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Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal
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Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal Xiaojun Sun∗, Fangni Chen, Jinzhi Wei, Fengming Zhang, Suyan Pang Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China
a r t i c l e
i n f o
Article history: Received 17 September 2015 Revised 12 March 2016 Accepted 24 May 2016 Available online xxx Keywords: Graphene oxide Triethylene tetramine CoFe2 O4 Adsorption Cr (VI)
a b s t r a c t A novel magnetic composite of CoFe2 O4 -Triethylene tetramine-Graphene oxide (CoFe2 O4 -TETA-GO) was firstly synthesized by hot reflux and hydrothermal method in this work, making separation of the adsorbents easy in the foreign magnetic field. And they were utilized to remove Cr (VI) from aqueous solution effectively. The prepared CoFe2 O4 -TETA-GO was characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Fourier Transforms Infrared Spectroscopy (FTIR), X-Ray Diffractions (XRD) and X-ray Photoelectron Spectrum (XPS). The characterization results showed that CoFe2 O4 TETA-GO were modified by magnetic nanoparticles CoFe2 O4 and TETA, existing the excellent separation characteristic. The influence of factors such as initial pH value in solution and contact time on the adsorption performance of Cr (VI) onto CoFe2 O4 -TETA-GO was studied. The adsorption kinetics fit pseudosecond-order model and the equilibrium adsorption were well described with Langmuir model. The saturated adsorption capacity of Cr (VI) was about 180.12 mg/g on the CoFe2 O4 -TETA-GO at pH = 2. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Toxic heavy metal ions widely exist in many modern industries, such as leather tanning, metal polishing, electroplating and textiles etc. [1,2]. Chromium (Cr), common heavy metal, has two oxidation states predominantly in natural environment, trivalent form Cr (III) and hexavalent form Cr (VI) [3]. It is necessary to remove Cr (VI) from wastewater because Cr (VI) is more toxic than Cr (III) [4,5]. The various removal approach of Cr (VI) from wastewater have been obtained such as adsorption [6], filtration [7], ion exchange [8], and chemical precipitation [9–11] etc. Among those methods, adsorption has been considered to be superior to others due to its high-efficiency, safety, economies and simplicity [12]. Graphene oxide (GO), a member of the carbonaceous nanomaterial family, is a two-dimensional wrinkle structure with one-atom-thick carbon sheet, presenting abundant O-containing functional groups on the edges and surfaces [13,14]. However, Cr (VI) exists in anionic group in solution, which cannot be adsorbed effectively by GO due to the electrostatic repulsion. Besides, GO cannot be easily separated from wastewater due to strong hydrophilicity [15]. Thus, it is necessary to develop a new material with positive charge to remove Cr (VI) [16].
∗
Corresponding author. Tel.: +86 451 8639 2701. E-mail address: [email protected] (X. Sun).
In order to solve the above problems, Triethylene tetramine (TETA) grafted to the surface of GO can provide a large amount of amino groups as active adsorption sites, which could enhance the adsorption ability for Cr (VI) [17]. GO could be grafted by the nucleophilic substitution reaction of the amine groups of TETA with the epoxide groups of GO [18]. Besides, the dispersion of CoFe2 O4 magnetic nanoparticles dispersed on GO will make the adsorbents separated easily from the wastewater with an external magnetic field [19]. CoFe2 O4 has stable chemical property as magnetic particles, which is difficult to be oxidized in solution [20]. In the paper, CoFe2 O4 -TETA-GO is prepared successfully and then is characterized by SEM, TEM, FTIR, XRD and XPS. The influence of factors, such as pH value and contact time on adsorption efficiency of Cr (VI) is investigated. Besides, the adsorption behavior of Cr (VI) is also studied, and the performance of CoFe2 O4 TETA-GO after regeneration is evaluated. Finally, the adsorption mechanism of Cr (VI) on CoFe2 O4 -TETA-GO is discussed.
2. Experiment 2.1. Materials Graphite, TETA and K2 Cr2 O7 are purchased from Sinopharm Chemical Reagent Co. Co (NO3 )2 ·6H2 O and Fe (NO3 )3 ·9H2 O are purchased from Qingdao Yage Chemical Reagent Co. All reagents used
http://dx.doi.org/10.1016/j.jtice.2016.05.040 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Sun et al., Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.05.040
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Fig. 1. SEM images of GO (a) and CoFe2 O4 -TETA-GO (b); TEM images of GO (c) and CoFe2 O4 -TETA-GO (d).
in the study are analytical grade, and all of the solution is prepared by double distilled water. 2.2. Preparation of the CoFe2 O4 -TETA-GO Graphene oxide is synthesized by a modified Hummers method [21]. 300 mg GO is well dispersed in 300 mL distilled water by sonification for 30 min. 3 mL TETA and 1.8 mL ammonia solution are added, and the solution is heated at 98 °C for 8 h in oil bath. The final product is TETA-GO [22]. 100 mg TETA-GO is dispersed to 100 mL distilled water, being sonicated for 30 min. The mass ratio of CoFe2 O4 to TETA-GO is 1:8. Co (NO3 )2 ·6H2 O and Fe (NO3 )3 ·9H2 O are added to the suspension. The solution is stirred for 12 h followed by a hydrothermal reaction at 180 °C for 24 h. The black products are washed and then dried at 50 °C in vacuum. The final product is signed as CoFe2 O4 -TETA-GO [23]. 2.3. Adsorption experiments Adsorption experiments are carried out with CoFe2 O4 -TETA-GO as adsorbent. All the adsorption experiments are performed in a constant temperature oscillation incubator (model SPX-250B-D). In 100 mL glass flasks 0.020–0.022 g adsorbents are added into 50 mL of Cr (VI) solution known concentration, they are kept shaking at 303 K during the preset time intervals. UV–vis spectrophotometer (T6-type) is used to determine the concentration of Cr(VI) solution at 540 nm after adsorption. The adsorption capacity (Qt ) is calculated according to the following equation:
QT = (Co − Ct )V/m
(1)
Where C0 (mg/L) is the initiating concentration of Cr(VI) in solution and Ct (mg/L) is the concentration at any time, V (L) is the volume of the solution and m (g) is the weight of the adsorbent. Given an enough adsorption period, QT will be equal to Qe (mg/g), the equilibrium adsorption capacity.
