Article pubs.acs.org/IECR
Preparation of Fe3O4@C@Layered Double Hydroxide Composite for Magnetic Separation of Uranium Xiaofei Zhang,†,‡ Jun Wang,*,†,‡ Rumin Li,†,‡ Qihui Dai,†,‡ Rui Gao,†,‡ Qi Liu,†,‡ and Milin Zhang†,‡ †
College of Material Science and Chemical Engineering and ‡The Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China S Supporting Information *
ABSTRACT: A three-component composite, Fe3O4@C@layered double hydroxide (Fe3O4@C@Ni−Al LDH), was prepared through a two-step layer-by-layer route. The carbon-coated Fe3O4 nanoparticles (Fe3O4@C) were first synthesized by a hydrothermal reaction, and then Ni−Al LDH nanosheets were anchored onto Fe3O4@C via an in situ growth method. The asobtained Fe3O4@C@Ni−Al LDH magnetic composite was applied as magnetic adsorbent for the extraction of uranium(VI) ions from aqueous solutions. The maximum adsorption capacity of the composite for uranium(VI) ion was 174.1 ± 0.2 mg g−1, showing a high efficiency for the removal of uranium(VI) from aqueous solution. In addition, the composite can be easily separated from the solution by a magnet after the adsorption process. The results indicate that Fe3O4@C@Ni−Al LDH composite can be used as a potential adsorbent for sorption uranium(VI) and also provide a simple, fast separation method for removal of uranium(VI) ions from aqueous solution.
1. INTRODUCTION Today’s world faces an unprecedented energy crisis. The traditional oil, coal, and other nonrenewable fossil fuels are being gradually consumed, and new sustainable energy is being sought. Nuclear energy as an economical and clean energy for sustainable development has become one of the alternatives to ease the energy shortage of the world.1 However, mining and processing of uranium mineral resources bring a large amount of uranium pollution. Uranium is one of the most dangerous heavy metals in the environment, due to its long half-life, high radioactivity, and biological toxicity.2,3 Thus, the removal, recovery, and purification of uranium are especially important. Up to now, several methods, such as chemical precipitation, solvent extraction, membrane separation, and adsorption,4−10 have been extensively applied for the removal of uranium(VI) from aqueous solutions. Among these methods, adsorption appears to be one of the most effective selections owing to its cost-effective, versatile, and simple features of operation for removing trace levels of ions.11 However, the separation process of adsorbents from aqueous solution is usually complex and time-consuming. In recent years, magnetic nanoparticles are a great topic for research on a wide range of applications, including targeted drug delivery,12 magnetic resonance imaging,13 catalysis,14,15 and environmental remediation.16,17 The magnetic separation technique has been shown to be a promising method for solid− liquid phase separation.18 It is easy to separate the sorbent from matrices with a magnetic field, which makes solid−liquid phase separation fast, simple, as well as highly effective. Das et al.19 have proved the sorption of uranium(VI) on magnetite (Fe3O4) nanoparticles. Yusan and co-workers20 have studied the adsorption of uranium from aqueous solution using αFeOOH. Their sorption capacities are relative small; therefore, they are usually modified with other materials to extend their applications. © 2013 American Chemical Society
Layered double hydroxides (LDHs), as mutlifunctional materials, have received considerable attention in recent years because of their special structures and potential applications, such as catalysts, sorbents, and medicines.21−25 LDHs consist of stacked layers of metal cations (M2+ and M3+) similar to brucite-like structures. The brucite-type layers are stacked on top of each other and held together by weak interactions between the H atoms.26 LDHs have the structural formula [M 2+1−xM3+x(OH) 2](A n−)x/n·mH2O, where M 2+ and M3+ denote divalent and trivalent metals and An− represents interlayer anions. LDHs can be applied as adsorbents because of their large interlayer space and high concentration of active sites.27 Compared to other adsorbents, LDHs have their advantages with respect to low cost, great adsorption capacity, and double layer structure. Recently, Park et al.28 have removed Cu2+ and Pb2+ ions from aqueous solutions by employing Mg/Al layered double hydroxide. Kulyukhin et al.29 have synthesized Mg, Al and Mg, Nd layered double hydroxides for the removal of uranium from aqueous solutions. To our best knowledge, there is no report about preparation and research of the sorption kinetic and thermodynamic of Fe3O4@C@layered double hydroxide (Fe 3 O4 @C@Ni−Al LDH) composite as adsorbent for removing uranium(VI) from aqueous solutions. In this paper, we prepared Fe3O4@C@Ni−Al LDH through a two-step layer-by-layer route. The use of an in situ growth method allows LDH nanosheets to become strongly and uniformly anchored onto the surface of Fe3O4@C. This work aims at preparing a low-cost and highly efficient magnetic composite adsorbent with high adsorption capacity. The Received: Revised: Accepted: Published: 10152
