Activated Carbons Derived from Teak Sawdust-Hydrochars for Efficient Removal of Methylene Blue, Copper, and Cadmium from Aqueous Solution

Round 1

Reviewer 1 Report

Questions and comments are following:  

1.Line 125.

Reviewer: What salts of copper (II) and cadmium (II) were used for adsorption experiments? Could the anions’ chemical origin change to the adsorption properties of the prepared carbon materials?

2.Lines 143-144.

Authors: “The results indicated that the particle sizes of the synthesized carbonaceous materials exceeded 10 μm”.

Reviewer: The particle size of the synthesized carbonaceous material should be determined NOT by SEM-images. These images can illustrate relatively rugged surface only.

3.Line 196. Table 2, row 3.

Reviewer: Why is there NO zinc content for the ACZ1175 sample in the Table?

4.Line 210. Table 3, rows 7-8.

Reviewer: As one can see, the total amount of oxygen-containing functional groups on ACZ1175 and ACZ1200, for example, differ by 11%.  What is the experimental relative error of the titration method? Perhaps, amounts of 2.5, 2.7, 2.4 mmol g^–1 are the same in magnitude.

5.Lines 281-282.

Authors:  “In Tables 5 and 6, the adsorptive amounts in mmol g^–1 of Cu(II) are higher than those of Cd(II)”.

Reviewer: The amount of Cd(II) adsorbed on WAC (equal to 3.71 mmol g^–1) is higher than the amount of adsorbed Cu(II) (equal to 2.30 mmol g^-1) in 1.6 times. How can this result be explained?

6.Lines 284-286.

Authors: “The ion radius of Cd(II) is higher than that of Cu(II).”

Reviewer: What are the radii (in nm) of these ions?

Authors: “When ion exchange was the sole adsorption mechanism, the adsorptive amount in mmol/g of Cu(II) should be higher than that of Cu(II) “.

Reviewer: Check the data with respect to ions.

7.Line 295.

Authors: “If ion exchange were the sole adsorption mechanism, the adsorptive amounts of MB should be approximately twice those of Cu(II) and Cd(II)”.

Reviewer: This statement should be clarified.

The adsorptive amounts are expressed in mg g^–1 and mmol g^–1. If the amounts in mmol g^–1 are the same, the amounts in mg g^–1 differ 3 and 5 times in accordance with their molecular weights (M.m. of methylene blue are 320, Cd – 112, Cu – 64)

8.Lines 311-312. Table 7.

Reviewer: It is recommended to greatly edit this table, for example, to shorten the descriptions, and also not to use abbreviations such as DIW, MHC, MAHC and so on. As a recommendation, the maximum adsorption amount and corresponding references can be presented in this table.

 

Typos should be corrected. For example, in Table 7 HNO₃ instead HNO3, [1] TiO₂ instead TiO2, [4] Bi₂WO₆ instead Bi2WO6,  [27] Adv.Catal instead Adv.Catal. and so on.

Author Response

Responses to the Reviewer 1 Comments

 

Point 1. Line 125.

Reviewer: What salts of copper (II) and cadmium (II) were used for adsorption experiments? Could the anions’ chemical origin change to the adsorption properties of the prepared carbon materials?

 

Response:

The Cu(II) and Cd(II) salts used in this study were Cu(NO₃)₂ (trihydrate) and Cd(NO₃)₂ (tetrahydrate), respectively, purchased from Sigma-Aldich. Stock solutions were prepared by dissolving the salts in distilled water (Lines 92-93 and 127-129).  The selected reagents are widely used in the research regarding the adsorption process. As a result, the anion chemical origin (i.e., nitrate in this case) could not affect the adsorption properties of the prepared ACs.

 

Point 2. Lines 143-144.

Authors: “The results indicated that the particle sizes of the synthesized carbonaceous materials exceeded 10 μm”.

Reviewer: The particle size of the synthesized carbonaceous material should be determined NOT by SEM-images. These images can illustrate relatively rugged surface only.

 

Response:

Thank you for the reminder. To avoid confusion, the sentence is removed from the revised manuscript. Only the observed result of SEM is described now (see Lines 150-153).

 

Point 3. Line 196. Table 2, row 3.

Reviewer: Why is there NO zinc content for the ACZ1175 sample in the Table?

 

Response:

Zinc content on the surface of the ACZ1175 sample was too low to be detected by XPS. That is why we could not show this data set in Table 2, row 3.

 

Point 4. Line 210. Table 3, rows 7-8.

Reviewer: As one can see, the total amount of oxygen-containing functional groups on ACZ1175 and ACZ1200, for example, differ by 11%.  What is the experimental relative error of the titration method? Perhaps, amounts of 2.5, 2.7, 2.4 mmol g–1 are the same in magnitude.

 

Response:

Thank you for the suggestion. In this study, each experiment analysis step was performed in duplicate, and the resulting data were averaged. We could not show the error bar and standard deviation in Table 3. If the bias of a repeated experiment exceeded 15%, a triplicate run was performed. The possible reason regarding bias might result from inhomogeneous constituent on the adsorbents.

 

Point 5. Lines 281-282.

Authors: “In Tables 5 and 6, the adsorptive amounts in mmol g^–1 of Cu(II) are higher than those of Cd(II)”.

Reviewer: The amount of Cd(II) adsorbed on WAC (equal to 3.71 mmol g^–1) is higher than the amount of adsorbed Cu(II) (equal to 2.30 mmol g^-1) in 1.6 times. How can this result be explained?

