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Adsorption of Cd(II) at the Interface of water and graphene oxide prepared from flaky graphite and amorphous graphite
Accepted Manuscript Title: Adsorption of Cd(II) at the Interface of water and graphene oxide prepared from flaky graphite and amorphous graphite Authors: Yu Zhang, Weijun Peng, Ling Xia, Shaoxian Song PII: DOI: Reference:
S2213-3437(17)30388-3 http://dx.doi.org/doi:10.1016/j.jece.2017.08.004 JECE 1799
To appear in: Received date: Revised date: Accepted date:
24-5-2017 2-8-2017 3-8-2017
Please cite this article as: Yu Zhang, Weijun Peng, Ling Xia, Shaoxian Song, Adsorption of Cd(II) at the Interface of water and graphene oxide prepared from flaky graphite and amorphous graphite, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Adsorption of Cd(II) at the Interface of water and graphene oxide prepared from flaky graphite and amorphous graphite
Yu Zhang1, Weijun Peng1, Ling Xia2, 3*, Shaoxian Song1,2*
1 School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China 2 Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China 3 Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China
Corresponding author: L. Xia([email protected]), S. Song([email protected])
1
ABSTRACT This study aimed to investigate the adsorption properties of Cd(II) on graphene oxide prepared from amorphous graphite and flaky graphite. The natural graphite was characterized by Scanning electron microscope, X-ray fluorescence spectrometer and X-ray diffraction. The equilibrium data were described well by both Langmuir model and Freundlich model, and the adsorption rate was fitted by Pseudo-second-order precisely. The GOs before and after adsorption were measured by Atomic force microscope, Fourier transform infrared spectrometer and Energy dispersive system. It was found that the larger adsorption capacity of Cd(II) on GO prepared from amorphous graphite was largely attributed to the more oxygenous functional groups appeared on GO-AG. Keywords: natural graphite; graphene oxide; Cd(II) removal; adsorbent
1. Introduction Heavy metal ions contamination has become a growing world concern in recent years. Cadmium is one of the most toxic elements, which can be found naturally in water, soil, air, ores and foodstuffs[1]. It’s well recognized for its adverse effects in the environment and it was accumulated readily in living systems[2]. According to the guideline of World Health Organization (WHO), a concentration of 0.005 mg L -1 of cadmium ions was set as the permitted concentration in drinking water[3]. Long term exposure to the Cd(II) in drinking water higher than the allowable concentration can lead to renal degradation, hepatotoxicity and carcinogenicity, lung insufficiency, and respiratory system injury[4, 5]. For protecting the human beings and the environment, it 2
is necessary to remove Cd(II) in water[6]. Currently, common methods for removing cadmium ions from aqueous solution included ion exchange[7], adsorption[8] and precipitation[9]. Among these techniques, adsorption is widely used due to the superior characteristics of speediness, economy and easy to operation[10]. Many materials have been reported as adsorbents for Cd(II) removal including yeast[11], fly ash[12], perlite[13], clay[14], activated carbon[15] and so on. Recently, GO (graphene oxide) has drawn increasing attention as an adsorbent for multivalent metal ions with enormous surface area, hydrophilicity and high negative charge density[16]. GO is the product of graphite by oxidation and exfoliation process, so that the characteristics of GO can be largely influenced by the raw graphite used. Flaky graphite (FG) and amorphous graphite (AG) are the most used natural graphite materials. While flaky graphite is the most used natural material for GO preparation due to the excellent layered structure, nano-sized holes and easy to expansion[17]. However, there are large reserves of natural graphite in China, of which the reserve of amorphous graphite is about 2 million 830 thousand tons. And the price of amorphous graphite is lower than that of flaky graphite. Amorphous carbon with a high fraction of diamond-like (sp3) bond can be used in many day-life applications (such as magnetic storage, optical storage, engine parts, wear protection and razor blades) [18, 19]. However, there is scarce literature focusing on GO preparation with amorphous graphite. In this work, GOs prepared from natural flaky graphite and amorphous graphite were used as adsorbents for removing Cd(II) from aqueous solution. Adsorption 3
performances were estimated by kinetic models and adsorption isotherms. SEM was used to study the surface feature of natural graphite. AFM, FT-IR and SEM-EDX were used to study the adsorption behavior. Then further analysis of the influence of graphite type on adsorption result was investigated.
2. Experiment 2.1. Materials and chemicals. Synthesize GOs with two kinds of natural graphite, amorphous and flaky graphite. The purity of flaky graphite was more than 99% while the purity of amorphous graphite was only 90%. Potassium permanganate (KMnO4), sodium hydroxide (NaOH), cadmium nitrate tetrahydrate (CdNO3·4HO2), sodium nitrate (NaNO3), 30% hydrogen peroxide aqueous solution (H2O2), 98% sulfuric acid (H2SO4) and 68% nitric acid (HNO3) used in the study were all of analytically pure grade. Deionized water used in the work was prepared by Milli-Q ultrapure water system (Direct-8, America). 2.2. Preparation of GO-AG and GO-FG GrOs (graphite oxide) were prepared by the Hummers method[20]. At first, 150 mL of H2SO4 were added into a conical flask and stirred strongly in ice-water. Then, 3.0 g of graphite and 1.5 g of NaNO3 were placed into the flask in sequence. After stirred for 10 min, 9.0 g of KMnO4 were added slowly into the mixture and the compound was keep stirring for 1 h at the temperature below 10 ℃. The next step was to put the compound into a pre-heated water bath at 35 ℃ and stirred continually for 4
another 2 h. Subsequently, add 150 mL of deionized water by drop in order to avoid the temperature rising sharply. After that, put the compound into a pre-heated water bath at 98 ℃ again and stir continually for 30 min. Add 100 mL of deionized water and 10mL of 30% H2O2 lastly. Centrifuge the mixture at 4000 rpm for 10 min with high speed centrifuge (Sorvall ST 16, Thermo Scientific, America). Wash the sediment with 5wt% hydrochloric acid solution for three times and followed by warm water (50 ℃). Finally, freeze-dried the sediment for 24 h in a vacuum, the powder was GrO. The GrOs prepared from flaky and amorphous graphite were referred to as GrO-FG and GrO-AG, respectively. Put 100 mg of GrO powder into 100 mL of deionized water. Then shear the mixture with FLUKO high-shear homogenizer (Germany) at 10000 rpm for 3 min. Exfoliate ultrasonically with a Cole-Parmer ultrasonic processor (CP505, American) subsequently. The amplitude and the time were set to 40% and 10 min, respectively. Centrifuge the colloidal suspension at 4000 rpm for 3 min and the supernatant was GO. The GOs prepared from GrO-FG and GrO-AG was referred as GO-FG and GO-AG, respectively. 2.3. Cd(II) adsorption Adsorption experiments were set up as triplicate. The 3 mg of GO and 100 mL Cd(II) aqueous solution were added into 150 mL conical flask. Then the suspension was shaken thoroughly by a water bath vibrator (SHA-B, China) with a speed of 150 rpm for 12 h at 25 ℃. For the effect of initial pH experiments, the range of pH value of initial solution was set within a range from 2 to7 with initial Cd(II) concentration 5
