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Equilibrium studies of adsorption of amino acids on NaZSM-5 zeolite
Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61 www.elsevier.com/locate/colsurfa
Equilibrium studies of adsorption of amino acids on NaZSM-5 zeolite E. Titus a, A.K. Kalkar a,*, V.G. Gaikar b a
b
Applied Physics Division, Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India Chemical Engineering Division, Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India Received 26 July 2002; accepted 14 March 2003
Abstract The adsorption of amino acids such as phenylalanine, alanine, tyrosine and tryptophan on highly hydrophobic NaZSM-5 zeolite were studied. Selective adsorption of phenylalanine and tyrosine were observed. The single component adsorption isotherms were fitted in Langmuir model. For the amino acids considered in this work, the isotherms are essentially independent of pH, but they vary significantly with the temperature. Heats of adsorption were obtained from van’t Hoff plots of the Henry’s law constant limit of the Langmuir isotherm and found to be higher for more hydrophobic solute. # 2003 Published by Elsevier B.V. Keywords: Aminoacids; Separation; Adsorption
1. Introduction Adsorption from liquids into molecular sieve zeolites is finding increasing applications in separation and purification processes. The ability of zeolites to adsorb water is known to decrease with decreasing concentration of aluminium in the zeolite frame-work [1 /4]. Because of the hydrophobicity of zeolite, the pore volume of high silica zeolite will be free from water and is accessible for the adsorbate. Therefore, the emergence of hydrophobic zeolites such as silicalite and ZSM-5 finds
* Corresponding author. E-mail addresses: [email protected], [email protected] (A.K. Kalkar). 0927-7757/03/$ - see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0927-7757(03)00131-6
application in the separation of carbohydrates and aminoacids. Here we present the first report of the adsorption of aminoacids on NaZSM-5 zeolite. Aminoacids are commonly separated by chromatographic or electrophoresis methods [5 /9]. Separation by distillation method is difficult due to their thermal instability. Adsorptive separations of amino acids using NaZSM-5 zeolite are advantageous because of the easy availability and high specificity originating from the molecular sieving characteristics of zeolites. Aminoacids have several active functional groups with very different acidities and could thus exhibit complex adsorption behaviour due to its different intermolecular interactions, including Van der waals forces, hydrogen bonding, coulombic interactions as well as the influence of
56
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
different molecule-adsorbent interactions. We have studied the molecule-adsorbent interactions in detail. Adsorption of aminoacids into ion exchange resins [10 /15], polymeric adsorbents [16 /18], and activated carbon [19] are reported earlier. The NaZSM-5 zeolite consists of a threedimensional channel system. The straight channels along the b -axis are defined by 10 membered oxygen rings with an elliptical cross section of ˚ . These channels are 5.7 :/5.8 by 5.1 :/5.2 A interconnected by sinusoidal channels along the a -axis defined also by 10 membered oxygen rings ˚. with nearly circular cross section of 5.49/0.2 A The Si/Al ratio of NaZSM-5 (encilite) used is 80.
2. Experimental The experiments were conducted with L-phenylalanine, L-tryptophan, L-alanine and L-tyrosine. L-Phenylalanine (purity 99%) was supplied by Sigma Chemical Company and all other amino acids were obtained from Sisco Research Laboratories Ltd., Mumbai. NaZSM-5 zeolite of particle size 20 mm was obtained from United Catalysts India Ltd., Baroda. The zeolites were activated at 400 8C for 2 h. Pre-weighed amounts of the zeolite adsorbent (2 g) were then placed in an adsorption cell containing a known volume of solution with a known initial solute concentration (concentrations were calibrated using UV spectrometer). The cells were then kept in locally made shaking mixer with a constant temperature water bath and shaken for a period of 12 h and was found to be sufficient to reach equilibrium. After equilibration, a sample of the liquid was withdrawn, and the residual solute concentration in the liquid phase was analysed using UV spectrophotometer. The batch studies were carried out for the single component at different temperatures and also for the binary mixtures at room temperature of 303 K. Experiments were carried out at different solution pH values, different temperatures and in water. Double distilled water was used for the experiment. A 20 mM each of trichloroacetate, acetate and phosphate buffer was used at pH 3, 6 and 9, respectively.
