1. bookVolume 35 (2017): Issue 1 (March 2017)
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2083-134X
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16 Apr 2011
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access type Open Access

Solvothermal synthesis and characterization of magnetic Fe3O4 nanoparticle by different sodium salt sources

Published Online: 24 Feb 2017
Volume & Issue: Volume 35 (2017) - Issue 1 (March 2017)
Page range: 50 - 57
Received: 10 Apr 2016
Accepted: 18 Nov 2016
Journal Details
License
Format
Journal
eISSN
2083-134X
First Published
16 Apr 2011
Publication timeframe
4 times per year
Languages
English
Abstract

Four different magnetic Fe3O4 nanoparticles were synthesized and characterized by solvothermal method based on different sodium salts. Sodium salts which were used to synthesize the nanoparticles were NaOAc, Na2CO3, a mixture of NaOAc and Na3Cit, and a mixture of NaOAc and Na2C2O4. The structural and optical properties of the synthesized nanoparticles were examined by XRF, XRD, SEM and FT-IR. The results estimated from XRD pattern and SEM image indicated that the second sample (Na2CO3) had the lowest average particle and crystallite size around 29 nm and 43 nm. It was also shown that the first (NaOAc) and second (Na2CO3) samples had the best FT-IR spectra, similar to the available commercial sample which was provided by Merck. At last, the prepared Fe3O4 nanoparticles were applied as sorbents to sorb uranium ions (U(VI)) from radioactive wastewater. The adsorption results showed that the highest U(VI) adsorption was obtained for the second sample in the solution with pH around 10.

Keywords

Introduction

Magnetic Fe3O4 nanoparticles as a kind of magnetic nanoparticle material have attracted significant interest due to their unique magnetic properties and feasibility of preparation [1]. The preparation feasibility of Fe3O4 nanoparticles is one of the key advantages of the magnetic nanoparticles. Using an external magnetic field can lead to easy and rapid separation of the Fe3O4 particles from their matrix or solution [2, 3]. Other advantages of the magnetic Fe3O4 nanoparticles, including scalable and non-toxic synthesis and economical and efficient properties, are the other reason to consider Fe3O4 nanoparticles for potential applications and fundamental research [1, 4, 5]. Magnetic Fe3O4 nanoparticles are supposed to be the main potential adsorbent for toxic and radioactive heavy metals such as Cu, Cr, Cd, Ni, As, Pb, and U, because of their good chemical and mechanical stability, high surface area and the unique advantage of easy separation [1, 4, 69]. Today, these nanoparticles are frequently used for removing heavy-metals from wastewater because of their high adsorption capacity for heavy-metal ions and organic pollutants [4]. The studies have shown that the higher adsorption capacity to remove metals can be achieved by the magnetic Fe3O4 nanoparticles with high ratio of surface-to-volume which is known as the finite-size effect [1, 4].

Although magnetite nanoparticles with different properties were prepared by many authors, they still inspire researchers to improve their characteristics by which a great significance in different fields can be obtained [10, 11]. There are several methods to synthesize nanostructured magnetic materials usually by co-precipitation, sol-gel method, thermal decomposition of organometallic compounds, hydrothermal synthesis, etc. [1216]. The hydrothermal or solvothermal method is well-established to synthesize many nanoparticle materials such as magnetite Fe3O4 nanoparticles. However, the kinetics is typically slow because of relatively low temperatures which are used in the experiment [2]. The first magnetite Fe3O4 microspheres (200 nm to 800 nm) were synthesized by the solvothermal method in the ethylene glycol (MEG) in 2005 [17].

Wang et al. [8] synthesized water-soluble Fe3O4 nanoparticles as adsorbents to remove heavy metals from waste water by a hydrothermal approach. The synthesized nanoparticles had high solubility and appropriate stability (at least for one month). They also showed excellent removal ability. Chen et al. [12] studied different parameters which influence the solvothermal synthesis of Fe3O4 microspheres in monoethylene glycol (MEG). They successfully synthesized 0.34 µm magnetic Fe3O4 microspheres. They also prepared EDTA-modified Fe3O4 microspheres as a potential magnetic adsorbent to remove heavy metals from waste water. Li et al. [2] used the microwave-solvothermal method to synthesize Fe3O4 magnetic nanoparticles in a simple reaction system. They succeeded in synthesizing Fe3O4 magnetic nanoparticles during a considerably shorter time than the other traditional methods such as solvothermal or hydrothermal methods. Also, Shen et al. [4] conducted research on the synthesis of magnetite Fe3O4 nanoparticles to remove Cd2+, Cr6+, Cu2+ and Ni2+ from waste water. They provided the magnetic Fe3O4 nanoparticles with different sizes.

