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Adsorption of alkylphenols onto microporous carbons prepared from coconut shell
Synthetic Metals 125 (2002) 207±211
Adsorption of alkylphenols onto microporous carbons prepared from coconut shell S. Iwasakia, T. Fukuharaa, I. Abea,*, J. Yanagib, M. Mourib, Y. Iwashimab, T. Tabuchic, O. Shinoharac a
Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan Takeda Chemical Industries, Ltd., 17-85, Jusohonmachi 2-Chome, Yodogawa-ku, Osaka 532-8686, Japan c Department of Civil Engineering, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan b
Received 14 May 2001; received in revised form 5 June 2001; accepted 9 August 2001
Abstract In order to obtain fundamental data on removal of the alkylphenol (AP) group of endocrine-disrupting chemicals, seven microporous carbons with speci®c surface area of 513±1828 m2/g were prepared from coconut shell, and the adsorption isotherms of APs with alkyl chains consisting of 5±9 carbon atoms were measured. The following results were obtained: the amount of 4-nonylphenol adsorbed at equilibrium concentrations above 20 mg/l increased with activation time; the amount adsorbed at 1 mg/l increased with activation time from 0.5 to 2.5 h, but decreased from 2.5 to 4.5 h. This decrease under prolonged activation is attributable to the effect of decreased dispersion force through pore enlargement being greater than the effect of increased speci®c surface area. The amount of APs adsorbed at above 10 mg/ l was higher for APs with a linear alkyl chain of 5±7 carbon atoms than for those with branched alkyl chains of 8±9 carbon atoms due to a molecular sieving effect in the latter. # 2001 Published by Elsevier Science B.V. Keywords: Activated carbon; Adsorption; Alkylphenol; Microporosity; Endocrine-disrupting chemicals
1. Introduction There is a growing concern over the possible harmful consequences of exposure to chemicals capable of modulating or disrupting the endocrine system in animals and humans. Endocrine-disrupting chemicals (EDCs) are commonly de®ned as exogenous substances (such as certain herbicides, fungicides, insecticides and industrial chemicals) that cause adverse health effects in an intact organism or its progeny as a consequence of changes in endocrine functions. Alkylphenol ethoxylates (APEs) are among the most widely used groups of surfactants. Worldwide, about 500,000 t are produced annually for use in detergents, paints, pesticides, textile and petroleum recovery chemicals, metal working ¯uids, and personal care products. Of total APE production, 80±85% is sold as nonylphenol ethoxylate and over 15% as octylphenol (OP) ethoxylate. APE surfactants have been around for a long time, despite charges by environmental activists that APE biodegradation intermediates are more toxic than APEs themselves. More recently, nonylphenol (NP), a biodegradation product of nonylphenol * Corresponding author. Tel.: 81-6-6963-8045; fax: 81-6-6963-8049. E-mail address: [email protected] (I. Abe).
0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 3 0 - 6
ethoxylates, has been reported to possess weak estrogen-like action. Effects on a variety of aquatic organisms with different levels of complexity have been suggested [1,2]. Alkylphenols (APs) such as NP and OP have frequently been detected in industrial ef¯uents and river water in many countries [3,4]. A removal technique for the very small quantities of APs in such waters is, therefore, urgently required. Abe [5] estimated the adsorbability of about 70 ECDs onto activated carbon from their chemical structures and demonstrated that adsorption by activated carbon was very effective for their removal. The same author had already shown that this method of adsorption was effective for removal of various nonionic surfactants [6,7]. Experiments to investigate the adsorption of NPs onto four activated carbons made from different raw materials have indicated that activated carbon made from coconut shell had high adsorption power even at the very low concentration of 1 mg/ l because it had the smallest mean pore diameter [8]. In the present study, seven activated carbons with different pore-size distributions were prepared from coconut shell and the adsorption isotherms of APs with different alkyl chain structures were measured. The relationship between adsorptivity and pore-size distribution is discussed.
208
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
of iodine adsorbed onto microporous carbon at C 2:5 g=l, was determined using the adsorption isotherm.
