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Preconcentration of Phenanthrene from Aqueous Solution by a Slightly Hydrophobic Nonionic Surfactant Jing-Liang Li,† Renbi Bai,† and Bing-Hung Chen*,†,‡ Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, and Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Received October 23, 2003. In Final Form: April 30, 2004
1. Introduction Surfactant-based extraction techniques have become more and more attractive, in which water is commonly utilized as the solvent,1-5 in contrast to the extensive usage of volatile organic solvents in the conventional liquidliquid extraction. Though it has been attempted for surfactant-based extraction to be performed in other media, such as the supercritical CO26 and amyl alcohol,7 the extraction still proceeds mainly in aqueous surfactant solutions and based on phase changes of such surfactant solutions, especially the clouding phenomena of the surfactant solutions.8,9 For such a process it is also referred as the cloud-point extraction (CPE) process. In the CPE process, the main extraction media are the micellar solutions, in general but not all, of nonionic surfactants. As micellar solutions are heated, the aqueous solubility of the surfactant molecules diminishes due to the break of hydrogen bonds between water and surfactant molecules.10 At a certain temperature, called the cloud point, the micellar solution separates into two immiscible phases: a surfactant-rich phase (L1 phase) containing most of the surfactants and an excess water phase (W phase) having surfactants with concentration near the critical micelle concentration (cmc). Upon phase separation, hydrophobic compounds originally bound to the micelles can be favorably extracted into the surfactant-rich phase usually possessing much smaller phase volume than that of the excess water phase. The CPE techniques offer several advantages over the conventional solvent extraction, including experimental convenience, low cost, pos* To whom correspondence may be addressed. Current address: National Cheng Kung University, Taiwan. Phone number: +8866-275-7575, ext. 62695. Fax number: +886-6-234-4496. E-mail address:
[email protected]. † National University of Singapore. ‡ National Cheng Kung University. (1) Quina, F. H.; Hinze, W. L. Ind. Eng. Chem. Res. 1999, 38, 41504168. (2) Materna, K.; Szymanowski, J. J. Colloid Interface Sci. 2002, 255, 195-201. (3) Kamei, D. T.; Wang, D. I. C.; Blankschtein, D. Langmuir 2002, 18 (8), 3047-3057. (4) Gazzaz, H. A.; Robinson, B. H. Langmuir 2000, 16 (23), 86858691. (5) Sakulwongyai, S.; Trakultamupatam, P.; Scamehorn, J. F.; Osuwan, S.; Christian, S. D. Langmuir 2000, 16 (22), 8226-8230. (6) Hanrahan, J. P.; Zigler, K. J.; Glennon, J. D.; Steytler, D. C.; Eastoe, J.; Dupont, A.; Holmes, J. D. Langmuir 2003, 19 (8), 31453150. (7) Pandit, P.; Basu, S. J. Colloid Interface Sci. 2002, 245, 208-214. (8) Li, J. L.; Chen, B.-H. J. Colloid Interface Sci. 2003, 263 (2), 625632. (9) Szymanowski, J.; Apostoluk, W. J. Colloid Interface Sci. 2000, 228, 178-181. (10) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989.
