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On Irreversible Adsorption of Electron-Donating Compounds in Aqueous Solution
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
177, 384–390 (1996)
0048
On Irreversible Adsorption of Electron-Donating Compounds in Aqueous Solution HAJIME TAMON, 1 MASANORI ATSUSHI,
AND
MORIO OK AZAKI
Department of Chemical Engineering, Kyoto University, Kyoto 606-01, Japan Received March 23, 1995; accepted June 9, 1995
In the adsorption of electron-donating compounds such as phenol, aniline, L-phenylalanine, and L-tyrosine from aqueous solution, irreversibility was observed on activated carbon and graphite. The compounds, except L-tyrosine, were reversibly adsorbed on a synthetic adsorbent. In the case where the carbonaceous adsorbents contacted the aqueous solution containing electron-donating compounds for a long time, the irreversible amount adsorbed increased with the contact time. A two-state adsorption model was used to explain why the irreversible adsorption of electron-donating compound appears in aqueous solution. First, the compound is adsorbed in the precursor state for irreversible adsorption, and then moves into its irreversible state over a potential energy barrier after a long contact time. The appearance of irreversible adsorption was qualitatively explained by the two-state adsorption model.
Grant and King have studied irreversible adsorption of phenolic compounds on activated carbon (12). They have determined that oxidative coupling of phenolic compounds on the carbon surface is a plausible explanation for irreversible adsorption. However, the appearance of irreversible adsorption for other electron-donating compounds has not yet been studied. In this article, we measure adsorption and desorption characteristics of some aromatic compounds in aqueous solution on carbonaceous and synthetic adsorbents. We apply a twostate adsorption model for chemisorption to the irreversible adsorption in aqueous solution, and discuss why irreversible adsorption appears.
q 1996 Academic Press, Inc.
Key Words: irreversible adsorption; aqueous solution; electrondonating compound; activated carbon; graphite; two-state adsorption model.
INTRODUCTION
Activated carbons and synthetic adsorbents have been used for liquid purification and wastewater treatment (1– 8). The feasibility of an adsorption process depends greatly on the cost of regeneration of spent adsorbents. It is well known that regeneration of activated carbons loaded with phenolic compounds is difficult. Although the adsorption capacity of synthetic adsorbents is generally less than that of carbons, they are advantageous for certain application because their regeneration is easy. Solvent regeneration of spent activated carbon has been studied (9–11). We have reported that the regeneration efficiency depends on substituent groups of aromatic compounds, and that electron-donating compounds are difficult to be desorbed from the carbon by use of ethanol (10). If irreversible adsorption occurs, regeneration of spent adsorbent is very difficult. Hence, it is very important to understand why irreversible adsorption appears in aqueous solution. 1
To whom correspondence should be addressed.
0021-9797/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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MATERIALS AND METHODS
Materials Activated carbon (CAL) was supplied by Calgon Co. Ltd., USA; its specific surface area is 9.27 1 10 5 m2 /kg. Synthetic adsorbent (HP-21) was produced by Mitsubishi Chemical Corp., Japan, and is a styrene divinylbenzene polymer. The specific surface area of the adsorbent is 5.83 1 10 5 m2 /kg. Graphite was supplied by Wako Pure Chemical Industries, Ltd., Japan; its specific surface area is 9.90 1 10 3 m2 /kg. Phenol, aniline, p-nitroaniline, nitrobenzene, L-phenylalanine and L-tyrosine were reagent grade chemicals from Wako Pure Chemical Industries, Ltd., Japan. Water used in the experiment was distilled after ion exchange, whose electric resistance was more than 5 1 10 4 V m and whose pH was 5.8. Methods Adsorption experiment. Adsorption isotherms of a single solute in aqueous solution were measured on activated carbon, graphite, and synthetic adsorbent. In 3 1 10 04 m3 Erlenmyer flasks with a screw cock, known weights of adsorbent particles were contacted with 2 1 10 04 m3 aqueous solutions, which had various concentrations of organic compounds. Adsorption experiments used 5.0 1 10 05 –2.0 1 10 03 kg of
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FIG. 1. Adsorption and desorption equilibria of aromatic compounds in aqueous solution on activated carbon at 308 K.
FIG. 2. Adsorption and desorption equilibria of aromatic compounds in aqueous solution on graphite at 308 K.
activated carbon, 5.0 1 10 04 –2.1 1 10 02 kg of graphite, and 0.5 1 10 04 –1.0 1 10 02 kg of synthetic adsorbent. The flasks were placed on a shaker and agitated at 308 K. After a contact time for adsorption, the amount adsorbed was determined from the initial and final concentrations of adsorbate in the liquid phase. The following six organic compounds were used as adsorbates; nitrobenzene, phenol, aniline, p-nitroaniline, L-phenylalanine, and L-tyrosine. The concentration of adsorbate in aqueous solution was measured by an ultraviolet spectrophotometer (Shimadzu Corp., Japan; UV-260) and a total organic carbon analyzer (Shimadzu Corp., Japan; TOC-5000). Desorption experiment. After adsorption, adsorbents were filtered from the solutions by the use of a 0.1 mm membrane filter (Toyo Roshi Co., Ltd., Japan; NC). In 3 1 10 04 m3 Erlenmyer flasks, adsorbates were then desorbed from the adsorbents by 2 1 10 04 m3 distilled water at 308 K. The concentration of desorbed compound was measured by the ultraviolet spectrophotometer, and the amount desorbed was determined. The solution remained on the surface of adsorbents and in their pores after the filtration. In this work, the amount of solution remained was much smaller than the amount of distilled water used in the desorption experiments.
