Synthesis of Magnetically Separable Porous Carbon Microspheres

May 14, 2008 - Magnetic measurements were carried out on a SQUID magnetometer (MPMS XL-7, Quantum. Design) at room temperature. A 57Co/Rh source ...
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J. Phys. Chem. C 2008, 112, 8623–8628

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Synthesis of Magnetically Separable Porous Carbon Microspheres and Their Adsorption Properties of Phenol and Nitrobenzene from Aqueous Solution Yufang Zhu,*,† Lingxia Zhang,‡ Falko M. Schappacher,§ Rainer Po¨ttgen,§ Jianlin Shi,‡ and Stefan Kaskel*,† Institut fu¨r Anorganische Chemie, Technische UniVersita¨t Dresden, Mommsenstrasse 6, Dresden, 01069, Germany, State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-xi Road, Shanghai 200050, People’s Republic of China, and Institut fu¨r Anorganische and Analytische Chemie, UniVersita¨t Mu¨nster, Corrensstrasse 30, 48149 Mu¨nster, Germany ReceiVed: February 5, 2008; ReVised Manuscript ReceiVed: March 14, 2008

Magnetic porous carbon microspheres have been synthesized by a carbonization process of chitosan microspheres containing iron precursors. The structure and morphology of the magnetic porous carbon microspheres were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), 57Fe Mo¨ssbauer spectroscopy, and N2 adsorption-desorption technique. Using phenol and nitrobenzene as model pollutants, the magnetic porous carbon microspheres showed good adsorption capacities of 35 and 97 mg/g, respectively, and they can be regenerated after release in ethanol. During the adsorption and desorption cycles, the porous carbon microspheres were easily separated by an external magnetic field. About 80 and 84% adsorption capacity for phenol and nitrobenzene can be retained after five consecutive operations. Therefore, these magnetic porous carbon microspheres could potentially be applied in separation processes. Introduction Phenolic derivatives are widely used as intermediates in the synthesis of plastics, colors, pesticides, insecticides, etc. Degradation of these substances causes the release of phenol and its derivatives in the environment, which results in serious environmental problems because of their toxicity, poor biodegradability, and accumulation potential in plants and animal tissues.1–3 Many methods have been developed to remove phenol and its derivatives, such as biological treatment,2 activated carbon adsorption, ion exchange,4 solvent extraction,5 and so on. Among these techniques, adsorption on activated carbon is the most effective and frequently used method due to its high surface area and fast adsorption kinetics. There has been an increasingly large amount of the literature devoted to the study of adsorption for the removal of aqueous-organic species using activated carbon.6–15 However, the small particle sizes of activated carbon often cause difficulties when trying to separate activated carbon in liquid-solid phase processes. At present, magnetic separation technology has been paid more and more attention due to the fact that it can be easily separated under an applied magnetic field. Recently, a series of magnetic composite materials have been successfully synthesized and used in many applications. For example, Pich and Huang reported immobilized laccase based on magnetic PS particles and chitosan microspheres.16,17 Lu demonstrated a kind of magnetically separable hydrogenation catalyst by using Co-containing mesoporous carbon.18 These examples showed excellent magnetic separation behavior. * Corresponding authors. Phone: +49-351-46334710(Y.Z.); +49-35146334885(S.K.). Fax: +49-351-46337287 (Y.Z., S.K.). E-mail: yufang.zhu@ chemie.tu-dresden.de (Y.Z.); [email protected] (S.K.). † Technische Universita ¨ t Dresden. ‡ Chinese Academy of Sciences. § Universita ¨ t Mu¨nster.

Figure 1. XRD patterns (Cu KR1 radiation) of magnetic porous carbon microspheres carbonized at different temperatures.

Chitosan, poly[β-(1-4)-linked-2-amino-2-deoxy-D-glucose], is the N-deacetylated product of chitin, which is a major component of arthropod and crustacean shells such as lobsters, crabs, shrimp, and cuttlefish.17 Due to the good availability of free amino groups in chitosan, it carries a positive charge and thus in turn reacts with many negatively charged species, which make it easy to prepare chitosan composite microspheres. Therefore, carbon composite microspheres also can be easily obtained after the carbonization process of the chitosan composite microspheres. Herein, we propose a strategy to synthesize magnetic porous carbon microspheres by using a simple carbonization process of chitosan microspheres adsorbing the negatively charged [Fe(C2O4)3]3-. These carbon microspheres combined the advantages of high surface area and good magnetic separability. Using phenol and nitrobenzene as model pollutants, magnetic porous carbon microspheres show good adsorption property, and the adsorbed phenol and nitrobenzene can be effectively released

