This paper studies the thermodynamic aspects of the processes of adsorption of phenol from dilute aqueous solutions on different commercial carbons, evaluating how to optimize the removal of this persistent contaminant. Two powdered activated carbons from two different companies were used: Tetrahedron Carbon (Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas Inc., USA). Both specific surface areas were measured by means of the BET method. The adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). Experimental isotherms were determined at 293 K, 303 K and 313 K. The Freundlich and Sips theoretical models were used to fit the experimental data. Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations. Thermodynamic parameters were calculated and correlated with the adsorption behaviours. The values of the thermodynamic parameters obtained indicate an exothermic and spontaneous process for both carbons, and mainly for Norit. This is due to the fact that there might be chemically activated regions on the surface of the Norit carbon, which give rise to combined mechanisms of physisorption and chemisorption.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Fertitta, Abel E.

Húmpola, P. D.

Odetti, H. S.

Vicente, José Luis

Servicio de Difusión de la Creación Intelectual

J. Chil. Chem. Soc., 58, Nº 1 (2013)1541THERMODYNAMIC ANALYSIS OF ADSORPTION MODELS OF PHENOL IN LIQUID PHASEON DIFFERENT ACTIVATED CARBONSP. D.  HÚMPOLA1 *, H. S. ODETTI1, A. E. FERTITTA2, J. L. VICENTE21Departamento de Química General e Inorgánica. Facultad de Bioquímica y Ciencias Biológicas. Universidad Nacional del Litoral, Casilla de Correo 242 (3000) Paraje “El Pozo”. Santa Fe. Argentina.  Teléfono/Fax: +54- 0342-4575212. 2Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), (UNLP – CIC – CONICET),Casilla de Correo 16, Sucursal 4 (1900) La Plata, Argentina.(Received: July 5, 2012 - Accepted: November 19, 2012)ABSTRACTThis paper studies the thermodynamic aspects of the processes of adsorption of phenol from dilute aqueous solutions on different commercial carbons, evaluating how to optimize the removal of this persistent contaminant. Two powdered activated carbons from two different companies were used: Tetrahedron Carbon (Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas Inc., USA). Both specific surface areas were measured by means of the BET method. The adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). Experimental isotherms were determined at 293 K, 303 K and 313 K. The Freundlich and Sips theoretical models were used to fit the experimental data. Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations. Thermodynamic parameters were calculated and correlated with the adsorption behaviours. The values of the thermodynamic parameters obtained indicate an exothermic and spontaneous process for both carbons, and mainly for Norit. This is due to the fact that there might be chemically activated regions on the surface of the Norit carbon, which give rise to combined mechanisms of physisorption and chemisorption. Key Words: adsorption, carbon, phenol, energy.1. INTRODUCTIONSince the 1920s, activated carbon has been used for the conventional treatment of water in order to improve its esthetic quality and to prevent potentially toxic compounds from getting to the consumer1. A number of organic compounds are normally found in contaminated water, and among them, phenols and molecules which contain phenolic groups are relatively frequent as contaminants2. Activated carbon adsorption has been suggested by the United States Environmental Protection Agency as one of the available technologies to control environmental contamination3. Adsorption of phenolic compounds on activated carbon surfaces is an important technology for water purification. Since adsorption on activated carbon can be affected by the surface properties of the carbons, the chemical composition of the surface groups can be a critical factor in determining the efficiency of the carbons in the removal of a given adsorbate in solution or their duration for use as adsorbents in a water treatment system4. Therefore, it is necessary to develop a rational framework for the selection of activated carbons in order to have a more thorough knowledge of the influence of the textural (i.e. pore sizes) and chemical carbon surface characteristics on the adsorption of organic contaminants from aqueous solutions5.It is also worth mentioning that the heats of adsorption or changes in enthalpies of adsorption (ΔH◦) have been considered as indicators of the energetic heterogeneity of an adsorbent in adsorption6, which depends on the distribution of micropores and mesopores of different shapes and sizes as well as on the distribution of adsorption sites of different chemical nature (polarity) within the pores. The difference between physisorption and chemisorption lies in the magnitude of the enthalpy and Gibbs free energy changes. Generally, the ΔG◦ value is in the range of 0 to −20 kJ.mol-1 and −80 to −400 kJ.mol-1 for physical and chemical adsorptions, respectively. The