Biochar Derived from Agricultural Wastes as a Means of Facilitating the Degradation of Azo Dyes by Sulfides

Round 1

Reviewer 1 Report

The present manuscript reports on the employment of biochar derived from rice chaff, beanstalk, and corn straw to promote the redox reaction between azo dyes and sulfides. The degradation experiments as well as biochar characterization were properly carried out to support the conclusions. However, some minor modifications should be applied to the article for its publication in Catalysts.

See details here below.

1. Further information on the state of the art of biochar should be added in order to emphasize the emerging applications of this material and the novelty of this work. Information regarding other applications in which biochar can be a valid and sustainable alternative to traditional carbon materials (carbon nanotubes, graphene, etc.) should be added. Please cite very recent papers reporting on sensing (DOI: 10.1016/j.microc.2020.105506), electrocatalytic (DOI: 10.1016/j.gee.2020.11.010) and energy storage (DOI: 10.1016/j.rser.2020.110464) application of biochar.
2. (Figure S1-S3) Please explain the presence of Au signal in all the EDS spectra.
3. (Line 18) Please define the acronym “RCB800” in the Abstract.
4. (Figure S4, Figure 2, Scheme 1) Please revise these plots in order to improve their readability.
5. (Line 104) Please define Energy Dispersive X-Ray Spectroscopy only in one of these forms in the text: “EDS” or “EDX”
6. (Lines 181, 199, 326, 474) Please correct “Compertz model” with “Gompertz model”. Please use at least one reference to state that such model is normally used for this kind of process (e.g. DOI: 10.1021/acs.est.8b06692, which is already cited in this manuscript).
7. (Line 268) Please define the acronym “AQDS”. In general, an acronym should be defined when mentioned for the first time in the text.

Author Response

Response to reviewer 1

Thank you for your rigorous comments concerning our manuscript entitled “Biochar derived from agricultural wastes as a means of facilitating the degradation of azo dyes by sulfides”. These suggestions are valuable and helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval.

1. Related descriptions and documents have been added at “Introduction”.
2. Due to the need to enhance the conductivity of the material during the SEM-EDS test, a layer of gold was sputtered on the surface of the biochar materials, so the signal of the gold element was observed, which has been explained in the appendix (Figure S1).
3. The definition of RCB800 has been added: “rice chaff biochar prepared at 800° C”.
4. Figure S4 (Figure S6 now), Figure 2 has been modified.

Scheme 1 briefly describes the mechanism and factors of biochar catalyzing the reduction of azo dyes by sulfides, including four processes: ① Sulfides lose electrons and are converted into polysulfides; ② Polysulfides lose electrons and are converted into sulfur; ③ The dye molecule gets electrons, and the azo bond is broken; ④ The elemental sulfur produced in step 2 is combined with excess sulfides to form polysulfides, which provide reactant for step 3. In addition, according to the characterization results, the figure depicts porous carbon materials with aromatic structure and oxygen-containing functional groups. The above content corresponds to part of the content in section 2.7.

5. Energy Dispersive X-Ray Spectroscopy is uniformly abbreviated as EDS.
6. The “Gompertz model has been changed to the correct form. Two documents (doi:10.1021/acs.est.8b06692, doi:10.1016/j.ijhydene.2009.11.007) have been serially numbered and marked at the end of the chapter "2.2.1 Calculation" before, now they have been advanced.
7. The definition of AQDS has been added: “anthraquinone-2,6-disulfonic acid disodium salt”.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this article, the authors prepared and tested biochars for potential catalytic degradation of organic dyes. In general, the article can be considered for publication after a major revision.  

1) The purity of all chemicals should be included for reproducibility purposes. What were the conditions for calcination - if vacuum then show the numbers,  if nitrogen or other gas - then show the flow rate.

2) Supply HRTEM of prepared biochars, such mesopores should be easily observed there. 

3) In general, the article has no statistical analysis, therefore, hard to discuss the reproducibility of the results. In particular, each point in Figure 4-7 should contain the error bar charts (measured at least 3 times per trial).

4) Discuss what is happening to sulfide ions after the reaction and if they remain, how one can remove them from the water?     

Author Response

Response to reviewer 2

Thank you for your rigorous comments concerning our manuscript entitled “Biochar derived from agricultural wastes as a means of facilitating the degradation of azo dyes by sulfides”. These suggestions are valuable and helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval.

