3.1 Basic indicators
The pH range was between 3.62 and 7.78, with an average of 5.09, and showed an increasing trend with the increase of profile depth. The acidic surface tailings were mainly caused by the oxidation of metal sulfides to produce an acidic environment. As the depth increases, the oxidation environment weakens, and the pH gradually rises and stabilizes. The content of MC was low, with an average of 0.56, and there was no obvious change with the increase of profile depth (Table 1).
Table 1
Basic indicators of tailings
Profile depth/ m | 0 | -0.5 | -1 | -1.5 | -2 | -2.5 | -3 | -3.5 | -4 | -4.5 | -5 |
pH | 3.62 | 3.66 | 3.67 | 3.87 | 3.75 | 4.25 | 4.89 | 5.34 | 6.19 | 7.66 | 7.78 |
MC | 0.42 | 0.43 | 0.55 | 0.52 | 0.56 | 0.61 | 0.59 | 0.62 | 0.61 | 0.58 | 0.64 |
Note: “MC”-moisture content/ % |
3.2 Particle size distribution
Tailings in different layers have different particle size distribution characteristics. The larger the particle size distribution is, the higher the particle content is in the tailings. Meanwhile, the degree of oxidation of metal sulfide is not only related to oxygen and water, but also has a great influence on the particle size distribution of tailings (Jakubek et al., 2018; Silveira et al., 2018; Kang et al., 2018). In this study, the particle-size classification was: coarse sand (1 ~ 0.5 mm), medium sand (0.5 ~ 0.25 mm), fine sand (0.25 ~ 0.125 mm), very fine sand (0.125 ~ 0.063 mm), silt sand (0.063 ~ 0.002 mm) and clay (< 0.002 mm) (Fan et al., 2021). As shown in Fig. 2, the particle size of tailings was mainly distributed in 0.002 ~ 0.5 mm, and the particle size distribution changed greatly with the depth of tailings. The particle size of tailings at 0m ~ 1m was mainly distributed between 100 µm and 1000 µm, of which coarse sand, medium sand and fine sand were mostly. The particle size of the tailings at 1m ~ 5m was mainly distributed between 10 µm and 100 µm, of which fine sand, very fine sand and silt were mostly. Larger particles make it easy for rain and air to react with minerals on the surface and make the surface harder.
3.3 Resistivity of tailings accumulation
Electrical resistivity tomography (ERT) is a physical technology. The RES2DINV two-dimensional inversion program is an inversion calculation program based on the smooth constrained least squares method. It can use the new optimization nonlinear least squares algorithm to greatly improve the resolution of the direct current method and better reflect the underground apparent resistivity (Hsu et al., 2010; Theoharatos., 2008). Figure 3 was the resistivity profile of accumulated tailings obtained by two linear survey lines. The detection depth of the parallel electrical method was about 12m, and the average resistivity of tailings was about 400Ω·m, and the average resistivity of tailings was generally low due to low water content. However, there was an obvious low-resistance anomaly area of about 1.2 m in the surface layer because the HMs on the surface of the tailings have been leached by rainwater for a long time, and the accumulation time was longer, which caused the continuous penetration of water and air, causing the tailings to continue to oxidize and form a hardened layer of oxidation. The formed hardened layer can limit the continuous infiltration of acid leachate and the oxidation of sulfide, reducing the oxidation rate of tailings (Ahn et al., 2011). In addition, the surface has a larger particle size (Fig. 2), high water permeability, and a high content of conductive minerals (sulfide) (Fig. 4). As the increase of depth profile, the particle size shows a decreasing trend, and the S content gradually decreases and then stabilizes. This is the reason why the resistivity of surface is low and bottom is high (Fig. 3). In the profile with decreasing granularity, fine metal sulfide minerals are not suitable for flotation, so fine materials can retain most of the water, and the vertical flow gradually slowed down and prevented the formation of large areas in strong oxidation zone (Nikonow et al., 2019). But in the process of moving to south resistivity changed, 30 m in horizontal direction at high resistance was unusual, this was because the artificial mining activities led to the consolidation or less tailings stacking the uneven and produce cracks, increased resistivity. Hazardous transmission of acid mine waste water (AMD) or HMs pollutants was present here, reflecting that may affect the structural stability of faults, or cracks (Fig. 3a). At the edge of the profile, the resistance was higher due to the weathering of the tailings, the evaporation of water and the lower ion concentration (Fig. 3b).
