3618
Ind. Eng. Chem. Res. 1998, 37, 3618-3624
Influence of Surface Characteristics on Liquid-Phase Adsorption of Phenol by Activated Carbons Prepared from Bituminous Coal Hsisheng Teng* and Chien-To Hsieh Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
Liquid-phase adsorption of phenol by activated carbons prepared from a bituminous coal was investigated. The carbon preparation consisted of carbonization of the oxidized or the unoxidized coal followed by activation in CO2 to various extents of burnoff. It was observed from the experimental results that BET surface area and pore volume are important factors in determining the adsorptive capacity of the activated carbons. The Langmuir model yields a fairly good fit to the adsorption isotherms, indicating a monolayer adsorption of phenol onto these carbons. The amount of phenol adsorbed per unit surface area, corresponding to complete coverage of the adsorptive sites, decreases with the extent of burnoff and with the particle size of the carbon. The decrease can be attributed to the increase in diffusion path. It was found that the adsorptive capacity decreases with the temperature for the carbon prepared from the unoxidized coal, while it increases for the carbons from the oxidized coal. This difference can be attributed to different populations of oxygen functional groups on the carbon surfaces. According to the Langmuir model, the adsorption of phenol on these carbons was found to be to an endothermic process. Introduction Phenol is a very common contaminant in industrial wastewaters. An undesirable feature of phenol is that in the chlorination for further purification of water reaction with the chlorine produces carcinogenic chlorinated compounds (La´szlo´ et al., 1997). Activated carbon is widely employed in water and wastewater treatment processes for removing organic contaminants, including phenol and phenolic compounds (Cooney and Xi, 1994; Caturla et al., 1988; Juang et al., 1996; Juang and Swei, 1996). Activated carbon possesses a large capacity for adsorption of organic matters, due to its large surface area and porosity (Singh et al., 1996; Stenzel, 1993; Ying et al., 1990). Pore structure, in terms of surface area and pore volume, is an important property of activated carbon products, which determines the performance of the carbon during adsorption. Activated carbons are produced from a variety of raw materials, the most common of which are coal, wood, peat, and coconut shells (Avon et al., 1997; MuozGuillena et al., 1992; Teng et al., 1996). Depending on the raw materials, activated carbons have different surface characteristics with surface functional groups, surface area, porosity, and pore size distribution. Activated carbons prepared from bituminous coals are widely provided in the market. These products have greater density, hardness, and abrasion resistance (Greenbank and Spotts, 1993), which are required properties in wastewater treatment. In this study, an Australian bituminous coal was chosen as the precursor of activated carbon. The activating process also plays an important role for determining the quality of activated carbon. Basically, there are two different processes for the preparation of activated carbon: physical and chemical activations (Wigmans, 1989). Physical activation with CO2 was employed in the present study. Phenol is a relatively small molecule (6.20 Å in molecular diameter) (Singh et al., 1996). These mol* To whom correspondence should be addressed. Tel.: 8866-2086969, ext. 62640. Fax: 886-6-2344496. E-mail: hteng@ mail.ncku.edu.tw.
ecules in aqueous solutions are able to completely fill the micropore volume at saturation (Barton, 1993). Although the surface properties of carbon samples cannot be studied by phenol adsorption only, the knowledge obtained from the present work should be applicable to the adsorption of phenolic compounds and other adsorbates which have structures similar to that of phenol. To have a better understanding of the adsorption behavior of phenol by activated carbon, a series of carbons were prepared from bituminous coal in the present study. It is well-known that oxidation of the coal precursor prior to the activation can affect the surface characteristics of the resulting carbons (Teng et al., 1996, 1997; Alvarez et al., 1994). The oxidation technique was employed and its influence on the liquid-phase adsorption behaviors of the carbons was investigated. The major interest of this paper is to explore the influence of surface characteristics of the activated carbons, which are produced from different preparation processes, on the liquid-phase adsorption of phenol. The adsorption isotherms were described by