Understanding Activation Effects on Low-Temperature Biochar for Optimization of Herbicide Sorption
Round 1
Reviewer 1 Report
In the paper "Understanding activation effects on low-temperature biochar for optimization of herbicide sorption" the Authors determine the effect of H2O2 treatment of low-temperature grape-seed biochar on clomazone and cyhalofop retention. The results suggest that such modified BC is suitable for the removal of weak acid herbicides and therefore may be potentially considered as an effective soil amendment. The manuscript is overall well-written and logical. The collected experimental data is straightforward and expected in its integrality. Presented results stay within the scope of the Agronomy journal.
Below please find few issues that need to be addressed.
Line 337-344 The Authors discuss the decrease of G-350 hydrophobicity based on Ea calculations. It would be more informative to supplement it in the Methods part as there is no association of this property with Ea there.
There is also a very simple quantitative method to directly measure the change of hydrophobicity - Water Drop Repellency Time test. For details please see the literature:
Gray, M., M. Johnson, M. Dragila, AND M. Kleber. Water uptake in biochars: The roles of porosity and hydrophobicity. BIOMASS AND BIOENERGY.(2014) 61:196-205.
Douglas, P., Hallin, I. L., Douglas, P., Doerr, S. H., & Bryant, R. The effect of addition of a wettable biochar on soil water repellency. European Journal of Soil Science, (2015) 66(6):1063-1073.
Ćwieląg-Piasecka, I., Medyńska-Juraszek, A., Jerzykiewicz, M. et al. Humic acid and biochar as specific sorbents of pesticides. J Soils Sediments (2018) 18: 2692.
In Fig. 5. what was the reason for the decreased clomazone sorption (RE) in case of G-500ºC in comparison to the lower temperature biochar?
Line 484-486 Which "favorable characteristics (i.e., those that improve soil fertility and nutrient availability) are preserved in G-350 H2O2?
Line 494. I would specify and add at the end of the sentence: ..."in case of the other weak acid herbicides".
A few grammar mistakes:
Line 312 gross! misinterpretations
Line 437 please delete one must
Line 455 select--> selected
Author Response
In the paper "Understanding activation effects on low-temperature biochar for optimization of herbicide sorption" the Authors determine the effect of H2O2 treatment of low-temperature grape-seed biochar on clomazone and cyhalofop retention. The results suggest that such modified BC is suitable for the removal of weak acid herbicides and therefore may be potentially considered as an effective soil amendment. The manuscript is overall well-written and logical. The collected experimental data is straightforward and expected in its integrality. Presented results stay within the scope of the Agronomy journal.
Below please find few issues that need to be addressed.
Line 337-344 The Authors discuss the decrease of G-350 hydrophobicity based on Ea calculations. It would be more informative to supplement it in the Methods part as there is no association of this property with Ea there.
RESPONSE: Regrettably, we can not perform this experiment since we have not more biochar samples left and we used them to carry out the sorption isotherms suggested by reviewer 2. We will consider the information provided by the Reviewer in future experiments.
In Fig. 5. what was the reason for the decreased clomazone sorption (RE) in case of G-500ºC in comparison to the lower temperature biochar?
RESPONSE: The statistical treatment of the data indicated that there was no significant differences (p > 0.05) between the RE of clomazone for the case of G500 and G350 as it is indicated with the same letter on the bars in the Figure 4 of the revised manuscript.
Line 484-486 Which "favorable characteristics (i.e., those that improve soil fertility and nutrient availability) are preserved in G-350 H2O2?
RESPONSE: This sentence has been removed since we did not explicit demonstrated that other characteristics were preserved.
Line 494. I would specify and add at the end of the sentence: ..."in case of the other weak acid herbicides".
RESPONSE: This sentence has been added. L.526-527.
A few grammar mistakes:
Line 312 gross! misinterpretations
RESPONSE: Gross has been deleted from the revised manuscript.
