Clarification of Clove Basil Extract Using Spinel Hollow Fiber Membranes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, the preparation of the hollow fiber membrane with alumina and dolomite as raw materials through a phase-inversion and sintering method, and its application in the filtration of clove basil aqueous extract are studied. The authors attempt to solve the problem of the high cost of conventional ceramic membrane. It is a significant attempt. This manuscript is clearly and concisely organized and written. The important technical point is also clearly pointed. The conclusions are provided by the data. After minor revision, this manuscript can be accepted for publishing.

 

Special suggestions:

 

Abstract Section:

The abstract should be shortened properly.

Line 14: DRX analyses, “DRX” should be “XRD”.

 

Results and discussion Section:

Regarding SEM images:

It would be better if the image with an entire shape of hollow fiber can be provided or added. The image of Figure 2a only shows multi-pore distribution.

 

Lines 233-236:

“a water permeability of 35.28 ± 2.46 L h^-1 m^-2 bar^-1. This value is lower than some of those reported in the literature for ceramic membranes [6,9,20,21]. This behavior is likely attributed to the densification of the sponge-like layer during the sintering process, which led to the packing of spinel grains and an overall increase in material density.”:

It would be better if the actual reason for the densification of the sponge-like layer is explained simply.

 

Author Response

Comments 1: Abstract Section: The abstract should be shortened properly.

Response: The abstract has been shortened to comply with the journal’s 200-word limit.

Comments 2: Line 14: DRX analyses, “DRX” should be “XRD”.

Response:  The term "DRX" has been corrected to "XRD" for consistency with standard terminology.

Comments 3: Results and discussion Section: Regarding SEM images: It would be better if the image with an entire shape of hollow fiber can be provided or added. The image of Figure 2a only shows multi-pore distribution.

Response: Regarding the SEM images, we have included a new image (Figure 2a) showing the entire shape of the hollow fiber, alongside the existing cross-section image (Figure 2b), which illustrates the multi-pore distribution. The term “multi-pore distribution” was included to describe the cross-section SEM image of the hollow fiber (Figure 2b).

Comments 4: Lines 233-236: “a water permeability of 35.28 ± 2.46 L h-1 m-2 bar-1. This value is lower than some of those reported in the literature for ceramic membranes [6,9,20,21]. This behavior is likely attributed to the densification of the sponge-like layer during the sintering process, which led to the packing of spinel grains and an overall increase in material density.”: It would be better if the actual reason for the densification of the sponge-like layer is explained simply.

Response: The description of the water permeability has been revised to clarify the densification mechanism: “The permeability value is probably associated with the fiber densification during the calcination process, caused by the shrinkage and rearrangement of spinel grains at high temperatures.”

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript investigates the clarification performance of spinel hollow fiber membranes for the aqueous extract of clove basil. Asymmetric spinel membranes were fabricated using a phase inversion and sintering method, and their pore structure, hydrophilicity, mechanical strength, and filtration performance were examined. The application potential of these membranes in the separation of bioactive substances was validated through fouling mechanism analysis. However, this research lacks innovation, as numerous studies have already reported the preparation of spinel hollow fiber membranes using the phase inversion method. Moreover, the experimental design of this study appears to be relatively simplistic, as it does not involve optimization of doping amounts or examination of the effects of phase inversion conditions on membrane structure and performance. More importantly, the study does not explore the impact of different sintering temperatures on the phase changes within the membrane. Additionally, it is observed from Figure 2a that the membrane does not exhibit an asymmetric structure.  these limitations, the originality and depth of the paper need improvement, and therefore, it is not recommended for publication.

