Efficient removal of Cs(I) from aqueous solution using graphene oxide

Progress in Nuclear Energy xxx (xxxx) xxx

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Efficient removal of Cs(I) from aqueous solution using graphene oxide Min Xing a, b, Shuting Zhuang a, Jianlong Wang a, c, * a

Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China Beijing Municipal Research Institute of Environmental Protection, Beijing 100084, PR China c Beijing Key Laboratory of Radioactive Waste Treatment, INET, Tsinghua University, Beijing 100084, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Graphene oxide Cesium Radionuclide Adsorption

In this study, graphene oxide was synthesized, characterized and applied for cesium removal from aqueous solution. The effect of various parameters, including adsorbent dosage (0.05–0.2 g L 1), pH (3–9) and ionic strength (NaNO3 0.001–0.1 mo L 1) on adsorption capacity, as well as its adsorption kinetics, isotherms, ther­ modynamics and mechanism were investigated. The experimental results indicated that the adsorption of cesium (I) by the graphene oxide was an exothermic, pH- and ionic strength-dependent process. The pseudo secondorder kinetic model and the Langmuir model fitted this process well, and the adsorption capacity was calcu­ lated to be 95.46 mg g 1. According to the analytical results of XRD, FTIR, and XPS, the adsorption mechanism for cesium ions should be attributed to the oxygen-containing functional groups of the graphene oxide.

1. Introduction Cesium is a major fission product, which is common in the radioac­ tive wastewater generated from the nuclear power plants, reprocessing of spent fuels, radionuclides production facilities, etc. In the Fukushima accident, cesium is one of the main radionuclides. Among various ra­ dionuclides, radio-cesium [t1/2(137Cs) ¼ 30 a, strong γ-emission] ex­ hibits more risks than the short-lived radionuclides in the later decay periods (Wang et al., 2018). Additionally, cesium ions are very difficult to be removed from aqueous solution in comparison with other nuclides or heavy metals (Alby et al., 2017; Chen and Wang, 2011, 2012a; 2012b; Zhu et al., 2012, 2014). Therefore, the removal of cesium ions is vital while challenging. So far, various kinds of technologies have been developed for cesium removal, such as chemical precipitation, membrane separation (Liu et al., 2019, 2018; Jia et al., 2017; Jia and Wang, 2017; Liu and Wang, 2013), solvent extraction, and adsorption (Wang and Zhuang, 2019a, 2019b). Among these methods, adsorption has received more attention due to its ease of operation, low cost, and less energy consumption (Wang and Chen, 2009, 2014; Wang and Zhuang, 2017). Several ad­ sorbents, such as clay minerals (Yang et al., 2016), ammonium molyb­ dophosphate (Park et al., 2010), Prussian blue and its analogues (Hwang et al., 2017; Wang et al., 2018), as well as biosorbents (Yin et al., 2017; Chen and Wang, 2016; Wang and Chen, 2006) have been applied for cesium removal. However, they have low or moderate adsorption

capacity or need extreme pH conditions for the practical radioactive wastewater. Carbon-based materials, such as activated carbon, have been widely applied in large scale wastewater treatments, however, they cannot meet the demand of specific application for cesium removal due to the low adsorption capacity and selectivity for cesium ions against other background ions (Wang and Wang, 2019; Caccin et al., 2013; Ofomaja et al., 2013). Recently, more attention has been given to graphene-based materials for heavy metal ions removal, especially the graphene oxide (Xu et al., 2011). Graphene oxide is synthesized from the oxidation of twodimensional carbon allotrope, graphene. It is known for extraordinary specific surface area and abundant O-containing functional groups, such as –COOH, and –OH. These characteristics offer graphene oxide with substantial adsorptive sites for various kinds of heavy metal ions (Duru et al., 2016) and organic pollutants. Previously, our research group has reported a series of graphene-based adsorbents (Xing et al., 2016; Zhuang et al., 2018; Zhuang and Wang, 2019). For example, the nano-scaled zero valent iron/graphene composite showed a high adsorption capacity of 134.27 mg g 1 (pH ¼ 5.7, T ¼ 30 � C) for cobalt ions (Xing and Wang, 2016), indicating the potential application of graphene oxide. The application of graphene oxide for pollutants removal has been related to its composites or derivatives in most cases. For example, the pectin-stabilized magnetic graphene oxide Prussian blue (PSMGPB)

