Effect of coal ash on sonochemical degradation of phenol in water

The influences of coal ash on the degradation of phenol in water were investigated under the stirring or ultrasonic irradiation conditions. Phenol solution (10mg/L, 100mL) was sonicated at 200 kHz and 200 W with or without coal ash (53-106 microm in particle size and concentration of 0.0-1.5 wt%). It was found that the sonochemical degradation of phenol in the presence of coal ash was faster than that in the absence of coal ash, and the optimum amount of coal ash was a maximum at 0.4-0.6 wt%. It was confirmed that the phenol degradation did not occur by the addition of hydrogen peroxide and nitric acid under the stirring conditions. The sonochemical degradation with coal ash was depressed by the addition of tertiary butyl alcohol as a radical scavenger. These results indicated that the coal ash accelerated the phenol degradation due to the increase in the amount of hydroxyl radicals under the ultrasonic irradiation. Since the coal ash used had a porous and uneven surface, which was observed by SEM, it was assumed that the coal ash led to the increase in the nucleation site for cavitation bubble due to its surface roughness.     

Direct sonication of acetic acid in aqueous solutions

Sonochemical oxidation has a promising future in the area of waste water treatment as one of the advanced oxidation methods. In this study, direct ultrasonic degradation of acetic acid was investigated in low powers (0.1-0.4 W) and in a frequency range of 30-100 kHz. An ultrasonic transducer was used for sonication. The results showed that there was an optimum frequency at 60 kHz for direct sonication of acetic acid and degradation rate increased up to a power of 0.2 W and then it decreased. Sonochemistry is associated with the bubble of cavitation which depends on the sound pressure field and nature of molecule. Therefore, the frequency and intensity have to be optimized for the minimization of energy requirement during waste water treatment with ultrasound.     

Influence of ultrasonic frequency on multibubble sonoluminescence

Computer simulations of bubble oscillations are performed under conditions of multibubble sonoluminescence (MBSL) in water for various ultrasonic frequencies. The range of the ambient bubble radius for sonoluminescing bubbles narrows as the ultrasonic frequency increases; at 20 kHz it is 0.1-100 microm while at 1 MHz it is 0.1-3 microm. At 1 MHz, any sonoluminescing bubble disintegrates into a mass of smaller bubbles in a few or a few tens of acoustic cycles, while at 20 kHz and 140 kHz some sonoluminescing bubbles are shape stable. The mechanism of the light emission also depends on the ultrasonic frequency. As the ultrasonic frequency increases, the amount of water vapor trapped inside bubbles at the collapse decreases. As a result, MBSL originates mainly in plasma emissions at 1 MHz while it originates in chemiluminescence of OH radicals and plasma emissions at 20 kHz.     

Oxygen-induced concurrent ultrasonic degradation of volatile and non-volatile aromatic compounds

Acoustic cavitation, induced by ultrasound, can be used to eliminate organic pollutants from water. This type of ultrasonic treatment of polluted water can be grouped with those generally referred to as advanced oxidative processes since it involves hydroxyl radicals. In this case these highly active species are generated from the dissociation of water and oxygen dissociation caused by cavitation bubble collapse. The cavitation induced degradation rates of organic compounds in water are mainly linked to their vapor pressure and solubility and here we will further explore these links by examining the degradation of a mixture of two materials with different physical properties, chlorobenzene and 4-chlorophenol. The results obtained when a dilute solution of a mixture of these compounds saturated with argon is subjected to sonication at 300 kHz, parallels previous observations achieved in an aerated aqueous medium at 500 kHz. The two compounds exhibit sequential degradation with the more volatile chlorobenzene entering the cavitation bubble and being destroyed first. The 4-chlorophenol degradation occurs subsequently only when the chlorobenzene has been completely destroyed. The two compounds exhibit different behavior when sonicated in water saturated with oxygen. Under these conditions the two compounds are degraded simultaneously, a remarkable result for which two explanations can be proposed, both of which are based on the formation of additional OH radical species: The ability to produce conditions for the simultaneous elimination of two organic compounds by the use of oxygen is of great importance in the developing field of ultrasonic water treatment.     

