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(2)O(2) formation, and the kinetics of diphenylmethane (DPhM) sonochemical darkening. Results revealed that the overall sonochemical reaction rates in H(2)O 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 eta=VG/S, where G is a sonochemical yield of H(2)O(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(2)O and DPhM depends strongly on the reaction vessel materials: the reaction rates decreased with the increase of the materials elasticity. Kinetic study of H(2)SO(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.     

Mechanism of W(CO)6 sonolysis in diphenylmethane

The present work analyses the mechanism of W₂C/C nanocomposite formation during sonolysis of W(CO)₆ in diphenylmethane (DPhM) solutions. Carbon supported WC_x nanoparticles attract much interest as an alternative fuel cell electrocatalysts. Sonolysis of neat DPhM under the effect of 20 kHz power ultrasound in argon at 80 °C yields a sonopolymer as a solid product and acetylene, hydrogen, methane, diacetylene and benzene as gaseous products. Diacetylene is formed due to the secondary sonochemical dimerisation of acetylene obtained at the primary stage of DPhM sonolysis. FTIR and μ-Raman studies show that the sonopolymer consists of a mixture of some polymeric partially oxidized aromatic species, and disordered carbon. Sonolysis of W(CO)₆ in diphenylmethane solutions follows the first order kinetics. This process yields monodispersed 2-3 nm X-ray amorphous WC_x nanoparticles embedded in amorphous sonopolymer. The annealing of air sensitive as-prepared solids in an inert atmosphere at 600 °C causes formation of stable W₂C/C nanocomposite with W₂C average particle size in the range of 4-7 nm and hexagonal carbon fine particles with the average size of 30-40 nm. Kinetic study revealed that tungsten carbide is formed inside the cavitation bubble due to the reaction of tungsten nanoparticles originated from primary sonolysis of W(CO)₆ with acetylene produced as a result of diphenylmethane sonochemical degradation.     

Mechanistic considerations for the degradation of methyl tert-butyl ether (MTBE) by sonolysis: effect of argon vs. oxygen saturated solutions

The ultrasonic degradation mechanism of methyl tert-butyl ether (MTBE) in aqueous solution is complex because of the competition between hydroxyl radical attack, pyrolysis, and hydrolysis reactions. A detailed investigation of degradation pathways using sonolysis has been performed using reaction byproducts identification. The observed bi-product distributions are rationalized in terms of hydroxyl radical (OH) mediated processes and pyrolysis. The role of oxygen mediated and pyrolytic pathways were assessed using O₂ and Ar saturated solutions. Chemical destruction by sonolysis is often rationalized using hydroxyl radical chemistry. Pyrolysis is unique to this advanced oxidation process, and is important in the case of MTBE because it transfers into the cavitating bubbles. While α-hydrogen abstraction by OH and low temperature pyrolysis was important, it was also shown that β-hydrogen abstraction leads, in some cases, to the same reaction byproducts, which emphasized the importance of α-hydrogen abstraction. High temperature pyrolysis resulted in minor degradation reactions based on the formation of reaction by-products.     

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.     

OH-radical formation by ultrasound in aqueous solution--Part II: Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield

Terephthalate and Fricke dosimetry have been carried out to determine the sonolytic energy yields of the OH free radical and of its recombination product H2O2 in aqueous solutions under various operating conditions (nature of operating gas, power, frequency, temperature). For example, in the sonolysis of Ar-saturated terephthalate solutions at room temperature, a frequency of 321 kHz, and a power of 170 W kg-1, the total yield [G(.OH) + 2 G(H2O2)], equals 16 x 10(-10) mol J-1. This represents the total of .OH that reach the liquid phase from gas phase of the cavitating bubble. The higher the solute concentration, the lower the H2O2 production as more of the OH free radicals are scavenged, in competition with their recombination. Fricke dosimetry, in the absence and presence of Cu2+ ions, shows that the yield of H atom reaching the liquid phase is much lower, with G(H.) of the order of 3 x 10(-10) mol J-1. These sonolytic yields are smaller in solutions that are at the point of gas saturation, and increase to an optimum as the initial sonication-induced degassing and effervescence subsides. The probing of the sonic field has shown that the rate of sonolytic free-radical formation may vary across the sonicated volume depending on frequency and power input.     