2.4. Characterization of the samples Fourier transformed infrared (FTIR) spectroscopy is measured by type-370 (Avatar Nicolet, USA) over a range from 400 to 40 0 0 cm–1 . Morphological structures of samples is characterized by scanning electron microscopy (FEI.SIRION, Philips Electronics, Netherlands) and transmission electron microscopy (TEM) which are obtained from a H-7650 microscope (HITASCHI, Japan). The samples are analyzed by a powder X-ray diffractometer (XRD) with ˚ operating at 45 kV, 40 mA and Cu Kα radiation (λ = 1.54178 A) step size of 0.05°. The samples are examined by X-ray photoelectron spectra (XPS) spectrometer (Probe VG USA) with a hemispherical electron analyzer and an Al Kα X-ray source. 2.5. Desorption and regeneration experiment Desorption study of the adsorbents is carried out with 2 wt % NaOH solution as eluent. Desorption is believed to be finished when the concentration change of the eluent is less than 3%. The adsorbent is washed thoroughly for next use. The regeneration performance of the adsorbents is proven in the subsequent adsorption–desorption cycles for 6 times. 3. Results and discussion 3.1. Characterization of CoFe2 O4 -TETA-GO 3.1.1. SEM and TEM The SEM images of GO and CoFe2 O4 -TETA-GO in Fig. 1 reveal the structure change of the samples. Fig. 1a shows the prepared GO presents ultrathin and larger lamellar structure with smooth surface and slight wrinkled edge. After decoration with TETA and CoFe2 O4 (Fig. 1b), the surface of GO becomes rouge and curly, which the nanoparticles of CoFe2 O4 disperses on. In Fig. 1, the morphology of GO and CoFe2 O4 -TETA-GO composite is displayed by the result of TEM. As shown in Fig. 1c,
Please cite this article as: X. Sun et al., Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.05.040
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the GO sample exhibits transparent and relatively thin layered structure with slight corrugated edge. As for the CoFe2 O4 -TETAGO sample (Fig. 1d), it can be seen that the surface of adsorbent pleated even more seriously. At the same time, the corrugated sheet is equably covered with some nanoparticles, which demonstrates that CoFe2 O4 nanoparticles had been assembled on the surface of the composite. 3.1.2. FTIR The functionalized groups on GO and CoFe2 O4 -TETA-GO have been revealed by FTIR in Fig. 2a. The characteristic peaks of GO at 1055, 1226, 1400, 1629 cm–1 can be ascribed to epoxy C–O–C stretching vibrations, alkoxy C–OH stretching vibrations, carboxyl O–H stretching vibrations and C–C stretching mode of the sp2 carbon skeletal network, respectively. The strong peaks at 1719 and 3409 cm–1 are due to the C=O stretching vibrations of the -COOH groups and O–H stretching vibration, respectively. The FTIR spectrum of GO is similar to a previous report in the literature [24]. The FTIR spectrum of GO demonstrates the presence of many oxygencontaining functional groups on the surface of GO sheets. Compared to the FTIR spectrum of GO, the characteristic peaks of the CoFe2 O4 -TETA-GO locating at 1101 and 1560 cm–1 represent the stretching vibration of secondary amine C–N and the bending vibration of secondary amine N–H, respectively. The peaks at 2920 and 2846 cm–1 are assigned to C–H stretching vibration of –CH2 in the TETA chain. Those results confirm that TETA is linked with GO sheets by chemical bonds, and TETA is successfully grafted on the surface of GO sheets [25]. Moreover, the peak of 584 cm–1 corresponds to Co/Fe-O [26], proving that the adsorbent contains the magnetic nanoparticles. 3.1.3. XRD Fig. 2b shows the XRD patterns of GO and the CoFe2 O4 -TETAGO. The intense and narrow peak of GO at 2θ = 10.3° corresponds to the primary diffraction of the (001) plane. The interlayer distance of (001) plane is estimated from Bragg’s law (2dsinθ = nλ), and the interlay spacing (d-spacing) of GO is 0.835 nm. Compared with the typical peak and the d-spacing of the natural graphite (2θ = 26.3° 0.335 nm, not shown here), the increased interlayer distance of GO sheets implies a large number of oxygen-containing functional groups on the surfaces of GO sheets by the oxidation treatment [27, 28]. A sharp peak of CoFe2 O4 -TETA-GO at 10.3° disappears, suggesting that the structure of GO is changed after being grafted with TETA. The feature peak of 30.3°, 35.5°, 43.2°, 57.3° and 62.6° is corresponding to crystal indexes of (220), (311), (400), (511), and (440), respectively. The peak positions of the nanoparticles can be indexed from the JCPDS data card (19-0629) for the inverse spinel crystal structure of CoFe2 O4 in Fig. 2b. The observation of XRD testifies that the magnetic nanoparticles have been successfully prepared on the adsorbent surface. 3.1.4. XPS The XPS spectra of GO and CoFe2 O4 -TETA-GO composites are given in Fig. 2c. The peaks of N 1 s, Fe 2p and Co 2p at about 400.10, 725.08 and 781.08 eV appears in the wide-scan XPS spectrum of the CoFe2 O4 -TETA-GO. The high-resolution C 1 s spectrum of GO (Fig. 2d) shows three peaks at 284.04, 286.20 and 288.20 eV, which correspond to non-oxygenated carbon (C-C/C = C), epoxy or hydroxyl carbon (C–O) and carbonyl (C=O) respectively [29]. As for the high-resolution C 1 s spectrum of the CoFe2 O4 -TETA-GO in Fig. 2e, a novel peak at 285.78 eV can be assigned to the C–N, and we find that the intensity of C–O peak decrease obviously, manifesting that the amino of TETA reacted with the epoxy of GO successfully [22, 30]. The result suggests the existence of CoFe2 O4 and TETA on the surface of GO. In addition, Fig. 2f shows that CoFe2 O4 -
3
