MD simulations of sorption processes on soil matrices

The interaction of soil material with organic compounds controls many important environmental processes. Adsorption on soil material is one of the key factors that detennine the mobility and the availability for biotransformation of organic solutes in the subsurface. The accessibility to and the number of sorption sites on minerals and organic matter are important factors, which influence the transport of pollutants in soil. Soil is composed of inorganic and organic substances. A mixture of organic (humic substances) and inorganic material (minerals) makes up the skeletal framework of soils. Representative model situations have to be constructed which can be treated with present computational techniques. Within the frame of this project we want to focus on two major aspects. One is the interaction of clay mineral surfaces with organic compounds in soil. We want to extend our studies to more complex and thus more realistic situations. In particular we want to study the interactions of pesticides, typical fragments of humic acids and solvated cations (e.g. K+, Mg2+, Ca2+) on clay surfaces. We want to apply molecular dynamics methods for the appropriate description of this large variety of molecular arrangements. These calculations allow the evaluation of dynamical properties of adsorption processes, in particular concerning pesticides. The interaction of the pesticide with the clay surface can be compared with the interactions between pesticides and humic substances, which allows the estimation of the distribution of pesticides between adsorption on the clay surface, of binding to humic substances or of being in a mobile status in the soil solution. The second part of the project aims at a detailed description of humic substances and their interactions with organic compounds. First, we want to extend our work on the interaction of HS models, which consist of series of organic functional moieties, and will investigate the influence of different external environments (aqueous or lipophilic) on the thermodynamic stability of the sorption. Therefore, free energy calculations will be conducted applying different dielectric constants. Second, the inclusion of a simple lipohilic HS model and water to the functional model part will give a more realistic representation of the system. The outcome of such simulations will allow us to estimate the distribution of a selected pesticide between a free state and an adsorbed state within the HS model. Both parts of the project will supply valuable and fundamental insights in sorption processes within soil matrixes, which can be used for a better understanding and better interpretation of experimental data on the fate and behavior of organic compounds within the environment.