Ind. Eng. Chem. Res. 2006, 45, 6383-6386
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RESEARCH NOTES Removal of Bisphenol A and Diethyl Phthalate from Aqueous Phases by Ultrasonic Atomization Hideo Maruyama,* Hideshi Seki, Yasuhiro Matsukawa, Akira Suzuki, and Norio Inoue Laboratory of Bioresources Chemistry, DiVision of Marine Biosciences, Graduate School of Fisheries Science, Hokkaido UniVersity, Minato 3-1-1, Hakodate 041-8611, Japan
An ultrasonic atomization technique was applied to remove endocrine disruptor chemicals (EDCs) from an aqueous environment. Bisphenol-A (BPA) and diethyl phthalate (DEP) were used as model EDCs. BPA or DEP could be transferred from the bulk liquid to the collected droplet liquid using this technique. Removal experiments were conducted, and the results showed that the removal efficiency was dependent on the initial concentration of EDCs. This technique was potentially effective to remove EDCs in the lower-concentration region (less than ca. 5.0 × 10-5 M). On the other hand, from the viewpoint of enrichment, the both plateau level and the increase region of the enrichment ratio could be observed experimentally, which was expected in our previous study [Suzuki et al., Ind. Eng. Chem. Res. 2006, 45, 830]. This fact suggests that our proposed enrichment mechanism via this technique is valid. Introduction Recently, influences of environmental distributed chemicalss the so-called endocrine disrupting chemicals (EDCs)son humans, vertebrates, etc. have been reported. Among EDCs, bisphenol-A (BPA) is well-known to influence hormonal signals and have an irreversible effect on the development of the reproductive organs. Especially, many reports of these influences on fish have appeared in the literature.1-9 Fish and shellfish are important to humans, not only as food or protein resources but also as resources of physiologically active and bioactive substances. For conservation and restoration of the aquatic environments, attempts have been made to remove these substances from aqueous environments. For this purpose, several techniques have been attempted and developed (i.e., adsorption method using solid adsorbents,10-14 degradation using ozone and ultraviolet light irradiation,15 degradation by catalyzed or enzymatic methods,16,17 degradation by ultrasonic sound irradiation,18 and so on). On the other hand, many EDCs have hydrophobic functional groups in their structure. The authors have focused on the hydrophobicity of EDCs and supposed that the adsorption phenomenon of EDCs at the liquid/atmosphere interface will be utilized to remove EDCs from aqueous environments. The authors have already reported an enrichment of amino acids, which have some degree of surface activity, via ultrasonic atomization.19 This technique is available for the enrichment/ removal of dilute dissolved surface-active substances20,21 and has some advantages (i.e., low energy requirements and no requirement of tedious treatments such as desorption or addition of any other chemicals). This technique will be one of the potential methods for the removal of these EDCs from aqueous environments. In this study, an ultrasonic atomization method was conducted with EDCs in aqueous solution. Bisphenol-A (BPA) and diethyl * To whom correspondence should be addressed. Tel: +81138-408813. Fax: +81-138-408811. E-mail: maruyama@ elsie.fish.hokudai.ac.jp.
Figure 1. Schematic drawing of the experimental setup for ultrasonic atomization: (1) liquid column, (2) oscillator, (3) thermostat heater, (4) oscillation circuit and power supply, (5) humidifier, (6) air pump, (7) sampling reservoir, and (8) reference reservoir.
