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.     

Wastewater treatment using hybrid treatment schemes based on cavitation and Fenton chemistry: A review

Advanced oxidation processes such as cavitation and Fenton chemistry have shown considerable promise for wastewater treatment applications due to the ease of operation and simple reactor design. In this review, hybrid methods based on cavitation coupled with Fenton process for the treatment of wastewater have been discussed. The basics of individual processes (Acoustic cavitation, Hydrodynamic cavitation, Fenton chemistry) have been discussed initially highlighting the need for combined processes. The different types of reactors used for the combined processes have been discussed with some recommendations for large scale operation. The effects of important operating parameters such as solution temperature, initial pH, initial pollutant concentration and Fenton’s reagent dosage have been discussed with guidelines for selection of optimum parameters. The optimization of power density is necessary for ultrasonic processes (US) and combined processes (US/Fenton) whereas the inlet pressure needs to be optimized in the case of Hydrodynamic cavitation (HC) based processes. An overview of different pollutants degraded under optimized conditions using HC/Fenton and US/Fenton process with comparison with individual processes have been presented. It has been observed that the main mechanism for the synergy of the combined process depends on the generation of additional hydroxyl radicals and its proper utilization for the degradation of the pollutant, which is strongly dependent on the loading of hydrogen peroxide. Overall, efficient wastewater treatment with high degree of energy efficiency can be achieved using combined process operating under optimized conditions, as compared to the individual process.     

Degradation of imidacloprid containing wastewaters using ultrasound based treatment strategies

The present work deals with application of sonochemical reactors for the treatment of imidacloprid containing wastewaters either individually or in combination with other advanced oxidation processes. Experiments have been performed using two different configurations of sonochemical reactors viz. ultrasonic horn (20 kHz frequency and rated power of 240 W) and ultrasonic bath equipped with radially vibrating horn (25 kHz frequency and 1 kW rated power). The work also investigates the effect of addition of process intensifying agents such as H₂O₂ and CuO, which can enhance the production of free radicals in the system. The combination studies with advanced oxidation process involve the advanced Fenton process and combination of ultrasound with UV based oxidation. The extent of degradation obtained using combination of US and H₂O₂ at optimum loading of H₂O₂ was found to be 92.7% whereas 96.5% degradation of imidacloprid was achieved using the combination of US and advanced Fenton process. The process involving the combination of US, UV and H₂O₂ was found to be the best treatment approach where complete degradation of imidacloprid was obtained with 79% TOC removal. It has been established that the use of cavitation in combination with different oxidation processes can be effectively used for the treatment of imidacloprid containing wastewater.     

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

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

Sonochemical effectiveness factor (eUS) in the reactors for wastewater treatment by sono-Fenton oxidation: Novel considerations

A comprehensive algorithm was recently proposed for calculation of the sonochemical effectiveness factor and wastewater treatment modeling. The presented approach implies that ultrasound is an auxiliary source of free radicals in Fenton type reactions; introduction of ultrasound represents an enhancement of pollutant degradation rates. The sonochemical effectiveness factor was introduced in kinetic models as the e_US factor (Gr?i? et al., 2012 [1]). As a substantial follow-up, this study presents novel considerations. The e_US factor was modeled as a function of employed frequency, actual cavitation-related power intensity of ultrasound and a portion of the cavitationally active zone, i.e. dimensionless active volume. The effect of temperature was disregarded in the present model considerations. Cavitationally active zone in reactors was determined based on the erosion of aluminum foil, resulting in cone-shaped space arising from transducer.In the present study, sonochemical treatment of industrial wastewater containing HCOONa as organic pollutant was performed using different equipment: ultrasonic baths (UB1, UB2 and UB3), cylindrical reactor with homogenizer (HCR) and three-frequency hexagonal cell, i.e. ultrasonic pilot reactor prototype (PP). Explored frequency range was from 20 to 120 kHz. Homogeneous and heterogeneous Fenton-type sonochemical processes, US/Fe(II)(FeSO₄,aq.)/H₂O₂ and US/Fe(II)(steel-plate)/H₂O₂, respectively, applied to industrial wastewater were investigated in terms of mineralization kinetics. Newly modeled e_US factor was introduced in corresponding kinetic models and the overall model was validated. Kinetic parameters of Fenton process were treated as independent of ultrasound, since e_US factor consists of cavitation-related phenomena responsible for the mineralization rate enhancement. In average, a 21% increase of mineralization efficiency was achieved using a single frequency, while more than 70% increase can be achieved by combining 20, 68 and 120 kHz in PP.     

