Preparation and characterization of cornstalk microspheric hydrochar and adsorption mechanism of mesotrione

In this study, cornstalk was pyrolysed to obtain hydrochar (HC), which was used to remove mesotrione from aqueous solutions. HC characterization and batch experiments were conducted to investigate mesotrione adsorption and the underlying mechanism. The characterization revealed microspheres on the HC surface. FT-IR spectra showed that the HC contained a large number of –OH groups, C=C bonds of aromatic rings, C–H groups in aromatic rings and phenolic C–O bonds. The adsorption results showed that the mesotrione adsorption ability gradually increased as the HC preparation temperature increased. The quasi-second-order kinetic equation (R2 ≥ 0.9860, p < 0.05) agreed well with the mesotrione adsorption process. The maximum monolayer adsorption capacity, which was obtained at pH 7 and 45°C with HC prepared at 240°C, was 3181.7 mg kg−1 with the Langmuir isotherm model (R2 ≥ 0.9491, p < 0.05). Van der Waals and dipole forces and hydrogen bonds were inferred as the main adsorption mechanisms. HC has potential as an effective and energy-saving adsorbent for mesotrione to reduce environmental pollution.

Results and discussion Line 140, was FTIR performed during the process and at what time periods, or after hydrothermal carbonization? Is there a reference to support the claim on lines 157-159 on these wavebands? Line 167, replace "the same" with constant Line 168-171 the author presents contradictory statements. Maximum adsorption capacity (3.01 at pH 3) and minimum (0.5 at pH) but authors also state that adsorption capacity increased from pH 3 to 9. Line 178-179, the authors suggest that there is no significant difference between the adsorption capacity at pH 9 and pH 11. Is this supported? What is the measure of the stability referred to? Clarify If the pKa is 3.2 what is nature (charge) of the mesotrione molecule at pH<3.2 (at pH 3) and pH>3.2 (5-11) and does this influence the observed trend? Line 181-183 the authors make reference to clofencet and salicylic acid pesticides which are not structurally related to the compound under study. Additionally, the study is on adsorption on soil which is very different from hydrochars. Is this citation helpful? Clarify Line 190-is 0-20 mg/L initial or equilibrium concentrations? Line 194-196 the authors state that adsorption is a spontaneous exothermic process? Is this statement objectively true? Are there no endothermic adsorption processes? Furthermore, states the "adsorption of mesotrione was not affected by temperature". Is this what is expected for an exothermic process? The authors then again state that the adsorption capacity decreased with an increase in temperature. These are contradictory statements. The authors should perform a statistical analysis of the variations observed to determine if the values are significantly different or not.
Line 196: How did they arrive at the statement that the process is physisorption and not otherwise? Lines 199-204: Can the authors find relevant materials such as biochars or carbonaceous materials for comparison? Figure 4 should be enlarged for improved visibility Lines 208-209: The reference 27 should be replaced by the original papers for Quasi-first-order and quasi-second-order kinetic models. These equations pre-existed reference 27. Lines 213-215: Its necessary to indicate whether the experimental conditions were the same for this comparison between hydrochar and biochar for proper interpretation Line 234: Is the unit for k2 min-1? Delete Lines 242-244 on R2 values. The information is in the Table What does fitting of the data to the quasi-second-order kinetic model mean for the adsorption process under study? An interpretation is lacking Line 268: References for these adsorption isotherms are missing The isotherms are presented in the text in their nonlinear form yet the data was processed using linear regressions. The equations should be presented in the linear form used. Figure 6 should be enlarged. The data in the graphs are not legible The line to 268 mentions linear regression coefficients, the equations presented are nonlinear, the figures plotted are linear and line 288 mentions that nonlinear regression coefficients are reported in the Table. This is confusing. What method was used in data analysis? Line 290-291-Error functions such as RMSE should be used to determine best fitting isotherm model just as in kinetics Line 299-300: The specific surfaces areas and CECs mentioned have not been presented anywhere in the manuscript Line 312 indicates the adsorption is chemical while line 196 indicated the adsorption is physical. Clarify Is it necessary to discuss the Temkin model if it was not the best fitting? Line 330: The units for distribution coefficient is lacking Line 340-341 indicates the adsorption process is endothermic with a positive enthalpy value. However, in lines 194-196 the authors stated that the adsorption was exothermic. Can this contradiction be clarified? Can this be harmonized with the variation in adsorption capacity with temperature? Equation 8 used the distribution coefficient which should have units (probably L/g). However, the units of ΔG in Table 3 is kJ/mol. If the parameters in equation 8 (R, T and LnK) are multiplied their units don't cancel out to give kJ/mol. The implication is Ln K must be dimensionless. The authors should convert the distribution coefficient to a dimensionless function and then determine ΔG. This will also affect the values used in equation 9 and the reported values in table 3 are in doubt. Thermodynamics parameters should be recalculated.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation? Reject
Comments to the Author(s) The authors reported "Preparation and characterization of cornstalks microspheric hydrochar and adsorption mechanism of mesotrione". After taking consideration, I do not suggest it for publication in this journal. My comments are listed as follows: 1.
I have prepared more hydrochar samples from lignocellulose materials. The microsphere is possible, but the microsphere is often not too much (~20%), not total compared to pure carbohydrates such as glucose. The SEM image only confirmed some part of the morphology of hydrochar, not total. 2.
The main problem of using hydrochar for water treatment is to contribute to increase the TOC of water. The water after adsorption often has slight dark colour because the release of dissolve organic compounds from hydrochar. 3.
The authors missed some important properties of the materials (i.e., zeta potential, BET surface area).

