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Adsorption of nicotine and tar from the mainstream smoke of cigarettes by oxidized carbon nanotubes
Applied Surface Science 252 (2006) 2933–2937 www.elsevier.com/locate/apsusc
Adsorption of nicotine and tar from the mainstream smoke of cigarettes by oxidized carbon nanotubes Zhigang Chen a,b, Lisha Zhang a, Yiwen Tang a,*, Zhijie Jia a a
Institute of Nano-science and Technology, Central China Normal University, Wuhan 430079, China b Institute of Advanced Materials, Fudan University, Shanghai 200433, China Received 23 February 2005; received in revised form 30 April 2005; accepted 30 April 2005 Available online 3 June 2005
Abstract The adsorption of nicotine and tar from the mainstream smoke (MS) by the filter tips filled respectively with oxidized carbon nanotubes (O-CNTs), activated carbon and zeolite (NaY) has been investigated. O-CNTs show exceptional removal efficiency and their adsorption mechanism is investigated. Capillary condensation of some ingredients from MS in the inner hole of OCNTs is observed and may be the primary reason for their superior removal efficiency. The effect of O-CNTs mass on the removal efficiencies is also studied and the results show that about 20–30 mg O-CNTs per cigarette can effectively remove most of nicotine and tar. # 2005 Elsevier B.V. All rights reserved. PACS: 68.43. h; 68.35. p Keywords: Carbon nanotubes; Cigarette smoke; Adsorption; Tar and nicotine
1. Introduction The pollution and health hazard caused by smoking have been an urgent problem in the world [1–3]. Tobacco smoke is a very complex mixture containing more than 3800 compounds, which are aerosol composed of volatile agents in the vapor phase and of semi- and non-volatile compounds [4,5]. Besides nicotine, the major inducer of tobacco * Corresponding author. Tel.: +86 27 67861185; fax: +86 27 67861185. E-mail address: [email protected] (Y. Tang).
dependence, cigarette smoke also contains various toxic compounds and notably carcinogenic agents like polycyclic aromatic hydrocarbons, commonly called ‘tar’ [6]. Both of them constitute a serious health risk. In the past years, great efforts have been made in the development of filter tips. It is an effective and convenient way to add some additives such as active carbon [7], zeolite [5,8] and NaClO3 [9] in the filter tips, utilizing their abilities to physisorb and/or chemisorb substances from the mainstream smoke (MS) of cigarette. However, increasingly stringent standard on the quality of cigarette has stimulated a growing effort on the
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.04.044
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improvement of removal efficiency of toxic compounds. Carbon nanotubes (CNTs), a fascinating new material, are attracting more and more attention since their discovery [10]. Their small sizes, large surface areas, hollow and nanosized layered structures, high mechanical strength and remarkable electrical conductivities make them have a wide range of promising applications, such as field emission [11], reinforcing materials in composites [12], nanoprobes [13] and chemical sensors [14]. Studies of CNTs used as absorbents have also been reported [15–19]. Li et al. [16] have found that CNTs show exceptional adsorption capability and high adsorption efficiency for cadmium (II) removal from aqueous solution. It has been known that surface oxidized CNTs can be used as the prime material for adsorption of gas such as H2 [17], O2 [18] and methane [19]. Thus, it is supposed that CNTs can be used as a candidate for removal of ingredients from MS. In this paper we compare the removal efficiencies of nicotine and tar from MS by the filter tips filled with oxidized carbon nanotubes (O-CNTs), activated carbon and zeolite (NaY). Adsorption mechanism of O-CNTs and the effect of O-CNTs mass on the removal efficiency are also investigated.
