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Modified Langmuir-like Model for Modeling the Adsorption from Aqueous Solutions by Activated Carbons Kirk A. VanDer Kamp,† Dongmei Qiang,‡ Aktham Aburub,* and Dale Eric Wurster* College of Pharmacy, University of Iowa, Iowa City, Iowa 52242 Received June 29, 2004. In Final Form: September 17, 2004 A new equation, a modified Langmuir-like equation (M-LLE), for describing adsorption from solution by activated carbons is proposed for the first time in this work. The M-LLE assumes that there are two types of interactions: (a) specific interactions which are typical, enthalpy-driven interactions and (b) nonspecific interactions driven by the loss of water structuring, upon adsorption (hydrophobic bonding), around the nonpolar parts of the drug. The proposed model was evaluated by studying the adsorption of three drugs: procaine, fluoxetine, and phenobarbital by four different activated carbons under different experimental conditions. As the hydrophobicity of the drug increased, the capacity constant representing the interactions driven by hydrophobic bonding (KHB, M-LLE equation) increased. Experimental conditions that decrease hydrophobic bonding, such as increased temperature and higher cosolvent concentration, resulted in a decrease in KHB. Salts that tend to increase water structuring and hydrophobic bonding caused an increase in KHB. All of these studies support the M-LLE, because they support the notion of hydrophobic-bonding-driven interactions.
1. Introduction Practical application of the adsorption process is based mainly on the selective uptake of individual components from their mixtures with other substances. Activated carbon has been widely used as an adsorbent for the removal of many toxic compounds because of its large specific surface area and high degree of surface activity.1-4 In pharmaceutical applications, orally administered activated carbon, either alone or in conjunction with other suitable procedures, has been effective in preventing the absorption of many accidentally or intentionally ingested overdoses of drugs.5-7 The surfaces of activated carbons are heterogeneous, being composed of both hydrophobic regions (bare carbon surface) and a variety of polar functional groups. The latter consist mainly of oxygen-containing functionalities.8,9 Different mechanisms have been found to be involved in adsorption by activated carbons, including van der Waals forces, hydrogen bonding,10-12 electrostatic interactions, * To whom correspondence should be addressed. E-mail:
[email protected] (D.E.W.);
[email protected] (A.A.). † Present address: Eli Lilly and Company, Indianapolis, IN 46208. ‡ Present address: Boehringer-Ingelheim Pharmaceuticals, Inc., Ridgefield, CT 06877. (1) Oeste, F. D.; Haas, R.; Kaminski, L. Environ. Sci. Pollut. Res. Int. 2000, 7, 5-6. (2) Menon, V. C.; Komarneni, S. J. Porous Mater. 1998, 5, 43-58. (3) Ryu, Y. K.; Lee, H. J.; Yoo, H. K.; Lee, C. H. J. Chem. Eng. Data 2002, 47, 1222-1225. (4) Saxer, R. Abstr. Pap. Am. Chem. Soc. 1995, 210, 6-SCHB. (5) Thabet, H.; Brahmi, N.; Amamou, M. Presse Med. 1999, 28, 955958. (6) Orisakwe, O. E.; Akintonwa, A. Human Exp. Toxicol. 1991, 10, 133-135. (7) Neuvonen, P. J.; Olkkola, K. T. Med. Toxicol. Adverse Drug Exp. 1988, 3, 33-58. (8) Boehm, H. P. Carbon 1994, 32, 759-769. (9) Leon, C. A.; Radovic, L. R. Chemistry and Physics of Carbon; Marcel Dekker, Inc.: New York, 1992; Vol. 24. (10) Nevskaia, D. M.; Guerrero-Ruiz, A. J. Colloid Interface Sci. 2001, 234, 316-321. (11) Franz, M.; Arafat, H. A.; Pinto, N. G. Carbon 2000, 38, 18071819.
ion-pairing and ion exchange,13,14 and hydrophobic bonding.15-20 Many drugs possess both hydrophilic and hydrophobic moieties. The polar parts of the molecule tend to interact with the surface polar functional groups via hydrogen bonding, the Keesom force, the Debye force, or ion-ion interactions, all of which are enthalpy-driven processes.21 The nonpolar portions of the molecule adsorb to the basal carbon surface via hydrophobic bonding, which is an entropy-driven process.22 The Langmuir-like equation (LLE)21 is probably the most extensively used equation for describing adsorption from solution by activated carbons. The Langmuir-like model assumes that only specific interactions, between a specific functional group on the surface and a particular adsorbate functional group, take place. In the case of activated carbon, a significant fraction of the surface is nonpolar (basal carbon planes). The possibility of nonspecific adsorption on this portion of the surface is not addressed in the Langmuir-like model. A new equation, a modified Langmuir-like equation (M-LLE), is introduced for the first time in this work to overcome the aforementioned shortcoming of the LLE. The validity of this new equation is tested by studying the adsorption of three different drugs, phenobarbital, (12) Burke, G. M.; Wurster, D. E.; Berg, M. J.; Veng-Pedersen, P.; Schottelius, D. D. Pharm. Res. 1992, 9, 126-130. (13) Gurses, A.; Yalcin, M.; Dogar, C. Fuel Process. Technol. 2003, 81, 57-66. (14) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219-304. (15) Tomlinson, A.; Scherer, B.; Taylor, S. E. Carbon 1999, 38, 1328. (16) Mazet, M.; Dusart, O.; Lafrance, P. J. Surf. Sci. Technol. 1989, 5, 345-353. (17) Ihara, Y. J. Appl. Polym. Sci. 1992, 44, 1837-1840. (18) Wang, J.; Lu, Y.; Thomas, R. K. Adsorpt. Sci. Technol. 1998, 16, 557-564. (19) Akaho, E.; Fukumori, Y. J. Pharm. Sci. 2001, 90, 1288-1297. (20) Wu, S. H.; Pendleton, P. J. Colloid Interface Sci. 2001, 243. (21) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6 ed.; John Wiley & Sons, Inc.: New York, 1997. (22) Tanford, C. The Hydrophobic Effect Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley & Sons, Inc.: New York, 1980.
