Development of Kraft Lignin Chemically Modified as a Novel Crosslinking Agent for the Synthesis of Active Hydrogels

Round 1

Reviewer 1 Report

This paper presents an interesting method to produce resins for adsorption of Pb and Cu. Currently, pollution of wastewater with heavy metals is an important issue and it is important to search for novel means to solve this problem. Novel means of using residues, such as kraft lignins, for this purpose are of high interest for the scientific community.

I have a couple of remarks regarding the manuscript, concerning mainly intorduction and discussion of results:

1) The introduction needs to be reworked in my opinion. At the very beginning the use of craft lignin for adhesive formulation is mentioned. Then crosslinking agents are mentioned. At the end it is stated that the goal is to demonstrate the sucess of the crosslinking procedure. However, it might be not clear for the reader why could it be potentially interesting. In the very beginning the introduction should definitely start by giving the context of the problem. I.e. a short paragraph about the problem of heavy metals pollution.

2) Then it would be in order to have a short pargraph about other methods of removal of Pb and Cu, presented in the published literature.

3) The use of craft lignin for adhesive formulation should definitely be mentioned, along with the other alterative ways for using lignin, competitive to the use for production of the use in this study, to give reader an idea regarding the value each of these competing utilisation routes could possibly add.

4) The sentence " The lignin has biodegradability properties and due this[2][3], its use is widely approach, but are not significatively explored the interaction of its functional groups to generated a modified lignin in order to use it as a chemically functionalized crosslinking monomer (crosslinking agent)[4]." should probably be rewritten, as the meaning is not clear.

5) In the description of FTIR method it is stated "...to get an accurate
measurement of the general and specific vibrations of principal functional groups". I'd suggest rephrasing, as FTIR indeed is a viable mean to identify functional groups. However, it should not be overlooked that the result of FTIR ATR alone is qualitative only (giving interesting information nonetheless).

6) It could be beneficial for the paper to extend the discussion of the obtainned results. E.g. for the SEM rit is mentioned "...SEM micrographs of hydrogels obtained are consistent with the SEM images obtained by Ji, X. at al in 2017, where they synthesized and characterized hydrogels based on alkaline lignin from ionic liquids." It would be good to clarify and state in what way they are consistent. What could be seen on SEM images here that could be also seen in the cited work of Ji et al.

7)   "Metal ions sorption probes" and "In this regard, we can observe that in case of the Cu2+ adsorption probes in all cases the inclusion of NMBA produces..." - by adsorption probes do authors mean adsorption experiments?

8) I think that the discussion of the results of Cu and Pb adsorption experiments should be significantly extended, as this seems to be the reason why the resins have been synthesised in the first place. There are a lot of results in the literature on sorption of Cu and Pb, using activated carbons prepared in various ways. This should give a solid basis for comparison of the performance of the sorbents.

Author Response

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Reviewer 2 Report

The manuscript presents a strategy to modify kraft lignin using a basic nucleophilic reaction (through acryloyl chloride) in order to be used as crosslinker to produce acrylamide based hydrogels for the absorption of metal ions. In my opinion, the manuscript presents a piece of good scientific work that suit to be published in this journal, however, the innovation of the presented results as well as some doubts that have been building up must be discussed. - The introduction does not accomplish its purpose: This section must present the exposition of an unsolved problem followed by an overview of the actual solutions and ended by the novelty of your approach. This is not a minor point and must be take in consideration. - As you surely know, using acryloyl chloride for the functionalization with methacrylate groups may produce acidification of the media through the hydrolysis of the ester group. This effect will produce pH changes modifying the Cu2+ and Pb2+ absorption. What has been your strategy regarding this phenomenon? - Due to the vacuum conditions of FE-SEM, the hydrogels must be dried using a well -designed protocol to ensure that the images of the hydrogel dried are similar that when the hydrogel has water inside. This protocol must be described in materials and methods. - I miss error bars in the plots presented in figure 7, figure 8 and figure 9. - Also I suggest to the authors to revise the english of some parts of the manuscript.

Author Response

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Reviewer 3 Report

This paper deals with the development of novel crosslinking agent from kraft lignin.   However, as pointed out below, the experimental methods and results seem to be contain some uncertain points, so I cannot recommend this manuscript to be published on Applied Science.

 

- It is know that the chemical structure of lignin is very complicated, however, if the lignin will be used as crosslinking agent, its functionality (how many -OH groups per one kraft lignin molecule) should be quantitatively clarified.

 

- Acryloyl chloride is highly reactive with water, therefore, the added acryloyl chloride in Scheme 1 seems to react with the solvent water before reacting with -OH of the kraft lignin.

 

- Reaction Scheme 1 shows the acryloyl chloride reacts with -OCH3 group to produce acrylic ester.   Did such kinds of reaction really occur in this experimental condition?

 

- As mentioned above, acryloyl chloride easily react with water, so the 1H NMR spectrum in Figure 3A) seems strange.

 

- In the water-swelling behavior of xerogel in Figure 7(A), polynominal fitting curves are indicated by solid lines.   However, the experimental data show that the swelling degree gradually reaches to the constant value after long time elapsed, which is a reasonable behavior.   Why the authors tried to fit the data by polynominal function?

 

- In order to discuss the results in Figure 9, swelling degree of the hydrogel at various pH seem to be necessary, but such data are not indicated.  

 

 

Author Response

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Reviewer 4 Report

The reviewed article concerns the use of kraft lignin modified using a basic nucleophilic substitution reaction (NSA) to the synthesis of acrylic acid-based hydrogels – to evaluate their retention potential of heavy metal ions. It is an interesting research idea with a significant practical dimension, due to the already known possibility of using modified lignin as an adsorbent of water and sewage pollutants (including the heavy metals removal).

The manuscript is of good quality; the results are consistent with the study design. The discussion was good and the bibliography was consistent. However, the degree of scientific novelty is average.

Comments:

1. In the abstract, complete the note to the results obtained (in the last sentences)
2. Edit the text thoroughly and number the chapters and subsections in accordance with the guidelines for authors, please.
3. Complete the methodology with all the necessary data (e.g. detailed data for the apparatus used, units in the formula descriptions), please.
4. Perform a comparative analysis of the results of your study (including individual analyzes such as NMR, FTIR, metal ions sorption, etc.) with the results obtained by other researchers. Enter descriptions in relevant parts of the text.
5. Complete the discussion on the possibilities of using the prepared materials - on the industrial scale (costs, quantities, predicted performance, comparison with a laboratory scale).

Author Response

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Round 2

Reviewer 3 Report

I think this manuscript is not appropriately revised against my comments.

 

2) The reaction of acryloyl chloride in the added reference 32 was conducted in the immiscible mixture of organic solvent (dichloromethane) and water.   The reaction system is different from the present manuscript.

 

4) Figure 7A was kindly revised, however, the fitting curve in Figure 8 was not changed.

 

Author Response

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Round 3

Reviewer 3 Report

This paper was appropriately revised.   I recommend publication of this manuscript on Applied Science.