Line 437 please delete one must
RESPONSE: Must has been deleted from the revised manuscript.
Line 455 select--> selected
RESPONSE: Selected has been added to the revised manuscript. L.488.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Summary:
This manuscript presents the results for a study investigating how biochar may be activated through chemical oxidation to enhance the removal efficiency of two different herbicides (cyhalofop and clomazone). The aim of the study was to investigate the effects of oxidation on biochar, evaluate the impact of oxidation on the sorption of herbicides, and determine potential mechanisms for sorption. The study produced biochar from grape wood pruning waste at 350, 500, and 900°C and the 350°C biochar underwent a chemical oxidation using 3% H2O2. Biochar properties were presented including elemental data (C, H, N, O, and ash), specific surface area, SEM, Zeta potential, and FTIR spectra. In general the oxidation treatment increased the O content (as well as oxygenated functional groups) and increased the SSA of the biochar. The sorption of the two herbicides were evaluated by combined biochar with a 1 mg L-1 herbicide solution and following 24 hours estimating the removal efficiency. The results indicated that the 900°C biochar had the greatest removal efficiency for both the herbicides. For the 350°C biochar, the activation significantly increased the removal efficiency of the biochar for the cyhalofop herbicide, but not the clomazone. Mechanisms for the enhanced uptake of cyhalofop were related to pH and increase in oxygenated functional groups resulting in charge based sorption.
Broad Comments:
Overall the paper is very well written and conclusions well supported. The authors conducted a lot of laboratory research to investigate the uptake of the herbicide to biochar and the methodologies, for the most part, are sound. However, that being said, the overall novelty of the paper I would not rank as high. Other studies have looked at uptake of herbicides and pesticides to biochar and conducted much more detailed experimental design to elicit sorption capacities and kinetics. This study simply looked at the removal efficiency of the herbicides at one concentration, which does not provide much information in terms of applying the biochar into application. Additionally, one of the aims of the paper was to elicit sorption mechanisms, and while there is supporting information from laboratory analysis, additional analysis may have provided more sound findings into the charge related sorption mechanisms. I have listed below my major comments along with suggestions for improvement of the manuscript and minor comments related to line numbers.
Major comments:
Why was only the 350°C biochar activated and not the other temperature biochars? Based on past studies the temperature of pyrolysis has an impact on the change in biochar characteristics (Suliman et al 2016; Sanford et al., 2019). Looking at chemical oxidation of biochar for nitrate uptake, oxidizing higher temperature biochars had a greater impact on the oxygen content and oxygenated functional groups of the biochar resulting in significantly higher sorption of nitrate. I would be curious if similar results would occur for the Cyhalofop herbicide. Ideally the authors would repeat the methods presented in the manuscript for the 500 and 900°C biochars then compare differences in terms of chemical changes and uptake efficiencies for different temperature biochar. Why was only the removal efficiency evaluated for? While this is interesting to know the removal efficiency, it would be much more beneficial to understand the sorption capacity and kinetics of the biochars for implementing it into treatment systems or the soil. Multiple other studies (Essandoh et al, 2017; Mandal et al., 2017; Herath et al., 2016) have looked at the sorption capacity and kinetics for other biochars and herbicides/pesticides, which provide a greater understanding of the mechanisms and potential end use applications for the biochar uptake of organic contaminate. Ideally the authors would evaluate the sorption capacity using at least 5 different concentrations of the herbicides and fit the data to the appropriate models (Langmuir, Freundlich, ect.) to estimate the biochar capacity for the two herbicides and also look at the kinetics by taking subsamples from one of the concentrations at different time intervals to model the kinetics (1st order pseudo, 2nd order pseudo, intraparticle diffusion, ect.), which would provide a better understanding of the required retention time needed for implementation into practice. This analysis would increase the quality of the manuscript and provide more beneficial information for potential end use applications. The study showed that the 900C biochar had a far greater removal efficiency than the 350 activated biochar, so why would it be necessary to activate at all if biochar can be produced at a higher temperature. I think it would be interesting to compare the characteristics of the 900C biochar to the others to see what properties/mechanisms enhanced the uptake. I think this could be achieved if the FTIR spectra for 500 and 900C biochar was added to figure 3 and discussed. Furthermore, the authors state that cationic bridging may be a potential mechanism. It would be beneficial if the authors conducted XPS analysis, as this would provide information on the potential cations present on the biochar. Additionally, they would be able to curve fit the carbon peak to identify which oxygenated functional groups may have been altered on the biochar which could provide more information on sorption mechanisms.
