Due to their abundance in nature, metal oxide minerals commonly interact with aqueous contaminants and naturally occurring compounds along the mineral-water interface. Understanding the interactions which occur in the aqueous environment is paramount to evaluating chemical processes which control the fate of these species but is increasingly complex as these reactions are often controlled by the surrounding chemical environment. Density Functional Theory (DFT) is a well-suited method for studying reaction energetics occurring along surfaces at the solid-gas interface, but its application to the solid-aqueous interface has been difficult. In this work, we present an extended DFT + Thermodynamics method for calculating the inner-sphere adsorption energetics occurring at the mineral-aqueous interface for three environmentally relevant systems. The approach allows for the incorporation of experimental parameters such as pH, temperature, concentration, and applied potential which allows for a more consistent comparison to experimental results and accurate description of adsorption energetics. The model was used to investigate the interactions between the (001), (012), and (110) surfaces of α-Al2O3 and α-Fe2O3 with three separate oxyanions (AsO43-/PO43-/SO42-), two small organic molecules (oxalate/catechol), and a metal cation (Pb). For each system, a series of binding sites were sampled and analyzed through structural and geometric properties, adsorption energetics, and vibrational modes. By comparison with experiment, the results indicate the method has been able to accurately predict binding modes of adsorbate complexes, capture trends in adsorption energetics due to varying pH, and calculate circumstances in which inner-sphere adsorption is unfavorable. The DFT + Thermodynamics method can readily be applied to various solid-aqueous surface reactions and shows improved results compared to standard DFT approaches.

DFT + thermodynamics method to study inner-sphere adsorption occurring at the mineral-water interface

ACS Spring Meeting 2021

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