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Sb2O5
Accepted Manuscript Strontium(II) adsorption on Sb(III)/Sb2O5 Lan Zhang, Jiying Wei, Xuan Zhao, Fuzhi Li, Feng Jiang, Meng Zhang PII: DOI: Reference:
S1385-8947(14)01592-7 http://dx.doi.org/10.1016/j.cej.2014.11.124 CEJ 12979
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
18 September 2014 22 November 2014 24 November 2014
Please cite this article as: L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, M. Zhang, Strontium(II) adsorption on Sb(III)/ Sb2O5, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.11.124
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Strontium(II) adsorption on Sb(III)/Sb2O5 Lan Zhang, Jiying Wei, Xuan Zhao*, Fuzhi Li, Feng Jiang, Meng Zhang
(Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 10084, China) * Corresponding author. Tel: +8610 62796428 Fax: +8610 62771150 Email: [email protected]. Item
Total number
Page
39
Table
4
Figure
11
1
Highlights A new sorbent Sb(III)/Sb2O5 for the removal of
90
Sr was prepared by sol-gel
method. Sr2+ adsorption performance onto Sb(III)/Sb2O5 was significant improved. The sorbent could be used under high salts condition and over a wide pH range of 2.2 to 12.0.
2
Abstract: The rapid development of the nuclear power plants (NPPs) in China leads to an increasing attention to the treatment of low-level radioactive wastewater (LLRW). One of the possibilities is the application of antimony pentoxide based ion exchange materials, which can exhibit effective adsorption of
90
Sr. In this paper, a
novel sorbent Sb(III)/Sb 2O5 is prepared by sol-gel method and its structure and surface properties are determined by X-Ray photoelectron spectra (XPS), X-ray diffraction (XRD), zeta potential, surface area and porosity analysis. The batch experiments demonstrate a very efficient and selective Sr(II) elimination over a wide pH range from 2.0 to 12.0. At Sb(III)/Sb (total) ratio of 0.41, the optimum adsorption can be achieved with Kd value of 8.8×108 mL/g. The co-existing calcium ions can affect the adsorption of strontium, since the Kd value drops to 10 3~102 mL/g with calcium concentration increase to 0.1 mol/L. Compared with divalent cations, the monovalent cations like Na+ and K+ have only minor influence. The strontium adsorption isotherm coincides very well with Freundlich model. The KF values are 10.7 mg/g at 283 K, 23.2 mg/g at 303 K, and 40.6 mg/g at 323 K. The thermodynamic studies reveal an endothermic and spontaneous process. The kinetic performance follows the pseudo-second-order adsorption model, with intra particle diffusion as the rate controlling step. Keywords: Low level radioactive waste water; adsorption; Strontium (II); Sb(III)/Sb2O5
3
1. Introduction The rapid development of nuclear energy in China will inevitably lead to large volumes of low level radioactive waste water (LLRW). Removal of radioactive nuclides from the LLRW would be necessary before discharge to environment. Radioactive strontium is the major contaminant to be removed in many cases, due to its high percentage in the nuclides of most LLRW, the long half-life and high solubility. There is evidence that in humans radioactive strontium uptake increases the risk of leukemia and bone neoplasms, and many other forms of cancer and autoimmune disease [1]. Because the radioactive strontium is in trace concentration and some LLRW solutions contain excessive ions such as Na+, K+ and NO3-, which are radiologically inactive, selective separation methods are necessary in many cases. Ion exchange resins can be used only for the purification of dilute solutions. In general, inorganic ion exchangers can offer selective separation of trace radioactive nuclides from high-salt solutions. In addition, the inorganic ion exchange material exhibits several superior properties, such as high thermal and radiation stability. In the past decades, various inorganic ion exchange materials have been investigated for elimination of
90
Sr, such as zeolites [2-6], minerals [7-12], hydrous
metal oxides and their mixtures [13-20]. However, the pH of LLRW solutions, varying in a wide range, is often a restrictive factor since most sorbents mentioned above exhibit poor performance in acidic solutions. It is well known that hydrous antimony pentoxide with pyrochlore structure (HAP, formula Sb2O5·nH2O) is an 4
effective ion-exchanger for Sr(II) in acidic solution with high selectivity and theoretical adsorption capacity of 5.1 meq/g [21]. In HAP structure, by corner sharing SbO6 groups, a three-dimensional Sb 2O6 network is built up and tunnels are formed in the direction of (100) and (112). Largely because of this tunnel structure, pyrochlore compounds display rapid ion transport properties and the protons located in the tunnels can be exchanged relatively easily by other cations, such as Sr(II) [22]. Replacing some Sb(V) ions (r = 0.060 nm) in pyrochlore structure by dopant ions with lower valence and ionic radius in range of 0.040 ~ 0.078 nm, can not only accurately regulate the size of tunnels to enhance the selectivity for Sr(II) ions, but also form some defects in the lattice and thus increase the quantity of exchangeable protons as charge balancing ions. MÖller and some other researchers have reported that ion-exchange properties of antimony pyrochlore based materials can be greatly enhanced by the substitution of Sb(V) with several ions, such as Si(IV) [21, 23], Ti(IV) [24], Zr(IV) [25, 26], Sn(IV) [27]. In this paper, a new type of self-doped antimonate Sb(III)/Sb2O5 is developed and its adsorption properties are evaluated using batch experiments. With respect to the preparation of antimonate material, particular attention has to be paid to the fact that chemicals like SbCl5 [21, 23] and aqua regia [28, 29] are toxic and corrosive. An alternative sol-gel method is developed by partially oxidizing trivalent antimony to pentavalent antimony in an absolute alcohol solvent. The Sb(III)/Sb 2O5 material with homogeneous solid solution structure can be achieved after the following hydrolysis 5
process. 2. Materials and methods 2.1. Chemicals and reagents All the chemicals were of analytical grade (AR), including antimony trichloride (SbCl3), strontium nitrate (Sr(NO3)2), hydrogen peroxide solution (H2O2, 30 wt%), absolute ethanol (C2H6O), ammonium hydroxide (NH4OH, 25%), nitric acid (HNO3, 65 w%), sodium hydroxide (NaOH) and ammonium nitrate (NH4NO3). 2.2. Preparation of antimony oxides Synthesis of Sb(III)/Sb2O5: 4.6 g SbCl3 was first dissolved in 20 mL absolute ethanol at 323 ~ 368 K, then 30 wt% hydrogen peroxide solution with certain volume (2 mL, 4 mL, 6 mL and 10 mL) was added to the solution under continuous stirring. After refluxing at 368 K for 3 hours, 50 mL deionized water was added to the solution and then was kept at 333 K for 10 hours. The precipitate was first washed with 1 mol/L ammonium nitrate solution to destroy the colloid structure, and then washed with deionized water several times. The final products were achieved after drying in an oven at 343 K. For comparison, antimony trioxide (Sb2O3) was synthesized in the following procedures: 3.0 g SbCl3 was dissolved in 20 mL absolute ethanol, and then 50 mL deionized water was added to the solution under stirring. The mixed solution was kept at 333 K for 10 hours. The precipitate was washed with 1 mol/L ammonium nitrate solution and deionized water, and the final products were achieved after drying in an 6
oven at 353 K. 2.3. Characterization of sorbent materials and analysis of water samples Phase characterization was carried out using X-ray diffractometer (D/max-RB, Rigaku, Japan) with Cu Ka radiation (γ=0.15406 nm at 40 kV and 45 mA). The valence of antimony in the product materials was determined by an X-Ray photoelectron spectrometer (EscaLab 250Xi, Thermo Scientific, Britain). All binding energy (BE) values were referenced to the C1s line at 284.8 eV from adventitious carbon. Software XPS peak 4.1 was used for data analysis. The binding energies of Sb3d3/2 were 539.6 eV for Sb 2O3 and 540.4 eV for Sb 2O5. Nitrogen adsorption and desorption isotherms were measured at 77 K with a surface area analyzer (NOVA4000, Quantachrome, USA). The BET method was used to determine the surface area. The pore-size distribution was determined by the Saito-Foley (SF) method and BJH method from the desorption branches of the isotherm. And the Zeta potential curves were determined by micro-electrophoresis (JS94H2, Powereach, China). The concentration of Sr(II) was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-MS) using Thermo ICP-MSXII based on the general rules for (JY/T 015-1996). 2.4. Batch experiments, raw water For the investigation of effect of Sb(III) doping on the uptake of strontium, the prepared sorbent (10 mg) was equilibrated with 40 mL Sr(NO3)2-bearing solutions. Their strontium concentrations amounted to approximately 4.0 mg/L. After vibration 7
in a thermostatic shaker for 40 hours at 303 K, the supernatant was filtrated by 0.10 µm filter membrane for analysis. For the investigation of effect of excessive inactive ions and pH on the uptake of strontium, the Sr(NO3)2-bearing solutions were additionally spiked with HNO3, NaOH, NaCl, NaNO3, KNO3, Mg(NO3)2 or Ca(NO3)2.The distribution coefficient Kd and adsorption percentage (ads, %), were calculated using the following equations: =
Ads(%) =
×
(1)
× 100
(2)
Where Ci (mg/L) and Ce (mg/L) are the initial and equilibrium concentration of strontium in aqueous solutions respectively, V (mL) is the aqueous volume and m (g) is the sorbent weight. For adsorption isotherms studies, samples of 10 mg Sb(III)/Sb2O5 (prepared with 4 mL H2O2 addition) were equilibrated with 40 mL Sr(NO3)2-bearing solutions. Their original strontium concentrations amounted to approximately 2~10 mg/L. The batch equilibrium experiments were performed at three different temperatures of 283 K, 303 K, and 323 K. After vibration in a thermostatic shaker for 40 h, the suspension was filtrated by 0.10 µm filter membrane for analysis. The sorbent loading of Qe (mg/g) was determined from the difference between initial and equilibrium Sr(II) concentrations in solution, and calculated using the following equation: =