2. Experimental 2.1. Preparation of microporous carbon The raw material for preparation of microporous carbons was coconut-shell char obtained from the Philippines. The granular size, packed density, volatile matter content, and ash content of the coconut-shell char were 9±32 mesh, 0.600 g/ml, 12.8, and 1.1%, respectively. A steam-based activation method was used to develop micropores in the char: 2000 g was inserted in a rotary kiln preheated to 9008C and then steam was introduced at the rate of 25 g/min for various periods to produce microporous carbon. Activation yield of microporous carbon was determined from the difference in the mass of carbon before and after activation. 2.2. Measurement of porosity Speci®c surface area and pore volume were determined from nitrogen adsorption isotherms using the Nihon Bell Belsorp 18SA (Osaka, Japan). Speci®c surface area (S) was calculated from BET plots in the relative pressure range 0.01±0.15 [9]. The volume of pores (V) was determined from the amount of nitrogen adsorbed at relative pressure 0.931 [10]. Mean pore diameter (D) was calculated from D 4V=S, the pore system was assumed to consist of uniform cylindrical nonintersecting capillaries [11].
2.4. Measurement of AP adsorption The following APs were used: 4-nonylphenol, 4-tertoctylphenol, 4-n-heptylphenol, 4-n-hexylphenol, and 4-npentylphenol (Tokyo Chemical Industry, Tokyo, Japan). It was con®rmed by thin-layer chromatography and nuclear magnetic resonance that 4-nonylphenol was a mixture of several branched isomers. The water used for AP dissolution was puri®ed by distillation, followed by treatment via activated carbon adsorption and ion-exchange. The adsorption isotherms of APs were obtained by adding various quantities of activated carbon to 100 ml vial tubes containing an aliquot of an AP solution of 100 mg/l, which were then capped and shaken for 15 h in a bath maintained at 258C. Preliminary experiments showed that adsorption was essentially completed within 15 h. After equilibration, a sample was taken from each tube and passed through a glass-®ber ®lter to remove any suspended carbon. AP content in the ®ltrate solution was measured using a spectro¯uorophotometer, Shimadzu RF-5300PC (Kyoto, Japan). The excitation and emission wavelengths were 221.0 and 304.0 nm, respectively. The equilibrium amount adsorbed was calculated using the following equation: X C0
2.3. Measurement of iodine adsorption capacity All carbon samples were ground to powder and passed through a 45 mm sieve. The powders were washed with distilled water and dried at 1108C for 24 h and kept in a desiccator containing silica gel. A certain amount of carbon was added to 50 ml of 0.05 M iodine solution, which was then shaken for 15 min at room temperature. After centrifugation, 10 ml of supernatant was titrated with 0.1 M sodium thiosulfate to determine equilibrium concentration of iodine C (g/l) and amount of iodine adsorbed onto carbon X (mg/g). The adsorption isotherm of iodine onto the microporous carbon was approximated by the Freundlich equation. Iodine adsorption capacity, de®ned as the amount
C
V W
(1)
where X is the equilibrium amount adsorbed per gram of activated carbon (mg/g), C0 and C the initial and equilibrium concentration (mg/l), V the volume of solution (l), and W the weight of activated carbon (mg). 3. Results and discussion 3.1. Preparation of microporous carbon Table 1 shows the results of steam activation of coconutshell char. The microporous carbons [MC2] to [MC6] have similar adsorption capacity to that of commercially available
Table 1 Porosity and adsorption capacity of microporous carbons prepared from coconut shell No.
Activation time (h)
Activation yield (%)
Packing density (g/ml)
Asha (%)
Hardnessa (%)
Specific surface area (m2/g)
Pore volume (ml/g)
Mean pore diameter (nm)
Acetonea adsorption capacity (%)
Iodinea adsorption capacity (mg/g)
[MC1] [MC2] [MC3] [MC4] [MC5] [MC6] [MC7]
0.5 1.5 2.5 3.0 3.5 4.0 4.5
84.0 67.6 53.8 47.0 41.6 37.4 27.8
0.630 0.571 0.487 0.465 0.440 0.412 0.367
1.4 3.0 2.6 3.3 4.3 5.3 6.0
98.6 98.1 97.4 96.8 96.1 95.7 94.2
513 794 1111 1276 1419 1599 1828
0.214 0.342 0.487 0.560 0.623 0.710 0.860
1.67 1.72 1.75 1.75 1.76 1.78 1.88
11.2 19.3 26.8 30.5 32.8 36.8 40.2
630 960 1220 1340 1380 1470 1530
a
Tested by JIS K1474.