TETA-GO can be separated quickly from the solution under additional magnetic field. 3.2. Effect of equilibrium pH The adsorption of Cr (VI) is impacted by equilibrium pH of the solution since it affects not only the ionic species of metals but also the adsorbent surface charge [31]. It is known that Cr2 O7 2– is the predominant species at low equilibrium pH (< 2), whereas Cr (VI) exists as HCrO4 – in equilibrium pH value from 2 to 4, and CrO4 2– is stable when equilibrium pH is more than 4 [32]. The effect of equilibrium pH value on the adsorption capacity of CoFe2 O4 -TETA-GO is studied and shown in Fig. 3a. The pH value of the solution is determined by precision pH meter. HAc-NaAc buffering solution and phosphate buffer solution is used to control the pH value of the solution. Obviously, the maximum adsorption amounts of Cr (VI) on CoFe2 O4 -TETA-GO appears in equilibrium pH from 1.85 to 2.9. When the equilibrium pH is 4.65, the adsorption values appear a significant reduction. It could be explained that there are abundant amine groups which is due to GO modified by TETA on the surface of CoFe2 O4 -TETA-GO. According to the zeta potential with different pH in Fig. 3b, the zeta potential of CoFe2 O4 -TETA-GO is positive at lower equilibrium pH. The result shows the protonation of amine groups makes the adsorbent surface to present positive charge, which is favorable for the adsorption of Cr (VI) anions due to electrostatic attraction. However, the zeta potential of CoFe2 O4 -TETA-GO is negative with the increase of pH. The concentration of OH− around CoFe2 O4 -TETA-GO will increase with the equilibrium pH increase, and it will compete with Cr (VI) anions during the adsorption process, resulting in the reduction of Cr (VI) anions adsorption capacity on CoFe2 O4 -TETA-GO. Therefore, equilibrium pH of 2.0 in solution is chosen for the following studies. 3.3. Adsorption time and adsorption kinetics Fig. 4a shows the impact of adsorption time on adsorption capacity on CoFe2 O4 -TETA-GO at Cr (VI) initial concentration of 100 mg/L. It is found that very fast adsorption appears during the first stage (0–1 h). Then adsorption capacity increases mildly during the second period (1–2 h). And during the last stage (2–12 h) the adsorption process achieves equilibrium. The fast adsorption in the first stage is attributed to the fact that the abundant adsorption sites are available, which facilitates the adsorption of Cr (VI). During the following period, a large number of adsorption sites on the surface of CoFe2 O4 -TETA-GO have been occupied, and the adsorption process becomes slow and tends to equilibrium. To investigate the controlling mechanism of the adsorption processes, the pseudo-first-order kinetic model and the pseudosecond-order kinetic model are used to analyze the kinetic adsorption data. The pseudo-first-order kinetic model is expressed as follows [33]:
Ln(Qe − QT ) = LnQe − K1 T
(2)
The pseudo-second-order kinetic model is given as follows [34]:
T /QT = 1/K2 Qe2 + T /Qe
(3)
Where Qe and QT are the adsorption amounts at the time of equilibrium and at time t. K1 is the pseudo-first-order rate constant (h–1 ). K2 is the pseudo-second-order rate constant of adsorption (g/mg·h). Fig. 4b, c and Table 1 show that the pseudo-second-order kinetic presents much higher correlation coefficient than that of pseudo-first-order. The pseudo-second-order kinetic can well describe the adsorption process, indicating that the rate-limiting step
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Fig. 2. (a) FTIR spectra of GO, CoFe2 O4 -TETA-GO. (b) XRD pattern of GO, CoFe2 O4 -TETA-GO and JCPDS- CoFe2 O4 . (c) XPS spectra of GO, CoFe2 O4 -TETA-GO; (d) High-resolution XPS spectra of C1s for GO; (e) High-resolution XPS spectra of C1s for CoFe2 O4 -TETA-GO; (f) The state of CoFe2 O4 -TETA-GO under additional magnetic field.
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Fig. 3. (a) Qe of CoFe2 O4 -TETA-GO for Cr (VI) in solution (100 mg/L) with different equilibrium pH. (b) Zeta potentials of CoFe2 O4 -TETA-GO with different equilibrium pH.
Fig. 4. (a) Qe of Cr(VI) adsorption on CoFe2 O4 -TETA-GO in different contact time; (b) Test of pseudo-first-order rate equation for the adsorption of Cr(VI) by CoFe2 O4 -TETAGO; (c) Test of pseudo-second-order rate equation for the adsorption of Cr(VI) by CoFe2 O4 -TETA-GO.
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X. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–8 Table 2 Adsorption isotherm parameters. Langmuir isotherm
Freundlich isotherm
Qm (mg/g)
KL (g/mg)
R2
n
KF (mg/g)
R2
180.12
5.47
0.992
2.363
24.11
0.975
Fig. 5. Test of the adsorption isotherm model for the adsorption of Cr(VI) on CoFe2 O4 -TETA -GO. Table 1 Adsorption kinetic parameters. Pseudo-first-order
Pseudo-second-order
K1 (h−1 )
Qe.cal (mg/g)
R2
K2 (g/mg·min)
Qe.cal (mg/g)
R2
Qe.exp (mg/g)
0.1995
130.389
0.924
0.0519
176.74
0.994
173.89
Table 3 Comparison of the saturated adsorption capacity of various adsorbents for Cr(VI).
of the adsorption may be intermolecular forces between the adsorbent and the adsorbate [35]. So we conclude that the main driving forces for the adsorption are ascribed to the electrostatic attraction between HCrO4 – and amino groups. Moreover, the experimental adsorption capacity (Qe exp : 173.89 mg/g) value for the pseudo-second-order kinetics is very close to that calculated (Qe cal : 175.74 mg/g). 3.4. Adsorption isotherm In order to investigate the adsorption equilibrium of the CoFe2 O4 -TETA-GO for Cr (VI), the adsorption isotherm is used in experiment. Langmuir isotherm and Freundlich isotherm are applied usually. The Langmuir isothermal assumes that adsorption takes place on monolayer on the active sites of the adsorbent [36], which is described as [35]:
Ce /Qe = 1/KL Qm + Ce /Qm
Adsorbent
Qm (mg/g)
Ref.