* Corresponding author. Energy Science Building, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.pnucene.2019.103167 Received 10 February 2019; Received in revised form 2 September 2019; Accepted 22 September 2019 0149-1970/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Min Xing, Progress in Nuclear Energy, https://doi.org/10.1016/j.pnucene.2019.103167

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nanocomposites (qm ¼ 1.609 mmol g 1), the polyaniline-grafted GO (qm ¼ 1.39 mmol g 1), and magnetic graphene oxide (qm ¼ 15.8 mg g 1) have been synthesized (Kadam et al., 2016; Sun et al., 2013; Wang and Yu, 2014). Both the Prussian blue and polyaniline are potential for ce­ sium removal. Additionally, the blending and grafting may deteriorate the adsorption capacity of graphene oxide owing to the consumption of active sites. Although high or moderate adsorption capacity has been achieved, the adsorption of cesium ions by graphene oxide has not yet fully understand. Kaewmee et al. (2017) and Tan et al. (2016) reported pristine graphene oxide for cesium removal. But quite different adsorption capacities have been achieved, varying from 528 mg g 1 to 40.00 mg g 1. Further research on the basic study of graphene oxide’s adsorption performance for cesium ions is of great importance to pro­ vide an insight into its modification strategies and further application. Herein, the graphene oxide was synthesized by the modified Hum­ mers’ method (Hummers and Offeman, 1958) and applied for the adsorption of cesium from aqueous solution. The effect of various con­ ditions (e.g. adsorbent dosage, pH, ionic strength, contact time, and initial concentration), together with different adsorption kinetic and isothermal models fitting, have been studied. Furthermore, its adsorp­ tion mechanism was studied by various analytical tools, including XRD, FTIR, XPS and SEM-EDX.

Fig. 1. N2 adsorption desorption isotherm and pore size distribution of graphite oxide using BJH method (desorption branch of the isotherm).

2. Materials and methods 2.1. Chemicals Graphene (EG-JF90-50N) was purchased from the Beijing Invention Biology Engineering & New Material Co., Ltd (China). H2O2 solution (30 wt %), potassium permanganate, sulfuric acid solution (98 wt %), and hydrochloric acid solution were all analytical grade and available at Sinopharm Chemical Reagent Co., Ltd (China). 2.2. Preparation of graphene oxide The graphene oxide was synthesized according to the modified Hummers and Offeman (1958) method. Firstly, graphene (1.0 g) and sulfuric acid solution (98 wt %, 106 mL) were mixed in a three-necked flask for 30 min with an ice-water bath. Then, potassium permanga­ nate (5 g) was slowly added into the mixture and stirred for 3 days at 25 � 1 � C. After the reaction, three batches of deionized water (40 mL) were orderly added into the mixture and stirred for 30 min at 30 � 1 � C, 60 � 1 � C and 90 � 1 � C, respectively. A small amount of H2O2 solution was applied to remove the remaining MnO4 . Subsequently, the precip­ itation was obtained by centrifuging at 12000 r min 1 for 30 min, and was washed with 1 M hydrochloric acid solution and deionized water. The graphene oxide was obtained by freeze-drying.

Fig. 2. Zeta potential of Graphene oxide.

qt ¼ ðC0

Ct Þ V=m

(1)

where m (g) stands for the mass of the graphene oxide; V (L) is the volume of the solution; Ct (mg L 1) represents the remaining concen­ tration of cesium ions at time t. All experiments were repeated three times.

2.3. Adsorption experiments A series of batch adsorption experiments were carried out to deter­ mine the effect of process variables on adsorption capacity. The initial pH of the solution was adjusted using NaOH or HCl solution. The sam­ ples were shaken at the rate of 150 r min 1. The other conditions of each experiment, except the controlled factor, were attached after the cor­ responding figures. The value of pH or ionic strength was fixed initially without further adjusting. Generally, a well-known dosage of the graphene oxide (0.05 g L 1 -0.2 g L 1) was put into a Cs(I)-containing solution with an initial con­ centration (C0) ranging from 5 mg L 1 to 200 mg L 1. The other opera­ tional conditions were studied at pH from 3.0 to 9.0, ionic strength (NaNO3) from 0 to 0.1 M, and temperature from 283 K to 313 K. After 24 h, the suspension was filtered by a 0.22 μm membrane and the con­ centration of cesium ions before and after adsorption was measured. The adsorption capacity (qt) can be calculated by the following equation:

2.4. Analytical methods The concentration of cesium ions before and after adsorption was quantified using a ZA3000 Polarized Zeeman Atomic Absorption Spec­ trophotometer (HITACHI, Japan). The Brunauer-Emmett-Teller (BET) measurement was carried out by a Micromeritics Tristar II 3020. Pore structural parameters were determined by the Barrett Joyner Halenda (BJH) method using the desorption branch of the isothermals. The Zeta potential was measured using a Horiba SZ-100Z. A field emission scanning electron microscopy (FE-SEM, JSM-7001F) equipped with an Oxford INCA energy-dispersive X-ray (EDX) spectrometer was used for the morphological and elemental analysis. The XRD spectra were recorded by a Bruker D8-Advance. The data were received from 5� to 90� with a scanning rate of 8� min 1, the voltage of 40 kV, and electric current of 40 mA. The FTIR spectra were measured using a Perkin Elmer 2

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Fig. 3. Effect of initial pH on the removal of Cs(I) by graphene oxide. C0 ¼ 10 mg L 1, m/V ¼ 0.2 g L 1, T ¼ 30 � C.

Fig. 5. Adsorption kinetics of Cs(I) on graphene oxide. C0 ¼ 10 mg L 1, m/ V ¼ 0.2 g L 1, pH ¼ 6, T ¼ 30 � C.

0.31 cm3 g 1, respectively. The pHpzc of the synthetic graphene oxide was measured by the pH drift method, as shown in Fig. 2. Results showed that the pHpzc was 1.0, which may be attributed to the deprotonation of hydroxyl and carboxyl groups in graphene oxide (Tan et al., 2016). When pH < pHpzc, the surface charge of the adsorbent was positive due to the protonation reaction (SOH þ Hþ→SOHþ 2 ); while pH > pHpzc, the surface charge of the adsorbent was negative due to the deprotonation reaction (SOH→SO þ Hþ), where S represented the surface of the adsorbent, and –OH represented the oxygen functional groups (Zhao et al., 2011). 3.2. Effect of pH The effect of initial pH on Cs(I) adsorption by graphene oxide was shown in Fig. 3. Adsorption capacity increased gradually with the in­ crease of pH at the range of 3.0–9.0. The species of the heavy metal ions and the surface charge of the adsorbent were dependent on pH value (Kula et al., 2008). The main species of Cs(I) was Cs(I) at this pH range (Wang and Zhuang, 2019b). As the pHpzc of graphene was 1.0 (Fig. 2), the surface of adsorbent was negative charged at this pH range (3.0–9.0), which was occupied by the positively charged co-cations (e.g. Naþ and Hþ) at the electrified interface to maintain its electrical neutrality. The adsorption space is always filled with solvent and solute molecules, and the displacement of solvent molecules is by the oncoming solute species. The ion exchange between Cs(I) and other cations accounted for the adsorption of cesium ions into graphene. At lower pH values, adsorption was hindered by the competition between Hþ and Cs(I) for adsorption sites, thus the adsorption capacity was relatively low (8 mg g 1 at pH 3.0). Similar results were also found in the adsorption of Cs(I) by activated carbon (Hanafi, 2010), CNTs (Yavari et al., 2011) and graphene oxide (Tan et al., 2016).

Fig. 4. Effect of NaNO3 ionic strength on the removal of Cs(I) by graphene oxide. C0 ¼ 10 mg L 1, m/V ¼ 0.2 g L 1, T ¼ 30 � C.

Spectrum GX from 400 cm 1 to 4000 cm 1. The XPS spectra were ob­ tained using a PHI Quantera SXM with the Kα of Al as the excitation source. 3. Results and discussion 3.1. Characterization of graphene oxide The physicochemical properties of the graphene oxide were studied by measuring the N2 adsorption desorption isotherm and pore size distribution of graphite oxide (Fig. 1), as well as the Zeta potential (Fig. 2). As shown in Fig. 1, the adsorption curve of the graphene oxide was mixture of type I and type IV isothermal of IUPAC classification with a H3 hysteresis loop (Sing et al., 1985; Thommes et al., 2015). The BET surface area was calculated to be 93.7 m2 g 1. The hysteresis loops of type H3 may be attributed to the slit-shaped mesopores derived from the stacking of plate-like graphene oxide (Ji et al., 2014). Pore size distri­ bution convinced the presence of mesopores on the graphite oxide (Sing et al., 1985). Its pore size and pore volume were about 2.43 nm and