magnitude of the ΔH◦ value lies in the range of -2.1 to -20.9 and -80 to -200 kJ.mol-1 for physical and chemical adsorptions, respectively. Positive ΔS◦ value suggests the organization of the adsorbate at the solid/solution interface becomes more random, while negative value suggests the opposite fact. It is also supposed that the change of ΔS◦ value is related to the displacement of the adsorbed water molecules by the adsorbate7. Heat is always released in the process, thus adsorption enthalpy is negative, which means that there will be a decrease of entropy in adsorption. This is due to the fact that a molecule in its gaseous state or in solution has more freedom of movement than one that is bonded to the surface8, 9, 10. The aim of this paper is to study the adsorption of phenol from dilute aqueous solutions on different commercial carbons, evaluating how to optimize the removal of this persistent contaminant, and correlating the adsorption behaviours to the thermodynamic analysis.2. EXPERIMENTAL2.1 Characterization of AdsorbentsEquilibrium isotherms for the adsorption of nitrogen (N2) in the gas phase were measured at 77.4 K11. This was done for two commercially available active carbon samples which were obtained from the Tetrahedron Carbon (Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas Inc., USA) companies. Tables 1 and 2 present information about these materials obtained from BET standard analysis and the characterization of the microporosity from Dubinin-Astakhov (DA). At relative lower pressures, i.e., x = p/p0 ≤ 0.01, the micropore filling process takes place. The DA equation has been used to describe the volume filling of micropores and the energetic heterogeneity of solids12. DA proposed to relate the volume V occupied by the adsorbate with the differential molar work of adsorption, A = RT log(1/x), via the following equation:                                                                                                                                       (1)According to the Polanyi-Dubinin theories, C1,1 is the micropore volume, θ = V/C1,1, RT ln(1/x) is the differential molar work of adsorption; C1,2 = β E0, where E0 is the characteristic energy, β is the function of the adsorbate, and C1,3 describes the surface heterogeneity. For simplicity, if Eq. (1) is written C1,3 = 2, the DA equation reduces to the Dubinin-Radushkevich (DR) equation, first proposed to describe solids with low degree of burn-off. The exponent values proposed are C1,3 > 4 for zeolites, C1,3 =2-3 for homogeneous carbons, and C1,3 <2 for heterogeneous active carbons.  The DA equation corresponds to the choice of arbitrary values in the Weibull distribution function. With this additional parameter in the adsorption isotherm equation, the DA equation provides flexibility in the description of adsorption data of many microporous solids ranging from a narrow to a wide micropore size distribution13. The gas phase isotherms were volumetrically determined employing Pyrex conventional equipment. The corresponding values for N2 at 77.4 K are shown in Figures 1-2. Pressures were determined using absolute capacitance manometers, MKS – Baratron 122 AA-00010AB, with 1.0 ´  10-3 Torr maximum error. Work temperatures were thermostatically controlled. Temperature was measured with a digital thermometer, Altronix, with a Pt-100 (DIN) sensor head, previously calibrated against an oxygen vapour pressure thermometer with 0.1 K precision14. Gases, provided by Matheson Gas Products, were employed without any previous treatment and they were of high purity, greater than 99%.e-mail: p_humpola@hotmail.comJ. Chil. Chem. Soc., 58, Nº 1 (2013)15422.2 Adsorption MeasurementsThe adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). Experimental isotherms from dilute aqueous solutions were determined twice in order to assess their reproducibility, at a constant temperature of 293 K, 303 K and 313 K with periodic manual agitation. The initial phenol concentrations ranged from 0.1 to 0.6 mol.L-1 and the mass of carbon was in the range of 0.3000 g ± 0.0005 g. Initial and equilibrium concentrations of phenol were determined by molecular absorption spectrophotometry at 272.2 nm using the UV-Visible CECIL CE 3021 SERIES 3000 spectrophotometer15. These data were used to calculate the quantity adsorbed per unit mass of the adsorbent.2.3 Adsorption Isotherms Data were fitted by using Freundlich and Sips theoretical models and SigmaPlot V. 10.0 software. Their mathematical representations are given below:Freundlich Isotherm3. RESULTS AND DISCUSSIONTable 1. Activated Carbon Characterization. Specific surface area [m2.g-1]Monolayer volume [mL.g-1]TETRAHEDRON 545.32 148.6NORIT 680.23 154.3Table 2. DA Parameter Values for a Low Pressure Range (N2 at 77.4 K).C1,1 [mL.g-1] E0 [kJ.mol-1] C1,3 TETRAHEDRON 140   23.8   3.0   NORIT 204   21.8   1.9         (2)where n is the adsorbed amount at equilibrium(mol.g−1), Ceq  the adsorbate equilibrium concentration (mol. L−1), KF is a constant indicative of the relative adsorption capacity of the adsorbent (mol1−(1/n) L1/n g−1) and nF is a constant indicative of the intensity of the adsorption. The Freundlich expression is an exponential equation and therefore, assumes that as the adsorbate concentration increases, the concentration of adsorbate on the adsorbent surface also increases. The magnitude of the exponent nF gives an indication on the favourability of adsorption. It is generally stated that values of nF in the range 2–10 represent good, 1–2 moderately difficult, and less than 1 poor adsorption characteristics. Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations16, 17, 18.Sips IsothermRecognizing the problem of the continuing increase in the adsorbed amount with an increase in concentration in the Freundlich equation, Sips pro-posed an equation similar in form to the Freundlich equation, but it has a finite limit when the concentration is sufficiently high. The Sips isotherm equation is sometimes called the Langmuir-Freundlich equation in the literature because it has the combined form of Langmuir and Freundlich equations.Fig. 1: Isotherms of  N2 adsorption – desorption on Tetrahedron Carbon at 77.4 K.Fig. 2: Isotherms of N2 adsorption – desorption on Norit Carbon at 77.4 K.              (3)where n is the adsorbed amount at equilibrium(mol.g−1), nmLF  the Langmuir–Freundlich maximum adsorption capacity (mol.g−1), Ceq the adsorbate equilibrium concentration (mol. L−1),  KLF  the equilibrium constant for a heterogeneous solid, and nLF  is the heterogeneity parameter, lying between 0 and 1. In form this equation resembles that of Langmuir equation. The difference between this equation and the Langmuir equation is the additional parameter “nLF” in the Sips equation. If this parameter nLF is unity, the Langmuir equation can be applicable for ideal surfaces. Hence the parameter nLF could be regarded as the parameter characterizing the system heterogeneity. The system heterogeneity could stem from the solid or the adsorbate or a combination of both. The parameter nLF is usually greater than unity, and therefore the larger this parameter the more heterogeneous the system19, 20. At low adsorbate concentrations, it reduces to Freundlich isotherm; while at high concentrations, it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm21. 2.4 Adsorption ThermodynamicsThe adsorption thermodynamics were studied to gain an insight into the adsorption behaviours. Parameters including Gibbs free energy change (ΔG◦), enthalpy change (ΔH◦) and entropy change (ΔS◦)22 are calculated according to the following thermodynamic equations: DG = -RTlnK    (4)  ln K = (∆S/R) – (DH/RT)   (5)where K (L.mol-1) is from Freundlich and Sips equations, R is the gas constant (8.314 J.mol-1.K-1) and T is the temperature in Kelvin. In the application of Eq. (5), the values of ln K are plotted against 1/T, the ΔH◦ and ΔS◦ values are calculated from the slope and intercept of the plot.J. Chil. Chem. Soc., 58, Nº 1 (2013)1543Fig. 3: Comparison of pore size distribution of Norit (full line) and Tetrahedron (dashed line) carbons from the isotherms of N2 adsorption at 77.4 K.Fig. 6: Adsorption Isotherm for Tetrahedron Carbon.  Sips h = nmLF (KLF Ceq)1/nLF/[1 + (KLF Ceq) 1/nLF ].Fig. 4: Adsorption Isotherm for Tetrahedron Carbon. Freundlich h = KF.Ceq1/nF.Fig. 5: Adsorption Isotherm for Norit Carbon. Freundlich h = KF.Ceq1/nF.Fig. 7: Adsorption Isotherm for Norit Carbon.  Sips h = hmLF (KLF Ceq) 1/nLF/[1 + (KLF Ceq) 1/nLF ].Table 3. Adsorption Energies Obtained from Freundlich Model.FREUNDLICH  n=KF.Ceq.1/nFTetrahedron Carbon Norit CarbonΔH (KJ.mol-1) -9.16 -13.11ΔS (KJ.mol-1.K-1) 0.075 0.083ΔG (KJ.mol-1) 293 K - 31.75 -35.38ΔG(KJ.mol-1) 303 K - 32.32 -36.21ΔG(KJ.mol-1) 313 K - 33.10 -37.03Table 4. Adsorption Energies Obtained from Sips Model.SIPS (n= nmLF. (KLF.Ceq.)nLF/1 + (KLF.Ceq.)nLF)Tetrahedron Carbon Norit CarbonΔH (KJ.mol-1) -78,87 -95.53ΔS (KJ.mol-1.K-1) 0.30 0.31ΔG (KJ.mol-1) 293 K -166.81 -187.42ΔG(KJ.mol-1) 303 K -169.80 -190.56ΔG(KJ.mol-1) 313 K -172.80 -193.69J. Chil. Chem. Soc., 58, Nº 1 (2013)1544Data for specific surface areas (table 1 and figures 1 and 2) show that Norit carbon has a higher surface area value than Tetrahedron, which indicates that the former will have a higher adsorption capacity for the same amount measured in equilibrium. Figures 1 and 2, which show the N2 isotherms at 77.4 K, present clear differences in the two carbons. Norit isotherm is reversible while the hysteresis loop of Tetrahedron (Figure 1) reflects capillary condensation. From Table 2, according to the DA fit (Figures 1 and 2), we note that N2 adsorption at 77.4 K shows that E0 for Tetrahedron (23.8 kJ.mol-1) is greater than for Norit (21.8 kJ.mol-1), whereas C1,1  is in the inverse order, Norit (204 mL.g-1) and Tetrahedron (140 mL.g-1). The parameter C1,3 is different for both carbons: 1.0 for Norit and 3 for Tetrahedron. As for their microporosity, both carbons present similar average diameters: L = 0.9 nm for Tetrahedron and L = 1.0 nm for Norit.Thus, in agreement with other authors, it is suggested that the Freundlich model is only valid at low equilibrium