Bioleaching Coal Gangue with a Mixed Culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans

Round 1

Reviewer 1 Report

Dear authors

 

The present paper demonstrated interesting results. I have few points:

 

- The authors use the word “heavy metals”. The IUPAC has abolished that word. Please, describe what metals you are talking about.

 

I understood that the leaching column was homemade, is it correct? If so, please, put a image of this equipment.

 

The text: “The culture solution was pumped continuously to flow through the column from top to bottom” what flow was used?

 

The results were well descripted. However, what those results impact on the environment?

I would like the authors to comment more on the impact of this study on environmental issues

Author Response

Thank you for your advice.

1.The term was removed and substituted with ‘Fe, Mn and Cr’ or ‘metals’ in the revised version. 

2.A schematic diagram was drawn and presented in the revised manuscript as Figure 1.

3.The flow rate was 10 mL/min. It is now indicated in the text. Line 112-114.

4.Discussion and comments on the environmental aspect were amended. For example, the potential risk of AMD, sulfate, high concentration of meals in leaching solution were discussed in the revised version. They can be found in Line 187-195, Line 198-199, Line 226-233, Line 521-255.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper entitled “Bioleaching coal gangue with sulfur oxidizing microbes Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidant” present an interesting concept of reclaiming metal values from coal gangue, which is a waste generated in high quantities. Bioleaching of metals is considered environment-friendly and cost-efficient way to extract metal values of waste of any kind, so this is a good step to perform research on this subject.

1. The paper itself contains well organized and collected data from well conducted experiments. However, the presentation of data, especially the grammar and spelling mistakes exclude the paper in current form to be published in Minerals. The paper HAS TO BE read and corrected by a native English speaker. Some examples of mistakes:
   1. Line 20: “leaching rate following”, should be “leaching rate as follows”
   2. Line 29: “sulfate sulfur desulfurization” – too much sulfur in one sentence, does not sound very professional
   3. Line 60, 61: “could accelerated”, should be “could accelerate”; the whole sentence is hardly comprehensible
   4. Line 196: “among of them”, should be “among them”
   5. Line 205: “often take place”, should be “often takes place”
2. In the title, and in whole text, Authors stubbornly use Acidithiobacillus thiooxidant, whereas the name of the strain is Acidithiobacillus thiooxidans.
3. I would not call A. ferrooxidans “sulfur-oxidizing bacteria”, since it does oxidize iron. Of course, the promotion of S oxidation by AF can be seen, but this is because the Fe gets oxidized e.g. in pyrite. Please avoid this expression.
4. Following previous remark, the SOB abbreviation is not quite useful anymore, but mixing FOB and SOB should give something like FSOB.
5. I also would avoid the term “heavy metal”, leaving it to music. I would rather use “trace element”, “metal”.
6. Leaching does not mean recovery. If Authors do not obtain ready-to-use metallic product at the end of their experiment, they should avoid the term “recovery” in the text. Instead, please use “leaching”, “extraction”, etc.
7. Table 1 – please use 3 significant digits in the expression of numeric values
8. Table 2 – I would rather use 0.xxx format instead of .xxx to present correlation coefficients
9. The Authors use terms “Fe-Mn oxide fraction”, “organic fraction” for metal binding forms in tested material. Maybe to make sure what binding forms are, the 3-step sequential extraction (BCR), followed by fourth step with aqua regia, would be helpful?
10. In introduction, Authors state that there are only few examples of microbial leaching of coal gangue, maybe they should be cited here? Also I do not see any literature on A. thiooxidans use for coal gangue, it may be cited in the introduction as well.

These points have to be clarified before considering the paper to be published in Minerals.

Author Response

Thank you for your advice.

Response:

1.1 Corrected accordingly. Line 23-24.

1.2 Corrected accordingly. Line 30.

1.3 Corrected accordingly. Line 77, Line 203.

1.4 Corrected accordingly. Line 209-210, Line 3.

2. FOB was used.

3. Now we use SOB and FOB so that the name can be more specifically represent the individual microorganism.

4. The term was avoided and substituted with ‘Fe, Mn and Cr’ or ‘metals’ in the revised version.

5.“Leaching” or “extraction” are used in the revised manuscript whenever possible.

