Sensitivity of free radicals production in acoustically driven bubble to the ultrasonic frequency and nature of dissolved gases

Ultrasonic destruction of surfactants

This study investigates the effectiveness of ultrasonic (US) treatment in removing and mineralizing surfactants in wastewater. It examines the complex mechanisms and variables (acoustic conditions, solution temperature, initial dose, etc.) that affect sonolytic processes. The effect of water matrix components (such as salts and the presence of secondary pollutants) on process performance is thoroughly investigated. Various treatments are analyzed through a detailed comparison of synergistic hybridization processes. The study also provides a comprehensive review of current environmental applications and explores potential directions for surfactant degradation using ultrasound. Insightful information is presented to advance sustainable wastewater treatment techniques. The literature review clearly reveals the promising future of sonotreatment for degrading various surfactants under different conditions. The use of multifrequency mechanisms and the integration of other advanced oxidation processes (AOPs) with the US process have significantly enhanced the energy efficiency of the sonochemical system. Additionally, the results highlight the need to focus on developing new sonoreactor designs, identifying degradation intermediates, and hybridizing the sonochemical system under innovative operating conditions.

Ultrasonic decomposition of endocrine disrupting Compounds - A review

Endocrine disrupting compounds (EDCs) need to be removed by efficient treatment methods as they are a major concern for both human and environmental health. To reduce the impact of EDCs in water, this review examines the use of ultrasonic degradation processes. Following an overview of EDCs and their origins, the basic concepts of sonochemistry are examined, highlighting the potential of ultrasound in chemical reactions. An in-depth analysis of the variables that affect the ultrasonic degradation of EDCs, such as frequency, intensity/power, temperature and solution chemistry, prepares the reader for a case study investigation focusing on specific EDCs. The study also looks at synergistic methods, emphasizing how hybrid ultrasonic systems can improve removal efficiency. The study provides a comprehensive overview of the use of sonochemistry in the treatment of EDCs by addressing current issues and suggesting future research directions. The aim of this review paper is to provide insightful analysis and useful suggestions for scientists working on EDC remediation projects.

Ammonia production by microbubbles: A theoretical analysis of achievable energy intensity

The present paper studies the energy intensity of ammonia production by a freely oscillating microbubble placed in an infinite domain of liquid. The initial content of the bubble is a mixture of hydrogen and nitrogen. The bubble is expanded isothermically to a maximum radius, then it is "released" and oscillates freely. The input energy is composed of the potential energy of the bubble at the maximum radius, the energy required to produce hydrogen, and the pumping work in case a vacuum is employed. The chemical yield is computed by solving the underlying governing equations: the Keller-Miksis equation for the radial dynamics, the first law of thermodynamics for the internal temperature and the reaction mechanism for the evolution of the concentration of the chemical species. The control parameters during the simulations are the equilibrium bubble size, initial expansion ratio, ambient pressure, the initial concentration ratio of hydrogen and the material properties of the liquid. At the optimal parameter setup, the energy intensity is 90.17GJ/t that is 2.31 times higher than the best available technology, the Haber-Bosch process. In both cases, the hydrogen is generated via water electrolysis.

Influence of interactions between bubbles on physico-chemical effects of acoustic cavitation

Ultrasound technology has been extensively used as one of the efficient and economic methodology to achieve the desired outcomes in many applications by harnessing the physico-chemical effects of acoustic cavitation. However, the cavitation-associated effects, primarily determined by the oscillatory dynamics of cavitation bubbles, are considerably complex and still remain poorly understood. The main objective of this study was to perform a numerical analysis of the acoustic cavitation (i.e., the cavitation dynamics, the resultant temperature, pressure and chemical yields within collapsing bubbles), particularly focusing on the influence of the interactions between bubbles. A comprehensive model was developed to simulate the acoustic cavitation dynamics via combining the influences of mass transfer, heat conduction and chemical reactions as well as the interaction effects between bubbles. The results demonstrated that only the large bubble exerts a greater impact on the small one in a two-bubble system. Specifically, within parameter ranges covered this study, there are noticeable decreases in the expansion ratio of the small bubble, the resultant temperature, pressure and molar yields of free radicals, hence weakening the cavitation intensity and cavitation- associated physico-chemical effects. Moreover, the influences of the interactions between bubbles were further assessed quantitatively under various parameters, such as the ultrasound amplitude PA and frequency f, the distance between bubbles d0, the initial radius of the large bubble R20, as well as the liquid properties (e.g., surface tension σ and viscosity μ). It was found that the suppression effect can be amplified when subjected to ultrasound with an increased PA and/or a decreased f, probably due to a stronger cavitation intensity under this condition. Additionally, the suppression effect is also enhanced with a decrease in d0, σ and μ, but with R20 increasing. This study can contribute to deepening knowledge about acoustic cavitation and the resultant physical and/or chemical effects, potentially further facilitating the ultrasound-assisted various applications involving acoustic cavitation.

Towards scaling up the sonochemical synthesis of Pt-nanocatalysts

Large scale production of electrocatalysts for electrochemical energy conversion devices such as proton exchange membrane fuel cells must be developed to reduce their cost. The current chemical reduction methods used for this synthesis suffer from problems with achieving similar particle properties such as particle size and catalytic activity when scaling up the volume or the precursor concentration. The continuous production of reducing agents through the sonochemical synthesis method could help maintain the reducing conditions (and also the particle properties) upon increasing the reactor volume. In this work we demonstrate that the reducing conditions of Pt-nanoparticles are indeed maintained when the reactor volume is increased from 200 mL to 800 mL. Similar particle sizes, 2.1(0.3) nm at 200 mL and 2.3(0.4) nm at 800 mL, and catalytic activities towards the oxygen reduction reaction (ORR) are maintained as well. The reducing conditions were assessed through TiOSO4 dosimetry, sonochemiluminesence imaging, acoustic power measurements, and Pt(II) reduction rate measurements. Cyclic voltammetry, CO-stripping, hydrogen evolution measurements, ORR measurements, and electron microscopy were used to evaluate the catalytic activity and particle size. The similar particle properties displayed from the two reactor volumes suggest that the sonochemical synthesis of Pt-nanoparticles is suitable for large scale production.

Numerical investigation on the influence of dual-frequency coupling parameters on acoustic cavitation and its analysis of the enhancement and attenuation effect

To understand the effect of coupling parameters between two ultrasonic waves on acoustic cavitation, in this work, Keller-Miksis equation was introduced to built a bubble dynamics model that was used to describe the dynamic evolution of bubble and to discuss the effect of dual-frequency coupling parameters, such as frequency difference f (5 ∼ 280 kHz), phase difference φ (0 ∼ 7π/4 rad), and power allocation ratio β (0 ∼ 9), on acoustic cavitation in the presence of two ultrasonic waves irradiation. The enhancement and attenuation effect of cavitation have also been analyzed in detail by comparing the different dual-frequency combinations with single-frequency mode. It was found that all coupling parameters have a significant impact on acoustic cavitation, where the smaller values of f and φ were employed when β = 1, the stronger cavitation intensity was observed. Nevertheless, as the power allocation ratio is increased from 1 to 9 at φ = 0 for different frequency differences, the acoustic cavitation exhibits an attenuation trend. When the total acoustic power is evenly distributed, namely β = 1, the largest maximum expansion ratio (i.e. 12.96) was obtained at φ = 0 and f = 5 kHz, which represents a strongest cavitation effect. In addition, for different frequency combinations, the enhancement effect is found under the mixture of low and low frequency, whereas attenuation effect is generated easily by the combination of high and low frequency. Moreover, the effect become more pronounced as the proportion of high frequency component increases.