×
(3)
The kinetic studies were conducted under static condition in a thermostat with a 8
stirrer at 300 rpm. 160 mg Sb(III)/Sb2O5 (prepared with 4 mL H2O2 addition) was covered with 1000 mL Sr(NO3)2-bearing solution with the initial strontium concentration of 4 mg/L. The samples taken equaled approximately 0.5% of the total volume of the original solution every 3~10 min. The instantaneous sorbent loading could be obtained by analysis of the taken samples. The general mathematical models, the pseudo-first-order, pseudo-second-order and intraparticle diffusion model, were used to simulate the adsorption kinetics. 3. Results and discussion 3.1 Characterizations To identify the composition of the prepared Sb(III)/Sb2O5, the chemical state of antimony was measured by XPS. As shown in Fig 1, the Sb 3d spectra exhibit asymmetrical peaks for both Sb 3d 3/2 and Sb 3d5/2. Due to the overlapping of O 1s and Sb 3d5/2 peaks around 531 eV [30, 31], only Sb 3d3/2 spectrum is deconvoluted and two antimony environments are detected: Sb(V) at 540.4 eV and Sb(III) at 539.5 eV. These assignments are in good agreement with previous work by Izquierdo and Zeng [32, 33]. The Sb 3d spectra also demonstrate that addition of H2O2 can increase the extent of Sb(III) oxidation to Sb(V). According to the XRD patterns shown in Fig. 2, all diffraction peaks of the Sb(III) doped samples are readily indexed to Sb 2O5 phase with pyrochlore structure (JPCDS No.49-0994), which are completely different with Sb 2O3 (JPCDS No. 11-0689). Despite of Sb(III)/Sb(Total) ratios varying till 0.76, all the samples show the same 9
structure as pure Sb2O5. The (111) diffraction peaks slightly shift to lower angles with elevation of Sb(III) proportion, that reveals the increase of lattice parameters according to Bragg’s law. XRD results indicate that Sb(III) ions can be well dissolved into the framework of Sb 2O5 and a solid solution with pyrochlore structure is formed. The expansion of (111) plane spacing may be attributed to the substitution of Sb(V) atoms (r = 0.060 nm) by Sb(III) with large atoms (r = 0.076 nm). The surface area and porosity of the sample (Sb(III)/Sb(Total) ratio = 0.41) were detected by nitrogen adsorption-desorption isotherm, as shown in Fig. 3. The steep increase at a relative pressure of 7.9×10-6 < P/P0 < 1.0×10 -2 and the hysteresis loop at 0.05 < P/P0 < 0.95 reveal both micro pore and meso pore structure in the antimony oxide material. The average diameters of micro and meso pore are calculated as 0.56 nm and 3.4 nm, respectively (Fig. 3 b and c). The BET surface area and pore volume are calculated as 88.2 m2/g and 0.153 cm3/g. 3.2. Strontium adsorption performance 3.2.1. Effect of Sb(III) doping Fig 4 demonstrates Kd values of the as-prepared materials with different Sb(III)/Sb(Total) ratios. Sb2O3 cannot adsorb Sr(II) effectively. Pure Sb2O5 offers a favorable uptake of Sr(II) ions with Kd value of 1.4×10 6 mL/g. An obvious increase of Kd value could be obtained by substitution of some Sb(V) for Sb(III). At Sb(III)/Sb (total) ratio of 0.41, the optimum adsorption can be achieved with Kd value of 8.8× 108 mL/g. 10
As evidenced by XRD results, the solid solution maintains a pyrochlore lattice like pure Sb 2O5 with some Sb(III) ions located in Sb(V) sites. Sb(III) substitution leads to an expansion of (111) plane spacing and thus regulates the size of (111) tunnels to accelerate Sr(II) ions transfer within the solid solution. Meanwhile, Sb(III) substitution can result in some defects of cations in the lattice and thus can increase the quantity of protons as charge balancing ions, which can be exchanged by Sr(II) ions. 3.2.2. Effect of excessive inactive ions The influence of co-existing nonradioactive ions on the adsorption of strontium was investigated. Fig.5 demonstrates that Ca2+ has an obvious influence on the amount of strontium adsorbed, since the Kd value drops from 10 7 mL/g to 103~10 2 mL/g with Ca2+ concentration increase from 0 to 0.1 mol/L. Compared with the divalent cations, the monovalent cations like Na+ and K+ have only minor influence. Fig. 5 also demonstrates the linear dependency between Kd values and concentrations of competing ions in log-log coordinate. For binary system, the slope of the linear equation is theoretically equal to – (zA/zB) when the adsorption is governed by ion exchange mechanism only (zA is the charge of ions to be adsorbed and zB is the charge of competing ions) [21]. The slopes of plots shown in Fig. 5 deviate from theoretical values, suggesting that other adsorption mechanisms might affect the strontium adsorption [23], such as the steric hindrance of pyrochlore structure. 3.2.3. Effect of pH 11
The adsorption of Sr (II) was tested at different pH values, as shown in Fig. 6. With the increase of pH value from 0.1 to 2.3, there is almost a linear dependency of strontium adsorption. Then a stable Sr(II) adsorption percentage of 99% can be observed at pH value of above 2.3. That could be explained by reaction balance equation equilibrium (4):
+ 2 + ↔ 2
(4)
In high acidic solution, Sr2+ ions will face the competition of H+ ions. With increasing pH, the uptake of H+ ions decreases, that can facilitate the uptake of Sr2+ ions. The zpc (zero point charge) value of Sb(III)/Sb 2O5 is determined at pH 2.1 as shown in Fig. 7. At pH above zpc, the sorbent surface is negatively charged, that facilitates the diffusion of Sr2+ in the electric double layer of solid surface and exchange with H+ ions. However, at pH above zpc, the positively charged surface repulses Sr2+ ions diffusion to the solid phase. The experimental results are consistent with the theoretical analysis. As a consequence, the as-prepared Sb(III)/Sb 2O5 can effectively adsorb Sr(II) ions over a wider pH range, compared with many other metal oxide sorbents [8, 13, 14, 16, 26, 34, 35], as summarized in Fig. 8. 3.3. Adsorption isotherms The relationships between the equilibrium concentration of strontium in the liquid phase and the equilibrium loading of the sorbent are shown in Fig 9. The adsorption isotherms are modeled either by Langmuir relationship or Freundlich relationship. 12
Langmuir isotherm model is expressed as equation (5): =
! "#
(5)
$"#
Where Qe is the amount of strontium ions adsorbed on the solid phase at equilibrium (mg/g), Ce is the equilibrium concentration of strontium ions in solution (mg/L), Qm is the maximum adsorption capacity of the sorbent (mg/g), KL is the constant of Langmuir isotherm (L/mg). The Freundlich isotherm is given by equation (6): '
= % × & (
(6)
Where KF is the Freundlich constant related to the adsorption capacity (mg/g), and 1/n is a constant related to adsorption intensity and surface heterogeneity. To estimate the adsorption mechanism, the experimental data were also fitted to the Dubinin-Radushkevich (D-R) isotherm equation (7): = × exp (
' 2 )) 1 32
(-./0($
)
(7)
Where KD is a constant related to the adsorption energy (mol2/kJ2), ɛ is the Polanyi potential expressed as ɛ = RTln(1+1/Ce), R is the universal gas constant (8.3145 J/mol/K), T is the absolute temperature (K), and E is the mean free energy change when 1 mole of ions were transferred to the surface from infinity. If the magnitude of E is between 8~16 kJ/mol, the adsorption process is governed by ion exchange, while for the values of E<8 kJ/mol, physical forces may affect the adsorption mechanism, and for values of E>16 kJ/mol, the adsorption may be dominated by particle diffusion [13]. 13
According to the correlation coefficients (R2) summarized in Table 1, the isotherms can be better described by the Freundlich relationship (0.90 < R2 < 0.97). In addition, the maximum adsorption capacity values (Qm) calculated by Langmuir relationship increase with the elevation of temperature, that conflicts with the model assumptions of sorbent with homogenous surface and finite adsorption capacity. As an empirical equation, Freundlich relationship is suitable for non-ideal adsorption on heterogeneous surfaces and multilayer adsorption. The values of 1/n < 1 indicate the heterogeneity of the sorbent surface and a favorable adsorption condition. The parameter KF is related to the adsorption capacity. Under our experimental condition, the KF values are 10.7 mg/g at 283 K, 23.2 mg/g at 303 K, and 40.6 mg/g at 323 K. The increase of KF value with temperature can be attributed to the fact that higher temperature can enlarge the solid surface area and provide more chance for Sr2+ to exchange with protons [36]. The calculated E values by the D-R model are employed to understand the adsorption mechanism. In this study, the calculated E value of 15.4 kJ/mol at 323 K, represents the adsorption controlled by ion exchange mechanism [2, 37]. The value of E is 18.2 kJ/mol at 303K, which is higher than 16 kJ/mol, suggesting that the adsorption might be dominated by particle diffusion [13, 15]. At 283 K, the calculated value of E of 1.58 kJ/mol, indicates a physical adsorption of Sr(II) ions due to the Van der Waals forces. However, according to Park and İnan, adsorption mechanism can not be judged only by the mean free energy in the D-R model [15, 35]. At temperature 14
of 283 K and 303 K we have observed the pH decrease after equilibrium, indicating H+-ion exchange contributed to the Sr2+ adsorption, therefore, ion exchange might also affect the adsorption mechanism Since Sr(II) adsorption capacity of Sb(III)/Sb2O5 varies with temperature and the equilibrium concentration, the comparison of adsorption capacity with other sorbents reported can only be made under a similar test condition. As shown in Table 2, the as-prepared Sb(III)/Sb 2O5 shows competitive adsorption capacity. 3.4. Thermodynamic analysis Thermodynamics were investigated for a deep insight into adsorption behaviors. Parameters, such as enthalpy change ∆Hº and entropy change ∆Sº, are calculated from the slope and intercept of the linear variation of ln Kd versus 1/T, using equation (8): ln =
∆7° -
−
∆:°
(8)
-.
