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)₄N⁺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)₄N⁺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)₄N⁺]. To model extraction at aqueous nitrate concentrations as high as 3 M, it was found necessary to include the Pitzer mixing parameter θ(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)₄N⁺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.