Kinetics and mechanism of imazosulfuron hydrolysis

Knowledge of the kinetics and pathways of hydrolytic degradation is crucial to the prediction of the fate and transport mechanism of chemicals. This w ork first describes the kinetics of the chemical hydrolysis of imazosulfuro n, a new sulfonylurea herbicide, and evaluates the results to propose a deg radation pathway. The hydrolysis of imazosulfuron has been studied in aqueo us buffers both within the pH range 1.9-12.3 at ambient temperature (thermo stated at 25 +/- 2 degreesC) and at pH 3.6 within the temperature range of 15-55 degreesC. The hydrolysis rate of imazosulfuron was characterized by a first-order kinetics, pH- and temperature-dependent, and accelerated by ac idic conditions and higher temperatures. The calculated half-lives at pH 4. 5 and 5.9 were 36.5 and 578 days, respectively. At pH 6.6, 7.4, 9.2, and 12 .3 no significant change in imazosulfuron concentration was observed after 150 days. Half-lives were much lower at pH < 4 (= imazosulfuron pK(a)), at which they ranged from 3.3 to 6.3 days. Moreover, a change in temperature f rom 15 to 25 degreesC in acidic conditions (pH 3.6) decreased the half-life of imazosulfuron by a factor of similar to4.0; in any case, a 3-5-fold inc rease in the rate of hydrolysis was found for each 10 degreesC increase in temperature. In acidic conditions the only hydrolysis products were the two molecules resulting from the cleavage of the sulfonylurea bridge.