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
Fig. 1. Differential pore-size distributions of microporous carbons prepared from coconut shell.
activated carbon produced from coconut shell. Fig. 1 shows differential pore-size distributions of microporous carbons. It was seen from Fig. 1 that pores with large diameter increased with activation time. Fig. 2 shows the relationship between activation time and activation yield, the latter decreased linearly with the increase of the former. This indicates that the reaction rate of carbon with steam is of zero order with respect to the mass of carbon, as expressed in the equation below [3]: dM=dt k
(2)
209
Fig. 3. Relationship of activation time to specific surface area and to mean pore diameter.
Fig. 3 shows the relationships of activation time to speci®c surface area and to mean pore diameter. The speci®c surface area and mean pore diameter increased linearly with activation time. The speci®c surface area and pore volume increased several times with activation time from 0.5 to 4.5 h, whereas the mean pore diameter indicated only about 10% augmentation. 3.2. Adsorption characteristics of 4-nonylphenol
where M is the mass of carbon, t the activation time, and k the rate constant. The same behavior was observed in charcoals from lauan and rubber trees [12,16].
Fig. 4 shows the adsorption isotherms of 4-nonylphenol at 258C onto microporous carbons prepared by steam-activating coconut-shell char for various lengths of time. The
Fig. 2. Relationship between activation time and activation yield.
Fig. 4. Adsorption isotherms of 4-nonylphenol onto microporous carbons.
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S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
Table 2 Freundlich adsorption constants of 4-nonylphenol onto microporous carbons Microporous carbon
[MC1] [MC2] [MC3] [MC4] [MC5] [MC6] [MC7]
Freundlich constants K
1/N
7.6 35 46 40 33 37 34
0.095 0.14 0.16 0.28 0.50 0.55 0.60
amount of 4-nonylphenol adsorbed at equilibrium concentrations above 20 mg/l increased with activation time, as prolongation of the activation time increases micropore volume and speci®c surface area. The adsorption isotherms were approximated using the Freundlich equation: 1 log X log K log C (3) N where K and 1/N are adsorption constants. The adsorption constants were estimated by linear regression analysis between log C and log X and are shown in Table 2. The constant 1/N increased with activation time. This constant expresses the affinity between adsorbate and adsorbent and decrease in value with increasing affinity. Adsorption onto carbon, whose surface is hydrophobic, is mainly based on the London dispersion force, which is a part of the van der Waals force. The more closely the adsorbate molecules in the pores are located to the surrounding pore walls, the higher the adsorption force [13]. It was concluded that the prolongation of activation time increased mean pore diameter, decreased adsorption force and thus increased the value of constant 1/N. The value of K was equal to the amount of 4-nonylphenol adsorbed at 1 mg/l, which increased with activation time from 0.5 to 2.5 h, but decreased from 2.5 to 4.5 h. The initial increase was due to increased speci®c surface area, the subsequent decrease under prolonged activation was attributable to the effect of decreased dispersion force through pore enlargement being greater than the effect of increased speci®c surface area. 3.3. Adsorption characteristics of APs The adsorption isotherms of APs with different alkylchain structures onto [MC3], which showed the greatest K value for 4-nonylphenol, are shown in Fig. 5. The amount of APs adsorbed at equilibrium concentrations above 10 mg/l was higher for APs with linear alkyl chains of 5±7 carbon atoms than for those with branched alkyl chains of 8±9 carbon atoms. In general, the adsorbability of organic compounds onto a given activated carbon from aqueous solution increases with
Fig. 5. Adsorption isotherms of APs onto [MC3].
increasing number of carbon atoms in the molecule, as such increase causes increased chemical potential, inducing the molecule to leave the solution [14,15]. Among APs with linear alkylchains, the amount adsorbed increased with the number of carbon atoms heptyl > hexyl > pentyl. Similar behavior was observed among APs with branched alkyl chains nonyl > octyl. The ®nding that the amount adsorbed was higher in APs with linear alkyl chains (pentyl-, hexyl- and heptyl-) than in those with branched alkyl chains (octyl- and nonyl-) can be accounted for in terms of a molecular sieving effect, as the branched alkyl chains are relatively bulky. Pores of smaller size than thickness of the bulky alkyl chain are not available for adsorption. In the case of nonylphenol and octylphenol, the adsorption-decreasing effect of molecular sieving is likely to be greater than the adsorption-increasing effect of increased chemical potential. The effect of molecular sieving is very great in microporous carbon prepared from coconut shell because of its small mean pore diameter. The Freundlich adsorption constants for the adsorption isotherms are shown in Table 3. The values of 1/N for nonyl and octylphenols were lower than for heptyl, hexyl and pentylphenols, indicating their greater af®nity for Table 3 Freundlich adsorption constants of alkylphenols onto [MC3] Alkylphenol
4-Nonylphenol 4-tert-Octylphenol 4-n-Heptylphenol 4-n-Hexylphenol 4-n-Pentylphenol
Freundlich constants K
1/N
46 39 56 30 39
0.16 0.15 0.33 0.47 0.33
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
microporous carbon, as the mean pore diameter of [MC3] was relatively smaller for APs with branched alkyl chains than for those with linear alkyl chains. This also suggests that in APs with branched alkyl chains, amounts adsorbed at very low concentration below 1 mg/l would be greater than in APs with linear alkyl chains.
