MINERALOGICAL

MAGAZINE,

A P R I L 1985, V O L . 49, P P . 203 210

The use of zeolites for the treatment of radioactive waste A. DYER AND K. Y. MIKI-IAIL Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, UK

ABSTRACT. The uptake of 137-Cs and 90-Sr on to synthetic zeolite A has been studied. The use of a combined barium exchange followed by a relatively mild heat treatment has been shown to fix the isotopes within a collapsed zeolite structure. Leaching experiments using deionized and simulated sea and pond waters were carried out on zeolite compacts and zeolite/cement composites. They demonstrated the effectiveness of the calcination and cement containment. Some comments on the interpretation of leach test data are made.

(Dyer et al., 1971), so t h a t this p r o p e r t y suggests a way of fixing isotopes in zeolites prior to disposal via the s t a n d a r d procedures of c o n t a i n m e n t into cements or glasses ( M a t s u z u r u a n d Ito, 1978; Siemens et al., 1982) p e r h a p s w i t h o u t a n expensive high t e m p e r a t u r e calcination step ( G o t o et al., 1982).

KEYWORDS: radioactive waste, zeolite, cement.

Materials The A zeolite used was supplied by Laporte Industries, Widnes, Cheshire. It was in its sodium form (4A), as a powder and as compacted zeolite crystals held by a day-binder in the form of spheres (1.4 2.0 ram). The isotopes used were as supplied by Amersham International. All other chemicals were of GPR purity or better.

T H E zeolites are a class of aluminosilicate minerals with u n i q u e ion-exchange selectivities a n d a k n o w n resistance to radiation. These properties have encouraged extensive studies into their use in the t r e a t m e n t of radioactive waste. Recently 'clean-up' investigations, concerned with zeolites, have c o n c e n t r a t e d largely on the application of n a t u r a l zeolites particularly those with high Si c o n t e n t like clinoptilolite (Mercer a n d Ames, 1978), b u t currently synthetic zeolites have been employed successfully in, for example, the Three Mile Island a f t e r m a t h (Collins et al., 1982). I n some ways synthetic zeolites can be the preferred exchanger, despite a higher cost, because the supplies of n a t u r a l minerals can vary in quality a n d m o r e i m p o r t a n t l y a variation in geologic occurrence can cause subtle selectivity alterations (Dyer a n d Mikhail, unpubl.). Synthetic zeolites offer also the potential to a d v a n t a g e o u s l y m a n i p u late their properties. This p a p e r considers simple chemical ways whereby radioactive Cs a n d Sr species m i g h t be fixed within a zeolite m a t r i x a n d the effectiveness in this t r e a t m e n t when isotopically loaded zeolites are c o n t a i n e d in cement composites. The effectiveness is quantified by leach experiments o n zeolites a n d zeolite/cement composites. T h e synthetic zeolite studied is zeolite A which is readily available in p o w d e r a n d pellet form a n d has been used successfully at T M I (Collins et al., 1982). This zeolite is k n o w n to lose stability w h e n exchanged with several cations, a n d with Ba in particular

(~) Copyright the Mineralogical Society

Experimental

Characterization As received zeolite A was characterized by X-ray powder diffraction (XRD) and by thermal analyses. These techniques were used to monitor changes in the zeolite caused by various treatments. Some elemental analyses were carried out by electron probe microanalysis. Experiments on powdered zeolite (i) Amounts of 4A powder were placed in contact with simulated pH 11.4 pond water (150 mg Na2CO3, 200 mg NaOH in 11 deionized water) maintaining a liquid/solid ratio of 10: 1. The pond water was labelled with 137-Cs or 90-Sr (carrier free). After 24 hrs. stirring at room temperature (RT) the samples were separated from the liquid phase, dried and then treated further with BaC12 solution of differing concentrations at R T for 24 hrs. (with stirring) and at a liquid/solid ratio of 20: 1. (ii) A series of unlabelled NaBaCs A zeolites was prepared in a similar manner to the above (using 0.01M CsC1) and the extent of exchange monitored by the appearance of Na in solution as determined by flame photometry. (iii) 'Back exchange' experiments were carried out on samples from (i) and (ii) which had been heated in air for 24 hrs. at different temperatures. In each case the uptake of 22-Na from 1.0 M NaC1 was taken as a measure of 'back-exchange'. Equilibrium times were 2 days at least. Recoveries of 137-Cs were monitored as appropriate.

