J. Cent. South Univ. (2012) 19: 2136-2141

DOI: 10.1007/s11771-012-1256-2

Cloud-point extraction and preconcentration of bisphenol A from water samples

TAN Zhi-jian(谭志坚)^{1, 2}, LI Fen-fang(李芬芳)^{1, 2}

1. Key Laboratory of Resources Chemistry of Nonferrous Metals of Ministry of Education

(Central South University), Changsha 410083, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: A novel cloud-point extraction (CPE) was successfully used in preconcentration of biphenol A (BPA) from aqueous solutions. Majority of BPA is extracted into the surfactant-rich phase. The parameters affecting the CPE such as concentration of surfactant and electrolyte, equilibration temperature and time and pH of sample solution were investigated. The samples were analyzed by high-performance liquid chromatography with ultraviolet detection. Under the optimized conditions, preconcentration of 10 mL sample gives a preconcentration factor of 11. The limit of detection (LOD) and limit of quantification (LOQ) are 0.1 μg/L and 0.33 μg/L, respectively. The linear range of the proposed method is 0.2-20 μg/L with correlation coefficients greater than 0.998 7 and the spiking recoveries are 97.96%-100.42%. The interference factor was tested and the extraction mechanism was also investigated. Thus, the developed CPE has proven to be an efficient, green, rapid and inexpensive approach for extraction and preconcentration of BPA from water samples.

Key words: biphenol A; cloud-point extraction; extraction mechanism; high-performance liquid chromatography; water samples

1 Introduction

Biphenol A (BPA) is mainly used as a monomer in the preparation of epoxide resins, polycarbonate plastics, phenolic resins and polyesters. The residual non- polymerized BPA easily migrates into the water and food during the thermal treatment process required for sterilization. BPA is detected in a diverse range of products including the interior coatings of cans, reinforced pipes, infant bottles, water main filters, nail polish and food packaging materials [1]. Recent researches indicate that BPA and its derivatives have high potential as endocrine disruptors in humans and wildlife. It can cause male reproductive dysfunction and disrupt thyroid hormone action [2]. Due to the widespread exposure to BPA, there is a need to monitor and remove BPA in the environment. In fact, some extraction methods have been developed to extract BPA from various samples such as Soxhlet extraction (SE) [3], liquid-liquid extraction (LLE) [4], solid-phase extraction (SPE) [5] and supercritical fluid extraction (SFE) [6]. These methods are effective in extraction of BPA. However, the SE and LLE consume a large sample volume and a lot of organic solvents and the SPE and SFE are time-consuming and expensive. Therefore, a simple, rapid and less labor-intensive extraction method is needed for environmental analysis. In the last decade, increasing interest on the use of aqueous micellar solutions had been found in the field of separation science. Aqueous solutions of many nonionic would become turbid when heated to the cloud point temperature. When cooled below this temperature, the systems became a homogeneous clear phase again. Above the cloud point, the solution separates into two distinct phases: one surfactant-rich phase (micellar phase) with a small volume and the other aqueous phase with a bulk volume containing low concentration of surfactant monomers, equal to the critical micellar concentration (CMC) [7]. Cloud-point extraction (CPE) is a kind of environmentally benign liquid–liquid extraction method. It offers several advantages such as low cost, high security and recoveries, easy disposal of surfactants and short time for phase separation [8]. CPE has been designed for extraction and preconcentration of a variety of substances including protein [9], metal ions [10] and natural organic compounds [11].

In this work, the CPE technique was chosen to preconcentrate BPA from natural water samples using Triton X-114 (TX-114) as non-ionic surfactant. BPA is hydrophobic, which is subsequently trapped in the surfactant-rich phase. The effects of equilibration temperature and time, concentrations of surfactant and inorganic electrolyte as well as the pH of sample solutions were studied. The optimal extraction conditions were decided.

