comparison of ultrasonic and conventional mechanical soil-washing processes for diesel-contaminated...

Published: January 13, 2011

r 2011 American Chemical Society 2400 dx.doi.org/10.1021/ie1016688 | Ind. Eng. Chem. Res. 2011, 50, 2400–2407

ARTICLE

pubs.acs.org/IECR

Comparison of Ultrasonic and Conventional Mechanical Soil-WashingProcesses for Diesel-Contaminated SandYounggyu Son,† Jihoon Cha,‡ Myunghee Lim,‡ Muthpandian Ashokkumar,† and Jeehyeong Khim*,‡

†School of Chemistry/Department of Chemical and Biomolecular Engineering, University of Melbourne, Melbourne,VIC 3010, Australia‡School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-701, Korea

ABSTRACT: The effect of ultrasound on the conventional mechanical soil-washing process was investigated. To determine theoptimal frequency for maximum efficiency, tests were conducted with aluminum foils under four frequencies including 35, 72, 110,and 170 kHz. It is known that the physical effects generated during acoustic cavitation damage the foil by causing pits and holes. Thesonication at 35 kHz resulted in maximum damage to the aluminum foil as compared to that observed at other frequencies. Based onthese results, 35 kHz was selected for the ultrasonic soil-washing processes in this study. The optimal washing time was found to be 1min, because there was no significant increase in the removal efficiency over 1 min for the three processes, mechanical, ultrasonic,and combined ultrasonic-mechanical. It was also found that the combined process enhanced the performance of the soil-washingprocess significantly as compared to other two processes in terms of (i) diesel removal efficiency, (ii) process time, (iii) consumptionof electric energy, and (iv) production of washing leachate. The efficiency of washing under ultrasonic processing conditions wassimilar to that observed with mechanical washing in the presence of small amounts of sodium dodecyl sulfate (SDS), suggesting thatthe ultrasonic washing process does not require external chemicals and can be considered as a “green” process.

1. INTRODUCTION

Ultrasound irradiation in aqueous phase leads to cavitationevents including the formation, oscillation/growth, and violentcollapse of bubbles. Extreme conditions generated inside thebubble are responsible for several sonochemical and sonophysi-cal effects.1-3 Even though it is not easy to distinguish sono-chemical effects from sonophysical effects in a specific ultrasonicreaction, and the observed effect may be as a result of both effects,it is generally known that sonochemical effects are highly relatedto the molecular transformation of chemical compounds such asdegradation of pollutants and synthesis of materials in a homo-geneous-like system, while sonophysical effects are involved withdesorption processes such as surface cleaning and extraction, andmicromixing such as emulsification in a multiphase system.4-11

In soil-washing processes, desorption of the organic/inorganicpollutants from the contaminated soil is a key step. Convention-ally, water or water with chemicals such as acids, bases, surfac-tants, and chelating agents are added as a washing liquid, and thenmechanical mixing is applied.12 This typical washing process canremove pollutants only from the surface of the soil particles.Residual pollutants that are strongly adsorbed on the surface ortrapped inside surface pores require higher mixing intensity,mixing time, or washing liquid.13 Sonophysical effects induced byultrasound irradiation can enhance desorption from the solidphase to liquid phase by violent action including microjet,microstreaming, and shock wave, thereby enhancing the removalefficiency significantly.3,14

The ultrasonic soil-washing processes using the sonophysicaleffects have not been widely studied. Newman et al. showed theway to use ultrasound in solids treatment processes using anultrasonic vibrating tray;15 Feng and Aldrich investigated various

factors including sonication duration, power intensity, particlesize, pH, KCl, and surfactant on ultrasonic soil-washing processesfor the removal of diesel.14 They also compared ultrasonicwashing to two mechanical washing methods and revealed thatultrasonic washing had advantages in the consumption of electricenergy and water;16 Kim and Wang, based on soil mechanics,found that oil removal efficiency by ultrasound was highly relatedto the effective grain size and the hydraulic gradient;17 Na et al.investigated the effects of stirring, surfactant, operation time,power intensity, and particle size;18 Shestha et al. examined threesoil materials for the removal of different organic pollutantsunder various conditions such as soil-water ratio, operationtime, and input power.19

