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Cesium selectivity of (Al+Na)-substituted tobermorite
Cement and Concrete Research. Vol. 24, No, 3, pp. 573-579. 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All fights re,served 0008-8846/94 $6.00 + .00
Pergamon
CESIUM SELECTIVITY
OF (AI+Na)-SUBSTITUTED
TOBERMORITE
O.P. S h r i v a s t a v a a n d S r i d h a r Komarneni* Materials R e s e a r c h Laboratory The P e n n s y l v a n i a St at e University University Park, PA 16802 (Refereed)
(ReceivedJune 17; in final form August 8, 1993) ABSTRACT Several s y n th e t ic t o b e r m o r i t e s with different levels of [Al+Na] s u b s t i t u t i o n were p r e p a r e d from two different types of starting materials an d their cation e x c h a n g e a n d c e s i u m selective properties were investigated. The s u b s t i t u t e d tobermorites were found to have high cation exchange capacities an d very high selectivities for Cs + ion. C e s i u m selectivity of the s u b s t i t u t e d tobermorites w as d e m o n s t r a t e d in the presence of divalent cations s u c h as Ca 2+, Mg2+, Ba 2+ an d univalent cations s u c h as Na +, K+ and Li+ which are one h u n d r e d times more concentrated t h a n the c e s i u m ion. The u p t a k e of Cs is m a x i m u m in the presence of highly hydrated Mg 2+ an d Li+ ions w h e r e a s it is m i n i m u m in the presence of less hydrated K+ and Ba 2+ ions d u e to steric limitations of the tobermorite structure.
INTRODUCTION S y n t h et i c tobermorites have now b e e n recognized as a new family of cation e x c h a n g e r s mainly in alkaline or nearly n e u t r a l m e d i u m [1-6]. Hydrated cal ci u m silicates r e s e m b l e 2:1 clay minerals in some respects [7]. Megaw a n d Kelsey [7] ascribed to tobermorite a pseudoo r t h o r h o m b i c u n i t cell containing identical layers parallel to (001). The [Si04] t e t r a h e d r a are in 'puckered' c h a i n s with H a t o m s attached to 02- anions of the t e t r a h e d r a parallel to "o.' The Ca-O octahedra show four 02- neighbors in complex sheets (4Ca2Si3 09) and two 02- in the sheet of the n e x t level. The crystal s t r u c t u r e of tobermorite was further refined by H am i d [8] w h o p o s t u l a t e d infinite Si3 (O/OH)9 c h a i n s r u n n i n g parallel to 'b' an d are linked together by Ca atoms. Ca(l), Ca(2), Ca(3) a n d Ca(4) are identically coordinated with 7 oxygen a t o m s forming a tetragonal pyramid. The octahedral coordination of Ca(5) an d Ca(6) is distorted d u e to w e a k Ca-O interaction. The ion exchange properties of u n s u b s t i t u t e d an d s u b s t i t u t e d tobermorites fall into two categories: the reversible exchange as sh o w n by alkali a n d alkaline e a r t h m e t a l c a ti o n s like Li+ , Na +, K+, Cs +, Sr 2+, Ba 2+, etc., in [Al+Na]-substituted tobermorites [9-11] an d the irreversible type reactions s h o w n by divalent cations like Ni, Co, etc., in u n s u b s t i t u t e d tobermorite a n d o t h e r c a l c i u m silicates [4-6]. The behavior of the u n s u b s t i t u t e d tobermorite a n d xonoflite t o wa r d s Mg 2+ is also close to the latter type. The reversible exchange is very similar to t h e e x c h a n g e exhibited by zeolites w h e r e the kinetics are comparatively fast a n d full exchange r e s u l t s in only a few m i n u t e s to h o u r s [9,10]. Although zeolites an d clay m i n e r a l s can be u s e d for Cs s e p a r a t i o n from radioactive waste solutions, [Al+Na]-substituted t o b e r m o r i t e s * Also with t h e D e p a r t m e n t of Agronomy. 573
574
O.P. Shrivastavaand $. Komameni
Vol. 24, No. 3
are well suited not only for Cs separation b u t also for s u b s e q u e n t immobilization in c e m e n t b e c a u s e they are t h e r m o d y n a m i c a l l y stable in c e m e n t e n v i r o n m e n t w h e r e a s zeolites a n d clays are not. S u b s t i t u t e d tobermorites have been prepared previously by u si n g expensive alumlnosilicate gels as the A1 source. The objectives here are (a) to synthesize s u b s t i t u t e d tobermorites from inexpensive AI sources s u c h as a zeolite or sodium a l u m i n a t e an d (2) to d e t e r m i n e a n d c o m p a r e the c e s i u m selectivity of these tobermorites from different solutions c o n t a i n i n g alkali a n d alkaline e a r t h cations.
