Photocatalytic Remediation of Harmful Alexandrium minutum Bloom Using Hybrid Chitosan-Modified TiO₂ Films in Seawater: A Lab-Based Study

Question/suggestion

Authors’ Response

This article is devoted to the preparation of chitosan-modified TiO2 hybrid films with their subsequent use as a photocatalyst for the restoration of harmful flowering of Alexandrium Minutum. The topic is relevant for some marine regions, but the synthesis process itself is interesting, and there is also a prospect of using this catalyst in other aspects of photochemistry. The title of the article fully reflects its content. In the "Abstract" section, the results are described quite fully, which allows you to quickly get acquainted with this work. The introduction clearly describes the problems that this work is aimed at solving. At the same time, I recommend the authors to refine the following points:

Thank you for the comments. We have refined the manuscript as suggested by Reviewer 1.

1. (TiO2) - purchased by the authors or was it personally synthesized by them? it is desirable to specify. If it is synthesized, then it is necessary to indicate the method of obtaining and/or a link to it.

The TiO₂ was synthesized by us. We have cited the paper that we had referred to accordingly. Please refer to reference [53].

2. Chitosan has a narrow range (pH) of solubility in water. It is desirable to indicate what value was in the course of this work.

Thank you for the valuable comment. We have included the weight difference and the physical appearance of the fresh and used films in supplementary data. A description of the data is given in Section 2.5.

3. Figure 1. Have the authors normalized the spectra?

Thank you for the questions. To the best of our knowledge, normalization is often done when there are significant changes in the peak position or the shape of the spectra.  The FTIR spectra and XRD diffractograms were not normalized since the shifting of the peaks was not significant. The FTIR was taken at different parts of the films, and a similar peak position was observed. The films from different batches were also tested and similar spectra were observed as well.

4. XRD diffractograms. When describing, it is desirable to make an estimate (or calculation) of the crystallinity of the starting materials and products obtained, as well as to describe it.

Thank you for the valuable comment. The physicochemical properties for the anatase TiO₂ are further elaborated upon in reference [53].

5. When describing the results obtained, it is desirable to add more comparisons with literary sources, this will add validity to some assumptions.

Thank you for the suggestions. We have compared the efficiency of the films with literary sources. Please refer to Table 2.

6. It is desirable to cite the work: 10.1016/j.molstruc.2021.131083.

Thank you for the recommendation. We have included the reference. Please refer to [13].

7. Photocatalytic Mitigation Studies. It also requires more comparison with known catalysts. This will help to draw an adequate conclusion about the efficiency of the resulting catalyst.

Thank you for the suggestions. We have compared the efficiency of the films with literary sources. Please refer to Table 2.

Question/suggestion

Authors’ Response

In the paper entitled „Photocatalytic Remediation of Harmful Alexandrium Minutum Bloom Using Hybrid Chitosan Modified TiO₂ Films in Seawater: A Lab-Based Study”, Ibrahim and co-workers presented studies of the titania/chitosan nanocomposite and potential photocytotoxicity against the selected algal bloom. The obtained photocatalyst was characterized by the use of SEM and XRD techniques. The results of the photocatalytic remediation seem to be interesting and worth publishing in Catalysts. However, a major revision needs to be performed. There are some issues that should be corrected/explained/investigated:

Thank you for the comments. We have refined the manuscript as suggested by the Reviewer 2.

1. Could you explain the non-linearity of obtained RE values presented in Fig. 4?

Thank you for the question. The non-linearity of obtained RE values is explained in Section 2.2. The main reason is due to the weak electrostatic attraction between the algae cells and the film and the swelling of the films.

2. Can you determine the amount of algae cells repelled from the film surface?

Thank you for the question. The amount of algae repelled can be estimated from the RE value. For example, if the RE value is 50%, then the amount of algae repelled is 50% of the total amount of cells. The efficiency of a mitigating agent is reported by the RE value not the expelled algae cell. Hence, the RE value was reported to provide an account of the efficiency of the films.

3. The best performance of TiO₂ nanoparticles in photodegradation studies was obtained when irradiated with UV (365nm) light. Please repeat photodegradation experiments with the use of a UV lamp instead of using visible light.

Thank you for the suggestion. We agree that the best performance of TiO₂ is under UV. Aside from performance, the UV irradiation may cause damage to the cells of A. Minutum. Under the lab conditions, the A. minutum cells need a light intensity of 70 umol photons m²s^-1 using a 16:8 hour (light:dark) photocycle in order to ensure survival. Hence, we have decided to conduct the mitigation study under the same light intensity and relevant conditions to support algae growth.

4. Can you determine the hydroxyl radicals generation efficiency of the obtained system (with the use of the same literature-known quenchers like methylene blue, DPBF, or other)?

Thank you for the comment. There are papers that have reported the use of hydroxyl scavengers to confirm the presence of hydroxyl species. However, depending on the species, we do feel like this method may yield inaccurate data. Adding hydroxyl scavenger into the cell mixture will change the medium conditions and can either suppress or even enhance the algae growth. Furthermore, The E_CB and E_VB indicate that hydroxyl radicals will be formed. Hence, we did not carry out the scavenging test due to the unknown biological effects of such scavenger systems. 

5. What about the stability of obtained nanosystem? Can you determine its reusability? Please perform the studies concerning the photostability of fabricated films.

Thank you for the questions. We did not carry out the reusability test because we noticed that the films turned to gel-like and experienced weight loss after first use. The weight loss indicates that the film disintegrated during the mitigation process. We have included a new section to explain this. Please refer to Section 2.5.