Heterogeneity of Polymer-Based Active Carbons in Adsorption of

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Heterogeneity of Polymer-Based Active Carbons in Adsorption of Aqueous Solutions of Phenol and 2,3,4-Trichlorophenol K. La´szlo´,*,† P. Podkosćielny,‡ and A. Da¸ browski*,‡ Department of Physical Chemistry, Budapest University of Technology and Economics, H-1521 Budapest, Hungary, and Faculty of Chemistry, Department of Theoretical Chemistry, Maria Curie-Skłodowska University, pl. M. Curie-Skłodowskiej 3, 20-031 Lublin, Poland Received October 28, 2002. In Final Form: April 23, 2003 Heterogeneity effects that accompany adsorption of phenolic compounds from water by polymer-based activated carbons are investigated at different values of solution pH. Activated carbons prepared from poly(ethylene terephthalate) and polyacrylonitrile (APET and APAN, respectively) were used to adsorb phenol and 2,3,4-trichlorophenol in acidic (pH ) 3), unbuffered, and basic (pH ) 11) aqueous solutions. A Langmuir-Freundlich adsorption-isotherm equation was used to estimate the parameters that characterize adsorption of phenols from dilute solutions on heterogeneous surfaces. Adsorption energy distribution functions were calculated by a regularization method. Analysis of these functions for the APET and APAN carbons provides comparative information about their heterogeneity.

Introduction The heterogeneity of active carbon surfaces stems from two sources, geometrical and chemical. Geometrical heterogeneity is the result of differences in size and shape of pores, as well as cracks, pits, and steps. Chemical heterogeneity is associated with different functional groups, mainly oxygen groups that are located most frequently at the edges of the turbostratic crystallites, as well as with various surface impurities. Both chemical and geometrical heterogeneities contribute to the unique sorption properties of active carbons. Functional groups and delocalized electrons of the graphitic structure determine the apparent chemical character of an activated carbon surface.1 Oxygen, for instance, may be present in various forms, such as carboxyls, carbonyls, phenols, lactones, aldehydes, ketones, quinines, hydroquinones, anhydrides, or ether structures. These groups may also interact among themselves. Some groups, e.g., carbonyl, carboxyl, phenolic, hydroxyl, and lactonic, are acidic, while pyrone, chromene, and quinone are basic.2 Granular or powdered activated carbons are prepared from a variety of raw materials, among which the most frequently employed are hard coal, lignite, lignocellulosic materials, and certain polymers.2-12 Polymer precursors * Authors to whom correspondence may be addressed: K. La´szlo´, tel +36-1-463-1893, fax +36-1-463-3767, e-mail klaszlo@ mail.bme.hu; A. Da¸ browski, tel +48-81-5375605, fax +48-815375685, e-mail [email protected]. † Budapest University of Technology and Economics. ‡ Maria Curie-Skłodowska University. (1) Leoń Y Leoń, C. A.; Radovic, L. R. In Chemistry and physics of carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; Vol. 24, p 214. (2) La´szlo´, K.; Szu¨cs, A. Carbon 2001, 39, 1945. (3) Mahajan, O. P.; Moreno-Castilla, C.; Walker, P. L., Jr. Sep. Sci. Technol. 1980, 15, 1733. (4) La´szlo´, K.; Bo´ta, A.; Nagy, L. G. Carbon 2000, 38, 1965. (5) Deryło-Marczewska, A.; Marczewski, A. W. Langmuir 1997, 13, 1245. (6) Bo´ta, A.; La´szlo´, K.; Nagy, L. G.; Copitzky, T. Langmuir 1997, 13, 6502. (7) La´szlo´, K.; Tombaćz, E.; Josepovits, K. Carbon 2001, 39, 1217. (8) La´szlo´, K.; Tombaćz, E.; Kerepesi, P. Colloids Surf, A, in press. (9) La´szlo´, K. Prog. Colloid Polym. Sci. 2001, 117, 5.

are used preferentially if carbon with low inorganic content is needed. In addition to the nature of the starting material, the preparation process, including the method of carbonization, activation, and/or further treatment, also have a significant effect on the final surface properties. Since carbon has a high adsorption capacity for organic compounds, it is the most commonly used adsorbent for removing these compounds from aqueous media.13-14 The presence of water further modifies the chemistry of a surface, as its interaction with the specific groups on the carbon surface may modify their chemical behavior. Owing to the amphoteric character of a carbon surface, i.e., to the acidic and/or basic functional groups, the surface properties may be influenced by the pH value of the coexisting bulk liquid phase. When a dissolved chemical species that is to be removed, e.g., by adsorption, bears an acidic and/or basic character, any acidic or basic sites on the carbon may also participate in the interaction. In this paper, the interaction of activated carbon with weak aromatic acids is addressed, since the question of heterogeneity in the adsorption of aromatic compounds from water has until now attracted little general attention.15-25 Weak aromatic acids such as phenolic compounds are, however, widespread in many industrial (10) Juang, R.-S.; Tseng, R.-L.; Wu, F.-C. Adsorption 2001, 7, 65. (11) Wu, F.-C.; Tseng, R.-L.; Juang, R.-S. J. Environ. Sci. Health 1999, A34, 1753. (12) Alaya, M. N.; Hourieh, M. A.; Youssef, A. M.; El-Sejariah, F. Adsorpt. Sci. Technol. 2000, 18, 27. (13) Streat, M.; Patrick, J. W.; Camporro Perez, M. J. Water Res. 1995, 29, 467. (14) Dvorak, B. I.; Lawler, D. F.; Speitel, G. E.; Jones, D. L.; Badway, D. Water Environ. Res. 1993, 65, 827. (15) Jaroniec, M.; Deryło, A. J. Colloid Interface Sci. 1981, 84, 191. (16) Jaroniec, M.; Madey, R. Physical Adsorption on Heterogeneous Solids; Elsevier: Amsterdam, 1988. (17) Marczewski, A. W.; Deryło-Marczewska, A.; Jaroniec, M. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2951. (18) Barton, S. S.; Evans, M. J. B.; MacDonald, J. A. F. Pol. J. Chem. 1997, 71, 651. (19) Michot, L. J.; Didier, F.; Villie´ras, F.; Cases, J. M. Pol. J. Chem. 1997, 71, 665. (20) Deryło-Marczewska, A.; Jaroniec, M.; Gelbin, D.; Seidel, A. Chem. Scr. 1984, 24, 239. (21) Jaroniec, M.; Deryło, A.; Marczewski, A. Monatsh. Chem. 1983, 114, 393.

