Combined Time-Resolved Laser Fluorescence Spectroscopy and Extended X?ray Absorption Fine Structure Spectroscopy Study on the Complexation of Trivalent Actinides with Chloride at T = 25?200 °C

摘要

The complexation of trivalent actinides (An(III)) with chloride is studied in the temperature range from 25 to 200 °C by spectroscopic methods. Time-resolved laser fluorescence spectroscopy (TRLFS) is applied to determine the thermodynamic data of Cm(III)?Cl? complexes, while extended X-ray absorption fine structure spectroscopy (EXAFS) is used to determine the structural data of the respective Am(III) complexes. The experiments are performed in a custom-built hightemperature cell which is modified for the respective spectroscopic technique. The TRLFS results show that at 25 °C the speciation is dominated mainly by the Cm~(3+) aquo ion. Only a minor fraction of the CmCl~(2+) complex is present in solution. As the temperature increases, the fraction of this species decreases further. Simultaneously, the fraction of the CmCl_2~+ complex increases strongly with the temperature. Also, the CmCl_3 complex is formed to a minor extent at T > 160 °C. The conditional stability constant log β′_2 is determined as a function of the temperature and extrapolated to zero ionic strength with the specific ion interaction theory approach. The log β°_2(T) values increase by more than 3 orders of magnitude in the studied temperature range. The temperature dependency of log β°_2 is fitted by the extended van’t Hoff equation to determine Δ_rH°_m, Δ_rS°_m, and Δ_rC°_(p,m). The EXAFS results support these findings. The results confirm the absence of americium(III) chloride complexes at T = 25 and 90 °C ([Am(III)] = 10~(?3) m, [Cl?] = 3.0 m), and the spectra are described by 9?10 oxygen atoms at a distance of 2.44? 2.48 ?. At T = 200 °C two chloride ligands are present in the inner coordination sphere of Am(III) at a distance of 2.78 ?.