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Sorption properties of clays are effective to control the mobility and availability of pesticides in soil. Adsorption/desorption mechanisms of a model pesticide (ethofumesate) on clays and organoclays were investigated to elucidate the impact of clay structural and chemical properties on clay-pesticide interactions in an aqueous environment. A representative panel of five commercial montmorillonites (MMTs) was studied: a natural sodium MMT, an organically modified montmorillonite (OMMT) with an apolar alkylammonium interlayer cation displaying OH groups and three OMMTs with very apolar alkylammonium interlayer cations. Batch equilibration experiments coupled with Freundlich modeling showed that the amount of adsorbed pesticide and the adsorption/ desorption mechanisms were mainly governed by the polarity and the chemical structure of the MMT interlayer organic cations. The lower the polarity of the MMT, the higher the adsorption capacity. Batch equilibration and batch release experiments both indicated that very little amount of ethofumesate was adsorbed on the edges and external surfaces of all MMTs through hydrogen bonding. For OMMTs, ethofumesate was mainly adsorbed within the gallery by the establishment of stronger hydrophobic interactions with the organic interlayer cation, leading to a rearrangement of alkylammonium chains with a potential basal spacing increase (for high steric hindrance interlayer cations), as confirmed by wide angle X-ray diffraction analysis (WAXS) and Fourier transform infrared spectroscopy (FTIR) analysis. Results provide important advances in the field of sequestration and/or controlled release system of pesticide in soil by using rational strategies of clay chemical modifications, as demonstrated by the slow release properties of organoclay-ethofumesate complexes if compared to commercial formulation pesticide.

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