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Carbon adsorbents from waste ion-exchange resins
Carbon 43 (2005) 1143–1150 www.elsevier.com/locate/carbon
Carbon adsorbents from waste ion-exchange resins V.M. Gunko a, R. Leboda
b,*
, J. Skubiszewska-Zie˛ba b, B. Charmas b, P. Oleszczuk
c
a
b
Institute of Surface Chemistry, 17 General Naumov street, 03164 Kiev, Ukraine Department of Chemical Physics, Faculty of Chemistry, Maria Curie-Skodowska University, M.C. Skodowska Sq. 3, 20-031 Lublin, Poland c Institute of Soil Science and Environmental Management, Agricultural University of Lublin, 20-069 Lublin, Poland Received 9 July 2002; accepted 24 September 2004
Abstract A series of activated carbons was prepared from different waste commercial ion-exchange resins and studied by means of adsorption, SEM and IR methods. Samples were additionally washed or washed/frozen. This resulted in increases in micro- and mesoporosity in comparison with initial activated carbons. For some samples, the latter treatment gives enhancement of mesoporosity but reduction of microporosity and vice versa comparing with only washed carbons due to different localization of water droplets in mesopores or micropores. Changes in the morphology of chars and activated samples depended on resin composition and history. Relatively high values of porosity (Vp 0.4 cm3/g) and specific surface area (SBET 600 m2/g) show that activated carbons prepared from waste ion-exchange resins can be utilized for different purposes, especially after additional treatment (such as washing, impregnation by certain compounds and subsequent thermal activation). 2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Resins; Adsorption; Microporosity; Microstructure
1. Introduction Carbon adsorbents are typically produced by carbonization and subsequent activation of biogenic-origin materials [1–4], synthetic polymers [5–16] and other precursors [17–20] under different conditions. Ion-exchange resins (IER) which can be utilized as carbon adsorbent precursors [5–11] become waste in 3–5 years of their exploitation and are practically undergradable in the environment. Therefore their utilization is of importance from the practical point of view. There are advantages of producing carbons from similar resins, such as controlled and developed micro- and/or mesoporosity. Also spherical shape of granules makes it possible to prepare adsorbent particles of appropriate hydrodynamic properties. How*
Corresponding author. Tel.: +48 81 537 5656; fax: +48 81 533 3348. E-mail address: [email protected] (R. Leboda).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.09.032
ever, there are problems as well: IER pyrolysis results in low char yields and/or destruction of spherical grains to a mechanically weak, shapeless carbon mass, which, of course, affects the choice of carbonization and activation conditions. In general, conditions determining the pyrolysis pathways are the chemical structure of precursors, their microand macrostructure, kind and amount of functional groups, as well as reaction temperature and atmosphere, and activation conditions [1–4]. The presence of such metals as iron, calcium and others and adsorbed mineral or organic substances in waste IERs should also be taken into account [5–11]. Moreover, the waste may contain mixtures of ion exchangers of different physicochemical properties, which make the problem of adsorbent preparation more difficult. This paper presents results of synthesis and studies of carbon adsorbents produced from several waste ion-exchange resins.
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V.M. Gunko et al. / Carbon 43 (2005) 1143–1150
2. Experimental 2.1. Materials Waste ion-exchange resins IR-120, IRA-68, IRA-402, IRC-84 (produced by Rohm Haas, USA), and Duolite C20 (Dia-Prosim, France) (Table 1) used in production of gelatin (for its deionization) were carbonized, activated and treated. For comparison, commercial ion-exchange resin Zerolit 225 · 20 characterized by significant cross-linking was also carbonized under the same conditions. Carbonization was carried out in a temperature-controlled flow quartz reactor (20–800 C at 9 C/min in a deoxygenated argon atmosphere). The chars were kept at 800 C for 30 min. It should be noted that 700–800 C is optimal carbonization temperature to produce adsorbents (from similar IER precursors) with maximum developed porosity [5–11]. Char activation was performed in a flow quartz reactor with water vapor as an activator. First nitrogen was passed through the bed (150 cm3/min). At 800 C, the nitrogen flow was stopped and water vapor was introduced at 0.6 cm3/min using a peristaltic pump (Cole Palmer) and a vaporizer heated to 300 C. Activation was carried out for 30 min and the reaction system was cooled in nitrogen atmosphere to room temperature. Activated carbons were also treated (by washing or washing/freezing by liquid nitrogen) to remove some of the ash and decomposed fragments. 2.2. Apparatus and methods Nitrogen adsorption–desorption isotherms (Fig. 1) were recorded at 77.4 K using a Micromeritics ASAP 2405N adsorption analyzer. The specific surface area SBET, pore volume Vp (estimated at p/p0 0.98, where p and p0 refer to the equilibrium and saturation pressures, respectively), and other parameters listed in Table 2 were determined using the nitrogen adsorption– desorption data [21]. For all the chars, a maximal sorption capacity for methanol (Table 3) was estimated using the titration method [22]. Methanol was used as a probe compound because of the small size of its molecule and
Fig. 1. Nitrogen adsorption–desorption isotherms of activated carbons, washed–dried (W), and washed–frozen-dried (F): (a) IER2, IER4, and IER7; and (b) IER3, IER5, and IER6.
