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Adsorption characteristics of benzene on resin-based activated carbon under humid conditions
Journal of Industrial and Engineering Chemistry 71 (2019) 242–249
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Adsorption characteristics of benzene on resin-based activated carbon under humid conditions Ji-Young Oha,c, Young-Woo Youa,b , Junbeam Parka,c , Ji-Sook Honga , Iljeong Heoa , Chang-Ha Leec, Jeong-Kwon Suha,* a
Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291, Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea c Department of Chemical and Biomolecular Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 May 2018 Received in revised form 14 November 2018 Accepted 16 November 2018 Available online 24 November 2018
VOCs removal under humid conditions is an important issue in the various industries. However, adsorption capacity of commercial coconut-shell–based activated carbon (CAC) is diminished considerably in high relative humidity. In this study, we prepared resin-based activated carbon (RAC) from strong cation-exchange resins consisting of polystyrene with divinylbenzene, and investigated benzene adsorption characteristics under humid conditions. The results of water isotherm and breakthrough experiments revealed that RAC adsorbed no water vapor in low P/P0 region and the amount of adsorbed benzene did not decrease significantly with the addition of water vapor, indicating high water resistance compared to CAC. This high resistance of RAC to water vapor can be contributed by the low content of hydrophilic sites (metal impurity and surface oxygen), confirmed by XPS and ICP results. The relationship between RAC porosity and benzene adsorption under humid conditions was also investigated. Benzene adsorption under humid conditions was influenced significantly by the narrow micropore volume of RAC. As the narrow micropore volume of RAC decreased, the adsorption of water vapor was inhibited, so that the decline in the time of breakthrough at relative humidity at 70% was considerably alleviated from 44 to 1.2%. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Adsorption of benzene Resin-based activated carbon Humid condition Water vapor
Introduction Volatile organic compounds (VOCs) cause various air pollution and human health problems [1–5]. Thus stringent regulations are currently implemented for VOC emissions, and effective technologies for VOC removal are needed [3,6]. VOCs can be removed by various technologies such as catalysis or thermal oxidation, adsorption, absorption and condensation, depending on the situations [2,4,7–15]. Among them, adsorption processes are regarded as the most economical and environmentally friendly technology due to their low operating cost, easy control and no side product [1,2,6]. The adsorbent for VOC removal in an adsorption process should have hydrophobic characteristics, since the exhaust gases generally include water vapor as well as VOCs [3,4,16,17]. This requirement has resulted in the extensive use of activated carbon as a VOC adsorbent, due to its inherent hydrophobicity. However,
* Corresponding author. E-mail address: [email protected] (J.-K. Suh).
the adsorption capacity of activated carbon decreases considerably under conditions of high humidity [18,19]. Water vapor can be adsorbed onto the hydrophilic sites, such as surface oxygen groups and metal impurities [20,21], and then this adsorbed water blocks the pores of activated carbon and deteriorates the performance of adsorption processes [18–24]. Therefore, many studies have been conducted on the deactivation of the hydrophilic sites of activated carbon. Some papers have described the application of heat treatments to remove surface oxygen groups. This method is quite simple, but it does not remove surface oxygen groups completely, and it can collapse the pore structure of the activated carbon due to the high operating temperature [19,25]. According to Jang et al. [26], C¼O groups in the conjugated ketone or quinone structures began to disappear at 1373 K. And surface area was decreased by 20% at this temperature. Other researchers have attempted to block the hydrophilic sites by coating the activated carbon with hydrophobic materials, such as siloxane. However, the coating material can also block the pores of activated carbon and reduce the overall VOC adsorption [24,27,28]. Kim et al. reported that coating activated carbon with PDMS decreased the specific surface
https://doi.org/10.1016/j.jiec.2018.11.032 1226-086X/© 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
J.-Y. Oh et al. / Journal of Industrial and Engineering Chemistry 71 (2019) 242–249
area by 7–58%, depending on the coating temperature [4]. Therefore, a new approach to reducing hydrophilicity while maintaining porosity of activated carbon is needed. Thus, in this study, we tried to improve the water vapor resistance of activated carbon by altering the activated carbon precursor rather than using the post-treatment methods such as heat treatment or coating. We prepared activated carbon using a strong cation-exchange resins consisting of polystyrene with divinylbenzene, as a precursor instead of coconut-shell which is the usual precursor of gas phase activated carbon [29–33]. Unlike the activated carbon prepared from coconut-shell, resin-based activated carbon (RAC) exhibited higher hydrophobicity because of the low oxygen content and absence of metal impurities. We also investigated the effect of the pore size distribution of RAC on benzene adsorption under humid conditions. The narrow micropore volume played an important role in determining the hydrophobicity of the activated carbon. Experimental Adsorbent In this study, two kinds of activated carbon were used; details are shown in Table 1. The activated carbon prepared from a spherical ion-exchange resin was termed RAC, and commercial granular coconut-shell–based activated carbon (SYG, Samchully Carbotech) was termed CAC. Both samples were sieved to the same size range (Mesh No. 30 to 40). The preparation method for RAC is as follows [34–37]. A commercial strong cation-exchange resin consisting of polystyrene with divinylbenzene (Dow Chemical) was used as a precursor of RAC. The resin was dried overnight in an oven at 383 K. The resin was then carbonized and activated in a quartz reactor. The carbonization step was conducted at 1173 K under a nitrogen atmosphere, and the
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activation step was carried out under a nitrogen flow containing steam at 25 vol%. The samples were activated for 1, 2, 3, and 6 h, and designated RAC-1, 2, 3, and 4, respectively. The spherical shape of the resin was maintained during the carbonization and activation steps, and any crack or hole was not observed in RAC (not shown). This is because that the polystyrene chains of the resin are cross-linked with divinylbenzene, and this cross-linked structure can make it possible to maintain the shape of the resin during the preparation of the activated carbon [34]. Characterization The porosity of the activated carbon was determined from N2 and CO2 adsorption isotherms at 77 K and 273 K, respectively, measured with a volumetric adsorption apparatus (3Flex, Micromeritics). The specific surface area was determined by the Brunauer-Emmet-Teller (BET) equation, and the total pore volume was measured at P/P0 = 0.99. Pore size distributions, narrow micropore volumes, and micropore volumes were determined using the non-local density functional theory (NLDFT) method. All samples were pretreated under vacuum for more than 3 h at 623 K before measurements. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an Axis Nova (Kratos) instrument to determine the amount of oxygen-containing groups on the surface of activated carbon. Monochromatic Al-Kα (1486.6 eV) was used as an X-ray source (15 kV, 10 mA) at a vacuum state of 109–108 T. Water adsorption isotherms were measured to evaluate the water resistance of the activated carbon samples. The samples were measured at 298 K using a volumetric adsorption apparatus (3Flex, Micromeritics). All samples were degassed under vacuum for more than 3 h at 623 K before measurement. The metal content of the activated carbon was determined by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) using an iCAP 7400 Duo instrument (Thermo Fisher Scientific).