of 400 mg L-1. In the equilibrium experiments, the rang of Cd(II) concentration was from 10 mg L-1 to 1100 mg L-1 and the concentration of GO was 30 mg L-1. In the kinetics experiments, the concentration of Cd(II) and GO was 10 mg L-1 and 60 mg L-1, respectively. Get the samples at different time interval (1-180 min). Finally, the suspension was filtered by 0.22 μm filter membrane and Cd(II) concentration in the filtrate was immediately examined by Atomic absorption spectrometer (A6880, Shimadzu, Japan) [21]. The adsorption capacity of Cd(II) was calculated by the following equation: qt =
C0 −Ct m
×V
(1)
Where qt is the adsorption capacity of the adsorbent (mg g-1) C0 and Ct are the initial and determined concentration of Cd(II) (mg L-1), respectively; V is the volume of solution (L) and m is the weight of the adsorbent (g). 2.4. Methods X-ray diffraction pattern was obtained by an X-Ray Diffraction (XRD, D8 Advance, Germany) with Cu-Kα radiation. The component analysis of graphite was conducted using an X-ray fluorescence spectrometer (XRF, Axios advanced, China) operating at a power of 4 kW. The morphology of the samples was detected by the scanning electron microscope (SEM, JSM-IT300, Japan) and Transmission electron microscope (TEM, JEM2100F, Japan). Defect of lattice was determined by Raman spectrometer (Raman, INVIA, the British). Oxygen-containing functional groups of graphene were characterized by X-ray photoelectron spectrometer (XPS, VG Multilab2000, America). To determine the functional groups of samples, GOs before 6
and after adsorption of Cd(II), were scanned by a Fourier transform infrared spectrometer (FT-IR, Nicolet6700, America). The wavelength ranged from 4000 to 400 cm-1. The distribution of GO sheet thickness was obtained by the atomic force microscope (AFM, Multimode 8 Bruker, America). To prepared the samples, solution with GOs before and after Cd(II) adsorption were dropped onto a clean mica, then let stand for 12 h to dry at room temperature. The cadmium’s distribution on the graphene oxide after adsorption was determined by energy X-ray detector (EDS, Hitachi S4800, Japan).
3. Results and discussion 3.1. Characterization of adsorbents Fig. 1 shows the XRD spectra of raw flaky and amorphous graphite. The characteristic peaks of both graphite were located at 2θ=26.5°. So the average d-spacing of the two samples were about 0.336 nm, which corresponded to the reflection plane of graphite[22]. The peak intensity of flaky graphite was stronger than that of amorphous graphite, which indicated flaky graphite had more integrated lattice structure. Fig. 2 shows the surface morphology of flaky graphite (a) and amorphous graphite (b). Flaky graphite exhibited lamellar structure. Its lateral dimension was about 1-3 μm. However, amorphous graphite exhibits grain structure and its average lateral dimension was approximately 0.5 μm. Table1 lists the content of main elements in AG and FG. The purity of flaky graphite is more than 99%, while the purity of amorphous graphite is only 90%. 7
Obviously, the amounts of metal impurities in amorphous graphite were much more than that in flaky graphite. The prepared GOs from two kinds of graphite have been characterized by XRD, TEM, Raman spectra, XPS and FT-IR. From Fig. 1, it was found that after two kinds of graphite were prepared to graphene oxides by oxidation and stripping, the characteristic peaks of graphite were replaced by the characteristic peaks of graphene oxides located at 2θ=10°. The average d-spacing of the two adsorbents were about 0.8 nm, wider than that of graphite. The reason may be due to the appearance of oxygenous functional groups between the layers. From TEM results in Fig. 3, it was obvious that graphene had a characteristic of single-laminated structure. The horizontal diameter of GO-FG was about 1.2 μm, while the horizontal diameter of GO-AG was just 0.3-0.5μm. Raman images of GOs and the corresponding parameters are shown in Fig. 4. There are two characteristic peaks located at 1350 cm-1 and 1580 cm-1 for both GOs, respectively. The peaks at 1580 cm-1 were the characteristic peak of crystal carbon, called G-band, while the peaks at 1350 cm-1 were used to estimate the disorder and the defect of lattice. ID/IG combining the two parameters is usually used to determine the disorder of material. In this study, ID/IG of AG-GO was greater than that of FG-GO, suggested larger defection which may be more favorable to the adsorption reaction with metal ions. Fig. 5 shows the AFM images and corresponding sectional height profiles of GO-AG and GO-FG. Generally, the thickness of monolayer graphene oxide is about 8
0.8-1.0 nm. In this work, the thickness of grapheme oxide synthesized by AG and FG is approximately 0.804 nm and 0.926 nm, respectively. However, the thickness is higher than the thickness of theoretical monolayer grapheme. This may be because there are many oxygen containing functional groups on the graphene sheets[24, 25]. Fig. 6 shows the XPS images of GO-AG (a) and GO-FG (b). The maps can be divided into three symmetrical peaks, which corresponding O-C=O, C-O, C-C/C=C, respectively. It indicated the abundant oxygen-containing functional groups loaded at both graphene oxides. Fig. 7 showed the FT-IR spectra of GOs before and after Cd(II) adsorption. The peaks at 3420-3388 cm-1 were assigned to -OH stretching vibration in hydroxyl and water molecule[26,
27]
. The peaks at 1724-1723 cm-1 can be attributed to C=O
stretching vibration in carbonyl and carboxylic groups[28, 29]. The peaks at 1621-1616 cm-1 were C=C characteristic peaks by C=C stretching vibration in unoxidized graphite domains. The peaks at 1054-1041 cm-1 were C-O characteristic peaks by C-O Stretching vibration. The above functional groups were found in both GOs. It was noted that the peak at 1225 cm-1 assigned to C-OH in phenol [29] only appeared in GO-AG. According to previous studies, amorphous graphite is easier to be oxidized than flaky graphite[28]. This may be the reason that oxygenous functional groups found in GO-AG. 3.2. Cd(II) adsorption 3.2.3 Effect of initial pH
9