Application of salicylic acid dosimetry to evaluate hydrodynamic cavitation as an advanced oxidation process

The generation of OH* radicals inside hydrodynamic cavitation bubbles was monitored using a salicylic acid dosimeter. The reaction of this scavenger with OH* produces 2,5-dihydroxybenzoic acid (2,5-DHB) and, to a lesser degree, 2,3-DHB. The former, is a specific reaction product that can be determined with a very high sensitivity using HPLC-IF. This method has been applied to study the influence of the flow-rate and the solution pH for a given cavitation chamber geometry. The salicylic dosimetry has proven especially suitable for the characteristic time scales of hydrodynamic cavitation (higher than those of ultrasonic cavitation), which usually gives rise to recombination of radicals before they can reach the liquid-phase. Working at low pH the hydrophobic salicylic acid migrates to the gas-liquid interface and reacts with the OH* radicals, increasing the trapping efficiency of the dosimeter. Hydrodynamic cavitation works as a very low frequency sonochemical reactor, and therefore its potential as an Advanced Oxidation Process might be limited to reactions at the gas-liquid interface and inner bubble (i.e. with volatiles and/or hydrophobic substances).     

Acoustic cavitation at low gas pressures in PZT-based ultrasonic systems

The generation of cavitation-free radicals through evanescent electric field and bulk-streaming was reported when micro-volumes of a liquid were subjected to 10 MHz surface acoustic waves (SAW) on a piezoelectric substrate [Rezk et al., J. Phys. Chem. Lett. 2020, 11, 4655–4661; Rezk et al., Adv. Sci. 2021, 8, 2001983]. In the current study, we have tested a similar hypothesis with PZT-based ultrasonic units (760 kHz and 2 MHz) with varying dissolved gas concentrations, by sonochemiluminescence measurement and iodide dosimetry, to correlate radical generation with dissolved gas concentrations. The dissolved gas concentration was adjusted by controlling the over-head gas pressure. Our study reveals that there is a strong correlation between sonochemical activity and dissolved gas concentration, with negligible sonochemical activity at near-vacuum conditions. We therefore conclude that radical generation is dominated by acoustic cavitation in conventional PZT-based ultrasonic reactors, regardless of the excitation frequency.     

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.     

Dosimetry in sonochemistry: the use of aqueous terephthalate ion as a fluorescence monitor

The generation of HO radicals by acoustic cavitation in water was monitored by their reaction with terephthalic acid (TA) anion to produce fluorescent hydroxyterephthalate ions using a cleaning bath (38kHz) and a probe system (20, 40 and 60 kHz) as different sources of ultrasound. When using the ultrasonic bath as a source of energy for sonochemical studies, the shape of the reaction vessel is important. In the case of HO production from water (50 cm³), reaction in a conical flask (100 cm³) produces 2.75 times more radicals than a round-bottomed flask of the same capacity. The fluorescence yield (fluorescence intensity/ultrasound dosage) obtained using the conical flask and ultrasonic bath was similar to that for a probe operating at 40 kHz on the same volume of solution. For a probe system operating at 20, 40 and 60 kHz the greatest sonochemical efficiency was attained at the highest of these frequencies (60 kHz). For the probe system the fluorescence yield is directly proportional to power input and the concentration of TA. The fluorescence yield decreases as the temperature is increased.     