Even though all researchers mentioned above showed thatultrasonic irradiation could be one of the promising techniques inthe soil-washing processes, the application of ultrasound to large-scale soil-washing processes still needs further research. Mostresearchers investigated the effect of ultrasound with a horn-typeultrasound generator, and this instrument was originally made forvigorous physical actions with a concentrated and high irradia-tion power in small volume reactors. Unfortunately, thosevigorous actions could not be made in large-scale reactors, andthe effect of ultrasound for engineering use might be over-estimated in previous research work.20

This study was designed to evaluate the performance of ultra-sonic soil-washing processes with mechanical mixing in a bath-type sonoreactor equipped with plate-type transducers. Both

Received: August 5, 2010Accepted: December 23, 2010Revised: December 2, 2010

2401 dx.doi.org/10.1021/ie1016688 |Ind. Eng. Chem. Res. 2011, 50, 2400–2407

Industrial & Engineering Chemistry Research ARTICLE

mechanical mixing and plate-type transducers used herein areapplicable for large-scale use. The specific objectives of thisresearch were to understand the effect of sonophysical effectsusing aluminum foil under various frequency conditions, to com-pare three washing processes (mechanical, ultrasonic, and com-bined ultrasonic-mechanical) in terms of operation time, elec-tric energy consumption, andwashing leachate production, in theabsence and presence of a surfactant.

2. METHODS AND MATERIALS

2.1. Sonoreactor. Figure 1 shows a schematic of the experi-mental setup. The pentagon-shaped sonoreactor consisted of astainless steel reactor and five ultrasonic transducer modules(Mirae Ultrasonic Tech.) that were placed on each side wall.Each transducer module contained three lead zirconate titanate(PZT) transducers (Tamura), which could be operated at one ofthe four frequencies, 35, 72, 110, or 170 kHz. A 400 mL pyrexvessel containing target materials was placed in the top of theultrasonic bath. The reactor was filled with 5 L of tap water, andthe water temperature was maintained at 25 ( 2 οC using arecirculation cooling system.The electric input power was measured by a multimeter (M-

4660M, METEX). The water temperature in the vessel wasmeasured during ultrasound irradiation using a thermometer(DTM-318, Tecpel), and effective calorimetric power in thevessel was obtained using the following equation:

PUS ¼ dTdt

cpM ð1Þwhere PUS is the calorimetric power, dT/dt is the rate oftemperature increase, cp is the specific heat of water underconstant pressure, and M is the mass of water in the vessel.20

The effective power levels delivered were 1.0, 2.5, 4.1, and 6.0 Wfor 100, 200, 300, and 400Wof electric input power, respectively.2.2. Aluminum Foil Tests. A 40mm� 40 mm aluminum foil

was placed at the bottom of the pyrex vessel in the sonoreactorand then irradiated ultrasonically for 1 min, which was equivalentto the time of washing processes in this study. The thickness ofaluminum foil was 16 μm. The foil was damaged continuouslyand perforated locally due to various physical effects of ultra-sound.

After ultrasound irradiation, the eroded foil was dried, scannedusing scanner (Scanjet G3010, Hewlett-Packard), and then theimage was digitized using Photoshop (Adobe Systems Inc.) forthe evaluation of ultrasonic damage. Because the damaged regionappeared bright while the undamaged region appeared dark, thedeformation ratio was defined as follows:

deformation ratio ð%Þ ¼ pixelnumber of bright areapixelnumber of total area

� 100

ð2Þ

2.3. Washing Processes. Joomunsin sand, commerciallyavailable in Korea, was sieved to the particle size of 0.4-0.6mm. The sieved soil was contaminated with diesel and then agedover 15 days in dark at room temperature. Diesel was purchasedfrom GS Caltex, one of the typical gas stations in Korea. Theinitial concentration of diesel in the soil was 7000mg/kg in termsof total petroleum hydrocarbons (TPH).For all soil-washing processes, the soil-water ratio was 1:3 (10 g

of soil was used), and a mechanical mixing using an agitator withTeflon bladewas applied at the speed of 60 rpm.The electric powerconsumed by the agitator was measured using a multimeter.After a 1 min washing process, the residual diesel concentra-

tion in TPH was analyzed according to the Korean standardmethod for soil pollution.21 The slurry sample in the vessel wasdehydrated using anhydrous sodium sulfate, and then residualdiesel was extracted from the soil using dichloromethane underultrasonic irradiation for 10 min. The extraction solution wasfiltered by disposable syringe filter and injected into a gas chro-matographer (Agilent Technologies 6890N) equipped with aflame ionization detector and a DB-TPH column (30 m � 0.32mm � 0.25 mm). It was found that there was no significant lossof diesel compounds in terms of TPH during the experimentsand analyses during the preliminary test. Diesel removal effi-ciency was calculated using the following equation:

diesel removal efficiency ð%Þ ¼ Ci -Cf

Ci� 100 ð3Þ

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

Figure 1. Schematic of the experimental setup.



