Facile preparation of amino functionalized graphene oxide decorated with Fe3O4 nanoparticles for the adsorption of Cr(VI)

Applied Surface Science 384 (2016) 1–9

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Facile preparation of amino functionalized graphene oxide decorated with Fe3 O4 nanoparticles for the adsorption of Cr(VI) Donglin Zhao a , Xuan Gao a , Changnian Wu a , Rong Xie a , Shaojie Feng a,∗ , Changlun Chen b,∗ a b

Key Laboratory of Functional Molecule Design and Interface Process, Anhui Jianzhu University, Hefei 230601, PR China Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China

a r t i c l e

i n f o

Article history: Received 2 April 2016 Received in revised form 2 May 2016 Accepted 3 May 2016 Available online 6 May 2016 Keywords: Fe3 O4 /graphene oxide composite Adsorption Cr(VI)

a b s t r a c t A novel ternary magnetic composite consisting of graphene oxide (GO), diethylenetriamine and Fe3 O4 nanoparticles (AMGO) were synthesized by a facile one-step reaction route and characterized. The AMGO was applied as a magnetic adsorbent for the Cr(VI) removal from aqueous solutions and the magnetic separation process only took 40 s. The maximum adsorption capacity of the AMGO for Cr(VI) was 123.4 mg/g, displaying a high efficiency for the removal of Cr(VI), which was much higher than that of MGO. The removal process was pH dependence, endothermic and spontaneous. The pseudo-second-order model described well the adsorption kinetic data and the Langmuir isotherm model fitted the experimental data better than the Freundlich isotherm model. XPS analysis revealed that the Cr(VI) was reduced to the low-toxicity Cr(III) during the adsorption process. Both the Cr(VI) adsorption and subsequent reduction of adsorbed Cr(VI) to Cr(III) contributed to the Cr(VI) removal. In addition, the excellent reproducibility indicate that the AMGO may be a promising adsorption material for the separation and preconcentration of Cr(VI) ions from aqueous solutions in environmental pollution cleanup. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Chromium is one of the heavy metal ions present in effluents produced from the aerospace, steel fabrication, electroplating, textile manufacturing, leather, mining and chemical manufacturing [1,2]. In recent years, great attention is given to chromium owing to its high toxicity to both the ecological environment and living organisms [3–7]. According to the World Health Organization, it has limited 0.05 mg/L as the maximum allowable emission standard of Cr(VI) on surface water [6]. Therefore, the removal of Cr(VI) to an acceptable threshold before being discharged is of the upmost importance. Several technologies have been developed to reduce/remove Cr(VI) from water such as adsorption, chemical treatment, ion exchange, membrane process and electrochemical reduction/precipitation [7–11]. Adsorption has been most widely utilized owing to availability, ease of operation, relatively low-cost and efficiency in comparison with other conventional methods. For this purpose, various materials, such as biomaterials [12], metal

∗ Corresponding authors. E-mail addresses: [email protected] (S. Feng), [email protected] (C. Chen). http://dx.doi.org/10.1016/j.apsusc.2016.05.022 0169-4332/© 2016 Elsevier B.V. All rights reserved.

oxides [13], activated carbon [14] and nanomaterials [15] have been applied to remove Cr(VI) from aqueous solutions with high adsorption capacity. Graphene oxide (GO), which is prepared from graphite by the Hummers method, can readily bring abundant functional groups, and is more useful for adsorption because of the presence of several active sites on its surface [16], and has been investigated as high efficient adsorbents to remove heavy metal ions and organic pollutants [17–22]. In comparison to other carbonaceous nanomaterials, GO may be more environmental friendly and have better biocompatibility [23]. However, it is difficult to separate GO from aqueous solution using traditional centrifugation and filtration methods during and after the adsorption process due to its small particle size [24]. Therefore, magnetic graphene-based adsorbents with large specific surface areas and magnetic separation have begun to be used in the field of environmental remediation. For example, Liu and Zhu et al. synthesized magnetic graphene composites for Cr(VI) removal [25,26]. However, one major challenge is related to the low adsorption capacity for Cr(VI). The mechanisms of metal ion adsorption onto adsorbents are usually owing to electrostatic interaction, complexation or ion exchange. It has been reported that high adsorption efficiency of Cr(VI) was achieved when the surface modification of the

