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Recovering phenanthrene from spiked sand by a combined remediation process of micellar solubilization and cloud-point extraction
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Journal of the Chinese Institute of Chemical Engineers 39 (2008) 337–342 www.elsevier.com/locate/jcice
Recovering phenanthrene from spiked sand by a combined remediation process of micellar solubilization and cloud-point extraction Jing-Liang Li a,b, Bing-Hung Chen c,* a
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Centre for Micro-Photonics, Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, Vic. 3122, Australia c Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
b
Received 14 December 2007; received in revised form 26 January 2008; accepted 29 January 2008
Abstract A remediation process, which combines the micellar solubilization and the cloud-point extraction technique by a nonionic surfactant Tergitol 15-S-7, was used to decontaminate phenanthrene, as a model hydrophobic pollutant, from spiked sand samples. A first-order kinetics model was employed to describe the solubilization behavior of phenanthrene well. It was observed that presence of surfactant decreased the mass-transfer coefficient of phenanthrene from sand surface to surfactant solutions, however, higher solubilization rate was obtained due to enhanced aqueous solubility and, thus, the larger driving force resulted from solublization. Cloud-point extraction was used to concentrate the phenanthrene solubilized in the washing solutions in an attempt to minimize the amount of wastewater. The extraction was carried out, subsequently, at room temperature by adding sodium sulfate to suppress the cloud-point low enough to induce phase-separation of the surfactant-rich phase with a minimal phase volume from the coexisting water phase. Recoveries higher than 93% were achieved in the combined process of micellar solubilization and cloud-point extraction on ultimate removal of immobilized phenanthrene sorbed on sands. The results showed that this combined process is efficient in recovering phenanthrene sorbed and immobilized on sands from contaminated sites, and produces only minimal amount of wastewater, i.e. less than 3% of its original volume. # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Nonionic surfactant; Polycyclic aromatic hydrocarbon (PAH); Micellar solubilization; Cloud-point extraction; Soil remediation
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a family of highly toxic chemicals. Their wide existence in the environment arouses many concerns because many of them are either carcinogens or mutagens (Brody et al., 2007; Rybicki et al., 2006). Moreover, their low aqueous solubility and high affinity to soils contribute to their persistence in the environment. Developing effective ways to remove these pollutants from contaminated sites has stimulated extensive research interests in recent years (Fiorenza et al., 2000). Surfactant-mediated remediation is a promising technique for decontamination of hydrocarbon-polluted soils (Fiorenza et al., 2000; Venditti et al., 2007). Surfactants can increase the solubility of hydrophobic compounds significantly through a
* Corresponding author. Tel.: +886 6 275 7575x62695; fax: +886 6 234 4496. E-mail addresses: [email protected], [email protected] (B.-H. Chen).
process known as solubilization (Li and Chen, 2002; Rosen, 2004). The enhanced solubilization of PAHs in presence of surfactants has been reported considerably (Dar et al., 2007; Grimberg et al., 1994, 1995; Hung et al., 2007). While much effort has been shown on how to increase the decontamination efficiency by surfactant flushing in soil remediation processes, relatively less attention has been paid on the proper treatment or disposal of the washing solution. In recent years, cloud-point extraction (CPE) techniques using proper surfactants have been widely utilized for efficiently concentrating and detecting trace toxic compounds including PAHs. High recoveries and preconcentration factors of PAHs using CPE have been also demonstrated (Bai et al., 2001; Hung et al., 2007; Li and Chen, 2003; Li et al., 2004). The CPE process is mainly based on the clouding phenomena of surfactants, especially that of nonionic surfactants. Micellar solutions of proper nonionic surfactants are homogeneous and isotropic at ambient temperature. Upon appropriate alteration of conditions, such as temperature and additives, the micellar solutions become turbid due to the loss in aqueous solubility of
0368-1653/$ – see front matter # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2008.01.005
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Nomenclature a C C* Cphen CS 0 Csoil
k kl Msoil Rm RCPE RT VW VS
the specific surface area of sand sample (m2) enveloped phenanthrene concentration in overall washing solution collected (mg/L) saturation concentration of phenanthrene in micellar solution (mg/L) phenanthrene concentration in washing solution at time t (mg/L) phenanthrene concentration of the surfactant phase (mg/L) initial concentration of PAH sorbed on soil (mg/g sand) mass-transfer coefficient (1/(min m2)) lumped mass-transfer coefficient (1/min) soil mass (g) recovery of phenanthrene in the micellar solubilization process (%) recovery of phenanthrene in cloud-point extraction process (%) total/ultimate recovery of phenanthrene (%) volume of overall micellar solution (mL) volume of surfactant phase after cloud-point phase-separation (mL)
surfactant molecules (Schott, 1997; Rosen, 2004). Consequently, the micellar solutions will separate into two coexisting phases. One is the surfactant-rich phase, which contains most of surfactant molecules, whereas the other is the water phase, in which surfactant concentration is low and close to its critical micelle concentration (CMC). This temperature at which the phase-separation occurs is called cloud-point (Rosen, 2004). Upon cloud-point phase-separation, the hydrophobic compounds initially present in the solution and favorably bound to the micelles will be preferentially extracted into the surfactantrich phase (Bai et al., 2001; Hung et al., 2007; Li and Chen, 2003; Li et al., 2004). Very often, the volume of the surfactantrich phase is much smaller than that of the coexisting water phase. Therefore, concentration of these compounds is enhanced by the CPE process. Namely, the CPE process can significantly increase the concentration of the extracts principally by reducing the phase volume of the contaminant-containing surfactant solution. This feature is conjectured in this work to be employed in a typical surfactant flushing remediation process to reduce the volume of the washing solutions generated in a soil remediation process. Moreover, the main solvent used in a typical CPE process is water, in contrast to the organic liquids in a liquid–liquid extraction process, which also gives CPE a great advantage in environmental benignity and compatibility. The CPE process has been employed in many applications to extract compounds of interests, such as proteins and enzymes (Lai et al., 2006; Soriano-Lopes et al., 2007), environmental pollutants from soils/sediments (Cheng and Sabatini, 2007; Hung et al., 2007; Li et al., 2004), heavy metal ions from effluents (Ferreira et al.,
