Comparison of the lipophilic redox-recyclable extractant [Fe(η5-C5H3(s-C7H15)2)2][NO3] with [N(n-C7H15)4][NO3] for liquid-liquid anion-exchange of aqueous 99TcO4-

The highly selective liquid-liquid extraction of the pertechnetate anion TcO4- from a nitrate matrix by the lipophilic redox-recyclable extractant HEP+NO3- [HEP = 1,1',3,3'-tetrakis(2-methyl-2-hexyl)ferrocene] has been investigated in comparison with the well known anion-exchange reagent tetra-n-heptyl ammonium nitrate (heptyl)(4)N+NO3-. Under conditions of low metal-anion loading, the distribution ratio D(TcO4-) was found to be insensitive to the alkali metal cation present as M+NO3- (M+ = Li+, Na+, K+) and the initial concentration of TcO4- in the aqueous phase for both extractants. Also, slope analysis of the distribution data revealed approximate linear relationships between D(TcO4-) and the concentration of extractant in the organic phase (slope 1.0) and the concentration of NaNO3 in the aqueous phase (slope = -1) plotted on a full logarithmic scale, suggesting that the extraction of TcO4- occurs by a simple anion-exchange mechanism. Graphical analysis of partial loading data confirmed that both HEP+NO3- and (heptyl)(4)N+NO3- are predominantly monomeric in chlorobenzene under conditions of low metal-anion loading. However, at an organic-phase TcO4- concentration of approximately 0.003 M, a sharp decrease in D(TcO4-) was observed, suggesting the formation of large aggregated species. At lower concentrations of TcO4- equilibrium modeling of the extraction data with the program SXLSQI was generally consistent with the simplest model of anion exchange, namely, that involving only the monomeric organic-phase ion pairs R+NO3- and R+TcO4- [R+ = HEP+ or (heptyl)(4)N+]. To model extraction at aqueous nitrate concentrations as high as 3 M, it was found necessary to include the Pitzer mixing parameter theta(NO3--TcO4-), whose value (0.0435 +/- 0.0039) upon refinement was found to be the same for both extractants. The logK(exch) values for TcO4-/NO3- anion exchange were found to be 3.84 +/- 0.01 for HEP+NO3- and 3.69 +/- 0.01 for (heptyl)(4)N+NO3-, indicating that the selectivity for TcO4- over NO3- is approximately the same for both extractants. This is consistent with the hypothesis, based on electrostatic principles, that the selectivity in the observed anion-exchange process is favored by the large (and comparable) sizes of the cationic extractants. However, our analysis suggests that a further increase in cation size would not significantly improve TcO4-/NO3- selectivity in chlorobenzene, though selectivity could be so increased in diluents of lower dielectric constant.