were conducted at least three times. The surfactant used herein wasSDS, purchased from Sigma-Aldrich.

3. RESULTS AND DISCUSSION

3.1. Evaluation of Aluminum Foil Erosion by Ultrasound.Aluminum foil erosion tests were conducted to evaluate thestrength of the physical effects of ultrasound and to decide theoptimal frequency. Figure 2 shows the damaged foil imagesunder various input power and frequency conditions, which weretrimmed in a circular shape with a diameter of 35 mm. The whitespots illustrate the cavitational damages by ultrasound irradia-tion. The bigger and brighter zones were perforated holes, whichrevealed the mass loss of the foil, while smaller and less brightzones were pits. In addition, the damage on the foil was notuniform because the ultrasound field was not uniform, and thefoil surface might have tiny damages including scratches, dirt, andpits before the test.22

The cavitational erosion process consisted of several periodsincluding incubation, acceleration, steady state, and attenuation.In the incubation period, no mass loss was observed, but subtledeformation like a pit was developed in a cluster. As cavitationaldamages were accumulated on the pits, mass loss of materialoccurred in the acceleration and succeeding periods.22,23 Thematerial could be eventually perforated when a very thin materiallike the aluminum foil was used.When a pit appeared on the foil, it enhanced the cavitation

events vigorously and made more pits near the first pit. As morepits appeared and gathered, the damage on the site was accumu-lated, and then a hole was generated. Because the edge of the holecould be another amplifier of cavitation, more erosion occurredaround the hole.22 As a result of this, the size of the hole increased

very fast as in burning a piece of paper from the inside. Once theaccumulated damage resulted in the formation of a hole, thedamage on that site could not be visualized anymore. However,the hole enhanced the cavitational erosion dramatically as explainedabove, and the damaged area was expanded enough to include thedamage on themissing region. Thus, this evaluationmethod can beconsidered for quantifying the erosion on the foil based on thedamaged area for short and long irradiation times.For the quantification of the erosion on the foil, the damaged

areas are compared to the total area of the foil by digitizing basedon the pixel using the images in Figure 2. The results for thequantification of the erosion using eq 2 are shown in Figure 3.At 35 kHz frequency, the damage on the foil was the most

severe among the four frequency conditions. As the appliedfrequency was increased, the magnitude of erosion decreasedsignificantly. No perforation hole was observed for 72, 110, and170 kHz, and the deformation ratio ranged only from zero to5.7% in these frequency conditions, while the ratio observed was7.6-27.0% for 35 kHz. From these observations, it can berevealed that the cavitational erosion under higher three fre-quency conditions did not proceed over the incubation period,and the damaging power of the sonophysical effects on thesurface was not high enough to make severe damages.23

It is well-known that the radial oscillation of acoustic bubbles issignificantly larger for the lower frequency. In other words, theratio between Rmax and Rmin during oscillations of the cavitationbubbles is higher for the lower frequency.24 It is also known that theresonance radius of the cavitation bubbles decreases with anincrease in the ultrasonic frequency. The resonant radius of bubbleis inversely proportional to applied frequency according to eq 4.25

Rr2 ¼ 3kP0

Fωr

2 ð4Þ

Figure 2. The damaged foil images caused by sonication at different frequencies and power levels.