10.1021/jp8010684 CCC: $40.75  2008 American Chemical Society Published on Web 05/14/2008

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Zhu et al. was added to the suspension, and the mixture was stirred for another 1 h at 40 °C. Then 6.0 mL of NaOH solution (1.0 M) was gradually added to the mixture, and the stirring was continued for another 2 h at 70 °C. Finally, the mixture of the products was filtered and washed with acetone and distilled water several times. The chitosan microspheres (CS) were obtained after drying for 12 h at 80 °C.

Figure 2. Experimental and simulated 57Fe Mo¨ssbauer spectra of the sample of CSFe-800 microspheres at 77 and 298 K.

TABLE 1: Fitting Parameters of 57Fe Mo¨ssbauer Spectroscopic Measurements of the Sample of CSFe-800a compound

δ/mm s-1

γ-Fe2O3 R-Fe Fe3C

298 K 0.18(1) -0.11(1) 0.07(1)

BHf/T

Γ/mm s-1

49.1(1) 33.2(1) 21.0(1)

0.35b 0.29(1) 0.35(1)

51.5(1) 33.6(1) 24.5(1)

0.35b 0.33(3) 0.39(1)

77 K γ-Fe2O3 R-Fe Fe3C

0.33(2) 0.01(1) 0.20(1)

a Numbers in parentheses represent the statistical errors in the last digit. δ, isomer shift; Γ, experimental line width; BHf, hyperfine field. b Parameter fixed during the least-squares fits.

TABLE 2: Structure Parameters of the Products Carbonized at Different Temperatures CSFe-1000 CSFe-900 CSFe-800 CSFe-700 CS-1000 SBET (m2/g) Vp (cm3/g)

286.1 0.284

261.6 0.260

247.1 0.238

226.2 0.196

23.7 0.002

in ethanol. Furthermore, the porous carbon microspheres can be easily separated by a magnet during the adsorption and desorption processes. Experimental Section Preparation of Chitosan (CS) Microspheres. Chitosan microspheres were synthesized using the reported method.19 A typical procedure was performed as follows: A 2.0 wt % solution of chitosan in 5% aqueous acetic acid (20 mL) was poured into 50 mL of toluene containing 1.0 mL of the emulsifier (Span80) at room temperature. During the process, the suspension was stirred for 30 min. A 3.0 mL volume of 25% glutaraldehyde

Preparation of Magnetic Porous Carbon Microspheres. Chitosan is a cationic polymer and has hydrophilic and swelling properties, which makes it possible to adsorb the anionic iron salts. For the preparation of magnetic porous carbon microspheres, 1.0 g of dried chitosan microspheres was immersed in 10 mL of (NH4)3[Fe(C2O4)3] solution (0.1 g/mL) for 24 h. Then chitosan microspheres were filtered and washed with distilled water. After drying for 12 h at 60 °C, the sample was carbonized under an Ar atmosphere at a certain temperature (700-1000 °C) for 4 h in steps of 3 °C/min. The magnetic porous carbon microsphere composites were obtained after cooling. The samples were named as CSFe-t (t is the carbonization temperature). Adsorption and Desorption Tests. A certain amount of dried magnetic porous carbon microspheres (CSFe-1000) and 30 mL of 0.1 mg/mL phenol solution (or 0.05 mg/mL nitrobenzene solution) were mixed, followed by stirring at room temperature for 10 h. The magnetic porous carbon microspheres with adsorbed phenol (or nitrobenzene) were separated by a magnet, and the concentration of the solution was analyzed by UV adsorption at a wavelength of 270 nm (268 nm for nitrobenzene). On the other hand, the separated magnetic porous carbon microspheres were immersed in 50 mL of ethanol at room temperature under stirring. The release medium (3 mL) solution was removed for analysis by UV absorption at given time intervals and replaced with the same volume of ethanol. For the recycling experiment, the CSFe-1000 microspheres were attracted on the bottom of the bottle by using a magnet after adsorption of phenol and nitrobenzene. The pollutant solution was removed and ethanol was added to the bottle. The release of pollutants from the CSFe-1000 microspheres in ethanol lasted for 12 h, followed by removal of the ethanol by using a magnet to attract CSFe-1000 microspheres on the bottom of the bottle. When the release operation was repeated three times, these CSFe-1000 microspheres were added in the phenol or nitrobenzene solution under stirring for adsorption. Characterization. The powder X-ray diffraction (XRD) patterns were obtained on a Stoe Stadi P powder diffractometer equipped with a curved germanium (111) monochromator and linear PSD using Cu KR1 radiation (1.5405 Å) in transmission geometry. Scanning electron microscopy (SEM) was carried out on a Zeiss DMS 982 Gemini field emission scanning electron microscope at 4.0 kV. The UV/vis absorbance spectra were measured using a Shimadzu UV-1650PC spectrophotometer. N2 adsorption-desorption isotherms were obtained on a Nova 2000 pore analyzer at 77 K under continuous adsorption conditions. BET and BJH analyses were used to determine the surface area, pore size, and pore volume. Magnetic measurements were carried out on a SQUID magnetometer (MPMS XL-7, Quantum Design) at room temperature. A 57Co/Rh source was available for the 57Fe Mo¨ssbauer spectroscopic investigations. The sample was placed within a Plexiglas container at a thickness of about 10 mg of Fe/cm2. The measurements were conducted in the usual transmission geometry at 77 and 298 K. The total counting time was 5 days per spectrum.