its good interaction with both hydrophilic and hydrophobic surfaces. The pore size distributions (PSD) were calculated as differential functions fV (x) using the overall isotherm equation based on the combination of the modified Kelvin equation and the statistical adsorbed film thickness [23] applied to a model of slit-like pores. The
Table 1 Characteristics of ion-exchange resins No
Resin
Type
IER1 IER2 IER3 IER4 IER5 IER6 IER7
IR-120 IRA-402 ZA Duolit C-20 IRA-68 IRA-68 Zerolit 225 · 20 with no waste
Gel resin with ST + 8% DVB (–SO 3) Gel resin with ST + 8% DVB (–NðCH3 Þþ 3) Ion-exchange resin from Puławy ST + DVB (–SO 3) Gel resin with acryl DVB (–N(R)2) Gel resin with acryl DVB (–N(R)2) ST + DVB (–SO 3)
Note: ST is styrene, and DVB is divinyl benzene; some active groups are shown.
V.M. Gunko et al. / Carbon 43 (2005) 1143–1150
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Table 2 Textural parameters of differently treated carbon samples Sample
SBET (m2/g)
Vp (cm3)
Rp (nm)
Smic (m2/g)
Smes (m2/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vmac (cm3/g)
Dwslit
DAJ
IER2 IER2w IER2f IER3 IER3w IER3f IER4 IER4w IER4f IER5 IER5w IER5f IER6 IER6w IER6f IER7 IER7w IER7f
526 535 554 131 345 320 492 566 576 270 260 286 89 149 156 624 641 608
0.248 0.249 0.260 0.107 0.344 0.311 0.326 0.383 0.407 0.152 0.142 0.159 0.137 0.159 0.171 0.319 0.330 0.318
0.38 0.38 0.38 0.90 1.00 0.95 0.53 0.53 0.54 0.45 0.43 0.43 1.55 1.13 1.10 0.40 0.53 0.53
507 516 532 92 153 146 407 458 456 246 235 260 24 71 70 535 544 510
19 19 22 39 192 174 85 108 120 24 25 26 65 78 86 89 97 98
0.224 0.220 0.235 0.041 0.077 0.071 0.179 0.201 0.202 0.111 0.107 0.117 0.014 0.035 0.036 0.235 0.242 0.233
0.022 0.020 0.025 0.058 0.259 0.238 0.144 0.179 0.202 0.041 0.036 0.042 0.124 0.122 0.135 0.084 0.089 0.087
0.002 0.009 0 0.007 0.008 0.003 0.003 0.003 0.003 0.009 0.001 0.003 0.004 0.017 0.004 0.010 0 0.011
0.173 0.202 0.169 0.301 0.474 0.467 0.307 0.246 0.297 0.147 0.219 0.154 0.443 0.496 0.504 0.379 0.434 0.432
2.960 2.961 2.957 2.762 2.676 2.680 2.859 2.840 2.836 2.913 2.916 2.914 2.600 2.707 2.691 2.886 2.880 2.868
Note. wWashed and ffrozen wetted samples. Specific surface area of micro- (Smic) and mesopores (Smes) and pore volume of micro- (Vmic), meso(Vmes) and macropores (Vmac) corresponding to pore half-width x < 1 nm, 1 < x < 25 nm, and x > 25 nm respectively were calculated by integration of the corresponding differential distribution functions. DAJ is the fractal dimension [28]. Table 3 Characteristics of chars and activated carbons Bulk density (g/cm3)
Sample
IER1 IER2 IER3 IER4 IER5 IER6 IER7
Methanol sorption capacity (cm3/g)
Before activation
After activation
Before activation
After activation
0.33 0.52 1.07 0.86 0.52 0.33 0.75
– 0.45 0.74 0.58 0.45 0.25 0.61
0.14 0.03 0.03 0.03 0.05 0.05 0.16
– 0.27 0.27 0.33 0.19 0.29 0.33
desorption data (as the overall isotherms) were used to compute the fV (x) distributions with a modified regularization/singular value decomposition procedure CONTIN [24] under non-negativity constrains for fV (x) (i.e., fV(x) P 0 at any x) at a fixed regularization parameter a = 0.01 [25]. The fV(x) distributions determined linked to pore volume can be transformed to the distributions fS(x) with respect to specific surface area using the corresponding (e.g., slit-like) model of pores [26,27] w Vp fV ðxÞ fS ðxÞ ¼ ; ð1Þ x x
Burn-off (wt.%)
Decrease in bulk density (%)
– 14.5 31.0 36.0 16.7 29.0 25.0
– 13.5 31.0 32.5 13.5 24.2 18.5
was used at xmin = 0.2 nm and xmax = 100 nm. The surface areas of micro-, meso- and macropores were calculated as follows S mic ¼
Z
1
fS ðxÞdx;
ð3Þ
0:2
S mes ¼
Z
25
fS ðxÞdx;
ð4Þ
1
S mac ¼
Z
100
fS ðxÞdx:
ð5Þ
25
where w = 1, 2, and 3 for slit-like, cylindrical, and spherical pores, respectively. For estimation of deviation of the pore shape from slit-shaped one Dwslit = SBET/ Ssum 1 (Table 2) where S sum ¼
Z
xmax
xmin
fS ðxÞdx ¼
Z
rmax rmin
1 Vp fV ðxÞ dx; x x
ð2Þ
For pictorial presentation the pore size distribution functions were calculated as incremental PSDs (IPSDs) fz;n ðxi Þ ¼ 0:5ðfz ðxi Þ þ fz ðxi1 ÞÞðxi xi1 Þ;
ð6Þ
where subscript z = V or S [26,27]. Calculation of the fractal dimension (DAJ) of the adsorbents was performed on the basis of the adsorption
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data (p/p0 < 0.85) using the Frenkel–Halsey–Hill equation [28]. SEM images of the chars were obtained using a TESLA BS 301 scanning microscope. The diffuse reflectance infrared (IR) spectra were recorded by means of a FTIR Perkin Elmer 1725X spectrometer using samples stirred with KBr.
