[en] Acoustic noise spectra were studied for the first time in overheated water using sono-hydrothermal reactor operating at 20 kHz ultrasound in the temperature range from 25 to 200 degrees C at the autogenic pressure of 1-14 bar. The obtained results highlighted a dominating role of stable cavitation during ultrasonic treatment of hot water. Heating of sonicated water results in the formation of large number of nonlinearly oscillating bubbles synchronous with the driving frequency. At 200 degrees C, the acoustic spectra also display strong subharmonic and multiple ultraharmonic bands. Moreover, cavitation bubbles formed at 200 degrees C exhibit chaotic and random motions. It has been shown that the addition of TiO₂ nanoparticles to hydrothermal water heated at 200 degrees C allows to eliminate subharmonic/ultraharmonic bands and stochastic oscillations as well. This effect was assigned to Pickering-like bubble stabilization due to the particle accumulation at the bubble surface. (author)

[en] Sonochemistry studies chemical and physical effects in liquids submitted to power ultrasound. These effects arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species. In principle, each cavitation bubble can be considered as a microreactor initiating chemical reactions at mild conditions. In addition, microjets and shock waves accompanied bubble collapse produce fragmentation, dispersion and erosion of solid surfaces or particles. Microbubbles oscillating in liquids also enable nucleation and precipitation of nanosized actinide compounds with specific morphology. This review focuses on the versatile sonochemical processes with actinide ions and particles in homogenous solutions and heterogenous systems. The redox reactions in aqueous solutions, dissolution or precipitation of refractory solids, synthesis of actinide nanoparticles, and ultrasonically driving decontamination are considered. The guideline for further research is also discussed.

[en] The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of micro-bubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species and to the emission of light, named sono-luminescence. In this manuscript, we describe the techniques allowing study of extreme intra-bubble conditions and chemical reactivity of acoustic cavitation in solutions. The analysis of sono-luminescence spectra of water sparged with noble gases provides evidence for nonequilibrium plasma formation. The photons and the 'hot' particles generated by cavitation bubbles enable to excite the non-volatile species in solutions increasing their chemical reactivity. For example the mechanism of ultra-bright sono-luminescence of uranyl ions in acidic solutions varies with uranium concentration: sono-photoluminescence dominates in diluted solutions, and collisional excitation contributes at higher uranium concentration. Secondary sono-chemical products may arise from chemically active species that are formed inside the bubble, but then diffuse into the liquid phase and react with solution precursors to form a variety of products. For instance, the sono-chemical reduction of Pt(IV) in pure water provides an innovative synthetic route for monodispersed nanoparticles of metallic platinum without any templates or capping agents. Many studies reveal the advantages of ultrasound to activate the divided solids. In general, the mechanical effects of ultrasound strongly contribute in heterogeneous systems in addition to chemical effects. In particular, the sono-lysis of PuO_2 powder in pure water yields stable colloids of plutonium due to both effects. (authors)

[en] Highlights: • Complete decontamination of Mg-based materials can be achieved under ultrasound in mild conditions. • Decontamination occurs through the controlled dissolution of the surface with a restrained generation of H₂. • Surface structuration of Mg-based materials is observed during ultrasonic treatment. • Insoluble precipitate may form with extended treatment and induce a progressive recontamination of the surface. UNGG cladding nuclear wastes constitute a huge volume of Mg-based materials that raises economic and safety concerns, particularly due to their radioactivity coupled to the potential generation of H₂ gas under deep underground disposal. Their significant decontamination would result in more secure and less expensive storage, with a better containment of the separated long-lived radioisotopes that could enter in a classical channel. Sonication of genuine UNGG cladding materials and simulants at 345 kHz in 0.01 M oxalic acid solution (20 °C) allowed the structuring of their surfaces with the observation of homogeneously distributed craters of 20–40 µm in diameter. After a thorough characterization and comparison of the ultrasound effects generated at the surface, the various samples were artificially contaminated and characterized before sonication. The complete and rapid sonochemical decontamination of Mg-based materials was then observed, in addition to the removal of the carbon layer promoting corrosion on the inner UNGG cladding. The extension of sonication allows the neo-formed brucite (Mg(OH)₂) and zirconium-based phases to accumulate on the surface, thus contributing in a slight but continuous surface recontamination process. This phenomenon results from the re-adsorption of uranyl cations from the solution which can be avoided by optimizing the duration of treatment.

[en] Knowledge on cavitation bubble size distribution, ambient radius of bubbles is of interest for many applications that include therapeutic and diagnostic medicine. It however becomes a hard quest when increasing the ultrasonic frequency, when direct observation of bubble dynamics is no longer possible. An indirect method based on the estimation of the bubble dissolution time under pulsed ultrasound (362 kHz) is used here under optimized conditions to derive ambient radii of cavitation bubbles in water saturated with He, Ar, Xe, O₂, N₂ and air: 3.0 μm for Ar, 1.2 μm for He, 3.1 μm for Xe, 2.8 μ m for O₂, around 1 μm for N₂ and air. If the pulse on-time is increased, bubble coalescence occurs, the extent of which is rather limited for Ar but extremely high for He or N₂. (authors)

