The Role of Soil Organic Matter and Hydration in the Sorption of Trifluorinated Pesticides.

Recent findings show that agricultural chemicals, such as pesticides, find their way to surface and groundwaters. The presence of these pesticides is detrimental to the health of the environment. Hence, a better mechanistic understanding of factors affecting pesticide mobility and transport is needed, and to achieve this goal, sorption-desorption experiments are highly useful. In our study, sorption of trifluorinated pesticides, namely: norflurazon, acifluorfen and flutolanil, to well characterized soils of low to high organic matter content are investigated. Results show that sorption-desorption isotherms adhere to Freundlich equation with R2 of 0.98-0.99. Organic carbon normalized sorption coefficient (Koc) values show the trend flutolanil>norflurazon>acifluorfen. These results will be discussed with respect to pesticide structure and polarity as well as the role of natural organic matter and clays in the sorption of hydrophobic pesticides. In addition, pesticide sorption to soils of varying hydration conditions (an initially dry, pre-hydrated, native state) reveals some differences indicating an important role of wetting/drying cycles. The use of sodium azide also results to hysteresis and a change in Koc, which hints a possible effect of sodium azide on NOM, apart from its microbial activity. Furthermore, 19F NMR spectra of these pesticides sorbed to model sorbents and soil will also be presented. Finally, a model of hydrophobic pesticide sorption to soils will be put forward.