Dalton Transactions

Dalton Transactions PAPER

Cite this: Dalton Trans., 2013, 42, 14710

Synthesis of porous Fe3O4 hollow microspheres/ graphene oxide composite for Cr(VI) removal Mancheng Liu,a Tao Wen,a,b Xilin Wu,b Changlun Chen,*a Jun Hu,a Jie Lia and Xiangke Wanga A composite of porous Fe3O4 hollow microspheres/graphene oxide (Fe3O4/GO) has been fabricated through a facile self-assembly approach. Driven by the mutual electrostatic interactions, the amine-functionalized Fe3O4 microspheres prepared by a hydrothermal method and then modified by 3-aminopropyltrimethoxysilane were decorated with negatively-charged GO sheets. The Fe3O4 microspheres were hollow with porous surfaces and the surfaces were successfully modified with the amine, which was confirmed by Fourier transform infrared spectroscopy. The specific saturation magnetization of Fe3O4/ GO was 37.8 emu g−1. The sorption performance of Fe3O4/GO for Cr(VI) was evaluated. The maximum

Received 11th April 2013, Accepted 10th May 2013

sorption capacity for Cr(VI) on Fe3O4/GO was 32.33 mg g−1, which was much higher than that of Fe3O4 microspheres. The GO sheets could not only prevent agglomeration of the Fe3O4 microspheres and enable a good dispersion of these oxide microspheres, but also substantially enhance the specific surface

DOI: 10.1039/c3dt50955a

area of the composite. The Fe3O4/GO composite may be a promising sorption material for the separation

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and preconcentration of heavy metal ions from aqueous solutions in environmental pollution cleanup.

1.

Introduction

Large amounts of Cr-contaminated wastewater are discharged into the environment due to its wide application in rapid industrialized processes, such as electronics, electroplating, wood preservation and leather tanning.1 Cr(VI) has a high mobility and carcinogenic and mutagenic effects, posing a serious threat to human health.2 According to the World Health Organization, it has limited 5 μg L−1 as the maximum allowable emission standard of Cr(VI). With better awareness of the health problems, various technologies are currently available to remove Cr from aqueous solutions, such as cyanide treatment,3 coprecipitation,4 reverse osmosis,5,6 ion exchange7 and sorption.8–12 Compared to the highly toxic intermediates and other toxic organochlorines of cyanide treatment,13 additional processes for the further treatment of the precipitate sludge involves high operational costs and the limited pH range of the reverse osmosis technique,6 and limited ion exchange.7,14 A sorption technique using various sorbents is an alternative favorable and feasible approach because of its high efficiency and low cost. In addition, sorption can

a Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, P.R. China. E-mail: [email protected]; Fax: +86-551-65591310; Tel: +86-551-65593308 b College of Nuclear Science and Technology, University of Science and Technology of China, Hefei, 230000, P.R. China

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effectively remove heavy metal ions in the wastewater at low concentrations.15 Different from traditional materials, graphene, a twodimensional carbon nanostructure with one-atom thickness, has attracted considerable attention in many fields.16 Owing to its large theoretical specific surface area (∼2630 m2 g−1), graphene or graphene oxide (GO) has been proven as an effective sorbent for the removal of heavy metal ions17 and organic pollutants.18 However, it is difficult to separate it from aqueous solutions because of its small particle size, causing serious health and environmental problems once it is discharged into the environment.19,20 The centrifugation method needs a very high rate and the traditional filtration method may cause blockages of the filters. Compared with traditional centrifugation and filtration methods, the magnetic separation method is considered as a rapid and effective technique for separating nano-materials from aqueous solutions.21 Hence, magnetite/graphene composites with large specific surface areas (enhancing the removal of water pollutants) and magnetic separation (facilitated by the recycling of the composites) have begun to be used in the field of environmental treatment.22–24 For example, Bhunia et al.25 designed a heterogeneous matrix of iron/iron oxide dispersed on reduced graphene oxide (RGO), which showed the highest specific surface area among the RGO–iron oxide and RGO–iron composites. The resultant porous RGO–Fe(0)/Fe3O4 material exhibited highest efficiency to adsorb heavy metal ions (such as Cr(VI), Hg(II), Pb(II), Cd(II), and As(III)) and was employed for catalytic