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Table 1. Average Relative Percentages of Functional States (XPS)24 and Specific Surface Areas (N2 Vapor Adsorption) for the Four Activated Carbonsa activated carbon SuperChar Darco KB-B Norit B Supra Norit USP XX a
specific surface area (m2/g)
CsH, CsC hydrocarbon
CsO hydroxyl
CdO carbonyl
OsCdO carboxylic acid or ester
73.8 (0.2) 78.5 (1.0) 78.3 (0.3) 85.7 (0.4)
16.4 (0.2) 13.0 (0.7) 15.1 (0.1) 8.73 (0.45)
5.90 (0.10) 6.30 (0.20) 3.30 (0.20) 3.33 (0.06)
3.93 (0.42) 2.23 (1.11) 3.30 (0.36) 2.23 (0.25)
2940 (21) 1580 (40) 1430 (34) 986 (6)
The numbers in parentheses are the standard deviations.
Figure 1. Typical C(1s) spectrum of SuperChar.24
procaine, and fluoxetine, by different activated carbons. The adsorption isotherms were fitted using the M-LLE and the LLE, and the fits were compared to show the superiority of the M-LLE over the LLE. To further test the M-LLE, the effects of temperature, solvent, and ionic strength (adjusted with different anions) on adsorption were examined to demonstrate that the added term in the M-LLE has a physical meaning that agrees with the main assumption of the model. 2. Experiment 2.1. Activated Carbon Samples. Four activated carbons, SuperChar (lot G812R, Gulf Bio-systems, Inc., Dallas, TX), Darco KB-B (lot 202.07A), Norit B Supra (lot 8003-4), and Norit USP XX (all from American Norit Company, Inc., Jacksonville, FL), were chosen as the adsorbents in this work. All samples were vacuum-dried at 100 °C and HPO42- > acetate > citrate3- > Cl- > NO3- > I- > ClO4-
Figure 20. Effect of temperature on the KHB values for phenobarbital adsorption by Norit USP XX and Norit B Supra from phosphate buffer (pH 7.5; 0.05 M, I ) 0.12 M).31
that in aqueous solutions. This might result from phenobarbital interacting with a different polar functional group on the activated carbon surface when water is replaced by toluene. 3.7. Effect of Temperature on Phenobarbital Adsorption. The adsorption of phenobarbital by Norit USP XX and Norit B Supra was also studied at different temperatures.31 The adsorption isotherms are shown in Figures 18 and 19, respectively. These adsorption isotherms were fitted using the M-LLE. It was found that the specific capacity constant (K2) remained virtually the same irrespective of the temperature at which the adsorption study was conducted. The extent of the specific interactions (K2) is determined by the number of specific adsorption sites, which is independent of temperature. Therefore, K2 was not affected by temperature. On the other hand, the nonspecific capacity constant (KHB) decreased systematically with temperature, as can be seen in Figure 20. As temperature increases, water structuring decreases. Therefore, the adsorption capacity due to hydrophobic bonding (KHB) decreases. Because increasing temperature affects water structuring in solution and has nothing to do with the surface, the change in KHB with temperature should be the same irrespective of the adsorbent used. It can be seen from Figure 20 that the slopes of the fitted correlation lines for KHB versus temperature for the two activated carbons are virtually the same, being less than 5% different. (30) Huang, L.-F. Ph.D. Dissertation, The University of Iowa, Iowa City, IA, 1993. (31) Aburub, A.; Wurster, D. E. Presented at American Association of Pharmaceutical Scientists (AAPS) Annual Meeting, Salt Lake City, UT, Nov 2003.
