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Formation of Analcime Film under HydrothermalConditionsHitoshi MIMURA a , Tatsuyuki TEZUKA a & Kenichi AKIBA aa Institute for Advanced Materials Processing, Tohoku University , Katahira, Aoba-ku,Sendai , 980-77Published online: 15 Mar 2012.

To cite this article: Hitoshi MIMURA , Tatsuyuki TEZUKA & Kenichi AKIBA (1995) Formation of AnalcimeFilm under Hydrothermal Conditions, Journal of Nuclear Science and Technology, 32:12, 1250-1258, DOI:10.1080/18811248.1995.9731848

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Jouriial of N~CLE.&R SCIENCE and TECHNOLOGY, 32 (121 , pp.1250-1258 (December 1995).

Formation of Analcime Film under Hydrothermal Conditions

Hitoshi MIMURA, Tatsuyuki TEZUKA+ and Kenichi AKIBA

Institute for Advanced Muterials Processing, Tohoku University

(Received June 30, 1995)

Polycrystalline analcime film was prepared from hydrogels under hydrothermal conditions. Tlie arialciine film of about 200 pni t.hickness built up on a porous alumina support at temper- atures above 160°C. and the size and crystallinity of analcime increased with temperature. The thickness of analciine film increased with incubation time, and the growth rate at 180'C was faster than that at 160°C. The analcime film obtained was thermally stable at temperatures up to 700"C, and no structural changes were observed after treatment in nitric acid solutions below 0.01 h i . Analciiiie had an ion-sieve ability to large sized Cs' ion, and Ag+ and Rb+ ions of small sizes were incorporated in analcime.

KEY WORDS: analcime film, porous support, hydrogel, hydrothermal condition, ion-sieve ability, temperature dependence, time dependence, grain size, crystallinity, rubidium ion, cesium ion, silver ion

I. INTRODUCTION Much attention has been denoted to

the sfparation of radionuclides in high-level liquid wastes (HLLWs) by the use of inorganic adsorbents")'''. Zeolites (e.g. analcime), a kind of inorganic ion exchangers with high resistance to radiation and with high ther- mal stability. have been widely used as useful adsorhents of radionuclides in HLLWs owing to their specific ion-exchange proper tie^(^)(^). The structures of zeolites are characterized by rigid three-dimensional frameworks con- taining narrow channels, and they have ion- sieving properties for larger sized hydrated cations: analcime with a small pore size of about 0.32nm wm found to be a suitable adsorbent for the separation of Na+, K+ and Rb+ ions from Cs+ ion'?). Most of the zeolites, however, were used in powdered or granulated forms for the separation of radionuclides in HLLWs.

Membrane separation technology has become of interest, and several types of in- organic membrane have been developed(')('). As for zeolite membrane, ZSM-5 and silicalite films have been prepared from hydrogels

under hydrothermal condition^(^)(^), and the separation of alcohol/water mixtures by the zeolite films has been attempted by Sano et ul.('O) These silica-rich zeolites were, however, ineffective for the separation of radioactive nuclides, because of their poor ion-exchange ability'"). The preparation of zeolite film with low Si/A1 ratio and high ion- exchange selectivity are desired for the sepa- ration of radionuclides; the film consisted of analcime seems to be an effective adsorbent for the separation of alkali metal ions from Cs+ ion.

The present study deals with the prepa- ration of analcime films on a porous support under hydrothermal conditions.

II. EXPERIMENTAL 1. Starting Materials and the

Preparation of Analcime Film Hydrothermal synthesis of analcime film

was performed as follows. Colloidal silica (Cataloid S-2OL; 20.0 wt% SiO,, 80.0 wt% water) and powdered natural clinoptilolite

* Kutuhiru, Aoba-ku, Sendui 980- 77. 'Present address: Tohokii Electric Power Co., Inc., Ichibun-cho, Aobu-ku, Sendui 980.

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Vol. 32, No. 12 (Dec. 1995) 1251

(200 mesh under, produced at Futatsui, Akita Pref., Japan; 67.97 wt% SiO,, 10.10 wt% A1,0,, 0.85 wt% Na,O) were added to a stirred mixture of sodium hydroxide and tetrapropylammonium bromide (TPABr) to give a hydrogel with a composition

.(Hz0)3,375(12). The hydrogel was transferred to a 30cm3 stainless-steel autoclave with an inner vessel of Teflon. A porous support of alumina plate (corundum type, 5 mmx5 mmx2 mm; 76.0 wt% Al,O,, 23.0 wt% sio,) was placed in the bottom of Teflon vessel as shown in Fig.1. The autoclave was then heated in an oven over the temperature range from 150'C to 180°C for 3 h to 3d . After appropriate incubation periods, the autoclave was cooled and the support was then taken out. The zeolite film formed on the support was washed with deionized water and airdried at 80°C for 1 d.