September 12, 2012 June 24, 2013 July 8, 2013 July 8, 2013 dx.doi.org/10.1021/ie3024438 | Ind. Eng. Chem. Res. 2013, 52, 10152−10159
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magnetic field and washed with deionized water to remove the unadsorbed uranium(VI) ions. The exhausted adsorbent was reintroduced to the desorption medium and agitated by a shaker for 180 min at 25 °C. After each cycle of recovery, regenerated Fe 3 O 4 @C@Ni−Al LDH was washed with deionized water and dried for reuse. 2.4. Characterization. X-ray diffraction (XRD) analysis was performed on a Rigaku D/max-IIIB diffractometer with Cu Kα irradiation (λ = 1.541 78 Å). The X-ray source was operated at 40 kV and 150 mA. Morphology was characterized using transmission electron microscopy (TEM, FEI Tecnai G2 20 STWIN). Fourier-transform infrared (FT-IR) spectrum was recorded with an AVATAR 360 FT-IR spectrophotometer using standard KBr pellets. The magnetic measurement was carried out with a vibrating sample magnetometer (VSM, Lanzhou University LakeShore 7304). The BET surface areas were obtained from nitrogen adsorption isotherms at 77 K using an Micromeritics ASAP 2010 analyzer.
influence of various experimental parameters on uranium(VI) adsorption, such as solution pH value, adsorbent dose, contact time, and the temperature have been studied. The adsorption kinetics and thermodynamics have also been investigated.
2. EXPERIMENTAL SECTION 2.1. Preparation of Samples. The Fe3O4 was prepared according to the previous report.30 Fe3O4 microspheres (0.4 g) were dispensed in 0.1 M HNO3, sonicated for 10 min, and then washed with deionized water. The microspheres were introduced in 80 mL of glucose aqueous solution (0.8 M) and dispersed ultrasonically for 5 min. The resulting black mixture was transferred into a Teflon-lined stainless steel autoclave, heated at 180 °C for 8 h, and then cooled down naturally to room temperature. The Fe3O4@C was isolated with a bar magnet, washed with deionized water and ethanol successively, and then dried in a vacuum oven at 50 °C for 12 h. The preparation of the boehmite by sol−gel method has been described previously.31 Boehmite (5.8 g) was added to 107 mL of deionized water with stirring at 85 °C for 1 h, and 9.5 mL of HNO3 (1.0 M) was slowly added dropwise to this solution, which refluxed for 6 h. The AlOOH primer sol was obtained after slow cooling to room temperature. Subsequently, the Fe3O4@C microspheres were dispersed in the AlOOH primer sol with vigorous agitation for 1 h. The Fe3O4@C@ AlOOH was isolated with a bar magnet and washed with ethanol. The resulting Fe3O4@C@AlOOH was dried in air for 30 min. The whole process (dispersion, withdrawing, drying) was repeated five times. In the following step, 0.01 mol of Ni(NO3)2·6H2O and 0.015 mol of NH4NO3 were dissolved in 70 mL of deionized water to form a solution. The Fe3O4@C@ AlOOH was placed in the above solution in an autoclave at 100 °C for 48 h. Finally, the resulting Fe3O4@C@ Ni−Al LDH was separated by a magnet, washed several times with ethanol, and dried in vacuum at 60 °C for 12 h. 2.2. Adsorption Experiments. A batch technique was applied to study the sorption of uranium(VI) from the prepared solutions by the Fe3O4@C@Ni−Al LDH. For each experiment, 0.05 g of Fe3O4@C@Ni−Al LDH was mixed with 50 mL of the metal nitrate solution in a conical flask, and then the conical flask was sealed and kept agitating in a shaking bath at 120 strokes min−1. At the end of the adsorption period, the solution was separated from the solids by magnetic separation. The initial and equilibrium concentrations of tested ions in supernatant were determined. The adsorption capacity Qe (mg g−1) and the removal efficiency (RE) were calculated using the following equation: Qe =
(C0 − Ce)V m
RE (%) =
C0 − Ce × 100 C0
3. RESULTS AND DISCUSSION 3.1. Characterization of Samples. Figure 1 shows the XRD patterns of Fe3O4, Fe3O4@C, and Fe3O4@C@Ni−Al
Figure 1. XRD patterns of Fe3O4 (a), Fe3O4@C (b), and Fe3O4@C@ Ni−Al LDH (c).
