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Adsorption and heterogeneous degradation of rhodamine B on the surface of magnetic bentonite material
Accepted Manuscript Title: Adsorption and heterogeneous degradation of rhodamine B on the surface of magnetic bentonite material Author: Dong Wan Guanghua Wang Wenbing Li Kun Chen Lulu Lu Qin Hu PII: DOI: Reference:
S0169-4332(15)01091-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.05.004 APSUSC 30318
To appear in:
APSUSC
Received date: Revised date: Accepted date:
19-1-2015 8-4-2015 2-5-2015
Please cite this article as: D. Wan, G. Wang, W. Li, K. Chen, L. Lu, Q. Hu, Adsorption and heterogeneous degradation of rhodamine B on the surface of magnetic bentonite material, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.05.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights: Magnetic bentonite was prepared by in situ precipitation oxidization method.
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The composite material has been used to adsorb and degrade rhodamine B.
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The adsorption and catalytic ability of Fe3O4 improved significantly after being dispersed in the
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bentonite.
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The magnetic bentonite is low-cost and environment-friendly and has a long-term reusability and stability.
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Adsorption and heterogeneous degradation of rhodamine B on the surface of magnetic
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bentonite material
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Dong Wan, Guanghua Wang, Wenbing Li, Kun Chen, Lulu Lu, Qin Hu
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Abstract: A kind of imitation enzyme catalyst, Fe3O4 nanoparticles decorated Al pillared bentonite (Fe3O4/Al-B), was
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successfully prepared by in situ precipitation oxidization method and then applied for the adsorption and degradation of
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rhodamine B (RhB) in the presence of H2O2. The catalyst was characterized by XRD, SEM, XPS, BET, VSM and FTIR
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spectroscopy. The effects of oxidant concentration, initial RhB concentration and iron leaching on the degradation of RhB
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were investigated. The surface interactions with RhB in the absence and the presence of oxidant could be well described
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by Langmuir and Langmuir–Hinshelwood models, respectively. The Fe3O4/Al-B showed higher ability of adsorption and
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degradation efficiency towards RhB than bare Fe3O4 in the batch experiments. The whole degradation process of RhB
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followed pseudo-first-order rate law and was mainly controlled by surface mechanism reaction. The enhanced
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degradation efficiency of Fe3O4/Al-B might relate to the enrichment of RhB molecules by Al-B in the vicinities of active
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sites. Furthermore, the catalyst showed stable catalytic activity and convenient recycling. Negligible iron leaching
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showed the reused Fe3O4/Al-B can withstood the oxidation.
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Key word: adsorption; magnetic bentonite; heterogeneous catalyst; rhodamine B
1. Introduction
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In recent years, Fe3O4 magnetic nanoparticles (MNPs) as a popular magnetic material have attracted significant
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interest of many researchers, because of their potential applications in catalysis, biotechnology, water purification and so
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on[1,2]. In 2007, Gao et al. found the Fe3O4 MNPs possessed intrinsic peroxidase-like activity, which was able to catalyse
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the oxidation of organic substrates in wastewater treatment[3]. Since then the peroxidase-like nature of Fe3O4 MNPs was
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widely explored for the applications in the degradation of some refractory organics such as phenol[4], aniline[5],
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2,4-dichlorophenol[6], p-nitrophenol[7] and dye pollutants[8]. Moreover, with an inverse spinel crystal structure, Fe3O4
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MNPs exhibit unique electric and magnetic properties, which based on the transfer of electrons between Fe2+ and Fe3+ in
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the octahedral sites, allowing the Fe species to be reversibly oxidized and reduced while keeping the same structure[9]. So 2
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it can be able to function steadily as a heterogeneous catalyst without substantial loss. In addition, Fe3O4 is magnetic and
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can be easily separated from the terminal solution by magnetic separation, which provide it a huge potential application in
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the advanced treatment of organic wastewater. However, several reports demonstrated that the H2O2-activating ability of Fe3O4 MNPs was not so strong for the
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practical removal of refractory organic pollutants from wastewaters and the coaggregation of Fe3O4 MNPs, especially in
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water solution, will decrease the effective surface active sites of Fe3O4 MNPs and thus reduce their catalytic
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activity[10-14]. Therefore, it is urgently necessary to improve the H2O2-activating ability and dispersity of Fe3O4 MNPs.
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Recently, immobilized Fe3O4 MNPs on solid supports during the preparation process were found to be effective to prevent
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the coaggregation and improve the catalytic activity. The supports were organic or inorganic materials, such as
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multiwalled carbon nanotubes[12-14], mesocellular carbon foam[15], poly(3,4-ethylene-dioxythiophene)[16] and
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activated carbon[17]. Given the economic aspect, however, the supports mentioned above are too expensive to apply in
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the practical wastewater treatment. Bentonite, as an abundance and low cost mineral with many excellent properties such
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as high stability, microporosity and larger surface area, has been widely used in a number of industrial branches[18]. After
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intercalated by polymeric inorganic oxocations, the stability and surface area of bentonite could be promoted effectively.
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Due to its smaller particle size, higher stability, larger surface area, sheet-like structure and most of all, low cost, pillared
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bentonite may be particularly suitable to be used as a support for the synthesis of Fe3O4 MNPs. The dispersity, adsorption
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capacity or catalytic ability of Fe3O4 MNPs may be improved after being dispersed in the bentonite.
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In the present study, Al pillared bentonite(Al-B) was chosen to immobilize Fe3O4 MNPs, which may prevent the
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coaggregation of Fe3O4 MNPs. One the other hand, the excellent adsorption property of pillared bentonite may have
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positive effect on the catalytic activity of Fe3O4 MNPs. Recently, Lian et al. had synthesised the magnetic bentonite
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material(Fe3O4@Al-B) by a solvothermal reaction[19]. Their study has confirmed that the magnetic bentonite has
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enhanced adsorptivity for organic pollutants than bare Fe3O4 or bentonite. However, they investigated only adsorption
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property instead of peroxidase-like catalytic activity of magnetic bentonite. Besides, the solvothermal process may also
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have difficult in practical application. In our research, we report another convenience and green method, in situ
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precipitation oxidization, to synthesis magnetic bentonite. In order to test the adsorption and catalytic activity, the
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material was further used for adsorbing and degrading organic pollutants in the present of H2O2 in water. As we known,
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rhodamine B (RhB) is a typical dye pollutant and has been selected as a model dyeing pollutant in many investigations
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because of its widespread application and recalcitrant nature[20-22]. In this study, the magnetic bentonite had been used
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in peroxidase-like process to adsorb and degrade RhB in water.
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2. Materials and methods
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2.1. Materials Natural bentonite was procured from Shanghai No.4 Reagent & H.V. Chemical Co., Ltd, China. Rhodamine B (RhB)
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was obtained from Shanghai Zhanyun Chemical Co., Ltd., China. All chemicals were of analytical grade and used as
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received. In the experiments, distilled water was used for preparing the solutions and suspensions.
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72 2.2. Preparation of magnetic bentonite (Fe3O4/Al-B)
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The Al pillared bentonite(Al-B) was prepared by pillaring the bentonite through cation-exchange process as
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described in literature [23]. The magnetic bentonite were synthesized by in situ precipitation oxidization method. 3.1 g
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Al-B was added into a 1000 mL flask containing 300 mL distilled water with constantly stirring. The solution was bubbled
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with N2 flow for 15 min to remove the dissolved oxygen and placed in a 95 °C water bath, then 11.1 g FeSO4·7H2O was
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added into the flask. Afterwards, 100 mL solution containing 3.2 g NaOH and 3.2 g NaNO3 added dropwise (10 mL·min-1)
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into the heating solution while keep vigorous stirring and stable N2 flow during the entire reaction period. After that, the
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solution was heated at 95 °C for another hour, then cooled to room temperature. The precipitate was isolated by a
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permanent magnet. After repeated washing with deionized water and absolute ethanol under ultrasonication, the formed
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Fe3O4/Al-B nanocomposites were dried in a vacuum oven at 100 °C for 12 h. Fe3O4 was synthesized as above procedure
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without adding Al-B. All the products were stored in desiccator under ambient temperature for further experiments.
84 2.3. Characterization
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X-ray diffraction (XRD) patterns of the samples were recorded by a Rigaku D/max-RB diffrac-tometer (Rigaku,
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Japan) at 40 kV and 30 mA using filtered Cu Kα radiation (λ= 0.15418 nm). Micrographs of the samples were taken by
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using a Nova400 Nano SEM (FEI, USA). Before observation of SEM, all samples were fixed on silicon wafer and coated
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with gold. X-ray photoelectron spectroscopy (XPS) was performed with a MULT1LAB2000 X-ray photoelectron
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spectrometer (VG, USA) with a monochromatized Al Kα X-ray source (25 eV) operated at 300 W to identify metal
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oxidation states of the nanocomposites. After shirley background subtraction, the spectra were analyzed by the XPSPeak
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4.1 program. Magnetic characterization of the samples were carried out using a JDAW-2000D vibrating sample
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magnetometer (VSM) at a temperature of 298 K with fields up to 6000 Oe. The Fourier transform infrared spectroscopy
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(FTIR) of the samples were recorded on a Nicolet 6700 FTIR spectrometer (Thermo Scientific, USA) in the 400~4000
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cm−1 range with a resolution of 2 cm-1. N2 adsorption–desorption isotherms of the samples were recorded on an ASAP
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2020-M micropore physisorption analyzer (Micromeritics, USA). Specific surface areas were calculated by the 4
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Brunauer–Emmett–Teller (BET) method.
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2.4. Sorption experiments In order to determine the kinetics of the sorption process, 200 mL 40 mg∙L-1 RhB solution was added into a 500 mL
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flask. The sorption experiments were subsequently started by adding 0.2 g Fe3O4/Al-B or Fe3O4 nanoparticles with
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mechanically stirring. At several points in time the concentration of the freely dissolved fraction of RhB was determined.
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Furthermore, the desorption kinetics of the process was studied. Samples of the adsorption experiment described above
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were allowed to equilibrate over a time period of 12 h, and then the samples were magnetic separated and the clear water
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phase was decanted. The desorption process was started by addition of 200 mL neutral fresh deionized water with
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mechanically stirring. After selected desorption time, the freely dissolved fraction of RhB was measured.
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All equilibrium sorption experiments were conducted at 25 °C in the dark at the initial pH of RhB solution. The solid
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samples were mixed with variable solute concentrations. Results of adsorption kinetic experiments indicated that sorption
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equilibrium was achieved within 2 h. Before analysis, the suspensions were magnetic separated and the clear water phase
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was decanted. Then the equilibrium concentrations of RhB in the aqueous phase was measured and sorbed concentrations
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were calculated by difference according to:
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qe
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where qe (mg∙g-1) is the sorbed concentration, Ci (mg∙L-1) is the initial aqueous phase concentration, Ce (mg∙L-1) is the
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equilibrium concentration in the aqueous phase, V (L) is the volume of solution, Ms (g) is the mass of solid sorbent.
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2.5. Degradation of RhB by heterogeneous catalytic experiments
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The experiments were carried out in a conical flask with a stopper (containing 200mL of reaction solution). All
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experiments were carried out under constantly stirring to make the catalyst good dispersion. The initial pH and
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temperature were as same as that in sorption experiments. Before reaction, the suspension containing magnetic bentonite
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and RhB was stirred for 2 h to achieve adsorption equilibrium. The degradation reaction was initiated when H2O2 was
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added to the solution.
