Ionic Liquid

The overarching goal of our research to investigate the suitability of room temperature ionic liquids (RTILs) in biphasic metal ion extraction for use in spent nuclear fuel (SNF) reclamation. RTILs have demonstrated high metal extraction efficiencies, however loss of the anionic or cationic components through ion exchange present a serious challenge. Therefore, the first step was to find a RTIL that was sufficiently hydrophobic and we used our facile biphasic electrochemistry at water|RTIL (w|RTIL) and water|organic (w|o) solvent interfaces in order to discriminate between different candidates. The technique we employed used a micro-interface housed at the tip of a pulled glass capillary; the aqueous phase inside the pipette and immersed in organic or RTIL phase. By using a micro- interface we greatly reduced the volume of material required. Since RTILs can be expensive, this made direct w|RTIL assessments more feasible. Many commercially available RTILs were tested, however none were adequate. So we prepared a novel RTIL in-house at a greatly reduced cost (more than 10 times cheaper). Throughout the RTIL synthesis we were able to introduce measures that both simplified the preparation procedure and allowed for facile, quick purification of the product at quantitative yields. It is these kinds of innovations which will help lower the cost of RTILs and make them more attractive to large scale industrial processes.

By adding ligands to the organic or RTIL phase the transfer of metal ions from water to these phases can be greatly increased. Using electrochemistry at the biphasic interface, we were able to quantify several metal-ligand complexation reactions, individually, so that the selectivity and efficiency could be compared. This also allowed me to show that RTILs were 100 000 times better than organic solvents in metal ion extraction. we also observed a greater selectivity for strontium versus uranium or cesium using two ligands common to contemporary SNF industrial reprocessing. 95% of SNF is useable uranium that has been contaminated with fission byproducts that, in turn, include high amounts of 90Sr and 137Cs isotopes; major contributors to its radioactivity. This metal ion differentiation result demonstrates the importance of this work.

These small scale experiments also served as a proof-of-concept that electrochemical, rather than the typical physical means of separation, is possible. Physical separation requires a great deal of engineering and mechanical moving parts; owing to the inherent radioactivity of the SNF being separated, maintenance of this equipment is prohibitively expensive. If an electrochemical method were to be used, the mechanical engineering could be reduced and thus the cost would go down. This would make reprocessing SNF more attractive and we could avoid long-term geological disposal which has all kinds of potentially nasty repercussions.

In conclusion, we were able to test and synthesize a host of RTILs and investigate their suitability towards metal ion extraction. Critically, the selectivity and efficiency of biphasic separations was quantified and it is this that should have a lasting impact on how we view SNF reprocessing.

  1. Stockmann, T. J.; Ding, Z., Tetraoctylphosphonium Tetrakis(pentafluorophenyl)Borate Room Temperature Ionic Liquid Towards Enhanced Physicochemical Properties for Electrochemistry. J Phys Chem B 2012, 116 (42), 12826-12834.
  2. Stockmann, T. J.; Montgomery, A.-M.; Ding, Z., Correlation of Stoichiometries for Rb+ Extraction Determined by Mass Spectrometry and Electrochemistry at Liquid|Liquid Interfaces. Anal Chem 2012, 84 (14), 6143-6149.
  3. Stockmann, T. J.; Zhang, J.; Wren, J. C.; Ding, Z., Hydrophobic alkylphosphonium ionic liquid for electrochemistry at ultramicroelectrodes and micro liquid|liquid interfaces. Electrochim Acta 2012, 62 (0), 8-18.
  4. Stockmann, T. J.; Ding, Z., Uranyl Ion Extraction with Conventional PUREX/TRUEX Ligands Assessed by Electroanalytical Chemistry at Micro Liquid/Liquid Interfaces. Anal Chem 2011, 83 (19), 7542-7549.
  5. Stockmann, T. J.; Lu, Y.; Zhang, J.; Girault, H. H.; Ding, Z., Interfacial Complexation of Sr2+ with CMPO for Its Extraction in Reprocessing Spent Nuclear Fuels. Chem Eur J 2011, 17 (47), 13206-13216.