LDH. The diffraction peaks of the Fe3O4 can be indexed to cubic Fe3O4 (JCPDS 65-3107). After coating with the carbon layer, the diffraction pattern of the resulting material (Figure 1b) shows a reflection characteristic of amorphous C as well as the Fe3O4 reflections. From the XRD pattern of the Fe3O4@ C@Ni−Al LDH, except the diffraction peaks of the Fe3O4 sample, the peaks marked with ∇ of (003), (006), (012), and (110) planes are typical ones for LDH material. The morphologies of Fe3O4, Fe3O4@C, and Fe3O4@C@Ni− Al LDH were characterized by TEM images. The TEM image shows that the Fe3O4 particles have a quasi-spherical shape and a mean diameter of 10 nm (Figure 2A,B). After being coated with a carbon layer, core−shell Fe3O4@C with a thin carbon layer ∼10 nm in thickness was obtained (Figure 2C). By an in situ growth procedure, Ni−Al LDH was formed on the surface of Fe3O4@C. From Figure 2D, it is found that LDH nanosheets are closely anchored on the surface of Fe3O4@C. Figure 3 displays the FT-IR spectra of Fe3O4, Fe3O4@C, and Fe3O4@C@Ni−Al LDH, respectively. In the spectrum of Fe3O4, as shown in Figure 3a, a peak at 571 cm−1 is assigned to the Fe−O bond vibration of Fe3O4. Compared with the spectrum of Fe3O4, there are some new peaks in the spectrum of Fe3O4@C, as shown in Figure 3b. The bands at 1721 and
(1)
(2)
where C0 and Ce (mg L−1) are the initial and equilibrium concentrations of uranium(VI) solutions; m is the weight of sorbent (g); V is the volume of the uranium(VI) solution (L). All the adsorption experiments were run in triplicate. 2.3. Desorption and Regeneration. In order to evaluate the reusability of Fe3O4@C@Ni−Al LDH, desorption experiments using different eluants were carried out. After saturated adsorption, the uranium(VI)-loaded Fe3O4@C@Ni−Al LDH particles were separated from the solution by an external 10153
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Table 1. Physical Properties of Fe3O4, Fe3O4@C, and Fe3O4@C@ Ni−Al LDH characteristics
Fe3O4
surface area (m2 g−1) pore volume (cm3 g−1) av pore radius (nm) saturation magnetization (emu g−1) coercive force (Oe) remanence (emu g−1)
74 0.097 5.2 70.06
Fe3O4@C Fe3O4@C@ Ni−Al LDH 13 0.034 26.0 8.25
77 0.243 10.9 2.20
21.58 4.13
52.80 0.78
69.15 0.19
value reflects that the as-synthesized Fe3O4@C@Ni−Al LDH is a soft magnet at room temperature. These results show that Fe 3 O 4 @C@Ni−Al LDH composite possesses magnetic responsivity and can be easily separated during the sorption experiments. 3.2. Equilibrium Studies. 3.2.1. Effect of Solution pH. pH is one of the important factors that affect the adsorption efficiency, and the pH change on the adsorption of uranium from aqueous solutions is shown in Table 2. The adsorption of Table 2. pH Change upon Adsorption of Uranium by Fe3O4@C@Ni−Al LDH
Figure 2. TEM images of Fe3O4 particles (A, B), Fe3O4@C (C), and Fe3O4@C@Ni−Al LDH (D).
