Ultrasonics Sonochemistry 19 (2012) 395–398

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Ultrasonics Sonochemistry

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Short Communication

Comparison of energy consumptions between ultrasonic, mechanical, andcombined soil washing processes

Younggyu Son a, Sanggeon Nam b, Muthupandian Ashokkumar a, Jeehyeong Khim b,⇑a School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australiab School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-701, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 June 2011Received in revised form 31 October 2011Accepted 8 November 2011Available online 23 November 2011

Keywords:Soil washing processesDieselUltrasoundMechanical mixingEnergy consumption

1350-4177/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ultsonch.2011.11.002

⇑ Corresponding author. Tel.: +82 2 32903318; fax:E-mail address: hyeong@korea.ac.kr (J. Khim).

Vigorous physical effects including micro-jet and micro-streaming can be induced in heterogeneous sys-tems by acoustic cavitation. This can be useful for the removal of pollutants from contaminated soil par-ticles. In this study, the diesel removal efficiencies in ultrasonic, mechanical, and combined soil washingprocesses have been compared considering the electrical energy consumptions for these processes. Thecombined process showed synergistic effects for both removal efficiency and effective volume also hasthe advantage of a short operation time compared to the sequential processes. Thus the ultrasonic soilwashing process with mechanical mixing is considered a promising technology for industrial use.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Acoustic cavitation in liquids can induce sonochemical andsonophysical effects. In environmental engineering sonochemicaleffects are related to degradation of organic pollutants using pyro-lysis and radical reactions [1–7], while sonophysical effects are in-volved in mixing and cleaning using micro-jet, micro-streaming,and shock waves. Generally these two effects occur simultaneouslyand sonophysical effects can be more violent in heterogeneous sys-tems involving solid and liquid phases [1,2,6,8–10].

Soil washing processes are operated in a slurry phase containingthe contaminated soil and washing liquid. The desorption/separa-tion of the contaminants from the soil is the main goal of thisprocess [9,11]. Acoustic cavitation can enhance the contaminantremoval efficiency in soil washing processes through vigoroussonophysical effects and may partly contribute the degradation ofdesorbed pollutants in leachate by sonochemical effects [9]. Previ-ous studies have reported that the ultrasound aided soil washingprocesses can achieve much higher performance than conventionalsoil washing processes. However most studies in ultrasonic soilwashing processes have been carried out in small lab-scale systemswith horn-type ultrasound generators and studies for industrial useare rare [12–16].

A horn-type sonicator can induce macro-scale mixing as well asmicro-scale mixing due to high power density from a very

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small-surface-area emitter [17]. However this violent physicalaction can only be obtained in a relatively small volume and ultra-sound energy from the tip can be attenuated drastically withina very short irradiation distance. As a result of this, the cavitationactivity can be observed only near the emitting tip [18]. Thus,a higher intensity of mechanical mixing is required for the use ofa horn-type ultrasound generator compared to that required for aplate-type ultrasound generator in large scale processing. Sutkarand Gogate also reported that horn type sonication systems cannotbe effective in large-scale sonoreactors due to the transmittancelimitation of the ultrasound [19].

The purpose of this study is to investigate the electrical energyconsumption in ultrasonic, mechanical, and combined soil washingprocesses under various input power conditions. The plate-typetransducer and mechanical stirrer were used as one of basic stepsfor the design of large-scale ultrasonic soil washing processes.Moreover a comparison between sequential and combined pro-cesses has been carried out.

2. Materials and methods

Joomusin sand, one of standard sands in South Korea, wassieved in the size range of 0.4–0.6 mm. The unified soil classifica-tion was SP. The effective grain size (D10) and the porosity were0.20 and 0.38 mm, respectively. The sand was contaminated withdiesel and then aged over 1 month in dark at room temperature.The initial concentration of diesel was 20,000 mg/kg in terms of

Total input power (W)0 2 4 6 8 10 12 14

Die

sel r

emov

al e

ffic

ienc

y (%

)

20

30

40

50

60

70

80

90

100

US (2.4/4.5/7.3/9.9 W)Mixing (2.0/2.6/3.34.3 W)US(2.4W)+Mixing(2.0/2.6/3.3W)US(4.5W)+Mixing(2.0/2.6/3.3W)US(7.3W)+Mixing(2.0/2.6/3.3W)US(9.9W)+Mixing(2.0/2.6/3.3W)

Fig. 1. Diesel removal efficiency under various electrical input power conditions inultrasonic, mechanical, and combined soil washing processes.