A UV/Vis spectrophotometer (Perkin Elmer Lamda 3B) with 1 cm path length quartz cuvettes was used to determine the concentration of the amino acids. The absorption maxima for the solutes, L-phenylalanine, L-tyrosine and L-tryptophan occur at the wavelengths 257, 270 and 280 nm, respectively. L-Alanine was analysed using Ninhydrin reaction. For the analysis of mixture we have used peakfit program to resolve the data.
3. Results and discussion Adsorption isotherms were obtained for different solutes at different temperatures, and pH. These isotherms were found to be ideal with Langmuir model: q
qs bC 1 bC
where qs b is the Henry’s law constant. The Langmuir parameters, qs and b were fitted to the data by a non-linear least squares fitting routine (mathcad package). From the batch studies for the single components, L-alanine and L-tryptophan at different pH solution and in water does not exhibit any adsorption, while L-phenylalanine and L-tyrosine exhibit significant values of adsorption on the zeolite NaZSM-5. Because of the small effect of pH, no attempt was made to fit data as a function of pH with different isotherm parameters. It appears that adsorption only slightly dependent on the net charge on the amino acid molecule The mean kinetic diameters of the amino acids (as obtained from Chem-X molecular modeling soft˚ ), L-phenyl alanine (5.8 ware) are L-alanine (4.8 A ˚ ˚ ˚ ). A), L-tyrosine (5.9 A) and L-tryptophan (6.5 A ˚. The channel diameter of NaZSM-5 zeolite is 5.8 A Since the kinetic diameter of tryptophan is higher than the zeolite channel diameter, there is no uptake of this solute by the zeolite. On the other hand, the kinetic diameter of alanine is smaller than the zeolite channel diameter, and hence adsorption was expected on the basis of size selectivity. But there is no adsorption of alanine at all. L-Alanine is more hydrophilic in nature as
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
compared with the other amino acids while the zeolite, which is having Si/Al ratio of 80, is strongly hydrophobic in nature. Thus, no driving force exists for the adsorption of alanine on NaZSM-5 type of zeolites. The preferential solvation of such hydrophilic amino acid like alanine by water excludes alanine from the hydrophobic NaZSM-5 zeolite. L-Phenylalanine and L-tyrosine exhibit, however, significant adsorption on the zeolite. The adsorption data of the two solutes was found to conform well into the Langmuir adsorption isotherm. Fig. 1 gives the Langmuir plots for phenylalanine. A good fit was obtained between the experimental and predicted values. The equilibrium constants and the loading capacities were obtained by fitting the adsorption data in the linear form of the Langmuir equation. The values obtained are reported in Table 1. The equilibrium constant values are very much higher for phenylalanine (five to six times) than tyrosine indicating that the strength of the interaction with the zeolite is comparatively higher for this solute. Phenylalanine is relatively more hydrophobic in nature than tyrosine (due to the presence of an additional /OH group in the case of tyrosine). Hence due to greater hydrophobic interactions between phenylalanine and the zeolite, the equilibrium constant values are higher for this solute. The loading capacity values indicate that the uptake of phenylalanine is almost three times higher than that of tyrosine. Since phenylalanine
57
has a molecular diameter slightly smaller than the channel diameter of the zeolite, the solute can enter into the channels and pack inside the zeolite channels easily. This more closely packed arrangement leads to a higher value of the loading capacity. But in the case of tyrosine, due to the presence of an additional /OH group, probably a greater steric hindrance prevents these molecules from packing closely together inside the channel, resulting in lower uptake values. From the experimental data, it is seen that the amount adsorbed decreases with increase in temperature. Adsorption is an exothermic process and hence with increase in temperature, the extent of adsorption decreases. Van’t Hoffs relation gives the effect of temperature. d ln K DH dT RT 2
(1)
The heats of adsorption estimated using the above relation are reported in Table 1. These low values indicate that the interaction between the zeolite and the solute are physical in nature. The slightly higher value in the case of phenylalanine indicates better interaction of the solute with the zeolite. Direct measurement of the adsorption at the liquid /solid interface is difficult. Hence, it is more usual to calculate the extent of adsorption from an accurate determination of the surface excess. The surface excess approach gives a better idea of the strength of the interaction between a solute and the adsorbent. Surface excess is a measure of the extent to which the bulk liquid is impoverished with respect to one component, because the surface layer is correspondingly enriched, or in other words, the measure of the specific adsorption from solution. If ns is the total adsorbed amount and xs and xl are the mole fractions of component 1 in the adsorbed phase and the bulk phase, respectively, then the surface excess ne can be expressed as: ne ns (xs xl )
(2)
Surface excess can be directly related to experimental measurements as follows: Fig. 1. Langmuir plots for phenylalanine.