where Rr is the resonant bubble radius, κ is the polytropic index, P0is the hydrostatic pressure, F is the density of the liquid medium,and ωr is the resonant frequency (applied frequency).Interestingly, the deformation ratio followed the trend of the

resonant radius of bubble for the applied frequency as shown inFigure 3. Because of the larger size of the bubbles at lowerfrequencies, the bubble oscillations are more effective in gen-erating physical effects, such as micro jet, turbulence, shear, etc.Thus, the physical effects that cause the damage to the particles ina sound field are more effective at low frequency under the sameinput power conditions, especially around 20-40 kHz.In our preliminary test, we also found that the diesel removal

efficiency in 170 kHz was less than 10%, and this was mainly dueto the exchange of the washing liquid. This will be discussed later.Thus, 35 kHz, the lowest frequency, was chosen as the optimalfrequency for ultrasonic washing processes among the fourapplied frequencies. On the other hand, many researchers reportedthat high frequencies are optimal to degrade organic pollutantssonochemically by various radical species, which are producedinside and near the cavitation bubbles and have strong oxidationpower.5,6,26-28 High frequency can be applied additionally to thewashing processes with the low frequency irradiation system for thetreatment of desorbed pollutants in washing leachate. It is possibleto increase the biodegradability of desorbed pollutants even thoughfull mineralization cannot be achieved in a single washing process.In the following section, however, only 35 kHzwas used to estimatedesorption of pollutants, the fundamental stage of soil washing,under various experimental conditions.3.2. Comparison of Soil-Washing Processes. Ultrasonic,

mechanical, and combined ultrasonic-mechanical washing pro-cesses were investigated at 35 kHz. The optimal mechanicalmixing speed was determined as 60 rpm in a preliminary test, andit was 2.5W in terms of electrical energy consumption. The ultra-sound input power was 2.5W in terms of a calorimetric power. Asshown in Figure 4, the diesel removal efficiency in the ultrasonicsoil-washing process with the mechanical mixing was muchhigher than that observed with individual washing processes.This is because ultrasound alone could not suspend the soil

particles and vigorous stirring was absent even though ultrasoundcould induce strong physical effects. When ultrasound wasirradiated to the slurry from the outside of the vessel, most soilparticles were settled at the bottom of vessel due to gravity, andonly a small fraction of settled particles located near the wall of

the vessel was exposed to the ultrasound directly. Theoretically,ultrasound can travel through the solids in the liquid medium;however, the power intensity decreases drastically as it passes,because boundary layer loss occurs, which is one of soundadsorption phenomena.29 As a result of this, cavitation eventsdid not occur noticeably. On the other hand, the mechanicalmixing suspended the soil matrix completely in macroscale andincreased a contact between contaminated soil and washingliquid according to the mixing intensity. This action enabledadsorbed pollutants on the surface of the soil to wash out easily,but pollutants trenched in the soil pores were barely desorbed.Feng and Aldrich suggested that the mechanical stirring induceddesorption only from the outer layers of diesel on the soilparticle.13

For the simultaneous application of ultrasound and themechanical stirring, both the macroscale mixing and the micro-scale mixing were induced, and it resulted in a much higherremoval efficiency due to the overall pollutants desorption fromnot only the surface of soil particles but also the inside of pores.Moreover, the mechanical mixing increased the exposure fre-quency of soil particles to high-intensity ultrasound and en-hanced the ultrasonic desorption of pollutants significantly.Thus, these results reveal that the mechanical mixing is essentialto increase the performance of the ultrasonic soil-washingprocess remarkably.When a horn-type ultrasound generator was used, ultrasound

could generate bothmacroscale as well as microscale mixing. Thetip with a small-surface area can emit concentrated-high-ultra-sonic energy like a jet of water from a nozzle. Dahlem et al.showed a snapshot of violent flow pattern originated from the tipof the laboratory horn-type generator.30 In our preliminary testusing the horn-type generator, it was also observed with thenaked eyes that soil particles in the vessel were moved veryvigorously along the water flow induced by the ultrasonic energyfrom the tip. The visible mixing flow for the soil particles was aspowerful as the flow made by mechanical stirring. Na et al.obtained relatively high removal efficiency in a horn-type ultra-sonic soil-washing system for diesel contaminated soil withoutadditional mechanical stirring.18 However, it seemed that themacroscale mixing by ultrasound from the horn-type sonicatorwas not so effective for the industrial-scale uses because thisphenomenon could be only observed in small-scale reactors.

Figure 3. Deformation ratio under various frequency and input powerconditions and resonant bubble radius for four frequencies. Figure 4. Variations of diesel removal efficiency under different soil-

washing conditions.



