Comparison of Ultrasonic and Conventional Mechanical Soil-Washing

ARTICLE pubs.acs.org/IECR

Comparison of Ultrasonic and Conventional Mechanical Soil-Washing Processes for Diesel-Contaminated Sand Younggyu 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 the optimal 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. The sonication at 35 kHz resulted in maximum damage to the aluminum foil as compared to that observed at other frequencies. Based on these results, 35 kHz was selected for the ultrasonic soil-washing processes in this study. The optimal washing time was found to be 1 min, 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-washing process significantly as compared to other two processes in terms of (i) diesel removal efficiency, (ii) process time, (iii) consumption of electric energy, and (iv) production of washing leachate. The efficiency of washing under ultrasonic processing conditions was similar to that observed with mechanical washing in the presence of small amounts of sodium dodecyl sulfate (SDS), suggesting that the 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 cavitation events including the formation, oscillation/growth, and violent collapse of bubbles. Extreme conditions generated inside the bubble are responsible for several sonochemical and sonophysical effects.1-3 Even though it is not easy to distinguish sonochemical effects from sonophysical effects in a specific ultrasonic reaction, and the observed effect may be as a result of both effects, it is generally known that sonochemical effects are highly related to the molecular transformation of chemical compounds such as degradation of pollutants and synthesis of materials in a homogeneous-like system, while sonophysical effects are involved with desorption processes such as surface cleaning and extraction, and micromixing such as emulsification in a multiphase system.4-11 In soil-washing processes, desorption of the organic/inorganic pollutants from the contaminated soil is a key step. Conventionally, water or water with chemicals such as acids, bases, surfactants, and chelating agents are added as a washing liquid, and then mechanical mixing is applied.12 This typical washing process can remove pollutants only from the surface of the soil particles. Residual pollutants that are strongly adsorbed on the surface or trapped inside surface pores require higher mixing intensity, mixing time, or washing liquid.13 Sonophysical effects induced by ultrasound irradiation can enhance desorption from the solid phase to liquid phase by violent action including microjet, microstreaming, and shock wave, thereby enhancing the removal efficiency significantly.3,14 The ultrasonic soil-washing processes using the sonophysical effects have not been widely studied. Newman et al. showed the way to use ultrasound in solids treatment processes using an ultrasonic vibrating tray;15 Feng and Aldrich investigated various r 2011 American Chemical Society

factors including sonication duration, power intensity, particle size, pH, KCl, and surfactant on ultrasonic soil-washing processes for the removal of diesel.14 They also compared ultrasonic washing to two mechanical washing methods and revealed that ultrasonic washing had advantages in the consumption of electric energy and water;16 Kim and Wang, based on soil mechanics, found that oil removal efficiency by ultrasound was highly related to 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 three soil materials for the removal of different organic pollutants under various conditions such as soil-water ratio, operation time, and input power.19 Even though all researchers mentioned above showed that ultrasonic irradiation could be one of the promising techniques in the soil-washing processes, the application of ultrasound to largescale soil-washing processes still needs further research. Most researchers investigated the effect of ultrasound with a horn-type ultrasound generator, and this instrument was originally made for vigorous physical actions with a concentrated and high irradiation power in small volume reactors. Unfortunately, those vigorous actions could not be made in large-scale reactors, and the effect of ultrasound for engineering use might be overestimated in previous research work.20 This study was designed to evaluate the performance of ultrasonic soil-washing processes with mechanical mixing in a bathtype sonoreactor equipped with plate-type transducers. Both Received: August 5, 2010 Accepted: December 23, 2010 Revised: December 2, 2010 Published: January 13, 2011 2400

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

Industrial & Engineering Chemistry Research

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Figure 1. Schematic of the experimental setup.

mechanical mixing and plate-type transducers used herein are applicable for large-scale use. The specific objectives of this research were to understand the effect of sonophysical effects using aluminum foil under various frequency conditions, to compare three washing processes (mechanical, ultrasonic, and combined ultrasonic-mechanical) in terms of operation time, electric energy consumption, and washing leachate production, in the absence and presence of a surfactant.

