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Environ. Sci. Technol. 2001, 35, 3936-3940

A Novel Cloud-Point Extraction Process for Preconcentrating Selected Polycyclic Aromatic Hydrocarbons in Aqueous Solution DONGSHUN BAI, JINGLIANG LI, S. B. CHEN, AND B.-H. CHEN* Department of Chemical and Environmental Engineering, The National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

A novel but simple cloud-point extraction (CPE) process is developed to preconcentrate the trace of selected polycyclic aromatic hydrocarbons (PAHs) with the use of the readily biodegradable nonionic surfactant Tergitol 15S-7 as extractant. The concentrations of PAHs, mixtures of naphthalene and phenanthrene as well as pyrene, in the spiked samples were determined with the new CPE process at ambient temperature (23 ˚C) followed by high performance liquid chromatography (HPLC) with fluorescence detection. More than 80% of phenanthrene and pyrene, respectively, and 96% of naphthalene initially present in the aqueous solutions with concentrations near or below their aqueous solubilities were recovered using this new CPE process. Importantly Tergitol 15-S-7 does not give any fluorometric signal to interfere with fluorescence detection of PAHs in the UV range. No special washing step is, thus, required to remove surfactant before HPLC analyses. Different experimental conditions were studied. The optimum conditions for the preconcentration and determination of these selected PAHs at ambient temperature have been established as the following: (1) 3 wt % surfactant; (2) addition of 0.5 M Na2SO4; (3) 10 min for equilibration time; and (4) 3000 rpm for centrifugal speed with duration of 10 min.

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) constitute a class of hazardous organic chemicals consisting of two or more fused benzene rings in linear, angular or cluster arrangements. They are ubiquitous in the environment and are released from anthropogenic or natural origins. PAHs form, for example, during petrochemical cracking processes, as a result of the incomplete combustion of fossil fuels, and during the disposal of coal tar as well as chemical wastes. These substances have been of great environmental concerns, as they are either known or suspected carcinogens or mutagens (1-3). Due to their carcinogenic and mutagenic characteristics, strict legal controls are now imposed to regulate their production, usage, and emission, in which the determination of trace of PAHs has to be addressed. PAHs are very hydrophobic and have very low aqueous solubility. For example, their solubility ranges from 32.5 ppm for naphthalene down to 0.14 ppm for pyrene (4-6). Such * Corresponding author phone: +65-874-4728; fax: +65-779-1936; e-mail: [email protected]. 3936

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low solubility necessitates some preconcentration procedures prior to the analytical determination of PAHs. Among the different preconcentration methods, cloud-point extraction (CPE) offers many advantages and becomes more and more attractive (7-10). Surfactants have long been known to human beings for their capability to enhance the solubility of hydrophobic materials (11). If the surfactant concentration exceeds a certain threshold, called a critical micelle concentration (CMC), at temperature higher than its Krafft temperature, surfactant monomers in aqueous solution will tend to aggregate to form micelles in colloidal-size. Under such a condition, the hydrophobic solubilizates are incorporated in the hydrophobic cores of the micelles. When a micellar solution of weakly polar surfactants, such as nonionic or zwiterionic surfactants, is heated above a certain temperature that is called cloud-point temperature, it becomes turbid. At the cloud-point the surfactant solution undergoes phase separation into a surfactant-rich liquid phase (L1 phase) and an almost micelle-free dilute solution (W phase) whose concentration is equal to CMC at that temperature. The phase separation is reversible; when the mixture is cooled to the temperature below the cloud-point temperature, these two phases merge to form a clear phase again. The phase separation at cloud-point temperature is believed to be due to the decrease in intermicellar repulsion and/or the sharp increase in aggregation number of the micelles except in some surfactant solutions, e.g., C12E8. As the cloud-point is approached, the solubilization of nonpolar solubilizates increases very rapidly, probably because of an increase in aggregation number of the micelles (12-14). The temperature at which clouding phenomena occur depends on the structure of surfactants. For example, for nearly pure polyoxyethylenated nonionic surfactants, the cloud-point temperatures are 10 °C for C12E4, 30 °C for C12E5, 52 °C for C12E6, and 62 °C for C12E7 (15-17). Huibers et al. (18) recently developed a general empirical relationship for estimating the cloud-point of pure nonionic surfactants of the alkyl ethoxylate class from their structures. Owing to the hydrophobic nature, the analytes in the present study will exist favorably in the surfactant-rich phase, whose volume is usually smaller. The small phase volume allows us to preconcentrate and to extract the analytes in one step. Compared with the traditional organic liquidliquid extraction, cloud-point extraction uses a very small amount of relatively nonflammable and nonvolatile surfactant, which is easy to dispose. In addition, CPE can lead to a high recovery and preconcentration factor and can minimize losses due to the sorption of analytes onto containers (19). The other advantage of CPE is the preferable use of water as the solvent in the micellar solution, which is benign to the environment, compared to the organic solvents still used in other preconcentration procedures. Additionally, benefit of using CEP arises from the good compatibility between surfactant-rich phase and the hygroorganic mobile phase in liquid chromatography, which offers great convenience to the analysis of the trace of hydrophobic materials. Great success in using the surfactant-mediated cloud-point extraction for the preconcentration procedures has been achieved not only in the systems having hydrophobic substances, such as PAHs (19-24) and other organic compounds (25-36), but also in the systems having low concentrations of metal ions (37-43). 10.1021/es0108335 CCC: $20.00

