Sonochemical and sonocatalytic degradation of monolinuron in water

The degradation of the phenylurea monolinuron (MLN) by ultrasound irradiation alone and in the presence of TiO2 was investigated in aqueous solution. The experiments were carried out at low and high frequency (20 and 800 kHz) in complete darkness. The degradation of MLN by ultrasounds occurred mainly by a radical pathway, as shown the inhibitory effect of adding tert-butanol and bicarbonate ions to scavenge hydroxyl radicals. However, CO3- radicals were formed with bicarbonate and reacted in turn with MLN. In this study, the degradation rate of MLN and the rate constant of H2O2 formation were used to evaluate the oxidative sonochemical efficiency. It was shown that ultrasound efficiency was improved in the presence of nanoparticles of TiO2 and SiO2 only at 20 kHz. These particles provide nucleation sites for cavitation bubbles at their surface, leading to an increase in the number of bubbles when the liquid is irradiated by ultrasound, thereby enhancing sonochemical reaction yield. In the case of TiO2, sonochemical efficiency was found to be greater than with SiO2 for the same mass introduced. In addition to the increase in the number of cavitation bubbles, activated species may be formed at the TiO2 surface that promote the formation of H2O2 and the decomposition of MLN.

► Monolinuron is degraded by ultrasounds by OHOH or CO3- radicals.
► Ultrasound efficiency was improved in the presence of nanoparticles of TiO2 and SiO2 only at 20 kHz.
► Activated species may be formed at the TiO2 surface that promote the formation of H2O2 and MLN decomposition.