ne nt (x0 xl )
(3)
58
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
Table 1 Equilibrium constants and loading capacities for the adsorption of the L-phenylalanine and L-tyrosine on NaZSM-5 zeolite Component
L-Phenylalanine L-Tyrosine
Equilibrium constant (kg mol 1)
Loading capacity (mol kg 1)
30 8C
40 8C
50 8C
30 8C
40 8C
50 8C
7.10 1.52
6.40 1.34
5.01 1.12
0.26 0.082
0.21 0.079
0.19 0.075
Where nt is the total moles of the solution at the beginning and x0 and xl are the mole fractions of the solute initially and at equilibrium, respectively. The specific adsorption from solution is given by the composite isotherm which is the result obtained by combining the individual isotherms for adsorption of each component (plot of the surface excess of component (1) versus the mole fraction of component (1) in liquid phase at equilibrium in a mixture of components (1) and (2)). Fig. 2 gives the surface excess plots for phenylalanine and tyrosine at different temperatures. The surface excess increases with increase in the concentration up to a particular value and thereafter decreases. This decrease in the surface excess values at higher concentrations can be attributed to the increased interaction between the solute molecules leading to a decrease in the interaction between the solute and the zeolite. The surface excess values of phenylalanine is four times greater than the values for tyrosine indicating that the interaction is very strong in the case of phenylalanine. This is in accordance with the equilibrium constant values. Also, the decrease in the surface excess values for phenylalanine occurs at higher concentrations, whereas that for tyrosine occurs at comparatively lower values. The surface excess can be used to estimate selectivity in adsorption of a component from a mixture. The selectivity with respect to the solvent is defined by a separation factor (ae ), which is determined from the surface excess using the Everett’s equation [20]. Everett’s equation is given by: xl1 xl2 xl1 1 ne1 ns ns (ae 1)
(4)
Where x1 and x2 are the mole fractions of solute
Heat of adsorption (kJ mol 1)
5.04 4.12
Fig. 2. Surface excess plots for (a) phenylalanine and (b) tyrosine.
and solvent at equilibrium in the liquid phase. ns are the adsorbed phase concentration. This equation is valid only when the following assumptions are satisfied:
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
1) The adsorption is of Langmuir type. 2) The separation factor remains constant over the entire concentration range. 3) The ratio of the activity coefficients can be approximated to unity. 4) The adsorbed components have similar molecular sizes in the adsorbed phase or the selective component covers the surface even at low concentrations. The separation factor for the adsorption of the solutes from water was calculated using the Everett’s equation. Fig. 3 gives the Everett’s plot for the adsorption of phenylalanine and tyrosine on NaZSM-5 zeolite. Table 2 gives the separation
Table 2 Separation factor of the solute with respect to water (solvent) Component
L-Phenylalanine L-Tyrosine
Separation factor 30 8C
40 8C
50 8C
453.20 411.32
437.40 354.42
319.14 256.39
factor values obtained from these plots. The separation factor values obtained were high for both the solutes with phenylalanine showing a slightly higher value indicating extremely high affinity of these solutes towards the solid zeolite phase as compared with the solvent, water. Based on the single component studies, the mixture studies were carried out at room temperature of 30 8C. For an ideal behaviour in the liquid phase and in the adsorbed phase, the experimental separation factor can also be defined by the following expression: a12
Fig. 3. Everett’s plot for the adsorption of (a) phenylalanine and (b) tyrosine.
