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carbon aerogel nanostructures with high performance for organic dye removal
Separation and Purification Technology 149 (2015) 74–81
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Design of composite maghemite/hematite/carbon aerogel nanostructures with high performance for organic dye removal Yi-Feng Lin ⇑, Chia-Yu Chang Department of Chemical Engineering, R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli 320, Taiwan, ROC
a r t i c l e
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Article history: Received 27 November 2014 Received in revised form 16 May 2015 Accepted 22 May 2015 Available online 22 May 2015 Keywords: Fe3O4/a-Fe2O3 c-Fe2O3/a-Fe2O3 Carbon aerogel Nanorods Mesoporous
a b s t r a c t Composite Fe3O4/a-Fe2O3 nanorods (180N) with a specific surface area of 16.2 m2/g were successfully synthesized via a facile hydrothermal reaction at 180 °C. To increase the specific surface area, a mesoporous carbon aerogel (CA) with a large specific surface area was added to the hydrothermal reaction to form mesoporous Fe3O4/a-Fe2O3/CA structures (CA-180) with a specific surface area of 267.7 m2/g instead of nanorods. The as-prepared 180N and CA-180 samples were further annealed at 400 °C to oxidize the Fe3O4 phase, resulting in the formation of composite c-Fe2O3/a-Fe2O3 nanorods (400N) with a specific surface area of 27.7 m2/g and mesoporous c-Fe2O3/a-Fe2O3/CA structures (CA-400) with a specific surface area of 551 m2/g. The CA-400 samples were further used to remove the dye Rhodamine B (RhB) from aqueous solution, and 98.2% of the RhB dye was removed using the CA-400 sample when the initial RhB concentration was 8 ppm. The as-prepared CA-400 sample exhibits strong potential for use in wastewater treatment applications, such as organic dye removal. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1-D) nanostructures such as nanowires [1–3], nanorods [4,5], nanotubes [6,7] and nanofibers [8,9] have drawn extensive research attention in recent years because of the novel properties that result from this special dimensionality [10]. A wide range of applications of 1-D nanomaterials have been investigated, such as field emitters [11,12], piezoelectric nanogenerators [13–15], solar cells [16,17], water-splitting systems [18], photocatalysts [19] and sensors [20,21]. Various important 1-D nanomaterials, such as ZnO [22], ZnS [23], CdS [11–14] and Si [24], have been successfully prepared through gas-phase or wet-chemistry approaches in recent years. Magnetic building blocks have been the subject of numerous studies because of their magnetic properties and their broad range of applications in many fields, including drug delivery [25], imaging [26], spintronic devices [27], recording devices [28], supercapacitors [29], photocatalysis [30] and the adsorption of heavy-metal ions [31]. Iron oxide is one such magnetic building block material. It has three different crystalline phases: magnetite (Fe3O4), maghemite (c-Fe2O3) and hematite (a-Fe2O3). As a result, relevant 1-D iron oxide nanostructures such as nanorods [32,33], nanowires [34,35], nanoneedles [36] and nanotubes [37,38] have been successfully
⇑ Corresponding author at: 200, Chungpei Rd., Chungli, Taoyaun, Taiwan, ROC. E-mail address: yfl[email protected] (Y.-F. Lin). http://dx.doi.org/10.1016/j.seppur.2015.05.025 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
developed in recent years. Numerous applications of 1-D iron oxide nanostructures have been discussed, including field emitters [36], degradation catalysts [39], wireless manipulation devices [40] and sensors [41]. To the best of our knowledge, few previous studies have examined the application of 1-D iron oxide nanostructures as adsorbents for the dye RhB. Carbon aerogels (CAs), one of mesoporous materials [42–44] and carbon materials [45,46], have been the subject of numerous studies because of their high specific surface area (400–1200 m2/g), high porosity (greater than 80%), high mechanical strength, mesoporous structures (2–50 nm) and low cost [41]. A wide range of applications of CAs, including hydrogen storage materials [47], capacitors [48], supercapacitors [29], ion-exchange resins [49] and metal-ion adsorbents [50], have been previously studied. For metal-ion adsorption applications, CAs require an additional separation step, such as centrifugation, to remove them from solution. In our previous study, mesoporous Fe/CA structures were successfully prepared for use as As(V)-ion adsorbents [51]. The resulting as-prepared Fe/CA structures could be easily separated from the solution using an external magnetic field because of their ferromagnetic property, and further separation steps, such as centrifugation, were not needed. Herein, Fe3O4/a-Fe2O3 nanorods (180N) were successfully prepared using a facile hydrothermal process at 180 °C, as shown in Scheme 1. The as-prepared 180N samples were further annealed at 400 °C under atmospheric conditions to form c-Fe2O3/a-Fe2O3
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Scheme 1. Preparation of mesoporous iron oxide/carbon aerogel composite nanostructures.
nanorods (400N). However, the specific surface area of the as-prepared nanorod samples was quite small. As a result, we attempted to add a CA with a high specific surface area to the reactant solution to prepare mesoporous Fe3O4/a-Fe2O3/CA structures (CA-180). The specific surface area of the CA-180 samples was greatly increased compared to the 180N samples. The CA-180 samples were also further annealed at 400 °C to form mesoporous c-Fe2O3/a-Fe2O3/CA structures (CA-400). We also investigated the ability of the four as-prepared samples (180N, 400N, CA-180 and CA-400) to adsorb RhB. The CA-400 samples exhibited better adsorption performances than the other three samples and good response to an external magnetic field, which indicates that the use of a magnetic field can replace the traditional centrifugal separation step used to remove adsorbents from solution, thereby reducing energy consumption. Furthermore, the CA-400 samples are also reusable for the adsorption of RhB dye, which makes them potentially useful for wastewater treatment, including organic dye removal.
