Journal of Radloanalytieal and Nuclear Chemistry, Articles, Vol. 159, No. 1 (1992) 47-62

THE REMOVAL OF THORIUM FROM AQUEOUS SOLUTIONS USING ZEOLITES A. DYER, L. C. JOZEFOWlCZ

Cockcroft Buildlng,Department of Chemistry ~td Applied Chemistry, University of Salfor~ Salford M5 4WT (UK)

(Received August 2, 1991)

The ability of zeolites to remove thorium-232 from naturally radioactive thorium nitrate solutions was studied by ion exchange, adsorption and surface phenomena. The best zeolites were a clinoptilolite from Mudhills, California, USA and a mordenite rich tuff from Eastgate, Nevada, USA. As the concentration of thorium in solution was increased the mechanism of thorium removal switches from that of partial ion-exchangu to a surface precipitation. This was confirmed by electron probe microanalysis (EPMA) in conjunction with XRD and Energy Disperaive X-ray analysis (EDX). A Langmuir-type of adsorption occurred with the thorium-232 daughter isotopes at the same time as partial exchange/precipitation of the thorium parent which is in regular equilibrium with its natural daughters. Some experiments with 234Th were carried out to confirm this hypothesis as well as column experiments to confirm the usefulness of the Th removal.

Introduction

Zeolites are aluminosilicate cation exchangers which have been widely studied for their use in aqueous nuclear waste treatment and have been proven in this capacity by clean-up from the Three Mile Island accident and in the Site Ion Exchange Plant (SIXEP) at British Nuclear Fuels site at Sellafie!d, UK.1,2 Thorium-232 appears as an impurity in the production of nuclear fuel and can be removed by solvent extraction or by the use of organic resins. 3 This work attempts to quantify the use of zcolites as an alternative to these processes. Both natural and synthetic zeolites have been used in a programme of work designed as follows: (1) initial experiments involving simple distribution coefficient measurements, kinetic measuremen~ to establish equilibrium conditions and the effect of completing cations, (2) isotherm experiments on the best candidate materials chosen on the basis of the initial experiments using homo ionic zeolites and binary cation mixtures, (3) Langmuir plots of the paren t and daughter isotopes constructed using 4 qeq/Ceq = 1/Kqm~,~+ Ce4qmax

(1)

Elsevier Sequoia S. A. Lausanne Akaddmlai Kiad6, Budapest

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF THORIUM where q~q, Ceq, K and qmax respectively represent equilibrium species in the solid, concentration of species in the equilibrium solution, a Langmuirian type constant, and the m a x i m u m equilibrium concentration in the solid, (4) fixed bed column work on the ' b e s t ' zeolites using various effluents, (5) surface analyses ( E P M A / E D X ) o f appropriate ' s p e n t ' zeolites from column experiments (together with XRD).

Experimental

Initial experiments The full list o f zeolites studied and their origins is in Table 1. The natural zeolites were mechanically ground in a pestle and mortar and sieved to a particle-size o f between 106 - 150 p.m. They were then sedimented in a 2 m column filled with deionized water. The fraction collected in the base o f the column after 30 Table t Zeolites and related materials used in preliminary work Zeolite Natural Analcime Brewsterite Clinoptil.olite Clinoptilolite (1010A) Chabazite Chabazite Mordenite**/ Clinoptilolite Phillipsite Palagonite Tuff Synthetic 4A 5A 13X NaY Zeolon 900 KL

Description (Source) Wikienp, Nevada Strontian, Scotland Mud Hills, California Death Valley Junction, California Bowie, Arizona Skye, Scotland Eastgate, Nevada (Eastgate 2020) Pine Valley, Nevada Skye, Scotland with binder with binder powder powder synthetic mordenite synthetic perlialite

