Hydroxyl radical production in liquid water by laser cavitation

Laser cavitation (LC) is an interesting novel method to the production of hydroxyl (·OH) radical. A setup of LC system was designed and methylene blue (MB) aqueous solution was chosen to detect the ·OH radical. The production and detection mechanism of ·OH radical were investigated combined with the evolution process of laser-induced cavitation bubble. The effects of operating parameters (laser energy, pulse frequency, temperature, pH value, water matrix and liquid state) on the production were discussed. Moreover, the energy consumption of LC was analyzed. The results indicate that the production of ·OH radical origins from the tearing of water molecules by the combined impact of laser shock wave, bubble collapse shock wave and water-jet, and the optimal concentration of MB is 5 mg/L. The production yield of ·OH radical increases with the rise of laser energy (25 mJ − 150 mJ) and pulse frequency (1 –5 Hz). The optimal temperature for the production yield is ranging from 35 ℃ to 45 ℃, and the conditions of flowing, acidic tap water are more conducive to the production of ·OH radical. The production of ·OH radicals by LC at 25 mJ laser energy and 1 Hz pulse frequency have the best production efficiency.