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. 2023 Oct 24;101:106659. doi: 10.1016/j.ultsonch.2023.106659

Effect of violent mixing on sonochemical oxidation activity under various geometric conditions in 28-kHz sonoreactor

Dukyoung Lee a, Jumin Kang a,b, Younggyu Son a,b,⁎
PMCID: PMC10630164  PMID: 39491264

Highlights

  • •

    Effect of violent mixing on sonochemical oxidation activity was investigated (28 kHz).

  • •

    Application of mixing resulted in significant enhancement in sonochemical activity.

  • •

    The enhancement was due to substantial change in the sonochemical active zone.

  • •

    Mixing technique could be one of promising options for large-scale applications.

Keywords: Sonochemical oxidation activity, Mixing, Geometric conditions, Stirrer, Homogenizer, Scale-up

Abstract

The effects of violent mixing and reactor geometric conditions were investigated using the overhead stirrer and high-speed homogenizer in 28-kHz sonoreactors. The sonochemical oxidation activity was quantified using the KI dosimetry method, and the sonochemical active zone was visually observed using the luminol method. Higher mixing rates resulted in a significant enhancement of the sonochemical oxidation activity, primarily due to a significant change in the sonochemical active zone. When using the overhead stirrer (0–2,000 rpm), the highest activity for 2λ and 3λ occurred at 500 rpm, whereas the highest activity for 4λ was obtained at 250 rpm. For the high-speed homogenizer (0–12,000 rpm), the highest activity was consistently obtained at 3,500 rpm across all liquid height conditions. The impact of mixing position (Top, Mid, and Bot positions) on sonochemical activity was analyzed. The results revealed that the lowest activity was obtained for the bottom position, likely attributed to significant ultrasound attenuation. The reactor size effect was investigated using the high-speed homogenizer in five cylindrical sonoreactors with different diameters (12–27 cm). It was found that very low activity could be observed due to unexpected geometric conditions, and the application of mixing (3,500 rpm in this study) could result in high sonochemical activity regardless of geometric conditions.

1. Introduction

The sonochemical oxidation activity has been extensively studied in various ultrasonic systems for the purpose of removing aqueous pollutants [1], [2], [3], [4], [5], [6]. Although acoustic cavitation can trigger potent oxidizing reactions without the need for additional chemicals, it has been observed that using ultrasound alone results in relatively low pollutant and TOC (Total Organic Carbon) removal efficiency [1], [6], [7]. Moreover, it has been noted that the novel sonochemical effects are primarily demonstrated in small-scale systems due to the limited transmission of ultrasonic energy further into the liquid medium [7], [8], [9], [10]. Consequently, there is a continuous demand for techniques that can enhance the sonochemical oxidizing power and scale up sonoreactors for industrial applications.

Previous researchers have explored various methods to enhance the sonochemical oxidizing power, including combining it with other advanced oxidation processes (AOPs) [1], [2], [4], [11], utilizing additional oxidizing agents and catalysts [1], [12], [13], [14], [15], employing novel gas saturation/sparging techniques [16], [17], [18], [19], [20], [21], [22], and using mechanical stirring [12], [13], [18], [23], [24], [25]. Ultrasound technology has demonstrated high synergistic effects when combined with other AOPs [1], [2], [4], and the most effective sonochemical oxidation activity has been achieved using gas mixtures of Ar and O2 [5], [8], [17], [21], [22], [26]. Recently, several researchers have investigated the impact of mechanical mixing on the sonochemical oxidation activity and the formation of the sonochemical active zone. It has been reported that mechanical mixing significantly enhances the sonochemical activity by noticeably changing the active zone due to strong flow induced by mixing [12], [13], [18], [23], [24], [25].

Kojima et al. reported approximately 1.7 times higher sonochemical oxidation activity when they applied mixing at 300 and 350 rpm in a 490-kHz sonoreactor [24]. In a 486-kHz sonoreactor, Yasuda et al. tested two different mixing devices and reported 1–1.5 times higher sonochemical activity for the application of the mixing (0–2,000 rpm) [25]. Bussemaker and Zhang conducted tests on various frequency sonoreactors (40, 376, 995, 1179-kHz) and found that the enhancement of sonochemical oxidation activity through the application of mixing was only observed for the low-frequency condition (40 kHz) [23]. Choi et al. conducted a study on the removal of rhodamine-B in a 28-kHz sonoreactor and found that the application of mixing at 200 rpm was most effective, resulting in 5 times higher 1st reaction constants compared to the case with no mixing [18]. The results reported thus far suggest that the application of mixing can be more effective in low-frequency sonoreactors, and the mechanical mixing technique shows promise as an option for scaling up sonoreactors.