sibility of using nontoxic and less dangerous reagents,1,11,12 and good compatibility in high-performance liquid chromatography (HPLC) analysis.13,14 There are many factors affecting the CPE performance. One of them is water content in the surfactant-rich phase, which influences directly on the phase volume and, hence, the concentration of solutes in that phase. The other is hydrophobic affinity between analytes and micelles that house the hydrophobic analytes before/after the phase separation. That is, the hydrophobic affinity and the solubilization capacity of the surfactants could have a profound influence on the performance of the CPE process.9,15 Coincidentally, it has been long known that some mesophases, such as the lamellar phase (LR), of surfactants have better solubilization capacity and higher hydrophobic affinity than the micellar phases.10 However, there is almost no report found in the open literature, in which surfactants at conditions forming mesophases were employed in the surfactant-based extraction techniques. It may be attributed to the fact that mesophases are usually too viscous to be separated from coexisting phases and to be injected into the HPLC directly. It is, thus, our attempt in this study to examine whether the introduction of such surfactants forming mesophases, i.e., the slightly hydrophobic surfactant, at proper conditions could really enhance the performance of the surfactant-based extraction process on polycyclic aromatic hydrocarbons (PAHs). In this work, phenanthrene was used as the model PAH, and a nonionic surfactant Tergitol 15-S-5 was carefully selected as an extracting agent. Tergitol 15-S-5 solution exhibits as a dispersion of the lamellar liquid crystalline phase (LR) in water at ambient temperature (ca. 22 °C). 2. Materials and Methods The nonionic surfactant Tergitol 15-S-5, supplied by Union Carbide (Charleston, WV), is a mixture of species with secondary ethoxylated alcohols located at various positions along a linear hydrocarbon chain having 11-15 carbon atoms and with an average ethylene oxide number of 5. Its averaged molecular weight and calculated HLB number are 415 g/mol and 10.6, respectively. Reagent grade phenanthrene, sodium sulfate, and sodium chloride as well as HPLC-grade methanol were purchased from Aldrich, Sigma and Merck. Deionized water from a Milli-Q purification system (Millipore, USA) having resistivity greater than18.2 MΩ cm was used in preparing samples and the mobile phase for the HPLC analysis. All chemicals were used as received. The phenanthrene concentration in extract and water phase was determined using a Shimadzu HPLC system that is described elsewhere.8,14,15 A mixture of methanol (80 vol %) and deionized water (20 vol %) was used as the mobile phase with a flow rate set at 1 mL/min. The excitation and emission wavelengths for phenanthrene are 248 and 395 nm, respectively. The detection limit of the fluorescence detector for phenanthrene is 1 ppb. Detailed experimental procedures are given in the Supporting Information. (11) Huddleston, J. G.; Willauer, H. D.; Griffin, S. T.; Rogers, R. D. Ind. Eng. Chem. Res. 1999, 38, 2523-2539. (12) Willauer, H. D.; Huddleston, J. G.; Rogers, R. D. Ind. Eng. Chem. Res. 2002, 41 (7), 1892-1904. (13) Pinto, C. G.; Pavo´n, J. L. P.; Cordero, B. M. Anal. Chem. 1994, 66, 874-881. (14) Bai, D. S.; Li, J. L.; Chen, S. B.; Chen, B.-H. Environ. Sci. Technol. 2001, 35, 3936-3940. (15) Li, J. L.; Chen, B.-H. Chem. Eng. Sci. 2002, 57 (14), 2825-2835.
10.1021/la035975m CCC: $27.50 © 2004 American Chemical Society Published on Web 05/29/2004
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Figure 1. Effect of NaCl concentration on preconcentration factor in different surfactant solutions at 22 °C (initial phenanthrene concentration ) 12 mg/L).
3. Results and Discussion To predict the extraction performance, a series of investigations on the aqueous phase behaviors of Tergitol 15-S-5 were carried out by visual observation with crossed polarizers and/or by optical microscopy equipped with crossed polarizers. At ambient temperature (22 °C), Tergitol 15-S-5 solution exhibits as the dispersion of the lamellar phase in water (LR + W). Slightly increasing the temperature to about 26.3 °C, 2 wt % surfactant solution separates into two distinct isotropic phases, W + L3 phases, in which L3 was used later as the surfactant-rich phase to extract phenanthrene from aqueous solution. Addition of NaCl and Na2SO4 could lower the coexisting temperature of W + L3 phases (Figures 2 and 3 in the Supporting Information). In comparison with other mesophases, the fluidic nature of the L3 phase eases the sample handling, especially in sample