Environ Sci Pollut Res (2014) 21:3756–3763 DOI 10.1007/s11356-013-2328-z
RESEARCH ARTICLE
Adsorption equilibrium and dynamics of gasoline vapors onto polymeric adsorbents Lijuan Jia & Weihua Yu & Chao Long & Aimin Li
Received: 10 August 2013 / Accepted: 4 November 2013 / Published online: 27 November 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract The emission of gasoline vapors is becoming a significant environmental problem especially for the population-dense area and also results in a significant economic loss. In this study, adsorption equilibrium and dynamics of gasoline vapors onto macroporous and hypercrosslinked polymeric resins at 308 K were investigated and compared with commercial activated carbon (NucharWV-A 1100). The results showed that the equilibrium and breakthrough adsorption capacities of virgin macroporous and hypercrosslinked polymeric resins were lower than virgin-activated carbon. Compared with origin adsorbents, however, the breakthrough adsorption capacities of the regenerated activated carbon for gasoline vapors decreased by 58.5 % and 61.3 % when the initial concentration of gasoline vapors were 700 and 1, 400 mg/L, while those of macroporous and hypercrosslinked resins decreased by 17.4 % and 17.5 %, and 46.5 % and 45.5 %, respectively. Due to the specific bimodal property in the region of micropore (0.5−2.0 nm) and meso-macropore (30–70 nm), the regenerated hypercrosslinked polymeric resin exhibited the comparable breakthrough adsorption capacities with the regenerated activated carbon at the initial concentration of 700 mg/L, and even higher when the initial
Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-013-2328-z) contains supplementary material, which is available to authorized users. L. Jia : W. Yu : C. Long (*) : A. Li State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, No. 163 Xianlin Avenue, Nanjing 210023, China e-mail: [email protected] W. Yu : C. Long : A. Li Nanjing University & Yancheng Academy of Environmental Protection Technology and Engineering, No. 188 Yingbin Avenue, Yancheng 22400, China
concentration of gasoline vapors was 1,400 mg/L. In addition, 90 % of relative humidity had ignorable effect on the adsorption of gasoline vapors on hypercrosslinked polymeric resin. Taken together, it is expected that hypercrosslinked polymeric adsorbent would be a promising adsorbent for the removal of gasoline vapors from gas streams. Keywords Gasoline vapors . Polymeric resin . Adsorption . Regeneration . Dynamic . VOCs . Activated carbon
Introduction Gasoline is a mixture of relatively volatile hydrocarbons with chains containing 4–12 carbons, including alkanes, cycloalkanes, alkenes, and aromatics. It is likely to be volatile for gasoline from distribution facilities and storage tanks at the ambient temperature. It is estimated that approximately 0.1 to 0.3 % of liquid gasoline is evaporated to the atmosphere when it is loaded from a storage tank to a tank truck (Cruz-Núñez et al. 2003; Chue et al. 2004). The emission of gasoline vapors into atmosphere is not only becoming a significant environmental problem especially for the population-dense area and also results in a significant economic loss. In China, the Ministry of Environmental Protection has set the emission limit of 25 mg total organic compounds (excluding methane) per liter of gasoline loaded for gasoline storage and distribution facilities. Consequently, the recovery and reuse of evaporated gasoline from loading, unloading, and other handling processes are of significant importance from both economic and environmental points of view. Various technologies have been introduced for recovering gasoline vapors, such as adsorption, condensation, and membranes separation (He et al. 2009; El-Sharkawy et al. 2008; Ryu et al. 2002a; Liu et al. 2000; Pezolt et al. 1997; Shie et al. 2003; Liu et al. 2006). It is well known that adsorbent-based
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separation processes for the recovery of VOCs vapor are one of the most promising and cost-effective methods and are becoming increasingly popular. The core of adsorption process technology is to develop suitable adsorbents with high specific surface, stable physical, chemical properties, regenerability on site, etc. Among the adsorbents commercially available for the removal of VOCs from gas streams, activated carbon had been considered as one of the most attractive adsorbents and had been studied extensively (He et al. 2009; El-Sharkawy et al. 2008; Ryu et al. 2002a; Boulinguiez and Le Cloirec 2009; Boulinguiez and Le Cloirec 2010). However, it has been recognized that activated carbons adsorption can encounter some problems such as poor regenerability, fire risk, low mechanical strength, and the influence of water vapor (Bansal and Goyal 2005; Inagaki 2009; Zerbonia et al. 2001; Brennan et al. 2001; Cosnier et al. 2006). Hence, some alternative adsorbents, such as hydrophobic silica gel (Chue et al. 2004) and dealuminated Y zeolite (Ryu et al. 2002b), had been investigated to separate and recover gasoline vapors. Over the past few decades, porous polymeric resin has emerged as a potential alternative to activated carbon due to its controllable pore structure and stable physical and chemical properties, as well as regenerability on site, and is frequently used as an adsorbent in purification and separation processes (Long et al. 2005, 2009; Fontanals et al. 2005; Valderrama et al. 2007). Some investigators including our research group have studied adsorption characteristics of a few VOCs vapor by polymeric adsorbents (Podlesnyuk et al. 1999; Simpson et al. 1996; Liu et al. 2009; Long et al. 2010, 2011, 2013; Wu et al. 2012). These studies have indicated that polymeric adsorbents had a good sorption capacity for VOCs from polluted gas streams. Although the adsorption properties of representative components of gasoline vapors including heptane, hexane, and benzene onto polymeric adsorbent had been investigated (Liu et al. 2009; Long et al. 2010; Wu et al. 2012), gasoline vapor is a mixture of hydrocarbons with chains containing 4–12 carbons, and little information is available in the literature concerning the adsorption of actual gasoline vapors on polymeric adsorbents. Therefore, much further research is still needed for a better understanding of adsorption equilibrium and dynamic characteristics of actual gasoline vapors on polymeric resins. Permanent porous polymeric resins may be classified into two categories: macroporous and hypercrosslinked (Podlesnyuk et al. 1999). Hypercrosslinked polymeric adsorbent is a typical adsorbent with a predominantly microporous structure in the regions of pore size 0.5–2 nm (Tai et al. 1999; Long et al. 2012), while the pore size of macaroporous resin was distributed predominantly in the regions of mesopore and macropore (Long et al. 2009). In this study, adsorption of actual gasoline vapors by macroporous and hypercrosslinked polymeric resins was studied and compared with a
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commercial activated carbon (NucharWV-A 1100). Thus, adsorption equilibrium of gasoline vapors onto polymeric adsorbents and activated carbon were investigated. In addition, the dynamic adsorption and desorption properties of polymeric adsorbents and activated carbon for gasoline vapors were evaluated. Such a basic experimental investigation is essential for the application of polymeric resins to recover gasoline vapors and the design of an efficient system as well.
