Remediation of Acid Mine Drainage in the Haizhou Open-Pit Mine through Coal-Gangue-Loaded SRB Experiments

Round 1

Reviewer 1 Report

(1) In the "2. Materials and Methods" section, provide some photos of the experimental system and samples. And it is recommended to write this section separately according to the experimental equipment, experimental methods, and experimental materials. Besides, please summarize the current research issues and deficiencies in the introduction.

(2) Most of the images in the manuscript are not clear enough, and the authors need to update them. In addition, if each image contains several subgraphs, please provide explanations for each subgraph in the figure title. What physical meaning does the vertical axis represent in Figure 2?

(3) In Figure 1, effect of coal gangue types was explored. The results indicate that there are differences in the treatment effects of coal gangue types on pollutants. What is the reason for this result? That is to say, what mechanism is it? Please explain clearly and make appropriate supplements in the original manuscript.

(4) The quadratic multiple regression model on the removal rate of SO42-, Fe2+, Mn2+ in AMD by coal gangue loaded SRB in Haizhou open-pit mine was obtained. What is the basis for fitting operation? We know that as long as the power is raised, almost all data can be fitted well through polynomials.

(5) The following references should be cited to support the last sentence of page 1. ① Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2022, 8, 82.; ② Natural Resources Research, 2023, https://doi.org/10.1007/s11053-023-10202-7.

(6) At the end of the introduction, it is mentioned in the manuscript that: In order to obtain a convenient, cheap and efficient AMD repair material, and provide a new technology for environmental pollution remediation in mining areas. Through research, has this goal been achieved? I don't see anything in the manuscript that mentions eventually getting a restorative material. please explain.

The language of the manuscript needs to be moderately polished. For example, the last sentence of the first page is confusing.

Author Response

Replies To Reviewer 1:

Thank you very much for your opinions. Combining with your opinions, we have revised the whole article. Your opinions are of great help to the improvement of all aspects of our paper. We marked the revisions in the article with yellow. Thank you very much for your opinion again. Best wishes. The following is our reply to your opinions one by one.

1. In the "2. Materials and Methods" section, provide some photos of the experimental system and samples. And it is recommended to write this section separately according to the experimental equipment, experimental methods, and experimental materials. Besides, please summarize the current research issues and deficiencies in the introduction.

Reply: (1) Thank you for your precious advice. The author had rewritten Section 2 ‘Materials and Methods’, and added some pictures of the samples. Meanwhile, the author made the format and made the experimental instruments, methods, materials separately. The details are shown as follows:

Coal gangue: the spontaneous combustion coal gangue from Gaode mine in Fuxin City (42°0′39.43″N, 121°42′4.85″E), Liaoning Province (1# CG), the spontaneous combustion coal gangue from Haizhou open-pit mine in Fuxin City (42°0′9.90″N, 121°41′12.36″E), Liaoning Province (2# CG), the coal gangue from Haizhou open-pit mine (3# CG) and the coal gangue from a mine in Jincheng City (112°43′E, 35°47′N), Shanxi Province (4# CG) were crushed and screened respectively, soaked with distilled water for three times, and dried at 80℃.

SRB: The wet mud from the lower part of the coal gangue hill in Haizhou Open-pit Mine (42°0′9.90″N, 121°41′12.36″E) was taken as the seed mud, and the SRB was anaerobically cultured using Starkey medium [25,28]. SRB after separation and purification was used in this study. The SRB used in this experiment belongs to Desulfotomaculum, NCBI accession number is MT804386. The main components of Starkey medium were K₂HPO₄ 0.5 g/L, NH₄Cl 1.0 g/L, MgSO₄·7H₂O 2.0 g/L, Na₂SO₄ 0.5 g/L, CaCl₂·H₂O 0.1 g/L, yeast extract 1.0 g/L, sodium lactate 4 mL/L,

(a) Sludge samples taken from the foot of coal gangue mountain	(b) Cultured SRB samples

 (NH₄)₂Fe(SO₄)₂·6H₂O 0.5 g/L, ascorbic acid 0.1 g/L, pH = 7.0.

Figure 1 Sludge samples and cultured SRB samples taken from the foot of coal gangue mountain.

2.2. Test methods

2.3. Test apparatus:
The main instruments used in the experiment were: thermostatic incubator (Changzhou Kaihang Instrument Co., Ltd., Changzhou, China, HZ-9811K type), ultra-clean bench (Suzhou, China, VD-650 type), CO2 anaerobic incubator (Shanghai Likang Co., Ltd., Shanghai, China, HF151 type), portable pressure steam sterilizer (Shanghai Boxun Biomedical Instrument Co., Ltd., Shanghai, China, YXQ-LS-18SI type), electronic balance (Shanghai Ligu Instrument Co., Ltd., Shanghai, China, BS-224-S type). Electric hot blast drying oven (Shanghai Boxun Medical Biological Instrument Co., Ltd., Shanghai, China, GZX-9246MBE type), PHS-3C pH meter (Shanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China), CT-8022 ORP meter (Shanghai Hechen Energy Technology Co., Ltd., Shanghai, China), V-1600PC visible spectrophotometer (Shanghai Yuanwang Liquid Level Meter Co., Ltd., Shanghai, China) and Z-2000 flame atomic spectrophotometer (Hitachi Co., Japan) and other instruments.

(2) The author added some contents in the ‘Introduction’ section, the details are shown as follows:

Acid mine drainage (AMD) is an acidic wastewater formed by the oxidation, weathering, rain and other physical and chemical effects of sulfur-containing minerals in mining areas. AMD contains high concentrations of SO₄^2-, Fe²⁺, Mn²⁺ and Al³⁺, which is the main environmental pollution problem faced by the global mining industry [1-3]. In addition, the instability of the water environment during the mining process can easily lead to geological disasters [4,5]. Especially, the exploitation of the Peșteana open-pit mine in Romania was affected by water injection speed, lasting time, AMD and other environmental factors, and it’s easy to cause geological disasters such as floods [6]. At present, the main methods to repair AMD include neutralization method, adsorption method, biological method and coagulation method [7,8]. The adsorption method is to adsorb the ions in AMD on the surface of porous materials. The adsorption effect on metal ions is good, but the adsorption effect on SO₄^2- is poor. Therefore, the repair of AMD containing SO₄^2-, Fe²⁺, Mn²⁺ and other metal ions is a serious challenge in the field of environmental treatment in the mining area [9]. In particular, it is necessary to focus on the remediation of AMD (including SO₄^2-, Fe²⁺ and Mn²⁺) pollution in the water environment of the mining area. Finding a cheap and efficient adsorption material is the key of mine water environment remediation.