211
7. The amount of APs adsorbed at equilibrium concentrations above 10 mg/l was higher for APs with linear alkyl chains of 5±7 carbon atoms than for those with branched alkyl chains of 8±9 carbon atoms due to the molecular sieving effect of the latter. 8. 1/N values for APs with branched alkyl chains were lower than that for those with linear alkyl chains.
4. Conclusions Seven microporous carbons with speci®c surface area of 513±1828 m2/g were prepared from coconut shell, the adsorption isotherms of APs with alkyl chains consisting of 5±9 carbon atoms measured, and the relationship between adsorption properties and pore-size distribution investigated. The following results were obtained: 1. Although specific surface area and pore volume increased linearly with activation time, the corresponding increase in mean pore diameter was small. 2. All APs had very high adsorbability by all microporous carbons, apart from the carbon with the lowest specific surface area. 3. The amount of 4-nonylphenol adsorbed at equilibrium concentrations above 20 mg/l increased with activation time as prolongation of the latter increases micropore volume and specific surface area. 4. The Freundlich constant K (equal to the amount adsorbed at 1 mg/l) increased with activation time from 0.5 to 2.5 h, but decreased from 2.5 to 4.5 h. This decrease under prolonged activation was attributed to the effect of decreased dispersion force through pore enlargement being greater than the effect of increased specific surface area. 5. Prolongation of activation time increased mean pore diameter, decreased adsorption force and thus increased the Freundlich constant 1/N. 6. In both APs with linear alkyl chains and those with branched alkyl chains, the amount adsorbed increased with the number of carbon atoms.
Acknowledgements This research was supported in part by the Grant-in-Aid for JSPS-RFTF96R11701 from the Japan Society for the Promotion of Science. References [1] E.J. Routledge, J.P. Sumpter, Environ. Toxicol. Chem. 15 (1996) 241. [2] S. Jobling, D. Sheahan, J.A. Osborne, P. Mathiessen, J.P. Sumpter, Environ. Toxicol. Chem. 15 (1996) 194. [3] T. Fujiwara, J. Jpn. Soc. Water Environ. 22 (1999) 624. [4] H. Tanaka, J. Jpn. Soc. Water Environ. 22 (1999) 629. [5] I. Abe, J. Water Waste 41 (1999) 43. [6] I. Abe, K. Hayashi, M. Kitagawa, Yukagaku 25 (1976) 145. [7] I. Abe, K. Hayashi, M. Kitagawa, Yukagaku 25 (1976) 151. [8] I. Abe, S. Iwasaki, T. Fukuhara, S. Nakanishi, N. Kawasaki, T. Nakamura, S. Tanada, Tanso (1998) 234. [9] I. Abe, Chem. Express 7 (1992) 97. [10] R.W. Cranston, F.A. Inkley, Adv. Catal. 9 (1957) 143. [11] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1967, p. 208. [12] Y. Sanada, M. Suzuki, K. Fujimoto, Activated Carbon Ð its Fundamentals and Application, Kodansha-sha, Tokyo, 1975, p. 50. [13] S. Kondo, T. Ishikawa, I. Abe, Science of Adsorption, Maruzen, Tokyo, 1991, p. 194. [14] I. Abe, K. Hayashi, M. Kitagawa, T. Urahata, Bull. Chem. Soc. Jpn. 52 (1979) 1899. [15] I. Abe, K. Hayashi, M. Kitagawa, T. Urahata, Bull. Chem. Soc. Jpn. 53 (1980) 1199. [16] H. Kitagawa Nippon Kagaku Kaishi (1974) 1336.
