After the closure of the Yudong coal mine, the pH value was approximately 3.0, and the Fe and Mn concentrations reached 380 and 69 mg/L, respectively, in the acid mine drainage (AMD), causing serious pollution to the water bodies in the nearby watershed. Combined with the formation conditions of AMD, the comprehensive treatment technology of source reduction–end treatment is adopted to treat the AMD. The treatment area of the goaf is 0.3 km3, the filling and grouting volume is about 6.7 m3, and the curtain grouting volume is 4,000 m3. Through the grouting and sealing treatment in the goaf, the water volume is reduced to less than 85% of the initial volume (100 m3/h). After the end treatment, the pH value of the effluent is around 7.0, the content of Fe and Mn is less than 0.1 mg/L, and the removal rate is above 99%. The project was subsequently operated at RMB 0.85 yuan/t. This project is aimed at the treatment of AMD from small coal mines in complex terrain conditions. It has the characteristics of low cost and high efficiency and can provide an effective treatment technology for AMD in southwestern China and areas with the same geological conditions.

  • Combining grouting and sealing technology with end technology to jointly treat AMD.

  • This technology reduces the amount of mine water by more than 85%, elevates the pH of AMD to neutral, and the removal rate of Fe and Mn is greater than 99%.

  • Provides an economical and effective treatment method for acidic mining wastewater treatment.

Coal is China's main energy source, with its production and consumption ranking first in the world (Dong et al. 2021). The distribution of coal resources in China is uneven, with most distributed in the north and west, and a few distributed in the south and east (Feng et al. 2014). In recent years, the Chinese government has implemented a policy of reducing coal production capacity and closed a large number of mines with depleted resources and outdated production capacity (Yuan 2019; Wang et al. 2021b). It is estimated that by 2030, the number of closed/abandoned mines in China will reach 15,000. China's coal mines mainly rely on underground mining. Closed coal mines not only leave a lot of mining space but also cause problems such as coal seam roof collapse and mine water pollution (Chen et al. 2023; Xia et al. 2023). Therefore, the pollution control of closed coal mines has attracted more attention (Acharya & Kharel 2020; Jiao et al. 2023). Coal resources are distributed on both sides of the Yudong River Basin in Kaili City, Guizhou Province, southwestern China, with a reserve of 81.42 million tons. The large-scale mining of coal resources began in the early 1980s, with over 80 recorded coal mines. Most of them have been closed, with only a small portion left for integration, and currently, all are in a state of shutdown. A large amount of acid mine drainage (AMD) from the closed coal mine wellhead flows into the river, and the pH, Fe, and Mn concentrations in the mine water seriously exceed the standard. The water body and riverbed in the basin are reddish brown, causing serious environmental pollution to the Yudong River Basin, affecting the production and life of villages along the river. It is imperative to find an economical and effective treatment technology to treat it.

Grouting technology is considered one of the most effective technologies to solve coal seam roof collapse (Jiang et al. 2018; Yuan et al. 2022). It is a method of grouting and transforming the goaf roof aquifer (impermeable layer) through ground construction drilling, to block the hydraulic connection between the roof aquifer and the goaf area. On the basis of exploring the development height of the water-conducting fracture zone in the coal seam roof, grouting transformation is carried out on the discovered water-conducting channels, primary dissolution gaps, structural fractures, etc., to complete regional governance (Zhang et al. 2018; Shi et al. 2021). In the 1990s, China developed a process to transform the underlying aquifer of coal mining faces into an impermeable layer, and widely used grouting technology for water control in coal mines and filling and reinforcing rock layers. The use of grouting technology to reinforce the roof has prevented many roof accidents caused by loose roof falling or broken roof leakage and achieved satisfactory economic results (Bai et al. 2023).