…..

Above all, coal gangue can be a kind of absorbing material, which could remedy the Fe²⁺, Mn²⁺ pollution in AMD, but with poor repairing effect on SO₄^2- in AMD. Aiming at the poor remediation effect on SO₄^2- in AMD by coal gangue, we could combine the coal gangue with other materials. SRB in microbial method can reduce SO₄^2- to sulfide [26] under anaerobic conditions, so as to achieve the purpose of repairing SO₄^2- pollution in AMD. Xianjun Wang et al. [27] showed that SRB can effectively remove SO₄^2- in AMD and improve the pH value of AMD. Junzhen Di et al. [28] showed that SRB can efficiently repair AMD pollution and metabolize SO₄^2- to form metal sulfide precipitation. However, SRB is vulnerable to environmental pH value and concentration of heavy metal ions [29]. Therefore, the combination of coal gangue and SRB can not only effectively remove metal ions in AMD, but also remove SO₄^2-, reducing environmental pollution of AMD. Coal gangue loaded SRB can not only improve the problem of poor removal of SO₄^2- in AMD by coal gangue, but also can decrease the adverse influence of surrounding environment on SRB growth by using the adsorption performance of coal gangue. At the same time, the coal gangue in this technology is the solid waste of the mining area, which is easy to obtain. AMD is a kind of polluted wastewater in mining area. Remediation of AMD by coal gangue loaded SRB can achieve the effect of ‘treating waste with waste’. Coal gangue loaded SRB is a convenient and cheap AMD repairing material.

 

2. Most of the images in the manuscript are not clear enough, and the authors need to update them. In addition, if each image contains several subgraphs, please provide explanations for each subgraph in the figure title. What physical meaning does the vertical axis represent in Figure 2?

Reply: Thank you for your precious advice. The author had changed all the figures into more clarity version. At the same time, the author added the explanations for each figure. The details are marked in color shown in the new version of the manuscript we submitted.

 

3. In Figure 1, effect of coal gangue types was explored. The results indicate that there are differences in the treatment effects of coal gangue types on pollutants. What is the reason for this result? That is to say, what mechanism is it? Please explain clearly and make appropriate supplements in the original manuscript.

Reply: Thank you for your advice for improving our paper. The author added the reason for the difference in the treatment effect of coal gangue types on pollutants. The details are shown as follows:

Previous studies have shown that the sulfur content in 1# CG and 4# CG was high, and the pH of 1# CG and 4# CG leachate was acidic, which is not conducive to the initial growth of SRB [34]. At the same time, previous studies have also shown that 2# CG and 3# CG contained more alkali metal compounds such as K, Na, Ca and Mg, and the release of some soluble alkaline substances made the leaching solution of 2# CG and 3# CG alkaline. The alkalinity of 3 # CG leachate was stronger than that of 2# CG [34]. The alkaline environment of 3# CG leaching solution was more conducive to the growth and propagation of SRB, which made the pH value of the effluent after 3# CG loaded SRB repaired AMD increased faster, the ORP value decreased faster, and the SO₄^2- removal was more obvious. According to the report of Kyoungkeun, the decrease of ORP value can reflect the activity of SRB and the efficiency of SRB metabolizing SO₄^2- [35]. The increase of pH value, the decrease of ORP value and the obvious removal of SO₄^2- can indicate that SRB lives well in 3# CG environment. Combined with the removal rate of SO₄^2-, Fe²⁺ and Mn²⁺, the overall effect of four kinds of coal gangue loaded with SRB to remove SO₄^2-, Fe²⁺ and Mn²⁺ was: 3# CG loaded SRB > 2# CG loaded SRB > 1# CG loaded SRB > 4# CG loaded SRB. Making comprehensive analysis of the effluent change of pH value, ORP value, and SO₄^2- removal rate, Fe²⁺ removal rate, Mn²⁺ removal rate after the four kinds of coal gangue loaded SRB to repairing AMD pollution Therefore, 3# CG was selected as the research material for subsequent tests.

 

4. The quadratic multiple regression model on the removal rate of SO₄^2-, Fe²⁺, Mn²⁺ in AMD by coal gangue loaded SRB in Haizhou open-pit mine was obtained. What is the basis for fitting operation? We know that as long as the power is raised, almost all data can be fitted well through polynomials.

Reply: Thank you for your precious advice. The author used the quadratic multiple regression model of the response surface test analysis software (Design-Expert 8.0) to fit the data of this study, and obtained the quadratic multiple regression model of the removal rate of SO₄^2-, Fe²⁺ and Mn²⁺ in AMD by coal gangue loaded SRB in Haizhou open-pit mine. Design-Expert 8.0 software will analyze the experimental results of the response surface based on the data, and the analysis results are quadratic multiple regression models with different coefficients. Combined with your advice the author modified the paper. The revised version is shown as follows:

The experimental parameters in Table 1 were evaluated by the quadratic multiple regression model of Design-Expert 8.0 software. The quadratic multiple regression model between the response value and the factors of the Design-expert 8.0 software is:

In the formula above: Y represents response value. β0 represents offset migration coefficient. βi represents independent variable linear offset coefficient. βii represents second-order offset coefficient of independent variable. βij represents independent variable interaction effect coefficient. Xi, Xj, XiXj represents level values of each factor were used to analyze the main effects and interaction effects of each factor.

Through the above Design-Expert 8.0 software model, fit and get the quadratic multiple regression model on the removal rate of SO₄^2-, Fe²⁺, Mn²⁺ in AMD by coal gangue loaded SRB in Haizhou open-pit mine were shown as follows:

 

5. The following references should be cited to support the last sentence of page 1.

① Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2022, 8, 82.; ②Natural Resources Research, 2023, https://doi.org/10.1007/s11053-023-10202-7.

