Reduction of High-Chromium-Containing Wastewater in the Leaching of Pyritic Waste Rocks from Coal Mines

Round 1

Reviewer 1 Report

Manuscript revision Sustainability- 1893011: Reduction of hexavalent chromium wastewater by leaching upon pyritic waste rock from coal mine

 

The authors report an interesting proposal to treat two hazardous materials (pyritic waste from the coal mining and the high-chromium-containing wastewater) together. Both hazardous materials are widely produced, and their correct disposal is often costly. Studies that present innovations to reduce the cost of treatment and disposal of industrial waste, especially encouraging the circular economy, should be shared with the scientific community concerned with environmental and sustainability issues.

The subject of the article is within the journal scope and may be considered for publication after some corrections:

 

1. Formatting and grammar proofing: there are words that are incorrectly separated at the end of the line. There are also some grammatical and concordance errors. I strongly recommend an English review.

 2. The title of the manuscript does not describe the real objective of the work, which is to enable the technological use of two highly polluting residues. It is necessary to review it so that both approaches are considered. I also suggest that the term “hexavalent chromium wastewater” be changed to “high-chromium-containing wastewater”

 3. Line 39: “In Brazilian carboniferous region of Santa Catarina, after extracted, the coal needs to be beneficial to attend the combustion standards in thermoelectric plants, as result of treatment are produced approximately 60% as solid waste with about 10% of amount with 65% as pyrite that must be discharged in waste deposits [1,8]”. The sample used, however, is from another region (Paraná). I suggest expanding the considerations made in this sentence in order to broaden the approach of coal production to the region where the material is collected.

 4. Lines 144, 161, 198 and 216: correct subscript or superscript. Line 198: correct ml to mL.

 5. Line 167: “The solution had pH below 2.0.” Is the pH kept constant throughout the test? If yes, how was the control done? If not, why was it not controlled and how might this affect (or not) the result?

 6. Line 174: “The solid material, resulting from the filtration process, was dried, grounded and calcined at 1100 °C”. Did you follow any methodology to determine this temperature value?

 7. Chromium was analyzed by which methodology described in the Standard Method for the Examination of Water and Wastewater? It is important to identify whether it was done by colorimetric method or atomic absorption. It is also important to highlight which parameters are analyzed for the effluent and which methods are used for this.

 8. Has the toxicity of the solid waste resulting from the effluent neutralization been evaluated?

 9. Why was the ignition loss test not done?

 10. Why did the authors choose the column feed in descending flow? Several works report a greater effectiveness of the contact between the solution and the particles bed when the upfeed method is adopted.

 11. A figura 3(a) mostra um pigmento industrial comercial? Na legenda fala que o Brown-red pigment produced by joint treatment coal waste. Talvez seja necessário rever a legenda para melhor entendimento.

 12. It would be interesting to present a figure with the concentrations of Cr³⁺ and Cr⁶⁺ and total Cr over time to better illustrate the kinetics of chromium reduction. It is expected that the total chromium concentration will remain constant during the process.

 13. Table 1. Chemical composition of material. Can the high chromium concentration be harmful or toxic for application in ceramic parts?

 14. Figure 9. I suggest removing this figure. The mass loss is insignificant and the analysis does not present information that adds to the work.

 15. Line 271: “To evaluate the differences of the particle size distribution between InPi and ChSlPi samples was performed the granulometric analyses. Results indicate that ChSlPi had particle biggest than InPi, it´s can indicate the worst dispersion during pigment application in ceramics”. Present proposals to mitigate the problem of dispersion, in order to promote the use of the material.

 16. Line 285: “Results of CIE L*a*b* and DEL*a*b* represents the color difference, thus the InPi is more light-colored. Parameter a* revel that it´s redder an b* revel that it´s less yellow. The global color difference is strong (DEL*a*b*), however the color differences above 1.5 can be seen by human eye [65,66]”. Considering that the material is a waste and not a material synthesized for this purpose, it is possible to evaluate this result as positive and highlight this result as positive. It is necessary, however, to present a strategy to mitigate this deviation for industrial application. By standardizing the particle size, is it possible to reduce this difference?

