Preparation of Magnetic Composite Hollow Microsphere and Its

Article pubs.acs.org/IECR

Preparation of Magnetic Composite Hollow Microsphere and Its Adsorption Capacity for Basic Dyes Honglei Zhang, Xiangcun Li,* Gaohong He, Jingjing Zhan, and Dan Liu State Key Laboratory of Fine Chemicals, The R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: Magnetic microspheres with an Fe3O4 core and a SiO2−TiO2 hybrid shell were prepared by a surfactant-assisted aerosol process and subsequent etching treatment. The core−shell spheres with robust and chemically stable Ti−O−Si shells exhibit excellent adsorption performance toward basic dyes. The maximum adsorption capacities were obtained at 147 mg/g for methylene blue (MB) and 124.6 mg/g for basic fuchsin. MB with an initial concentration of 20 mg/L can be completely removed in 5 min at a dosage of 0.5 mg/L, and the equilibrium time is 90 min in the MB concentration range 20−250 mg/L. The adsorption kinetics follows the pseudo-second-order model. Furthermore, the dye saturated microspheres can be easily recycled by an external magnetic field and regenerated using 1−3 wt % NaOH aqueous solution. After six recycle runs, 98% of the adsorption capacity was still retained. The low-cost magnetic hollow spheres with good adsorption capacity are a promising candidate for water treatment. fields and environmental remediation.29,30 In this work, we exploited this idea to determine if the hollow spheres with a SiO2 −TiO2 hybrid shell and magnetic core could be constructed by a facile method. Our proposal was to introduce Fe3O4 nanoparticles (∼20 nm) into the porous SiO2−TiO2 spheres by a surfactant-aided aerosol process first and then to calcine the composite particles with controlled temperature and time. Thus, porous particles with Ti−O−Si shell and Si−O−Si core framework can be obtained, and magnetic Fe3O4 particles can be encapsulated in the composite matrix. The Ti−O−Si shell is chemically stable, while the interior Si−O−Si structure can be easily removed by the etching solvent introduced by the pores. The collapse and removal of the interior framework of the calcined particles can result in the formation of hollow spheres with a Ti−O−Si shell structure and an Fe3O4 core. An additional advantage of using such a Ti−O−Si shell coating is the fact that the large and stable negative zeta (ζ) potential (−33.8 to −38 mV) of the spheres can provide a strong electrostatic adsorption force to the positively charged dyes, and the surface silanol groups can also serve as functional sites to graft some molecules. Furthermore, the magnetic spheres can be easily recycled via an external magnetic field and reused several times without a decrease in their adsorption efficiency. The combination of the hollow spheres with magnetism endows them with great potential use in adsorption processes.

1. INTRODUCTION Dye wastewater with high color intensity and complex components is largely yielded from the printing and dyeing industries, which can induce serious water pollution without appropriate treatment before its discharge.1,2 Thus, different methods, including adsorption, membrane processes,3 flocculation or sedimentation,4,5 photocatalytic degradation,6−9 and biological treatment,10,11 have been explored to remove dyes from the wastewater. Especially adsorption has been proven to be one of the most effective methods for removing a wide variety of pollutants in wastewater due to its high efficiency and simple operation process.12,13 Therefore, different adsorbents such as activated carbon (AC),14−18 mesoporous SiO2,19,20 polymer microspheres or networks,21 and alumina22 were synthesized and their adsorption properties were studied. AC has been reported to be widely used and efficient to treat dyes in wastewater among these materials, while the strict synthesis condition (600−900 °C) and high production cost are disadvantages of the adsorbent.23−25 Furthermore, recycling the suspended fine particles from water and regeneration of the adsorbent remain issues to be improved urgently.26 Therefore, it is desirable to design novel kinds of adsorbents with recyclable property, high adsorptive capacity, and low producing cost. Recently, a novel kind of hollow TiO2−SiO2 microsphere with TiO2 nanofibers on the surface was prepared by an aerosol assisted process in our laboratory.27,28 The hollow capsule exhibits chemical stability in a strong acid or alkaline solution due to its robust Ti−O−Si shell structures, providing its promising applications in many fields such as water treatment, bioseparation, and drug delivery systems. Applying ceramic coatings or encapsulating magnetic nanoparticles within a silica/titania matrix has been demonstrated to be an effective way to enhance the stability of the functional particles, and the core−shell particles present extensive applications in biological © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Tetraethyl orthosilicate (TEOS) and titanium isopropoxide (TIP) were purchased from SigmaAldrich and used without further treatment. All of the other Received: Revised: Accepted: Published: 16902

