Selective Adsorption of Pb²⁺ in the Presence of Mg²⁺ by Layer-by-Layer Self-Assembled MnO₂/Mxene Composite Films

Round 1

Reviewer 1 Report

Hongjing Qu et al reported a research paper on "Selective adsorption of Pb²⁺ by layer-by-layer self-assembled MnO₂/MXene composite films". Film morphology was analyzed by SEM and the structural characterization was done using powder XRD, XPS. Adsorption performance of the MnO₂/MXene composite films were carried out. Effect of pH, adsorption time on adsorption performance were studied. Adsorption selectivity of Pb²⁺ over Mg²⁺ also reported. 

Overall good research work and this research work fits the aim and scope of the Journal "Processes". However there are certain deficiencies in the article which need to be improved through the revision.

Major Comments:

(1) Authors gave significance of heavy metal adsorption in the introduction. In practical application view point,  industrial water contains combination of heavy and light metals. 

Reviewer suggest authors to compare and contrast the selectivity and adsorption capacity of different heavy metal ions (for example: Cd, Hg, As, Cu etc) under similar conditions so that the selectivity of the films for Pb2+ over other heavy metal ions will be determined and apt for the title of the article.

(2) Authors claim that  MnO₂/MXene composite films are more fluppy and have more adsorption site than MXene films. 

Reviewer suggest authors should use same scale bar ( for example: use 5 micron scale bar to compare the two films) so that readers and easily identify and appreciate the difference between the morphology of the films.

Minor comments:

(1) Line 42: what is meant by "MAX parent material"

(2) Include the full form of "SDS" surfactant.

(3) Line 74: Authors gave the instrument list for the characterization. Reviewer recommends to include a statement that prepared MnO₂/MXene composite films were characterized using the below mentioned instruments.

 

Author Response

We would like to express our sincere thanks to the referees for the critical and careful review of the entire manuscript. Their constructive suggestions and comments are helpful and inspiring for us to reword some of the statements in the text to make it precise, clear and easy to read. Listed below are our point-by-point responses to their questions/concerns.

 

Reviewer 1:

 

(1) Authors gave significance of heavy metal adsorption in the introduction. In practical application view point, industrial water contains combination of heavy and light metals.

Reviewer suggest authors to compare and contrast the selectivity and adsorption capacity of different heavy metal ions (for example: Cd, Hg, As, Cu etc) under similar conditions so that the selectivity of the films for Pb²⁺ over other heavy metal ions will be determined and apt for the title of the article.

Response: Thanks for the comment. The adsorption in this article is reflected in the competitive adsorption of the heavy metal ion Pb²⁺ with other ions, such as Mg²⁺. Beside, Figure 12 and the associated text description in the article illustrate the selective adsorption of MnO₂/Mxene.

(2) Authors claim that MnO₂/MXene composite films are more fluppy and have more adsorption site than MXene films.

Reviewer suggest authors should use same scale bar ( for example: use 5 micron scale bar to compare the two films) so that readers and easily identify and appreciate the difference between the morphology of the films.

Response：Thanks for the kind comment. The scale bar of the figure 1 has been revised in the new version.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript presented the selective adsorption of Pb2+ by layer-by-layer self-assembled 2 MnO2/MXene composite films were evaluated, there are a few comments needed to be addressed as following:

1. Abstract needs to be rewritten, the results are not presented well.
2. In introduction, provide the different methods available for treatments of those pollutants and what is the novelty of this work.

1. The introduction is too simple to draw readers' attention quickly. At least, the authors should detail the reason why they want to start this work.

4. Synthesis of monolayer manganese oxide nanosheets and preparation of MnO2/MXene composite films must be supported with references.

