Geochemical mechanisms of fluorine migration and transformation during deep geological sequestration of high-salinity coal mine water in the ordos basin

Deep geological sequestration is an emerging strategy for managing high-salinity coal mine water. However, the migration behavior of fluoride (F−) during sequestration poses significant risks to groundwater safety and system stability. Elucidating the mechanisms of F− migration and transformation is therefore critical for ensuring the long-term safety of sequestration projects. In this study, static batch experiments were conducted to investigate F− migration and retention in sequestration formations with different lithologies. Three representative rock types were collected from the Liujiagou Formation (fine sandstone–muddy sandstone and fine sandstone–mudstone) and the Shiqianfeng Formation (medium sandstone–mudstone) in the Ordos Basin. The experiments were carried out under simulated formation temperature (55 C) using synthetic high-salinity water with varying initial F− concentrations (20–150 mg/L) and pH conditions (2.0–10.0). Adsorption kinetics and isotherms were analyzed using pseudo-first-order, pseudo-second-order, and Freundlich models, while water–rock interaction mechanisms were evaluated through ionic ratio analysis, chloro-alkaline indices, and sodium adsorption ratio. Results showed that F− removal suggests dominance of chemical adsorption, following a rapid–slow–equilibrium pattern, and was well described by the pseudo-second-order kinetic model and Freundlich isotherm (R2 > 0.999). F− migration was likely governed by coupled processes of cation exchange, competitive adsorption, and mineral precipitation–dissolution. Fine sandstone–mudstone exhibited the strongest F− fixation capacity (462.7 mg/kg) due to synergistic adsorption and precipitation. Sensitivity analysis identified pH as the most influential factor. These findings provide a experimental basis for predicting F− pollution and support the safe implementation of deep geological sequestration in high-salinity coal mine water management.