Clarification of regimes determining sonochemical reactions in solid particle suspensions

Classification of regimes determining ultrasonic cavitation erosion in aqueous solutions containing dissolved air

Ultrasonic cavitation is crucial for wastewater treatment, as it enhances the oxidation and degradation of contaminants. However, cavitation erosion of acoustic horn tips poses a significant challenge due to reduced treatment efficiency and increased operational costs. Since the air dissolved in liquids not only affects sonochemical yields but also complicates ultrasonic cavitation erosion, the quantitative analysis of dissolved air affecting cavitation erosion is required. This study experimentally investigated the effect of dissolved air on erosion by increasing the dissolved oxygen content from 1.18 to 18.30 mg·L-1 via pre-supersaturation and degasification methods. Employing ultrasonic vibrations, material removal results indicated that erosion was initially aggravated and then alleviated, with an increase in dissolved air shifting the state of the solution from an undersaturated to a supersaturated state. The most severe erosion was observed when the dissolved oxygen content reached 4 mg·L-1, which corresponded to a half-saturated state. Theoretical examinations of heterogeneous nucleation rates and energy dissipation following asymmetrical bubble collapse revealed four regimes: homogeneous nucleation, vaporous and gaseous cavitation bubble nucleation, and microbubble formation. With increasing dissolved air, accelerated vaporous cavitation aggravates erosion, while gaseous cavitation and microbubble formation alleviate erosion, which provides a classification of regimes determining cavitation erosion affected by dissolved air. These findings highlight the significant effect of dissolved air on ultrasonic cavitation erosion and, with a better understanding of these regimes, can aid in optimizing the design and operation of sonoreactors used for wastewater treatment.

Comparison of sonochemical and sonophysical activity under various geometric conditions in 28 kHz Double-Bath sonoreactors

Sonochemical and sonophysical activities were investigated under various geometric conditions using 28 kHz double-bath-type sonoreactors. Sonochemical activity was quantified using KI dosimetry (triiodide ion concentration), while sonophysical activity was measured via lightweight expanded clay aggregate (LECA) desorption tests (turbidity). Thirty-five different geometric conditions were tested, comprising seven distances between the sonoreactor bottom and vessel bottom (L2) and five water levels (L3). The optimal conditions for sonochemical activity [(L2/L3, unit: mm): 70/76, 90/116, 70/116, 110/146, and 70/96] and sonophysical activity [(L2/L3, unit: mm): 30/78, 70/88, 150/168, 30/68, and 30/38] did not match, and no conditions showing high values in both activities were found. The presence of a thin-walled vessel did not affect ultrasound transmission. However, the presence of LECAs caused a large attenuation of ultrasound and the formation of different cavitational active zones in the vessel. This was identified as the main reason for the mismatch in optimal geometric conditions for sonochemical and sonophysical activities. The sonochemical activity could be enhanced by the presence of LECAs under conditions; however, the enhanced sonochemical activity was much lower than the five highest sonochemical activities. In addition, the movement of LECAs induced by ultrasound irradiation significantly enhanced the sonophysical activity.

Tetracycline degradation by dual-frequency ultrasound combined with peroxymonosulfate

Tetracycline has received a great deal of interest for the harmful effects of substance abuse on ecosystems and humanity. The effects of different processes on the degradation of tetracycline were compared, with dual-frequency ultrasound (DFUS) in combination with peroxymonosulfate (PMS) being the most effective for the tetracycline degradation. Free radical scavenging experiments showed that O2∙-,SO4∙- and •OH were the main reactive radicals in the degradation of tetracycline. According to the major intermediates of tetracycline degradation identified, three possible degradation pathways were proposed, which are of significance for translational studies of tetracycline degradation. Notably, these intermediates were found to be significantly less toxicity. The number of active bubbles in the degradation vessel was calculated using a semi-empirical formula, and a higher value of 1.44 × 108 L-1s-1 of bubbles was obtained when using dual-frequency ultrasound at 20 kHz (210 W/L) and 80 kHz (85.4 W/L). Therefore, compared to 20 kHz, although the yield of strong oxidizing substances from individual active bubbles decreased slightly, a significant increment of the number of active bubbles still resulted in a higher synergistic effect, and the combination of DFUS and PMS should be effective in promoting the generation of reactive free radicals and mass transfer processes within the degradation vessel, which provides a method for efficient removal of tetracycline from wastewater.

Joint effects of gas bubbles and solid particles on sonochemical inhibition in sonicated aqueous solutions

Wastewater is a multicomponent and multiphase mixture. Gas bubbles and solid particles in the dispersed phase influence sonochemical efficiency during ultrasonic treatment of wastewater, sometimes unfavorably; however, the influencing factors and mechanisms remain unclear. In this paper, the influence of argon gas bubbles (1.2 mm) and monodisperse silica particles (0.1 mm) on sonochemical effects in an aqueous system using a horn-type reactor (20 kHz) is reported. Triiodide formation decreased with an increase in the volume fraction of either or both phases. The two phases started inhibiting sonoreactions as the total volume fraction approached 3.0-4.0 vol% compared to pure water. The effect of the gas-to-solid ratio is also considered. We propose an acoustic attenuation model, which incorporates the scattering effect of solid particles and the thermal effect of gas bubbles. The agreement between the modeling and experimental results demonstrates that the two phases are jointly responsible for sonochemical inhibition by increasing ultrasound attenuation. This enhances the understanding of sonochemistry in gas-solid-liquid systems and helps regulate gases and solids in sonochemical reactors.

Critical Roles of Impurities and Imperfections in Various Phases of Materials

In many materials, impurities and imperfections play a critical role on the physical and chemical properties. In the present review, some examples of such materials are discussed. A bulk nanobubble (an ultrafine bubble) is stabilized against dissolution by hydrophobic impurities attached to the bubble surface. An acoustic cavitation threshold in various liquids decreases significantly by the presence of impurities such as solid particles, etc. The strength of brittle ceramics is determined by the size and number of pre-existing microcracks (imperfections) in the specimen. The size effect of a BaTiO3 nanocrystal is influenced by the amount and species of adsorbates (impurities) on its surface as adsorbate-induced charge-screening changes the free energy. The dielectric constant of an assembly of BaTiO3 nanocubes is influenced by a small tilt angle (imperfection) between two attached nanocubes, which induces strain inside a nanocube, and is also influenced by the spatial strain-relaxation due to defects and dislocations (imperfections), resulting in flexoelectric polarization.