Where Kd is the distribution coefficient (mL/g), T is the absolute temperature (K), and R is the gas constant (8.314 J/mol/K). ∆Sº and ∆Hº can be obtained from the intercept and slope of the linear graphs of log Kd and 1/T. The Gibbs free energy change ∆Gº is obtained from equation (9) as follows: ∆;° = ∆° − <∆°
(9)
The thermodynamic parameters for strontium adsorption are summarized in Table 3. The positive values of ∆Hº indicate the endothermic adsorption. The values of ∆Gº are negative and decrease with temperature increase, implying that the adsorption is spontaneous and more favorable at high temperature [26]. In addition, 15
the positive ∆Sº values suggest that there is an increase in the randomness at the solid-solution interface during adsorption [26, 36], due to the increasing mobility of Sr2+ ions and the surrounding water molecules caused by the dehydration of Sr2+ ions [15]. 3.5. Kinetic study of adsorption process For dilute solution, the distribution coefficient (Kd) and adsorption rate are considered to be the most important parameters rather than maximum adsorption capacity, since adsorption in dilute solution is mainly controlled by diffusion and mass transfer and far below saturated adsorption [39]. As shown in Fig. 10, the uptake of Sr(II) increases rapidly at the initial stage and then slow down gradually until the equilibrium. Compared with Sr(II) adsorption onto Sb 2O5 [40], doping Sb(III) into Sb2O5 can improve the Sr(II) adsorption rate, which is probably due to the enlarged surface area and pore size in Sb(III)/Sb2O5 (the BET areas of Sb2O5 and Sb(III)/Sb 2O5 are 56.3 m2/g and 88.2 m2/g, the SF pore diameters are 0.45 nm and 0.56 nm, respectively). In order to investigate the adsorption kinetic performance, pseudo-first-order and pseudo-second-order kinetic models are used to interpret the experimental data in Fig. 10. The linear form of pseudo-first-order equation is expressed by equation (10): log( − ? ) = log −
@'
.BCB
D
(10)
And the linearized equation of pseudo-second-order equation is given by equation 16
(11): ?
E
=
$ @2 2
+
$
D
(11)
Where t is the contact time, Qe is the amount of strontium ions adsorbed on the solid phase at equilibrium (mg/g), Qt is the amount of strontium ions adsorbed on the solid phase at contact time t (mg/g), k1 is the pseudo-first-order rate constant (min-1), and k2 is the pseudo-second-order rate constant (g mg-1min-1). The results summarized in Table 4 demonstrate that Sr(II) adsorption on the as-prepared sorbent can be described by pseudo-second-order model better than pseudo-first-order model. The model given by Boyd et al. is employed to identify the rate-controlling step [12]: E
F
$
0 = 1 − G2 ∑I 0J$ 02 K
2 L?
(12)
Where B is a time constant and n is an integer (1, 2, 3, etc.). When 0.85 < Qt/Qe < 1, equation (12) can be simplified as: E
=1−
F
G2
K L?
(13)
It can be also written as: G2
MD = −NO P F × (1 −
E
)Q
(14)
When 0 < Qt/Qe ≤ 0.85, equation (12) can be simplified as equation (15), and transferred to equation (16) to express Bt in terms of Qt/Qe: E
=
F
√GS
B
√MD − G2 MD
MD = 2π − U B E − 2U V1 −
(15) G× E B
(16) 17
The linearity of the plot of Bt versus t is applied to identify whether film diffusion or intraparticle diffusion governs the adsorption. As shown in Fig.11, The straight line passing through the origin indicates that the adsorption rate is controlled by intraparticle diffusion [14]. The result is consistent with results of D-R isotherm model analysis. 4. Conclusions The antimony pentoxide-based ion exchange material is considered as an effective sorbent to extract trace amount of
90
Sr from LLRW of nuclear plants with
high selectivity. In this paper, a kind of solid solution Sb(III)/Sb 2O5 with pyrochlore structure is prepared. The Sr(II) adsorption capacity and adsorption rate can be enhanced remarkably by substituting some Sb(V) atoms by Sb(III) atoms in the framework. The as-prepared sorbent can offer a favorable selective removal of Sr(II) both in high salts conditions and over a wide pH range of 2~12. The adsorption isotherms coincide well with Freundlich model. The adsorption process of Sr(II) ions can be simulated by pseudo-second-order adsorption model, with intra particle diffusion as the rate-limiting step. The thermodynamic study reveals that Sr(II) adsorption is a spontaneous process and can be accelerated by the temperature elevation within the range of 283 K~323 K.
Acknowledgments The project was supported by the National Energy Sci &Tech Project (Grant No. 18
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experimental design, Chem. Eng. J. 168 (2011) 1263-1271. [16] T.P. Valsala, A. Joseph, N.L. Sonar, M.S. Sonavane, J.G. Shah, K. Raj, V. Venugopal, Separation of strontium from low level radioactive waste solutions using hydrous manganese dioxide composite materials, J. Nucl. Mater. 404 (2010) 138-143. [17] R. Sureda, X. Martinez-Llado, M. Rovira, J. de Pablo, I. Casas, J. Gimenez, Sorption of strontium on uranyl peroxide: implications for a high-level nuclear waste repository, J. Hazard. Mater. 181 (2010) 881-885. [18] A. Nilchi, M.R. Hadjmohammadi, S. Rasouli Garmarodi, R. Saberi, Studies on the adsorption behavior of trace amounts of 90Sr2+, 140La3+, 60Co2+, Ni2+ and Zr4+ cations on synthesized inorganic ion exchangers, J. Hazard. Mater. 167 (2009) 531-535. [19] B. El-Gammal, S.A. Shady, Chromatographic separation of sodium, cobalt and europium on the particles of zirconium molybdate and zirconium silicate ion exchangers, Colloids Surf., A 287 (2006) 132-138. [20] G. Gurboga, H. Tel, Preparation of TiO2-SiO2 mixed gel spheres for strontium adsorption, J. Hazard. Mater. 120 (2005) 135-142. [21] T. Möller, R. Harjula, M. Pillinger, A. Dyer, J. Newton, E. Tusa, S. Amin, M. Webb, A. Araya, Uptake of 85Sr, 134Cs and 57Co by antimony silicates doped with Ti4+, Nb 5+, Mo6+ and W6+, J. Mater. Chem. 11 (2001) 1526-1532. [22] V. Luca, C.S. Griffith, M.G. Blackford, J.V. Hanna, Structural and ion exchange properties of nanocrystalline Si-doped antimony pyrochlore, J. Mater. Chem. 15 (2005) 564. [23] T. Möller, A. Clearfield, R. Harjula, Preparation of hydrous mixed metal oxides of Sb, Nb, Si, Ti and W with a pyrochlore structure and exchange of radioactive cesium and strontium ions into the materials, Microporous Mesoporous Mater. 54 (2002) 187-199. [24] T. Möller, R. Harjula, P. Kelokaski, K. Vaaramaa, P. Karhu, J. Lehto, Titanium antimonates in various Ti: Sb ratios: ion exchange properties for radionuclide ions, J. Mater. Chem. 13 (2003) 535-541. [25] S. Inan, E. Nostar, Structure and Ion Exchange Behavior of Zirconium Antimonates for Strontium, Sep. Sci. Technol. 48 (2013) 1364-1369. [26] P. Cakir, S. Inan, Y. Altas, Investigation of strontium and uranium sorption onto zirconium-antimony oxide/polyacrylonitrile (Zr-Sb oxide/PAN) composite using experimental design, J. Hazard. Mater. 271 (2014) 108-119. [27] R. Koivula, R. Harjula, J. Lehto, Structure and ion exchange properties of tin antimonates with various Sn and Sb contents, Microporous Mesoporous Mater. 55 (2002) 231-238. [28] I.M. Ali, Sorption studies of 134Cs, 60Co and 152+154Eu on phosphoric acid activated silico-antimonate crystals in high acidic media, Chem. Eng. J. 155 (2009) 580-585. [29] I.M. Ali, E.S. Zakaria, S.A. Shama, I.M. El-Naggar, Synthesis, properties and effect of ionizing radiation on sorption behavior of iron silico-antimonate, J. 20
Radioanal. Nucl. Chem. 285 (2010) 239-245. [30] J. Gurgul, M.T. Rinke, I. Schellenberg, R. Pöttgen, The antimonide oxides REZnSbO and REMnSbO (RE = Ce, Pr) – An XPS study, Solid State Sci. 17 (2013) 122-127. [31] O.E. Linarez Pérez, M.D. Sánchez, M. López Teijelo, Characterization of growth of anodic antimony oxide films by ellipsometry and XPS, J. Electroanal. Chem. 645 (2010) 143-148. [32] R. Izquierdo, E. Sacher, A. Yelon, X-ray photoelectron spectra of antimony oxides, Appl. Surf. Sci. 40 (1989) 175-177. [33] D.W. Zeng, B.L. Zhu, C.S. Xie, W.L. Song, A.H. Wang, Oxygen partial pressure effect on synthesis and characteristics of Sb2O3 nanoparticles, Materials Science and Engineering: A 366 (2004) 332-337. [34] O.I. Pendelyuk, T.V. Lisnycha, V.V. Strelko, S.A. Kirillov, Amorphous MnO2–TiO2 Composites as Sorbents for Sr2+ and UO22+, Adsorption 11 (2005) 799-804. [35] Y. Park, Y.-C. Lee, W.S. Shin, S.-J. Choi, Removal of cobalt, strontium and cesium from radioactive laundry wastewater by ammonium molybdophosphate–polyacrylonitrile (AMP–PAN), Chem. Eng. J. 162 (2010) 685-695. [36] W. Guan, J. Pan, H. Ou, X. Wang, X. Zou, W. Hu, C. Li, X. Wu, Removal of strontium(II) ions by potassium tetratitanate whisker and sodium trititanate whisker from aqueous solution: Equilibrium, kinetics and thermodynamics, Chem. Eng. J. 167 (2011) 215-222. [37] M.T. Yagub, T.K. Sen, H.M. Ang, Equilibrium, Kinetics, and Thermodynamics of Methylene Blue Adsorption by Pine Tree Leaves, Water Air Soil Pollut. 223 (2012) 5267-5282. [38] S. İnan, Y. Altaş, Adsorption of Strontium from Acidic Waste Solution by Mn–Zr Mixed Hydrous Oxide Prepared by Co-Precipitation, Sep. Sci. Technol. 45 (2010) 269-276. [39] T. Sangvanich, V. Sukwarotwat, R.J. Wiacek, R.M. Grudzien, G.E. Fryxell, R.S. Addleman, C. Timchalk, W. Yantasee, Selective capture of cesium and thallium from natural waters and simulated wastes with copper ferrocyanide functionalized mesoporous silica, J. Hazard. Mater. 182 (2010) 225-231. [40] M.V. Sivaiah, K.A. Venkatesan, R.M. Krishna, P. Sasidhar, G.S. Murthy, Ion exchange properties of strontium on in situ precipitated polyantimonic acid in amberlite XAD-7, Sep. Purif. Technol. 44 (2005) 1-9.