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D. Zhao et al. / Applied Surface Science 384 (2016) 1–9

adsorbent with varied functional groups including amine, thiol, and carboxyl [27,28]. To solve the low adsorption capacity for Cr(VI) onto magnetic graphene-based adsorbents (MGO), amino groups are introduced onto the surface of GO because nitrogen-containing functional group can form stable chelates with heavy metal ions. The objective of this paper focused on improving the Cr(VI) removal ability by decorating Fe3 O4 /graphene oxide composites with diethylenetriamine. In this study, amino functionalized graphene oxide decorated with Fe3 O4 nanoparticles (AMGO) was prepared using a simple one-step reaction route. The morphologies, structures and properties of the resultant composite were studied. The adsorption behavior onto the ternary composites was investigated under various conditions, including pH, time and temperature. The mechanism of the interaction of Cr(VI) on the AMGO was also studied by XPS analysis. 2. Experimental section 2.1. Materials Graphite powder (99.95% purity, average diameter of 20 mm, Qingdao Graphite Co. Ltd., China), potassium dichromate (K2 Cr2 O7 ), hydrogen peroxide (H2 O2 , 30%), potassium permanganate (KMnO4 ), sodium acetate (NaAc), iron (III) chloride hexahydrate (FeCl3 ·6H2 O), diethylenetriamine and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co. Ltd. Milli-Q (Millipore, Billerica, MA, USA) water was used in all experiments. 2.2. Preparation of AMGO The GO was prepared by the modified Hummers method through oxidation of graphite powder [29]. Typically, 1.5 g graphite powder was added in a concentrated H2 SO4 (80 mL) solution at 0 ◦ C. While maintaining vigorous stirring, 4.0 g KMnO4 was slowly added to the mixture and the temperature was carefully controlled to keep the reaction temperature below 20 ◦ C. The mixture was stirred at 35 ◦ C for 24 h, and then diluted with Milli-Q water. Then 14 mL of 30% H2 O2 solution was slowly added into the mixture until the color of the mixture changed into yellow. For purification, the mixture was washed by rinsing and centrifugation (10,000 rpm for 20 min) with 0.2 M HCl followed by Milli-Q water for several times. After filtration and drying at 30 ◦ C under vacuum, GO was obtained as a solid. A hydrothermal method was carried out to prepare AMGO [29,30]. Concretely, 0.5 g FeCl3 ·6H2 O, 3 g NaAc and 30 mL diethylenetriamine were dissolved in GO EG solution at ambient temperature. After stirring vigorously for 40 min, the mixture was transferred into a 100 mL teflon lined stainless-steel autoclave and reacted at 200 ◦ C for 6 h followed by cooling to room temperature naturally. The product was washed with ethanol and ultrapure water for several times, and then dried in vacuum oven at 60 ◦ C for 24 h. 2.3. Characterization The morphology of AMGO was obseved by field emission scanning electron microscopy (SEM) (JEOL JSM-6700, Tokyo, Japan) and tranmission electron microscopy (TEM) (JEOL-2010, Tokyo, Japan). The powder X-ray diffraction (XRD) patterns were measured on a X’Pert PRO diffractometer with Cu K␣ radiation (␭ = 0.154 nm). Fourier transform infrared spectroscopy (FTIR) analysis was performed using a Nexus 670 FTIR spectrometer (Thermo Nicolet, Madison) equipped with a KBr beam splitter (KBr, FTIR grade). The structural information of AMGO was evaluated by a Raman Spectrometer (Model Nanofinder 30R., Tokyo Instruments Inc., Tokyo,

Japan). The atomic force microscopy (AFM) image was obtained in air by using a Digital Instrumental Nanoscope III (Veeco, American) in tapping mode. X-ray photoelectron spectroscopy (XPS) analysis was performed on ESCALAB 250 (Thermo Fisher Scientific, USA). Magnetization measurements were carried out using a vibrating sample magnetometer (VSM, Lakeshore 7404) under applied magnetic field at room temperature. Brunauer-Emmett-Teller (BET) surface area was measured at Micromeritics ASAP 2010 at 77 K by N2 adsorption-desorption isotherms. The zeta potential was measured at various pH with a Zetasizer Nano ZS instrument (Malvern Instrument Co., UK) at 25 ◦ C as a function of pH.

