Energy & Fuels 2009, 23, 2197–2204
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Production of Activated Carbon from Pine Cone and Evaluation of Its Physical, Chemical, and Adsorption Properties Gozde Duman,† Yunus Onal,†,‡ Cagdas Okutucu,† Sermin Onenc,† and Jale Yanik*,† Faculty of Science, Department of Chemistry, Ege UniVersity, 35100 BornoVa, Izmir, Turkey and Faculty of Engineering, Department Of Chemical Engineering, Inonu UniVersity, 44280 Malatya, Turkey ReceiVed NoVember 13, 2008. ReVised Manuscript ReceiVed January 16, 2009
In this study, the activated carbons were prepared from pine cone by chemical activation. Boehm titration and FT-IR analysis were conducted to determine the surface groups of the activated carbons while N2 adsorption (77 K) was carried out to evaluate their pore characteristics. Zinc chloride produced activated carbon with a higher surface area and micropore volume compared to that produced by phosphoric acid activation. The amount of activating agent used strongly influenced the porous texture; considerable lossing in microporosity accompanies the increasing of activating agent. The potential application of activated carbons obtained from pine cone as adsorbents for removal of water pollutants have been checked for phenol, methylene blue, and Cr(VI). The surface functional groups were reflected in the capacity of the carbons to adsorb different species from solution. The adsorption capacity of activated carbon prepared with phosphoric acid was more than that of activated carbon prepared using zinc chloride. In conclusion, the results indicated that the activated carbon made from pine cone had remarkable mesopore surface areas and notable adsorption capacities for phenol, methylene blue, and Cr(VI).
1. Introduction The availability of activated carbon for industrial use has much to do with accessing resources, renewing resources, and processing to rigid specifications to control specific industrial applications. In practice, coal and lignocellulosic materials are two main sources for the production of commercial activated carbons. Of particular interest is the preparation of activated carbons based on biomass from forestry and agricultural wastes. A large number of agricultural byproducts, such as coconut shells,1 palm-kernel shells, wood chips,2 sawdust,3 corn cobs,4 seeds,5 etc. have been successfully used in the preparation of activated carbon. The characteristics of activated carbons depend on the activation methods as well as the properties of the starting materials. There are activation methods in production of activated carbons: physical and chemical. The physical activation method involves carbonization of the raw material and the subsequent activation at high temperature in a carbon dioxide or steam atmosphere. The chemical activation method involves the carbonization of the raw material previously impregnated with a chemical agent such as zinc chloride, phosphoric acid, potassium hydroxide, etc. The advantage of chemical activation over physical activation is that it can be performed in only one step and at a relatively low temperature.6 In addition, chemical * To whom correspondence should be addressed. E-mail: jale.yanik@ ege.edu.tr. † Ege University. ‡ Inonu University. (1) Laine, J.; Yunes, S. Carbon 1992, 30 (4), 601–4. (2) Jagtoyen, M.; Derbyshire, F. Carbon 1993, 31 (7), 1185–92. (3) Ferraz, M. C. A. Fuel 1998, 67, 1237–41. (4) Tsai, W. T.; Chang, C.; Lee, S. L. Bioresour. Technol. 1998, 64, 211–17. (5) Gergova, K.; Petrov, N.; Eser, S. Carbon 1994, 32, 693–702. (6) Ahmadpour, A.; Do, D. D. Carbon 1997, 35 (12), 1723–32.