The zeolite film obtained was identified by powder X-ray diffractometer (XRD, Rigaku RAD-B) and infrared spectrometer (IR, Hitachi 260-50). The density of zeolite film was measured by Archimedes' method (Roller- Smith Coop., Bermann Density Balance). The surface and the cross section of the film was characterized by scanning electron mi- croscope (SEM) and electron probe micro analyzer (EPMA, Hitachi X-650s).

2. Adsorption Experiments The zeolite film formed on the support

plate was contacted with a 50cm3 solution of either 0.1 M (=mol/dm3) CsNO,, 0.1M RbNO, or 0.1 M AgNO, at 25°C for 3 d. The adsorption of these cations on the film was examined by EPMA. In the case of Ag+, the

of (Na,0)6,.5.(A1,03),.,.(Si02)~7.3.(TPABr)1.~

Fig.1 Autoclave for preparation of analcime film under hydrothermal conditions

adsorption percentage was examined by mea- suring the concentration decrease of Ag+ in the aqueous phase by an ion electrode method (Ion meter, TOA Electronics Ltd., IM-40s).

Ill. RESULTS AND DISCUSSION 1. Effect of Treatment Temperature

on the Growth of Analcime Film Crystallization of zeolite depends on pro-

cessing temperature in a closed hydrothermal system. The effect of temperature (up to 180°C) on the growth of analcime film was examined by XRD, SEM and EPMA.

(1) Identification of Crystallized Phase Figures 2(a) and (b) show the XRD

patterns of the film samples obtained by in- cubating at 150°C and 160°C for 1 d . For the sample obtained at 150"C, both phases of analcime (NaA1Si,06~H,0)('3) and alumina (A1,0,) were detected, indicating that the sufrace was incompletely covered with anal-

0 Alumina ( A l 2 0 ~ )

0 Analcime

(NaAl S i20, - H20) k

0.9 i 2 0.45

n

u)

0 0 1

0 5 10 20 30 40 50 60

2e(C~-&,dWl

(b) 160°C

Fig.2 XRD paterns of zeolite film formed on alumina support after crystallization at different temperatures for 1 d

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1252 J. Nucl. Sci. Technol.,

cime film. On the other hand, for the sample obtained above 160°C, only analcime phase with higher crystallinity was detected on the surface of the support. The analcime forma- tion was also confirmed by IR spectrometry. The density of analcime formed at 170°C was determined to be 2.21 cm3/g, which is close to a theoretical value of 2.25 cm3/g(13).

(2) Surface Morphology of Analcime Film Photographs l(a)-(c) show the surface

morphology of the samples formed at different temperatures up to 180°C for 1 d . Spherical particles of analcime were partly formed on the support at 150°C, and a great number of trapezohedron-type crystals of analciine built up over the support at temperatures above 160°C. Analcime crystal is a 30-hedron with the faces (100) and {211}(14) (Photo. l(c)). The size of analcime crystals obtained was al- most constant at 16O-17O0C, while the size became large at higher temperature of 180 “C .

(3) Depth Profiles of Materials in

Cross-sections of the film samples were submitted to SEM and EPMA in order to evaluate the film thickness and the degree of alteration of the support.

Photographs 2(a) and (b) show the SEM images of the cross sections of the samples. An example of line analysis for Si (Si-Ka) is also superposed on the SEM micrograph,

Analcime Film

where a horizontal line marks the trail of analyzing electron beam. The explanatory inscriptions on upper margin indicate the po- sitions of analcime film formed on the altered support. The film thickness was evaluated to be about 240 pm formed at 160°C and about 200pm formed at 180°C by line analysis of Si. As for the degree of packing of anal- cime crystals, the film formed at 180°C com- prised more densely packed crystals compared to that formed at 160°C.

The surface layer of the treated sample was roughly divided into four parts with depth as illustrated in Photo. 2(b); the porous support (“D”) overlaid with an altered layer ( “C”), inner altered particles (“B”) and an outermost analcime film (“A”). Figure 3 illustrates EDS (Energy Dispersive Spec- troscopy) spectra for the positions shown in Photo.2(b). The ratio of Na:Al:Si at the position “A” was determined from quantita- tive EDS analysis using ZAF correction to be 1.0:1.0:2.3, which is close to that of analcime (NaAlSi,O,). On the other hand, the ratios at the positions “B” and “C” were different from that at position “D” in the support; the content of Si at the position “B” was higher than that at the position “D”, while the Si content was considerably lowered at the posi- tion “C” . The formation of altered layer (“C” ) was probably due to the selective dissolution