Author Response

Comments 1: This manuscript investigates the clarification performance of spinel hollow fiber membranes for the aqueous extract of clove basil. Asymmetric spinel membranes were fabricated using a phase inversion and sintering method, and their pore structure, hydrophilicity, mechanical strength, and filtration performance were examined. The application potential of these membranes in the separation of bioactive substances was validated through fouling mechanism analysis. However, this research lacks innovation, as numerous studies have already reported the preparation of spinel hollow fiber membranes using the phase inversion method. Moreover, the experimental design of this study appears to be relatively simplistic, as it does not involve optimization of doping amounts or examination of the effects of phase inversion conditions on membrane structure and performance. More importantly, the study does not explore the impact of different sintering temperatures on the phase changes within the membrane. Additionally, it is observed from Figure 2a that the membrane does not exhibit an asymmetric structure.  these limitations, the originality and depth of the paper need improvement, and therefore, it is not recommended for publication.

Response 1: We appreciate the reviewer’s thorough evaluation and valuable comments regarding our manuscript. We respectfully address the concerns as follows:

We acknowledge that the fabrication of spinel hollow fiber membranes via the phase inversion and sintering method has been previously reported. However, we believe the main innovation of our work lies in the application of these membranes for the clarification and concentration of bioactive compounds from clove basil extract—a novel and underexplored area. This extends the relevance of spinel-based membranes to a new type of feed solution, which is rich in natural compounds of pharmacological interest.

Regarding the study of sintering temperature effects, we agree that such analysis provides important insights into the structural evolution of ceramic membranes. However, these effects were comprehensively investigated in our previous work (Bessa et al.), where we analyzed phase changes in the spinel structure as a function of sintering temperature. To avoid redundancy, we focused this manuscript on evaluating the membrane’s performance in a practical separation application.

We also accept the reviewer’s observation that the membrane morphology does not exhibit a well-defined asymmetric structure. The term “asymmetric pore size distribution” has been removed from the revised version to better reflect the actual structural characteristics observed in the SEM images (Figure 2a).

While we recognize that the experimental design could be further expanded—for instance, by including parameter optimization or additional structural analyses—we believe that the current scope provides a valuable initial assessment of the membrane’s separation performance in a real-world context, contributing to the growing body of literature on natural extract purification using ceramic membranes.

We hope these clarifications address the reviewer’s concerns and demonstrate the relevance and originality of our work.

Reviewer 3 Report

Comments and Suggestions for Authors

1. Title Clarity: The title could be rephrased for better clarity—for example: “…for clarification of clove basil aqueous extract.”
2. Line 13 - Terminology: The term calcination is more accurate than sintering here, as the solid-state reaction due to calcination occurs prior to the sintering of particles.
3. Line 15 - Terminology Clarification: Linking asymmetric pore structure to a hydrophilic surface is confusing. The term asymmetric pore structure typically refers to a membrane with smaller pores on the surface and larger pores underneath, which is a structural description rather than a surface chemistry property.
4. Introduction - Scientific Rationale: The Introduction lacks discussion on how the spinel structure influences membrane separation performance. In reality, the crystalline structure of a ceramic membrane generally has minimal effect on the purification of aqueous systems compared to its porous structure and surface properties.
5. Reference Overlap: The third and fourth authors of Reference 1 are also coauthors of this manuscript. That reference discusses purification of a similar shrub extract using a porous, non-spinel alumina membrane. However, this manuscript does not cite a relevant publication that involved an asymmetric spinel hollow fiber membrane for treating shrub extract (DOI: 10.1111/jfpp.14948), which also shares the same third author. As such, it remains unclear whether the current asymmetric spinel HF membrane performs better than the previously reported one.
6. Introduction - Structural Relevance: The final paragraph of the Introduction lacks discussion on how the membrane structure was specifically tailored for the feed solution—in this case, basil aqueous extract.
7. Section 2.1 - Phase Inversion and Morphology: Given that the suspension used for extrusion contained predominantly ceramic powders (with sizes in the micron scale), the typical phase-inversion effect—common in polymer-based HF membrane fabrication—would be significantly reduced. In Figure 2, the denser outer surface compared to the bulk is typical for sintered ceramics. The presence of PES likely acted as a pore former in the bulk. Therefore, the observed morphology differences cannot attributable to the impact of phase inversion.
8. Figures 3 and 4 - Relevance: Figures 3 and 4 are not essential, as they provide only limited support for the separation results. This is particularly due to the absence of control samples for proper comparison.
9. Section 2.2 - Feed Characterization: The mesh size of the stainless-steel strainer should be specified. Additionally, more detailed characterization of the feed post-centrifugation is necessary, including:
   a) Whether it is a colloidal dispersion, and if so, the particle content and particle size distribution.
   b) The concentration of soluble phenolic compounds, which could be determined using UV-Vis spectroscopy.
10. Mechanism of Separation: Based on Figures 2 and 6, it can be inferred that separation is initially achieved through mechanical blocking of flocculants. Subsequently, membrane fouling likely captures soluble species such as phenolic compounds. Please provide your perception on this aspect.

