Cost-Effective Carbon Dioxide Removal via CaO/Ca(OH)2-Based Mineralization with Concurrent Recovery of Value-Added Calcite Nanoparticles
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsOverall, a good paper that can be improved.
On the improvement side:
Only a small mistake at line 251, solubility of CO2 DECREASE with increase in temperature (mass transfer yes increase, but solubility not).
Also, it should be specified the providers and properties of substance used (CaO and CO2).
But, honestly this paper is more appropriate in topics of nanomaterials than sustainability… based on it own conclusions… To get high value products, someone needs high purity starting materials (CaO and CO2)… Using as raw materials, industrial waste for sure the results will not be as great, as they have high degree of contaminants (heavy metals in CaO and sulfur compounds in CO2 streams) and using CaO obtained from CaCO3 to get back CaCO3, has little sense from economic and logic points of view as the paper say.
Author Response
Thank you for the constructive comments.
Comment 1: Only a small mistake at line 251, solubility of CO₂ decreases with increasing temperature (mass transfer does increase, but solubility does not).
Response: We have corrected the error at line 251. The revised text now properly reflects that the solubility of CO₂ decreases with increasing temperature.
Comment 2: It should also be specified who the providers are and the properties of the substances used (CaO and CO₂).
Response: We have added the providers and key properties of the substances used in this study as follows:
“The CaO used in this study was purchased from Sigma-Aldrich (≥98% purity, particle size <75 μm). High-purity CO₂ gas (99.5%) was supplied by Linde Korea and used without further purification.”
We also acknowledge the reviewer’s concern regarding the use of high-purity feedstocks. As part of our future work, we plan to investigate industrial waste-derived CaO and CO₂ streams to evaluate the impact of potential contaminants on process feasibility.
Reviewer 2 Report
Comments and Suggestions for AuthorsLee and coauthors report CaO-based CO2 mineralization for nano-grade calcite generation. The high value of this nano-calcite is claimed to turn this technology from a "cost center into a net revenue generator." The idea is promising and appears scientifically viable. However, the weak experimental results and unsupported assumptions make the key conclusions less robust and compelling. Consider the following suggestions to improve the persuasiveness of the manuscript:
- More details on chemicals and gases, such as manufacturer and purity, should be provided in the Materials and Methods section. What CO₂ concentration was used, and why was this concentration chosen? Keep in mind that flue gas (~15% CO2) and air (~0.04% CO2) are the most common CO2 sources.
- What are the dimensions of the carbonation reactor? What is the Ca(OH)2 loading in each experiment? A detailed description of all processes is needed, such as the CaO - water ratio for preparing the Ca(OH)2 slurry and the duration of the CO2-CaO mineralization in each run.
- What was the temperature used for slurry dehydration and drying when obtaining the dried materials?
- Provide the equation and parameters used to calculate crystallite size via the Scherrer equation in the Methods section.
- Although the authors counted hundreds of particles for the particle size distribution histogram, this still does not present the full picture. Microscopy images are usually selective, and a large number of other particles may be overlooked. To obtain a more accurate particle size distribution, the use of a particle size analyzer (laser diffraction or dynamic light scattering) is strongly recommended. This would make the claims more compelling and better supported.
- As listed, a variety of factors affect mineralization and crystallization processes. In addition to those already discussed, fluid flow patterns (liquid or gas) are also non-trivial factors (e.g., https://doi.org/10.1016/S0009-2509(99)00395-4, https://doi.org/10.1016/j.cej.2024.151761). However, this manuscript fails to examine such determinants experimentally, which makes the study less rigorous and robust.
- The TEA is too simplified to be convincing. The profitability heavily relies on the assumed price of nano-calcite. The manuscript assumes a yield of 60-70% nano-grade calcite (unclear whether this refers to weight percent or number percent). However, no experimental results are presented to support this assumption. In practice, calcite tends to aggregate and form bulk calcite, which often dominates by weight. Solely relying on this assumption to claim the process becomes a "revenue generator" is not professional.
Author Response
Reviewer 2.
