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Kinetics and thermodynamics of Eu(III) adsorption onto synthetic monoclinic pyrrhotite
Journal of Molecular Liquids 218 (2016) 565–570
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Kinetics and thermodynamics of Eu(III) adsorption onto synthetic monoclinic pyrrhotite Yuke Zhu, Haibo Liu, Tianhu Chen ⁎, Bin Xu, Ping Li School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China
a r t i c l e
i n f o
Article history: Received 30 December 2015 Received in revised form 22 January 2016 Accepted 23 January 2016 Available online xxxx Keywords: Pyrrhotite Eu(III) Adsorption kinetics Thermodynamics XPS
a b s t r a c t The adsorption of Eu(III) from aqueous solutions onto monoclinic pyrrhotite (M-Pyr) was studied by batch experiments under various experimental conditions. M-Pyr was synthesized and characterized by XRD, FT-IR, SEM and potentiometric titration. The macroscopic results showed that the adsorption of Eu(III) on M-Pyr was independent of ionic strength, indicating that inner-sphere surface complexation predominated their adsorption. The kinetic adsorption process was well described by pseudo-second-order model with high correlation coefficient (R2 N 0.999). The adsorption isotherms indicated that the adsorption of Eu(III) on M-Pyr fitted by Langmuir model was better than Freundlich model. The maximum adsorption capacity of Eu(III) on M-Pyr calculated from Langmuir model was 10.03 mg/g at pH 5.0 and 293 K. The thermodynamic parameters suggested that the adsorption of Eu(III) on M-Pyr was a spontaneous and endothermic processes. According to XPS analysis, the adsorption of Eu(III) on M-Pyr was mainly dominated by ion exchange and inner-sphere surface complexation at low pH and high pH conditions, respectively. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With the peaceful utilization of nuclear-related industries, a large amount of radioactive wastes is transferred into sub-environments. For work safety in the nuclear industry and for human health, the removal of a variety of radionuclides from nuclear waste solutions is an important environmental concern in nuclear waste management [1]. Europium (Eu(III)) as chemical analogue of trivalent lanthanides has been extensively studied in the past few years. Numerous studies have investigated the removal of europium on various materials such as clay minerals [2–4] and iron oxides [5–7]. Iron sulfides, including pyrite, pyrrhotite and mackinawite, are the widespread and abundant natural minerals. Pyrrhotite (Fe1 − XS, 0 b X b 0.125) displays various superstructures caused by ordered Fe vacancy geometries in the nonstoichiometric composition with an average Fe–S distance of 2.50 Å [8–10]. Owing to magnetic characteristics, pyrrhotite as a natural adsorbent has been extensively employed to remove a variety of environmental contaminants such as chromium [11], gold [12], copper [13–15], cadmium [16], and phosphorus [17]. Widler et al. [12] found that the maximum adsorption capacity of gold on pyrrhotite from aqueous solutions containing up to 40 mg/kg at 25 °C and 0.1 mol/L ionic strength. Li et al. [17] also demonstrated that the adsorption capacity of pyrrhotite for phosphate was 1.15 mg/g at ⁎ Corresponding author. E-mail address: [email protected] (T. Chen).
http://dx.doi.org/10.1016/j.molliq.2016.01.100 0167-7322/© 2016 Elsevier B.V. All rights reserved.
29 °C according to the Langmuir isotherm. To the authors' knowledge, the few studies on the removal of radionuclides on natural pyrrhotite are available [18,19]. The objectives of this study are to (1) synthesize pyrrhotite nanoparticles and characterize it by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) and potentiometric titration; (2) investigate the effect of water chemistries (reaction time, pH, ionic strength, initial concentration and temperature) on Eu(III) adsorption onto pyrrhotite by batch technique; (3) determine the adsorption mechanism between Eu(III) and pyrrhotite by using X-ray photoelectron spectroscopy (XPS) analysis. The results are helpful to understand the application of pyrrhotite as a promising adsorbent in nuclear waste management. 2. Experimental 2.1. Synthesis of pyrrhotite Pyrrhotite was synthesized by the calcinations of pyrite (Lujiang of Anhui, China) under N2 atmosphere at 923 K for 1.5 h [20]. Briefly, natural pyrite was soaked with 10% HCl for 2 h, and then was washed with Milli-Q water for 4 times. The purified pyrite was obtained by vacuum dried at room temperature. Sequentially, 3.0 g of purified pyrite were placed into the quartz tube, then sample was heated to 923 K for 1.5 h. The pyrrhotite was obtained by vacuum dried at room temperature. Eu(III) stock solution (0.1 mol/L) was prepared from Eu2O3 (purity 99.99%)
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after dissolution and dilution with 0.01 mol/L HClO 4 solution. All chemicals used in this experiment were analytical-grade, and all the solutions were prepared with Milli-Q water. 2.2. Characterization The identification of pyrrhotite was recorded by XRD (Dandonghaoyuan 2700, Cu target, electric tension of 40 kV, electric current of 30 mA, scan rate of 4°/min). The surface functional groups of pyrrhotite were determined by FT-IR (VERTEX-70) and the morphology of pyrrhotite was characterized by SEM (FE-SEM, Sirion 200). The potentiometric acid–base titration was performed to determine the chemical properties of pyrrhotite by use of a computer controlled titration system (DL50 automatic titrator, Mettler Toledo). Briefly, 0.3 of pyrrhotite was spiked into 0.01 mol/L NaClO4 background electrolyte at T = 20.0 ± 0.5 °C, and the sample was purged with argon gas for 2 h to exclude atmospheric CO2 (g). The initial pH of suspension was adjusted to pH 3.0 by adding 0.01 mol/L of HClO4 with vigorous stirring for 1 h, and then the suspension was slowly titrated to pH 11.0 with 0.05 mol/L NaOH titrant at a variable increment (0.008 up to 0.15 mL). The pHPZC (pH at point of zero charge) was obtained by fitting the data sets of potentiometric titration. The XPS measurements were conducted with a Thermo Escalab 250 electron spectrometer using 150 W Al-Kα radiations. 