21
Tables: Table 1 Langmuir, Freundlich and D-R model parameters for Sr2+ adsorption onto as-prepared Sb(III) /Sb 2O5 Table 2 The Sr(II) adsorption capacity of various sorbents. Table3 Thermodynamic parameters for Sr2+ adsorption on as-prepared Sb(III)/Sb 2O5. (solution volume: 40 mL; solution pH: 4.0; contact time: 10h) Table 4 Kinetics constants for pseudo-first-order model and pseudo-second-order model. (Sorbent dose: 0.16 g; solution volume: 1000 mL; solution pH: 4.0)
22
Table 1 Adsorption isotherm
Constants
283K
303K
323K
Qm(mg/g)
15.2
23.6
41.8
KL(L/mg)
1.5
888.5
572.5
R2
0.83
0.55
0.63
KF(mg/g)
10.7
23.2
40.6
1/n
0.134
0.046
0.081
R2
0.91
0.97
0.90
Qm(mg/g)
14.6
24.0
43.1
E(kJ/mol)
1.58
17.6
14.6
R2
0.80
0.80
0.91
models
Langmuir model
Freundlich model
D-R model
23
Table 2 Sorbent Sb(III)/Sb2O5 Zr-Sb oxide Zr–Mn oxide ZrO2 ZrO2-TiO2 SiO2-TiO2 11 A˚ tobermorite PTW STW natural clinoptilolite zeolite A
adsorption capacity (mg/g)
Temp. (K)
pH
Equilibrium Sr(II) concentration (mg/L)
Ref.
25.7 22.2 30.9 7.1 16.0 12.0
303 298 303 303 303 303
4 4 4.1 9 9 10.6
~6 ~165 150 40 ~14
[25] [38] [13] [13] [20]
1.5*
293
-
-
[11]
27.5 21.0
308 308
6 6
~50 ~50
[36] [36]
9.8*
293
5
303*
298
6
* Maximum adsorption capacity
24
[4] >700
[3]
Table 3 T (K)
∆Hº (kJ/mol)
∆Sº (J/mol/K)
283 303
183.5
724.1
323
25
∆Gº (kJ/mol)
R2
-19.9
0.94
-39.4
0.94
-48.4
0.94
Table 4 model
pseudo-first-order
pseudo-second-order
parameter
value
Qe (mg/g)
11.55
k1 (min-1)
0.0086
R2
0.95
Qe (mg/g)
11.55
k2 (g mg-1 min-1)
0.031
R2
1
26
Figures: Fig. 1.Sb 3d spectra of the prepared samples: (a) Sb2O3; (b)-(e) Sb(III)/Sb2O5 prepared with H2O2 addition of 2 mL, 4 mL, 6 mL and 10 mL, respectively. Fig. 2. XRD patterns of sorbents with Sb(III)/Sb (Total) ratios of (a) 1, (b) 0.76, (c) 0.41, (d) 0.21 and (e) 0. Fig. 3. Porosity analysis of the prepared sample with Sb(III)/Sb(Total) ratio of 0.41: (a) nitrogen adsorption-desorption isotherm; (b) meso pore size distribution; (c) micro pore size distribution. Fig. 4. Effect of Sb(III) doping on Sr(II) adsorption of the as-prepared sample (initial Sr2+ concentration is 4 mg/L, pH = 4.0, at 303 K) Fig. 5. The effect of co-existing ions on strontium adsorption Fig. 6. The effect of pH on Sr(II) adsorption onto as-prepared Sb(III)/Sb 2O5 (initial Sr2+ concentration is 4 mg/L, at 303K) Fig. 7. Zeta potential data of as-prepared Sb(III)/Sb 2O5 Fig. 8. Effect of pH on Sr(II) adsorption onto sorbents: (a) Ta-doped hex-WO3 [14]; (b) Sb(III)-Sb 2O5; (c) MnO2-PMMA [16]; (d) Zr-Sb oxide/PAN [26]; (e) hydroxyapatite [8]; (f) MnO2-TiO2 [34]; (g) ZrO2-TiO2 [13] and (h) AMP-PAN [35]. Fig. 9. Adsorption isothermson as-prepared Sb(III)/Sb2O5 Fig. 10. Effect of contact time on Sr(II) adsorption capacity of as-prepared Sb(III)/Sb2O5 (initial Sr2+ concentration is 4 mg/L, at pH 4.0, at 303K) . 27
Fig. 11.Plot of Bt vs. time for Sr(II) ions adsorption on as-prepared sorbent.
28
Intensity 545
a
Sb(III)/Sb(Total)=1
b
Sb(III)/Sb(Total)=0.76
c
Sb(III)/Sb(Total)=0.41
d
Sb(III)/Sb(Total)=0.21
e
Sb(III)/Sb(Total)=0
540
535
B.E. (eV)
Fig. 1
29
530
a
Intensity (a.u.)
b c d e 10
20
30
40
2 Thea (°)
Fig. 2
30
50
60
3
Volume adsorbed (cm /g)
0 .000
0.00 2
0.004
0.006
0.008
0.0 10
a 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
c
3
dV/dD (cm /nm/g)
3
dV/dD (cm /nm/g)
b
0
20
40
60
80
100
120
140
Pore diameter (nm)
160
0.0
0.5
1.0
1.5
2.0
2.5
Pore diameter (nm)
Fig. 3
31
3.0
3.5
4.0
8
7
Log(Kd)
6
5
4
3
2 0.0
0.2
0.4
0.6
Sb(III)/Sb(Total) ratio
Fig. 4
32
0.8
1.0
Fig. 5
33
100
99%
Ads (%)
80
60
40
20
0
2
4
6
pH value
Fig. 6
34
8
10
12
40 20
pH=2.1 Zeta potential (mV)
0 1
2
3
4
-20
pH
-40 -60 -80 -100
Fig. 7
35
5
6
Qe 0
2
4
6
8
10
d
h
c
g
b
f
a
e
12
14 0
pH value
2
4
6
8
pH value
Fig. 8
36
10
12
14
50
323K 303K 283K
45 40
Qe(mg/g)
35 30 25 20 15 10 -1
0
1
2
3
4
Ce(mg/L)
Fig. 9
37
5
6
7
8
12
Qt (mg/g)
10
8
6
4
2 0
200
400
600
Time (min)
Fig. 10
38
800
1000
8 7 6
Bt
5 4 3 2 1 0 0
200
400
600
Time (min)
Fig. 11
39
800
1000
Highlights A new sorbent Sb(III)/Sb2O5 for the removal of
90
Sr was prepared by sol-gel
method. Sr2+ adsorption performance onto Sb(III)/Sb2O5 was significant improved. The sorbent could be used under high salts condition and over a wide pH range of 2.2 to 12.0.
40
S1385-8947(14)01592-7 http://dx.doi.org/10.1016/j.cej.2014.11.124 CEJ 12979
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
18 September 2014 22 November 2014 24 November 2014
Please cite this article as: L. Zhang, J. Wei, X. Zhao, F. Li, F. Jiang, M. Zhang, Strontium(II) adsorption on Sb(III)/ Sb2O5, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.11.124
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Strontium(II) adsorption on Sb(III)/Sb2O5 Lan Zhang, Jiying Wei, Xuan Zhao*, Fuzhi Li, Feng Jiang, Meng Zhang
(Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 10084, China) * Corresponding author. Tel: +8610 62796428 Fax: +8610 62771150 Email: [email protected]. Item
Total number
Page
39
Table
4
Figure
11
1
Highlights A new sorbent Sb(III)/Sb2O5 for the removal of
90
Sr was prepared by sol-gel
method. Sr2+ adsorption performance onto Sb(III)/Sb2O5 was significant improved. The sorbent could be used under high salts condition and over a wide pH range of 2.2 to 12.0.
