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Fabrication and dye removal performance of magnetic CuFe2O4@CeO2 nanofibers
Accepted Manuscript Title: Fabrication and dye removal performance of magnetic CuFe2 O4 @CeO2 nanofibers Author: Lianli Zou Qiuju Wang Xiangqian Shen Zhou Wang Maoxiang Jing Zhou Luo PII: DOI: Reference:
S0169-4332(15)00217-2 http://dx.doi.org/doi:10.1016/j.apsusc.2015.01.176 APSUSC 29613
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-12-2014 21-1-2015 22-1-2015
Please cite this article as: L. Zou, Q. Wang, X. Shen, Z. Wang, M. Jing, Z. Luo, Fabrication and dye removal performance of magnetic nanofibers, Applied Surface Science (2015), CuFe2 O4 @CeO2 http://dx.doi.org/10.1016/j.apsusc.2015.01.176 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.
Fabrication and dye removal performance of magnetic CuFe2O4@CeO2 nanofibers Lianli Zou, Qiuju Wang, Xiangqian Shen*, Zhou Wang, Maoxiang Jing, Zhou Luo
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Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, China *Corresponding author: Prof. Xiangqian Shen
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E-mail address: [email protected]
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Tex/fax: +86-511-88791964
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Abstract: Novel magnetic adsorbents with CeO2 nanoparticles (about 20 nm) coated on CuFe2O4 nanofibers were fabricated by combining electrospinning technique and chemical precipitation methods. The prepared CuFe2O4@CeO2 composite nanofibers show a diameter of 200 nm with a high specific
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surface area of 64.12 m2/g. These composite nanofibers exhibit a typical soft-magnetic materials behavior with a specific saturation magnetization (Ms) of 20.51 Am2/kg. The adsorption performances
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of these composite nanofibers were evaluated by column bed studies for methyl orange (MO) removal
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from aqueous solution. The effect of pH, flow rate and dye concentration on adsorption performances were investigated. The results show that the adsorption capacity decreases with increase of pH. The
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largest adsorption capacity of the column beds shows about 100 g/mL under the condition of C0=0.05
mg/mL, F=2.0 mL/min and pH=4.0. The kinetic process is described by Thomas model. The rate constant decreases with the extension of reaction time and decreasing pH. The desorption behaviors are also studied in 0.5 M NaCl solution, ethyl alcohol and deionized water respectively, which show the adsorbed MO molecules can be easily desorbed from CuFe2O4@CeO2 composite nanofibers in NaCl
solution. The adsorption mechanism of ionic interaction, formation of hydrogen bonds and pore diffusion is rationally proposed. Key words: electrospinning, CuFe2O4@CeO2 nanofibers, adsorption, column beds, mechanism
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1.
Introduction
Organic dyes have extensively used in the production of papers, leather, food, cosmetics, clothes and so on, but lots of dyes or pigments are toxic in nature especially the high concentration of dye effluents
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released into water, which can cause skin irritation, cancer and mutation [1-2]. Therefore, it is necessary to remove the toxic organic dyes from wastewater. Among a series of methods for the
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treatment of dye effluents, i.e. adsorption, flocculation, photodegradation, biodegradation and many other technologies [3-5], the adsorption is considered to a promising way to remove these toxic
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substances [6]. Recent years, many researchers have paid much attentions on the preparation of mesoporous materials, which are usually used as adsorbents to remove dyes and heavy metal ions from
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wastewater owing to their large specific surface area, high pore volume and high removal efficiency [7-11]. For example, the mesoporous natural clay showed a high removal efficiency for Reactive
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Yellow 2 [8]. The malachite green and methyl blue could be efficiently absorbed by the mesoporous aluminophosphate and barium phosphate nano-flake, respectively [10-11]. Mesoporous CeO2-based materials with nano size are widely used as oxygen storage materials [12],
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luminescent materials [13], gas sensors [14] and catalyst [15]. Some investigations showed the CeO2 is
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one of the excellent materials for removing dyes and heavy ions in wastewater. For example, Fe2O3-CeO2-TiO2/γ -Al2O3 and CeO2–TiO2 showed a high removal efficiency for azo dyes by catalytic
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wet air oxidation [16,17]; the porous CeO2-ZrO2 nanospheres showed an adsorption capacity of 27.1
and 9.2 mg/g for As(V) and As(III), respectively, at an equilibrium arsenic concentration of 0.01 mg/L [18]; Ji et al. reported that CeO2 was a good photocatalyst for azo dye degradation under visible light
irradiation [19]. What’s more, the equilibrium and kinetic for fluoride adsorption onto CeO2/Al2O3 composites were investigated by different adsorption isotherm and kinetic models [20]. Though CeO2 is one of the most active rare earth metal oxides and has been extensively used in wastewater treatment, the separation and recycling of these rare earth metal oxides like cerium oxide from the mixture is still inefficient. If the cerium oxide nanoparticles can’t be completely separated from the mixture, they will cause the secondary pollution and be not economical in applications. It is a simple, convenient, high effective and low cost way to separate the adsorbents from aqueous solution by magnetism [21]. Therefore, a composite of CeO2 and magnetic matters, especially the magnetic nanofibers, which show a better degradation and recycle performance for water treatment 2
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than that for the counter particles [22,23], can be a competitive adsorbent candidate. While the spinel-type CuFe2O4 can be a good candidate for core in the composite magnetic material since it has a good stability and a relatively low specific saturation magnetization resulting in a very low aggregation of nanofibers [24]. Herein, the CuFe2O4@CeO2 composite nanofibers with the structure of CuFe2O4 as
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the core and CeO2 nanoparticles as the outer layer were prepared in this study. Furthermore, these
showed an outstanding adsorption performance for dye removal. 2.
Experiment
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2.1 Synthesis of magnetic CuFe2O4@CeO2 composite nanofibers
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composite nanofibers were prepared as a porous fixed column bed for wastewater treatment and
The preparation of CuFe2O4 nanofibers was similar to the process described in our previous paper for
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Fe2O3 nanofibers [25]. In a typical procedure, 1.0 g poly(vinylpyrrolidone) (PVP, Mw=1,300,000, Aldrich) was dissolved in a mixture of ethanol (9.0 g) and deionized water (2.0 g), followed by
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magnetic stirring for about 5 hours to ensure the dissolution of PVP. Then 1.6160 g ferric nitrate (Fe(NO3)3•9H2O, AR) and 0.3993 g copper acetate (Cu(CH3COO)2•9H2O, AR) with the molar ratio of
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2:1 were added into the above solution and magnetically stirred about 24 hours at room temperature. The solution was loaded into a plastic syringe with a stainless steel needle and the feeding speed of 0.6
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mL/h was controlled by a syringe pump during the electrospinning process. While the applied voltage
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was 17 kV between the positive and negative electrode with the distance of 18 cm. The obtained precursor nanofibers were dried at 90 oC for 20 hours and subsequently calcined at 600 oC with a heating rate of 2 oC/min for 2 hours at ambient atmosphere to form magnetic CuFe2O4 nanofibers.
CuFe2O4@CeO2 composite nanofibers were synthesized via a chemical precipitation method. First, about 0.40 g CuFe2O4 nanofibers and 1.0 g Ce(NO3)3•6H2O were dispersed in 100 mL deionized water
and then sonicated for 30 min, followed by adding 200 mL 0.05 mol/L NaOH under continuously stirring with the flow rate of 1.0 mL/min to form the precipitate. After vacuum filtration, the precipitate was washed with deionized water and ethanol for several times, and then dried at 120 oC for 4 hours.
The composite nanofibers were formed by calcination of the dried precipitate at 600ºC for 1 hour in ambient atmosphere.
2.2 Characterization of the composite nanofibers The phases of prepared nanofibers were determined by an X-ray diffractometer (Rigaku 3
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D/Mmax2500PC) using Cu-kα radiation. The morphologies of these nanofibers were studied by field emission scanning electron microscopy (FESEM, JSM-7001F), and the surface area was determined by the Brunauer–Emmett–Teller (BET) method with the instrument of NOVA 2000e and the pore size distribution was calculated from the adsorption branch of the isotherm based on BJH method. The
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Fourier transform infrared (FT-IR) measurement of sample was recorded by a Nexu670 spectrometer in a wavenumber range of 400-4000 cm-1 at a resolution of 4 cm-1 by using KBr as the pellets.