The commonly used end-treatment technologies for acidic mine drainage include neutralization, sulfurization, constructed wetlands, adsorption, ion exchange, and microbial methods (Range & Hawboldt 2019; Ali Redha 2020; Petronijevic et al. 2020; Wang et al. 2021a). The neutralization method is the most commonly used technology for treating AMD, which mainly relies on an acid–base neutralization reaction to reduce the acidity of the water body and cause metal ions to form hydroxide precipitates for removal from the water body, or under the joint action of microorganisms, metal ions can be removed in the form of sulfides, hydroxides, and carbonate precipitates (Kalin et al. 2006; Naidu et al. 2019; Skousen et al. 2019; Tong et al. 2021). In engineering applications, the neutralization method has made significant improvements, with a wide range of water quality applications, and the treated water meets the discharge standards. The commonly used neutralization materials include lime, limestone, sodium hydroxide, and sodium carbonate (Li et al. 2018). Permeable reaction walls were originally used for in-situ treatment of groundwater pollution by excavating ditches and filling them with active materials with certain permeability (Fytas 2010) (such as organic solid mixtures, limestone, or gravel), but the reaction tank must be built underground and the wall-filling materials need to be replaced regularly in practical use. Research has shown that mixed organic matter reactors such as sawdust and straw also have significant removal effects on AMD treatment (Song et al. 2012). Constructed wetland is a type of land constructed and operated by humans, similar to swamps. It mainly utilizes the physical, chemical, and biological synergistic effects of soil, artificial media, plants, and microorganisms to treat wastewater (Woulds & Ngwenya 2004). It has the characteristics of good pollutant removal efficiency, low infrastructure and operating costs, simple process equipment, and convenient maintenance and management (Skousen et al. 2017; Pat-Espadas et al. 2018).

In recent years, there has been significant progress in theory and technology in the treatment of acidic wastewater from coal mines (Park et al. 2019; Yang et al. 2023). However, due to the limitations of the geological environment, economics, and resource conditions, many regions still use traditional wastewater treatment technologies, resulting in unsatisfactory treatment results (Park et al. 2019). Adopting source control combined with end treatment technology, the treatment of acidic wastewater from closed coal mines in the Yudong River Basin, boldly applying the injection technology of coal mine prevention and control water to the blockage of wastewater sources. Based on the characteristics of local karst landforms, the process of using abandoned tunnels to fill with lime and organic matter as pollutants is selected for reduction. Constructed wetlands are used as the end for deep purification, the traditional high-cost and low-efficiency end treatment mode has been transformed, providing new technologies and ideas for the treatment of AMD.

The Yudong River Basin is located in Longchang Town, northwest of Kaili City, Qiandongnan Prefecture, Guizhou Province, China (Figure 1). Its geographical coordinates are 26°35′29.03″–26°44′28.22″N and 107°43′55.91″–107°58′55.82″E. It has been severely affected by AMD. In 2017, the General Administration of Coal Geology of China discovered a total of 17 mine water inflow points within a 234 km2 basin, with a total inflow of 2,112.5 m3/h (dry season) −7,720 m3/h (wet season). The annual inflow of mine water was 43,066,000 m3, including 33.813 million m3 in the wet season and 9.253 million m3 in the dry season. Most of the mine water exceeded the discharge standards for total iron and manganese. The Xiayuan Qiaotou Coal Mine is one of the coal mines in the Yudong River Basin. The designated mining area of the coal mine is a quadrilateral shape, with a mining area of 0.7898 km2 and a mining scale of 30,000 t/year. It adopts inclined shafts and adits for development and adopts a longwall retreat full caving method for coal mining. Due to the thin thickness of the coal seam, manual mining and combined transportation of labor and machinery are used. There are four shafts in total: the main shaft is located in the western part of the well field, with an elevation of 685 m at the wellhead, responsible for transportation tasks. The auxiliary well is located in the eastern part of the well field, with an elevation of 808 m, and it is responsible for pedestrian tasks. The main air shaft is located 30 m south of the main shaft in the western part of the well field, with an elevation of 694 m at the wellhead. The auxiliary air shaft is located 60 m north of the auxiliary shaft in the eastern part of the well field, with an elevation of 812 m at the wellhead. It was integrated and closed in 2008.
Figure 1

Map showing the location of Yudong River Basin.

Figure 1

Map showing the location of Yudong River Basin.

