Sonochemical Yield Research Articles

Influence of micron-sized air bubbles on sonochemical reactions in aqueous solutions exposed to combined ultrasonic irradiation and aeration processes

Sonochemical reactors are promising for wastewater treatment applications. However, the combination of ultrasonic irradiation and aeration processes raises questions regarding the extent to which entrained air bubbles influence sonochemistry. In this study, the effect of micron-sized air bubbles (20 and 70 µm, 0.1–5.0 vol%) on sonochemical activity, based on KI dosimetry, was tested in a horn-type reactor (20 kHz). Bubbles were generated using the pressurization–depressurization method. The results showed that low bubble concentrations increased sonochemical yields but higher concentrations tended to decrease the yields. Compared with the bubble-free case, 20-μm bubbles enhanced sonochemistry at all concentrations, while 70-μm bubbles inhibited sonochemistry at the highest concentration (5.0 vol%). Two regimes, acoustic attenuation and nucleus introduction, were identified as inhibiting and enhancing sonochemical effects, respectively, and their dependence on bubble characteristics was examined. These results are significant as investigation into these regimes will allow a deeper understanding of the key factors controlling sonochemistry in the presence of micron-sized air bubbles and could be used to design sonoreactors equipped with aerators.

Importance of sonoluminescence and sonochemistry for understanding and optimizing ultrasound applications

Ultrasound has shown to improve a spectrum of applications such as crystallization, degradation, synthesis, extraction, and cell disruption. These improvements largely stem from the cavitation induced extreme physical and/or chemical effects. Cavitation activities are often quantified using sonoluminescence intensity or sonochemical yield and are dependent on sonication conditions. For example, frequencies less than 100 kHz produces stronger physical effects and higher sonoluminescence per bubble, whereas frequencies between 200 and 800 kHz give rise to higher sonochemical yields. Similarly, solution conditions are equally important as 50% air saturation or addition of 1 mM anionic surfactant (sodium dodecyl sulfate) have shown to enhance sonoluminescence intensity. Often frequency and power are varied to assess its impact on a given application. However, this often do not reveal the underlying mechanisms to clearly assess whether the system could be further optimized or better designed to tailor for a particular application. This presentation aims to show the importance of complimentary evaluation of the sonoluminescence and sonochemical yield and also provide case studies on sonocrystallization, degradation of pollutants, and sono-grafting on membranes.

Clarification of regimes determining sonochemical reactions in solid particle suspensions

Although there has been extensive research on the factors that influence sonochemical reactions in solid particle suspensions, the role that solid particles play in the process remains unclear. Herein, the effect of monodisperse silica particles (10–100 μm, 0.05–10 vol%) on the sonochemical activity (20 kHz) was investigated using triiodide formation monitoring and luminol tests. The results demonstrate that, in the particle size range considered, the sonochemical yields were enhanced in dilute suspensions (0.05–1 vol%), while further particle addition in semi-dilute suspensions (1–10 vol%) decreased the yields. Two regimes, namely the site-increasing regime and sound-damping regime, are identified in respect of the enhancing and inhibiting effects of the particles, respectively, and their dependence on particle characteristics is analyzed. Both regimes are confirmed based on the cavitation erosion test results or cavitation noise analysis. The clarification of the two regimes provides a better understanding of the dominant factors controlling sonochemistry in the presence of solid particles, as well as a guide for sonochemical efficiency prediction.

The oxidation study of N,N-diethyl-meta-toluamide in dual frequency ultrasonic reactor

