Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the manuscript, “Synthesis and Characterization of Titania-Coated Hollow Mesoporous Hydroxyapatite Composites for Photocatalytic Degradation of Methyl red Dye in water", the authors presented the fabrication of a photocatalyst active under UV irradiation. Photocatalysis and photodegradation of wastewater has been the subject of numerous studies already. Majority of which is shifting towards visible light active catalysts. While the subject of this paper is quite interesting, the use of UV light degradation is less appealing. The fabrication process and the subsequent material characterization seemed rigorous and sufficient. However, the photodegradation component of the paper has some flaws as described below:

- Show the spectrum of methyl red and the choice of the 437 nm peak.

- Show the spectrum of the UV light used. What is the irradiance?

- How was the photodegradation conducted? Did the authors use a calibration curve? This should be explained in the methods section.

- Photodegradation using nanomaterials is very tricky. To draw conclusions, multiple runs should be carried out. The plots in Fig. 7 do not show replicates of the photodegradation tests. Is the data based on only one run?

- Spectrophotometric methods to quantify a target analyte and obtain its kinetic profile should be analyzed carefully, especially when dealing with real matrices and/or advanced oxidation/reduction technologies. It is possible to have spectral interferences that may absorb radiation at the wavelength of the target analyte absorption maximum. Therefore, it is recommended to use chromatographic methods for the quantification of the test analyte. Can the authors verify that the absorbance data used in the photodegradation do not have any interference?

- Fig. 7(c), report the R2 values. Is it first-order or pseudo-first-order kinetics?

- The title in Section 3.3.3 is wrong. This should read “Post-photodegradation FTIR analysis and mineralization study of MR dye”.

 

Comments on the Quality of English Language

English is acceptable.

Author Response

Reviewers' comments:

Reviewer #1:

Comment: 1. Show the spectrum of methyl red and the choice of the 437 nm peak.

Response: First of all, I would like to thank you for your valuable time, expertise and critical comments. We appreciate the reviewer's request for clarification regarding the UV-Vis absorption spectrum of methyl red and the choice of the 437 nm peak for our analysis. We provide a detailed explanation for selecting the 437 nm peak.

The UV-Vis absorption spectrum of methyl red shows two primary absorption peaks, typically around 430-440 nm and 520-540 nm, corresponding to different protonation states of the dye.

In our study, we monitored the peak at 437 nm, which corresponds to the deprotonated form of methyl red, typically observed in neutral to slightly basic pH conditions. Methyl red is basically a pH indicator, and its absorption spectrum changes with pH due to structural changes in the molecule. In acidic solutions, methyl red appears red with a maximum absorption around 520-540 nm, whereas in neutral to basic solutions, it appears yellow with a maximum absorption around 430-440 nm [1]. The UV spectra of methyl red at different pH is also provided in Fig. S4, showing the change of  in acidic and basic conditions.

During our photocatalytic degradation experiments, the pH of the solution was maintained near neutral conditions (pH 6) to optimize the photocatalytic activity of TiO₂/HM-HAP composites. As it can also be seen in Fig. 7(d) that maximum degradation observed at pH 6. Under these conditions, the 437 nm peak is more prominent and stable. Monitoring the 437 nm peak allowed us to accurately track the concentration of methyl red, ensuring consistent and reliable measurements throughout the degradation process. Similar method or procedure were reported in

Nitin Kumar’s paper published in 2014 (10.1080/19443994.2014.942380), as well as R. Badri Narayan’s paper published in 2016 (10.1016/j.jclepro.2019.119088), and “Ihsan Ullah’s paper published in 2024 (10.21203/rs.3.rs-4316790/v1).

 

[1]       R. Lafi, L. Abdellaoui, I. Montasser, W. Mabrouk, A. Hafiane, The effect of head group of surfactant on the adsorption of methyl red onto modified coffee residues, J. Mol. Struct. 1249 (2022) 131527. https://doi.org/10.1016/J.MOLSTRUC.2021.131527.

 

Comment: 2. Show the spectrum of the UV light used. What is the irradiance?

Response: Respected reviewer, thank you for your valuable comment. The UV light source used in our experiment is a bulb that primarily emits at a wavelength of 254 nm. The UV light source used in our experiment is a 24-32 W bulb manufactured by Hangzhou Qiwei Instruments Co., Ltd (杭州齐威仪器有限公司). According to the manufacturer's specifications, the bulb is designed to emit primarily at a wavelength of 254 nm. While we do not have the spectral graph directly from our measurements, we rely on the detailed technical specifications provided by the manufacturer. Hangzhou Qiwei Instruments Co., Ltd. has provided assurance that the bulb has a peak emission at 254 nm. This specification is documented in the product datasheet, ensuring that the bulb's primary output wavelength is suitable for our experimental requirements.

 

Comment: 3. How was the photodegradation conducted? Did the authors use a calibration curve? This should be explained in the methods section.

Response: Respected reviewer, thank you for your valuable comment. Yes, to quantify the degradation of MR dye, a calibration curve was used, as presented in Fig. S2 of the supporting information. The calibration curve was prepared by measuring the absorbance of a series of MR dye solutions with known concentrations, allowing us to correlate absorbance with dye concentration accurately. By using this calibration curve, the concentration of MR dye at different irradiation times was determined, enabling the calculation of the photodegradation rate and efficiency.

Fig. S2. Beer-Lambert law validation for MR at λ = 437 nm.

Revision in manuscript (page 9)

In order to measure the extent of degradation of the MR dye, a calibration curve was employed, which can be seen in Fig. S2 in the supplementary information. The calibration curve was generated by measuring the absorbance of a range of MR dye solutions with predetermined concentrations, enabling us to establish a precise relationship between absorbance and dye concentration. Applying this calibration curve, the concentration of MR dye at various irradiation times was established, facilitating the calculation of the photodegradation rate and efficiency.

 

Comment: 4. Photodegradation using nanomaterials is very tricky. To draw conclusions, multiple runs should be carried out. The plots in Fig. 7 do not show replicates of the photodegradation tests. Is the data based on only one run?

Response: Respected reviewer, thank you for your valuable comment. Photodegradation experiments using nanomaterials involve inherent variability, and to ensure reliable and conclusive results, multiple runs are essential. The photodegradation tests in our study were completed in triplicate to ensure experimental repeatability and validate the observed patterns.
The experiment was replicated three times with the aim of reducing random mistakes and ensuring consistent results, while maintaining identical settings. The data depicted in Figure 7 accurately portrays the mean performance observed in three separate trials, thereby offering a reliable indication of the photocatalytic efficacy of the nanomaterials. In Figure 7(b), error bars were initially omitted for each replicate in order to accommodate space limitations. Figure 7(b) has been updated and now includes error bars.