reaction. The hydrothermal reaction was performed at 180 °C for 24 h. After cooling to room temperature, the product was washed with ethanol and then dried at 50 °C for 24 h. The 180N samples were further annealed at 400 °C under atmospheric conditions for 7 h to form the c-Fe2O3/a-Fe2O3 nanorods (400N). To prepare the mesoporous Fe3O4/a-Fe2O3/CA structures (CA-180), FeSO4 (ca. 1.67 g) and CA (ca. 0.3 g) were added to 60 ml of DI water and stirred for 24 h. The resulting solution was centrifuged to obtain black powders, and the black powders were further dried at 50 °C for 24 h. The as-obtained black powders and HMTA (ca. 0.84 g) were added to 60 ml of DI water and stirred for 2 h. The solution was then transferred to a 100-ml Teflon-lined autoclave, and the autoclave was heated at 180 °C for 24 h. After cooling to room temperature, the product was washed with ethanol and then dried at 50 °C for 24 h. The CA-180 samples were
2. Experimental 2.1. Chemicals Ferrous sulfate (FeSO4), hexamethylenetetramine (HMTA) and DI water were used to prepare the Fe3O4/a-Fe2O3 nanorods (180N) and c-Fe2O3/a-Fe2O3 nanorods (400N). The synthesis of carbon aerogel (CA) was based on the procedure described in our previous report [52]; the specific surface area and average pore diameter of the CA were approximately 702 m2/g and 10 nm, respectively. The RhB dye was dissolved in DI water to an initial concentration of 8 ppm. 2.2. Preparation of iron oxide and iron oxide/carbon aerogel structures To prepare the Fe3O4/a-Fe2O3 nanorods (180N), FeSO4 (ca. 1.67 g) and HMTA (ca. 0.84 g) were dissolved in 60 ml of DI water. After stirring for 2 h, the solution mixture was transferred to a 100-ml Teflon-lined autoclave for the subsequent hydrothermal
Fig. 1. XRD patterns of reference (a) Fe3O4, (b) a-Fe2O3 crystals, (c) 180N and (d) CA-180 samples. The inset shows the corresponding XRD patterns at the diffraction angles between 34° and 37°.
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Fig. 2. SEM images of the (a) 180N and (b) CA-180 samples.
further annealed at 400 °C under atmospheric conditions for 7 h to form the mesoporous c-Fe2O3/a-Fe2O3/CA structures (CA-400).
2.3. Adsorption and desorption of the RhB dye The as-prepared 180N, 400N, CA-180 and CA-400 samples were used for RhB dye adsorption studies. RhB dye adsorption was performed by immersing 10 mg of 180N, 400N, CA-180 or CA-400 into 100 ml of RhB dye solution with a RhB dye concentration of 8 ppm under gentle stirring for 12 h to achieve adsorption equilibrium. After magnetic separation, the remaining concentration of RhB dye was determined by UV–visible spectrophotometry to calculate the percentage of RhB dye adsorbed.
2.4. Characterization The surface morphology and the crystalline phases of the as-prepared samples were examined via field-emission scanning electron microscopy (FESEM, Hitachi, S-4800N) and powder X-ray diffraction (PXRD, PANalytical X’Pert PRO, PW3040/60), respectively. The pore size distribution and the specific surface area of the as-synthesized samples were measured based on N2 adsorption/desorption isotherms (BET, Micromeritics ASAP 2020). The magnetic properties of the as-prepared CA-400 samples were studied using superconducting quantum interference device (SQUID) magnetometry (Quantum Design MPMS7). The concentration of RhB dye in solution was measured via UV–visible spectrophotometry (Hitachi, U-3900/U-3900H).
Fig. 3. The N2 adsorption and desorption curves of the (a) 180N and (c) CA-180 samples and the pore size distributions of the (b) 180N and (d) CA-180 samples.
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Fig. 4. XRD patterns of reference (a) c-Fe2O3, (b) a-Fe2O3 crystals, (c) 400N and (d) CA-400 samples. The inset shows the corresponding XRD patterns at diffraction angles between 34° and 38°.
3. Results and discussion The crystalline structures of the as-prepared 180N and CA-180 samples were investigated by XRD, as shown in Fig. 1(c) and (d), respectively. The reference patterns of Fe3O4 and a-Fe2O3 crystals are included in the figure for comparison. The diffraction peaks of the 180N and CA-180 samples appear to fit well with the peaks of the reference a-Fe2O3 crystals, implying that 180N and CA-180 are composed of a-Fe2O3. However, the peak at approximately 35.5° in the patterns of the 180N and CA-180 samples fits well with the reference Fe3O4 crystals (inset in Fig. 1), indicating that both the 180N and CA-180 samples are composite Fe3O4/a-Fe2O3 nanomaterials. The surface morphologies of the 180N and CA-180 samples were further studied using FESEM, as shown in Fig. 2(a) and (b), respectively. In the case of the 180N samples, rod-like materials with a length of several hundred nanometers were observed, as shown in Fig. 2(a), indicating that the composite Fe3O4/a-Fe2O3 nanorods were formed in the absence of CA. However, particle-like structures instead of nanorods were observed in the mesoporous CA samples, as shown in Fig. 2(b). The mesoporous CA restricts the growth of 1-D nanorods to form the mesoporous composite Fe3O4/a-Fe2O3/CA nanostructures observed in the CA-180 samples. These two different nanostructures – nanorods and mesoporous structures – are attributed to the presence or absence of the mesoporous CA. The specific surface areas and pore size of the 180N and CA-180 samples were measured based on the N2 adsorption and
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desorption isotherms obtained via the BET method, as shown in Fig. 3. Both the 180N and CA-180 samples exhibited a type-IV isotherm, as shown in Fig. 3(a) and (c), respectively. The type-IV isotherm indicates the existence of mesoporous structures in the as-prepared 180N and CA-180 samples, which is in good agreement with the average pore sizes of approximately 2.2 and 8.9 nm, respectively. The specific surface areas of the 180N and CA-180 were approximately 16.2 and 267.7 m2/g, implying that the addition of mesoporous CA not only changes the structures but also increases the specific surface areas. The as-prepared 180N and CA-180 samples were further annealed at 400 °C for 7 h; the resulting samples are referred to as 400N and CA-400, respectively. The crystal structures, surface morphologies and specific surface areas of the 400N and CA-400 samples are presented in Figs. 4–6. The crystalline structures of the 400N and CA-400 samples were first investigated by XRD, as shown in Fig. 4. The XRD patterns of the 400N and CA-400 samples are presented in Fig. 4(c) and (d), respectively. The diffraction patterns of the reference c-Fe2O3 and a-Fe2O3 crystals are also included in Fig. 4(a) and (b). The diffraction peaks of the 400N and CA-400 samples match well with the peaks of the reference a-Fe2O3 material. However, after carefully examining the diffraction region from 34° to 38°, we observed that the peak at approximately 35.6° in the patterns of the 400N and CA-400 samples fits well with the peak for the reference c-Fe2O3 material, as shown in the inset of Fig. 4. This result indicates that the 400N and CA-400 samples are composite c-Fe2O3/a-Fe2O3 nanomaterials. Although the compositions of the 400N and CA-400 samples changed, their surface morphologies remained the same as those for the 180N and CA-180 samples, respectively. This result indicates that c-Fe2O3/a-Fe2O3 nanorods and mesoporous c-Fe2O3/a-Fe2O3/CA structures were obtained in the 400N and CA-400 samples, as shown in Fig. 5(a) and (b), respectively. The specific surface areas and pore diameters of the 400N and CA-400 samples were also measured based on their N2 adsorption and desorption isotherms, as shown in Fig. 6. Both the 400N and CA-400 samples exhibited a type-IV isotherm, as shown in Fig. 6(a) and (c), respectively. These type-IV isotherms indicate the presence of mesopores in the as-prepared 400N and CA-400 samples, which is in good agreement with the average pore sizes of approximately 14.9 and 6 nm, respectively. The specific surface areas of the 400N and CA-400 were approximately 27.7 and 551 m2/g, which are larger than the specific surface areas of the 180N (16.2 m2/g) and CA-180 (267.7 m2/g) samples, respectively. The specific surface area of the CA-400 samples was greatly increased from 267.7 to 551 m2/g when the CA-180 samples were annealed at 400 °C, indicating that annealing at 400 °C not only changed the composition but also increased the specific surface area of the material. The magnetic properties of the as-prepared CA-400 samples were analyzed using a superconducting quantum inference device
Fig. 5. SEM images of (a) 400N and (b) CA-400 samples.
