Selective separation of samarium(III) by synergistic extraction with β-diketone and methylphenylphenanthroline carboxamide

Abstract

Synergistic extraction of trivalent lanthanides (Lns(III)) with pivaloyltrifluoroacetone (HA) and N-methyl-N-phenyl-1,10-phenanthroline-2-carboxamide (MePhPTA) was evaluated across the Ln series. The distribution ratio (D) of Sm(III) under an identical condition was the largest among all Lns(III). The separation factor (SF) between Sm(III) and Nd(III) (SF=DSm/DNd) was 2.0 and SF between Sm(III) and Eu(III), (DSm/DEu) was 1.4. Upon analyzing the extraction data in detail on the basis of mass balance, it was found that the dominant extracted species of light Lns(III) was a stable ternary complex consisting of Ln(III), HA, and MePhPTA (B), namely, LnA3B, while the dominant extracted species of heavy Lns(III) was the ion pair, [LnA2B]+ClO4−.The complex for Pr(III) was very stable (the stability constant, β¯, denoted as [LnA3B]o[LnA3]o−1[B]o−1, was 108.3). It suggests that LnA3 can form two 5-membered rings with MePhPTA, and the size of Pr(III) matches to the distance between the donor atoms in MePhPTA. Although the stability constant decreased with increasing Ln atomic number, the synergistic extraction constant (Kex31=[LnA3B]o[H+]3[Ln3+]−1[HA]o−3[B]o−1) was the largest for Sm(III). Since the constant, Kex31, is given by Kex31=Kex30×β¯ where Kex30=[LnA3]o[H+]3[Ln3+]−1[HA]o−3, the largest Kex31 of Sm(III) is attributable to the difference of the degree of the variation of Kex30 between the light and the heavy Lns(III); the increment of extraction constant of LnA3 (logKex30) for light Lns is larger than the decrement of the stability constant of LnA3B (log⁡β¯), while the increment of logKex30 of post-Sm lessens than the decrement of log⁡β¯. From these results, it is concluded that selective separation of a particular Ln(III) among all Lns(III) is possible using synergistic extraction with a suitable combination of a multidentate β-diketone and a Lewis base.