Stirring-Assisted Cloud-Point Extraction of ... - ACS Publications

Ind. Eng. Chem. Res. 2008, 47, 3949–3956

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Stirring-Assisted Cloud-Point Extraction of Polycyclic Aromatic Hydrocarbons Bingjia Yao and Li Yang* School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, China

A novel cloud-point extraction (CPE) process, namely, stirring-assisted cloud-point extraction (S-CPE), was developed with a stirring rate over 380 rpm in a PEG/PPG-18-18 dimethicone aqueous solution at a temperature over its cloud point. Compared with a general CPE process with centrifugation (C-CPE), the stirring process successfully accelerated the phase separation, where the whole process was able to be finished in 15 min and a lower water content in the surfactant-rich phase was also obtained, e.g., the water content was as low as 52 wt % at a 2 wt % surfactant solution, indicating a higher resulting distribution coefficient. The phase separation of the process was studied with dynamic lighting scattering. Because of the increasing contact chances between micelles and extractable species, the stirring operation is also in favor to increase the extractability of the CPE process, where higher recoveries of polycyclic aromatic hydrocarbons (PAHs) were obtained even with a lower surfactant concentration, e.g., a 98.9% recovery of anthracene was obtained in a 1 wt % surfactant solution. What is more important is that the stirring operation is suitable for a scaling-up process, and the surfactant-rich phase floating upon the solution is more easily collected and removed. In addition, the extractant used in the S-CPE, PEG/PPG-18-18 dimethicone, has little UV absorbance, avoiding the disturbance to the signal of PAHs in UV or fluorescence detector, so it is convenient to determine PAHs concentration in every phase during S-CPE process by high-performance liquid chromatography (HPLC) directly. The stirring operation successfully avoids the low phase-separation efficiency like in the CPE process with heating and has no treatment capacity limitation like in C-CPE. Therefore, S-CPE offers an efficient possibility for scaling up a typical CPE process to be applied in the separation of PAHs in the water treatment. 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are carcinogens and mutagenic, and they distribute widely in the water, air, and earth. Thus, the monitoring and separation of PAHs were paid more and more attention in recent years. Because they are strongly hydrophobic, PAHs exist in the water in a trace level, which largely limits the efficiency of traditional separation methods, e.g., liquid-liquid extraction, in the purification or preconcentration of the water polluted by PAHs.1–4 Cloud-point extraction (CPE) is a micelle-mediated separation technology based on the phase-separation phenomenon of a surfactant solution. Because the whole process is free from organic solvents, the CPE process is low-cost and friendly to the environment, compared with conventional separation processes.5–16 Over the past few years, CPE has been successfully applied as a promising preconcentration process in the treatment of the water containing trace-level PAHs.17–22 However, all these studies are in the analysis field, where the treatment capacity was limited in 10 mL test tubes; only one scaling-up CPE experiment was reported till now,23,24 and no practical application of CPE in water treatment was established. Time-consuming and uncompleted phase separation induced by the simple heating process is thought to be a main factor barring the real application of CPE.25 Although addition of salts and high-speed centrifugation are both able to accelerate the phase separation in a typical CPE process,17,26–28 the former results in a high salt content in the treated aqueous phase and an additional desalinization process is required after the CPE process; in the case of centrifugation, because the process is performed in two pieces of equipment, incubator and centrifuge, low recoveries are frequently obtained because the temperature goes down during the transfer and centrifugation procedures, * To whom correspondence should be addressed. Phone: 862154748917. Fax: 862154741297. E-mail: [email protected].

