Liquid–Liquid Extraction of Uranyl by TBP: The TBP and Ions Models and Related Interfacial Features Revisited by MD and PMF Simulations

Metastable precipitation and ion-extractant transport in liquid-liquid separations of trivalent elements

The extractant-assisted transport of metal ions from aqueous to organic environments by liquid-liquid extraction has been widely used to separate and recover critical elements on an industrial scale. While current efforts focus on designing better extractants and optimizing process conditions, the mechanism that underlies ionic transport remains poorly understood. Here, we report a nonequilibrium process in the bulk aqueous phase that influences interfacial ion transport: the formation of metastable ion-extractant precipitates away from the liquid-liquid interface, separated from it by a depletion region without precipitates. Although the precipitate is soluble in the organic phase, the depletion region separates the two and ions are sequestered in a long-lived metastable state. Since precipitation removes extractants from the aqueous phase, even extractants that are sparingly soluble in water will continue to be withdrawn from the organic phase to feed the aqueous precipitation process. Solute concentrations in both phases and the aqueous pH influence the temporal evolution of the process and ionic partitioning between the precipitate and organic phase. Aqueous ion-extractant precipitation during liquid-liquid extraction provides a reaction path that can influence the extraction kinetics, which plays an important role in designing advanced processes to separate rare earths and other minerals.

Recent progress in application of surface X-ray scattering techniques to soft interfacial films

X-ray reflection (XR) and surface grazing incidence X-ray diffraction GIXD) techniques have traditionally been used to evaluate the structure of soft interfacial films. In recent years, the use of synchrotron radiation and two-dimensional detectors has enabled high resolution and high speed measurements of interfacial films, which makes it possible to evaluate more detailed and complex interfacial film structures and adsorption dynamics. In this review, we will provide an overview of recent progress in structural characterization of simple oil/water interfaces, interfacial films of biologically relevant materials, oil/water interfaces for extraction of rare metal ions, and adsorption of nanoparticles. Examples of the application of time-resolved XR methods and surface sensitive techniques such as GISAXS and surface X-ray fluorescence analysis will also be presented.

Aqueous Interfaces in Chemical Separations

Chemical separations play a vital role in refinery and reprocessing of critical materials, such as platinum group metals, rare earths, and actinides. The choice of separation system─whether it is liquid-liquid extraction (LLE), sorbents, or membranes─depends on specific needs and applications. In almost all separation processes, the desired metal ions adsorb or transfer across an aqueous interface, such as the solid/liquid interface in sorbents or oil/water interfaces in LLE. Despite these separation technologies being extensively used for decades, our understanding of the molecular-scale mechanisms governing ion adsorption and transport at interfaces remains limited. This knowledge gap presents a significant challenge in meeting the increasing demands for these critical materials due to their growing use in advanced technologies. Fortunately, recent advancements in surface-specific experimental and computational techniques offer promising avenues to bridge this gap and facilitate the development of next-generation separation systems. Interestingly, unanswered questions regarding interfacial phenomena in chemical separations hold great relevance to various fields, including energy storage, geochemistry, and atmospheric chemistry. Therefore, the model interfacial systems developed for studying chemical separations, such as amphiphilic molecules assembled at a solid/water, air/water, or oil/water interface, may have far-reaching implications, extending beyond separations and opening doors to addressing a wide range of scientific inquiries. This perspective discusses recent interfacial studies elucidating amphiphile-ion interactions in chemical separations of metal ions. These studies provide direct, molecular-scale information about solute and solvent behavior at aqueous interfaces, including multivalent and complex ions in highly concentrated solutions, which play key roles in LLE of critical materials.

