Degradation of phenyltrifluoromethylketone in water by separate or simultaneous use of TiO2 photocatalysis and 30 or 515 kHz ultrasound

TiO2 photocatalysis and ultrasound are emerging technologies for the mineralization of pollutants in water. To further investigate these technologies and to assess whether advantages and synergy can be expected from their differences, phenyltrifluoromethylketone (PTMK) was selected as a test compound for pollutants generating CF3COOH, an undesirable final product. The PTMK first-order removal rate constant k was ca. 14 times higher when the ultrasound frequency was 515 kHz instead of 30 kHz for the same energy, and ca. 2.5 times higher when a TiO2 sample we synthesized was used instead of TiO2 Degussa P25. On simultaneous photocatalytic and ultrasonic treatment an increase in k by a factor between 1.4 and 1.9, depending on the TiO2 sample, was observed at 30 kHz but not at 515 kHz. On the basis of catalase enzymatic effect upon k, these observations are tentatively explained by a photocatalytic OH• radical production from sonochemically formed H2O2, provided that the H2O2 residence time on TiO2 is sufficient. PTMK ultrasonic pyrolysis was demonstrated by product analysis. The amount of CF3COOH was ca. 8 times lower in sonicated solutions than in UV-irradiated TiO2 suspensions, for both frequencies and both TiO2 samples. Therefore, because of a higher k value, a high frequency ultrasonic (pre)treatment is preferable to minimize CF3COOH formation.