July 26, 2013 October 15, 2013 October 25, 2013 October 25, 2013 dx.doi.org/10.1021/ie402404z | Ind. Eng. Chem. Res. 2013, 52, 16902−16910

Industrial & Engineering Chemistry Research

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chemical reagents were of analytical grade and used without further purification, including FeSO4·7H2O (Shenyang Lianbang Chemical Reagent Co., 99.5%), FeCl3·6H2O (Tianjin Bangdi Chemical Reagent Co. Ltd.), absolute ethanol (Tianjin Xingmake Chemical Reagent Co. Ltd., ≥99.5%), cetyltrimethylammonium bromide (CTAB), and methylene blue (Tianjin Kermel Chemical Reagent Co. Ltd.) 2.2. Preparation. 2.2.1. Preparation of Fe3O4. A 150 mL volume of deionized water was placed in a round-bottom flask; subsequently the water was deoxygenated by bubbling N2 gas for 30 min. Then, 2.1 g of FeSO4·7H2O, 2.7 g FeCl3·6H2O, and 1 g of PEG 4000 were added and the mixture was stirred for 10 min at 50 °C using mechanical stirring at 600 rpm. Afterward, 10 mL of ammonia was slowly added to the mixture. The solution eventually became black, and then was mechanically stirred for about 60 min at 50 °C. The resulting precipitates were separated by a magnet and washed with distilled water and absolute ethanol several times and then dried for about 10 h at 60 °C. 2.2.2. Preparation of TiO 2/SiO 2/Fe3O 4 and Hollow Magnetic Microspheres. The porous TiO2 /SiO 2 /Fe 3 O 4 (TSF) particles were prepared by an aerosol based process. The mixture with a molar ratio of TIP/TEOS/FeCl3/CTAB/ HCl/EtOH/Fe3O4 = 1.1:2.4:1.0:0.5:0.03:42.2:0.14 was used as the aerosol solution, which was atomized into aerosol droplets and then sent through a heating zone (200 °C) using N2 as a carrier gas, where solvent evaporation, hydrolysis, and condensation of silica/titania occurred. The TiO2/SiO2/ Fe3O4 (TSF) particles collected by a filter membrane were calcined in air at 500 °C for 3 h to remove the surfactant and solvent. In order to obtain the TiO2/SiO2/Fe3O4 hollow magnetic microspheres (TSF-HMMS), the TSF particles were further etched in 5 wt % NaOH solution for 2.5 h to remove the SiO2. 2.3. Characterization and Apparatus. The microstructure of the magnetic composite microspheres was taken using a Hitachi-4800 field emission scanning electron microscope (FE-SEM) and a Tecnai-20 transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra of KBr-pressed pellets were recorded on a Model EQUINOX 55 spectrometer, and X-ray diffraction (XRD) patterns were obtained with a D/MAX-2400 diffractometer (Cu Kα radiation, 0.154 nm). Magnetic measurements of samples were performed on a Quantum Design SQUID (MPMS XL-7) magnetometer. Nitrogen adsorption−desorption isotherms at 77 K for the materials were tested using a Micromeritics AUTOSORB-1-MP (Supporting Information, Figure S1). The surface area was given by the Barrett−Joyner−Halenda (BJH) method, and the particles showed high surface areas and pore volumes (TSF 180.9 m2/g, 0.299 cm3/g; TSF-HMMS 231.2 m2/g, 0.243 cm3/ g). 2.4. Adsorption Experiments. All adsorption experiments were carried out at room temperature (25 ± 2 °C). The suspension solution containing 20 mg/L methylene blue (MB) and 0.5 g/L adsorbent was magnetically stirred at 150 rpm for 50 min. Then the suspension was recycled by an external magnetic field and the clarified solution was analyzed using a UV−vis spectrophotometer (7504 PC). The experiments were repeated three times, and the average values were used. The adsorption rate R (%) and the equilibrium adsorption capacity Qe (mg/g) were calculated with eqs 1 and 2, respectively. R (%) = (C0 − Ct )/C0·100%