204

A. D Y E R

A N D K . Y. M I K H A I L

'~176 f

o/~ INTENSITY

O

NoA powder

f

Na BoC~A

3

I NaBoCs A

6

I

o

sol-

NaBoC$ A IO

oF

I 210

3Is

so

DEGREES 2 0 C u K ~

FIG. 1. X R D patterns o f N a Ba Cs A powder prepared by using 3, 6, and 10 ml of 0.5 M BaCI 2 per gram zeolite (referred to as 3, 6, and 10 in subsequent diagrams).

3

EXO

l

6 f

~E

IO

j

~ w

ENDO

IOO

I 200

I 300 ToC

FIG. 2. DSC traces of samples prepared as fig. 1.

I 400

500

ZEOLITE

TREATMENT

Experiments on zeolite spheres (i) Breakthrough curves were constructed to measure the uptake of 137-Cs and 90-Sr on to spheres from simulated pond water. The columns contained 3 g of spheres in a glass column (6.75 mm i.d.) and the flow rate was 24 ml hr-1. (ii) Experiments were carried out to discover the optimal conditions for Ba treatment of the spheres beyond breakthrough to minimize 137-Cs loss. These were column experiments and the pH was varied as well. (iii) Spheres containing isotopes were heated in air and vacuum, after various treatments, to check the validity of the powder experiments.

Containment experiments (i) Zeolites from column experiments were calcined at 300 ~ and then mixed with cement (OPC) to a ratio of water/cement at 0.4 (see Table I). After sufficient mixing the cement paste was poured into a polythene container and left for 24 hrs. at room temperature (RT). It was then taken from the container and air cured for 24 days. Compressive strengths and density measurements were made in triplicate.

OF RADIOACTIVE

pond water. Tests were also made on zeolite spheres loaded with 137-Cs without cement. Some electron probe microanalyses on leached samples were carried out. (iii) Measurement of radioactivity. All measurements were carried out by determining Cerenkov radiation from aqueous solution in the tritium channel of a Nuclear Chicago Mark II liquid scintillation counter. Polythene vials were used throughout.

Results and Discussion Zeolite powder experiments The preliminary experiments described showed t h a t 0.5 M BaC12 solution was suitable for treating 137-Cs-containing zeolites in t h a t a m i n i m u m of isotope was displaced even w h e n the initial Cs exchange was 0.01 M. Test experiments d e m o n strated t h a t sodium was also exchanged from the zeolite by the Ba 2 + treatment.

Table III.

Effect of replacemen~ of Na from Na CS A by Ba

ml g 1 of 0,55~ Table I.

Properties of zeolite/cement composites (water/cement ratio = 0.4) cured in air for 28 days

Zeolite/zeolite + cement ratio

Density8

gcm-

Compressive strength kg m -2 iO I

O.lO

1.88 -+ 0.02

321 + 15

0,15

1.87 -+ 0.02

308 • 17

0.20

1.86 + 0.02

235 • 17

Cement control

1.90 + 0.03

440 •

8

(ii) Leach tests were carried out, by a modified IAEA method (Hespe, 1971), on composites containing a 0.15 zeolite/zeolite and cement weight ratio. Cured samples were immersed in 100 200 ml leachant in plastic vessels held, with frequent agitation, at 25 ~ in a water bath. The leachants were renewed daily for 3 days, weekly for 3 weeks, and then once per month for 3 months. The leachants were deionized water, synthetic sea water (Taylor and Kuwairi, 1978), see Table II, and simulated

Table II.

Composition of synthetic sea water

Component NaCI

Content (wt, %) 2.3480

MgCI 2

0.4981

Na2SO 4

0.3917

CaCI 2

O.1102

KCI

0.0664

NaIICO3

O.O192

KBr

0.0096

205

WASTE

BaCI 2 used

~ Xa replaced

% Wt.

loss

to 5OO~

(TC)

Cndotherm 2" ~ from DSC

1.99

44.2

17.9

208

2.99

54.3

17.0

205 192

3.98

64.9

16.8

5.98

72.7

16.5

175

7.98

73.4

16.O

170

9.95

74,4

15.5

165

The effect of t r e a t m e n t was m o n i t o r e d by X R D (fig. 1) a n d t h e r m a l analysis (see fig. 2 for D S C a n d Table III for T G results). T h e loss of intensity in the X R D p h o t o g r a p h s was a n artefact of the presence of Ba (a high X-ray absorber) a n d n o t a consequence of lattice collapse. This p h e n o m e n o n has been discussed in detail elsewhere (Dyer et al., 1970) a n d the 'back-exchange' experiments carried o u t here confirmed t h a t N a could replace B a from the treated powders. Fig. 3 shows X R D p a t t e r n s after heat treatments. These results, a n d further characterization b y D S C (fig. 4), showed t h a t lattice collapse h a d occurred. This was confirmed by 'back-exchange' experim e n t s as can be seen from figs. 5(a) a n d (b). These experiments d e m o n s t r a t e d t h a t 137-Cs could be fixed into A zeolite b y b a r i u m exchange followed by heat treatment. They were used to predetermine the conditions in which zeolite spheres were used to take up b o t h 137-Cs a n d 90-Sr.