Supplier Minerals Research USA *

Phelps Dodge, USA AnacondaMinerals, USA Letcher Assoc, USA * Anaconda Minerals, USA Minerals Research, USA * Laporte Inorganics, UK Laporte Inorganics, UK Laporte Iuorganics, UK Laporte Inorganies, UK Norton Co, UK Laporte Inorganics, UK

*Collected by Zeolite Research Group, Universily of Salford - supplied by O. M. WILSON. **This mixture will be referred to as 'Eastgate' for convenience. minutes was removed and oven dried (110 ~ for 12 hours. The zcolites were screened for Th uptake and percentage removal o f 13-activity from 23Zl'h daughter isotopes 228Ac and 228Ra. 48

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF TttORIUM

Thorium analyses was by colourimetry using Morin reagent (see later) and radioisotope analysis by Cerenkov counting. The tests were K~ measurements with one week being allowed for equilibration. Measurements were on two concentrations of thorium nitrate (0.025 and 0.0025M). Those zeolites with high K d values from those initial tests ( > 103) were subjected to a further series of trial experiments to test uptakes in the presence of Na + and K + ( 1 0 4 and 102 ppm respectively). From all these preliminary tests Mud Hills clinoptilolite and the Eastgate mixture of mordenite/clinoptilolite were chosen for further work and it is with these two materials that the rest of this work is concerned. Isotherms A set of batch-uptake experiments were conducted on 0.1 g portions of zeolite, to elucidate the maximum equilibration period for the uptake of thorium from 10 ml aqueous thorium nitrate solutions of various concentrations. From these preliminary experiments, an equilibration period of 2 weeks was deemed suitable. The homo-ionic sodium- and potassium-forms of clinoptilolite and Eastgate were prepared by refluxing with the respective solution of sodium or potassium nitrate solution (AnataR, 1M, 100 ml) for 20 hours. The zeolite was filtered, washed and the reflux carried out with fresh nitrate solution. In this way, 5 successive exchanges were conducted. After the final reflux, the solid was filtered, washed with deionixed water and dried in an over at 110 ~ for 12 hours. The solid was then transferred to a d e s i c c a t o r , c o n t a i n i n g s a t u r a t e d sodium chloride solution, for a 2 - w e e k water-equiiibration period, prior to use in isotherm experiments. Binary mixtures of thorium nitrate/potassium nitrate, and thorium nitrate/sodium nitrate were prepared using AnalaR reagents. (Thorium nitrate contained 232Th in secular equilibrium with its daughters, and was supplied by Koch-Light. Potassium and sodium nitrates were supplied by BDH). The total normalities of these stock solutions were set at 0.2N, 0.02N and 0.002N. Solid to liquid proportions were set at 0.2 g to 20 ml. The samples were agitated for 2 weeks at 18 ~ centrifuged, and a portion of the supernatant was taken for thorium analysis by colourimetry. The colourimetric method used Morin as the reagent, using the absorbance at 410 nm on a Pye-b_Jnicam SP1800 UV spectrometer in 1.0 cm quartz glass cuvette at pH 2.0. These absorbances were compared with a linear calibration curve of absorbance 'vs.' standard concentration of thorium. This approach was adapted from the method of PERKINS et al. s It proved superior to di-sodium salt EDTA titmtions, using xylenol orange indicator (at a pH of 2.6) and the Morin method was sensitive to concentrations below 3 ppm Th. Isotherms are presented in Figs 1, 2 and 3. A,:, As are the equivalent fractions of Th 4+ in solid and solution phases respectively. Some samples were chosen to assess 49

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF THORIUM A

~I 0.6

0.2

f

i

I 0.02

i

I -% i 0.04 0.06

,

I

,

~

0.08

0.10

J

y.

0.12

f

0.14

I i.,. 0.16 Ac

Fig. 1. K/Th isotherm on elinoptilolite (0.01 N)

~.