Materials and methods Materials and characterization The commercial macroporous polymeric resin (Macro-resin) and hypercrosslinked polymeric resin (Hyper-resin) were supplied by N&G Environmental Technology Co. Ltd. (Jiangsu, China). The granular activated carbon (Nuchar WV-A 1100) is commercially available (MeadWestvaco, USA), which is especially designed for hydrocarbon vapors adsorption. The pore texture of adsorbents was determined by N2 isotherms data at 77 K, using an adsorption analyzer ASAP 2010 (Micromeritics Instrument Co., USA). Their specific surface area (S BET), micropore volume (V micro), and mesopore volume (V meso) were calculated from the N2 isotherm data at 77 K by Brunauer–Emmett–Teller (BET), Dubinin–Astakov (DA), and Barrett–Joyner–Halena (BJH) methods, respectively. The mechanical strength means the anti-damage performance of adsorbent under mechanical force and is signified by abrasion resistance usually. The abrasion resistance was measured by methods of GB/T12598-2001 and GB/T12496.61999 (in Chinese), respectively. Adsorption experiments The adsorption of gasoline vapor was determined by the column adsorption method. The detailed experimental apparatus and adsorption procedure have been described previously (Liu et al. 2009; Wu et al. 2012). Briefly, adsorbents was precisely weighed out and charged into the adsorption column (φ 14.0 mm×150 mm) made of glass. The adsorption temperature was maintained at 308 K using thermostatic water bath. The carrier gas containing a scheduled concentration of gasoline vapors with a flow rate of 25 mL/min was passed through the column until the gasoline vapors concentration became constant and stable; the gasoline vapors concentration in the effluent steam was measured using gas chromatography by methods of “stationary source emission–determination of nonmethane hydrocarbons–gas chromatography” (HJ/T 38–1999, in Chinese). The breakthrough curves were obtained by recording the concentration of gasoline vapors consecutively at the outlet of adsorption
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column. In this study, it was recognized that the adsorption equilibrium was reached when the exit concentration became equal to the inlet concentration and stable over 60 min. So, the equilibrium amount adsorbed was equal to the weight change of adsorbent before and after the adsorption process. Regeneration of gasoline vapors loaded adsorbents was carried out in the same fixed-bed column configuration and in the down-flow mode. After the adsorption procedure, the adsorption column was connected to a vacuum pump and regenerated under vacuum at 0.005 Mpa. Here, a high-precision microbalance (BS224S, Sartorius, Germany) was adopted as the weighing device. In addition, the influence of water vapor on the adsorption of gasoline vapors on the three adsorbents was evaluated through the analysis of breakthrough curves. The experiments were performed in the same fixed-bed column configuration. Two nitrogen flows were conducted to the bubble saturator containing gasoline or liquid water, respectively. Then, these two streams were diluted with third nitrogen stream to attain
Environ Sci Pollut Res (2014) 21:3756–3763 Table 1 Selected properties of three adsorbents Adsorbents
WV-A 1100
Macro-resin
Hyper-resin
S BET (m2/g) S meso (m2/g) V micro (mL/g) V meso (mL/g) Mean diameter (nm) Abrasion resistancea (%) Abrasion resistanceb (%)
1,644.9 531.6 0.742 0.628 2.77 15.3 37.5
850.3 536.9 0.125 1.291 7.1171 87.6 89.1
1,194.6 140.8 0.547 0.185 2.5350 93.7 95.4
a
By the measuring method of GB/T12598-2001
b
By the measuring method of GB/T12496.6-1999
a given gasoline vapor concentration and water vapor concentration. The carrier gas containing a scheduled concentration of water and gasoline vapor was passed through the adsorption column.
Results and discussion Characteristics of adsorbents
Fig. 1 a The N2 adsorption–desorption isotherms at 77 K and b pore size distributions of three adsorbents
The N2 adsorption–desorption isotherms at 77 K of three adsorbents are demonstrated in Fig. 1a. According to IUPAC classification, the adsorption isotherm of Hyperresin is close to type I, reflecting the domination of micropores in the pore structure; the accelerated uptake at p/p0≈1 means that multilayer adsorption and capillary condensation were formed, and thus Hyper-resin contains the larger-sized mesopores or macropores while the N2 adsorption isotherm of Macro-resin was of type IV with a remarkable hysteresis loop, indicating a predominant pore size distribution in the mesoporous region. In comparison with two polymeric adsorbents, the N2 adsorption– desorption isotherms of WV-A 1100 is typeII, which is typical of adsorbents with mixed micro- and mesoporous structure. The pore size distributions of adsorbents calculated by applying the density functional theory are shown in Fig. 1b. It is clearly shown that Hyper-resin is typical of micropore adsorbent with a small amount of mesomacropore (30–70 nm), and the pore size of Macro-resin is mainly distributed in the regions of mesopores while WV-A 1100 contained well-developed pore structure in both regions of micropore and mesopore. Detailed pore structural properties of three adsorbents are presented in Table 1. In addition, it can be learned from Table 1 that the abrasion resistance of polymeric resins is much higher than that of WV-A 1100, indicating that polymeric adsorbents can be used for longer-lasting service than activated carbon.
