Mechanism and In Situ Prevention of Oxidation in Coal Gangue Piles: A Review Aiming to Reduce Acid Pollution
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors· - Manuscript, entitled “Mechanism and In-Situ Prevention of Oxidation in Coal Gangue Piles: A Review Aiming to Reduce Acid Pollution”, summarised the in-situ remediation represents a more cost-efficient and resource-effective approach, for major coal-producing countries worldwide. While, application of neutralization, precipitation, adsorption, ion exchange, membrane technology, biological treatment, and electrochemical technology to long-term pollutants with low concentration found in coal gangue piles might be costly and unsustainable. There are shortcomings and modifications that should be included in order to enhance the final manuscript for the readers.
· - Please write the complete name “acid mine drainage” of (AMD) abbreviation at line 15.
· - Fig (1) needs to show the coal gangue piles locations on fig.
· - Would you please be more specific about sentence (However, in-situ remediation technology for acid pollution in coal gangue piles has not been specifically reported yet, especially the remediation measures based on oxidation mechanism.) in introduction section lines 77, 78 and 79.
· - Please checked the sentence (pyrite is oxidized to generate Fe2O3 and elemental sulfur (Eq. (1)), and SO2 is released in the presence of sufficient oxygen (Eq. (2)).) in pyrite oxidation section Lines 100 and 101 with Fig. 2. I think (Eq. 1) replaced with (Eq. 2).
· - Would you please be paraphrasing the sentence (The result showed that the proper amount of SiO2 nanoparticles significantly improved the oxidation inhibition rate of PropS-SH, which is increased from 53.3% to 81.1 % of 2 wt% SiO2 nanoparticles combined with 3% (v/v) PropS-SH solution) in passivation coating section Lines 332-334. There is no experiment in the manuscript, so there is no result.
· - At the end of each discussed method of in-situ prevention, please refer to a real’s application and where exactly was applied (example), whenever possible.
Author Response
Comments 1: Please write the complete name “acid mine drainage” of (AMD) abbreviation at line 15.
Response 1: I have added the full name of AMD on line 15.
Comments 2: Fig (1) needs to show the coal gangue piles locations on fig.
Response 2: I have specified the location of the coal gangue pile in Fig.1.
Comments 3: Would you please be more specific about sentence (However, in-situ remediation technology for acid pollution in coal gangue piles has not been specifically reported yet, especially the remediation measures based on oxidation mechanism.) in introduction section lines 77, 78 and 79.
Response 3: In response to your revision suggestions, We have carefully revised the wording here. We realized that this sentence was not specific enough for the current summary of in-situ remediation technologies for acidic pollution, so We specifically proposed that the current review of in-situ remediation technologies mainly focused on the oxidation mechanism of pyrite oxidation. Thus, the focus of this article will also be on summarizing in-situ remediation measures for more comprehensive oxidation mechanisms, such as microbial action, low-temperature oxidation of coal, free radical action, and so on.
Comments 4: Please checked the sentence (pyrite is oxidized to generate Fe2O3 and elemental sulfur (Eq. (1)), and SO2 is released in the presence of sufficient oxygen (Eq. (2)).) in pyrite oxidation section Lines 100 and 101 with Fig. 2. I think (Eq. 1) replaced with (Eq. 2).
Response 4: We have carefully considered the issue you mentioned when writing the paper. However, considering that the oxygen in coal gangue piles is often insufficient, so we believe that the generation of elemental sulfur when oxygen is insufficient should be highlighted.
Comments 5: Would you please be paraphrasing the sentence (The result showed that the proper amount of SiO2 nanoparticles significantly improved the oxidation inhibition rate of PropS-SH, which is increased from 53.3% to 81.1 % of 2 wt% SiO2 nanoparticles combined with 3% (v/v) PropS-SH solution) in passivation coating section Lines 332-334. There is no experiment in the manuscript, so there is no result.
Response 5: We have adopted your suggestion and supplemented the experimental conditions and process for obtaining the optimal ratio of PropS-SH and SiO2 based on the reference literature, namely, the performance of PropS-SH/SiO2 nanocomposite coatings with a different content of SiO2 nanoparticles on pyrite oxidation inhibition was evaluated through electrochemical measurements and chemical leaching testing by Liu et al., combined with the experimental results in the manuscript, forming the complete study.
Comments 6: At the end of each discussed method of in-situ prevention, please refer to a real’s application and where exactly was applied (example), whenever possible.
