Synthesis and adsorption properties of spongelike porous MnFe2O4

Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 1–7

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Synthesis and adsorption properties of spongelike porous MnFe2 O4 Xiangyu Hou, Jing Feng ∗ , Yueming Ren, Zhuangjun Fan, Milin Zhang Key Laboratory of Superlight Materials & Surface Technology of Ministry of Education, Harbin Engineering University, Harbin, 150001, PR China

a r t i c l e

i n f o

Article history: Received 5 January 2010 Received in revised form 25 February 2010 Accepted 14 March 2010 Available online 20 March 2010 Keywords: Sol–gel MnFe2 O4 Spongelike porous Magnetism Adsorption property

a b s t r a c t Spongelike porous MnFe2 O4 (SPM) was synthesized by a sol–gel method with egg white. The obtained SPM was characterized and applied for the removal of methylene blue (MB) from aqueous solution in the batch system. The morphologies of SPM were spongelike porous bulks and the pore size could be controlled by the dosage of egg white. SPM showed good magnetic property at room temperature. In addition, SPM was suitable for adsorption due to its porous structure and high BET surface areas. The pseudosecond-order model described the adsorption kinetics well. FT-IR analysis suggested that –N+ (CH3 )2 of MB cations and the Fe–O bond of SPM were responsible for good adsorption. The adsorption equilibrium data fit Langmuir isotherm equation well with a maximum MB adsorption capacity of 20.67 mg/g. Moreover, SPM could be separated conveniently under a magnetic field (recovery ratio >98%) and reused seven cycles keeping a high activity (>96%). MB removal efficiencies of the last three cycles (99.5%) were even higher than that of the first four cycles. The results suggested that the SPM was a promising reusable adsorbent to remove MB form wastewater. © 2010 Published by Elsevier B.V.

1. Introduction The removal of dyes from wastewater was one of the most important issues due to their health hazards and environmental pollution. Ozonation [1,2], photooxidation [3], adsorption [4], membrane filtration [5] and flocculation [6] were often applied for removing organic pollutants from dye wastewater. Among these methods, adsorption was an efficient and promising way of removing organic dyes. Various adsorbents, such as active carbon [7], composite adsorbent [8], molecular sieves [9] and chitosan [10] have been widely used. However, the difficult separation from water or high cost hindered the application of these adsorbents. Recently, various magnetic adsorbents had been investigated to solve the separation difficulty. For instance, magnetic beads containing magnetic nanoparticles and activated carbon were applied for methylene blue and methyl orange [11], bromide-coated magnetic nanoparticles were applied for the preconcentration of phenolic compounds [12] and magnetic polymer nanospheres adsorbent for dye molecules removal [13]. Nevertheless, the adsorption capacity or magnetism would be lost for the magnetic composite adsorbents and coated adsorbents. Find a single phase magnetic adsorption was a valid way to conquer this problem. Ferrite had good magnetism at room temperature [14]. However, there were few works about single phase spinel ferrite MFe2 O4 (M = Mn, Co, Ni, Zn) as adsorbent to remove MB from dye

∗ Corresponding author. Tel.: +86 451 82569890; fax: +86 451 82533026. E-mail address: [email protected] (J. Feng). 0927-7757/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2010.03.016

wastewater [15,16]. The particle ferrite was not suitable for adsorption, but porous structure materials were generally considered as excellent adsorbents [17–19]. In addition, egg white would be an available ligand and template to synthesize porous materials for two reasons. First, the main component of egg white was protein, and the protein could react with metal ions as ligand due to its active organic groups (–NH2 and –COOH) and colloidal properties. For instance, Maensiri et al. [20] synthesized nanoparticle NiFe2 O4 with egg white as ligand in a sol–gel process. Second, egg white could be used as a template to form porous oxides [21], because the macro molecule structure of the egg white could occupy large space at low temperature and burn off at high temperature. However, there were few reports about the preparation of porous ferrite MnFe2 O4 using egg white. In this work, spongelike porous MnFe2 O4 (SPM) was synthesized by a sol–gel route with egg white. The adsorption kinetics, isotherm, FT-IR analysis and the reuse of adsorbent were investigated to make a full understanding of adsorption mechanism of the SPM. 2. Experiments 2.1. Synthesis of SPM SPM was synthesized by a sol–gel method with egg white. Mn(NO3 )2 (A.R), Fe(NO3 )3 ·9H2 O (A.R) and fresh eggs (purchased) were used as raw materials. Firstly, 40 mL fresh egg white was stirred for 30 min to form a homogeneous solution. Then, 3.542 g Mn(NO3 )2 and 8.013 g Fe(NO3 )3 ·9H2 O were added slowly into the