tion, and Cs is the solubility of compound in water. The values of Cs for the compounds used in this work are shown in Table 1. Open and filled symbols denote adsorption and desorption data. It can be seen that the desorption greatly depends on substituent groups of aromatic compounds. For aromatic compounds substituted by electron-donating groups, hysteresis of adsorption and desorption was observed at low relative concentrations. For example, aniline, which has an amino group ( –NH2 ), and phenol, which has a hydroxyl group ( –OH), belong to this category. On the other hand, the compounds with electron-attracting groups show no hysteresis. The nitro group ( 0NO2 ) is in this category. It is found that the electronic state of the adsorbate influences desorption characteristics in aqueous solution. Adsorption and desorption isotherms of the compounds on the synthetic adsorbent (HP-21) are presented in Fig. 3. Experimental results show no hysteresis in the desorption of aniline, phenol, nitrobenzene, and p-nitroaniline. Reproducibility of the amount of phenol, nitrobenzene, or p-nitroaniline adsorbed was fairly good, within an error of about 5% on the activated carbon. However, the adsorption data for aniline on the activated carbon were scattered as shown in Fig. 1 because adsorption equilibrium was not achieved for 7 days, as described later. Since the amount
RESULTS AND DISCUSSION
TABLE 1 Solubility of Aromatic Compounds in Water
Desorption Characteristics
Compound
Figures 1 and 2 show adsorption and desorption isotherms of aromatic compounds on the activated carbon (CAL) and the graphite. A contact time for adsorption or desorption was 7 days. The ordinate of these figures is the amount adsorbed, and the abscissa is the relative concentration C/ Cs . Here, C is the adsorbate concentration in aqueous solu-
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Phenol Nitrobenzene Aniline p-nitroaniline L-phenylalanine L-tyrosine
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Solubility [mol/m3] 1.07 1.78 3.95 6.74 3.10 6.05
1 103 1 10 1 102 1 104 1 102
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FIG. 3. Adsorption and desorption equilibria of aromatic compounds in aqueous solution on synthetic adsorbent HP-21 at 308 K.
FIG. 5. Adsorption and desorption data of aniline in aqueous solution on graphite at 308 K.
adsorbed on the graphite and the synthetic adsorbent was small, the accuracy of the adsorption data became low and maximum errors of 30% and 13%, respectively, were observed on the graphite and the synthetic adsorbent, as shown in Figs. 2 and 3.
The hysteresis of adsorption and desorption for electrondonating compounds in aqueous solution was observed on the activated carbon and the graphite as shown in Figs. 1 and 2. We suppose that irreversible adsorption appears in the adsorption of electron-donating compounds. If irreversible adsorption occurs, the contact time of 7 days may be not enough for adsorption. Hence, long-time adsorption experiments were conducted. Figure 4 shows adsorption data of aniline on the activated
carbon. This figure shows that the amount of aniline adsorbed increases with the contact time. We suppose that the long contact time more than 37 days is necessary to achieve the equilibrium. Figure 5 shows adsorption data of aniline on the graphite. In this case, the amount of aniline adsorbed also increased with the contact time for adsorption. The isotherm asymptotically approaches to the rectangular isotherm after 95 days of contact. After aniline had been adsorbed on the graphite for 95 days, the desorption of aniline from the graphite was conducted by distilled water for 28 days. One can see that the amount of aniline desorbed is small and that irreversible adsorption occurs. Adsorption data of L-phenylalanine on the activated carbon and the graphite are shown in Fig. 6. It can be seen that the amount adsorbed increases with the contact time. Adsorption of L-phenylalanine on the graphite seems to be expressed by the rectangular isotherm after 42 days. Figure
FIG. 4. Adsorption data of aniline in aqueous solution on activated carbon at 308 K.
FIG. 6. Adsorption data of L-phenylalanine in aqueous solution on activated carbon and graphite at 308 K.
Adsorption Characteristics of Aromatic Compounds Containing an Amino Group
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FIG. 7. Adsorption data of L-tyrosine in aqueous solution on activated carbon and graphite at 308 K.
7 shows adsorption data of L-tyrosine on the carbon and the graphite. The amount of L-tyrosine adsorbed also increases with the contact time, and the rectangular isotherm is observed on the graphite after 28 days of contact. Although we conducted long-time experiments to measure the adsorption of electron-donating compounds, it might take longer contact time to achieve the true equilibrium. Hence, the experimental data were scattered as shown in Figs. 4 to 7. Peel et al. have suggested that it takes a very long contact time to achieve the equilibrium because the diffusion rate greatly decreases in micropores of activated carbons (13). On the other hand, in this work, a very long contact time is required to achieve equilibrium on the nonporous graphite. Since the physisorption rate is very high on nonporous materials, our experimental results suggest that an elementary process of adsorption is very slow at the graphite surface. Adsorption data for L-phenylalanine and L-tyrosine on the synthetic adsorbent (HP-21) are shown in Fig. 8. Although the data are scattered, one can see that L-phenylalanine achieves equilibrium within 7 days. On the other hand, a long contact time, more than 28 days, is necessary to achieve equilibrium in the adsorption of L-tyrosine. In the present work, the TOC concentration of the solution was measured by the total organic carbon analyzer after adsorption. On the other hand, the concentration measured by the ultraviolet spectrophotometer was converted to the TOC concentration. Both values were equal. From this result, one can see that aniline, L-phenylalanine, and L-tyrosine were not converted to other compounds during the adsorption experiments. The above experimental results indicate the following. As for the adsorption of electron-attracting compounds on carbonaceous adsorbents, the adsorption is reversible. On the other hand, in the adsorption of electron-donating compounds such as phenol, aniline, L-tyrosine, and L-phenylala-
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FIG. 8. Adsorption data of L-phenylalanine and L-tyrosine in aqueous solution on synthetic adsorbent HP-21 at 308 K.