3. Results and discussion SEM images show (Fig. 2) that carbonization of IERs and subsequent activation results in partial (major or minor) destruction of granules. Some granules keep their shape (Fig. 2c and e) but others strongly change the morphology (Fig. 2a, b, and d) up to full decomposition on activation (IER1) (Table 1). These results depend on the IER origin (Table 1) and history, since the preparation technique was the same for all the samples. Thus, the morphology of chars (as well as activated carbons) prepared from different IERs varies from spherical granules (characterized by different levels of deformation) to flakes or particles of an irregular shape, which is of importance for subsequent application of these materials. As follows from Table 1, the initial ion-exchange resins differ in their chemical composition and skeleton structure; for example, gel resins with styrene (ST) and divinyl benzene (DVB) used in different proportions (IR-120, IRA-402, IRA-68) or acrylic ICR-84. The initial IERs differ in the density due to variations in polymer packing in grains of ion exchangers. The grain sizes affect the apparent density of chars obtained from these IERs, and the greater the apparent density of the initial resin, the greater the bulk density of the char (Table 3). Polymer shrinking decreases with increasing content of divinyl benzene but char yield increases (Table 3). In the case of active carbons, the mechanical strength of the grains increases as apparent density increases and pore volume decreases [4]. A high apparent density corresponds to favorable conditions for further activation of such materials to 50% burn-off or more with respect to the initial IERs (which is important from both economical and technological points of view). This refers mainly to resins preserving a spherical shape with the exception of IRA-402 or ICR-84 depolymerized under applied thermal conditions. To obtain spherical carbon adsorbents from similar resins, a more complex pyrolysis procedure should be applied [5–11,29,30]. Additional treatment of IER7 by oxidizing with 30% H2O2 solution (before thermal activation), impregnating with Ca(CH3COO)2 Æ H2O at a Ca(II) concentration of 1 wt.% and subsequently activating it enhances mesopore contribution up to 2.5 cm3/g at SBET = 1200 m2/g [29]; application of KOH to activate carbons prepared from IERs gives similar results [30].
Fig. 2. SEM photographs of (a and c) chars and (b, d, e) activated carbons of (a and b) IER6 (·250 and ·500), (c and d) IER4 (·100 and ·50), and (e) IER7 (·400).
The chars possess a small pore volume (Table 3) and a low specific surface area (not shown here) with the exception of IR-120 having SBET = 35 m2/g and Vp = 0.062 cm3/g at pore half-width x = 3.6 nm. Their nitrogen adsorption isotherms had a shape typical for mesoporous adsorbents. As follows from Fig. 1 and the data in Table 2, activation with water vapor causes significant development of porous structure (as IER1 was destroyed upon activation, its textural characteristics are not given). The shapes of the adsorption isotherms, hysteresis loops (Fig. 1) and the pore size distributions (Figs. 3 and 4) re-
V.M. Gunko et al. / Carbon 43 (2005) 1143–1150
veal a complex texture. The type of isotherms does not strongly change due to washing or washing/freezing of activated carbons; however, the adsorption values increase significantly for IER3 (Fig. 4b) and IER4 (Fig. 4c, Table 2). However, the porosity type does not change on these additional treatments and only some portion of pores is emptied due to washing (compare corresponding PSDs in Fig. 4). Washing/freezing has slightly greater effect on porosity enlargement (as ice has a larger volume than water and these volume changes on freezing remain in the textural ‘‘memory’’ of carbons) [31] for certain samples (Fig. 4c and e). Washing leads to emptying of micropores (Fig. 4e) and narrow mesopores (Fig. 4c) or pores over a broader range (Fig. 4b). However, for certain samples, e.g., IER2 (Fig. 4a), IER5 (Fig. 4d) and IER7 (Fig. 4f) especially for IER2 (see its textural parameters in Table 2), this effect is small. It should be noted that IER2 has the lowest value of burn-off (Table 3). Changes in burn-off correspond to porosity enlargement tendency after washing (Table 2, Fig. 4). These changes in micro- and mesopore volume can differ due to localization of water droplets mainly in mesopores of relatively hydrophobic carbon
Fig. 3. Incremental pore size distributions of activated carbons with respect to (a) pore volume and (b) specific surface area.
