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Activated Carbons Chemically Modified by Concentrated H2SO4 for the Adsorption of the Pollutants from Wastewater and the Dibenzothiophene from Fuel Oils Zongxuan Jiang, Yan Liu, Xiuping Sun, Fuping Tian, Fuxia Sun, Changhai Liang, Wansheng You, Chongren Han, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, P.R. China Received July 24, 2002. In Final Form: October 22, 2002 The surface properties, porosities, and adsorption capacities of activated carbons (AC) are modified by the oxidation treatment using concentrated H2SO4 at temperatures 150-270 °C. The modified AC was characterized by N2 adsorption, base titration, FTIR, and the adsorption of iodine, chlorophenol, methylene blue, and dibenzothiophene. The treatment of AC with concentrated H2SO4 at 250 °C greatly increases the mesoporous volume from 0.243 mL/g to 0.452 mL/g, specific surface areas from 393 m2/g to 745 m2/g, and acidic surface oxygen complexes from 0.071 meq/g to 1.986 meq/g as compared with the unmodified AC. The base titration results indicate that the amount of acidic surface oxygen groups on the modified AC increases with increasing the treatment temperatures and carboxyls and phenols are the most abundant carbon-oxygen functional groups. The carboxyl groups, COO- species, and hydroxyl groups are detected mainly for the sample treated at 250 °C. The mesoporous properties of the AC modified by concentrated H2SO4 were further tested by the adsorption of methylene blue and dibenzothiophene. The AC modified by concentrated H2SO4 at 250 °C has much higher adsorption capacities for large molecules (e.g., methylene blue and dibenzothiophene) than the unmodified AC but less adsorption capacities for small molecules (e.g., iodine). The adsorption results from aqueous solutions have been interpreted using Freundlich adsorption models.
Introduction Activated carbons (AC) are widely used as adsorbents because of their high specific surface areas, well-developed porosities, and tunable surface oxygen-containing complexes. The specific surface areas and porosities of AC are greatly affected by the precursors of carbonaceous materials and methods of preparation.1-4 The adsorption capacity and the adsorption rate of an AC are directly associated with the specific surface areas and the pore size distribution of the AC.5 In general, the larger the specific surface area is, the greater the adsorption capacity. However, for the adsorption of larger molecules, the adsorption capacity and the adsorption rate are largely dependent on the mesoporous (and macroporous) volumes.6-9 The selective adsorption properties of inorganic materials on AC are strongly influenced by the amount and the types of surface oxygen complexes (for instance, when the AC used for the adsorption of heavy metal ions from aqueous solutions).10-14 * Corresponding auther. Fax: 86-411-4694447. E-mail: canli@ dicp.ac.cn. Homepage: http://www.canli.dicp.ac.cn. (1) Ismadji, S.; Bhatia, S. K. Carbon 2001, 39, 1237. (2) Daguerre, E.; Guillot, A.; Py, X. Carbon 2000, 38, 59. (3) Sabio, M. M.; Rodriguez, R. F.; Caturla, F.; Selles, M. J. Carbon 1996, 34 (4), 457. (4) Jagtoyen, M.; Derbyshire, F. Carbon 1993, 31 (7), 1185. (5) Chiang, H. L.; Huang, C. P.; Chiang, P. C.; You, J. H. Carbon 1999, 37, 1919. (6) Hu, Z. H.; Srinivasan, M. P.; Ni, Y. M. Adv. Mater. 2000, 12 (1), 62. (7) Costas, P.; Vernon, L. S. Carbon 2000, 38, 1423. (8) Costas, P.; Vernon, L. S. Carbon 2001, 39, 3925. (9) Hsieh, C. T.; Teng, H. S. Carbon 2000, 38, 863. (10) Rivera-Utrilla, J.; Ferro-Garcı´a, M. A. Adsorpt. Sci. Technol. 1986, 3, 293. (11) Jia, Y. F.; Thomas, K. M. Langmuir 2000, 16, 1118. (12) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. J. Colloid Interface Sci. 1992, 151, 433.
The surface oxygen complexes of AC can be created by different modification methods, for example, dry and wet oxidations. Dry oxidation is a modification method involving reactions with oxidizing gases (such as steam, CO2, etc.) at high temperatures (>700 °C).15,16 The surface oxygen complexes (e.g., carbonyl groups) are unstable and decompose at high temperature, and this leads to the loss of some surface oxygen-containing species. Wet oxidation is a modification method involving reactions between the carbon surfaces and oxidizing solutions (such as aqueous solutions of HNO3, H2O2, NaOCl, (NH4)2S2O8, etc.) under relatively mild reaction conditions (e.g., 20-100 °C).17,18 HNO3 is often used for the wet oxidation since its oxidizing properties can be controlled by adjusting the concentration and temperature.17 However, HNO3 also brings about some problems: (1) reducing the specific surface areas and porosities of AC;19 (2) giving off NO2 gas during the reactions. Most other aqueous oxidants also have similar drawbacks and damage the mesoporous structures at higher temperatures (e.g. >80 °C).20 The study on modification of AC with concentrated H2SO4 has been reported in the literature,21-24 while the modification of AC with liquid H2SO4 at temperatures (13) Bandosz, T. J.; Jagiello, J.; Schwarz, J. A. Langmuir 1993, 9, 2518. (14) Biniak, S.; Pakula, M.; Szymanski, G. S.; Swiatkowski, A. Langmuir 1999, 15, 6121. (15) Smisek, M.; Cerny, S. Active Carbon: manufacture, properties and applications; Elsevier: Amsterdam, 1970. (16) Leboda, R.; Zieba, J. S.; Bogillo, V. I. Langmuir 1997, 13, 1211. (17) Boehm, H. P. Carbon 2002, 40, 145. (18) Moreno-Castilla, C.; Ferro-Garcı´a, M. A.; Joly, J. P.; BautistaToledo, I.; Cassasco-Marı´n, F.; Rivera-Utrilla, J. Langmuir 1995, 11 (11), 4386. (19) Gil, A.; Puente, G. de la; Grange, P. Microporous Mater. 1997, 12, 51. (20) Moreno-Castilla, C.; Lo´pez-Ramo´n, M. V.; Carrasco-Marin, F. Carbon 2000, 38, 1995. (21) Terzyk, A. P.; Rychlicki, G. Colloids Surf., A 2000, 163, 135.
