Kinetics and mechanism of Eu(III) transfer in tributyl phosphate microdroplet/HNO3 aqueous solution system revealed by fluorescence microspectroscopy

In this study, we reveal an Eu(III) extraction mechanism at the interface between HNO₃ and tributyl phosphate (TBP) solutions using fluorescence microspectroscopy. The mass transfer rate constant at the interface is obtained from the analysis of fluorescence intensity changes during the forward and backward extractions at various HNO₃ and TBP concentrations to investigate the reaction mechanism. This result indicates that one nitrate ion reacts with Eu(III) at the interface, whereas TBP molecules are not involved in the interfacial reaction, which is different from the results obtained using the NaNO₃ solution in our previous study. We demonstrate that the chemical species of Eu(III) complex with nitrate ion and TBP in the aqueous solution play an important role for the extraction mechanism. The rate constants of the interfacial reactions in the forward and backward extractions are (4.0-5.0) × 10^-7 m M^-1 s^-1 and (3.2-3.3) × 10^-6 m s^-1, respectively. We expect that our revealed mechanism provides useful and fundamental knowledge for actual solvent extraction.

Polarization analysis for small-angle neutron scattering with a 3He spin filter at a pulsed neutron source

Neutron polarization analysis (NPA) for small-angle neutron scattering (SANS) experiments using a pulsed neutron source was successfully achieved by applying a 3He spin filter as a spin analyzer for the neutrons scattered from the sample. The cell of the 3He spin filter gives a weak small-angle scattering intensity (background) and covers a sufficient solid angle for performing SANS experiments. The relaxation time of the 3He polarization is sufficient for continuous use for approximately 2 days, thus reaching the typical duration required for a complete set of SANS experiments. Although accurate evaluation of the incoherent neutron scattering, which is predominantly attributable to the extremely large incoherent scattering cross section of hydrogen atoms in samples, is difficult using calculations based on the sample elemental composition, the developed NPA approach with consideration of the influence of multiple neutron scattering enabled reliable decomposition of the SANS intensity distribution into the coherent and incoherent scattering components. To date, NPA has not been well established as a standard technique for SANS experiments at pulsed neutron sources such as the Japan Proton Accelerator Research Complex (J-PARC) and the US Spallation Neutron Source. It is anticipated that this work will contribute significantly to the accurate determination of the coherent neutron scattering component for scatterers in various types of organic sample systems in SANS experiments at J-PARC, particularly for systems involving competition between the coherent and incoherent scattering intensity.

Competitive extraction of Li, Na, K, Mg and Ca ions from acidified aqueous solutions into chloroform layer containing diluted alkyl phosphates

Alkyl phosphates were extensively used in liquid-liquid extraction of lanthanides and actinides, but to a lesser extent for alkali and alkaline earth metals. The high amount of alkyl phosphate, which is usually used in the organic layer (>40 wt%), is not favoured due to its corrosive effect and toxicity. In the present work, diluted chloroform solutions (20.0 mM) of tri-n-butyl phosphate (TBP), tris(2-ethylhexyl) phosphates (TRIS) and bis(2-ethylhexyl) phosphate (BIS) were investigated for their extraction of Li, Na, K, Mg and Ca ions. The extraction experiments were conducted on 7.0 M HNO₃ aqueous solutions containing 60.0 mM of metal ions in binary (Li⁺ and Mg²⁺), ternary (Li⁺, Na⁺ and K⁺) and quinary (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺) mixtures. The Li⁺ selectivity over Mg²⁺ was very high in the binary system. Remarkably, increasing HNO₃ concentration in the aqueous layer had opposing effect on the extraction of Li⁺ (positive) and Mg²⁺ (negative). However, the selectivity for Li⁺ became less dramatic in the case of ternary and quinary system, though the selectivity varied with initial metal concentrations. The amounts of water and NO₃^- transferred into the organic layer demonstrated their synergistic effect on extracting metal ions. In the ternary and quinary systems, the total concentrations of metal ions in the organic layer (ranged from 49 to 85 mM) were higher than the concentration of ligand in the organic layer (20.0 mM), suggesting that metal ions may be extracted into water/ligand/NO₃^- aggregates in the organic layer. TBP, TRIS and BIS do not have significant difference in their extraction behaviour. The FTIR results indicated formation of P⁺-O^-M⁺/M²⁺ in the solid TBP/metal complex.

Modeling of the Competition between Uranyl Nitrate and Nitric Acid upon Extraction with Tri-n-butyl Phosphate

Uranium is a strategic element and plays an important role in energy resources. A H₂O-HNO₃-UO₂(NO₃)₂-TBP (tri-n-butyl phosphate)-diluent system is commonly used for uranium separation and purification in liquid-liquid extraction. Uranyl nitrate is promoted by the existence of nitrate at low HNO₃ concentrations but is inhibited at high HNO₃ concentrations. Considering the competitive extraction between HNO₃ and UO₂(NO₃)₂, a generic extraction model is developed. The activities of components in the aqueous phase were estimated using Pitzer models. The thermodynamic equilibrium constants and Pitzer parameters were regressed by experimental data. The resulting model was able to successfully predict uranyl nitrate, nitric acid, and water extraction over a large range of conditions (U, 0-1.8 mol/L; HNO₃, 0-10 mol/L; TBP, 5-100 vol %) within average absolute relative deviations of 11.2, 15.7, and 23.8%, respectively. The predicted results show that water and nitric acid were extracted as di-solvates HNO₃·(TBP)₂·H₂O and (TBP)₂·2H₂O at low nitric acid concentrations, with the formation of mono-solvates HNO₃·TBP and HNO₃·TBP·H₂O as the acid concentration increased. Uranyl nitrate was shown to be rejected from the organic phase as the formation of HNO₃·TBP and HNO₃·TBP·H₂O in acid was extracted at high acid concentrations.

Effects of temperature on the extraction of U(VI) and Pu(IV) by tris(2-methylbutyl) phosphate from nitric acid media

Extraction behavior of tris(2-methylbutyl) phosphate (T2MBP), a higher homologue of tri-n-butyl phosphate (TBP), in the extraction of U(VI) and Pu(IV) from nitric acid media has been evaluated in the present study. Distribution ratios for the extraction of these metal ions by T2MBP were measured as a function of equilibrium aqueous phase nitric acid concentration, extractant concentration and temperature to understand the temperature effects on extraction. Enthalpies of extraction of T2MBP/n-dodecane-U(VI)-HNO3 and T2MBP/n-dodecane-Pu(IV)-HNO3 systems were calculated from the distribution ratio data using “second law” method. The data reveal that the extraction of U(VI) by T2MBP is exothermic in nature, whereas that of Pu(IV) is endothermic.

The supramolecular speciation: a key for improved understanding and modelling of chemical reactivity in complex systems

The objective of this paper is to show through various examples that the "supramolecular" speciation is a key for the understanding and modelling of phenomena as varied as unusually rapid mass transfers in solvent extraction, organic phase splitting in liquid-liquid systems and complex behaviours of the organic matter in the environment.