Q e = (C0 − Ct )V /m

(2)

where C0 (mg/L) is the initial concentration, Ct (mg/L) is the concentration of dye after time t, V (L) is the solution volume, and m (g) is the mass of the adsorbent. The widely used adsorption isotherms are the Langmuir isotherm and the Freundlich isotherm for wastewater treatment applications, which are described in eqs 3 and 4, respectively.31 Ce C 1 = + e Qe bQ m Qm log Q e = log KF +

(3)

1 log Ce n

(4)

where Qm (mg/g) is the theoretical maximum monolayer sorption capacity and b is a constant related to the energy of adsorption. KF ((mg/g)/(mg/L)n) is indicative of the adsorption capacity, and its great value means high adsorption capacity of an adsorbent. The adsorption kinetics of MB on the TSF-HMMS was fitted by the pseudo-first-order model, pseudo-second-order model, and Weber−Morris intraparticle diffusion model, which are described in eqs 5, 6 and 7, respectively.12 log(Q e − Q t ) = log Q e −

K1 t 2.303

(5)

t 1 t = + 2 Qt Qe Q e K2

(6)

Q t = C + K idt 1/2

(7)

2.5. Regeneration Experiments of the Adsorbent. In order to test the regeneration ability of the adsorbent, hollow spheres with full adsorption of dye molecules were treated with different concentrations of NaOH solution (1, 3, 5 wt %) or calcination at 400 °C for 2 h. Then the regenerated adsorbents were reused for further adsorption experiments.

3. RESULTS AND DISCUSSION 3.1. Characterization of TSF-HMMS. It can be seen from Figure 1a,b that the Fe3O4 particles are monodispersed with a narrow size distribution of about 20−40 nm, and their shapes are almost spheres. The small size of the magnetic particles is advantageous to their incorporation into the porous TiO2− SiO2 microspheres. Figure 1c,d shows the representative SEM and TEM images of the macroporous TiO2/SiO2/Fe3O4 (TSF) spheres prepared by an aerosol-aided process. Compared with the porous TiO2−SiO2 particles (Supporting Information, Figure S2a), the macropores in the TSF spheres cannot be easily distinguished from the TEM image due to the incorporation of Fe3O4 nanoparticles in the framework (Figure 1d). The TSF spheres have a smooth surface but also many macropores of ∼50 nm on the surface. The macropores templated by ferric colloid decorated CTAB micelles in the aerosol droplets can provide an easy entry to the particle interior in potentially diffusion limited situations. Figure 1e shows an SEM image of TiO2−SiO2 hollow spheres containing Fe3O4 nanoparticles obtained by etching the porous TSF particles in alkaline solution. The surface of the hollow microspheres is decorated by sharp-ended titania nanofibers with diameters of about 10 nm, and the evolution process and formation mechanism of the hollow TiO2−SiO2 spheres with titania nanofibers on the surface were described in our previous

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Figure 2. FTIR spectra of (a) Fe3O4 nanoparticles, (b) TSF porous precursor particles, and (c) TSF-HMMS obtained by etching TSF in NaOH solution.

the composite framework. The peak at 1499 cm−1 is a typical Ti−OH or Si−OH stretching vibration (Figure 2b,c), and the weak bands at 800 and 463 cm−1 can be assigned to the symmetric stretching and deformation modes of Si−O−Si, respectively.34 The peak at 1065 cm−1 is attributed to the asymmetric stretching vibration of Si−O−Si, while the band at 960 cm−1 is a typical asymmetric Si−O−Ti vibration, indicating the formation of silica and titania matrix on the surface of TSF precursor particles during the calcination process. However, for the TSF-HMMS hollow spheres, the Si−O−Si stretching vibration at 1065 cm−1 disappears completely (Figure 2c), and only a characteristic peak of Ti−O−Si at 956 cm−1 is observed.28 The result reveals that the silica with the Si−O− Si structure is removed from the TSF precursor particles during the etching process and the residual SiO2 combines with TiO2 by a Ti−O−Si bond to construct the shell of the hollow spheres. Actually, the Fe3O4 nanoparticles here can be indexed to the cubic inverse spinel structure (Figure 3).35 It can be found that the diffraction peaks of TSF and TSF-HMMS are similar to those of the parent Fe3O4 particles except for the appearance of titania in the precursor and etched samples. The results suggest that the phase structure of the magnetic particles is well retained even through the calcining and etching processes due to the shielding effect of the SiO2−TiO2 matrix.