EXPERIMENTAL Materials an d methods: (Al+Na)-substituted tobermorites were synthesized h y d r o t h e r m a l l y by Kalousek's m e t h o d [12]. U n s u b s t i t u t e d tobermorite of expected c om p o s i t i o n Ca 5 Si 6 O16.(OH) 2 .4H20 (C/S = 0.80) was synthesized in teflon lined Parr b o m b s by t r e a t m e n t at 175"C for 72 h r s u n d e r s a t u r a te d s team pressure. Similarly the (AI+Na) s u b s t i t u t e d tobermorites of 4 different compositions, i.e., 3.1, 6.2, 12.4 and 20 mole % (AI+Na) for Si 4+ were prepared. Two different sources of (AI+Na) were used; namely, zeolite m o l e c u l a r sieve type 4A (600 mesh) a n d sodium aluininate trihydrate. The calculated quantities of zeolite 4A a n d Na20.A1203.3H20 respectively were mixed uniformly with CaO an d SiO2 to make a slurry in double distilled deionized a n d decarbonated water (solid:water ratio = 1:4). The Ca/Si+AI+Na ratio w a s m a i n t a i n e d at 0.8 by adding calculated quantity of SiO2. The chemical composition of the m o l e c u l a r sieve 4A is as follows: Si02 = 34.0%; A1203 = 28.5°/0; CaO < 0.05; N a 2 0 = 17.6O/o; K20 < 0.05; H 2 0 = 20.12%. The parr b o m b s containing the reaction mixture in appropriate q u a n t i t y were heated at 175"-180" for 72 hrs. After h y d r o t h e r m a l t r e a t m e n t the solid an d solution were separated by centrifugation. The solids were w a s h e d 3-4 times with d e c a r b o n a t e d distilled water a n d dried at 100"C. Cation exchange capacity (CEC) m e a s u r e m e n t s: The total CECs of the samples were determined by K ~ Cs exchange following the standard procedure [13]. A known weight of the sample was equilibrated with 0.5N KCI solution followed by 5 washings with 0.01N KCI to remove the excess of KCI a n d to prevent hydrolysis. The K+ was then displaced by Cs on repeated w a s h i n g s with 0.2N CsCI. The solution was collected an d analyzed for K+ by atomic e m i s s i o n s p e c t r o m e t r y (AES) a n d total CEC was determined after m a k i n g corrections for excess 0.01N KCI which was found by accurate weighing. Three replicates were used for these measurements. The selective Cs exchange was m e a s u r e d by the following general procedure: 25 ml of 0.02N solution of Mn+ containing 0.0002N CsCI was added to 25 mg of sample in 60 ml glass bottle. The mixture w a s equilibrated for 24 hrs at 25"C. Triplicates were u s e d for each sample. After equilibration, the solid a n d solutions were separated. Cesium w as analyzed in solution by atomic absorption s p e c tr o p h o t o m e tr y (AAS) u s i n g a Perkin Elmer PE 703 i n s t r u m e n t . The Na, Ca, Si a n d A1 ions in solution were analyzed by AES u si n g SpectraMetrics S p e c t r a S p a n Ill DC p l a s m a e m i s s i o n spectrometer. Solid p h a s e was characterized by powder x-ray diffraction (XRD) on c o m p u t e r - i n t e r f a c e d Scintag USA x-ray diffractometer u si n g nickel-faltered CuKa radiation.
RESULTS AND DISCUSSION Powder x-ray diffraction analysis of the synthetic p r o d u c t s showed t h a t all the s a m p l e s except the one with 20% [AI+Na] substitution were single p h ase preparations of 1.13 n m tobermorite. S c a n n i n g electron microscopy showed t h a t the s a m p l e s consisted of lath-like crystals of 0.2-0.5 btm length a n d 0.2-1 ~tm width. The diffraction p a t t e r n s an d micrographs are in good a g r e e m e n t with those of earlier preparations of synthetic tobermorites [14-16]. Figure 1 c o m p a r e s the x-ray p a t t e r n s of the (Al+Na)-substituted tobermorites with t h a t of the u n s u b s t i t u t e d one. The d(002) in 12.4o/5 substituted sample is at 11.596Awhich is about 0.3/~ greater t h a n the corresponding value of the u n s u b s t i t u t e d sample (Table 1; Figure 1). This
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Cs SELECTIVITY,SUBSTITUTEDTOBERMORITE
575
e xp an s i o n u p o n s u b s t i t u t i o n of Al for Si h a s been well established by previous studies [1-3, 16] a nd is d u e to t h e larger ionic r a d i u s of Al compared to Si. Single p h a s e p r e p a r a t i o n s were achieved u p to 12.4% (Al+Na) s u b s t i t u t i o n b u t w h e n the reaction m i x t u r e co n t ai n ed 20 mole % (Na+AI), the p r o d u c t w a s no longer a single p h a s e as is evident from the x-ray p a t t e m w h i c h shows additional p e a k s d u e to s o d i u m aluminosllicate (Fig. 1). Table 1 s u m m a r i z e s the positions of d(002) of different p r e p a r a t i o n s as a function of A1 substitution.