Considering that cadmium ions begin to precipitate in water under the condition of pH 8, the intimal pH was set within a pH range of 2 to 7. As presented in Fig. 8, the adsorption capacity was significantly affected by the initial solution pH value. The adsorption capacity of Cd(II) was fluctuated with pH value, and showed a maximum value at pH 4. Given that the prepared GOs without any adjustment were of pH 3-4, the subsequent experiments were conducted without any initial pH adjustment. 3.2.2. Adsorption isotherm The experiments of adsorption of Cd(II) on GOs prepared from AG and FG as a function of initial Cd(II) concentrations were conducted and the results were fitted by two isotherm models, Langmuir model (2) and Freundlich model (3), respectively. Langmuir adsorption model is often used to describe a simple monolayer adsorption process. It hypothesizes there is a certain number of adsorption sites on the surface of the adsorbent in the theory, and each adsorption site only adsorbed by one atom or molecule. The linear isotherm equation is presented as following Eq. (2). The Freundlich model is often used to describe the process of layers adsorption. It assumes that the active sites on the surface of the adsorbent are exponential in the theory, and its linear equation is as following Eq. (3). Ce qe
=
Ce qm
+
1
(2)
KL ×qm 1
ln q e = ln K F + n × ln Ce
(3)
Where qe is the equilibrium adsorption capacity (mg g-1); Ce is the equilibrium concentration of Cd(II) in solution (mg L-1); qm is maximum adsorption capacity (mg
10
g-1); KL (L mg-1) and KF (mol1-n Ln g-1) are Langmuir and Freundlich adsorption constant, respectively; and n represents adsorption intensity. The fitting parameters for adsorption of Cd(II) on GO-AG and GO-FG fitted by Langmuir model and Freundlich model were listed in Table 2. The separation factors (RL) of GOs were both 0.29, which is between 0 and 1. So the reaction was more likely to occur. The Langmuir and Freundlich adsorption constants on GO-FG were 0.003 L mg-1 (KL) and 8.08 mol1-n Ln g-1 (KF), respectively, and the correlation coefficient R2 were 0.973 and 0.971, respectively. So the experimental data of adsorption on GO-FG were fitted well with both Langmuir model and Freundlich model. The Langmuir and Freundlich adsorption constants on GO-AG were 0.003 L mg-1 (KL) and 1.73 mol1-n Ln g-1 (KF), respectively, and the correlation coefficient R2 were 0.993 and 0.992, respectively. So the experimental data of adsorption on GO-AG were also fitted well with both Langmuir model and Freundlich model. According to Langmuir simulation, the maximum adsorption capacity of GO-AG was 1792.6 mg g-1, which was much higher than that of GO-FG (1531.7 mg g-1). The reasons may be the extra C-OH functional groups only appear on GO-AG. And the adsorption intensities in the parameters of Freundlich model on GO-FG and GO-AG were 1.63 and 5.26, respectively, which indicated cadmium ions were easier attached to GO-AG than GO-FG. It was worth mentioning that the adsorption capacity of Cd(II) on either GO-AG or GO-FG was much larger than most reported adsorbents as listed in Table 4. 3.2.2. Adsorption kinetics 11
Adsorption kinetics is generally used to describe the adsorption rate of adsorbents to adsorbates. Through the adsorption kinetics model, the equilibrium adsorption quantity and the main dynamics diffusion process of adsorbates in the environmental adsorption can be obtained. It can also offer a lot of help for the treatment of pollutants in the environment. Pseudo-first order kinetics model is suitable for describing the dynamics process controlled by the physical diffusion mechanism. The change of energy is basically not involved in the diffusion process. Pseudo-second order kinetics model is built on the basis of rate control steps during chemical reactions or chemical adsorption, including external liquid film diffusion, surface adsorption and diffusion in the granules. It can reflect real adsorption process. Intra-particle pore diffusion model is often used to describe the process of ion diffusion in the particles. It reflects the ion diffusion transfer mechanism of adsorption process. It can get kinetics parameters by fitting the experimental of adsorption capacity as a function of adsorption time by Pseudo-first-order (4)， Pseudo-second-order kinetics (5) and Intra-particle pore diffusion models (6), respectively: ln(q e − q t ) = ln q e − K1 t t qt
=
1 K2 ×q2e
+
(4)
t
(5)
qe
𝑞𝑡 = c + 𝐾𝑛 × 𝑡 0.5
(6)
where K1 (min-1), K2 (g mg-1 min-1) and Kn (mg g-1 m-0.5) are the pseudo-first-order, pseudo-second-order and intra-particle pore diffusion models rate constant,
12
respectively; qe (mg g-1) and qt (mg g-1) are the adsorption capacity of Cd(II) on graphene at equilibrium time and at time t (min), respectively. The fitting parameters of pseudo-first-order, pseudo-second-order kinetics and
intra-particle pore diffusion models for the adsorption of Cd(II) on GOs were listed in Table 3. The pseudo-first-order, pseudo-second-order and intra-particle pore diffusion models rate constants on GO-FG were 0.006 min-1 (K1), 0.04 g mg-1 min-1 (K2) and 0.204 mg g-1 m-0.5 (Kn), respectively. Initial adsorption rate (h) of adsorbate was 140.23 mg g-1 min-1. In addition, the pseudo-first-order, pseudo-second-orderand intra-particle pore diffusion models rate constants on GO-AG were 0.008 min-1 (K1), 0.005 g mg-1 min-1 (K2) and 0.509 mg g-1 m-0.5 (Kn), respectively. Initial adsorption rate (h) of adsorbate was 14.47 mg g-1 min-1. By comparing the correlation coefficients of the three models, the adsorption of Cd(II) on both GO-AG and GO-FG were best fitted by Pseudo-second-order kinetics model. Meanwhile, the experimental rate is fast. The whole reaction process finished within 2 hours. The GOs after adsorption of Cd(II) were measured by SEM-EDS (Fig. 11), AFM (Fig. 5) and FT-IR (Fig. 7). From the SEM-EDS images, cadmium ions were uniformly adsorbed on graphene oxides. The element energy diagram indicated the main element was cadmium on graphene oxide after adsorption in addition to carbon and oxygen. From AFM images in Fig. 12, the thickness of GO-AG and GO-FG after adsorption increased to 1.17 nm and 1.30 nm, respectively. The reason may be the surface of GOs loaded with cadmium and graphene oxide agglomerates easily after 13
adsorption. From the FT-IR spectra in Fig. 7, the -OH characteristic peaks shifted for both GO-AG and GO-FG, which may be because -OH groups participated in the complexation reaction with Cd(II). After adsorption, the peaks located at 1724-1723 cm-1 disappeared for both GOs, indicating that carboxylic groups were involved in the adsorption of Cd(II) by complexation[30]. And after adsorption, it was also noted that the peaks at 1225 cm-1 disappeared. It indicated the C-OH in phenol was involved in the adsorption reaction. The reaction only appeared on GO-AG, which caused the higher maximum adsorption capacity of GO-AG than that of GO-FG.
4. Conclusions (1) The maximum adsorption capacity of graphene oxide prepared with amorphous graphite was 1792.6 mg g-1, which was higher than that with flaky graphite (260.9 mg g-1). The reason may be that C-OH in phenol on appeared in GO-AG was involved in the adsorption reaction of Cd(II). The equilibrium data were fitted well with both Langmuir model and Freundlich model, and the adsorption rate was fitted with Pseudo-second-order precisely. (2) It was found that although the purity of amorphous graphite is worse than that of the flaky graphite, the GO-AG has greater adsorption capacity. So it is promising to use amorphous graphite as the raw material for graphene oxide synthesis instead of flaky graphite.
