review of the desorption/regeneration performance of the adsorbents. Additionally, briefly discuss the safe disposal strategies for spent adsorbents (such as solidification or resource utilization).

4. Table Formatting and Data Alignment Issues

The layout of some tables in the manuscript is somewhat disorganized, which severely affects the reading experience. For example, there are obvious misalignments and line breaks between the column names and data in Table 4 (e.g., in the Ref column, [121 and ] are split into different lines). 

Suggestion: You need to readjust the width and line-breaking logic of tables to ensure that the headers strictly align with the data.

Author Response

We sincerely thank the Reviewer for the constructive comments and suggestions. The manuscript has been revised accordingly, with particular attention to the visualization of adsorption mechanisms, real-water matrix limitations, regeneration and reuse, sustainability considerations, and table formatting.

Comment 1

Lack of Intuitive Visual Representation for Core Adsorption Mechanisms. When discussing different modification strategies, the manuscript repeatedly mentions interfacial interaction mechanisms, such as enhanced ion exchange from zeolite modification, as well as π-π interactions and hydrogen bonding introduced by carbon-based materials. However, the main text relies entirely on textual descriptions and lacks intuitive mechanism diagrams. Suggestion: It is highly recommended that you should draw and include high-quality mechanism diagrams and a graphical abstract. Specifically, professional plotting software could be used to construct micro-scale models that visually demonstrate the specific interaction processes—such as complexation, electrostatic attraction, and π-π stacking—of multi-component pollutants (e.g., Pb2+, Cu2+, or organic dyes) on the surface of modified geopolymers.

Response: We thank the Reviewer for this useful suggestion. A new mechanism diagram has been added as Figure 6 in the revised manuscript. The figure schematically represents the main adsorption pathways involved in geopolymer-based adsorbents. In particular, it distinguishes between pristine/zeolite-functionalized geopolymers, where ion exchange, electrostatic attraction, surface complexation, precipitation, and pore filling are dominant, and carbon-functionalized composites, where additional physical adsorption pathways such as micropore filling, π–π stacking, hydrogen bonding, van der Waals interactions, and interactions with oxygen-containing groups become relevant. The accompanying text in Section 4 has also been expanded to directly link these mechanisms to geopolymer composition, zeolitic phases, carbonaceous domains, functional groups, pH, and pollutant chemistry. This addition provides a more intuitive and mechanistic interpretation of the adsorption behavior of functionalized geopolymer adsorbents.

Comment 2

Insufficient Discussion on Anti-Interference Mechanisms in Real Engineering Water Matrices. Although you correctly point out in the conclusion that most existing studies are limited to ideal single-pollutant batch experiments, the main text does not delve into the “anti-interference mechanisms” of modified geopolymers in complex water environments. In actual industrial wastewater detoxification or municipal tailwater advanced purification projects, high concentrations of co-existing ions and dissolved organic matter will intensely compete with the adsorbents. Suggestion: It is suggested to add a dedicated subsection in Section 4 to specifically discuss competitive adsorption behaviors in complex matrices (such as tailwater or industrial wastewater). Additional literature should be supplemented to demonstrate the targeted selectivity under different modification strategies (e.g., the anti-interference capabilities of MOF/COF-derived carbon materials or specific zeolite phases in complex water quality scenarios) to enhance the article's reference value for real-world engineering applications.

Response: We agree with the Reviewer that competitive adsorption and matrix interference are crucial for real wastewater applications. Since the available literature on geopolymer adsorbents tested in complex real matrices remains limited and highly heterogeneous, we considered that a dedicated extensive subsection could be speculative. However, we have strengthened the discussion in Sections 4 and 5 to explicitly address this issue. The revised manuscript now states that adsorption performance is strongly affected by pH, water matrix composition, competing ions, dissolved organic matter, and multi-pollutant conditions. We also clarify that real effluents may alter the relative contribution of ion exchange, electrostatic interactions, pore filling, and surface complexation, depending on pollutant class and material composition. In Section 5, we further emphasize that most available studies are still based on single-pollutant batch experiments and that future research should validate functionalized geopolymers in real wastewater matrices, including industrial effluents, municipal tailwaters, and mixed-contaminant systems. This issue has also been included among the main future research priorities in the revised Conclusions.

Comment 3

Absence of Regeneration and Life Cycle Assessment. As a paper submitted to Sustainability, the “sustainability” of the materials cannot merely stop at the use of solid waste precursors like fly ash or red mud. The service life, desorption/regeneration efficiency, and safe disposal of the adsorbents after saturation are decisive factors in evaluating their engineering feasibility, yet the article barely touches upon these aspects. Suggestion: You should add a systematic review of the desorption/regeneration performance of the adsorbents. Additionally, briefly discuss the safe disposal strategies for spent adsorbents (such as solidification or resource utilization).

Response: We thank the Reviewer for this important comment. We agree that the sustainability of geopolymer adsorbents cannot be inferred only from the use of waste-derived precursors. In the revised manuscript, Section 5 has been expanded to include a specific discussion on regeneration and reuse. We now emphasize that regeneration efficiency, adsorbent reuse, mechanical integrity after repeated cycles, leaching behaviour, and management of spent sorbents are essential parameters for evaluating engineering feasibility. The revised text also explains that regeneration protocols depend on the pollutant class and adsorption mechanism, and that treatments such as acid/salt/chelating solutions, pH adjustment, solvent washing, oxidative treatment, or thermal regeneration may restore part of the capacity but can also affect the geopolymer framework, pore accessibility, or functional phase stability.

Regarding life-cycle assessment, we agree that this is a key issue for a Sustainability paper. However, directly comparable peer-reviewed LCA studies specifically focused on functionalized geopolymer adsorbents for wastewater treatment are still scarce. For this reason, we avoided introducing a speculative quantitative LCA comparison. Instead, we added a critical discussion emphasizing that future assessments should consider activator production, functional additives, regeneration efficiency, adsorption performance under realistic conditions, and end-of-life management of spent adsorbents. The revised manuscript also identifies techno-economic and life-cycle assessments based on pollutant-removal functional units as important priorities for future research.

Comment 4

Table Formatting and Data Alignment Issues. The layout of some tables in the manuscript is somewhat disorganized, which severely affects the reading experience. For example, there are obvious misalignments and line breaks between the column names and data in Table 4 (e.g., in the Ref column, [121 and ] are split into different lines). Suggestion: You need to readjust the width and line-breaking logic of tables to ensure that the headers strictly align with the data.

Response: We thank the Reviewer for pointing out this formatting issue. The tables have been carefully revised to improve readability and alignment. Column widths and line breaks were adjusted, and the reference column was reformatted to avoid inappropriate splitting of citation numbers. Particular attention was paid to Table 4, where the alignment between headers and data has been corrected. We also reviewed the other tables to ensure a more consistent layout throughout the manuscript.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accepted