•The effect of ultrasound on mechanical soil washing processes was investigated.•The ultrasonically treated concentrations of heavy metals were highly satisfactory.•Ultrasound played a more important ...role in less acidic or washing liquid conditions.
In order to determine the optimal operating conditions of full-scale soil washing processes for the removal of heavy metals, the effect of high-power ultrasound on the conventional mechanical soil washing process was investigated in a large lab-scale 28kHz sonoreactor. The soil samples were obtained from an abandoned railway station site in Seoul, Korea, which was contaminated with Cu (242.7±40.0mg/kg), Pb (441.3±49.8mg/kg), and Zn (358.0±35.7mg/kg). The treated concentrations of three heavy metal species in each process were compared with the regulation levels. It was found that higher performance, satisfying the regulation levels, was obtained in the ultrasonic/mechanical process due to the combined effects of macroscale mixing and microscale sonophysical effects. Moreover ultrasound played a more important role in less favorable conditions for the mechanical washing process (less acidic or less washing liquid conditions). Considering the application of the ultrasonic/mechanical soil washing process in real contaminated sites, the optimal conditions for the reactor with the bottom area of 15×15cm2 and the input ultrasound power of 250W were determined as follows: (1) the amount of soil per an operation was a 300g; (2) the ratio of soil and liquid was 1:3; (3) the concentration of acidic washing liquid was 0.5M HCl.
•Zeroth- and First-order sonochemical reactions were compared in a 300 kHz sonoreactor.•KI dosimetry and BPA degradation were employed under various liquid levels/volumes.•Gas saturation and sparging ...were applied using Ar:O2 (75:25) and N2:O2 (75:25).•Gas sparging was effective for higher liquid level/volume conditions.•A comparison of the required reaction times is suggested for 1st-order reaction kinetics.
The sonochemical oxidation activity was investigated for gas saturation and gas sparging under various liquid levels and volumes in 300 kHz sonoreactors. The liquid levels and volumes ranged from 5λ (25 mm, 0.47 L) to 50λ (250 mm, 4.30 L) and two gas mixtures, Ar:O2 (75:25) and N2:O2 (75:25), were used. Two types of reaction kinetics were observed to quantitatively analyze the sonochemical oxidation reactions: zero-order (KI dosimetry: C0 = 60.2 mM) and first-order (Bisphenol A (BPA) degradation: C0 = 0.043 mM). The masses of the sonochemical oxidation reactions were calculated and compared rather than the concentrations to more accurately compare the sonochemical oxidation activity under different liquid volume conditions. First, as the liquid level or volume increased for the zero-order reactions, the concentration of I3- ions representing the volume-averaged activity decreased substantially for gas saturation owing to the increase in liquid volume. However, gas sparging substantially enhanced sonochemical oxidation activity, and the mass of I3- ions representing the total activity remained constant as the liquid level increased from 20λ because of the improved liquid mixing and a shift in the sonochemical active zone. Second, as evidenced by the zero-order reactions, the concentration of BPA decreased considerably as the liquid level or volume increased in the first-order reactions. When gas sparging was used, higher reaction constants were obtained for both gas mixtures, ranging from 40λ to 50λ. However, a comparison of the sonochemical oxidation activity in terms of the degraded mass of BPA was inapplicable as the concentration of BPA decreased substantially and a lack of reactants occurred for the lower liquid level and volume conditions as the irradiation time elapsed. Instead, using the first-order reaction constant, a comparison of the required reaction times for a specific removal efficiency (30%, 60%, and 90%) was proposed. Gas sparging can substantially reduce the reaction time required for a liquid level of 40λ or higher.
•Effects of gas saturation/sparging were studied for sonochemical NO2−/NO3− generation.•Twelve gas conditions of Ar, O2, N2, and binary mixtures were tested.•N2:Ar (25:75) resulted in the highest ...NO2− and NO3− concentrations under all conditions.•H2O2/NO2−/NO3− related OH radical activity was suggested.•Gas sparging enhanced the generation of NO2− and NO3− for O2:N2, N2, and N2:Ar.
