Kinetics of the riboflavin-sensitized degradation of pyrethroid insecticides

Abstract

The Riboflavin (Rf) photosensitized degradation of pyrethroid insecticides was studied under visible-light conditions. The pyrethroid insecticides studied (Pyrs) were Bioallethrin (Bio), Ethyl chrysanthemate (EC), Permethrin (Per), and Cypermethrin (Cyp). Riboflavin, one of the natural photosensitizers widely distributed in nature, was used as a photosensitizer in order to mimic the natural environment. Pyrs degradation would occur due to the reaction of the insecticide with the excited electronic states of the Rf and/or the reactive oxygen species (ROS) generated mainly from the triplet excited state of the Rf (3Rf*). Singlet oxygen (1O2) and superoxide anion (O2?−) are the main ROS photogenerated from 3Rf*. Only EC and Bio react towards 1O2 with reactive rate constants of 3.1 × 106 and 2.4 × 106 M−1s−1, respectively. Rf-sensitized photolysis of Pyrs in the presence and absence of superoxide dismutase indicates that O2?− does not participate in Pyrs degradation. Per and Cyp are poor quenchers of 3Rf* (3kq < 107 M−1s−1), whereas Bio and EC quench 3Rf* with rate constants of 1.5 × 107 and 5.2 × 108 M−1s−1, respectively. For the Pyrs, the possible contribution of Rf to the sensitized photodegradation of Pyrs in natural waters strongly depends on the Pyrs chemical structure.