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Fig. 6. The N2 adsorption and desorption curves of the (a) 400N and (c) CA-400 samples and the pore size distributions of the (b) 400N and (d) CA-400 samples.
Fig. 7. Magnetization curve of the (a) CA-400 sample and (b) the same curve magnified near the origin. The inset in (a) is a photograph of the sample solution under an external magnetic field.
(SQUID) magnetometer. The magnetic hysteresis loop (Fig. 7(a) and (b)) for the as-prepared CA-400 samples measured at room temperature in an applied magnetic field of up to 50,000 Oe indicates ferrimagnetic behavior in the as-prepared mesoporous composite c-Fe2O3/a-Fe2O3/CA structures. The magnetization saturation of the mesoporous c-Fe2O3/a-Fe2O3/CA structures was 4.6 emu/g, which is substantially lower than that of bulk c-Fe2O3 materials (75 emu/g) [53]. The presence of antiferromagnetic a-Fe2O3 and CA materials and the reduced size of the
c-Fe2O3 nanoparticles may be responsible for the lower magnetization saturation. Fig. 7(b) shows the magnetic hysteresis loop in an applied magnetic field between 400 and 400 Oe, which indicates that the remaining magnetization and coercivity are 0.85 emu/g and 141 Oe, respectively. When the mesoporous c-Fe2O3/a-Fe2O3/CA structures dispersed in solution were subjected to a magnetic field, they were completely separated from the solution, as shown in the inset of Fig. 7(a). The composite c-Fe2O3/a-Fe2O3/CA structures exhibit good response to an
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Table 1 Regression parameters (Qmax, b and R2) of the fitted Langmuir isotherm model for the as-prepared 180N, 400N, CA-180 and CA-400 samples, respectively. Langmuir model
180IM 400N CA-180 CA-400
Qmax
b
R2
1.52 1.74 45.67 151.52
4.41 17.94 2.11 2.44
0.997 0.995 0.994 0.994
Table 2 Comparison of RhB dye adsorption capacity with other adsorbents.
Fig. 8. Plots of the RhB dye removal capacity (Qe) as a function of time for the 400N and CA-400 samples.
Adsorbents
Adsorption capacity (mg/g)
References
MPGC-900 Fe3O4/RGO(M2) OPBC Coffee ground powder Carbonaceous adsorbent CA400
73.0 142.86 69.86 5.26 91.1 151.5
54 55 56 57 58 This study
Fig. 9. RhB dye removal using the 180N, 400N, CA-180 and CA-400 samples in DI water solution with an initial RhB dye concentration of 8 ppm. Fig. 11. Three cycles of RhB dye removal using the CA-400 sample at the initial RhB dye concentration of 8 ppm.
Fig. 10. Plots of the RhB dye removal capacity (Qe) as a function of the RhB dye equilibrium concentration (Ce) for the 180N, 400N, CA-180 and CA-400 samples.
external magnetic field, which indicates that the use of a magnetic field can replace the traditional centrifugal separation step used to remove adsorbents from solution, thereby reducing energy consumption.