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Dalton Transactions oxidation reactions. Sun et al.22 reported a simple one step solvothermal method for producing magnetite/RGO composites as a sorbent for dye pollutants. The solvothermal method not only effectively lowers the oxygen content and defect level in graphene sheets,26,27 but also obtains high-quality superparamagnetic Fe3O4 nanoparticles. Herein, a composite of Fe3O4 hollow microspheres/graphene oxide (Fe3O4/GO) was fabricated by mutual electrostatic interactions between positively charged Fe3O4 hollow microspheres and negatively charged GO sheets and was characterized by scanning and transmission electron microscopy (SEM and TEM), Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements. The application of Fe3O4/GO for the removal of Cr(VI) from aqueous solutions was investigated by a batch experiment.

2.

Experimental details

2.1

Materials

All reagents were of analytical reagent grade and used as received. Milli-Q water was used in all the experiments. Graphite powder (20 μm, 99.95% purity) was purchased from Qingdao Tianhe Graphite Co. Ltd, China. 2.2

Preparation of GO

GO was prepared from graphite by a modified Hummers method.18,28 In brief, a certain amount of graphite, NaNO3 and H2SO4 were mixed and stirred, and then KMnO4 was gradually added. The stirring was continued for 2 h in an icewater bath and continually stirred for 5 days at room temperature. Then, Milli-Q water was added and the solution was stirred for 30 min. After the temperature was reduced to 60 °C, H2O2 (30%) was added dropwise. For purification, the mixture was washed by rinsing and centrifugation with 5% H2SO4 and then Milli-Q water several times. After filtration and drying under vacuum, the dark brown GO was obtained. 2.3

Preparation of the Fe3O4 hollow microspheres

Fe3O4 hollow microspheres were synthesized by a hydrothermal method.29 In a typical synthesis process, 5 mmol of ferric chloride hexahydrate (FeCl3·6H2O) was dissolved in 40 mL of ethylene glycol with magnetic stirring to obtain a clear solution. Then, a certain amount of urea was added into the above solution. The mixture was stirred vigorously for 30 min until it became homogeneous and was then transferred into a 50 mL Teflon-lined stainless steel autoclave at 200 °C for 12 h. After they was cooled to room temperature naturally, the precipitated products were centrifuged and washed with Milli-Q water and ethanol several times and then dried at 60 °C in a vacuum oven overnight. 2.4

Electrostatic assembly of Fe3O4/GO

The fabrication and the magnetic sorption process of Fe3O4/ GO are shown in Scheme 1. The Fe3O4 hollow microspheres


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Scheme 1 Synthesis route of the Fe3O4/GO composite and the magnetic sorption process for Cr(VI) ions.

were then modified with 3-aminopropyltrimethoxysilane to render them with NH2 groups.30,31 Afterward, the NH2–Fe3O4 hollow microspheres were connected with negatively-charged GO sheets. Typically, 50 mg of the Fe3O4 hollow microspheres was added to a freshly prepared solution of 1% APTS in Milli-Q water and the mixture was ultrasonicated for 5 min before it was stirred for 18 h at 60 °C. After the reaction, the modified Fe3O4 microspheres were centrifuged and washed with Milli-Q water several times and then dried in a vacuum oven. The Fe3O4/GO composite was fabricated via mutual electrostatic interactions between GO and the positively charged Fe3O4 microspheres. A 0.6 mg mL−1 GO dispersion was added into 3 mg mL−1 of the modified Fe3O4 under mild magnetic stirring. The mixture was stirred at room temperature for 10 h and then the Fe3O4/GO composite was obtained after centrifugation, washed with Milli-Q water and finally dried in a vacuum at 30 °C. In the synthetic procedure of the composite materials, the weight ratio of Fe3O4 to GO was 5 : 1. 2.5

