Reactivity of Cesium and Fly Ash under Hydrothermal Conditions with NaOH Solution

Transcription

1 Trans. Mat. Res. Soc. Japan 41[1] (216) Reactivity of Cesium and Fly Ash under Hydrothermal Conditions with NaOH Solution Toshinari Kori 1, Seiichi Sato 1, Kenji Nagata 2* and Takehiko Yokomine 3 1 Tokushima Prefectural Industrial Technology Center, 11-2 Nishibari, Saiga-cho, Tokushima , Japan 2 Department of Materials Science & Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya , Japan 3 Department of Nuclear Engineering, Graduate School of Engineering, Kyoto University, Kyoto daigakukatsura, Nishikyo-ku, Kyoto , Japan * Corresponding author: Fax: , nagata.kenji@nitech.ac.jp The reactivity of Cs and fly ash and the immobilization of Cs in the reaction products were investigated under hydrothermal conditions with NaOH solution. Crystalline phases of reaction products varied with hydrothermal conditions such as reaction temperature, concentration of NaOH solution and reaction time. The amount of immobilized Cs increased with the crystallization of pollucite, but its value decreased with the formation of other phases as Cs.84Na 2.16Al 3Si 3O 12.32H 2O, hydroxysodalite and hydroxycancrinite. A part of Cs was exchanged to Na on Na-P1 zeolite, which was formed in a dilute NaOH solution at low temperature for a long reaction time. The synthesized pollucite was the solid solution between pollucite and analcime, and the atomic ratio of Cs/(Cs+Na) in obtained products was larger than the initial ratio of Cs and NaOH solution in the system. This result suggests that Cs is more easily immobilized in the structure of synthesized pollucite than Na under hydrothermal treatments. Keywords: cesium, hydrothermal, pollucite, Cs.84Na2.16Al3Si3O12.32H2O, Na-P1 zeolite 1. INTRODUCTION A large quantity of radioactive cesium( 137 Cs) and strontium( 9 Sr) were scattered in the ocean [1, 2] and on land [1] by the Fukushima Dai-ichi Nuclear Power Plant accident. Cs that dropped on the ground was ionized by rainwater, and then Cs ion was strongly adsorbed on clay minerals in the soil. However, Cs ion fixed on clay minerals is exchanged by highly concentrated cations such as K + and NH4 + present in fertilizer and the manure of cattle [3], and dissolved Cs ion is absorbed into the plants and the trees. Because radioactive pollution that spread over a wide area has an influence over a long period, the isolation of contaminated substances from the environment and final disposal in stable form are required. As a part of decontamination activities, contaminated soil was stripping off from farmland, and fallen leaves and lower twigs of the forest were collected even now. These wastes are expected to be stored at the Temporary Storage Site around the Fukushima Dai-ichi Nuclear Power Plant, since the final disposal method is not decided at the current stage. On the other hand, many studies on the stabilization processing of high-level radioactive wastes were carried out in the 197s through the 198s. In these studies, various Cs immobilization materials such as borosilicate glass, Cs-doped BaAl2Ti6O16 [4], pollucite (CsAlSi2O6) [5-7], CsAlSiO4 [5, 7], CsAlSi5O12 [8, 9] and CsZr2(PO4)3 [1] have been proposed because Cs content in high-level radioactive wastes is high, and the solubility of Cs in water is great. These compounds are normally calcined at high temperature above 1,1 K, and accordingly, the volatile elements such as Cs in waste vaporize into the atmosphere. The synthesis of cesium aluminosilicate such as pollucite and CsAlSiO4 from various natural minerals under hydrothermal condition was reported, but the detailed examination about the fixed ratio of Cs was not performed [5, 11-14]. Taylor [15] and Komarneni [8] reported that pollucite is more stable than CsAlSiO4 and CsAlSi5O12 in aqueous solution and various brines at elevated temperature. Hydrothermal treatment has been industrially used in the synthesis of zeolite, which is crystalline aluminosilicate with group I or II elements as counterions, and the single crystal growth of quartz at low temperature less than 6K. Especially, it is well known for the zeolite synthesis that the crystalline phases of obtained products under hydrothermal conditions vary with reaction temperature and time, atomic ratio of the Si/Al of starting materials and the concentration of alkaline solution [16-2]. In addition, the crystallinity of the raw material has an influence on the reactivity because the solubility of the amorphous materials in alkaline solution condition is greater than that of the crystalline materials. Recently, fly ash by-product from power plants has become available for use as a raw material of zeolite synthesis because its chemical composition is similar to volcanic material and natural zeolite and its mineralogical composition is composed of large amounts of amorphous (glass) phase [21]. In Japan, the operation of all nuclear power plants was suspended by the Fukushima nuclear accident. Consequently, 29