and sometimes the system even became monophasic.29 In addition, high energy cost and limited treatment capacity also make the centrifugation operation unsuitable for practical application in water treatment. On the other hand, merely dependent on thermal motion, the typical CPE process was not able to provide enough contact chances between micelles and extracted species, where their solubilization into the micelles was limited, especially in the treatment of the water containing trace-level extracted species. The deficiency usually resulted in a sharp decrease of extractability when the initial concentration of extracted species was higher than a critical value.30 Therefore, a CPE process with higher efficiency both in phase separation and extractant procedure is required for its future application in the treatment of the water polluted by species like PAHs. In this study, the stirring operation was applied into a PEG/ PPG-18/18 dimethicone surfactant micelle solution, and a very quick phase-separation phenomenon was observed when the stirring speed was over 380 rpm and at a temperature over its cloud point. It was the first time to introduce a stirring operation into a CPE process, instead of a conventional centrifugation operation, as a new method to accelerate the phase separation. The silicone surfactant, PEG/PPG-18/18 dimethicone, had a relative low cloud point (below 313 K), and a high recovery of PAHs in a conventional CPE process was reported in our previous study.31 Compared with traditional nonionic surfactants, the silicone surfactant contained more flexible polysilone chains and was able to offer a more compact micelle structure containing lower water content, which led to a higher distribution coefficient even in the conventional CPE process with centrifugation.31 What’s more, PEG/PPG-18/18 dimethicone is degradable and has no phenol moieties in its structure, which guaranteed its environmental friendliness. Its advantage of no signal in UV and fluorescence detector also avoided the overlapping with PAHs in chromatographs27,32–34 and made the determination of the PAHs concentration in every phase more

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Figure 1. Structure of PEG/PPG-18/18 dimethicone (x, y, m, and n are the numbers of the respective group in the bracket).

convenient during the acquisition of the efficiency or the monitoring of the state of the CPE process. In the stirring-assisted CPE process (S-CPE), the introduction of stirring was thought to strengthen the collision between the micelles of PEG/PPG-18/18 dimethicone at a temperature higher than its cloud point, which sped the aggregation of micelles and the final phase separation, where the process was able to be accomplished within 15 min; meanwhile, the water content in the surfactant-rich phase was also efficiently lowered. In addition, quick stirring was also presumed to increase contact chances between micelles and extracted species during the solubilization, and higher recoveries were achieved. In this paper, the phase separation was studied with dynamic lighting scattering (DLS), water content, and density of the surfactantrich phase; the recoveries of three selected PAHs, anthracene (Anth), pyrene (Pyre), and phenanthrene (Phen), were used to evaluate the performance of the S-CPE process. The influences of surfactant concentration, stirring speed, time, and PAHs concentration on the performance were discussed, where a detailed comparison with the CPE process with centrifugation was performed. 2. Experimental Section 2.1. Reagents. Dow Corning DC-190 (PEG/PPG-18/18 dimethicone) (INCI), also known as silicone-ethylene oxide/ propylene oxide copolymer, was supplied by Dow Corning,35 and its structure is shown in Figure 1 The compound will be referred to in the text as DC-190. Its gravity and viscosity are 1.037 g/mL and 2000 cP, respectively, at ambient temperature. Its cloud point is 311 K, and the weight-average molecular weight was 1310 determined by VPO (K-7000, Knauer). Unfortunately, no information on the detailed molecular structure, i.e., the values of x, y, m, and n, and percent active were available from the manufacturers. In fact, some impurities were found in its high-performance liquid chromatograph (HPLC) or UV determination, but no interference to our research was found, so the surfactant is used without any purification. Anthracene (99%), pyrene (98%), and phenanthrene (98%) were purchased from Acros Organics. Triton X-114 (TX-114, (1,1,3,3tetramethylbutyl)phenylpolyethylene glycol, t-Oct-C6H4-(OCH2OCH2)xOH, x )7-8) and HPLC-grade acetonitrile were from Fluka and Calendon Laboratories Ltd., respectively, and the deionized water used in the mobile phase was with a resistivity of 18.2 MΩ · cm. All the other reagents were of analytical grade and used without further treatment. 2.2. Apparatus. PE-series 200 HPLC system (Perkin-Elmer, U.S.A.), including pump, degasser, autoinjector, column oven, ultraviolet detector, and a TOSOH ODS gel column (4.6*150 mm), was used to determine the amount of PAH. 2.3. Procedure. 2.3.1. Stirring-Assisted Cloud-Point Extraction Procedure. A desired aqueous solution with different PAHs initial concentrations was obtained by mixing the surfactant aqueous solution and the stock solution of PAHs at an appropriate volume ratio, where the stock PAHs solution was prepared by dissolving the solid specimen (around 0.1 g) into 50 mL of acetonitrile. Solution (40 mL) was contained into a

