Synthesis of Magnetic Fe3O4@hierarchical Hollow Silica

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Synthesis of Magnetic Fe3O4@hierarchical Hollow Silica Nanospheres for Efficient Removal of Methylene Blue from Aqueous Solutions Jinxi Zhang, Baoshan Li,* Wanliang Yang,‡ and Jianjun Liu State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, Peoples Republic of China ABSTRACT: Magnetic Fe3O4@hierarchical hollow silica spheres (Fe3O4@HHSS) with high specific surface area (SBET) were successfully synthesized by the microemulsification method. The microstructure and properties of Fe3O4@HHSS were studied by SEM, HRTEM, XRD, BET, and VSM techniques. The inner wall of Fe3O4@HHSS is composed of small hollow nanospheres and Fe3O4 nanoparticles, and the small hollow nanospheres and Fe3O4 nanoparticles are covered with an external silica wall. As a magnetic material, the Fe3O4@HHSS showed good magnetic response and could be easily recovered by an external magnet. The Fe3O4@HHSS exhibited an excellent ability to adsorb methylene blue (MB) from aqueous solutions with maximum MB adsorption capacity of 71.45 mg/g. The adsorption process was chemisorption in nature, while the adsorption isotherm data were well fitted to the Langmuir model and the kinetic data were well fitted to the pseudo-second-order kinetic model. Furthermore, the dye saturated Fe3O4@HHSS could be regenerated by using acidic ethanol solution, and the Fe3O4@HHSS showed excellent reusability.

1. INTRODUCTION As one kind of organic pollutants coming from leather, textile, printing, paper making, pharmaceutical, plastic, and food, the organic dyes in wastewater undergo chemical and biological changes, increasing the chemical oxygen demand (COD) and reducing light penetration and visibility.1,2 Furthermore, some of the dyes and their degradation counterparts are potentially carcinogenic and toxic, and their presence in water will cause serious threats to ecological environment and human life.3 Therefore, a great many techniques have been explored to remove organic dyes from water, such as photocatalytic degradation, biological treatment, chemical oxidation, adsorption, and coagulation.4−7 Among them, adsorption of dyes has been widely studied due to its merits of high efficiency, simplicity, and economy. Activated carbons, zeolites, clays, biomass industrial byproducts, agricultural wastes, and polymeric materials have been used as adsorbents.8−12 However, the applications of these adsorbents are limited because of their separation inconvenience. Thus, it is necessary to explore new promising adsorbents. Recently, hollow spheres have received significant attention due to their potential applications in wide areas such as drug delivery,13,14 photonics,15 gas sensor,16 catalysis,17−19 and adsorbance.20 Among these hollow sphere materials, the hollow silica spheres are of especial interest in drug delivery and adsorbance due to their greater drug loading volumes and better adsorption abilities compared with conventional silica materials.21,22 Most of the hollow silica spheres were synthesized by hard template-assisted routes, which include polymers, metals, ceramics, and composites with various diameters and wall thicknesses. As the cost of the hard template removal is relatively high, the preparations of hollow silica spheres without a hard template have attracted more and more attention.23,24 On the other hand, magnetic nanoparticles have attracted significant interest in magnetic separation, magnetic resonance © XXXX American Chemical Society

imaging contrast enhancement, and drug delivery, due to their excellent superparamagnetism and low toxicity. However, the bare magnetic nanoparticles are liable to aggregation and rapid biodegradation coupled with a limited carrying capacity. Thus, it is quite necessary to combine magnetic nanoparticles with carriers to achieve better adsorption capacity and stability. Various polymeric and inorganic materials have been reported as carriers of magnetic materials.25−27 Among these different carriers, silica as an inorganic carrier has been proved to be an ideal supporting material.28 Recently, preparations of hollow magnetic silica particles have been studied due to their unique properties, such as large surface area, hollow structure, and high stability.29−32 In this work, we report a microemulsification method for synthesizing a magnetic Fe3O4@hierarchical hollow silica sphere (Fe3O4@HHSS). The Fe3O4@HHSS possesses high specific surface area, huge interior space, and robust magnetic properties. The inner wall of Fe3O4@HHSS is composed of small hollow nanospheres and Fe3O4 nanoparticles, and the small hollow nanospheres and Fe3O4 nanoparticles are covered with an external silica wall. The adsorption performance of the Fe3O4@HHSS for the removal of organic dye from aqueous solution was evaluated by choosing methylene blue (MB) as a model dye. The adsorption of MB was analyzed by using the adsorption kinetics and isotherm models. The impact of pH on adsorption and the reuse ability of Fe3O4@HHSS were also investigated. Owing to its large cavity structures and magnetic properties, the resultant Fe3O4@HHSS can realize perfect absorption and magnetic separation of MB. Received: January 5, 2014 Revised: June 5, 2014 Accepted: June 10, 2014

