Analysis of Influencing Characteristics of Biochars for Ammonium Adsorption

Round 1

Reviewer 1 Report

The main objective of his study was to compare the effect of various physical and chemical properties on the adsorption of ammonia on various biochars. Authors used statistical analysis for the processing of the data from a plenty of publications in the literature in order to determine the key parameters for the ammonia adsorption. Overall, this study contributes to the state of art of the adsorption of ammonia on the biochar, which has an importance not only for the ammonia recovery from wastewaters, but also for the preparation of fertilizers.

The manuscript is well organized and the description of the methods, as well as the discussion of obtained results, are proper, but nonetheless, I have a few comments regarding this study:

1.      Page 2, line 77. Authors state that they had to exclude 20 publications from their analysis due to contradictory or missing information. Could the authors state particular examples of such information?

2.      Page 5, The captions in the graphs on Figure 3 may be adjusted in order to be more readable

3.      Page 8, line 261. How do the authors explain the negative correlation between adsorption capacity and contact time? The adsorption capacity usually grows with the contact time until the equilibrium is reached.

Author Response

Thank you for your constructive feedback. Answers to your comments can be found below:

1. Most of the papers that were excluded were because of missing data in the experimental section that prevented fact checking, though a few failed the fact check that was performed (reported adsorption capacity was higher than the maximum theoretical capacity). We have added a few examples in lines 78-86.
2. Font sizes have been adjusted on Figure 1 page 5 to make the graph more legible.
3. We chose not to comment on the relationships between adsorption capacity and parameters that had correlation coefficients less than 0.3 for significance reasons. It is possible that the relationship between contact time and adsorption capacity was not adequately determined from the data we analyzed, because it does go against the logical relationship. Assuming the negative correlation is correct, we think that the relationship is influenced by another factor that is not present in this analysis (like biochar material) which could influence the length of time required for equilibrium to be reached which could then skew the data.

Reviewer 2 Report

Dear Authors, Dear Editor,

the work is another proof that statistics can almost everything. I'm terribly sorry but I cannot recommend this work for publication.  

I suggest, cooperation with the Carbon-materials specialist, before the next submission.

The BET surface area CAN NOT be the decisive parameter during adsorption from solution!!!! Because it is determined from N2 low-temp. adsorption. In the solution, the presence of ions (electrostatic interactions contrary to VDV), coordination zone (size of the complexes), etc. etc..., makes the situation completely different.

More, "accurately model the adsorption of ammonium onto biochar for full scale implementation of nitrogen recovery in wastewater treatment" is absolutely wasting of time and money. You need to look closer to "water treatment stations" where carbons work as adsorbents. Their adsorption capacities are far exceeded and they still work properly, because of the parameter you didn't consider in your studies.

Finally, I agree this is extremely challenging to embrace the biochars, as they are a very wide group of materials. Their properties depend on many factors: starting from the source material, and ending on the formed granule.

Summing, after reading the Abstract I expected something...

Author Response

Thank you for your thoughtful comments. Please see our response below:

It is true that BET surface area is determined from gaseous adsorption of nitrogen in low temperature, but what it represents is the available surface area for the biochar (including porosity, micropore volume etc.) which may be related to ammonium adsorption from liquid. In theory, if the surface area is higher, there should be more adsorption sites available for ammonium to adhere to, though as we have noted this is not always a direct correlation. As you have stated there are many interactions in solution which make ammonium adsorption complicated, which is most probably why we have not been able to make a direct correlation between surface area and ammonium adsorption capacity. That does not mean that it does not play any role in the mechanism of ammonium capture though.

It is unclear what parameter you are referring to that we did not include in our studies, but we would be open to hearing more about it. We are promoting a circular economy scenario of using biochar to recover ammonium to be reused as a solid fertilizer – which is different than replacing biological nitrogen removal from the wastewater treatment plants. As we have stated, the capacities of biochar have not been shown to be substantial enough to handle the full load of a wastewater treatment plant. This may have been unclear in our paper, and so the sentence you have quoted has been revised to better reflect our point.

We agree that it is extremely challenging to embrace biochars in this context due to their widely varying properties, though we hope that our article sheds some light on the potential for the materials with better standards and reporting.

Reviewer 3 Report

The authors of the paper which title is “Investigation of influencing characteristics of biochars for ammonium adsorption” evaluated the biochars from literature in the field of ammonium adsorption from aqueous solutions to check the effect of parameters in ammonium adsorption. Paper is interesting but in need to address the following comments before publishing in the Journal of Applied Science.

