Europium speciation by time-resolved laser-induced fluorescence
Introduction
Knowledge of speciation (i.e. the repartition of the different chemical forms of one element) is of prime importance for environmental studies and, in particular, in relation to nuclear activities, present and future [1], [2], [3]. Among the ligands present in groundwaters that could complex radionuclides are inorganic anions such as hydroxide and carbonate, and also organic matter such as humic and fulvic acids [4].
To study the speciation of an element, the technique used must possess, at least, three properties. The technique must firstly be selective (since complex matrixes are likely to be encountered in natural systems), secondly, sensitive (in relation with solubility aspects and also to be representative of the concentrations observed in natural systems) and, thirdly, non-intrusive (in order not to modify the composition of the solution). One of these techniques is Time-resolved laser-induced fluorescence (TRLIF) spectroscopy which has often been used to study the speciation of many actinides and lanthanides (U, Cm, Eu, Sm, …) [5], [6], [7]. Its principle is based on pulsed laser excitation followed by temporal resolution of the fluorescence signal which leads to the elimination of short lifetime parasital fluorescence. The other great advantage of TRLIF is its triple resolution: (1) excitation resolution by the proper choice of the laser wavelength (tripled or quadrupled Nd-YAG, O.P.O., N2, …); (2) emission fluorescence which gives characteristic spectra of the fluorescent cation (free or complexed); and (3) fluorescence lifetime which is characteristic of its environment (complexation, quenching). These two latter types of data provide useful information on the chemical species present in solution and is quite useful for complexation studies.
In this work, europium(III) has been chosen as a chemical analogue of actinides such as americium(III) or curium(III) and for its simplicity of use [8].
TRLIF has been used to identify spectrally (peak position, full width at mid height) and temporally (lifetime) the different species of europium present in aqueous solutions. Lifetime measurement can be used for the determination of water molecules surrounding the fluorescent species [9]. The determination of water molecules in the first co-ordination sphere of europium is very important in terms of complexation studies since the number of water molecules removed allows one to determine the nature of the complex. The number of water molecules around europium can be found by the following equation (obtained by measuring europium lifetimes in H2O/D2O solutions, which can vary between around 110 μs in pure H2O solution and 2300 μs in pure D2O solution) [10]:where τ is in ms.
TRLIF spectroscopy has already been used to study europium in different systems (in aqueous solutions, for interaction studies with organic materials or mineral surfaces, …) [11], [12], [13], [14], [15] but only a few studies have been carried out on europium carbonate and hydroxide complexes [11], [16], [17].
Fig. 1 presents the energy diagram and fluorescence spectrum of free europium as Eu3+[18]. This spectrum, characteristic of Eu3+, presents fluorescence in the red with the strongest lines around 580, 593, 617, 650 and 700 nm. These emission lines come from transitions from the excited state at 17374 cm−1 () to ground states at 0, 374, 1036, 1888, 2866 cm−1 (, J=0–4) [18]. The transition at 617 nm () is hypersensitive and, thus, is very important in complexation studies since its intensity is enhanced in the case of complexation with a ligand relative to its intensity obtained in an acidic medium (where free europium is predominant).
To determine the spectrum and the lifetime of each europium species, the procedure is:
- •
to find chemical conditions (pH, ionic strength, presence of salts, working atmosphere) in which there is only one main species, and
- •
if such chemical conditions cannot be found, to find spectroscopic conditions (excitation wavelength, temporal delay and gatewidth) which favour the fluorescence of one species.
This paper, therefore, will present spectroscopic data (spectra, lifetimes and number of water molecules in the first co-ordination sphere) of different complexes of europium with the ligands carbonate, hydroxide and humic substances.
Section snippets
Apparatus
Time-resolved laser-induced fluorescence: a Nd-YAG laser (Model Minilite, Continuum, Santa Clara, CA), operating at 266 nm (quadrupled) and delivering about 2.5 mJ of energy in a 4 ns pulse with a repetition rate of 15 Hz, was used as the excitation source. The laser output energy was monitored by a laser power meter (Scientech, Boulder, CO). The laser beam was directed into the 4 ml quartz cell of the spectrofluorometer “FLUO 2001” (Dilor, Lille, France). The radiation coming from the cell was
Results and discussion
To determine chemical conditions where a single species is the main component, it is possible to build different speciation diagrams with the help of literature data (interaction constants β, solubility products Ks, …) (Table 1) [19], [20], [21], [22], [23], [24], [25]. Two examples are given in Fig. 2, one calculated for an atmospheric pressure of CO2 (pCO2=3.16×10−4 atm) in which carbonate complexes can be seen and one calculated for a null CO2 pressure in which only hydroxide complexes can be
Conclusion
Time-resolved laser-induced fluorescence has been successfully used for the spectroscopic characterisation of different species of europium (free europium, and its carbonate, hydroxide and humate complexes). With the use of very well characterised chemical conditions (pH, carbonate concentration) and spectral deconvolution, the acquisition of precise fluorescence spectra and lifetimes for these different complexes proved to be feasible. This spectroscopic fluorescence database is particularly
Acknowledgements
The authors would like to thank DEN/DDIN (Maı̂trise des Risques and HAVL) and DEN/DSOE (R&D).
References (38)
- et al.
Appl. Geochem.
(1992) - et al.
J. Alloys Comp.
(1994) - et al.
J. Alloys Comp.
(1994) - et al.
Anal. Chim. Acta
(1989) - et al.
Anal. Chim. Acta
(1999) - et al.
Anal. Chim. Acta
(1999) - et al.
Talanta
(1991) - et al.
J. Inorg. Nucl. Chem.
(1978) Sci. Tot. Environ.
(1991)- G.R. Choppin, B. Allard, in: A.J. Freeman, C. Keller (Eds.), Handbook on the Physics and Chemistry of the Actinides,...
J. Rad. Nucl. Chem.
Anal. Chem.
Radiochim. Acta
J. Am. Chem. Soc.
Anal. Chem.
Radiochim. Acta
Cited by (95)
Magnetic separation-enhanced photoluminescence detection of dipicolinic acid and quenching detection of Cu(II) ions
2024, Spectrochimica Acta - Part A: Molecular and Biomolecular SpectroscopyDiblock versus block-random copolymer architecture effect on physical properties of Gd<sup>3+</sup>-based hybrid polyionic complexes
2023, Journal of Colloid and Interface ScienceHydration states of europium(III) adsorbed on silicas with nano-sized pores
2023, Applied GeochemistryInfluence of coordinating environment on photophysical properties of UV excited sharp red emitting material: Judd Ofelt analysis
2022, Journal of Photochemistry and Photobiology A: ChemistryCitation Excerpt :For the last decades, a lot of research has been done for the synthesis of luminescent lanthanide (Ln) complexes due to their importance in biomedical field [1], optical fiber [2], lasers [3], sensors [4], solar concentrators [5] and organic light emanating devices [6–9].
Sorption of europium on diatom biosilica as model of a “green” sorbent for f-elements
2021, Applied Geochemistry