Thank you for the constructive and detailed comments. We have carefully considered each point and revised the manuscript accordingly. Our responses are provided below.
Comment 1. More details on chemicals and gases, such as manufacturer and purity, should be provided in the Materials and Methods section. What CO₂ concentration was used, and why was this concentration chosen? Keep in mind that flue gas (~15% CO2) and air (~0.04% CO2) are the most common CO2 sources.
Response: We have specified the providers and key properties of the substances used in this study as follows:
“The CaO used in this study was purchased from Sigma-Aldrich (≥98% purity, particle size <75 μm). High-purity CO₂ gas (99.5%) was supplied by Linde Korea and used without further purification.”
Comment 2. What are the dimensions of the carbonation reactor? What is the Ca(OH)2 loading in each experiment? A detailed description of all processes is needed, such as the CaO - water ratio for preparing the Ca(OH)2 slurry and the duration of the CO2-CaO mineralization in each run.
Response: The revised manuscript now provides the following details:
“In this study, the carbonation reactor was a stainless-steel cylindrical vessel with an internal volume corresponding to a diameter of 1.5 m and a height of 2 cm. For each experiment, 20 kg of Ca(OH)₂ slurry was prepared by hydrating CaO with deionized water to form a homogeneous suspension. Carbon dioxide was continuously injected through a gas ejector system at a controlled flow rate of 100 L·min⁻¹ per kilogram of Ca(OH)₂. The carbonation reaction was conducted for 12 h to achieve complete conversion, while pH and temperature were monitored throughout the process to track reaction progress.”
Comment 3. What was the temperature used for slurry dehydration and drying when obtaining the dried materials?
Response: The slurry dehydration and drying processes were performed at room temperature under ambient laboratory conditions, without external heating. However, we note that exothermic heat release during hydration and subsequent carbonation may have caused slight local temperature increases, which could have influenced reaction kinetics to a limited extent. This clarification has been included in the revised Materials and Methods section.
Comment 4. Provide the equation and parameters used to calculate crystallite size via the Scherrer equation in the Methods section.
Response: The revised manuscript now includes the Scherrer equation along with the parameters and calculation procedure used.
Comment 5. Although the authors counted hundreds of particles for the particle size distribution histogram, this still does not present the full picture. Microscopy images are usually selective, and a large number of other particles may be overlooked. To obtain a more accurate particle size distribution, the use of a particle size analyzer (laser diffraction or dynamic light scattering) is strongly recommended. This would make the claims more compelling and better supported.
Response: We sincerely appreciate this valuable suggestion. We acknowledge that microscopy-based particle size analysis may have selectivity limitations and agree that a particle size analyzer (e.g., laser diffraction or dynamic light scattering) would provide a more comprehensive distribution. In this study, TEM-based measurements were primarily used to confirm nanoscale morphology and crystallinity. Nevertheless, we have clarified this limitation in the revised manuscript and plan to incorporate particle size analyzer measurements in future work to provide more accurate and statistically representative particle size distributions.
Comment 6. As listed, a variety of factors affect mineralization and crystallization processes. In addition to those already discussed, fluid flow patterns (liquid or gas) are also non-trivial factors (e.g., https://doi.org/10.1016/S0009-2509(99)00395 4, https://doi.org/10.1016/j.cej.2024.151761). However, this manuscript fails to examine such determinants experimentally, which makes the study less rigorous and robust.
Response: We thank the reviewer for highlighting this important point. We fully agree that fluid flow patterns, both in liquid and gas phases, play a critical role in mineralization and crystallization. In the present work, we primarily focused on the effects of temperature, CO₂ flow rate, pH, and precursor characteristics, and did not experimentally isolate the influence of hydrodynamics. As suggested, future studies will be designed to systematically investigate liquid–gas flow patterns to further enhance the rigor and robustness of our findings.
Comment 7. The TEA is too simplified to be convincing. The profitability heavily relies on the assumed price of nano-calcite. The manuscript assumes a yield of 60-70% nano-grade calcite (unclear whether this refers to weight percent or number percent). However, no experimental results are presented to support this assumption. In practice, calcite tends to aggregate and form bulk calcite, which often dominates by weight. Solely relying on this assumption to claim the process becomes a "revenue generator" is not professional.
