Exploring the effects of pulsed ultrasound at 205 and 616 kHz on the sonochemical degradation of octylbenzene sulfonate

Compared to continuous wave (CW) ultrasound, pulsed wave (PW) ultrasound has been shown to result in enhanced sonochemical degradation of octylbenzene sulfonate (OBS). However, pulsed ultrasound was investigated under limited pulsing conditions. In this study, pulse-enhanced degradation of OBS was investigated over a broad range of pulsing conditions and at two ultrasonic frequencies (616 and 205 kHz). The rate of OBS degradation was compared to the rate of formation of 2-hydroxyterephthalic acid (HTA) following sonolysis of aqueous terephthalic acid (TA) solutions. This study shows that sonication mode and ultrasound frequency affect both OBS degradation and HTA formation rates, but not necessarily in the same way. Unlike TA, OBS, being a surface active solute, alters the cavitation bubble field by adsorbing to the gas/solution interface of cavitation bubbles. Enhanced OBS degradation rates during pulsing are attributed to this adsorption process. However, negative or smaller pulse enhancements compared to enhanced HTA formation rates are attributed to a decrease in the high-energy stable bubble population and a corresponding increase in the transient bubble population. Therefore, sonochemical activity as determined from TA sonolysis cannot be used as a measure of the effect of pulsing on the rate of degradation of surfactants in water. Over relatively long sonolysis times, a decrease in the rate of OBS degradation was observed under CW, but not under PW conditions. We propose that the generation and accumulation of surface active and volatile byproducts on the surface and inside of cavitation bubbles, respectively, during CW sonolysis is a contributing factor to this effect. This result suggests that there are practical applications to the use of pulsed ultrasound as a method to degrade surface active contaminants in water.     

Sonochemical degradation of surfactants with different charge types: Effect of the critical micelle concentration in the interfacial region of the cavity

Ionic surfactants tend to accumulate in the interfacial region of ultrasonic cavitation bubbles (cavities) because of their surface active properties and because they are difficult to evaporate in cavitation bubbles owing to their extremely low volatilities. Hence, sonolysis of ionic surfactants is expected to occur in the interfacial region of the cavity. In this study, we performed sonochemical degradation of surfactants with different charge types: anionic, cationic, zwitterionic, and nonionic. We then estimated the degradation rates of the surfactants to clarify the surfactant behavior in the interfacial region of cavitation bubbles. For all of the surfactants investigated, the degradation rate increased with increasing initial bulk concentration and reached a maximum value. The initial bulk concentration to obtain the maximum degradation rate had a positive correlation with the critical micelle concentration (cmc). The initial bulk concentrations of the anionic surfactants were lower than their cmcs, while those of the cationic surfactants were higher than their cmcs. These results can be explained by the negatively charged cavity surface and the effect of the coexisting counterions of the surfactants.     

Sonochemical and sonocatalytic degradation of monolinuron in water

The degradation of the phenylurea monolinuron (MLN) by ultrasound irradiation alone and in the presence of TiO(2) was investigated in aqueous solution. The experiments were carried out at low and high frequency (20 and 800 kHz) in complete darkness. The degradation of MLN by ultrasounds occurred mainly by a radical pathway, as shown the inhibitory effect of adding tert-butanol and bicarbonate ions to scavenge hydroxyl radicals. However, CO(3)(-) radicals were formed with bicarbonate and reacted in turn with MLN. In this study, the degradation rate of MLN and the rate constant of H(2)O(2) formation were used to evaluate the oxidative sonochemical efficiency. It was shown that ultrasound efficiency was improved in the presence of nanoparticles of TiO(2) and SiO(2) only at 20 kHz. These particles provide nucleation sites for cavitation bubbles at their surface, leading to an increase in the number of bubbles when the liquid is irradiated by ultrasound, thereby enhancing sonochemical reaction yield. In the case of TiO(2), sonochemical efficiency was found to be greater than with SiO(2) for the same mass introduced. In addition to the increase in the number of cavitation bubbles, activated species may be formed at the TiO(2) surface that promote the formation of H(2)O(2) and the decomposition of MLN.     

Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics

This paper tries to discern the mechanistic features of sonochemical degradation of recalcitrant organic pollutants using five model compounds, viz. phenol (Ph), chlorobenzene (CB), nitrobenzene (NB), p-nitrophenol (PNP) and 2,4-dichlorophenol (2,4-DCP). The sonochemical degradation of the pollutant can occur in three distinct pathways: hydroxylation by OH radicals produced from cavitation bubbles (either in the bubble–bulk interfacial region or in the bulk liquid medium), thermal decomposition in cavitation bubble and thermal decomposition at the bubble–liquid interfacial region. With the methodology of coupling experiments under different conditions (which alter the nature of the cavitation phenomena in the bulk liquid medium) with the simulations of radial motion of cavitation bubbles, we have tried to discern the relative contribution of each of the above pathway to overall degradation of the pollutant. Moreover, we have also tried to correlate the predominant degradation mechanism to the physico-chemical properties of the pollutant. The contribution of secondary factors such as probability of radical–pollutant interaction and extent of radical scavenging (or conservation) in the medium has also been identified. Simultaneous analysis of the trends in degradation with different experimental techniques and simulation results reveals interesting mechanistic features of sonochemical degradation of the model pollutants. The physical properties that determine the predominant degradation pathway are vapor pressure, solubility and hydrophobicity. Degradation of Ph occurs mainly by hydroxylation in bulk medium; degradation of CB occurs via thermal decomposition inside the bubble, degradation of PNP occurs via pyrolytic decomposition at bubble interface, while hydroxylation at bubble interface contributes to degradation of NB and 2,4-DCP.     

Effect of polymer surface activity on cavitation nuclei stability against dissolution

The persistence of acoustic cavitation in a pulsed wave ultrasound regime depends upon the ability of cavitation nuclei, i.e., bubbles, to survive the off time between pulses. Due to the dependence of bubble dissolution on surface tension, surface-active agents may affect the stability of bubbles against dissolution. In this study, measurements of bubble dissolution rates in solutions of the surface-active polymer poly(propyl acrylic acid) (PPAA) were conducted to test this premise. The surface activity of PPAA varies with solution pH and concentration of dissolved polymer molecules. The surface tension of PPAA solutions (55-72 dynes/cm) that associated with the polymer surface activity was measured using the Wilhelmy plate technique. Samples of these polymer solutions then were exposed to 1.1 MHz high intensity focused ultrasound, and the dissolution of bubbles created by inertial cavitation was monitored using an active cavitation detection scheme. Analysis of the pulse echo data demonstrated that bubble dissolution time was inversely proportional to the surface tension of the solution. Finally, comparison of the experimental results with dissolution times computed from the Epstein-Plesset equation suggests that the radii of residual bubbles from inertial cavitation increase as the surface tension decreases.     

Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. “Transient” cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, “stable” cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble–bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both “stable” and “transient” cavitation bubbles which pulsate nonlinearly with the period of ultrasound.     

Theoretical estimation of sonochemical yield in bubble cluster in acoustic field

In order to learn more about the physical phenomena occurring in cloud cavitation, the nonlinear dynamics of a spherical cluster of cavitation bubbles and cavitation bubbles in cluster in an acoustic field excited by a square pressure wave are numerically investigated by considering viscosity, surface tension, and the weak compressibility of the liquid.The theoretical prediction of the yield of oxidants produced inside bubbles during the strong collapse stage of cavitation bubbles is also investigated. The effects of acoustic frequency, acoustic pressure amplitude, and the number of bubbles in cluster on bubble temperature and the quantity of oxidants produced inside bubbles are analyzed. The results show that the change of acoustic frequency, acoustic pressure amplitude, and the number of bubbles in cluster have an effect not only on temperature and the quantity of oxidants inside the bubble, but also on the degradation types of pollutants, which provides a guidance in improving the sonochemical degradation of organic pollutants.     

Effect of ultrasonic frequency and surfactant addition on microcapsule destruction

In a previous study, we found that cavitation bubbles cause the ultrasonic destruction of microcapsules containing oil in a shell made of melamine resin. The cavitation bubbles can be smaller or larger than the resonance size; smaller bubbles cause Rayleigh contraction, whereas larger bubbles are not involved in the sonochemical reaction. The activity in and around the bubble (e.g., shear stress, shock wave, microjet, sonochemical reaction, and sonoluminescence) varies substantially depending on the bubble size. In this study, we investigated the mechanism of the ultrasonic destruction of microcapsules by examining the correlations between frequency and microcapsule destruction rate and between microcapsule size and cavitation bubble size. We evaluated the bubbles using multibubble sonoluminescence and the bubble size was changed by adding a surfactant to the microcapsule suspension. The microcapsule destruction was frequency dependent. The main cause of microcapsule destruction was identified as mechanical resonance, although the relationship between bubble size and microcapsule size suggested that bubbles smaller than or equal to the microcapsule size may also destroy microcapsules by applying shear stress locally.     

