Hydration of Cm³⁺ in Aqueous Solution from 20 to 200 °C. A Time-Resolved Laser Fluorescence Spectroscopy Study

Abstract

Time-resolved laser fluorescence spectroscopy (TRLFS) is used to study the hydration of the Cm3+ ion in acidified (0.1 M perchloric acid) H2O and D2O from 20 to 200 degrees C. Strong temperature dependency is found for several of the spectroscopic quantities associated with the 6D'(7/2) --> 8S'(7/2) photoemission spectra, with similar relative changes in both solvents. The emission band shifts to lower energy with increasing temperature, which is attributed to an equilibrium between hydrated Cm3+ ions with different numbers of water molecules in the first coordination sphere, namely [Cm(H2O)9]3+ and [Cm(H2O)8]3+. Comparison with crystalline reference compounds and the analysis of hot bands corroborates the assignment of these species. The molar fraction of the octahydrated species increases from approximately 10% at room temperature to approximately 40% at 200 degrees C, indicating an entropy driven reaction. The corresponding thermodynamic parameters are obtained as Delta H degrees = + 13.1 +/- 0.4 kJ mol(-1), Delta S degrees = + 25.4 +/- 1.2 J mol(-1) K(-1), and Delta G298 = + 5.5 +/- 0.6 kJ mol(-1). Both the emission intensity and lifetime decrease with increasing temperature. The temperature dependency of the nonradiative decay rate of the emitting 6D'(7/2) level follows an Arrhenius equation with the activation energy 26.5 kJ mol(-1) (2250 cm(-1)) in both H2O and D2O, which is somewhat lower than the energy gap between 6D'(7/2) and 6P'(5/2) exited state levels.

Europium retention onto clay minerals from 25 to 150°C: Experimental measurements, spectroscopic features and sorption modelling

“…Under acidic conditions (pH 3), the corresponding recorded lifetimes also decrease with temperature, from 110 ± 5 ls at 25°C to 95 ± 5 ls at 150°C. As mentioned by Kimura et al (2002) for free aqueous europium, Lindqvist-Reis et al (2005) for aqueous curium and Kirishima et al (2004) for aqueous uranyl, both the fluorescence intensity and lifetime show a decrease with increasing temperature. Indeed, Kimura et al (2002) 3+ .…”

“…To our knowledge, only two published studies have addressed the acquisition and interpretation of fluorescence spectra of aqueous trivalent cations (Eu 3+ or Cm 3+ ) at high temperature: Kimura et al (2002) up to 227°C (500 K) and Lindqvist-Reis et al (2005) up to 200°C. We therefore began by recording fluorescence spectra and lifetimes of aqueous europium between 25 and 150°C in 0.5 M NaClO 4 solutions.…”

“…Even in the spectroscopic field, there are very few studies characterizing sorbed lanthanides at elevated temperature. To our knowledge, only three research groups have developed specific experimentation for TRLFS measurements (time resolved laser fluorescence spectroscopy) at high temperature (see Kimura et al, 2002;Bauer et al, 2005;Lindqvist-Reis et al, 2005). For experiments in aqueous solution, the first and second group show clearly that the change in hydration of trivalent cations with increasing temperature can be observed via the fluorescence properties of these elements.…”

Americium(III) coordination chemistry: An unexplored diversity of structure and bonding

“…Although the range of investigation here (from Pu to Cm, DZ is only equal to 2) is very small, the stability of the capping distances justifies the assumption of a statistical decrease of coordination number as for the lanthanide case. In solution, from optical spectra and EXAFS data, americium and curium were found as mostly nine-coordinated at room temperature [18,20].…”

Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 1: Batch sorption and time-resolved laser fluorescence spectroscopy experiments

“…In solid form those hydrates are known to form tricapped trigonal prisms and each metal ion is coordinated to 9 H 2 O molecules. Coordination numbers of around 9 have been experimentally determined also in solution (Rizkalla and Choppin, 1992;Stumpf et al, 2004a;Lindqvist-Reis et al, 2005). For surface sorbed species much less information on the coordination of actinide and lanthanide ions is available.…”