Transformation of carbendazim induced by the H2O2/UV system in the presence of hydrogenocarbonate ions : involvement of the carbonate radical

The transformation of the fungicide carbendazim by hydroxyl radicals generated by the photolysis (λexc. = 254 nm) of hydrogen peroxide in aqueous solution has been studied in the absence and in the presence of hydrogenocarbonate ions. In the presence of high concentrations of hydrogen peroxide, the second-order rate constant of the reaction of HO˙ radicals with carbendazim has been determined to be equal to (2.2 ± 0.3) × 109 L.mol−1.s−1. The identification of the main degradation by-products shows the existence of two different reaction sites for carbendazim induced degradation: the benzene ring and the methyl group. Good simulations of carbendazim disappearance have been obtained by kinetic modelling over a wide range of initial H2O2 concentrations. In the presence of hydrogenocarbonate ions, a quenching effect is observed and the simulations lead to an underestimation of the carbendazim disappearance. This is because of the involvement of the carbonate radicals, which react with carbendazim with a second-order rate constant evaluated to be equal to (6 ± 2) × 106 L.mol−1.s−1 by kinetic modelling. When the starting concentration of HCO3− is high enough, the elimination of carbendazim by CO3˙ becomes the major route of carbendazim transformation.