2007; Pyrzynska, 2007), improved analytical methods (Keith et al., 2007), etc. The performance of the CPE process has been approved in these studies. However, drawbacks of CPE processes are often reflected on the high extraction temperatures that intrinsically arise from surfactants themselves. In practice, it is both convenient and economic to facilitate the extraction process proceeding at ambient temperatures. As aforementioned, smaller phase-volume of surfactant-rich phases are also desired in field applications. To achieve these, surfactants with lower cloud-point temperatures and strategy to put in salting out additives should be preferentially employed (Bai et al., 2001; Li et al., 2004). In this study, not only the development of the CPE process that can possibly remove more contaminants from aquifer by surfactant flushing, but also design of a CPE process producing a minimal quantity of the waste washing solution is favored. To our best knowledge, this work is the first report ever in the open literature on the concept of combining a remediation process by surfactant flushing and solubilization, and a CPE process to minimize the waste effluents. The efficiency of such a process is also examined on the decontamination of pollutant-spiked sands that simulate a contaminated aquifer. That is, the efficiency of the combined process in terms of the recoveries of the micellar solubilization process and the cloud-point extraction process, as well as the overall recovery of phenanthrene of the process were studied. Phenanthrene, a common hydrophobic PAH, was selected as a model pollutant, along with a commercial grade of readily biodegradable nonionic surfactant, Tergitol 15-S-7. The solubilization kinetics of phenanthrene in presence and absence of the surfactant was also investigated. Tergitol 15-S-7 was especially chosen in this work, based on its low cloud-point, about 38 8C in 1 wt% solution, and its high extraction capacity for PAHs (Bai et al., 2001; Li and Chen, 2003). Sodium sulfate was applied to enhance the phase-separation of Tergitol 15-S-7 by further lowering its cloud-point, reducing the volume of the surfactant-rich phase and increasing the density of the aqueous phase. As a result, overall recovery efficiency higher than 93% to recuperate immobilized phenanthrene from sands could be achieved. Moreover, the phase volume of the surfactant washing solution has been reduced to less than 3% of its original volume. 2. Materials and method 2.1. Materials The nonionic surfactant, Tergitol 15-S-7 supplied by Dow Chemical (USA), is a mixture of species with an alcohol group located at various positions along a linear chain of 11–15 carbon atoms and with an average ethylene oxide number of 7.3. It has a low critical micelle concentration of 39 ppm at 25 8C. Moreover, this surfactant is readily biodegradable and has been accepted by the Food Safety and Inspection Service of the U.S. Department of Agriculture for general-purpose cleaning or as an ingredient of general-purpose cleaner for use in federally inspected meat and poultry processing plants.
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Reagent grade of phenanthrene was obtained from Aldrich. Sodium sulfate is of analytical grade and purchased from Merck. HPLC-grade methanol was acquired from Sigma. Deionized water from a Milli-Q purification system (Millipore, USA) having resistivity greater than 18.2 MV cm was used in preparing samples. All chemicals were used as received.
rate at 1 mL/min. The excitation and emission wavelengths used in the fluorescence detector on phenanthrene were 248 and 395 nm, respectively. The detection limit of the fluorescence detector for phenanthrene was 1 ppb.
2.2. Preparation and characteristics of sand spiked with phenanthrene
The recovery of phenanthrene from sands is schematically described in Fig. 1, which consists of two main steps, i.e. the micellar solubilization process and the cloud-point extraction process.
Sand used in the extraction experiments was obtained from the High Performance Concrete Laboratory of The National University of Singapore. It was sieved with a size range from 500 to 800 mm. The porosity of common sand packing in this experiment is found at ca. 0.35. Before being spiked, sand was washed with acetone thoroughly to get rid of any organic contents and then dried at 500 8C for 48 h. Phenanthrene-spiking sand was prepared by soaking sand in acetone with dissolved phenanthrene for a week. Afterwards, acetone was removed by evaporation and spiked sand was, subsequently, washed completely by deionized water. The phenanthrene content of spiked sand was determined by the total organic carbon (TOC) instrument. The measured value indicated that, on average, 0.6 mg of phenanthrene was successfully immobilized on per gram of sand sample. An Eppendorf 5810R (Hamburg, Germany) centrifuge was used to accelerate phase-separation in cloud-point extraction. A Shimadzu TOC-5000A total organic carbon analyzer equipped with an ASI-5000A auto sampler was used to determine phenanthrene contents on spiked sand samples. 2.3. HPLC analysis of phenanthrene concentration A Shimadzu high-performance liquid chromatography equipped with a fluorescence detector was used for analysis of phenanthrene concentration. The details of the system have been provided elsewhere (Li and Chen, 2002). An Agilent PAH C18 column (250 mm 4.6 mm) was used. It was packed with 5 mm particles and connected with the Guard cartridge holder (Agilent 79918PH-100). Mobile phase was a mixture of methanol (80 vol%) and ultrapure water (20 vol%), with a flow
2.4. Experimental procedure
2.4.1. Micellar solubilization process Spiked sands of 10 g were added to a 500-mL Erlenmeyer flask. Subsequently, 200 mL of micellar solution of Tergitol 15S-7 at various concentrations was poured into the Erlenmeyer flask, which was agitated continuously on an orbital shaker operated at 150 rpm. To measure the solubilization kinetics, aliquots of 0.5 mL solution were withdrawn at certain time intervals. The samples were then centrifuged for HPLC analysis. From previous studies, the solubilization rate of phenanthrene can be described by a first-order mass-transfer model (Grimberg et al., 1994, 1995; Li and Chen, 2002). dCphen ¼ kaðC C phen Þ ¼ kl ðC C phen Þ dt
(1)
where Cphen is the phenanthrene concentration in washing solution at time t; C* is the saturation concentration of phenanthrene; k is the mass-transfer coefficient; a is the specific surface area of sand sample; and kl is the lumped mass-transfer coefficient. Since the specific surface area of the sand sample is difficult to be precisely determined, instead, the lumped masstransfer coefficients at different surfactant concentrations were preferentially involved in this model. Consequently, the recovery by micellar solubilization process, Rm can be expressed as follows: Rm ð%Þ ¼
C VW 0 Csoil M soil
100
(2)
Fig. 1. Schematic description of the procedures combined with micellar solubilization and cloud-point extraction in recovering phenanthrene from spiked sands in aquifer.
