Sonochemical reactor characterization in the presence of cylindrical and conical reflectors

Abstract

Ultrasonic systems must be able to produce an acoustic field with the highest possible energy concentration in sonochemical reactors to accomplish maximum efficacy in the sonolytic degradation of water contaminants. In the present study, the impact of cylindrical and conical stainless-steel reflectors placed on the liquid surface on the sonochemical oxidation activity of ultrasonication reactors was investigated. The amount of effective acoustic power transferred to the ultrasonicated medium without and with reflectors was measured by calorimetric characterization of the sono-reactors at diverse ultrasonication frequencies in the interval of 300–800 kHz and different electrical powers in the range of 40–120 W. Iodide dosimetry without and with reflectors at diverse ultrasonication conditions (300–800 kHz and 40–120 W) and various aqueous solution volumes in the range of 300–500 mL was used to assess the sonochemical oxidation activity, i.e., the generation of oxidative species (mainly hydroxyl radicals). Sonochemiluminescence (SCL) imaging was used to study the active acoustic cavitation bubbles distribution in the sono-reactors without and with reflectors. Significant impacts of the position and shape of the reflectors on the active acoustic cavitation bubble distribution and the sonochemical oxidation activity were observed due to remarkable modifications of the ultrasonic field by directing and focusing of the ultrasonic waves. A significant augmentation in the triiodide formation rate was obtained in the presence of the conical reflector, especially at 630 kHz and 120 W (60.5% improvement), while iodide oxidation was quenched in the presence of the cylindrical reflector at all ultrasonication frequencies and powers. The SCL images show a noteworthy modification in the ultrasonic field and the acoustic cavitation bubble population when reflectors were used. The sonochemical oxidation activity was improved by the conical reflector when placed in the Fresnel zone (near field region).

1. Introduction

The effective use of ultrasonic energy for acoustic cavitation-induced transformations has been intensively investigated [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. High-frequency ultrasonic irradiation vibrates the molecules it passes through as it travels through a medium as a pressure wave. In the liquid through which it passes, the ultrasonic wave causes cavitation bubbles to form. Existing gas nuclei in solutions will grow by rectification diffusion and coalescence under the influence of Bjerknes forces during ultrasonication, if the acoustic pressure is high enough, until they reach the active size at which an adiabatic collapse occurs. The bubbles collapse resulting in the formation of chemically active species and the release of heat. The direct causes of the chemical effects are the extraordinarily high pressures and temperatures attained in the gas cavities, as the bubbles are reduced in size by many orders of magnitude in a few µs [5], [11], [12], [13], [14], [15]. Water vapor is thermally dissociated into hydrogen atoms and reactive hydroxyl radicals due to the extraordinarily high pressures and temperatures developed by collapsing acoustic cavitation bubbles in the aqueous phase. The radical species produced have three possible reactions: recombination, reactions with other gaseous entities inside the bubble, and diffusion into the bulk liquid media where they can interact with the molecules present.

Reactive species yields serve as representative indices for understanding the chemical consequences of sonolytic cavitation. Therefore, these yields have been evaluated in several experimental settings [16], [17], [18], [19], [20], [21], [22], [23] and in theoretical and computational studies [23], [24], [25], [26], [27], [28], [29], [30]. The impact of additives has also been investigated [31]. Sonochemical efficiencies are sometimes defined as yields normalized to the actual ultrasonication power and/or ultrasonic energy density. Due to the complexity of acoustic cavitation, which can be influenced by several factors, even when only one parameter is changed, the characteristics of acoustic active bubbles remain unclear despite the active analysis of these yields and efficiencies under various operating conditions.

Sono-reactors need the ability to generate an ultrasonication field with maximum energy concentration. In order to maximize energy utilization, an ultrasonic reactor with reflectors may be the appropriate solution. In medical ultrasound imaging, reflectors have been intensively investigated. However, in sonochemical studies, there is a paucity of work on the impact of reflectors on the sonochemical oxidation activity of sono-reactors [32], [33], [34], [35], [36]. Leong et al. [32] have determined the ultrasound pressures and homogeneity obtained in the aqueous phase by high-frequency transducers in the absence and presence of reflector plates. They have shown that placing a reflector plate 50 cm from the transducer increases the homogeneity of the ultrasound pressure of 2 MHz ultrasonic irradiation. Moreover, 400 kHz ultrasonic wave produced a more uniform ultrasound pressure distribution notwithstanding without or with reflector. Additionally, the influence of a stainless-steel reflector placed on the aqueous solution surface on the oxidation of potassium iodide was studied by Asakura and Yasuda [33]. They reported that for all sample volumes and sonication frequencies, a quenching of the oxidation rate was observed regardless of the increase in ultrasonication power, with the exception of 1960 kHz. The ultrasonication power at which quenching happened augmented with volume and ultrasonication frequency. The quenching effect was influenced by placing a reflector on the water surface. Without a reflector, quenching was abrupt, but with the reflector it was progressive. The distribution of cavitation in water, mapped using a device with an acoustic radiation force balancer, has been studied by Yin et al. [34]. The device consisted of a precisely shaped aluminum alloy reflector and a focused ultrasonic transducer. Their results show that at a certain electrical power of 650 W, the reflector can prevent the directional displacement of the bubbles caused by the acoustic irradiation force. Consequently, the sonochemiluminescence intensity of the pre-focal zone was higher than that of the normal group. Most of the bubbles moved in the acoustic stream when the electric power was higher than or equal to 70 W. Both theoretical and experimental research has been carried out by Moussatov et al. [35] to analyze the acoustic cavitation produced in a tiny liquid film stuck between a reflector and a sonotrode surface. They have indicated that this design causes a significant amplification of the acoustic pressure, allowing cavitation to be generated at low power or over a broad interval of frequencies. Finally, Kauer et al. [36] investigated different visualization techniques to observe the generation, development and collapse of acoustic cavitation bubbles on solid surfaces. The active acoustic cavitation bubble distribution on the solid surface in the sono-reactor was found to be influenced by the reflector orientation and position with respect to the transducer, in addition to its material characteristics.

