Modeling sorption of soil organic matter on mineral surfaces: wood-derived polymers on mica

Molecular mechanics and molecular dynamics calculations were carried out on model organo–mineral composites typical of both paper products and soil. Sorption energy of neutral model linear chains of lignin, carbohydrate, and humic substances on muscovite mica depends on the mutual orientation of organic and mineral components, spatial organization and functionality of the organic chain, and the presence of metal cations. Carbohydrates are characterized by their higher affinity towards the mineral surface than the more flexible helical aromatic polymers. In the model calculations, sorption energies are twice as high. Oxidation of lignin into respective humic acids does not lead to better sorption. Unlike metal cations, water molecules interlacing between organic and mineral surfaces do not act as bridges and sharply decrease sorption energy. Flexible linear polymers may undergo drastic conformational changes when approaching the mineral surface, to ensure a gain in the interaction energy that more than compensates a loss in the conformational energy proper.