The selective binding of fulvic acid on goethite and kaolinite faces: molecular-level insights

Context Mineral binding plays a crucial role in soil carbon stabilization, with mineral structures and surface reactions significantly influencing carbon storage. Aim This study investigates the molecular interactions between organic matter (fulvic acid, FA) and two common soil minerals (goethite and kaolinite). Methods In situ attenuated total reflectance Fourier transform infrared and two-dimensional correlation spectra were explored to study the binding sequence of FA fractions on the crystal faces of minerals. Additionally, X-ray photoelectron spectroscopy was used to validate the binding process by analyzing changes in relevant binding energy. Key results Carboxyl, hydroxyl, and phenolic hydroxyl fractions of FA were found to bind onto the crystal faces of both goethite and kaolinite, with pH playing a critical role in these interactions. Carboxylic fractions preferentially bound onto goethite faces via ligand exchange, forming Fe–C bonds. At pH 8.0, hydrophobic aromatic fractions preferentially bound onto kaolinite, and for polysaccharide-like fractions at pH 4.0. The binding sequence on goethite indicated that carboxylic preceded the deprotonated carboxyl. In contrast, binding onto kaolinite exhibited greater heterogeneity, likely due to its complex surface properties. Conclusion Goethite demonstrated a greater capacity for carbon sequestration compared to kaolinite, with the bound carbon of goethite appearing more stable. Kaolinite exhibited superior selectivity for binding hydrophobic organic carbon. Implications This study provides molecular-level insights into organic matter–mineral interactions, offering new perspectives on carbon stabilization through mineral interactions in soils and sediments.