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effluents. Besides being carcinogenic, they cause an unpleasant taste and odor, even at low concentrations, in drinking water/water supplies. Another undesirable consequence is that in the chlorinating process commonly employed to purify drinking water, phenol reacts with chlorine to produce carcinogenic (poly)-chlorinated compounds. The high toxicity of phenol and its derivatives, together with the difficulties inherent in their biological degradation, are a strong driving force to develop new methods of treatment. Nonetheless, the best prospects for overall treatment appear to reside in adsorption. Our main object in this paper is to investigate the effects of heterogeneity of polymer-based activated carbons upon the adsorption of phenolic compounds from water, at different pH values of the solution. The heterogeneous properties of solid adsorbents can essentially be described by their energy-distribution function. To obtain this function, the Fredholm integral equation of the first kind must be solved. This integral equation, however, is numerically ill-posed; i.e., small changes in the overall adsorption isotherm caused by experimental errors can significantly influence the distribution function.26,27 Exclusive use of the least-squares method for solving illposed problems can thus lead to distorted results. In this work, a regularization method was therefore employed, which takes into account the ill-posed character of the integral equation.26-31 In our study, activated carbons prepared from poly(ethylene terephthalate) (PET)2 and from polyacrylonitrile (PAN)8 were used to adsorb both phenol and a potential chlorinated derivative, 2,3,4-trichlorophenol, dissolved in aqueous solutions that were acidic (pH ) 3), unbuffered, and basic (pH ) 11), respectively. In our previous papers,2,8 the Langmuir adsorptionisotherm equation was used to describe adsorption of phenols. Most of isotherms analyzed were of type L in Giles’s classification,32 and the Langmuir equation provided a good fit. The surfaces of carbons are, however, highly heterogeneous, and the real energy distribution for interactions between phenol and active carbon cannot be determined on the basis of the Langmuir adsorptionisotherm equation. A more advanced model should therefore be used instead of the popular Langmuir form, originally derived to describe adsorption on homogeneous surfaces. A model based on the Langmuir-Freundlich (LF) adsorption-isotherm equation was therefore chosen. The LF equation has been used successfully for single (or multi) solute adsorption from dilute solutions on heterogeneous surfaces.16,20-23,25 It should be emphasized that the present approach is valid mainly for adsorption of phenols from unbuffered (22) Deryło-Marczewska, A.; Marczewski, A. W. In Adsorption Science and Technology; Do, D. D., Ed.; World Scientific: Singapore, 2000; p 174. (23) Deryło-Marczewska, A.; Marczewski, A. W. Appl. Surf. Sci. 2002, 196, 264. (24) Stoeckli, F.; Lo´pez-Ramoń, M. V.; Moreno-Castilla, C. Langmuir 2001, 17, 3301. (25) Podkosćielny, P.; Da¸ browski, A.; Marijuk, O. V. Appl. Surf. Sci. 2003, 205, 297. (26) von Szombathely, M.; Braüer, P.; Jaroniec, M. J. Comput. Chem. 1992, 13, 17. (27) Heuchel, M.; Braüer, P.; von Szombathely, M.; Messow, U.; Einicke, W. D.; Jaroniec, M. Langmuir 1993, 9, 2547. (28) Heuchel, M.; Jaroniec, M.; Gilpin, R. K.; Braüer, P.; von Szombathely, M. Langmuir 1993, 9, 2537. (29) Heuchel, M.; Jaroniec, M. Langmuir 1995, 11, 1297. (30) Podkosćielny, P.; Da¸ browski, A.; Bu¨low, M. Appl. Surf. Sci. 2002, 196, 312. (31) Da¸ browski, A.; Podkosćielny, P.; Bu¨low, M. Colloids Surf., A 2003, 212, 109. (32) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J. Chem. Soc. 1960, 3974.

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and acidic solutions, because phenols do not dissociate under these conditions. At pH ) 11, however, phenols do dissociate, i.e., pH > pKa for phenol or 2,3,4-trichlorophenol, and the related mechanism of adsorption can be explained by double electric layer theory.33 Nevertheless, for consistency with the previous results based on the Langmuir model,2,8 which used data for pH ) 11, the present approach, in which the LF equation and the regularization method are used to derive the energy distribution functions, will be applied to the case pH ) 11 as well. Theoretical Section The isotherm equations for single-solute adsorption from dilute solutions may be deduced from those used for adsorption of binary liquid mixtures. Sircar34 was the first to propose an approach based on the competitive character of adsorption for dilute solutions. One component of the “1 + 2” binary liquid mixture is considered as the solute (1), whereas the second component is treated as the solvent (2). When the concentration of solute is very small, the excess adsorption of the solute, n1σ(n), is identical to the absolute adsorbed amount of this solute, n1, i.e., n1σ(n) = n1 for x1l = 0. Additionally, the mole fraction x1l may be replaced by the solute equilibrium concentration, c1. The overall isotherm equations for single-solute monolayer adsorption may be divided into two main groups.16 The first group (generalized Langmuir equation) has the form

