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Sorption of Prochloraz on Primary Soil Organomineral Size Separates

Danish Inst. of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark;
Environmental Eng. Lab., Dep. of Civil Eng., Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark.

^* Corresponding author (Lis.W.de.Jonge{at}agrsci.dk)

ABSTRACT

The adsorption and desorption of the fungicide prochloraz (N-propyl-N-[2-(2,4,6-trichlorophenoxy)-ethyl]-1-H-imidazol-1-carboxamid) to two whole soils and their sand, silt, and day-size separates were investigated in batch experiments. The size separates were isolated using ultrasonic dispersion in water and repeated gravity sedimentation. For both soils, clay (<2 µm) and silt (2–20 µm) sorbed more prochloraz per kg of soil than did sand (20–2000 µm) and whole soil. Based on unit mass of organic matter, however, whole soils and size separates were equally effective adsorbers. On a unit surface area (BET N₂) basis, the silt- and sand-size separates were the most efficient sorbents. Adsorption and desorption isotherms, obtained after 1 and 24 h equilibration, were best described by the Freundlich model. Sorption-desorption nonsingularity was observed for all size separates, and in general the degree of nonsingularity increased with contact time. Several sorption parameters were well correlated to soil organic C content and to particle porosity, the latter likely representing the pore volume available for diffusion of pesticide toward sorption sites. Adsorption kinetics and adsorption-desorption nonsingularity could be well described by a two stage-two rate model. Adsorption kinetics were characterized by a rapid initial adsorption with most of the pesticide being bound within 10 rain followed by a slow adsorption period. When using pesticide transport models that accounts for particle-mediated transport, it is important to consider that the potentially mobile clay- and silt-size separates typically have higher adsorption capacity than the average capacity of the whole soil.

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