activation produces a much higher yield than the physical activation and leads to a decrease in the mineral matter content.7 In this work, the results obtained on the preparation of activated carbons from cone of Stone pine (Pinus pinea L.) are reported. Pinus pinea L. is an economically important tree in the Mediterranean area, mainly in Spain, Portugal, Italy, Greece, Albania, and Turkey and has a significant role in soil conservation, landscape architecture, and for its edible seeds.8 Its plantation has been highly promoted by European policy of the afforestation of croplands.9 Turkey has 35 000 ha of stone pine forest, and pine cones are widely used as a domestic fuel in forest villages or are retained in forest. In wastewater treatments, special attention is given to removal of dangerous inorganic material such as the heavy metals and resistant organic compounds. Wastes containing chromium are created in many branches of industry such as tannery, paint, ink, dye, and aluminum manufacturing industries, etc. Phenol and its derivatives such as methyl phenols, ethyl phenols, and dimethyl phenols constitute a group of pollutants that are invariably present in the effluents from industries engaged in the manufacture of a variety of chemical compounds such as plastics, dyes and plants used for coal gasification, and petrochemical units. Many of these compounds are carcinogenic, even when present in low concentrations. Thus, investigations relating to the removal of phenols from water have engaged the attention of a large number of investigators. Activated carbons, because of their large area and a high degree of surface reactivity, have a high efficiency for the removal of phenolic compounds from water. (7) Lillo-Rodenas, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2003, 41, 267–75. (8) Orda´s, R. J.; Alonso, P.; Cuesta, C.; Cortızo, M.; Rodrı´guez, A.; Ferna´ndez, B. Protocols for Micropropagation of Woody Trees and Fruits; Jain, S. M.,Ha¨ggman, H. Eds.; Springer: 2007; Ch. 4, 33-39. (9) Herrero, B.; Gutıe´rrez, J. Acta Bot. Croat. 2006, 65 (2), 117–25.
10.1021/ef800510m CCC: $40.75 2009 American Chemical Society Published on Web 02/27/2009
2198 Energy & Fuels, Vol. 23, 2009 Table 1. Properties of Pine Cone moisture ash volatile C H N S
proximate analysis, wt % (as received) 9.6 0.9 77.8
elemental analysis, wt % (on dry basis) 42.62 5.56 0.76 0.05
main constituents, wt % (on dry basis) lignin 24.9 hemicellulose 37.6 cellulose 32.7 extractives 4.8
Industrial effluents from dyes, textile, and pulp and paper industries are highly colored due to the presence of residual dyes. These dyes cause microtoxicity to aquatic life and slow down self-purification of streams by reducing light penetration. The color in water also generates public resentment. Therefore, stringent standards are being fixed by the regulating agencies for the removal of dyes before the effluent is discharged into rivers and lakes. The methods that have generally been used for the removal of dyes from wastewater are flocculation and coagulation using metallic compounds. However, these methods introduce metallic impurities and produce a large quantity of sludge, and the disposal of the sludge is another environmental problem. The sludge-free treatments are, therefore, gaining importance. Activated carbon adsorption is one such method that has a potential for the removal of dyes from wastewater. Consequently, a considerably amount of research has been carried out in this direction. The objective of the present work was to investigate the feasibility of producing activated carbons from the cone of Turkish stone pine by chemical activation and their ability to remove hexavalent chromium phenol and metylene blue from dilute aqueous solutions. Different preparation variables on the characteristics of activated products were studied to find the optimum conditions for making activated carbons with welldeveloped porosity. 2. Materials and Methods 2.1. Material. The cones of Pinus pinea L. supplied from a forest in the Izmir area were ground below 1 mm and air-dried for several days. Some properties and composition of the cone are given in Table 1. Although stone cone has very high volatile content, low ash content and a reasonable amount of carbon make it a suitable carbon precursor. 2.2. TG Analysis. Thermo gravimetric analysis of raw stone cone and impregnated stone cone was performed in a thermogravimetric (TG) analyzer (Perkin-Elmer Diamond TG/DTA) under a N2 atmosphere. Impregnated samples were prepared as follows. Cone was impregnated with ZnCl2 or H3PO4 solution by the wet method. After impregnation, the samples were dried overnight at 105 °C. The sample amount (particle size 150%) the microporosity decreases at a high rate. There are two competing mechanisms of pore evolution in the carbon structure.32 The first one is the micropore formation, which starts with the addition of chemicals. H3PO4 or ZnCl2 incorporation to the lignocellulosic structure of cone seems to be the cause of the creation of micropores, and the second one is the pore widening that is the result of chemical effects inside the opened pores; therefore, it starts acting when the chemical ratio is reasonably high. Similar trends in evolution of porosity have been described for other lignocellulosic materials. In the literature, the pore structure of activated carbons was also found to be related to the concentration of the impregnation agents. Several researchers reported that activated carbons obtained at low impregnation ratios were essentially microporous and that, as the amount of impregnation agent increases, the activated carbons become predominantly mesoporous.21,23,33-36 (29) Timur, S.; Kantarli, I. C.; Ikizoglu, E.; Yanik, J. Energy Fuels 2006, 20, 2636–41. (30) Caturla, F.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Carbon 1991, 29, 999–1007. (31) Ahmadpour, A.; Do, D. D. Carbon 1996, 34, 471–9. (32) Rodriguez-Reinoso, F.; Molina-Sabio, M. Carbon 1992, 30, 1111–9. (33) Girgis, B. S.; Ishak, M. F. Mater. Lett. 1999, 39, 107–14. (34) Molina-Sabio, M.; Rodriguez-Reinoso, F. Colloids Surface, A 2004, 241, 15–25. (35) Baquero, M. C.; Giraldo, L.; Moreno, J. C.; Suarez-Garcia, F.; Martinez-Alonso, A.; Tascon, J. M. D. J. Anal. Appl. Pyrol. 2003, 70, 779– 84.
