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The sequestration of Sr(II) and Cs(I) from aqueous solutions by magnetic graphene oxides
Journal of Molecular Liquids 209 (2015) 508–514
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The sequestration of Sr(II) and Cs(I) from aqueous solutions by magnetic graphene oxides Deming Li a,b,⁎, Bo Zhang b, Fengqin Xuan b a b
School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230000, PR China Department of Chemical Engineering, Anhui Vocational and Technical College, Hefei 230011, PR China
a r t i c l e
i n f o
Article history: Received 8 April 2015 Received in revised form 3 June 2015 Accepted 6 June 2015 Available online xxxx Keywords: Sequestration Magnetic graphene oxides Fe3O4 Radionuclides Mechanism
a b s t r a c t The adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was investigated by batch techniques. The adsorption kinetics indicated that adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be satisfactorily fitted by pseudo-second-order kinetic model. The adsorption of Sr(II) and Cs(I) on magnetic graphene oxides increased with increasing pH from 2.0 to 6.0, then high level adsorption of Sr(II) and Cs(I) was observed at pH N 6.0. The maximum adsorption capacity of magnetic graphene oxides calculated from Langmuir model at pH 4.0 and 293 K was 14.706 and 9.259 mg/g for Sr(II) and Cs(I), respectively. The thermodynamic parameters showed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was an exothermic and spontaneous process. According to surface complexation modeling, the adsorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides was cation exchange, whereas the inner-sphere surface complexation dominated the sorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides. The finding suggested that magnetic graphene oxides can be one of the promising adsorbents for the immobilization and preconcentration of radionuclides from aqueous solutions in environmental cleanup. © 2015 Published by Elsevier B.V.
1. Introduction The removal of radionuclides (e.g., uranium, strontium and cesium) has been recently issued at nuclear waste management facilities, uranium mining, and milling sites [1]. Among them, strontium (e.g., Sr-90) is a contaminant from the production of nuclear weapons that is found in soil, sediment, and/or groundwater. Cesium (e.g., Cs-137) in liquid wastes is considered as important radioactive contaminants because of destructive effect on the environment [2]. Therefore, it is requester to decrease the permit of the National Interim Drinking Water Regulations [3]. Kim et al. [4] investigated the adsorption of Cs on kaolinite, montmorillonite, corundum, and gibbsite. Previous studies investigated the adsorption of Sr(II) and Cs(I) from aqueous solutions as a function of pH, ionic strength, temperature and solution concentrations by a variety of adsorbents such as clay minerals and Fe/Al hydrous oxides [5–12]. In these studies, the effect of water chemistry (i.e., reaction time, pH, ionic strength) on Sr(II) and Cs(I) adsorption was elucidated by batch experiments. However, the low adsorption capacities of these adsorbents limited the practical application of radionuclide adsorption from aqueous solutions by large volumes.
⁎ Corresponding author at: School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230000, PR China. E-mail address: [email protected] (D. Li).
http://dx.doi.org/10.1016/j.molliq.2015.06.022 0167-7322/© 2015 Published by Elsevier B.V.
Owing to a variety of oxygenated functional groups, graphene oxides display the excellent adsorption performance for organic contaminants [13–15] and heavy metals [16–18]. Sun et al. [19] found that the maximum adsorption capacities of graphene oxides supported polyaniline composites at pH 3.0 and T = 298 K were calculated to be 1.03, 1.65, 1.68, and 1.9 mmol/g for U(VI), Eu(III), Sr(II), and Cs(I), respectively. The authors investigated that the high affinities of radionuclides (such as Eu(III), U(VI) and Sr(II)) to graphene oxides was due to the hydroxyl and carboxyl groups attaching to the edges of graphene oxides by XPS analysis. More recently, the interaction mechanism of Sr(II) and Cs(I) at water–solid interface was demonstrated by surface complexation modeling [20–22]. In general, Sr(II) and Cs(I) adsorption occurred by ion exchange at low pH conditions, whereas surface complexation predominated their adsorption at high pH conditions. Marmier and Fromage demonstrated that the sorption of Cs(I) on magnetite has satisfactorily been qualified by constant capacity modeling (CCM) and diffuse layer modeling (DLM) [23]. To the authors' knowledge, few studies on the fitting of Sr(II) and Cs(I) adsorption onto magnetic graphene oxide were available. The aims of this study were to (1) synthesize magnetic graphene oxides and characterize their morphology and nanostructures; (2) investigate the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides as a function of reaction time, pH, ionic strength and temperature by batch experiments; (3) simulate the Sr(II) and Cs(I) adsorption by
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using surface complexation modeling. This paper highlights the interaction mechanism between Sr(II)/Cs(I) and magnetic graphene oxides by using batch experiments and surface complexation modeling.
(ICP-AES). Additional control experiments with no magnetic graphene oxides present were done to check for Sr(II)/Cs(I) adsorption to vessel walls and for precipitation of Sr(OH)2 (s) or CsOH (s). The adsorption capacity (Qe, mg/g) can be calculated by Eq. (1):
2. Materials and methods
Qe ¼ V C 0 −C eq =m
ð1Þ
2.1. Synthesis of magnetic graphene oxides Graphene oxides were synthesized by modified Hummer's method [24]. Briefly, 2.0 g flake graphite (200 mesh, 99.8% purity, Qingdao Tianhe Graphite Co., Ltd.) and 1.0 g NaNO3 (Analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) were added in 100 mL concentrated H2SO4 (98%, analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) under continuous stirring and ice-water batch conditions, then 6.0 g of KMnO4 was slowly added into the aforementioned suspensions at room temperature for 5 days. At last, 12 mL H2O2 (30 wt.%, analytical reagent) was added to remove the residual MnO− 4 ions. It should be noted that the suspension of graphene oxides was obtained by centrifugating it at 2300 rpm for 60 min. The solid phase was re-dispersed under vigorous stirring and bath ultrasonication for 30 min at the power of 140 W. The graphene oxide solution was purified by repeated ultrasonication and dialysis for one week. The magnetic graphene oxides were obtained by chemical co-precipitation method. Typically, 0.2 g graphene oxides were dispersed in 450 mL water under vigorously stirring for 30 min. Then 0.92 g FeCl3 · 6H2O and 0.52 g FeSO4 · 7H2O (Analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) was dropwise added to graphene oxides solution at room temperature under N2 conditions. 30% ammonia solution was added until the pH 10 for 4 h. The suspensions were washed with water and ethanol several times, and the magnetic graphene oxides were obtained by drying it in vacuum oven overnight. The specific surface area of magnetic graphene oxides determined by BET nitrogen gas adsorption was 114 m2/g, which was significantly lower than the theoretical data (approximately 2630 m2/g) due to the incomplete exfoliation and aggregation [25]. The stock Sr(II) and Cs(I) solution (1.0 × 10−3 mol/L) was prepared by dissolving SrCl2 · 6H2O(s) (Aldrich, spectroscopic reagents) and CsCl · 6H2O(s) (Aldrich, spectroscopic reagents) in deionized water, respectively.
where C0 (mg/L) and Ceq (mg/L) are initial concentration and equilibrated concentration after sorption, respectively. m (g) and V (mL) are the mass of adsorbents and the volume of the suspension, respectively. All experimental data were the average of triplicate determinations and the error bars (within ± 5%) were provided. 2.4. Surface complexation modeling The surface complexation modeling was employed to simulate the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides with an aid of FITEQL 4.0 code [26]. The protonation and deprotonation constants (log K+ and log K− values) can be given by Eqs. (2) and (3), respectively: SOH þ Hþ ¼ SOH2 þ SOH ¼ SO‐ þ Hþ
þ logK ¼ log SOH2 þ = ½SOH Hþ ð2Þ log K‐ ¼ log SO‐ Hþ =SOH :
ð3Þ
The values of log K+ and log K− can be obtained by fitting the potentiometric titration of magnetic graphene oxides in the presence of 0.01 mol/L NaClO4 solutions. In this study, diffuse double layer model (DDLM) was employed to simulate the pH-edge adsorption of Sr(II) and Cs(I) on magnetic graphene oxides with ion exchange (XH) and surface complexation sites (SOH). The surface complexation reactions can be presented by Eqs. (4) and (5): nXH þ Mnþ ¼ Xn M þ nHþ
log K XH
ð4Þ
SOH þ Mnþ ¼ SOMðn‐1Þþ þ nHþ
log K SOH :
ð5Þ
These chemical equilibrated constants (log KXH and log KSOH) were obtained by fitting the pH-dependent adsorption data.