0.20

0.15

0.10

0.05

0

I 0

0.02

0.04

0.06

0.08

i

I i I 0.10 0.12

Fig. 2. K/Th isotherm on clinoptilolite (0.001 N)

i 0.14

I 0.16 Ac

A 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

I 0.5

i 1.0

] 1.5

I 2.0

1, ~. 2.5 Ac

Fig. 3. Na/Th isotherm on clinoptilolite (0.1 N)

50

A. DYER, L. C. JOZEFOWICZ: THE R E M O V A L OF T H O R I U M

the reversibility of the thorium-exchange and also the repeated-exchange capacity after the method of DYER, ENAMY and TOWNSEND. 6

The production of Langmuir-plots A portion of the equilibrated isotherm supernatants for clinoptilolite were analysed for thorium by colourimetry as before to construct Langmuir-type plots (Eastgate samples were not used in these experiments because of its mixed zeolite content). The solutions were also analysed by the Cerenkov radiochemical counting technique 7 to quantitatively analyse the behaviour of the natural thorium isotope daughters, 228Ac and 228Ra. 5.0 ml of the solution to be analysed was placed in a

"7

A 35-30-25

u

20 15 10 5 0

0

100

200

300

I 500

400

Ceq , m g . [ -1

Fig. 4. Langmuir plot 232Th on K clinoptilolite &

1.0_

0.8-

Q60.~" 02~1 0

I 0.2

I

I 0.4

i

I 0.6

I

t 0.8

Ceq,orbitrary

i

I ~ 1.0 units

Fig. 5. Langmuir plot 228Ae, 228Ra on K clinoptilolite (0.1 N thorium nitrate) Normalised x axis

51

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF THORIUM

low-background, polyethylene counting vial and 5.0 ml of deionized water was added. The vial was placed in a Beckman LS-233 Automatic Liquid Scintillation Counter and counted twice for 100 minutes in the tritium channel of the counter. A background count from a vial containing 10.0 ml deionized water was subtracted from the result J

g 0.30 13"

g 0.25

(o

O.20 0.15 0.10 0.05 0

0.2

0.4

0.6

0.8

1.0

Ceq, a r b i t r a r y units

Fig. 6. Langmuir plot 2Z8Ac, 228Ra on Na clinoptilolite (0.1 N thorium nitrate) Normalised x-axis

in every case. The count rate did not decrease appreciably over the time period of the experiments. Quenching was not apparent from the test-results of experiments that were conducted using stock solutions of thorium nitrate and solutions containing varying concentrations of possible quencher ions (i.e. K +, Na + and AP +, the latter which was thought to be leached from the zeolite by the mildly acidic thorium solutions). The behaviour of these daughter isotopes, considered from a Langmuirian-type of view, are shown in Figs 4, 5 and 6. It is important to note that these resuts were obtained in conjunction with the isotherm results for pure 232Th.

Column experiments A pyre x glass column was made from a 10.0 cm length of (1.0 cm inside diameter) precision glass tubing. A glass condenser jacket was fitted around the tube and a medium-course glass sinter was fixed at one end of the glass tuber. The column experiment arrangement is shown in Fig. 7. The volume of the column and all the connecting-tubing was determined by filling the void spaces with deionized water and releasing this into a measurisng cylinder. Zeolite (3.0 g) was slurried into the column with deionized water and deionized water was pumped through the zeolite bed overnight to ensure a compact bed. 52

A. DYER, L. C. JOZEFOWICZ: THE REMOVAL OF THORIUM

CoLumn

war

~

Cramp ~>:~

J,

:~"

Water in /

out

Zeolite Sinter

Fraction coLLector

]

