ARTICLE pubs.acs.org/IECR
Adsorption of Low-Level CO2 Using Modified Zeolites and Activated Carbon Kyung-Mi Lee,† Yun-Hui Lim,† Chan-Jung Park,‡ and Young-Min Jo*,† †
Department of Environmental Science and Engineering, Center for Environmental Studies, Kyung Hee University, Gyeonggi-Do, Korea, 446-701 ‡ Development Team 2, Environmental Technology Institute, Woongjin Coway Co., Ltd., Woongjin R&D center, San 4, Bongchun 7-dong, Gwanak-gu, Seoul, South Korea, 151-818 ABSTRACT: To control the levels of indoor CO2 in public spaces, we investigated adsorption at room temperature onto zeolites and activated carbon (AC) modified by alkali and alkaline earth metals. A fixed-bed adsorption apparatus was used to obtain more information about the effects of impregnated cations. Cations impregnated into the supports had a significant influence on the adsorption of CO2. In particular, sorbents impregnated with Ca had the highest adsorption capacity. Moreover, modified zeolites (for all cations) had greater adsorption capacities than ACs, despite their smaller surface areas, because of the electrostatic interforces between zeolites. The presence of moisture in the mixed gas flow caused decreases in adsorption capacity. The results of the equations of Sips and Toth matched well with the CO2 adsorptions of the present test sorbents. The presence of cations induced heterogeneous interactions between CO2 and the sorbents.
’ INTRODUCTION Indoor air quality (IAQ) has become an area of concern because it is directly related to human health. Controlling ambient concentrations of CO2 is the recommended method for maintaining adequate indoor air quality, although the specific relationships between CO2 and air quality are not often determined.1 High concentrations of CO2 can cause symptoms such as headaches, dizziness, and nausea. Air ventilation is important because it dilutes indoor contaminants. However, the recent practice of thoroughly insulating buildings to conserve or reflect heat also results in decreases of IAQ. Currently, a few general methods are being used to purify polluted air, including adsorption, absorption, membrane, and cryogenic gas cleaning techniques.24 Adsorption has been considered to be a promising method for controlling low concentrations of CO2 because of its simplicity, low energy requirements, and cost effectiveness.5 Zeolite and activated carbon have been used for years in air cleaning applications. Zeolite is an easily attainable, commercially available adsorbent with high adsorption affinities for various gases. Comprised of aluminum and silicon oxides, zeolite is a microporous crystalline material possessing adsorptive and ion exchange properties, which influence the attractive forces between adsorbents and adsorbates. Activated carbon is an extremely porous material with a high ratio of surface area to unit weight and is currently the most widely used adsorbent. Capturing indoor air pollutants using activated carbon at normal temperatures is considered economically feasible.6 However, activated carbon is a physical, nonpolar adsorbent that sometimes exhibits low selectivity with polar contaminants. In recent years, modified activated carbon was considered a potential adsorbent due to its increased selectivity. Chemically modified activated carbons can be used to control certain pollutant gases. In this study, two adsorbents, activated carbon (AC) and zeolite 13X, were modified with alkali and alkaline earth metal r 2011 American Chemical Society
solutions to improve their adsorption capacities. Adsorption tests for CO2 were conducted to investigate the interactions between CO2 and the surfaces of modified sorbents. Experimental investigations studied different adsorption isotherms to correlate the adsorption data. The adsorption isotherm models used in this study were those of Langmuir and Freundlich (two parameters) and of Sips and Toth (three parameters). To study the effects of moisture on the adsorption capacities of the modified sorbents, CO2/air was passed through distilled water. Thus, the adsorption characteristics of two sorbents were investigated and compared to aid in the selection of the most appropriate method for CO2 removal.