θ1,t(c1) )

(K h c1)η

(1)

[1 + (K h c1)κ]η/κ

where K h is the equilibrium constant for a heterogeneous solid, η and κ are the heterogeneity parameters, θ1,t(c1) is the fractional coverage of the adsorbent surface, and θ1,t(c1) ) n1/no1, where no,1 is the monolayer-adsorption capacity of the solute. The equilibrium constant K h has the form

h 12) K h ) Ko exp(E

(2)

h 12 is the where Ko is a pre-exponential factor and E characteristic energy that determines the position of the distribution function on the energy axis. At special values of the heterogeneity parameters, eq 1 transforms into various known adsorption-isotherm equations. For example, for η ) κ, eq 1 becomes the Langmuir-Freundlich (LF) type equation. For η ) 1, eq 1 has the form of the Jossens equation,35 and for η ) κ ) 1, eq 1 becomes the standard Langmuir equation, which describes single-solute adsorption on a homogeneous solid. The second group of adsorption-isotherm equations may be written as follows16

{ [ ( )] }

˜ j kT ln θ1,t(c1) ) exp -B

c1sol c1

j

(3)

for (33) Mu¨ller, G.; Radke, C. J.; Prausnitz, J. M. J. Colloid Interface Sci. 1985, 103, 466 and 484. (34) Sircar, S.; Myers, A. L.; Molstad, M. C. Trans. Faraday Soc. 1970, 66, 2354. (35) Jossens, L.; Prausnitz, J. M.; Fritz, W.; Schlu¨nder, E. U.; Myers, A. L. Chem. Eng. Sci. 1978, 33, 1097.

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c1 < c1sol

be written in a more general form as

where c1sol is a parameter that is often identified with the solubility of the solute, B ˜ j is a heterogeneity parameter, k is the Boltzmann constant, and T is the absolute temperature. For j ) 1, eq 3 becomes the Freundlich-type isotherm, but for j ) 2 it becomes the Dubinin-Radushkevich (DR) isotherm equation. The LF equation has been used with success to estimate the monolayer capacity in the case of adsorption, for binary nonelectrolyte liquid mixtures on solid surfaces.36,37 It was also found to be appropriate for describing adsorption from dilute solutions on heterogeneous surfaces; cf., for example, refs 16, 20-23, and 25. It has the following form

g(y) )

θ1,t(c1) )

(K h LFc1)η

(4)

1 + (K h LFc1)η

where K h LF denotes the mean equilibrium constant and η is the heterogeneity parameter, 0 < η < 1. Values of the monolayer-adsorption capacity, no,1, as well as of the parameters η and K h LF were calculated by the MINUIT procedure.38 The associated square deviation takes the standard form

F ˆ )

(n1(exp) - n1(theor)) ∑ i)1

(5)

where n1(exp) denotes the experimental values of amount of adsorbed solute, i.e., phenols, and n1(theor) are the theoretical ones. Calculation of the Adsorption-Energy Distribution Function. For adsorption of a “1 + 2” liquid mixture on a heterogeneous solid surface, each adsorption site is characterized by the energy difference E12 ) E1 - E2 of the two components, i.e., in our case, phenol (or 2,3,4trichlorophenol) (1) and water (2). For dilute solutions, the total fractional coverage of solute, θ1,t(c1), in the surface phase may be expressed as27,29

θ1,t(c1) )

∫EE

12max

12min

Φx exp(E12/RT) 1 + Φx exp(E12/RT)

F(E12) dE12

(6)

where Φ ) Φ(c1,θ1,t) is a model-dependent function,27 F(E12) is the normalized distribution function, which characterizes the adsorbent heterogeneity in terms of E12 ) E1 E2, x ) c1/c1sol, where c1sol is the solubility of the solute, R is the gas constant, and T is the absolute temperature. The function, Φ, accounts for all molecular interactions in the bulk and surface phases. It depends on the model assumed for those phases and on the topography of adsorption sites on the solid surface. If a lattice model is used to describe molecular interactions in both phases, Φ can be represented by the ratio of the molecular partition functions for the surface and bulk solutions.27 In our calculations, this function was assumed to be equal to unity.29 The program INTEG,26 based on the regularization method, was used for inverting eq 6 with respect to the energy distribution function, F(E12). Equation 6 is a linear Fredholm integral equation of the first kind, which can (36) Podkosćielny, P.; Da¸ browski, A. Colloids Surf., A 2000, 162, 215. (37) Podkosćielny, P.; Da¸ browski, A.; Leboda, R. Colloids Surf., A 2001, 182, 219. (38) James, F.; Roos, M. Comput. Phys. Commun. 1975, 10, 343.