In the present work, the influence of cylindrical and conical reflectors placed on the water surface on the sonochemical oxidation activity of ultrasonic reactors operating at diverse ultrasonication frequencies in the interval of 300–800 kHz and various electrical powers in the range of 40–120 W was examined. Calorimetric characterization of the sono-reactors at various ultrasonication frequencies and electrical powers was performed to quantify the amount of effective acoustic power that is transferred to the ultrasonicated medium without and with reflectors. The sonochemical oxidation activity in the investigated sono-reactors was studied using potassium iodide dosimetry and sonochemiluminescence imaging. The effect of the liquid volume in the reactor on potassium iodide dosimetry and sonochemiluminescence without and with the conical reflector was examined. The degradation of propylparaben in water by sonolysis was also investigated without and with the reflectors.

2. Experimental

2.1. Chemicals

Propylparaben (PPB, Propyl 4-hydroxybenzoate, analytically pure) was purchased from Fluka and used as received. Acetonitrile for high-performance liquid chromatography (HPLC) analysis and potassium iodide used in the iodide dosimetry studies were procured from Riedel-de Haën. Luminol and sodium hydroxide used in the sonochemiluminescence experiments were acquired from Sigma-Aldrich. Ultrapure water was used to make all solutions, which was also used to prepare the mobile phase for HPLC analyses.

2.2. Ultrasonication reactors and reflectors

A high-frequency transducer was placed at the bottom of a cylindrical jacketed glass reactor (500 mL capacity) with an internal diameter of 6.9 cm. The 4 cm diameter piezoelectric ceramic disk was attached to a 5 cm diameter Pyrex plate surface forming the transducer. Three different transducers operating at ultrasonication frequencies of 300, 630 and 800 kHz were used. The power supply for the transducers is an AG series amplifier from T&C Power Conversion, Inc. A Lauda RE 630 SN chiller was used in all experiments to maintain the solution temperature at 25 ± 1 °C during ultrasonication by flowing cooling solution throughout the sono-reactor envelope.

The reflectors used in this work were made of stainless steel. This material was chosen due to its high durability, resistance to corrosion, and ability to withstand the intense ultrasonic energy without degradation. The first reflector was cylindrical with a height of 2.2 cm and a diameter of 5.8 cm. The second was a cylindrical-conical reflector with a total height of 3.5 cm and a diameter of 5.8 cm, with a conical part height of 3.2 cm. For the cylindrical-conical reflector, only the conical part was submerged in the solution. Since only the conical part of the cylindrical-conical reflector was submerged in the reacting medium, it will be referred to as the conical reflector throughout the study. The cylindrical reflector was placed on the top of the ultrasonically treated liquid medium.

A schematic of the ultrasonication setups without and with the cylindrical and conical reflectors was shown in Fig. 1.

Fig. 1.

Schematic of the ultrasonication setups without and with the cylindrical and conical reflectors.

2.3. Calorimetric characterization

To quantify the quantity of effective calorimetric power that is transported to the ultrasonicated medium, a thermocouple (type K) was held at half the water level and the temperature rise was recorded over a 5 min period [9], [37]. Assuming that all the energy transported by the ultrasonication wave and involved in the cavitation process is recuperated as heat, this increase in solution temperature is attributed to ultrasonication. The thermocouple remained in place throughout the calorimetric runs. To verify the reproducibility of the results, each measurement was repeated five times. Finally, the cooling liquid in the reactor jacket was evacuated to reduce heat loss.

2.4. Iodide dosimetry

To explore and compare the influence of reflectors in the three sono-reactors studied, i.e., 300, 630 and 800 kHz, at various electrical powers in the interval of 40–120 W, iodide dosimetry was examined using 0.1 M KI solution. Samples of the reacting medium were collected in a 300 µL microvolume quartz cuvette (1 cm path length) at various times for analysis. The triiodide production rate is time-independent, i.e., constant, and follows a zero-order kinetics law. Therefore, only 14 min of ultrasonication of the potassium iodide solution was required to generate sufficient experimental results to correctly determine the rate constant. Samples were returned to the sono-reactor after analysis in order to maintain a constant ultrasonic power density delivered to the solution.

Triiodide anion formation measurements were performed in the 300, 630, and 800 kHz sonoreactors with different volumes of 300, 400, and 500 mL of 0.1 M KI solution in the reactors in the absence and presence of the conical reflector at 80 W electrical power level.