1-     The abstract part of the paper needs to be revised. Authors must bring some quantitative results to make the paper more understandable. Moreover, abstract is not suitable to recommend the research community for the next steps of the research study. Please re-write the abstract and make it more scientific.

2-     In general, it seems that the paper is a review paper, but the authors used Python and GraphPad Prism to analyze the previous data; however, the title of the paper is misleading. Please change the title of the paper to cover the goal of the paper much better. Authors can add the term of data analysis or systematic analysis.

3-     The introduction part also needs to be improved. In terms of biochar processing and optimization process of ammonium adsorption, authors must bring the latest study in the field, report their results and then, highlight the novelty and goals of present study.   

4-     In terms of ammonium adsorption of biochar, are the authors considered the different types of biochar in terms of preparation, location of the source of the biochar and also the pyrolysis process?

5-     What is the Mann-Whitney test? Please explain and cite the references of this technique.

6-     In terms of conclusion part and the correlation matrix, what pre-treatment and biochar modification authors suggest to improve the quality of biochar for different industries?  

7-     English language of the paper also need to be improved, please check the type and grammatical problem of the paper.

Author Response

Thank you for your thoughtful comments. We have addressed your feedback and believe the manuscript has been improved as a result of your input.

1. The wording of the abstract has been changed to remove the recommendation. The purpose of the paper was to comment on the lack of consistency in experimental methods and biochar production, and to determine whether the current data available is enough to make statistical conclusions about adsorption capacity and its relationship to various char characteristics. We have improved our summary of the main findings of the paper in the abstract by highlighting the factors influencing adsorption suggested by literature which the meta-analysis supports. We have also better explained that the disagreement of the meta-analysis with expected trends in literature highlights the difficulty of usefully comparing different studies. We have added to the abstract that an example of such a comparison – informed by clusters identified by the analysis we perform – is shown in the manuscript. Finally, we have also integrated the conclusions of the study into the abstract more clearly.
2. The wording of the title has been changed from “investigation” to “analysis” to better cover the scope of the paper.
3. The introduction has been expanded to include the range of ammonium adsorptions found and more citations for the use of biochar in ammonium adsorption applications.
4. In the data analysis we have recorded the general material type and specific material (as seen in Figure 1) as well as the pyrolysis temperature for biochar production. The location and specific source of the biochar feedstock was not recorded but is inherent to the study that is referenced for each data point. Biochar preparation also has more details than only pyrolysis temperature, but this is the most reported parameter, so it was used for the analysis. The methods (like the location of biochar feedstock) can be tied back to the study referenced for each data point. An attempt was made to determine if there was a correlation between study and adsorption capacity to determine if differing methods were obvious enough between studies but there was too little data from each specific class for meaningful comparisons.
5. Mann-Whitney test (also called the Wilcoxon-Mann-Whitney test) is a hypothesis test used to determine whether the mean of one group of data is the same as the mean of another group of data. If the means are statistically the same, it can be said that the two groups are part of the same, larger data set. It is similar to a t-test but is has less assumptions about the data sets and therefore can be applied in situations where little is known about the distribution of the data. A reference has been added to the text for clarification.
6. We did not discuss pre- or post- pyrolysis treatment for biochars in this paper because the data available is even more scarce than the data for untreated biochars. As such we cannot provide any recommendations for treatments of the biochars that would increase the ammonium adsorption.
7. The paper has been reviewed for grammar and language by a native English speaker.

Reviewer 4 Report

"Investigation of influencing characteristics of biochars for ammonium adsorption" is a new convincing approach to proper investigation through data normalization.

There are minor changed to be made

1) Line 26: What do you mean by species?

2) Line 29: (made, to decrease) make proper punctuations

3) Line 56-58: Hard to understand. Rewrite it clearly

4) Line 67-68: If possible, please mention the year until (the reference article you've collected).

5) Line 70: mention the package you used in python (for statistical analyses and EM)

6) Line 142: Please refer to a research article that successfully implemented the imputed data. Since this biplot is considered one of the key results, you should clearly mention the selected dimensions and the parameters of this data

7) Methods: Please provide two or three subtopics. It looks complicated to read the methods without subtopics.

8) Line 134: Punctuation - For example, 

9) Figure 3: Hard to understand the correlation map without a colour scale. elaboration for colour is needed.

10) Line 277-279: The statement is not clear. write it clearly

11) Line 327: I assume it is Figure 5

12) Tables 3 & 5: Justify whether the comparison between the experiments from the same research article is considerable in your research article. It was already analysed, compared and published in that article.