This study aimed to investigate the impact of mechanical mixing on sonochemical oxidation activity in rectangular 28-kHz sonoreactors. Two mixing devices, namely an overhead stirrer (0–2,000 rpm) and a high-speed homogenizer (0–12,000 rpm), were employed. The sonochemical oxidation activity was quantified using the KI dosimetry, whereas the sonochemical active zone was visually observed using the sonochemiluminescence (SCL) method. Additionally, to explore the effect of the sonoreactor size under mixing application, five cylindrical sonoreactors with different diameters (12–27 cm) were used. Furthermore, the influence of mixing position was investigated, considering three positions: top, middle, and bottom.

2. Materials and methods

2.1. Chemicals

Junsei Chemical Co. Ltd. (Tokyo, Japan) supplied KI. Sigma–Aldrich Co. (USA) provided BPA and luminol (3-aminophthalhydrazide, C8H7N3O2). Samchun Pure Chemical Co., Ltd. (Korea) supplied the NaOH. All chemicals were used exactly as received.

2.2. Ultrasonic system

An acrylic rectangular sonoreactor (L × W × H: 130 mm × 130 mm × 300 mm) was employed in this study, with a 28-kHz transducer module (Mirae Ultrasonic Tech., Bucheon, Korea) positioned at the bottom of the sonoreactor. The investigation focused on the effect of mixing rate on sonochemical oxidation activity under various liquid height/volume conditions. In our preliminary tests, the application of violent mixing was more advantageous in low-frequency sonoreactors, than high-frequency sonoreactors as reported in previous research [23]. Two types of mixing apparatus were used: an overhead stirrer (X10, Ystral, Germany) and a high-speed homogenizer (Hei-TORQUE Value 100, Heidolph, Germany). The overhead stirrer was equipped with a stainless-steel dissolver impeller (diameter: 50 mm), and the mixing rate ranged from 0 to 2,000 rpm. The high-speed homogenizer (rotor–stator mixer) was equipped with a dispersing tool (stator diameter: 25 mm), and the mixing rate ranged from 0 to 12,000 rpm. For reference, Fig. 1 illustrates a schematic of the ultrasonic system used in this study.

Fig. 1.

Fig. 1

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Schematic of the 28 kHz ultrasonic system used in this study.

The liquid height/volume was determined based on the wavelength of the applied frequency; the wavelength was calculated using Equation (1):

λ=cf (1)

where λ is the wavelength, c is the speed of sound in water (1500 m/s), and f is the applied frequency (28 kHz). The tested liquid height/volume ranged from 2λ (107 mm, 1.8 L) to 4λ (214 mm, 3.7 L). The temperature of the liquid body was maintained at 20 ± 2 °C. The working electrical power ranged from 65 to 70 W, as measured using a power meter (HPM-300A, ADPower, Korea). The ultrasonic power, also known as calorimetric power, was calculated as follows using Equation (2):

Pcal=dTdtCPM (2)

where Pcal is the ultrasonic or calorimetric energy, dT/dt is the rate of increase in liquid temperature, Cp is the specific heat capacity of the liquid (4.184 J/(g·K) for water), and M is the mass of the liquid. Regarding the combination of ultrasound and mechanical mixing, the ultrasonic power was obtained by subtracting the calorimetric power for the “mixing” from the calorimetric power for the “US/mixing.”

2.3. Quantification of sonochemical oxidation reactions

Sonochemical oxidation reactions were quantified using KI dosimetry. The initial concentration of the KI solution was 10 g/L, and the irradiation time was 20 min. The final sonochemical product, triiodide (I3−), was detected using a UV–Vis spectrophotometer (Libra S60; Biochrom Ltd., UK).

2.4. Visualization of sonochemical oxidation reactions

The sonochemically active zone was visualized using a luminol solution (0.1 g/L luminol and 1 g/L NaOH) in a completely dark room [17], [27]. SCL images were captured using an exposure-controlled digital camera (α58; Sony Corp., Japan). The exposure time was set at 60 s.