After ultrasound irradiation, the eroded foil was dried, scanned using scanner (Scanjet G3010, Hewlett-Packard), and then the image was digitized using Photoshop (Adobe Systems Inc.) for the evaluation of ultrasonic damage. Because the damaged region appeared bright while the undamaged region appeared dark, the deformation ratio was defined as follows: def ormation ratio ð%Þ ¼

2. METHODS AND MATERIALS

pixelnumber of bright area 100 pixelnumber of total area

ð2Þ

2.1. Sonoreactor. Figure 1 shows a schematic of the experi-

mental setup. The pentagon-shaped sonoreactor consisted of a stainless 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 of the four frequencies, 35, 72, 110, or 170 kHz. A 400 mL pyrex vessel containing target materials was placed in the top of the ultrasonic bath. The reactor was filled with 5 L of tap water, and the water temperature was maintained at 25 ( 2 οC using a recirculation cooling system. The electric input power was measured by a multimeter (M4660M, METEX). The water temperature in the vessel was measured during ultrasound irradiation using a thermometer (DTM-318, Tecpel), and effective calorimetric power in the vessel was obtained using the following equation: dT cp M ð1Þ PUS ¼ dt where PUS is the calorimetric power, dT/dt is the rate of temperature increase, cp is the specific heat of water under constant 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 W for 100, 200, 300, and 400 W of electric input power, respectively. 2.2. Aluminum Foil Tests. A 40 mm 40 mm aluminum foil was placed at the bottom of the pyrex vessel in the sonoreactor and then irradiated ultrasonically for 1 min, which was equivalent to the time of washing processes in this study. The thickness of aluminum foil was 16 μm. The foil was damaged continuously and perforated locally due to various physical effects of ultrasound.

2.3. Washing Processes. Joomunsin sand, commercially available in Korea, was sieved to the particle size of 0.4-0.6 mm. The sieved soil was contaminated with diesel and then aged over 15 days in dark at room temperature. Diesel was purchased from GS Caltex, one of the typical gas stations in Korea. The initial concentration of diesel in the soil was 7000 mg/kg in terms of 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 with Teflon blade was applied at the speed of 60 rpm. The electric power consumed by the agitator was measured using a multimeter. After a 1 min washing process, the residual diesel concentration in TPH was analyzed according to the Korean standard method for soil pollution.21 The slurry sample in the vessel was dehydrated using anhydrous sodium sulfate, and then residual diesel was extracted from the soil using dichloromethane under ultrasonic irradiation for 10 min. The extraction solution was filtered by disposable syringe filter and injected into a gas chromatographer (Agilent Technologies 6890N) equipped with a flame ionization detector and a DB-TPH column (30 m 0.32 mm 0.25 mm). It was found that there was no significant loss of diesel compounds in terms of TPH during the experiments and analyses during the preliminary test. Diesel removal efficiency was calculated using the following equation: Ci - Cf 100 ð3Þ diesel removal efficiency ð%Þ ¼ Ci

where Ci is the initial diesel concentration of soil in TPH, and Cf is the final diesel concentration of soil in TPH. All the experiments 2401

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


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

were conducted at least three times. The surfactant used herein was SDS, 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 the strength of the physical effects of ultrasound and to decide the optimal frequency. Figure 2 shows the damaged foil images under various input power and frequency conditions, which were trimmed in a circular shape with a diameter of 35 mm. The white spots illustrate the cavitational damages by ultrasound irradiation. The bigger and brighter zones were perforated holes, which revealed the mass loss of the foil, while smaller and less bright zones were pits. In addition, the damage on the foil was not uniform because the ultrasound field was not uniform, and the foil surface might have tiny damages including scratches, dirt, and pits before the test.22 The cavitational erosion process consisted of several periods including incubation, acceleration, steady state, and attenuation. In the incubation period, no mass loss was observed, but subtle deformation like a pit was developed in a cluster. As cavitational damages were accumulated on the pits, mass loss of material occurred in the acceleration and succeeding periods.22,23 The material could be eventually perforated when a very thin material like 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 more pits appeared and gathered, the damage on the site was accumulated, and then a hole was generated. Because the edge of the hole could be another amplifier of cavitation, more erosion occurred around 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 the accumulated damage resulted in the formation of a hole, the damage on that site could not be visualized anymore. However, the hole enhanced the cavitational erosion dramatically as explained above, and the damaged area was expanded enough to include the damage on the missing region. Thus, this evaluation method can be considered for quantifying the erosion on the foil based on the damaged 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 based on the pixel using the images in Figure 2. The results for the quantification 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 applied frequency was increased, the magnitude of erosion decreased significantly. No perforation hole was observed for 72, 110, and 170 kHz, and the deformation ratio ranged only from zero to 5.7% in these frequency conditions, while the ratio observed was 7.6-27.0% for 35 kHz. From these observations, it can be revealed that the cavitational erosion under higher three frequency conditions did not proceed over the incubation period, and the damaging power of the sonophysical effects on the surface was not high enough to make severe damages.23 It is well-known that the radial oscillation of acoustic bubbles is significantly larger for the lower frequency. In other words, the ratio between Rmax and Rmin during oscillations of the cavitation bubbles is higher for the lower frequency.24 It is also known that the resonance radius of the cavitation bubbles decreases with an increase in the ultrasonic frequency. The resonant radius of bubble is inversely proportional to applied frequency according to eq 4.25 3kP0 2 ωr Rr 2 ¼ ð4Þ F 2402



