Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titani... more Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titanium substrates in the presence and in the absence of an ultrasound field. The films obtained by mechanical agitation on glassy carbon are strongly improved when the electrodeposition process is carried out with the ultrasound field, providing adherent deposits free from nodules and stress, but with pores appearing occasionally. These enhanced properties were not achieved by mechanical conditions, even when optimization of temperature, current density, additives and geometrical aspects was attempted. The best practical anodes were obtained by sonoelectrodeposition using specially treated titanium as substrate, providing comparable behavior to commercial electrodes.
Advances in Water Treatment and Pollution Prevention, 2012
ABSTRACT Sonochemical oxidation is one of the advanced oxidation techniques that are widely used ... more ABSTRACT Sonochemical oxidation is one of the advanced oxidation techniques that are widely used to decompose various organic contaminants in aqueous environment. Recent studies have suggested that the use of hybrid techniques is more effective compared to individual techniques for the decomposition of organic contaminants. The combination of more than one oxidation technique overcomes the disadvantages of individual techniques. This chapter deals with the instrumentation aspects of sonochemistry on its own and its combination with photocatalysis and electrochemistry. Various experimental parameters such as ultrasound frequency, power, lab-scale and large-scale equipment used for the sonochemical oxidation of organic contaminants have been analyzed using several examples available in the literature.
The effect of ultrasonic power on the characteristics of lowfrequency ultrasound assisted electro... more The effect of ultrasonic power on the characteristics of lowfrequency ultrasound assisted electrodeposited Ni coatings from an additivefree Watts bath has been evaluated by different methods. XRD analysis showed that, while mechanical agitation favoured the electrocrystallization of Ni in the [211] direction, ultrasound promoted the electrodeposition of Ni with a [100] preferred orientation. FIBSEM images of the surface of Ni deposits not only indicated that the surface structure agreed to some extent with the XRD results, but also that ultrasound refined, to a certain extent, some of the grains of the surface of the coatings. FIBSEM images of the crosssection of some of the coatings confirmed this effect of ultrasound on the microstructure of the deposits. Such change in the microstructure of Ni, along with work hardening effect of ultrasound, resulted in an increase in the hardness of the deposits. The characteristics of the deposits depended on the ultrasonic power employed, and it was found that Ni coatings electrodeposited using an ultrasonic power of 0.124 W/cm 3 presented the higher proportion of crystals with a [100] preferred orientation, the highest degree of grain refinement in the surface and the highest microhardness values. Nevertheless, these deposits also presented visible erosion marks on the surface of the coatings due to the formation of transient bubble structures near the surface of the cathode during the electrodeposition. These erosion marks might be considered the main drawback to the use of ultrasound during the electrodeposition.
ABSTRACT The electrodeposition of multifunctional composite coatings has rapidly emerged in the l... more ABSTRACT The electrodeposition of multifunctional composite coatings has rapidly emerged in the last decade due to the enhanced mechanical properties and corrosion resistance that such composite coatings exhibit compared to electroplated single metal and alloy deposits. Many studies have indicated that the implementation of ultrasound in composite electroplating processes can bring about many benefits, not only as a tool to improve the dispersion and de-agglomeration of particles in the electroplating bath, but also to enhance the incorporation of finely dispersed and uniformly distributed particles into the metal matrix. The present paper summarizes the fundamentals of the use of ultrasound and acoustic cavitation and how it may influence the electrodeposition of composite coatings with particles by commenting on some of the most significant works on this topic presented by the scientific community in the last 10 years. This paper will review these investigations and discuss how the ultrasonic parameters may affect the dispersion of the particles in the electrolyte and its effect on the characteristics of the composite coatings, generally resulting in the enhancement of the mechanical properties and corrosion resistance of the composite coatings. In addition, this paper will review some of the issues that may arise when using ultrasound in such processes and the pros and cons of the different transducer systems available, highlighting the need for detailed information regarding the ultrasonic parameters and equipment used when utilizing sonication.
ABSTRACT In this work, we present a comparative analysis of the degradation of aqueous solutions ... more ABSTRACT In this work, we present a comparative analysis of the degradation of aqueous solutions of PCE by sonochemical, electrochemical and sonoelectrochemical treatment, pointing out the advantages and drawbacks of the different approaches, and also the synergic effects of the simultaneous applications of both energetic fields: ultrasound and electricity. Prior to these studies, not only were specific researches carried out in order to develop stable components (anodic materials) under high power ultrasound fields, but the experimental devices used during this study were also characterized. In summary, the sonochemical method presents serious deficiencies not only from an environmental point of view but also in its energetic requirements. The electrochemical method presents competitive costs and feasible technically processes (using optimized filter-press reactors), but it does not suit the environmental requirements. The sonoelectrochemical treatment has provided the best results from technical and environmental points of view but economical issues must be improved. Further research lines are suggested on the basis of the obtained results.
