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Cs-leaching behavior of Cs-titanosilicate in NaCl solution
Materials Letters 65 (2011) 314–316
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Cs-leaching behavior of Cs-titanosilicate in NaCl solution Ikuo Yanase ⁎, Taiki Takahashi, Motoyoshi Tomizawa, Hidehiko Kobayashi Saitama University, Faculty of Engineering, Department of Applied Chemistry, 255 Shimoohkubo, Sakura-ku, Saitama-shi, Saitama, 338-8570 Japan
a r t i c l e
i n f o
Article history: Received 25 March 2010 Accepted 3 October 2010 Available online 8 October 2010 Keywords: Titanosilicate Cesium ion Leaching behavior Zeta potential
a b s t r a c t Cs-titanosilicate, which is a pollucite-related phase, was synthesized and the Cs-leaching behavior of Cs-titanosilicate was evaluated in a NaCl aqueous solution and ion-exchanged water. CsNO3, TiO2 and SiO2 powders were mixed in ethanol by ball-milling and then the mixed powder was heated at temperatures of 600 to 900 °C under atmospheric and reduced pressures of air. Under reduced pressure, the Cs-titanosilicate phase was crystallized at 700 °C, which was lower than that under atmospheric pressure because CsNO3 decomposition was promoted under reduced pressure. The Cs-leaching ratio of the Cs-titanosilicate in a NaCl aqueous solution is higher than that in ion-exchanged water. On the other hand, the Cs-leaching ratio of the Cs-titanosilicate synthesized under the reduced pressure was lower than that under atmospheric pressure. It was considered that the lower negative zeta potential of the Cs-titanosilicate synthesized under the reduced pressure diminished the amount of Na+ ion adsorbed on the particles surface of the Cs-titanosilicate, which resulted in the suppression of Cs-leaching. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cs-titanosilicate, CsTiSi2O6.5, has the crystal structure composed of 48 corner-sharing (Ti,Si)O4 tetrahedra with 16 Cs+ ions occupying its cavities in the unit cell. The space group of CsTiSi2O6.5 belongs to Ia-3d with cubic symmetry, which is the same space group as pollucite, CsAlSi2O6, the crystal structure of which has 48 corner-sharing (Al,Si)O4 tetrahedra. A feature of CsTiSi2O6.5, different from pollucite is that eight extra oxide ions are present to compensate the charge valance for 16 Cs+ ions in the unit cell [1–3]. Two kinds of extra oxide ions coordinate with Cs+ in the cage and Ti4+ of TiO4 to form TiO5 [4–6]. Titanosilicates [7–10], minerals [11,12], and other ceramics [13,14] available to incorporate Cs+ ions, have been expected as a candidate for trapping Cs+ ions in radioactive nuclear wastes. Some studies on Cs immobilization and exchangeability of CsTiSi2O6.5 have been reported [15–17]. However, reports on the Cs-leaching behavior of CsTiSi2O6.5 in aqueous solutions with alkaline ions have been rare. Considering that CsTiSi2O6.5 must be exposed in radioactive waste solutions with Na+ ions in Cs-trapping [11], it is important to investigate the Cs-leaching behavior of CsTiSi2O6.5. Furthermore, the extra oxide ions of CsTiSi2O6.5 seem to play an important role on the Cs-leaching behavior, because they are thought to influence the interaction between the framework structure of (Ti,Si)O4 tetrahedra and cations, owing to the negative charge of oxide ions. However, there has been no report on the effect of the extra oxide ions on the Cs-leaching behavior of CsTiSi2O6.5.