Figure 1. (a, c, e) SEM and (b, d, f) TEM images of Fe3O4, TSF, and TSF-HMMS hollow spheres. (a, b) Fe3O4 nanoparticles, (c, d) porous TiO2/SiO2/Fe3O4 (TSF) particles, and (e, f) TiO2−SiO2 hollow spheres containing Fe3O4 nanoparticles (TSF-HMMS) obtained by etching the TSF particles in 5 wt % NaOH solution.

work.28 The hollow spheres encapsulating Fe3O4 particles in the core are fully proved by the TEM image in Figure 1f (black dots, compared with the hollow TiO2−SiO2 sphere in Figure S2b in the Supporting Information). The complete removal of the interior of the TSF particles leads to a hollow sphere with a thin shell thickness of about 20 nm and Fe3O4 particles in its hollow space. Elemental analysis of the TSF-HMMS (X-ray energy dispersive spectroscopy, Supporting Information, Figure S3a) reveals that the mass ratio of silica to titanium to iron is 1.29/8.82/16.47, while the corresponding mass ratio for the precursor TSF particle is 15.79/6.14/12.83 (Supporting Information, Figure S3b). The results prove that the removal of silica from the precursor results in the collapse of its interior framework and leaves the integrity of the Ti−O−Si shell and the encapsulation of Fe3O4 nanoparticles in the hollow space. Figure 2 shows the FTIR spectra of Fe3O4, the precursor TSF particles, and the TSF-HMMS hollow spheres. The peaks at 1631 and 3427 cm−1 can be attributed to the bending vibration of O−H and the stretching vibration of −OH, respectively, for adsorbed water molecules on the particle surface. The strong bond at 587 cm−1 in Figure 2a is assigned to the Fe−O vibration, and the peak intensity becomes weaker in Figure 2b,c, which may be due to the shielding effect of the SiO2−TiO2 coating layer on the magnetic particles or the interaction between the core and the layer.32,33 The result reveals that Fe3O4 particles have been successfully wrapped in

Figure 3. XRD spectra of (a) prepared Fe3O4 nanoparticles, (b) TSF porous precursor particles, and (c) TSF-HMMS obtained by etching TSF in NaOH solution. 30.2, 35.6, 43.2, 53.6, 57.3, and 62.7° are indexed to the (220), (311), (400), (422), (511), and (440) planes of the Fe3O4 cubic inverse spinel structure. 16904



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TSF-HMMS, the removal efficiency of the dye was 91% at 3 min and about 99% of the total amount was removed when the contacting time was continued to 5 min, larger than the removal efficiency of the TSF sample (∼90%) at the same time. The removal efficiency for the TSF can be increased to 95% within 20 min, while it remains almost constant for the TSFHMMS. Therefore, the contacting time of the tested dye on the TSF and TSF-HMMS is considered to be 20 and 5 min, respectively, under the conditions. The high adsorption rate of MB onto the TSF-HMMS may be due to the adequate free activated adsorptive sites on the microsphere surface, indicating potentially lower capital and operational costs for its practical applications of the novel absorbent. The contacting time for equilibrium adsorption in this study is shorter than the reported values for dye adsorption onto other adsorbents.37−40 More importantly, the dye saturated adsorbent can be separated effectively from the solution by an external magnetic field (Figure 5a, inset). Figure 5b shows the equilibrium adsorption isotherm of MB on the TSF-HMMS. It can be seen that the adsorption capacity increases dramatically at first, suggesting a high driving force for the adsorption process. Then, the amount adsorbed reaches a plateau at a high equilibrium solution concentration, reflecting the saturated adsorption of the dye. The maximum adsorption capacity of the dye on the TSF-HMMS is about 147 mg/g, which is comparable to or higher than the adsorption abilities of some carbon based absorbents on MB in Table 1.12,41−44

The magnetic hysteresis loop of TSF-HMMF shows its ferromagnetic behavior at room temperature in Figure 4. The

Figure 4. Magnetization curves of Fe3O4 nanoparticles, TSF precursor particles, and TSF-HMMS adsorbent.

magnetic saturation value of the adsorbent is about 7.5 emu/g, higher than the reported 3.04 emu/g of Fe3O4 nanoparticles embedded in TiO2/SiO2 matrix, which may be due to the smaller shielding effect of the thin TiO2−SiO2 shell than the thick TiO2/SiO2 solid coatings.36 3.2. Adsorption Performance of TSF-HMMS. Figure 5a shows the adsorption capacity of MB over TSF and TSFHMMS microspheres as a function of contacting time. For