Herein, we synthesized and characterized magnetite Fe3O4 nanoparticles by solvothermal method based on different sodium salts. The prepared Fe3O4 nanoparticles were applied as sorbents to sorb U(VI) ions from radioactive wastewater. The batch sorption technique was used to assess the absorbability of magnetite Fe3O4 nanoparticles from the wastewater.

Experimental
Chemicals

Iron source and solvent which were used in this study, were FeCl3·6H2O and MEG, respectively. To study the effect of anions on the characteristics of the synthesized magnetite Fe3O4 nanoparticles, different sodium salts were used including CH3COONa (NaOAc), Na2CO3, trisodium citrate dihydrate (Na3Cit), sodium oxalate (Na2C2O4). To study uranium adsorption, 1000 ppm uranium standard (UO2(NO3)2) was also used to prepare a sample solution. All chemicals were purchased from Merck Co. The uranium adsorption experiments were performed to perform a comparative study of uranium adsorption by different sodium salts based synthesized magnetite Fe3O4 nanoparticles. The adsorbent concentration in all adsorption test was 1g/L and uranium aqueous solution concentration was 50 ppm. A 50 ppm aqueous uranium solution was prepared by diluting 25 mL of 1000 ppm uranium standard (UO2(NO3)2, 1000 ppm) in deionized water (500 mL). The adsorption experiments were carried out by using the batch method. Negligible volumes of diluted nitric acid or sodium hydroxide solution were used to adjust the solution pH.

Procedures of synthesizing magnetite Fe3O4 nanoparticles in MEG

Firstly, a yellow solution was prepared by dissolving 4 g FeCl3·6H2O in 10 mL MEG. The process was followed by adding an appropriate amount of each basic sodium salt or a mixture of two of them. Then, the resulted mixture was vigorously stirred for about 30 min to 60 min to homogenize it. After that, the final solution was transferred to a Teflon-lined autoclave with a stainless-steel cover. The solution was heated in the autoclave at temperature of 200 °C for 24 h. Then, the resultant product was cooled to room temperature (RT). The final black product was separated by centrifugation, washed in water and ethanol, and dried at 70 °C in vacuum environment for 4 h.

Characterization

Four available systems, XRF, XRD, SEM and FT-IR were used to characterize and compare the synthesized magnetite Fe3O4 nanoparticles. XRD, Bruker D8 Advance diffractometer was used to collect XRD patterns at 30 kV and 20 mA, and CuKα radiation (λ = 0.1540598 nm). The particle sizes of synthesized nanoparticles were measured by FE-SEM (HITACHI S-4160). The crystallite size of the synthesized magnetic Fe3O4 nanoparticles was estimated based on the Scherrer equation by using X-ray line broadening method [18]. Bruker Optics TENSOR 27 FT-IR spectrometer was also used to record FT-IR spectra of synthesized samples.

Adsorption tests

Uranium adsorption experiments were performed by preparing 50 ppm aqueous uranium solutions with various solution pH. The uranium concentrations were determined before treatment and after 2.5 hours. The percent of metal removal [%] was calculated using the following equation: Removal(%)=C0CeC0×100$${\mathop{\rm R}\nolimits} {\rm{emove}}\left( \% \right) = \frac{{{C_0} - {C_e}}}{{{C_0}}} \times 100$$ where C0 is the initial uranium concentration and Ce is the uranium concentration at equilibrium, after treatment with synthesized Fe3O4 nanoparticles [7].

