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Fe@Fe2O3 Core-Shell Nanowires as the Iron Reagent. 2. An Efficient and Reusable Sono-Fenton System Working at Neutral pH Zhihui Ai,† Lirong Lu,† Jinpo Li,† Lizhi Zhang,*,† Jianrong Qiu,‡ and Minghu Wu§ Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, National Coal Combustion Laboratory, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China, and Department of Biology and Chemistry, Xianning College, Xianning 437100, People’s Republic of China ReceiVed: January 17, 2007; In Final Form: February 22, 2007
In this study, we developed an efficient and reusable sono-Fenton system with Fe@Fe2O3 core-shell nanowires as the iron reagent. Similar to our previous sono-Fenton system working at pH ) 2, this system could also much more effectively degrade rhodamine B than those with Fe2+, Fe2+/Fe2O3, or commercial zerovalent iron particles as the iron reagents at neutral pH. The nature of the Fe@Fe2O3 nanowire Fenton iron reagent was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). It was found that this novel Fenton iron reagent could be recycled in this sono-Fenton system working at neutral pH. More importantly, the efficiency of this neutral-pH sono-Fenton system with adding 0.001 mol L-1 of Fe2+ even reached 92% for degrading RhB at neutral pH. This efficiency was close to that of our previous sono-Fenton system working at pH ) 2. We proposed a possible mechanism for the sono-Fenton degradation of RhB over Fe@Fe2O3 core-shell nanowires at neutral pH, which involved an in situ recycling of iron species (Fe0 f Fen+ f Fe2O3). We believe that this economical and facile sono-Fenton system based on an Fe@Fe2O3 core-shell nanowire iron reagent may be applied to practical wastewater treatment.
Introduction Since the 1990s, Fenton reactions have been widely utilized to degrade organic compounds in industrial wastewater. Their effectiveness results from the generated hydroxyl radicals (•OH) that are highly reactive and nonselective to decompose various organic compounds. However, pH is a critical operating parameter in wastewater treatment by using the traditional Fenton process because these Fenton systems can only work efficiently under highly acidic conditions (pH ) 2-3).1 In most cases, strong acids have to be added into wastewater to meet this pH requirement before the application of traditional Fenton reactions. Meanwhile, Fe2+ (or Fe3+) ions in traditional Fenton systems cannot be recycled, which brings a new contamination. These drawbacks limit further application of the traditional Fenton process in wastewater treatment. Therefore, the development of active heterogeneous systems to enable the Fenton reaction to operate at near-neutral pH is of great significance because it could offer advantages including not needing acid to adjust the pH value of the wastewater, no sludge generation, and the possibility of recycling the iron reagent.2 Many efforts have been made in developing new techniques to broaden the working pH range of the Fenton reactions toward neutral pH.3-10 Recently, Nafion membrane-supported or -immobilized Fenton catalysts were used to extend the working range of the Fenton reaction to a 2-10 pH range effectively.4,11-15 However, the Nafion membranes are too expensive to use on a large scale. Hence, researchers turn to explore new heteroge* To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn. Tel/Fax: +86-27-6786-7535. † Central China Normal University. ‡ Huazhong University of Science and Technology. § Xianning College.