Reply: Thank you for your precious advice, the author cited the two papers above in the ‘Introduction’ section. The revised version is shown as follows:

Acid mine drainage (AMD) is an acidic wastewater formed by the oxidation, weathering, rain and other physical and chemical effects of sulfur-containing minerals in mining areas. AMD contains high concentrations of SO₄^2-, Fe²⁺, Mn²⁺ and Al³⁺, which is the main environmental pollution problem faced by the global mining industry [1-3]. In addition, the instability of the water environment during the mining process can easily lead to geological disasters [4,5]. Especially, the exploitation of the Peșteana open-pit mine in Romania was affected by water injection speed, lasting time, AMD and other environmental factors, and it’s easy to cause geological disasters such as floods [6]. At present, the main methods to repair AMD include neutralization method, adsorption method, biological method and coagulation method [7,8]. The adsorption method is to adsorb the ions in AMD on the surface of porous materials. The adsorption effect on metal ions is good, but the adsorption effect on SO₄^2- is poor. Therefore, the repair of AMD containing SO₄^2-, Fe²⁺, Mn²⁺ and other metal ions is a serious challenge in the field of environmental treatment in the mining area [9]. In particular, it is necessary to focus on the remediation of AMD (including SO₄^2-, Fe²⁺ and Mn²⁺) pollution in the water environment of the mining area. Finding a cheap and efficient adsorption material is the key of mine water environment remediation.

 

6. At the end of the introduction, it is mentioned in the manuscript that: In order to obtain a convenient, cheap and efficient AMD repair material, and provide a new technology for environmental pollution remediation in mining areas. Through research, has this goal been achieved? I don't see anything in the manuscript that mentions eventually getting a restorative material. please explain.

Reply: Thank you for your precious advice. We achieved this goal by studying. Combined with your advice, we modified our ‘conclusion’ section. The details are shown as follows:

Aiming at the pollution problem of acid mine drainage (AMD) with high content of Fe²⁺, Mn²⁺ and SO₄^2-. In this study, coal gangue and SRB were used as materials to prepare coal gangue loaded SRB materials, which solved the pollution problem of coal mine acid wastewater with high content of Fe²⁺, Mn²⁺ and SO₄^2-. Coal gangue loaded SRB can not only improve the problem of poor SO₄^2- removal in AMD by coal gangue, but also reduce the adverse influence of surrounding environment on SRB growth by using the adsorption performance of coal gangue. At the same time, the coal gangue in this technology is the solid waste of the mining area, which is easy to obtain. AMD is a kind of polluted wastewater in mining area. Remediation of AMD by coal gangue loaded SRB can achieve the effect of 'treating waste with waste'. Coal gangue loaded SRB is a convenient, cheap and efficient AMD repairing material. The remediation technology of coal gangue loaded SRB for other types of contaminated wastewater still needs to be further explored in the future.

 

Author Response File: Author Response.docx

Reviewer 2 Report

 

1.     Did you use SRB directly with microorganisms in wet mud during the experiment or did you separate SRB? If all strains of bacteria in wet mud were used together without separation, how did you determine that SRB was working instead of other strains of bacteria? Also, if you separated SRB from wet mud, is your SRB a mixed strain or a single strain? If it is a mixed strain, how do you determine which microorganism is more effective? If you have a single strain of bacteria, please write out the process of purifying the bacterial strain, how to identify it, and write out the name of the bacterial strain. It is recommended that you rewrite the SRB part according to the questions mentioned earlier.

2.     The article only mentions the code name of the analytical instrument. Please write out the full name of the analytical instrument.

3.     Did you buy Starkey medium directly or configure Starkey medium yourself? If you bought Starkey medium directly, you should write down the name of the manufacturer. If you configured Starkey medium yourself, please write down the complete formula.

4.     The author should indicate the location of the wet mud containing the strains on a map, and the distribution maps of each mining area should also be drawn.

5.     In the removal rate formula, A, B, C are not mentioned by the author. The author should clearly write down the definition of A, B, C.

6.     The author should write down the formulas for the kinetic equation and isothermal equation.

7.     Most of the figures in the article are blurry, and the author should improve the clarity of these figures.

8.     The author’s conclusion is that SRB combined with 3# CG has the best effect because the wet mud containing the strains comes from 3# CG. The author should collect strains from 1# CG, 2# CG and 4# CG for experimentation and comparison in order to make the conclusion that SRB combined with 3# CG has the best effect. Please supplement this part of the research and analysis.

 

In my opinion, this article can be published in this journal if the authors explain all the unclear parts and offer enough new data. Also, the authors should recheck and correct the errors of typos and grammar before sending out this manuscript. By all the aspects enumerated above, the author should make a major revision of the paper before its publication.

 

Comments for author File: Comments.pdf

The authors should recheck and correct the errors of typos and grammar before sending out this manuscript.

Author Response

Replies To Reviewer 2:

Thank you very much for your opinions. Combining with your opinions, we have revised the whole article. Your opinions are of great help to the improvement of all aspects of our paper. We marked the revisions in the article with yellow. Thank you very much for your opinion again. Best wishes. The following is our reply to your opinions one by one.

1. Did you use SRB directly with microorganisms in wet mud during the experiment or did you separate SRB? If all strains of bacteria in wet mud were used together without separation, how did you determine that SRB was working instead of other strains of bacteria? Also, if you separated SRB from wet mud, is your SRB a mixed strain or a single strain? If it is a mixed strain, how do you determine which microorganism is more effective? If you have a single strain of bacteria, please write out the process of purifying the bacterial strain, how to identify it, and write out the name of the bacterial strain. It is recommended that you rewrite the SRB part according to the questions mentioned earlier.

Reply: Thank you for your precious advice. The SRB used in this paper was a single strain. Combined with your advice, the author added some contents in ‘Materials and Methods’ section. The details are shown as follows:

SRB: The wet mud from the lower part of the coal gangue hill in Haizhou Open-pit Mine (42°0′9.90″N, 121°41′12.36″E) was taken as the seed mud, and the SRB was anaerobically cultured using Starkey medium [25,28]. SRB after separation and purification was used in this study. The SRB used in this experiment belongs to Desulfotomaculum, NCBI accession number is MT804386.

 

2. The article only mentions the code name of the analytical instrument. Please write out the full name of the analytical instrument.