3.3. Effect of ionic strength The effect of ionic strength on the adsorption of cesium ions by graphene oxide was presented in Fig. 4. NaNO3 was chosen as the electrolyte due to its minimal complexation with heavy metals (Criscenti and Sverjensky, 1999), and the comparable ionic radii between Naþ and Kþ, as well as the abundance of Naþ in seawater (Wang and Zhuang, 2019b). As shown in Fig. 4, pH 3.0, pH 6.0 and pH 9.0 were chosen to present the acidic, neutral and alkaline environment. The results indi­ cated that the adsorption capacity decreased with increase of ionic 3

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Table 1 Parameters of adsorption kinetics for Cs(I) adsorption by graphene oxide. Kinetic models

Parameters

Pseudo-first order Pseudo-second order

k

qe (mg g 1)

R2

9.58 � 3.47 2.53 � 0.93

5.08 � 0.27 5.35 � 0.23

0.849 0.923

strength at all given pH values. The similar results were also observed previously (Wang and Yu, 2014; Yavari et al., 2011). With the increase of the ionic strength, the activity coefficient of cesiun ions decreased, thus hindering the migration of ions to the adsorbent surface. In addi­ tion, the decrease of adsorption capacity may also be due to the competition between Cs(I) and Naþ for adsorption sites. Compared with the control group without addition of NaNO3, the effect of pH on cesium adsorption capacity was different. The results showed that in the presence of NaNO3, pH ¼ 6 seemed to be the optimal pH condition for cesium removal. Similar results were also observed by Tan et al. (2016) using 0.001–0.1 M NaClO4 to study the ionic strength effect. 3.4. Effect of contact time and adsorption kinetics The adsorption capacity at different contacting time was investi­ gated. As shown in Fig. 5, adsorption capacity increased rapidly within the first 4 h, then increased slowly and reached equilibrium within 8 h. To further identify the adsorption performance, it is important to study the adsorption rate, which can provide the information on the equilibrium time, as well as the possible rate-controlling steps (Guo and Wang, 2019a, 2019b). In this study, the pseudo first-order and pseudo second-order rate equations were applied for fitting the kinetic data. Their linear mathematic equations were given in Eq. (2) and Eq. (3), respectively. logðqe

qt Þ ¼ log qe

k1 t

t 1 t ¼ þ q k2 qe 2 qe

(2) (3)

where qe (mg g 1) and qt (mg g 1) are the adsorbed amounts at equi­ librium and at time t, respectively; and k1 (min 1) and k2 (g⋅mg 1⋅min 1) are the rate constant of the pseudo first-order model and the pseudo second-order model, respectively. The fitting results were also given in Fig. 5 and Table 1. The pseudo second-order equation fitted better with the adsorption data (R2 ¼ 0.923), and qe (5.35 mg g 1) calculated by the pseudo secondorder equation was closer to that of experimental qe. The pseudo firstorder kinetics assumed that adsorption rate was controlled by single process or mechanism, while most adsorption process contained multi­ ple processes and mechanisms. The pseudo second-order kinetic model also indicated that chemical reaction was the rate-controlling mecha­ € �lu et al., 2011). nisms (Ozero g

Fig. 6. The Langmuir (top) and Freundlich (bottom) isotherm model fitted for the adsorption of Cs(I) on graphene oxide at different temperatures C0¼(5–200) mg L 1, m/V ¼ 0.2 g L 1, t ¼ 24 h, pH ¼ 6.

