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Ind. Eng. Chem. Res. 2006, 45, 830-833
Enrichment of Amino Acids by Ultrasonic Atomization Akira Suzuki, Hideo Maruyama,* Hideshi Seki, Yasuhiro Matsukawa, and Norio Inoue Laboratory of Bioresources Chemistry, DiVision of Marine Biosciences, Graduate School of Fisheries Sciences, Hokkaido UniVersity, Minato 3-1-1 Hakodate 041-8611, Japan
Dilute aqueous solutions of two amino acids, tryptophan (Trp) and phenylaramine (Phe), were atomized ultrasonically, and it was found that the amino acids were enriched in the collected mist droplets. The enrichment ratio increased with decreasing initial concentration of amino acids. An enrichment model was proposed and was verified with experimental results. The mechanism is analogous to that of nonfoaming adsorptive bubble separation (NFBS) reported previously. The model explained well the experimental results, and the specific surface area of the mist droplets was estimated. In the case of ultrasonic atomization, the estimated specific surface areas for both Trp and Phe were larger than those of NFBS. This suggests that smaller droplets will be formed by ultrasonic atomization. The present method will be available to separate gas-liquid interfaces where surface-active substances adsorb and to enrich them from dilute aqueous solution. Introduction Ultrasonic atomization has been used to produce very fine particles of liquid. This technique has been utilized in the fields of combustion, humidification, and so on. Recently, enrichment operations by this technique have been reported for ethyl alcohol1-4 and surface-active agents.5,6 This technique is available for the enrichment of dilute dissolved surface-active substances and has some advantages, i.e., low energy requirements and no requirement of tedious treatments such as desorption or addition of any other chemicals. For the optimal design and operation of industrial processes, it is necessary to clarify the enriching mechanism of this technique. We previously reported the enrichment mechanism of nonfoaming adsorptive bubble separation (NFBS) techniques.7,8 It seems that the enrichment mechanism of ultrasonic atomization can be expressed analogously to that of NFBS, because both techniques involve separating the gas-liquid interface where surface-active substances adsorb from the bulk liquid and are enriched in the collected droplets. We consider that the enriching mechanism will be greatly affected by two factors: (i) the adsorption equilibrium relationship between the bulk liquid and droplet surface of objective substances and (ii) the specific surface area of the atomized droplets. The former is governed by physicochemical properties of the objective substances, and the latter is influenced by liquid properties and operating conditions. In this study, enrichment experiments were conducted with amino acids, which are produced in large amounts worldwide for use in the pharmaceutical and food industries, among others. Enrichment of amino acids is expected upon ultrasonic atomization, because amino acids have some degree of surface activity. To clarify the enrichment mechanism of ultrasonic atomization, a simple model is proposed and verified with the experimental results. Enrichment Mechanism Taking into account that the objective surface-active substances contained in the atomized droplets originate from the
adsorbed molecules at the gas-liquid interface and the molecules dissolved in the bulk liquid, the mass balance can be expressed as
Cm ) (AmX + VmCb)/Vm
(1)
where Cb and Cm are the concentration of the bulk liquid and liquid drop, respectively. Am, Vm, and X are the surface area and volume of a liquid droplet and the surface density of the objective substance at the surface of a liquid droplet, respectively. The enrichment ratio, E, can be defined as
E ≡ Cm/Cb ) 1 + (Am/Vm)(X/Cb)
(2)
The term (Am/Vm) in the right side of eq 2 corresponds to the specific surface area, Sd, of the droplets, so eq 2 can also be expressed as
E ≡ Cm/Cb ) 1 + Sd(X/Cb)
(3)
The adsorption equilibrium of amino acid between in the bulk liquid and at the gas interface is subjected to the Langmuir adsorption isotherm expressed as
X)
KγCb 1 + KCb
(4)
In this equation, K and γ represent the adsorption equilibrium constant and the saturated surface density at the liquid-gas interface, respectively. Elimination of X from eqs 1 and 4 gives
K E ) 1 + Sdγ 1 + KCb
(5)
For instance, typical calculation profiles of log(E - 1) are shown in Figure 1 as functions of the adsorption equilibrium constant, K; the saturated surface density, γ; and the specific surface area, Sd. The maximum value of E is affected by all three parameters. The bending point of the curve is shifted to more less Cb, and the plateau level of E increases with increasing K. The experimental value of E can be verified with the calculated value, if Sd, γ, and K are known. Materials and Methods
* To whom correspondence should be addressed. Tel.: +81-138-408813. Fax:+81-138-408811. E-mail: maruyama@ elsie.fish.hokudai.ac.jp.