2
Abstract: The rapid development of the nuclear power plants (NPPs) in China leads to an increasing attention to the treatment of low-level radioactive wastewater (LLRW). One of the possibilities is the application of antimony pentoxide based ion exchange materials, which can exhibit effective adsorption of
90
Sr. In this paper, a
novel sorbent Sb(III)/Sb 2O5 is prepared by sol-gel method and its structure and surface properties are determined by X-Ray photoelectron spectra (XPS), X-ray diffraction (XRD), zeta potential, surface area and porosity analysis. The batch experiments demonstrate a very efficient and selective Sr(II) elimination over a wide pH range from 2.0 to 12.0. At Sb(III)/Sb (total) ratio of 0.41, the optimum adsorption can be achieved with Kd value of 8.8×108 mL/g. The co-existing calcium ions can affect the adsorption of strontium, since the Kd value drops to 10 3~102 mL/g with calcium concentration increase to 0.1 mol/L. Compared with divalent cations, the monovalent cations like Na+ and K+ have only minor influence. The strontium adsorption isotherm coincides very well with Freundlich model. The KF values are 10.7 mg/g at 283 K, 23.2 mg/g at 303 K, and 40.6 mg/g at 323 K. The thermodynamic studies reveal an endothermic and spontaneous process. The kinetic performance follows the pseudo-second-order adsorption model, with intra particle diffusion as the rate controlling step. Keywords: Low level radioactive waste water; adsorption; Strontium (II); Sb(III)/Sb2O5
3
1. Introduction The rapid development of nuclear energy in China will inevitably lead to large volumes of low level radioactive waste water (LLRW). Removal of radioactive nuclides from the LLRW would be necessary before discharge to environment. Radioactive strontium is the major contaminant to be removed in many cases, due to its high percentage in the nuclides of most LLRW, the long half-life and high solubility. There is evidence that in humans radioactive strontium uptake increases the risk of leukemia and bone neoplasms, and many other forms of cancer and autoimmune disease [1]. Because the radioactive strontium is in trace concentration and some LLRW solutions contain excessive ions such as Na+, K+ and NO3-, which are radiologically inactive, selective separation methods are necessary in many cases. Ion exchange resins can be used only for the purification of dilute solutions. In general, inorganic ion exchangers can offer selective separation of trace radioactive nuclides from high-salt solutions. In addition, the inorganic ion exchange material exhibits several superior properties, such as high thermal and radiation stability. In the past decades, various inorganic ion exchange materials have been investigated for elimination of
90
Sr, such as zeolites [2-6], minerals [7-12], hydrous
metal oxides and their mixtures [13-20]. However, the pH of LLRW solutions, varying in a wide range, is often a restrictive factor since most sorbents mentioned above exhibit poor performance in acidic solutions. It is well known that hydrous antimony pentoxide with pyrochlore structure (HAP, formula Sb2O5·nH2O) is an 4
effective ion-exchanger for Sr(II) in acidic solution with high selectivity and theoretical adsorption capacity of 5.1 meq/g [21]. In HAP structure, by corner sharing SbO6 groups, a three-dimensional Sb 2O6 network is built up and tunnels are formed in the direction of (100) and (112). Largely because of this tunnel structure, pyrochlore compounds display rapid ion transport properties and the protons located in the tunnels can be exchanged relatively easily by other cations, such as Sr(II) [22]. Replacing some Sb(V) ions (r = 0.060 nm) in pyrochlore structure by dopant ions with lower valence and ionic radius in range of 0.040 ~ 0.078 nm, can not only accurately regulate the size of tunnels to enhance the selectivity for Sr(II) ions, but also form some defects in the lattice and thus increase the quantity of exchangeable protons as charge balancing ions. MÖller and some other researchers have reported that ion-exchange properties of antimony pyrochlore based materials can be greatly enhanced by the substitution of Sb(V) with several ions, such as Si(IV) [21, 23], Ti(IV) [24], Zr(IV) [25, 26], Sn(IV) [27]. In this paper, a new type of self-doped antimonate Sb(III)/Sb2O5 is developed and its adsorption properties are evaluated using batch experiments. With respect to the preparation of antimonate material, particular attention has to be paid to the fact that chemicals like SbCl5 [21, 23] and aqua regia [28, 29] are toxic and corrosive. An alternative sol-gel method is developed by partially oxidizing trivalent antimony to pentavalent antimony in an absolute alcohol solvent. The Sb(III)/Sb 2O5 material with homogeneous solid solution structure can be achieved after the following hydrolysis 5
process. 2. Materials and methods 2.1. Chemicals and reagents All the chemicals were of analytical grade (AR), including antimony trichloride (SbCl3), strontium nitrate (Sr(NO3)2), hydrogen peroxide solution (H2O2, 30 wt%), absolute ethanol (C2H6O), ammonium hydroxide (NH4OH, 25%), nitric acid (HNO3, 65 w%), sodium hydroxide (NaOH) and ammonium nitrate (NH4NO3). 2.2. Preparation of antimony oxides Synthesis of Sb(III)/Sb2O5: 4.6 g SbCl3 was first dissolved in 20 mL absolute ethanol at 323 ~ 368 K, then 30 wt% hydrogen peroxide solution with certain volume (2 mL, 4 mL, 6 mL and 10 mL) was added to the solution under continuous stirring. After refluxing at 368 K for 3 hours, 50 mL deionized water was added to the solution and then was kept at 333 K for 10 hours. The precipitate was first washed with 1 mol/L ammonium nitrate solution to destroy the colloid structure, and then washed with deionized water several times. The final products were achieved after drying in an oven at 343 K. For comparison, antimony trioxide (Sb2O3) was synthesized in the following procedures: 3.0 g SbCl3 was dissolved in 20 mL absolute ethanol, and then 50 mL deionized water was added to the solution under stirring. The mixed solution was kept at 333 K for 10 hours. The precipitate was washed with 1 mol/L ammonium nitrate solution and deionized water, and the final products were achieved after drying in an 6
oven at 353 K. 2.3. Characterization of sorbent materials and analysis of water samples Phase characterization was carried out using X-ray diffractometer (D/max-RB, Rigaku, Japan) with Cu Ka radiation (γ=0.15406 nm at 40 kV and 45 mA). The valence of antimony in the product materials was determined by an X-Ray photoelectron spectrometer (EscaLab 250Xi, Thermo Scientific, Britain). All binding energy (BE) values were referenced to the C1s line at 284.8 eV from adventitious carbon. Software XPS peak 4.1 was used for data analysis. The binding energies of Sb3d3/2 were 539.6 eV for Sb 2O3 and 540.4 eV for Sb 2O5. Nitrogen adsorption and desorption isotherms were measured at 77 K with a surface area analyzer (NOVA4000, Quantachrome, USA). The BET method was used to determine the surface area. The pore-size distribution was determined by the Saito-Foley (SF) method and BJH method from the desorption branches of the isotherm. And the Zeta potential curves were determined by micro-electrophoresis (JS94H2, Powereach, China). The concentration of Sr(II) was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-MS) using Thermo ICP-MSXII based on the general rules for (JY/T 015-1996). 2.4. Batch experiments, raw water For the investigation of effect of Sb(III) doping on the uptake of strontium, the prepared sorbent (10 mg) was equilibrated with 40 mL Sr(NO3)2-bearing solutions. Their strontium concentrations amounted to approximately 4.0 mg/L. After vibration 7
in a thermostatic shaker for 40 hours at 303 K, the supernatant was filtrated by 0.10 µm filter membrane for analysis. For the investigation of effect of excessive inactive ions and pH on the uptake of strontium, the Sr(NO3)2-bearing solutions were additionally spiked with HNO3, NaOH, NaCl, NaNO3, KNO3, Mg(NO3)2 or Ca(NO3)2.The distribution coefficient Kd and adsorption percentage (ads, %), were calculated using the following equations: =
Ads(%) =
×
(1)
× 100
(2)
Where Ci (mg/L) and Ce (mg/L) are the initial and equilibrium concentration of strontium in aqueous solutions respectively, V (mL) is the aqueous volume and m (g) is the sorbent weight. For adsorption isotherms studies, samples of 10 mg Sb(III)/Sb2O5 (prepared with 4 mL H2O2 addition) were equilibrated with 40 mL Sr(NO3)2-bearing solutions. Their original strontium concentrations amounted to approximately 2~10 mg/L. The batch equilibrium experiments were performed at three different temperatures of 283 K, 303 K, and 323 K. After vibration in a thermostatic shaker for 40 h, the suspension was filtrated by 0.10 µm filter membrane for analysis. The sorbent loading of Qe (mg/g) was determined from the difference between initial and equilibrium Sr(II) concentrations in solution, and calculated using the following equation: =
×
(3)
The kinetic studies were conducted under static condition in a thermostat with a 8
stirrer at 300 rpm. 160 mg Sb(III)/Sb2O5 (prepared with 4 mL H2O2 addition) was covered with 1000 mL Sr(NO3)2-bearing solution with the initial strontium concentration of 4 mg/L. The samples taken equaled approximately 0.5% of the total volume of the original solution every 3~10 min. The instantaneous sorbent loading could be obtained by analysis of the taken samples. The general mathematical models, the pseudo-first-order, pseudo-second-order and intraparticle diffusion model, were used to simulate the adsorption kinetics. 3. Results and discussion 3.1 Characterizations To identify the composition of the prepared Sb(III)/Sb2O5, the chemical state of antimony was measured by XPS. As shown in Fig 1, the Sb 3d spectra exhibit asymmetrical peaks for both Sb 3d 3/2 and Sb 3d5/2. Due to the overlapping of O 1s and Sb 3d5/2 peaks around 531 eV [30, 31], only Sb 3d3/2 spectrum is deconvoluted and two antimony environments are detected: Sb(V) at 540.4 eV and Sb(III) at 539.5 eV. These assignments are in good agreement with previous work by Izquierdo and Zeng [32, 33]. The Sb 3d spectra also demonstrate that addition of H2O2 can increase the extent of Sb(III) oxidation to Sb(V). According to the XRD patterns shown in Fig. 2, all diffraction peaks of the Sb(III) doped samples are readily indexed to Sb 2O5 phase with pyrochlore structure (JPCDS No.49-0994), which are completely different with Sb 2O3 (JPCDS No. 11-0689). Despite of Sb(III)/Sb(Total) ratios varying till 0.76, all the samples show the same 9
