Review Reports - Estimating the Reducing Power of Carbon Nanotubes and Granular Activated Carbon Using Various Compounds

Round 1

Reviewer 1 Report

This study aimed to estimate the reducing power of carbon nanotubes and granular activated carbon. Moreover, the effect of pH and dosage was also investigated. Overall, the paper has an interesting topic. However, A major revision was needed before publication.

The author mentioned that the poresizedistribution was related to the reaction time, but the effect of surface chemistry property should also be considered.
As an electron donor, where is the electron from?
Why the carbon materials cannot reduce the compounds with reduction potentials in the range +0.218 V to +0.054 V?

Author Response

The author mentioned that the poresizedistribution was related to the reaction time, but the effect of surface chemistry property should also be considered.

→ Response: The authors sincerely acknowledge the valuable comments provided by the reviewer. As we explained in line 152~157, pore size distribution/pore structure is likely to affect the reduction reaction time. With pore size distribution, the oxygen functional groups on carbon-based materials are also an important factor affecting the redox reactions (Kemmou et al., 2018; Wu et al., 2018a, b). As Lu et al., 2018, there is little difference on the number of oxygen functional groups between CNTs and GAC as the attached figure. Therefore, we thought that pore size distribution/pore structure may be a more determining factor than the surface chemistry showing different reduction reaction time between two materials.

2. As an electron donor, where is the electron from?

→ Response: Carbon-based materials such as graphite and carbon nanotubes have been commonly used as anodes due to the number of free electrons available on their surface which were produced via their exothermic production process (Caizán-Juanarena et al., 2020). Therefore, carbon-based materials function as electron donors providing freely moving electrons which were held on their surface.

3. Why the carbon materials cannot reduce the compounds with reduction potentials in the range +0.218 V to +0.054 V?

→ Response: The objective of this study is to estimate the reducing capacity of CNTs and GAC using various compounds having different reduction potentials. The reasons why CNTs and GAC assessed failed to reduce the compounds of which reduction potentials are in range of +0.218 V to 0.054 V is that the difference on their reduction potentials between CNTs/GAC and the target compounds (PW₁₂O₄₀^3- and SiW₁₂O₄₀^4-) was not enough to occur electron transfer. Therefore, there was no reduction reaction which is electron transfer from CNTs/GAC to the PW₁₂O₄₀^3- and SiW₁₂O₄₀^4-. This description was reflected in the conclusions in line 247-248. .

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and suggestions to authors

Authors: Heesoo Woo , Ilho Kim , and Saerom Park

Manuscript title:

Estimating the reducing power of carbon nanotubes and granu- 2 lar activated carbon using various compounds

You have very interesting results that I considered important for the scientific discipline. To support the publication process I have made some suggestions below:

Lines 39-42

How large is the specific surface area of CNTs? Have any relevant studies been carried out?

Line 45

The "s" in CNTs is missing

Line 65

Should be 2.1. Reagents

Lines 80-82

How long and at what temperature were the carbonaceous materials dried?

Line 100

Should be 1,5-diphenylcarbazide

Comments for author File: Comments.pdf

Author Response

Reviewer #2.

Lines 39-42: How large is the specific surface area of CNTs? Have any relevant studies been carried out?

→ Response: The range of specific surface area (SSA) of carbon-based materials in water purification is very wide. In general, GACs with various sizes of pores have the largest SSA, followed by graphene-based materials such as SWCNT and MWCNT because graphene has a large SSA of 2,629 m²/g, theoretically. The large SSA of MWCNT and GAC used in this study were also confirmed with BET analysis, and the results are presented in Table 1 in the manuscript. The table below summarizes the SSA of various carbon-based materials.

Carbon-based materials	Specific surface area (m²/g)	References
Graphite	17.56	Chen et al., 2004
Carbonised beet pulp	47.5	Dursun et al., 2005
Biochar (sludge based)	70.12	Wang et al., 2017
Nanodiamond	244	Yang et al., 2021
Graphene oxide	497	Ersan et al., 2016
Single-walled carbon nanotubes	541	Yang et al., 2006

Line 45: The "s" in CNTs is missing

→ Response: Corrected

Line 65: Should be 2.1. Reagents

→ Response: Corrected

Lines 80-82: How long and at what temperature were the carbonaceous materials dried?

→ Response: The detail description on the drying method was added in line 82.

Line 100: Should be 1,5-diphenylcarbazide
→ Response: Corrected

Author Response File: Author Response.docx

Reviewer 3 Report

I prepared the manuscript review Estimating the reducing power of carbon nanotubes and granu-2 lar activated carbon using various compounds written by Heesoo Woo, Ilho Kim2 and Saerom Park

The study is clear and adequately described. In my opinion this manuscript is acceptable for publication.

Author Response

Reviewer#3.

I prepared the manuscript review Estimating the reducing power of carbon nanotubes and granu-2 lar activated carbon using various compounds written by Heesoo Woo, Ilho Kim2 and Saerom Park

The study is clear and adequately described. In my opinion this manuscript is acceptable for publication.

-> Response: I sincerely appreciate to your decision.

Reviewer 4 Report

The manuscript under review presents results on the reducing capability of CNTs and GAC using the reduction degree of several compounds. From the view of scientific research it is OK to publish in the journal of “Water”. However I doubt the economical efficiency for the using of CNTs as these five compounds can easily be reduced with other type of reductants.

Please introduce the necessity of the CNTs and GAC and the economical analysis of this technology.

The reason why these five compounds were selected to study the reducing capability of CNTs and GAC should be state.

Author Response

Please introduce the necessity of the CNTs and GAC and the economical analysis of this technology.

→ Response: Carbon is the most common chemical element and can be obtained in an eco-friendly and ultra-low-cost procedure (Kolodyŉska et al., 2018; Khiari et al., 2020; Gonzaga et al., 2020). In comparison with other reductants in terms of cost, although carbon-based materials may be not the best beneficial materials, this study is able to provide possibility of carbon-based materials use in many applications such as reductants and catalysts. Among the carbon-based materials, GAC is a lower cost material than CNT as the below table (Gao et al., 2020).

(Gao et al., 2020)

The reason why these five compounds were selected to study the reducing capability of CNTs and GAC should be state.

→ Response: First of all, as the objective of this study, we considered different reduction potentials which the target compounds have. For Chromium, it was selected due to its toxicity which means requiring the removal/reduction of Cr(VI) to Cr(III). Fe was selected due to its easy-accessibility. For other polyoxometalates, they have unique chemical structure, polynuclear metal-oxo clusters with discrete and chemically modifiable structures. Therefore, the intention of choosing these compounds was to assess if CNTs and GAC are able to reduce these compounds regardless of their unique chemical structures.