A photophysical approach to investigate the photooxidation mechanism of pesticides: Hydroxyl radical versus electron transfer

The role of hydroxyl radical in different solar photocatalytic processes, namely photo-Fenton, titanium dioxide-based oxidation and organic photocatalysis (triphenylpyrylium) has been investigated. Using a methodology based on the flash photolytic generation of OH from N-hydroxypyridine-2(1H)-thione, followed by t-stilbene trapping, the reaction rate constants of different pesticides (dimethoate, methidathion, alachlor and pyrimethanyl) with this reactive oxygen species in deaerated acetonitrile have been determined. The results obtained under photo-Fenton conditions are in reasonable agreement with the estimated rate constants; hence the assumption that the photo-Fenton reaction mainly involves participation of the hydroxyl radical seems plausible. The oxidation pattern using titanium dioxide was also investigated; however, under these conditions no clear correlation could be found due to participation of an alternative electron transfer mechanism.Finally, pyrimethanil was the most reactive pesticide when photodegradation was carried out in the presence of TPP, in spite of its low reactivity towards hydroxyl radical. Laser flash photolysis experiments evidenced that the reaction occurs via photoinduced electron transfer within a ground state complex; this is in good correlation with the relative amount of pyranyl radical formation.

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► The role of OH has been investigated by laser flash photolysis.
► The reaction rate constants of five pesticides with OH have been determined.
► Results under photo-Fenton conditions are in agreement with OH involvement.
► The oxidation pattern using TiO2 suggests a mixed mechanism.
► Using TPP, photoinduced electron transfer occurs within a ground state complex.