ActiVated Carbon from Pine Cone
The surface areas of the activated carbons produced using phosphoric acid were somewhat lower than those produced using zinc chloride. Besides this, micropore volumes of activated carbons prepared by ZnCl2 are higher than that of activated carbon obtained using H3PO4. This observation is in agreement with the results of other studies.34,37,38 The comparison in the mechanisms for the activation by each chemical has been described by Molina-Sabio.34 They suggested that the small size of the ZnCl2 or its hydrates leds to the formation of small and uniform size of the micropores, whereas H3PO4 developed large mesopores and even macropores due to the presence of polyphosphoric acids, which occupy a wide range of pore sizes. Comparison may be made of the surface areas found for pine cone used in this study with more traditionally used lignocellulosic materials. Activated carbons having a surface area ranging between 1267 and 1070 m2 g-1 were produced from several kinds of nutshells by chemical activation with H3PO4 at 450 °C followed by activation with air at 300 °C.39 On the other hand, the activated carbon obtained from macadamia nutshell using ZnCl2 in a 1:1 impregnation ratio at 500 °C had a surface area of 1718 m2 g-1.21 In the preparation of activated carbon from lignin using ZnCl2 and H3PO4, with activation at a impregnation ratio of 1:1 and 600 °C carbonization temperature, the surface areas were around 1000 and 1300 m2 g-1, respectively.40 Generally, the activated carbons having the surface area ranging from 1032 to 1333 m2 g-1 have been obtained from several kinds of lignocellulosic precursors using H3PO4 at the impregnation ratios between 1 and 3 at the carbonization temperatures of 450-500 °C.33,36,41-43 3.3. Surface Chemistry. The adsorptive properties of activated carbon not only depend on its surface area and porous structure but also on its chemical composition. Activated carbons have considerable amounts of heteroatoms, and the presence of these atoms, especially oxygen, in surface groups has an appreciable effect on the adsorptive properties of the activated carbon, since they introduce active sites on the carbon surface. So, carbons having the same surface area but prepared by different methods may show different adsorption characteristics due to their surface chemistry. In this study, the amount of surface oxygen groups on the activated carbons having acidic and basic properties has been determined by Boehm titration method. The concentrations of acidic surface oxygen groups (carboxylic, lactonic, and phenolic) and basic surface oxygen groups (chromene and pyrone) of activated carbons having the highest surface area from different activating agents are shown in Table 3. The activated carbons obtained with 150 wt % ZnCl2 and 200 wt % H3PO4 were designated as A-Z and A-P, respectively. The results presented in Table 3 revealed that the predominant functions at the surface of the activated carbons are acidic, and activated carbon prepared using H3PO4 has the higher concen(36) Suarez-Garcia, F.; Martinez-Alonso, A.; Tascon, J. M. D. Carbon 2001, 39 (7), 1111–5. (37) Rodriguez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21, Ch. 1. (38) Guo, J.; Lua, C. J. Porous Mater. 2000, 7, 491–7. (39) Toles, C. A.; Marshall, W. E.; Johns, M. M. J. Chem. Technol. Biotechnol. 1998, 72, 255–63. (40) Hayashi, J.; Kazehaya, A.; Muroyama, K.; Watkinson, A. P. Carbon 2000, 38, 1873–8. (41) Girgis, B. S.; Yunis, S. S.; Soliman, A. M. Mater. Lett. 2002, 57, 164–72. (42) Ahmedna, M.; Marshall, W. E.; Rao, R. M. Bioresour. Technol. 2000, 71, 113–23. (43) Dastgheib, S. A.; Rockstraw, D. A. Carbon 2001, 39, 1849–55.