concentrations of the adsorbate in solution, as the Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations. Sips circumvents the limitation of the rising adsorbate concentration, associated with the Freundlich isotherm model. As to the Sips model, it can also be observed that the KLF parameter presents higher values for the Norit carbon –which corresponds to its higher surface area value– for the whole range of equilibrium concentrations of phenol. nF values  are in the range of 2-10 and nLF are greater than unity. For the three temperature values (293, 303 and 313 K), adsorption decreases with increasing temperature on both carbons (figures 4, 5, 6 and 7). This finding correlates with the evolution of the parameters KF and KLF: as the temperature increases, these values decrease, thus indicating lower adsorption capacity of both carbons. Negative ΔG◦ values are obtained in all cases, revealing the spontaneous nature of these adsorptions. The ΔH◦ values are negative, demonstrating the exothermic nature of these adsorptions, which is in agreement with the experimental observations (tables 3 and 4). There are also important differences in the thermodynamic parameters as determined by both fitting models. The greater values in the measurement of ΔH◦ obtained with Sips indicate that this model is more appropriate to interpret phenol adsorption. This process seems to occur by physisorption and chemisorption due to certain heterogeneity on the surface associated with the surface groups. This tendency, more marked in the Norit carbon, can be correlated with the behaviour observed for this carbon in the DA heterogeneity parameter. 4. CONCLUSIONSAdsorption isotherms are important to describe the interaction between the adsorbates and the activated carbon and are crucial to optimize the use of the latter as an adsorbent. The fact that the Sips model gave a better fit indicates that this is a useful approach to the heterogeneity of the surface and to study the behaviour of the phenol on the surface for the whole range of equilibrium concentrations suggested. Although the Freundlich model is appropriate to describe the region of low concentrations, it is not valid for higher concentrations, where the interactions of the phenol molecules in the solution are likely to have a more important role, thus reducing the affinity of phenol towards the surface.Current results in our laboratory for adsorption of phenolic compounds from aqueous solutions using the two above mentioned activated carbons, have shown different behaviours, which can be attributed to micropore filling, and meso and macropore adsorption. In these cases, the presence of surface chemical groups will certainly increase the degree of complexity and heterogeneity, which is also coherent with the hereby presented description (the process of micropore filling favours carbon Norit over Thetrahedron). The values of the thermodynamic parameters obtained indicate an exothermic and spontaneous process for both carbons, and mainly for Norit. This is due to the fact that there might be chemically activated regions on the surface of the Norit carbon, which give rise to combined mechanisms of physisorption and chemisorption. 5. REFERENCES1. Newcombe, G. in Adsorption by Carbon. Adsorption from Aqueous Solutions: Water Purification. Edited by Bottani-Tascón. Elsevier Amsterdam, 2008. 2. Nevskaia D.M., Castillejos-López E., Guerrero-Ruiz A., Muñoz V, Carbon, 42, 653, (2004).3. Moreno-Castilla, C., Carbon, 42, 83, (2004).4. MacDonald JAF., Evans MJB., Carbon, 40, 703, (2002).5. Lei L., Quinlivan PA., Knappe Detlef RU., Carbon, 40, 2085, (2002).6. Hernández-Huesca R, Aguilar-Armenta G., Journal of the Mexican Chemical Society, 46, 109, (2002).7. Qing-Song, L.; Tong, Z.; Peng, W.; Ji-Ping, J.; Nan, L., Chem. Eng. J., 157, 348, (2010).8. Khan AR, Ataullah R, Al-Haddad A., J. Colloid Interface Sci., 194, 154, (1997).  9. Singh KP., Malik A, Sinha S, Ojha P., J. Hazard. Mater., 150, 626, (2008).10.  Srihari V, Das A., Desalination, 225, 220, (2008).11. Antilén, M; Förster, J.; Del Cofetto, Sylvie; Rodier Elizabeth, Fudym Oliver; Venezia, Anna M.; Deganello G and Escudey, M., J. Chil. Chem. Soc., 49, 313, (2004).12. Gil, A.; Grange, P, Colloids Surf. A, 113, 39, (1996). 13. Rychlicki, G.; Terzyk, A.P.; Lukaszewicz, J.P., Colloids Surf. A. 96, 105, (1995).14. Albesa AG, Llanos JL, Vicente JL., Langmuir, 24, 3836, (2008).15. Siva Kumar, N.; Min K. J., Chil. Chem. Soc., 56, 539, (2011).16.  Hamdaoui, O.; Naffrechoux, E., J. Hazard. Mater., 147, 381, (2007).17. Mourao, P.A.M.; Carrott, P.J.M.; Ribeiro Carrott, M.M.L., Carbon, 44, 2422, (2006).18.  Chung-Hai, Y., J. Colloid Interface Sci., 208, 379, (1998).19. Hamdaoui, O.; Naffrechoux, E.; J., J. Hazard. Mater., 147, 401, (2007).20. Duong, D. Do. in Adsorption Analysis: Equilibria and Kinetics. Practical Approaches of Pure Component Adsorption Equilibria. Imperial College Press, 1998.21. Foo, K.Y.; Hameed, B.H., Chem. Eng. J., 156, 2, (2010).22. Kanwal, F: Rehman, R.; Mahumud, T; Anwar, J.; Ilyas, R., J. Chil. Chem. Soc, 57, 1058, (2012).

