Thermodynamics of Alkali Metal Uranoborates

Abstract

The standard enthalpies of formation of crystalline A I BUO 5 (A I = Li, Na, K, Rb, Cs) at 298.15 K were determined by reaction calorimetry. For these compounds, the heat capacities in the range 10 3300 K were determined by adiabatic vacuum calorimetry, and the thermodynamic functions were calculated. The standard entropies and Gibbs energies of formation at 298.15 K and the standard thermodynamic functions of solution of these compounds were calculated.Here we report on a thermodynamic study of the compounds A I BUO 5 (A I = Li, Na, K, Rb, or Cs).The compounds A I BUO 5 were prepared by a solidphase reaction between UO 3 , B 2 O 3 , and alkali metal carbonate or nitrate ( Table 1). The syntheses were performed in a platinum crucible, which was charged with the reactants (for the atomic ratio, see Table 1). The charge was calcined at 75031000oC for 8 h. Excess boron and alkali metal were washed out with boiling distilled water in the form of water-soluble borates, and the precipitate was dried in air for 24 h. Hydration of anhydrous alkali metal uranoborates to monohydrates in the case of Na, K, Rb, and Cs and to sesquihydrate in the case of Li was performed under hydrothermal conditions at 200oC in a hermetically sealed 50% filled pressure vessel for 15 h.The phase purity of the compounds was checked by X-ray diffraction (DRON-3.0 diffractometer, CoK a radiation). The elemental composition was determined photometrically using standard procedures. Boron(III) was determined by the reaction with quinalizarin in concentrated sulfuric acid (l max = 625 nm) [1]. The U(VI) content was determined from the absorption of its complex with arsenazo III (l max = 650 nm, pH 3) [2]. The alkali metal content w(A 2 O) (%) was determined from the difference: w(A 2 O) = 2[100% 3 w(UO 3 )] 3 w(B 2 O 3 ). The measurements were performed with a KFK-3 photocolorimeter and a pH-140 pH meter. According to elemental analysis, the compounds had the formula A I BUO 5 .According to the results of X-ray phase analysis, alkali metal uranoborates have a layered structure. As reported by Gasperin [3,4] for the lithium and sodium derivatives, the [BUO 5 ] n 2i layers are formed by pentagonal bipyramids UO 7 linked into infinite chains through common edges and by BO 3 triangles linking the chains into two-dimensional layers. The axial bonds in the uranium polyhedron are shortened and directed toward the interlayer space, in which the alkali metal ions are located. THERMOCHEMISTRY OF ALKALI METAL URANOBORATESThe experimental thermochemical data were obtained with an upgraded calorimeter of S.M. Skuratov's design [5]. All the experiments were performed in a thin-walled Teflon ampule consisting of two vessels: internal (V = 5 cm 3 ) into which weighed portions of the samples were placed and external (V = 40 cm 3 filled with an HF solution. The reactants were mixed by breaking the bottom of the internal vessel. The temperature in the experiments were measured with a platinum resistivity thermometer and an Shch-516 digital voltmeter. The adiabatic con...

Interaction of Nd(iii) and Cm(iii) with borate in dilute to concentrated alkaline NaCl, MgCl2and CaCl2solutions: solubility and TRLFS studies

“…The authors observed no enhanced release of radionuclides in spite of the high borate concentration in solution (B10 À2 mol kg À1 ). Chernorukov, Nipruk and co-workers[9][10][11][12][13][14][15][16][17][18][19] conducted a very comprehensive series of thermochemical and solubility experiments with M I,II -U(VI)-B solid phases, with M I = Li, Na, K, Rb, Cs and M II = Mg, Ca, Sr, Ba, Mn, Co, Ni, Zn. The uranoborates were prepared by a combination of hydrothermal and ion-exchange approaches.…”