Application of ultrasonic irradiation for the degradation of chemical contaminants in water

The sonochemical degradation of a variety of chemical contaminants in aqueous solution has been investigated. Substrates such as chlorinated hydrocarbons, pesticides, phenols, explosives such as TNT, and esters are transformed into short-chain organic acids, CO₂, and inorganic ions as the final products. Time scales of treatment in simple batch reactors over the frequency range of 20 to 500 kHz are reported to range from minutes to hours for complete degradation. Ultrasonic irradiation appears to be an effective method for the rapid destruction of organic contaminants in water because of localized high concentrations of oxidizing species such as hydroxyl radical and hydrogen peroxide in solution, high localized temperatures and pressures, and the formation of transient supercritical water.

The degradation of chemical compounds by acoustic cavitation is shown to involve three distinct pathways: 1) oxidation by hydroxyl radicals, 2) pyrolytic decomposition and 3) supercritical water oxidation. Detailed reaction mechanisms for the degradation of p-nitrophenol, carbon tetrachloride, parathion, p-nitrophenyl acetate and trinitrotoluene are presented.     

Effect of irradiation distance on degradation of phenol using indirect ultrasonic irradiation method

Ultrasound is used as degradation of hazardous organic compounds. In this study, indirect ultrasonic irradiation method was applied to the degradation process of phenol, the model hazardous organic compound, and the effects of irradiation distance on radical generation and ultrasonic power were investigated. The chemical effect estimated by KI oxidation dosimetry and ultrasonic power measured by calorimetry fluctuated for the irradiation distance, and there was a relationship between the period of the fluctuation of ultrasonic effect and the wavelength of ultrasound. The degradation of phenol was considered to progress in the zero-order kinetics, before the decomposition conversion was less than 25%. Therefore, the simple kinetic model on degradation of phenol was proposed, and there was a linear relation in the degradation rate constant of phenol and the ultrasonic power inside the reactor. In addition, the kinetic model proposed in this study was applied to the former study. There was a linear relation in the degradation rate constant of phenol and ultrasonic energy in the range of frequency of 20-30 kHz in spite of the difference of equipment and sample volume. On the other hand, the degradation rate constant in the range of frequency of 200-800 kHz was much larger than that of 20-30 kHz in the same ultrasonic energy, and this behaviour was agreed with the former investigation about the dependence of ultrasonic frequency on chemical effect.     

A new development of dyestuffs degradation system using ultrasound

Dyestuffs are often present in industrial wastewaters and can consist of hazardous substances which have a serious impact on the environment and personal health. This report describes a system developed to degrade these substances using sonochemical reactions. Ultrasonic frequencies of 118, 224, 404 and 651 kHz and power input values of 11.4, 29.0 and 41.5 W were tested on Rhodamine B and Orange II dyestuff solutions in order to find the best degradation conditions. The ultrasonic irradiation of air-saturated solutions produces free radicals that combine and generates hydrogen peroxide, and compared to the production of hydrogen peroxide when irradiating water, a decrease was found during the irradiation to dyestuff solutions, indicating that some of the free radicals were consumed in the dyestuffs degradation process. The effects of the ultrasonic irradiation conditions on the pH, nitric and nitrous acid formations as well as the total organic carbon value (TOC) were also investigated. For the ultrasonic frequencies of 224, 404 and 651 kHz, the degradation rates were very similar, however, the 118 kHz system presented a degradation rate of about one-third that of the higher frequencies for both dyestuffs. The Rhodamine B solutions were decolorized within 2 h of ultrasonic irradiation for all systems with the exception of the 118 kHz one. For Orange II, except for the 118 kHz system, all solutions were decolorized within 4 h of ultrasonic irradiation. All reactions were carried out at 25 degrees C and the total ultrasonic irradiation time was 10 h.     

Sonolysis of chlorobenzene in Fenton-type aqueous systems

The influence of ultrasounds (200 kHz frequency) on the decomposition of chlorobenzene (CB) in a water solution (around 100 ppm concentration) containing iron or palladium sulfates was investigated. The intermediates of the sonolysis were identified, thus allowing a deeper insight into the degradation mechanism. It was established that CB degradation starts by pyrolysis inside the cavitation bubbles. The initial sonolysis product is benzene, formed in a reaction occurring outside the cavitation from phenyl radicals and the hydrogen atoms sonolytically generated from the water. Polyphenols as products of the CB sonochemical degradation are reported for the first time. The palladium salt was found to be a useful and sensitive indicator for differentiating the sites and mechanisms of the product formation. An alternative mechanism for the CB sonolysis is advanced, explaining the formation of phenols, polyphenols, chlorophenols and benzene.     