2. Experimental CNTs were fabricated by catalytic pyrolysis of the propylene (C3H6) at about 750 8C in a ceramic tube with Fe particles as the catalysts. The as-prepared CNTs were dispersed in concentrated nitric acid and refluxed at 140 8C to remove most of the catalyst particles and obtain oxidized carbon nanotubes (O-CNTs). Adsorption and desorption of nitrogen on O-CNTs was measured at 77 K in a volumetric system in the whole relative pressure range. From the isotherms, the BET surface area and the pore size distribution of samples were determined. Pore size distributions were calculated by the Barrett–Joyner–Halenda method [20]. Zeolite NaY with a Si/Al ratio of 2.86 and activated carbon are commercially available powders. The properties of these materials and O-CNTs can be seen from Table 1. The absorbents of O-CNTs, activated carbon and zeolite (NaY) were added in the filter tips of cigarettes respectively. Prior to smoking, all thus
Table 1 Relevant parameters of the adsorbents Adsorbent
Pore size (nm)
SBET (m2/g)
Zeolite (NaY) Activated carbon O-CNTs
0.74 3.0–7.0 3–40
766 904 151
The data of zeolite and activated carbon were given by the supplier as seen in Ref. [5].
treated cigarettes were kept in a chamber of 60 3% relative humidity at 295 K for at least 24 h. They were then machine-smoked under standard conditions [4]. MS first passed through the absorbents, then the smoke condensates were collected on Cambridge filter pads and weighed. The filter pads were extracted with a solvent and analyzed immediately by combined gas chromatography–mass spectrometry (GC–MS) for nicotine and tar under standard conditions ISO10315 [21] and ISO-4387 [22], respectively. The O-CNTs before and after adsorbing MS were characterized by high resolution transmission electron microscope (HRTEM) (Hitachi-H8000, Japan) and Nicolet Nexus 470 FT-IR spectrometer.
3. Results and discussion The reduction results of nicotine and tar by OCNTs, activated carbon and zeolite are shown in Table 2. According to Table 2, the adsorption capacity of O-CNTs for nicotine (up to 0.56 mg/cigarette) and tar (up to 13.0 mg/cigarette) is higher than that of zeolite and even higher than that of activated carbon. Zeolite, whose adsorption capacity is determined by its pore size and surface area, can physisorb some ingredients from MS. For example, zeolite (NaY) with ˚ can remove large amounts of the pore size of 7.4 A ˚ such as some ingredients with molecular size <7.4 A ˚ ˚ ) and naphthalene (7.3 A), acenaphthylene (7.2 A ˚ anthracene (7.3 A), but it can hardly reduce nicotine ˚ ) and some other ingredients of tar such as (7.8 A ˚ ), benzofluoranthene (9.5 A ˚) benzoanthracene (9.3 A ˚ and chrysene (9.3 A) [8]. So when using zeolite as the adsorbent material, the removal efficiency of nicotine is very low and the removal efficiency of tar is also not high. The removal efficiency of nicotine and tar by activated carbon is higher, which results from the fact that activated carbon has more suitable pore size and
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Table 2 Comparison of the removal capability of O-CNTs, activated carbon and zeolite Adsorbent
Mo (mg/cigarette)
Mnicotine (mg/cigarette)
Blank Zeolite (NaY) Activated carbon O-CNTs
0 50.5 50.5 50.5
1.1 1.01 0.72 0.54
hnicotine (%) 8.2 34.5 50.9
ynicotine (g/g)
Mtar (mg/cigarette)
htar (%)
ytar (g/g)
0.0018 0.0075 0.0111
16.0 9.4 6.3 3.0
41.3 60.6 81.3
0.131 0.192 0.257
Mo: the mass of the adsorbent filled in one cigarette; Mnicotine and Mtar: the mass of nicotine and tar in MS after absorbing by the adsorbent; hnicotine and htar: the removal efficiency of nicotine and tar for one cigarette by the adsorbent; hnicotine = (1.1 Mnicotine)/1.10 100%; htar = (16.0 Mtar)/16.0 100%; ynicotine and ytar: the removal mass of nicotine and tar for one cigarette through unit mass of adsorbent; ynicotine = (1.1 Mnicotine)/Mo; ytar = (16.0 Mtar)/Mo.
