Some Mechanisms of Extraction of M(II), (III), (IV), and (VI) Metals by Acidic Organophosphorus Extractants

In general, the mono-acidic phosphates and phosphonates are dimeric and the di-acidic phosphates and phosphonic acids are polymeric in the diluents commonly employed in metal extraction studies. Therefore, they may be symbolized, respectively, as (HY)₂ and (H₂Y)_x. The extraction of tracer-level M(III) actinides and lanthanides from a dilute mineral acid by representatives of these two classes of extractants in toluene diluent may be represented, respectively, as: where the subscripts A and O refer to mutually equilibrated aqueous and organic phases. However, in an alcohol diluent the H₂Y extractants appear to be monomeric, and they extract M⁺³ cations with a third-power extractant dependency. In toluene diluent, the HY extractants function as dimers, the extractant dependencies for selected M⁺² and M⁺⁴ cations being: (2-power); Ca⁺², Sr⁺², Ba⁺² (2.5-power, 3-power); Th⁺⁴ (3-power). In certain systems, Th⁺⁴ is extracted as a species containing one or two nitrate groups. Structures of the extracted species are postulated.