RESUMEN
We investigated the binary Cm-citrate system using time-resolved laser fluorescence spectroscopy (TRLFS), parallel factor analysis (PARAFAC), and quantum chemical calculations. Evidence collectively suggests the stepwise coordination and deprotonation of citrate alcohol groups in Cm-cit complexes with two bound citrate moieties upon increasing pH, which is supported by a bathochromic shift in emission spectra, an observed increase in lifetime measurements, and lower energy minima for citrate alcohol involvement versus hydrolysis of the Cm-citrate species. Our PARAFAC results agree with a 3-component model for the Cm-citrate system and offer pure component decompositions, yielding fraction species across the studied pH range that have a correlated slope = 1 as a function of pH. For the first time, evidence of ternary Ca-Cm-citrate complexes was revealed by TRLFS with increasing calcium concentration at fixed pHm. The formation of these ternary complexes was substantiated with density functional theory (DFT) calculations on simple model systems of the complexes. Shared citrate carboxylate groups between calcium and curium were proposed for all three ternary Ca-Cm-cit complexes based on DFT-determined Ca-O and Cm-O distances. Moreover, we found that the ternary complex with both alcohol groups deprotonated is most stable when it shares both two carboxylate and two alcohol groups between Ca and Cm. The presence of shared functional groups highlights the enhanced stability of these ternary complexes. Additional work is warranted to further constrain the stoichiometry, stability constants and dependence on ionic strength of these complexes for purposes of thermodynamic modeling of repository settings.
RESUMEN
We explored the speciation and kinetics of the Pu(VI)-citrate and Pu(III)-citrate systems (pHm = 2.5-11.0, I = 0.1 M NaCl, T = 23 °C, O2(g) < 2 ppm) using ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometry, solvent extraction, and PHREEQC modeling. Formation constants were determined for PuO2(HcitH)(aq) (logâ¯K°1,1 = 1.09 ± 0.05) and PuO2(HcitH)(citH)3- (logâ¯K°1,2 = -0.20 ± 0.07), and evidence for (PuO2)m(citH-k)n(OH)x2m(3+k)n-x was identified under alkaline conditions. Pu(VI) species were found to be less stable in the presence of citrate than in the absence of citrate (t ≤ 168 days); the rate of reduction increased with increasing pH. The direct reduction of Pu(VI) to Pu(IV) was required to fit experimental data in the presence of citrate but did not improve the fit for Pu in the absence of citrate. We also observed increased Pu(III) stability in the presence of citrate (t ≤ 293 days), with higher concentrations of Pu(III) favored at lower pH. Finally, we provide evidence of a radiolysis-driven mechanism for the citrate-mediated reduction of plutonium that involves electron transfer from the oxidative breakdown of citrate. Our work highlights the need to investigate the redox effect of organic ligands on plutonium oxidation states under repository-relevant conditions.