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Adsorption of methylene blue onto humic acid-coated Fe3O4 nanoparticles
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Adsorption of methylene blue onto humic acid-coated Fe3 O4 nanoparticles Xian Zhang a , Panyue Zhang b,∗ , Zhen Wu b , Ling Zhang c,∗ , Guangming Zeng b , Chunjiao Zhou d a
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China Key Laboratory of Environmental Biology and Pollution Control (Ministry of Education), Hunan University, Changsha 410082, China c College of Civil Engineering, Hunan University, Changsha 410082, China d College of Science, Hunan Agricultural University, Changsha 410128, China b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
HA-Fe3 O4 nanoparticles showed a high saturation magnetization of 77 emu/g. HA-Fe3 O4 nanoparticles had a high capacity for methylene blue adsorption. Sorption kinetics could be described by a pseudo-second-order equation. Sorption isotherm agreed well with Langmuir equation. Desorption of HA-Fe3 O4 was conducted with a mixture of methanol and acetic acid.
a r t i c l e
i n f o
Article history: Received 6 September 2012 Received in revised form 5 December 2012 Accepted 21 December 2012 Available online xxx Keywords: Humic acid Fe3 O4 nanoparticles Adsorption Methylene blue Desorption
a b s t r a c t Humic acid-coated Fe3 O4 (HA-Fe3 O4 ) nanoparticles as magnetic adsorbents were prepared with coprecipitation of humic acids and Fe3 O4 nanoparticles. TEM analysis indicated that the average diameter of the spherical HA-Fe3 O4 core was about 15 nm. TGA characterization showed that the HA-Fe3 O4 nanoparticles contained about 50% (w/w) HA. The characteristic absorption of HA at 1604/cm and 1701/cm was observed in the FIRT spectra of HA-Fe3 O4 nanoparticles. The HA-Fe3 O4 nanoparticles exhibited a typical superparamagnetic characteristic with a saturation magnetization of 77 emu/g, which resulted in an easy solid–liquid separation with an external magnet. The HA-Fe3 O4 nanoparticles were applied for methylene blue (MB) adsorption and results showed that the HA-Fe3 O4 nanoparticles possessed much higher adsorbed amount of MB than the bare Fe3 O4 nanoparticles and HA powders. The HA-Fe3 O4 nanoparticles remained stable in a broad pH range of 3–11. The adsorption kinetics can be described by a pseudosecond-order equation, and the time when 50% of the MB was adsorbed (t1/2 ) was 7 min. The adsorption isotherm of the HA-Fe3 O4 nanoparticles agreed well with Langmuir adsorption equation, and the maximum adsorbed amount of MB was 0.291 mmol/g. Desorption of the saturated HA-Fe3 O4 nanoparticles was easily carried out with a mixture of methanol and acetic acid with a volume ratio of 9:1. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Yuelushan, Changsha 410082, China. Tel.: +86 15001255497; fax: +86 731 88823701. E-mail addresses: [email protected] (P. Zhang), [email protected] (L. Zhang).
Removal of organic dyestuffs from wastewater is a challenge in industry wastewater treatment, since most dyes are persistent organic molecules, stable to light, heat and oxidizing agents [1,2]. People have tried to develop different ways for dye wastewater purification for some years. Several methods have been proposed last few years to separate dye pollutants from wastewater, such as adsorption with easily separated adsorbents, which can be regenerated and reused.
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Currently, nanoparticles such as nano-carbon materials and nano-metal oxides have been studied as powerful adsorbents for their large specific surface area and small internal diffusion resistance [3,4]. Nevertheless, there are more and more observations on health and environmental risks of manufactured nanoparticles in water [5]. In order to separate the fine particles in aqueous solution quickly and easily, magnetic materials, usually in the form of magnetite Fe3 O4 or its oxidized product ␥-Fe2 O3 have been developed and utilized [6]. The magnetic particles can adsorb contaminants from aqueous solution and then be separated from the water with a simple magnetic process. The integrated process, which couples magnetic separation together with surface complexation adsorption, ionic exchange and solvent extraction, is usually called magnetically assisted chemical separation (MACS) [7]. Synthesis of the magnetic nanoparticles is a mature technology in material science, but the magnetic nanoparticle application in pollution control is a newly emerged concept. The magnetic nanoparticles can be used in MACS processes, without or with a specific coating by complexing species, or through being entrapped in a polymer matrix [7]. Hu et al. used bare Fe3 O4 /␥-Fe2 O3 nanoparticles for Cr(VI) removal and recovery from wastewater [8,9]. More works have been focused on modification of the Fe3 O4 /␥-Fe2 O3 nanoparticles to provide a better surface specificity. A wide variety of organic, inorganic and biological substances, such as polyacrylic acid [10], thiol [11], cetyltrimethylammonium bromide [12], multiwall carbon nanotube [13], zeolite [14], alginate [15], gum Arabic [16], yeast [17] and chitosan [18] have been used to combine with the magnetic nanoparticles for contaminant removal. Substances like alginate, chitosan, etc. are excellent choice for modifying the magnetic nanoparticles due to their low price, wide sources and environmental friendliness. Humic acid (HA) is a kind of abundant natural organic macromolecules on earth, which usually originated from decomposition of plants and animal residues [19]. The HA shows a high reaction activity due to its unique amorphous structure, which possesses a large polycyclic aromatic hydrocarbons as framework and a lot of carboxyl, phenolic hydroxyl, carbonyl, methoxyl, alcoholic hydroxyl, ether and amino on the framework [20,21]. It is widely accepted that the HA has a predominant affinity with hydrophobic organic compounds (HOCs) [22], however, the separation of the HA from water is troublesome. It is a good tentative idea to combine the HA adsorption to iron oxide and the magnetic separation together for contamination removal. Humic acid-coated Fe3 O4 (HA-Fe3 O4 ) nanoparticles for efficient removal of heavy metals in water was tested by Liu et al., and it is expected that the as-prepared nanoparticles have wide applicability in the removal of heavy metals from various raw waters [23]. Nevertheless, abundantly existing organic pollutants were rarely removed with the MACS process. Moreover, the regeneration of the HA-Fe3 O4 nanoparticles is not involved in previous reports. In this study, a low-cost and easy synthesis of a kind of magnetic adsorbent, HA-Fe3 O4 nanoparticles, was developed. The physical and chemical characterization of the synthesized HA-Fe3 O4 nanoparticles were conducted by modern analytical methods. The adsorption behavior of the dye methylene blue (MB) onto HA-Fe3 O4 nanoparticles was investigated, and the main adsorption mechanisms were discussed. Regeneration and reuse of the saturated adsorbents were tested and a practical method was proposed.