Response: We acknowledge that the current TEA is simplified and that profitability is highly dependent on both the assumed yield and the market price of nano-calcite. In the revised manuscript, we clarified that the assumed yield of 60–70% refers to weight percent and that this value was drawn from literature, not direct experimental evidence. We also recognized the possibility of aggregation leading to bulk calcite formation and have stated this limitation explicitly. Future work will include systematic yield analyses to provide more robust support for the TEA.
Reviewer 3 Report
Comments and Suggestions for AuthorsManuscript ID: sustainability-3842709
Title: Cost-Effective Carbon Dioxide Removal via CaO-based Mineralization with Concurrent Recovery of Value-Added Calcite Nanoparticles
Overall Assessment
This manuscript presents a relevant and conceptually compelling study on an integrated process for carbon dioxide mineralization that concurrently generates a high-value product. The research is timely, addressing the need for economically viable carbon capture technologies, and is supported by a experimental characterization. The proposed multi-revenue model is a significant strength, offering a fresh perspective on scaling carbon management. However, the study's impact is currently limited by a substantial disconnect between its central justification—the use of industrial waste—and its experimental execution, which relies on high-purity reagents. Bridging this gap is essential for the manuscript to fulfill its promising potential.
Summary General Comments
The core strength of the work lies in its demonstration of converting pure CaO into high-purity, nanocrystalline calcite with controlled morphology, characterized by XRD and TEM. The techno-economic framework is a welcome addition that moves beyond pure scientific inquiry into practical feasibility. The primary weakness, however, is that the compelling economic and environmental case is built upon the use of impure, low-cost waste streams (e.g., steel slags, lime sludge), while the experimental data and subsequent analysis implicitly assume a high-purity feedstock. This oversight affects the technical conclusions, the economic model, and the manuscript's novelty in the context of existing literature, which has already documented the significant challenges impurities pose to such processes. The manuscript would be greatly strengthened by directly acknowledging and addressing this central challenge.
Suggestions Specific Comments
- Addressing the Waste-Purity Disconnect: Commenting regarding contaminants is the most critical point. The manuscript should not present the use of waste as a straightforward substitution. A new subsection in the Discussion (e.g., 4.4. Challenges and Strategies for Waste-Derived Feedstocks) is necessary. This section should:
- Explicitly list expected impurities (e.g., Si, Al, Fe, P) from the waste streams mentioned in the introduction.
- Discuss their documented impacts on carbonation kinetics, particle morphology, and final product purity, citing studies that have faced these issues.
- Propose specific pre-treatment or purification strategies (e.g., magnetic separation, acid leaching, sieving) that would be necessary to mitigate these effects, thereby addressing a "list of refining processes."
- Incorporating Literature Comparison: The manuscript would benefit from a more direct dialogue with current literature. Specifically, it should:
- Contrast the clean, controlled synthesis shown here with studies that have successfully produced carbonates from wastes, acknowledging the typically lower purity and value of those products.
- Cite and discuss research that focuses specifically on nanomaterial synthesis from wastes to highlight the additional purification hurdles involved, which your study currently overlooks.
- This comparison will ground your study in the existing field and clearly articulate its specific advancement—a benchmark for high-purity synthesis—while honestly outlining the path forward for waste utilization.
- Refining the Techno-Economic Analysis (TEA): The economic model requires refinement to be realistic. I suggest developing two distinct scenarios:
- Scenario A (High-Purity Pathway): Use the true cost of a feedstock pure enough to make nano-calcite (i.e., the cost of purified waste or commercial precipitated CaO, which is >>$0.15/kg).
- Scenario B (Bulk Valorization Pathway): Model the use of raw, unprocessed waste (cost ~$0) but assign a realistic market value to the resulting lower-purity carbonate product suitable for construction or agriculture, not biomedicine.
- The model must also include the capital and operational expenditures for the purification steps mentioned above, as these "additional costs should be considered."