⁎ Corresponding author. Tel./fax: + 81 048 858 3720. E-mail address: [email protected] (I. Yanase). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.010
In this study, two kinds of Cs-titanosilicates with a pollucite-related phase have been synthesized under atmospheric and reduced pressure of air. Cs-leaching behaviors of the synthesized Cs-titanosilicates have been investigated in a NaCl aqueous solution and ion-exchanged water. Furthermore, the relationship between the zeta potential and the Cs-leaching ratio of the synthesized Cs-titanosilicates has been investigated. 2. Experimental CsNO3 (99%, High Purity), anatase-type TiO2 (Ultrafine, Idemitsu), and amorphous SiO2 powder prepared from SiO2 sol (SnowtexO, Nissan Chemical) were used as the starting raw materials to synthesize Cs-titanosilicate, CsTiSi2O6.5. These powders were mixed for 48 h in ethanol by ball-milling with Al2O3 balls. The mixed powders were heated at temperatures of 600 to 900 °C for 5 h with a heating rate of 20 °C/min under atmospheric pressure of air with a flow rate of 200 ml/min to induce the extra oxide ions in the crystal structure. Similarly, the mixed powders were heated under a reduced pressure of 103 Pa of air to remove the extra oxide ions from the crystal structure. Crystalline phases of the heated powder were examined by X-ray diffraction (XRD; Rad-C, CuKα, 40 kV, 30 mA, Rigaku) analysis. The lattice constant of the synthesized single-phase Cs-titanosilicate compound was refined by the least-squares method using the diffraction peaks for (332), (440), (532), (631) and (732) planes. Si powder was used as an external standard substance. The synthesized Cs-titanosilicate was stirred at 20 °C for 1, 2, 3, 5, 10, 20, and 24 h in 0.001 M and 0.01 M NaCl aqueous solutions and ion-exchanged water to investigate the Cs-leaching behavior of the synthesized Cs-titanosilicate. The filtrated solution after stirring was analyzed by atomic absorption
(633)
(631) (444)
(431)
(521) (440)
(332)
(321)
(420)
45
(b) (a) 20
25
30
35
50
2θ / ° (CuK α) Fig. 2. XRD patterns of powders heated at 873 K (a), 973 K (b), 1073 K (c), and 1173 K (d) under reduced pressure of air. Circles: CTS; squares: CsNO3.
CTS-v. It was therefore considered that aluminium atoms contamination resulted in the smaller lattice constants of our samples in comparison with Balmer's samples [4]. Cs-leaching behaviors of CTS-a and CTS-v were investigated in a NaCl aqueous solution and ion-exchanged water. The results are shown in Fig. 3. CTS-a had the Cs-leaching ratio of 2 to 3% in ion-exchanged water over the leaching time range of 5 to 24 h. The ratio of CTS-a was larger than that reported [15]. This seems to be due to the Cs-excessive composition of the Cs-titanosilicates of CTS-a and CTS-v in comparison with CsTiSi2O6.5, as described above. In the case of the 0.001 M NaCl solution, the Cs-leaching ratio of CTS-a was higher than that in ion-exchanged water, increased with increasing leaching time up to 10 h, and became constant beyond 15 h, suggesting that Cs-leaching behavior almost reached its equilibrium. Furthermore, in the case of the 0.01 M NaCl solution, the Cs-leaching ratio became higher than that in
(633)
(631) (444)
(532)
(521) (440)
(431)
14 12
Cs leached / %
Intensity / a.u.
(d)
40
(c)
16
(420) (332)
(321)
(400)
The XRD patterns of the heated powders are shown in Fig. 1. The samples heated at temperatures of 600 and 700 °C had an amorphous phase indicating that CsNO3 was almost completely decomposed at temperatures ranging from 600 to 700 °C. Diffraction peaks were recognized for the powders heated at 800 °C, which indicates that the Cs-titanosilicate phase (hereafter, CTS-a) was crystallized. Finally, a single phase of well-crystallized CTS-a could be obtained after heating at 900 °C for 5 h, and was indexed as a pollucite-related phase referring to the JCPDS card of 29-0407. Fig. 2 shows the XRD patterns of the powders obtained after heating at temperatures of 600 to 900 °C for 5 h under the reduced pressure of 103 Pa of air. Cs-titanosilicate was crystallized when the powders were heated at 700 °C. The crystallization temperature was lower than that in the case of atmospheric pressure. It was considered that the lower crystallization temperature was caused by the promotion of the thermal decomposition of CsNO3 under the reduced pressure. The amount of amorphous phase decreased with increasing temperature, and the Cs-titanosilicate single phase without an amorphous phase was obtained at 800 and 900 °C (hereafter CTS-v). The average of the chemical compositions in molar ratio for CTS-a and CTS-v were Cs: Ti: Si = 1.13: 1.05: 1.95 and Cs: Ti: Si = 1.14: 1.09: 1.91, respectively, estimated from EDX results, and show that there is no difference in chemical composition between CTS-a and CTS-v. On the other hand, the lattice constants of CTS-a and CTS-v were 1.3681(1) nm and 1.3674(1) nm, respectively. It seems that the lower lattice constant of CTS-v compared with CTS-a affected the decrease in the number of extra oxide ions in the unit cell of CTS-v. In addition, similar amount of Al; Al/Cs = 0.01 in molar ratio, was detected for both samples of CTS-a and
(532)
3. Results and discussion
(d) Intensity / a.u.
