Noboru Aoyagi

Organic phase demixing remains a major issue of liquid-liquid extraction processes due to an incomplete understanding of its underlying mechanisms. In the context of uranium extraction with aliphatic amines diluted in linear alkanes, we investigated this phase separation by characterizing the multiscale structure of the demixed phases. We determined the extractant phase stability threshold, defined as the maximum concentration of extractant before macroscopic phase splitting occurs. Using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), we further analyzed the microstructure of the two resulting organic phases. With all aliphatic diluents, the heavier phase usually named as the third phase, consists of weak water-in-oil (w/o) aggregates with interdigitated hydrocarbon chains, while the lighter phase is mainly composed of monomeric amines and of amines aggregates containing water. Even after centrifugation, SANS detects residual deuterated diluent molecules in the third phase. A fraction of these w/o aggregates contains uranium(VI) cations (U-loaded aggregates), which exhibit stronger attractive interactions than unloaded aggregates. Phase demixing is therefore the results of the segregation of U-loaded aggregates into large clusters, detected as a Porod-type decay in SAXS at low q-values.

Dissipative structures often appear as an unstable counterpart of ordered structures owing to fluctuations that do not form a homogeneous phase. Even a multiphase mixture may simultaneously undergo one chemical reaction near equilibrium and another one that is far from equilibrium. Here, we observed in real time crystal seed formation and simultaneous nanocrystal aggregation proceeding from Ce^IV complexes to CeO₂ nanoparticles in an acidic aqueous solution, and investigated the resultant hierarchical nanoarchitecture. The formed particles exhibited two very different size ranges, resulting in further pattern formation with opalescence. The hierarchically assembled structures in solutions were CeO₂ colloids, viz. primary core clusters (1–3 nm) of crystalline ceria and secondary clusters (20–30 nm) assembled through surface ions. Such self-assembly is widespread in multi-component complex fluids, paradoxically moderating hierarchical reactions. Stability and instability are not only critical but also complementary for co-optimisation around the nearby free energy landscape prior to bifurcation.

There is an ever-increasing body of evidence that metallic complexes involving amphiliphic ligands do not form normal solutions in organic solvents. Instead, they form complex fluids with intricate structures. For example, the metallic complexes may aggregate into clusters, and these clusters themselves may aggregate into superclusters. To gain a deeper insight into the mechanisms at play, we have used an improved force field to conduct extensive molecular dynamics simulations of a system composed of zirconium nitrate, water, nitric acid, tri-n-butyl phosphate, and n-octane. The important new finding is that a dynamic equilibrium between the cis and trans isomers of the metal complex is likely to play a key role in the aggregation behavior. The isolated cis and trans isomers have similar energies, but simulation indicates that the clusters consist predominantly of cis isomers. With increasing metal concentration, we hypothesize that more clustering occurs and the chemical equilibrium shifts toward the cis isomer. It is possible that such isomeric effects play a role in the liquid-liquid extraction of other species and the inclusion of such effects in flow sheet modeling may lead to a better description of the process.

Dissipative structures often appear as an unstable counterpart of ordered structures owing to fluctuations that do not form a homogeneous phase. Even a multiphase mixture may simultaneously undergo one chemical reaction near equilibrium and another one that is far from equilibrium. Here, we observed in real time crystal seed formation and simultaneous nanocrystal aggregation proceeding from CeIV complexes to CeO2 nanoparticles in an acidic aqueous solution, and investigated the resultant hierarchical nanoarchitecture. The formed particles exhibited two very different size ranges. The hierarchically assembled structures in solutions were CeO2 colloids, viz. primary core clusters (1–3 nm) of crystalline ceria and secondary clusters (20–30 nm) assembled through surface ions. Such self-assembly is widespread in multi-component complex fluids, paradoxically moderating hierarchical reactions. Stability and instability are not only critical but also complementary for co-optimisation around the nearby free energy landscape prior to bifurcation.

We compare the extraction and separation performance of a newly developed fluorous extractant, tris(4,4,5,5,6,6,7,7,7-nonafluoroheptyl) phosphate (TFP) in a fluorinated diluent perfluorohexane, for trivalent lanthanide ions (Ln^III) with a conventional organic extractant, tri-n-butyl phosphate (TBP) in n-octane. Interestingly, the fluorinated extractant TFP exhibits a superior extraction performance for Ln^III than the organic extractant TBP, despite the tendency of hydrocarbon fluorination to reduce the extraction performance. Furthermore, the high separation factors among Ln^III by TFP suggest that the separation performance of TFP is superior to that of TBP. We proved the superior extraction and separation performance of TFP by comparing the coordination environment of the extracted TFP and TBP complexes, the fluorous phase, and the organic phase. From a microscopic perspective, luminescence lifetime measurements and extended X-ray absorption fine structure spectroscopy revealed the near coordination environment of Eu^III in the extracted complex of the extracting phase. Hydrating water scarcely exists in the Eu^III-TFP complex, while the Eu^III-TBP complex has at least one hydrating water molecule. On the other hand, from a macroscopic perspective, water and nitrate anion transport into the extracting phase during Eu^III extraction are revealing. The large amounts of water and NO₃ ⁻ anions in the organic phase compared to those in the fluorous phase suggest that the extraction of water and NO₃ ⁻ anions somehow impairs the extraction of Eu^III into the organic phase. Hence, TFP in a fluorinated diluent can be used as an alternative extraction and separation system for Ln^III.