2 3 Reactivity of Cesium and Fly Ash under Hydrothermal Conditions with NaOH Solution Table I Chemical composition of fly ash (mass %) Na2O Al2O3 SiO2 CaO TiO2 Fe2O3 Other Oxide Ig. Loss because the operation of many power plants was resumed from the viewpoint of stabilized power supply, the quantity of fly ash discharged from there increased. Since the amount of contaminated soil which is stored in the Temporary Storage Site is enormous, the immobilization process of Cs should be performed as much as possible in short time and low temperature by a simple method. In this study, we investigated the reactivity of Cs and fly ash and the immobilization of Cs into reaction products under hydrothermal conditions using NaOH solution as alkaline media in detail. 2. EXPERIMENTAL Fly ash of less than 2 µm in diameter obtained from Tachibana Bay Power Plant (Tokushima, Japan) was used as a starting material. The chemical composition and mineralogical analysis of the fly ash are listed in Tables I and II, respectively. Table II Mineralogical analysis of fly ash (mass %) Mullite (Al6Si2O13) 2±2 -Quartz (SiO2) 6±2 Magnetite (Fe3O4) 3.4 Non-crystalline aluminosilicate 69±4 Unburnt Carbon 2 The fly ash and CsNO3 (Kanto Chemical Co., Inc., 4N) were put into a 7 dm 3 autoclave with NaOH solution. The autoclave was heated to a specified temperature at a rate of 3 K/min, and the contents in the autoclave were stirred at the same temperature during the reaction period. After hydrothermal treatment, the products in the autoclave were separated into solid and liquid phases by filtration. The crystalline structure of the solid phase was identified by X-ray powder diffraction method (Rigaku Co., RINT-Ultima III) and its chemical composition was analyzed with an X-ray fluorescence analyzer (Rigaku Co., ZSX- Primus II). The morphology of reaction products was observed by scanning electron microscopy (Hitachi Ltd., S-43). The Cs fixed ratio in the reaction products was calculated from the difference between quantity of added Cs and Cs ion content measured by an atomic absorption spectrophotometer (Hitachi Ltd., Z-5) in the liquid phase. 3. RESULTS AND DISCUSSION 3.1 Reactivity of Cs and fly ash To investigate the reactivity of Cs (CsNO3: 1.5 mmol) and fly ash (1 g), hydrothermal treatments were carried out as functions of concentration of NaOH solution (.5-4 M: 3 dm 3 ) and reaction temperature (423 K, 473 K and 523 K) for.5 h. Crystalline phases of the reaction products identified by XRD varied with NaOH concentration and reaction temperature (Table III). No Cs compounds were formed at 423 K, but pollucite (CsAlSi2O6) was identified with 1 M and.2 M NaOH solution at 473 K and 523 K, respectively. Regardless of reaction temperature, hydroxylsodalite (Na1.8Al2Si1.6O H2O) and hydroxycancrinite (Na14Al12Si13O51 6H2O) were crystallized with NaOH solution of more than 2 M. F u r t h e r, Cs.84Na2.16Al3Si3O12.32H2O [22], which is partially Cs exchanged nepheline hydrate (Na3Al3Si3O12.5H2O), was synthesized at 473 K and 523 K. These results indicate that both Cs and Na ion in the system react with fly ash under hydrothermal conditions. The crystalline phases of the obtained products were a similar trend as that reported by Querol [23] except that analcime replaced to pollucite. When CsNO3 was not added to raw materials, nepheline hydrate was not formed under the same conditions. On the other hand, in the case of using CsOH instead of NaOH, CsAlSiO4 was crystallized. Table III Crystalline phases of reaction products Treatment conditions Temp. (K) NaOH (M) Crystalline phases Q, M Q, M Q, M, S S, C Q, M P N, S C, N Q, M Q, M Q, M, P P P N, S C, N Q: -Quartz, M: Mullite, S: Hydroxysodalite, C: Hydroxycancrinite, P: Pollucite, N: Cs.84Na2.16Al3Si3O12.32H2O Cs fixed ratio (%) K 473K 523K Conc. of NaOH (M) Fig. 1 Cs fixed ratio into reactance.

