Removal of gaseous VOCs by ultrasonic irradiation of water

The decomposition of gaseous toluene as a representative volatile organic compound using 200 kHz ultrasonic waves was investigated as a new air cleaning method that does not require an ultraviolet light source, in accordance with the Minamata Convention on Mercury. Upon applying ultrasonic irradiation to water in a batch reactor, the removal ratio of gaseous toluene above the water reached 91.0% at 10 ppm and 92.7% at 40 ppm. The addition of t-BuOH, a radical scavenger that suppresses ultrasonic decomposition, inhibited the removal of gaseous toluene, indicating that the ultrasonic irradiation not only accelerated the dissolution of toluene into the water but also mediated its decomposition. Further experiments in a flow system revealed that the removal ratio of gaseous toluene decreased with increasing toluene flow rate, whereas the total mass of toluene removed increased, suggesting that sufficient amounts of the active species, such as •OH radicals, required for the initial decomposition of toluene were formed in the liquid phase upon ultrasonic irradiation.