Characterization

Field emission SEM (FESEM) images were obtained with a JEOL JSM-6330F microscope. TEM and high-resolution TEM (HRTEM) were performed on a JEOL-2010 microscope. The XRD patterns were measured on a (Philips X’Pert Pro Super X-ray) diffractometer with a Cu–Kα source (λ = 1.541 Å). FTIR spectroscopy measurements were mounted using a PerkinElmer 100 spectrometer over a range from 400 to 4000 cm−1. The XPS measurements were conducted with a VG Scientific ESCALAB Mark II system. Raman spectra were recorded with a Renishew Raman spectrometer. Magnetic measurements were performed in a MPMS-XL SQUID magnetometer with an applied magnetic field of 20 kOe. 2.6

Batch sorption experiment

Analytical-grade K2Cr2O7 was employed to prepare a Cr(VI) stock solution. All the sorption experiments were carried out according to the batch technique in a series of 10 mL polyethylene test tubes. For Cr(VI) sorption, stock suspensions of Fe3O4/ GO, KNO3 and Cr(VI) were added into the test tubes and the difference in the total volume of the mixtures was compensated by Milli-Q water to achieve the desired concentrations of

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the different components. The pH values of the suspensions were adjusted by adding negligible volumes of 0.01 mol L−1 HNO3 or KOH solutions. After the suspensions were shaken for 24 h, the solid phase was separated from the solution using a conventional magnet. The concentrations of Cr(VI) in the filtrate were determined by atomic absorption spectroscopy. The amount of Cr(VI) sorbed onto Fe3O4/GO was calculated from the equation: qe ¼

ðC0 Ce Þ V m

ð1Þ

where C0 is the initial concentration (mg L−1), Ce is the equilibrium concentration (mg L−1), qe is the amount of Cr(VI) sorbed onto Fe3O4/GO at the equilibrium time (mg g−1), V is the solution volume (L) and m is the mass of the sorbent (g).

3.

Results and discussion

3.1

Characterization of Fe3O4/GO

The morphologies of the Fe3O4 microspheres and Fe3O4/GO were observed by SEM and TEM (Fig. 1). Fig. 1A shows that the Fe3O4 microspheres are monodisperse and uniform on a large scale and have a spherical morphology and hollow cavity inside, with diameters from 300 to 400 nm. Observed from the inset of Fig. 1A, it is clear that the surface of the Fe3O4 microspheres is relatively rough and uneven and has some broken holes. The image of Fe3O4/GO (shown in Fig. 1B) synthesized via electrostatic interactions between the NH2-modified Fe3O4 microspheres and GO sheets indicates that the GO sheets are tightly bound onto the Fe3O4 microspheres. Such submicronlevel Fe3O4 spheres greatly help in preventing the restacking of the GO sheets, avoiding the loss of a highly active surface area.32 Furthermore, due to the good exfoliation of graphite, the obtained GO layers are apparently transparent even under

Fig. 1 SEM image of the Fe3O4 hollow microspheres (A), the inset is the HRSEM image of the Fe3O4 hollow microspheres, SEM image of Fe3O4/GO (B), TEM images of Fe3O4/GO (C), and (D), the inset is the HRTEM image taken on the marked part.