spectroscopy (AA-670, Shimadzu) to investigate the Cs-leaching ratio of Cs-titanosilicate. The Cs-leaching ratio was calculated as the ratio of Cs for the chemical composition of CsTiSi2O6.5. The zeta potential of the Cs-titanosilicate in ion-exchanged water was investigated using a zeta potential analyzer (ZEECOM ZC2000, Microtec). Here, average for three samples was used to determine zeta potentials. The chemical composition of the Cs-titanosilicate was examined by energy dispersive X-ray (EDX; Quantax400-125S, Bruker) analysis. Here, average for three samples was used to determine chemical compositions.
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I. Yanase et al. / Materials Letters 65 (2011) 314–316
(c)
10 8 6 4
(b) 2
(a) 20
0
25
30
35
2θ / ° (CuK α)
40
45
50
Fig. 1. XRD patterns of powders heated at 873 K (a), 973 K (b), 1073 K (c), and 1173 K (d) under atmospheric pressure of air. Circles: CTS; triangles: TiO2; squares: CsNO3.
0
5
10
15
20
25
30
Time / hrs Fig. 3. Cs-leaching behaviors in NaCl aqueous solution and ion-exchanged water for CTS synthesized at 1173 K under atmospheric pressure of air (white circles: 0.01 M NaCl; triangles: 0.001 M; squares: ion-exchanged water) and CTS synthesized under reduced pressure of air (black circles: 0.01 M NaCl).
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I. Yanase et al. / Materials Letters 65 (2011) 314–316
Table 1 Zeta potential and pH in ion-exchanged water for CTS-a and CTS-v powders. Sample
Zeta potential (mV)
pH
CTS-a CTS-v
− 103 − 82
6.47 6.12
the case of the 0.001 M NaCl solution, whereas the Cs-leaching ratio of CTS-v in the 0.01 M NaCl aqueous solution was obviously lower than that of CTS-a in the 0.01 M and 0.001 M NaCl solutions. The zeta potentials of the synthesized CTS-a and CTS-v particles and the pH in ion-exchanged water were investigated. Here, the zeta potentials and pH were measured after the CTS particles were dispersed for 1 h in ion-exchanged water. The results are shown in Table 1. The zeta potential and pH for CTS-v were lower than those for CTS-a. It was considered that the adsorption of H+ ions on CTS-v was suppressed owing to the lower zeta potential than that on CTS-a, which resulted in the lowering of the pH of CTS-v in ion-exchanged water [18]. As the negative charge of the particle surface attracts positively charged ions such as Na+ ions in solution, it was assumed that the adsorption of Na+ ions onto the surface of CTS-v was suppressed in comparison with that of CTS-a. As a result, the Cs-leaching ratio of CTS-v became lower than that of CTS-a. This is because the adsorption of cations onto particles in solution is one of the important factors in exchanging cations in host compounds [19]. Such a low zeta potential of CTS-v seems to be due to the smaller number of extra oxide ions in the unit cell, which is a result of it being synthesized under a reduced pressure of air. 4. Conclusions Mixed raw powders of CsNO3, TiO2, and SiO2 were heated at temperatures of 600 to 900 °C for 5 h under both conditions of atmospheric and reduced pressure of air. The crystallization temperature of Cs-titanosilicate decreased when the mixed powder was heated under the reduced pressure. The Cs-leaching ratio of the Cs-titanosili-
cate synthesized under reduced pressure in NaCl aqueous solution was clearly lower than that under atmospheric pressure. The zeta potential of the Cs-titanosilicate synthesized under the reduced pressure was smaller than that under atmospheric pressure. It was considered that the negative zeta potential of the Cs-titanosilicate was one of the important factors that influenced the Cs-leaching ratio in the NaCl aqueous solution. Acknowledgement This study was financially supported by the Young Research Project of Saitama University in Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
