Photodegradation of the Acaricide Abamectin: A Kinetic Study

JUAN PABLO ESCALADA, JOSÉ GIANOTTI, ADRIANA PAJARES, WALTER A. MASSAD, FRANCISCO AMAT-GUERRI, NORMAN A. GARCÍA.

JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY

AMER CHEMICAL SOC

Año: 2008 vol. 56 p. 7355 - 7359

0021-8561

The acaricide abamectin is a mixture of two colorless homologues in a molar ratio of at least 4:1 with the same structure of macrocyclic lactone. The kinetics of its degradation under direct (254 nm) and dye-sensitized (>400 nm) photoirradiation in methanol solution has been studied by UV-vis spectrophotometry, potentiometric detection of dissolved oxygen, stationary fluorescence, laser flash photolysis, and time-resolved detection of singlet molecular oxygen (O2(1ƒ¢g)) phosphorescence. The results indicate that the degradation is very efficient under direct irradiation with UV light (254 nm), with a quantum yield of 0.23. On the contrary, under visible-light irradiation, using the natural pigment riboflavin or the synthetic dye rose bengal as sensitizers, the degradation is very inefficient and proceeds through a O2(1ƒ¢g)-mediated mechanism, with a bimolecular rate constant for the overall O2(1ƒ¢g) quenching (the sum of physical and chemical quenching) of 5.5 ~ 105 M-1 s-1. This value is similar to those reported for the rate constants of the reactions of O2(1ƒ¢g) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.>400 nm) photoirradiation in methanol solution has been studied by UV-vis spectrophotometry, potentiometric detection of dissolved oxygen, stationary fluorescence, laser flash photolysis, and time-resolved detection of singlet molecular oxygen (O2(1ƒ¢g)) phosphorescence. The results indicate that the degradation is very efficient under direct irradiation with UV light (254 nm), with a quantum yield of 0.23. On the contrary, under visible-light irradiation, using the natural pigment riboflavin or the synthetic dye rose bengal as sensitizers, the degradation is very inefficient and proceeds through a O2(1ƒ¢g)-mediated mechanism, with a bimolecular rate constant for the overall O2(1ƒ¢g) quenching (the sum of physical and chemical quenching) of 5.5 ~ 105 M-1 s-1. This value is similar to those reported for the rate constants of the reactions of O2(1ƒ¢g) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.2(1ƒ¢g)) phosphorescence. The results indicate that the degradation is very efficient under direct irradiation with UV light (254 nm), with a quantum yield of 0.23. On the contrary, under visible-light irradiation, using the natural pigment riboflavin or the synthetic dye rose bengal as sensitizers, the degradation is very inefficient and proceeds through a O2(1ƒ¢g)-mediated mechanism, with a bimolecular rate constant for the overall O2(1ƒ¢g) quenching (the sum of physical and chemical quenching) of 5.5 ~ 105 M-1 s-1. This value is similar to those reported for the rate constants of the reactions of O2(1ƒ¢g) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.2(1ƒ¢g)-mediated mechanism, with a bimolecular rate constant for the overall O2(1ƒ¢g) quenching (the sum of physical and chemical quenching) of 5.5 ~ 105 M-1 s-1. This value is similar to those reported for the rate constants of the reactions of O2(1ƒ¢g) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.2(1ƒ¢g) quenching (the sum of physical and chemical quenching) of 5.5 ~ 105 M-1 s-1. This value is similar to those reported for the rate constants of the reactions of O2(1ƒ¢g) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.2(1ƒ¢g) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.