Adsorption of alkylphenols onto microporous carbons prepared from coconut shell S. Iwasakia, T. Fukuharaa, I. Abea,*, J. Yanagib, M. Mourib, Y. Iwashimab, T. Tabuchic, O. Shinoharac a
Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan Takeda Chemical Industries, Ltd., 17-85, Jusohonmachi 2-Chome, Yodogawa-ku, Osaka 532-8686, Japan c Department of Civil Engineering, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan b
Received 14 May 2001; received in revised form 5 June 2001; accepted 9 August 2001
Abstract In order to obtain fundamental data on removal of the alkylphenol (AP) group of endocrine-disrupting chemicals, seven microporous carbons with speci®c surface area of 513±1828 m2/g were prepared from coconut shell, and the adsorption isotherms of APs with alkyl chains consisting of 5±9 carbon atoms were measured. The following results were obtained: the amount of 4-nonylphenol adsorbed at equilibrium concentrations above 20 mg/l increased with activation time; the amount adsorbed at 1 mg/l increased with activation time from 0.5 to 2.5 h, but decreased from 2.5 to 4.5 h. This decrease under prolonged activation is attributable to the effect of decreased dispersion force through pore enlargement being greater than the effect of increased speci®c surface area. The amount of APs adsorbed at above 10 mg/ l was higher for APs with a linear alkyl chain of 5±7 carbon atoms than for those with branched alkyl chains of 8±9 carbon atoms due to a molecular sieving effect in the latter. # 2001 Published by Elsevier Science B.V. Keywords: Activated carbon; Adsorption; Alkylphenol; Microporosity; Endocrine-disrupting chemicals
1. Introduction There is a growing concern over the possible harmful consequences of exposure to chemicals capable of modulating or disrupting the endocrine system in animals and humans. Endocrine-disrupting chemicals (EDCs) are commonly de®ned as exogenous substances (such as certain herbicides, fungicides, insecticides and industrial chemicals) that cause adverse health effects in an intact organism or its progeny as a consequence of changes in endocrine functions. Alkylphenol ethoxylates (APEs) are among the most widely used groups of surfactants. Worldwide, about 500,000 t are produced annually for use in detergents, paints, pesticides, textile and petroleum recovery chemicals, metal working ¯uids, and personal care products. Of total APE production, 80±85% is sold as nonylphenol ethoxylate and over 15% as octylphenol (OP) ethoxylate. APE surfactants have been around for a long time, despite charges by environmental activists that APE biodegradation intermediates are more toxic than APEs themselves. More recently, nonylphenol (NP), a biodegradation product of nonylphenol * Corresponding author. Tel.: 81-6-6963-8045; fax: 81-6-6963-8049. E-mail address: [email protected] (I. Abe).
0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 3 0 - 6
ethoxylates, has been reported to possess weak estrogen-like action. Effects on a variety of aquatic organisms with different levels of complexity have been suggested [1,2]. Alkylphenols (APs) such as NP and OP have frequently been detected in industrial ef¯uents and river water in many countries [3,4]. A removal technique for the very small quantities of APs in such waters is, therefore, urgently required. Abe [5] estimated the adsorbability of about 70 ECDs onto activated carbon from their chemical structures and demonstrated that adsorption by activated carbon was very effective for their removal. The same author had already shown that this method of adsorption was effective for removal of various nonionic surfactants [6,7]. Experiments to investigate the adsorption of NPs onto four activated carbons made from different raw materials have indicated that activated carbon made from coconut shell had high adsorption power even at the very low concentration of 1 mg/ l because it had the smallest mean pore diameter [8]. In the present study, seven activated carbons with different pore-size distributions were prepared from coconut shell and the adsorption isotherms of APs with different alkyl chain structures were measured. The relationship between adsorptivity and pore-size distribution is discussed.
208
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
of iodine adsorbed onto microporous carbon at C 2:5 g=l, was determined using the adsorption isotherm.
