Pore-scale transport and adsorption behavior of Mn(II) in CT-reconstructed adsorptive permeable filling materials for mine water pollution control

Constructing underground reservoirs in coal mine goafs can effectively mitigate surface subsidence and facilitate large-scale utilization and purification of mine water. However, seepage characteristics within backfill bodies and the migration–adsorption behavior of Mn 2+ remain insufficiently visualized and characterized. In this study, X-ray CT scanning, Avizo-based three-dimensional reconstruction, and COMSOL Multiphysics simulation were employed to establish realistic pore-scale models of adsorptive permeable filling materials (AP-FM) for coal mines. The effects of coal gangue particle size on the seepage characteristics and Mn 2+ migration–adsorption behavior were systematically investigated. The results show that the saturated Mn 2+ adsorption capacities of AP-FM with particle sizes of 5–10 mm, 10–15 mm, and 15–20 mm were 0.971, 0.859, and 0.733 mol/m 3 , respectively. Increasing coal gangue particle size increased the mean flow velocity within the model and resulted in nonuniform hydraulic pressure transmission. The model with a particle size of 15–20 mm exhibited the highest mean flow velocity of 1.76 × 10 −4 m/s, followed by those with particle sizes of 10–15 mm and 5–10 mm, with mean flow velocities of 1.34 × 10 −4 and 1.32 × 10 −4 m/s, respectively. Cross-sectional porosity was negatively correlated with average pore flow velocity, while pore connectivity was also identified as a key factor governing the distribution and magnitude of water flow velocity within the model. The adsorption behavior of the materials conformed to the Langmuir model, and adsorption capacity decreased with increasing particle size. This study provides theoretical guidance for purification-oriented utilization of mine water in coal mines.