Revision in manuscript (page 23)

 

Fig. 7(b) Time curve of % degradation at pH = 6, and 25 °C temperature (b) degradation kinetics of MR at pH = 6, and 25 °C temperature

 

Comment: 5. Spectrophotometric methods to quantify a target analyte and obtain its kinetic profile should be analyzed carefully, especially when dealing with real matrices and/or advanced oxidation/reduction technologies. It is possible to have spectral interferences that may absorb radiation at the wavelength of the target analyte absorption maximum. Therefore, it is recommended to use chromatographic methods for the quantification of the test analyte. Can the authors verify that the absorbance data used in the photodegradation do not have any interference?

Response: Respected reviewer, thank you for your valuable comment. The spectrophotometric method employed in our study to quantify the target analyte during photodegradation experiments involved measuring absorbance at the wavelength corresponding to the analyte's absorption maximum. Specifically, absorbance was measured at 437 nm for methyl red (MR) dye, as detailed in the Methods section.

To address potential spectral interferences, several precautions were taken:

Blank Corrections: Blank samples, which included all components except the MR dye, were assessed and the absorbance results were adjusted by subtracting them to account for background absorption.

Spectral Overlap: According to our understanding and examination of literature, no significant spectral overlaps were identified between MR dye and common interferents present in our experimental matrix. The accuracy of our spectrophotometric method was confirmed through thorough validation procedures, which involved calibrating with standard solutions of MR dye concentrations and comparing with alternative analytical methods when possible.

We are confident that the absorbance data utilized in our photodegradation investigations precisely depict the degradation of MR dye when subjected to the photocatalytic materials employed.

 

Comment: 6. Fig. 7(c), report the R² values. Is it first-order or pseudo-first-order kinetics?

Response: Respected reviewer, thank you for your valuable comment. The R² values have been added in Fig. 7(c). And it’s pseudo-first-order kinetics as mentioned in “Section 2.7. Kinetics of MR degradation”.

Revision in manuscript (page 23)

 

Fig. 7(c) Degradation kinetics of MR

 

Comment: 7. The title in Section 3.3.3 is wrong. This should read “Post-photodegradation FTIR analysis and mineralization study of MR dye”.

Response: Respected reviewer, thank you for your valuable comment. The title of section 3.3.3 has been corrected as “Post-photodegradation FTIR analysis and mineralization study of MR dye”.

Revision in manuscript (page 23)

3.3.3 Post-photodegradation FTIR analysis and mineralization study of MR dye

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

MS No:	coatings-3074300
Title:	Synthesis and Characterization of Titania-Coated Hollow Mesoporous Hydroxyapatite Composites for Photocatalytic Degradation of Methyl red Dye in water
Authors:	Farishta Shafiq, Simiao Yu, Yongxin Pan and Weihong Qiao

 

Evaluation:

The present manuscript deal with the synthesis and physicochemical characterization of Titania-Coated Hollow Mesoporous Hydroxyapatite Composites. Their efficiency is tested towards Photocatalytic Degradation of Methyl red Dye under UV light in water.  Overall, the paper is well written and contains useful information concerning materials synthesis and characterization. The detailed and in-depth physicochemical characterization of the synthesized materials witnesses the strong background of the research group in this field. On the other hand, the main drawback is the fact that an azodye, methyl Red, is used as a model compound and not a substance from the group of emerging contaminants, like pharmaceuticals, or personal care products.   

In my opinion this paper can be published at Coatings after major revision.

·       Avoid the use of abbreviations in the Title.

·       I would advise the Authors to use different keywords than the one mentioned in the Title.

·       The Authors should point out the novelty of the present manuscript in the Introduction.

·       Introduction needs revision in order to contain more updated information on the topic.

·       The Authors state that they used UV light irradiation (λ=365nm). They should perform a chemical actinometry in order to find out the light intensity entering their photoreactor.

·       Data considering the efficiency of the present photocatalytic materials towards emerging contaminants degradation should be added to the revised manuscript.

·       What about the stability of the as prepared materials? The Authors should present data considering their efficiency in consecutive experimental runs. Moreover, physicochemical characterization of the used catalyst should be added to the revised manuscript.

·       Photoluminescence analysis would be useful in order to emphasize more on the photocatalytic mechanism.

·       The Authors should carry out tests with the addition of suitable scavengers to the system in order to find out the reactive oxygen species (ROS) that are present to their system.

 

 

Author Response

Reviewer #2:

Comment: 1.  Avoid the use of abbreviations in the Title.

Response: First of all, I would like to thank you for your valuable time, expertise and critical comments. We have double checked the title and made sure the existence of no abbreviations in the title.

 

Comment: 2.  I would advise the Authors to use different keywords than the one mentioned in the Title.

Response: Respected reviewer, thank you for your valuable comment. The keywords have been changed.

Revision in manuscript (page 2)

Keywords: TiO₂-HAP composites, hydrothermal synthesis, photodegradation, azo dye removal, degradation kinetics, degradation mechanism

 

Comment: 3.  The Authors should point out the novelty of the present manuscript in the Introduction.

Response: Respected reviewer, thank you for your valuable comment. The introduction has been revised and the novelty of this work has been added.

Revision in manuscript (page 4 & 5)

Prior research on TiO₂/HAP composites has mainly concentrated on using rod-shaped or quasi-spherical hydroxyapatite (HAP) structures. However, these structures often face difficulties such as particle agglomeration and ineffective TiO₂ loading [37–39]. These problems might result in a decrease in the number of active sites on the surface, which in turn reduces the effectiveness of the composite in interacting with contaminants. Our research presents a novel method that utilizes a template technique to create hollow mesoporous HAP structures, which are then coated with TiO₂. This innovative approach effectively solves the issues of aggregation that are typically seen with traditional HAP forms, while also greatly improving the surface area and ease of access to the active areas inside the composite. By employing hollow mesoporous hydroxyapatite (HAP) in this distinct morphology, the effectiveness and durability of the TiO₂/HAP composite for photocatalytic and environmental purposes are significantly enhanced. This study is the first known case of employing hollow mesoporous HAP templates for TiO₂ coating. This opens up new prospects for improving the efficiency and adaptability of composite materials in sustainable technologies. The unique structure of spherical hollow hydroxyapatite particles not only prevents aggregation but also allows for a greater surface area owing to the hollow interior. This makes them very appropriate for coating TiO₂ and conducting investigations on photocatalysis.

