Revi,ta Mexicana de Física 38, Suplemento 1 (1992) 237-2.1

Sorption of radioactive material on zeolitesp, BOSCII

Instituto Nacional de Investigaciones NuclearesApartado postal 18-1027, 11801 México, D.F., México

Uniuersidad Autónoma Metropolitana.IztapalapaApartado postal 55-532, 09340 México, D.F., México

AND

S. BULBULIANInstituto Nacional de Investigaciones Nucleares

Apartado postal 18-1027, 11801 México, D.F., MéxicoRecibido el 16 de agosto de 1991; aceptado el 30 de enero de 1992

A BSTRACT. Zcolites are crystalline aluminosilicates. They are incorporatcd to detergcnts as wellas te craking eatalysts because oí their sorptive properties. In this work, their utilization inthe scparation oC radioactivc material is discusscd. This review includes topies as d¡frecent a.~the separation of technetium from molybdenum with Y zeolíte and zeolíte barr;ers. In this lastapplica.t.ion the water can pass through the barrier but radionuclides 137Cs, 60Co and 90Sr arerciained.

RESUMEN. Las zeolitas son aluminosilicaios cristalinos cuyas propiedades sortivas justifican suincorporación tanto en los detergentes como en el catalizador ue craqueo. En este trabajo sediscute Sil utilización para retener material radiactivo. Esta revisión abarca temas tan diversoscomo el uso de las zeolitas Y para separar el Tc del Mo o COIllO la construcción de barreras dezcolitas que permitan el paso del agua pero que retengan radionúclidos como el 137es, el 60Co yel 90Sr.

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l. INTRODUCTION

Sorptioll processes are widely used in the chemical, bioehemical and petroleum industries,both for removal of traee impllrities and for bulk separations. These proeesses have beenin operation for many years for air and water treatment, sugar deeolorization, ele.

The primary requirement to develop a sorption separation proeess is a sorbent that hassufficient seleetivity, capacíty and stability to aehieve the required separation over a longtime. In this paper, sorne of the important applieations of zeolites as selective sorbents inradiochemical proeesses are reviewed.

2. ZEOLITES MARKET

Zeolites are a group of hydrated erystalline aluminosilieates that may be either obtainedfrom natllrally oecurring deposits or manllfactured synthetieally by olle or several different

238 P. Bosel! ANOS. BULBULlAN

processes. For instance, a high purity natural erionite has been found in Agua Prieta,Sonora in the north of Mexico, but ZSM5 is a zeolite which has not been found in nature.Synthetic zeolites are produced in 13 countries by at least 39 companies. Their totalworld production capacity is around 1,000,000 tons although actual world synthetic zeoliteproduction is likely to be less than half as very few producers operate at full capacity [1].

Approximately 80 to 90% of total synthetic zeolite consumption is in detergent builders.This application makes use of the ion-exchange properties of synthetic Zeolite A to softenwashing water and therefore the effectiveness of a detergent is increased. Zeolite A, aswell as many other zeolites, is often used as a molecular sieve.

As far as natural zeolites are concemed, the building and construction industry isthought to be the largest consumer, principal uses including lightweight aggregates andbuilding stone (~ 80%). In Mexico clinoptilolite fram Etla is often used in Oaxaca asbuilding materia!.

3. ZEOLITE CHEMICALCOMPOSITION

To describe a Zeolite we have chosen the definition proposed by Smith [21 and by Breck [3]:zeolites are aluminosilicate structures that possess exchangeable cations and reversiblyadsorb and desorb water. A representative empirical formula for a zeolite is written as

M represents the exchangeable cations, generally from the group I or II ions, althoughother metal, non metal, and organic cations mal' also be used to balance the frameworkcharge, and n represents the cation valence. These cations are present either d uringsynthesis or through post-synthesis ion.exchange. Therefore, structurally, the zeolite isa crystalline aluminosilicate with a framework based on an extensive three-dimensionalnetwork of oxygen ions [41.The previous definition, which seems so restrictive, comprisesover 150 synthetic zeolites and 40 natural zeolites.

Theoretical studies of zeolite structures and structure types indicate that onl)' a smallfraction of the possible configurations for polymeric aluminosilieates have been prepared.

4. ION EXCIlANGE

The extra eharge in aluminum tetrahedra has to be balaneed by eations principally Na+and Ca++ and many others in smaller proportions. Those cations confer to zeolites theirion exchanger features.

Because of the unique mieroporous framework strueture of natural zeolites, watermolecules can pass through the channcl~ and pores aJlowing cations present in solulionto be exehanged for cations in the zeo!lte strueture. Each mineral has a eharaeteristicion-exchange seleetivity and eapacity, although this propert)' can be affeeted b)' faetorssueh as pH, solution strength, temperature and the presenee of other ions in solution.This highly seleetive cation exchange eapacity of zeolites makes them particularl)' userulin eontrolling specifie eation levels in water, agrieulture and other areas.

SORPTION OF RADIOACTIVE MATERIAL ON ZEOLITES 239

The cations are not a part of the framework and that is why under the right conditionsthey can be exchanged for other cations. Such an exchange has no effect on the crystalstructure.This ion exchange property of zeolites has been used in radiochemistry processes. Sorne

of them are reviewed in the following pages.

