Photocatalytic degradation of the fungicide Fenhexamid in aqueous TiO2 suspensions: Identification of intermediates products and reaction pathways

Liquid-chromatography interfaced with time-of-flight mass spectrometry (LC–TOF/MS) was used to separate and characterize the transformation products arising from TiO2-photocatalytic degradation of the fungicide Fenhexamid (FEX) in aqueous solution under simulated solar irradiation. Prior to identification, irradiated solutions of FEX (10 mg L−1) were concentrated by solid-phase extraction. Assignments of the mass spectra ions were aided by elemental composition calculations, comparison of structural analogues and available literature, and acquired knowledge regarding mass spectrometry of related heterocyclic compounds. The primary transformation intermediates identified were hydroxyl and/or keto-derivatives. Several positional isomers are typically produced as a consequence of the non-selectivity of the OH radical attack. Moreover, products resulted from the cleavage of the amide and NHdichlorophenol bonds were formed. Finally, cyclic – benzo[d]oxazole intermediates are also formed through an intramolecular photocyclization process and cleavage of halogen – carbon bond. In the case of the hydroxy and/or keto-derivatives, the generic fragmentation scheme obtained from the interpretation of the ESI–TOF–MS data cannot be diagnostic to precisely localize the position of the entering substituent on the FEX molecule, and thus to characterize all its possible oxygenated derivatives by assigning a plausible structure with confidence. On the basis of identified products different pathways of photocatalytic degradation of FEX were proposed and discussed.

Research highlights
► In this study the elucidation of the TPs arising from the TiO2 treatment of Fenhexamid was described.
► LC–TOF/MS was demonstrated to be a very powerful approach for the identification of TPs.
► The main TPs identified were hydroxy and/or keto-derivatives.
► TPs formed from dechlorination, cyclization, cleavage of amide and NHdichlorophenol bonds were also detected.
► More than one isomer were detected.