The sonochemical generation of NO2− and NO3− is considered to be one of the reasons for the low sonochemical oxidation activity in the presence of N2 in the liquid phase. In this study, the generation characteristics of NO2− and NO3− were investigated using the same 28 kHz sonoreactor and the 12 gas conditions used in Part I of this study. Three gas modes, saturation/closed, saturation/open, and sparging/closed, were applied. N2:Ar (25:75), N2:Ar (50:50), and O2:N2 (25:75) in the saturation/closed mode generated the three highest values of NO2− and NO3−. Ar and O2 were vital for generating relatively large concentrations of NO2− and NO3−. The absorption of N2 from the air resulted in high generation of NO2− and NO3− for Ar 100 % and Ar/O2 mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO2− and NO3− for N2:Ar (25:75), O2:N2 (25:75), and N2 significantly because of the change in the sonochemically active zone and the increase in the mixing intensity in the liquid phase, as discussed in Part I. The ratio of NO3− to NO2− was calculated using their final concentrations, and a ratio higher than 1 was obtained for the condition of Ar 100 %, Ar/O2 mixtures, and O2 100 %, wherein a relatively high oxidation activity was detected. From a summary of the results and findings of previous studies, it was revealed that the observations of NO2− + NO3− could be more appropriate for investigating the NO2− and NO3− generation characteristics. In addition, H2O2/NO2−/NO3− related activity rather than H2O2 activity was suggested to quantify the OH radical activity more appropriately in the presence of N2.
Dissolved gases have a substantial influence on acoustic cavitation and sonochemical oxidation reactions. Little research on the changes in dissolved gases and the resultant changes in sonochemical ...oxidation has been reported, and most studies have focused only on the initial dissolved gas conditions. In this study, the dissolved oxygen (DO) concentration was measured continuously during ultrasonic irradiation using an optical sensor in different gas modes (saturation/open, saturation/closed, and sparging/closed modes). Simultaneously, the resulting changes in sonochemical oxidation were quantified using KI dosimetry. In the saturation/open mode using five gas conditions of Ar and O2, the DO concentration decreased rapidly when O2 was present because of active gas exchange with the atmosphere, and the DO concentration increased when 100% Ar was used. As a result, the order of the zero-order reaction constant for the first 10 min (k0-10) decreased in the order Ar:O2 (75:25) > 100% Ar ≈ Ar:O2 (50:50) > Ar:O2 (25:75) > 100% O2, whereas that during the last 10 min (k20-30) when the DO concentration was relatively stable, decreased in the order 100% Ar > Ar:O2 (75:25) > Ar:O2 (50:50) ≈ Ar:O2 (20:75) > 100% O2. In the saturation/closed mode, the DO concentration decreased to approximately 70–80% of the initial level because of ultrasonic degassing, and there was no influence of gases other than Ar and O2. Consequently, k0-10 and k20-30 decreased in the order Ar:O2 (75:25) > Ar:O2 (50:50) > Ar:O2 (25:75) > 100% Ar > 100% O2. In the sparging/closed mode, the DO concentration was maintained at approximately 90% of the initial level because of the more active gas adsorption induced by gas sparging, and the values of k0-10 and k20-30 were almost the same as those in the saturation/closed mode. In the saturation/open and sparging/closed modes, the Ar:O2 (75:25) condition was most favorable for enhancing sonochemical oxidation. However, a comparison of k0-10 and k20-30 indicated that there would be an optimal dissolved gas condition that was different from the initial gas condition. In addition, the mass-transfer and ultrasonic-degassing coefficients were calculated using changes in the DO concentration in the three modes.
•20-kHz sonicator systems were investigated under various geometric conditions.•Sonochemical oxidation activities and SCL images were compared in large-scale vessels.•The change in the probe position ...could change sonochemical activity significantly.•The enhancement of activity could be attributed to strong reflections at the bottom and side wall.
