Complex formation of Cm(III) with formate studied by time-resolved laser fluorescence spectroscopy. (October 2015)

• Record Type: Journal Article
• Authors: Fröhlich, Daniel R.; Skerencak-Frech, Andrej; Panak, Petra J.
• Is Part Of: Applied geochemistry. Volume 61(2015:Oct.)
• Page Start: 312
• Page End: 317
• Publication Date: 2015-10
• Subjects: Curium -- Formate -- Complexation -- Thermodynamics -- Saline solution -- Elevated temperature; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2015.06.015
• Languages: English
• ISSNs: 0883-2927

Abstract

Highlights: Cm(III) complexation with formate is investigated at T = 20–90 °C. The impact of formate on Cm(III) speciation increases with increasing temperature. The complexation reactions of Cm(III) with formate are endothermic/entropy-driven. Abstract: Pore waters of natural clays, which are investigated as potential host rock formations for high-level nuclear waste, are known to contain large amounts of low-molecular weight organic compounds. These small organic ligands might impact the aqueous geochemistry of the stored radionuclides and, thus, their migration behavior. In the present work, the complexation of Cm(III) with formate in aqueous NaCl solution is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as a function of the ionic strength (0.5–3.0 mol/kg), the ligand concentration (0–0.2 mol/kg) and the temperature (20–90 °C). The Cm(III) speciation is determined by deconvolution of the emission spectra. The obtained distribution of Cm(III) species is used to calculate the conditional stability constants (log K ′( T )) at a given temperature and ionic strength which are extrapolated to zero ionic strength by using the specific ion interaction theory (SIT). Thus, the thermodynamic log K 0 n ( T ) values for the formation of [Cm(Form) n ] (3− n )+ ( n = 1, 2) and the ion interaction coefficients ( ε ( i, k )) for [Cm(Form) n ] (3− n )+ ( n = 1, 2) with Cl − are obtained. The log K 0 1 ( T ) (2.11 (20 °C)–2.49 (90 °C)) and log K 0 2 ( T ) values (1.17 (30 °C–2.01 (90 °C)) increase continuously with increasing temperature. The log K 0 n ( T ) values are used to derive the standard reaction enthalpies and entropies ( Δr H 0 m, Δr S 0 m ) of the respective complexation reactions according to the Van't Hoff equation. In all cases, positive Δr H 0 m and Δr S 0 m values are obtained. Thus, both complexation steps are endothermic and entropy-driven.