Zeolite sphere experiments Fig. 6 shows the effect of (i) volume of 0.01 M CsC1 passed t h r o u g h a c o l u m n (ii) volume of 0.5 M

206

A. D Y E R

AND

K. Y. M I K H A I L

[•I.0 I

f r [ r F

iOOO C

s~

0/0 ~37c~

-

removed by NclClfr~ v11 ~ YA 20O NaBaCs A vA powder 30 - ~AVA v~ IOO~vacuumCv~ VA 15O

v~

20

6

80

SO

200

2SO*C

O0

150

200

I00 ~ I

I

~50

] l

z i.-

~

~o -VJ

!

'if

~

IIA v .i VA

v 200" lSO

.f

M Bee12used

[] o.s

i0o

i

l~

*mNo

200

back~xch~n~eL~'j

v

ISO v 200 v

i

2

VA

0

10

IOOOC 150 2OO

I

o

r r r r r

ISOv 200 v

31S DEGREES2#CuK=

70

3OO~

FIG. 5. (a) Percentage 137-Cs removed from N a Ba Cs A powders prepared with both 1.0 M and 0.5 M BaCI 2 solution and preheated to various temperatures. (b) Percentage Na back exchanged from the same samples.

lOOv

i -

5O

20

210

FIG. 3. Effect of heat and vacuum treatment on Na Ba Cs A powders as shown by X R D patterns.

3 -200~ EXO

J

6-2OOOC I 0-2

~E

J

~

O 0

3- 2OO~247

/

J

f

6 -2OO~ 1 0-200~

v

V

EN DO

0

I

I

I

I

IO0

200

300

400

yoc

FIG. 4. DSC traces of Na Ba Cs A powders after heat and vacuum treatments.

5 O0

ZEOLITE TREATMENT 40

A 30

OF RADIOACTIVE WASTE

207

-pH

"/

Table

11.4

Typical

IV.

column o p e r a t i n g

conditions

lnfluent solutions (a) l a b e l l e d O.01H CsCI, ~

B

Or c a r r i e r

20

lP)

deionized

free water

c a r r i e r Tree 13T-Ca

90-Sr wash

(c) 30 ml of O.SH Sac12 IO pH

[,fluent

11.4

l n f l u e n t f l o w rate O

06

3

I0

6 3

106

3

temperature

ambient

Wt. of z e o l i t e spheres

3 9

Column d f m e n s i o n

pH

9.5

30 B

1 3 7 Cs

removed

(i.d.)

6,15 mm

Free column v o l u m e

40

o/~

25 ml hr -1 (12 ml hr "1 for BaC121

by 2 0

Barium I0

106

3

108

3

106

3

1.5 ml

little effect and that treatment at 200 ~ caused lattice collapse. It was decided to use 300~ without vacuum to treat zeolites from the column experiments prior to containment (fig. 7). The breakthrough curves observed are shown in fig. 8. The free column volumes were the void space observed in the bed of 3,0 g of zeolite pellets packed into the column of internal diameter quoted. T h a t of 137-Cs is satisfactory but those for 90-Sr not so. In fact most of the activity detected in the effluent was later shown to be 90-Y and further work is in progress on this system.

Leach experiments 30 A

pH

(i) Analyses of leached samples showed that leaching with pond and sea waters had little effect on the samples (Table V) as judged by the SiO2/ A120 3 ratio. The calcined sample seemed to show

6.0

B

2O

10 Table

V.

Composition

of the

Zeolites a f t e r

Pond

Water and

Sea Water T r e a t m e n t

IO 6 3 106 3 ml.of O.S M BoCl2used p e r 9. of

zeolite

I0

6

3 Content.

Z

NaBaCs k

Calcined

NaOaCs A

spheres

FIG. 6. Effect of various parameters on 137-Cs removal from Na Ba Cs A spheres (A = 150, B = 75, and C = 15 mls of 0.01 M CsCI passed through the bed of zeolite).