Graphene oxide Fe@Fe2 O3 core-shell nanowires Fe3 O4 /Graphene oxide CTAB modified graphene Magnetic cyclodextrin-Graphene oxide Magnetic chitosan-Graphene oxide-ionic liquid CoFe2 O4 -GO CoFe2 O4 -TETA-GO
74 7.78 32.33 21.57 120.19 145.35 96.15 180.12
This work [39] [40] [41] [42] [43] This work This work
adsorb Cr (VI) by electrostatic attraction. Qm value for Cr (VI) adsorption is 180.12 mg/g. Compared with Qm value for Cr (VI) adsorption on other adsorbents listed in Table 3, CoFe2 O4 -TETA-GO presents higher adsorption capacity than other adsorbents. However, Adsorption amount of CoFe2 O4 - GO, which TETA does not modify, is much lower than CoFe2 O4 -TETA-GO. The result shows that TETA is helpful to improve the adsorption performance of CoFe2 O4 -TETA-GO.
(4)
Where Ce is the equilibrium concentration of Cr (VI) in solution (mg/L), Qm is the saturated adsorption capacity (mg/g), and KL is the Langmuir adsorption equilibrium constant (L/mg). The Freundlich isothermal on behalf of the multilayer adsorption [37] can be used as empirical formula, which is expressed as [38]:
LnQe = Ln KF + (1/n )Ln Ce
Fig. 6. The regeneration performance of CoFe2 O4 -TETA-GO.
(5)
KF and n are the Freundlich constants, respectively. As shown in Fig. 5a, b and Table 2, comparison of the correlation coefficients for Cr (VI) adsorption indicates that the Langmuir isotherm model fits better than the Freundlich isotherm model, suggesting that Cr (VI) adsorption on the CoFe2 O4 -TETA-GO is monolayer coverage. This can be ascribed to the amino groups distributing on the surface of CoFe2 O4 -TETA-GO, which contributes to
3.5. Regeneration of saturate adsorbents The results of regeneration and recycling use of CoFe2 O4 -TETAGO are shown in Fig. 6. It can be found that the adsorption capacity of Cr (VI) slightly decreases to 158.56 mg/g after regeneration for 1 times. The adsorption capacity keep 78% of the original adsorption capacity after regeneration for 5 times. the result is better than that reported in the reference [44]. The phenomenon can be explained that Cr (VI)-adsorbed CoFe2 O4 -TETA-GO may not be desorbed completely by the eluent. 3.6. Removal mechanism To further investigate the mechanism of Cr (VI) removal, the existing forms of Cr (VI) on CoFe2 O4 -TETA-GO after adsorption are analyzed by XPS. The high-resolution Cr 2p spectra (Fig. 7a) can
Please cite this article as: X. Sun et al., Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.05.040
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Fig. 7. (a) High-resolution XPS spectra of Cr 2p for Cr-laden CoFe2 O4 -TETA-GO; (b) Proposed mechanism of Cr (VI) removal by CoFe2 O4 -TETA-GO.
be curve-fitted two components at binding energies of Cr 2p3/2 (577.12 eV) and Cr 2p1/2 (587.67 eV). The peaks at 577.12 eV and 587.67 eV can be regarded as the signals of Cr (III) and Cr (VI), respectively [45]. The results suggest that there are Cr (VI) and Cr (III) on the surface of the CoFe2 O4 -TETA-GO. Therefore, we conclude that at low pH value Cr (III) has formed following the Eq. (6) [43].
HCrO4 – + 7H+ +3e– = Cr3+ +4H2 O
(6)
Based on the above analysis, it is deduced that the removal mechanism of Cr (VI) by CoFe2 O4 -TETA-GO potentially consists of the following three steps (Fig. 7b): (1) The protonation happens between groups of -NH2 and -OH, forming -NH3 + and -OH2 + , and then the negative Cr (VI) group binds to CoFe2 O4 -TETA-GO by the electrostatic attraction; (2) A portion of the negative Cr (VI) group is reduced to cation Cr (III) with the help of electrons on the carbocyclic six-membered ring of CoFe2 O4 -TETA-GO; (3) The protonated amino and hydroxyl groups liberate the cation Cr (III) by electrostatic repulsion. Moreover, the negatively charged groups (-COO– ) of CoFe2 O4 -TETA-GO possibly combine with the cation Cr (III) by the electrostatic interaction. 4. Conclusions In summary, CoFe2 O4 -TETA-GO is synthesized via nucleophilic substitution and hydrothermal method. SEM and TEM results reveal that the obtained CoFe2 O4 -TETA-GO is covered by CoFe2 O4 nanoparticles with rough surface and sheet-like structure. CoFe2 O4 -TETA-GO displays a remarkable adsorption performance for Cr (VI) groups with maximum adsorption capacity (180.12 mg/g) and fast adsorption ability in two hours at pH value of 2.0. In addition, it can be separated by external magnetic field. The adsorption process fits the pseudo-second-order kinetics model well, and the equilibrium experiment fits the Langmuir isotherm model. At the same time, some Cr (VI) groups adsorbed by CoFe2 O4 -TETA-GO is reduced to Cr (III) at low pH, and CoFe2 O4 TETA-GO can be used repeatedly. Acknowledgment This work was financially supported by Harbin special funds outstanding subject leader innovative talents of science and tech-
nology research project (2015RAXXJ006), the National Natural Science Foundation of China (51208159).