2. Materials and methods 2.1. Synthesis of magnetic nanoparticles The HA powders used are a pure chemical, purchased from Institute of Tianjin Fine Chemicals and used without any further
isolation and purification. The other reagents are all of analytical grade. All water used for synthesis of HA-Fe3 O4 magnetic nanoparticles is ultrapure water. The HA-Fe3 O4 magnetic nanoparticles were synthesized with a co-precipitation method. Firstly, 60 ml aqueous solution with 8.33 g/L HA and 1.00 g/L NaOH was heated to 101 ◦ C under stirring and refluxing, while the solution was purged with argon to remove oxygen. Secondly, 5 ml aqueous solution with 216 g/L FeCl3 ·6H2 O and 112 g/L FeSO4 ·7H2 O was rapidly injected into the hot HA solution. Then the mixture was kept refluxing at 101 ◦ C for 2 h, followed by the cooling to room temperature. Black precipitates were collected with magnetic field, then washed to neutral with ultrapure water and air-dried for use as the magnetic HA-Fe3 O4 nanoparticles. The bare Fe3 O4 nanoparticles were prepared as control in a similar way except that no HA was added by Fe3 O4 precipitation. 2.2. Characterization of HA-Fe3 O4 nanoparticles Transmission electron microscopy (TEM) images were obtained using a JEOL JEM 3010 microscope operated at 200 kV. Thermogravimetric analyses (TGA) were carried out on a NETZSCH STA-409 apparatus with an air flow and a heating rate of 10 ◦ C/min. Fourier transform infrared spectrums (FTIR) were performed on a WQF 410 FTIR spectrometer with KBr pellet and a resolution of 4.1/cm. Magnetic properties were measured using a LAKESHORE VSM-7310 at 27 ◦ C. Hydrodynamic size was detected using a MALVERN Zetasizer 3000HS analyzer. 2.3. Batch MB adsorption experiments Adsorption experiments were carried out at room temperature (about 20 ◦ C) using batch process on a thermostated shaker. Unless described specifically, 2 g/L of the as-prepared HA-Fe3 O4 nanoparticles were added into 100 ml solution with a MB concentration of 1 mmol/L. Glass flasks containing the MB solution and adsorbents were shaken at 180 r/min for 2 h, followed by the phase separation using an external magnet. The residual MB concentration was determined with a SHIMADZU UV-Vis-2550 spectrophotometer at 660 nm. The MB concentration was directly proportional to absorbance at 665 nm, and the correlation coefficient was 0.9999. The adsorbed amount (Qeq ) was expressed in mmol MB per gram HA-Fe3 O4 nanoparticles, as Eq. (1): Qeq =
C0 − Ceq V m
(1)
where C0 (mmol/L) represents the initial MB concentration, Ceq (mmol/L) is the equilibrium MB concentration in solution after adsorption, V (L) is the volume of the aqueous solution and m (g) is the mass of HA-Fe3 O4 nanoparticles. Comparison experiments of the MB adsorption onto HA-Fe3 O4 nanoparticles, HA powders and bare Fe3 O4 nanoparticles were conducted. The MB adsorption onto the bare Fe3 O4 nanoparticles and HA powders was carried out in the same way as onto the HAFe3 O4 nanoparticles, in which the HA powders were directly added into the solution without any pretreatment. The adsorbents of 4 g/L were added into the MB solution with a MB concentration of 0.2 mmol/L at pH 6.4 and pH 8.4. After adsorption, the bare Fe3 O4 nanoparticles were separated with an external magnet, while the HA powders were separated with centrifugation. To evaluate the influence of the pH on the MB adsorption, the pH of the MB solutions were firstly adjusted using 0.1 mmol/L diluted HCl or NaOH solution before addition of the as-prepared HA-Fe3 O4 nanoparticles. The final pH of the solution after adsorption and solid–liquid separation was measured and there were no significant differences were found compared with the initial pH of MB solution.