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Magnetization was measured using a Quantum Design MPMS5 SQUID magnetometer with a maximum field of ±10000 Oe at room temperature. The point of zero charge (PZC) of nanofibers was
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evaluated by the mass titration method. With a continuous magnetic stirring, the nanofibers were slowly added into the deionized water until the pH value showed no changes. In order to reduce the
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discrepancies, the tests were repeated for three times and the average pH value of 7.8 was determined
2.3 Dye removal on column bed studies
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as the PZC in this work.
The adsorption performance of magnetic CuFe2O4@CeO2 composite nanofibers was investigated by
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the column bed studies. The fixed column bed was formed through the following process. At first, 0.3g
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CuFe2O4@CeO2 composite nanofibers were added into 50mL deionized water and ultrasonic dispersed for 30 min to form a uniform suspension. Driven by a peristaltic pump, the suspension was injected
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into the top of a glass column at the flow rate of 0.5 mL/min, the glass column with a diameter of 8 mm and a length of 15 cm. Then, the suspension of composite nanofibers formed a fixed column bed of 30 mm at the bottom of glass column. Subsequently, the column bed was thoroughly washed with deionized water overnight to ensure a closely packed structure without large voids or channels. With a peristaltic pump, the dye solution with a initial concentration (C0) was injected to the top of the column
bed at a desired flow rate. At given intervals, about 1.5 mL effluent flows were collected and the absorbance of remaining concentration solution was analyzed using a UV/Vis spectrophotometer at a maximum wave-length of 465 nm. When the dye concentration (Ct) of column effluent approached 90% of C0, the adsorption process was completed. The time (at breakthrough point, tb; at 0.5 and 0.9 loading, t0.5 and tex) and volume of dye solution treated (Vb, V0.5 and Vex) at different stages were used to value the adsorption ability of the column bed at different conditions. 3.
Results and discussion 4
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3.1 Characterization of nanofibers Fig. 1 shows the XRD patterns of the magnetic nanofibers, (a) CuFe2O4 and (b) CuFe2O4@CeO2. As shown in Fig. 1(a), the reflection peaks can be indexed as cubic CuFe2O4 (JCPDS No. 25-0283), however, there is still a little impurity phase of orthorhombic CuFe2O4 (JCPDS No. 34-0425).
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Compared with Fig. 1(a), after coated with CeO2 nanoparticles, the peaks of orthorhombic CuFe2O4 disappear, while the diffraction peaks of CeO2 (JCPDS No. 34-0394) occur, as shows in Fig. 1(b). The
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disappearance of orthorhombic CuFe2O4 phase can be attributed to a further calcination at high temperature. The average grain size (as shown in Table 1), which is calculated by using Scherrer’s
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equation, is about 9.5 nm, 23.5 nm for CeO2 and CuFe2O4, respectively. Obviously, the average grain size of CuFe2O4 has a slightly increase with a further heat treatment as expected.
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Fig. 2 shows the SEM morphologies of the composite nanofibers. From these SEM images, it is obvious that the nanofibers show a coarse and porous surface with a diameter about 200 nm, the CeO2
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nanoparticles are evenly distributed on the fiber surface and have a close contact to CuFe2O4 nanofibers, as shown in Fig.2c and Fig.2d. It is different from the nanoparticles, these nanofibers can effectively
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prevent the aggregation of nanoparticles, and the composite nanofibers band together to form abundant meso/macropores. Theoretically, these porous structures will be helpful for adsorption and storage
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enhancement, with a high adsorption capacity of adsorbents.
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To measure the specific surface area (SBET) and pore structure of the CuFe2O4@CeO2 composite nanofibers, the nitrogen adsorption-desorption isotherm was investigated. As shown in Fig.3, the isotherm curve with a H3 hysteresis loop belongs to the type-V shape, which is attributed to the accumulation of nitrogen molecules and indicates the presence of pore structures in material [26]. The calculated specific surface area, pore volume and average pore diameter are 64.12 m2/g, 0.15 cc/g and 4.74 nm, respectively (presented in Table 1). The pore size distribution of the sample is described in the insert picture of Fig. 3. It can be seen that the dominant pore size is ranged from 50 to 150 Å, which confirms that the CuFe2O4@CeO2 composite nanofibers have a structure of mesopore and contain a very high pore volume. This is consistent with the SEM observations in Fig.2. The magnetic properties of the nanofibers are characterized by the magnetic parameters such as the specific saturation magnetization (Ms), remanent magnetism (Mr) and coercivity (Hc). The hysteresis loops of CuFe2O4 and CuFe2O4@CeO2 nanofibers are showed in Fig.4, which exhibits a typical soft-magnetic materials behavior. As shown in table 1, with Ms of 28.32 Am2/kg, the CuFeO4 5
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nanofibers show a Mr of 12.85 Am2/kg and Hc of 25.11 kA/m, while the CuFe2O4@CeO2 composite nanofibers have a Ms of 20.51 Am2/kg, Mr of 7.24 Am2/kg and Hc of 8.18 kA/m, respectively. After coated CeO2 nanoparticles, the Hc value of the CuFe2O4 nanofibers decreases about 67.4%, while the
capacity due to a higher specific surface area and a larger amount of mesopores. 3.2 Dye removal by magnetic CuFe2O4@CeO2 composite nanofibers
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3.2.1 Column bed study
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Ms shows a little reduction of about 27.6%. The coated CeO2 nanoparticles will enhance the adsorption
The dye removal capacity of these magnetic CuFe2O4@CeO2 composite nanofibers was examined by
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the column bed studies and the methyl orange (MO) solution was chosen as the simulation wastewater. The experimental processes of column bed studies were described in section 2.3 and the parameters
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such as pH of solution, the flow rate of dye solution (F) and the initial dye concentration (C0) were considered. The breakthrough curves are showed in Fig. 5. Basically, the curves show the same typical
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S-shape, indicating that the adsorption capacity reaches equilibrium in liquid [27]. Fig. 5a shows the adsorption capacity of column beds at different pH values, and decreases gradually
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with the increase of pH. The similar effect of pH on MO adsorption was observed in other researches [28], implying the electrostatic interaction between the dye molecules and adsorbents is a main
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adsorption mechanism during the process. As pH increase from 4.0 to 10.0, the breakpoint decreases
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from 262 min to 11min, t0.5 reduced from 357 min to 20 min, respectively. Fig. 5b shows the effect of F (2.0, 3.5 and 5.0 mL/min) on the breakthrough curves at a constant pH=4 and C0 =0.025 mg/mL. The
breakpoint tb and t0.5 decrease at a high flow rate. As F increases from 2.0 to 5.0 mg/mL, the effluent solution volume increases, with tb decreased from 262 min to 49 min and t0.5 decreased from 357 min to
118 min. Fig.5c shows the effect of dye concentrations on adsorption ability and the concentrations of 0.025, 0.05 and 0.1mg/mL are studied in this part. Some performance parameters of column beds with different pH, F and C0 obtained from the breakthrough curves are listed in Table 2. As shown in Table 2, the breakpoint appears faster with increasing influent F and C0, which is due to
the decrease in contact time between the dye and adsorbents. Decreasing the contact time will result in premature breakthrough to occur, thus reduce the service time of the column bed [27]. The totally adsorbed MO quantity (Qe; mg/g) in the column bed for a given pH, F and C0 can be calculated from Eq.(3.1):[29,30]
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Qe =
T
0
(C0 - Ct ) Fdt m
(3.1)
Where Qe is the capacity for a given column bed (mg/g), Ct is the outlet MO concentration, T is the service time (min), and m is the weight of adsorbent used (g). Through graphical integration in Origin
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8.5, the capacity value Qe can be obtained and the results are showed in table 2. The adsorption capacity decreases with the increasing pH, the largest adsorption capacity of the column beds reaches about 100 g/mL under the conditions of C0=0.05 mg/mL, F=2.0 mL/min and pH=4.0.
