Research Article pubs.acs.org/journal/ascecg
Spectroscopic and Modeling Investigation of Eu(III)/U(VI) Sorption on Nanomagnetite from Aqueous Solutions Mengxue Li,† Yubing Sun,*,‡ Haibo Liu,*,† Tianhu Chen,† Tasawar Hayat,§ Njud S. Alharbi,⊥ and Changlun Chen‡,§,⊥ †
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China Key Laboratory of New Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, Anhui People’s Republic of China § NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ⊥ Department of Biological Sciences, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡
ABSTRACT: Nanomagnetite was synthesized by heating natural siderite in nitrogen conditions and was characterized using XRD, XPS, TEM, FT-IR and acid−base titration. Characteristic results illustrated that the reactive site density (17.91 sites/nm2) of nanomagnetite was significantly higher than that of natural siderite (3.63 sites/nm2), whereas average pore size (14.9 nm) of nanomagnetite decreased compared to natural siderite (52.5 nm). Effects of different ambient conditions (i.e., pH, contact time, temperatures and ionic strength) on removal behaviors of Eu(III)/U(VI) on nanomagnetite were conducted by batch experiments. Removal isotherms and kinetics of Eu(III)/U(VI) on nanomagnetite fitted well by models of Langmuir and pseudo-second kinetic, respectively. Additionally, the max sorption capacity of nanomagnetite with Eu(III) (11.95 mg/g) at pH = 2.5 and T = 328 K was significantly higher than sorption capacity of U(VI) (4.93 mg/g). The XPS analysis demonstrated that the surface oxygen groups of nanomagnetite played an important role in the sorption process of Eu(III)/U(VI) via inner surface complexation. The sorption of Eu(III)/U(VI) on nanomagnetite fitted satisfactorily using surface complexation modeling with two and three inner-sphere surface complexation sites, respectively. These findings are crucial for the evaluation of radioactive nuclides at ultralow pH conditions. KEYWORDS: Nanomagnetite, Radionuclides, XPS analysis, Surface complexation modeling, Adsorption mechanism
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INTRODUCTION The discharge of radioactive wastes (e.g., europium, uranium, technetiumand selenium) from nuclear industries into subsurface environments has been a hot spot of the around world.1−4 These radioactive wastes may lead to the long-term potential threat to soils, groundwater and human beings through food chain.5,6 Owing to the widespread distribution, carcinogenicity, and long half-life radiation of radionuclides, a burning interest regarding the adsorption behaviors of these radionuclides from aqueous solutions has been attracted at nowadays. In recent years, a lot of researchers investigated the sorption of radionuclides onto various adsorbents especially for clay sorbents,7−10 metal oxides11−16 and carbon-based sorbents.17−19 Among these various adsorbents, magnetite has been paid more attentions owing to the prominent properties such as easy separation, low toxicity and strong magnetism.20,21 Magnetite (Fe3O4) presented a kind of crystals such as octahedron and rhombic dodecahedron.22 The technique of magnetic separation has been considered as a convenient and effective solid−liquid separation measure in government of wastewater. In view of the convenient separation process with an external magnetic field, magnetite was regarded as a © 2017 American Chemical Society
prospective sorbent for efficient and quick sorption of radionuclides.18,23,24 According to previous reports that U(VI) sorption onto nanomagnetite included simultaneous sorption and reduction.25−27 Generally, the nanomagnetite was synthesized by adding Fe(II) and Fe(III) salts into the basic solutions under the N2 conditions, the complex procedures inhibited its widespread application in environmental cleanup.14,20,28−32 In this study, the magnetite was synthesized by heating natural siderite under N2 conditions, which was the simple, cheap and convenient method. The purposes of this investigation were (1) to synthesize nanomagnetite and characterize it using XRD,TEM, FT-IR, XPS and acid−base titration; (2) to explore the impact of environmental factors (temperature, pH, connect time, and ionic strength etc.) on the sorption of Eu(III)/U(VI) on nanomagnetite using batch techniques; (3) to ascertain the reaction mechanism between nanomagnetite and Eu(III)/ U(VI) using XPS analysis and surface complexation modeling. Received: March 19, 2017 Revised: April 19, 2017 Published: May 16, 2017 5493