McCready DE, Balmer ML, Keefer KD. Powder Diffr 1997;12:40–6. Xu H, Navrotsky A, Balmer ML, Su Y. J Am Ceram Soc 2002;85:1235–42. Xu H, Navrotsky A, Balmer ML, Su Y, Bitten ER. J Am Ceram Soc 2001;84:555–60. Balmer ML, Huang Q, Wong-Ng W, Roth RS, Santoro A. J Solid State Chem 1997;130: 97–120. Hess NJ, Su Y, Balmer ML. J Phys Chem B 2001;105:97–102. Hess NJ, Balmer ML, Bunker BC. J Solid State Chem 1997;129:206–13. Moller T, Harjula R, Letho J. Sep Purif Technol 2002;28:13–23. Nyman M, Gu BX, Wang LM, Ewing RC, Nenoff TM. Micropor Mesopor Mater 2000;40: 115–25. Tripathi A, Medvedev DG, Delgado J, Clearfield A. J Solid State Chem 2004;117: 2903–15. Cherry BR, Nyman M, Alam TM. J Solid State Chem 2004;117:2079–93. Bosch P, Caputo D, Liguori B, Colella C. J Nucl Mater 2004;324:183–8. Cho Y, Komarneni S. Appl Clay Sci 2009;43:401–7. Moller T, Clearfield A, Harjula R. Micropor Mesopor Mater 2002;54:187–99. Drabarek E, McLeod TI, Hanna JV, Griffith CS, Luca V. J Nucl Mater 2009;384: 119–29. Balmer ML, Su Y, Grey IE, Santro A, Roth R, Huang Q, et al. Mat Res Soc Symp Proc 1997;465:449–55. Su Y, Balmer ML, Bunker BC. Mat Res Soc Symp Proc 1996;465:457–64. Su Y, Balmer ML, Wang L, Bunker BC, Nyman M, Nenoff T, et al. Mat Res Soc Symp Proc 1999;556:77–84. Ersoy B, Mehmet SC. Micropor Mesopor Mater 2002;55:305–12. Su LLF, Zhao XS. Micropor Mesopor Mater 2007;101:355–62.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Cs-leaching behavior of Cs-titanosilicate in NaCl solution Ikuo Yanase ⁎, Taiki Takahashi, Motoyoshi Tomizawa, Hidehiko Kobayashi Saitama University, Faculty of Engineering, Department of Applied Chemistry, 255 Shimoohkubo, Sakura-ku, Saitama-shi, Saitama, 338-8570 Japan
a r t i c l e
i n f o
Article history: Received 25 March 2010 Accepted 3 October 2010 Available online 8 October 2010 Keywords: Titanosilicate Cesium ion Leaching behavior Zeta potential
a b s t r a c t Cs-titanosilicate, which is a pollucite-related phase, was synthesized and the Cs-leaching behavior of Cs-titanosilicate was evaluated in a NaCl aqueous solution and ion-exchanged water. CsNO3, TiO2 and SiO2 powders were mixed in ethanol by ball-milling and then the mixed powder was heated at temperatures of 600 to 900 °C under atmospheric and reduced pressures of air. Under reduced pressure, the Cs-titanosilicate phase was crystallized at 700 °C, which was lower than that under atmospheric pressure because CsNO3 decomposition was promoted under reduced pressure. The Cs-leaching ratio of the Cs-titanosilicate in a NaCl aqueous solution is higher than that in ion-exchanged water. On the other hand, the Cs-leaching ratio of the Cs-titanosilicate synthesized under the reduced pressure was lower than that under atmospheric pressure. It was considered that the lower negative zeta potential of the Cs-titanosilicate synthesized under the reduced pressure diminished the amount of Na+ ion adsorbed on the particles surface of the Cs-titanosilicate, which resulted in the suppression of Cs-leaching. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cs-titanosilicate, CsTiSi2O6.5, has the crystal structure composed of 48 corner-sharing (Ti,Si)O4 tetrahedra with 16 Cs+ ions occupying its cavities in the unit cell. The space group of CsTiSi2O6.5 belongs to Ia-3d with cubic symmetry, which is the same space group as pollucite, CsAlSi2O6, the crystal structure of which has 48 corner-sharing (Al,Si)O4 tetrahedra. A feature of CsTiSi2O6.5, different from pollucite is that eight extra oxide ions are present to compensate the charge valance for 16 Cs+ ions in the unit cell [1–3]. Two kinds of extra oxide ions coordinate with Cs+ in the cage and Ti4+ of TiO4 to form TiO5 [4–6]. Titanosilicates [7–10], minerals [11,12], and other ceramics [13,14] available to incorporate Cs+ ions, have been expected as a candidate for trapping Cs+ ions in radioactive nuclear wastes. Some studies on Cs immobilization and exchangeability of CsTiSi2O6.5 have been reported [15–17]. However, reports on the Cs-leaching