pH initial
pH final
pH initial
pH final
2.00 4.00 5.18 6.00
6.89 8.18 8.39 8.63
8.00 10.00 12.00
9.15 10.81 12.52
uranium(VI) on Fe3O4@C@Ni−Al LDH was studied by varying pH in the range of 2.00−12.00. The pH value of uranyl nitrate solution was adjusted with 0.1 M HNO3 or NaOH solution. It can be seen from Figure 4 that the
Figure 3. FT-IR spectra of Fe3O4 (a), Fe3O4@C (b), and Fe3O4@C@ Ni−Al LDH (c).
1595 cm−1 are associated with the CO vibration and CC vibration, respectively, indicating the carbonization of glucose during hydrothermal reaction.32,33 In the spectrum of Fe3O4@ C@Ni−Al LDH, besides the peaks of the Fe3O4@C sample, an intense and broad peak at 3473 cm−1 is ascribed to the stretching vibration of hydroxyl groups of LDH layers and interlayer water molecules. An intense absorption band at about 1379 cm−1 is assigned to an asymmetric stretching of CO32− in the interlayer.34 The magnetic properties were investigated at room temperature (300 K). The values of the corresponding magnetic parameters, including saturation magnetization, coercivity, and remanence are listed in Table 1. The saturation magnetizations of Fe3O4@C and Fe3O4@C@Ni−Al LDH are 8.25 and 2.20 emu g−1 respectively, which are lower than that of Fe3O4 particles (Ms = 70.06 emu g−1). The reduced saturation magnetization is mainly owed to the presence of diamagnetic C and Ni−Al LDH surrounding the Fe3O4 cores. Although the saturation magnetization decreases, the saturation magnetization is enough to enable Fe3O4@C@Ni−Al LDH to be manipulated by the conventional magnets.35 The low coercivity
Figure 4. Effect of pH value on adsorption property of Fe3O4@C@ Ni−Al LDH; initial uranium(VI) concentration 200 mg L−1, pH 2.00− 12.00, 25 °C, and 0.05 g of Fe3O4@C@Ni−Al LDH.
maximum adsorption occurs at pH 6.00; hence, pH 6.00 is used in further studies. The pH effect on uranium sorption can be explained by the surface characteristics of the adsorbents and speciation of the solute. At lower pH, UO22+ is the predominant species of uranium(VI). As the solution pH is increased, the hydrolysis products such as UO 2 OH + , (UO2)2(OH)22+, and (UO2)3(OH)5+ are formed.36 When the pH value is lower, the surfaces of the adsorbent exhibits positive characteristics, leading to the low adsorption ability of adsorbent for uranium(VI). Besides, the H+ ions in acidic 10154
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solution will also compete with positively charged uranium species for the sorption sites, resulting in the reduced uptake of uranium(VI). Along with the increase of pH value, the surface of adsorbent becomes negatively charged due to the deprotonation process, so that the Coulombic attraction between adsorbent and uranium(VI) probably strengthens the interaction between each other. However, with further increase of the pH value, the adsorption capacity decreases. The decrease in removal of uranium(VI) likely results from the formation of different uranyl hydroxide and carbonate complexs that are negatively charged, resulting in the decrease in the uptake of uranium(VI) ions.37,38 3.2.2. Effect of Adsorbent Dose. The adsorbent dose is another factor that influences the adsorption equilibrium. In order to examine the effect of adsorbent dose on the uranium(VI) removal, adsorption experiments were set up with various amounts of Fe3O4@C@Ni−Al LDH from 0.005 to 1.5 g. Figure 5 shows the effect of adsorbent dose for the
Figure 6. Adsorption isotherm of Fe3O4@C@Ni−Al LDH for uranium(VI) at different temperatures for pH 6.00, 25−55 °C, and 0.05 g of Fe3O4@C@Ni−Al LDH.