4.
Many important adsorption data were missed such as effect of NaCl, desorption study…..

5.
Adsorption mechanism needs to be proposed based on the experimental data, not from the literature. 6.
The problem of using the liner method for calculating the parameters of kinetics and isotherm models has intensively discussed in the literature. The authors repeated this problem.

7.
In the same data, but the authors represented different figures (Figures 4, 6, and 7). This is defined as redundant presentation and should be avoided in any ISI paper. 8.
The authors only concluded the data of adsorption were described by models (i.e., PSO) without any further giving information and strong discussion. 9.
The authors did not prepare this work carefully. As a result, there are a lot of mistakes, such as KL (L/g) in Table 2 There are a lot of uncertain or incorrect conclusions, such as + "The physical adsorption and desorption rates were extremely fast and were not generally affected by temperature." this is not true for all cases. + "FT-IR spectra showed that the HC contained a large number of -OH groups, C=C bonds of aromatic rings, C-H groups in aromatic rings and phenolic C-O bonds." Any hydrochar material contains those functional groups. The large number of those groups cannot be obtained by FTIR.

Decision letter (RSOS-202209.R0)
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Dear Professor Yang:
Title: Preparation and characterization of cornstalk microspheric hydrochar and adsorption mechanism of mesotrione Manuscript ID: RSOS-202209 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: Major Revision RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Yang and co-workers reported a mild hydrothermal method to acquire the microspherical structure of hydrochar from cornstalk and investigated the adsorption capacity of hydrochar. The article is well organized and the presentation is also good. However, some minor issues still need to be improved: 1. For the SEM images, the figure 1(c) should provide with the same magnification and scale compared with figure 1(a)(b). 2. The adsorption capacity and removal rate decreased or tended to become stable when pH > 9. Please illustrate why the optimum initial pH= 7 was selected for the subsequent experiments.
Reviewer: 2 Comments to the Author(s) Grammatical mistakes and spelling: post-emergence line 46 The claim in line 47 requires reference The thoughts on line 47 and 48 needs proper connection The threat of the compound's presence in water has not been highlighted In line 55 the authors contrast pyrolysis and hydrochar production (hydrothermal carbonization) but in the abstract, they claim hydrochar was produced by pyrolysis. Clarify. Line 71, delete and absorption kinetics Preparation of hydrochar The collection or source of cornstalk is not mentioned Batch equilibrium studies The model of pH measuring device is missing Line 96, is the initial concentrations denoted by Ce? Are concentrations listed in bracket equilibrium concentrations? Line 100, was the adsorption capacity data the only data analyzed using Origin software?
Results and discussion Line 140, was FTIR performed during the process and at what time periods, or after hydrothermal carbonization? Is there a reference to support the claim on lines 157-159 on these wavebands? Line 167, replace "the same" with constant Line 168-171 the author presents contradictory statements. Maximum adsorption capacity (3.01 at pH 3) and minimum (0.5 at pH) but authors also state that adsorption capacity increased from pH 3 to 9. Line 178-179, the authors suggest that there is no significant difference between the adsorption capacity at pH 9 and pH 11. Is this supported? What is the measure of the stability referred to? Clarify If the pKa is 3.2 what is nature (charge) of the mesotrione molecule at pH<3.2 (at pH 3) and pH>3.2 (5-11) and does this influence the observed trend? Line 181-183 the authors make reference to clofencet and salicylic acid pesticides which are not structurally related to the compound under study. Additionally, the study is on adsorption on soil which is very different from hydrochars. Is this citation helpful? Clarify Line 190-is 0-20 mg/L initial or equilibrium concentrations? Line 194-196 the authors state that adsorption is a spontaneous exothermic process? Is this statement objectively true? Are there no endothermic adsorption processes? Furthermore, lines 193-194 states the "adsorption of mesotrione was not affected by temperature". Is this what is expected for an exothermic process? The authors then again state that the adsorption capacity decreased with an increase in temperature. These are contradictory statements. The authors should perform a statistical analysis of the variations observed to determine if the values are significantly different or not.