2.2. Characterization 3. Results and discussion The morphology and nanostructures of graphene oxides and magnetic graphene oxides were synthesized by SEM (JSM-6700 F Field Emission Scanning Electron Microscope, JEOL) and TEM (JEOL-2010 Transmission Electron Microscope, Japan). The samples used for the SEM and TEM observation were prepared by dropping the graphene oxide suspension on copper foil. A variety of oxygenated functional groups of magnetic graphene oxides were determined by FTIR (JASCO FT-IR 410 spectrophotometer) with the KBr pellet technology and XPS (ESCALAB 250Xi XPS analyzer, Thermo Scientific) with monochromatic Al Kα X-ray source (hν = 1486.6 eV) at a vacuum below 10−7 Pa. The oxidation extent of magnetic graphene oxides was also demonstrated by Raman spectra (LabRam HR Raman spectrometer, France) at 514.5 nm with Ar+ laser. 2.3. Batch experiments The adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was prepared in the presence of atmospheric CO2 at 293 K from pH 2.0 to 11.0. Briefly, magnetic graphene oxides were mixed with a freshly prepared Sr(II) or Cs(I) and 0.01 mol/L NaClO4 solution in polycarbonate test tubes. After the pH was adjusted, the tubes were sealed and then reacted for 2 days in a constant temperature over-end-over shaker. The pH in aqueous solutions was adjusted by adding 0.1 mol/L NaOH and HCl (Fisher Scientific). The concentration of Sr(II) and Cs(I) solution was analyzed by inductively coupled plasma atomic emission spectrometry
3.1. Characterization The morphology and nanostructure of graphene oxide and magnetic graphene oxides were characterized by SEM techniques. As shown in Fig. 1A, the aggregated and randomly accumulated graphene oxide nanosheets were observed, which were consistent with previous studies [16,27]. However, magnetic graphene oxide showed the wrinkled nanosheets with accumulated magnetic nanoparticles (Fig. 1B). The oxygenated functional groups of magnetic graphene oxides were determined by FTIR spectra. The peaks at approximately 1730, 1620, 1220, and 1100 cm−1 were corresponded to the stretching vibration of the C = O, skeletal C = C, carboxyl O = C–O, and alkoxy C–O bond, respectively [19,28]. The widen peaks at 3450 cm−1 (data not shown) was assigned to the –OH stretching vibration of adsorbed water [29]. The deconvolution of the C 1 s peak of magnetic graphene oxide was shown in Fig. 2E. As illustrated in Fig. 2E, the main peaks at 284.7, 286.6, 287.9, and 289.8 eV were ascribed to the C–C, C–O, C = O, and O–C = O, respectively [30–32]. In the Raman spectra (Fig. 1F), two main peaks at 1350 cm−1 (D band, the stretching vibration of sp3 carbon atoms) and 1580 cm−1 (G band, the stretching vibration of sp2 carbon atoms) were observed. The D and G bands were related with the defects/disorders and first-order scattering of the E2g mode, respectively [29]. The low intensity ratio of the D band and G band (ID/IG) (0.83) of magnetic graphene oxides indicated the defects/disorders of magnetic
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A
C
E
B
D
F
Binding Energy (eV) Fig. 1. The characterization of magnetic graphene oxide. A and B: SEM images of graphene oxides and magnetic graphene oxides; C: TEM image; D: FTIR spectra; E: XPS spectra; F: Raman spectra.
graphene oxides [33,34]. Based on the characteristic results, it was demonstrated that magnetic graphene oxides presented a variety of oxygencontaining functional groups such as epoxy, carboxyl, carbonyl, and hydroxyl groups. 3.2. Adsorption kinetics The adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene oxide were investigated by batch techniques. As shown in Fig. 2, the
adsorption of Sr(II) on magnetic graphene oxides was higher than that of Cs(I), which were presumably due to Sr(II) preferentially adsorbed to adsorbents [35–37]. Yavari et al. also indicated that cesium uptake process to achieve equilibrium was faster than strontium [36]. The authors demonstrated that over 99% of cesium and strontium was washed out of column by using 4 mol/L NH4Cl solution. However, Sahai et al. found that the Sr(II) adsorption was lower than that of Cs(I) presumably due to different adsorbents used [38]. The adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene oxides were fitted by
D. Li et al. / Journal of Molecular Liquids 209 (2015) 508–514
oxide can be satisfactorily fitted by pseudo-second-order kinetic model with high correlation coefficient (R2 N 0.999) compared to pseudo-firstorder kinetic model (R2 b 0.995).
100 90 80 0.35
70
3.2. pH effect
0.30
60
0.25
50
0.20 t/Qt
Amount of adsorbed radionuclides (%)
511
40
0.15
30
0.10
20
0.05
10
Sr(II)
0.00
Cs(I)
0
3
6
9
0 0
3
6
9 12 15 Reaction time (h)
12
15 18
18
21
Time (h)
21
24
24
Fig. 2. The adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene oxide, pH 4.0, I = 0.01 mol/L NaClO4, C0 = 10.0 mg/L, m/v = 1.2 g/L, T = 293 K.
The pH dependent adsorption of Sr(II) and Cs(I) on magnetic graphene oxides were shown in Fig. 3. As shown in Fig. 3A, the adsorption of Cs(I) on magnetic graphene oxides was minimal at pH b 3.0, whereas the adsorption of Cs(I) on magnetic graphene oxides significantly increased with increasing pH from 4.0 to 6.0. The high-level adsorption was observed at pH N 6.5. Approximately 80% of Cs(I) was removed by magnetic graphene oxides at pH 7.0. The adsorption behavior of Sr(II) on magnetic graphene oxides was similar to Cs(I) over a wide range of pH from 2.0 to 11.0 (Fig. 3B), whereas the adsorption of Sr(II) on magnetic graphene oxides was significantly higher than that of Cs(I). 3.3. Effect of ionic strength
ln ðqe −qt Þ ¼ ln qe −k f t
ð6Þ
The effect of ionic strength on Sr(II) and Cs(I) adsorption on magnetic graphene oxides was also showed in Fig. 3. One can see that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides significantly decreased with increasing ionic strength at pH b 5.0, whereas the Sr(II) and Cs(I) adsorption was independent of ionic strength at pH N 5.0. Sun et al. [19] found that the adsorption of U(VI) on graphene oxides was independent of pH. It is demonstrated that the outer-sphere surface complexation was sensitive to the effect of ionic strength, whereas the inner-sphere surface complexation was independent of ionic strength [41]. The ionic strength-dependent adsorption indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was outersphere surface complexation at pH b 5.0, whereas the inner-sphere surface complexation dominated the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides at pH N 5.0.
