The Effect of Activation on the Structure of Biochars Prepared from Wood and from Posidonia Oceanica: A Spectroscopic Study

Round 1

Reviewer 1 Report

Reviewers Comments

Manuscript ID:  Physchem -1736105

Title The Effect of Activation on the Structure of Two Biochars: a Spectroscopic Study

Journal: Physchem

 

General comments

The study reports synthesis and characterizations of biochar and its KOH chemical activated form using Spectroscopic Study. The manuscript is quite interesting and overall similarity index of 15%, is good and well acceptable. Though the manuscript is well written and presents significant vital synthesis and characterizations results and analysis, yet the authors failed to clearly establish the manuscript novelty and potential areas of applications of the adsorbents reported. A major revision is needed prior to the acceptance of manuscript as per the following comments provided below 

 

Specific comments

 

1.     The title of the article is too generic. The main source of the biochar should be clearly stated

2.     The abstract ignores to provide important characteristics of the biochars

3.     Introduction part not sufficient information is provided. A more comprehensive review on the subject matter with relevant and most updated references is needed.

4.     There are lot other chemicals used for activation such as H2SO4, ZnCl, NaOH etc., why did the author select KOH? What are the merits and demerits? A comprehensive review on activation chemicals that can be used in Table format must be provided and discussed comparing these chemicals in terms of the quality of the biochar/adsorbent produced. Please refer to this reference  https://doi.org/10.3390/ijerph14101094

5.     The manuscript novelty is clearly missing

6.     The manuscript did well in characterization, yet full potentials of the biochar especially in water and wastewater as well as gaseous pollutant capture can be further understood with TGA, BET, SEM, as well as surface charges (zeta potential, point of zero charge) characterizations which are missing

7.      The article failed to provide potential areas of applications (such as pollutant uptake from aqueous and air) of the biochars and example results to demonstrate the applicability

Author Response

Reviewer 1

1. The study reports synthesis and characterizations of biochar and its KOH chemical activated form using Spectroscopic Study. The manuscript is quite interesting and overall similarity index of 15%, is good and well acceptable. Though the manuscript is well written and presents significant vital synthesis and characterizations results and analysis, yet the authors failed to clearly establish the manuscript novelty and potential areas of applications of the adsorbents reported. A major revision is needed prior to the acceptance of manuscript as per the following comments provided below

Answer: A structural study on biochars and activated carbons is presented, using non-standard complementary characterization methods and pointing to the effect of activation on the structure (as clarified in the title). The novelty of the paper resides in this structural characterization. This is clearly stated in the Introduction. Physchem is a journal devoted to physical chemistry, which includes the structural characterization of materials. Assessing potential areas of application is beyond of the scope of the journal.

However, considering that part of the readership of the paper may include material scientists preparing carbonaceous materials, characterizing them with standard methods, and testing them in possible applications, we revised the Conclusions adding a paragraph on potential areas of application of the carbons activated with the two different KOH:biochar ratios (last paragraph of the Conclusions).

 

Specific comments

 

2. The title of the article is too generic. The main source of the biochar should be clearly stated

Answer: according to the reviewer’s request, we modified the title of the article from “The Effect of Activation on the Structure of Two Biochars: a Spectroscopic Study” to “The Effect of Activation on the Structure of Biochars Prepared from Wood and from Posidonia Oceanica: a Spectroscopic Study”.

 

3. The abstract ignores to provide important characteristics of the biochars

Answer: in the abstract only the characteristics investigated with our methods were mentioned so that the reader’s attention is focused on the properties studied in this manuscript.

 

4. Introduction part not sufficient information is provided. A more comprehensive review on the subject matter with relevant and most updated references is needed.

Answer: according to the reviewer’s request, we added some references on applications using porous carbon materials (refs 2, 50, and 51 in the revised version of the manuscript).

 

5. There are lot other chemicals used for activation such as H2SO4, ZnCl, NaOH etc., why did the author select KOH? What are the merits and demerits? A comprehensive review on activation chemicals that can be used in Table format must be provided and discussed comparing these chemicals in terms of the quality of the biochar/adsorbent produced. Please refer to this reference https://doi.org/10.3390/ijerph14101094

Answer: the merits of KOH have been already given in the Introduction (Lines 43-47 in the revised version of manuscript) and a recent review containing a discussion on chemical activation was cited in the old version of the manuscript (Paragraph 3.2, ref 27). We moved this reference to the Introduction, when chemical activation is introduced, and cross-referenced to it in paragraph 3.2. We think that providing a review on activation chemicals and a discussion on them in our paper would be outside the scope of the work.

