Visible Light Driven Photocatalytic Decolorization and Disinfection of Water Employing Reduced TiO₂ Nanopowders

Round 1

Reviewer 1 Report

Manuscript ID: catalysts-1072240

Type of manuscript: Article

Title: Visible light driven photocatalytic decolorization and disinfection of water employing reduced TiO2 nanopowders

Authors: Xiaolan Kang, Chrysanthi Berberidou, Augustinas Galeckas, Einar Sagstuen, Truls Norby, Ioannis Poulios, Athanasios Chatzitakis

The manuscript deals with both preparation and characterisation of engineered titania nanopowders useful for photocatalytic applications.

The authors report interesting results and propose quite clear interpretations as well, even though few explanations/additions are still needed in my opinion.

In particular, please consider the following:

1. As for the morphological characterisation, a HRTEM characterisation of the nanopowders could add some useful insights about the presence/absence of lattice fringes (i.e, in defining the ultimate crystalline nature) of the external sphere of the obtained material(s), in order to confirm the indications coming from the XRD patterning about the transformation from anatase to rutile as well;
2. In the XRD characterisation, the authors quote the same JCPSD card to identify both anatase (on line 123, card 21-1272) and rutile (on line 132, card 21-1272) crystal phases: the latter might be emended with the correct card indication;
3. A few references by different authors, rather than only those quoted in the 6-10 reference list, should be added to enrich the referencing, as other research groups published interesting and important papers on the topic dealing with titania doping to improve its inability to apply in visible light applications. Consider, for instance, the papers by both prof. C. L. Bianchi’s and prof. Ohtani’s groups.

In my opinion the manuscript should be reconsidered after these minor revisions before being published in Catalysts.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript was written by Xiaolan Kang et al. reports on the synthesis of the reduced TiO₂ nanopowders for the application of photocatalytic decolorization and disinfection of water by a sol-gel method. From my point of view, this manuscript contains information that can be of interest to the scientific community, and I recommend its publication. However, amendments must be made before the final publication. Below are listed my observations.

1. In this study, the surface area is a significant parameter. Therefore, I strongly suggest that the BET surface area measurement should be added for all of the samples in the revised manuscript or supporting information.
2. Recycling experiments should be provided, and reduced TiO₂ nanopowders after photocatalytic reaction should be characterized by XRD or XPS to check the stability of the catalyst.
3. The performance data of test photocatalytic activity should be added to the error bar, which can ensure the reliability of the data.
4. As a comparison, the degradation performance of colorless Bisphenol A (BPA) or phenol could be provided.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

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Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript submitted by Kang and Berberidou et al., entitled Visible light driven photocatalytic decolorization and disinfection of water employing reduced TiO₂ nanopowders, deals with the important topic of titanium dioxide use as a photoactive material. Authors prepared four different TiO₂ particles, characterized them and assessed their potential as photoremediation catalysts and as photosensitizers in photodisinfection. The article fits the scope of the Catalysts.

The study is rather well designed and the manuscript well written. Use of appropriate figures makes it easier to follow the flow of text.

However, there are some issues that need to be addressed before the article can be considered for publishing:

1. My biggest concern is the assessment of the photoactivity of the materials – both in photodecomposition of pollutants and photodisinfection. For the experiments in decolorization of malachite green (MG)-polluted water you used only filtration through 0.45 µm filters. Was this sufficient to separate the catalyst? Also, using other techniques than only UV-Vis (i.e. total organic carbon - TOC) could tell more about what happens to MG upon the experiment – is it decomposed to other products or if mineralization occurs. This has a tremendous effect on the subsequent toxicity of the solution after remediation, as the AOPs are known to often increase the toxicity of the pollutants. Also, an assessment of the acute toxicity of the solutions would be useful for evaluation of the experiment.
2. In case of photodisinfection, you do not report the so-called dark toxicity and no such information is given in the discussion. Were the materials tested for their effect on the endospores without irradiation? Even without light, nanomaterials, including titania, might cause some changes and reduce the viability of cells (https://doi.org/10.1016/j.fct.2008.09.012, https://doi.org/10.1111/ijac.13128). This can be strongly marked in case of N-TiO₂, as the annealing process could introduce amino groups, known for their interaction with negatively charged bacterial membranes (doi.org/10.1128/AAC.46.4.1073-1079.2002). As the materials after the irradiation are incubated with the spores for 24 h, the light-independent toxicity would have a significant effect on the viability results. Also, the lack of activity of certain materials could be probably explained by the lack of the absorption of the used light, but such information is not included in the discussion.
3. The photodisinfection and photodegradation is not discussed.

Less significant remarks:

4. Did you consider using the Scherrer equation to calculate the crystallite size? The signals in XRD spectra of A-TiO₂ and C-TiO₂ are visibly broader than the others, so their size is visibly smaller than of the other materials.
5. The name Bacillus stearothermophilus is an old name, currently these bacteria are called Geobacillus stearothermophilus. I also wonder, wouldn’t testing not only on G. stearothermophilus but on Gram-negative strain as well show the full potential of the studied materials?
6. What was the hydrodynamic diameter of the materials? Did you assess the size distribution in water? It might affect the internalization of the materials into cells.
7. Please consider adding error bars in figure 6.
8. The light intensity for photodisinfection was measured at ~2 mW/cm². At what wavelength?
9. Section 2.5 is mentioned in lines 399 and 440, but there is no such section in the manuscript.
10. Minor spelling and style mistakes: “have detected” (line 233); “holes much be” (line 292); “added” (line 392); “visible light activate” (line 452); too many commas (“then,” line 58; “is, also,” line 77; “were, then,” line 411).

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I would like to recommend this paper for publication, as it meets the publication criterion.

Reviewer 4 Report

The manuscript improved significantly and all the comments were fully answered. I recommend the article to be accepted for publication.