With the increasing concern of emerging contaminants (ECs), advanced oxidation processes (AOPs) have been widely investigated to fulfill the drinking water quality because of the potential adverse health effects of ECs. Accordingly, N,N-diethyl-meta-toluamide (DEET) is selected as a model compound belonging ECs to monitor its ultrasonic oxidation which is one of the most popular AOPs in a dual frequency ultrasonic reactor (DFUR) using low-frequency probe (20 KHz) and high-frequency transducer (640 KHz) type sources. DFUR was calorimetrically optimized in terms of power densities of both ultrasonic sources in order to provide the highest sonochemical yield with efficient energy output. Pseudo-first order kinetic equation was applied to results by measuring the concentration decreasing during the oxidation reactions. The pseudo-first-order rate constants, k, increased from 7.8x10-3 min-1 (640 kHz, R2=0.930) to 13.5x10-3 min-1 (DFUR, R2=0.990), by contrast, the rate constant was only 0.7x10-3 min-1 (R2=0.281) for 20 kHz low-frequency ultrasonic source. DEET oxidation was evaluated with the presence of different gas saturation (Ar, Air, O2, and N2); addition of hydrogen peroxide (PO), persulfate (PS) and monoperoxysulfate (MPS) and PO concentration effect (molar ratio of DEET:PO; 1:1, 1:2, 1:5, 1:10 and 1:20). The DEET oxidation rate was calculated as 35.8 x10-3 min-1 (R2=0.994) in the presence of Argon gas saturation, while it was 13.5 x10-3 min-1 (R2=0.990) when no gas bubbling. Therefore, the main degradation pathway was predicted as pyrolysis taking place inside the cavitation bubble where DEET molecules can reach. On the other side, a lower degradation rate in the presence of PO, PS, and MPS than that of no additives has indicated that the bulk phase degradation pathway for some part of DEET molecules are still occurred.

Theoretical estimation of sonochemical yield in bubble cluster in acoustic field**Project supported by the National Natural Science Foundation of China (Grant No. 11674207).

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.

Optimized dispersion quality of aqueous carbon nanotube colloids as a function of sonochemical yield and surfactant/CNT ratio

In this paper, we propose and verify a theoretical model of the development of dispersion quality of aqueous carbon nanotube (CNT) colloid as a function of sonochemical yield of the sonication process. Four different surfactants; Triton X-100, Pluronic F-127, CTAB and SDS were studied. From these four SDS had the lowest dispersion performance which was surprising. Optical dispersion quality results fits well with proposed theoretical model.

Multibubble Sonochemistry and Sonoluminescence at 100 kHz: The Missing Link between Low- and High-Frequency Ultrasound.

Ultrasonic frequency is one of the most important parameters that decides the characteristics of acoustic cavitation. Low- (16-50 kHz) and high- (≥200 kHz) frequency ultrasounds present opposite physical and chemical behaviors and have been extensively studied, yet frequencies in between are poorly characterized. In this study, acoustic cavitation at the intermediate ultrasonic frequency of 100 kHz is compared with that at 20 kHz and at 362 kHz by different experimental investigations: sonochemical yield (H2O2), images of sonochemiluminescence and sonoluminescence, as well as sonoluminescence spectra in aqueous media saturated with Ar or Ar/(20 vol %)O2. The chemical activity (H2O2 yield) of cavitation bubbles at 100 kHz presents a transitional behavior between low and high frequencies. The active cavitation zone distributes in the whole sonicated volume, similarly to high-frequency ultrasound and much further than at 20 kHz. The spectral shape of 100 kHz spectra is similar to that at 20 kHz. On the contrary, 100 kHz ultrasound provides the dissociation of O2 and N2 molecules inside the bubble, which is more typical for high-frequency ultrasound. This faculty is explained by the more extreme conditions reached at collapse compared with 20 kHz. Rovibronic temperatures of OH (A2Σ+) excited radicals derived from spectroscopic simulations confirm this interpretation.

Intensification of sonochemical reactions in solid-liquid systems under fully suspended condition

Process intensification of sonochemical reactions due to the addition of solids is studied using very high solids concentrations (up to 0.5 (v/v)). The impacts of particle size, surface roughness, and solids concentration on sonochemical yields were studied experimentally by measuring the concentration of I3− formed in the cavitation of potassium iodide solution in an agitated baffled tank fitted with an ultrasonic generator (20kHz, 131W). Solids used were cation exchange resin (625μm), sand (303μm), and spherical glass beads (207, 551 and 1290μm). It was found that, due to the net effect of wave attenuation and increased number of nucleation sites available, cavitation level initially decreases with increasing solids concentration up to 0.1 (v/v), then increases up to 0.4 (v/v), followed by a further decrease. Cavitation activity increases with increasing particle diameter due to the reduction in the liquid tensile strength in the presence of larger particles, which decrease the cavitation threshold. The cavitation level is found to be enhanced due to an increase in the surface roughness. The results imply that particle size, concentration, and surface roughness all play important roles in the formation and subsequent collapse of cavities, which cumulatively influence sonochemical reaction yields.