t=qt ¼ 1= ks qe 2 þ t=qe
ð7Þ
3.4. Adsorption isotherms
Table 1 Kinetic parameters of Sr(II) and Cs(I) adsorption on magnetic graphene oxides. Adsorbent
Sr(II) Cs(I)
Pseudo-first-order
Pseudo-second-order
qe(mg/g)
kf(h−1)
R2
qe(mg/g)
Ks(g / (mg h))
R2
0.3750 0.0382
0.064 0.209
0.752 0.860
4.500 2.974
6.101 8.850
0.999 0.999
pseudo-first-order and pseudo-second-order kinetic model. Their linear equations were described by Eqs. (6) [39] and (7) [40]:
where qe and qt (mg/g) are the amount of Sr(II) and Cs(I) adsorbed at equilibrium and at time t, respectively. kf and ks are the pseudo-firstorder and pseudo-second-order kinetic rate constant, respectively. The fitted parameters of pseudo-first-order and pseudo-secondorder kinetic models were summarized in Table 1. As shown in Table 1, the adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene
Amount of adsorbed radionuclides (%)
100
Fig. 4 showed the adsorption isotherms of Sr(II) and Cs(I) on magnetic graphene oxides at different temperature conditions. As illustrated in Fig. 4, the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides obviously increased with increasing initial concentration, then maintained high-level adsorption at higher concentration conditions. The adsorption isotherms of Sr(II) and Cs(I) on magnetic graphene oxides
A
90
B
80 70 60 50 40 30 20 10
Cs(I)_0.01 mol/L Cs(I)_0.1 mol/L
Sr(II)_0.01 mol/L Sr(II)_0.1 mol/L
0 1
2
3
4
5
6
pH
7
8
9
10 11 1
2
3
4
5
6
7
8
9
10
pH
Fig. 3. The effect of pH on Cs(I) (A) and Sr(II) (B) on magnetic graphene oxides, C0 = 10.0 mg/L, m/v = 1.2 g/L, T = 293 K.
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293 K 313 K 323 K
293 K 313 K 323 K
16 14
Qe (mg/g)
12 10 8 6 4 2 1
2
3
4
5
6
7
2
Ce (mg/L)
4
6
8
10
12
14
Ce (mg/L)
Fig. 4. The adsorption isotherms of Sr(II) (A) and Cs(I) (B) on magnetic graphene oxides, I = 0.01 mol/L NaClO4, pH 4.0, m/v = 1.2 g/L, T = 293 K.
were simulated by Langmuir and Freundlich models. Their linear formation can be described by Eqs. (8) [42] and (9) [43]: C e =Q e ¼ C e =Q max þ 1=ðK L Q max Þ
ð8Þ
ln Q e ¼ ln K F þ 1=n ln C e
ð9Þ
where Ce (mg/L) and Qe (mg/g) are the concentration of uranium after equilibrium at solution and solid-phase, respectively; KL (L/mg) and KF ((mg/g)/(mg/L)1/n) are the Langmuir and Freundlich adsorption coefficient, respectively. Qmax (mg/g) is maximum adsorption capacity, and 1/n is the heterogeneity of the adsorption sites and an indicator of isotherm nonlinearity. The fitted results of Sr(II) and Cs(I) adsorption on magnetic graphene oxides were showed in Fig. 4 and Table 2. As listed in Table 2, the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be better fitted by Langmuir model (R2 N 0.995) compared to Freundlich model (R2 b 0.99). The maximum adsorption capacities of magnetic graphene oxides calculated from Langmuir model at pH 4.0 and 293 K were 9.259 and 14.706 mg/g for Cs(I) and Sr(II), respectively. The fitted results indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was attributed to the monolayer adsorption. 3.5. Thermodynamic parameters
where R and T are universal gas constant (8.314 J/mol K) and Kelvin temperature, respectively. As shown in Fig. 4A and B, the adsorption equilibrium constant ln Keq can be calculated by plotting ln Kd versus Ce and then extrapolating Ce to zero. The adsorption enthalpy change (ΔH0, kJ/mol) and adsorption entropy change (ΔS0, kJ/(mol K)) can be calculated from the Eq. (11): ΔG0 ¼ ΔH0 −TΔS0 : Based on Eqs. (10)–(11), therefore ln K eq ¼ ΔS0 =R−ΔH 0 =RT:
0
ΔG ¼ −RT ln K eq
ð10Þ
Table 3 Thermodynamic parameters of Sr(II) and Cs(I) adsorption on magnetic graphene oxides at 293, 313, and 333 K.
Sr(II)
Cs(I)
293 K 313 K 333 K 293 K 313 K 333 K
ΔG0 kJ · mol−1
ΔH0 kJ · mol−1
ΔS0 J.(mol · K)−1
−7.72 −9.78 −10.32 −5.73 −7.38 −7.86
7.113
64.12
4.718
34.76
Table 4 The optimized parameters of surface complexation modeling for Sr(II) and Cs(I) adsorption on magnetic graphene oxides. Samples
Table 2 Parameters for Langmuir and Freundlich models of Sr(II) and Cs(I) adsorption on magnetic graphene oxides at pH 4.0 and T = 293 K. Samples
Sr(II) Cs(I)
Langmuir model
Freundlich model
b (mg/g)
Qmax (L/mg)
R2
LnkF (mg1-nLn/g)
1/n
R2
0.462 0.124
14.706 9.259
0.994 0.996
1.718 0.262
0.37 0.592
0.977 0.983
ð12Þ
The ΔH0 and ΔS0 values can be calculated from the slope and intercept of plot of lnKeq versus 1/T, respectively. The corresponding parameters were summarized in Table 3. As show in Table 3, The ΔG0 values of Sr(II) and Cs(I) on magnetic graphene oxides at 293 K were calculated to −7.72 and −5.73 kJ/mol,
Samples
As shown in Fig. 4, it is observed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides increased with increasing temperature from 303 to 333 K, suggesting that the adsorption Sr(II) and Cs(I) on magnetic graphene oxides were promoted at higher temperature. The thermodynamic parameters of Sr(II) and Cs(I) on magnetic graphene oxides were calculated by temperature-dependent adsorption isotherms. The adsorption Gibbs free energy change (ΔG0, kJ/mol) can be expressed by Eq. (10):
ð11Þ
Protonation and deprotonation Magnetic graphene oxides
Surface complexation modeling Sr(II) Cs(I)
Equations
Log K
SOH + H+ = SOH+ 2 SOH = SO− + H+
3.6 −4.4
2XH + Sr2+ = X2Sr + 2H+ SOH + Sr2+ = SOSr+ + H+ XH + Cs+ = XCs + H+ SOH + Cs+ = SOCs + H+
2.2 6.1 2.8 5.5
D. Li et al. / Journal of Molecular Liquids 209 (2015) 508–514
Amount of adsorbed radionuclide (%)
100
B
A
90
513
80 +
70
SOSr
SOCs
60 50 40 30 20 10
X2Sr
XCs
0 1
2
3
4
5
pH
6
7
8
9
1
2
3
4
5
pH
6
7
8
9
10
Fig. 5. The fitted results of Sr(II) (A) and Cs(I) (B) adsorption on magnetic graphene oxides by surface complexation modeling, C0 = 10.0 mg/L, I = 0.01 mol/L NaClO4, m/v = 1.2 g/L, T = 293 K.
respectively, which was comparable to previous study [44]. The negative values of ΔG0 showed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was a spontaneous process. It is also observed that the value of ΔG0 of Sr(II) on magnetic graphene oxides was higher than that of Cs(I), suggesting that Sr(II) was more prone to be adsorbed on magnetic graphene oxides compared to Cs(I), which was consistent with the pH-dependent adsorption. The positive values of ΔH0 (7.11 and 4.71 kJ/mol for Sr(II) and Cs(I), respectively) indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was an endothermic process. The endothermic nature of adsorption processes suggested that chemisorptions were the predominant mechanism [45]. The positive values of ΔS0 (64.12 and 34.76 J/mol/K for Sr(II) and Cs(I), respectively) revealed the increased randomness at the solid– liquid interface. Thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was an endothermic and spontaneous process.
groups. The adsorption kinetics revealed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be satisfactorily fitted pseudo-second-order kinetic model with high correlation coefficients. Results from the batch adsorption indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was ion-exchange at low pH conditions, whereas the inner-sphere surface complexation was observed at pH 5.0, primarily as a hydrated surface complex. The result from surface complexation modeling indicated that the pH-edge adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be fitted by ionic exchange and surface complexation sites very well. The observation presented in the study provides the implication for the preconcentration and the removal of radionuclides from environment cleanup applications. Acknowledgments Financial supports from the Foundation of Anhui Vocational and Technical College (No. 2014zrkx08) are acknowledged.