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During all the oxidation reactions, 5 mL aliquots were withdrawn and clarified quickly by an outer strong permanent
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magnet at selected time intervals. The aqueous phase was sampled for analysis. The solid catalyst separated from aqueous
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phase was rinsed by 5 mL ethanol for three times. The rinsed liquid was mixed for analysis. The residual RhB amount is
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the sum of that in aqueous and solid phase. After the oxidation reaction, the catalyst was repeated washing and dried for 5
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reusing. All experimental runs were performed in the absence of light. Each experiment was achieved in triplicates. All
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results were expressed as a mean value of the 3 experiments.
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2.6. Analyses At given intervals of degradation, a sample was analyzed by UV-Vis spectroscopy (Ultrospec 3300 pro, GE
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Healthcare Bio-Sciences China Ltd) at a wavelength of 554 nm, which is the maximum absorption wavelength of
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RhB[20]. The concentration of RhB was converted through the standard curve method of dyes. The concentration of iron
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leaching in the solution was measured by the 1,10-phenatroline spectrophotometric method[24]. The chemical oxygen
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demand (CODCr) was determined by dichromate method. To eliminate the interference of H2O2 with CODCr
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measurements, the reaction was finally blocked by raising the pH to 9~10, adding MnO2 and allowing the samples to sit
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overnight[25].
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3. Results and discussion
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3.1. Characterization of catalyst
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The X-ray diffraction patterns of the samples are shown in Fig.1. Diffraction peaks assigned to bentonite at
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2θ=5.8°[18] can be seen for Al-B and Fe3O4/Al-B, indicating that the bentonite structure was not destroyed after
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treatment by pillaring process and chemical precipitation of Fe3O4. As shown in Fig.1b and c, eight characteristic peaks
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for Fe3O4 (2θ=18, 30, 35.5, 37, 43, 53.4, 57 and 62.5°) are observed for the synthesized Fe3O4 and Fe3O4/Al-B. And no
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other peaks corresponding to the hematite are detected in the XRD patterns, indicating that the Fe3O4 nanoparticles in the
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composites are pure Fe3O4 with inverse spinel structure[26]. In addition, no other peaks are observed in the XRD pattern
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of reused Fe3O4/Al-B (Fig. 1d). This indicated that there was no obvious change of structure and component of the
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catalyst after oxidation reaction.
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The morphologies of Al-B and catalysts were characterized by SEM. The SEM image in Fig. 2a shows that the
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pillared bentonite is layer structure and had a coarse porous surface, which is essential for the adsorption of organic
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pollutant. The synthesized Fe3O4 nanoparticles (Fig. 2b) with diameters ranging from 40 to 100 nm are dramatically
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co-aggregated together. The SEM image in Fig. 2c shows that the Fe3O4 nanoparticles growing on the Al-B surface with
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better dispersing and less co-aggregation. Although the products have experienced repeated washing in water and
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methanol under ultrasonication before SEM measurements, almost all magnetite nanoparticles are still found on the
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bentonite surface. This indicates the strong interaction between Al-B and Fe3O4 nanoparticles. The image of Fe3O4/Al-B
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nanoparticles reused for three times in Fig. 1d shows no obvious change of structure of the catalyst after oxidation 6
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reaction. The XPS spectrums of Fe3O4 and Fe3O4/Al-B are taken over the entire energy range showed in Fig. 3. The
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photoelectron lines at binding energy of about 284, 530, and 711 eV are attributed to C 1s, O 1s, and Fe 2p, respectively. A
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detail of the Fe peaks can be seen in the inset of Fig. 3. The peaks of Fe 2p1/2 and Fe 2p3/2 locate at 710.9 and 724.5 eV,
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respectively. The results agree with literature data for magnetite and substantiate the XRD results, thus further confirm
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that the oxide in the samples are Fe3O4[26-29].
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Fig. 4 exhibits the hysteresis loop of the as-prepared samples at room temperature. It can be seen that the saturation
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magnetization of Fe3O4/Al-B (30.9 emu∙g-1) is much smaller than that of the unmodified Fe3O4 (82.9 emu∙g-1). The
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decrease may result from the existence of nonmagnetic bentonite. A superparamagnetic behavior (i.e., zero coercive field
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and zero remnant magnetization) is observed for all the samples. The result shows that they can be manipulated by an
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external magnetic field, such as a magnet, thus providing a potential advantage for the separation, recovery and reuse of
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adsorbents as well as catalysts.
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Nitrogen adsorption/desorption isotherms of the Fe3O4 and Fe3O4/Al-B material showed in Fig. 5 display type II
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isotherms. Wide hysteresis area of N2 adsorption/desorption isotherms can be clearly seen in the case of Fe3O4 and
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Fe3O4/Al-B, which suggest the occurrence of capillary condensation and the wide distributions of pores in both
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cases[12,19]. The specific surface areas calculated by using a BET equation are found to be 53.02 and 4.68 m2∙g-1 for
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Fe3O4/Al-B and Fe3O4, respectively. The nanocomposites display much higher adsorption quantity than pure Fe3O4
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because the porosity and good adsorbability of the Al-B used as support.
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3.2. Sorption of RhB
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A sorption/desorption experiment was performed to make a comparison of the timescale of degradation reaction
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with that of sorption and desorption of RhB on Fe3O4/Al-B and Fe3O4 nanoparticles (Fig. 6). As can be seen, the
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sorption process of RhB on the two kinds of nanoparticles closely approach equilibrium within 10 min. No significant
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changes are observed after 60 min and 240 min. About 10% and 80% RhB are sorbed on Fe3O4 (1.0 g∙L-1) and
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Fe3O4/Al-B (1.0 g∙L-1), respectively. The desorption experiment shows the re-equilibration was almost completed within
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7 min for Fe3O4 and 8 min for Fe3O4/Al-B.
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RhB aqueous solutions with different initial concentrations varying from 5 mg∙L-1 to 400 mg∙L-1 were used for the
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adsorption experiment. Sorption isotherms were determined to investigate the effect of contaminant sorption on the
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catalytic oxidation(Fig. 7). The experimental isotherm data were fitted to the equations of Langmuir and Freundlich by
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applying linear regression analysis. On the basis of the statistic analysis, the isotherms can be well described with the 7
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Langmuir model which assumed that the single adsorbate binds to a single site on the adsorbent and that all surface sites
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on the adsorbents have the same affinity for the adsorbate and no interactions between the adsorbates. The linear form
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of the Langmuir equation is given by:
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qe
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qm(mg·g-1) is the monolayer adsorption capacity, and Ka (L·mg-1) is the Langmuir constant. The fitted Langmuir
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equations are qe = 4.276×0.029Ce/(1 + 0.029Ce) for Fe3O4 (R2 = 0.9937) and qe = 62.154×0.121Ce/(1 + 0.121Ce) for
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Fe3O4/Al-B (R2 = 0.9902), respectively. Good correlations imply that the adsorption of RhB on the catalysts are
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monolayer adsorption. Isotherms for RhB show that the sorption affinity of Fe3O4/Al-B is much higher than that of
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Fe3O4. The adsorption capacities of RhB onto Fe3O4/Al-B and Fe3O4 are calculated to be 62.15 and 4.28 mg·g-1, which
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are in conformity with the specific surface area.
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3.3.1. RhB degradation experiments by heterogeneous catalytic reaction
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The catalytical degradation efficiencies of RhB by Fe3O4/Al-B and Fe3O4 were evaluated under different
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experimental conditions (Fig. 8). It can be seen from Fig. 8 that no degradation of RhB happens when there is only
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Fe3O4/Al-B in the solution without the addition of H2O2. The experiment was also carried out to evaluate the ability of
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H2O2 to eliminate RhB without the addition of any heterogeneous catalyst. Fig. 8 shows that the degradation of RhB is
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almost negligible in the presence of H2O2 only, which may due to its low oxidation potential compared with hydroxyl
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radicals. The catalytic ability of Fe3O4/Al-B in presence of H2O2 shows a significant RhB reduction achieving conversion
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rate of 97.8% after 300 min, which is much more efficient than bare Fe3O4 and H2O2 (58.3%). The catalytic ability of Al-B
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was also evaluated in the presence of H2O2. The very low degradation percentage of RhB indicates that the support is
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insignificant catalytically active. Therefore, the contribution of the direct catalysis of Al-B to the improved degradation
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performance with Fe3O4/Al-B is very limited.
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It is reported in several works that the synergistic effect resulting by the adsorption property of support caused the
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improvement in the rate of substrate degradation[12-15]. Herein we have similar results. Since Al-B is a good adsorbent,
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the adsorption capacity of Fe3O4/Al-B for RhB is enhanced dramatically compared with that of bare Fe3O4, which has
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been discussed in Section 3.2. Thus, Al-B provides more available and effective contact surface areas between reactants
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and active sites, and the adsorbed RhB molecules in the immediate vicinity of immobilized Fe-ions are easily attacked by
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the generated •OH. On the other hand, the dispersity of Fe3O4 nanoparticles is promoted after immobilized on Al-B, 8
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which can been seen from the SEM photograph(Section 3.1). More active sites of Fe3O4 nanoparticles are exposed, which
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also result in an increase of the generation of •OH. The synergistic effect between Fe3O4 and Al-B is obvious in the
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degradation of RhB through heterogeneous catalytic progress.
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According to previous study, hydroxyl radical(•OH) could be generated by the reaction between H2O2 and Fe3O4
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leading to a degradation of RhB, which was known as the Haber–Weiss mechanism[20-22]. The degradation of the dye by
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•OH can be typically described as a second-order reaction:
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dC kC[OH] dt
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where C and [•OH] are concentrations of RhB and hydroxyl radical, respectively. k is the second-order rate constant, and
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t is the reaction time. By assuming that •OH instantaneous concentration is constant, the kinetics of degradation of RhB in
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water can be described according to the pseudo-first-order equation as given below[20]:
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−ln(Ct/C0) = kappt
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kapp = k[•OH]
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where C0 is the initial concentration of RhB and kapp is the apparent pseudo-first-order rate constant (min-1). The kapp
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constants are obtained from the slopes of the straight lines by plotting −ln(Ct/C0) as a function of time t, through
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regression. Good correlation coefficients (R2 ~ 0.99) are obtained in our system (Fig. 9).
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The kinetic constants rate kapp are determined at different H2O2 concentrations (Fig. 9a). It is observed that the
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degradation rate of RhB increases observably when H2O2 concentration increased from 0 to 150 mmol∙L-1. Since the RhB
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degradation is directly related to the concentration of the •OH produced by the catalytic decomposition of H2O2, more
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RhB decomposition is expected with a higher increase of H2O2 concentration. However, a significant improvement is not
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seen when the H2O2 concentration increased to 250 mmol∙L-1, because of the H2O2 was adsorption saturation on the active
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sites of catalyst. It is well accepted that, in the peroxidase-like catalytic reaction, the kinetic constant rate will tend to be
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steady when the concentration of reactant continually increasing, but in this system, a decrease in the values of kapp is
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observed at a much higher H2O2 concentration. The occurrence of this maximum H2O2 concentration for the effective
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degradation of RhB can be explained by scavenging effect of hydroxyl radicals by hydrogen peroxide[27]:
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H2O2 + •OH → H2O + •OOH
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The •OH radicals preferentially attack the RhB molecules at low H2O2 concentration, whereas at much higher H2O2
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concentration, there is a competitive reaction between the RhB and H2O2. Moreover, the oxidation potential of generated
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radicals •OOH is much smaller than that of the •OH species. Therefore, it makes the degradation of RhB slowdown.