Reply: Thank you for your precious advice. Combined with your advice, the author added the full name of the experimental instruments in ‘Materials and Methods’ section. The details are shown as follows:

2.3. Test apparatus:
The main instruments used in the experiment were: thermostatic incubator (Changzhou Kaihang Instrument Co., Ltd., Changzhou, China, HZ-9811K type), ultra-clean bench (Suzhou, China, VD-650 type), CO2 anaerobic incubator (Shanghai Likang Co., Ltd., Shanghai, China, HF151 type), portable pressure steam sterilizer (Shanghai Boxun Biomedical Instrument Co., Ltd., Shanghai, China, YXQ-LS-18SI type), electronic balance (Shanghai Ligu Instrument Co., Ltd., Shanghai, China, BS-224-S type). Electric hot blast drying oven (Shanghai Boxun Medical Biological Instrument Co., Ltd., Shanghai, China, GZX-9246MBE type), PHS-3C pH meter (Shanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China), CT-8022 ORP meter (Shanghai Hechen Energy Technology Co., Ltd., Shanghai, China), V-1600PC visible spectrophotometer (Shanghai Yuanwang Liquid Level Meter Co., Ltd., Shanghai, China) and Z-2000 flame atomic spectrophotometer (Hitachi Co., Japan) and other instruments.

 

3. Did you buy Starkey medium directly or configure Starkey medium yourself? If you bought Starkey medium directly, you should write down the name of the manufacturer. If you configured Starkey medium yourself, please write down the complete formula.

Reply: Thank you for your precious advice. The medium used in this paper was configured by the author. And the composition of the medium was added in ‘Materials and Methods’ section, which is also shown as follows:

The main components of Starkey medium were K₂HPO₄ 0.5 g/L, NH₄Cl 1.0 g/L, MgSO₄·7H₂O 2.0 g/L, Na₂SO₄ 0.5 g/L, CaCl₂·H₂O 0.1 g/L, yeast extract 1.0 g/L, sodium lactate 4 mL/L, (NH₄)₂Fe(SO₄)₂·6H₂O 0.5 g/L, ascorbic acid 0.1 g/L, pH = 7.0.

 

4. The author should indicate the location of the wet mud containing the strains on a map, and the distribution maps of each mining area should also be drawn.

Reply: Thank you for your precious advice. Combined with your advice, the author provided the latitude and longitude of sampling points for each material. The details are shown as follows:

Coal gangue: the spontaneous combustion coal gangue from Gaode mine in Fuxin City (42°0′39.43″N, 121°42′4.85″E), Liaoning Province (1# CG), the spontaneous combustion coal gangue from Haizhou open-pit mine in Fuxin City (42°0′9.90″N, 121°41′12.36″E), Liaoning Province (2# CG), the coal gangue from Haizhou open-pit mine (3# CG) and the coal gangue from a mine in Jincheng City (112°43′E, 35°47′N), Shanxi Province (4# CG) were crushed and screened respectively, soaked with distilled water for three times, and dried at 80℃.

SRB: The wet mud from the lower part of the coal gangue hill in Haizhou Open-pit Mine (42°0′9.90″N, 121°41′12.36″E) was taken as the seed mud, and the SRB was anaerobically cultured using Starkey medium [25,28].

 

5. In the removal rate formula, A, B, C are not mentioned by the author. The author should clearly write down the definition of A, B, C.

Reply: Thank you for your precious advice. Combined with your advice, the author added the definition of A, B, C after the formula. The revised version is shown as follows:

SO₄^2- removal rate (%) = 88.16 + 3.42 × A + 4.62 × B + 3.97 × C - 1.48 × A × B + 4.07 × A × C + 2.66 × B × C - 9.35 × A² - 5.76×B² - 8.37×C², R²=0.9730.

Fe²⁺ removal rate (%) = 98.81 + 1.39 × A + 2.14 × B + 1.82 × C - 0.44 × A × B + 0.87 × A × C - 0.59 × B × C - 3.42 × A² - 0.45×B² - 1.72 × C², R²=0.8982.

Mn²⁺ removal rate (%) = 86.45 + 0.77 × A + 1.27 × B + 7.07 × C - 0.51 × A × B - 2.62 × A × C + 0.64 × B × C - 0.66 × A² - 4.86 × B² - 10.53 × C², R²=0.9565.

In the formula above, A represents coal gangue dosage whose unite is g, B represents SRB inoculation amount whose unit is %, C represents temperature whose unit is ℃.

 

6. The author should write down the formulas for the kinetic equation and isothermal equation.

Reply: Thank you for your precious advice. Combined with your advice, the author has supplemented the formulas for the kinetic equation and isothermal equation. The details are shown as follows:

(1) Langmuir and Freundlich adsorption isotherm models were used to fit the Fe²⁺ and Mn²⁺ adsorption by coal gangue loaded SRB in Haizhou open-pit mine, and the fitting results were shown in Figure 4.

Langmuir adsorption model:

Freundlich adsorption model:

In the formula above, q_e represents the equilibrium adsorption capacity, and its unit is mg/g. c_e represents equilibrium concentration, and its unit is mg/L. k_L is Langmuir adsorption constant. q_m is the saturated adsorption capacity. k_f is Freundlich adsorption equilibrium constant, and it is related to the type and characteristics of adsorbents. n is Freundlich adsorption equilibrium constant.

(2) Lagergren's first-order adsorption kinetic equation, Lagergren's second-order adsorption kinetic equation, origin 2023 software and particle expansion model were used to analyze the adsorption kinetic process of Fe²⁺ and Mn²⁺ by coal gangue loaded SRB in Haizhou open-pit mine. The results are shown in Figure 6 and Table 2.

Lagergren first-order adsorption kinetic model: ln(q_e – q_t) = lnq_e – k₁t

and the formula can be changed into: q_t = q_e(1 - e^-k₁^t)

Lagergren second-order adsorption kinetic model:

Intraparticle diffusion model: q_t = k₃ · t^1/2 + c

In the formula above, q_t represents the adsorption capacity at time t, mg/L; t represents adsorption time, min; k₁ represents first-order kinetic rate constant, 1/min; k₂ represents second-order kinetic rate constant second-order kinetic rate constant, mg/(g·min); k₃ represents intraparticle diffusion rate constant, mg/(g·min^1/2).

(3) Adopting the fitting models of zero-order kinetics and first-order kinetics respectively, use Origin 2023 software and analyse the reduction process of SO₄^2- by coal gangue loaded SRB in Haizhou open-pit mine, and the results was shown in Figure 7.