behavior of CsTiSi2O6.5 in aqueous solutions with alkaline ions have been rare. Considering that CsTiSi2O6.5 must be exposed in radioactive waste solutions with Na+ ions in Cs-trapping [11], it is important to investigate the Cs-leaching behavior of CsTiSi2O6.5. Furthermore, the extra oxide ions of CsTiSi2O6.5 seem to play an important role on the Cs-leaching behavior, because they are thought to influence the interaction between the framework structure of (Ti,Si)O4 tetrahedra and cations, owing to the negative charge of oxide ions. However, there has been no report on the effect of the extra oxide ions on the Cs-leaching behavior of CsTiSi2O6.5.
⁎ Corresponding author. Tel./fax: + 81 048 858 3720. E-mail address: [email protected] (I. Yanase). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.010
In this study, two kinds of Cs-titanosilicates with a pollucite-related phase have been synthesized under atmospheric and reduced pressure of air. Cs-leaching behaviors of the synthesized Cs-titanosilicates have been investigated in a NaCl aqueous solution and ion-exchanged water. Furthermore, the relationship between the zeta potential and the Cs-leaching ratio of the synthesized Cs-titanosilicates has been investigated. 2. Experimental CsNO3 (99%, High Purity), anatase-type TiO2 (Ultrafine, Idemitsu), and amorphous SiO2 powder prepared from SiO2 sol (SnowtexO, Nissan Chemical) were used as the starting raw materials to synthesize Cs-titanosilicate, CsTiSi2O6.5. These powders were mixed for 48 h in ethanol by ball-milling with Al2O3 balls. The mixed powders were heated at temperatures of 600 to 900 °C for 5 h with a heating rate of 20 °C/min under atmospheric pressure of air with a flow rate of 200 ml/min to induce the extra oxide ions in the crystal structure. Similarly, the mixed powders were heated under a reduced pressure of 103 Pa of air to remove the extra oxide ions from the crystal structure. Crystalline phases of the heated powder were examined by X-ray diffraction (XRD; Rad-C, CuKα, 40 kV, 30 mA, Rigaku) analysis. The lattice constant of the synthesized single-phase Cs-titanosilicate compound was refined by the least-squares method using the diffraction peaks for (332), (440), (532), (631) and (732) planes. Si powder was used as an external standard substance. The synthesized Cs-titanosilicate was stirred at 20 °C for 1, 2, 3, 5, 10, 20, and 24 h in 0.001 M and 0.01 M NaCl aqueous solutions and ion-exchanged water to investigate the Cs-leaching behavior of the synthesized Cs-titanosilicate. The filtrated solution after stirring was analyzed by atomic absorption
(633)
(631) (444)
(431)
(521) (440)
(332)
(321)
(420)
45
(b) (a) 20
25
30
35
50
2θ / ° (CuK α) Fig. 2. XRD patterns of powders heated at 873 K (a), 973 K (b), 1073 K (c), and 1173 K (d) under reduced pressure of air. Circles: CTS; squares: CsNO3.
CTS-v. It was therefore considered that aluminium atoms contamination resulted in the smaller lattice constants of our samples in comparison with Balmer's samples [4]. Cs-leaching behaviors of CTS-a and CTS-v were investigated in a NaCl aqueous solution and ion-exchanged water. The results are shown in Fig. 3. CTS-a had the Cs-leaching ratio of 2 to 3% in ion-exchanged water over the leaching time range of 5 to 24 h. The ratio of CTS-a was larger than that reported [15]. This seems to be due to the Cs-excessive composition of the Cs-titanosilicates of CTS-a and CTS-v in comparison with CsTiSi2O6.5, as described above. In the case of the 0.001 M NaCl solution, the Cs-leaching ratio of CTS-a was higher than that in ion-exchanged water, increased with increasing leaching time up to 10 h, and became constant beyond 15 h, suggesting that Cs-leaching behavior almost reached its equilibrium. Furthermore, in the case of the 0.01 M NaCl solution, the Cs-leaching ratio became higher than that in
(633)
(631) (444)
(532)
(521) (440)
(431)
14 12
Cs leached / %
Intensity / a.u.