(7)

where g(y) is a known function, calculated from the experimental adsorption isotherm, the integral kernel, K(z,y), represents the local isotherm of eq 6, and f(z) ) F(E12) denotes the energy-distribution function. The regularization method requires the discretization of the integral equation by a quadrature method. Equation 7 must therefore be transformed into a system of linear equations, g ) Af. The one-dimensional matrixes g and f respectively represent the functions g and f, and A is a two-dimensional matrix that represents the kernel, K(z,y). Regularization consists of replacing the ill-posed problem of minimizing the functional, ||Af - g||2, by a well-posed one, which smoothes the distribution function and distorts it only insignificantly. This can be done by adding a term, γ||Cf||2, to the minimization functional26

S(f) ) ||Af - g||2 + γ||Cf||2

(8)

Above, γ is the regularization parameter, which is a measure for weighting both terms in eq 8. Usually,26-31 the following condition holds

||Cf||2 ≈

n

2

∫ab K(z,y) f(z) dz

∫ab f2(z) dz

(9)

Regularization methods make no assumption about the shape of the energy distribution curve. Experimental Section I.1. Preparation and Characterization of APET Carbon. The granular activated carbon (APET) was obtained from poly(ethylene terephthalate) (PET). The preparation and the physical properties of this carbon, as well as the analysis of the nitrogenadsorption data, were reported earlier.2,6 The BET specific surface area, SBET, of this activated carbon, deduced from the nitrogenadsorption isotherm, amounts to 1170 m2/g, and the average pore radius calculated for cylindrical pore geometry as 2Vtot/ SBET is 1.07 nm. According to the pore-size distribution, it contains both micro- and mesopores. The total pore volume, Vtot ) 0.625 cm3/g, of which 0.425 cm3/g can be ascribed to the micropore volume, is found from a t-plot.9 The surface chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS) using an XR3E2 (VG Microtech) twin anode X-ray source and a Clam2 (X-ray photoelectron spectroscopy) hemispherical electron energy analyzer.2,7 High-resolution spectra of the O1s and C1s signals were recorded in 0.05 eV steps with a pass energy of 20 eV. After subtraction of the linear baseline, curve fitting was performed assuming a Gaussian peak shape. The defined number of peaks is identified by deconvoluting the spectrum. The specified binding energy (electronvolts) corresponds to each Gaussian peak, and it indicates the type of surface group(s) in which a given element (i.e., C or O) is present. The additional information obtained from XP spectra is the element percentage in a given functional group (element structure). The distributions of the carbon and oxygen structures (atom %, (0.5-0.8%) derived from the XP C1s and XP O1s spectra, respectively, were published recently in tabulated form.2,7 The XPS results showed that the surface of the APET carbon contains 95.7 atom % carbon (of which 60 atom % of surface carbon atoms are present in graphitic form) and 4.3 atom % oxygen, which leads to an O/C ratio of 0.045. The carbon surface reveals a basic surface character as the initial pH value of the carbon suspension (0.5 g of carbon/100 cm3 0.01 M NaCl solution under CO2-free conditions) was found to be 8.1.7 The Boehm titration method was used to determine the number of oxygenated surface groups. 81.6% of the functional groups are basic. As carboxylic groups were not detected within the experimental error of the Boehm titration method,2 it follows

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that the acidic character arises from the phenolic (81.1%) and lactonic (18.9%) groups. I.2. Preparation and Characterization of APAN Carbon. The APAN carbon used in this work was obtained from polyacrylonitrile (PAN) by a two-step physical activation process.6,8 The preparation and the physical properties of this carbon, including the analysis of nitrogen adsorption and small-angle X-ray scattering data, were reported elsewhere.4,6,7 This carbon is highly microporous, the BET surface area being 544 m2/g and Vtot ) 0.278 cm3/g. The average pore radius is 1.02 nm. The total micropore volume, determined from a two-term DR equation,4 was W0 ) 0.266 cm3/g. The surface chemical composition of the APAN carbon samples was also determined by XPS. The distributions of carbon, oxygen, and nitrogen structures (atom %) from the XP C1s, XP O1s, and XP N1s spectra, respectively, were published recently in tabulated form.7,8 According to the analysis of the N1s spectrum, several forms of nitrogen (e.g., quaternary nitrogen, pyrrolic-N, pyridinicN) exist on the surface of this carbon sample. These sites reveal a basic character, and their protonation results in the formation of positive charges.8 The XPS results showed that the surface of the APAN carbon contains 89.4 atom % carbon (of which 50.4 atom % in the form of graphitic carbon), 5.3 atom % oxygen, and about 5.3 atom % nitrogen. The initial pH of the carbon suspension, determined in the same way as above, was found to be 7.8. Boehm titration was used to determine the number of surface groups of the carbon studied.8 About 75% of the functional groups are basic and about 25% are acidic. II. Sorption from Dilute Aqueous Solutions of Phenols on APET and APAN Carbons. Solutions of phenol and 2,3,4trichlorophenol were prepared using doubly distilled water, or the appropriate buffer solutions (Titrisol pH ) 3, and Titrisol pH ) 11, Merck). Carbon (0.05 g) was shaken with 5-60 mL of phenol (5 mmol/L) or 2,3,4-trichlorophenol (2 mmol/L) solutions for 24 h and 7 days, respectively, in sealed vials at room temperature. Contact times were determined from preliminary kinetic measurements.39 Initial and equilibrium concentrations were determined by measuring the UV absorption of the aromatic solutes.2,8

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Figure 1. Adsorption isotherms from aqueous phenol solutions of various pH on APET carbon (ambient temperature). Symbols are the measured values; lines are the theoretical isotherms calculated from the LF equation.