1. Heavy metal ion adsorption test needs to be rewritten with more details.
2. In Figure 1. Adjust the magnification of thses figures.
3. Line 145, modify Figure 4. to be Figure 5.
4. In section, 3.2 Study on adsorption performance
5. What is your reference for drawing this picture 6, and if so, link the mechanism of interaction and results to the Figure and transfer the conditions of the experiment from line 157-161 to the methods section.
6. Fig. 7 should be redrawn into separate figures.
7.  show bar in all the adsorption graphs
8.  In pH factor, why don’t apply pH after 7 value.
9. Please provide the experimental conditions: adsorbent mass, solution volume, temperature, pH, time, mixing rpm, all the factors which are applicable, in every figure caption.

14. The adsorbent need to assess by FTIR spectra.
15. In section, study on adsorption performance, please provide with more discussion.
16. Where the equations for adsorption removal and capacity in whole manuscript, the authors must be supported it with references.
17. Similarly, the effect of initial concentration, please provide this study with the isotherm adsorption models.
18. Similar to the effect of time, please provide this study with the kinetics models.
19. In case of the variation in the initial adsorbate concentration, the Y-axis should be the amount adsorbed. If you compare % adsorption or % removal, obviously the value decreases with the increase in the dose.
20. There are no discussions about the mechanism of the adsorption, was the adsorption made by chemical and/or physical forces? This needs to be fully addressed in the manuscript.
21. In general, the discussion section should give an interpretation of the significance of the results obtained with reference to similar works done by other authors.
22. Pleas this manuscript with graphical abstract.
23. In conclusion section, should be support by the some results.
24. Please, explain the most important recommendations and suggestion to improve the quality of these materials to benefit more from it in the treatment process.

                                Finally, I wish you all the best and success

 

 

 

 

Author Response

We would like to express our sincere thanks to the referees for the critical and careful review of the entire manuscript. Their constructive suggestions and comments are helpful and inspiring for us to reword some of the statements in the text to make it precise, clear and easy to read. Listed below are our point-by-point responses to their questions/concerns.

 

 

Reviewer 2:

 

(1) Line 42: what is meant by "MAX parent material"

 

Response: Thanks for the comment. "MAX parent material" has been modified to "MAX phase material"

 

• Include the full form of "SDS" surfactant.

 

Response: Thanks for the comment. SDS has been labeled as sodium dodecyl sulfate, where first mentioned in the text (Line 9)

 

• Line 74: Authors gave the instrument list for the characterization. Reviewer recommends to include a statement that prepared MnO₂/MXene composite films were characterized using the below mentioned instruments.

Response: Thanks for the comment. The statement has been added in the “2.3 Characterization test” part, and this part has been revised to “Field emission scanning electron microscope (SEM, Zeiss SIGMA, Carl Zeiss AG), and transmission electron microscope (TEM, JEM-2100, JEOL) were applied to the morphological characterisation of the MnO₂/MXene composite film, whiel the X-ray diffraction patterns were recorded on the X-ray diffractometer (XRD, XPert Pro, PANalytical Corporation), fully automatic rapid comparison Surface area and porosity analyzer (BET, ASAP 2020, McMerritik (Shanghai) Instrument Co., Ltd.) The photoelectron spectra were recorded on X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher, USA) ), continuous light source atomic absorption spectrometer (contrAA700, Analytik Jena AG, Germany), while the IR spectra were recorded on a Thermo FT-IR 5700 IR spectrometer (KBr)”

 

（4）There are a few typing mistakes as well, and authors are advised to carefully proof-read the text. PH instead of pH, no space between number and unit in line 63. Missing unit in line 116. There are 2 "figure 4".

 

Response: Thanks for the comment. All the typing mistakes have been modified, and the whole text has been carefully checked.

 

More figure legend labels are needed in Figure 3 B; Figure 4 (should be figure 5) D. Regarding figure 7 and 8, is there any duplications performed? If so, please provide the error bar in the figures.  If not, what is the reproducibility of removal experiments? 

Response: Thanks for the comment. The figure legend labels are added in Figure 3B, and we have been modified the Figure 4 to Figure 5. The error bar in the figures has been added.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors of the manuscript "Selective adsorption of Pb2+ by layer-by-layer self-assembled MnO2/MXene composite films" presents a very interesting study in environmental chemistry. The article is written in a clear and understandable way. The selection of literature is appropriate and logical, there are no unnecessary citations.