Minor Comments:
Line 69: Citation issue, skipped 13.
Line 77: Citation issue, should switch 13 and 14 from line 69. Please check rest of references to ensure they are in correct order.
Line 92-93: Awkward sentence, specifically “is proposed by its treatment with”, suggest it be rephrased.
Line 151-155: Discussed in Major comments above. At the very least please provide justification on why oxidation was only done on 350C biochar. Please also discuss why you chose the 3% H2O2 solution, and compare results to other studies that have used higher concentrations of H2O2 to see differences in characteristics (possibly Wang et al., 2015 or others).
Line 157: We the samples sent out for analysis or run in the lab with equipment from the company. If the latter, it should be “name of instrument (manufacturer, city, state, country)”. This applied to next few as well (Line 161, 163, 166, ect).
Line 159: I have had issues with past reviewers of my own publications regarding this methodology to determine oxygen content. It would be beneficial to conduct XPS analysis, which would also be beneficial for explaining sorption mechanisms. However, if the editor and other reviewers do not have an issue, I think the methodology is sufficient.
Line 164-167: Please provide the number of scans the spectra was obtained over, the scan resolution, and wavenumber range for the FTIR analysis. Also please discuss reasoning for using powder instead of a KBr pellet for analysis. Many past papers have used KBr pellets (Cheng et al., 2006; Joseph et al, 2018), which helps clean up the noise from the spectra, but looking at figure 3 your spectra seems pretty clear so KBr pellet may not be necessary.
Line 168-171: Typically this method involves buffering the solutions and obtaining the zeta potential at the same pH. Can you please provide if this was done, and if not please provide your reasoning, as it would greatly impact the zeta potential.
Line 178-190: All the equations seems a bit unnecessary. Typically I just see papers state BET analysis was conducted and cite methodology.
Line 191-206: Discussed concerns in major comments. Would have liked to see more detailed analysis conducted in order to determine sorption capacity and kenitics. At the very least, please provide reasoning that the RE was the only thing evaluated and add detail on the potential benefits of other analysis.
Line 193-194: Please cite preliminary studies if possible. If this is your own data it may be nice to add into the a supplemental file.
Line 194: Please add shaker RPM and temperature.
Line 220: Please provide reasoning one of the samples was not conducted in triplicate.
Line 372-374: If the 900C biochar resulted in a much more significant uptake of why would one bother with activation at all? Additionally, I think the characteristics of the 900C biochar should be compared with the other temperatures and activated biochar to see if any properties stand out to suggest potential mechanisms for greater uptake of 900C biochar. Possibly adding the 500C and 900C biochar to the FTIR figure (fig. 3) would elicit some potential differences.
Line 438: Awkward sentence, specifically “but must their effects must”, please rephrase.
Line 444-446: cation bridging has been shown in nitrogen studies to be a significant mechanisms for nitrate binding (Kannmann et al, 2015; Joseph et al., 2019; Sanford et al., 2019). It would be nice to compare characteristics as similar mechanisms are suggested. Additionally, adding XPS analysis would provide greater information on the potential minerals bound to the biochar and how they may have impacted uptake of the herbicide.
