Organic and Aqueous Redox Speciation of Cu(III) Periodate Oxidized Transuranium Actinides

Abstract

A hexavalent group actinide separation process could streamline used nuclear fuel recycling and waste management. The limiting factor to such a process compatible with current fuel dissolution practices is obtaining and maintaining hexavalent Am, AmO2 2+, in molar nitric acid because of the high reduction potential of the Am­(VI)/Am­(III) couple (1.68 V vs SCE). Two strong oxidants, sodium bismuthate and Cu3+ periodate, have demonstrated quantitative oxidation of Am under molar acid conditions, and better than 50% recovery by diamyl amylphosphonate (DAAP) is possible under these same conditions. This work considers the use of Cu3+ periodate to oxidize Np­(V) to Np­(VI) and Pu­(IV) to Pu­(VI) and to recover these elements by extraction with DAAP. A metal:oxidant ratio of 1:1.2 and 1:3 was necessary to quantitatively oxidize Np­(V) and Pu­(IV), respectively, to the hexavalent state. Extraction of hexavalent Np, Pu, and Am by 1 M DAAP in n-dodecane was measured using ultraviolet–visible (PuO2 2+, AmO2 2+) and near-infrared (NpO2 2+) spectroscopy. Distribution values of AmO2 2+ were found to match previous tracer level studies. The organic phase spectra of Np, Pu, and Am are presented, and molar absorptivities are calculated for characteristic peaks. Hexavalent Pu was found to be stable in the organic phase, while NpO2 2+ showed some reduction to NpO2 +; Am was present as Am3+, AmO2 +, and AmO2 2+ species in aqueous and organic phases during the extraction experiments. These results demonstrate, for the first time, the ability to recover macroscopic amounts of americium that would be present during fuel reprocessing and are the first characterization of Am organic phase oxidation state speciation relevant to a hexavalent group actinide separation process under acidic conditions.

Chemical Understanding of Actinide Separations

“…A group actinide(VI) extraction seeks to oxidize U, Np, Pu, and Am to the 6+ oxidation state and extract all four actinides into an organic phase as their respective actinyl cations by a solvating ligand or by a cocrystallization method. [53][54][55][56][57][58][59][60] The primary challenge in the group actinide(VI) extraction concept is oxidizing americium(III) to the 6+ oxidation state. An Am(VI)/Am(III) redox potential of 1.68 V vs. SCE requires strong oxidizers to generate the americyl cation).…”

“…Solvent extraction of AmO2 2+ oxidized by Cu 3+ periodate was demonstrated with DAAP and monoamide ligands. 56,58,59 Although separation by 1 M DAAP was achieved, short contact times less than 10 seconds were required. 56,58 Distribution values decreased with extended contact times.…”

“…56,58,59 Although separation by 1 M DAAP was achieved, short contact times less than 10 seconds were required. 56,58 Distribution values decreased with extended contact times. The trend was observed both in NaBiO3 and Cu 3+ periodate oxidation schemes.…”

“…Studies showed that upon contact AmO2 2+ began to reduce to less extractable species (AmO2 + and Am 3+ ). 58 The initial challenge to group actinide(VI) separations was to oxidize Am in molar nitric acid solutions required for liquid-liquid extraction process like PUREX. The ability to oxidize and extract americium(VI) in molar nitric acid has been demonstrated.…”

Photocatalytic Conversion of Am(III) to Am(VI) Using a TiO2 Electrode

“…Am­(VI) is difficult to generate given the high Am­(IV/III) couple ( E 1/2 = 2.62 V vs SHE) . A variety of chemical oxidants have been developed for Am­(III) oxidation, including persulfate, sodium bismuthate, , copper periodate, silver ions in conjunction with ozone, even noble gas compounds . From a fuel cycle prospective, many of these oxidants have drawbacks due to downstream complications such as in the case of persulfate, where the sulfate product inhibits vitrification of the waste, or engineering complications as in the design of a filtration step to remove insoluble sodium bismuthate.…”