Results and discussion

One of the important parameters which has a significant influence on the synthesis of magnetite Fe3O4 nanoparticles is the effect of anions of sodium salts [12]. In general, the synthesis of magnetite Fe3O4 nanoparticles from FeCl3·6H2O is affected by coordination ability (e.g., CH3COO, CO32 $\rm CO^{2-}_3$ ) of sodium salts. The anions with the weaker coordination ability usually promote the synthesis. In order to synthesize magnetic Fe3O4 nanoparticles from FeCl3·6H2O, the adequate amount of OH ions should be provided to reduce Fe3+ by MEG. However, the anions with stronger coordination ability, such as oxalate and citrate, are not appropriate for the synthesis of magnetite Fe3O4 nanoparticles in MEG. In this case, stable Fe3+ complexes are formed which inhibit Fe(OH)3 formation and reduce Fe3+ by MEG, and consequently prevent the formation of magnetite Fe3O4 nanoparticles in MEG. So, for the Fe3O4 nanoparticles, synthesis by the anions with stronger coordination ability, extra base, such as NaOH, and NaOAc, is still required [12]. In this study, different sodium sources, including CH3COONa (NaOAc), Na2CO3, trisodium citrate dihydrate (Na3Cit), sodium oxalate (Na2C2O4) were used to synthesize magnetite Fe3O4 nanoparticles. The experimental results showed that the solution provided by trisodium citrate dihydrate (Na3Cit) and sodium oxalate (Na2C2O4) separately did not lead to the synthesis of magnetic Fe3O4 nanoparticles. Four types of magnetic Fe3O4 nanoparticles were synthesized by using different sodium additions, including (1) CH3COONa (NaOAc), (2) Na2CO3, (3) mixture of NaOAc and trisodium citrate dihydrate (Na3Cit), and (4) mixture of NaOAc and sodium oxalate (Na2C2O4). The XRF analysis of the four synthesized nanoparticle types indicated that Fe–O contribution in the samples was 85 % to 92 %. The XRF patterns of the four synthesized magnetic Fe3O4 nanoparticles are shown in the Fig. 1. As shown in the figure, the characteristic lines of Fe–O contributions are related to three important lines of Fe: K-Alpha, Fe, L-Alpha and Fe, K-Beta. The other peaks in the XRF patterns are related to the other X-ray fluorescence which comes from the X-ray source. The amounts of different sodium sources which were used to synthesize Fe3O4 in the four sample types were: (1) 3.642 g of NaOAc, (2) 2.35 g of Na2CO3, (3) 2.73 g of NaOAc and 1.088 g of Na3Cit, and (4) 2.73 g of NaOAc and 0.745 g of Na2C2O4. The amounts of FeCl3·6H2O, and MEG were 4 g and 10 mL in all the sample solutions, respectively. Their values were calculated based on optimum Na/Fe molar ratio, which was 3:1 [12]. The experimental results indicated that the sodium sources with anions of stronger coordination ability, such as oxalate and citrate, are not appropriate for the synthesis of magnetic Fe3O4 nanoparticles in MEG.

XRF patterns of magnetite Fe3O4 nanoparticles synthesized by different sodium salts (a) Na2CO3, (b) NaAc, (c) NaAc-CIT and (d) NaAc-oxallat.

Fig. 2 shows the XRD patterns of the four synthesized magnetite Fe3O4 nanoparticles. The observed characteristic peaks are (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (6 2 2). These peaks are the most important peaks which confirm that the resultant particles are pure Fe3O4. They were observed for the Sample 1 (NaOAc) at the diffraction angles of 2θ = 18.34°, 30.15°, 35.5°, 43.13°, 53.49°, 56.99°, 62.57°, 70.97°, 74.03°, respectively. The peak positions for the other samples are listed in the Table 1. As can be seen, their position and intensities of almost all of peaks are well matched to those of the other experimental results and the standard PDF Cards for Fe3O4 nanoparticles (JCPDS Card No. 19-0629) [1922]. The synthesized samples are in a good agreement with the other experiments [16, 23, 24]. There is an additional peak between (1 1 1) and (2 2 0) in the XRD pattern of NaAc-oxalate, which can be due to the Fe2O3 or alpha-Fe2O3 particles in the synthesized magnetic Fe3O4 nanoparticles [21, 22]. It can be one of the most important reasons that the synthesized magnetic Fe3O4 nanoparticles based on NaAc-oxalate have the lowest uranium adsorption. The results show a slight variation in the peak positions for different samples which can be due to their structure formation.

