Elsevier

Ultrasonics Sonochemistry

Volume 58, November 2019, 104599
Ultrasonics Sonochemistry

Cavitational activity in heterogeneous systems containing fine particles

https://doi.org/10.1016/j.ultsonch.2019.05.016Get rights and content

Highlights

  • Calorimetric energy and cavitational activity measurements were carried out in heterogeneous systems.

  • Different sonochemiluminescence trends were observed depending up on the presence and size of beads.

  • Ultrasonic soil washing for the remediation of clay-sized soils was used as a case study.

Abstract

Ultrasound has been increasingly used in various processes containing a variety of homogeneous and heterogeneous systems. For largescale applications, a high energy efficiency of the process is required. With this view, the calorimetric energy and cavitational activity measurements were carried out in heterogeneous systems consisting of both liquid and solid phases (fine particles) in a 28-kHz double-bath sonoreactor. Ultrasonic soil washing for the remediation of clay-sized soils (∼75 µm), contaminated with metals (Cu, Pb, and Zn), was used as a case study. As the liquid height/volume in the inner vessel increased under the same input electrical power, the inner vessel calorimetric energy also increased, whereas the total calorimetric energy between the inner vessel and the outer reactor remained approximately constant. No significant differences in calorimetric energies were observed for both with and without soil conditions. The chemical activity under similar experimental conditions was evaluated using sonochemiluminescence. Different sonochemiluminescence trends were observed depending up on the presence and size of beads. The highest total sonochemiluminescence intensity with a uniform spatial distribution was obtained from fine beads (#200, 75 µm) suspended in the vessel. Ultrasound application significantly enhanced the removal efficiency of heavy metals when combined with mechanical agitation. The enhanced removal efficiency of the combined processes was attributed to a significant removal of metals from the residual (F5) fraction. It has been concluded that ultrasound has enough extraction power to be comparable to methods that employ extremely powerful acids for washing fine particles.

Introduction

Acoustic cavitation has been widely studied in chemical and environmental engineering processes for decades. Previous studies have reported the effectiveness of ultrasound arising mainly from sonochemical effects, like pyrolysis and radical oxidation/reduction reactions, in homogeneous systems containing liquids [1], [2], [3], [4], [5], [6], [7]. Recently, some researchers have focused on the application of ultrasound to heterogeneous systems containing both liquid and solid phases. The solid-to-liquid ratio of the system can be determined on micro- (∼10−6) to deci- (∼10−1) scales depending on the application, such as catalytic processes, nano/micro material syntheses, and washing/cleaning processes [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. It is well known that solid materials in a liquid phase can be adequately dispersed, damaged, and cleaned via sonophysical effects like microjets, shockwaves, and microstreaming.

The enhancement of cavitational effects resulting from the addition of fine particles has been reported in previous studies. Tuziuti et al. found that a larger amount of radical oxidation, cavitation noise, and calorimetric energy resulted from the addition of alumina particles (d: 1–80 µm) compared with a system without particle addition. This enhancement via particle addition was attributed to an increased number of nucleation sites and active cavitation bubbles [12]. They also reported a higher sonochemiluminescence (SCL) intensity upon the addition of both Teflon particles (powder, d: 10 µm) and alumina particles (d: 10 µm) [11]. Kim et al. investigated the effect of biochar powder addition on the sonochemical degradation of aqueous pollutants. They observed an enhancement in degradation that arose from a combination of adsorption and radical oxidation on the biochar surface [13]. Vinodgopal et al. used high-frequency ultrasound to achieve a stable suspension and dispersion of graphene particles and metal nanoparticles and, as a result, successfully synthesized graphene-Au nanocomposites [10]. Furthermore, Neppolian et al. suggested an ultrasound-assisted synthesis of nano-photocatalysts consisting of dispersed TiO2 and Pt particles on graphene oxide sheets [8].

In previous studies on large solid materials, low-frequency (20–40 kHz) ultrasound technology has been widely applied for washing, cleaning, and extraction processes. Choi et al. equipped a conventional washing machine with an ultrasonic system to wash contaminated textiles and reported a higher washing efficiency in the combined process than the individual processes [16]. Gotoh et al. also tested ultrasound and mechanical cleaning processes for various contaminated fabrics and they found that the combined processes were effective with less damage on the fiber surfaces [21]. Balachandran et al. applied ultrasound technology to a supercritical extraction process to obtain a high extraction efficiency and yield [22].

Some studies have demonstrated the advantages of ultrasound in soil washing processes for the remediation of diesel- and metal-contaminated sand-sized (∼2 mm) soils [15], [18], [23], [24]. These work have focused mainly on the optimization of such ultrasonic soil washing processes and have achieved higher performances in terms of pollutant removal efficiency, washing time, energy and chemical consumption, and washing leachate production. However, very little research has been reported on calorimetric energy and cavitational activity regarding ultrasonic soil washing systems.

In this study, we investigated the calorimetric energy and SCL in a heterogeneous system consisting of glass beads in various solid-to-liquid ratios to elucidate their energy distributions and cavitational activities in a double-bath ultrasonic reactor. In addition, heavy-metal (Cu, Pb, and Zn) removal was attempted in ultrasonic soil washing processes under optimal ultrasonic washing conditions for fine-grained soils (less than 75 µm).

Section snippets

Sonoreactor

Fig. 1 is a schematic of the double-bath ultrasonic reactor used in this study. The rectangular stainless-steel washing vessel (15 cm × 15 cm × 15 cm) was submerged in a rectangular sonoreactor (20 cm × 20 cm × 20 cm), which was equipped with an ultrasonic transducer module underneath. The washing vessel was placed 2 cm above the bottom of the sonoreactor, and the thickness of the bottom vessel plate was 5 mm. The sonoreactor was filled with 2 L of water, and the temperature was maintained at

Calorimetric energy

Fig. 2 shows the calculated calorimetric energies measured under various experimental conditions. The calorimetric energies were determined from the heat increase in the water outside of the washing vessel, the volume of which was 2 L for all cases. The mass of water and fine soils (∼75 μm) in the washing vessel changed depending on the L:S ratio and the presence of soils. The calorimetric energies for the systems containing soil were determined from the heat increase observed in water/soil

Conclusions

The ultrasonic effects of calorimetric energy and SCL in a 28-kHz heterogeneous system consisting of a liquid phase and a solid phase (fine particles) were investigated in a double-bath sonoreactor. For both with and without soil systems, the calorimetric energy in the inner vessel increased as the liquid height increased, whereas the total calorimetric energy in both the inner vessel and the outer sonoreactor remained approximately constant. In the presence of clay-sized glass beads (75 µm),

Acknowledgments

This work was supported by the Korea Ministry of Environment (MOE) as “SEM (Subsurface Environment Management)” Program [project No. 2018002480009] and the National Research Foundation of Korea [Grant No. NRF-2018R1D1A1B07048124].

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