Fate of herbicides influenced by biotic and abiotic interactions

The interactions between 2,4-D and simazine with inorganic and organic soil colloids affected by adsorption (abiotic) and enzymatic mechanisms (biotic) were investigated. Complex model systems which closely simulate those encountered in soils [i.e. pure montmorillonite, montmorillonite covered by different amounts of OH-Al species (chlorite-like complexes)], were used as adsorbents. 2,4-D gave a negligible adsorption on pure montmorillonite, whereas it was significantly adsorbed on chlorite-like complexes. The larger the OH-Al amounts on clay surfaces, the greater the quantity of 2,4-D adsorbed. An opposite behaviour was observed with simazine, which adsorbed much more on montmorillonite than on chlorite-like complexes. A well characterised laccase from the fungal strain Cerrena unicolor was used as biotic catalyst. The enzyme displayed a lower ability to oxidise 2,4-D and simazine whereas it was extremely efficient in the oxidation of 2,4-DCP (a derivative of 2,4-D) and catechol. The presence of soil colloids or reactive co-substrates differently influenced the enzymatic transformation of 2,4-DCP. The results obtained in this study provided useful information for predicting the behaviour of a xenobiotic substance when entered into soil.