’ EXPERIMENTAL SECTION Preparation of Test Sorbents. The test sorbents used in this study were the commercial zeolite 13X (UOP, USA) and activated carbon (Calgon, USA). These sorbents were chemically modified through ion-exchange and wet-impregnation methods, respectively. In the ion-exchange method, zeolite powder was dispersed in 1 M metal solutions for 24 h under constant stirring at 6070 °C. Precursor metals were prepared by dissolving LiCl or MCH3COOH (M = K+, Ca2+, Mg2+) in distilled water. Then, the slurry was filtered via vacuum suction with repeated washings, and the collected particles were dried at 120 °C for 2 h. The dry powder was pelletized with an organic binder of dextrin (5 wt %) and was calcined at 500 °C to produce a spherical pellet 34 mm in diameter. In the wet-impregnation method, activated carbon particles (45 mm) were immersed in 1 M metal solutions at room temperature for 24 h and dried at 120 °C for 2 h. Received: June 27, 2011 Accepted: November 14, 2011 Revised: October 10, 2011 Published: November 14, 2011 1355
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Figure 1. Schematic diagram of the experimental apparatus.
The original zeolite and activated carbon were designated as zeo-raw and AC-raw respectively, and the modified sorbents were designated as zeo-ion and AC-ion. Characterization of Test Sorbents. The chemical compositions of all modified sorbents were examined using ICP-AES (LEEMAN). Acid digestion was used to remove any organic materials in the test samples before measurement. In this study, powdery samples were dissolved in a sub-boiled acidic solution (HCl/HNO3 = 3:1). X-ray diffraction (XRD) with a Rigaku instrument indicated the phase structure of the test sorbents in the range of 2θ within 565° at a scanning rate of 4°/min. Specific surface area was evaluated using the BET method (Quantachrome, MS-22). Samples were outgassed at 400 °C for 2 h for zeolitic sorbents and at 120 °C for activated carbon, and N2 adsorption/desorption isotherms were performed in liquid N2 (196 °C). Examination of CO2 Adsorption. Figure 1 shows the experimental setup used in this work. The CO2 adsorption properties of the sorbent were closely examined in a fixed bed (36 mm inner diameter) at room temperature. The mass gas flow rates of CO2 and air were adjusted using mass flow controllers (MFC). The concentration of CO2 introduced into the fixed column was 3000 ppm throughout the test, and the feed flow rate was fixed at 2/min. Concentrations were measured in real-time by a continuous CO2 analyzer (SenseAir, ASEN ALARM). Before the adsorption test, sorbents were predried at 120 °C for 2 h. Meanwhile, to ascertain the effects of moisture, water vapor was generated by a water bubbler, and relative humidity was measured using a hygrometer (RBR-ecom, M-3309). Each experiment was continued until saturation, which was indicated by an outlet CO2 concentration equal to the inlet CO2 concentration.
’ RESULTS AND DISCUSSION The prepared adsorbents with chemical modification were characterized in terms of specific surface area and chemical composition. Adsorption amounts of CO2 at low concentrations were evaluated under various reaction pressure and humidity. The experimental data were fit to four isotherm models: Langmuir, Freundlich, Sips and Toth relations.
Table 1. Major Compositions of the Modified Sorbents: (a) Zeolite- and (b) Activated Carbon-Based (a) Zeolite-Based Sorbent [ppm] K
Li
Na
zeo-raw
M
4560
zeo-Li (1 M) zeo-K (1 M) zeo-Ca (1 M)
Ca
3015
1259
9280
1590 812
zeo-Mg (1 M)
2110
1860
1740
(b) Activated Carbon-Based Sorbent [ppm] K
Li
Ca
Mg
AC-raw AC-K (1 M) AC-Li (1 M) AC-Ca (1 M) AC-Mg (1 M)
983 745 2650 1875
Characterization of Modified Sorbents. The chemical characteristics of an sorbent relate closely to the adsorption behaviors of the gas molecules.7 The present modified sorbents were expected to achieve better selectivity for the specific adsorbate, CO2. ICP analysis confirmed the degree of ion impregnation, as summarized in Table 1. Raw zeolite contains a large proportion of sodium on a structural base of aluminum and silicon. The analysis results revealed an obvious exchange of cations. Alkali metal ions replaced the sodium ions of the zeolite. The introduction of electropositive cations enhanced the adsorption of acidic gases, and the addition of alkali metal ions resulted in a significant increase in the CO2 adsorption capacity.8 Monovalent cations, such as sodium, contained in Zeolite 13X can be better exchanged with monovalent alkali metal elements (Li+, K+) than divalent alkali-earth metal ions (Ca2+, Mg2+). The amounts of impregnated cations seemed to affect the morphological properties of the sorbent. The BET surface area of the modified sorbents was evaluated and is presented in Table 2. 1356