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Characterization of nanomagnetite (A and B) TEM images of nanomagnetite and nature siderite, respectively; (C) XRD patterns; (D) N2 adsorption−desorption isotherm; (E) total pore size distribution of nanomagnetite. (F) Potentiometric titration. Pore size Analyzer. The acid−base titration of nanomagnetite at 0.01 mol/L NaCl solution was carried out by a titration apparatus (DL50 automatic titrator, Mettler Toledo).Typically, 0.04 g of nanomagnetite was put into 0.01 mol/L NaCl (40 mL) background electrolyte at T = 25 ± 1 °C, then purged the suspension with argon gas for 0.5 h to exhaust CO2(g). Adjusted the original pH of suspension to 2.0 using 0.5 mol/L HCl, and consequently the suspension was gently titrated to pH 11.0 using 0.5 mol/L NaOH titrant (0.001−0.1 mL). The surface reactive site density was acquired by fitting titration data by FITEQL v2.6 mode.33 Batch Experiments. The batch experiments were proceeded with 1.0 g/L nanomagnetite with 10 mg/L Eu(III) and U(VI) solutions in the presence of 0.01 mol/L NaCl. The stock solutions of Eu(III) and U(VI) were prepared by dissolving Eu2O3 (purity 99.99%) and UO2(NO 3) 2 (spectrographic purity) with concentrated HNO 3 solution, respectively. Briefly, the nanomagnetite and NaCl solutions were pre-equilibrated 12 h, and then aliquot Eu(III)/U(VI) solution was mixed into aforementioned suspension and was reacted under vigorous stirring conditions for 24 h. The preliminary experiment demonstrated that 24 h was enough to achieve reaction equilibrium for the suspension. The pH of the suspension was adjusted using
The highlight of this study was the application of nanomagnetite into the sorption of radionuclides in environmental remediation.
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EXPERIMENTAL SECTION
Preparation and Characterization of Nanomagnetite. The nanomagnetite was synthesized by heating the natural siderite ( 4.5. The total surface reactive site concentration (CSOH, mol/g) could also be informed by eq 5:39−41
where C0 (mg/L) is original and Ceq (mg/L) is after sorption concentration of Eu(III)/U(VI), respectively. The m (g) is the mass of nanomagnetite and V (mL) is the suspension volume, respectively. Surface Complexation Modeling. The sorption data was fitted by surface complexation modeling using Visual MINTEQ 2.6 mode.33 The reactions of protonation and deprotonation of nanomagnetite can be depicted by eqs 3 and 4, respectively: SOH + H+ = SOH 2+
adsorbents
average pore size (nm)
CSOH = 0.5 × ([H+]sample + water − [H+]water ) +
(5)
+
where [H ]sample+water and [H ]water stand for the concentration of consumed H+ with and without samples, respectively. As shown in Table 1, the total surface reactive site concentration of nanomagnetite was calculated to be 17.91 sites/nm, which was significantly higher than that of natural metal oxides and sediments (∼2.3 sites/nm2).39 The results of potentiometric titration indicated that nanomagnetite presented more surface reactive sites compared with natural metal oxides. Impact of pH and Ionic Strength. Figure 2A,B shows the impact of ionic strength on Eu(III) and U(VI) removal by nanomagnetite, respectively. Observed in Figure 2A, the slight sorption of Eu(III) onto nanomagnetite was appeared at pH 2.0−5.0, whereas sorption of Eu(III) promptly increased with increasing pH ranging from 5.0 to 7.0, then kept a comparatively higher removal rate (∼90%) at pH > 7.0. Nevertheless, the significant increased sorption of U(VI) onto nanomagnetite was appeared at pH 2.0−6.0, whereas sorption of U(VI) significantly decreased at pH > 7.0. Although the pHedge sorption of UVI) on nanomagnetite was shifted to lower pH opposed to Eu(III) sorption, the removal amount of U(VI) at pH 7.0 (∼70%) was remarkably lower than the amount of Eu(III) (∼85%). These results could be