Al substitution regulates humic acid interactions with hematite and organic carbon stabilization

Mineral-organic interactions at environmental interfaces play a critical role in regulating the persistence and transformation of organic carbon in soils and sediments. Iron (oxy)hydroxides are among the most important reactive mineral phases responsible for organic matter retention; however, the influence of structural substitutions within iron oxides on organic carbon stabilization mechanisms remains poorly understood. This study investigated how aluminum (Al) substitution alters the surface reactivity of hematite and regulates humic acid (HA) binding behavior using adsorption experiments, in situ attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), and CD-MUSIC surface complexation modeling. The results showed that Al substitution altered the composition and reactivity of hematite surface hydroxyl sites, with an increased contribution of AlOH^0.5- sites relative to FeOH^0.5- sites. While the overall adsorption capacity for PAHA (purified Aldrich humic acid) on the Al substituted hematite is higher than that on pure hematite, the introduced AlOH^0.5- sites exhibit superior binding affinity towards carboxyl (R-COOH) and hydroxyl (R-OH) functional groups, and promote the formation of more stable inner-sphere complexes-particularly with COOH groups. These results reveal an Al-driven interfacial mechanism that enhances both the selectivity for specific organic functional group and the stability of organo-mineral associations. Our findings provide a fundamental mechanistic understanding of how cation-substituted iron oxides selectively stabilize reactive organic carbon fractions at the molecular level, with significant implications for carbon persistence in natural environments.