Selective Adsorption of Lead in Mixed Metals Wastewater System by Lignin-Carbon-Supported Titanate Nanoflower BC@TNS Adsorbent: Performance and Mechanism
, Lei Zhong 1, Zekun Yang 1, Chak-Yin Tang 2, Wing-Cheung Law 2, Ruchun Wu 1,* and Fengwei Xie 3,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDear authors,
The paper reports the synthesis of a composite based on biochar@sodium tri-titanate nanosheets for heavy metals adsorption, varying different parameters, and elucidating the mechanism involved in the process. The lignin-carbon biochar, compositing the material, is derived from the pyrolysis of the lignin-carbon. Titanium hydroxide - Ti(OH)x – was further incorporated into the biochar using the hydrothermal method to obtain BC-supported sodium tri-titanate for further characterization and applications as adsorbents of heavy metals. The authors showed that the composite presents selectivity for Pb(II) removal compared to other metals, such as Zn(II), Cd(II), Cu(II), Ni(II), etc. Although the manuscript is interesting, the functionalization of the biochar, derived from lignin-carbon, with sodium tri-titanate is not evident since the authors did not show results for control samples, such as for pristine biochar or tri-titanate itself. The effect of the titanate nanosheets in the adsorbent should be better discussed, and further characterization is necessary to understand the material's behavior, otherwise, it is simply speculative. Some important characterization should be supplemented, so the present state should be revised before acceptance.
1) The title is not adequate. The authors should highlight the “mixed metals” wastewater system” in the title.
2) The abstract is well-structured and informative, but it could benefit from a brief mention of the kinetic and adsorption models employed in the study, also the authors should highlight that biochar is derived from lignin-carbon pyrolysis.
3) The introduction is thorough and provides a clear rationale for the study.
4) The novelty of the formation of sodium tri-titanate and its effect on the composite behavior should be highlighted more.
5) The introduction is thorough and provides a clear rationale for the study. However, consider adding more details on the formation of the biochar preparation and tri-titanate synthesis at the end of the introduction as part of the objective of the work.
6) There are several format errors throughout the manuscript. For instance, please refer to the Results and Discussion section, lines 188 to 206. Please correct!
7) In the SEM/EDS data, the authors did not mention whether the results are related to the adsorbent before or after adsorption. From the results, I believe these are related to the adsorbent analyzed after the adsorption process. In my opinion, it is necessary to include SEM/EDS data for the adsorbent before adsorption in order to confirm variation in elemental dispersion and mechanism involved in the adsorption claimed by the authors.
8) The authors mentioned BET surface area for the adsorbent, but they did not show any result for this. Please add N2 isotherm for composites and control samples before adsorption.
9) Regarding the XRD patterns, how can the authors ensure that sodium tri-titanate is formed? Please include the ICSD for the corresponding sodium tri-titanate phase and plot it together with the XRD patterns of the composites.
10) Please correct the number of all the figures in the text. The order is not correct!
11) As stated by the authors, titanate nanosheets play a role in the adsorption capacity of the prepared composite. However, they did not present any control experiments to confirm this. If this is the case, the authors must include Ti 2p XPS spectra before and after adsorption and discuss accordingly. This data may confirm such a statement on the mechanism involved in adsorption.
12) Concerning the adsorption capacity of the composite and the effect of titanate nanosheets (TNS), it would be useful to add data on the ability of BC and TNS itself to adsorb the metal ions. Is the improved behavior because the synthesis alters the BC particle size and surface area, or because the presence of Ti alters the surface chemistry?
13) The tables should mention the standard deviations of the values presented.
14) The adsorption equilibrium was tested considering adsorption models such as Langmuir, Freundlich, and Dubinin-Radush-Kevich. What if the Temkin model was applied in the current study?
Other remarks:
- Provide high resolution images.
- There are several minor grammatical / spelling errors.
- Please define all the acronyms at their first appearance.
- Please check and correct the scientific units.
- Please, make the correct assignment of the vibration bands in the IR spectra shown in Figure 1b
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is dealing with the synthesis of BC@Ti(OH)X composite for the selective removal of Pb from synthetic solution.
The method of the paper is not well described.
Synthetic of BC@Ti(OH)X composite and BC@ titanate nanosheets composite (BC@TNS) are not well described. Please provide more details.
Explain the hydrothermal treatment with TBOT procedure.
Line -119 BC@Ti(OH)X were fabricated by a one-step hydrothermal method adapted from previous studies [26,28]. Please provide more details. Indicate the conditions and the material used…
Line 120 BC suspension. What do you mean by suspension.?
Line 149 Why authors have use pH 3?
Please check the references for figures and tables in the discussion section (Error! Reference source not found.).
Please check order of figures and table and their correct citation in the manuscript.
Line 204-206 Moreover, the elemental mapping image ( ) and EDS spectrum ( (c), (d), (e), (f), (g), (h)) confirmed the presence of C, O, Ti, Pb, Cd and Zn, verifying the successful synthesis of the BC@TNS. Please mention that these results correspond to the composite after adsorption.
Line 214 Figure 1 (a) showed the XRD spectra of BC@TNS before and after Pb(II), Cd(II), and Zn(II) adsorption. Please correct order of figure
Ions such as Na+ and H+ positioned in the interlayer spaces, acting as exchangeable 227 counterions. Did authors determine Na+ and the pH of the solution after adsorption?
Line 233- 234 Accordingly, the TNS lattice was in an unstable state and more prone to structural changes. Subsequently, the uptake of larger metal cations (Pb(II), Cd(II), and Zn(II)) induced a stretching of the TNS lattice structure. Why authors have not provided SEM image at 300 nm this could be helpful to confirm this fact.
The kinetics and isotherms models could be described in the method section
3.8. Recycling and regeneration. The authors have not provided any results about desorption rate for each metal. Especially that Pb was assumed to be adsorbed through chemisorption process which make the desorption difficult to be achieved.
Comments on the Quality of English LanguageThe English could be improved to more clearly express the research.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsPlease find attached the review report.
Comments for author File: 
Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsDear authors,
I would like to congratulate for the effort made to improve the manuscript. The manuscript is scientifically sound and, in my opinion, it can be accepted for publication.
Best regards
Reviewer 3 Report
Comments and Suggestions for AuthorsRespected Authors,
The manuscript could be published in this form.