Chloropropiophenone ( -Cl-ppone) reduction in dimethylformamide (DMF) on platinum electrode has b... more Chloropropiophenone ( -Cl-ppone) reduction in dimethylformamide (DMF) on platinum electrode has been studied using potassium perchlorate (KClO 4 ) or tetra-n-butylammonium tetrafluoroborate ((Bu n ) 4 NBF 4 ) as background electrolyte. Voltammetric and surface sensitive Fourier transform IR spectroscopy (FTIR) experiments have been carried out and it has pointed out the presence of an inactivation process in the reduction of the -chloropropiophenone on a Pt electrode. Although the inactivation process occurs in both cases, the use of KClO 4 or (Bu n ) 4 NBF 4 presents a significant different behavior in the reduction process. The influence of same variables such as -chloropropiophenone, water or oxygen concentration have been studied and rotating electrode experiments have been made in order to conclude the best reaction conditions. The inactivation process is strongly determined by the electrode potential. The inactivation of the electrode surface is held at initial potential, including under forced convection conditions, however it disappears at high electrode potential, in contact with atmosphere and at open circuit during long or shorter time with the help of convection. The effect of a high power, low frequency ultrasonic field in the reduction process has been investigated. The intensity of the ultrasonic field and the influence of the working electrodeemitter surface gap have been studied but did not avoid the inactivation process.
Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emer... more Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emerged in the last 15 years. This paper summarizes some of the most important works on this topic presented in the past, along with the diverse numerical works that have been published since then, reviewing the state of the art from a qualitative point of view. In this sense, we illustrate and discuss some of the models recently developed by the scientific community to deal with some of the complex events that take place in a sonochemical reactor such as the vibration of the reactor walls and the nonlinear phenomena inherent to the presence of ultrasonic cavitation. In addition, we point out some of the upcoming challenges that must be addressed in order to develop a reliable tool for the proper designing of efficient sonoreactors and the scale-up of sonochemical processes. 1 Sonochemistry [1] is the area of high-energy chemistry 2 which studies chemical reactions and processes involving 3 acoustic cavitation formed by the application of an ultrasonic 4 field in a frequency range which commonly varies between 20 5 kHz and 10 MHz. It allows chemists to increase the conversion, 6 improve the yield, initiate and change the reaction pathways in 7 all sorts of biological, chemical or electrochemical processes 8 [2], becoming a prominently used technique in a wide variety of 9 research areas, including: (i) material science [3] (ii) synthetic 10 chemistry [4, 5], (iii) water remediation [6, 7], (iv) biotechno-11 logical applications [8], (v) electrochemical processes [9], (vi) 12 food technology [10], and (vii) spent nuclear fuel reprocessing 13 [11], among others. The versatility of the ultrasonic permits 14 its combination with other technologies such as photocataly-15 sis [12] or microwaves [13], proving the enormous potential of 16 Sonochemistry. 17 Despite this extensive research at laboratory scale, a limited 18 number of applications have been industrially scaled-up due to 19 two main reasons: (i) the lack of expertise in diverse areas such 20 as ultrasonics or sonochemical engineering, and (ii) the lack of 21 proper reactor designing strategies. Related to this, Sutkar and 22 Gogate have stated that understanding the cavitational activity 23 and its distribution would yield efficiently designed sonochem-24 ical reactors and systems [14], and for this purpose, theoreti-25 cal analysis of the cavitational activity distribution with proper 26 * Corresponding author: [email protected] (I. Tudela) 1 Deceased experimental validation could be used for the optimization of 27 sonochemical processes taking into account building materials, 28 geometry of the reactor and working frequency of the sono-29 chemical system. A correct understanding of the acoustic field 30 structure inside a sonochemical reactor is therefore needed to 31 proceed with its optimization and scale-up in order to design 32 efficient large scale reactors [15]. 33 The numerical simulation of the spatial distribution of the 34 acoustic pressure inside sonochemical reactors has widely 35 emerged in the last 15 years to shed new light on this issue, 36 and quite a few groups around the world have tried to model 37 the acoustic field inside sonoreactors with the aim of predict-38 ing the cavitation events within the reactor. To our knowledge, 39 the most recent review on this topic found in the literature was 40 published more than 10 years ago [16], and no exhaustive lit-41 erature revisions are usually found in most of the papers that 42 deal with the simulation of the acoustic field in a sonoreactor. 43 Therefore, the goal of the present paper is to introduce numer-44 ical methods for the development of sonochemical reactors to 45 a wider audience of scientists by summarizing the continuous 46 development of numerical methods employed by the scientific 47 community from the late 1990's until now. In this paper, basic 48 methodologies and results from many works are briefly com-49 mented, pointing out the strong and weak points in the most 50 representative cases from a qualitative point of view. And new 51 trends and future challenges on the problem are also discussed. 