nine, the amount adsorbed increases with the contact time for adsorption. The isotherms asymptotically approach rectangular isotherms for extremely long contact times, and irreversible adsorption is observed. We consider that the very slow adsorption is a result of the slow rate of the elementary process on adsorption sites because slow adsorption is observed even on nonporous graphite. The compounds, except L-tyrosine, induce reversible adsorption on the synthetic adsorbent. Thus, one can see that the slow process on adsorption sites is closely related to the chemical structure of adsorbent surface. Mechanism of Irreversible Adsorption Two-state adsorption model. A classical adsorption model has been used to interpret chemisorption in the gas phase (14 and 15). To explain why irreversibility appears
FIG. 9. Two-state adsorption model.
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FIG. 10. Change of liquid-phase concentration and amount of L-tyrosine adsorbed on graphite with contact time for desorption at 308 K: sn, initial adsorbed amount Å 1.86 1 10 03 mol/kg; lm, initial adsorbed amount Å 2.37 1 10 03 mol/kg.
in the adsorption of electron-donating compounds, we apply the model to adsorption of organic compounds in aqueous solution. The two-state adsorption model used in this work is shown in Fig. 9. As the adsorbate approaches the surface its energy falls as it becomes physisorbed into the precursor state for chemisorption, that is, the reversible state. We can expect there to be a potential energy barrier separating the precursor state and the irreversible state. Since the barrier is too high for the adsorption of electronattracting compounds to move over the barrier, the adsorption stops in the precursor state. The adsorptive interaction of electron-donating compounds and the carbonaceous surface is stronger in aqueous solution, and the potential energy barrier is lower than that for electron-attracting compounds. This interpretation seems to be reasonable by considering the Hammond postulate (16). This postulate shows that the structure of the transition state will resemble the product more closely than the reactant for endothermic processes whereas the opposite is true for exothermic reactions. Consequently, electron-donating compounds adsorbed in the precursor state move into the irreversible state after very long time. Experimental verification of model. The two-state adsorption model is experimentally verified by the adsorption of L-tyrosine on the nonporous graphite. The graphite that had absorbed L-tyrosine for one day was filtered by the use of a 0.1 mm membrane filter, and then L-tyrosine was desorbed by distilled water at 308 K. The amount of distilled water was 2 1 10 04 m3 and was same as the amount of the solution used in adsorption experiments. The changes of the concentration of L-tyrosine in the solution and the amount of L-tyrosine adsorbed with the contact time for desorption are shown in Fig. 10. The liquid-phase concentration of Ltyrosine increases and the amount adsorbed decreases for one day just after the start of desorption. After then, the concentration decreases and the adsorption of L-tyrosine occurs even in the desorption process. Figure 11 shows the
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FIG. 11. Change of liquid-phase concentration and amount of aniline adsorbed on activated carbon with contact time for desorption at 308 K: sn, initial adsorbed amount Å 1.53 mol/kg; lm, initial adsorbed amount Å 1.34 mol/kg.
effect of the contact time for desorption on the liquid-phase concentration and the amount of aniline adsorbed on the activated carbon. As seen from this figure, although aniline is desorbed in the initial stage of desorption, aniline is adsorbed again for the long contact time. These results suggest that the adsorbate in the precursor state gradually moves into the irreversible state. L-tyrosine was adsorbed on graphite for some contact times and was instantly desorbed by distilled water. The amount desorbed by this operation was defined as the reversible amount adsorbed, and the difference between the amount adsorbed before desorption and the reversible amount was regarded as the irreversible amount adsorbed. The change of the reversible, irreversible and total amounts of L-tyrosine adsorbed with the contact time for adsorption is shown in Fig. 12. At 12 min of contact, the reversible amount of L-tyrosine adsorbed is 98.5% of the total amount adsorbed. The reversible amount, however, decreases to 21.2% of the total amount after 60 h.
FIG. 12. Change of irreversible amount of L-tyrosine adsorbed with contact time for adsorption in aqueous solution on graphite at 308 K.
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FIG. 13. Reversible adsorption equilibria of L-tyrosine in aqueous solution on graphite at 308 K.
