Acid Mine Drainage Treatment Using a Process Train with Laterite Mine Waste, Concrete Waste, and Limestone as Treatment Media

Round 1

Reviewer 1 Report

Dear Authors,

  The manuscript is wonderfully written, although there is scope for improvement in reducing the text in results and improving conclusions section, only provide key points, that you need the readers to know, which is missing. I have attached pdf file containing 7 specific comments. I hope this improves your manuscript. Well done. 

 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

GENERAL IMPRESSION

Various configurations to treat AMD were applied and interpreted in this paper. Interesting parameters seem to be pH, Fe, Ni, and sulfate, where Fe²⁺ and Fe³⁺ should also be considered. Among them, when considering previous studies, the meaningful finding seems to be that the laterite mine waste (LMW) is efficient to remove Ni and sulfate. Nevertheless, the relevant mechanisms were not discussed and studied enough. Especially, Fe oxyhydroxides are generally not efficient in removing sulfate and Ni from column experiments of previous studies. As the efficiency of initial dry LMW can be overestimated due to the initial sorption in the batch experiments in this paper, column tests with a continuous flow are recommended.

 

GENERAL COMMENTS

1. To model precipitation as carbonates which is an important component, alkalinity should have been measured. Otherwise, the limitation of geochemical modeling should be stated and Section 3.4 loses significance.
2. Subsection 4.1 (Whole Rock Chemistry) and 4.2 (Mineralogy) should be combined and both XRD and XRF results should be concurrently interpreted to elucidate minerals and their concentrations. For example, the substrate may not consist of oxides which were explained in subsection 4.1.
3. Lines 409–423: The difference of pH increase between limestone and concrete waste needs more discussion. Different Ca compounds in limestone and concrete waste will affect the results. Lines 414–415 with an affirmative conclusion should also be revised, as the conclusion seems to need more evidence. In Lines 420–421, wasn’t the concrete waste affected by armoring? Lines 443–454 also should be reconsidered accordingly
4. Lines 478–498: Increasing pH by alkaline material generally result in decreasing Eh, regarding the negative relationship between pH and Eh in the pH-Eh diagram. ‘Oxidizing’ and ‘reducing’ agents should be used with caution. In this paper, only Fe removal should be discussed with Eh if necessary. Discussion on Eh, EC, and TDS can be substantially reduced as they don’t have significance.
5. In many cases, unwashed limestone can increase pH more than expected due to the existing precipitates such as efflorescence on the surface. This should be considered if necessary.
6. Fe²⁺ should also be measured to conduct geochemical modeling for SI with respect to iron phases.
7. The high efficiency of Ni and sulfate removal by the laterite mine waste may be due to the initially high sorption capacity. Relevant discussion should be focused on.
8. A large part of descriptions and explanations can be and should be reduced throughout the manuscript (e.g., Line 222 a third-party laboratory; Lines 239–240; Lines 425–430).
9. The whole manuscript should be checked for grammar (e.g., Line 23 Six analyses; Line 120 Concrete, Limestone), structure (e.g., Line 25 ‘and’ heavy metal ‘and’ sulfate), mistypes (e.g., Lines 140 and 309 EH; Lines 253 and 267 sulfide-sulfate; Line 255 Fe₃H₁₂KO₁₄S₂^-3 (the standard expression of formula should be also checked again throughout the manuscript); Line 264 ALOHSO₄; Line 731 ration) and accordingly revised.
10. Element symbols (e.g., Ni in Line 108) should be consistently used throughout the manuscript.

 

SPECIFIC COMMENTS

1. Lines 105–106: As goethite does not generate net alkalinity due to the release of Fe³⁺ after dissolution, this expression should be revised.
2. Line 108: The citation of [21] should be placed here.
3. Lines 114–116: Although (i) and (ii) indicate parallel structure, present and past tenses were used.
4. Lines 116–117: The phrase ‘the potential of process trains’ seems to be too general. This should be specific to the present study.
5. Line 126: The title of this subsection should include analytical methods or analysis.
6. As sizes of reactors are substantially different from each other, Fig. 1 should be revised. In the present figure, although 400 mL is repeatedly sampled, the amount of solution seems to be constant.
7. Please compare Eq. 7 with its explanation (Lines 200–207) again.
8. 6: Usage of ‘sampling’ and ‘sample’ points should be consistent. Please check for ‘sulfates concentration,’ too.
9. Lines 406–408: This paragraph seems to repeat the relevant results of the results section.
10. Lines 410–411: The significant numbers should be considered again regarding the reliability of the model.
11. Line 853: Please reconsider using ‘thru.’