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3.3.3. Effect of initial RhB concentration The pseudo-first-order apparent rate constant was determined at various initial RhB concentration (Fig. 9b). It can be
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seen that the kapp constant decreases when the initial concentrations of RhB increased from 10 to 60 mg·L-1. Increasing
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amount of pollutant may occupy a greater number of iron active sites, which becomes unavailable for H2O2 and results in
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lower •OH generation rate. More RhB are sorbed on the catalyst surface and less H2O2 are interacted with iron surface,
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leading to less hydroxyl radical formed at the surface. The Langmuir–Hinshelwood model, which is widely used in
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heterogeneous catalysis, was tested [29]. In this case, the species present in the reaction compete with each other for
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adsorption on a fixed number of active sites. This Langmuir–Hinshelwood model postulates that the rate of reaction of
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two species adsorbed on the surface is the ratelimiting step:
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1 1 1 [RhB]i kapp kint kint K s
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where kapp is the initial pseudo-first-order rate constant (min-1), kint is the intrinsic reaction rate constant (mg·L-1·min-1),
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and Ks is the adsorption constant of RhB over catalyst surface (L·mg-1). The linear correlation (R2 ~ 0.99) between 1/kapp
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and [RhB]i is relatively good, indicating that surface reactions of RhB, including sorption and oxidation by surface •OH,
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play an important role in determining rate of the whole reaction. kint and Ks are obtained as 0.788 mg·L-1·min-1 and 0.04
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L·mg-1. This Ks value is lower than the sorption constant determined in the absence of oxidant by Langmuir model (Ka =
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0.121 L·mg-1, in Section 3.2), suggesting an adsorption competition for the catalyst surface.
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3.3.4. Removal of RhB in aqueous or sorbed phase
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Harber–Weiss circle indicated that the generation of •OH from H2O2 is the key step of the whole degradation
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process[9,12]. The formation of radicals from H2O2 and iron oxides has been proposed in the literature[30]. The
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mechanism of H2O2 activation by Fe3O4/Al-B may involve the initial formation of a complex assigned as ≡FeII(H2O2)
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(Eq. (8)). The initially generated ≡FeII(H2O2) species can produce •OH by intramolecularly electron transfer, then a
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chain of reactions occurring on the catalyst surface:
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≡FeII + H2O2 → ≡FeII(H2O2)
(8)
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≡FeII(H2O2) → ≡FeIII + OH- + •OH
(9)
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≡FeIII + H2O2 → ≡FeIII(H2O2)
(10)
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≡FeIII(H2O2) → ≡FeII + H+ + •OOH
(11) 10
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≡FeIII + •OOH → ≡FeII + O2 +H+
(12)
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On the other hand, the RhB molecules are mostly sorbed on the surface of Al-B, which has been discussed in
275
Section 3.2. The interactions between RhB and Al-B can be described as equilibrium reaction between sorbed and
276
aqueous species:
277
Al-B + RhBe ↔ Al-B(RhB)s
278
The hydroxyl radicals formed by the reaction between hydrogen peroxide and iron surface can probably attack the
279
sorbed species as well as the aqueous species leading to the degradation of RhB:
280
•OH + Al-B(RhB)s → Al-B + RhB by-products → ···CO2 + H2O
(14)
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•OH + RhBe → RhB by-products → ···CO2 + H2O
(15)
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IR spectras show the RhB sorbed on the surface of Fe3O4/Al-B along the oxidation reaction (Fig. 10a). Prior to IR
283
analysis, oxidation experiment of RhB was conducted versus time in 1 g·L-1 of Fe3O4/Al-B with the optimum value of
284
H2O2 dose. During the oxidation reaction, a few milligrams of catalyst were sampled from oxidation reactor at selected
285
time intervals and immediately analyzed by IR spectroscopy (Fig. 10a).
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As can be seen from Fig. 10a, in the Fe3O4/Al-B samples, the peak at 1044 cm-1 assigns to Si–O–Si stretching
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vibrations in montmorillonite lattice[31]. The band at 572 cm-1 attributes to the vibrations of Fe–O, which is the
288
characteristic peak of Fe3O4[26]. There are some new peaks appear on the IR spectra of Fe3O4/Al-B-sorbed RhB
289
compare with that of the Fe3O4/Al-B alone. The peaks at 1607 cm-1 corresponds to aromatic ring vibration, while the
290
1341 cm-1 peak and 1415 cm-1 peak attribute to C–aryl bond and C–N stretching vibration respectively, which can be
291
assigned to RhB [20,21]. The sorbed RhB disappear from the surface of Fe3O4/Al-B at 180 min treatment time, which is
292
corresponding to the result in Fig. 10b. The adsorbed amount of RhB was calculated according to the residual
293
concentration of RhB in aqueous solution. It can be seen from the Fig. 10b that the decrease of RhB in aqueous solution
294
is quite slow during the reaction process, which indicates that the main decomposition of RhB is occurred in the
295
adsorbed phase on the surface of Fe3O4/Al-B, thus confirming with the RhB degradation process discussed above.
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However, even the sorbed RhB disappear from the surface of catalyst, there may be some intermediates are adsorbed on
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catalyst instead of RhB, which are further broken into CO2 and H2O by •OH. On the other hand, the adsorbed amount of
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RhB in degradation process is lower than that of sorption experiment (Section 3.2), which indicates the adsorption
299
competition between RhB and H2O2 or intermediates. Therefore, we can conclude that both desorption and degradation
300
processes contribute to the disappearance of RhB from catalyst surface at the first stage of reaction. In the case of
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desorption, H2O2 competes with RhB for the fixation on active sites as previously shown by Langmuir–Hinshelwood
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model, and displaces it from surface to aqueous phase. In the case of degradation, the •OH radicals generated from the
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surface reaction between hydrogen peroxide and surface active sites may preferentially react with surface sorbed RhB,
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which confirming with the results in Section 3.3.1. The enhanced adsorption of RhB has a dramatically promotion for its
305
degradation. The CODCr removal rate of RhB solution during the degradation process was recorded (Fig. 10b). Five hours after
307
the reaction started, the CODCr removal rate of RhB solution is about 82.8%, which means the RhB are effectively
308
degraded in our system. The IR spectra of RhB before and after degradation has also been analyzed (Fig. 10a). The
309
main wavelength of the absorption peaks of RhB is range from 2000–1000 cm-1. As can be seen, after degradation, most
310
bands greatly have lessened in intensity or disappeared, indicating that drastic destruction happened. However, some
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bands are still not disappeared, related to the uncompleted destruction of some aromatic group and inadequate
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mineralization, which well corresponds with the COD result.
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3.4. The catalyst stability and influence of homogeneous catalytic reaction
It is crucial to evaluate the stability of a heterogeneous catalyst. The action of H2O2 on the oxide surface can
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transform the oxide particles into an only FeIII-bearing mineral or into an amorphous iron oxide which may be less stable
317
and more soluble. Subsequently, a different kinetic behavior and degradation rate of organic compound may be observed
318
owing to a substantial change in the surface characteristics of the catalyst. But in this study, the very low dissolved iron
319
concentration measured (< 0.5 mg·L-1) did not expect the formation of more soluble iron oxide. In addition, the XRD
320
diffraction pattern of the used Fe3O4/Al-B was found to be same as that recorded before reaction (Fig. 1c and d) and the
321
SEM image of the used catalyst was also found to be similar to that of the fresh particles (Fig. 2d). XPS is sensitive to the
322
outermost layer of solid sample and provides both semi-quantitative surface composition and chemical state information.
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The details of the Fe 2p peaks (Fe 2p1/2 and Fe 2p3/2) of the Fe3O4/Al-B samples before and after use during the RhB
324
degradation are presented in Fig. 11. The Fe 2p peaks at binding energies (BE) of 711.2 and 725.0 eV with a satellite
325
signal at 719.0 eV are characteristic of Fe3+, while the peaks at BE of 709.9 and 723.4 eV with a satellite signal at 715.5
326
eV are characteristic of Fe2+[10,27]. The components of two oxidation states of surface iron species are consistent with
327
the binding energy values[12]. The major component is found to be Fe3+ which contributes to 67.6% of the total iron
328
surface atoms, while 32.4% of the total iron surface atoms are in the Fe2+ state. This result corresponds with the Fe3O4
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crystal structure. For the sample after RhB degradation, the Fe2+ is found to be 20.8% and Fe3+ 79.2%. This indicates that
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part of the Fe2+ in the outermost layer of the catalyst are oxidized into Fe3+ during oxidation reaction, which has been
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discussed in Section 3.3.4.
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The reusability of the catalyst has been evaluated under the reaction conditions which were identical with the first 12
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oxidation cycle. The catalyst was reused without any regeneration. As shown in Fig. 12, the degradation efficiencies of
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RhB are 98.7%, 97.7%, 97.3% and 96.2% after 300 min for cycle 1, 2, 3 and 4, respectively. The RhB degradation
335
performances on the reused catalyst show a slight difference with the first oxidation cycle, which indicates the high
336
stability of the catalytic activity during oxidation reactions. The homogeneous catalytic experiments were also performed using iron salts in similar concentration (1.0 mg·L-1
338
Fe3+) based on the maximal amount of iron leaching from the catalyst after the oxidation cycle. It can be seen from the Fig.
339
12 that RhB degradation efficiency is only 21% after 300 min. Therefore, the homogeneous reaction just makes a very
340
small contribution to the RhB degradation. As discussed above, the heterogeneous catalytic reaction of RhB occurre
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mostly on the catalyst surface. The •OH are generated from the surface of the solid catalyst. Thus, RhB adsorbed on the
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surface of catalyst can be attacked by the •OH and further to be degraded without experiencing the homogeneous pathway
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induced by dissolving iron species. Iron species are therefore recycled directly on the catalyst without significant
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diffusion into the solution phase.
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345 4. Conclusion
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In this study the magnetic bentonite material (Fe3O4/Al-B) has been successfully prepared by means of in situ
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growth of Fe3O4 nanoparticles on Al pillared bentonite surface through a precipitation oxidization process. Adsorption
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isotherms of RhB on Fe3O4/Al-B and Fe3O4 were studied, showing that Fe3O4/Al-B has a high ability for the adsorption of
350
RhB in aqueous solution. The Fe3O4/Al-B can be used as an efficient heterogeneous catalyst to adsorb and degrade RhB in
351
aqueous solution by peroxidase-like catalytic reaction with H2O2. The whole degradation process follows
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pseudo-first-order rate law and is mainly controlled by surface mechanism reaction. The enhanced catalytic activity of
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Fe3O4/Al-B in heterogeneous catalytic system is contributed by the positive effect of Al-B by adsorption of dye molecules
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facilitating the rate of degradation. This magnetic bentonite material will be of huge potential application in
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heterogeneous catalysis for removal of organic contaminants due to its good structural stability, low-cost,
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environment-friendly, simple separation, stable catalytic activity and no need to be regeneration.