Scavenging of OH(*) radicals produced from H(2)O sonolysis with nitrate ions

The scavenging of OH(?) radicals formed during H(2)O sonolysis with nitrate-ions was studied in HNO(3)/NaNO(3) mixture at the constant NO(3)(-) ions concentration ([HNO(3)]+[NaNO(3)])=1 M in Ar atmosphere. Small amounts of N(2)H(5)NO(3) was added to solutions to avoid HNO(2) accumulation due to HNO(3) sonolysis. It was shown that the increase of [H(+)] causes the increase of H(2)O(2) formation rate (W(H(2)O(2)). (W(H(2)O(2)) values reach the plateau at [HNO(3)] approximately 1 M. The (W(H(2)O(2)) ratio in solution with [H(+)]=1 M and pure water was found to be equal to 2.4+/-0.4. It was assumed that (W(H(2)O(2)) increase in nitric acid medium is related to the changing of H(2)O(2) formation mechanism. In pure water H(2)O(2) is formed due to the OH(*) radicals recombination. In HNO(3)+NaNO(3) mixture the mechanism of H(2)O(2) formation consists in conversion of OH(*) radicals to NO(3)(*) radicals followed by NO(3)(*) radicals hydrolysis. Results obtained show that OH(*) radicals recombination mainly occurs in the liquid phase surrounding the cavitating bubble.     

Sonolysis of metal beta-diketonates in alkanes

The kinetics of metal beta-diketonates sonolysis was studied in hexadecane solutions using a UV/VIS spectrophotometric technique. The following complexes were prepared and studied: Cu(HFAA)(2), Cu(DPM)(2), Fe(ACAC)(3), Ni(DPM)(2), Er(DPM)(3), Nd(DPM)(3), Th(DPM)(4), UO(2)(BTFA)(2).TOPO, and Np(HFAA)(4), where HHFAA is hexafluoroacetylacetone, HDPM is dipivaloylmethane, HACAC is acetylacetone, HBTFA is benzoyltrifluoroacetone, and TOPO is trioctylphosphine oxide. Sonolysis was performed under the following conditions: ultrasonic frequency 22 kHz, intensity of ultrasound 3-5 Wcm(-2), temperature 70-92 degrees C, Ar atmosphere. The kinetic behavior of the studied complexes are interpreted using a two-site model of the sonochemical processes. In the case of metal beta-diketonates with high vapor pressure the sonochemical reactions tend to occur in the gaseous phase of the cavitating bubbles. The sonolysis of less volatile complexes first occur in the liquid reaction zone surrounding the bubbles. Sonication of the studied complexes results in the formation of X-ray amorphous products consisted of a mixture of metal beta-diketonates partial degradation products. Heating of as-prepared sonication products in air yields nanocrystalline oxides of corresponding metals.     

Sonochemical degradation of martius yellow dye in aqueous solution

The sonolytic degradation of the textile dye martius yellow, also known as either naphthol yellow or acid orange 24, was studied at various initial concentrations in water. The degradation of the dye followed first-order kinetics under the conditions examined. Based on gas chromatographic results and sonoluminescence measurements of sonicated aqueous solutions of the dye, it is concluded that pyrolysis does not play a significant role in its degradation. The chromatographic identification of hydroxy added species indicates that an OH radical induced reaction is the main degradation pathway of the dye. Considering the non-volatility and surface activity of the dye, the degradation of the dye most probably takes place at the bubble/solution interface. The quantitative and qualitative formation of the degradation intermediates and final products were monitored using HPLC and ESMS. The analytical results suggest that the sonolytic degradation of the dye proceeds via hydroxylation of the aryl ring and also by C-N bond cleavage of the chromophoric ring, either through OH radical attack or through another unidentified process. The identification of various intermediates and end products also imply that the degradation of martius yellow proceeds through multiple reaction pathways. Total organic carbon (TOC) analyses of the dye solutions at various times following sonication revealed that sonolysis was effective in the initial degradation of the parent dye but very slow in achieving mineralization. The slow rate of mineralization is likely to be due to the inability of many of the intermediate products such as, the carboxylic acids, to accumulate at the bubble (air/water) interface and undergo decomposition due to their high water solubility (low surface activity).     

Luminescence of Tb3+ and Gd3+ ions in sonolysis under the conditions of a single bubble moving in aqueous solutions of TbCl3 and GdCl3

Luminescence bands of Tb³⁺ and Gd³⁺ ions are detected during sonolysis in the regime of a moving single bubble in aqueous solutions of TbCl₃ and GdCl₃ salts with concentration 1–2 mol/L. Saturation with argon, low temperatures of solutions (?5°C), and a high concentration of salts are the factors facilitating sonoluminescence of the metal. Comparison with the characteristics of sonoluminescence of lanthanide ions studied earlier in the regimes of multibubble and single-bubble sonolysis with a stationary bubble shows that the electron excitation of metal ions in the given case is associated with translational displacements of the bubble. Our results confirm the validity of the sonochemical model of microdroplet injection, which explains the penetration of nonvolatile salts into cavitation bubbles as a result of their deformation during intense movements.     

Estimation of chemical and physical effects of cavitation by analysis of cavitating single bubble dynamics

Cavitation events create extreme conditions in a localized ‘bubble collapse’ region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb’s Free Energy method using simulated collapse conditions. OHG values as high as 1x10¹² OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.     

