Photodegradation of the herbicide Norflurazon sensitised by Riboflavin. A kinetic and mechanistic study

摘要

The kinetics and mechanism of the Riboflavin (Rf)-promoted photochemical degradation with visible light of the herbicide Norflurazon (NF) has been studied by time-resolved and stationary techniques. Using light of wavelength higher than 400 nm-a region where NF is totally transparent-and with concentrations of Rf and NF of ca. 0.02 and 1 mM, respectively, only the excited triplet state of Rf ((3)Rf*) is quenched by NF, in competition with dissolved ground state triplet oxygen, O-2((3)Sigma(g)(-)). NF degradation mainly occurs by reaction with superoxide radical anion O-2(.-) formed through two electron transfer steps: from NF to (3)Rf*, yielding Rf radical anion, and from this anion to O-2((3)Sigma(g)(-)), regenerating ground state Rf. Although singlet molecular oxygen is also produced, NF only quenches this oxidative species in a physical mode. The global result is the photoprotection of the sensitiser and the photodegradation of NF. (C) 2004 Elsevier Ltd. All rights reserved.