Please cite this article in press as: X. Zhang, et al., Adsorption of methylene blue onto humic acid-coated Fe3 O4 nanoparticles, Colloids Surf. A: Physicochem. Eng. Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2012.12.056
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A kinetic experiment was performed at an initial MB concentration of 1 mmol/L and an initial pH of 8.4 by measuring the adsorption at different time intervals. One adsorption isotherm was measured in the same way at an initial pH of 8.4 using initial MB concentrations varying from 0.2 to 5 mmol/L. 2.4. Regeneration and reuse experiments Regeneration of the adsorbents was conducted with a mixture of methanol and acetic acid with a volume ratio of 9:1 [24]. The saturated HA-Fe3 O4 nanoparticles were added into the mixture of methanol and acetic acid with a ratio of 5 g/L, and the HA-Fe3 O4 nanoparticles were generated with a magnetic stirrer for 10 min. After regeneration, the HA-Fe3 O4 nanoparticles were separated with an external magnet, and the MB concentration in liquid was measured with a SHIMADZU UV-Vis-2550 spectrophotometer at 660 nm to estimate the desorbed amount of MB. The same step was repeated for three times until the MB concentration in the liquid was lower than 0.002 mmol/L. Then the nanoparticles were washed with ultrapure water and air-dried. The adsorption/desorption of the MB onto/from the HA-Fe3 O4 nanoparticles were assessed in eight consecutive cycles. 3. Results and discussion 3.1. Characterization of as-synthesized HA-Fe3 O4 nanoparticles TEM analysis showed that the roughly spherical HA-Fe3 O4 nanoparticles were with an average diameter of about 15 nm. A high resolution TEM image revealed the satisfactory crystallinity of the HA-Fe3 O4 nanoparticles with an interplanar spacing of about 0.48 nm [25], which corresponded to the (1 1 1) lattice plane of the cubic Fe3 O4 (0.483 nm). Laser particle size analysis showed a relatively uniform distribution of the HA-Fe3 O4 nanoparticles with an average hydrodynamic size of 122 nm, showing that the HAFe3 O4 nanoparticles tended to aggregate in aqueous solution due to the small size effect. TGA measurement is shown in Fig. 1a. When the temperature increased to 105 ◦ C, the water would be firstly evaporated; then the HA will be carbonated and incinerated with further increasing the temperature until about 600 ◦ C; the inorganic materials would be left. The bare Fe3 O4 nanoparticles contained about 6% water besides the inorganic Fe3 O4 . The HA powders contained about 10% water and about 2% ash besides the organic HA. The HA-Fe3 O4 nanoparticles contained about 5% water and about 45% inorganic Fe3 O4 , so the loss on ignition was about 50%, which mainly originated from HA. Illés and Tombácz testified that the natural HA coated on the magnetic nanoparticles, the colloidal stability of magnetite nanoparticles in aqueous medium was enhanced [26]. FTIR spectra (Fig. 1b) of the HA-Fe3 O4 nanoparticles showed the characteristic absorption of the HA at 1604/cm and 1701/cm, which were attributed to C O bond stretching vibration in carboxylic salt and free carboxylic acid, respectively; the peak at 586/cm resulted from the characteristic absorption of Fe O bond stretching vibration. The HA-Fe3 O4 nanoparticles exhibited a typical superparamagnetic characteristic with a saturation magnetization of 77 emu/g from magnetic hysteresis loops, which led to an easy solid–liquid separation with an external magnet.
Fig. 1. TGA curves (a) and FTIR spectra (b) of magnetic HA-Fe3 O4 nanoparticles.
3.2. Comparison of MB adsorption onto HA-Fe3 O4 nanoparticles, HA powders and bare Fe3 O4 nanoparticles Comparison of the MB adsorption onto HA-Fe3 O4 nanoparticles, HA powders and bare Fe3 O4 nanoparticles is shown in Fig. 2. The HA-Fe3 O4 nanoparticles had much higher adsorbed amount of MB than the HA powders and bare Fe3 O4 nanoparticles at the given conditions. The bare Fe3 O4 nanoparticles showed a quite weak
Fig. 2. Adsorbed amount of HA-Fe3 O4 nanoparticles compared with bare Fe3 O4 nanoparticles and HA powders.
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Fig. 4. Influence of contact time on MB adsorption onto HA-Fe3 O4 nanoparticles under neutral condition. Fig. 3. Influence of initial pH on MB adsorption onto HA-Fe3 O4 nanoparticles.
adsorbed amount of MB. The difference of the adsorbed amount between the bare Fe3 O4 nanoparticles and HA-Fe3 O4 nanoparticles illustrated that the HA coated on the surface of the HA-Fe3 O4 nanoparticles and provided the main activated sites for the MB adsorption. Liu et al. reported that O and N content in the HAFe3 O4 nanoparticles was significantly higher than that in the raw HA materials [23], and the small size fractions of HA enriched in polar functional moieties were found to adsorb on Fe3 O4 surface preferentially [26,27]. So, the change in surface groups and selective adsorption of HA during the co-precipitation of HA and Fe3 O4 may be the main reasons why the observed adsorbed amount of MB onto the HA-Fe3 O4 nanoparticles was much higher than that onto the HA powders. 3.3. Influence of solution pH on MB adsorption onto HA-Fe3 O4 nanoparticles According to Fig. 3, it can be seen that the adsorbed amount of MB increased with increasing the pH of MB solution. The weaker MB adsorption under lower solution pH may be explained by a protonation of the acidic functional groups like carboxylate of HA [28,29] and competition adsorption between the MB and hydrogen ions. It is generally recognized that the HA is more hydrophilic under a basic condition. Hydrophilicity of the HA may play a positive role for dye adsorption, because water penetrating into the HA structure may create new adsorption sites [30]. Moreover, the HA is usually negative charged and the zeta potential of the HA decreased with increasing the solution pH [31]. The negatively charged HA can better react with cationic MB trough electrostatic attraction, and the electrostatic attraction increases with pH increasing. Overall, electrostatic interaction and complex adsorption may be the main mechanisms of the MB adsorption onto HA-Fe3 O4 nanoparticles. Previous reports indicated that the decomposition of magnetic particles might occur under a strong acidic environment, and the Fe3 O4 nanoparticles were oxidized and quickly lost their magnetism or even dissolved when pH was below 4.0 [17]. Nevertheless, no significant influence of acidic environment on decomposition of HA-Fe3 O4 was observed in this study, because the hydrophobicity of the firmly enlaced HA might protect the Fe3 O4 core under acidic condition [32]. On the other hand, it was observed that the HA slightly escaped from the Fe3 O4 core at a pH of 11 and even were completely solubilized when the pH was over 12. Illés and Tombácz also found that an increase in pH results in decreasing adsorption of HA on Fe3 O4 nanoparticles [33]. The hydrophilicity of
