Kinetic Investigations on the Reduction of Plutonium (VII) in Alkaline Solutions

Abstract

Plutonium ( VII) / Reduction kinetics /Hydrolysis /Mechanism Experimental AbstractReduction of Pu(VII) to Pu(VI) in various alkaline solutions ranging from 0.3 to 1.96 Μ at an ionic strength of 2 Μ has been kinetically investigated. The observed first order rate constant, ^obs has been found to depend on [OH ] where η = 1 and 2 when the concentration of hydroxide ion is in the range of 1 to 1.96 Μ and 0.3 to 0.62 Μ respectively. The activation energy (^act) the overall reaction has been found to vary from 8 ± 0.4 kcal/mol to 29 ± 0.7 kcal/mol when the concentration of hydroxide ion varied from 0.5 to 1 Μ at the same ionic strength, μ = 2 Μ. A kinetic reaction mechanism has been proposed wherein hydrolytic reactions (2) and (3) followed by rate controlling reactions (4) and (5) and subsequent fast reactions of Pu(V) and Pu(VII) are involved.

Preparation, Stability, and Structural Characterization of Plutonium(VII) in Alkaline Aqueous Solution

“…Our results are consistent with the original, quantitative observations about the kinetics of Pu(VII) autoreduction , and failure to stabilize Pu(VIII) in either a solid material or in solution . Although the Pu(VII) lifetime is hours under alkaline solution conditions, even then, the reduction of Pu(VII) by H 2 O is spontaneous according to the reaction of eq with a positive cell potential, +0.55 V, calculated from the standard reduction potentials for the half-reactions …”

“…The same cell and conditions were employed for in situ optical experiments with a Cary-14 Olis-conversion spectrophotometer. The optical spectra for the alkaline Pu(VI) and Pu(VII) species, provided as Supporting Information, are in agreement with the numerous spectra found in the literature. ,,,,,− …”

“…It is possible that the XAFS spectrum, both the XANES and EXAFS components, from a Pu(VIII) moiety is indistinguishable from that obtained from the heptavalent species. Coupled with the expected low yield of the octavalent Pu species, 42 and their calculated transitory stabilities, 79 Our results are consistent with the original, quantitative observations about the kinetics of Pu(VII) autoreduction 60,79 and failure to stabilize Pu(VIII) in either a solid material or in solution. 41 Although the Pu(VII) lifetime is hours under alkaline solution conditions, even then, the reduction of Pu(VII) by H 2 O is spontaneous according to the reaction of eq 3 with a positive cell potential, +0.55 V, calculated from the standard reduction potentials for the half-reactions.…”

Redox reactions of actinides in carbonate and alkaline solutions

“…The addition of a NaClO solution to an alkaline solution of neptunium(VI) containing iridium hydroxides leads to the slow formation of Np(VII) because iridium is transformed into higher oxidation states under the action of the ClO 7 ions. The ClO 27. radical ions that are formed react with Np(VI) [see the reaction (138)].…”

“…Hence, it was concluded 138 that Pu(VII) involved in the complex PuO 4 (OH) 3À 2 anion eliminates 1 / 2 O 2 and is transformed into Pu(V), which rapidly reacts with Pu(VII). However, no explanation was given as to how one-half of the oxygen molecule was eliminated.…”