\ Reservoir

--~. Direction of ftow

Fig. 7. Arrangement

for column

experiments

&

1.2LA

1.0 0.8 0.6 0.4 0.2 0

100

200

300

400

500

600

700

Eluted volume, mL

Fig. 8. Breakthrough curve,

thorium on

Eastgate

Thorium nitrate solution was prepared (1000 ppm Th) and was placed in the column reservoir. A Gilson Minipuls 2, 4-channel high precision peristaltic pump was used to pump the solution into the column at a flow-rate of 20 ml per hour. 5.0 cm 3 of effluent was collected per fraction and each sample was analysed for Th by colourimetry. The dead volume of the column piping was subtacted from the results, which are shown in Figs 8 and 9. In certain cases, the influents to the columns were changed after total thorium breakthrough was achieved, in order to obtain information 53

A. DYER, L. C. JOZEFOWICZ: THE REMOVAL OF THORIUM

A 1.0

u

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A 9

0.8 0.6 0.1+

./

0.2

0

I

1

I

I

I

100

200

300

400

500

E t u t e d v o l u m e , mt

Fig. 9. Breakthrough curve, thorium on clinoptilollte

J

41 Inftuent :

r

270mr HCt I

3

2 -

I

11+5mt deionized

1 ii.~,w"~'u

0~.._.~--=,-o- t 0

200

1 400

J

u~

I 600

275mt KNO3

~ ~ap..:~ r- 9 . . . . i 800

1000

1200

~,. 1l+00

E l u t e d v o l u m e , mt

Fig. 10. Breakthrough curve, Eastgate with change of influent solutions

about the type o f thorium uptake involved. For example, for the Eastgate column, after breakthrough, the influent was changed successively to: (a) 145 ml of deionized water, i.e. until no more Th could be eluted, (b) potassium nitrate (1M, 275 ml), and (c) hydrochloric acid (1%, 270 ml). The results from these experiments are shown in Fig. 10. C, Co are the activities per ml -t of fraction collected and initial solution respectively. Some experiments were also conducted in order to assess the suitability of clinoptilolite to remove thorium from 0.5M nitric acid solutions. Two such experiments were performed. A 234Th tracer solution was prepared by dissolving uraniuim ore concentrate (UOC) in nitric acid (6M) and radiochemically solvent-extracting the 2Z4Th from the 2~U decay chain equilibrium solution with 30% tert-butyl phosphate, in odourless kerosene. With suitable evaporation and dilution, a solution of approximately 54

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF THORIUM

A

o 1.2

L) b

End of thorium influent

/

1.0 0.8

F ~ 5 0 ml deionised woter influent

~

o o

o o

0.6 0.4 I M NaNO 3 in~luent

0.2

r

200

,

400

80O IO00Etuted v o l u m e , ml

600

Fig. 11. Breakthrough curve, clinoptilolite, ~4Th in 0.5M nitric acid with changes of influence solutions radiometric method A

u

14f 12 I 0

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.8 f

0

-

~

=

0

100

200

l 300

u

9

9

9

l

J

I 500

400

J

I I,600

Eluted v o l u m e , ml Fig. 12. Breakthrough curve, clinoptilolite, Th (1000 pm in 0.5 nitric acid) ' colourimetric method

1000 ppm z-~4Thwas obtained in 0.5M nitric acid. This solution was pumped through a clinoptilolite column, and after breakthrough of thorium the influent was changed to: (a) deionized water (50 ml) and (b) sodium nitrate solution (1M, 400 ml). The fractions of effluent were analysed for Z-~Th by Cerenkov counting. The results are in Fig. 11. The second experiment of this type employed an influent of approximately 100 ppm z~2Th in 0.5M nitric acid and the effluent was analysed colourimetrically using a slightly different procedure to that mentioned previously. To obtain accurate results for thorium concentrations in such acidic media, the indicator chosen was Thoron after the 55

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF THORIUM

method of FLETCHER and MILKEY. 8 The pH of the sample solutions was always between 0.4 and 1.2, which is the pH range over which the thoron complex is colour-stable, and the absorbance was read at 545 nm in 1.0 cm quartz glass cuvettes. The results of this experiment were compared to those obtained for the 234Th experiment, in order to verify the two different methods of analysis (compare Fig. 12 to Fig. 11).