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Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal Xiaojun Sun∗, Fangni Chen, Jinzhi Wei, Fengming Zhang, Suyan Pang Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China
a r t i c l e
i n f o
Article history: Received 17 September 2015 Revised 12 March 2016 Accepted 24 May 2016 Available online xxx Keywords: Graphene oxide Triethylene tetramine CoFe2 O4 Adsorption Cr (VI)
a b s t r a c t A novel magnetic composite of CoFe2 O4 -Triethylene tetramine-Graphene oxide (CoFe2 O4 -TETA-GO) was firstly synthesized by hot reflux and hydrothermal method in this work, making separation of the adsorbents easy in the foreign magnetic field. And they were utilized to remove Cr (VI) from aqueous solution effectively. The prepared CoFe2 O4 -TETA-GO was characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Fourier Transforms Infrared Spectroscopy (FTIR), X-Ray Diffractions (XRD) and X-ray Photoelectron Spectrum (XPS). The characterization results showed that CoFe2 O4 TETA-GO were modified by magnetic nanoparticles CoFe2 O4 and TETA, existing the excellent separation characteristic. The influence of factors such as initial pH value in solution and contact time on the adsorption performance of Cr (VI) onto CoFe2 O4 -TETA-GO was studied. The adsorption kinetics fit pseudosecond-order model and the equilibrium adsorption were well described with Langmuir model. The saturated adsorption capacity of Cr (VI) was about 180.12 mg/g on the CoFe2 O4 -TETA-GO at pH = 2. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Toxic heavy metal ions widely exist in many modern industries, such as leather tanning, metal polishing, electroplating and textiles etc. [1,2]. Chromium (Cr), common heavy metal, has two oxidation states predominantly in natural environment, trivalent form Cr (III) and hexavalent form Cr (VI) [3]. It is necessary to remove Cr (VI) from wastewater because Cr (VI) is more toxic than Cr (III) [4,5]. The various removal approach of Cr (VI) from wastewater have been obtained such as adsorption [6], filtration [7], ion exchange [8], and chemical precipitation [9–11] etc. Among those methods, adsorption has been considered to be superior to others due to its high-efficiency, safety, economies and simplicity [12]. Graphene oxide (GO), a member of the carbonaceous nanomaterial family, is a two-dimensional wrinkle structure with one-atom-thick carbon sheet, presenting abundant O-containing functional groups on the edges and surfaces [13,14]. However, Cr (VI) exists in anionic group in solution, which cannot be adsorbed effectively by GO due to the electrostatic repulsion. Besides, GO cannot be easily separated from wastewater due to strong hydrophilicity [15]. Thus, it is necessary to develop a new material with positive charge to remove Cr (VI) [16].
∗
Corresponding author. Tel.: +86 451 8639 2701. E-mail address: [email protected] (X. Sun).
In order to solve the above problems, Triethylene tetramine (TETA) grafted to the surface of GO can provide a large amount of amino groups as active adsorption sites, which could enhance the adsorption ability for Cr (VI) [17]. GO could be grafted by the nucleophilic substitution reaction of the amine groups of TETA with the epoxide groups of GO [18]. Besides, the dispersion of CoFe2 O4 magnetic nanoparticles dispersed on GO will make the adsorbents separated easily from the wastewater with an external magnetic field [19]. CoFe2 O4 has stable chemical property as magnetic particles, which is difficult to be oxidized in solution [20]. In the paper, CoFe2 O4 -TETA-GO is prepared successfully and then is characterized by SEM, TEM, FTIR, XRD and XPS. The influence of factors, such as pH value and contact time on adsorption efficiency of Cr (VI) is investigated. Besides, the adsorption behavior of Cr (VI) is also studied, and the performance of CoFe2 O4 TETA-GO after regeneration is evaluated. Finally, the adsorption mechanism of Cr (VI) on CoFe2 O4 -TETA-GO is discussed.
2. Experiment 2.1. Materials Graphite, TETA and K2 Cr2 O7 are purchased from Sinopharm Chemical Reagent Co. Co (NO3 )2 ·6H2 O and Fe (NO3 )3 ·9H2 O are purchased from Qingdao Yage Chemical Reagent Co. All reagents used
http://dx.doi.org/10.1016/j.jtice.2016.05.040 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Sun et al., Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.05.040
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Fig. 1. SEM images of GO (a) and CoFe2 O4 -TETA-GO (b); TEM images of GO (c) and CoFe2 O4 -TETA-GO (d).
in the study are analytical grade, and all of the solution is prepared by double distilled water. 2.2. Preparation of the CoFe2 O4 -TETA-GO Graphene oxide is synthesized by a modified Hummers method [21]. 300 mg GO is well dispersed in 300 mL distilled water by sonification for 30 min. 3 mL TETA and 1.8 mL ammonia solution are added, and the solution is heated at 98 °C for 8 h in oil bath. The final product is TETA-GO [22]. 100 mg TETA-GO is dispersed to 100 mL distilled water, being sonicated for 30 min. The mass ratio of CoFe2 O4 to TETA-GO is 1:8. Co (NO3 )2 ·6H2 O and Fe (NO3 )3 ·9H2 O are added to the suspension. The solution is stirred for 12 h followed by a hydrothermal reaction at 180 °C for 24 h. The black products are washed and then dried at 50 °C in vacuum. The final product is signed as CoFe2 O4 -TETA-GO [23]. 2.3. Adsorption experiments Adsorption experiments are carried out with CoFe2 O4 -TETA-GO as adsorbent. All the adsorption experiments are performed in a constant temperature oscillation incubator (model SPX-250B-D). In 100 mL glass flasks 0.020–0.022 g adsorbents are added into 50 mL of Cr (VI) solution known concentration, they are kept shaking at 303 K during the preset time intervals. UV–vis spectrophotometer (T6-type) is used to determine the concentration of Cr(VI) solution at 540 nm after adsorption. The adsorption capacity (Qt ) is calculated according to the following equation:
QT = (Co − Ct )V/m
(1)
Where C0 (mg/L) is the initiating concentration of Cr(VI) in solution and Ct (mg/L) is the concentration at any time, V (L) is the volume of the solution and m (g) is the weight of the adsorbent. Given an enough adsorption period, QT will be equal to Qe (mg/g), the equilibrium adsorption capacity.