 

Response:

The authors appreciate the comment. It is a typo. The number in Table 5 should have been (and has been corrected to) “1.01” (Line 262, Table 5, row 3).

 

Point 6. Lines 284-286.

Authors: “The ion radius of Cd(II) is higher than that of Cu(II).”

Reviewer: What are the radii (in nm) of these ions?

Authors: “When ion exchange was the sole adsorption mechanism, the adsorptive amount in mmol/g of Cu(II) should be higher than that of Cu(II) “.

Reviewer: Check the data with respect to ions.

 

Response:

Thank you for the comment. The ionic radii of the metal ions are 0.97 Å (Cd²⁺) and 0.73 Å (Cu²⁺), which has been indicated in the manuscript (Line 296). Some unclear description was found in the manuscript regarding the adsorption mechanism. The sentence has been rephrased (Lines 289-302).

 

Point 7. Line 295.

Authors: “If ion exchange were the sole adsorption mechanism, the adsorptive amounts of MB should be approximately twice those of Cu(II) and Cd(II)”.

Reviewer: This statement should be clarified.

The adsorptive amounts are expressed in mg g^–1 and mmol g^–1. If the amounts in mmol g^–1 are the same, the amounts in mg g^–1 differ 3 and 5 times in accordance with their molecular weights (M.m. of methylene blue are 320, Cd – 112, Cu – 64)

 

Response:

The text has been edited to clarify the statement (Lines 303-311).

 

 

Point 8. Lines 311-312. Table 7.

Reviewer: It is recommended to greatly edit this table, for example, to shorten the descriptions, and also not to use abbreviations such as DIW, MHC, MAHC and so on. As a recommendation, the maximum adsorption amount and corresponding references can be presented in this table.

 

Response:

Thank you for the suggestion and correction is made (see Table 7).

 

Point 9.

Typos should be corrected. For example, in Table 7 HNO₃ instead HNO3, [1] TiO₂ instead TiO2, [4] Bi₂WO₆ instead Bi2WO6,  [27] Adv.Catal instead Adv.Catal. and so on.

 

Response:

Thank you for the suggestion. Corrections have been made.

Author Response File: Author Response.docx

Reviewer 2 Report

The use of wastematerial to produce activated carbon is very interesting, and is often described in literature. The authors state that to their knowledge the use of sawdust from teak has not been described. When I searched for it in Scopus, however, I found 92 papers describing the use of sawdust to produce activated carbon, 3 of them describing sawdust from teak. Besides, I found 18 papers on activated carbon made from teak! So, I would suggest that the authors have another look at literature.

The paper is clearly written and the adsorption effects have been thoroughly studied. However, no pH values are mentioned, and these are crucial for adsorption processes, especially if O-containing surface groups like acids are involved. Mostly, adsorption studies are carried out at different pH-values, but at least the actual pH value should be mentioned.

 

Furthermore, the goal is to use this type of activated carbon in wastewater treatment for the removal of heavy metals. Wastewater contains a high concentration of organic matter. Do the authors have an idea whether competition by organic matter would hinder this removal of heavy metals?

Author Response

Responses to the Reviewer 2 Comments

 

Point 1:

The use of waste material to produce activated carbon is very interesting, and is often described in literature. The authors state that to their knowledge the use of sawdust from teak has not been described. When I searched for it in Scopus, however, I found 92 papers describing the use of sawdust to produce activated carbon, 3 of them describing sawdust from teak. Besides, I found 18 papers on activated carbon made from teak! So, I would suggest that the authors have another look at literature.

 

Response:

Thank you for your suggestion. The sentence on “Introduction” has been rephrased (Lines 76-79). Especially, in this study we emphasized sawdust from teak (Tectona grandis) - a waste product of a furniture manufacturing factory to synthesize activated carbon samples (ACs) with different processing through a two-step reaction: (i) hydrothermal carbonization, followed by (ii) chemical activation. It is worth noting that we would like to share the knowledge of these adsorption mechanisms which have not been discussed in-depth in the literature.

 

Point 2:

The paper is clearly written and the adsorption effects have been thoroughly studied. However, no pH values are mentioned, and these are crucial for adsorption processes, especially if O-containing surface groups like acids are involved. Mostly, adsorption studies are carried out at different pH-values, but at least the actual pH value should be mentioned.

 

Response:

Thank you for the comment. The sentence has been rephrased (Lines 132-134). The effect of pH on MB and metal species biosorption has been studied by many researchers, and the results indicated that pH of the solution exerts an effect on the uptake of metal ions. In this work, the authors intended to discuss adsorption mechanisms through comparison of adsorptive amounts between the adsorbates. Because precipitation of heavy metal ions occurs at pH 6.0, adsorption experiments of MB, Cd^2+, and Cu²⁺ were set as pH 5.0. The initial pH of the solution adjusted to 5.0 ± 0.1 by 0.1 mol/L NaOH and HCl solution.

 

 

Point 3:

Furthermore, the goal is to use this type of activated carbon in wastewater treatment for the removal of heavy metals. Wastewater contains a high concentration of organic matter. Do the authors have an idea whether competition by organic matter would hinder this removal of heavy metals?

 

Response:

Thank you for your suggestion and reminder. In the present study, we only focused on the adsorption capacity of the AC for a specific organic or metal adsorbate. While we have not tacked it, the competitive effect resulting from co-existing multi-components is for sure critical in a real-world system and warrants for future investigations, as theoretically, the existence of organic matter would inhibit the heavy metal adsorption by competing for the active sites.

 

Author Response File: Author Response.docx