 

Comment: 4.   Introduction needs revision in order to contain more updated information on the topic.

Response: Respected reviewer, thank you for your valuable comment. The whole introduction has been revised and modified.

 

Comment: 5.  The Authors state that they used UV light irradiation (λ=365nm). They should perform a chemical actinometry in order to find out the light intensity entering their photoreactor.

Response: Respected reviewer, thank you for your valuable comment. We appreciate your suggestion regarding the need for chemical actinometry to determine the light intensity entering our photoreactor during the photodegradation experiments. We would like to first clarify that there was a typographical error in our initial submission. The photodegradation experiments were actually conducted using UV light at λ=254 nm, not λ=365 nm as previously stated.

We acknowledge the importance of chemical actinometry for accurately measuring the light intensity. However, due to time constraints and resource limitations, we were unable to perform chemical actinometry. Instead, we ensured consistent experimental conditions by employing pseudo-first-order kinetics to determine the rate of photodegradation of methyl red dye under UV irradiation at λ=285 nm. This kinetic approach involves monitoring the decrease in dye concentration over time, based on the assumption that the degradation process follows pseudo-first-order kinetics with respect to the dye concentration. While this method allowed us to reliably assess the photocatalytic efficiency of our materials, we recognize that chemical actinometry could provide a more precise measurement of the light intensity. We plan to incorporate this method in future studies to enhance the accuracy of our experimental setup.

 

Comment: 6.  Data considering the efficiency of the present photocatalytic materials towards emerging contaminants degradation should be added to the revised manuscript.

Response: Respected reviewer, thank you for your valuable comment. We value the reviewer's perceptive recommendation to incorporate data regarding the efficiency of our photocatalytic materials in addressing emerging pollutants. While we have not yet conducted specific experiments on emerging contaminants due to time constraints, we provide a discussion on the potential applicability of our TiO₂/HM-HAP composites based on their demonstrated performance with methyl red dye and relevant literature.

The TiO₂/HM-HAP composites synthesized in our study have shown excellent photocatalytic activity in the degradation of methyl red dye. This high efficiency can be attributed to the key factors: such as,

• High surface area: The hollow mesoporous structure of HAP offers a large surface area, enhancing the adsorption of contaminants and increasing the availability of active sites for photocatalytic reactions.
• Effective UV-Light absorption: The TiO₂-coating ensures efficient absorption of UV light, which is crucial for generating reactive oxygen species (ROS) that drive the degradation process.
• Strong Photocatalytic Activity: TiO₂ is well-known for its strong photocatalytic properties, and its combination with HAP supports improves its stability and dispersion, further enhancing performance.

Several studies have reported the effective use of TiO₂-based photocatalysts for the degradation of various emerging contaminants. For instance:

• Kutuzova et al. (2021) (3390/catal11060728) demonstrated the antibiotics (Sulfamethoxazole, Trimethoprim and Ciprofloxacin) degradation using TiO₂-Based photocatalysts.
• Nguyen et al. (2018) (1016/j.jclepro.2018.08.110) reported the degradation of methylene blue and methyl orange by palladium-dopped TiO₂.
• Cheng et al. (2021) (1016/j.matlet.2016.03.120) showed the efficient degradation of nitrophenol by using ZnO-TiO₂.

Considering the resemblances in the photocatalytic mechanisms and the types of reactive oxygen species (ROS) involved, it is reasonable to expect that our TiO₂/HM-HAP composites would demonstrate comparable effectiveness in degrading various emerging pollutants. We recognize the significance of conducting experimental tests to confirm the efficiency of our materials in addressing new pollutants. Therefore, we intend to carry out these trials in future research in order to gather extensive data on the suitability of our photocatalysts for a wider variety of contaminants.

 

Comment: 7.  What about the stability of the as prepared materials? The Authors should present data considering their efficiency in consecutive experimental runs. Moreover, physicochemical characterization of the used catalyst should be added to the revised manuscript.

By combining TiO₂ with HAP, we take use of the natural stability of HAP, which enhances the durability of the resulting photocatalyst. This stability is not simply asserted, but it is supported by thorough examination, as demonstrated in multiple studies. The articles provide FTIR and TGA analyses that demonstrate the stability of HAP at various temperatures, confirming the consistent nature of our photocatalyst.

We acknowledge the importance of providing comprehensive data on the stability and efficiency of our photocatalytic materials over multiple runs. We plan to conduct further consecutive experimental runs and include detailed stability data in future studies. These additional experiments will help to fully validate the long-term performance of the TiO₂/HM-HAP composites.

Revision in manuscript (Page 18)

 

Comment: 8.  Photoluminescence analysis would be useful in order to emphasize more on the photocatalytic mechanism.

Response: Many thanks to the reviewer. We appreciate the suggestion to include photoluminescence. (PL) analysis to further explore the photocatalytic mechanism. In our study, we have employed a comprehensive set of analytical techniques to investigate the behavior of our TiO₂/HM-HAP composites during the degradation of methyl red dye. UV-Vis spectroscopy was used to monitor the degradation process under UV light, while LC-MS analysis confirmed the breakdown of dye molecules. Post-photodegradation FTIR analysis provided insights into surface chemistry changes, and Total Organic Carbon (TOC) measurements quantified the extent of mineralization. Based on these analyses, we have developed a detailed mechanism highlighting the generation of reactive oxygen species (ROS) and the adsorption-degradation process on the composite surface. These findings collectively support the effectiveness of our photocatalytic materials and their mechanism of action, demonstrating their potential for environmental remediation applications.

 

Comment: 9.  The Authors should carry out tests with the addition of suitable scavengers to the system in order to find out the reactive oxygen species (ROS) that are present to their system.

Response: Many thanks to the reviewer. We appreciate your recommendation to carry out tests with the addition of suitable scavengers to identify the reactive oxygen species (ROS) present in our system. In response to this comment, we have conducted additional experiments using isopropanol as a hydroxyl radical (*OH) scavenger. The findings have been incorporated into the revised manuscript.

Revision in Manuscript (Page 25 & 26)

In addition, an examination was conducted on the impact of an OH scavenger on photocatalysis. Isopropanol was employed as an *OH scavenger for the experiment. The catalyst's catalytic efficiency decreases when a scavenger is present. This has been illustrated using a graph, presented in Fig. S. The predominant catalyst in this process is now clearly identified as the *OH radical [53].

Revision in Supplementary Information (Page S7)

 

Fig. S7 Plot representing effect of scavenger on photolysis, adsorption, and photolysis experiment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper of Qiao et al. reports the development of titania-coated hollow mesoporous hydroxyapatites for photocatalytic degradation of methyl red in water.