3.6. Surface complexation modeling References The pH-edge adsorption data of Sr(II) and Cs(I) on magnetic graphene oxides were simulated by using DDLM with aid of FITEQL v 4.0 code [46]. The values of parameter derived from surface complexation modeling were obtained by optimizing the fitted results under different pH conditions (Table 4). As shown in Fig. 5, the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be simulated by DDLM model with XH and SOH sites very well. As summarized in Table 4, the log KSOH value of Sr(II) (6.1) was higher than that of Cs(I) (5.5), whereas the log KXH value of Cs(I) (2.8) was higher than that of Sr (2.2). The results showed that Sr(II) was more available to bond with surface complexation sites of magnetic graphene oxides compared to Sr(I), which was consistent with the batch experiments. According to surface complexation modeling, the adsorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides was cation exchange, whereas the inner-sphere surface complexation dominated the sorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides. 4. Conclusions Based on the characteristic results, it was demonstrated that the magnetic graphene oxides presented a variety of oxygen-containing functional groups such as epoxy, carboxyl, carbonyl, and hydroxyl
[1] Y.B. Sun, J.X. Li, X.K. Wang, Geochim. Cosmochim. Acta 140 (2014) 621–643. [2] J.O. Kim, S.M. Lee, C. Jeon, Chem. Eng. Res. Des. 92 (2014) 368–374. [3] R.G. Riley, J.M. Zachara, F.J. Wobber, DOE/ER-0547T, U.S. Department of Energy, Office of Energy Research, 1992. [4] Y. Kim, R.T. Cygan, R.J. Kirkpatrick, Geochim. Cosmochim. Acta 60 (1996) 1041–1052. [5] M.S. Murali, J.N. Mathur, J. Radioanal. Nucl. Chem. 254 (2002) 129–136. [6] S. Kasar, S. Kumar, A. Kar, R.K. Bajpai, C.P. Kaushik, B.S. Tomar, J. Radioanal. Nucl. Chem. 300 (2014) 71–75. [7] A.M. EI-Kamash, J. Hazard. Mater. 151 (2008) 432–445. [8] R.G. Anthony, R.G. Dosch, D. Gu, C.V. Philip, Ind. Eng. Chem. Res. 33 (1994) 2702–2705. [9] W.P. Ma, P.W. Brown, S. Komarneni, J. Am. Ceram. Soc. 79 (1996) 1707–1710. [10] H. Mimura, K. Akiba, J. Nucl. Sci. Technol. 30 (1993) 436–443. [11] P.N. Chiang, M.K. Wang, P.M. Huang, J.J. Wang, C.Y. Chiu, J. Environ. Radioact. 101 (2010) 472–481. [12] A.J. Fisher, P.E. Blochl, Phys. Rev. Lett. 70 (1993) 3263. [13] J.N. Tiwari, K. Mahesh, N.H. Le, K.C. Kemp, R. Timilsina, R.N. Tiwari, K.S. Kim, Carbon 56 (2013) 173–182. [14] Y.B. Sun, S.B. Yang, G.X. Zhao, Q. Wang, X.K. Wang, Chem. Asian. J. 8 (2013) 2755–2761. [15] J. Wang, Z.M. Chen, B.L. Chen, Environ. Sci. Technol. 48 (2014) 4817–4825. [16] Y.B. Sun, Q. Wang, C.L. Chen, X.K. Wang, Environ. Sci. Technol. 46 (2012) 6020–6027. [17] Y.B. Sun, C.C. Ding, W.C. Cheng, X.K. Wang, J. Hazard. Mater. 280 (2014) 399–408. [18] X. Wang, J.T. Yu, J. Radioanal. Nucl. Chem. 303 (2015) 807–813. [19] Y. Sun, D. Shao, C. Chen, S. Yang, X. Wang, Environ. Sci. Technol. 47 (2013) 9904–9910. [20] Y.J. Park, K.K. Park, M.Y. Suh, A.K. Yoon, K.S. Choi, K.Y. Jee, W.H. Kim, J. Korean Chem. Soc. 44 (2000) 305–310. [21] S.A. Carroll, S.K. Roberts, L.J. Criscenti, P.A. O'Day, Geochem. Trans. 9 (2008) 2–28.
514 [22] [23] [24] [25]
[26] [27] [28] [29] [30] [31] [32] [33]
D. Li et al. / Journal of Molecular Liquids 209 (2015) 508–514 N. Marmier, A. Delisee, F. Fromage, J. Colloid Interface Sci. 212 (1999) 228–233. N. Marmier, F. Fromage, J. Colloid Interface Sci. 223 (2000) 83–88. W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339-1339. R.L.D. Whitby, V.M. Gunko, A. Korobeinyk, R. Busquets, A.B. Cundy, K. Laszlo, J. Skubiszewska-Zieba, R. Leboda, E. Tombacz, I.Y. Toth, K. Kovacs, S.V. Mikhalovsky, ACS Nano 6 (2012) 3967–3973. D.A. Dzombak, F.M.M. Morel, Wiley, New York, 1990. Y.B. Sun, S.B. Yang, C.C. Ding, W.C. Cheng, Z.X. Jin, RSC Adv. 5 (2015) 24886–24892. J. Xu, L. Wang, Y. Zhu, Langmuir 28 (2012) 8418–8425. Y.B. Sun, S.B. Yang, Y. Chen, C.C. Ding, W.C. Cheng, X.K. Wang, Environ. Sci. Technol. 49 (2015) 4255–4262. T. Ramanathan, F.T. Fisher, R.S. Ruoff, L.C. Brinson, Chem. Mater. 17 (2005) 1290–1295. X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, F. Zhang, Adv. Mater. 20 (2008) 4490–4493. Z.H. Sheng, L. Shao, J.J. Chen, W.J. Bao, F.B. Wang, X.H. Xia, ACS Nano 5 (2011) 4350–4358. M.C. Hsiao, S.H. Liao, M.Y. Yen, P.I. Liu, N.W. Pu, C.A. Wang, C.C.M. Ma, ACS Appl. Mater. Interfaces 2 (2010) 3092–3099.
[34] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y.Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558–1565. [35] H. Faghihian, M. Moayed, A. Firooz, M. Iravani, J. Colloid Interface Sci. 393 (2013) 445–451. [36] R. Yavari, Y.D. Huang, S.J. Ahmadi, G. Bagheri, J. Radioanal. Nucl. Chem. 286 (2010) 223–229. [37] N. Sahai, S.A. Corroll, S. Roberts, P.A. O'Day, J. Colloid Interface Sci. 222 (2000) 198–212. [38] P.R. Duncan, Idaho National Engineering National Laboratory, Biotechnology Group, 1995. [39] S. Lagergren, Handlingar 24 (1989) 1–39. [40] Y.S. Ho, G. Mckay, Process Chem. 34 (1999) 451–465. [41] Y.B. Sun, S.T. Yang, G.D. Sheng, Z.Q. Guo, X.L. Tan, J.Z. Xu, X.K. Wang, Sep. Purif. Technol. 83 (2011) 193–203. [42] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361–1403. [43] H.M.F. Freundlich, J. Phys. Chem. 57 (1906) 385–470. [44] E. Metwally, R.O. Abdel Rahman, R.R. Ayoub, Radiochim. Acta 95 (2009) 409–416. [45] S.A. Khan, R. Ur-Reman, M.A. Khan, J. Radioanal. Nucl. Chem. 190 (1995) 81–96. [46] A. Herbelin, J. C. Westall, Dept. Chem., Oregon St. Univ., Corvallis, OR, USA. 1999.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The sequestration of Sr(II) and Cs(I) from aqueous solutions by magnetic graphene oxides Deming Li a,b,⁎, Bo Zhang b, Fengqin Xuan b a b
School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230000, PR China Department of Chemical Engineering, Anhui Vocational and Technical College, Hefei 230011, PR China
a r t i c l e
i n f o
Article history: Received 8 April 2015 Received in revised form 3 June 2015 Accepted 6 June 2015 Available online xxxx Keywords: Sequestration Magnetic graphene oxides Fe3O4 Radionuclides Mechanism
a b s t r a c t The adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was investigated by batch techniques. The adsorption kinetics indicated that adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be satisfactorily fitted by pseudo-second-order kinetic model. The adsorption of Sr(II) and Cs(I) on magnetic graphene oxides increased with increasing pH from 2.0 to 6.0, then high level adsorption of Sr(II) and Cs(I) was observed at pH N 6.0. The maximum adsorption capacity of magnetic graphene oxides calculated from Langmuir model at pH 4.0 and 293 K was 14.706 and 9.259 mg/g for Sr(II) and Cs(I), respectively. The thermodynamic parameters showed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was an exothermic and spontaneous process. According to surface complexation modeling, the adsorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides was cation exchange, whereas the inner-sphere surface complexation dominated the sorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides. The finding suggested that magnetic graphene oxides can be one of the promising adsorbents for the immobilization and preconcentration of radionuclides from aqueous solutions in environmental cleanup. © 2015 Published by Elsevier B.V.