Table 1. Adsorption Capacity of MB on Various Adsorbents adsorbenta

ads concn (g/L)

equilib time (min)

Qm (mg/g)

TSF-HMMS

0.5

90

147.0

CNS/OH AC/OH HEMA−EGDMA graphene GO/calcium alginate graphene oxide garlic peel graphene−carbon halloysite

2.5 2.5 5.0 0.5 0.5 0.5 3.0 0.4 3.0

1440 1440 120 255 300 300 210 180 180

682.0 185.0 197.5 153.8 140.8 135.4 82.6 81.9 84.3

ref this work 24 24 47 41 12 12 42 43 44

a

CNS/OH, colloidal carbon nanospheres activated by NaOH solution; AC/OH, activated carbon treated by NaOH solution; HEMA−EGDMA, poly(hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate) beads.

Compared to the carbon based adsorbents with long equilibrium time, sample loss, and high energy consumption for recycling via centrifugation, the equilibrium time here to get the maximum adsorption capacity is only 90 min regardless of the initial concentration of the dye (20−250 mg/L), which is largely shorter than the reported values for other absorbents. Additional advantages of our absorbent are the fact that the suspended particles can be easily recycled by an external magnetic field and its easy preparation process, demonstrating its promising application in wastewater treatment. The collective performance for the TSF-HMMS may be due to its special surface structure. The loose and porous TiO2−SiO2 shell structure and the TiO2 nanofibers on the shell can provide the absorbent with a high surface area to volume ratio and large active adsorption sites, which enormously improves its response time and adsorption ability on the dye. Different from the

Figure 5. (a) Removal efficiency and (b) adsorption isotherm of MB on TSF porous precursor particles and TSF-HMMS hollow spheres. The equilibrium time is 90 min in the adsorption isotherm curve regardless of the initial concentration of MB (20−250 mg/L). 16905



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Table 2. Parameters of Langmuir and Freundlich Adsorption Isotherm Models Langmuir model

Freundlich model

Qm(mg/g)

b(L/mg)

RL

R2

KF (mg/g·(mg/L)n)

n

R2

150

3.1

0.0013

0.999

10 000

7.9

0.518

Table 3. Parameters of the Pseudo-First-Order, Pseudo-Second-Order, and Weber−Morris Models for Adsorption of MB on TSF-HMMS pseudo-first-order model

pseudo-second-order model 2

K1 (1/min)

Qe (mg/g)

R

0.191

10 000

0.91

Weber−Morris model 2

K2 (g/mg·min)

Qe (mg/g)

R

0.002

158

0.99

Kid (mg/g·min1/2)

C

R2

10.4

76.4

0.96

Figure 6. Molecular structures of different dyes and their estimated molecular sizes: (a) methylene blue (MB), (b) basic fuchsin (BF), (c) methyl orange (MO), and (d) Congo red (CR).

previous porous absorbents, the hollow magnetic microspheres here provide a profitable space for the encapsulation or accommodation of targeted objects in practical applications. To investigate the role of Fe3O4 during the adsorption process, TiO2−SiO2 hollow microspheres without Fe3O4 core were prepared and their adsorption property was studied. It was found that the adsorption rate of MB on the TiO2−SiO2 hollow microsphere was similar to those of TSF and TSF-HMMS (Supporting Information, Figure S4). The maximum adsorp-

tion capacity of the hollow spheres was tested to be 151.26 mg/ g, comparable with that of TSF-HMMS (147.0 mg/g). The results reveal that the Fe3O4 core has very little effect on the adsorption capacity of the TSF-HMMS, while plays a dominant role in its recycling process of the adsorbent. In order to further study the adsorption mechanism, the isotherm data were fitted to the Langmuir and Freundlich isotherms according to eqs 3 and 4, respectively. It is clearly exhibited that the value of the linearity of the Langmuir 16906



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adsorption isotherm model is greater than that of the Freundlich (Supporting Information, Figure S5), and the parameters obtained from linear regression are given in Table 2. The maximum adsorption capacity for MB is estimated to be 150 mg/g from the Langmuir model, which agreed with the experimental results. The dimensionless separation factor, RL, which is an essential characteristic of the Langmuir isotherm to define the favorability of an adsorption process, is expressed as RL =