The article relates to the preparation, characterization and testing of materials and it is rich of results.

However, before publication, some points should be considered.

- Pzc is not cited in the abstract. Why? It should be mentioned with all other characterizations.

- In the introduction, recent literature regarding dyes must be added and some others related to the TiO2 use in different applications (for example https://doi.org/10.3390/ma16031304, and others).

- I think that the use of HAP is well-known in the literature for several applications. The interesting point related to its use is its environmental feature, since it can also come from wastes. This point must be more stressed in the paper by citing additional recent literature.

- Is TiO2 (line 97) a commercial sample or previously synthesized?

- In paragraph 2.4 you should rearrange the order of characterizations. For example, LC-MS, TOC and UV-visible for monitoring the reactions must be moved after the description of the photocatalytic testing.

- If for each sampling you take 3 ml from the starting 25 ml, it is not a lot? You did more experiments in parallel and from each you take an aliquot for minimizing the error? These details must be properly indicated.

- line 155: PSS? Specify the full name the first time you are mentioning it.

-XRD results: Miller indexes are necessary in both description (main text) and figure.

-Table 1: surface area values must be reported without commar. Which is the added value of growing Ti02 onto HAP from a surface area point of view? N2 ads/des isotherm of the bare HAP must be added to enrich the description.

- PZC: you correctly discussed the correlation between PZC and dye ads, but recently references about this approach should be added (lines 322-329).

- Remove line 265-266 since there is all the description in the following paragraph.

-Photolysis experiment must be provided.

-Are you sure that the activity you saw is exclusively related to photodegradation of the pollutant? The exclusive adsorption test and results must be provided, added in SI and described in the main text.

- In general, experimental conditions about photodegradation tests must be provided in the captions of figures related to this topic.

-Photodegradation tests of the bare TiO2 and HAP must be added and properly explained in the text to demonstrate the added value of preparing the composite.

-From a literature point of view, the bare HAP is a well-known adsorbent material. Is it able to adsorb the studied pollutants?

- TiO2/HAP is a material already largely studied in the literature. Which is the added point of this research? This concept must be more stressed in the full paper. In addition, according to this premise, the addition of a paragraph indicating a literature comparison about the obtained performances and those reported in the literature is strongly recommended.

-English lexicon should be generally improved.

Author Response

Reviewer #3:

Comment: 1.  Pzc is not cited in the abstract. Why? It should be mentioned with all other characterizations.

Response: First of all, I would like to thank you for your valuable time, expertise and critical comments. The abstract has been revised and modified according to your suggestion and the PZC has been mentioned with all the other characterization techniques.

Revision in manuscript (page 1, Line 13)

Utilizing SEM, XRD, XPS, BET, FTIR, EDS, and UV–vis DRS spectroscopy, and point of zero charge (PZC) analysis, the coating morphological and physicochemical parameters of the produced samples were analyzed.

Comment: 2.  In the introduction, recent literature regarding dyes must be added and some others related to the TiO₂ use in different applications (for example https://doi.org/10.3390/ma16031304, and others).

Response: Many thanks to the reviewer. The introduction has been revised and recent literature regarding dyes and applications of TiO₂ has been added. The reference you suggested was of great importance and has been cited in the paper.

 

Revision in manuscript (page 3)

Recent research has demonstrated the capability of TiO₂-based composites to improve photocatalytic activity. For example, Sukhadeve et al. (2023) revealed the effective degradation of methylene blue dye using Ag-Fe co-doped TiO₂ nanoparticles [17]. Furthermore, Ali et al. (2023) reviewed the catalytic activity of Ag and Zn-doped TiO₂ nano-catalysts for the removal of methylene blue and methyl orange dyes [18]. Liza et al. (2024) conducted an additional study that examined the impact of Ag-doping on the morphology, band gap, and photocatalytic activity of TiO₂ nanoparticles in the context of textile dye degradation [19]. Moreover, TiO₂ has been implemented in a variety of applications beyond wastewater treatment, including antibacterial remedies [20], protective coatings for metal prostheses [21], and self-cleaning surfaces for air purification [22].

 

Comment: 3.  I think that the use of HAP is well-known in the literature for several applications. The interesting point related to its use is its environmental feature, since it can also come from wastes. This point must be more stressed in the paper by citing additional recent literature.

Response: Many thanks to the reviewer. We appreciate the importance of highlighting the environmental benefits of using hydroxyapatite (HAP), particularly its potential to be sourced from waste materials. The introduction has been revised and additional recent literature has been cited to support the discussion.

Revision in manuscript (page 4)

Hydroxyapatite (HAP) is also extensively recognized for its applications in a variety of areas, such as the environmental, biomedical, and industrial sectors [31,32]. HAP's ecological friendliness is one of its most intriguing features, as it can be produced from waste materials, rendering it a sustainable and environmentally responsible option. Recent research has investigated the potential of HAP, which is derived from sources of waste, such as egg shells, fly ash, and fish bones to reduce environmental impact and promote sustainable practices [33–35].

 

Comment: 4.  Is TiO₂ (line 97) a commercial sample or previously synthesized?

Response: Many thanks to the reviewer. The TiO₂ used in the experiments was a commercially available sample, purchased from MACKLIN.

 

Comment: 5.  In paragraph 2.4 you should rearrange the order of characterizations. For example, LC-MS, TOC and UV-visible for monitoring the reactions must be moved after the description of the photocatalytic testing.

Response: Many thanks to the reviewer. The paragraph 2.4 has been revised and modified.

Revision in manuscript (page 7 & 8)

UV-vis diffuse reflectance spectra (DRS) of the synthesized materials were measured by a UV-vis spectrophotometer (Lambda 1050+) utilizing BaSO₄ as a reference material. Spectroscopic study utilizing a UV-Visible spectrophotometer carry-100 (Malaysia), was used to evaluate the absorbance of methyl red dye throughout its photocatalytic degradation. The absorbance spectra were recorded between 350 and 750 nm wavelength. pH measurement was performed using a PHS-3C pH meter with an integrated temperature sensor for temperature regulation. The examination of degradation products was conducted using liquid chromatography mass spectrometry (LC-MS, G6230, USA). The TOC of before and after decolorization of dye solution was quantified by organic element analyzer (UNICUBE).

 

Comment: 6.  If for each sampling you take 3 ml from the starting 25 ml, it is not a lot? You did more experiments in parallel and from each you take an aliquot for minimizing the error? These details must be properly indicated.