the HA was enhanced under basic condition [32], and Zhang and Bai believed that molecular structure or composition of the polymers may change at the solid/solution interface under extreme solution pH [34]. For all that, the as-prepared HA-Fe3 O4 displayed stable structure in a broad pH range of 3–11. 3.4. Adsorption kinetics Adsorption kinetics of the MB onto HA-Fe3 O4 nanoparticles is presented in Fig. 4. A rapid adsorption of the MB occurred within the first minutes, and then followed by a slow adsorption until the adsorbed MB reached the equilibrium value. The pseudo-second-order equation, in which the chemical reaction rate is proportional to the square of reactant’s concentration, is often successfully used to describe the kinetics of the pollutant adsorption, as Eq. (2) [28]: dQt = K(Qeq − Qt )2 dt
(2)
where Qt (mmol/g) is the amount of dye adsorbed at a certain time t (min), K (g/mmol min) is the second-order rate constant. A well fitted kinetics curve is shown in Fig. 4, and the correlation coefficient R2 was 0.9992 (Table 1). The Eq. (2) was integrated and linearized with the boundary conditions t = 0 (Qt = 0) to teq (Qt = Qeq ), as Eq. (3): t 1 1 = t+ 2 Qt Qeq KQeq
(3)
A linear relationship with high correlation coefficients is observed between t/Qt and t indicating the applicability of the pseudo-second-order model to describe the adsorption process. It can be seen from Table 1 that R2 was 0.9992 and the calculated equilibrium sorption capacity Qeq, calc of the HA-Fe3 O4 nanoparticles was 0.253 mmol/g, showing a good agreement with the experimental value of Qeq, exp (0.247 mmol/g). t1/2 corresponded to the time when 50% of the MB was adsorbed, and was calculated from the linearized equation at the condition of Qt = 0.5 Qeq , and the T1/2 of the HA-Fe3 O4 nanoparticles was 7.0 min. The high adsorption rate Table 1 Adsorption kinetic parameters of MB onto HA-Fe3 O4 nanoparticles by a pseudosecond-order equation (t1/2 corresponded to Qt = 0.5 Qeq ). Qeq , exp (mmol/L)
Qeq , calc (mmol/L)
K (g/mmol min)
R2
t1/2 (min)
0.247
0.253
0.564
0.9992
7.0
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Table 2 Adsorption isotherm parameters of MB onto HA-Fe3 O4 nanoparticles by Langmuir equation and Freundlich equation. Langmuir Qeq =
KL Qmax Ceq 1+KL Ceq
n Freundlich Qeq = KF Ceq
Qmax (mmol/g)
KL
R2
n
KF
R2
0.291
9.89
0.9987
0.1982
0.2366
0.9776
of the MB onto HA-Fe3 O4 nanoparticles may result from not only the fast electrostatic and/or complexation reaction between the MB and HA-Fe3 O4 nanoparticles, but also the low diffusion resistance of MB onto the nano-scale HA-Fe3 O4 particles. 3.5. Adsorption isotherms In order to quantitatively describe the adsorption of MB on to HA-Fe3 O4 nanoparticles, Langmuir and Freundlich adsorption equations were both fitted to the adsorption results. The Langmuir and Freundlich equations were expressed in Table 2 and their linear form can be represented as Eq. (4) and Eq. (5): Ceq 1 1 = Ceq + Qeq Qmax Qmax KL
(4)
log Qeq = log KF + n log Ceq
(5)
where Qmax (mmol/g) is defined as the maximum capacity of adsorbent, KL and KF (L/mg) is the Langmuir and Freundlich adsorption constant respectively, and n (dimensionless) is the Freundlich exponential coefficient. The experimental data and the fitted isotherms of MB adsorption onto HA-Fe3 O4 nanoparticles were represented in Fig. 5. Isotherms parameters for MB adsorption were calculated from the slope and intercept of the Eq. (4) and Eq. (5) (shown in Table 2). It is found that the correlation coefficients are satisfactory (R2 > 0.96) for both equations, and the data are better fitted by Langmuir model. For analyzing the adsorption isotherms, a wide MB concentration from 0.2 to 5 mmol/L was tested, and the Freundlich adsorption equation became deviated because of the high MB concentration. The asprepared HA-Fe3 O4 nanoparticles showed a maximum adsorption amount of 0.291 mmol/g, which show a great potential of HA-Fe3 O4 nanoparticle application in MB pollution control compared with the other adsorbents [35].
Fig. 6. Desorption and reuse of HA-Fe3 O4 nanoparticles.
3.6. Regeneration and reuse Reuse of the adsorbents is important due to economic and resource reasons. Thermal desorption, pH modification and eluting with different kinds of organic solution (ethanol, methanol, acetic acid and a mixture of methanol and acetic acid with a volume ratio of 9:1) were examined in this study. The regeneration ability with the mixture of methanol and acetic acid was found optimum. The saturated HA-Fe3 O4 nanoparticles were. From Fig. 6, it can be seen that the adsorbed amount of MB onto the as-prepared HAFe3 O4 nanoparticles was 0.267 mmol/g, and the adsorbed amount of MB onto the regenerated HA-Fe3 O4 nanoparticles, which were regenerated with the mixture of methanol and acetic acid, hardly decreased even after the 8th recycling. The methanol, acetic acid and MB can be recovered through distillation. In addition, it was observed that the equilibrium time of adsorption reduced after the regeneration, because the HA-Fe3 O4 nanoparticles might be highly dispersed through marinated in methanol–acetic acid solution. This also indicates that the treatment of the as-prepared particles before the adsorption is carried out is important. 4. Conclusion
Fig. 5. Adsorption isotherms of MB onto HA-Fe3 O4 nanoparticles under neutral condition.