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SEM observation. The TEM image of Fe3O4/GO is shown in Fig. 1C, in which the hollow and surface-broken nature of the Fe3O4 microspheres is clearly revealed and the shell thickness is approximately 40 nm. In addition, to prevent the aggregation of the Fe3O4 microspheres, the GO sheets are partly tiled around the surface of the Fe3O4 microspheres. The HRTEM image (Fig. 1D) confirms that the extremely thin and flexible GO sheets are firmly attached to the Fe3O4 microspheres. The inset shows the lattice fringes of Fe3O4 in the boxed area, where the interplane distance of 2.51 Å corresponds to the (311) plane of the Fe3O4 structure. The XRD pattern was obtained and analyzed for the assynthesized Fe3O4/GO. The main peaks in Fig. 2A at 2θ = 30.21° (220), 35.71° (311), 43.31° (400), 53.7° (422), 57.35° (511) and 62.72° (440) show the characteristics of iron oxides in Fe3O4/GO.33 The XRD pattern is in good agreement with the standard profile of a cubic magnetite structure (JCPDS 65-3107). Compared with the pure GO diffracted signals, there are no diffracted signals of the GO sheets in Fe3O4/GO, which is consistent with previous reports.23 There may be two reasons why no carbon peaks are observed. First, the Fe3O4 microspheres reduce the aggregation of the GO sheets, which results in more fewer-layered GO and leads to the observation of weaker peaks from carbon. Secondly, the strong signals of the iron oxides tend to overwhelm the weak carbon peaks. Fig. 2B shows the Raman spectra of GO and Fe3O4/GO. The Raman spectrum of GO demonstrates two sharp bands at 1360 and 1596 cm−1 that are assigned to the D band and G band, respectively. Usually, the D band corresponds to the disorder of GO originating from defects associated with vacancies, grain boundaries and amorphous carbon species, whereas the G band is ascribed to the first-order scattering of the E2g mode observed for sp2 carbon domains.34 The intensity ratio of the D to G band (ID/IG) is generally accepted to reflect the graphitization degree of carbonaceous materials and the defect density.35 After the electrostatic interactions with the NH2– Fe3O4 hollow microspheres, the Raman spectrum of Fe3O4/GO shows typical bands of Fe3O4, the characteristic D and G bands of GO shift to 1344 and 1584 cm−1 and the intensity ratio of the D to G band is remarkably increased, which can be attributed to the significant decrease of the ordered graphite crystal structure of GO after conjugating with NH2–Fe3O4.36 Therefore, we can coarsely estimate the disorder degree in GO by the rule: the higher the D/G value, the more defects exist.37 Furthermore, the appearance of the 2D band (2725 cm−1) and the D + G band (2932 cm−1) also shows a substantial increase in the disorder degree in the GO sheets of the composite. Detailed information about the composition of the asprepared samples is provided by the XPS measurements in Fig. 2C. The sharp peaks in the full scan spectra reveal the presence of carbon, oxygen and iron elements at binding energies of about 285, 530 and 711 eV in Fe3O4/GO. In the Fe2p spectrum (the inset of Fig. 2C), the peaks of Fe2p3/2 and Fe2p1/2 are located at 711.29 and 724.82 eV, which are indicative of the formation of a Fe3O4 phase in Fe3O4/GO, and not at


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Fig. 2 XRD patterns of GO and Fe3O4/GO (A), Raman spectra of GO and Fe3O4/GO (B), FTIR spectra of GO and Fe3O4/GO (C), XPS wide spectrum of Fe3O4/GO, and the inset is the Fe2p spectrum of Fe3O4/GO (D), XPS spectrum of C1s of Fe3O4/GO (E), and the magnetization curve of Fe3O4/GO, the bottom inset is the magnetic separation of Fe3O4/GO (F).