Response: Respected reviewer, thank you for your valuable comment. We appreciate your concern regarding the potential impact of sampling on the accuracy of our results. To address this, we conducted multiple parallel experiments under identical conditions. This allowed us to take 3 mL aliquots from different samples at each time point, thereby minimizing the impact of volume reduction on our results. These details have been included in the revised methods “Section 2.6” to clarify our approach and ensure transparency in our experimental procedure.

Revision in manuscript (page 8 & 9)

2.6. Photocatalytic studies of MR

The photocatalytic activity of the samples was evaluated by observing the degradation of MR dye as the pollutant target under ultraviolet irradiation. The chemical structure and other characteristics of the employed dye are detailed in Table S1. Initially, 25 mL of MR solution (10 mg/L) and 30 mg of catalyst were vigorously agitated for 30 min in the dark to produce an adsorption/desorption equilibrium. The solution was then agitated at room temperature under UV light irradiation (λ = 254 nm) for several minutes. Periodically, 3 mL of solution were collected and filtered to eliminate the solid phase. Although this decreases the initial volume, parallel experiments were carried out to ensure uniformity and eliminate potential mistakes resulting from volume reduction. More precisely, a number of 25 mL samples were prepared and subjected to the same circumstances. This enabled the extraction of small portions from various samples at each specific time interval. This method guaranteed that the decrease in volume did not impact the accuracy of the outcomes. After that, the absorbance of the filtrates was measured spectroscopically at 437 nm for MR dye. In order to measure the extent of degradation of the MR dye, a calibration curve was employed, which can be seen in Fig. S2 in the supplementary information. The calibration curve was generated by measuring the absorbance of a range of MR dye solutions with predetermined concentrations, enabling us to establish a precise relationship between absorbance and dye concentration. Applying this calibration curve, the concentration of MR dye at various irradiation times was established, facilitating the calculation of the photodegradation rate and efficiency.

 

Comment: 7.  line 155: PSS? Specify the full name the first time you are mentioning it.

Response: Many thanks to the reviewer. The full name of the PSS has been added.

Revision in manuscript (page 10)

In addition, PSS (sodium poly (styrene sulfonate)) was used as a crystal growth additive to accelerate the transformation of CaCO₃ from calcite to vaterite during the CaCO₃ manufacturing process.

 

Comment: 8.  XRD results: Miller indexes are necessary in both description (main text) and figure.

Response: Many thanks to the reviewer. We have revised the XRD results portion and mentioned the miller indexes in both the description and also in figure.

Revision in manuscript (page 11 & 12)

The XRD patterns of TiO₂, HM-HAP, and TiO₂-coated HM-HAP composites are shown in Fig. 2(b). The diffraction peaks at 25.2°, 37.1°, 53.9°, 55.8°, 62.7°, 68.5°, and 70.3° correspond to the (101), (004), (105), (211), (204), (116), and (220) planes of anatase-TiO₂, respectively (JCPDS no. 21-1272) [36]. In case of pure HM-HAP, all diffraction peaks and their relative intensities correspond to the standard diffraction data of pure hexagonal phase HAP (JCPDS, 09-0432) [37,38].  Specifically, the peaks at 2 values of 31.9°, 32.1°, and 33.1° corresponds to the (211), (112), and (300) planes of HAP, respectively. These indices confirm the successful synthesis of hydroxyapatite with a well-defined crystalline structure.

 

Fig. 2(b) XRD spectra of TiO₂, HM-HAP and TiO₂-coated HM-HAP (10%, 20%, 30%, 40% and 50%) composites

 

Comment: 9.  Table 1: surface area values must be reported without commas. Which is the added value of growing TiO₂ onto HAP from a surface area point of view? N₂ ads/des isotherm of the bare HAP must be added to enrich the description.

Response: Many thanks to the reviewer. We have revised Table 1 to ensure that all surface area values are reported without commas for consistency and clarity.

Revision in manuscript (page 14 & 16)

The pore-volume, pore-size and Brunauer-Emmett-Teller (BET) specific surface area acquired from N₂ adsorption-desorption for all the TiO₂-coated HM-HAP composites are presented in Table 1. Fig. 5(b) depicts the isotherms and corresponding pore size distribution histograms, indicating the mesoporous structure of the TiO₂-coated HM-HAP composites. According to the IUPAC system, all of the synthesized samples have type IV isotherms, which have a H3 hysteresis loop. This is because particle aggregation makes slit-shaped pores [50]. The BET-specific surface area of 10% TiO₂-coated HM-HAP was 56 m²/g and declined as the TiO₂ concentration increased, while the corresponding pore-diameter increased gradually. A possible explanation for this result is that when the number of TiO₂ molecules increases, agglomeration may occur, resulting in no greater pore occupancy. Thus, as a result, a decreasing trend in the specific surface area occurred. Moreover, the coating of TiO₂ onto HAP greatly increases the surface area of the resulting composite material (Table 1). The augmentation in surface area is essential as it offers a greater number of active sites for photocatalytic reactions, hence enhancing the efficacy of the photodegradation process. More precisely, the TiO₂-coated HM-HAP composite has a greater surface area in comparison to pure HAP, as specified in Table 1. The improvement can be ascribed to the synergistic interplay between TiO₂ and HAP, with HAP serving as a supporting framework that hinders the clumping of TiO₂ nanoparticles, hence ensuring a large surface area.

 

Table 1. BET-specific surface area and pore diameter analysis of synthesized composites.

Sample	BET specific surface area (m2/g)	Pore size (nm)
HM-HAP	44	12.47
10% TiO2/HM-HAP	56	27.82
20% TiO2/HM-HAP	56	28.19
30% TiO2/HM-HAP	47	29.28
40% TiO2/HM-HAP	46	31.94
50% TiO2/HM-HAP	38	33.43

 

 

 

Comment: 10.  PZC: you correctly discussed the correlation between PZC and dye ads, but recently references about this approach should be added (lines 322-329).

Response: Many thanks to the reviewer. The manuscript has been revised and references about the approach discussed in lines 322-329 has been added.

Revision in manuscript (page 23)

Fig. 7(d) shows the photocatalytic behavior of the synthesized composite towards the degradation of a solution of MR dye at different solution pH carried at room temperature. From Fig. 7(d), it’s clear that the maximum photocatalytic degradation happened at pH 6. The explanation for this behavior is that the PZC of the TiO₂/HM-HAP coated composites is in the range of 7.5-8.1. Thus, the material’s surface is positively charged when the pH is < 7.5 and negatively charged when the pH is > 8.1. Therefore, the dye was in anionic form, and the surface of the material at pH 6 was positively charged [46,60,61].

 

Comment: 11.  Remove line 265-266 since there is all the description in the following paragraph.