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Acknowledgements
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This project was supported in part by the Natural Science Foundation of Hubei Province (No.2014CFB810), Specialized
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Research Fund for the Doctoral Program of Higher Education of China (20114219110002) and Educational Commission
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of Hubei Province of China (D20131107).
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References
364
[1] A.H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application,
371 372 373 374 375 376 377 378 379
ip t
cr
370
peroxidase-like activity of ferromagnetic nanoparticles, Nat. Nanotechnol. 2 (2007) 577–583.
[4] J. Zhang, J. Zhuang, L. Gao, Y. Zhang, N. Gu, J. Feng, D. Yang, J. Zhu, X. Yan, Decomposing phenol by the hidden talent of ferromagnetic nanoparticles, Chemosphere 73 (2008) 1524–1528.
us
369
[3] L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, T. Wang, J. Feng, D. Yang, S. Perrett, X. Yan, Intrinsic
[5] S. Zhang, X. Zhao, H. Niu, Y. Shi, Y. Cai, G. Jiang, Superparamagnetic Fe3O4 nanoparticles as catalysts for the catalytic oxidation of phenolic and aniline compounds, J. Hazard. Mater. 167 (2009) 560–566.
an
368
38–49.
[6] L. Xu, J. Wang, Fenton-like degradation of 2,4-dichlorophenol using Fe3O4 magnetic nanoparticles, Appl. Catal. B: Environ. 123–124 (2012) 117–126.
[7] S.P. Sun, A.T. Lemley, p-Nitrophenol degradation by a heterogeneous Fenton-like reaction on nano-magnetite:
M
367
[2] R.D. Ambashta, M. Sillanpää, Water purification using magnetic assistance: A review, J. Hazard. Mater. 180 (2010)
Process optimization, kinetics, and degradation pathways, J. Mol. Catal. A-Chem. 349 (2011) 71–79.
[8] N. Wang, L. Zhu, M. Wang, D. Wang, H. Tang, Sono-enhanced degradation of dye pollutants with the use of H 2O2
d
366
Angew. Chem. Int. Ed. 46 (2007) 1222–1244.
activated by Fe3O4 magnetic nanoparticles as peroxidase mimetic, Ultrason. Sonochem. 17 (2010) 78–83.
te
365
[9] R.C.C. Costa, M.F.F. Lelis, L.C.A. Oliveira, J.D. Fabris, J.D. Ardisson, R.R.V.A. Rios, C.N. Silva, R.M. Lago,
381
Novel active heterogeneous Fenton system based on Fe3−xMxO4 (Fe, Co, Mn, Ni): The role of M2+ species on the
382
reactivity towards H2O2 reactions, J. Hazard. Mater. B129 (2006) 171–178.
Ac ce p
380
383
[10] N. Wang, L. Zhu, D. Wang, M. Wang, Z. Lin, H. Tang, Sono-assisted preparation of highly-efficient peroxidase-like
384
Fe3O4 magnetic nanoparticles for catalytic removal of organic pollutants with H2O2, Ultrason. Sonochem. 17 (2010)
385
526–533.
386
[11] X. Liang, Z. He, Y. Zhong, W. Tan, H. He, P. Yuan, J. Zhu, J. Zhang, The effect of transition metal substitution on the
387
catalytic activity of magnetite in heterogeneous Fenton reaction: In interfacial view, Colloid. Surface A 435 (2013)
388
28–35.
389
[12] X. Hu, B. Liu, Y. Deng, H. Chen, S. Luo, C. Sun, P. Yang, S. Yang, Adsorption and heterogeneous Fenton
390
degradation of 17α-methyltestosterone on nano Fe3O4/MWCNTs in aqueous solution, Appl. Cataly. B-Environ. 107
391
(2011) 274–283.
392
[13] M.F. Variava, T.L. Church, A.T. Harris, Magnetically recoverable FexO y–MWNT Fenton’s catalysts that show 14
Page 14 of 30
395 396 397 398 399
[14] V. Cleveland, J.P. Bingham, E. Kan, Heterogeneous Fenton degradation of bisphenol A by carbon nanotube-supported Fe3O4, Sep. Purif. Technol. 133 (2014) 388–395.
[15] J. Chun, H. Lee, S.H. Lee, S.W. Hong, J. Lee, C. Lee, J. Lee, Magnetite/mesocellular carbon foam as a magnetically recoverable Fenton catalyst for removal of phenol and arsenic, Chemosphere 89 (2012) 1230–1237.
ip t
394
enhanced activity at neutral pH, Appl. Catal. B: Environ. 123–124 (2012) 200–207.
[16] S. Shin, H. Yoon, J. Jang, Polymer-encapsulated iron oxide nanoparticles as highly efficient Fenton catalysts, Cataly. Commun. 10 (2008) 178–182.
cr
393
[17] M.H. Do, N.H. Phan, T.D. Nguyen, T.T.S. Pham, V.K. Nguyen, T.T.T. Vu, T.K.P. Nguyen, Activated carbon/Fe3O4
401
nanoparticle composite: Fabrication, methyl orange removal and regeneration by hydrogen peroxide, Chemosphere
402
85 (2011) 1269–1276.
407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422
an
removal of microcystin-LR in water, Appl. Surf. Sci. 289 (2014) 245– 251.
M
406
[19] L. Lian, X. Cao, Y. Wu, D. Sun, D. Lou, A green synthesis of magnetic bentonite material and its application for
[20] X. Xue, K. Hanna, N. Deng, Fenton-like oxidation of Rhodamine B in the presence of two types of iron (II, III) oxide, J. Hazard. Mater. 166 (2009) 407–414.
d
405
water, Chem. Eng. J. 200–202 (2012) 97–103.
[21] P.P. Gan, S.F.Y. Li, Efficient removal of Rhodamine B using a rice hull-based silica supported iron catalyst by
te
404
[18] J. Ma, J. Qi, C. Yao, B. Cui, T. Zhang, D. Li, A novel bentonite-based adsorbent for anionic pollutant removal from
Fenton-like process, Chem. Eng. J. 229 (2013) 351–363.
[22] X. Wang, Y. Pan, Z. Zhu, J. Wu, Efficient degradation of rhodamine B using Fe-based metallic glass catalyst by
Ac ce p
403
us
400
Fenton-like process, Chemosphere 117 (2014) 638–643.
[23] J. Carriazo, E. Guélou, J. Barrault, J.M. Tatibouët, R. Molina, S. Moreno, Synthesis of pillared clays containing Al, Al-Fe or Al-Ce-Fe from a bentonite: Characterization and catalytic activity, Catal. Today 107-108 (2005) 126–132.
[24] L. Chen, C. Deng, F. Wu, N. Deng, Decolorization of the azo dye Orange II in a montmorillonite/H 2O2 system, Desalination 281 (2011) 306–311.
[25] M. Neamţu, C. Zaharia, C. Catrinescu, A. Yediler, M. Macoveanu, A. Kettrup, Fe-exchanged Y zeolite as catalyst for wet peroxide oxidation of reactive azo dye Procion Marine H-EXL, Appl. Catal. B: Environ. 48 (2004) 287–294.
[26] Y. Liu, W. Jiang, Y. Wang, X.J. Zhang, D. Song, F.S. Li, Synthesis of Fe3O4/CNTs magnetic nanocomposites at the liquid–liquid interface using oleate as surfactant and reactant, J. Magn. Magn. Mater. 321 (2009) 408–412.
[27] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. 15
Page 15 of 30
425 426 427 428 429 430
temperature, Appl. Surf. Sci. 303 (2014) 6–13.
[29] X. Xue, K. Hanna, M. Abdelmoula, N. Deng, Adsorption and oxidation of PCP on the surface of magnetite: Kinetic experiments and spectroscopic investigations, Appl. Catal. B: Environ. 89 (2009) 432–440.
[30] E.G. Garrido-Ramírez, B.K.G Theng, M.L. Mora, Clays and oxide minerals as catalysts and nanocatalysts in Fenton-like reactions — A review, Appl. Clay Sci. 47 (2010) 182–192.
ip t
424
[28] D. Wilson, M.A. Langell, XPS analysis of oleylamine/oleic acid capped Fe3O4 nanoparticles as a function of
[31] S. Zuo, R. Zhou, Al-pillared clays supported rare earths and palladium catalysts for deep oxidation of low
cr
423
concentration of benzene, Appl. Surf. Sci. 253 (2006) 2508–2514.
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Fig. 1. XRD patterns of Al-B (a), Fe3O4 (b), fresh Fe3O4/Al-B (c) and reused Fe3O4/Al-B (d).
433 434
Fig. 2. SEM images of Fe3O4 (a), Al-B (b), fresh Fe3O4/Al-B (c) and reused Fe3O4/Al-B (d).
435 Fig. 3. XPS spectra of Fe3O4 (a) and Fe3O4/Al-B (b) (the inset shows the high-resolution scan of Fe 2p region).
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Fig. 4. Hysteresis loop of Fe3O4 (a) and Fe3O4/Al-B (b).
439 Fig. 5. Nitrogen adsorption/desorption isotherms for Fe3O4 and Fe3O4/Al-B.
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Fig. 6. Adsorption and desorption kinetics for RhB on Fe3O4 and Fe3O4/Al-B. (C0 = 40 mg∙L-1, [adsorbent] = 1 g·L-1; T =
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25 °C)
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Fig. 7. Isothermal adsorption of RhB on Fe3O4 and Fe3O4/Al-B. ([adsorbent] = 1 g·L-1; T = 25 °C)
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Fig. 8. Degradation of RhB along time under different conditions. (C0 = 40 mg∙L-1, [catalyst] = 1 g·L-1, [H2O2] = 200
448
mmol∙L-1, T = 25 °C)
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Fig. 9. The effect of initial H2O2 (a) and RhB (b) concentration on the degradation efficiency of RhB. Inset: correlation of
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the apparent pseudo-first-order rate constant with H2O2 or RhB concentration. (C0 = 40 mg∙L-1, [Fe3O4/Al-B] = 1 g·L-1,
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[H2O2] = 200 mmol∙L-1, T = 25 °C)
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Fig. 10. (a) IR spectra of RhB before and after degradation. (b) RhB removal and CODCr elimination in the RhB
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degradation. (C0 = 40 mg∙L-1, [Fe3O4/Al-B] = 1 g·L-1, [H2O2] = 200 mmol∙L-1, T = 25 °C)
456 457
Fig. 11. XPS spectrum of Fe on Fe3O4/Al-B surface after and before degradation of RhB (Fe 2p line).
458 459
Fig. 12. Effect of recycling the catalyst and homogeneous catalysis on the degradation of RhB. (C0 = 40 mg∙L-1,
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[Fe3O4/Al-B] = 1 g·L-1, [H2O2] = 200 mmol∙L-1, T = 25 °C)
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S0169-4332(15)01091-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.05.004 APSUSC 30318
To appear in:
APSUSC
Received date: Revised date: Accepted date:
19-1-2015 8-4-2015 2-5-2015
Please cite this article as: D. Wan, G. Wang, W. Li, K. Chen, L. Lu, Q. Hu, Adsorption and heterogeneous degradation of rhodamine B on the surface of magnetic bentonite material, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.05.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2
Highlights: Magnetic bentonite was prepared by in situ precipitation oxidization method.