Zero-order dynamic model: c_t = c₀ – k₀ · t

First-order kinetic fitting model: lnc_t = lnc₀ – k’₁ · t

In the formula above, c₀ represents initial SO₄^2- concentration, mg/L. c_t represents the concentration of SO₄^2- is at any time, mg/L. k₀ represents the rate constant of zero order reaction, mg/(L min). k’₁ represents the rate constant of the first order reaction, 1/min.

 

7. Most of the figures in the article are blurry, and the author should improve the clarity of these figures.

Reply: Thank you for your precious advice. The author had changed all the figures into more clarity version. At the same time, the author added the explanations for each figure. The details are marked in color shown in the new version of the manuscript we submitted.

 

8. The author’s conclusion is that SRB combined with 3# CG has the best effect because the wet mud containing the strains comes from 3# CG. The author should collect strains from 1# CG, 2# CG and 4# CG for experimentation and comparison in order to make the conclusion that SRB combined with 3# CG has the best effect. Please supplement this part of the research and analysis.

Reply: Thank you for your precious advice. The author’s conclusion is: The effect of SRB combined with 3 # CG is the best, and the wet mud containing bacteria should not come from 3 # CG. 2 # CG and 3 # CG are coal gangue from Haizhou open-pit mine (latitude N:42°0′9.90′′, longitude E:121°41′12.36′′). Among them, 2#CG is the spontaneous combustion coal gangue of Haizhou open-pit mine, 3#CG is the coal gangue of Haizhou open-pit mine. From the figures in ‘3.1. Analysis of single factor test results’ section we know that, the repairing effect on AMD by 2#CG loaded SRB and 3#CG loaded SRB is very different. It shows that under the condition that the wet mud sampling point containing bacteria is consistent with the coal gangue sampling point, different types of coal gangue have different effects on the treatment of AMD by coal gangue loaded SRB. Combined with your advice, the author added some contents in ‘3.1. Analysis of single factor test results’ section, which are shown as follows:

Previous studies have shown that the sulfur content in 1# CG and 4# CG was high, and the pH of 1# CG and 4# CG leachate was acidic, which is not conducive to the initial growth of SRB [34]. At the same time, previous studies have also shown that 2# CG and 3# CG contained more alkali metal compounds such as K, Na, Ca and Mg, and the release of some soluble alkaline substances made the leaching solution of 2# CG and 3# CG alkaline. The alkalinity of 3 # CG leachate was stronger than that of 2# CG [34]. The alkaline environment of 3# CG leaching solution was more conducive to the growth and propagation of SRB, which made the pH value of the effluent after 3# CG loaded SRB repaired AMD increased faster, the ORP value decreased faster, and the SO₄^2- removal was more obvious. According to the report of Kyoungkeun, the decrease of ORP value can reflect the activity of SRB and the efficiency of SRB metabolizing SO₄^2- [35]. The increase of pH value, the decrease of ORP value and the obvious removal of SO₄^2- can indicate that SRB lives well in 3# CG environment. Combined with the removal rate of SO₄^2-, Fe²⁺ and Mn²⁺, the overall effect of four kinds of coal gangue loaded with SRB to remove SO₄^2-, Fe²⁺ and Mn²⁺ was: 3# CG loaded SRB > 2# CG loaded SRB > 1# CG loaded SRB > 4# CG loaded SRB. Making comprehensive analysis of the effluent change of pH value, ORP value, and SO₄^2- removal rate, Fe²⁺ removal rate, Mn²⁺ removal rate after the four kinds of coal gangue loaded SRB to repairing AMD pollution Therefore, 3# CG was selected as the research material for subsequent tests.

 

Author Response File: Author Response.docx

Reviewer 3 Report

Issue 1

The paper discusses a method of treating acid mine drainage, which is a type of polluted water containing high levels of Fe2+, Mn2+, and SO42-. The authors combined coal gangue with sulfate-reducing bacteria to remediate the polluted water. They conducted experiments to determine the optimal conditions for the treatment process and to understand the mechanism behind the removal of pollutants. Results showed that coal gangue loaded with sulfate reducing bacteria was effective in removing pollutants from acid mine drainage, and the optimal conditions for the treatment process were determined. The study also revealed the factors that affect the removal of different pollutants and the kinetic models that describe the removal process.

This study is important because acid mine drainage (AMD) is a significant environmental issue that can cause severe pollution and harm to aquatic life. AMD is often highly acidic and contains high levels of heavy metals, which can cause long-term damage to the environment if not treated properly. This study presents a novel approach using coal gangue and sulfate reducing bacteria to remediate AMD, which is a cost-effective and environmentally friendly solution. The study also provides insights into the mechanism behind the removal of pollutants, which can inform the development of future treatment technologies. The findings of this study have the potential to contribute to the development of sustainable solutions for the remediation of AMD and the protection of the environment.

 

Issue 2

The introductory section of this paper requires improvement in order to better engage and inform the reader about the topic at hand.

 

Issue 3

Important issues must be discussed in your research (indicate lines):

1. What is the optimal coal gangue dosage, SRB inoculation amount, and temperature for the remediation of acid mine drainage (AMD) using coal gangue loaded SRB in Haizhou Open-Pit Mine?

2. What is the order of factors that affect the removal of SO42-, Fe2+, and Mn2+ from AMD by SRB loaded on coal gangue?

3. How effective is the coal gangue loaded with SRB in repairing AMD in Haizhou Open-Pit Mine compared to other types of coal gangue loaded with SRB?

4. What is the removal mechanism of SO42-, Fe2+, and Mn2+ in AMD by coal gangue loaded SRB in Haizhou Open-Pit Mine?

5. Which kinetic models describe the adsorption and reduction kinetics of Fe2+, Mn2+, and SO42- by coal gangue loaded SRB in Haizhou Open-Pit Mine during the remediation of AMD?

 

Issue 4

Methodology:

1. How did the researchers determine the optimal conditions for the remediation of AMD using coal gangue loaded SRB in Haizhou Open-Pit Mine?

2. What type of experiments were conducted to understand the removal mechanism of SO42-, Fe2+, and Mn2+ in AMD by coal gangue loaded SRB in Haizhou Open-Pit Mine?

