Time-resolved Laser Fluorescence Spectroscopy Research Articles

Complexation of Cm(iii) and Eu(iii) with CyMe4-BTPhen and CyMe4-BTBP studied by time resolved laser fluorescence spectroscopy

The complexation of Cm(III) and Eu(III) with 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BTPhen) and 6,6'-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2'-bipyridine (CyMe4-BTBP) in methanolic solution was investigated by TRLFS. For both ligands, the 1:2 complex with the particular metal ion is the only species observed in equilibrated samples. The species distribution for various ligand concentrations was determined and stability constants of the 1:2 complexes were derived (log β2 = 13.8 ± 0.2 (Cm(III)-CyMe4-BTPhen), log β2 = 11.6 ± 0.4 (Eu(III)-CyMe4-BTPhen), log β2 = 12.4 ± 0.3 (Cm(III)-CyMe4-BTBP) and log β2 = 11.3 ± 0.3 (Eu(III)-CyMe4-BTBP)). Biphasic experiments in combination with TRLFS studies on the organic phase revealed the formation of ternary complexes with two CyMe4-BTPhen or -BTBP molecules and additional coordination of a nitrate anion as species formed during the extraction process.

Uranium determination in groundwater using laser spectroscopy

AbstractThe aim of this work is to review the laser-based analytical techniques for the quantitative determination of uranium in aqueous solutions. Among the various types of laser-based analytical techniques, two different spectroscopic techniques based on the measurement of laser-induced luminescence of hexavalent uranium ions [U(VI)] in groundwater are reviewed in detail. In the first technique, called time-resolved laser fluorescence spectroscopy, the time-resolved laser-induced luminescence intensities of U(VI) as a function of uranium concentration are measured to obtain the calibration curve. In the second technique, which is based on the simultaneous measurement of the U(VI) luminescence and Raman scattering of water, the calibration curve is obtained by measuring the ratio of the luminescence intensity of U(VI) to the Raman scattering intensity of water for the quantitative determination of uranium. A limit of detection of 0.03 μg/l was achieved at an excitation wavelength of 266 nm using these laser spectroscopic techniques. The results determined using these techniques are in good agreement with the results determined by inductively coupled plasma mass spectrometry (ICPMS). Thus, these laser-based analytical techniques will be useful to personnel requiring a rapid determination of uranium before the shipment of samples to an accredited laboratory, where relatively intricate and expensive apparatuses, such as ICPMS and radiochemical spectrometry, are used.

A spectroscopic study on the formation of Cm(iii) acetate complexes at elevated temperatures

The complexation of Cm(III) with acetate is studied by time resolved laser fluorescence spectroscopy (TRLFS) as a function of ionic strength, ligand concentration, temperature and background electrolyte (NaClO4, NaCl and CaCl2 solution). The speciation of Cm(III) is determined by peak deconvolution of the emission spectra. To obtain the thermodynamic stability constants (log K) for the formation of [Cm(Ac)n](3-n) (n = 1-3), the experimental data are extrapolated to zero ionic strength according to the specific ion interaction theory (SIT). The results show a continuous increase of the stability constants with increasing temperature (20-90 °C). The standard reaction enthalpies and entropies (ΔrH, ΔrS) of the respective reactions are derived from the integrated Van't Hoff equation. The results show that all complexation steps are endothermic and thus entropy driven (ΔrH and ΔrS > 0).

Judd–Ofelt parameters of diglycolamide-functionalized calix[4]arene Eu3+ complexes in room temperature ionic liquid for structural analysis: Effects of solvents and ligand stereochemistry

The complexation behavior of several diglycolamide (DGA)-functionalized calix [4]arene ligands (C4DGA) with Eu3+ in C8mimNTf2 (1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) was studied by time resolved laser fluorescence spectroscopy (TRLFS). The high intensity emission peaks of Eu3+ at 614 and 619nm, which are due to the 5D0→7F2 transition, sensitive to the local environment around Eu3+, suggest that Eu3+ sits in a highly asymmetric environment. The metal-ligand stoichiometry of the complexes is 1:1, while there was no inner sphere water molecule around Eu3+. The splitting pattern of the individual transitions of Eu3+ indicates that the local site symmetry around Eu3+ is C4V. Though the emission characteristics of the complexes in C8mimNTf2 and a 1:5 acetonitrile-water mixture were similar, the smaller linewidth in C8mimNTf2 can be attributed to the restricted motion of the Eu3+ complex. Judd and Ofelt parameters (Ω2, Ω4) were calculated for all Eu3+–C4DGA complexes. Steric crowding around the ligating sites decreases the proximity of Eu3+ with the carbonyl oxygens. This is evident from a decrease in the Ω2 value, which is a signature of the increase in the bond length and concomitant decrease in the covalent interactions. Higher transition probabilities (Amd, A2ed, A4ed) of the Eu3+–C4DGA complexes in C8mim·NTf2 compared to a 1:5 acetonitrile-water mixture point to more mixing of the metal f orbitals with the ligand orbitals. The transition probability of the electric-dipole transition of 5D0→7F2 is three times more than the magnetic-dipole transition probability and twice that of the electric-dipole transition of 5D0→7F4. Alkyl substituents on the amidic nitrogen atoms increase the quantum efficiency as it decreases the additional non-radiative pathway.