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Adsorption equilibrium The adsorption equilibrium data of gasoline vapors onto WVA 1100, Macro-resin, and Hyper-resin at 308 K were obtained. Langmuir and Freundlich equations are used to correlate the experimental equilibrium data of gasoline vapors. Langmuir Equation q ¼ qm
kLC 1 þ kLC
Freundlich Equation q ¼ k F C 1=n
equation. The parameters k F and n varied with different adsorbents; however, there was no apparent correlation between k F, n, and the adsorbent properties (surface area and
ð1Þ
ð2Þ
where q is the amount adsorbed (milligrams per gram), C is the equilibrium concentration (milligrams per liter), q m, k L,k F, and n are the model parameters. From Table S1 (Electronic supplementary material) and Fig. 2a, it is clear that the Freundlich equation has a better correlation with experimental data than the Langmuir
Fig. 2 Adsorption isotherms of gasoline vapors plotted as: a unit massbased adsorbed concentration and b unit surface area-based adsorbed concentration versus gas-phase concentration on WV-A 1100, Hyperresin, and Macro-resin at 308 K
Fig. 3 Adsorption breakthrough and desorption curves of gasoline vapors on three adsorbents (the amounts of three adsorbents are 4.9423, 5.5490, and 4.2279 g for WV-A 1100, Macro-resin, and Hyper-resin, respectively; the initial concentrations are 700 mg/L; adsorption temperature is 308 K)
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porosity) within the examined concentration ranges. In the present work, the adsorption isotherms are used mainly for comparing adsorption capacities between different adsorbents. It is clearly shown in Fig. 2a that the adsorption capacities of the WV-A 1100 and Hyper-resin increased sharply with the increase of gasoline vapors concentrations at lower concentrations, manifesting the existence of microporous structure inside the two adsorbents while those on Macro-resin slowly increased with the increment of concentration without obvious inflection point, which is typical of adsorption in a mesoporous adsorbent. It is well-known that the adsorption energy in the micropores is much larger than in the mesopores due to the overlapping of adsorption forces from the opposite walls of the micropores. Therefore, the adsorption capacities of WV-A 1100 and Hyper-resin for the low concentration of gasoline vapors were much higher than Macro-resin. By comparison of the adsorption capacities of gasoline vapors on three adsorbents in the experimental concentration range, it is found that, at the lower concentration of the gasoline vapor, Hyper-resin has the comparable adsorption capacities with activated carbon (WV-A 1100). The possible reason is that the micropore volume of Hyper-resin (0.547 mL/g) is close to that of WV-A 1100 (0.742 mL/g). However, WV-A 1100 exhibited better adsorption capacities than Hyper-resin at higher concentrations. The results were mainly due to the fact that WV-A 1100 has the larger BET surface area and well-developed mesopores. Among the three adsorbent, Macro-resin has the lowest adsorption capacities for gasoline vapors, which is consistent with its lowest values of micropore volume and surface area. To further understand the effect of pore size distribution on the adsorption of gasoline vapors, the surface area normalized adsorption data of three adsorbents are presented in Fig. 2b. Compared with Macro-resin, the larger normalized adsorption capacities of WV-A 1100 and Hyper-resin for lower concentration of gasoline vapors could be primarily caused by the micropore-filling mechanism. Probably due to the formation of capillary condensation, however, Macro-resin has the highest adsorption capacities for higher concentration of gasoline vapors among three adsorbents. Although the surface area and micropore volume of WV-A 1100 are larger than Table 2 Breakthrough adsorption characteristics of gasoline vapors onto three adsorbents
those of Hyper-resin, it should be noted that the surface areanormalized adsorption of gasoline vapors is higher on Hyperresin than on WV-A 1100. The reason for this result is as follows. From Fig. 1 and Table 1, it is clearly learned that the micropore is mainly responsible for the surface area of Hyperresin and WV-A 1100; however, WV-A 1100 is highly microporous and has more ultramicropores (width less than 0.5 nm) compared with Hyper-resin and probably invokes the size exclusion effect on adsorption of gasoline vapors. Hence, the large surface areas of WV-A 1100 cannot be completely utilized in adsorption of gasoline vapors, resulting in lower surface area-normalized adsorption capacities. In fact, due to the close proximity of the walls of micropores, the overlapping of adsorption potential fields from the opposite walls of the micropores may occur and enhance adsorption force. Thus, the highly microporous size distribution is also mainly responsible for the low regeneration efficiency of activated carbons, which is one of the main shortcomings of using microporous activated carbons as adsorbents in VOCs treatment. Dynamic adsorption–desorption properties In this study, consecutive column runs were carried out to understand the adsorption dynamic behavior in a fixed bed, in between two adsorptions, adsorbent column was regenerated. Figure 3 shows the breakthrough curves of gasoline vapors adsorbed on virgin and regenerated WV-A 1100 and two polymeric adsorbents at the initial concentrations of 700 mg/L. Since the breakthrough curves of the third and fourth adsorption cycles overlapped with that of the second adsorption, those curves are not shown in Fig. 3 (left). Evidently, a slight change of the breakthrough curves of gasoline vapors onto virgin and regenerated Macro-resin would be found; however, the regenerated WV-A 1100 and Hyper-resin showed obviously shorter adsorption breakthrough time for gasoline vapors than theirs virgin form. These results indicate that Macro-resin had higher regeneration efficiency than WV-A 1100 and Hyper-resin, which has been clearly shown in Fig. 3 (right). Furthermore, the breakthrough behaviors of 1,400 mg/L of gasoline vapors on three adsorbents were also investigated and
Initial concentration (mg/L)
Adsorbent
Equilibrium capacity (mg/g)
Breakthrough capacity (first) (mg/g)
Breakthrough capacity (second) (mg/g)
700
WV-A 1100 Hyper-resin Macro-resin WV-A 1100 Hyper-resin Macro-resin
445.9 353.4 232.1 524.1 398.9 315.7
317.9 253.9 139.1 332.8 309.3 157.3
185.1 150.1 125.4 145.2 167.2 130.8
1,400
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C/mg .L
-1
350 300
hyper-resin
250