structure as pure Sb2O5. The (111) diffraction peaks slightly shift to lower angles with elevation of Sb(III) proportion, that reveals the increase of lattice parameters according to Bragg’s law. XRD results indicate that Sb(III) ions can be well dissolved into the framework of Sb 2O5 and a solid solution with pyrochlore structure is formed. The expansion of (111) plane spacing may be attributed to the substitution of Sb(V) atoms (r = 0.060 nm) by Sb(III) with large atoms (r = 0.076 nm). The surface area and porosity of the sample (Sb(III)/Sb(Total) ratio = 0.41) were detected by nitrogen adsorption-desorption isotherm, as shown in Fig. 3. The steep increase at a relative pressure of 7.9×10-6 < P/P0 < 1.0×10 -2 and the hysteresis loop at 0.05 < P/P0 < 0.95 reveal both micro pore and meso pore structure in the antimony oxide material. The average diameters of micro and meso pore are calculated as 0.56 nm and 3.4 nm, respectively (Fig. 3 b and c). The BET surface area and pore volume are calculated as 88.2 m2/g and 0.153 cm3/g. 3.2. Strontium adsorption performance 3.2.1. Effect of Sb(III) doping Fig 4 demonstrates Kd values of the as-prepared materials with different Sb(III)/Sb(Total) ratios. Sb2O3 cannot adsorb Sr(II) effectively. Pure Sb2O5 offers a favorable uptake of Sr(II) ions with Kd value of 1.4×10 6 mL/g. An obvious increase of Kd value could be obtained by substitution of some Sb(V) for Sb(III). At Sb(III)/Sb (total) ratio of 0.41, the optimum adsorption can be achieved with Kd value of 8.8× 108 mL/g. 10
As evidenced by XRD results, the solid solution maintains a pyrochlore lattice like pure Sb 2O5 with some Sb(III) ions located in Sb(V) sites. Sb(III) substitution leads to an expansion of (111) plane spacing and thus regulates the size of (111) tunnels to accelerate Sr(II) ions transfer within the solid solution. Meanwhile, Sb(III) substitution can result in some defects of cations in the lattice and thus can increase the quantity of protons as charge balancing ions, which can be exchanged by Sr(II) ions. 3.2.2. Effect of excessive inactive ions The influence of co-existing nonradioactive ions on the adsorption of strontium was investigated. Fig.5 demonstrates that Ca2+ has an obvious influence on the amount of strontium adsorbed, since the Kd value drops from 10 7 mL/g to 103~10 2 mL/g with Ca2+ concentration increase from 0 to 0.1 mol/L. Compared with the divalent cations, the monovalent cations like Na+ and K+ have only minor influence. Fig. 5 also demonstrates the linear dependency between Kd values and concentrations of competing ions in log-log coordinate. For binary system, the slope of the linear equation is theoretically equal to – (zA/zB) when the adsorption is governed by ion exchange mechanism only (zA is the charge of ions to be adsorbed and zB is the charge of competing ions) [21]. The slopes of plots shown in Fig. 5 deviate from theoretical values, suggesting that other adsorption mechanisms might affect the strontium adsorption [23], such as the steric hindrance of pyrochlore structure. 3.2.3. Effect of pH 11
The adsorption of Sr (II) was tested at different pH values, as shown in Fig. 6. With the increase of pH value from 0.1 to 2.3, there is almost a linear dependency of strontium adsorption. Then a stable Sr(II) adsorption percentage of 99% can be observed at pH value of above 2.3. That could be explained by reaction balance equation equilibrium (4):
+ 2 + ↔ 2
(4)
In high acidic solution, Sr2+ ions will face the competition of H+ ions. With increasing pH, the uptake of H+ ions decreases, that can facilitate the uptake of Sr2+ ions. The zpc (zero point charge) value of Sb(III)/Sb 2O5 is determined at pH 2.1 as shown in Fig. 7. At pH above zpc, the sorbent surface is negatively charged, that facilitates the diffusion of Sr2+ in the electric double layer of solid surface and exchange with H+ ions. However, at pH above zpc, the positively charged surface repulses Sr2+ ions diffusion to the solid phase. The experimental results are consistent with the theoretical analysis. As a consequence, the as-prepared Sb(III)/Sb 2O5 can effectively adsorb Sr(II) ions over a wider pH range, compared with many other metal oxide sorbents [8, 13, 14, 16, 26, 34, 35], as summarized in Fig. 8. 3.3. Adsorption isotherms The relationships between the equilibrium concentration of strontium in the liquid phase and the equilibrium loading of the sorbent are shown in Fig 9. The adsorption isotherms are modeled either by Langmuir relationship or Freundlich relationship. 12
Langmuir isotherm model is expressed as equation (5): =
! "#
(5)
$"#
Where Qe is the amount of strontium ions adsorbed on the solid phase at equilibrium (mg/g), Ce is the equilibrium concentration of strontium ions in solution (mg/L), Qm is the maximum adsorption capacity of the sorbent (mg/g), KL is the constant of Langmuir isotherm (L/mg). The Freundlich isotherm is given by equation (6): '
= % × & (
(6)
Where KF is the Freundlich constant related to the adsorption capacity (mg/g), and 1/n is a constant related to adsorption intensity and surface heterogeneity. To estimate the adsorption mechanism, the experimental data were also fitted to the Dubinin-Radushkevich (D-R) isotherm equation (7): = × exp (
' 2 )) 1 32
(-./0($
)
(7)
Where KD is a constant related to the adsorption energy (mol2/kJ2), ɛ is the Polanyi potential expressed as ɛ = RTln(1+1/Ce), R is the universal gas constant (8.3145 J/mol/K), T is the absolute temperature (K), and E is the mean free energy change when 1 mole of ions were transferred to the surface from infinity. If the magnitude of E is between 8~16 kJ/mol, the adsorption process is governed by ion exchange, while for the values of E<8 kJ/mol, physical forces may affect the adsorption mechanism, and for values of E>16 kJ/mol, the adsorption may be dominated by particle diffusion [13]. 13
According to the correlation coefficients (R2) summarized in Table 1, the isotherms can be better described by the Freundlich relationship (0.90 < R2 < 0.97). In addition, the maximum adsorption capacity values (Qm) calculated by Langmuir relationship increase with the elevation of temperature, that conflicts with the model assumptions of sorbent with homogenous surface and finite adsorption capacity. As an empirical equation, Freundlich relationship is suitable for non-ideal adsorption on heterogeneous surfaces and multilayer adsorption. The values of 1/n < 1 indicate the heterogeneity of the sorbent surface and a favorable adsorption condition. The parameter KF is related to the adsorption capacity. Under our experimental condition, the KF values are 10.7 mg/g at 283 K, 23.2 mg/g at 303 K, and 40.6 mg/g at 323 K. The increase of KF value with temperature can be attributed to the fact that higher temperature can enlarge the solid surface area and provide more chance for Sr2+ to exchange with protons [36]. The calculated E values by the D-R model are employed to understand the adsorption mechanism. In this study, the calculated E value of 15.4 kJ/mol at 323 K, represents the adsorption controlled by ion exchange mechanism [2, 37]. The value of E is 18.2 kJ/mol at 303K, which is higher than 16 kJ/mol, suggesting that the adsorption might be dominated by particle diffusion [13, 15]. At 283 K, the calculated value of E of 1.58 kJ/mol, indicates a physical adsorption of Sr(II) ions due to the Van der Waals forces. However, according to Park and İnan, adsorption mechanism can not be judged only by the mean free energy in the D-R model [15, 35]. At temperature 14
of 283 K and 303 K we have observed the pH decrease after equilibrium, indicating H+-ion exchange contributed to the Sr2+ adsorption, therefore, ion exchange might also affect the adsorption mechanism Since Sr(II) adsorption capacity of Sb(III)/Sb2O5 varies with temperature and the equilibrium concentration, the comparison of adsorption capacity with other sorbents reported can only be made under a similar test condition. As shown in Table 2, the as-prepared Sb(III)/Sb 2O5 shows competitive adsorption capacity. 3.4. Thermodynamic analysis Thermodynamics were investigated for a deep insight into adsorption behaviors. Parameters, such as enthalpy change ∆Hº and entropy change ∆Sº, are calculated from the slope and intercept of the linear variation of ln Kd versus 1/T, using equation (8): ln =
∆7° -
−
∆:°
(8)
-.
Where Kd is the distribution coefficient (mL/g), T is the absolute temperature (K), and R is the gas constant (8.314 J/mol/K). ∆Sº and ∆Hº can be obtained from the intercept and slope of the linear graphs of log Kd and 1/T. The Gibbs free energy change ∆Gº is obtained from equation (9) as follows: ∆;° = ∆° − <∆°
(9)
The thermodynamic parameters for strontium adsorption are summarized in Table 3. The positive values of ∆Hº indicate the endothermic adsorption. The values of ∆Gº are negative and decrease with temperature increase, implying that the adsorption is spontaneous and more favorable at high temperature [26]. In addition, 15
the positive ∆Sº values suggest that there is an increase in the randomness at the solid-solution interface during adsorption [26, 36], due to the increasing mobility of Sr2+ ions and the surrounding water molecules caused by the dehydration of Sr2+ ions [15]. 3.5. Kinetic study of adsorption process For dilute solution, the distribution coefficient (Kd) and adsorption rate are considered to be the most important parameters rather than maximum adsorption capacity, since adsorption in dilute solution is mainly controlled by diffusion and mass transfer and far below saturated adsorption [39]. As shown in Fig. 10, the uptake of Sr(II) increases rapidly at the initial stage and then slow down gradually until the equilibrium. Compared with Sr(II) adsorption onto Sb 2O5 [40], doping Sb(III) into Sb2O5 can improve the Sr(II) adsorption rate, which is probably due to the enlarged surface area and pore size in Sb(III)/Sb2O5 (the BET areas of Sb2O5 and Sb(III)/Sb 2O5 are 56.3 m2/g and 88.2 m2/g, the SF pore diameters are 0.45 nm and 0.56 nm, respectively). In order to investigate the adsorption kinetic performance, pseudo-first-order and pseudo-second-order kinetic models are used to interpret the experimental data in Fig. 10. The linear form of pseudo-first-order equation is expressed by equation (10): log( − ? ) = log −
@'
.BCB
D
(10)
And the linearized equation of pseudo-second-order equation is given by equation 16
(11): ?
