Sorption and aging of corn and soybean pesticides in tropical soils of Brazil

The efficacy and fate of pesticides in soil strongly depend on sorption reversibility that is known to decrease with increasing contact time (aging). We elucidated the aging dynamics of eight different pesticides in two contrasting agricultural soils of tropical Brazil (Ustox and Psamments), using batch equilibrium experiments and sequential extractions of field samples. Adsorption was best described by Langmuir isotherms for the entire and by Freundlich equations for the lower concentration range. In field samples, water extractable pesticide fractions mostly dissipated at least twice as fast as did the solvent extractable fractions. Hence, in comparison to 0 days K-OC values, measured field K-OC values were higher by a factor of 2.6-38 for the clayey Ustox and 2.1-72 for the sandy Psamments toward the end of the experimental period at 80 days after application. The extent of such aging basically increased with increasing polarity of the pesticide. An absolute enrichment of polar pesticides within the final exhaustive acetone-ethyl acetate-water extracts was observed during the experimental period, so that we may deduce that pesticides had redistributed into stronger sorption sites. The time course of the K-OC development was well-described using a semiempirical equation, which had been employed for temperate soils before.