Rare-earth (RE) phosphates often appear as an accessory phase in igneous or metamorphic rocks; however, these rocks are composed of myriad chemical elements and nuclides that interfere with the qualitative or quantitative analyses of the RE phosphates over a range of concentrations in the absence of a pretreatment. In addition, the limit of each analytical methodology constrains the approach as well as the usefulness of the results in geoscience applications. Here, we report the specific mineral characterization of RE-containing ores from Yen Phu mine, Vietnam, using a range of state-of-the-art spectroscopic techniques in conjunction with microscopy: Mössbauer spectroscopy, infrared microspectroscopy, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Because the distribution of each element in the deposit differs, such combinatorial works are necessary and could lead to more plausible answers to questions surrounding the point of origin of RE elements. The results of our Mössbauer spectroscopic analysis indicate that the three ores sampled at different locations all contain magnetite-like, hematite-like, and iron(III) salts other than hematite. In addition, we confirmed the presence of phosphate around the grain boundary in the magnetite-like mineral phase by infrared microspectroscopic analysis. The present analytical findings of trace amounts of europium(III) using TRLFS suggest that the europium ions generate identical luminescence spectra despite being embedded in three different matrices of iron minerals. This demonstration highlights the benefits of combinatorial spectroscopic analyses to gain insights into the effects of the environment of REs on their solid-state chemistry and shows the potential utility of TRLFS as a resource mining tool. Further applications of this approach in the analytical screening of rocks and minerals are feasible.

Unacceptably high uranium concentrations in decentralized and remote potable groundwater resources, especially those of high hardness (e.g. high Ca2+, Mg2+ and CO32- concentrations), are a common worldwide problem. The complexation of alkali earth metals, carbonate and uranium(VI) results in the formation of thermodynamically stable ternary aqueous species that are predominantly neutrally-charged (e.g. Ca2(UO2)(CO3)30). The removal of the uncharged (non-adsorbing) complexes is a problematic issue for many water treatment technologies. As such, we have evaluated the efficacy of a recently-developed electrochemical technology, termed flow-electrode capacitive deionization (FCDI), to treat a synthetic groundwater, the composition of which is comparable to groundwater resources in the Northern Territory, Australia (and elsewhere worldwide). Theoretical calculations and time-resolved laser fluorescence spectroscopy analyses confirmed that Ca2(UO2)(CO3)30 was the primary aqueous species followed by Ca(UO2)(CO3)32− (at circumneutral pH values). Results under different operating conditions demonstrated that FCDI is versatile in reducing uranium concentrations to < 10 μg L−1 with low electrical consumption (e.g. ~ 0.1 kWh m−3). It is concluded that the capability of FCDI to remove uranium under these common conditions depends on the dissociation kinetics of the Ca2(UO2)(CO3)30 complex in the electrical field. The subsequent formation of the negatively-charged Ca(UO2)(CO3)32− species results in the efficient transport of uranium across the anion exchange membrane followed by immobilization on the positively-charged flow (anode) electrode.

The speciation and coordination geometries of M(III)-citrate complexes in aqueous solutions, where M denotes Eu, Tb, Lu, or Cm, are studied using potentiometric titration, ¹H, and ¹³C nuclear magnetic resonance spectroscopies. Their photophysical properties are also characterized by time-resolved fluorescence spectra. The formation constants of mononuclear, dinuclear, and trinuclear Lu-citrate species, i.e., [Lu(Cit)₂]³⁻, [Lu₂(OH)₂(Cit)₂]²⁻, and [Lu₃(OH)₄(Cit)₄]⁷⁻, were determined as 9.78 ± 0.30, 3.60 ± 0.30, and 1.02 ± 0.03, respectively, by potentiometric titration in 3.00 M NaClO₄ aqueous media. Terminal carboxylic conformation in trinuclear complexes comprised both the five- and six-membered rings at different exchanging rates. Hydration states evaluated for Eu³⁺ ions are the chemical formula of [Eu(Cit)₂(H₂O)_2.5]³⁻ and [Eu₃(OH)₄(Cit)₄(H₂O)_3.4]⁷⁻. These complexes in aqueous solution have geometrical similarity to the crystal structures in the literature. Furthermore, the entity of the hetero-trinuclear complex induces the intramolecular energy transfer from Tb³⁺ to Eu³⁺. The incorporation of Cm³⁺ into these homo/hetero-trinuclear citrate complexes proved to be a successful trial to probe the formation of actinide polymer at a trace level.