2.4. Batch adsorption experiments Analytical-grade K2 Cr2 O7 was employed to prepare a Cr(VI) stock solution. The Cr(VI) adsorption from aqueous solutions was carried out by the batch experiment. The pH of the suspension was adjusted by adding negligible volumes of 0.01 M HCl or NaOH solutions. The adsorbent dosage of AMGO applied in this work was 0.2 g/L. The test tubes were shaken for 12 h to reach equilibrium (preliminary experiments found that this was adequate for the suspension to obtain equilibrium). The suspension was mixed thoroughly by a mechanical shaker at an agitation speed of 150 rpm. After the mixture was shaken for 12 h, the solid and liquid was separated by the magnetic separation method. The concentration of total Cr was determined by using inductively coupled plasma mass spectrometry (ICP) (Optima 8000, USA). The amount of Cr(VI) adsorbed on AMGO was calculated from the difference between the initial concentration (C0 ) and the equilibrium concentration (Ce ) (kd = qe /Ce and qe = (C0 − Ce ) × V/m), where qe is the amount of Cr(VI) adsorbed on AMGO, V is the volume of the suspension, and m is the mass of AMGO. All of the experiments were conducted in triplicate, and the error values were less than 5%.

3. Results and discussion 3.1. Characteristics Fig. 1A shows the typical SEM image of the GO, displaying that the product consists of randomly aggregated, thin and crumpled sheets, showing the flower shape [31]. The TEM and HRTEM images of the GO (Fig. 1B and C) show that the prepared GO nanosheets are a few layers thick with lateral dimensions of several micrometers. The SEM image of the AMGO (Fig. 1D) shows that the Fe3 O4 spheres are uniformly decorated and firmly anchored on the wrinkled graphene layers with a high density [32,33]. Some wrinkles are found on the surface of AMGO, which may be important for preventing aggregation of graphene and maintaining high surface area with a particular advantage of loading magnetic nanoparticles [34]. As can be seen from Fig. 1E and F, the Fe3 O4 nanoparticles were well deposited on graphene which were a flexible interleaved structure. The AFM image (shown in Fig. 1G and H) reveals that the thickness of the AMGO is approximately 1.5 nm for the selected place in the distribution graph. XRD is used to confirm the phase and structure of the prepared samples. Fig. 2A shows the XRD patterns of the prepared GO and AMGO. As can be seen that the intense peaks of AMGO at 2␪ values of 30.4◦ , 35.3◦ , 43.2◦ , 53.2◦ , 57.1◦ and 62.6◦ indexed to (220) (311) (400) (422) (511) and (440), respectively, which are consistent with the standard XRD data for the cubic phase Fe3 O4 and indicating the existence of Fe3 O4 nanoparticles in the as-obtained AMGO composite [35]. Moreover, the diffraction peak of GO is observed at about 2␪ = 10.1◦ , the results agree with typical graphite oxide feature [36].

D. Zhao et al. / Applied Surface Science 384 (2016) 1–9

3

Fig. 1. SEM(A) and TEM (B) and HRTEM (C) of GO, SEM (D) and TEM (E, F) of AMGO, AFM image of AMGO (G), (G) section analysis of (H).

GO

10

20

30

40

50

60

70

AMGO

4000

3000

1050

1000

Wavelength(cm )

C

1589 GO AMGO

500 1000 1500 2000 2500 3000 -1

Raman shift (cm )

Zeta Potential(mV)

1342

2000 -1

2 Theta(degree) 1344 1597

1210

1730 1621

440

1574

511 422

400

3434

220

Transmission(%)

Intensity(a.u.)

AMGO

B

GO

580

A

311

25 20 15 10 5 0 -5 -10 -15 -20 -25

D

2

4

6

8

pH

Fig. 2. (A) XRD patterns of GO and AMGO, (B) FTIR spectra of GO and AMGO, (C) Raman spectra of GO and AMGO, (D) Zeta potential of AMGO.