higher surface area for adsorption of nicotine and tar. What is more, the adsorption capability of activated carbon has been improved by the functional groups introduced by oxidation [23]. Though the surface area of O-CNTs is much lower than that of zeolite and activated carbon as shown in Table 1, O-CNTs has the highest adsorption capability, which results from the special structure and properties of CNTs. Fig. 1 shows the TEM images of O-CNTs before (a) and after (b) adsorbing MS. As is shown in Fig. 1(a), O-CNTs are usually curved with length ranging from hundreds of nanometers to micrometers and no catalyst particles are displayed. Furthermore, it can be seen that heat treatment in the presence of concentrated nitric acid also results in opening caps at the ends of O-CNTs, which is in good agreement with the earlier reports [24,25]. The curved O-CNTs form many aggregated pores with 3–40 nm pore size which are suitable for adsorption all kinds of molecules from MS. On the other hand, MS composed of volatile agents in the vapor phase and of semi- and non-volatile compounds are easily adsorbed or condensed on surfaces of materials due to their low saturated vapor pressure. Therefore, a lot of substances, which are mainly polycyclic aromatic hydrocarbons, are observed at the exterior of O-CNTs after adsorbing MS, as shown in Fig. 1(b). It is interesting that the hollow core of O-CNTs is partially or fully filled with ingredients from MS as shown in Fig. 1(b). Fig. 2 presents the HRTEM image of O-CNTs after adsorbing MS, from which the filling phenomenon can be seen clearly. It is caused by the interaction between MS and CNTs walls leading to phase transitions and capillary condensation, which experimentally verifies the theoretical prediction of T.W. Ebbesen about capillary condensation in CNTs [26]. When MS pass through the open-ended CNTs,
parts of ingredients can enter the inner hole of the open-ended CNTs. Incorporation of atoms or molecules inside nanotubes occurs through either capillary action or via charge transfer effects due to the suitable about 4.5 nm inner diameter (obtained from Fig. 2) of
Fig. 1. TEM images of O-CNTs before (a) and after (b) adsorbing the mainstream smoke.
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Fig. 2. HRTEM image of O-CNTs after adsorbing the mainstream smoke.
O-CNTs and relatively high pressure of ingredients of MS. What is more, the interior surfaces of open-ended CNTs also exhibit a stronger binding energy for adsorbing molecules compared to the planar carbon surfaces [25]. So some ingredients from MS are drawn in the hollow core of CNTs. We also infer that some amounts of nicotine and tar may be adsorbed in the inter-layers of CNTs due to the same reasons mentioned above. Therefore, O-CNTs can be used as a container, and the hollow cores and inter-layers of the open-ended CNTs can adsorb considerable amounts of nicotine and tar. It is the primary reason why the adsorption capacity of O-CNTs for nicotine and tar is higher than that of other materials that have no such structure. In addition, it has been well known that heat treatment in the presence of concentrated nitric acid can oxidize CNTs and introduce many functional groups such as hydroxyl, carboxyl and carbonyl on the surface of CNTs. These functional groups also improve the adsorption capability of CNTs [16]. In order to make out the interaction between O-CNTs and MS, the IR spectroscopy is used to identify the form of chemical binding between them. Fig. 3 shows the IR spectra of the O-CNTs before (a) and after (b) adsorbing MS. Both of the spectra show a peak at about 1590 cm 1, corresponding to the IR active phonon mode of CNTs [27]. The spectrum (a) shows a peak at about 1720 cm 1, corresponding to the stretch mode of carboxylic acid groups [28], which indicates that O-CNTs are introduced carboxylic acid groups at both ends and some sites of the sidewalls. Contrast to the spectrum (a), a noticeable characteristic in the
Fig. 3. FTIR spectra of the O-CNTs before (a) and after (b) adsorbing the mainstream smoke.
spectrum (b) is that the C O stretching mode of the carboxylic acid groups shifts from about 1720 to about 1740 cm 1. The about 20 cm 1 peak shift to higher frequency can be explained by adsorption and binding between surface functional groups of some ingredients of MS and carboxylic acid groups on O-CNTs [29]. It supplies strong evidence for chemical adsorption between O-CNTs and smoke. In order to estimate the optimal amount of O-CNTs filled in the filter tips of cigarettes, O-CNTs mass (Mo) dependence of htar and ytar was investigated. Here we only study htar and ytar in that most of carcinogenic compounds exist in tar. The results are shown in Fig. 4.