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dx.doi.org/10.1021/ie5000533 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Materials. All solvents and reagents were commercially available and were used as purchased without further purification: tetraethyl orthosilicate (TEOS, A.R.), aqueous ammonia solution (NH4OH, 28 wt %), oleic acid and ethanol (Xilong Chemical Company, China); cetyltrimethylammonium bromide (CTAB, C.R.), n-octane (A.R.), and methylene blue (MB, Tianjin Fuchen Chemical Reagents Factory, China); FeCl3·6H2O (99 wt %), FeSO4·7H2O (99 wt %) (Tianjing Chemical Reagent Co., China). 2.2. Preparation. 2.2.1. Preparation of Oleic Acid Modified Fe3O4 Nanoparticles. The Fe3O4 nanoparticles were prepared by coprecipitation reaction procedure and the typical process was described as follows:33 4.31 g of FeSO4· 7H2O and 9.5 g of FeCl3·6H2O were dissolved in 50 mL of deionized water with nitrogen protection. The solution was heated at 353 K and stirred for 30 min, and then 40 mL of NH4OH (28 wt % aqueous solution) was added quickly. After being stirred for 15 min, 1 g of oleic acid was added. The dispersion was further heated at 353 K and stirred for 3 h. After the dispersion cooled to the room temperature, the oleic acid modified Fe3O4 nanoparticles (OA-Fe3O4) were collected with use of a magnet and washed to neutralize with deionized water. The OA-Fe3O4 was further purified with ethanol three times. 2.2.2. Preparation of Fe3O4@HHSS. Fe3O4@HHSS was synthesized by a microemulsification method. The microemulsification method has been employed to synthesize hierarchical hollow silica sphere in our previous researchs.14,17 In a typical procedure, OA-Fe3O4 was dispersed in a solution composed of 1.38 g of CTAB, 66 g of deionized water, 14 mL of NH4OH (3 mol/L, aqueous solution), and 20 mL of noctane. The suspension was irradiated in a water bath of an ultrasonic cleaner at a frequency of 40 kHz for 20 min (JL60DTH, J&L Shanghai Ultrasonics). The ultrasonic output was kept to 60 W. The suspension was further vigorously mechanically stirred for 30 min to form an emulsion. Then 7.2 mL of TEOS was dropped into the mixture. The resulting mixture was stirred for another 0.5 h. The mixture was transferred into a Teflon-lined autoclave and heated at 373 K for 24 h. The solid product was washed and collected by three cycles of redispersion and magnetic separation in ethanol. Finally, the collected product was dried at 333 K for 12 h and calcined at 823 K for 6 h in air to obtain the Fe3O4@HHSS. 2.3. Characterization. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2500 VBZ+/PC diffractometer using Cu Kα radiation (λ = 0.1541 nm). N2 adsorption−desorption isotherms were obtained with use of a Micromeritics ASAP2020 M instrument. The materials were degassed in a vacuum at 573 K for 6 h before the measurements. The specific surface area (SBET) was estimated by using the Brunauer−Emmett−Teller (BET) equation. The pore size distribution was calculated from the desorption branch of the isotherm by using the Barrett−Joyner−Halenda (BJH) method. Morphologies of the sample were examined by high-resolution transmission electron microscope (HRTEM) on a JEM-3010 (Japan) with an accelerating voltage of 200 kV. Scanning electron microscope (SEM) photographs of the samples were obtained with use of a Hitachi S-4700 electron microscope. The elemental analysis was performed by an energy dispersive X-ray spectroscope (EDS) attached to SEM. Magnetic measurement was carried out with a vibrating sample magnetometer (VSM, LakeShore 7407, USA) at room