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In order to understand the adsorption process, the FT-IR was carried out to detect the functional groups
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of the adsorbents before and after used, and the results are showed in Fig.6. As shown in Fig.6(a), the strong absorption band around 563 cm−1 can be attributed to the stretching mode of tetrahedral complexes [24]. The broad absorption band around 3407 cm−1 represents the stretching mode of H2O
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molecules and OH groups. The band at 1638 cm−1 corresponds to the bending mode of H2O molecules [31]. The absorption bands in MO spectrum in Fig.6(b), reveal the existence of –SO3- at 1201 cm−1 as
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well as a stretching vibration at 1039 cm−1 for C–N. Whereas the band located at about 1361cm−1 represents –N=N– stretching vibration of azo group and the band around 1601cm−1 is assigned to vCC
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vibrations in the aromatic rings [6,31].
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By comparing the spectrum of (a) and (b) with the FT-IR spectrum of methyl orange (c), there is no new absorption band occurred, which indicates that the MO molecules are physically adsorbed on
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CuFe2O4@CeO2 nanofibers. While the intensity of adsorption bands between 600 cm−1 and 1601 cm−1 becomes weaker after MO being adsorbed on magnetic composite nanofibers, owing to the protonation of function groups in MO reacting with the surface and edges of the adsorbents. It should be noted that the band at 1201 cm−1 is nearly disappeared, implying the –SO3- group combined with the positively
charged surface sites on the adsorbents, such as Ce4+, Fe3+ and Cu2+. 3.2.2 Kinetics analysis
Thomas model is a simple and widely used model to predict the dynamic behavior of the column bed, and has successfully described and predicted dye adsorptions for various adsorbents. The equation can be represented as Eq.(3.2, 3.3): [29,33]
Ct 1 = C0 1+exp(kq0 m
(3.2)
F
- C0 t )
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ln(
C0 kq m 1) 0 - kC0t Ct F
(3.3)
Where k is the adsorption rate constant (mL/(mg min)), q0 is the capacity for a given bed (mg/g), F is the flow rate of dye solution through column bed (mL/min), t is the service time (min), and m is the
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weight of adsorbent used (g).
According to the experimental data Ce, C0 and t at different conditions, the software package Origin 8.5
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is used to fit the data and the fitted results are plotted in Fig. 7. It can be seen that the plots of
ln[(C0/Ct)-1] against t are multilinear, with at least two linear segments. Although, similar results were
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observed by Öztürk et al. [29] and Tor et al. [30], this phenomenon was not completely analyzed. The adsorption rate constant ki and correlation coefficient values Ri2 (i=1,2) obtained in this work are
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presented in Table 3. The high value of Ri2 indicates that the Thomas model can be used to depict the adsorption process of MO on this column bed. The rate constant, k1>k2, suggests the reaction rate tend
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to decrease with time. The decrease of reaction rate may come from two parts, one is the amount of active sites on the adsorbents decreased as the dye molecules absorbed on the magnetic nanofibers, the
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other is the diffusion of dye molecules between the micro- and mesopores. A similar phenomenon was observed in Sethuraman’s research that the dye adsorption by montmorillonite showed a steady
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decrease from 10 mg•g-1min-1 in the first 5 min to 0.55 mg•g-1min-1 over the next hour, and then to 0.07
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mg•g-1min-1 for the final hour [34]. It is interesting that the rate constant increases with increase of pH, F and C0 at the first stage, but decreases with increase of F and C0 during the following adsorption
period. As we know, decreasing the mass transfer resistance results in an increase of the adsorption rate constant [35], so increasing the flow rate will increase the rate constant because of a bigger driven force. As the total active sites on the adsorbents surface are almost a constant, if the adsorption saturation occurs on the surface of adsorbent, its adsorption ability will be largely decreased as well as the decreasing of rate constant. Then, increasing F will lead the dyes more difficult to be adsorbed on the fibers due to a shorter time to penetrate and diffuse into the inner pores of the adsorbent. This is the reason why the rate constant increases with increasing of F at the first stage, but then decreased with increasing of F. As for the effect of dye concentration, the courses are similar to F, an increasing C0 diminishes the breakthrough time by a faster saturation of the column bed and results in an increase of rate constant [36]. At the condition of C0=0.1 mg/mL, the active sites on the adsorbent are quickly occupied by the dye molecules, which leads a rate constant declined sharply. While in a low C0, the rate 8
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constant shows a steady decrease and declines more slowly than that at a high C0. Because of the multilinear form of plots, the theoretical predictions of the adsorption capacity qi are quite different to the calculated adsorption capacity by integrals, which is not deeply investigated in this paper.
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3.2.3 Regeneration of column bed
The regeneration of the column bed is important for the application from the engineering view. The
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desorption test was carried out in 0.5 M NaCl solution, ethyl alcohol and deionized water, respectively. Fig. 8 shows the desorption behavior of MO in different solvents at an input flow rate of 2.0 mL/min.
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The MO saturated column beds were carried out at the condition of F=3.5 mg/mL for about 12 hours. As shown in Fig.8, the NaCl solution shows the best ability for MO desorption among these three
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solvents, the order of desorption activities is NaCl > CH3CH2OH > H2O. While in deionized water, there is almost no MO molecules desorbed from the nanofibers, which indicates the electrostatic
further study.
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3.3 Adsorption mechanism discussion
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interaction plays a significant role in the adsorption process. The detailed desorption process needs a
It is worth noting that the adsorption mechanism at different conditions will be affected by many
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variables. The ionic interaction, hydrogen bonds and pore diffusion can play an essential factor for MO
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molecules adsorption [11,29]. Based on the effects of pH and desorption process, the charge state between the interface of adsorbents and solution is a main factor for adsorption capacity, which indicates the surface ionic interaction and formation of hydrogen bonds will play a major role. Additionally, the column bed is a micro-filter in fact, which has a hierarchical pore structure with multi-channels, consisting of the large pores coming from the stacking of nanofibers, the macropores and mesopores formed during the preparation process. So the dye molecules can be easily absorbed on the inner pore surface though diffusion, which has been proved by the kinetics analysis. As shown in Fig. 9(a) and (b), the reaction process can be divided into two major steps: firstly, the MO molecules are absorbed onto the surface or external pore surface of the adsorbents when the dye solution passed through the porous column bed. In this process, the dye molecules will interact with the vacant active sites presented in the surface and external pores. Because the composite nanofibers have a positively charged surface below pH 7.8 (point of zero potential), the MO anions can be easily absorbed on the positively charged surface of adsorbents by electrostatic interactions. Secondly, when the activated sites 9
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reach saturation, the remaining vacant surface sites are difficult to occupy because of the repulsive forces between the dye molecules on the nanofibers and the bulk phase. Then the diffusion process plays a dominant role, as showed in Fig. 9(b), the dye molecules diffuse through the porous channel and interface of CeO2 and then absorbed onto the inner pore surface. The combinations of MO and
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composite nanofibers are presented in Fig. 9(c), the negative groups –SO3- react with the positively charged Ce4+, Fe3+ and Cu2+ on the adsorbents surface, which is evidenced from the FT-IR results.
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While the hydrogen bonds formed between O2- and positively charged of protonation amino groups in acid solution is equally important in the adsorption process.
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4. Conclusions
1. Novel magnetic adsorbents of CuFe2O4@CeO2 composite nanofibers were fabricated by combining
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electrospinning technique and chemical precipitation methods. The ultrafine CeO2 nanoparticles (about 20 nm with a grain size of 9.5 nm) are evenly coated on the magnetic CuFe2O4 nanofibers with a
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diameter of about 200 nm.
2. The CuFe2O4@CeO2 composite nonofibers have a high specific surface area of 64.12 m2/g and Ms of
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20.51 Am2/kg. These composite nanofibers show a mesoporous structure and can form a porous micro-filter like column bed for dye removal.
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3. The porous column beds consisting of the magnetic composite nanofibers show a high adsorption
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capacity for MO molecules. The highest adsorption capacity is about 100 mg/g at the condition of pH=4.0, F=2.0 mg/min and C0=0.05 mg/mL.