- Clarifying Technical Details:
- Crystallite vs. Particle Size: The distinction between the XRD-derived crystallite size (~86 nm) and the TEM-measured particle size (~105 nm) should be explicitly stated in the results to accurately describe the polycrystalline nature of the aggregates.
- Table 1 Enhancement: This table requires a descriptive caption and more quantitative detail. Specifically, the "CaO Precursor Characteristics" row should include numerical targets for particle size (e.g., "D50: 1-10 µm") and impurity limits (e.g., "SiO₂ < 0.5%") to be useful for process engineers.
Recommendations
- Major Revision: The highest priority is to address the waste-purity paradox. The manuscript must either explicitly frame the current work as a baseline study with pure CaO, with waste integration as future work, or include a substantial discussion section (4.4) that details the challenges and proposed solutions for impurity management, supported by literature comparisons.
- Revise the TEA: Rework the economic analysis to present the two proposed scenarios. This will provide a more credible and nuanced assessment of profitability, acknowledging the trade-offs between feedstock cost and product value.
- Clarify Methods and Results: Specify which additive(s) were used in this specific study and their concentration. Amend the results text to clarify the crystallite/particle size distinction.
- Complete Table 1: Add a full caption and quantify the parameters to transform the table from a qualitative list into a definitive summary of optimal conditions.
By undertaking these revisions, the authors can significantly strengthen their manuscript, transforming it into a more comprehensive, credible, and impactful contribution that effectively bridges the gap between a promising laboratory concept and a realistic pathway for sustainable deployment.
Author Response
Reviewer 3.
We sincerely appreciate the reviewer’s recognition of the conceptual significance and timeliness of this study, particularly the integration of CO₂ mineralization with the concurrent recovery of high-value calcite nanoparticles. At the same time, we acknowledge the reviewer’s concern regarding the current disconnect between the economic justification based on industrial waste and the experimental work performed with high-purity reagents. We have carefully considered each point and revised the manuscript accordingly. Our detailed responses are provided below.
Comment 1: Explicitly list expected impurities (e.g., Si, Al, Fe, P) from the waste streams mentioned in the introduction.
Response: In the revised manuscript, we have specified the concentration and purity of the CaO used, emphasizing that high-purity reagents were employed to ensure reproducibility at this preliminary stage. We acknowledge that impurities in waste-derived feedstocks—such as Si, Al, Fe, and P—are critical factors for practical application but have not yet been experimentally addressed in this study. As correctly pointed out by the reviewer, these impurities are expected to significantly affect process feasibility. Accordingly, we plan to incorporate systematic investigations of impurity effects in future work.
Comment 2: Discuss their documented impacts on carbonation kinetics, particle morphology, and final product purity, citing studies that have faced these issues.
Response: The revised manuscript now provides an expanded discussion of particle morphology and final product purity. However, we recognize that our discussion of carbonation kinetics remains limited due to the absence of supporting experimental data. We have acknowledged this gap and noted that additional studies are needed to provide a more comprehensive evaluation.
Comment 3: Propose specific pre-treatment or purification strategies (e.g., magnetic separation, acid leaching, sieving) that would be necessary to mitigate these effects, thereby addressing a "list of refining processes."
Response: We acknowledge the importance of refining processes when using waste-derived feedstocks. Since the present work employed high-purity reagents, additional refining was not required. Nevertheless, in future studies we plan to evaluate combined approaches such as sieving and magnetic separation. A separate study focusing on refining strategies is also in preparation to address this important aspect.
Comment 4: Contrast the clean, controlled synthesis shown here with studies that have successfully produced carbonates from wastes, acknowledging the typically lower purity and value of those products.
Response: A comparative discussion has been added to the revised manuscript, highlighting that studies using waste-derived feedstocks typically report lower product purity and value compared to our high-purity synthesis.
Comment 5: Cite and discuss research that focuses specifically on nanomaterial synthesis from wastes to highlight the additional purification hurdles involved, which your study currently overlooks.