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The composite material has been used to adsorb and degrade rhodamine B.
4
The adsorption and catalytic ability of Fe3O4 improved significantly after being dispersed in the
6
bentonite.
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The magnetic bentonite is low-cost and environment-friendly and has a long-term reusability and stability.
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Adsorption and heterogeneous degradation of rhodamine B on the surface of magnetic
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bentonite material
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Dong Wan, Guanghua Wang, Wenbing Li, Kun Chen, Lulu Lu, Qin Hu
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11 College of Chemical Engineering and Technology, Wuhan University of Science and Technology, Wuhan 430081, China.
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Abstract: A kind of imitation enzyme catalyst, Fe3O4 nanoparticles decorated Al pillared bentonite (Fe3O4/Al-B), was
15
successfully prepared by in situ precipitation oxidization method and then applied for the adsorption and degradation of
16
rhodamine B (RhB) in the presence of H2O2. The catalyst was characterized by XRD, SEM, XPS, BET, VSM and FTIR
17
spectroscopy. The effects of oxidant concentration, initial RhB concentration and iron leaching on the degradation of RhB
18
were investigated. The surface interactions with RhB in the absence and the presence of oxidant could be well described
19
by Langmuir and Langmuir–Hinshelwood models, respectively. The Fe3O4/Al-B showed higher ability of adsorption and
20
degradation efficiency towards RhB than bare Fe3O4 in the batch experiments. The whole degradation process of RhB
21
followed pseudo-first-order rate law and was mainly controlled by surface mechanism reaction. The enhanced
22
degradation efficiency of Fe3O4/Al-B might relate to the enrichment of RhB molecules by Al-B in the vicinities of active
23
sites. Furthermore, the catalyst showed stable catalytic activity and convenient recycling. Negligible iron leaching
24
showed the reused Fe3O4/Al-B can withstood the oxidation.
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Key word: adsorption; magnetic bentonite; heterogeneous catalyst; rhodamine B
1. Introduction
29
In recent years, Fe3O4 magnetic nanoparticles (MNPs) as a popular magnetic material have attracted significant
30
interest of many researchers, because of their potential applications in catalysis, biotechnology, water purification and so
31
on[1,2]. In 2007, Gao et al. found the Fe3O4 MNPs possessed intrinsic peroxidase-like activity, which was able to catalyse
32
the oxidation of organic substrates in wastewater treatment[3]. Since then the peroxidase-like nature of Fe3O4 MNPs was
33
widely explored for the applications in the degradation of some refractory organics such as phenol[4], aniline[5],
34
2,4-dichlorophenol[6], p-nitrophenol[7] and dye pollutants[8]. Moreover, with an inverse spinel crystal structure, Fe3O4
35
MNPs exhibit unique electric and magnetic properties, which based on the transfer of electrons between Fe2+ and Fe3+ in
36
the octahedral sites, allowing the Fe species to be reversibly oxidized and reduced while keeping the same structure[9]. So 2
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it can be able to function steadily as a heterogeneous catalyst without substantial loss. In addition, Fe3O4 is magnetic and
38
can be easily separated from the terminal solution by magnetic separation, which provide it a huge potential application in
39
the advanced treatment of organic wastewater. However, several reports demonstrated that the H2O2-activating ability of Fe3O4 MNPs was not so strong for the
41
practical removal of refractory organic pollutants from wastewaters and the coaggregation of Fe3O4 MNPs, especially in
42
water solution, will decrease the effective surface active sites of Fe3O4 MNPs and thus reduce their catalytic
43
activity[10-14]. Therefore, it is urgently necessary to improve the H2O2-activating ability and dispersity of Fe3O4 MNPs.
44
Recently, immobilized Fe3O4 MNPs on solid supports during the preparation process were found to be effective to prevent
45
the coaggregation and improve the catalytic activity. The supports were organic or inorganic materials, such as
46
multiwalled carbon nanotubes[12-14], mesocellular carbon foam[15], poly(3,4-ethylene-dioxythiophene)[16] and
47
activated carbon[17]. Given the economic aspect, however, the supports mentioned above are too expensive to apply in
48
the practical wastewater treatment. Bentonite, as an abundance and low cost mineral with many excellent properties such
49
as high stability, microporosity and larger surface area, has been widely used in a number of industrial branches[18]. After
50
intercalated by polymeric inorganic oxocations, the stability and surface area of bentonite could be promoted effectively.
51
Due to its smaller particle size, higher stability, larger surface area, sheet-like structure and most of all, low cost, pillared
52
bentonite may be particularly suitable to be used as a support for the synthesis of Fe3O4 MNPs. The dispersity, adsorption
53
capacity or catalytic ability of Fe3O4 MNPs may be improved after being dispersed in the bentonite.
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In the present study, Al pillared bentonite(Al-B) was chosen to immobilize Fe3O4 MNPs, which may prevent the
55
coaggregation of Fe3O4 MNPs. One the other hand, the excellent adsorption property of pillared bentonite may have
56
positive effect on the catalytic activity of Fe3O4 MNPs. Recently, Lian et al. had synthesised the magnetic bentonite
57
material(Fe3O4@Al-B) by a solvothermal reaction[19]. Their study has confirmed that the magnetic bentonite has
58
enhanced adsorptivity for organic pollutants than bare Fe3O4 or bentonite. However, they investigated only adsorption
59
property instead of peroxidase-like catalytic activity of magnetic bentonite. Besides, the solvothermal process may also
60
have difficult in practical application. In our research, we report another convenience and green method, in situ
61
precipitation oxidization, to synthesis magnetic bentonite. In order to test the adsorption and catalytic activity, the
62
material was further used for adsorbing and degrading organic pollutants in the present of H2O2 in water. As we known,
63
rhodamine B (RhB) is a typical dye pollutant and has been selected as a model dyeing pollutant in many investigations
64
because of its widespread application and recalcitrant nature[20-22]. In this study, the magnetic bentonite had been used
65
in peroxidase-like process to adsorb and degrade RhB in water.
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66 3
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67
2. Materials and methods
68
2.1. Materials Natural bentonite was procured from Shanghai No.4 Reagent & H.V. Chemical Co., Ltd, China. Rhodamine B (RhB)
70
was obtained from Shanghai Zhanyun Chemical Co., Ltd., China. All chemicals were of analytical grade and used as
71
received. In the experiments, distilled water was used for preparing the solutions and suspensions.
ip t
69
72 2.2. Preparation of magnetic bentonite (Fe3O4/Al-B)
cr
73
The Al pillared bentonite(Al-B) was prepared by pillaring the bentonite through cation-exchange process as
75
described in literature [23]. The magnetic bentonite were synthesized by in situ precipitation oxidization method. 3.1 g
76
Al-B was added into a 1000 mL flask containing 300 mL distilled water with constantly stirring. The solution was bubbled
77
with N2 flow for 15 min to remove the dissolved oxygen and placed in a 95 °C water bath, then 11.1 g FeSO4·7H2O was
78
added into the flask. Afterwards, 100 mL solution containing 3.2 g NaOH and 3.2 g NaNO3 added dropwise (10 mL·min-1)
79
into the heating solution while keep vigorous stirring and stable N2 flow during the entire reaction period. After that, the
80
solution was heated at 95 °C for another hour, then cooled to room temperature. The precipitate was isolated by a
81
permanent magnet. After repeated washing with deionized water and absolute ethanol under ultrasonication, the formed
82
Fe3O4/Al-B nanocomposites were dried in a vacuum oven at 100 °C for 12 h. Fe3O4 was synthesized as above procedure
83
without adding Al-B. All the products were stored in desiccator under ambient temperature for further experiments.
84 2.3. Characterization
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85
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74
86
X-ray diffraction (XRD) patterns of the samples were recorded by a Rigaku D/max-RB diffrac-tometer (Rigaku,
87
Japan) at 40 kV and 30 mA using filtered Cu Kα radiation (λ= 0.15418 nm). Micrographs of the samples were taken by
88
using a Nova400 Nano SEM (FEI, USA). Before observation of SEM, all samples were fixed on silicon wafer and coated
89
with gold. X-ray photoelectron spectroscopy (XPS) was performed with a MULT1LAB2000 X-ray photoelectron
90
spectrometer (VG, USA) with a monochromatized Al Kα X-ray source (25 eV) operated at 300 W to identify metal
91
oxidation states of the nanocomposites. After shirley background subtraction, the spectra were analyzed by the XPSPeak
92
4.1 program. Magnetic characterization of the samples were carried out using a JDAW-2000D vibrating sample
93
magnetometer (VSM) at a temperature of 298 K with fields up to 6000 Oe. The Fourier transform infrared spectroscopy
94
(FTIR) of the samples were recorded on a Nicolet 6700 FTIR spectrometer (Thermo Scientific, USA) in the 400~4000
95
cm−1 range with a resolution of 2 cm-1. N2 adsorption–desorption isotherms of the samples were recorded on an ASAP
96
2020-M micropore physisorption analyzer (Micromeritics, USA). Specific surface areas were calculated by the 4
Page 4 of 30
97
Brunauer–Emmett–Teller (BET) method.
98 99
2.4. Sorption experiments In order to determine the kinetics of the sorption process, 200 mL 40 mg∙L-1 RhB solution was added into a 500 mL
101
flask. The sorption experiments were subsequently started by adding 0.2 g Fe3O4/Al-B or Fe3O4 nanoparticles with
102
mechanically stirring. At several points in time the concentration of the freely dissolved fraction of RhB was determined.
103
Furthermore, the desorption kinetics of the process was studied. Samples of the adsorption experiment described above
104
were allowed to equilibrate over a time period of 12 h, and then the samples were magnetic separated and the clear water
105
phase was decanted. The desorption process was started by addition of 200 mL neutral fresh deionized water with
106
mechanically stirring. After selected desorption time, the freely dissolved fraction of RhB was measured.
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100
All equilibrium sorption experiments were conducted at 25 °C in the dark at the initial pH of RhB solution. The solid
108
samples were mixed with variable solute concentrations. Results of adsorption kinetic experiments indicated that sorption
109
equilibrium was achieved within 2 h. Before analysis, the suspensions were magnetic separated and the clear water phase
110
was decanted. Then the equilibrium concentrations of RhB in the aqueous phase was measured and sorbed concentrations
111
were calculated by difference according to:
112
qe
113
where qe (mg∙g-1) is the sorbed concentration, Ci (mg∙L-1) is the initial aqueous phase concentration, Ce (mg∙L-1) is the
114
equilibrium concentration in the aqueous phase, V (L) is the volume of solution, Ms (g) is the mass of solid sorbent.
116
M
d
(1)
te
(Ci Ce )V Ms
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an
107
2.5. Degradation of RhB by heterogeneous catalytic experiments
117
The experiments were carried out in a conical flask with a stopper (containing 200mL of reaction solution). All
118
experiments were carried out under constantly stirring to make the catalyst good dispersion. The initial pH and
119
temperature were as same as that in sorption experiments. Before reaction, the suspension containing magnetic bentonite
120
and RhB was stirred for 2 h to achieve adsorption equilibrium. The degradation reaction was initiated when H2O2 was
121
added to the solution.