Table 1 Characteristics of RAC-2 and CAC. Sample
Precursor
Shape
Packing density (g/cm3)
Metal content K (ppm)
Ca (ppm)
RAC series CAC
Ion exchange resin Coconut shell
Spherical Crushed
0.47–0.74 0.50
0 8020
0 570
Fig. 1. Schematic diagram of the breakthrough experimental.
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Breakthrough experiments The breakthrough experiments were conducted using a continuous flow system under dry conditions or a relative humidity (RH) of 50, 70, and 90% (Fig. 1, Fig. S1). The system consisted of an adsorption bed, detector (GC-FID), and humidity control parts. A stainless steel tube (inner-diameter: 7.75 mm, outer-diameter: 9.53 mm) was used as an adsorption bed, and its temperature was maintained at 298 K using a circulator. Each adsorbent was packed to a length of 5 cm of the bed. The packing weight was different depending on the packing density of the adsorbent (RACs: 1.11–1.75 g, CAC: 1.18 g). The packed adsorbent was fixed with metal sieve (100 mesh) and quartz wool. The concentrations of the inlet and outlet gases were detected using a gas chromatograph equipped with a flame ionization detector (GCFID, YL6100 GC, Younglin) and a capillary column (DB-WAX, Agilent Technologies). The relative humidity was controlled by flowing a feed stream into a bubbler filled with distilled water. The bubbler was adjusted to the desired temperature using a circulator during the breakthrough experiments. The adsorbent bed was pretreated at 623 K and purged with helium under vacuum before the experiment. Then, the bed was saturated with He initially at atmospheric pressure. After stabilization of the circulator temperature and GC-FID signal, a constant amount of benzene gas (500 mol ppm in air) was flowed through a mass flow controller (MFC). Results and discussion Comparison of hydrophobicity of RAC and CAC Comparison of porosity between RAC-2 and CAC RAC-2 had a similar specific surface area to the commercial CAC adsorbent, so RAC-2 was selected for comparison with CAC from RAC series. The porosities of RAC-2 and CAC were determined from their N2 adsorption/desorption isotherms (77 K). As shown in Fig. 2, both RAC-2 and CAC shows the type I isotherms with little hysteresis, indicating that micropores is dominant in both adsorbents. This was further confirmed by the ratio of the micropore volumes, which were 90% and 88% for RAC-2 and CAC, respectively, as shown in Table 2. The microporous nature of the adsorbents means that RAC-2 and CAC are suitable for VOC removal, since most VOCs are adsorbed primarily on the micropores of activated carbon [2,38,39]. The specific surface area of RAC-2 is 1286 m2/g, which is about 10% higher than that of CAC. The total pore volume and micropore volume are also about 10% higher in RAC-2 (Table 2). However, narrow micropore volume (<0.7 nm) shows the opposite tendency. The narrow micropore volume of CAC (0.17 cm3/g) was larger than that of RAC-2 (0.15 cm3/g). Detailed information on the pore size distribution was obtained by NLDFT calculation (Fig. 3). The overall pore size distributions of RAC-2 and CAC are similar, but the narrow micropore volume is slightly higher in CAC than in RAC-2, while the opposite situation
Fig. 2. N2 adsorption/desorption isotherms of RAC-2 and CAC at 77 K; adsorption (filled symbols) and desorption (open symbols).
occurs for micropores larger than 0.7 nm. The pore size is known to influence the adsorption of benzene and water significantly, and this will be discussed in later Section [2,40]. Comparison of surface functional groups between RAC-2 and CAC Oxygen-containing groups on the surface of activated carbon are known to adsorb water readily, and this blocks the pores and reduces the amount of VOCs adsorbed [41,42]. The oxygencontaining groups of RAC-2 and CAC were compared by measuring the surface C1 s XPS for each material. Before the discussion of XPS results, we measured X-ray diffraction analysis (XRD) to confirm the crystallite structure of the adsorbents. As can be seen Fig. S2, the XRD patterns of two adsorbents are identical, indicating the same crystallite structure. This XRD pattern means the amorphous carbon structure, the typical pattern of activated carbon. The C1s XPS peaks of the samples were separated into four Gaussian peaks of 284.8 (C–C bond), 285.8 (C–O bond), 286.8 (C¼O bond), and 288.6 (O–C¼O bond) [43], as shown in Fig. 4 and Table 3. The total amounts of oxygen-containing groups were 21.5% for RAC-2 and 26.9% for CAC, indicating that the amount of oxygencontaining groups was about 20% smaller in RAC-2 than in CAC. In particular the amount of C–O groups was about 40% lower in RAC2. XPS of O1s was also measured to verify the result of C1s XPS (Fig. S3). The shape of the XPS data is similar between both adsorbents. However, it can be seen that C–O groups are more in CAC than RAC, and C¼O groups more in RAC than CAC, coincided with the result of C1s XPS. Thus, C1s XPS data is reliable to determine the surface oxygen groups. This lower amount of oxygen-containing groups in RAC-2 could reflect the lower oxygen content of the ion-exchange resin precursor used to formulate RAC-2 when compared to coconut-shell. In addition, the oxygen of the resin can be removed easily by the form of SO2 during the heat treatment.
Table 2 Porosity of RAC-2 and CAC. Sample
SBET (m2/g)
PVt (cm3/g)
PV0.7 nm (cm3/g)
PV2 nm (cm3/g)
PV2 nm/PVt (%)
RAC-2 CAC
1286 1153
0.53 0.49
0.15 0.17
0.48 0.43
90 88
SBET: Specific surface area. PVt: Total pore volume. PV2 nm: Cumulative pore volume (<2 nm). PV0.7 nm: Cumulative pore volume (<0.7 nm) based on CO2 adsorption isotherm.
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Fig. 3. Pore size distribution of RAC-2 and CAC calculated by NLDFT method; based on CO2 isotherm (273 K) (a) and N2 isotherm (77 K) (b).
Water adsorption isotherm of RAC-2 and CAC Fig. 5(a) shows the water adsorption isotherm measured at 298 K. The amount of water adsorbed by RAC-2 increased rarely in the low pressure range, but increased sharply after P/P0 = 0.55. Conversely, the water adsorption isotherm of CAC increased gradually from the low pressure region and increased rapidly at a lower value of P/ P0 = 0.45 than in RAC-2. Therefore, the equilibrium state indicates that RAC-2 is more resistant to water than CAC. The difference in the adsorption of water by RAC-2 and CAC at low pressure can be explained by the differences in metal content and oxygen-containing groups. Oxygen-containing groups and metal impurities of the activated carbon surface adsorb water at low pressure due to their high affinity for water, which results in the formation of water clusters. The formed clusters attract water from the bulk phase and these clusters gradually grow in size. When the pressure exceeds a specific pressure, the pores of the activated carbon are filled with water. This leads to decline of the amount of VOC adsorbed significantly. Tables 1 and 3 show that CAC has a relatively high content of oxygen-containing groups and metal impurities, such as K and Ca. Conversely, RAC-2 is low in oxygencontaining groups and does not contain any metals. Therefore, these two factors result in adsorption of water vapor in CAC at low pressure. Removal of the metal impurities of CAC (by treatment with 35 wt.% HCl, stirring for 1 h, thorough washing and heat treatment at 1073 K, 2 h) reduced the K level from 8020 to 250 ppm and Ca from 570 to 90 ppm, according to ICP analysis. The water adsorption isotherm before and after metal removal was represented in Fig. 5. The amount of saturated water adsorption of CAC increased after acid treatment. This might be because that blocked pore due to metal impurity is
Fig. 4. XPS spectra C1s peaks of samples; (a) RAC-2 and (b) CAC.