According to the reviewer’s request, we added the reference https://doi.org/10.3390/ijerph14101094 in the Introduction, when applications using porous carbon materials are mentioned.

 

6. The manuscript novelty is clearly missing

Answer: see answer to point 1

 

7. The manuscript did well in characterization, yet full potentials of the biochar especially in water and wastewater as well as gaseous pollutant capture can be further understood with TGA, BET, SEM, as well as surface charges (zeta potential, point of zero charge) characterizations which are missing.

Answer: As already stated in point 1, the aim of our study was a structural characterization of biochars and activated carbons pointing to the effect of activation on the structure. Gaining information on potential fields of application of biochars was not our main objective. However, some comments on the potential application of the different materials on the basis of the structural characterization are made in the Conclusions. N2 adsorption/desorption data, not interpreted here using BET model but the BJH one, are provided in the manuscript. SEM images of wAC and pAC_micro were reported in the paper by Pedicini et al published on Int. J. Hydrogen Energy 2020, 45, 14038-14047 and coauthored by some of us.

 

8. The article failed to provide potential areas of applications (such as pollutant uptake from aqueous and air) of the biochars and example results to demonstrate the applicability.

Answer: The intent of the paper is not to provide example results to demonstrate the applicability of the materials. As declared in the title, this is a study on the structure of biochars prepared from two sources and of activated carbons derived from them. That is why we submitted the paper to a physical chemistry journal and not to a journal focusing on the application of materials. However, as stated in the answer to comment 7, some suggestions on possible application fields on the basis of the structure are given in the Conclusions.

 

Reviewer 2 Report

The area of improvement the authors should consider pertains the conclusion section.  The manuscript evaluates 2 biochars and subsequent activations with KOH.  Biochar and chemical activation are not unique.  The authors use a plethora of analytical instrumentation to evaluate the materials.  The conclusion section is written more similar to a summary.  What did the authors learn from the results?  Pyrolysis and activation conditions were not modified.  Did the results provide insight to how variables could have been modified to provide different properties is interesting to learn.

Author Response

Reviewer 2

The area of improvement the authors should consider pertains the conclusion section. The manuscript evaluates 2 biochars and subsequent activations with KOH. Biochar and chemical activation are not unique. The authors use a plethora of analytical instrumentation to evaluate the materials. The conclusion section is written more similar to a summary. What did the authors learn from the results? Pyrolysis and activation conditions were not modified. Did the results provide insight to how variables could have been modified to provide different properties is interesting to learn.

Answer: We thank the reviewer for this comment. We modified the structure of the manuscript, separating the Results from the Discussion. In the Discussion paragraph, the results from the different methods are discussed together and, in order to make the discussion easier to follow, a table was added listing the data/elements emerged from each characterization technique.

Consequently, the Conclusions were modified with the aim of making clear what we learnt from our results.

Reviewer 3 Report

Physchem

Manuscript ID: physchem-1736105

TITLE:  The Effect of Activation on the Structure of Two Biochars: a Spectroscopic Study

AUTHORS: Silvia Pizzanelli*, Susanna Maisano, Calogero Pinzino, Antonella Manariti, Vitaliano Chiodo, Emanuela Pitzalis and Claudia Forte

The authors report on a spectroscopic study of the structure of two biochars and of their activated carbons via EPR, solid state NMR and transmission FTIR, supported by X-ray diffraction and nitrogen adsorption/desorption measurements together with elemental composition. The long-term aim of this study is to understand the activation mechanism of porous carbon, usually generated by carbonization of the original biomass through pyrolysis, which is extremely important for the design of carbon materials with desirable properties.

The work is interesting; the goal and the methods presented by the authors to achieve this goal are to some extent sufficient. The arguments used, however, to support their conclusions from the various techniques have not been clearly developed, nor have they been largely combined, making the article precarious about its publication in Physchem.

My expertise is mainly materials characterization. Nevertheless, I made a great effort to understand how it is implemented and what the application of each technique provides / contributes. I would prefer to see a reaction mechanism, apparently based on experimental findings; this would facilitate the understanding of the procedures and emphasize their importance. The previous work of the co-authors (Ref. 8) could be used as a guide.