122
During all the oxidation reactions, 5 mL aliquots were withdrawn and clarified quickly by an outer strong permanent
123
magnet at selected time intervals. The aqueous phase was sampled for analysis. The solid catalyst separated from aqueous
124
phase was rinsed by 5 mL ethanol for three times. The rinsed liquid was mixed for analysis. The residual RhB amount is
125
the sum of that in aqueous and solid phase. After the oxidation reaction, the catalyst was repeated washing and dried for 5
Page 5 of 30
126
reusing. All experimental runs were performed in the absence of light. Each experiment was achieved in triplicates. All
127
results were expressed as a mean value of the 3 experiments.
128 129
2.6. Analyses At given intervals of degradation, a sample was analyzed by UV-Vis spectroscopy (Ultrospec 3300 pro, GE
131
Healthcare Bio-Sciences China Ltd) at a wavelength of 554 nm, which is the maximum absorption wavelength of
132
RhB[20]. The concentration of RhB was converted through the standard curve method of dyes. The concentration of iron
133
leaching in the solution was measured by the 1,10-phenatroline spectrophotometric method[24]. The chemical oxygen
134
demand (CODCr) was determined by dichromate method. To eliminate the interference of H2O2 with CODCr
135
measurements, the reaction was finally blocked by raising the pH to 9~10, adding MnO2 and allowing the samples to sit
136
overnight[25].
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138
3. Results and discussion
139
3.1. Characterization of catalyst
M
137
The X-ray diffraction patterns of the samples are shown in Fig.1. Diffraction peaks assigned to bentonite at
141
2θ=5.8°[18] can be seen for Al-B and Fe3O4/Al-B, indicating that the bentonite structure was not destroyed after
142
treatment by pillaring process and chemical precipitation of Fe3O4. As shown in Fig.1b and c, eight characteristic peaks
143
for Fe3O4 (2θ=18, 30, 35.5, 37, 43, 53.4, 57 and 62.5°) are observed for the synthesized Fe3O4 and Fe3O4/Al-B. And no
144
other peaks corresponding to the hematite are detected in the XRD patterns, indicating that the Fe3O4 nanoparticles in the
145
composites are pure Fe3O4 with inverse spinel structure[26]. In addition, no other peaks are observed in the XRD pattern
146
of reused Fe3O4/Al-B (Fig. 1d). This indicated that there was no obvious change of structure and component of the
147
catalyst after oxidation reaction.
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148
The morphologies of Al-B and catalysts were characterized by SEM. The SEM image in Fig. 2a shows that the
149
pillared bentonite is layer structure and had a coarse porous surface, which is essential for the adsorption of organic
150
pollutant. The synthesized Fe3O4 nanoparticles (Fig. 2b) with diameters ranging from 40 to 100 nm are dramatically
151
co-aggregated together. The SEM image in Fig. 2c shows that the Fe3O4 nanoparticles growing on the Al-B surface with
152
better dispersing and less co-aggregation. Although the products have experienced repeated washing in water and
153
methanol under ultrasonication before SEM measurements, almost all magnetite nanoparticles are still found on the
154
bentonite surface. This indicates the strong interaction between Al-B and Fe3O4 nanoparticles. The image of Fe3O4/Al-B
155
nanoparticles reused for three times in Fig. 1d shows no obvious change of structure of the catalyst after oxidation 6
Page 6 of 30
156
reaction. The XPS spectrums of Fe3O4 and Fe3O4/Al-B are taken over the entire energy range showed in Fig. 3. The
158
photoelectron lines at binding energy of about 284, 530, and 711 eV are attributed to C 1s, O 1s, and Fe 2p, respectively. A
159
detail of the Fe peaks can be seen in the inset of Fig. 3. The peaks of Fe 2p1/2 and Fe 2p3/2 locate at 710.9 and 724.5 eV,
160
respectively. The results agree with literature data for magnetite and substantiate the XRD results, thus further confirm
161
that the oxide in the samples are Fe3O4[26-29].
ip t
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Fig. 4 exhibits the hysteresis loop of the as-prepared samples at room temperature. It can be seen that the saturation
163
magnetization of Fe3O4/Al-B (30.9 emu∙g-1) is much smaller than that of the unmodified Fe3O4 (82.9 emu∙g-1). The
164
decrease may result from the existence of nonmagnetic bentonite. A superparamagnetic behavior (i.e., zero coercive field
165
and zero remnant magnetization) is observed for all the samples. The result shows that they can be manipulated by an
166
external magnetic field, such as a magnet, thus providing a potential advantage for the separation, recovery and reuse of
167
adsorbents as well as catalysts.
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Nitrogen adsorption/desorption isotherms of the Fe3O4 and Fe3O4/Al-B material showed in Fig. 5 display type II
169
isotherms. Wide hysteresis area of N2 adsorption/desorption isotherms can be clearly seen in the case of Fe3O4 and
170
Fe3O4/Al-B, which suggest the occurrence of capillary condensation and the wide distributions of pores in both
171
cases[12,19]. The specific surface areas calculated by using a BET equation are found to be 53.02 and 4.68 m2∙g-1 for
172
Fe3O4/Al-B and Fe3O4, respectively. The nanocomposites display much higher adsorption quantity than pure Fe3O4
173
because the porosity and good adsorbability of the Al-B used as support.
175
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168
3.2. Sorption of RhB
176
A sorption/desorption experiment was performed to make a comparison of the timescale of degradation reaction
177
with that of sorption and desorption of RhB on Fe3O4/Al-B and Fe3O4 nanoparticles (Fig. 6). As can be seen, the
178
sorption process of RhB on the two kinds of nanoparticles closely approach equilibrium within 10 min. No significant
179
changes are observed after 60 min and 240 min. About 10% and 80% RhB are sorbed on Fe3O4 (1.0 g∙L-1) and
180
Fe3O4/Al-B (1.0 g∙L-1), respectively. The desorption experiment shows the re-equilibration was almost completed within
181
7 min for Fe3O4 and 8 min for Fe3O4/Al-B.
182
RhB aqueous solutions with different initial concentrations varying from 5 mg∙L-1 to 400 mg∙L-1 were used for the
183
adsorption experiment. Sorption isotherms were determined to investigate the effect of contaminant sorption on the
184
catalytic oxidation(Fig. 7). The experimental isotherm data were fitted to the equations of Langmuir and Freundlich by
185
applying linear regression analysis. On the basis of the statistic analysis, the isotherms can be well described with the 7
Page 7 of 30
186
Langmuir model which assumed that the single adsorbate binds to a single site on the adsorbent and that all surface sites
187
on the adsorbents have the same affinity for the adsorbate and no interactions between the adsorbates. The linear form
188
of the Langmuir equation is given by:
189
qe
190
qm(mg·g-1) is the monolayer adsorption capacity, and Ka (L·mg-1) is the Langmuir constant. The fitted Langmuir
191
equations are qe = 4.276×0.029Ce/(1 + 0.029Ce) for Fe3O4 (R2 = 0.9937) and qe = 62.154×0.121Ce/(1 + 0.121Ce) for
192
Fe3O4/Al-B (R2 = 0.9902), respectively. Good correlations imply that the adsorption of RhB on the catalysts are
193
monolayer adsorption. Isotherms for RhB show that the sorption affinity of Fe3O4/Al-B is much higher than that of
194
Fe3O4. The adsorption capacities of RhB onto Fe3O4/Al-B and Fe3O4 are calculated to be 62.15 and 4.28 mg·g-1, which
195
are in conformity with the specific surface area.
(2)
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qm K a Ce 1 K a Ce
an
196 3.3. Degradation of RhB
198
3.3.1. RhB degradation experiments by heterogeneous catalytic reaction
M
197
The catalytical degradation efficiencies of RhB by Fe3O4/Al-B and Fe3O4 were evaluated under different
200
experimental conditions (Fig. 8). It can be seen from Fig. 8 that no degradation of RhB happens when there is only
201
Fe3O4/Al-B in the solution without the addition of H2O2. The experiment was also carried out to evaluate the ability of
202
H2O2 to eliminate RhB without the addition of any heterogeneous catalyst. Fig. 8 shows that the degradation of RhB is
203
almost negligible in the presence of H2O2 only, which may due to its low oxidation potential compared with hydroxyl
204
radicals. The catalytic ability of Fe3O4/Al-B in presence of H2O2 shows a significant RhB reduction achieving conversion
205
rate of 97.8% after 300 min, which is much more efficient than bare Fe3O4 and H2O2 (58.3%). The catalytic ability of Al-B
206
was also evaluated in the presence of H2O2. The very low degradation percentage of RhB indicates that the support is
207
insignificant catalytically active. Therefore, the contribution of the direct catalysis of Al-B to the improved degradation
208
performance with Fe3O4/Al-B is very limited.
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209
It is reported in several works that the synergistic effect resulting by the adsorption property of support caused the
210
improvement in the rate of substrate degradation[12-15]. Herein we have similar results. Since Al-B is a good adsorbent,
211
the adsorption capacity of Fe3O4/Al-B for RhB is enhanced dramatically compared with that of bare Fe3O4, which has
212
been discussed in Section 3.2. Thus, Al-B provides more available and effective contact surface areas between reactants
213
and active sites, and the adsorbed RhB molecules in the immediate vicinity of immobilized Fe-ions are easily attacked by
214
the generated •OH. On the other hand, the dispersity of Fe3O4 nanoparticles is promoted after immobilized on Al-B, 8
Page 8 of 30
215
which can been seen from the SEM photograph(Section 3.1). More active sites of Fe3O4 nanoparticles are exposed, which
216
also result in an increase of the generation of •OH. The synergistic effect between Fe3O4 and Al-B is obvious in the
217
degradation of RhB through heterogeneous catalytic progress.
218 3.3.2. Effect of the hydrogen peroxide concentration
ip t
219
According to previous study, hydroxyl radical(•OH) could be generated by the reaction between H2O2 and Fe3O4
221
leading to a degradation of RhB, which was known as the Haber–Weiss mechanism[20-22]. The degradation of the dye by
222
•OH can be typically described as a second-order reaction:
223
dC kC[OH] dt
224
where C and [•OH] are concentrations of RhB and hydroxyl radical, respectively. k is the second-order rate constant, and
225
t is the reaction time. By assuming that •OH instantaneous concentration is constant, the kinetics of degradation of RhB in
226
water can be described according to the pseudo-first-order equation as given below[20]:
227
−ln(Ct/C0) = kappt
(4)
228
kapp = k[•OH]
(5)
229
where C0 is the initial concentration of RhB and kapp is the apparent pseudo-first-order rate constant (min-1). The kapp
230
constants are obtained from the slopes of the straight lines by plotting −ln(Ct/C0) as a function of time t, through
231
regression. Good correlation coefficients (R2 ~ 0.99) are obtained in our system (Fig. 9).