1. Materials. Tryptophan (Trp) and phenylalanine (Phe) were purchased from Kanto Chemical Co. Inc. (Tokyo, Japan) and
10.1021/ie0506771 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005
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Figure 3. Typical relationship between the initial concentration, Ci, in the column and the concentration, Cm, of mist droplets. The experiments were conducted at pH 10. The solid line corresponds to the diagonal (Cm ) Ci).
Figure 1. Variation of log(E - 1) as functions of (a) the saturated surface density, γ; (b) the equilibrium adsorption constant, K; and (c) the specific surface area, Sd. Values of parameters used in the calculation are as follows: (a) K ) 1×107 cm3/mol, Sd ) 2 × 103 cm2/cm3; (b) γ ) 1 × 10-10 mol/cm2, Sd ) 2 × 103 cm2/cm3; (c) K ) 1 × 107 cm3/mol, γ ) 1 × 10-10 mol/cm2.
Figure 2. Schematic drawing of experimental setup for ultrasonic atomization: (1) pump, (2) humidifier, (3) vessel, (4) atomizer, (5) power supply, (6) sample reservoir, (7) reference reservoir.
were used without further purification. HCl and NaOH were also purchased from Kanto Chemical Co. Inc. (Tokyo, Japan) for use in adjusting amino acid solutions to several desired values. 2. Experimental Setup. Figure 2 presents a schematic drawing of the experimental setup. The atomization vessel was cylindrical and made of transparent acrylic pipe. The inside diameter was 44 mm, and the height was 100 mm. An ultrasonic oscillator was set at the center of the vessel bottom. The ultrasonic frequency was 1.6 MHz, and the electric power was 24 W. The carrier gas inlet and outlet were attached to the wall of the vessel. The diameter of the carrier gas inlet and outlet was 5 mm. Air was used as a carrier gas and was made to flow through the vessel to accompany the droplets. To minimize droplet drying, the inlet glass tube was installed at the wall of the vessel to induce humidified air. 3. Experimental Procedure. The atomization vessel was placed in 25 °C water bath. Then, 100 cm3 of the amino acid solution was prepared at a desired concentration and charged into the vessel. The liquid height was 6.6 cm. After that, the
air as a carrier gas was flowed into the vessel by pumping, and atomization was started. The gas flow rate was 4 cm3/s. A great deal of fine droplets were induced into a sampling reservoir by the aid of the 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 in the sampling and reference tubes due to drying was observed. 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 nearly equal to that of water. Each run was conducted for 20 min. In almost every experiment, ca. 0.3-0.5 cm3 was collected in the sampling tube for 20 min. The concentration of the amino acid solution was measured spectrophotometrically (Hitachi U-1500 or JASCO Ubest-30) at 280 nm for tryptophan and at 220 or 260 nm for phenylalanine. The concentration of mist droplets, Cm, was determined by spectrophotometric measurement of the liquid in the sampling tube and by correction for mist droplet dilution in the sampling tube. All experiments were carried out at room temperature and under atmospheric pressure. Results and Discussion As typical experimental results, Figure 3 shows the effect of the initial concentration, Ci, of amino acid on the concentration of mist droplets, Cm. The solid line in Figure 3 corresponds to the diagonal (Cm ) Ci). As seen in Figure 3, the difference between log Cm and log Ci became larger as Ci decreased. This tendency suggests that the enrichment ratio, E, becomes larger at lower concentrations. Figure 4 shows the relationship between the enrichment ratio, E, and Cb. These results were obtained at pH 10. Our previous study9 showed that estimated values of the adsorption equilibrium constant and the saturated adsorption density at the liquidair interface for phenylalanine were the highest at pH 10 in the experimental range; that is, at this pH, phenylalanine was enriched well. On the other hand, these values for tryptophan did not vary significantly with pH. Therefore, the present experiment was conducted at pH 10. According to eq 2, the ratio, E, was defined as the ratio of the concentration of mist droplets to the bulk concentration, Cb. However, Ci can be regarded as the bulk concentration, because the volume of the
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Figure 4. Change in the enrichment ratio, E, of the amino acids (a) phenylalanine and (b) tryptophan with the initial bulk concentration. Experiments were conducted at pH 10.