The 20-kHz probe-type sonicator systems were investigated for the enhancement of the cavitational oxidation activity under various geometric conditions including vertical and horizontal probe positions and vessel sizes/volumes as a following study to our previous study. The sonochemical oxidation activity (mass-based I3- ion generation rate) increased significantly for all vessel size conditions as the probe was placed close to the vessel bottom, owing to the expansion of the sonochemical active zone induced by the reflections of ultrasound at the bottom and the reactor wall. A concentric circular active zone is observed at positions close to the bottom. The highest sonochemical activity was obtained at 1 cm (vertical position) in the 20 cm vessels (input power: 50 %). At the vertical positions of 11 cm to 7 cm, no significant difference in the sonochemical activity was observed for all input power conditions (25, 50, and 75 %) because no meaningful reflections occurred. Higher sonochemical activities were obtained at an input power of 75 % owing to the increased power and strong reflection. The highest cavitational yield considering the energy efficiency was obtained at 6 cm (vertical position) for 75 % of all power and geometric conditions. Horizontal probe position tests showed that the asymmetric formation of the sonochemical active zone could significantly enhance the sonochemical activity. The highest activity was obtained at 1 cm (vertical position) and 2.5 cm (horizontal position) in the 20 cm vessel.
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•Effects of gas saturation and sparging were studied for sonochemical H2O2 generation.•Gas conditions of Ar, O2, N2, and binary mixtures (12 conditions) were tested.•Highest H2O2 ...generation for saturation and sparging conditions occurred at Ar:O2 (75:25).•Higher H2O2 generation was obtained for gas sparging under all gas conditions.•A bulb-shaped active zone was observed for Ar/O2 mixture sparging conditions.
Cavitational/sonochemical activity can be significantly enhanced or reduced depending on the gases dissolved in the liquid. Although many researchers have suggested the order of importance of dissolved gas conditions that affect the degree of sonoluminescence (SL), sonochemiluminescence (SCL), and compound degradation, the most suitable gas condition for sonochemical oxidation reactions is currently unknown. In this study (Part I), the effects of gas saturation and sparging on the generation of H2O2 were investigated in a 28-kHz sonoreactor system. Four gas modes, saturation/closed, saturation/open, sparging/closed, and sparging/open, were applied to Ar, O2, N2, and binary gas mixtures. The change in dissolved oxygen (DO) concentration during ultrasonic irradiation was measured and was used as an indicator of whether the gaseous exchange between liquid and air altered the gas content of the liquid. Considerable difference in the DO concentration was observed for the gas saturation/open mode, ranging from –11.5 mg/L (O2 100 %) to +4.3 mg/L (N2 100 %), while no significant difference was observed in the other gas modes. The change in the gas content significantly reduced the linearity for H2O2 generation, which followed pseudo-zero-order kinetics, and either positively or negatively affected H2O2 generation. Ar:O2 (75:25) and Ar:O2 (50:50) resulted in the highest and second-highest H2O2 generation for both gas saturation and sparging, respectively. In addition, gas sparging resulted in much higher H2O2 generation for all gas conditions compared to gas saturation; this was because of the significant change in the cavitational active zone and concentrated ultrasonic energy, which formed a bulb-shaped active zone, especially for the Ar/O2 mixtures adjacent to the transducer at the bottom. The sparging flow rate and position also significantly affected H2O2 generation; the highest H2O2 generation was obtained when the sparger was placed at the bottom adjacent to the transducer, with a flow rate of 3 L/min.
In Part II, the generation of nitrogen oxides, including nitrite (NO2–) and nitrate (NO3–), was investigated using the same ultrasonic system with three gas modes: saturation/open, saturation/closed, and sparging/closed.