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Summary and general comments.

In this work, authors treated simulated acid mine drainage effluent using mixture of limestone, laterite mine waste and concrete waste + setting up with different combination. Water treatment mechanism works by precipitation through the change of pH.

The work is interesting, however please find my comments below to further improve the quality of this work.

Specific comment

 

1. The introduction did not provide sufficient background. What is exactly is laterite mine waste? What is a concrete waste? The leftover? What are in these LMW and concrete waste that help in the water treatment?
2. What is process train?
3. Table 1. Are these reagents added to 20 L of water?
4. Do mention the reagent grade in the methodology section
5. Do mention the manufacturer and model number of the ICP-AES
6. Concentration of copper missing in Table 2.
7. In figure 1, what do authors meant by 0.75mL AMD/ g media? 0.75 mL AMD for 1 g of media? Since 400 mL AMD is used, does that means, the solid media contain up to 533 g?
8. How exactly are these train set up? Plastic containers?
9. Section 2.5. Is there is any setting for PHREEQC software, authors need to mention the set up to ensure simulation can be reproduced.
10. For eqn7, from where authors obtain the “standard value”.
11. Section 3.4 Authors listed the potential precipitation of different mineral for several runs, What does it mean when LS and CW produce the same precipitated minerals? Does this mean they both perform equally well? Can the degree or level of precipitation predict by the software?
12. Do recheck the header numbering. There are two section 3.5.
13. For the first section 3.5. These are the data obtained from simulation. Did author verify if this trend can be obtained experimentally?
14. Figures 3 and 4 can be merged.
15. Why is pH plot for LMW not showed?
16. Do recheck journal format for figures. Usually, all information should be included in the figure captions, not inside the plot.
17. The values in Table 7 are strange. What does negative % removal means? The element leaching from the solid media?
18. Is there any repeat carried out for each process train?
19. Sections 3.6 and 3.7 need more description and scientific interpretation.
20. The graphs for TDS and EC are exactly the same. are these values are obtained using the same instrument?
21. Rather than layering, does it make more sense to mix the three wastes together?

 

 

 

 

 

Author Response

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Round 2

Reviewer 2 Report

GENERAL IMPRESSION

I can check some revision according to the previous comments. Nevertheless, removed parts of the revised manuscript are not easy to be found. Only added parts are indicated in red characters.

 

GENERAL COMMENTS

1. The discussion on the cause of higher efficiency at concrete waste is still lacking (Lines 531–538). Different Ca compounds in limestone (CaCO₃) and concrete waste (Ca oxides and others) should be discussed more. Anorthite has a low ability for neutralization (Lines 537–538). Both limestone and concrete waste may undergo armoring due to the precipitation of Fe (hydr)oxides on the surface. Furthermore, solubility and kinetics of dissolution should be discussed comparing both substrates.
2. Lines 563–583: I cannot find the revised results of the following previous comments. “Increasing pH by alkaline material generally result in decreasing Eh, regarding the negative relationship between pH and Eh in the pH-Eh diagram.” and “‘Oxidizing’ and ‘reducing’ agents should be used with caution.” (e.g., CaO is not a general reducing agent (Line 578))
3. In this paper, only Fe removal should be discussed with Eh if necessary. Discussion on Eh, EC, and TDS can be substantially reduced as they don’t have significance.
4. Although sulfate was efficiently removed by the laterite mine waste in the experiment, this effect can be limited in the initial stage of sorption especially for sulfate, and the limitations also should be commented.

SPECIFIC COMMENTS

1. The sizes of the 2^nd and 3^rd reactors were not reduced. Please refer to the previous comment: “As sizes of reactors are substantially different from each other, Fig. 1 should be revised. In the present figure, although 400 mL is repeatedly sampled, the amount of solution seems to be constant.”

Author Response

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Reviewer 3 Report

Significant improvement observed after revision

Author Response

The manuscript has been revised accordingly