2.3. Adsorption experiments Adsorption experiments were conducted with 2 g/L pyrrhotite and 10 mg/L Eu(III) under N2 conditions at T = 293 K in the presence of 0.01 mol/L NaClO4. The pH of suspension was adjusted to be in the range 2.0–10.0 by adding negligible volume of 0.1 or 0.05 mol/L HClO4 or NaOH solution, the adsorption of Eu(III) without pyrrhotite was carried out under the same experimental conditions. Eu(III) stock solution and radiotracer Eu(III) were spiked into the bulk suspension gradually in order to avoid formation of Eu(OH)3(s) precipitate at pH N 7.0. The suspensions were shaken for 24 h to ensure that the adsorption reaction could achieve adsorption equilibrium. Subsequently an aliquot of the suspension (5.0 mL) was removed and the total activity of Eu(III) was determined. The solid phases were separated from liquid phases by centrifugation at 9500 rpm for 10 min, and then the supernatant was poured into a syringe and filtered through a 0.22 μm membrane. The concentration of Eu(III) in supernatant was analyzed by liquid scintillation counting on a Packard 3100 TR/AB liquid scintillation analyzer(Perkin-Elmer) with the scintillation cocktail (Ultima Gold AB, Packard). The removal percentage of Eu(III) (adsorption (%)) and adsorption capacity (Qs, mg/ g) can be expressed as Eqs. (1) and (2), respectively: Adsorption ð%Þ ¼ C0 −Ceq 100%=C0
ð1Þ
Qs ¼ V C0 −Ceq =m
ð2Þ
where C0 (mg/L) and Ceq (mg/L) are initial concentration and concentration after adsorption, respectively. m (g) and V (mL) are the mass of pyrrhotite and the volume of the suspension, respectively. All experimental data were the average of triplicate determinations and the relative errors were within ± 5%. 3. Results and discussion 3.1. Characterization The identification of the pyrrhotite is conducted by XRD pattern. As shown in Fig. 1A, the characteristic peaks at 2θ = 30.03, 33.98, 44.13, 53.36 and 57.71° are indexed to the (220), (−224), (228), (620) and (2212) plane of monoclinic pyrrhotite (Fe7S8, M-Pyr), respectively [21]. Fig. 1B shows the FT-IR spectrum of pyrrhotite. The peak at
3420 cm− 1 is related to the O\\H stretching, whereas the peaks at 1622 and 1467 cm− 1 refer to the H\\O\\H bending [22]. The strong bands at 1088 and 470 cm−1 represent the Si\\O\\Si groups of the tetrahedral sheet [4]. The peaks at 800 and 625 cm−1 are attributed to the bending and stretching vibration of S\\O, respectively [22,23]. The SEM image of M-Pyr is shown in the Fig. 1C, one can see that M-Pyr is closely agglomerated together by randomly aggregated nanosheets at submicrons level. The M-Pyr is polyporous and poorly crystalline. The pHPZC (pH at point of zero charge) is measured to be 6.0 by potentiometric acid–base titration (Fig. 1D). 3.2. Adsorption kinetics Kinetic experiments of Eu(III) adsorption on M-Pyr at pH 3.0 and 5.0 are shown in Fig. 2. As shown in Fig. 2, the adsorption of Eu(III) adsorption on M-Pyr significantly increases with increasing reaction time from 0 to 2 h, then slight enhancement of Eu(III) adsorption is observed after 2 h. The fast adsorption of Eu(III) on M-Pyr indicates that the instantaneous adsorption process may be primarily dominated by ion exchange or chemical adsorption [24–26]. Approximately 80% (pH 3.0) and 100% (pH 5.0) of Eu(III) are adsorbed on M-Pyr at 24 h, respectively. The pseudo-first-order and pseudo-second-order kinetic models are employed to simulate the adsorption kinetic of Eu(III) on M-Pyr. Their linear forms are given in Eqs. (3) [27] and (4) [28], respectively: ln ðQ e −Q t Þ ¼ ln Q e −K 1 t
ð3Þ
t=Q t ¼ 1= K 2 Q e 2 þ t=Q e
ð4Þ
where Qt (mg/g) and Qe (mg/g) are the adsorption concentration of Eu(III) at time t and at equilibrium time, respectively. K1 (g/(mg·min)) and K2 (g/(mg·min)) are the adsorption constants of pseudo-first-order and pseudo-second-order kinetic models, respectively. The fitted parameters of pseudo-first-order and pseudo-second-order kinetic models are summarized in Table 1. One can see that the adsorption kinetics of Eu(III) adsorption on M-Pyr can be satisfactorily simulated by pseudosecond-order kinetic model (R2 N 0.999) compared to pseudo-first order kinetic model (R2 b 0.98). 3.3. Effect of pH and ionic strength The effect of pH on the adsorption of Eu(III) on M-Pyr is showed in Fig. 3A. As shown in Fig. 3A, the amounts of Eu(III) adsorbed increased from approximately 20% to 100% at pH 2.0–7.0. As showed in Fig. 1D, the surface charge of M-Pyr is positive at pH b 6.0, whereas the surface of M-Pyr presents massive negative surface charge at pH N 6.0. Therefore, the low adsorption at pH b 6.0 could be due to the electrostatic repulsion between positive charged Eu3+ and positive charged M-Pyr surface. At pH N 6.0, the positively charged Eu(III) ions [Eu(OH)2 + species] can be easily adsorbed on the negatively charged M-Pyr because of the strong electrostatic interaction [29]. However, the increased adsorption of Eu(III) on M-Pyr at pH 2.0–6.0 is inconsistent with electrostatic interaction, which is presumably due to the surface complexation [30–33]. Fig. 3A also shows the effect of ionic strength on Eu(III) adsorption on M-Pyr. The increasing ionic strength has no effect on the adsorption of Eu(III) on M-Pyr throughout a wide pH range. According to previous studies, the ion exchange or outer-sphere surface complexation was obviously sensitive to the variations of ionic strength [34,35], while the inner-sphere surface complexation was not influenced by ionic strength [36,37]. Therefore, inner-sphere surface complexation dominates Eu(III) adsorption on M-Pyr [38]. This adsorption behavior is similar to the results of Eu(III) adsorption on graphene oxide nanosheets [29] and graphene oxide-supported polyamine [39].