Temperature-Dependent Hydrolysis Reactions of U(VI) Studied by TRLFS

Temperature-dependent hydrolysis behaviors of aqueous U(VI) species were investigated with time-resolved laser fluorescence spectroscopy (TRLFS) in the temperature range from 15 to 75°C. The formation of four different U(VI) hydrolysis species was measured at pHs from 1 to 7. The predominant presence of UO2 2+ , (UO2)2(OH)2 2+ , (UO2)3(OH)5 + , and (UO2)3(OH)7 - species were identified based on the spectroscopic properties such as fluores- cence wavelengths and fluorescence lifetimes. With an increasing temperature, a remarkable decrement in the flu- orescence lifetime for all U(VI) hydrolysis species was observed, representing the dynamic quenching behavior. Furthermore, the increase in the fluorescence intensity of the further hydrolyzed U(VI) species was clearly observed at an elevated temperature, showing stronger hydrolysis reactions with increasing temperatures. The formation constants of the U(VI) hydrolysis species were calculated to be log K0 2,2 = -4.0 ‚ 0.6 for (UO2)2(OH)2 2+ , log K 0 3,5 = -15.0 ‚ 0.3 for (UO2)3(OH)5 + , and log K 0 3,7 = -27.7 ‚ 0.7 for (UO2)3(OH)7 - at 25°C and I = 0 M. The specific ion interaction theory (SIT) was applied for the extrapolation of the formation constants to infinitely dilut- ed solution. The results of temperature-dependent hydrolysis behavior in terms of the U(VI) fluorescence were compared and validated with those obtained using computational methods (DQUANT and constant enthalpy equa- tion). Both results matched well with each other. The reaction enthalpies and entropies that are vital for the com- putational methods were determined by a combination of the van't Hoff equation and the Gibbs free energy equa- tion. The temperature-dependent hydrolysis reaction of the U(VI) species indicates the transition of a major U(VI) species by means of geothermal gradient and decay heat from the radioactive isotopes, representing the necessity of deeper consideration in the safety assessment of geologic repository.

Trace concentration – Huge impact: Nitrate in the calcite/Eu(III) system

The interactions of trivalent lanthanides and actinides with secondary mineral phases such as calcite is of high importance for the safety assessment of deep geological repositories for high level nuclear waste (HLW). Due to similar ionic radii, calcium-bearing mineral phases are suitable host minerals for Ln(III) and An(III) ions. Especially calcite has been proven to retain these metal ions effectively by both surface complexation and bulk incorporation. Since anionic ligands (e.g., nitrate) are omnipresent in the geological environment and due to their coordinating properties, their influence on retentive processes should not be underestimated. Nitrate is a common contaminant in most HLW forms as a result of using nitric acid in fuel reprocessing. It is also formed by microbial activity under aerobic conditions. In this study, atomic force microscopy investigations revealed a major influence of nitrate upon the surface of calcite crystals. NaNO3 causes serious modifications even in trace amounts (<10−7M) and forms a soft surface layer of low crystallinity on top of the calcite crystal. Time-resolved laser fluorescence spectroscopy of Eu(III) showed that, within this layer, Eu(III) ions are incorporated, while losing most of their hydration shell. The results show that solid solution modelling for actinides in calcite must take into account the presence of nitrate in pore and ground waters.