XRD and surface studies Samples of the thorium-treated zeolites taken from column experiments, with 1000 ppm Th influent, were washed with deionized water and dried in a desiccator. These samples were then taken for XRD analysis along with a sample of the natural untreated zeolite.

Eastgate o

100

-

O O c ca

-g

0

0

0

loL

L .AL

. . . .

58,690 >

< 2000

2 0 , degrees Fig. 13. XRD pattern for Eastgate - as received

'IlSe X-ray powder diffractcograms were recorded on a Siemens D500 X-ray dfffractometer, with facilities for background subtraction and overlay (see Figs 13 - 15). Electron probe microanalysis was conducted on a Jeol 733 Electron Probe Microanalyser, which was equipped with a Link 860 analytical energy dispersive X-ray detector. The electron beam condition used to obtain the X-ray spectra was 15 kV with a probe current of 3 9 10.9 A. A thorium-treated clinoptilolite was sent for energy dispersive X-ray analysis and the results may be seen in Figs 17 and 18. The second figure shows the spectrum of 56

A. DYER, L. C. JOZEFOWICZ: THE REMOVALOF THORIUM Th eastgate 100' -O0

>

L

5.240

57,620 >

2e, degrees Fig. 14. XRD pattern for Eastgale after column experiments it

Ctinoptiioiite

(Mudhifls)

Ca ~a .>

g:

0 <

~

I

i

1

I

5.000

I

I

I

1 I 55.000>

Fig. 15. XRD pattern for clinoptilolite - as received 20, degrees one o f the bright areas on the surface of a particle as shown by the SEM scan o f the Electron Probe. The E D X spectrum was also taken of an acid-washed sample of Eastgate (Fig. 19).

Discussion Figures 1 - 2 show that K clinoptilolite has a strong preference for thorium at low concentrations with about 15 - 20% o f the available cation sites being taken up. At higher concentrations (e.g. Na/Th at 0.1N Fig. 3) this preference was not shown. 57

A. DYER, L. C. JOZEFOWICZ: THE REMOVAL OF THORIUM A

Th Ctinoptitotite

~"~~176 I

.c TO

0 < 5.000

i

I

I

1

I

I

I

I

i

L

55.000> 28 , d e g r e e s

Fig. 16. XRD pattern for cUnoptilolite after column experiments

Si

0.2

5.3 FR87

Fig. 17. EDX of clinoptilolite from column experiments (unwashed)

The Langmuir isotherms for clinoptflolite were non linear for thorium (Fig. 4) but those for the 228Ac, 228Ra daughters were closer to those representative of an adsorption process (Figs 5 and 6). From the isotherm data for dinoptilolite, and analysis of the exchange-solutions, it can be suggested that ion-exchange was responsible for only part of the thorium uptake. 58

A. DYER, L C. JOZEFOWICZ: THE REMOVAL OF THORIUM Th 5i

/l 0,2

I

l

I 5.3 FR 87

Fig. 18. EDX of clinoptilolite from column experiments (unwashed). 'Bright' area on zeolite particle

Si

0.2

5.3 FR 87

Fig. 19. EDX of Eastgate from column experiments (acid washed column)

59

A. DYER, L. C..1OZEFOWlCZ: THE REMOVALOF THORIUM At low concentrations of thorium, exchange was probably occurring as Th(OH)~ + or ThO 2+, and at higher concentrations, it was more likely that surface precipitation of the thorium, as hydrolysis products, was occurring. The fact that the 13-emitter daughters o f 23zI'h were obeying Langmuirian-type adsorption at the same time as the sorption of thorium itself was occurring onto clinoptilolite, suggests that binary Table 2 Chemical composition of zeolites*

Clinoptilolite Compound Oxide SiO2 AI203 Na20 K20 CaO Fe203 MgO H20**

Mordenite'/Clinoptilolite

(Mudhills)

(Eastgate)

