Metal oxide materials interact with organic matter through a variety of mechanisms, wherein the surface-organic interactions are influenced by the surrounding environmental conditions. Collaborative experimental work has shown that organic molecule adsorption trends on hematite nanoparticle surfaces are dependent on the identity of the organic molecule, where the binding mode is influenced by solution pH and the presence of electrolytes. To obtain an atomic-level mechanistic understanding of these interactions, density functional theory (DFT) calculations and thermodynamics were used to study the adsorption of small organic molecules on hematite nanoparticle surfaces. The DFT + Thermodynamics model incorporates experimental parameters such as pH, applied potential, and ion concentration, allowing for more consistent comparisons to experimental findings and a more accurate description of adsorption energetics. Both inner- and outer-sphere adsorption calculations were performed to study how oxalate and pyrocatechol interact with the (001), (012) and (110) α-Fe<sub>2</sub>O<sub>3</sub> surfaces at varying pH values. A series of surface binding sites were sampled and studied in terms of adsorption energetics, adsorption complex structural stability, and vibrational modes. Results indicate that as pH decreases, oxalate changes binding preference from outer- to inner-sphere, forming a stable five-membered ring with surface iron atoms. Alternatively, over the pH range studied, pyrocatechol consistently prefers inner-sphere adsorption with the formation of a six-membered ring with surface iron atoms. Vibrational analysis of the adsorbed molecules was also conducted, and the predicted vibrational modes are in good agreement with experimental results. This method can be extended to study additional organic molecules and sorbent minerals, useful to predict binding mode and adsorption affinity.

Inner- and outer-sphere adsorption of small organic molecules on hematite surfaces: Insights from density functional theory

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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