Water Interfaces Group – UIC Department of Physics

Solvent Extraction – Separating Ions from Aqueous Solutions

Selective extraction of metal ions from a complex aqueous mixture into an organic phase is used to separate toxic or radioactive metal ions from polluted aqueous environments and nuclear waste, as well as to produce industrially relevant metals, such as rare earth ions. Selectivity arises from the choice of an extractant amphiphile, dissolved in the organic phase, which interacts preferentially with the target metal ion. The extractant-mediated process of ion transport from an aqueous to an organic phase takes place at the aqueous-organic interface; nevertheless, little is known about the molecular mechanism of this process in spite of its importance. Although we have shown that state-of-the-art X-ray scattering is uniquely capable of probing molecular ordering at a liquid-liquid interface with sub-nanometer spatial resolution, utilizing this capability to investigate interfacial dynamical processes of short temporal duration remains a challenge.

Recently, we demonstrated that a temperature-driven adsorption transition can switch the extraction on and off by controlling adsorption and desorption of extractants at the oil-water interface [1]. Lowering the temperature through this transition immobilizes a supramolecular ion-extractant complex at the interface during the extraction (Figure 1), providing us with the opportunity to investigate an intermediate molecular state of the extraction process. In our experiments, we measured X-ray reflectivity to determine the electron density variation with depth at the oil-water interface, providing information about the interfacial composition and arrangement of extractants, ions, and other molecules. We also measured x-ray fluorescence near-total reflection, allowing us to count the number of erbium atoms at the interface.

Solvent Extraction

Figure 1. Supramolecular Er-extractant (erbium-DHDP) complexes that form at the interface between an aqueous ErBr3 solution and a dodecane solution of DHDP (dihexadecyl phosphate) in the midst of the solvent extraction process. The blue dashed line illustrates the complex consisting of 3 molecules of DHDP, 1 erbium ion, and a few water molecules. The condensed layer of complexes below the adsorption transition forms the inverted bilayer illustrated here.

Combining the results of these two types of X-ray measurements, we assembled the model of the extractant-metal complex at the interface illustrated in Figure 1 [1]. Although we were expecting a monolayer of extractants, which is the common arrangement of amphiphiles at interfaces at equilibrium, instead we found an inverted bilayer, an unusual structure consisting of an inner layer of erbium ions sandwiched between two layers of extractants. The ratio of extractant to metal ion was three to one, allowing each negatively charged extractant to shield one of erbium's three positive charges. This observation suggested that tiny micelles — 3 extractants, 1 metal ion, and a few water molecules — form at the interface and shuttle the metals across the oil-water interface. X-ray spectroscopy verified that these micelles exist in the bulk liquid phase after extraction takes place.

We have also investigated other metals, such as strontium, to demonstrate how different ion charges affect the mechanism of solvent extraction [2], as well as measurements of yttrium, which has the same charge as erbium, and also forms an inverted bilayer [work in progress]. The molecular picture of the extraction mechanism that is being developed by these experiments should lead to improved processes for both removing toxic elements from water, as well as improving the extraction of valuable metals for further technological use.

For an introduction to X-ray scattering techniques used for studying liquid surfaces and interfaces, see this link.

For a more detailed description of these techniques, see the book described here.


[1] Observation of a Rare Earth Ion-Extractant Complex Arrested at the Oil-Water Interface During Solvent Extraction, Wei Bu, Hao Yu, Guangming Luo, Mrinal K. Bera, Binyang Hou, Adam W. Schuman, Binhua Lin, Mati Meron, Ivan Kuzmenko, Mark R. Antonio, L. Soderholm, Mark L. Schlossman, Journal of Physical Chemistry B 118, 10662-10674 (2014).

[2] X‐ray Studies of Interfacial Strontium−Extractant Complexes in a Model Solvent Extraction System, Wei Bu, Miroslav Mihaylov, Daniel Amoanu, Binhua Lin, Mati Meron, Ivan Kuzmenko, L. Soderholm, Mark L. Schlossman, Journal of Physical Chemistry B 118, 12486-12500 (2014).