initial solution is much larger than that of the collected mist droplets. Thus, E was calculated by Cm/Ci hereafter. The lower the initial concentration, the larger the enrichment ratio. This fact suggests that Phe and Trp adsorb at the liquid-air interface of the droplets generated by ultrasonic atomization. In NFBS, the bubble residence time for equilibrium was ca. 3-4 s. In this study, ultrasonic atomization apparently only causes the surface area at the liquid-air interface to increase. In this system, bubbles considered sorbents in NFBS did not exist. We suppose that the adsorption time required for adsorption equilibrium is very short in comparison to the case of NFBS. The bending point expected from eq 5 was not observed with the present experimental conditions. According to eq 5, the enrichment ratio, E, approaches 1 + SdγK as the value of KCb becomes much smaller than 1. In contrast, E also approaches 1 + Sdγ/Cb as the value of KCb becomes much larger than 1. Thus, eq 5 was approximated as follows
E - 1 ) SdγK E - 1 ) Sdγ/Cb
(KCb , 1) (KCb . 1)
(6a) (6b)
Equations 6a and 6b indicate that E - 1 will be constant, at the value SdγK, in the region of KCb , 1, and have a slope of -1 in the plot of log(E - 1) vs Cb and log Cb in the region of KCb . 1. Equation 6b was chosen for data fitting, because the plateau region expected from eq 5 was not observed in the present experimental conditions, which should correspond to the range of KCb . 1. Figure 5 shows the results of fitting eq 6b to Figure 3. The solid lines in Figure 5 represent the calculated values. In Figure 5b, some data in the range Ci J 5 × 10-4 M for Trp of NFBS deviates from the dotted line (slope -1), because the values of E did not vary and were almost constant at ca. 1.1 as a result of slight foaming within the column in NFSB experiment. Only Sd was determined by a least-squares regression. Values of K and γ for Phe and Trp were obtained by the nonfoaming adsorptive bubble separation method (NFBS)9 and were employed for the calculation. These parameters and the calculated
Figure 5. Fitting of the data to eq 6b for (a) phenylalanine and (b) tryptophan. Table 1. Parameters Used for Fitting of Data to Eq 6b and Sd Determined for Ultrasonic Atomization K (cm3/mol) tryptophan phenylalanine
γ (mol/cm2)
NFBS 5.35 × 105 4.27 × 10-12 4.77 × 107 1.95 × 10-11 This Study
tryptophan phenylalanine
Sd (cm2/cm3) 5.75 × 103 3.18 × 104 1.60 × 105 5.84 × 104
values of Sd are summarized in Table 1. In addition, E obtained by NFBS were also shown in Figure 5 and the dotted lines represent the calculated values. The experimental results for both this study and NFBS were explained by the present model. This suggests that the enrichment mechanism of the present system is fundamentally analogous to that of NFBS. E obtained by ultrasonic atomization was larger than that obtained by NFBS. This fact would be caused by difference in droplet size and its distribution between NFBS and the present system. Especially Sd of Trp in this study is larger than that in NFSB. Considering each droplet generated by ultrasonic atomization as a homogeneous sphere, the droplet diameter was estimated as 1.03 µm for Phe and 0.38 µm for Trp from Table. 1. In the experimental range of Ci for Phe and Trp (from 1.0 × 10-6 to 1 × 10-2 M), the surface tension of Phe solution decreased from 72 to 66.5 dyn/cm and that of Trp solution decreased from 72 to 63 dyn/cm.10 Lang11 has proposed a correlation equation describing the effects of ultrasonic frequency, f (Hz); surface tension, σ (N/m); and the liquid density, F (kg/m3), on the atomized droplet diameter, d (m)
d ) 0.34[8πσ/(Ff 2)]1/3
(7)
According to Lang’s equation, the diameter of the droplet for both Trp and Phe was ca. 3 µm under the present conditions. The value of the diameter calculated from Sd for Trp is especially lower than that estimated by Lang’s equation. A clear explanation for this result cannot be given here. In a further study, we will attempt to measure the atomized droplet size.