Experimental study of the ultrasound attenuation in chemical reactors

Ultrasound is used in different domains, and in sonochemistry particularly, for different purposes and in various flow configurations: monophasic, two-phase and polyphasic reactors. In order to optimize and to design sonochemical reactors, it is important to describe the ultrasonic intensity space and time distribution. In addition, it is important to study the different parameters influencing the intensity profiles of the ultrasonic wave. In this work, a thermoelectric probe has been used to measure the ultrasonic intensity. This procedure has shown that the ultrasound propagation is influenced by the presence of cavitation bubbles, the flow regime and the presence of solid particles.     

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

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

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.     

Spatiotemporal measurement using L-band ESR-CT system for water sonolysis in the presence of 1-Hydroxy-2,2,5,5-tetramethyl-3-imidazoline-3-oxide

When using ESR to measure the radicals generated by ultrasound, it is necessary to extract a solution and place it in the ESR system. To avoid this process, we incorporated an ultrasonic reaction cell in an L-band ESR-CT system, producing a system that allows the detection of the concentration of radicals during ultrasonic irradiation. This system was used to measure the time and space dependences of OH radicals generated by ultrasonic irradiation. When a 10 ml aqueous solution of 1-hydroxy-2,2,5,5-tetramethyl-3-imidazoline-3-oxide (HTIO) was irradiated with ultrasound, it was found that the generation of radicals was clearly shown in a CT image after a period of 10 min. It was also found that continued irradiation resulted in an increased concentration of radicals. In addition to this system, an X-band ESR system was also used to measure the concentration of OH radicals generated, and the results of both systems were then compared. Both results are very similar, showing that the proposed system, which was realized by incorporating an ultrasonic irradiation cell in the L-band ESR-CT system, operated properly. Because this system allows the measurement of sonochemical reactions in an opaque cell or an opaque solution such as blood and industrial wastewater, it is a very useful measurement system for achieving the applying of sonochemistry to the medical engineering field.     

Shock-wave model of acoustic cavitation

Shock-wave model of liquid cavitation due to an acoustic wave was developed, showing how the primary energy of an acoustic radiator is absorbed in the cavitation region owing to the formation of spherical shock-waves inside each gas bubble. The model is based on the concept of a hypothetical spatial wave moving through the cavitation region. It permits using the classical system of Rankine-Hugoniot equations to calculate the total energy absorbed in the cavitation region. Additionally, the model makes it possible to explain some newly discovered properties of acoustic cavitation that occur at extremely high oscillatory velocities of the radiators, at which the mode of bubble oscillation changes and the bubble behavior approaches that of an empty Rayleigh cavity. Experimental verification of the proposed model was conducted using an acoustic calorimeter with a set of barbell horns. The maximum amplitude of the oscillatory velocity of the horns' radiating surfaces was 17 m/s. Static pressure in the calorimeter was varied in the range from 1 to 5 bars. The experimental data and the results of the calculations according to the proposed model were in good agreement. Simple algebraic expressions that follow from the model can be used for engineering calculations of the energy parameters of the ultrasonic radiators used in sonochemical reactors.     

声化学新发展——纳米材料的超声制备

声空化所引发的特殊的物理，化学环境为制备具有特殊性能的新型材料提供了一条重要的途径，近年来，声化学处理已成为制备纳米材料的一种十分有效的技术，文章综合介绍了超声法制备纳米材料的主要类型，其中包括超声声解法，声化学还原法，超声共沉淀法，超声微乳液法等，并着重阐述了超声的作用原理和各种方法的特点。     

Characterisation of mixing rate due to high power ultrasound

For meaningful assessment of results from laboratory and pilot plant trials, it is often necessary to know the mixing characteristics within the ultrasonic reactors. Previous workers have used conductivity measurements in an attempt to characterize the residence time distribution in ultrasonic reactors, but these results do not provide direct data on the mixing within the high power region adjacent to the ultrasonic probe tip. We overcome this difficulty through direct visualization of the mixing process within the high energy region close to the tip of the ultrasonic probe. Our analysis proceeds by determining an approximate turbulent diffusivity in a batch reactor arrangement for different values of ultrasonic energy input. For input electrical power levels between 70 and 120 W and a processing volume of 30 ml, the effective turbulent diffusivity varied from about 0.2 x 10(-3) to 0.7 x 10(-3)m(2)/s. We demonstrate that such results can be coupled to a suitable dispersion model to estimate the actual residence time distribution in flow-though arrangements when the through-put adds little additional mixing energy. Therefore, coupling the effective turbulent diffusivity identified in a batch reactor with a suitable dispersion model for the reactor offers an alternative approach to the deduction of RTD when determining the actual RTD in the high intensity zone of steady flow sonochemical reactors is problematic.     

