Coal-based solid waste from the Panyi Mine in the Huainan mining area: Fine-scale chemical structure and ecological risk assessment of heavy metals

The utilization of clean coal-based solid waste is a pressing issue in the ecological construction of mining areas. To address this issue, the key is to ascertain the structures and morphology of heavy metals in coal-based solid waste. This study investigated the coal gangue and fly ash samples from the Panyi Mine in the Huainan mining area, Anhui Province. First, this study characterized the coal-based solid waste using microanalysis methods, such as X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). Then, it examined the fine-scale chemical structure and heavy metal dissemination modes of coal-based solid waste. Finally, this study assessed the potential ecological risks of heavy metals by combining the risk assessment code (RAC). The key findings are as follows: (1) The coal gangue, dominated by clay particles (0-5 μm) and coarse-grained silts (10-50 μm), exhibits irregular particle sizes and large spatial distribution spacings. Its main mineral is quartz (SiO₂), with the bending vibrations of AlO₄ and SiO₄ presence in its infrared (IR) spectra. In contrast, the fly ash, dominated by coarse-grained silts (10-50 μm) and sandy gravels (50-250 μm), manifests spherical inclusions and porous particles on the surface, varying in particle size. Its main mineral is mullite (Al₆Si₂O₁₃), with symmetric and antisymmetric stretching vibrations of Si-O-Si within organosilicon present in its IR spectra. (2) In the coal gangue, Ni and Pb are primarily organic bound or bound to Fe-Mn oxides, while Cr, Cd, and As predominantly occur in the residual form. In the fly ash, As principally occurs in residual and organic bound forms, while other heavy metals occur in the residual form. (3) As indicated by the RAC-based ecological risk assessment, Cr and As in both coal gangue and fly ash are at a low risk level, and other heavy metals in them do not pose a risk. The results of this study hold practical significance for the reutilization of coal-based solid waste and the eco-environmental protection of mining areas.