Single, simultaneous and sequential applications of ultrasonic frequencies for the elimination of ibuprofen in water

The study is about the assessment of single and multi-frequency operations for the overall degradation of a widely consumed analgesic pharmaceutical-ibuprofen (IBP). The selected frequencies were in the range of 20–1130kHz emissions coming from probes, baths and piezo-electric transducers attached to plate-type devices. Multi-frequency operations were applied either simultaneously as “duals”, or sequentially at fixed time intervals; and the total reaction time in all operations was 30-min. The work also covers evaluation of the effect of zero-valent iron (ZVI) on the efficiency of the degradation process and the performance of the reaction systems. It was found that low-frequency probe type devices especially at 20kHz were ineffective when applied singly and without ZVI, and relatively more effective in combined-frequency operations in the presence of ZVI. The power efficiencies of the reactors and/or reaction systems showed that 20-kHz probe was considerably more energy intensive than all others, and was therefore not used in multi-frequency operations. The most efficient reactor in terms of power consumption was the bath (200kHz), which however provided insufficient mineralization of the test chemical. The highest percentage of TOC decay (37%) was obtained in a dual-frequency operation (40/572kHz) with ZVI, in which the energy consumption was neither low nor exceptionally too high. A sequential operation (40+200kHz) in that respect was more efficient, because it required much less energy for a similar TOC decay performance (30%). In general, the degradation of IBP increased with increased power consumption, which in turn reduced the sonochemical yield. The study also showed that advanced Fenton reactions with ZVI were faster in the presence of ultrasound, and the metal was very effective in improving the performance of low-frequency operations.

How to design silent control experiments for ultrasound-assisted oil sands extraction and upgrading: A computational study

Abstract The major concern in estimating sonochemical yield and efficiency in ultrasound-assisted processes is in defining a “silent” control experiment, without cavitation effects. To estimate the potential benefit of the ultrasonic treatment as compared to conventional heating, we propose that the effects should be compared at the same power input, when the energy in a silent experiment is dissipated as heat. Our calculations of possible temperature increase under the silent conditions for oil sands extraction and upgrading showed necessity of such approach.

Multibubble sonoluminescence as a tool to study the mechanism of formic acid sonolysis

Sonoluminescence spectra collected from 0.1 to 3.0M aqueous solutions of formic acid sparged with argon show the OH(A2Σ+−X2Πi) and C2(d3Πg→a3Πu) emission bands and a broad continuum typical for multibubble sonoluminescence. The overall intensity of sonoluminescence and the sonochemical yield of HCOOH degradation vary in opposite directions: the sonoluminescence is quenched while the sonochemical yield increases with HCOOH concentration. By contrast, the concentration of formic acid has a relatively small effect on the intensity of C2 Swan band. It is concluded that C2 emission originates from CO produced by HCOOH degradation rather than from direct sonochemical degradation of HCOOH. The intensity of C2 band is much stronger at high ultrasonic frequency compared to 20kHz ultrasound which is in line with higher yields of CO at high frequency. Another product of HCOOH sonolysis, carbon dioxide, strongly quenches sonoluminescence, most probably via collisional non-radiative mechanism.

A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 1. Overhead stirring

The effect of flow in an ultrasonic reactor is an important consideration for practical applications and for the scale-up of ultrasonic processing. Previous literature on the influence of flow on sonochemical activity has reported conflicting results. Therefore, this work examined the effect of overhead stirring at four different frequencies, 40, 376, 995 and 1179kHz, in two different reactor configurations. Comparable power settings were utilised to elucidate the underlying mechanisms of interactions between the flow and sonochemical activity. The sonochemical activity was determined by the yield of hydrogen peroxide, measured by iodide dosimetry, and the active region was visualised with sonochemiluminescence imaging. The overhead stirring in the low frequency reactor altered the yield of hydrogen peroxide so it produced the maximum yield out of the four frequencies. The increase in hydrogen peroxide yield was attributed to a reduction in coalescence at 40kHz. However at the higher frequencies, coalescence was not found to be the main reason behind the observed reductions in sonochemical yield. Rather the prevention of wave propagation and the reduction of the standing wave portion of the field were considered.

