Efficient Extraction of Methylene Blue from Aqueous Solution Using Phosphine-Based Deep Eutectic Solvents with Carboxylic Acid

Round 1

Reviewer 1 Report

The ms “Efficient Extraction of Methylene Blue from Aqueous Solution 2 Using Phosphine-based Deep Eutectic Solvents with Carbox- 3 ylic Acid Anions 4” describes the use of different DES for the extraction of MB from aqueous solutions. 

The ms work is well described, but the lack of details in the DES preparation and characterization makes me concerned. 

My concerns are specially related to DES water content, solubility, and characterisation:

The experimental details for the preparation of DES must include the amount of water that is present in the DES. Even if the authors didn’t add water, the starting material can have it. During the preparation of the DES, hygroscopic, water can capture and present at the final solvent composition. I believe that the full characterization of the DES should be mandatory to present in papers, I recommend the characterization through NMR and FTIR. These techniques will show the water amount and any possible impurity. 

What was the ratio of aqueous solution vs Des used? Until what ratio was possible to observe phase separation?

How many times were the experiments repeated? The error bars should be included in the graphs, to assess the meaning of the result.

It will be important to check the pH variation without MB in the aqueous solution. This experiment will give information about the interaction of DES with salts.

What is the solubility of these DES in water?

Author Response

Thank you for your review, please check the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

In the present manuscript, the authors investigated the extraction of methylene blue from aqueous solution using Deep Eutectic Solvents.

In page 3 lines 98-99, page 6 line 161, page 6 line 172, page 6 lines 176-177, page 7 line 186, pages 8 lines 199-200, page 8 line 211, page 9 line 218, pages 10 lines 239-240, page 11 lines 262-263, page 12 lines 276-277, page 12 line 295, to check: Error! Reference source not found.

In page 5 line 143 “in the range 0.3-2.5 m”, what the meaning of m?

Use the same format for references: In Introduction [1] and in section 3.2 (Ahmedi et al. 2019, Xu et al. 2020, Li et al. 2020).

In page 6 line 169, use “dm^-3” instead of “dm-3”.

In page 7 line 182, use “dm^-3” instead of “dm-3”.

In page 8 line 196, use “dm^-3” instead of “dm-3”.

In page 9 lines 230 and 231, use “g/L” instead of “g/l”

In page 10 line 235, use “g/L” instead of “g/l”

In page 12 line 297, to check la reference “Kermanioryani et al.”

In page 13 line 326, the grant number is missing.

Improve Figure 11.

According to the results analysis, in my opinion, the best conditions for extraction of methylene blue (MB) the studied conditions are presented, but those are not strictly the optimal conditions. To obtain the optimal conditions, a mathematical or statistical analysis would be required.

Author Response

Thank you for your review. Please check the attachment

Author Response File: Author Response.docx

Reviewer 3 Report

（1） there are some strange words in the manuscript: Error! Reference source not found.

(2) DES might not be chemically stable at the conditions for absorbing pollutant, the authors could prove it.

(3) It seems DESs are hard to be recycled. It is not good.

(4) Apart from MB, other pollutants should also be tested.

Author Response

Thank you for your review. Please check the attachment

Author Response File: Author Response.docx

Reviewer 4 Report

General comments:

1. The abstract should be revised, commonly, it is presented in order of background or objective, methods, results and findings, and conclusions.

2. Is it techno-economic feasible to use DES for MB removal? The 1:10 ratio is too low to be applied on a pilot scale or industrial scale.

3. Was the experiment of MB removal using DESs conducted in duplicate? Please label the standard deviation, since the repeatability is very important for other scientists to duplicate.

4. Did you do the recovery of DES and reuse it for MB removal? I wonder if it is simple to separate DES from MB and if the recovered DES still remains robust efficiency.

5. In the section on Results and Discussion, the authors presented dominantly data description without expanding the reasons and mechanisms.

Author Response

Thank you for your review. Please check the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors,

Thank you for your reply. I am clarified with your information. I just want to reenforce that authors should include as much details as possible in the experimental part, in particular in the preparation of DES.  

Best regards.

Reviewer 2 Report

Accept in present form

Reviewer 4 Report

The authors addressed my concerns, please accept it as it is.