Close modal

The water quality survey results of the Xiayuan Qiaotou Coal Mine from July 2017 to November 2018 are shown in Table 1. The pH, Fe, and Mn concentrations in the mine water significantly exceed the limit values of pH 6–9, total iron 6 mg/L, and total manganese 4 mg/L in the ‘Coal Industry Pollutant Discharge Standard’ (GB20426-2006). It is a typical AMD with low pH, high , and Fe concentrations. Compared with the dry season, the water quality during the wet season is better, mainly due to the large influx of rainfall into the goaf during the rainy season, diluting the concentration of pollutants and promoting the discharge of a large amount of mine water. During the dry season, because of the reduced evaporation, the solubility of various metals in soluble salts can be reduced, but the impact on pollutant concentration is very small. The Qiaotou Coal Mine is the main discharge mode of pollution in the Yudong River Basin, as it is directly discharged from the mine mouth (inclined shaft, adit, air shaft, drainage roadway, etc.) by gravity. This type of discharge method is greatly affected by rainfall and changes significantly with seasons. The water volume during the high-water period increases significantly, while the water volume during the low-water period gradually decreases. The average water inflow of the mine during the high-water period is about 100 m3/h, and the average water inflow of the mine during the low-water period is about 20 m3/h. It can be seen that seasonal precipitation has a significant impact on the water inflow of the mine, and the rainy season precipitation becomes the main supplement to the water inflow of the mine. When atmospheric precipitation seeps into the goaf and accumulates, and the water level elevation is higher than the wellbore, groundwater flows out along the wellbore. Due to its large cross-section, smooth flow, and large discharge volume, it becomes the main channel for mine water discharge.

Table 1

Water inflow and water quality characteristics of the Xiayuan Qiaotou Coal Mine

Water quality parameters2017–2007 (wet season)2017–2012 (dry season)2018–2003 (dry season)2018–2008 (wet season)2018–2011 (normal period)China Standard (GB20426-2006)
pH 2.78 3.05 2.98 3.20 3.49 6–9 
(mg/L) 316.00 340.00 351.00 302.00 366.00 – 
Cl (mg/L) 14.84 17.46 16.72 21.38 16.52 – 
F (mg/L) 1.00 0.90 0.80 1.20 0.86 10.00 
Fe (mg/L) 287.70 258.37 252.01 292.10 380.73 6.00 
Mn (mg/L) 69.00 85.00 75.00 58.00 68.00 4.00 
Mg (mg/L) 52.65 44.58 40.90 61.64 45.80 – 
Average flow (m³/h) 120 12 35 118 10 – 
Water quality parameters2017–2007 (wet season)2017–2012 (dry season)2018–2003 (dry season)2018–2008 (wet season)2018–2011 (normal period)China Standard (GB20426-2006)
pH 2.78 3.05 2.98 3.20 3.49 6–9 
(mg/L) 316.00 340.00 351.00 302.00 366.00 – 
Cl (mg/L) 14.84 17.46 16.72 21.38 16.52 – 
F (mg/L) 1.00 0.90 0.80 1.20 0.86 10.00 
Fe (mg/L) 287.70 258.37 252.01 292.10 380.73 6.00 
Mn (mg/L) 69.00 85.00 75.00 58.00 68.00 4.00 
Mg (mg/L) 52.65 44.58 40.90 61.64 45.80 – 
Average flow (m³/h) 120 12 35 118 10 – 

Kaili City belongs to the temperate and humid climate zone of the central subtropical zone, which is a typical monsoon climate. Affected by the monsoon, continuous rainy and low-temperature frost weather often occurs in winter, and alternating cold and warm air in spring often leads to strong winds, heavy rain, and low-temperature cloudy and rainy weather. In early summer, it often has heavy rainstorms, while in midsummer, it is often sunny or rainy. In autumn, it is often sunny and dry. According to data from Kaili Meteorological Station, the average annual precipitation for many years has been 1,264.7 mm, with uneven distribution and concentrated from May to October. The average annual precipitation from May to October is 835.7 mm, accounting for 66.1% of the average annual precipitation. The average annual precipitation days (daily precipitation ≥0.1 mm) are 171 days, and the measured maximum daily precipitation is 256.5 mm (12 July 1970).

The study area is affected by the overall uplift of the Yunnan–Guizhou Plateau and river cutting. Steep U-shaped valleys are formed on both sides of the Yudong River. The natural slope of the terrain is more than 70°. Some areas are nearly vertical, forming high and steep cliffs. The terrain is mainly composed of medium to low mountain terrain, with higher levels in the northwest, southwest, and southeast, and lower levels in the central and northeast. The altitude is mostly between 800 and 1,020 m, with a relative height difference of generally 50–400 m. The terrain is mainly composed of dissolution erosion hills and valleys, with deep valleys and narrow and deep river channels. The two banks are mainly composed of Zhongshan hills, with significant differences in terrain fluctuations.