The as-prepared HA-Fe3 O4 nanoparticles exhibited a typical superparamagnetic characteristic and high reactive activity for MB adsorption. The adsorption kinetics can be described by pseudo-second-order equation. The t1/2 corresponded to the time when 50% of MB was adsorbed and was 7 min. The adsorption isotherm of the HA-Fe3 O4 nanoparticles agreed well with Langmuir sorption equation, and the maximum adsorption amount at the given conditions was 0.291 mmol/g. Furthermore, the HAFe3 O4 nanoparticles showed a high capacity for recycling. The adsorbed MB was well desorbed after elution with a mixture of methanol and acetic acid, and the adsorbed amount of the regenerated HA-Fe3 O4 nanoparticles hardly decreased even after the 8th recycling.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Adsorption of methylene blue onto humic acid-coated Fe3 O4 nanoparticles Xian Zhang a , Panyue Zhang b,∗ , Zhen Wu b , Ling Zhang c,∗ , Guangming Zeng b , Chunjiao Zhou d a
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China Key Laboratory of Environmental Biology and Pollution Control (Ministry of Education), Hunan University, Changsha 410082, China c College of Civil Engineering, Hunan University, Changsha 410082, China d College of Science, Hunan Agricultural University, Changsha 410128, China b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
HA-Fe3 O4 nanoparticles showed a high saturation magnetization of 77 emu/g. HA-Fe3 O4 nanoparticles had a high capacity for methylene blue adsorption. Sorption kinetics could be described by a pseudo-second-order equation. Sorption isotherm agreed well with Langmuir equation. Desorption of HA-Fe3 O4 was conducted with a mixture of methanol and acetic acid.
a r t i c l e
i n f o
Article history: Received 6 September 2012 Received in revised form 5 December 2012 Accepted 21 December 2012 Available online xxx Keywords: Humic acid Fe3 O4 nanoparticles Adsorption Methylene blue Desorption
a b s t r a c t Humic acid-coated Fe3 O4 (HA-Fe3 O4 ) nanoparticles as magnetic adsorbents were prepared with coprecipitation of humic acids and Fe3 O4 nanoparticles. TEM analysis indicated that the average diameter of the spherical HA-Fe3 O4 core was about 15 nm. TGA characterization showed that the HA-Fe3 O4 nanoparticles contained about 50% (w/w) HA. The characteristic absorption of HA at 1604/cm and 1701/cm was observed in the FIRT spectra of HA-Fe3 O4 nanoparticles. The HA-Fe3 O4 nanoparticles exhibited a typical superparamagnetic characteristic with a saturation magnetization of 77 emu/g, which resulted in an easy solid–liquid separation with an external magnet. The HA-Fe3 O4 nanoparticles were applied for methylene blue (MB) adsorption and results showed that the HA-Fe3 O4 nanoparticles possessed much higher adsorbed amount of MB than the bare Fe3 O4 nanoparticles and HA powders. The HA-Fe3 O4 nanoparticles remained stable in a broad pH range of 3–11. The adsorption kinetics can be described by a pseudosecond-order equation, and the time when 50% of the MB was adsorbed (t1/2 ) was 7 min. The adsorption isotherm of the HA-Fe3 O4 nanoparticles agreed well with Langmuir adsorption equation, and the maximum adsorbed amount of MB was 0.291 mmol/g. Desorption of the saturated HA-Fe3 O4 nanoparticles was easily carried out with a mixture of methanol and acetic acid with a volume ratio of 9:1. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Yuelushan, Changsha 410082, China. Tel.: +86 15001255497; fax: +86 731 88823701. E-mail addresses: [email protected] (P. Zhang), [email protected] (L. Zhang).
Removal of organic dyestuffs from wastewater is a challenge in industry wastewater treatment, since most dyes are persistent organic molecules, stable to light, heat and oxidizing agents [1,2]. People have tried to develop different ways for dye wastewater purification for some years. Several methods have been proposed last few years to separate dye pollutants from wastewater, such as adsorption with easily separated adsorbents, which can be regenerated and reused.
0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.12.056
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Currently, nanoparticles such as nano-carbon materials and nano-metal oxides have been studied as powerful adsorbents for their large specific surface area and small internal diffusion resistance [3,4]. Nevertheless, there are more and more observations on health and environmental risks of manufactured nanoparticles in water [5]. In order to separate the fine particles in aqueous solution quickly and easily, magnetic materials, usually in the form of magnetite Fe3 O4 or its oxidized product ␥-Fe2 O3 have been developed and utilized [6]. The magnetic particles can adsorb contaminants from aqueous solution and then be separated from the water with a simple magnetic process. The integrated process, which couples magnetic separation together with surface complexation adsorption, ionic exchange and solvent extraction, is usually called magnetically assisted chemical separation (MACS) [7]. Synthesis of the magnetic nanoparticles is a mature technology in material science, but the magnetic nanoparticle application in pollution control is a newly emerged concept. The magnetic nanoparticles can be used in MACS processes, without or with a specific coating by complexing species, or through being entrapped in a polymer matrix [7]. Hu et al. used bare Fe3 O4 /␥-Fe2 O3 nanoparticles for Cr(VI) removal and recovery from wastewater [8,9]. More works have been focused on modification of the Fe3 O4 /␥-Fe2 O3 nanoparticles to provide a better surface specificity. A wide variety of organic, inorganic and biological substances, such as polyacrylic acid [10], thiol [11], cetyltrimethylammonium bromide [12], multiwall carbon nanotube [13], zeolite [14], alginate [15], gum Arabic [16], yeast [17] and chitosan [18] have been used to combine with the magnetic nanoparticles for contaminant removal. Substances like alginate, chitosan, etc. are excellent choice for modifying the magnetic nanoparticles due to their low price, wide sources and environmental friendliness. Humic acid (HA) is a kind of abundant natural organic macromolecules on earth, which usually originated from decomposition of plants and animal residues [19]. The HA shows a high reaction activity due to its unique amorphous structure, which possesses a large polycyclic aromatic hydrocarbons as framework and a lot of carboxyl, phenolic hydroxyl, carbonyl, methoxyl, alcoholic hydroxyl, ether and amino on the framework [20,21]. It is widely accepted that the HA has a predominant affinity with hydrophobic organic compounds (HOCs) [22], however, the separation of the HA from water is troublesome. It is a good tentative idea to combine the HA adsorption to iron oxide and the magnetic separation together for contamination removal. Humic acid-coated Fe3 O4 (HA-Fe3 O4 ) nanoparticles for efficient removal of heavy metals in water was tested by Liu et al., and it is expected that the as-prepared nanoparticles have wide applicability in the removal of heavy metals from various raw waters [23]. Nevertheless, abundantly existing organic pollutants were rarely removed with the MACS process. Moreover, the regeneration of the HA-Fe3 O4 nanoparticles is not involved in previous reports. In this study, a low-cost and easy synthesis of a kind of magnetic adsorbent, HA-Fe3 O4 nanoparticles, was developed. The physical and chemical characterization of the synthesized HA-Fe3 O4 nanoparticles were conducted by modern analytical methods. The adsorption behavior of the dye methylene blue (MB) onto HA-Fe3 O4 nanoparticles was investigated, and the main adsorption mechanisms were discussed. Regeneration and reuse of the saturated adsorbents were tested and a practical method was proposed.