The kinetic study of RhB dye adsorption using 400N and CA-400 samples was firstly studied in Fig. 8. Fig. 8 is the plot of the RhB dye removal capacity (Qe) as a function of the adsorption time. The RhB dye removal capacity (Qe) reaches a stable value after the adsorption time of 12 h. This result indicates the adsorption equilibrium is achieved for the adsorption time of 12 h. Consequently, the adsorption time is 12 h for the other RhB dye adsorption experiments. The removal percentage of RhB dye using the as-prepared 180N, 400N, CA-180 and CA-400 samples is demonstrated in Fig. 9, where the initial concentration of the RhB dye was 8 ppm. The percentages of RhB dye removed were 3.3%, 4.1%, 78.1% and 98.2% using the as-prepared 180N, 400N, CA-180 and CA-400 samples, respectively. The addition of mesoporous CA restricts the formation of 1-D Fe3O4/a-Fe2O3 nanorods (180N samples) with a smaller specific surface area (16.2 m2/g) in favor of the mesoporous Fe3O4/a-Fe2O3/CA structures with a larger specific surface area (267.7 m2/g). This increased specific surface area results in increased RhB dye removal. However, the efficiency of RhB dye removal by the as-prepared CA-400 samples is still greater than the efficiency of the CA-180 samples because of their larger specific surface area. One may argue that the surface charges of CA-180 and
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CA-400 samples should be another important factor for the RhB dye adsorption capacity. As a result, the surface charges at pH 7 of the CA-180 and CA-400 samples are also carried out. The surface charges of the CA-180 and CA-400 samples at pH 7 are approximately 44.5 and 45.1 mV, respectively. Since the surface charges of CA-180 and CA-400 samples are almost the same, the specific surface area of CA-180 and CA-400 samples is the significant factor for RhB dye adsorption capacity. CA-400 sample has larger specific surface area (551 m2/g) than CA-180 sample (267.7 m2/g), leading to the better RhB dye adsorption capacity than CA-180 sample. The adsorption ability of the 180N, 400N, CA-180 and CA-400 samples is further determined from the adsorption isotherms. Fig. 10 is the plots of RhB dye adsorption capacity (Qe) versus the equilibrium concentration of RhB dyes (Ce) using the as-prepared 180N, 400N, CA-180 and CA-400 samples, respectively. The RhB dye adsorption capacity using the as-prepared CA-400 samples is greatly higher than the as-prepared 180N, 400N and CA-180 samples due to the largest specific surface area of the CA-400 samples (551 m2/g), which is consistent with the results of Fig. 9. The adsorption data were further used to fit the Langmuir isotherm 1 e þ QCmax . Here, Ce is the equilibrium concentration model: QC ee ¼ Q max b of RhB dyes, Qe is the RhB dye adsorption capacity, Qmax and b are the monolayer capacity of the adsorbent and the adsorption constant, respectively. The value of the regression parameters (Qmax, b and R2) are shown in Table 1. The fitting coefficients (R2) of the 180N, 400N, CA-180 and CA-400 samples are 0.997, 0.995, 0.994 and 0.994, respectively, indicating that the Langmuir model fits the adsorption data of the 180N, 400N, CA-180 and CA-400 samples well. The RhB dye adsorption capacity of the as-prepared CA-400 samples was also compared with other adsorbents, as shown in Table 2. The results indicate that the as-prepared CA-400 samples in this study are with better RhB dye adsorption capacity than other adsorbents, such as MPGC-900 [54], Fe3O4/RGO [55], OPBC [56], coffee ground powder [57] and carbonaceous adsorbent [58]. The reuse of the CA-400 sample was also investigated over three cycles, as shown in Fig. 11. The RhB removal percentage reaches a stable value of approximately 98% after the third adsorption cycle of the RhB dyes. This result indicates that the as-prepared CA-400 samples are reusable for RhB dye removal. As a result, the as-prepared CA-400 samples have great potentials for the RhB dye removal in industrial applications.
4. Conclusions In this study, composite Fe3O4/a-Fe2O3 nanorods were successfully prepared using a hydrothermal process at 180 °C, and composite c-Fe2O3/a-Fe2O3 nanorods were obtained after the Fe3O4/a-Fe2O3 nanorods were annealed at 400 °C. This material was obtained because the Fe3O4 phase was oxidized to c-Fe2O3 during the annealing process. The specific surface areas of the composite Fe3O4/a-Fe2O3 and c-Fe2O3/a-Fe2O3 nanorods were 16.2 and 27.7 m2/g, respectively. To further increase the specific surface area of the as-prepared materials, a mesoporous CA with a high specific surface area was added to the reactant solution for the hydrothermal reaction at 180 °C. As a result, mesoporous composite Fe3O4/a-Fe2O3/CA structures with a specific surface area of 267.7 m2/g were successfully prepared when mesoporous CA was added. Mesoporous composite c-Fe2O3/a-Fe2O3/CA structures with a larger specific surface area of 551 m2/g were further obtained after the mesoporous Fe3O4/a-Fe2O3/CA structures were annealed at 400 °C. The RhB dye removal performance of the mesoporous composite c-Fe2O3/a-Fe2O3/CA structures was better than the dye removal performance of the other three structures because
of the larger specific surface area of 551 m2/g. As a result, the mesoporous composite c-Fe2O3/a-Fe2O3/CA structures have strong potential for use in wastewater treatment, including organic dye removal. Acknowledgement The authors thank the Ministry of Science and Technology (project number: 101-2221-E-033-055-MY3 and 102-2622-E033-007-CC3) for financial support. References [1] W. Li, Y.X. Yin, S. Xin, W.G. Song, T.G. Guo, Low-cost and large-scale synthesis of alkaline earth metal germanate nanowires as a new class of lithium ion battery anode material, Energy Environ. Sci. 5 (2012) 8007–8013. [2] R.H. Gonçalves, L.D.T. Leite, E.R. Leite, Colloidal WO3 nanowires as a versatile route to prepare a photoanode for solar water splitting, ChemSusChem 5 (2012) 2341–2347. [3] Y.F. Lin, H.W. Chang, S.Y. Lu, C.W. Liu, Preparation, characterization, and electrophysical properties of nanostructured BiPO4 and Bi2Se3 derived from a structurally characterized single-source precursor Bi[Se2P(OiPr)2]3, J. Phys. Chem. C 111 (2007) 18538–18544. [4] J. Huang, K. Ding, Y. Hou, X. Wang, X. Fu, Synthesis and photocatalytic activity of Zn2GeO4 nanorods for the degradation of organic pollutants in water, ChemSusChem 1 (2008) 1011–1019. [5] C.C. Chen, W.Y. Cheng, S.Y. Lu, Y.F. Lin, Y.J. Hsu, K.S. Chang, C.H. Kang, K.L. Tung, Growth of zirconia and yttria-stabilized zirconia nanorod arrays assisted by phase transition, CrystEngComm 12 (2010) 3664–3669. [6] C. Yu, A. Murali, K. Choi, Y. Ryu, Air-stable fabric thermoelectric modules made of N- and P-type carbon nanotubes, Energy Environ. Sci. 5 (2012) 9481–9486. [7] G. Centi, S. Perathoner, Carbon nanotubes for sustainable energy applications, ChemSusChem 4 (2011) 913–925. [8] S. Ren, R. Prakash, D. Wang, V.S.K. Chakravadhanula, M. Fichtner, Fe3O4 anchored onto helical carbon nanofibers as high-performance anode in lithium-ion batteries, ChemSusChem 5 (2012) 1397–1400. [9] Y.F. Lin, C.S. Wang, C.C. Ko, C.H. Chen, K.S. Chang, K.L. Tung, K.R. Lee, Polyvinylidene fluoride/siloxane nanofibrous membranes for long-term continuous CO2-capture with large absorption-flux enhancement, ChemSusChem 7 (2014) 604–609. [10] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Onedimensional nanostructures: synthesis, characterization, and applications, Adv. Mater. 