Lines 51-53: Is the “KOH:biochar mass ratios …1:8” correct? “In this last case, KOH:biochar mass ratios ranging between 1:8 and 1:1 were explored with carbonization temperatures in the range 400-800 °C.” Apparently, yes (Ref-2)! Therefore, why did you choose to utilize 4:1 to 2:1 activating agent/carbon ratios in your work (the same holds for your previous work (Ref-8)? Please keep the mass ratios with a specific hierarchy, either KOH:biochar or biochar:KOH, throughout the MS.

There is need of a table listing the data/elements that have emerged from each of the characterization techniques that additionally could reveal their complementarity along with any possible disagreement. This could also help reveal the potential of the different characterization techniques and their combination in understanding the activation mechanism of biochars (you could also use data of other works).

I would also suggest the use of Raman spectroscopy as it is considered a key technique for the analysis of various carbonaceous materials. (e.g. 10.3390/c1010077)

The quality of the FTIR absorption spectra could be improved utilizing diffuse reflectance configuration.

Other comments:

Please give explicitly the MAS: Magic Angle Spinning  

Lines 380-381: “The spectrum of the dry sample (Figure 7e, black line) is quite similar to that of wAC (Figure 6b, black line),…” Is it correct? Most probably is similar to the Figure 7c, black line).

Lines 397-398: “The fact that no aliphatic C-H signals are detected in the NMR spectra is probably due to the lower sensitivity of NMR compared to IR and to their very low content.” Somewhat arbitrary.

Lines 415-419: “The pB spectrum differs from the wB and the wAC spectra, showing a relatively 415 strong and broad band at 1485 cm−1 and a narrow peak at 875 cm−1. These peaks have been observed in submicrometric calcite particles [44]. The calcite signals are absent in the activated carbons prepared from Posidonia, that is, pAC_meso and pAC_micro, indicating that activation removes calcite.” It is not clear to me.

Lines 462-464: “Overall, the data point to a key role of the KOH:biochar ratio in affecting aromatic network size, functional groups bound to the aromatic planes, and porosity, with non-significant effects of the type of the original biomass.” Aromatic network size like what? What kind of functional groups?

Author Response

Reviewer 3

1. The authors report on a spectroscopic study of the structure of two biochars and of their activated carbons via EPR, solid state NMR and transmission FTIR, supported by X-ray diffraction and nitrogen adsorption/desorption measurements together with elemental composition. The long-term aim of this study is to understand the activation mechanism of porous carbon, usually generated by carbonization of the original biomass through pyrolysis, which is extremely important for the design of carbon materials with desirable properties.

The work is interesting; the goal and the methods presented by the authors to achieve this goal are to some extent sufficient. The arguments used, however, to support their conclusions from the various techniques have not been clearly developed, nor have they been largely combined, making the article precarious about its publication in Physchem.

My expertise is mainly materials characterization. Nevertheless, I made a great effort to understand how it is implemented and what the application of each technique provides / contributes. I would prefer to see a reaction mechanism, apparently based on experimental findings; this would facilitate the understanding of the procedures and emphasize their importance. The previous work of the co-authors (Ref. 8) could be used as a guide.

Answer: The understanding of the activation mechanism, as also recognized by the reviewer, is a long-term aim just defining the context in which our study is set. We do not have a reaction mechanism to propose. In order to propose a mechanism, an investigation of the only solid fraction, as that presented here, would not be sufficient, as the reviewer certainly knows since he is an expert in materials characterizations. As stated in the title, our goal is gaining information on structural features of biochars and activated carbons of two different biomasses.

According to the reviewer’s suggestion, we revised the manuscript with the aim of combining the arguments from the different techniques and developing them to support our conclusions. In particular, we modified the structure of the manuscript, separating the Results from the Discussion. In the latter paragraph the results from the different methods are discussed together and, in order to make the discussion easier to follow, we added a table listing the data/elements emerged from each characterization technique.

Consequently, the Conclusions were modified. In this paragraph, we pointed out and made clear what we learnt from our results.