C0=700mg/L
200 150 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
100 50 0 0
20
40
60
80
100
120
140
600
hyper-resin C/mg .L
-1
500 C0=1400mg/L
400 300 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
200 100 0 0
20
40
60
80
100
t/min
C/mg .L
-1
350 300
macro-resin
250
C0=700mg/L
200 150 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
100 50 0 0
10
20
30
40
50
60
70
80
700
C/mg .L
-1
600
macro-resin
500
C0=1400mg/L
400 300
first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
200 100 0 0
10
20
30
40
50
60
t/min
C/mg .L
-1
350 300
WV-A 1100
250
C0=700mg/L
200 150 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
100 50 0 0
20
40
60
80
100
120
140
160
900
C/mg .L
-1
800
WV-A 1100
700
C0=1400mg/L
600 500 400
first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
300 200 100 0 0
20
40
60
t/min
80
100
120
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Fig. 4
The effect of relative humidity on the breakthrough curves of gasoline vapors on three adsorbents (the amounts of three adsorbents are 4.9423, 5.5490, and 4.2279 g for WV-A 1100, Macro-resin, and Hyperresin, respectively; the initial concentrations are 700 and 1,400 mg/L, respectively; adsorption temperature is 308 K)
shown in Fig. S1 (Electronic supplementary material). The results are similar to those obtained for the removal of 700 mg/L of gasoline vapors. For the sake of comparison quantitatively, the adsorption breakthrough capacities based on the breakthrough time were calculated and shown in Table 2. The breakthrough time is calculated as the time at which the concentration in the outlet is 25 mg/L. It is learned that the breakthrough adsorption capacities of gasoline vapors onto the regenerated Macro-resin decreased only by about 17.4 % and 17.5 % compared with virgin resin when the initial concentration of gasoline vapor was 700 and 1,400 mg/L respectively, while 58.5 % and 61.3 % onto activated carbon and 46.5 % and 45.5 % onto Hyper-resin. Such result was mainly attributed to different pore structure of adsorbents. It is clearly learned that Hyper-resin and activated carbon are typical microporous adsorbents with a certain amounts of macropore and mesopore, while the pore population of Macro-resin is mainly distributed in the region of mesopores. It is well known that the adsorption energy in the micropores is much larger than that in the mesopores. Compared with Macroresin which adsorbs gasoline vapor mainly by capillary condensation of mesopores, the strong adsorption force acting on adsorbate molecule in the micropores makes it ineffective to regenerate activated carbon and Hyper-resin. However, the absence of micropore led to lower breakthrough adsorption capacities of Macro-resin, as compared with WV-A 1100 and Hyper-resin. On the other hand, it should be noted that, although the equilibrium adsorption capacities of Hyper-resin for gasoline vapors are 92.5 and 125.2 mg/g less than WV-A 1100 at the initial concentrations of 700 and 1,400 mg/L, respectively, Hyper-resin exhibited the similar breakthrough adsorption capacities to WV-A 1100 at the initial concentration of 700 mg/L and even higher when the initial concentration is 1,400 mg/L. The good breakthrough adsorption capacities of gasoline vapors on Hyper-resin are probably due to its specific bimodal property in the region of micropore (0.5–2.0 nm) and meso-macropore (30–70 nm). The larger micropore volume of Hyper-resin accounts for the enhanced adsorption capacities; the mesopores and macropores of Hyper-resin act as transport pores for the effective diffusion of gasoline vapors molecules to the internal surfaces and into the micropores and therefore improve the adsorption kinetics. Taken together, it is expected that Hyper-resin would be a promising adsorbent for the removal of gasoline vapors from gas streams.
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Influence of RH on gasoline vapor adsorption Many studies have pointed out that water vapor could be adsorbed competitively with VOCs on commercial activated carbon, resulting in a diminished capacity and slow adsorption kinetics for the targeted adsorbates, especially when VOCs are not hydrosoluble (Rodriguez-Mirasol et al. 2005; Kim et al. 2004; Marbán and Fuertes 2004; Sager and Schmidt 2010; Kaplan et al. 2006). Therefore, it is necessary to study the influence of relative humidity on gasoline vapors adsorption. The influence of relative humidity on breakthrough adsorption of gasoline vapors is shown in Fig. 4. It is found that relative humidity (RH=90 %) has almost negligible effect on the adsorption of gasoline vapors on polymeric adsorbents (Macro-resin and Hyper-resin) and activated carbon (WV-A 1100) whether the initial concentration of gasoline vapors is 700 or 1,400 mg/L. The probable reason is that, at high VOCs concentrations, the adsorption of water would be much slower than that of the VOCs (Cosnier et al. 2006); hence, water has such a low influence on VOCs adsorption under these conditions. Therefore, the effect of humidity on adsorption of gasoline vapors by polymeric adsorbents could be ignored in practical applications at these concentrations.
Conclusions Adsorption equilibrium and dynamics of gasoline vapors onto macroporous and hypercrosslinked polymeric resins (Macroresin and Hyper-resin) were evaluated and compared with commercial activated carbon (NucharWV-A 1100). It is concluded that hypercrosslinked polymeric resin (Hyper-resin) will be an efficient and competitive adsorbent for gasoline vapors recovery. Due to the specific bimodal property in the region of micropore (0.5–2.0 nm) and meso-macropore (30– 70 nm), hypercrosslinked polymeric resin (Hyper-resin) exhibits the good breakthrough adsorption capacities. Compared with activated carbon, the results of consecutive column adsorption–desorption confirmed that the regenerated hypercrosslinked polymeric resin had the comparable breakthrough adsorption capacity for lower concentration of gasoline vapor (700 mg/L), but higher when the initial concentration of gasoline vapor is 1,400 mg/L. In addition, the moisture (RH=90 %) had little effect on the adsorption of gasoline vapor on hypercrosslinked polymeric resin and on activated carbon, whether the initial concentration of gasoline vapor is 700 or 1,400 mg/L. Acknowledgments This research was financially funded by National Natural Science Foundation of China (Grant No 51078180), New Century Excellent Talents in University (Grant No NCET-11-0230), Qing Lan Project of Jiangsu Province, and Program for Changjiang Scholars Innovative Research Team in University.