Figure 3. Deformation ratio under various frequency and input power conditions and resonant bubble radius for four frequencies.

where Rr is the resonant bubble radius, κ is the polytropic index, P0 is 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 in Figure 3. Because of the larger size of the bubbles at lower frequencies, the bubble oscillations are more effective in generating physical effects, such as micro jet, turbulence, shear, etc. Thus, the physical effects that cause the damage to the particles in a sound field are more effective at low frequency under the same input 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 due to the exchange of the washing liquid. This will be discussed later. Thus, 35 kHz, the lowest frequency, was chosen as the optimal frequency for ultrasonic washing processes among the four applied frequencies. On the other hand, many researchers reported that high frequencies are optimal to degrade organic pollutants sonochemically by various radical species, which are produced inside and near the cavitation bubbles and have strong oxidation power.5,6,26-28 High frequency can be applied additionally to the washing processes with the low frequency irradiation system for the treatment of desorbed pollutants in washing leachate. It is possible to increase the biodegradability of desorbed pollutants even though full mineralization cannot be achieved in a single washing process. In the following section, however, only 35 kHz was used to estimate desorption 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 processes were investigated at 35 kHz. The optimal mechanical mixing speed was determined as 60 rpm in a preliminary test, and it was 2.5 W in terms of electrical energy consumption. The ultrasound input power was 2.5 W in terms of a calorimetric power. As shown in Figure 4, the diesel removal efficiency in the ultrasonic soil-washing process with the mechanical mixing was much higher 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 ultrasound could induce strong physical effects. When ultrasound was irradiated to the slurry from the outside of the vessel, most soil particles were settled at the bottom of vessel due to gravity, and only a small fraction of settled particles located near the wall of

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Figure 4. Variations of diesel removal efficiency under different soilwashing conditions.

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 sound adsorption phenomena.29 As a result of this, cavitation events did not occur noticeably. On the other hand, the mechanical mixing suspended the soil matrix completely in macroscale and increased a contact between contaminated soil and washing liquid according to the mixing intensity. This action enabled adsorbed 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 induced desorption only from the outer layers of diesel on the soil particle.13 For the simultaneous application of ultrasound and the mechanical stirring, both the macroscale mixing and the microscale mixing were induced, and it resulted in a much higher removal efficiency due to the overall pollutants desorption from not only the surface of soil particles but also the inside of pores. Moreover, the mechanical mixing increased the exposure frequency of soil particles to high-intensity ultrasound and enhanced the ultrasonic desorption of pollutants significantly. Thus, these results reveal that the mechanical mixing is essential to increase the performance of the ultrasonic soil-washing process remarkably. When a horn-type ultrasound generator was used, ultrasound could generate both macroscale as well as microscale mixing. The tip with a small-surface area can emit concentrated-high-ultrasonic energy like a jet of water from a nozzle. Dahlem et al. showed a snapshot of violent flow pattern originated from the tip of the laboratory horn-type generator.30 In our preliminary test using the horn-type generator, it was also observed with the naked eyes that soil particles in the vessel were moved very vigorously along the water flow induced by the ultrasonic energy from the tip. The visible mixing flow for the soil particles was as powerful as the flow made by mechanical stirring. Na et al. obtained relatively high removal efficiency in a horn-type ultrasonic soil-washing system for diesel contaminated soil without additional mechanical stirring.18 However, it seemed that the macroscale mixing by ultrasound from the horn-type sonicator was not so effective for the industrial-scale uses because this phenomenon could be only observed in small-scale reactors. 2403



