Surface Charge Heterogeneity and Mechanisms of Organic Binding Modes on an Iron Oxyhydroxide

Mineral-associated organic matter involving iron oxyhydroxide minerals is important to the preservation and transformation of organic matter in soils and sediments. Largely lacking is a quantitative evaluation of different binding mechanisms in relation to the mineral surface charges. Here, with ferrihydrite, we investigated complexes with organic compounds of various charges and structures, including a ribonucleotide, a sugar, a phenolic acid, and amino acids with different side chains. After constructing model ferrihydrite nanoparticles using reported iron-oxygen coordination, we mapped theoretically the spatial distribution of positive and negative charges, corroborated experimentally by atomic force microscopy. With these variable charges due to protonation extent of surface hydroxyls, molecular dynamics simulations revealed binding mechanisms of organic moieties with opposite charges, confirmed experimentally by infrared spectroscopy. For electrostatic interactions, quantum mechanics-calculated energies determined the order of binding strength consistent with our adsorption data: ester-linked phosphate > protonated primary amine ≥ carboxylate attached to phenyl ring = carboxylate attached to alkyl group. Ligand exchange, which was more thermodynamically favorable than electrostatic interactions despite the energy barrier to the transition state, was driven by the stability of the product. We obtained a quantitative rationale for the binding of ribonucleotide phosphate through ligand exchange versus binding of carboxylate and amino groups through electrostatic interactions, thus informing mechanistic frameworks for mineral-organic associations.