MULTICOMPONENT ADSORPTION EQUILIBRIA OF AMINO ACIDS ON POLYMER RESINS

WANG GEUN SHIM1, JAE WOOK LEE2, WOO CHUL YANG1 AND HEE MOON1- *

'Faculty of Applied Chemistry, Chonnam National University, Gwangju 500-75, Korea. 2 Department of Chemical Engineering, Seonam University, Namwon 590-170, Korea.

1 Introduction

Amino acids are widely used in many fields such as food, chemical, pharmaceutical, agricultural, medicine and cosmetic industries and are mainly produced by microbial fermentation. To improve the efficiency of recovery, separation, and purification from fermentation broths, several separation techniques have been employed. When the energy efficiency, selectivity, and cost are considered, the adsorption-based separation technique is a promising method for the separation of the amino acids. Several studies have been reported in the literature for amino acid adsorption on various materials including activated carbon, silica, ion exchangers, alumina and polymeric resins. Of these, polymeric sorbents are more attractive because of their regeneration characteristics and have been extensively used for the removal of organic contaminants from dilute aqueous mixtures and air streams and also for the recovery of biochemicals from dilute liquid solutions.

The main objective of the present study was to acquire accurate information on adsorption and desorption behaviors of amino acids on polymeric sorbents, XAD-4 and XAD-16. Desorption can be assumed to be a competitive adsorption between an amino acid and a desorbate. In order to confirm this postulate, several binary equilibrium data between amino acids and organic solvents were obtained and compared with predicted results. For calculation, Extended Langmuir (EL) and Ideal Adsorbed Solution Theory (IAST) based on the Langmuir equation as a single-component isotherm were employed to represent the competitive adsorption equilibria.

2 Experimental

The polymeric adsorbents used were macroreticular and spherical polystyrene resins cross-linked with divinylbenzene XAD-4 and XAD-16 (see Table 1) supplied by Rohm and Haas Co. (USA). Nitrogen adsorption-desorption was measured using an ASAP 2010 volumetric adsorption apparatus (Micrometrics) at 77.4 K. The surface area was calculated by using the BET method. The pore diameter was obtained from the BJH pore size distribution method. L-phenylalanine (Phe) and L-trytopan (Trp), penicillin G (Pen-G) and cephalosporin C (CPC), were used as model amphoteric compounds. Amino acids and Pen-G were from Junsei Co. (Japan), and a sodium CPC was from Cheil Food & Chemicals Inc. (Korea) and were used without further

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purification. Isoproply alchol (IPA) and methanol (purity > 99%) were from Carlo Erba (USA).

Table 1. Properties of polymeric sorbents

Adsorbent Unit XAD-4 XAD-16

Chemical structure - Polystyrene Polystyrene

Particle size urn 490-690 560-710

Moisture holding capacity % 58.27 68.96

Surface area ( BET ) n r g-1 830 920

Average pore diameter nm 4.76 8.54

Equilibrium experiments were carried out at different pH values, temperatures and organic solvents. The experiments were carried out by contacting a given amount of adsorbent with adsorbate solution of 1-25 mol m'3 and organic solvents solution of 200-1,600 mol m"3 in incubator shaking at a constant temperature (298.15 K). The solution pH was adjusted by using HC1 (0.1 mol dm-3) and NaOH (0.1 mol dm"3). After equilibrium was reached, excess amino acids and antibiotics in solutions were analyzed by using UV spectrometry (Varian, model DMS 100s). IPA and methanol concentrations were measured by a GC (Shimadzu, model GC14B), equipped with a hydrogen flame ionization detector. The adsorption capacity of the polymeric adsorbent was determined from material balance.

3 Results and discussion

Adsorption isotherm is the most fundamental and informative data on an adsorption system. It is also very important in model prediction for analyzing and designing an adsorption process. Adsorption onto synthetic polymer sorbent is generally driven by the dispersed force between the adsorbate and the resin. Thus, the adsorption capacity depends on the property of the sorbate. Moreover, the adsorption capacity is also influenced by other factors such as temperature, solution pH, and the amount of impurities contained in the solution. Fig. 1 shows the adsorption isotherms for Phe, Trp, CPC and Pen-G on XAD-4 and XAD-16 at 298.15 K. The adsorption capacity of these sorbates was similar although differences in the surface area and average pore size of XAD-4 and XAD-16 was observed. The adsorption decreased in the order, Pen-G > Trp ) Phe ) CPC according to the hydrophobicity. Grzegorczyk and Carta similarly reported that adsorption capacity is dependent on the hydrophobicity of the sorbate. This means that adsorption is relevant to the dispersion and hydrophobic interactions between the non-ionic polymeric sorbents and the hydrophobic character of sorbates. Also, it is known that the number of hydrophilic groups and their positions may have a negative effect on the adsorption of polymeric adsorbents. The experimental data for Phe, Trp, CPC and Pen-G obtained in the present work were correlated using the Langmuir, Freundlich and Sips isotherm equations. All isotherms were found adequately to describe the adsorption of the two amino acids and the two antibiotics.