the well-known Langmuir and Freundlich models and a comparison was made of the applicability of these models. The effect of temperature on the phenol adsorption was examined, and the thermodynamic data were also evaluated. Experimental Section An Australian bituminous coal, Mt. Thorley (MT), was used as the starting material for the preparation of activated carbons for the present study. The proximate and ultimate analyses and the ash composition of the raw coal are shown in Table 1. The ash composition of the coal precursor may have its contribution to the adsorption properties of the resulting activated carbons, but this is not explored in the present study. The asreceived coal was ground and sieved to a desired particle size before being treated. Carbonization of the asreceived coal was implemented in a vertical cylindrical furnace (25 mm i.d.) with an N2 flow at 100 mL/min. In
S0888-5885(97)00796-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/12/1998
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3619 Table 1. Analysis of Mt. Thorley Bituminous Coal carbon nitrogen hydrogen
Ultimate (wt %, dry-ash-free basis) 78.9 oxygen 3.3 sulfur 5.4
Proximate (wt %, as-received basis) moisture 3.1 fixed carbon volatile matters 33.1 ash SiO2 Al2O3 Fe2O3 TiO2 CaO
Ash Composition (wt %) 67 MgO 22 Na2O 6.1 K2O 1.2 P2O5 0.64
11.8 0.6
Table 2. Variations in the Surface Area, Pore Volume, Pore Size Distribution, and Average Pore Diameter with the Extent of Burnoff for the MT and the MTO Carbons from the Unoxidized and the Oxidized Coals, Respectively
BET SA (m2/g)
MT21 MT34 MT40 MT51
321 449 528 634
0.17 0.25 0.29 0.34
89 85 85 88
11 15 15 12
21 22 22 22
MTO12 MTO20 MTO34 MTO48 MTO57
191 411 629 780 950
0.096 0.21 0.33 0.42 0.50
94 94 90 88 91
6 6 10 12 9
20 20 21 22 21
carbon 0.58 0.45 0.85 0.26
the process of carbonization, the samples were heated at 30 °C/min from room temperature to a maximum heat treatment temperature of 900 °C. It has been reported that oxidation of a bituminous coal before carbonization is able to reduce the fluidity and caking of the coal and results in an increase in char microporosity (Alvarez et al., 1994; Teng et al., 1997). This technique was also employed in preparing activated carbons in the present study. The oxidation of the coal was implemented in the cylindrical furnace under a stream of air at about 185 °C for a period of 24 h. Following the oxidation process, the oxidized samples were then carbonized in N2, at 30 °C/min from 185 °C to the maximum heat treatment temperature. Following the carbonization process, the resulting chars from both the unoxidized and the oxidized coals were gasified, also in the furnace, in a stream of CO2 at the maximum heat treatment temperature. Activated carbons with various extents of burnoff were prepared. Unless otherwise specified, the particle sizes of the carbon samples used in the adsorption experiments were within a range of 0.42-1.0 mm. Specific surface areas and porosities of these carbon adsorbents were determined by gas adsorption. An automated adsorption apparatus (Micromeritics, ASAP 2000) was employed for these measurements. Adsorption of N2, as a probe species, was performed at -196 °C. Before any such analysis the sample was degassed at 300 °C in a vacuum at about 10-3 Torr. Surface areas and micropore volumes of the samples were determined from the application of the Brunauer-Emmett-Teller (BET) and Dubinin-Radushkevich (D-R) equations, respectively, to the adsorption isotherms at relative pressures between 0.06 and 0.2. The amount of N2 adsorbed at pressures near unity corresponds to the total amount adsorbed at both micropores and mesopores; and consequently, the subtraction of the micropore volume (from the D-R equation) from the total amount (determined at p/p0 ) 0.98 in this case) will provide the volume of the mesopore (Rodrı´guez-Reinoso et al., 1995). The average pore diameter can be determined according to the surface area and total pore volume (the sum of the micropore and mesopore volumes), if the pores are assumed to be parallel and cylindrical. Phenol, which has a molecular weight of 94, was employed as the adsorbate in adsorption experiments. An aqueous solution with a 500 mg/L concentration was prepared by mixing an appropriate amount of phenol with distilled water. Adsorption experiments were conducted by placing a fixed amount of carbon adsorbent (0.2-0.8 g) and 100 mL of the aqueous solution in a 250 mL glass-stoppered flask. The flask was then put in a
pore size distribution micro meso (%) (%)
pore volume (cm3/g)
55.0 8.8
average pore diameter (Å)
