Solvent extraction of first-row transition metals by thiosubstituted organophosphinic acids.

Abstract

Organophosphorus compounds are well known as solvent-extraction reagents. Two new reagents in this class are Cyanex 302 and Cyanex 301, the respective mono- and dithio analogs of the commercial extractant, Cyanex 272 (bis(2,4,4-trimethylpentyl)-phosphinic acid). The replacement of oxygen by sulfur in these reagents decreases their pKₐ, and enables extraction to be carried out at much lower pH than previously attained. A comparative characterization of Cyanex 272, Cyanex 302, and Cyanex 301 is undertaken. The aggregation and partitioning behavior of these reagents is determined. A comparison of the solvent extraction behavior of first-row transition-metal ions from manganese to zinc in acidic sulfate solution by these reagents is reported. Distribution coefficients shift to lower pH with increasing sulfur substitution of the extractant, the greatest effect being observed for soft Lewis acids. These reagents are found to be strong extractants for the transition metals examined, but poor extractants for alkaline-earth ions and manganese(II). Certain metal ions, in particular copper(II) and silver(I), are, however, extremely difficult to strip, which may mitigate against potential applications. Stoichiometry of the extraction reactions and the nature of the metal complexes formed are postulated based on slope-analysis techniques and spectroscopic and molecular-mass measurements. Molecular modelling of the extractants and extracted species is presented. Some reasons for the enhanced stability of the complexes formed with certain metals are discussed in terms of d-electron effects, donor-/acceptor-atom interactions, ligand-field observations, and steric effects associated with the extractants.