2 Experimental

2.1 Materials

Triton X-114 (C₃₀H₆₄O₉, relative molar mass is 558.75 g/mol, CMC is 0.35 mmol/L at 25 ℃), purchased from Union Carbide Company (Germany), was used without further purification; BPA was purchased from Sigma-Aldrich Laborchemikalien GmbH (Seelze, Germany); standard stock solutions (1 mg/mL) was prepared in methanol and stored at 4 ℃; the solution was further diluted to different concentrations for analysis. Other reagents were of analytical grade and purchased from China National Medicine Corporation Ltd. The water samples were centrifuged and then filtrated by a microporous membrane (0.45 μm) and stored at 4 ℃.

2.2 Preparation of CPE system

Each CPE experiment was conducted by using a  15 mL tube containing different concentrations of TX-114 and salt. The tube was incubated in the thermostatically controlled bath at 40 ℃ for 20 min  then was for complete separation by centrifugation at   4 000 r/min for 5 min. The phases were cooled down and the volume of each phase was noted. The surfactant-rich phase after CPE was diluted with distilled water for HPLC (Shimadzu, two LC-20 AT pumps coupled with a UV detector) analysis. The whole flow chart of preconcentration of BPA is shown in Fig. 1.

2.3 Liquid chromatography analysis

BPA is hydrophobic and slightly soluble in water, and surfactants have long been known to human-beings for their capability to enhance the solubility of hydrophobic materials. Owing to the hydrophobic nature, BPA tends to preferentially distribute in surfactant-rich phase. Aliquots of surfactant-rich phase were drawn out by a syringe for HPLC (Shimadzu, two LC-20 AT pumps coupled with a UV detector) analysis. A symmetry C-18 column (150 mm×4.6 mm ID, 5 μm) was used for LC separation. The mobile phase contained acetonitrile and 0.15% acetic (50:50, V/V). The column oven temperature was 40 ℃, and the flow rate was   1.0 mL/min. 5 μL of the surfactant-rich phase was injected into the HPLC system. 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 detection wavelength was set as 210 nm.

The preconcentration factor is defined as: F_c=C_s/ C_initial, where C_initial and C_s are concentrations of the analyte in the initial aqueous solution before and after the CPE process in the surfactant-rich phase, respectively. The phase volume ratio is defined as: R=V_a/V_s, where V_a and V_s are the aqueous phase volume and surfactant-rich phase volume, respectively. The extraction recovery (E) of BPA in the surfactant-rich phase is determined from Eq. (1):

          (1)

where C₀ represents the BPA concentration in the initial crude sample before being added to the phases, and V₀ represents the volume added to the phases.

3 Results and discussion

3.1 Effect of TX-114 concentration

TX-114 can form CPE system with an ambient cloud point (23-30 ℃) at different concentrations. It was chosen as an extraction agent due to its low toxicological properties and low cost, as well as easy phase separation by centrifugation [12]. The surfactant content in the range of 0.5%-12% (w/φ) of the extraction system was studied. The variation of phase volume ratio and recovery of CPE process with respect to the content of TX-114 is shown in Fig. 2. At a fixed sample volume, the phase volume ratio decreases with the increase of TX-114 content. When the TX-114 content is above 3% (m/V), the recovery is close to the maximum and has slight further increase with the increase of TX-114 content, so this content is chosen for further studies.

Fig. 1 Flow chart of extraction and preconcentration of BPA

Fig. 2 Effect of TX-114 content on phase volume ratio and recovery (Phases are composed of 1 mL BPA solution     (100 μg/L) and incubated at 40 ℃ for 15 min)

3.2 Effect of equilibration temperature and time

It is desirable to employ the lowest possible equilibration temperature and the shortest time without a significant compromise in extraction efficiency. Theoretically, the optimal equilibration temperature for the extraction occurs when the temperature is 15-20 ℃ higher than the cloud point temperature of surfactant [13]. And 40 ℃ is chosen as the optimal temperature in this work.

Numerous studies on the partitioning of analytes in CPE processes involving non-ionic surfactants had reported optimal equilibration time ranging from 10 to 20 min [14-15]. In this work, the recovery of BPA was measured at 5, 10, 15, 20 and 30 min equilibration time. Through the experiments, it is found that the equilibration time of 15 min is enough for achieving quantitative extraction, and the extraction efficiency has no considerable enhancement for longer incubation time.