3. Results and discussion

3.1. Effect of mixing rate

The study investigated the impact of mixing rate on the sonochemical oxidation activity in a 28-kHz rectangular sonoreactor under different liquid height conditions, namely 2λ (1.8 L), 3λ (2.8 L), and 4λ (3.7 L). For both the overhead stirrer and the high-speed homogenizer, the dissolver impeller and the dispersing tool were placed in the top region adjacent to the liquid surface. The sonochemical oxidation activity was quantified and compared by measuring the mass of sonochemically generated I3- ions under various liquid height/volume conditions [27]. Fig. 2 displays the mass of I3- ions under different mixing rates for the application of the overhead stirrer (0, 100, 250, 500, 1,000, and 2,000 rpm). In the condition of “US/0 rpm”, the stirrer was placed adjacent to the liquid surface and did not work. So the stirrer could inhibit the ultrasound transmission in the liquid. Additionally, Fig. 3 shows the SCL images and corresponding real images for the liquid height of 4λ. In cases where no mixing was applied (“US” and “US/0 rpm”), relatively low sonochemical oxidation activity was observed. However, as the mixing rate increased, the sonochemical oxidation activity showed a significant improvement. Notably, the highest activity for liquid heights of 2 and 3λ was observed at 500 rpm, whereas the highest activity for 4λ was observed at 250 rpm.

Fig. 2.

Fig. 2

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Sonochemical oxidation activity (mass of I3− ions) using the overhead stirrer (0–2,000 rpm) for liquid level conditions of 2λ–4λ.

Fig. 3.

Fig. 3

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Sonochemiluminescence images for the mixing conditions of Fig. 2.

The study revealed that the significant enhancement in sonochemical oxidation activity was due to the substantial change in the sonochemical active zone due to the application of violent mixing, as depicted in Fig. 3. Visually, the cases of “US”, “US/0 rpm” and “US/100 rpm” displayed weak light, indicating low sonochemical oxidation activity. However, higher mixing rates (250 – 2,000 rpm) exhibited bright and strong light, representing very high sonochemical activity. With the increase in mixing rate, the stability of the liquid surface decreased, and the number of small bubbles, generated by the violent mixing, increased. As the mixing rates increased, the standing wave field gradually diminished, primarily due to the heightened instability of the liquid surface. Consequently, the active zone became concentrated in a small region adjacent to the transducer at the bottom, owing to the excessive presence of bubbles. At the maximum mixing rate of 2,000 rpm, no standing wave field was observed, and a cone-shaped active zone formed at the bottom. This occurred because a large number of mixing-generated bubbles hindered the upward transmission of ultrasound. For the SCL image of “US/500 rpm”, the active zone consisted of the standing wave field adjacent to the liquid surface, and the concentrated zone adjacent to the bottom. Their combination might have led to the highest sonochemical oxidation activity observed in this study.

Previous researchers have extensively studied the effect of mechanical mixing on sonochemical oxidation activity, considering various frequencies (28–1,179 kHz), mixing rates (0–2,000 rpm), and geometric conditions (0.5–11.2 L) [13], [18], [23], [24], [25]. Their findings revealed that the enhancement of sonochemical oxidation activity induced by mechanical mixing was obtained mainly under low-frequency conditions (28, 35, and 40 kHz) [13], [18], [23]. It might be due to the significant change in sonochemical active zone as shown in this study. However, for higher frequency conditions, the application of mechanical mixing significantly reduced the sonochemical oxidation activity [13], [23], primarily because high-frequency ultrasound in the range of hundreds of kHz possesses low physical transmission power [8]. Kojima et al. reported enhanced sonochemical oxidation activity through the application of mechanical mixing for 490-kHz sonoreactor. This enhancement might be attributed to the careful adjustment of mixing to avoid significant inhibition of ultrasonic irradiation in the relatively large sonoreactor [24]. Although no enhancement was observed due to mixing, Bussemaker and Zhang visually observed a concentrated active zone at high mixing rates (600 and 900 rpm) for high-frequency conditions in their SCL images [23]. In addition, it has been reported that continuous gas sparging can induce similar results, leading to the formation of a concentrated active zone adjacent to the transducer and a significant enhancement of sonochemical oxidation activity [17], [28], [29].