1. Introduction The removal of radionuclides (e.g., uranium, strontium and cesium) has been recently issued at nuclear waste management facilities, uranium mining, and milling sites [1]. Among them, strontium (e.g., Sr-90) is a contaminant from the production of nuclear weapons that is found in soil, sediment, and/or groundwater. Cesium (e.g., Cs-137) in liquid wastes is considered as important radioactive contaminants because of destructive effect on the environment [2]. Therefore, it is requester to decrease the permit of the National Interim Drinking Water Regulations [3]. Kim et al. [4] investigated the adsorption of Cs on kaolinite, montmorillonite, corundum, and gibbsite. Previous studies investigated the adsorption of Sr(II) and Cs(I) from aqueous solutions as a function of pH, ionic strength, temperature and solution concentrations by a variety of adsorbents such as clay minerals and Fe/Al hydrous oxides [5–12]. In these studies, the effect of water chemistry (i.e., reaction time, pH, ionic strength) on Sr(II) and Cs(I) adsorption was elucidated by batch experiments. However, the low adsorption capacities of these adsorbents limited the practical application of radionuclide adsorption from aqueous solutions by large volumes.
⁎ Corresponding author at: School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230000, PR China. E-mail address: [email protected] (D. Li).
http://dx.doi.org/10.1016/j.molliq.2015.06.022 0167-7322/© 2015 Published by Elsevier B.V.
Owing to a variety of oxygenated functional groups, graphene oxides display the excellent adsorption performance for organic contaminants [13–15] and heavy metals [16–18]. Sun et al. [19] found that the maximum adsorption capacities of graphene oxides supported polyaniline composites at pH 3.0 and T = 298 K were calculated to be 1.03, 1.65, 1.68, and 1.9 mmol/g for U(VI), Eu(III), Sr(II), and Cs(I), respectively. The authors investigated that the high affinities of radionuclides (such as Eu(III), U(VI) and Sr(II)) to graphene oxides was due to the hydroxyl and carboxyl groups attaching to the edges of graphene oxides by XPS analysis. More recently, the interaction mechanism of Sr(II) and Cs(I) at water–solid interface was demonstrated by surface complexation modeling [20–22]. In general, Sr(II) and Cs(I) adsorption occurred by ion exchange at low pH conditions, whereas surface complexation predominated their adsorption at high pH conditions. Marmier and Fromage demonstrated that the sorption of Cs(I) on magnetite has satisfactorily been qualified by constant capacity modeling (CCM) and diffuse layer modeling (DLM) [23]. To the authors' knowledge, few studies on the fitting of Sr(II) and Cs(I) adsorption onto magnetic graphene oxide were available. The aims of this study were to (1) synthesize magnetic graphene oxides and characterize their morphology and nanostructures; (2) investigate the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides as a function of reaction time, pH, ionic strength and temperature by batch experiments; (3) simulate the Sr(II) and Cs(I) adsorption by
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509
using surface complexation modeling. This paper highlights the interaction mechanism between Sr(II)/Cs(I) and magnetic graphene oxides by using batch experiments and surface complexation modeling.
(ICP-AES). Additional control experiments with no magnetic graphene oxides present were done to check for Sr(II)/Cs(I) adsorption to vessel walls and for precipitation of Sr(OH)2 (s) or CsOH (s). The adsorption capacity (Qe, mg/g) can be calculated by Eq. (1):
2. Materials and methods
Qe ¼ V C 0 −C eq =m
ð1Þ
2.1. Synthesis of magnetic graphene oxides Graphene oxides were synthesized by modified Hummer's method [24]. Briefly, 2.0 g flake graphite (200 mesh, 99.8% purity, Qingdao Tianhe Graphite Co., Ltd.) and 1.0 g NaNO3 (Analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) were added in 100 mL concentrated H2SO4 (98%, analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) under continuous stirring and ice-water batch conditions, then 6.0 g of KMnO4 was slowly added into the aforementioned suspensions at room temperature for 5 days. At last, 12 mL H2O2 (30 wt.%, analytical reagent) was added to remove the residual MnO− 4 ions. It should be noted that the suspension of graphene oxides was obtained by centrifugating it at 2300 rpm for 60 min. The solid phase was re-dispersed under vigorous stirring and bath ultrasonication for 30 min at the power of 140 W. The graphene oxide solution was purified by repeated ultrasonication and dialysis for one week. The magnetic graphene oxides were obtained by chemical co-precipitation method. Typically, 0.2 g graphene oxides were dispersed in 450 mL water under vigorously stirring for 30 min. Then 0.92 g FeCl3 · 6H2O and 0.52 g FeSO4 · 7H2O (Analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) was dropwise added to graphene oxides solution at room temperature under N2 conditions. 30% ammonia solution was added until the pH 10 for 4 h. The suspensions were washed with water and ethanol several times, and the magnetic graphene oxides were obtained by drying it in vacuum oven overnight. The specific surface area of magnetic graphene oxides determined by BET nitrogen gas adsorption was 114 m2/g, which was significantly lower than the theoretical data (approximately 2630 m2/g) due to the incomplete exfoliation and aggregation [25]. The stock Sr(II) and Cs(I) solution (1.0 × 10−3 mol/L) was prepared by dissolving SrCl2 · 6H2O(s) (Aldrich, spectroscopic reagents) and CsCl · 6H2O(s) (Aldrich, spectroscopic reagents) in deionized water, respectively.
where C0 (mg/L) and Ceq (mg/L) are initial concentration and equilibrated concentration after sorption, respectively. m (g) and V (mL) are the mass of adsorbents and the volume of the suspension, respectively. All experimental data were the average of triplicate determinations and the error bars (within ± 5%) were provided. 2.4. Surface complexation modeling The surface complexation modeling was employed to simulate the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides with an aid of FITEQL 4.0 code [26]. The protonation and deprotonation constants (log K+ and log K− values) can be given by Eqs. (2) and (3), respectively: SOH þ Hþ ¼ SOH2 þ SOH ¼ SO‐ þ Hþ
þ logK ¼ log SOH2 þ = ½SOH Hþ ð2Þ log K‐ ¼ log SO‐ Hþ =SOH :
ð3Þ
The values of log K+ and log K− can be obtained by fitting the potentiometric titration of magnetic graphene oxides in the presence of 0.01 mol/L NaClO4 solutions. In this study, diffuse double layer model (DDLM) was employed to simulate the pH-edge adsorption of Sr(II) and Cs(I) on magnetic graphene oxides with ion exchange (XH) and surface complexation sites (SOH). The surface complexation reactions can be presented by Eqs. (4) and (5): nXH þ Mnþ ¼ Xn M þ nHþ
log K XH
ð4Þ
SOH þ Mnþ ¼ SOMðn‐1Þþ þ nHþ
log K SOH :
ð5Þ
These chemical equilibrated constants (log KXH and log KSOH) were obtained by fitting the pH-dependent adsorption data.