Fig. 4. The plot of the removal efficiencies of tar (htar) (A) and the removal mass of tar through unit mass of adsorbent (ytar) (B) as a function of O-CNTs mass (Mo).
Z. Chen et al. / Applied Surface Science 252 (2006) 2933–2937
With the increasing Mo, the value of htar goes up rapidly, but the value of ytar decreases dramatically. Apparently, Mo between 20 and 30 mg makes both htar and ytar at relative high level and is therefore considered as an appropriate additive mass for one cigarette. Even if the introduction of O-CNTs in the cigarette filter would bring about some additional cost, it would be justified by the reduction of most toxic compounds. Therefore, we suppose that O-CNTs may become an extremely potential material to reduce toxic compounds from MS. Additionally, the great reduction of nicotine may affect the flavor when using O-CNTs as adsorbent material, but we can compensate its losing with other materials such as Chinese traditional medicine. The research is under progress. 4. Conclusions In conclusion, the adsorption capability of O-CNTs for nicotine and tar from MS is much higher than that of activated carbon and zeolite (NaY). Nicotine and tar may be adsorbed in the inner hole, at the interior of the tube wall, in the inter-layers and at the exterior of the tube wall of O-CNTs. In addition, capillary condensation of some ingredients of MS in the inner hole of O-CNTs may be the primary reason for their exceptional removal efficiency. When about 20– 30 mg O-CNTs per cigarette is filled in the filter tips of cigarettes, it increases few costs but can reduce most of nicotine and tar. Therefore, it can be predicted that CNTs can become promising materials for the application in cigarette industry. References [1] The World Health Report 1995, World Health Organization, Geneva, 1995, p. 34. [2] Tobacco Smoking, International Agency for Research on Cancer, Monograph 38, World Health Organization, Geneva, 1986. [3] D. HoVmann, I. HoVmann, J. Toxicol. Environ. Health 50 (1997) 307.
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[4] S.E. Atawodi, R. Preussmann, B. Spiegelhalder, Cancer Lett. 97 (1995) 1. [5] Y. Xu, J. Zhu, L. Ma, A. Ji, Y. Wei, X. Shang, Microp. Mesop. Mater. 60 (2003) 125. [6] T.A. Chepiga, M.J. Morton, P.A. Murphy, J.T. Avalos, B.R. Bombick, D.J. Doolittle, M.F. Borgerding, J.E. Swauger, Food Chem. Toxicol. 38 (2000) 949. [7] D.W. bombick, B. Reed Bombick, P.H. Ayres, K. Putnam, J. Avalos, M.F. Borgerding, D.J. Doolittle, Fundam. Appl. Toxicol. 39 (1997) 11. [8] W.M. Meier a, K. Siegmann, Microp. Mesop. Mater. 33 (1999) 307. [9] Yamauchi, Akira Japan, Antioxidant for removal of tar in tobacco smoking [P], JP 11 243,938 (14 September 1999). [10] S. Iijima, Nature 354 (1991) 56. [11] W. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [12] P.M. Ajayan, O. Stephan, C. Colliex, D. Trauth, Science 265 (1994) 1212. [13] H.J. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [14] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622. [15] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [16] Y.H. Li, S. Wang, J. Wei, X. Zhang, C. Xu, Z. Luan, D. Wu, B. Wei, Chem. Phys. Lett. 336 (2001) 205. [17] F.L. Dark, P. Malbrunot, G.P. Tartaglia, Int. J. Hydrogen Energy 27 (2002) 193. [18] X.Y. Zhu, S.M. Lee, Y.H. Lee, T. Frauenheim, Phys. Rev. Lett. 85 (2000) 2757. [19] X. Zhang, W. Wang, Fluid Phase Equilibr. 194 (2002) 289. [20] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [21] Cigarettes-determination of nicotine in smoking condensates gas-chromatographic method. ISO10315-1991. [22] Cigarettes-determination of total and nicotine-free dry particulate matter using a routine analytical smoking machine. ISO4387-1991. [23] A. Dabrowski, Adsorption and its Applications in Industry and Environmental Protection, Elsevier, Amsterdam, 1999. [24] P.M. Ajayan, T.W. Ebbesent, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Naturre (London) 362 (1993) 522. [25] A. Kuznetsova, D.B. Mawhinney, V. Naumenko, J.T. Yates, J. Liu, R.E. Smalley, Chem. Phys. Lett. 321 (2000) 292. [26] T.W. Ebbesen, Phys. Today 6 (1996) 26. [27] M.A. Hamon, H. Hu, P. Bhowmik, S. Niyogi, B. Zhao, M.E. Itkis, R.C. Haddon, Chem. Phys. Lett. 347 (2001) 8. [28] J. Chen, M.A. Hamon, H. Hu, Y. Chen, A.M. Rao, P.C. Eklund, R.C. Haddon, Science 282 (1998) 95. [29] Z. Kang, Y. Wang, E. Wang, S. Lian, L. Gao, W. You, C. Hu, L. Xu, Solid State Commun. 129 (2004) 559.
Adsorption of nicotine and tar from the mainstream smoke of cigarettes by oxidized carbon nanotubes Zhigang Chen a,b, Lisha Zhang a, Yiwen Tang a,*, Zhijie Jia a a
Institute of Nano-science and Technology, Central China Normal University, Wuhan 430079, China b Institute of Advanced Materials, Fudan University, Shanghai 200433, China Received 23 February 2005; received in revised form 30 April 2005; accepted 30 April 2005 Available online 3 June 2005
Abstract The adsorption of nicotine and tar from the mainstream smoke (MS) by the filter tips filled respectively with oxidized carbon nanotubes (O-CNTs), activated carbon and zeolite (NaY) has been investigated. O-CNTs show exceptional removal efficiency and their adsorption mechanism is investigated. Capillary condensation of some ingredients from MS in the inner hole of OCNTs is observed and may be the primary reason for their superior removal efficiency. The effect of O-CNTs mass on the removal efficiencies is also studied and the results show that about 20–30 mg O-CNTs per cigarette can effectively remove most of nicotine and tar. # 2005 Elsevier B.V. All rights reserved. PACS: 68.43. h; 68.35. p Keywords: Carbon nanotubes; Cigarette smoke; Adsorption; Tar and nicotine
1. Introduction The pollution and health hazard caused by smoking have been an urgent problem in the world [1–3]. Tobacco smoke is a very complex mixture containing more than 3800 compounds, which are aerosol composed of volatile agents in the vapor phase and of semi- and non-volatile compounds [4,5]. Besides nicotine, the major inducer of tobacco * Corresponding author. Tel.: +86 27 67861185; fax: +86 27 67861185. E-mail address: [email protected] (Y. Tang).