Moreover, as depicted in Fig. 2, the highest activity for each liquid height condition was 5.6, 17.3, and 13.3 times higher than that of “US” for 2, 3, and 4λ, respectively. Overall, the highest activities were achieved at 3λ for mixing rates of 500 rpm. It appears that the application of mechanical mixing was more effective for higher mixing rate and liquid height (larger liquid volume) conditions when considering the activity compared to “US”. This efficacy could be attributed to a more effective inhibition of ultrasound transmission and a more rapid movement of reactants (I- ions) to the sonochemical active zone [18], [23], [24], [28]. Further research for higher/larger liquid height/volume conditions (5λ ∼ ) is required to establish the optimal relationship between liquid height/volume in sonoreactors and the mechanical mixing rates/intensities.

Fig. 4 presents the mass of I3- ions obtained using the high-speed homogenizer at various mixing rates (0, 3,500, 7,000, and 12,000 rpm). Additionally, Fig. 5 displays the SCL images along with corresponding real images for the liquid height of 4λ. It should be noted that the mixing intensities for the homogenizer (0–2,000 rpm) were not significantly higher than those for the stirrer (0–12,000 rpm), despite the higher mixing rates. This disparity is due to the different mechanisms by which the two mixing devices induce violent mixing in the liquid phase.

Fig. 4.

Fig. 4

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Sonochemical oxidation activity (mass of I3− ions) using the high-speed homogenizer (0–12,000 rpm) for liquid level conditions of 2λ–4λ.

Fig. 5.

Fig. 5

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Sonochemiluminescence images for the mixing conditions of Fig. 4.

Similar to the results obtained with the overhead stirrer, the application of liquid mixing with the high-speed homogenizer resulted in a meaningful enhancement of sonochemical oxidation activity, primarily due to the significant change in the sonochemical active zone. However, no significant activity was observed at the maximum mixing condition (only mixing: 12,000 rpm), and the highest sonochemical oxidation activity was achieved at 3,500 rpm for all three liquid heights.

Notably, the highest activity for each liquid height condition was 6.5, 18.8, and 10.2 times higher than that of “US” for 2λ, 3λ, and 4λ, respectively. From the SCL images, it appeared that the mixing intensity for 3,500, 7,000, and 12,000 rpm of the homogenizer was very similar to that for 500, 1,000, and 2,000 rpm of the stirrer, respectively.

For the liquid height of 4λ, the calorimetric power, also known as ultrasonic power, was determined by measuring the temperature increase for the “US/mixing” and “mixing” cases. The working electrical power of the ultrasonic device ranged from 65 to 70 W for both “US” and “US/mixing” cases. For the application of the overhead stirrer, the calculated calorimetric power was 20.7 ± 2.6, 34.9 ± 2.2, 39.7 ± 2.2, 34.2 ± 1.8, 44.2 ± 0.6, and 25.6 ± 4.2 W for US, US/100 rpm, US/250 rpm, US/500 rpm, US/1,000 rpm, and US/2,000 rpm, respectively. For the application of high-speed homogenizer, the calorimetric power was found to be 20.7 ± 2.6, 34.7 ± 5.1. 44.0 ± 3.0, and 31.4 ± 3.2 W for US, US/3,500 rpm, US/7,000 rpm, and US/12,000 rpm, respectively.

Comparatively, higher calorimetric power was observed for the “US/mixing” cases, where higher sonochemical oxidation activity was obtained, as compared to “US” alone. However, no meaningful relationship between the calorimetric power and sonochemical oxidation activity was found for the “US/mixing” cases. The effect of mechanical mixing on the calorimetric power during ultrasound irradiation has rarely been reported [23], [24].

In bath-type sonoreactors equipped with the transducer module at the bottom, the sonochemical active zone primarily forms in the standing wave field adjacent to the liquid surface. However, higher liquid height from the transducer to the liquid surface can lead to larger ultrasound attenuation and reduced sonochemical activity [30]. These phenomena pose challenges in scaling up sonoreactors. Consequently, we believe that the formation of a concentrated sonochemical active zone through the application of mechanical mixing, while considering the characteristics of mixing devices, holds promise as a technique for large-scale applications of low-frequency sonoreactors [31].