opened during the acid treatment. The initial amount of water adsorbed in CAC was considerably reduced after metal removal. Therefore, the initial increase in the amount of water adsorbed in CAC was caused by its metal impurities. HCl treatment could introduce the additional surface oxygen groups. However, the effect of additional surface oxygen groups might be minimal, since the heat treatment was conducted to the activated carbon after HCl treatment, and RAC and CAC were processed under the same conditions. However, even though the metal impurities were almost completely removed, CAC still adsorbed water vapor at a lower pressure when compared to RAC. This is attributed to the higher content of oxygen-containing groups in CAC, as mentioned above. In summary, RAC-2 was more resistant to water vapor than CAC, due to the lower content of oxygen-containing groups and the absence of metal impurities. The selectivity for benzene over water was also discussed. As known, carbon material is inherently hydrophobic. Thus, activated carbon can adsorb benzene more selectively in low humidity region. However, as can be seen in the water isotherm, water vapor Table 3 Relative amount of oxygen containing groups in C1s peak of samples (%). Sample
C–C 284.8 eV
C–O 285.8 eV
C¼O 286.8 eV
O–C¼O 288.6 eV
Total oxygen groups
RAC-2 CAC
78.5 73.1
8.2 14.3
10.9 8.7
2.4 3.9
21.5 26.9
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Breakthrough Experiments for RAC-2 and CAC Table 4 shows the breakthrough experimental results of RAC-2 and CAC by the relative humidity. CAC shows longer breakthrough time per adsorbent weight under dry conditions. Lillo-Ródenas reported that the amount of benzene adsorbed onto activated carbon depends on the narrow micropore volume (<0.7 nm), rather than the total micropore volume [2]. Since the kinetic diameter of benzene is 5.85 Å, a narrow micropore close to benzene diameter greatly influences the amount of benzene adsorbed. Therefore, although CAC has low the specific surface area and total micropore volume, CAC shows a higher amount of benzene adsorbed per unit weight due to its higher narrow micropore volume. However, in terms of unit bed volume, the breakthrough time was about 18% longer for RAC-2 than for CAC. Table 1 shows that this is attributed to the high packing density of RAC-2, caused by the perfect spherical shape of the adsorbent [34]. The performance of both adsorbents declined with increases in relative humidity, but the decline ratio is different. At 50% RH, the breakthrough time for RAC-2 did not decrease substantially when compared to the dry condition (from 119 to 113 min), whereas the breakthrough time of CAC decreased by 12.4% (from 97 to 85 min). The decline in the ratio of breakthrough time was also lower for RAC-2 (33.6%) than for CAC (44.3%) at 70% RH. However, the breakthrough time decreased significantly for both adsorbents (RAC-2: 53.8%, CAC: 58.8%) at close to the saturation pressure (90% RH). These phenomena can be explained by the water adsorption isotherms of the two adsorbents. Both adsorbents are saturated at 90% RH, which results in a similar decrease ratio of breakthrough time in both adsorbents. However, the water adsorption isotherm of CAC increased from the low relative pressure and then increased rapidly at a relatively lower pressure than was observed for RAC-2, due to the high amount of oxygen-containing groups and the metal impurities. For these reasons, RAC-2 showed a lesser decline than CAC in the breakthrough time up to 70% RH. Fig. 5. Water adsorption isotherm of RAC-2 and CAC at 298 K; (a) fresh and (b) acid treated sample.
was adsorbed rapidly at the specific high humidity due to the condensation. Therefore, the benzene selectivity of the activated carbon will be decreased significantly in high humidity region. This speculation was confirmed in the breakthrough experiments.
Adsorption of benzene under humid conditions in the RAC series prepared with different activation times Comparison of porosity among the RAC series materials Table 5 shows the pore characteristics of the RAC samples prepared by varying the activation time. As the activation time
Table 4 Benzene breakthrough times at different relative humidity of RAC–2 and CAC. Sample
RH (%)
Breakthrough time (min)
Breakthrough time/Packed mass (min/g)
RAC-2
0 50 70 90
119 113 (5.0) 79 (33.6) 55 (53.8)
79 75 53 36
CAC
0 50 70 90
97 85 (12.4) 54 (44.3) 40 (58.8)
81 71 46 33
Breakthrough time: The time when C/C0 is 1%.
Table 5 Physical characteristics of RAC series. Sample
Activation time (h)
SBET (m2/g)
PVt (cm3/g)
PV0.7nm (cm3/g)
PV2nm (cm3/g)
PV2nm/PVt (%)
Packing density (g/cm3)
RAC-1 RAC-2 RAC-3 RAC-4
1 2 3 6
1006 1286 1477 1924
0.41 0.53 0.64 0.92
0.17 0.15 0.13 0.11
0.37 0.48 0.56 0.66
91 90 87 72
0.74 0.64 0.57 0.47
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Fig. 6. N2 adsorption/desorption isotherms of RAC series at 77 K; adsorption (filled symbols) and desorption (open symbols).
increased, the specific surface area of the samples also increased from 1006 m2/g to 1924 m2/g, the total pore volume from 0.41 cm3/ g to 0.92 cm3/g, and the micropore volume from 0.37 cm3/g to 0.66 cm3/g. However, the narrow micropore volume (<0.7 nm) and the ratio of the micropore volume decreased with increasing activation time, as also confirmed by the N2 isotherm (Fig. 6). A longer activation time resulted in a change in the N2 isotherm of the sample from Type I to Type V, and the hysteresis phenomenon was clear. This indicates that the micropores grow in size and develop into mesopores [44,45]. The micropore size distribution of the RAC samples is shown in Fig. 7. The overall shape of the pore size distribution was similar among the samples, but the narrow micropore volume (<0.7 nm) decreased from 0.17 to 0.11 cm3/g as the activation time increased. Water adsorption isotherms of the RAC series The water adsorption isotherms of the RAC series measured at 298 K are shown in Fig. 8. The amount of saturated water adsorption is related to total pore volume of adsorbents [46]. Thus amount of saturated water adsorption was higher at the longer activated RAC due to its higher total pore volume. Notably, water vapor adsorbed at the higher pressure as the activation time increased. In other words, a lower narrow micropore volume resulted in a higher resistance to water vapor. Many research papers have reported that the rapid adsorption of water is related to the presence of narrow micropores (<0.7 nm) [18,46,47]. Activated carbon is inherently hydrophobic, so it does not strongly adsorb water. However, if the pore of activated carbon is sufficiently small, then the adsorption energy increases and water can be adsorbed. The adsorbed water molecules interact with other surrounding water molecules to form water clusters, which in turn interact with surrounding clusters and increase in size. When 8–10 clusters are bundled to 1 nm in size, the water molecules change from hydrophilic to hydrophobic and become more stable in the pores [40,46–48]. Therefore, in this study, water adsorption was suppressed in longer activated samples due to its low narrow micropore volume.