396 Y. Son et al. / Ultrasonics Sonochemistry 19 (2012) 395–398

total petroleum hydrocarbons (TPH). The viscosity of diesel oil is5–7 cSt at 25 �C.

For ultrasonic soil washing processes with a mechanical mixing,a Pyrex vessel (d = 85 mm) containing 10 g of contaminated soiland 30 mL of water was placed in a pentagon-shaped sonoreactor(Mirae Ultrasonic Tech.) operating at 36 kHz with maximumpower of 500 W. The solution pH in the vessel was 5–6. Thesonoreactor was filled with 5 L of tap water and maintained at25 ± 2 �C using recirculation cooling system. The ultrasound irradi-ation time was 1 min. An agitator with Teflon blade was applied atthe speed of 50, 100, 150, and 200 rpm for the mechanical mixingand the corresponding electrical powers for each speed were 2.0,2.6, 3.3, and 4.3 W, respectively. The electrical input power wasmeasured using a multi-meter (M-4660M, METEX). The energytransfer efficiency between the electrical input energy and thecalorimetric heat energy was averagely 35% for four input powerincluding 100, 200, 300, and 400 W.

The position of the vessel in the sonoreactor was determined bythe analysis of the ultrasound energy distribution using hydropho-ne and diesel removal efficiency at various points in the prelimin-ary test. The blade propeller was positioned at the place where thespatter of the slurry (soil and water) was minimized to maximizethe ultrasound effect to the slurry.

The ultrasonic power was measured using the following calo-rimetry equation:

PUS ¼dTdt

CpM ð1Þ

where PUS is the calorimetric power, dT/dt is the rate of temperatureincrease, Cp is the specific heat of water under constant pressure,and M is the mass of water in the vessel. The measured calorimetricpowers were 0.8, 1.6, 2.6, and 3.5 W for 100, 200, 300, and 400 Welectrical input powers, respectively.

The diesel concentration of the soil in terms of TPH was mea-sured according to the Korean standard method for soil pollution.The sample consisting of soil and water was taken and dehydratedusing anhydrous sodium sulfate and extracted using dichlorometh-ane under ultrasonic irradiation for 10 min. The extraction solutionwas filtered by disposable syringe filter and 2 lL of sample was in-jected into a gas chromatograph (Agilent Technologies 6890N)equipped with a flame ionization detector and a DB-TPH column(30 m � 0.32 mm � 0.25 mm). The oven temperature was main-tained at 45 �C for 2 min, and then ramped from 45 to 310 �C at10 �C/min. The temperatures of the injector and the detector were280 and 300 �C, respectively. The carrier gas was nitrogen and theflow rate was 2 mL/min. All experiments were conducted at leastthree times in order to check the reproducibility. Diesel removalefficiency was calculated using following equation:

Diesel removal efficiency ð%Þ ¼ Ci � Cf

Ci� 100 ð2Þ

where Ci is the initial diesel concentration of soil in TPH, and Cf isthe final diesel concentration of soil in TPH.

3. Results and discussion

Fig. 1 shows the effect of input power on diesel removal effi-ciency in ultrasonic, mechanical, and combined soil washing pro-cesses. Higher removal efficiency was observed in the combinedprocess compared to the individual processes. The mechanical stir-ring induces macro-scale mixing and results in the desorption ofcontaminants only from the surface of the soil particles [12].Whereas, low frequency ultrasound irradiation results in micro-scale mixing, originated from sonophysical effects such as microjet, micro-streaming and shock wave that results in the removal

of contaminants not only from the soil surface but also from insidethe pores. However the removal of contaminant by ultrasound iseffective only in the relatively outer region of the soil matrix be-cause ultrasound could not penetrate deeply inside the soil matrixdue to large attenuation. Thus it is hard to achieve a high removalefficiency in the individual processes [9].

In the combined process, on the other hand, the mechanicalstirring increases the exposure of ultrasound to the soil particlesby the severe agitation of the soil matrix due to macro-scale mix-ing and it can significantly enhance the removal efficiency. Thusultrasonic soil washing process with mechanical mixing seems tobe promising for industrial use due to the synergistic effects forboth higher removal efficiency and larger ultrasound-effectivevolume.