temperature. The hydrodynamic diameter distribution of Fe3O4@HHSS was determined by dynamic light scattering (DLS) measurements (Zetasizer Malvern 30). 2.4. Adsorption Experiments. The adsorption studies of MB on the Fe3O4@HHSS were carried out at room temperature by adding 50 mg of Fe3O4@HHSS into 50 mL of different concentrations of MB aqueous solutions. The mixtures were continuously shaken for different times and were separated through a magnetic field immediately. The concentration of MB was analyzed by using a UV−vis spectrometer (Shimadzu UV1700) at 664 nm. The adsorption efficiency of the material for MB was calculated based on the concentrations before and after adsorption: adsorption efficiency =

A 0 − Ae 100% A0

(1)

where A0 and Ae are the UV−vis absorbency of the initial dye solution and the residual dye solution, respectively. 2.5. Desorption and Reuse Experiments. For the desorption study, 50 mg of the Fe3O4@HHSS was added to 50 mL of MB solution (40 mg/L) and the mixture was shaken for 120 min. The MB-adsorbed Fe3O4@HHSS was separated by magnetic field and the supernatant dye solution was analyzed by UV−vis spectra. Then, the MB-adsorbed Fe3O4@ HHSS was dispersed into 40 mL of acidic ethanol (HCl/ ethanol) with a pH of 2.0 and shaken for 120 min. The Fe3O4@ HHSS was collected by magnetic field and reused for adsorption again. The recycles of adsorption experiments were conducted five times.

3. RESULTS AND DISCUSSION 3.1. Characterization of Fe3O4@HHSS. The schematic illustration of the preparation of Fe3O4@HHSS is shown as Scheme 1. The preparation procedure mainly includes two steps: The first step was the preparation of OA-Fe3O4 by the classical coprecipitation method. The second step was the preparation of Fe3O4@HHSS by the microemulsification method. In the first step, Fe3 O4 was synthesized by coprecipitating Fe2+ and Fe3+ with use of NH4OH. By adding Scheme 1. Schematic Illustration of the Preparation of Fe3O4@HHSS

B



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The chemical composition of the Fe3O4@HHSS sample was investigated by EDS and the result is shown in Figure 2. The

oleic acid and stirring continuously, the surface of the Fe3O4 nanoparticles was modified by oleic acid. After the redundant oleic acid was washed with water and ethanol, OA-Fe3O4 was obtained. In the second step, OA-Fe3O4 was well dispersed by ultrasonic vibration in a solution of deionized water, NH4OH, octane, and CTAB. During a further 30 min of vigorous mechanical stirring, the octane molecular came into the CTAB micelle, and the carbon chain of octane was matched with the carbon chain of the CTAB molecule to form a bigger spherical micelle denoted as octane@CTAB spherical micelles. When the silica precursor TEOS was added and underwent hydrolysis by NH4OH, assembly occurred because of the resulting negatively charged silicate anions on the cationic surface of the octane@ CTAB spherical micelles. Condensation of self-assembled silicate anions led to the formation of small hollow silica spheres. Furthermore, the excess of n-octane and water formed an oil-in-water microemulsion, and the small hollow silica spheres and OA-Fe3O4 particles gathered at the droplet surface to form a Pickering emulsion. The excess of TEOS ions deposited on the surface of the droplet to form an external silica shell covering the aggregated small hollow silica spheres and OA-Fe3O4 particles. After drying and calcination, Fe3O4@ HHSS was obtained. The SEM image of Fe3O4@HHSS is shown in Figure 1a. It can be seen that the Fe3O4@HHSS sample is consisted of a

Figure 2. EDS result of Fe3O4@HHSS.

result shows the presence of O, Si, and Fe in the Fe3O4@HHSS sample and the atom ratio of Si to Fe is 7.7:1. Calculated from the EDS results, the Fe3O4 content in Fe3O4@HHSS is 14.3 wt %. The XRD pattern of the Fe3O4@HHSS sample is shown in Figure 3, which is in agreement with the standard Fe3O4 spinel

Figure 1. SEM (a) and HRTEM (b) images of Fe3O4@HHSS.