4. The kinetic study shows the plot of ln[(C0/Ct)-1] against t are multilinear. The rate constant decreases with the decrease of pH and the extension of time, while its value increases with increasing the inlet flow rate and initial concentration at the first stage. 5. The diffusion process and electrostatic interaction play a significant role in the adsorption process, the negative groups –SO3- can combine with the positively charged Ce4+, Fe3+ and Cu2+ on the adsorbents surface, the hydrogen bonds will form between O2- and positively charged of protonation amido groups. 6. The order of desorption activity is NaCl > CH3CH2OH > H2O. The magnetic adsorbents can be easily separated and recycled from the mixture solution by magnetism. Acknowledgments 10
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This work was financially supported by the National Natural Science Foundation of China (Grant No. 51274106, 51474113, 51474037), the Natural Science Foundation of Jiangsu Provincial Higher
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Education of China (Grant No. 12KJA430001), the Science and Technology Support Program of Jiangsu Province of China (Grant No. BE2012143, BE2013071), the Jiangsu Province's Postgraduate
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Cultivation and Innovation Project of China (Grant CXZZ13_0662), and the Priority Academic
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Program Development of Jiangsu Higher Education Institutions.
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te
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study via a kinetics investigation to the degradation of acid orange 7, Applied Catalysis B: Environmental 121– 122 (2012) 223– 229.
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Y. Liu, D.Z. Sun, Development of Fe2O3-CeO2-TiO2/γ-Al2O3 as catalyst for catalytic wet air
oxidation of methyl orange azo dye under room condition, Applied Catalysis B: Environmental 72 (2007) 205–211.
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B.X. Zhao, B.C. Shi, X.L. Zhang, X. Cao, Y.Z. Zhang, Catalytic wet hydrogen peroxide oxidation of H-acid in aqueous solution with TiO2-CeO2 and Fe/TiO2-CeO2 catalysts, Desalination 268 (2011) 55 –59.
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W.H. Xu, J. Wang, L. Wang, G.P. Sheng, J.H. Liu, H.Q. Yu, X.J. Huang, Enhanced arsenic removal from water by hierarchically porous CeO2-ZrO2 nanospheres: Role of surface-and structure-dependent properties, Journal of Hazardous Materials 260 (2013) 498– 507.
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P.F. Ji, J.L. Zhang, F. Chen, M. Anpo, Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation, Applied Catalysis B: Environmental 85 (2009) 12
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148–154. [20]
T. Zhang, Q.R. Li, Y. Liu, Y.L. Duan, W.Y. Zhang, Equilibrium and kinetics studies of fluoride ions adsorption on CeO2/Al2O3 composites pretreated with non-thermal plasma, Chemical Engineering Journal 168 (2011) 665–671. Q. Dai, J. Wang, J. Yu, J. Chen, J. Wang, J. Chen, Catalytic ozonation for the degradation of
ip t
[21]
acetylsalicylic acid in aqueous solution by magnetic CeO2 nanometer catalyst particles, Applied
[22]
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Catalysis B, Environmental 144 (2014) 686–693.
P. Zhang, C.L. Shao, M.Y. Zhang, Z.C. Guo, J.B. Mu, Z.Y. Zhang, X. Zhang, Y.C. Liu, Bi2MoO6
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ultrathin nanosheets on ZnTiO3 nanofibers: A 3D open hierarchical heterostructures synergistic system with enhanced visible-light-driven photocatalytic activity, Journal of Hazardous Materials
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217–218 (2012) 422–428.
P. Zhang, C.L. Shao, M.Y. Zhang, Z.C. Guo, J.B. Mu, Z.Y. Zhang, X. Zhang, P.P. Liang, Y.C. Liu,
M
Controllable synthesis of Zn2TiO4@carbon core/shell nanofibers with highphotocatalytic performance, Journal of Hazardous Materials 229–230 (2012) 265–272. W. Ponhan, S. Maensiri, Fabrication and magnetic properties of electrospun copper ferrite
d
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(CuFe2O4) nanofibers, Solid State Sciences 11 (2009) 479–484. Q.J. Wang, R.J. Liu, X.Q. Shen, L.L. Zou, D.M. Wu, Mesoporous Iron Oxide Nanofibers and
te
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Their Loading Capacity of Curcumin, Journal of Nanoscience and Nanotechnology 14 (2014) 2871-2877.
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H.Y. Zhu, R. Jiang, L. Xiao, W. Li, A novel magnetically separable γ-Fe2O3/crosslinked chitosan adsorbent: Preparation, characterization and adsorption application for removal of hazardous azo dye, Journal of Hazardous Materials 179 (2010) 251–257
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J. Feng, L. Su, Y.H. Ma, C.L. Ren, Q. Guo, X.G. Chen, CuFe2O4 magnetic nanoparticles: A simple and efficient catalyst for the reduction of nitrophenol, Chemical Engineering Journal 221 (2013)
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16–24.
L. Ayed, E. Khelifi, H.B. Jannet, H. Miladi, A. Cheref, S. Achour, A. Bakhrouf, Response surface
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methodology for decolorization of azo dye Methyl Orange by bacterial consortium: Produced enzymes and metabolites characterization, Chemical Engineering Journal 165 (2010) 200–208.
M.F.F. Sze, V.K.C. Lee, G. McKay, Simplified fixed bed column model for adsorption of organic
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pollutants using tapered activated carbon columns, Desalination 218 (2008) 323-333. N.K. Goel, V. Kumar, S. Pahan, Y.K. Bhardwaj, S. Sabharwal, Development of adsorbent from
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M
of Hazardous Materials 193 (2011) 17–26
kinetic adsorption of dyes, Journal
V.K.C. Lee, J.F. Porter, G. McKay, Development of fixed-bed adsorber correlation
[36]
d
models, Industrial & Engineering Chemistry Research, 39 (2000) 2427-2433. A.A. Ahmad, B.H. Hameed, Fixed-bed adsorption of reactive azo dye onto granular activated
Ac ce p
te
carbon prepared from waste, Journal of Hazardous Materials 175 (2010) 298-303.
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Figure caption
Ac ce p
te
d
M
an
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cr
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Figure1 XRD patterns of the prepared magnetic nanofibers: (a) CuFe2O4 and (b) CuFe2O4@CeO2.
15
Page 15 of 25
Ac ce p
te
d
M
an
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Figure 2 SEM morphologies of the prepared nanofibers: CuFe2O4 (a, b), CuFe2O4@CeO2 (c, d).
16
Page 16 of 25
Figure 3 Nitrogen adsorption-desorption isotherm and pore sizes distribution of CuFe2O4@CeO2
Ac ce p
te
d
M
an
us
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composite nanofibers.
17
Page 17 of 25
Ac ce p
te
d
M
an
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Figure 4 The hysteresis loops of CuFe2O4 and CuFe2O4@CeO2 nanofibers.
18
Page 18 of 25
Figure 5 The breakthrough curves for adsorption of MO onto CuFe2O4@CeO2 nanofibers at different
Ac ce p
te
d
M
an
us
cr
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pH (a), flow rates (b) and inlet dye concentrations (c).
19
Page 19 of 25
Ac ce p
te
d
M
an
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Figure 6 FT-IR spectra of adsorbents before and after used (a, b), Methyl Orange (c).
20
Page 20 of 25
Figure 7 Plots of ln[( C0/Ct)−1] vs. t by the Thomas equation at different pH (a), flow rates (b) and inlet
Ac ce p
te
d
M
an
us
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dye concentrations (c).
21
Page 21 of 25
Figure 8 Desorption curves of MO in 0.5 M NaCl solution (a), ethyl alcohol (b) and deionized water
Ac ce p
te
d
M
an
us
cr
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(c).