explained by electrostatic interaction of distribution of Eu(III)/U(VI) speciation at various pH and the surface property of nanomagnetite.42−44 Figure 2C,D shows Eu(III)/U(VI) species distribution in solutions, respectively. Eu(III) mainly exited with Eu3+ species at pH < 7.0, whereas the species of hydrolyzed mononuclear (i.e., Eu(OH)2+) increased with increasing pH > 8.0.45 Meanwhile, the positive and negative charged of nanomagnetite was observed at pH < 4.5 and pH > 4.5, respectively. Consequently, slight increased sorption of Eu(III) onto nanomagnetite at pH < 5.0 probably ascribed to electrostatic repulsion between of positive nanomagnetite and Eu3+ species. The increased sorption of Eu(III) at pH 5.0−7.0 probably owing to the electrostatic attraction of positive Eu(III) species (Eu3+/Eu(OH)2+ species) and negatively charged nanomagnetite. Compared with Eu(III), the similar trends of U(VI) sorption was observed at pH < 7.0. As displayed in Figure 2D, U(VI) species were existed in complicated forms mainly existed in solution with UO22+ at pH < 4.0, and some complex hydrolyzed species (e.g., UO2OH+, (UO2)3(OH)5+ and (UO2)4(OH)7+ species) were appeared at pH 4.0−8.0, whereas the negative U(VI) species (i.e., (UO2)3(OH)7− and UO2(OH)3−) were observed at pH > 8.0. Therefore, the inhibited sorption of U(VI) onto nanomagnetite at pH > 8.0 was attributed to electrostatic repulsion between negative nanomagnetite and U(VI) species (e.g., (UO2)3(OH)7− and UO2(OH)3−).46
where SOH is the amphoteric surface sites. The equilibrium constants of protonation and deprotonation reactions (log K+ and log K− values) were gained by optimizing the titration data by double layer model. The more details regarding the surface complexation modeling were provided in the previous studies.34−36
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RESULTS AND DISCUSSION Characterization. Figure 1A,B shows the TEM images of nanomagnetite and natural siderite, respectively. It is observed that the as-prepared magnetite was nanoparticles with massive nanopores (Figure 1A), whereas the natural siderite displayed the sheet structure (Figure 1B). An inset in Figure 1A,B shows that nanomagnetite presented the good dispersibility and magnetic separation. The mineralogy of nanomagnetite before and after U(VI)/Eu(III) adsorption was recorded by XRD pattern (Figure 1C). The reflections at 2θ = 30.1, 35.4, 43.1, 53.4, 57.1 and 62.5° corresponded to the crystal planes of (220), (311), (400), (422), (511) and (440) of the cubic Fe3O4 phase, respectively.13 The broad diffraction bands with low relative intensities revealed the relative low crystallinity of nanoparticles. Furthermore, no significant differences of nanomagnetite were observed after sorption process. The XRD patterns illustrated that the mineralogy of nanomagnetite did not change after Eu(III)/U(VI) sorption.2 To gain further insight into the porous property of nanomagnetite, the N2-BET and pore size distribution of the sample were explored by BET measurements. As shown in Figure 1D, the N2 adsorption/ desorption isotherm of nanomagnetite displayed the typical IVtype isotherms with H3-hysteresis loops, which suggested the presence of mesopores at the surface of nanomagnetite.37,38 As shown by total pore size distribution (Figure 1E, the pore size of nanomagnetite presented a distribution of multipeaks, including basically mesopores, a small quantity of micropores and no macropores. It is observed that more than 90% of the mesopores sizes were located at ∼17 nm (Figure 1E). The N2BET of nanomagnetite calculated by multipoints N2-BET method was 12.4 m2/g. As summarized in Table 1, the total pore volume and average pore diameter were 4.62 × 10−2 cc/g and 14.9 nm, respectively. Figure 1F shows the potentiometric acid−base titration of nanomagnetite at pH starting from 2.0 to 5495
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
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Figure 2. Effect of pH and ionic strength on the adsorption of Eu(III) (A) and U(VI) (B) onto nanomagnetite, C0 = 10 mg/L, m/V = 1.0 g/L, T = 298 K. Distribution of Eu(III) (C) and U(VI) (D) species in aqueous solutions, C0 = 10 mg/L, T = 298 K.