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egg white solution with vigorous stirring. Subsequently, the mixed solution was evaporated in a water bath at 80 ◦ C, calcined at 500 ◦ C for 5 h, and then cooled with a mixture of ice and water. The final product of sample SPM40 was obtained after drying. The other samples of SPM60, SPM80, SPM100 and SPM120 were prepared in a similar way with the egg white dosages of 60 mL, 80 mL, 100 mL and 120 mL, respectively. 2.2. Characterizations The crystal phase identification was characterized by powder XRD using a TTR-III diffractometer (Rigaku, Japan) in 2 range of 10–80◦ with Cu K␣ radiation ( = 0.15418 nm). The morphologies were characterized by SEM (HITACHI S-4800, Japan). The specific surface area was measured by Brunauer–Emmett–Teller (BET) method using a Builder SSA-4200 specific surface area measuring instrument (China). Magnetic properties were measured at room temperature with JDM-14D vibrating sample magnetometer (China). The interactions between organic cations of MB and the adsorbent were tested by FT-IR (PerkinElmer Spectrum 100, USA). The concentration of MB was analyzed by using 721 UVVisible Spectrophotometer (China) with the maximum absorption wavelength of MB at 665 nm. 2.3. Adsorption–desorption experiments SPM adsorbent (0.08 g) and MB solutions (7.0 mg/L) were added in 100 mL conical flask to investigate the adsorptions of MB in batch system at room temperature. The effects of pH (2.5–12.5) and pore size (0–2.0 ␮m) on the adsorption properties were determined. The kinetics experiments and the experiments of reutilization were determined under the condition of pH 3.5. The adsorption isotherm was tested over the concentration range 2–48 mg/L of MB solutions at pH 3.5. The concentration of MB in the solution was analyzed by standard methods. Adsorption capacity and removal efficiency were calculated according to Eqs. (1) and (2) [22]: q=

(C0 − Ce )V W

(1)

E=

(C0 − Ce ) × 100% C0

(2)

where q (mg/g) is adsorption capacity; E (%) is removal efficiency; C0 (mg/L) and Ce (mg/L) are initial and equilibrated MB concentrations, respectively; V (L) is the volume of added solution and W (g) is the mass of the adsorbent (dry). The adsorbed SPM was washed in 50 mL anhydrous ethanol three times for desorbing MB. Then the SPM was dried at 60 ◦ C for 4 h for adsorption in the succeeding cycle. The adsorption–desorption procedure was repeated seven cycles and the recovery ratio was calculated by Eq. (3): Recovery ratio =

mass of regenerated adsorbent (mg) × 100% initial mass of adsorbent (mg) (3)

3. Results and discussion

Fig. 1. XRD patterns of all SPM samples.