Response 6: We have carefully listened to your suggestions and realized that there were some issues with the previous manuscript, and have spent a lot of time making corresponding modifications. Firstly, based on the theoretical and mechanistic review in Section 3, I supplemented the application effects of the technology widely used in coal gangue piles and evaluated the effectiveness of the technology with long-term monitoring. Secondly, for chapters with insufficient literature support, such as chapter 3.2, relevant literature has been supplemented to support the discussion.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis paper mainly summarizes the oxidation mechanism of coal gangue accumulation and its in-situ prevention technology to reduce acid pollution. This paper discusses four aspects of coal gangue oxidation and several in-situ remediation technologies to control the oxidation process of coal gangue, and provides a scientific and systematic in-situ remediation method with detailed contents. The technology has a strong application prospect and has guiding significance for actual environmental governance. However, there are some problems, which must be solved before it is considered for publication. If the following problems are well-addressed.
1. The article lacks practical application cases and data support.
The technologies in Section 3 are only an introduction to the theory, which can't fully explain the applicability and effects of each technology in different environments. It is suggested to add corresponding references to the application effects of technologies.
For example, L260-271: How to solve the technical difficulties and challenges by scientific theoretical methods, and the description of numerical simulation methods is too general, increase the references.
2. Lack of detailed evaluation data on the long-term effects and stability of various technologies.
The discussion of various technical effects is too general, and only the long-term benefits in theory are put forward, lacking the support of long-term monitoring data, so it is suggested to increase the corresponding references.
3. There is insufficient discussion on the possible secondary pollution caused by in-situ remediation technology and its impact on the environment and human health.
The discussion on environmental impact is too brief, so it is suggested to systematically and comprehensively analyze the potential environmental risks. The discussion on the impact of coal gangue acidification pollution on human health is limited to a general description, lacking specific data and related research explanations in references.
Comments on the Quality of English LanguageThe English language quality is acceptable, the language of some paragraphs can be more refined to avoid verbosity and repetition and improve the readability of the article. A professional language editing service is recommended for a final review to ensure the highest standard of language quality.
Author Response
Comments 1:
The article lacks practical application cases and data support.
The technologies in Section 3 are only an introduction to the theory, which can't fully explain the applicability and effects of each technology in different environments. It is suggested to add corresponding references to the application effects of technologies.
For example, L260-271: How to solve the technical difficulties and challenges by scientific theoretical methods, and the description of numerical simulation methods is too general, increase the references.
Response 1:
I have carefully listened to your suggestions and realized that there were some issues with the previous manuscript, and have spent a lot of time making corresponding modifications. Firstly, based on the theoretical and mechanistic review in Section 3, I supplemented the application effects of the technology widely used in coal gangue piles and evaluated the effectiveness of the technology with long-term monitoring. Secondly, for chapters with insufficient literature support, such as chapter 3.2, relevant literature has been supplemented to support the discussion.
In addition, you specifically mentioned that there was incomplete discussion in L260-271. On the basis of supplementing references, I have readjusted the logic of this paragraph. Firstly, I introduced the commonly used surface covering materials for acid pollution in coal gangue piles, and put forward the viewpoint that single structure materials lacked durability affected by multiple factors. Then, suitable solutions have been proposed to address this phenomenon, and the long-term monitoring effect under the action of the solution has been introduced.
Comments 2:
Lack of detailed evaluation data on the long-term effects and stability of various technologies.
The discussion of various technical effects is too general, and only the long-term benefits in theory are put forward, lacking the support of long-term monitoring data, so it is suggested to increase the corresponding references.
Response 2:
Firstly, I focused on supplementing the effects and long-term monitoring data of two widely used technologies, Mixed Co-Disposal and Coverage Barrier.
Secondly, the bactericides targeting oxidizing bacteria and microbial technologies that exert reducing effects are currently in the early stages of technological maturity and application, lacking on-site application and long-term monitoring. The existing data in the manuscript is basically effective data that can be referenced at present. These two technologies are also the important research directions of our research group at present. Based on the national key research and development project, we have successfully applied bactericides and reducing bacteria to demonstration sites in Taiyuan, Shanxi in 2023. In the future, we will also strengthen the monitoring of pollutants and evaluate the on-site application effects of these two technologies.
Finally, unfortunately, I was unable to effectively supplement the long-term monitoring data of the on-site application effects of Passivation Coating and Coal Oxidation Inhibitors, as these two technologies are currently in the laboratory validation and mechanism characterization stage, providing prospects for application in coal gangue piles.
Comments 3:
There is insufficient discussion on the possible secondary pollution caused by in-situ remediation technology and its impact on the environment and human health.