This indicates that ClO4- has less ability to decrease water structuring than Cl-. In other words, water molecules have a more ordered structure around fluoxetine molecules in an aqueous solution that contains ClO4- than in an aqueous solution that contains Cl-. Adsorption of fluoxetine onto the basal carbon surface from an aqueous solution containing ClO4- will lead to a greater loss of water structuring and a greater entropy gain. Therefore, the nonspecific capacity for fluoxetine was greater in the ClO4- solution than in the Cl - solution. This anion effect on fluoxetine adsorption also supports the idea that the nonspecific adsorption to the carbon basal plane is driven by the entropy gain associated with the loss of water structuring. The specific capacity constant (K2) systematically increased with ionic strength, as shown in Table 4. In addition, for any given ionic strength, the anion effect on K2 followed the order Cl- < Br- ≈ NO3- < ClO4-. Snoeyink found that the adsorption capacity of protons on activated carbon surfaces increased as the ionic strength increased and followed the order Cl- < NO3- < ClO4-.32 He proposed that adsorbed anions on activated carbon surfaces might act as specific sites for the adsorption of cationic solutes. The effect of anions on fluoxetine adsorption was conducted at pH 2.0. At that pH, most of the fluoxetine (pKa ) 8.7) exists as the conjugate acid (positively charged). Therefore, a similar mechanism for the adsorption of fluoxetine as the one proposed by Snoeyink explains the anion effects on K2. 4. Conclusion (1) A new model for describing adsorption from solution by activated carbons is proposed. The adsorption of (32) Snoeyink, V. L. Adsorption of Strong Acids, Phenol, and 4-Nitrophenol from Aqueous Solution by Activated Carbon in Agitated Nonflow Systems; Department of Civil Engineering, University of Michigan: Ann Arbor, 1968; p 166.
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procaine, fluoxetine, and phenobarbital from aqueous solution by four different activated carbons was studied. The M-LLE and the LLE were then used to fit the adsorption data. Unlike the LLE, which showed systematic deviations from the experimental data, the M-LLE, which considers both the specific and nonspecific interactions between the drug and the activated carbon surface, did not show any significant systematic deviations. (2) The fractions of the nonspecific capacities for procaine, fluoxetine, and phenobarbital were found to be linearly related to the relative percentages of bare carbon surface (XPS). This supports the notion that the bare carbon surface is the region for the nonspecific adsorption of drugs to the activated carbon surface. (3) The apparent area occupied per procaine molecule, as obtained from the specific capacity, was found to be linearly related to the sum of the relative percentages of surface sCOOH and CsO functional states. This indicates that both -COOH and C-O functional states are acting as the specific adsorption sites for procaine on the activated carbon surface. (4) The apparent area occupied per phenobarbital molecule, as obtained from the specific capacity, was found to be linearly related to the relative percentage of surface C-O functional state. This indicates that the C-O site is acting as the specific adsorption site for phenobarbital on the activated carbon surface. (5) The apparent area occupied per fluoxetine molecule, as obtained from the specific capacity, was found to be linearly related to the sum of the relative percentages of surface sCOOH and CdO groups. This indicates that both sCOOH and CdO groups are acting as the specific adsorption sites for fluoxetine on the activated carbon surface. (6) The strong linear correlation between the nonspecific capacities (KHB) of different drugs and the logarithms of the octanol-water partition coefficients for these drugs agrees very well with the M-LLE. As the hydrophobicity of the drug increases, the water structuring around the drug increases. Thus, the driving force for the nonspecific adsorption increases and KHB increases. (7) The nonspecific capacity decreased as the concentration of ethanol in solution was increased and became negligible in toluene. This is also in agreement with the
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concept of water structuring being responsible for the nonspecific adsorption. Ethanol decreases water structuring around the drug and, therefore, decreases the driving force for the nonspecific adsorption. In toluene solution, there is no water structuring around the solute molecules, so there is negligible nonspecific adsorption. (8) The nonspecific capacity was also found to be influenced by temperature. As temperature increased, the nonspecific capacity decreased. As temperature increases, water structuring around the solute molecules decreases. Thus, the entropy gain upon the loss of water structuring is less at higher temperatures than at lower temperatures. As the extent of hydrophobic bonding decreases with increasing temperature, lower KHB values are observed. Importantly, the specific capacity constant, K2, remained virtually constant as the temperature was changed. (9) In the presence of anions from neutral salts, the nonspecific capacity constant (KHB) had values which increased in the order Cl- < Br- < NO3- < ClO4-. This behavior further supports the notion of hydrophobic bonding being responsible for the KHB term, because the order can be attributed to the abilities of these anions to decrease water structuring around the solute molecules. Among these anions, Cl- has the highest ability to decrease water structure around the solute molecules. This means that when Cl- is present in solution, the entropy gain due to the loss of water structuring is lower than for the other ionic species tested. (10) All of these adsorption studies support the notion that the M-LLE is a more appropriate model than the LLE for describing solute adsorption from solution by activated carbons. KHB has physical meaning and represents the entropically driven nonspecific adsorption of drugs and other compounds to the basal carbon surface. Acknowledgment. The authors thank Drs. Rama Abu Shmeis, Khouloud Alkhamis, Gerald M. Burke, Lian-Feng Huang, and William M. Kolling for helpful discussions. One of the authors (D. E. W.) acknowledges the Obermann Center for Advanced Studies, whose facilities greatly contributed to the completion of this paper. LA040093O
