Research paper Adsorption/desorption of fungicides in natural clays from Southeastern Spain José Antonio Rodríguez-Liébana, Alberto López-Galindo, Concepción Jiménez de Cisneros, Antonia Gálvez, Marisa Rozalén, Rita Sánchez-Espejo, Emilia Caballero, Aránzazu Peña ⁎ Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Avenida de las Palmeras 4, 18100-Armilla, Granada, Spain abstract article info Article history: Received 21 April 2016 Received in revised form 16 June 2016 Accepted 11 July 2016 Available online 16 July 2016 Nine natural clay samples, collected in the eastern Andalusian region (South of Spain), were evaluated for the re- tention of two widely used non-ionic fungicides, metalaxyl and fludioxonil. The clays showed different mineral- ogical 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 pes- ticide 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. © 2016 Elsevier B.V. All rights reserved. Keywords: Pesticide Spanish clay Adsorption Release stability Seed coating 1. Introduction Natural clays have been proposed for the remediation of environ- mental waters contaminated with organic pollutants, like pesticides (Sidhoum et al., 2013; Ouali et al., 2015; Azarkan et al., 2016), because they can be used as cheap and readily available retention barriers. The presence of pesticides in environmental waters is highly dependent on pesticide properties, mainly on their solubility and persistence (Gustafson, 1989). For instance, metalaxyl, a highly soluble and moder- ately persistent fungicide, has been detected in monitoring programs in different countries (Bermúdez-Couso et al., 2013; Triantafyllidis et al., 2013; Allinson et al., 2015), while no record has been found in the liter- ature about the presence in water of the hydrophobic fludioxonil. On the contrary, residues of both fungicides have been detected in various crops (Dedola et al., 2014; Wanwimolruk et al., 2015). On the other hand, natural clays have been also considered as car- riers of seed treatments (Nisar et al., 2009), which could release to the seeds, in the early stages of development, pesticides, nutrients, or my- corrhizal fungi, among others (Taylor and Salanenka, 2012; Colla et al., 2015; Adhikari et al., 2016). Seed treatment, apart from providing added value to the seeds, is directed to the optimization of plant sowing. In this sense, the inclusion of pesticides in the clays would result in (a) a reduction in seed quality deterioration during storage due to pests or diseases; (b) a decrease of pesticide application doses, depending on planting density; (c) a decline in environmental risks, including surface water, groundwater, soil, or air pollution; (d) a minimization of the destruction of beneficial fauna and (e) an enhancement of safety condi- tions for seed handlers, regarding traditional treatments. The incorporation of pesticides in the clay minerals, in comparison with conventional pesticide applications, has the advantage of retaining them in the clay matrix, enabling accurate and uniform pesticide appli- cation, and providing controlled availability of the active ingredient to the seedlings in the initial development. Therefore, pesticide availability for the seeds in their immediate surrounding would be maintained for longer periods of time and the released pesticide concentration would be kept to a safe value, reducing the potential pesticide leaching and its availability for non-target organisms. At the same time, when adsorbed on the clays in the seed coating, pesticides are protected from dissipation processes, such as photodegradation or volatilization (El-Nahhal et al., 2001; Si et al., 2004). Clay minerals are negatively charged and hydrophyllic, therefore their use has been mainly oriented to the retention of cationic pesti- cides, such as paraquat (Sánchez-Jiménez et al., 2012; Sidhoum et al., Applied Clay Science 132–133 (2016) 402–411 ⁎ Corresponding author. E-mail address: aranchaph@iact.ugr-csic.es (A. Peña). http://dx.doi.org/10.1016/j.clay.2016.07.006 0169-1317/© 2016 Elsevier B.V. All rights reserved. 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