59
xs1 =xl1 xs2 =xl2
(5)
where x ’s are pseudo mole fractions of the adsorbing species. For mixtures of alanine/phenylalanine, alanine/ tyrosine, phenylalanine/tryptophan and tyrosine/ tryptophan, only phenylalanine and tyrosine were adsorbed from their respective mixtures with absolutely no adsorption of either alanine or tryptophan. In the case of the mixture of phenylalanine/ tyrosine, phenylalanine was selectively adsorbed on the zeolite. Fig. 4 shows the surface excess plot for the adsorption of phenylalanine from its mixture with tyrosine and water. This plot clearly shows that phenylalanine was selectively adsorbed on the zeolite. The separation factor is strongly dependent on the composition of the mixture as shown (Fig. 5). For a typical Langmuir adsorbed components, the separation factor should be constant and independent of the composition. Since separation factor is not constant, it is possible that the adsorbed phase may show a non-ideal behaviour. The interaction energy between two different adsorbed molecules
60
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
where (cW/2kT ) is a measure of the interaction between the adsorbed species. A plot of ln(a12) versus (1 /2xs1) gives a straight line with slope (cW /2kT ) which can be taken as the measure of the interaction energy. A zero slope indicates no interaction between the adsorbed molecules. The non-zero slope of the plots is the indication of strong interaction among the adsorbed species. The values of the interaction energies, however, could not be obtained since the experimental values were found not to fit the above equation linearly.
4. Conclusion
Fig. 4. Surface excess plot for the adsorption of phenylalanine from its mixture with tyrosine and water.
Single component isotherm study of alanine, phenylalanine, tyrosine and tryptophan on NaZSM-5 shows no adsorption of alanine and tryptophan. There was preferential adsorption of phenylalanine and tyrosine. High separation factors with respect to water shows no adsorption of water. Higher equilibrium constant and heat of adsorption for phenylalanine in comparison with tyrosine indicates that adsorption is because of hydrophobic interactions. The low value of heat of adsorption shows physical adsorption. From the mixture studies, there was selective adsorption of phenylalanine over tyrosine due to differences in their hydrophobic nature. Separation factors were high and there was interaction between the adsorbed solute molecules.
References
Fig. 5. Separation factor for the mixture of phenylalanine and tyrosine.
can be obtained from the following equation [21]: K o ln a12 (1 2xs1 )
cW 0 2kT
(6)
[1] E.M. Flanigen, E.M. Beent, R.W. Grose, J.P. Cohen, R.L. Patten, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512. [2] C.G. Pope, J. Colloid Interf. Sci. 116 (1987) 221. [3] D.H. Olson, W.O. Haag, M.R. Lago, J. Catal. 61 (1980) 390. [4] H. Nakamoto, H. Takakashi, Zeolites 2 (1982) 67. [5] P. Jander, V. Backovska, A. Felinger, J. Chromatogr. 919 (2001) 67. [6] M. Estami, P. Hashmi, M.N. Sarbolouk, J. Chromatogr. 31 (1993) 480. [7] A. Berthod, T.L. Xiao, Y. Liu, W.S. Jenks, D.W. Armstrong, J. Chromatogr. A 955 (2002) 53. [8] E. Skocir, J. Vinde Vogel, P. Sandra, J. Chromatogr. 39 (1994) 7.