In this dosimetry, sonolysis-generated reactive species oxidize iodide anions to iodine (I₂). The excess iodide anion reacts with the iodine formed to yield triiodide anion (I₃⁻), which absorbs at 350 nm with 26,000 L/mol·cm molar extinction coefficient [9], [38], [39].

2.5. Sono-oxidation of propylparaben

The sono-oxidation of PPB (10⁻⁴ M, 300 mL) in aqueous phase was carried out with the sono-reactor operating at the intermediate ultrasonic frequency used in this work (630 kHz and 120 W). The impact of reflectors on the sonolytic decomposition of PPB was investigated (Fig. 1). In order to compare the experimental results without and with the reflectors, the substrate solution was subjected to ultrasonic treatment for 30 min.

2.6. Analytical techniques

A Supelcosil LC-18 column was used in a Waters Associates 590 instrument for the quantitative analysis of PPB. A 20 µL loop was used to inject the samples with a Rheodyne injector. Detection was performed using a Model 440 UV detector adjusted at 254 nm. Water-acetonitrile mixture (50/50) was used as the mobile phase in isocratic mode.

For iodide dosimetry, the optical density was determined at 350 nm using a spectrophotometer (Agilent 8453 UV–visible).

2.7. Sonochemiluminescence

Sonochemiluminescence (SCL) in the absence and presence of cylindrical and conical reflectors was utilized to visualize the active acoustic cavitation regions in the sono-reactor. For SCL experiments, ultrasonication at 630 kHz and electrical powers of 60 and 100 W was applied to a solution containing 1 mM luminol and 0.1 M sodium hydroxide [36], [40]. A Nikon digital SLR camera with an AF 18–55 mm zoom lens was used for SCL imaging. Exposure times were 30 s and 60 s.

3. Results and discussion

3.1. Calorimetric characterization

Calorimetric characterizations have been most commonly used to measure the ultrasonication power dissipated in sono-reactors.

In Fig. 2(a)–(c), the calorimetric powers were shown for the sono-reactors in the absence and presence of reflectors under the same operating and ultrasonication conditions. From these figures, it can be observed that the calorimetric powers in the sono-reactors were of the same order of magnitude in the presence and absence of reflectors. In general, the ultrasonication powers determined without reflectors were slightly higher than those calculated in the presence of reflectors. This is due to the reflection and scattering of ultrasonication waves by the reflectors, leading to a reduction in energy transfer to the reacting medium. When ultrasonic waves are generated in the sono-reactor, they propagate and interact with the reflectors. The conical and cylindrical shape of the reflectors can cause the waves to be deflected, reflected, or scattered in various directions. This dispersion of ultrasonic energy results in a reduction of the overall calorimetry in the system. The calorimetric powers with the cylindrical reflector were slightly higher than those with the conical reflector. The piezoelectric ceramic disk vibrates, thereby converting the electrical energy from the generator into mechanical energy. The liquid molecules then oscillate about their mean position as a result of the mechanical energy being converted to acoustic energy by ultrasound waves traveling through a liquid medium, until the liquid separates because the mean distance between them is greater than the critical molecular distance required to maintain contact. Cavitation bubbles are created during the process, meaning that the energy transfer goes from acoustic to cavitation and then to heat, which is measured by tracking the temperature evolution rate as a function of time [41]. The rise in liquid temperature is mainly due to [2], [41], [42], [43]: (i) viscous interactions between the liquid molecules, (ii) heat transmission from the collapsing bubbles to the bulk of the liquid, (iii) absorption of ultrasound waves by the liquid, and (iv) friction between water and the oscillating bubbles. The decrease in ultrasonic power in the presence of reflectors was similar to that obtained by Asakura and Yasuda [33], who indicated that when the effective electrical power at a sonication frequency of 1018 kHz is below 135 W, the calorimetric power in the absence of the reflector is higher than that in its presence.

Fig. 2.

Relationship between the electrical power and the ultrasonic power as measured by calorimetry in the absence and presence of cylindrical and conical reflectors at different frequencies: (a) 300 kHz, (b) 630 kHz and (c) 800 kHz (conditions: volume = 300 mL, electrical power = 40–120 W).

Taking into account the electrical power supplied to the sono-reactors, which ranges from 120 to 40 W, the effective energy conversion efficiency without reflector is respectively from 47.75% to 60% at 300 kHz, from 45.14% to 49.23% at 630 kHz and from 45.88% to 56.43% at 800 kHz. In the presence of the cylindrical reflector, the effective energy conversion efficiency is from 46.67% to 57.5% at 300 kHz, from 44.17% to 50% at 630 kHz, and from 44.58% to 55% at 800 kHz. With the conical reflector, the effective energy conversion efficiency is from 46.29% to 55% at 300 kHz, from 44.10% to 52.05% at 630 kHz, and from 44.09% to 54.23% at 800 kHz. The effective energy conversion efficiency decreased as the electrical power increased, regardless of whether the reflectors were present or absent.