Fig. 2 showed SEM pictures at high magnification for each obtained SPM. There were few pores in SPM40, but there were plenty of pores in SPM60, SPM80 and SPM100 with the average pore size from 0.5 ␮m to 2.0 ␮m (detected by SEM images), as listed in Table 1. However, the porous structure disappeared in SPM120 due to the collapse of the exceedingly large pores. The SEM image at lower magnification of SPM60 was shown in Fig. 3. It was clear that SPM60 looked like large spongelike bulks covered with dense pores. The SEM images at low magnification of other porous samples were similar to SPM60. The BET surface areas SBET calculated from nitrogen adsorption isotherm plots were listed in Table 1. A maximum SBET was found to be 29.80 m2 /g for SPM60. The SBET of SPM60, SPM80 and SPM100 were much higher than that of MnZn-ferrite (2.68 m2 /g) reported by Skołyszewska et al. [23]. The large SBET further confirmed the porous structure, which agreed with the results of SEM images. Fig. 4 showed that all the porous SPM samples had good magnetic properties at room temperature. The values of saturation magnetizations Ms (as shown in Table 1) were higher than or close to those of magnetic photocatalysts and adsorbents in other reports. For instance, Ms of TiO2 coated Mn–Zn ferrite was 8.2 emu/g [24] and Ms of Fe3 O4 -chitosan nanoparticles was 21.5 emu/g [25]. The good magnetic properties made SPM susceptible to the external magnetic field, which improved the reuse of the SPM. 3.2. Properties of adsorption 3.2.1. Effect of pH The degree of MB adsorption onto the adsorbent surface was primarily influenced by the surface charge on the adsorbent, so the pH value was an important monitoring parameter in the process of adsorption. The effect of initial pH (2.5–12.5) on the removal of MB was shown in Fig. 5A. The removal efficiencies at equilibrium state were larger than 85% in the wide range of pH 3.5–12.5 and the maximum adsorption occurred at pH 3.5. The effect of pH could be explained by the competition between H+ and cation groups on the MB, because the adsorption of MB was an organic cations

3.1. Characters of SPM The XRD patterns of all SPM samples in Fig. 1 showed that the synthesized ferrites were pure phase spinel MnFe2 O4 . All of the detectable peaks could be identified as characteristic diffraction of spinel MnFe2 O4 standard data, as shown in JCPDS: 38-0430. The crystal structures were tetragonal and the space groups were P42m/nnm.

Table 1 The pore size, saturation magnetizations (Ms ) and BET surface areas (SBET ) of SPM. Samples

SPM40

SPM60

SPM80

SPM100

SPM120

Pore size (␮m) Ms (emu/g) SBET (m2 /g)

– – 22.90

0.5 27.9 29.80

1.0 41.5 27.40

2.0 45.2 26.50

– – 6.03

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Fig. 2. SEM images of all SPM samples at high magnification.

Fig. 3. SEM image of SPM60 at lower magnification.

Fig. 4. Magnetic hysteresis loops of porous SPM at room temperature.

adsorption. Lower adsorption of MB at pH 2.5 was probably due to the presence of excess H+ ions, which competed with the cation groups on the MB for adsorption sites. As the surface charge density was lower at higher pH (pH 3.5–12.5), the electrostatic repulsions

between the positively charged dye (MB) and the surface charge of SPM were lowered. This change resulted in the removal efficiencies at pH 3.5–12.5 were larger than that at pH 2.5. Therefore, it was supposed that coulombic interaction plays an important role

Fig. 5. Effects of pH (A) and pore size of SPM (B) on the removal efficiency of MB.

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Fig. 6. Adsorption kinetics of SPM60 for MB (A), linear plot of log(qe − qt ) vs. t (B) and linear plot of t/qt vs. t (C).

in the adsorption of MB onto SPM. Similar trends were reported in the literature for the adsorption of basic dyes methylene blue onto carbon [26].

The kinetic data were further analyzed using pseudo-secondorder kinetics model [22], expressed as Eq. (5). It was assumed that the adsorption capacity of adsorbent was proportional to the number of active sites on its surface.

3.2.2. Effect of adsorbent’s pore size Fig. 5B showed that the pore size of SPM influenced the removal efficiency of MB. The samples with porous structure presented higher removal efficiencies (>90%) than that of the samples (SPM40, SPM120) without porous structure. This could be explained by the variation of the SBET of SPM. SPM60 with the largest SBET (29.80 m2 /g) showed the highest removal efficiency (98.5%).