The reversible adsorption equilibrium is instantly achieved because the physisorption rate is very high on nonporous materials. Thus, we measured the amount of L-tyrosine adsorbed after 10 min of contact and obtained the reversible adsorption equilibrium. The reversible adsorption isotherm of L-tyrosine is shown in Fig. 13. The reversible amounts adsorbed after various contact times are also presented in this figure. One can see that the reversible amount adsorbed depends on the liquid-phase concentration of Ltyrosine, and that it is independent of the contact time for adsorption. The reversible adsorption isotherm is expressed by the Freundlich isotherm as shown in Fig. 13. The irreversible equilibrium, on the other hand, is the rectangular isotherm shown in Fig. 7. From Figs. 7 and 13, it is found that the reversible amount of L-tyrosine adsorbed is around 1 to 10% of the equilibrium amount adsorbed. As seen from Figs. 10 and 12, the reversible amount decreases with the contact time for adsorption or desorption and the irreversible amount increases with the time. Hence, L-tyrosine adsorbed in the precursor state (reversible state) gradually moves into the irreversible state. The result suggests that a potential energy barrier exists between the precursor and the irreversible state. In the adsorption of electron-donating compounds from aqueous solution on carbonaceous adsorbents, the irreversible amount adsorbed depends on the contact time as described above. In order to desorb the compounds from spent carbons efficiently, the short cycle of adsorption and desorption may be advantageous. Since the feasibility of adsorption process greatly depends on the cost of regeneration, the result obtained in this work is useful to improve the regeneration efficiency of spent activated carbons. The surface chemical groups of activated carbon seem to influence irreversible adsorption in aqueous solution. The authors have studied the influence of surface oxides of car-
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bonaceous adsorbents on adsorption and desorption characteristics of aromatic compounds in aqueous solution (11). When carbonaceous adsorbents are oxidized, the efficiency of ethanol regeneration of spent adsorbents loaded with phenolic compounds is improved. On the other hand, the compounds with amino group such as aniline, give lower regeneration efficiency on the oxidized carbons than on the original carbons. The result has been explained by the condensation reaction between the aromatic compounds with amino group and the surface oxides of carbons. Hence, we consider that the surface chemical groups influence the adsorption irreversibility. Adsorption experiments at different values of pH are also important (17). In order to elucidate the mechanism of irreversible adsorption in aqueous solution, we should study the influence of pH of solution and surface chemical groups of adsorbents on irreversible adsorption in the future. SUMMARY
Adsorption and desorption equilibria of aniline, phenol nitrobenzene, p-nitroaniline, L-phenylalanine and L-tyrosine in aqueous solution were determined on activated carbon, graphite, and synthetic adsorbent. In the adsorption of electron-donating compounds such as phenol, aniline, L-tyrosine and L-phenylalanine on carbonaceous adsorbents, irreversible adsorption was observed. The compounds except L-tyrosine were reversibly adsorbed on the synthetic adsorbent. We used a two-state adsorption model to explain why irreversibility appears in the adsorption of electron-donating compounds in aqueous solution. After adsorbates are stabilized in the precursor state (reversible state), they move into the irreversible state over a potential energy barrier after a long contact time. The effect of contact time for desorption was experimentally estimated on the amount of L-tyrosine desorbed, and the result was qualitatively explained by the model. The irreversible amount of L-tyrosine adsorbed was also determined. It was found that the irreversible amount of L-tyrosine adsorbed increased with the contact time for adsorption. The result indicated there to be a potential energy barrier between the reversible and the irreversible state. ACKNOWLEDGMENT The authors are grateful to Masashi Nishigaki and Koji Aburai for their help in the experimental work and to Mitsubishi Chemical Corporation for supplying the synthetic adsorbent.
REFERENCES 1. Hassler, J. W., ‘‘Activated Carbon,’’ 2nd ed. Chem. Publ., New York, 1963.
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2. Kippling, J. J., ‘‘Adsorption from Solution of Non-Electrolytes.’’ Academic Press, London, 1965. ˇ erny´, S., ‘‘Active Carbon.’’ Elsevier, New York, 3. Smı´sˇek, M., and C 1970. 4. Mattson, J. S., and Mark, H. B., Jr., ‘‘Activated Carbon: Surface Chemistry and Adsorption from Solution.’’ Dekker, New York, 1971. 5. McGuire, M. J., and Suffet, I. H., Eds., ‘‘Activated Carbon Adsorption of Organics from the Aqueous Phase.’’ Ann Arbor Science, Ann Arbor, MI, 1980. 6. Suzuki, M., ‘‘Adsorption Engineering.’’ Kodansha, Tokyo, 1990. 7. Jankowska, H., Swiatkowski, A., and Choma, J., ‘‘Active Carbon.’’ Ellis Horwood, New York, 1991. 8. Noll, K. E., Gounaris, V., and Hou, W.-S., ‘‘Adsorption Technology for Air and Water Pollution Control.’’ Lewis, Chelsea, MI, 1991.
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9. Ferro-Carcia, M. A., Utrera-Hidalgo, E., Rivera-Utrilla, J., and Moreno-Castilla, C., Carbon 31, 857 (1993). 10. Tamon, H., Saito, T., Kishimura, M., Okazaki, M., and Toei, R., J. Chem. Eng. Jpn. 23, 426 (1990). 11. Tamon, H., and Okazaki, M., in ‘‘Fundamentals of Adsorption’’ (M. Suzuki, Ed.), p. 663. Kodansha, Tokyo, 1993. 12. Grant, T. M., and King, C. J., Ind. Eng. Chem. Res. 29, 264 (1990). 13. Peel, R. G., Benedek, A., and Crowe, C. M., AIChE J. 27, 26 (1981). 14. Hayward, D. O., and Trapnell, B. M. W., ‘‘Chemisorption.’’ Butterworths, London, 1964. 15. Atkins, P. W., ‘‘Physical Chemistry,’’ 4th ed. Oxford Univ. Press, London, 1990. 16. Hammond, G. S., J. Am. Chem. Soc. 77, 334, 1955. 17. Rivera-Utrilla, J., Utrera-Hidalgo, E., Ferro-Carcia, M. A., and Moreno-Castilla, C., Carbon 29, 613, 1991.