22
Page 22 of 25
Ac ce p
te
d
M
an
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Figure 9 The schematic illustration of MO adsorption on CuFe2O4@CeO2 composite nanofibers
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Page 23 of 25
Table caption Table 1 The average grain size (D), specific surface area (SBET), total pore volume (Pv), average pore size (Pa), coercivity (Hc), specific saturation magnetization (Ms) and remanence (Mr) of nanofibers. D (nm)
SBET
Pv
Pa
Hc
Ms
Mr
2
CeO2
(m /g)
(cc/g)
(nm)
(kA/m)
(Am /kg)
(Am2/kg)
CuFe2O4
18.3
--
8.44
2.13×10-3
5.04
25.11
28.32
12.85
CuFe2O4@CeO2
23.5
9.5
64.12
0.15
4.74
8.18
20.51
7.24
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Table 2 Parameters in the adsorption of MO at various operation conditions. C0=0.025 mg/mL
C0=0.05 mg/mL
C0=0.1 mg/mL
F=3.5 ml/min
F=5.0 mL/min
F=2.0 mL/min
F=2.0 mL/min
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F=2.0 mL/min
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CuFe2O4
cr
2
pH=4.0
pH=5.0
pH=6.0
pH=8.0
pH=10.0
pH=4.0
pH=4.0
pH=4.0
pH=4.0
tb(min)
262
129
93
80
11
180
49
110
16
t0.5(min)
357
251
206
161
20
316
tex(min)
581
387
319
241
129
Vb(mL)
514
258
186
160
22
V0.5(mL)
714
502
412
322
40
Vex(mL)
1162
774
638
482
258
Qe(mg/g)
65.04
42.73
35.24
27.91
7.52
96.89
243
26
479
478
393
630
245
220
32
1106
590
486
52
1866
2395
956
786
52.32
100.11
78.91
Ac ce p
te
d
M
118
533
Table 3 Model parameters for the adsorption of MO by using CuFe2O4@CeO2 composite nanofibers at
various operation conditions. C0=0.025 mg/mL
pH=4.0
R1
2
k1(L/mg•min)
pH=5.0
0.9845 1.13×10
1.41×10
F=2.0
F=3.5
F=5.0
F=2.0
F=2.0
ml/min
mL/min
mL/min
mL/min
pH=4.0
pH=4.0
pH=4.0
pH=4.0
pH=8.0
0.9819 -3
C0=0.1 mg/mL
mL/min
pH=6.0
0.9830 -3
C0=0.05 mg/mL
1.61×10
pH=10.0
0.9673 -3
1.90×10
0.9789 -3
1.17×10
0.9581 -2
1.79×10
0.9357 -3
1.28×10
0.9390 -2
8.41×10
0.8809 -4
3.31×10-3
R22
0.9337
0.9859
0.9849
0.9813
0.9666
0.9858
0.9541
0.9577
0.9684
k2(L/mg•min)
4.39×10-4
6.47×10-4
6.56×10-4
8.61×10-4
4.77×10-3
4.14×10-4
1.95×10-4
2.31×10-4
7.05×10-5
24
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1. The magnetic adsorbents with CeO2 nanoparticles coated on CuFe2O4 nanofibers were prepared. 2. The nanofibers had a diameter of 200 nm, specific surface area of 64.12 m2/g and Ms of 20.51 Am2/kg. 3. The plots of ln[(C0/Ce)-1] vs. t were multilinear when fitted by Thomas models.
Ac ce p
te
d
M
an
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cr
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4. The magnetic adsorbents can be easily separated and recycled from mixture solution by magnetism.
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S0169-4332(15)00217-2 http://dx.doi.org/doi:10.1016/j.apsusc.2015.01.176 APSUSC 29613
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-12-2014 21-1-2015 22-1-2015
Please cite this article as: L. Zou, Q. Wang, X. Shen, Z. Wang, M. Jing, Z. Luo, Fabrication and dye removal performance of magnetic nanofibers, Applied Surface Science (2015), CuFe2 O4 @CeO2 http://dx.doi.org/10.1016/j.apsusc.2015.01.176 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.
Fabrication and dye removal performance of magnetic CuFe2O4@CeO2 nanofibers Lianli Zou, Qiuju Wang, Xiangqian Shen*, Zhou Wang, Maoxiang Jing, Zhou Luo
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Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, China *Corresponding author: Prof. Xiangqian Shen
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E-mail address: [email protected]
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Tex/fax: +86-511-88791964
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Abstract: Novel magnetic adsorbents with CeO2 nanoparticles (about 20 nm) coated on CuFe2O4 nanofibers were fabricated by combining electrospinning technique and chemical precipitation methods. The prepared CuFe2O4@CeO2 composite nanofibers show a diameter of 200 nm with a high specific
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surface area of 64.12 m2/g. These composite nanofibers exhibit a typical soft-magnetic materials behavior with a specific saturation magnetization (Ms) of 20.51 Am2/kg. The adsorption performances
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of these composite nanofibers were evaluated by column bed studies for methyl orange (MO) removal
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from aqueous solution. The effect of pH, flow rate and dye concentration on adsorption performances were investigated. The results show that the adsorption capacity decreases with increase of pH. The
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largest adsorption capacity of the column beds shows about 100 g/mL under the condition of C0=0.05
mg/mL, F=2.0 mL/min and pH=4.0. The kinetic process is described by Thomas model. The rate constant decreases with the extension of reaction time and decreasing pH. The desorption behaviors are also studied in 0.5 M NaCl solution, ethyl alcohol and deionized water respectively, which show the adsorbed MO molecules can be easily desorbed from CuFe2O4@CeO2 composite nanofibers in NaCl
solution. The adsorption mechanism of ionic interaction, formation of hydrogen bonds and pore diffusion is rationally proposed. Key words: electrospinning, CuFe2O4@CeO2 nanofibers, adsorption, column beds, mechanism
1
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1.
Introduction
Organic dyes have extensively used in the production of papers, leather, food, cosmetics, clothes and so on, but lots of dyes or pigments are toxic in nature especially the high concentration of dye effluents
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released into water, which can cause skin irritation, cancer and mutation [1-2]. Therefore, it is necessary to remove the toxic organic dyes from wastewater. Among a series of methods for the
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treatment of dye effluents, i.e. adsorption, flocculation, photodegradation, biodegradation and many other technologies [3-5], the adsorption is considered to a promising way to remove these toxic
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substances [6]. Recent years, many researchers have paid much attentions on the preparation of mesoporous materials, which are usually used as adsorbents to remove dyes and heavy metal ions from
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wastewater owing to their large specific surface area, high pore volume and high removal efficiency [7-11]. For example, the mesoporous natural clay showed a high removal efficiency for Reactive
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Yellow 2 [8]. The malachite green and methyl blue could be efficiently absorbed by the mesoporous aluminophosphate and barium phosphate nano-flake, respectively [10-11]. Mesoporous CeO2-based materials with nano size are widely used as oxygen storage materials [12],
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luminescent materials [13], gas sensors [14] and catalyst [15]. Some investigations showed the CeO2 is
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one of the excellent materials for removing dyes and heavy ions in wastewater. For example, Fe2O3-CeO2-TiO2/γ -Al2O3 and CeO2–TiO2 showed a high removal efficiency for azo dyes by catalytic
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wet air oxidation [16,17]; the porous CeO2-ZrO2 nanospheres showed an adsorption capacity of 27.1
and 9.2 mg/g for As(V) and As(III), respectively, at an equilibrium arsenic concentration of 0.01 mg/L [18]; Ji et al. reported that CeO2 was a good photocatalyst for azo dye degradation under visible light
irradiation [19]. What’s more, the equilibrium and kinetic for fluoride adsorption onto CeO2/Al2O3 composites were investigated by different adsorption isotherm and kinetic models [20]. Though CeO2 is one of the most active rare earth metal oxides and has been extensively used in wastewater treatment, the separation and recycling of these rare earth metal oxides like cerium oxide from the mixture is still inefficient. If the cerium oxide nanoparticles can’t be completely separated from the mixture, they will cause the secondary pollution and be not economical in applications. It is a simple, convenient, high effective and low cost way to separate the adsorbents from aqueous solution by magnetism [21]. Therefore, a composite of CeO2 and magnetic matters, especially the magnetic nanofibers, which show a better degradation and recycle performance for water treatment 2
Page 2 of 25
than that for the counter particles [22,23], can be a competitive adsorbent candidate. While the spinel-type CuFe2O4 can be a good candidate for core in the composite magnetic material since it has a good stability and a relatively low specific saturation magnetization resulting in a very low aggregation of nanofibers [24]. Herein, the CuFe2O4@CeO2 composite nanofibers with the structure of CuFe2O4 as
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the core and CeO2 nanoparticles as the outer layer were prepared in this study. Furthermore, these
showed an outstanding adsorption performance for dye removal. 2.