The amount of heat lost to the environment must be taken into account when using the calorimetric technique, but in small scale sono-reactors the heat loss can be assumed to be insignificant. Heat losses through the glass walls of the sono-reactor can be neglected because the conductivity coefficient of glass is very low (∼0.8 W/m·K). However, the heat lost by conduction with the reflector surface can be considered. With a conductivity coefficient of ∼ 15 W/m·K, stainless steel has one of the lowest values of any metal; copper’s thermal conductivity is on average 20 times that of stainless steel. The stainless-steel material of the reflectors can absorb and dissipate some of the ultrasonic energy. Stainless steel can act as a sink for the ultrasonic energy, reducing its effectiveness in heating the reacting medium. The calorimetric energies measured in the sono-reactors with reflectors may be higher than those without reflectors if the energy lost by conduction through contact with the reflectors is added. Taking into account the receiving area of the reflectors, which is proportional to the heat flux as stated by Fourier’s law, the heat losses with the conical reflector (39.34 cm²) were higher than with the cylindrical reflector (26.42 cm²). This may explain the slightly lower ultrasonic powers determined with the conical reflector as compared to the cylindrical reflector.

Stainless steel is a good reflector of ultrasound waves (discussed in more detail in Section 3.2), which means that it will bounce back a significant portion of the ultrasound energy that comes in contact with it. This can result in reduced penetration depth and less energy reaching the target area. The presence of the stainless-steel reflector can cause shadowing where the ultrasound waves are blocked by the reflector. This can result in reduced or uneven distribution of ultrasound energy to the surrounding areas. Standing waves can occur when ultrasonic waves hit the stainless-steel reflector. Standing waves occur when the reflected and incident ultrasound waves interfere with each other, resulting in areas of low and high intensity within the sono-reactor. These variations in intensity can affect the performance and consistency of the sono-reactor. Additionally, the presence of the stainless-steel reflector can disturb the generation and behavior of acoustic cavitation bubbles. Cavitation is a critical phenomenon for sonochemical reactions, and variations in cavitation patterns due to the reflector can disturb the distribution and level of ultrasonic energy in the sono-reactor.

It is important to note that the behavior of ultrasonication waves with the stainless-steel reflectors can be complex and dependent on various factors. Therefore, experimental validation and optimization are typically necessary when working with such configurations in sono-reactors.

3.2. Iodide dosimetry

Fig. 3(a)–(c) show the impact of the cylindrical and conical reflectors on the rate of triiodide anion formation at diverse ultrasonication frequencies, i.e., 300, 630 and 800 kHz, and various electrical powers in the interval of 40–120 W. These figures show that regardless of the frequency and in the presence or absence of reflectors, the triiodide generation rate increased with increasing power applied to the transducer disk. The triiodide production rate increase with ultrasonication power is explained by an increase in the amount of acoustic energy delivered to the sono-reactor and an augmentation in the acoustic cavitation bubble population [44], [45], [46]. Due to the higher acoustic cavitation bubble population and the rapid collapse and pulsation with the higher acoustic energy, a high concentration of reactive radicals is generated in the reacting medium, which led to an intensification of the triiodide formation rate in the aqueous solution [47], [48]. It was also mentioned that the velocity of the bubble wall increased abruptly throughout the final collapse phase [48], [47]. Stronger collapses are caused by these high implosion velocities, which raise the pressure and temperature inside the bubbles to extremely high levels. These harsh conditions create unique chemical reaction environments [47].

Fig. 3.

Triiodide formation rate in the absence and presence of cylindrical and conical reflectors at different frequencies: (a) 300 kHz, (b) 630 kHz and (c) 800 kHz and various powers (conditions: volume = 300 mL, electrical power = 40–120 W).

An important improvement in the triiodide formation rate was noticed in the presence of the conical reflector at all ultrasonication frequencies and electrical powers tested (Fig. 3(a)−(c)). The highest average improvement (37.6%) was obtained at 630 kHz, followed by 800 kHz (32%), and finally 300 kHz (11.2%). At 630 kHz with the conical reflector, the intensification of triiodide generation rate was 60.5% at 120 W, 39.8% at 80 W, 36% at 60 W, 29.6% at 100 W, and 22.2% at 40 W. The improvements at 800 kHz were 73.2% at 80 W, 34.7% at 120 W, 26.3% at 100 W, 21.5% at 60 W, and 4.4% at 40 W. At 300 kHz, the enhancements were 20.9% at 60 W, 15.5% at 80 W, 7.6% at 100 W, 6.2% at 40 W, and 6.0% at 120 W. In the presence of the conical reflector, a portion of an ultrasonic wave is reflected back into the first medium (aqueous solution) at the same speed when it contacts an interface between two media whose dimensions are greater than its wavelength [49]. The remaining portion of the wave propagates at the speed of that medium after being refracted or transmitted into the medium outside the interface. The angles of reflection and incidence are identical for reflection, whereas they are typically unequal for transmission. If the ultrasonication wavelength equals or exceeds the reflector size, the incident beam will be scattered in all directions [49]. The ratio of the specific impedance of the two media on either side of an interface determines how much of the incident ultrasonication wave is refracted, reflected, or transmitted. The product of the density of a medium and the speed of ultrasound in that medium gives its specific acoustic impedance (Z). The calculated specific acoustic impedances for stainless steel and water are, respectively, Z_SS = 44.7 × 10⁶ kg/m²·s (Rayls) and Z_W = 1.5 × 10⁶ kg/m²·s (Rayls). The amount of ultrasonication energy reflected or transmitted at an interface separating two continuous isotropic media depends on the fraction of the specific acoustic impedance of the media. As this impedance ratio approaches unity, less energy is reflected from the interface and further energy is transferred to the second medium. The impedance ratio for the stainless steel-water interface is close to 30, indicating that much energy is reflected from the interface into the reacting medium (aqueous solution) and less energy is transmitted into the stainless-steel reflector. By analogy with electromagnetic waves, the reflection at a material interface can be determined using the following equation:

$$\mathrm{\Gamma }=\frac{{\mathrm{Z}}_{\mathrm{S}\mathrm{S}}-{\mathrm{Z}}_{\mathrm{W}}}{{\mathrm{Z}}_{\mathrm{S}\mathrm{S}}+{\mathrm{Z}}_{\mathrm{W}}}$$ (1)

where $\mathrm{\Gamma }$ is the reflection coefficient and characterizes the fraction of the amplitude of the ultrasonic field of the reflected wave to that of the incident wave, and Z_W and Z_SS are, respectively, the acoustic impedances of water and stainless-steel.

The calculated reflection coefficient ($\mathrm{\Gamma }$ ≈ 0.94) at the interface between the reacting medium (aqueous solution) and the stainless-steel reflector indicates that approximately 94% of the incident energy is reflected into the reacting medium, while the remainder (∼6%) is transmitted into the stainless-steel reflector. This reflected energy enhances the generation of reactive radicals, leading to a higher rate of triiodide formation in the reacting medium with the conical reflector.

In the presence of the conical stainless-steel reflector, the acoustic cavitation bubble population can be influenced in several ways. Firstly, the conical shape of the reflector can help focus and concentrate the ultrasound waves, resulting in a higher intensity of the ultrasonic field in the vicinity of the reflector. This increased intensity promotes the nucleation and growth of cavitation bubbles. The conical shape can also act as a constructive interference zone, where the reflected ultrasound waves and incident waves converge and reinforce each other. This reinforcement enhances the intensity of the acoustic field and creates a favorable environment for cavitation bubble formation. Additionally, the conical reflector can provide a surface that promotes bubble nucleation. Microscopic imperfections on the reflector's surface can serve as nucleation sites for cavitation bubble formation. These imperfections create small air pockets or localized turbulence, which facilitate the formation of gas bubbles when subjected to the intense acoustic field. Furthermore, the conical shape can help retain and confine the cavitation bubbles near the reflector. As the cavitation bubbles are generated, the converging nature of the reflector geometry can prevent them from drifting away from the region of interest. This confinement allows for a higher acoustic cavitation bubble population nearby the reflector, leading to enhanced sonochemical oxidation activity. Overall, the conical stainless-steel reflector in an ultrasonic reactor can enhance the acoustic cavitation bubble population by focusing and intensifying the acoustic field, providing nucleation sites for bubble generation, and trapping the bubbles near the reflector. These factors contribute to increased cavitation activity and can enhance the effectiveness of sonolytic reactions, leading to an intensification of the rate of triiodide formation.

A traveling ultrasonic wave moves away from the transducer and toward a reflector or the water surface. The reflector or water surface then reflects the traveling wave back to the transducer in the case of the cylindrical reflector, and to the entire sono-reactor in the case of the conical reflector. Only when the reflected and traveling waves propagate without attenuation can the interference between the two waves result in a standing wave. Traveling wave and standing wave components coexist in the ultrasonic field when the reflected wave is less attenuated than the traveling wave. The difference between the traveling wave and the reflected wave is used to describe the traveling wave component. The traveling wave component of the ultrasonic field moving from the transducer to the reflector or water surface is superimposed on the standing wave of the ultrasonic field. Both the fundamental Bjerknes force, a form of radiation force generated in a standing wave, and the traveling wave component from the transducer to the reflector or water surface impose irradiation forces on the bubbles [33]. The primary Bjerknes force is stronger than the irradiation force resulting from the traveling wave component when the difference between the reflected and traveling waves is minimal. As a result, the primary Bjerknes force traps a high number of bubbles in the antinode of the standing wave sound pressure. Standing wave field is the term used to describe this ultrasonic field. Few bubbles are trapped in the standing wave and instead flow from the transducer to the reflector or water surface if the reflected wave is smaller than the traveling wave and the irradiation force caused by the traveling wave component is greater than the primary Bjerknes force. A standing wave field is produced because the power of the reflected wave and the moving wave are nearly equal close to the reflector or water surface. As a result, bubbles trapped in the standing wave create a chemical reaction field. On the other hand, because the reflected wave is approximately equal to the traveling wave, the traveling wave field is produced close to the reflector. The bubbles then advance toward the transducer surface as a result of the creation of a tiny chemical reaction field [50].