Line 457: Think you forgot to add some information after e.g. “(e.g. ____, [42,44])”
References discussed:
Cheng, C.-H., Lehmann, J., Thies, J. E., Burton, S. D., & Engelhard, M. H. (2006). Oxidation of black carbon by biotic and abiotic processes. Organic Geochemistry, 37(11), 1477–1488.
Essandoh, M., Wolgemuth, D., Pittman, C. U., Mohan, D., & Mlsna, T. (2017). Phenoxy herbicide removal from aqueous solutions using fast pyrolysis switchgrass biochar. Chemosphere, 174, 49–57. https://doi.org/10.1016/j.chemosphere.2017.01.105
Essandoh, M., Wolgemuth, D., Pittman, C. U., Mohan, D., & Mlsna, T. (2017). Adsorption of metribuzin from aqueous solution using magnetic and nonmagnetic sustainable low-cost biochar adsorbents. Environmental Science and Pollution Research, 24(5), 4577–4590. https://doi.org/10.1007/s11356-016-8188-6
Herath, I., Kumarathilaka, P., Al-Wabel, M. I., Abduljabbar, A., Ahmad, M., Usman, A. R. A., & Vithanage, M. (2016). Mechanistic modeling of glyphosate interaction with rice husk derived engineered biochar. Microporous and Mesoporous Materials, 225, 280–288. https://doi.org/10.1016/j.micromeso.2016.01.017
Joseph, S., Kammann, C. I., Shepherd, J. G., Conte, P., Schmidt, H., Hagemann, N., … Graber, E. R. (2018). Microstructural and associated chemical changes during the composting of a high temperature biochar : Mechanisms for nitrate , phosphate and other nutrient retention and release. Science of the Total Environment, 618, 1210–1223. https://doi.org/10.1016/j.scitotenv.2017.09.200
Kammann, C. I., Schmidt, H.-P., Messerschmidt, N., Linsel, S., Steffens, D., Müller, C., … Stephen, J. (2015). Plant growth improvement mediated by nitrate capture in co-composted biochar. Scientific Reports, 5, 1–12. https://doi.org/10.1038/srep11080
Mandal, S., Sarkar, B., Igalavithana, A. D., Ok, Y. S., Yang, X., Lombi, E., & Bolan, N. (2017). Mechanistic insights of 2,4-D sorption onto biochar: Influence of feedstock materials and biochar properties. Bioresource Technology, 246, 160–167. https://doi.org/10.1016/j.biortech.2017.07.073
Sanford, J. R., Larson, R. A., & Runge, T. (2019). Nitrate sorption to biochar following chemical oxidation. Science of The Total Environment, 669, 938–947. https://doi.org/10.1016/j.scitotenv.2019.03.061
Suliman, W., Harsh, J. B., Abu-Lail, N. I., Fortuna, A. M., Dallmeyer, I., & Garcia-Perez, M. (2016). Modification of biochar surface by air oxidation: Role of pyrolysis temperature. Biomass and Bioenergy, 85, 1–11. https://doi.org/10.1016/j.biombioe.2015.11.030
Wang, B., Lehmann, J., Hanley, K., Hestrin, R., & Enders, A. (2015). Adsorption and desorption of ammonium by maple wood biochar as a function of oxidation and pH. Chemosphere, 138, 120–126. https://doi.org/10.1016/j.chemosphere.2015.05.062
Author Response
RESPONSE: As it has been indicated in the introduction section, biochars prepared at low temperature provide more agronomic benefits due to their higher nutrient retention capability than those prepared at higher temperatures. For that reason, we treated G350 with H2O2. It is expected that the agronomic benefits provided for the low biochar relative to the bioavailability of nutrients would be maintained. Regarding the sorption mechanism, we have performed, as the reviewer suggested, the sorption isotherms for cyhalofop and clomazone on G350 and G350-H2O2. Unfortunately, it was not possible to address any other aspect, such as the sorption kinetics or XPS analysis since we have no more samples remained. The sorption isotherms confirmed the results obtained for the RE and now, this information appears in the revised manuscript.