710.35 and 724.0 eV, which are for γ-Fe2O3.38 In Fig. 2D, the high resolution C1s peak of Fe3O4/GO can be deconvoluted into three components: (1) a characteristic peak of nonoxygenated C (284.6 eV); (2) surface-adsorbed hydrocarbons and their oxidative forms (e.g., C–OH and epoxide) (286.2 eV); (3) the carbon element in association with the oxygen in the carbonate ions (288.6 eV).39 The functionalization of GO with Fe3O4 hollow microspheres was further confirmed by the FTIR spectra presented in Fig. 2E. In the spectrum of GO, the peaks at 1726, 1618 and 1047 cm−1 correspond to stretching of the CvO bond of carboxyl groups, the skeletal CvC vibrations of unoxidized graphene domains and the vibrations of alkoxy C–O, respectively. After the electrostatic reaction, the FTIR spectrum of Fe3O4/GO differs from that of GO, as evidenced by the new peak at 594 cm−1, which can be related to the vibration of Fe–O functional groups, and other two new characteristic peaks of the amide carbonyl group for Fe3O4/GO at 1629 cm−1 (–CONH amide band I) and 1454 cm−1 (C–N stretch of amide) also appear, implying that the Fe3O4 hollow microspheres are linked to the GO surface by covalent bonding.40,41


Fig. 2F depicts the magnetization hysteresis loop of Fe3O4/ GO at room temperature. The saturation magnetization of the obtained adsorbent is about 37.8 emu g−1, which is strong enough to ensure convenient magnetic separation. Additionally, as seen in the inset, Fe3O4/GO is conveniently attracted to the wall of vessel. This experimental result of simple magnetic separation confirms that Fe3O4/GO is magnetic and can be used as a magnetic sorbent to enrich pollutants adsorbed from large volumes of aqueous solutions. 3.2

Different effects on Cr(VI) sorption to Fe3O4/GO

Fig. 3A shows the sorption of Cr(VI) on Fe3O4/GO as a function of contact time. The Cr(VI) sorption occurs in two steps: an initial fast step which lasts for 2 h followed by the slower sorption process, which continued until the equilibrium is reached within 5 h. A further increase in the contact time does not show an increase in sorption. The result that the sorption is quick to achieve equilibrium is important for the application of Fe3O4/GO to remove metal ions from aqueous solutions in real applications. 24 h is selected to ascertain the adsorption equilibrium in the following experiments.

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Fig. 3 (A) Effect of pH and ionic strength on Cr(VI) sorption on Fe3O4/GO, CCr(VI)initial = 10.0 mg L−1, m/V = 0.2 g L−1, and T = 293 K. (B) Time-dependence of the Cr(VI) sorption on Fe3O4/GO at pH = 4.5 ± 0.1, CCr(VI) = 10.0 mg L−1, m/V = 0.2 g L−1, I = 0.01 M KNO3 and T = 293 K, the inset is the pseudo-secondorder sorption kinetics plot of Cr(VI) on Fe3O4/GO; (C) Effect of FA on Cr(VI) sorption onto Fe3O4/GO, CCr(VI) initial = 10.0 mg L−1, CFA initial = 10.0 mg L−1, m/V = 0.2 g L−1, I = 0.01 M KNO3 and T = 293 K. (D) Effect of the sorbent content on Cr(VI) sorption on Fe3O4/GO at pH = 4.5 ± 0.1, CCr(VI)initial = 10.0 mg L−1, I = 0.01 M KNO3 and T = 293 K.

The data of the Cr(VI) sorption on Fe3O4/GO was simulated with the pseudo-first-order and pseudo-second-order models in Fig. 3A. The pseudo-first-order equation is expressed as follows:42 qt ¼ qe ð1 ek1 t Þ

ð2Þ

The pseudo-second-order equation is given as eqn (3).43 qt ¼

qe 2 k 2 t 1 þ qe k 2 t

ð3Þ

where qe (mg g−1) is the equilibrium sorption capacity, qt (mg g−1) is the amount of Cr(VI) adsorbed on Fe3O4/GO at time t (h), K1 (g mg−1 h−1) is the pseudo-first-order rate constant of sorption and K2 (g mg−1 h−1) is the pseudo-second-order rate constant of sorption. The parameters of the two models are listed in Table 1. The correlation coefficient of the pseudo-second-order kinetic model for the linear plots is much higher than that of the pseudo-first-order model, due to the close agreement between the experimental qe (mg g−1) values and the estimated qe (mg g−1) values from the pseudo-second-order kinetic model.

