Theoretical investigations into the interaction between soil organic carbon and soil minerals

Organic carbon (OC) is an essential component of soil; its presence in soil is important for growing crops, for maintaining the stability of soils, and more generally for capturing CO2 from the atmosphere. Sorption of OC to oxide mineral surfaces is considered a key process in the soil preservation due to its ability to protect OC from microbial degradation. To understand the sorption of OC in soils, it is important to obtain a quantitative description of the binding of organic molecules to soil minerals and soil metal cations. In this work, we conducted theoretical modelling investigations on the binding of water and small organic molecules, typical building blocks of OC, on α-Al2O3 (alumina), a common soil mineral. Alumina was modelled both bare and hydroxylated using (0001)-oriented periodic slabs, using density functional theory (DFT) calculations and empirical dispersion. Adsorption energies and binding energies were calculated. The amine, amide and carboxylic acid functional groups were found to adsorb more strongly than water during our small molecule adsorption investigations on the bare and hydroxylated α-Al2O3 (0001) surface. These investigations illustrated the key functional groups required for strong OC binding in soils. With knowledge gained from this first stage of investigations, we conducted our biopolymer adsorption investigations. Several biopolymers such as, cellulose and chitin were adsorbed to the hydroxylated alumina surface. Pectin was the most strongly adsorbing of the biopolymers and hydrogen bonding was a major contributor to its stability. Finally, we also investigated the binding of small organic molecules to the hydrated [Al(H2O)6]3+ and [Fe(H2O)6]3+ complexes to investigate the binding of cations in the soil solution. In the cation investigations we calculated the Gibbs energy of reaction for the products of the ligand substitution process. Acetate, amine and amide ligands yielded the most energetically favourable substitution products for both the Al3+ and Fe3+ studies. Fe3+ produced more energetically favoured products compared to its Al3+ counterpart. Overall, many organic functional groups, particularly, the amine, amide and acid bind strongly to the bare and hydroxylated alumina surface and metal cations. Hence, they are likely to remain adsorbed on alumina and cations in soils.