Noboru Aoyagi

Application of Parallel Factor Analysis for Time-Resolved Laser Fluorescence Spectroscopy: Implication for Metal Speciation Study

Time-resolved laser fluorescence spectroscopy (TRLFS) is an analytical technique capable of discriminating different chemical species of a fluorescent metal ion such as UO(2)(2+), Cm(3+), and lanthanides. Although TRLFS has been widely used to investigate the speciation of the fluorescent metal ions, extracting quantitative and structural information from multiple TRLFS data measured as a function of chemical and physical parameters is not a simple task. The purpose of this study is to apply parallel factor analysis (PARAFAC) for the interpretation of a series of TRLFS data. PARAFAC is a robust technique because it utilizes the entire information contained in a multiway TRLFS data set. The complexation of Eu(3+) by acetate was studied as a test case for the PARAFAC decomposition. It is shown that three factors are necessary and sufficient to explain the systematic variations in the original data set. The resulting spectra, decay, and relative concentrations of the factors were all in agreement with the fluorescent properties and the complexation behaviors of Eu(3+)-acetate complexes. Based on these results, it was concluded that PARAFAC is a promising data analysis tool for TRLFS used for the speciation studies of fluorescent metal ions.

Applications of Time-Resolved Laser Fluorescence Spectroscopy to the Environmental Biogeochemistry of Actinides

Time-resolved laser fluorescence spectroscopy (TRLFS) is a useful means of identifying certain actinide species resulting from various biogeochemical processes. In general, TRLFS differentiates chemical species of a fluorescent metal ion through analysis of different excitation and emission spectra and decay lifetimes. Although this spectroscopic technique has largely been applied to the analysis of actinide and lanthanide ions having fluorescence decay lifetimes on the order of microseconds, such as UO , Cm, and Eu, continuing development of ultra-fast and cryogenic TRLFS systems offers the possibility to obtain speciation information on metal ions having room-temperature fluorescence decay lifetimes on the order of nanoseconds to picoseconds. The main advantage of TRLFS over other advanced spectroscopic techniques is the ability to determine in situ metal speciation at environmentally relevant micromolar to picomolar concentrations. In the context of environmental biogeochemistry, TRLFS has principally been applied to studies of (i) metal speciation in aqueous and solid phases and (ii) the coordination environment of metal ions sorbed to mineral and bacterial surfaces. In this review, the principles of TRLFS are described, and the literature reporting the application of this methodology to the speciation of actinides in systems of biogeochemical interest is assessed. Significant developments in TRLFS methodology and advanced data analysis are highlighted, and we outline how these developments have the potential to further our mechanistic understanding of actinide biogeochemistry.

Surface speciation of Eu3+ adsorbed on kaolinite by time-resolved laser fluorescence spectroscopy (TRLFS) and parallel factor analysis (PARAFAC)

Time-resolved laser fluorescence spectroscopy (TRLFS) is an effective speciation technique for fluorescent metal ions and can be further extended by the parallel factor analysis (PARAFAC). The adsorption of Eu(3+) on kaolinite as well as gibbsite as a reference mineral was investigated by TRLFS together with batch adsorption measurements. The PAFAFAC modeling provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu(3+) surface complexes with kaolinite; an outer-sphere (factor A) complex and two inner-sphere (factors B and C) complexes. Their intensity profiles qualitatively explained the measured adsorption of Eu(3+). Based on the TRLFS results in varied H(2)O/D(2)O media, it was shown that the outer-sphere complex exhibited more rapid fluorescence decay than Eu(3+) aquo ion, because of the energy transfer to the surface. Factor B was an inner-sphere complex, which became dominant at relatively high pH, high salt concentration and low Eu(3+) concentration. Its spectrum and lifetime were similar to those of Eu(3+) adsorbed on gibbsite, suggesting its occurrence on the edge face of the gibbsite layer of kaolinite. From the comparison with the spectra and lifetimes of crystalline or aqueous Eu(OH)(3), factor C was considered as a poly-nuclear surface complex of Eu(3+) formed at relatively high Eu(3+) concentration.