(d)
40
(c)
16
(420) (332)
(321)
(400)
The XRD patterns of the heated powders are shown in Fig. 1. The samples heated at temperatures of 600 and 700 °C had an amorphous phase indicating that CsNO3 was almost completely decomposed at temperatures ranging from 600 to 700 °C. Diffraction peaks were recognized for the powders heated at 800 °C, which indicates that the Cs-titanosilicate phase (hereafter, CTS-a) was crystallized. Finally, a single phase of well-crystallized CTS-a could be obtained after heating at 900 °C for 5 h, and was indexed as a pollucite-related phase referring to the JCPDS card of 29-0407. Fig. 2 shows the XRD patterns of the powders obtained after heating at temperatures of 600 to 900 °C for 5 h under the reduced pressure of 103 Pa of air. Cs-titanosilicate was crystallized when the powders were heated at 700 °C. The crystallization temperature was lower than that in the case of atmospheric pressure. It was considered that the lower crystallization temperature was caused by the promotion of the thermal decomposition of CsNO3 under the reduced pressure. The amount of amorphous phase decreased with increasing temperature, and the Cs-titanosilicate single phase without an amorphous phase was obtained at 800 and 900 °C (hereafter CTS-v). The average of the chemical compositions in molar ratio for CTS-a and CTS-v were Cs: Ti: Si = 1.13: 1.05: 1.95 and Cs: Ti: Si = 1.14: 1.09: 1.91, respectively, estimated from EDX results, and show that there is no difference in chemical composition between CTS-a and CTS-v. On the other hand, the lattice constants of CTS-a and CTS-v were 1.3681(1) nm and 1.3674(1) nm, respectively. It seems that the lower lattice constant of CTS-v compared with CTS-a affected the decrease in the number of extra oxide ions in the unit cell of CTS-v. In addition, similar amount of Al; Al/Cs = 0.01 in molar ratio, was detected for both samples of CTS-a and
(532)
3. Results and discussion
(d) Intensity / a.u.
spectroscopy (AA-670, Shimadzu) to investigate the Cs-leaching ratio of Cs-titanosilicate. The Cs-leaching ratio was calculated as the ratio of Cs for the chemical composition of CsTiSi2O6.5. The zeta potential of the Cs-titanosilicate in ion-exchanged water was investigated using a zeta potential analyzer (ZEECOM ZC2000, Microtec). Here, average for three samples was used to determine zeta potentials. The chemical composition of the Cs-titanosilicate was examined by energy dispersive X-ray (EDX; Quantax400-125S, Bruker) analysis. Here, average for three samples was used to determine chemical compositions.
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(400)
I. Yanase et al. / Materials Letters 65 (2011) 314–316
(c)
10 8 6 4
(b) 2
(a) 20
0
25
30
35
2θ / ° (CuK α)
40
45
50
Fig. 1. XRD patterns of powders heated at 873 K (a), 973 K (b), 1073 K (c), and 1173 K (d) under atmospheric pressure of air. Circles: CTS; triangles: TiO2; squares: CsNO3.
0
5
10
15
20
25
30
Time / hrs Fig. 3. Cs-leaching behaviors in NaCl aqueous solution and ion-exchanged water for CTS synthesized at 1173 K under atmospheric pressure of air (white circles: 0.01 M NaCl; triangles: 0.001 M; squares: ion-exchanged water) and CTS synthesized under reduced pressure of air (black circles: 0.01 M NaCl).