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Figure captions
Fig. 1. XRD spectra of raw graphite and corresponding graphene oxide Fig. 2. SEM images of FG (a) and AG (b) amplified 5000 times Fig. 3. TEM images of GO-FG (a) and GO-AG (b) amplified 500 times Fig. 4. Raman spectra of GOs and corresponding parameters Fig. 5. (a) AFM tapping mode image of GO-AG; (b) the sectional height profile corresponding to (a); (c) AFM tapping mode image of GO-FG; (d) the sectional height profile corresponding to (c). Fig. 6. XPS images of GO-AG (a) and GO-FG (b) Fig. 7. FT-IR spectra of GO-FG (a), GO-FG after Cd(II) adsorption (b), GO-AG (c) and GO-AG after Cd(II) adsorption (d). Fig. 8. Adsorption capacity of Cd(II) on GO-FG and GO-AG as a function of pH Fig. 9. Langmuir and Freundlich isotherm models for adsorption of Cd(II) on GO-AG and GO-FGGO-AG (d), respectively and Intra-particle pore diffusion model for adsorption of Cd(II) on GO-FG (e) and GO-AG (f), respectively. Fig. 11. (a) SEM-EDS images for adsorption of Cd(II) on GO-FG; (b) element energy diagram corresponding to (a); (c) SEM-EDS images for adsorption of Cd(II) on GO-AG; (d) element energy diagram corresponding to (c). Fig. 12. (a) AFM tapping mode image of GO-AG after Cd(II) adsorption; (b) section height profile corresponding to (a); (c) AFM tapping mode image of GO-FG after Cd(II) adsorption; (d) section height profile corresponding to (c).
18
Intensity (a.u.)
FG
AG GO-FG GO-AG
10
20 30 40 50 Diffraction angle (2)
60
Fig. 1.XRD spectra of raw graphite and corresponding graphene oxide
(a)
(b)
Fig. 2. SEM images of FG (a) and AG (b) amplified 5000 times
19
(a)
(b)
Intensity (a.u.)
Fig. 3. TEM images of GO-FG (a) and GO-AG (b) amplified 500 times
500
GO-FG
ID/IG=0.89;La=49.36
GO-AG
ID/IG=0.93;La=47.56
1000 1500 2000 Roman shift (cm^(-1))
2500
Fig. 4. Roman spectra of GOs and corresponding parameters
20
(a)
(b)
Thickness (nm)
1.0
0.5
0.0
-0.5 0.0
0.4
0.8 Length (μm)
1.2
(c)
Thickness (nm)
1.0
(d)
0.5
0.0
-0.5 0.0
0.3
0.6 Length (m)
0.9
1.2
Fig. 5. (a) AFM tapping mode image of GO-AG; (b) the sectional height profile corresponding to (a); (c) AFM tapping mode image of GO-FG; (d) the sectional height profile corresponding to (c).
Fig. 6. XPS images of GO-AG (a) and GO-FG (b)
21
1041 1051
1399 1384
1048 1054
a
1400 1384
1618 1723 1621
3420
b
1225
1724 1618 1616
3402 3395
c
3388
Transmittance(a.u.)
C=O C-OH C=C C-O OH
OH
d
4000 3500 3000 2500 2000 1500 1000 500 Wavelength(cm^(-1)) Fig. 7. FT-IR spectra of GO-FG (a), GO-FG after Cd(II) adsorption (b), GO-AG (c) and GO-AG after Cd(II) adsorption (d).
1200 GO-FG GO-AG
1000
qe (mg/g)
800 600 400 200 0
2
3
4
5
6
7
pH
Fig. 8. Adsorption capacity of Cd(II) on GO-FG and GO-AG as a function of pH
22
Fig. 9. Langmuir and Freundlich isotherm models for adsorption of Cd(II) on GO-AG and GO-FG
23
Fig. 10. Pseudo-first-order kinetics model for adsorption of Cd(II) on GO-FG (a) and GO-AG (b), respectively; Pseudo-second-order kinetics model for adsorption of Cd(II) on GO-FG (c) and GO-AG (d), respectively and Intra-particle pore diffusion model for adsorption of Cd(II) on GO-FG (e) and GO-AG (f), respectively.
24
(a)
(b)
(c)
(d)
Fig. 11. (a) SEM-EDS images for adsorption of Cd(II) on GO-FG; (b) element energy diagram corresponding to (a); (c) SEM-EDS images for adsorption of Cd(II) on GO-AG; (d) element energy diagram corresponding to (c).
25
2.0
(a)
(b) Thickness (nm)
1.5 1.0 0.5 0.0 -0.5
0
100
200 300 Length (m)
400
2.0
(c)
(d)
Thickness (nm)
1.5 1.0 0.5 0.0 -0.5
0
100
200 300 Length (m)
400
Fig. 12. (a) AFM tapping mode image of GO-AG after Cd(II) adsorption; (b) section height profile corresponding to (a); (c) AFM tapping mode image of GO-FG after Cd(II) adsorption; (d) section height profile corresponding to (c).
26
Table captions
Table 1 Composition analysis of flaky and amorphous graphite (wt%).
Elements Sample C
Mn
NaO
MgO
K2O
CaO
Zn
Pb
AG
90.502
0.011
0.046
0.09
0.202
0.158
0.006
0.002
FG
99.990
0
0.002
0
0
0.002
0
0
Table 2 The fitting parameters for adsorption of Cd(II) on GO-AG and GO-FG by Langmuir model and Freundlich model
Langmuir qm
Freundlich
KL
KF R2
nF
R2
(mg g-1)
(L mg-1)
(mol1-n Ln g-1)
GO-AG
1792.6
0.003
0.993
1.73
5.26
0.992
GO-FG
1531.7
0.003
0.973
8.08
1.63
0.971
27
Table 3 The first-order, second-order kinetics and intra-particle pore diffusion models parameters for the adsorption of Cd(II) on GOs.