Author Response

Comments 1: Title Clarity: The title could be rephrased for better clarity—for example: “…for clarification of clove basil aqueous extract.”

Response 1: We appreciate the reviewer’s detailed feedback. The title has been revised to "Clarification of Clove Basil Extract Using Spinel Hollow Fiber Membranes" for clarity.

 

Comments 2: Line 13 - Terminology: The term calcination is more accurate than sintering here, as the solid-state reaction due to calcination occurs prior to the sintering of particles.

Response 2: The term "sintering" has been replaced with "calcination" where appropriate, as suggested.

 

Comments 3: Line 15 - Terminology Clarification: Linking asymmetric pore structure to a hydrophilic surface is confusing. The term asymmetric pore structure typically refers to a membrane with smaller pores on the surface and larger pores underneath, which is a structural description rather than a surface chemistry property.

Response 3: The sentence was changed to: “The spinel hollow fiber membrane presented a hydrophilic surface (water contact angle of 74.5°), moderate roughness (144.31 nm), and high mechanical strength.”

 

Comments 4: Introduction - Scientific Rationale: The Introduction lacks discussion on how the spinel structure influences membrane separation performance. In reality, the crystalline structure of a ceramic membrane generally has minimal effect on the purification of aqueous systems compared to its porous structure and surface properties.

Response 4: The introduction has been expanded to clarify the relationship between membrane structure and performance. We added: “The pore size, pore size distribution, and porosity of the membrane are critical for determining its separation capabilities. Membranes with tailored pore structures can achieve permeation fluxes and favorable mechanical and surface properties. While the spinel structure provides a robust framework, the membrane performance in aqueous separations is primarily dictated by its ability to selectively allow certain molecules to pass through its pores, which is mainly governed by the pore size distribution and surface interactions.”

 

Comments 5: Reference Overlap: The third and fourth authors of Reference 1 are also coauthors of this manuscript. That reference discusses purification of a similar shrub extract using a porous, non-spinel alumina membrane. However, this manuscript does not cite a relevant publication that involved an asymmetric spinel hollow fiber membrane for treating shrub extract (DOI: 10.1111/jfpp.14948), which also shares the same third author. As such, it remains unclear whether the current asymmetric spinel HF membrane performs better than the previously reported one.

Response 5: The reference (DOI: 10.1111/jfpp.14948) was already cited in the manuscript: “Paraíso et al. [1] applied spinel hollow fiber to clarify hibiscus extract, reducing tea cream formation by 43.1%.”

 

Comments 6: Introduction - Structural Relevance: The final paragraph of the Introduction lacks discussion on how the membrane structure was specifically tailored for the feed solution—in this case, basil aqueous extract.

Response 6: We appreciate the reviewer’s insightful comment regarding the importance of aligning membrane structure with the characteristics of the feed solution. In response, we have revised the final paragraph of the Introduction to clarify how the membrane was tailored for processing clove basil aqueous extract.