neous catalysts with a lower cost for the Fenton system. For instance, Fe-containing solids, such as goethite, hematite, clay minerals, iron hydroxide, and iron supported on silica and on alumina,3,6,16-23 have been investigated in heterogeneous Fenton systems. These systems could operate at near-neutral pH, and the iron catalysts could be recycled. More recently, it was reported that a heterogeneous Fenton system based on Fe0/Fe3O4 composites could work at near-neutral pH.24-26 Recently, we synthesized Fe@Fe2O3 core-shell nanowires simply by the reduction of Fe3+ ions with sodium borohydride in aqueous solution at ambient atmosphere.27 These Fe@Fe2O3 core-shell nanowires were used as an iron reagent to efficiently degrade rhodamine B (RhB) in a sono-Fenton system at pH ) 2.28 However, we found these novel iron reagents were partially dissolved in such an acidic solution. In order to overcome this drawback, herein, we establish a novel sono-Fenton system which can work at neutral pH and realize the recycle of Fe@Fe2O3 core-shell nanowires. Experimental Section Synthesis of Fe@Fe2O3 Nanowires. All of the chemicals were of analytical grade and used as received without further purification. The Fe@Fe2O3 nanowires were synthesized by the reaction between ferric chloride and sodium borohydride described previously.27,28 In a typical procedure, 0.15 g of FeCl3‚ 6H2O and 0.3 g of NaBH4 were dissolved in 50 and 20 mL of deionized water, respectively. The resulting NaBH4 solution was then dropped into the FeCl3‚6H2O solution in a 150 mL flask. The addition rate of NaBH4 was about 0.2 mL/s. The whole synthesis process was performed at ambient atmosphere, without the protection of inert gases or vacuum. The flask was shaken by hand (magnetic stirring could not be used in order to avoid
10.1021/jp070412v CCC: $37.00 © 2007 American Chemical Society Published on Web 04/21/2007
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Figure 1. Sono-Fenton degradation of RhB with Fe@Fe2O3 nanowires at neutral pH as a function of reaction time. The inset is the UV-vis spectral changes of RhB with reaction time.
magnetically induced aggregation of the resultant iron particles) during the addition. The solution was bubbling plenty of gas with the addition of the NaBH4 solution, accompanied by fluffy black precipitates that appeared on the surface of the solution. The fluffy black precipitates were collected, washed with deionized water and ethanol, and finally dried under nitrogen flow for characterization and use. The whole procedure could be scaled up to produce Fe@Fe2O3 core-shell nanowires in tens of grams quantity Procedure of Sono-Fenton Degradation of RhB. The sonoFenton experiments were carried out in an ultrasound clean bath with a frequency of 25 kHz (100 W, KQ-100A, China). Then, 0.9 mmol of Fe@Fe2O3 nanowires (the molecular weight of the nanowires was assumed as 56) was introduced into 50 mL of 5 mg‚L-1 RhB solution in a 100 mL glass cell with a water cooling jacket outside. Air was bubbled (0.1 m3‚h-1) into the RhB solution during the sono-Fenton degradation. As a comparison, experiments with ultrasound irradiation alone, a sonoFenton process with 0.9 mmol of commercial Fe0, Fe2+, or Fe3+ as the iron sources were conducted under the same conditions. The concentration of RhB was monitored by colorimetry with a U-3310 UV-vis spectrometer (HITACHI) at an interval of 10 min. Characterization of the Freshly Prepared and Used Fe@Fe2O3 Nanowires. X-ray powder diffraction patterns were obtained on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å). Scanning electron microscopy images were obtained on a LEO 1450VP scanning electron microscope. X-ray photoemission spectroscopy was recorded on a Kratos ASIS-HS X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al KR) operated at 150 W (15 kV, 10 mA). FTIR spectra were recorded on a Nicolet Nexus spectrometer with the standard KBr pellet method. Results and Discussion Sono-Fenton Degradation of RhB by Using Fe@Fe2O3 Core-Shell Nanowires as the Iron Reagent at Neutral pH. It was found that without ultrasound, Fe@Fe2O3 core-shell
Figure 2. Sono-Fenton degradation of RhB with Fe0, Fe2+, or Fe2+/ Fe2O3 at neutral pH as a function of reaction time.
nanowires only caused slight a decrease of the RhB concentration in the aqueous solution at neutral pH. This slight decrease may be attributed to adsorption of RhB over nanowires. Meanwhile, our previous study showed that RhB could not be efficiently degraded only in the presence of ultrasound irradiation produced by the cleaning bath.28 In contrast, sono-Fenton degradation of RhB in the presence of Fe@Fe2O3 nanowires at neutral pH was very obvious (Figure 1). It was found that the maximum absorption peak of RhB at 555 nm gradually diminished upon the sono-Fenton process (inset of Figure 1). No blue shift of the peak at 555 nm suggested that the degradation of RhB was attributed to the decomposition of the conjugated xanthene ring in RhB.14 About 61% of RhB was degraded in 60 min, accompanied by the diminishing of the red color. This result suggested that the Fe@Fe2O3 core-shell nanowires are effective to degrade organic pollutants such as dye pollutants in the Fenton system even at neutral pH. For comparison, sono-Fenton processes with commercial Fe0, Fe2+, or Fe2+/Fe2O3 as the iron sources were also used to degrade RhB at neutral pH (Figure 2). We found that RhB could not be
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Figure 3. XRD patterns of the Fe@Fe2O3 nanowires after 60 min of the sono-Fe@Fe2O3 reaction at neutral pH.