Fig. 2B shows the FTIR spectra of GO and AMGO. In the spectrum of GO, the bands at 1050 cm−1 is due to the C O bending mode. The band at 1621 cm−1 is assigned to the ␦H O H bending mode, another band at 1730 cm−1 is assigned to the stretching vibration of carbonyl (C O) groups. The broad band between at ∼3427 and 3250 cm−1 are attributed to the O H stretching vibration of the car-

boxylic acids groups of GO [35]. In the spectrum of AMGO, the band at 580 cm−1 is attributed to the Fe O bond vibration of Fe3 O4 in AMGO [37]. FTIR spectrum of AMGO exhibits the peak at 1210 cm−1 corresponding to C N [38]. The band at 1574 cm−1 is caused by the stretching vibration and bending vibration of N H bond, which is ascribed to amino groups on the surface of grapheme [39]. The

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D. Zhao et al. / Applied Surface Science 384 (2016) 1–9

A

Magnetization(emu/g)

30 15 0 -15

before adsorption

-30

after adsorption

-11000

-5500

0

5500

11000

0.0020

B Adsorption Desorption

C

0.0016

3

80 70 60 50 40 30 20 10 0 0.0

Pore volume (cm /g.nm)

3

Quantity absorbed (cm /g)

Applied magnetic field (Oe)

0.0012 0.0008 0.0004

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

20

40

60

80

100

120

Pore diameter (nm)

Fig. 3. Magnetic hysteresis loop of AMGO before and after adsorption of Cr(VI) (A), BET pattern (B) and particles size distribution of AMGO (C).

broad band observed around 3434 cm−1 might ascribes to the O H stretching vibration from the adsorbed H2 O on the surface of AMGO component [40]. According to the Raman spectra of the AMGO composites and GO in Fig. 2C, the obvious peaks at 1344 and 1597 cm−1 can be attributed to the disordered structure (D band, sp3 carbon atoms of disorders and defects) and graphite structure (G band, sp2 carbon atoms in graphitic sheets) of GO, respectively [41]. Comparing with pristine GO, the ratio of D and G peaks of AMGO becomes higher, suggesting a higher level of disorder of the graphene layers during the functionalization process [34,42]. In carbonaceous material, G-band in the Raman spectrum corresponds to sp2 carbon atoms stretching modes. Compared to GO, it is clear that G-band of AMGO occurs at 1589 cm−1 , which is down shifted by 8 cm−1 . The Raman shifts of the G band for AMGO provide evidence for the charge transfer between GO and Fe3 O4 , which shows a strong interaction between GO and Fe3 O4 [34]. The zeta potential of the AMGO is investigated at room temperature. pHzpc is an important parameter to evaluate the surface charge properties of adsorbent and it determines the electrophoretic mobility where the net total particle charge is zero [43]. As depicted in Fig. 2D, the zeta potential gradually becomes negative with the increase of pH. The pHzpc of the AMGO is 3.4 and the surface of AMGO is negatively charged when the pH higher than 3.4. The pH of intrinsic water usually ranges from 7 to 9, the as-prepared AMGO is negatively charged in most of natural water environment, which is prone to form stable complex compounds with positively charged metal ions on its surfaces [44]. Fig. 3A depicts the magnetization hysteresis loops of the AMGO before and after adsorption of Cr(VI) at room temperature with an applied magnetic field of 10 kOe. The saturation magnetization of the AMGO composite is 30.2 emu g−1 , indicating that the AMGO composite has a high magnetism. The magnetization hysteresis loop of the AMGO after adsorption of Cr(VI) has not apparent variation in performance and can meet the requirements for magnetic separation. BET analysis was reported as an effective tool for investigating the specific surface area of nanomaterials. The N2 adsorption/desorption isotherm of as-synthesized AMGO exhibits typical IV-type isotherms with H3 -hysteresis loops (Fig. 3B), indicating the presence of mesopores with many slit holes, which might generate from the gaps between magnetic beads and GO sheets or

the gaps between GO sheets [34,45]. As shown in Fig. 3C, the average pore diameters of AMGO are 16.89 nm. According to the N2 adsorption analysis the BET surface area of AMGO is 57.55 m2 /g and the micropore volume of AMGO is 0.38 cm3 / g.