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where C is the enveloped phenanthrene concentration in total washing solution collected (mg/L); VW is the volume of overall 0 washing solution (mL); Csoil is the initial concentration of phenanthrene immobilized on soil (mg/g); and Msoil is the mass of the soil (g). 2.4.2. Cloud-point extraction process After micellar solubilization arrives at equilibrium, the washing solution in the flask was transferred to 30 mL centrifuging tubes. Proper amount of sodium sulfate was added to these tubes to have a final concentration at 0.5 M. After sodium sulfate is completely dissolved, the tubes were centrifuged at 4000 rpm for 20 min to speed up the phaseseparation process. Samples from the surfactant-rich phase were withdrawn for HPLC analysis. The percentage of phenanthrene, RCPE, extracted from the washing solution into the surfactant phase by the CPE process could be estimated as follows: RCPE
CS V S ¼ C VW
(3)
where VS is the volume of surfactant phase and CS is the phenanthrene concentration in the surfactant phase after cloudpoint phase-separation. The ultimate recovery of the phenanthrene, RT, by the combined process could then be calculated with Eq. (4), accordingly. RT ð%Þ ¼
CS V S 0 Csoil M soil
100 ¼
Rm RCPE 100
(4)
3. Results and discussions 3.1. Solubilization kinetics of phenanthrene Equilibrium solubilization, but not solubilization kinetics, of PAHs, including phenanthrene, by micellar solutions of Tergitol 15-S-7 has been reported earlier by Li and Chen (2002). The solubilization extent of PAHs is linearly proportional to the concentration of surfactant beyond its CMC. Figs. 2 and 3 show the solubilization kinetics of phenanthrene in absence and presence of surfactant. A firstorder kinetics model could describe it well on the solubilization behavior of phenanthrene in surfactant solutions. The saturation concentration of phenanthrene and the lumped mass-transfer coefficients obtained from properly fitting of experimental data to Eq. (1) are given in Table 1. The aqueous solubility of phenanthrene found from Fig. 2 is around 1.02 ppm. It is of note to mention that, in the experiment shown as Fig. 2, only about 3.4% of total phenanthrene spiked on sands was dissolved into water (Table 1). Solubilization of phenanthrene sorbed on sands into surfactant solutions usually involves three steps (Yeom et al., 1996). The first step is the matrix diffusion of phenanthrene from sands, followed by the mass-transfer at the interface, i.e. film diffusion, and finally partitioning into the
Fig. 2. Solubilization of phenanthrene immobilized on sands in aquifer into water in the absence of surfactant.
micellar phase. The last step is usually much faster than other two (Gehlen and DeSchryver, 1993). Hence, solubilization kinetics of pollutants from contaminated soils really depends on many factors. The most important one is the contamination history of the soil. For weathered soil with a long history of contamination, the matrix diffusion commonly controls the solubilization process. As a result, solubilization behavior cannot be simply described by a firstorder kinetics model. In contrast, in the case of artificially contaminated soil or soil with a short contamination history, the film diffusion is more important. Hence, a first-order kinetics model might be adequate. Table 1 also shows that the lumped mass-transfer coefficient decreased with an increase in surfactant concentration. If the specific surface areas of the sand samples in each experiment could be regarded as the same, the mass-transfer coefficient k decreases with an increase in surfactant concentration. This assumption is valid in this work only when washing solutions with surfactant concentration less than 0.1 wt% were employed (Fig. 4). The discrepancy at higher surfactant concentrations will be discussed later in this section.
Fig. 3. Solubilization of phenanthrene immobilized on sands in aquifer into surfactant solutions. Concentrations of surfactants range from 0.05 to 0.2 wt%.
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Table 1 Fitted parameters obtained from micellar solubilization process of phenanthrene and recoveries of phenanthrene from a combined process of micellar solubilization and cloud-point extraction Surfactant concentration (wt%)
0 0.05 0.1 0.2
Saturated phenanthrene concentration, C* (mg/L) Measured value
Fitted value
1.018 11.795 28.561 29.811
1.010 11.650 28.014 29.547
kl (1/min)
Rm (%)
RCPE (%)a
RT (%)
0.201 0.179 0.043 0.044
3.4 39.3 95.2 99.4
– – 98.1 0.2 98.0 0.3
– – 93.4 97.4
–, cloud-point extraction was not carried out. a Na2SO4 were added into washing solution with a final concentration at 0.5 M.
