Leaching characteristics and pollution risk assessment of potentially harmful elements from coal gangue exposed to weathering at different time

Longqing Shi Shandong University of Science and Technology Jinfeng Peng Shandong University of Science and Technology Dongjing Xu (  xudongjinggg@126.com ) Shandong University of Science and Technology https://orcid.org/0000-0002-8815-0681 Jinjin Tian Shandong University of Science and Technology Tianhao Liu Shandong University of Science and Technology Binbin Jiang Water Resource Protection and Utilization in Coal Mining Facai Zhang Etuoke Banner Great Wall No.3 Mining Co.,Ltd


Introduction
China is the largest coal mining country. The annual production of raw coal was 3.90 billion tons in 2020, accounting for nearly 50% of the global coal production. Coal remains an important source of fuel and raw materials (Yang and Xu 2021). Coal gangue is a carbon-containing solid waste produced during the process of coal mining and washing. It accounts for approximately 10-20% of the coal produced. It is kept piled up (4. To address the problem of coal gangue pollution, the Jizhong Energy Group has successfully developed two new mining technologies (new domestic mining technology for studying coal gangue back lling the goaf). One of the methods involves the use of ground gangue to back ll the underground. The other method involves the process where coal gangue is not poured into the well but directly back lled. The gangue produced by mining is lled into the permanent coal pillar under the building. When the gangue is not raised in the well, it can be used to replace the coal resources that could not be mined following the original method (Xu 2009). The most e cient method of waste management involves the reduction in the production of wastes and the reuse of solid wastes. The commonly used methods include hydraulic lling, wind lling, mechanical lling, and self-slipping lling, among which the rst two methods are more e cient and effective than the others (Bian et al. 2008;Zhang et al. 2013).
The technology of using coal gangue as a lling material to back ll goaf not only helps ll the collapsed area but also helps reduce the accumulation of solid wastes. However, there are a large number of pollutants in the gangue. These toxic substances may be transferred to the adjacent environment, causing pollution (see Figure 1).
The oxidation of sul de minerals in gangue results in the production of acidic wastewater containing toxic heavy metals (Novikova and Gas'kova 2013). Due to the weak a nity of Mn with other heavy metal elements, the sulfur in the gangue also affects the content of Mn (Gao et al. 2021), indirectly causing environmental pollution .Tang et al. found that the concentrations of Cu, Pb, and Zn increased in the reclaimed soil in the subsidence area lled with coal gangue in the Huainan mining area (Tang et al. 2018). When a large amount of the pollutants are released, they not only infect the surrounding soil but also in ltrate the groundwater through the surface water.
This pollutes the vadose zone and endangers human health through the food chain ). Through environmental risk assessment, Hussain R et al. found that coal and coal wastes pose a high risk in their research on coal mining pollution in Hancheng County, Shanxi Province, and food and plants face medium to high risks (Hussain R et al. 2018).
There are many researches on coal gangue, these studies mainly focus on the utilization of coal gangue, and lack of attention to environmental risks (Li and Wang 2019). Fresh and weathered coal gangue were analyzed to compare and study the environmental pollution caused by the two types of coal gangue. The coal gangue obtained from the gangue hill and used for our studies was weathered for 1 year. The fresh coal gangue obtained from the goaf roadway was used as the experimental materials. The static immersion experiment was conducted using deep karst water as the immersion liquid to study the release law of Fe 3+ , Mn 2+ , SO 4 2− , and other major pollutants present in the two kinds of coal gangue. The change in pH of the soaking solution were also discussed and analyzed. The geo-accumulation index method was used to study the pollution caused by heavy metals. The results can help in the selection of the lling materials required during the production process of coal. Further, this approach seems to be promising in the prevention and control of groundwater pollution in mining areas. It might also help in achieving the effective implementation of water environmental protection strategies in fragile ecological areas.
2 Sample Collection And Experiment

Overview of the sampling area
The sampling area is located in Jining City and Tai'an City in the southwestern Shandong Province. The three mining areas of Baizhuang, Hongqi, and Zhaizhen ( Figure 2), all of which belong to the Luxi Stratigraphic Division of the North China Stratigraphic Region, were chosen. The terrain of this area dips slowly from southwest to northeast, with the vast Yellow River ooding the alluvial plain. It is characterized by a semi-humid climate with hot summers, long frost-free periods, and a high average annual temperature (13-14°C). It is the region with the most abundant heat resources. The average annual rainfall ranges between 600 and 800 mm. The coal-bearing strata in this area are located mainly in the Upper Carboniferous and Lower Permian regions. The Permian coal resources are rich in coal. The coal-bearing strata and coal seam thickness are stable. Gas coal is the main type of coal present, and the base of the coal strata is composed of Ordovician limestone, which is rich in water and has a high groundwater phreatic level. Most of the coal mines in the southwestern Shandong region are of the low-sulfur non-spontaneous combustion type. The gangue is alkaline and is primarily composed of siltstone, mudstone, and carbonaceous mudstone. The gangue lithology is primarily sandstone, mudstone, limestone, and carbonaceous mudstone, containing pyrite, kaolinite, and coal debris. In addition to SiO 2 , Al 2 O 3 , Fe, Mg, Ca, and other constant elements, other heavy metals such as Pb, Ag, Cd, Cr, As, and Mn (toxic heavy metal elements) were also present (Shen 2020).