AP: Colloid
177, 384–390 (1996)
0048
On Irreversible Adsorption of Electron-Donating Compounds in Aqueous Solution HAJIME TAMON, 1 MASANORI ATSUSHI,
AND
MORIO OK AZAKI
Department of Chemical Engineering, Kyoto University, Kyoto 606-01, Japan Received March 23, 1995; accepted June 9, 1995
In the adsorption of electron-donating compounds such as phenol, aniline, L-phenylalanine, and L-tyrosine from aqueous solution, irreversibility was observed on activated carbon and graphite. The compounds, except L-tyrosine, were reversibly adsorbed on a synthetic adsorbent. In the case where the carbonaceous adsorbents contacted the aqueous solution containing electron-donating compounds for a long time, the irreversible amount adsorbed increased with the contact time. A two-state adsorption model was used to explain why the irreversible adsorption of electron-donating compound appears in aqueous solution. First, the compound is adsorbed in the precursor state for irreversible adsorption, and then moves into its irreversible state over a potential energy barrier after a long contact time. The appearance of irreversible adsorption was qualitatively explained by the two-state adsorption model.
Grant and King have studied irreversible adsorption of phenolic compounds on activated carbon (12). They have determined that oxidative coupling of phenolic compounds on the carbon surface is a plausible explanation for irreversible adsorption. However, the appearance of irreversible adsorption for other electron-donating compounds has not yet been studied. In this article, we measure adsorption and desorption characteristics of some aromatic compounds in aqueous solution on carbonaceous and synthetic adsorbents. We apply a twostate adsorption model for chemisorption to the irreversible adsorption in aqueous solution, and discuss why irreversible adsorption appears.
q 1996 Academic Press, Inc.
Key Words: irreversible adsorption; aqueous solution; electrondonating compound; activated carbon; graphite; two-state adsorption model.
INTRODUCTION
Activated carbons and synthetic adsorbents have been used for liquid purification and wastewater treatment (1– 8). The feasibility of an adsorption process depends greatly on the cost of regeneration of spent adsorbents. It is well known that regeneration of activated carbons loaded with phenolic compounds is difficult. Although the adsorption capacity of synthetic adsorbents is generally less than that of carbons, they are advantageous for certain application because their regeneration is easy. Solvent regeneration of spent activated carbon has been studied (9–11). We have reported that the regeneration efficiency depends on substituent groups of aromatic compounds, and that electron-donating compounds are difficult to be desorbed from the carbon by use of ethanol (10). If irreversible adsorption occurs, regeneration of spent adsorbent is very difficult. Hence, it is very important to understand why irreversible adsorption appears in aqueous solution. 1
To whom correspondence should be addressed.
0021-9797/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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MATERIALS AND METHODS
Materials Activated carbon (CAL) was supplied by Calgon Co. Ltd., USA; its specific surface area is 9.27 1 10 5 m2 /kg. Synthetic adsorbent (HP-21) was produced by Mitsubishi Chemical Corp., Japan, and is a styrene divinylbenzene polymer. The specific surface area of the adsorbent is 5.83 1 10 5 m2 /kg. Graphite was supplied by Wako Pure Chemical Industries, Ltd., Japan; its specific surface area is 9.90 1 10 3 m2 /kg. Phenol, aniline, p-nitroaniline, nitrobenzene, L-phenylalanine and L-tyrosine were reagent grade chemicals from Wako Pure Chemical Industries, Ltd., Japan. Water used in the experiment was distilled after ion exchange, whose electric resistance was more than 5 1 10 4 V m and whose pH was 5.8. Methods Adsorption experiment. Adsorption isotherms of a single solute in aqueous solution were measured on activated carbon, graphite, and synthetic adsorbent. In 3 1 10 04 m3 Erlenmyer flasks with a screw cock, known weights of adsorbent particles were contacted with 2 1 10 04 m3 aqueous solutions, which had various concentrations of organic compounds. Adsorption experiments used 5.0 1 10 05 –2.0 1 10 03 kg of
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FIG. 1. Adsorption and desorption equilibria of aromatic compounds in aqueous solution on activated carbon at 308 K.
FIG. 2. Adsorption and desorption equilibria of aromatic compounds in aqueous solution on graphite at 308 K.
activated carbon, 5.0 1 10 04 –2.1 1 10 02 kg of graphite, and 0.5 1 10 04 –1.0 1 10 02 kg of synthetic adsorbent. The flasks were placed on a shaker and agitated at 308 K. After a contact time for adsorption, the amount adsorbed was determined from the initial and final concentrations of adsorbate in the liquid phase. The following six organic compounds were used as adsorbates; nitrobenzene, phenol, aniline, p-nitroaniline, L-phenylalanine, and L-tyrosine. The concentration of adsorbate in aqueous solution was measured by an ultraviolet spectrophotometer (Shimadzu Corp., Japan; UV-260) and a total organic carbon analyzer (Shimadzu Corp., Japan; TOC-5000). Desorption experiment. After adsorption, adsorbents were filtered from the solutions by the use of a 0.1 mm membrane filter (Toyo Roshi Co., Ltd., Japan; NC). In 3 1 10 04 m3 Erlenmyer flasks, adsorbates were then desorbed from the adsorbents by 2 1 10 04 m3 distilled water at 308 K. The concentration of desorbed compound was measured by the ultraviolet spectrophotometer, and the amount desorbed was determined. The solution remained on the surface of adsorbents and in their pores after the filtration. In this work, the amount of solution remained was much smaller than the amount of distilled water used in the desorption experiments.