t /qt = 1/(K 2 × Q e 2) + t /Q e
The impact of ionic strength is shown in Figure 2A,B, respectively. Hardly any effect on Eu(III)/U(VI) sorption on nanomagnetite was observed. Previous studies indicated that outer-sphere surface complexation was strongly dependent on impact of ionic strength, on the contrary, inner-sphere surface complexation was insensitive of it.47,48 Hence, inner-sphere surface complexation dominated the sorption experiments by chemical bonds of adsorbate with amphoteric groups. Adsorption Kinetics. Figure 3A shows the kinetics of Eu(III) and U(VI) sorption onto nanomagnetite at pH 2.5. Although the sorption trends observably increased with increasing pH values (Figure 2), the pH values of practical wastewater from spent nuclear fuel was generally low (pH 2.0− 3.0); therefore, it is meaningful to evaluate the adsorption capacity of nanomagnetite at low pH conditions. Obviously, the sorption rate increased with the increasing reaction time from 5 min to 9 h and then the sorption process was reached equilibrium at reaction time more than 8 h. The removal capacity of Eu(III) onto nanomagnetite was higher than U(VI), which demonstrated that the high affinity of Eu(III) onto nanomagnetite. To determine the underlying mechanisms in experimental process, pseudo-first-order and pseudo-secondorder kinetic models were used for fitting sorption kinetics data. The linear forms of kinetic models could be expressed in eqs 6 and 7, respectively: ln(Q e − Q t ) = ln Q e − K1 × t
(7)
where Q e (mg/g) and Q t (mg/g) are the sorption concentration at equilibrium and time t, respectively. K1 and K2 are kinetic rate constants. The parameters obtained by fitting kinetic models are listed in Table 2. As shown in Figure 3B, the sorption kinetics of Eu(III)/U(VI) onto nanomagnetite simulated well by pseudo-second model due to the high values of correlation coefficients (R2 > 0.999) (Table 2). Adsorption Isotherms. As an effective technique to evaluate and calculate the adsorption capacities, adsorption isotherms was used to evaluate the sorption capacity of nanomagnetite for Eu(III) and U(VI) ions. Figure 3C,D shows isothermal sorption of Eu(III) and U(VI) ions onto nanomagnetite at various temperatures, respectively. Observed in Figure 3C,D, the sorption of Eu(III) and U(VI) ions onto nanomagnetite significantly increased with increasing solution concentration at low concentration, whereas the sorption trends were decreased at high concentration. Langmuir and Freundlich models were applied to simulate the data of sorption isotherms. Langmuir suggested that sorption occurred in a monolayer with all sorption sites identical and energetically equivalent, whereas Freundlich was the equation of exponential curve with the assumption of the heterogeneous adsorbent surface.49 The linear forms are expressed in eqs 8 and 9, respectively:
(6) 5496
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
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Figure 3. Adsorption kinetics (A) of Eu(III) and U(VI) on nanomagnetite and corresponding fitting of pseudo-second-order kinetic model (B), C = 10 mg/L, m/V = 1.0 g/L, I = 0.01 mol/L NaCl, T = 298 K, pH= 2.5. Adsorption isotherms of Eu(III) (A) and U(VI) (B) on nanomagnetite at different temperatures, pH = 2.5, m/V = 1.0 g/L, I = 0.01 mol/L NaCl.