Line 196: How did they arrive at the statement that the process is physisorption and not otherwise? Lines 199-204: Can the authors find relevant materials such as biochars or carbonaceous materials for comparison? Figure 4 should be enlarged for improved visibility Lines 208-209: The reference 27 should be replaced by the original papers for Quasi-first-order and quasi-second-order kinetic models. These equations pre-existed reference 27. Lines 213-215: Its necessary to indicate whether the experimental conditions were the same for this comparison between hydrochar and biochar for proper interpretation Line 234: Is the unit for k2 min-1? Delete Lines 242-244 on R2 values. The information is in the Table What does fitting of the data to the quasi-second-order kinetic model mean for the adsorption process under study? An interpretation is lacking Line 268: References for these adsorption isotherms are missing The isotherms are presented in the text in their nonlinear form yet the data was processed using linear regressions. The equations should be presented in the linear form used. Figure 6 should be enlarged. The data in the graphs are not legible The line to 268 mentions linear regression coefficients, the equations presented are nonlinear, the figures plotted are linear and line 288 mentions that nonlinear regression coefficients are reported in the Table. This is confusing. What method was used in data analysis? Line 290-291-Error functions such as RMSE should be used to determine best fitting isotherm model just as in kinetics Line 299-300: The specific surfaces areas and CECs mentioned have not been presented anywhere in the manuscript Line 312 indicates the adsorption is chemical while line 196 indicated the adsorption is physical. Clarify Is it necessary to discuss the Temkin model if it was not the best fitting? Line 330: The units for distribution coefficient is lacking Line 340-341 indicates the adsorption process is endothermic with a positive enthalpy value. However, in lines 194-196 the authors stated that the adsorption was exothermic. Can this contradiction be clarified? Can this be harmonized with the variation in adsorption capacity with temperature? Equation 8 used the distribution coefficient which should have units (probably L/g). However, the units of ΔG in Table 3 is kJ/mol. If the parameters in equation 8 (R, T and LnK) are multiplied their units don't cancel out to give kJ/mol. The implication is Ln K must be dimensionless. The authors should convert the distribution coefficient to a dimensionless function and then determine ΔG. This will also affect the values used in equation 9 and the reported values in table 3 are in doubt. Thermodynamics parameters should be recalculated.
Reviewer: 3 Comments to the Author(s) The authors reported "Preparation and characterization of cornstalks microspheric hydrochar and adsorption mechanism of mesotrione". After taking consideration, I do not suggest it for publication in this journal. My comments are listed as follows: 1. I have prepared more hydrochar samples from lignocellulose materials. The microsphere is possible, but the microsphere is often not too much (~20%), not total compared to pure carbohydrates such as glucose. The SEM image only confirmed some part of the morphology of hydrochar, not total. 2. The main problem of using hydrochar for water treatment is to contribute to increase the TOC of water. The water after adsorption often has slight dark colour because the release of dissolve organic compounds from hydrochar. 3. The authors missed some important properties of the materials (i.e., zeta potential, BET surface area). 4. Many important adsorption data were missed such as effect of NaCl, desorption study….. 5. Adsorption mechanism needs to be proposed based on the experimental data, not from the literature. 6. The problem of using the liner method for calculating the parameters of kinetics and isotherm models has intensively discussed in the literature. The authors repeated this problem. 7. In the same data, but the authors represented different figures (Figures 4, 6, and 7). This is defined as redundant presentation and should be avoided in any ISI paper.
8. The authors only concluded the data of adsorption were described by models (i.e., PSO) without any further giving information and strong discussion. 9. The authors did not prepare this work carefully. As a result, there are a lot of mistakes, such as KL (L/g) in Table 2? k2 (min-1) page 234 ? Quasi-first-order?? Quasi-second-order ?? 10. There are a lot of uncertain or incorrect conclusions, such as + "The physical adsorption and desorption rates were extremely fast and were not generally affected by temperature." this is not true for all cases. + "FT-IR spectra showed that the HC contained a large number of -OH groups, C=C bonds of aromatic rings, C-H groups in aromatic rings and phenolic C-O bonds." Any hydrochar material contains those functional groups. The large number of those groups cannot be obtained by FTIR. + "The positive value of ΔH indicated an endothermic process between the adsorbate and the adsorbent [35]. Mesotrione molecules entered the surface or interlayer of the hydrochar from the aqueous phase"?? + "The maximum adsorption capacities of the hydrochar at equilibrium were 1.947, 3.015, 3.442, 3.564, and 3.694 mg g-1, corresponding to initial mesotrione concentrations of 10, 20, 30, 40, and 50 mg L-1, respectively.?? The information is repeated the data from Figure? and they are also not called as the maximum adsorption capacities.

See Appendix A.
Decision letter (RSOS-202209.R1) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Professor Yang: Title: Preparation and characterization of cornstalk microspheric hydrochar and adsorption mechanism of mesotrione Manuscript ID: RSOS-202209.R1 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Decision letter (RSOS-202209.R2)
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Dear Professor Yang: Title: Preparation and characterization of cornstalk microspheric hydrochar and adsorption mechanism of mesotrione Manuscript ID: RSOS-202209.R2 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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