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Figure 3. N2 adsorption-desorption isotherms and corresponding pore size distributions of magnetic porous carbon microspheres carbonized at different temperatures.

Figure 4. SEM images of samples of CS (a, b) and CSFe-1000 (c, d) microspheres.

Figure 5. (a) TEM image of the CSFe-1000 microspheres; (b) EDX spectrum taken from the particle in (a).

Results and Discussion Characterization of Magnetic Porous Carbon Microspheres. The crystalline structures of the products carbonized at different temperatures were characterized by powder X-ray diffraction (XRD). As shown in Figure 1, the peaks at 2θ ) 26.2° in all patterns can be attributed to the diffraction of the (002) plane of the graphite structure, which is due to the

carbonization of chitosan in the presence of iron giving carbon structures with some degree of graphitic order.20 Besides graphite, the pattern also includes γ-Fe2O3, Fe3C, and R-Fe according to the reflection peak positions and relative intensities at the carbonization temperature of 700 °C. With the increase of the carbonization temperature, the intensity of γ-Fe2O3 decreases, while the intensities of Fe3C and R-Fe increase. When

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Figure 6. (a) Magnetization curve for CSFe-1000 microspheres measured at room temperature; (b) photographs of CSFe-1000 microspheres in polluted water under an external magnetic field.

the carbonization temperature reaches 1000 °C, the pattern only shows graphite, Fe3C, and R-Fe. Because the diffraction peaks of γ-Fe2O3 and Fe3O4 are quite similar, 57Fe Mo¨ssbauer spectroscopy is further used to analyze the sample. Figure 2 shows the 57Fe Mo¨ssbauer spectra of the sample of CSFe-800 recorded at 77 and 298 K together with transmission integral fits. The corresponding fitting parameters are listed in Table 1. The spectra could be well reproduced assuming three signals, which could be attributed to γ-Fe2O3, R-Fe, and cementite, Fe3C. The experimental data (isomer shifts, line widths, and hyperfine fields) are in good agreement with the literature.21–27 Especially the magnetic hyperfine fields of R-Fe and cementite are close to the reference data. The 57Fe Mo¨ssbauer spectra allowed distinguishing for the iron oxide. In the upper part of Figure 2, we present simulations of the γ-Fe2O3 and Fe3O4 (magnetite, data taken from ref 26) spectra. Since magnetite has two iron substructures, the spectrum is composed of two spectral components which can be attributed to Fe2+ and Fe3+. Based on the 57Fe Mo¨ssbauer spectra, this possibility can be ruled out for our sample. Therefore, the sample of CSFe-800 contains γ-Fe2O3 and no Fe3O4. Hirano reported the synthesis of magnetite dispersed carbon by pressure pyrolysis of divinylbenzene-vinylferrocene.28 Here it also can be explained that the pyrolysis of chitosan promoted the reduction of some iron precursors to maghemite at high temperature under an Ar atmosphere. When the carbonization temperature increases, it facilitates the formation of Fe3C through the reaction of maghemite with carbon and the further decomposition of Fe3C into metallic Fe.29 Therefore, the iron compound containing carbon composites can be obtained by carbonizing chitosan microspheres containing iron precursors. Figure 3shows the N2 adsorption-desorption isotherms and corresponding pore size distributions of the carbonized products, and the data of the surface area and pore volume are listed in Table 2. The data for the carbonized products exhibit a type IV isotherm, typical of a mesoporous structure.30 It can be observed that the N2 adsorption amounts, the surface area, and pore volume increase with the increase of the carbonization temper-