(a) 150°C (b) 160’C (c ) 180’C

Photo. 1 Surface morphology of film formed at different temperatures for 1 d

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Vol. 32, No. 12 (Dec. 1995) 1253

Photo. 2 Depth profile of film formed at different temperatures for 1 d

A Si

c A ‘

D -4 Na

( 0 KEV XES 10.24KEV)

graph. The EDS spectra at three positions, “B1” , “B2” and “B3”, are also illustrated. The outer part of the particle (position “Bl”) was identified by quantitative EDS analysis to be an analcime phase. On the other hand, in the inner part of the boundary, a large num- ber of fine particles of alumina (“B2”) were formed and dispersed in the analcime phase (“B3”); a part of the porous support incor- porating Na+ was converted into analcime, resulting that fine particles of aluminum were left as a residue.

On the basis of these analytical data, a schematic diagram for the formation of polycrystalline analcime film is illustrated in Fig.4. Table 1 summarizes the growth of the analcime film and alteration of the sup- port at different temperatures. The analcime film built up on the surface of the support by the hydrothermal reaction of mixed hydro- gels, and in the inner part of the boundary,

Fig.3 EDS spectra at different parts in surface layer of sample treated for 1 d at 180°C

of Si from the support. In order to clarify the alteration process a t

the film-support boundary, the altered parti- cle (“B”) was magnified as shown in Photo. 3. Here, the film-support boundary is clearly observed in the center of the SEM photo-

both phases of analcime and alumina were formed by hydrothermal alteration of the sup- port.

2. Effect of Reaction Time on the Growth of Analcime Film

Hydrothermal treatment of the hydrogels

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Temp.+ Film thicknesstt Shape and size of analcime particlett

(P) (pm) - Spherical, 42

("C) 150 160 235 Trapezohedron, 34 170 258 Trapezohedron, 30 180 196 Trapezohedron, 67

J. Nucl. Sci. Technol.,

Size of altered Thickness of particlett altered layertt (P) (pm)

64 50 130 63 129 50 243

-

Film-support boundary 4

2i I ( 0 KEV XES 10.24KEV

Si

Photo. 3 Close-up of film-support boundary and EDS spectra for sample treated for I d at 180°C

was performed for different periods (2. e. , from 3 h to 3 d), and the growth rate of analcime film and alteration of the support were eval- uated by surface analyses.

(1) Surface Morphology Photographs 4(a)-(c) show the surface

morphology of the sample treated at 180°C for different periods of time. In the early stage of crystallization (after 3 h), several ag- gregates of tiny spherical particles (-5 pm) of analcime were formed as deposits. Rela- tively large spherical analcime particles about 90 pm in diameter were formed on the support after 10 h, but the sutface of the support was partly covered with analcime. After crystal- lization for 2 d, whole surface became covered

cime. Fig.4 Schematic diagram for formation of anal- with a large number of well-crystallized anal-

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Vol. 32, No. 12 (Dec. 1995) 1255

(4 3 h (b) 10h ( c ) 2d Photo. 4 Surface morphology of sample treated for different periods of time at, 180°C

( 2 ) Depth Profile Photographs 5(a) and (b) show the SEM

images of the cross sections of the film sam- ples after crystallization for 10 h and 2 d at 180°C. The thickness of the film tended to increase with crystallization time. After crys- tallization for 10 h, a small number of parti- cles of analcime were formed on the altered particles in the altered layer of about 40pm in thickness. After crystallization for 2d, a thick analcime film of about 370 pm built up

on the altered particles, and the size of anal- cime also increased with Crystallization time. These findings suggest that crystalline eni- bryos of analcime were at first formed by the alteration of the support and then analcime crystals successively grew on the altered par- ticles from the hydrogel phase. This is more clearly revealed in Fig.5, showing the change in weight of the sample treated for different periods of time. The weight loss in the early stage within 10 h is probably due to the dis-

(a) 10h jb) 2d

Photo. 5 Depth profile of film formed after treatment for different periods of time at 180°C

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1256 J. Nucl. Sci. Technol.,

Tm (d)

Fig.5 Change in weight of sample treated for different periods of time at 180°C

solution of Si from the support, resulting in the formation of the altered layer with low content of Si. After treatment over 10 h, suc- cessive accumulation of analcime crystals led to an increase in weight.

Figure 6 shows the growth rate of the film at different temperatures. At 160°C, a relatively thick film was rapidly formed after 14 h and its growth rate was fairly fast within 1 d, while the rate tended to level off over 1 d. A marked growth of the film was observed at 180°C ; the growth rate at 180°C is much faster than that at 160°C , and the film thick- ness goes up to over 400 pm.