3. What models were used to describe the adsorption and reduction kinetics of Fe2+, Mn2+, and SO42- by coal gangue loaded SRB in Haizhou Open-Pit Mine during the remediation of AMD, and how were these models derived?

 

Issue 5

The literature review is presented quite well, but includes mainly the experience of authors from China. I strongly recommend that you consider the works of other authors.

Please consider below mentioned paper.

Lazar, M.; Apostu, I.; Faur, F.; Rotunjanu, I. Factors influencing the flooding process of former coal open-pits. Min. Miner. Depos., 2021, 15, 124-133. https://doi.org/10.33271/mining15.02.124 I believe it worth considering in your paper as it present an important issues regarding flooding. It is very important factor but not discussed in your research. What is or can be the impact of the open-pit flooding on acid mine drainage?

 

Issue 6

Please provide a short description of further research.

 

 Issue 7

In the conclusion section of the paper, it is crucial to highlight the uniqueness and importance of the research. Doing so will help readers comprehend the study's originality and potential impact on future research and practical applications. It is, therefore, necessary to clearly and concisely convey the innovative elements of the research in the conclusion section of the paper.

 

 Issue 8

 

The article reviewed has made a positive impression, and after taking into account the recommended revisions and suggestions, it will be recommended for publication in the "Sustainability" journal.

Author Response

Replies To Reviewer 3:

Thank you very much for your opinions. Combining with your opinions, we have revised the whole article. Your opinions are of great help to the improvement of all aspects of our paper. We marked the revisions in the article with yellow. Thank you very much for your opinion again. Best wishes. The following is our reply to your opinions one by one.

Issue 2

The introductory section of this paper requires improvement in order to better engage and inform the reader about the topic at hand.

Reply: Thank you for your precious advice. The author has modified the ‘Introduction’ section in the paper. The revised version is shown as bellow:

……In addition, the instability of the water environment during the mining process can easily lead to geological disasters [4,5]. Especially, the exploitation of the Peșteana open-pit mine in Romania was affected by water injection speed, lasting time, AMD and other environmental factors, and it’s easy to cause geological disasters such as floods [6].

……In particular, it is necessary to focus on the remediation of AMD (including SO₄^2-, Fe²⁺ and Mn²⁺) pollution in the water environment of the mining area.

……Above all, coal gangue can be a kind of absorbing material, which could remedy the Fe²⁺, Mn²⁺ pollution in AMD, but with poor repairing effect on SO₄^2- in AMD. Aiming at the poor remediation effect on SO₄^2- in AMD by coal gangue, we could combine the coal gangue with other materials.

……Coal gangue loaded SRB can not only improve the problem of poor removal of SO₄^2- in AMD by coal gangue, but also can decrease the adverse influence of surrounding environment on SRB growth by using the adsorption performance of coal gangue. At the same time, the coal gangue in this technology is the solid waste of the mining area, which is easy to obtain. AMD is a kind of polluted wastewater in mining area. Remediation of AMD by coal gangue loaded SRB can achieve the effect of ‘treating waste with waste’. Coal gangue loaded SRB is a convenient and cheap AMD repairing material.

 

Issue 3

Important issues must be discussed in your research (indicate lines):

1. What is the optimal coal gangue dosage, SRB inoculation amount, and temperature for the remediation of acid mine drainage (AMD) using coal gangue loaded SRB in Haizhou Open-Pit Mine?
2. What is the order of factors that affect the removal of SO₄^2-, Fe²⁺, and Mn²⁺ from AMD by SRB loaded on coal gangue?
3. How effective is the coal gangue loaded with SRB in repairing AMD in Haizhou Open-Pit Mine compared to other types of coal gangue loaded with SRB?
4. What is the removal mechanism of SO₄^2-, Fe²⁺, and Mn²⁺ in AMD by coal gangue loaded SRB in Haizhou Open-Pit Mine?
5. Which kinetic models describe the adsorption and reduction kinetics of Fe²⁺, Mn²⁺, and SO₄^2- by coal gangue loaded SRB in Haizhou Open-Pit Mine during the remediation of AMD?

Reply: Thank you for your advice. Combined with all your suggestions, the author added ‘3.4 Discussion’ in section ‘3 Results and Discussion’. The details are shown as bellow:

3.4. Discussion

AMD formed by mining activities has the most serious impact on the water environment of the mining area. In particular, open-pit mining has caused serious damage to water, soil and plants in the mining area, resulting in large-scale diffusion of pollutants in AMD in the mining area. In this study, coal gangue and SRB were used as materials to prepare coal gangue loaded SRB materials by combining adsorption and microbial methods to solve the pollution problem of coal mine acid wastewater with high content of Fe²⁺, Mn²⁺ and SO₄^2-. As a waste, coal gangue can be used to remove heavy metals from water mixtures, but its adsorption capacity is not high. Therefore, many scholars have improved the adsorption capacity of coal gangue to heavy metals by means of modification and compounding, and have achieved outstanding results. Ramin Mohammadi prepared alginate-combustion coal gangue (ACCG) composites. The maximum adsorption capacities of ACCG for zinc and manganese were 77.68 mg/g and 64.29 mg/g, respectively. The adsorption process followed the Langmuir isotherm model and the intraparticle diffusion model [42]. Gao Xiang et al. used coal gangue as the main raw material, chitosan and calcined coal gangue were mixed to prepare ceramsite, and the removal rate of Cr (VI) by ceramsite can reach 75.6% [43]. Zhanhu Zhang et al. [44] prepared the modified coal gangue adsorbent by calcination-chelator modification. Under the condition of adsorption time of 2h and dosage of 0.8 g/L, the optimum removal rates of 50 mg/L Cu²⁺ and Pb²⁺ in mineral processing wastewater were 69.34 % and 79.98 %, respectively. An adsorbent analcime-activated carbon (ANA-AC) was synthesized from coal gangue. The adsorption of Pb²⁺ in solution by ANA-AC conformed to Langmuir isotherm model and pseudo-second-order kinetic model [45]. Qizheng Qin et al. [46] used coal gangue as raw material, and coal gangue was modified based on low temperature combined roasting-dewatering leaching method. The adsorption capacity of modified coal gangue adsorption material for lead ion was higher than that of original coal gangue and kaolinite. Cheng Wang et al. [47] used coal gangue as raw material, synthesizing sodium silicate by hydrothermal method. The maximum adsorption capacities of sodium silicate for Cd²⁺ and methylene-blue were 60 mg/g and 17.3 mg/g, respectively. Above all, the adsorption capacity of coal gangue to heavy metals can be improved by modification and compounding. Compared with the above research, the coal gangue loaded SRB material proposed in this study can not only remove metal ions such as Fe²⁺, Mn²⁺, but also can remove SO₄^2-. Especially in AMD, such a special pollution of wastewater where the SO₄^2- is higher. Therefore, coal gangue loaded SRB can not only improve the problem of poor removal of SO₄^2- in AMD by coal gangue, but also reduce the adverse influence of surrounding environment on SRB growth by using the adsorption performance of coal gangue, and improve the removal effect of heavy metals.