Zhu et al. ature. At the carbonization temperature of 1000 °C, the surface area of the product reached 286.1 m2/g as calculated from the linear part of the BET plot. The single point adsorption total volume at P/P0 ) 0.896 was 0.284 cm3/g. The pore size distribution profile indicates that most of the nanopores are less than 11 nm in all samples. The hysteresis loops at P/P0 between 0.4 and 0.9 are the characteristics of an H4-type adsorption. This phenomenon may be attributed to the fact that the carbonized products contain interconnected macropores and mesopores.31,32 Therefore, these carbonized products have a porous structure. If only chitosan microspheres were carbonized at 1000 °C, the carbonized products had low surface area and were nonporous. Therefore, the presence of both mesopores and macropores in the porous carbon microspheres should have resulted from the cracking and the expansion of gases, such as H2O and CO2 that are formed through the carbonization of chitosan and the reduction process of iron precursors at high temperature under an Ar atmosphere.29,33 Figure 4 shows the scanning electron microscropy (SEM) images of the samples of CS and CSFe-1000. The magnetic porous carbon composites inherited the spherical morphology of the chitosan microspheres after the carbonization process. The sizes of the magnetic porous carbon microspheres decreased due to shrinkage during the carbonization process. Some iron compound particles are distributed on the surface of the microspheres because iron salts tend to migrate out of the pores and aggregate outside during the heat treatment.34 Interestingly, there exist a large number of macropores on the surface of the porous carbon microsphere composites, which is consistent with the results obtained from the N2 adsorption measurements. Figure 5a shows the transmission electron microscopy (TEM) image of CSFe-1000. It can be observed that fewer particles are located on the edge of the microspheres, but more particles are distributed inside the microspheres. The energy-dispersive X-ray (EDX) spectrum (Figure 5b) (the particle is indicated with an arrow in Figure 5a) gives proof of the existence of Fe and C. Therefore, most of the iron compound particles are entrapped in carbon microspheres. The sizes of most of the iron compound particles range from 20 to 80 nm. However, some aggregates on the surface of the microspheres can reach to 250 nm (Figure 4d). The magnetization curve measured at room temperature for CSFe-1000 is shown in Figure 6a. A large magnetic hysteresis loop can be observed, and the saturation magnetization value is about 13.9 emu/g. This indicates that the porous carbon microspheres have ferromagnetic properties and their movement in liquid media can be controlled by magnetic fields. The magnetic separation property of the porous carbon microspheres was measured in phenol and nitrobenzene aqueous solutions by placing a magnet near the glass bottle. Figure 6b shows the photographs of CSFe-1000 microspheres separated from the solution under an external magnetic field. It can be found that the magnetic porous carbon microspheres were quickly attracted toward the magnet within seconds. Therefore, this provides an easy and efficient way to separate the magnetic porous carbon microspheres from phenol and nitrobenzene aqueous solutions. Phenol and Nitrobenzene Adsorption on CSFe-1000 Microspheres. Here we chose CSFe-1000 as the sorbent to test the adsorption capacity of phenol and nitrobenzene since CSFe1000 has the highest surface area and the largest pore volume among these samples. To determine the adsorption capacities of the CSFe-1000 microspheres for phenol and nitrobenzene, equilibrium adsorption experiments were introduced. Figure 7 shows the equilibrium adsorption curves of phenol and ni-

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Figure 7. Equilibrium adsorption isotherms of CSFe-1000 microspheres for (a) phenol and (b) nitrobenzene.

Figure 10. Relative recycling adsorption capacities of CSFe-1000 microspheres for phenol and nitrobenzene in water at room temperature after different cycles. Figure 8. Adsorption amounts of phenol and nitrobenzene on CSFe1000 microspheres as a function of time.