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[32–36], whose enlargement on freezing results in elimination of micropores (differently filled by water in comparison with mesopores) (Table 2). As a consequence of different water distribution in micro- or mesopores of activated carbons, pre-adsorbed water can differently affect adsorption of organic molecules under static or dynamic conditions [32–36]. Deviation of pore shape from slit-like (Table 2, Dwslit) is from 15% (IER5 with the main contribution of narrow pores, Fig. 4d) to 50% (IER6 with a broad PSD, Fig. 4e). However, IER7 possessing narrow distribution of pores demonstrates a large deviation Dwslit. The shape of its adsorption–desorption isotherm and open hysteresis loop (Fig. 1) suggests that this carbon has pores of complex shape and connectivity. Notice that IER2, IER4 and IER5 have open hysteresis loops too due to an insufficient connectivity of the pore network, which inhibits desorption. However, their isotherms have different shapes since IER2 is microporous, while IER4 and IER5 are micro/mesoporous. Consequently, insufficient development of pore network connectivity may be characteristic for certain micro- and micro/mesoporous carbons studied here. It follows from the data in Table 2 that, under the same activation conditions, the susceptibility of chars to burn-off changes from 15% to 36%, which is accompanied by changes in apparent density. The adsorbent prepared from pure Zerolit 225 · 20 (IER7) possesses the largest SBET value and its char possesses a large apparent density, which persists even after activation. The IR spectra (Fig. 5) indicate that the carbons possess different surface structures, e.g., aliphatic, aromatic, cyclic [37], as one can observe the bands at 1460 cm1 and over the 1320–1100 cm1 range (e.g., 1145 cm1 for CH3–CH–CH3, 1220 cm1 for (CH3–CH–CH2), and 1255 cm1 for (CH3)3C–). All the analyzed spectra possess a low peak at 2840 cm1 showing the presence of CH2 groups. The band at 1640 cm1 corresponds to alkene fragments or planar and non-planar deformation vibrations of OH groups. At the same time, the peaks at 1145, 1220 and 1255 cm1 suggest the existence of C–O and C–O–C bonds (ethers, esters, and lactones). The peaks at 1680 cm1 come from C@C, N–C@C, C@N, or C@O groups and coupled polyenes. A low-intensity band at 3600 cm1 can be connected to –OH groups and adsorbed water, which can take part in hydrogen bonding. The bands at 1300–1020 cm1 could be connected with C–O–C and C–O groups (ethers, acids, lactones). In the case of aliphatic esters, there is a strong band at 1150 cm1 caused by asymmetric valence vibrations of C–O–C. The bands at 1740 cm1 can originate from valence vibrations of C@O and C–O in esters and lactones. There are also bands over the 700– 600 cm1 range related to valence vibrations of C–S bonds but they are slightly formed.
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Fig. 4. Pore size distributions of activated carbons (initial), washed–dried and washed–frozen-dried: (a) IER2, (b) IER3, (c) IER4, (d) IER5, (e) IER6, and (f) IER7 with respect to pore volume.
4. Conclusions The surfaces of activated carbons prepared from ionexchange resins are complex both with respect to their texture (characterized by developed micro- and mesoporosity and a high value of the fractal dimension, up to
2.96) and chemistry. The surfaces of these carbon adsorbents possess a polyfunctional character, as both specific (e.g., oxygen-containing groups such as phenolic, carboxylic and lactonic) and non-specific adsorption sites (e.g., on basal graphite planes) are present. These adsorbents show complex porous structures over micro- and
V.M. Gunko et al. / Carbon 43 (2005) 1143–1150
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Fig. 5. IR spectra of IER chars.
mesopore ranges, which change on washing or washing/ freezing of wetted carbons. The values of specific surface area and pore volume suggest that the synthesized activated carbons can be utilized for different purposes. Additional specific treatments can allow one to significantly enhance their porosity and specific surface area.
Acknowledgments This research was supported by NATO (grant no. EST.CLG.976890). R.L. is grateful to the Foundation for Polish Science for financial support. The authors thank Prof. L.R. Radovic for useful remarks and comments.
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[4] Jankowska H, S´wia˛tkowski A, Choma J. Active carbon. New York: Ellis Horwood; 1991. [5] Horikawa T, Hayashi J, Muroyama K. Carbon 2002;40:709–14. [6] Hayashi J, Matsuzaki K, Hiraumi T, Muroyama K. Carbon 1996;34:273–4. [7] Nagawa H, Watanabe K, Harada Y, Miura K. Carbon 1999;37:1455–61. [8] Teng H, Wang S. Carbon 2000;38:817–24. [9] Miura K, Nakagawa H, Yukiyama N. In: International symposium on carbon. Tokyo; 1998. p. 66–7. [10] Miura K, Nakagawa H, Watanable K. Tanso 1999;186:25–9. [11] Lenghaus K, Qiao GGH, Solomon DH, Gomez C, RodriguezReinoso F, Sepulveda-Escribano A. Carbon 2002;40:743–9. [12] Li Z, Jaroniec M. Carbon 2001;39:2080–2. [13] Kocirik M, Brych J, Hradil J. Carbon 2001;39:1919–28. [14] Zippi EM, Kabalka GW. Carbon 1996;34:1539–42. [15] Neffe A, S´wia˛tkowski A, Zie˛tek S, Michalovskij SV, Puzij AM. Przem Chem 1991;70(2):75–7. [16] Puziy AM, Poddubnaya OI. Carbon 1998;36:45–50. [17] Putyera K, Jagiełło J, Bandosz T, Schwarz JA. Carbon 1995;33:1047–52. [18] Bo´ta A, Laszlo´ K, Subklew G, Schlimpfer H, Schwuger MJ. Adsorption 1996;3:81–92. [19] Laszlo´ K, Bota A, Nagy LG. Carbon 2000;38:1965–76. [20] Gierak A, Leboda R, Hubicki Z. Material. I Kongresu Technologii Chemicznej, Szczecin, Politechnika Szczecin´skaya, September 19–22, 1995. p. 807–10 (in Polish). [21] Gregg SJ, Sing KSW. Adsorption, surface area and porosity. London: Academic Press; 1982.