Thermodynamic analysis of adsorption models of phenol in liquid phase on different activated carbons

SEDICI - Repositorio de la UNLP

J. Chil. Chem. Soc., 58, Nº 1 (2013)
1541
THERMODYNAMIC ANALYSIS OF ADSORPTION MODELS OF PHENOL IN LIQUID PHASE
ON DIFFERENT ACTIVATED CARBONS
P. D.  HÚMPOLA1 *, H. S. ODETTI1, A. E. FERTITTA2, J. L. VICENTE2
1Departamento de Química General e Inorgánica. Facultad de Bioquímica y Ciencias Biológicas. Universidad Nacional del Litoral, Casilla de Correo 242 
(3000) Paraje “El Pozo”. Santa Fe. Argentina.  Teléfono/Fax: +54- 0342-4575212. 
2Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), (UNLP – CIC – CONICET),
Casilla de Correo 16, Sucursal 4 (1900) La Plata, Argentina.
(Received: July 5, 2012 - Accepted: November 19, 2012)
ABSTRACT
This paper studies the thermodynamic aspects of the processes of adsorption of phenol from dilute aqueous solutions on different commercial carbons, 
evaluating how to optimize the removal of this persistent contaminant. Two powdered activated carbons from two different companies were used: Tetrahedron 
Carbon (Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas Inc., USA). Both specific surface areas were measured by means of the BET 
method. The adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). Experimental isotherms were determined at 293 K, 303 K and 313 K. The Freundlich 
and Sips theoretical models were used to fit the experimental data. Freundlich isotherm slightly diverges with the experimental results for higher equilibrium 
concentrations. Thermodynamic parameters were calculated and correlated with the adsorption behaviours. The values of the thermodynamic parameters obtained 
indicate an exothermic and spontaneous process for both carbons, and mainly for Norit. This is due to the fact that there might be chemically activated regions on 
the surface of the Norit carbon, which give rise to combined mechanisms of physisorption and chemisorption. 
Key Words: adsorption, carbon, phenol, energy.
1. INTRODUCTION
Since the 1920s, activated carbon has been used for the conventional 
treatment of water in order to improve its esthetic quality and to prevent 
potentially toxic compounds from getting to the consumer1. A number of 
organic compounds are normally found in contaminated water, and among 
them, phenols and molecules which contain phenolic groups are relatively 
frequent as contaminants2. Activated carbon adsorption has been suggested by 
the United States Environmental Protection Agency as one of the available 
technologies to control environmental contamination3. 
Adsorption of phenolic compounds on activated carbon surfaces is an 
important technology for water purification. Since adsorption on activated 
carbon can be affected by the surface properties of the carbons, the chemical 
composition of the surface groups can be a critical factor in determining the 
efficiency of the carbons in the removal of a given adsorbate in solution or 
their duration for use as adsorbents in a water treatment system4. Therefore, 
it is necessary to develop a rational framework for the selection of activated 
carbons in order to have a more thorough knowledge of the influence of the 
textural (i.e. pore sizes) and chemical carbon surface characteristics on the 
adsorption of organic contaminants from aqueous solutions5.
It is also worth mentioning that the heats of adsorption or changes in 
enthalpies of adsorption (ΔH◦) have been considered as indicators of the 
energetic heterogeneity of an adsorbent in adsorption6, which depends on the 
distribution of micropores and mesopores of different shapes and sizes as well 
as on the distribution of adsorption sites of different chemical nature (polarity) 
within the pores. 