3.3 Effect of pH

BPA has two phenolic hydroxyl groups whose disassociation constants pK_a is from 9.6 to 10.2 [16]. The pH of the system was adjusted in the range of 1.0-12.0. Through the experiments, it is found that the phase volume ratio is hardly influenced by pH. The effect of pH on recovery is shown in Fig. 3. The recovery has small changes in the pH range of 1.0-7.0 and the maximal extraction yield is obtained at pH 4.0. However, the recovery decreases sharply at pH above 7.0. This is because when the pH of system is basic, BPA dissociates to form H⁺ and corresponding anions (—O^-), which can form a stronger hydrogen bond with water molecules. So, BPA becomes more hydrophilic, tending to be separated in aqueous phase. When the system is acid, it exists in the form of molecule. TX-114 can form a hydrogen bond with BPA, so BPA is nearly completely separated in surfactant phase.

Fig. 3 Effect of pH on recovery (Phases are composed of 3% (m/V) TX-114 and 1 mL BPA solution (100 μg/L), equilibrium temperature and time are 40 ℃ and 15 min, respectively)

3.4 Effect of inorganic electrolyte

The effect of salt was investigated with adding NaCl in the range of 1%-7% (m/V) to the system. It was well reported that the salt decreased the cloud point of the surfactant [17]. The cloud-point was determined by visual observation, as shown in Fig. 4. The cloud point decreases with the increase of NaCl concentration. The salting-in and salting-out effects can be used to interpret the electrolyte effects on the cloud points of non-ionic surfactant [18].

Fig. 4 Effect of sodium chloride on cloud point (Phases are composed of 3% (m/V) TX-114 and 1 mL BPA solution (100 μg/L))

As shown in Fig. 5, the phase volume ratio increases with the increase of salt concentration. This is because addition of salt can facilitate phase separation by increasing the density of the aqueous phase. The salt increases the incompatibility between the water structures in hydration and surfactant macromolecules, which can reduce the concentration of “free water” in surfactant-rich phase, resulting in the decrease of surfactant-rich phase volume. Furthermore, NaCl can also enhance the extraction efficiency and the maximal recovery is obtained when 3% (m/V) NaCl is added. The salt concentration is increased, and the aggregation number increases, but the CMC remains constant [19-20]. The salt promotes the dehydration of ethoxy groups on the outer surface of the micelles. As a result, the salts enhance the micellar size and concentration in surfactant-rich phase [21]. In addition, BPA may become less soluble in the aqueous solution at higher salt concentration. All these factors lead to the increase of extraction ability of TX-114. However, when the salt concentration is higher than the critical point, the surfactant-rich phase will be on the surface of the solution, which will make it more difficult to separate the extraction solvent into two phases, and the accuracy and reproducibility will probably be not so satisfied.

Fig. 5 Effect of sodium chloride on phase volume ratio and recovery (Phases are composed of 3% (m/V) TX-114 and 1 mL BPA solution (100 μg/L), equilibrium temperature and time are 40 ℃ and 15 min, respectively, and pH is 4.0)

3.5 Analytical performance

In order to validate the proposed methodology, the developed procedure was applied to the determination of tap water, river water and waste water samples. For this purpose, 10 mL of each sample was preconcentrated under the optimal condition. The linear range of the proposed method is 0.2-20 μg/L with correlation coefficients greater than 0.998 7 and the spiking recoveries are 97.96%-100.42%. The results are shown in Table 1, which have confirmed the validity of the proposed method. Under the optimal conditions, the maximal preconcentration factor of 11 is obtained.