3. Optimal Conditions for Analcime Film Preparation

Figure 7 illustrates the ranges of tem- perature and treating time for the formation of the analcime film. Here, the intermediate range of "anal~irne'~ indicates the range where analcime crystals were partly formed on the support. It was found that higher temper- ature and longer periods of time were nec- essary for the formation of analcime. The size and crystallinity of analcime crystals were also found to increase with temperature and

180'C

160c

1 2 3 4 5 Time (d)

Fig.6 Growth of film at different temperatures

200 - Analcime film - .o a 1 -y +\;: E 150- : :\:

Analcirne

100 10' 1 02 Time (h)

X : No analcime was observed A: Analcime particles were observed 0: Analcime film was formed

Fig.7 Formation of analcime film under hydrothermal conditions

treating time. Taking into account the alter- ation of the support and the thermal dura- bility of the inner Teflon vessel, apporpriate experimental conditions for the preparation of analcime film were found to be 170-180°C for 1-2 d.

4. Thermal Stability and Acid

Analcime film calcined at temperatures up to 1,OOO"C for 1 h was examined by XRD. Figure 8 shows the effect of calcining tem- perature on XRD intensity of the maximum peak (hkl 400) of analcime. The structure of analcime was kept up to 700°C ( 1 3 ) , while above 900°C it was converted into amorphous phase.

The acid resistance of analcime film was also checked by varying HNO, concentration up to 5 ~ . The analcime film was contacted with HNO, solutions a t 25°C for 1 d. The XRD intensity of the maximum peak showed only a little change at HNO, concentration

Resistance of Analcime Film

x I > , , , , , ,+&x

0 500 loo0 Calcining temp.(t)

Fig.8 Effect of calcining temperature on structural change of analcime film

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Vol. 32, No. 12 (Dec. 1995) 1257

below 0.01 M, while above 0.1 M the intensity decreased gradually with increasing acidity, indicating relatively poor resistance to acids.

5. Ion-Sieve Ability of Analcime

The separation of Rb+ from Cs+ is of importance in the processing of HLLWs; in order to obtain the highly purified Cs+, the other alkali metals are required to be sep- arated from Cs+. Since analcime consists of three-dimensional cage structure with an effective pore size of about 0.32 nm(5), this zeolite is expected to have an ion-sieve ability to Cs+ ion of which the size is larger than 0.32 nm. In order to check the ion-sieve effect, the degree of incorporation of Rb+ and Cs+ ions into the analcime film was examined by EPMA. In EDS spectrum, only Rb+ ions with 0.30 nm in diameter were detected in the film, but no adsorption of Cs+ (0.34 nm in di- ameter) was observed on the Large differences in adsorption ability of analcime film can be applied to the mutual separation of alkali metal ions from Cs+ in the processing of HLLWs.

Figure 9 shows the adsorption rate of Ag+ (0.25nm in diameter) on the analcime film. The adsorption of Ag+ reached an equilibrium state after 15 h-shaking, yield- ing the adsorption percentage over 99%“”). The incorporation of Ag+ into the analcime film was also observed by EPMA. This Ag+- impregnated film would be available for the

Film

100 ^ -

0 500 1MM 1500 Tme (rnin)

Conditions Analcime film: 0.5766 g Initial concentration of Ag+: 10 ppm Volume of Ag+ solution: 50 cm3

Reaction temperature: 25OC pH, 4.67, pH, 4.58

Fig.9 Adsorption rate of Ag+ on analcime film

removal of radioactive iodine such as

IV. CONCLUSIONS Analcime film was synthesized on a porous

alumina support from hydrogels under hy- drothermal conditions.

After an incubation for 1 d, spherical particles of analcime was partly formed at 150°C, and polycrystalline analcime film of about 200pm in thickness built up on the support above 160°C. The analcime crystal constituting the film was 30-hedron and its size and crystallinity increased with tempera- ture. The surface alteration of the support also took place above 150°C. The altered layer consisted of fine alumina particles and analcime phase. The formation of analcime phase inside the support probably caused the successive accumulation of analcime crystals from the hydrogel phase. The growth rate of the film at 180°C was much faster than that at 160°C after crystallization time over I d.

The analcime film was thermally stable at temperatures up to 7OO0C, while it has rel- atively poor resistance to acids. The spe- cific ion-sieve effect of analcime can be ap- plied to the separation of alkali metal ions from Cs+. The analcime film also adsorbed quantitatively Ag+.

ACKNOWLEDGMENT The authors express appreciation to Mr. Y. Sato for his helpful discussion of EPMA data.

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