1 1 t = + t qt qe k2 q2e

3.2.3. Adsorption kinetics Fig. 6A showed the adsorption kinetics of SPM60 for MB removal. The adsorption capacity increased rapidly in the first 3 h, because the available active sites on the adsorbents were abundant. Then, the speed of increasing slowed down for the gradual decrease of active sites, and the equilibrium time was about 5 h. In order to investigate the mechanism of adsorption kinetics, pseudo-first-order and pseudo-second-order kinetics models were applied to interpret absorption dynamics. The pseudo-first-order kinetic model [27] was given as Eq. (4): log(qe − qt ) = log qe −

 k  1 2.303

t

(5)

where k2 ((g/mg)/min) is the second-order rate constant at the equilibrium. Plotting t/qt against t (min), the values of k2 (slope2 /intercept) and qe (1/slope) could be determined from the intercept and slope of the revealed plot. The linear regression and adsorption kinetic constants were shown in Fig. 6C and Table 2. The correlation coefficient was up to 0.9998 and the theoretical qtheory fit experimental values qe (12.672 mg/g) well. The rate constant k2 determined was 0.00323 (g/mg)/min. These results suggested that the pseudo-second-order adsorption mechanism was predominant for this adsorbent system, and it could be concluded that the overall rate of MB adsorption process was controlled by chemical reaction. The possible interaction between MB and the SPM were illustrated by FT-IR spectrums of SPM60 before and after adsorption, as shown in Fig. 7. The peak at 573 cm−1 (Fe–O, typical of spinel ferrite) inferred the obtainment of SPM. The appearances of three new peaks at 2365 cm−1 , 1261 cm−1 and 800 cm−1 after adsorption could be attributed to the formation of new C O and N O, and the

(4)

where qe (mg/g) and qt are the amounts of MB adsorbed onto adsorbent at equilibrium and any time t (min), respectively, k1 (h−1 ) is the first-order rate constant at equilibrium. Plotting the experiment data in the form of log(qe − qt ) versus t, a straight line would be obtained if the pseudo-first-order kinetic model was a suitable expression. The first-order rate constant (k1 ) could be obtained from the slope of the line. The linear regression and parameters were presented in Fig. 6B and Table 2. The correlation coefficient was as low as 0.9132, indicating that pseudo-first-order kinetics could not be used to describe the adsorption behaviors of MB onto the SPM.

Table 2 Kinetics parameters of the pseudo-order rate equation for MB adsorption on SPM60. Experimental

qe (mg/g) −1

12.672

Pseudo-first-order rate equation

k1 (h ) qtheory (mg/g) R12 S.D.

0.405 60.146 0.9132 0.1755

Pseudo-second-order rate equation

k2 ((g/mg)/min) qtheory (mg/g) R22 S.D.

0.00323 12.945 0.9998 0.3887

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Table 3 RL values in Langmuir isotherm of SPM60. C0 (mg/L)

RL (×10−2 )

2.0 4.0 6.0 10.0 12.0 24.0 48.0

8.056 4.187 2.838 1.722 1.144 0.725 0.364

We could assume that all the binding sites on the sorbent were free sites and ready to accept the adsorbate from solution. The affinity between MB and SPM could be predicted using Langmuir dimensionless separation factor RL given by the relation as Eq. (7) [22]. Fig. 7. FT-IR spectrums of SPM60 before and after adsorption.

RL =

1 1 + KL C0

(7)

deformation of H–O, respectively. Besides, the other peaks shifted or strengthened after reaction. These changes indicated the variations of chemical bonds, which predicted that –N+ (CH3 )2 of MB cations and Fe–O bond of SPM were responsible for the electrostatic attraction onto adsorbent [28–34].

where C0 (mg/L) is the initial MB concentration. Benhammou et al. [35] had shown that RL indicated the shape of the isotherm, 0 < RL < 1 represented the favorable adsorption and RL > 1 represented unfavorable adsorption. The values of RL in our test were all 0 < RL < 1 (listed in Table 3) indicated a favorable adsorption.

3.2.4. Adsorption isotherm Langmuir adsorption isotherm was used to describe the adsorption of MB onto SPM60 at room temperature in this work. Langmuir adsorption isotherm assumed monolayer coverage of the adsorption surface [22]. It was expressed as Eq. (6):

