Differences in the sorption kinetics of various non-ionisable pesticides in a limited number of agricultural soils from the Mediterranean basin

Adsorption in soil of organic contaminants, such as pesticides, is a time-dependent process, which can be relevant for understanding and predicting the potential pollution risk of different water sources. The adsorption behavior of six different pesticides with a wide range of physicochemical properties (log K 1.26–5.8) was evaluated in up to three different soils with low organic carbon (OC) content (≤1.2%). Pesticide sorbed amounts were fitted to several mathematical models to unravel the mechanisms involved in the adsorption process. The linear distribution constants revealed that pendimethalin and the pyrethroid insecticides were strongly retained in soil, whereas the other three compounds were moderately or weakly adsorbed. In the three soils, the pseudo second order model described more accurately the sorption kinetics of all the contaminants. The more hydrophobic pesticides (log K ≥ 4.6) presented lower kinetic rates as compared with the other compounds under study. Both Elovich and intraparticle diffusion models reflected a strong contribution of a rapid initial adsorption on soil surface for thiacloprid, dimethenamid and fenarimol. For the hydrophobic pesticides this contribution was moderate according to the intraparticle diffusion model. Therefore, slower diffusion into the soil micropores was more relevant for the more hydrophobic compounds and for the bigger molecules, and less significant for the more polar pesticides because almost 90% of the total amount adsorbed was achieved in the rapid initial stage.We acknowledge funding from the Spanish Ministry of Science and Innovation (project CGL2007-60355 ) co-funded by European Feder funds. JARL thanks the scholarship received from CSIC (reference JAEPre_09_01596_2 ), co-funded by ES