3.4 Distribution characteristics of major and trace elements
As shown in Fig. 4, and the relevant data was shown in Table S1. The sequence of sulfide minerals was Fe2O3 > SiO2 > CaO > Al2O3 > MgO > K2O > Na2O. The contents of Fe, Si and Al were 47.11%, 29.04% and 3.11%, respectively, which was consistent with the results of XRD analysis (Fig. 6), indicating that the composition of iron ore was mainly composed of pyrite, magnetite, pyrrhotite, goethite and other iron oxide and quartz, and consists of a small part of aluminum silicate. The average content of Fe2O3 in hardened layer was 41.41%, and that in weak oxide layer and loose layer was 32.05%, and the content decreased with the increase of depth. The average content of S reached 4.97%, which was mainly concentrated in hardened layer and tends to be stable with the profile depth, and was the main element for producing acidic wastewater. The contents of SiO2 and CaO at different depths in the profile was stable, and the average content respectively was 24.24% and 17.4%. Tailings containing a lot of SiO2 and CaO could be used as brick materials (Kim et al., 2019), which could not only make full use of mineral resources, but also be an important means to protect the ecological environment. The content of MgO in hardened layer was lower than that in loose layer, which was mainly related to the leaching degree of surface runoff, while the content of other compounds did not change much with the increase of profile depth.
The elements in Table S2 were included in the list of priority pollutants of USEPA. It showed that the relative abundance of the average HMs contents in tailings profile followed the sequence: Cu > Zn > Hg > As > Pb > Cr > Ni > Cd, and the average contents of Cu, Cd, Zn and As respectively were 1865.30 mg/kg, 1.39 mg/kg, 774.39 mg/kg and 78.34 mg/kg, reaching 37.31 times, 4.63 times, 3.87 times and 1.96 times of the risk screening value in GB15618-2018. The average contents of Pb was 68.84 mg/kg and Ni was 42.95 mg/kg, which was lower than the risk screening value. It can be seen from Fig. 5 that the content of Hg and As in hardened layer was higher than that in loose layer, exposure of sulfide to air and water would cause the acidic leaching solution to release high concentrations of Hg and As that were easily adsorbed and precipitated by iron hydroxide, and are then enriched in oxidized hardened layer. The content of Cd in the hardened layer (0.66 mg/kg) was significantly lower than that in the non-oxidized layer (1.81 mg/kg), the main reason was that Cd was a sulfide associated element, and the oxidation process led to the consumption of Cd, Co and other sulfide associated elements in the tailing (Alakangas et al., 2006). The contents of Cu, Zn, Cr and Ni in the hardened layer were lower than that in the loose layer. This was because the HMs migrate to the surrounding area through rainwater leaching, which led to pollute groundwater, surface water and river water, and thus affected the health of the residents below the dam through the aquatic food chain.
HMs pollutants associated with primary sulfur in tailings, and the oxidation of tailings can be released to the surroundings of trace metals, and the S accelerated leaching of HMs and the acidic waste water produced in the surface layer and local enrichment. In addition, the formation of secondary minerals affected the migration and transformation of HMs, its surface tended to adsorption of HMs, which aggravated the pollution of surrounding soil and water (Rodríguez et al., 2009; Hamberg et al., 2016). Meanwhile, the particle-size of tailings affected the migration of HMs, and the larger surface particle size led to less dust generated under the action of wind. Therefore, the pollution of the tailings to the surrounding environment was mainly the acidic mine waste water generated by rainwater scouring and the migration of HMs.
