Molecular Fingerprints and Reactivity Evolution of Dissolved Organic Matter Leached from Coal under Geothermal Conditions

Understanding the thermal behavior of coal-derived dissolved organic matter (DOM) is essential for assessing its environmental reactivity in deep mining environments. Accordingly, in this study, gas coal (GC) and anthracite, representing distinct coal ranks, were subjected to temperature-controlled static incubation experiments at 25 and 50 °C to investigate the temperature-induced release dynamics and molecular evolution of Coal-DOM under simulated subsurface conditions. A multianalytical strategy combining excitation-emission matrix spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and machine learning (XGBoost-SHapley Additive exPlanations [SHAP]) was applied to characterize the molecular-level transformations of Coal-DOM. The results indicated that elevated temperature substantially enhanced DOM release, with dissolved organic carbon concentrations increasing by approximately 54% (to 10.3 ± 0.3 mg-C/L) for GC-DOM and 52% (to 12.3 ± 0.2 mg-C/L) for ANT-DOM compared to 25 °C, and promoted the generation of more oxidized, aromatic, and nitrogen-rich compounds, particularly in GC-DOM. FT-ICR-MS analysis revealed significant molecular alterations: the number of assigned molecular formulas increased from 5,593 to 8,470 for GC-DOM and from 4140 to 8107 for ANT-DOM upon heating to 50 °C, accompanied by an increase in O/C ratio (from 0.264 to 0.285 for GC-DOM). Further, Van Krevelen analysis revealed a shift toward lipid- and lignin-like structures at 50 °C. Meanwhile, SHAP analysis identified the nominal oxidation state of carbon, modified aromaticity index and double bond equivalence per carbon atom as the most influential predictors of thermally generated compounds. Additionally, mass difference network analysis indicated that oxidation and dealkylation were the predominant transformation pathways, with +3O additions being the most frequent. Overall, this study elucidates the temperature-mediated release mechanisms and structural evolution of Coal-DOM across coal ranks, offering molecular-level insight into its reactivity, mobility, and potential environmental risks in subsurface mining settings.