Solubilization of phenanthrene in presence of surfactant includes its molecular dissolution by bulk solution and phenanthrene solubilized from the interface to micelles. The decreased mass-transfer coefficient can be attributed to the slower mass-transfer of phenanthrene through palisade layer of surfactant micelles. However, apparent saturated solubility of phenanthrene molecules, i.e. C* appearing in Eq. (1), in the aqueous phase is much smaller than that in micellar solutions. Hence, enhanced solubilization capacity of phenanthrene with increasing surfactant concentration compensates the loss in the mass-transfer coefficient of phenanthrene, which, in turn, generally results in an overall enhancement in solubilization rates of phenanthrene. Similar results were obtained for the solubilization of palmitic acid from solid surface to the micellar solutions of Tergitol 15-S-7 (Lim et al., 2005). Furthermore, the saturated solubility of phenanthrene and the recovery efficiency of phenanthrene by surfactants are also plotted as Fig. 4. As aforementioned, apparent solubilities of hydrophobic solubilizates are often increased linearly with surfactant concentrations in micellar solutions. However, in this work, solubilization behavior of phenanthrene in 0.2 wt% Tergitol 15-S-7, i.e. at ca. 50 CMC of Tergitol 15-S-7, deviates from the linear prediction on surfactant effect to equilibrium solubilization (Li and Chen, 2003; Lim et al., 2005). That is, packing of sands starts to influence significantly
the solubilization behaviors, as it dwindles drastically on the specific surface area on sands available to phenanthrene molecules for dissolution into bulk solutions of surfactant (Fiorenza et al., 2000). 3.2. Recovery of phenanthrene by micellar solubilization The recovery of phenanthrene by micellar solubilization at various surfactant concentrations is given in Table 1. In general, recovery efficiency of phenanthrene by micellar solubilization increases significantly with an increase in surfactant concentration. Moreover, as surfactant concentration increases to 0.2 wt%, almost all phenanthrene initially spiked on sands was completely solubilized into surfactant washing solutions. With 0.1 or 0.2 wt% surfactant solutions, the nearly complete solubilization of phenanthrene takes about 2 h. Certainly, as mentioned in previous section, porosity of sand packings and mobility of surfactant solutions in these pores could also affect the solubilization kinetics as well (Fiorenza et al., 2000). With surfactant solutions at lower surfactant concentration, the recovery efficiency of phenanthrene by micellar solubilization is linearly proportional to surfactant concentration (Fig. 4). As the recovery efficiency of CPE processes using Tergitol 15-S-7 is generally high enough, the micellar solubilization process is indeed the limiting step in the combined process in terms of the overall recovery efficiency of phenanthrene. 3.3. Cloud-point extraction of phenanthrene from washing solution
Fig. 4. Effect of surfactant concentration on apparent solubility of phenanthrene in micellar solution and recovery efficiency of phenanthrene by solubilization in this micellar solution.
The phenanthrene in surfactant solution after micellar solubilization was extracted by adding sodium sulfate to facilitate the extraction process proceeding at ambient temperature. The final salt concentration is 0.5 M. The sodium sulfate was used to reduce the cloud-point of the surfactant solution and to accelerate the cloud-point separation by increasing the density of the aqueous phase. Indeed, the presence of suitable salting-out salts in the micellar solution can significantly reduce the volume of the surfactant-rich phase after cloud-point separation by dehydrating surfactant molecules. Previous work showed that at the surfactant concentration of 1 wt% and sodium sulfate concentration of 0.5 M, the phase volume of surfactant-rich phase has been reduced by 30 times, compared to the original bulk phase (Li and Chen, 2003). Namely, phenanthrene can be
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further concentrated into a small volume of surfactant-rich phase. This makes the subsequent handling and treatment process of phenanthrene easier by reducing the amount of the wastewater to be processed. Detailed work on the effects of sodium sulfate on the phase volume of the surfactant phase and on the recovery of PAHs has been reported in our previous works (Li and Chen, 2003). The recovery of phenanthrene during the cloud-point extraction and the total recovery of phenanthrene spiked on the sand samples are given in Table 1. For surfactant solutions with surfactant concentrations at 0.1 and 0.2 wt%, over 93% and 97% of phenanthrene could be recovered by the combined process of micellar solubilization and cloud-point extraction. Furthermore, the quantity of wastewater from surfactant flushing process can be greatly reduced as well. Finally, this combined process can certainly be employed in the remediation process of soil contaminated by toxic and hydrophobic compounds. Water recycled after the CPE process can be again used in the remediation process. In general, the performance of the combined process might still be limited by the geophysical and geochemical factors of the aquifers, which is always inevitable in any soil remediation methods using surfactants (Fiorenza et al., 2000). Albeit, this combined process is still very promising in soil remediation. 4. Conclusions The process, which combines micellar solubilization and cloud-point extraction using Tergitol 15-S-7 is an efficient approach for the remediation of contaminated soil. The solubilization of phenanthrene from the spiked sand samples follows a first-order solubilization model. The effectiveness of this process depends mainly on the efficiency of micellar solubilization. High recoveries of phenanthrene were observed for the cloud-point extraction process. Applying cloud-point extraction is a good way to collect the phenanthrene in the washing solution. The small volume of the surfactant-rich phase makes it easy to handle the pollutant. The surfactant-rich phase can be separated from the water phase for further treatment such as incineration as the surfactant-rich phase is combustible. The water solution left, which contains surfactant and pollutant at very low concentrations, can be recycled for solubilization applications. Acknowledgements The authors would like to thank the National University of Singapore and the National Cheng Kung University for the support. References Bai, D. S., J. L. Li, S. B. Chen, and B. H. Chen, ‘‘A Novel Cloud-point Extraction Process for Preconcentrating Selected Polycyclic Aromatic Hydrocarbons in Aqueous Solution,’’ Environ. Sci. Technol., 35, 3936 (2001).
