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Quantitative understanding of uranium transport by high temperature fluids is crucial for confident assessment of its migration in a number of natural and artificially induced contexts, such as hydrothermal uranium ore deposits and nuclear waste stored in geological repositories. An additional recent and atypical context would be the seawater inundated fuel of the Fukushima Daiichi Nuclear Power Plant. Given its wide applicability, understanding uranium transport will be useful regardless of whether nuclear power finds increased or decreased adoption in the future. The amount of uranium that can be carried by geofluids is enhanced by the formation of complexes with inorganic ligands. Carbonate has long been touted as a critical transporting ligand for uranium in both ore deposit and waste repository contexts. However, this paradigm has only been supported by experiments conducted at ambient conditions. We have experimentally evaluated the ability of carbonate-bearing fluids to dissolve (and therefore transport) uranium at high temperature, and discovered that in fact, at temperatures above 100 °C, carbonate becomes almost completely irrelevant as a transporting ligand. This demands a re-evaluation of a number of hydrothermal uranium transport models, as carbonate can no longer be considered key to the formation of uranium ore deposits or as an enabler of uranium transport from nuclear waste repositories at elevated temperatures. Carbonate species may play a role in uranium transport in natural systems and nuclear waste repositories at high temperatures. Here uranyl carbonate species are shown to decrease in stability under hydrothermal conditions, suggesting that these species may not mediate transport after all. [ABSTRACT FROM AUTHOR]

Oxidized actinide species are often more mobile than reduced forms. Bioremediation strategies have been developed to exploit this chemistry and stabilize actinides in subsurface environments. We investigated the ability of metal-reducing bacteria Geobacter metallireducens and Shewanella oneidensis to enzymatically reduce Np(V) and Np(V) citrate, as well as the toxicity of Np(V) to these organisms. A toxic effect was observed for both bacteria at concentrations of ≥4.0 mM Np(V) citrate. Below 2.0 mM Np(V) citrate, no toxic effect was observed and both Fe(III) and Np(V) were reduced. Cell suspensions of S. oneidensis were able to enzymatically reduce unchelated Np(V) to insoluble Np(IV)(s), but cell suspensions of G. metallireducens were unable to reduce Np(V). The addition of citrate enhanced the Np(V) reduction rate by S. oneidensis and enabled Np(V) reduction by G. metallireducens. The reduced form of neptunium remained soluble, presumably as a polycitrate complex. Growth was not observed for either organism when Np(V) or Np(V) citrate was provided as the sole terminal electron acceptor. Our results show that bacteria can enzymatically reduce Np(V) and Np(V) citrate, but that the immobilization of Np(IV) may be dependent on the abundance of complexing ligands. [ABSTRACT FROM AUTHOR]

The bioavailability and mobility of Pu species can be profoundly affected by siderophores and other oxygen-rich organic ligands. Pu(IV)(siderophore) complexes are generally soluble and may constitute with other soluble organo-Pu(IV) complexes the main fraction of soluble Pu(IV) in the environment. In order to understand the impact of siderophores on the behavior of Pu species, it is important to characterize the formation and redox behavior of Pu(siderophore) complexes. In this work, desferrioxamine B (DFO-B) was investigated for its capacity to bind Pu(IV) as a model siderophore and the properties of the complexes formed were characterized by optical spectroscopy measurements. In a 1:1 Pu(IV)/DFO-B ratio, the complexes Pu(IV)(H2DFO•B)4+, Pu(IV)(H1DFO-B)3+, Pu(IV)(DFO-B)2+, and Pu(IV)(DFO-B)(OH)+ form with corresponding thermodynamic stability constants log β1,1,2 = 35.48, log β1,1,1 = 34.87, log β1.1,0 = 33.98, and log β1,1,-1 = 27.33, respectively. In the presence of excess DFO-B, the complex Pu(IV)H2(DFO-B)22+ forms with the formation constant log β2,1,2 = 62.30. The redox potential of the complex Pu(IV)H2(DFO-B)22+ was determined by cyclic voltammetry to be E½ = -0.509 V, and the redox potential of the complex Pu(IV)(DFO-B)2+ was estimated to be E½ = -0.269 V. The redox properties of Pu(IV)(DFO-B)2+ complexes indicate that Pu(III)(siderophore) complexes are more than 20 orders of magnitude less stable than their Pu(IV) analogues. This indicates that under reducing conditions, stable Pu(siderophore) complexes are unlikely to persist. [ABSTRACT FROM AUTHOR]

A 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 Pu4+/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 β110 = 26.44. At higher pH, the hydrolysis species, PuY(OH)- and PuY(OH)22-, form with the corresponding overall stability constants log β11-1 = 21.95 and log β11-2 = 15.29. The redox potential of the complex PuY at pH = 2.3 was determined to be E½ 342 mV. The correlation between redox potential, pH, and the protonation state of PuY- was derived to estimate the redox potential of the Pu4+/3+Y complex as a function of pH. Under conditions of neutral pH and excess EDTA relative to Pu4+, PuY24- forms with an overall formation constant of log β120 = 35.39. In the presence of ancillary ligands, mixed-ligand complexes form, as exemplified by the citrate and carbonate complexes PuY(citrate)3- (log β1101 = 33.45) and PuY(carbonate)2- (log β1101 = 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. [ABSTRACT FROM AUTHOR]

Examines the mechanism of the proton-driven ligand dissociation of siderophore iron complexes with mono- and dihydroxamic acid ligands in the presence of chloride, bromide and nitrate ions. Effect of chloride ion on rate acceleration of ligand dissociation; Time-dependent absorbance decay for hydrogen-driven dissociation; Microscopic kinetic parameters for dissociation; Effect of iron-iron distance.

Describes the kinetics and mechanism of sidephore ligand dissociation from their fully chelated Iron(III) complexes from cyclic tetradentate alcaligin and rhodotorulic acid in aqeuous solution. Influence of the level of ligand preorganization on the available ligand paths and intermediates; Structure of bis and tris Iron(III) complexes.

Looks at a study which was conducted on a new hexadentate tripodal ligand (O-TRENSOX) in relation to the thermodynamic stability of Fe(II) complexes. How O-TRENSOX is formed; Methodology used to conduct the study; Results of the study.