'd'~, 14.72
5.53
6
16
5.42
26
2.49
136
1.97
1.64
46
56
DEGREES TWO THETA (CuKa) Fig. 1. X-ray diffraction p a t t e m s of various s u b s t i t u t e d an d u n s u b s t i t u t e d tobermorites. The diffraction p a t t e m s of the s a m p l e s were u n a l t e r e d after e x c h a n g e reaction showing t h a t t h e crystallinity of the e x c h a n g e r r e m a i n e d intact after the u p t a k e of Cs +. Figure 2 d e m o n s t r a t e s the effect of (Al+Na) s u b s t i t u t i o n on the total CECs of the tobermorites. The u n s u b s t i t u t e d tobermorite showed a total CEC of 37.1 m e q / 1 0 0 g which is in a g r e e m e n t with the reported data [1]. The total CEC was raised here by (Al+Na) substitution up to a significantly high v a l u e of 114.1 m e q / 1 0 0 g. The CEC v a l u e s of different p r e p a r a t i o n s are c o m p a r a b l e with the c o r r e s p o n d in g theoretical values calculated on the b asi s of their Na contents. Figures 3 and 4 a n d Tables 2 a n d 3 demonstrate the effect of substitution on c e s i u m e x c h a n g e capacity a n d distribution coefficients (Kd) in the presence of different co m p et i n g cations. The exchange data (Tables 2 and 3) show that, though the p a t t e m of u p t a k e in the p r e p a r a t i o n s u s i n g two different [AI+Na] s o u r c e s i.e., zeolite 4A an d NaAIO2.3H20 is similar in Table i. Effect of Al s u b s t i t u t i o n on d(002) spacings of synthetic tobermorites. % S u b s t i t u t i o n of Si 4+ with AI+Na 0 3.1 6.2 12.4 20.0
d(002) (nm) I. 1302 i. 1408 1.1492 i. 1596 i. 1597
576
Vol. 24, No. 3
O.P. Shrivastava and S. Komameni
120
I
I
I X
I00 -
~
/ /
80~
-
, / /
60-
ce of AI + Na x Sodium aluminate as source of AI -I- No z. Calculated CEC's on the basis of AI content
I 20['-J
I I
I
I
5.1
I
6.2 %
12.4
(AI + No) Substitution
Fig. 2. D e p e n d e n c e of t o t a l c a t i o n e x c h a n g e c a p a c i t y o n t h e e x t e n t of [AI+Na] s u b s t i t u t i o n .
m a n y r e s p e c t s , it is n o t e x a c t l y i d e n t i c a l . T h i s d i f f e r e n c e s e e m s to reflect t h e r e a c t i v i t y of t h e t w o d i f f e r e n t [Al+Na] s o u r c e s . M a x i m u m selective u p t a k e of C s + is a b o u t 19 m e q / 1 0 0 g i n t h e p r e s e n c e of Mg 2+ a n d Li + w h e r e a s t h e u p t a k e is m i n i m u m i n t h e c a s e of K + a n d B a 2+ ions. T h i s s h o w s t h a t t h e K + is t h e m o s t c o m p e t i n g a n d Mg 2+ is t h e l e a s t c o m p e t i n g c a t i o n for C s + o n t h e s e s u b s t i t u t e d t o b e r m o r i t e s . C e s i u m s e l e c t i v i t y i n t h e p r e s e n c e of d i f f e r e n t c o m p e t i n g c a t i o n s i n c r e a s e s a s follows: Mg > Li > C a > Na > B a > K.
20,
,
,
,
I
I
'
'
'
'
I
'
I
,
,
hSo -~- ~
^
~. ~
/
o ,ol•~ ~ /
¢J¢0 I
w
/ T ,'.~'f
-
x Cs/Ba
/ / ~ //,~
o C,/K
_
o Cs/Na
II/
- - ~o c s / , i
0 0
5 %
(AI
I0 +
Na)
15
Substitution
Fig. 3. D e p e n d e n c e of selective C s + e x c h a n g e o n t h e e x t e n t of [ A l + N a ] - s u b s t i t u t i o n of t o b e r m o r i t e s m a d e f r o m Zeolite 4 A a s t h e AI+Na s o u r c e .
Vol. 24, No. 3
Cs SELECTIVITY,SUBSTITUTEDTOBERMOR1TE
577
Table 2. C e s i u m selectivity of tobermorites from different cationic solutions.
Ion m i x t u r e
Cs taken up, meq/100 G
Kd*
U n s u b s t i t u t e d Tobermorite
Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
1.65 1.65 2.10 0.22 2.63 2.63
85 85 111 11 142 142
3.1% [Al+Nal-Substituted Tobermorite from Zeolite 4A Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
9.92 12.25 5.79 1.73 9.92 13.68
891 1393 379 89 806 1947
6.2% [Al+Na}-Substituted Tobermorite from Zeolite 4A Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
12.78 14.33 6.09 3.01 13.61 15.71
1545 1413 407 166 1828 2943
12.4% {Al+Na)-Substituted Tobermorite from Zeolite 4A
Cs/Ca C s / Mg
Cs/Ba Cs/K Cs/Na Cs/Li
17.52 19.1 5.41 4.06 16.62 18.08
5065 9732 346 238 3745 6088
,K d = f r a c t i o n o f c a t i o n o n i o n e x c h a n g e r × m I o f s o l u t i o n fraction o fcation in solution g o fion exchanger
An e x a m i n a t i o n of c e s i u m selectivity data in light of the tobermorite s t r u c t u r e [7,8, I0] shows t h a t the u p t a k e is related to the hydration s t a t u s of the cations an d the steric limitations of the s t r u c t u r e [11]. Highly hydrated Mg2+ and Li+ ions are excluded while less hydrated Cs + and K+ are preferentially t a k e n up in the interlayers. The u p t a k e of c e s i u m is a reversible ion e x c h a n g e reaction [11] resulting in release of equivalent Na + ions from the interlayers of tobermorite. T h u s the cation exchange a n d selectivity properties of t o b e r m o r i t e s are a n a l o g o u s to those of zeolites a nd clay minerals.