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The specific surface areas of the zeolites and activated carbons ranged from 301.9 to 462.9 m2/g and from 652.9 to 1,215.2 m2/g respectively, and zeo-Ca showed the lowest surface area, 301.9 m2/g, as shown in part a of Table 2. Although the ionic radii of divalent cations are greater than monovalents, the net area occupied by the divalent cations is expected to be less than that Table 2. Surface Areas of the Modified Sorbents: (a) Zeoliteand (b) Activated Carbon-Based (a) Zeolite-Based Sorbent surface area (m2/g) zeo-raw
453.3
Zeo-Li (1 M)
462.9
Zeo-K (1 M)
399.1
Zeo-Ca (1 M)
301.9
Zeo-Mg (1 M)
360.2 (b) Activated Carbon-Based Sorbent
AC-raw
surface area (m2/g) 1215.2
AC-Li(1 M)
911.5
AC-K(1 M)
1002.9
AC-Ca(1 M)
652.9
AC-Mg(1 M)
718.3
by sodium because two monovalent ions are replaced by a single divalent ion. However, pelletization of the sorbents with heat treatment did not influence the adsorption properties of these microporous materials. This is because thermal treatment after ion-exchange shrinks the surface areas of the sorbents, and at the same time the escape of organic binders by heating attributes to the formation of tiny holes both in the surface and inner structures of the pellets. Raw activated carbon showed the largest surface area due to a well-developed systematic pore structure. The impregnation process used in this work resulted in decreased surface area, possibly by blocking open pores due to the penetration of small metal ions. Therefore, sorbents impregnated by Ca2+ or Mg2+ resulted in significant reductions in surface area due to the pores filling with large ions. Figure 2 shows the XRD patterns of the sorbents that were chemically modified with 1 M alkali metal solutions. The peaks of the zeolite-based sorbents indicated a typical zeolite framework, with peak patterns very similar to that of zeo-raw, in which distinctive phases were not observed on the external surface of the zeolite. Therefore, the framework, even after ion-exchange, retained the intrinsic property of raw zeolite. However, the intensities of the diffraction peaks in zeo-Ca and zeo-Mg decreased, implying a loss of crystallinity, whereas zeo-K and zeo-Li showed more crystalline peaks. The relatively small surface areas of zeo-Ca and zeo-Mg could be due to the decreases in crystallinity indicated by the XRD patterns. In particular, excess alkalis most likely exist in the zeolite pores in the forms of alkali
Figure 2. XRD patterns of the modified sorbents: (a) zeolite- and (b) activated carbon-based. (Marks (b) reflect faujasite). 1357
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Industrial & Engineering Chemistry Research metal oxides, as indicated from the examination of XRD patterns. This conversion may therefore cause the breakage of SiOSi and SiOAl linkages, which induces a slight loss of crystallinity over time. Activated carbon-based sorbents showed a similar tendency as the zeolite-based sorbents, as shown in part b of Figure 2. Some peaks disappeared or decreased in intensity, implying destruction of the framework. However, ion-modified sorbents showed similar XRD patterns to those of the pure sorbents. No evidence of newly developed crystalline compounds with alkali or alkaline earth metals was observed on the external surfaces of the test sorbents. Adsorption of Carbon Dioxide. The areal capacities of the prepared sorbents, which represent the adsorption amount per unit surface area of the sorbent, were evaluated and are summarized in Figure 3. These specific adsorption characteristics enabled us to estimate the ionic effects on chemical interactions between CO2 gas molecules and the effective sites of the sorbent. The areal capacity was more than 10 times greater in zeo-Ca than it was in AC-Ca. The sorbent area determines the number of physical potential sites for adsorption. Nevertheless, the results indicated that the adsorption amount was not always proportional to the specific surface area and that it also depended on the type or quantity of impregnated cations. Larger areal capacities in the modified zeolites were probably due to the electrostatic intensities of the chemical elements. The carbon atom in a CO2 molecule exhibits electrophilicity because of electron migration to the oxygen side.9 Thus, acidic CO2 gas possesses