 

2. Lines 51-53: Is the “KOH:biochar mass ratios …1:8” correct? “In this last case, KOH:biochar mass ratios ranging between 1:8 and 1:1 were explored with carbonization temperatures in the range 400-800 °C.” Apparently, yes (Ref-2)! Therefore, why did you choose to utilize 4:1 to 2:1 activating agent/carbon ratios in your work (the same holds for your previous work (Ref-8)?

Answer: the aim of the cited work was to study the mechanism. Typically, higher KOH:biochar ratios are used for activation.

 

3. Please keep the mass ratios with a specific hierarchy, either KOH:biochar or biochar:KOH, throughout the MS.

Answer: done

 

4. There is need of a table listing the data/elements that have emerged from each of the characterization techniques that additionally could reveal their complementarity along with any possible disagreement. This could also help reveal the potential of the different characterization techniques and their combination in understanding the activation mechanism of biochars (you could also use data of other works).

Answer: see answer to point 1.

 

5. I would also suggest the use of Raman spectroscopy as it is considered a key technique for the analysis of various carbonaceous materials. (e.g. 10.3390/c1010077)

Answer: We do not have the expertise nor the instrumentation for this type of measurement. Indeed, the potential of Raman spectroscopy in the structural characterization of carbonaceous materials is well known, although, in the case of amorphous materials, the spectral interpretation is not so straightforward and there may be uncertainties in the type of deconvolution used.

 

6. The quality of the FTIR absorption spectra could be improved utilizing diffuse reflectance configuration.

Answer: An attempt to register FTIR spectra with diffuse reflectance technique was made, but the spectra showed very weak absorption bands compared to those obtained with KBr pellets and a greater baseline distortion due to specular reflection of radiation. Other attempts were made in ATR mode using germanium crystal, as commonly used for carbonaceous substances, once again obtaining very weak signals.

 

Other comments:

7. Please give explicitly the MAS: Magic Angle Spinning.

Answer: done

 

8. Lines 380-381: “The spectrum of the dry sample (Figure 7e, black line) is quite similar to that of wAC (Figure 6b, black line),…” Is it correct? Most probably is similar to the Figure 7c, black line).

Answer: The comment is appropriate. There was a typo. We substituted “Figure 6b” with “Figure 7b”.

 

9. Lines 397-398: “The fact that no aliphatic C-H signals are detected in the NMR spectra is probably due to the lower sensitivity of NMR compared to IR and to their very low content.” Somewhat arbitrary.

Answer: We cannot find other plausible reasons for the observed discrepancy. As a matter of fact, NMR is intrinsically less sensitive compared to IR. The aliphatic C-H groups, although clearly detectable in the IR spectrum, are probably present in extremely low amounts, lower than the limit of detection for NMR.

 

10. Lines 415-419: “The pB spectrum differs from the wB and the wAC spectra, showing a relatively strong and broad band at 1485 cm−1 and a narrow peak at 875 cm−1. These peaks have been observed in submicrometric calcite particles [44]. The calcite signals are absent in the activated carbons prepared from Posidonia, that is, pAC_meso and pAC_micro, indicating that activation removes calcite.” It is not clear to me.

Answer: we substituted the sentence “indicating that activation removes calcite” with “indicating that the activation process followed by washing with an HCl aqueous solution removes calcite originally present in pB sample”.

The point is discussed again in the paragraph “Discussion”, sub-paragraph 4.2. where the following sentence is written “It is worth to point out that the activation process followed by washing with an HCl aqueous solution removes calcite, as well as Mn²⁺ and Fe³⁺ ions from pB sample. The removal of calcite is evident from the comparison of the FT-IR spectrum of pB with those of pAC_meso and pAC_micro, whereas the removal of the ions is guessed considering the EPR spectra of pB and of pAC_meso.”

 

11. Lines 462-464: “Overall, the data point to a key role of the KOH:biochar ratio in affecting aromatic network size, functional groups bound to the aromatic planes, and porosity, with non-significant effects of the type of the original biomass.” Aromatic network size like what? What kind of functional groups?

Answer: we expanded this part in the new paragraph 4 “Discussion” and in the new Table 3, trying to make our points clearer. Regarding the size of the network, unfortunately with the present data we can only state that wB and pAC_meso samples are characterized by condensed aromatic structures smaller than the graphenic structures in wAC and pAC_micro, with no values for the size.

Round 2

Reviewer 3 Report

The authors have adequately responded to the reviewer's comments and the article is now eligible for publication in PhysChem