Figure 3. XRD pattern of Fe3O4@HHSS. The green lines represented standard PDF card 75−1610.

large amount of hollow spheres with diameter range from 90 to 150 nm. Most of the hollow spheres remain spherical and intact. From the SEM image of a broken sphere at higher magnification (inset of Figure 1a), the hollow interior with large void size is clearly observed. It also obviously shows that the interior surface of the hollow sphere is rough and the inner wall is consisted of aggregated small nanospheres. On the contrary, the outer surfaces of the hollow spheres are relatively smooth, indicating that the aggregated small nanospheres are covered with an external silica wall. Figure 1b shows the typical HRTEM image of Fe3O4@HHSS sample. It can be observed that Fe3O4@HHSS possesses hollow sphere structure and the Fe3O4 nanoparticles (black dots) are encapsulated dispersedly in the HHSS matrix. The inset of Figure 1b is the HRTEM image of a broken hollow sphere of Fe3O4@HHSS, which reveals that the small nanospheres also possess hollow structure. From SEM and TEM images, it can be concluded that the inner wall of Fe3O4@HHSS is composed of small hollow nanospheres and Fe3O4 nanoparticles, and the small hollow nanospheres and Fe3O4 nanoparticles are covered with an external silica wall.

structure (JCPDS No. 75−1610), with characteristic diffraction peaks of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0), indicating the existence of Fe3O4. The broad peak from 20° to 28° corresponds to the amorphous SiO2. The nitrogen adsorption−desorption isotherms of Fe3O4@ HHSS are shown as Figure 4a. The Fe3O4@HHSS sample exhibits type IV pattern with two capillary condensation steps occurring in the high relative pressure (P/P0) range of 0.7− 0.95, indicating bimodal large pores. An H3-type of hysteresis loop at P/P0 above 0.5 is present, suggesting the adsorption of nitrogen molecules in the hollow voids.34 SBET of the Fe3O4@ HHSS sample is as high as 451.6 m2/g. Figure 4b shows the pore size distribution of Fe3O4@HHSS. It reveals that the Fe3O4@HHSS sample contains large pores (above 30 nm) and small pores (7.5 nm), corresponding to the hollow cavities of Fe3O4@HHSS and the small hollow nanospheres, respectively. Compared with previous magnetic materials,35,36 the Fe3O4@ HHSS possesses higher specific surface area, special hierarchical hollow structure, and large void size, which will bring about a better application in wastewater treatment as magnetic adsorbent. C



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Figure 4. N2 adsorption isotherm (a) and the pore size distribution of Fe3O4@HHSS (b).

Figure 5. Magnetization hysteresis loops at room temperature of OA-Fe3O4 and Fe3O4@HHSS (a) and the separation of Fe3O4@HHSS from solution under an external magnetic field (b).

Figure 5a shows the magnetic hysteresis loops of OA-Fe3O4 and Fe3O4@HHSS samples. It can be seen that the OA-Fe3O4 and Fe3O4@HHSS are both magnetic at room temperature, and no hysteresis loop can be observed. The saturation magnetization of the OA-Fe3O4 is 53.4 emu/g. As expected, the magnetization of Fe3O4@HHSS was reduced to 7.0 emu/g, due to the nonmagnetic HHSS matrix. The temperature-dependent zero-field-cooling (ZFC) and field-cooling (FC) magnetization curve of Fe3O4@HHSS are shown in the inset of Figure 5a (the ZFC-FC magnetization curve was gained under an applied magnetic field of 100 Oe). Two curves coincide at high temperature (above 115 K) and begin to separate as the temperature decreases (below 115 K). The ZFC curve shows a maximum at 115 K (blocking temperature) corresponding to the signature of the superparamagnetic behavior of Fe3O4@ HHSS. The separation ability was studied through the separation and redispersion experiment under an external magnetic field (Figure 5b). The magnetic Fe3O4@HHSS nanoparticles were attracted toward the magnet within 2 min, while being dispersed again if shaking occurs shortly after withdrawing the magnet, indicating a good magnetic response and dispersibility. Figure 6 shows the hydrodynamic diameter distribution of the Fe3O4@HHSS sample after dispersing in deionized water for 2 h with magnetic stirring. The size distribution of Fe3O4@ HHSS displays a unimodal size distribution and the average hydrodynamic diameter is about 132 nm, which is consistent with the diameter of Fe3O4@HHSS in SEM image, indicating a good colloidal stability of Fe3O4@HHSS in deionized water.