52 2. Numerical simulations of the acoustic field inside sono-53 chemical reactors using linear-based models 54 2.1. Basic linear-based models 55 The vast majority of the works dealing with the simulation 56 of the acoustic field inside a sonochemical reactor rely on the 57 resolution of the equations that describe the linear propagation 58 of sound in a liquid. Such equations, which are derived from the 59 linearization of the Euler equations [17], yield the well-known 60 Helmholtz equation for the linear propagation of sound waves: 61 ∇ 2 P + k 2 P = 0 (1) being P the acoustic pressure and k = (ω/c l ) the wave number, 62 where ω is the angular frequency and c l is the sound speed of 63 the liquid. The Helmholtz equation can be easily solved by 64 setting the next boundary conditions: 65 • P = 0 for 'infinitely soft' boundaries. 66 • ∇P · n = 0 for 'infinitely hard' boundaries, being n the 67 normal vector pointing outward the liquid. 68 • P = P 0 at the emitter surface of the ultrasonic transducer, 69 being P 0 the amplitude of the wave. 70 The simplicity of the Helmholtz equation and the boundary 71 conditions defined above facilitate the task of calculating the 72 linear propagation of sound in a liquid by numerical methods. 73 In the last years, the wider availability of commercial FEM (Fi-74 nite Element Methods) software packages presenting acoustic 75 modules based on the Helmholtz equation have allowed an in-76 creasing number of different research groups to employ numer-77 ical simulations as a powerful tool to design and characterize 78 sonochemical systems. 79 Among the different commercial software codes available, 80 COMSOL Multiphysics, formerly known as FEMLAB, has 81 probably been the most employed in recent years, not only be-82 cause the Helmoltz equation and the different boundary con-83 ditions are implemented, but also because it does not require 84 deep knowledge on either advanced acoustics modelling or nu-85 merical methods. With this software, Sáez et al. [18] tried to 86 characterize a 20 kHz sonochemical reactor by considering the 87 vessel boundaries as infinitely rigid walls and putting special ef-88 forts on the discretization of the domain following the rules by 89 Ihlenburg, Babûska and co-workers [19, 20]. In their work, the 90 simulations were compared with aluminum foil experiments, 91 observing in both cases a main active zone just along the axial 92 direction located at the emitter center. Their numerical results 93 also indicated a gradual decrease in the ultrasonic field activ-94 ity with the distance from the emitter, which was roughly con-95 firmed by aluminum foil experiments. Klíma et al. [21] faced 96 the optimization of a 20 kHz sonochemical reactor based on the 97 three-dimensional simulation of the acoustic pressure with the 98 same software. Despite the limitations of the linear Helmholtz 99 equation, which does not take into account non-linear wave 100 propagation and generation of transversal elastic waves, and the 1 + iωµ l /ρ l c 2 l (5) where µ l and ρ l are the viscosity and density of the liquid, re-138 spectively. These expressions were used by Sutkar et al. in 139 order to simulate the acoustic field in diverse low [28] and 140 high frequency [29] sonoreactors. A similar approach was re-141 cently followed by Xu et al. [30], although in this latter case 142 k c = (ω/c l ) − iα and Z c = ρ l c l , where α is an attenuation coeffi-143 cient analogous to used by other authors later mentioned in this 144 related to the bubble formation process [85]. In this mainly the-595 oretical latter work (all the calculations are in one-dimensional 596 domain), although they neglected physical phenomena such as 597 primary and secondary Bjerknes forces and acoustic stream-598 ing, and they defined small bubbles which acted as nuclei in 599 865 up possibility for sonochemical reactions, Chem. Eng. Sci. 54 (1999) 866 2829-2838. 867 [35] L.K. Zarembo, Acoustic streaming, in: L.D. Rozember (Ed.), High-868 intensity ultrasonic fields, Plenum Press, New York-London, 1971. 869 [36] J.-L. Laborde, C. Bouyer, J.-P. Caltagirone, A. Gérard, Acoustic cavita-870 tion field prediction at low and high frequency ultrasounds, Ultrasonics 871 36 (1998) 581-587. 872 [37] L. van Wijngaarden, Linear and nonlinear dispersion of pressure pulses in 873 liquid bubble mixtures, in: R.D. Cooper, S.W. Doroff (Eds.), 6th Sympo-874
The effects of operational parameters upon the hydroxyl radical generation of sonochemical reacto... more The effects of operational parameters upon the hydroxyl radical generation of sonochemical reactors are critical to optimize this technology for wastewater treatment purposes. Ultrasonic wave characteristics are usually considered as the main parameter to be taken into account. Nevertheless, it is the interaction of these waves with the liquid medium and the reactor what really affects the process. Therefore, the characterization of sonochemical reactors should be based on the effective pressure distribution in the reactor, which not only includes the wave characteristics but also the propagation and reflection of these ultrasonic oscillations. The pressure field can be characterized using different parameters, such as maximum pressure amplitude or volumetric integration of pressure in the reactor. This study intends to find a correlation between such calculated pressure-distributionrelated parameters and experimental measurements of hydroxyl radicals in the process. Both experiments and calculations are run varying the tip-bottom distance (keeping the rest of parameters constant), creating different reflection effects with the reactor walls and therefore different pressure distributions across the reactor. The hydroxyl radical measurements are performed with salicylic acid dosimetry, applying a specific developed method for biphasic cavitating systems. On the other hand, the pressure distribution was calculated simulating the different configurations with the computational tool COMSOL.