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The kinetic constants rate kapp are determined at different H2O2 concentrations (Fig. 9a). It is observed that the
233
degradation rate of RhB increases observably when H2O2 concentration increased from 0 to 150 mmol∙L-1. Since the RhB
234
degradation is directly related to the concentration of the •OH produced by the catalytic decomposition of H2O2, more
235
RhB decomposition is expected with a higher increase of H2O2 concentration. However, a significant improvement is not
236
seen when the H2O2 concentration increased to 250 mmol∙L-1, because of the H2O2 was adsorption saturation on the active
237
sites of catalyst. It is well accepted that, in the peroxidase-like catalytic reaction, the kinetic constant rate will tend to be
238
steady when the concentration of reactant continually increasing, but in this system, a decrease in the values of kapp is
239
observed at a much higher H2O2 concentration. The occurrence of this maximum H2O2 concentration for the effective
240
degradation of RhB can be explained by scavenging effect of hydroxyl radicals by hydrogen peroxide[27]:
241
H2O2 + •OH → H2O + •OOH
242
The •OH radicals preferentially attack the RhB molecules at low H2O2 concentration, whereas at much higher H2O2
243
concentration, there is a competitive reaction between the RhB and H2O2. Moreover, the oxidation potential of generated
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(6)
9
Page 9 of 30
244
radicals •OOH is much smaller than that of the •OH species. Therefore, it makes the degradation of RhB slowdown.
245 246
3.3.3. Effect of initial RhB concentration The pseudo-first-order apparent rate constant was determined at various initial RhB concentration (Fig. 9b). It can be
248
seen that the kapp constant decreases when the initial concentrations of RhB increased from 10 to 60 mg·L-1. Increasing
249
amount of pollutant may occupy a greater number of iron active sites, which becomes unavailable for H2O2 and results in
250
lower •OH generation rate. More RhB are sorbed on the catalyst surface and less H2O2 are interacted with iron surface,
251
leading to less hydroxyl radical formed at the surface. The Langmuir–Hinshelwood model, which is widely used in
252
heterogeneous catalysis, was tested [29]. In this case, the species present in the reaction compete with each other for
253
adsorption on a fixed number of active sites. This Langmuir–Hinshelwood model postulates that the rate of reaction of
254
two species adsorbed on the surface is the ratelimiting step:
255
1 1 1 [RhB]i kapp kint kint K s
256
where kapp is the initial pseudo-first-order rate constant (min-1), kint is the intrinsic reaction rate constant (mg·L-1·min-1),
257
and Ks is the adsorption constant of RhB over catalyst surface (L·mg-1). The linear correlation (R2 ~ 0.99) between 1/kapp
258
and [RhB]i is relatively good, indicating that surface reactions of RhB, including sorption and oxidation by surface •OH,
259
play an important role in determining rate of the whole reaction. kint and Ks are obtained as 0.788 mg·L-1·min-1 and 0.04
260
L·mg-1. This Ks value is lower than the sorption constant determined in the absence of oxidant by Langmuir model (Ka =
261
0.121 L·mg-1, in Section 3.2), suggesting an adsorption competition for the catalyst surface.
263
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3.3.4. Removal of RhB in aqueous or sorbed phase
264
Harber–Weiss circle indicated that the generation of •OH from H2O2 is the key step of the whole degradation
265
process[9,12]. The formation of radicals from H2O2 and iron oxides has been proposed in the literature[30]. The
266
mechanism of H2O2 activation by Fe3O4/Al-B may involve the initial formation of a complex assigned as ≡FeII(H2O2)
267
(Eq. (8)). The initially generated ≡FeII(H2O2) species can produce •OH by intramolecularly electron transfer, then a
268
chain of reactions occurring on the catalyst surface:
269
≡FeII + H2O2 → ≡FeII(H2O2)
(8)
270
≡FeII(H2O2) → ≡FeIII + OH- + •OH
(9)
271
≡FeIII + H2O2 → ≡FeIII(H2O2)
(10)
272
≡FeIII(H2O2) → ≡FeII + H+ + •OOH
(11) 10
Page 10 of 30
273
≡FeIII + •OOH → ≡FeII + O2 +H+
(12)
274
On the other hand, the RhB molecules are mostly sorbed on the surface of Al-B, which has been discussed in
275
Section 3.2. The interactions between RhB and Al-B can be described as equilibrium reaction between sorbed and
276
aqueous species:
277
Al-B + RhBe ↔ Al-B(RhB)s
278
The hydroxyl radicals formed by the reaction between hydrogen peroxide and iron surface can probably attack the
279
sorbed species as well as the aqueous species leading to the degradation of RhB:
280
•OH + Al-B(RhB)s → Al-B + RhB by-products → ···CO2 + H2O
(14)
281
•OH + RhBe → RhB by-products → ···CO2 + H2O
(15)
282
IR spectras show the RhB sorbed on the surface of Fe3O4/Al-B along the oxidation reaction (Fig. 10a). Prior to IR
283
analysis, oxidation experiment of RhB was conducted versus time in 1 g·L-1 of Fe3O4/Al-B with the optimum value of
284
H2O2 dose. During the oxidation reaction, a few milligrams of catalyst were sampled from oxidation reactor at selected
285
time intervals and immediately analyzed by IR spectroscopy (Fig. 10a).
an
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(13)
As can be seen from Fig. 10a, in the Fe3O4/Al-B samples, the peak at 1044 cm-1 assigns to Si–O–Si stretching
287
vibrations in montmorillonite lattice[31]. The band at 572 cm-1 attributes to the vibrations of Fe–O, which is the
288
characteristic peak of Fe3O4[26]. There are some new peaks appear on the IR spectra of Fe3O4/Al-B-sorbed RhB
289
compare with that of the Fe3O4/Al-B alone. The peaks at 1607 cm-1 corresponds to aromatic ring vibration, while the
290
1341 cm-1 peak and 1415 cm-1 peak attribute to C–aryl bond and C–N stretching vibration respectively, which can be
291
assigned to RhB [20,21]. The sorbed RhB disappear from the surface of Fe3O4/Al-B at 180 min treatment time, which is
292
corresponding to the result in Fig. 10b. The adsorbed amount of RhB was calculated according to the residual
293
concentration of RhB in aqueous solution. It can be seen from the Fig. 10b that the decrease of RhB in aqueous solution
294
is quite slow during the reaction process, which indicates that the main decomposition of RhB is occurred in the
295
adsorbed phase on the surface of Fe3O4/Al-B, thus confirming with the RhB degradation process discussed above.
296
However, even the sorbed RhB disappear from the surface of catalyst, there may be some intermediates are adsorbed on
297
catalyst instead of RhB, which are further broken into CO2 and H2O by •OH. On the other hand, the adsorbed amount of
298
RhB in degradation process is lower than that of sorption experiment (Section 3.2), which indicates the adsorption
299
competition between RhB and H2O2 or intermediates. Therefore, we can conclude that both desorption and degradation
300
processes contribute to the disappearance of RhB from catalyst surface at the first stage of reaction. In the case of
301
desorption, H2O2 competes with RhB for the fixation on active sites as previously shown by Langmuir–Hinshelwood
302
model, and displaces it from surface to aqueous phase. In the case of degradation, the •OH radicals generated from the
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11
Page 11 of 30
303
surface reaction between hydrogen peroxide and surface active sites may preferentially react with surface sorbed RhB,
304
which confirming with the results in Section 3.3.1. The enhanced adsorption of RhB has a dramatically promotion for its
305
degradation. The CODCr removal rate of RhB solution during the degradation process was recorded (Fig. 10b). Five hours after
307
the reaction started, the CODCr removal rate of RhB solution is about 82.8%, which means the RhB are effectively
308
degraded in our system. The IR spectra of RhB before and after degradation has also been analyzed (Fig. 10a). The
309
main wavelength of the absorption peaks of RhB is range from 2000–1000 cm-1. As can be seen, after degradation, most
310
bands greatly have lessened in intensity or disappeared, indicating that drastic destruction happened. However, some
311
bands are still not disappeared, related to the uncompleted destruction of some aromatic group and inadequate
312
mineralization, which well corresponds with the COD result.
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306
314
an
313
3.4. The catalyst stability and influence of homogeneous catalytic reaction
It is crucial to evaluate the stability of a heterogeneous catalyst. The action of H2O2 on the oxide surface can
316
transform the oxide particles into an only FeIII-bearing mineral or into an amorphous iron oxide which may be less stable
317
and more soluble. Subsequently, a different kinetic behavior and degradation rate of organic compound may be observed
318
owing to a substantial change in the surface characteristics of the catalyst. But in this study, the very low dissolved iron
319
concentration measured (< 0.5 mg·L-1) did not expect the formation of more soluble iron oxide. In addition, the XRD
320
diffraction pattern of the used Fe3O4/Al-B was found to be same as that recorded before reaction (Fig. 1c and d) and the
321
SEM image of the used catalyst was also found to be similar to that of the fresh particles (Fig. 2d). XPS is sensitive to the
322
outermost layer of solid sample and provides both semi-quantitative surface composition and chemical state information.
323
The details of the Fe 2p peaks (Fe 2p1/2 and Fe 2p3/2) of the Fe3O4/Al-B samples before and after use during the RhB
324
degradation are presented in Fig. 11. The Fe 2p peaks at binding energies (BE) of 711.2 and 725.0 eV with a satellite
325
signal at 719.0 eV are characteristic of Fe3+, while the peaks at BE of 709.9 and 723.4 eV with a satellite signal at 715.5
326
eV are characteristic of Fe2+[10,27]. The components of two oxidation states of surface iron species are consistent with
327
the binding energy values[12]. The major component is found to be Fe3+ which contributes to 67.6% of the total iron
328
surface atoms, while 32.4% of the total iron surface atoms are in the Fe2+ state. This result corresponds with the Fe3O4
329
crystal structure. For the sample after RhB degradation, the Fe2+ is found to be 20.8% and Fe3+ 79.2%. This indicates that
330
part of the Fe2+ in the outermost layer of the catalyst are oxidized into Fe3+ during oxidation reaction, which has been
331
discussed in Section 3.3.4.
332
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315
The reusability of the catalyst has been evaluated under the reaction conditions which were identical with the first 12
Page 12 of 30
333
oxidation cycle. The catalyst was reused without any regeneration. As shown in Fig. 12, the degradation efficiencies of
334
RhB are 98.7%, 97.7%, 97.3% and 96.2% after 300 min for cycle 1, 2, 3 and 4, respectively. The RhB degradation
335
performances on the reused catalyst show a slight difference with the first oxidation cycle, which indicates the high
336
stability of the catalytic activity during oxidation reactions. The homogeneous catalytic experiments were also performed using iron salts in similar concentration (1.0 mg·L-1
338
Fe3+) based on the maximal amount of iron leaching from the catalyst after the oxidation cycle. It can be seen from the Fig.
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12 that RhB degradation efficiency is only 21% after 300 min. Therefore, the homogeneous reaction just makes a very
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small contribution to the RhB degradation. As discussed above, the heterogeneous catalytic reaction of RhB occurre
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mostly on the catalyst surface. The •OH are generated from the surface of the solid catalyst. Thus, RhB adsorbed on the
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surface of catalyst can be attacked by the •OH and further to be degraded without experiencing the homogeneous pathway
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induced by dissolving iron species. Iron species are therefore recycled directly on the catalyst without significant
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diffusion into the solution phase.
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345 4. Conclusion
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In this study the magnetic bentonite material (Fe3O4/Al-B) has been successfully prepared by means of in situ
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growth of Fe3O4 nanoparticles on Al pillared bentonite surface through a precipitation oxidization process. Adsorption
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isotherms of RhB on Fe3O4/Al-B and Fe3O4 were studied, showing that Fe3O4/Al-B has a high ability for the adsorption of
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RhB in aqueous solution. The Fe3O4/Al-B can be used as an efficient heterogeneous catalyst to adsorb and degrade RhB in
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aqueous solution by peroxidase-like catalytic reaction with H2O2. The whole degradation process follows
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pseudo-first-order rate law and is mainly controlled by surface mechanism reaction. The enhanced catalytic activity of
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Fe3O4/Al-B in heterogeneous catalytic system is contributed by the positive effect of Al-B by adsorption of dye molecules
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facilitating the rate of degradation. This magnetic bentonite material will be of huge potential application in
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heterogeneous catalysis for removal of organic contaminants due to its good structural stability, low-cost,
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environment-friendly, simple separation, stable catalytic activity and no need to be regeneration.
