Introduction

China is a big coal producer and has the highest coal output in the world. Coal gangue is the solid waste discharged from the process of coal mining and coal washing, and it contains a certain amount of combustible materials such as pyrite. And it is prone to slow oxidation and spontaneous combustion at high temperature (Liang et al. 2016). Worst of all, some harmful heavy metals, such as chromium, arsenic, mercury, and lead (Liu et al. 2017; Shi & Schulin 2018; Zhang et al. 2018) can transfer via atmospheric dust fall or rainwater into the soil and will endanger human health via food chains.

Leaching test is an effective method to simulate the process and assess the impact. Chen et al. (1988) analyzed the release rate of heavy metal by simulating the interaction process of low-sulfur coal gangue and water. Dang et al. (1998) studied the environmental geochemistry effect and change regularity of heavy metal in coal gangue in process of weathering and found that pH was an important factor influencing the dissolution of heavy metals in the static soaking of coal gangue. Zhou et al. (2014) designed a leaching test of the coal gangues to study the migration mechanism and regularity of the harmful metals and found that metal concentrations dissolved out and the release rate from coal gangue was related to the pH of leachate and the element concentrations in coal gangues. The damage done to the environment by coal gangue or other solid waste can be evaluated by ecological risk assessment. Hakanson (1980) proposed the potential ecological risk index which was widely used. Heavy metal background, loads, and bioavailability (Chen et al. 2018; Deng et al. 2017; Zhang et al. 2017; Zhou et al. 2017) were considered in the potential ecological index.

Nowadays, clay bricks have been banned in China, so a widely used alternative choice is to explore the gangue hills. So coal gangue has been considered as resource in China. In the process of excavation and utilization of the coal gangue hill, gangue at different weathering degrees was exposed to the environment, which can be harmful to the surroundings. During the field investigation, it was found that an old gangue, which was stocked for about 60 years, was excavating and formed a relatively complete profile where the gangue with different weathering degrees was exposed. What is the difference in the release characteristics of heavy metals in these gangues at different weathering degrees? What is the possible reason for these differences? What is their impact on the environment? These interesting questions are worth answering. In this work, we gathered the coal gangue at different weathering degree, including red gangue (RG), brown gangue (BG), gray gangue (GG), light black gangue (LB), and deep black gangue (DB), from Suncun mine, a coal mine of Xinwen mining field located in Tai’an, Shandong Province of China, and made leaching test and sequential extraction process to study the characteristics of heavy metal distribution, leaching rule, and potential risk to the environment.

Materials and methods

Sampling and preprocessing

Suncun coal mine (35° 52′ N, 117° 40′ 46″ E) is located in Tai’an city in Shandong Province of China. The coal mine has a long history of mining of more than 100 years; many huge gangue hills have been left there. A coal gangue hill about 60 years old has been excavated to reuse gangue in bricks making, road paving, and cement producing. The gangue in the hills has undergone a longtime weathering and its color changed a lot. According to the weathering degree and color of the coal gangue, samples were collected from a profile in five categories (Fig. 1). All gangue samples were stored and taken to the laboratory within 48 h. The samples were broken by crusher, and then were ground and sieved through a 100-mesh sieve, and finally were stored in sterile plastic packages.

Fig. 1
figure 1

Photos of gangue samples and sampling site. a Gangue hill. b Sketch map sampling profile. c Photos of sample

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Sample processing and heavy metal determination

Three experiments were carried out including total metal content determination, chemical species analysis, and leaching test.

Before total content of heavy metal determination, gangue samples were microwave digested in procedure as follows: 0.2 g sample, 8.0 mL nitric acid, and 5.0 mL hydrofluoric acid were mixed and then processed in a microwave digestion tank. After digestion and acid driven, the solutions in the tank were washed out with 2% (v/v) nitric acid and settled in a 50-mL volumetric flask.

