Effect of Biochar Addition on Mechanism of Heavy Metal Migration and Transformation in Biogas Residue Aerobic Compost
Round 1
Reviewer 1 Report
I have reviewed the manuscript Effect of biochar addition on mechanism of heavy metal migration and transformation in biogas residue aerobic compost. Next, I share my comments:
1. Authors claim: Studies have shown that aerobic composting of biogas residue cannot eliminate and reduce the toxic effect of heavy metals. Please add some references to support this affirmation.
2. It is mandatory to include a deeper revision of the previous works related, in order to clarify the contribution of the work.
3. Information related to the procedures to obtain the information in Table 1 must be added.
4. How the treatments percentages were defined?
5. Statistical anlysis is missing.
Author Response
Response to Reviewer 1 Comments
Point 1: Authors claim: Studies have shown that aerobic composting of biogas residue cannot eliminate and reduce the toxic effect of heavy metals. Please add some references to support this affirmation.
Response 1: Thank you to the reviewers and editors for the pertinent comment. As reviewer said, this paper may be lack of some references to support the idea that aerobic composting of biogas residue cannot eliminate and reduce the toxic effect of heavy metals. According to the comment of reviewer, we add the corresponding references in line 57 to support this view.
Point 2: It is mandatory to include a deeper revision of the previous works related, in order to clarify the contribution of the work.
Response 2: Thank you to the reviewers and editors for the instructive comment. According to the comment of reviewer, we have added relevant earlier work and conclusions, about compost temperature, moisture content, etc., which was added in lines 73 to 81 to clarify the contribution of the work.
Point 3: Information related to the procedures to obtain the information in Table 1 must be added.
Response 3: Thank you to the reviewers and editors for the instructive comment. According to the comment of reviewer, we have added a paragraph (2.2. Analytical methods) on lines 141 through 151 that contains information and literature related to the analysis in Table 1. The modifications were as follows: The moisture content was measured by the drying method (105 °C, 12h). The pH was determined by pH potentiometer method, EC was determined by conductivity meter, and specific surface area was determined by specific surface and pore size analyzer. Total nitrogen content was determined using an EA 3000 elemental analyzer. Total organic carbon content was determined by potassium dichromate external heating method and organic carbon analyzer. The content of organic matter was determined by Muffle furnace burning. The contents of total phosphorus and total potassium were determined by ammonium molybdate spectrophotometry and flame atomic absorption spectrometry.
Point 4: How the treatments percentages were defined?
Response 4: Thank you to the reviewers and editors for the sincere comment. The rules/proportions of each treatment were set according to the amount of biochar added (0% to 10% of the total compost material mass), where T1 was 2.5% biochar added, T2 was 5% biochar added, T3 was 7.5% biochar added, T4 was 10% biochar added, and CK was no biochar added. The quality of biogas residue in each treatment was unchanged, and C/N and moisture content were ensured by changing the amount of corn stover and pig manure supplementary materials.
Point 5: Statistical anlysis is missing.
Response 5: Thank you to the reviewers and editors for the sincere comment. According to the comment of the reviewer, we have added a paragraph for statistical analysis in lines 179-184. The modifications were as follows: The experimental data were expressed in the form of "mean ± standard error". The data were sorted by Excel 2013 software, and the drawing was made by Origin 9.0 software. One-way analysis of variance (ANOVA), Duncan's multiple comparisons, correlation analysis, PCA and CA were analyzed by SPSS 25.0 software, and P<0.05 was considered as the significant difference level.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Manuscript ID: Fermentation-1932657
Title: Effect of biochar addition on mechanism of heavy metal migration and transformation in biogas residue aerobic compost
The manuscript explores an interesting arena of research where biochar could be utilized as additive to enhance the passivation effects of heavy metals in the biogas residue. The following corrections can be implemented before the manuscript is accepted for publication.
Reviewer Comments
1 Mention the units of each component mentioned in Table 2 for all the treatments.
What was the reason for selecting the specific compositions of corn stalk and pig manure during the treatment as shown in Table 2? Also, if the biogas residue was obtained from the anaerobic digestion of corn stalk and pig manure, why was it again added during the process of composting?
Reframe the sentence “The initial C/N and water……………….60-65% respectively” in lines 102-103 in Pg. 3 of the manuscript.
At several places in the manuscript, authors have written CD as a representation of cadmium, kindly change it to Cd, throughout the manuscript.
5 Authors must mention the formulae used to calculate the passivation activity of heavy metals as mentioned in section 2.2 of the manuscript.
6. Authors have mentioned that continuous degradation of the compost during the composting process, the moisture and dry matter quality of the compost gradually decreased, which caused the concentration effect (Line 166-167, Pg. No. 4), however opposite results have been shown in Table 3 (Pg. 5) [reduction in Cu after treatment] of the manuscript. Authors must check and edit the discrepancy. Also, if continuous degradation of compost results in decline in moisture and its quality resulting in increased Cu concentration, won’t the same phenomenon be there with increased biochar addition. Please add a note to this. Discuss in detail, what is meant by “dry matter quality” in compost.
7. Authors must discuss the reasons for the difference in the extent of passivation effect of different metals during the composting residue.
8. Apart from variation in biochar content, what other factors like feedstock used for biochar formation, temperature of composting etc. can influence the passivation of heavy metals must also be explained in the manuscript.
9. The manuscript must be thoroughly read for grammatical and typographical errors. For example: Delete the word “It” in line 259.
1 There is no mention regarding the mechanism of passivation of heavy metals throughout the manuscript. A brief discussion related to the above-mentioned effect must be added to further improvise the manuscript.
