Desorption Mechanism of a Pesticide from a Hydrated Calcium Montmorillonite Unraveled by Molecular Dynamics Simulations

Fenhexamid, N-(2,3-dichloro-4-hydroxyphenyl)-1-methylcyclohexanecarboxamide,a fungicide, is authorized in the European Union to be used in agriculture.Active molecule behavior in the environment and its transfer to environmentalcomponents (soil, water, and air) are primordial topics to study tolimit environmental pollution. Following a step by step approach,we studied fenhexamid. In previous works, we worked on the fenhexamidpesticide isolated and then on its complexes with one or two Ca2+ and Na+, the most abundant cations in soils,in the gas phase. We also reported theoretical calculations aboutfenhexamid in interaction with a clay surface: Ca-montmorillonite.In the present paper, potentials of mean force describing overallfenhexamid desorption from a Ca-montmorillonite surface inthe presence of water were computed from molecular dynamics simulationscombined with umbrella sampling. Several mechanisms were envisaged.The results allow to quantitatively estimate the free energy of desorptionand to provide a detailed insight into the hydration sphere of Ca2+ during the desorption process.