Hydrocarbon Degradation by Contact with Anoxic Water Microdroplets

Oils are hydrophobic, but their degradation is frequently found to be accelerated in the presence of water microdroplets. The direct chemical consequences of water-oil contact have long been overlooked. We show that aqueous microdroplets in emulsified water-hexadecane (C16H34) mixtures can spontaneously produce CO2, •H, H2, and short-chain hydrocarbons (mainly C1 and C2) as detected by gas chromatography, electron paramagnetic resonance spectroscopy, and mass spectrometry. This reaction results from contact electrification at the water-oil microdroplet interface, in which reactive oxygen species are produced, such as hydrated hydroxyl radicals and hydrogen peroxide. We also find that the H2 originates from the water microdroplet and not the hydrocarbon it contacts. These observations highlight the potential of interfacial contact electrification to produce new chemistry.

Sonochemical reactor characterization in the presence of cylindrical and conical reflectors

Ultrasonic systems must be able to produce an acoustic field with the highest possible energy concentration in sonochemical reactors to accomplish maximum efficacy in the sonolytic degradation of water contaminants. In the present study, the impact of cylindrical and conical stainless-steel reflectors placed on the liquid surface on the sonochemical oxidation activity of ultrasonication reactors was investigated. The amount of effective acoustic power transferred to the ultrasonicated medium without and with reflectors was measured by calorimetric characterization of the sono-reactors at diverse ultrasonication frequencies in the interval of 300-800 kHz and different electrical powers in the range of 40-120 W. Iodide dosimetry without and with reflectors at diverse ultrasonication conditions (300-800 kHz and 40-120 W) and various aqueous solution volumes in the range of 300-500 mL was used to assess the sonochemical oxidation activity, i.e., the generation of oxidative species (mainly hydroxyl radicals). Sonochemiluminescence (SCL) imaging was used to study the active acoustic cavitation bubbles distribution in the sono-reactors without and with reflectors. Significant impacts of the position and shape of the reflectors on the active acoustic cavitation bubble distribution and the sonochemical oxidation activity were observed due to remarkable modifications of the ultrasonic field by directing and focusing of the ultrasonic waves. A significant augmentation in the triiodide formation rate was obtained in the presence of the conical reflector, especially at 630 kHz and 120 W (60.5% improvement), while iodide oxidation was quenched in the presence of the cylindrical reflector at all ultrasonication frequencies and powers. The SCL images show a noteworthy modification in the ultrasonic field and the acoustic cavitation bubble population when reflectors were used. The sonochemical oxidation activity was improved by the conical reflector when placed in the Fresnel zone (near field region).

Sonochemical formation of peroxynitrite in water: Impact of ultrasonic frequency and power

There is a lack of literature on peroxynitrite formation due to sonolysis of aerated water. In this work, the impact of sonication parameters, frequency and power, on ultrasonic peroxynitrite production in aerated alkaline water was investigated. Peroxynitrite formation was clearly established with undeniable evidence at all the tested frequencies in the range of 516-1140 kHz with a typical G-value (energy-specific yield) of 0.777 × 10-10, 0.627 × 10-10, 0.425 × 10-10 and 0.194 × 10-10 mol/J at 516, 558, 860 and 1140 kHz, respectively. The ultrasonication frequency has a direct impact on the sonochemical peroxynitrite production. Increasing the ultrasonication frequency in the interval 321-1140 kHz reduces peroxynitrite formation. The most practical sonochemistry dosimetries, including hydrogen peroxide production, triiodide dosimetry, Fricke dosimetry, and 4-nitrocatechol formation, were compared with the sonochemical efficiency of the reactors used to produce peroxynitrite. The G-value, energy specific yield, for the tested dosimetries was higher than that for peroxynitrite formation, regardless of frequency. For all chemical dosimetries investigated, the same trend of frequency dependence was found as for peroxynitrite generation. The influence of ultrasonication power on peroxynitrite formation by sonication at diverse frequencies in the interval 585-1140 kHz was studied. No peroxynitrite was formed at lower acoustic power levels, regardless of frequency. As the frequency increases, more power is required for peroxynitrite formation. The production of peroxynitrite increased as the acoustic power increased, despite the frequency of ultrasonic waves. Ultrasonic power is a key factor in the production of peroxynitrite by sonolysis. Since peroxynitrite is uniformly distributed in the bulk solution, peroxynitrite-sensitive solutes can be transformed both in the bulk of the solution and in the surfacial region (shell) of the cavitation bubble. The formation of peroxynitrite should be taken into account in sonochemistry, especially at higher pH values. Ultrasonic peroxynitrite formation in alkaline solution (pH 12) can be considered as a kind of chemical dosimetry in sonochemistry.

The Reducing Agents in Sonochemical Reactions without Any Additives

It has been experimentally reported that not only oxidation reactions but also reduction reactions occur in aqueous solutions under ultrasound without any additives. According to the numerical simulations of chemical reactions inside an air or argon bubble in water without any additives under ultrasound, reducing agents produced from the bubbles are H, H2, HO2 (which becomes superoxide anion (O2-) in liquid water), NO, and HNO2 (which becomes NO2- in liquid water). In addition, H2O2 sometimes works as a reducing agent. As the reduction potentials of H and H2 (in strongly alkaline solutions for H2) are higher than those of RCHOH radicals, which are usually used to reduce metal ions, H and H2 generated from cavitation bubbles are expected to reduce metal ions to produce metal nanoparticles (in strongly alkaline solutions for H2 to work). It is possible that the superoxide anion (O2-) also plays some role in the sonochemical reduction of some solutes. In strongly alkaline solutions, hydrated electrons (e-aq) formed from H atoms in liquid water may play an important role in the sonochemical reduction of solutes because the reduction potential is extremely high. The influence of ultrasonic frequency on the amount of H atoms produced from a cavitation bubble is also discussed.

Sonochemical advanced oxidation process for the degradation of furosemide in water: Effects of sonication's conditions and scavengers

The intensive consumption of pharmaceuticals and drugs in the last decades has led to their increased concentrations in wastewaters from industrial sources. The present paper deals, for the first time, with the sonochemical degradation and mineralization of furosemide (FSM) in water. FSM is a potent loop diuretic used to treat fluid build-up due to heart failure, liver scarring, or kidney disease. The influence of several operating parameters such as acoustic intensity, ultrasonic frequency, initial FSM concentration, solution's pH, nature of the dissolved gas (Ar, air and N2) and radical scavengers (2-propanol and tert-butanol) on the oxidation of FSM was assessed. The obtained results showed that the degradation rate of the drug increased significantly with the increase of the acoustic intensity in the range of 0.83 to 4.3 W cm-2 and decreased with the augmentation of the frequency in the range of 585-1140 kHz. It was also found that the initial rate of the sonolytic degradation of FSM increased with the increase of its initial concentration (2, 5, 10, 15 and 20 mg/L). The most significant degradation was achieved in acidic conditions at pH 2, while in terms of saturating gas, the rate of FSM degradation decreased in the order of Ar > air > N2. The FSM degradation experiments with radical scavengers showed that the diuretic molecule degraded mainly at the interfacial region of the bubble by hydroxyl radical attack. Additionally, in terms of acoustic conditions, the sono-degradation of 30.24 µmol L-1 of FSM solution demonstrate an optimal performance at 585 kHz and 4.3 W/cm2, the results indicated that even if the ultrasonic action eliminated the total concentration of FSM within 60 min, a low degree of mineralization was obtained due to the by-products formed during the sono-oxidation process. The ultrasonic process transforms FSM into biodegradable and environmentally friendly organic by-products that could be treated in a subsequent biological treatment. Besides, the efficiency of the sonolytic degradation of FSM in real environmental matrices such as natural mineral water and seawater was demonstrated. Consequently, the sonochemical advanced oxidation process represent a very interesting technique for the treatment of water contaminated with FSM.