Table 1 Comparison between the pseudo-first-order and pseudo-secondorder kinetic models for Cr(VI) sorption onto Fe3O4/GO

Pseudo-first-order model

Pseudo-second-order model

qe (mg g−1)

K1 (g mg−1 h−1)

R2

qe (mg g−1)

K2 (g mg−1 h−1)

R2

27.86

1.20

0.952

27.28

0.15

0.999

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These facts suggests that the kinetic sorption is very well described by the pseudo-second-order equation. Fig. 3B shows the Cr(VI) sorption on Fe3O4/GO as a function of pH in different KNO3 solutions. Maximum Cr(VI) sorption is observed at pH 2–3 and then the sorption starts to decrease dramatically as the initial solution pH increases, in agreement with the earlier reports.44 The pH dependence of Cr(VI) sorption can be related to the type and ionic state of the functional groups present on the sorbent material and the species of Cr(VI) in solution. At low pH, the predominant Cr(VI) species is HCrO4− but as the pH increases, this form shifts to Cr2O72−. The high sorption efficiency at low pH can also be attributed to the fact that the surface of Fe3O4/GO is surrounded by high quantities of hydronium ions (H+), which will promote the approach of negatively charged HCrO4− or Cr2O72−. The more positive the surface charge of Fe3O4/GO, the faster the sorption rate of Cr(VI) in the solution, since the binding of anionic Cr(VI) ion species is enhanced through the electrostatic attraction. An increase in the pH in the solution will make the surface negatively charged, greatly weakening the electrostatic attraction between Fe3O4/GO and the negatively charged Cr(VI) anions, thus reducing the sorption efficiency. Moreover, the sorption efficiency became less significant at higher pH values due to the competition between OH− ions and negatively charged Cr(VI).45 Ionic strength, besides pH, is also an important factor that influences sorption. Fig. 3B shows the sorption percentage of Cr(VI) on Fe3O4/GO as a function of pH (2–10) in 0.001, 0.01 and 0.1 mol L−1 KNO3 solutions. It is clear that Cr(VI) sorption on Fe3O4/GO is strongly dependent on the ionic strength. This may be explained by the fact that the ionic strength can influence the interface potential and double layer thickness and thereby, can affect the binding of the sorbed species. These results are in agreement with the sorption of Cr(VI) on α-alumina.46 Fulvic acid (FA) has a strong complexation ability with metal ions and plays an important role in metal ion species in natural water. Therefore, we studied the sorption performance of Cr(VI) on the Fe3O4/GO composite as a function of pH in the presence of organic substances and the results are shown in Fig. 3C. It can be seen that the presence of FA reduces the Cr(VI) sorption below pH 7.5. FA consists of heterogeneous components with a wide range of molecular weights and different chemical moieties. The sorption of lower molecular weight FA fractions with more acidic functional groups onto adsorbents is higher than that of higher molecular weight fractions.47 When Cr(VI) is added to a suspension of Fe3O4/GO–FA, FA, consisting of lower molecular weight FA fractions and more hydrophobic fractions, bound Fe3O4/GO composite (through strong π–π interactions between FA and GO) and occupies the complexation sites of Cr(VI). In addition, FA in the solution with higher molecular weight FA fractions, which have strong complexation with Cr(VI), will lead to the presence of Cr(VI) in solution. At pH > 7.5, the weak effect of FA on the sorption of Cr(VI) on Fe3O4/GO may be due to the fact that the OH− ions make the Fe3O4/GO surfaces more negative, which leads to the slight complexation between Fe3O4/GO and FA.


