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 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), Ni­(II), and Fe­(III). X-ray fluorescence near total reflection and tensiometry are used to characterize ion adsorption behavior 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), as well as adsorption to a monolayer of water-insoluble extractant dihexadecyl phosphoric acid (DHDP) at the aqueous–vapor interface. Competitive adsorption of Ni­(II) and Fe­(III) utilizing either HDEHP or DHDP illustrates the essential feature of the recent lanthanide studies that the ion, which is preferentially extracted in liquid–liquid extraction, Fe­(III), is found preferentially adsorbed to the water–vapor interface only in the presence of the water-insoluble extractant DHDP. A more subtle competition produces 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 with a monolayer of DHDP reveal that Co­(II) is preferentially adsorbed to the surface. This preference for Co­(II) is also supported by molecular dynamics simulations of the potential of mean force of ions interacting with the soluble extractants in water. These results highlight the possibility that complexation of extractants and ions in the aqueous phase can alter selectivity in the solvent extraction of critical elements.

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.

Solvent extraction of trivalent rare earth metal ions by organophosphorus extractants proceeds via binding of phosphoric acid headgroups to the metal ion. Water molecules in the tightly bound first hydration shell of the metal ions must be displaced by oxygen atoms from phosphoric acid headgroups. Here, we use classical MD simulations to explore the event in which a fully hydrated Er3+ binds to its first phosphoric acid headgroup. Approach of the headgroup into the region between the first and second hydration shells leads to a fast ejection of a water molecule that is accompanied by reordering of the hydration water molecules, including discretization of their angular positions and collective rotation about the metal ion. The water molecule ejected from the first shell is located diametrically opposite from the binding oxygen. Headgroup binding places a headgroup oxygen closer to Er3+ than its first hydration shell and creates a loosely bound water which subsequently exchanges between the first shell and its environment. This second exchange of water also occurs at discrete angular positions. This geometrical aspect of binding may be of relevance to understanding the binding and transport of ion-extractant complexes that are expected to occur at the organic-aqueous liquid-liquid interface used in solvent extraction processes.