1 1 + bCm

(8)

where b is the Langmuir constant and Cm (mg/L) is the maximum initial concentration of MB. The value of RL indicates the type of the isotherm as either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1).45 In our work, the RL value is calculated to be 0.0013 for the adsorption of MB, which indicates that the process of MB adsorption on TSF-HMMS is favorable. The adsorption kinetics of MB on TSF-HMMS was fitted by the pseudo-first-order model, pseudo-second-order model, and Weber−Morris intraparticle diffusion model, respectively (Supporting Information, Figure S6).46 The relevant parameters were calculated and are listed in Table 3. From the correlation coefficients (R2) in Table 3, the adsorption of MB onto TSF-HMMS fits the pseudo-second-order rate model well, and the calculated value of Qe (158 mg/g) is in accordance with the experiment value. This suggests that the adsorption process of MB seems to be controlled by the chemical process via sharing of electrons or by covalent forces through exchanging of electrons between the adsorbate and adsorbent, where the adsorption ability of the absorbent is related to the adsorption active sites. However, the linear plot (Supporting Information, Figure S6c) for the Weber−Morris intraparticle diffusion model does not pass through the origin; such a deviated straight line from the origin point indicates that the pore diffusion is also a rate-controlling step in the adsorption process. Combing the above analysis, we can conclude that the adsorption process in this study is actually controlled by a chemical interaction process and intraparticle diffusion. Since the isoelectric point of TSF-HMMS is around pH 3.2, the surface of the adsorbent layer at neutral pH is negatively charged (the ζ potential of TSF-HMMS was tested to be −33.8 mV in aqueous solution at pH 6.82). Therefore, the chemical interaction can be ascribed to the electrostatic attraction between the negatively charged surface of the adsorbent and the positively charged MB molecules in this study, because the dye molecule is positively charged due to the dissociation of chloride ion (Figure 6a). Furthermore, the bound effect of MB and TSF-HMMS can also be possible by the formation of hydrogen bonding between the N in (dye molecule) and the proton atom in the Ti−OH or Si−OH group (adsorbent surface), as shown in Figure 7.21 It can be found that the maximum adsorption capacity is 142.9 mg/g in pH 10.70 aqueous solution, which is comparable to that of 147.0 mg/g at pH 6.82 condition (Table 4). However, the adsorption capacity decreases gradually with decreasing the pH value of the aqueous solution, and the result can be due to the fact that the negative charge on the adsorbent surface is neutralized by the protons (H+) at low pH value and the electrostatic driving force on MB decreases dramatically. According to the discussion, the adsorption capacity of basic fuchsin (BF, Figure 6b) on TSF-HMMS was also tested to be

Figure 7. Adsorption of MB molecules on TSF-HMMS hollow microsphere surface through H-bonding and electrostatic interaction.

Table 4. Adsorption Capacity of MB on TSF-HMMS at Different pH Values pH Qe (mg/g)

10.70 142.9

6.82 147.0

4.95 129.2

3.07 108.3

0.99 5.3

124.6 mg/g in pH 6.82 aqueous solution. BF has a similar functional group as MB due to the dissociation of chloride ion, and the slightly lower adsorption ability than MB can be ascribed to its relatively large molecule size (1.15 nm × 1.01 nm × 0.56 nm) which can produce a steric effect on the surface adsorption process. However, the adsorption capacities of methyl orange (MO) and Congo red (CR) on the adsorbent are only 8.5 and 18.4 mg/g, respectively, in pH 6.82 aqueous solution because of the presence of negatively charged surfonic acid groups in the dye molecules resulting from the dissociation of sodium ion (Figure 6c,d). These results further reveal the important role of electrostatic attraction for the adsorption process. On the other hand, the mesopores with diameters of ∼3 nm on the surface of TSF-HMMS (Figure 1f) may permit the entry of the smaller dye molecules (MB) into the sphere interior, while its pore confinement effect on large molecules (BF, CR) combined with the spatial effect of the nanofibers on molecular adsorption should contribute to the intraparticle diffusion model in this study. Accordingly, the electrostatic chemical interaction and Weber−Morris intraparticle diffusion model are responsible for the adsorption mechanism of MB, and the former plays a dominant role. 3.3. Regeneration Experiments. The recycling and regeneration abilities of the adsorbent are crucial for its practical application. The magnetic property supports that the hollow TSF-HMMF magnetic nanocomposite can be separated from the suspension using an external magnetic field (Figure 5a, inset). Accordingly, the adsorbent with full adsorption of dye molecules was recycled easily and regenerated using NaOH solution as the desorption agent. For studying their adsorption stabilities, the regenerated adsorbents were utilized again to adsorb the same concentration of MB solution. As shown in Figure 8a,b, the adsorption efficiency of TSF-HMMS is still higher than 98% after regeneration six times, indicating that this material has excellent adsorption performance and stability. The ζ potential of the regenerated adsorbent using 1 wt % NaOH solution after every recycling was tested to be in the 16907