Figure 6. Hydrodynamic diameter distribution of Fe3O4@HHSS.

3.2. Adsorption and Recycling Studies. The adsorption ability of Fe3O4@HHSS for organic dye from aqueous solution was examined by using MB as a model. Figure 7 shows the effects of time on the adsorption of MB by Fe3O4@HHSS at different initial concentrations of MB (pH 7). It is clear that the adsorption rate of MB on Fe3O4@HHSS is rapid within the first 20 min for all the concentrations. Thereafter, the adsorption rate is decreases with time. The adsorption equilibrium was almost reached within 60 min. The removal of MB within 120 min can be up to 97.6%, 95.5%, 93.7%, and 91.5% for the initial concentrations of MB of 10, 20, 30, and 40 mg/L, respectively, exhibiting the good MB adsorption ability of Fe3O4@HHSS in aqueous solution. D



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The adsorption isotherm of MB on Fe3O4@HHSS was obtained after adsorption for 12 h at room temperature (pH 7). Figure 9a shows the result of the adsorption isotherm at 298 K and the adsorption amounts of MB on Fe3O4@HHSS increased with MB concentration increasing. The equilibrium isotherm equation is usually used to describe the experimental adsorption data. Thus, the obtained experimental adsorption data were analyzed by the Langmuir model:

Ce C 1 = e + qe qmax qmax KL

where Ce and qe are the equilibrium concentration of adsorbate in liquid phase (mg/L) and on the solid phase (mg/g), respectively; qmax and KL are Langmuir constants related to the maximum MB sorption capacity (71.45 mg/g) and affinity parameter (0.437 L/mg), respectively, which can be calculated from the slope (1/qmax) and intercept (1/qmaxKL) of the linear plot (Figure 9b). The calculated R2 is 0.999, which shows that the Langmuir model fit the result well. To investigate inherent energetic changes associated with adsorption, thermodynamic parameters are estimated. The Gibbs energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) changes can be calculated from the following equations:38 q Kc = e Ce (4)

Figure 7. Effect of contact time on the adsorption capacity of Fe3O4@ HHSS at different initial concentrations of MB. The error bars represent the standard deviations (n = 5).

The kinetic study of adsorption processes provides important data about the efficiency of adsorption. Herein, the kinetic data were analyzed by using Ho’s pseudo-second-order kinetics:37 t 1 t = + qt qe k psqe2 (2) where qt and qe are the amount of MB adsorbed at time t (min) and at equilibrium, respectively; kps is the rate constant of second-order adsorption (g/mg min). Figure 8 shows the plots of t/qt versus t, and the calculated qe, kps and the corresponding linear regression correlation

ΔG 0 = −RT ln Kc

(5)

ΔG 0 = ΔH 0 − T ΔS 0

(6)

ΔS 0 ΔH 0 − R RT

(7)

ln Kc =

where Kc is the distribution coefficient, qe is the equilibrium amount of MB adsorbed on the adsorbent per liter of the solution, and Ce is the equilibrium concentration of MB in the solution. The initial part of the adsorption isotherms is used to estimate Kc. In a narrow temperature range 298 to 328 K, ΔH0 and ΔS0 can be regarded as virtually constant and temperature independent.38 The plot of ln Kc versus 1/T according to eq 7 is shown in Figure 10. The ΔH0 and ΔS0 can be calculated from the slope and intercept of the plot and the adsorption thermodynamic paramaters are listed in Table 2. The enthalpy of adsorption is relatively high, indicating that interaction between sorbent and MB molecules is not only physical but chemical. The pH value of the dye solution plays an important role in the adsorption process. The dyes are usually ionic and the ionization degree and structural change of dye can be influenced by the pH value.39 The pH value also can influence the surface charge and dissociation of functional groups of an adsorbent. As MB is a cationic dye, the adsorption of MB is highly dependent on the pH value. The adsorption performance of the material was studied over the pH range of 3−11 within 120 min with a solution of 40 mg/L MB. The result is illustrated in Figure 11. It is clear that the adsorption performance of MB by Fe3O4@HHSS is better at high pH than at low pH. The reason can be explained by the reaction formulas 8−11. There are silanol groups on the surface of silica material Fe3O4@HHSS and the silanol groups could interact with MB molecules. At low pH values, there is competition of

Figure 8. Pseudo-second-order kinetics plots of MB adsorption on Fe3O4@HHSS.