Sample collection
The samples used in the experimental study consist of two parts. The rst part involves the collection of coal gangue samples from the Hongqi Coal Mine. In the mine outside the gangue pile, 10 sample points were selected from top to bottom using the "snake sampling method". Each sample point contained the same amount of gangue which was put in a clean polypropylene bag for storage to form the weathered coal gangue sample. Following this, 10 samples of equal amounts of coal gangue were collected at equal intervals in the 3112 working face roadway buried 358 meters deep. The samples were immediately put into clean polypropylene bags to form the fresh coal gangue samples. The nal samples of coal gangue were collected separately. The pretreatment method was followed by crushing. The quarter method was used to screen out coal gangue samples of particle sizes in the range of 0.45-3.2 mm.

Immersion experiments
The pre-processed and crushed gangue samples were divided into two parts: one was analyzed using the X-ray diffraction (

Dissolution of the main pollutants in coal gangue
The X-ray uorescence spectrometry was used to determine the chemical composition of fresh gangue and weathered gangue collected from the Hongqi Coal Mine. The ICP-MS technique and the ion chromatograph ICS-600 were used to analyze the three types of karst water collected to determine the various parameters (Table 1 and The concentrations of the polluting elements in the leachate change with an increase in time, and the analysis of the graph drawn from the experimental data shows that the dissolved concentration of Fe 3+ changes signi cantly with an increase in the soaking time (Fig. 4). In each group, the trend in the changes in the Fe 3+ ions is roughly similar, and the solubility decreases signi cantly. The solubility rises again after two peak dissolution values. The leaching solution of both groups reached the rst peak dissolution value on day 1. Following this, the extent of dissolution gradually decreased. A low peak of dissolution was observed on day 4, and a new peak appeared on day 7. This can be attributed to the static immersion method. During the initial process of the experiment, the water-gangue action can only activate and cause the rapid release of the metal on the surface. During the experimental period, the two types of gangue Fe 3+ presented "wave-like" behavior. The total solubility of the weathered gangue in the leaching solution is higher than that of the fresh gangue. This is because the oxidation product of pyrite in weathered gangue (Fe 3+ ) is mainly precipitated on the surface in the form of amorphous hydrated oxides. This accelerates the rate of release of Fe 3+ during the soaking process (Wu et al.

