NOTES Adsorption Behavior of Some Amino Acids on Chemically Pretreated Alumina, 11 INTRODUCTION Chromatographic aluminas normally possess high concentrations of surface hydroxyls (1), and a few studies (25) have shown that chemical pretreatment of chromatographic alumina evokes its ion-exchange nature. It is known that amino acids are ampholytes, and change of pH beyond their isoelectric point in aqueous solution has a signifcant effect on the net charge. This, in turn, can play an important role in their adsorption behavior on polar adsorbents, like alumina. Few empirical studies (6) have shown the significance of pH in the adsorption of amino acids on alumina. Aspartic acid was chosen for a close study. EXPERIMENTAL Alumina samples ofpH 1.0-11.0 were prepared as described earlier (2, 3). From the stock solution of aspartic acid (BDH) in water (1.0 × 10-3 M), solutions of 5 to 80 ml were withdrawn, diluted to 100 ml, and estimated colorimetrically at 570 nm (7). Adsorption experiments were conducted by equilibrating alumina (0.10 g)with aqueous amino acid solutions (10 ml) of known concentration. The variation of adsorption as a function of time (10 min-6 hr) was studied with the substrates o f p H 2.010.0 at 30°C. In 10 min only, 70-80% adsorption occurs. However, the equilibrium is almost attained in 4 hr. The temperature effect studies (30-60°C) with alumina (pH 2.0-10.0) show the process to be almost athermic. The process is found to be pH-dependent (Figs. 1 and 2); it increases in the pH range 1.0 - 4.5, maximum at pH 4.5 and then decreases in the range ofpH 4.5-11.0. The isotherms are of L-type with a tendency to assume S-shape at pH >/7.0. The influence of electrolytes (NaC1, Na2SO4, Na3PO4) on the degree of adsorption was studied by taking alumina of pH 4.0-10.0. The concentration of the solute and the electrolytes was thesame, i.e., 1.0 × 10-3 and 2.0 × 10-4 M. It is observed that adsorption decreases significantly (PO3- > SO~- > C1-). Desorption of the amino acid adsorbed (pH 4.0-10.0, 30°C, 6 hr) from aqueous solutions, was studied with aqueous inorganic electrolytes (0.1-0.001 M) NaCI, CH3COONa, Na2SO4, Na2C204, and Na3PO4). The studies reveal that (PO43- (0.1-0.01 M) desorbs the amino acids
Presented at the 69th Session of the "Indian Science Congress", (3-8th Jan. 1982), The Abstract, IV, 86, p. 177. Mysore, India.
quantitatively. The desorption efficacy is in the order (PO43- > C2042- > 802- :> CH3COO- > C1-. It is also observed that desorption gradually increases with increase in the pH of alumina and concentration of the electrolyte. RESULTS AND DISCUSSION Fuller (8) attributes the apparent ion-exchange capacity of alumina due to amphoteric reaction of hydroxyl groups, contained in the structure of alumina as shown below >AI--OH ~ >AI + + OHI L >AIO- + H + H+
>A1--OH ~ >A1 - OHm OH- 1L >A10- + H20 It is clear that the upper reactions producing anions, are favored at low pH; whereas the lower reactions result in cation-exchange characteristics. Aspartic acid is monoamino dicarboxylic acid with pKI = 1.88, pK2 = 3.65 (--COOH), and pK3 = 9.66 (--NH~) at 25°C. The isoelectric point (pI) is at pH 2.77. Hence at pH >/ 3.0, it will be negatively charged [-OOC-CH2.CH(NH3) +. COO-]. The anionic form of amino acid in its favorable range of adsorption (pH 3.0-8.0) appears to be the principal adsorbing species. At pH < 4.5, concentration of the chloride ions on the substrate surface increases, and the ionic species responsible for adsorption decrease. The behavior is so prominent, that the acid has only 100 adsorption on alumina at pH 1.0. At pH > 4.5 the proportion of exchangable chloride decreases and hence the adsorption goes on decreasing gradually. At pH > 9.5, the adsorption decreases significantly, which may be due to change in the nature of species. The pK3 value is 9.66 (--NH~-). It appears that the rapid exchange of the solute anions occurs at pH ~< 7.0 followed by a slow exchange at sites within the alumina. Ion-exchange nature of adsorption is also indicated by athermic nature of the process and fast adsorption. The isotherms are of I., and S-type. The L-type curves below pH 7.0 indicate monolayer and stronger affinity of the solute for the substrate. The S-type isotherms of the acid on alumina ofpH 7.0-11.0 show a change in the nature of species being adsorbed. However, at pH >/7.0, the adsorbate-adsorbent interaction becomes prominent resulting in cooperative adsorption and hence, the S-type isotherms (9, 10). The abrupt change in the nature of curves at pH 3.0-2.0 (Fig. i) and pH 9.5-10.0 (Fig. 2) also points to change in the nature of solute species. This is further indicated by the PK1, pK2, and pK3 values of the solute which lie close to those pH values.