Sonochemical Effect Using Ultrasonic Atomizer at 2.4 MHz

Sonochemical reactions were demonstrated using a commercial ultrasonic atomizer at 2.4 MHz. The influences of experimental conditions, bottom shape and glass thickness of reactors, irradiation method, and liquid height on the sonochemical yield were discussed. The sonochemical effect was evaluated by potassium iodide dosimetry and degradation of methylene blue. Direct and indirect irradiations were examined. The former had the highest yield. In the latter case, sonochemical yield decreased in the solution because glass prevented the transmission of ultrasonic waves. Poly film, on the other hand, could transmit ultrasonic waves very well without damage.

Acoustic Bubble Sizes, Coalescence, and Sonochemical Activity in Aqueous Electrolyte Solutions Saturated with Different Gases

Acoustic bubble sizes, coalescence behavior, and sonochemical activity have been investigated in water in the presence of various electrolyte additives (KCl, HCl, and NaNO(3)) and saturating gases-helium, air, and argon. A strong correlation was identified between the bubble radius and the dissolved gas concentration in the cavitation medium. The extent of bubble coalescence for each gas was also studied in different electrolyte solutions. A causal relationship between coalescence and bubble size was inferred. Importantly, the effects of the different electrolytes could be completely attributed to their "salting out" effect on the dissolved gas, providing valuable insight into the contentious issue of ion-specific coalescence inhibition. Extrapolation of the bubble size data to conditions where bubble coalescence is minimal, i.e., zero gas concentration and zero ultrasound exposure time, yielded a bubble radius of 1.5 +/- 0.5 microm at an acoustic frequency of 515 kHz. In addition, the effects of electrolyte concentration and gas type on sonochemical activity were investigated. Sonochemical yields were increased by up to 1 order of magnitude at high electrolyte concentrations. This has been attributed to reduced gas and vapor content in the bubble core prior to collapse and a lower clustering density.

Effect of reaction vessel diameter on sonochemical efficiency and cavitation dynamics

The influence of reaction vessel diameter on the sonochemical yield was investigated by using reaction vessels with five different diameters. It was revealed that the formation of H 2O 2 and chloride ion, from the sonolysis of pure water and 1,2,4-trichlorobenzene aqueous solution, was affected by the reaction vessel diameter. That is, these yields increased as the reaction vessel diameter increased up to ø 90 mm and then decreased over ø 90 mm. From the analyses of the measurement of sonochemiluminescence and the calorimetry, it was suggested that active cavitation bubbles were formed at certain zones. In the case of a larger diameter reaction vessel, it was suggested that bubble nuclei that have not grown up to the resonance size, escaped from the sonication zone to the non-sonication zone and dissolved away. As a result, the number of active cavitation bubbles and the yields of H 2O 2 and chloride ion would decrease in the case of a larger diameter reaction vessel.

Sonochemistry and Sonoluminescence under Simultaneous High- and Low-Frequency Irradiation

Sonoluminescence emission, sonochemiluminescence from luminol and the sonochemical yield of hydrogen peroxide were studied under a dual frequency field combining low-frequency (20 kHz) and high-frequency (355 kHz) ultrasound. Under certain conditions it was found that a synergistic enhancement of both sonoluminescence and sonochemical processes could be attained. Ultrasound parameters including pulsing conditions and sonication time were found to sensitively influence the efficiency of the dual-frequency mode operation. Possible mechanisms responsible for the dual-frequency effects are discussed.

