Release behavior of dissolved carbon species from coal gangue during alkaline hydrothermal treatment

Coal gangue is produced in large quantities and may release dissolved carbon during long-term weathering and leaching, yet its carbon mobilization behavior under alkaline hydrothermal conditions remains insufficiently understood. This study investigated the effects of extractant type, particle size, KOH concentration, liquid-to-solid ratio, hydrothermal temperature, and reaction time on the release of dissolved carbon (DC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) from coal gangue. Among the tested extractants, KOH showed the highest extraction capacity and was selected as the alkaline medium for subsequent experiments. Orthogonal optimization identified 180 °C, 10 mL g^-1 liquid-to-solid ratio, 0.75 mol per L KOH, and 18 h as the optimal conditions, under which DC, DOC, and DIC releases reached 24.58, 18.43, and 6.15 mg g^-1, respectively. Range analysis and ANOVA indicated that hydrothermal temperature and liquid-to-solid ratio were the dominant factors within the investigated range, while KOH provided the alkaline environment required for carbon mobilization. EEM analysis showed that the released DOC mainly exhibited humic-like and fulvic-like fluorescence characteristics. Structural characterization further revealed aluminosilicate framework transformation, pore-structure enlargement, surface chemical changes, and the formation of K-bearing silicate/aluminosilicate crystalline phases after alkaline hydrothermal treatment. These results indicate that dissolved carbon release from coal gangue is controlled by the coupled effects of alkaline attack, mineral phase transformation, pore-structure opening, and organic carbon liberation. This study provides new insight into dissolved carbon mobilization from coal gangue and offers a basis for regulating carbon release during the hydrothermal utilization of coal-based solid wastes.