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Fig. 1. Characterization of M-Pyr. A: XRD patterns; B: FTIR spectrum; C: SEM image; D: Potentiometric titration.
Fig. 2. Adsorption kinetics (A) and corresponding fitting of pseudo-second-order kinetic model (B) of Eu(III) adsorption on M-Pyr, pH 5.0, m/V = 2.0 g/L, I = 0.01 mol/L NaClO4, and T = 293 K.
is promoted at higher temperature. The adsorption data are fitted by Langmuir and Freundlich model. The Langmuir isotherm model is used to describe monolayer adsorption processes onto a surface [40,41], whereas Freundlich isotherm model is used to describe heterogeneous adsorption [42]. The linear forms of Langmuir and Freundlich equation can be expressed as Eqs. (5) and (6), respectively:
3.4. Adsorption isotherms Fig. 4A and B show the adsorption isotherms of Eu(III) on M-Pyr at different pH and at different temperatures, respectively. As shown in Fig. 4A, the adsorption of Eu(III) on M-Pyr at pH 5.0 is significantly higher than that of Eu(III) adsorption at pH 3.0. As shown in Fig. 4B, the adsorption of Eu(III) on M-Pyr obviously increases with the increasing of the reaction temperature. Adsorption capacity is highest at T = 333 K and lowest at T = 293 K, which shows that Eu(III) adsorption on M-Pyr
Ce =Q e ¼ 1=ðKL Q m Þ þ Ce =Q m
ð5Þ
Table 1 Parameters of pseudo-first-order and pseudo-second-order kinetic models. Pseudo-first-order
pH = 3.0 pH = 5.0
Pseudo-second-order
Qe (mg/g)
K1 (g/(mg·min))
R2
Qe (mg∙g−1)
K2 (g/(mg·min))
R2
4.0646 4.8875
1.5137 1.6965
0.976 0.912
4.1736 4.9801
0.5889 0.6128
0.9996 0.9993
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Fig. 3. A: Effect of pH and ionic strength on adsorption of Eu(III) on M-Pyr, CEu(III) = 10 mg/L, m/V = 2.0 g/L, and T = 293 K; B: The relative distribution of Eu(III) as a function of pH in aqueous solutions, CEu(III) = 10 mg/L, I = 0.01 mol/L NaClO4, and T = 293 K.
lgQ e ¼ 1=n lgCe þ lgK F
ð6Þ
where Qm (mg/g) is the maximum adsorption capacity of adsorbent at complete monolayer coverage. KL (L/mg) is a Langmuir constant, which is related to the free energy of sorption. 1/n is the heterogeneity of the adsorption sites; KF represents equilibrium coefficient, which describes the partitioning of the adsorbate between the solid and liquid phases over the concentration range studied. As shown in Fig. 4C and D, one can see that the adsorption behaviors of Eu(III) on M-Pyr can be fitted by Langmuir model well. As shown in Table 2, the maximum adsorption capacity (Qmax ) of Eu(III) on M-Pyr calculated from Langmuir model is 10.03 mg/g at pH 5.0 and 293 K. Comparing to qmax values
of Eu(III) adsorption on other adsorbents such as TiO2 [2], ZSM-5 zeolite [43], one can see that M-Pyr presents the higher adsorption capacity for Eu(III) compared to common materials. 3.5. Thermodynamic parameters The thermodynamic parameters (i.e., standard free energy change (ΔG°), standard enthalpy change (ΔH°) and the standard entropy change (ΔS°)) can be calculated according to Eqs. (7) and (8): ΔG ° ¼ −RT lnK °
ð7Þ
Fig. 4. Adsorption isotherms of Eu(III) adsorption on M-Pyr. A: Effect of pH, m/V = 2.0 g/L, I = 0.01 mol/L NaClO4, and T = 293 K. B: Effect of temperatures, m/V = 2.0 g/L, I = 0.01 mol/L NaClO4, and pH = 5.0. C and D: the fitting of adsorption isotherms of Eu(III) on M-Pyr by Langmuir model and Freundlich model, respectively.