Fig. 7. Micropore size distribution of RAC series calculated by NLDFT method; based on CO2 isotherm (273 K) (a) and N2 isotherm (77 K) (b).
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Fig. 8. Water adsorption isotherm of RAC series at 298 K.
Table 6 Benzene breakthrough times at different relative humidity of RAC series. Sample
RAC-1 RAC-2 RAC-3 RAC-4
Breakthrough time (min) RH 0%
RH 50%
RH 70%
RH 90%
120 119 108 85
103 (14.2) 113 (5.0) 108 (0) 85 (0)
67 79 87 84
46 55 59 61
(44.2) (33.6) (19.4) (1.2)
(61.7) (53.8) (45.4) (28.2)
(): (BTRH0% BTRHx%)/BTRH0% 100.
Breakthrough experiments with the RAC series The breakthrough results of the RAC samples are summarized in Table 6. The breakthrough time for RAC-4, the longest activated sample, was the shortest in the dry condition. This is because RAC4 had the lowest ratio of micropore volume and packing density, as shown in Table 5. Under humid conditions, a sample with longer activation showed an enhanced water resistance. As the relative humidity increased to 50, 70 and 90%, the decline in the ratio of the breakthrough time of RAC-1 increased sharply to 14.2, 44.2, and 61.7%, respectively. Conversely, RAC-4 showed little change in breakthrough time up to 70% RH and only a 28.2% decrease at 90% RH. This tendency toward water resistance is consistent with the results of the water adsorption isotherm, as described in Section “Water adsorption isotherms of the RAC series”. As the activation time increased, the decline in the breakthrough time was alleviated due to the decrease in the narrow micropore volume, where water vapor adsorption occurs easily. For practical applications of activated carbon, the absolute value of the breakthrough time, based on unit volume, can be more important than the decline ratio of the breakthrough time. In the low humidity range, the samples activated for a short time showed a long breakthrough time due to their large narrow micropore volume and high packing density. By contrast, although the samples activated for a long time adsorbed a lower amount of benzene in the dry condition, the absolute amount of benzene adsorbed was higher in the high humidity range due to their high water resistance. Therefore, activated carbon should be selected with an appropriate pore size distribution depending on the humidity condition. Conclusions In this work, the benzene adsorption characteristics of resinbased activated carbon were explored under humid conditions.
The XPS results showed that RAC had about 20% less surface oxygen-containing groups when compared to CAC. In addition, the ICP results showed that, unlike CAC, RAC contains no metal impurities. For these two reasons, RAC showed higher water resistance characteristics, as determined by water isotherm and breakthrough experiments. The effect of porosity on benzene adsorption under humid conditions was examined by controlling the porosity of RAC by changing the activation time. The RAC water adsorption isotherms showed that samples given a long-term activation were more resistant to water vapor. Increases in activation time increased the specific surface area, total pore volume, and micropore volume of the activated carbon, but the narrow micropore volume (<0.7 nm) decreased gradually. Water preferentially adsorbs in the narrow micropores, which have high adsorption energy; therefore, water adsorption was suppressed in the long-term activated samples. The same trend was observed in the breakthrough experiments, where the decline in benzene adsorption under humid conditions was alleviated in the long-term activated samples. RAC-1, activated for 1 h, showed a breakthrough time reduction of about 44%, while RAC-4, activated for 6 h, showed a breakthrough time reduction of only 1.2% at 70% RH. Therefore, improvement in the performance under humid conditions will require removal of the narrow micropores from activated carbon, as the narrow micropores readily adsorb water vapor. For practical applications, the absolute breakthrough time of the samples was compared based on unit volume. The high adsorption capacity of benzene under dry conditions and high packing density led to superior performance of the sample with less activation under low relative humidity. However, the resistance to water is more important than the other factors in the high humid conditions; thus, a sample with high activation is more appropriate. Therefore, the choice of activated carbon with suitable porosity should take into account the humidity conditions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.11.032. References [1] M.C. Huang, C.H. Chou, H. Teng, AIChE J. 48 (2002) 1804. [2] M.A. Lillo-Ródenas, D. Cazorla-Amorós, A. Linares-Solano, Carbon 43 (2005) 1758. [3] S. Xian, Y. Yu, J. Xiao, Z. Zhang, Q. Xia, H. Wang, Z. Li, RSC Adv. 5 (2015) 1827. [4] K.D. Kim, E.J. Park, H.O. Seo, M.G. Jeong, Y.D. Kim, D.C. Lim, Chem. Eng. J. 200– 202 (2012) 133. [5] D.G. Lee, J.H. Lee, C.H. Lee, Sep. Purif. Technol. 77 (2011) 312. [6] L. Li, S. Liu, J. Liu, J. Hazard. Mater. 192 (2011) 683. [7] Bo Pan, Baoshan Xing, Environ. Sci. Technol. 42 (2008) 9005. [8] L.F. Liotta, Appl. Catal. B 100 (2010) 403. [9] H. Wu, L. Wang, J. Zhang, Z. Shen, J. Zhao, Catal. Commun. 12 (2011) 859. [10] T. Guo, Z. Bai, C. Wu, T. Zhu, Appl. Catal. B 79 (2008) 171. [11] J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Atmos. Environ. 43 (2009) 2229. [12] Tawfik A. Saleh, Salawu Omobayo Adio, Mohammad Asif, H. Dafall, J. Cleaner Prod. 182 (2018) 960. [13] Amani M. Alansi, Waed Z. Alkayali, Maha H. Al-qunaibit, Talal F. Qahtan, Tawfik A. Saleh, RSC Adv. 5 (2015) 71441. [14] Tawfik A. Saleh, Desalin. Water Treat. 57 (2016) 10730. [15] Tawfik A. Saleh, J. Water Supply Res. T 64 (2015) 892. [16] T. Horikawa, N. Sakao, Carbon 56 (2013) 183. [17] J.H. Kim, C.H. Lee, W.S. Kim, J.S. Lee, J.T. Kim, J.K. Suh, J.M. Lee, J. Chem. Eng. Data. 48 (2003) 137. [18] H.B. Liu, B. Yang, N.D. Xue, J. Hazard. Mater. 318 (2016) 425. [19] F. Cosnier, A. Celzard, G. Furdin, D. Bégin, J.F. Marêché, O. Barrés, Carbon 43 (2005) 2554. [20] A.A. Alswat, M.B. Ahmad, Tawfik A. Saleh, J. Water Supply Res. T 65 (2016) 465. [21] Hannatu A. Sani, Mansor B. Ahmad, Tawfik A. Saleh, RSC Adv. 6 (2016) 108819. [22] T. Horikawa, T. Sekida, J. Hayashi, M. Katoh, D.D. Do, Carbon 49 (2011) 416. [23] N.J. Foley, K.M. Thomas, P.L. Forshaw, D. Stanton, P.R. Norman, Langmuir 13 (1997) 2083.