Powder X-ray diffraction (XRD) patterns of magnetite Fe3O4 nanoparticles: (a) NaAc as sodium salt, (b) Na2CO3 which shows the lowest average crystallite size, (c) NaAc-Cit and (d) NaAc-oxallat. The red bars represent the standard pattern of Fe3O4 (PDF Card No. 19-0629).

Crystallite size values of magnetite Fe3O4 nanoparticles synthesized by different sodium salts for different peaks.

Dh k l = Calculated crystallize size [nm]

h k lSample 1Sample 2Sample 3Sample 4
FWHMDh k lFWHMDh k lFWHMDh k lFWHMDh k l
(1 1 1)18.340.1846.7217.520.302817.350.3027.9918.410.1846.72
(2 2 0)30.150.1271.6430.360.2435.8430.190.2435.8230.180.1271.65
(3 1 1)35.500.1848.4235.600.3029.0635.610.3029.6135.550.1848.43
(4 0 0)43.130.1849.5943.320.3029.7743.200.3029.7643.190.3029.76
(4 2 2)53.490.1851.6453.680.1851.6753.520.1851.6453.520.3030.99
(5 1 1)56.990.1278.757.270.2439.4057.240.2439.4057.060.1278.73
(4 4 0)62.570.1280.9362.860.2440.5362.900.2440.5462.650.2440.48
(6 2 0)70.970.1856.6371.390.3034.0671.180.1285.0571.030.1856.65
(6 2 2)74.030.2443.3174.330.2443.3974.560.2443.4674.080.1286.64

The microstructures of the synthesized magnetic Fe3O4 particles were also characterized by SEM. Fig. 3 shows the SEM images for the four synthesized magnetic Fe3O4 particles. The SEM images show that the synthesized products are composed of spherical nanoparticles which have the minimum average diameter of about 45 nm for sample 2 (Na2CO3) and the maximum average diameter of about 190 nm for sample 1 (NaAc).

SEM images of the synthesized magnetic Fe3O4 particles: (a) NaAc, (b) Na2CO3, (c) NaAc-CIT, and (d) NaAc-oxallat.

A comparative study which can be done on the basis of XRD patterns is determination of the average crystallite size of different synthesized samples. The average crystallite size (Dhkl) of magnetite Fe3O4 nanoparticles is estimated by the Scherrer equation as below: Dhkl=0.89λ/βcosθ$$\begin{align}{D_{h \ k \ l}} = 0.89\lambda /\beta \cos \theta\end{align}$$ Here, β is the full-width at half-maximum (FWHM) of XRD diffraction lines, the wavelength λ = 0.1540598 nm and θ is the half diffraction angle of 2θ [10, 17, 23]. The estimated average crystallite sizes of the synthesized magnetite Fe3O4 nanoparticles for the (311) diffraction peak are 48.42 nm, 29.06 nm, 29.61 nm, and 48.4 nm for sample 1 to sample 4, respectively. The average crystallite size for the other peaks have been calculated and summarized in Table 1. As can be seen, sample 2 (Na2CO3) has approximately the lowest average crystallite size in the almost all of the peaks. The XRD pattern of sample 2 is shown in the Fig. 2b.

Fig. 4(a) shows the FT-IR spectra for different synthesized magnetite Fe3O4 nanoparticles. The spectra illustrate a broad band around 580 cm−1, which is associated with the stretching vibrations of F–O bond (v1) or the tetrahedral groups (Fe3+–O2−) in Fe3O4. Also the peak at about 390 cm1 is observed, which belongs to v2 of the Fe–O bond of the bulk magnetite. The results show that the FT-IR spectra of sample 1 and sample 2 are in a good agreement with the available commercial sample. The FT-IR spectrum of sample 2 is also shown in the Fig. 4(b). There are also the broad bands at around 1630 cm−1 and 3436 cm−1 in the spectrum, which correspond to the H2O molecules or O–H adsorbed on the surface of Fe3O4 [8, 16, 25].

FT-IR spectra of magnetite Fe3O4 nanoparticles: (a) NaAc sodium salts, (b) Na2CO3, (c) NaAc-Cit, (d) NaAc-oxallat.