578
O.P. Shrivastavaand S. Komameni
2O
I
I
io I
/
......
. . . .
-°'"
...
+
~g
Fig. 4. Dependence of selective Cs + exchange on the extent of [Al+Na]s u b s t i t u t i o n of tobermorites m a d e from sodium a l u m i n a t e as the AI+Na source.
oE "g
I0
g., O
0
®.0 o~ -.-C
Vol. 24, No. 3
a 0o
~"
/
~'
~
CslMg x CslBa
+
~
/
o Cs/K
~ /
o c_~iN9 I
O
I
A
Cs/Li
3.1 6.2 % (AI + No) Substitution
I
12.4
Table 3. C e s i u m selectivity of tobermorites from different cationic solutions.
Ion m i x t u r e
Cs taken up, meq/i00 G
Kd*
3 / 1 % (Al+Na)-Substituted Tobermorite from S o d i u m A l u m i n a t e Trihydrate
Cs/Ca C s / Mg Cs/Ba Cs/K Cs/Na Cs/Li
11.35 12.30 10.50 3.61 13.30 13.53
1227 1291 1897 196 1439 1636
6.2% (Al+Na)-Substituted Tobermorite from S o d i u m A l u m i n a t e Trihydrate
Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
16.61 14.38 11.5 5.26 16.18 18.10
4170 1575 2788 314 2529 4918
12.4% [Al+Na}-Substituted Tobermorite from S o d i u m A l u m i n a t e Trihydrate Cs/Ca C s / Mg Cs/Ba Cs/K Cs/Na Cs/Li
16.76 16.24 11.6 7.96 18.35 19.32
4372 2880 703 573 4357 7788
Vol. 24, No. 3
Cs SELECTIVITY,SUBSTITUTEDTOBERMOR1TE
579
CONCLUSION [Al+Na]-substituted tobermorites c a n be prepared u s i n g either a zeolite or NaAIO2 as the Al source. T h e s e tobermorites are similar a n d exhibit high selectivity for Cs a n d this selectivity i n c r e a s e s in different cationic solutions as follows: Mg2+ > Li+ > Ca 2+ > Na + > Ba 2+ > K+. The high Cs selectivity of s u b s t i t u t e d tobermorites can be u s e d in partitioning of radioactive Cs from n u c l e a r w a s t e s containing n u m e r o u s cations.
Acknowledgements This r e s e a r c h w a s s u p p o r t e d by the S e p a r a t i o n Processes Program, Division of Chemical a n d Process Engineering, National Science Foundation, u n d e r Grant No. CBT-8619064. One of the a u t h o r s (OPS) is thankful to Dr. H.S. Gour University, Sagar, India, for granting h i m st u d y leave.
REFERENCES i. S. Komameni, D.M. Roy and R. Roy, Cem. Concr. Res. I__22,773 (1982). 2. S. K o m a m e n i a n d D.M. Roy, Science 220, 647 (1983). 3. S. K o m a m e n i a n d D.M. Roy, Scientific Basis for Nuclear Waste Management, Vol. 5, D.G. Brookins (ed.), Elsevier, New York, pp. 55-62 (1983). 4. S. Komameni, R. Roy a n d D.M. Roy, Cem. Concr. Res. 1_.66,47 (1986). 5. O.P. S h r i v a s t a v a a n d F.P. Glasser, Reactivity of Solids 2, 261 (1986). 6. O.P. Shrivastava a n d F.P. Glasser, J. Mat. Sci. 4, 1122 (1985). 7. H.D. Megaw and C. Kelsey, Nature 177, 390 (1956). 8. S_A.. Hamid, Z. Krist. 154, 189 (1981). 9. S. Komarneni, Clays a n d Clay Minerals 3._3_3(2), 145 (1985). 10. M. TsuJi an d S. Komarneni, J. Mat. Res. 4 (3), 698 (1989). 11. S. Komarneni a n d M. Tsuji, J. Am. Ceram. Soc. 72, 1668 (1989). 12. G,L. Kalousek, J. Am. Ceram. Soc. 40, 74 (1956). 13. M.L. J a c k s o n , Clays a n d Clay Minerals 1_!1, 29 (1963). 14. J.C.P.D.S., Inorganic Phases, 1989, Mineral Section, File No. 19-1364. 15. S.A.S. EI-Hemaly, T. Mitsuda a n d H.F.W. Taylor, Cem. Concr. Res. 7, 429 (1977). 16. S. Diamond, J.L. White a n d W.L. Dolch, Am. Mineral. 5 j , 388 (1960).
