Facet- and pH-Dependent Binding Mechanisms of Natural Organic Matter on Hematite

Natural organic matter (NOM) plays a crucial role in biogeochemical processes, influencing contaminant transport, carbon cycling, and metal speciation in the environment. However, the molecular-level interaction mechanisms between NOM and mineral surfaces remain poorly understood due to the structural complexity of NOM and the facet-dependent reactivity of minerals. In this study, we investigate the binding mechanisms of NOM on hematite (α-Fe₂O₃) surfaces using dynamic force spectroscopy (DFS) and classical density functional theory (cDFT) simulations. The adsorption behavior of four key NOM functional groups, carboxyl, phosphonate, hydroxyl, and amine, was examined for three hematite facets: (001), (012), and (113). Our results reveal a range of binding strengths, with ligand exchange and electrostatic interactions playing dominant roles in adsorption. The (001) facet exhibited the strongest interactions with amine and hydroxyl, while the (113) facet favored carboxyl and phosphonate binding. The effect of pH on binding forces varied by functional groups, highlighting the interplay of electrostatic interactions, hydration forces, and surface coordination chemistry. These findings provide detailed insights into NOM-mineral interactions that could be used to parametrize surface complexation models used in reactive transport simulations for a better understanding of the mobility and bioavailability of contaminants in natural environments.