phthalate (DEP) were used as model EDCs, which have mostly the same molecular weight and different values of the n-octanol/ water partition coefficient, respectively. The authors will report that this technique is potentially effective to remove EDCs from the aqueous phase and provide further verification of our proposed model.19 Materials and Methods 1. Materials. Bisphenol-A (BPA) and diethyl phthalate (DEP) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan) and were used without further purification. These compounds were dissolved in distilled water that contained 1.0 wt % NaCl. The solutions were used in experiments in this study. All experiments were conducted in a water bath (25 °C) and under atmospheric pressure. 2. Experimental Setup. Figure 1 represents a schematic drawing of the experimental setup. The atomization vessel was cylindrical and comprised of transparent acrylic resin. The inside diameter was 7 cm, and the height was 11.5 cm. An ultrasonic
10.1021/ie060353s CCC: $33.50 © 2006 American Chemical Society Published on Web 07/26/2006
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oscillator (Honda Electronics Co., Ltd., Japan) with a diameter of 2 cm was set at the center of the bottom of the vessel. The ultrasonic frequency was 2.4 MHz, and the electric power was 13.2 W. The inlet and outlet of the carrier gas were attached to the wall of the vessel. The diameter of the gas inlet and outlet of the carrier gas was 5 mm. Air was used as a carrier gas, which was made to flow through the vessel to accompany the droplets. To minimize droplet drying, the inlet glass tubes was installed at the wall of the vessel to induce humidified air. The humidifier consisted of an Erlenmeyer flask and a gas dispersion tube. A quantity (800 cm3) of distilled water was loaded in a 1000-cm3 Erlenmeyer flask. Gas dispersion tubes, which were fused in a fritted glass disk (with a mean pore diameter of 100160 µm), and a glass tube were inserted in a silicon rubber stopper. The Erlenmeyer flask was plugged with a silicon rubber stopper. Air was introduced from the gas dispersion tube, passed through distilled water, and entered into the atomization vessel. 3. Experimental Procedure. The atomization vessel was placed in a water bath at a temperature of 25 °C. A quantity (200 cm3) of the EDCs solution was prepared at a desired concentration and it was charged into the vessel. Air, as a carrier gas, then was induced into the vessel by pumping and the atomization then began. The gas flow rate was 4 cm3/s. Many fine droplets were induced into a sampling reservoir with the aid of an air stream. To avoid any error caused by sample drying, another reference reservoir was connected to the sampling reservoir in series. Equal amounts of distilled water were placed in the two reservoirs. The volume of sampled liquid was calibrated by the volume decrease of precharged water in the reference reservoir. In preliminary experiments, essentially no decrease in the precharged water was observed in the sampling and reference tubes due to drying. Therefore, the increase in liquid weight in the sampling tube could be regarded as the weight of collected mist droplets, assuming that the density of the liquid in the mist droplets was almost equal to that of water. Each run was conducted for 30-60 min. The concentration of EDCs solution was measured spectrophotometrically (Hitachi model U-1500 or Jasco model Ubest-30, Japan) at wavelengths of 285 nm for BPA and 245 or 280 nm for DEP, respectively. All experiments were performed at room temperature and under atmospheric pressure. Results and Discussion In the preliminary experiments, ultraviolet (UV) spectra of the aqueous solutions of EDCs (BPA and DEP) were obtained before and after ultrasonic irradiation (for 1 h). Both were unchanged. Therefore, the authors should suppose that a degradation of BPA and DEP via ultrasonic irradiation did not occur in the present system. As typical experimental results, Figure 2 shows the time course of a concentration profile of the bulk liquid, Cb, within the column for BPA and DEP. The ordinate corresponds to the dimensionless concentration (Cb/Ci), and the initial concentration (Ci) values of BPA and DEP are 1.03 × 10-4 M and 5.16 × 10-5 M, respectively. As seen in Figure 2, the value of Cb decreased linearly as time increased in the removal process. A relationship between Ci and Cb/Ci is shown in Figure 3. The results were obtained using ultrasonic irradiation for 30 min. The value of the plots used the mean value of several runs at the same Ci value. Especially because the obtained Cb values were scattered in Ci < 2.5 × 10-5 M, the upper and lower error bars are shown at the plot points in Figure 3. In the experimental higher-concentration region, the difference in the Cb/Ci value between BPA and DEP was not observed, however, in the
Figure 2. Typical time course of concentration Cb of the bulk liquid within the column for (O) bisphenol-A (BPA) and (4) diethyl phthalate (DEP). The initial concentrations were Ci ) 1.03 × 10-4 M for BPA and Ci ) 5.16 × 10-5 M for DEP.