13) Conclusion: Make it clear and precise brief. (Important characteristic suggestions)

14) Proper grammar check is needed

Author Response

Thank you for your thoughtful comments. Responses to your notes can be seen below:

1) Species is synonymous with type or kind, in this case we mean that the most popular form that nitrogen is present in would be bonded with four hydrogen to form NH4+.

2) This punctuation has been removed.

3) These sentences have been rewritten for clarification.

4) Articles used for analysis were published from 2013-2022. This information has been added to the paper.

5) We have added that EM was implemented by the authors and that scikit-learn was used for PCA.

6) Added a reference to the article establishing EM as a method for performing PCA which is naturally accommodating of missing data.

We have specified that the dimensionality of the problem is reduced from 40 – using all the collected data – to 9 when considering only the factors identified in Table 1.

7) Two subsections have been added to the methods section for better organization.

8) The punctuation in this sentence has been revised. 

9) We respectfully disagree that a color bar/color scale would aid clarity. Given that the exact correlation values are shown in the matrix, the value of each color is already present on the diagram. Adding a color bar would not provide the reader with any clearer indication of the value of each cell in the matrix.

However, we have added clarification in the figure caption that positive correlations in the matrix are colored red and negative correlations are colored blue.

10) This sentence has been rewritten for clarification.

11) The correct figure for this sentence is Figure 4. The text has been corrected.

12) The examples listed in Tables 3&5 were to illustrate the differences in the data between different studies, to comment on the fact that trends in adsorption have been conflicting. The tables have been combined and the text edited to better explain this point.

13) Most of the text has been moved to the discussion section to make the conclusion more concise.

14) Grammar and language check was performed by a native English speaker.

Round 2

Reviewer 2 Report

Dear Authors,  I'm terribly sorry but I’m forced to maintain my previous statement, thus I cannot recommend this work for publication:

It is true that BET surface area is determined from gaseous adsorption of nitrogen in low temperature, but what it represents is the available surface area for the biochar (including porosity, micropore volume etc.)  – NOT TRUE

which may be related to ammonium adsorption from liquid – NOT TRUE

In theory, if the surface area is higher, there should be more adsorption sites available for ammonium to adhere to, – NOT TRUE

… there are many interactions in solution which make ammonium adsorption complicated, which is most probably why we have not been able to make a direct correlation between surface area and ammonium adsorption capacity. – Then, why do you state that BET is the most important index?

That does not mean that it does not play any role in the mechanism of ammonium capture though. – This is partially true; but you can always say that, especially for hard, non-elastic materials 

It is unclear what parameter you are referring to that we did not include in our studies, but we would be open to hearing more about it. We are promoting a circular economy scenario of using biochar to recover ammonium to be reused as a solid fertilizer – which is different than replacing biological nitrogen removal from the wastewater treatment plants. As we have stated, the capacities of biochar have not been shown to be substantial enough to handle the full load of a wastewater treatment plant. This may have been unclear in our paper, and so the sentence you have quoted has been revised to better reflect our point. – did you try to calculate (compare) these benefits with the environmental costs (or simple products) incurred during biochars production (biomass pyrolysis)?

Your statement: “Biochar can be used for carbon sequestration and is estimated to be able to reduce net emissions of carbon dioxide, methane and nitrous oxide by 12% per year when used in soil [5]" based on the paper where carbon sequestration wasn't the topic, and "carbon sequestration" was mentioned 2-times, is an example of the situation I'd like to fight with "it was published - thus it must be a truth"

We agree that it is extremely challenging to embrace biochars in this context due to their widely varying properties, though we hope that our article sheds some light on the potential for the materials with better standards and reporting. – I’m sorry but in my opinion, it unnecessarily overclouds the overall picture

I strongly recommend cooperation with the Carbon-materials specialist, before the next correction of the text. For example p.12 ll. 394-416

-        The text was written too fast and wasn’t thought out

-        “Looking at BET surface area for example; the first five entries in Table 3 differ only in initial ammonium concentration, but the adsorption capacity varies from 9.14 mgNH4+/g biochar to 37.63 mgNH4+/g biochar, corresponding to a difference of 19% of the maximum adsorption capacity” – dear Authors, this is not surprising, this is basic physical chemistry that during adsorption isotherms determination the adsorption depends on the adsorbate concentration.

-        Why line 8 in Tab 3 was excluded from “the first five entries” this is the same material, the same work [19]!!!

-        What is the “percentage of maximum capacity”?? in the source, there is “removal efficiency” – this is not the same!!! (please, physical chemistry)

-        The data collected in Table 3 are in general too pure; you cannot compare adsorption from slurry with NH4Cl solution! More Kizito et al. prove in their work that SA is not a key parameter!