degraded by the sono-Fenton process with Fe2+ as the iron reagent at neutral pH, while the sono-Fenton process with Fe0 or Fe2+/Fe2O3 as the iron reagent could only degrade 32 or 16% of RhB in 60 min at neutral pH, respectively. These comparisons revealed the high activity of the sono-Fenton system based on Fe@Fe2O3 core-shell nanowires at neutral pH and the possibility to extend the working pH range of the sono-Fenton reaction to neutral pH. XRD Pattern and SEM Images of the Used Fe@Fe2O3 Nanowires after the Sono-Fenton Process Working at Neutral pH. XRD was used to characterize the used Fe@Fe2O3 core-shell nanowires after the sono-Fenton reactions at neutral pH. As shown in Figure 3, the Fe2O3 (hematite, JCPDS file No. 73-603) phase and the metallic iron phase (JCPDS file No. 85-1410) coexisted in the used Fe@Fe2O3 iron reagent. This revealed that the components of the used Fe@Fe2O3 nanowires were similar to those of the as-prepared Fe@Fe2O3 nanowires.28 However, the content of Fe2O3 in the used nanowires was much higher than that in the as-prepared Fe@Fe2O3 nanowires, according to the XRD peak ratio. The morphology of the used Fe@Fe2O3 nanowires was examined by SEM (Figure 4). The wire-like shape of the iron reagent was kept well after the sonoFenton reactions (Figure 4a). However, different from the asprepared core-shell nanowires,27,28 these used nanowires possessed a much rougher surface (Figure 4b), which was attributed to the formation of a new Fe2O3 layer on the surface of the nanowires during sono-Fenton degradation. This is consistent with XRD results. Obviously, corrosion of the iron cores in water during the sono-Fenton reaction at neutral pH resulted in the deposition of Fe2O3 on the surface of the Fe@Fe2O3 core-shell nanowires. This corrosion of the iron cores involves the transport of oxygen-bearing species to the metal cores and the diffusion of the resulting iron ions outward to the solution phase.29 In this case, oxygen-bearing species, such as O2 dissolved in the solution or H2O2 produced by ultrasound irradiation, first transported from aqueous solution to the interfaces between Fe cores and Fe2O3 shells. Then iron ions (Fe2+/Fe3+, Fen+) could be produced through electron transfer from Fe cores to these oxygen-bearing species (electron acceptors). When the concentration of the iron ions became saturated near the surface of Fe@Fe2O3 core-shell nanowires, these iron ions would nucleate and grow into new Fe2O3 layers on the surface of Fe@Fe2O3 core-shell nanowires. XPS Spectra of the Fe@Fe2O3 Nanowires. X-ray photoelectron spectroscopy was further used to study the surface
Figure 4. SEM images at low magnification (a) and high magnification (b) of the Fe@Fe2O3 nanowires after 60 min of the sono-Fenton process at neutral pH.