However, the article needs some improvements from the current version.

1. There are a few typing mistakes as well, and authors are advised to carefully proof-read the text. PH instead of pH, no space between number and unit in line 63. Missing unit in line 116. There are 2 "figure 4".
2. More figure legend labels are needed in Figure 3 B; Figure 4 (should be figure 5) D
3. Regarding figure 7 and 8, is there any duplications performed? If so, please provide the error bar in the figures.  If not, what is the reproducibility of removal experiments?  

Author Response

We would like to express our sincere thanks to the referees for the critical and careful review of the entire manuscript. Their constructive suggestions and comments are helpful and inspiring for us to reword some of the statements in the text to make it precise, clear and easy to read. Listed below are our point-by-point responses to their questions/concerns.

 

Reviewer 3:

1. Abstract needs to be rewritten, the results are not presented well.

Response: Thanks for the comment. The abstract has been rewritten.

 

2. In introduction, provide the different methods available for treatments of those pollutants and what is the novelty of this work.

Response: Thanks for the comment. The introduction has been revised and the different methods available for treatments of heavy metal have been added.

 

3. The introduction is too simple to draw readers' attention quickly. At least, the authors should detail the reason why they want to start this work.

Response: Thanks for the comment. The introduction has been revised, and the reason why we start this work is added.

4. Synthesis of monolayer manganese oxide nanosheets and preparation of MnO2/MXene composite films must be supported with references.

Response: Thanks for the comment. We have added the reference of synthesis of monolayer manganese oxide nanosheets and preparation of MnO₂/MXene composite films as reference 23 and 24 in the revised manuscript.

 

5. Heavy metal ion adsorption test needs to be rewritten with more details.

Response: Thanks for the comment. The adsorption test has been rewritten with more details such as the relevant formula to calculate the adsorption capacity and adsorption efficiency as below.

Adsorption capacity was calculated by using the mass balance equation for the adsorption [1]:

  [1]

And the adsorption efficiency (%) was calculated from the formula [2]：

  [2]

where q is the adsorption capacity (mg/g) at equilibrium,  and  are the initial and the equilibrium concentrations (mg/L), respectively. V is the volume (L) of the solution, and m is the mass (g) of the adsorbent used.

6. In Figure 1. Adjust the magnification of thses figures.

Response: Thanks for the comment. We have adjusted the magnification of Figure 1 in the revised manuscript.

7. Line 145, modify Figure 4. to be Figure 5.

Response: Thanks for the comment. The mistake has been modified.

 

8. What is your reference for drawing this picture 6, and if so, link the mechanism of interaction and results to the Figure and transfer the conditions of the experiment from line 157-161 to the methods section.

Response: Thanks for the comment. Figure 6 has been revised to Figure 7, and the reference has been added: Ren, Y., et al. (2012). "Adsorption mechanism of copper and lead ions onto graphene nanosheet/δ-MnO₂." Materials Chemistry and Physics 136(2): 538-544.

 

9. Fig. 7 should be redrawn into separate figures.

Response: Thanks for the comment. Fig. 7 has been redrawn and separated to figure 8~11.

 

10. show bar in all the adsorption graphs

Response: Thanks for the comment. All the adsorption graphs have been revised.

 

11. In pH factor, why don’t apply pH after 7 value.

Response: Thanks for the comment. When the pH is greater than 7, heavy metal cations (e.g. Pb²⁺, Cu²⁺) in solution can easily combine with OH- to form a precipitate.

 

12. Please provide the experimental conditions: adsorbent mass, solution volume, temperature, pH, time, mixing rpm, all the factors which are applicable, in every figure caption.

Response: Thanks for the comment. The experimental conditions has been added in every figure caption.