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

The results shown in Figure 4 also indicate that after a 1 min washing operation, the increase of the removal efficiency was constant for all three processes. In a previous study, it was observed that the removal efficiency of diesel was not increased after 5 min due to readsorption of the diesel.13 When desorption of diesel is maximized in each process, the washing liquid should be separated from the treated soil to prevent readsoption. Thus, 1 min was decided as the optimal washing time for ultrasonic soilwashing process in the presence of mechanical stirring for further experiments. 3.3. Ultrasonic Soil-Washing Process with Mechanical Mixing. To enhance the diesel removal and compare the performance in the ultrasonic soil-washing process with mechanical mixing and the mechanical soil-washing process in terms of other factors rather than the removal efficiency, washing liquid (tap water was used) was exchanged after a 1 min mechanical soilwashing process, and then the concentration of the residual diesel in the soil was analyzed to obtain removal efficiency. This procedure was repeated five times. Figure 5 shows that the removal efficiency increases significantly until the fourth washing process, whereas the fifth washing process does not show enhancement in the removal efficiency. It seemed that desorption of diesel occurred mainly from the surface of soil during the first four washing processes, and the removal efficiency did not increase significantly when the rest of the diesel was trapped inside the pores. Remarkably, the first ultrasonic soil-washing process in the presence of mechanical mixing showed removal performance similar to that of the fourth mechanical soil-washing process. As explained above, ultrasound with the mechanical mixing could induce the macro- and microscale mixing in the slurry and synergistic actions on the removal of diesel from not only the surface but also from inside the pore of soil. Na et al. have reported that the mass transfer constant was increased by 60% when sonication was combined with stirring as compared to that observed for stirring only during naphthalene desorption from soil.18 After the second ultrasonic soil-washing process, the removal efficiency reached close to 90%. The removal efficiency of 91% after second ultrasonic soil washing in this study left a residual diesel concentration of 644 mg/kg, and it could meet the Worrisome Level of Soil Contamination for Area 2 (800 mg/kg) in Korea. It can be expected that a

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third washing process would meet the regulation for Area 1 (500 mg/kg), which is the strongest regulation for contaminated soils in Korea.31 From the results discussed so far, it is evident that the ultrasonic soil-washing process in the presence of mechanical mixing has several advantages. First, this process could achieve high removal efficiency in a single attempt. Second, it could decrease the operating time markedly. In this study, the addition of 35 kHz ultrasound irradiation to the conventional soil-washing process enabled the washing time to decrease from 4 to 1 min for the removal of 75% diesel from the contaminated soil. Third, it could reduce energy consumption significantly. Total electric energy 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 h and mechanical mixing energy of 0.042 W h, while the electric energy of 0.17 W h was consumed for the 4 min mechanical soilwashing process. Even though ultrasound irradiation energy was calculated using calorimetry, an indirect measurement, and cooling energy for the whole sonoreactor was not considered, the ultrasonic soil-washing process in the presence of mechanical mixing could be an energy-efficient process as compared to the conventional processes. The direct comparison of energy consumption on each process for real processes cannot be made because the experiments were carried out in small scale. However, this can be considered as one of the references for comparing the processes in terms of economy. Fourth, the amount of washing leachate could be decreased remarkably. Generally, washing leachate contained various desorbed pollutants and fine particles. The characteristics of the washing leachate depended on the characteristics of target contaminated soil, the soil-water ratio, and chemicals added in washing processes, but it is clear that post treatment processes such as biological treatments, conventional physicochemical treatments, and advanced oxidation processes are always required. Thus, the less washing leachate the process made, the more competitive the process could be for engineering use. Herein, the amount of washing leachate in the ultrasonic soil-washing process in the presence of mechanical mixing was only 25% of the amount of conventional washing to achieve a removal efficiency of 75%. Feng and Aldrich also investigated the effect of the exchange of the washing liquid, and it was found that the increase of the removal efficiency was only less than 2% on average despite three times rinsing after each process.13 This ineffectiveness might be due to their washing system. They used the horn-type ultrasound generator without mechanical mixing for the soil-washing process, and this could not induce macroscale mixing properly as mentioned early. Table 1 shows the effect of input power on the washing efficiency of the ultrasonic process in the presence of mechanical stirring. As the input power increases, the removal efficiency increases. However, the relative increase in the efficiency is relatively low as compared to the relative increase in the power. The removal efficiency increase by only 4.4% when the input power is increased from 2.5 to 6.0 W. This means that a higher removal efficiency can be achieved at relatively lower acoustic power levels. Feng and Aldrich and Na et al. also reported that there were not significant increases in the removal efficiency as compared to the increase of the input power or power intensity.13,18 It was also suggested that the replacement of the washing liquid would result in higher removal efficiency than increasing washing time or input power in the ultrasonic soilwashing process with mechanical mixing as discussed earlier. 2404