3 Toshinari Kori et al. Trans. Mat. Res. Soc. Japan 41[1] (216) 31 These results suggest that pollucite transforms to nepheline hydrate zeolite rather than CsAlSiO4 because there is more Na ion than Cs in the system, and Cs.84Na2.16Al3Si3O12.32H2O was formed by ion exchange of Cs and Na. The fixed ratio of Cs in reaction products was slight at 423 K (Fig. 1). Its ratio rapidly increased with the increasing concentration of NaOH solution and reaction temperature until pollucite was identified by XRD. Conversely, the highly concentrated NaOH solution brought a decline of the fixed ratio with the formation of Cs.84Na2.16Al3Si3O12.32H2O, hydroxylsodalite and hydroxycancrinite. Figure 2 shows the SEM photographs of the fly ash and reaction products at 523 K. The fly ash (Fig. 2(a)) consisted of spherical particles interspersed with many small fractions on the surface. Non-crystalline aluminosilicate glass was dissolved in alkaline solution from the surface of the fly ash, and a small amount of pollucite crystals, which could not be identified by XRD, formed on the surface (Fig. 2(b)). With the increase of concentration of NaOH solution, fine pollucite crystals covered the surface (Fig. 2(c)), and the crystal growth of pollucite and the transformation to a thin lath-like crystal of Cs.84Na2.16Al3Si3O12.32H2O were observed (Fig. 2(d)). Pollucite was the superior material for immobilization of Cs because it changed to other crystals by highly concentrated NaOH and elevated temperature treatments under the hydrothermal condition. Table IV Crystalline phases of reaction products Treatment conditions Temp. (K) NaOH (M) Time (h) Crystalline phases Q, M Q, M Q, M Q, M, NaP P, NaP1, Q, M P, NaP1, Q, M P, NaP1, Q, M Q, M Q, M Q, M, NaP P P Q, M Q, M P, Q, M P P P P Q: -Quartz (SiO2), M: Mullite (Al6Si2O13), NaP1: NaP1 zeolite (Na6Al6Si1O32 12H2O), P: Pollucite (CsAlSi2O6) hours, and each of XRD intensities of pollucite and NaP1 zeolite did not change after 4 hours with.5 M NaOH solution at 423 K. The fixed ratio of Cs in reaction products increased with the formation of pollucite, and its value was nearly constant at over 9%. When NaP1 zeolite coexisted with pollucite in reaction products, its ratio increased approximately 5% compared with the case in which only pollucite was synthesized. Cs ion remaining in the solution without immobilized as pollucite crystal was taken into NaP1 zeolite, because NaP1 zeolite has a high cation-exchange capacity and exhibits a high selectivity for Cs ion [25-27]. 1 Fig. 2 SEM photographs of reaction products at 523 K. NaOH concentrations were (a) fly ash, (b).1, (c).2 and (d) 1 M. 3.2 Effect of reaction time The effect of reaction time on synthesis of pollucite was examined with dilute NaOH solutions (.5 M and 1 M) at low temperature (423 K and 473 K). The crystalline phases of the obtained products are summarized in Table IV, and the fixed ratio of Cs is shown in Figure 3. Pollucite and NaP1 zeolite (Na6Al6Si1O32 12H2O), which is characterized by two channels of.31 x.44nm 2 and.28 x 4.9nm 2 [24], were formed by the reaction of Cs and fly ash. The crystallization of pollucite progressed in both elevated temperature and highly concentrated NaOH treatments, and pollucite was crystallized in a shorter time at elevated temperature. On the other hand, the synthesis of NaP1 zeolite required more than 3 Fix ratio of Cs (%) :423K,.5M :423K, 1.M :473K, 1.M Time (h) Fig. 3 Effects of reaction time for fixed ratio of Cs. 3.3 Abundance ratio of Cs and Na in synthesized pollucite Pollucite and analcime (NaAlSi2O6 H2O), which