a quadrupole moment with a symmetric molecular structure. However, interforces between CO2 and the adsorbent may induce a weak dipole within the internal structure. This partial polarization creates an electrostatic gradient that can facilitate gas adsorption. However, physical adsorption onto activated carbon particles may not be attractive for highly polar or quadrupole gases such as CO2 because activated carbon is not likely to induce an electric field. However, cation-containing zeolites form an excellent structure for CO2 adsorption, resulting in enhancement of the electrostatic potential. Impregnated cations provide preferred adsorption sites to polar or easily polarizable molecules, but cation type will also affect the adsorption of nonpolar molecules due to the induced electrostatic interactions with the ionic surface. The extent or strength of adsorption of gas molecules onto the zeolite surface can be dominated by the interaction intensity of the adsorbate with the cation-induced electrostatic field. However, the acidbase properties of the zeolite framework can also play an important role in determining adsorption properties. The intensity of the electrostatic field, and of the charge density in particular, increases in the sequence of K+ < Na+ < Li+, resulting in enhanced electrostatic fields and greater CO2 adsorption capacities. Thus, the weak interforces between CO2 molecules and the potassium ions of zeo-K result in relatively lower adsorption capacity. Alkaline earth ions in the zeolite-based sorbents created higher charge densities and ultimately provided increased adsorption amounts compared to those with alkali ions. As shown in part b of Figure 3, the cation-impregnated sorbents, in particular AC-Ca and AC-Mg, showed higher adsorption capacities than did the raw AC sorbent. AC-raw exhibited an adsorption capacity of 0.16 103 mmol/m2 at 1 atm, which increased to 0.51 103 mmol/m2 after impregnation with the alkaline earth metal Ca. Chemisorption seems to play an additional role over pure physisorption onto the AC-raw.
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Figure 3. Specific adsorption amounts according to unit surface area: (a) zeolite- and (b) activated carbon-based sorbents.
In other words, the high adsorption capacities in the modified sorbents may arise from the combined interactions of physical and chemical adsorptions by the loaded metal ion groups. Those metal ions present more active sites to attract CO2 molecules. Thus, the introduction of alkali metal or alkaline earth metal cations to the surfaces of solid adsorbents forms a number of basic sites with strong affinities for acidic gas molecules such as CO2; calcium and magnesium oxides enhanced CO2 adsorption. Effects of Moisture. The presence of moisture in an indoor environment may influence the CO2-capture performance of an adsorbent. To investigate the effects of moisture on the modified sorbents, the CO2-containing gas flow was directed to a distilled water column before entering the adsorption packed bed. Figures 4 and 5 show the adsorption amounts of CO2 in terms of relative humidity (RH) on the modified zeolites and activated carbon, respectively. The data given in the figures were obtained through many repeated tests and the representative mean values are displayed. Moisture diminished the adsorption capacity for all test sorbents, but the degree of reduction varied depending on the sorbent. Moisture covered the metal cations, resulting in a reduction of the electric field intensity. As shown in Figure 4, the CO2 adsorption capacities of zeo-raw, K, Li, Ca and Mg at 1 atm under RH 70% decreased to 25%, 34%, 13%, 17%, and 26% compared to those at RH 20%, respectively. Because moisture interferes with CO2 attachment on the sorbent surface, the 1358
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Figure 4. Adsorbed CO2 amounts as a function of relative humidity: (a) zeo-raw, (b) zeo-K, (c) zeo-Li, (d) zeo-Ca, and (e) zeo-Mg.
humidity of the indoor atmosphere is a critical factor for CO2 control in a dry adsorption process. Highly polar water can bind strongly with cations, thus reducing the intensity and heterogeneity of the electric field on the sorbent surface. Although highly porous materials may capture a considerable amount of gas molecules, adsorption capacity can often be significantly reduced in the presence of water vapor. Zeolites have both hydrophilic and hydrophobic sites, but the zeolitic sorbents in this work, which consisted mainly of Al2O3 and other metal cations, were more hydrophilic.