Energy & Fuels, Vol. 23, 2009 2201 Table 3. Amounts of Various Oxygen-containing Functional Groups of Activated Carbons (mmol g-1)
A-P A-Z
phenolic groups
lactones
carboxylic groups
total acidic groups
total basic groups
1.01 0.18
0.06 0.29
1.49 0.42
2.56 0.89
1.74 0.58
tration of surface groups than the activated carbon obtained by ZnCl2 activation. In addition to Boehm titration, FTIR spectra of activated carbons were taken in order to qualitatively evaluate the chemical structure of activated carbons (Figure 2). However, it was not easy to get good spectra because carbons are black materials that absorbed almost all of the radiation in the visible spectrum, and the peaks obtained were usually a sum of the interactions of different types of groups. Similar shapes of the spectra for both samples was obtained. The strong band appears at 1384 cm-1 and is due to carboxylic and/or carboxylate groups. The band at 3400 cm-1 was assigned to -OH in phenolic and carboxylic groups. The broad band around 1000 cm-1 is usually assigned to CsO stretching vibrations in ethers and alcohols. The bonds at 1560 and 1695 cm-1 show the presence of quinones and lactones. 3.4. Adsorption Results. The potential application of activated carbons obtained from pine cone as adsorbents for removal of water pollutants have been checked for three target species chosen as representative of toxic organic (phenol), dye, and inorganic [Cr(VI)] contaminants. Because the activated carbons with highest surface area had no microporosity, adsorption tests were performed with the activated carbons obtained with 150 wt % ZnCl2 and H3PO4, which were designated as A-Z and A-P, respectively. 3.4.1. Phenol. Figure 3 shows the adsorption isotherms obtained for phenol at 25 °C with the activated carbons prepared from pine cone. For practical operation, correlation of the isotherms using a theoretical or empirical equation is desired. Also, the parameters in Freundlich and Langmuir equations are very useful for predicting adsorption capacities and also for incorporating into mass transfer relationships in the design of contacting equipment. The Langmuir and Freundlich models were used to fit the adsorption isotherms and to evaluate the isotherm parameters. The differences observed on the phenol adsorption capacities of A-P and A-Z can not be explained from their different BET surface area and micropore volume. In fact, the A-Z sample, which presents the higher surface area and micropore volume than the A-P, yields a considerable lower capacity. It seems that the phenol adsorption is highly related to the oxygencontaining surface functional groups. The influence of basic surface oxygen groups on the adsorption of phenol by activated carbons as already pointed out by some researcher.44,45 As a conclusion, due to the higher basic functional groups, the activated carbon A-P showed higher phenol adsorption. Results from the Langmuir and Freundlich analyses of the adsorption of phenol on the activated carbons are reported in Table 4. Langmuir isotherm theory is based on the assumption of monolayer adsorption onto a surface containing finite number of adsorption sites of uniform energies of adsorption with no transmigration of adsorbate in the pores of the adsorbent surface. On the other hand, the Freundlich isotherm describes adsorption where the adsorbate has a heterogeneous surface with adsorption (44) Gonzalez-Serranoa, E.; Corderoa, T.; Rodriguez-Mirasola, J.; Cotorueloa, L.; Rodriguezb, J. J. Water Res. 2004, 38, 3043–50. (45) Moreno-Castilla, C.; Rivera-Utrilla, J.; Lopez-Ramon, M. V.; Carrasco-Marin, F. Carbon 1995, 35, 845–51.
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Figure 2. IR spectra of activated carbons.
sites that have different energies of adsorption. For the activated carbon A-Z, the experimental data was well fitted to both the Freundlich and Langmuir isotherms. In the case of A-P, the Freundlich isotherm fits the adsorption data slightly better than the Langmuir model. The values of 1/n are also found to be less than 1, which suggests favorable adsorption behavior of phenol onto activated carbons.
The adsorption capacity of A-P for phenol is comparable to that of commercial activated carbons or activated carbons derived from biomass. Phenol adsorption capacity values were 238 mg g-1 for activated carbon from kraft lignin,46 137 mg g-1 for commercial activated carbon,46 94 mg g-1 for activated (46) Fierro, V.; Torne’-Fernandez, V.; Montane, D.; Celzard, A. Microporous Mesoporous Mater. 2008, 111, 276–84.
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Figure 6. Adsorption isotherm of Cr(VI) onto activated carbons.
