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.