3.2. Effect of mixing position

Fig. 6 illustrates the impact of mixing position on the sonochemical oxidation activity for liquid heights of 2λ and 4λ, and corresponding SCL and real images are shown in Fig. 7. The overhead stirrer and the high-speed homogenizer were operated at mixing rates of 500 rpm and 3,500 rpm, respectively, as these rates yielded the highest activity in previous investigations. Conventionally, it is recommended to position the stirring part, such as the impeller, at the bottom region in stirred vessels [32]. In this study, as the immersion depth of the mixing device increased, the active zone underwent changes, resulting in significant variations in the sonochemical oxidation activity.

Fig. 6.

Fig. 6

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Sonochemical oxidation activity (mass of I3− ions) using the overhead stirrer (500 rpm) and high-speed homogenizer (3,500 rpm) for various mixing positions and liquid level conditions.

Fig. 7.

Fig. 7

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Sonochemiluminescence and real images for the mixing conditions of Fig. 6.

For 2λ, the lowest activity was observed when the mixing was applied at the bottom position (very close to the transducer) for both mixing devices. In the SCL images, the active zone shrank, the standing wave field diminished, and high activity was primarily observed at the bottom region. This phenomenon could be attributed to the large ultrasound attenuation caused by the blockage of ultrasound transmission by the impeller and dispersing tool, as well as the device-induced flow [18].

For 4λ, a similar level of activity was obtained for each mixing device, although the visually observed active zone varied significantly for the three mixing positions. When the mixing was applied at the bottom, the sonochemical active zone shrank and concentrated at the bottom region, similar to the cases observed for 2λ. However, a larger and more concentrated active zone was formed, resulting in relatively high activity. Therefore, it was found that the mixing effect on the sonochemical oxidation activity could vary significantly depending on the mixing position and liquid height/volume in the sonoreactor.

Previous researchers have applied the mixing device at different regions in the sonoreactors, such as the top region close to the liquid surface [23], [24], the middle region [25], or the bottom region [12], [13], [18], and they did not change the mixing position during their experiments. It has been reported that the highest sonochemical oxidation activity was obtained when gas sparging was applied at the bottom adjacent to the transducer, and the activity decreased significantly as the gas sparger moved away from the bottom to the liquid surface. Moreover, very concentrated active zones, similar to those observed in this study, were formed when the sparger was placed at the bottom [17], [28].

3.3. Effect of reactor size

In our previous research, we observed a significant increase in sonochemical oxidation activity as the liquid height was increased from 1λ to 5λ for low frequency conditions (36 kHz) under the same input power condition [9], [28], [33]. Asakura et al. also reported a similar trend, finding that the sonochemical oxidation activity increased with increasing liquid height/volume for various frequency conditions (45–490 kHz) under the same input power condition. They obtained the highest activity at liquid heights of 9.5λ (29 mm), 12.2λ (79 mm), 13.1λ (152 mm), and 15.0λ (500 mm) for 490, 231, 129, and 45 kHz, respectively [30]. The studies mentioned above primarily used cylindrical sonoreactors equipped with the transducer at the bottom. The enhancement observed for higher liquid height/volume conditions can be attributed to the stable formation of the sonochemical active zone, including the standing wave field, over certain liquid height conditions [9], [30]. However, in this study, a noticeable decrease in sonochemical activity was observed as the liquid height increased from 2λ to 4λ for the “US” cases, as shown in Fig. 2, Fig. 4. This could be attributed to the geometric effect of the sonoreactor. Hence, the investigation focused on the effect of lateral boundary conditions, perpendicular to the ultrasound transmission, of the sonoreactor on the sonochemical oxidation activity using cylindrical sonoreactors with various diameters and the high-speed homogenizer. Fig. 8 presents the sonochemical oxidation activity for various reactor sizes/volumes (liquid height: 4λ; diameter: 12, 17, 18, 22, and 27 cm) and mixing conditions (at the top region: 0–12,000 rpm), and Fig. 9 displays the corresponding SCL images. Real images for the SCL images are shown in Fig. 1S.

Fig. 8.

Fig. 8

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Sonochemical oxidation activity (mass of I3− ions) using the high-speed homogenizer for various reactor size conditions. The liquid height was 4λ.

Fig. 9.

Fig. 9

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Sonochemiluminescence images for the mixing rate and reactor size conditions of Fig. 8.