Figure 3. Adsorption isotherm of phenol onto activated carbons.
carbon is dependent not only on the BET surface area but also on the interaction of positive ions with surface functional groups of activated carbons. Comparison may be made of the adsorption capacity found for activated carbons prepared from pine cone with the activated carbons prepared from other wastes. Methylene blue adsorption capacity values were 556 mg g-1 for the activated carbon prepared from pinewood,48 225.64 mg g-1 for activated carbon from jute fiber,49 42.04 mg g-1 for activated carbon from activated sewage sludge,50 200 mg g-1 for activated carbon from buffing dust,51 333 mg g-1 for activated carbon from oreganum stalk,29 200 mg g-1 for activated carbon from Guava Seeds,52 and 130 mg g-1 for activated desert plant.53 3.4.3. Cr(VI). In the adsorption of Cr(VI) on activated carbon, two mechanism, namely, ion exchange mechanism and oxidization mechanism, can simultaneously occur. These two mechanism, according to Liu et al, are shown in the following diagram.54
Figure 4. Adsorption isotherm of methylene blue onto activated carbons.
Figure 5. Effect of pH on percent removal of Cr(VI) by A-P.
carbon from oreganum stalk,29 264 mg g-1 for activated carbon (Sigma-Aldrich),47 and 240 mg g-1 for activated carbon from pinewood.48 3.4.2. Methylene Blue. Figure 4 shows the adsorption isotherms obtained for methylene blue with the activated carbons prepared from pine cone. The equilibrium data obtained for methylene blue indicate that both A-Z and A-P have identical adsorption capacity, although the former has a larger surface area. This may be explained by the fact that methylene blue is a cationic dye and dissociates in aqueous solution. These ions are adsorbed on the activated carbon by both physical and chemical mechanisms. Hence, adsorption capacity of activated (47) Roostaei, N.; Tezel, F. H. J. EnViron. Manage. 2004, 70, 157–64. (48) Tseng, R. L.; Wu, F. C.; Juang, R. S. Carbon 2003, 41, 487–95.
Some researcher reported that the adsorption mechanism seems to be mainly related to the reduction of Cr(VI) and the adsorption of Cr(III), and the decrease of pH leads to an increase of Cr(III) formation.55,56 However, Liu et al. stated that the ion exchange mechanism was dominant for the removal of Cr(VI) at low concentrations.54 On the other hand, the pH value of the solution is an important controlling parameter in the Cr(VI) adsorption process. At pH ranging from 1 to 14, there are four soluble Cr(VI) species, which are CrO24-, Cr2O27-, HCrO4-, and H2CrO4. From pH 1 to pH 6, most chromium species exist in solution in the form HCrO4-, whereas from pH 6 to pH 10, (49) Senthilkumaar, S.; Varadarajan, P. R.; Porkodi, K.; Subbhuraam, C. V. J. Colloid Interface Sci. 2005, 284 (1), 78–82. (50) Otero, M.; Rozada, F.; Calvo, L. F.; Garcia, A. I.; Moran, A. Dyes Pigments 2003, 57, 55–65. (51) Yilmaz, O.; Kantarli, I. C.; Yuksel, M.; Saglam, M. Yanik. J. Resour., ConserV. Recycl. 2007, 49, 436–48. (52) Rahman, I. A.; Saad, B. Malays. J. Chem. 2003, 5 (1), 8–14. (53) Bestani, B.; Benderdouche, N.; Benstaali, B.; Belhakem, M.; Addou, A. Bioresour. Technol. 2008, 99, 8441–4. (54) Liu, S. X.; Chen, X.; Chen, X. Y.; Liu, Z. F.; Wang, H. L. J. Hazard. Mater. 2007, 141, 315–19. (55) Di Natale, F.; Lancia, A.; Molino, A.; Musmarra, D. J. of Hazard. Mater. 2007, 145, 381–90. (56) Gardea-Torresdey, J. L.; Tiemann, K. J.; Armendariz, V.; BessOberto, L.; Chianelli, R. R.; Rios, J.; Parsons, J. G.; Gamez, G. J. Hazard. Mater. B 2000, 80, 175–88.
