Ultrasonic frequency is a key parameter determining multibubble sonoluminescence (MBSL) spectra of water saturated with Ar/O2 gas mixtures. At 20 kHz, the MBSL is quenched by oxygen. By contrast, at high-frequency ultrasound the maximal MBSL intensity is observed in the presence of Ar/20%O2 gas mixture. Nevertheless, oxygen has no influence on the shape of MBSL spectra. The effect of oxygen on MBSL is explained by oxygen dissociation inside the collapsing bubble which is much more effective at high ultrasonic frequency compared to 20 kHz ultrasound. In contrast to MBSL, a higher yield of H2O2 is observed in Ar/20%O2 gas mixture whatever the ultrasonic frequency. At 20°C and 20% of oxygen the maximal yield of H2O2 is observed at 204-362 kHz. The maximal yield of H2O2 is shifted to 613kHz when the bulk temperature is raised up to 40°C. Coupling of high-frequency ultrasound with mechanical stirring and intensive Ar/O2 bubbling improves H2O2 production. Comparison of MBSL and sonochemistry allowed to conclude that H2O2 is formed from non-excited OH (X(2)Π) and HO2 radicals. Finally, it was shown that at the studied conditions the efficiency of ultrasonic degassing is hardly influenced by frequency.

Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO₂ suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO₂ particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO₂ and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu L_III-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO₂ nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO₂ nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO₂ cores and hydrolyzed Pu(IV) moieties at the surface shell.

This review summarizes recent studies of multibubble sonoluminescence (MBSL) in aqueous media in order to highlight new insights into the origin of the sonochemical activity. The observation of OH(C2Σ+-A2Σ+) emission band and a spectroscopic analysis of OH(A2Σ+-X2Πi) emission band in MBSL of water pre-equilibrated with noble gases revealed the formation of a nonequilibrium plasma inside the collapsing bubble (Te>Tv>Tr, where Te is an electron temperature, Tv is a vibrational temperature and Tr is a rotational (gas) temperature). The Te and Tv estimated using OH(A2Σ+-X2Πi) emission band increase with ultrasonic frequency. In Xe the Tv of OH(A2Σ+) state is much higher than in Ar most probably due to the lower ionization potential of Xe. The MBSL of C2∗ Swan band (d3Πg-a3Πu) measured in aqueous tert-butanol (t-BuOH) solutions correlates with the data obtained for OH(A2Σ+-X2Πi) emission band. Analysis of the gaseous products of t-BuOH sonolysis revealed a significant sonochemical activity even at high t-BuOH concentration when MBSL is totally quenched, indicating that drastic intrabubble conditions (plasma) are not necessarily accompanied by sonoluminescence. The nonequilibrium plasma model of cavitation allows to explain the reverse carbon isotope effect observed during the sonolysis of water in the presence of Ar/CO gas mixture.

XRD, EXAFS and HR-TEM study reveals a strong correlation between AnO ₂ (An = Th and Pu) nanoparticle size and An( iv ) local structure.

The industrial treatment of spent nuclear fuel is based upon a hydrometallurgical process in nitric acid medium. In order to minimize the volume of radioactive waste it seems interesting to generate the reactive species in situ in such solutions using ultrasonic irradiation without addition of salt-forming reagents. This review summarizes for the first time the versatile sonochemical processes with uranium, neptunium and plutonium in homogeneous nitric acid solutions and heterogeneous systems. The dissolution of refractory solids, ultrasonically driven liquid-liquid extraction and the sonochemical degradation of the volatile products of organic solvent radiolysis issued from PUREX process are considered. Also the guidelines for required further work to ensure successful application of the studied processes at industrial scale are discussed.