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61 [9] M. Kresak, E.C. Moreno, R.T. Zanradnik, D.I. Hay, J. Colloid Interf. Sci. 59 (1977) 283. [10] M. Saunders, J. Virow, G. Carta, AIChE J. 35 (1989) 53. [11] L. Hanak, T. Szanya, G. Simon, Ninth International Symposium on Preparative and Industrial Chromatography, Nancy, 1992, p. 185. [12] A. Kitakawa, T. Yonemoto, T. Tadaki, Trans. IChem E. 72 (C) (1994) 201. [13] G. Simon, L. Hanak, G. Grevillot, T. Szanya, G. Marton, J. Chromatogr. 664 (1995) 17. [14] G. Simon, L. Hanak, G. Grevillot, T. Szanya, G. Marton, Chem. Eng. Sci. 52 (1997) 467. [15] Q.J. Yu, N.-H.L. Wang, 6 (1987) 33.
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[16] D.S. Gizegorczyck, G. Carta, Chem. Eng. Sci. 51 (1996) 807. [17] D.S. Gizegorczyck, G. Carta, Chem. Eng. Sci. 51 (1996) 819. [18] S. Diez, A. Leitao, L. Ferreira, A. Rodrigues, Sep. Purif. Technol. 13 (1998) 25. [19] O. Ferrandon, H. Bouabane, Comptes Rendus de l’Acade´mie des Sciences-Series IIC-Chemistry 4 (2001) 12. [20] D.H. Everett, J. Chem. Soc. Faraday Trans. 60 (1964) 1803. [21] S. Ihm, H. Lee, in: Y. Murakui (Ed.), Elsevier, Amsterdam, 1986, pp. 571.
Equilibrium studies of adsorption of amino acids on NaZSM-5 zeolite E. Titus a, A.K. Kalkar a,*, V.G. Gaikar b a
b
Applied Physics Division, Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India Chemical Engineering Division, Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India Received 26 July 2002; accepted 14 March 2003
Abstract The adsorption of amino acids such as phenylalanine, alanine, tyrosine and tryptophan on highly hydrophobic NaZSM-5 zeolite were studied. Selective adsorption of phenylalanine and tyrosine were observed. The single component adsorption isotherms were fitted in Langmuir model. For the amino acids considered in this work, the isotherms are essentially independent of pH, but they vary significantly with the temperature. Heats of adsorption were obtained from van’t Hoff plots of the Henry’s law constant limit of the Langmuir isotherm and found to be higher for more hydrophobic solute. # 2003 Published by Elsevier B.V. Keywords: Aminoacids; Separation; Adsorption
1. Introduction Adsorption from liquids into molecular sieve zeolites is finding increasing applications in separation and purification processes. The ability of zeolites to adsorb water is known to decrease with decreasing concentration of aluminium in the zeolite frame-work [1 /4]. Because of the hydrophobicity of zeolite, the pore volume of high silica zeolite will be free from water and is accessible for the adsorbate. Therefore, the emergence of hydrophobic zeolites such as silicalite and ZSM-5 finds
* Corresponding author. E-mail addresses: [email protected], [email protected] (A.K. Kalkar). 0927-7757/03/$ - see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0927-7757(03)00131-6
application in the separation of carbohydrates and aminoacids. Here we present the first report of the adsorption of aminoacids on NaZSM-5 zeolite. Aminoacids are commonly separated by chromatographic or electrophoresis methods [5 /9]. Separation by distillation method is difficult due to their thermal instability. Adsorptive separations of amino acids using NaZSM-5 zeolite are advantageous because of the easy availability and high specificity originating from the molecular sieving characteristics of zeolites. Aminoacids have several active functional groups with very different acidities and could thus exhibit complex adsorption behaviour due to its different intermolecular interactions, including Van der waals forces, hydrogen bonding, coulombic interactions as well as the influence of
56
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
different molecule-adsorbent interactions. We have studied the molecule-adsorbent interactions in detail. Adsorption of aminoacids into ion exchange resins [10 /15], polymeric adsorbents [16 /18], and activated carbon [19] are reported earlier. The NaZSM-5 zeolite consists of a threedimensional channel system. The straight channels along the b -axis are defined by 10 membered oxygen rings with an elliptical cross section of ˚ . These channels are 5.7 :/5.8 by 5.1 :/5.2 A interconnected by sinusoidal channels along the a -axis defined also by 10 membered oxygen rings ˚. with nearly circular cross section of 5.49/0.2 A The Si/Al ratio of NaZSM-5 (encilite) used is 80.
