Modelling of Pesticide-Soil Interactions

Soil is a very complex system containing organic and inorganic components. Humic substances (HS) represent most of the organic part. The inorganic material consists mainly of silicates and other oxides (like Fe2O3). Among different silicate minerals, clays play an important role due to their high chemical surface activity. The main aim of our project was to describe interactions between organic molecules (pesticides) with selected soil constituents on a molecular level. Due to the high complexity of the system, we introduced several idealizations and simplifications for the construction of appropriate models. We have focused on three particular problems: - behavior of the free Al3+ cation in soil solution - structure and dynamics of hydroxyl groups of the kaolinite group minerals and adsorption of selected model molecules on mineral surfaces of these minerals - investigation of interactions between typical functional groups of humic substances and pesticides The investigation of complexes between aluminium-hexaquo complexes with organic acids (acetate, oxalate, citrate) showed a distinct influence of the specific organic acid and the complex species itself (monodentate, bidentate) on the interaction energies. Generally, it was found that the most stable complexes in the water solution are complexes with citrate ligands. We have chosen dickite as one of the classical 1:1 clay minerals as basic model for a clay-pesticide system. For adsorption studies of small molecules on the surface we developed an ONIOM cluster model. It could be shown that water and acetic acid interact only weakly with the tetrahedral side of the clay mineral, whereas the adsorption and interaction with hydrogen bonds on the octahedral side was significant. This study was repeated with the pesticide 2,4-D. Similar results were obtained as for the acetate ion. The most complicated task in our project was to develop models for interaction of pesticides with humic acids, which is - from the practical point of view - one of the most important processes determining the behaviour of pesticides in soil. As humics are a highly complex group of substances and not a single molecular structure can be defined, we did not attempt to define a single humic model, but chose a completely different approach. We concentrated on the investigation of specific interactions of a model pesticide (2,4-D) with functional groups, which are known to be present in humic substances. These investigations were performed under varying chemical environment (from polar to unpolar). Basic results were obtained on the influence of the polarity of the environment on different interactions. Results, which are potentially very important to explain the phenomena of "bound residues" of pesticides in soil. In the following project we try to extend this approach to more realistic systems.