3.2. Removal of Cr(VI) The solution pH is one of the most important parameters for the metal solution because of its remarkable effect on the speciation of metal ions, the surface charge and the binding sites of the sorbent. The effect of solution pH on the adsorption of Cr(VI) onto AMGO is presented in Fig. 4A. It is evident that the adsorption capacity was highly dependent on the pH with maximum adsorption at pH 2. It is reported that the speciation of Cr(VI) in aqueous solution is strongly pH-dependent. At acidic pH values, the predominant Cr(VI) species are H2 CrO4 0 , HCrO4 − , CrO4 2− and Cr2 O7 2− [45,46]. The NH2 groups on the surface of the AMGO can either be protonated to form NH3 + at low pH (R-NH2 + H+ = R-NH3 + ) or be deprotonated to form NH2 · · ·OH− at high pH (R-NH2 + OH− = RNH2 · · ·OH− ).The pHzpc (pH zero point charge, Fig. 2D) of AMGO is 3.4, at pH < pHzpc the surface charge of AMGO is postive, the negatively charged HCrO4 − and Cr2 O7 2− are easily to be adsorbed to the positively charged AMGO owing to the electronic attraction. Higher pH decreasing the adsorption capacity may be explained by the decrease in the extent of protonation of the amine and the positively charged on the AMGO. At pH 1, Cr(VI) exists as H2 CrO4 and HCrO4 − , while at pH 2Cr(VI) exists mostly as HCrO4 − , thus higher Cr(VI) adsorption is expected. Similar phenomena has also been reported by Avila et al. [5]. Besides electrostatic interaction, the Cr(VI) removal by the AMGO composite may still be involved in chemical reduction process. The pH values after Cr(VI) adsorption were measured, and the results indicated the pH values increase, which is due to that the Cr(VI) reduction is involved in the H+ consumption. In order to confirm this speculation, the detailed discussion was presented in the following section. The effect of contact time on the adsorption of Cr(VI) ions is investigated and the results are shown in Fig. 4B. It can be observed that Cr(VI) adsorption on AMGO is fast within the first 50 min, then it rises slowly and reaches equilibrium. The adsorption is higher in the beginning because of greater number of reaction sites available

D. Zhao et al. / Applied Surface Science 384 (2016) 1–9

140

A

120

100

qt(mg/g)

80

qe(mg/g)

GO

B

120

100

5

60 40

AMGO

80 60 40

MGO Fe3O4

20 0

20 0

1

2

3

4

5

6

7

8

9

0

50

100

150

pH

250

300

160

140

C

120

140

D

120

80

qe(mg/g)

100

qt(mg/g)

200

t(min)

33mg/L 45mg/L

60 40

100 298K 308K 318K

80 60 40

20

20

0 0

50

100

150

200

250

5

300

10

15

20

25

30

Ce(mg/L)

t(min)

Fig. 4. Effect of (A) initial pH on AMGO, and (B) Effect of contact time on the adsorption of Cr(VI) ions on different sorbents ((CCr(VI)initial = 33 mg/L, T = 298 K, and m/V = 0.2 g/L, (A) shaking time 12 h, (B) pH = 2.0 ± 0.1)), (C) Effect of contact time on Cr(VI) adsorption on AMGO at different concentrations (pH = 2.0 ± 0.1, T = 298 K and m/V = 0.2 g/L), (D)Adsorption isotherm of AMGO for Cr(VI) at different temperatures (pH = 2.0 ± 0.1, m/V = 0.2 g/L and shaking time 12 h). Table 1 Kinetic parameters for the adsorption of Cr(VI) onto AMGO. C0 (Cr(VI))

Pseudo-first-order

(mg/L)

K1 (min−1 )

qe,mod (mg/g)

R2

K2 (g/min/mg)

Pseudo-second-order qe,mod (mg/g)

R2

33 45

6.7E-02 2.5E-02

137.6 152.3

0.871 0.884

3.2E-03 1.9E-03

125.0 142.8

0.991 0.998

for the adsorption of Cr(VI) ions. According to the above results, the shaking time is fixed at 12 h in the following experiments to make sure that the adsorption reaction can achieve complete equilibrium. Compared with pure GO (Fig. 4B), the AMGO composite exhibits the slight lower adsorption capacity. The Cr(VI) adsorption equilibrium time on AMGO is 50 min, but it is 150 min for GO. The adsorption of Cr(VI) onto Fe3 O4 is remained below 5.2 mg/g, however, it could separate graphene sheets against the aggregation, which is beneficial to improve the adsorption rate [33]. Experiments are performed at two different initial adsorptive concentrations, and data is collected within the time period 5–300 min. Results obtained are depicted in Fig. 4C. The initial concentration has no significant effect on the equilibrium time [47]. The adsorption capacity of Cr(VI) onto AMGO increases as the increase in the initial Cr(VI) concentrations. This may be attributed to a higher chance of collision between Cr(VI) ions and active sites on the adsorbent surface and a better driving force, which lowers the mass transfer resistance [6]. To identify the rate-controlling mechanisms during the adsorption of Cr(VI), two simplified models were applied to estimate the experimental data. The pseudo-first-order and the pseudo-second-order equations are generally expressed as follows [48]: k1 log(qe − qt ) = log qe − t 2.303