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Journal of Colloid and Interface Science. Vol. 97, No. 2, February 1984
I00 I~H = 4.5
~ H = 4,O
pH = 3 " 0
h H = 2.O
< 3o 20 I~H=
End Concn (M} X IO S
FIG. l . Change i n the n a t u r e o f a d s o r p t i o n o f aspartic acid f r o m aqueous solutions o n H C l - t r e a t e d
alumina of varying pH. Time: 6 hr, temperature: 30°C.
I00 PH = 5 . 0 PH = 6 " 0 PH = 7 " 0 PH = 8"0
T % x o~
PH = 9.0 r
PH = 9"5
PH= I 0 " 0 P H : U.O
Concn ( M ) X I O S ,,
FIG. 2. Change in the nature of adsorption' of aspartic acid from aqueous solutions on HC1- and NaOHtreated alumina of varying pH. Time: 6 hr, temperature: 30°C. Journal of Colloid and Interface Science, Vol. 97, No. 2, February 1984
We thank Professor S. G. Tandon of Ravishankar Univelsity for his valuable suggestions and laboratory facilities, and to the CSIR, New Delhi, for awarding fellowship to one of the authors (S.M.). REFERENCES 1. Snyder, L. R., "Principles of Adsorption Chromatography," p. 165. Dekker, New York, 1968. 2. Mundhara, G. L., and Tiwari, J. S., J. Ind. Chem. Soc. 55, 1032 (1978). 3. Tiwari, M. P., Tiwari, J. S., and Mundhara, G. L., J. Ind. Chem. Soc. 56, 798 (1979); 57(3), 306 (1980); 57(4), 404 (1980). 4. Giles, C. H., and Datye, K. V., Trans. Inst. Met. Finishing 40, 113 (1963); 57, 48 (1979). 5. Mundhara, G. L., and Tiwari, J. S., J. Ind. Chem. Soc. 56, 737 (1979). 6. Lederer, E., and Lederer, M., "Chromatography," pp. 291-296, 2nd ed. Elsevier, Amsterdam/New York, 1967.
Journal of Colloid and Interface Science, Vol. 97, No. 2, February 1984
7. Snell, F. D., and Snell, C. T., "Colorimetric Methods of Analysis," Vol. IVA, pp. 216-225. Van Nostrand, Princeton, N. J., 1967. 8. Fuller, M. J., Chromatogr. Rev. 14, 45 (1971). 9. Giles, C. H., Smith, D., and Huitson, A., J. Colloid Interface Sci. 47(3), 755 (1974). 10. Giles, C. H., Anthony, P., D'Silva, and Easton, I. A., J. Colloid Interface Sci. 47(3), 766 (1974). SHIPRA MOITRA
G. L. MUNDHARA2 J. S. TIWARI3 RAMESH K . M~SnRA
Department of Chemistry Ravishankar University Raipur 492 010 (M.P.), India Received July 27, 1982; accepted July 15, 1983 2 To whom correspondence should be addressed. 3 Present address: Department of Chemistry, Government Girls' College, Raipur (M.P.), India.