Modelling the strength of mineral-organic binding: organic molecules on the α-Al₂O₃(0001) surface

Organic carbon (OC) is an essential component of soil. Sorption of OC to oxide mineral surfaces is a key process in soil preservation due to its ability to protect OC from microbial degradation. To understand the sorption of OC in soils and obtain a quantitative description of the binding of organic molecules to soil minerals, we investigated the binding of water and small organic molecules, typical building blocks of OC, on α-Al₂O₃, a common soil mineral. α-Al₂O₃ was modelled using (0001)-oriented periodic slabs, using density functional theory calculations with empirical dispersion correction. For water, dissociative adsorption was energetically preferred to molecular adsorption. Amine, amide and carboxylic acid functional groups were found to bind more strongly to this surface compared to water. Alcohol, ether, thiol and ester functional groups had adsorption energies very similar to that of water, while hydrocarbons were found to bind less strongly. Carboxylic acids were the strongest bound surface adsorbates in this study. Dissociated adsorption configurations (where allowed by the molecules' chemical nature) were usually more favourable than molecular adsorption. Hydrogen bonding was found to be a major contributor to the stability of adsorption configurations. This work shows that a number of organic functional groups, in particular amine, amide and carboxylic acids, bind to the α-Al₂O₃(0001) surface more strongly than water; thus they are likely to be adsorbed on this mineral surface under ambient conditions and to provide stability of adsorbed OC.