Figure 5 shows the adsorption amounts of CO2 with different water contents using activated carbon-based sorbents. The results showed a linear decrease in the adsorption amount with increasing moisture content. Compared to RH 20%, the adsorption amount of AC-raw, K, Li, Ca, and Mg at 1 atm for CO2 at RH 70% decreased to 13%, 15%, 17%, 18%, and 19%, respectively. Activated carbon is composed mostly of pure carbon and nonpolar CC bonds. Therefore, the behavior of water in porous activated carbon particles is associated with physical properties such as surface area and pore structure. Despite the 1359
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Figure 5. Adsorbed CO2 amounts as a function of relative humidity: (a) AC-raw, (b) AC-K, (c) AC-Li, (d) AC-Ca, and (e) AC-Mg.
hydrophobic property of AC sorbents, in high humidity conditions, water vapor may form a thin film along the particle surface, resulting in hydrophilicity. Moreover, high levels of moisture results in the development of a water film or bridge over the pore channels, frequently filling the pores.10 Thus, the AC sorbent provides some hydrophilic sites that could competitively affect CO2 adsorption. Consequently, the existence of moisture in the air stream had a negative effect on overall CO2 adsorption capacity. Therefore,
the lifetime of the adsorbent and the optimum operating conditions for indoor air management can be achieved by regulating humidity. Model Analysis. Adsorption isotherms with theoretical and empirical parameters foster the understanding of surface properties and affinities between adsorbents and adsorbates at a given operating temperature. Thus, a precise mathematical evaluation of the equilibrium isotherms, preferably based on a reasonable adsorption mechanism, is essential for an effective design of the 1360
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Figure 6. Correlation of experimental data for CO2 adsorption isotherms: (a) modified zeolites, (b) modified AC; L: Langmuir equation, F: Freundlich equation, S: Sips equation, T: Toth equation.
adsorption process. A plot of the adsorption amount versus the equilibrium pressure provides the main parameters of the isotherm equation. Isotherm models involving two and three parameters were used in this work. Langmuir and Freundlich models (two parameter models) are the most commonly used models in gas adsorption studies due to their simplicity, comprehensive physical condition, easy interpretation, and reliable establishment.11 Langmuir and Freundlich models and the three-parameter equations of the Sips and Toth models were comparatively reviewed through the application of experimental equilibrium data. The optimization procedure required an error function to fit the experimental data to the equation. This study assessed the average relative errors (ARE) using the following equation: " # N jqj:exp qj:cal j 100 ð1Þ ARE% ¼ N qj:exp j¼1
∑
where N is the number of experimental data points, and the subscripts cal and exp denote estimated and experimental values, respectively.
Adsorption isotherms for the test sorbents are displayed in Figure 6, and the model parameters obtained from the best fit of the experimental data are summarized in Table 3, according to the ARE. The simplest isotherm equation describing local adsorption molecules on active sites is the Langmuir model (eq 2):12 q bP ¼ qm 1 þ bP
ð2Þ
where q is the adsorbed amount, qm is the monolayer capacity, P is pressure, and b is the Langmuir constant. The Langmuir model assumes that binding to the sorbent surface relies primarily on physical forces, and is derived from the assumption that all adsorption sites possess equal affinities for the specific gas adsorbate. Parameter b in the Langmuir equation is called the affinity constant and is a measure of the interaction forces between the gas molecules and the adsorbent surface.13 The zeolite sorbent impregnated with Ca (zeo-Ca) had a high affinity, which led to strong electrostatic interactions between 1361
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Table 3. Isotherm Parameters Obtained by Fitting the Isotherm Models to the Experimental Data Langmuir two-parameter models
Freundlich ARE
b
qm
(%)
ARE k
n
(%)
zeo-raw
2.81
1.21
3.59
0.91
2.24
2.51
zeo-Li zeo-K
1.02 0.85
1.31 0.47
6.03 3.47
1.16 0.22
1.56 1.53
5.77 3.18
zeo-Ca
4.86
2.51
3.63
2.66
2.92
2.61
zeo-Mg
3.74
2.48
7.77
2.21
2.67
6.65
AC-raw
0.35
0.32
7.41
0.18
0.81
5.75
AC-Li
0.50
0.39
8.18
0.41
0.21
5.93
AC-K
0.65
0.16
7.11
0.23
0.27
6.44
AC-Ca
0.45
0.58
5.10
2.21
0.32
3.08
AC-Mg
0.44
0.49
7.17
2.02
0.30
4.52
Sips ARE
three-parameter models zeo-raw
Toth
B m qm 0.38 1.67 2.93
(%) 2.14
ARE K t qm 3.23 0.45 2.96
(%) 1.95
zeo-Li
0.40 1.20 3.09
2.12
2.62 0.66 3.81
2.08
zeo-K
0.23 1.17 1.55
2.36
0.31 0.49 1.80
1.98
zeo-Ca
0.90 1.80 5.32
1.98
8.66 0.37 5.01
1.91
zeo-Mg AC-raw
0.80 2.05 3.97 0.15 1.35 0.68
2.46 3.19
7.20 0.28 4.81 0.38 0.36 0.79
2.30 2.18
AC-Li
0.22 1.15 0.59
4.64
1.07 0.49 0.59
3.28
AC-K
0.12 0.83 0.36
4.85
0.16 0.55 0.51
3.44
AC-Ca
