The Effect of Na-Bearing Thermal Solutions on Ca-Mg-Bentonite Sorption of Sr and Cs
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THE EFFECT OF Na-BEARING THERMAL SOLUTIONS ON Ca-MgBENTONITE SORPTION OF Sr AND Cs Alexander P. MUKHAMET-GALEYEV, Alexey V. KUDRIN, Boris S. NIKONOV, Nadezhda D. SHIKINA Institute of Geology of Ore Deposits,Petrography,Mineralogy, and Geochemistry,Russian Academy of Sciences, Staromonetnyi 35, 109017 Moscow, Russia
ABSTRACT The sorption of Sr and Cs has been investigated on Ca-Mg-bentonite preliminary treated by 0.1-1.0 m NaCI solutions at 25, 170-250'C during 0.5-7 months. The bentonite remained stable in these conditions. Moreover, hydrothermal treatment has raised the sorption activity of the bentonite. The treated bentonite has shown a strong selectivity for Sr and less pronounced one for Cs. The sorption of the elements increased with temperature and decreased with NaCI content more significant for Sr, than for Cs. The distribution ratio Rd was nearly independent on ion loading for Cs and strongly increased up to 104.5 mL/g in 10-2.0-10-0.5 meq/g ion loading interval for Sr, dropping sharply at more concentrations.
INTRODUCTION
A great part of radioactive wastes have been solidified into Al-P-Na glass matrix in Russia. Sodium is the most soluble and mobile component of the matrix (1). Ca-Mg-bentonite, a common mineral of apobasite weathering crusts on a possible repository area, is a very suitable material for an engineered barriers because of its great sorption and swelling properties. Thus, the goal of this paper was to investigate the impact of heated Na-bearing solutions on Ca-Mgbentonite sorption of Sr and Cs, which belong to the most principal mobile fission products. EXPERIMENTAL TECHNIQUES The Ca-Mg-bentonite, from the Cherkassy deposit (Cherkassy oblast, Ukrain), was selected as a good example of a well swelling mineral showing high sorption properties (70-80 meq/100g) (2). It consist of 85% smectite, 12% quartz, and 3% kaolinite. The smectite analysis corrected for impurities yielded the following formula: Ko.o0Nao.o0Cao.o 9Mgo.o6(Ali.36Tio.o4Feo.46Mgo.1 4)(Si 3.78Alo.22)0oo(OH) 2*2.9H20, which shows that the interlayer sites were generally occupied by Ca and Mg, and the tetrahedral net charge dominates. Bentonite-sodium solution interaction was investigated in autoclaves at 170, 210 and 250'C and the vapour saturation pressure as well as at ambient condition as the reference state. Batches of 1000, 650 and 380 mg were used in runs with 25 ml of 0.1, 0.3 and 1.0 m Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
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NaCl solutions, respectively in order to allow sufficient buffer capacity of the solutions with respect to the ion exchange reaction. The quenched solutions were centrifuged, separated by filtration, and analyzed for Ca, Mg, Al by atomic absorption spectrometry, for Si - by spectrophotometry, and for IT - by potentiometry. The bentonite was investigated by XRD and TEM with simultaneous X-Ray energydispersion analysis. The sorption of Sr and Cs by the bentonite samples, preliminary treated by sodium-bearing solutions at elevated temperatures, were studied by the
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