Specific Cooperative Effect of a Macrocyclic Receptor for Metal Ion Transfer into an Ionic Liquid

An intramolecular cooperative extraction system for the removal of strontium cations (Sr(2+)) from water by use of a novel macrocyclic receptor (H(2)βDA18C6) composed of diaza-18-crown-6 and two β-diketone fragments in ionic liquid (IL) is reported, together with X-ray spectroscopic characterization of the resulting extracted complexes in the IL and chloroform phases. The covalent attachment of two β-diketone fragments to a diazacrown ether resulted in a cooperative interaction within the receptor for Sr(2+) transfer, which remarkably enhanced the efficiency of Sr(2+) transfer relative to a mixed β-diketone and diazacrown system. The intramolecular cooperative effect was observed only in the IL extraction system, providing a 500-fold increase in extraction performance for Sr(2+) over chloroform. Slope analysis and potentiometric titration confirmed that identical extraction mechanisms operated in both the IL and chloroform systems. Extended X-ray absorption fine structure spectroscopy revealed that the average distance between Sr(2+) and O atoms in the Sr(2+) complex was shorter in IL than in chloroform. Consequently, Sr(2+) was held by H(2)βDA18C6 more rigidly in IL than in chloroform, representing an important factor dominating the magnitude of the intramolecular cooperative effect of H(2)βDA18C6 for Sr(2+). Furthermore, competitive extraction studies with alkaline earth metal ions revealed that the magnitude of the intramolecular cooperative effect depended on the suitability between metal ion size and the cavity size of H(2)βDA18C6. Sr(2+) was successfully recovered from IL by controlling the pH in the receiving phase, and the extraction performance of H(2)βDA18C6 in IL was maintained after five repeated uses.

Uranium Binding Mechanisms of the Acid-Tolerant Fungus Coniochaeta fodinicola

The uptake and binding of uranium [as (UO2)(2+)] by a moderately acidophilic fungus, Coniochaeta fodinicola, recently isolated from a uranium mine site, is examined in this work in order to better understand the potential impact of organisms such as this on uranium sequestration in hydrometallurgical systems. Our results show that the viability of the fungal biomass is critical to their capacity to remove uranium from solution. Indeed, live biomass (viable cells based on vital staining) were capable of removing ∼16 mg U/g dry weight in contrast with dead biomass (autoclaved) which removed ∼45 mg U/g dry weight after 2 h. Furthermore, the uranium binds with different strength, with a fraction ranging from ∼20-50% being easily leached from the exposed biomass by a 10 min acid wash. Results from X-ray absorption spectroscopy measurements show that the strength of uranium binding is strongly influenced by cell viability, with live cells showing a more well-ordered uranium bonding environment, while the distance to carbon or phosphorus second neighbors is similar in all samples. When coupled with time-resolved laser fluorescence and Fourier transformed infrared measurements, the importance of organic acids, phosphates, and polysaccharides, likely released with fungal cell death, appear to be the primary determinants of uranium binding in this system. These results provide an important progression to our understanding with regard to uranium sequestration in hydrometallurgical applications with implications to the unwanted retention of uranium in biofilms and/or its mobility in a remediation context.

Time-resolved laser-induced fluorescence spectroscopy combined with parallel factor analysis: a robust speciation technique for UO2 2+

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) is a powerful speciation technique for fluorescent metal ions and can be further improved by combining with multi-mode factor analysis such as parallel factor analysis (PARAFAC). This study demonstrates the applicability of TRLFS combined with PARAFAC for the speciation of uranyl (UO22+) in the presence of silicic acid (Si(OH)4). A series of TRLFS data with varied Si(OH)4 concentration was processed by PARAFAC, resulting in three factors corresponding to free UO22+, UO2SiO(OH)3+, and UO2OH+. The stability constant of UO2SiO(OH)3+ was further optimized, based on the intensity profiles of the factors.

Role of Tf2N− anions in the ionic liquid–water distribution of europium(III) chelates