From Fig. 3(a)–(c), it was also observed that the rate of triiodide sonolytic formation with the cylindrical reflector was inferior than that without reflector and with the conical reflector, regardless of the ultrasonication frequency and electrical power. Compared to the results without reflector, the triiodide production rate at 300 kHz with the cylindrical reflector is reduced by 42.3%, 37.7%, 10.4%, 18.9%, and 7.7% at 40, 60, 80, 100, and 120 W, respectively. At 630 kHz, these reductions are 22.5%, 27.8%, 18.9%, 36.5%, and 30.8% at 40, 60, 80, 100, and 120 W, respectively. At 800 kHz, the rate of triiodide formation is lessened by 32.1% at 40 W, 29.4% at 60 W, 11% at 80 W, 19.7% at 100 W and 7.2% at 120 W. The highest average decrease was 27.3% at 800 kHz, followed by 300 kHz (23.4%) and finally 630 kHz (19.9%). The lower rate of triiodide formation with the cylindrical reflector, which is parallel to the transducer disk, is due to the reflection of the acoustic field back to the transducer conducting to the attenuation of the acoustic field and quenching of cavitation. The concentrated ultrasound scattering and attenuation by the bubbles causes the reflected wave from the transducer surface to become weaker. Additionally, the cylindrical reflector can dislodge cavitation bubbles from the pressure antinode, thereby reducing cavitation activity and consequently triiodide yield. Overall, by scattering and diffusing the ultrasonic waves, the cylindrical reflector can disrupt the generation and growth of cavitation bubbles, resulting in a decrease in sonochemical oxidation activity.

3.3. Impact of volume on iodide dosimetry

The impact of different volumes of potassium iodide solution in the range of 300–500 mL on the sonolytic production of triiodide anion was evaluated without and with the conical reflector at various ultrasonication frequencies (300, 630 and 800 kHz) and at an electrical power of 80 W. This power level was chosen because it provides the highest intensification of the triiodide production rate at the three frequencies tested. The results obtained were shown in Fig. 4. Without the conical reflector, the ultrasonic formation rate of triiodide anion decreased significantly as the aqueous solution volume increased, regardless of frequency. The rate of triiodide production decreased with increasing ultrasonication frequency for a given volume of liquid. At 300 kHz, the triiodide production rate decreased from 3.19 to 2.08 and 1.30 µM/min by increasing the liquid volume from 300 to 400 and 500 mL, respectively. The triiodide generation rate passes from 2.45 to 1.74 and 0.92 µM/min, respectively, with increasing the volume from 300 to 400 and 500 mL at an ultrasonication frequency of 630 kHz. When the liquid volume augmented from 300 to 400 and 500 mL at 800 kHz, the rate of triiodide formation diminished from 1.97 to 1.37 and 0.72 µM/min, respectively. This is due to the decrease in energy density as the liquid volume increases because the energy is diluted over a larger volume. This occurs because the same amount of energy is distributed over a larger volume of liquid. As a result, the energy delivered per unit volume decreases, resulting in a lower energy density.

Fig. 4.

Triiodide formation rate in the absence and presence of conical reflector at different frequencies: 300, 630 and 800 kHz (conditions: volume = 300 mL, electrical power = 80 W).

In the presence of the conical reflector, for a specified ultrasonication frequency, the triiodide production rate was significantly reduced as the volume of aqueous solution increased. The volume of the reaction mixture determines the efficiency of mixing in the ultrasonic reactor. A smaller volume can result in better mixing and an increased collision frequency between reactant molecules, resulting in a higher triiodide formation rate. The energy dissipated by the ultrasonic waves depends on the volume of the reaction mixture. A larger volume requires more energy to achieve the same energy density as a smaller volume. Therefore, a larger volume may require a longer exposure time to achieve the desired triiodide formation rate. The presence of the conical stainless-steel reflector can enhance the transmission of ultrasonic energy within the reaction mixture. However, the volume of the reaction mixture can affect the reflection and absorption of ultrasonic waves. As the volume increases, there is a greater likelihood of wave attenuation due to increased reflection and absorption, resulting in a decreased triiodide formation rate.

For a liquid volume of 300 mL, the triiodide generation rate increased by 15.5%, 39.8%, and 73.2% in the presence of the conical reflector at ultrasonication frequencies of 300, 630, and 800 kHz, respectively. At 630 and 800 kHz, the triiodide formation rate increased by 38.5% and 57.3% with the conical reflector for an aqueous solution volume of 400 mL, whereas this rate decreased by 33.2% at a frequency of 300 kHz. The same behavior was observed for a liquid volume of 500 mL, where the triiodide production rate augmented by 51.5% at 630 kHz and by 57.0% at 800 kHz, but decreased by 33.7% at 300 kHz. It seems that the intensification of sonochemical oxidation activity in the presence of the conical reflector may be related to its position in the near field (Fresnel region) or in the far field (Fraunhofer region). This will be discussed in detail below.

To determine the wavelength of the applied frequency, Eq. (2) was used:

$$\lambda =C/\nu$$ (2)

where λ is the wavelength of the ultrasonication irradiation, ν is the ultrasonication frequency and C (1500 m/s) is the speed of the ultrasonic wave in water.