ature, indicating the endothermic nature of the process. In order to understand the adsorption behavior of the adsorbents, the equilibrium data were further evaluated and found to conform best to the Langmuir model. According to the Langmuir model, adsorption occurs uniformly on the active sites of the sorbent, and once a sorbate occupies a site, no further sorption can take place at the same site.39 Its form can be described by the following equation Ce C 1 = + e Qe bQ m Qm
(3)
−1
where Ce (mg L ) is the equilibrium concentration of uranium(VI), Qe (mg g−1) is the equilibrium adsorption amount of uranium(VI), Qm (mg g−1) is the Langmuir constant representing the maximum monolayer capacity of uranium(VI), and b is Langmuir constant related to the energy of adsorption. According to eq 3, a plot of Ce/Qe versus Ce is obtained at different temperatures, as shown in Figure 7. The values of Qm
Figure 5. The effect of adsorbent dose on the uptake of uranium(VI) by Fe3O4@C@Ni−Al LDH; (●) theoretical value of Qm; initial uranium(VI) concentration 200 mg L−1, pH 6.00, 25 °C, and 0.005− 0.15 g of Fe3O4@C@Ni−Al LDH.
removal efficiency, adsorption capacity, and the theoretical maximum adsorption capacity of uranium(VI). The theoretical maximum adsorption capacity of uranium(VI) can be calculated from Figure S1 in Supporting Information. It can be seen from Figure 5 that the removal efficiency of uranium(VI) increases rapidly with the increasing dosage of Fe3O4@C@Ni−Al LDH and then approaches equilibrium, while the adsorption capacity decreases. With an increase in adsorption dose, m, the value of Ce will be gradually reduced to zero. Considering the mass balance equation and the Langmuir isotherm equation, the adsorption quantity Q is proportional to (C0 − Ce) and inversely proportional to m. Therefore, the adsorption capacity of uranium(VI) increases with an increase of adsorbent dose at the initial stages and then decreases. Furthermore, with increasing Fe3O4@C@Ni−Al LDH content, it provides more sorption sites to adsorb uranium(VI) ions, thereby resulting in the increased removal efficiency. The decrease in sorbent capacity may be due to aggregation resulting from high sorbent dose, leading to a decrease in total surface area of the sorbent. In addition, it may be also due to interference between binding sites and the insufficiency of metal ions in solution with respect to available binding sites. 3.2.3. Effect of Temperature. Figure 6 shows the adsorption isotherms of uranium(VI) on Fe3O4@C@Ni−Al LDH at different temperatures. The adsorption curves show that the uptake of uranium(VI) increases with the increased temper-
Figure 7. Langmuir isotherms plots for the adsorption of uranium(VI) onto Fe3O4@C@Ni−Al LDH at different temperatures.