analysis by HPLC. The recovery characteristics of phenanthrene by Tergitol 15-S-5 surfactant were studied with respect to various factors, such as the concentration of the added salts, the surfactant concentrations, the experimental temperature, and the initial concentration of the phenanthrene. Preconcentration factors and recovery efficiency of phenanthrene by Tergitol 15-S-5 solutions were investigated as a function of added NaCl concentrations (Figure 1). The initial concentrations of phenanthrene were 1 and 12 mg/ L, respectively, near its aqueous solubility.15 Insignificant effects of initial phenanthrene concentration were found on both preconcentration factors and recovery efficiency. Consequently, only those that resulted from the use of 12 mg/L phenanthrene are presented in Figure 1. In general, at the same level of added NaCl, a higher preconcentration factor takes place in less concentrated surfactant solutions. In contrast, the recovery efficiency seems almost independent of the NaCl concentration (Figure 4 in the Supporting Information). For instance, the preconcentration factor near 15 in 2 wt % Tergitol 15-S-5 solution with 1 M NaCl was found, whereas it increases about 2-fold in the presence of 4 M NaCl, instead. The increase in the preconcentration factor can be attributed to the lesser water content and, hence, smaller volume of surfactant-rich phase when more NaCl is added. The preconcentration factor can be significantly improved by decreasing surfactant concentration, which indicates that phenanthrene in surfactant-rich phase is not saturated yet. An increase in surfactant concentration will enlarge the phase-volume ratios and reduce the preconcentration factors (Table 1 in the Supporting
Figure 2. Effect of temperature on preconcentration factor and recovery efficiency of phenanthrene. Experiments were carried out in 2 wt % Tergitol 15-S-5 solution with addition of 3 M NaCl and initial concentration of phenanthrene at 12 mg/L.
Information). That is, to achieve a higher preconcentration factor, a less concentrated surfactant solution but having more NaCl added is desired. For example, a high preconcentration factor near 75, estimated from the phasevolume ratio, can be achieved in 1 wt % Tergitol 15-S-5 solution with addition of NaCl more than 4 M. To further examine and validate the effectiveness of the extraction process, phenanthrene at a very low initial concentration of 1 ppb was extracted using 1 wt % Tergitol 15-S-5 solution containing NaCl at 4 M. The phenanthrene concentration in the surfactant-rich phase from triplicate analysis is 74 ( 3 ppb. That is, the measured preconcentration factor is about 74 ( 3, consistent with that predicted above. For comparison, a maximum preconcentration factor near 40 was obtained in the 1 wt % Tergitol 15-S-7 solution with 0.6 M Na2SO4 added.8 Since phase behaviors of nonionic surfactants are strongly influenced by temperature,10,16 it is imperative to study the effect of temperature difference between experimental temperature and phase-separation temperature, on extraction performance (Figure 2). For convenience, phenanthrene of 12 mg/L initially present in 2 wt % Tergitol 15-S-5 solution with 3 M NaCl added, which renders phase separation at 4 °C, was employed for demonstration of the temperature effect. Increasing the temperature difference could enhance the preconcentration factors prominently but only slightly the recovery efficiency. The increase in preconcentration factors can be once again attributed in line with the fact that surfactant aggregates are dehydrated more at an elevated temperature because of weaker hydrogen bondings between unassociated water molecules and ether oxygens on surfactant moieties resulted from larger thermal fluctuactions,10,17 which yields smaller water contents and phase volume in the surfactant-rich phase. At 30 °C, the preconcentration factor is augmented to ca. 50. Effect of initial phenanthrene concentration on extraction performance by 2 wt % Tergitol 15-S-5 solution at 22 °C with addition of NaCl and Na2SO4 were studied, respectively, and are presented in the Figures 3 and 4, accordingly. Both chloride and sulfate ions are capable of dehydrating surfactant aggregates and bringing down the phase-separation temperature.7 Moreover, sulfate ions (16) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975-1000. (17) Materna, K.; Milosz, I.; Miesiac, I.; Cote, G.; Szymanowski, J. Environ. Sci. Technol. 2001, 35, 2341-2346.
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Figure 3. Effect of initial phenanthrene concentration on preconcentration factor in 2 wt % Tergitol 15-S-5 solution with addition of NaCl at 22 °C.