Sonochemical degradation of chlorophenols in water

Sonochemical degradation of dilute aqueous solutions of 2-, 3- and 4-chlorophenol and pentachlorophenol has been investigated under air or argon atmosphere. The degradation follows first-order kinetics in the initial state with rates in the range 4.5-6.6 microM min-1 under air and 6.0-7.2 microM min-1 under argon at a concentration of 100 microM of chlorophenols. The rate of OH radical formation from water is 19.8 microM min-1 under argon and 14.7 microM min-1 under air in the same sonolysis conditions. The sonolysis of chlorophenols is effectively inhibited, but not completely, by the addition of t-BuOH, which is known to be an efficient OH radical scavenger in aqueous sonolysis. This suggests that the main degradation of chlorophenols proceeds via reaction with OH radicals; a thermal reaction also occurs, although its contribution is small. The addition of appropriate amounts of Fe(II) ions accelerates the degradation. This is probably due to the regeneration of OH radicals from hydrogen peroxide, which would be formed from recombination of OH radicals and which may contribute a little to the degradation. The ability to inhibit bacterial multiplication of pentachlorophenol decreases with ultrasonic irradiation.     

Sonolysis of aqueous 4-nitrophenol at low and high pH

The sonolysis of 4-nitrophenol in argon-saturated aqueous solution has been studied at 321 kHz. In order to evaluate separately the effect of OH radicals that are formed in the cavitational bubble and part of which react in the aqueous phase with this substrate, radiolytic studies in N2O-saturated solutions were carried out for comparison. A detailed product study of the sonolysis of 4-nitrophenol solutions shows that at pH 10, where 4-nitrophenol is deprotonated (pKa = 7.1), its sonolytic degradation is fully accounted for by OH-radical-induced reactions in the aqueous phase. At this pH, the sonolytic yield of H2O2 resulting from OH radical recombination in the solution, measured as a function of the 4-nitrophenol concentration, is reduced in line with the scavenging capacity of the 4-nitrophenolate. In contrast, at pH 4 the formation of H2O2 is already fully suppressed when the solution is 7 x 10(-4) mol dm-3 in 4-nitrophenol, and oxidative-pyrolytic degradation predominates, as exemplified by the large yields of CO and CO2 which are accompanied by a large H2 yield. The basis of this difference in behavior is a hydrophobic enrichment of 4-nitrophenol (which is undissociated at pH 4) at the interface of the cavitational bubble by a factor of about 80. The pH dependence of the yields of the pyrolytic products reflects the hydrolytic equilibrium concentration of 4-nitrophenol. The paper also demonstrates that the complexity of this sonochemical system precludes its use a gauge to determine the temperature in the interior of the cavitational bubble.     

绕水翼初生游离型空化流动结构

为了研究初生空化流动形态及其紊流流场结构,采用高速录像技术观察了绕Clark-Y型水翼初生空化的空化形态,应用LDV分别测量了无空化和初生空化条件下的紊流流场分布.结果表明,绕水翼小攻角无分离流动区域的初生空化形态呈游离发夹涡型空泡团结构,但其具有和单泡相同的发展过程;初生空化和无空化紊流流场的速度和紊流强度没有发现有规律性的差异,初生游离型空穴的形成与发展过程,对雷诺平均流场没有显著的影响.     

A reaction kinetics model of water sonolysis in the presence of a spin-trap

The time development of the concentration of a spin-trapped OH radical was studied by electron spin resonance at various sound intensities and various 5,5-dimethyl-1-pyrroline N-oxide (DMPO) concentrations in water sonolysis. The lifetime of the spin-trapped OH radical was also studied, and factors governing sonolysis are discussed. We found that the production of spin-trapped OH radical increases with increasing ultrasound intensity. The lifetime of a spin-trapped OH radical decreases linearly with increase in sonication time. This result suggests that an unknown scavenger is produced by ultrasound. Based on the above results, we suggested a model of the reaction kinetics and estimated the production rate of OH radical from this model.     

A statistical thermodynamic approach to sonochemical reactions

The calculation of the equilibrium constants K of the sonolysis reactions of CO2 into CO and O atom, the recombination of O atoms into O2 and the formation of H2O starting with H and O atoms, has been studied by means of statistical thermodynamic. The constants have been calculated at 300 kHz versus the pressure and the temperature according to the extreme conditions expected in a cavitation bubble, e.g. in the range from ambient temperature to 15200 K and from ambient pressure to 300 bar. The decomposition of CO2 appears to be thermodynamically favored at 15200 K and 1 bar with a constant K1=1.52 x 10(6), whereas the formation of O2 is not expected to occur (K2=1.8 x10(-8) maximum value at 15200 K and 300 bar) in comparison to the formation of water (K3=3.4 x 10(47) at 298 K and 300 bar). The most thermodynamic favorable location of each reactions is then proposed, the surrounding shell region for the thermic decomposition of CO2 and the wall of the cavitation bubble for the formation of water. Starting from a work of Henglein on the sonolysis of CO2 in water at 300 kHz, the experimental amount of CO formed (7.2 x 10(20)molecules L(-1)) is compared to the theoretical CO amount (1.4 x 10(27)molecules L(-1)) which can be produced by the sonolysis of the same starting amount CO2. With the help of the literature data, the number of cavitation bubble has been evaluated to 6.2 x 10(15) bubbles L(-1) at 300 kHz, in 15 min. This means that about 1 bubble on 1900000 is efficient for undergoing the sonolysis of CO2.     