Y. Zhu et al. / Journal of Molecular Liquids 218 (2016) 565–570 Table 2 Langmuir model and Freundlich model of Eu(III) adsorption on M-Pyr. Temperature (K)
pH
293 293 313 333
3.0 5.0 5.0 5.0
Langmuir model
Freundlich model
Qm (mg/g)
KL (L/mg)
R2
KF (mg/g)/(mg·L)−n)
1/n
R2
7.90 10.03 11.84 13.30
0.15 0.19 0.24 0.31
0.9986 0.9989 0.9986 0.9992
0.65 1.17 1.85 2.79
0.89 0.89 0.82 0.81
0.9848 0.9532 0.9509 0.9527
569
increasing temperature indicates that the adsorption of Eu(III)on M-Pyr is more favorable at higher temperature [44]. The positive ΔH° value (15.68 kJ/mol) suggests that Eu(III) adsorption on M-Pyr is an endothermic process [45]. It is determined that the dehydration of Eu(III) from aqueous Eu(III) complex ion is an endothermic process, but attachment of Eu(III) to the surface of endothermic is an exothermic processes [46]. It is plausible to assume that the energy of dehydration exceeds the exothermicity of the Eu(III) ions attached to the surface of M-Pyr. 3.6. Adsorption mechanism
Table 3 Thermodynamic parameters for the adsorption of Eu(III) on M-Pyr. Temperature (K)
ΔG/(kJ/mol)
ΔS/(J/(mol·K))
ΔH/(kJ/mol)
293 313 333
−11.57 −12.72 −13.92
58.78
15.68
ln K ° ¼ ΔS °=R−ΔH °=RT
ð8Þ
where R is the universal constant (8.314 J/(mol∙K)) and T is the temperature in Kelvin. The optimized parameters of ΔG°, ΔH°, ΔS° are listed in Table 3. As shown in Table 3, negative ΔG° values (−11.57 kJ/mol at 293 K, −13.93 kJ/mol at 333 K) indicate that the adsorption of Eu(III) on M-Pyr is a spontaneous process, and the decrease of ΔG° with
The XPS spectroscopy technique is used to determine the interaction mechanism of the adsorption of Eu(III) on M-Pyr. Fig. 5 shows the XPS spectra of survey and high resolution scans for the Eu 3d O 1s and S 2p of M-Pyr after adsorption at different pH conditions. The occurrences of Eu 3d5/2 are observed at pH 6.0 and pH 10.0 in terms of the survey spectra (Fig. 5B), which indicates that Eu(III) is chemically adsorbed onto the surface of M-Pyr [47,48]. The relative intensity and binding energies of O 1s for M-Pyr samples of Eu(III) adsorption in basic solution are significantly lower than that of samples in acid solution, whereas the relative intensity and binding energies of S 2p in basic solution are higher than that of samples in acid solution. Fig. 5C shows the high resolution scans of O 1s of the three samples. One can see that the relative energies of O 1s slightly decrease with increasing pH value. As illustrated in Table 4, the binding energies of O 1s at pH 4.0 and 10.0 are 531.15 and 530.89 eV, respectively. The lower binding energy of O 1s at high pH conditions indicates the retention of radionuclides by M-Pyr related to
Fig. 5. The survey and high resolution scans of XPS spectra of Eu(III) adsorption on M-Pyr. (A) Total survey scans; (B) Eu 3d peaks, (C) O 1s peaks, (D) S 2p peaks, m/V = 10 g/L, I = 0.01 mol/L NaClO4, T = 293 K.
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Table 4 Binding energies of XPS analysis of M-Pyr after adsorption at different pH. pH 4.0 6.0 10.0
Eu 3d5/2
O 1s
S 2p
1135.63 1135.63
531.15 531.03 530.89
162.74 163.20 163.38
the oxygen-containing functional groups [39,49–51]. As show in Fig. 5D, the binding energy of S 2p is observed, and the binding energy of S 2p under basic condition is higher than the acid condition (163.38 eV at pH 10.0 and 162.74 eV at pH 4.0), which is due to the reaction between ≡S − H and Eu(III) [52]. These evidences reveal that Eu(III) is available to bind with hydroxyl and `S\\H with pH N 6.0. The results of XPS analysis indicate that the adsorption mechanism of Eu(III) on M-Pyr could be attributed to the oxygen- and sulfur-containing functional groups of M-Pyr. 4. Conclusions Monoclinic pyrrhotite was successfully synthesized by the calcinations of pyrite at 923 K for 1.5 h under N2 gas conditions. The batch adsorption results indicated that the adsorption of Eu(III) on M-Pyr significantly increased with increasing pH from 2.0–7.5, and then high-level adsorption remained at pH N 7.5. No effect of ionic strength on the adsorption of Eu(III) on M-Pyr throughout a wide pH range, indicating that inner-sphere surface complexation dominated Eu(III) adsorption on M-Pyr. The adsorption kinetics and adsorption isotherms of Eu(III) on M-Pyr can be well described by pseudo-second-order model and Langmuir model, respectively. The maximum adsorption capacity (Qmax) of Eu(III) on M-Pyr calculated from Langmuir model was 10.03 mg/g at pH 5.0 and 293 K. The thermodynamic analysis indicated that the adsorption process of Eu(III) on M-Pyr was an endothermic and spontaneous processes. According to the results of XPS analysis, the adsorption of Eu(III) on M-Pyr was inner-sphere surface complexation at pH N 6.0 by the combination of Eu(III) and sulfur-containing functional groups. The results indicated that M-Pyr can be used a suitable adsorbent for the preconcentration and removal of Eu(III) from aqueous solutions. Acknowledgments This study was financially supported by the Natural Science Foundation of China (Nos. 41402030, 41172048, and 41572029). References [1] T. Rabung, H. Geckeis, J.I. Kim, H.P. Beck, Radiochim. Acta 82 (1998) 243–248. [2] M. Bouby, J. Lutzenkirchen, K. Dardenne, T. Preocanin, M.A. Denecke, R. Klenze, H. Geckeis, J. Colloid Interface Sci. 350 (2010) 551–561. [3] Q.H. Fan, X.L. Tan, J.X. Li, X.K. Wang, W.S. Wu, G. Montavon, Environ. Sci. Technol. 43 (2009) 5776–5782. [4] Y.B. Sun, J.X. Li, X.K. Wang, Geochim. Cosmochim. Acta 140 (2014) 621–643. [5] Y.B. Sun, C.L. Chen, X.L. Tan, D.D. Shao, J.X. Li, G.X. Zhao, S.B. Yang, Q. Wang, X.K. Wang, Dalton Trans. 41 (2012) 13388–13394. [6] C.C. Ding, W.C. Cheng, Y.B. Sun, J. Hazard. Mater. 295 (2015) 127–137.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Kinetics and thermodynamics of Eu(III) adsorption onto synthetic monoclinic pyrrhotite Yuke Zhu, Haibo Liu, Tianhu Chen ⁎, Bin Xu, Ping Li School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China