(1)

t 1 t = + 2 qt q k2 qe e

(2)

where qe and qt are the capacity of Cr(VI) adsorbed (mg/g) at equilibrium and time t (min), respectively. K1 is the pseudo-first-order

rate constant (min−1 ), and K2 is the pseudo-second-order rate constant (g/(mg min)). The values of constants of kinetic models obtained from the plots for adsorption of Cr(VI) onto AMGO at 298 K are shown in Table 1. The values of the correlation coefficient show the applicability of the pseudo-second-order model for describing the experimental results to a higher degree of accuracy for all studied chromium concentrations. To evaluate the interaction between an adsorbate and an adsorbent and to design and operate a adsorption system successfully, equilibrium adsorption isotherm data is very important. The adsorption isotherms for Cr(VI) ions onto AMGO at 298, 308 and 318 K are shown in Fig. 4D. The adsorption isotherm is the highest at T = 318 K and is the lowest at T = 298 K. To analyze the experimental equilibrium adsorption data in the present work, two typical isotherm models, such as the Langmuir and Freundlich isotherm model are used to describe the equilibrium data and the non-linear forms are expressed as follows: Ce 1 Ce = + qe qmax bqmax

(3)

log qe = log kF + n log Ce

(4)

where Ce is the equilibrium concentration of Cr(VI) ions remained in the solution after adsorption equilibration (mg/L), qe is the amount of Cr(VI) ions adsorbed on per weight unit of solid after equilibrium (mg/g), qmax , the maximum adsorption capacity, is the amount of sorbate at complete monolayer coverage (mg/g), b (L/mg) is a constant that relates to the heat of sorption, kF (mg1−n Ln /g) represents the adsorption capacity and n represents the degree of dependence

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D. Zhao et al. / Applied Surface Science 384 (2016) 1–9

Table 2 The parameters for the Langmuir and Freundlich adsorption isotherms of Cr(VI) on AMGO at different temperatures. T(K)

Langmuir qmax (mg/g)

298 308 318

124.5 138.3 151.6

Freundlich b (L/mg)

R2

0.58 0.67 0.44

0.939 0.949 0.960

KF (mg1−n Ln /g) 13.52 19.76 30.83

n

R2

0.753 0.655 0.544

0.835 0.866 0.911

of adsorption at equilibrium concentration.The relative parameters values calculated from the two models are listed in Table 2. The adsorption isotherms are fitted better by the Langmuir model than by the Freundlich model, suggesting that Cr(VI) adsorption onto the AMGO are adsorbed with homogeneous binding sites, equivalent adsorption energies, no interaction between adsorbed species, and monolayer coverage [49]. The values of some of these Cr(VI) adsorbents were given in Table 3 which was compared with the results obtained in this study [25,50–57]. The adsorption thermodynamic behavior has been studied via different thermodynamic parameters, including the Gibb’s free energy (G), entropy (S) and enthalpy (H) changes, which have been calculated according to the following equation: G◦ = −RT lnK ◦

(5) (H0 )

and the standard entropy The standard enthalpy change (S0 ) are then obtained from the linear plot of lnK◦ versus 1/T for Cr(VI) ions adsorption on the AMGO in the following equations [58]: 0

lnkd

S H − R RT

0

(6)