 

Comments 7: Section 2.1 - Phase Inversion and Morphology: Given that the suspension used for extrusion contained predominantly ceramic powders (with sizes in the micron scale), the typical phase-inversion effect—common in polymer-based HF membrane fabrication—would be significantly reduced. In Figure 2, the denser outer surface compared to the bulk is typical for sintered ceramics. The presence of PES likely acted as a pore former in the bulk. Therefore, the observed morphology differences cannot attributable to the impact of phase inversion.

Response 7: Regarding phase inversion, we agree that the classical mechanism differs for ceramic suspensions. As correctly pointed out, the denser outer layer observed in Figure 2 is more likely a result of sintering dynamics, particularly the packing and densification of spinel particles near the surface. The use of PES as a binder and pore former contributed to the formation of macropores in the inner structure upon burnout during sintering, which explains the more porous bulk morphology. In light of the reviewer’s comment, we have revised the text in Section 2.1 to clarify that the observed morphological features result primarily from calcination effects and the role of organic additives.

 

Comments 8: Figures 3 and 4 - Relevance: Figures 3 and 4 are not essential, as they provide only limited support for the separation results. This is particularly due to the absence of control samples for proper comparison.

Response 8: We appreciate the reviewer’s observation regarding the limited contribution of Figures 3 and 4 to the discussion of separation performance. We agree that the contact angle image (Figure 3) is not essential, as the numerical value is already reported in the text and adequately discussed. Accordingly, we have removed this figure from the revised manuscript. However, we chose to retain the AFM image (Figure 4) because it provides a visual representation of the membrane's outer surface morphology, which supports the discussion on fouling behavior observed during filtration. Surface roughness plays a key role in fouling tendencies, and the AFM image complements the quantitative roughness value by offering a spatial context for interpreting the fouling mechanism.

 

Comments 9: Section 2.2 - Feed Characterization: The mesh size of the stainless-steel strainer should be specified.

Response 9: The following information was indicated: “Extracts were pretreated by filtration through a 10-mesh stainless-steel strainer and a quantitative grade disc filter paper (particle retention: 20 - 25 µm, Unifil, Brazil), followed by a centrifugation step (Beckman Coulter Avanti J25, USA) at 8000 rpm for 20 min.”

 

Comments 10: Additionally, more detailed characterization of the feed post-centrifugation is necessary, including:

1. Whether it is a colloidal dispersion, and if so, the particle content and particle size distribution.
2. b) The concentration of soluble phenolic compounds, which could be determined using UV-Vis spectroscopy.

Response 10: We included DLS results of crude and centrifuged extract to report the participle size distribution. Results of TPC, antioxidant activity, soluble solids and color parameters of crude extract were included in Table 2.

 

Comments 11: Mechanism of Separation: Based on Figures 2 and 6, it can be inferred that separation is initially achieved through mechanical blocking of flocculants. Subsequently, membrane fouling likely captures soluble species such as phenolic compounds. Please provide your perception

Response 11: The following text was indicated in the manuscript to provide a discussion about these topics: “The best fit was obtained for n = 0, indicating that cake formation is the dominant fouling mechanism in the filtration process. This suggests that flow resistance primarily results from the accumulation of solid materials on the membrane surface rather than from pore blocking or other fouling phenomena. The complexity of the crude clove basil extract probably contributed to cake formation during filtration. Additionally, the coalescence of spinel particles on the membrane outer surface after the calcination process, as evidenced in the SEM image (Figure 2), further reduced permeate flux and intensified cake layer formation.”