The difference between physisorption and chemisorption lies in the 
magnitude of the enthalpy and Gibbs free energy changes. Generally, the ΔG◦ 
value is in the range of 0 to −20 kJ.mol-1 and −80 to −400 kJ.mol-1 for physical 
and chemical adsorptions, respectively. The magnitude of the ΔH◦ value lies 
in the range of -2.1 to -20.9 and -80 to -200 kJ.mol-1 for physical and chemical 
adsorptions, respectively. Positive ΔS◦ value suggests the organization of the 
adsorbate at the solid/solution interface becomes more random, while negative 
value suggests the opposite fact. It is also supposed that the change of ΔS◦ 
value is related to the displacement of the adsorbed water molecules by the 
adsorbate7. Heat is always released in the process, thus adsorption enthalpy is 
negative, which means that there will be a decrease of entropy in adsorption. 
This is due to the fact that a molecule in its gaseous state or in solution has more 
freedom of movement than one that is bonded to the surface8, 9, 10. 
The aim of this paper is to study the adsorption of phenol from dilute 
aqueous solutions on different commercial carbons, evaluating how to optimize 
the removal of this persistent contaminant, and correlating the adsorption 
behaviours to the thermodynamic analysis.
2. EXPERIMENTAL
2.1 Characterization of Adsorbents
Equilibrium isotherms for the adsorption of nitrogen (N2) in the gas phase 
were measured at 77.4 K11. This was done for two commercially available 
active carbon samples which were obtained from the Tetrahedron Carbon 
(Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas 
Inc., USA) companies. Tables 1 and 2 present information about these 
materials obtained from BET standard analysis and the characterization of the 
microporosity from Dubinin-Astakhov (DA). At relative lower pressures, i.e., 
x = p/p0 ≤ 0.01, the micropore filling process takes place. The DA equation 
has been used to describe the volume filling of micropores and the energetic 
heterogeneity of solids12. DA proposed to relate the volume V occupied by the 
adsorbate with the differential molar work of adsorption, A = RT log(1/x), via 
the following equation:
                                                                                                                                       (1)
According to the Polanyi-Dubinin theories, C1,1 is the micropore volume, 
θ = V/C1,1, RT ln(1/x) is the differential molar work of adsorption; C1,2 = β E0, 
where E0 is the characteristic energy, β is the function of the adsorbate, and C1,3 
describes the surface heterogeneity. For simplicity, if Eq. (1) is written C1,3 = 
2, the DA equation reduces to the Dubinin-Radushkevich (DR) equation, first 
proposed to describe solids with low degree of burn-off. The exponent values 
proposed are C1,3 > 4 for zeolites, C1,3 =2-3 for homogeneous carbons, and C1,3 
<2 for heterogeneous active carbons.  
The DA equation corresponds to the choice of arbitrary values in the 
Weibull distribution function. With this additional parameter in the adsorption 
isotherm equation, the DA equation provides flexibility in the description of 
adsorption data of many microporous solids ranging from a narrow to a wide 
micropore size distribution13. 
The gas phase isotherms were volumetrically determined employing 
Pyrex conventional equipment. The corresponding values for N2 at 77.4 K are 
shown in Figures 1-2. Pressures were determined using absolute capacitance 
manometers, MKS – Baratron 122 AA-00010AB, with 1.0 ´  10-3 Torr maximum 
error. Work temperatures were thermostatically controlled. Temperature was 
measured with a digital thermometer, Altronix, with a Pt-100 (DIN) sensor 
head, previously calibrated against an oxygen vapour pressure thermometer 
with 0.1 K precision14. Gases, provided by Matheson Gas Products, were 
employed without any previous treatment and they were of high purity, greater 
than 99%.
e-mail: p_humpola@hotmail.com
J. Chil. Chem. Soc., 58, Nº 1 (2013)
1542
2.2 Adsorption Measurements
The adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). 
Experimental isotherms from dilute aqueous solutions were determined twice 
in order to assess their reproducibility, at a constant temperature of 293 K, 303 
K and 313 K with periodic manual agitation. The initial phenol concentrations 
ranged from 0.1 to 0.6 mol.L-1 and the mass of carbon was in the range of 
0.3000 g ± 0.0005 g. Initial and equilibrium concentrations of phenol were 
determined by molecular absorption spectrophotometry at 272.2 nm using the 
UV-Visible CECIL CE 3021 SERIES 3000 spectrophotometer15. These data 
were used to calculate the quantity adsorbed per unit mass of the adsorbent.
2.3 Adsorption Isotherms 
Data were fitted by using Freundlich and Sips theoretical models and 
SigmaPlot V. 10.0 software. Their mathematical representations are given 
below:
Freundlich Isotherm
3. RESULTS AND DISCUSSION
Table 1. Activated Carbon Characterization. 
Specific surface 
area [m2.g-1]
Monolayer 
volume [mL.g-1]
TETRAHEDRON 545.32 148.6
NORIT 680.23 154.3
Table 2. DA Parameter Values for a Low Pressure Range (N2 at 77.4 K).