Experiment
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2.1 Synthesis of magnetic CuFe2O4@CeO2 composite nanofibers
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composite nanofibers were prepared as a porous fixed column bed for wastewater treatment and
The preparation of CuFe2O4 nanofibers was similar to the process described in our previous paper for
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Fe2O3 nanofibers [25]. In a typical procedure, 1.0 g poly(vinylpyrrolidone) (PVP, Mw=1,300,000, Aldrich) was dissolved in a mixture of ethanol (9.0 g) and deionized water (2.0 g), followed by
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magnetic stirring for about 5 hours to ensure the dissolution of PVP. Then 1.6160 g ferric nitrate (Fe(NO3)3•9H2O, AR) and 0.3993 g copper acetate (Cu(CH3COO)2•9H2O, AR) with the molar ratio of
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2:1 were added into the above solution and magnetically stirred about 24 hours at room temperature. The solution was loaded into a plastic syringe with a stainless steel needle and the feeding speed of 0.6
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mL/h was controlled by a syringe pump during the electrospinning process. While the applied voltage
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was 17 kV between the positive and negative electrode with the distance of 18 cm. The obtained precursor nanofibers were dried at 90 oC for 20 hours and subsequently calcined at 600 oC with a heating rate of 2 oC/min for 2 hours at ambient atmosphere to form magnetic CuFe2O4 nanofibers.
CuFe2O4@CeO2 composite nanofibers were synthesized via a chemical precipitation method. First, about 0.40 g CuFe2O4 nanofibers and 1.0 g Ce(NO3)3•6H2O were dispersed in 100 mL deionized water
and then sonicated for 30 min, followed by adding 200 mL 0.05 mol/L NaOH under continuously stirring with the flow rate of 1.0 mL/min to form the precipitate. After vacuum filtration, the precipitate was washed with deionized water and ethanol for several times, and then dried at 120 oC for 4 hours.
The composite nanofibers were formed by calcination of the dried precipitate at 600ºC for 1 hour in ambient atmosphere.
2.2 Characterization of the composite nanofibers The phases of prepared nanofibers were determined by an X-ray diffractometer (Rigaku 3
Page 3 of 25
D/Mmax2500PC) using Cu-kα radiation. The morphologies of these nanofibers were studied by field emission scanning electron microscopy (FESEM, JSM-7001F), and the surface area was determined by the Brunauer–Emmett–Teller (BET) method with the instrument of NOVA 2000e and the pore size distribution was calculated from the adsorption branch of the isotherm based on BJH method. The
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Fourier transform infrared (FT-IR) measurement of sample was recorded by a Nexu670 spectrometer in a wavenumber range of 400-4000 cm-1 at a resolution of 4 cm-1 by using KBr as the pellets.
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Magnetization was measured using a Quantum Design MPMS5 SQUID magnetometer with a maximum field of ±10000 Oe at room temperature. The point of zero charge (PZC) of nanofibers was
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evaluated by the mass titration method. With a continuous magnetic stirring, the nanofibers were slowly added into the deionized water until the pH value showed no changes. In order to reduce the
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discrepancies, the tests were repeated for three times and the average pH value of 7.8 was determined
2.3 Dye removal on column bed studies
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as the PZC in this work.
The adsorption performance of magnetic CuFe2O4@CeO2 composite nanofibers was investigated by
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the column bed studies. The fixed column bed was formed through the following process. At first, 0.3g
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CuFe2O4@CeO2 composite nanofibers were added into 50mL deionized water and ultrasonic dispersed for 30 min to form a uniform suspension. Driven by a peristaltic pump, the suspension was injected
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into the top of a glass column at the flow rate of 0.5 mL/min, the glass column with a diameter of 8 mm and a length of 15 cm. Then, the suspension of composite nanofibers formed a fixed column bed of 30 mm at the bottom of glass column. Subsequently, the column bed was thoroughly washed with deionized water overnight to ensure a closely packed structure without large voids or channels. With a peristaltic pump, the dye solution with a initial concentration (C0) was injected to the top of the column
bed at a desired flow rate. At given intervals, about 1.5 mL effluent flows were collected and the absorbance of remaining concentration solution was analyzed using a UV/Vis spectrophotometer at a maximum wave-length of 465 nm. When the dye concentration (Ct) of column effluent approached 90% of C0, the adsorption process was completed. The time (at breakthrough point, tb; at 0.5 and 0.9 loading, t0.5 and tex) and volume of dye solution treated (Vb, V0.5 and Vex) at different stages were used to value the adsorption ability of the column bed at different conditions. 3.
Results and discussion 4
Page 4 of 25
3.1 Characterization of nanofibers Fig. 1 shows the XRD patterns of the magnetic nanofibers, (a) CuFe2O4 and (b) CuFe2O4@CeO2. As shown in Fig. 1(a), the reflection peaks can be indexed as cubic CuFe2O4 (JCPDS No. 25-0283), however, there is still a little impurity phase of orthorhombic CuFe2O4 (JCPDS No. 34-0425).
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Compared with Fig. 1(a), after coated with CeO2 nanoparticles, the peaks of orthorhombic CuFe2O4 disappear, while the diffraction peaks of CeO2 (JCPDS No. 34-0394) occur, as shows in Fig. 1(b). The
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disappearance of orthorhombic CuFe2O4 phase can be attributed to a further calcination at high temperature. The average grain size (as shown in Table 1), which is calculated by using Scherrer’s
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equation, is about 9.5 nm, 23.5 nm for CeO2 and CuFe2O4, respectively. Obviously, the average grain size of CuFe2O4 has a slightly increase with a further heat treatment as expected.
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Fig. 2 shows the SEM morphologies of the composite nanofibers. From these SEM images, it is obvious that the nanofibers show a coarse and porous surface with a diameter about 200 nm, the CeO2
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nanoparticles are evenly distributed on the fiber surface and have a close contact to CuFe2O4 nanofibers, as shown in Fig.2c and Fig.2d. It is different from the nanoparticles, these nanofibers can effectively
d
prevent the aggregation of nanoparticles, and the composite nanofibers band together to form abundant meso/macropores. Theoretically, these porous structures will be helpful for adsorption and storage
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enhancement, with a high adsorption capacity of adsorbents.
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To measure the specific surface area (SBET) and pore structure of the CuFe2O4@CeO2 composite nanofibers, the nitrogen adsorption-desorption isotherm was investigated. As shown in Fig.3, the isotherm curve with a H3 hysteresis loop belongs to the type-V shape, which is attributed to the accumulation of nitrogen molecules and indicates the presence of pore structures in material [26]. The calculated specific surface area, pore volume and average pore diameter are 64.12 m2/g, 0.15 cc/g and 4.74 nm, respectively (presented in Table 1). The pore size distribution of the sample is described in the insert picture of Fig. 3. It can be seen that the dominant pore size is ranged from 50 to 150 Å, which confirms that the CuFe2O4@CeO2 composite nanofibers have a structure of mesopore and contain a very high pore volume. This is consistent with the SEM observations in Fig.2. The magnetic properties of the nanofibers are characterized by the magnetic parameters such as the specific saturation magnetization (Ms), remanent magnetism (Mr) and coercivity (Hc). The hysteresis loops of CuFe2O4 and CuFe2O4@CeO2 nanofibers are showed in Fig.4, which exhibits a typical soft-magnetic materials behavior. As shown in table 1, with Ms of 28.32 Am2/kg, the CuFeO4 5
Page 5 of 25
nanofibers show a Mr of 12.85 Am2/kg and Hc of 25.11 kA/m, while the CuFe2O4@CeO2 composite nanofibers have a Ms of 20.51 Am2/kg, Mr of 7.24 Am2/kg and Hc of 8.18 kA/m, respectively. After coated CeO2 nanoparticles, the Hc value of the CuFe2O4 nanofibers decreases about 67.4%, while the
capacity due to a higher specific surface area and a larger amount of mesopores. 3.2 Dye removal by magnetic CuFe2O4@CeO2 composite nanofibers
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3.2.1 Column bed study
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Ms shows a little reduction of about 27.6%. The coated CeO2 nanoparticles will enhance the adsorption
The dye removal capacity of these magnetic CuFe2O4@CeO2 composite nanofibers was examined by
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the column bed studies and the methyl orange (MO) solution was chosen as the simulation wastewater. The experimental processes of column bed studies were described in section 2.3 and the parameters
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such as pH of solution, the flow rate of dye solution (F) and the initial dye concentration (C0) were considered. The breakthrough curves are showed in Fig. 5. Basically, the curves show the same typical
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S-shape, indicating that the adsorption capacity reaches equilibrium in liquid [27]. Fig. 5a shows the adsorption capacity of column beds at different pH values, and decreases gradually
d
with the increase of pH. The similar effect of pH on MO adsorption was observed in other researches [28], implying the electrostatic interaction between the dye molecules and adsorbents is a main
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adsorption mechanism during the process. As pH increase from 4.0 to 10.0, the breakpoint decreases
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from 262 min to 11min, t0.5 reduced from 357 min to 20 min, respectively. Fig. 5b shows the effect of F (2.0, 3.5 and 5.0 mL/min) on the breakthrough curves at a constant pH=4 and C0 =0.025 mg/mL. The
breakpoint tb and t0.5 decrease at a high flow rate. As F increases from 2.0 to 5.0 mg/mL, the effluent solution volume increases, with tb decreased from 262 min to 49 min and t0.5 decreased from 357 min to
118 min. Fig.5c shows the effect of dye concentrations on adsorption ability and the concentrations of 0.025, 0.05 and 0.1mg/mL are studied in this part. Some performance parameters of column beds with different pH, F and C0 obtained from the breakthrough curves are listed in Table 2. As shown in Table 2, the breakpoint appears faster with increasing influent F and C0, which is due to
the decrease in contact time between the dye and adsorbents. Decreasing the contact time will result in premature breakthrough to occur, thus reduce the service time of the column bed [27]. The totally adsorbed MO quantity (Qe; mg/g) in the column bed for a given pH, F and C0 can be calculated from Eq.(3.1):[29,30]
6
Page 6 of 25
Qe =
T
0
(C0 - Ct ) Fdt m
(3.1)
Where Qe is the capacity for a given column bed (mg/g), Ct is the outlet MO concentration, T is the service time (min), and m is the weight of adsorbent used (g). Through graphical integration in Origin
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8.5, the capacity value Qe can be obtained and the results are showed in table 2. The adsorption capacity decreases with the increasing pH, the largest adsorption capacity of the column beds reaches about 100 g/mL under the conditions of C0=0.05 mg/mL, F=2.0 mL/min and pH=4.0.