Minor Comments:
Line 69: Citation issue, skipped 13.
RESPONSE: We have ordered all the references cited in the revised manuscript.
Line 77: Citation issue, should switch 13 and 14 from line 69. Please check rest of references to ensure they are in correct order.
RESPONSE: We have ordered all the references cited in the revised manuscript.
Line 92-93: Awkward sentence, specifically “is proposed by its treatment with”, suggest it be rephrased.
RESPONSE: this sentence has been rewritten in the revised manuscript. L.90-91.
Line 151-155: Discussed in Major comments above. At the very least please provide justification on why oxidation was only done on 350C biochar. Please also discuss why you chose the 3% H2O2 solution, and compare results to other studies that have used higher concentrations of H2O2 to see differences in characteristics (possibly Wang et al., 2015 or others).
RESPONSE: This section has been improved and more information has been added. We have clarified why BC L 146-150. Comparison of our data with other previous studies are discussed in the results section. L 150-154.
Line 157: We the samples sent out for analysis or run in the lab with equipment from the company. If the latter, it should be “name of instrument (manufacturer, city, state, country)”. This applied to next few as well (Line 161, 163, 166, ect).
RESPONSE: OK. This is indicated in the section.
Line 159: I have had issues with past reviewers of my own publications regarding this methodology to determine oxygen content. It would be beneficial to conduct XPS analysis, which would also be beneficial for explaining sorption mechanisms. However, if the editor and other reviewers do not have an issue, I think the methodology is sufficient.
RESPONSE: Thank you for your comment. As it has been indicated above, we have no access to the XPS analysis nor enough amount of biochar to perform the analysis.
Line 164-167: Please provide the number of scans the spectra was obtained over, the scan resolution, and wavenumber range for the FTIR analysis. Also please discuss reasoning for using powder instead of a KBr pellet for analysis. Many past papers have used KBr pellets (Cheng et al., 2006; Joseph et al, 2018), which helps clean up the noise from the spectra, but looking at figure 3 your spectra seems pretty clear so KBr pellet may not be necessary.
RESPONSE: This information has been included in the revised version, appearing in L 167-168. The equipment used for the analysis by FTIR allows direct measurements. In this case, a ATR cell was coupled to the system and Mercury-cadmium-telluride (MCT) detector which avoided the used of KBr, as long as the samples were well defined.
Line 168-171: Typically this method involves buffering the solutions and obtaining the zeta potential at the same pH. Can you please provide if this was done, and if not please provide your reasoning, as it would greatly impact the zeta potential.
RESPONSE: As the reviewer indicates, Z potential measurements are influenced by the pH of the solution. Here, we performed the analysis in deionized water (DI) since it is the medium where the sorption experiments were done. Consequently, we emulated the same background for the Z potential analysis.
Line 178-190: All the equations seems a bit unnecessary. Typically I just see papers state BET analysis was conducted and cite methodology.
RESPONSE: We have removed from the text the equation for determine the BET and we cited the methodology.
Line 191-206: Discussed concerns in major comments. Would have liked to see more detailed analysis conducted in order to determine sorption capacity and kenitics. At the very least, please provide reasoning that the RE was the only thing evaluated and add detail on the potential benefits of other analysis.
RESPONSE: For this purpose, the sorption isotherms for both pesticides have been performed for the biochar prepared at 350 and its activated form.
Line 193-194: Please cite preliminary studies if possible. If this is your own data it may be nice to add into the a supplemental file.
RESPONSE: Preliminary studies have been cited in the revised manuscript.
Line 194: Please add shaker RPM and temperature.
RESPONSE: This information has been included in the revised manuscript. L190.
Line 220: Please provide reasoning one of the samples was not conducted in triplicate.
RESPONSE: Due to the small amount of sample and that replicates were very similar duplicates were used in this case.