a r t i c l e
i n f o
Article history: Received 30 December 2015 Received in revised form 22 January 2016 Accepted 23 January 2016 Available online xxxx Keywords: Pyrrhotite Eu(III) Adsorption kinetics Thermodynamics XPS
a b s t r a c t The adsorption of Eu(III) from aqueous solutions onto monoclinic pyrrhotite (M-Pyr) was studied by batch experiments under various experimental conditions. M-Pyr was synthesized and characterized by XRD, FT-IR, SEM and potentiometric titration. The macroscopic results showed that the adsorption of Eu(III) on M-Pyr was independent of ionic strength, indicating that inner-sphere surface complexation predominated their adsorption. The kinetic adsorption process was well described by pseudo-second-order model with high correlation coefficient (R2 N 0.999). The adsorption isotherms indicated that the adsorption of Eu(III) on M-Pyr fitted by Langmuir model was better than Freundlich model. The maximum adsorption capacity of Eu(III) on M-Pyr calculated from Langmuir model was 10.03 mg/g at pH 5.0 and 293 K. The thermodynamic parameters suggested that the adsorption of Eu(III) on M-Pyr was a spontaneous and endothermic processes. According to XPS analysis, the adsorption of Eu(III) on M-Pyr was mainly dominated by ion exchange and inner-sphere surface complexation at low pH and high pH conditions, respectively. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With the peaceful utilization of nuclear-related industries, a large amount of radioactive wastes is transferred into sub-environments. For work safety in the nuclear industry and for human health, the removal of a variety of radionuclides from nuclear waste solutions is an important environmental concern in nuclear waste management [1]. Europium (Eu(III)) as chemical analogue of trivalent lanthanides has been extensively studied in the past few years. Numerous studies have investigated the removal of europium on various materials such as clay minerals [2–4] and iron oxides [5–7]. Iron sulfides, including pyrite, pyrrhotite and mackinawite, are the widespread and abundant natural minerals. Pyrrhotite (Fe1 − XS, 0 b X b 0.125) displays various superstructures caused by ordered Fe vacancy geometries in the nonstoichiometric composition with an average Fe–S distance of 2.50 Å [8–10]. Owing to magnetic characteristics, pyrrhotite as a natural adsorbent has been extensively employed to remove a variety of environmental contaminants such as chromium [11], gold [12], copper [13–15], cadmium [16], and phosphorus [17]. Widler et al. [12] found that the maximum adsorption capacity of gold on pyrrhotite from aqueous solutions containing up to 40 mg/kg at 25 °C and 0.1 mol/L ionic strength. Li et al. [17] also demonstrated that the adsorption capacity of pyrrhotite for phosphate was 1.15 mg/g at ⁎ Corresponding author. E-mail address: [email protected] (T. Chen).
http://dx.doi.org/10.1016/j.molliq.2016.01.100 0167-7322/© 2016 Elsevier B.V. All rights reserved.
29 °C according to the Langmuir isotherm. To the authors' knowledge, the few studies on the removal of radionuclides on natural pyrrhotite are available [18,19]. The objectives of this study are to (1) synthesize pyrrhotite nanoparticles and characterize it by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) and potentiometric titration; (2) investigate the effect of water chemistries (reaction time, pH, ionic strength, initial concentration and temperature) on Eu(III) adsorption onto pyrrhotite by batch technique; (3) determine the adsorption mechanism between Eu(III) and pyrrhotite by using X-ray photoelectron spectroscopy (XPS) analysis. The results are helpful to understand the application of pyrrhotite as a promising adsorbent in nuclear waste management. 2. Experimental 2.1. Synthesis of pyrrhotite Pyrrhotite was synthesized by the calcinations of pyrite (Lujiang of Anhui, China) under N2 atmosphere at 923 K for 1.5 h [20]. Briefly, natural pyrite was soaked with 10% HCl for 2 h, and then was washed with Milli-Q water for 4 times. The purified pyrite was obtained by vacuum dried at room temperature. Sequentially, 3.0 g of purified pyrite were placed into the quartz tube, then sample was heated to 923 K for 1.5 h. The pyrrhotite was obtained by vacuum dried at room temperature. Eu(III) stock solution (0.1 mol/L) was prepared from Eu2O3 (purity 99.99%)
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after dissolution and dilution with 0.01 mol/L HClO 4 solution. All chemicals used in this experiment were analytical-grade, and all the solutions were prepared with Milli-Q water. 2.2. Characterization The identification of pyrrhotite was recorded by XRD (Dandonghaoyuan 2700, Cu target, electric tension of 40 kV, electric current of 30 mA, scan rate of 4°/min). The surface functional groups of pyrrhotite were determined by FT-IR (VERTEX-70) and the morphology of pyrrhotite was characterized by SEM (FE-SEM, Sirion 200). The potentiometric acid–base titration was performed to determine the chemical properties of pyrrhotite by use of a computer controlled titration system (DL50 automatic titrator, Mettler Toledo). Briefly, 0.3 of pyrrhotite was spiked into 0.01 mol/L NaClO4 background electrolyte at T = 20.0 ± 0.5 °C, and the sample was purged with argon gas for 2 h to exclude atmospheric CO2 (g). The initial pH of suspension was adjusted to pH 3.0 by adding 0.01 mol/L of HClO4 with vigorous stirring for 1 h, and then the suspension was slowly titrated to pH 11.0 with 0.05 mol/L NaOH titrant at a variable increment (0.008 up to 0.15 mL). The pHPZC (pH at point of zero charge) was obtained by fitting the data sets of potentiometric titration. The XPS measurements were conducted with a Thermo Escalab 250 electron spectrometer using 150 W Al-Kα radiations. 