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McDaniel MP, Hottovy TD. J Colloid Interface Sci 1980;78:31–6. Nguyen C, Do DD. Langmuir 1999;15:3608–15. Provencher SW. Comput Phys Commun 1982;27:213–27. Gunko VM, Do DD. Colloids Surf A 2001;193:71–83. Gunko VM, Mikhalovsky SV. Carbon 2004;42:843–9. Gunko VM, Turov VV, Skubiszewska-Zie˛ba J, Charmas B, Leboda R. Adsorption 2004;10:5–18. Avnir D, Jaroniec M. Langmuir 1989;5:1431–4. Leboda R, Skubiszewska-Zie˛ba J, Tomaszewski W, Gunko VM. J Colloid Interface Sci 2003;263:533–41. Park SJ, Jung WY. J Colloid Interface Sci 2002;250: 196–200. Leboda R, Skubiszewska-Zie˛ba J, Gunko VM, Turov VV, Palijczuk D, Zie˛tek S. Polish patent P 354146 (2002).
[32] Leboda R, Turov VV, Tomaszewski W, Gunko VM, Skubiszewska-Zie˛ba J. Carbon 2002;40:389–96. [33] Palijczuk V, Gunko VM, Leboda R, Skubiszewska-Zie˛ba J, Zie˛tek S. J Colloid Interface Sci 2002;250:5–17. [34] Gunko VM, Turov VV, Skubiszewska-Zie˛ba J, Leboda R, Tsapko MD, Palijczuk D. Appl Surf Sci 2003;214:178–89. [35] Tomaszewski W, Gunko VM, Leboda R, Skubiszewska-Zie˛ba J. J Colloid Interface Sci 2003;266:388–402. [36] Turov VV, Gunko VM, Leboda R, Bandosz TJ, SkubiszewskaZie˛ba J, Palijczuk V, et al. J Colloid Interface Sci 2002;253:23– 34. [37] Zawadski J. Infrared spectroscopy in surface chemistry of carbons. In: Thrower PA, editor. Chemistry and physics of carbon, vol. 21. New York: Marcel Dekker; 1988.
Carbon adsorbents from waste ion-exchange resins V.M. Gunko a, R. Leboda
b,*
, J. Skubiszewska-Zie˛ba b, B. Charmas b, P. Oleszczuk
c
a
b
Institute of Surface Chemistry, 17 General Naumov street, 03164 Kiev, Ukraine Department of Chemical Physics, Faculty of Chemistry, Maria Curie-Skodowska University, M.C. Skodowska Sq. 3, 20-031 Lublin, Poland c Institute of Soil Science and Environmental Management, Agricultural University of Lublin, 20-069 Lublin, Poland Received 9 July 2002; accepted 24 September 2004
Abstract A series of activated carbons was prepared from different waste commercial ion-exchange resins and studied by means of adsorption, SEM and IR methods. Samples were additionally washed or washed/frozen. This resulted in increases in micro- and mesoporosity in comparison with initial activated carbons. For some samples, the latter treatment gives enhancement of mesoporosity but reduction of microporosity and vice versa comparing with only washed carbons due to different localization of water droplets in mesopores or micropores. Changes in the morphology of chars and activated samples depended on resin composition and history. Relatively high values of porosity (Vp 0.4 cm3/g) and specific surface area (SBET 600 m2/g) show that activated carbons prepared from waste ion-exchange resins can be utilized for different purposes, especially after additional treatment (such as washing, impregnation by certain compounds and subsequent thermal activation). 2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Resins; Adsorption; Microporosity; Microstructure
1. Introduction Carbon adsorbents are typically produced by carbonization and subsequent activation of biogenic-origin materials [1–4], synthetic polymers [5–16] and other precursors [17–20] under different conditions. Ion-exchange resins (IER) which can be utilized as carbon adsorbent precursors [5–11] become waste in 3–5 years of their exploitation and are practically undergradable in the environment. Therefore their utilization is of importance from the practical point of view. There are advantages of producing carbons from similar resins, such as controlled and developed micro- and/or mesoporosity. Also spherical shape of granules makes it possible to prepare adsorbent particles of appropriate hydrodynamic properties. How*
Corresponding author. Tel.: +48 81 537 5656; fax: +48 81 533 3348. E-mail address: [email protected] (R. Leboda).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.09.032
ever, there are problems as well: IER pyrolysis results in low char yields and/or destruction of spherical grains to a mechanically weak, shapeless carbon mass, which, of course, affects the choice of carbonization and activation conditions. In general, conditions determining the pyrolysis pathways are the chemical structure of precursors, their microand macrostructure, kind and amount of functional groups, as well as reaction temperature and atmosphere, and activation conditions [1–4]. The presence of such metals as iron, calcium and others and adsorbed mineral or organic substances in waste IERs should also be taken into account [5–11]. Moreover, the waste may contain mixtures of ion exchangers of different physicochemical properties, which make the problem of adsorbent preparation more difficult. This paper presents results of synthesis and studies of carbon adsorbents produced from several waste ion-exchange resins.