Uranium adsorption by different synthesized magnetite Fe3O4 nanoparticles as a function of the solution pH is shown in the Fig. 5a. As shown in this figure, the best uranium adsorption is obtained for sample 2 at the solution pH around 10. It can be due to its lowest average particle size. The adsorption of U(VI) by Fe3O4 was greatly affected by the solution pH. The adsorption increased from about 5 % to 85 % when the pH value increased from 2.0 to 10.0. The results also indicate that the optimum solution pH is different for various samples. Based on Fig. 5, the graph can be divided into three parts, (1) pH between 1 and 6, (2) pH between 6 and 10, (2) pH higher than 10. In part 1, in which the solution pH is lower than 6, UO22+${\rm{UO}}_2^{2 + }$ is the dominant species of uranium ions in the solution which have to compete with the high abundance of H+ and H3O+ ions for the binding sites on the adsorbent surface. In part 2, in which solution pH is between 6 and 10, the uranium adsorption increases greatly due to fewer H+ and H3O+ ions, and also higher number of adsorptive sites for uranium ions. However, when pH values are higher than 10 (part 3), the amount of uranium ions adsorbed decreases with increasing pH. This is due to the repulsion of the anions UO2OH3${\rm{U}}{{\rm{O}}_2}\left( {{\rm{OH}}} \right)_3^ -$ and UO3OH7,${\rm{U}}{{\rm{O}}_3}\left( {{\rm{OH}}} \right)_7^ -,$ which is caused by the hydrolysis of U(VI)) to the negatively charged surface of Fe3O4 at high pH [1, 26]. Fig. 5b shows the main extraction of Fe3O4 nanoparticles dispersion in the solution and separation by an external magnetic field.

(a) Effect of solution pH on uranium adsorption efficiency by different magnetite Fe3O4 nanoparticles, and (b) photographs of Fe3O4 dispersion and magnetic separation.

The sorption isotherms of uranium ions onto magnetite Fe3O4 nanoparticles synthesized based on Na2CO3 as the selected sodium salt, are shown in the Fig. 6. The experimental results were simulated by the Langmuir and Freundlich models. Langmuir model: 1qe=1qm+1KLqm1Ce$$\begin{align}\frac{1}{{{q_{e}}}} = \frac{1}{{{q_m}}} + \frac{1}{{{K_L}{q_m}}} \cdot \frac{1}{{{C_e}}}\end{align}$$ where qe is the equilibrium adsorption of uranium ions, in mg/g, Ce is the equilibrium concentration of ions, in mg/L, qm is the maximum adsorption capacity, in mg/g and KL is the Langmuir constant which is related to the energy of adsorption.

Freundlich model: lnqe=lnKF+1nlnCe$$\begin{align}\ln \left( {{q_e}} \right) = \ln \left( {{K_F}} \right) + \left( {\frac{1}{n}} \right)\,\ln \left( {{C_e}} \right)\end{align}$$ where qe is the amount of uranium sorbed on Fe3O4, in mg/g, Ce is the equilibrium concentration of uranium ions in mg/L, KF (mg1–n·Ln/g) is the sorption capacity of the equilibrium concentration of ions when it equals 1, and n is the degree of dependence of sorption on equilibrium concentration. The results show that the Freundlich model fits the uranium ion sorption isotherms better than Langmuir model, which indicates that multilayer sorption occurred. This model has the best fit for the aqueous solution with low uranium concentration [16, 27].

(a) Adsorption percentages, (b) Freundlich and (c) Langmuir isotherms for the adsorption of uranium ions onto magnetite Fe3O4 nanoparticles synthesized based on the Na2CO3 as the selected sodium salt.

Conclusions

In this study, the magnetic Fe3O4 nanoparticles were synthesized successfully by the solvothermal method. The nanoparticles were synthesized by four different sodium sources including (1) CH3COONa (NaOAc), (2) Na2CO3, (3) mixture of NaOAc and trisodium citrate dihydrate (Na3Cit), and (4) mixture of NaOAc and sodium oxalate (Na2C2O4). The structural and optical properties of the synthesized magnetic Fe3O4 nanoparticles were examined by XRF, XRD, SEM and FT-IR at room temperature. The SEM and XRD patterns indicated that using Na2CO3 as sodium salt lead to obtaining the lowest average particle and crystallite size around 43 nm and 29 nm, respectively. The adsorption test showed that magnetic Fe3O4 nanoparticles are a suitable adsorbent for removal U(VI) from aqueous solutions and the best adsorption result was achieved for sample 2 at solution pH around 10.