1. Purity of chemicals and nitrogen flow rate have been added in the text
2. We considered that the diameter range of the mesopores is 2-50 nm, it seems that HRTEM (High Resolution Transmission Electron Microscope) with high-precision is not needed, and FETEM (Field emission transmission electron microscope) is sufficient to observe the mesoporous structure, so we proceeded FETEM test. As we wished, the mesoporous structure was observed.
3. The figures in the manuscript and supporting information have been repaint, and error bars have been added.
4. Chapter 2.7 and Scheme.1 explain that sulfide will be oxidized into polysulfide and sulfur element in the degradation process, and the participation of sulfide can be observed in the EDS spectrum of RCB800-2. The conversion of sulfide in sewage into elemental sulfur and polysulfide facilitates further precipitation and flocculation treatment, and reduces the volatilization of sulfide into the atmosphere. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Review Comments

This manuscript reports “ Through the action of different biochars, sulfides can rapidly decompose and transform oxidizing dyes. The biochar material RCB800, derived from rice chaff, was observed to have the best effect, with a degradation rate of 96.6% in 40 min and 100% in 50 min”. Such research effort and publication in biochar research should be encouraged.

However, this manuscript needs to be revised to address some deficiencies to better reflect the current state of the biochar research.  For example, this manuscript does not report/discuss any measurement of biochar cation exchange capacity (CEC) that is a key property central to water filtration.  The authors seem not aware of the latest publications on biochar surface oxygenation for CEC improvement:

Kharel et al (2019) Biochar surface oxygenation by ozonization for super high cation exchange capacity, ACS Sustainable Chem. Eng. 2019, 7(19): 16410-16418.

The manuscript could be better if the authors could discuss the possible mechanisms such as whether the oxygen-functional groups on the biochar surfaces are required for catalytic reduction of the azo dyes by sulfide?  Is it possible to enhance the effect? Note, currently the effect (Fig. 4) seems could only reduce the time from 80 min (control) to 40 min (MO+S2+ RCB800), which is quite limited.

Finally, many of terms such as RCB800, BSB800, CSB800 and MO etc. need to be defined before use.

Therefore, if the authors could objectively address these issues, this reviewer would support the publication of their revised manuscript.

Author Response

Response to reviewer 3

Thank you for your rigorous comments concerning our manuscript entitled “Biochar derived from agricultural wastes as a means of facilitating the degradation of azo dyes by sulfides”. These suggestions are valuable and helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval.

1. Thank you for providing us with some new ideas about the current state of the biochar research, and we have the following views on the cation exchange capacity (CEC). 1. Improving soil fertility and catalyzing the degradation of pollutants are both important functions of biochar materials. It is generally recognized that cation exchange capacity is often an important indicator of the ability of biochar to improve soil fertility, while catalytic degradation, from this perspective, CEC analysis may not be necessary. 2. CEC value may be used as a measure of biochar adsorption of cations, especially heavy metal cations, but the dyes and sulfides involved in this article are all anions, and the degradation is mainly achieved through redox reactions. In addition, CEC data may not directly explain the experimental phenomena or results of this article. 3. Although CEC is related to the oxygen-containing functional groups on the surface of the material, the specific surface area, etc., these data have been explained by XPS, FTIR, and BET in this article, and are consistent with the experimental results. Therefore, whether CEC is no longer needed in this article What about the test?
2. The characterization methods EA and FTIR were used to demonstrate that biochar has abundant oxygen-containing functional groups, and combined with the discussion about the catalytic performance of AQDS and the O1s in the XPS spectrum, it proved that oxygen-containing functional groups such as quinone and carboxyl groups play a crucial role in the catalytic process. In terms of the catalytic process in this study, the biochar material could double the catalytic effect, and the amount of catalyst used was rarely only 100mg/L, so we believe that the effect of the biochar materials in this study is nice.
3. The terms have been defined.

Author Response File: Author Response.pdf

Reviewer 4 Report

This review concerns the manuscript catalysts-1135793-peer-review-v1 titled "Biochar derived from agricultural wastes as a means of facilitating the degradation of azo dyes by sulfides." The work describes the preparation of biochar materials and their application in dye degradation upon the adsorption of sulfides. The paper needs editing for grammar and punctuation, along with improvements for conciseness and clarity. The manuscript seemed all over the place as it is intended to study biochar materials but has utilized Carbon Nanotubes and Zeolites, too. The literature on this topic is significant, and the authors need to be precise (in both abstract and introduction) about their work's novelty. This should consider both materials preparation and application. This referee would recommend the publication upon major revision if these issues were the only problem. However, there are severe flaws in the characterization presented (see below). Thereby, this referee finds the paper unsuitable to be published in MDPI Catalysts in its current state.

The textural characterization of these materials has serious flaws, yet it is an essential characterization technique for these materials. The low-temperature nitrogen sorption measurements were not conducted correctly, as evidenced by the quality of the data displayed in Figure S4. To start, the first data point at low relative pressure reaches 55 to 65 cm³/g. That indicates the presence of micropores (quite common for carbons), which were not resolved by the analysis. Thus, a micropore analysis is required for these materials to understand their porosity. More aggravating is that both adsorption and desorption branches do not close at 0.4 P/P_o – this is unphysical [see ref. 1]. As an expert in the field, this referee guarantees that these measurements must be repeated as they have no use in their current form. It is hard to say what went wrong during analysis for sure, but it is recommended to use a 5 cm³/g of incremental doses and have the Dewar clean from ice and full prior analysis. Please use DFT for slit-like pores to calculate the pore size distribution [see ref. 2]. Both micropore and mesopore volume and size should be given. Moreover, the specific surface area of microporous materials is complex; please follow the IUPAC guidelines for calculations and discussion [see ref. 3]. The specific surface area should be presented with no more than three significant figures; that is, no decimals for this case.