2.4. Characterization of the samples Fourier transformed infrared (FTIR) spectroscopy is measured by type-370 (Avatar Nicolet, USA) over a range from 400 to 40 0 0 cm–1 . Morphological structures of samples is characterized by scanning electron microscopy (FEI.SIRION, Philips Electronics, Netherlands) and transmission electron microscopy (TEM) which are obtained from a H-7650 microscope (HITASCHI, Japan). The samples are analyzed by a powder X-ray diffractometer (XRD) with ˚ operating at 45 kV, 40 mA and Cu Kα radiation (λ = 1.54178 A) step size of 0.05°. The samples are examined by X-ray photoelectron spectra (XPS) spectrometer (Probe VG USA) with a hemispherical electron analyzer and an Al Kα X-ray source. 2.5. Desorption and regeneration experiment Desorption study of the adsorbents is carried out with 2 wt % NaOH solution as eluent. Desorption is believed to be finished when the concentration change of the eluent is less than 3%. The adsorbent is washed thoroughly for next use. The regeneration performance of the adsorbents is proven in the subsequent adsorption–desorption cycles for 6 times. 3. Results and discussion 3.1. Characterization of CoFe2 O4 -TETA-GO 3.1.1. SEM and TEM The SEM images of GO and CoFe2 O4 -TETA-GO in Fig. 1 reveal the structure change of the samples. Fig. 1a shows the prepared GO presents ultrathin and larger lamellar structure with smooth surface and slight wrinkled edge. After decoration with TETA and CoFe2 O4 (Fig. 1b), the surface of GO becomes rouge and curly, which the nanoparticles of CoFe2 O4 disperses on. In Fig. 1, the morphology of GO and CoFe2 O4 -TETA-GO composite is displayed by the result of TEM. As shown in Fig. 1c,
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the GO sample exhibits transparent and relatively thin layered structure with slight corrugated edge. As for the CoFe2 O4 -TETAGO sample (Fig. 1d), it can be seen that the surface of adsorbent pleated even more seriously. At the same time, the corrugated sheet is equably covered with some nanoparticles, which demonstrates that CoFe2 O4 nanoparticles had been assembled on the surface of the composite. 3.1.2. FTIR The functionalized groups on GO and CoFe2 O4 -TETA-GO have been revealed by FTIR in Fig. 2a. The characteristic peaks of GO at 1055, 1226, 1400, 1629 cm–1 can be ascribed to epoxy C–O–C stretching vibrations, alkoxy C–OH stretching vibrations, carboxyl O–H stretching vibrations and C–C stretching mode of the sp2 carbon skeletal network, respectively. The strong peaks at 1719 and 3409 cm–1 are due to the C=O stretching vibrations of the -COOH groups and O–H stretching vibration, respectively. The FTIR spectrum of GO is similar to a previous report in the literature [24]. The FTIR spectrum of GO demonstrates the presence of many oxygencontaining functional groups on the surface of GO sheets. Compared to the FTIR spectrum of GO, the characteristic peaks of the CoFe2 O4 -TETA-GO locating at 1101 and 1560 cm–1 represent the stretching vibration of secondary amine C–N and the bending vibration of secondary amine N–H, respectively. The peaks at 2920 and 2846 cm–1 are assigned to C–H stretching vibration of –CH2 in the TETA chain. Those results confirm that TETA is linked with GO sheets by chemical bonds, and TETA is successfully grafted on the surface of GO sheets [25]. Moreover, the peak of 584 cm–1 corresponds to Co/Fe-O [26], proving that the adsorbent contains the magnetic nanoparticles. 3.1.3. XRD Fig. 2b shows the XRD patterns of GO and the CoFe2 O4 -TETAGO. The intense and narrow peak of GO at 2θ = 10.3° corresponds to the primary diffraction of the (001) plane. The interlayer distance of (001) plane is estimated from Bragg’s law (2dsinθ = nλ), and the interlay spacing (d-spacing) of GO is 0.835 nm. Compared with the typical peak and the d-spacing of the natural graphite (2θ = 26.3° 0.335 nm, not shown here), the increased interlayer distance of GO sheets implies a large number of oxygen-containing functional groups on the surfaces of GO sheets by the oxidation treatment [27, 28]. A sharp peak of CoFe2 O4 -TETA-GO at 10.3° disappears, suggesting that the structure of GO is changed after being grafted with TETA. The feature peak of 30.3°, 35.5°, 43.2°, 57.3° and 62.6° is corresponding to crystal indexes of (220), (311), (400), (511), and (440), respectively. The peak positions of the nanoparticles can be indexed from the JCPDS data card (19-0629) for the inverse spinel crystal structure of CoFe2 O4 in Fig. 2b. The observation of XRD testifies that the magnetic nanoparticles have been successfully prepared on the adsorbent surface. 3.1.4. XPS The XPS spectra of GO and CoFe2 O4 -TETA-GO composites are given in Fig. 2c. The peaks of N 1 s, Fe 2p and Co 2p at about 400.10, 725.08 and 781.08 eV appears in the wide-scan XPS spectrum of the CoFe2 O4 -TETA-GO. The high-resolution C 1 s spectrum of GO (Fig. 2d) shows three peaks at 284.04, 286.20 and 288.20 eV, which correspond to non-oxygenated carbon (C-C/C = C), epoxy or hydroxyl carbon (C–O) and carbonyl (C=O) respectively [29]. As for the high-resolution C 1 s spectrum of the CoFe2 O4 -TETA-GO in Fig. 2e, a novel peak at 285.78 eV can be assigned to the C–N, and we find that the intensity of C–O peak decrease obviously, manifesting that the amino of TETA reacted with the epoxy of GO successfully [22, 30]. The result suggests the existence of CoFe2 O4 and TETA on the surface of GO. In addition, Fig. 2f shows that CoFe2 O4 -
3