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0.8

0.6

JM

0.0

XAD-16 /

• tjT

M jg^2^r^'

O Phe

D Trp

0 CPC

A Pen-G

Langmuir

20 12

C (mol m4)

Fig. 1. Adsorption isotherms of Phe, Trp, CPC and Pen-G on XAD-4 and XAD-16 at 298.15 K.

8 12

C (mol mJ)

16

3.1 Effect of organic solvent and its concentration

From an economic point of view, the success of an adsorption system greatly depends on the efficient regeneration of the sorbent. The choice of desorption method will depend on the physical and chemical characteristics of both sorbates and sorbents. For nonionic polymeric sorbents, the solvent regeneration technique has been considered superior to other methods since the attractive forces binding the solute to the resin surface are physical in nature. Grzegorczyk and Carta suggested that desorption of solutes by the addition of organic solvents is carried out either by the competitive adsorption of the adsorbate surface or by the effect of the organic solvent on the activity of the solute in the fluid phase.

Fig.2 shows the effect of organic solvents of IPA on the adsorption isotherms of Phe and Trp on XAD-16 at 298.15K. In all cases the adsorption amount of Phe and Trp decreased significantly as the alcohol concentration increased due to the differences of hydrogen bonding. The data show that higher concentrations of organic solvent are appropriate for regeneration. However, since the alcohols should be removed after regeneration, lower concentration, if possible, is preferable. These results show that as for pH value, solvent concentration is an important factor for adsorption-desorption processes and chromatographic separation of amino acids on polymeric sorbents.

3.2 Binanry adsorption equilibria

For efficient separation of valuable amino acids from dilute heterogeneous aqueous solutions, it is essential to understand the physical and thermodynamic characteristics of their adsorption and desorption on polymeric sorbents. When dealing with multicomponent adsorption systems, reliable predictions of the adsorption equilibria are required to properly analyze the dynamic behaviors of the adsorptive separation processes. To design and operate adsorption-based separation and purification systems, multicomponent equilibria are generally required over a broad range of composition. Fig. 3 shows the binary adsorption equilibrium data at 298.15 K used in investigating the competitive adsorption of Trp-IPA mixtures. Total concentration 30 mol m"3 of binary mixtures is fixed. Since the surface of a nonionic polymeric sorbent seems to be

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energetically homogeneous, EL and IAST were employed to predict binary equilibrium data. As shown in Fig.3, the solid lines are the result of the EL (thin lines) and the IAST (thick lines) predictions. Both predictions were obtained by using the pure component isotherm parameters of the Langmuir equation. The EL and the IAST predictions are in good agreement with the experimental data. In the cyclic adsorption process, accuracy of multicomponent equilibria and the computation time are important. In a sense, the EL and the IAST models used in this work are adequate and numerically simple forms for describing the competitive adsorption between amino acids and organic solvents.

0.4 ! 1 0.6

0.3

0.2

0.1

0.0

Phe IPA

A 0% A 6% A 10%

A A

' * • * A A • A

~0 .4 •

0.2

0.0

a D

a a

. D

a -i • 3 iQr

a

Trp

a

IPA D 0% • 5% • 10 %

5 10 15 20 C (mol m"3)

25 25 0 5 10 15 20 C (mol m"3)

Fig. 2. Effect of organic solvents on adsorption isotherms of Phe and Trp on XAD-16 at 298.15 K.

0.8

0.6

0.4

0.2

\ / ^ ^ \jr

XX ° Trp

ff\\ a IPA

1.0

0.8 •

0.0 0.2 0.4 0.6 0.8 1.0 0.4 0.6 YTrp y T , p

Fig. 3. Comparison of the EL and IAST predictions with experimental

data for the adsorption of Trp-IPAon XAD-16.

References 1. Grzegorczyk DJ and Carta G, Adsorption of amino acids on porous polymeric

adsorbents - 1 . Equilibrium. Chem Engng Sci 51: 807 - 817 (1996). 2. Yang WC, Shim WG, Lee JW and Moon H, Adsorption and desorption dynamics

of amino acids in a nonionic polymeric sorbent XAD-16 column. Korean J. Chem EnglO: 922-929 (2003).

3. Lee JW, Shim WG, Yang WC and Moon H, Adsorption equilibrium of amino acids and antibiotics on non-ionic polymeric sorbents. J Chem Tech and Biotech accepted for publication (2004).

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