NONBONDED ORGANO-MINERAL INTERACTIONS AND SORPTION OF ORGANIC-COMPOUNDS ON SOIL SURFACES - A MODEL APPROACH

Molecular mechanics and molecular dynamics calculations have been perf ormed on organo-mineral composites that model the sorption of high-mol ecular-weight humic polymers on mineral surfaces and the sorption of l ow-molecular-weight organic contaminants on both mineral and organic s urfaces in soil. Muscovite mica was chosen as a mineral model; an oxid ized topological lignin-carbohydrate complex was chosen as a humic mod el; benzene, sodium benzoate, atrazine, and DDT represent different cl asses of contaminants. Sorption energies were estimated based on molec ular mechanics calculations. Flexible linear polymers undergo drastic conformational changes when approaching the mineral surface, to ensure a gain in the interaction energy that outweighs a loss in the conform ational energy proper. Therefore, the gas-phase conformation compositi on of environmental organic polymers is not directly related to their spatial organization in soil composites. Molecular dynamics simulation suggests high stability of the organic polymer coatings of mineral su rfaces in the environment. Low-molecular-weight organic molecules demo nstrate much less affinity for the mineral surface, which implies unhi ndered exchanges between the surface and its near environment. Ionizab le compounds, e.g. salts of organic acids, are different, because they can form strong associations with a mineral surface through cation br idges. Sorption energies are compound-specific and depend on the sorba te-sorbent orientation. The calculations suggest some preference for t he edges of a model muscovite sheet in comparison with the basal oxyge n surface as a sorption site. Coating of mineral surfaces with organic polymers does not hinder the sorption of organic molecules except in the special case of organic ions. (C) 1998 Elsevier Science B.V.