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 [ $${({\rm{C}}}_{8}{)}_{3}{{\rm{C}}}_{1}{{\rm{N}}}^{+}{{\rm{DS}}}^{-}]$$ . 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 $${\rm{Pu}}({\rm{N}}{{\rm{O}}}_{3}{)}_{2}^{2+}$$ (~80%) and $${\rm{Pu}}{({\rm{N}}{{\rm{O}}}_{3})}^{3+}\,( \sim 11 \% )$$ , leading to the formation of ML2 and ML3 species viz. ( $${\rm{Pu}}({\rm{N}}{{\rm{O}}}_{3}{)}_{2}^{2+}\,({\rm{D}}{{\rm{S}}}^{-}{)}_{2}$$ and $${\rm{Pu}}({\rm{N}}{{\rm{O}}}_{3}{)}_{2}^{2+}\,({\rm{D}}{{\rm{S}}}^{-}{)}_{3}$$ ). 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 $$({({\rm{C}}}_{8}{)}_{3}{{\rm{C}}}_{1}{{\rm{N}}}^{+}{)}_{2}{\rm{Pu}}({\rm{N}}{{\rm{O}}}_{3}{)}_{6}^{2-}$$ , $${\rm{Pu}}({\rm{N}}{{{\rm{O}}}_{3})}_{4}.\,{({\rm{C}}}_{8}{)}_{3}{{\rm{C}}}_{1}{{\rm{N}}}^{+}{{\rm{DS}}}^{-}$$ and $${\rm{Pu}}({\rm{N}}{{\rm{O}}}_{3}{)}_{2}^{2+}\,({\rm{D}}{{\rm{S}}}^{-}{)}_{2}$$ , 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: $$({\rm{U}}{{\rm{O}}}_{2}^{2+})\,({\rm{D}}{{\rm{S}}}^{-}{)}_{2}$$ and $${\rm{U}}{{\rm{O}}}_{2}({\rm{N}}{{\rm{O}}}_{3}{)}_{2}.\,2{({\rm{C}}}_{8}{)}_{3}{{\rm{C}}}_{1}{{\rm{N}}}^{+}{{\rm{DS}}}^{-}$$ 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.