Photooxidation of three spirocyclic acid derivative insecticides in aqueous suspensions as catalyzed by titanium and zinc oxides

• Photocatalytic oxidation of spirodiclofen, spiromesifen and spirotetramat in water.
• Different semiconductor oxides were studied under solar and artificial irradiation.
• ZnO was the most efficient for the removal of these insecticides.
• The best photocatalytic efficiency was obtained under sunlight irradiation.
• The main phototransformation intermediates (enol-derivatives) were identified.

The photocatalyzed degradation of tetramic (spirotetramat) and tetronic (spirodiclofen and spiromesifen) spirocyclic acid (ketoenol) derivative insecticides has been studied in aqueous suspensions using different semiconductor oxides. The photodegradation was studied by monitoring the change in compound concentration using HPLC-MS analysis and by the decrease in the dissolved organic carbon (DOC) content as a function of irradiation time. Laboratory scale experiments under UVA irradiation were performed to investigate the kinetics degradation of these compounds using different binary (ZnO, TiO2 and TiO2 Kronos vlp 7000) and ternary oxides (Zn2TiO4 and ZnTiO3) of zinc and titanium. Highest degradation rates were observed for ZnO and TiO2 P25 Degussa while with ternary oxides smaller photodecomposition rates were assessed, especially with ZnTiO3. New photocatalytic experiments were then performed under natural sunlight using ZnO, TiO2 P25 Degussa and TiO2 Kronos vlp 7000 leading nearly complete degradation of the studied insecticides when ZnO and TiO2 P25 Degussa were used. The photodegradation processes followed first-order reaction kinetics. Comparison of these catalysts showed that ZnO is the most efficient for the removal of these insecticides at constant volumetric rate of photon absorption in the photoreactor. The main photocatalytic intermediates (enol-derivatives) were detected during the degradation in all cases as a consequence of the cleavage of alkyl-oxygen bond of the parent compounds.

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