A fundamental study on the degradation of paracetamol under single- and dual-frequency ultrasound

The degradation of paracetamol, a widely found emerging pharmaceutical contaminant, was investigated under a wide range of single-frequency and dual-frequency ultrasonic irradiations. For single-frequency ultrasonic irradiation, plate transducers of 22, 98, 200, 300, 400, 500, 760, 850, 1000, and 2000 kHz were employed and for dual-frequency ultrasonic irradiation, the plate transducers were coupled with a 20 kHz ultrasonic horn in opposing configuration. The sonochemical activity was quantified using two dosimetry methods to measure the yield of HO• and H₂O₂ separately, as well as sonochemiluminescence measurement. Moreover, the severity of the bubble collapses as well as the spatial and size distribution of the cavitation bubbles were evaluated via sonoluminescence measurement. The paracetamol degradation rate was maximised at 850 kHz, in both single and dual-frequency ultrasonic irradiation. A synergistic index higher than 1 was observed for all degrading frequencies (200 – 1000 kHz) under dual-frequency ultrasound irradiation, showing the capability of dual-frequency system for enhancing pollutant degradation. A comparison of the results of degradation, dosimetry, and sonoluminescence intensity measurement revealed the stronger dependency of the degradation on the yield of HO• for both single and dual-frequency systems, which confirms degradation by HO• as the main removal mechanism. However, an enhanced degradation for frequencies higher than 500 kHz was observed despite a lower HO• yield, which could be attributed to the improved mass transfer of hydrophilic compounds at higher frequencies. The sonoluminescence intensity measurements showed that applying dual-frequency ultrasonic irradiation for 200 and 400 kHz made the bubbles larger and less uniform in size, with a portion of which not contributing to the yield of reactive oxidant species, whereas for the rest of the frequencies, dual-frequency ultrasound irradiation made the cavitation bubbles smaller and more uniform, resulting in a linear correlation between the overall sonoluminescence intensity and the yield of reactive oxidant species.     

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.     

Degradation of aqueous solution of potassium iodide and sodium cyanide in the presence of carbon tetrachloride

The degradation of potassium iodide, carbon tetrachloride and sodium cyanide has been studied using an ultrasounic probe of 20 kHz frequency. In the case of potassium iodide and sodium cyanide, the rate of degradation was much higher in presence of CCl4. The location of the ultrasonic horn showed a significant effect in the degradation of CCl4.     

Acoustic noise spectra under hydrothermal conditions

Acoustic noise spectra were studied for the first time in overheated water using sonohydrothermal reactor operating at 20 kHz ultrasound in the temperature range from 25 to 200 °C at the autogenic pressure of 1–14 bar. The obtained results highlighted a dominating role of stable cavitation during ultrasonic treatment of hot water. Heating of sonicated water results in the formation of large number of nonlinearly oscillating bubbles synchronous with the driving frequency. At 200 °C, the acoustic spectra also display strong subharmonic and multiple ultraharmonic bands. Moreover, cavitation bubbles formed at 200 °C exhibit chaotic and random motions. It has been shown that the addition of TiO₂ nanoparticles to hydrothermal water heated at 200 °C allows to eliminate subharmonic/ultraharmonic bands and stochastic oscillations as well. This effect was assigned to Pickering-like bubble stabilization due to the particle accumulation at the bubble surface.     

Sonochemical degradation of phenol in water: a comparison of classical equipment with a new cylindrical reactor

Cavitation due to ultrasonic waves produces highly reactive oxidising species in water. As a result, it can be used to oxidise organic pollutants such as aromatic compounds in dilute aqueous solutions. Recent studies have demonstrated that reactors operating in the high frequency range (e.g. 500 kHz) are more efficient than reactors working at lower frequency (20 kHz) for the destruction of these kinds of contaminants. Our study describes the degradation of phenol with the help of a cylindrical ultrasonic apparatus that operates at 35 kHz (Sonitube-SODEVA). To date, the use of this type of reactor has not been reported. The reaction rates thus obtained were compared to those obtained at the same ultrasonic power (50 W) with more classical devices operating at 20 and 500 kHz. The general result is that in aqueous solution, the rate of phenol destruction is higher at 500 kHz than at 35 or 20 kHz. Addition of hydrogen peroxide and copper sulphate to the medium provides a different oxidative system that proceeds more efficiently at 35 kHz; the time of destruction was about one-third of the time needed at 500 kHz. It was also observed that the intermediate organic compounds are eliminated much faster at 35 kHz in comparison with the two frequencies. The observation of such different behaviour is not necessarily a pure frequency effect, but can be due to a response to other parameters such as the acoustic field and intensity.     