The role of bis(trifluoromethanesulfonyl)imide (Tf2N−) anions in the ionic liquid–water distribution systems of Eu(III) chelates with 2-thenoyltrifluoroacetone (Htta) was investigated by liquid–liquid distribution and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The extraction constants of neutral Eu(tta)3 and anionic Eu(tta)4− chelates in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][Tf2N]) were determined by analyzing the distribution equilibrium. The effect of the ionic liquids on the distribution constant of the neutral Eu(tta)3 chelate was evaluated by the regular solution theory. The distribution constant of Eu(tta)3 in [Cnmim][Tf2N] was increased dramatically by the solvation effects of Eu(tta)3 in [Cnmim][Tf2N]. TRLFS for [Eu(tta)3(H2O)3] synthesized revealed that the Eu(tta)3 chelate was almost completely dehydrated in a series of [Cnmim][Tf2N] (n = 2–10). The Eu(tta)3 chelate exists as di- or tri-hydrates in 1-ethyl-3-methylimidazolium perchlorate ([C2mim][ClO4]) containing 20 mol dm−3 water, whereas mono-hydrated chelate was formed in [C2mim][Tf2N, ClO4] in the presence of 0.50 mol dm−3 Tf2N− and 20 mol dm−3 water. These results show that the coordinated water molecules of [Eu(tta)3(H2O)3] were replaced by the Tf2N− anions. In fact, an anionic adduct, [Eu(tta)3(Tf2N)]−, was observed by electrospray ionization mass spectrometry in the presence of [C4mim][Tf2N].

Europium binding to humic substances extracted from deep underground sedimentary groundwater studied by time-resolved laser fluorescence spectroscopy

ABSTRACT Humic substances (HSs) are ubiquitous in various environments including deep underground and play an important role in the speciation and mobility of radionuclides. The binding of Eu3+, a chemical homologue of trivalent actinide ions, to HSs isolated from sedimentary groundwater at −250 m below the surface was studied by time-resolved laser fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) as a function of pH and salt concentration. PARAFAC modeling reveals the presence of multiple factors that correspond to different Eu3+ species. These factors resemble those observed for Eu3+ binding to HSs from surface environments; however, detailed comparison shows that there are some particularities in Eu3+ binding to the deep groundwater HSs. The distribution coefficients (Kd) of Eu3+ binding to the HSs calculated from the PARAFAC modeling exhibits a rather strong salt effect. At 0.01 M NaClO4, the Kd values are relatively large and comparable to those to the surface HSs; they are decreaed at 0.1 M NaClO4 by more than an order of the magnitude. The Kd values are larger for the humic acid fraction of the deep underground HSs than the fulvic acid fraction over the entire range of pH and salt concentration investigated in this study.

Sorption of Eu3+ on Na-montmorillonite studied by time-resolved laser fluorescence spectroscopy and surface complexation modeling

The influence of pH and concentrations of Eu3+ and NaNO3 on the sorption of Eu3+ to Na-montmorillonite were investigated through batch sorption measurements and time-resolved laser fluorescence spectroscopy. The pH had little effect on the distribution coefficients (Kd) in the range of pH 4–7 at 0.01 M NaNO3, which indicates that the cation exchange reaction is a dominant sorption process. Meanwhile, the Kd strongly depended on pH at 1 M NaNO3, suggesting the formation of inner-sphere surface complexes. A cation exchange model combined with a one-site non-electrostatic surface complexation model was successfully applied to the measured Kd data. Linear free-energy relationship was used to estimate the formation constants of the surface species from those of the corresponding aqueous hydrolyzed species. The TRLFS spectra of Eu3+ sorbed on Na-montmorillonite were processed by parallel factor analysis, which provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu3+ surface species. These species corresponded to one outer-sphere (factor A) and two inner-sphere (factors B and C) complexes. It turned out that factors A and B correspond to Eu3+ sorbed by ion exchange to permanent charge sites of Na-montmorillonite and inner-sphere complexation with surface hydroxyl groups of the edge faces. Factor C became dominant at relatively high pH and ionic strength and likely corresponded to the precipitation of Eu(OH)3 on the surface of Na-montmorillonite.

Luminescence and Potentiometric Studies on the Complexation of Europium(III) by Picolinate in an Aqueous Solution

The complexation of Eu3+ aquo ions by picolinic acid in an aqueous solution was investigated to describe the formation of polynuclear complexes that are uncommon for trivalent lanthanides. Potentiometric titration indicated that no polynuclear complexes formed via hydroxide bridges, even at high pH, and that the monomeric form [Eu(Pic)4_(H2O)]− was present when a large excess of a picolinate ligand was used. In addition, lifetime analysis of Eu3+ luminescence via time-resolved laser-induced fluorescence spectroscopy was performed under conditions where Eu(pic)2+ appears as the secondary dominant species. Strong quenching of the luminescence was detected, suggesting that polymeric complexes ([Eu(Pic)]m2m+) form, even at low pH, owing to a vicinal Eu–Eu interaction.