Rapid photodegradation of clethodim and sethoxydim herbicides in soil and plant surface model systems

Photolysis is an important degradation process to consider when evaluating the persistence of a pesticide in the field. In this work, photolytic behavior of clethodim and sethoxydim herbicides under simulated solar radiation was examined in organic solvents, on glass disk and silica gel plates as leaf and soil model surfaces. The photodegradation was characterized by determination of their half-lives (t1/2), dissipation rate constant (k) and identification of degradation products by means of HPLC-Qtof-MS. Photolytic degradation of clethodim and sethoxydim was very rapid. The photodegradation rate was enhanced in leaf model than in water with half-lives that ranged from 6.3 ± 0.5 to 10.1 ± 0.4 min for clethodim and from 8.0 ± 0.3 to 20.5 ± 0.5 min for sethoxydim. The fastest rate of degradation was obtained on silica gel plates with half-lives of 1.8 and 5.0 min for clethodim and sethoxydim respectively. Photoreduction of the oxime ether moiety was the main transformation processes giving rise to a photostable product, the corresponding dealkoxylated derivative. Isomerization of oxime ether bond and oxidation of sulfur atom to form Z-isomer and the corresponding sulfoxides were the others reactions involved. The different environments tested influenced the concentrations of photoproducts formed during the irradiation of both herbicides. This result suggests that photolysis will be an important pathway of dissipation of the two herbicides. On the basis of these findings, further studies could be desirable to estimate the effects of transformation products on the environment.