Speaker
Description
N,N'-dimethyl,N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) is used to separate An(III) and Ln(III) from fission products in several liquid-liquid extraction processes (DIAMEX, EXAm, GANEX)$^{1-2}$ that aim at recycling actinides. One key to understand these processes is the stoichiometry of the complexes formed during extraction. Time-resolved laser fluorescence spectroscopy (TRLFS) poses, in contrast to extraction experiments, a technique to study the inner sphere coordination of fluorescent metal ions (e.g. Cm(III)). Therefore, the stoichiometry of the complexes can be determined. The presented work focuses on the complexation of Cm(III) with DMDOHEMA studied by TRLFS in mono- and biphasic systems. First, stability constants of the Cm(III)-DMDOHEMA complexes are determined in 1-octanol. The formation of [Cm(DMDOHEMA)$_n$]$^{3+}$ with log$\beta$’$_1$ = 2.63 ± 0.33, log$\beta$’$_2$ = 3.99 ± 0.48, log$\beta$’$_3$= 4.34 ± 0.52 is confirmed. Fluorescence lifetime measurements indicate the formation of a 1:4 complex. Furthermore, biphasic experiments are performed with Cm(III) and DMDOHEMA in dependence of the DMDOHEMA, nitric acid and nitrate concentration. TRLFS measurements of the different organic phases reveal a predominance of two species: (I) at $\lambda_{max}$ = 601.3 nm, and (II) 604.2 nm. These are identified as [Cm(DMDOHEMA)$_3$(NO$_3$)(H$_2$O)]$^{2+}$ (I) and [Cm(DMDOHEMA)$_4$(H$_2$O)]$^{3+}$ (II) by comparison with the monophasic experiments. Species II is dominant at low acid concentrations. This trend is explained by the free DMDOHEMA concentration, which decreases due to the extraction of both nitric acid and water. To support the proposed stoichiometries, vibrational sideband spectroscopy is performed. This laser-based technique allows the observation of vibrations of functional groups coordinated to the fluorescent metal ion. Clear differences are obtained for species I and II in the range of 900 – 1400 cm$^{-1}$. Moreover, vibrational spectra for species I and II calculated by density functional theory (DFT) are in good agreement with the experimental data, confirming the proposed stoichiometries.
References:
1. Reprocessing and Recycling of Spent Nuclear Fuel. Taylor, R. J., Ed. Woodhead Publishing: 2015.
2. Rostaing, C.; Poinssot, C.; Warin, D.; Baron, P.; Lorrain, B., Development and Validation of the EXAm Separation Process for Single Am Recycling. Procedia Chemistry 2012, 7, 367-373.