The impact of methanol mass transport on its conversion for the production of hydrogen and oxygenated reactive species in sono-irradiated aqueous solution

This study aims principally to assess numerically the impact of methanol mass transport (i.e., evaporation/condensation across the acoustic bubble wall) on the thermodynamics and chemical effects (methanol conversion, hydrogen and oxygenated reactive species production) of acoustic cavitation in sono-irradiated aqueous solution. This effect was revealed at various ultrasound frequencies (from 213 to 1000 kHz) and acoustic intensities (1 and 2 W/cm2) over a range of methanol concentrations (from 0 to 100%, v/v). It was found that the impact of methanol concentration on the expansion and compression ratios, bubble temperature, CH3OH conversion and the molar productions inside the bubble is frequency dependent (either with or without consideration of methanol mass transport), where this effect is more pronounced when the ultrasound frequency is decreased. Alternatively, the decrease in acoustic intensity decreases clearly the effect of methanol mass transport on the bubble sono-activity. When methanol mass transfer is eliminated, the decrease of the bubble temperature, CH3OH conversion and the molar yield of the bubble with the rise of methanol concentration was found to be more amortized as the wave frequency is reduced from 1 MHz to 213 kHz, compared to the case when the mass transport of methanol is taken into account. Our findings indicate clearly the importance of incorporating the evaporation and condensation mechanisms of methanol throughout the numerical simulations of a single bubble dynamics and chemical activity.

Optimum scavenger concentrations for sonochemical nanoparticle synthesis

Maintaining nanoparticle properties when scaling up a chemical synthesis is challenging due to the complex interplay between reducing agents and precursors. A sonochemical synthesis route does not require the addition of reducing agents as they are instead being continuously generated in-situ by ultrasonic cavitation throughout the reactor volume. To optimize the sonochemical synthesis of nanoparticles, understanding the role of radical scavengers is paramount. In this work we demonstrate that optimum scavenger concentrations exist at which the rate of Ag-nanoparticle formation is maximized. Titanyl dosimetry experiments were used in conjunction with Ag-nanoparticle formation rates to determine these optimum scavenger concentrations. It was found that more hydrophobic scavengers require lower optimum concentrations with 1-butanol < 2-propanol < ethanol < methanol < ethylene glycol. However, the optimum concentration is shifted by an order of magnitude towards higher concentrations when pyrolytic decomposition products contribute to the reduction. The reduction rate is also enhanced considerably.

Effect of Organic Ions on The Formation and Collapse of Nanometric Bubbles in Ionic Liquid/Water Solutions: A Molecular Dynamics Study

Molecular dynamics simulation is applied to investigate the effect of two ionic liquids (IL) on the nucleation and growth of (nano)cavities in water under tension and on the cavities' collapse following the release of tension. Simulations of the same phenomena in two pure water samples of different sizes are carried out for comparison. The first IL, i.e., tetra-ethylammonium mesylate ([Tea][Ms]), is relatively hydrophilic and its addition to water at 25 wt % concentration decreases its tendency to nucleate cavities. Apart from quantitative details, cavity formation and collapse are similar to those taking place in water and qualitatively follow the Rayleigh-Plesset (RP) equation. The second IL, i.e., tetrabutyl phosphonium 2,4-dimethylbenzenesulfonate ([P₄₄₄₄][DMBS]), is amphiphilic and forms nanostructured solutions with water. At 25 wt % concentrations, [P₄₄₄₄][DMBS] favors the nucleation of bubbles that tend to form at the interface between water-rich and IL-rich domains. Cavity collapse in [P₄₄₄₄][DMBS]/water solutions are greatly hindered by a shell of ions decorating the interface between the solution and the vapor phase. A similar effect is observed for the equilibration of a population of bubbles of different sizes. The drastic slowing down of the bubbles' relaxation processes suggests ways to produce long-lived nanometric cavities in the liquid phase that could be useful for nanotechnology and drug delivery.

Critical Analysis of Hydrogen Production by Aqueous Methanol Sonolysis

Recently, several experimental and theoretical studies have demonstrated the feasibility of enhancing the sonochemical production of hydrogen via methanol pyrolysis within acoustic cavitation bubbles (i.e. sonolysis of aqueous methanol solution). This review includes both the experimental and theoretical achievements in the field of hydrogen production by methanol sonolysis. Additionally, the limits of the process's applicability and plausible solutions are highlighted. The impact of different parameters influencing the process performance is discussed. Finally, the effects of methanol concentration on the size distribution of active cavitation bubbles are analyzed.

Penetration of hydroxyl radicals in the aqueous phase surrounding a cavitation bubble

In the sonochemical degradation of nonvolatile compounds, the free radicals must be delivered into the aqueous solution from the cavitation bubble to initiate reduction-oxidation reactions. The penetration depth in the liquid becomes an important parameter that influences the radical delivery efficiency and eventual treatment performance. However, the transport of radicals in the liquid phase is not well understood yet. In this paper, we focus on the most reactive OH radical and numerically simulate its penetration behavior. This is realized by solving the coupled equations of bubble dynamics, intracavity chemistry, and radical dispersion in the aqueous phase. The results present both the local and global penetration patterns for the OH radicals. By performing simulations over a wide range of acoustic parameters, we find an undesirable phenomenon that the penetration can be adversely suppressed when strengthening the radical production. A mechanistic analysis attributes this to the excessively vigorous recombination reactions associated with high radical concentrations near the bubble interface. In this circumstance, the radicals are massively consumed and converted into molecular species before they can appreciably diffuse away. Our study sheds light on the interplay between radical production inside the bubble and dispersion in the outside liquid. The derived conclusions provide guides for sonochemical applications from a new perspective.

Estimation of the number density of active cavitation bubbles in a sono-irradiated aqueous solution using a thermodynamic approach

An alternative semi-empirical technique is developed to determine the number density of active cavitation bubbles (N) formed in sonicated solutions. This was achieved by relating the acoustic power supplied to the solution (i.e., determined experimentally) to the released heat by a single bubble. The energy dissipation via heat exchange is obtained by an advanced cavitation model accounting for the liquid compressibility and viscosity, the non-equilibrium condensation/evaporation of water vapor, and heat conduction across the bubble wall and heats of chemical reactions resulting within the bubble at the collapse. A good concordance was observed between our results and those found in the literature. It was found that the number of active bubbles increased proportionally with a rise in ultrasound frequency. Additionally, the increase of acoustic intensity increases the number of active bubbles, whatever the sonicated solution's volume. On the other hand, it was observed that the rise of the irradiated solution volume causes the number of active bubbles to be reduced even when the acoustic power is increased. A decrease in acoustic energy accelerates this negative impact.

Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems, Part I: H2O2 generation

Cavitational/sonochemical activity can be significantly enhanced or reduced depending on the gases dissolved in the liquid. Although many researchers have suggested the order of importance of dissolved gas conditions that affect the degree of sonoluminescence (SL), sonochemiluminescence (SCL), and compound degradation, the most suitable gas condition for sonochemical oxidation reactions is currently unknown. In this study (Part I), the effects of gas saturation and sparging on the generation of H₂O₂ were investigated in a 28-kHz sonoreactor system. Four gas modes, saturation/closed, saturation/open, sparging/closed, and sparging/open, were applied to Ar, O₂, N₂, and binary gas mixtures. The change in dissolved oxygen (DO) concentration during ultrasonic irradiation was measured and was used as an indicator of whether the gaseous exchange between liquid and air altered the gas content of the liquid. Considerable difference in the DO concentration was observed for the gas saturation/open mode, ranging from -11.5 mg/L (O₂ 100 %) to +4.3 mg/L (N₂ 100 %), while no significant difference was observed in the other gas modes. The change in the gas content significantly reduced the linearity for H₂O₂ generation, which followed pseudo-zero-order kinetics, and either positively or negatively affected H₂O₂ generation. Ar:O₂ (75:25) and Ar:O₂ (50:50) resulted in the highest and second-highest H₂O₂ generation for both gas saturation and sparging, respectively. In addition, gas sparging resulted in much higher H₂O₂ generation for all gas conditions compared to gas saturation; this was because of the significant change in the cavitational active zone and concentrated ultrasonic energy, which formed a bulb-shaped active zone, especially for the Ar/O₂ mixtures adjacent to the transducer at the bottom. The sparging flow rate and position also significantly affected H₂O₂ generation; the highest H₂O₂ generation was obtained when the sparger was placed at the bottom adjacent to the transducer, with a flow rate of 3 L/min. In Part II, the generation of nitrogen oxides, including nitrite (NO₂^-) and nitrate (NO₃^-), was investigated using the same ultrasonic system with three gas modes: saturation/open, saturation/closed, and sparging/closed.

Merits and Demerits of ODE Modeling of Physicochemical Systems for Numerical Simulations

In comparison with the first-principles calculations mostly using partial differential equations (PDEs), numerical simulations with modeling by ordinary differential equations (ODEs) are sometimes superior in that they are computationally more economical and that important factors are more easily traced. However, a demerit of ODE modeling is the need of model validation through comparison with experimental data or results of the first-principles calculations. In the present review, examples of ODE modeling are reviewed such as sonochemical reactions inside a cavitation bubble, oriented attachment of nanocrystals, dynamic response of flexoelectric polarization, ultrasound-assisted sintering, and dynamics of a gas parcel in a thermoacoustic engine.

Production and dispersion of free radicals from transient cavitation Bubbles: An integrated numerical scheme and applications

As an advanced oxidation process with a wide range of applications, sonochemistry relies on acoustic cavitation to induce free radicals for degrading chemical contaminants. The complete process includes two critical steps: the radical production inside the cavitation bubble, and the ensuing dispersion of these radicals into the bulk solution. To grasp the physicochemical details in this process, we developed an integrated numerical scheme with the ability to quantitatively describe the radical production-dispersion behavior. It employs coupled simulations of bubble dynamics, intracavity chemical reactions, and diffusion-reaction-dominated mass transport in aqueous solutions. Applying this method to the typical case of argon and oxygen bubbles, the production mechanism for the main radicals is revealed. Moreover, the temporal-spatial distribution of the radicals in the liquid phase is presented. The results demonstrate that the enhanced radical production observed in oxygen bubbles can be traced to the initiation reaction O₂ + H₂O → OH+HO₂, which requires relatively low activation energy. In the outside liquid region, the dispersion of radicals is limited by robust recombination reactions. The simulated penetration depth of OH is around 0.2 μm and agrees with reported experimental measurements. The proposed numerical approach can be employed to better capture the radical activity and is instrumental in optimizing the engineering application of sonochemistry.

Production of O Radicals from Cavitation Bubbles under Ultrasound

In the present review, the production of O radicals (oxygen atoms) in acoustic cavitation is focused. According to numerical simulations of chemical reactions inside a bubble using an ODE model which has been validated through studies of single-bubble sonochemistry, not only OH radicals but also appreciable amounts of O radicals are generated inside a heated bubble at the violent collapse by thermal dissociation of water vapor and oxygen molecules. The main oxidant created inside an air bubble is O radicals when the bubble temperature is above about 6500 K for a gaseous bubble. However, the concentration and lifetime of O radicals in the liquid water around the cavitation bubbles are unknown at present. Whether O radicals play some role in sonochemical reactions in the liquid phase, which are usually thought to be dominated by OH radicals and H₂O₂, should be studied in the future.

Use of mesoporous BiOI microspheres for sonocatalytic degradation of tetracycline hydrochloride

Self-assembled mesoporous BiOI microsphere with nanosheets were prepared by a solvothermal method and used as sonocatalysts. The sonocatalytic performances of the BiOI microspheres were evaluated in terms of the degradation rate of tetracycline hydrochloride (TCH) as a model pollutant. We designed three comparative experiments to explore the degradation of TCH solution under natural light, namely with sonication, with BiOI alone, and with ultrasound (US)/BiOI synergy. The degradation rate of TCH with US/BiOI synergy was 227 times higher than that achieved with sonication and 83 times higher than that achieved with BiOI alone. The maximum TCH degradation rate was 93.0%. The synergistic effect was therefore significant, and the synergy factor was estimated to be 61. Many factors such as the ultrasonic duty cycle, applied power, catalyst concentration, and initial TCH dye concentration may affect the ultrasonic degradation efficiency. Box-Behnken design of the response surface method were used to optimize the parameters and to study the effects of the catalyst concentration, ultrasonic duty cycle, and applied power. Analysis of variance confirmed that the quadratic response surface model for predicting the sonocatalytic efficiency was good for the corresponding parameters (R² = 0.9936 and adjusted R² = 0.9854). The optimization results were verified by replicate experiments. The high TCH degradation rate may be related to the generation of reactive oxygen species at the end of cavitation bubble collapse, which can improve the chemical yields.

Frequency controlled agglomeration of pt-nanoparticles in sonochemical synthesis

Optimizing the surface area of nanoparticles is key to achieving high catalytic activities for electrochemical energy conversion devices. In this work, the frequency range (200 kHz-500 kHz) for maximum sonochemical radical formation was investigated for the sonochemical synthesis of Pt-nanoparticles to assess whether an optimum frequency exists or if the entire range provides reproducible particle properties. Through physical and electrochemical characterization, it was found that the frequency dependent mechanical effects of ultrasound resulted in smaller, more open agglomerates at lower frequencies with agglomerate sizes of (238 ± 4) nm at 210 kHz compared to (274 ± 2) nm at 326 kHz, and electrochemical surface areas of (12.4 ± 0.9) m2g-1 at 210 kHz compared to (3.4 ± 0.5) m2g-1 at 326 kHz. However, the primary particle size (2.1 nm) and the catalytic activity towards hydrogen evolution, (19 ± 2) mV at 10mA cm2,remained unchanged over the entire frequency range. Highly reproducible Pt-nanoparticles are therefore easily attainable within a broad range of ultrasonic frequencies for the sonochemical synthesis route.