2.3. Adsorption experiments Adsorption experiments were conducted with 2 g/L pyrrhotite and 10 mg/L Eu(III) under N2 conditions at T = 293 K in the presence of 0.01 mol/L NaClO4. The pH of suspension was adjusted to be in the range 2.0–10.0 by adding negligible volume of 0.1 or 0.05 mol/L HClO4 or NaOH solution, the adsorption of Eu(III) without pyrrhotite was carried out under the same experimental conditions. Eu(III) stock solution and radiotracer Eu(III) were spiked into the bulk suspension gradually in order to avoid formation of Eu(OH)3(s) precipitate at pH N 7.0. The suspensions were shaken for 24 h to ensure that the adsorption reaction could achieve adsorption equilibrium. Subsequently an aliquot of the suspension (5.0 mL) was removed and the total activity of Eu(III) was determined. The solid phases were separated from liquid phases by centrifugation at 9500 rpm for 10 min, and then the supernatant was poured into a syringe and filtered through a 0.22 μm membrane. The concentration of Eu(III) in supernatant was analyzed by liquid scintillation counting on a Packard 3100 TR/AB liquid scintillation analyzer(Perkin-Elmer) with the scintillation cocktail (Ultima Gold AB, Packard). The removal percentage of Eu(III) (adsorption (%)) and adsorption capacity (Qs, mg/ g) can be expressed as Eqs. (1) and (2), respectively: Adsorption ð%Þ ¼ C0 −Ceq 100%=C0
ð1Þ
Qs ¼ V C0 −Ceq =m
ð2Þ
where C0 (mg/L) and Ceq (mg/L) are initial concentration and concentration after adsorption, respectively. m (g) and V (mL) are the mass of pyrrhotite and the volume of the suspension, respectively. All experimental data were the average of triplicate determinations and the relative errors were within ± 5%. 3. Results and discussion 3.1. Characterization The identification of the pyrrhotite is conducted by XRD pattern. As shown in Fig. 1A, the characteristic peaks at 2θ = 30.03, 33.98, 44.13, 53.36 and 57.71° are indexed to the (220), (−224), (228), (620) and (2212) plane of monoclinic pyrrhotite (Fe7S8, M-Pyr), respectively [21]. Fig. 1B shows the FT-IR spectrum of pyrrhotite. The peak at
3420 cm− 1 is related to the O\\H stretching, whereas the peaks at 1622 and 1467 cm− 1 refer to the H\\O\\H bending [22]. The strong bands at 1088 and 470 cm−1 represent the Si\\O\\Si groups of the tetrahedral sheet [4]. The peaks at 800 and 625 cm−1 are attributed to the bending and stretching vibration of S\\O, respectively [22,23]. The SEM image of M-Pyr is shown in the Fig. 1C, one can see that M-Pyr is closely agglomerated together by randomly aggregated nanosheets at submicrons level. The M-Pyr is polyporous and poorly crystalline. The pHPZC (pH at point of zero charge) is measured to be 6.0 by potentiometric acid–base titration (Fig. 1D). 3.2. Adsorption kinetics Kinetic experiments of Eu(III) adsorption on M-Pyr at pH 3.0 and 5.0 are shown in Fig. 2. As shown in Fig. 2, the adsorption of Eu(III) adsorption on M-Pyr significantly increases with increasing reaction time from 0 to 2 h, then slight enhancement of Eu(III) adsorption is observed after 2 h. The fast adsorption of Eu(III) on M-Pyr indicates that the instantaneous adsorption process may be primarily dominated by ion exchange or chemical adsorption [24–26]. Approximately 80% (pH 3.0) and 100% (pH 5.0) of Eu(III) are adsorbed on M-Pyr at 24 h, respectively. The pseudo-first-order and pseudo-second-order kinetic models are employed to simulate the adsorption kinetic of Eu(III) on M-Pyr. Their linear forms are given in Eqs. (3) [27] and (4) [28], respectively: ln ðQ e −Q t Þ ¼ ln Q e −K 1 t
ð3Þ
t=Q t ¼ 1= K 2 Q e 2 þ t=Q e
ð4Þ
where Qt (mg/g) and Qe (mg/g) are the adsorption concentration of Eu(III) at time t and at equilibrium time, respectively. K1 (g/(mg·min)) and K2 (g/(mg·min)) are the adsorption constants of pseudo-first-order and pseudo-second-order kinetic models, respectively. The fitted parameters of pseudo-first-order and pseudo-second-order kinetic models are summarized in Table 1. One can see that the adsorption kinetics of Eu(III) adsorption on M-Pyr can be satisfactorily simulated by pseudosecond-order kinetic model (R2 N 0.999) compared to pseudo-first order kinetic model (R2 b 0.98). 3.3. Effect of pH and ionic strength The effect of pH on the adsorption of Eu(III) on M-Pyr is showed in Fig. 3A. As shown in Fig. 3A, the amounts of Eu(III) adsorbed increased from approximately 20% to 100% at pH 2.0–7.0. As showed in Fig. 1D, the surface charge of M-Pyr is positive at pH b 6.0, whereas the surface of M-Pyr presents massive negative surface charge at pH N 6.0. Therefore, the low adsorption at pH b 6.0 could be due to the electrostatic repulsion between positive charged Eu3+ and positive charged M-Pyr surface. At pH N 6.0, the positively charged Eu(III) ions [Eu(OH)2 + species] can be easily adsorbed on the negatively charged M-Pyr because of the strong electrostatic interaction [29]. However, the increased adsorption of Eu(III) on M-Pyr at pH 2.0–6.0 is inconsistent with electrostatic interaction, which is presumably due to the surface complexation [30–33]. Fig. 3A also shows the effect of ionic strength on Eu(III) adsorption on M-Pyr. The increasing ionic strength has no effect on the adsorption of Eu(III) on M-Pyr throughout a wide pH range. According to previous studies, the ion exchange or outer-sphere surface complexation was obviously sensitive to the variations of ionic strength [34,35], while the inner-sphere surface complexation was not influenced by ionic strength [36,37]. Therefore, inner-sphere surface complexation dominates Eu(III) adsorption on M-Pyr [38]. This adsorption behavior is similar to the results of Eu(III) adsorption on graphene oxide nanosheets [29] and graphene oxide-supported polyamine [39].