Reviewer 4 Report

Comments and Suggestions for Authors

This study examines the effectiveness of asymmetric spinel (MgAl₂O₄) hollow fiber membranes in clarifying clove basil (Ocimum gratissimum L.) aqueous extract, which is rich in bioactive compounds. The membranes were created using a phase-inversion and sintering method at 1350 °C with alumina and dolomite. The manuscript is a continuation of several studies previously conducted by these authors, and the data obtained lacks novelty, since filtration was carried out only for one extract, which does not show the prospects for using these membranes. This manuscript will be ready for publication after some corrections and additional experiments:

1. DRX analyses are worth deciphering.
2. Is it worth presenting data in an abstract with such a precise error? For example, for measuring contact angles, the error is at the level of 1 to tens of degrees.
3. The novelty of the work should be indicated in the introduction and abstract, since previously published work by Bessa et al. [9] includes the development of these membranes.
4. “This work introduces an innovative approach by exploring the potential of spinel-based membranes in the filtration of plant extracts, providing valuable insights into their application for bioactive compound isolation.” If such a novelty is declared, then it is worth conducting testing using various extracts as an example.
5. The description of the reagents analysis methods used should be highlighted in separate subsections Materials and Characterization, respectively.
6. For all reagents and devices, it is worth indicating the company, city, state (for USA) and country of manufacture.
7. On what basis was this ratio of f alumina (42.8 wt.%) and dolomite(14.3 wt.%) chosen?
8. The abstract indicates permeate fluxfor a steady-stateregime, but the filtration description describes a crossflow system for testing. This point is worth clarifying and explaining.
9. It is worth characterizing the obtained clove basil extractand its purity.
10. It is worth avoiding the use of personal pronoun
11. It is worth presenting a comparison of the obtained filtration data with other membranes in a separate section.
12. The quality of the Figures needs to be significantly improved.
13. The conclusions should be rewritten with an analysis of the effects and data obtained.

Author Response

Comments 1: DRX analyses are worth deciphering.

Response 1: The peaks in Figure 1 were identified. Also, discussion about XRD analyses were improved. “Figure 1 presents the X-ray diffraction spectra of pure alumina, pure dolomite, and the alumina: dolomite mixture at a mass ratio of 3:1, all sintered at 1350 °C. The XRD patterns of pure alumina exhibited characteristic diffraction peaks at 25.6°, 35.1°, 37.8°, 43.4°, 52.5°, 57.5°, 66.5°, and 68.2° [14,15]. These peaks confirm the high purity and crystalline nature of the alumina raw material, which is critical for achieving mechanical stability in membranes. Meanwhile, pure dolomite material displayed a strong diffraction peak near 31° [16]. After calcination of the alumina-dolomite mixture at 1350 °C, the XRD spectrum reveals the formation of spinel (MgAl₂O₄), evidenced by peaks matching the spinel phase. The absence of residual alumina or dolomite peaks suggests complete reaction, yielding a homogeneous spinel structure. The spinel phase is advantageous due to its high thermal/chemical stability and mechanical strength, which are superior to pure alumina at lower sintering temperatures.”

 

Comments 2: Is it worth presenting data in an abstract with such a precise error? For example, for measuring contact angles, the error is at the level of 1 to tens of degrees.

Response 2: We apologize for our mistake in reporting errors with excessive precision (e.g., contact angle: 74.5° ± [small error]). In the revised version we rounded values to reflect instrument precision (e.g., "74.5°" → "~75°" or "74 ± 1°"). We retain high precision only for critical metrics.

 

Comments 3: The novelty of the work should be indicated in the introduction and abstract, since previously published work by Bessa et al. [9] includes the development of these membranes.

Response 3: The following text was included in the revised version to indicate the novelty of the work: “This investigation extends the application of spinel hollow fiber membranes [9] to the concentration of bioactive compounds from clove basil extract.”

 

Comments 4: “This work introduces an innovative approach by exploring the potential of spinel-based membranes in the filtration of plant extracts, providing valuable insights into their application for bioactive compound isolation.” If such a novelty is declared, then it is worth conducting testing using various extracts as an example.

Response 4: While testing multiple extracts would strengthen the study, we focused on clove basil as a model system and recommend broader validation in future work. 

 

Comments 5: The description of the reagents analysis methods used should be highlighted in separate subsections Materials and Characterization, respectively.

This section was included in the revised version of the manuscript.

Response 5: For all reagents and devices, it is worth indicating the company, city, state (for USA) and country of manufacture. This information was included in the revised version of the manuscript.