2. Experimental 2.1. Preparation of microporous carbon The raw material for preparation of microporous carbons was coconut-shell char obtained from the Philippines. The granular size, packed density, volatile matter content, and ash content of the coconut-shell char were 9±32 mesh, 0.600 g/ml, 12.8, and 1.1%, respectively. A steam-based activation method was used to develop micropores in the char: 2000 g was inserted in a rotary kiln preheated to 9008C and then steam was introduced at the rate of 25 g/min for various periods to produce microporous carbon. Activation yield of microporous carbon was determined from the difference in the mass of carbon before and after activation. 2.2. Measurement of porosity Speci®c surface area and pore volume were determined from nitrogen adsorption isotherms using the Nihon Bell Belsorp 18SA (Osaka, Japan). Speci®c surface area (S) was calculated from BET plots in the relative pressure range 0.01±0.15 [9]. The volume of pores (V) was determined from the amount of nitrogen adsorbed at relative pressure 0.931 [10]. Mean pore diameter (D) was calculated from D 4V=S, the pore system was assumed to consist of uniform cylindrical nonintersecting capillaries [11].
2.4. Measurement of AP adsorption The following APs were used: 4-nonylphenol, 4-tertoctylphenol, 4-n-heptylphenol, 4-n-hexylphenol, and 4-npentylphenol (Tokyo Chemical Industry, Tokyo, Japan). It was con®rmed by thin-layer chromatography and nuclear magnetic resonance that 4-nonylphenol was a mixture of several branched isomers. The water used for AP dissolution was puri®ed by distillation, followed by treatment via activated carbon adsorption and ion-exchange. The adsorption isotherms of APs were obtained by adding various quantities of activated carbon to 100 ml vial tubes containing an aliquot of an AP solution of 100 mg/l, which were then capped and shaken for 15 h in a bath maintained at 258C. Preliminary experiments showed that adsorption was essentially completed within 15 h. After equilibration, a sample was taken from each tube and passed through a glass-®ber ®lter to remove any suspended carbon. AP content in the ®ltrate solution was measured using a spectro¯uorophotometer, Shimadzu RF-5300PC (Kyoto, Japan). The excitation and emission wavelengths were 221.0 and 304.0 nm, respectively. The equilibrium amount adsorbed was calculated using the following equation: X C0
2.3. Measurement of iodine adsorption capacity All carbon samples were ground to powder and passed through a 45 mm sieve. The powders were washed with distilled water and dried at 1108C for 24 h and kept in a desiccator containing silica gel. A certain amount of carbon was added to 50 ml of 0.05 M iodine solution, which was then shaken for 15 min at room temperature. After centrifugation, 10 ml of supernatant was titrated with 0.1 M sodium thiosulfate to determine equilibrium concentration of iodine C (g/l) and amount of iodine adsorbed onto carbon X (mg/g). The adsorption isotherm of iodine onto the microporous carbon was approximated by the Freundlich equation. Iodine adsorption capacity, de®ned as the amount
C
V W
(1)
where X is the equilibrium amount adsorbed per gram of activated carbon (mg/g), C0 and C the initial and equilibrium concentration (mg/l), V the volume of solution (l), and W the weight of activated carbon (mg). 3. Results and discussion 3.1. Preparation of microporous carbon Table 1 shows the results of steam activation of coconutshell char. The microporous carbons [MC2] to [MC6] have similar adsorption capacity to that of commercially available
Table 1 Porosity and adsorption capacity of microporous carbons prepared from coconut shell No.
Activation time (h)
Activation yield (%)
Packing density (g/ml)
Asha (%)
Hardnessa (%)
Specific surface area (m2/g)
Pore volume (ml/g)
Mean pore diameter (nm)
Acetonea adsorption capacity (%)
Iodinea adsorption capacity (mg/g)
[MC1] [MC2] [MC3] [MC4] [MC5] [MC6] [MC7]
0.5 1.5 2.5 3.0 3.5 4.0 4.5
84.0 67.6 53.8 47.0 41.6 37.4 27.8
0.630 0.571 0.487 0.465 0.440 0.412 0.367
1.4 3.0 2.6 3.3 4.3 5.3 6.0
98.6 98.1 97.4 96.8 96.1 95.7 94.2
513 794 1111 1276 1419 1599 1828
0.214 0.342 0.487 0.560 0.623 0.710 0.860
1.67 1.72 1.75 1.75 1.76 1.78 1.88
11.2 19.3 26.8 30.5 32.8 36.8 40.2
630 960 1220 1340 1380 1470 1530
a
Tested by JIS K1474.