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higher than 150 °C has not been reported in the literature until now. Terzyk et al.21 and Teng et al.22 investigated the modification of AC with fuming H2SO4 and concentrated H2SO4 at relatively low temperatures ( GH, while the trend of k is GH > GHS250. The different change trends may imply that the interaction mechanisms between the two carbon materials and iodine molecules may not be the same. The results from k show that GH has the stronger affinity for iodine and the larger adsorption capacity. For methylene blue adsorption on (44) Tanada, S.; Kawasaki, N.; Nakamura, T.; Araki, M.; Isomura, M. J. Colloid Interface Sci. 1999, 214, 106.
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Jiang et al. Table 4. Amount of Adsorbed Dibenzothiophene (DBT) on the Selected Activated Carbons during a Period of 4 h
Figure 8. Variance of sulfur removal with adsorption time.
the selected adsorbents, the both 1/n and k have the same change order, GHS250 > GH, suggesting that GHS250 having the larger amount of mesopores has the stronger affinity and the larger adsorption capacity. Adsorption of Dibenzothiophene (DBT) in Heptane on the AC Modified by H2SO4. Figure 8 illustrates the variance of sulfur removal for the selected activated carbons with adsorption time. The removal of sulfur existing in the fuel oils has attracted much attention because of the very stringently environmental regulations. Although the conventional hydodesulfurization (HDS) processes can remove the majority of sulfur present as simple sulfides, disulfides, mercaptans, and thiophenes, some thiophene compounds, especially benzothiophene, dibenzothiophene, and their derivatives are very difficult to be removed via the HDS process. The operation at high temperatures and high pressure is inevitably required to remove these refractory sulfur compounds to approach the ultradeep desulfurization of fuel oils (less than 50 ppm). This certainly brings about the following problems: high investment, high operating cost, reduction of catalyst cycle length, increase of hydrogen consumption because of the hydrogenation of olefins and aromatics present in fuel oils, the reduction of octane number for gasoline, and so forth. Thus, some petroleum companies and research groups all over the world are exploring the new desulfurization processes.45-49 The desulfurization with adsorption is considered to be one of the most promising methods of desulfurization. At present, the main restriction of the (45) Savage, D. W.; Kaul, B. K.; Dupre, G. D.; O’Bara, J. T.; Wales, W. E.; Ho, T. C. U.S. Patent 5,454,933, 1995. (46) Irvine, R. L.; Bensen, B. A.; Varraveto, D. M.; Frye, R. A. NPRA AM-99-42; National Petrochemical and Refiners Association: Washington, DC, 1999. (47) Meille, V.; Schulz, E.; Vrinat, M.; Lemaire, M. Chem. Commun. 1998, 305. (48) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Chem. Commun. 1998, 2601. (49) Bo _smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494.
sample
amount of adsorbed DBT (g of DBT/g of adsorbent)
GH GHS250
0.154 0.295
process to industrial application is the low adsorption capacities of adsorbents. As dibenzothiophene is a relatively large molecule, its adsorption capacity is greatly affected by the mesopore volume. As can be seen from Figure 8, the 100% sulfur removal time of the AC modified by concentrated H2SO4 at 250 °C (GHS250) prolongs 2 h as compared with unmodified activated carbon (GH). This result indicates that the 100% sulfur removal time is directly related to the mesopore volumes. On comparing mesopore volumes (Table 1) and the amount of dibenzothiophene adsorbed on the carbons (Table 4), it can be seen that the GHS250 has the larger mesopore volume and hence its adsorption capacity for dibenzothiophene is higher than GH. The amount of dibenzothiophene adsorbed on GHS250 is approximately 2 times the amount on GH. It is also seen from Figure 8 that the decline trend of sulfur removal for GHS250 with time is faster than the GH. This implies that the mesopores can speed up the adsorption rate of DBT on the AC. Conclusion A modification method is found to increase the specific areas and pore volumes (especially mesopores) and to produce the surface oxygen complexes of AC through the reactions between the AC and concentrated H2SO4 at relatively high temperatures (>150 °C). The increase of mesopore volume is attributed to the opening of micropores through the reactions between the ACs and the concentrated H2SO4 at relatively high temperatures. By changing the treatment temperatures and time, the specific surface areas, porosities, and the amount of surface oxygen complexes can be tuned in a wide range. The amount and types of surface oxygen complexes (such as carboxyl groups, phenolic groups, etc.) on the modified AC are verified by base titration and FTIR spectroscopy. The AC modified by concentrated H2SO4 shows much higher adsorption capacities of methylene blue and dibenzothiophene as compared with the unmodified AC. The adsorption capacity of the AC reaches the maximum after the modification by H2SO4 at 250 °C. The amount of dibenzothiophene adsorbed on the GHS250 is approximately 2 times that on the unmodified AC. It is proposed that the significant increase of the adsorption capacity for larger molecules is mainly due to the increase of mesopore volume. The modification with the concentrated H2SO4 has a negative effect on the adsorption of small molecules (such as iodine) because of the larger adsorption capacity on the unmodified AC as compared with the AC chemically modified by concentrated H2SO4. LA020670D