2014).
Fe 3+ +3H 2 O⇌Fe(OH) 3 ↓+3H + (1) Coal gangue contains a large number of harmful trace elements such as Mn 2+ , which causes serious environmental pollution (Cai et al. 2008). The trend in the solubility of Mn 2+ contained in coal gangue is presented in Figure 5. During the experimental period, the solubility of the sulfate ions changed (Fig. 6). In karst water No.1, the concentration of SO 4 2− decreased till day 6 days, but the overall trend appeared upward. After day 6, three phases of opposite dissolution trends were observed. Little oating was observed under these conditions. The overall solubility of SO 4 2− in the weathered gangue was approximately twice that observed in the fresh group. The changes in the karst water of No.3 were similar to the changes in No.1. The concentration of SO 4 2− in FS3 rapidly declined during 0-1 d (Fig. 6c). The dissolution trend observed after 4 d was almost the same as the trend observed with weathered gangue. The solubility of the weathered group was higher than that of the fresh group.
The two groups of change images were similar to "radical sign", roughly for the rst rise. Following this, equilibrium is achieved as the soaking solution contains more dissolved oxygen at the beginning of the experiment. Pyrite was oxidized in the short term and accelerated the dissolution of SO 4 2− . As the experimental time was increased, the extent of dissolution of dissolved oxygen in the soaking solution increased. The rate of mineral decomposition decreased. The concentration of dissolved SO 4 2− did not uctuate much in the middle and late stages. The concentration of SO 4 2− in FS3 dropped rapidly and then increased suddenly, giving rise to a "V"shaped uctuation after 1 day. This can be related to the presence of carbon-containing organic matter in fresh coal gangue (Fan and Lu 1999). During soaking, it adsorbs the sul de minerals in the coal gangue to reduce solubility. A new dissolved state is reached following stirring.
In karst water No.2, the changes in the SO 4 2− concentration in both groups followed the rising-declining-risingdeclining cycle. The image of ion concentration in this group presents two "high points" and three "low points", and the overall change resembles the "M" shape (Fig. 6b). The oxidation mechanism (Pandey et al. 2007) revealed that Fe 3+ (an additional oxidant) exerts an oxidizing effect on sul des. This prompted the release of SO 4 2− in the gangue. Fe 3+ could be easily hydrolyzed. It was precipitated as iron hydroxide. Its redox potential and oxidizing capacity were reduced. The solubilities of the sul des decreased in the absence of Fe 3+ .
The concentration of SO 4 2− in the three groups of karst water was always higher in the weathered group than the concentration observed in the fresh group. This can be attributed to the loose structure of the weathered gangue. Under the effect of long-term weathering, the internal structure of the gangue was destroyed. The ions in the mineral lattice were decomposed and freed. From the original chemical state to the free state, the solubility of pollutants also increased (Xiao et al. 2006). According to the data, the weathered gangue contains a large number of sul des oating on the gangue surface. This can be attributed to the role of atmospheric rainfall and surface water immersion. The pyrite contained in the open-air coal gangue is formed following the weathering Hydrolysis reaction as follows: FeS 2 (s) +3.5O 2 +H 2 O=Fe 3+ +2SO 4 2− +2H + +e − (6) Therefore, the concentration of the dissolved SO 4 2− in weathered coal gangue is higher than that in fresh coal gangue. A comparison of the graph of No.1, 2, and 3 soaking liquids reveals that the SO 4 2− detection value for the weathering group is greater than that of fresh coal gangue at each stage of the experiment.
It can be concluded that the concentrations of dissolved Fe 3+ , Mn 2+ , and SO 4 2− were higher in the weathered gangue. The data in the table presents the maximum leaching amount of pollutant ions in the soaking solution of each group. The concentration of Fe 3+ , Mn 2+ , and SO 4 2− in the soaking solution of the weathering group is higher than the concentration of each ion in the fresh group. The results revealed that the amount of pollutants in the weathered coal gangue is signi cantly higher than that in the unweathered gangue.
A part of the sample was in an anoxic state during the experiment. Following the process of intermittent stirring, the gangue samples came in full contact with the immersion solution. Intermittent stirring increased the movement of the molecules in the solution to a certain extent. This also hindered the process of adsorption and the process of forming colloids (He et al. 2014). Both in uence the rate of dissolution of each ion.