[1] https://0-doi-org.brum.beds.ac.uk/10.1021/cm0101069

[2] https://0-doi-org.brum.beds.ac.uk/10.1016/j.carbon.2012.12.011

[3] https://0-doi-org.brum.beds.ac.uk/10.1515/pac-2014-1117

Author Response

Response to reviewer 4

Thank you for your rigorous comments concerning our manuscript entitled “Biochar derived from agricultural wastes as a means of facilitating the degradation of azo dyes by sulfides”. These suggestions are valuable and helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval.

In this study, the raw material of the material is biomass with a wide range of sources and low cost, which is conducive to waste recycling. Through material characterization and control experiments, it is proved that the mesoporous structure and oxygen-containing functional groups contained in the material play a positive role in the redox reaction process, as well as the important role of polysulfide in the reaction process. This manuscript has already received certain grammatical and punctuation improvements before submission. We humbly accept the reviewer’s suggestions on any language details in the manuscript.

Thank you for referee’s detailed and rigorous suggestions on the characterization results in the manuscript. As the referee said, according to the characterization results in the nitrogen adsorption and desorption experiment in the manuscript, the material does have micropores. However, an important factor in the process of biochar catalyzing dyes and sulfides in this study is the presence and amount of mesopores. According to the discussion in Chapter 2.7, the mesoporous structure of biochar and zeolite contributes to the presence of polysulfides. Another important influencing factor is the specific surface area of ​​the material. Only a certain amount of specific surface area can the biochar material carry enough oxygen-containing functional groups to promote the oxidation-reduction reaction between dyes and sulfides. The purpose of the nitrogen adsorption and desorption experiment is to find whether the biochar material contains mesopores and to measure the specific surface area of ​​the material. The results show that the material has a rich mesoporous structure and a large specific surface area, which is sufficient to catalyze the degradation reaction. At present, we have not yet known the role of the microporous structure in the production and occurrence of polysulfides.

According to the following two latest research results (https://0-doi-org.brum.beds.ac.uk/10.1007/s10853-021-05778-5,https://0-doi-org.brum.beds.ac.uk/10.1016/j.jhazmat.2020.124951), the nitrogen adsorption and desorption curve similar to this article may also be regarded as type-IV curve. The effective figure of the specific surface area data has been changed.

The above points of view have been discussed in batch trials and discussions. Thank you again for the suggestions of the referee. We sincerely hope that our views and conclusions can be recognized by the referee.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

No more comments, the revised manuscript can be considered for publication

Author Response

Thanks again for your review and suggestions.

Reviewer 4 Report

The textural characterization of these materials has serious flaws, and the authors have not corrected the problem. Unfortunately, this can only be done by repeating the analysis in question – the low-temperature nitrogen sorption. This referee is an expert in the field with over ten years of experience and has seen this problem occur. Instead of repeating the analysis, the authors justified the erroneous data with even the worst data. The data shown in the reference provided (https://0-doi-org.brum.beds.ac.uk/10.1007/s10853-021-05778-5,https://0-doi-org.brum.beds.ac.uk/10.1016/j.jhazmat.2020.124951) shows a straight line of the amount of nitrogen uptake versus relative pressure. If the x- and y-axes are labeled correctly in that paper, the authors made a mistake during cold free space measurements. The problem in that paper is different from the situation in the article discussed herein.

Herein, the adsorption and desorption branches do not match at the relative pressure of 0.4. This is unphysical and signifies that a mistake was made. Likely, the level of nitrogen was too low when the desorption curve was being analyzed.

Considering the serious experimental flaws presented in this work, this referee advises against this manuscript's publication in Materials or any other journal.

Author Response

Thanks again to the referee for patient, detailed and responsible review and suggestions. Due to our lack of relevant experience in the field of nitrogen adsorption and desorption testing, the data in this study is abnormal, which is different from the general rule. We deeply regret and apologize for this. The problems the referee pointed out gave us a lot of enlightenment. Because of experimental conditions, materials and time constraints, we may not be able to obtain accurate, reasonable and interpretable experimental data in a short time. For this reason, we decided to remove unreasonable adsorption isotherm related data and analysis, and utilize other characterization and analysis to support our conclusions. This study is innovative in terms of waste utilization, sulfides and dyes degradation, and related mechanisms. We appeal to the referee for the support to the publication of this manuscript in Catalysts.