 2001 American Chemical Society Published on Web 08/23/2001

Despite many benefits of using cloud-point extraction, the choices of the surfactants often bring the nuisance to the analysis of analytes using HPLC system with fluorescence detection. The reason can be ascribed to the large UV absorbance and fluorometric signals, especially in the UV ranges, of surfactants containing double-bond structures, such as phenyl group in the surfactants of polyoxyethylene alkylphenyl ether (7, 24). The typical surfactants of this type commonly used in the cloud-point separation include Triton series (Union Carbide), Igepal series (Rhodia), and PONPE series (poly(ethylene glycol) mono-4-nonylphenyl ethers). To alleviate this problem, researchers have attempted alternative extractants including polymers (10, 44-45), anionic surfactants such as sodium dodecyl sulfate (SDS) (19, 21-23), and primary ethoxylated alcohols such as Brij series (ICI) (25) and Genapol series (Clariant) (20, 26). However, little success has been achieved in using polymers as extractants to preconcentrate PAHs (45). In addition, the use of anionic surfactants as effective extractants in the cloudpoint separation often requires addition of salts and adjustments of pH, usually to a very low value (19, 22). Moreover, the Brij and Genapol surfactants would not undergo phase separation unless at a higher temperature, for example, 70 °C for Brij 97 and 90 °C for Brij 30 (22). It is thus the purpose of this work to attempt to develop a simple CPE process that can be operated at ambient temperature and use the environmentally friendly surfactants. A nonionic surfactant of secondary ethoxylated alcohol Tergitol 15-S-7 was chosen as an extraction agent because of its low cloud-point temperature and high capability to dissolve significant amounts of large triglyceride oils such as triolein (46-48). Different experimental conditions were studied, and the optimal extraction conditions were decided.