Osteopontin: A Uranium Phosphorylated Binding‐Site Characterization

Herein, we describe the structural investigation of one possible uranyl binding site inside a nonstructured protein. This approach couples spectroscopy, thermodynamics, and theoretical calculations (DFT) and studies the interaction of uranyl ions with a phosphopeptide, thus mimicking a possible osteopontin (OPN) hydroxyapatite growth-inhibition site. Although thermodynamical aspects were investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) and isothermal titration calorimetry (ITC), structural characterization was performed by extended X-ray absorption fine structure (EXAFS) at the U LIII -edge combined with attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. From the vibrational and fluorescence spectra, several structural models of a UO2 (2+) /peptide complex were developed and subsequently refined by using theoretical calculations to fit the experimental EXAFS obtained. The structural effect of the pH value was also considered under acidic to moderately acidic conditions (pH 1.5-5.5). Most importantly, the uranyl/peptide coordination environment was similar to that of the native protein.

Pressure-induced phase transitions in sodium europium carbonate (Na3Eu(CO3)3) and europium-doped yttrium sesquioxide (Y2O3:Eu3+) by time-resolved laser fluorescence spectroscopy

Time-resolved laser fluorescence spectroscopy has been used to investigate pressure-induced phase transitions in Y2O3:Eu3+ and Na3Eu(CO3)3. Na3Eu(CO3)3 shows a reversible structural phase transformation at 10.5 GPa that can be monitored by fluorescence spectroscopy, fluorescence lifetime measurements and X-ray diffraction. The bulk modulus of Na3Eu(CO3)3 was determined to be B=55(2) GPa from synchrotron X-ray diffraction data. The pressure-induced phase transformations of Y2O3:Eu3+ were characterised by fluorescence lifetime measurements of the hypersensitive 5D0→7F2 transition, which allowed us to monitor the pressure-induced increase in the coordination number.

Complexation of Cm(III) and Eu(III) with 2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine and 2-bromohexanoic acid studied by time-resolved laser fluorescence spectroscopy

AbstractThe complexation of Cm(III) and Eu(III) with 2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine (C5-BPP) and 2-bromohexanoic acid as lipophilic anion has been investigated by time-resolved laser fluorescence spectroscopy. Upon increasing ligand concentration three different Cm(III)-C5-BPP species with emission bands at 604.1 nm, 607.9 nm and 611.4 nm, respectively, are found and attributed to complexes with one, two and three C5-BPP molecules in the inner coordination sphere. Comparison with results of TRLFS experiments without 2-bromohexanoic acid shows that the C5-BPP ligand is able to completely displace the lipophilic anion from the inner coordination sphere, forming [Cm(C5-BPP)3]3+complexes. This complex is also found in the organic phase of an extraction experiment performed with Cm(III), demonstrating that the lipophilic anion required for the extraction is not directly coordinated to the metal ion in the species formed during extraction. In case of Eu(III) the number of different species formed cannot be determined accurately. Nevertheless, the formation of the complex [Eu(C5-BPP)3]3+in the presence of 2-bromohexanoic acid is confirmed.

Highly Efficient Diglycolamide‐Based Task‐Specific Ionic Liquids: Synthesis, Unusual Extraction Behaviour, Irradiation, and Fluorescence Studies

Two new diglycolamide-based task-specific ionic liquids (DGA-TSILs) were evaluated for the extraction of actinides and lanthanides from acidic feed solutions. These DGA-TSILs were capable of exceptionally high extraction of trivalent actinide ions, such as Am(3+), and even higher extraction of the lanthanide ion, Eu(3+) (about 5-10 fold). Dilution of the DGA-TSILs in an ionic liquid, C(4)mim(+)·NTf(2)(-), afforded reasonably high extraction ability, faster mass transfer, and more efficient stripping of the metal ion. The nature of the extracted species was studied by slope analysis, which showed that the extracted species contained one NO(3)(-) anion, along with the participation of two DGA-TSIL molecules. Time-resolved laser fluorescence spectroscopy (TRLFS) analysis showed a strong complexation with no inner-sphere water molecule in the Eu(III)-DGA-TSIL complexes in the presence and absence of C(4)mim(+)·NTf(2)(-) as the diluent. The very high radiolytic stability of DGA-TSIL 6 makes it one of the most-efficient solvent systems for the extraction of actinides under acidic feed conditions.

Characterization of powellite-based solid solutions by site-selective time resolved laser fluorescence spectroscopy

We present a comprehensive study of the solid solution system Ca2(MoO4)2-NaGd(MoO4)2 on the molecular scale, by means of site-selective time resolved laser fluorescence spectroscopy (TRLFS). Eu(3+) is used as a trace fluorescent probe, homogeneously substituting for Gd(3+) in the solid solution crystal structure. Site-selective TRLFS of a series of polycrystalline samples covering the whole composition range of the solid solution series from 10% substitution of Ca(2+) to the NaGd end-member reveals it to be homogeneous throughout the whole range. The trivalent ions are incorporated into the powellite structure in only one coordination environment, which exhibits a very strong ligand-metal interaction. Polarization-dependent measurements of a single crystal of NaGd(Eu)(MoO4)2 identify the coordination geometry to be of C2v point symmetry. The S4 symmetry of the Ca site within the powellite lattice can be transformed into C2v assuming minor motion in the first coordination sphere.

