Retention partly controls the behaviour of pesticides in the environment, and is usually characterized with sorption and desorption isotherms. However, this approach suffers several limitations: pesticide sorption in soils is time dependent, and desorption-related parameters are scant and difficult to use. The objective of this study was to maximize the exploitation of sorption and/or desorption data to characterize desorption along with the effect of aging on retention. The experiments involved three soils and five herbicides (a broad-spectrum herbicide, glyphosate, and four commonly used selective herbicides, trifluralin, metazachlor, metamitron and sulcotrione). Sorption isotherms were not linear and herbicide desorption was markedly hysteretic. Desorption was inversely related to adsorption, being small when sorption was great (glyphosate and trifluralin), and great when sorption was small (metazachlor, metamitron and sulcotrione). Single, different desorption isotherms are obtained that depend on initial sorbed herbicide concentration. A theoretical approach allowed calculation of adapted desorption parameters for different sorption concentrations from only one desorption isotherm. Soil retention of metazachlor, metamitron and sulcotrione increased with time, while that of trifluralin remained stable. The sorption parameter values increased with time, which is explained by a decrease in the herbicide concentration due to degradation, the non-linear nature of the sorption isotherms, hysteretic desorption, and the formation of bound residues. The relative contributions of these phenomena, which depend on the nature of herbicide and soil type, were estimated. Generalized equations were derived to describe sorption and desorption, and these equations could be implemented in pesticide-fate models to take into account sorption and desorption parameters as well as their time dependence.
Désorption des herbicides et dépendance temporelle de leur sorption dans les sols.