Relevance of surface adsorption and aqueous complexation for the separation of Co(II) and Ni(II)

During the solvent extraction of metal ions from an aqueous to an organic phase, organic-soluble extractants selectively target aqueous-soluble ions for transport into the organic phase. In the case of extractants that are also soluble in the aqueous phase, our recent studies of lanthanide ion-extractant complexes at the surface of aqueous solutions have suggested that ion-extractant complexation in the aqueous phase can hinder the solvent extraction process. Here, we investigate a similar phenomenon relevant to the separation of Co(II) and Ni(II) with supporting experiments that probe the separation of Fe(III) and Ni(II). X-ray fluorescence near total reflection and tensiometry are used to characterize the adsorption behavior of Co(II) and Ni(II) at the surface of aqueous solutions containing water-soluble extractants, either bis(2-ethylhexyl) phosphoric acid (HDEHP) or 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP). Consistent with our earlier studies of lanthanides, we observe a comparable adsorption behavior of Co(II) and Ni(II) at the surfaces of both HDEHP and HEHEHP aqueous solutions in spite of the known preference for Co(II) under solvent extraction conditions. Comparison experiments that utilized the water-insoluble extractant di-hexadecyl phosphoric acid (DHDP), confined to a monolayer on the water surface, reveal that Co(II) is preferentially adsorbed to the surface, as expected. This preference for Co(II) is also supported by molecular dynamics simulations of the potential of mean force for the ions interacting with the soluble extractants in water. These results highlight the possibility that complexation of extractants and ions in the aqueous phase can hinder the desired selectivity in the solvent extraction of critical elements.