15 (2003) 353. [11] Y.F. Lin, Y.J. Hsu, S.Y. Lu, S.C. Kung, Non-catalytic and template-free growth of aligned CdS nanowires exhibiting high field emission current densities, Chem. Commun. (2006) 2391–2393. [12] Y.F. Lin, Y.J. Hsu, S.Y. Lu, K.T. Chen, T.Y. Tseng, Well-aligned ternary Cd1xZnxS nanowire arrays and their composition-dependent field emission properties, J. Phys. Chem. C 111 (2007) 13418–13426. [13] Y.F. Lin, J. Song, D. Yong, S.Y. Lu, Z.L. Wang, Alternating the output of CdSnanowire nanogenerator by white-light stimulated optoelectronic effect, Adv. Mater. 20 (2008) 3127–3130. [14] Y.F. Lin, J. Song, D. Yong, S.Y. Lu, Z.L. Wang, Piezoelectric nanogenerator using CdS nanowires, Appl. Phys. Lett. 92 (2008) 022105. [15] Z.L. Wang, J. Song, Piezoelectric nanogenerators based on zinc oxide nanowire array, Science 312 (2006) 242–246. [16] J. Wang, J. Wu, J. Lin, M. Huang, Y. Huang, Z. Lan, Y. Xiao, G. Yue, S. Yin, T. Sato, Application of Y2O3:Er3+ nanorods in dye-sensitized solar cells, ChemSusChem 5 (2012) 1307–1312. [17] W.Y. Cheng, Y.F. Lin, S.Y. Lu, Nanowires improved charge separation and light utilization in metal-oxide solar cells, Appl. Phys. Lett. 99 (2011) 063107. [18] R. Morrish, M. Rahman, J.M.D. MacElroy, C.A. Wolden, Activation of hematite nanorod arrays for photoelectrochemical water splitting, ChemSusChem 4 (2011) 474–479. [19] J. Shi, X. Wang, Hierarchical TiO2–Si nanowire architecture with photoelectrochemical activity under visible light illumination, Energy Environ. Sci. 5 (2012) 7918–7922. [20] T.Y. Wei, C.T. Huang, B.J. Hansen, Y.F. Lin, L.J. Chen, S.Y. Lu, Z.L. Wang, Large enhancement in photon detection sensitivity via schottky-gated CdS nanowire nanosensors, Appl. Phys. Lett. 96 (2010) 013508. [21] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Science 293 (2001) 1289–1292. [22] R. Yu, C. Pan, Z.L. Wang, High performance of ZnO nanowire protein sensors enhanced by the piezotronic effect, Energy Environ. Sci. 6 (2013) 494–499. [23] Y.F. Lin, Y.J. Hsu, S.Y. Lu, W.S. Chiang, One-step formation of core–shell sulfideoxide nanorod arrays from a single precursor, Nanotechnology 17 (2006) 4773–4782. [24] H.Y. Lee, J. Ryu, J.H. Kim, S.H. Lee, C.B. Park, Biocatalyzed artificial photosynthesis by hydrogen-terminated silicon nanowires, ChemSusChem 5 (2009) 2129–2132.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Design of composite maghemite/hematite/carbon aerogel nanostructures with high performance for organic dye removal Yi-Feng Lin ⇑, Chia-Yu Chang Department of Chemical Engineering, R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli 320, Taiwan, ROC
a r t i c l e
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Article history: Received 27 November 2014 Received in revised form 16 May 2015 Accepted 22 May 2015 Available online 22 May 2015 Keywords: Fe3O4/a-Fe2O3 c-Fe2O3/a-Fe2O3 Carbon aerogel Nanorods Mesoporous
a b s t r a c t Composite Fe3O4/a-Fe2O3 nanorods (180N) with a specific surface area of 16.2 m2/g were successfully synthesized via a facile hydrothermal reaction at 180 °C. To increase the specific surface area, a mesoporous carbon aerogel (CA) with a large specific surface area was added to the hydrothermal reaction to form mesoporous Fe3O4/a-Fe2O3/CA structures (CA-180) with a specific surface area of 267.7 m2/g instead of nanorods. The as-prepared 180N and CA-180 samples were further annealed at 400 °C to oxidize the Fe3O4 phase, resulting in the formation of composite c-Fe2O3/a-Fe2O3 nanorods (400N) with a specific surface area of 27.7 m2/g and mesoporous c-Fe2O3/a-Fe2O3/CA structures (CA-400) with a specific surface area of 551 m2/g. The CA-400 samples were further used to remove the dye Rhodamine B (RhB) from aqueous solution, and 98.2% of the RhB dye was removed using the CA-400 sample when the initial RhB concentration was 8 ppm. The as-prepared CA-400 sample exhibits strong potential for use in wastewater treatment applications, such as organic dye removal. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1-D) nanostructures such as nanowires [1–3], nanorods [4,5], nanotubes [6,7] and nanofibers [8,9] have drawn extensive research attention in recent years because of the novel properties that result from this special dimensionality [10]. A wide range of applications of 1-D nanomaterials have been investigated, such as field emitters [11,12], piezoelectric nanogenerators [13–15], solar cells [16,17], water-splitting systems [18], photocatalysts [19] and sensors [20,21]. Various important 1-D nanomaterials, such as ZnO [22], ZnS [23], CdS [11–14] and Si [24], have been successfully prepared through gas-phase or wet-chemistry approaches in recent years. Magnetic building blocks have been the subject of numerous studies because of their magnetic properties and their broad range of applications in many fields, including drug delivery [25], imaging [26], spintronic devices [27], recording devices [28], supercapacitors [29], photocatalysis [30] and the adsorption of heavy-metal ions [31]. Iron oxide is one such magnetic building block material. It has three different crystalline phases: magnetite (Fe3O4), maghemite (c-Fe2O3) and hematite (a-Fe2O3). As a result, relevant 1-D iron oxide nanostructures such as nanorods [32,33], nanowires [34,35], nanoneedles [36] and nanotubes [37,38] have been successfully
⇑ Corresponding author at: 200, Chungpei Rd., Chungli, Taoyaun, Taiwan, ROC. E-mail address: yfl[email protected] (Y.-F. Lin). http://dx.doi.org/10.1016/j.seppur.2015.05.025 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
developed in recent years. Numerous applications of 1-D iron oxide nanostructures have been discussed, including field emitters [36], degradation catalysts [39], wireless manipulation devices [40] and sensors [41]. To the best of our knowledge, few previous studies have examined the application of 1-D iron oxide nanostructures as adsorbents for the dye RhB. Carbon aerogels (CAs), one of mesoporous materials [42–44] and carbon materials [45,46], have been the subject of numerous studies because of their high specific surface area (400–1200 m2/g), high porosity (greater than 80%), high mechanical strength, mesoporous structures (2–50 nm) and low cost [41]. A wide range of applications of CAs, including hydrogen storage materials [47], capacitors [48], supercapacitors [29], ion-exchange resins [49] and metal-ion adsorbents [50], have been previously studied. For metal-ion adsorption applications, CAs require an additional separation step, such as centrifugation, to remove them from solution. In our previous study, mesoporous Fe/CA structures were successfully prepared for use as As(V)-ion adsorbents [51]. The resulting as-prepared Fe/CA structures could be easily separated from the solution using an external magnetic field because of their ferromagnetic property, and further separation steps, such as centrifugation, were not needed. Herein, Fe3O4/a-Fe2O3 nanorods (180N) were successfully prepared using a facile hydrothermal process at 180 °C, as shown in Scheme 1. The as-prepared 180N samples were further annealed at 400 °C under atmospheric conditions to form c-Fe2O3/a-Fe2O3
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Scheme 1. Preparation of mesoporous iron oxide/carbon aerogel composite nanostructures.