2. Materials and methods 2.1. Synthesis of magnetic nanoparticles The HA powders used are a pure chemical, purchased from Institute of Tianjin Fine Chemicals and used without any further
isolation and purification. The other reagents are all of analytical grade. All water used for synthesis of HA-Fe3 O4 magnetic nanoparticles is ultrapure water. The HA-Fe3 O4 magnetic nanoparticles were synthesized with a co-precipitation method. Firstly, 60 ml aqueous solution with 8.33 g/L HA and 1.00 g/L NaOH was heated to 101 ◦ C under stirring and refluxing, while the solution was purged with argon to remove oxygen. Secondly, 5 ml aqueous solution with 216 g/L FeCl3 ·6H2 O and 112 g/L FeSO4 ·7H2 O was rapidly injected into the hot HA solution. Then the mixture was kept refluxing at 101 ◦ C for 2 h, followed by the cooling to room temperature. Black precipitates were collected with magnetic field, then washed to neutral with ultrapure water and air-dried for use as the magnetic HA-Fe3 O4 nanoparticles. The bare Fe3 O4 nanoparticles were prepared as control in a similar way except that no HA was added by Fe3 O4 precipitation. 2.2. Characterization of HA-Fe3 O4 nanoparticles Transmission electron microscopy (TEM) images were obtained using a JEOL JEM 3010 microscope operated at 200 kV. Thermogravimetric analyses (TGA) were carried out on a NETZSCH STA-409 apparatus with an air flow and a heating rate of 10 ◦ C/min. Fourier transform infrared spectrums (FTIR) were performed on a WQF 410 FTIR spectrometer with KBr pellet and a resolution of 4.1/cm. Magnetic properties were measured using a LAKESHORE VSM-7310 at 27 ◦ C. Hydrodynamic size was detected using a MALVERN Zetasizer 3000HS analyzer. 2.3. Batch MB adsorption experiments Adsorption experiments were carried out at room temperature (about 20 ◦ C) using batch process on a thermostated shaker. Unless described specifically, 2 g/L of the as-prepared HA-Fe3 O4 nanoparticles were added into 100 ml solution with a MB concentration of 1 mmol/L. Glass flasks containing the MB solution and adsorbents were shaken at 180 r/min for 2 h, followed by the phase separation using an external magnet. The residual MB concentration was determined with a SHIMADZU UV-Vis-2550 spectrophotometer at 660 nm. The MB concentration was directly proportional to absorbance at 665 nm, and the correlation coefficient was 0.9999. The adsorbed amount (Qeq ) was expressed in mmol MB per gram HA-Fe3 O4 nanoparticles, as Eq. (1): Qeq =
C0 − Ceq V m
(1)
where C0 (mmol/L) represents the initial MB concentration, Ceq (mmol/L) is the equilibrium MB concentration in solution after adsorption, V (L) is the volume of the aqueous solution and m (g) is the mass of HA-Fe3 O4 nanoparticles. Comparison experiments of the MB adsorption onto HA-Fe3 O4 nanoparticles, HA powders and bare Fe3 O4 nanoparticles were conducted. The MB adsorption onto the bare Fe3 O4 nanoparticles and HA powders was carried out in the same way as onto the HAFe3 O4 nanoparticles, in which the HA powders were directly added into the solution without any pretreatment. The adsorbents of 4 g/L were added into the MB solution with a MB concentration of 0.2 mmol/L at pH 6.4 and pH 8.4. After adsorption, the bare Fe3 O4 nanoparticles were separated with an external magnet, while the HA powders were separated with centrifugation. To evaluate the influence of the pH on the MB adsorption, the pH of the MB solutions were firstly adjusted using 0.1 mmol/L diluted HCl or NaOH solution before addition of the as-prepared HA-Fe3 O4 nanoparticles. The final pH of the solution after adsorption and solid–liquid separation was measured and there were no significant differences were found compared with the initial pH of MB solution.
