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

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=D(Sm)/D(Nd)) was 2.0 and SF between Sm(III) and Eu(III), (D(Sm)/D(Eu)) 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, LnA(3)B, while the dominant extracted species of heavy Lns(III) was the ion pair, LnA(2)BClO(4)(-). The complex for Pr(III) was very stable (the stability constant, β¯, denoted as LnA(3)BLnA(3)(-1)B(-1), was 10(8.3)). It suggests that LnA(3) 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 (K(ex31)=LnA(3)BH(+)Ln(3+)HA(-3)B(-1)) was the largest for Sm(III). Since the constant, K(ex31,) is given by K(ex31)=K(ex30)×β¯ where K(ex30)=LnA(3)H(+)Ln(3+)HA(-3), the largest K(ex31) of Sm(III) is attributable to the difference of the degree of the variation of K(ex30) between the light and the heavy Lns(III); the increment of extraction constant of LnA(3) (logK(ex30)) for light Lns is larger than the decrement of the stability constant of LnA(3)B (logβ¯), while the increment of logK(ex30) of post-Sm lessens than the decrement of logβ¯. 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.