dependence, cigarette smoke also contains various toxic compounds and notably carcinogenic agents like polycyclic aromatic hydrocarbons, commonly called ‘tar’ [6]. Both of them constitute a serious health risk. In the past years, great efforts have been made in the development of filter tips. It is an effective and convenient way to add some additives such as active carbon [7], zeolite [5,8] and NaClO3 [9] in the filter tips, utilizing their abilities to physisorb and/or chemisorb substances from the mainstream smoke (MS) of cigarette. However, increasingly stringent standard on the quality of cigarette has stimulated a growing effort on the
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.04.044
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improvement of removal efficiency of toxic compounds. Carbon nanotubes (CNTs), a fascinating new material, are attracting more and more attention since their discovery [10]. Their small sizes, large surface areas, hollow and nanosized layered structures, high mechanical strength and remarkable electrical conductivities make them have a wide range of promising applications, such as field emission [11], reinforcing materials in composites [12], nanoprobes [13] and chemical sensors [14]. Studies of CNTs used as absorbents have also been reported [15–19]. Li et al. [16] have found that CNTs show exceptional adsorption capability and high adsorption efficiency for cadmium (II) removal from aqueous solution. It has been known that surface oxidized CNTs can be used as the prime material for adsorption of gas such as H2 [17], O2 [18] and methane [19]. Thus, it is supposed that CNTs can be used as a candidate for removal of ingredients from MS. In this paper we compare the removal efficiencies of nicotine and tar from MS by the filter tips filled with oxidized carbon nanotubes (O-CNTs), activated carbon and zeolite (NaY). Adsorption mechanism of O-CNTs and the effect of O-CNTs mass on the removal efficiency are also investigated.
2. Experimental CNTs were fabricated by catalytic pyrolysis of the propylene (C3H6) at about 750 8C in a ceramic tube with Fe particles as the catalysts. The as-prepared CNTs were dispersed in concentrated nitric acid and refluxed at 140 8C to remove most of the catalyst particles and obtain oxidized carbon nanotubes (O-CNTs). Adsorption and desorption of nitrogen on O-CNTs was measured at 77 K in a volumetric system in the whole relative pressure range. From the isotherms, the BET surface area and the pore size distribution of samples were determined. Pore size distributions were calculated by the Barrett–Joyner–Halenda method [20]. Zeolite NaY with a Si/Al ratio of 2.86 and activated carbon are commercially available powders. The properties of these materials and O-CNTs can be seen from Table 1. The absorbents of O-CNTs, activated carbon and zeolite (NaY) were added in the filter tips of cigarettes respectively. Prior to smoking, all thus
Table 1 Relevant parameters of the adsorbents Adsorbent
Pore size (nm)
SBET (m2/g)
Zeolite (NaY) Activated carbon O-CNTs
0.74 3.0–7.0 3–40
766 904 151
The data of zeolite and activated carbon were given by the supplier as seen in Ref. [5].
treated cigarettes were kept in a chamber of 60 3% relative humidity at 295 K for at least 24 h. They were then machine-smoked under standard conditions [4]. MS first passed through the absorbents, then the smoke condensates were collected on Cambridge filter pads and weighed. The filter pads were extracted with a solvent and analyzed immediately by combined gas chromatography–mass spectrometry (GC–MS) for nicotine and tar under standard conditions ISO10315 [21] and ISO-4387 [22], respectively. The O-CNTs before and after adsorbing MS were characterized by high resolution transmission electron microscope (HRTEM) (Hitachi-H8000, Japan) and Nicolet Nexus 470 FT-IR spectrometer.