•Ultrasonic activity was investigated in polar and nonpolar organic solvents.•Ultrasonic/mechanical soil washing for PCBs removal was also investigated.•Ultrasonic washing had higher washing ...efficiency compared to conventional process.•Degradation of PCBs in the liquid phase was investigated in photolytic processes.
Ultrasonic soil washing processes using organic solvents were investigated for the development of novel remediation technologies for persistent organic pollutants (POPs)- contaminated soils. Aluminum foil erosion was first tested to understand sonophysical activity in water, methanol (polar) and n-hexane (nonpolar) in a 28 kHz double-bath-type sonoreactor. Significant sonophysical damage on the aluminum foil was observed at the antinodes for all solvents, and the order of degree of sonophysical damage was as follows: water > methanol > n-hexane. Subsequently, conventional (mechanical mixing only) and ultrasonic soil washing (mechanical mixing and ultrasound) techniques were compared for the removal of polychlorinated biphenyls (PCBs) from soil. Two types of contaminated soils, fresh (Soil A, C0 = 2.5 mg/kg) and weathered (Soil B, C0 = 0.5 mg/kg), were used and the applied soil-to-liquid (S:L) ratio was 1:5 and 1:10 for methanol and n-hexane, respectively. The polar solvent significantly increased washing efficiencies compared to the nonpolar solvent, despite the nonpolar nature of the PCBs. Washing efficiency was significantly enhanced in ultrasonic soil washing compared to conventional washing, owing to macro- and micro-scale sonophysical actions. The highest washing efficiencies of 90% for Soil A and 70% for Soil B were observed in the ultrasonic washing processes using methanol. Additionally, a single operation of the ultrasonic washing process was superior to two sequential processes with conventional mixing in terms of washing efficiency, consumption of washing agents, treatment of washing leachate, and operation time. Finally, the removal of PCBs in an organic solvent (methanol) was investigated in photolytic and sonolytic processes for the post-treatment of soil washing leachate. A photolysis efficiency of 80% was obtained within 60 min of UV exposure for intensities of 1.0, 2.0, and 4.0 W/cm2. The primary mechanism of PCBs degradation is photolytic dechlorination. In contrast, no degradation was detected in the sonolytic process, as the excess organic solvent acted as a strong radical scavenger.
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•Sonochemical and sonophysical activities were investigated in 20 kHz probe systems.•These two activities were enhanced when the probe was placed close to the bottom.•Sonophysical ...desorption primarily occurred when the painted beads were suspended.
Even though acoustic cavitation has been widely investigated, only few researchers focused on the relationship between sonochemical and sonophysical activities and on the enhancement of sonophysical activity. In this study, sonochemical and sonophysical activities were investigated in a heterogeneous system to understand the relationship between these two activities and to suggest optimal conditions for ultrasonic desorption/extraction processes comprising milli-sized glass beads. The sonochemical activity was quantitatively analyzed using potassium iodide dosimetry in homogeneous and heterogeneous systems. Sonophysical activity was quantitatively and qualitatively analyzed using paint-coated bead desorption tests and aluminum foil erosion tests under three probe positions of “T” (1 cm below the liquid surface), “B” (1 cm above the vessel bottom), and “M” (midpoint between “T” and “B”). Three different sizes of glass beads (diameter: 0.2, 1.0, and 4.0 mm) were used in this study. The highest sonochemical activity was obtained at “B” in both homogeneous and heterogeneous systems. However, three times lower sonochemical activity was observed in the heterogeneous system than in the homogeneous system because significant attenuation and unstable reflection of ultrasound occurred in the bead layer and suspension. Higher sonophysical activity was observed, when the bead size decreased and the probe approached the bottom. However, no significant sonophysical activity was detected when the beads were attached to the bottom. Therefore, the sonophysically active region was the zone around the probe body, opposite to the ultrasound irradiation tip, and only suspended beads could undergo severe cavitational actions. This was confirmed via aluminum foil tests. Several erosion marks on the foil were observed in the area around the probe body, whereas no severe damage was observed at the bottom. Moreover, the degree of sonophysical activity did not change for various saturating gases. This might be due to the different thresholds of sonochemical and sonophysical activities.