BaC12 solution subsequently passed (iii) variation in p H of 0.01 M CsC1. It can be seen that p H variation has little effect and that the replacement of 137-Cs is a function of BaC12 treatment as anticipated. The conditions used to prepare samples for leach testing are summarized in Table IV. Heat and vacuum treatments of the spheres monitored by D S C showed that again vacuum had

kl2O 3

21.5

23.2

22.8

21.7

SiO 2

33.6

36,2

35,2

33.5

K20

11.47

oao

15.5

0,65 16.9

1.06 16.6

0.81 15.9

cao

1.23

1.41

1.32

1.30

Ts 2

0.16

0.10

0.16

0.17 0.3S

Fe203

0.00

0.Sk

0.43

H90

Q.11

1.35

1,21

1.20

NaO

3.27

5.57

3.05

3.55

2.66

2.65

2.62

2.63

Ss

Samples a, c treated w i t h pond w a t e r

Samples b. d t r e a t e d w i t h sea w a t e r

208

A. D Y E R A N D K. Y. M I K H A I L

3 O0 ~ EXO

T l

AE

ENDO

t 0

I

I O0

I

200

I

300

400

500

ToC

FIG. 7. DSC traces of Na Ba Cs A spheres (30~ breakthrough) after various heat treatments.

I oo

o/o

0 90Sr

0 c a r r i e r free

5O

BREAKTHROUGH POND WATER 50

0

4O

0 . 0 1 M CsCI

I

2

3

137Ct

carv;er

4

5

FREE C O L U M N V O L U M E

free

CUMULATIVE FRACT I ON 137 C$ LEAC HEO X 10 ~

6

X 103

Fro. 8. Breakthrough curves for 137-Cs and 90-Sr/Y on

NaA spheres.

little or n o ion u p t a k e as evidenced by the lack of increase in N a a n d M g c o n t e n t with sea water treatment. (ii) Fig. 9 summarizes the sea water leaches of 137-Cs-containing species. T h e effect of calcination is seen clearly a n d c o n t a i n m e n t also reduces the leach rate. P o n d water treatments (fig. 10) follow a similar p a t t e r n except t h a t the leach from calcined

20

I 0

~~4 2

4

6,

8

tO

~s Fro. 9. Sea-water leach of Na Ba Cs* A spheres (75 ~. breakthrough) and zeolite/cement composites (prepared using 0.01 M CsC1). (1) Uncalcined zeolite. (2) Calcined zeolite. (3) Calcined zeolite/cement. (4) Uncalcined zeolite/ cement.

209

Z E O L I T E T R E A T M E N T O F R A D I O A C T I V E WASTE

2

40

3O

CUMULATIVE FRACTION 137C$ LEACHED 20 X 10 . 2

/

IO

0

2

4

6

8

I0

or~9-Ays

FIG. 10. Pond water leach ofNa Ba Cs* A spheres (75% breakthrough) and zeolite/cement composites (prepared using 0.01 CsC1). (1) Uncalcined zeolite. (2) Calcined zeolite. (3) Calcined zeolite/cement. (4) Uncalcined zeolite/ cement.

83 ~ Cs

CUMULATIVE FRACTION 137 LEACHED 6 X I O -2

2

0

4

0

I

I

2

4

I 6

I 8

i

I0

DT6~g

N a B a C s , A spheres is quite high. This may reflect the well-known solubility of zeolite A at high pH as reported by Guth et al. (1980) although the analyses and other evidence presented here do not show any large solubility effects, presumably the appearance of tracer 137-Cs is a good indication of a small solubility. Deionized water leaches (fig. 11) show that release of activity is proceeding beyond 3 months except in the calcined-contained sample which has reached a form of equilibrium. These results confirm recent criticisms by Roy (1981) of the 'absurdity' of leach data arising from deionized water treatment. Fig. 12 shows the similar effects observed on samples prepared under carrier free conditions. Despite the poor breakthrough curve observed for 90-Sr uptake containment and leach tests were made on A containing 90-Sr. The results are summarized in fig. 13 and show a significantly lower leach rate than those for 137-Cs. The effect of

4 CUMULATIVE FRAC T I O N 137Cs LEACHED 2 X I 0 "2

0

2

4

6

8

IO

4 bAYS

FIG. 1l. Deionized water leach of Na Ba Cs* A spheres (75 % breakthrough) and zeolite/cement composites (prepared using 0.01 CsC1).(1) Uncalcined zeolite. (2) Calcined zeolite. (3) Calcined zeolite/cement. (4) Uncalcined zeolite/ cement.