Results and Discussion The experimental adsorption isotherms of phenol and 2,3,4-trichlorophenol on APET and APAN active carbons at room temperature in unbuffered, acidic (pH ) 3) and basic (pH ) 11) aqueous solutions are presented in Figures 1-4, respectively. They are the same as those reported in refs 2 and 8, but the theoretical adsorption isotherms were estimated previously from the standard Langmuir equation. Here, the symbols denote the experimental points, while the curves are the theoretical isotherms calculated using the LF model (eq 4). Generally speaking, 2,3,4-trichlorophenol data are always more scattered than those of phenol. This can be attributed both to the weaker net interaction energies (see below) and to the 0.63-0.72 nm2 cross-sectional area of the trichlorinated phenol molecule (typically 0.43 nm2 for phenol).2 The extremely narrow average pore size suggests that the introductory meso- and macropores are practically absent. Access for the larger solute molecules is thus limited, resulting in strong scatter of the measured data. The phenol adsorption isotherms in Figures 1-4 belong to type L in Giles’s classification,32 except for the unbuffered 2,3,4-trichlorophenol curve (H-type) (cf. Figures 2 and 4). The shape of the isotherm provides qualitative information on the nature of the solute-surface interaction. Type L means that the aromatic ring adsorbs parallel to the surface and that no strong competition exists between the adsorbate and the solvent to occupy the (39) Bo´ta, A.; La´szlo´, K.; Nagy, L. G.; Subklew, G.; Schlimper, H.; Schwuger, M. J. Adsorption 1996, 3, 81.

Figure 2. Adsorption isotherms from aqueous 2,3,4-trichlorophenol solutions of various pH on APET carbon (ambient temperature). Symbols are the measured values; lines are the theoretical isotherms calculated from the LF equation.

adsorption sites; H curves, however, are associated with strong adsorbate-surface interactions. The results of calculations for the LF equation are summarized in Table 1 for APET carbon and in Table 2 for APAN carbon, respectively. The tables include the values of monolayer capacity no,1, the heterogeneity parameter η, the mean equilibrium constant K h LF, and the function F ˆ (eq 5), respectively. The last column shows the values of surface area available for one adsorbate molecule ωLF. This parameter is calculated using the relation ωLF ) SBET/(no,1NA), where NA is Avogadro’s number. As mentioned in the Introduction, the Langmuir model was used previously2,8 to describe phenol and 2,3,4trichlorophenol adsorption on APET and APAN active carbons. It can be seen that the theoretical Langmuir (cf. refs 2 and 8) and LF isotherms are similar to each other. This is due to fact that the heterogeneity parameters η are close to 1 (cf. Tables 1 and 2, 0.799 < η < 0.988) and that for η ) 1, the LF isotherm reduces to the Langmuir form.


Figure 3. Adsorption isotherms from aqueous phenol solutions of various pH on APAN carbon (ambient temperature). Symbols are the measured values; lines are the theoretical isotherms calculated from the LF equation.

Figure 4. Adsorption isotherms from aqueous 2,3,4-trichlorophenol solutions of various pH on APAN carbon (ambient temperature). Symbols are the measured values; lines are the theoretical isotherms calculated from the LF equation.

Moreover, it has been proved40 that when the phenol molecules are adsorbed in a flat position, they may change the apparent heterogeneity of the solid surface, because they may occupy adsorption sites of different energies. The values of the function F ˆ (cf. Tables 1 and 2) for the LF equation are smaller by almost 1 order of magnitude than the analogous values calculated with the Langmuir equation (unpublished data). This is not surprising, since fitting of experimental data with a three-parameter equation, viz., the LF equation, yields better results. In phenol adsorption on APET carbon (Table 1), the order of the monolayer capacities is pH ) 11 < pH ) 3 < unbuffered, while in the case of 2,3,4-trichlorophenol the sequence is pH ) 11 < unbuffered < pH ) 3. As for the values of the heterogeneity parameters, η, the order for phenol is unbuffered < pH ) 3 < pH ) 11. This means that the surface has the most heterogeneous character for the unbuffered solution and the least for solution with pH ) 11 (η is closer to one). In the case of 2,3,4trichlorophenol, the sequence of η is as for phenol adsorption. (40) Da¸ browski, A.; Jaroniec, M. Adv. Colloid Interface Sci. 1990, 31, 155.

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In phenol adsorption on APAN carbon (Table 2), the order of the monolayer capacities is pH ) 11 < unbuffered < pH ) 3, and in the case of trichlorophenol, the order of capacities is the same. The sequence of heterogeneity parameters, η, for phenol is unbuffered < pH ) 11 < pH ) 3. This means that the surface appears most heterogeneous for the unbuffered solution (η ) 0.841) and least heterogeneous for the solution with pH ) 3 (η ) 0.962). In the case of trichlorophenol the sequence of η is unbuffered < pH ) 3 < pH ) 11. We can conclude that pH influences both the adsorption capacity and the surface heterogeneity. Not only the electrolytic dissociation, and thus the solubility of the dissolved molecules, but the states of the surface functionalities are modified by the pH of the aqueous solution.2,7,8 Moreover, the numerical values of K h LF are 1 or 2 orders of magnitude higher in trichlorophenol than in phenolic systems. This phenomenon can be explained by the significantly smaller solubility of 2,3,4-trichlorophenol in water in comparison with phenol.2 The electron withdrawal effect of the chlorine atoms reduces the electron density of the aromatic ring, which is reflected in the lower pKa value as well. As illustrated in Figures 1-4 and confirmed by the fits to the LF equation, adsorption from solution is limited, and no,1 corresponds to a monolayer. The data of Tables 1 and 2 also suggest that the quantity of phenol, no,1, adsorbed by the solids corresponds to a smaller equivalent volume than that required to fill the micropores. Using the molar volume of solid phenol at 293 K, Vm ) 89 cm3,24 yields the equivalent volumes V eq ) no,1Vm, which are always smaller than the micropore volume. The values of Veq are 0.198, 0.231, and 0.114 cm3/g for pH ) 3, unbuffered, and pH ) 11, respectively, as opposed to a micropore volume of 0.425 cm3/g in the case of the APET carbon. For the APAN carbon the equivalent volumes are equal to 0.073, 0.072, and 0.066 cm3/g for pH ) 3, unbuffered, and pH ) 11, respectively, compared to a micropore volume of 0.266 cm3/g. Although the above calculations do not take into account phenol adsorption on the external surface, it may be concluded that adsorption of phenols by activated carbons takes place concurrently with the adsorption of the solvent.2 Recently, Stoeckli et al.24 pointed out that molar enthalpy values for the transfer of phenol from solution to active carbon surfaces (PLW) or to the carbon black surface (N234-G) are closely similar. They amount to 30.1 kJ/mol for the active carbon PLW and 34.34 kJ/mol for carbon black N234-G. The conclusion was drawn that the adsorption processes on porous and nonporous carbons are similar. The adsorption-energy distribution functions obtained by inversion of eq 6 are shown in Figures 5 and 6 for APET carbon and in Figures 7 and 8 for APAN carbon, respectively. Numerically stable functions were obtained with the value of the regularization parameter, γ ) 0.1. Figure 5 shows the calculated values of the adsorptionenergy distribution functions for phenol on APET active carbon in acidic (pH ) 3), unbuffered, and basic (pH ) 11) aqueous solutions, denoted by open circles and diagonal and upright crosses, respectively. The single peak of the energy-distribution function for (pH ) 11) shows a maximum at about E12max ) E1 - E2 ) 18.9 kJ/mol. That for (pH ) 3) is lower, broader (∆E12 ) 9.6 kJ/mol), and shifted toward slightly higher energy (E12max ) 19.3 kJ/ mol) compared to the (pH ) 11) peak. The lowest and the broadest peak, covering the widest range of adsorption energies E12 (∆E12 ) 10.2 kJ/mol), is that for the unbuffered solution.