Ultrasound/chlorine sono-hybrid-advanced oxidation process: Impact of dissolved organic matter and mineral constituents

In this work, after exploring the first report on the synergism of combining ultrasound (US: 600 kHz) and chlorine toward the degradation of Allura Red AC (ARAC) textile dye, as a contaminant model, the impact of various mineral water constituents (Cl-, SO42-, NO3-, HCO3- and NO2-) and natural organic matter, i.e., humic acid (HA), on the performance of the US/chlorine sono-hybrid process was assessed for the first time. Additionally, the process effectiveness was evaluated in a real natural mineral water (NMW) of a known composition. Firstly, it was found that the combination of ultrasound and chlorine (0.25 mM) at pH 5.5 in cylindrical standing wave ultrasonic reactor (f = 600 kHz and Pe = 120 W, equivalent to PA ∼ 2.3 atm) enhanced in a drastic manner the degradation rate of ARAC; the removal rate being 320% much higher than the arithmetic sum of the two separated processes. The source of the synergistic effect was attributed to the effective implication of reactive chlorine species (RCS: Cl, ClO and Cl2-) in the degradation process. Radical probe technique using nitrobenzene (NB) as a specific quencher of the acoustically generated hydroxyl radical confirmed the dominant implication of RCS in the overall degradation rate of ARAC by US/chlorine system. Overall, the presence of humic acid and mineral anions decreased the efficiency of the sono-hybrid process; however, the inhibition degrees depend on the type and the concentration of the selected additives. The reaction of these additives with the generated RCS is presumably the reason for the finding results. The inhibiting effect of Cl-, SO42-, NO3- and NO2- was more pronounced in US/chlorine process as compared to US alone, whereas the inverse scenario was remarked for the effect of HA. These outcomes were associated to the difference in the reactivity of HA and mineral anions toward RCS and OH oxidizing species, in addition to the more selective character of RCS than hydroxyl radical. The displacement of the reaction zone with increasing the additive concentration may also be another influencing factor that favors competition reactions, which subsequently reduce the available reactive species in the reacting medium. The NMW exerted reductions of 43% and 10% in the process efficiency at pH 5.5 and 8, respectively, thereby confirming the RCS-quenching mechanism by the water matrix constituents. Hence, this work provided a precise understanding of the overall mechanism of chlorine activation by ultrasound to promote organic compounds degradation in water.

The importance of chemical mechanisms in sonochemical modelling

A state-of-the-art chemical mechanism is introduced to properly describe chemical processes inside a harmonically excited spherical bubble placed in water and saturated with oxygen. The model uses up-to-date Arrhenius-constants, collision efficiency factors and takes into account the pressure-dependency of the reactions. Duplicated reactions are also applied, and the backward reactions rates are calculated via suitable thermodynamic equilibrium conditions. Our proposed reaction mechanism is compared to three other chemical models that are widely applied in sonochemistry and lack most of the aforementioned modelling issues. In the governing equations, only the reaction mechanisms are compared, all other parts of the models are identical. The chemical yields obtained by the different modelling techniques are taken at the maximum expansion of the bubble. A brief parameter study is made with different pressure amplitudes and driving frequencies at two equilibrium bubble sizes. The results show that due to the deficiencies of the former reaction mechanisms employed in the sonochemical literature, several orders of magnitude differences of the chemical yields can be observed. In addition, the trends along a control parameter can also have dissimilar characteristics that might lead to false optimal operating conditions. Consequently, an up-to-date and accurate chemical model is crucial to make qualitatively and quantitatively correct conclusions in sonochemistry.

An alternative technique for determining the number density of acoustic cavitation bubbles in sonochemical reactors

The present paper introduces a novel semi-empirical technique for the determination of active bubbles' number in sonicated solutions. This method links the chemistry of a single bubble to that taking place over the whole sonochemical reactor (solution). The probe compound is CCl₄, where its eliminated amount within a single bubble (though pyrolysis) is determined via a cavitation model which takes into account the non-equilibrium condensation/evaporation of water vapor and heat exchange across the bubble wall, reactions heats and liquid compressibility and viscosity, all along the bubble oscillation under the temporal perturbation of the ultrasonic wave. The CCl₄ degradation data in aqueous solution (available in literature) are used to determine the number density through dividing the degradation yield of CCl₄ to that predicted by a single bubble model (at the same experimental condition of the aqueous data). The impact of ultrasonic frequency on the number density of bubbles is shown and compared with data from the literature, where a high level of consistency is found.

Synergistic Effect of Ultrasound Cavitation and Gas in the Water Disinfection

The paper considers water purification processes from Bacillus bacteria type under the conditions of gases bubbling only (argon, helium, oxygen, and carbon dioxide), cavitation and combined action of gas and cavitation. The synergistic effect was found under conditions of simultaneous action of gas and cavitation (kd(gas/US ) >kd(gas) + kd(US) almost double) and it was shown that kd(gas/US) >kd(gas) by almost an order of magnitude. Relative series of effective destruction of microbial cells was established: Ar/US > О2/US >Не/US > СО2/US. Destruction degree of the cells reaches 70 %at the short-term Ar/US exposure (~8 min), which is 7 times more active than cavitation action and 13.5 times more than bubbling of Aralone.

Ultrasound-mediated radical cascade reactions: Fast synthesis of functionalized indolines from 2-(((N-aryl)amino)methyl)acrylates

Novel functionalized indolines were synthesized from 2-(((N-aryl)amino)methyl)acrylates and formamides under ultrasonic irradiation for the first time. Aiming to develop a straightforward and easy-to-implement methodology for the synthesis of indolines, an instrumentation setup was designed, including ultrasound (US) equipment (Ultrasonic Horn; tip diameter of 12.7 mm, 20 kHz, maximum power of 400 W), an open reaction flask, and an inexpensive and green catalyst (1 mol%; FeSO4·7H2O; CAS: 7782-63-0) without the need for anhydrous conditions. The use of the sono-Fenton process in the presence of formamides and 2-(((N-aryl)amino)methyl)acrylates afforded a broad range of functionalized indolines within 60 s in high yields. Several experimental parameters of the ultrasound-assisted reaction were evaluated, such as amplitude (40-80%), sonication time (15-60 s), and pulsed ultrasonic irradiation. A 60 s silent reaction did not produce the desired indoline. The optimized conditions for US-mediated reactions allowed the production of functionalized indolines in high isolated yields (up to 99%, 60 s reaction, pulse ration 1 s:1 s, US amplitude 60 %).

Numerical simulations for sonochemistry

Numerical simulations for sonochemistry are reviewed including single-bubble sonochemistry, influence of ultrasonic frequency and bubble size, acoustic field, and sonochemical synthesis of nanoparticles. The theoretical model of bubble dynamics including the effect of non-equilibrium chemical reactions inside a bubble has been validated from the study of single-bubble sonochemistry. By the numerical simulations, it has been clarified that there is an optimum bubble temperature for the production of oxidants inside an air bubble such as OH radicals and H2O2 because at higher temperature oxidants are strongly consumed inside a bubble by oxidizing nitrogen. Unsolved problems are also discussed.