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Adsorption characteristics of benzene on resin-based activated carbon under humid conditions Ji-Young Oha,c, Young-Woo Youa,b , Junbeam Parka,c , Ji-Sook Honga , Iljeong Heoa , Chang-Ha Leec, Jeong-Kwon Suha,* a
Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291, Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea c Department of Chemical and Biomolecular Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 May 2018 Received in revised form 14 November 2018 Accepted 16 November 2018 Available online 24 November 2018
VOCs removal under humid conditions is an important issue in the various industries. However, adsorption capacity of commercial coconut-shell–based activated carbon (CAC) is diminished considerably in high relative humidity. In this study, we prepared resin-based activated carbon (RAC) from strong cation-exchange resins consisting of polystyrene with divinylbenzene, and investigated benzene adsorption characteristics under humid conditions. The results of water isotherm and breakthrough experiments revealed that RAC adsorbed no water vapor in low P/P0 region and the amount of adsorbed benzene did not decrease significantly with the addition of water vapor, indicating high water resistance compared to CAC. This high resistance of RAC to water vapor can be contributed by the low content of hydrophilic sites (metal impurity and surface oxygen), confirmed by XPS and ICP results. The relationship between RAC porosity and benzene adsorption under humid conditions was also investigated. Benzene adsorption under humid conditions was influenced significantly by the narrow micropore volume of RAC. As the narrow micropore volume of RAC decreased, the adsorption of water vapor was inhibited, so that the decline in the time of breakthrough at relative humidity at 70% was considerably alleviated from 44 to 1.2%. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Adsorption of benzene Resin-based activated carbon Humid condition Water vapor
Introduction Volatile organic compounds (VOCs) cause various air pollution and human health problems [1–5]. Thus stringent regulations are currently implemented for VOC emissions, and effective technologies for VOC removal are needed [3,6]. VOCs can be removed by various technologies such as catalysis or thermal oxidation, adsorption, absorption and condensation, depending on the situations [2,4,7–15]. Among them, adsorption processes are regarded as the most economical and environmentally friendly technology due to their low operating cost, easy control and no side product [1,2,6]. The adsorbent for VOC removal in an adsorption process should have hydrophobic characteristics, since the exhaust gases generally include water vapor as well as VOCs [3,4,16,17]. This requirement has resulted in the extensive use of activated carbon as a VOC adsorbent, due to its inherent hydrophobicity. However,
* Corresponding author. E-mail address: [email protected] (J.-K. Suh).
the adsorption capacity of activated carbon decreases considerably under conditions of high humidity [18,19]. Water vapor can be adsorbed onto the hydrophilic sites, such as surface oxygen groups and metal impurities [20,21], and then this adsorbed water blocks the pores of activated carbon and deteriorates the performance of adsorption processes [18–24]. Therefore, many studies have been conducted on the deactivation of the hydrophilic sites of activated carbon. Some papers have described the application of heat treatments to remove surface oxygen groups. This method is quite simple, but it does not remove surface oxygen groups completely, and it can collapse the pore structure of the activated carbon due to the high operating temperature [19,25]. According to Jang et al. [26], C¼O groups in the conjugated ketone or quinone structures began to disappear at 1373 K. And surface area was decreased by 20% at this temperature. Other researchers have attempted to block the hydrophilic sites by coating the activated carbon with hydrophobic materials, such as siloxane. However, the coating material can also block the pores of activated carbon and reduce the overall VOC adsorption [24,27,28]. Kim et al. reported that coating activated carbon with PDMS decreased the specific surface
https://doi.org/10.1016/j.jiec.2018.11.032 1226-086X/© 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
J.-Y. Oh et al. / Journal of Industrial and Engineering Chemistry 71 (2019) 242–249
area by 7–58%, depending on the coating temperature [4]. Therefore, a new approach to reducing hydrophilicity while maintaining porosity of activated carbon is needed. Thus, in this study, we tried to improve the water vapor resistance of activated carbon by altering the activated carbon precursor rather than using the post-treatment methods such as heat treatment or coating. We prepared activated carbon using a strong cation-exchange resins consisting of polystyrene with divinylbenzene, as a precursor instead of coconut-shell which is the usual precursor of gas phase activated carbon [29–33]. Unlike the activated carbon prepared from coconut-shell, resin-based activated carbon (RAC) exhibited higher hydrophobicity because of the low oxygen content and absence of metal impurities. We also investigated the effect of the pore size distribution of RAC on benzene adsorption under humid conditions. The narrow micropore volume played an important role in determining the hydrophobicity of the activated carbon. Experimental Adsorbent In this study, two kinds of activated carbon were used; details are shown in Table 1. The activated carbon prepared from a spherical ion-exchange resin was termed RAC, and commercial granular coconut-shell–based activated carbon (SYG, Samchully Carbotech) was termed CAC. Both samples were sieved to the same size range (Mesh No. 30 to 40). The preparation method for RAC is as follows [34–37]. A commercial strong cation-exchange resin consisting of polystyrene with divinylbenzene (Dow Chemical) was used as a precursor of RAC. The resin was dried overnight in an oven at 383 K. The resin was then carbonized and activated in a quartz reactor. The carbonization step was conducted at 1173 K under a nitrogen atmosphere, and the
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activation step was carried out under a nitrogen flow containing steam at 25 vol%. The samples were activated for 1, 2, 3, and 6 h, and designated RAC-1, 2, 3, and 4, respectively. The spherical shape of the resin was maintained during the carbonization and activation steps, and any crack or hole was not observed in RAC (not shown). This is because that the polystyrene chains of the resin are cross-linked with divinylbenzene, and this cross-linked structure can make it possible to maintain the shape of the resin during the preparation of the activated carbon [34]. Characterization The porosity of the activated carbon was determined from N2 and CO2 adsorption isotherms at 77 K and 273 K, respectively, measured with a volumetric adsorption apparatus (3Flex, Micromeritics). The specific surface area was determined by the Brunauer-Emmet-Teller (BET) equation, and the total pore volume was measured at P/P0 = 0.99. Pore size distributions, narrow micropore volumes, and micropore volumes were determined using the non-local density functional theory (NLDFT) method. All samples were pretreated under vacuum for more than 3 h at 623 K before measurements. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an Axis Nova (Kratos) instrument to determine the amount of oxygen-containing groups on the surface of activated carbon. Monochromatic Al-Kα (1486.6 eV) was used as an X-ray source (15 kV, 10 mA) at a vacuum state of 109–108 T. Water adsorption isotherms were measured to evaluate the water resistance of the activated carbon samples. The samples were measured at 298 K using a volumetric adsorption apparatus (3Flex, Micromeritics). All samples were degassed under vacuum for more than 3 h at 623 K before measurement. The metal content of the activated carbon was determined by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) using an iCAP 7400 Duo instrument (Thermo Fisher Scientific).