 

13. The adsorbent need to assess by FTIR spectra.

Response: Thanks for the comment. The characterization experiment of FTIR has been carried out and the relevant figure and discussion has been added into the “3.1 Morphology and structure characterization” part. In the MnO₂/MXene composite film, the wide band at 1640 and 3292 cm^-1 represents the bending vibration of water molecules and O-H contraction vibration of water molecules, respectively, while the characteristic peaks of Mn-O and Ti - C are found between 600 and 700 cm^-1.

 

14. In section, study on adsorption performance, please provide with more discussion.

Response: Thanks for the comment. A discussion of adsorption models has been added and a discussion of adsorption properties has been enriched,

 

15. Where the equations for adsorption removal and capacity in whole manuscript, the authors must be supported it with references.

Response: Thanks for the comment. The equations and relevant reference for adsorption removal and capacity ave been added in the revised manuscript. Adsorption capacity was calculated by using the mass balance equation for the adsorption [1]:

  [1]

And the adsorption efficiency (%) was calculated from the formula [2]：

  [2]

where  is the adsorption capacity (mg/g) at equilibrium,  and  are the initial and the equilibrium concentrations (mg/L), respectively,  is the volume (L) of the solution, and  is the mass (g) of the adsorbent used.

 

16. Similarly, the effect of initial concentration, please provide this study with the isotherm adsorption models.

Response: Thanks for the comment. Adsorption kinetics models has been added in “3.2.6. Adsorption kinetic of MnO₂/Mxene composite films” part.

17. Similar to the effect of time, please provide this study with the kinetics models.

Response: Thanks for the comment. Modeling of adsorption kinetics was conducted by using pseudo-second-order models with linear fitting (Fig. 1, 2, 3):

where k is the pseudo-second-order adsorption rate constant (g mmol−1 min−1); t is the adsorption time, qe and qt  are the adsorption rate (mmol g-1) at equilibrium and at time t, respectively.

Table 1 Kinetic parameters for the pseudo-second-order models for the adsorption

	qe	k	R2
	116.53	0.433	0.98462

 

 

18. In case of the variation in the initial adsorbate concentration, the Y-axis should be the amount adsorbed. If you compare % adsorption or % removal, obviously the value decreases with the increase in the dose.

Response: Thanks for the comment. The Y-axis of the figure 10 has revised to the adsorption capacity.

 

19. There are no discussions about the mechanism of the adsorption, was the adsorption made by chemical and/or physical forces? This needs to be fully addressed in the manuscript.

Response: Thanks for the comment. The mechanism of the adsorption contains both physical adsorption and chemical adsorption, and the long adsorption equilibrium time also indicates that MnO₂/MXene composite film on Pb²⁺ is the chemical adsorption mechanism plays a decisive role in the adsorption rate of the reaction.

 

20. In general, the discussion section should give an interpretation of the significance of the results obtained with reference to similar works done by other authors.

 

Response: Thanks for the comment. We have added the results obtained with reference to similsr works done by others in the revised manuscript. Table 1. Comparison of the maximum adsorption capacities of various MnO2-based adsorbents for Pb(II).

Adsorbent	Adsorption capacity (mg/g)	Reference
ε-MnO2 nanoflowers	239.7 mg/g	28
g-C3N4/MnO2 composite	204 mg/g	29
α-MnO2	124.87 mg/g	25
MnO2 modified magnetic graphitic carbon nitride composite	187.6 mg/g	30
graphene nanosheet/δ-MnO2	781 µmol/g (161.6 mg/g)	27
MnO2/MXene composite films	1235 µmol/g (255.6 mg/g)	This work

 

 

22. Pleas this manuscript with graphical abstract.

Response: Thanks for the comment. We have provided the graphical abstract as below.

 

23. In conclusion section, should be support by the some results.

Response: Thanks for the comment. The conclusion section has been revised as below.

“ In this paper, the MnO₂/Mxene composite films were prepared by a layer-by-layer self-assembly process using SDS as a soft template and applied to the adsorption of Pb²⁺ in water. The effects of solution pH, adsorption time, initial concentration, and impurity ions on the adsorption performance of Pb²⁺ were investigated.