Table 2. Kinetic Models of Eu(III) and U(VI) Adsorption on Nanomagnetite U(VI) Eu(III)
Qe (mg·g−1)
K1 g/(mg·min)
R2
Qe (mg·g−1)
K2 g/(mg·min)
R2
1.97 2.13
0.43 0.21
0.9963 0.9930
5.46 3.64
0.74 0.55
0.9994 0.9992
Table 3. Langmuir and Freundlich Model of Eu(III) and U(VI) Adsorption on Nanomagnetite Langmuir Eu(III)
U(VI)
Freundlich
D-R isotherm
temperature (K)
qm (mg/g)
KL (L/mg)
R2
KF (mg/g)/ (mg·L)−n)
1/n
R2
qm (mg/g)
β
R2
293 313 328 293 313 328
5.75 7.56 11.94 2.46 3.28 4.93
0.10 0.17 0.51 0.08 0.14 0.18
0.9989 0.9985 0.9977 0.9933 0.9958 0.9951
0.27 0.69 1.02 0.22 0.46 0.61
0.92 0.75 0.93 0.68 0.55 0.64
0.9722 0.9616 0.9628 0.9823 0.9828 0.9278
1.70 1.85 2.18 1.03 1.30 1.60
3.58 1.91 0.79 1.46 1.21 1.24
0.9805 0.9262 0.9153 0.7299 0.8859 0.8912
Ce/Q e = 1/(KL ·qm) + Ce/qm
(8)
log Q e = (1/n)log C e + log K F
(9)
ln Q e = ln qm − β ∂ 2
where Qe and qm refer to defined above, β represents the activity coefficient related to the mean sorption energy (mol2 /kJ2), ∂ stands for the Polanyi potential, which is equal to
where qm (mg·g−1) refer to the max sorption capacity of sorbent. KL (L·mg−1) stands for a Langmuir constant. 1/n is the heterogeneity of the sorption sites; KF represents equilibrium coefficient. In addition, another frequently used model D-R isotherm model was used to simulate the sorption process.50,51 The D-R isotherm model could be expressed as eq 10: Q e = qmexp(−K (RT ln(1 + 1/Ce))2 )
(11)
∂ = RT ln(1 + 1/Ce)
(12)
where R is the ideal gas constant (8.314 J/(mol·K)), and T is the absolute temperature in Kelvin (K). The relative parameters of three models are listed in Table 3. As shown in Table 3, Langmuir model (R2 > 0.9933) fitted excellent to the sorption of Eu(III)/U(VI) onto nanomagnetite compared with Freundlich (R2 < 0.9828) and D-R isotherm model (R2 < 0.8912). The result indicated that Eu(III) and
(10)
Equation 10 could be expressed in linear form: 5497
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
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ACS Sustainable Chemistry & Engineering Table 4. Thermodynamic Parameters for Eu(III) and U(VI) Adsorption on Nanomagnetite Eu(III)
U(VI)
temperature (K)
ΔG° (kJ·mol−1)
298 313 328 298 313 328
−4.36 −4.57 −5.03 −1.92 −2.31 −2.49
ΔS° (J mol−1·K−1)
ΔH° (kJ·mol−1)
97.82
4.52
82.64
3.87
Figure 4. (A) FTIR spectra of nanomagnetite before and after adsorption. (B−F) Survey and high resolution scans of XPS spectra of Eu(III) and U(VI) adsorption on nanomagnetite: (B) total survey, (C) O 1s, (D) Fe 2p, (E) Eu 3d, (F) U 4f.
and 4.93 mg/g for Eu(III) and U(VI), respectively. The results indicated that the sorption capacity of the nanomagnetite to Eu(III) was observably higher than U(VI), which was corresponded to the result of kinetics. Besides, the values of KL were increased with increased temperatures intimating a
U(VI) ions were adsorbed on the surface of the nanomagnetite with homogeneous binding sites, equivalent sorption energies, hardly any interaction between adsorbed species, and monolayer coverage. As tabulated in Table 3, the max sorption capacities of nanomagnetite at pH 2.5 and T = 328 K are 11.94 5498
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
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Figure 5. Surface complexation modeling of Eu(III) (A) and U(VI) (B) on nanomagnetite, C0 = 10 mg/L, I = 0.01 mol/L NaCl, m/V = 1.0 g/L, T = 298 K.