The thermodynamic parameters calculated from the adsorption isotherms at pH 2.0 and different temperatures from the above equation are tabulated in Table 4. The positive values of H0 confirm an endothermic nature of adsorption, while the positive values of S0 reflect an increase in randomness at the solid/solution interface during the Cr(VI) adsorption onto the AMGO. The negative values of G0 indicate that the adsorption follows a spontaneous and feasible trend. And the values of G0 are found to be decreasing with rise in temperature, which shows that the higher temperature may facilitate the adsorption of Cr(VI) ions on AMGO because of a greater driving force of adsorption. 3.3. Desorption and regeneration study The regeneration ability of the spent adsorbent material is an important factor to estimate the cost effectiveness. For the regeneration, the Cr(VI)-adsorbed AMGO are immerged in 7 mL of NaOH solution (0.1 M) for 3 h and then washed several times with MilliQ water to remove adsorbed alkali. The adsorption−desorption experiments with 0.1 M NaOH are repeated for five cycles. We utilize the same batch of AMGO to adsorb Cr(VI) for five consecutive cycles (Fig. 5A) and find that the adsorption capacity is 123.4 mg/g at the first cycle and then slightly decease to 106.5 mg/g at the fifth cycle. Moreover, it is worth noting that the AMGO almost keeps its original structure after five successive cycles of adsorption–desorption (Fig. 5B), compared to Fig. 1E, which is Table 4 Values of thermodynamic parameters for the adsorption of Cr(VI) on AMGO. T(K)

G◦ (kJ/mol)

S◦ (J/(mol K)

H◦ (kJ/mol)

298 308 318

−6.48 −7.51 −9.75

161.87

41.94

Fig. 5. (A) Adsorption capacity of Cr(VI) on the AMGO composite in five successive cycles of desorption–adsorption (pH = 2.0 ± 0.1, m/V = 0.2 g/L, T = 298 K and shaking time 12 h), (B) TEM of AMGO composite after five successive cycles of desorption–adsorption.

believed to be responsible for its high cycling stability shown above [51]. This result indicates that the adsorbent has sufficient chemical stability for the recovery of Cr(VI) from aqueous solution. 3.4. XPS spectra analysis and adsorption mechanism To further analyze the mechanism of Cr(VI) adsorption by AMGO, XPS (Fig. 6A) is employed to investigate the existing forms of Cr(VI) on AMGO and their interactions after adsorption process. The Fe 2p spectrum simulation peaks at 710.7 and 724.6 eV (Fig. 6B), which are assigned to Fe 2p3/2 and Fe 2p1/2 , respectively. The position of the Fe 2p3/2 peak at 710.7 eV is indicative of the existence of Fe3 O4 in graphene nanosheets, which is in accordance with the result of XRD analysis [44]. The presence of Cr 2p on the surface of AMGO clearly confirms the successful adsorption of Cr(VI). Two energy bands at about 576.7 and 586.8 eV corresponding to the binding energies of Cr 2p3/2 and Cr 2p1/2 , which are consistent with Cr(III) and Cr(VI) (Fig. 6C), respectively [32,45,46]. The XPS analysis indicates that Cr is adsorbed on the AMGO as Cr(VI) and Cr(III), which also shows that some of the adsorbed Cr(VI) anions are reduced to Cr(III) on the surface of AMGO. These results indicate that both Cr(VI) and Cr(III) exist on the AMGO. In strongly acidic medium, Cr(VI) binds to the surface of AMGO by electrostatic attraction between protonated amine groups and HCrO4 − groups. Subsequently, Cr(VI) is reduced to Cr(III) on the surface of the AMGO with the aid of ␲ electrons on carbocyclic six-membered ring [59]. The reduction process can be partly followed by the reaction chemistry: HCrO4 − + 7H+ + 3e → Cr3+ + 4H2 OE0 = +1.35V

(7)

D. Zhao et al. / Applied Surface Science 384 (2016) 1–9

7

Table 3 Comparison of Cr(VI) adsorption capacity using AMGO with other reported adsorbents. Sorbents

Solution chemistryconditions

qmax (mg/g)

Refs.

Fe3 O4 /GO Magnetite nanoparticles Magnetic cyclodextrin/chitosan/GO Magnetic ␤-cyclodextrin/GO NZVI/Fe3 O4 /graphene Hydrous zirconium oxide ED/DMF/rGO Activated carbon Oxidized multiwalled carbon nanotubes AMGO

pH = 4.5 pH = 2.5 pH = 3.0 pH = 3.0 pH = 3.0 pH = 3.0 pH = 2.0 pH = 4.0 pH = 2.6 pH = 2.0