Figure 2. Chromatograms of three PAHs obtained in the surfactant-rich phase by S-CPE in a 1 wt % DC-190 solution containing 2 ppm PAHs.

200 mL beaker and then heated to a certain temperature (318 K, except where indicated). A speed-controllable agitator with stirring arms (model S-212, Shanghai Shen Sheng Biotech Co. Ltd., China) was used to provide stirring at a certain speed in the turbid solution. After the stirring operation, the solution was filtered by a strainer of 200 mesh, and the continuous phase floating upon the solution was left in the strainer. A certain amount of surfactant-rich phase was dried at 353 K until no loss of mass was observed, and the water content in the surfactant-rich phase was obtained from the net loss of water in mass. Then the remaining surfactant-rich phase with a definite weight (around 0.1 g) was dissolved by 1 mL of acetonitrile and injected into the HPLC directly for the determination of recovery. Every data point reported in this study was an average of three experiments. The summation of the PAHs concentrations in both phases after phase separation of S-CPE was compared with the initial amount to reassure the credibility of the data. 2.3.2. Cloud-Point Extraction Procedure with Centrifugation. After an incubation for 20 min at a prescribed temperature (318 K), the surfactant solution containing PAHs, which was same as the one used in S-CPE process, was contained into a 10 mL graduated centrifuge tube and treated by a centrifugation at 3000 rpm for 30 min. Because of its high viscosity (reported in our previous study31), the surfactant-rich phase was obtained by simply leaning the tube slowly to remove the upper water phase without ice bath. The water content in the surfactantrich phase and the recovery of anthracene were determined in the same methods described above. 2.3.3. Analytical Procedure. The samples obtained from the two cloud-point extraction processes (5 µL aliquots) were injected into the HPLC system, which was equilibrated with 75% acetonitrile/water at a flow rate of 1.0 mL/min at 308 K. UV absorbance at 255 nm was monitored by a UV detector. The detection limits (3-fold the standard deviation of the background noise) of the HPLC are 2, 5, and 10 µg/L for anthracene, pyrene, and phenanthrene, respectively. The chromatogram of the surfactant-rich phase obtained by a S-CPE process with 1wt % DC-190 in 2 ppm PAHs solution was shown in Figure 2, where the silicone surfactant had no signal in the chromatogram and no interference to the three PAHs, so the surfactant-rich phase obtained by the S-CPE with DC-190 was able to be examined by HPLC directly without any further cleanup treatment. The dynamic light scattering experiments were performed on a micelle size analyzer, model Zetasizer Nano S (Malvern instruments, U.K.). A He-Ne laser with a power of 4.0 mW was used as a light source, and the temperature was able to be

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Figure 3. Pictorial steps involved in the stirring-assisted cloud-point extraction. Step I: A certain amount of surfactant was added into the water containing PAHs to form a micelle solution. Step II: After the solution was heated to 318 K, the solution turned turbid. Step III: A stirring set stirred in a certain speed, and some white small surfactant aggregates appeared in the solution. Step IV: With the stirring, small surfactant aggregates combined into large ones; both volume and number increased quickly, and they rose to the top layer of the solution because of the density difference while the solution turned clear gradually. Step V: After the stirring operation, all the surfactant aggregates had combined into one large continuous phase, as the surfactant-rich phase, and the water phase turned clearer. Step VI: Two phases were easily separated by filtering the solution through a strainer of 200 mesh. Then the continuous surfactant aggregates phase collected in the strainer was sent to the determination of water content or injected into the HPLC after being dissolved with acetonitrile in a certain proportion.