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RESEARCH ARTICLE
Adsorption equilibrium and dynamics of gasoline vapors onto polymeric adsorbents Lijuan Jia & Weihua Yu & Chao Long & Aimin Li
Received: 10 August 2013 / Accepted: 4 November 2013 / Published online: 27 November 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract The emission of gasoline vapors is becoming a significant environmental problem especially for the population-dense area and also results in a significant economic loss. In this study, adsorption equilibrium and dynamics of gasoline vapors onto macroporous and hypercrosslinked polymeric resins at 308 K were investigated and compared with commercial activated carbon (NucharWV-A 1100). The results showed that the equilibrium and breakthrough adsorption capacities of virgin macroporous and hypercrosslinked polymeric resins were lower than virgin-activated carbon. Compared with origin adsorbents, however, the breakthrough adsorption capacities of the regenerated activated carbon for gasoline vapors decreased by 58.5 % and 61.3 % when the initial concentration of gasoline vapors were 700 and 1, 400 mg/L, while those of macroporous and hypercrosslinked resins decreased by 17.4 % and 17.5 %, and 46.5 % and 45.5 %, respectively. Due to the specific bimodal property in the region of micropore (0.5−2.0 nm) and meso-macropore (30–70 nm), the regenerated hypercrosslinked polymeric resin exhibited the comparable breakthrough adsorption capacities with the regenerated activated carbon at the initial concentration of 700 mg/L, and even higher when the initial
Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-013-2328-z) contains supplementary material, which is available to authorized users. L. Jia : W. Yu : C. Long (*) : A. Li State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, No. 163 Xianlin Avenue, Nanjing 210023, China e-mail: [email protected] W. Yu : C. Long : A. Li Nanjing University & Yancheng Academy of Environmental Protection Technology and Engineering, No. 188 Yingbin Avenue, Yancheng 22400, China
concentration of gasoline vapors was 1,400 mg/L. In addition, 90 % of relative humidity had ignorable effect on the adsorption of gasoline vapors on hypercrosslinked polymeric resin. Taken together, it is expected that hypercrosslinked polymeric adsorbent would be a promising adsorbent for the removal of gasoline vapors from gas streams. Keywords Gasoline vapors . Polymeric resin . Adsorption . Regeneration . Dynamic . VOCs . Activated carbon
Introduction Gasoline is a mixture of relatively volatile hydrocarbons with chains containing 4–12 carbons, including alkanes, cycloalkanes, alkenes, and aromatics. It is likely to be volatile for gasoline from distribution facilities and storage tanks at the ambient temperature. It is estimated that approximately 0.1 to 0.3 % of liquid gasoline is evaporated to the atmosphere when it is loaded from a storage tank to a tank truck (Cruz-Núñez et al. 2003; Chue et al. 2004). The emission of gasoline vapors into atmosphere is not only becoming a significant environmental problem especially for the population-dense area and also results in a significant economic loss. In China, the Ministry of Environmental Protection has set the emission limit of 25 mg total organic compounds (excluding methane) per liter of gasoline loaded for gasoline storage and distribution facilities. Consequently, the recovery and reuse of evaporated gasoline from loading, unloading, and other handling processes are of significant importance from both economic and environmental points of view. Various technologies have been introduced for recovering gasoline vapors, such as adsorption, condensation, and membranes separation (He et al. 2009; El-Sharkawy et al. 2008; Ryu et al. 2002a; Liu et al. 2000; Pezolt et al. 1997; Shie et al. 2003; Liu et al. 2006). It is well known that adsorbent-based
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separation processes for the recovery of VOCs vapor are one of the most promising and cost-effective methods and are becoming increasingly popular. The core of adsorption process technology is to develop suitable adsorbents with high specific surface, stable physical, chemical properties, regenerability on site, etc. Among the adsorbents commercially available for the removal of VOCs from gas streams, activated carbon had been considered as one of the most attractive adsorbents and had been studied extensively (He et al. 2009; El-Sharkawy et al. 2008; Ryu et al. 2002a; Boulinguiez and Le Cloirec 2009; Boulinguiez and Le Cloirec 2010). However, it has been recognized that activated carbons adsorption can encounter some problems such as poor regenerability, fire risk, low mechanical strength, and the influence of water vapor (Bansal and Goyal 2005; Inagaki 2009; Zerbonia et al. 2001; Brennan et al. 2001; Cosnier et al. 2006). Hence, some alternative adsorbents, such as hydrophobic silica gel (Chue et al. 2004) and dealuminated Y zeolite (Ryu et al. 2002b), had been investigated to separate and recover gasoline vapors. Over the past few decades, porous polymeric resin has emerged as a potential alternative to activated carbon due to its controllable pore structure and stable physical and chemical properties, as well as regenerability on site, and is frequently used as an adsorbent in purification and separation processes (Long et al. 2005, 2009; Fontanals et al. 2005; Valderrama et al. 2007). Some investigators including our research group have studied adsorption characteristics of a few VOCs vapor by polymeric adsorbents (Podlesnyuk et al. 1999; Simpson et al. 1996; Liu et al. 2009; Long et al. 2010, 2011, 2013; Wu et al. 2012). These studies have indicated that polymeric adsorbents had a good sorption capacity for VOCs from polluted gas streams. Although the adsorption properties of representative components of gasoline vapors including heptane, hexane, and benzene onto polymeric adsorbent had been investigated (Liu et al. 2009; Long et al. 2010; Wu et al. 2012), gasoline vapor is a mixture of hydrocarbons with chains containing 4–12 carbons, and little information is available in the literature concerning the adsorption of actual gasoline vapors on polymeric adsorbents. Therefore, much further research is still needed for a better understanding of adsorption equilibrium and dynamic characteristics of actual gasoline vapors on polymeric resins. Permanent porous polymeric resins may be classified into two categories: macroporous and hypercrosslinked (Podlesnyuk et al. 1999). Hypercrosslinked polymeric adsorbent is a typical adsorbent with a predominantly microporous structure in the regions of pore size 0.5–2 nm (Tai et al. 1999; Long et al. 2012), while the pore size of macaroporous resin was distributed predominantly in the regions of mesopore and macropore (Long et al. 2009). In this study, adsorption of actual gasoline vapors by macroporous and hypercrosslinked polymeric resins was studied and compared with a
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commercial activated carbon (NucharWV-A 1100). Thus, adsorption equilibrium of gasoline vapors onto polymeric adsorbents and activated carbon were investigated. In addition, the dynamic adsorption and desorption properties of polymeric adsorbents and activated carbon for gasoline vapors were evaluated. Such a basic experimental investigation is essential for the application of polymeric resins to recover gasoline vapors and the design of an efficient system as well.