Author Response
Response to Reviewer 2 Comments
Point 1: Mention the units of each component mentioned in Table 2 for all the treatments.
Response 1: Thank you to the reviewers and editors for the sincere comment. According to the comment of reviewer, we have added the units for each component in each treatment group in Table 2.
Point 2: What was the reason for selecting the specific compositions of corn stalk and pig manure during the treatment as shown in Table 2? Also, if the biogas residue was obtained from the anaerobic digestion of corn stalk and pig manure, why was it again added during the process of composting?
Response 2: Thank you to the reviewers and editors for the pertinent comment. Firstly, the biogas residue in this experiment came from the anaerobic fermentation of corn stalk and pig manure, which has no direct correlation with the subsequent addition of corn stalk/pig manure. Secondly，we believe that single biogas residue is not balanced in nutritional elements（The C/N ratio of digester residue was 19.5:1.）, so we need to add quantitative corn stalk and pig manure as auxiliary materials with relatively small amount, which can not only adjust the dry base mass, adjust the C/N to 25:1, and maintain the moisture content of 60%~65%.
Point 3: Reframe the sentence “The initial C/N and water……………….60-65% respectively” in lines 102-103 in Pg. 3 of the manuscript.
Response 3: Thank you reviewers and editors for the instructive comment. According to the comment of reviewer, we have made the correction on lines 101-103. The modifications were as follows: The auxiliary materials were a small amount of crushed corn straw and pig manure. In order to adjust the C/N ratio and water content in the compost materials, they were made to be 25:1 and 60%-65%, respectively.
Point 4: At several places in the manuscript, authors have written CD as a representation of cadmium, kindly change it to Cd, throughout the manuscript.
Response 4: Thank you reviewers and editors for the sincere comment. According to the comment of the reviewer, we have changed the Cd to the representative of cadmium in the whole paper. Thanks again for the reviewer's reminding.
Point 5: Authors must mention the formulae used to calculate the passivation activity of heavy metals as mentioned in section 2.2 of the manuscript.
Response 5: Thank you to the reviewers and editors for the sincere comment. According to the comment of the reviewer, we have added formula 2 for the passivation activity of heavy metals in lines 169-173. The distribution rate of various forms of heavy metals and the passivation effect are determined according to the calculation formula 2.
Point 6: Authors have mentioned that continuous degradation of the compost during the composting process, the moisture and dry matter quality of the compost gradually decreased, which caused the concentration effect (Line 166-167, Pg. No. 4), however opposite results have been shown in Table 3 (Pg. 5) [reduction in Cu after treatment] of the manuscript. Authors must check and edit the discrepancy. Also, if continuous degradation of compost results in decline in moisture and its quality resulting in increased Cu concentration, won’t the same phenomenon be there with increased biochar addition. Please add a note to this. Discuss in detail, what is meant by “dry matter quality” in compost.
Response 6: Thank you to the reviewers and editors for the sincere comment. Sorry, due to our mistake, the "Reduced state" of the second heavy metal in Table 3, Table 4, Table 5, Table 6 was mistakenly written as "Residual state". Secondly, in line 166-167 of the original text, we want to express that after composting, the content of reduced Cu in each treatment only increased in T2, while the content of reduced Cu in other treatments decreased. We speculate that the reason may be that the overall mass of the heap decreases gradually in the process of composting. Since the calculation of heavy metal content is calculated by dividing the amount of heavy metal substance by the weight of all the compost heap, the heap volume decreases and the overall content increases if the amount of reduced Cu remains unchanged. For the explanation here, we have carried out a detailed check and revised in the original article. In addition, we explained that "the same phenomenon would not occur with the addition of biochar". As the amount of biochar added increased, the conversion of reduced Cu to residue state was promoted. Therefore, as shown in Table 3, the content of reduced Cu in T3 and T4 decreased after composting. Dry matter quality means that the quality of the remaining material after drying water. Perhaps we did not make it clear that the meaning of "dry matter mass" in compost is explained in the text.
Point 7: Authors must discuss the reasons for the difference in the extent of passivation effect of different metals during the composting residue.
Response 7: Thank you to the reviewers and editors for the sincere comment. According to the comment of the reviewer, we have added a section to discuss the reasons for the different passivation effects of different metals in compost residue in lines 351-380.
Point 8: Apart from variation in biochar content, what other factors like feedstock used for biochar formation, temperature of composting etc. can influence the passivation of heavy metals must also be explained in the manuscript.
Response 8: Thank you to the reviewers and editors for the sincere comment. According to the comment of the reviewer, we have added a discussion of the physicochemical properties of biochar on compost, including temperature, germination index, electrical conductivity, etc., in lines 73-77 of the main text.
Point 9: The manuscript must be thoroughly read for grammatical and typographical errors. For example: Delete the word “It” in line 259.
Response 9: Thank you to the reviewers and editors for the sincere comment. According to the comment of the reviewer, we checked the whole text carefully to avoid grammatical and typographical mistakes.
Point 10: There is no mention regarding the mechanism of passivation of heavy metals throughout the manuscript. A brief discussion related to the above-mentioned effect must be added to further improvise the manuscript.
Response 5: Thank you to the reviewers and editors for the sincere comment. According to the comment of the reviewer, we have added the mechanism of heavy metals and related literature in lines 315 to 350. The mechanism of heavy metal passivation is described from three aspects of physics, chemistry and biology.
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I have reviewed the new version of the manuscript. All my previous concerns have been addressed.