TETA-GO can be separated quickly from the solution under additional magnetic field. 3.2. Effect of equilibrium pH The adsorption of Cr (VI) is impacted by equilibrium pH of the solution since it affects not only the ionic species of metals but also the adsorbent surface charge [31]. It is known that Cr2 O7 2– is the predominant species at low equilibrium pH (< 2), whereas Cr (VI) exists as HCrO4 – in equilibrium pH value from 2 to 4, and CrO4 2– is stable when equilibrium pH is more than 4 [32]. The effect of equilibrium pH value on the adsorption capacity of CoFe2 O4 -TETA-GO is studied and shown in Fig. 3a. The pH value of the solution is determined by precision pH meter. HAc-NaAc buffering solution and phosphate buffer solution is used to control the pH value of the solution. Obviously, the maximum adsorption amounts of Cr (VI) on CoFe2 O4 -TETA-GO appears in equilibrium pH from 1.85 to 2.9. When the equilibrium pH is 4.65, the adsorption values appear a significant reduction. It could be explained that there are abundant amine groups which is due to GO modified by TETA on the surface of CoFe2 O4 -TETA-GO. According to the zeta potential with different pH in Fig. 3b, the zeta potential of CoFe2 O4 -TETA-GO is positive at lower equilibrium pH. The result shows the protonation of amine groups makes the adsorbent surface to present positive charge, which is favorable for the adsorption of Cr (VI) anions due to electrostatic attraction. However, the zeta potential of CoFe2 O4 -TETA-GO is negative with the increase of pH. The concentration of OH− around CoFe2 O4 -TETA-GO will increase with the equilibrium pH increase, and it will compete with Cr (VI) anions during the adsorption process, resulting in the reduction of Cr (VI) anions adsorption capacity on CoFe2 O4 -TETA-GO. Therefore, equilibrium pH of 2.0 in solution is chosen for the following studies. 3.3. Adsorption time and adsorption kinetics Fig. 4a shows the impact of adsorption time on adsorption capacity on CoFe2 O4 -TETA-GO at Cr (VI) initial concentration of 100 mg/L. It is found that very fast adsorption appears during the first stage (0–1 h). Then adsorption capacity increases mildly during the second period (1–2 h). And during the last stage (2–12 h) the adsorption process achieves equilibrium. The fast adsorption in the first stage is attributed to the fact that the abundant adsorption sites are available, which facilitates the adsorption of Cr (VI). During the following period, a large number of adsorption sites on the surface of CoFe2 O4 -TETA-GO have been occupied, and the adsorption process becomes slow and tends to equilibrium. To investigate the controlling mechanism of the adsorption processes, the pseudo-first-order kinetic model and the pseudosecond-order kinetic model are used to analyze the kinetic adsorption data. The pseudo-first-order kinetic model is expressed as follows [33]:
Ln(Qe − QT ) = LnQe − K1 T
(2)
The pseudo-second-order kinetic model is given as follows [34]:
T /QT = 1/K2 Qe2 + T /Qe
(3)
Where Qe and QT are the adsorption amounts at the time of equilibrium and at time t. K1 is the pseudo-first-order rate constant (h–1 ). K2 is the pseudo-second-order rate constant of adsorption (g/mg·h). Fig. 4b, c and Table 1 show that the pseudo-second-order kinetic presents much higher correlation coefficient than that of pseudo-first-order. The pseudo-second-order kinetic can well describe the adsorption process, indicating that the rate-limiting step
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Fig. 2. (a) FTIR spectra of GO, CoFe2 O4 -TETA-GO. (b) XRD pattern of GO, CoFe2 O4 -TETA-GO and JCPDS- CoFe2 O4 . (c) XPS spectra of GO, CoFe2 O4 -TETA-GO; (d) High-resolution XPS spectra of C1s for GO; (e) High-resolution XPS spectra of C1s for CoFe2 O4 -TETA-GO; (f) The state of CoFe2 O4 -TETA-GO under additional magnetic field.
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Fig. 3. (a) Qe of CoFe2 O4 -TETA-GO for Cr (VI) in solution (100 mg/L) with different equilibrium pH. (b) Zeta potentials of CoFe2 O4 -TETA-GO with different equilibrium pH.
Fig. 4. (a) Qe of Cr(VI) adsorption on CoFe2 O4 -TETA-GO in different contact time; (b) Test of pseudo-first-order rate equation for the adsorption of Cr(VI) by CoFe2 O4 -TETAGO; (c) Test of pseudo-second-order rate equation for the adsorption of Cr(VI) by CoFe2 O4 -TETA-GO.
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X. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–8 Table 2 Adsorption isotherm parameters. Langmuir isotherm
Freundlich isotherm
Qm (mg/g)
KL (g/mg)
R2
n
KF (mg/g)
R2
180.12
5.47
0.992
2.363
24.11
0.975
Fig. 5. Test of the adsorption isotherm model for the adsorption of Cr(VI) on CoFe2 O4 -TETA -GO. Table 1 Adsorption kinetic parameters. Pseudo-first-order
Pseudo-second-order
K1 (h−1 )
Qe.cal (mg/g)
R2
K2 (g/mg·min)
Qe.cal (mg/g)
R2
Qe.exp (mg/g)
0.1995
130.389
0.924
0.0519
176.74
0.994
173.89
Table 3 Comparison of the saturated adsorption capacity of various adsorbents for Cr(VI).
of the adsorption may be intermolecular forces between the adsorbent and the adsorbate [35]. So we conclude that the main driving forces for the adsorption are ascribed to the electrostatic attraction between HCrO4 – and amino groups. Moreover, the experimental adsorption capacity (Qe exp : 173.89 mg/g) value for the pseudo-second-order kinetics is very close to that calculated (Qe cal : 175.74 mg/g). 3.4. Adsorption isotherm In order to investigate the adsorption equilibrium of the CoFe2 O4 -TETA-GO for Cr (VI), the adsorption isotherm is used in experiment. Langmuir isotherm and Freundlich isotherm are applied usually. The Langmuir isothermal assumes that adsorption takes place on monolayer on the active sites of the adsorbent [36], which is described as [35]:
Ce /Qe = 1/KL Qm + Ce /Qm
Adsorbent
Qm (mg/g)
Ref.