Y. Zhu et al. / Journal of Molecular Liquids 218 (2016) 565–570
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Fig. 1. Characterization of M-Pyr. A: XRD patterns; B: FTIR spectrum; C: SEM image; D: Potentiometric titration.
Fig. 2. Adsorption kinetics (A) and corresponding fitting of pseudo-second-order kinetic model (B) of Eu(III) adsorption on M-Pyr, pH 5.0, m/V = 2.0 g/L, I = 0.01 mol/L NaClO4, and T = 293 K.
is promoted at higher temperature. The adsorption data are fitted by Langmuir and Freundlich model. The Langmuir isotherm model is used to describe monolayer adsorption processes onto a surface [40,41], whereas Freundlich isotherm model is used to describe heterogeneous adsorption [42]. The linear forms of Langmuir and Freundlich equation can be expressed as Eqs. (5) and (6), respectively:
3.4. Adsorption isotherms Fig. 4A and B show the adsorption isotherms of Eu(III) on M-Pyr at different pH and at different temperatures, respectively. As shown in Fig. 4A, the adsorption of Eu(III) on M-Pyr at pH 5.0 is significantly higher than that of Eu(III) adsorption at pH 3.0. As shown in Fig. 4B, the adsorption of Eu(III) on M-Pyr obviously increases with the increasing of the reaction temperature. Adsorption capacity is highest at T = 333 K and lowest at T = 293 K, which shows that Eu(III) adsorption on M-Pyr
Ce =Q e ¼ 1=ðKL Q m Þ þ Ce =Q m
ð5Þ
Table 1 Parameters of pseudo-first-order and pseudo-second-order kinetic models. Pseudo-first-order
pH = 3.0 pH = 5.0
Pseudo-second-order
Qe (mg/g)
K1 (g/(mg·min))
R2
Qe (mg∙g−1)
K2 (g/(mg·min))
R2
4.0646 4.8875
1.5137 1.6965
0.976 0.912
4.1736 4.9801
0.5889 0.6128
0.9996 0.9993
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Fig. 3. A: Effect of pH and ionic strength on adsorption of Eu(III) on M-Pyr, CEu(III) = 10 mg/L, m/V = 2.0 g/L, and T = 293 K; B: The relative distribution of Eu(III) as a function of pH in aqueous solutions, CEu(III) = 10 mg/L, I = 0.01 mol/L NaClO4, and T = 293 K.
lgQ e ¼ 1=n lgCe þ lgK F
ð6Þ
where Qm (mg/g) is the maximum adsorption capacity of adsorbent at complete monolayer coverage. KL (L/mg) is a Langmuir constant, which is related to the free energy of sorption. 1/n is the heterogeneity of the adsorption sites; KF represents equilibrium coefficient, which describes the partitioning of the adsorbate between the solid and liquid phases over the concentration range studied. As shown in Fig. 4C and D, one can see that the adsorption behaviors of Eu(III) on M-Pyr can be fitted by Langmuir model well. As shown in Table 2, the maximum adsorption capacity (Qmax ) of Eu(III) on M-Pyr calculated from Langmuir model is 10.03 mg/g at pH 5.0 and 293 K. Comparing to qmax values
of Eu(III) adsorption on other adsorbents such as TiO2 [2], ZSM-5 zeolite [43], one can see that M-Pyr presents the higher adsorption capacity for Eu(III) compared to common materials. 3.5. Thermodynamic parameters The thermodynamic parameters (i.e., standard free energy change (ΔG°), standard enthalpy change (ΔH°) and the standard entropy change (ΔS°)) can be calculated according to Eqs. (7) and (8): ΔG ° ¼ −RT lnK °
ð7Þ
Fig. 4. Adsorption isotherms of Eu(III) adsorption on M-Pyr. A: Effect of pH, m/V = 2.0 g/L, I = 0.01 mol/L NaClO4, and T = 293 K. B: Effect of temperatures, m/V = 2.0 g/L, I = 0.01 mol/L NaClO4, and pH = 5.0. C and D: the fitting of adsorption isotherms of Eu(III) on M-Pyr by Langmuir model and Freundlich model, respectively.