0.38 1.66 0.97
2.46
2.25 0.28 2.24
2.16
AC-Mg
0.29 1.63 0.85
2.86
1.34 0.29 2.18
2.81
zeo-Ca and CO2 molecules. The parameter qm is the amount of CO2 adsorbed in a monolayer, and the coverage of which is generally high on divalent cations such as Ca and Mg. Thus, to act as a good sorbent, the values of b and qm must be sufficiently high. Therefore, zeo-Ca and zeo-Mg exhibited good CO2 adsorption compared to the other sorbents. However, Figure 6 shows a relatively poor correlation between the Langmuir model and the experimental data compared to that with the Freundlich model because the Langmuir model was developed with the assumption of homogeneous adsorption with no interaction between the adsorbate molecules. The Freundlich equation, a type of empirical relation, can be used to analyze a heterogeneous surface for nonideal adsorptions. It assumes that the energy of the adsorption sites decays exponentially, as in eq 3:14 q ¼ kP1=n
ð3Þ
where k and n are Freundlich parameters. The parameter k roughly indicates the adsorption capacity, which may reach the maximum value after impregnation with divalent cations (Ca2+ or Mg2+). This means that the high binding force provides a high mutual affinity between the CO2 and sorbent, as indicated in Table 3. Compared to that of the Langmuir model, the ARE value was relatively low in the Freundlich isotherm model, which takes into account the vigorous interactions between the gaseous adsorbates and the heterogeneous surface of the sorbent.
Therefore, the Freundlich equation presents a more satisfactory description of reality. Whereas the Freundlich equation was well fitted to higher concentrations,15 it did not show a close match with concentrations less than 3000 ppm. The Sips model assumes that the surface of the adsorbent is heterogeneous, as does the Freundlich equation, which is generally used at relatively high pressures, except under certain conditions under high pressure.16,17 The parameters of B, m, and qm in eq 4 are given in Table 3. The equation of the Sips model is as follows: q BP1=m ¼ qm 1 þ BP1=m
ð4Þ
where B and m are the Sips parameters. As seen from Figure 6 and Table 3, the Sips equation offered a very satisfactory description of CO2 adsorption. The parameter B, which indicates the adsorption force between the adsorbate and the adsorbent, was high for divalent molecules: 0.290.90. Table 3 indicates that the sorbents modified with Ca had a stronger interforce between CO2 and sorbents than did the others. Differing from the Langmuir model, the Sips model involves an additional parameter, m, which characterizes the system heterogeneity. In general, a homogeneous adsorbent exhibits a constant value of m as adsorbate loading is increased. When the parameter m approaches 1, the Sips equation is reduced to the Langmuir equation. Thus, the distance of m from unity may be considered as a measure of the deviation from the Langmuir isotherm. The parameter m varied in the range of 0.83 to 2.05, indicating that the adsorption data were more in line with the Freundlich form rather than the Langmuir form. The heterogeneities of the sorbents used in this study depended mainly on the distribution of cations over the various sites in the sorbent. The Toth isotherm (eq 5) is a semiempirical expression which has proven useful in describing adsorption in heterogeneous systems18 and is represented using q KP ¼ qm ½1 þ ðKPÞt 1=t
ð5Þ
where K and t represent the Toth parameters. The monolayer capacity represented by qm can be obtained from the correlation of the experimental data to the model equation. The adsorption capacity based on monolayer coverage was larger in Ca- and Mg-impregnated sorbents than it was in the others. A similar tendency for qm was also found in the Langmuir and Sips models, although the qm estimated by the Toth model was slightly higher than that predicted by the Sips model. The heterogeneity parameter, t, ranged from 0.2 to 0.7, indicating that the sorbent surface was relatively heterogeneous. For t = 1, this isotherm reduces to the Langmuir isotherm model. The parameter K is related to the interaction of CO2 with surface characteristics, with a higher value of K indicating a strong interaction between CO2 and the sorbent. Cations of Ca contributed to the increased adsorption force of CO2. In addition, it could be presumed that the acidic CO2 preferred basic sites. The ARE values were estimated between 1.91 and 8.18%. On the basis of the correlation coefficients, as well as the ARE, the Toth isotherm equation corresponded better to practical adsorption than did the Sips isotherm. Analyses using the isotherm models showed more accurate fitting in the order of Toth > Sips > Freundlich > Langmuir. 1362
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Industrial & Engineering Chemistry Research In addition, the isotherm models with three parameters achieved more precise interpretations compared to those with two parameters. At the same time, the adsorption of CO2 did not occur on the surface monolayer.