Aiming at the treatment of wastewater containing iron and manganese, some scholars have analyzed the treating effect of various adsorption materials on iron and manganese by using coal gangue combined with other materials, bentonite and new composite materials. Liping Zhang et al. [48] showed that the use of coal gangue, sandy soil and clay (mass ratio 45:4:1) as goaf filling material can effectively remove iron and manganese in coal mining drainage. The maximum adsorption capacity of iron and manganese were 163.79 μg and 15.25 μg, respectively. Bentonite can adsorb and remove Fe (II). The maximum removal rate of Fe (II) by 0.5 g bentonite was more than 98% [49]. Maria K. Doula synthesized a high surface area clinoptilolite-iron oxide system (Clin-Fe) using Clinoptilolite, and found that the adsorption capacity of Clinoptilolite for Mn was 7.69 mg/g, while the adsorption capacity of Clin-Fe for Mn was 27.12 mg/g [50]. Ibrahim M. El-Sherbiny et al. [51] combined sodium alginate with different proportions of calcareous soil to form new composite microparticles (NCM). The removal rate of 0.5-16.0 mg/L Fe by NCM was almost 100 %, and the removal rate of 0.5 mg/L Mn (II) was about 89%. Xuying Guo et al. [24,25] showed that the dosage of spontaneous combustion coal gangue is 8 g/100 mL, which can adsorb Fe²⁺ and Mn²⁺ in AMD. The removal efficiency of Fe²⁺ and Mn²⁺ in AMD by NaOH modified spontaneous combustion coal gangue was obviously better than that of spontaneous combustion coal gangue. The average removal rates of Fe²⁺ and Mn²⁺ by NaOH modified spontaneous combustion coal gangue were 97.73% and 44.82%, respectively [24,25]. Compared with the above research, the removal rate of SO₄^2-, Fe²⁺ and Mn²⁺ by coal gangue loaded SRB material in Haizhou open-pit mine can be maintained at a high level. Under the conditions of coal gangue dosage of 52 g, SRB inoculation amount of 11.7% and temperature of 33-34°C, the removal rates of SO₄^2-, Fe²⁺ and Mn²⁺ in AMD by coal gangue loaded SRB were 89.76 %, 99.66 % and 87.95% after treating AMD for 4 days. Zhuxiang Liu reported that under the condition of initial pH=6.0 and initial SO₄^2- concentration of 2000 mg/L, the removal efficiency of Fe by SRB with expanded granular sludge bed reactor was 95.2-100% [52]. Mingliang Zhang et al. [29] reported that the maximum reduction rate of sulfate in AMD by SRB immobilized particles prepared with SRB as the main component was 2.67 g/(L·d). It was difficult to remove manganese in AMD, and the manganese removal efficiency was only 42%-99%. Compared with the above studies on the treatment of AMD by SRB, the process of building an expanded granular sludge bed reactor and preparing SRB immobilized particles was relatively complicated. The coal gangue loaded SRB material prepared in this study only needed to mix coal gangue with microorganisms in a certain proportion, and the preparation process was relatively simple. In summary, coal gangue loaded SRB can achieve the effect of ‘treating waste with waste’, which is a kind of convenient and cheap AMD repair material. This study only focused on the removal effect of SO₄^2-, Fe²⁺ and Mn²⁺ by coal gangue loaded SRB, and the removal effect of other heavy metals in wastewater by coal gangue loaded SRB will be further analyzed in the future. At the same time, the application effect of coal gangue loaded SRB material in other industrial fields is analyzed, and the application range of coal gangue loaded SRB material is further promoted

 

Issue 4

Methodology:

1. How did the researchers determine the optimal conditions for the remediation of AMD using coal gangue loaded SRB in Haizhou Open-Pit Mine?

Reply: Thank you for your advice. The author made response surface test based on the single factor test results. Response surface experiment is a statistical experimental design for optimizing biological processes. This method is used to establish a continuous variable surface model, and evaluate the factors affecting biological processes and their interactions, and determine the optimal level range. The number of test groups required is relatively small, which can save manpower and material resources. Therefore, this method has been successfully applied to various biological process optimization.

In this study, the response surface test was used to determine the optimal conditions for the repair of AMD by coal gangue loaded SRB in Haizhou Open-pit Mine: adding 3#CG 50 g, inoculating 10% of SRB, at 35°C, 100 r/min.

The author showed that RSM can assure the optimum conditions in section ‘2 Materials and Methods’. The details are shown as follows:

“Response surface methodology (RSM): Based on complete randomolized design results, three factors, the amount of coal gangue, the amount of SRB inoculation and the culture temperature, were selected, combining with the removal rate changes of SO₄^2-, Fe²⁺, Mn²⁺ after repairing the AMD for 4 d by coal gangue loaded SRB in Haizhou open-pit mine, assuring the optimum conditions of repairing AMD by coal gangue loaded SRB in Haizhou open-pit mine.”

 

2. What type of experiments were conducted to understand the removal mechanism of SO₄^2-, Fe²⁺, and Mn²⁺ in AMD by coal gangue loaded SRB in Haizhou Open-Pit Mine?

Reply: Thank you for your advice. Combined with you advice, the author modified the section ‘Materials and Methods’, and the revised version is shown as below:

To reveal the mechanism of Haizhou open-pit coal gangue loaded SRB repairing Fe²⁺, Mn²⁺, SO₄^2- in AMD, we made adsorption isotherm test of the Haizhou open-pit coal gangue loaded SRB repairing Fe²⁺, Mn²⁺, adsorption kinetics test, and reduction kinetics test of Haizhou open-pit coal gangue load SRB repairing SO₄^2- in AMD, respectively. The specific methods are as follows:

 

3. What models were used to describe the adsorption and reduction kinetics of Fe²⁺, Mn²⁺, and SO₄^2- by coal gangue loaded SRB in Haizhou Open-Pit Mine during the remediation of AMD, and how were these models derived?

