Source reduction and end treatment of acid mine drainage in closed coal mines of the Yudong River Basin

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a policy of reducing coal production capacity and closed a large number of mines with depleted resources and outdated production capacity (Yuan 2019; Wang et al. 2021b).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 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 parameters .	2017–2007 (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 parameters .	2017–2007 (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	–

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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 (S_2-3wn); Devonian Yaosuo Group (D_3y); Carboniferous Lower Pendulum Group (C₁b); Permian Lower Liangshan Group (P₂l), Qixia Group (P₂q), Maokou Group (P₂m); and Cenozoic Fourth Series (Q).

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).

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 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.

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 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).

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.

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.

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.