3. Results and discussion The reduction results of nicotine and tar by OCNTs, activated carbon and zeolite are shown in Table 2. According to Table 2, the adsorption capacity of O-CNTs for nicotine (up to 0.56 mg/cigarette) and tar (up to 13.0 mg/cigarette) is higher than that of zeolite and even higher than that of activated carbon. Zeolite, whose adsorption capacity is determined by its pore size and surface area, can physisorb some ingredients from MS. For example, zeolite (NaY) with ˚ can remove large amounts of the pore size of 7.4 A ˚ such as some ingredients with molecular size <7.4 A ˚ ˚ ) and naphthalene (7.3 A), acenaphthylene (7.2 A ˚ anthracene (7.3 A), but it can hardly reduce nicotine ˚ ) and some other ingredients of tar such as (7.8 A ˚ ), benzofluoranthene (9.5 A ˚) benzoanthracene (9.3 A ˚ and chrysene (9.3 A) [8]. So when using zeolite as the adsorbent material, the removal efficiency of nicotine is very low and the removal efficiency of tar is also not high. The removal efficiency of nicotine and tar by activated carbon is higher, which results from the fact that activated carbon has more suitable pore size and
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Table 2 Comparison of the removal capability of O-CNTs, activated carbon and zeolite Adsorbent
Mo (mg/cigarette)
Mnicotine (mg/cigarette)
Blank Zeolite (NaY) Activated carbon O-CNTs
0 50.5 50.5 50.5
1.1 1.01 0.72 0.54
hnicotine (%) 8.2 34.5 50.9
ynicotine (g/g)
Mtar (mg/cigarette)
htar (%)
ytar (g/g)
0.0018 0.0075 0.0111
16.0 9.4 6.3 3.0
41.3 60.6 81.3
0.131 0.192 0.257
Mo: the mass of the adsorbent filled in one cigarette; Mnicotine and Mtar: the mass of nicotine and tar in MS after absorbing by the adsorbent; hnicotine and htar: the removal efficiency of nicotine and tar for one cigarette by the adsorbent; hnicotine = (1.1 Mnicotine)/1.10 100%; htar = (16.0 Mtar)/16.0 100%; ynicotine and ytar: the removal mass of nicotine and tar for one cigarette through unit mass of adsorbent; ynicotine = (1.1 Mnicotine)/Mo; ytar = (16.0 Mtar)/Mo.
higher surface area for adsorption of nicotine and tar. What is more, the adsorption capability of activated carbon has been improved by the functional groups introduced by oxidation [23]. Though the surface area of O-CNTs is much lower than that of zeolite and activated carbon as shown in Table 1, O-CNTs has the highest adsorption capability, which results from the special structure and properties of CNTs. Fig. 1 shows the TEM images of O-CNTs before (a) and after (b) adsorbing MS. As is shown in Fig. 1(a), O-CNTs are usually curved with length ranging from hundreds of nanometers to micrometers and no catalyst particles are displayed. Furthermore, it can be seen that heat treatment in the presence of concentrated nitric acid also results in opening caps at the ends of O-CNTs, which is in good agreement with the earlier reports [24,25]. The curved O-CNTs form many aggregated pores with 3–40 nm pore size which are suitable for adsorption all kinds of molecules from MS. On the other hand, MS composed of volatile agents in the vapor phase and of semi- and non-volatile compounds are easily adsorbed or condensed on surfaces of materials due to their low saturated vapor pressure. Therefore, a lot of substances, which are mainly polycyclic aromatic hydrocarbons, are observed at the exterior of O-CNTs after adsorbing MS, as shown in Fig. 1(b). It is interesting that the hollow core of O-CNTs is partially or fully filled with ingredients from MS as shown in Fig. 1(b). Fig. 2 presents the HRTEM image of O-CNTs after adsorbing MS, from which the filling phenomenon can be seen clearly. It is caused by the interaction between MS and CNTs walls leading to phase transitions and capillary condensation, which experimentally verifies the theoretical prediction of T.W. Ebbesen about capillary condensation in CNTs [26]. When MS pass through the open-ended CNTs,
parts of ingredients can enter the inner hole of the open-ended CNTs. Incorporation of atoms or molecules inside nanotubes occurs through either capillary action or via charge transfer effects due to the suitable about 4.5 nm inner diameter (obtained from Fig. 2) of
Fig. 1. TEM images of O-CNTs before (a) and after (b) adsorbing the mainstream smoke.
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Fig. 2. HRTEM image of O-CNTs after adsorbing the mainstream smoke.