Pergamon
CESIUM SELECTIVITY
OF (AI+Na)-SUBSTITUTED
TOBERMORITE
O.P. S h r i v a s t a v a a n d S r i d h a r Komarneni* Materials R e s e a r c h Laboratory The P e n n s y l v a n i a St at e University University Park, PA 16802 (Refereed)
(ReceivedJune 17; in final form August 8, 1993) ABSTRACT Several s y n th e t ic t o b e r m o r i t e s with different levels of [Al+Na] s u b s t i t u t i o n were p r e p a r e d from two different types of starting materials an d their cation e x c h a n g e a n d c e s i u m selective properties were investigated. The s u b s t i t u t e d tobermorites were found to have high cation exchange capacities an d very high selectivities for Cs + ion. C e s i u m selectivity of the s u b s t i t u t e d tobermorites w as d e m o n s t r a t e d in the presence of divalent cations s u c h as Ca 2+, Mg2+, Ba 2+ an d univalent cations s u c h as Na +, K+ and Li+ which are one h u n d r e d times more concentrated t h a n the c e s i u m ion. The u p t a k e of Cs is m a x i m u m in the presence of highly hydrated Mg 2+ an d Li+ ions w h e r e a s it is m i n i m u m in the presence of less hydrated K+ and Ba 2+ ions d u e to steric limitations of the tobermorite structure.
INTRODUCTION S y n t h et i c tobermorites have now b e e n recognized as a new family of cation e x c h a n g e r s mainly in alkaline or nearly n e u t r a l m e d i u m [1-6]. Hydrated cal ci u m silicates r e s e m b l e 2:1 clay minerals in some respects [7]. Megaw a n d Kelsey [7] ascribed to tobermorite a pseudoo r t h o r h o m b i c u n i t cell containing identical layers parallel to (001). The [Si04] t e t r a h e d r a are in 'puckered' c h a i n s with H a t o m s attached to 02- anions of the t e t r a h e d r a parallel to "o.' The Ca-O octahedra show four 02- neighbors in complex sheets (4Ca2Si3 09) and two 02- in the sheet of the n e x t level. The crystal s t r u c t u r e of tobermorite was further refined by H am i d [8] w h o p o s t u l a t e d infinite Si3 (O/OH)9 c h a i n s r u n n i n g parallel to 'b' an d are linked together by Ca atoms. Ca(l), Ca(2), Ca(3) a n d Ca(4) are identically coordinated with 7 oxygen a t o m s forming a tetragonal pyramid. The octahedral coordination of Ca(5) an d Ca(6) is distorted d u e to w e a k Ca-O interaction. The ion exchange properties of u n s u b s t i t u t e d an d s u b s t i t u t e d tobermorites fall into two categories: the reversible exchange as sh o w n by alkali a n d alkaline e a r t h m e t a l c a ti o n s like Li+ , Na +, K+, Cs +, Sr 2+, Ba 2+, etc., in [Al+Na]-substituted tobermorites [9-11] an d the irreversible type reactions s h o w n by divalent cations like Ni, Co, etc., in u n s u b s t i t u t e d tobermorite a n d o t h e r c a l c i u m silicates [4-6]. The behavior of the u n s u b s t i t u t e d tobermorite a n d xonoflite t o wa r d s Mg 2+ is also close to the latter type. The reversible exchange is very similar to t h e e x c h a n g e exhibited by zeolites w h e r e the kinetics are comparatively fast a n d full exchange r e s u l t s in only a few m i n u t e s to h o u r s [9,10]. Although zeolites an d clay m i n e r a l s can be u s e d for Cs s e p a r a t i o n from radioactive waste solutions, [Al+Na]-substituted t o b e r m o r i t e s * Also with t h e D e p a r t m e n t of Agronomy. 573
574
O.P. Shrivastavaand $. Komameni
Vol. 24, No. 3
are well suited not only for Cs separation b u t also for s u b s e q u e n t immobilization in c e m e n t b e c a u s e they are t h e r m o d y n a m i c a l l y stable in c e m e n t e n v i r o n m e n t w h e r e a s zeolites a n d clays are not. S u b s t i t u t e d tobermorites have been prepared previously by u si n g expensive alumlnosilicate gels as the A1 source. The objectives here are (a) to synthesize s u b s t i t u t e d tobermorites from inexpensive AI sources s u c h as a zeolite or sodium a l u m i n a t e an d (2) to d e t e r m i n e a n d c o m p a r e the c e s i u m selectivity of these tobermorites from different solutions c o n t a i n i n g alkali a n d alkaline e a r t h cations.
