Efficient extraction of tetravalent actinide from nitric acid feeds using tri-N-octyl methyl ammonium N-dodecyl sulphate functionalized task-specific ionic liquid

An attempt was made to demonstrate the efficient and selective extraction of the tetravalent plutonium from aqueous nitric acid medium using a novel anion functionalized task-specific ionic liquid: tri-n-octyl methyl ammonium n-dodecyl sulfate ({C}₈) ₃{C}₁{N}^+{DS}^-. The maximum separation factor for Pu (IV) over U (VI) was found to be more than 103; whereas that for Pu (IV) over Am (III) was more than 105. At 4 M HNO3, the majority of Pu existed as Pu (N{O}₃) ₂^2+ (~80%) and Pu ({N{O}₃) }^3+\, (11 \%), leading to the formation of ML2 and ML3 species viz. (Pu (N{O}₃) ₂^2+\, (D{S}^-) ₂ and Pu (N{O}₃) ₂^2+\, (D{S}^-) ₃). On the other hand, in 8 M HNO3, plutonium existed as cationic, neutral, and anionic species with the relative composition: Pu (NO3) 22+ ~16%, Pu (NO3) 62- (~42%), and Pu (NO3) 4 ~ 40%. Hence, the extraction of Pu (IV) proceeded via the ( ({C}₈) ₃{C}₁{N}^+) ₂Pu (N{O}₃) ₆^2-, Pu (N{{O}₃) }₄. \, ({C}₈) ₃{C}₁{N}^+{DS}^- and Pu (N{O}₃) ₂^2+\, (D{S}^-) ₂, species getting transferred into the ionic liquid phase. In the case of uranium, such speciation analyses were performed as well, which revealed the predominance of the ML2 species: (U{O}₂^2+) \, (D{S}^-) ₂ and U{O}₂ (N{O}₃) ₂. \, 2 ({C}₈) ₃{C}₁{N}^+{DS}^- during its extraction from 4 M HNO3 and 8 M HNO3, respectively. The sluggishness in the extraction of Pu (IV) and U (VI) was attributed to the viscosity-induced slow diffusion of the actinides and the actinide-ionic liquid complexes. The extraction processes were exothermic. The extent of exothermicity was found to be greater for Pu (IV) as compared to that for U (VI), and it was more for the extraction from 8 M HNO3. The solvent systems exhibited good radiation stability. However, a gradual deterioration in the extraction performance with increasing gamma ray exposure was evidenced. Multiple contacts with a solution of 10 mM of oxalic acid were found to be effective for quantitative back extraction of Pu (IV) from the loaded ionic liquid phase.