Table 1 Characteristics of RAC-2 and CAC. Sample
Precursor
Shape
Packing density (g/cm3)
Metal content K (ppm)
Ca (ppm)
RAC series CAC
Ion exchange resin Coconut shell
Spherical Crushed
0.47–0.74 0.50
0 8020
0 570
Fig. 1. Schematic diagram of the breakthrough experimental.
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Breakthrough experiments The breakthrough experiments were conducted using a continuous flow system under dry conditions or a relative humidity (RH) of 50, 70, and 90% (Fig. 1, Fig. S1). The system consisted of an adsorption bed, detector (GC-FID), and humidity control parts. A stainless steel tube (inner-diameter: 7.75 mm, outer-diameter: 9.53 mm) was used as an adsorption bed, and its temperature was maintained at 298 K using a circulator. Each adsorbent was packed to a length of 5 cm of the bed. The packing weight was different depending on the packing density of the adsorbent (RACs: 1.11–1.75 g, CAC: 1.18 g). The packed adsorbent was fixed with metal sieve (100 mesh) and quartz wool. The concentrations of the inlet and outlet gases were detected using a gas chromatograph equipped with a flame ionization detector (GCFID, YL6100 GC, Younglin) and a capillary column (DB-WAX, Agilent Technologies). The relative humidity was controlled by flowing a feed stream into a bubbler filled with distilled water. The bubbler was adjusted to the desired temperature using a circulator during the breakthrough experiments. The adsorbent bed was pretreated at 623 K and purged with helium under vacuum before the experiment. Then, the bed was saturated with He initially at atmospheric pressure. After stabilization of the circulator temperature and GC-FID signal, a constant amount of benzene gas (500 mol ppm in air) was flowed through a mass flow controller (MFC). Results and discussion Comparison of hydrophobicity of RAC and CAC Comparison of porosity between RAC-2 and CAC RAC-2 had a similar specific surface area to the commercial CAC adsorbent, so RAC-2 was selected for comparison with CAC from RAC series. The porosities of RAC-2 and CAC were determined from their N2 adsorption/desorption isotherms (77 K). As shown in Fig. 2, both RAC-2 and CAC shows the type I isotherms with little hysteresis, indicating that micropores is dominant in both adsorbents. This was further confirmed by the ratio of the micropore volumes, which were 90% and 88% for RAC-2 and CAC, respectively, as shown in Table 2. The microporous nature of the adsorbents means that RAC-2 and CAC are suitable for VOC removal, since most VOCs are adsorbed primarily on the micropores of activated carbon [2,38,39]. The specific surface area of RAC-2 is 1286 m2/g, which is about 10% higher than that of CAC. The total pore volume and micropore volume are also about 10% higher in RAC-2 (Table 2). However, narrow micropore volume (<0.7 nm) shows the opposite tendency. The narrow micropore volume of CAC (0.17 cm3/g) was larger than that of RAC-2 (0.15 cm3/g). Detailed information on the pore size distribution was obtained by NLDFT calculation (Fig. 3). The overall pore size distributions of RAC-2 and CAC are similar, but the narrow micropore volume is slightly higher in CAC than in RAC-2, while the opposite situation
Fig. 2. N2 adsorption/desorption isotherms of RAC-2 and CAC at 77 K; adsorption (filled symbols) and desorption (open symbols).
occurs for micropores larger than 0.7 nm. The pore size is known to influence the adsorption of benzene and water significantly, and this will be discussed in later Section [2,40]. Comparison of surface functional groups between RAC-2 and CAC Oxygen-containing groups on the surface of activated carbon are known to adsorb water readily, and this blocks the pores and reduces the amount of VOCs adsorbed [41,42]. The oxygencontaining groups of RAC-2 and CAC were compared by measuring the surface C1 s XPS for each material. Before the discussion of XPS results, we measured X-ray diffraction analysis (XRD) to confirm the crystallite structure of the adsorbents. As can be seen Fig. S2, the XRD patterns of two adsorbents are identical, indicating the same crystallite structure. This XRD pattern means the amorphous carbon structure, the typical pattern of activated carbon. The C1s XPS peaks of the samples were separated into four Gaussian peaks of 284.8 (C–C bond), 285.8 (C–O bond), 286.8 (C¼O bond), and 288.6 (O–C¼O bond) [43], as shown in Fig. 4 and Table 3. The total amounts of oxygen-containing groups were 21.5% for RAC-2 and 26.9% for CAC, indicating that the amount of oxygencontaining groups was about 20% smaller in RAC-2 than in CAC. In particular the amount of C–O groups was about 40% lower in RAC2. XPS of O1s was also measured to verify the result of C1s XPS (Fig. S3). The shape of the XPS data is similar between both adsorbents. However, it can be seen that C–O groups are more in CAC than RAC, and C¼O groups more in RAC than CAC, coincided with the result of C1s XPS. Thus, C1s XPS data is reliable to determine the surface oxygen groups. This lower amount of oxygen-containing groups in RAC-2 could reflect the lower oxygen content of the ion-exchange resin precursor used to formulate RAC-2 when compared to coconut-shell. In addition, the oxygen of the resin can be removed easily by the form of SO2 during the heat treatment.
Table 2 Porosity of RAC-2 and CAC. Sample
SBET (m2/g)
PVt (cm3/g)
PV0.7 nm (cm3/g)
PV2 nm (cm3/g)
PV2 nm/PVt (%)
RAC-2 CAC
1286 1153
0.53 0.49
0.15 0.17
0.48 0.43
90 88
SBET: Specific surface area. PVt: Total pore volume. PV2 nm: Cumulative pore volume (<2 nm). PV0.7 nm: Cumulative pore volume (<0.7 nm) based on CO2 adsorption isotherm.
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Fig. 3. Pore size distribution of RAC-2 and CAC calculated by NLDFT method; based on CO2 isotherm (273 K) (a) and N2 isotherm (77 K) (b).
Water adsorption isotherm of RAC-2 and CAC Fig. 5(a) shows the water adsorption isotherm measured at 298 K. The amount of water adsorbed by RAC-2 increased rarely in the low pressure range, but increased sharply after P/P0 = 0.55. Conversely, the water adsorption isotherm of CAC increased gradually from the low pressure region and increased rapidly at a lower value of P/ P0 = 0.45 than in RAC-2. Therefore, the equilibrium state indicates that RAC-2 is more resistant to water than CAC. The difference in the adsorption of water by RAC-2 and CAC at low pressure can be explained by the differences in metal content and oxygen-containing groups. Oxygen-containing groups and metal impurities of the activated carbon surface adsorb water at low pressure due to their high affinity for water, which results in the formation of water clusters. The formed clusters attract water from the bulk phase and these clusters gradually grow in size. When the pressure exceeds a specific pressure, the pores of the activated carbon are filled with water. This leads to decline of the amount of VOC adsorbed significantly. Tables 1 and 3 show that CAC has a relatively high content of oxygen-containing groups and metal impurities, such as K and Ca. Conversely, RAC-2 is low in oxygencontaining groups and does not contain any metals. Therefore, these two factors result in adsorption of water vapor in CAC at low pressure. Removal of the metal impurities of CAC (by treatment with 35 wt.% HCl, stirring for 1 h, thorough washing and heat treatment at 1073 K, 2 h) reduced the K level from 8020 to 250 ppm and Ca from 570 to 90 ppm, according to ICP analysis. The water adsorption isotherm before and after metal removal was represented in Fig. 5. The amount of saturated water adsorption of CAC increased after acid treatment. This might be because that blocked pore due to metal impurity is
Fig. 4. XPS spectra C1s peaks of samples; (a) RAC-2 and (b) CAC.