and b can be obtained by the slope and intercept of the line and are given in Table 3. In the figure, the Langmuir model appears to be the fitting model for uranium(VI) sorption with a high related coefficient R2 (>0.98). The temperature dependence of adsorption process is associated with changes in several thermodynamic parameters such as standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) of adsorption, which are calculated using the following equations40 ΔG° = −RT ln(b) 10155
(4)
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concentration of the uranium(VI) led to a high adsorption rate. With the occupation of the active sites and the decrease of the uranium(VI) concentration, the uptake rate decreases until equilibrium is obtained. To study the controlling mechanism of the adsorption process, the adsorption data were treated as pseudo-first-order and pseudo-second-order.41−43 The pseudofirst-order kinetic equation is given as
Table 3. Langmuir Constants for the Sorption Uranium(VI) onto Fe3O4@C@Ni−Al LDH at Different Temperatures Langmuir constants b (L mg−1) Qm (mg g−1) R2
25 °C
35 °C
45 °C
55 °C
0.09 ± 0.02 227 ± 15
0.10 ± 0.02 238 ± 12
0.12 ± 0.01 251 ± 5
0.22 ± 0.03 257 ± 18
0.98
0.99
0.99
0.99
ln(qe − qt ) = ln qe − k1t
ΔH ° ⎛ 1 1⎞ ln(b2 /b1) = − ⎜ − ⎟ R ⎝ T2 T1 ⎠
(5)
ΔG° = ΔH ° − T ΔS°
(6)
(7)
where k1 is the rate constant of pseudo-first-order adsorption, and qe and qt (mg g−1) refer to the amount of uranium(VI) ions adsorbed at equilibrium and at time (t), respectively. The values of qe and k1 are calculated from the intercepts and slopes of the plot (Figure 9A) corresponding to eq 7, which are
From eq 4, the ΔG° data at different temperatures can be obtained. The values of ΔG° are found to be −11.2 ± 0.6, −11.7 ± 0.5, −12.6 ± 0.1, and −14.6 ± 0.7 kJ mol−1 at 25, 35, 45, and 55 °C, respectively. The negative value of ΔG° indicates that the adsorption reaction is spontaneous. The positive value of ΔH° (22.4 ± 0.9 kJ mol−1) shows the endothermic nature of the adsorption process. The positive value of ΔS° (111 ± 2 J mol−1 K−1) suggests the increased randomness at the solid/liquid interface during the adsorption of uranium(VI) on Fe3O4@C@Ni−Al LDH. The positive value of ΔH° and decrease in the value of ΔG° with rising in temperature show that the adsorption is more favorable at high temperature. 3.3. Kinetic Studies. In order to establish the equilibrium time for maximum uptake and to uncover the kinetics of adsorption process, uranium adsorption on Fe3O4@C@Ni−Al LDH was investigated as a function of contact time, as given in Figure 8. This figure shows that the adsorption rate of
Figure 9. Pseudo-first-order (A) and pseudo-second-order (B) plot for the removal of uranium(VI) by Fe3O4@C@Ni−Al LDH; initial uranium(VI) concentration 200 mg L−1, pH 6.00, and 0.05 g of Fe3O4@C@Ni−Al LDH.
given in Table 4. The low related coefficients R2 and the great difference between the calculated values of adsorption capacity and the experimental values indicate that the sorption mechanism of uranium(VI) on Fe3O4@C@Ni−Al LDH does not follow the pseudo-first-orde kinetic model well. The pseudo-second-order adsorption kinetic model is expressed as t 1 t = + qt qe k 2qe 2 (8) where k2 is the rate constant of pseudo-second-order adsorption. Figure 9B shows straight lines of the pseudosecond-order model with correlation coefficients of 0.99. Furthermore, the calculated values of adsorption capacity are very close to the experimental values. Therefore, adsorption of uranium(VI) on Fe3O4@C@Ni−Al LDH is consistent with the pseudo-second-order model. The pseudo-second-order rate constants listed in Table 4 are used to estimate the adsorption activation energy using Arrhenius equation:
Figure 8. Effect of contact time on uranium(VI) adsorption at different temperatures; initial uranium(VI) concentration 200 mg L−1, pH 6.00, and 0.05 g of Fe3O4@C@Ni−Al LDH.