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I. Yanase et al. / Materials Letters 65 (2011) 314–316
Table 1 Zeta potential and pH in ion-exchanged water for CTS-a and CTS-v powders. Sample
Zeta potential (mV)
pH
CTS-a CTS-v
− 103 − 82
6.47 6.12
the case of the 0.001 M NaCl solution, whereas the Cs-leaching ratio of CTS-v in the 0.01 M NaCl aqueous solution was obviously lower than that of CTS-a in the 0.01 M and 0.001 M NaCl solutions. The zeta potentials of the synthesized CTS-a and CTS-v particles and the pH in ion-exchanged water were investigated. Here, the zeta potentials and pH were measured after the CTS particles were dispersed for 1 h in ion-exchanged water. The results are shown in Table 1. The zeta potential and pH for CTS-v were lower than those for CTS-a. It was considered that the adsorption of H+ ions on CTS-v was suppressed owing to the lower zeta potential than that on CTS-a, which resulted in the lowering of the pH of CTS-v in ion-exchanged water [18]. As the negative charge of the particle surface attracts positively charged ions such as Na+ ions in solution, it was assumed that the adsorption of Na+ ions onto the surface of CTS-v was suppressed in comparison with that of CTS-a. As a result, the Cs-leaching ratio of CTS-v became lower than that of CTS-a. This is because the adsorption of cations onto particles in solution is one of the important factors in exchanging cations in host compounds [19]. Such a low zeta potential of CTS-v seems to be due to the smaller number of extra oxide ions in the unit cell, which is a result of it being synthesized under a reduced pressure of air. 4. Conclusions Mixed raw powders of CsNO3, TiO2, and SiO2 were heated at temperatures of 600 to 900 °C for 5 h under both conditions of atmospheric and reduced pressure of air. The crystallization temperature of Cs-titanosilicate decreased when the mixed powder was heated under the reduced pressure. The Cs-leaching ratio of the Cs-titanosili-
cate synthesized under reduced pressure in NaCl aqueous solution was clearly lower than that under atmospheric pressure. The zeta potential of the Cs-titanosilicate synthesized under the reduced pressure was smaller than that under atmospheric pressure. It was considered that the negative zeta potential of the Cs-titanosilicate was one of the important factors that influenced the Cs-leaching ratio in the NaCl aqueous solution. Acknowledgement This study was financially supported by the Young Research Project of Saitama University in Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
McCready DE, Balmer ML, Keefer KD. Powder Diffr 1997;12:40–6. Xu H, Navrotsky A, Balmer ML, Su Y. J Am Ceram Soc 2002;85:1235–42. Xu H, Navrotsky A, Balmer ML, Su Y, Bitten ER. J Am Ceram Soc 2001;84:555–60. Balmer ML, Huang Q, Wong-Ng W, Roth RS, Santoro A. J Solid State Chem 1997;130: 97–120. Hess NJ, Su Y, Balmer ML. J Phys Chem B 2001;105:97–102. Hess NJ, Balmer ML, Bunker BC. J Solid State Chem 1997;129:206–13. Moller T, Harjula R, Letho J. Sep Purif Technol 2002;28:13–23. Nyman M, Gu BX, Wang LM, Ewing RC, Nenoff TM. Micropor Mesopor Mater 2000;40: 115–25. Tripathi A, Medvedev DG, Delgado J, Clearfield A. J Solid State Chem 2004;117: 2903–15. Cherry BR, Nyman M, Alam TM. J Solid State Chem 2004;117:2079–93. Bosch P, Caputo D, Liguori B, Colella C. J Nucl Mater 2004;324:183–8. Cho Y, Komarneni S. Appl Clay Sci 2009;43:401–7. Moller T, Clearfield A, Harjula R. Micropor Mesopor Mater 2002;54:187–99. Drabarek E, McLeod TI, Hanna JV, Griffith CS, Luca V. J Nucl Mater 2009;384: 119–29. Balmer ML, Su Y, Grey IE, Santro A, Roth R, Huang Q, et al. Mat Res Soc Symp Proc 1997;465:449–55. Su Y, Balmer ML, Bunker BC. Mat Res Soc Symp Proc 1996;465:457–64. Su Y, Balmer ML, Wang L, Bunker BC, Nyman M, Nenoff T, et al. Mat Res Soc Symp Proc 1999;556:77–84. Ersoy B, Mehmet SC. Micropor Mesopor Mater 2002;55:305–12. Su LLF, Zhao XS. Micropor Mesopor Mater 2007;101:355–62.