2. Experimental The experiments were conducted with L-phenylalanine, L-tryptophan, L-alanine and L-tyrosine. L-Phenylalanine (purity 99%) was supplied by Sigma Chemical Company and all other amino acids were obtained from Sisco Research Laboratories Ltd., Mumbai. NaZSM-5 zeolite of particle size 20 mm was obtained from United Catalysts India Ltd., Baroda. The zeolites were activated at 400 8C for 2 h. Pre-weighed amounts of the zeolite adsorbent (2 g) were then placed in an adsorption cell containing a known volume of solution with a known initial solute concentration (concentrations were calibrated using UV spectrometer). The cells were then kept in locally made shaking mixer with a constant temperature water bath and shaken for a period of 12 h and was found to be sufficient to reach equilibrium. After equilibration, a sample of the liquid was withdrawn, and the residual solute concentration in the liquid phase was analysed using UV spectrophotometer. The batch studies were carried out for the single component at different temperatures and also for the binary mixtures at room temperature of 303 K. Experiments were carried out at different solution pH values, different temperatures and in water. Double distilled water was used for the experiment. A 20 mM each of trichloroacetate, acetate and phosphate buffer was used at pH 3, 6 and 9, respectively.
A UV/Vis spectrophotometer (Perkin Elmer Lamda 3B) with 1 cm path length quartz cuvettes was used to determine the concentration of the amino acids. The absorption maxima for the solutes, L-phenylalanine, L-tyrosine and L-tryptophan occur at the wavelengths 257, 270 and 280 nm, respectively. L-Alanine was analysed using Ninhydrin reaction. For the analysis of mixture we have used peakfit program to resolve the data.
3. Results and discussion Adsorption isotherms were obtained for different solutes at different temperatures, and pH. These isotherms were found to be ideal with Langmuir model: q
qs bC 1 bC
where qs b is the Henry’s law constant. The Langmuir parameters, qs and b were fitted to the data by a non-linear least squares fitting routine (mathcad package). From the batch studies for the single components, L-alanine and L-tryptophan at different pH solution and in water does not exhibit any adsorption, while L-phenylalanine and L-tyrosine exhibit significant values of adsorption on the zeolite NaZSM-5. Because of the small effect of pH, no attempt was made to fit data as a function of pH with different isotherm parameters. It appears that adsorption only slightly dependent on the net charge on the amino acid molecule The mean kinetic diameters of the amino acids (as obtained from Chem-X molecular modeling soft˚ ), L-phenyl alanine (5.8 ware) are L-alanine (4.8 A ˚ ˚ ˚ ). A), L-tyrosine (5.9 A) and L-tryptophan (6.5 A ˚. The channel diameter of NaZSM-5 zeolite is 5.8 A Since the kinetic diameter of tryptophan is higher than the zeolite channel diameter, there is no uptake of this solute by the zeolite. On the other hand, the kinetic diameter of alanine is smaller than the zeolite channel diameter, and hence adsorption was expected on the basis of size selectivity. But there is no adsorption of alanine at all. L-Alanine is more hydrophilic in nature as
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
compared with the other amino acids while the zeolite, which is having Si/Al ratio of 80, is strongly hydrophobic in nature. Thus, no driving force exists for the adsorption of alanine on NaZSM-5 type of zeolites. The preferential solvation of such hydrophilic amino acid like alanine by water excludes alanine from the hydrophobic NaZSM-5 zeolite. L-Phenylalanine and L-tyrosine exhibit, however, significant adsorption on the zeolite. The adsorption data of the two solutes was found to conform well into the Langmuir adsorption isotherm. Fig. 1 gives the Langmuir plots for phenylalanine. A good fit was obtained between the experimental and predicted values. The equilibrium constants and the loading capacities were obtained by fitting the adsorption data in the linear form of the Langmuir equation. The values obtained are reported in Table 1. The equilibrium constant values are very much higher for phenylalanine (five to six times) than tyrosine indicating that the strength of the interaction with the zeolite is comparatively higher for this solute. Phenylalanine is relatively more hydrophobic in nature than tyrosine (due to the presence of an additional /OH group in the case of tyrosine). Hence due to greater hydrophobic interactions between phenylalanine and the zeolite, the equilibrium constant values are higher for this solute. The loading capacity values indicate that the uptake of phenylalanine is almost three times higher than that of tyrosine. Since phenylalanine
