Soil hydrophobicity: a contribution of diuron sorption experiments

Retention processes play a major role in the fate and impact of organic contaminants in soils. The main goal of this study was to determine the influence of soil hydrophobic properties on the retention of diuron by using plots of a long-term experiment in Versailles. We selected seven plots with pH 3.4 to 8.2 and low organic content. Sorption isotherms were obtained on soil slurries and kinetic measurements of diuron sorption were performed on undisturbed soil samples. The results showed that the Freundlich coefficient k(f) decreased as pH increased and that the K(oc) coefficient, k(f)/TOC, was linearly related to the contact angle measured on the clay fraction over a wide range of pH. A low initial adsorption rate and low adsorption equilibrium were observed for the plot treated with NaNO(3). In this case, the structure in micro-aggregates was assumed to limit the accessibility of adsorption sites. The CaCO(3) and CaO treated plots had similar organic matter contents, pHs, CECs and bulk densities, although their < 2 mu m:C ratio differed. The higher retention of diuron in the CaCO(3) plot is attributed to the higher hydrophobicity of the < 2 mu m fraction, probably masking part of the permanent clay negative charges. Thus, in addition to the different treatments, organic matter composition and distribution should also be included as specific soil properties. We conclude that the sorption properties of pesticides such as diuron can be a good tool to obtain greater understanding of soil properties and the degree of soil hydrophobicity/hydrophilicity in particular.