4 32 Reactivity of Cesium and Fly Ash under Hydrothermal Conditions with NaOH Solution belong to the same zeolite family, form a solid solution [28], and the general chemical formula of natural pollucite was expressed as (CsxNay)[Alx+ySi48-xyO96] (16-x)H2O by Beger [29], in which 2y 16-x y. Therefore, to investigate the abundance ratio of Cs and Na in synthesized pollucite, hydrothermal treatments were performed by changing the additional amount of Cs (-4.5 mmol) with 1 g of fly ash and 1 dm 3 of.5 M NaOH solution at 523 K for.5 h. XRD patterns of obtained products and pollucite, which was synthesized using CsOH instead of NaOH under the same condition, are shown in Figure 4. In the case of absence of Cs and using CsOH solution, each XRD pattern was identified with analcime and pollucite single-phases. With increasing Cs, the peak intensities at 2θ=16 (hkl=211), 2θ=18 (hkl=22) and 2θ=33 (hkl=431) for analcime decreased; those at 2θ=24 (hkl=321) and 2θ=37 (hkl=44) for pollucite increased adversely. The fixed ratio of Cs and the atomic ratio of Cs/(Cs + Na) in reaction products are shown in Figures 5 and 6, respectively. When the amount of Cs addition was less than 2.25 mmol, more than 98% of additional Cs was immobilized in the reaction compounds; however, its Fig. 4 XRD patterns of hydrothermal reaction products. Fixed ratio of Cs (%) Cs (mmol) Fig. 5 Fixed amount of Cs in reactance. Cs/(Cs+Na) Cs (mmol) Fig. 6 Atomic ratio of Cs/(Cs + Na) of reaction products. value gradually decreased with the increase of the additional quantity of Cs. On the other hand, the atomic ratio of Cs/(Cs + Na) in the product drastically increased, and the ratio was larger than the initial ratio of these reactants (Fig. 6, dashed line) in the reaction system. It was reported by Yanagisawa [12] that there appears to be a significant preference for Cs in pollucite which was synthesized with CsNO3 in NaOH solution, but the mechanism is not clear. The single-phase pollucite and analcime described above were treated in 1 dm 3 of.5 M NaOH and CsOH under the same hydrothermal condition, respectively. Atomic ratio of Cs/(Cs+Na) in the product obtained by NaOH treatment of pollucite was not changed. However, its ratio provided by CsOH treatment of analcime considwrably was increased to.84. This indicates that transformation from analcime to pollucite is easier than reverse conversion. In other word, silicate and aluminate ions that dissolved from fly ash in alkaline solution more easily react with Cs ion than Na ion under hydrothermal condition. CONCLUSION For the investigation of stabilization process of contaminated soil generated by the Fukushima nuclear accident, hydrothermal treatment of Cs and fly is performed. Hydrothermal conditions such as reaction temperature, concentration of NaOH solution and reaction time had a very important influence on the crystallization of reaction products and the immobilization of Cs. Pollucite and Cs.84Na2.16Al3Si3O12.32H2O were formed as Cs compounds, and a part of Cs exchanged to Na on NaP1 zeolite which crystalized at low temperature. The amount of immobilized Cs increased with the formation of pollucite. The synthesized pollucite was the solid solution between pollucite and analcime, and Cs was more easily immobilized in the crystal structure than Na. If aluminosilicate in the contaminated soil is dissolved by hydrothermal treatment using alkali solution such as NaOH, most of the radioactive Cs will be immobilized in pollucite structure.