2. Experimental Section 2.1. Reagents. The commercial nonionic surfactant Tergitol 15-S-7 supplied by Union Carbide (USA) is a mixture of species with an alcohol group located at various positions along a chain of 11-15 carbon atoms and with an average ethylene oxide number of 7.3. The cloud-point and pH value of 1 wt % Tergitol 15-S-7 micellar solution is around 37 °C and near 6.8, as reported by the manufacturer. Its critical micelle concentration is as low as 0.0039 wt % at 25 °C. Tergitol 15-S-7 is readily biodegradable and has been accepted by the U.S. Department of Agriculture for general-purpose cleaning or as an ingredient of general-purpose cleaners for use in federally inspected meat and poultry processing plants. It has been reported to have great solubilization capacities of large triglyceride oils and fatty alcohols (46-48). Solubilities of phenanthrene and pyrene in 1 wt % Tergitol 15-S-7 solution are measured to be 307 and 149 ppm, respectively (49). Reagent grade of naphthalene, phenanthrene, and pyrene were obtained from Aldrich. Sodium sulfate as well as other inorganic salts are of analytical grade and purchased from Merck and Sigma. HPLC-grade methanol was acquired from Sigma. Deionized water from a Milli-Q purification system (Millipore, USA) having resistivity greater than 18.2 MΩ-cm was used in preparing samples and the mobile phase. All chemicals were used as received. 2.2. Apparatus. The separation and quantification of the PAH analytes in the extract was carried out by using a Shimadzu HPLC system consisting of one LC-10ATVP pump, two DGU-14A degasers, an SIL-10ADVP autoinjector, a CTO10ASVP column oven, an SCL-10AVP system controller, and an RF-10AXL fluorescence detector. PAH concentration was obtained from data processed with Shimadzu software ClassVP 5.03. The stationary-phase column was an Agilent PAH column (250 × 4.6 mm i.d.) packed with 5 µm particles and connected with a Guard cartridge (Agilent 79918PH-534) and the Guard cartridge holder (Agilent 79918PH-100). At least

triplicate samples from experiments of the same condition were drawn to determine the PAH concentration in aqueous solution. Measurement of cloud-point temperature was conducted using a waterbath (Polyscience) with a good temperature control of (0.1 °C. An Eppendorf refrigerated centrifuge (model 5810R) was also used to speed up the complete phase separation of turbid bulk solution above the cloud-point. 2.3. Procedure. 2.3.1. Cloud-Point and Determination of Preconcentration Factor. The cloud-point of aqueous surfactant solution was determined by heating 10 mL of such micellar solution in screw-capped glass culture tubes. The rate of temperature increase in the water bath was set at 1 °C/min or less, if necessary. The cloud-point was determined by visual observation at the temperature at which the solution became obviously turbid. All the cloud-points reported in this work were the average of at least triplicate measurements. The measurement of cloud-point temperature is reproducible within 0.2 °C. The preconcentration factor was defined as the ratio of the volume of bulk solution before phase separation to that of the surfactant-rich phase after phase separation. They can be regarded as an indicator on the increases of the concentrations of analytes present in the surfactant-rich phase, while their concentrations in the excess water phase (W) exist as little as their aqueous solubilities, due to the cloud-point extraction. The preconcentration factor was determined with calibrated glass tubes following the same condition of the preconcentration step described in next section. 2.3.2. Cloud-Point Preconcentration. The preconcentration step was achieved by using nonionic surfactant Tergitol 15-S-7 as the extractant. Stock solutions of PAH mixtures were prepared from dissolving the known amounts of PAHs into 50 mL of HPLC-grade methanol. In a typical extraction experiment, a suitable amount of Tergitol 15-S-7 was added in about 10 mL of deionized water along with 100 µL of stock solution having the analytes. The initial PAH concentrations in typical samples under the study are 14.1, 10.2, and 3.7 ppm, respectively, for naphthalene, phenanthrene, and pyrene. The cloud-point separation was initiated by addition of 0.711 g of Na2SO4. The final volume of bulk solution was ensured at 10 mL, and the concentration of Na2SO4 is 0.5 M. The samples were then allowed to equilibrate for 10 min and followed by centrifugation for 10 min at 3500 rpm to achieve the complete separation of the two phases. The interface between the top surfactant-rich phase (L1) and the bottom excess water phase (W) is very sharp. It is worth mentioning that the surfactant-rich phase is not necessary to be in the top, as it depends on the densities of the two phases. A 30 µL aliquot from the surfactant rich phase was transferred to the HPLC sample vials and diluted with 120 µL deionized water to reduce the viscosity. A 10 µL diluted sample was then injected directly into the HPLC system by the automatic sampler. Data reported in this work were averaged from at least three experimental trials under the same condition. 2.3.4. Liquid Chromatography Analysis with Fluorescence Detection. After extraction of adequate volume of surfactant-rich phase, separation of PAHs was performed using a mobile phase consisting of 80/20 methanol/water by volume. The flow rate of the mobile phase is 1 mL/min. Since the surfactant-rich phase was compatible with the mobile phase, no special washing step was required to remove the surfactant from the column. The excitation and emission wavelengths were set at 310 and 370 nm, respectively. This was previously determined by using solution containing a known concentration of PAH and surfactant with concentration above its critical micellar concentration. VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Cloud point of Tergitol 15-S-7 solution as a function of surfactant concentration.