nanorods (400N). However, the specific surface area of the as-prepared nanorod samples was quite small. As a result, we attempted to add a CA with a high specific surface area to the reactant solution to prepare mesoporous Fe3O4/a-Fe2O3/CA structures (CA-180). The specific surface area of the CA-180 samples was greatly increased compared to the 180N samples. The CA-180 samples were also further annealed at 400 °C to form mesoporous c-Fe2O3/a-Fe2O3/CA structures (CA-400). We also investigated the ability of the four as-prepared samples (180N, 400N, CA-180 and CA-400) to adsorb RhB. The CA-400 samples exhibited better adsorption performances than the other three samples and good response to an external magnetic field, which indicates that the use of a magnetic field can replace the traditional centrifugal separation step used to remove adsorbents from solution, thereby reducing energy consumption. Furthermore, the CA-400 samples are also reusable for the adsorption of RhB dye, which makes them potentially useful for wastewater treatment, including organic dye removal.
reaction. The hydrothermal reaction was performed at 180 °C for 24 h. After cooling to room temperature, the product was washed with ethanol and then dried at 50 °C for 24 h. The 180N samples were further annealed at 400 °C under atmospheric conditions for 7 h to form the c-Fe2O3/a-Fe2O3 nanorods (400N). To prepare the mesoporous Fe3O4/a-Fe2O3/CA structures (CA-180), FeSO4 (ca. 1.67 g) and CA (ca. 0.3 g) were added to 60 ml of DI water and stirred for 24 h. The resulting solution was centrifuged to obtain black powders, and the black powders were further dried at 50 °C for 24 h. The as-obtained black powders and HMTA (ca. 0.84 g) were added to 60 ml of DI water and stirred for 2 h. The solution was then transferred to a 100-ml Teflon-lined autoclave, and the autoclave was heated at 180 °C for 24 h. After cooling to room temperature, the product was washed with ethanol and then dried at 50 °C for 24 h. The CA-180 samples were
2. Experimental 2.1. Chemicals Ferrous sulfate (FeSO4), hexamethylenetetramine (HMTA) and DI water were used to prepare the Fe3O4/a-Fe2O3 nanorods (180N) and c-Fe2O3/a-Fe2O3 nanorods (400N). The synthesis of carbon aerogel (CA) was based on the procedure described in our previous report [52]; the specific surface area and average pore diameter of the CA were approximately 702 m2/g and 10 nm, respectively. The RhB dye was dissolved in DI water to an initial concentration of 8 ppm. 2.2. Preparation of iron oxide and iron oxide/carbon aerogel structures To prepare the Fe3O4/a-Fe2O3 nanorods (180N), FeSO4 (ca. 1.67 g) and HMTA (ca. 0.84 g) were dissolved in 60 ml of DI water. After stirring for 2 h, the solution mixture was transferred to a 100-ml Teflon-lined autoclave for the subsequent hydrothermal
Fig. 1. XRD patterns of reference (a) Fe3O4, (b) a-Fe2O3 crystals, (c) 180N and (d) CA-180 samples. The inset shows the corresponding XRD patterns at the diffraction angles between 34° and 37°.
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Fig. 2. SEM images of the (a) 180N and (b) CA-180 samples.
further annealed at 400 °C under atmospheric conditions for 7 h to form the mesoporous c-Fe2O3/a-Fe2O3/CA structures (CA-400).
2.3. Adsorption and desorption of the RhB dye The as-prepared 180N, 400N, CA-180 and CA-400 samples were used for RhB dye adsorption studies. RhB dye adsorption was performed by immersing 10 mg of 180N, 400N, CA-180 or CA-400 into 100 ml of RhB dye solution with a RhB dye concentration of 8 ppm under gentle stirring for 12 h to achieve adsorption equilibrium. After magnetic separation, the remaining concentration of RhB dye was determined by UV–visible spectrophotometry to calculate the percentage of RhB dye adsorbed.
2.4. Characterization The surface morphology and the crystalline phases of the as-prepared samples were examined via field-emission scanning electron microscopy (FESEM, Hitachi, S-4800N) and powder X-ray diffraction (PXRD, PANalytical X’Pert PRO, PW3040/60), respectively. The pore size distribution and the specific surface area of the as-synthesized samples were measured based on N2 adsorption/desorption isotherms (BET, Micromeritics ASAP 2020). The magnetic properties of the as-prepared CA-400 samples were studied using superconducting quantum interference device (SQUID) magnetometry (Quantum Design MPMS7). The concentration of RhB dye in solution was measured via UV–visible spectrophotometry (Hitachi, U-3900/U-3900H).
Fig. 3. The N2 adsorption and desorption curves of the (a) 180N and (c) CA-180 samples and the pore size distributions of the (b) 180N and (d) CA-180 samples.
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Fig. 4. XRD patterns of reference (a) c-Fe2O3, (b) a-Fe2O3 crystals, (c) 400N and (d) CA-400 samples. The inset shows the corresponding XRD patterns at diffraction angles between 34° and 38°.