The geomorphology of Kaili City is a low-middle mountainous landscape of erosion and dissolution origin, with many internal basins and gentle slopes, and the development of Karst landforms. The terrain is generally high in the southeast and low in the northwest, with relatively low-lying gullies. The overall form of tectonics in the area is a northeast-directional complex oblique tectonic system, with north-southeast-west folds and faults, mainly the Dafengdong reverse fault, the Yudong positive fault, the Dapaomu oblique, and the Yudong oblique. The outcrops in the treatment area range from old to new: Silurian Middle-Upper Weng Xiang Group (S2-3wn); Devonian Yaosuo Group (D3y); Carboniferous Lower Pendulum Group (C1b); Permian Lower Liangshan Group (P2l), Qixia Group (P2q), Maokou Group (P2m); and Cenozoic Fourth Series (Q).

The field geological profile of the coal seam roof is shown in Figure 2. The Permian Lower Liangshan Group is the main ore-bearing strata, and the lithology consists of quartz sandstone, carbonaceous shale, coal seam, and bauxite interspersed with pyrite agglomerates or nodules. The development of karst in the overlying Permian Qixia Maokou Group of the Liangshan Group has caused a large amount of acidic wastewater with extremely strong erosion ability to be discharged resulting in serious environmental pollution. Due to the distribution in karst areas and the influence of climatic and geological conditions, the discharge of mine wastewater, at many points, is wide, scattered, and persistent, causing serious pollution to the water environment, the soil environment, and the ecological environment (Li 2018).
Figure 2

Field geological profile of coal seam roof.

Figure 2

Field geological profile of coal seam roof.

Close modal

Water sample collection

Continuous monitoring of the drainage flow and water quality at the wellhead was conducted from July 2019 to August 2021. Field measurements were made of pH using a portable pH meter (HI9124, HANNA). An acidified sample (50% nitric acid) was analyzed for total concentrations of Fe, Mn, and other metals by inductively coupled plasma emission spectrometry (ICAP-QC, Thermo Fisher, USA).

For the mine hydraulic discharge using the V-notch Weir of actual measurement (h is over the weir depth), the calculation formula is as follows:
formula
(1)

Governance plan and theoretical basis

The karst fissures are developed in the carbonate rocks of the coal bed roof. Based on the concept of regional water management in advanced and integrated water source–channel treatment (China Coal Science and Engineering Group Wuhan Design and Research Institute Co.), the roof regional treatment technology is used to construct directional branch holes in the destination layer, seal the fissures and hidden water-conducting channels in the aquifer by grouting, and cutoff the vertical hydraulic connection between them (Wu et al. 2019). The aquifer is then transformed into a relative aquiclude, thus serving the purpose of cutting rainwater and karst water into the mine. At the same time, to prevent the lateral runoff recharge of groundwater, combined with the vertical area treatment technology, a curtain is formed in the direction of its incoming water to block the lateral recharge of groundwater. The roof is combined with the curtain to form a relatively closed isolation space to minimize the recharge in the goaf. Using the filling grouting method, the mine is targeted to be sealed and filled within the influence area of the goaf according to a certain hole spacing and arrangement.

The selection of the grouting target layer requires the following conditions: (1) the selected layer has injectability. The development of certain cracks is conducive to the diffusion of the slurry. (2) The selected layer has a certain strength. The location of drilling and grouting should avoid broken or weak rock layers such as water-conducting fracture zones, to ensure the final treatment effect of the target layer. (3) The selected layer should have a certain slurry holding capacity. According to the investigation results, the height of the water-conducting fracture zone is 10–20 times the mining height, and the general mining height of the coal seam is 2.0 m. It is estimated that the height of the water-conducting fracture zone is 20.0–40.0 m. Therefore, the grouting layer is determined as the lower part of the carbonate rocks in the Qixia and Maokou Formations, and the limestone layer, within the range of 5.0–50.0 m above the coal seam roof (Figure 3).
Figure 3

Schematic diagram of grouting layers Li et al. (2021b).

Figure 3

Schematic diagram of grouting layers Li et al. (2021b).