’ CONCLUSIONS We presented a comparative study of the adsorption characteristics of CO2 with modified zeolite sorbents and activated carbons. Despite small surface areas, the zeolite-based sorbents showed higher adsorption capacities than those with AC bases. For example, zeo-Ca and AC-Ca had surface areas of 301.9 m2/g and 652.9 m2/g, respectively. Nevertheless, the CO2 adsorption capacity of zeo-Ca was about four times higher than that of ACCa, implying that the adsorption capacity was associated closely with interactions between the chemical constituents of the sorbent and the CO2 molecules. Moisture in the feed gas flow reduced the gas adsorption capacity, with the adsorbed amounts of zeo-Ca and AC-Ca decreasing from 1.6 and 0.33 mmol/g at 20% RH to 1.3 and 0.27 mmol/g at 70% RH, respectively. Moisture broadly covers the cations and may fill the pore channels, causing a reduction of the electrical interforces. The adsorption mechanism of CO2 was studied based on equilibrium isotherms. An error analysis of ARE revealed that the Toth model corresponded most closely to the experimental results. Conversely, CO2 molecules were adsorbed onto the sorbent surface with heterogeneous coverage. These results also suggest that the interactions between CO2 and the sorbents are affected by impregnated cations.
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(9) Tatsuro, H.; Hiroaki, H.; Tadehisa, F.; Yukio, K.; Masakazu, H.; Kenzi, S.; Toshiaki, M. Effect of added basic metal oxides on CO2 adsorption on alumina at elevated temperature. Appl. Catal., A 1998, 167, 195–202. (10) Foley, N. J.; Thomas, K. M.; Forshaw, P. L.; Stanton, D.; Norman, P. R. Kinetics of water vapor adsorption on activated carbon. Langmuir 1997, 13, 2083–2089. (11) Muthuswamy, S.; Arthur, R. B.; Kuppusamy, V.; Yun, S. I. Two and three-parameter isothermal modeling for liquid-phase sorption of porcion blue H-B inactive mycelia biomass of panus fulvus. J. Chem. Tech. Biotech. 2007, 82, 389–398. (12) Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 1918, 40, 1361–1403. (13) Toth, J. Uniform interpretation of gas/solid adsorption. Adv. Colloid Interface Sci. 1995, 55, 1–239. (14) Freundlich, H. Over the adsorption in solution. J. Phys. Chem. 1906, 57, 385–470. (15) Chilton, N.; Losso, J. N.; Marshal, W. E.; Rao, R. M. Freundlich adsorption isotherms of agricultural by-product-based powdered activated carbons in a geosminwater system. Bioresour. Technol. 2002, 85, 131–135. (16) Sips, R. On the structure of catalyst surface. J. Chem. Phys. 1948, 16, 490–495. (17) Wei, S.; Zhang, L.; Li, L.; Lee, R. L. Adsorption of CO2 and N2 on synthesized NaY zeolite at high temperatures. Adsorption 2009, 15, 497–505. (18) Toth, J. State equations of the solid gas interface layer. Acta Chim. Acad. Sci. Hung. 1971, 69, 311–317.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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