抄録
Irradiation of TiO_2 particles under ultraviolet light has been known to be a source of OH radicals in the disinfection or oxidizing systems. However, a new system was devised in this study for production of OH radicals by replacing UV with ultrasonic system (US). Formation of OH radicals was investigated in a TiO_2/US system to study the mechanism of formation of OH radicals as well as enhancement of cavitation effect in the ultrasonic system. Formation of OH radicals in the irradiating solution was studied by the reaction of OH radicals with salicylate. 2,3-dihydroxybenzoicacid (2,3-DHBA) and 2,5-dihydroxybenzoicacid (2,5-DHBA) can be formed during this reaction and they can be measured by HPLC with an electro chemical detector (HPLC-ECD; Eicom). Formation of DHBAs could be detected during the irradiation of salicylate solution under ultrasonic system and in the presence of TiO_2. It was seen that the concentration of DHBAs was increased within 20 minutes irradiation and then decreased in spite of high concentration of salicylate. It was likely due to the oxidation and degradation of DHBA in the ultrasonic system. Therefore, for investigating the oxidation of DHBA in the presence of salicylate, more salicylate was added after 20 minutes reaction to the irradiating mixture consecutively. The concentration of DHBA was increased for three times by the addition of salicylate concentration in the solution. Enhancement of cavitation energy was also investigated by the addition of various gases (Ar, O2, N2) into irradiation solution. The ratio of the specific heats at constant pressure and constant volume (Cp/Cv) of the gas and vapor inside the cavity is known to be one of the most crucial parameters that determine the final temperature inside cavitation bubbles. Monoatomic gases, such as argon has a high value of Cp/Cv, while diatomic gases, such as N_2, O_2 have a lower values. Therefore, addition of argon to the irradiating solution induced higher concentration of OH radicals due to higher cavitation energy. Our results indicated that the addition of O_2 and N_2 gases at the same conditions resulted lower oxidation power. Finally, the competitive oxidation of salicylate and DHBAs in the irradiating solutions was more investigated in this research to develop a more descriptive mechanism of generation of hydroxyl radicals.