Close modal
The layout of the filling and grouting space is shown in Figure 4. It has a radius of grout diffusion of about 30 m, a drilling distance of less than the sum of the radius of the two holes, and a distance of 50 m. Altogether 14 straight holes and 40 inclined holes are designed, with a depth of 200 m for the square holes and 400 m for the inclined holes, with a total drilling volume of 18,800 m. Nine holes are motorized holes for inspection and play a supplementary grouting role. According to the goaf of 0.3 km2 and the average thickness of the coal seam of 0.8 m, the estimated grouting volume is 67,000 m3, and the specific gravity of slurry is 1.5, including 42,000 m3 of water, 45,000 t of cement, and 237 t of quick-setting agent water glass.
Figure 4

Engineering layout of quantities in coal mine goaf of Xiayuan Qiaotou.

Figure 4

Engineering layout of quantities in coal mine goaf of Xiayuan Qiaotou.

Close modal

The curtain wall will bear a certain head pressure, therefore, to ensure the curtain wall is effective in the long term, the thickness of the curtain wall needs to be determined and analyzed. The thickness of the curtain wall in soluble rock strata should not be less than 10 m. To achieve the minimum safety thickness of the curtain wall, the drilling spacing is set to double the minimum safety thickness of the curtain wall, taking into account the construction cost and efficiency (Dong et al. 2020). From Figure 4, we can see the layout of curtain management boreholes, the hole spacing of straight holes is 50 m, with two rows of staggered holes, and one inspection hole is set every 80 m. Horizontal boreholes are arranged side by side, horizontal sections are staggered, the upper and lower spacing of each horizontal hole is 20 m, and the hole depth is 80 m. The total volume of boreholes is 8,000 m, and the total volume of grouting is 4,000 m3, including 2,800 m3 of water and 3,000 t of cement.

Filling ditch combined with constructed wetland as end treatment

The end-treatment process is shown in Figure 5 and the layout is shown in Figure 6. Due to the low pH of mine drainage, alkaline neutralization needs to be generated, and a lime–organic matter mixed material filling ditch is the simplest technology to passively generate alkali Li et al. (2021a). Based on target loads and budgeted pollutant removal rates, the lime–organic matter mixed material filling ditch-constructed wetland system was used at the wellhead to treat the mine water in the workings. Mine effluent is collected and distributed to the lime–organic matter mixed material filling ditch through a catchment canal for acid neutralization and partial metal removal, and then it enters the constructed wetland for aerobic removal of residual metals, with the hydraulic residence time being 20 and 24 h, respectively.
Figure 5

Schematic diagram of the end-treatment process.

Figure 5

Schematic diagram of the end-treatment process.

Close modal
Figure 6

Layout of the end-treatment process.

Figure 6

Layout of the end-treatment process.

Close modal

Lime–organic matter mixed material filling ditch treatment unit: the size of the lime–organic matter mixed material filling ditch is 102 × 1.2 × 1 m, and the upper layer is a 0.2 m thick water layer that can provide a certain head for system flushing, the lower layer is a 0.8 m thick lime–organic matter mixed layer. The lime–organic matter mixed layer contains limestone with a particle size of 10–15 mm (content ≥90%)(Bhattacharya et al. 2008) and organic fertilizer (corn cob 70%, chicken manure 20%, straw 5%, and bran 5%), with the organic fertilizer containing 56% organic matter. The mixed layer raises the pH by dissolving limestone while reducing the Fe and Mn concentrations in the effluent by generating hydroxide precipitation and providing nutrients for microorganisms to promote the growth of sulfate-reducing bacteria, which consume oxygen and sulfate and effectively produce alkalinity. To mitigate the impact of system blockage on system operation, rough, and inert materials with high-specific surface area (such as shavings, sawdust, etc.) are blended in the mixed layer to increase the porosity of the mixed layer and slow down the occurrence of system blockage.

Constructed wetland treatment unit: the constructed wetland consists of four tandem pools, and the elevation difference between each level is 10 cm. The overall water flow is in the form of folded surface flow. The bottom of the wetland is a concrete water barrier with a thickness of 15 cm. Each pool is designed with a length of 10 m, a width of 9 m, an average effective water depth of 1 m, and a maximum design capacity of 360 m3. The wetland is filled from top to bottom with materials such as Mn sand, volcanic rock, and limestone, and the surface is planted with native aquatic plants such as calamus, reeds, and knotty grass.