Table 1. Adsorption Kinetic Parameters of MB Adsorption on Fe3O4@HHSS concn of MB (mg/L)

qe (mg/g)

kps (g/mg min)

R2

10 20 30 40

9.89 19.39 28.74 37.43

0.0578 0.0225 0.0096 0.0056

0.9999 0.9998 0.9992 0.9964

(3)

coefficient (R2) values are summarized in Table 1. The results show that there is a good agreement between calculated and experimental qe values. R2 for the second-order kinetics model under four different concentrations are all greater than 0.996, showing that the pseudo-second-order model is applicable for description of MB adsorption. E



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Figure 9. Adsorption isotherm curve (a) and Langmuir plot of the isotherm (b) of Fe3O4@HHSS.

higher pH, which is in favor of cationic MB adsorption (reaction formulas 10 and 11).40 −SiOH + H+ ⇌ −SiOH 2+

(8)

−SiOH + MB+ ⇌ − Si−O−MB + H+

(9)

−

−

−SiOH + OH ⇌ −SiO + H 2O −

+

−SiO + MB ⇌ −Si−O−MB

Table 2. Thermodynamic Parameters of MB Adsorption on Fe3O4@HHSS ΔG0 (kJ/mol)

ΔH0 (kJ/mol)

ΔS0 (J/mol K)

R2

298 308 318 328

−8.52 −7.27 −6.24 −5.14

−41.9

−112

0.998

(11)

The regeneration and recycling abilities of the adsorbent are crucial for its practical application. The recycling ability of the Fe3O4@HHSS was investigated. The adsorbed MB can be efficiently desorbed from Fe3O4@HHSS by using acidic ethanol (pH 2) and simultaneously the adsorbent was regenerated. As shown in Figure 12, there is only a very slight decrease of the removal after five recycles, indicating the excellent recycling abilities of Fe3O4@HHSS for the removal of MB.

Figure 10. A plot of ln Kc versus 1/T.

temp (K)

(10)

Figure 12. Removal efficiency of MB on Fe3O4@HHSS in different recycles.

4. CONCLUSION The magnetically separable Fe3O4@HHSS nanocomposite was successfully prepared via a microemulsification method. The inner wall of Fe3O4@HHSS is composed of small hollow nanospheres and Fe3O4 nanoparticles, and the small hollow nanospheres and Fe3O4 nanoparticles are covered with an external silica wall. The specific surface area of the sample is 451.6 m2/g, and the magnetic moment of the sample is 7.0 emu/g. The material has good adsorption ability for MB in aqueous solutions with a maximum adsorption quantity of 71.45 mg/g. Furthermore, the MB-adsorbed Fe3O4@HHSS can be easily separated by a magnetic field from the solution, the

Figure 11. Effect of pH on the adsorption of MB on Fe3O4@HHSS.

the hydrogen ion with cationic MB and the repulsive force between the MB and positively charged Fe3O4@HHSS surface (reaction formula 8). With the pH increasing, there would be more silanol groups on the surface of Fe3O4@HHSS which could interact with MB (reaction formulas 9). Additionally, the number of negatively charged adsorption sites increased at F



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adsorbed MB could be efficiently desorbed from Fe3O4@HHSS by acidic ethanol, and the material possesses an excellent recycling ability.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-64445611. Fax: +86-10-64445611. E-mail: bsli@ mail.buct.edu.cn. Present Address

‡ Wanliang Yang: College of Chemical Engineering, Guizhou University, Guiyang, 550025, China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science of Foundation of China (No. 21271017). REFERENCES

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Synthesis of Magnetic Fe3O4@hierarchical Hollow Silica

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