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Journal of the Chinese Institute of Chemical Engineers 39 (2008) 337–342 www.elsevier.com/locate/jcice
Recovering phenanthrene from spiked sand by a combined remediation process of micellar solubilization and cloud-point extraction Jing-Liang Li a,b, Bing-Hung Chen c,* a
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Centre for Micro-Photonics, Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, Vic. 3122, Australia c Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
b
Received 14 December 2007; received in revised form 26 January 2008; accepted 29 January 2008
Abstract A remediation process, which combines the micellar solubilization and the cloud-point extraction technique by a nonionic surfactant Tergitol 15-S-7, was used to decontaminate phenanthrene, as a model hydrophobic pollutant, from spiked sand samples. A first-order kinetics model was employed to describe the solubilization behavior of phenanthrene well. It was observed that presence of surfactant decreased the mass-transfer coefficient of phenanthrene from sand surface to surfactant solutions, however, higher solubilization rate was obtained due to enhanced aqueous solubility and, thus, the larger driving force resulted from solublization. Cloud-point extraction was used to concentrate the phenanthrene solubilized in the washing solutions in an attempt to minimize the amount of wastewater. The extraction was carried out, subsequently, at room temperature by adding sodium sulfate to suppress the cloud-point low enough to induce phase-separation of the surfactant-rich phase with a minimal phase volume from the coexisting water phase. Recoveries higher than 93% were achieved in the combined process of micellar solubilization and cloud-point extraction on ultimate removal of immobilized phenanthrene sorbed on sands. The results showed that this combined process is efficient in recovering phenanthrene sorbed and immobilized on sands from contaminated sites, and produces only minimal amount of wastewater, i.e. less than 3% of its original volume. # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Nonionic surfactant; Polycyclic aromatic hydrocarbon (PAH); Micellar solubilization; Cloud-point extraction; Soil remediation
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a family of highly toxic chemicals. Their wide existence in the environment arouses many concerns because many of them are either carcinogens or mutagens (Brody et al., 2007; Rybicki et al., 2006). Moreover, their low aqueous solubility and high affinity to soils contribute to their persistence in the environment. Developing effective ways to remove these pollutants from contaminated sites has stimulated extensive research interests in recent years (Fiorenza et al., 2000). Surfactant-mediated remediation is a promising technique for decontamination of hydrocarbon-polluted soils (Fiorenza et al., 2000; Venditti et al., 2007). Surfactants can increase the solubility of hydrophobic compounds significantly through a
* Corresponding author. Tel.: +886 6 275 7575x62695; fax: +886 6 234 4496. E-mail addresses: [email protected], [email protected] (B.-H. Chen).
process known as solubilization (Li and Chen, 2002; Rosen, 2004). The enhanced solubilization of PAHs in presence of surfactants has been reported considerably (Dar et al., 2007; Grimberg et al., 1994, 1995; Hung et al., 2007). While much effort has been shown on how to increase the decontamination efficiency by surfactant flushing in soil remediation processes, relatively less attention has been paid on the proper treatment or disposal of the washing solution. In recent years, cloud-point extraction (CPE) techniques using proper surfactants have been widely utilized for efficiently concentrating and detecting trace toxic compounds including PAHs. High recoveries and preconcentration factors of PAHs using CPE have been also demonstrated (Bai et al., 2001; Hung et al., 2007; Li and Chen, 2003; Li et al., 2004). The CPE process is mainly based on the clouding phenomena of surfactants, especially that of nonionic surfactants. Micellar solutions of proper nonionic surfactants are homogeneous and isotropic at ambient temperature. Upon appropriate alteration of conditions, such as temperature and additives, the micellar solutions become turbid due to the loss in aqueous solubility of
0368-1653/$ – see front matter # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2008.01.005
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Nomenclature a C C* Cphen CS 0 Csoil
k kl Msoil Rm RCPE RT VW VS
the specific surface area of sand sample (m2) enveloped phenanthrene concentration in overall washing solution collected (mg/L) saturation concentration of phenanthrene in micellar solution (mg/L) phenanthrene concentration in washing solution at time t (mg/L) phenanthrene concentration of the surfactant phase (mg/L) initial concentration of PAH sorbed on soil (mg/g sand) mass-transfer coefficient (1/(min m2)) lumped mass-transfer coefficient (1/min) soil mass (g) recovery of phenanthrene in the micellar solubilization process (%) recovery of phenanthrene in cloud-point extraction process (%) total/ultimate recovery of phenanthrene (%) volume of overall micellar solution (mL) volume of surfactant phase after cloud-point phase-separation (mL)
surfactant molecules (Schott, 1997; Rosen, 2004). Consequently, the micellar solutions will separate into two coexisting phases. One is the surfactant-rich phase, which contains most of surfactant molecules, whereas the other is the water phase, in which surfactant concentration is low and close to its critical micelle concentration (CMC). This temperature at which the phase-separation occurs is called cloud-point (Rosen, 2004). Upon cloud-point phase-separation, the hydrophobic compounds initially present in the solution and favorably bound to the micelles will be preferentially extracted into the surfactantrich phase (Bai et al., 2001; Hung et al., 2007; Li and Chen, 2003; Li et al., 2004). Very often, the volume of the surfactantrich phase is much smaller than that of the coexisting water phase. Therefore, concentration of these compounds is enhanced by the CPE process. Namely, the CPE process can significantly increase the concentration of the extracts principally by reducing the phase volume of the contaminant-containing surfactant solution. This feature is conjectured in this work to be employed in a typical surfactant flushing remediation process to reduce the volume of the washing solutions generated in a soil remediation process. Moreover, the main solvent used in a typical CPE process is water, in contrast to the organic liquids in a liquid–liquid extraction process, which also gives CPE a great advantage in environmental benignity and compatibility. The CPE process has been employed in many applications to extract compounds of interests, such as proteins and enzymes (Lai et al., 2006; Soriano-Lopes et al., 2007), environmental pollutants from soils/sediments (Cheng and Sabatini, 2007; Hung et al., 2007; Li et al., 2004), heavy metal ions from effluents (Ferreira et al.,