1144
V.M. Gunko et al. / Carbon 43 (2005) 1143–1150
2. Experimental 2.1. Materials Waste ion-exchange resins IR-120, IRA-68, IRA-402, IRC-84 (produced by Rohm Haas, USA), and Duolite C20 (Dia-Prosim, France) (Table 1) used in production of gelatin (for its deionization) were carbonized, activated and treated. For comparison, commercial ion-exchange resin Zerolit 225 · 20 characterized by significant cross-linking was also carbonized under the same conditions. Carbonization was carried out in a temperature-controlled flow quartz reactor (20–800 C at 9 C/min in a deoxygenated argon atmosphere). The chars were kept at 800 C for 30 min. It should be noted that 700–800 C is optimal carbonization temperature to produce adsorbents (from similar IER precursors) with maximum developed porosity [5–11]. Char activation was performed in a flow quartz reactor with water vapor as an activator. First nitrogen was passed through the bed (150 cm3/min). At 800 C, the nitrogen flow was stopped and water vapor was introduced at 0.6 cm3/min using a peristaltic pump (Cole Palmer) and a vaporizer heated to 300 C. Activation was carried out for 30 min and the reaction system was cooled in nitrogen atmosphere to room temperature. Activated carbons were also treated (by washing or washing/freezing by liquid nitrogen) to remove some of the ash and decomposed fragments. 2.2. Apparatus and methods Nitrogen adsorption–desorption isotherms (Fig. 1) were recorded at 77.4 K using a Micromeritics ASAP 2405N adsorption analyzer. The specific surface area SBET, pore volume Vp (estimated at p/p0 0.98, where p and p0 refer to the equilibrium and saturation pressures, respectively), and other parameters listed in Table 2 were determined using the nitrogen adsorption– desorption data [21]. For all the chars, a maximal sorption capacity for methanol (Table 3) was estimated using the titration method [22]. Methanol was used as a probe compound because of the small size of its molecule and
Fig. 1. Nitrogen adsorption–desorption isotherms of activated carbons, washed–dried (W), and washed–frozen-dried (F): (a) IER2, IER4, and IER7; and (b) IER3, IER5, and IER6.
its good interaction with both hydrophilic and hydrophobic surfaces. The pore size distributions (PSD) were calculated as differential functions fV (x) using the overall isotherm equation based on the combination of the modified Kelvin equation and the statistical adsorbed film thickness [23] applied to a model of slit-like pores. The
Table 1 Characteristics of ion-exchange resins No
Resin
Type
IER1 IER2 IER3 IER4 IER5 IER6 IER7
IR-120 IRA-402 ZA Duolit C-20 IRA-68 IRA-68 Zerolit 225 · 20 with no waste
Gel resin with ST + 8% DVB (–SO 3) Gel resin with ST + 8% DVB (–NðCH3 Þþ 3) Ion-exchange resin from Puławy ST + DVB (–SO 3) Gel resin with acryl DVB (–N(R)2) Gel resin with acryl DVB (–N(R)2) ST + DVB (–SO 3)
Note: ST is styrene, and DVB is divinyl benzene; some active groups are shown.
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Table 2 Textural parameters of differently treated carbon samples Sample
SBET (m2/g)
Vp (cm3)
Rp (nm)
Smic (m2/g)
Smes (m2/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vmac (cm3/g)
Dwslit
DAJ
IER2 IER2w IER2f IER3 IER3w IER3f IER4 IER4w IER4f IER5 IER5w IER5f IER6 IER6w IER6f IER7 IER7w IER7f
526 535 554 131 345 320 492 566 576 270 260 286 89 149 156 624 641 608
0.248 0.249 0.260 0.107 0.344 0.311 0.326 0.383 0.407 0.152 0.142 0.159 0.137 0.159 0.171 0.319 0.330 0.318
0.38 0.38 0.38 0.90 1.00 0.95 0.53 0.53 0.54 0.45 0.43 0.43 1.55 1.13 1.10 0.40 0.53 0.53
507 516 532 92 153 146 407 458 456 246 235 260 24 71 70 535 544 510
19 19 22 39 192 174 85 108 120 24 25 26 65 78 86 89 97 98
0.224 0.220 0.235 0.041 0.077 0.071 0.179 0.201 0.202 0.111 0.107 0.117 0.014 0.035 0.036 0.235 0.242 0.233
0.022 0.020 0.025 0.058 0.259 0.238 0.144 0.179 0.202 0.041 0.036 0.042 0.124 0.122 0.135 0.084 0.089 0.087
0.002 0.009 0 0.007 0.008 0.003 0.003 0.003 0.003 0.009 0.001 0.003 0.004 0.017 0.004 0.010 0 0.011
0.173 0.202 0.169 0.301 0.474 0.467 0.307 0.246 0.297 0.147 0.219 0.154 0.443 0.496 0.504 0.379 0.434 0.432
2.960 2.961 2.957 2.762 2.676 2.680 2.859 2.840 2.836 2.913 2.916 2.914 2.600 2.707 2.691 2.886 2.880 2.868
Note. wWashed and ffrozen wetted samples. Specific surface area of micro- (Smic) and mesopores (Smes) and pore volume of micro- (Vmic), meso(Vmes) and macropores (Vmac) corresponding to pore half-width x < 1 nm, 1 < x < 25 nm, and x > 25 nm respectively were calculated by integration of the corresponding differential distribution functions. DAJ is the fractal dimension [28]. Table 3 Characteristics of chars and activated carbons Bulk density (g/cm3)