opened during the acid treatment. The initial amount of water adsorbed in CAC was considerably reduced after metal removal. Therefore, the initial increase in the amount of water adsorbed in CAC was caused by its metal impurities. HCl treatment could introduce the additional surface oxygen groups. However, the effect of additional surface oxygen groups might be minimal, since the heat treatment was conducted to the activated carbon after HCl treatment, and RAC and CAC were processed under the same conditions. However, even though the metal impurities were almost completely removed, CAC still adsorbed water vapor at a lower pressure when compared to RAC. This is attributed to the higher content of oxygen-containing groups in CAC, as mentioned above. In summary, RAC-2 was more resistant to water vapor than CAC, due to the lower content of oxygen-containing groups and the absence of metal impurities. The selectivity for benzene over water was also discussed. As known, carbon material is inherently hydrophobic. Thus, activated carbon can adsorb benzene more selectively in low humidity region. However, as can be seen in the water isotherm, water vapor Table 3 Relative amount of oxygen containing groups in C1s peak of samples (%). Sample
C–C 284.8 eV
C–O 285.8 eV
C¼O 286.8 eV
O–C¼O 288.6 eV
Total oxygen groups
RAC-2 CAC
78.5 73.1
8.2 14.3
10.9 8.7
2.4 3.9
21.5 26.9
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Breakthrough Experiments for RAC-2 and CAC Table 4 shows the breakthrough experimental results of RAC-2 and CAC by the relative humidity. CAC shows longer breakthrough time per adsorbent weight under dry conditions. Lillo-Ródenas reported that the amount of benzene adsorbed onto activated carbon depends on the narrow micropore volume (<0.7 nm), rather than the total micropore volume [2]. Since the kinetic diameter of benzene is 5.85 Å, a narrow micropore close to benzene diameter greatly influences the amount of benzene adsorbed. Therefore, although CAC has low the specific surface area and total micropore volume, CAC shows a higher amount of benzene adsorbed per unit weight due to its higher narrow micropore volume. However, in terms of unit bed volume, the breakthrough time was about 18% longer for RAC-2 than for CAC. Table 1 shows that this is attributed to the high packing density of RAC-2, caused by the perfect spherical shape of the adsorbent [34]. The performance of both adsorbents declined with increases in relative humidity, but the decline ratio is different. At 50% RH, the breakthrough time for RAC-2 did not decrease substantially when compared to the dry condition (from 119 to 113 min), whereas the breakthrough time of CAC decreased by 12.4% (from 97 to 85 min). The decline in the ratio of breakthrough time was also lower for RAC-2 (33.6%) than for CAC (44.3%) at 70% RH. However, the breakthrough time decreased significantly for both adsorbents (RAC-2: 53.8%, CAC: 58.8%) at close to the saturation pressure (90% RH). These phenomena can be explained by the water adsorption isotherms of the two adsorbents. Both adsorbents are saturated at 90% RH, which results in a similar decrease ratio of breakthrough time in both adsorbents. However, the water adsorption isotherm of CAC increased from the low relative pressure and then increased rapidly at a relatively lower pressure than was observed for RAC-2, due to the high amount of oxygen-containing groups and the metal impurities. For these reasons, RAC-2 showed a lesser decline than CAC in the breakthrough time up to 70% RH. Fig. 5. Water adsorption isotherm of RAC-2 and CAC at 298 K; (a) fresh and (b) acid treated sample.
was adsorbed rapidly at the specific high humidity due to the condensation. Therefore, the benzene selectivity of the activated carbon will be decreased significantly in high humidity region. This speculation was confirmed in the breakthrough experiments.
Adsorption of benzene under humid conditions in the RAC series prepared with different activation times Comparison of porosity among the RAC series materials Table 5 shows the pore characteristics of the RAC samples prepared by varying the activation time. As the activation time
Table 4 Benzene breakthrough times at different relative humidity of RAC–2 and CAC. Sample
RH (%)
Breakthrough time (min)
Breakthrough time/Packed mass (min/g)
RAC-2
0 50 70 90
119 113 (5.0) 79 (33.6) 55 (53.8)
79 75 53 36
CAC
0 50 70 90
97 85 (12.4) 54 (44.3) 40 (58.8)
81 71 46 33
Breakthrough time: The time when C/C0 is 1%.
Table 5 Physical characteristics of RAC series. Sample
Activation time (h)
SBET (m2/g)
PVt (cm3/g)
PV0.7nm (cm3/g)
PV2nm (cm3/g)
PV2nm/PVt (%)
Packing density (g/cm3)
RAC-1 RAC-2 RAC-3 RAC-4
1 2 3 6
1006 1286 1477 1924
0.41 0.53 0.64 0.92
0.17 0.15 0.13 0.11
0.37 0.48 0.56 0.66
91 90 87 72
0.74 0.64 0.57 0.47
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Fig. 6. N2 adsorption/desorption isotherms of RAC series at 77 K; adsorption (filled symbols) and desorption (open symbols).
increased, the specific surface area of the samples also increased from 1006 m2/g to 1924 m2/g, the total pore volume from 0.41 cm3/ g to 0.92 cm3/g, and the micropore volume from 0.37 cm3/g to 0.66 cm3/g. However, the narrow micropore volume (<0.7 nm) and the ratio of the micropore volume decreased with increasing activation time, as also confirmed by the N2 isotherm (Fig. 6). A longer activation time resulted in a change in the N2 isotherm of the sample from Type I to Type V, and the hysteresis phenomenon was clear. This indicates that the micropores grow in size and develop into mesopores [44,45]. The micropore size distribution of the RAC samples is shown in Fig. 7. The overall shape of the pore size distribution was similar among the samples, but the narrow micropore volume (<0.7 nm) decreased from 0.17 to 0.11 cm3/g as the activation time increased. Water adsorption isotherms of the RAC series The water adsorption isotherms of the RAC series measured at 298 K are shown in Fig. 8. The amount of saturated water adsorption is related to total pore volume of adsorbents [46]. Thus amount of saturated water adsorption was higher at the longer activated RAC due to its higher total pore volume. Notably, water vapor adsorbed at the higher pressure as the activation time increased. In other words, a lower narrow micropore volume resulted in a higher resistance to water vapor. Many research papers have reported that the rapid adsorption of water is related to the presence of narrow micropores (<0.7 nm) [18,46,47]. Activated carbon is inherently hydrophobic, so it does not strongly adsorb water. However, if the pore of activated carbon is sufficiently small, then the adsorption energy increases and water can be adsorbed. The adsorbed water molecules interact with other surrounding water molecules to form water clusters, which in turn interact with surrounding clusters and increase in size. When 8–10 clusters are bundled to 1 nm in size, the water molecules change from hydrophilic to hydrophobic and become more stable in the pores [40,46–48]. Therefore, in this study, water adsorption was suppressed in longer activated samples due to its low narrow micropore volume.
