Degradation intermediates and reaction pathway of pyraclostrobin with TiO2 photocatalysis

The present study deals with the photocatalytic degradation of the strobilurin pyraclostrobin in the presence of titanium dioxide (TiO2) as a photocatalyst and UV light irradiation. The obtained results show a complete degradation of pyraclostrobin at pH 6.2, temperature at 20 ± 1 °C and 0.5 g L−1 of catalyst after 60 min irradiation. Photodegradation of pyraclostrobin exhibited pseudo-first-order reaction kinetics. The rate of photodecomposition of pyraclostrobin was measured using high performance liquid chromatography–diode array detector (HPLC–DAD). The effect of solution pH in the 2–10 range was investigated as well as the reaction intermediates formed during degradation. To obtain a better understanding of the mechanistic details of this TiO2-assisted photodegradation of pyraclostrobin with UV-irradiation, the intermediates of the processes were concentrated, separated and identified by the solid-phase extraction (SPE) and liquid chromatography/mass spectrometry (LC–MS/MS) technique. The probable photodegradation pathways were proposed and discussed. The main steps involved: hydroxylation of the aromatic rings (chloro-phenyl, phenyl and pyrazol) followed by the loss of the N-methoxy group, substitution of chloride atom by a hydroxyl group, rupture of the pyrazol and phenyl bond and the scission of the oxygen and pyrazol bond.

Figure optionsDownload as PowerPoint slideHighlights
► Pyraclostrobin was totally degraded in an aqueous TiO2 suspension in the dark after 60 min.
► The rate constant for the reaction of pyraclostrobin was calculated as kobs = 1.4 × 10−2 min−1.
► The photodegradation rate was found to increase along with decreasing pH.
► Eleven intermediates were identified by HPLC–APCI–MS/MS.
► A mechanism of degradation has been tentatively proposed.