4486 Znd. Eng. zyxwvutsr Chem. Res. 1995,34, zyxwvu 4486-4493 Evaluation of Kinetic and Thermodynamic Parameters of Amino Acids on Modified zyxw Divinylbenzene-Polystyrene Resins Using a Liquid Chromatography Technique Mercedes Martinez, Alejandro Carrancio, Jose Luis Casillas, and Jose Aracil* Departamento de Zngenieria Quimica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain Liquid chromatography has been used to evaluate the sorption and diffusion characteristics of DL-tryptophan, DL-phenylalanine, L-DOPA, DL-glUtamiC acid, and DL-lySine in modified divinyl- benzene-polystyrene resins. All amino acids adsorbed quite rapidly, but there were significant differences in both equilibrium constants and diffusivities. On the modified resins, the values of equilibrium constants decreased with decreasing molecular weight. Diffisivity values increased with temperature and decreased with molecular weight for both resins. 1. Introduction There is a high demand for biologically-produced molecules. Amino acids, peptides, and proteins'play a very important role in industry and are among the main products of biotechnology. These compounds often have an extremely high value and must be produced with considerable purity. Nowadays selective fermentation with genetically selected bacterial strains is the main production route. Fermentation products generally undergo a sequence of separation steps in which cell mass and high molecular weight compounds are re- moved first and more difficult separations are carried out last. The industrial-scale separation and purification of fermentation products using modified resins have been previously examined (Aracil et al., 1992). The adsorp- tion capacity and selectivity of the resin were modified by changing the chemical properties and structure of the polymer surface. A particular type of these resins was selected from chromatography studies on their adsorption properties. The present study reports the extension of the liquid chromatography previously developed for the study of liquid adsorption and diffusion in divinylbenzene- polystyrene materials using the moment method. The use of the chromatographic method makes it possible to determine both thermodynamic and kinetic transport parameters. From the first moment the following parameters can be obtained: adsorption equilibrium constant and heat of adsorption. From the second moment the pore mass transfer coefficient is calculated. The chromatographic method has been applied to adsorbate-adsorbent systems that produced elution peaks with a shape close to Gaussian and with only a small degree of tailing. The parameters obtained this way allow one to simplify the rigorous methods to design the purification operation in both batch and fmed-bed processes. As a result, the computing time necessary for the calculations is considerably reduced. For highly asymmetric peaks, the method of moments has not been applied because the evaluation of the second moment is subject to significant error. There are two main objectives in the present study. First objective is to further verify the applicability and reliability of the liquid chromatography in the deter- * Author to whom correspondence is addressed. Fax: 34.1.13944114. E-mail: JAMl@EUCMAX.SIM.UCM.ES. 0888-5885l95I2634-4486~Q9.QQlQ mination of kinetic and thermodynamic parameters of adsorption in two different porous adsorbents. The second aim of this work is to investigate the effect of both the chemical nature of the liquid adsorbate and the physical properties of the porous adsorbent on the adsorption parameters. We have studied the thermodynamics and kinetics of adsorption of DL-tryptophan,DL-phenylalanine,L-DOPA, DL-glutamic acid, and DL-lysine. An evaluation was carried out to investigate the influence of polarity on the process over modified Amberlite XAD-2 and XAD-4 resins, XAD-2-1 and XAD-4-1 (Aracil et al., 1992). 2. Experimental Section 2.1. Apparatus. A liquid chromatography metering pump (Perkin-Elmer LC-100 column oven) provided a steady flow of eluent through a liquid sampling valve (Rheodyne), and hence to the adsorbent column and Perkin-Elmer LC-75 spectrophotometric detector with variable W wavelength equipped with flow cells. The high-pressure pump was equipped with flow controllers and a digital flow meter, which could provide flow rates up to 5 cm3 min-l at pressures up to 1.0 x lo7 Pa. 2.2. Column Preparation and Temperature Con- trol. The XAD-2 and XAD-4 modified resins, XAD-2-1 and XAD-4-1, synthesized in our laboratory by halogen- ation of commercial divinylbenzene-polystyrene resins and the experimental procedure have been previously described (Martinez et al., 1988). The resins were ground and wet-sieved to obtain suitable portions of particles of diameter between 30 and 45 pm. The resin thus prepared was slurried in neutrally dense acetic acid in a high-pressure vessel and packed into a column (20 x 0.22 cm) by sweeping the slurry in a stream of high-pressure distilled deaerated water into the column containing a 4 ,um frit at one end. The resin temperature was controlled by the column oven. To ensure that the eluent temperature at the bed entrance did not differ significantly from the bed tem- perature, the eluent was routed through a preheat tube, held in a constant temperature water bath upstream of the injection valve. 2.3. Reagents and Buffer. The Amberlite XAD-2 and XAD-4 were supplied by Rohm and Haas Co. The monomer trapped in the resin pores was removed by Soxhlet extraction using methanol as a solvent. The eluent of controlled pH (3.0) and ionic strength (0.1 M) was prepared by dissolving known quantities 0 1995 American Chemical Society