The discussion on environmental impact is too brief, so it is suggested to systematically and comprehensively analyze the potential environmental risks. The discussion on the impact of coal gangue acidification pollution on human health is limited to a general description, lacking specific data and related research explanations in references.
Response 3:
Firstly, I fully agree with your suggestion that secondary pollution, environmental benefits, and health risks must inevitably be considered in environmental science and engineering. Only by fully considering the above three issues can it be considered a complete study. Therefore, I am working hard to search for information and attempting to supplement and improve it. However, I have to admit that the length of my revision for your suggestion is not as large as the first two suggestions, because research on secondary pollution caused by these technologies is very rare, and most studies focus on mechanisms and effects. Below, I will explain the issues I have discovered.
(1) Mixed Co Disposal. I have added the issue of secondary pollution/ by-products, as it is essentially a neutralization reaction that inevitably produces a certain amount of sludge. Previous scholars only raised the issue of sludge generation in a general manner, but did not specifically study its components and response strategies.
(2) Coverage Barrier. When using natural materials as a cover layer, the issue of secondary pollution could be ignored to some extent. When studying materials such as sludge, industrial waste, and organic materials, the issue of secondary pollution should be given attention due to the introduction of new pollutants. However, no research on secondary pollution has been found in the literature.
(3) Passivation Coating and Coal Oxidation Inhibitors. Similar to the above, both technologies introduce new pollutants, mostly substances that are not originally present in the environment. However, no research on secondary pollution has been found in the literature.
(4) Bactericides. At present, the bactericides used in coal gangue piles are environmentally friendly substances that have been screened and have the characteristic of easy degradation, so the problem of secondary pollution could be ignored. The research and application of inorganic bactericides containing heavy metals must consider secondary pollution, but it is currently not common. Moreover, there is currently no research on the evolution of microbial communities in the native environment under the action of fungicides.
In summary, the problem of secondary pollution caused by the in-situ restoration technology of coal gangue piles has not been fully studied, and it is hoped that it can be further improved in the future.
Comments 4: The English language quality is acceptable, the language of some paragraphs can be more refined to avoid verbosity and repetition and improve the readability of the article. A professional language editing service is recommended for a final review to ensure the highest standard of language quality.
Response 1: I have proofread some of the language, grammar and formatting.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe article is devoted to an important review of existing technologies and methods for reducing oxidation in coal mining waste, existing in the form of dumps.
I have a number of additions that would improve the review and presentation of this scientific problem for readers.
1. It is necessary to expand the introduction a little:
- Regarding monitoring of spontaneous combustion processes and their elimination. This is important from the point of view of assessing the sustainable development of such territories;
- In addition, these dumping areas generate quite a lot of dust and create a number of problems in the surrounding area;
- Use of waste rock dumps, including as natural filters for influent water from quarries (Authors Lesin, Tyulenev and other);
This broad context of discussion is important for understanding the complexity of such objects as waste rock dumps from coal mining.
2. It is true that clays, other soils and other materials can be good isolates, but it is difficult to maintain their stability/integrity since waste rock dumps are often dynamic, especially in the early stages of their life. Dynamism leads to subsidence, sliding and, accordingly, a break in the continuity of the insulation. You should cite studies that address these issues to support your conclusions.
3. The dynamics of the dump can lead to the destruction of a single vegetation cover, due to the formation of a new ravine network, and oxidation processes can begin again.
4. Biological remediation should be considered a little more fully as an important step in the formation of a surface acid barrier.
Author Response
Comments 1:
It is necessary to expand the introduction a little:
- Regarding monitoring of spontaneous combustion processes and their elimination. This is important from the point of view of assessing the sustainable development of such territories;
- In addition, these dumping areas generate quite a lot of dust and create a number of problems in the surrounding area;
- Use of waste rock dumps, including as natural filters for influent water from quarries (Authors Lesin, Tyulenev and other);
This broad context of discussion is important for understanding the complexity of such objects as waste rock dumps from coal mining.
Response 1:
Thank you very much for your valuable feedback. Below is my response to your suggestion of appropriately expanding the introduction.