As one of several types of pollutants in water, chlorinated compounds have been routinely subject... more As one of several types of pollutants in water, chlorinated compounds have been routinely subjected to sonochemical analysis to check the environmental applications of this technology. In this review, an extensive study of the influence of the initial concentration, ultrasonic intensity and frequency on the kinetics, degradation efficiency and mechanism has been analyzed. The sonochemical degradation follows a radical mechanism which yields a very wide range of chlorinated compounds in very low concentrations. Special attention has been paid to the mass balance comparing the results from several analytical techniques. As a conclusion, sonochemical degradation alone is not an efficient treatment to reduce the organic pollutant level in waste water.
The sonoelectrochemical treatment of aqueous solutions of trichloroacetic acid (TCAA) has been sc... more The sonoelectrochemical treatment of aqueous solutions of trichloroacetic acid (TCAA) has been scaledup from the voltammetric analysis to pre-pilot stage. The degradation in absence of ultrasound field has yield to a poor performance which has been improved in presence of ultrasound. The sonovoltametry study has provided the range of potentials and/or current densities to be used with the lowest current efficiency penalty. Sonoelectrolyses at batch scale (carried out with a horn-transducer 24 kHz positioned at about 3 cm from the surface of the electrode) achieved little improvement in the degradation. However, when a specifically designed sonoelectrochemical reactor (not optimized) was used during the scale-up, the presence of ultrasound field provided better results (fractional conversion 97%, degradation efficiency 26%, selectivity 0.92 and current efficiency 8%) at lower ultrasonic intensities and volumetric flow.
Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titani... more Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titanium substrates in the presence and in the absence of an ultrasound field. The films obtained by mechanical agitation on glassy carbon are strongly improved when the electrodeposition process is carried out with the ultrasound field, providing adherent deposits free from nodules and stress, but with pores appearing occasionally. These enhanced properties were not achieved by mechanical conditions, even when optimization of temperature, current density, additives and geometrical aspects was attempted. The best practical anodes were obtained by sonoelectrodeposition using specially treated titanium as substrate, providing comparable behavior to commercial electrodes.
The chemical effects of acoustic cavitation are obtained in sono-reactors built-up from a vessel ... more The chemical effects of acoustic cavitation are obtained in sono-reactors built-up from a vessel and an ultrasonic source. In this paper, simulations of an existing sono-reactor are carried out, using a linear acoustics model, accounting for the vibrations of the solid walls. The available frequency range of the generator (19 kHz-21 kHz) is systematically scanned. Global quantities are plotted as a function of frequency in order to obtain response curves, exhibiting several resonance peaks. The attenuation coefficient of the wave is taken as a variable parameter, in absence of the precise knowledge of the bubble size distribution, and its influence is studied. The concepts of acoustic energy, intensity and active power are recalled, along with the general balance equation for acoustic energy. The latter is used as a convergence check of the simulations. Finally, it is shown that the interface between the liquid and the solid walls cannot be correctly represented by the simple approximations of either infinitely soft, or infinitely hard boundaries. Moreover, the liquid-solid coupling allows the cooling jacket to receive a noticeable part of the input power, although it is not in direct contact with the sonotrode. It may therefore undergo cavitation and this feature opens the perspective to design sono-reactors which avoid direct contact between the working liquid and the sonotrode.
A preliminary study of the 20 kHz sonoelectrochemical degradation of perchloroethylene in aqueous... more A preliminary study of the 20 kHz sonoelectrochemical degradation of perchloroethylene in aqueous sodium sulfate has been carried out using controlled current density degradation sonoelectrolyses in batch mode. An important improvement in the viability of the sonochemical process is achieved when the electrochemistry is implemented, but the improvement of the electrochemical treatment is lower when the 20 kHz ultrasound field is simultaneously used. A fractional conversion of 100% and degradation efficiency around 55% are obtained independently of the ultrasound power used. The current efficiency is also enhanced compared to the electrochemical treatment and a higher speciation is also detected; the main volatile compounds produced in the electrochemical and sonochemical treatment, trichloroethylene and dichloroethylene, are not only totally degraded, but also at shorter times than in the sonochemical or electrochemical treatments.
Fluid-solid reactions are common chemical processes that take place in a heterogeneous media. The... more Fluid-solid reactions are common chemical processes that take place in a heterogeneous media. These processes are of considerable importance, especially in chemical and metallurgical industries (1, 2). Fluid-solid processes in which the solid particle size does not effectively change are the roasting or oxidation of sulfide ores to yield the metal oxides and the nitrogenation of calcium carbide to obtain cyanamide, among others. Other fluid-solid processes involve the variation of particle dimensions such as combustion of solid fuels, gasification of coal or oil shale, and the production of sodium thiosulfate from sulfur and sodium sulfite.
Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titani... more Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titanium substrates in the presence and in the absence of an ultrasound field. The films obtained by mechanical agitation on glassy carbon are strongly improved when the electrodeposition process is carried out with the ultrasound field, providing adherent deposits free from nodules and stress, but with pores appearing occasionally. These enhanced properties were not achieved by mechanical conditions, even when optimization of temperature, current density, additives and geometrical aspects was attempted. The best practical anodes were obtained by sonoelectrodeposition using specially treated titanium as substrate, providing comparable behavior to commercial electrodes.
Advances in Water Treatment and Pollution Prevention, 2012
ABSTRACT Sonochemical oxidation is one of the advanced oxidation techniques that are widely used ... more ABSTRACT Sonochemical oxidation is one of the advanced oxidation techniques that are widely used to decompose various organic contaminants in aqueous environment. Recent studies have suggested that the use of hybrid techniques is more effective compared to individual techniques for the decomposition of organic contaminants. The combination of more than one oxidation technique overcomes the disadvantages of individual techniques. This chapter deals with the instrumentation aspects of sonochemistry on its own and its combination with photocatalysis and electrochemistry. Various experimental parameters such as ultrasound frequency, power, lab-scale and large-scale equipment used for the sonochemical oxidation of organic contaminants have been analyzed using several examples available in the literature.
The effect of ultrasonic power on the characteristics of lowfrequency ultrasound assisted electro... more The effect of ultrasonic power on the characteristics of lowfrequency ultrasound assisted electrodeposited Ni coatings from an additivefree Watts bath has been evaluated by different methods. XRD analysis showed that, while mechanical agitation favoured the electrocrystallization of Ni in the [211] direction, ultrasound promoted the electrodeposition of Ni with a [100] preferred orientation. FIBSEM images of the surface of Ni deposits not only indicated that the surface structure agreed to some extent with the XRD results, but also that ultrasound refined, to a certain extent, some of the grains of the surface of the coatings. FIBSEM images of the crosssection of some of the coatings confirmed this effect of ultrasound on the microstructure of the deposits. Such change in the microstructure of Ni, along with work hardening effect of ultrasound, resulted in an increase in the hardness of the deposits. The characteristics of the deposits depended on the ultrasonic power employed, and it was found that Ni coatings electrodeposited using an ultrasonic power of 0.124 W/cm 3 presented the higher proportion of crystals with a [100] preferred orientation, the highest degree of grain refinement in the surface and the highest microhardness values. Nevertheless, these deposits also presented visible erosion marks on the surface of the coatings due to the formation of transient bubble structures near the surface of the cathode during the electrodeposition. These erosion marks might be considered the main drawback to the use of ultrasound during the electrodeposition.
ABSTRACT The electrodeposition of multifunctional composite coatings has rapidly emerged in the l... more ABSTRACT The electrodeposition of multifunctional composite coatings has rapidly emerged in the last decade due to the enhanced mechanical properties and corrosion resistance that such composite coatings exhibit compared to electroplated single metal and alloy deposits. Many studies have indicated that the implementation of ultrasound in composite electroplating processes can bring about many benefits, not only as a tool to improve the dispersion and de-agglomeration of particles in the electroplating bath, but also to enhance the incorporation of finely dispersed and uniformly distributed particles into the metal matrix. The present paper summarizes the fundamentals of the use of ultrasound and acoustic cavitation and how it may influence the electrodeposition of composite coatings with particles by commenting on some of the most significant works on this topic presented by the scientific community in the last 10 years. This paper will review these investigations and discuss how the ultrasonic parameters may affect the dispersion of the particles in the electrolyte and its effect on the characteristics of the composite coatings, generally resulting in the enhancement of the mechanical properties and corrosion resistance of the composite coatings. In addition, this paper will review some of the issues that may arise when using ultrasound in such processes and the pros and cons of the different transducer systems available, highlighting the need for detailed information regarding the ultrasonic parameters and equipment used when utilizing sonication.
ABSTRACT In this work, we present a comparative analysis of the degradation of aqueous solutions ... more ABSTRACT In this work, we present a comparative analysis of the degradation of aqueous solutions of PCE by sonochemical, electrochemical and sonoelectrochemical treatment, pointing out the advantages and drawbacks of the different approaches, and also the synergic effects of the simultaneous applications of both energetic fields: ultrasound and electricity. Prior to these studies, not only were specific researches carried out in order to develop stable components (anodic materials) under high power ultrasound fields, but the experimental devices used during this study were also characterized. In summary, the sonochemical method presents serious deficiencies not only from an environmental point of view but also in its energetic requirements. The electrochemical method presents competitive costs and feasible technically processes (using optimized filter-press reactors), but it does not suit the environmental requirements. The sonoelectrochemical treatment has provided the best results from technical and environmental points of view but economical issues must be improved. Further research lines are suggested on the basis of the obtained results.