Preliminary results on the effect of power ultrasound on nitrogen oxide and dioxide atmosphere in nitric acid solutions.

The effect of ultrasound (20 kHz, 3 W cm-2) on the kinetics of HNO2 and H2O2 formation was investigated in a 1 M HNO3 medium for NO2-Ar and NO-Ar gas mixtures in various volume fractions (f(NO2) < 1.7 vol% and f(NO) < 1.1 vol%, respectively). The H2O2 formation rate measured in 1 M HNO3 in the presence of N2H5NO3 was observed to be much lower than that of HNO2 without N2H5NO3, and was relatively independent of the NO2 or NO gas volume fractions in the argon atmosphere. The HNO2 formation rate increased under ultrasound, and was higher with NO than with NO2. The induction period observed without ultrasound disappeared when ultrasound was applied. The first step in the sonochemical mechanism of HNO2 formation in the presence of NO2 involves thermal decomposition of NO2 into NO within the cavitation bubble. In the second step of HNO2 formation, NO reacts either with HNO3 in the cavitation bubble, or with NO2 in the cavitation bubble or at the bubble/solution interface.     

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.     

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu²⁺ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.     

A comparative sonochemical reaction that is independent of the intensity of ultrasound and the geometry of the exposure apparatus

Sonolysis of aqueous solutions of n-alkyl anionic surfactants results in the formation of secondary carbon-centered radicals (-*CH-). The yield of -*CH- depends on the bulk surfactant concentration up to a maximum attainable radical yield (the 'plateau yield') where an increasing surfactant concentration (below the critical micelle concentration) no longer affects the -*CH- yield. In an earlier study it was found that the ratio of -*CH- detected following sonolysis of aqueous solutions of sodium pentane sulfonate (SPSo) to that of sodium dodecyl sulfate (SDS) (i.e. CH(SPSo)/CH(SDS)) depended on the frequency of sonolysis, but was independent of the ultrasound intensity, at the plateau concentrations [J.Z. Sostaric, P. Riesz, Adsorption of surfactants at the gas/solution interface of cavitation bubbles: an ultrasound intensity-independent frequency effect in sonochemistry, J. Phys. Chem. B 106 (2002) 12537-12548]. In the current study, it was found that the CH(SPSo)/CH(SDS) ratio depended only on the ultrasound frequency and did not depend on the geometry of the ultrasound exposure apparatus considered.     

Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this “bubble streamers” is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.     

Effects of surface tension on the dynamics of a single micro bubble near a rigid wall in an ultrasonic field

Acoustic cavitation is a very important hydrodynamic phenomenon, and is often implicated in a myriad of industrial, medical, and daily living applications. In these applications, the effect mechanism of liquid surface tension on improving the efficiency of acoustic cavitation is a crucial concern for researchers. In this study, the effects of liquid surface tension on the dynamics of an ultrasonic driven bubble near a rigid wall, which could be the main mechanism of efficiency improvement in the applications of acoustic cavitation, were investigated at the microscale level. A synchronous high-speed microscopic imaging method was used to clearly record the temporary evolution of single acoustic cavitation bubble in the liquids with different surface tension. Meanwhile, the bubble dynamic characteristics, such as the position and time of bubble collapse, the size and stability of the bubbles, the speed of bubble boundaries and the micro-jets, were analyzed and compared. In the case of the single bubbles near a rigid wall, it was found that low surface tension reduces the stability of the bubbles in the liquid medium. Meanwhile, the bubbles collapse earlier and farther from the rigid wall in the liquids with lower surface tension. In addition, the surface tension has no significant influence on the speed of the first micro-jet, but it can substantially increase the speed of second and the third micro-jets after the first collapse of the bubble. These effects of liquid surface tension on the bubble dynamics can explain the mechanism of surfactants in numerous fields of acoustic cavitation for facilitating its optimization and application.     

Sonolysis of surfactants in aqueous solutions: an accumulation of solute in the interfacial region of the cavitation bubbles

The sonolysis of surfactants (such as sodium dodecylbenzenesulfonate (DBS), sodium dodecylsulfate (SDS), and polyethylene glycol monostearate), sodium 4-toluenesulfonate (STS), and 1-hexanol in aqueous solutions was investigated under an argon atmosphere with ultrasound of 200 kHz in order to compare the scavenging efficiency of the hydroxyl radical and the accumulation in the gas-liquid interfacial region of the cavitation bubbles. The degradation rate of the solute follows the order 1-hexanol > DBS and SDS > STS. The scavenging efficiency of the hydroxyl radical by non-volatile surfactants was much greater than that of the non-volatile and hydrophilic solute (e.g., STS). The surfactant was accumulated in a relatively high ratio in the interfacial region. The degradation of surfactants occurred by reaction with the hydroxyl radical and also by pyrolysis at high temperature. On the other hand, STS, due to its non-volatile and hydrophilic properties, was principally present in the bulk solution and the degradation by pyrolysis was not observed at the investigated concentration ranges.     