Materials and methods Materials and characterization The commercial macroporous polymeric resin (Macro-resin) and hypercrosslinked polymeric resin (Hyper-resin) were supplied by N&G Environmental Technology Co. Ltd. (Jiangsu, China). The granular activated carbon (Nuchar WV-A 1100) is commercially available (MeadWestvaco, USA), which is especially designed for hydrocarbon vapors adsorption. The pore texture of adsorbents was determined by N2 isotherms data at 77 K, using an adsorption analyzer ASAP 2010 (Micromeritics Instrument Co., USA). Their specific surface area (S BET), micropore volume (V micro), and mesopore volume (V meso) were calculated from the N2 isotherm data at 77 K by Brunauer–Emmett–Teller (BET), Dubinin–Astakov (DA), and Barrett–Joyner–Halena (BJH) methods, respectively. The mechanical strength means the anti-damage performance of adsorbent under mechanical force and is signified by abrasion resistance usually. The abrasion resistance was measured by methods of GB/T12598-2001 and GB/T12496.61999 (in Chinese), respectively. Adsorption experiments The adsorption of gasoline vapor was determined by the column adsorption method. The detailed experimental apparatus and adsorption procedure have been described previously (Liu et al. 2009; Wu et al. 2012). Briefly, adsorbents was precisely weighed out and charged into the adsorption column (φ 14.0 mm×150 mm) made of glass. The adsorption temperature was maintained at 308 K using thermostatic water bath. The carrier gas containing a scheduled concentration of gasoline vapors with a flow rate of 25 mL/min was passed through the column until the gasoline vapors concentration became constant and stable; the gasoline vapors concentration in the effluent steam was measured using gas chromatography by methods of “stationary source emission–determination of nonmethane hydrocarbons–gas chromatography” (HJ/T 38–1999, in Chinese). The breakthrough curves were obtained by recording the concentration of gasoline vapors consecutively at the outlet of adsorption
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column. In this study, it was recognized that the adsorption equilibrium was reached when the exit concentration became equal to the inlet concentration and stable over 60 min. So, the equilibrium amount adsorbed was equal to the weight change of adsorbent before and after the adsorption process. Regeneration of gasoline vapors loaded adsorbents was carried out in the same fixed-bed column configuration and in the down-flow mode. After the adsorption procedure, the adsorption column was connected to a vacuum pump and regenerated under vacuum at 0.005 Mpa. Here, a high-precision microbalance (BS224S, Sartorius, Germany) was adopted as the weighing device. In addition, the influence of water vapor on the adsorption of gasoline vapors on the three adsorbents was evaluated through the analysis of breakthrough curves. The experiments were performed in the same fixed-bed column configuration. Two nitrogen flows were conducted to the bubble saturator containing gasoline or liquid water, respectively. Then, these two streams were diluted with third nitrogen stream to attain
Environ Sci Pollut Res (2014) 21:3756–3763 Table 1 Selected properties of three adsorbents Adsorbents
WV-A 1100
Macro-resin
Hyper-resin
S BET (m2/g) S meso (m2/g) V micro (mL/g) V meso (mL/g) Mean diameter (nm) Abrasion resistancea (%) Abrasion resistanceb (%)
1,644.9 531.6 0.742 0.628 2.77 15.3 37.5
850.3 536.9 0.125 1.291 7.1171 87.6 89.1
1,194.6 140.8 0.547 0.185 2.5350 93.7 95.4
a
By the measuring method of GB/T12598-2001
b
By the measuring method of GB/T12496.6-1999
a given gasoline vapor concentration and water vapor concentration. The carrier gas containing a scheduled concentration of water and gasoline vapor was passed through the adsorption column.
Results and discussion Characteristics of adsorbents
Fig. 1 a The N2 adsorption–desorption isotherms at 77 K and b pore size distributions of three adsorbents
The N2 adsorption–desorption isotherms at 77 K of three adsorbents are demonstrated in Fig. 1a. According to IUPAC classification, the adsorption isotherm of Hyperresin is close to type I, reflecting the domination of micropores in the pore structure; the accelerated uptake at p/p0≈1 means that multilayer adsorption and capillary condensation were formed, and thus Hyper-resin contains the larger-sized mesopores or macropores while the N2 adsorption isotherm of Macro-resin was of type IV with a remarkable hysteresis loop, indicating a predominant pore size distribution in the mesoporous region. In comparison with two polymeric adsorbents, the N2 adsorption– desorption isotherms of WV-A 1100 is typeII, which is typical of adsorbents with mixed micro- and mesoporous structure. The pore size distributions of adsorbents calculated by applying the density functional theory are shown in Fig. 1b. It is clearly shown that Hyper-resin is typical of micropore adsorbent with a small amount of mesomacropore (30–70 nm), and the pore size of Macro-resin is mainly distributed in the regions of mesopores while WV-A 1100 contained well-developed pore structure in both regions of micropore and mesopore. Detailed pore structural properties of three adsorbents are presented in Table 1. In addition, it can be learned from Table 1 that the abrasion resistance of polymeric resins is much higher than that of WV-A 1100, indicating that polymeric adsorbents can be used for longer-lasting service than activated carbon.