constant-temperature shaker bath, with a shaker speed of 100 rpm. Preliminary experiments of the solution had shown the adsorption process to attain equilibrium in 24 h for all the carbon samples used in the present study. Upon equilibration, all samples were filtered through nylon filters prior to analysis, to minimize interference of the carbon fines with the analysis. The concentrations of the adsorbate in the residual solutions were determined spectrophotometrically at a wavelength of 268 nm, using a UV/visible spectrophotometer (Shimadzu, Model UV-1201), by comparing the light absorbance of the sample solutions against a calibration curve in an appropriate range of concentrations. The equilibrium adsorption capacities (qe) at different phenol concentrations were determined according to mass balance on adsorbate:
qe ) (Ci - Ce)V/m where Ci is the initial concentration, Ce is the residual or equilibrium concentration, V is the volume of the liquid phase, and m is the mass of the activated carbon. Results and Discussion Surface Structures of the Activated Carbon. The surface characteristics of the activated carbons prepared from the unoxidized and the oxidized MT coals are given in Table 2. The carbons prepared from the unoxidized MT coal are designated as MT carbons and those from the oxidized coal are MTO carbons. Each final carbon has been designated by using the nomenclature of the carbon type followed by the burnoff level reached in CO2 gasification. These carbons are mainly microporous, as indicated from the results of the pore size distribution. It clearly shows that the developments of the surface area, pore volume, and pore size distribution are functions of the burnoff level for all the samples. The surface area and pore volume unanimously increase upon activation. The increase can be attributed to both the opening of closed pores and the enlarging of the micropores resulting from carbon gasification (Walker and Almagro, 1995). The proportion of mesopore volume and the average pore diameter generally increase with the burnoff level for both MT and MTO carbons, but the variations are not obvious. There appears to be more change in pore size distribution with MTO carbons. The reason for this is probably due to the smaller average pore diameter with the MTO
3620 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
Figure 2. Equilibrium MT carbon adsorption of phenol as a function of equilibrium phenol concentration (Ce) at 30 °C.
Figure 1. Variations of the amount of phenol adsorbed on the carbons with time during the courses of liquid-phase adsorption on (a) the MT carbons and (b) the MTO carbons. The amount of the activated carbon used in each experiment was 0.2 g and the adsorption was performed at 30 °C.
carbons at low burnoff levels, which would provide more opportunities for pore widening during carbon gasification. It can be obviously seen that the MTO carbons have better surface characteristics (higher surface area and porosity) than the MT carbons at the same levels of burnoff. This can be explained by the fact that oxygen functional groups induced from oxidation promote crosslinking between coal structures and thus destroy the caking properties of the coal to facilitate the development of a microporous structure in the char during carbonization (Alvarez et al., 1994; Teng et al., 1997). Liquid-Phase Adsorption Behaviors. Typical rates of phenol adsorption by the activated carbons are revealed in Figure 1. In the experiments for Figure 1, about 8 flasks were prepared in the same fashion for each type of carbon sample. The flasks were put in the constant-temperature shaker bath maintained at 30 °C. At a desired time, a flask was removed from the bath and the liquid sample was taken for measuring the phenol concentration. The results show that the equilibrium adsorption can be reached in 24 h, regardless of the carbon type. However, it should be noted that although phenol solutions of the same initial concentration were used, the equilibrium concentrations (after 24 h of adsorption) were different for different carbons, and therefore, the final amounts of phenol adsorbed shown in Figure 1 cannot provide a fair comparison of the adsorption capacities of these carbons. The adsorption capacities of activated carbons are normally compared by use of their adsorption isotherms. Figures 2 and 3 show the adsorption isotherms of phenol onto the MT and the MTO carbons, respectively; i.e., the relations between the amount of phenol adsorbed per unit mass of carbon adsorbent and their remaining concentrations in the aqueous solutions. Since the carbons employed in the present study are
Figure 3. Equilibrium MTO carbon adsorption of phenol as a function of equilibrium phenol concentration (Ce) at 30 °C.
