The Preparation of a Lignosulfonate/Chitosan–Graphene Oxide Hydrogel Biosorbent to Effectively Remove Cr(VI) from Wastewater: Adsorption Performance and Mechanisms
Round 1
Reviewer 1 Report (Previous Reviewer 2)
The manuscript “Preparation of a lignosulfonate/chitosan-graphene oxide hydro-gel biosorbent for effec-tive removal of Cr(VI) from wastewater: Adsorption performance and mechanisms” by Caohui Han et al. should be improved in accordance with the following comments:
1. Deconvolution of peaks of XPS spectra on Fig. 2 c, d, e is not match well with the experimental data. Especially, the simulated and experimental peaks near 286-287 eV are significantly shifted. The presented deconvolution does not allow to make strong conclusions about relative contents of C‒C/C=C, C‒O, C=O, and O‒C=O (section 3.1.2). It is necessary to provide proper fitting of experimental curves and show the sum of the calculated peaks of the Figure 2.
2. In section “3.1.3. XRD” authors declare that “the peak of the composite without SL was much higher and narrower than the composite without CS” and “it was obvious that the diffraction peak of CS/SL/GO = 1:10:10 was wide and low, indicating that the addition of SL and CS affect the crystalline structure and amorphous structure of LCGH composite”. These statements must be validated. Fitting of the experimental peaks and calculating the width of the peaks are needed to distinguish which peak is “narrower”. Comparison of intensities of XRD peaks of the different samples is possible if standard is added into the samples. It is not clear how authors indicated effect on crystalline and amorphous structure. The relative intensity of amorphous “halo” should be described in the text.
3. The discussion of adsorption mechanisms in section 3.4. is based entirely on infrared spectroscopy data. An properly scaled inset should be added on the Figure 7 to clearly show the shifts of IR bands (i.e., 1760 to 1753, 1650 to 1660, 1015 to 1009 cm-1 and other mentioned in the text).
Author Response
Please see the attachment
Author Response File: 
Author Response.docx
Reviewer 2 Report (Previous Reviewer 1)
The reviewer has reviewed this manuscript earlier. The quality is improved after revision. Still, the reviewer's major concern is the missing of experiments for Cr(VI) removal from real wastewater. It might be OK to use simplified solution for initial assessment of the adsorbing material, but claiming the prepared material can effectively remove Cr(VI) will not be fully supported. Therefore, changing the wording in the title to "to effectively remove Cr(VI) from wastewater" would be more accurate. In addition, changing the wording of such statements in the main text is also necessary. The reviewer would like to recommend acceptance of this paper after minor revision as described above.
Author Response
Please see the attachment
Author Response File: Author Response.docx
Reviewer 3 Report (New Reviewer)
The suggestion and comments are attached in Word file
Comments for author File: 
Comments.pdf
Author Response
Please see the attachment
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report (New Reviewer)
Accept in present form
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
This manuscript presented the fabrication of a composite material, its efficiency in removing Cr(VI) from water by absorption, and discussion of the absorption mechanisms. Good results were obtained in this research, indicating the potential prospect of such material for wastewater treatment in terms of removing toxic metal of Cr(VI). Overall, the research design and experimental results were in fine quality. The reviewer would like recommend publication of this manuscript after major revision to solve some deficiencies as detailed below:
1) While the ultimate application was removing Cr(VI) from wastewater, the solution used in this study was Cr(VI) solution with a few more salts and metals, which was too simplified to represent wastewater with Cr(VI) pollution. It might be OK to use such solution for initial assessment of the adsorbing material, given that conditions of such solution can be adjusted in a convenient manner. However, using real wastewater for the Cr(VI) removal is essential. Otherwise, it is difficult to justify the good removal efficiency from experiments using over-simplified solutions was even tenable for real wastewater with much more complicated matrix.
2) Rearrangement and reorganization of figures are strongly recommended. A) Too many figures/tables (15) were presented in the manuscript. While each item might be delivering some information, not all items were equally important towards the core value of this paper. Therefore, the reviewer would like to suggest moving some figures/tables into supplemental information. B) Changing “Scheme” to “Figure 1” might be a better idea, given that there was not a second “Scheme”. C) Reorganizing “Figure 1” and “Figure 5” to two-column style might be a better idea.
3) While the English writing was fine overall, numerous small mistakes were found throughout the manuscript. Please carefully go through the paper, and correct/polish the writing to improve the quality of the manuscript.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript contains discussions based on misinterpretation of experimental data, misunderstanding of the literature, and unsubstantiated conclusions (see comments). The publication of an article containing such significant mistake is unacceptable in Molecules.
In addition, the text is unclear in some places and the figures contain data presentation errors. It seems that the manuscript was not checked by the authors before it was submitted to the editors.
The manuscript should be rejected.