Ce (mg L 1) represents the equilibrium concentration of Cs(I); n is the Freundlich constant related to the adsorption intensity; KL (L mg 1) and KF (mg g 1) are constants for the Langmuir related to the affinity of binding sites and the Freundlich model related to the sorption capacity, respectively. The fitting results of these two models were presented in Fig. 6 and the parameters were summarized in Table 2. From the comparison of the determined correlation coefficients (R2), the Langmuir model (R2 > 0.95) fitted the data better than that of the Freundlich model (0.86 < R2 < 0.94) in all given conditions. With the increase of the temperature from 283 K to 313 K, the adsorption capacity (qm) calculated from the Langmuir model decreased and the maximal adsorption capacity (95.46 mg g 1) was achieved at 283 K. It should be noted that the goodness-of-fit is not a sufficient condition to maintain that the model describes well the phenomenon studied. To fit a particular polynomial expression (i.e., the Langmuir’s polynomial) to the experimental data cannot guarantee that the adsorption mecha­ nism is consistent with model. It is really necessary to understand that the Langmuir model

3.5. The adsorption isotherms and thermodynamics Adsorption isotherms are vital for the identification of adsorption capacity and possible mechanism. Monolayer adsorption model, the Langmuir model, as well as the experimental Freundlich model, were employed in this study, and their mathematic equations were given as follows: Langmuir model : qe ¼

q m KL Ce 1 þ KL Ce

Freundlich model : qe ¼ KF Ce1=n

(4) (5)

where qe (mg g 1) stands for the equilibrium amount of Cs(I) adsorbed; 4

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Table 2 Isotherm parameters for the adsorption of Cs into graphene oxide. T (K)

Langmuir qm (mg g

283 293 303 313

Freundlich 1

)

KL (L mg

95.46 � 6.62 86.54 � 2.19 95.38 � 6.08 77.35 � 6.63

1

2

)

0.026 � 0.005 0.032 � 0.002 0.024 � 0.004 0.028 � 0.007

KF (L mg

0.971 0.995 0.983 0.952

8.28 � 2.26 11.07 � 2.49 7.36 � 2.25 6.83 � 2.38

ΔG0 (kJ mol

283 293 303 313

19.45 20.01 19.97 20.53

1

)

ΔH0 (kJ mol 10.45

1

)

ΔS0 (J mol

1

K 1)

32.00

describes, on a molecular level, an equilibrium between a non-adsorbed molecule and an unoccupied surface site. In the case of adsorption from solution, the phenomenon is always competitive. This is an exchange process, because: (1) the adsorption space is always filled with solvent and solute molecules (the displacement of solvent molecules by the oncoming solute species), (2) when the phenomenon takes the form of ion exchange, the adsorption of one type of ions (e.g., Cs(I) ions) is accompanied by the desorption of the equivalent amount of the co-ions pre-adsorbed at the electrified interface to maintain its electrical neutrality. The thermodynamic parameters of the adsorption were calculated from the data at varying temperatures by the following equations (Van’t Hoff equation): ΔS0 R

ΔH 0 RT

(6)

ΔG0 ¼ ΔH 0

TΔS0

(7)

ln K0 ¼

)

n

R2

0.449 � 0.060 0.375 � 0.050 0.467 � 0.067 0.445 � 0.075

0.914 0.939 0.912 0.869

The values of ΔH0 and ΔS0 were 10.45 kJ mol 1 and 32.00 J mol 1 K 1, respectively. It demonstrated that the whole adsorption process was exothermic and randomness decreasing process. The negative values of the ΔG0 at the given temperatures also indicated the sponta­ neity of the adsorption process. As far as the Van’t Hoff method is concerned (i.e., eq. (6)), it should be realized that the thermodynamic parameters are inferred from the individual adsorption isotherms for Cs(I) ions only. Different from gas adsorption, adsorption from solvents is accompanied by the mechanism of retention of Cs(I) ions at the interface, as well as other partial mechanisms of desorption, rehydration/dehydration of ions, dewetting of the surface, some of them making an endothermic contribution to the overall effect. That is why there is always a great difference between the enthalpy effects measured directly by the appropriate calorimetry techniques and those inferred from the Van’t Hoff procedure. It should be noted that the positive value of entropy obtained in Table 3. Generally speaking, the adsorption of one species leads to a decrease in its degrees of freedom because it is partially immobilized at the surface (e.g., localized adsorption involved in the Langmuir model). The entropy change upon adsorption should be thus negative! However, we can find that in the previous studies, some reported values of entropy of adsorption are positive, whilst others are negative. More discussion and examples can be found in our previous review paper (Xu and Wang, 2017). Similar results were observed in our previous studies. In this study, the parameter K0 was based on the calculation of distribution coeffi­ cient. This parameter was the result of the overall effect. Therefore, the explanation of thermodynamic parameters should take into account the overall effect. That is the reason we say “the adsorption was exothermic”. In the whole system, the attractive forces of GO and sorbates do positive work and turn into heat, which is a process of declining energy quality and an increase in entropy, which can explain the positive value of ΔS in the whole adsorption system.