C1,1 [mL.g
-1] E0 [kJ.mol
-1] C
1,3
 
TETRAHEDRON 140   23.8   3.0   
NORIT 204   21.8   1.9   
      (2)
where n is the adsorbed amount at equilibrium(mol.g−1), Ceq  the adsorbate 
equilibrium concentration (mol. L−1), KF is a constant indicative of the relative 
adsorption capacity of the adsorbent (mol1−(1/n) L1/n g−1) and nF is a constant 
indicative of the intensity of the adsorption. The Freundlich expression is an 
exponential equation and therefore, assumes that as the adsorbate concentration 
increases, the concentration of adsorbate on the adsorbent surface also increases. 
The magnitude of the exponent nF gives an indication on the favourability of 
adsorption. It is generally stated that values of nF in the range 2–10 represent 
good, 1–2 moderately difficult, and less than 1 poor adsorption characteristics. 
Freundlich isotherm slightly diverges with the experimental results for higher 
equilibrium concentrations16, 17, 18.
Sips Isotherm
Recognizing the problem of the continuing increase in the adsorbed 
amount with an increase in concentration in the Freundlich equation, Sips pro-
posed an equation similar in form to the Freundlich equation, but it has a finite 
limit when the concentration is sufficiently high. The Sips isotherm equation is 
sometimes called the Langmuir-Freundlich equation in the literature because it 
has the combined form of Langmuir and Freundlich equations.
Fig. 1: Isotherms of  N2 adsorption – desorption on Tetrahedron Carbon 
at 77.4 K.
Fig. 2: Isotherms of N2 adsorption – desorption on Norit Carbon at 77.4 K.
              (3)
where n is the adsorbed amount at equilibrium(mol.g−1), nmLF  the 
Langmuir–Freundlich maximum adsorption capacity (mol.g−1), Ceq the 
adsorbate equilibrium concentration (mol. L−1),  KLF  the equilibrium constant 
for a heterogeneous solid, and nLF  is the heterogeneity parameter, lying between 
0 and 1. In form this equation resembles that of Langmuir equation. The 
difference between this equation and the Langmuir equation is the additional 
parameter “nLF” in the Sips equation. If this parameter nLF is unity, the Langmuir 
equation can be applicable for ideal surfaces. Hence the parameter nLF could be 
regarded as the parameter characterizing the system heterogeneity. The system 
heterogeneity could stem from the solid or the adsorbate or a combination of 
both. The parameter nLF is usually greater than unity, and therefore the larger 
this parameter the more heterogeneous the system19, 20. At low adsorbate 
concentrations, it reduces to Freundlich isotherm; while at high concentrations, 
it predicts a monolayer adsorption capacity characteristic of the Langmuir 
isotherm21. 
2.4 Adsorption Thermodynamics
The adsorption thermodynamics were studied to gain an insight into the 
adsorption behaviours. Parameters including Gibbs free energy change (ΔG◦), 
enthalpy change (ΔH◦) and entropy change (ΔS◦)22 are calculated according to 
the following thermodynamic equations:
 DG = -RTlnK    (4)
 
 ln K = (∆S/R) – (DH/RT)   (5)
where K (L.mol-1) is from Freundlich and Sips equations, R is the gas 
constant (8.314 J.mol-1.K-1) and T is the temperature in Kelvin. In the 
application of Eq. (5), the values of ln K are plotted against 1/T, the ΔH◦ and 
ΔS◦ values are calculated from the slope and intercept of the plot.
J. Chil. Chem. Soc., 58, Nº 1 (2013)
1543
Fig. 3: Comparison of pore size distribution of Norit (full line) and 
Tetrahedron (dashed line) carbons from the isotherms of N2 adsorption at 77.4 
K.
Fig. 6: Adsorption Isotherm for Tetrahedron Carbon.  Sips h = nmLF (KLF 
Ceq)
1/nLF/[1 + (KLF Ceq) 
1/nLF ].
Fig. 4: Adsorption Isotherm for Tetrahedron Carbon. Freundlich h = 
KF.Ceq
1/nF.
Fig. 5: Adsorption Isotherm for Norit Carbon. Freundlich h = KF.Ceq1/nF.
Fig. 7: Adsorption Isotherm for Norit Carbon.  Sips h = hmLF (KLF Ceq) 1/
nLF/[1 + (KLF Ceq) 
1/nLF ].
Table 3. Adsorption Energies Obtained from Freundlich Model.
FREUNDLICH  n=KF.Ceq.1/nF
Tetrahedron Carbon Norit Carbon
ΔH (KJ.mol-1) -9.16 -13.11
ΔS (KJ.mol-1.K-1) 0.075 0.083
ΔG (KJ.mol-1) 293 K - 31.75 -35.38
ΔG(KJ.mol-1) 303 K - 32.32 -36.21
ΔG(KJ.mol-1) 313 K - 33.10 -37.03
Table 4. Adsorption Energies Obtained from Sips Model.
