Oxidative dissolution of Tc(IV)O2 . nH2O colloids by sonolysis

Redox reactions of Tc in aqueous solutions by sonolysis were investigated. Aqueous pertechnetate (Tc(VII)O4-) solutions and Tc(IV)O2 . nH2O colloids solutions were sonicated (200 kHz, 200 W) in a glass cell with flat bottom under Ar or He atmosphere at 20 C. No reduction of TcO4- was observed in this study. However, we found that the TcO2 . nH2O colloids dispersed in an aqueous solution were completely dissolved by ultrasonic irradiation within 30 min under Ar atmosphere, and TcO4- was eventually produced. The production of TcO4- was considerably suppressed in the presence of t-butyl alcohol (an effective scavenger of OH radicals), indicating that Tc(IV) was oxidized by OH radicals (produced by dissociation of water molecules) in hot cavitation bubbles created by ultrasound. The formation rate of TcO4- under He atmosphere was smaller than that under Ar atmosphere. This can be attributed to a difference of the effective maximum temperature in the collapsing bubbles. Because thermal conductivity of He is much higher than Ar, thermal transport effectively occurs from the bubbles to the surrounding liquid. Thus, temperature of cavitation bubbles filled with He should be lower than those filled with Ar, resulting in the smaller OH formation rate. (orig.)