EXPERIMENTAL Materials an d methods: (Al+Na)-substituted tobermorites were synthesized h y d r o t h e r m a l l y by Kalousek's m e t h o d [12]. U n s u b s t i t u t e d tobermorite of expected c om p o s i t i o n Ca 5 Si 6 O16.(OH) 2 .4H20 (C/S = 0.80) was synthesized in teflon lined Parr b o m b s by t r e a t m e n t at 175"C for 72 h r s u n d e r s a t u r a te d s team pressure. Similarly the (AI+Na) s u b s t i t u t e d tobermorites of 4 different compositions, i.e., 3.1, 6.2, 12.4 and 20 mole % (AI+Na) for Si 4+ were prepared. Two different sources of (AI+Na) were used; namely, zeolite m o l e c u l a r sieve type 4A (600 mesh) a n d sodium aluininate trihydrate. The calculated quantities of zeolite 4A a n d Na20.A1203.3H20 respectively were mixed uniformly with CaO an d SiO2 to make a slurry in double distilled deionized a n d decarbonated water (solid:water ratio = 1:4). The Ca/Si+AI+Na ratio w a s m a i n t a i n e d at 0.8 by adding calculated quantity of SiO2. The chemical composition of the m o l e c u l a r sieve 4A is as follows: Si02 = 34.0%; A1203 = 28.5°/0; CaO < 0.05; N a 2 0 = 17.6O/o; K20 < 0.05; H 2 0 = 20.12%. The parr b o m b s containing the reaction mixture in appropriate q u a n t i t y were heated at 175"-180" for 72 hrs. After h y d r o t h e r m a l t r e a t m e n t the solid an d solution were separated by centrifugation. The solids were w a s h e d 3-4 times with d e c a r b o n a t e d distilled water a n d dried at 100"C. Cation exchange capacity (CEC) m e a s u r e m e n t s: The total CECs of the samples were determined by K ~ Cs exchange following the standard procedure [13]. A known weight of the sample was equilibrated with 0.5N KCI solution followed by 5 washings with 0.01N KCI to remove the excess of KCI a n d to prevent hydrolysis. The K+ was then displaced by Cs on repeated w a s h i n g s with 0.2N CsCI. The solution was collected an d analyzed for K+ by atomic e m i s s i o n s p e c t r o m e t r y (AES) a n d total CEC was determined after m a k i n g corrections for excess 0.01N KCI which was found by accurate weighing. Three replicates were used for these measurements. The selective Cs exchange was m e a s u r e d by the following general procedure: 25 ml of 0.02N solution of Mn+ containing 0.0002N CsCI was added to 25 mg of sample in 60 ml glass bottle. The mixture w a s equilibrated for 24 hrs at 25"C. Triplicates were u s e d for each sample. After equilibration, the solid a n d solutions were separated. Cesium w as analyzed in solution by atomic absorption s p e c tr o p h o t o m e tr y (AAS) u s i n g a Perkin Elmer PE 703 i n s t r u m e n t . The Na, Ca, Si a n d A1 ions in solution were analyzed by AES u si n g SpectraMetrics S p e c t r a S p a n Ill DC p l a s m a e m i s s i o n spectrometer. Solid p h a s e was characterized by powder x-ray diffraction (XRD) on c o m p u t e r - i n t e r f a c e d Scintag USA x-ray diffractometer u si n g nickel-faltered CuKa radiation.
RESULTS AND DISCUSSION Powder x-ray diffraction analysis of the synthetic p r o d u c t s showed t h a t all the s a m p l e s except the one with 20% [AI+Na] substitution were single p h ase preparations of 1.13 n m tobermorite. S c a n n i n g electron microscopy showed t h a t the s a m p l e s consisted of lath-like crystals of 0.2-0.5 btm length a n d 0.2-1 ~tm width. The diffraction p a t t e r n s an d micrographs are in good a g r e e m e n t with those of earlier preparations of synthetic tobermorites [14-16]. Figure 1 c o m p a r e s the x-ray p a t t e r n s of the (Al+Na)-substituted tobermorites with t h a t of the u n s u b s t i t u t e d one. The d(002) in 12.4o/5 substituted sample is at 11.596Awhich is about 0.3/~ greater t h a n the corresponding value of the u n s u b s t i t u t e d sample (Table 1; Figure 1). This
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Cs SELECTIVITY,SUBSTITUTEDTOBERMORITE
575
e xp an s i o n u p o n s u b s t i t u t i o n of Al for Si h a s been well established by previous studies [1-3, 16] a nd is d u e to t h e larger ionic r a d i u s of Al compared to Si. Single p h a s e p r e p a r a t i o n s were achieved u p to 12.4% (Al+Na) s u b s t i t u t i o n b u t w h e n the reaction m i x t u r e co n t ai n ed 20 mole % (Na+AI), the p r o d u c t w a s no longer a single p h a s e as is evident from the x-ray p a t t e m w h i c h shows additional p e a k s d u e to s o d i u m aluminosllicate (Fig. 1). Table 1 s u m m a r i z e s the positions of d(002) of different p r e p a r a t i o n s as a function of A1 substitution.