Chloropropiophenone ( -Cl-ppone) reduction in dimethylformamide (DMF) on platinum electrode has b... more Chloropropiophenone ( -Cl-ppone) reduction in dimethylformamide (DMF) on platinum electrode has been studied using potassium perchlorate (KClO 4 ) or tetra-n-butylammonium tetrafluoroborate ((Bu n ) 4 NBF 4 ) as background electrolyte. Voltammetric and surface sensitive Fourier transform IR spectroscopy (FTIR) experiments have been carried out and it has pointed out the presence of an inactivation process in the reduction of the -chloropropiophenone on a Pt electrode. Although the inactivation process occurs in both cases, the use of KClO 4 or (Bu n ) 4 NBF 4 presents a significant different behavior in the reduction process. The influence of same variables such as -chloropropiophenone, water or oxygen concentration have been studied and rotating electrode experiments have been made in order to conclude the best reaction conditions. The inactivation process is strongly determined by the electrode potential. The inactivation of the electrode surface is held at initial potential, including under forced convection conditions, however it disappears at high electrode potential, in contact with atmosphere and at open circuit during long or shorter time with the help of convection. The effect of a high power, low frequency ultrasonic field in the reduction process has been investigated. The intensity of the ultrasonic field and the influence of the working electrodeemitter surface gap have been studied but did not avoid the inactivation process.
Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emer... more Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emerged in the last 15 years. This paper summarizes some of the most important works on this topic presented in the past, along with the diverse numerical works that have been published since then, reviewing the state of the art from a qualitative point of view. In this sense, we illustrate and discuss some of the models recently developed by the scientific community to deal with some of the complex events that take place in a sonochemical reactor such as the vibration of the reactor walls and the nonlinear phenomena inherent to the presence of ultrasonic cavitation. In addition, we point out some of the upcoming challenges that must be addressed in order to develop a reliable tool for the proper designing of efficient sonoreactors and the scale-up of sonochemical processes. 1 Sonochemistry [1] is the area of high-energy chemistry 2 which studies chemical reactions and processes involving 3 acoustic cavitation formed by the application of an ultrasonic 4 field in a frequency range which commonly varies between 20 5 kHz and 10 MHz. It allows chemists to increase the conversion, 6 improve the yield, initiate and change the reaction pathways in 7 all sorts of biological, chemical or electrochemical processes 8 [2], becoming a prominently used technique in a wide variety of 9 research areas, including: (i) material science [3] (ii) synthetic 10 chemistry [4, 5], (iii) water remediation [6, 7], (iv) biotechno-11 logical applications [8], (v) electrochemical processes [9], (vi) 12 food technology [10], and (vii) spent nuclear fuel reprocessing 13 [11], among others. The versatility of the ultrasonic permits 14 its combination with other technologies such as photocataly-15 sis [12] or microwaves [13], proving the enormous potential of 16 Sonochemistry. 17 Despite this extensive research at laboratory scale, a limited 18 number of applications have been industrially scaled-up due to 19 two main reasons: (i) the lack of expertise in diverse areas such 20 as ultrasonics or sonochemical engineering, and (ii) the lack of 21 proper reactor designing strategies. Related to this, Sutkar and 22 Gogate have stated that understanding the cavitational activity 23 and its distribution would yield efficiently designed sonochem-24 ical reactors and systems [14], and for this purpose, theoreti-25 cal analysis of the cavitational activity distribution with proper 26 * Corresponding author: [email protected] (I. Tudela) 1 Deceased experimental validation could be used for the optimization of 27 sonochemical processes taking into account building materials, 28 geometry of the reactor and working frequency of the sono-29 chemical system. A correct understanding of the acoustic field 30 structure inside a sonochemical reactor is therefore needed to 31 proceed with its optimization and scale-up in order to design 32 efficient large scale reactors [15]. 33 The numerical simulation of the spatial distribution of the 34 acoustic pressure inside sonochemical reactors has widely 35 emerged in the last 15 years to shed new light on this issue, 36 and quite a few groups around the world have tried to model 37 the acoustic field inside sonoreactors with the aim of predict-38 ing the cavitation events within the reactor. To our knowledge, 39 the most recent review on this topic found in the literature was 40 published more than 10 years ago [16], and no exhaustive lit-41 erature revisions are usually found in most of the papers that 42 deal with the simulation of the acoustic field in a sonoreactor. 43 Therefore, the goal of the present paper is to introduce numer-44 ical methods for the development of sonochemical reactors to 45 a wider audience of scientists by summarizing the continuous 46 development of numerical methods employed by the scientific 47 community from the late 1990's until now. In this paper, basic 48 methodologies and results from many works are briefly com-49 mented, pointing out the strong and weak points in the most 50 representative cases from a qualitative point of view. And new 51 trends and future challenges on the problem are also discussed. 