Passive Treatment of Acid Mine Drainage Effluents Using Constructed Wetlands: Case of an Abandoned Iron Mine, Morocco

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study evaluated two vertical subsurface flow constructed wetlands (CWs) for treating acid mine drainage (AMD) from an abandoned mine in Morocco—one with plants (PCW) and one without (CCW). Both systems successfully increased the pH of highly acidic water and removed heavy metals, with slight differences in efficiency. The planted wetland improved sulfate reduction and metal stabilization in sediments. Overall, the study highlights the effectiveness of CWs, especially with vegetation, in AMD remediation using limestone, organic substrates, and sulfate-reducing bacteria. The following comments are intended to guide the authors in refining and enhancing their work for successful publication:

1.     The study evaluates metal(loid) removal mechanisms but does not comprehensively distinguish between biotic and abiotic processes. Could the authors clarify the relative contributions of plant uptake, microbial activity (e.g., sulfate-reducing bacteria), and abiotic sorption/precipitation in the constructed wetlands?

2.     The paper discusses metal(loid) precipitation but does not address the long-term stability of immobilized metals. How do environmental fluctuations (e.g., pH, Eh, organic matter degradation) influence the potential remobilization of previously bound contaminants?

3.       The study focuses on CWs but does not compare their effectiveness against other passive or active AMD treatment methods. Could the authors provide a comparative analysis or discuss the advantages and limitations of CWs in contrast to alternative treatment approaches?

4.      The HRT is set at 10 days, but the impact of varying HRT on treatment efficiency is not explored. Have the authors considered testing different HRTs to optimize metal removal efficiency and determine the minimum required residence time?

5.      The sequential chemical extraction (SCE) method identifies metal(loid) associations, but the bioavailability of these metals under dynamic environmental conditions is not discussed. Can the authors elaborate on how metal fractionation influences potential ecological risks and metal uptake by plants?

6.     The study uses Juncus effusus for phytoremediation but does not assess its interactions with microbial communities. Have the authors considered analyzing microbial diversity in the substrate to understand plant-microbe synergies in AMD remediation?

7.     The study is conducted at a laboratory scale, but its applicability to field conditions remains unclear. Could the authors discuss potential challenges in scaling up the system, such as hydraulic dynamics, seasonal variations, and maintenance requirements?

8.     The study acknowledges that long-term retention capacity may be influenced by substrate accumulation and preferential pathways. However, no data on the long-term stability of metal(loid) immobilization or wetland performance is provided. Can the authors discuss potential strategies to mitigate long-term efficiency losses?

9.     The study highlights an increase in Mn concentration post-treatment, attributing this to weak adsorption and reductive dissolution. However, Mn removal mechanisms are not sufficiently explored. Could the authors provide further clarification on Mn behavior and potential optimization strategies to enhance its removal?

10.  The study mentions that seasonal changes (temperature, precipitation) influence contaminant removal efficiency, but this aspect was not assessed. Can the authors discuss how variations in temperature and hydraulic loading might affect system performance?

11.  The article mentions sulfate reduction as a key mechanism for metal(loid) removal but does not provide supporting data on sulfate-reducing bacterial activity or organic carbon availability. Could the authors include microbial analysis or additional sulfate speciation data to support this claim?

12.  The study states that redox potential values ranged from 36.5 to 73 mV, suggesting that reducing conditions were not dominant. Given the importance of redox-sensitive processes in metal immobilization, how do the authors explain metal precipitation under such conditions?

13.  The article discusses plant influence on pH regulation and organic acid exudation but does not present direct evidence of metal(loid) uptake by vegetation. Have the authors considered plant tissue analysis to quantify bioaccumulation and differentiate between plant uptake vs. substrate adsorption?

14.  The study utilizes sequential extraction to determine metal(loid) partitioning, but the discussion lacks confirmation of specific mineral phases. Can the authors incorporate X-ray absorption spectroscopy (XAS) or other mineralogical techniques to validate the speciation of retained metals?

 

15. Figures 2 & 3 in the manuscript can be greatly improved in the revision.

 

Comments on the Quality of English Language

None

Author Response

Please see our responses in the attached file.

Thank you.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a study on the remediation of acid mine drainage (AMD) using constructed wetlands in an abandoned iron mine in Morocco. The study provides valuable insights into the effectiveness of vertical flow constructed wetlands (CWs) for treating AMD, particularly regarding metal removal, pH neutralization, and microbial sulfate reduction. This article has some strengths and weaknesses.

Strengths:

- The topic is relevant to environmental remediation and sustainable water treatment, aligning well with the scope of Water (MDPI).

The experimental setup is well-detailed, including substrate composition, water quality monitoring, and analytical methods such as ICP-MS and XRD, ensuring robust data collection.

- The study employs statistical analyses (ANOVA, Kruskal-Wallis) to validate results, improving scientific reliability.

- The findings highlight the potential for using constructed wetlands with limestone substrates for passive AMD treatment, which is valuable for policymakers and environmental engineers.

- The manuscript follows a clear format, presenting the problem, methodology, results, and conclusions systematically.

Weaknesses:

The manuscript contains grammatical and typographical errors that need revision for better readability. Examples include:

-- Line 26: "Both types of CWS, (PCW) and u (CCW), showed similar efficiency in metal removal." (The wording is unclear.)

--Line 133: "Miracle-Gro N:P 24:8:16 was add to the PCW system" (should be "added").

- The manuscript sometimes uses "CWW" instead of "CCW," leading to confusion.

- Figure 1 lacks clear labeling of experimental setups (e.g., specifying substrate layers).

- Some graphs (e.g., Figure 3) need better axis labels and legends for clarity.

- The discussion on Mn removal is insufficient. The manuscript acknowledges negative removal rates but does not explore potential mechanisms in depth (e.g., competitive adsorption, microbial interactions).

- The role of sulfate-reducing bacteria (SRB) in long-term sulfate removal is briefly mentioned but could be better substantiated with references.

- While p-values are provided, confidence intervals should be included for better interpretation.

- The study does not discuss the longevity of the constructed wetlands or potential clogging and substrate saturation over time.

- More explanation and literature support are needed for Mn removal and SRB role.

 

Comments on the Quality of English Language

A thorough language review is necessary.

Author Response

Please, see our responses in the attached file.

Thank you very much.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Manuscript titled Mineralogic study of the remediation of acid mine drainage with constructed wetlands of an abandoned iron mine in Morocco touches sensitive but needed to discuss part of modern mining industry.

Authors developed two small-scale CWs systems providing an adequate surface and volume to the experiment. The experimental cells were contained a layer of neutralizing agent, specifically limestone, covered with a mix of river gravel and a source of organic carbon.

Methodology and materials were adequately designed and selected. Results are clear and present high scientific value. Presented results have been clearly presented and discussed by Authors.

 It is important to perform study and research relationship between production and mining sites transformation by scholars and industry leaders to put academic ideas and results into application.

In the Reviewer opinion - the presented manuscript can be published as it is.

Presented topic is original and relevant to the scientific field. There is a novelty in the text - considering methods and materials which were used during studies.

Conclusions consistent with the evidence and arguments presented in the manuscript. All the important results have been discussed and presented. The purpose of the research, methodology, materials and discussion were provided.

The references are appropriate.

Good quality, citation, and results.

Author Response

Thanks for your comments.

Please see our response in the file attached.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revision (water-3465002-peer-review-v2) is acceptable.

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for revising the article based on the proposed suggestions.