Y. Zhu et al. / Journal of Molecular Liquids 218 (2016) 565–570 Table 2 Langmuir model and Freundlich model of Eu(III) adsorption on M-Pyr. Temperature (K)
pH
293 293 313 333
3.0 5.0 5.0 5.0
Langmuir model
Freundlich model
Qm (mg/g)
KL (L/mg)
R2
KF (mg/g)/(mg·L)−n)
1/n
R2
7.90 10.03 11.84 13.30
0.15 0.19 0.24 0.31
0.9986 0.9989 0.9986 0.9992
0.65 1.17 1.85 2.79
0.89 0.89 0.82 0.81
0.9848 0.9532 0.9509 0.9527
569
increasing temperature indicates that the adsorption of Eu(III)on M-Pyr is more favorable at higher temperature [44]. The positive ΔH° value (15.68 kJ/mol) suggests that Eu(III) adsorption on M-Pyr is an endothermic process [45]. It is determined that the dehydration of Eu(III) from aqueous Eu(III) complex ion is an endothermic process, but attachment of Eu(III) to the surface of endothermic is an exothermic processes [46]. It is plausible to assume that the energy of dehydration exceeds the exothermicity of the Eu(III) ions attached to the surface of M-Pyr. 3.6. Adsorption mechanism
Table 3 Thermodynamic parameters for the adsorption of Eu(III) on M-Pyr. Temperature (K)
ΔG/(kJ/mol)
ΔS/(J/(mol·K))
ΔH/(kJ/mol)
293 313 333
−11.57 −12.72 −13.92
58.78
15.68
ln K ° ¼ ΔS °=R−ΔH °=RT
ð8Þ
where R is the universal constant (8.314 J/(mol∙K)) and T is the temperature in Kelvin. The optimized parameters of ΔG°, ΔH°, ΔS° are listed in Table 3. As shown in Table 3, negative ΔG° values (−11.57 kJ/mol at 293 K, −13.93 kJ/mol at 333 K) indicate that the adsorption of Eu(III) on M-Pyr is a spontaneous process, and the decrease of ΔG° with
The XPS spectroscopy technique is used to determine the interaction mechanism of the adsorption of Eu(III) on M-Pyr. Fig. 5 shows the XPS spectra of survey and high resolution scans for the Eu 3d O 1s and S 2p of M-Pyr after adsorption at different pH conditions. The occurrences of Eu 3d5/2 are observed at pH 6.0 and pH 10.0 in terms of the survey spectra (Fig. 5B), which indicates that Eu(III) is chemically adsorbed onto the surface of M-Pyr [47,48]. The relative intensity and binding energies of O 1s for M-Pyr samples of Eu(III) adsorption in basic solution are significantly lower than that of samples in acid solution, whereas the relative intensity and binding energies of S 2p in basic solution are higher than that of samples in acid solution. Fig. 5C shows the high resolution scans of O 1s of the three samples. One can see that the relative energies of O 1s slightly decrease with increasing pH value. As illustrated in Table 4, the binding energies of O 1s at pH 4.0 and 10.0 are 531.15 and 530.89 eV, respectively. The lower binding energy of O 1s at high pH conditions indicates the retention of radionuclides by M-Pyr related to
Fig. 5. The survey and high resolution scans of XPS spectra of Eu(III) adsorption on M-Pyr. (A) Total survey scans; (B) Eu 3d peaks, (C) O 1s peaks, (D) S 2p peaks, m/V = 10 g/L, I = 0.01 mol/L NaClO4, T = 293 K.
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Table 4 Binding energies of XPS analysis of M-Pyr after adsorption at different pH. pH 4.0 6.0 10.0
Eu 3d5/2
O 1s
S 2p
1135.63 1135.63
531.15 531.03 530.89
162.74 163.20 163.38
the oxygen-containing functional groups [39,49–51]. As show in Fig. 5D, the binding energy of S 2p is observed, and the binding energy of S 2p under basic condition is higher than the acid condition (163.38 eV at pH 10.0 and 162.74 eV at pH 4.0), which is due to the reaction between ≡S − H and Eu(III) [52]. These evidences reveal that Eu(III) is available to bind with hydroxyl and `S\\H with pH N 6.0. The results of XPS analysis indicate that the adsorption mechanism of Eu(III) on M-Pyr could be attributed to the oxygen- and sulfur-containing functional groups of M-Pyr. 4. Conclusions Monoclinic pyrrhotite was successfully synthesized by the calcinations of pyrite at 923 K for 1.5 h under N2 gas conditions. The batch adsorption results indicated that the adsorption of Eu(III) on M-Pyr significantly increased with increasing pH from 2.0–7.5, and then high-level adsorption remained at pH N 7.5. No effect of ionic strength on the adsorption of Eu(III) on M-Pyr throughout a wide pH range, indicating that inner-sphere surface complexation dominated Eu(III) adsorption on M-Pyr. The adsorption kinetics and adsorption isotherms of Eu(III) on M-Pyr can be well described by pseudo-second-order model and Langmuir model, respectively. The maximum adsorption capacity (Qmax) of Eu(III) on M-Pyr calculated from Langmuir model was 10.03 mg/g at pH 5.0 and 293 K. The thermodynamic analysis indicated that the adsorption process of Eu(III) on M-Pyr was an endothermic and spontaneous processes. According to the results of XPS analysis, the adsorption of Eu(III) on M-Pyr was inner-sphere surface complexation at pH N 6.0 by the combination of Eu(III) and sulfur-containing functional groups. The results indicated that M-Pyr can be used a suitable adsorbent for the preconcentration and removal of Eu(III) from aqueous solutions. Acknowledgments This study was financially supported by the Natural Science Foundation of China (Nos. 41402030, 41172048, and 41572029). References [1] T. Rabung, H. Geckeis, J.I. Kim, H.P. Beck, Radiochim. Acta 82 (1998) 243–248. [2] M. Bouby, J. Lutzenkirchen, K. Dardenne, T. Preocanin, M.A. Denecke, R. Klenze, H. Geckeis, J. Colloid Interface Sci. 350 (2010) 551–561. [3] Q.H. Fan, X.L. Tan, J.X. Li, X.K. Wang, W.S. Wu, G. Montavon, Environ. Sci. Technol. 43 (2009) 5776–5782. [4] Y.B. Sun, J.X. Li, X.K. Wang, Geochim. Cosmochim. Acta 140 (2014) 621–643. [5] Y.B. Sun, C.L. Chen, X.L. Tan, D.D. Shao, J.X. Li, G.X. Zhao, S.B. Yang, Q. Wang, X.K. Wang, Dalton Trans. 41 (2012) 13388–13394. [6] C.C. Ding, W.C. Cheng, Y.B. Sun, J. Hazard. Mater. 295 (2015) 127–137.
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