52 2. Numerical simulations of the acoustic field inside sono-53 chemical reactors using linear-based models 54 2.1. Basic linear-based models 55 The vast majority of the works dealing with the simulation 56 of the acoustic field inside a sonochemical reactor rely on the 57 resolution of the equations that describe the linear propagation 58 of sound in a liquid. Such equations, which are derived from the 59 linearization of the Euler equations [17], yield the well-known 60 Helmholtz equation for the linear propagation of sound waves: 61 ∇ 2 P + k 2 P = 0 (1) being P the acoustic pressure and k = (ω/c l ) the wave number, 62 where ω is the angular frequency and c l is the sound speed of 63 the liquid. The Helmholtz equation can be easily solved by 64 setting the next boundary conditions: 65 • P = 0 for 'infinitely soft' boundaries. 66 • ∇P · n = 0 for 'infinitely hard' boundaries, being n the 67 normal vector pointing outward the liquid. 68 • P = P 0 at the emitter surface of the ultrasonic transducer, 69 being P 0 the amplitude of the wave. 70 The simplicity of the Helmholtz equation and the boundary 71 conditions defined above facilitate the task of calculating the 72 linear propagation of sound in a liquid by numerical methods. 73 In the last years, the wider availability of commercial FEM (Fi-74 nite Element Methods) software packages presenting acoustic 75 modules based on the Helmholtz equation have allowed an in-76 creasing number of different research groups to employ numer-77 ical simulations as a powerful tool to design and characterize 78 sonochemical systems. 79 Among the different commercial software codes available, 80 COMSOL Multiphysics, formerly known as FEMLAB, has 81 probably been the most employed in recent years, not only be-82 cause the Helmoltz equation and the different boundary con-83 ditions are implemented, but also because it does not require 84 deep knowledge on either advanced acoustics modelling or nu-85 merical methods. With this software, Sáez et al. [18] tried to 86 characterize a 20 kHz sonochemical reactor by considering the 87 vessel boundaries as infinitely rigid walls and putting special ef-88 forts on the discretization of the domain following the rules by 89 Ihlenburg, Babûska and co-workers [19, 20]. In their work, the 90 simulations were compared with aluminum foil experiments, 91 observing in both cases a main active zone just along the axial 92 direction located at the emitter center. Their numerical results 93 also indicated a gradual decrease in the ultrasonic field activ-94 ity with the distance from the emitter, which was roughly con-95 firmed by aluminum foil experiments. Klíma et al. [21] faced 96 the optimization of a 20 kHz sonochemical reactor based on the 97 three-dimensional simulation of the acoustic pressure with the 98 same software. Despite the limitations of the linear Helmholtz 99 equation, which does not take into account non-linear wave 100 propagation and generation of transversal elastic waves, and the 1 + iωµ l /ρ l c 2 l (5) where µ l and ρ l are the viscosity and density of the liquid, re-138 spectively. These expressions were used by Sutkar et al. in 139 order to simulate the acoustic field in diverse low [28] and 140 high frequency [29] sonoreactors. A similar approach was re-141 cently followed by Xu et al. [30], although in this latter case 142 k c = (ω/c l ) − iα and Z c = ρ l c l , where α is an attenuation coeffi-143 cient analogous to used by other authors later mentioned in this 144 related to the bubble formation process [85]. In this mainly the-595 oretical latter work (all the calculations are in one-dimensional 596 domain), although they neglected physical phenomena such as 597 primary and secondary Bjerknes forces and acoustic stream-598 ing, and they defined small bubbles which acted as nuclei in 599 865 up possibility for sonochemical reactions, Chem. Eng. Sci. 54 (1999) 866 2829-2838. 867 [35] L.K. Zarembo, Acoustic streaming, in: L.D. Rozember (Ed.), High-868 intensity ultrasonic fields, Plenum Press, New York-London, 1971. 869 [36] J.-L. Laborde, C. Bouyer, J.-P. Caltagirone, A. Gérard, Acoustic cavita-870 tion field prediction at low and high frequency ultrasounds, Ultrasonics 871 36 (1998) 581-587. 872 [37] L. van Wijngaarden, Linear and nonlinear dispersion of pressure pulses in 873 liquid bubble mixtures, in: R.D. Cooper, S.W. Doroff (Eds.), 6th Sympo-874
The effects of operational parameters upon the hydroxyl radical generation of sonochemical reacto... more The effects of operational parameters upon the hydroxyl radical generation of sonochemical reactors are critical to optimize this technology for wastewater treatment purposes. Ultrasonic wave characteristics are usually considered as the main parameter to be taken into account. Nevertheless, it is the interaction of these waves with the liquid medium and the reactor what really affects the process. Therefore, the characterization of sonochemical reactors should be based on the effective pressure distribution in the reactor, which not only includes the wave characteristics but also the propagation and reflection of these ultrasonic oscillations. The pressure field can be characterized using different parameters, such as maximum pressure amplitude or volumetric integration of pressure in the reactor. This study intends to find a correlation between such calculated pressure-distributionrelated parameters and experimental measurements of hydroxyl radicals in the process. Both experiments and calculations are run varying the tip-bottom distance (keeping the rest of parameters constant), creating different reflection effects with the reactor walls and therefore different pressure distributions across the reactor. The hydroxyl radical measurements are performed with salicylic acid dosimetry, applying a specific developed method for biphasic cavitating systems. On the other hand, the pressure distribution was calculated simulating the different configurations with the computational tool COMSOL.