FIGURE 2. Effect of added salts on cloud point of 1 wt % Tergitol 15-S-7 solution.

3. Results and Discussion 3.1. Cloud-Point and Preconcentration Factor. Figure 1 displays the cloud-point of Tergitol 15-S-7 micellar solution as a function of surfactant concentration ranging from 1 to 10 wt %. The cloud-point of Tergitol 15-S-7 increases monotonically with increasing surfactant concentration. In micellar solutions comprising nonionic surfactants of primary ethoxylated alcohols, such as C12E5 and C12E6, it has been observed that the cloud-point temperature decreases rapidly with increasing surfactant concentration from the CMC to about 1 wt % and increases again monotonically until reaching the phase boundary with a further increase in surfactant concentration (15-17). Since Tergitol 15-S-7 is a mixture of secondary alcohol ethoxylates, similar phase behavior would be expected. The addition of electrolytes to the surfactant micellar solution can often increase or decrease its cloud-point temperature. The phenomenon can be possibly explained from the changes of intermicellar repulsions or the sizes of the micelles due to the adsorptions of ions to these micelles. Various salts (LiCl, NaCl, KCl, Na2SO4, NaBr, NaI, MgCl2, BaCl2, and BaCl2) were added to the micellar solutions of Tergitol 15-S-7 for the study on their effects to change the cloudpoint temperatures. It is clearly demonstrated in Figure 2 that addition of salts, except sodium iodide, decreases the cloud-points. Among these salts, sodium sulfate was found to be the most effective in decreasing the cloud point temperature at the same concentration of added salt. The effect on the decrease of cloud-point temperature becomes more obvious with increasing concentration of sodium sulfate. To ensure the cloud-point temperature of the Tergitol 15-S-7 solution containing PAH mixtures of interest below ambient temperature (23 °C) and to avoid the unwanted 3938