E
=
$ @2 2
+
$
D
(11)
Where t is the contact time, Qe is the amount of strontium ions adsorbed on the solid phase at equilibrium (mg/g), Qt is the amount of strontium ions adsorbed on the solid phase at contact time t (mg/g), k1 is the pseudo-first-order rate constant (min-1), and k2 is the pseudo-second-order rate constant (g mg-1min-1). The results summarized in Table 4 demonstrate that Sr(II) adsorption on the as-prepared sorbent can be described by pseudo-second-order model better than pseudo-first-order model. The model given by Boyd et al. is employed to identify the rate-controlling step [12]: E
F
$
0 = 1 − G2 ∑I 0J$ 02 K
2 L?
(12)
Where B is a time constant and n is an integer (1, 2, 3, etc.). When 0.85 < Qt/Qe < 1, equation (12) can be simplified as: E
=1−
F
G2
K L?
(13)
It can be also written as: G2
MD = −NO P F × (1 −
E
)Q
(14)
When 0 < Qt/Qe ≤ 0.85, equation (12) can be simplified as equation (15), and transferred to equation (16) to express Bt in terms of Qt/Qe: E
=
F
√GS
B
√MD − G2 MD
MD = 2π − U B E − 2U V1 −
(15) G× E B
(16) 17
The linearity of the plot of Bt versus t is applied to identify whether film diffusion or intraparticle diffusion governs the adsorption. As shown in Fig.11, The straight line passing through the origin indicates that the adsorption rate is controlled by intraparticle diffusion [14]. The result is consistent with results of D-R isotherm model analysis. 4. Conclusions The antimony pentoxide-based ion exchange material is considered as an effective sorbent to extract trace amount of
90
Sr from LLRW of nuclear plants with
high selectivity. In this paper, a kind of solid solution Sb(III)/Sb 2O5 with pyrochlore structure is prepared. The Sr(II) adsorption capacity and adsorption rate can be enhanced remarkably by substituting some Sb(V) atoms by Sb(III) atoms in the framework. The as-prepared sorbent can offer a favorable selective removal of Sr(II) both in high salts conditions and over a wide pH range of 2~12. The adsorption isotherms coincide well with Freundlich model. The adsorption process of Sr(II) ions can be simulated by pseudo-second-order adsorption model, with intra particle diffusion as the rate-limiting step. The thermodynamic study reveals that Sr(II) adsorption is a spontaneous process and can be accelerated by the temperature elevation within the range of 283 K~323 K.
Acknowledgments The project was supported by the National Energy Sci &Tech Project (Grant No. 18
NY20120102). References [1] V. Höllriegl, H. Z. München, Strontium in the Environment and Possible Human Health Effects, in: J.O. Nriagu (Ed.), Encyclopedia of Environmental Health, Elsevier, Burlington, 2011, pp. 268-275. [2] R.O.A. Rahman, H.A. Ibrahim, M. Hanafy, N.M.A. Monem, Assessment of synthetic zeolite Na A–X as sorbing barrier for strontium in a radioactive disposal facility, Chem. Eng. J. 157 (2010) 100-112. [3] A.M. El-Kamash, Evaluation of zeolite A for the sorptive removal of Cs+ and Sr2+ ions from aqueous solutions using batch and fixed bed column operations, J. Hazard. Mater. 151 (2008) 432-445. [4] I. Smičiklas, S. Dimović, I. Plećaš, Removal of Cs1+, Sr2+ and Co2+ from aqueous solutions by adsorption on natural clinoptilolite, Appl. Clay Sci. 35 (2007) 139-144. [5] N.V. Elizondo, E. Ballesteros, B.I. Kharisov, Cleaning of liquid radioactive wastes using natural zeolites, Appl. Radiat. Isot. 52 (2000) 27-30. [6] M. Atkins, F.P. Glasser, J.J. Jack, Zeolite-P in Cements - Its Potential for Immobilizing Toxic and Radioactive-Waste Species, Waste Manage. (Oxford) 15 (1995) 127-135. [7] S.H. Tan, X.G. Chen, Y. Ye, J. Sun, L.Q. Dai, Q. Ding, Hydrothermal removal of Sr2+ in aqueous solution via formation of Sr-substituted hydroxyapatite, J. Hazard. Mater. 179 (2010) 559-563. [8] I. Smiciklas, A. Onjia, S. Raicevic, D. Janackovic, M. Mitric, Factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 152 (2008) 876-884. [9] T. Missana, M. Garcia-Gutierrez, U. Alonso, Sorption of strontium onto illite/smectite mixed clays, Phys. Chem. Earth. 33 (2008) S156-S162. [10] T. Missana, M. García-Gutiérrez, Adsorption of bivalent ions (Ca(II), Sr(II) and Co(II)) onto FEBEX bentonite, Phys. Chem. Earth. 32 (2007) 559-567. [11] N.J. Coleman, D.S. Brassington, A. Raza, A.P. Mendham, Sorption of Co2+ and Sr2+ by waste-derived 11 Å tobermorite, Waste Manage. (Oxford) 26 (2006) 260-267. [12] D.C. Liu, C.N. Hsu, C.L. Chuang, Ion-Exchange and Sorption Kinetics of Cesium and Strontium in Soils, Appl. Radiat. Isot. 46 (1995) 839-846. [13] H. Tel, Y. Altaş, M. Eral, Ş. Sert, B. Çetinkaya, S. İnan, Preparation of ZrO2 and ZrO2–TiO2 microspheres by the sol–gel method and an experimental design approach to their strontium adsorption behaviours, Chem. Eng. J. 161 (2010) 151-160. [14] X. Li, W. Mu, X. Xie, B. Liu, H. Tang, G. Zhou, H. Wei, Y. Jian, S. Luo, Strontium adsorption on tantalum-doped hexagonal tungsten oxide, J. Hazard. Mater. 264 (2014) 386-394. [15] S. İnan, Y. Altaş, Preparation of zirconium–manganese oxide/polyacrylonitrile (Zr–Mn oxide/PAN) composite spheres and the investigation of Sr(II) sorption by 19
experimental design, Chem. Eng. J. 168 (2011) 1263-1271. [16] T.P. Valsala, A. Joseph, N.L. Sonar, M.S. Sonavane, J.G. Shah, K. Raj, V. Venugopal, Separation of strontium from low level radioactive waste solutions using hydrous manganese dioxide composite materials, J. Nucl. Mater. 404 (2010) 138-143. [17] R. Sureda, X. Martinez-Llado, M. Rovira, J. de Pablo, I. Casas, J. Gimenez, Sorption of strontium on uranyl peroxide: implications for a high-level nuclear waste repository, J. Hazard. Mater. 181 (2010) 881-885. [18] A. Nilchi, M.R. Hadjmohammadi, S. Rasouli Garmarodi, R. Saberi, Studies on the adsorption behavior of trace amounts of 90Sr2+, 140La3+, 60Co2+, Ni2+ and Zr4+ cations on synthesized inorganic ion exchangers, J. Hazard. Mater. 167 (2009) 531-535. [19] B. El-Gammal, S.A. Shady, Chromatographic separation of sodium, cobalt and europium on the particles of zirconium molybdate and zirconium silicate ion exchangers, Colloids Surf., A 287 (2006) 132-138. [20] G. Gurboga, H. Tel, Preparation of TiO2-SiO2 mixed gel spheres for strontium adsorption, J. Hazard. Mater. 120 (2005) 135-142. [21] T. Möller, R. Harjula, M. Pillinger, A. Dyer, J. Newton, E. Tusa, S. Amin, M. Webb, A. Araya, Uptake of 85Sr, 134Cs and 57Co by antimony silicates doped with Ti4+, Nb 5+, Mo6+ and W6+, J. Mater. Chem. 11 (2001) 1526-1532. [22] V. Luca, C.S. Griffith, M.G. Blackford, J.V. Hanna, Structural and ion exchange properties of nanocrystalline Si-doped antimony pyrochlore, J. Mater. Chem. 15 (2005) 564. [23] T. Möller, A. Clearfield, R. Harjula, Preparation of hydrous mixed metal oxides of Sb, Nb, Si, Ti and W with a pyrochlore structure and exchange of radioactive cesium and strontium ions into the materials, Microporous Mesoporous Mater. 54 (2002) 187-199. [24] T. Möller, R. Harjula, P. Kelokaski, K. Vaaramaa, P. Karhu, J. Lehto, Titanium antimonates in various Ti: Sb ratios: ion exchange properties for radionuclide ions, J. Mater. Chem. 13 (2003) 535-541. [25] S. Inan, E. Nostar, Structure and Ion Exchange Behavior of Zirconium Antimonates for Strontium, Sep. Sci. Technol. 48 (2013) 1364-1369. [26] P. Cakir, S. Inan, Y. Altas, Investigation of strontium and uranium sorption onto zirconium-antimony oxide/polyacrylonitrile (Zr-Sb oxide/PAN) composite using experimental design, J. Hazard. Mater. 271 (2014) 108-119. [27] R. Koivula, R. Harjula, J. Lehto, Structure and ion exchange properties of tin antimonates with various Sn and Sb contents, Microporous Mesoporous Mater. 55 (2002) 231-238. [28] I.M. Ali, Sorption studies of 134Cs, 60Co and 152+154Eu on phosphoric acid activated silico-antimonate crystals in high acidic media, Chem. Eng. J. 155 (2009) 580-585. [29] I.M. Ali, E.S. Zakaria, S.A. Shama, I.M. El-Naggar, Synthesis, properties and effect of ionizing radiation on sorption behavior of iron silico-antimonate, J. 20
