The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

Abstract

IntroductionAfter the lithium isotope separation by ion exchange chromatography by Taylor and Urey [1], different systems, such as solvent extraction [2], amalgam/aqueous solution and amalgam/organic solution [3,4], ion exchange electromigration and ion exchange displacement chromatography [5][6][7], were applied to study the isotope effects by chemical exchange reactions. Regardless the chemical exchange system used, the molecular vibrations were believed to be the main, factor that affect the isotope exchange reactions of heavy elements. This belief was based on the relationship between the equilibrium constant of the isotopic exchange reactions and the reduced partition function ratios, RPFR, shown by Waldmann and by Bigeleisen and Mayer [8] in 1943 and in 1947, respectively. But during the systematic experimental and theoretical investigations on the isotope effects in chemical exchange systems carried out through the last two decades this belief became doubtful. The anomalous 235 U-238 U isotope effect in U(IV)-U(VI) chemical exchange shown by the present authors (M. N. and Y. F.) could not be explained by the molecular vibration theory [9]. Later on, the same trend was found in the case of 233 U isotope effects [10]. It was suggested that the isotope effects in the U(IV)-U(VI) exchange system arise mainly from the interaction between the nuclei and the electrons in the isotopes. Nishizawa has suggested anomalous mass effects in lighter elements such as Zn [11]. This was attributed to the isotope shift in the orbital energy. Bigeleisen has shown that the field shift becomes the major effect in the isotopic chemical exchange systems of uranium [12]. Recently, the anomaly of 155 Gd isotope effects in ligand exchange reactions observed by ion exchange chromatography were studied and found to be due to the shape and size of the nucleus [13].Studying the temperature effect on the equilibrium constant of isotopic chemical exchange reactions is a powerful tool for analyzing the isotope effects. It can help in determining which is the major factor that affects the isotope effects, molecular vibrations or the field shift. If the equilibrium constant was found to be inversely proportional to the square of temperature, the molecular vibration is considered as the main factor. On the other hand, the field shift would be considered as one of the main factors if the equilibrium constant was found to increase with the temperature. This trend was observed in the case of europium [7] and barium [3].By studying the temperature effect, a better understanding of the origin of the isotope effects would be achieved. So, this work was carried out to study the effect of temperature on the separation of uranium isotopes by using ion exchange chromatography based on a uranium-ligand exchange system. The uranium isotope effect in the exchange system uranyl(VI)-malate ligand at 288-343 K has been studied by ion exchange displacement chromatography. At all temperatures 235 U is enriched at the front of the uranium band. The single st...

Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions

“…Since the uranium adsorption capacities of the experimental systems were not so much changed among the experimental conditions of the tested temperatures in the reported work, 16) the distributions of the uranium chemical species in two phase are also estimated to be not so largely changed. In such cases, as previously mentioned, the temperature dependence of " shows the same pattern as that of " o .…”

“…The isotope separation coefficients theoretically calculated from the frequency of 3 , however, give much smaller values than the experimentally observed ones. 15) Recently temperature dependence of isotope effects in U(VI)-malate complex ligand exchange has been studied in the temperature range from 15-70 C. 16) The results show that the separation coefficient is increased with increase in temperature in the tested temperature range. The pattern of temperature dependence is found to be given by Eq.…”

Anomaly of 155Gd and 157Gd Isotope Effects in Ligand Exchange Reactions Observed by Ion Exchange Chromatography

“…The high performance liquid chromatography technique was used for separating the isotopes of Sm, Ce, and Eu, while the electron exchange system was used for studying the isotope separation of Ce and Eu. The ligand exchange system was applied to Gd, 9 Eu, 10 and U [11][12][13] isotope separation.…”