Figure 3. Change in the bulk concentration Cb of the bulk liquid within the column for (O) bisphenol-A (BPA) and (4) diethyl phthalate (DEP). The results were obtained using ultrasonic irradiation for 30 min.
lower-concentration region (Ci < 5 × 10-5 M); the Cb/Ci value for BPA was lower than that for DEP. The reason for this phenomenon could be considered to be the fact that the adsorption equilibrium constant, K, for BPA is different from that for DEP. Thus, in the lower-concentration region, the amount of adsorbed BPA on the atomized droplet surface should be larger than that of DEP. The detail of the adsorption behavior of BPA and DEP will be mentioned in the following section. This experimental facts suggest that this technique will be potentially able to remove EDCs in the lower-concentration region. The authors have already proposed an enrichment mechanism of the ultrasonic atomization technique.19 The enrichment ratio (E) can be expressed as
E≡
(
)
Cm Kγ ) 1 + Sd Ci 1 + KCb
(1)
where Cm, K, γ, and Sd denote the concentration of liquid droplet, the adsorption equilibrium constant, the saturated adsorption density, and the specific surface area of the atomized droplets, respectively. Moreover, eq 1 is approximated as follows:
E - 1 ) SdγK E-1)
Sdγ Cb
(for KCb , 1) (for KCb . 1)
(2a) (2b)
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expressed in units of kg/m3) on the atomized droplet diameter d (given in meters):
d ) 0.34
Figure 4. Fitting of the data to eq 1 for (O) bisphenol-A (BPA) and (4) diethyl phthalate (DEP). The results were obtained using ultrasonic irradiation for 30 min.
In the previous study, the authors demonstrated an enrichment of amino acids (tryptophan and phenylalanine) via ultrasonic atomization. However, the authors applied eq 2b to explain the experimental results, because the plateau and increase region of E and the bending point were not observed experimentally.19 Figure 4 shows the change in the enrichment ratio E of BPA and DEP with the initial bulk concentration Ci. As seen in Figure 4, the plateau level and increase region of E in the lower- and higher-concentration regions and the bending point can be recognized for both BPA and DEP. This fact supports that our proposed enriching mechanism of ultrasonic atomization technique is fundamentally valid. By fitting the data to eq 1, two curves were obtained in Figure 4, for BPA and DEP, respectively. The values of K and γ were determined using the nonfoaming adsorptive bubble separation (NFBS) method.22-25 The values of K and γ were 2.04 × 105 cm3/g and 1.35 × 10-8 g/cm2, respectively, for BPA and 9.14 × 104 cm3/g and 1.79 × 10-8 g/cm2, respectively, for DEP.25,26 In the case of the NFBS method, Ci can be regarded as the bulk concentration, because the volume of the initial solution is much larger than that of the collected mist droplets (pseudo-continuous system).25,26 As described in the previous paragraph, one reason for the results in Figure 3 is considered to be the fact that the adsorption equilibrium constant (K) for BPA is approximately 2-fold larger than that for DEP. Thus, in the lower-concentration region, the amount of adsorbed BPA on the atomized droplet surface should be larger than that of DEP. For a comparison of the amino acids and EDCs, Ci should be used to calculate eq 1, although the Ci value of EDCs decreased considerably in the lower-concentration region in this system, as observed in Figure 3. The values of Sd were determined by a least-squares method and were 5.46 × 104 and 5.07 × 104 cm2/cm3 for BPA and DEP, respectively. Each curve agrees well with the experimental data, although, in the higher-concentration region, deviation is observed for both BPA and DEP. If the droplet that is generated by