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
Fig. 1. Differential pore-size distributions of microporous carbons prepared from coconut shell.
activated carbon produced from coconut shell. Fig. 1 shows differential pore-size distributions of microporous carbons. It was seen from Fig. 1 that pores with large diameter increased with activation time. Fig. 2 shows the relationship between activation time and activation yield, the latter decreased linearly with the increase of the former. This indicates that the reaction rate of carbon with steam is of zero order with respect to the mass of carbon, as expressed in the equation below [3]: dM=dt k
(2)
209
Fig. 3. Relationship of activation time to specific surface area and to mean pore diameter.
Fig. 3 shows the relationships of activation time to speci®c surface area and to mean pore diameter. The speci®c surface area and mean pore diameter increased linearly with activation time. The speci®c surface area and pore volume increased several times with activation time from 0.5 to 4.5 h, whereas the mean pore diameter indicated only about 10% augmentation. 3.2. Adsorption characteristics of 4-nonylphenol
where M is the mass of carbon, t the activation time, and k the rate constant. The same behavior was observed in charcoals from lauan and rubber trees [12,16].
Fig. 4 shows the adsorption isotherms of 4-nonylphenol at 258C onto microporous carbons prepared by steam-activating coconut-shell char for various lengths of time. The
Fig. 2. Relationship between activation time and activation yield.
Fig. 4. Adsorption isotherms of 4-nonylphenol onto microporous carbons.
210
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
Table 2 Freundlich adsorption constants of 4-nonylphenol onto microporous carbons Microporous carbon
[MC1] [MC2] [MC3] [MC4] [MC5] [MC6] [MC7]
Freundlich constants K
1/N
7.6 35 46 40 33 37 34
0.095 0.14 0.16 0.28 0.50 0.55 0.60
amount of 4-nonylphenol adsorbed at equilibrium concentrations above 20 mg/l increased with activation time, as prolongation of the activation time increases micropore volume and speci®c surface area. The adsorption isotherms were approximated using the Freundlich equation: 1 log X log K log C (3) N where K and 1/N are adsorption constants. The adsorption constants were estimated by linear regression analysis between log C and log X and are shown in Table 2. The constant 1/N increased with activation time. This constant expresses the affinity between adsorbate and adsorbent and decrease in value with increasing affinity. Adsorption onto carbon, whose surface is hydrophobic, is mainly based on the London dispersion force, which is a part of the van der Waals force. The more closely the adsorbate molecules in the pores are located to the surrounding pore walls, the higher the adsorption force [13]. It was concluded that the prolongation of activation time increased mean pore diameter, decreased adsorption force and thus increased the value of constant 1/N. The value of K was equal to the amount of 4-nonylphenol adsorbed at 1 mg/l, which increased with activation time from 0.5 to 2.5 h, but decreased from 2.5 to 4.5 h. The initial increase was due to increased speci®c surface area, the subsequent decrease under prolonged activation was attributable to the effect of decreased dispersion force through pore enlargement being greater than the effect of increased speci®c surface area. 3.3. Adsorption characteristics of APs The adsorption isotherms of APs with different alkylchain structures onto [MC3], which showed the greatest K value for 4-nonylphenol, are shown in Fig. 5. The amount of APs adsorbed at equilibrium concentrations above 10 mg/l was higher for APs with linear alkyl chains of 5±7 carbon atoms than for those with branched alkyl chains of 8±9 carbon atoms. In general, the adsorbability of organic compounds onto a given activated carbon from aqueous solution increases with
Fig. 5. Adsorption isotherms of APs onto [MC3].
increasing number of carbon atoms in the molecule, as such increase causes increased chemical potential, inducing the molecule to leave the solution [14,15]. Among APs with linear alkylchains, the amount adsorbed increased with the number of carbon atoms heptyl > hexyl > pentyl. Similar behavior was observed among APs with branched alkyl chains nonyl > octyl. The ®nding that the amount adsorbed was higher in APs with linear alkyl chains (pentyl-, hexyl- and heptyl-) than in those with branched alkyl chains (octyl- and nonyl-) can be accounted for in terms of a molecular sieving effect, as the branched alkyl chains are relatively bulky. Pores of smaller size than thickness of the bulky alkyl chain are not available for adsorption. In the case of nonylphenol and octylphenol, the adsorption-decreasing effect of molecular sieving is likely to be greater than the adsorption-increasing effect of increased chemical potential. The effect of molecular sieving is very great in microporous carbon prepared from coconut shell because of its small mean pore diameter. The Freundlich adsorption constants for the adsorption isotherms are shown in Table 3. The values of 1/N for nonyl and octylphenols were lower than for heptyl, hexyl and pentylphenols, indicating their greater af®nity for Table 3 Freundlich adsorption constants of alkylphenols onto [MC3] Alkylphenol
4-Nonylphenol 4-tert-Octylphenol 4-n-Heptylphenol 4-n-Hexylphenol 4-n-Pentylphenol
Freundlich constants K
1/N
46 39 56 30 39
0.16 0.15 0.33 0.47 0.33
S. Iwasaki et al. / Synthetic Metals 125 (2002) 207±211
microporous carbon, as the mean pore diameter of [MC3] was relatively smaller for APs with branched alkyl chains than for those with linear alkyl chains. This also suggests that in APs with branched alkyl chains, amounts adsorbed at very low concentration below 1 mg/l would be greater than in APs with linear alkyl chains.