 

Comments 6: On what basis was this ratio of alumina (42.8 wt.%) and dolomite(14.3 wt.%) chosen?

Response 6: The following text was included in the revised version of the manuscript: “At first, PES (5.7 wt.%) was completely dissolved in DMSO (36.8 wt.%) under magnetic stirring. Then, the ceramic powder composed of alumina (42.8 wt.%) and dolomite (14.3 wt.%) was slowly added to the mixture. The ceramic suspension was mixed under ball milling for 48 h. At last, PVP (0.4 wt.%) was added to the mixture and the ball milling process was kept for a further 48 h. The material proportions suggested by Bessa et al. [9] were slightly adjusted to achieve an appropriate visual viscosity for the extrusion process.”

 

Comments 7: The abstract indicates permeate fluxfor a steady-stateregime, but the filtration description describes a crossflow system for testing. This point is worth clarifying and explaining.

Response 7: The term "steady-state flux" has been replaced with "final flux" to accurately reflect the crossflow filtration mode.

 

Comments 8: It is worth characterizing the obtained clove basil extract and its purity.

Response 8: Extract characterization data (TPC, antioxidant capacity, etc.) have been added to Table 2. 

 

Comments 9: It is worth avoiding the use of personal pronoun

Response 9: Personal pronouns have been removed throughout the text

 

Comments 10: It is worth presenting a comparison of the obtained filtration data with other membranes in a separate section.

Response 10. Comparison data were included in the revised version of the manuscript.

 

Comments 11: The quality of the Figures needs to be significantly improved.

Response 11: The quality of the figures was improved.

 

Comments 12: The conclusions should be rewritten with an analysis of the effects and data obtained.

Response 12: Some statements were included in the conclusion section to report the effects of the membrane characteristics on the data obtained.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed some of the issues I previously raised. However, I still have a minor question regarding lines 137-145, where the authors state: "The membrane precursors were laid out on a planar substrate to achieve proper orientation." By aligning the membrane precursors in a parallel arrangement, is there no risk of the hollow structure collapsing or deforming during sintering due to the elevated temperature? A slow heating process could soften the membrane, potentially compromising its original hollow morphology. Could the authors please provide a more detailed explanation of how the membrane maintains its hollow structure during high-temperature sintering? The authors should provide SEM images of the membrane surface and cross-section before and after sintering, and a more thorough discussion of these aspects would strengthen the manuscript’s credibility regarding structural stability under thermal processing.

 

Author Response

Comments 1: The authors have addressed some of the issues I previously raised. However, I still have a minor question regarding lines 137-145, where the authors state: "The membrane precursors were laid out on a planar substrate to achieve proper orientation." By aligning the membrane precursors in a parallel arrangement, is there no risk of the hollow structure collapsing or deforming during sintering due to the elevated temperature? A slow heating process could soften the membrane, potentially compromising its original hollow morphology. Could the authors please provide a more detailed explanation of how the membrane maintains its hollow structure during high-temperature sintering? 

Response 1: We thank the reviewer for their careful review of our manuscript. During extrusion, the fibers are continuously discharged into a water bath, resulting in a helical orientation. Prior to calcination, the fibers are carefully aligned using rectangular rulers without applying pressure to avoid deformation. During calcination, the fibers are placed unrestrained in ceramic holders. These clarifications have been incorporated into the revised manuscript.

Comments 2: The authors should provide SEM images of the membrane surface and cross-section before and after sintering, and a more thorough discussion of these aspects would strengthen the manuscript’s credibility regarding structural stability under thermal processing.

Response 2: SEM images of the fiber before calcination were included and discussed in the revised version of the manuscript.

Reviewer 4 Report

Comments and Suggestions for Authors

The author has revised the manuscript and is ready for acceptance.

Author Response

Comments 1: The author has revised the manuscript and is ready for acceptance.

We sincerely thank the reviewer for their insightful comments and constructive feedback on our work.