GO-AG
GO-FG
59.67
61.42
K1 (min-1)
0.008
0.006
qe (mg g-1)
19.05
9.05
R2 h
0.68 14.47
0.66 140.23
K2 (g mg-1 min-1)
0.005
0.04
qe (mg g-1)
53.79
59.21
R2
0.98
0.99
Kn(mg g-1 m-0.5)
0.509
0.204
R2
0.54
0.38
qe (mg g-1) Pseudo-first kinetics
Pseudo-second kinetics
Intra-particle pore diffusion
28
Table 4 The comparative study on adsorption of Cd(II) Maximum adsorbed equilibrium time Adsorbent
amount, qmax
Refs (min)
(mg
g-1)
PANI/CoHCF/NC
27.17
400
[31]
Pumice
5.28
4320
[32]
Zeolite
13.26
4320
[32]
Vermiculite
16.07
4320
[32]
GO-FG
1551.06
120
In the study
GO-AG
2079.72
120
In the study
29
S2213-3437(17)30388-3 http://dx.doi.org/doi:10.1016/j.jece.2017.08.004 JECE 1799
To appear in: Received date: Revised date: Accepted date:
24-5-2017 2-8-2017 3-8-2017
Please cite this article as: Yu Zhang, Weijun Peng, Ling Xia, Shaoxian Song, Adsorption of Cd(II) at the Interface of water and graphene oxide prepared from flaky graphite and amorphous graphite, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Adsorption of Cd(II) at the Interface of water and graphene oxide prepared from flaky graphite and amorphous graphite
Yu Zhang1, Weijun Peng1, Ling Xia2, 3*, Shaoxian Song1,2*
1 School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China 2 Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China 3 Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China
Corresponding author: L. Xia([email protected]), S. Song([email protected])
1
ABSTRACT This study aimed to investigate the adsorption properties of Cd(II) on graphene oxide prepared from amorphous graphite and flaky graphite. The natural graphite was characterized by Scanning electron microscope, X-ray fluorescence spectrometer and X-ray diffraction. The equilibrium data were described well by both Langmuir model and Freundlich model, and the adsorption rate was fitted by Pseudo-second-order precisely. The GOs before and after adsorption were measured by Atomic force microscope, Fourier transform infrared spectrometer and Energy dispersive system. It was found that the larger adsorption capacity of Cd(II) on GO prepared from amorphous graphite was largely attributed to the more oxygenous functional groups appeared on GO-AG. Keywords: natural graphite; graphene oxide; Cd(II) removal; adsorbent
1. Introduction Heavy metal ions contamination has become a growing world concern in recent years. Cadmium is one of the most toxic elements, which can be found naturally in water, soil, air, ores and foodstuffs[1]. It’s well recognized for its adverse effects in the environment and it was accumulated readily in living systems[2]. According to the guideline of World Health Organization (WHO), a concentration of 0.005 mg L -1 of cadmium ions was set as the permitted concentration in drinking water[3]. Long term exposure to the Cd(II) in drinking water higher than the allowable concentration can lead to renal degradation, hepatotoxicity and carcinogenicity, lung insufficiency, and respiratory system injury[4, 5]. For protecting the human beings and the environment, it 2
is necessary to remove Cd(II) in water[6]. Currently, common methods for removing cadmium ions from aqueous solution included ion exchange[7], adsorption[8] and precipitation[9]. Among these techniques, adsorption is widely used due to the superior characteristics of speediness, economy and easy to operation[10]. Many materials have been reported as adsorbents for Cd(II) removal including yeast[11], fly ash[12], perlite[13], clay[14], activated carbon[15] and so on. Recently, GO (graphene oxide) has drawn increasing attention as an adsorbent for multivalent metal ions with enormous surface area, hydrophilicity and high negative charge density[16]. GO is the product of graphite by oxidation and exfoliation process, so that the characteristics of GO can be largely influenced by the raw graphite used. Flaky graphite (FG) and amorphous graphite (AG) are the most used natural graphite materials. While flaky graphite is the most used natural material for GO preparation due to the excellent layered structure, nano-sized holes and easy to expansion[17]. However, there are large reserves of natural graphite in China, of which the reserve of amorphous graphite is about 2 million 830 thousand tons. And the price of amorphous graphite is lower than that of flaky graphite. Amorphous carbon with a high fraction of diamond-like (sp3) bond can be used in many day-life applications (such as magnetic storage, optical storage, engine parts, wear protection and razor blades) [18, 19]. However, there is scarce literature focusing on GO preparation with amorphous graphite. In this work, GOs prepared from natural flaky graphite and amorphous graphite were used as adsorbents for removing Cd(II) from aqueous solution. Adsorption 3
performances were estimated by kinetic models and adsorption isotherms. SEM was used to study the surface feature of natural graphite. AFM, FT-IR and SEM-EDX were used to study the adsorption behavior. Then further analysis of the influence of graphite type on adsorption result was investigated.
2. Experiment 2.1. Materials and chemicals. Synthesize GOs with two kinds of natural graphite, amorphous and flaky graphite. The purity of flaky graphite was more than 99% while the purity of amorphous graphite was only 90%. Potassium permanganate (KMnO4), sodium hydroxide (NaOH), cadmium nitrate tetrahydrate (CdNO3·4HO2), sodium nitrate (NaNO3), 30% hydrogen peroxide aqueous solution (H2O2), 98% sulfuric acid (H2SO4) and 68% nitric acid (HNO3) used in the study were all of analytically pure grade. Deionized water used in the work was prepared by Milli-Q ultrapure water system (Direct-8, America). 2.2. Preparation of GO-AG and GO-FG GrOs (graphite oxide) were prepared by the Hummers method[20]. At first, 150 mL of H2SO4 were added into a conical flask and stirred strongly in ice-water. Then, 3.0 g of graphite and 1.5 g of NaNO3 were placed into the flask in sequence. After stirred for 10 min, 9.0 g of KMnO4 were added slowly into the mixture and the compound was keep stirring for 1 h at the temperature below 10 ℃. The next step was to put the compound into a pre-heated water bath at 35 ℃ and stirred continually for 4
another 2 h. Subsequently, add 150 mL of deionized water by drop in order to avoid the temperature rising sharply. After that, put the compound into a pre-heated water bath at 98 ℃ again and stir continually for 30 min. Add 100 mL of deionized water and 10mL of 30% H2O2 lastly. Centrifuge the mixture at 4000 rpm for 10 min with high speed centrifuge (Sorvall ST 16, Thermo Scientific, America). Wash the sediment with 5wt% hydrochloric acid solution for three times and followed by warm water (50 ℃). Finally, freeze-dried the sediment for 24 h in a vacuum, the powder was GrO. The GrOs prepared from flaky and amorphous graphite were referred to as GrO-FG and GrO-AG, respectively. Put 100 mg of GrO powder into 100 mL of deionized water. Then shear the mixture with FLUKO high-shear homogenizer (Germany) at 10000 rpm for 3 min. Exfoliate ultrasonically with a Cole-Parmer ultrasonic processor (CP505, American) subsequently. The amplitude and the time were set to 40% and 10 min, respectively. Centrifuge the colloidal suspension at 4000 rpm for 3 min and the supernatant was GO. The GOs prepared from GrO-FG and GrO-AG was referred as GO-FG and GO-AG, respectively. 2.3. Cd(II) adsorption Adsorption experiments were set up as triplicate. The 3 mg of GO and 100 mL Cd(II) aqueous solution were added into 150 mL conical flask. Then the suspension was shaken thoroughly by a water bath vibrator (SHA-B, China) with a speed of 150 rpm for 12 h at 25 ℃. For the effect of initial pH experiments, the range of pH value of initial solution was set within a range from 2 to7 with initial Cd(II) concentration 5