2.2. Characterization 3. Results and discussion The morphology and nanostructures of graphene oxides and magnetic graphene oxides were synthesized by SEM (JSM-6700 F Field Emission Scanning Electron Microscope, JEOL) and TEM (JEOL-2010 Transmission Electron Microscope, Japan). The samples used for the SEM and TEM observation were prepared by dropping the graphene oxide suspension on copper foil. A variety of oxygenated functional groups of magnetic graphene oxides were determined by FTIR (JASCO FT-IR 410 spectrophotometer) with the KBr pellet technology and XPS (ESCALAB 250Xi XPS analyzer, Thermo Scientific) with monochromatic Al Kα X-ray source (hν = 1486.6 eV) at a vacuum below 10−7 Pa. The oxidation extent of magnetic graphene oxides was also demonstrated by Raman spectra (LabRam HR Raman spectrometer, France) at 514.5 nm with Ar+ laser. 2.3. Batch experiments The adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was prepared in the presence of atmospheric CO2 at 293 K from pH 2.0 to 11.0. Briefly, magnetic graphene oxides were mixed with a freshly prepared Sr(II) or Cs(I) and 0.01 mol/L NaClO4 solution in polycarbonate test tubes. After the pH was adjusted, the tubes were sealed and then reacted for 2 days in a constant temperature over-end-over shaker. The pH in aqueous solutions was adjusted by adding 0.1 mol/L NaOH and HCl (Fisher Scientific). The concentration of Sr(II) and Cs(I) solution was analyzed by inductively coupled plasma atomic emission spectrometry
3.1. Characterization The morphology and nanostructure of graphene oxide and magnetic graphene oxides were characterized by SEM techniques. As shown in Fig. 1A, the aggregated and randomly accumulated graphene oxide nanosheets were observed, which were consistent with previous studies [16,27]. However, magnetic graphene oxide showed the wrinkled nanosheets with accumulated magnetic nanoparticles (Fig. 1B). The oxygenated functional groups of magnetic graphene oxides were determined by FTIR spectra. The peaks at approximately 1730, 1620, 1220, and 1100 cm−1 were corresponded to the stretching vibration of the C = O, skeletal C = C, carboxyl O = C–O, and alkoxy C–O bond, respectively [19,28]. The widen peaks at 3450 cm−1 (data not shown) was assigned to the –OH stretching vibration of adsorbed water [29]. The deconvolution of the C 1 s peak of magnetic graphene oxide was shown in Fig. 2E. As illustrated in Fig. 2E, the main peaks at 284.7, 286.6, 287.9, and 289.8 eV were ascribed to the C–C, C–O, C = O, and O–C = O, respectively [30–32]. In the Raman spectra (Fig. 1F), two main peaks at 1350 cm−1 (D band, the stretching vibration of sp3 carbon atoms) and 1580 cm−1 (G band, the stretching vibration of sp2 carbon atoms) were observed. The D and G bands were related with the defects/disorders and first-order scattering of the E2g mode, respectively [29]. The low intensity ratio of the D band and G band (ID/IG) (0.83) of magnetic graphene oxides indicated the defects/disorders of magnetic
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A
C
E
B
D
F
Binding Energy (eV) Fig. 1. The characterization of magnetic graphene oxide. A and B: SEM images of graphene oxides and magnetic graphene oxides; C: TEM image; D: FTIR spectra; E: XPS spectra; F: Raman spectra.
graphene oxides [33,34]. Based on the characteristic results, it was demonstrated that magnetic graphene oxides presented a variety of oxygencontaining functional groups such as epoxy, carboxyl, carbonyl, and hydroxyl groups. 3.2. Adsorption kinetics The adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene oxide were investigated by batch techniques. As shown in Fig. 2, the
adsorption of Sr(II) on magnetic graphene oxides was higher than that of Cs(I), which were presumably due to Sr(II) preferentially adsorbed to adsorbents [35–37]. Yavari et al. also indicated that cesium uptake process to achieve equilibrium was faster than strontium [36]. The authors demonstrated that over 99% of cesium and strontium was washed out of column by using 4 mol/L NH4Cl solution. However, Sahai et al. found that the Sr(II) adsorption was lower than that of Cs(I) presumably due to different adsorbents used [38]. The adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene oxides were fitted by
D. Li et al. / Journal of Molecular Liquids 209 (2015) 508–514
oxide can be satisfactorily fitted by pseudo-second-order kinetic model with high correlation coefficient (R2 N 0.999) compared to pseudo-firstorder kinetic model (R2 b 0.995).
100 90 80 0.35
70
3.2. pH effect
0.30
60
0.25
50
0.20 t/Qt
Amount of adsorbed radionuclides (%)
511
40
0.15
30
0.10
20
0.05
10
Sr(II)
0.00
Cs(I)
0
3
6
9
0 0
3
6
9 12 15 Reaction time (h)
12
15 18
18
21
Time (h)
21
24
24
Fig. 2. The adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene oxide, pH 4.0, I = 0.01 mol/L NaClO4, C0 = 10.0 mg/L, m/v = 1.2 g/L, T = 293 K.
The pH dependent adsorption of Sr(II) and Cs(I) on magnetic graphene oxides were shown in Fig. 3. As shown in Fig. 3A, the adsorption of Cs(I) on magnetic graphene oxides was minimal at pH b 3.0, whereas the adsorption of Cs(I) on magnetic graphene oxides significantly increased with increasing pH from 4.0 to 6.0. The high-level adsorption was observed at pH N 6.5. Approximately 80% of Cs(I) was removed by magnetic graphene oxides at pH 7.0. The adsorption behavior of Sr(II) on magnetic graphene oxides was similar to Cs(I) over a wide range of pH from 2.0 to 11.0 (Fig. 3B), whereas the adsorption of Sr(II) on magnetic graphene oxides was significantly higher than that of Cs(I). 3.3. Effect of ionic strength
ln ðqe −qt Þ ¼ ln qe −k f t
ð6Þ
The effect of ionic strength on Sr(II) and Cs(I) adsorption on magnetic graphene oxides was also showed in Fig. 3. One can see that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides significantly decreased with increasing ionic strength at pH b 5.0, whereas the Sr(II) and Cs(I) adsorption was independent of ionic strength at pH N 5.0. Sun et al. [19] found that the adsorption of U(VI) on graphene oxides was independent of pH. It is demonstrated that the outer-sphere surface complexation was sensitive to the effect of ionic strength, whereas the inner-sphere surface complexation was independent of ionic strength [41]. The ionic strength-dependent adsorption indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was outersphere surface complexation at pH b 5.0, whereas the inner-sphere surface complexation dominated the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides at pH N 5.0.