Liquid-Liquid Extraction of the Eu(III) Cation by BTP Ligands into Ionic Liquids: Interfacial Features and Extraction Mechanisms Investigated by MD Simulations

What happens at the ionic-liquid (IL)/water interface when the Eu³⁺ cation is complexed and extracted by bis(dimethyltriazinyl) pyridine "BTP" ligands has been investigated by molecular dynamics and potential of mean force simulations on the interface crossing by key species: neutral BTP, its protonated BTPH⁺ form, Eu³⁺, and the Eu(BTP)₃³⁺ complex. At both the [BMI][Tf₂N]/water and [OMI][Tf₂N]/water interfaces, neither BTP nor Eu(BTP)₃³⁺ are found to adsorb. The distribution of Eu(BTP)₂³⁺ and Eu(BTP)³⁺ precursors of Eu(BTP)₃³⁺, and of their nitrate adducts, implies the occurrence of a stepwise complexation process in the interfacial domain, however. The analysis of the ionic content of the bulk phases and of their interface before and after extraction highlights the role of charge buffering by interfacial IL cations and anions, by different amounts depending on the IL. Comparison of ILs with octanol as the oil phase reveals striking differences regarding the extraction efficiency, the affinity of Eu(BTP)₃³⁺ for the interface, the effects of added nitric acid and of counterions (NO₃^- vs Tf₂N^-), charge neutralization mechanisms, and the extent of "oil" heterogeneity. Extraction into octanol is suggested to proceed via adsorption at the surface of water pools, nanoemulsions, or droplets, with marked counterion effects.

Influence of uranyl complexation on the reaction kinetics of the dodecane radical cation with used nuclear fuel extraction ligands (TBP, DEHBA, and DEHiBA)

Specialized extractant ligands - such as tri-butyl phosphate (TBP), N,N-di-(2-ethylhexyl)butyramide (DEHBA), and N,N-di-2-ethylhexylisobutryamide (DEHiBA) - have been developed for the recovery of uranium from used nuclear fuel by reprocessing solvent extraction technologies. These ligands must function in the presence of an intense multi-component radiation field, and thus it is critical that their radiolytic behaviour be thoroughly evaluated. This is especially true for their metal complexes, where there is negligible information on the influence of complexation on radiolytic reactivity, despite the prevalence of metal complexes in used nuclear fuel reprocessing solvent systems. Here we present a kinetic investigation into the effect of uranyl (UO₂²⁺) complexation on the reaction kinetics of the dodecane radical cation (RH˙⁺) with TBP, DEHBA, and DEHiBA. Complexation had negligible effect on the reaction of RH˙⁺ with TBP, for which a second-order rate coefficient (k) of (1.3 ± 0.1) × 10¹⁰ M^-1 s^-1 was measured. For DEHBA and DEHiBA, UO₂²⁺ complexation afforded an increase in their respective rate coefficients: k(RH˙⁺ + [UO₂(NO₃)₂(DEHBA)₂]) = (2.5 ± 0.1) × 10¹⁰ M^-1 s^-1 and k(RH˙⁺ + [UO₂(NO₃)₂(DEHiBA)₂]) = (1.6 ± 0.1) × 10¹⁰ M^-1 s^-1. This enhancement with complexation is indicative of an alternative RH˙⁺ reaction pathway, which is more readily accessible for [UO₂(NO₃)₂(DEHBA)₂] as it exhibited a much larger kinetic enhancement than [UO₂(NO₃)₂(DEHiBA)₂], 2.6× vs. 1.4×, respectively. Complementary quantum mechanical calculations suggests that the difference in reaction kinetic enhancement between TBP and DEHBA/DEHiBA is attributed to a combination of reaction pathway (electron/hole transfer vs. proton transfer) energetics and electron density distribution, wherein attendant nitrate counter anions effectively 'shield' TBP from RH˙⁺ electron transfer processes.

