Enhanced Breaking of Lignin and Mesopore Formation in Zinc Chloride Assisted Hydrothermal Carbonization of Waste Biomasses

Round 1

Reviewer 1 Report

This study focuses on the hydrochars prepared by different biowaste materials in the presence of aqueous salt solutions for dyes adsorption, which was compared to the hydrochars prepared in pure water. Although the research is not innovative enough, it has a good adsorption effect on dyes. The subject of this manuscript is in the scope of Journal of Carbon Research. I think that the paper is valuable for the researchers related to hydrochar for pollutant removal. I recommend its publication in Journal of Carbon Research after minor revision.

1. “Hydrochars with increased carbon contents, pore volume and surface areas were especially formed from lignin-rich coconut shell powder” in lines 11-12, the word lignin-rich is not suitable. Because this study did not show the lignocellulosic composition of three kinds of biomass and did not use three components respectively to test and compare whether the characteristics of carbon contents, pore volume and surface areas were mainly affected by lignin. So, the word lignin-rich is not accurate here.
2. The reasons for the change of C content had not been clearly explained. Why does steam activation have different effects on the C content of HTC prepared from different raw materials, such as dandelion and wood chips?
3. “The variation of temperature has more influence on the carbon content than the time” in lines 175-176 is inaccurate here. In your experiment, the selection of temperature interval was large, while the time interval was small, so it cannot be accurately said that “the temperature has more influence on the carbon content than the time”. It is impossible to accurately compare the influence of time and temperature on the change of C content, but it can only be said that temperature has a more significant impact on C content under your experimental conditions.
4. “The presence of a more stable morphology is a good requisite for the introduction of porosity into these structures” was described in lines 205-206. However, in some studies, lignocellulose was degraded and then repolymerized to form new structures, for example, crosslinked carbon microspheres, which also can increase the specific surface area and form many pores. Therefore, a more stable morphology is not necessary for the introduction of porosity.
5. Why ZnCl₂ can destroy the structure of lignin? Is it the specific function of metal or can ZnCl₂ be used as a catalyst? Without ZnCl₂, if the reaction time is prolonged or the reaction temperature is increased, whether the lignin structure can also be destroyed.
6. The word residual ZnCl₂ in line 368 is inaccurate here. This should not be residual ZnCl₂, but the ZnCl₂ you added to the system, which had the catalytic effect during the hydrothermal carbonization of biomass.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This is an excellent research, with clear novelty aspect and contribution to the field. Firstly it shows that addition of salt has influence on HTC, regarding production of sorbents, using HTC. Secondly, it shows the importance of chemisorption, in comparison to physisorption, in terms of removal of dyes, such as methylene blue and methyl orange, which could have significant impact, when it comes to development of new, more effective sorbents for wastewater purification. I’d like to recomment its publication, after minor revision:

• In the introduction, alternative use of hydrothermal cabonisation process for biomass, eg. for valorisation of biomass to solid fuels, should also be mentioned.
• In the introduction, specific surface area is mentioned for hydrochars produced from glucose and hydrochar produced in saline conditions. It would be good if readers could get a full overview of what’s possible with HTC, regarding surface of different low quality biomass. Examples could be found in work of Magdziarz et al. https://doi.org/10.1016/j.renene.2021.05.018
• I’m a bit confused regarding HTC process conditions for this investigation. In lines 71 – 72 there is residence time 4 – 20 hours, whereas in lines 89 – 90 there is residence time of 12 hours. Please integrate HTC description in sub-section 2.2. Might be sensible to make a table with experimental matrix if it’s too inconvenient to make description that would be easy to read.
• In line 91 and 93 grinding is mentioned. Was the first grinding preformed on wet material? Depending on the grinding pronciple, it could have (to some extent) influence on the surface, especially second grinding of dried material (if material is brittle after HTC and drying). For the sake of scutiny please briefly mention the type of grinder/grinding principle (ball mill, roller mill, knife mill, etc.).
• I’d suggest rephrasing “The samples were ground one last time and selected over a sieve with a mesh size of 0.63 mm”. My guess is that the particles bigger than 0.63 mm were used for further experiments/characterisation. The term “selected over a sieve” does not seem suitble and may cause confusion.
• I’d suggest some corrections to Figure 2. Firstly the area for coal does not seem “typical”, please see https://doi.org/10.3390/su131810480, https://doi.org/10.1016/j.biortech.2014.07.010 (cited by the authors – [16]) or https://doi.org/10.1016/j.biortech.2011.06.099 010 (cited by the authors – [17]). There would be a “pistol” kind of shape for hardcoal. For biomass, it’s hard to say about “typical” and areas marked as biomass in different papers vary, but I haven’t seen extensive variations, regarding hard coal. So I wouldn’t be to strict with the area marked as biomass, but in the current version Coal reaches 1.2 for H/C, which seems to much to me.
• Lines 214 – 223: Many functional groups are mentioned in the sentence: “Signals of several functional groups in the coconut precursor, namely hydroxyl, ester, ether, aldehyde, keto, carbonyl, and carboxylic groups, as well as aromatic C=C stretching vibrations become sharper after hydrothermal carbonization in pure water (Cn-HTC), indicating a relatively increasing amount of these groups in the samples.” Please include corresponding peaks along with references in this sentence as well as in subsequent sentences, where identified functional groups are mentioned.
• In figure 4 I guess that the top row of pictures is SEM and the bottom is TEM. However, the reader shouldn’t need to guess – that should be explicitly stated in the caption.
• Lines 404 – 406: “Micropore formation occurs by oxidation of surface carbon compounds combined with small local explosions caused by a rapid desorption of the formed CO2.” – is it possible to back it up with a citation? Especially eplosion caused by desorption of CO2 should be backed up with strong evidence. Otherwise it might be sensible to “soften” the claim by saying that “Micropore formation might occur…”
• Line 526: A typo – instead of “In was proven…” should be “It was proven…”
• Line 529: A typo – instead of “This could by concluded…” should be “This could be concluded…”
• Line 542 – 544: “The dye adsorption strongly depends on the presence of suitable oxygen containing adsorption sites as seen from the pseudo-second order kinetics and takes place via external as well as intraparticle diffusion.” – How does pseudo-second order kinetic demonstrate the presence of oxygen containing adsorption sites? I suppose the basis of this conclusion is not just the kinetics, but all the experimental data in this work, including sorption capacity, functional groups, pore sixe distribution etc.
• Looking at the evidence, I think it would be plausible to generalise in the “Conclusions” section that chemisorption is far more important than physisorption in terms of removal of dyes, such as methylene blue and methyl orange. Highlighting of such information would be important for any further studies in the area.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The authors study the effect of Zn salts addition on the properties of different hydrochars made from sustainable resources. In general terms, the aspects dealing with the hydrothermal carbonization and the characterisation of the materials are well covered, including the use of RMN and IR spectroscopy, very powerful but not very often used for these purposes. From this point of view, the paper deserves publication.

However, the adsorption experiments must be more thoroughly treated. It would be interesting to know about the adsorption equilibrium, for example, if Langmuir or Freundlich model can apply.

On the other hand, kinetic experiments are not fully discussed…What are the aspects determining the kinetic constant?.

Other minor points are the following:

• Some explanations about the raw materials must be added. For example, the properties of the sawdust. Different woods present very different properties.
• Please revise the English use. Several expressions are not usual.

Author Response

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Author Response File: Author Response.pdf