GPU accelerated numerical investigation of the spherical stability of an acoustic cavitation bubble excited by dual-frequency

The spherical stability of an acoustic cavitation bubble under dual-frequency excitation is investigated numerically. The radial dynamics is described by the Keller-Miksis equation, which is a second-order ordinary differential equation. The surface dynamics is modelled by a set of linear ordinary differential equation according to Hao and Prosperetti (1999), which takes into account the effect of vorticity by boundary layer approximation. Due to the large amount of investigated parameter combinations, the numerical computations were carried out on graphics processing units. The results showed that for bubble size between R_E=2μm and 4μm, the combination of a low and a high frequency, and the combination of two close but not equal frequencies are important to prevent the bubble losing its shape stability, while reaching the chemical threshold (R_max/R_E=3) (Kalmár et al., 2020). The phase shift between harmonic components of dual-frequency excitation has no effect on the shape stability.

A critical analysis of sono-hybrid advanced oxidation process of ferrioxalate system for degradation of recalcitrant pollutants

The emerging contaminants in wastewater discharged from numerous chemical process industries, pharmaceutical industries, textile, and wineries have attracted the attention of the scientific community due to their toxicity and persistence in the environment. The conventional techniques are incompetent to treat many of such recalcitrant toxic pollutants. To achieve high mineralization, advanced oxidation processes (AOPs) are found to be more efficient for the degradation of these organic pollutants without producing secondary pollutants with no/less amount of sludge. The primary oxidation agents for AOPs are in-situ generated free radicals, which are highly reactive and effective oxidants for degrading any type of organic molecules present in the wastewater. In the past decades, the combination of AOPs or simultaneous application of more than one AOP has been investigated extensively for wastewater treatment and these hybrid-AOPs have been reported to be beneficial for high-level mineralization of organic pollutants. This paper presented the characteristics, properties and influence of parameters in sono-photo-ferrioxalate system. The primary operating parameters in sono-photo-ferrioxalate system that affect the kinetics are defined as the solution pH, temperature, molar ratio of Fe³⁺/C₂O₄^2-, H₂O₂ concentration, source of light, ultrasound intensity, dissolved gases, and size of cavitation bubble. In this process, several oxidizing radicals are generated such as HO^•, HO₂^•, C₂O₄^•-, CO₂^•- and O₂^•- which are also responsible for degradation. In this review, we have mainly addressed the degradation of recalcitrant pollutants using the sono-photo-ferrioxalate system and a critical analysis of process parameters that influence mineralization efficiency.

An inverse method to fast-track the calculation of phase diagrams for sonoluminescing bubbles

A sound driven air bubble can be transformed into an argon bubble emitting light pulses stably. The very foundation to investigate the sonoluminescing bubble is to accurately determine the ambient radius and gas composition in the interior. The conventional approach is to model the air-to-argon transformation process through a large number of bubble dynamics simulations to obtain the physical parameters of the ultimate argon bubble. In this paper, we propose a highly efficient method to pinpoint this information in a phase diagram. The method is based on the diffusive equilibrium for each species inside the bubble and derives the ambient radius and composition inversely. To calculate the former parameter, the bisection algorithm is employed to consecutively narrow down the searching range until the equilibria is approached. Afterward, several cycles of full dynamics simulations are conducted to refine the composition. The method is validated using published experimental data. The calculated ambient radii deviate from the test results by less than 1 μm, which falls within the margin of measurement error. The advantages of this method over the semi-analytical approach reported by Hilgenfeldt et al. [J. Fluid Mech. 365 (1998)] are also discussed. Our study provides a standard procedure to calculate the ambient radius and composition and is beneficial for the numerical simulation of sonoluminescing bubbles.

A comprehensive numerical analysis of heat and mass transfer phenomenons during cavitation sono-process

The present study treats the effects of mass transport, heat transfer and chemical reactions heat on the bubble dynamics by spanning a range of ambient bubble radii. The thermodynamic behavior of the acoustic bubble was shown for three wave frequencies, 355, 515 and 1000 kHz. The used acoustic amplitude ranges from 1 to 3 atm. It has been demonstrated that the ambient bubble radius, R₀, of the maximal response (i.e., maximal bubble temperature and pressure, T_max and P_max) is shifted toward lower values if the acoustic amplitude (at fixed frequency) or the ultrasonic frequency (at fixed amplitude) are increased. The range of the ambient bubble radius narrows as the ultrasonic frequency increases. Heat exchange at the bubble interface was found to be the most important mechanism within the bubble internal energy balance for acoustic amplitudes lower than 2.5 and 3 atm for ultrasonic frequencies of 355 and 515 kHz, respectively. For acoustic amplitudes greater or equal to 2.5 and 3 atm, corresponding to 355 and 515 kHz, respectively, mass transport mechanism (i.e., evaporation and condensation of water vapor) becomes dominant compared to the other mechanisms. At 1000 kHz, the mechanism of heat transfer persists to be dominant for all the used acoustic amplitudes (from 1 to 3 atm). Practically, all the above observations were maintained for bubbles at and around the optimum bubble radius, whereas no significant impact of the three energetic mechanisms was observed for bubbles of too lower and too higher values of R₀ (limits of the investigated ranges of R₀).

Insight into the impact of excluding mass transport, heat exchange and chemical reactions heat on the sonochemical bubble yield: Bubble size-dependency

Numerical simulations have been performed on a range of ambient bubble radii, in order to reveal the effect of mass transport, heat exchange and chemical reactions heat on the chemical bubble yield of single acoustic bubble. The results of each of these energy mechanisms were compared to the normal model in which all these processes (mass transport, thermal conduction, and reactions heat) are taken into account. This theoretical work was carried out for various frequencies (f: 200, 355, 515 and 1000 kHz) and different acoustic amplitudes (PA: 1.5, 2 and 3 atm). The effect of thermal conduction was found to be of a great importance within the bubble internal energy balance, where the higher rates of production (for all acoustic amplitudes and wave frequencies) are observed for this model (without heat exchange). Similarly, the ignorance of the chemical reactions heat (model without reactions heat) shows the weight of this process into the bubble internal energy, where the yield of the main species (OH, H, O and H2) for this model was accelerated notably compared to the complete model for the acoustic amplitudes greater than 1.5 atm (for f = 500 kHz). However, the lowest production rates were registered for the model without mass transport compared to the normal model, for the acoustic amplitudes greater than 1.5 atm (f = 500 kHz). This is observed even when the temperature inside bubble for this model is greater than those retrieved for the other models. On the other hand, it has been shown that, at the acoustic amplitude of 1.5 atm, the maximal production rates of the main species (OH, H, O and H2) for all the adopted models appear at the same optimum ambient-bubble size (R0 ~ 3, 2.5 and 2 µm for, respectively, 355, 500 and 1000 kHz). For PA = 2 and 3 atm (f = 500 kHz), the range of the maximal yield of OH radicals is observed at the range of R0 where the production of OH, O and H2 is the lowest, which corresponds to the bubble temperature at around 5500 K. The maximal production rate of H, O and H2 is shifted toward the range of ambient bubble radii corresponding to the bubble temperatures greater than 5500 K. The ambient bubble radius of the maximal response (maximal production rate) is shifted toward the smaller bubble sizes when the acoustic amplitude (wave frequency is fixed) or the ultrasound frequency (acoustic power is fixed) is increased. In addition, it is observed that the increase of wave frequency or the acoustic amplitude decrease cause the range of active bubbles to be narrowed (scenario observation for the four investigated models).