The wavelengths of the ultrasonication frequencies used were 5, 2.381 and 1.875 mm for 300, 630 and 800 kHz, respectively. The liquid volumes examined, 300, 400 and 500 mL, correspond, respectively, to 16 × λ, 21.4 × λ and 26.74 × λ at 300 kHz, 33.6 × λ, 44.94 × λ and 56.15 × λ at 630 kHz and 42.6 × λ, 57.06 × λ and 71.31 × λ at 800 kHz. These volumes (300, 400 and 500 mL) correspond to liquid heights (levels) in the ultrasonication reactor of 8.02, 10.70 and 13.37 cm, respectively.

The Fresnel zone, also known as the near field, is where the overall size of the ultrasonic beam is generally constant as it is transmitted from the transducer. However, there are some intensity variations within the zone itself, both up and down the beam axis. This is followed by the far field, also called the Fraunhofer region, where the beam diverges and becomes more uniform. Eq. (3) can be used to determine the distance of the Fresnel zone (near field) from the transducer.

$$L_{\mathit{NF}}=D^2/(4\lambda )$$ (3)

where D is the transducer diameter and λ the wavelength of the ultrasonication irradiation.

The calculated near field lengths (L_NF) are 8 cm, 16.8 cm, and 21.33 cm from the transducer surface at 300, 630, and 800 kHz, respectively. As the ultrasonic wavelength gets shorter, the near field for a given transducer radius becomes more complicated (has more maxima and minima). The Fraunhofer zone (far field) of the transducer lies beyond this distance L_NF. The far field acoustic intensity of a transducer is inversely proportional to the acoustic pressure squared. Far field beam directivity depends on diffraction; higher beam directivity results in higher ultrasonic frequency for a given transducer size.

As discussed above, the three aqueous solution volumes examined correspond to liquid heights of 8.02, 10.70 and 13.37 cm, indicating that the conical reflector is located in the Fresnel zone for ultrasonication frequencies of 630 and 800 kHz, while at 300 kHz, only for 300 mL (liquid level of 8.02 cm), the reflector is placed at the boundary of the near field zone, though for the higher volumes it is positioned in the far field region. In conclusion, the sonochemical oxidation activity at ultrasonication frequencies of 300, 630 and 800 kHz was intensified by the conical reflector when placed in the near field region.

Fig. 4 also shows that the triiodide formation rate in the presence of the conical reflector was improved by 73.2%, 57.3% and 57.0% for liquid volumes of 300, 400 and 500 mL, respectively, at 800 kHz and by 39.8%, 38.5% and 51.5% for solution volumes of 300, 400 and 500 mL, respectively, at 630 kHz. An enhancement of 15.5% was obtained at 300 kHz and 300 mL, while inhibition was observed for the other volumes studied (400 and 500 mL). The enhancement effect with the conical reflector was inversely proportional to ultrasonication frequency in the range investigated.

3.4. Sono-oxidation of propylparaben

A significant improvement in the triiodide production rate was achieved with the conical reflector at 630 kHz ultrasonication frequency and 120 W electrical power (60.5% increase). Consequently, the sonolytic degradation experiments of an aqueous PPB solution with an initial concentration of 10⁻⁴ M, performed with the sono-reactor operating at 630 kHz ultrasonication frequency and 120 W electrical power in the absence and presence of cylindrical and conical stainless-steel reflectors, were shown in Fig. 5. The efficiency of the sonolytic decomposition of the contaminant was enhanced in the presence of the conical reflector compared to that without the reflector. With the cylindrical reflector, a minor increase in the initial oxidation rate of the contaminant was noted, especially at beginning times (insert in Fig. 5). The presence of a reflector enhances the acoustic cavitation process, resulting in increased PPB decomposition efficiency. Additionally, the shape of the reflector affects the distribution and intensity of the ultrasound waves. Conical and cylindrical shapes are particularly effective in directing and focusing the ultrasound waves. The conical shape allows for better focusing of the waves towards the entire reactor, while the cylindrical shape guarantees a more uniform distribution of the waves. The reflector acts as a focusing element, directing and focusing the ultrasonication waves in order to improve the cavitation process and therefore the generation of reactive species, particularly ^•OH radicals, which are highly reactive and critical to contaminant degradation. By focusing the energy, more acoustic cavitation bubbles are generated in the sono-reactor, resulting in more efficient degradation by enhancing the generation of ^•OH radicals. In conclusion, the use of conical and cylindrical stainless-steel reflectors can increase the efficiency of ultrasonic degradation of PPB by enhancing the cavitation process and generating more hydroxyl radicals.

Fig. 5.

Sono-oxidation of propylparaben in the absence and presence of cylindrical and conical reflectors at an ultrasonic frequency of 630 kHz and an electrical power of 120 W (conditions: volume = 300 mL, PPB initial concentration = 10⁻⁴ M). The insert is the initial degradation rate of PPB calculated at different time intervals.

3.5. Sonochemiluminescence

Fig. 6 shows SCL images of luminol solutions taken at 630 kHz without reflector at two electrical powers of 60 and 100 W using two different liquid volumes of 300 and 500 mL. Without reflector, the top of the reactor and areas near the liquid surface exhibit the strongest SCL emission. By changing the liquid volume and electrical power, the SCL emission pattern can be modified. The standing waves are parallel to the surface of the transducer.

Fig. 6.

SCL images obtained without reflector in the sono-reactor operating at a frequency of 630 kHz and electrical powers of 60 and 100 W, and for two liquid volumes of 300 and 500 mL.