5 Toshinari Kori et al. Trans. Mat. Res. Soc. Japan 41[1] (216) 33 REFERENCES [1] S. Hirao, H. Yamazawa and T. Nagae, J. Nucl. Sci. Technol., 5, (213). [2] T. Kobayashi, H. Nagai, M. Chino and H. Kawamura, J. Nucl. Sci. Technol., 5, (213). [3] B. L. Sawhney, Clay Clay Miner., 2, 93-1(1972). [4] A. E. Ringwood, S. E. Kesson, N. G. Ware, W. O. Hibberson and A. Major, Nature, 278, (1979). [5] S. Komarneni, Clays Clay Miner., 33, (1985). [6] S. Komarneni and R. Roy, Nucl. Chem. Waste Management, 2, (1981). [7] H. Mimura and T. Kanno, J. Nucl. Sci. Technol., 22, (1985). [8] S. Komarneni and R. Roy, J. Am. Ceram. Soc., 66, (1983). [9] T. Adl and E. R. Vance, J. Mater. Sci., 17, (1982). [1] R. Roy, E. R. Vance and J. Alamo, Mater. Res. Bull., 17, (1982). [11] S. Komarneni and W. B. White, Clays Clay Miner., 29, (1981). [12] K. Yanagisawa, M. Nishioka and N. Yamasaki, J. Nucl. Sci. Technol., 24, 51-6(1987). [13] H. Mimura, M. Shibata and K. Akiba, J. Nucl. Sci. Technol., 27, (199). [14] Y. Yokomori, et al., Sci. Rep., 4, 4195(214). [15] P. Taylor, S. D. DeVaal and D. G. Owen, Can. J. Chem., 67, 76-81(1989). [16] R. M. Barrer and E. A. D. White, J. Chem. Soc., 1952, (1952). [17] R. M. Barrer and J. W. Baynham, J. Chem. Soc., 1956, (1956). [18] R. M. Barrer, J. W. Baynham, F. W. Bultitude and W. M. Meier, J. Chem. Soc., 1959, (1959). [19] R. M. Barrer and N. McCallum, J. Chem. Soc., 1953, (1953). [2] R. M. Barrer, J. Chem. Soc., 195, (195). [21] X. Querol, N. Moreno, J. C. Umaña, A. Alastuey, E. Hernádez, A. López-Soler and F. Plana, Int. J. Coal Geol., 5, (22). [22] S. Hansen and L. Fälth., Z. Kristallogr., 164, (1983). [23] X. Querol, F. Plana, A. Alastuey and A. López-Soler, Fuel, 76, (1997). [24] D. W. Breck, Zeolite molecular sieves: structure, chemistry, and use, John Wiley and Sons, New York, [25] H. Mimura and K. Akiba, J. Nucl. Sci. Technol., 3, (1993). [26] W. Ma, P. W. Brown and S. Komarneni, J. Mater. Res., 13, 3-7(1998). [27] R. P. Penilla, A. G. Bustos and S. G. Elizalde, Fuel, 85, (26). [28] D. Coombs, et al., Can. Mineral., 35, (1997). [29] R. M. Beger, Z. Kristallogr., 129, 28-32(1969). (Received April 6, 215; Accepted July 7, 215)