There is only one stable cesium isotope with a mass number of 133 in nature. However, an artificial radioactive cesium isotope - cesium-137 is the most radiationally hazardous and practically important. Its main source is the fission reaction of uranium nuclei. The efficacy of separation of 137Cs from nuclear waste depends on the chemical and structural compatibility of uranium (VI) and cesium especially in aqueous media.
Cesium uranate Cs2U6O19•10H2O was synthesised for the first time via a reaction between synthetic schoepite UO3•2.25H2O and aqueous solution of CsOH. The composition and structure of the compound was studied using X-ray fluorescence spectrometry, X-ray powder diffraction, IR-spectrometry, differential thermal analysis and thermogravimetry.
Elemental composition of the cesium uranate was established via XRF after dissolving the compound in aqueous solution of nitric acid. Following spectral lines were used in the analysis: CsLα and ULα. The amount of water in the formula unit was determined by heating the sample to 600°С. The compound was established to be a hydrate with a formula: Cs2U6O19•10H2O.
X-ray powder diffraction shows that the compound is an individual crystalline phase. Reflections at small 2θ angles and pinacoidal peaks indicate layered structure of the studied compound.
Four types of absorption bands are found on the IR-spectrum. They correspond to the molecular water, the uranyl moiety UO2δ+, UOH bonds, and U-Oeq bonds in the coordination polyhedron of U. Characteristic band for δ(H2O) is shifted from 1595 cm-1, as observed for gaseous Η2Ο molecules, to 1624 cm-1 due to the forming of hydrogen bonds. Due to the same reason both symmetrical and asymmetrical stretching νas(H2O) and νs(H2O) are not resolved into the constituents and give a wide integral band at 3508 cm-1. A band at 925 cm-1 is assigned to asymmetrical stretching ν2(UO2δ+). Absence of symmetrical stretching band indicates that the uranyl moiety has a linear configuration and equal bond lengths. A band at 996 cm-1 and a shoulder at 3315 cm-1 are assigned to bending δ(UOH) and stretching vibrations ν(UOH) of uranium hydroxide fragment respectively. Bands at 400 to 600 cm-1 can be assigned to planar vibrations of U-Oeq.
The hexauranate is not thermally stable. Six molecules of H2O leave the Cs2U6O19•10H2O by 160°C. Upon further heating to 310°C the remaining 4 molecules of H2O evaporate and the compound becomes amorphous. New crystalline structure forms at 600°C and remains virtually the same in the temperature interval from 700°C to 1000°C.
Conducted experiments allow us to predict the structure of the synthesised cesium hexauranate. Oxygenous coordination polyhedra of U form the negatively charged layers, while Cs+ cations and water molecules are situated in interlayer space. They along with hydroxide groups bind the layers together creating a three dimensional lattice.
All experiments were conducted using analytical instruments made by Shimadzu Corp.