-         What pH means here? Why do you use 8.15 (in [19] there is 8.0–8.3)?

-        Based on the literature e.g. [19] you should conclude that the biomass source material is much more important

-        Etc. etc…

In fact (it looks like a shoot into the knee) from results collected in Table 3 one can conclude that BET absolutely does not correlate with ammonia adsorption!!! This is what I wrote previously “good statistic analysis” can EVERYTHING even calculate 3 legs (please, do not remove this)

Author Response

Dear Reviewer, please see our responses below in blue after your comments (in black).

Dear Authors,  I'm terribly sorry but I’m forced to maintain my previous statement, thus I cannot recommend this work for publication:

We appreciate the time you have taken to provide feedback on our work, however, we find your feedback and comments lacking the necessary substance and support to enable us to take a corrective action. In spite of this, we have reviewed your feedback and addressed the majority of your concerns to the best of our abilities; both in terms of comments in response, as well as revisions to our work based on your critiques.

It is true that BET surface area is determined from gaseous adsorption of nitrogen in low temperature, but what it represents is the available surface area for the biochar (including porosity, micropore volume etc.)  – NOT TRUE

BET is the go-to analytical technique that is used widely in “…the determination of the overall… …specific external and internal surface area of disperse (e.g., nano-powders) or porous solids by measuring the amount of physically adsorbed gas according to the Brunauer, Emmett and Teller (BET) method…” (see ISO 9277:2010[en]). Even though it is understood that BET provides an estimation of the true specific surface area and there may be an over-estimation of the surface area due to the enhanced adsorption that is possible (via micropores, etc.), an absolute dismissal of employing such a widely accepted technique for the purpose of estimating specific surface areas is unjustified.

which may be related to ammonium adsorption from liquid – NOT TRUE

BET is employed to estimate specific surface area, with the understanding that an increase in available surface area correlates strongly to an increase in the available adsorption potential if a holistic understanding that all other parameters associated with adsorption potential is accounted for as well.

In theory, if the surface area is higher, there should be more adsorption sites available for ammonium to adhere to, – NOT TRUE

This is partly the case if only BET were under consideration, which it is obviously not from the manuscript. This correlation between BET and adsorption capacity is strongly proportional when all other parameters are considered as well.

… there are many interactions in solution which make ammonium adsorption complicated, which is most probably why we have not been able to make a direct correlation between surface area and ammonium adsorption capacity. – Then, why do you state that BET is the most important index?

This is a misunderstanding. We have not concluded that BET is the most important index for surface area and ammonium adsorption capacity in isolation and for all materials (see 436-439). Rather, it is shown to be important with some materials and in combination with other parameters inherent in the materials themselves which is difficult to separate due to lack of analysis. In order to perform future analysis, we suggest that ammonium in solution, CEC and surface area should continue to be analyzed and reported, though additional analysis on the material itself such as ultimate and proximate analysis etc. should be considered. Currently there is not enough analysis performed on the material itself to narrow down what specific characteristics are affecting the ammonium adsorption capacity.

That does not mean that it does not play any role in the mechanism of ammonium capture though. – This is partially true; but you can always say that, especially for hard, non-elastic materials

Unless the biochar undergoes graphitization at 3600 °C, it is unlikely that any biochar would be free of oxygenated substituents and electrostatic charge distributions at the surface. If graphite (a hard, non-elastic material) is used for adsorption tests, it fails to perform sufficiently compared to biochars that have not undergone any treatment at all. However, when biochar’s surface area is increased by some oxidation route, adsorption increases some ten-fold or more with a similar increase in specific surface area! – it is the magnitude of the “gain” between the adsorption observed and the severity of activation undergone by biochars that is much larger than other changes observed (such as change in species concentrations, porosity, pose size, pH, temperature, etc.) that not only “qualifies” this statement, but also “quantifies” it. IT is not possible to achieve these considerable increases to adsorption without an increase in surface area via mild oxidative activation.

It is unclear what parameter you are referring to that we did not include in our studies, but we would be open to hearing more about it. We are promoting a circular economy scenario of using biochar to recover ammonium to be reused as a solid fertilizer – which is different than replacing biological nitrogen removal from the wastewater treatment plants. As we have stated, the capacities of biochar have not been shown to be substantial enough to handle the full load of a wastewater treatment plant. This may have been unclear in our paper, and so the sentence you have quoted has been revised to better reflect our point. – did you try to calculate (compare) these benefits with the environmental costs (or simple products) incurred during biochars production (biomass pyrolysis)?

