The dissolution of metal oxide in ionic liquid (IL) medium is an important and challenging issue for the development of alternative solvent systems for carrying out non-aqueous chemistry operations of metal ions in the nuclear field where IL could be a potential alternative to the high-temperature molten salt for non-aqueous processing of metal ions. This review article emphasizes on the developments in the field of dissolution of metal oxides in IL medium particularly that of the of oxides of uranium and few other metal oxides including those of lanathanides which have relevance in the spent nuclear fuel (SNF) reprocessing. The dissolution of metal oxides in ILs is broadly divided on the basis of different factors affecting the dissolution. The chloroaluminate-based ILs although reported to have dissolved many oxides including inert oxides suffer from the air and moisture sensitivity issues whereas the dissolution of metal oxides in acid and water stable ILs are limited by the solubility of acid and water in the ILs. Task-specific ionic liquids (TSIL) containing Brønsted acid functional groups are reported to have dissolved many oxides depending on the pKa of the functional acid group and the complexing ability of the conjugate base. This review article will help the readers to understand the basic problem of dissolution of metal oxides in ILs and recent development in the field of dissolution.
The extraction of Np(IV) and Pu(IV) was studied using a tripodal amide ligand (N,N,N′,N′,N″,N″-hexa-n-octyl nitrilotriacetamide termed as HONTA) in an ionic liquid (C4mim·NTf2) as well as in a molecular diluent mixture (95% n-dodecane +5% iso-decanol) from an aqueous feed containing diluted nitric acid. The extraction efficiency of Pu(IV) was higher than that of Np(IV), while the RTIL-based solvent system extracted the metal ions better than the molecular diluent-based solvent system. The extracted species were ascertained by slope analysis as M(NO3)5·L(LH) and M(NO3)4·L2 for the ionic liquid- and the molecular diluent-based solvent systems, respectively. The stripping of the metal ions from the organic extracts was achieved efficiently when a buffered complexing medium of 0.05 M EDTA and 1 M guanidine carbonate was used. Cyclic voltammetry was done for the Np(IV) extract in the ionic liquid-based solvent system in view of the wide electrochemical window experienced.
Dissolution of uranium oxide was carried out using a solution of HD2EHP in C8mim·NTf2, which was apparently facilitated by the in situ generation of water during the complex formation reaction. The dissolved complex in the ionic liquid phase led to splitting of the latter into a light phase and a heavy phase where the former contained predominantly the UO2(HL2)2 complex (HL = HD2EHP), while the latter contained the ionic liquid as supported by FTIR and UV–Visible spectral analyses. The complexation of the uranyl ion was suggested to take place in the equatorial plane where two dimeric units of the H-bonded HD2EHP molecules took part in complexation. An increase in temperature facilitated the dissolution rate with an activation energy of 31.0 ± 2.8 kJ/mol. The cyclic voltammetry studies indicated potential chances of recovery of the dissolved uranium by electrodeposition at the cathode. The proposed dimeric structure of HD2EHP in the complexation with U(VI) was supported by DFT studies also.
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