As justification for the decision, please see the list of comments:
1. Unfounded discussion
The experimental results reported by the authors of the manuscript show a decrease in the adsorption capacity of Cr(VI) in the presence of Cu and Zn cations. Further discussion of these results in section 3.2 is unsubstantiated. There is text in the manuscript that is a tracing of the source [50]:
Manuscript (443-448) - "In particular, in the binary system of Cu(II) and Cr(VI), Cu(II) could occupy a complete complexing site on LCGH, and acted as a cationic bridge in a single Cr(VI) solution and aggregated by electrostatic attraction with more Cr(VI) anions [50]. Subsequently, because the hydroxyl group in LCGH triggered a strong reduction reaction of Cr(VI), more Cr(III) was produced and fixed on the surface of LCGH in situ, reducing the available adsorption sites and resulting in a decrease in adsorption capacity."
Reference [50] (section 3.2.5. Effect of coexisting ions) - "On the contrary, in the binary system of Cu(II) and Cr(VI), Cu(II) that could occupy the complete complexing sites on NZVI-SDBC acted as a cation bridge in single Cr(VI) solution, concentrating more Cr(VI) anions through electrostatic attraction. After that, Cr(VI) was initiated an intense reduction reaction due to NZVI or hydroxyl groups of NZVI-SDBC, generating more Cr(III) that immobilized in situ on biochar surface."
However, the authors miss the point that in the cited article this phrase describes an increase in the adsorption capacity of Cr (VI) in the presence of Cu. It is logical that the concentration on the surface due to the formation of bridges leads to a more efficient removal of ions from the solution. In the manuscript, however, the mentioned text is used to describe the decrease in the adsorption capacity of Cr (VI). Thus, the discussion of the mechanism of action of coexisting ions in the manuscript makes no sense.
2. Unfounded discussion
The authors of the manuscript showed that increasing the ionic strength of the solution leads to a decrease in the adsorption capacity of Cr (VI) using NaCl, KCl and CaCl2. However, it remains unclear whether the ionic strength was kept constant in the experiments with Cu and Zn cations. What is the contribution of the change in ionic strength to the effect of Cu and Zn cations on the adsorption capacity?
3. Unfounded discussion
(205-206) It can be seen that SL could insert into the GO layer and prevents the graphene sheets from severe agglomeration. – It is not clearly seen on micrograph. Comparison of unmodified GO and composites are needed.
(208-209) The CS could also crosslink with the SL/GO sheet and increase the surface area of the biosorbent. – There are no data of measurements of composite surface area in the manuscript. Therefore, authors cannot make this conclusion.
4. Misattributions
A lot of misattributions of IR absorption bands are made in the manuscript.
Some examples of wrong // right attributions:
‒CN stretching 1153 cm─1 (manuscript, 148-149) // 1380 cm─1 (ref. [24])
‒NH stretching 1380 cm─1 (manuscript, 148-149) // ‒NH bending 1602 cm─1 and ‒NH stretching about 3430 cm─1 (ref. [24])
5. Inaccuracy in data interpretation
On Fig. 1a and Fig. 6 the bands marked “3440” are obviously near 3250 cm-1, not 3440 cm-1.
Also shifts of absorption bands about 5-10 cm-1 mentioned in the section 3.4 (407-414) of the manuscript are not clearly visible on Fig. 6. Taking into account the authors' carelessness in interpreting IR spectra, we cannot be sure that the shifts indicated in the text take place.
6. Unfounded discussion
The discussion of adsorption mechanisms in section 3.4. is based entirely on infrared spectroscopy data. The authors' misinterpretation and unclear presentation of the IR spectra render the discussion meaningless (403-417) and make the conclusion about the adsorption mechanisms unfounded (485-486).
7. Inaccuracy in data interpretation
Deconvolution of peaks of XPS spectra on Fig. 1 c, d, e is not match well with the experimental data. Especially, the simulated and experimental peaks near 286-287 eV are significantly shifted. The inaccurate deconvolution does not allow to make strong conclusions about relative contents of C‒C/C=C, C‒O, C=O, and O‒C=O (section 3.1.2)
8. Unclear text and figures
(3.3.2. Analysis of adsorption isotherms, 335) The maximum adsorption capacity for Cr(VI) could reach to 564.2 mg/g in the range of the experimental concentration. – At which temperature?
Fig. 3a. Percentage removal is proportional to absorption capacity. However, on the Fig. 3a data of samples 1:10:5 and 2:10:10 are inversed.
Absence of axis description on Fig. 4a and Fig. 5a.
9. Multiple misprints (for instance, FI-TR, 13) and repetitions in the text (for instance, …the higher the initial concentration of Cr(VI), the more the Cr(VI) was adsorbed by LCGH. … the sharp increase in adsorption capacity was observed …As the concentration of Cr(VI) increased, more Cr(VI) were adsorbed onto the surface, 331-334).
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
While revisions did improve the quality of the manuscript, the most important issue pointed out by the reviewer stayed unaddressed. That is, not testing the adsorption efficiency in real wastewater is unacceptable for materials designed for Cr(VI) removal from wastewater. Unless this issue was addressed by adding experiments accordingly, the reviewer cannot recommend publication of this manuscript in the journal of Molecules.
Reviewer 2 Report
The revised version of manuscript can be accepted for publication in Molecules.
