Molecular Fingerprint-Dependent Retention and Fractionation of Combustion-Derived Dissolved Organic Matter on Kaolinite and Montmorillonite

Abstract The environmental fate of combustion-derived dissolved organic matter (CDOM) at clay mineral interfaces remains poorly understood. We applied FT-ICR-MS, XPS, and other techniques to investigate the interfacial behavior of CDOM from coal, charcoal, pinewood, and maize straw on kaolinite and montmorillonite. Two distinct mechanisms were identified: ligand exchange on kaolinite (increasing binding energies by 1.2 ∼ 1.7 eV) and Ca2+/Mg2+-mediated cation bridging on montmorillonite (decreasing binding energies by 0.3 ∼ 1.8 eV). Maize straw-CDOM and charcoal-CDOM exhibited the highest adsorption on both clays, while pinewood-CDOM showed mineral-selective retention (montmorillonite only) and coal-CDOM showed negligible adsorption. Coal-CDOM (−SO3H-rich) with negligible adsorption was due to electrostatic repulsion and hydration steric hindrance. Pinewood-CDOM (phenolic) was retained only on montmorillonite via cation bridging, as its phenolic groups cannot undergo ligand exchange with kaolinite. Charcoal-CDOM (−NH2-rich) was retained on both clays primarily through electrostatic attraction of –NH3+, with additional contributions from Ca2+/Mg2+-mediated cation bridging and cation–π interactions on montmorillonite. Maize straw-CDOM (mixed –COOH, –NH2, –SO3H) was retained through synergistic ligand exchange, cation bridging, and electrostatic/H-bonding. Concentration-dependent fractionation shifted from H-bonding/electrostatic attraction (small polar molecules) to hydrophobic/cation–π interactions (large aromatic molecules) with increasing concentration. This framework provides mechanistic context for pyrogenic carbon sequestration and contaminant mobility in clay-rich environmental matrices.