ultrasonic atomization is a homogeneous sphere, the droplet diameter (d) can be estimated from 6/Sd. The value of d in the case of the Sd values previously described correspond to 1.10 and 1.18 µm for BPA and DEP, respectively. If Sd could be given for best fitting to the experimental E value in the higherconcentration region, for example, in the case of Ci ) 2 × 10-4 M, the values of d are estimated as 1.73 and 1.98 µm for BPA and DEP, respectively. These calculations suggest that the d value also increases slightly as the value of Ci increases. Lang27 has proposed a correlation equation that describes the effects of the ultrasonic frequency (f, given in units of Hz), surface tension (σ, expressed in units of N/m), and liquid density (F,
( ) 8πσ Ff 2
1/3
(3)
For example, according to the literature,28 the surface tension of the DEP solution was varied from 71.9 dyn/cm to 69.6 dyn/ cm in the Ci range of 1.0 × 10-5 M e Ci e 2 × 10-4 M. According to eq 3, the d value for DEP decreased from 2.31 µm to 2.29 µm in the Ci range of 1.0 × 10-5 M e Ci e 2 × 10-4 M; this change in d is very slight. Both variations of d (calculated from the present result and from the Lang equation) show different tendencies. Liquid viscosity is not considered in the Lang equation. One of the reasons for this deviation in Figure 4 can be affected by a change in the liquid viscosity for both the BPA and DEP solutions. The viscosities of the solutions were measured using Ostwald’s viscometer and were varied from 8.97 × 10-3 g cm-1 s-1 to 1.10 × 10-2 g cm-1 s-1 in the Ci range of 1.0 × 10-6 e Ci e 1.0 × 10-4 M. Moreover, in the same concentration range, the volume of mist droplets collected in the sampling tube for 30 min decreased from ca. 0.7 cm3 to 0.25 cm3 for both BPA and DEP. These changes should affect the value of Sd. The authors considered Sd in eq 1 to be a constant value. In further studies, however, it will be necessary to clarify the dependence of Sd on the liquid properties and the operating conditions. Conclusion The removal of bisphenol-A (BPA) and diethyl phthalate (DEP) was performed via ultrasonic atomization. They could be removed from the aqueous phase using this technique, and this technique was determined to be potentially effective in the lower-concentration region. On the other hand, from the viewpoint of enrichment, both BPA and DEP were surely adsorbed on the surface of the mist droplet and were taken away from the bulk liquid within the column. In addition, both the plateau and the increase region of the enrichment ratio could be observed experimentally, which have been expected in our previous study,19 and our proposed mechanism was verified by the experimental results. Notation Cb ) concentration of bulk liquid (kg/m3) Ci ) initial concentration of bulk liquid (kg/m3) Cm ) concentration of liquid droplet (kg/m3) d ) diameter of atomized liquid droplet (m) E ) enrichment ratio f ) ultrasonic frequency (Hz) K ) equilibrium adsorption constant (m3/kg) Sd ) specific surface area based on volume (m2/m3) γ ) saturated surface density (kg/m2) F ) liquid density (kg/m3) σ ) surface tension (N/m) Literature Cited (1) Sohoni, P.; Tyler, C. R.; Hurd, K.; Caunter, J.; Hetheridge, M.; Williams, T.; Woods, C.; Evans, M.; Toy, R.; Gargas, M.; Sumpter, J. P. Reproductive Effects of Long-Term Exposure to Bisphenol A in the Fathead Minnow (Pimephales promelas). EnViron. Sci. Technol. 2001, 35, 2917. (2) Belfroid, A.; Velzen, M.; Horst, B.; Vethaak, D. Occurrence of Bisphenol A in Surface Water and Uptake in Fish: Evaluation of Field Measurements. Chemosphere 2002, 49, 97.
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ReceiVed for reView March 23, 2006 ReVised manuscript receiVed June 28, 2006 Accepted July 13, 2006 IE060353S