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Table 1. Parameters for Linear Form of the LF Equation (eq 4) for APET Carbon solute phenol 2,3,4-trichlorophenol

medium

no,1 (mmol/g)

η

K h LF

F ˆ

ωLF (nm2)

pH ) 3 unbuffered pH ) 11 pH ) 3 unbuffered pH ) 11

2.227 2.590 1.282 3.475 3.064 1.851

0.941 0.892 0.980 0.931 0.799 0.988

1.90 5.35 2.56 18.37 731.8 51.40

0.0103 0.0381 0.0035 0.0814 1.9956 0.0373

0.873 0.750 1.516 0.559 0.634 1.050

Table 2. Parameters for the Linear Form of the LF Equation (eq 4) for APAN Carbon solute phenol 2,3,4-trichlorophenol

medium

no,1 (mmol/g)

η

K h LF

F ˆ

ωLF (nm2)


0.822 0.815 0.742 1.007 0.973 0.657

0.962 0.841 0.911 0.886 0.845 0.888

0.602 7.56 4.99 27.05 166.7 10.11

0.0002 0.0073 0.0004 0.0089 0.2576 0.0019

1.099 1.109 1.218 0.897 0.929 1.37T

Figure 5. Adsorption-energy distribution functions for phenol on APET active carbon.

Figure 6. Adsorption-energy distribution functions for 2,3,4trichlorophenol on APET active carbon.

The characteristics of the adsorption-energy distribution functions for APET and APAN active carbon are given in Tables 3 and 4, respectively. The tables list the values of peak maximum, E12max, peak height, the range of peak locations, and the characteristic 1/e width, ∆E12, respectively. The direction of these changes correlates well with the order of heterogeneity parameters, η, i.e., unbuffered < pH ) 3 < pH ) 11 in the LF equation. Otherwise we know that the smaller the value of the heterogeneity parameter η, the broader is the adsorption-distribution function; i.e., it spreads over a wider range of possible adsorption energies. Figure 6 shows the adsorption-energy distribution functions for 2,3,4-trichlorophenol on APET active carbon in acidic (pH ) 3), unbuffered, and basic (pH ) 11) aqueous solutions, denoted as for phenol. The sequence of peaks

Figure 7. Adsorption-energy distribution functions for phenol on APAN active carbon.

Figure 8. Adsorption-energy distribution functions for 2,3,4trichlorophenol on APAN active carbon.

is the same as that for phenol (see also Table 3), correlating well with the sequence of the heterogeneity parameters, η, for the LF equation (see Table 1). Figure 7 exhibits the energy-distribution functions for phenol on APAN active carbon in acidic, unbuffered, and basic aqueous solutions, denoted as before. The sequence of the peaks in the direction of increasing energy E12 correlates again very well with the sequence of the values of the heterogeneity parameter, η. Figure 8 shows the energy-distribution functions for 2,3,4-trichlorophenol on APAN carbon. The sequence of the peaks is the same as for the 2,3,4-trichlorophenol on APET carbon, and correlation with the values of heterogeneity parameters (for the LF equation) is maintained. The calculated F(E12) distributions suggest that the adsorption energy of phenol on APET carbon is stronger by about 20 kJ/mol than that of water. For 2,3,4-


Langmuir, Vol. 19, No. 13, 2003 5293

Table 3. Characterization of Adsorption-Energy Distribution Functions for Phenol and 2,3,4-Trichlorophenol on APET Active Carbon solute phenol 2,3,4-trichlorophenol

medium

E12max (kJ/mol)

peak height (mol/kJ)

peak location (kJ/mol)

∆E12 (kJ/mol)


19.3 19.9 18.9 13.0 15.4 11.1

0.151 0.124 0.160 0.164 0.054 0.172

2.4-30.2 2.3-36.0 2.3-27.4 0-25.5 0-28.3 0-22.4

9.6 10.2 8.7 9.8 10.3 8.4

Table 4. Characterization of Adsorption-Energy Distribution Functions for Phenol and 2,3,4-Trichlorophenol on APAN Active Carbon solute phenol 2,3,4-trichlorophenol

medium

E12max (kJ/mol)

peak height (mol/kJ)

peak location (kJ/mol)