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Table 1. Effect of Input Power for the Removal Efficiency in the Ultrasonic Soil-Washing Process with Mechanical Mixing calorimetric input power

1.0 W

2.5 W

4.1 W

6.0 W

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

Interestingly, the trend of diesel removal under four input power conditions in ultrasonic soil-washing tests did not match with the result in aluminum foil tests. The damage by sonophysical effects on the surface of aluminum foil increased significantly as input power increased from 1.0 to 6.0 W. The damage included pits and holes. However, the removal efficiency in ultrasonic soil washing did not increase noticeably under the same input power conditions. It is because ultrasound should penetrate many heterogeneous layers consisting of soil particles and liquid for desorption of pollutants, and the transmitted ultrasound energy could decrease greatly due to the existence of many solid/liquid boundaries. Thus, only the degree of desorption in the outest layer of contaminated soil could be predictable from the result of aluminum foil tests, and it seemed that sonophysical effects inside were not effective enough to occur vigorous desorption like at the outest layer of soil. Even though mechanical mixing could help soil to expose more to highpower ultrasound and enhance desorption of pollutants, it did not make 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 organic compounds from the soil in slurry phase by two mechanisms including micellar solubilization over the critical micelle concentration (CMC) and mobilization due to hydrophobic interaction.32,33 SDS is one of the common surfactants in soil-washing processes, and it was considered a model surfactant in this study.13,18,34-38 Figure 6 shows the effect of SDS on the diesel removal efficiency in the mechanical soil-washing process and the ultrasonic soil-washing process with mechanical mixing. The maximal value of removal efficiency was obtained at the concentration of 5 mM for each washing process, but the removal efficiency did not increase noticeably as compared to that at the concentration of 1 mM. At 10 mM, a slightly higher concentration than CMC, the removal efficiency decreased, and then there was no significant change of removal efficiency at higher concentrations of SDS for both cases. This result was partly consistent with previous results, which showed that surfactant could enhance the removal efficiency of organic pollutants remarkably in the mechanical and ultrasonic soil-washing processes and the removal efficiency was not changed significantly above CMC.13,18 However, the removal efficiency herein could be maximized at 1 mM, which was approximately only 10% of CMC of SDS, while most previous researchers reported that the surfactant-enhanced washing processes could be optimized above CMC due to an increase of solubility of organic pollutants. The reason why the removal efficiency decreased above CMC might be the formation of foam due to the presence of high concentration of surfactant. When the mechanical mixing was applied to the slurry where the concentration of SDS was above the CMC, a white foam (like soap foam) formed, surrounded the soil particles, and then impeded desorption of pollutants by the 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 and Lee revealed that equilibrium solubility of naphthalene increased significantly as mixing rate increased.34 For ultrasound irradiation, foam could block the transmission of acoustic energy to the soil particles, which occurs in various sonophysical effects. It

