Photochemical behavior of the fungicide carbendazim in dilute aqueous solution

The direct phototransformation of the fungicide carbendazim has been studied upon monochromatic irradiation at 254 nm. In aqueous solution, carbendazim presents the properties of weak base. The pKa of carbendazim has been determined equal to 4.53±0.07 (by UV-Vis spectrophotometric measurements). Both forms of carbendazim present an emission of fluorescence. The wavelength for the maximum of emission are 393 and 305 nm for the protonated and the neutral form respectively. The fluorescence quantum yields have been measured and values equal to 5×10−3 and 9×10−3 have, respectively been obtained for the NH+ and N form (λexc=254 nm). Upon irradiation at 254 nm, the quantum yield of carbendazim phototransformation has been determined. It is respectively equal to 2.9×10−3 and 2.3×10−3 for the protonated and the neutral form. The increase of oxygen concentration leads to an increase of the phototransformation rate at pH 8.4. On the contrary, the addition of bicarbonate ions inhibits the phototransformation process. This inhibition is consistent with a hypothesis of a Stern–Volmer quenching. The Stern–Volmer quenching constant has been evaluated to 400 M−1. Degradation products have been identified by liquid chromatography coupled with mass spectrometry. The major one was aminobenzimidazole and its formation could be explained by a photohydrolysis mechanism.