∆E12 (kJ/mol)


15.4 21.0 20.5 9.4 13.1 7.3

0.145 0.121 0.136 0.134 0.082 0.138

1.1-24.3 11.1-35.1 6.5-32.2 0-22.1 0-29.6 0-17.3

9.8 9.9 8.7 11.2 17.3 9.0

trichlorophenol, the particular values are ca. 11-15 kJ/ mol (cf. Table 3 and Figures 5 and 6). The interaction energy values for APAN are somewhat higher for phenol (e.g., 21 kJ/mol for unbuffered solution) and less for 2,3,4trichlorophenol (cf. Table 4 and Figures 7 and 8). A comparison of the resulting F(E12) distributions can be made with calorimetric data in the literature for the E12max values in unbuffered phenol solutions and for available molar enthalpies in phenol/water systems on nonmodified active carbons. Unfortunately, no calorimetric data were available for adsorption of 2,3,4-trichlorophenol. The enthalpy for the transfer of phenol from solution to PLW24 active carbon surfaces found by Stoeckli et al. was 30.1 kJ/mol, while the enthalpy change was ca. 30.0 kJ/mol for another active carbon.24 Other data for PLW active carbon are given in ref 24; e.g., Stotal ) 1097 m2/g and total micropore volume W0 ) 0.45 cm3/g (from the Dubinin-Astakhow (DA) equation). These enthalpies are similar to the energy 25 kJ/mol found by Salvador and Merchań41 for a typical active carbon (SBET ) 621 m2/g), on the basis of kinetic experiments (thermally programmed adsorption and desorption). Barton et al.18 found the molar enthalpy change to be equal to 22 kJ/mol for the phenol/water system on to ARBPL porous carbon (SBET ) 1088 m2/g). In that paper, only the total number of acidic sites and basic sites were reported. About 75% of the functional groups were basic in nature. For the APET carbon of the present investigation, SBET was 1170 m2/g, and 81.6% of the total functional groups were basic.2 Unfortunately, this comparison is incomplete, since only a few characteristics are available. As to the above values of enthalpy changes, the interaction energies (20-21 kJ/mol) correspond to E12max values, and they have similar values. The reason for the discrepancy between these E12max values and enthalpy values from the literature is simple; they arise from differences in porosity of the carbons studied as well as from a different surface chemistry, i.e., different basic sites interact with phenols with different energy. Generally it was found2,8 that the possible interactions between the carbon surface and phenols are (a) electron donor-acceptor interaction between the aromatic phenolic ring and the basic surface oxygens, (b) dispersion effect between the aromatic phenolic ring and the π electrons (41) Salvador, F.; Merchań, M. D. Carbon 1996, 34, 1543.

of the graphitic structure, and (c) electrostatic attraction and repulsion when ions are present. An accurate analysis of the mechanism of adsorption can be carried out by investigating the chemistry of the adsorbed layer on a microscopic scale using, for example, infrared spectroscopy.42 The nature of the specific interaction between phenol, m-nitrophenol, and p-nitrophenol and the surface of an active carbon, when adsorbed from aqueous solution, was studied by Mattson et al.43 using infrared reflection spectroscopy. They concluded that neither of the substituent groups was involved directly in the interaction with the surface but they contribute to the electron acceptor characteristics of the aromatic ring of the solute. A nitro group as well as a -Cl group13,43,44 acts as a strong electron-withdrawing group in reducing the overall electron density in the π-system of the ring. The interaction (attraction) with the carbon surface is thus enhanced with respect to the interaction of phenol with the surface. It is suggested that phenolic compounds adsorb on active carbon via a donor-acceptor complex in which basic surface oxygen groups act as electron donor and the aromatic ring of the solute acting as acceptor. Because of the π-system interaction, it is expected that the solute molecules will adsorb in planar orientation. Moreover, another type of interaction is possible. There are basic sites on the surface of the carbons, probably located at π-electron-rich regions within the basal planes of carbon crystallites (away from the crystallite edges), which interact by dispersion forces with the π-electrons in phenol.45,46 Experimental support for this view was given by Mahajan et al.,3 who studied phenol adsorption on graphite and boron-doped graphite samples. The results obtained showed that substitution of boron into the lattice of polycrystalline graphite, with accompanying removal of π-electrons from graphite, resulted in a decrease of phenol uptake from water. The adsorption mechanism of phenol and 2,3,4-trichlorophenol on APET and APAN active carbons expected from the chemical behavior is in agreement with the (42) Parfitt, G. D.; Rochester, C. H. In Adsorption from solution at the solid/liquid interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; p 3. (43) Mattson, J. S.; Mark, H. B., Jr.; Malbin, M. D.; Weber, W. J., Jr.; Critenden, J. C. J. Colloid Interface Sci. 1969, 31, 116. (44) Moreno-Castilla, C.; Rivera-Utrilla, J.; Lopez-Ramon, M. V.; Carrasco-Marin, F. Carbon 1995, 33, 845. (45) Couglin, R. W.; Ezra, F. S. Environ. Sci. Technol. 1968, 2, 291. (46) Leoń Y Leoń, C. A.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992, 30, 797.

