Time dependency of triticonazole fungicide sorption and consequences for diffusion in soil

Diffusion is a key process regarding the redistribution in soils of systemic pesticides applied in seed treatments, We investigated the influence of nonequilibrium sorption on the diffusivity of triticonazole fungicide [(1RS)-(E)-5-(4-chlorophenyl-methylen)-2,2-dimethyl-1-(1H-1,2,4-triazol-1-ylmethyl)-cyclopentan-1-ol] in a loam clay soil from Grignon, France (Typic Eutrochrept). Triticonazole diffusion coefficients were measured in saturated soil columns fbr time periods ranging from 1 to 25 d. Nonequilibrium sorption was evaluated from measurement of apparent desorption coefficients, and from estimation of apparent partition coefficients from a diffusivity model. Triticonazole availability was also evaluated based on successive aqueous and methanol extractions, and measurement of the nonextractable residues fraction, Triticonazole diffusion in soil was characterized by a strong time-dependence. Apparent diffusion coefficient decreased from 0.05 cm(2) d(-1) at 2 d to 0.02 cm(2) d(-1) at 25 d, Triticonazole apparent sorption coefficient increased with time of diffusion, from 5 L kg(-1) at 1 d to 10.5 L kg(-1) at 25 d, which was greater than the batch equilibrium value of 4.35 L kg(-1). The measured and calculated results were very close, showing that nonequilibrium sorption was the key process responsible for the decrease in triticonazole apparent diffusion rate. The availability of the diffusing residues diminished constantly with time. The water-extractable fraction corresponding to the more available residues decreased from 31% at 1 d to 17% at 25 d, while a resistant fraction was rapidly formed representing 9% after 25 d. Triticonazole slow desorption behavior was attributed to rate-limiting intrasorbent diffusion.