211
7. The amount of APs adsorbed at equilibrium concentrations above 10 mg/l was higher for APs with linear alkyl chains of 5±7 carbon atoms than for those with branched alkyl chains of 8±9 carbon atoms due to the molecular sieving effect of the latter. 8. 1/N values for APs with branched alkyl chains were lower than that for those with linear alkyl chains.
4. Conclusions Seven microporous carbons with speci®c surface area of 513±1828 m2/g were prepared from coconut shell, the adsorption isotherms of APs with alkyl chains consisting of 5±9 carbon atoms measured, and the relationship between adsorption properties and pore-size distribution investigated. The following results were obtained: 1. Although specific surface area and pore volume increased linearly with activation time, the corresponding increase in mean pore diameter was small. 2. All APs had very high adsorbability by all microporous carbons, apart from the carbon with the lowest specific surface area. 3. The amount of 4-nonylphenol adsorbed at equilibrium concentrations above 20 mg/l increased with activation time as prolongation of the latter increases micropore volume and specific surface area. 4. The Freundlich constant K (equal to the amount adsorbed at 1 mg/l) increased with activation time from 0.5 to 2.5 h, but decreased from 2.5 to 4.5 h. This decrease under prolonged activation was attributed to the effect of decreased dispersion force through pore enlargement being greater than the effect of increased specific surface area. 5. Prolongation of activation time increased mean pore diameter, decreased adsorption force and thus increased the Freundlich constant 1/N. 6. In both APs with linear alkyl chains and those with branched alkyl chains, the amount adsorbed increased with the number of carbon atoms.
Acknowledgements This research was supported in part by the Grant-in-Aid for JSPS-RFTF96R11701 from the Japan Society for the Promotion of Science. References [1] E.J. Routledge, J.P. Sumpter, Environ. Toxicol. Chem. 15 (1996) 241. [2] S. Jobling, D. Sheahan, J.A. Osborne, P. Mathiessen, J.P. Sumpter, Environ. Toxicol. Chem. 15 (1996) 194. [3] T. Fujiwara, J. Jpn. Soc. Water Environ. 22 (1999) 624. [4] H. Tanaka, J. Jpn. Soc. Water Environ. 22 (1999) 629. [5] I. Abe, J. Water Waste 41 (1999) 43. [6] I. Abe, K. Hayashi, M. Kitagawa, Yukagaku 25 (1976) 145. [7] I. Abe, K. Hayashi, M. Kitagawa, Yukagaku 25 (1976) 151. [8] I. Abe, S. Iwasaki, T. Fukuhara, S. Nakanishi, N. Kawasaki, T. Nakamura, S. Tanada, Tanso (1998) 234. [9] I. Abe, Chem. Express 7 (1992) 97. [10] R.W. Cranston, F.A. Inkley, Adv. Catal. 9 (1957) 143. [11] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1967, p. 208. [12] Y. Sanada, M. Suzuki, K. Fujimoto, Activated Carbon Ð its Fundamentals and Application, Kodansha-sha, Tokyo, 1975, p. 50. [13] S. Kondo, T. Ishikawa, I. Abe, Science of Adsorption, Maruzen, Tokyo, 1991, p. 194. [14] I. Abe, K. Hayashi, M. Kitagawa, T. Urahata, Bull. Chem. Soc. Jpn. 52 (1979) 1899. [15] I. Abe, K. Hayashi, M. Kitagawa, T. Urahata, Bull. Chem. Soc. Jpn. 53 (1980) 1199. [16] H. Kitagawa Nippon Kagaku Kaishi (1974) 1336.