The equation (n=6) for the calibration curve is linear: Y=2 269.67+1 553.39X, where Y is integrated absorbance, and X is the BPA concentration, μg/L. The limit of detection (LOD) is the lowest analyte concentration that could be detectable above the noise level of the system, typically three times the standard deviations of the blank (n=10). The LOD is defined as C_L=3S_b/m, where C_L, S_b and m are limit of detection, standard deviation of blanks and slope of the calibration curve, respectively. The limit of quantification (LOQ) is the lowest concentration of analyte that could be accurately measured. The LOQ is defined as ten times the standard deviations of the blank (n=10). The LOD and LOQ of this method are 0.1 μg/L and 0.33 μg/L, respectively.

Table 1 Determination of BPA in different water samples

3.6 Interference factors

During the CPE procedure, some organics and metals ions could be extracted into the surfactant-rich phase. The effect of interfering ions with different concentrations was studied. Various amounts of foreign species were considered as interferent and added to a solution containing BPA of 100 μg/L. An error of less than or equal to 5% in the absorbance reading was considered tolerable. The results presented in Table 2 show the good selectivity of this procedure. Two similar compounds, phenol and p-benzenediol, are also tolerable up to ratios of 10 and 5, respectively. Moreover, Fig. 6 shows the liquid chromatogram of BPA before and after CPE, and it is found that the interferents do not affect the LC separation of the water samples and the recovery of BPA.

Table 2 Effect of species ions on determination of 100 μg/L of BPA

Fig. 6 Liquid chromatogram of BPA before and after CPE procedure: (a) Xiangjiang river water before CPE; (b) Xiangjiang river water withdrawn from diluted TX-114 phase after CPE

3.7 Mechanism of CPE of BPA

There are some explanations for the extraction mechanism for the CPE procedure, such as the dehydration process occurs in the external layer of micelles and the inter micellar acting force is strengthened and micellar size is increased (increasing aggregation number) with the increase of temperature [22]. It was reported that the core of non-ionic surfactant is surrounded by a mantle of aqueous hydrophilic chains, the materials may be absorbed in both the core and the mantle, and non-polar compounds are absorbed in the core of micelles, while polar solutes are located on the mantle [23]. The structure of BPA is symmetrical without polarity, so it is absorbed in the core of micelles. BPA can form strong hydrogen bond with the ether bond (—O—) and hydroxyl group at the end of long hydrophobic chain in TX-114.

Figure 7 shows the UV spectra of BPA in aqueous solution with different contents of TX-114 solution. The two characteristic absorption peaks of 227 and 277 nm are observed in aqueous solution. The absorption peak at 227 nm has a small blue shift with the increase of TX-114 concentration. However, this phenomenon is not observed at 277 nm. Furthermore, the absorbance strength of all the absorption peaks is enhanced with the increase of TX-114 concentration. These phenomena may be linked with the change of hydrogen bond between BPA and micelles. BPA can form more strong hydrogen bonds with micelle molecules when more surfactant is added. This fact supports that BPA resides on the core of the micelles.

Fig. 7 Effect of X-114 solution on UV spectra of BPA with different contents of TX-114 solution added to BPA solution (100 μg/L)

4 Conclusions

1) CPE is successfully used in preconcentration of BPA from natural water samples. The affecting factors to CPE are studied in detail and temperature and pH are the critical factors affecting the distribution of BPA.

2) The proposed methodology is easy, safe and inexpensive. In addition, it provides a lower limit of detection. Some possible interference cannot affect the determination of BPA in real water samples. The extraction mechanism is demonstrated and the results show that BPA is just absorbed on the core of the micelles.

3) The application of cloud-point extraction of BPA will offer a new practical approach for preconcentration and determination of other hazardous materials from water samples.

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(Edited by HE Yun-bin)

Foundation item: Project(20956001) supported by the National Natural Science Foundation of China; Project(CX2011B083) supported by Hunan Provincial Innovation Foundation for Postgraduate, China; Project(K1104026-11) supported by Project of Changsha Science and Technology Bureau, China

Received date: 2011-07-01; Accepted date: 2011-10-18

Corresponding author: LI Fen-fang, Professor, PhD; Tel: +86-731-88836961; E-mail: lfflqq@mail.csu.edu.cn