As for the chemical species, gangue sample was processed by Tessier sequential extraction (Tessier et al. 1979). The chemical species was related to five binding forms: exchangeable fraction (EXC), carbonate fraction (CAR), Fe/Mn oxide-bound fraction (OXI), organic fraction (ORG), and residual fraction (RES). The main processing steps were as follows:

  1. (1)

    Exchangeable fraction (EXC), the solid sample (1.0 g) was extracted by 1 M MgCl2 at an initial pH 7 (30 mL) and was shaken at 250 rpm for 1 h at room temperature.

  2. (2)

    Carbonate-bound fraction (CAR), the residue from (1) was extracted by 1 M CH3COONa (45 mL) buffered to pH 5 with CH3COOH and was shaken at 250 rpm for 6 h at room temperature.

  3. (3)

    Fe/Mn oxide-bound fraction (OXI), the residue from (2) was extracted by 0.04 M hydroxylamine hydrochloride (in 25% (v/v) CH3COOH) at initial pH 2 (45 mL) and was shaken at 250 rpm for 5 h at 96 ± 3 °C.

  4. (4)

    Organic matter/sulfide-bound fraction (ORG), the residue from (3) was extracted by 0.02 M HNO3 (10 mL) and 30% (v/v) H2O2 (10 mL) at initial pH 2 and was shaken at 250 rpm for 3 h at 85 ± 2 °C. Then, 10 mL of 30% H2O2 was added into the same tube with continuous shaking for 2 h. Finally, 15 mL of 3.2 M CH3COONH4 in 20% HNO3 was added into the tube, and then it was shaken at 250 rpm for 0.5 h at room temperature.

  5. (5)

    Residual fraction (RES), the residue from (4) was microwave digested with HNO3, HF and HCl.

The leaching tests were designed to investigate the release of metals at the solid:liquid ratio 1:4 (g/mL) and at pH = 4, 5, 6, 7. The process steps of the experiments were as follows. One hundred gram of gangue sample was mixed with 400 mL DI water (or DI water that the pH was adjusted by HNO3 and/or NaOH) in a 500-mL conical bottle, and shaking it on a thermostatic water bath oscillator at 25 °C. After a certain shaking time (0.5 h, 1 h, 3 h, 6 h, 12 h, or 24 h), the oscillator was temporary stopped and 10 mL solution was taken out from the conical bottle and then was centrifugally separated; the solution was preserved for determination and named as leaching solution in this work.

Heavy metals arsenic (As) and mercury (Hg) were determined with non-dispersive atomic fluorescence spectrometer (PF6-2), and heavy metals copper (Cu), lead (Pb), nickel (Ni), chromium (Cr), cadmium (Cd), and zinc (Zn) were determined with atomic absorption spectrometer (TAS-986). All chemicals in the extraction experiments were guaranteed reagent (GR), and water was DI water (purified by Milli-Q Academic, the electrical conductivity was 0.055 μS/cm). The quality for analytical accuracy was guaranteed by setting reagent blanks and inserting standard soil samples (China Standard Soil ESS-2 Brown Soil). Standard adding method was also used in leaching test, and the recovery varied from 93.7 to 106.1%.

Biological risk assessment method

The potential ecological risk index which was proposed by Swedish scientist Hakanson (1980) as a diagnostic method for water pollution control is still widely used (Chen et al. 2018; Deng et al. 2017; Zhang et al. 2017). Heavy metal background, loads, and bioavailability were considered in the potential ecological index (Huang et al. 2018). The formula is shown as the following:

  1. (1)

    Single heavy metal pollution index

$$ {C}_f^i=\raisebox{1ex}{${C}^i$}\!\left/ \!\raisebox{-1ex}{${C}_n^i$}\right. $$
(1)

where \( {C}_f^i \) indicates single heavy metal pollution index, Ci is the measured concentration of a heavy metal in gangue, \( {C}_n^i \) is the background value (BGV) of heavy metal, and the superscript i indicates the specific pollutant.

  1. (2)

    Potential ecological risk index

$$ {E}_r^i={T}_r^i\times {C}_f^i $$
(2)

where \( {E}_r^i \) is the potential ecological risk factors for each heavy metal and \( {T}_r^i \) is the toxicity factor of heavy metal i and is determined for Cu = Ni = Pb = 5, Cr = 2, Cd = 30, and Zn = 1.

