Unraveling dissolved organic matter in Chinese coal mine drainage: Compositions, sources, and environmental implications for potentially toxic elements

Dissolved organic matter (DOM) in coal mine drainage (CMD) interacts with potentially toxic elements (PTEs), posing environmental risks across diverse coal-bearing basins in China. CMD from 37 mines were classified hydrogeochemically into three categories and were analyzed by multi-spectroscopy. CMD in Category II (pH = 5.78) confronted with severe PTEs enrichment, with Fe concentration reaching 275.23 mg·L^-1 (917 ×standard) alongside uniquely elevated Tl (502 ×standard). Mn, Al, Hg, Cd, Pb, B, Co, and Mo also exhibited category-specific risks in CMD. DOM was predominantly composed of lignin-derived polyphenols (C3, 59.36 %). Category I showed the highest microbial-derived humic-like fluorescence intensity (C2) and the strongest aromaticity (SUVA₂₅₄ = 0.31 L·(mg·m)^-1), while Category II displayed prominent gallic acid-like components (C1), peak chromophoric DOM (a(350) = 8.18 m^-1), and relatively the highest molecular weight. DOM composition in CMD was controlled by hydrogeochemical and environmental characteristics, such as pH and TDS, exhibiting autochthonous sources and weak humification. Redundancy analysis identified Ba, significantly correlated with DOC, as the primary driver of DOM spectral variation (54.20 % explained). DOM composition in CMD was significantly influenced by coal, with both systems dominated by lignin (53.09 %-55.78 %) and CHO compounds (53.94 %-64.20 %). Predicted transformation pathways for coal-derived molecules entering CMD involved decarboxylation (25.91 %), oxygen addition (25.85 %), and dealkyl group (22.64 %), which mainly occurred in the lignin/CRAM and were dominated by the reduced states, resulting in obvious disappearances of CHOS compounds. XGBoost-SHAP modeling confirmed that the nominal oxidation state of carbon was the key feature predicting molecular reactivity (mean |SHAP| = 1.43), followed by the O/C ratio (1.17). These findings provided molecular-level insights for environmental management in coal mines.