(1) The pH value of the solution, adsorption time, and initial concentration all significantly affect the adsorption of Pb²⁺. The removal of Pb²⁺ by the MnO₂/Mxene composite film reached 98.3% at pH=6 and reached adsorption equilibrium at 8 hours. Increasing the content of Pb²⁺ in the initial solution can enhance the adsorption capacity of the MnO₂/Mxene composite film, which can reach 1235 µmol/g when the initial concentration of Pb²⁺ is 90 mg/L.

(2) The MnO₂/MXene composite film has a particular selectivity for Pb²⁺ adsorption, and the removal rate of Pb²⁺ can still reach 61.7% in the solution with high Mg²⁺ content (Mg²⁺/Pb²⁺=10:1).

(3) The MnO₂/Mxene composite film has the property of easy recovery, and after five cycles of testing, its Pb²⁺ removal rate can still reach 96.4%.

(4) The process of adsoprtion of Pb²⁺ by MnO₂/Mxene composite film is consistent with the pseudo-second-order models.”

24. Please, explain the most important recommendations and suggestion to improve the quality of these materials to benefit more from it in the treatment process.

Response: Thanks for the comment. In this work, we obtained MnO₂/MXene composite film synthesized by a simple self-assembly process, and the adsorption capacity can reach 1235 µmol/g due to the rich adsorption sites from the film, besides the MnO₂/MXene composite film still has a selectivity on the adsorption of Pb²⁺, proving its promising prospects for practical wastewater treatment applications.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Quality of the paper has been improved after the revision. However, the title of the article say "Selective adsorption of Pb²⁺ by layer-by layer self assembled MnO₂/MXene composite films". Which is a broad statement in the selectivity of Pb²⁺ adsorption. Wheras, authors only tested films selectivity in presence of single cation (Mg²⁺). Reviewer suggest authors to change the title to include the term "Selective adsorption of Pb²⁺ in presence of Mg²⁺" . If authors want to stay with the current title, authors need to include experimental evidence in presence of common heavy and other metal cations under similar conditions.

Author Response

Response to the Reviewer 1’ comments

 

We would like to express our sincere thanks to the referees for the critical and careful review of the entire manuscript. Their constructive suggestions and comments are helpful and inspiring for us to reword some of the statements in the text to make it precise, clear and easy to read. Listed below are our point-by-point responses to their questions/concerns.

 

Reviewer 1:

Major Comments:

Quality of the paper has been improved after the revision. However, the title of the article say "Selective adsorption of Pb²⁺ by layer-by layer self assembled MnO₂/MXene composite films". Which is a broad statement in the selectivity of Pb²⁺ adsorption. Whereas, authors only tested films selectivity in presence of single cation (Mg²⁺). Reviewer suggest authors to change the title to include the term "Selective adsorption of Pb²⁺ in presence of Mg²⁺" . If authors want to stay with the current title, authors need to include experimental evidence in presence of common heavy and other metal cations under similar conditions.

Response: Thanks for the comment. The title of the article has been revised to "Selective adsorption of Pb²⁺ in presence of Mg²⁺ by layer-by-layer self-assembled MnO₂/MXene composite films".

 

Author Response File: Author Response.docx

Reviewer 2 Report

Accept in present form after correcting Equation No. 2 and review the manuscript.

Author Response

Response to the Reviewer 2’ comments

 

We would like to express our sincere thanks to the referees for the critical and careful review of the entire manuscript. Their constructive suggestions and comments are helpful and inspiring for us to reword some of the statements in the text to make it precise, clear and easy to read. Listed below are our point-by-point responses to their questions/concerns.

 

Reviewer 2:

Major Comments:

Accept in present form after correcting Equation No. 2 and review the manuscript.

Response: Thanks for the comment. The Equation No.2 has been revised. Specifically, the description of the Equation No.2 has changed to “the removal rate (%) was calculated from the equation (2): ”, and the equation has also been corrected.

Author Response File: Author Response.docx