Fig. 7. Micropore size distribution of RAC series calculated by NLDFT method; based on CO2 isotherm (273 K) (a) and N2 isotherm (77 K) (b).
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Fig. 8. Water adsorption isotherm of RAC series at 298 K.
Table 6 Benzene breakthrough times at different relative humidity of RAC series. Sample
RAC-1 RAC-2 RAC-3 RAC-4
Breakthrough time (min) RH 0%
RH 50%
RH 70%
RH 90%
120 119 108 85
103 (14.2) 113 (5.0) 108 (0) 85 (0)
67 79 87 84
46 55 59 61
(44.2) (33.6) (19.4) (1.2)
(61.7) (53.8) (45.4) (28.2)
(): (BTRH0% BTRHx%)/BTRH0% 100.
Breakthrough experiments with the RAC series The breakthrough results of the RAC samples are summarized in Table 6. The breakthrough time for RAC-4, the longest activated sample, was the shortest in the dry condition. This is because RAC4 had the lowest ratio of micropore volume and packing density, as shown in Table 5. Under humid conditions, a sample with longer activation showed an enhanced water resistance. As the relative humidity increased to 50, 70 and 90%, the decline in the ratio of the breakthrough time of RAC-1 increased sharply to 14.2, 44.2, and 61.7%, respectively. Conversely, RAC-4 showed little change in breakthrough time up to 70% RH and only a 28.2% decrease at 90% RH. This tendency toward water resistance is consistent with the results of the water adsorption isotherm, as described in Section “Water adsorption isotherms of the RAC series”. As the activation time increased, the decline in the breakthrough time was alleviated due to the decrease in the narrow micropore volume, where water vapor adsorption occurs easily. For practical applications of activated carbon, the absolute value of the breakthrough time, based on unit volume, can be more important than the decline ratio of the breakthrough time. In the low humidity range, the samples activated for a short time showed a long breakthrough time due to their large narrow micropore volume and high packing density. By contrast, although the samples activated for a long time adsorbed a lower amount of benzene in the dry condition, the absolute amount of benzene adsorbed was higher in the high humidity range due to their high water resistance. Therefore, activated carbon should be selected with an appropriate pore size distribution depending on the humidity condition. Conclusions In this work, the benzene adsorption characteristics of resinbased activated carbon were explored under humid conditions.
The XPS results showed that RAC had about 20% less surface oxygen-containing groups when compared to CAC. In addition, the ICP results showed that, unlike CAC, RAC contains no metal impurities. For these two reasons, RAC showed higher water resistance characteristics, as determined by water isotherm and breakthrough experiments. The effect of porosity on benzene adsorption under humid conditions was examined by controlling the porosity of RAC by changing the activation time. The RAC water adsorption isotherms showed that samples given a long-term activation were more resistant to water vapor. Increases in activation time increased the specific surface area, total pore volume, and micropore volume of the activated carbon, but the narrow micropore volume (<0.7 nm) decreased gradually. Water preferentially adsorbs in the narrow micropores, which have high adsorption energy; therefore, water adsorption was suppressed in the long-term activated samples. The same trend was observed in the breakthrough experiments, where the decline in benzene adsorption under humid conditions was alleviated in the long-term activated samples. RAC-1, activated for 1 h, showed a breakthrough time reduction of about 44%, while RAC-4, activated for 6 h, showed a breakthrough time reduction of only 1.2% at 70% RH. Therefore, improvement in the performance under humid conditions will require removal of the narrow micropores from activated carbon, as the narrow micropores readily adsorb water vapor. For practical applications, the absolute breakthrough time of the samples was compared based on unit volume. The high adsorption capacity of benzene under dry conditions and high packing density led to superior performance of the sample with less activation under low relative humidity. However, the resistance to water is more important than the other factors in the high humid conditions; thus, a sample with high activation is more appropriate. Therefore, the choice of activated carbon with suitable porosity should take into account the humidity conditions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.11.032. References [1] M.C. Huang, C.H. Chou, H. Teng, AIChE J. 48 (2002) 1804. [2] M.A. Lillo-Ródenas, D. Cazorla-Amorós, A. Linares-Solano, Carbon 43 (2005) 1758. [3] S. Xian, Y. Yu, J. Xiao, Z. Zhang, Q. Xia, H. Wang, Z. Li, RSC Adv. 5 (2015) 1827. [4] K.D. Kim, E.J. Park, H.O. Seo, M.G. Jeong, Y.D. Kim, D.C. Lim, Chem. Eng. J. 200– 202 (2012) 133. [5] D.G. Lee, J.H. Lee, C.H. Lee, Sep. Purif. Technol. 77 (2011) 312. [6] L. Li, S. Liu, J. Liu, J. Hazard. Mater. 192 (2011) 683. [7] Bo Pan, Baoshan Xing, Environ. Sci. Technol. 42 (2008) 9005. [8] L.F. Liotta, Appl. Catal. B 100 (2010) 403. [9] H. Wu, L. Wang, J. Zhang, Z. Shen, J. Zhao, Catal. Commun. 12 (2011) 859. [10] T. Guo, Z. Bai, C. Wu, T. Zhu, Appl. Catal. B 79 (2008) 171. [11] J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Atmos. Environ. 43 (2009) 2229. [12] Tawfik A. Saleh, Salawu Omobayo Adio, Mohammad Asif, H. Dafall, J. Cleaner Prod. 182 (2018) 960. [13] Amani M. Alansi, Waed Z. Alkayali, Maha H. Al-qunaibit, Talal F. Qahtan, Tawfik A. Saleh, RSC Adv. 5 (2015) 71441. [14] Tawfik A. Saleh, Desalin. Water Treat. 57 (2016) 10730. [15] Tawfik A. Saleh, J. Water Supply Res. T 64 (2015) 892. [16] T. Horikawa, N. Sakao, Carbon 56 (2013) 183. [17] J.H. Kim, C.H. Lee, W.S. Kim, J.S. Lee, J.T. Kim, J.K. Suh, J.M. Lee, J. Chem. Eng. Data. 48 (2003) 137. [18] H.B. Liu, B. Yang, N.D. Xue, J. Hazard. Mater. 318 (2016) 425. [19] F. Cosnier, A. Celzard, G. Furdin, D. Bégin, J.F. Marêché, O. Barrés, Carbon 43 (2005) 2554. [20] A.A. Alswat, M.B. Ahmad, Tawfik A. Saleh, J. Water Supply Res. T 65 (2016) 465. [21] Hannatu A. Sani, Mansor B. Ahmad, Tawfik A. Saleh, RSC Adv. 6 (2016) 108819. [22] T. Horikawa, T. Sekida, J. Hayashi, M. Katoh, D.D. Do, Carbon 49 (2011) 416. [23] N.J. Foley, K.M. Thomas, P.L. Forshaw, D. Stanton, P.R. Norman, Langmuir 13 (1997) 2083.
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