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

seemed to be similar to the formation of a dense cloud of cavitation bubbles near the tip of the horn-type sonicator, resulting in low 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 be another reason because the concentration of SDS reported here is the bulk concentration. It was suggested that a higher concentration of the surfactant (than that for CMC in bulk phase) was required to achieve the effective CMC in slurry phase.39 Zheng and Obbard reported that effective CMC was 4-5 times higher than normal CMC using nonionic surfactants in soil/aqueous system and the effective CMC increased as the soil portion in the slurry increased due to the surfactant adsorption.40 However, any significant increase of the removal efficiency did not occur over 20 mM, which was 2 times higher than the CMC, and anionic surfactants such as SDS are less adsorbed to the soil than are nonionic surfactants.33 Our ongoing study also showed that adsorption of SDS in the slurry phase was negligible (data not shown). There was no formation of foam in both processes at the concentration of 1 and 5 mM, below CMC. Moreover, Khalladi et al. reported that the surface tension in the liquid reached a minimum value at ∼2 mM of SDS with and without soil. They also revealed that minerals in soil acting as electrolyte in solution enhanced the reduction of surface tension by SDS, and there was no effective loss of surfactant due to the presence of the soil particles.37 These enabled the removal efficiency to be higher below CMC without micellar solubilization. Table 2 shows the removal efficiency for the mechanical and the ultrasonic-mechanical soil-washing processes in the presence and absence of SDS (5 mM) after 1 min operation. The addition of SDS enhanced the removal efficiency significantly in the mechanical soilwashing process, and this process seemed to be relatively efficient in terms of an energy assumption because only one-half of electric energy was consumed in the mechanical soil-washing process with SDS as compared to the combined ultrasonic-mechanical soilwashing process. Yet additional treatment processes would be required to remove or recover the used surfactant, and difficulty for a recovery would result in large consumption of the surfactant.33 On the other hand, it seemed that SDS was not necessary to operate the ultrasonic-mechanical soil-washing process because 2405



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

mechanical soil-washing process

mechanical soil-washing process with SDS

ultrasonic soil-washing process

ultrasonic soil-washing process with SDS

diesel removal efficiency (%)

31.0 ( 5.4

64.2 ( 5.1

74.7 ( 7.6

76.1 ( 3.8

the removal efficiency was the same with and without SDS. No SDS would be required for high performance in this process. Thus, it is clear that ultrasound enables conventional soilwashing processes to be a green process.

4. CONCLUSION The ultrasonic soil-washing process was compared to the conventional mechanical soil-washing process. In addition, the effect of combining mechanical mixing with ultrasonic process was also investigated to evaluate the removal efficiency of diesel, the operation time of process, the consumption of electric energy, and the production of washing leachate. It was found that the ultrasonic soil washing with the mechanical mixing was the best process among three processes in all aspects considered herein because of macro- and micromixing effects. The addition of SDS resulted in an increase in removal efficiency for the mechanical soil-washing process as well as the ultrasonic soilwashing process combined with mechanical mixing. Even though SDS increased the removal efficiency, the addition of SDS in the ultrasonic soil-washing process combined with mechanical mixing was not necessary. Thus, the combined process produces less contaminated washing leachate, or no treatment process for the recovery of SDS is required. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ82 2 32903318. Fax: þ82 2 9287656. E-mail: hyeong@ korea.ac.kr.

’ ACKNOWLEDGMENT We acknowledge financial support from the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2008-313-D00576). Y.S. also acknowledges an Endeavour Research Fellowship. ’ REFERENCES (1) Adewuyi, Y. G. Sonochemistry: Environmental science and engineering applications. Ind. Eng. Chem. Res. 2001, 40, 4681–4715. (2) Thompson, L. H.; Doraiswamy, L. K. Sonochemistry: Science and engineering. Ind. Eng. Chem. Res. 1999, 38, 1215–1249. (3) Mason, T. J.; Lorimer, J. P. Applied Sonochemistry-The Uses of Power Ultrasound in Chemistry and Processing; Wiley-VCH Verlag GmbH: Weinheim, 2002. (4) Lim, M.; Kim, S.; Kim, Y.; Khim, J. Sonolysis of chlorinated compounds in aqueous solution. Ultrason. Sonochem. 2007, 14, 93–98. (5) Torres, R. A.; Petrier, C.; Combet, E.; Carrier, M.; Pulgarin, C. Ultrasonic cavitation applied to the treatment of bisphenol A. Effect of sonochemical parameters and analysis of BPA by-products. Ultrason. Sonochem. 2008, 15, 605–611. (6) Inoue, M.; Masuda, Y.; Okada, F.; Sakurai, A.; Ichiro, T.; Sakakibara, M. Degradation of bisphenol-A using sonochemical reactions. Water Res. 2008, 42, 1379–1386. (7) Teo, B. M.; Chen, F.; Hatton, T. A.; Grieser, F.; Ashokkumar, M. Novel one-pot synthesis of magnetite latex nanoparticles by ultrasound irradiation. Langmuir 2009, 25, 2593–2595.

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Comparison of Ultrasonic and Conventional Mechanical Soil-Washing

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