chemical compositions of the Fe@Fe2O3 core-shell nanowires before and after the sono-Fenton reaction (Figure 5). Figure 5a shows the survey spectra of Fe@Fe2O3 nanowires before and after the sono-Fenton process at neutral pH. Elements of Fe, O, and adventitious C coexisted in both the as-prepared and used Fe@Fe2O3 nanowires. The core level spectra of Fe 2p regions were also investigated by XPS. Figure 5b shows the Fe 2p peaks at binding energies of 710.9 and 724.6 eV, with a shakeup satellite at 719.2 eV. The dominant peak at 710.9 eV and the satellite signal at 719.2 eV can be attributed to Fe2O3.30,31 The shoulder peak at 724.6 eV can be assigned to Fe in Fe2O3.32 The Fe 2p core level features of Fe@Fe2O3 nanowires before and after reactions match well with those of Fe2O3 and Fe reported in the literature.30,32 Generally, pure Fe gives peaks at around 707 eV, which appears slightly in Figure 5b. Although it is difficult to validate the existence of metallic Fe phases in the samples according to XPS, the XRD results definitely confirms the presence of a pure Fe phase. As the core parts of the Fe@Fe2O3 core-shell nanowires, it was reasonable for there to be no signal of metallic Fe in XPS analysis when the surface was covered with thick Fe2O3 layers. This is because XPS can only detect signals within the upper 10 nm thickness on the surface, while the XRD can detect signals in micro-order thickness. Meanwhile, O 1s spectra of Fe@Fe2O3 before and after the reaction were also recorded (Figure 5c). The broad peak of O 1s can be fitted by two peaks at binding energies of 530.0 and 531.4 eV. The dominant
Fe@Fe2O3 Core-Shell Nanowires as the Iron Reagent
Figure 5. XPS spectra of the Fe@Fe2O3 nanowires before and after the sono-Fenton process at neutral pH; (a) survey of the sample, (b) Fe 2p, and (c) O 1s.
peak at 530.0 eV is characteristic of oxygen in a metal oxide such as Fe2O3, and the other peak at around 531.4 eV suggests the presence of other components such as OH, H2O, and carbonate species adsorbed on the surface. It is obvious that the content of oxide in Fe@Fe2O3 nanowires increases, while the amount of surface hydroxyl groups decreases after the sono-Fenton process. This is consistent with XRD and SEM results.
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7433 Infrared Spectra of the Fe@Fe2O3 Nanowires after the Sono-Fenton Process at Neutral pH. The FTIR spectrum was used to identify the functional groups on the surface of the Fe@Fe2O3 iron reagent. Figure 6 represents the spectra of the as-prepared and used Fe@Fe2O3 nanowires. The IR spectra display broad bands at 3363-3406 cm-1, which are believed to be associated with the stretching vibrations of hydrogenbonded surface water molecules and hydroxyl groups. In addition, the bands at 1387, 1262, and 1019 cm-1 imply the existence of residual hydroxyl groups. The band at 1262 cm-1 only appears in the IR spectra of the as-prepared nanowires, and the bands at 1387 and 1019 cm-1 of the used nanowires become weak. It reveals that the sono-Fenton reactions can reduce the amount of surface hydroxyl groups on Fe@Fe2O3 nanowires, consistent with XPS measurement. The weak adsorption at 1631 cm-1 is assigned to the bending mode of water. The bands at less than 600 cm-1 represent the Fe-O transverse vibration mode of iron oxides. The peak at 563 and 446 cm-1 are attributed to the Fe-O bond vibration of the Fe@Fe2O3 core-shell nanowires. Comparing the two spectra, we found that the transverse vibrations of the used nanowires became apparent because more Fe2O3 was deposited on the surfaces of the used Fe@Fe2O3 nanowires. This result agrees with XRD and SEM observations. No organic groups were found to be adsorbed on the surface on the basis of IR spectra.33 Stability of Fe@Fe2O3 Core-Shell Nanowires in Multiple Runs of the Degradation of RhB. The stability of a catalyst is a key issue for its application. Although XRD and XPS revealed the formation of an Fe2O3 layer on the surface of the nanowires during the sono-Fenton degradation of RhB, we found that this formation of a new Fe2O3 layer did not obviously influence the activity of the nanowires after recycling the used Fe@Fe2O3 nanowires (Figure 7). This result suggests that the Fe@Fe2O3 core-shell nanowires are promising for application in wastewater treatment because there is no need to preadjust the solution pH and reusability of the catalyst. Sono-Fenton Degradation Mechanism of RhB over Fe@Fe2O3 Nanowires at Neutral pH. Recently, Moura and co-workers proposed a heterogeneous Fenton mechanism for the degradation of organics on Fe0/Fe3O4 composites. They thought that Fe2+ species could be created by electron transfer from Fe0 to Fe3+ species of Fe0/Fe3O4 during the Fenton reaction.24,25 On the basis of the above results, we proposed a possible mechanism for the sono-Fenton degradation of RhB on the Fe@Fe2O3 core-shell nanowires (Scheme 1). We have previously mentioned that iron ionic species (Fen+) could form from the core parts of nanowires through the electron transfer from Fe0 to H2O2 produced by ultrasound irradiation during the sonoFenton reaction. This electron transfer follows a Haber-Weisslike mechanism.34 Because of the core-shell structures of our new Fenton iron reagent, these free iron ions could be adsorbed by the oxide shell of the nanowires, preventing their complete leaching into the RhB solution. In this sono-Fenton system, these iron ions (Fen+) are believed to initiate the Fenton reaction on the surface of the nanowire iron reagent, accelerating the decomposition of H2O2 to form •OH radicals.24 Meanwhile, these iron ionic species could also be transferred into a new Fe2O3 layer in the presence of ultrasound irradiation and oxygen. These oxides would deposit on the surfaces of these nanowires, as mentioned previously. Therefore, an in situ recycling of iron species (Fe0 f Fen+ f Fe2O3) could be realized (Scheme 1). These newly formed iron reagent-containing iron ions, zerovalent iron and iron oxides (Fe2O3), served as heterogeneous iron
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Figure 6. Infrared spectra of the Fe@Fe2O3 nanowires before (a) and after (b) the sono-Fenton process at neutral pH.
