Enhanced Ga2O3-photocatalyzed and photochemical degradation of the Fipronil insecticide by UVC irradiation in mixed aqueous/organic media under an inert atmosphere

Abstract

Agrochemicals such as the insecticide Fipronil that bear fluoro groups are generally fat-soluble and nearly insoluble in water, so that their photodegradation in a heterogeneous aqueous gallium oxide dispersion presents some challenges. This article examined the photodegradation of this insecticide by solubilizing it through the addition of organic solvents (EtOH, MeOH, THF, 1,4-dioxane and ethylene glycol) to an aqueous medium and then subjecting the insecticide to 254 nm UVC radiation under photocatalytically inert (Ga₂O₃/N₂) and air-equilibrated (Ga₂O₃/O₂) conditions, as well as photochemically in the absence of Ga₂O₃ but also under inert and air-equilibrated conditions. Defluorination, dechlorination, desulfonation and denitridation of Fipronil were examined in mixed aqueous/organic media (10, 25 and 50 vol% in organic solvent). After 3 h of UVC irradiation (50 vol% mixed media) defluorination with Ga₂O₃/N₂ was ∼65% greater than in aqueous media, and ca. 80% greater than the direct photolysis of Fipronil under inert (N₂) conditions; under air-equilibrated conditions both Ga₂O₃-photocatalyzed and photochemical defluorination were significantly lower than in aqueous media. Dechlorination of Fipronil was ∼160% (Ga₂O₃/N₂) and 140% (photochemically, N₂) greater than in aqueous media; under air-equilibrated conditions, both photocatalyzed and photochemical formation of Cl⁻ ions in mixed media fell rather short relative to aqueous media. The photocatalyzed (Ga₂O₃/N₂) and photochemical (N₂) conversion of the sulfur group in Fipronil to SO₄²⁻ ions was ca. 20% and 30% greater, respectively, in mixed media, while under air-equilibrated conditions photocatalyzed desulfonation was nearly twofold less than in the aqueous phase; direct photolysis showed little variations in mixed media. Denitridation of the nitrogens in Fipronil occurred mostly through the formation of ammonia (as NH₄⁺) under all conditions with negligible quantities of NO₃⁻; again mixed media offered enhanced denitridation, particularly under inert N₂ conditions. Time-of-flight electrospray (TOF-MS/ESI⁻) mass spectrometry revealed a fairly large number of intermediates formed in the degradation of Fipronil, particularly under photocatalytic conditions. Only a couple of intermediates were identified in the photodegradation and the presence of Ga₂O₃ enhanced the complexity of an already cumbersome problem owing to the involvement of organic solvents.