mainly microporous, the influence on capacity measurement caused by hysteresis is assumed to negligible. In the range of equilibrium concentrations shown in the figures, the equilibrium adsorption capacity at a specified concentration generally increases with the extent of carbon burnoff, and at similar burnoff levels the MTO carbons have a higher equilibrium adsorption of phenol than the MT. It has been reported that the BET surface area of adsorbents is an important factor in determining the adsorption capacity of the adsorbents (Noll et al., 1992). Comparison of the results of Figures 2 and 3 with the surface characteristics of the carbons in Table 2 indicates that, as expected, the equilibrium adsorption capacity generally increases with the BET surface area and pore volume. Therefore, considering the removal of phenol from wastewater, oxidation pretreatment is very beneficial in producing activated carbons from bituminous coals. To facilitate the estimation of the adsorption capacities at various liquid-phase concentrations, the two wellknown equilibrium adsorption isotherm models, Freundlich and Langmuir, were employed. The Freundlich equation is an empirical one employed to describe the isotherm data by
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3621
qe ) KF(Ce)1/n
(1)
where KF and n are empirical constants that determine the curvature and steepness of the isotherm (Akgerman and Zardkoohi, 1996). A plot of ln qe against ln Ce would give the values of n and KF from the slope and the intercept, respectively. The adsorption isotherms in Figures 2 and 3 are employed to deduce the parameters of the Freundlich equation. The parameters, together with the correlation coefficient (r2), are listed in Table 3. It can be seen that the values of 1/n are similar for different carbons, indicating that the liquid-phase concentration dependences are similar for different carbon surfaces (Bhattacharya and Venkobachar, 1982). Since the values of 1/n are less than unity, the isotherms can be characterized by a concave Freundlich isotherm, indicating that significant adsorption takes place at low concentrations, but the increase in the amount adsorbed with concentration become less significant at higher concentrations. Although the Freundlich equation is an empirical one, the parameter KF can serve as a measure of the relative adsorptive capacity of different carbons at the same equilibrium concentrations Table 3 shows that the values of KF generally increase with the extent of carbon burnoff, reflecting the fact that the BET surface area is an important factor in determining the adsorptive capacity of the carbons. The other model, the Langmuir, can be represented as
θ ) qe/qm ) KLCe/(1 + KLCe)
(2)
where θ is the fractional coverage, qm is the amount adsorbed per unit mass of adsorbent corresponding to complete coverage of the adsorptive sites, and KL is the Langmuir constant. The values of qm are designated as the adsorptive capacities of the carbons in the following discussion. A plot of (1/qe) against (1/Ce) would give KL and qm from the slope and intercept. These parameters, together with the correlation coefficient (r2), for the adsorption isotherms are also listed in Table 3, to compare with those determined from the Freundlich model. The results in Table 3 show that both models give fairly good linear fits to the adsorption data, according to the values of r2. However, the Langmuir equation (r2 g 0.992) yields a somewhat better fit than the Freundlich (r2 between 0.942 and 0.996). It is wellknown that the Langmuir equation is intended for a homogeneous surface. A good fit of this equation reflects monolayer adsorption (Juang and Swei, 1996). Therefore, the results of the present study indicate a monolayer adsorption of phenol onto these carbons. As a matter of fact, the adsorption isotherms shown in Figures 2 and 3 reflect a monolayer adsorption; i.e., the knees of the isotherms are sharp, inferred from the extrapolation of the isotherm at low concentrations, and the plateaus are fairly horizontal. The Freundlich equation frequently gives an adequate description of adsorption data over a restricted range of concentration, even though it is not based on a theoretical background. Apart from homogeneous surfaces, the Freundlich equation is also suitable for a highly heterogeneous surface and an adsorption isotherm lacking a plateau, indicating a multilayer adsorption (Juang et al., 1996). Since the experimental evidence indicates that a monolayer adsorption model is suitable for the present
Table 3. Parameters for the Freundlich and Langmuir Adsorption Isotherms Determined at 30 °C Freundlich isotherm
Langmuir isotherm
carbon
KFa
1/n
r2
qm (mg/g)
KL (L/mg)
r2
MT21 MT34 MT40 MT51
31.2 43.7 50.1 45.9
0.195 0.209 0.180 0.219
0.981 0.955 0.971 0.952
103 123 131 138
0.0326 0.104 0.108 0.115
0.992 0.995 0.999 0.999
MTO12 MTO20 MTO34 MTO48 MTO57
26.8 39.2 42.7 57.5 67.2
0.219 0.172 0.280 0.228 0.214
0.980 0.977 0.942 0.991 0.996
93 125 170 189 213
0.0439 0.0451 0.0857 0.0681 0.0603
0.999 0.992 0.996 0.992 0.997
a
In units of mg1-(1/n) L1/n/g.