After sorption, the bands presented at 2887 and 2953 cm−1 were probably ascribed to the impurity peaks of nanomagnetite. The total survey and high resolution XPS spectra are exhibited in Figure 4B−F. As shown in Figure 4B, the main bands of C 1s, O 1s and Fe 2p for magnetite were observed, and the U 4f/Eu 3d peaks were presented after sorption of Eu(III) and U(VI), which demonstrated that Eu(III)/U(VI) were successfully adsorbed onto nanomagnetite.53 The high resolution scans for O 1s spectra are shown in Figure 4C. O 1s spectra could be disintegrated into two main peaks appeared at 530 and 531 eV, which were attributed to the bond of Fe−O− Fe and Fe−O−H, respectively. The binding energies of O 1s for magnetite-Eu and magnetite-U were slightly shifted to lower binding energies compared to that of magnetite, which illustrated that Eu(III)/U(VI) were bound with oxygenated groups.53 In addition, the relative intensities of Fe−O−H for nanomagnetite-Eu and nanomagnetite-U were significantly higher than intensities of nanomagnetite, indicating Eu(III)/ U(VI) sorption on nanomagnetite by chemisorbed OH− groups.54 The further evidence was provided by high resolution Fe 2p spectra of nanomagnetite. As depicted in Figure 4D, the Fe 2p spectra could be resolved into two main peaks occurring at 710.84 and 713.89 eV, which were represented for Fe2+ and Fe3+, respectively.55,56 A slight decrease in relative intensities of Fe2+ was observed for magnetite-Eu and magnetite-U, which was corresponded to the partly oxidation of Fe2+ to Fe3+ during experiments. Meanwhile, the binding energies of Fe3+ were shifted to the higher energies, which attributed to the sorption of Eu(III)/U(VI) and oxidation of Fe2+ ion. Figure 4E,F shows the high resolution scans for Eu 3d and U 4f, respectively. Two doublets peaks, including Eu 3d5/2 and Eu 3d3/2 peaks, U 4f7/2 and U 4f5/2, were owing to the multiple splitting of electrons with unpaired spins in the atomic shells.53 XPS spectral analysis demonstrated a high effective sorption of Eu(III)/U(VI) at such low pH conditions probably ascribed to the oxygenated functional groups of nanomagnetite. Surface Complexation Modeling. Figure 5A,B shows the surface complexation modeling of Eu(III) and U(VI) sorption onto nanomagnetite by using MINTEQ software, respectively. In recent years, MINTEQ software has been extensively employed to similar the adsorption behavior of adsorbate on the various adsorbents.57−60 The experiments of ionic strength effect demonstrated that Eu(III)/U(VI) adsorbed on nanomagnetite was inner-sphere surface complexation. Therefore, an initial attempt to simulate the adsorption edges of Eu(III) onto
more prominent sorption capacity of Eu(III)/U(VI) onto nanomagnetite. Thermodynamic Parameters. The assessment of thermodynamic parameters supplied an insight into the reaction mechanism of Eu(III)/U(VI) sorption onto nanomagnetite. The impact of temperature is shown in Figure 3C,D, respectively. The sorption capacity was the highest at T = 328 K and the lowest at T = 298 K, indicating that a higher temperature could contributed to the sorption process for both Eu(III) and U(VI) ions on nanomagnetite. To understand the thermodynamic properties of adsorption reaction, the thermodynamic parameters (ΔG0, ΔH0 and ΔS0) can be computed using eqs 13 and 14, respectively: ΔG° = ΔH ° − T ΔS°
(13)
ln K ° = ΔS°/R − ΔH °/RT
(14)
where K° refers to the adsorption equilibrium constant, which could be estimated by plotting ln Kd versus Ce and extrapolating Ce to zero. The corresponding parameters for the sorption of Eu(III)/U(VI) onto nanomagnetite are summarized in Table 4. As shown in Table 4, the negative values of ΔG° for Eu(III) (−4.36 kJ/mol at 298 K) and U(VI) (−1.92 kJ/mol at 298 K) illustrating a spontaneous process for sorption process. The positive ΔS° values (82.64 and 97.82 kJ/mol for U(VI) and Eu(III), respectively) implied that molecular arrangements became more confusion in reaction process, leading to an increase in the disorder of the solid-solution system.52,53 The positive ΔH° values (3.87 and 4.52 kJ/mol for U(VI) and Eu(III), respectively), suggesting an endothermic process for the sorption of Eu(III)/U(VI) on nanomagnetite. Adsorption Mechanism. To identify the retention mechanism and molecular structures, the FT-IR and XPS spectra of nanomagnetite for Eu(III) and U(VI) sorption were displayed in Figure 4 (sorption of Eu(III) and U(VI) ions onto nanomagnetite noted as magnetite-Eu and magnetite-U, respectively). As exhibited by FT-IR spectra in Figure 4A, the relative intensities of characteristic bands of magnetite-Eu and magnetite-U at 3643 and 1660 cm−1 were significantly higher than that of magnetite, which could be contributed to the O−H stretching vibration of adsorbed water in the lattice matrices.2,14 Furthermore, the bands presented at 1587 and 1660 cm−1 were caused by the deformation of hydroxyl vibration of water. Compared with magnetite, these peak intensities of magnetiteEu and magnetite-U increased significantly, which probably ascribed to the sorption of Eu(III)/U(VI) onto nanomagnetite. 5499
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
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accurate evaluation of radioactive nuclides under different environmental conditions.