5. SORBENT BARRIERS

Low-level waste management program groups [51, are developing sorbent materials toprevent the migration or radionuclides from these sites. Unlike impermeable barriers, sor-bent barriers allow moisture to pass while selectively sorbing contaminants. This preventsfilling the waste sites with water, referred to as the bathtub effect. The sorptive behaviorof these barriers is similar to the ion-exchange properties that many soils possess forcertain radionuclides. However the degree of sorption of radionuclides is dependent on thetype of soil, the specific radionuclide, and the presence of competing ions. Therefore, theuse of sorptive additives as radionuclide barriers is recommended to prevent radionuclidemigration from the waste site. The experimental work discussed in this paper was directedtoward identifying and evaluating sorbent materials for three radionuclides of major con-cem: 137Cs, 60Co and 90Sr. No single material was found to be effective for sorbing allthree radionuclides of interest. Therefore, the authors revealed formulations containing AZeolite and clinoptilolite for sorbing cesium, and activated charcoal for sorbing cobalto

6. NOBLE GAS TRAPPING

Penzhom and collaborators [6] discovered that the noble gases Ar, Kr and Xe can betrapped firmly in type A Zeolite as well as in mordenite and chabazite. The zeolitesare hydrothermally vitrified at temperatures between 340 and 650°C in the presenceof pressurized gas. The occurrence of trapping in an amorphous non zeolite species wasdemonstrated by X-ray diffraction, neutron scattering and electron scattering. The changein structure of loaded zeolites was also manifested by a substantialloss in sorption capacityfor water as well as an almost complete disappearance of t he specific surface area. The gasrelease rates al temperatures below 550°C were found to be extremely low. The estimateddiffusion coefficients were exceedingly smal!. The trapped gas was neither liberated by hotwater nor by liquid Rb, at 150°C.

7. TIIE THREE MILE ISLAND ACCIDENT

Zeolite A was used successfully in the clean up operation at the Three Miles Island,U.S.A., accident. Radioactive water was pumped through a series of columns containinga 10 cubic metre mixture of the zeolites, thus removing the radioactive cesium and stron-tium particlcs. As cach column becarD£' saturated with radioactive cations a new one ",as

240 P. Boscn ANOS. BULBULIAN

inserted and in this way around 0.6 million gallons of contaminated water was treatedand about 0.5 million curies of radiation removed [I].

8. REMOVAL OF LONG LIVED FISSION PRODUCTS

Radioisotopes of Cs are prod uced in nuclear fission amI hence, they are a common con-stituent of liquid radioactive wastes arising from nnclear facilities. Both clinoptilolite andchabazite have been used in the removal of radioactive lJ7CS and 90Sr from high levelradioactive processing wastes at the llenford AtOlllic Energy Project in the USA. Zeoliteion exchangers have also been used in processing low level lJ7Cs cont.aminated waste att.he plant since 1960.Gradev [7] studied the possibilit.y of using sorne cation forms of the clinoptilolit.es

as sorbent for radioactive iodine extraction from solnt.ions. 1Iis results showed that thesilver form of clinoptilolit.e is the most promising for extraction of radioactive iodinefrom liquid radioactive wastes. 1Ie used silver (1), thallium (1), mercury (1) and lead (11)forms of natural calcium-sodium clinoptilolite. The results showed the following order:Ag+ » Pb2+ > Tl2 > 1Ig2+, which can be explained by the energetically more favorablesubstitution of silver cat.ions with sodium ions. Compared to the thallium, mercury andlead ions, the silver cations possess ion radius and hydration energy closest to sodiumcations.

9. 99Mo-LABELED ZEOLITES

A and Y Zeolites were cation exchanged with 99Mo labeled molybdenum [8]' t.he molyb-deoum load for A zeolit.e being very low (0.08 wt%). 99Mo decay t.o 99mTc lead, t.o t.heloss of one electron (oxidation by one unit), the valence equilibrium in t.he zeolite wasdisrupt.ed, and t.echnetium det.ached from the framework of t.he zeolite. Therefore, whenthe labeled zeolites were heated in the presence of a fiow of chlorine gas, technetium elut.edpreferentially from the solido The striking differences to be observed in 99mTc separat.ion,when A or Y zeolites are used, has to be correlat.ed to the different sites occupied bymolybdenum atoms and to the basic strength of oxygen framework in the zeolit.e,.

ACKNOWLEDGEMENT

P.ll. Thanks the Cyted-D project for financial suppor!..

REFERENCES

1. Roskill, Zeolites Economy, 1st. Ed. Informat.ion Services Ud. (August 1988).2. J.V. Smith, Zcolites 4 (1984) 309.3. W. Breck, Zcolite Molecular Sieves, Wiley Iuterseieuce, New York (1974).4. R. Szostak, Molecular sieves, Van Nordstrand, New York (1989).

SORI'TION OF RADIOACTIVE MATERIAL ON ZEOLITES 241

5. S.J. Mitchell, II.D. Freeman, Pacific Northwest Labs., Richland, 54 p. 75-76. Annual meetingof the American Nuclear Society. Dalias, Tx (USA). 7-11 Jun. 1987, supported by the USDept. of Energy.

6. R.D. Penzhorn, P. Schuster, M. Leitzing and II.E. Noppel, Kernforschungszentrum Karlsruhe,G.m.b.II., Germany, F.R., Inst. fur Radiochemie., Ber Hunscnges. Phys. Chem. 185 N 0005-9021. HDPCA., Nov. 1982, v. 86 (11) p. 1077-1082.

7. G.D. Gradev, J. o/ Radioanal. Nud. Chem. Artides 116, 2 (1978) 341-346.8. S. Bulbulian, M.T. Olguín, P. !losch, Zeolites 10 (1990) 588-592.