Title
Photodegradation of carbendazim sensitized by aromatic ketones
Author
Dolors Jornet, Mª Angeles Castillo, Mª Consuelo Sabater, Rosa Tormos, Miguel A. Miranda
Year
2013
Journal
Journal of Photochemistry and Photobiology A: Chemistry
Abstract
Carbendazim (1) is a benzimidazole extensively used as post-harvest fungicide on fruits and vegetables. The aim of the present work is to study the photodegradation of 1sensitized by aromatic ketones, with special attention to mechanistic aspects and to the possible detoxification associated with photochemical treatment. Laser flash photolysis (LPF) λexc = 355 nm of xanthone (XA) and anthraquinone (AQ) was performed in MeCN solutions, in the presence of 1. A new transient absorbing at λmax 500 and 320 nm was obtained and assigned to the semioxidized radical cation 1+radical dot. An exergonic thermodynamics for electron transfer quenching was confirmed by means of the Rehm–Weller equation. The same species was observed by direct LFP of 1 at 266 nm in polar solvents. Conversely, when a deoxygenated solution of 1 was submitted to LFP in cyclohexane the transient spectrum presented a band with maximum at 380 nm; it was assigned to the triplet excited state (31*) on the basis of energy transfer to oxygen and β-carotene. The photodegradation of 1 was achieved using XA and AQ as electron acceptors in a solar-simulator, in aerated aqueous medium; the reaction was faster with XA. Formation of a new photoproduct was initially observed; its structure was assigned as carbendazim N-C5 dimer (2). A balance of the total organic carbon (TOC) after prolonged irradiation indicated that mineralization does not occur to a significant extent, pointing to oxidative fragmentation of 1 and 2 to give a variety of low molecular weight products. To check whether the observed photodegradation of 1 results in a decreased toxicity, biological assays were performed using an established model based on the inhibition of mobility of Daphnia magna. The results demonstrate that photodegradation leads to a diminished toxicity, indicating that the photoproducts are less toxic than the parent compound.
Instrument
FP-8500
Keywords
Fluorescence, Quantum yield, Agricultural & environmental