Method for determining the chemically active zones in a high-frequency ultrasonic reactor

In order to determine the optimal geometry of the high-frequency sonoreactors to be used for degradation of organic compounds in aqueous solutions, a method based on the chemiluminescence of luminol (which reacts specifically with OH radicals) is applied to visualize the zones where the sonochemical reactions are the most active. The spatial distribution of the light intensity emitted by these reactions is measured inside the reactor with an optical fibre fitted on a photomultiplier. To determine the relative ultrasonic energy distribution, the temperature is measured at different points of the reactor with a thermocouple probe embedded in an absorbing material. The zones where the energy density is the highest were shown not to correspond with those where chemiluminescence reactions are the most efficient.     

Degradation of dichlorvos containing wastewaters using sonochemical reactors

The present work deals with application of sonochemical reactors for the degradation of dichlorvos containing wastewaters. The sonochemical reactor used in the work is a simple ultrasonic horn type operating at 20 kHz with a power rating of 270 W. The effect of different operating parameters such as operating pH, temperature and power density on the extent of degradation has been investigated initially followed by intensification studies using additives such as hydrogen peroxide, Fenton's reagent and CCl(4). It has been observed that low frequency sonochemical reactors can be effectively used for treatment of pesticide wastewaters and acidic conditions and optimum values of temperature and power dissipation favors the degradation of dichlorvos. The efficacy of sonochemical reactors can be further enhanced by using different additives at optimized loadings. Complete removal of the pesticide at the given loading has been obtained using an optimized combination of ultrasound and Fenton's chemistry. The controlling mechanism for the sonochemical degradation has been confirmed to be the free radical attack based on the studies involving radical scavengers. The novelty of the present work is clearly established as there have been no earlier studies dealing with degradation of dichlorvos pesticide using sonochemical reactors operating at low frequency which offers distinct advantage in terms of cost and the stability of the reactor.     

碟形超声变幅杆的设计

优化超声变幅杆的形状结构可有效地提高水域声场分布和空化区域,提升对水域超声空化效果。通过模拟分析发现传统超声变幅杆在水域中具有声场分布均匀性差、变幅杆端部声压高等特征,不利于声波在水域中传播。基于此,提出并优化设计了一种具有碟形结构的变幅杆,位于变幅杆的最大振幅处的碟形结构,有更大的振动位移;模拟表明其水域声场和声压均衡度显著优于传统变幅杆,铝箔空化腐蚀实验进一步证实了其水域中的声压分布均匀性。同时,实验通过铝箔的空化腐蚀、KI剂量测定及工件表面油渍去除对比了传统变幅杆和碟形变幅杆,分析表明碟形变幅杆所在水域中有较大的空化腐蚀区域,腐蚀速率明显提升,声化学反应速率提高,油渍去除程度增强,说明了设计的碟形变幅杆能够促进空化泡的产生,增加水域空化区域。     

Ultrasound and polar homogeneous reactions

The effect of ultrasound on the rates of homogeneous heterolytic reactions not switched to a free radical pathway can be explained by the perturbation of the molecular organization of or the solvation in the reacting system. A quantitative analysis of the sonochemical acceleration on the basis of the microreactor concept was carried out. It was found that (1) the Diels-Alder reaction cannot be accelerated by ultrasound except when SET or free radical processes are promoted, (2) the rectified diffusion during cavitation cannot be responsible for the acceleration of reactions, and (3) the sonochemical acceleration of polar homogeneous reactions takes place in the bulk reaction medium. This implies the presence of a 'sound-field' sonochemistry besides the 'hot-spot' sonochemistry. The occurrence of a sonochemical deceleration effect can be predicted.