'd'~, 14.72
5.53
6
16
5.42
26
2.49
136
1.97
1.64
46
56
DEGREES TWO THETA (CuKa) Fig. 1. X-ray diffraction p a t t e m s of various s u b s t i t u t e d an d u n s u b s t i t u t e d tobermorites. The diffraction p a t t e m s of the s a m p l e s were u n a l t e r e d after e x c h a n g e reaction showing t h a t t h e crystallinity of the e x c h a n g e r r e m a i n e d intact after the u p t a k e of Cs +. Figure 2 d e m o n s t r a t e s the effect of (Al+Na) s u b s t i t u t i o n on the total CECs of the tobermorites. The u n s u b s t i t u t e d tobermorite showed a total CEC of 37.1 m e q / 1 0 0 g which is in a g r e e m e n t with the reported data [1]. The total CEC was raised here by (Al+Na) substitution up to a significantly high v a l u e of 114.1 m e q / 1 0 0 g. The CEC v a l u e s of different p r e p a r a t i o n s are c o m p a r a b l e with the c o r r e s p o n d in g theoretical values calculated on the b asi s of their Na contents. Figures 3 and 4 a n d Tables 2 a n d 3 demonstrate the effect of substitution on c e s i u m e x c h a n g e capacity a n d distribution coefficients (Kd) in the presence of different co m p et i n g cations. The exchange data (Tables 2 and 3) show that, though the p a t t e m of u p t a k e in the p r e p a r a t i o n s u s i n g two different [AI+Na] s o u r c e s i.e., zeolite 4A an d NaAIO2.3H20 is similar in Table i. Effect of Al s u b s t i t u t i o n on d(002) spacings of synthetic tobermorites. % S u b s t i t u t i o n of Si 4+ with AI+Na 0 3.1 6.2 12.4 20.0
d(002) (nm) I. 1302 i. 1408 1.1492 i. 1596 i. 1597
576
Vol. 24, No. 3
O.P. Shrivastava and S. Komameni
120
I
I
I X
I00 -
~
/ /
80~
-
, / /
60-
ce of AI + Na x Sodium aluminate as source of AI -I- No z. Calculated CEC's on the basis of AI content
I 20['-J
I I
I
I
5.1
I
6.2 %
12.4
(AI + No) Substitution
Fig. 2. D e p e n d e n c e of t o t a l c a t i o n e x c h a n g e c a p a c i t y o n t h e e x t e n t of [AI+Na] s u b s t i t u t i o n .
m a n y r e s p e c t s , it is n o t e x a c t l y i d e n t i c a l . T h i s d i f f e r e n c e s e e m s to reflect t h e r e a c t i v i t y of t h e t w o d i f f e r e n t [Al+Na] s o u r c e s . M a x i m u m selective u p t a k e of C s + is a b o u t 19 m e q / 1 0 0 g i n t h e p r e s e n c e of Mg 2+ a n d Li + w h e r e a s t h e u p t a k e is m i n i m u m i n t h e c a s e of K + a n d B a 2+ ions. T h i s s h o w s t h a t t h e K + is t h e m o s t c o m p e t i n g a n d Mg 2+ is t h e l e a s t c o m p e t i n g c a t i o n for C s + o n t h e s e s u b s t i t u t e d t o b e r m o r i t e s . C e s i u m s e l e c t i v i t y i n t h e p r e s e n c e of d i f f e r e n t c o m p e t i n g c a t i o n s i n c r e a s e s a s follows: Mg > Li > C a > Na > B a > K.
20,
,
,
,
I
I
'
'
'
'
I
'
I
,
,
hSo -~- ~
^
~. ~
/
o ,ol•~ ~ /
¢J¢0 I
w
/ T ,'.~'f
-
x Cs/Ba
/ / ~ //,~
o C,/K
_
o Cs/Na
II/
- - ~o c s / , i
0 0
5 %
(AI
I0 +
Na)
15
Substitution
Fig. 3. D e p e n d e n c e of selective C s + e x c h a n g e o n t h e e x t e n t of [ A l + N a ] - s u b s t i t u t i o n of t o b e r m o r i t e s m a d e f r o m Zeolite 4 A a s t h e AI+Na s o u r c e .
Vol. 24, No. 3
Cs SELECTIVITY,SUBSTITUTEDTOBERMOR1TE
577
Table 2. C e s i u m selectivity of tobermorites from different cationic solutions.
Ion m i x t u r e
Cs taken up, meq/100 G
Kd*
U n s u b s t i t u t e d Tobermorite
Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
1.65 1.65 2.10 0.22 2.63 2.63
85 85 111 11 142 142
3.1% [Al+Nal-Substituted Tobermorite from Zeolite 4A Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
9.92 12.25 5.79 1.73 9.92 13.68
891 1393 379 89 806 1947
6.2% [Al+Na}-Substituted Tobermorite from Zeolite 4A Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
12.78 14.33 6.09 3.01 13.61 15.71
1545 1413 407 166 1828 2943
12.4% {Al+Na)-Substituted Tobermorite from Zeolite 4A
Cs/Ca C s / Mg
Cs/Ba Cs/K Cs/Na Cs/Li
17.52 19.1 5.41 4.06 16.62 18.08
5065 9732 346 238 3745 6088
,K d = f r a c t i o n o f c a t i o n o n i o n e x c h a n g e r × m I o f s o l u t i o n fraction o fcation in solution g o fion exchanger
An e x a m i n a t i o n of c e s i u m selectivity data in light of the tobermorite s t r u c t u r e [7,8, I0] shows t h a t the u p t a k e is related to the hydration s t a t u s of the cations an d the steric limitations of the s t r u c t u r e [11]. Highly hydrated Mg2+ and Li+ ions are excluded while less hydrated Cs + and K+ are preferentially t a k e n up in the interlayers. The u p t a k e of c e s i u m is a reversible ion e x c h a n g e reaction [11] resulting in release of equivalent Na + ions from the interlayers of tobermorite. T h u s the cation exchange a n d selectivity properties of t o b e r m o r i t e s are a n a l o g o u s to those of zeolites a nd clay minerals.