of 400 mg L-1. In the equilibrium experiments, the rang of Cd(II) concentration was from 10 mg L-1 to 1100 mg L-1 and the concentration of GO was 30 mg L-1. In the kinetics experiments, the concentration of Cd(II) and GO was 10 mg L-1 and 60 mg L-1, respectively. Get the samples at different time interval (1-180 min). Finally, the suspension was filtered by 0.22 μm filter membrane and Cd(II) concentration in the filtrate was immediately examined by Atomic absorption spectrometer (A6880, Shimadzu, Japan) [21]. The adsorption capacity of Cd(II) was calculated by the following equation: qt =
C0 −Ct m
×V
(1)
Where qt is the adsorption capacity of the adsorbent (mg g-1) C0 and Ct are the initial and determined concentration of Cd(II) (mg L-1), respectively; V is the volume of solution (L) and m is the weight of the adsorbent (g). 2.4. Methods X-ray diffraction pattern was obtained by an X-Ray Diffraction (XRD, D8 Advance, Germany) with Cu-Kα radiation. The component analysis of graphite was conducted using an X-ray fluorescence spectrometer (XRF, Axios advanced, China) operating at a power of 4 kW. The morphology of the samples was detected by the scanning electron microscope (SEM, JSM-IT300, Japan) and Transmission electron microscope (TEM, JEM2100F, Japan). Defect of lattice was determined by Raman spectrometer (Raman, INVIA, the British). Oxygen-containing functional groups of graphene were characterized by X-ray photoelectron spectrometer (XPS, VG Multilab2000, America). To determine the functional groups of samples, GOs before 6
and after adsorption of Cd(II), were scanned by a Fourier transform infrared spectrometer (FT-IR, Nicolet6700, America). The wavelength ranged from 4000 to 400 cm-1. The distribution of GO sheet thickness was obtained by the atomic force microscope (AFM, Multimode 8 Bruker, America). To prepared the samples, solution with GOs before and after Cd(II) adsorption were dropped onto a clean mica, then let stand for 12 h to dry at room temperature. The cadmium’s distribution on the graphene oxide after adsorption was determined by energy X-ray detector (EDS, Hitachi S4800, Japan).
3. Results and discussion 3.1. Characterization of adsorbents Fig. 1 shows the XRD spectra of raw flaky and amorphous graphite. The characteristic peaks of both graphite were located at 2θ=26.5°. So the average d-spacing of the two samples were about 0.336 nm, which corresponded to the reflection plane of graphite[22]. The peak intensity of flaky graphite was stronger than that of amorphous graphite, which indicated flaky graphite had more integrated lattice structure. Fig. 2 shows the surface morphology of flaky graphite (a) and amorphous graphite (b). Flaky graphite exhibited lamellar structure. Its lateral dimension was about 1-3 μm. However, amorphous graphite exhibits grain structure and its average lateral dimension was approximately 0.5 μm. Table1 lists the content of main elements in AG and FG. The purity of flaky graphite is more than 99%, while the purity of amorphous graphite is only 90%. 7
Obviously, the amounts of metal impurities in amorphous graphite were much more than that in flaky graphite. The prepared GOs from two kinds of graphite have been characterized by XRD, TEM, Raman spectra, XPS and FT-IR. From Fig. 1, it was found that after two kinds of graphite were prepared to graphene oxides by oxidation and stripping, the characteristic peaks of graphite were replaced by the characteristic peaks of graphene oxides located at 2θ=10°. The average d-spacing of the two adsorbents were about 0.8 nm, wider than that of graphite. The reason may be due to the appearance of oxygenous functional groups between the layers. From TEM results in Fig. 3, it was obvious that graphene had a characteristic of single-laminated structure. The horizontal diameter of GO-FG was about 1.2 μm, while the horizontal diameter of GO-AG was just 0.3-0.5μm. Raman images of GOs and the corresponding parameters are shown in Fig. 4. There are two characteristic peaks located at 1350 cm-1 and 1580 cm-1 for both GOs, respectively. The peaks at 1580 cm-1 were the characteristic peak of crystal carbon, called G-band, while the peaks at 1350 cm-1 were used to estimate the disorder and the defect of lattice. ID/IG combining the two parameters is usually used to determine the disorder of material. In this study, ID/IG of AG-GO was greater than that of FG-GO, suggested larger defection which may be more favorable to the adsorption reaction with metal ions. Fig. 5 shows the AFM images and corresponding sectional height profiles of GO-AG and GO-FG. Generally, the thickness of monolayer graphene oxide is about 8
0.8-1.0 nm. In this work, the thickness of grapheme oxide synthesized by AG and FG is approximately 0.804 nm and 0.926 nm, respectively. However, the thickness is higher than the thickness of theoretical monolayer grapheme. This may be because there are many oxygen containing functional groups on the graphene sheets[24, 25]. Fig. 6 shows the XPS images of GO-AG (a) and GO-FG (b). The maps can be divided into three symmetrical peaks, which corresponding O-C=O, C-O, C-C/C=C, respectively. It indicated the abundant oxygen-containing functional groups loaded at both graphene oxides. Fig. 7 showed the FT-IR spectra of GOs before and after Cd(II) adsorption. The peaks at 3420-3388 cm-1 were assigned to -OH stretching vibration in hydroxyl and water molecule[26,
27]
. The peaks at 1724-1723 cm-1 can be attributed to C=O
stretching vibration in carbonyl and carboxylic groups[28, 29]. The peaks at 1621-1616 cm-1 were C=C characteristic peaks by C=C stretching vibration in unoxidized graphite domains. The peaks at 1054-1041 cm-1 were C-O characteristic peaks by C-O Stretching vibration. The above functional groups were found in both GOs. It was noted that the peak at 1225 cm-1 assigned to C-OH in phenol [29] only appeared in GO-AG. According to previous studies, amorphous graphite is easier to be oxidized than flaky graphite[28]. This may be the reason that oxygenous functional groups found in GO-AG. 3.2. Cd(II) adsorption 3.2.3 Effect of initial pH
9
