Enhancement of photocatalytic degradation of phenol and chlorophenols by ultrasound

The synergistic effect due to the application of ultrasound on the photocatalytic degradation of phenol and chlorophenols was demonstrated in the presence of Hombikat TiO2 suspensions in our sonophotocatalytic reactor. A noticeable enhancement of the reaction rate was found when ultrasound was combined with the photocatalysis of phenol in a volume of 100 mL as compared with UV light photocatalysis alone. By reducing the reaction volume or increasing the average ultrasonic power density within a reasonable range, the enhancement could be intensified considerably. The average ultrasonic power density of around 0.7 W/mL seemed to be the optimal usage of ultrasonic energy in our system for achieving efficient sonophotocatalytic degradation of phenol. The influence of ionic strength and anions on sonophotucatalysis was evaluated. Our results show that chloride anions (Cl-) inhibit the action of sonophotocatalysis at low pH values. Chlorophenols, namely, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol, were involved in this investigation as well, leading to the suggestions that (1) the generation of chloride radicals (Cl-.) might assist the degradation of the organic compounds before substantial chloride anions (Cl-) were formed to inhibit the photocatalysis and (2) the solubility of the examined organic compounds has a significant influence on the performances of the photocatalytic and sonochemical reactions.