Supported ionic liquid phases for extraction and separation of medical radiolanthanides

Van de Voorde Michiel

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

Van Hecke Karen
karen.van.hecke@sckcen.be
+32 14 33 32 75

Expert group

Radiochemistry

PhD started

2015-10-01

Short project description

Several radioisotopes of elements of the lanthanides series find applications in the field of therapeutic radiopharmaceuticals, mainly for treatment of different types of cancer. Examples include 143Pr, 153Sm, 149Pm, 166Ho and 177Lu. These radioisotopes are ß- emitters. In many cases, the radiolanthanides of element Z are prepared by neutron irradiation of targets containing element Z-1 in the lanthanide series [1]. A (n,γ) reaction, followed by ß- decay delivers the wanted radioisotope. An example is 142Ce(n,γ)143Ce(ß-)143Pr. Sometimes, the irradiation target does not consist of element Z-1, but Z. For instance: 152Sm(n,γ)153Sm. In this particular case, 155Eu is also formed (natural samarium also contains stable 154Sm): 154Sm(n,γ)155Sm(ß-)155Eu. To prepare pure radiolanthanides, they have to be separated from the target materials. Also unwanted products have to be removed (for instance 155Eu in the case of 153Sm). This translated into the research question: how can small amounts of a lanthanide be separated from large amounts of a neighbouring lanthanide. This is quite a challenge, taking into account the very similar chemical properties of the elements of the lanthanide series. In contrast to separations of lanthanides (rare earths) on an industrial scale, one has to deal with much lower concentrations of lanthanides. Moreover, one has to deal with the issues related to the handling of radioactive materials and the required radiation resistance of the chemicals that are used in the separation process.

A highly innovative approach in this project is the use of supported ionic liquid phases (SILPs) for the separation of pairs of lanthanides. In SILPs, an ionic liquid phase is immobilized on a high-surface area solid support by impregnation or by covalent attaching of the ionic liquid cation [2]. Ionic liquids (ILs) are solvents that consist entirely of ions [3]. 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 solubilise 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 [4-10]. 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. By using ionic liquids with long alkyl chains, water-immiscible ionic liquids with chloride or nitrate ions can be obtained. 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. However, it is also possible to get with ionic liquids selectivities for solvent extraction processes that are much higher than those observed for solvent extraction processes with conventional organic solvents. Ionic liquids also show a good potential in the reprocessing of spent nuclear fuel, as an alternative for the solvents used in the PUREX process [11,12]. The SILPs combine the advantages of ionic liquid solvent extraction systems and ion-exchange resins, and are very useful for the recovery of metals from diluted aqueous streams, as the ionic liquid can be functionalized into acting as a specific metal extractant. They are typically applied in chromatographic columns.

Very recently, ionic liquids are also envisaged for the processing of medical radioisotopes produced in nuclear reactors [13,14]. The irradiated targets are highly radioactive which means that processing agents (e.g. extractants) with an acceptable radiation resistance are required. 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 [15,16]. 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. Other radiation resistant ionic liquids are benzimidazolium, pyridinium and (iso)quinolinium salts.

Objective

The objective of this project is the development of SILPs containing radiation resistant ionic liquids that are suitable for the separation of lanthanides, and to apply these ionic liquids for the extraction and separation of radiolanthanides with a high separation yield. After a literature study, several lanthanide systems will be selected as target mixtures. Possible mixtures are the Yb/Lu and Sm/Eu systems in view of the 177Lu and 153Sm medical radioisotopes currently obtained via neutron irradiation of ytterbium and samarium respectively. Since 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) imidazolium, 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 dialkylphosphates. Some of these anions can acts as extractants themselves. Otherwise, molecular extractants (e.g. crown ethers) have to be dissolved in the ionic liquid phase. No hexafluorophosphate ionic liquids will be prepared because the hexafluorophosphate anion is not resistant to hydrolysis. Different porous solid supports (silica, alumina,...) will be investigated for the development of SILPs and the stability of the resulting SILPs against leaching of the ionic liquid phase from the support will be tested. For the separation of the Sm/Eu couple, advantage is taken of the fact that Eu3+ can easily be reduced to Eu2+, whereas reduction of Sm3+ to Sm2+ requires more drastic conditions. Divalent europium and trivalent samarium have quite different chemical properties, so that this facilitates the Sm/Eu separation. In the same way, advantage is taken of the fact that Yb3+ can be reduced to Yb2+ in organic solvents, whereas Lu3+ cannot. Separation of the couple Yb2+/Lu3+ is much easier than separation of the couple Yb3+/Lu3+.

This PhD project is envisaged as a first feasibility study to investigate the possible implementation of supported ionic liquid phases for separation for radiolanthanides. The development of a full separation and purification technology towards the production of pure medical radioisotopes is beyond the scope of this project.

In a first phase, ionic liquids will be developed and tested on non-radioactive lanthanide systems. In a second phase, the radiation stability of selected ionic liquids will be tested and extraction tests on radioactive systems will be performed. The radioactive lanthanide systems will be produced via irradiation of the respective target lanthanide oxide or nitrate in the BR1 reactor. Sample quantities and the required neutron fluxes are low to ensure further sample manipulations in a glove box environment. Such irradiation experiments in BR1 are common practice at SCK•CEN. However, the possibility and compatibility towards larger scale experiments in BR2 will be kept in mind. The irradiated targets will be dissolved in an acid aqueous solution (in the case when lanthanide oxides were used) or water (in the case when lanthanide nitrates were used) followed by dedicated extraction tests.

 

References

[1] "Manual for reactor produced radioisotopes", IAEA-TECDOC-1340, IAEA, Vienna, 2003.

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[10] Vander Hoogerstraete, T.; Onghena, B.; Binnemans, K., "Homogeneous liquid-liquid extraction of metal ions with a functionalized ionic liquid", The Journal of Physical Chemistry Letters 2013, 4, 1659−1663.

[11] Binnemans, K., "Lanthanides and actinides in ionic liquids", Chemical Reviews 2007, 107, 2592-2614.

[12] Sun, X.Q.; Luo, H.M.; Dai. S., "Ionic Liquids-Based Extraction: A Promising Strategy for the Advanced Nuclear Fuel Cycle", Chemical Reviews, 2012, 112, 2100–2128.

[13] Luo, H. M.; Boll, R. A.; Bell, J. R.; Dai, S., "Facile solvent extraction separation of Th-227 and Ac-225 based on room-temperature ionic liquids", Radiochimica Acta 2012, 100, 771-777.

[14] Luo, H. M.; Boll, R. A.; Bell, J. R.; Dai, S., "Methods for separating medical isotopes using ionic liquids", U.S. Patent 2014/0072485 A1, Mar. 13, 2014.

[15] Bosse, E.; Berthon, L.; Zorz, N.; Monget, J.; Berthon, C.; Bisel, I.; Legand, S.; Moisy, P., "Stability of [MeBu3N][Tf2N] under gamma irradiation", Dalton Transactions 2008, 924-931.

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