3. Results and discussion The crystalline structures of the as-prepared 180N and CA-180 samples were investigated by XRD, as shown in Fig. 1(c) and (d), respectively. The reference patterns of Fe3O4 and a-Fe2O3 crystals are included in the figure for comparison. The diffraction peaks of the 180N and CA-180 samples appear to fit well with the peaks of the reference a-Fe2O3 crystals, implying that 180N and CA-180 are composed of a-Fe2O3. However, the peak at approximately 35.5° in the patterns of the 180N and CA-180 samples fits well with the reference Fe3O4 crystals (inset in Fig. 1), indicating that both the 180N and CA-180 samples are composite Fe3O4/a-Fe2O3 nanomaterials. The surface morphologies of the 180N and CA-180 samples were further studied using FESEM, as shown in Fig. 2(a) and (b), respectively. In the case of the 180N samples, rod-like materials with a length of several hundred nanometers were observed, as shown in Fig. 2(a), indicating that the composite Fe3O4/a-Fe2O3 nanorods were formed in the absence of CA. However, particle-like structures instead of nanorods were observed in the mesoporous CA samples, as shown in Fig. 2(b). The mesoporous CA restricts the growth of 1-D nanorods to form the mesoporous composite Fe3O4/a-Fe2O3/CA nanostructures observed in the CA-180 samples. These two different nanostructures – nanorods and mesoporous structures – are attributed to the presence or absence of the mesoporous CA. The specific surface areas and pore size of the 180N and CA-180 samples were measured based on the N2 adsorption and
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desorption isotherms obtained via the BET method, as shown in Fig. 3. Both the 180N and CA-180 samples exhibited a type-IV isotherm, as shown in Fig. 3(a) and (c), respectively. The type-IV isotherm indicates the existence of mesoporous structures in the as-prepared 180N and CA-180 samples, which is in good agreement with the average pore sizes of approximately 2.2 and 8.9 nm, respectively. The specific surface areas of the 180N and CA-180 were approximately 16.2 and 267.7 m2/g, implying that the addition of mesoporous CA not only changes the structures but also increases the specific surface areas. The as-prepared 180N and CA-180 samples were further annealed at 400 °C for 7 h; the resulting samples are referred to as 400N and CA-400, respectively. The crystal structures, surface morphologies and specific surface areas of the 400N and CA-400 samples are presented in Figs. 4–6. The crystalline structures of the 400N and CA-400 samples were first investigated by XRD, as shown in Fig. 4. The XRD patterns of the 400N and CA-400 samples are presented in Fig. 4(c) and (d), respectively. The diffraction patterns of the reference c-Fe2O3 and a-Fe2O3 crystals are also included in Fig. 4(a) and (b). The diffraction peaks of the 400N and CA-400 samples match well with the peaks of the reference a-Fe2O3 material. However, after carefully examining the diffraction region from 34° to 38°, we observed that the peak at approximately 35.6° in the patterns of the 400N and CA-400 samples fits well with the peak for the reference c-Fe2O3 material, as shown in the inset of Fig. 4. This result indicates that the 400N and CA-400 samples are composite c-Fe2O3/a-Fe2O3 nanomaterials. Although the compositions of the 400N and CA-400 samples changed, their surface morphologies remained the same as those for the 180N and CA-180 samples, respectively. This result indicates that c-Fe2O3/a-Fe2O3 nanorods and mesoporous c-Fe2O3/a-Fe2O3/CA structures were obtained in the 400N and CA-400 samples, as shown in Fig. 5(a) and (b), respectively. The specific surface areas and pore diameters of the 400N and CA-400 samples were also measured based on their N2 adsorption and desorption isotherms, as shown in Fig. 6. Both the 400N and CA-400 samples exhibited a type-IV isotherm, as shown in Fig. 6(a) and (c), respectively. These type-IV isotherms indicate the presence of mesopores in the as-prepared 400N and CA-400 samples, which is in good agreement with the average pore sizes of approximately 14.9 and 6 nm, respectively. The specific surface areas of the 400N and CA-400 were approximately 27.7 and 551 m2/g, which are larger than the specific surface areas of the 180N (16.2 m2/g) and CA-180 (267.7 m2/g) samples, respectively. The specific surface area of the CA-400 samples was greatly increased from 267.7 to 551 m2/g when the CA-180 samples were annealed at 400 °C, indicating that annealing at 400 °C not only changed the composition but also increased the specific surface area of the material. The magnetic properties of the as-prepared CA-400 samples were analyzed using a superconducting quantum inference device
Fig. 5. SEM images of (a) 400N and (b) CA-400 samples.
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Fig. 6. The N2 adsorption and desorption curves of the (a) 400N and (c) CA-400 samples and the pore size distributions of the (b) 400N and (d) CA-400 samples.
Fig. 7. Magnetization curve of the (a) CA-400 sample and (b) the same curve magnified near the origin. The inset in (a) is a photograph of the sample solution under an external magnetic field.
(SQUID) magnetometer. The magnetic hysteresis loop (Fig. 7(a) and (b)) for the as-prepared CA-400 samples measured at room temperature in an applied magnetic field of up to 50,000 Oe indicates ferrimagnetic behavior in the as-prepared mesoporous composite c-Fe2O3/a-Fe2O3/CA structures. The magnetization saturation of the mesoporous c-Fe2O3/a-Fe2O3/CA structures was 4.6 emu/g, which is substantially lower than that of bulk c-Fe2O3 materials (75 emu/g) [53]. The presence of antiferromagnetic a-Fe2O3 and CA materials and the reduced size of the
c-Fe2O3 nanoparticles may be responsible for the lower magnetization saturation. Fig. 7(b) shows the magnetic hysteresis loop in an applied magnetic field between 400 and 400 Oe, which indicates that the remaining magnetization and coercivity are 0.85 emu/g and 141 Oe, respectively. When the mesoporous c-Fe2O3/a-Fe2O3/CA structures dispersed in solution were subjected to a magnetic field, they were completely separated from the solution, as shown in the inset of Fig. 7(a). The composite c-Fe2O3/a-Fe2O3/CA structures exhibit good response to an
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Table 1 Regression parameters (Qmax, b and R2) of the fitted Langmuir isotherm model for the as-prepared 180N, 400N, CA-180 and CA-400 samples, respectively. Langmuir model
180IM 400N CA-180 CA-400
Qmax
b
R2
1.52 1.74 45.67 151.52
4.41 17.94 2.11 2.44
0.997 0.995 0.994 0.994
Table 2 Comparison of RhB dye adsorption capacity with other adsorbents.