Sample
IER1 IER2 IER3 IER4 IER5 IER6 IER7
Methanol sorption capacity (cm3/g)
Before activation
After activation
Before activation
After activation
0.33 0.52 1.07 0.86 0.52 0.33 0.75
– 0.45 0.74 0.58 0.45 0.25 0.61
0.14 0.03 0.03 0.03 0.05 0.05 0.16
– 0.27 0.27 0.33 0.19 0.29 0.33
desorption data (as the overall isotherms) were used to compute the fV (x) distributions with a modified regularization/singular value decomposition procedure CONTIN [24] under non-negativity constrains for fV (x) (i.e., fV(x) P 0 at any x) at a fixed regularization parameter a = 0.01 [25]. The fV(x) distributions determined linked to pore volume can be transformed to the distributions fS(x) with respect to specific surface area using the corresponding (e.g., slit-like) model of pores [26,27] w Vp fV ðxÞ fS ðxÞ ¼ ; ð1Þ x x
Burn-off (wt.%)
Decrease in bulk density (%)
– 14.5 31.0 36.0 16.7 29.0 25.0
– 13.5 31.0 32.5 13.5 24.2 18.5
was used at xmin = 0.2 nm and xmax = 100 nm. The surface areas of micro-, meso- and macropores were calculated as follows S mic ¼
Z
1
fS ðxÞdx;
ð3Þ
0:2
S mes ¼
Z
25
fS ðxÞdx;
ð4Þ
1
S mac ¼
Z
100
fS ðxÞdx:
ð5Þ
25
where w = 1, 2, and 3 for slit-like, cylindrical, and spherical pores, respectively. For estimation of deviation of the pore shape from slit-shaped one Dwslit = SBET/ Ssum 1 (Table 2) where S sum ¼
Z
xmax
xmin
fS ðxÞdx ¼
Z
rmax rmin
1 Vp fV ðxÞ dx; x x
ð2Þ
For pictorial presentation the pore size distribution functions were calculated as incremental PSDs (IPSDs) fz;n ðxi Þ ¼ 0:5ðfz ðxi Þ þ fz ðxi1 ÞÞðxi xi1 Þ;
ð6Þ
where subscript z = V or S [26,27]. Calculation of the fractal dimension (DAJ) of the adsorbents was performed on the basis of the adsorption
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data (p/p0 < 0.85) using the Frenkel–Halsey–Hill equation [28]. SEM images of the chars were obtained using a TESLA BS 301 scanning microscope. The diffuse reflectance infrared (IR) spectra were recorded by means of a FTIR Perkin Elmer 1725X spectrometer using samples stirred with KBr.
3. Results and discussion SEM images show (Fig. 2) that carbonization of IERs and subsequent activation results in partial (major or minor) destruction of granules. Some granules keep their shape (Fig. 2c and e) but others strongly change the morphology (Fig. 2a, b, and d) up to full decomposition on activation (IER1) (Table 1). These results depend on the IER origin (Table 1) and history, since the preparation technique was the same for all the samples. Thus, the morphology of chars (as well as activated carbons) prepared from different IERs varies from spherical granules (characterized by different levels of deformation) to flakes or particles of an irregular shape, which is of importance for subsequent application of these materials. As follows from Table 1, the initial ion-exchange resins differ in their chemical composition and skeleton structure; for example, gel resins with styrene (ST) and divinyl benzene (DVB) used in different proportions (IR-120, IRA-402, IRA-68) or acrylic ICR-84. The initial IERs differ in the density due to variations in polymer packing in grains of ion exchangers. The grain sizes affect the apparent density of chars obtained from these IERs, and the greater the apparent density of the initial resin, the greater the bulk density of the char (Table 3). Polymer shrinking decreases with increasing content of divinyl benzene but char yield increases (Table 3). In the case of active carbons, the mechanical strength of the grains increases as apparent density increases and pore volume decreases [4]. A high apparent density corresponds to favorable conditions for further activation of such materials to 50% burn-off or more with respect to the initial IERs (which is important from both economical and technological points of view). This refers mainly to resins preserving a spherical shape with the exception of IRA-402 or ICR-84 depolymerized under applied thermal conditions. To obtain spherical carbon adsorbents from similar resins, a more complex pyrolysis procedure should be applied [5–11,29,30]. Additional treatment of IER7 by oxidizing with 30% H2O2 solution (before thermal activation), impregnating with Ca(CH3COO)2 Æ H2O at a Ca(II) concentration of 1 wt.% and subsequently activating it enhances mesopore contribution up to 2.5 cm3/g at SBET = 1200 m2/g [29]; application of KOH to activate carbons prepared from IERs gives similar results [30].
Fig. 2. SEM photographs of (a and c) chars and (b, d, e) activated carbons of (a and b) IER6 (·250 and ·500), (c and d) IER4 (·100 and ·50), and (e) IER7 (·400).
