Cavitational activity in heterogeneous systems containing 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].
References (43)
- et al.
Oxygen-induced concurrent ultrasonic degradation of volatile and non-volatile aromatic compounds
Ultrason. Sonochem.
(2007) Simple design strategy for bath-type high-frequency sonoreactors
Chem. Eng. J.
(2017)- et al.
A parametric review of sonochemistry: control and augmentation of sonochemical activity in aqueous solutions
Ultrason. Sonochem.
(2017) - et al.
The effects of liquid height/volume, initial concentration of reactant and acoustic power on sonochemical oxidation
Ultrason. Sonochem.
(2014) - et al.
Graphene oxide based Pt–TiO2 photocatalyst: ultrasound assisted synthesis, characterization and catalytic efficiency
Ultrason. Sonochem.
(2012) - et al.
Sonophotocatalytic degradation of 4-chlorophenol using Bi2O3/TiZrO4 as a visible light responsive photocatalyst
Ultrason. Sonochem.
(2011) - et al.
Peat moss-derived biochar for sonocatalytic applications
Ultrason. Sonochem.
(2018) - et al.
Sonocatalytic degradation of Reactive Yellow 39 using synthesized ZrO2 nanoparticles on biochar
Ultrason. Sonochem.
(2017) - et al.
Ultrasonic and mechanical soil washing processes for the removal of heavy metals from soils
Ultrason. Sonochem.
(2017) - et al.
Ultrasonic washing of textiles
Ultrason. Sonochem.
(2016)