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phase, such as liquid crystalline phase, from forming, the concentration of sodium sulfate is preferably chosen at 0.5 M throughout this study. With addition of 0.5 M Na2SO4, the cloud-point temperatures for the micellar solutions containing 1 to 4 wt % Tergitol 15-S-7 are found to be 12, 13.5, 14.2, and 14.8 °C, respectively. Lesser concentration of Na2SO4 cannot reduce the cloud-point sufficiently lower than the ambient temperature. In case that Na2SO4 concentration is too high (greater than 0.75 M), it is found that the surfactant rich phase will be no longer clear and become milky-white. The stable emulsion phase, presumably the LR phase, is observed as floating on top of the aqueous phase. Addition of too much Na2SO4 will decrease the cloud point to a very low temperature, and, thus, the formation of the unwanted and viscous liquid crystalline phase, usually the LR lamellar phase appearing first, is inevitable (15-17). This made it more difficult to separate surfactant-rich phase accurately. Cloud-point temperatures of the micellar solutions can also be effectively reduced by addition of other hydrophobic substances, such as long-chain hydrocarbons and alcohols except ethanol (29, 44, 46-48). In general, the phosphates and sulfates lower the cloud point, whereas the thiocynanates and idodides increase it. The cloud-point and temperature needed to achieve phase separation can be easily manipulated in either direction by adding various substances. The preconcentration factors for the Tergitol 15-S-7 micellar solutions having from 1 wt %, 2 wt %, 3 wt %, and 4 wt % surfactants with 0.5 M Na2SO4 are obtained at 35.7, 22.2, 13.3, and 9.6, respectively. That is, the concentration of the hydrophobic analytes in the surfactant-rich phase will increase for almost 20-fold than that in the bulk solution by using 2 wt % Tergitol 15-S-7 solution for the cloud-point extraction. The result shows an increase in the extracted volume of surfactant-rich phase with the increasing surfactant concentration initially present in bulk solution. From the viewpoint in concentrating the analytes present in aqueous solution, the larger preconcentration factor, e.g., the smaller phase volume in the surfactant-rich phase, is desired. It is of note to mention that the phase volume of the surfactant-rich phase, which forms after phase separation from the bulk micellar solution having the fixed amount of surfactant, in general, decreases with an increasing temperature difference above the cloud-point, e.g., addition of too much sulfates into micellar solutions. The smaller phase volume often causes great difficulty to collect it, unless significant amounts of analytes and surfactant are used. Due to the difficulty to collect the small volume of the surfactantrich phase and subsequently the unacceptable experimental error, the concentration of Tergitol 15-S-7 used in this study was chosen to be greater than or equal to 2 wt %. The recovery of the PAHs in the surfactant-rich phase is calculated from the measured concentrations of PAHs by HPLC and the phase volume from the preconcentration factor measurements. Indeed, the presence of very low concentrations of PAHs will not alter the cloud-point and consequently the preconcentration factors. That is, the preconcentration factors are not different from those obtained from the PAHfree micellar solutions. There are many different factors affecting the extraction process. Some of them are surfactant concentration, ionic strength, pressure, temperature, and so on (9). It is very important to optimize them in order to obtain the good recovery strategy. In the case of PAHs, some of the factors, such as pH, have slight or no influence on the recovery percentages, because these compounds do not present ionic forms. In this work, the effect of surfactant concentration, equilibration time, initial concentration of analyte, and centrifugal speed were carefully studied for the decision on the optimal operation conditions.

FIGURE 3. Chromatograms of PAH mixtures obtained by the injection of surfactant-rich phases after cloud-point separation from 3 wt % Tergitol 15-S-7 solution with 10 min for equilibration and 10 min for centrifugation at 3500 rpm.

FIGURE 4. Effect of surfactant concentration on PAH recovery. 3.2. Effect of Surfactant Concentration on PAH Recovery. Figure 3 displays the chromatograms of PAH mixtures obtained by the injection of surfactant-rich phases after cloud-point separation using 3 wt % Tergitol 15-S-7 solution with 10 min for equilibration and 10 min for centrifugation at 3500 rpm. The retention times for naphthalene, phenanthrene, and pyrene are found at 4.5, 7.0, and 11.1 min, respectively. It clearly shows that the surfactant Tergitol 15S-7 does not render any peak of UV absorbance and any fluorometric signal in the UV ranges, in contrast to those chromatographic peaks of Triton X-114 observed by Ferrer et al. on cloud-point separation of PAHs (24). That is, the complicated cleanup procedures and undesirably masking the UV absorbance peaks and the fluorometric signals, especially in the UV ranges, of the PAHs in effluent could be avoided by using Tergitol 15-S-7, instead. Figure 4 shows that the concentration of the surfactant has a considerable effect on the recovery of naphthalene. These experiments were conducted at ambient temperature (near 23 °C) with 10 min for equilibration time and 10 min for centrifugation at 3500 rpm. In micellar solution having 3 wt % or 4 wt % surfactant, the recovery of naphthalene is about 96%. On the contrary, the recovery of naphthalene is only about 86% in a 2 wt % surfactant solution. However, for phenanthrene and pyrene, the effect of surfactant concentration is insignificant. The recovery of phenanthrene and pyrene is about 86% and 80%, respectively. The recovery percentage reported in this work is reproducible within 2%. The recovery of phenanthrene by using 3 wt % Tergitol 15S-7 solution is comparable to those reported, while the recovery of phenanthrene and pyrene are satisfactorily compared to those reported values which range from near 60% to about 110% (7, 19-24, 45). It is interesting to note that the recovery of pyrene and phenanthrene in this work is slightly less than 100%. It is possibly owing to the sorption of PAHs onto containers, which is also noted by Sicilia et al. (19).