The chars possess a small pore volume (Table 3) and a low specific surface area (not shown here) with the exception of IR-120 having SBET = 35 m2/g and Vp = 0.062 cm3/g at pore half-width x = 3.6 nm. Their nitrogen adsorption isotherms had a shape typical for mesoporous adsorbents. As follows from Fig. 1 and the data in Table 2, activation with water vapor causes significant development of porous structure (as IER1 was destroyed upon activation, its textural characteristics are not given). The shapes of the adsorption isotherms, hysteresis loops (Fig. 1) and the pore size distributions (Figs. 3 and 4) re-
V.M. Gunko et al. / Carbon 43 (2005) 1143–1150
veal a complex texture. The type of isotherms does not strongly change due to washing or washing/freezing of activated carbons; however, the adsorption values increase significantly for IER3 (Fig. 4b) and IER4 (Fig. 4c, Table 2). However, the porosity type does not change on these additional treatments and only some portion of pores is emptied due to washing (compare corresponding PSDs in Fig. 4). Washing/freezing has slightly greater effect on porosity enlargement (as ice has a larger volume than water and these volume changes on freezing remain in the textural ‘‘memory’’ of carbons) [31] for certain samples (Fig. 4c and e). Washing leads to emptying of micropores (Fig. 4e) and narrow mesopores (Fig. 4c) or pores over a broader range (Fig. 4b). However, for certain samples, e.g., IER2 (Fig. 4a), IER5 (Fig. 4d) and IER7 (Fig. 4f) especially for IER2 (see its textural parameters in Table 2), this effect is small. It should be noted that IER2 has the lowest value of burn-off (Table 3). Changes in burn-off correspond to porosity enlargement tendency after washing (Table 2, Fig. 4). These changes in micro- and mesopore volume can differ due to localization of water droplets mainly in mesopores of relatively hydrophobic carbon
Fig. 3. Incremental pore size distributions of activated carbons with respect to (a) pore volume and (b) specific surface area.
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[32–36], whose enlargement on freezing results in elimination of micropores (differently filled by water in comparison with mesopores) (Table 2). As a consequence of different water distribution in micro- or mesopores of activated carbons, pre-adsorbed water can differently affect adsorption of organic molecules under static or dynamic conditions [32–36]. Deviation of pore shape from slit-like (Table 2, Dwslit) is from 15% (IER5 with the main contribution of narrow pores, Fig. 4d) to 50% (IER6 with a broad PSD, Fig. 4e). However, IER7 possessing narrow distribution of pores demonstrates a large deviation Dwslit. The shape of its adsorption–desorption isotherm and open hysteresis loop (Fig. 1) suggests that this carbon has pores of complex shape and connectivity. Notice that IER2, IER4 and IER5 have open hysteresis loops too due to an insufficient connectivity of the pore network, which inhibits desorption. However, their isotherms have different shapes since IER2 is microporous, while IER4 and IER5 are micro/mesoporous. Consequently, insufficient development of pore network connectivity may be characteristic for certain micro- and micro/mesoporous carbons studied here. It follows from the data in Table 2 that, under the same activation conditions, the susceptibility of chars to burn-off changes from 15% to 36%, which is accompanied by changes in apparent density. The adsorbent prepared from pure Zerolit 225 · 20 (IER7) possesses the largest SBET value and its char possesses a large apparent density, which persists even after activation. The IR spectra (Fig. 5) indicate that the carbons possess different surface structures, e.g., aliphatic, aromatic, cyclic [37], as one can observe the bands at 1460 cm1 and over the 1320–1100 cm1 range (e.g., 1145 cm1 for CH3–CH–CH3, 1220 cm1 for (CH3–CH–CH2), and 1255 cm1 for (CH3)3C–). All the analyzed spectra possess a low peak at 2840 cm1 showing the presence of CH2 groups. The band at 1640 cm1 corresponds to alkene fragments or planar and non-planar deformation vibrations of OH groups. At the same time, the peaks at 1145, 1220 and 1255 cm1 suggest the existence of C–O and C–O–C bonds (ethers, esters, and lactones). The peaks at 1680 cm1 come from C@C, N–C@C, C@N, or C@O groups and coupled polyenes. A low-intensity band at 3600 cm1 can be connected to –OH groups and adsorbed water, which can take part in hydrogen bonding. The bands at 1300–1020 cm1 could be connected with C–O–C and C–O groups (ethers, acids, lactones). In the case of aliphatic esters, there is a strong band at 1150 cm1 caused by asymmetric valence vibrations of C–O–C. The bands at 1740 cm1 can originate from valence vibrations of C@O and C–O in esters and lactones. There are also bands over the 700– 600 cm1 range related to valence vibrations of C–S bonds but they are slightly formed.
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Fig. 4. Pore size distributions of activated carbons (initial), washed–dried and washed–frozen-dried: (a) IER2, (b) IER3, (c) IER4, (d) IER5, (e) IER6, and (f) IER7 with respect to pore volume.
4. Conclusions The surfaces of activated carbons prepared from ionexchange resins are complex both with respect to their texture (characterized by developed micro- and mesoporosity and a high value of the fractal dimension, up to
2.96) and chemistry. The surfaces of these carbon adsorbents possess a polyfunctional character, as both specific (e.g., oxygen-containing groups such as phenolic, carboxylic and lactonic) and non-specific adsorption sites (e.g., on basal graphite planes) are present. These adsorbents show complex porous structures over micro- and
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Fig. 5. IR spectra of IER chars.
mesopore ranges, which change on washing or washing/ freezing of wetted carbons. The values of specific surface area and pore volume suggest that the synthesized activated carbons can be utilized for different purposes. Additional specific treatments can allow one to significantly enhance their porosity and specific surface area.
Acknowledgments This research was supported by NATO (grant no. EST.CLG.976890). R.L. is grateful to the Foundation for Polish Science for financial support. The authors thank Prof. L.R. Radovic for useful remarks and comments.
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