Light induced transformations of fungicides on titanium dioxide. Pathways and by-products evaluation using the LC-MS technique

Azoxystrobin, carbendazim, and mepanipyrim, widely used heterocyclic fungicides, have been photocatalytically degraded in aqueous solution on TiO2. Several concomitant pathways occur, concerning reductive and/or oxidative attacks, thus leading to numerous intermediates, identified and characterized through an MSn spectra analysis. Multiple stage mass spectrometry of redox degradation compounds was simpler than that of parent fungicides, whose strong structures sometimes caused deactivated fragmentation pathways. Noteworthy were the different fragmentation ways that enabled hydroxylation positions to be located. Azoxystrobin is easily degraded and, within a few hours of irradiation, complete mineralization is achieved. For mepanipyrim and carbendazim, instead of an initial marked degradation, a lack in both carbon and nitrogen mineralization is observed. This is linked to the formation in both cases of guanidine, the only species persistently observed in the investigated time and still containing bound nitrogen.