流体控制方程的超声空化泡动力学模拟*

本文基于流体动力学控制方程和VOF模型,在FLUENT 14.5软件环境下对超声空化泡进行数值模拟。首先研究了超声空化泡一个周期内的形态变化,并且利用空化泡形态变化的最大面积、最小面积、膨胀时间、收缩时间等数值结果分析超声参数对空化效果的影响。同时探究了双频超声作用下空化泡运动的变化,计算结果表明:在其他条件相同的情况下,在1~5MPa范围内,超声声压幅值为3MPa时空化效果最好;当超声频率大于20kHz时,空化效果随着超声频率的增大而降低。对于频率相同的双频超声,较声压幅值为其两倍的单频超声有更好的空化效果;对于频率不同的双频超声,空化效果受到频率差的影响。     

Effect of additives on ultrasonic degradation of phenol

Sonication for phenol degradation has proved to be an attractive process over the years at least on a laboratory scale but the rates of phenol degradation under sonication have always been quite low. The present work investigates the use of simple additives such as salt and carbon tetrachloride as process intensifying parameters with an aim of reduction in the treatment times and hence the cost of operation. The intermediates formed in the degradation process have been analyzed and it has been observed that these intermediates degrade faster as compared to phenol. A hybrid technique of ozonation coupled with cavitation has also been investigated with an objective of finding the optimum conditions for the combination of ozonation and cavitation for synergistic effects. Analysis of the intermediates for the combination treatment scheme also indicates that the intermediates (hydroquinone, catechol, resorcinol, maleic acid, acetic acid, oxalic acid, formic acid, etc.) are more biodegradable prompting a possible combination of cavitation with aerobic oxidation for large scale treatment of phenol containing waste.     

Generation and control of acoustic cavitation structure

The generation and control of acoustic cavitation structure are a prerequisite for application of cavitation in the field of ultrasonic sonochemistry and ultrasonic cleaning. The generation and control of several typical acoustic cavitation structures (conical bubble structure, smoker, acoustic Lichtenberg figure, tailing bubble structure, jet-induced bubble structures) in a 20–50 kHz ultrasonic field are investigated. Cavitation bubbles tend to move along the direction of pressure drop in the region in front of radiating surface, which are the premise and the foundation of some strong acoustic cavitation structure formation. The nuclei source of above-mentioned acoustic cavitation structures is analyzed. The relationship and mutual transformation of these acoustic cavitation structures are discussed.     

Piezo/sono-catalytic activity of ZnO micro/nanoparticles for ROS generation as function of ultrasound frequencies and dissolved gases

We report an accurate study on sonocatalytic properties of different ZnO micro and nanoparticles to enhance OH radical production activated by cavitation. In order to investigate some of the still unsolved aspects related to the piezocatalytic effect, the degradation of Methylene Blue and quantification of radicals production have been evaluated as function of different ultrasonic frequencies (20 kHz and 858 kHz) and dissolved gases (Ar, N₂ and air). The results shown that at low frequency the catalytic effect of ZnO particles is well evident and influenced by particle dimension while at high frequency a reduction of the degradation efficiency have been observed using larger particles. An increase of radical production have been observed for all ZnO particles tested while the different saturating gases have poor influence. In both ultrasonic set-up the ZnO nanoparticles resulted the most efficient on MB degradation revealing that the enhanced radical production may arise more from bubbles collapse on particles surface than the discharge mechanism activate by mechanical stress on piezoelectric particles. An interpretation of these effects and a possible mechanism which rules the sonocatalytic activity of ZnO will be proposed and discussed.     