The formation and source of AMD

Figure 7 shows the schematic diagram of the mine water supplement. The coal mines in Guizhou mainly contain sulfur in pyrite. When exposed to humid air, pyrite will interact with H2O and O2, releasing H+ and Fe2+, and bacteria act as catalysts to accelerate the reaction of Fe2+ being oxidized by O2 to Fe3+. As the pH decreases, the Fe3+ activity increases and further oxidizes the pyrite with the promotion of ferrous sulfur oxide bacteria. In general, the formation of AMD is the result of the combined action of ferrous sulfur oxide bacteria, Fe3+, and O2, with ferrous sulfur oxide bacteria playing a decisive role (Zheng & Cai 2007). The reaction equation is shown in (2)–(4):
formula
(2)
formula
(3)
formula
(4)
Figure 7

Schematic diagram of the mine water supplement.

Figure 7

Schematic diagram of the mine water supplement.

Close modal

The reactions produce a large amount of sulfate and Fe while maintaining the mine water at a lower pH. Although the alkaline material in the mine water flowing through the rocks can neutralize some of the acidity, it is generally believed that only acidic wastewater can be produced when the sulfur content in coal is above 3%. The sulfur content in the Late Permian Guizhou coal mining area of the South China coal-bearing region is 4.92% (Tang et al. 2015), and pyrite sulfur accounts for more than 80% of the total sulfur in the high-sulfur coal of the Liangshan Group (Wu et al. 2010).

Effect of water reduction

In the past, the average discharge from the mine was about 100 m3/h during periods of abundant water and about 20 m3/h during periods of dry water. The drainage flow rate of the pithead was continuously monitored from July 2019 to August 2021, and it can be seen from Figure 8 that the range of gushing water flow in the treated mine was 8.29–17.50 m3/h, with an average of 11.37 m3/h. The flow rate fluctuated less, and there was no significant difference between the flow rate during the dry and abundant water periods. The reverse proves that the recharge water source of the untreated mining area is mainly precipitation. Through roof curtain grouting and water plugging treatment, the reduction of mine water recharge and discharge from the goaf reaches more than 85%, effectively isolating the recharge of each water source to the goaf from the source, reducing the flow of polluted mine water to the surface, and reducing the difficulty for subsequent AMD treatment.
Figure 8

Changes in water yield of the mine before and after grouting.

Figure 8

Changes in water yield of the mine before and after grouting.

Close modal

Effect of end treatment

pH increased to neutral

The variation of system inlet and outlet pH over time for different sampling periods is shown in Figure 9. The system inlet pH is stable in the range of 3.01–3.75, and the outlet pH rises from 6.94 to 7.85 after treatment by the system. During the operation period from July 2019 to October 2020, the outlet pH was less stable relative to the inlet water, fluctuating between 7.0 and 8.0. From October 2020, the fluctuations narrowed to between 7.0 and 7.5 and the curve leveled off. pH changes in the lime–organic matter mixed material filling ditch were mainly enhanced by dissolving limestone in the wastewater for neutralization. Filling ditch treatment reduces the intensity of constructed wetland treatment of AMD, while the constructed wetland supplements the neutralization of acidity by alkaline substrates and microbial metabolites. The stability of the pH value of the system effluent shows that the lime can dissolve better under the condition of this parameter and produce enough alkaline material. The lime–organic matter mixed material filling ditch is combined with the constructed wetland treatment to get a better treatment effect, and the overall effluent pH is maintained at about neutral.
Figure 9

pH change at inlet and outlet.

Figure 9

pH change at inlet and outlet.

Close modal

Fe and Mn effectively reduced

The influent and effluent concentrations of total Fe and Mn at each stage in the combined lime–organic matter mixed material filling ditch-constructed wetland system are shown in Figure 10. The range of total Fe concentration in the influent water of the system from July 2019 to August 2021 is 190–255 mg/L, with an average of 218.58 mg/L. The range of concentration in the effluent water of the filling ditch is 29.8–56.8 mg/L, with an average of 43.27 mg/L. The removal rate of total Fe in this stage ranges from 71.31 to 86.45%, and the average concentration of total Fe in the effluent of the constructed wetland is less than 0.1 mg/L. It can be seen that the influent concentration fluctuates more than the effluent concentration, indicating that seasonal changes have little effect on the treatment effect (Hedin et al. 2010). The removal of Fe in the filling ditch is mainly through the adsorption of limestone and the generation of hydroxide precipitation, while the removal of Fe in the constructed wetland involves complex biochemical reactions, where Fe is oxidized by oxidizing bacteria to generate hydroxide precipitation and it can also be reduced by microorganisms to Fe sulfide and deposited into the substrate. The two stages of total Fe removal showed good results and proved that the system had a good and stable operation. The combination of lime–organic matter mixed material filling ditch and constructed wetland not only makes the total Fe removal rate as high as 99% but also effectively treats about 625,000 m3 of coal mine wastewater per year.
Figure 10

Variation of effluent quality in each stage of the end-treatment process.