57
has a molecular diameter slightly smaller than the channel diameter of the zeolite, the solute can enter into the channels and pack inside the zeolite channels easily. This more closely packed arrangement leads to a higher value of the loading capacity. But in the case of tyrosine, due to the presence of an additional /OH group, probably a greater steric hindrance prevents these molecules from packing closely together inside the channel, resulting in lower uptake values. From the experimental data, it is seen that the amount adsorbed decreases with increase in temperature. Adsorption is an exothermic process and hence with increase in temperature, the extent of adsorption decreases. Van’t Hoffs relation gives the effect of temperature. d ln K DH dT RT 2
(1)
The heats of adsorption estimated using the above relation are reported in Table 1. These low values indicate that the interaction between the zeolite and the solute are physical in nature. The slightly higher value in the case of phenylalanine indicates better interaction of the solute with the zeolite. Direct measurement of the adsorption at the liquid /solid interface is difficult. Hence, it is more usual to calculate the extent of adsorption from an accurate determination of the surface excess. The surface excess approach gives a better idea of the strength of the interaction between a solute and the adsorbent. Surface excess is a measure of the extent to which the bulk liquid is impoverished with respect to one component, because the surface layer is correspondingly enriched, or in other words, the measure of the specific adsorption from solution. If ns is the total adsorbed amount and xs and xl are the mole fractions of component 1 in the adsorbed phase and the bulk phase, respectively, then the surface excess ne can be expressed as: ne ns (xs xl )
(2)
Surface excess can be directly related to experimental measurements as follows: Fig. 1. Langmuir plots for phenylalanine.
ne nt (x0 xl )
(3)
58
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
Table 1 Equilibrium constants and loading capacities for the adsorption of the L-phenylalanine and L-tyrosine on NaZSM-5 zeolite Component
L-Phenylalanine L-Tyrosine
Equilibrium constant (kg mol 1)
Loading capacity (mol kg 1)
30 8C
40 8C
50 8C
30 8C
40 8C
50 8C
7.10 1.52
6.40 1.34
5.01 1.12
0.26 0.082
0.21 0.079
0.19 0.075
Where nt is the total moles of the solution at the beginning and x0 and xl are the mole fractions of the solute initially and at equilibrium, respectively. The specific adsorption from solution is given by the composite isotherm which is the result obtained by combining the individual isotherms for adsorption of each component (plot of the surface excess of component (1) versus the mole fraction of component (1) in liquid phase at equilibrium in a mixture of components (1) and (2)). Fig. 2 gives the surface excess plots for phenylalanine and tyrosine at different temperatures. The surface excess increases with increase in the concentration up to a particular value and thereafter decreases. This decrease in the surface excess values at higher concentrations can be attributed to the increased interaction between the solute molecules leading to a decrease in the interaction between the solute and the zeolite. The surface excess values of phenylalanine is four times greater than the values for tyrosine indicating that the interaction is very strong in the case of phenylalanine. This is in accordance with the equilibrium constant values. Also, the decrease in the surface excess values for phenylalanine occurs at higher concentrations, whereas that for tyrosine occurs at comparatively lower values. The surface excess can be used to estimate selectivity in adsorption of a component from a mixture. The selectivity with respect to the solvent is defined by a separation factor (ae ), which is determined from the surface excess using the Everett’s equation [20]. Everett’s equation is given by: xl1 xl2 xl1 1 ne1 ns ns (ae 1)
(4)
Where x1 and x2 are the mole fractions of solute
Heat of adsorption (kJ mol 1)
5.04 4.12
Fig. 2. Surface excess plots for (a) phenylalanine and (b) tyrosine.