uranium(VI) is rapid during the initial stages of the adsorption process. Then, uptake rate slowly declines and reaches equilibrium. Kinetics of adsorption on uranium(VI) consists of three phases: an initial rapid phase, a slower second phase, and a equilibrium phase. The fast uranium(VI) removal rate in the initial phase is interpreted to be the instantaneous adsorption stage or external surface adsorption. The slower second phase is attributed to the intraparticle diffusion of uranium(VI) into the inner surface of Fe3O4@C@Ni−Al LDH or the ion exchange in the inner surface of Fe3O4@C@Ni−Al LDH. At 25 °C, the adsorption of uranium(VI) reaches equilibrium after 180 min. Moreover, at the initial stage of the sorption process, a great amount of active sites and the high
k = Ae−(E / RT )
(9) −1
where E is the activation energy of adsorption (kJ mol ), R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature (K), and A is an empirical constant. A plot of ln(k) versus 1/T yields a straight line, with slope −E/R (Figure 10). For diffusion-controlled processes, the activation energy of adsorption is less than 20 kJ mol−1.44,45 The calculated value of E from Figure 10 was 11.4 ± 0.2 kJ mol−1, indicating that the process of uranium(VI) removal is mainly controlled by the diffusion. 10156
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Table 4. Kinetic Parameters of Different Models for Uranium(VI) Ions Adsorption onto Fe3O4@C@Ni−Al LDH Composite kinetic models and parameters
25 °C
qe (exp) (mg L−1)
174.1 ± 0.2
qe (calc) (mg L−1) k1 (min−1) R2
66 ± 1 0.035 ± 0.001 0.94
qe (calc) (mg L−1) k2 (g mg−1 min−1) R2
178.4 ± 0.2 0.0014 ± 0.0001 0.99
35 °C 179.8 ± 0.3 Pseudo-First-Order 77 ± 1 0.043 ± 0.001 0.93 Pseudo-Second-Order 185.9 ± 0.2 0.0016 ± 0.0002 0.99
45 °C
55 °C
184.0 ± 0.3
188.3 ± 0.4
68 ± 1 0.045 ± 0.001 0.99
62 ± 3 0.049 ± 0.001 0.94
191.6 ± 0.4 0.0018 ± 0.0003 0.99
194.9 ± 0.1 0.0021 ± 0.0001 0.99
Figure 10. Plot of ln(k) against 1/T for uranium(VI) adsorption on Fe3O4@C@Ni−Al LDH.
Figure 11. Adsorption isotherm of Fe3O4, Fe3O4@C, and Fe3O4@C@ Ni−Al LDH for uranium(VI); pH 6.00, 25 °C, and 0.05 g of adsorbents.
3.4. Desorption and Regeneration Studies. The regeneration of adsorbent is the most important aspect of an economical technology. In order to estimate the reusability of Fe3O4@C@Ni−Al LDH, desorption of uranium(VI) from Fe3O4@C@Ni−Al LDH was investigated. Several different eluants, such as 0.1 M NaOH, 0.1 M NaHCO3, 0.1 M Na2CO3, 0.1 M Na2SO4, HNO3 (pH 4) as well as the distilled water, were used for desorption of uranium. The percentage desorption for corresponding desorbing agents were 44.2 ± 0.9, 79.8 ± 0.9, 88.3 ± 0.6, 8.7 ± 0.9, 50.6 ± 0.9, and 6.3 ± 0.7%, respectively. Among them, uranium(VI)-loaded Fe3O4@ C@Ni−Al LDH particles showed the highest desorption yield by using 0.1 M Na2CO3. The effect of concentration of Na2CO3 solution on desorption of uranium showed that 0.5 M Na2CO3 solution had the maximum yield (92.6 ± 0.8%). To assess the reusability of the adsorbent, the adsorption− desorption experiment with 0.5 M Na2CO3 was repeated for three cycles. After three cycles, the sorption capacity of the Fe3O4@C@Ni−Al LDH decreased from 174.1 ± 0.2 to 138.2 ± 0.5 mg g−1. This result shows that the adsorbent has sufficient chemical stability for the recovery of uranium(VI) from aqueous media. 3.5. Mechanism. By using the three particles of Fe3O4, Fe3O4@C, and Fe3O4@C@Ni−Al LDH, isothermal adsorption experiments were carried out for uranium(VI) at 25 °C (Figure 11). Fe3O4 shows low adsorption capacity for uranium, indicating that Fe3O4 has a rare contribution for uranium removal in the composite. In contrast, Fe3O4@C and Fe3O4@ C@Ni−Al LDH have high adsorption capacity for uranium, meaning that C and Ni−Al LDH mostly contribute to uranium removal in the composite. The saturated adsorption capacity of Fe3O4@C for uranium(VI) ion gets up to 122.3 ± 0.6 mg g−1. After coating with the carbon layer, surface area and pore