As one of several types of pollutants in water, chlorinated compounds have been routinely subject... more As one of several types of pollutants in water, chlorinated compounds have been routinely subjected to sonochemical analysis to check the environmental applications of this technology. In this review, an extensive study of the influence of the initial concentration, ultrasonic intensity and frequency on the kinetics, degradation efficiency and mechanism has been analyzed. The sonochemical degradation follows a radical mechanism which yields a very wide range of chlorinated compounds in very low concentrations. Special attention has been paid to the mass balance comparing the results from several analytical techniques. As a conclusion, sonochemical degradation alone is not an efficient treatment to reduce the organic pollutant level in waste water.
The sonoelectrochemical treatment of aqueous solutions of trichloroacetic acid (TCAA) has been sc... more The sonoelectrochemical treatment of aqueous solutions of trichloroacetic acid (TCAA) has been scaledup from the voltammetric analysis to pre-pilot stage. The degradation in absence of ultrasound field has yield to a poor performance which has been improved in presence of ultrasound. The sonovoltametry study has provided the range of potentials and/or current densities to be used with the lowest current efficiency penalty. Sonoelectrolyses at batch scale (carried out with a horn-transducer 24 kHz positioned at about 3 cm from the surface of the electrode) achieved little improvement in the degradation. However, when a specifically designed sonoelectrochemical reactor (not optimized) was used during the scale-up, the presence of ultrasound field provided better results (fractional conversion 97%, degradation efficiency 26%, selectivity 0.92 and current efficiency 8%) at lower ultrasonic intensities and volumetric flow.
Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titani... more Practical lead dioxide anodes have been obtained by electrodeposition on glassy carbon and titanium substrates in the presence and in the absence of an ultrasound field. The films obtained by mechanical agitation on glassy carbon are strongly improved when the electrodeposition process is carried out with the ultrasound field, providing adherent deposits free from nodules and stress, but with pores appearing occasionally. These enhanced properties were not achieved by mechanical conditions, even when optimization of temperature, current density, additives and geometrical aspects was attempted. The best practical anodes were obtained by sonoelectrodeposition using specially treated titanium as substrate, providing comparable behavior to commercial electrodes.
The chemical effects of acoustic cavitation are obtained in sono-reactors built-up from a vessel ... more The chemical effects of acoustic cavitation are obtained in sono-reactors built-up from a vessel and an ultrasonic source. In this paper, simulations of an existing sono-reactor are carried out, using a linear acoustics model, accounting for the vibrations of the solid walls. The available frequency range of the generator (19 kHz-21 kHz) is systematically scanned. Global quantities are plotted as a function of frequency in order to obtain response curves, exhibiting several resonance peaks. The attenuation coefficient of the wave is taken as a variable parameter, in absence of the precise knowledge of the bubble size distribution, and its influence is studied. The concepts of acoustic energy, intensity and active power are recalled, along with the general balance equation for acoustic energy. The latter is used as a convergence check of the simulations. Finally, it is shown that the interface between the liquid and the solid walls cannot be correctly represented by the simple approximations of either infinitely soft, or infinitely hard boundaries. Moreover, the liquid-solid coupling allows the cooling jacket to receive a noticeable part of the input power, although it is not in direct contact with the sonotrode. It may therefore undergo cavitation and this feature opens the perspective to design sono-reactors which avoid direct contact between the working liquid and the sonotrode.
A preliminary study of the 20 kHz sonoelectrochemical degradation of perchloroethylene in aqueous... more A preliminary study of the 20 kHz sonoelectrochemical degradation of perchloroethylene in aqueous sodium sulfate has been carried out using controlled current density degradation sonoelectrolyses in batch mode. An important improvement in the viability of the sonochemical process is achieved when the electrochemistry is implemented, but the improvement of the electrochemical treatment is lower when the 20 kHz ultrasound field is simultaneously used. A fractional conversion of 100% and degradation efficiency around 55% are obtained independently of the ultrasound power used. The current efficiency is also enhanced compared to the electrochemical treatment and a higher speciation is also detected; the main volatile compounds produced in the electrochemical and sonochemical treatment, trichloroethylene and dichloroethylene, are not only totally degraded, but also at shorter times than in the sonochemical or electrochemical treatments.
Fluid-solid reactions are common chemical processes that take place in a heterogeneous media. The... more Fluid-solid reactions are common chemical processes that take place in a heterogeneous media. These processes are of considerable importance, especially in chemical and metallurgical industries (1, 2). Fluid-solid processes in which the solid particle size does not effectively change are the roasting or oxidation of sulfide ores to yield the metal oxides and the nitrogenation of calcium carbide to obtain cyanamide, among others. Other fluid-solid processes involve the variation of particle dimensions such as combustion of solid fuels, gasification of coal or oil shale, and the production of sodium thiosulfate from sulfur and sodium sulfite.
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