2007; Pyrzynska, 2007), improved analytical methods (Keith et al., 2007), etc. The performance of the CPE process has been approved in these studies. However, drawbacks of CPE processes are often reflected on the high extraction temperatures that intrinsically arise from surfactants themselves. In practice, it is both convenient and economic to facilitate the extraction process proceeding at ambient temperatures. As aforementioned, smaller phase-volume of surfactant-rich phases are also desired in field applications. To achieve these, surfactants with lower cloud-point temperatures and strategy to put in salting out additives should be preferentially employed (Bai et al., 2001; Li et al., 2004). In this study, not only the development of the CPE process that can possibly remove more contaminants from aquifer by surfactant flushing, but also design of a CPE process producing a minimal quantity of the waste washing solution is favored. To our best knowledge, this work is the first report ever in the open literature on the concept of combining a remediation process by surfactant flushing and solubilization, and a CPE process to minimize the waste effluents. The efficiency of such a process is also examined on the decontamination of pollutant-spiked sands that simulate a contaminated aquifer. That is, the efficiency of the combined process in terms of the recoveries of the micellar solubilization process and the cloud-point extraction process, as well as the overall recovery of phenanthrene of the process were studied. Phenanthrene, a common hydrophobic PAH, was selected as a model pollutant, along with a commercial grade of readily biodegradable nonionic surfactant, Tergitol 15-S-7. The solubilization kinetics of phenanthrene in presence and absence of the surfactant was also investigated. Tergitol 15-S-7 was especially chosen in this work, based on its low cloud-point, about 38 8C in 1 wt% solution, and its high extraction capacity for PAHs (Bai et al., 2001; Li and Chen, 2003). Sodium sulfate was applied to enhance the phase-separation of Tergitol 15-S-7 by further lowering its cloud-point, reducing the volume of the surfactant-rich phase and increasing the density of the aqueous phase. As a result, overall recovery efficiency higher than 93% to recuperate immobilized phenanthrene from sands could be achieved. Moreover, the phase volume of the surfactant washing solution has been reduced to less than 3% of its original volume. 2. Materials and method 2.1. Materials The nonionic surfactant, Tergitol 15-S-7 supplied by Dow Chemical (USA), is a mixture of species with an alcohol group located at various positions along a linear chain of 11–15 carbon atoms and with an average ethylene oxide number of 7.3. It has a low critical micelle concentration of 39 ppm at 25 8C. Moreover, this surfactant is readily biodegradable and has been accepted by the Food Safety and Inspection Service of the U.S. Department of Agriculture for general-purpose cleaning or as an ingredient of general-purpose cleaner for use in federally inspected meat and poultry processing plants.
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Reagent grade of phenanthrene was obtained from Aldrich. Sodium sulfate is of analytical grade and purchased from Merck. HPLC-grade methanol was acquired from Sigma. Deionized water from a Milli-Q purification system (Millipore, USA) having resistivity greater than 18.2 MV cm was used in preparing samples. All chemicals were used as received.
rate at 1 mL/min. The excitation and emission wavelengths used in the fluorescence detector on phenanthrene were 248 and 395 nm, respectively. The detection limit of the fluorescence detector for phenanthrene was 1 ppb.
2.2. Preparation and characteristics of sand spiked with phenanthrene
The recovery of phenanthrene from sands is schematically described in Fig. 1, which consists of two main steps, i.e. the micellar solubilization process and the cloud-point extraction process.
Sand used in the extraction experiments was obtained from the High Performance Concrete Laboratory of The National University of Singapore. It was sieved with a size range from 500 to 800 mm. The porosity of common sand packing in this experiment is found at ca. 0.35. Before being spiked, sand was washed with acetone thoroughly to get rid of any organic contents and then dried at 500 8C for 48 h. Phenanthrene-spiking sand was prepared by soaking sand in acetone with dissolved phenanthrene for a week. Afterwards, acetone was removed by evaporation and spiked sand was, subsequently, washed completely by deionized water. The phenanthrene content of spiked sand was determined by the total organic carbon (TOC) instrument. The measured value indicated that, on average, 0.6 mg of phenanthrene was successfully immobilized on per gram of sand sample. An Eppendorf 5810R (Hamburg, Germany) centrifuge was used to accelerate phase-separation in cloud-point extraction. A Shimadzu TOC-5000A total organic carbon analyzer equipped with an ASI-5000A auto sampler was used to determine phenanthrene contents on spiked sand samples. 2.3. HPLC analysis of phenanthrene concentration A Shimadzu high-performance liquid chromatography equipped with a fluorescence detector was used for analysis of phenanthrene concentration. The details of the system have been provided elsewhere (Li and Chen, 2002). An Agilent PAH C18 column (250 mm 4.6 mm) was used. It was packed with 5 mm particles and connected with the Guard cartridge holder (Agilent 79918PH-100). Mobile phase was a mixture of methanol (80 vol%) and ultrapure water (20 vol%), with a flow
2.4. Experimental procedure
2.4.1. Micellar solubilization process Spiked sands of 10 g were added to a 500-mL Erlenmeyer flask. Subsequently, 200 mL of micellar solution of Tergitol 15S-7 at various concentrations was poured into the Erlenmeyer flask, which was agitated continuously on an orbital shaker operated at 150 rpm. To measure the solubilization kinetics, aliquots of 0.5 mL solution were withdrawn at certain time intervals. The samples were then centrifuged for HPLC analysis. From previous studies, the solubilization rate of phenanthrene can be described by a first-order mass-transfer model (Grimberg et al., 1994, 1995; Li and Chen, 2002). dCphen ¼ kaðC C phen Þ ¼ kl ðC C phen Þ dt
(1)
where Cphen is the phenanthrene concentration in washing solution at time t; C* is the saturation concentration of phenanthrene; k is the mass-transfer coefficient; a is the specific surface area of sand sample; and kl is the lumped mass-transfer coefficient. Since the specific surface area of the sand sample is difficult to be precisely determined, instead, the lumped masstransfer coefficients at different surfactant concentrations were preferentially involved in this model. Consequently, the recovery by micellar solubilization process, Rm can be expressed as follows: Rm ð%Þ ¼
C VW 0 Csoil M soil
100
(2)
Fig. 1. Schematic description of the procedures combined with micellar solubilization and cloud-point extraction in recovering phenanthrene from spiked sands in aquifer.
