Fundamental Study Of Uranyl Phosphonates For Prevention Of Radioactive Contamination

Owing primarily to the dual pressure from the shortage of fossil fuel and the threat of global warming, energy sources such as nuclear, wind, and solar ones are being vigorously developed. Among them, nuclear energy has received extensive attention by its virtue of high energy density and low emission of greenhouse gases. However, spent nuclear fuel contains a lot of unreacted uranium, together with fission products and neutron capture products that are highly radioactive. Therefore, efficient recovery of uranium from the spent fuel as well as separation of those products from nuclear reaction to reduce the long term storage pressure have been one of the most important challenges in the development of nuclear industries. Phosphonate are a family of ligand with functional groups of P-O and P=O that could strongly bond to actinides, and are widely used in used nuclear fuel reprocesses such as the TBP ligand in PUREX process and diphosphonic acid in Diphonix Resin. As a consequence, systematic studies regarding the interaction between actinides(especially uranium) with phosphonate at various conditions are of significant importance for deep and straightforward understanding of the spent nuclear fuel separation processes.Herein this work, a series of uranium phosphonate compounds with complex structures have been synthesized under hydrothermal and ionothermal conditions via variation of reaction parameters and choices of ionic liquids. Those compounds were characterized with single crystal X-ray diffraction experiments in order to obtain the local coordination mode between uranium and phosphorus oxygen functional groups, and absorption spectra and fluorescence spectra, EDS and X-ray powder diffraction. The results are as follows:1) By adjusting the reaction time and pH of the hydrothermal reaction systems, new uranium phosphonates were successfully synthesized with various coordination modes. Results show that under acidic condition, only P=O oxygen atoms are bonded to uranium centers, whereas P-O ones are involved in bonding in basic systems. Additionally, increasing the reaction time would favor the thermodynamically stable compounds with P-O-U bondings. Further analysis of the UV-Vis-NIR absorption and fluorescence spectra show that the relative intensity of the peaks is closely related to the local coordination geometry of the uranyl units.2) From ionothermal reactions, a group of uranium phosphonate compounds were crystallized from six different ionic liquids that act both as solvent and structural template, providing experimental results of the role ionic liquids played in the bonding between uranium and phosphonate ligands. It was found that the cation of ionic liquids and/or its decomposition product mainly work as a template that filled in the cavity or the interlayer space, whereas small anion of F- can bound to uranyl unit and large anions such as PF6- does not. In addition, the temperature dependent absorption and fluorescence spectra show that all oxo-bonded uranyl units exhibit negative correlation between peak intensity and temperature, whereas the trend of the F-bonded uranyl units is reversed.