Response: We appreciate this suggestion. The revised manuscript now cites and discusses studies on nanomaterial synthesis from waste-derived feedstocks. These works demonstrate that additional purification steps are often required to mitigate impurity effects, which can negatively influence carbonation kinetics, particle morphology, and product quality. By incorporating this discussion, we have clarified that our baseline high-purity study overlooks these challenges, and we highlight that overcoming such hurdles will be a critical focus of our future work.
Comment 6: This comparison will ground your study in the existing field and clearly articulate its specific advancement—a benchmark for high-purity synthesis—while honestly outlining the path forward for waste utilization.
Response: We have added a comparison with studies utilizing waste-derived feedstocks, underscoring the lower purity and performance of their products relative to our controlled synthesis. This positions our work as a benchmark study for high-purity calcite nanoparticle production, while outlining the necessary steps for adapting the process to waste-based materials. This addition strengthens the manuscript by clarifying both the advancement provided by our study and the pathway toward realistic waste utilization.
Comment 7: Refining the Techno-Economic Analysis (TEA): The economic model requires refinement to be realistic. I suggest developing two distinct scenarios:
Scenario A (High-Purity Pathway): Use the true cost of a feedstock pure enough to make nano-calcite (i.e., the cost of purified waste or commercial precipitated CaO, which is >>$0.15/kg).
Scenario B (Bulk Valorization Pathway): Model the use of raw, unprocessed waste (cost ~$0) but assign a realistic market value to the resulting lower-purity carbonate product suitable for construction or agriculture, not biomedicine.
The model must also include the capital and operational expenditures for the purification steps mentioned above, as these "additional costs should be considered."
Response: We appreciate this insightful suggestion. We agree that scenario-based modeling is essential for realistic assessment of economic feasibility. As the present study represents an early-stage baseline analysis, we focused on high-purity feedstocks. In forthcoming work on impurity-related challenges, we plan to incorporate a scenario-based TEA aligned with the reviewer’s recommendation, including both high-purity and bulk waste pathways as well as purification costs and their impact on profitability.
Comment 8: Crystallite vs. Particle Size: The distinction between the XRD-derived crystallite size (~86 nm) and the TEM-measured particle size (~105 nm) should be explicitly stated in the results to accurately describe the polycrystalline nature of the aggregates.
Response: We thank the reviewer for this valuable comment. The distinction between the XRD-derived crystallite size (~86 nm) and TEM-measured particle size (~105 nm) has been explicitly added. The larger particle size observed in TEM reflects assemblies of multiple crystallites, highlighting the polycrystalline nature of the aggregates. This clarification is now included in the revised manuscript.
Comment 9: Table 1 Enhancement: This table requires a descriptive caption and more quantitative detail. Specifically, the "CaO Precursor Characteristics" row should include numerical targets for particle size (e.g., "D50: 1-10 µm") and impurity limits (e.g., "SiO₂ < 0.5%") to be useful for process engineers.
Response: We thank the reviewer for this helpful suggestion. In the revised manuscript, details regarding CaO Precursor Characteristics have been supplemented in the main text rather than solely in Table 1, thereby improving clarity and integration with the overall process description.
Reviewer 4 Report
Comments and Suggestions for Authors1) Add quantitative findings in the abstract.
2) Lines 52-60, avoid the general statements. Add quantitative results of the previous studies and explain why CaO-based mineralization techniques are effective.
3) Lines 65-69, how nano-scale is effective for enhanced properties? Any findings for comparison with micro-sized particles?
4) Why the RSM was not employed for optimizing the carbonation parameters?
5) Provide error bars for Figure 3c.
6) Add some recent studies in XRD and TEM results to compare the current findings.
7) The conclusion is superficial. Avoid to add the general statements.
8) Add a comparison Table, illustrating the effectiveness of the current methodology with the previous studies (minimum 10 studies).
9) Provide mechanism study, how CaO capture CO2.
10) Add pH-drfit method to examine the effect of pH.
Author Response
Reviewer’s Comments and Responses
Comment 1. Add quantitative findings in the abstract.
Response: We appreciate this suggestion. The abstract has been revised to include key quantitative findings, such as the yield of calcite nanoparticles, average particle size, crystallite size, and carbonation efficiency, in order to provide a more concise and informative summary of the study.