system, the parameters of the Langmuir model will be employed in the following discussion on the adsorption behavior. Table 3 reveals that qm increases with the extents of carbon burnoff for both types of carbons. Again, the BET surface area or pore volume of the carbon determines the limiting number of sites available for the adsorption of phenol, since the surface area and pore volume also increase with the burnoff level. As stated previously, the creation of surface area during carbon activation may result from opening up of the closed pores and enlarging, either widening or deepening, of the existing micropores (Walker and Almagro, 1995). The accessibility of phenol molecules to the new surfaces should play an important role in determining the adsorptive capacity in liquid phase. If the carbon were constant in character and all the micropores were utilized in adsorption (i.e., the surface is fully accessible and has a uniform distribution of adsorptive sites), the amount of phenol adsorption would increase proportionally with the increasing surface area. On the other hand, if the micropores created during carbon activation cannot be fully accessed by phenol molecules in liquid phase, then the adsorptive capacity increase will not be proportional to surface area during adsorption, and as a result, the ratio of adsorptive capacity to surface area would decrease with the increasing extent of carbon burnoff. The accessibility of the carbon surface to phenol was examined, by calculating the ratio of qm to BET surface area for the activated carbons with different extents of burnoff. The calculated results show that the qm/BET ratios are similar for both types of carbons with similar extents of burnoff. However, the ratios show a significant decrease with the burnoff level. This result strongly indicates that the carbons with different burnoff levels do not have constant surface characteristics for phenol adsorption. One explanation for this is that the increase in surface area with the increase of burnoff may be due to the creation of more micropores having average diameter less than 6.2 Å that are not accessible to phenol. However, this situation is unlikely to occur, since the increase in burn-off normally causes the widening of pores, as indicated in Table 2 that the average pore diameter of the carbons increases with the extent of burnoff. Assuming that the adsorbates are spherical and have a close-packed hexagonal arrangement on the carbon surface (Lowell and Shields, 1991), the cross-sectional area of a phenol molecule was calculated to be 30.2 × 10-20 m2 (Singh et al., 1996). Under this assumption the fraction of area covered by the amount of phenol corresponding to complete coverage of the adsorptive
3622 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
Figure 4. Fraction of area covered by the amount of phenol corresponding to complete coverage, Xm, at different extents of carbon burnoff. The adsorption were performed at 30 °C.
sites, Xm, can thus be estimated, and the results are shown in Figure 4. It clearly shows that most of the BET surface area is accessible to phenol for carbons with a low extent of burnoff (more than 90% for MTO12); however, the accessibility decreases with increasing burnoff level, as depicted by the trend of Xm with the burnoff level. One should notice that upon activation the mean diffusion path for phenol molecules in the carbon micropores would increase with the extent of carbon burnoff because of the opening of closed pores and the deepening of the original pores. Under these circumstances, it is very likely that, according to experimental evidence, the increased diffusion path restricts the accessibility of the carbon surface to phenol. To verify that the length of the diffusion path influences the adsorptive capacity, adsorption experiments were carried out with carbons of different particle sizes. This was based on the reason that the diffusion path increases with the size of carbon particles. The adsorption isotherms of phenol on the MT34 and MTO34 carbons with different particle sizes are shown in Figure 5 and the values of qm determined from the Langmuir equation are listed in Table 4. It can be seen from Figure 5 that, at the same equilibrium concentration in liquid phase, the amount of phenol adsorbed decreases with the particle size of the activated carbons. Data in Table 4 also exhibits that with the increase in particle size from 0.21-0.42 to 1-2 mm, the amount adsorbed for complete coverage, qm, decreases from 187 to 153 mg/g for the MTO carbon and from 134 to 104 mg/g for the MT. Therefore, the results reflect that the phenol molecules may be unable to penetrate all the internal pores within the carbon particles. The longer the diffusion path, which is an increasing function of particle size, the more difficult becomes complete saturation of the particle, since the greater will be the probability that the solute molecules will come up against pores too small for the solute to penetrate (McKay et al., 1985). Therefore, the results from the experiments with different particle sizes support the inference that the increase in the diffusion path caused by carbon burnoff results in a decrease in the fraction of carbon surface accessible to phenol molecules, as depicted in Figure 4.
Figure 5. Adsorption isotherms for phenol on the carbons with different particle sizes at 30 °C: (a) MT34; (b) MTO34. Table 4. Influence of Particle Size on the Adsorptive Capacities of the MT34 and MTO34 Carbonsa qm (mg/g)
a
particle size (mm)
MT34
MTO34
1.0-2.0 0.42-1.0 0.21-0.42
104 123 134
153 170 187
Adsorption performed at 30 °C.
Table 5. Influence of Adsorption Temperature on the Parameters of the Langmuir Isotherm for Different Carbons MT21
MTO20
MTO48
temp (°C)
qm (mg/g)
KL (L/mg)
qm (mg/g)
KL (L/mg)
qm (mg/g)
KL (L/mg)
30 40 50
103 91.6 78.2
0.0326 0.0479 0.0750
125 132 137
0.0451 0.0518 0.0580
189 193 200
0.0681 0.0750 0.0771
Effects of Adsorption Temperature. The effect of temperature on the system was investigated by performing experiments at temperatures with a range of 30-50 °C. The MT21, MTO20, and MTO48 carbons were employed to explore the temperature effects. It was found that, according to the adsorption isotherms, the amount of phenol adsorbed on the MT21 carbon decreases with the adsorption temperature, whereas those on the MTO20 and the MTO48 carbon increase. The Langmuir parameters for the adsorption isotherms obtained from different temperatures were calculated, and the results are shown in Table 5. It can be seen that, as expected from the experimental results, the value of qm decreases with the adsorption temperature for the MT carbon, whereas the value increases for the MTO carbons.
