Reduction of Plutonium(VI) to (V) by Hydroxamate Compounds at Environmentally Relevant pH

Abstract

Natural organic matter is known to influence the mobility of plutonium (Pu) in the environment via complexation and reduction mechanisms. Hydroxamate siderophores have been specifically implicated due to their strong association with Pu. Hydroxamate siderophores can also break down into di and monohydroxamates and may influence the Pu oxidation state, and thereby its mobility. In this study we explored the reactions of Pu(VI) and Pu(V) with a monohydroxamate compound (acetohydroxamic acid, AHA) and a trihydroxamate siderophore desferrioxamine B (DFOB) at an environmentally relevant pH (5.5-8.2). Pu(VI) was instantaneously reduced to Pu(V) upon reaction with AHA. The presence of hydroxylamine was not observed at these pHs; however, AHA was consumed during the reaction. This suggests that the reduction of Pu(VI) to Pu(V) by AHA is facilitated by a direct one electron transfer. Importantly, further reduction to Pu(IV) or Pu(III) was not observed, even with excess AHA. We believe that further reduction of Pu(V) did not occur because Pu(V) does not form a strong complex with hydroxamate compounds at a circum-neutral pH. Experiments performed using desferrioxamine B (DFOB) yielded similar results. Broadly, this suggests that Pu(V) reduction to Pu(IV) in the presence of natural organic matter is not facilitated by hydroxamate functional groups and that other natural organic matter moieties likely play a more prominent role.

Characterization of Americium and Curium Complexes with the Protein Lanmodulin: A Potential Macromolecular Mechanism for Actinide Mobility in the Environment

“…Among the small ligands that have been studied for actinide binding in the environment, the siderophores have garnered significant attention since they are ubiquitous and are strong metal chelators. In particular, the tris-hydroxamate, desferrioxamine B (DFOB), has been extensively studied ,− and is often taken as a representative molecule of this class of ligands because it is widespread in nature, and it forms very strong complexes with plutonium ,, as well as with the trivalent lanthanides and actinides , (Table S6). It has been proposed that the DFOB complexes could represent as much as ∼30% of the speciation of trivalent lanthanides in seawater, even if present at subnanomolar concentration .…”

Plutonium Redox Transformation in the Presence of Iron, Organic Matter, and Hydroxyl Radicals: Kinetics and Mechanistic Insights

“…The Pu stock solution was then purified using anion exchange resin (BioRad AG 1 × 8, 100–200 mesh) preconditioned with 8 M HNO 3 . The Pu was loaded on the resin in 8 M HNO 3 , washed with three column volumes of 8 M HNO 3 , and then eluted in 0.1 M HCl. , The final Pu­(IV) concentration in the stock solution was determined to be 8.84 × 10 –8 M. A Pu­(VI) stock was prepared using an electrochemistry method described in previous studies. , Briefly, the purified Pu­(IV) stock was added to an electrochemical cell with 0.1 M NaNO 3 electrode solution at 1.9 V for 5 days.…”

“…Nevertheless, these organic compounds can influence the fate and transport of Pu through redox and complexation reactions . Pu­(V/VI) can be rapidly reduced to Pu­(III/IV) by NOM, depending on the nature of the organic matter. , In addition, NOM will complex Pu via carboxylate functionalities and nitrogen groups in both colloidal and particulate phases . Santschi et al revealed that the majority of the remobilized Pu from contaminated soils to streams in storm runoffs, pond discharge, and wind dispersion events at Rocky Flats National Wildlife Refuge was associated with organic macromolecules rather than the more abundant inorganic colloids (iron oxides and clays) .…”

Open questions on the environmental chemistry of radionuclides

“…2). While other recent studies 8,9 have shown that synthetic multidentate chelators can drive the oxidation state of lanthanides and actinides to +IV, Morrison et al 7 demonstrated that even a small bidentate ligand, like acetohydroxamic acid, reduces Pu(VI) to Pu(V) under environmentally relevant conditions, and that the reduction reaction does not produce Pu(IV) or Pu(III). These results raise many questions about the role of commonly occurring natural chelators in environmental samples that are often organic-rich mixtures containing trace amounts of radionuclides.…”

“…A reference spectra of Pu(IV) DFOB is included showing that the reaction of Pu(VI) with DFOB does not produce any spectral features associated with a Pu(IV) DFOB complex. Adapted with permission from ref 7. .…”