A spectroscopic study of Cm(iii) complexation with propionate in saline solutions at variable temperatures

The complexation of Cm(III) with propionate is studied by time resolved laser fluorescence spectroscopy (TRLFS) in saline solutions (NaCl and CaCl2) as a function of ionic strength, ligand concentration, and temperature. The molar fractions of the different Cm(III) species are determined by peak deconvolution of the measured fluorescence spectra. By using the specific ion interaction theory (SIT), the stability constants at zero ionic strength (log K0n) of the first and second complexation steps are determined. The stability constants of the mono- and dipropionate complexes increase continuously with increasing temperature between 20–90 °C in both background electrolytes. The log K0n values are linearly correlated to the reciprocal temperature, indicating ΔrH0m = const. and ΔrC0p,m = 0. Therefore, the thermodynamic constants (ΔrH0m, ΔrS0m, ΔrG0m) for the formation of the [Cm(Prop)]2+ and [Cm(Prop)2]+ complexes are derived from the integrated van't Hoff equation. The results show that both reactions are entropy driven. Furthermore, neither the formation of ternary species including Ca2+ nor a complexing effect of Cl− at elevated temperatures is observed under the chosen experimental conditions. Lastly, the ion–ion interaction coefficients of both complexed species with Cl− are derived for the first time.

Spectroscopic Studies on the Thermodynamics of the Complexation of Trivalent Curium with Propionate in the Temperature Range from 20 to 90 °C

The thermodynamics of the stepwise complexation reaction of Cm(III) with propionate was studied by time resolved laser fluorescence spectroscopy (TRLFS) and UV/Vis absorption spectroscopy as a function of the ligand concentration, the ionic strength and temperature (20–90 °C). The molar fractions of the 1:1 and 1:2 complexes were quantified by peak deconvolution of the emission spectra at each temperature, yielding the log10 \( K_{n}^{\prime } \) values. Using the specific ion interaction theory (SIT), the thermodynamic stability constants log10 \( K_{n}^{0} (T) \) were determined. The log10 \( K_{n}^{0} (T) \) values show a distinct increase by 0.15 (n = 1) and 1.0 (n = 2) orders of magnitude in the studied temperature range, respectively. The temperature dependency of the log10 \( K_{n}^{0} (T) \) values is well described by the integrated van’t Hoff equation, assuming a constant enthalpy of reaction and \( \Updelta_{\text{r}} C^\circ_{{p,{\text{m}}}} = 0, \) yielding the thermodynamic standard state \( \left( {\Updelta_{\text{r}} H^\circ_{\text{m}} ,\Updelta_{\text{r}} S^\circ_{\text{m}} ,\Updelta_{\text{r}} G^\circ_{\text{m}} } \right) \) values for the formation of the \( {\text{Cm(Prop)}}_{n}^{3 - n} \), n = (1, 2) species.

Complexation of trivalent lanthanides and actinides with several novel diglycolamide-functionalized calix[4]arenes: solvent extraction, luminescence and theoretical studies

Several diglycolamide-functionalized calix[4]arenes (DGA–Calix) were evaluated for actinide extraction from acidic feeds. The ligands with four diglycolamide (DGA) pendent arms are significantly more effective extractants than those with two DGA pendent arms. The ligands have a preference for the extraction of Eu3+, a representative trivalent lanthanide ion, as compared to Am3+, a commonly encountered trivalent actinide ion. The role of organic diluents on the metal ion extraction was investigated and the results were compared with the widely studied DGA-based extractant TODGA (N,N,N′,N′-tetra-n-octyl diglycolamide). Time resolved laser fluorescence spectroscopy (TRLFS) studies showed a strong complexation with no inner-sphere water molecules in the Eu(III)–DGA–Calix complexes and the complex formation constants (log β) were calculated. Ab initio density functional calculations were carried out to explain the higher stability of the Eu-complex of the DGA–Calix ligand with four pendent arms as compared to the one with two pendent arms.