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where C is the enveloped phenanthrene concentration in total washing solution collected (mg/L); VW is the volume of overall 0 washing solution (mL); Csoil is the initial concentration of phenanthrene immobilized on soil (mg/g); and Msoil is the mass of the soil (g). 2.4.2. Cloud-point extraction process After micellar solubilization arrives at equilibrium, the washing solution in the flask was transferred to 30 mL centrifuging tubes. Proper amount of sodium sulfate was added to these tubes to have a final concentration at 0.5 M. After sodium sulfate is completely dissolved, the tubes were centrifuged at 4000 rpm for 20 min to speed up the phaseseparation process. Samples from the surfactant-rich phase were withdrawn for HPLC analysis. The percentage of phenanthrene, RCPE, extracted from the washing solution into the surfactant phase by the CPE process could be estimated as follows: RCPE
CS V S ¼ C VW
(3)
where VS is the volume of surfactant phase and CS is the phenanthrene concentration in the surfactant phase after cloudpoint phase-separation. The ultimate recovery of the phenanthrene, RT, by the combined process could then be calculated with Eq. (4), accordingly. RT ð%Þ ¼
CS V S 0 Csoil M soil
100 ¼
Rm RCPE 100
(4)
3. Results and discussions 3.1. Solubilization kinetics of phenanthrene Equilibrium solubilization, but not solubilization kinetics, of PAHs, including phenanthrene, by micellar solutions of Tergitol 15-S-7 has been reported earlier by Li and Chen (2002). The solubilization extent of PAHs is linearly proportional to the concentration of surfactant beyond its CMC. Figs. 2 and 3 show the solubilization kinetics of phenanthrene in absence and presence of surfactant. A firstorder kinetics model could describe it well on the solubilization behavior of phenanthrene in surfactant solutions. The saturation concentration of phenanthrene and the lumped mass-transfer coefficients obtained from properly fitting of experimental data to Eq. (1) are given in Table 1. The aqueous solubility of phenanthrene found from Fig. 2 is around 1.02 ppm. It is of note to mention that, in the experiment shown as Fig. 2, only about 3.4% of total phenanthrene spiked on sands was dissolved into water (Table 1). Solubilization of phenanthrene sorbed on sands into surfactant solutions usually involves three steps (Yeom et al., 1996). The first step is the matrix diffusion of phenanthrene from sands, followed by the mass-transfer at the interface, i.e. film diffusion, and finally partitioning into the
Fig. 2. Solubilization of phenanthrene immobilized on sands in aquifer into water in the absence of surfactant.
micellar phase. The last step is usually much faster than other two (Gehlen and DeSchryver, 1993). Hence, solubilization kinetics of pollutants from contaminated soils really depends on many factors. The most important one is the contamination history of the soil. For weathered soil with a long history of contamination, the matrix diffusion commonly controls the solubilization process. As a result, solubilization behavior cannot be simply described by a firstorder kinetics model. In contrast, in the case of artificially contaminated soil or soil with a short contamination history, the film diffusion is more important. Hence, a first-order kinetics model might be adequate. Table 1 also shows that the lumped mass-transfer coefficient decreased with an increase in surfactant concentration. If the specific surface areas of the sand samples in each experiment could be regarded as the same, the mass-transfer coefficient k decreases with an increase in surfactant concentration. This assumption is valid in this work only when washing solutions with surfactant concentration less than 0.1 wt% were employed (Fig. 4). The discrepancy at higher surfactant concentrations will be discussed later in this section.
Fig. 3. Solubilization of phenanthrene immobilized on sands in aquifer into surfactant solutions. Concentrations of surfactants range from 0.05 to 0.2 wt%.
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Table 1 Fitted parameters obtained from micellar solubilization process of phenanthrene and recoveries of phenanthrene from a combined process of micellar solubilization and cloud-point extraction Surfactant concentration (wt%)
0 0.05 0.1 0.2
Saturated phenanthrene concentration, C* (mg/L) Measured value
Fitted value
1.018 11.795 28.561 29.811
1.010 11.650 28.014 29.547
kl (1/min)
Rm (%)
RCPE (%)a
RT (%)
0.201 0.179 0.043 0.044
3.4 39.3 95.2 99.4
– – 98.1 0.2 98.0 0.3
– – 93.4 97.4
–, cloud-point extraction was not carried out. a Na2SO4 were added into washing solution with a final concentration at 0.5 M.