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Adsorption equilibrium The adsorption equilibrium data of gasoline vapors onto WVA 1100, Macro-resin, and Hyper-resin at 308 K were obtained. Langmuir and Freundlich equations are used to correlate the experimental equilibrium data of gasoline vapors. Langmuir Equation q ¼ qm
kLC 1 þ kLC
Freundlich Equation q ¼ k F C 1=n
equation. The parameters k F and n varied with different adsorbents; however, there was no apparent correlation between k F, n, and the adsorbent properties (surface area and
ð1Þ
ð2Þ
where q is the amount adsorbed (milligrams per gram), C is the equilibrium concentration (milligrams per liter), q m, k L,k F, and n are the model parameters. From Table S1 (Electronic supplementary material) and Fig. 2a, it is clear that the Freundlich equation has a better correlation with experimental data than the Langmuir
Fig. 2 Adsorption isotherms of gasoline vapors plotted as: a unit massbased adsorbed concentration and b unit surface area-based adsorbed concentration versus gas-phase concentration on WV-A 1100, Hyperresin, and Macro-resin at 308 K
Fig. 3 Adsorption breakthrough and desorption curves of gasoline vapors on three adsorbents (the amounts of three adsorbents are 4.9423, 5.5490, and 4.2279 g for WV-A 1100, Macro-resin, and Hyper-resin, respectively; the initial concentrations are 700 mg/L; adsorption temperature is 308 K)
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porosity) within the examined concentration ranges. In the present work, the adsorption isotherms are used mainly for comparing adsorption capacities between different adsorbents. It is clearly shown in Fig. 2a that the adsorption capacities of the WV-A 1100 and Hyper-resin increased sharply with the increase of gasoline vapors concentrations at lower concentrations, manifesting the existence of microporous structure inside the two adsorbents while those on Macro-resin slowly increased with the increment of concentration without obvious inflection point, which is typical of adsorption in a mesoporous adsorbent. It is well-known that the adsorption energy in the micropores is much larger than in the mesopores due to the overlapping of adsorption forces from the opposite walls of the micropores. Therefore, the adsorption capacities of WV-A 1100 and Hyper-resin for the low concentration of gasoline vapors were much higher than Macro-resin. By comparison of the adsorption capacities of gasoline vapors on three adsorbents in the experimental concentration range, it is found that, at the lower concentration of the gasoline vapor, Hyper-resin has the comparable adsorption capacities with activated carbon (WV-A 1100). The possible reason is that the micropore volume of Hyper-resin (0.547 mL/g) is close to that of WV-A 1100 (0.742 mL/g). However, WV-A 1100 exhibited better adsorption capacities than Hyper-resin at higher concentrations. The results were mainly due to the fact that WV-A 1100 has the larger BET surface area and well-developed mesopores. Among the three adsorbent, Macro-resin has the lowest adsorption capacities for gasoline vapors, which is consistent with its lowest values of micropore volume and surface area. To further understand the effect of pore size distribution on the adsorption of gasoline vapors, the surface area normalized adsorption data of three adsorbents are presented in Fig. 2b. Compared with Macro-resin, the larger normalized adsorption capacities of WV-A 1100 and Hyper-resin for lower concentration of gasoline vapors could be primarily caused by the micropore-filling mechanism. Probably due to the formation of capillary condensation, however, Macro-resin has the highest adsorption capacities for higher concentration of gasoline vapors among three adsorbents. Although the surface area and micropore volume of WV-A 1100 are larger than Table 2 Breakthrough adsorption characteristics of gasoline vapors onto three adsorbents
those of Hyper-resin, it should be noted that the surface areanormalized adsorption of gasoline vapors is higher on Hyperresin than on WV-A 1100. The reason for this result is as follows. From Fig. 1 and Table 1, it is clearly learned that the micropore is mainly responsible for the surface area of Hyperresin and WV-A 1100; however, WV-A 1100 is highly microporous and has more ultramicropores (width less than 0.5 nm) compared with Hyper-resin and probably invokes the size exclusion effect on adsorption of gasoline vapors. Hence, the large surface areas of WV-A 1100 cannot be completely utilized in adsorption of gasoline vapors, resulting in lower surface area-normalized adsorption capacities. In fact, due to the close proximity of the walls of micropores, the overlapping of adsorption potential fields from the opposite walls of the micropores may occur and enhance adsorption force. Thus, the highly microporous size distribution is also mainly responsible for the low regeneration efficiency of activated carbons, which is one of the main shortcomings of using microporous activated carbons as adsorbents in VOCs treatment. Dynamic adsorption–desorption properties In this study, consecutive column runs were carried out to understand the adsorption dynamic behavior in a fixed bed, in between two adsorptions, adsorbent column was regenerated. Figure 3 shows the breakthrough curves of gasoline vapors adsorbed on virgin and regenerated WV-A 1100 and two polymeric adsorbents at the initial concentrations of 700 mg/L. Since the breakthrough curves of the third and fourth adsorption cycles overlapped with that of the second adsorption, those curves are not shown in Fig. 3 (left). Evidently, a slight change of the breakthrough curves of gasoline vapors onto virgin and regenerated Macro-resin would be found; however, the regenerated WV-A 1100 and Hyper-resin showed obviously shorter adsorption breakthrough time for gasoline vapors than theirs virgin form. These results indicate that Macro-resin had higher regeneration efficiency than WV-A 1100 and Hyper-resin, which has been clearly shown in Fig. 3 (right). Furthermore, the breakthrough behaviors of 1,400 mg/L of gasoline vapors on three adsorbents were also investigated and
Initial concentration (mg/L)
Adsorbent
Equilibrium capacity (mg/g)
Breakthrough capacity (first) (mg/g)
Breakthrough capacity (second) (mg/g)
700
WV-A 1100 Hyper-resin Macro-resin WV-A 1100 Hyper-resin Macro-resin
445.9 353.4 232.1 524.1 398.9 315.7
317.9 253.9 139.1 332.8 309.3 157.3
185.1 150.1 125.4 145.2 167.2 130.8
1,400
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C/mg .L
-1
350 300
hyper-resin
250
C0=700mg/L