Formation of ternary CaUO2(CO3)32− and Ca2UO2(CO3)3(aq) complexes under neutral to weakly alkaline conditions

The chemical behavior of ternary Ca-UO2-CO3 complexes was investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) in combination with EDTA complexation at pH 7-9. A novel TRLFS revealed two distinct fluorescence lifetimes of 12.7 ± 0.2 ns and 29.2 ± 0.4 ns for uranyl complexes which were formed increasingly dependent upon the calcium ion concentration, even though nearly indistinguishable fluorescence peak shapes and positions were measured for both Ca-UO2-CO3 complexes. For identifying the stoichiometric number of complexed calcium ions, slope analysis in terms of relative fluorescence intensity versus calcium concentration was employed in a combination with the complexation reaction of CaEDTA(2-) by adding EDTA. The formation of CaUO2(CO3)3(2-) and Ca2UO2(CO3)3(aq) was identified under given conditions and their formation constants were determined at I = 0.1 M Na/HClO4 medium, and extrapolated to infinitely dilute solution using specific ion interaction theory (SIT). As a result, the formation constants for CaUO2(CO3)3(2-) and Ca2UO2(CO3)3(aq) were found to be log β113(0) = 27.27 ± 0.14 and log β213(0) = 29.81 ± 0.19, respectively, providing that the ternary Ca-UO2-CO3 complexes were predominant uranium(vi) species at neutral to weakly alkaline pH in the presence of Ca(2+) and CO3(2-) ions.

TRLFS of Eu3+ and Cm3+ doped La2Zr2O7: A comparison of defect fluorite to pyrochlore structures

Time-resolved laser fluorescence spectroscopy (TRLFS) was used to probe the local environment of Eu3+ and Cm3+ doped into La2Zr2O7 in both the defect fluorite and pyrochlore structures. It is evident that each structure has a dominant environment for the trivalent dopant ion. This makes it possible to characterize the fluorescence emission spectrum and lifetime for each Eu3+ and Cm3+ species. This data can be used for a relative quantification of the Eu3+ or Cm3+ in each local environment. This technique has application to verifying single phase material and probing radiation damage, and has been shown to be more sensitive than the current evaluation method of X-ray diffraction. The defect fluorite structure has a poorly defined site due to random oxygen vacancies and a short fluorescence lifetime, while the pyrochlore structure has a well defined emission and long fluorescence lifetime. The Cm3+ fluorescence from the pyrochlore structure was found to have a greater bathochromic shift (red shift) and larger ground state splitting of the 8S7/2 than previously reported in any system.

New insight into Cm(III) interaction with kaolinite – Influence of mineral dissolution

Cm(III) speciation in natural kaolinite (St. Austell, UK) suspensions under alkaline conditions was studied by time-resolved laser fluorescence spectroscopy (TRLFS). The spectroscopic investigations were performed under argon atmosphere (O2<1ppm) using a constant curium concentration, ionic strength and mineral content of 2×10−7M, 1mM NaClO4 and 0.25g/L, respectively, throughout the study. The impact of kaolinite mineral dissolution on the speciation of the trivalent actinide was investigated in oversaturation experiments where excess amounts of aluminum and/or silicon were added to alkaline kaolinite suspensions. Only silicon addition was found to influence the curium ligand-field under the experimental conditions indicating the formation of a curium-silicate complex in the kaolinite environment. In experiments with 10−3M added silicon but no solid phase for curium attachment only the hydrolysis species Cm(OH)2+ could be detected at pH 10. Thus, the formation of colloidal silicate species for the attachment of curium could be excluded and the observed species in alkaline kaolinite environments could be assigned to a ternary kaolinite/curium/silicate complex forming between adsorbed curium at the mineral surface and dissolved silicates in solution. A similar curium-silicate complex with identical spectroscopic features was also found in investigations with α-alumina as sorbent phase upon addition of silicon to the mineral suspensions, suggesting that silicon complexation with surface-bound curium is independent of the sorbent material.