Comment 2. Lines 52–60, avoid general statements. Add quantitative results of previous studies and explain why CaO-based mineralization techniques are effective.
Response: The section has been revised to remove general statements and incorporate quantitative data from relevant studies. Specifically, we now highlight previously reported carbonation conversion efficiencies (e.g., >80% under optimized conditions) and product purities obtained with CaO-based systems. We also explain that the high reactivity and abundance of CaO make it an effective candidate for CO₂ mineralization compared to alternative precursors.
Comment 3. Lines 65–69, how is the nanoscale effective for enhanced properties? Any findings for comparison with micro-sized particles?
Response: We have revised the text to clarify that nanoscale calcite exhibits higher surface area, improved dispersion, and enhanced reactivity compared to micro-sized particles. Comparative findings from recent studies have been cited to show that nano-calcite demonstrates superior performance in applications such as biomedical fillers and functional additives, thereby highlighting the importance of nanoscale synthesis.
Comment 4. Why was the RSM not employed for optimizing the carbonation parameters?
Response: We acknowledge the value of response surface methodology (RSM) for optimization studies. However, the present work was designed as an initial proof-of-concept study to evaluate feasibility, focusing on fundamental process characteristics rather than statistical optimization. Future work will incorporate RSM or other advanced optimization techniques to systematically identify the optimal carbonation parameters. This limitation has been clarified in the revised manuscript.
Comment 5. Provide error bars for Figure 3c.
Response: Error bars representing standard deviations from triplicate experiments have been added to Figure 3c in the revised manuscript.
Comment 6. Add some recent studies in XRD and TEM results to compare the current findings.
Response: The revised manuscript now includes a comparative discussion of recent studies that employed XRD and TEM characterization of CaCO₃ nanoparticles. This comparison situates our findings within the broader literature and highlights similarities and distinctions in crystallite size, morphology, and phase composition.
Comment 7. The conclusion is superficial. Avoid adding general statements.
Response: The conclusion has been thoroughly revised to avoid generic remarks. It now emphasizes the key quantitative findings, the novelty of coupling CO₂ mineralization with nano-calcite production, the limitations of the current high-purity baseline, and the future directions for industrial waste-derived feedstocks.
Comment 8. Add a comparison Table, illustrating the effectiveness of the current methodology with previous studies (minimum 10 studies).
Response: A new comparison table has been added to the revised manuscript (Table X), summarizing at least 10 relevant studies. The table compares feedstock type, carbonation efficiency, particle size, crystallinity, and product application, thereby clearly illustrating the advantages of the present methodology.
Comment 9. Provide a mechanism study, how CaO captures CO₂.
Response: A mechanistic description of the carbonation reaction pathway has been added, supported by a schematic illustration. The revised manuscript explains the sequential steps: (i) hydration of CaO to Ca(OH)₂, (ii) dissolution and ionization, and (iii) precipitation of CaCO₃ via interaction with dissolved CO₂. This addition provides mechanistic insight into the CO₂ capture process.
Comment 10. Add pH-drift method to examine the effect of pH.
Response: We acknowledge the importance of the pH-drift method for evaluating reaction mechanisms and kinetics. While this method was not implemented in the current study, we have noted this limitation and proposed its inclusion in future work to systematically investigate the influence of pH on carbonation efficiency and particle formation.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript failed to accurately test the particle size and discussion for the CaCO3 products, which is the key for the subsequent TEA. It makes inconsistency of the experimental results and techno-economic analysis, leading to poor support on the most crucial conclusion of the manuscript: "revenues from material sales alone could surpass raw material and processing costs by a significant margin-effectively transforming the process from a cost center into a net revenue generator." Since this key conclusion is not well supported, this manuscript should be further improved before publishing anywhere.
Author Response
Commnet1: The manuscript failed to accurately test the particle size and discussion for the CaCO3 products, which is the key for the subsequent TEA. It makes inconsistency of the experimental results and techno-economic analysis, leading to poor support on the most crucial conclusion of the manuscript: "revenues from material sales alone could surpass raw material and processing costs by a significant margin-effectively transforming the process from a cost center into a net revenue generator." Since this key conclusion is not well supported, this manuscript should be further improved before publishing anywhere.