578
O.P. Shrivastavaand S. Komameni
2O
I
I
io I
/
......
. . . .
-°'"
...
+
~g
Fig. 4. Dependence of selective Cs + exchange on the extent of [Al+Na]s u b s t i t u t i o n of tobermorites m a d e from sodium a l u m i n a t e as the AI+Na source.
oE "g
I0
g., O
0
®.0 o~ -.-C
Vol. 24, No. 3
a 0o
~"
/
~'
~
CslMg x CslBa
+
~
/
o Cs/K
~ /
o c_~iN9 I
O
I
A
Cs/Li
3.1 6.2 % (AI + No) Substitution
I
12.4
Table 3. C e s i u m selectivity of tobermorites from different cationic solutions.
Ion m i x t u r e
Cs taken up, meq/i00 G
Kd*
3 / 1 % (Al+Na)-Substituted Tobermorite from S o d i u m A l u m i n a t e Trihydrate
Cs/Ca C s / Mg Cs/Ba Cs/K Cs/Na Cs/Li
11.35 12.30 10.50 3.61 13.30 13.53
1227 1291 1897 196 1439 1636
6.2% (Al+Na)-Substituted Tobermorite from S o d i u m A l u m i n a t e Trihydrate
Cs/Ca Cs/Mg Cs/Ba Cs/K Cs/Na Cs/Li
16.61 14.38 11.5 5.26 16.18 18.10
4170 1575 2788 314 2529 4918
12.4% [Al+Na}-Substituted Tobermorite from S o d i u m A l u m i n a t e Trihydrate Cs/Ca C s / Mg Cs/Ba Cs/K Cs/Na Cs/Li
16.76 16.24 11.6 7.96 18.35 19.32
4372 2880 703 573 4357 7788
Vol. 24, No. 3
Cs SELECTIVITY,SUBSTITUTEDTOBERMOR1TE
579
CONCLUSION [Al+Na]-substituted tobermorites c a n be prepared u s i n g either a zeolite or NaAIO2 as the Al source. T h e s e tobermorites are similar a n d exhibit high selectivity for Cs a n d this selectivity i n c r e a s e s in different cationic solutions as follows: Mg2+ > Li+ > Ca 2+ > Na + > Ba 2+ > K+. The high Cs selectivity of s u b s t i t u t e d tobermorites can be u s e d in partitioning of radioactive Cs from n u c l e a r w a s t e s containing n u m e r o u s cations.
Acknowledgements This r e s e a r c h w a s s u p p o r t e d by the S e p a r a t i o n Processes Program, Division of Chemical a n d Process Engineering, National Science Foundation, u n d e r Grant No. CBT-8619064. One of the a u t h o r s (OPS) is thankful to Dr. H.S. Gour University, Sagar, India, for granting h i m st u d y leave.
REFERENCES i. S. Komameni, D.M. Roy and R. Roy, Cem. Concr. Res. I__22,773 (1982). 2. S. K o m a m e n i a n d D.M. Roy, Science 220, 647 (1983). 3. S. K o m a m e n i a n d D.M. Roy, Scientific Basis for Nuclear Waste Management, Vol. 5, D.G. Brookins (ed.), Elsevier, New York, pp. 55-62 (1983). 4. S. Komameni, R. Roy a n d D.M. Roy, Cem. Concr. Res. 1_.66,47 (1986). 5. O.P. S h r i v a s t a v a a n d F.P. Glasser, Reactivity of Solids 2, 261 (1986). 6. O.P. Shrivastava a n d F.P. Glasser, J. Mat. Sci. 4, 1122 (1985). 7. H.D. Megaw and C. Kelsey, Nature 177, 390 (1956). 8. S_A.. Hamid, Z. Krist. 154, 189 (1981). 9. S. Komarneni, Clays a n d Clay Minerals 3._3_3(2), 145 (1985). 10. M. TsuJi an d S. Komarneni, J. Mat. Res. 4 (3), 698 (1989). 11. S. Komarneni a n d M. Tsuji, J. Am. Ceram. Soc. 72, 1668 (1989). 12. G,L. Kalousek, J. Am. Ceram. Soc. 40, 74 (1956). 13. M.L. J a c k s o n , Clays a n d Clay Minerals 1_!1, 29 (1963). 14. J.C.P.D.S., Inorganic Phases, 1989, Mineral Section, File No. 19-1364. 15. S.A.S. EI-Hemaly, T. Mitsuda a n d H.F.W. Taylor, Cem. Concr. Res. 7, 429 (1977). 16. S. Diamond, J.L. White a n d W.L. Dolch, Am. Mineral. 5 j , 388 (1960).