32.33 120.0 21.6 120.0 101.0 67.7 92.2 15.47 2.88 123.4

[25] [50] [51] [52] [53] [54] [55] [56] [57] Present study

O1s Cr2p

C1s

Fe2p

A

Fe2p

Fe2p

N1s

Fe 2p3/2

B

Fe 2p1/2

after adsorption

before adsorption 150 300 450 600 750 900 1050

707

Cr2p Cr 2p 3/2

714

721

728

735

Binding Energy(eV)

Binding Energy(eV)

C

Eb=399.91 eV

N 1s

D

W=2.65 eV after adsorption

Cr 2p1/2

Eb=399.73 eV W=2.31 eV before adsorption 570

575

580

585

590

595

396

398

400

402

404

406

408

Binding Energy(eV)

Binding Energy(eV)

Fig. 6. XPS spectrum of (A) AMGO before and after the adsorption of Cr(VI), high-resolution scan of (B) Fe 2p, (C) Cr 2p, (D) N 1s before and after the adsorption of Cr(VI).

The presence of Cr(III) on the AMGO surface suggests that some fraction of adsorbed Cr(VI) was reduced to Cr(III) by a reduction process. Therefore, it can be inferred that both Cr(VI) and Cr(III) are simultaneously existing on the surface of the AMGO composite after adsorption process. Since Cr(III) bonded on the AMGO composite can not be dissolved, no Cr(III) concentration was detected. Further, the nitrogen species of diethylenetriamine play crucial important roles in the reduction process. To verify this speculation, the N 1s XPS spectrum of AMGO composite before and after adsorption is shown in Fig. 6D. Reaction with Cr(VI) under the condition of pH 2.0 results in a moderate shift of N 1s to higher binding energy (Eb ). The largest difference between Cr(VI) adsorption on AMGO surfaces before and after reaction is the width of the peak. As shown in Fig. 6D, the peak width (W) of N 1s of before adsorption (2.31 eV) is lower than that of after adsorption (2.65 eV). The changes in binding energy of N 1s (0.18 eV) and in the peak width of N 1s (0.34 eV) after Cr adsorption indicate that N is involved in binding Cr(VI) [2,5]. The amine groups (−NH−, −N < , −NH2 ) were protonated at pH values below eight and adsorbed anionic hexavalent chromium (CrO4 2− , HCrO4 − , and Cr2 O7 2− ) via electrostatic attraction (−NH2 as the representative species): − + −NH+ + HCrO− 4 → −NH3 − HCrO4 3

(8)

−NH+ + CrO2− → −NH+ − Cr2 O2− 4 7 3 3

(9)

2− + −NH+ + Cr2 O2− 7 → −NH3 − Cr2 O7 3

(10)

The converted Cr(III) species may be chelated on the amine groups on the PANI/LDHs surface. At different pH values, different Cr(III) species exist in solution [60]. Therefore, the possible adsorption mechanisms may be as follows (−NH2 as a representative): −NH2 + Cr3+ → −NH2 Cr3+ 2+

−NH2 + Cr(OH)

→ −NH2 Cr(OH)

(11) 2+

+ −NH2 + Cr(OH)+ 2 → −NH2 Cr(OH)2

(12) (13)

To sum up, the Cr(VI) removal mechanism can be inferred as follows: First, Cr(VI) anion binds to the AMGO by electrostatic interaction between the negatively charged Cr(VI) species and the protonated amine groups. Subsequently, Cr(VI) is reduced to Cr(III) with the aid of ␲ electrons on carbocyclic six-membered ring [59]. 5. Conclusion AMGO was synthesized using a simple, cost effective and environmentally friendly method for the removal of Cr(VI) from aqueous solution. Compared to the MGO, AMGO with amine groups exhibited higher adsorption efficiency. The adsorption process was pH dependence. The optimum pH for total Cr removal was 2.0 and the maximum adsorption capacity of the AMGO for Cr(VI) was 123.4 mg/g. Cr(VI) adsorption did not only include the electrostatic attraction on the surface of AMGO, but also involved the reduction process from Cr(VI) to Cr(III). The synergistic contribution from

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each component in AMGO played an important role in the Cr(VI) adsorption. In addition, the AMGO not only displays large adsorption capacity but also can be easily separated by a magnet. All the results shows that AMGO is a very attractive adsorbent for efficient Cr(VI) removal from contaminated aqueous media.

Acknowledgments Financial support from the National Natural Science Foundation of China (21377005, 21477133 and 41273134) is acknowledged.

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