6. We have 2 digits for major elements and three for trace elements with much lower concentrations.

7. Corrected accordingly.Table 2.

8. In present work, we mainly focused on our work on total metals concentration. The fraction of Fe or other metal were not discussed. Organic fraction was changed to residual fraction. Line 219-221.

9.Two references regarding the bioleaching using A. thiooxidans were cited in the Introduction. Line 54 and 69.

However, to the best of our knowledge, there is only one paper published by our group, studying coal gangue with A. thiooxidans (He, H.; Han, Y.T.; Shi, K.Y.; Hong, F.F.; Zhu, H.W.; Leng, Y.W.; Tao, X.X.; Zheng, L.; Ma, C.Y.; Zhao, Y.D. Variation of sulfur speciation of coal gangue leaching by acidophilic ferrous / sulfur oxidizing microbes. Journal of China Coal Society. 2017, 42(5), 1304 -1310, doi:10. 13225 / j. cnki. jccs. 2016. 1169. (In Chinese)), which is already cited in the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript presents novel results regarding application of bioleaching based approach. Thus, the article is appropriate for Minerals and Metal Recovery and Environment Remediation by Bioleaching Technology special issue. Despite this, the manuscript should be improved. 

1. Editing of English language and style required as the manuscript contains incorrect sentences and phrases. For example:

1.1 Line 14: Two strains of A. ferrooxidans SOB-1 and A. thiooxidant SOB-2 isolated from coal gangue  dump WERE MIXED CULTURE and USED TO BIOLEACHING the coal gangue in a column reactor

1.2 Line 196: Among of them…

1.3 The leaching, weathering and biological catalysis of coal gangue can  release heavy metals and SULFIDES [sulfates are usually released into the liquid medium due to bioweathering, while sulfides are components of the solid phase]  into the environment.

2. The description of some methods should be clarified:

2.1  Line 137. By the end of the bioleaching of coal gangue, 50 g coal gangue from both test and control groups were collected from the column. The samples were washed 3 times with deionized water, vacuum freeze-dried for 10 hours, and ground to a particle size of 0.074  mm for further analyses.

In the experiments, you loaded two layers of the coal gangue. Did you take into the account this fact when collected the samples? The samples from two layers could significantly differed in composition.

2.2 Line 308 The total sulfur in coal gangue samples were 2.06 % (raw gangue), 1.585% (control  sample) and 1.181% (tested sample). The total desulfurization rate was 23.06% and 57.33%  for control and bioleaching treatment, respectively.

The mass residue yield [(mass of the residue)/(initial mass of the gangue) × 100%] should be taken into account when calculating leaching rate. In the present form, the calculation is incorrect! Leaching of other elements should also be calculated.

In general, authors should make a conclusion about possibility of application of bioleaching to recovery valuable metals and/or decrease harmful effect of the gangue studied taking into the account leaching rate and metal content in the gangue.

Author Response

Thank you for your advice.

Response:

1.1Corrected accordingly. Line 18.

1.2 Corrected accordingly. Line 203.

1.3 Corrected accordingly. Line 42.

2.1 The setup is for practical purposes like sand filtration (fine sand and coarse sand) in the water treatment system so that the aqueous phase can drain properly. The leaching solution was driven exclusively by gravity. Besides, when coal gangue in the column were recovered for analyses, only the top layer was sampled.

2.2 The rates were re-calculated, particular the individual sulfur species. Line 290-291. Line 300-308.

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

After corrections, the paper is more appropriate for publication in Minerals. However, the text has to be read again by a native speaker because of minor language mistakes, for example Line 39 - the first sentence of Introduction, Line 44 - has been concerns. Also, abbreviations have to be deciphered at first use in the whole text (example - Line 142).

Reviewer 3 Report

The article may be accepted as the authors improved the manuscript.