XRF patterns of magnetite Fe3O4 nanoparticles synthesized by different sodium salts (a) Na2CO3, (b) NaAc, (c) NaAc-CIT and (d) NaAc-oxallat.
XRF patterns of magnetite Fe3O4 nanoparticles synthesized by different sodium salts (a) Na2CO3, (b) NaAc, (c) NaAc-CIT and (d) NaAc-oxallat.

Powder X-ray diffraction (XRD) patterns of magnetite Fe3O4 nanoparticles: (a) NaAc as sodium salt, (b) Na2CO3 which shows the lowest average crystallite size, (c) NaAc-Cit and (d) NaAc-oxallat. The red bars represent the standard pattern of Fe3O4 (PDF Card No. 19-0629).
Powder X-ray diffraction (XRD) patterns of magnetite Fe3O4 nanoparticles: (a) NaAc as sodium salt, (b) Na2CO3 which shows the lowest average crystallite size, (c) NaAc-Cit and (d) NaAc-oxallat. The red bars represent the standard pattern of Fe3O4 (PDF Card No. 19-0629).

SEM images of the synthesized magnetic Fe3O4 particles: (a) NaAc, (b) Na2CO3, (c) NaAc-CIT, and (d) NaAc-oxallat.
SEM images of the synthesized magnetic Fe3O4 particles: (a) NaAc, (b) Na2CO3, (c) NaAc-CIT, and (d) NaAc-oxallat.

FT-IR spectra of magnetite Fe3O4 nanoparticles: (a) NaAc sodium salts, (b) Na2CO3, (c) NaAc-Cit, (d) NaAc-oxallat.
FT-IR spectra of magnetite Fe3O4 nanoparticles: (a) NaAc sodium salts, (b) Na2CO3, (c) NaAc-Cit, (d) NaAc-oxallat.

(a) Effect of solution pH on uranium adsorption efficiency by different magnetite Fe3O4 nanoparticles, and (b) photographs of Fe3O4 dispersion and magnetic separation.
(a) Effect of solution pH on uranium adsorption efficiency by different magnetite Fe3O4 nanoparticles, and (b) photographs of Fe3O4 dispersion and magnetic separation.

(a) Adsorption percentages, (b) Freundlich and (c) Langmuir isotherms for the adsorption of uranium ions onto magnetite Fe3O4 nanoparticles synthesized based on the Na2CO3 as the selected sodium salt.
(a) Adsorption percentages, (b) Freundlich and (c) Langmuir isotherms for the adsorption of uranium ions onto magnetite Fe3O4 nanoparticles synthesized based on the Na2CO3 as the selected sodium salt.

Crystallite size values of magnetite Fe3O4 nanoparticles synthesized by different sodium salts for different peaks.

Dh k l = Calculated crystallize size [nm]

h k lSample 1Sample 2Sample 3Sample 4
FWHMDh k lFWHMDh k lFWHMDh k lFWHMDh k l
(1 1 1)18.340.1846.7217.520.302817.350.3027.9918.410.1846.72
(2 2 0)30.150.1271.6430.360.2435.8430.190.2435.8230.180.1271.65
(3 1 1)35.500.1848.4235.600.3029.0635.610.3029.6135.550.1848.43
(4 0 0)43.130.1849.5943.320.3029.7743.200.3029.7643.190.3029.76
(4 2 2)53.490.1851.6453.680.1851.6753.520.1851.6453.520.3030.99
(5 1 1)56.990.1278.757.270.2439.4057.240.2439.4057.060.1278.73
(4 4 0)62.570.1280.9362.860.2440.5362.900.2440.5462.650.2440.48
(6 2 0)70.970.1856.6371.390.3034.0671.180.1285.0571.030.1856.65
(6 2 2)74.030.2443.3174.330.2443.3974.560.2443.4674.080.1286.64

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