This was not within the scope of this work and therefore was not analyzed.

Your statement: “Biochar can be used for carbon sequestration and is estimated to be able to reduce net emissions of carbon dioxide, methane and nitrous oxide by 12% per year when used in soil [5]" based on the paper where carbon sequestration wasn't the topic, and "carbon sequestration" was mentioned 2-times, is an example of the situation I'd like to fight with "it was published - thus it must be a truth"

This was a mistake on our part of citing an article that used this figure instead of citing the article where this number originated. The original article focused on using biochar as a mitigation technique for climate change. This has been corrected in the text.

We agree that it is extremely challenging to embrace biochars in this context due to their widely varying properties, though we hope that our article sheds some light on the potential for the materials with better standards and reporting. – I’m sorry but in my opinion, it unnecessarily overclouds the overall picture

I strongly recommend cooperation with the Carbon-materials specialist, before the next correction of the text.

If agricultural soils have carbon present in the form of biochar, they perform significantly better than soils that do not contain biochar. This has been consistently observed in the terra preta and terra mulata soils of the Amazon region for decades. Our research has a specific goal as communicated in the introduction and any other considerations are supplementary to this. The involvement of a carbon specialist is an extreme deviation for this and would likely not achieve the objectives and scope of the project.

For example p.12 ll. 394-416

-        The text was written too fast and wasn’t thought out

What about the text gave the impression that it was written in haste? Please provide evidence or examples that we may address.

-        “Looking at BET surface area for example; the first five entries in Table 3 differ only in initial ammonium concentration, but the adsorption capacity varies from 9.14 mgNH4+/g biochar to 37.63 mgNH4+/g biochar, corresponding to a difference of 19% of the maximum adsorption capacity” – dear Authors, this is not surprising, this is basic physical chemistry that during adsorption isotherms determination the adsorption depends on the adsorbate concentration.

There may have been other factors such as using wetted biochar samples which may have diluted the broth. Whatever reasons for these deviations that either we or anyone else including the reviewers may provide would amount to speculation at best. All that can be done is to use these data together with a larger sample set and accept that there would be variances unaccounted for by the measured variables. Additionally we would like to point out that the study by Kizito et al. had a higher measured adsorption capacity with increasing ammonium concentration in solution, but less efficient adsorption (lower percentage of maximum theoretical capacity).

-        Why line 8 in Tab 3 was excluded from “the first five entries” this is the same material, the same work [19]!!!

This was done intentionally, as we wanted to draw two different comparisons using the data from reference 19. The first five entries are to be compared to each other, and the last entry for reference 19 was to be compared with reference 43 as it used a different pH than the first five entries and has a similar concentration to the tests done in 43. We concede your point that it would be better organized all together and thus have made this change in the table.

-        What is the “percentage of maximum capacity”?? in the source, there is “removal efficiency” – this is not the same!!! (please, physical chemistry)

As part of the reorganization of the table, percentage of maximum capacity was added as another way to look at the efficiency when compared with the theoretical adsorption capacity. We have added some text in the paper to explain how this was calculated (lines 96-99).

-        The data collected in Table 3 are in general too pure; you cannot compare adsorption from slurry with NH4Cl solution! More Kizito et al. prove in their work that SA is not a key parameter!

This point was not clear in the last version, but it is likely that the difference in performance for the same material in the two different tests may have been that one was in pure solution and the other was in a slurry. We have added more to the text to make this point, as we have shown that there is a statistical difference in the adsorption capacity for mixed versus pure solution. As far as your point about Kizitio et al. there have been numerous examples of studies that have shown that surface area was not an important factor for ammonium adsorption, but there have also been numerous studies showing the opposite. The conclusion we have drawn is that in the data we have analyzed it is important, along with other parameters that are inherent to the biochar material itself.

-         What pH means here? Why do you use 8.15 (in [19] there is 8.0–8.3)?

The pH value is the pH of the ammonium solution used in the adsorption test. For simplicity averages were taken when a range was given in the original article.

-        Based on the literature e.g. [19] you should conclude that the biomass source material is much more important

This is among our conclusions, as we have noted in lines 436-439. We have added a line in the conclusion to this effect as well (454-455).

-        Etc. etc…

In fact (it looks like a shoot into the knee) from results collected in Table 3 one can conclude that BET absolutely does not correlate with ammonia adsorption!!! This is what I wrote previously “good statistic analysis” can EVERYTHING even calculate 3 legs (please, do not remove this)

Reviewer 3 Report

Paper is ready to publish in the present format.

Author Response

Thank you for your time and consideration. We are please that you agree that the manuscript is ready for publication.