The results shown in Figure 4 also indicate that after a 1 minwashing operation, the increase of the removal efficiency wasconstant for all three processes. In a previous study, it wasobserved that the removal efficiency of diesel was not increasedafter 5 min due to readsorption of the diesel.13 When desorptionof diesel is maximized in each process, the washing liquid shouldbe separated from the treated soil to prevent readsoption. Thus, 1min was decided as the optimal washing time for ultrasonic soil-washing process in the presence of mechanical stirring for furtherexperiments.3.3. Ultrasonic Soil-Washing Process with Mechanical

Mixing. To enhance the diesel removal and compare the perfor-mance in the ultrasonic soil-washing process with mechanicalmixing and the mechanical soil-washing process in terms of otherfactors rather than the removal efficiency, washing liquid (tapwater was used) was exchanged after a 1 min mechanical soil-washing process, and then the concentration of the residual dieselin the soil was analyzed to obtain removal efficiency. This pro-cedure was repeated five times. Figure 5 shows that the removalefficiency increases significantly until the fourth washing process,whereas the fifth washing process does not show enhancement inthe removal efficiency. It seemed that desorption of dieseloccurred mainly from the surface of soil during the first fourwashing processes, and the removal efficiency did not increasesignificantly when the rest of the diesel was trapped inside thepores.Remarkably, the first ultrasonic soil-washing process in the

presence of mechanical mixing showed removal performancesimilar to that of the fourth mechanical soil-washing process. Asexplained above, ultrasound with the mechanical mixing couldinduce the macro- and microscale mixing in the slurry andsynergistic actions on the removal of diesel from not only thesurface but also from inside the pore of soil. Na et al. havereported that the mass transfer constant was increased by 60%when sonication was combined with stirring as compared to thatobserved for stirring only during naphthalene desorption fromsoil.18 After the second ultrasonic soil-washing process, theremoval efficiency reached close to 90%.The removal efficiency of 91% after second ultrasonic soil

washing in this study left a residual diesel concentration of 644mg/kg, and it could meet the Worrisome Level of Soil Contam-ination for Area 2 (800mg/kg) in Korea. It can be expected that a

third washing process would meet the regulation for Area 1(500 mg/kg), which is the strongest regulation for contaminatedsoils in Korea.31

From the results discussed so far, it is evident that theultrasonic soil-washing process in the presence of mechanicalmixing has several advantages. First, this process could achievehigh removal efficiency in a single attempt. Second, it coulddecrease the operating time markedly. In this study, the additionof 35 kHz ultrasound irradiation to the conventional soil-washingprocess enabled the washing time to decrease from 4 to 1 min forthe removal of 75% diesel from the contaminated soil. Third, itcould reduce energy consumption significantly. Total electricenergy consumption for this 1 min process was only 0.083 W h,which is the sum of the ultrasound irradiation energy of 0.042 W hand mechanical mixing energy of 0.042 W h, while the electricenergy of 0.17 W h was consumed for the 4 min mechanical soil-washing process. Even though ultrasound irradiation energy wascalculated using calorimetry, an indirect measurement, andcooling energy for the whole sonoreactor was not considered,the ultrasonic soil-washing process in the presence of mechanicalmixing could be an energy-efficient process as compared to theconventional processes. The direct comparison of energy con-sumption on each process for real processes cannot be madebecause the experiments were carried out in small scale. How-ever, this can be considered as one of the references forcomparing the processes in terms of economy. Fourth, theamount of washing leachate could be decreased remarkably.Generally, washing leachate contained various desorbed pollu-tants and fine particles. The characteristics of the washingleachate depended on the characteristics of target contaminatedsoil, the soil-water ratio, and chemicals added in washing pro-cesses, but it is clear that post treatment processes such asbiological treatments, conventional physicochemical treatments,and advanced oxidation processes are always required. Thus, theless washing leachate the process made, the more competitive theprocess could be for engineering use. Herein, the amount ofwashing leachate in the ultrasonic soil-washing process in thepresence of mechanical mixing was only 25% of the amount ofconventional washing to achieve a removal efficiency of 75%.Feng and Aldrich also investigated the effect of the exchange ofthe washing liquid, and it was found that the increase of theremoval efficiency was only less than 2% on average despite threetimes rinsing after each process.13 This ineffectiveness might bedue to their washing system. They used the horn-type ultrasoundgenerator without mechanical mixing for the soil-washing pro-cess, and this could not induce macroscale mixing properly asmentioned early.Table 1 shows the effect of input power on the washing