  1. (3)

    Comprehensive ecological risk index

$$ \mathrm{RI}=\sum {E}_r^i $$
(3)

where RI is the comprehensive ecological risk index.

The definition and classification of ecological risk levels are listed in Table 1 (Wang et al. 2017a, b).

Table 1 Potential ecological risk index level
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Results and discussion

Total content of heavy metal in coal gangue

The total content of metal in gangue of different weathering degree is shown in Table 2. Besides zinc, the total amount of heavy metal was more than local background values of Shandong. Especially cadmium was 11 times in LB and 35 times in BG over the level. With the deepening of the weathering degree, the content of Pb and Cu initially declined, but then increased. It indicated the possibility of release and contamination to the surrounding soil and water by gangue.

Table 2 Contents of heavy metals in gangue (mg/kg)
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Chemical species of heavy metals in gangue

The assumption that all forms of a given metal have an equal impact on the environment is clearly untenable (Tessier et al. 1979; Zhang et al. 2016; Zhou et al. 2017). Chemical species of metals are more suitable rather than the total concentration for assessment of the potential effects or analysis of the behavior of the heavy metals in solid waste. The heavy metal in solid material can be partitioned into five fractions. Hydrogen ion will be produced because of acid rain and the exchangeable fraction was easy to migrate and release into the environment in this condition (Ji et al. 2018). The carbonate speciation of metals can form carbonate or precipitates such as calcium carbonate. Heavy metal will be absorbed on the surface of precipitates. Under acid rain conditions, exchangeable fraction and carbonate bound were easier to release into the environment compared with other three speciation (Wang et al. 2013).

In gangues, the sum of EXC and CAR in Ni, Cd, As, and Hg (Fig. 2e–h) was only a small proportion of the total. It meant that the four heavy metals were difficult to be released into the environment and absorbed by the organism. The sum of exchangeable and carbonate fraction of zinc or copper was about 20% of the total; it means zinc or copper was easily released into the environment. It was very interesting that the organic fraction of zinc or copper gradually increased in the order of BG < BG < GG < LB < DB (Fig. 2 c, d), and it was exactly the order in which samples were collected from the surface to the inner layers. Zinc and copper are typically thiophilic and are often associated with pyrite. The sulfide can be preserved well under the anoxic condition. It means that weathering intensity gradually weakened in the order of BG < BG < GG < LB < DB.

Fig. 2
figure 2

Heavy metal speciation of five kinds of gangue. EXC, exchangeable fraction; CAR, carbonate fraction; OXI, iron manganese oxide bound; ORG, organic fraction; RES, resident fraction (a Cr, b Pb, c Zn, d Cu, e Ni, f Cd, g As, and h Hg)

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Cr and Pb had also attracted widespread attention due to their physical (Ni et al. 2017) and chemical (Yang et al. 2017) properties and toxicity (Zhou et al. 2017). The Cr and Pb in gangue were a large proportion of the total (Fig. 2a, b). The change of cationic exchangeable and carbonated bound in chromium was fluctuant and the sum of the two fractions increases with the deepening of weathering. While the EXC and total amount of lead increased first and then decreased, it can be speculated that it was the result of enrichment first and then leaching. It was due to the oxidation of the organic carbon and lattice destruction of the minerals. The part of Cr in gangue was mainly associated with exchangeable fraction (EXC) and carbonate fraction (CAR), which could be released into the environment under acidic conditions (Yin et al. 2016). Compared with the other four fractions, the residual fraction was the main part in BG, GG, and LB, which was relevant with silicate (He et al. 2016). The harm of Cr and Pb should arouse great attention as the accumulation of the elements is toxic to human health (Tang et al. 2017).

Results of leaching tests

Heavy metals in gangue had caused ecological pollution and influenced human health under the condition of the sun, rainfall, weathering, and other factors (Li et al. 2017). Leaching test was an effective method to simulate the process and influence (Imoto et al. 2018). The results showed that Cu cannot be released in neutral condition (Table 3). With the deepening of gangue, the separating metal content of Zn (Table 3) and Ni (Table 4) increased first and then declined and it reached the maximum when the degree of weathering was LB. From the above, it could be concluded the EXC and CAR of Cr and Pb were a large proportion of the total and should attract great attention.