% 64.23 10.61 2.71 2.84 2.42 0.72 0.16 14.11

% 57.43 13.78 1.49 2.46 5.45 1.97 2.45 13.37

*From suppliers data sheet. **From thermogravimetric analysis to 600 ~ XRD analysis confirmed that Eastgate was a mixture of mordenite and clinoptilolite.

ion-exchange was impossible in the systems used and therefore only limited information can be deduced from the isothersm. Column experiments show that the Eastgate zeolite has a larger capacity for the uptake of thorium compared to that of clinoptilolite, and that breakthrough is more gradual. Hence, the parameter of eluted volume at 50% breakthrough concentration for Eastgate is about 375 ml compared to about 70 ml for clinoptilolite. The similarity in the results obtained by the two different methods of analysis of the effluent in the column experiments shows that the counting method for z-~4Th and the colourimetric analysis for 232Th solutions in nitric acid are comparable (see Figs 11 and 12). Interestingly, the clinoptilolite columns exposed to solutions of thorium in nitric acid removed more thorium from solution that did those exposed to solutions which did not contain nitric acid. This was thought to be due to the acid dissolving impurities and some .amorpous material in the clinoptilolite, thus making accessibility for the thorium to the pore structure easier. The breakthrough volume parameter, at 50% breakthrough, for the acid solution increased to over 100 ml (see Figs 9 and 12). 60

A. DYER, L. C. JOZEFOWICZ: THE REMOVAL OF THORIUM

Figures 10 and 11 show the results from columns treated with various solutions to try to remove thorium after loading. The results show that, for the case of clinoptilolite and acidic thorium solution (Fig. 11), little of the thorium was removed by water, thus indicating that some ion-exchange had occurred and that thorium was in an insoluble form. Very little thorium was removed by exchange with Na +. The Eastgate column (Fig. 10) shows similar results, however, acid treatment removed most of the thorium from the zeolite column. Figures 17 to 19 show the EDX analysis of the two zeolites from columns loaded with Th and in the case of Fig. 19, of an acid-washed Eastgate column. An overall EMPA scan of the clinoptilolite revealed that a small percentage of the thorium was on the surface of the zeolite in localised areas (Fig. 18). However, these localised areas, which showed up as "bright areas" on the EPMA scan, were found to be areas of relatively very high, thorium concentration (Fig. 18). The Eastgate column sample had been acid-washed, and in agreement with the column results in Fig. 10, only a negligible amount of thorium was left on the surface (Fig. 19). XRD data for Eastgate shows that the crystallinity of the zeolite was mostly retained after treatment with normal thorium solutions (compare Figs 13 and 15). The increase of intensity of a peak (A) at a 20 value of 43.15 ~ corresponding to a d-spacing of 1.88 ~,, was close to those observed in the literature for thorium oxide peaks. 9 Figure 13 characterised the Eastgate zeolite as being a mixed zeolite of good crystallinity. The major peaks correspond to those found in the powder diffractograms of mordenite and clinoptilolite. 1~ Figures 14 and 16 show the X-ray powder diffractograms for clinoptilolite and a thorium-treated sample respectively. It can be seen that some of the crystallinity of the zeolite was lost after the thorium-treatment. Three new peaks (A, B and C) appeared at 20 values of 22.40 ~ 30.24 ~ and 48.32 ~ respectively. These all corresponded to thoria peaks. The loss of crystallinity was more severe than with the Eastgate sample. No more information pertaining to thorium could be obtained from these diffractograms as the analysis for heavy metals by this method is difficult due to beam scattering and the fact that such species a s Th(on)4 are amorphous to X-rays.