Influence of dissolved-air concentration on spatial distribution of bubbles for sonochemistry

The pulsation of ultrasonic cavitation bubbles at various dissolved-air concentration in a sonochemical reaction field of standing-wave type is investigated experimentally by laser-light scattering. When a thin light sheet, finer than half the wavelength of sound, is introduced into the cavitation bubbles at an antinode of sound pressure, the scattered light intensity oscillates. The peak-to-trough light intensity is correlated with the number of bubbles that contribute to the sonochemical reaction. It is shown that as the dissolved air concentration becomes higher, the weighted center of the spatial distribution of the peak-to-trough intensity tends to shift towards the liquid surface. At higher concentration of the dissolved air, a great deal of bubbles with size distribution generated due to coalescence between bubbles disturbs sound propagation to change the sound phase easily. A standing wave to trap tiny oscillating bubbles is established only at the side which is nearer to the liquid surface. Also at higher concentration, liquid flow induced by drag motion of bubbles by the action of radiation force becomes apparent and position-unstable region of bubble is enlarged from the side of sound source towards the liquid surface. Therefore, the position of oscillating bubbles active for sonochemical reaction is limited at the side which is nearer to the liquid surface at higher concentration of the dissolved air.     

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.     

Bubbles in an acoustic field: an overview

Acoustic cavitation is the fundamental process responsible for the initiation of most of the sonochemical reactions in liquids. Acoustic cavitation originates from the interaction between sound waves and bubbles. In an acoustic field, bubbles can undergo growth by rectified diffusion, bubble-bubble coalescence, bubble dissolution or bubble collapse leading to the generation of primary radicals and other secondary chemical reactions. Surface active solutes have been used in association with a number of experimental techniques in order to isolate and understand these activities. A strobe technique has been used for monitoring the growth of a single bubble by rectified diffusion. Multibubble sonoluminescence has been used for monitoring the growth of the bubbles as well as coalescence between bubbles. The extent of bubble coalescence has also been monitored using a newly developed capillary technique. An overview of the various experimental results has been presented in order to highlight the complexities involved in acoustic cavitation processes, which on the other hand arise from a simple, mechanical interaction between sound waves and bubbles.     

球状泡群内气泡的耦合振动

振动气泡形成辐射场影响其他气泡的运动, 故多气泡体系中气泡处于耦合振动状态. 本文在气泡群振动模型的基础上, 考虑气泡间耦合振动的影响, 得到了均匀球状泡群内振动气泡的动力学方程, 以此为基础分析了气泡的非线性声响应特征. 气泡间的耦合振动增加了系统对每个气泡的约束, 降低了气泡的自然共振频率, 增强了气泡的非线性声响应. 随着气泡数密度的增加, 振动气泡受到的抑制增强; 增加液体静压力同样可抑制泡群内气泡的振动, 且存在静压力敏感区(1–2 atm, 1 atm=1.01325×10⁵ Pa); 驱动声波对气泡振动影响很大, 随着声波频率的增加, 能够形成空化影响的气泡尺度范围变窄. 在同样的声条件、泡群尺寸以及气泡内外环境下, 初始半径小于5 μm 的气泡具有较强的声响应. 气泡耦合振动会削弱单个气泡的空化影响, 但可延长多气泡系统空化泡崩溃发生的时间间隔和增大作用范围, 整体空化效应增强.     

Low-MHz frequency effect on a sonochemical reaction determined by an electrical method

The frequency effect of low-MHz ultrasound was studied by using the electrical detection method. The experimental data and theoretical analysis results showed that an appropriate frequency value of ultrasound can be chosen to make a sonochemical reaction give its maximum yield according to the distribution of bubbles in liquid. A Gaussian shape distribution of gas bubble radii is to be expected in a water sample with a normal air atmosphere. In addition, the experimental data indicated that comparison of the effect of frequency on the sonochemical efficiency should be carried out under conditions of not only the same sonic power but also the same sonic intensity.