3.2.5. Reuse of SPM Seven adsorption–desorption cycles were repeated to detect the reuse of SPM. SPM could be separated easily and thoroughly from solution by applying an external magnet with a recovery ratio of about 98% (shown in Fig. 9A). As shown in Fig. 9B, SPM kept high adsorption efficiencies (>96%) during all the seven adsorption–desorption cycles. The removal

qe =

qm KL Ce 1 + KL Ce

(6)

where qe (mg/g) is the amount of MB adsorbed per unit weight of adsorbent at equilibrium, Ce (mg/L) is the equilibrium concentration of MB in solution, qm (mg/g) is the maximum adsorption capacity of the adsorbent, KL (L/mg) is the affinity constant. The values of qm and KL could be obtained from the intercept and slope of the line of Ce /qe against Ce . Fig. 8 showed the results of linear fitting in Langmuir isotherm for SPM60. The correlation coefficient (R2 ) was 0.9998, which indicated that the Langmuir adsorption model applied successfully in this affinity adsorbent system. The values of qm and KL were 20.670 mg/g and 5.7064 L/mg. The max adsorption capacity qm was higher than that obtained from pseudosecond-order kinetics model (12.672 mg/g) due to the lower MB initial concentration in the kinetics experiment.

Fig. 8. Linear isotherm plot of Langmuir isotherm for SPM60.

Fig. 9. The separation of SPM60 from solution by a magnet (A) and removal efficiencies of MB during adsorption–desorption cycles for SPM60 (B).

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Fig. 10. SEM pictures of SPM60 for the first cycle (A) and the fourth cycle (B).

efficiencies reduced from 98% to 96% in the first three cycles. However, the removal efficiencies kept about 99.5% in the last three cycles. This might interpreted as that the large spongelike bulks broke into small ones, leading more active sites were exposed during the repeated cycles (illustrated in Fig. 10). Therefore, it could be concluded that the SPM could be reused several times keeping significantly high adsorption capacities. 4. Conclusions SPM was synthesized by a sol–gel route with egg white. The SPM had good magnetic properties and their pore size could be affected by the dosage of egg white. In addition, the obtained SPM showed good absorption properties to MB and kept high adsorption efficiencies for several cycles. The pseudosecond-order model described adsorption kinetics well. And FT-IR analysis suggested that –N+ (CH3 )2 of MB cations and Fe–O bond of SPM were responsible for the good adsorption. The adsorption equilibrium data fitted Langmuir isotherm equation well with a maximum adsorption capacity of 20.67 mg/g and KL was 5.7064 L/mg at room temperature. The results suggested that the as-prepared SPM was a kind of efficient and convenient reusable adsorbents. Acknowledgments We appreciate the financial support of Research Fund for the Doctoral Program of Higher Education of China (No. 20070217060), the Natural Science Foundation of Heilongjiang Province (B2007-8), and The Harbin City Scientific and Technological Innovation Fund of China (Grant No. 2008RFQXG034). References [1] W.J. Huang, G.C. Fang, C.C. Wang, A nanometer-ZnO catalyst to enhance the ozonation of 2,4,6-trichlorophenol in water, Colloids Surf. A. 260 (2005) 45–51. [2] R. Rosal, A. Rodríguez, J. Antonio, P. Melón, A. Petre, E. García-Calvo, Oxidation of dissolved organic matter in the effluent of a sewage treatment plant using ozone combined with hydrogen peroxide (O3 /H2 O2 ), Chem. Eng. J. 149 (2009) 311–318. [3] Z.Y. Wang, C. Chen, F.Q. Wu, B. Zou, M. Zhao, J.X. Wang, C.H. Feng, Photodegradation of rhodamine B under visible light by bimetal codoped TiO2 nanocrystals, J. Hazard. Mater. 164 (2009) 615–620. [4] L. Chia-Hung, W. Jeng-Shiou, C. Hsin-Chieh, S. Shing-Yi, H.C. Khim, Removal of anionic reactive dyes from water using anion exchange membranes as adsorbers, Water Res. 41 (2007) 1491–1500. [5] P.K. Krishna, M.S. Venkata, S. Sridhar, B.R. Pati, P.N. Sarma, Laccase-membrane reactors for decolorization of an acid azo dye in aqueous phase: process optimization, Water Res. 43 (2009) 3647–3658. [6] Y. Zhou, Y. Gan, E.J. Wanless, G.J. Jameson, G.V. Franks, Interaction forces between silica surfaces in aqueous solutions of cationic polymeric flocculants: effect of polymer charge, Langmuir 24 (2008) 10920–10928.

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