Effect of exogenous dissolved organic matter on fenhexamid adsorption by soils

Fenhexamid (N-(2,3-dichloro-4-hydroxyphenyl)-1-methyl-cyclohexanecarboxamide, Teldor®) is a fungicide representative of the new chemical class of hydroxanilides. It is a locosystemic compound with excellent activity against B. cinerea. Dissolved organic matter (DOM) plays an important role on the mobility of pollutants in environment. The aim of this study was to investigate fenhexamid adsorption on soils after addition of DOM coming from different composts. Two soils were chosen, a calcareous soil with low organic matter (OM) content and an acidic soil rich in OM. The empirical Freundlich equation describes well the adsorption behaviour of fenhexamid on soils. The adsorption isotherms on calcareous soil resemble L-type isotherms characteristic of a high affinity of the fungicide for the adsorption sites. The adsorption on acidic soil shows a C-type isotherm suggesting a constant partitioning of adsorbate between solution and adsorbent sites. In general, the DOM addition to soils increases fenhexamid adsorption. In fact, the Freundlich constant (KF) increases from 4.86 to 6.70-8.16 on calcareous soil and from 35.2 to 44.1-49.2 on acidic soil. FT-IR spectra indicate that the fungicide does not undergo significant changes as a consequence of the adsorption. These findings are indicative of a weak interaction like a hydrophobic bond. The fungicide adsorption on soils is hysteretic and the amount of fenhexamid desorbed by water decreases as the soil OM content increases. Fenhexamid is scarcely soluble in water because of its scarcely polar character, therefore water is not suitable as extracting solvent. A strong increase of desorbed fenhexamid is obtained using dichloromethane as desorbing solvent.