Line 372-374: If the 900C biochar resulted in a much more significant uptake of why would one bother with activation at all? Additionally, I think the characteristics of the 900C biochar should be compared with the other temperatures and activated biochar to see if any properties stand out to suggest potential mechanisms for greater uptake of 900C biochar. Possibly adding the 500C and 900C biochar to the FTIR figure (fig. 3) would elicit some potential differences.
RESPONSE: The FTIR spectra for G-500 and G-900 have been included in the revised version (Figure 3) and compared with G-350 and G-350-H2O2. L312-317.
Line 438: Awkward sentence, specifically “but must their effects must”, please rephrase.
RESPONSE: Must has been removed from this sentence.
Line 444-446: cation bridging has been shown in nitrogen studies to be a significant mechanisms for nitrate binding (Kannmann et al, 2015; Joseph et al., 2019; Sanford et al., 2019). It would be nice to compare characteristics as similar mechanisms are suggested. Additionally, adding XPS analysis would provide greater information on the potential minerals bound to the biochar and how they may have impacted uptake of the herbicide.
RESPONSE: Unfortunately, it was not possible to perform the XPS analysis and further investigations are needed to clarify this issue.
Line 457: Think you forgot to add some information after e.g. “(e.g. ____, [42,44])”
RESPONSE: The sentence has been completed in the revised manuscript. L490.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The article has been improved. I appreciate all the extra work put in by the authors. I only have a few minor comments regarding the text. Considering the manuscript quality, along with the journals aims/scope and impact factor, I would recommended this manuscript following minor revisions.
Minor comments:
Line 76: Define KOH.
Line 88 and 96: Change BC to biochar as that is what you have done throughout for the most part.
L197: You went from equation 2 to 4. Also either use [] or ().
L208-209: Present equation similar to others (i.e. [4] Cs=…). Define parameters and units.
L303/Figure 3: The G-350-H2O2 spectra is hard to see and needs to be improved. Maybe differentiate with solid lines with different color?
L356: Referencing wrong figure? Guessing you pulled from your dissertation and it should be figure 2 instead of figure 4.2.
L380: Define Koc
L398/Figure 5: What is the shaded area for G350 H2O2? Please explain or remove.
L400/Table 2: In the table description please explain what the values in the parenthesis and +/- are.
L409: Chen et al 2018 should be done to journal requirements. Thus, Chen et al [10].
L412/Figure 6: Could you use lowercase and capital letters as you did in figure 4 instead of the asterisk?
Line 473: fully instead of full?
Author Response
Line 76: Define KOH.
Response: OK. L76
Line 88 and 96: Change BC to biochar as that is what you have done throughout for the most part.
Response: OK. L89 and L97.
L197: You went from equation 2 to 4. Also either use [] or ().
Response: The number has been changed. L210
L208-209: Present equation similar to others (i.e. [4] Cs=…). Define parameters and units.
Response: The equation has been numbered and the parameters defined.
L303/Figure 3: The G-350-H2O2 spectra is hard to see and needs to be improved. Maybe differentiate with solid lines with different color?
Response: the line color of G-350-H2O2 spectra has been changed in Figure 3.
L356: Referencing wrong figure? Guessing you pulled from your dissertation and it should be figure 2 instead of figure 4.2.
Response: Now Figure 2 appears in the revised manuscript L349.
L380: Define Koc
Response: Koc has been defined in the revised manuscript L373.
L398/Figure 5: What is the shaded area for G350 H2O2? Please explain or remove.
Response: We have removed the shaded area, it was a problem with the pdf.
L400/Table 2: In the table description please explain what the values in the parenthesis and +/- are.
Response: The description has been added to Table 2.
L409: Chen et al 2018 should be done to journal requirements. Thus, Chen et al [10].
Response: OK.
L412/Figure 6: Could you use lowercase and capital letters as you did in figure 4 instead of the asterisk?
Response: OK.
Line 473: fully instead of full?
Response: Now, fully has been written.