O-CNTs and relatively high pressure of ingredients of MS. What is more, the interior surfaces of open-ended CNTs also exhibit a stronger binding energy for adsorbing molecules compared to the planar carbon surfaces [25]. So some ingredients from MS are drawn in the hollow core of CNTs. We also infer that some amounts of nicotine and tar may be adsorbed in the inter-layers of CNTs due to the same reasons mentioned above. Therefore, O-CNTs can be used as a container, and the hollow cores and inter-layers of the open-ended CNTs can adsorb considerable amounts of nicotine and tar. It is the primary reason why the adsorption capacity of O-CNTs for nicotine and tar is higher than that of other materials that have no such structure. In addition, it has been well known that heat treatment in the presence of concentrated nitric acid can oxidize CNTs and introduce many functional groups such as hydroxyl, carboxyl and carbonyl on the surface of CNTs. These functional groups also improve the adsorption capability of CNTs [16]. In order to make out the interaction between O-CNTs and MS, the IR spectroscopy is used to identify the form of chemical binding between them. Fig. 3 shows the IR spectra of the O-CNTs before (a) and after (b) adsorbing MS. Both of the spectra show a peak at about 1590 cm 1, corresponding to the IR active phonon mode of CNTs [27]. The spectrum (a) shows a peak at about 1720 cm 1, corresponding to the stretch mode of carboxylic acid groups [28], which indicates that O-CNTs are introduced carboxylic acid groups at both ends and some sites of the sidewalls. Contrast to the spectrum (a), a noticeable characteristic in the
Fig. 3. FTIR spectra of the O-CNTs before (a) and after (b) adsorbing the mainstream smoke.
spectrum (b) is that the C O stretching mode of the carboxylic acid groups shifts from about 1720 to about 1740 cm 1. The about 20 cm 1 peak shift to higher frequency can be explained by adsorption and binding between surface functional groups of some ingredients of MS and carboxylic acid groups on O-CNTs [29]. It supplies strong evidence for chemical adsorption between O-CNTs and smoke. In order to estimate the optimal amount of O-CNTs filled in the filter tips of cigarettes, O-CNTs mass (Mo) dependence of htar and ytar was investigated. Here we only study htar and ytar in that most of carcinogenic compounds exist in tar. The results are shown in Fig. 4.
Fig. 4. The plot of the removal efficiencies of tar (htar) (A) and the removal mass of tar through unit mass of adsorbent (ytar) (B) as a function of O-CNTs mass (Mo).
Z. Chen et al. / Applied Surface Science 252 (2006) 2933–2937
With the increasing Mo, the value of htar goes up rapidly, but the value of ytar decreases dramatically. Apparently, Mo between 20 and 30 mg makes both htar and ytar at relative high level and is therefore considered as an appropriate additive mass for one cigarette. Even if the introduction of O-CNTs in the cigarette filter would bring about some additional cost, it would be justified by the reduction of most toxic compounds. Therefore, we suppose that O-CNTs may become an extremely potential material to reduce toxic compounds from MS. Additionally, the great reduction of nicotine may affect the flavor when using O-CNTs as adsorbent material, but we can compensate its losing with other materials such as Chinese traditional medicine. The research is under progress. 4. Conclusions In conclusion, the adsorption capability of O-CNTs for nicotine and tar from MS is much higher than that of activated carbon and zeolite (NaY). Nicotine and tar may be adsorbed in the inner hole, at the interior of the tube wall, in the inter-layers and at the exterior of the tube wall of O-CNTs. In addition, capillary condensation of some ingredients of MS in the inner hole of O-CNTs may be the primary reason for their exceptional removal efficiency. When about 20– 30 mg O-CNTs per cigarette is filled in the filter tips of cigarettes, it increases few costs but can reduce most of nicotine and tar. Therefore, it can be predicted that CNTs can become promising materials for the application in cigarette industry. References [1] The World Health Report 1995, World Health Organization, Geneva, 1995, p. 34. [2] Tobacco Smoking, International Agency for Research on Cancer, Monograph 38, World Health Organization, Geneva, 1986. [3] D. HoVmann, I. HoVmann, J. Toxicol. Environ. Health 50 (1997) 307.
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