Experimental investigation of cavitational bubble dynamics under multi-frequency system

Acoustic emission spectra measurements have been carried out under mono and multi-frequency acoustic sources to understand the fundamental difference in bubble/cavity dynamics. The effect of introducing the dual and triple frequency acoustic waves of different frequency on the sono-chemical yield has also been investigated experimentally. The introduction of a second wave has increased the number of cavitating bubbles and as well as the collapsing intensity of cavities resulting into higher sono-chemical yield, and better effective utilization of reactor volume with a large number of resonating cavitating bubbles. To get the information about the intensity of each of the cavity oscillating events, decomposition of the pressure signal measured by the hydrophone in the frequency domain of the FFT power spectrum has been carried out. Inverse fourier reconstruction technique, has been used to elaborate the dynamics of the cavitating bubbles in the multi-frequency system.

Mechanistic approach to enhancement of the yield of a sonochemical reaction

AbstractIn this study, a mechanistic approach has been taken to enhance yield of a sonochemical reaction. Formation of highly reactive free radicals due to the transient collapse of cavitation bubbles is the primary mechanism of a sonochemical reaction. A physical (reduction in dissolved gas concentration) and a chemical (increasing the reactant concentration) technique is used for enhancing yield of a sonochemical reaction using those techniques, which influence the phenomenon of radical formation by the cavitation bubbles. A bubble dynamics model is used for explaining the sonochemical phenomena. In a degassed medium, the ultrasound wave undergoes lesser attenuation; moreover, equilibrium size of a bubble shrinks due to rectified diffusion. Because of this, a bubble undergoes more violent collapse, resulting in greater production of radicals that give higher yield. On the other hand, increasing the initial reactant concentration shows an adverse effect on the sonochemical yield. This is ascribed to reduction in water vapor flux in the bubble due to reduction of vapor pressure of the medium. This study, therefore, demonstrates as how macroscopic manifestation (the sonochemical yield) of the microscopic phenomena (transient collapse of cavitation bubble) is a complicated function of several physical processes. The results of this study shed light on the complex and multifaceted physical mechanism of a sonochemical reaction, which may be useful in maximization of yield of other sonochemical systems. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Comparative study of sonochemical reactors with different geometry using thermal and chemical probes

Laboratory scale 20 kHz sonochemical reactors with different geometries have been tested using thermal probes, the kinetics of H 2O 2 formation, and the kinetics of diphenylmethane (DPhM) sonochemical darkening. Results revealed that the overall sonochemical reaction rates in H 2O and DPhM are driven by the total absorbed acoustic energy and roughly independent the geometry of the studied reactors. However, the sonochemical efficiency, defined as η = VG/ S, where G is a sonochemical yield of H 2O 2, V is a volume of sonicated liquid, and S is a surface of the sonotrode, was proved to increase with the decrease of S. This phenomenon was explained by growing of the maximum cavitating bubble size with ultrasonic intensity and its independence towards the specific absorbed acoustic power. For the cleaning bath reactor the kinetics of the sonochemical reactions in H 2O and DPhM depends strongly on the reaction vessel materials: the reaction rates decreased with the increase of the materials elasticity. Kinetic study of H 2SO 4 sonolysis using a sonoreactor without direct contact with titanium sonotrode showed that sulphate anion is an effective scavenger of OH radicals formed during water sonolysis.

Acoustic cavitation and sonoluminescence

The violent collapse of a bubble in an acoustic field can lead to the emission of light, a phenomenon called sonoluminescence (SL). The study of sonoluminescence has led to refinements in understanding cavitation. This talk will focus on three areas of research in which the application of sonoluminescence is used to investigate cavitation: (1) In the field of single-bubble SL, techniques have been improved to study the dynamical motion of the bubble and, with comparisons to SL, models of the bubble dynamics have been refined. In particular, thermal conduction and the trapping of water vapor during the bubble collapse contribute significantly to the internal temperatures and to the dynamics of the cavitation bubble. (2) In lithotripsy, intense shock waves generate cavitation fields which play a role in kidney stone communition. In vitro studies have shown that SL can be correlated with a bubble’s expansion and collapse within the field. (3) In sonochemistry, high-intensity ultrasound is used to initiate or enhance chemical reactions. SL is often observed under these conditions, and in some cases, there is a correspondence between SL intensity and sonochemical yield. Although often overlooked, it can be important to account for the physics and chemistry at the bubble surface.