Mechanism of the pore and molecular structure evolution of coal exposed to acid mine drainage (AMD)

In the post-extraction epoch, wastewater from mining activities, particularly acid mine drainage (AMD) residing in sulfur-laden coal terrains, assumes a pivotal role in the safety stewardship of decommissioned coal mines. This research aims to investigate the mechanism behind coal characteristic deterioration from prolonged exposure to AMD. Immersion assays were performed on coal samples across pH 2 to 5 to assess the impact of acid mine drainage. Subsequently, the pore and molecular architecture was appraised using microscopic methodologies. Computed Tomography (CT) findings elucidate that post-immersion, the porosity, and fissures proliferated longitudinally along the coal strata, engendering a marked amplification in surface porosity contiguous to pre-existing pores. This escalation in surface porosity was further accentuated in correlation with the intensification of AMD acidity. Nuclear Magnetic Resonance (NMR) data indicated a marginal augmentation in the content of both micropores and macropores within a tepid AMD milieu. However, in a more virulent AMD context, the proportion of micropores diminished, whereas that of macropores and pore throat size (PTS) experienced an upswing, thereby transmuting adsorptive pores into permeable conduits and consequently enhancing coal permeability. Scanning Electron Microscopy (SEM) corroborated the NMR outcomes; as AMD acidity transitioned from mild to severe, the coal matrix manifested many erosive pores, matrix layer disintegration, and an expansion in cleat width. Therefore, the microscopic pore evolution can be succinctly encapsulated as follows: in a mild AMD environment, dissolution of minerals predominates, generating erosive pores, whereas, in a more acidic AMD milieu, the matrix undergoes partial contraction, thereby augmenting pore volume, enhancing permeability, and inducing structural degradation. Additionally, Fourier Transform Infrared Spectroscopy (FTIR) analysis substantiated that AMD compromised the pore architecture and catalyzed the disintegration of coal macromolecules into lower molecular weight constituents. Therefore, AMD degrades coal macromolecules into smaller compounds, heightening matrix layer porosity and impairing coal characteristics. This research yields vital insights for the security and efficient management of abandoned mine excavations.