Table 3 Variation of Cu and Zn in coal gangue of different weathering degrees in leaching tests
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Table 4 Variation of Ni in coal gangue of different weathering degrees in leaching tests
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It can be concluded that Cr had different releasing patterns under different weathering conditions affected by pH (Fig. 3). The solubility of Cr in gangue was minimal at pH = 7 (2 mg/L) and was maximal at pH = 5. In RG (Fig. 3a), the release concentrations showed a trend of decline. While in GG (Fig. 3c), it presented a steady trend. Under the condition of pH = 7, the Cr has been washed out completely with a straight line. Observing the release curve of RG and GG (Fig. 3c), it can be found that the change of Cr was not obvious, that was, in the initial stage of soaking, Cr had been released for the most. In the other three kinds of gangue of different weathering degrees, the precipitation of Cr was mostly increased with time, and the concentration at pH = 5 was the largest, at pH = 7 the lowest. For red and light black gangues, with time changes, the curve had a large fluctuation.

Fig. 3
figure 3

Variation of Cr in coal gangue of different weathering degrees in leaching tests (a RG, b BG, c GG, d LB, and e DB)

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Under different weathering conditions, it could be found that a special releasing pattern of Pb was shown which was relative to pH (Fig. 4). The same as Cr, the solubility of Pb in gangue was minimal at pH = 7 (≤ 0.7 mg/L) and was maximal at pH = 5. This indicated that the Pb was easy to precipitate in neutral conditions and could be precipitated out in weak acidic conditions. In RG (Fig.4a) at pH = 4, 5, and 6, the maximum concentration of the solution was nearly equal, while the curve at pH = 5 and 7 was similar, which may be related to its chemical properties. Observing the releasing curve of gangue with Pb, it could be showed that there was not much change in it and the release rate was a smooth curve. It meant that in a short period of time, Pb could continue to precipitate out to the environment.

Fig. 4
figure 4

Variation of Pb in coal gangue of different weathering degrees in leaching tests (a RG, b BG, c GG, d LG, and e DB)

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The elements Cd, As, and Hg are more poisonous than the other four elements. However, these three trace elements were not detected in the leaching test. The same phenomenon can also be found in Fig. 2 (heavy metal speciation of five kinds of gangue) which showed the exchangeable and carbonate fractions of the Cd, As, or Hg were almost all not detected. It means that release of these three trace heavy metals was too little to be detected in leaching experiments.

After weathering for years, the lattice structure of minerals (e.g., the silicate minerals) was destroyed and a large amount of metal oxides were produced which was easier to release heavy metals (Yang et al. 2018). The information from the species of heavy metals described as above section also indicates the same law. In the process of leaching, the exchangeable fraction and carbonate fraction can be released into the water. So weathering degree should be concerned in the process of gangue hill excavation and heavy metal pollution controlling in the mining area.