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The effect of the Fe3O4/GO content on the Cr(VI) sorption from an aqueous solution is shown in Fig. 3D. It can be seen clearly that the sorption of Cr(VI) increases rapidly with increasing Fe3O4/GO content. This tendency may be explained by the fact that the number of functional groups on the Fe3O4/ GO surfaces increase with the increasing solid content and hence, more surface active sites are available to form complexes with Cr(VI) at the surface of Fe3O4/GO. Fig. 3D shows the distribution coefficient (Kd) of Cr(VI) on the Fe3O4/GO composite decreases gradually with the increase of the solid content, which is similar to Cu(II) removal by GO nanosheets decorated with Fe3O4 nanoparticles.25 The decrease of the Kd value is ascribed to the fact that the surface active sites are entirely exposed for sorption and the surface is saturated faster at a low adsorbent content. However, at a higher sorbent content, the higher energy sites of Fe3O4/GO are occupied by a larger fraction of lower energy sites. At the same time, a decrease in the total surface area and an increase in the diffusion path length caused by the probability of collision between the solid particles and therefore creating particle aggregation, contribute to the decrease in the distribution coefficient of Cr(VI). 3.3

Cr(VI) sorption isotherms and thermodynamic analysis

The sorption isotherms of Cr(VI) onto Fe3O4/GO at T = 293, 313 and 333 K are shown in Fig. 4A. The sorption isotherm is the highest at T = 333 K and is the lowest at T = 293 K, indicating that the process of Cr(VI) sorption onto Fe3O4/GO is favored at high temperature. To gain a better understanding of the sorption mechanisms and to quantify the sorption data, the Langmuir and Freundlich models were used to simulate the experimental data. The Langmuir isotherm model can be expressed by the following equation: Cs ¼

bCs max Ce 1 þ bCe

ð4Þ

where Cs max (mg g−1), the maximum sorption capacity, is the amount of Cr(VI) at complete monolayer coverage and b (L mg−1) is the constant that relates to the heat of sorption. The Freundlich isotherm model can be represented by the following equation: Cs ¼ K F Ce n

ð5Þ

Table 2 The parameters for the Langmuir and Freundlich models at different temperatures

Langmuir

Freundlich

T (K)

Cs max (mg g−1)

b (L mg−1)

R2

kF (mol1−n Ln g−1)

n

R2

293 313 333

32.33 47.62 54.32

0.77 0.49 0.54

0.994 0.993 0.992

11.81 15.70 21.95

0.38 0.41 0.37

0.961 0.963 0.943

where KF (mol1−n Ln g−1) represents the sorption capacity when the metal ion equilibrium concentration equals 1 and n represents the degree of dependence of the sorption with the equilibrium concentration. The parameters of the Langmuir and Freundlich models calculated from the sorption isotherms are listed in Table 2. The high value of the correlation coefficients indicates that the Langmuir model fits the experimental data better than the Freundlich model. The maximum sorption capacities (Cs max) of Cr(VI) on Fe3O4/GO at room temperatures (293 K) is 32.33 mg g−1, which is much higher than that of Fe3O4 (shown in Fig. 4B). This is maybe ascribed to their different specific surface areas. The specific surface area of Fe3O4/GO (43.56 m2 g−1) is much higher than that of Fe3O4 (18.84 m2 g−1). In addition, the sorption of Cr(VI) on the GO sheets also enhances the Cs max of Cr(VI). A comparison of the sorption capacity of Fe3O4/GO for the removal of Cr(VI) with those of other sorbents is shown in Table 3. Although a direct comparison of Fe3O4/GO with other sorbents is difficult due to the different experimental conditions applied, it can be seen that Fe3O4/GO has the highest adsorption capacity. Moreover, the convenience of magnetic separation makes Fe3O4/GO an attractive sorbent for the disposal of Cr(VI)-contaminated wastewater. The thermodynamic parameters (ΔG0, ΔS0, and ΔH0) for Cr(VI) sorption on Fe3O4/GO can be calculated from the temperature dependent sorption isotherms. The free energy change (ΔG0) is derived from the relationship: ΔG 0 ¼ RT ln K d