tion, and Cs is the solubility of compound in water. The values of Cs for the compounds used in this work are shown in Table 1. Open and filled symbols denote adsorption and desorption data. It can be seen that the desorption greatly depends on substituent groups of aromatic compounds. For aromatic compounds substituted by electron-donating groups, hysteresis of adsorption and desorption was observed at low relative concentrations. For example, aniline, which has an amino group ( –NH2 ), and phenol, which has a hydroxyl group ( –OH), belong to this category. On the other hand, the compounds with electron-attracting groups show no hysteresis. The nitro group ( 0NO2 ) is in this category. It is found that the electronic state of the adsorbate influences desorption characteristics in aqueous solution. Adsorption and desorption isotherms of the compounds on the synthetic adsorbent (HP-21) are presented in Fig. 3. Experimental results show no hysteresis in the desorption of aniline, phenol, nitrobenzene, and p-nitroaniline. Reproducibility of the amount of phenol, nitrobenzene, or p-nitroaniline adsorbed was fairly good, within an error of about 5% on the activated carbon. However, the adsorption data for aniline on the activated carbon were scattered as shown in Fig. 1 because adsorption equilibrium was not achieved for 7 days, as described later. Since the amount
RESULTS AND DISCUSSION
TABLE 1 Solubility of Aromatic Compounds in Water
Desorption Characteristics
Compound
Figures 1 and 2 show adsorption and desorption isotherms of aromatic compounds on the activated carbon (CAL) and the graphite. A contact time for adsorption or desorption was 7 days. The ordinate of these figures is the amount adsorbed, and the abscissa is the relative concentration C/ Cs . Here, C is the adsorbate concentration in aqueous solu-
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Phenol Nitrobenzene Aniline p-nitroaniline L-phenylalanine L-tyrosine
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Solubility [mol/m3] 1.07 1.78 3.95 6.74 3.10 6.05
1 103 1 10 1 102 1 104 1 102
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FIG. 3. Adsorption and desorption equilibria of aromatic compounds in aqueous solution on synthetic adsorbent HP-21 at 308 K.
FIG. 5. Adsorption and desorption data of aniline in aqueous solution on graphite at 308 K.
adsorbed on the graphite and the synthetic adsorbent was small, the accuracy of the adsorption data became low and maximum errors of 30% and 13%, respectively, were observed on the graphite and the synthetic adsorbent, as shown in Figs. 2 and 3.
The hysteresis of adsorption and desorption for electrondonating compounds in aqueous solution was observed on the activated carbon and the graphite as shown in Figs. 1 and 2. We suppose that irreversible adsorption appears in the adsorption of electron-donating compounds. If irreversible adsorption occurs, the contact time of 7 days may be not enough for adsorption. Hence, long-time adsorption experiments were conducted. Figure 4 shows adsorption data of aniline on the activated
carbon. This figure shows that the amount of aniline adsorbed increases with the contact time. We suppose that the long contact time more than 37 days is necessary to achieve the equilibrium. Figure 5 shows adsorption data of aniline on the graphite. In this case, the amount of aniline adsorbed also increased with the contact time for adsorption. The isotherm asymptotically approaches to the rectangular isotherm after 95 days of contact. After aniline had been adsorbed on the graphite for 95 days, the desorption of aniline from the graphite was conducted by distilled water for 28 days. One can see that the amount of aniline desorbed is small and that irreversible adsorption occurs. Adsorption data of L-phenylalanine on the activated carbon and the graphite are shown in Fig. 6. It can be seen that the amount adsorbed increases with the contact time. Adsorption of L-phenylalanine on the graphite seems to be expressed by the rectangular isotherm after 42 days. Figure
FIG. 4. Adsorption data of aniline in aqueous solution on activated carbon at 308 K.
FIG. 6. Adsorption data of L-phenylalanine in aqueous solution on activated carbon and graphite at 308 K.
Adsorption Characteristics of Aromatic Compounds Containing an Amino Group
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FIG. 7. Adsorption data of L-tyrosine in aqueous solution on activated carbon and graphite at 308 K.
7 shows adsorption data of L-tyrosine on the carbon and the graphite. The amount of L-tyrosine adsorbed also increases with the contact time, and the rectangular isotherm is observed on the graphite after 28 days of contact. Although we conducted long-time experiments to measure the adsorption of electron-donating compounds, it might take longer contact time to achieve the true equilibrium. Hence, the experimental data were scattered as shown in Figs. 4 to 7. Peel et al. have suggested that it takes a very long contact time to achieve the equilibrium because the diffusion rate greatly decreases in micropores of activated carbons (13). On the other hand, in this work, a very long contact time is required to achieve equilibrium on the nonporous graphite. Since the physisorption rate is very high on nonporous materials, our experimental results suggest that an elementary process of adsorption is very slow at the graphite surface. Adsorption data for L-phenylalanine and L-tyrosine on the synthetic adsorbent (HP-21) are shown in Fig. 8. Although the data are scattered, one can see that L-phenylalanine achieves equilibrium within 7 days. On the other hand, a long contact time, more than 28 days, is necessary to achieve equilibrium in the adsorption of L-tyrosine. In the present work, the TOC concentration of the solution was measured by the total organic carbon analyzer after adsorption. On the other hand, the concentration measured by the ultraviolet spectrophotometer was converted to the TOC concentration. Both values were equal. From this result, one can see that aniline, L-phenylalanine, and L-tyrosine were not converted to other compounds during the adsorption experiments. The above experimental results indicate the following. As for the adsorption of electron-attracting compounds on carbonaceous adsorbents, the adsorption is reversible. On the other hand, in the adsorption of electron-donating compounds such as phenol, aniline, L-tyrosine, and L-phenylala-
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FIG. 8. Adsorption data of L-phenylalanine and L-tyrosine in aqueous solution on synthetic adsorbent HP-21 at 308 K.