200 150 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
100 50 0 0
20
40
60
80
100
120
140
600
hyper-resin C/mg .L
-1
500 C0=1400mg/L
400 300 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
200 100 0 0
20
40
60
80
100
t/min
C/mg .L
-1
350 300
macro-resin
250
C0=700mg/L
200 150 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
100 50 0 0
10
20
30
40
50
60
70
80
700
C/mg .L
-1
600
macro-resin
500
C0=1400mg/L
400 300
first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
200 100 0 0
10
20
30
40
50
60
t/min
C/mg .L
-1
350 300
WV-A 1100
250
C0=700mg/L
200 150 first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
100 50 0 0
20
40
60
80
100
120
140
160
900
C/mg .L
-1
800
WV-A 1100
700
C0=1400mg/L
600 500 400
first run(RH=0%) first run(RH=90%) second run(RH=0%) second run(RH=90%)
300 200 100 0 0
20
40
60
t/min
80
100
120
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Fig. 4
The effect of relative humidity on the breakthrough curves of gasoline vapors on three adsorbents (the amounts of three adsorbents are 4.9423, 5.5490, and 4.2279 g for WV-A 1100, Macro-resin, and Hyperresin, respectively; the initial concentrations are 700 and 1,400 mg/L, respectively; adsorption temperature is 308 K)
shown in Fig. S1 (Electronic supplementary material). The results are similar to those obtained for the removal of 700 mg/L of gasoline vapors. For the sake of comparison quantitatively, the adsorption breakthrough capacities based on the breakthrough time were calculated and shown in Table 2. The breakthrough time is calculated as the time at which the concentration in the outlet is 25 mg/L. It is learned that the breakthrough adsorption capacities of gasoline vapors onto the regenerated Macro-resin decreased only by about 17.4 % and 17.5 % compared with virgin resin when the initial concentration of gasoline vapor was 700 and 1,400 mg/L respectively, while 58.5 % and 61.3 % onto activated carbon and 46.5 % and 45.5 % onto Hyper-resin. Such result was mainly attributed to different pore structure of adsorbents. It is clearly learned that Hyper-resin and activated carbon are typical microporous adsorbents with a certain amounts of macropore and mesopore, while the pore population of Macro-resin is mainly distributed in the region of mesopores. It is well known that the adsorption energy in the micropores is much larger than that in the mesopores. Compared with Macroresin which adsorbs gasoline vapor mainly by capillary condensation of mesopores, the strong adsorption force acting on adsorbate molecule in the micropores makes it ineffective to regenerate activated carbon and Hyper-resin. However, the absence of micropore led to lower breakthrough adsorption capacities of Macro-resin, as compared with WV-A 1100 and Hyper-resin. On the other hand, it should be noted that, although the equilibrium adsorption capacities of Hyper-resin for gasoline vapors are 92.5 and 125.2 mg/g less than WV-A 1100 at the initial concentrations of 700 and 1,400 mg/L, respectively, Hyper-resin exhibited the similar breakthrough adsorption capacities to WV-A 1100 at the initial concentration of 700 mg/L and even higher when the initial concentration is 1,400 mg/L. The good breakthrough adsorption capacities of gasoline vapors on Hyper-resin are probably due to its specific bimodal property in the region of micropore (0.5–2.0 nm) and meso-macropore (30–70 nm). The larger micropore volume of Hyper-resin accounts for the enhanced adsorption capacities; the mesopores and macropores of Hyper-resin act as transport pores for the effective diffusion of gasoline vapors molecules to the internal surfaces and into the micropores and therefore improve the adsorption kinetics. Taken together, it is expected that Hyper-resin would be a promising adsorbent for the removal of gasoline vapors from gas streams.
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Influence of RH on gasoline vapor adsorption Many studies have pointed out that water vapor could be adsorbed competitively with VOCs on commercial activated carbon, resulting in a diminished capacity and slow adsorption kinetics for the targeted adsorbates, especially when VOCs are not hydrosoluble (Rodriguez-Mirasol et al. 2005; Kim et al. 2004; Marbán and Fuertes 2004; Sager and Schmidt 2010; Kaplan et al. 2006). Therefore, it is necessary to study the influence of relative humidity on gasoline vapors adsorption. The influence of relative humidity on breakthrough adsorption of gasoline vapors is shown in Fig. 4. It is found that relative humidity (RH=90 %) has almost negligible effect on the adsorption of gasoline vapors on polymeric adsorbents (Macro-resin and Hyper-resin) and activated carbon (WV-A 1100) whether the initial concentration of gasoline vapors is 700 or 1,400 mg/L. The probable reason is that, at high VOCs concentrations, the adsorption of water would be much slower than that of the VOCs (Cosnier et al. 2006); hence, water has such a low influence on VOCs adsorption under these conditions. Therefore, the effect of humidity on adsorption of gasoline vapors by polymeric adsorbents could be ignored in practical applications at these concentrations.
Conclusions Adsorption equilibrium and dynamics of gasoline vapors onto macroporous and hypercrosslinked polymeric resins (Macroresin and Hyper-resin) were evaluated and compared with commercial activated carbon (NucharWV-A 1100). It is concluded that hypercrosslinked polymeric resin (Hyper-resin) will be an efficient and competitive adsorbent for gasoline vapors recovery. Due to the specific bimodal property in the region of micropore (0.5–2.0 nm) and meso-macropore (30– 70 nm), hypercrosslinked polymeric resin (Hyper-resin) exhibits the good breakthrough adsorption capacities. Compared with activated carbon, the results of consecutive column adsorption–desorption confirmed that the regenerated hypercrosslinked polymeric resin had the comparable breakthrough adsorption capacity for lower concentration of gasoline vapor (700 mg/L), but higher when the initial concentration of gasoline vapor is 1,400 mg/L. In addition, the moisture (RH=90 %) had little effect on the adsorption of gasoline vapor on hypercrosslinked polymeric resin and on activated carbon, whether the initial concentration of gasoline vapor is 700 or 1,400 mg/L. Acknowledgments This research was financially funded by National Natural Science Foundation of China (Grant No 51078180), New Century Excellent Talents in University (Grant No NCET-11-0230), Qing Lan Project of Jiangsu Province, and Program for Changjiang Scholars Innovative Research Team in University.
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