How do dissolved gases affect the sonochemical process of hydrogen production? An overview of thermodynamic and mechanistic effects - On the "hot spot theory"

Although most of researchers agree on the elementary reactions behind the sonolytic formation of molecular hydrogen (H₂) from water, namely the radical attack of H₂O and H₂O₂ and the free radicals recombination, several recent papers ignore the intervention of the dissolved gas molecules in the kinetic pathways of free radicals, and hence may wrongly assess the effect of dissolved gases on the sonochemical production of hydrogen. One may fairly ask to which extent is it acceptable to ignore the role of the dissolved gas and its eventual decomposition inside the acoustic cavitation bubble? The present opinion paper discusses numerically the ways in which the nature of dissolved gas, i.e., N₂, O₂, Ar and air, may influence the kinetics of sonochemical hydrogen formation. The model evaluates the extent of direct physical effects, i.e., dynamics of bubble oscillation and collapse events if any, against indirect chemical effects, i.e., the chemical reactions of free radicals formation and consequently hydrogen emergence, it demonstrates the improvement in the sonochemical hydrogen production under argon and sheds light on several misinterpretations reported in earlier works, due to wrong assumptions mainly related to initial conditions. The paper also highlights the role of dissolved gases in the nature of created cavitation and hence the eventual bubble population phenomena that may prevent the achievement of the sonochemical activity. This is particularly demonstrated experimentally using a 20 kHz Sinaptec transducer and a Photron SA 5 high speed camera, in the case of CO₂-saturated water where degassing bubbles are formed instead of transient cavitation.

Acoustic frequency and optimum sonochemical production at single and multi-bubble scales: A modeling answer to the scaling dilemma

The present work consists of an innovative approach aiming to address the scalability dilemma of the sonochemical activity dependency of acoustic frequency. The study originates from the discordance of observations between the theoretical investigations of the sonochemical activity of the single acoustic cavitation bubble in function of the acoustic frequency, in one hand, and the experimental findings regarding the optimal frequency condition, mainly in terms of pollutant degradation, in the other hand. A single bubble and an up-scaled model of the sonochemical activity are suggested and simulations were conducted based on both of them over the frequencies 20, 200, 300, 360, 443, 500, 600 and 800 kHz under an oxygen atmosphere. The results reveal that the sonochemical production at single bubble scale is monotonously decreasing with the increase of frequency, while all the products demonstrate an absolute optimum of sonochemical production at 200 kHz, except HO^• that attains its maximum molar yield under 300 kHz. Besides, the production of the predominant species, namely HO₂^•, HO^• and O₃, manifests a clear rebound at 500 kHz. All the present results were compared to and confirmed by experimental findings, while the scalability of the concentrations of sonochemically produced species was discussed using a parameter we introduced as "the mass focusing factor".

Effect of different gases on the sonochemical Cr(VI) reduction in the presence of citric acid

The sonochemical (850 kHz) Cr(VI) reduction (0.30 mM, pH 2) in the presence of citric acid (Cit, 2 mM) was analyzed under different working atmospheres: reactor open to air without sparging (ROAWS), and Ar, O₂, air and N₂ sparging. Hydrogen peroxide formation in pure water at pH 2 and KI dosimetry were also measured. Zero-order kinetics was observed in all cases. A complete Cr(VI) reduction after 180 min insonation was obtained with the ROAWS and under Ar, while a lower Cr(VI) reduction efficiency was achieved under the other conditions. The Cr(VI) reduction and H₂O₂ formation rates followed the order ROAWS ≅ Ar > air ≅ O₂ ≫ N₂, while for KI dosimetry the order was ROAWS ≫ O₂ ≅ air > Ar ≫ N₂. This indicates that H₂O₂ formation rate is a better measure of the system reactivity for Cr(VI) reduction. For air, O₂ and N₂, once the sparging was stopped, Cr(VI) reduction rate increased up to approximately the same value obtained for the ROAWS, suggesting that the sparging decreased the generation of reactive species and, thus, the Cr(VI) reduction rate. Nitrate production was measured at low concentrations (micromolar range) in the ROAWS, air and N₂ systems. Formic and acetic acids were detected as Cit degradation products. Reaction mechanisms were proposed. It can be concluded that the best condition for Cr(VI) removal is with the ROAWS because of a higher Cr(VI) reduction rate, no atmosphere control is required, and it is a less expensive system.

GPU accelerated study of a dual-frequency driven single bubble in a 6-dimensional parameter space: The active cavitation threshold

The active cavitation threshold of a dual-frequency driven single spherical gas bubble is studied numerically. This threshold is defined as the minimum intensity required to generate a given relative expansion (R_max-R_E)/R_E, where R_E is the equilibrium size of the bubble and R_max is the maximum bubble radius during its oscillation. The model employed is the Keller-Miksis equation that is a second order ordinary differential equation. The parameter space investigated is composed by the pressure amplitudes, excitation frequencies, phase shift between the two harmonic components and by the equilibrium bubble radius (bubble size). Due to the large 6-dimensional parameter space, the number of the parameter combinations investigated is approximately two billion. Therefore, the high performance of graphics processing units is exploited; our in-house code is written in C++ and CUDA C software environments. The results show that for (R_max-R_E)/R_E=2, the best choice of the frequency pairs depends on the bubble size. For small bubbles, below 3μm, the best option is to use just a single frequency of a low value in the giant response region. For medium sized bubbles, between 3μm and 6μm, the optimal choice is the mixture of low frequency (giant response) and main resonance frequency. For large bubbles, above 6μm, the main resonance dominates the active cavitation threshold. Increasing the prescribed relative expansion value to (R_max-R_E)/R_E=3, the optimal choice is always single frequency driving with the lowest value (20kHz here). Thus, in this case, the giant response always dominates the active cavitation threshold. The phase shift between the harmonic components of the dual-frequency driving (different frequency values) has no effect on the threshold.

Impact of seawater salinity on the sonochemical removal of emerging organic pollutants

The results presented in this study illustrate the multiple roles of seawater salinity toward the sonochemical degradation, at variable frequencies (300-1700 kHz), of several hazardous substances, i.e. propylparaben (PPR) endocrine disruptor and several synthetic dyes: naphthol blue black (NBB), malachite green (MG), basic red 29 (BR29), acid orange 7 (AO7), Rhodamine B (RhB) and basic fuchsin (BF). Sonochemical treatment degraded all pollutants in seawater at faster rates than in deionized water. The seawater-salts through increasing the ionic strength of the solution act as a potential pusher of hydrophilic pollutants toward the reactive interfacial area of cavitation bubbles. Additionally, the salts reduce the bubble coalescence, which yields higher number of active bubbles in the irradiating media. Analysing the degradation rate of PPR and NBB with two heterogeneous models based on Langmuir kinetics mechanism indicated that the bubble interfacial area was the preferred reaction zone for the ultrasonic degradation of PPR and NBB in seawater.