Figure 7. Sono-Fenton degradation of RhB with the recycled Fe@Fe2O3 nanowires at neutral pH.
SCHEME 1: Proposed Mechanism for Sono-Fenton Degradation of RhB over Fe@Fe2O3 Core-Shell Nanowires at Neutral pH
catalysts to generate hydroxyl radicals from sonochemically produced H2O2 to attack RhB adsorbed on the surface of the nanowires. RhB was then decomposed into CO2 and H2O.26 In
our previous study, we already used atom absorption spectrometry analysis to detect iron ionic species with very low concentration (0.2 mg‚L-1) in this sono-Fenton system based
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Figure 8. Degradation of RhB by sono-Fenton with Fe2+ (0.001 mol L-1) and Fe@Fe2O3 (0.018 mol L-1) in the neutral pH range as a function of reaction time. The inset is the UV-vis spectral changes of RhB versus reaction time.
on Fe@Fe2O3 core-shell nanowires at neutral pH.28 The participation of iron ionic species in the sono-Fenton degradation of RhB was further proved by a reference experiment as follows. We found that the addition of tiny Fe2+ (5 mg‚L-1) to this new sono-Fenton system based on Fe@Fe2O3 core-shell nanowires could significantly enhance the degradation rate of RhB (Figure 8). Over 92% of RhB was degraded by the sono-Fenton process with Fe2+ and Fe@Fe2O3 for 60 min at neutral pH. This efficiency was even close to that of our previous sono-Fenton system working at pH ) 2.28 The enhancement of efficiency by the addition of Fe2+ should be attributed to fast establishment of the similar in situ recycling of iron species (Fe0 f Fen+ f Fe2O3) at the beginning of the sono-Fenton degradation. Conclusions This study reported an effective, reusable, and economical sono-Fenton system based on an Fe@Fe2O3 core-shell nanowire iron reagent. This system worked well at neutral pH. It was found that Fe@Fe2O3 core-shell nanowires showed significant higher catalytic activity than Fe0, Fe2+, and Fe2+/ Fe2O3 for the degradation of RhB at neutral pH in this sonoFenton system. We proposed a possible mechanism for the sonoFenton degradation of RhB with Fe@Fe2O3 core-shell nanowires, which involved an in situ recycling of iron species (Fe0 f Fen+ f Fe2O3). More importantly, the efficiency of this neutral-pH sono-Fenton system with the addition of 0.001 mol L-1 of Fe2+ even reached 92% for degrading RhB at neutral pH. This efficiency was close to that of our previous sono-Fenton system working at pH ) 2. Because of its friendly working condition and reusability of catalyst, we believe this novel sono-Fenton system based on an Fe@Fe2O3 core-shell nanowire iron reagent may be applied to practical wastewater treatment. Acknowledgment. This work was supported by the National Science Foundation of China (Grants 20503009 and 20673041), Outstanding Young Research Award of National Natural Science Foundation of China (Grant 50525619), and the Open Fund of Hubei Key Laboratory of Catalysis and Materials Science (Grants CHCL0508 and CHCL06012).
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