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3623
Figure 6. Temperature dependence of the equilibrium constant for the formation of adsorptive sites, KA. Table 6. Heats of Adsorptive Site Formation (∆HA) and Phenol Adsorption (∆HL) for Different Carbons carbon
∆HA (kJ/mol)
∆HL (kJ/mol)
MT21 MTO20 MTO48
-25 9.7 4.4
34 10 5.1
For the convenience of representing the temperature influence, the apparent variation of qm with temperature can be described by a reaction model in which the formation of the adsorptive sites on the carbon surface is represented as
C T C′
(R1)
where C represents the sites that are not ready for phenol adsorption and C′ is the adsorptive sites for phenol adsorption. This process is expected to be fast in reaching equilibrium, as compared with that for the adsorption of phenol molecules on the adsorptive sites. The equilibrium population of C′ on the carbon surface, in units of mmol/g, is equivalent to the value of qm divided by the molecular weight of phenol. The equilibrium constant for R1 can be expressed as
KA ) [C′]/[C] ) qm/(q0 - qm)
(3)
where q0 is the amount required for monolayer coverage of the whole BET surface area by phenol. The value of KA at different adsorption temperatures were thus determined, and the results are revealed in Figure 6, showing that the value of KA is a decreasing function of the adsorption temperature for the MT carbon and an increasing function for the MTO carbons. To explore the energetics of the process, consider that
d(ln KA)/d(1/T) ) -∆HA/R
(4)
where ∆HA is the apparent heat of formation of C′. The values of ∆HA were determined and are listed in Table 6. As expected, the process for the MT carbon was apparently exothermic (negative ∆HA) and that for the MTO was apparently endothermic (positive ∆HA). The difference in the temperature dependence of qm for the MT and MTO carbons can be attributed to the difference in the population of oxygen functional groups
on the carbon surface. The decrease of qm with temperature for the MT carbon is expected, since the affinity due to the dispersion forces, which account for the major part of the adsorbate-adsorbent potential (Lowell and Shields, 1991), decreases with temperature. As for the MTO carbons, they have a larger amount of oxygen functional groups on the surface due to the oxidation treatment in the preparation process. As the polarity of water is larger than that of phenol, the water molecules are adsorbed selectively to the oxygen functional groups of the carbons. The adsorbed water molecules become secondary adsorption centers that retain other water molecules by means of hydrogen bonds (Couglin and Ezra, 1968; Asakawa and Ogino, 1984). Under the circumstances, some of the pore entrances are blocked by the complexes of associated water, preventing the adsorption of phenol to the surface within the blocked pores. Since the affinity resulting from the polarity and hydrogen bonding decreases with temperature, the numbers of water complexes become fewer at higher adsorption temperatures, allowing higher amounts of phenol uptake on carbon. Therefore, the formation of the complexes of associated water explains why the amount of adsorptive sites increases with the adsorption temperature for the MTO carbons. In the adsorption process the Langmuir adsorption model employed in the phenol adsorption on the adsorptive sites can be expressed in thermodynamic terms as
C* + Ph T C*(Ph)
(R2)
where Ph represents the phenol molecule in liquid phase, C* is the available adsorptive site for phenol adsorption, and C*(Ph) represents the sites occupied by phenol. The sum of the amounts of C* and C*(Ph) is equal to that of C′. The Langmuir constant, KL, represents the equilibrium constant for reaction R2. The values of KL at different adsorption temperatures were determined, and the results are shown in Table 5. The results show that KL is an increasing function of the adsorption temperature for different carbons. The increase of KL with temperature indicates that the affinity for phenol molecules is favored by high temperature, and therefore this adsorption process is apparently endothermic (Lo´pez-Delgado et al., 1996). The heat of adsorption can be calculated, considering that
d(ln KL)/d(1/T) ) -∆HL/R
(5)
where ∆HL is the heat of adsorption of phenol on the adsorptive sites. The values of ∆HL for different carbons are listed in Table 6. In general, the enthalpy change due to chemical adsorption is considerably larger than that due to physical adsorption (