nanomagnetite was satisfactory using two inner-sphere surface complexation sites, whereas the results underestimated the sorption edge of U(VI) onto nanomagnetite at high pH conditions. Therefore, three kinds of inner-sphere surface complexation sites were taken into consideration for fitting of U(VI) on nanomagnetite. These apparent equilibrium constants (log K values) were obtained by altering them repeatedly to be consistent with experimental data. The best set of parameters of Eu(III)/U(VI) sorption on nanomagnetite based on a goodness-of-fit criterion is provided in Table 5. As shown
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Corresponding Authors
*
[email protected] (Y.S.); Tel. (Fax): 86-551-65593308. *
[email protected] (H.L.) ORCID
Yubing Sun: 0000-0003-4931-8039 Notes
Table 5. Optimized Parameters of Surface Complexation Modeling of Eu(III) and U(VI) Adsorption on Nanomagnetite nanomagnetite Eu(III) U(VI)
reactions
log K
SOH + H+ = SOH2+ SOH = SO− + H+ SOH + Eu3+ = SOEu2+ + H+ 2SOH + Eu3+ + 2H2O = (SO)2Eu(OH)2− + 4H+ SOH + UO22+ = SOUO2+ + H+ 2SOH + UO22+ + 2H2O = (SO)2UO2(OH)22− + 4H+ SOH + UO22+ + 3CO32− = SOUO2(CO3)35− + H+
5.67 −6.51 2.69 −3.64 2.11 −3.97
AUTHOR INFORMATION
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial supports from National Natural Science Foundation of China (No. 41402030, 41172048, 41572029) and the Fundamental Research Funds for the Central Universities (JZ2017HGTB0196) are acknowledged.
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REFERENCES
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Figure 5A, Eu(III) sorption onto nanomagnetite fitted satisfactorily by double layer model with two inner-sphere surface complexation sites (SOEu2+ and (SO)2Eu(OH)2− species). The main adsorption sites of SOEu2+ and (SO)2Eu(OH)2− species were observed at pH < 4.0 and pH > 5.0, respectively. However, U(VI) sorption onto nanomagnetite fitted well by three inner-sphere surface complexation sites (SOUO2+, (SO)2UO2(OH)22− and SOUO2(CO3)35− species). As shown Figure 5B, the main adsorption sites were SOUO2+, (SO)2UO2(OH)22− and SOUO2(CO3)35− species at pH < 4.0, pH = 4.0−7.0 and pH > 7.0, respectively. The results of surface complexation modeling illustrated that Eu(III)/U(VI) adsorbed onto nanomagnetite was ascribed to inner-sphere surface complexation, whereas carbonato-uranyl complexes dominated U(VI) sorption at high pH conditions.
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CONCLUSIONS The nanomagnetite was satisfactorily synthesized by thermal treatment method. According to the characterization results, the nanomagnetite presented the more nanoporous size and more surface reactive sites. The batch sorption experiments illustrated that the inner-sphere surface complexation predominated the sorption process of Eu(III)/U(VI) sorption on nanomagnetite. In addition, sorption kinetics and isotherms of Eu(III)/U(VI) on nanomagnetite fitted satisfactorily using pseudo-second kinetic and Langmuir model, respectively. The max sorption capacity of nanomagnetite at pH 2.5 and T = 328 K was 11.94 mg/g for Eu(III) and 4.93 mg/g for U(VI). Thermodynamic investigation demonstrating an endothermic and spontaneous processes for Eu(III)/U(VI) sorption. Concluded from XPS analysis, the sorption of Eu(III) and U(VI) were ascribed to the oxygenated functional groups (i.e., Fe−OH) of nanomagnetite. Surface complexation modeling showed that Eu(III)/U(VI) sorption on nanomagnetite fitted satisfactorily by double layer model with the inner-sphere surface complexation sites. These results were crucial for the 5500
DOI: 10.1021/acssuschemeng.7b00829 ACS Sustainable Chem. Eng. 2017, 5, 5493−5502
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