In the case of “US,” the sonochemical oxidation activity decreased as the reactor size/volume increased. It is worth noting that no significant activity was observed for the 17 cm reactor (the experiment was repeated five times for this condition), leading us to conduct an additional test with an 18 cm reactor. Interestingly, a much higher activity was observed for the 18 cm reactor compared to the 17 cm reactor. The lack of significant activity for the 17 cm reactor might be attributed to the lateral boundary of the reactor, suggesting that most of the ultrasound energy was absorbed by the reactor wall. The difference in the formation of the active zone according to the reactor size was evident in the SCL images for the “US”. Bussemaker and Zhang also reported no sonochemical oxidation activity (no SCL) for several cases [23]. Consequently, it was found that the 12 cm reactor was determined as optimal for the sonochemical oxidation activity among the tested reactors without the application of the mixing.

Significantly higher activities were observed for all cases with mixing compared to the “US,” indicating a positive transformation of the active zone due to the violent mixing. Particularly for the 17 cm reactor, the activity was 39 – 69 times higher than that of the “US,” where no significant activity was observed. Thus, the application of mixing techniques proved to be effective in mitigating the low sonochemical activity caused by geometric conditions, which could not have been anticipated before conducting the experiments.

At a mixing rate of 12,000 rpm, similar active zones in the form of cone shapes adjacent to the transducer were observed for all reactor size conditions. The same trend was observed for 7,000 rpm, with the main difference being the intensity of the standing wave field. However, despite the formation of similar active zones, the measured sonochemical oxidation activity (mass of I3- ions) varied for these cases. This discrepancy can be attributed to a drawback of the SCL method, which involves 2-dimensional activity analysis [17]. However, it should be noted that the sonochemical activities in terms of the mass of I3- ions as shown in Fig. 8 were not significantly different for the various reactors considering the difference in the reactor size and liquid volumes.

The effect of the mixing position for various reactor size conditions was also investigated at 3,500 rpm, and the results are shown in Fig. 2S (mass of I3- ions) and Fig. 3S (SCL and real images). With the exception of the smallest reactor condition, the top position was determined to be the optimal position for the application of the high-speed homogenizer (3,500 rpm) in this study.

Consequently, it was revealed that the application of mixing techniques could significantly enhance the sonochemical activity regardless of the geometric conditions of the sonoreactors. However, no general rule for optimizing the application of mixing techniques to achieve higher sonochemical activity under various geometric conditions has been established yet. Further research is needed, involving the application of various mixing devices in sonoreactors with diverse geometric conditions and corresponding computational 3D simulations to study the formation of the sonochemical active zone and liquid flow pattern.

4. Conclusion

The effect of violent mixing induced by the overhead stirrer (0–2,000 rpm) and high-speed homogenizer (0–12,000 rpm) on the sonochemical oxidation activity (measured using KI dosimetry) was investigated for various geometric conditions of 28-kHz sonoreactors. The application of vigorous mixing significantly affected the formation of the sonochemical active zone, which was visualized using the luminol method (SCL images). This resulted in a substantial enhancement of the sonochemical activity. The mixing technique proved to be more advantageous for higher mixing rate and liquid height conditions, with the optimal mixing positions identified as the Top and Mid positions in the reactor. Notably, under a specific geometric condition (17 cm cylindrical sonoreactor without mixing in this study), no sonochemical oxidation activity was observed due to the significant absorption of ultrasound energy by the reactor boundaries. However, it was found that the application of violent mixing could ensure high levels of the sonochemical oxidation activity under various geometric conditions. Therefore, mechanical mixing techniques appear to be one of the promising options for achieving higher performance in sonochemical processes and scaling up sonoreactors.

CRediT authorship contribution statement

Dukyoung Lee: Methodology, Validation, Investigation, Writing – review & editing, Funding acquisition, Formal analysis. Jumin Kang: Methodology, Validation, Investigation, Formal analysis, Writing – original draft. Younggyu Son: Conceptualization, Methodology, Writing – review & editing, Funding acquisition, Writing – original draft, Visualization, Supervision.

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.

Acknowledgments

This work was supported by the National Research Foundation of Korea [NRF-2021R1A2C1005470, NRF-2022R1A6A3A13064210] and by the Korea Ministry of Environment (MOE) as “Subsurface Environment Management (SEM)” Program [project No. 2021002470001]. Dr. Y.G. Ahn (Korea Basic Science Institute, Western Seoul Center) is gratefully acknowledged for the help with data analysis.

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2023.106659.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Supplementary data 1
mmc1.docx (2.7MB, docx)

Data availability

No data was used for the research described in the article.

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