and solvent at equilibrium in the liquid phase. ns are the adsorbed phase concentration. This equation is valid only when the following assumptions are satisfied:
E. Titus et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 55 /61
1) The adsorption is of Langmuir type. 2) The separation factor remains constant over the entire concentration range. 3) The ratio of the activity coefficients can be approximated to unity. 4) The adsorbed components have similar molecular sizes in the adsorbed phase or the selective component covers the surface even at low concentrations. The separation factor for the adsorption of the solutes from water was calculated using the Everett’s equation. Fig. 3 gives the Everett’s plot for the adsorption of phenylalanine and tyrosine on NaZSM-5 zeolite. Table 2 gives the separation
Table 2 Separation factor of the solute with respect to water (solvent) Component
L-Phenylalanine L-Tyrosine
Separation factor 30 8C
40 8C
50 8C
453.20 411.32
437.40 354.42
319.14 256.39
factor values obtained from these plots. The separation factor values obtained were high for both the solutes with phenylalanine showing a slightly higher value indicating extremely high affinity of these solutes towards the solid zeolite phase as compared with the solvent, water. Based on the single component studies, the mixture studies were carried out at room temperature of 30 8C. For an ideal behaviour in the liquid phase and in the adsorbed phase, the experimental separation factor can also be defined by the following expression: a12
Fig. 3. Everett’s plot for the adsorption of (a) phenylalanine and (b) tyrosine.
59
xs1 =xl1 xs2 =xl2
(5)
where x ’s are pseudo mole fractions of the adsorbing species. For mixtures of alanine/phenylalanine, alanine/ tyrosine, phenylalanine/tryptophan and tyrosine/ tryptophan, only phenylalanine and tyrosine were adsorbed from their respective mixtures with absolutely no adsorption of either alanine or tryptophan. In the case of the mixture of phenylalanine/ tyrosine, phenylalanine was selectively adsorbed on the zeolite. Fig. 4 shows the surface excess plot for the adsorption of phenylalanine from its mixture with tyrosine and water. This plot clearly shows that phenylalanine was selectively adsorbed on the zeolite. The separation factor is strongly dependent on the composition of the mixture as shown (Fig. 5). For a typical Langmuir adsorbed components, the separation factor should be constant and independent of the composition. Since separation factor is not constant, it is possible that the adsorbed phase may show a non-ideal behaviour. The interaction energy between two different adsorbed molecules
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where (cW/2kT ) is a measure of the interaction between the adsorbed species. A plot of ln(a12) versus (1 /2xs1) gives a straight line with slope (cW /2kT ) which can be taken as the measure of the interaction energy. A zero slope indicates no interaction between the adsorbed molecules. The non-zero slope of the plots is the indication of strong interaction among the adsorbed species. The values of the interaction energies, however, could not be obtained since the experimental values were found not to fit the above equation linearly.
4. Conclusion
Fig. 4. Surface excess plot for the adsorption of phenylalanine from its mixture with tyrosine and water.
Single component isotherm study of alanine, phenylalanine, tyrosine and tryptophan on NaZSM-5 shows no adsorption of alanine and tryptophan. There was preferential adsorption of phenylalanine and tyrosine. High separation factors with respect to water shows no adsorption of water. Higher equilibrium constant and heat of adsorption for phenylalanine in comparison with tyrosine indicates that adsorption is because of hydrophobic interactions. The low value of heat of adsorption shows physical adsorption. From the mixture studies, there was selective adsorption of phenylalanine over tyrosine due to differences in their hydrophobic nature. Separation factors were high and there was interaction between the adsorbed solute molecules.
References
Fig. 5. Separation factor for the mixture of phenylalanine and tyrosine.
can be obtained from the following equation [21]: K o ln a12 (1 2xs1 )
cW 0 2kT
(6)
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