Response: Many thanks to the reviewer. The line 265-266 has been removed.

 

 

Comment: 12.  Photolysis experiment must be provided.

Response: Many thanks to the reviewer. We appreciate the reviewer's suggestion to include a photolysis experiment to further clarify the contribution of direct UV light irradiation to the degradation of methyl red dye. Here, we provide the details and results of the photolysis experiment conducted under the same conditions as our photocatalytic studies. To assess the effect of UV light alone on the degradation of methyl red dye, a control photolysis experiment was conducted as follows:

First, a solution of methyl red dye (10 mg/L) was prepared without the addition of any photocatalyst. The solution was exposed to UV light at λ=254 nm under identical conditions used for the photocatalytic experiments (same light intensity, distance, and duration). The samples were collected at regular intervals and filtered to remove any potential impurities. The absorbance of the samples was than measured spectroscopically at 437 nm. The results of the photolysis experiment showed minimal degradation of methyl red dye under direct UV light irradiation alone, indicating that the contribution of direct photolysis to the overall degradation process is negligible. This confirms that the observed degradation in our photocatalytic experiments is primarily due to the activity of the Anatase TiO₂ photocatalyst.

Revision in manuscript (page 25)

Moreover, the results of the photolysis experiment (Fig. S7) demonstrated that there was minimal degradation of the MR dye when exposed just to direct UV light. This suggests that the direct photolysis has a small impact on the overall degradation process.  Furthermore, the findings from adsorption experiment, presented in the Fig. S7, demonstrate that the catalyst was responsible for 20% of the overall removal of MR by adsorption. This demonstrates that although adsorption plays a role in the overall process, the main mechanism for removing dye under UV irradiation is definitely photodegradation. HAP possesses a significant number of hydroxyl groups, which contribute to its surface having a primarily negative charge. Methyl red (MR) is a negatively charged dye, and because of the like charges, there is a repulsive force that decreases its ability to stick to the HAP surface. While HAP does have some sites with positive charge due to calcium ions, the overall negative charge from the many hydroxyl groups is more prominent.

Revision in supplementary information (page S7)

 

Fig. S7 Plot representing effect of scavenger on photolysis, adsorption, and photolysis experiment.

 

Comment: 13.  Are you sure that the activity you saw is exclusively related to photodegradation of the pollutant? The exclusive adsorption test and results must be provided, added in SI and described in the main text.

Response: Respected reviewer, thank you for your valuable comment. To address your concern about whether the observed activity is exclusively related to photodegradation, we conducted an exclusive adsorption test. This involved stirring the methyl red dye solution with the catalyst in the dark, under identical conditions to the photodegradation experiments. The concentration of the dye was monitored over time using UV-vis spectrophotometry at 437 nm.

The results, which are provided in the Supplementary Information (Fig. S7), indicate that the adsorption of methyl red onto the catalyst accounted for 20% of the total dye removal. This confirms that while adsorption contributes to the overall process, the primary mechanism for dye removal under UV irradiation is indeed photodegradation. HAP contains abundant hydroxyl groups, which impart a predominantly negative charge to its surface. Methyl red (MR) is an anionic dye, and due to the similar charges, there is a repulsion effect that reduces its adsorption onto the HAP surface. Although HAP does possess some positively charged sites originating from the calcium ions, the overall negative charge from the abundant hydroxyl groups is more predominant. Consequently, the adsorption capacity of HAP for anionic dyes like MR is limited.

 We have included these results and a detailed description of the adsorption test in the main text and the Supplementary Information.

Revision in manuscript (page 25)

Moreover, the results of the photolysis experiment (Fig. S7) demonstrated that there was minimal degradation of the MR dye when exposed just to direct UV light. This suggests that the direct photolysis has a small impact on the overall degradation process.  Furthermore, the findings from adsorption experiment, presented in the Fig. S7, demonstrate that the catalyst was responsible for 20% of the overall removal of MR by adsorption. This demonstrates that although adsorption plays a role in the overall process, the main mechanism for removing dye under UV irradiation is definitely photodegradation. HAP possesses a significant number of hydroxyl groups, which contribute to its surface having a primarily negative charge. Methyl red (MR) is a negatively charged dye, and because of the like charges, there is a repulsive force that decreases its ability to stick to the HAP surface. While HAP does have some sites with positive charge due to calcium ions, the overall negative charge from the many hydroxyl groups is more prominent.

 

Revision in supplementary information (page S7)

 

Fig. S7 Plot representing effect of scavenger on photolysis, adsorption, and photolysis experiment.

Comment: 14.  In general, experimental conditions about photodegradation tests must be provided in the captions of figures related to this topic.

Response: Respected reviewer, thank you for your valuable comment. The caption of the figures has been revised and modified. The experimental conditions have been added.

Revision in manuscript (page 23)

Fig. 7 Degradation efficiency of MR over the synthesized composites at pH = 6, 25 °C, and 60 min of irradiation time (a), Time curve of % degradation at pH = 6, and 25 °C temperature (b) degradation kinetics of MR at pH = 6, and 25 °C temperature (c) and effect of pH on % degradation of MR at 25 °C, and 60 min of irradiation time (d).

 

Comment: 15.  Photodegradation tests of the bare TiO₂ and HAP must be added and properly explained in the text to demonstrate the added value of preparing the composite.

Response: Respected reviewer, thank you for your valuable comment. We appreciate your suggestion to provide detailed photodegradation tests for bare TiO₂ and HAP. We have conducted these tests and compared the results with those of the TiO₂/HM-HAP composite. The results clearly indicate that the 20% TiO₂/HM-HAP composite achieved up to almost 88% degradation efficiency, which is significantly higher compared to the pure TiO₂ and HM-HAP, which exhibited degradation efficiencies of 42% and 20%, respectively.

The improved performance can be due to the distinctive characteristics of HAP, such as its hydroxyl groups (-OH) and the presence of adsorbed H₂O molecules on its surface. These traits enable more effective adsorption and activation of the dye molecules. The composite material efficiently integrates the benefits of both TiO₂ and HAP, leading to enhanced charge separation and an increased number of active sites for the degradation reaction.

We have included these detailed results and explanations in the revised manuscript to better demonstrate the added value of preparing the composite.