Graphene oxide Fe@Fe2 O3 core-shell nanowires Fe3 O4 /Graphene oxide CTAB modified graphene Magnetic cyclodextrin-Graphene oxide Magnetic chitosan-Graphene oxide-ionic liquid CoFe2 O4 -GO CoFe2 O4 -TETA-GO
74 7.78 32.33 21.57 120.19 145.35 96.15 180.12
This work [39] [40] [41] [42] [43] This work This work
adsorb Cr (VI) by electrostatic attraction. Qm value for Cr (VI) adsorption is 180.12 mg/g. Compared with Qm value for Cr (VI) adsorption on other adsorbents listed in Table 3, CoFe2 O4 -TETA-GO presents higher adsorption capacity than other adsorbents. However, Adsorption amount of CoFe2 O4 - GO, which TETA does not modify, is much lower than CoFe2 O4 -TETA-GO. The result shows that TETA is helpful to improve the adsorption performance of CoFe2 O4 -TETA-GO.
(4)
Where Ce is the equilibrium concentration of Cr (VI) in solution (mg/L), Qm is the saturated adsorption capacity (mg/g), and KL is the Langmuir adsorption equilibrium constant (L/mg). The Freundlich isothermal on behalf of the multilayer adsorption [37] can be used as empirical formula, which is expressed as [38]:
LnQe = Ln KF + (1/n )Ln Ce
Fig. 6. The regeneration performance of CoFe2 O4 -TETA-GO.
(5)
KF and n are the Freundlich constants, respectively. As shown in Fig. 5a, b and Table 2, comparison of the correlation coefficients for Cr (VI) adsorption indicates that the Langmuir isotherm model fits better than the Freundlich isotherm model, suggesting that Cr (VI) adsorption on the CoFe2 O4 -TETA-GO is monolayer coverage. This can be ascribed to the amino groups distributing on the surface of CoFe2 O4 -TETA-GO, which contributes to
3.5. Regeneration of saturate adsorbents The results of regeneration and recycling use of CoFe2 O4 -TETAGO are shown in Fig. 6. It can be found that the adsorption capacity of Cr (VI) slightly decreases to 158.56 mg/g after regeneration for 1 times. The adsorption capacity keep 78% of the original adsorption capacity after regeneration for 5 times. the result is better than that reported in the reference [44]. The phenomenon can be explained that Cr (VI)-adsorbed CoFe2 O4 -TETA-GO may not be desorbed completely by the eluent. 3.6. Removal mechanism To further investigate the mechanism of Cr (VI) removal, the existing forms of Cr (VI) on CoFe2 O4 -TETA-GO after adsorption are analyzed by XPS. The high-resolution Cr 2p spectra (Fig. 7a) can
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Fig. 7. (a) High-resolution XPS spectra of Cr 2p for Cr-laden CoFe2 O4 -TETA-GO; (b) Proposed mechanism of Cr (VI) removal by CoFe2 O4 -TETA-GO.
be curve-fitted two components at binding energies of Cr 2p3/2 (577.12 eV) and Cr 2p1/2 (587.67 eV). The peaks at 577.12 eV and 587.67 eV can be regarded as the signals of Cr (III) and Cr (VI), respectively [45]. The results suggest that there are Cr (VI) and Cr (III) on the surface of the CoFe2 O4 -TETA-GO. Therefore, we conclude that at low pH value Cr (III) has formed following the Eq. (6) [43].
HCrO4 – + 7H+ +3e– = Cr3+ +4H2 O
(6)
Based on the above analysis, it is deduced that the removal mechanism of Cr (VI) by CoFe2 O4 -TETA-GO potentially consists of the following three steps (Fig. 7b): (1) The protonation happens between groups of -NH2 and -OH, forming -NH3 + and -OH2 + , and then the negative Cr (VI) group binds to CoFe2 O4 -TETA-GO by the electrostatic attraction; (2) A portion of the negative Cr (VI) group is reduced to cation Cr (III) with the help of electrons on the carbocyclic six-membered ring of CoFe2 O4 -TETA-GO; (3) The protonated amino and hydroxyl groups liberate the cation Cr (III) by electrostatic repulsion. Moreover, the negatively charged groups (-COO– ) of CoFe2 O4 -TETA-GO possibly combine with the cation Cr (III) by the electrostatic interaction. 4. Conclusions In summary, CoFe2 O4 -TETA-GO is synthesized via nucleophilic substitution and hydrothermal method. SEM and TEM results reveal that the obtained CoFe2 O4 -TETA-GO is covered by CoFe2 O4 nanoparticles with rough surface and sheet-like structure. CoFe2 O4 -TETA-GO displays a remarkable adsorption performance for Cr (VI) groups with maximum adsorption capacity (180.12 mg/g) and fast adsorption ability in two hours at pH value of 2.0. In addition, it can be separated by external magnetic field. The adsorption process fits the pseudo-second-order kinetics model well, and the equilibrium experiment fits the Langmuir isotherm model. At the same time, some Cr (VI) groups adsorbed by CoFe2 O4 -TETA-GO is reduced to Cr (III) at low pH, and CoFe2 O4 TETA-GO can be used repeatedly. Acknowledgment This work was financially supported by Harbin special funds outstanding subject leader innovative talents of science and tech-
nology research project (2015RAXXJ006), the National Natural Science Foundation of China (51208159).
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Please cite this article as: X. Sun et al., Preparation of magnetic triethylene tetramine-graphene oxide ternary nanocomposite and application for Cr (VI) removal, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.05.040