Unexpected inverse correlations and cooperativity in ion-pair phase transfer

Liquid/liquid extraction is one of the most widely used separation and purification methods, where a forefront of research is the study of transport mechanisms for solute partitioning and the relationships that these have to solution structure at the phase boundary. To date, organized surface features that include protrusions, water-fingers, and molecular hinges have been reported. Many of these equilibrium studies have focused upon small-molecule transport – yet the extent to which the complexity of the solute, and the competition between different solutes, influence transport mechanisms have not been explored. Here we report molecular dynamics simulations that demonstrate that a metal salt (LiNO₃) can be transported via a protrusion mechanism that is remarkably similar to that reported for H₂O by tri-butyl phosphate (TBP), a process that involves dimeric assemblies. Yet the LiNO₃ out-competes H₂O for a bridging position between the extracting TBP dimer, which in-turn changes the preferred transport pathway of H₂O. Examining the electrolyte concentration dependence on ion-pair transport unexpectedly reveals an inverse correlation with the extracting surfactant concentration. As [LiNO₃] increases, surface adsorbed TBP becomes a limiting reactant in correlation with an increased negative surface charge induced by excess interfacial NO₃⁻, however the rate of transport is enhanced. Within the highly dynamic interfacial environment, we hypothesize that this unique cooperative effect may be due to perturbed surface organization that either decreases the energy of formation of transporting protrusion motifs or makes it easier for these self-assembled species to disengage from the surface.

A forefront of research in separations science (specifically liquid–liquid extraction) is the study of transport mechanisms for solute partitioning, and the relationships that these have to solution structure at the phase boundary.

Speciation analysis the complexation of uranyl nitrate with tri-n-butyl phosphate in supercritical CO2

The complexation of solid uranyl nitrate with tri-n-butyl phosphate (TBP) in supercritical CO₂ is quite different from that of a liquid–liquid extraction system because fewer water molecules are involved. Here, the complexation mechanism was investigated by molecular dynamics simulation, emphasising on speciation distribution analysis. In the anhydrous uranyl nitrate system, poly-core uranyl-TBP species [UO₂(NO₃)₂]₂·3TBP and [UO₂(NO₃)₂]₃·3TBP were formed in addition to the predominant [UO₂(NO₃)₂]·1TBP and [UO₂(NO₃)₂]·2TBP species. The poly-core species was mainly constructed via the linkage of U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O⋯U contributed by pre-developed [UO₂(NO₃)₂]·1TBP species. However, in the hydrated uranyl nitrate system, TBP·[UO₂(NO₃)₂]·H₂O species form, preventing the formation of the poly-core species. The complexation developed differently depending on the TBP to the uranyl nitrate ratio, the solute densities and the participation of water. It suggested that the kinetically favoring species would gradually convert into the thermodynamically stable species [UO₂(NO₃)₂]·2TBP by ligand exchange.

Uranyl nitrate complex with TBP in supercritical CO₂ could form 1 : 1 and 1 : 2 species, and further generate 2 : 3 and 3 : 3 poly-core uranyl species.

Molecular Dynamics Studies on the Effect of Surface Roughness and Surface Tension on the Thermodynamics and Dynamics of Hydronium Ion Transfer Across the Liquid/Liquid Interface

Molecular dynamics simulations are used to examine the effect of surface roughness and surface tension on the transfer of the classical hydronium ion (H₃O⁺) across the water/1,2-dichloroethane interface. Free energy of transfer, hydration structure, and dynamics as a function of the ion location along the interface normal are calculated with six different values of a control parameter whose variation modifies the surface tension without impacting the bulk properties of the two solvents. Transfer of the classical hydronium ion across the water/1,2-dichloroethan interface involves the cotransfer of three hydration shell water molecules, independent of the surface tension. However, as the interaction between the two liquids weakens, a rise in interfacial tension and decrease in intrinsic water fingering and capillary fluctuations result in fewer water molecules cotransported with the ion in the second shell and a reduction in the length of the finger that the ion is attached to, consistent with the reduced size of the second hydration shell. First shell water residence time and lateral ion diffusion constants vary with the surface tension in a way that is consistent with the abovementioned structural insight.