Revision in manuscript (page 20)

As illustrated in Fig. 7(a), when catalyzed by a TiO₂-coated HM-HAP composite with UV irradiation (60 min), the degradation efficiency increased significantly, and in the presence of 20% TiO₂/HM-HAP coating, up to almost 88% of degradation efficiency was achieved. In contrast, the pure HM-HAP and TiO₂ demonstrated degradation efficiencies of 20% and 42%, respectively (Fig. 7(a)). This enhanced performance can be attributed to the unique properties of HAP, specifically its hydroxyl groups (^-OH) and adsorbed H₂O molecules on the surface. These hydroxyl groups are instrumental in interacting with h⁺ to generate hydroxyl radicals (˙OH), a highly reactive species known for its effectiveness in photocatalysis. Furthermore, the photocatalytic process involves intriguing electron phase shifts within the PO₄³⁻ groups on the HAP surface. This phenomenon leads to the formation of ˙O₂ radicals, further augmenting the catalytic efficiency of the composite material [46]. Although the bare TiO₂ exhibits photocatalytic activity, its efficiency is restricted by the rapid recombination of photogenerated electron-hole pairs. Conversely, HM-HAP alone offers adsorption sites but does not possess the catalytic efficiency of TiO₂. The TiO₂/HM-HAP composite optimizes charge separation and increases the number of active sites for the degradation reaction by integrating the benefits of both materials. Hence, integrating the advantages of HAP and TiO₂ not only results in the enhancement of TiO₂’s capability to absorb contaminants but also mitigates the recombination of photogenerated electron–hole pairs. This synergy between HAP and TiO₂ not only improves the material's adsorption capacity but also amplifies its overall photocatalytic performance.

 

Comment: 16.  From a literature point of view, the bare HAP is a well-known adsorbent material. Is it able to adsorb the studied pollutants?

Response: Respected reviewer, thank you for your valuable comment. We recognize that bare hydroxyapatite (HAP) is a widely recognized adsorbent material because of its many hydroxyl groups, which give its surface a mostly negative charge. The presence of this negative charge is extremely efficient in attracting and retaining cationic contaminants, including methylene blue (MB) dye. Nevertheless, methyl red (MR) is a negatively charged dye, and because of the like charges, there is a repulsive force that decreases its ability to bind to the HAP surface. While HAP does have some sites with positive charge due to calcium ions, the overall negative charge from the many hydroxyl groups is more prominent. Therefore, the ability of HAP to adsorb anionic dyes such as MR is limited.

In our study, the TiO₂-coated HM-HAP composite showed enhanced photocatalytic activity, which is attributed not only to the adsorption sites provided by HAP but also to the photocatalytic properties of TiO2. This synergy between adsorption and photocatalysis leads to a more efficient degradation process for MR dye under UV irradiation.

Comment: 17.  TiO₂/HAP is a material already largely studied in the literature. Which is the added point of this research? This concept must be more stressed in the full paper. In addition, according to this premise, the addition of a paragraph indicating a literature comparison about the obtained performances and those reported in the literature is strongly recommended.

Response: Respected reviewer, thank you for your valuable comment. The manuscript has been revised and the suggested part has been added, also a comparison paragraph showing comparison of the performance of the synthesized material with others in the literature has also been added.

Revision in manuscript (page 4 & 5)

Prior research on TiO₂/HAP composites has mainly concentrated on using rod-shaped or quasi-spherical hydroxyapatite (HAP) structures. However, these structures often face difficulties such as particle agglomeration and ineffective TiO₂ loading [37–39]. These problems might result in a decrease in the number of active sites on the surface, which in turn reduces the effectiveness of the composite in interacting with contaminants. Our research presents a novel method that utilizes a template technique to create hollow mesoporous HAP structures, which are then coated with TiO₂. This innovative approach effectively solves the issues of aggregation that are typically seen with traditional HAP forms, while also greatly improving the surface area and ease of access to the active areas inside the composite. By employing hollow mesoporous hydroxyapatite (HAP) in this distinct morphology, the effectiveness and durability of the TiO₂/HAP composite for photocatalytic and environmental purposes are significantly enhanced. This study is the first known case of employing hollow mesoporous HAP templates for TiO₂ coating. This opens up new prospects for improving the efficiency and adaptability of composite materials in sustainable technologies. The unique structure of spherical hollow hydroxyapatite particles not only prevents aggregation but also allows for a greater surface area owing to the hollow interior. This makes them very appropriate for coating TiO₂ and conducting investigations on photocatalysis.

Revision in manuscript (page 26)

In comparison to other recently researched sorbents, the adsorption capability of the Sa-modified hydroxyapatite as synthesized demonstrated promising findings. Zenefar et al. observed that the photodegradation potential of HAp-TiO₂-ZnO photocatalyst for methylene blue (MB) and methyl orange (MO) dye was 95% and 45% after 2 h, respectively [67]. Hap-TiO₂ nanocomposite demonstrated a degradation efficiency of 80% for methyl orange (5 mg/mL), as stated by Sharifat et al [38]. Furthermore, Anmin et al. reported the degradation efficiency of titanium substituted HAp for methylene blue to be 17-37% under visible light and 39-50% under UV light [68]. This literature review shows that TiO₂-coated HM-HAP composite is an effective catalyst for removal of dyes from water.

 

Comment: 18.  English lexicon should be generally improved.

Response: Respected reviewer, thank you for your valuable comment. The manuscript has been revised and all the grammar and syntax error have been corrected.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Shafiq et al. synthesized HM-HAP particles via a hydrothermal method using a CaCO3 template, followed by coating with titania to create TiO2/HM-HAP composites. Comprehensive characterization using SEM, XRD, XPS, BET, FTIR, EDS, UV–vis DRS spectroscopy, and PZC analysis elucidated the morphological and physicochemical properties of the composites. Evaluations showed that the 20% TiO2/HM-HAP composite exhibited effective photocatalytic degradation of methyl red (MR) dye under UV irradiation, following pseudo-first-order kinetics with a rate constant of 0.033. pH analysis revealed optimal degradation at pH 6, and LC-MS and FTIR analyses confirmed degradation products, highlighting HM-HAP as a promising support material for TiO2 coatings in water treatment applications.

Comments:

1. Authors should show the structure of hydroxyapatite and TiO2 and explain the interaction between them in TiO2/HM-HAP composites.

2. Authors should remove the numbers in y-axis scale in Figure 2a because these numbers are not obtained experimentally.

3. Authors should check the title of y-axis in Figure 8. Is it absorbance or transmittance?

 

4. Do holes or electrons play the most effective role in dye degradation?

Author Response

Comment: 1. Authors should show the structure of hydroxyapatite and TiO₂ and explain the interaction between them in TiO₂/HM-HAP composites.

Response: First of all, I would like to thank you for your valuable time, expertise and critical comments. The structure of hydroxyapatite (HAP) is characterized by a hexagonal lattice composed of calcium ions, phosphate groups, and hydroxyl groups. HAP has a high surface area with abundant hydroxyl groups, which impart negative charge and make it an excellent adsorbent material.