volume decrease from 74 m2 g−1 and 0.097 cm3 g−1 to 13 m2 g−1 and 0.034 cm3 g−1, respectively. The decrease of surface area may be attributed to the encapsulation shell of the carbon layer and partly to aggregation of particles during encapsulation process. The results show that the uranium(VI) sorption capacity of Fe3O4@C does not have a direct correlation with its specific surface area, specific pore volume, and mean pore diameter. The FTIR results (Figure 3) show qualitatively that the total amount of functional groups on the surface are increased after coating with the carbon layer. The increase in sorption capacity may be caused by the increased number of functional groups due to the surface modification of the Fe3O4. There is evidence in the literature that the uranium(VI) sorption capacity of carbon material depends on their number of functional groups and does not have a direct correlation with their specific surface area, specific pore volume, and mean pore diameter.46 In addition, it is worth mentioning that without the C layer between Fe3O4 and LDH, the LDH nanoplatelets do not closely anchor on the surface of Fe3O4, which is likely to have low affinity. The maximum adsorption capacity of Fe3O4@ C@Ni−Al LDH for uranium(VI) ion is found to be 174.1 ± 0.2 mg g−1 when the initial uranium(VI) concentration is kept as 200 mg L−1 at 25 °C, which is higher than that of the Fe3O4@C obtained under the same conditions. These results indicate that modification of Fe3O4@C with Ni−Al LDH improves the adsorption capacity for the uranium(VI) due to the higher concentration of active sites on Fe3O4@C@Ni−Al LDH. Removal of uranium(VI) using Ni−Al LDH can be facilitated via intercalation and surface adsorption. Uranyl ions have various forms in the water. Intercalation of the ions species involves a network of hydrogen bonds between the ions species and the hydroxyl layers. Surface adsorption involves the bonding of ion species to the hydroxyl units on the external 10157
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surface of the LDH. By the synergism of the intercalation and the surface adsorption, a mass of uranium(VI) will be attracted to the surfaces of Ni−Al LDH and then adsorbed, leading to the high adsorption capacity. On the basis of the above discussion, it is concluded that the surface adsorption or intercalation mechanism acts on Fe3O4@C@Ni−Al LDH in the adsorption process of uranium. Further studies are needed to more precisely characterize the detailed adsorption mechanism.
4. CONCLUSIONS We prepared Fe3O4@C@Ni−Al LDH composite particles and successfully applied them as a novel and effective adsorbent material for the removal of uranium(VI) ions from aqueous solution. The Fe3O4@C@Ni−Al LDH hybrids not only display large adsorption capacity but also can be easily separated by a magnet. It provides a rapid and effective way for removing the suspension from aqueous solution after the adsorption process. In this study, adsorption experiments were conducted by a batch technique, and the conditions have been optimized to be a pH value of 6.0, uranium concentration of 200 mg L−1, and contact time of 180 min. The adsorption of uranium(VI) on Fe3O4@C@Ni−Al LDH is quick, and the kinetic adsorption is fitted well by the pseudo-second-order model. The thermodynamic parameters reveal the process is endothermic and spontaneous in nature. The results illustrate that Fe3O4@C@ Ni−Al LDH could be a perfect candidate as an adsorbent for the removal of uranium(VI) from aqueous solutions.
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ASSOCIATED CONTENT
* Supporting Information S
Langmuir isotherms plots for the adsorption of uranium(VI) onto Fe3O4@C@Ni−Al LDH with various adsorbent doses. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86 451 8253 3026. Fax: +86 451 8253 3026. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Special Innovation Talents of Harbin Science and Technology (2011RFQXG016), Fundamental Research Funds of the Central University (HEUCFZ), Key Program of the Natural Science Foundation of Heilongjiang Province, Program of International S&T Cooperation special project (S2013ZR0649), Special Innovation Talents of Harbin Science and Technology (2012RFXXG104).
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