nine, the amount adsorbed increases with the contact time for adsorption. The isotherms asymptotically approach rectangular isotherms for extremely long contact times, and irreversible adsorption is observed. We consider that the very slow adsorption is a result of the slow rate of the elementary process on adsorption sites because slow adsorption is observed even on nonporous graphite. The compounds, except L-tyrosine, induce reversible adsorption on the synthetic adsorbent. Thus, one can see that the slow process on adsorption sites is closely related to the chemical structure of adsorbent surface. Mechanism of Irreversible Adsorption Two-state adsorption model. A classical adsorption model has been used to interpret chemisorption in the gas phase (14 and 15). To explain why irreversibility appears
FIG. 9. Two-state adsorption model.
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FIG. 10. Change of liquid-phase concentration and amount of L-tyrosine adsorbed on graphite with contact time for desorption at 308 K: sn, initial adsorbed amount Å 1.86 1 10 03 mol/kg; lm, initial adsorbed amount Å 2.37 1 10 03 mol/kg.
in the adsorption of electron-donating compounds, we apply the model to adsorption of organic compounds in aqueous solution. The two-state adsorption model used in this work is shown in Fig. 9. As the adsorbate approaches the surface its energy falls as it becomes physisorbed into the precursor state for chemisorption, that is, the reversible state. We can expect there to be a potential energy barrier separating the precursor state and the irreversible state. Since the barrier is too high for the adsorption of electronattracting compounds to move over the barrier, the adsorption stops in the precursor state. The adsorptive interaction of electron-donating compounds and the carbonaceous surface is stronger in aqueous solution, and the potential energy barrier is lower than that for electron-attracting compounds. This interpretation seems to be reasonable by considering the Hammond postulate (16). This postulate shows that the structure of the transition state will resemble the product more closely than the reactant for endothermic processes whereas the opposite is true for exothermic reactions. Consequently, electron-donating compounds adsorbed in the precursor state move into the irreversible state after very long time. Experimental verification of model. The two-state adsorption model is experimentally verified by the adsorption of L-tyrosine on the nonporous graphite. The graphite that had absorbed L-tyrosine for one day was filtered by the use of a 0.1 mm membrane filter, and then L-tyrosine was desorbed by distilled water at 308 K. The amount of distilled water was 2 1 10 04 m3 and was same as the amount of the solution used in adsorption experiments. The changes of the concentration of L-tyrosine in the solution and the amount of L-tyrosine adsorbed with the contact time for desorption are shown in Fig. 10. The liquid-phase concentration of Ltyrosine increases and the amount adsorbed decreases for one day just after the start of desorption. After then, the concentration decreases and the adsorption of L-tyrosine occurs even in the desorption process. Figure 11 shows the
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FIG. 11. Change of liquid-phase concentration and amount of aniline adsorbed on activated carbon with contact time for desorption at 308 K: sn, initial adsorbed amount Å 1.53 mol/kg; lm, initial adsorbed amount Å 1.34 mol/kg.
effect of the contact time for desorption on the liquid-phase concentration and the amount of aniline adsorbed on the activated carbon. As seen from this figure, although aniline is desorbed in the initial stage of desorption, aniline is adsorbed again for the long contact time. These results suggest that the adsorbate in the precursor state gradually moves into the irreversible state. L-tyrosine was adsorbed on graphite for some contact times and was instantly desorbed by distilled water. The amount desorbed by this operation was defined as the reversible amount adsorbed, and the difference between the amount adsorbed before desorption and the reversible amount was regarded as the irreversible amount adsorbed. The change of the reversible, irreversible and total amounts of L-tyrosine adsorbed with the contact time for adsorption is shown in Fig. 12. At 12 min of contact, the reversible amount of L-tyrosine adsorbed is 98.5% of the total amount adsorbed. The reversible amount, however, decreases to 21.2% of the total amount after 60 h.
FIG. 12. Change of irreversible amount of L-tyrosine adsorbed with contact time for adsorption in aqueous solution on graphite at 308 K.
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IRREVERSIBLE ADSORPTION IN AQUEOUS SOLUTION
FIG. 13. Reversible adsorption equilibria of L-tyrosine in aqueous solution on graphite at 308 K.