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A kinetic experiment was performed at an initial MB concentration of 1 mmol/L and an initial pH of 8.4 by measuring the adsorption at different time intervals. One adsorption isotherm was measured in the same way at an initial pH of 8.4 using initial MB concentrations varying from 0.2 to 5 mmol/L. 2.4. Regeneration and reuse experiments Regeneration of the adsorbents was conducted with a mixture of methanol and acetic acid with a volume ratio of 9:1 [24]. The saturated HA-Fe3 O4 nanoparticles were added into the mixture of methanol and acetic acid with a ratio of 5 g/L, and the HA-Fe3 O4 nanoparticles were generated with a magnetic stirrer for 10 min. After regeneration, the HA-Fe3 O4 nanoparticles were separated with an external magnet, and the MB concentration in liquid was measured with a SHIMADZU UV-Vis-2550 spectrophotometer at 660 nm to estimate the desorbed amount of MB. The same step was repeated for three times until the MB concentration in the liquid was lower than 0.002 mmol/L. Then the nanoparticles were washed with ultrapure water and air-dried. The adsorption/desorption of the MB onto/from the HA-Fe3 O4 nanoparticles were assessed in eight consecutive cycles. 3. Results and discussion 3.1. Characterization of as-synthesized HA-Fe3 O4 nanoparticles TEM analysis showed that the roughly spherical HA-Fe3 O4 nanoparticles were with an average diameter of about 15 nm. A high resolution TEM image revealed the satisfactory crystallinity of the HA-Fe3 O4 nanoparticles with an interplanar spacing of about 0.48 nm [25], which corresponded to the (1 1 1) lattice plane of the cubic Fe3 O4 (0.483 nm). Laser particle size analysis showed a relatively uniform distribution of the HA-Fe3 O4 nanoparticles with an average hydrodynamic size of 122 nm, showing that the HAFe3 O4 nanoparticles tended to aggregate in aqueous solution due to the small size effect. TGA measurement is shown in Fig. 1a. When the temperature increased to 105 ◦ C, the water would be firstly evaporated; then the HA will be carbonated and incinerated with further increasing the temperature until about 600 ◦ C; the inorganic materials would be left. The bare Fe3 O4 nanoparticles contained about 6% water besides the inorganic Fe3 O4 . The HA powders contained about 10% water and about 2% ash besides the organic HA. The HA-Fe3 O4 nanoparticles contained about 5% water and about 45% inorganic Fe3 O4 , so the loss on ignition was about 50%, which mainly originated from HA. Illés and Tombácz testified that the natural HA coated on the magnetic nanoparticles, the colloidal stability of magnetite nanoparticles in aqueous medium was enhanced [26]. FTIR spectra (Fig. 1b) of the HA-Fe3 O4 nanoparticles showed the characteristic absorption of the HA at 1604/cm and 1701/cm, which were attributed to C O bond stretching vibration in carboxylic salt and free carboxylic acid, respectively; the peak at 586/cm resulted from the characteristic absorption of Fe O bond stretching vibration. The HA-Fe3 O4 nanoparticles exhibited a typical superparamagnetic characteristic with a saturation magnetization of 77 emu/g from magnetic hysteresis loops, which led to an easy solid–liquid separation with an external magnet.
Fig. 1. TGA curves (a) and FTIR spectra (b) of magnetic HA-Fe3 O4 nanoparticles.
3.2. Comparison of MB adsorption onto HA-Fe3 O4 nanoparticles, HA powders and bare Fe3 O4 nanoparticles Comparison of the MB adsorption onto HA-Fe3 O4 nanoparticles, HA powders and bare Fe3 O4 nanoparticles is shown in Fig. 2. The HA-Fe3 O4 nanoparticles had much higher adsorbed amount of MB than the HA powders and bare Fe3 O4 nanoparticles at the given conditions. The bare Fe3 O4 nanoparticles showed a quite weak
Fig. 2. Adsorbed amount of HA-Fe3 O4 nanoparticles compared with bare Fe3 O4 nanoparticles and HA powders.
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Fig. 4. Influence of contact time on MB adsorption onto HA-Fe3 O4 nanoparticles under neutral condition. Fig. 3. Influence of initial pH on MB adsorption onto HA-Fe3 O4 nanoparticles.
adsorbed amount of MB. The difference of the adsorbed amount between the bare Fe3 O4 nanoparticles and HA-Fe3 O4 nanoparticles illustrated that the HA coated on the surface of the HA-Fe3 O4 nanoparticles and provided the main activated sites for the MB adsorption. Liu et al. reported that O and N content in the HAFe3 O4 nanoparticles was significantly higher than that in the raw HA materials [23], and the small size fractions of HA enriched in polar functional moieties were found to adsorb on Fe3 O4 surface preferentially [26,27]. So, the change in surface groups and selective adsorption of HA during the co-precipitation of HA and Fe3 O4 may be the main reasons why the observed adsorbed amount of MB onto the HA-Fe3 O4 nanoparticles was much higher than that onto the HA powders. 3.3. Influence of solution pH on MB adsorption onto HA-Fe3 O4 nanoparticles According to Fig. 3, it can be seen that the adsorbed amount of MB increased with increasing the pH of MB solution. The weaker MB adsorption under lower solution pH may be explained by a protonation of the acidic functional groups like carboxylate of HA [28,29] and competition adsorption between the MB and hydrogen ions. It is generally recognized that the HA is more hydrophilic under a basic condition. Hydrophilicity of the HA may play a positive role for dye adsorption, because water penetrating into the HA structure may create new adsorption sites [30]. Moreover, the HA is usually negative charged and the zeta potential of the HA decreased with increasing the solution pH [31]. The negatively charged HA can better react with cationic MB trough electrostatic attraction, and the electrostatic attraction increases with pH increasing. Overall, electrostatic interaction and complex adsorption may be the main mechanisms of the MB adsorption onto HA-Fe3 O4 nanoparticles. Previous reports indicated that the decomposition of magnetic particles might occur under a strong acidic environment, and the Fe3 O4 nanoparticles were oxidized and quickly lost their magnetism or even dissolved when pH was below 4.0 [17]. Nevertheless, no significant influence of acidic environment on decomposition of HA-Fe3 O4 was observed in this study, because the hydrophobicity of the firmly enlaced HA might protect the Fe3 O4 core under acidic condition [32]. On the other hand, it was observed that the HA slightly escaped from the Fe3 O4 core at a pH of 11 and even were completely solubilized when the pH was over 12. Illés and Tombácz also found that an increase in pH results in decreasing adsorption of HA on Fe3 O4 nanoparticles [33]. The hydrophilicity of