t=qt ¼ 1= ks qe 2 þ t=qe
ð7Þ
3.4. Adsorption isotherms
Table 1 Kinetic parameters of Sr(II) and Cs(I) adsorption on magnetic graphene oxides. Adsorbent
Sr(II) Cs(I)
Pseudo-first-order
Pseudo-second-order
qe(mg/g)
kf(h−1)
R2
qe(mg/g)
Ks(g / (mg h))
R2
0.3750 0.0382
0.064 0.209
0.752 0.860
4.500 2.974
6.101 8.850
0.999 0.999
pseudo-first-order and pseudo-second-order kinetic model. Their linear equations were described by Eqs. (6) [39] and (7) [40]:
where qe and qt (mg/g) are the amount of Sr(II) and Cs(I) adsorbed at equilibrium and at time t, respectively. kf and ks are the pseudo-firstorder and pseudo-second-order kinetic rate constant, respectively. The fitted parameters of pseudo-first-order and pseudo-secondorder kinetic models were summarized in Table 1. As shown in Table 1, the adsorption kinetics of Sr(II) and Cs(I) on magnetic graphene
Amount of adsorbed radionuclides (%)
100
Fig. 4 showed the adsorption isotherms of Sr(II) and Cs(I) on magnetic graphene oxides at different temperature conditions. As illustrated in Fig. 4, the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides obviously increased with increasing initial concentration, then maintained high-level adsorption at higher concentration conditions. The adsorption isotherms of Sr(II) and Cs(I) on magnetic graphene oxides
A
90
B
80 70 60 50 40 30 20 10
Cs(I)_0.01 mol/L Cs(I)_0.1 mol/L
Sr(II)_0.01 mol/L Sr(II)_0.1 mol/L
0 1
2
3
4
5
6
pH
7
8
9
10 11 1
2
3
4
5
6
7
8
9
10
pH
Fig. 3. The effect of pH on Cs(I) (A) and Sr(II) (B) on magnetic graphene oxides, C0 = 10.0 mg/L, m/v = 1.2 g/L, T = 293 K.
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293 K 313 K 323 K
293 K 313 K 323 K
16 14
Qe (mg/g)
12 10 8 6 4 2 1
2
3
4
5
6
7
2
Ce (mg/L)
4
6
8
10
12
14
Ce (mg/L)
Fig. 4. The adsorption isotherms of Sr(II) (A) and Cs(I) (B) on magnetic graphene oxides, I = 0.01 mol/L NaClO4, pH 4.0, m/v = 1.2 g/L, T = 293 K.
were simulated by Langmuir and Freundlich models. Their linear formation can be described by Eqs. (8) [42] and (9) [43]: C e =Q e ¼ C e =Q max þ 1=ðK L Q max Þ
ð8Þ
ln Q e ¼ ln K F þ 1=n ln C e
ð9Þ
where Ce (mg/L) and Qe (mg/g) are the concentration of uranium after equilibrium at solution and solid-phase, respectively; KL (L/mg) and KF ((mg/g)/(mg/L)1/n) are the Langmuir and Freundlich adsorption coefficient, respectively. Qmax (mg/g) is maximum adsorption capacity, and 1/n is the heterogeneity of the adsorption sites and an indicator of isotherm nonlinearity. The fitted results of Sr(II) and Cs(I) adsorption on magnetic graphene oxides were showed in Fig. 4 and Table 2. As listed in Table 2, the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be better fitted by Langmuir model (R2 N 0.995) compared to Freundlich model (R2 b 0.99). The maximum adsorption capacities of magnetic graphene oxides calculated from Langmuir model at pH 4.0 and 293 K were 9.259 and 14.706 mg/g for Cs(I) and Sr(II), respectively. The fitted results indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was attributed to the monolayer adsorption. 3.5. Thermodynamic parameters
where R and T are universal gas constant (8.314 J/mol K) and Kelvin temperature, respectively. As shown in Fig. 4A and B, the adsorption equilibrium constant ln Keq can be calculated by plotting ln Kd versus Ce and then extrapolating Ce to zero. The adsorption enthalpy change (ΔH0, kJ/mol) and adsorption entropy change (ΔS0, kJ/(mol K)) can be calculated from the Eq. (11): ΔG0 ¼ ΔH0 −TΔS0 : Based on Eqs. (10)–(11), therefore ln K eq ¼ ΔS0 =R−ΔH 0 =RT:
0
ΔG ¼ −RT ln K eq
ð10Þ
Table 3 Thermodynamic parameters of Sr(II) and Cs(I) adsorption on magnetic graphene oxides at 293, 313, and 333 K.
Sr(II)
Cs(I)
293 K 313 K 333 K 293 K 313 K 333 K
ΔG0 kJ · mol−1
ΔH0 kJ · mol−1
ΔS0 J.(mol · K)−1
−7.72 −9.78 −10.32 −5.73 −7.38 −7.86
7.113
64.12
4.718
34.76
Table 4 The optimized parameters of surface complexation modeling for Sr(II) and Cs(I) adsorption on magnetic graphene oxides. Samples
Table 2 Parameters for Langmuir and Freundlich models of Sr(II) and Cs(I) adsorption on magnetic graphene oxides at pH 4.0 and T = 293 K. Samples
Sr(II) Cs(I)
Langmuir model
Freundlich model
b (mg/g)
Qmax (L/mg)
R2
LnkF (mg1-nLn/g)
1/n
R2
0.462 0.124
14.706 9.259
0.994 0.996
1.718 0.262
0.37 0.592
0.977 0.983
ð12Þ
The ΔH0 and ΔS0 values can be calculated from the slope and intercept of plot of lnKeq versus 1/T, respectively. The corresponding parameters were summarized in Table 3. As show in Table 3, The ΔG0 values of Sr(II) and Cs(I) on magnetic graphene oxides at 293 K were calculated to −7.72 and −5.73 kJ/mol,
Samples
As shown in Fig. 4, it is observed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides increased with increasing temperature from 303 to 333 K, suggesting that the adsorption Sr(II) and Cs(I) on magnetic graphene oxides were promoted at higher temperature. The thermodynamic parameters of Sr(II) and Cs(I) on magnetic graphene oxides were calculated by temperature-dependent adsorption isotherms. The adsorption Gibbs free energy change (ΔG0, kJ/mol) can be expressed by Eq. (10):
ð11Þ
Protonation and deprotonation Magnetic graphene oxides
Surface complexation modeling Sr(II) Cs(I)
Equations
Log K
SOH + H+ = SOH+ 2 SOH = SO− + H+
3.6 −4.4
2XH + Sr2+ = X2Sr + 2H+ SOH + Sr2+ = SOSr+ + H+ XH + Cs+ = XCs + H+ SOH + Cs+ = SOCs + H+
2.2 6.1 2.8 5.5
D. Li et al. / Journal of Molecular Liquids 209 (2015) 508–514
Amount of adsorbed radionuclide (%)
100
B
A
90
513
80 +
70
SOSr
SOCs
60 50 40 30 20 10
X2Sr
XCs
0 1
2
3
4
5
pH
6
7
8
9
1
2
3
4
5
pH
6
7
8
9
10
Fig. 5. The fitted results of Sr(II) (A) and Cs(I) (B) adsorption on magnetic graphene oxides by surface complexation modeling, C0 = 10.0 mg/L, I = 0.01 mol/L NaClO4, m/v = 1.2 g/L, T = 293 K.
respectively, which was comparable to previous study [44]. The negative values of ΔG0 showed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was a spontaneous process. It is also observed that the value of ΔG0 of Sr(II) on magnetic graphene oxides was higher than that of Cs(I), suggesting that Sr(II) was more prone to be adsorbed on magnetic graphene oxides compared to Cs(I), which was consistent with the pH-dependent adsorption. The positive values of ΔH0 (7.11 and 4.71 kJ/mol for Sr(II) and Cs(I), respectively) indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was an endothermic process. The endothermic nature of adsorption processes suggested that chemisorptions were the predominant mechanism [45]. The positive values of ΔS0 (64.12 and 34.76 J/mol/K for Sr(II) and Cs(I), respectively) revealed the increased randomness at the solid– liquid interface. Thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was an endothermic and spontaneous process.
groups. The adsorption kinetics revealed that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be satisfactorily fitted pseudo-second-order kinetic model with high correlation coefficients. Results from the batch adsorption indicated that the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides was ion-exchange at low pH conditions, whereas the inner-sphere surface complexation was observed at pH 5.0, primarily as a hydrated surface complex. The result from surface complexation modeling indicated that the pH-edge adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be fitted by ionic exchange and surface complexation sites very well. The observation presented in the study provides the implication for the preconcentration and the removal of radionuclides from environment cleanup applications. Acknowledgments Financial supports from the Foundation of Anhui Vocational and Technical College (No. 2014zrkx08) are acknowledged.