The reversible adsorption equilibrium is instantly achieved because the physisorption rate is very high on nonporous materials. Thus, we measured the amount of L-tyrosine adsorbed after 10 min of contact and obtained the reversible adsorption equilibrium. The reversible adsorption isotherm of L-tyrosine is shown in Fig. 13. The reversible amounts adsorbed after various contact times are also presented in this figure. One can see that the reversible amount adsorbed depends on the liquid-phase concentration of Ltyrosine, and that it is independent of the contact time for adsorption. The reversible adsorption isotherm is expressed by the Freundlich isotherm as shown in Fig. 13. The irreversible equilibrium, on the other hand, is the rectangular isotherm shown in Fig. 7. From Figs. 7 and 13, it is found that the reversible amount of L-tyrosine adsorbed is around 1 to 10% of the equilibrium amount adsorbed. As seen from Figs. 10 and 12, the reversible amount decreases with the contact time for adsorption or desorption and the irreversible amount increases with the time. Hence, L-tyrosine adsorbed in the precursor state (reversible state) gradually moves into the irreversible state. The result suggests that a potential energy barrier exists between the precursor and the irreversible state. In the adsorption of electron-donating compounds from aqueous solution on carbonaceous adsorbents, the irreversible amount adsorbed depends on the contact time as described above. In order to desorb the compounds from spent carbons efficiently, the short cycle of adsorption and desorption may be advantageous. Since the feasibility of adsorption process greatly depends on the cost of regeneration, the result obtained in this work is useful to improve the regeneration efficiency of spent activated carbons. The surface chemical groups of activated carbon seem to influence irreversible adsorption in aqueous solution. The authors have studied the influence of surface oxides of car-
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bonaceous adsorbents on adsorption and desorption characteristics of aromatic compounds in aqueous solution (11). When carbonaceous adsorbents are oxidized, the efficiency of ethanol regeneration of spent adsorbents loaded with phenolic compounds is improved. On the other hand, the compounds with amino group such as aniline, give lower regeneration efficiency on the oxidized carbons than on the original carbons. The result has been explained by the condensation reaction between the aromatic compounds with amino group and the surface oxides of carbons. Hence, we consider that the surface chemical groups influence the adsorption irreversibility. Adsorption experiments at different values of pH are also important (17). In order to elucidate the mechanism of irreversible adsorption in aqueous solution, we should study the influence of pH of solution and surface chemical groups of adsorbents on irreversible adsorption in the future. SUMMARY
Adsorption and desorption equilibria of aniline, phenol nitrobenzene, p-nitroaniline, L-phenylalanine and L-tyrosine in aqueous solution were determined on activated carbon, graphite, and synthetic adsorbent. In the adsorption of electron-donating compounds such as phenol, aniline, L-tyrosine and L-phenylalanine on carbonaceous adsorbents, irreversible adsorption was observed. The compounds except L-tyrosine were reversibly adsorbed on the synthetic adsorbent. We used a two-state adsorption model to explain why irreversibility appears in the adsorption of electron-donating compounds in aqueous solution. After adsorbates are stabilized in the precursor state (reversible state), they move into the irreversible state over a potential energy barrier after a long contact time. The effect of contact time for desorption was experimentally estimated on the amount of L-tyrosine desorbed, and the result was qualitatively explained by the model. The irreversible amount of L-tyrosine adsorbed was also determined. It was found that the irreversible amount of L-tyrosine adsorbed increased with the contact time for adsorption. The result indicated there to be a potential energy barrier between the reversible and the irreversible state. ACKNOWLEDGMENT The authors are grateful to Masashi Nishigaki and Koji Aburai for their help in the experimental work and to Mitsubishi Chemical Corporation for supplying the synthetic adsorbent.
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2. Kippling, J. J., ‘‘Adsorption from Solution of Non-Electrolytes.’’ Academic Press, London, 1965. ˇ erny´, S., ‘‘Active Carbon.’’ Elsevier, New York, 3. Smı´sˇek, M., and C 1970. 4. Mattson, J. S., and Mark, H. B., Jr., ‘‘Activated Carbon: Surface Chemistry and Adsorption from Solution.’’ Dekker, New York, 1971. 5. McGuire, M. J., and Suffet, I. H., Eds., ‘‘Activated Carbon Adsorption of Organics from the Aqueous Phase.’’ Ann Arbor Science, Ann Arbor, MI, 1980. 6. Suzuki, M., ‘‘Adsorption Engineering.’’ Kodansha, Tokyo, 1990. 7. Jankowska, H., Swiatkowski, A., and Choma, J., ‘‘Active Carbon.’’ Ellis Horwood, New York, 1991. 8. Noll, K. E., Gounaris, V., and Hou, W.-S., ‘‘Adsorption Technology for Air and Water Pollution Control.’’ Lewis, Chelsea, MI, 1991.
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9. Ferro-Carcia, M. A., Utrera-Hidalgo, E., Rivera-Utrilla, J., and Moreno-Castilla, C., Carbon 31, 857 (1993). 10. Tamon, H., Saito, T., Kishimura, M., Okazaki, M., and Toei, R., J. Chem. Eng. Jpn. 23, 426 (1990). 11. Tamon, H., and Okazaki, M., in ‘‘Fundamentals of Adsorption’’ (M. Suzuki, Ed.), p. 663. Kodansha, Tokyo, 1993. 12. Grant, T. M., and King, C. J., Ind. Eng. Chem. Res. 29, 264 (1990). 13. Peel, R. G., Benedek, A., and Crowe, C. M., AIChE J. 27, 26 (1981). 14. Hayward, D. O., and Trapnell, B. M. W., ‘‘Chemisorption.’’ Butterworths, London, 1964. 15. Atkins, P. W., ‘‘Physical Chemistry,’’ 4th ed. Oxford Univ. Press, London, 1990. 16. Hammond, G. S., J. Am. Chem. Soc. 77, 334, 1955. 17. Rivera-Utrilla, J., Utrera-Hidalgo, E., Ferro-Carcia, M. A., and Moreno-Castilla, C., Carbon 29, 613, 1991.
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