Acta Chim. Slov. 2003, 50, 593-596. 593 LEACHING OF Cs FROM SPENT ION EXCHANGE RESINS IN CEMENT- BENTONITE CLAY MATRTX Ilija Plecas Vinca Institute of Nucleare Sciences, P.O.BOX 522 11001 Belgrade, Serbia and Montenegro Received 26-03-2003 Abstract The leaching rate of 137Cs from the spent cation exchange resins in cement-bentonite matrix has been studied. The solidification matrix was a standard Portland cement mixed with 290-350 (kg/m3) spent cation exchange resins, with or without 2-5% of bentonite clay. The leaching rates from the cement-bentonite matrix for137Cs:(3.2-6.6)×l0"4(cm/d), after 125 days were measured. From the leaching data the apparent diffusivity of cesium in cement-bentonite clay matrix with a waste load of 290-350 (kg/m3) spent cation exchange resins was measured for 137Cs: (l,5-26)×l0"5(cm2/d), after 125 days. The results presented in this paper are part of the results obtained in a 20-year mortar and concrete testing project, which will influence the design of radioactive waste management for a future radioactive waste disposal center. Introduction Ion exchange may be used most successfully for the removal of radioactive ions from dilute solutions. This process produces deionized water, thus the radioactive ions are removed together with non-radioactive ones. Ion exchangers are resins that are polvmers with cross-linking (connections between long carbon chains in a polvmer). The resin has active groups in the form of electricallv charged sites. At these sites ions of opposite charge are attracted but may be replaced by other ions depending on their relative concentrations and affinities for the sites. Spent cation exchange resins containing Cs represent a major portion of the solid radioactive waste in nuclear technology. Cement is used as a solidification material for the storage of intermediate-level radioactive waste. However, the retention of radionuclides, especially cesium, in the cement matrix is negligible. The sorption of cesium on cement is low and diffusivity of cesium in the hydrated cement is high. ' Only when the cement is mixed with a material having a significant sorption capacity, normally bead or powdered ion exchange resins, is the leachability of cesium and cobalt from the cement matrix low enough to be acceptable.3'4'5'6 I. Plecas: Leaching of Cs From Spent Ion Exchange Resins in Cement-Bentonite Clay Matrix 594 Acta Chim. Slov. 2003, 50, 593-596. The objectives of immobilization are to convert the waste into forms, which are: - leach resistant so that the release of radionuclides will be slow even though flowing water may contact them, - mechanically, physically and chemically stable for handling, transport and disposal. Although cement has several unfavorable characteristics as a solidifying material, i.e. low volume reduction and relatively high leachability, it possesses many practical advantages: good mechanical characteristics, low cost, easy operation and radiation and thermal stability. It is generally assumed that the cement leachability of Cs and other radionuclides can be reduced by adding minerals like bentonite, vermiculite and zeolite. Whereas zeolite was excluded for reasons of economy and availability, out of the above and other minerals, a natural bentonite is especially preferable in our leaching tests. ' Results and discussion The cement specimens were prepared from construction cement which is basically a standard Portland cement . The cement was mixed with saturated wet cation exchange resins,(100gr. of dry resins +100 gr. of water containing Cs) and bentonite clay(63% SiC>2; 18% AI2O3; 4% Fe203; 2.6% MgO and 3.3% CaO). The mixtures were čast into 50 mm diameter cylindrical molds with a height of 50 mm, which were then sealed and cured for 28 days prior to the leaching experiments. More then 100 different formulations of mortar form were examined to optimize their mechanical and sorption properties. In this paper we discuss eight representative formulations. Grout composition formulas are shown in Table I. Table 1. Grout Composition (calculated as grams for 1000 cm3 of mixtures). Formula Materials (g) Mi M2 M3 M4 M5 M6 M7 M8 cation exchange 350 350 350 350 290 290 290 290 resines Portland cement 1270 1280 1315 1270 1340 1335 1380 1340 Water 280 275 260 258 308 320 300 308 Bentonite clay 26 38 15 20 54 66 0 0 Initial activity (137Cs), A0 = 8.0×107 Bq/per sample. I. Plecas: Leaching of Cs From Spent Ion Exchange Resins in Cement-Bentonite Clay Matrix Acta Chim. Slov. 2003, 50, 593-596. 