Considering that cadmium ions begin to precipitate in water under the condition of pH 8, the intimal pH was set within a pH range of 2 to 7. As presented in Fig. 8, the adsorption capacity was significantly affected by the initial solution pH value. The adsorption capacity of Cd(II) was fluctuated with pH value, and showed a maximum value at pH 4. Given that the prepared GOs without any adjustment were of pH 3-4, the subsequent experiments were conducted without any initial pH adjustment. 3.2.2. Adsorption isotherm The experiments of adsorption of Cd(II) on GOs prepared from AG and FG as a function of initial Cd(II) concentrations were conducted and the results were fitted by two isotherm models, Langmuir model (2) and Freundlich model (3), respectively. Langmuir adsorption model is often used to describe a simple monolayer adsorption process. It hypothesizes there is a certain number of adsorption sites on the surface of the adsorbent in the theory, and each adsorption site only adsorbed by one atom or molecule. The linear isotherm equation is presented as following Eq. (2). The Freundlich model is often used to describe the process of layers adsorption. It assumes that the active sites on the surface of the adsorbent are exponential in the theory, and its linear equation is as following Eq. (3). Ce qe
=
Ce qm
+
1
(2)
KL ×qm 1
ln q e = ln K F + n × ln Ce
(3)
Where qe is the equilibrium adsorption capacity (mg g-1); Ce is the equilibrium concentration of Cd(II) in solution (mg L-1); qm is maximum adsorption capacity (mg
10
g-1); KL (L mg-1) and KF (mol1-n Ln g-1) are Langmuir and Freundlich adsorption constant, respectively; and n represents adsorption intensity. The fitting parameters for adsorption of Cd(II) on GO-AG and GO-FG fitted by Langmuir model and Freundlich model were listed in Table 2. The separation factors (RL) of GOs were both 0.29, which is between 0 and 1. So the reaction was more likely to occur. The Langmuir and Freundlich adsorption constants on GO-FG were 0.003 L mg-1 (KL) and 8.08 mol1-n Ln g-1 (KF), respectively, and the correlation coefficient R2 were 0.973 and 0.971, respectively. So the experimental data of adsorption on GO-FG were fitted well with both Langmuir model and Freundlich model. The Langmuir and Freundlich adsorption constants on GO-AG were 0.003 L mg-1 (KL) and 1.73 mol1-n Ln g-1 (KF), respectively, and the correlation coefficient R2 were 0.993 and 0.992, respectively. So the experimental data of adsorption on GO-AG were also fitted well with both Langmuir model and Freundlich model. According to Langmuir simulation, the maximum adsorption capacity of GO-AG was 1792.6 mg g-1, which was much higher than that of GO-FG (1531.7 mg g-1). The reasons may be the extra C-OH functional groups only appear on GO-AG. And the adsorption intensities in the parameters of Freundlich model on GO-FG and GO-AG were 1.63 and 5.26, respectively, which indicated cadmium ions were easier attached to GO-AG than GO-FG. It was worth mentioning that the adsorption capacity of Cd(II) on either GO-AG or GO-FG was much larger than most reported adsorbents as listed in Table 4. 3.2.2. Adsorption kinetics 11
Adsorption kinetics is generally used to describe the adsorption rate of adsorbents to adsorbates. Through the adsorption kinetics model, the equilibrium adsorption quantity and the main dynamics diffusion process of adsorbates in the environmental adsorption can be obtained. It can also offer a lot of help for the treatment of pollutants in the environment. Pseudo-first order kinetics model is suitable for describing the dynamics process controlled by the physical diffusion mechanism. The change of energy is basically not involved in the diffusion process. Pseudo-second order kinetics model is built on the basis of rate control steps during chemical reactions or chemical adsorption, including external liquid film diffusion, surface adsorption and diffusion in the granules. It can reflect real adsorption process. Intra-particle pore diffusion model is often used to describe the process of ion diffusion in the particles. It reflects the ion diffusion transfer mechanism of adsorption process. It can get kinetics parameters by fitting the experimental of adsorption capacity as a function of adsorption time by Pseudo-first-order (4)， Pseudo-second-order kinetics (5) and Intra-particle pore diffusion models (6), respectively: ln(q e − q t ) = ln q e − K1 t t qt
=
1 K2 ×q2e
+
(4)
t
(5)
qe
𝑞𝑡 = c + 𝐾𝑛 × 𝑡 0.5
(6)
where K1 (min-1), K2 (g mg-1 min-1) and Kn (mg g-1 m-0.5) are the pseudo-first-order, pseudo-second-order and intra-particle pore diffusion models rate constant,
12
respectively; qe (mg g-1) and qt (mg g-1) are the adsorption capacity of Cd(II) on graphene at equilibrium time and at time t (min), respectively. The fitting parameters of pseudo-first-order, pseudo-second-order kinetics and
intra-particle pore diffusion models for the adsorption of Cd(II) on GOs were listed in Table 3. The pseudo-first-order, pseudo-second-order and intra-particle pore diffusion models rate constants on GO-FG were 0.006 min-1 (K1), 0.04 g mg-1 min-1 (K2) and 0.204 mg g-1 m-0.5 (Kn), respectively. Initial adsorption rate (h) of adsorbate was 140.23 mg g-1 min-1. In addition, the pseudo-first-order, pseudo-second-orderand intra-particle pore diffusion models rate constants on GO-AG were 0.008 min-1 (K1), 0.005 g mg-1 min-1 (K2) and 0.509 mg g-1 m-0.5 (Kn), respectively. Initial adsorption rate (h) of adsorbate was 14.47 mg g-1 min-1. By comparing the correlation coefficients of the three models, the adsorption of Cd(II) on both GO-AG and GO-FG were best fitted by Pseudo-second-order kinetics model. Meanwhile, the experimental rate is fast. The whole reaction process finished within 2 hours. The GOs after adsorption of Cd(II) were measured by SEM-EDS (Fig. 11), AFM (Fig. 5) and FT-IR (Fig. 7). From the SEM-EDS images, cadmium ions were uniformly adsorbed on graphene oxides. The element energy diagram indicated the main element was cadmium on graphene oxide after adsorption in addition to carbon and oxygen. From AFM images in Fig. 12, the thickness of GO-AG and GO-FG after adsorption increased to 1.17 nm and 1.30 nm, respectively. The reason may be the surface of GOs loaded with cadmium and graphene oxide agglomerates easily after 13
adsorption. From the FT-IR spectra in Fig. 7, the -OH characteristic peaks shifted for both GO-AG and GO-FG, which may be because -OH groups participated in the complexation reaction with Cd(II). After adsorption, the peaks located at 1724-1723 cm-1 disappeared for both GOs, indicating that carboxylic groups were involved in the adsorption of Cd(II) by complexation[30]. And after adsorption, it was also noted that the peaks at 1225 cm-1 disappeared. It indicated the C-OH in phenol was involved in the adsorption reaction. The reaction only appeared on GO-AG, which caused the higher maximum adsorption capacity of GO-AG than that of GO-FG.
4. Conclusions (1) The maximum adsorption capacity of graphene oxide prepared with amorphous graphite was 1792.6 mg g-1, which was higher than that with flaky graphite (260.9 mg g-1). The reason may be that C-OH in phenol on appeared in GO-AG was involved in the adsorption reaction of Cd(II). The equilibrium data were fitted well with both Langmuir model and Freundlich model, and the adsorption rate was fitted with Pseudo-second-order precisely. (2) It was found that although the purity of amorphous graphite is worse than that of the flaky graphite, the GO-AG has greater adsorption capacity. So it is promising to use amorphous graphite as the raw material for graphene oxide synthesis instead of flaky graphite.
