Radiation-resistant ionic liquids for extraction of minor actinides

Zsabka Peter

Promoter

Binnemans Koen, (KULeuven), koen.binnemans@chem.kuleuven.be

SCK•CEN Mentor

Cardinaels Thomas
thomas.cardinaels@sckcen.be
+32 14 33 32 00

SCK•CEN Co-mentor

Verwerft Marc
marc.verwerft@sckcen.be
+32 14 33 30 48

Expert group

Radiochemistry

PhD started

2015-12-09

Short project description

Ionic liquids (ILs) are being studied as novel solvents for the sustainable extraction and separation of metal ions. Ionic liquids are solvents that consist entirely of ions. Typically, they are organic salts with a low melting point (< 100 °C). Ionic liquids have several properties that make them attractive as potential solvents for improved separation processes: wide liquidus ranges, high thermal stabilities, a negligible vapour pressure (and thus a very low volatility) and the ability to solubilize a wide range of solutes, including metal salts and complexes. A considerable variation is possible in both the cationic and anionic part of the ionic liquid. Typical organic cations are 1-alkyl-3-methylimidazolium (abbreviated as [Cnmim]+), N-alkylpyridinium, N,N-dialkylpyrrolidinium, tetraalkylammonium and tetraalkylphosphonium. Hexafluorophosphate (PF6-), and bis(trifluoromethylsulfonyl)imide (bistriflimide, Tf2N- or (CF3SO2)2N-) are used as anions in water-immiscible ionic liquids. The main rationale for using ionic liquids as the organic phase in liquid-liquid extraction processes is their low volatility and the low flammability. The replacement of organic diluents in liquid-liquid extractions by ionic liquids could lead to more sustainable extraction processes. Ionic liquids are also of interest for the processing of spent nuclear fuel. Studies showed that 1,3-dialkylimidazolium ionic liquids with chloride and nitrate anions are relatively radiation resistant and do not undergo significant decomposition by radiolysis upon exposure to high radiation doses. The stability of the ionic liquids against high radiation doses is comparable to that of benzene, but is much higher than that of the TBP/kerosene mixtures used in the PUREX process. The relatively high radiation resistance of imidazolium ionic liquids can be attributed to the presence of the aromatic imidazolium ring. Aromatic compounds have a higher stability against irradiation than non-aromatic compounds, because the aromatic ring can absorb radiation energy and can relax non-dissociatively. Moreover, mixtures of aromatic and non-aromatic compounds undergo less radiolytic decomposition than what is expected on the basis of the concentration of the non-aromatic compound, because of energy transfer to the aromatic compound. Analysis of the radiolysis products of 1,3-dialkylimidazolium chloride and nitrate ionic liquids show that the ionic liquids behave like a combination of an aromatic compound, an alkane and a salt.

Objective

The objective of this work is to develop radiation-resistant ionic liquids that can be used for the extraction of minor actinides from acidic aqueous solutions with a high separation yield. The design hypothesis is that ionic liquids with benzene-like aromatic cores will have a higher radiation resistance than imidazolium salts. Because the ionic liquids have to be used for extraction of metal ions from an aqueous phase to an ionic liquid phase, water-immiscible ionic liquids are required. The targeted ionic liquids consist of (substituted) benzimidazolium, pyridinium and (iso)quinolinium cations. Immiscibility with water will be achieved by making the alkyl chain of the ionic liquid cation sufficiently long and/or to use strongly hydrophobic anions. Examples of anions are bis(trifluoromethylsulfonyl)imide, higher branched alkanoates and b-diketonates. No hexafluorophosphate ionic liquids will be prepared because the hexafluorophosphate anion is not resistant to hydrolysis. Neodymium will be used as a non-radiative simulant for the minor actinides because of the similarity of ionic radius between Nd3+, Am3+ and Cm3+. The procedures developed will lead to a co-extraction of the lanthanides and the minor actinides to the ionic liquid phase. Because many of the fission product lanthanide nuclides strongly absorb neutrons, the minor actinides and lanthanides will have to be separated in a separate step.