Solubilization of phenanthrene in presence of surfactant includes its molecular dissolution by bulk solution and phenanthrene solubilized from the interface to micelles. The decreased mass-transfer coefficient can be attributed to the slower mass-transfer of phenanthrene through palisade layer of surfactant micelles. However, apparent saturated solubility of phenanthrene molecules, i.e. C* appearing in Eq. (1), in the aqueous phase is much smaller than that in micellar solutions. Hence, enhanced solubilization capacity of phenanthrene with increasing surfactant concentration compensates the loss in the mass-transfer coefficient of phenanthrene, which, in turn, generally results in an overall enhancement in solubilization rates of phenanthrene. Similar results were obtained for the solubilization of palmitic acid from solid surface to the micellar solutions of Tergitol 15-S-7 (Lim et al., 2005). Furthermore, the saturated solubility of phenanthrene and the recovery efficiency of phenanthrene by surfactants are also plotted as Fig. 4. As aforementioned, apparent solubilities of hydrophobic solubilizates are often increased linearly with surfactant concentrations in micellar solutions. However, in this work, solubilization behavior of phenanthrene in 0.2 wt% Tergitol 15-S-7, i.e. at ca. 50 CMC of Tergitol 15-S-7, deviates from the linear prediction on surfactant effect to equilibrium solubilization (Li and Chen, 2003; Lim et al., 2005). That is, packing of sands starts to influence significantly
the solubilization behaviors, as it dwindles drastically on the specific surface area on sands available to phenanthrene molecules for dissolution into bulk solutions of surfactant (Fiorenza et al., 2000). 3.2. Recovery of phenanthrene by micellar solubilization The recovery of phenanthrene by micellar solubilization at various surfactant concentrations is given in Table 1. In general, recovery efficiency of phenanthrene by micellar solubilization increases significantly with an increase in surfactant concentration. Moreover, as surfactant concentration increases to 0.2 wt%, almost all phenanthrene initially spiked on sands was completely solubilized into surfactant washing solutions. With 0.1 or 0.2 wt% surfactant solutions, the nearly complete solubilization of phenanthrene takes about 2 h. Certainly, as mentioned in previous section, porosity of sand packings and mobility of surfactant solutions in these pores could also affect the solubilization kinetics as well (Fiorenza et al., 2000). With surfactant solutions at lower surfactant concentration, the recovery efficiency of phenanthrene by micellar solubilization is linearly proportional to surfactant concentration (Fig. 4). As the recovery efficiency of CPE processes using Tergitol 15-S-7 is generally high enough, the micellar solubilization process is indeed the limiting step in the combined process in terms of the overall recovery efficiency of phenanthrene. 3.3. Cloud-point extraction of phenanthrene from washing solution
Fig. 4. Effect of surfactant concentration on apparent solubility of phenanthrene in micellar solution and recovery efficiency of phenanthrene by solubilization in this micellar solution.
The phenanthrene in surfactant solution after micellar solubilization was extracted by adding sodium sulfate to facilitate the extraction process proceeding at ambient temperature. The final salt concentration is 0.5 M. The sodium sulfate was used to reduce the cloud-point of the surfactant solution and to accelerate the cloud-point separation by increasing the density of the aqueous phase. Indeed, the presence of suitable salting-out salts in the micellar solution can significantly reduce the volume of the surfactant-rich phase after cloud-point separation by dehydrating surfactant molecules. Previous work showed that at the surfactant concentration of 1 wt% and sodium sulfate concentration of 0.5 M, the phase volume of surfactant-rich phase has been reduced by 30 times, compared to the original bulk phase (Li and Chen, 2003). Namely, phenanthrene can be
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further concentrated into a small volume of surfactant-rich phase. This makes the subsequent handling and treatment process of phenanthrene easier by reducing the amount of the wastewater to be processed. Detailed work on the effects of sodium sulfate on the phase volume of the surfactant phase and on the recovery of PAHs has been reported in our previous works (Li and Chen, 2003). The recovery of phenanthrene during the cloud-point extraction and the total recovery of phenanthrene spiked on the sand samples are given in Table 1. For surfactant solutions with surfactant concentrations at 0.1 and 0.2 wt%, over 93% and 97% of phenanthrene could be recovered by the combined process of micellar solubilization and cloud-point extraction. Furthermore, the quantity of wastewater from surfactant flushing process can be greatly reduced as well. Finally, this combined process can certainly be employed in the remediation process of soil contaminated by toxic and hydrophobic compounds. Water recycled after the CPE process can be again used in the remediation process. In general, the performance of the combined process might still be limited by the geophysical and geochemical factors of the aquifers, which is always inevitable in any soil remediation methods using surfactants (Fiorenza et al., 2000). Albeit, this combined process is still very promising in soil remediation. 4. Conclusions The process, which combines micellar solubilization and cloud-point extraction using Tergitol 15-S-7 is an efficient approach for the remediation of contaminated soil. The solubilization of phenanthrene from the spiked sand samples follows a first-order solubilization model. The effectiveness of this process depends mainly on the efficiency of micellar solubilization. High recoveries of phenanthrene were observed for the cloud-point extraction process. Applying cloud-point extraction is a good way to collect the phenanthrene in the washing solution. The small volume of the surfactant-rich phase makes it easy to handle the pollutant. The surfactant-rich phase can be separated from the water phase for further treatment such as incineration as the surfactant-rich phase is combustible. The water solution left, which contains surfactant and pollutant at very low concentrations, can be recycled for solubilization applications. Acknowledgements The authors would like to thank the National University of Singapore and the National Cheng Kung University for the support. References Bai, D. S., J. L. Li, S. B. Chen, and B. H. Chen, ‘‘A Novel Cloud-point Extraction Process for Preconcentrating Selected Polycyclic Aromatic Hydrocarbons in Aqueous Solution,’’ Environ. Sci. Technol., 35, 3936 (2001).
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