Effects of Substituting Activated Carbon with Titanium-Dioxide-Coated Cenospheres in Conventional Aquarium Filters

Round 1

Reviewer 1 Report

In my opinion, the paper could be accepted only after a deeply description of the preparation of TiO2 cenospheres within some characterizations of the investigated materials. these should be added to explain the results and why this filter acts better than carbon filter.

english language is ok

Author Response

Thank you for your comments. We have added material characterization of the cenospheres including TPA assay, Raman characterization and microscopy (SEM). We have also added explanations as to the cenospheres being a better filter in the results and discussion section.

Reviewer 2 Report

The study illustrates the effectiveness of TiO2 cenospheres in reducing the concentrations of three harmful compounds - ammonium, nitrate, and nitrite - in fish aquariums. However, before the paper can be approved for publication, several significant issues need to be addressed:

·         Please present TiO2 in the correct format.

·         The authors should provide insights into the photocatalytic activity of TiO2, which was not adequately explained in the current version of the paper.

·         Clarification and further explanation are needed concerning the reduction of nitrite levels via oxidation.

·         Figure 3 is challenging to discern due to its small size.

·         Additional details and explanation are required for Figure 5.

There are minor mistakes across the manuscript, the author may need to proof read it carefully. Otherwise, it is well-written.

Author Response

Thank you for your comments. We have added material characterization of the cenospheres including TPA assay, Raman characterization and microscopy (SEM). We have also added explanations as to the cenospheres being a better filter in the results and discussion section. We have added information how nitrite decreases and nitrates increase so the overall result is that the cenospheres help with nitrogen degradation. Have changed the formatting of Figure 3 to make it bigger and easier to read. Additional details and explanations have been added regarding Figure 5.

Reviewer 3 Report

The article has a certain scientific novelty and practical significance.

There are a number of serious remarks in the text of the work.

1. It is not clear whether the process can be scaled up to open water bodies.

2. Would you like to see the cost of the proposed material compared to conventional filter media?

3. There are no mechanisms for removing ammonium and nitrite ions. The ammonium ion is extremely weakly oxidized even by strong oxidizing agents (for example, ozone), which is why the mechanism is not clear.

4. There are no data on the possibility of the entry into water of hydroxyl radicals generated on the surface of titanium dioxide under the action of UV.

5. There are no data on the possibility of leaching of titanium dioxide nanoparticles into water (carcinogenic effect).

Author Response

Thank you for your thoughtful and insightful comments.

1. Our focus is on aquarium style fish tanks and tough we can look at increasing the size of the tank sizes, we do not envision the use of our cenospheres for open water bodies. However, on the larger scale, similar research has been conducted by company H2Nano for oil sands purification using sunlight and without the need for UV reactors.
2. We have not performed a techno-economic analysis for this study however we have added the sustainability and cost overtime benefits of the ccenospheres in the main text as follows: “Cenospheres are a sustainable alternative to ceramics and carbon filters as they     are a by-product of the oil refining process. Though the initial cost of this method may           seem high to implement in larger scale cases, cenospheres are a circular economy           alternative to other filters as shown in this work”
3. We have added the following explanation as the cenospheres act akin to a biofilter rather than a photocatalytic effect

“Here, our cenospheres act akin to an alternative biological filter. We choose to use small       floating species that mix well in aqueous solutions and a more likely to receive light. The                 cenospheres are comparable to that of a ceramic filter, wherein our solution is cheaper than               conventional ceramic filters. The bacterial growth is what leads to compound breakdown and          causes oxidation reactions to occur”

4. We have added TPA assay data to confirm the coated cenospheres are working as they should.
5. We have added that the minnow fish did not die during the 30 days of study. Although we did not look at toxicity in this work previous literature showcases that cenospheres are non-toxic https://doi.org/10.1016/j.cej.2013.03.077

Reviewer 4 Report

This paper shows the remove of three common harmful compounds, ammonium, nitrate, and nitrite in fish aquariums by activated carbon with titanium dioxide coated cenospheres. The mechanisms about the processes of removing ammonium, nitrate, and nitrite from water by activated carbon with titanium dioxide coated cenospheres were demonstrated in the context.

The studies were quite systematic and the resulted were well organized by the authors. I’d like to recommend the publication of this paper in Environments after revision.

1.      The crystalline of titanium dioxide and activated carbon should be provided to understand the adsorbed light range.

2.      The authors should provide the surface area to understand the pore properties.

3.      The duration of the sample should be verified by authors.

Author Response

Thank you for your comments. We have added material characterization of the cenospheres including TPA assay, Raman characterization and microscopy (SEM). We have also added explanations as to the cenospheres being a better filter in the results and discussion section. We have added surface area and composition information of the cenospheres provided by the manufacturer. We did not study the sample degradation past the 30 days of the study however work by H2Nano has done a similar study with a longer test period so we have added that reference to the discussion (https://doi.org/10.1021/acsestwater.3c00326)

Round 2

Reviewer 3 Report

good