Evidence for liquid phase reactions during single bubble acoustic cavitation

We extended the recent experiment by Lepoint et al. [Sonochemistry and Sonoluminescence, NATO ASI Series, Series C 524, Kluwer Academic Publishers, Dordrecht/Boston/London, 1999, p. 285], involving a so-called single bubble sonochemistry process, to a three-phase system. We have found experimental evidence that a single cavitating bubble can activate the oxidation of I- ions after the injection of a CCl4 liquid drop in the bubble trapping apparatus. The solvent drop (CCl4 is almost water insoluble) is pushed towards the bubble position and forms a thin film on the bubble surface. When the acoustic pressure drive is increased above 100 kPa, the three-phase system gives rise to a dark filament, indicating the complexation reaction between starch (added to the water phase) and I2. I2 species is the product of surface reactions involving bubble-induced decomposition of CCl4. Further increase of the acoustic drive causes the thin CCl4 film to separate from the bubble and stops I2 production. The study of the chemical activity of this three-phase system could give new advances on dynamics of the bubble collapse.     

Optical and acoustic monitoring of bubble cloud dynamics at a tissue-fluid interface in ultrasound tissue erosion

Short, high-intensity ultrasound pulses have the ability to achieve localized, clearly demarcated erosion in soft tissue at a tissue-fluid interface. The primary mechanism for ultrasound tissue erosion is believed to be acoustic cavitation. To monitor the cavitating bubble cloud generated at a tissue-fluid interface, an optical attenuation method was used to record the intensity loss of transmitted light through bubbles. Optical attenuation was only detected when a bubble cloud was seen using high speed imaging. The light attenuation signals correlated well with a temporally changing acoustic backscatter which is an excellent indicator for tissue erosion. This correlation provides additional evidence that the cavitating bubble cloud is essential for ultrasound tissue erosion. The bubble cloud collapse cycle and bubble dissolution time were studied using the optical attenuation signals. The collapse cycle of the bubble cloud generated by a high intensity ultrasound pulse of 4-14 micros was approximately 40-300 micros depending on the acoustic parameters. The dissolution time of the residual bubbles was tens of ms long. This study of bubble dynamics may provide further insight into previous ultrasound tissue erosion results.     

Sonolysis of an aqueous mixture of trichloroethylene and chlorobenzene

The effect of the initial concentration on the ultrasonic degradation of two volatile organic compounds trichloroethylene (TCE) and chlorobenzene (CB) was investigated. At higher concentrations, slower sonolysis rates were obtained due to the lowering of the average specific heat ratio gamma of the gas inside the cavitation bubbles. Furthermore, the effect of different concentrations of CB on the sonolysis of 3.34 mM TCE and the effect of different concentrations of TCE on the sonolysis of 3.44 mM CB was examined. The presence of CB lowered the sonolysis rate of TCE, while the sonolysis rate of CB did not decrease by TCE addition. An even higher sonolysis rate was obtained for 3.44 mM CB in the presence of 0.84 mM TCE than without TCE. The explanation for the different effects of both volatile organics on each other's sonolysis rate is thought to be the difference in reaction rate of TCE and CB with the radicals formed during sonolysis. The effect of TCE on the sonolysis rate of CB by lowering the gamma value is compensated by an increased indirect degradation of CB by radicals formed out of TCE. The decreased thermal degradation and the increased indirect radical degradation of CB in the presence of TCE is demonstrated by determining the kinetics of the degradation products styrene and dichlorobenzene.     

基于汽相/液相界面跟踪方法的弯管内空化流动的数值预测

采用发展的基于Reynolds-Averaged Navier-Stokes方程和液相/气相界面跟踪方法的单相空化模型和数值迭代算法,数值预测了二维弯管内空化卒泡的形状和相应的压力系数分布.将发展的二维空化模型和算法推广到i维空化流动的数值模拟,预测了三维非对称弯管内的三维空泡形状和流动特性.数值预测三维非对称弯管内空化流动时的卒泡位置和相应压力系数分布符合理论分析结论.研究结果证明了所发展的空化模型和算法能够应用于三维空化流动时的数值预测.     

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.