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Table 4. The Freundlich and the Langmuir Isotherm Constants for Phenol, Methylene Blue and Cr(VI) Adsorption adsorbates phenol MB Cr(VI)
Langmuir constants
Freundlich constants
activated carbons
SM (mg g-1)
KL (L mg-1)
R2
1/n
KF
R2
A-Z A-P A-Z A-P A-Z A-P
117.65 500.00 370.37 370.37 66.67 81.30
0.033 0.048 1.500 2.250 0.206 0.459
0.97 0.95 0.99 0.99 0.99 0.98
0.359 0.819 0.094 0.092 0.311 0.298
16.29 4.08 261.76 271.89 19.74 30.388
0.99 0.99 0.98 0.88 0.99 0.98
the CrO24- species become predominate in solution.57 As can be seen from Figures 5 and 6, the adsorption capacity of Cr(VI) onto activated carbon decreases significantly with increasing pH. Similar behavior has also been reported by other researchers.55,58 It is seen that pH 2.0 is the optimal pH for the Cr(VI) uptake for both activated carbons. The variation of adsorption of chromium ions can be explained by taking into account the surface charge of the carbon and the existing forms of chromium species at different pH values. Under acidic conditions, the surface of the activated carbons becomes highly protonated and favors the uptake of Cr(VI) in the anionic form. The equilibrium adsorption measurement of Cr(VI) has been carried out at pH 2.0. The equilibrium data fits both the Langmuir and Freundlich equations well for two activated carbons (Table 4). It is clearly seen that the adsorption capacity of A-P for Cr(VI) was significantly higher than that of A-Z. The reason for this is the fact that Cr(VI) adsorption is also related to the acidic surface functional groups besides pH.54,59,60 Due to the higher acidic functional groups, Cr(VI) was more adsorbed on A-P. Moreover, the obtained activated carbons obtained from pine cone has noticeable Cr(VI) adsorption capacity. Comparatively, the maximum adsorption capacity of commercial activated carbons were 101.4 mg g-1, 69.3 mg g-1,61 of commercial coconut activated carbon 7.6138 mg g-1,54 of activated carbon based waste tire 48.0 mg g-1,62 of sawdust acvtivated carbon 59.2 mg g-163 and of activated charcoal 12.87 mg g-1.58 4. Conclusions In this study, the activated carbons were prepared from cones of Stone pine by the chemical activation method. Phosphoric (57) Yang, L.; Chen, J. P. Bioresour. Technol. 2008, 99, 297–307. (58) Mor, S.; Ravindra, K.; Bishnoi, N. R. Bioresour. Technol. 2007, 98, 954–57.
acid and zinc chloride were used as the activating agent. Activated carbons having a surface area ranging between 819 and 1816 m2 g-1 were obtained by chemical activation with ZnCl2 at the impregnation ratios between 50 and 200 wt %, while ranging between 1148 and 1597 m2 g-1 with H3PO4. As surface areas of activated carbons increased with an increase in the concentration of activation agent, the activated carbons become predominantly mesoporous. The micropore volume of activated carbon prepared by ZnCl2 is higher than that of activated carbon obtained using H3PO4. Although FTIR spectrums of each activated carbon are quite similar, activated carbon prepared using H3PO4 has the higher concentration of surface groups than the activated carbon obtained by ZnCl2 activation. The aqueous adsorption tests showed that the activated carbons from Stone pine had a notable adsorption capacity for Cr(VI), methylene blue, and phenol. For Cr(VI) and phenol, the adsorption capacity of activated carbon prepared with H3PO4. was larger than that of activated carbon obtained using ZnCl2. On the basis of the results obtained from aqueous adsorption, it has been concluded that the role of carbon surface chemistry is also important as well as surface area and porosity. In conclusion, the present investigation showed that “cone of pine”, which is the product from widely planted tree in the Mediterranean area, can be effectively used as a raw material for the preparation of activated carbon by chemical activation. EF800510M (59) Babel, S.; Kurniawan, T. A. Chemosphere 2004, 54, 951–96. (60) Park, S. J.; Jang, Y. S. J. Colloid Interface Sci. 2002, 249, 458– 63. (61) Hu, Z.; Lei, L.; Li, Y.; Ni, Y. Sep. Purif. Technol. 2003, 31, 13– 18. (62) Hamadi, N. K.; Chen, X. D.; Farid, M. M.; Lu, M. G. Q. Chem. Eng. J. 2001, 84, 95–105. (63) Karthikeyan, T.; Rajgopal, S.; Miranda, L. R. J. Hazard. Mater. 2005, 124 (1-3), 192–9.