3.5 Mineralogical analysis of representative tailing samples
The color of the tailings hardened layer was brownish yellow, and the loose layer was gray-green. The mineralogy of tailings profile was evaluated by SEM, polarized light microscope and XRD. As shown in Fig. 6, the tailings in the hardened layer had a large particle-size, and the mineral phase was observed to be covered by a large number of tailings particles of different shapes and secondary minerals, and the rounded edges of the debris showed signs of physical or chemical wear (Fig. 6a). The main primary minerals were gypsum and quartz, the secondary minerals were jarosite, which produces alteration along the grain margin, and Muscovite (aluminium silicate) (Fig. 6b, c). The formation of gypsum minerals was attributed to the oxidation of sulfide minerals, and which released sulfate ions (Cihangir et al., 2018). Meanwhile, the external oxidation conditions affect the mineral composition and form of tailings. The important indexes of the oxidation process of tailings were goethite and hematite, and jarosite sediments can be formed through mineral dissolution and reprecipitation to promote the formation of oxidizing crusts (Tang et al., 2018; Redwan et al., 2012; Regelink et al., 2014). Jarosite was the main reason that causes the surface layer of tailings to be brownish yellow, and its content decreases with the decrease of oxidation degree. Mineral crystals in the loose layer were relatively complete and grow in layers (Fig. 6d). The primary minerals were gypsum, quartz, pyrite and feldspar. In addition to jarosite and muscovite, the secondary minerals also include pyrrhotite, goethite, sphalerite, and no obvious corrosion was observed on the surface of mineral particles (Fig. 6e, f). The presence of Al, K and Na in tailings was attributed to muscovite, jarosite and feldspar, and the diffraction peak parameters were shown in Table S3. The metal sulfide mainly contains iron ore, which was related to the processing technology of concentrator (Zheng et al., 2019b).
The existence of metal colloid is limited by the mineralogy and geochemical composition of the tailings solid, but the fluidity in the pore water of tailings is controlled by the secondary sedimentation-dissolution and adsorption-desorption reactions as well as the biogeochemical redox process (Nordstrom., 2011). For example, Fe, Al, Zn and Cu will show higher dissolution concentration and greater mobility in acidic environment, while As, Se, Mo, Sb and other elements forming (hydrogen) oxygen anions can move in alkaline pore water (Majzlan et al., 2014). Calcite, pyrite and goethite and other mineral characteristics peak were not seen in hardened layer, but the peak gradually appeared with the increase of depth. It also indicated that surface silicate minerals showed strong characteristic peaks, while carbonate minerals and metal sulfide minerals appeared weak or even zero characteristic peaks, reflecting opposite characteristics with the increase of profile depth. This was because there is more free oxygen and free water on the surface, S and Fe mainly enriched in the surface layer, metal sulfide oxidation produce acid to dissolve primary metal minerals in acidic conditions to generate high acidity of pore water, is characterized by a high concentration of dissolved Fe. Carbonate minerals such as calcite react with acid to be consumed, resulting in less metal sulfide and carbonate minerals in the surface tailings. As the depth increases, the oxidation weakens and the characteristic peak becomes stronger. Subsequent dissolution of aluminosilicate minerals in the sulfide oxidation zone also contributed to the dissolution of Al, and acidic pore water migrated downward and was neutralized by dissolution of carbonate minerals (Moncur et al., 2005).
3.6 Chemical fraction and risk assessment
The environmental risk of tailings was predicted by using BCR sequential extraction and TCLP. The chemical fraction distribution of HMs in representative composite tailings was showed in Fig. 7a, Tables S4 and Table S5, and the different fractions of HMs were distributed as: Ni and Cr were F4 > F2 > F1 > F3, Cd was F4 > F1 > F2 > F3, Pb and Zn were F4 > F2 > F3 > F1, Cu was F4 > F3 > F1 > F2. From the perspective of fraction transformation behavior, most of HMs are in the residual (F4), especially arsenic. This fraction (F4) is considered unusable and not easy to release. The mobility of HMs is closely related to the content of bioavailable states. Therefore, the RAC index is established on the basis of the bioavailable state, and the ratio of bioavailable content to total content is divided into five levels to judge its risk level (Fig. 7b). Usually the first fraction (F1) has the greatest instability and environmental fluidity in the environment (Singh et al., 2005). According to RAC, Cr and As in tailings are no risk, Cu, Zn, and Pb are low risk, Ni is medium risk, and Cd is high risk. Pb and Cd with certain risks may limit the resource utilization of tailings. The identification standard values of hazardous waste under the supervision level of China and USEPA are listed in Table S6 (Zhang et al., 2021; Zhou et al., 2021). According to the TCLP results (Fig. 7b), the leaching content of Cu, Zn, Pb, Cr, Ni, As is below the limit, while the leaching content of Cd is higher than the limit, which is a hazardous material. Therefore, except for Cd, the tailings are relatively safe for humans and the environment. However, there is still a risk of HMs pollution to the surrounding environment, and additional attention should be paid to Cd pollution.