Designing and characterizing a multi-stepped ultrasonic horn for enhanced sonochemical performance

The commonly used ultrasonic horn generates localized cavitation below its converging tip resulting in a dense bubble cloud near the tip and limiting diffusion of reactive components into the bubble cloud or reactive radicals out of the bubble cloud. To improve contact between reactive components, a novel ultrasonic horn design was developed based on the principles of the dynamic wave equation. The horn, driven at 20 kHz, has a multi-stepped design with a cone-shaped tip increasing the energy-emitting surface areas and creating multiple reactive zones. Through different physical and chemical experiments, performance of the horn was compared to a typical horn driven at 20 kHz. Hydrophone measurements showed high acoustic pressure areas around the horn neck and tip. Sonochemiluminescence experiments verified multiple cavitation zones consistent with hydrophone readings. Calorimetry and dosimetry results demonstrated a higher energy efficiency (31.3%) and a larger hydroxyl radical formation rate constant (0.36 μM min⁻¹) compared to typical horns. In addition, the new horn degraded naphthalene faster than the typical horn tested. The characterization results demonstrate that the multi-stepped horn configuration has the potential to improve the performance of ultrasound as an advanced oxidation technology by increasing the cavitation zone in the solution.     

Physical features of ultrasound assisted enzymatic degradation of recalcitrant organic pollutants

This work has attempted to provide answer to the interaction of sonolysis and enzymatic treatment on degradation of recalcitrant dyes in a combined treatment. The model system comprises of two dyes, acid red and malachite green as model pollutants, along with horseradish peroxidase as a model enzyme and ultrasound of 20 kHz frequency. A dual approach of coupling experimental results with simulations of cavitation bubble dynamics has been adopted. Utilization of oxidation potential of horseradish peroxidase has been found to be a function of convection level in the medium. Cavitation phenomenon is found to have an adverse effect on enzyme action due to generation of high amplitude shock waves, which denature the enzyme. Degradation of dye at high static pressure increases due to absence of cavitation and high energy interaction (or collisions) between enzyme and dye molecules, which are beneficial towards enzymatic oxidation of the latter. High intensity convection generated by ultrasound also obviates need for an external shielding agent such as PEG that prevents attachment of the phenoxy radicals to enzyme that blocks the active sites of the enzyme.     

Flow effects on phenol degradation and sonoluminescence at different ultrasonic frequencies

Current literature shows a direct correlation between the sonochemical (SC) process of iodide oxidation and the degradation of phenol solution. This implies phenol degradation occurs primarily via oxidisation at the bubble surface. There is no work at present which considers the effect of fluid flow on the degradation process. In this work, parametric analysis of the degradation of 0.1 mM phenol solution and iodide dosimetry under flow conditions was undertaken to determine the effect of flow. Frequencies of 44, 300 and 1000 kHz and flow rates of 0, 24, 228 and 626 mL/min were applied with variation of power input, air concentration, and surface stabilisation. Phenol degradation was analysed using the 4-aminoantipyrine (4-AAP) method, and sonoluminescence (SL) images were evaluated for 0.1, 20 and 60 mM phenol solutions. Flow, at all frequencies under certain conditions, could augment phenol degradation. At 300 kHz there was excellent correlation between phenol degradation and dosimetry indicating a SC process, here flow acted to increase bubble transience, fragmentation and radical transfer to solution. At 300 kHz, although oxidation is the primary phenol degradation mechanism, it is limited, attributed to degradation intermediates which reduce OH radical availability and bubble collapse intensity. For 44 and 1000 kHz there was poor correlation between the two SC processes. At 44 kHz (0.01 mM), there was little to suggest high levels of intermediate production, therefore it was theorised that under more transient bubble conditions additional pyrolytic degradation occurs inside the bubbles via diffusion/nanodroplet injection mechanisms. At 1000 kHz, phenol degradation was maximised above all other systems attributed to increased numbers of active bubbles combined with the nature of the ultrasonic field. SL quenching, by phenol, was reduced in flow systems for the 20 and 60 mM phenol solutions. Here, where the standing wave field was reinforced, and bubble localisation increased, flow and the intrinsic properties of phenol acted to reduce coalescence/clustering. Further, at these higher concentrations, and in flow conditions, the accumulation of volatile phenol degradation products inside the bubbles are likely reduced leading to an increase SL.