Observation of a Rare Earth Ion–Extractant Complex Arrested at the Oil–Water Interface During Solvent Extraction

Abstract

Selective extraction of metal ions from a complex aqueous mixture into an organic phase is used to separate toxic or radioactive metals from polluted 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 despite its importance. Although state-of-the-art X-ray scattering is uniquely capable of probing molecular ordering at a liquid-liquid interface with subnanometer spatial resolution, utilizing this capability to investigate interfacial dynamical processes of short temporal duration remains a challenge. We show that a temperature-driven adsorption transition can be used to turn the extraction on and off by controlling adsorption and desorption of extractants at the oil-water interface. Lowering the temperature through this transition immobilizes a supramolecular ion-extractant complex at the interface during the extraction of rare earth erbium ions. Under the conditions of these experiments, the ion-extractant complexes condense into a two-dimensional inverted bilayer, which is characterized on the molecular scale with synchrotron X-ray reflectivity and fluorescence measurements. Raising the temperature above the transition leads to Er ion extraction as a result of desorption of ion-extractant complexes from the interface into the bulk organic phase. XAFS measurements of the ion-extractant complexes in the bulk organic phase demonstrate that they are similar to the interfacial complexes.

High purity silicon materials prepared through wet-chemical and electrochemical approaches

“…Solvent extraction has been widely employed for the separation of metals such as Cu, Ni, Co, and rare earth elements, as well as in the field of nuclear fuel technology like actinide recovery and purification, radionuclide production [7,8,9]. It is basically composed to mixing for extraction, settling and stripping procedure in each stage.…”

X-ray Studies of Interfacial Strontium–Extractant Complexes in a Model Solvent Extraction System

“…The same temperature protocol was used for these experiments on SrCl 2 to facilitate comparison with the earlier experiments on ErCl 3 . 9 To ensure that X-rays probe the high density phase of DHDP at the dodecane−water interface, the experimental temperature T X-ray was typically 5−8°C below the transition temperature T o for each sample. Figure 5a illustrates X-ray reflectivity normalized to the Fresnel reflectivity, R/R F , from interfaces between 10 −4 M DHDP in dodecane and 10 −5 M SrCl 2 in water for all values of pH studied.…”

“…The electron density ρ 1 in the head group is due to a combination of water molecules, Sr 2+ ions, phosphate head group (PO 4 ) of DHDP, which may be protonated at lower pH (see Chart 1). Assuming that there is no unoccupied volume in slab 1, the total number of electrons in slab 1 is given by 8) and the volume of slab 1 is 9) where N j is the number of each species per DHDP molecule in slab 1 (N PO 4 = 1). …”

“…9 This structure has been rationalized by considering that Er ions at the water surface, upon complexation with DHDP molecules approaching from the oil side of the interface, undergo a dynamic interfacial instability that drags them out of the water to form an ion−extractant complex at the interface. The previously mentioned experiments were designed to trap ion−extractant complexes at the interface in the midst of this dynamical process.…”

Structural characterization of soft interfaces by standing-wave fluorescence with X-rays and neutrons

“…In addition, the attenuation of incident and fluorescence radiation by the liquid media and the requirement of horizontal sample geometries pose limitations to the fluorescence detection efficiency. Only recently, Bu et al reported the quantification by XSW fluorescence under total reflection of heavy ion amounts immobilized at oil/water interfaces via extractant molecules [64,65]. Their measurement configuration corresponds to Fig.…”