5294


distribution functions obtained. By setting the pH of the solution, the protonation of both the surface and the phenols may be controlled. The majority of the functional groups are basic for the carbons studied, according to both the pH measurement and the titration results. However, there is no direct relation between the numbers of the titrated acidic/basic groups and the pH value.47 Part of the basic character traces back to the delocalized electrons of the graphitic structure. At pH ) 3, both the functional groups on the carbon surface and the phenolic compounds are in nonionized forms (pH < pKa). The surface groups are either neutral or positively charged.24,44,48 As can be deduced from the values of K h LF, the interaction between the carbon surface and the phenolic compounds is the weakest in this case and can be attributed to dispersion effects. The weak interaction in the case of phenol results in a competitive adsorption of water molecules, yielding a lower surface concentration than that obtained with 2,3,4-trichlorophenol. The enhanced interaction in the case of trichlorophenol is due to the electron-withdrawal phenomenon of the three chlorine substituents, as they reduce the overall electron density of the aromatic ring. At pH ) 11, the phenols dissociate, forming phenolate anions, while the surface functional groups are either neutral or negatively charged. The electrostatic repulsion between the identical charges lowers the adsorption capacities, no,1, in the case of phenol and 2,3,4-trichlorophenol. Besides, the phenolate anions are more soluble in aqueous solution, and stronger adsorbate-water bonds must be broken before adsorption can take place.48,49 However, some dispersion interaction may occur. The oxygen atoms of the nondissociated basic groups may form a donor-acceptor relation, as may be deduced from the elevated value of K h LF, especially in the case of the trichlorophenol. Competitive adsorption of water molecules must also be considered, because the surface area per adsorbed unit, ωLF, exhibits the highest value at pH ) 11 in the case of both phenols. If adsorption takes place from unbuffered solutions, both the phenols and the surface groups coexist in their protonated and deprotonated forms, depending on their pKa values (pKa ) 9.89 for phenol and 7.59 for 2,3,4trichlorophenol at room temperature). All three types of surface-phenol interactions may occur simultaneously. In the case of phenol, dissociation of the phenol molecules is negligible. Competition with water molecules still takes place, as indicated by the relatively high value of the molecular area ωLF. The larger dissociation constant value, Ka, of the 2,3,4-trichlorophenol results in dissociated trichlorophenolate ions, so that ionic attraction acts between the trichlorophenolate anions and the surface sites that are positively charged. The few strong ionic interactions are reflected by the H-type of the isotherm and the large values of K h LF. For APAN carbon the surface coverage is similar to that prevailing at pH ) 3, which lends support to the idea that part of the adsorbed species is in a position that is not parallel to the graphene layer. The comparison can be interesting between the extent of adsorption of phenols for pH ) 3 and for unbuffered (47) Tessmer, C. H.; Vidic, R. D.; Uranowski, L. J. Environ. Sci. Technol. 1997, 31, 1872. (48) Liu, X.; Pinto, N. G. Carbon 1997, 35, 1387. (49) Snoeyink, V. L.; Weber, W. J.; Mark, H. B. Environ. Sci. Technol. 1969, 3, 918.

La´ szlo´ et al.

solutions (see Figures 1-4). We mentioned that, as can be deduced from the K h LF values, the interactions for pH ) 3 between the carbon surface and the nondissociated phenolic compounds are the weakest in this case and can be attributed to dispersion effects (interactions between the aromatic phenolic rings and the π electrons of the graphitic structure). In the direction from pH ) 3 to the pH of the unbuffered solution, the surface protonation effect is reduced and the resultant surface charge decreases. If adsorption takes place from unbuffered solutions, the three above-mentioned types of surface-phenol interactions may occur simultaneously, increasing the interactions in comparison with those for pH ) 3 and thus enhancing the adsorption of phenols. For the adsorption of trichlorophenolate anions, the strong ionic attractions are reflected in the H-type of the isotherm (Figures 2 and 4) and the large values of K h LF, but suitable peaks (Figures 6 and 8) are the “broadest” and shifted toward higher energy E12. Conclusions The present investigation, based on the LangmuirFreundlich adsorption-isotherm model, shows that this expression provides a good description of phenol adsorption from dilute solutions on heterogeneous surfaces. Analysis of the heterogeneity parameter, η, of the LF equation indicates that the surface has the most heterogeneous character (smallest values of η) for the unbuffered solution for the adsorption both of phenol and of 2,3,4trichlorophenol on the APET and APAN polymer based carbons. The energy-distribution functions obtained by the regularization method are useful for comparing heterogeneities of different carbons with respect to a given liquid mixture (e.g., phenol or 2,3,4-trichlorophenol), because uncertainties in evaluating these functions are analogous for all the systems studied. The sequence of the energydistribution peaks in order of increasing energy E12 correlates well with the sequence of the parameters of heterogeneity values, η, for the LF equation. The peaks are the “lowest” and the “broadest” (greatest ∆E12) for unbuffered solutions, as is especially evident for 2,3,4trichlorophenol adsorption, which suggests that the largest spread of interactions between the carbon surface and phenols occurs with these solutions. The adsorption of the phenols studied takes place simultaneously with the adsorption of the solvent. Literature data on enthalpy values for the transfer of phenol from solution to surfaces of active carbons confirm the correctness of the adsorption energies obtained in this study. Acknowledgment. This research was supported by the National Research Fund (OTKA, Grant No. T 025581) and the National Research and Development Programs (NKFP 3/043/2001). The technical assistance of Emese Fu¨lo¨p and Gyo¨rgy Bosznai is gratefully acknowledged. Many thanks to Edina Csibi and Erik Geissler for fruitful discussions.The authors are indebted to Professor Martin Bu¨low (BOC Gases Technology, NJ) for contributing many important suggestions which have been included in this paper. LA026761S

Heterogeneity of Polymer-Based Active Carbons in Adsorption of

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