Table 3 Thermodynamic parameters for the adsorption of Cs into graphene oxide. T (K)

1

R

where ΔS0 (J mol 1 K 1), ΔH0 (kJ mol 1), and ΔG0 (kJ mol 1) stand for standard entropy change, standard enthalpy change, and Gibbs free energy change, respectively; R is the molar gas constant (8.314 J K 1 mol 1). K0 can be calculated by plotting ln Kd versus Ce, and extrapo­ lating Ce to zero (Kd ¼ qe/Ce) (Zhao et al., 2011; Tran et al., 2016). The thermodynamic parameters were also summarized in Table 3.

Fig. 7. The SEM and EDX spectra of graphene oxide after adsorption of Cs(I). 5

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Fig. 8. The XRD spectra of GO after adsorption of Cs(I).

Fig. 9. The FTIR spectra of GO before and after adsorption of Cs(I).

All these indicated that adsorption from solution was a really com­ plex phenomenon and cannot be simplified to match with the Langmuir and Van’t Hoff patterns. 3.6. Adsorption mechanism 3.6.1. SEM and EDX The SEM and EDX spectra of the graphene oxide after adsorption of Cs(I) were presented in Fig. 7. A typical sheet-like structure of stacking graphene oxide was observed in Fig. 7(a). Besides, the EDX showed the elements of the adsorbent (C, N, and Na) and the element of adsorbent (Cs), indicating the adsorption of cesium ions from solution into the adsorbent, graphene oxide. Fig. 10. The XPS spectra of GO after adsorption of Cs(I).

3.6.2. XRD spectra Fig. 8 showed the XRD spectra of graphene oxide before and after adsorption of cesium ions. The graphene oxide had a characteristic diffraction peak at about 10� . After adsorption of cesium ions, this peak became weaken, indicating the deterioration of the crystallization of graphene oxide. Additionally, a new wide peak at around 23o-30� could be observed after adsorption, possibly due to the presence of CsO2

(JCPDS 65–2662) and CsOH (JCPDS 40–1066) (Band et al., 2004). In addition, the c-axis spacing of graphene oxide after adsorption decreased to 7.39 nm, which may increase its aggregation.

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Table 4 The adsorption capacity of Cs(I) by carbon-based adsorbents. Surface area (m2 g 1)

Dosage (g L 1)

Initial concentration (mg L 1)

t (h)

pH

T (K)

Ionic strength

q (mg g 1)

Reference

1150

20–100 mg

10–30

24

~6

296.15

–

0.76

4.39

10

RT

0.5

1.0–10.0

50–250

CNT As produced MWCNTs

Caccin et al. (2013) Ofomaja et al. (2013)

63

0.2–2.2

5–75

24

1–10

298

Oxidized MWCNTs

83.5

0.2–2.2

5–75

24

1–10

298

CoFC/alginate/CNT

–

–

80–190

12

2–10

50 beads/ 50 mL

150–280

3.5

Activated carbon Coconut shell activated carbon (Norit) Pine cone

~1.3 1.63

RT

0.001–0.05 M NaNO3 0.001–0.05 M NaNO3 0–0.035 M

2–10

RT

–

142.84

300

528

298–338

1–10 mM for K; 50–1,000 mM for Na 0.001–0.1 M

12.75 133.33

PB-encapsulated alginate/ calcium beads GO Pristine graphene oxide

–

2

10-10,000

100 min

Graphene oxide

139.5

0.5

~0.5–12

~12

1-12 (12) 2.0–11.0

Fe3O4/GO

–

0.2

2.0–20.0

1

2–10

283–303

0.001–0.1 M NaCl

16.4

Graphene oxide

93.7

0.05–0.2

5–200

24

3–9

283–313

95.46

Biochar Oxidized bamboo charcoal

0–0.1 M NaNO3

347.72

10

20–1000

6

2–12

298

0.75–12 mM Na or K

3.44

Raw bamboo charcoal

312.5

3.3–16.7

20–800

6

2–12

RT

–

14.85

Concentrated nitric acid–modified bamboo charcoal

2.3

3.3–16.7

20–800

6

2–12

RT

–

45.87

40.00

Yavari et al. (2011) Yavari et al. (2011) Vipin et al. (2014) (Vipin et al., 2014) Kaewmee et al. (2017) Tan et al. (2016) Wang and Yu (2014) This article Khandaker et al. (2018) Khandaker et al. (2017) Khandaker et al. (2017)

Where RT stands for room temperature.