SIPS (n= nmLF. (KLF.Ceq.)nLF/1 + (KLF.Ceq.)nLF)
Tetrahedron Carbon Norit Carbon
ΔH (KJ.mol-1) -78,87 -95.53
ΔS (KJ.mol-1.K-1) 0.30 0.31
ΔG (KJ.mol-1) 293 K -166.81 -187.42
ΔG(KJ.mol-1) 303 K -169.80 -190.56
ΔG(KJ.mol-1) 313 K -172.80 -193.69
J. Chil. Chem. Soc., 58, Nº 1 (2013)
1544
Data for specific surface areas (table 1 and figures 1 and 2) show that 
Norit carbon has a higher surface area value than Tetrahedron, which indicates 
that the former will have a higher adsorption capacity for the same amount 
measured in equilibrium. 
Figures 1 and 2, which show the N2 isotherms at 77.4 K, present clear 
differences in the two carbons. Norit isotherm is reversible while the hysteresis 
loop of Tetrahedron (Figure 1) reflects capillary condensation. 
From Table 2, according to the DA fit (Figures 1 and 2), we note that N2 
adsorption at 77.4 K shows that E0 for Tetrahedron (23.8 kJ.mol-1) is greater 
than for Norit (21.8 kJ.mol-1), whereas C1,1  is in the inverse order, Norit (204 
mL.g-1) and Tetrahedron (140 mL.g-1). The parameter C1,3 is different for both 
carbons: 1.0 for Norit and 3 for Tetrahedron. As for their microporosity, both 
carbons present similar average diameters: L = 0.9 nm for Tetrahedron and L 
= 1.0 nm for Norit.
Thus, in agreement with other authors, it is suggested that the Freundlich 
model is only valid at low equilibrium concentrations of the adsorbate in 
solution, as the Freundlich isotherm slightly diverges with the experimental 
results for higher equilibrium concentrations. Sips circumvents the limitation 
of the rising adsorbate concentration, associated with the Freundlich isotherm 
model. As to the Sips model, it can also be observed that the KLF
 parameter 
presents higher values for the Norit carbon –which corresponds to its higher 
surface area value– for the whole range of equilibrium concentrations of phenol. 
nF values  are in the range of 2-10 and nLF are greater than unity. For the three 
temperature values (293, 303 and 313 K), adsorption decreases with increasing 
temperature on both carbons (figures 4, 5, 6 and 7). This finding correlates with 
the evolution of the parameters KF and KLF: as the temperature increases, these 
values decrease, thus indicating lower adsorption capacity of both carbons. 
Negative ΔG◦ values are obtained in all cases, revealing the spontaneous 
nature of these adsorptions. The ΔH◦ values are negative, demonstrating 
the exothermic nature of these adsorptions, which is in agreement with the 
experimental observations (tables 3 and 4). 
There are also important differences in the thermodynamic parameters 
as determined by both fitting models. The greater values in the measurement 
of ΔH◦ obtained with Sips indicate that this model is more appropriate to 
interpret phenol adsorption. This process seems to occur by physisorption 
and chemisorption due to certain heterogeneity on the surface associated with 
the surface groups. This tendency, more marked in the Norit carbon, can be 
correlated with the behaviour observed for this carbon in the DA heterogeneity 
parameter. 
4. CONCLUSIONS
Adsorption isotherms are important to describe the interaction between 
the adsorbates and the activated carbon and are crucial to optimize the use 
of the latter as an adsorbent. The fact that the Sips model gave a better fit 
indicates that this is a useful approach to the heterogeneity of the surface and 
to study the behaviour of the phenol on the surface for the whole range of 
equilibrium concentrations suggested. Although the Freundlich model is 
appropriate to describe the region of low concentrations, it is not valid for 
higher concentrations, where the interactions of the phenol molecules in the 
solution are likely to have a more important role, thus reducing the affinity of 
phenol towards the surface.
Current results in our laboratory for adsorption of phenolic compounds 
from aqueous solutions using the two above mentioned activated carbons, 
have shown different behaviours, which can be attributed to micropore 
filling, and meso and macropore adsorption. In these cases, the presence of 
surface chemical groups will certainly increase the degree of complexity and 
heterogeneity, which is also coherent with the hereby presented description 
(the process of micropore filling favours carbon Norit over Thetrahedron). The 
values of the thermodynamic parameters obtained indicate an exothermic and 
spontaneous process for both carbons, and mainly for Norit. This is due to 
the fact that there might be chemically activated regions on the surface of the 
Norit carbon, which give rise to combined mechanisms of physisorption and 
chemisorption. 
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Thermodynamic analysis of adsorption models of phenol in liquid phase on different activated carbons

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