Changes in the pH of the soaking solution under the in uence of different types of coal gangue
The pH of the soaking solution of the three groups was weakly alkaline during the whole experiment (Fig. 7). The detected values were not much different. With an increase in the soaking time, the nal pH of the fresh group increased and became higher than that of the weathered group. The trend of pH change in the fresh group and the weathering group was similar, and the overall image was similar to the "logarithmic curve". The graph recorded for the change in the pH of solution No.3 presents a decreasing tendency, which is opposite to the trend presented by the graph recorded for the change in pH of solutions No.1 and 2. The pH value of the soaking solution in the three groups increased and decreased signi cantly before and after 6 days. Following this, the change occurred within a small range (8.0-8.5) till the end of the experiment. Alkaline pH is reached due to the presence of a large amount of SiO 2 and Al 2 O 3 in the gangue and the presence of a large amount of calcium, magnesium, and aluminum ions and salts in the karst water samples (Table 1 and 2). The uctuations in the pH of the soaking solution decrease as the solution balances with the single, multiple weak acids and weak alkalis. Under the action of oxygen, moisture, and microorganisms in the air, the reducing sul de minerals on the surface of the coal gangue are rst oxidized. Following this, a certain oxide layer is formed on the surface of the coal gangue. The oxide layer contains reducing sulfur oxidation products that can signi cantly reduce the pH of the soaking liquid (Srace et al., 2004). Under these conditions, the pH of the weathering group is lower than that of the fresh group.
The overall pH was weakly alkaline, and the oxidation and dissolution of sul de minerals contained in the gangue were inhibited under the closed state. This affected the acidic release of the gangue. With the passage of time, the pH value of the fresh gangue and weathered gangue soaking solution was nally stabilized between 8.0-8. 5. This was in line with the surface water environmental quality standard. The maximum concentration of each of the dissolved elements is listed in the following table with reference to the water quality standard (Class III; Environmental Quality Standard for Surface Water (GB3838-2002)): As can be seen from Table 1, the maximum concentration of SO 4 2− in the soaking solution of the two groups exceeds the value presented in the water quality standard of type III. The concentration of Fe 3+ is close to that presented in the water quality standard. The concentration of Mn 2+ in the soaking solution of the weathering group exceeds that presented in the water quality standard (2) in the soaking solution of the weathered gangue. The solubility of each substance is higher than that in the fresh gangue group. In general, the environment is affected, and in comparison, the harm caused by the weathered gangue to the environment is greater than the harm caused by fresh gangue.
Correlation analysis can measure the closeness of the correlation between the two variable factors. Pearson's correlation coe cients for each ion in the leachate and soaking solution were calculated separately for the fresh and weathered groups using SPSS Statistics 26 software. The correlation heat range was further plotted using Origin 2021, as shown in Figure 8.
It can be seen from Figure 8 that the pH of the soaking solution of each group has a signi cant correlation with SO 4 2− , indicating that SO 4 2− , as the main pollutants present predominantly and has a certain in uence on the uctuation of pH. The oxidation of pyrite in coal gangue produces acid, and the corresponding chemical reaction is presented as follows: 2FeS 2 +7O 2 +2H 2 O→2Fe 2+ +4SO 4 2− +4H + (7) When the H + concentration increases, the pH of the soaking solution decreases. The SO 4 2− p-value in Figure 8 shows that the content in the weathering group is higher than the content in the fresh group, indicating that the more the content of SO 4 2− in the gangue, the lower the pH of the soaking solution. This is in line with the analysis results presented in Figure 7. The pH of the weathering gangue soaking solution is slightly lower than the pH of the fresh gangue soaking solution.
It was found that the pH of soaking solution No. To further explore the in uence of the pollution causing ability of the dissolved ions in the leaching solution, the geo-accumulative index method was used to evaluate and analyze the samples taking into account not only the environmental geochemical background values but also the in uence of the anthropogenic factors and natural diagenesis to determine the degree of contamination caused by certain heavy metals at the sampling site. The following equation was used: I geo = log 2 (C n /kB n ) 8, where I geo is the cumulative pollution index of heavy metals, C n is the measured concentration of pollutants, B n is the geochemical environmental background value of the pollutants, and K is the correction coe cient (K=1.5).
According to the standard of the cumulative index, the pollution degree is classi ed as shown in Table 5. relatively high. The mechanism of oxidation of pyrite is shown in Figure 10. During the process of coal gangue back lling the goaf, attention should be paid to groundwater pollution caused by weathered coal gangue and Fe 3+ . As the degree of weathering of coal gangue increases, the pollution index of Fe 3+ and Mn 2+ increases, causing water pollution.
The toxicity caused by heavy metals is related to the total content and the form of existence. Different forms of existence have different environmental effects, which directly affect the toxicity, migration, and circulation of pollutants in nature. This paper studies the release of two types of pollutants from fresh and weathered coal gangues. Karst water also contains a certain amount of mineral ions. There is still a lack of detailed research on the speci c quantitative analysis of mineral dissolution in gangue and ions in an aqueous solution. Therefore, the potential pollution risks caused during the back lling of goafs by coal gangue under different weathering degrees need to be further explored. This study can provide a certain reference value for groundwater protection in mining areas.

Conclusion
The release of major pollutants from fresh and weathered gangue in the Hongqi Coal Mine was studied. The amount of pollutants released by weathered gangue in the soaking solution was higher than that by fresh gangue. The contrast difference was maximum for Mn 2+ (~ 7 times). The amount of SO 4 2− dissolved in the three main pollutants was the maximum (as high as 1442.96 mg/L). Throughout the experiment, the solubility of the contaminants varied inconsistently. The solubility of Fe 3+ showed a "wave-like" change, and that of Mn 2+ was curved up (weathering group) and down (fresh group). The graph representing the solubility of SO 4 2− was similar to the "radical sign".
The pH of the two types of coal gangue soaking solution was slightly alkaline (8.0-8.5) and the pH of the weathered gangue soaking solution was lower than that of the fresh group. This could be attributed to the presence of SO 4 2− . The results of geo-accumulative risk assessment show that weathered gangue causes more pollution than fresh gangue. Both Fe 3+ and Mn 2+ in the two types of coal gangue cause pollution. Among them, the pollution level of Fe was signi cantly higher than that of Mn 2+ . The level increased with the weathering years.
It may have a certain degree of impact on the groundwater environment. There is a lack of research on the correlation between the elements and the occurrence of the elements. The effect of the minerals present in karst water on the release of coal gangue pollutants should also be studied.

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Competing interests
The authors declare that they have no competing interests. Funding