Conclusions The Mudhills clinoptilolite used in this work had an effective pore size of 4 ~ thus, provided that the thorium species removed from the solution had a diameter of less than approximately 4 .~, ion-exchange would be expected to occur. At pH values of less than 3, one would expect the thorium ion to be in the 4 + state, Th 4+, as the pH 61

A. DYER, L C. JOZEFOWICZ:THE REMOVALOF THORIUM increases and the solutions become less acidic, one expects the presence of hydrolysis products, some of which may be polymeric. The Th - O ionic bond length is 2.54 .~.12 Hydrolysis products such as Th(OH)~ + and ThO 2+ are likely in this case as the presence of terminating hydroxyl groups at the zeolite surface 13 render it rather alkaline

( p n ~ 12). The Eastgate zeolite was superior to cliinoptilolite in ability to remove thorium from solution. It was found to be a mixture of mordenite and clinoptilolite. Such Eastgate deposits exist but arc not common and Eastgate zeolites usually tend to be chabazite-erionite mixtures. 10 The mechanisms of thorium deposition has been found to be ion-exchange in low concentration thorium solutions and surface precipitation for higher thorium concentrations with respect to natural zeolites of high silica/alumina ratio. The mechanism of zeolite surface precipitation has been found to be in agreement with earlier suggestions by B O L T O N 14 and more recent work with clays. 15 Nevertheless both zeolites provide excellent scavengers for Th species from aqueous solutions. Repeated treatments showed that the Eastgate material especially had a very high capacity for continued thorium removal by a precipitation method; with the daughter products also being retained on the zeolite when 232Th was being scavenged. lit

Sincere thanks are due to Dr. H. Enamy for helpfuldiscussion,D. Park (Shell ThorntonResearch Centre) for EPMA/EDX measurementsand to O. M. WILSON for zeolite samples.

Financialsupport and technicalhelp (Dr. H. ECCLES and Dr. J. F. ELLIS) from BNF Ltd, Springfields is gratefullyacknowledged. References 1. c. G. HtTZ, K. J. HOFSTETI'ER,in: The Mile Island Accident,L. M. TOTH,A. P. MALINANKAS,G. R. EIDAM, H. M. BURTON(Eds), ACS Symp. Ser., 293 (1986) p. 250. 2. M. HOWDEN,J. PILOT, in: Ion-ExchangeTechnology,D. NADEN, M. STREAT,(Eds), Ellis Horwood, UK, 1984, p. 698. 3. H. ECCLES, in: Ion-ExchangeTechnology,D. NADEN, M. STREAT,(Eds), Ellis Horwood, UK, 1984, p. 698. 4. I. YOSI-I/NOG,KaknrikenKenkyuHokoku (Tohoko Diagaku),21 (1988)71. 5. R. W. PERKINS, D. R. KALKWARF,Anal. Chem., 28 (1956) 1989. 6. A. DYER, H. ENAMY, R. P. TOWNSEND,Sep. Sci. Teehnol.,16 (1981) 173. 7. A. DYER, Liquid Sci/ltillationCountingPractice Heyden,London, 1980. 8. M. H. FLETCHER, R. G. MILKEY, Anal. Chem., 28 (1956) 1402. 9. InorganicPhases (JPCDS) Joint Committee on Powder DiffractionStandards, No. 35-103, 1984. 10. G. GOTTARDI,E. GALLI,Natural Zeolites,Springer-Verlag,1985, p. 208. 11. Phelps Dodge Zeolite Company,MudhillsClinoptiloliteData Sheet. 12. G. T. SEABORG, J. J. KATZ, The Actinide Elements,ist ed., McGraw Hill, 1954, p. 787. 13. D. FREUDE, M. HUNGER, H. PFEIFER, W. SCHWEIGER,Chem. Phys. Letts., 128 (1986) 62. 14. A. P. BOLTON, in: ExperimentalMethods in CatalyticResearch, Vol. 2, Eds: R. B. ANDERSON, P. T. DAWSON, (Eds), Academic Press, New York, 1976, p. 18. 15. C. A. SIKALIDIS,C. ALEXIADES,P. MISAELIDES,Toxicol. Environ,Chem., 20-21, (1989) 175. 62