ð6Þ −1

−1

where R is the universal gas constant (8.314 J mol K ), T is the temperature in Kelvin, Kd, the sorption equilibrium constant, can be calculated by plotting ln Kd versus Ce and extrapolating Ce to zero. Table 3 Comparison of the Cr(VI) sorption capacity of Fe3O4/GO with other sorbents

Fig. 4 (A) Sorption isotherms of Cr(VI) on Fe3O4/GO at three different temperatures. (B) Sorption isotherms of Cr(VI) on Fe3O4 and Fe3O4/GO at pH = 4.5 ± 0.1, m/V = 0.2 g L−1, I = 0.01 M KNO3 and T = 293 K.


Sorbents

pH

Cs max (mg g−1)

References

Ice straw Kaolinite Hematite Activated alumina Oxidized MWCNTs Mesoporous-Fe2O3 CTAB modified graphene Fe3O4/GO

2 4 8 4 4.28 2.5 2 4.5

3.15 0.45 2.30 7.44 1.18 15.6 21.57 32.33

44 48 46 49 50 51 52 This work

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Dalton Transactions electrostatic interactions between the positively charged porous Fe3O4 hollow microspheres and the negatively-charged GO sheets. The products are characterized by SEM, TEM, XRD, Raman, XPS, FTIR and VSM, and the magnetic Fe3O4/GO composite has been employed as a sorbent for the removal of Cr(VI) ions from water. The experimental results show that the composite has a higher sorption capacity (32.33 mg g−1). Therefore, the combination of the magnetic properties of the Fe3O4 microspheres and the superior properties of the GO sheets makes a powerful separation material to deal with environmental pollution.

Fig. 5 Linear plot of ln K° vs. 1/T for the sorption of Cr(VI) on Fe3O4/GO at 293, 313 and 333 K. m/V = 0.2 g L−1, pH = 4.5, I = 0.01 M KNO3.

Table 4 GO

Values of the thermodynamic parameters for Cr(VI) sorption on Fe3O4/

T (K)

ΔG0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

ΔH0 (kJ mol−1)

293 323 333

−13.22 −15.42 −18.35

— 128.36 —

24.41 24.77 23.13

The average standard enthalpy change (ΔH0) and standard entropy change (ΔS0) can be calculated from the slope and Y axis intercept of a plot of ln Kd versus 1/T (Fig. 5) using the van’t Hoff equation:53 ln K d ¼

ΔH 0 ΔS þ R RT

ð7Þ

The thermodynamic parameters calculated from the sorption isotherms and different temperatures from eqn (5) and (6) are tabulated in Table 4. The positive values of ΔH0 indicate that the sorption of Cr(VI) ions on Fe3O4/GO is an endothermic process. The explanation to the endothermic process of Cr(VI) sorption is that Cr(VI) ions are well solvated in water and denuded of their hydration sheath, and this dehydration process needs energy. This energy exceeds the exothermicity of Cr(VI) ions attached to the solid surface of Fe3O4/GO. Therefore, the higher the temperature, the more favorable the sorption process. The values of the Gibbs free energy change (ΔG0) become more negative with the increase in temperature, indicating more efficient sorption on the solid surface, which may be due to the fact that Cr(VI) ions are readily desolvated and hence, their sorption becomes more favorable. The positive values of the entropy change reflect the affinity of Fe3O4/GO toward Cr(VI) ions in aqueous solutions and may suggest some structure changes in the sorbents.

4.

Conclusions

In summary, we have developed a simple and effective method to synthesize a magnetic Fe3O4/GO composite by mutual

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Acknowledgements Financial support from the 973 project of MOST (2011CB933700), the National Natural Science Foundation of China (91126020, 21071147, 21107115, 21007074, 21077107 and 21225730), and the General Financial Grant from the China Postdoctoral Science Foundation (NO 2012M511432) are acknowledged.

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