Immobilization of uranium in biofilm microorganisms exposed to groundwater seeps over granitic rock tunnel walls in Olkiluoto, Finland

In an underground rock characterization facility, the ONKALO tunnel in Finland, massive 5–10-mm thick biofilms were observed attached to tunnel walls where groundwater was seeping from bedrock fractures at a depth of 70m. In laboratory experiments performed in a flow cell with detached biofilms to study the effect of uranium on the biofilm, uranium was added to the circulating groundwater (CGW) obtained from the fracture feeding the biofilm. The final uranium concentration in the CGW was adjusted to 4.25×10−5M, in the range expected from a leaking spent nuclear fuel (SNF) canister in a future underground repository. The effects were investigated using microelectrodes to measure pH and Eh, time-resolved laser fluorescence spectroscopy (TRLFS), energy-filtered transmission electron microscopy (EF-TEM), and electron energy-loss spectroscopy (EELS) studies and thermodynamic calculations were utilized as well. The results indicated that the studied biofilms constituted their own microenvironments, which differed significantly from that of the CGW. A pH of 5.37 was recorded inside the biofilm, approximately 3.5units lower than the pH observed in the CGW, due to sulfide oxidation to sulfuric acid in the biofilm. Similarly, the Eh of +73mV inside the biofilm was approximately 420mV lower than the Eh measured in the CGW. Adding uranium increased the pH in the biofilm to 7.27 and reduced the Eh to −164mV. The changes of Eh and pH influenced the bioavailability of uranium, since microbial metabolic processes are sensitive to metals and their speciation. EF-TEM investigations indicated that uranium in the biofilm was immobilized intracellularly in microorganisms by the formation of metabolically mediated uranyl phosphate, similar to needle-shaped autunite (Ca[UO2]2[PO4]2·2–6H2O) or meta-autunite (Ca[UO2]2[PO4]2·10–12H2O). In contrast, TRLFS studies of the contaminated CGW identified aqueous uranium carbonate species, likely (Ca2UO2[CO3]3), formed due to the high concentration of carbonate in the CGW. The results agreed with thermodynamic calculations of the theoretically predominant field of uranium species, formed in the uranium-contaminated CGW at the measured geochemical parameters.This investigation clearly demonstrated that biological systems must be considered as a part of natural systems that can significantly influence radionuclide behavior. The results improve our understanding of the mechanisms of biofilm response to radionuclides in relation to safety assessments of SNF repositories.

Temperature effect on fluorescence and UV–vis absorption spectroscopic properties of Dy(III) in molten LiCl–KCl eutectic salt

The spectroscopic properties of Dy(III) in molten LiCl–KCl eutectic salt at high temperature were investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and UV–vis absorption spectroscopy. For the first time, a visible fluorescence of Dy(III) in high-temperature LiCl–KCl eutectic salt was measured due to the electronic transitions from 4I13/2 and 4F9/2 to 6HJ/2 (J=7,…,15). The effect of temperature on hypersensitivity for the electronic transitions from the 4I13/2 excited state (∼25700cm−1) of Dy(III) was confirmed by altering temperature in chloride eutectic salt in accord with optical absorption measurements. The molar absorptivity of 4I13/2←6H15/2 was enhanced with increasing temperature. The fluorescence intensity of Dy(III) followed a simple mono-exponential decay curve, suggesting the formation of a single chemical species in high-temperature LiCl–KCl molten salt.

Speciation of Eu3+ bound to humic substances by time-resolved laser fluorescence spectroscopy (TRLFS) and parallel factor analysis (PARAFAC)

The bioavailability and toxicity of metal ions including radionuclides in the biosphere are greatly influenced by their speciation. Humic substances (HSs) are important constituents of various soil and water systems and have significant impact on the speciation and mobility of metal ions because of their high affinity to metal ions.In this study, the speciation of europium (Eu3+), a chemical homologue of trivalent actinides, with HSs collected from various origins was investigated by time-resolved laser fluorescence spectroscopy (TRLFS). The difficulties associated with the separation of the contribution of different Eu3+ species due to overlapping spectra or similar fluorescence lifetimes were addressed and mitigated by applying a multi-mode factor analysis, parallel factor analysis (PARAFAC), which resulted in the number, spectra, decay curves and relative fluorescence intensity profiles of different Eu3+ species. Subsequently, the interpretation of the Eu3+ species, was tackled by principal component analysis (PCA) and partial linear square (PLS) regression to deduce the nature of the Eu3+ species by taking into account the physicochemical properties of the HSs.Three factors corresponding to different Eu3+ species were obtained at 70μM Eu3+ for all HSs investigated except for one humic acid. One of the factors corresponded to free Eu3+ ion interacting with HSs via diffusion. The remaining two factors were thought to be Eu3+ bound to HSs: one bound to acidic functional groups of HSs and the other to the sites of HSs influenced by the carbon backbone structures. It was also found that the latter factor exhibited strong energy transfer from the excited Eu3+ center to HSs. At lower Eu3+ concentration (10μM), two factors having similar fluorescent characteristics to those of the second and third factors were obtained.