Fig. 8. Plots of the RhB dye removal capacity (Qe) as a function of time for the 400N and CA-400 samples.
Adsorbents
Adsorption capacity (mg/g)
References
MPGC-900 Fe3O4/RGO(M2) OPBC Coffee ground powder Carbonaceous adsorbent CA400
73.0 142.86 69.86 5.26 91.1 151.5
54 55 56 57 58 This study
Fig. 9. RhB dye removal using the 180N, 400N, CA-180 and CA-400 samples in DI water solution with an initial RhB dye concentration of 8 ppm. Fig. 11. Three cycles of RhB dye removal using the CA-400 sample at the initial RhB dye concentration of 8 ppm.
Fig. 10. Plots of the RhB dye removal capacity (Qe) as a function of the RhB dye equilibrium concentration (Ce) for the 180N, 400N, CA-180 and CA-400 samples.
external magnetic field, which indicates that the use of a magnetic field can replace the traditional centrifugal separation step used to remove adsorbents from solution, thereby reducing energy consumption.
The kinetic study of RhB dye adsorption using 400N and CA-400 samples was firstly studied in Fig. 8. Fig. 8 is the plot of the RhB dye removal capacity (Qe) as a function of the adsorption time. The RhB dye removal capacity (Qe) reaches a stable value after the adsorption time of 12 h. This result indicates the adsorption equilibrium is achieved for the adsorption time of 12 h. Consequently, the adsorption time is 12 h for the other RhB dye adsorption experiments. The removal percentage of RhB dye using the as-prepared 180N, 400N, CA-180 and CA-400 samples is demonstrated in Fig. 9, where the initial concentration of the RhB dye was 8 ppm. The percentages of RhB dye removed were 3.3%, 4.1%, 78.1% and 98.2% using the as-prepared 180N, 400N, CA-180 and CA-400 samples, respectively. The addition of mesoporous CA restricts the formation of 1-D Fe3O4/a-Fe2O3 nanorods (180N samples) with a smaller specific surface area (16.2 m2/g) in favor of the mesoporous Fe3O4/a-Fe2O3/CA structures with a larger specific surface area (267.7 m2/g). This increased specific surface area results in increased RhB dye removal. However, the efficiency of RhB dye removal by the as-prepared CA-400 samples is still greater than the efficiency of the CA-180 samples because of their larger specific surface area. One may argue that the surface charges of CA-180 and
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CA-400 samples should be another important factor for the RhB dye adsorption capacity. As a result, the surface charges at pH 7 of the CA-180 and CA-400 samples are also carried out. The surface charges of the CA-180 and CA-400 samples at pH 7 are approximately 44.5 and 45.1 mV, respectively. Since the surface charges of CA-180 and CA-400 samples are almost the same, the specific surface area of CA-180 and CA-400 samples is the significant factor for RhB dye adsorption capacity. CA-400 sample has larger specific surface area (551 m2/g) than CA-180 sample (267.7 m2/g), leading to the better RhB dye adsorption capacity than CA-180 sample. The adsorption ability of the 180N, 400N, CA-180 and CA-400 samples is further determined from the adsorption isotherms. Fig. 10 is the plots of RhB dye adsorption capacity (Qe) versus the equilibrium concentration of RhB dyes (Ce) using the as-prepared 180N, 400N, CA-180 and CA-400 samples, respectively. The RhB dye adsorption capacity using the as-prepared CA-400 samples is greatly higher than the as-prepared 180N, 400N and CA-180 samples due to the largest specific surface area of the CA-400 samples (551 m2/g), which is consistent with the results of Fig. 9. The adsorption data were further used to fit the Langmuir isotherm 1 e þ QCmax . Here, Ce is the equilibrium concentration model: QC ee ¼ Q max b of RhB dyes, Qe is the RhB dye adsorption capacity, Qmax and b are the monolayer capacity of the adsorbent and the adsorption constant, respectively. The value of the regression parameters (Qmax, b and R2) are shown in Table 1. The fitting coefficients (R2) of the 180N, 400N, CA-180 and CA-400 samples are 0.997, 0.995, 0.994 and 0.994, respectively, indicating that the Langmuir model fits the adsorption data of the 180N, 400N, CA-180 and CA-400 samples well. The RhB dye adsorption capacity of the as-prepared CA-400 samples was also compared with other adsorbents, as shown in Table 2. The results indicate that the as-prepared CA-400 samples in this study are with better RhB dye adsorption capacity than other adsorbents, such as MPGC-900 [54], Fe3O4/RGO [55], OPBC [56], coffee ground powder [57] and carbonaceous adsorbent [58]. The reuse of the CA-400 sample was also investigated over three cycles, as shown in Fig. 11. The RhB removal percentage reaches a stable value of approximately 98% after the third adsorption cycle of the RhB dyes. This result indicates that the as-prepared CA-400 samples are reusable for RhB dye removal. As a result, the as-prepared CA-400 samples have great potentials for the RhB dye removal in industrial applications.
4. Conclusions In this study, composite Fe3O4/a-Fe2O3 nanorods were successfully prepared using a hydrothermal process at 180 °C, and composite c-Fe2O3/a-Fe2O3 nanorods were obtained after the Fe3O4/a-Fe2O3 nanorods were annealed at 400 °C. This material was obtained because the Fe3O4 phase was oxidized to c-Fe2O3 during the annealing process. The specific surface areas of the composite Fe3O4/a-Fe2O3 and c-Fe2O3/a-Fe2O3 nanorods were 16.2 and 27.7 m2/g, respectively. To further increase the specific surface area of the as-prepared materials, a mesoporous CA with a high specific surface area was added to the reactant solution for the hydrothermal reaction at 180 °C. As a result, mesoporous composite Fe3O4/a-Fe2O3/CA structures with a specific surface area of 267.7 m2/g were successfully prepared when mesoporous CA was added. Mesoporous composite c-Fe2O3/a-Fe2O3/CA structures with a larger specific surface area of 551 m2/g were further obtained after the mesoporous Fe3O4/a-Fe2O3/CA structures were annealed at 400 °C. The RhB dye removal performance of the mesoporous composite c-Fe2O3/a-Fe2O3/CA structures was better than the dye removal performance of the other three structures because
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