Reply: Thank you for your precious advice. Combined with your advice, the author has supplemented the formulas for the kinetic equation and isothermal equation. The details are shown as follows:

(1) Langmuir and Freundlich adsorption isotherm models were used to fit the Fe²⁺ and Mn²⁺ adsorption by coal gangue loaded SRB in Haizhou open-pit mine, and the fitting results were shown in Figure 4.

Langmuir adsorption model:

Freundlich adsorption model:

In the formula above, q_e represents the equilibrium adsorption capacity, and its unit is mg/g. c_e represents equilibrium concentration, and its unit is mg/L. k_L is Langmuir adsorption constant. q_m is the saturated adsorption capacity. k_f is Freundlich adsorption equilibrium constant, and it is related to the type and characteristics of adsorbents. n is Freundlich adsorption equilibrium constant.

(2) Lagergren's first-order adsorption kinetic equation, Lagergren's second-order adsorption kinetic equation, origin 2023 software and particle expansion model were used to analyze the adsorption kinetic process of Fe²⁺ and Mn²⁺ by coal gangue loaded SRB in Haizhou open-pit mine. The results are shown in Figure 6 and Table 2.

Lagergren first-order adsorption kinetic model: ln(q_e – q_t) = lnq_e – k₁t

and the formula can be changed into: q_t = q_e(1 - e^-k₁^t)

Lagergren second-order adsorption kinetic model:

Intraparticle diffusion model: q_t = k₃ · t^1/2 + c

In the formula above, q_t represents the adsorption capacity at time t, mg/L; t represents adsorption time, min; k₁ represents first-order kinetic rate constant, 1/min; k₂ represents second-order kinetic rate constant second-order kinetic rate constant, mg/(g·min); k₃ represents intraparticle diffusion rate constant, mg/(g·min^1/2).

(3) Adopting the fitting models of zero-order kinetics and first-order kinetics respectively, use Origin 2023 software and analyse the reduction process of SO₄^2- by coal gangue loaded SRB in Haizhou open-pit mine, and the results was shown in Figure 7.

Zero-order dynamic model: c_t = c₀ – k₀ · t

First-order kinetic fitting model: lnc_t = lnc₀ – k’₁ · t

In the formula above, c₀ represents initial SO₄^2- concentration, mg/L. c_t represents the concentration of SO₄^2- is at any time, mg/L. k₀ represents the rate constant of zero order reaction, mg/(L min). k’₁ represents the rate constant of the first order reaction, 1/min.

 

Issue 5

The literature review is presented quite well, but includes mainly the experience of authors from China. I strongly recommend that you consider the works of other authors.

Please consider below mentioned paper.

Lazar, M.; Apostu, I.; Faur, F.; Rotunjanu, I. Factors influencing the flooding process of former coal open-pits. Min. Miner. Depos., 2021, 15, 124-133. https://doi.org/10.33271/mining15.02.124

I believe it worth considering in your paper as it presents an important issues regarding flooding. It is very important factor but not discussed in your research. What is or can be the impact of the open-pit flooding on acid mine drainage?

Reply: Thank you for your precious advice, the author cited the paper above in the ‘Introduction’ section. At the same time, the author also added ‘2.4 Discussion’ section. The revised version is shown as follows:

Especially, the exploitation of the Peșteana open-pit mine in Romania was affected by water injection speed, lasting time, AMD and other environmental factors, and it’s easy to cause geological disasters such as floods [6].

 

Issue 6

Please provide a short description of further research.

Reply: Thank you for your precious advice. Combined with your advice, the author added some contents in the paper, which are shown as bellow:

(1) In the discussion section, the author added: In summary, coal gangue loaded SRB can achieve the effect of ‘treating waste with waste’, which is a kind of convenient and cheap AMD repair material. This study only focused on the removal effect of SO₄^2-, Fe²⁺ and Mn²⁺ by coal gangue loaded SRB, and the removal effect of other heavy metals in wastewater by coal gangue loaded SRB will be further analyzed in the future. At the same time, the application effect of coal gangue loaded SRB material in other industrial fields is analyzed, and the application range of coal gangue loaded SRB material is further promoted.

(2) In the conclusion section, the author added: The remediation technology of coal gangue loaded SRB for other types of contaminated wastewater still needs to be further explored in the future.

 

Issue 7

In the conclusion section of the paper, it is crucial to highlight the uniqueness and importance of the research. Doing so will help readers comprehend the study's originality and potential impact on future research and practical applications. It is, therefore, necessary to clearly and concisely convey the innovative elements of the research in the conclusion section of the paper.

Reply: Thank you for your precious advice. Combined with your advice, the author added some contents in the paper, which are shown as bellow:

Aiming at the pollution problem of acid mine drainage (AMD) with high content of Fe²⁺, Mn²⁺ and SO₄^2-. In this study, coal gangue and SRB were used as materials to prepare coal gangue loaded SRB materials, which solved the pollution problem of coal mine acid wastewater with high content of Fe²⁺, Mn²⁺ and SO₄^2-. Coal gangue loaded SRB can not only improve the problem of poor SO₄^2- removal in AMD by coal gangue, but also reduce the adverse influence of surrounding environment on SRB growth by using the adsorption performance of coal gangue. At the same time, the coal gangue in this technology is the solid waste of the mining area, which is easy to obtain. AMD is a kind of polluted wastewater in mining area. Remediation of AMD by coal gangue loaded SRB can achieve the effect of 'treating waste with waste'. Coal gangue loaded SRB is a convenient, cheap and efficient AMD repairing material. The remediation technology of coal gangue loaded SRB for other types of contaminated wastewater still needs to be further explored in the future.

 

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript can be considered to be accepted for publication now. The manuscript quality is fine now. 

Now, the quality of the manuscript language has been greatly improved. The manuscript language is fine now.

Reviewer 2 Report

The authors already revised their paper point-by-point according to the suggestions of the reviewers. In my opinion, this article can be published in this journal.

Reviewer 3 Report

It's a perfect revision that was made by the authors.