3.6. Surface complexation modeling References The pH-edge adsorption data of Sr(II) and Cs(I) on magnetic graphene oxides were simulated by using DDLM with aid of FITEQL v 4.0 code [46]. The values of parameter derived from surface complexation modeling were obtained by optimizing the fitted results under different pH conditions (Table 4). As shown in Fig. 5, the adsorption of Sr(II) and Cs(I) on magnetic graphene oxides can be simulated by DDLM model with XH and SOH sites very well. As summarized in Table 4, the log KSOH value of Sr(II) (6.1) was higher than that of Cs(I) (5.5), whereas the log KXH value of Cs(I) (2.8) was higher than that of Sr (2.2). The results showed that Sr(II) was more available to bond with surface complexation sites of magnetic graphene oxides compared to Sr(I), which was consistent with the batch experiments. According to surface complexation modeling, the adsorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides was cation exchange, whereas the inner-sphere surface complexation dominated the sorption mechanism of Sr(II) and Cs(I) on magnetic graphene oxides. 4. Conclusions Based on the characteristic results, it was demonstrated that the magnetic graphene oxides presented a variety of oxygen-containing functional groups such as epoxy, carboxyl, carbonyl, and hydroxyl
[1] Y.B. Sun, J.X. Li, X.K. Wang, Geochim. Cosmochim. Acta 140 (2014) 621–643. [2] J.O. Kim, S.M. Lee, C. Jeon, Chem. Eng. Res. Des. 92 (2014) 368–374. [3] R.G. Riley, J.M. Zachara, F.J. Wobber, DOE/ER-0547T, U.S. Department of Energy, Office of Energy Research, 1992. [4] Y. Kim, R.T. Cygan, R.J. Kirkpatrick, Geochim. Cosmochim. Acta 60 (1996) 1041–1052. [5] M.S. Murali, J.N. Mathur, J. Radioanal. Nucl. Chem. 254 (2002) 129–136. [6] S. Kasar, S. Kumar, A. Kar, R.K. Bajpai, C.P. Kaushik, B.S. Tomar, J. Radioanal. Nucl. Chem. 300 (2014) 71–75. [7] A.M. EI-Kamash, J. Hazard. Mater. 151 (2008) 432–445. [8] R.G. Anthony, R.G. Dosch, D. Gu, C.V. Philip, Ind. Eng. Chem. Res. 33 (1994) 2702–2705. [9] W.P. Ma, P.W. Brown, S. Komarneni, J. Am. Ceram. Soc. 79 (1996) 1707–1710. [10] H. Mimura, K. Akiba, J. Nucl. Sci. Technol. 30 (1993) 436–443. [11] P.N. Chiang, M.K. Wang, P.M. Huang, J.J. Wang, C.Y. Chiu, J. Environ. Radioact. 101 (2010) 472–481. [12] A.J. Fisher, P.E. Blochl, Phys. Rev. Lett. 70 (1993) 3263. [13] J.N. Tiwari, K. Mahesh, N.H. Le, K.C. Kemp, R. Timilsina, R.N. Tiwari, K.S. Kim, Carbon 56 (2013) 173–182. [14] Y.B. Sun, S.B. Yang, G.X. Zhao, Q. Wang, X.K. Wang, Chem. Asian. J. 8 (2013) 2755–2761. [15] J. Wang, Z.M. Chen, B.L. Chen, Environ. Sci. Technol. 48 (2014) 4817–4825. [16] Y.B. Sun, Q. Wang, C.L. Chen, X.K. Wang, Environ. Sci. Technol. 46 (2012) 6020–6027. [17] Y.B. Sun, C.C. Ding, W.C. Cheng, X.K. Wang, J. Hazard. Mater. 280 (2014) 399–408. [18] X. Wang, J.T. Yu, J. Radioanal. Nucl. Chem. 303 (2015) 807–813. [19] Y. Sun, D. Shao, C. Chen, S. Yang, X. Wang, Environ. Sci. Technol. 47 (2013) 9904–9910. [20] Y.J. Park, K.K. Park, M.Y. Suh, A.K. Yoon, K.S. Choi, K.Y. Jee, W.H. Kim, J. Korean Chem. Soc. 44 (2000) 305–310. [21] S.A. Carroll, S.K. Roberts, L.J. Criscenti, P.A. O'Day, Geochem. Trans. 9 (2008) 2–28.
514 [22] [23] [24] [25]
[26] [27] [28] [29] [30] [31] [32] [33]
D. Li et al. / Journal of Molecular Liquids 209 (2015) 508–514 N. Marmier, A. Delisee, F. Fromage, J. Colloid Interface Sci. 212 (1999) 228–233. N. Marmier, F. Fromage, J. Colloid Interface Sci. 223 (2000) 83–88. W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339-1339. R.L.D. Whitby, V.M. Gunko, A. Korobeinyk, R. Busquets, A.B. Cundy, K. Laszlo, J. Skubiszewska-Zieba, R. Leboda, E. Tombacz, I.Y. Toth, K. Kovacs, S.V. Mikhalovsky, ACS Nano 6 (2012) 3967–3973. D.A. Dzombak, F.M.M. Morel, Wiley, New York, 1990. Y.B. Sun, S.B. Yang, C.C. Ding, W.C. Cheng, Z.X. Jin, RSC Adv. 5 (2015) 24886–24892. J. Xu, L. Wang, Y. Zhu, Langmuir 28 (2012) 8418–8425. Y.B. Sun, S.B. Yang, Y. Chen, C.C. Ding, W.C. Cheng, X.K. Wang, Environ. Sci. Technol. 49 (2015) 4255–4262. T. Ramanathan, F.T. Fisher, R.S. Ruoff, L.C. Brinson, Chem. Mater. 17 (2005) 1290–1295. X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, F. Zhang, Adv. Mater. 20 (2008) 4490–4493. Z.H. Sheng, L. Shao, J.J. Chen, W.J. Bao, F.B. Wang, X.H. Xia, ACS Nano 5 (2011) 4350–4358. M.C. Hsiao, S.H. Liao, M.Y. Yen, P.I. Liu, N.W. Pu, C.A. Wang, C.C.M. Ma, ACS Appl. Mater. Interfaces 2 (2010) 3092–3099.
[34] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y.Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558–1565. [35] H. Faghihian, M. Moayed, A. Firooz, M. Iravani, J. Colloid Interface Sci. 393 (2013) 445–451. [36] R. Yavari, Y.D. Huang, S.J. Ahmadi, G. Bagheri, J. Radioanal. Nucl. Chem. 286 (2010) 223–229. [37] N. Sahai, S.A. Corroll, S. Roberts, P.A. O'Day, J. Colloid Interface Sci. 222 (2000) 198–212. [38] P.R. Duncan, Idaho National Engineering National Laboratory, Biotechnology Group, 1995. [39] S. Lagergren, Handlingar 24 (1989) 1–39. [40] Y.S. Ho, G. Mckay, Process Chem. 34 (1999) 451–465. [41] Y.B. Sun, S.T. Yang, G.D. Sheng, Z.Q. Guo, X.L. Tan, J.Z. Xu, X.K. Wang, Sep. Purif. Technol. 83 (2011) 193–203. [42] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361–1403. [43] H.M.F. Freundlich, J. Phys. Chem. 57 (1906) 385–470. [44] E. Metwally, R.O. Abdel Rahman, R.R. Ayoub, Radiochim. Acta 95 (2009) 409–416. [45] S.A. Khan, R. Ur-Reman, M.A. Khan, J. Radioanal. Nucl. Chem. 190 (1995) 81–96. [46] A. Herbelin, J. C. Westall, Dept. Chem., Oregon St. Univ., Corvallis, OR, USA. 1999.