Fig. 1 Structure of hydroxyapatite (HAP).

Titanium dioxide (TiO₂), on the other hand, typically exists in anatase form for photocatalytic applications, with a tetragonal structure that provides active sites for photocatalytic reactions.

In the TiO₂/HM-HAP composites, the interaction between TiO₂ and HAP occurs through the coating process. The TiO₂ particles are deposited on the surface of HM-HAP, leading to the formation of TiO₂-coated HM-HAP composites. This interaction enhances the stability and dispersion of TiO₂ particles, while the hydroxyl groups on HAP provide additional adsorption sites for the pollutants. Consequently, the composite material benefits from the high surface area and adsorption capacity of HAP, combined with the photocatalytic properties of TiO₂. This synergistic interaction between TiO₂ and HAP in the composite material significantly improves the photocatalytic degradation of pollutants like methyl red dye under UV irradiation, as demonstrated in our study.

This explanation has also been added to the supplementary information:

 

Revision in Supplementary Information (page S7 & S8)

Section 1: Hydroxyapatite (HAP) has a hexagonal lattice structure consisting of calcium ions, phosphate groups, and hydroxyl groups. HAP possesses a substantial surface area and is rich in hydroxyl groups, which contribute to its negative charge and render it an exceptional adsorbent material.

Fig. S8 Structure of hydroxyapatite (HAP).

Titanium dioxide (TiO₂), on the other hand, typically exists in anatase form for photocatalytic applications, with a tetragonal structure that provides active sites for photocatalytic reactions.

In the TiO₂/HM-HAP composites, the interaction between TiO₂ and HAP occurs through the coating process. The TiO₂ particles are applied to the surface of HM-HAP, forming TiO₂-coated HM-HAP composites. This interaction improves the stability and distribution of TiO₂ particles, while the hydroxyl groups on HAP offer additional sites for the pollutants to be adsorbed. As a result, the composite material takes advantage of the photocatalytic properties of TiO₂ and HAP’s high surface area and adsorption capacity. The combined effect of TiO₂ and HAP in the composite material greatly enhances the process of breaking down contaminants, such as methyl red dye, using UV light.

 

Comment: 2.  Authors should remove the numbers in y-axis scale in Figure 2a because these numbers are not obtained experimentally.

Response: Respected reviewer, thank you for your valuable comment. The Fig. 2a has been revised and the numbers in y-axis scale has been removed.

Revision in manuscript (page 12)

Fig. 2 FTIR spectra of as-synthesized samples (a).

 

Comment: 3. Authors should check the title of y-axis in Figure 8. Is it absorbance or transmittance?

Response: Respected reviewer, thank you for your valuable comment. The y-axis title in Fig. 8 has been checked and corrected. It was mistakenly labeled as "absorbance" but has now been corrected to “% transmittance”.

Revision in manuscript (page 25)

Fig. 8 FTIR spectra of MR dye before degradation (a), and after degradation (b).

 

Comment: 4. Do holes or electrons play the most effective role in dye degradation?

Response: Respected reviewer, thank you for your valuable comment. According to our experimental findings and analysis, it seems that both holes (h⁺) and electrons (e⁻) have significant roles in the process of dye degradation. Our study (effect of scavengers) reveals that holes play a pivotal role in the degradation process of methyl red dye. The use of isopropanol as a scavenger (Fig. S7) provides support for this hypothesis, as it particularly targets hydroxyl radicals (·OH). The addition of isopropanol results in a notable reduction in the efficiency of degradation. This indicates that the predominant reactive species engaged in the photocatalytic process are hydroxyl radicals, which are mostly produced by photogenerated holes. This indicates that the presence of holes is crucial in enabling the generation of these radicals, therefore playing a pivotal part in the process of degradation.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

In my review report, I advised the Authors to conduct chemical actinometry, consecutive experimental runs, photoluminesence analysis and additional tests with emerging contaminants. However, apart from enriching the discussion, no further data were added to the revised manuscript. Based on this and considering the fact that the novelty of the presenet study is low I can't recommend it for publication on Coatings. 

Author Response

Comment: In my review report, I advised the Authors to conduct chemical actinometry, consecutive experimental runs, photoluminesence analysis and additional tests with emerging contaminants. However, apart from enriching the discussion, no further data were added to the revised manuscript. Based on this and considering the fact that the novelty of the present study is low I can't recommend it for publication on Coatings.

Response: Respected Reviewer, we appreciate your thorough feedback and recommendations for additional experiments. However, we would like to address the points raised regarding the suggested analyses and the scope of our study: While we understand the importance of chemical actinometry in characterizing light properties, our research primarily focuses on material synthesis, coating processes, and the photocatalytic degradation of pollutants. Chemical actinometry falls outside the core scope of our study. Our aim was to explore the efficiency and stability of the synthesized TiO₂/HM-HAP composites, not the light chemistry involved in the process. Although photoluminescence analysis was suggested, we have conducted comprehensive characterizations for post-photodegradation analyses using LC-MS and FTIR. These methods effectively elucidate the degradation of the dye and its mechanisms and validate the photocatalytic efficiency of our composites. Additionally, we have conducted scavenger experiments as requested, which demonstrated the role of hydroxyl radicals (·OH) in the degradation process. These findings further explain the mechanism of dye degradation and support the overall conclusions of our study.

Conducting additional tests for consecutive runs and emerging contaminants would require an extensive timeframe (20-25 days), which is not feasible within the given revision period. However, we have already provided a detailed discussion on the stability of our materials and supported it with literature demonstrating their effectiveness against other pollutants.

The novelty of our research lies in the innovative use of hollow mesoporous hydroxyapatite (HAP) structures, which address the common issues of particle agglomeration and ineffective TiO₂ loading observed in prior studies using rod-shaped or quasi-spherical HAP. This novel approach significantly enhances the surface area and accessibility to active sites, thereby improving the photocatalytic and environmental performance of the TiO₂/HAP composites. This is the first study to employ hollow mesoporous HAP templates for TiO₂ coating, opening new avenues for sustainable technologies.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors modified the paper according to the suggestions, improving its quality, so the work is suitable for the publication.

Comments on the Quality of English Language

Minor English revision is required.

Author Response

The authors modified the paper according to the suggestions, improving its quality, so the work is suitable for the publication.

Comment: 1. Minor English revision is required.

Response: First of all, I would like to thank you for your valuable time, expertise and critical comments. The whole manuscript has been double checked and all the grammar and syntax errors have been corrected.