Research on Release Law and Characteristics of Heavy Metals in Coal Gangue under Soaking and Leaching

In order to explore the release law and characteristics of heavy metals in natural coal gangue under the rainwater soaking and leaching, scanning electron microscope (SEM) was used to observe the microscopic morphology of natural coal gangue and X-ray diffraction (XRD) was used to determine the mineral composition and changes before and after leaching. The release law and characteristics of heavy metals in coal gangue were investigated by continuous static soaking and dynamic leaching tests with simulated precipitation under different solid-liquid ratio, particle size, pH and soil medium conditions. The results show that: (1) SEM and XRD analysis results reveal that the surface of natural coal gangue has a layered structure with high smoothness, and after leaching, the surface of coal gangue presents more concave-convex structures. There is small change in the content of each mineral phase, therefore the coal gangue has high stability during natural rainfall leaching process. (2) The simulated continuous static soaking test shows that the leaching content of As and Mn is higher, and the release rates are ranked as As>Cd>Cu≈Ni>Mn>Cr>Zn>Pb. Among them, the cumulative release of As poses a higher risk of adverse effects on the surrounding soil environment and groundwater, and the leaching of Cu has two stages: rapid release and stable release. Zn, Pb, and Cd belong to the late-stage release elements; Cr and As belong to indirect release elements; Mn belongs to fast release elements; and Ni belongs to slow release elements. (3) The continuous dynamic leaching test shows that the cumulative release of Mn, Cu, Zn and Ni is significantly higher at pH 3.0, and the cumulative release of As and Cr under neutral conditions is higher than that under acidic conditions. After the introduction of the soil medium, the cumulative release of Cr and As decreased significantly, indicating that both heavy metals are adsorbed the soil. The research shows that for the same stack of coal gangue, the release of heavy metals is higher with high rainfall. The maximum release of As can reach 2.822 μg/L at a solid-liquid ratio of 1∶20. Because Cr and As are accumulated in the soil, after leaching of the coal gangue, Cr and As pose potential risk of polluting the soil, and the impact on surrounding groundwater should also be considered.