Figure 10

Variation of effluent quality in each stage of the end-treatment process.

Close modal

The range of total Mn concentration in the system influent is 45.2–67.0 mg/L, with an average of 54.24 mg/L. The range of concentration in the effluent of lime–organic matter mixed material filling ditch is 11.4–25.0 mg/L, with an average of 16.73 mg/L, and the range of total Mn removal rate in this stage is 45.65–81.62%. The total Mn concentration of the constructed wetland effluent was less than 0.1 mg/L, and the total Mn removal rate was in the range of 99%. The removal of Mn2+ from wastewater by limestone is a complex chemical reaction process in which Mn2+ reacts with OH to form white Mn(OH)2 flocs, while Mn(OH)2 is easily oxidized by dissolved oxygen in the water to brown MnO2-H2O (Cheng et al. 2012). In addition, Mn2+ can be removed by the adsorption of Ca(OH)2 generated by the lime application and the flocs such as Fe(OH)2 and Fe(OH)3 generated by Fe2+ and Fe3+. In constructed wetlands, the substrate provides a good living environment for plants and microorganisms, and its large specific surface area is also very favorable for heavy metal adsorption. By the same principle as Fe removal, Mn in constructed wetland systems is removed by ion exchange or chemisorption on the filler surface. The combination of lime–organic matter mixed material filling ditch and constructed wetland can effectively remove Fe and Mn.

Applicability analysis

The Yudong River Basin AMD Treatment Project applies coal mine water prevention and control technology to wastewater treatment, transforming the high-cost traditional end-treatment mode. The engineering construction is combined with the rugged karst terrain conditions in the southern region, and the abandoned tunnels are fully utilized for the discharge process control of AMD. Finally, using constructed wetlands as deep purification not only ensures the efficiency of treatment but also has environmental and ecological benefits. The comprehensive treatment technology of source control and end treatment in this project can provide a new method for the treatment of AMD in areas where coal mining is mainly small, distributed in karst mountainous areas, affected by climate conditions, and has severe acid wastewater discharge problems.

Cost evaluation

The direct engineering part of the investment is RMB 6,452,500,000 yuan, of which the cost of mine water treatment in the goaf is RMB 55,460,000,000 yuan accounting for 70.13% of the total investment. According to the treatment volume of 20 m3/h, the traditional AMD treatment using the limestone neutralization method, the cost of treatment for a ton of water is generally between RMB 2.0–4.5 yuan. Using the lime–organic matter mixed material filling ditch-constructed wetland system treatment method, the alkaline volume of the filler is 360 m3, the volume of the volcanic ash ceramic is 8 m3, and the volume of the organic matter is 36 m3. The replacement cycle of the filler is 12 months and the cost is about RMB 150,000 yuan, which results in an estimated operating cost of RMB 0.85 yuan/t of water.

In the two years since the Yudong mine adopted the source reduction of grouting to seal the roof and curtain combined with the combination process of lime–organic matter mixed material filling ditch at the constructed wetland at the wellhead to treat AMD from closed coal mines, the system has been operating stably. The average discharge volume at the wellhead was reduced from 100 to 10.81 m3/h, and the difference between the discharge volume in the wet season and the dry season is very small, indicating that the treatment of the mining area has greatly reduced the recharge of mine water by rainfall, and the reduction of discharge volume has reached more than 85%. The pH value of the treated effluent rose to 6.94–7.85, and the overall effluent was stabilized at about neutral. Total Fe and total Mn were removed in large quantities mainly in the lime–organic matter mixed material filling ditch, and the average concentration of the effluent further purified by the constructed wetland was less than 0.1 mg/L, with the removal rate reaching 99%, and no seasonal influence on the treatment effect was found.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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