adjusted from 275 to 363 K. All the measurements were done at a scattering angle of 173°. 3. Results and Discussion 3.1. Phase Separation of Stirring-Assisted Cloud-Point Extraction. Figure 3 depicts the process of the stirring-assisted cloud-point extraction. During S-CPE, a new phase-separation phenomenon was observed. At the beginning of stirring, some small white surfactant aggregates (SAs) in the shape of a bubble appeared and rose to the top layer of the solution. Then, under the effect of stirring, the amount of the SA increased gradually and came together to form larger SAs, floating upon the solution, whereas the solution turned much clearer at the same time. After the stirring operation, SAs had already combined into one large continuous phase, referring to the photo in Figure 4. The continuous phase was made up of surfactant in a special bubblelike shape (it seems to be composed of many bubbles, with lots of holes) and was thought to be a new form of surfactant-rich phase. Because temperature has a big influence on the growing speed of micelle size to form SAs and the phase separation of S-CPE was strongly dependent on the aggregation speed of SAs to form surfactant-rich phase quickly, a higher temperature than cloud point was found as the critical temperature (318 K), with a stirring over 380 rpm, to satisfy the requirement of the S-CPE process, while below the temperature, no phase separation occurred. So the process was different from foam flotation or emulsion, in which air streams or a temperature below cloud point were required, respectively. The critical temperature higher than cloud point was also one of main proofs for S-CPE belonging to cloud-point extraction process.36,37 The stirring paddles made from different materials including glass,

metal, and PTFE (poly(tetrafluoroethylene)) were attempted in the stirring-assisted CPE process. All the paddles with different materials were able to induce the phase separation successfully, and no obvious difference of the phenomena and performances were found, which means that the phase-separation mechanism has no relation with the nature of the paddle. Different from the CPE process with centrifugation (C-CPE), the surfactantrich phase obtained by S-CPE was subtransparent in a white color and close to solid state in some dense part, indicating the low water content in it. However, the quick stirring made the continuous phase full of air, which enlarged the volume. Although the volume of the surfactant-rich phase was able to be determined by immersing the phase into a nonsolvent and observing the volume change of the nonsolvent, it is inaccurate to calculate the preconcentration factor or distribution coefficient based on the volume of the phase full of air. But the recovery of PAHs was able to be determined by dissolving the surfactantrich phase into acetonitrile; thus, it was used to evaluate the performance of S-CPE in the separation of PAHs in this study. The S-CPE process was able to be finished within 15 min, and a higher speed was able to be obtained at a higher stirring speed in the range of our study (1 g/mL, the density of the aqueous solution should also be >1 g/mL. However, the densities of the surfactant-rich phases obtained by three concentrations were all 380 rpm. With a speed below the critical value, no bubblelike aggregates appeared during the stirring because of lacking collision chances between micelles with the insufficient stirring effect, so the phase separation would not happened either, although the solution was also turbid due to the temperature being higher than the cloud point. Figure 9 shows the variation of the recoveries of the three PAHs at a speed that ranged from 390 to 600 rpm after 15 min of stirring operation; the effects of operating time on the recoveries are

Figure 10. Effect of time on the recovery of PAHs by S-CPE (stirring speed ) 450 rpm, temperature ) 318 K, surfactant concentration ) 1 wt %, and initial PAHs concentration ) 2 ppm).