cr
In order to understand the adsorption process, the FT-IR was carried out to detect the functional groups
us
of the adsorbents before and after used, and the results are showed in Fig.6. As shown in Fig.6(a), the strong absorption band around 563 cm−1 can be attributed to the stretching mode of tetrahedral complexes [24]. The broad absorption band around 3407 cm−1 represents the stretching mode of H2O
an
molecules and OH groups. The band at 1638 cm−1 corresponds to the bending mode of H2O molecules [31]. The absorption bands in MO spectrum in Fig.6(b), reveal the existence of –SO3- at 1201 cm−1 as
M
well as a stretching vibration at 1039 cm−1 for C–N. Whereas the band located at about 1361cm−1 represents –N=N– stretching vibration of azo group and the band around 1601cm−1 is assigned to vCC
d
vibrations in the aromatic rings [6,31].
te
By comparing the spectrum of (a) and (b) with the FT-IR spectrum of methyl orange (c), there is no new absorption band occurred, which indicates that the MO molecules are physically adsorbed on
Ac ce p
CuFe2O4@CeO2 nanofibers. While the intensity of adsorption bands between 600 cm−1 and 1601 cm−1 becomes weaker after MO being adsorbed on magnetic composite nanofibers, owing to the protonation of function groups in MO reacting with the surface and edges of the adsorbents. It should be noted that the band at 1201 cm−1 is nearly disappeared, implying the –SO3- group combined with the positively
charged surface sites on the adsorbents, such as Ce4+, Fe3+ and Cu2+. 3.2.2 Kinetics analysis
Thomas model is a simple and widely used model to predict the dynamic behavior of the column bed, and has successfully described and predicted dye adsorptions for various adsorbents. The equation can be represented as Eq.(3.2, 3.3): [29,33]
Ct 1 = C0 1+exp(kq0 m
(3.2)
F
- C0 t )
7
Page 7 of 25
ln(
C0 kq m 1) 0 - kC0t Ct F
(3.3)
Where k is the adsorption rate constant (mL/(mg min)), q0 is the capacity for a given bed (mg/g), F is the flow rate of dye solution through column bed (mL/min), t is the service time (min), and m is the
ip t
weight of adsorbent used (g).
According to the experimental data Ce, C0 and t at different conditions, the software package Origin 8.5
cr
is used to fit the data and the fitted results are plotted in Fig. 7. It can be seen that the plots of
ln[(C0/Ct)-1] against t are multilinear, with at least two linear segments. Although, similar results were
us
observed by Öztürk et al. [29] and Tor et al. [30], this phenomenon was not completely analyzed. The adsorption rate constant ki and correlation coefficient values Ri2 (i=1,2) obtained in this work are
an
presented in Table 3. The high value of Ri2 indicates that the Thomas model can be used to depict the adsorption process of MO on this column bed. The rate constant, k1>k2, suggests the reaction rate tend
M
to decrease with time. The decrease of reaction rate may come from two parts, one is the amount of active sites on the adsorbents decreased as the dye molecules absorbed on the magnetic nanofibers, the
d
other is the diffusion of dye molecules between the micro- and mesopores. A similar phenomenon was observed in Sethuraman’s research that the dye adsorption by montmorillonite showed a steady
te
decrease from 10 mg•g-1min-1 in the first 5 min to 0.55 mg•g-1min-1 over the next hour, and then to 0.07
Ac ce p
mg•g-1min-1 for the final hour [34]. It is interesting that the rate constant increases with increase of pH, F and C0 at the first stage, but decreases with increase of F and C0 during the following adsorption
period. As we know, decreasing the mass transfer resistance results in an increase of the adsorption rate constant [35], so increasing the flow rate will increase the rate constant because of a bigger driven force. As the total active sites on the adsorbents surface are almost a constant, if the adsorption saturation occurs on the surface of adsorbent, its adsorption ability will be largely decreased as well as the decreasing of rate constant. Then, increasing F will lead the dyes more difficult to be adsorbed on the fibers due to a shorter time to penetrate and diffuse into the inner pores of the adsorbent. This is the reason why the rate constant increases with increasing of F at the first stage, but then decreased with increasing of F. As for the effect of dye concentration, the courses are similar to F, an increasing C0 diminishes the breakthrough time by a faster saturation of the column bed and results in an increase of rate constant [36]. At the condition of C0=0.1 mg/mL, the active sites on the adsorbent are quickly occupied by the dye molecules, which leads a rate constant declined sharply. While in a low C0, the rate 8
Page 8 of 25
constant shows a steady decrease and declines more slowly than that at a high C0. Because of the multilinear form of plots, the theoretical predictions of the adsorption capacity qi are quite different to the calculated adsorption capacity by integrals, which is not deeply investigated in this paper.
ip t
3.2.3 Regeneration of column bed
The regeneration of the column bed is important for the application from the engineering view. The
cr
desorption test was carried out in 0.5 M NaCl solution, ethyl alcohol and deionized water, respectively. Fig. 8 shows the desorption behavior of MO in different solvents at an input flow rate of 2.0 mL/min.
us
The MO saturated column beds were carried out at the condition of F=3.5 mg/mL for about 12 hours. As shown in Fig.8, the NaCl solution shows the best ability for MO desorption among these three
an
solvents, the order of desorption activities is NaCl > CH3CH2OH > H2O. While in deionized water, there is almost no MO molecules desorbed from the nanofibers, which indicates the electrostatic
further study.
d
3.3 Adsorption mechanism discussion
M
interaction plays a significant role in the adsorption process. The detailed desorption process needs a
It is worth noting that the adsorption mechanism at different conditions will be affected by many
te
variables. The ionic interaction, hydrogen bonds and pore diffusion can play an essential factor for MO
Ac ce p
molecules adsorption [11,29]. Based on the effects of pH and desorption process, the charge state between the interface of adsorbents and solution is a main factor for adsorption capacity, which indicates the surface ionic interaction and formation of hydrogen bonds will play a major role. Additionally, the column bed is a micro-filter in fact, which has a hierarchical pore structure with multi-channels, consisting of the large pores coming from the stacking of nanofibers, the macropores and mesopores formed during the preparation process. So the dye molecules can be easily absorbed on the inner pore surface though diffusion, which has been proved by the kinetics analysis. As shown in Fig. 9(a) and (b), the reaction process can be divided into two major steps: firstly, the MO molecules are absorbed onto the surface or external pore surface of the adsorbents when the dye solution passed through the porous column bed. In this process, the dye molecules will interact with the vacant active sites presented in the surface and external pores. Because the composite nanofibers have a positively charged surface below pH 7.8 (point of zero potential), the MO anions can be easily absorbed on the positively charged surface of adsorbents by electrostatic interactions. Secondly, when the activated sites 9
Page 9 of 25
reach saturation, the remaining vacant surface sites are difficult to occupy because of the repulsive forces between the dye molecules on the nanofibers and the bulk phase. Then the diffusion process plays a dominant role, as showed in Fig. 9(b), the dye molecules diffuse through the porous channel and interface of CeO2 and then absorbed onto the inner pore surface. The combinations of MO and
ip t
composite nanofibers are presented in Fig. 9(c), the negative groups –SO3- react with the positively charged Ce4+, Fe3+ and Cu2+ on the adsorbents surface, which is evidenced from the FT-IR results.
cr
While the hydrogen bonds formed between O2- and positively charged of protonation amino groups in acid solution is equally important in the adsorption process.
us
4. Conclusions
1. Novel magnetic adsorbents of CuFe2O4@CeO2 composite nanofibers were fabricated by combining
an
electrospinning technique and chemical precipitation methods. The ultrafine CeO2 nanoparticles (about 20 nm with a grain size of 9.5 nm) are evenly coated on the magnetic CuFe2O4 nanofibers with a
M
diameter of about 200 nm.