efficiency of the ultrasonic process in the presence of mechanicalstirring. As the input power increases, the removal efficiencyincreases. However, the relative increase in the efficiency isrelatively low as compared to the relative increase in the power.The removal efficiency increase by only 4.4% when the inputpower is increased from 2.5 to 6.0 W. This means that a higherremoval efficiency can be achieved at relatively lower acousticpower levels. Feng and Aldrich and Na et al. also reported thatthere were not significant increases in the removal efficiency ascompared to the increase of the input power or powerintensity.13,18 It was also suggested that the replacement of thewashing liquid would result in higher removal efficiency thanincreasing washing time or input power in the ultrasonic soil-washing process with mechanical mixing as discussed earlier.

Figure 5. Comparison of removal efficiency for mechanical soil washingand ultrasonic soil washing with mechanical mixing after exchange ofwashing liquid.



Interestingly, the trend of diesel removal under four inputpower conditions in ultrasonic soil-washing tests did not matchwith the result in aluminum foil tests. The damage by sonophy-sical effects on the surface of aluminum foil increased significantlyas input power increased from 1.0 to 6.0 W. The damage includedpits and holes. However, the removal efficiency in ultrasonic soilwashing did not increase noticeably under the same input powerconditions. It is because ultrasound should penetrate many hetero-geneous layers consisting of soil particles and liquid for desorptionof pollutants, and the transmitted ultrasound energy could decreasegreatly due to the existence of many solid/liquid boundaries. Thus,only the degree of desorption in the outest layer of contaminatedsoil could be predictable from the result of aluminum foil tests, andit seemed that sonophysical effects inside were not effective enoughto occur vigorous desorption like at the outest layer of soil. Eventhough mechanical mixing could help soil to expose more to high-power ultrasound and enhance desorption of pollutants, it did notmake large increments as in aluminum tests.3.4. Effect of Surfactant on Soil-Washing Processes. It is

well-known that surfactant can enhance desorption of organiccompounds from the soil in slurry phase by two mechanismsincluding micellar solubilization over the critical micelle concen-tration (CMC) and mobilization due to hydrophobic inter-action.32,33 SDS is one of the common surfactants in soil-washingprocesses, and it was considered a model surfactant in thisstudy.13,18,34-38 Figure 6 shows the effect of SDS on the dieselremoval efficiency in themechanical soil-washing process and theultrasonic soil-washing process with mechanical mixing. Themaximal value of removal efficiency was obtained at the con-centration of 5 mM for each washing process, but the removalefficiency did not increase noticeably as compared to that at theconcentration of 1 mM. At 10 mM, a slightly higher concentra-tion than CMC, the removal efficiency decreased, and then therewas no significant change of removal efficiency at higher concen-trations of SDS for both cases. This result was partly consistentwith previous results, which showed that surfactant could enhancethe removal efficiency of organic pollutants remarkably in themechanical and ultrasonic soil-washing processes and the removalefficiency was not changed significantly above CMC.13,18 However,the removal efficiency herein could be maximized at 1 mM, whichwas approximately only 10% of CMC of SDS, while most previousresearchers reported that the surfactant-enhanced washing pro-cesses could be optimized above CMC due to an increase ofsolubility of organic pollutants.The reason why the removal efficiency decreased above CMC

might be the formation of foam due to the presence of highconcentration of surfactant. When the mechanical mixing wasapplied to the slurry where the concentration of SDS was abovethe CMC, a white foam (like soap foam) formed, surroundedthe soil particles, and then impeded desorption of pollutants bythe mechanical mixing and ultrasound. For mechanical mixing,the presence of foam resulted in the decrease of the shear force,and it was similar to a slower mixing rate was applied. Huang andLee revealed that equilibrium solubility of naphthalene increasedsignificantly as mixing rate increased.34 For ultrasound irradia-tion, foam could block the transmission of acoustic energy to thesoil particles, which occurs in various sonophysical effects. It

seemed to be similar to the formation of a dense cloud of cavi-tation bubbles near the tip of the horn-type sonicator, resulting inlow performance of ultrasound when high power was applied.Low concentration of SDS in the liquid phase due to the

