Binding of organic molecules on oxide mineral surfaces is a key process in soil preservation due to its ability to protect soil organic carbon from microbial degradation. To understand the nature and strength of sorption of organic carbon in soil, we investigated the binding of small organic molecules and larger polysaccharide biomolecules (typical building blocks of soil organic carbon) on α-Al2O3, a common soil mineral which is known to strongly bind organic matter. The calculations of adsorption on the α-Al2O3 (0001) surface were carried out using density functional theory (DFT) with empirical dispersion correction. First, adsorption of small organic molecules containing of a variety of functional groups was investigated and compared to the binding of water. Amine, amide and carboxylic acid functional groups were found to bind to this surface more strongly than water. Alcohol, ether, ester and thiol groups' binding energies were very similar to water, while hydrocarbons were found to bind less strongly. Carboxylic acids were the most strongly bound adsorbates in this study. Dissociated adsorption configurations were usually more favourable than molecular adsorption; hydrogen bonding was also found to make a significant contribution to the strength of adsorption. Thus, a number of organic functional groups were identified, such as amine, amide and carboxylic acids, which are able to bind to the α-Al2O3 (0001) surface more strongly than water and are likely to adsorb on this mineral surface under ambient conditions. Informed by these insights into adsorption of small molecules, we modelled adsorption of biomolecules - fragments of polysaccharides which naturally occur in soil: cellulose, chitin, chitosan and pectin. Pectin, which contains hydroxyl and carboxylic groups, and chitosan, which contains hydroxyl, amine and amide groups, were found to bind strongly, consistent with the strong binding of the functional groups present in these molecules. Cellulose, which contains only ether and hydroxyl groups, showed the least strong adsorption among the biomolecules studied. Our results show that biomolecules containing acid, amine and amide groups, such as pectin and chitosan, bind strongly to soil minerals and therefore are likely to be stable in soil.

Modelling mineral-organic interactions: Adsorption of organic molecules and biomolecules on the α-Al2O3 (0001) surface

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