EDTA and Mixed-Ligand Complexes of Tetravalent and Trivalent Plutonium

Abstract

EDTA forms stable complexes with plutonium that are integral to nuclear material processing, radionuclide decontamination, and the potentially enhanced transport of environmental contamination. To characterize the aqueous Pu(4+/3+)EDTA species formed under the wide range of conditions of these processes, potentiometry, spectrophotometry, and cyclic voltammetry were used to measure solution equilibria. The results reveal new EDTA and mixed-ligand complexes and provide more accurate stability constants for previously identified species. In acidic solution (pH < 4) and at 1:1 ligand to metal ratio, PuY (where Y4- is the tetra-anion of EDTA) is the predominant species, with an overall formation constant of log beta110 = 26.44. At higher pH, the hydrolysis species, PuY(OH)- and PuY(OH)(2)2-, form with the corresponding overall stability constants log beta(11 - 1) = 21.95 and log beta(11 - 2) = 15.29. The redox potential of the complex PuY at pH = 2.3 was determined to be E(1/2) = 342 mV. The correlation between redox potential, pH, and the protonation state of PuY- was derived to estimate the redox potential of the Pu(4+/3+)Y complex as a function of pH. Under conditions of neutral pH and excess EDTA relative to Pu4+, PuY(2)4- forms with an overall formation constant of log beta120 = 35.39. In the presence of ancillary ligands, mixed-ligand complexes form, as exemplified by the citrate and carbonate complexes PuY(citrate)3- (log beta1101 = 33.45) and PuY(carbonate)2- (log beta1101 = 35.51). Cyclic voltammetry shows irreversible electrochemical behavior for these coordinatively saturated Pu4+ complexes: The reduction wave is shifted approximately -400 mV from the reduction wave of the complex PuY, while the oxidation wave is invariant.

Understanding the Solution Behavior of Minor Actinides in the Presence of EDTA4–, Carbonate, and Hydroxide Ligands

“…[13][14][15][16] However, few studies have observed the formation of ternary complexes of the actinides or lanthanides with carbonate. 17 Actinide complexes of EDTA 4-can be soluble at both acidic and alkaline pHs, and may be used as a model to understand the migration and speciation of radionuclides in the environment, as well as the behaviour of actinides in various effluents and matrices. 18 Carbonate is an environmentally important and ubiquitous ligand, being found in ground and surface waters at concentrations between 10 -2 M to 10 -5 M. 19 The carbonate ion forms strong ionic interactions, due to its bidentate oxygen donors, with cations of the felements.…”

Trace metal complexation by the triscatecholate siderophore protochelin: structure and stability

“…Protochelin is also associated with the specific rejection of toxic W (Wichard et al 2008), and the ability to form complexes with many metals coupled with selectivity of the uptake may function as a detoxification mechanism to exclude toxic metals (Kraepiel et al 2009). Coupled with a growing body of work that has explored the geochemistry and biology of non-ferric metal interactions with other siderophores (Duckworth et al 2008b;Duckworth and Sposito 2005;Parker et al 2004Parker et al , 2007Boukhalfa et al 2004Boukhalfa et al , 2007Neu et al 2005;Harris et al 2007;Mishra et al 2009;Dahlheimer et al 2007;Frazier et al 2005;Szabo and Farkas 2011), work with protochelin has led to the suggestion that siderophores may be involved with the Fig. 1 Structures of the catecholate siderophores enterobactin, petrobactin, azotochelin, aminochelin, and protochelin transport of non-ferric metals (Kraepiel et al 2009) and have a diverse chemistry with a variety of environmentally relevant metals (Duckworth et al 2009a).…”

Purification and characterization of rhodobactin: a mixed ligand siderophore from Rhodococcus rhodochrous strain OFS

“…9). These data suggest that the binding strength of the siderophore is near the binding strength of EDTA, which has a formation constant with Pu(IV) of log b = 26.44 (Boukhalfa et al 2004). The stoichiometry of the Fe (III) complex was also verified by obtaining mass spectra of the Fe-siderophore species.…”