14
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Figure captions
Fig. 1. XRD spectra of raw graphite and corresponding graphene oxide Fig. 2. SEM images of FG (a) and AG (b) amplified 5000 times Fig. 3. TEM images of GO-FG (a) and GO-AG (b) amplified 500 times Fig. 4. Raman spectra of GOs and corresponding parameters Fig. 5. (a) AFM tapping mode image of GO-AG; (b) the sectional height profile corresponding to (a); (c) AFM tapping mode image of GO-FG; (d) the sectional height profile corresponding to (c). Fig. 6. XPS images of GO-AG (a) and GO-FG (b) Fig. 7. FT-IR spectra of GO-FG (a), GO-FG after Cd(II) adsorption (b), GO-AG (c) and GO-AG after Cd(II) adsorption (d). Fig. 8. Adsorption capacity of Cd(II) on GO-FG and GO-AG as a function of pH Fig. 9. Langmuir and Freundlich isotherm models for adsorption of Cd(II) on GO-AG and GO-FGGO-AG (d), respectively and Intra-particle pore diffusion model for adsorption of Cd(II) on GO-FG (e) and GO-AG (f), respectively. Fig. 11. (a) SEM-EDS images for adsorption of Cd(II) on GO-FG; (b) element energy diagram corresponding to (a); (c) SEM-EDS images for adsorption of Cd(II) on GO-AG; (d) element energy diagram corresponding to (c). Fig. 12. (a) AFM tapping mode image of GO-AG after Cd(II) adsorption; (b) section height profile corresponding to (a); (c) AFM tapping mode image of GO-FG after Cd(II) adsorption; (d) section height profile corresponding to (c).
18
Intensity (a.u.)
FG
AG GO-FG GO-AG
10
20 30 40 50 Diffraction angle (2)
60
Fig. 1.XRD spectra of raw graphite and corresponding graphene oxide
(a)
(b)
Fig. 2. SEM images of FG (a) and AG (b) amplified 5000 times
19
(a)
(b)
Intensity (a.u.)
Fig. 3. TEM images of GO-FG (a) and GO-AG (b) amplified 500 times
500
GO-FG
ID/IG=0.89;La=49.36
GO-AG
ID/IG=0.93;La=47.56
1000 1500 2000 Roman shift (cm^(-1))
2500
Fig. 4. Roman spectra of GOs and corresponding parameters
20
(a)
(b)
Thickness (nm)
1.0
0.5
0.0
-0.5 0.0
0.4
0.8 Length (μm)
1.2
(c)
Thickness (nm)
1.0
(d)
0.5
0.0
-0.5 0.0
0.3
0.6 Length (m)
0.9
1.2
Fig. 5. (a) AFM tapping mode image of GO-AG; (b) the sectional height profile corresponding to (a); (c) AFM tapping mode image of GO-FG; (d) the sectional height profile corresponding to (c).
Fig. 6. XPS images of GO-AG (a) and GO-FG (b)
21
1041 1051
1399 1384
1048 1054
a
1400 1384
1618 1723 1621
3420
b
1225
1724 1618 1616
3402 3395
c
3388
Transmittance(a.u.)
C=O C-OH C=C C-O OH
OH
d
4000 3500 3000 2500 2000 1500 1000 500 Wavelength(cm^(-1)) Fig. 7. FT-IR spectra of GO-FG (a), GO-FG after Cd(II) adsorption (b), GO-AG (c) and GO-AG after Cd(II) adsorption (d).
1200 GO-FG GO-AG
1000
qe (mg/g)
800 600 400 200 0
2
3
4
5
6
7
pH
Fig. 8. Adsorption capacity of Cd(II) on GO-FG and GO-AG as a function of pH
22
Fig. 9. Langmuir and Freundlich isotherm models for adsorption of Cd(II) on GO-AG and GO-FG
23
Fig. 10. Pseudo-first-order kinetics model for adsorption of Cd(II) on GO-FG (a) and GO-AG (b), respectively; Pseudo-second-order kinetics model for adsorption of Cd(II) on GO-FG (c) and GO-AG (d), respectively and Intra-particle pore diffusion model for adsorption of Cd(II) on GO-FG (e) and GO-AG (f), respectively.
24
(a)
(b)
(c)
(d)
Fig. 11. (a) SEM-EDS images for adsorption of Cd(II) on GO-FG; (b) element energy diagram corresponding to (a); (c) SEM-EDS images for adsorption of Cd(II) on GO-AG; (d) element energy diagram corresponding to (c).
25
2.0
(a)
(b) Thickness (nm)
1.5 1.0 0.5 0.0 -0.5
0
100
200 300 Length (m)
400
2.0
(c)
(d)
Thickness (nm)
1.5 1.0 0.5 0.0 -0.5
0
100
200 300 Length (m)
400
Fig. 12. (a) AFM tapping mode image of GO-AG after Cd(II) adsorption; (b) section height profile corresponding to (a); (c) AFM tapping mode image of GO-FG after Cd(II) adsorption; (d) section height profile corresponding to (c).
26
Table captions
Table 1 Composition analysis of flaky and amorphous graphite (wt%).
Elements Sample C
Mn
NaO
MgO
K2O
CaO
Zn
Pb
AG
90.502
0.011
0.046
0.09
0.202
0.158
0.006
0.002
FG
99.990
0
0.002
0
0
0.002
0
0
Table 2 The fitting parameters for adsorption of Cd(II) on GO-AG and GO-FG by Langmuir model and Freundlich model
Langmuir qm
Freundlich
KL
KF R2
nF
R2
(mg g-1)
(L mg-1)
(mol1-n Ln g-1)
GO-AG
1792.6
0.003
0.993
1.73
5.26
0.992
GO-FG
1531.7
0.003
0.973
8.08
1.63
0.971
27
Table 3 The first-order, second-order kinetics and intra-particle pore diffusion models parameters for the adsorption of Cd(II) on GOs.
GO-AG
GO-FG
59.67
61.42
K1 (min-1)
0.008
0.006
qe (mg g-1)
19.05
9.05
R2 h
0.68 14.47
0.66 140.23
K2 (g mg-1 min-1)
0.005
0.04
qe (mg g-1)
53.79
59.21
R2
0.98
0.99
Kn(mg g-1 m-0.5)
0.509
0.204
R2
0.54
0.38
qe (mg g-1) Pseudo-first kinetics
Pseudo-second kinetics
Intra-particle pore diffusion
28
Table 4 The comparative study on adsorption of Cd(II) Maximum adsorbed equilibrium time Adsorbent
amount, qmax
Refs (min)
(mg
g-1)
PANI/CoHCF/NC
27.17
400
[31]
Pumice
5.28
4320
[32]
Zeolite
13.26
4320
[32]
Vermiculite
16.07
4320
[32]
GO-FG
1551.06
120
In the study
GO-AG
2079.72
120
In the study
29