Hierarchical phenomena in multicomponent liquids: simulation methods, analysis, chemistry

Complex, multicomponent, solutions have often been studied solely through the lens of specific applications of interest. Yet advances to both simulation methodologies (enhanced sampling, etc.) and analysis techniques (network analysis algorithms and others), are creating a trove of data that reveal transcending characteristics across vast compositional phase space. This perspective discusses technical considerations of the reliable and accurate simulations of complex solutions, followed by the advances to analysis algorithms that elucidate coupling of different length and timescale behavior (hierarchical phenomena). The different manifestations of hierarchical phenomena are presented across an array of solution environments, emphasizing fundamental and ongoing science questions. With a more advanced molecular understanding in hand, a quintessential application (solvent extraction) is discussed, where significant opportunities exist to re-imagine the technical scope of an established technology.

Key Factors Determining Efficiency of Liquid-Liquid Extraction: Implications from Molecular Dynamics Simulations of Biphasic Behaviors of CyMe4-BTPhen and Its Am(III) Complexes

CyMe₄-BTPhen (2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline, denoted as L) has been considered as a promising extractant in lanthanide(III)/actinide(III) separation. Vast endeavors in its application put forward a compelling need on the understanding of the underlying mechanism in the liquid-liquid extraction. To address the issue of its dynamics in biphasic systems, we carried out molecular dynamics (MD) simulations of L and its complexes with a heavy f-block metal ion, americium(III) (Am³⁺) in "oil"/water binary solvents. Two types of organic phases have been considered, differing in the presence of octanol in the bulk n-dodecane or not, and the distribution of the solutes and their interfacial behaviors have been investigated. Two of the key factors that determine the efficiency of a liquid-liquid extraction protocol were delineated and discussed, that is, the appropriate ligand to enhance the lipophilicity of AmL complexes and appropriate way to form ion pairs to minimize the attraction between the complexes and aqueous phase. The simulations showed that the charge states of both ligand and AmL complexes were strongly correlated with their phase behavior, and the migration of neutral species was driven by van der Waals interactions while that of charged species by electrostatic interactions, indicating stronger lipophilicity of the former than the latter. The presence of octanol facilitated the migration of the ligand from the interface to the organic phase via hydrogen bond between its polar head and the ligand or the AmL complexes and constituted a polar core in the organic phase. This work bridged the widely used liquid-liquid extraction technique in chemistry to a fundamental chemical concept, that is, minimization of hydrophilicity and maximization of lipophilicity to facilitate phase transfer from the aqueous phase to the organic phase, and is expected to improve the understanding of dynamics of ligands and their complexes with metal ions and to contribute to the development of efficient protocols for phase transfer of target species.

Microscopic Behaviors of Tri- n-Butyl Phosphate, n-Dodecane, and Their Mixtures at Air/Liquid and Liquid/Liquid Interfaces: An AMBER Polarizable Force Field Study

In solvent extraction processes for recovering metal ions from used nuclear fuel, as well as other industrial applications, a better understanding of the metal complex phase transfer phenomenon would greatly aid ligand design and process optimization. We have approached this challenge by utilizing the classical molecular dynamics simulations technique to gain visual appreciation of the vapor/liquid and liquid/liquid interface between tri- n-butyl phosphate (TBP) and n-dodecane with air and water. In this study, we successfully reparameterized polarizable force fields for TBP and n-dodecane that accurately reproduced several of their thermophysical properties such as density, heat of vaporization, and dipole moment. Our models were able to predict the surface and interfacial tension of different systems when compared to experimental results that were also performed by us. Through this study, we gained atomistic understanding of the behaviors of TBP and n-dodecane at the interface against air and water, useful in further computational studies of such systems. Finally, our studies indicate that the initial configuration of a simulation may have a large effect on the final result.

UO22+ structure in solvent extraction phases resolved at molecular and supramolecular scales: a combined molecular dynamics, EXAFS and SWAXS approach

We developed a polarizable force field for unraveling the UO₂²⁺ structure in both aqueous and solvent extraction phases.