Reaction kinetics of sonochemical oxidation of potassium hexacyanoferrate(II) in aqueous solutions

We studied sonochemical reactions resulting from ultrasonic treatment of potassium hexacyanoferrate(II) in aqueous solutions using a custom-built apparatus working at 536 kHz. We concluded that primary reactions are completely dominated by oxidation of Fe(II) to Fe(III) and did not find any evidences for degradation of cyanide. At the highest concentration used in the present study (0.1 M) we detected formation of pentacyanoaquaferrate(II) complex, which is most probably formed in reactions between hexacyanoferrate(III) anions and hydrogen atoms or hydrated electrons formed in sonochemical processes. We also determined that hydroxyl radicals formation rate in our system, (8.7 ± 1.5)∙10^-8 M∙s^-1, is relatively high compared to other reported experiments. We attribute this to focusing of the ultrasonic wave in the sample vessel. Finally, we suggest that oxidation rate of hexacyanoferrate(II) anions can be a convenient benchmark of efficiency of sonochemical reactors.

A theoretical model to estimate inactivation effects of OH radicals on marine Vibrio sp. in bubble-shock interaction

A theoretical model for estimating inactivation effects on marine Vibrio sp. is developed from the viewpoint of the chemical action of the OH radicals induced by interaction of bubbles with shock waves. It consists of a biological probability model for cell viability and a bubble dynamic model for its collapsing motion due to the shock pressures. The biological probability model is built by defining a sterilized space of the OH radicals. To determine the radius of the sterilized space, the Herring equation is solved in the bubble dynamic model in consideration of the effect of the heat conductivity and mass transportation. Furthermore, the pressure waveform of incident shock wave used in the model is obtained with the pressure measurement. On the other hand, a bio-experiment of marine Vibrio sp. is carried out using a high-voltage power supply in a cylindrical water chamber. Finally, the viability ratio of marine bacteria estimated by the theoretical model is examined under the experimental conditions of this study. In addition, we also discuss the influence of bubble initial size for predicting the inactivation effects.

Study of non-spherical bubble oscillations under acoustic irradiation in viscous liquid

The effect of dissipation on the shape stability of a harmonically excited bubble is investigated. The employed liquid is the highly viscous glycerine. The rate of the dissipation is controlled through the alteration of viscosity of the liquid by varying its temperature. The mean radius of the bubble during its radial oscillation is described by the Keller-Miksis equation. Two approaches are used to describe the surface oscillations. The first model solves the surface dynamics equations of each mode together with the transport equation of the vorticity in the liquid domain. The second model approximates the transport equation, which is a partial differential equation, with a boundary layer approximation reducing the required computational resources significantly. The comparison of the surface models shows qualitative agreement at low dissipation rate; however, at high viscosity the application of the full transport equation is mandatory. The results show that an increasing rate of dissipation can significantly extend the shape stable domains in the excitation frequency-pressure amplitude parameter plane. Nevertheless, the collapse strength is decreasing due to the highly damped oscillations. It has been found that an optimal range of dissipation rate in terms of temperature can be defined expressing a good compromise between the collapse strength and surface stability. The computations are carried out by an in-house GPU accelerated initial value problem solver.

Sonochemical and sonoelectrochemical production of hydrogen

Reserves of fossil fuels such as coal, oil and natural gas on earth are finite. The continuous use and burning of these fossil fuel resources in the industrial, domestic and transport sectors has resulted in the extremely high emission of greenhouse gases, GHGs (e.g. CO₂) and solid particulates into the atmosphere. Therefore, it is necessary to explore pollution free and more efficient energy sources in order to replace depleting fossil fuels. The use of hydrogen (H₂) as an alternative fuel source is particularly attractive due to its very high specific energy compared to other conventional fuels and its zero GHG emission when used in a fuel cell. Hydrogen can be produced through various process technologies such as thermal, electrolytic, photolytic and biological processes. Thermal processes include gas reforming, renewable liquid and biooil processing, biomass and coal gasification; however, these processes release a huge amount of greenhouse gases. Production of electrolytic hydrogen from water is an attractive method to produce clean hydrogen. It could even be a more promising technology when combining water electrolysis with power ultrasound to produce hydrogen efficiently where sonication enhances the electrolytic process in several ways such as enhanced mass transfer, removal of hydrogen and oxygen (O₂) gas bubbles and activation of the electrode surface. In this review, production of hydrogen through sonochemical and sonoelectrochemical methods along with a brief description of current hydrogen production methods and power ultrasound are discussed.

Sono-nano chemistry: A new era of synthesising polyhydroxylated carbon nanomaterials with hydroxyl groups and their industrial aspects

The main objective of this review is to derive the salient features of previously developed ultrasound-assisted methods for hydroxylating graphene and Buckminsterfullerene (C₆₀). The pros and cons associated to ultrasound-assisted synthesis of hydroxy-carbon nanomaterials in designing the strategical methods for the industrial bulk production are also discussed. A guideline on the statistical methods has also been considered to further provide the scopes towards the application of the previously reported methods. Irrespective of many useful methods that have been developed in order to functionalize C₆₀ and graphene by diverse oxygenated functional groups e.g. epoxide, hydroxyl, carboxyl as well as metal/metal oxide via a combination of organic chemistry and sonochemistry, there is no report dealing exclusively on the application of ultrasonic cavitation particularly to synthesising polyhydroxylated carbon nanomaterials. On this context, this review emphasizes in investigating the critical aspects of sono-nanochemistry and the statistical approaches to optimize the variables in the sonochemical process towards a large-scale synthesis of polyhydroxylated graphene and C₆₀.

Sonocatalytic removal of methylene blue from water solution by cobalt ferrite/mesoporous graphitic carbon nitride (CoFe2O4/mpg-C3N4) nanocomposites: response surface methodology approach

In this study, cobalt ferrite/mesoporous graphitic carbon nitride (CoFe₂O₄/mpg-C₃N₄) nanocomposites were successfully synthesized by using a two-step protocol. Firstly, monodispersed CoFe₂O₄ nanoparticles (NPs) were synthesized via thermal decomposition of metal precursors in a hot surfactant solution and then they were assembled on mpg-C₃N₄ via a liquid phase self-assembly method. The sonocatalytic performance of as-synthesized CoFe₂O₄/mpg-C₃N₄ nanocomposites was evaluated on the methylene blue (MB) removal from water under ultrasonic irradiation. For this purpose, response surface methodology (RSM) based on central composite design (CCD) model was successfully utilized to optimize the MB removal over CoFe₂O₄/mpg-C₃N₄ nanocomposites. Analysis of variance (ANOVA) was applied to investigate the significance of the model. The results predicted by the model were obtained to be in reasonable agreement with the experimental data (R² = 0.969, adjusted R² = 0.942). Pareto analysis demonstrated that pH of the solution was the most effective parameter on the sonocatalytic removal of MB by CoFe₂O₄/mpg-C₃N₄ nanocomposites. The optimum catalyst dose, initial dye concentration, pH, and sonication time were set as 0.25 g L^-1, 8 mg L^-1, 8, and 45 min, respectively. The high removal efficiency of MB dye (92.81%) was obtained under optimal conditions. The trapping experiments were done by using edetate disodium, tert-butyl alcohol, and benzoquinone. Among the reactive radicals, ^•OH played a more important role than h⁺ and [Formula: see text] in the MB dye removal process. Moreover, a proposed mechanism was also presented for the removal of MB in the presence of CoFe₂O₄/mpg-C₃N₄ nanocomposites under the optimized sonocatalytic conditions. Finally, a reusability test of the nanocomposites revealed a just 9.6% decrease in their removal efficiency after five consecutive runs.