FIGURE 5. Effect of equilibrium time on PAH recovery using 3 wt % Tergitol 15-S-7 and 0.5 M Na2SO4. The optimal surfactant concentration is, therefore, chosen at 3 wt %. All the studies reported in the following sections are based on the use of 3 wt % Tergitol 15-S-7 micellar solution. 3.3. Effect of Other Parameters on PAH Recovery. As aforementioned, the samples were left 10 min for equilibration after initiating the preconcentration step, e.g., addition of Na2SO4, before proceeding to assisted phase separation by the centrifuge. Here too, the samples were left for 20, 180, and 720 min, respectively, to examine the effect of the equilibration time on the PAH recovery efficiency. Figure 5 illustrates that this parameter has no or slight influence, at least within experimental error, on the recovery of the analytes under study for equilibration time greater than 10 min. On the contrary, the equilibration time affects, to different degrees, the recoveries of fulvic and humic acids using Triton X-100 as extractant (33) and the recoveries of PAHs from humic acid solution using 1% sodium dodecane sulfonic acid (SDSA) and 4 M HCl for CPE (22). That is, it is sufficient to take 10 min for samples reaching equilibrium before centrifugation. It is noteworthy to mention that the use of anionic surfactant, sodium dodecyl sulfate (SDS), with addition of HCl in preconcentrating PAHs required waiting for 24 h before centrifugation for the surfactant-rich phase to be formed (19). In contrast, the use of Tergitol 15-S-7 is able to shorten the equilibration time to only 10 min. To further examine the effect of the initial analyte concentration, solutions containing different concentrations of PAH mixtures were subjected to the same CPE procedures. The experimental conditions are fixed and include the use of 3 wt % Tergitol 15-S-7 solution with addition of 0.5 M Na2SO4 and 10 min, respectively, for equilibration time and for centrifugation at 3500 rpm at ambient temperature. As the concentrations of analytes vary over an order of magnitude, the recovery efficiency remains practically constant (shown as Figure 6). The initial concentrations of these PAHs have no effect, practically, on the recovery percentages when nonionic surfactant Tergitol 15-S-7 is employed as extractant under the aforementioned conditions. It is interesting to note that the recovery of pyrene in 1 wt % SDS solution with addition of 4.2 M HCl was found to be dependent on the initial concentration of pyrene itself. Increasing its initial concentration in from 0.1 to 2.5 ppm, the recovery drops from 95% to 83% (19). Figure 7 exhibits the effect of centrifugal force in terms of rpm on the recovery efficiency of PAHs. The time needed for complete separation of these two coexisting phases is proportional to the viscosity but inversely proportional to the density difference and the centrifugal force (50). That is, at fixed conditions, higher centrifugal speed will reduce the duration time in need. The results confirm that 3000 rpm, VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Effect of initial PAH concentration on its recovery using 3 wt % Tergitol 15-S-7 and 0.5 M Na2SO4.

FIGURE 7. Effect of centrifugal speed on PAH recovery using 3 wt % Tergitol 15-S-7 and 0.5 M Na2SO4. instead of 3500 rpm previously used, and duration of 10 min are sufficient enough for the CPE procedure to preconcentrate PAHs using 3 wt % Tergitol 15-S-7 solutions at ambient temperature (23 °C). In summary, through the study of PAH recovery efficiency under different experimental conditions, the optimum conditions for CPE procedures on recovering naphthalene, phenanthrene, and pyrene in aqueous solution using Tergitol 15-S-7 can be obtained at (1) 3 wt % surfactant; (2) addition of 0.5 M Na2SO4; (3) 10 min for equilibration time; and (4) 3000 rpm for centrifugal speed with duration of 10 min. The operation can be carried out at ambient temperature (about 23 °C), and no special washing step is required to remove surfactant before HPLC analyses.

Acknowledgments The authors gratefully acknowledge financial support from the National University of Singapore (Project No. R-279-000056-112).

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Received for review April 9, 2001. Revised manuscript received July 6, 2001. Accepted July 13, 2001. ES0108335