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Acknowledgements
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This project was supported in part by the Natural Science Foundation of Hubei Province (No.2014CFB810), Specialized
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Research Fund for the Doctoral Program of Higher Education of China (20114219110002) and Educational Commission
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of Hubei Province of China (D20131107).
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References
364
[1] A.H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application,
371 372 373 374 375 376 377 378 379
ip t
cr
370
peroxidase-like activity of ferromagnetic nanoparticles, Nat. Nanotechnol. 2 (2007) 577–583.
[4] J. Zhang, J. Zhuang, L. Gao, Y. Zhang, N. Gu, J. Feng, D. Yang, J. Zhu, X. Yan, Decomposing phenol by the hidden talent of ferromagnetic nanoparticles, Chemosphere 73 (2008) 1524–1528.
us
369
[3] L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, T. Wang, J. Feng, D. Yang, S. Perrett, X. Yan, Intrinsic
[5] S. Zhang, X. Zhao, H. Niu, Y. Shi, Y. Cai, G. Jiang, Superparamagnetic Fe3O4 nanoparticles as catalysts for the catalytic oxidation of phenolic and aniline compounds, J. Hazard. Mater. 167 (2009) 560–566.
an
368
38–49.
[6] L. Xu, J. Wang, Fenton-like degradation of 2,4-dichlorophenol using Fe3O4 magnetic nanoparticles, Appl. Catal. B: Environ. 123–124 (2012) 117–126.
[7] S.P. Sun, A.T. Lemley, p-Nitrophenol degradation by a heterogeneous Fenton-like reaction on nano-magnetite:
M
367
[2] R.D. Ambashta, M. Sillanpää, Water purification using magnetic assistance: A review, J. Hazard. Mater. 180 (2010)
Process optimization, kinetics, and degradation pathways, J. Mol. Catal. A-Chem. 349 (2011) 71–79.
[8] N. Wang, L. Zhu, M. Wang, D. Wang, H. Tang, Sono-enhanced degradation of dye pollutants with the use of H 2O2
d
366
Angew. Chem. Int. Ed. 46 (2007) 1222–1244.
activated by Fe3O4 magnetic nanoparticles as peroxidase mimetic, Ultrason. Sonochem. 17 (2010) 78–83.
te
365
[9] R.C.C. Costa, M.F.F. Lelis, L.C.A. Oliveira, J.D. Fabris, J.D. Ardisson, R.R.V.A. Rios, C.N. Silva, R.M. Lago,
381
Novel active heterogeneous Fenton system based on Fe3−xMxO4 (Fe, Co, Mn, Ni): The role of M2+ species on the
382
reactivity towards H2O2 reactions, J. Hazard. Mater. B129 (2006) 171–178.
Ac ce p
380
383
[10] N. Wang, L. Zhu, D. Wang, M. Wang, Z. Lin, H. Tang, Sono-assisted preparation of highly-efficient peroxidase-like
384
Fe3O4 magnetic nanoparticles for catalytic removal of organic pollutants with H2O2, Ultrason. Sonochem. 17 (2010)
385
526–533.
386
[11] X. Liang, Z. He, Y. Zhong, W. Tan, H. He, P. Yuan, J. Zhu, J. Zhang, The effect of transition metal substitution on the
387
catalytic activity of magnetite in heterogeneous Fenton reaction: In interfacial view, Colloid. Surface A 435 (2013)
388
28–35.
389
[12] X. Hu, B. Liu, Y. Deng, H. Chen, S. Luo, C. Sun, P. Yang, S. Yang, Adsorption and heterogeneous Fenton
390
degradation of 17α-methyltestosterone on nano Fe3O4/MWCNTs in aqueous solution, Appl. Cataly. B-Environ. 107
391
(2011) 274–283.
392
[13] M.F. Variava, T.L. Church, A.T. Harris, Magnetically recoverable FexO y–MWNT Fenton’s catalysts that show 14
Page 14 of 30
395 396 397 398 399
[14] V. Cleveland, J.P. Bingham, E. Kan, Heterogeneous Fenton degradation of bisphenol A by carbon nanotube-supported Fe3O4, Sep. Purif. Technol. 133 (2014) 388–395.
[15] J. Chun, H. Lee, S.H. Lee, S.W. Hong, J. Lee, C. Lee, J. Lee, Magnetite/mesocellular carbon foam as a magnetically recoverable Fenton catalyst for removal of phenol and arsenic, Chemosphere 89 (2012) 1230–1237.
ip t
394
enhanced activity at neutral pH, Appl. Catal. B: Environ. 123–124 (2012) 200–207.
[16] S. Shin, H. Yoon, J. Jang, Polymer-encapsulated iron oxide nanoparticles as highly efficient Fenton catalysts, Cataly. Commun. 10 (2008) 178–182.
cr
393
[17] M.H. Do, N.H. Phan, T.D. Nguyen, T.T.S. Pham, V.K. Nguyen, T.T.T. Vu, T.K.P. Nguyen, Activated carbon/Fe3O4
401
nanoparticle composite: Fabrication, methyl orange removal and regeneration by hydrogen peroxide, Chemosphere
402
85 (2011) 1269–1276.
407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422
an
removal of microcystin-LR in water, Appl. Surf. Sci. 289 (2014) 245– 251.
M
406
[19] L. Lian, X. Cao, Y. Wu, D. Sun, D. Lou, A green synthesis of magnetic bentonite material and its application for
[20] X. Xue, K. Hanna, N. Deng, Fenton-like oxidation of Rhodamine B in the presence of two types of iron (II, III) oxide, J. Hazard. Mater. 166 (2009) 407–414.
d
405
water, Chem. Eng. J. 200–202 (2012) 97–103.
[21] P.P. Gan, S.F.Y. Li, Efficient removal of Rhodamine B using a rice hull-based silica supported iron catalyst by
te
404
[18] J. Ma, J. Qi, C. Yao, B. Cui, T. Zhang, D. Li, A novel bentonite-based adsorbent for anionic pollutant removal from
Fenton-like process, Chem. Eng. J. 229 (2013) 351–363.
[22] X. Wang, Y. Pan, Z. Zhu, J. Wu, Efficient degradation of rhodamine B using Fe-based metallic glass catalyst by
Ac ce p
403
us
400
Fenton-like process, Chemosphere 117 (2014) 638–643.
[23] J. Carriazo, E. Guélou, J. Barrault, J.M. Tatibouët, R. Molina, S. Moreno, Synthesis of pillared clays containing Al, Al-Fe or Al-Ce-Fe from a bentonite: Characterization and catalytic activity, Catal. Today 107-108 (2005) 126–132.
[24] L. Chen, C. Deng, F. Wu, N. Deng, Decolorization of the azo dye Orange II in a montmorillonite/H 2O2 system, Desalination 281 (2011) 306–311.
[25] M. Neamţu, C. Zaharia, C. Catrinescu, A. Yediler, M. Macoveanu, A. Kettrup, Fe-exchanged Y zeolite as catalyst for wet peroxide oxidation of reactive azo dye Procion Marine H-EXL, Appl. Catal. B: Environ. 48 (2004) 287–294.
[26] Y. Liu, W. Jiang, Y. Wang, X.J. Zhang, D. Song, F.S. Li, Synthesis of Fe3O4/CNTs magnetic nanocomposites at the liquid–liquid interface using oleate as surfactant and reactant, J. Magn. Magn. Mater. 321 (2009) 408–412.
[27] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. 15
Page 15 of 30
425 426 427 428 429 430
temperature, Appl. Surf. Sci. 303 (2014) 6–13.
[29] X. Xue, K. Hanna, M. Abdelmoula, N. Deng, Adsorption and oxidation of PCP on the surface of magnetite: Kinetic experiments and spectroscopic investigations, Appl. Catal. B: Environ. 89 (2009) 432–440.
[30] E.G. Garrido-Ramírez, B.K.G Theng, M.L. Mora, Clays and oxide minerals as catalysts and nanocatalysts in Fenton-like reactions — A review, Appl. Clay Sci. 47 (2010) 182–192.
ip t
424
[28] D. Wilson, M.A. Langell, XPS analysis of oleylamine/oleic acid capped Fe3O4 nanoparticles as a function of
[31] S. Zuo, R. Zhou, Al-pillared clays supported rare earths and palladium catalysts for deep oxidation of low
cr
423
concentration of benzene, Appl. Surf. Sci. 253 (2006) 2508–2514.
us
431
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Fig. 1. XRD patterns of Al-B (a), Fe3O4 (b), fresh Fe3O4/Al-B (c) and reused Fe3O4/Al-B (d).
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Fig. 2. SEM images of Fe3O4 (a), Al-B (b), fresh Fe3O4/Al-B (c) and reused Fe3O4/Al-B (d).
435 Fig. 3. XPS spectra of Fe3O4 (a) and Fe3O4/Al-B (b) (the inset shows the high-resolution scan of Fe 2p region).
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Fig. 4. Hysteresis loop of Fe3O4 (a) and Fe3O4/Al-B (b).
439 Fig. 5. Nitrogen adsorption/desorption isotherms for Fe3O4 and Fe3O4/Al-B.
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Fig. 6. Adsorption and desorption kinetics for RhB on Fe3O4 and Fe3O4/Al-B. (C0 = 40 mg∙L-1, [adsorbent] = 1 g·L-1; T =
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25 °C)
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Fig. 7. Isothermal adsorption of RhB on Fe3O4 and Fe3O4/Al-B. ([adsorbent] = 1 g·L-1; T = 25 °C)
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Fig. 8. Degradation of RhB along time under different conditions. (C0 = 40 mg∙L-1, [catalyst] = 1 g·L-1, [H2O2] = 200
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mmol∙L-1, T = 25 °C)
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Fig. 9. The effect of initial H2O2 (a) and RhB (b) concentration on the degradation efficiency of RhB. Inset: correlation of
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the apparent pseudo-first-order rate constant with H2O2 or RhB concentration. (C0 = 40 mg∙L-1, [Fe3O4/Al-B] = 1 g·L-1,
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[H2O2] = 200 mmol∙L-1, T = 25 °C)
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Fig. 10. (a) IR spectra of RhB before and after degradation. (b) RhB removal and CODCr elimination in the RhB
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degradation. (C0 = 40 mg∙L-1, [Fe3O4/Al-B] = 1 g·L-1, [H2O2] = 200 mmol∙L-1, T = 25 °C)
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Fig. 11. XPS spectrum of Fe on Fe3O4/Al-B surface after and before degradation of RhB (Fe 2p line).
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Fig. 12. Effect of recycling the catalyst and homogeneous catalysis on the degradation of RhB. (C0 = 40 mg∙L-1,
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[Fe3O4/Al-B] = 1 g·L-1, [H2O2] = 200 mmol∙L-1, T = 25 °C)
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