•Ultrasonic/mechanical mixing soil washing for the removal of heavy metals was investigated.•With the aid of ultrasound, higher removal efficiency was obtained.•Ultrasound was more effective in the ...less extreme washing conditions.•A new novel method was suggested to understand the ultrasonic desorption mechanism.
Ultrasound-assisted soil washing processes were investigated for the removal of heavy metals (Cu, Pb, and Zn) in real contaminated soils using HCl and EDTA. The ultrasound-assisted soil washing (US/Mixing) process was compared with the conventional soil washing (Mixing) process based on the mechanical mixing. High removal efficiency (44.8% for HCl and 43.2% for EDTA) for the metals was obtained for the most extreme conditions (HCl 1.0 M or EDTA 0.1 M and L:S = 10:1) in the Mixing process. With the aide of ultrasound, higher removal efficiency (57.9% for HCl and 50.0% for EDTA) was obtained in the same extreme conditions and similar or higher removal efficiency (e.g., 54.7% for HCl 0.5 M and L:S = 10:1 and 50.5% for EDTA 0.05 M and L:S = 5:1) was achieved even in less extreme conditions (lower HCl or EDTA concentration and L:S ratio). Therefore, it was revealed that the US/Mixing was advantageous over the conventional Mixing processes in terms of metal removal efficiency, consumption of chemicals, amount of generated washing leachate, and volume/size of washing reactor. In addition, the heavy metals removal was enhanced for the smaller soil particles in the US/Mixing process. It was due to more violent movement of smaller particles in slurry phase and more violent sonophysical effects. In order to understand the mechanism of ultrasonic desorption, the desorption test was conducted using the paint-coated beads with three sizes (1, 2, and 4 mm) for the free and attached conditions. It was found that no significant desorption/removal of paint from the beads was observed without the movement of beads in the water including floatation, collision, and scrubbing. Thus, it was suggested that the simultaneous application of the ultrasound and mechanical mixing could enhance the physical movement of the particles significantly and the very high removal/desorption could be attained.
•Effects of air sparging & mechanical mixing on sonochemical activity were examined.•Sonochemical activity was enhanced at higher sparging/mixing rates.•Deformation of standing wave-field was ...attributed to the sonochemical enhancement.
The effects of air sparging (0–16 L min−1) and mechanical mixing (0–400 rpm) on enhancing the sonochemical degradation of rhodamine B (RhB) was investigated using a 28 kHz sonoreactor. The degradation of RhB followed pseudo first-order kinetics, where sparging or mixing induced a large sonochemical enhancement. The kinetic constant varied in three stages (gradually increased → increased exponentially → decreased slightly) as the rate of sparging or mixing increased, where the stages were similar for both processes. The highest sonochemical activity was obtained with sparging at 8 L min−1 or mixing at 200 rpm, where the standing wave field was significantly deformed by sparging and mixing, respectively. The cavitational oxidation activity was concentrated at the bottom of the sonicator when higher sparging or mixing rates were employed. Therefore, the large enhancement in the sonochemical oxidation was attributed mainly to the direct disturbance of the ultrasound transmission and the resulting change in the cavitation-active zone in this study. The effect of the position of air sparging and mixing was investigated. The indirect inhibition of the ultrasound transmission resulted in less enhancement of the sonochemical activity. Moreover, the effect of various sparging gases including air, N2, O2, Ar, CO2, and an Ar/O2 (8:2) mixture was compared, where all gases except CO2 induced an enhancement in the sonochemical activity, irrespective of the concentration of dissolved oxygen. The highest activity was obtained with the Ar/O2 (8:2) mixture. Therefore, it was revealed that the sonochemical oxidation activity could be further enhanced by applying gas sparging using the optimal gas.
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