Single-Molecule Determinations of the Phase- and Facet-Dependent Adsorption of Alginate on Iron Oxide

The formation of iron oxide mineral–organic associations can regulate long-term preservation of soil organic matter (SOM) by providing shelter for SOM from degradation through adsorption on different crystalline/amorphous phase surfaces. However, little is known about the energetic basis for the phase- and facet-dependent SOM–iron oxide interactions at the molecular scale. Here, we use a representative organic molecule, alginate, which is present ubiquitously in soils, and investigate its adsorption on the surfaces of amorphous iron oxides (FexOy) or hematite (α-Fe2O3) by calculating the Kelvin potential difference and measuring the binding energy (ΔGB) between an alginate molecule and a specific surface of the hematite or amorphous FexOy phase by atomic force microscopy (AFM). Single-molecule determination of alginate–amorphous FexOy/hematite interactions provides mechanisms of the origin of the phase- or facet-dependent adsorption by systematically changing the pH and ionic strength/compositions of environmentally relevant reaction solutions. We show that the adsorption capacities of alginate follow the order amorphous FexOy > the (100) > (110) > (0001) faces of hematite. These molecular-scale investigations may improve our understanding of the phase-dependent or face-specific adsorption of SOM on other soil minerals in aqueous environments and potentially predict the preservation mechanisms and the fate of SOM.