Fig. 7 depicts SCL images of luminol solutions acquired at 630 kHz ultrasonication frequency and 60 and 100 W electrical power in the presence of the cylindrical reflector and two diverse solution volumes of 300 and 500 mL. The ultrasound waves interact with the cylindrical reflector placed on the surface of the aqueous luminol solution in the center of the sono-reactor. In the case of the cylindrical reflector, increased SCL activity is seen due to reflection (and decreased transmission) of the ultrasonic waves. The correspondence of the beam profile with the lateral activity distribution of the reflector surface shows a specific concentration towards the center of the reflector. In general, a uniform horizontal light distribution over the reflector was not achieved. This was predicted from the SCL distribution in the sono-reactor without reflector and appears to be caused by the concentrating characteristics of the fields, i.e., sturdier intensity.

Fig. 7.

SCL images obtained with the cylindrical reflector in the sono-reactor operating at a frequency of 630 kHz and electrical powers of 60 and 100 W, and for two liquid volumes of 300 and 500 mL.

Fig. 8 displays SCL pictures of luminol solutions recorded at 630 kHz in the presence of the conical reflector at electrical powers of 60 W and 120 W and for two different liquid volumes of 300 and 500 mL. In the case of the conical reflector, the reflected waves scatter specularly. A beam can be seen traveling toward the reflector when liquid volume and electrical power were at their highest. Closer to the reflector, the stripes form an obliquely traversed design that changes to horizontal lines toward the transducer. A significant portion of the wave is always returned, resulting in a feebler SCL pattern near the reflector. The maximum of emission is shifted to an area under the reflector as a result of reflections, and there is a greater imbalance in cavitational activity near the reflector. Additionally, SCL images obtained with half of the conical reflector in the sono-reactor operating at a sonication frequency of 630 kHz and electrical powers of 60 and 100 W, and for two different liquid volumes of 300 and 500 mL, shown in Fig. 9, demonstrate very interesting results and encourage to study in depth the effect of reflector shape and size.

Fig. 8.

SCL images obtained with the conical reflector in the sono-reactor operating at a frequency of 630 kHz and electrical powers of 60 and 100 W, and for two liquid volumes of 300 and 500 mL.

Fig. 9.

SCL images obtained with half of the conical reflector in the sono-reactor operating at a frequency of 630 kHz and electrical powers of 60 and 100 W, and for two liquid volumes of 300 and 500 mL.

Only the cylindrical reflector and without reflector allow the observation of SCL stripes representing standing wave patterns; however, standing waves have been observed in the presence of the conical reflector (half and totality) at 100 W electrical power. The reflector always engenders attenuation of the transmitted wave and back reflections, but the intensity of these phenomena will vary depending on the distance between the reflector and the transducer as well as the power. While the transmission loss is relatively small at the higher reflector height, the maximum wave blockage occurs at the lowest reflector height (liquid volume).

4. Conclusion

The impact of cylindrical and conical reflectors placed on the water surface on the sonochemical oxidation activity of ultrasonic reactors operating at different ultrasonication frequencies (300, 630 and 800 kHz) and electrical powers (40–120 W) was studied. The amounts of calorimetric power in the sono-reactors were of the same order of magnitude in the presence and absence of reflectors. The effective energy conversion efficiency, independent of frequency and electrical power, was in the range of 45.14%−60%, 44.17%−57.5% and 44.10%−55% without reflector, with the cylindrical reflector and with the conical reflector, respectively. The triiodide sonolytic formation rate was drastically enhanced with the conical reflector compared to that without reflector, while it was decreased with of the cylindrical reflector, regardless of the ultrasonication frequency and electrical power. With the conical reflector, the highest average improvement (37.6%) was obtained at 630 kHz, followed by 800 kHz (32%), and finally 300 kHz (11.2%). Both without and with the conical reflector, the triiodide ultrasonic formation rate decreased significantly as the aqueous solution volume increased from 300 to 500 mL, regardless of frequency. The sonochemical oxidation activity at the ultrasonication frequencies examined was intensified by the conical reflector when placed in the near field region (Fresnel zone). SCL images of luminol solutions taken at 630 kHz showed an increased SCL activity due to reflections, and there is a greater imbalance in cavitational activity near the reflector, but the intensity of these effects will vary depending on the distance between the reflector and the transducer as well as the power.

Overall, the conical reflector showed better performance. The reflectors induce back reflections and attenuation of the transmitted wave, but the strength of these impacts depends on the geometrical characteristics of the reflector, including its height and shape. The reflector reflects the traveling wave back to the transducer in the case of the cylindrical reflector, and to the entire sono-reactor in all directions (180°) in the case of the conical reflector. With the cylindrical reflector, the wave reflected back to the transducer causes attenuation of the transmitted wave. The conical reflector induces an efficient reflection of the wave in all directions of the sono-reactor, which bounces back a significant portion of the ultrasound energy, resulting in better performance.

The present work demonstrates that the stainless-steel reflectors can be used to efficiently increase the sonochemical oxidation activity in sono-reactors if the operating and ultrasonication conditions, as well as the position and shape of the reflectors, are appropriately selected.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.