the HA was enhanced under basic condition [32], and Zhang and Bai believed that molecular structure or composition of the polymers may change at the solid/solution interface under extreme solution pH [34]. For all that, the as-prepared HA-Fe3 O4 displayed stable structure in a broad pH range of 3–11. 3.4. Adsorption kinetics Adsorption kinetics of the MB onto HA-Fe3 O4 nanoparticles is presented in Fig. 4. A rapid adsorption of the MB occurred within the first minutes, and then followed by a slow adsorption until the adsorbed MB reached the equilibrium value. The pseudo-second-order equation, in which the chemical reaction rate is proportional to the square of reactant’s concentration, is often successfully used to describe the kinetics of the pollutant adsorption, as Eq. (2) [28]: dQt = K(Qeq − Qt )2 dt
(2)
where Qt (mmol/g) is the amount of dye adsorbed at a certain time t (min), K (g/mmol min) is the second-order rate constant. A well fitted kinetics curve is shown in Fig. 4, and the correlation coefficient R2 was 0.9992 (Table 1). The Eq. (2) was integrated and linearized with the boundary conditions t = 0 (Qt = 0) to teq (Qt = Qeq ), as Eq. (3): t 1 1 = t+ 2 Qt Qeq KQeq
(3)
A linear relationship with high correlation coefficients is observed between t/Qt and t indicating the applicability of the pseudo-second-order model to describe the adsorption process. It can be seen from Table 1 that R2 was 0.9992 and the calculated equilibrium sorption capacity Qeq, calc of the HA-Fe3 O4 nanoparticles was 0.253 mmol/g, showing a good agreement with the experimental value of Qeq, exp (0.247 mmol/g). t1/2 corresponded to the time when 50% of the MB was adsorbed, and was calculated from the linearized equation at the condition of Qt = 0.5 Qeq , and the T1/2 of the HA-Fe3 O4 nanoparticles was 7.0 min. The high adsorption rate Table 1 Adsorption kinetic parameters of MB onto HA-Fe3 O4 nanoparticles by a pseudosecond-order equation (t1/2 corresponded to Qt = 0.5 Qeq ). Qeq , exp (mmol/L)
Qeq , calc (mmol/L)
K (g/mmol min)
R2
t1/2 (min)
0.247
0.253
0.564
0.9992
7.0
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Table 2 Adsorption isotherm parameters of MB onto HA-Fe3 O4 nanoparticles by Langmuir equation and Freundlich equation. Langmuir Qeq =
KL Qmax Ceq 1+KL Ceq
n Freundlich Qeq = KF Ceq
Qmax (mmol/g)
KL
R2
n
KF
R2
0.291
9.89
0.9987
0.1982
0.2366
0.9776
of the MB onto HA-Fe3 O4 nanoparticles may result from not only the fast electrostatic and/or complexation reaction between the MB and HA-Fe3 O4 nanoparticles, but also the low diffusion resistance of MB onto the nano-scale HA-Fe3 O4 particles. 3.5. Adsorption isotherms In order to quantitatively describe the adsorption of MB on to HA-Fe3 O4 nanoparticles, Langmuir and Freundlich adsorption equations were both fitted to the adsorption results. The Langmuir and Freundlich equations were expressed in Table 2 and their linear form can be represented as Eq. (4) and Eq. (5): Ceq 1 1 = Ceq + Qeq Qmax Qmax KL
(4)
log Qeq = log KF + n log Ceq
(5)
where Qmax (mmol/g) is defined as the maximum capacity of adsorbent, KL and KF (L/mg) is the Langmuir and Freundlich adsorption constant respectively, and n (dimensionless) is the Freundlich exponential coefficient. The experimental data and the fitted isotherms of MB adsorption onto HA-Fe3 O4 nanoparticles were represented in Fig. 5. Isotherms parameters for MB adsorption were calculated from the slope and intercept of the Eq. (4) and Eq. (5) (shown in Table 2). It is found that the correlation coefficients are satisfactory (R2 > 0.96) for both equations, and the data are better fitted by Langmuir model. For analyzing the adsorption isotherms, a wide MB concentration from 0.2 to 5 mmol/L was tested, and the Freundlich adsorption equation became deviated because of the high MB concentration. The asprepared HA-Fe3 O4 nanoparticles showed a maximum adsorption amount of 0.291 mmol/g, which show a great potential of HA-Fe3 O4 nanoparticle application in MB pollution control compared with the other adsorbents [35].
Fig. 6. Desorption and reuse of HA-Fe3 O4 nanoparticles.
3.6. Regeneration and reuse Reuse of the adsorbents is important due to economic and resource reasons. Thermal desorption, pH modification and eluting with different kinds of organic solution (ethanol, methanol, acetic acid and a mixture of methanol and acetic acid with a volume ratio of 9:1) were examined in this study. The regeneration ability with the mixture of methanol and acetic acid was found optimum. The saturated HA-Fe3 O4 nanoparticles were. From Fig. 6, it can be seen that the adsorbed amount of MB onto the as-prepared HAFe3 O4 nanoparticles was 0.267 mmol/g, and the adsorbed amount of MB onto the regenerated HA-Fe3 O4 nanoparticles, which were regenerated with the mixture of methanol and acetic acid, hardly decreased even after the 8th recycling. The methanol, acetic acid and MB can be recovered through distillation. In addition, it was observed that the equilibrium time of adsorption reduced after the regeneration, because the HA-Fe3 O4 nanoparticles might be highly dispersed through marinated in methanol–acetic acid solution. This also indicates that the treatment of the as-prepared particles before the adsorption is carried out is important. 4. Conclusion
Fig. 5. Adsorption isotherms of MB onto HA-Fe3 O4 nanoparticles under neutral condition.
The as-prepared HA-Fe3 O4 nanoparticles exhibited a typical superparamagnetic characteristic and high reactive activity for MB adsorption. The adsorption kinetics can be described by pseudo-second-order equation. The t1/2 corresponded to the time when 50% of MB was adsorbed and was 7 min. The adsorption isotherm of the HA-Fe3 O4 nanoparticles agreed well with Langmuir sorption equation, and the maximum adsorption amount at the given conditions was 0.291 mmol/g. Furthermore, the HAFe3 O4 nanoparticles showed a high capacity for recycling. The adsorbed MB was well desorbed after elution with a mixture of methanol and acetic acid, and the adsorbed amount of the regenerated HA-Fe3 O4 nanoparticles hardly decreased even after the 8th recycling.
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Please cite this article in press as: X. Zhang, et al., Adsorption of methylene blue onto humic acid-coated Fe3 O4 nanoparticles, Colloids Surf. A: Physicochem. Eng. Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2012.12.056