Figure 9. Release curves of phenol and nitrobenzene from CSFe-1000 microspheres in ethanol.

trobenzene on CSFe-1000 microspheres at room temperature. The adsorption capacities for phenol and nitrobenzene increase with increasing equilibrium concentrations. Up to a certain equilibrium concentration, no more phenol or nitrobenzene can be adsorbed on the CSFe-1000 microspheres. Therefore, it can be estimated that the adsorption capacities of the CSFe-1000 microspheres for phenol and nitrobenzene are about 35 and 97 mg/g, respectively. This indicates that the CSFe-1000 microspheres have good adsorption capacities for pollutants of phenol and nitrobenzene, which is attributed to the high surface area and the porous structure. Figure 8 illustrates the adsorption amounts of phenol and nitrobenzene on the CSFe-1000 microspheres as a function of

time. It can be seen that the adsorption amounts of phenol and nitrobenzene on the CSFe-1000 microspheres increase quickly at the initial stage and then increase further but slowly afterward. Equilibrium adsorption is achieved for both phenol and nitrobenzene after approximately 10 h. It is interesting that the adsorbed phenol and nitrobenzene can be released from the magnetic porous carbon microspheres in ethanol. Figure 9 shows the release curves of phenol and nitrobenzene from the CSFe-1000 microspheres in ethanol. It can be found that burst releases were obtained within 1 h from CSFe-1000 microspheres, and then a steadily decreasing release followed. After 12 h, the released amounts of phenol and nitrobenzene reached 89 and 92% of the adsorbed amounts, respectively. Therefore, these magnetic porous carbon microspheres exhibit good regeneration ability. Because the magnetic porous carbon microspheres have the advantages of magnetic separation properties and good regeneration ability, it is possible to remove pollutants in water by adsorption and desorption cycles. Figure 10 shows the recycling of CSFe-1000 microspheres for the adsorption of phenol and nitrobenzene in water at room temperature. The magnetic porous carbon microspheres retained about 80 and 84% adsorption capacities for phenol and nitrobenzene, respectively, after five consecutive operations. The result indicates that the magnetic porous carbon microspheres have a good reusability. Conclusion In summary, we have synthesized magnetically separable porous carbon microspheres after the carbonization process of chitosan microspheres containing iron precursors. The magnetic porous carbon microspheres have a high surface area and a porous structure. Furthermore, the magnetic porous carbon

8628 J. Phys. Chem. C, Vol. 112, No. 23, 2008 microspheres can adsorb phenol and nitrobenzene from aqueous solution with relatively high capacities of 35 and 97 mg/g, respectively. The adsorbed phenol and nitrobenzene can also be released in ethanol at room temperature. In addition, the magnetic porous carbon microspheres can retain about 80% and 84% adsorption capacities for phenol and nitrobenzene, respectively, after five consecutive operations. More importantly, the magnetic porous carbon microspheres were easily separated by an external magnetic field during the adsorption and desorption cycles. Acknowledgment. We gratefully acknowledge the support of this research by the Alexander von Humboldt Foundation, the National Nature Science Foundation of China (No. 50702072), and the Deutsche Forschungsgemeinschaft. References and Notes (1) Dabrowski, A.; Podkoscielny, P.; Hubicki, M.; Barczak, M. Chemosphere 2005, 58, 1049. (2) Aksu, Z.; Yener, J. Waste Manage 2001, 21, 695. (3) Fang, H. H.; Chen, O. Waste Res. 1997, 31, 2229. (4) Kojima, T.; Nishijima, K.; Matsukata, M. J. Membr. Sci. 1995, 102, 43. (5) Kujawski, W.; Warszawski, A.; Ratajczak, W.; Porebski, T.; Capala, W.; Ostrowska, I. Desalination 2004, 163, 287. (6) Rege, S. U.; Yang, R. T.; Cain, C. A. Separations 1998, 44, 1519. (7) Rengaraj, S.; Moon, S.-H.; Sivabalan, R.; Arabindoo, B.; Murugesan, V. Waste Manage. 2002, 22, 543. (8) Nadia, R.; Tezel, F. H. J. EnViron. Manage. 2004, 70, 157. ¨ zkaya, B. J. Hazard. Mater. B 2006, 129, 158. (9) O (10) Dursun, G.; Cicek, H.; Dursun, A. Y. J. Hazard. Mater. B 2005, 125, 175. (11) Sabio, E.; Gonza´lez, E.; Gonza´lez, J. F.; Gonza´lez-Garcı´a, C. M.; Ramiro, A.; Ganan, J. Carbon 2004, 42, 2285. (12) Tseng, R.-L.; Wu, F.-C.; Juang, R.-S. Carbon 2003, 41, 487. ¨ .; Cecen, F. J. Chem. Technol. Biotechnol. 2006, 81, 94. (13) Aktas, O

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