Atomic-scale modelling of organic matter in soil: adsorption of organic molecules and biopolymers on the hydroxylated α-Al2O3 (0001) surface

Binding of organic molecules on oxide mineral surfaces is a key process which impacts the fertility and stability of soils. Aluminium oxide and hydroxide minerals are known to strongly bind organic matter. 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 on alpha-Al2O3 (corundum). We modelled the hydroxylated alpha-Al2O3 (0001) surface, since these minerals' surfaces are hydroxylated in the natural soil environment. Adsorption was modelled using density functional theory (DFT) with empirical dispersion correction. Small organic molecules (alcohol, amine, amide, ester and carboxylic acid) were found to adsorb on the hydroxylated surface by forming multiple hydrogen bonds with the surface, with carboxylic acid as the most favourable adsorbate. A possible route from hydrogen-bonded to covalently bonded adsorbates was demonstrated, through co-adsorption of the acid adsorbate and a hydroxyl group to a surface aluminium atom. Then we modelled the adsorption of biopolymers, fragments of polysaccharides which naturally occur in soil: cellulose, chitin, chitosan and pectin. These biopolymers were able to adopt a large variety of hydrogen-bonded adsorption configurations. Cellulose, pectin and chitosan could adsorb particularly strongly, and therefore are likely to be stable in soil.This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Automated summary

The binding of organic molecules on oxide mineral surfaces has a significant impact on soil fertility and stability. Aluminium oxide and hydroxide minerals have a strong affinity for organic matter. This study investigated the adsorption of small organic molecules and larger polysaccharide biomolecules on a hydroxylated alpha-Al2O3 surface using density functional theory (DFT) with empirical dispersion correction. The results showed that organic molecules form multiple hydrogen bonds with the surface, with carboxylic acid being the most favorable adsorbate. Biopolymers such as cellulose, chitin, chitosan, and pectin were also found to strongly adsorb on the surface.