595 Leachant was exchanged and analyzed for radioactivity after 1,2,3,4,5,6,7 days, and thereafter every week for 1 month and from there on every month. After each leaching period the radioactivity in the leachant was measured by gamma counting. The volume of the leachant in every leaching period was 200 mL. The results are expressed by incremental leaching rates Rn(cm/d): 2an VI R = :==--------=— (cm/d) (1) A0 s 2> where: an - the radioactivity of leached constituent during each leaching interval (Bq) A0 - the specific radioactivity initially present in the specimen (Bq) S - the exposed surface area of the specimen (cm2) V - the sample volume (cm3) t - the duration of the leaching period (d). The apparent diffusivity De is calculated from equation: 71 2 V — m —; 4 S ? 2 V 2 De = m 2 (cm /d) (2) where: m is slope of the straight line from a plot of ?an/Ao versus ??tn (d-1/2). Testing of compressive strength is a classical method, which is practiced in civil engineering. Cube shaped mortar samples 10×10×10 cm were used. Compressive strength (M) is expressed in MPa. The results of the leaching tests of immobilized spent cation exchange resins are given as the incremental leaching rate, Rn(cm/d), after 125 days. Testing of mechanical characteristics of cement matrix was performed with each of the eight samples. Table II gives the results of incremental leach rate Rn(cm/d) and apparent diffusivity De(cm /d) after 125 days, keeping in mind the decay of Cs. Table 2. Incremental leach rate R^cm/d) and apparent diffusivity De(cm2/d) of 137Cs after 125 days. Formula Radionuclide Mi M2 M3 M4 M5 M6 M7 M8 Rn137Cs ×l04 5.20 5.00 5.10 5.40 3.20 3.60 6.50 6.60 De137Cs ×l05 14.00 1.50 7.00 7.50 5.20 5.20 24.00 26.00 I. Plecas: Leaching of Cs From Spent Ion Exchange Resins in Cement-Bentonite Clay Matrix 596 Acta Chim. Slov. 2003, 50, 593-596. Conclusion A significant difference in leaching and in apparent diffusivity of Cs in immobilized spent cation exchange resins was observed. Analysis of the results presented in Table II shows that the values are very similar to the literature data ' ' ' and proves that cement-bentonite matrix permits secure preservation of radionuclides for more than 300 years. Results presented in this paper are part of the results obtained in 10-year grout and concrete testing project, which will influence the design of a radioactive waste management for a future radioactive waste disposal center. Acknowledgements Work supported by the Ministry of Science, Technologies and Development of the Republic Serbia under Contract No. 1985, “Research and Development of processes and materials for treatment of radioactive and hazardous waste and environmental hazard assessment”. References 1. K. Andersson, B. Torstenfelt, B. Allard. Diffusion of Cesium in Concrete. In Scientific Basis for Nuclear Waste Management; J. G. Moore, Ed.; Vol. 3, Plenum Press, New York, 1981, pp 235-242. 2. A. Atkinson, A. K. Nickerson, Nucl. Technol. 1988, 81, 100-113. 3. H. Christensen, Waste Management'81, 1981, 545-548. 4. H. Christensen, Nuclear and Chemical Waste Management, 1982, 3, 105-109. 5. E. D. Hespe,^4fo/w. Energy Rev. 1971, 9, 195-207. 6. I. Plecas, Lj. Mihajlovic, A. Kostadinovic, Radioactive Waste Management and Nuclear Fuel Cycle 1985, 6, 161-175. 7. I. Plecas, J. Drljaca, A. Peric, A. Kostadinovic, S. Glodic, Radioactive Waste Management and Nuclear Fuel Cycle 1990, 14, 195-205. 8. B. Torstenfeld, G. Hedin, Scientific Basis for Nuclear Waste Management 1988, 127, 495-499. 9. I. Plečaš, A. Perić, A. Kostadinović, J. Drljača, S. Glodić, Cement and Concrete Research an International Journal 1992, 22, 937-940. Povzetek Določili smo hitrosti izluževanja 137Cs iz utrjenih izrabljenih smol ionskih izmenjevalcev v cementno bentonitnih zmeseh. Za utrditev smo uporabili različne betonske zmesi Portland cementa z ali brez 2 - 5% dodatka bentonita in z umešanimi določenimi količinami izrabljenih smol ionskih izmenjevalcev (290 - 350 kg/m3-zmesi). Imerjene hitrosti izluževanja (Rn) v izluževalnih testih po 125 dneh so bile za 137Cs: (3,2 - 6,6) x 10"4 (cm/dan), in iz teh meritev izračunane difuzivnosti (De) za 137Cs: (1,5 - 26) x 10"5(cm2/dan). Te meritve so del 20 letnega projekta preizkušanja betonskih in maltnih materijalov za utrjevanje radioaktivnih odpadkov in so med drugim podlaga za načrtovanje ravnanja z radioaktivnimi odpadki v bodočem centralnem odlagališču. I. Plecas: Leaching of Cs From Spent Ion Exchange Resins in Cement-Bentonite Clay Matrix