Adsorption/desorption of fungicides in natural clays from Southeastern Spain

Abstract Nine natural clay samples, collected in the eastern Andalusian region (South of Spain), were evaluated for the retention of two widely used non-ionic fungicides, metalaxyl and fludioxonil. The clays showed different mineralogical composition and granulometry, had high Ca content, and presented medium–low cation exchange capacity and specific surface area and low organic carbon (OC) content (≤ 0.8%). Various equations were used for the fitting of kinetic and adsorption data. The pseudo-second-order kinetic model was successfully fitted to the experimental kinetic data. The Freundlich equation was better suited for the fitting of metalaxyl adsorption, while both Freundlich and Langmuir equations could be used to explain the retention of fludioxonil. Adsorption experiments, carried out using the batch approach, indicated that metalaxyl, a more polar fungicide, was always retained to a greater extent than fludioxonil, probably through electrostatic interactions. The retention of both fungicides was related with the OC content of the clays and their granulometry. The removal of clay OC led to a greater decrease in fludioxonil adsorption, probably due to its higher hydrophobicity. The retention extent of metalaxyl on selected clays, at 10 and 20 °C, was maintained up to 6 months, reflecting the high stability of pesticide adsorption on the selected adsorbents. A preliminary study at two pesticide concentrations showed that pilloring onion seeds with metalaxyl or fludioxonil may cause phytotoxicity to seedling growth when compared with control seeds.