(1) You proposed to describe the monitoring and elimination of spontaneous combustion process in the introduction section. I agree with your opinion, because the essence of acidic pollutants generated in coal gangue piles is the oxidation of coal gangue, and the oxidation reaction is inevitably accompanied by heat release. Therefore, it is necessary to monitor the spontaneous combustion of coal gangue piles that produce acidic pollutants. In the introduction, I also pointed out the phenomenon of heat release from coal gangue oxidation, the production of gaseous pollutants, and spontaneous combustion under unfavorable thermal conductivity conditions. However, as the focus of this article is on the mechanism of acid pollution caused by coal gangue and in-situ remediation measures, I subsequently discussed the definition, migration pathways, environmental hazards, and health risks of acid pollution, without further discussing the monitoring and elimination strategies of spontaneous combustion.
(2) You mentioned in the introduction that the coal gangue pile generates a large amount of dust and poses a threat to the surrounding area. This is indeed a major problem that has been troubling residents around coal mines. The oxidation of coal gangue is accompanied by the release of gaseous pollutants such as SO2, NOX, CO, CO2, and atmospheric particulate matter, which seriously affects the quality of life and human health of surrounding residents. In this article, I summarize all of the above components as "atmospheric pollutants". However, I did not elaborate on the harm of each air pollutant to the environment and human health, as I wanted to devote more energy to expressing why I needed to review the oxidation mechanism and corresponding in-situ remediation measures. I apologize for my unclear expression, and I have made appropriate modifications to the language and logical relationships in the introduction section.
(3) If I understand correctly, you introduced an artificial filtering array technology, which mainly involves using waste rock piles or other natural filters to treat the leachate containing pollutants discharged from the yard. I have carefully read the two articles you provided as authors, titled “Increasing the Reliability of the Work of Artificial Filtering Arrays for the Purification of Quarry Waste Water” and “The Experience of Using Innovative Artificial Filter Arrays on South Kuzbass Open Pit: Case Study”, and believe that this is a technique suitable for engineering remediation. This technology utilizes suitable natural or waste materials to treat leachate on site. I have added the paper as a reference to the introduction.
Comments 2: It is true that clays, other soils and other materials can be good isolates, but it is difficult to maintain their stability/integrity since waste rock dumps are often dynamic, especially in the early stages of their life. Dynamism leads to subsidence, sliding and, accordingly, a break in the continuity of the insulation. You should cite studies that address these issues to support your conclusions.
Response 2: You mentioned that settlement and sliding often occur during the initial stage of pile accumulation, resulting in incomplete coverage and making it difficult to ensure oxygen isolation performance. Based on your feedback, I have carefully improved the content of Section 3.2. On the basis of supplementing references, I have readjusted the logic of this paragraph. Firstly, I introduced the commonly used surface covering materials for acid pollution in coal gangue piles, and put forward the viewpoint that single structure materials lacked durability affected by multiple factors. Then, suitable solutions have been proposed to address this phenomenon, and the long-term monitoring effect under the action of the solution has been introduced. The capillary barrier layer can cope with the deformation of the stack to a certain extent. In addition, some studies have proposed cover materials that can resist deformation. However, it will not be summarized in this article, because the research is still in its infancy.
Comments 3: The dynamics of the dump can lead to the destruction of a single vegetation cover, due to the formation of a new ravine network, and oxidation processes can begin again.
Response 3: Your suggestion is indeed a problem faced in the vegetation restoration of acidic gangue piles. The deformation caused by natural settlement of the pile and the combustion holes generated by spontaneous combustion of coal gangue can easily lead to poor vegetation restoration results. Therefore, multiple technologies should be scientifically coupled to provide favorable geological conditions and soil fertility for vegetation restoration. Here, I propose the following two solutions. (1) When constructing the surface cover layer, synchronously compacting the gangue pile appropriately can effectively avoid deformation of loose piles; (2) Multiple technologies collaborate to suppress the oxidation of coal gangue mountains, thereby reducing geological instability caused by spontaneous combustion.
Comments 4: Biological remediation should be considered a little more fully as an important step in the formation of a surface acid barrier.
Response 4: Microbial remediation of acidic pollution in coal gangue does have great prospects and is currently widely studied by scholars. The two most representative populations in current research on acid pollution caused by coal gangue dumps are iron reducing bacteria and sulfate reducing bacteria. The microbial technology that exert reducing effects are currently in the early stages of technological maturity and application, lacking on-site application and long-term monitoring. The existing data in the manuscript is basically effective data that can be referenced at present. And this technology is also one of the important research directions of our research group at present. Based on the national key research and development project, we have successfully applied reducing bacteria to demonstration sites in Taiyuan, Shanxi in 2023. In the future, we will also strengthen the monitoring of pollutants and evaluate the on-site application effects of this technology.
Round 2
Reviewer 1 Report
Comments and Suggestions for Authorsthanks,
