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 alcohol 1-4 and surface-active agents. 5,6 This technique is avail- able for the enrichment of dilute dissolved surface-active substances and has some advantages, i.e., low energy require- ments 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 non- foaming 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 atomi- zation, 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 sub- stances contained in the atomized droplets originate from the adsorbed molecules at the gas-liquid interface and the mol- ecules dissolved in the bulk liquid, the mass balance can be expressed as where C b and C m are the concentration of the bulk liquid and liquid drop, respectively. A m , V m , 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, respec- tively. The enrichment ratio, E, can be defined as The term (A m /V m ) in the right side of eq 2 corresponds to the specific surface area, S d , of the droplets, so eq 2 can also be expressed as 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 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 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, S d . The maximum value of E is affected by all three parameters. The bending point of the curve is shifted to more less C b , and the plateau level of E increases with increasing K. The experimental value of E can be verified with the calculated value, if S d , γ, and K are known. Materials and Methods 1. Materials. Tryptophan (Trp) and phenylalanine (Phe) were purchased from Kanto Chemical Co. Inc. (Tokyo, Japan) and * To whom correspondence should be addressed. Tel.: +81-138-408813. Fax:+81-138-408811. E-mail: maruyama@ elsie.fish.hokudai.ac.jp. C m ) (A m X + V m C b )/V m (1) E ≡ C m /C b ) 1 + (A m /V m )(X/C b ) (2) E ≡ C m /C b ) 1 + S d (X/C b ) (3) X ) KγC b 1 + KC b (4) E ) 1 + S d γ K 1 + KC b (5) 830 Ind. Eng. Chem. Res. 2006, 45, 830-833 10.1021/ie0506771 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005