shown in Figure 10. Because of the higher hydrophobicity of Anth, its recovery was higher than another two PAHs in both figures; the relationship of hydrophobicity and recovery was the same as in other CPE processes.17,31 Higher stirring speed and longer time were both able to increase the recoveries of the PAHs, and the maximum recoveries were achieved when the stirring operation was performed at a speed of 460 rpm and for 15 min, respectively. The positive correlation between stirring speed and recovery confirmed our assumption that the increased contact chances between the micelles and PAHs by stirring operation are favorable to the higher recoveries. In a conventional C-CPE process, the centrifugation operation was usually performed for at least 15 min, and an additional incubation for 15 min prior to the centrifugation process was also necessary to guarantee a high recovery.40 In contrast, for the same recovery, S-CPE was able to complete the whole process within 15 min, which was ascribed to the quick aggregation between micelles strengthened by stirring. 3.4.3. Effect of the Initial Concentration of PAHs on the Recovery of PAHs. To compare the behaviors of S-CPE and C-CPE in a high PAHs concentration, the recoveries of anthracene with different initial concentrations were studied by the two CPE processes in the same surfactant concentration (Figure 11). In C-CPE, the recovery decreased quickly with the increase of PAHs concentration and already fell to 85% when the PAHs concentration was 2 ppm using a 1 wt % surfactant solution. A similar tendency of recovery was also reported in some other research using C-CPE.20 However, an opposite tendency was found in the S-CPE process, i.e., the recovery of PAHs increased with the PAHs concentration, although the recovery was not so high at a low PAHs initial concentration. The difference between two CPE processes, shown in Figure 11, illustrated that the low recoveries in traditional CPE processes were mainly caused by the insufficient contact chances for micelles to trap PAH molecules, especially in the solution with a high PAHs concentration; the deficiency was able to be compensated by introducing an additional stirring operation. Higher surfactant concentration of 2 wt % increased the recovery largely in the S-CPE process, but did not change the fact of a lower recovery in a low PAHs concentration. The phenomenon was thought to be ascribed to that the stirring operation disturbed the solubilization of PAHs in the surfactant micelles and decreased the corresponding recovery when the PAHs concentration was low, but the negative effect was decreased with the

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Literature Cited

Figure 11. Comparison of the recoveries of anthracene with S-CPE and C-CPE in different initial concentrations (stirring speed ) 450 rpm, temperature ) 318 K, and time ) 15 min).

increase of PAHs initial concentration and turned to be a positive one with a further increase of the PAHs concentration. 4. Conclusion In this research, a stirring operation was introduced into a cloud-point extraction process with silicone surfactant, PEG/ PPG-18/18 dimethicone, and the performance of the developed process, including the phase separation and the recoveries of PAHs, was discussed. In the stirring-assisted cloud-point extraction (S-CPE) process, when the surfactant solution was heated over the critical temperature (higher than cloud point) at a stirring speed over 380 rpm, the surfactant aggregates of PEG/PPG-18/18 dimethicone came out of solution quickly, combined into a continuous phase, and formed the final surfactant-rich phase, where the phase separation was proved through dynamic lighting scattering study. Compared with the conventional CPE process with centrifugation (C-CPE), the S-CPE process offered several advantages as follows: (1) Lower water content was obtained in the surfactant-rich phase, leading to a higher preconcentration capability of PAHs. (2) The whole procession was finished in one container, which is significant in the continuous water treatment with CPE process. (3) Higher recoveries of PAHs were able to be obtained in a lower surfactant concentration, even in the solution with a high PAHs initial concentration. (4) Quicker and simpler phase separation were obtained, improving the efficiency of the whole process. (5) The stirring operation was feasible for a scaling-up process, and the surfactant-rich phase formed the top layer of the solution after phase separation, which is convenient to the removal operation followed. (6) The silicone surfactant used in the S-CPE process has no UV signal, which offered an easy operation in acquiring the efficiency of the S-CPE process. These characteristics showed that the S-CPE could probably be used in a practical separation of PAHs in polluted water on a large scale. Acknowledgment The authors thank the Research Center of Analysis and Measurement of Shanghai Jiao Tong University for the help in the DLS measurements.

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ReceiVed for reView November 27, 2007 ReVised manuscript receiVed February 20, 2008 Accepted February 25, 2008 IE071618L