2. The CuFe2O4@CeO2 composite nonofibers have a high specific surface area of 64.12 m2/g and Ms of
d
20.51 Am2/kg. These composite nanofibers show a mesoporous structure and can form a porous micro-filter like column bed for dye removal.
te
3. The porous column beds consisting of the magnetic composite nanofibers show a high adsorption
Ac ce p
capacity for MO molecules. The highest adsorption capacity is about 100 mg/g at the condition of pH=4.0, F=2.0 mg/min and C0=0.05 mg/mL.
4. The kinetic study shows the plot of ln[(C0/Ct)-1] against t are multilinear. The rate constant decreases with the decrease of pH and the extension of time, while its value increases with increasing the inlet flow rate and initial concentration at the first stage. 5. The diffusion process and electrostatic interaction play a significant role in the adsorption process, the negative groups –SO3- can combine with the positively charged Ce4+, Fe3+ and Cu2+ on the adsorbents surface, the hydrogen bonds will form between O2- and positively charged of protonation amido groups. 6. The order of desorption activity is NaCl > CH3CH2OH > H2O. The magnetic adsorbents can be easily separated and recycled from the mixture solution by magnetism. Acknowledgments 10
Page 10 of 25
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51274106, 51474113, 51474037), the Natural Science Foundation of Jiangsu Provincial Higher
ip t
Education of China (Grant No. 12KJA430001), the Science and Technology Support Program of Jiangsu Province of China (Grant No. BE2012143, BE2013071), the Jiangsu Province's Postgraduate
cr
Cultivation and Innovation Project of China (Grant CXZZ13_0662), and the Priority Academic
us
Program Development of Jiangsu Higher Education Institutions.
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14
Page 14 of 25
Figure caption
Ac ce p
te
d
M
an
us
cr
ip t
Figure1 XRD patterns of the prepared magnetic nanofibers: (a) CuFe2O4 and (b) CuFe2O4@CeO2.
15
Page 15 of 25
Ac ce p
te
d
M
an
us
cr
ip t
Figure 2 SEM morphologies of the prepared nanofibers: CuFe2O4 (a, b), CuFe2O4@CeO2 (c, d).
16
Page 16 of 25
Figure 3 Nitrogen adsorption-desorption isotherm and pore sizes distribution of CuFe2O4@CeO2
Ac ce p
te
d
M
an
us
cr
ip t
composite nanofibers.
17
Page 17 of 25
Ac ce p
te
d
M
an
us
cr
ip t
Figure 4 The hysteresis loops of CuFe2O4 and CuFe2O4@CeO2 nanofibers.
18
Page 18 of 25
Figure 5 The breakthrough curves for adsorption of MO onto CuFe2O4@CeO2 nanofibers at different
Ac ce p
te
d
M
an
us
cr
ip t
pH (a), flow rates (b) and inlet dye concentrations (c).
19
Page 19 of 25
Ac ce p
te
d
M
an
us
cr
ip t
Figure 6 FT-IR spectra of adsorbents before and after used (a, b), Methyl Orange (c).
20
Page 20 of 25
Figure 7 Plots of ln[( C0/Ct)−1] vs. t by the Thomas equation at different pH (a), flow rates (b) and inlet
Ac ce p
te
d
M
an
us
cr
ip t
dye concentrations (c).
21
Page 21 of 25
Figure 8 Desorption curves of MO in 0.5 M NaCl solution (a), ethyl alcohol (b) and deionized water
Ac ce p
te
d
M
an
us
cr
ip t
(c).
22
Page 22 of 25
Ac ce p
te
d
M
an
us
cr
ip t
Figure 9 The schematic illustration of MO adsorption on CuFe2O4@CeO2 composite nanofibers
23
Page 23 of 25
Table caption Table 1 The average grain size (D), specific surface area (SBET), total pore volume (Pv), average pore size (Pa), coercivity (Hc), specific saturation magnetization (Ms) and remanence (Mr) of nanofibers. D (nm)
SBET
Pv
Pa
Hc
Ms
Mr
2
CeO2
(m /g)
(cc/g)
(nm)
(kA/m)
(Am /kg)
(Am2/kg)
CuFe2O4
18.3
--
8.44
2.13×10-3
5.04
25.11
28.32
12.85
CuFe2O4@CeO2
23.5
9.5
64.12
0.15
4.74
8.18
20.51
7.24
us
Table 2 Parameters in the adsorption of MO at various operation conditions. C0=0.025 mg/mL
C0=0.05 mg/mL
C0=0.1 mg/mL
F=3.5 ml/min
F=5.0 mL/min
F=2.0 mL/min
F=2.0 mL/min
an
F=2.0 mL/min
ip t
CuFe2O4
cr
2
pH=4.0
pH=5.0
pH=6.0
pH=8.0
pH=10.0
pH=4.0
pH=4.0
pH=4.0
pH=4.0
tb(min)
262
129
93
80
11
180
49
110
16
t0.5(min)
357
251
206
161
20
316
tex(min)
581
387
319
241
129
Vb(mL)
514
258
186
160
22
V0.5(mL)
714
502
412
322
40
Vex(mL)
1162
774
638
482
258
Qe(mg/g)
65.04
42.73
35.24
27.91
7.52
96.89
243
26
479
478
393
630
245
220
32
1106
590
486
52
1866
2395
956
786
52.32
100.11
78.91
Ac ce p
te
d
M
118
533
Table 3 Model parameters for the adsorption of MO by using CuFe2O4@CeO2 composite nanofibers at
various operation conditions. C0=0.025 mg/mL
pH=4.0
R1
2
k1(L/mg•min)
pH=5.0
0.9845 1.13×10
1.41×10
F=2.0
F=3.5
F=5.0
F=2.0
F=2.0
ml/min
mL/min
mL/min
mL/min
pH=4.0
pH=4.0
pH=4.0
pH=4.0
pH=8.0
0.9819 -3
C0=0.1 mg/mL
mL/min
pH=6.0
0.9830 -3
C0=0.05 mg/mL
1.61×10
pH=10.0
0.9673 -3
1.90×10
0.9789 -3
1.17×10
0.9581 -2
1.79×10
0.9357 -3
1.28×10
0.9390 -2
8.41×10
0.8809 -4
3.31×10-3
R22
0.9337
0.9859
0.9849
0.9813
0.9666
0.9858
0.9541
0.9577
0.9684
k2(L/mg•min)
4.39×10-4
6.47×10-4
6.56×10-4
8.61×10-4
4.77×10-3
4.14×10-4
1.95×10-4
2.31×10-4
7.05×10-5
24
Page 24 of 25
1. The magnetic adsorbents with CeO2 nanoparticles coated on CuFe2O4 nanofibers were prepared. 2. The nanofibers had a diameter of 200 nm, specific surface area of 64.12 m2/g and Ms of 20.51 Am2/kg. 3. The plots of ln[(C0/Ce)-1] vs. t were multilinear when fitted by Thomas models.
Ac ce p
te
d
M
an
us
cr
ip t
4. The magnetic adsorbents can be easily separated and recycled from mixture solution by magnetism.
25
Page 25 of 25