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Figure 8. Adsorption capacity of recycled TSF-HMMS regenerated with different methods: (a) 1 wt % NaOH solution, (b) 3 wt % NaOH solution, (c) 5 wt % NaOH solution, and (d) calcined at 400 °C for 2 h.

range −35.7 to −38.0 mV (Table 5), revealing the stable composition and strong electrostatic attraction force on the

smooth through high temperature treatment (Supporting Information, Figure S7d), indicating the structural damage of the TSF-HMMS spheres under this condition. The above discussions reveal the feasibility of recycling and regenerating of the TSF-HMMS adsorbent by using an external magnetic field and an alkaline solution in its practical water treatment applications.

Table 5. ζ-Potential Values of the Regenerated TSF-HMMS after Different Cycles cycle times ζ potential (mV)

1 −33.8

2 −38.0

3 −35.7

4 −37.6

5 −36.6

6 −37.4

surface of the regenerated adsorbent. When 5 wt % NaOH was applied, the removal efficiency decreased to 85% after six cycles of the adsorption−desorption process (Figure 8c). This may be because some functional groups and porous structure on the spheres were damaged in such a high concentration of NaOH solution, leading to lower adsorption performance. The removal efficiency of the regenerated adsorbents calcined by 400 °C decreases rapidly (Figure 8d); this may be because the functional group such as Ti−OH or Si−OH and mesopores were destroyed under such a high calcination temperature, and also the ashes were not removed effectively, which occupied the adsorption sites on the surface of the adsorbents. However, the excellent desorption performance at alkaline solution (1−3 wt % NaOH solution) is because excessive OH− ions compete with the cationic MB molecules for the activated adsorption sites on TSF-HMMS, and replace the activated adsorption sites through substance exchange resulting in desorbing MB from TSF-HMMS. Accordingly, the adsorbent was regenerated. We further observe that the regenerated adsorbent treated with NaOH solution retains its morphology well compared with the original hollow spheres (Supporting Information, Figure S7a− c), while the surface of the adsorbent become a little more

4. CONCLUSION TSF-HMMS hollow magnetic microspheres with well-defined core−shell structure and strong magnetic response have been successfully prepared by an aerosol process and subsequent etching treatment. The TSF-HMMS has good adsorption capacity and removal efficiency for MB from aqueous solution. Moreover, it can be separated effectively by an external magnetic field. The adsorption process follows the Langmuir isotherm equation with a maximum monolayer adsorption capacity of 147 mg/g, and the kinetics of the adsorption process follows the pseudo-second-order kinetic model. The regeneration experiments show that TSF-HMMS can be successfully regenerated by 1−3 wt % NaOH solution treatment. The adsorption efficiency of the regenerated adsorbent for methylene blue is still 98% after regeneration six times. It is expected that the hollow sphere with strong magnetic response is a promising candidate for applications in water treatment, catalysis, biochemistry, and miniature devices. 16908



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ASSOCIATED CONTENT

S Supporting Information *

Nitrogen adsorption−desorption curves and pore size distributions of TSF and TSF-HMMS (Figure S1), porous TiO2− SiO2 particles and the corresponding hollow spheres without the introduction of Fe3O4 (Figure S2), EDX spectra of TSF and TSF-HMMS (Figure S3), adsorption properties of TiO2−SiO2 hollow microsphere (without Fe3O4 core) (Figure S4), Langmuir and Freundlich isotherm models (Figure S5), kinetics plots of MB adsorption on TSF-HMMS (Figure S6), and SEM images of the regenerated TSF-HMMS adsorbent (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-0411-84986291. Fax: +86-0411-84986291. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (21006008, 21206014), the Fundamental Research Funds for the Central Universities (DUT12JN07), the Open Fund of Key Laboratory (2012LNSP02), and the National Science Fund for Distinguished Young Scholars of China (21125628).

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Preparation of Magnetic Composite Hollow Microsphere and Its

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