adsorption of SDS onto the surface of soil particles could beanother reason because the concentration of SDS reported hereis the bulk concentration. It was suggested that a higher concen-tration of the surfactant (than that for CMC in bulk phase) wasrequired to achieve the effective CMC in slurry phase.39 Zhengand Obbard reported that effective CMC was 4-5 times higherthan normal CMC using nonionic surfactants in soil/aqueoussystem and the effective CMC increased as the soil portion in theslurry increased due to the surfactant adsorption.40 However, anysignificant increase of the removal efficiency did not occur over20 mM, which was 2 times higher than the CMC, and anionicsurfactants such as SDS are less adsorbed to the soil than arenonionic surfactants.33 Our ongoing study also showed thatadsorption of SDS in the slurry phase was negligible (data notshown).There was no formation of foam in both processes at the

concentration of 1 and 5 mM, below CMC. Moreover, Khalladiet al. reported that the surface tension in the liquid reached aminimum value at ∼2 mM of SDS with and without soil. Theyalso revealed that minerals in soil acting as electrolyte in solutionenhanced the reduction of surface tension by SDS, and there wasno effective loss of surfactant due to the presence of the soilparticles.37 These enabled the removal efficiency to be higherbelow CMC without micellar solubilization.Table 2 shows the removal efficiency for the mechanical and the

ultrasonic-mechanical soil-washing processes in the presence andabsence of SDS (5mM) after 1min operation. The addition of SDSenhanced the removal efficiency significantly in themechanical soil-washing process, and this process seemed to be relatively efficient interms of an energy assumption because only one-half of electricenergy was consumed in the mechanical soil-washing process withSDS as compared to the combined ultrasonic-mechanical soil-washing process. Yet additional treatment processes would berequired to remove or recover the used surfactant, and difficultyfor a recovery would result in large consumption of the surfactant.33

On the other hand, it seemed that SDS was not necessary tooperate the ultrasonic-mechanical soil-washing process because

Table 1. Effect of Input Power for the Removal Efficiency inthe Ultrasonic Soil-Washing Process with Mechanical Mixing

calorimetric input power 1.0W 2.5W 4.1W 6.0W

diesel removal efficiency (%) 72.2( 4.3 74.7( 7.6 78.4( 4.3 79.1( 5.8

Figure 6. The effect of surfactant concentration for the diesel removalefficiency in the mechanical soil-washing process and the ultrasonic soil-washing process with mechanical mixing.



the removal efficiency was the same with and without SDS. NoSDS would be required for high performance in this process.Thus, it is clear that ultrasound enables conventional soil-washing processes to be a green process.

4. CONCLUSION

The ultrasonic soil-washing process was compared to theconventional mechanical soil-washing process. In addition, theeffect of combining mechanical mixing with ultrasonic processwas also investigated to evaluate the removal efficiency of diesel,the operation time of process, the consumption of electricenergy, and the production of washing leachate. It was foundthat the ultrasonic soil washing with the mechanical mixing wasthe best process among three processes in all aspects consideredherein because of macro- and micromixing effects. The additionof SDS resulted in an increase in removal efficiency for themechanical soil-washing process as well as the ultrasonic soil-washing process combined with mechanical mixing. Even thoughSDS increased the removal efficiency, the addition of SDS in theultrasonic soil-washing process combined with mechanical mix-ing was not necessary. Thus, the combined process produces lesscontaminated washing leachate, or no treatment process for therecovery of SDS is required.

’AUTHOR INFORMATION

Corresponding Author*Tel.: þ82 2 32903318. Fax: þ82 2 9287656. E-mail: [email protected].

’ACKNOWLEDGMENT

We acknowledge financial support from the Korea ResearchFoundation Grant funded by the Korean Government(MOEHRD) (KRF-2008-313-D00576). Y.S. also acknowledgesan Endeavour Research Fellowship.

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Table 2. Removal Efficiency for theMechanical Soil-Washing Process and the Ultrasonic-Mechanical Soil-Washing Process withMechanical Mixing in the Presence and Absence of SDS (5 mM)

processes mechanical soil-washing processmechanical soil-washingprocess with SDS

ultrasonic soil-washingprocess

ultrasonic soil-washingprocess with SDS

diesel removal efficiency (%) 31.0 ( 5.4 64.2 ( 5.1 74.7 ( 7.6 76.1 ( 3.8



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comparison of ultrasonic and conventional mechanical soil-washing processes for diesel-contaminated...

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