Mechanism of phase transfer of uranyl ions: a vibrational sum frequency generation spectroscopy study on solvent extraction in nuclear reprocessing

VSFG study on interfaces of TBP/uranyl aqueous solutions shows that uranyl ion does not form complexes with TBP at the interface, and proposes a new extraction mechanism.

Determination of the Structures of Uranyl-Tri-n-butyl-Phosphate Aggregates by Coupling Experimental Results with Molecular Dynamic Simulations

The complex structure of a plutonium uranium refining by extraction (PUREX) process organic phase was characterized by combining results from experiments and molecular dynamics simulations. For the first time, the molecular interactions between tri-n-butyl phosphate (TBP) and the extracted solutes, as well as TBP aggregation after the extraction of water and/or uranyl nitrate, were described and analyzed concomitantly. Coupling molecular dynamics simulations with small- and wide-angle X-ray scattering (SWAXS) experiments can lead to simulated organic solutions that are representative of the experimental ones, even for high extractant and solute concentrations. Furthermore, this coupling is well adapted for the interpretation of SWAXS experiments without preliminary hypothesis on the size or shape of aggregates. The results link together previous literature studies obtained for each level of depiction separately (complexation or aggregation). Without uranium, or at low metal concentration, almost no aggregation was observed. At high uranium concentration, organic phases contain small [UO₂ (NO₃ )₂ (TBP)₂ ]_n polymetallic aggregates (with n=2 to 4), in which the 1:2 U/TBP stoichiometry is preserved.

Quantifying Dimer and Trimer Formation by Tri-n-butyl Phosphates in n-Dodecane: Molecular Dynamics Simulations

Tri-n-butyl phosphate (TBP), a representative of neutral organophosphorous ligands, is an important extractant used in the solvent extraction process for the recovery of uranium and plutonium from spent nuclear fuel. Microscopic pictures of TBP isomerism and its behavior in n-dodecane diluent were investigated utilizing MD simulations with previously optimized force field parameters for TBP and n-dodecane. Potential mean force (PMF) calculations on a single TBP molecule show seven probable TBP isomers. Radial distribution functions (RDFs) of TBP suggest the existence of TBP trimers at high TBP concentrations in addition to dimers. 2D PMF calculations were performed to determine the angle and distance criteria for TBP trimers. The dimerization and trimerization constants of TBP in n-dodecane were obtained and match our own experimental values using the FTIR technique. The new insights into the conformational behaviors of the TBP molecule as a monomer and as part of an aggregate could greatly aid in the understanding of the complexation between TBP and metal ions in a solvent extraction system.

Solvent Extraction: Structure of the Liquid-Liquid Interface Containing a Diamide Ligand

Knowledge of the (supra)molecular structure of an interface that contains amphiphilic ligand molecules is necessary for a full understanding of ion transfer during solvent extraction. Even if molecular dynamics already yield some insight in the molecular configurations in solution, hardly any experimental data giving access to distributions of both extractant molecules and ions at the liquid-liquid interface exist. Here, the combined application of X-ray and neutron reflectivity measurements represents a key milestone in the deduction of the interfacial structure and potential with respect to two different lipophilic ligands. Indeed, we show for the first time that hard trivalent cations can be repelled or attracted by the extractant-enriched interface according to the nature of the ligand.

Liquid–liquid extraction of alkali cations by 18-crown-6: complexation and interface crossing studied by MD and PMF simulations

The 18C6/M⁺Pic⁻complexes form and adsorb “right at the nano-interface” where 18C6 prefers the K⁺guest.

Passage of TBP–uranyl complexes from aqueous–organic interface to the organic phase: insights from molecular dynamics simulation

Water/organic interface representing TBP orientation for neutral versus acidic interface and occurrence of UO₂²⁺–TBP–NO₃⁻ species in various stoichiometry.

Uranyl extraction by N,N-dialkylamide ligands studied using static and dynamic DFT simulations

DFT/MM-MD simulations highlight the structure and dynamics of mixed uranyl/nitrato/monoamides (L) complexes at an “oil”/water interface.