[en] The presence of salts in a solution is known to affect sonochemistry, but until now no consensus has been reached in the literature on how and why a salt influences sonochemistry. The present study focuses on the effect of NaCl on sonochemical activity and sonoluminescence at 362-kHz frequency in aqueous solutions saturated with He and Ar. It is shown that the presence of salt has a multiple impact: the global population of active bubbles decreases due to the decreasing gas solubility, new chemical reactions involving Na-center dot and Cl-center dot atoms occur that influence hydrogen and hydrogen peroxide yields and the standing wave component of the US wave is enhanced, favoring sonoluminescence emission. Interestingly, the effect of salt greatly depends on the nature of the saturating gas: for instance, strong acidification occurs under He, while it is limited under Ar. (authors)

[en] The influence of the sample morphology and experimental conditions towards the sonochemical dissolution of nanoscale ThO₂ samples in sulfuric acid media is described. Significant sonochemical dissolution rates and yields are observed at 20 kHz under Ar/O₂ atmosphere in dilute 0.5 M H₂SO₄ at room temperature, contrasting with the generally-reported high refractory behavior for ThO₂. The dissolution of ThO₂ combines the physical effects driven by acoustic cavitation phenomenon, the complexing affinity of Th(IV) in sulfuric medium and the sonochemical generation of H₂O₂. These sonochemical conditions further allow the observation of the partial conversion of ThO₂ into a scarce Th(IV) peroxo sulfate with 1D morphology resulting from one or both following processes: dissolution/reprecipitation or formation of an intermediate Th(IV) surface complex. (author)

[en] This review summarizes recent studies of multi bubble sono-luminescence (MBSL) in aqueous media in order to highlight new insights into the origin of the sono-chemical activity. The observation of OH (C²Σ⁺-A²Σ⁺) emission band and a spectroscopic analysis of OH(A²Σ⁺- X²Π_i) emission band in MBSL of water pre-equilibrated with noble gases revealed the formation of a nonequilibrium plasma inside the collapsing bubble (T_e ≥ T_v ≥ T_r, where T_e is an electron temperature, T_v is a vibrational temperature and T_r is a rotational (gas) temperature). The T_e and T_v estimated using A²Σ⁺- X²Π_i) emission band increase with ultrasonic frequency. In X_e the T_v of OH(A²Σ⁺) state is much higher than in Ar most probably due to the lower ionization potential of X_e. The MBSL of C_⁺2 Swan band (d³Π_g-a³Π_u) measured in aqueous tert-butanol (t-BuOH) solutions correlates with the data obtained for OH(A²Σ⁺-X²Π_i) emission band. Analysis of the gaseous products of t-BuOH sono-lysis revealed a significant sono-chemical activity even at high t-BuOH concentration when MBSL is totally quenched, indicating that drastic intra-bubble conditions (plasma) are not necessarily accompanied by sono-luminescence. The nonequilibrium plasma model of cavitation allows to explain the reverse carbon isotope effect observed during the sono-lysis of water in the presence of Ar/CO gas mixture. (authors)

[en] The dissolution of ceria is studied through a catalytic reduction process involving platinum nanoparticles in mild conditions at near ambient temperature. The deposition of platinum nanoparticles is made by sonication (Ar, 18 W cm^-2, 20 kHz), and further dissolution is studied as a function of different parameters such as stirring, sonication, dissolution media and temperature. The dissolution is evaluated using UV-vis spectrophotometry, ICP-AES, and SEM. The quantitative dissolution of ceria can be performed in HNO₃-HCOOH-[N₂H₅][NO₃], HNO₃-[N₂H₅][NO₃] or H₂SO₄-HCOOH mixtures at 40 degrees C. Nevertheless, it is shown that the combined use of ultrasound with nitric media in the presence of platinum nanoparticles can lead to passivating phenomena resulting in a decrease of the dissolution rate. (authors)

[en] Sono-luminescence spectra in relation with sono-chemical activity of water sparged with Ar/N₂ gas mixtures were systematically studied at two ultrasonic frequencies (20 and 359 kHz). At 20 kHz, solely the molecular emission of OH (A²Σ⁺-X²Π_i) is observed in addition to a broad continuum typical for multi-bubble sono-luminescence. On the contrary, at high frequency a second emission band is present around 336 nm which is assigned to the NH (A³Π-X³Σ^--) system. In addition, the sono-lysis of a 0.2 M NH₃.H₂O solution at 359 kHz in the presence of pure Ar yields the emission bands of NH (A³Π -X³Σ^--) (336 nm) and NH (C¹Π-A¹Δ) (322 nm) systems confirming the sono-chemical production of NH radicals. The N₂ (C³Π_u-B³Π_g) emission band is absent at both frequencies. This uncommon phenomenon can be explained by the quenching of the N₂ (C³Π_u) excited state with water molecules inside the bubbles. The sono-luminescence of NH radicals at 359 kHz indicates more effective intra-bubble dissociation of N₂ molecules at high ultrasonic frequency compared to low-frequency (20 kHz) ultrasound. Its absence at 20 kHz may also be related to strong quenching, e.g., by water molecules. The kinetic study of the formation of principal sono-chemical products (H₂, H₂O₂, HNO₃, HNO₂) confirmed the more drastic conditions produced during bubble collapse at higher ultrasonic frequency. (authors)