Notes
estimated by measuring the change in phenanthrene concentration after equilibrium in these containers for more than 48 h. Tergitol 15-S-5 solutions (1 wt %) were used, and initial concentrations of phenanthrene range from 2 to 20 mg/L. No detectable losses of phenanthrene were observed. It indicates that loss of phenanthrene due to adsorption onto containers could be negligible in the presence of this surfactant. The concentration of the phenanthrene left over in the excess water phase after phase separation was low as well. The phenanthrene concentrations in the excess water phases were found only about a few parts per billion, when 2 and 20 mg/L phenanthrene were initially spiked in the bulk phases. On the basis of the mass balance of phenanthrene present in the bulk and the excess water phases, the recovery efficiency of phenanthrene higher than 99% could be achieved in all cases. However, it is of note that the primary quantity to be measured is the phenanthrene concentration in the surfactant-rich phase, not that in the excess water phase. The extraction techniques are developed to enhance the detectability of phenanthrene in trace levels by preconcentration. It has been shown that recovery efficiency ranging from 85 to 110% was attained, though most of them are centered on 100% (Figures 4-6 in the Supporting Information,). Finally, the use of Tergitol 15-S-5 as an efficient extraction agent to recover phenanthrene from aqueous solutions has been studied and shown as feasible. 4. Conclusions
Figure 4. Effect of initial phenanthrene concentration on preconcentration factor in 2 wt % Tergitol 15-S-5 solution in the presence of Na2SO4 at 22 °C.
have been shown as one of the most prominent species in this aspect.14 Addition of 0.6 M Na2SO4 to 2 wt % Tergitol 15-S-5 has almost the same effectiveness as that of 3 M chloride ions in terms of reducing phase-separation temperature. Coincidently, preconcentration factors and recovery efficiency of these two systems are almost the same. Therefore, the phase separation temperature could be used as a rule of thumb on predicting the extraction performance. The preconcentration factor in the cloudpoint extraction process using 2 wt % Tergitol 15-S-7 with 0.6 M Na2SO4 added at 22 °C for phenanthrene was found at about 22,8 compared to that of 35 in the system using Tergitol 15-S-5, instead, under the same conditions (at 22 °C and 0.6 M Na2SO4 added; see Figure 3). Estimating the loss of analytes due to its adsorption onto the container as well as the experimental uncertainty is essential in evaluating the efficiency of the extraction process. To reduce the loss of the analytes in the traditional solvent-solvent extraction processes, a large dose of the solvent has to be used. In contrast, the loss of analytes due to the adsorption can be minimized in the surfactantbased aqueous extraction processes with the presence of the surfactants.13,18 In this work, loss of phenanthrene due to adsorption on centrifuge tubes and glass vials was (18) Sicilia, D.; Rubio, S.; Perez-Bendito, D. Anal. Chim. Acta 2002, 460 (1), 13-22.
The slightly hydrophobic nonionic surfactant, Tergitol 15-S-5, was used successfully in preconcentrating phenanthrene, as a model PAH, in the aqueous solution. Similar to the cloud-point extraction (CPE) process using the hydrophilic surfactants, the use of this surfactant provides not only better preconcentration factors as well as the comparatively higher recovery efficiency but also the ease in handling the sample prior to the HPLC analysis. A higher preconcentration factor can be achieved by increasing the concentration of added salt and/or decreasing the surfactant concentration. Sodium sulfate works more effectively in enhancing the preconcentration process. A relatively high preconcentration factor of ca. 75 was attained at 22 °C in 1 wt % surfactant solution with NaCl added at concentrations above 4 M. In general, the use of the Tergitol 15-S-5 as an efficient extraction agent to recover phenanthrene from aqueous solutions has been studied and shown as feasible. Acknowledgment. The authors gratefully acknowledge the National University of Singapore and the National Science Council of Taiwan for the financial support. Supporting Information Available: Description of Tergitol 15-S-5, experimental procedures, table of water content and phase-volume ratios, chromatograms of PAHs solubilized by various surfactants, coexisting temperature of W + LR phases as a function of Terfgitol 15-S-5 concentration, effect of NaCl concentration on the coexisting temperature effects of NaCl concentration on phenanthrene recovery, effect of initial phenanthrene concentration on recovery of Tergitol 15-S-5, and effect of initial phenanthrene concentration on recovery efficiency. This material is available free of charge via the Internet at http://pubs.acs.org. LA035975M