3.6.3. FTIR spectra To identify functional groups involving in the adsorption, FTIR spectra were applied for the comparison, as shown in Fig. 9. Results showed that a characteristic band at around 1734 cm 1, which should be – O vibration (Zhuang et al., 2017), obviously attributed to the C– decreased after adsorption, as well as the band at about 1226 cm 1 (C–O–C) and 1051 1 (C–O) (Li et al., 2016). Additionally, compared with graphene oxide, the bands at around 3400 cm 1 after adsorption had some degree of red-shift, suggesting that the –COOH and –OH were affected (Chen et al., 2015), which indicated that O-containing func­ tional groups played a major role in the adsorption process.

responsible for the adsorption. 3.7. Comparison of adsorption capacity by various carbon-based materials Table 4 summarized the adsorption capacity of cesium ions by various kinds of carbon-based adsorbents. The most widely used carbonbased adsorbent, activated carbon with an adsorption capacity of less than 2 mg g 1 (Caccin et al., 2013; Ofomaja et al., 2013), cannot satisfy the practical demands for cesium ions removal. The synthetic MWCNT also did not have a satisfying adsorption capacity (1.63 mg g 1), although after modification its adsorption capacity greatly improved (12.75 mg g 1). Results showed that the blending of Prussian blue and its analogues (Vipin et al., 2014) could greatly enhance its affinity for cesium ions. Additionally, our synthetic graphene oxide also showed higher adsorption capacity of cesium ions than that of biochar derived from bamboo (Khandaker et al., 2017, 2018). Therefore, compared with pristine activated carbon and CNT, the synthetic graphene oxide showed an extraordinary adsorption capacity for cesium ions. Compared with other reported graphene oxide, our synthetic adsorbent also proved competitive in adsorption of cesium ions.

3.6.4. XPS spectra The XPS spectra of graphene oxide were shown in Fig. 10. It can be seen from the wide scan spectra (Fig. 10(a)) that graphene oxide had characteristic peaks at 285 eV and 532 eV, which were assigned to be C 1s and O 1s, respectively. After adsorption of cesium ions, the signal of cesium ions could be detected at the binding energy of 725 eV and 740 eV, as shown in Fig. 10 (a) and (b). The previous analysis showed that the O-containing functional groups played an important role in the adsorption process. Therefore, the O 1s, as well as the C 1s, were further used to delineate its states, as shown in Fig. 10 (c). For C 1s spectra, the peaks at the binding energy of 284.8 eV, 286.1 eV, 286.9 eV, 287.8 eV, – C(C–C), C–O, C–O–C, and 288.8 eV should be owned to the C in the C– – O, and O–C– – O, respectively (Yang et al., 2009); For O 1s spectra, the C– peaks at the binding energy of 531.7 eV, 532.7 eV, and 533.6 eV should – O, C–O–C, and –COOH, respectively be attributed to the O in the C– (Ganguly et al., 2011). After adsorption of cesium ions, the peaks of – O disappeared. Additionally, the peaks of C–O–C and C– –O O–C– became weaker. However, only small difference of the C 1s spectra could be observed. That further confirmed our previous consumption that O-containing functional groups of the graphene oxide should be

4. Conclusion In this study, graphene oxide was synthesized and applied for cesium ions removal, its adsorption performance was examined. According to the characterization results, its surface area of 93.7 m2 g 1, pore size of 2.43 nm, pore volume of 0.31 cm3 g 1, and pHpzc of 1, were all deter­ mined. The optimal pH was about 6 when NaNO3 ionic strength was at the range of 0.001 mol L 1 to 0.1 mol L 1. Moreover, the adsorption equilibrium was obtained within 8 h and the maximal adsorption ca­ pacity was 95.46 mg g 1 according to the Langmuir model. 7

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Furthermore, the main adsorptive sites of O-containing functional groups were attributed to the cesium ions adsorption, which were verified by the analyses of XRD, FTIR, and XPS.

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