 17. Line 296: “The Brown-red pigment from sludge was submitted to industrial test in a ceramic factory (Figure 12)”. Was pre-processing done to correct the particle size before application to the ceramic part? It would also be important to compare with the application of the prepared pigment with the industrial pigment.

18. Check the value of Total solids (mg/L) in CONAMA 430.

19. The solution pH value shown in Table 3 was obtained after neutralization and sludge removal. What is the pH value after the reduction process, before neutralization?

20. What to do with the pyritic waste after application in the chromium reduction process? How was your chemical characterization?

Author Response

Dear Reviewer,

Initially I would like to thank you for your contributions to improve our paper. The answers to the considerations are in the attached file

Author Response File: Author Response.docx

Reviewer 2 Report

Dear editor,

Thanks for inviting me to review this manuscript titled “Reduction of hexavalent chromium wastewater by leaching upon pyritic waste rock from coal mine”. In this manuscript, a leaching and treatment device was developed and waste coal mines containing pyrite as a reducing agent to convert Cr⁶⁺ to Cr³⁺. It is informative and at the same time has profound research implications, thus, I recommend this manuscript be considered for publication after some revisions.

-- In lines 80-81, what are the results of the "different concentrations" mentioned in the article, and how does the concentration affect the degradation of pyrite at constant pH?

--The experimental procedure is not detailed enough, there should be data on the concentration of Cr6+, the temperature of the experiment, etc.

--In line 220, the number of experiments should be stated to make the data accurate and convincing. For example, how many hours (or how many times) was the cyclic experiment done.

 

--In lines 136-138, what are the advantages and disadvantages of chromium sludge obtained from the filtration process as a pigment compared to industrial pigments?

--In lines 230, what causes the solution to change from orange (Cr6+) to green (Cr3+) and whether it is directly related to the change in pH, please make a detailed explanation.

--What are the limits of the capacity of the treatment plant for pyrite use? What is the maximum processing load? Please provide a description of this.

--What are the disadvantages of this treatment device in use, or what challenges should be faced in the future, and does it meet the requirements of green treatment technology promoted by the state?

 

Author Response

Initially I would like to thank you for your contributions to improve our paper. The answers to the considerations are in the attached file

Author Response File: Author Response.docx

Reviewer 3 Report

Work is good

some notes:

please adjust the aim/objective and set in one sentence so the readers can briefly see what is all about

improvement of literature analysis. Please add some review reports for Sustainable materials and mining like Vincevica-Gaile et al on innovative materials, sustainable sorption studies by Ozola-Davidane et al.

At the final stage of the paper please add some sentences on practical pathway of the use in applied industry

 

Author Response

Initially I would like to thank you for your contributions to improve our paper. The answers to the considerations are in the attached file

Author Response File: Author Response.docx

Reviewer 4 Report

Title: Reduction of hexavalent chromium wastewater by leaching  upon pyritic waste rock from coal mine.

##Overallcomments 

The paper describes the reduction of chromium (VI) to Chromium (III) using pyrite leaching. The presentation of the report is acceptable. The article is interesting to me. After a minor correction, the paper could be accepted for publication in this journal.  

##Comments on the title, Abstract, and reference

Title is ok

The abstract is ok.

References need correction according to the journal format.

##Comments on Introduction

Line 44-49: It is hard to understand. Please correct.

Line 57 to 62: Please use numerical for numbering equations. Moreover, authors should use ChemDraw software or any to present equations correctly. 

Please avoid the use of bulk references. e.g. [9,33,40,42,43], [35, 44-47].

##Comments on Materials and Methods

line: 157: chloride tube mm?

##Comments on Results and discussion

The result and discussion part are ok. The presentation of figures is acceptable.

Table 3. value should be 0.00 decimal places.

##Comments on Conclusion.

line 335-339: needs corrections. The meaning is not clear.

Correct authors contributions: Eg. Authors full name: conceptualization, Formal analysis, writing original draft & Editing, Review draft and Editing.

 

Author Response

Initially I would like to thank you for your contributions to improve our paper. The answers to the considerations are in the attached file

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The requested revisions were satisfactorily carried out by the authors. The article can be published.