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Conclusion
Literature Cited
Enrichment of the amino acids tryptophan and phenylalanine was carried out by ultrasonic atomization. They were enriched in atomized droplets, and the enrichment ratio was increased with decreasing initial concentration. The model for the enrichment mechanism proposed in this study was verified with the experimental results and was able to express the enrichment profile. This suggests that the enrichment mechanism of the present system is fundamentally analogous to that of nonfoaming adsorptive bubble separation. Enrichment operations by ultrasonic atomization are desirable to remove the surface enrichment of the objective substances as much as possible and the entrained bulk liquid as little as possible. More detail and a quantitative discussion about operating factors will be provided in an article to follow.
(1) Sato, M.; Matsuura, K.; Fujii, T. Ethanol Separation from EthanolWater Solution by Ultrasonic Atomization and its Proposed Mechanism Based on Parametric Decay Instability of Capillary Wave. J. Chem. Phys. 2001, 114 (5), 2382. (2) Kirpalani, D. M.; Toll, F. Revealing the Physicochemical Mechanism for Ultrasonic Separation of Alcohol-Water Mixtures. J. Chem. Phys. 2002, 117 (8), 3874. (3) Yasuda, K.; Tanaka, N.; Rong, L.; Nakamura, M.; Li, L.; Oda, A.; Kawase, Y. Effects of Carrier Gas Conditions on Concentration of Alcohol Aqueous Solution by Ultrasonic Atomization. Jpn. J. Appl. Chem. Phys. 2003, 42 (5B), 2956. (4) Yasuda, K.; Bando, Y.; Yamaguchi, S.; Nakamura, M.; Oda, A.; Kawase Y., Analysis of Concentration Characteristics in Ultrasonic Atomization by Droplet Diameter Distribution. Ultrason. Sonochem. 2005, 12 (1), 37. (5) Rassokhin, D. N. Accumulation of Surface-Active Solutes in the Aerosol Particles Generated by Ultrasound. J. Phys. Chem. B 1998, 102 (22), 4337. (6) Takaya, H.; Nii, S.; Kawaizumi, F.; Takahashi, K. Enrichment of Surfactant from its Aqueous Solution Using Ultrasonic Atomization. Ultrason. Sonochem. 2005, 12 (6), 483. (7) Suzuki, A.; Maruyama, H.; Seki, H.; Hayashi, T. Application of Nonfoaming Bubble Separation to Enrichment of Dilute Dye Solution. J. Chem. Eng. Jpn. 1995, 28 (1), 115. (8) Suzuki, A.; Maruyama, H.; Seki, H. Adsorption Behavior of Organic Substances onto Bubble Surface in Nonfoaming Bubble Separation. J. Chem. Eng. Jpn. 1996, 29 (5), 794. (9) Furuya, K. Fundamental Study of Amino Acids Recovery from Aqueous Environment by Nonfoaming Adsorptive Bubble Separation Technique. M.C. Thesis, Department of Chemistry, Hokkaido University, Hakodate, Japan, 1997 (in Japanese). (10) Ogino, K.; Yamauchi, H.; Shibayama, T. Interfacial Properties of Some Amino Acids in Aqueous Solutions. Yukagagaku 1982, 31 (12), 1009 (in Japanese). (11) Lang, R. J. Ultrasonic Atomization of Liquids J. Acoust. Soc. Am. 1962, 34, 6.
Acknowledgment The authors gratefully express their thanks to Dr. M. Sato, Honda Electronics Company, Ltd., Japan, for his advice about the ultrasonic vibrator. Notation Am ) surface area of liquid drop generated by atomization (m2) Cb ) concentration of bulk liquid (mol/m3) Cm ) concentration of liquid drop (mol/m3) d ) diameter of atomized liquid droplet (m) E ) enrichment ratio f ) ultrasonic frequency (Hz) K ) equilibrium adsorption constant (m3/mol) Sd ) specific surface area based on volume (m2/m3) Vm ) volume of liquid drop generated by atomization (m3) X ) surface density (mol/m2) γ ) saturated surface density (mol/m2) F ) liquid density (kg/m3) σ ) surface tension (N/m)
ReceiVed for reView June 10, 2005 ReVised manuscript receiVed October 26, 2005 Accepted November 7, 2005 IE0506771