Ecological risk assessment

The concentration of heavy metal in coal gangue at different weathering degrees was set as Ci and the soil environment background values of heavy metals in Shandong Province as \( {\boldsymbol{C}}_{\boldsymbol{n}}^{\boldsymbol{i}} \)(Isimekhai et al. 2017). The main influencing factors changed with the degree of weathering (Table 5). For the fresh gangue, DB, Cr, Ni, and Cu contributed to the value of potential ecological risk index, but its RI was only 46.21, that was, at slight level. With the weathering degree deepening, in LB and DB, Cd had the most contribution to RI (96%, 95%). Because of the Cd, the ecological risk was at strong level (LB) and was up to very strong level (DB) respectively. Cr had contributed the most for GG, which reached 71%. When the gangue was weathered to its maximum, Cu, Pb, and Ni had almost the same contribution to RI for RG with about 30% respectively. The results showed that the ecological risk level of heavy metal in different weathering degree was not the same and the weathering degree of gangue had a great influence on the pollution level of heavy metal. It can be concluded the form of heavy metal in gangue changed due to weathering and lead to the difference of the leaching characteristic and risk. It can be explained from geochemical processes. In the process of exploitation, the gangue can cause different levels of risk to environment. The gangue will spontaneously ignite after a long stacking (Tan et al. 2016). The spontaneous combustion of gangue was an extremely complex physical and chemical reaction process, which was the result of the interaction between internal and external factors (Wang et al. 2017a, b; Wu et al. 2017). One of the theories is pyrite oxidation (Fig. 5). It shows that pyrite in coal gangue has strong reducibility. After the contact with O2, the pyrite can cause a series of redox reactions, releasing heat which accumulates in gangue pill. When the temperature reaches the burning point of the carbon in gangue, the spontaneous combustion occurs and SO2 emitted (Querol et al. 2011). On the one hand, the SO2 reacts with the water and oxygen in the air and forming sulfuric acid and then precipitated to the ground. On the other hand, pyrite directly reacts with water and oxygen; then, ferrous sulfate and acidic water formed (Wang et al. 2016). The Thiobacillus thiooxidans (Tt) (Li et al. 2010) and other oxidative bacteria play a catalytic role during the reactions. In this way, heavy metals associated with pyrite are released into the environment, while the carbonate fraction of heavy metal will be transferred to the environment because of the reactions with the acidic water (Shi et al. 2018).

Table 5 Evaluation of potential risk of heavy metal in coal gangue of different weathering degrees
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Fig. 5
figure 5

The mechanism of sulfur cycle and the release of heavy metal in coal gangue hill

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The prevention and control of pollution with gangue should focus on leachate produced in the course of excavation. Leachate can cause heavy metal pollution to the soil and water. Therefore, the leachate should be collected and treated rather than discharged directly. Meanwhile, planting trees or plants on the surface of gangue hill, which can be a rainwater-resisting layer and the oxygen transmission-obstructing layer, to reduce the volume of leachate is also a good method to prevent the leachate from polluting the surrounding environment.

Conclusion

In this study, the coal gangue at various weathering degrees was gathered from Suncun mine which located in Shandong Province of China. Research on sequential extraction, leaching test, and potential ecological risk leads to the following conclusions.

  1. (1)

    Total amount of almost all heavy metal was more than local background values of Shandong; especially cadmium was 11 times in LB and 35 times in BG over the level. But results of chemical species analysis showed that the sum of EXC and CAR in Ni, Cd, As, and Hg was only a small proportion of the total; the sum of exchangeable and carbonate fractions of zinc or copper was about 20% of the total; the organic fraction of zinc or copper increased in the order of BG < BG < GG < LB < DB; and the part of Cr in gangue was mainly associated with exchangeable fraction and carbonate fraction. Compared with the other four fractions, the residual fraction was the main part in BG, GG, and LB.

  2. (2)

    The results of leaching test showed various leaching patterns. Cu cannot be released in neutral condition. With the deepening of gangue, the separating metal content of Zn increased first and then declined and it reached the maximum when the degree of weathering was LB. The solubility of Cr and Pb in gangue was minimal at pH = 7 and was maximal at pH = 5.

  3. (3)

    The ecological risk level of heavy metal at various weathering degrees was not the same and the weathering degree of gangue had a significant influence on the risk level. For the fresh gangue, DB, Cr, Ni, and Cu contributed to the value of potential ecological risk index which was at slight level. For LB and DB, Cd had the most contribution to RI (96%, 95%). Because of the Cd, the ecological risk was at strong level (LB) and was up to very strong level (DB) respectively. Cr had contributed the most for GG, which reached 71%.

In summary, there were differences in the release characteristics of heavy metals in gangues at different weathering degrees. The species of heavy metal in gangue changed due to weathering and lead to the difference of the leaching characteristic and risk. Weathering degree should be concerned in the process of gangue hill excavation and heavy metal pollution controlling in the mining area. The prevention and control of pollution should focus on the leachate which should be collected and treated, and should take measures to reduce the volume of leachate.