Radioanal. Nucl. Chem. 285 (2010) 239-245. [30] J. Gurgul, M.T. Rinke, I. Schellenberg, R. Pöttgen, The antimonide oxides REZnSbO and REMnSbO (RE = Ce, Pr) – An XPS study, Solid State Sci. 17 (2013) 122-127. [31] O.E. Linarez Pérez, M.D. Sánchez, M. López Teijelo, Characterization of growth of anodic antimony oxide films by ellipsometry and XPS, J. Electroanal. Chem. 645 (2010) 143-148. [32] R. Izquierdo, E. Sacher, A. Yelon, X-ray photoelectron spectra of antimony oxides, Appl. Surf. Sci. 40 (1989) 175-177. [33] D.W. Zeng, B.L. Zhu, C.S. Xie, W.L. Song, A.H. Wang, Oxygen partial pressure effect on synthesis and characteristics of Sb2O3 nanoparticles, Materials Science and Engineering: A 366 (2004) 332-337. [34] O.I. Pendelyuk, T.V. Lisnycha, V.V. Strelko, S.A. Kirillov, Amorphous MnO2–TiO2 Composites as Sorbents for Sr2+ and UO22+, Adsorption 11 (2005) 799-804. [35] Y. Park, Y.-C. Lee, W.S. Shin, S.-J. Choi, Removal of cobalt, strontium and cesium from radioactive laundry wastewater by ammonium molybdophosphate–polyacrylonitrile (AMP–PAN), Chem. Eng. J. 162 (2010) 685-695. [36] W. Guan, J. Pan, H. Ou, X. Wang, X. Zou, W. Hu, C. Li, X. Wu, Removal of strontium(II) ions by potassium tetratitanate whisker and sodium trititanate whisker from aqueous solution: Equilibrium, kinetics and thermodynamics, Chem. Eng. J. 167 (2011) 215-222. [37] M.T. Yagub, T.K. Sen, H.M. Ang, Equilibrium, Kinetics, and Thermodynamics of Methylene Blue Adsorption by Pine Tree Leaves, Water Air Soil Pollut. 223 (2012) 5267-5282. [38] S. İnan, Y. Altaş, Adsorption of Strontium from Acidic Waste Solution by Mn–Zr Mixed Hydrous Oxide Prepared by Co-Precipitation, Sep. Sci. Technol. 45 (2010) 269-276. [39] T. Sangvanich, V. Sukwarotwat, R.J. Wiacek, R.M. Grudzien, G.E. Fryxell, R.S. Addleman, C. Timchalk, W. Yantasee, Selective capture of cesium and thallium from natural waters and simulated wastes with copper ferrocyanide functionalized mesoporous silica, J. Hazard. Mater. 182 (2010) 225-231. [40] M.V. Sivaiah, K.A. Venkatesan, R.M. Krishna, P. Sasidhar, G.S. Murthy, Ion exchange properties of strontium on in situ precipitated polyantimonic acid in amberlite XAD-7, Sep. Purif. Technol. 44 (2005) 1-9.
21
Tables: Table 1 Langmuir, Freundlich and D-R model parameters for Sr2+ adsorption onto as-prepared Sb(III) /Sb 2O5 Table 2 The Sr(II) adsorption capacity of various sorbents. Table3 Thermodynamic parameters for Sr2+ adsorption on as-prepared Sb(III)/Sb 2O5. (solution volume: 40 mL; solution pH: 4.0; contact time: 10h) Table 4 Kinetics constants for pseudo-first-order model and pseudo-second-order model. (Sorbent dose: 0.16 g; solution volume: 1000 mL; solution pH: 4.0)
22
Table 1 Adsorption isotherm
Constants
283K
303K
323K
Qm(mg/g)
15.2
23.6
41.8
KL(L/mg)
1.5
888.5
572.5
R2
0.83
0.55
0.63
KF(mg/g)
10.7
23.2
40.6
1/n
0.134
0.046
0.081
R2
0.91
0.97
0.90
Qm(mg/g)
14.6
24.0
43.1
E(kJ/mol)
1.58
17.6
14.6
R2
0.80
0.80
0.91
models
Langmuir model
Freundlich model
D-R model
23
Table 2 Sorbent Sb(III)/Sb2O5 Zr-Sb oxide Zr–Mn oxide ZrO2 ZrO2-TiO2 SiO2-TiO2 11 A˚ tobermorite PTW STW natural clinoptilolite zeolite A
adsorption capacity (mg/g)
Temp. (K)
pH
Equilibrium Sr(II) concentration (mg/L)
Ref.
25.7 22.2 30.9 7.1 16.0 12.0
303 298 303 303 303 303
4 4 4.1 9 9 10.6
~6 ~165 150 40 ~14
[25] [38] [13] [13] [20]
1.5*
293
-
-
[11]
27.5 21.0
308 308
6 6
~50 ~50
[36] [36]
9.8*
293
5
303*
298
6
* Maximum adsorption capacity
24
[4] >700
[3]
Table 3 T (K)
∆Hº (kJ/mol)
∆Sº (J/mol/K)
283 303
183.5
724.1
323
25
∆Gº (kJ/mol)
R2
-19.9
0.94
-39.4
0.94
-48.4
0.94
Table 4 model
pseudo-first-order
pseudo-second-order
parameter
value
Qe (mg/g)
11.55
k1 (min-1)
0.0086
R2
0.95
Qe (mg/g)
11.55
k2 (g mg-1 min-1)
0.031
R2
1
26
Figures: Fig. 1.Sb 3d spectra of the prepared samples: (a) Sb2O3; (b)-(e) Sb(III)/Sb2O5 prepared with H2O2 addition of 2 mL, 4 mL, 6 mL and 10 mL, respectively. Fig. 2. XRD patterns of sorbents with Sb(III)/Sb (Total) ratios of (a) 1, (b) 0.76, (c) 0.41, (d) 0.21 and (e) 0. Fig. 3. Porosity analysis of the prepared sample with Sb(III)/Sb(Total) ratio of 0.41: (a) nitrogen adsorption-desorption isotherm; (b) meso pore size distribution; (c) micro pore size distribution. Fig. 4. Effect of Sb(III) doping on Sr(II) adsorption of the as-prepared sample (initial Sr2+ concentration is 4 mg/L, pH = 4.0, at 303 K) Fig. 5. The effect of co-existing ions on strontium adsorption Fig. 6. The effect of pH on Sr(II) adsorption onto as-prepared Sb(III)/Sb 2O5 (initial Sr2+ concentration is 4 mg/L, at 303K) Fig. 7. Zeta potential data of as-prepared Sb(III)/Sb 2O5 Fig. 8. Effect of pH on Sr(II) adsorption onto sorbents: (a) Ta-doped hex-WO3 [14]; (b) Sb(III)-Sb 2O5; (c) MnO2-PMMA [16]; (d) Zr-Sb oxide/PAN [26]; (e) hydroxyapatite [8]; (f) MnO2-TiO2 [34]; (g) ZrO2-TiO2 [13] and (h) AMP-PAN [35]. Fig. 9. Adsorption isothermson as-prepared Sb(III)/Sb2O5 Fig. 10. Effect of contact time on Sr(II) adsorption capacity of as-prepared Sb(III)/Sb2O5 (initial Sr2+ concentration is 4 mg/L, at pH 4.0, at 303K) . 27
Fig. 11.Plot of Bt vs. time for Sr(II) ions adsorption on as-prepared sorbent.
28
Intensity 545
a
Sb(III)/Sb(Total)=1
b
Sb(III)/Sb(Total)=0.76
c
Sb(III)/Sb(Total)=0.41
d
Sb(III)/Sb(Total)=0.21
e
Sb(III)/Sb(Total)=0
540
535
B.E. (eV)
Fig. 1
29
530
a
Intensity (a.u.)
b c d e 10
20
30
40
2 Thea (°)
Fig. 2
30
50
60
3
Volume adsorbed (cm /g)
0 .000
0.00 2
0.004
0.006
0.008
0.0 10
a 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
c
3
dV/dD (cm /nm/g)
3
dV/dD (cm /nm/g)
b
0
20
40
60
80
100
120
140
Pore diameter (nm)
160
0.0
0.5
1.0
1.5
2.0
2.5
Pore diameter (nm)
Fig. 3
31
3.0
3.5
4.0
8
7
Log(Kd)
6
5
4
3
2 0.0
0.2
0.4
0.6
Sb(III)/Sb(Total) ratio
Fig. 4
32
0.8
1.0
Fig. 5
33
100
99%
Ads (%)
80
60
40
20
0
2
4
6
pH value
Fig. 6
34
8
10
12
40 20
pH=2.1 Zeta potential (mV)
0 1
2
3
4
-20
pH
-40 -60 -80 -100
Fig. 7
35
5
6
Qe 0
2
4
6
8
10
d
h
c
g
b
f
a
e
12
14 0
pH value
2
4
6
8
pH value
Fig. 8
36
10
12
14
50
323K 303K 283K
45 40
Qe(mg/g)
35 30 25 20 15 10 -1
0
1
2
3
4
Ce(mg/L)
Fig. 9
37
5
6
7
8
12
Qt (mg/g)
10
8
6
4
2 0
200
400
600
Time (min)
Fig. 10
38
800
1000
8 7 6
Bt
5 4 3 2 1 0 0
200
400
600
Time (min)
Fig. 11
39
800
1000
Highlights A new sorbent Sb(III)/Sb2O5 for the removal of
90
Sr was prepared by sol-gel
method. Sr2+ adsorption performance onto Sb(III)/Sb2O5 was significant improved. The sorbent could be used under high salts condition and over a wide pH range of 2.2 to 12.0.
40