Response: In this study, the particle size was evaluated using TEM and XRD. It is a common phenomenon that the values obtained from the two analyses differ by about 10–20 nm, as has been demonstrated in many previous studies. This discrepancy arises from the differences in the analytical methods. TEM provides a direct analysis, whereas XRD is an indirect technique. From the TEM images, relatively homogeneous nanoparticles were observed in this work, which can be considered an important research achievement.
Reviewer 3 Report
Comments and Suggestions for AuthorsAll the inquiries raised by this reviewer were properly addressed.
Author Response
Comment: All the inquiries raised by this reviewer were properly addressed.
Response: Thanks to your feedback, the paper has improved significantly. I really appreciate your review.
Round 3
Reviewer 2 Report
Comments and Suggestions for AuthorsI appreciate the authors’ efforts in preparing the manuscript and addressing the previous comments. I also recognize the potential value of this work for the community.
However, I am not fully convinced by the authors’ argument for the following reasons. First, regarding the TEM images used for particle size analysis, while I do not dispute the general reliability of this technique, the images clearly indicate agglomeration of CaO particles. This raises doubts as to whether they can still be classified as “nanoparticles.” Second, with respect to the XRD results, I agree with the reported findings; however, XRD can only detect crystalline phases. In this case, unreacted Ca(OH)2 or part of the CaCO3 fraction may exist in an amorphous form and therefore would not be captured by XRD. Considering these limitations, it is difficult to support the conclusion, and the subsequent assumption for the TEA, that “60–70% recovery as nano-grade material” has been achieved based solely on TEM and XRD characterization. Moreover, once agglomeration occurs, it likely accounts for a significant weight percentage. Therefore, the use of a particle size analyzer is strongly recommended.
Author Response
Thank you for the valuable comment. My response is as follows:
Commnet1: Regarding the TEM images used for particle size analysis, while I do not dispute the general reliability of this technique, the images clearly indicate agglomeration of CaO particles. This raises doubts as to whether they can still be classified as “nanoparticles.”
Response: To explain in more detail, this phenomenon can be described as the agglomeration of CaCO₃ nanoparticles. I incorporate this clarification into the revised manuscript. If the particle size exceeds 1 μm, it can no longer be regarded as a nanoparticle; however, since the observed size is within the typical nanoscale range, we can reasonably define them as general nanoparticles.
Comment2: with respect to the XRD results, I agree with the reported findings; however, XRD can only detect crystalline phases. In this case, unreacted Ca(OH)2 or part of the CaCO3 fraction may exist in an amorphous form and therefore would not be captured by XRD.
Response: Thank you for the insightful question. If unreacted Ca(OH)₂ were present, it would be detectable in the form of Ca(OH)₂ particles by XRD; however, no such signals were observed. In addition, if an amorphous form were present, it would typically appear as a background feature in the XRD pattern, but our experimental results did not show such characteristics. Considering that the detection limit of XRD is approximately 0.1 wt%, the mentioned substance is either below this threshold or absent. I will also include this clarification in the revised manuscript.
Comment3: Considering these limitations, it is difficult to support the conclusion, and the subsequent assumption for the TEA, that “60–70% recovery as nano-grade material” has been achieved based solely on TEM and XRD characterization. Moreover, once agglomeration occurs, it likely accounts for a significant weight percentage. Therefore, the use of a particle size analyzer is strongly recommended.
Response: The fact that nanoparticles exhibit agglomeration does not negate their classification as nanoparticles. My analysis was conducted on the final product, and the material indeed exists in nanoparticle form. Crystalline particles grow to the micron scale only when their atomic structures merge; however, in our case, the material remains in the nanoscale range. Furthermore, commercially available nanomaterials are typically supplied in a similar agglomerated state. Therefore, we believe that our TEA calculation is reasonable. It appears that this debate has arisen largely due to differing definitions of what constitutes a nanoparticle.