It can be seen in Fig. 1 that the removal efficiency in the com-bined process increases with an increase in input power. However,the relative increase in the removal efficiency in the combined pro-cess is much smaller than that observed with the individual pro-cesses. It is because the contaminant on the surface of soilparticles could be easily desorbed whereas desorption of the con-taminant from the pores required much higher energy. To increasethe removal efficiency from 65% to 79%, two times larger total en-ergy was required in the combined process. It was noticed that thediesel removal from the pores occurred after reaching a removalefficiency of �65%. In the mechanical soil washing process a re-moval efficiency of over 70% might be achieved by extrapolationof the data shown in Fig. 1 to a total input power of 7 W. However,the increase of the mixing speed over 200 rpm (3.3 W) is notappropriate due to the limitation of the very-high-speed mechan-ical mixing in a real washing system. Moreover, the application oflowest speed (50 rpm/2.0 W, 50 rpm/2.6 W, and 50 rpm/3.3 W) inthe combined process was less effective in terms of the removalefficiency. An optimal mechanical mixing rate is required to max-imize the exposure of soil particles to ultrasound irradiation andminimize the total input energy. In this study, 100 rpm speedwas considered as the optimal condition for the mechanical mixingin the combined process. In order to increase the removal effi-ciency to a larger degree, the exchange of the washing liquid couldbe carried out, which required a longer operation time. In addition,the process would generate more washing leachate, which needs asecondary treatment process [9,12]. Hence, the input energy forultrasound should be determined considering the removal effi-ciency, operating time, and the production of leachate when look-ing at the economy of the process.

Fig. 2. Diesel removal efficiency in sequential processes (ultrasonic soil washing ?mechanical soil washing and mechanical soil washing ? ultrasonic soil washing)and the combined soil washing process.

Y. Son et al. / Ultrasonics Sonochemistry 19 (2012) 395–398 397

In order to compare the efficiency of the sequential and com-bined processes, experiments were carried out with ultrasonic soilwashing process for 1 min followed by the mechanical soil wash-ing process for 1 min (ultrasonic soil washing ? mechanical soilwashing), the mechanical soil washing process for 1 min followedby the ultrasonic soil washing process for 1 min (mechanical soil

Fig. 3. SEM images of the soil particles’ surface after ultrasound irradiation at 100�

washing ? ultrasonic soil washing), and the combined processfor 1 min. The results are compared in terms of the removal effi-ciency in Fig. 2 The input powers for ultrasound irradiation andmechanical mixing were 9.9 and 4.3 W, respectively. It can be seenthat the combined process is more effective in terms of not onlythe removal efficiency but also the operation time. In addition,the maintenance of stable slurry conditions (suspension of solidparticles in the liquid) was a critical parameter during the sequen-tial processes. In the process of mechanical washing followed byultrasonic washing, the soil particles were well dispersed aftermechanical mixing and hence ultrasound irradiation was moreeffective. In the process of ultrasonic washing followed by mechan-ical washing, on the other hand, the soil particles were not welldispersed in water during ultrasonic processing and hence onlymechanical mixing was not effective. The turbidities in the suspen-sion of vessel were measured at 520 nm after each process and theturbidities for the ultrasonic washing process, the mechanicalwashing process, and the combined process were 0.1389, 0.2513,and 0.3328 abs, respectively.

Fig. 3 shows SEM (scanning electron microscope) images of thesurface of the soil particles after ultrasound irradiation. Themechanical mixing did not result in any significant damage onthe surface of the soil particles. In addition, there was no significantdifference in the SEM images for the ultrasonic and combined pro-cesses. Thus it was found that the soil particle surface was mainlydamaged by sonophysical effects which could increase the removalof the contaminant from soil. Extended exposure of the particles toultrasound irradiation resulted in severe breakage of the surface.

magnification (left-side images) and 10,000� magnification (right-side images).

398 Y. Son et al. / Ultrasonics Sonochemistry 19 (2012) 395–398

Thus it seemed that diesel removal in soil washing processes couldbe achieved by two ways: one is direct desorption of contaminantsfrom the surface of particles and the other is breakage/separationof fine particles with contaminants from larger mother particles.For fine particles and waste washing liquid, post treatments shouldbe required.

However, it should be noted that this very violent action oc-curred only in outer shell of the soil matrix and the mechanicalmixing is required to distribute the ultrasonic effect to the particlesinside the soil matrix.

4. Conclusion

The performance of ultrasonic, mechanical, and combined soilwashing processes under various input power conditions wasinvestigated in terms of diesel removal efficiency. The combinedprocesses showed higher removal efficiency than the individualprocesses and a larger energy was required to increase the removalefficiency from a certain level (from 65% in this study). It is re-vealed that the combined process is more effective compared tothat could be achieved by applying the individual processessequentially.

Acknowledgements

The authors acknowledge the financial support from Mid-careerResearcher program through NRF grant funded by the MEST (KRF-2009-0092799). Y.S. also acknowledges an Endeavour ResearchFellowship.

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