FIG. 12. Leaches of Na Ba Cs* A sphere (30~ breakthrough) and zeolite/cement composites (prepared from 'carrier free' solution). (1) Uncalcined zeolite/cement sea water leach. (2) Calcined zeolite/cement sea water leach. (3) Uncalcined zeolite/cement deionized water leach. (4) Calcined zeolite/cement deionized water leach. (5) Uncalcined zeolite/cement pond water leach. (6) Calcined zeolite/cement pond water leach.

barium treatment with a calcination step is again evident. Interpretation of leach curves The interpretation of curves such as those in fig. 9 often has been based upon its elucidation as a fast process followed by a slow process. This approach can be misleading and any attempt to apply this reasoning to the results in fig. 11 will clearly demonstrate this. One problem seems to arise from the calculation of 'diffusion' rates from cement composites, using the dimensions of the composite to feed into a suitable equation (Matsuzuru and Ito, 1978). We would argue that this is inappropriate if the rate controlling step is from the zeolite particle. The diffusion of species from zeolites has been studied widely (e.g. Dyer and Townsend, 1973) and their kinetics interpreted on the basis of the application of the C a r m a n - H a u l equation describing the movement of a species from a sphere of known volume into a limited, well-stirred, volume. With this premise in mind certain of the results herein were analysed by a computer program written for solutions to the C a r m a n - H a u l equation and they are presented in fig. 14. The results show that, in one of the cases examined, the diffusion coefficients are little affected by the presence of cement as expected if the

210

A. D Y E R A N D K. Y. M I K H A I L

9

I00

"M

leach rate of the isotopes from b o t h the zeolite a n d equivalent zeolite/cement composites. Acknowledgements. Laporte Industries are thanked for the gift of the zeolites used in the study. Mr G. Craig is thanked for help with electron probe microanalysis and one of us (KYM) gratefully acknowledges financial support from the Ministry of Higher Education, Iraq.

CUMULATIVE FRACT ION 90 S r / Y LEACHED X I O 13 60

40

REFERENCES

20

3%

O

2

4

6

8

IO

D.r'~-AuS

FIG. 13. Leaches of Na Ba Sr* A (75% breakthrough) zeolite/cement composites (prepared from 'carrier free' solution). (1) Uncalcined sea water leach. (2) Calcined sea water leach. (3) Calcined deionized water leach. X uncalcined de-ionized water leach; v uncalcined pond water leach; [] calcined pond water leach.

rate d e t e r m i n i n g step was release from the zeolite alone. I n the other case a discrepancy is n o t e d which can be explained as arising from a cement/ zeolite interaction which does not, necessarily, m e a n t h a t the premise is incorrect. F u r t h e r work in this area is in progress. Conclusion The t r e a t m e n t of zeolite A c o n t a i n i n g b o t h 137-Cs a n d 90-Sr by a b a r i u m solution followed by a calcination at 300~ successfully reduces the

Collins, E. D., Campbell, D. O., King, L. J., and Knaues, J. B. (1982) A.LCh.E. Syrup. Series, No. 213, 78, 9-15. Dyer, A., Celler, W. Z., and Shute, M. (1971) ACS Symposium Series 101, 436-42. - - G e t t i n s , R. B., and Brown, J. G. (1970) J. Inorg. Nucl. Chem. 32, 2389 94. and Townsend, R. P. (1973) Ibid. 35, 3001. Goto, Y., Matsuzawa, J., and Matsuda, S. (1982) Developments in Sedimentology, 35, 789 980. Guth, J. L., Caullet, P., and Wey, R. (1980) In Proe. 5th Int. Conf. on Zeolites (L. V. C. Rees, ed.), Heyden and Son, 30-9. Hespe, E. D. (1971) Atomic Energy Review, 9, 195 207. Matsuzuru, H., and Ito, A. (1978) Health Physics, 34, 643 8. Mercer, B. W., and Ames, L. L. (1978) In Natural Zeolites: Occurrence, Properties, Uses (L. B. Sand and F. A. Mumpton, eds.), Pergamon Press, 451 62. Roy, Rustum (1981) Ber. Kernforschungsanlage Juelich (Conf.) Juel-Conf-42, (Vol. 42) Proc. Int. Semin. Chem. Process Eng. High-Level Liquid Waste Solidification, 576-602. Siemens, D. H., Knowlton, D. E., and Shupe, M. W. (1982) A.1.Ch.E. Syrup. Series, No. 213, 78, 41 4. Taylor, M. A., and Kuwairi, A. (1978) Cement and Concrete Res. 8, 491-500.

12cement+

zeolite

13-

zeolite /A -

---'--I

cement + zeolite

ze olite

LOG

[~

I0

15 16

/A/~/A 9

/,4

A

sea

I

A

B

water

de~onlzed water

B

FIG. 14. Diffusion coefficients calculated by Carman-Haul equation for (A) calcined Na Ba Cs* A and (B) calcined natural zeolite loaded with 90-Sr.