Behavior of Np(VI) and Np(V) Ions in NaHCO3 Solutions Containing H2O2

Abstract

The behavior of Np(VI) and Np(V) in NaHCO 3 and NaHCO 3 + Na 2 CO 3 solutions containing H 2 O 2 was studied spectrophotometrically. In 0.7531.0 M NaHCO 3 , hydrogen peroxide oxidizes Np(V) to Np(VI). The kinetics curves of Np(V) oxidation into Np(VI) have a complex shape and are characterized either by an induction period of up to tens of minutes or by a period of steady-state Np(VI) concentration, followed by an increase in the Np(VI) concentration. When Np(VI) initially exists in the solution, the induction period is lacking. The process character changes when the bicarbonate concentration decreases, or when Na 2 CO 3 is added. In 1.0 M Na 2 CO 3 , 0.5 M NaHCO 3 + 0.5 M Na 2 CO 3 , or 0.013 0.5 M NaHCO 3 , hydrogen peroxide completely reduces Np(VI) into Np(V). The probable mechanisms of this process were discussed. Accumulation of Np(VI) in NaHCO 3 solutions can be accounted for by assuming that Np(VI) itself participates in the transformations. Initially, the reaction of Np(VI) with H 2 O 2 yields the excited *Np(V) ion. Then it reacts with another H 2 O 2 molecule and forms a carbonate3peroxide complex. In the collision of the latter with unexcited Np(V), two electrons from two Np(V) ions are transferred onto the O 2 2! ligand with formation of two Np(VI) ions.It was found in [1] that, in the course of g-radiolysis of aerated or Ar-saturated NaHCO 3 solutions containing 2 0 10 34 M Np(V), neptunium(VI) is formed with the yield G Np(VI) of about 8 ions per 100 eV. This value exceeds the yield of the oxidant species, OH radicals (G OH = 2.9 particles per 100 eV [2]) and is close to the total yield of all primary products of water radiolysis, i.e., G OH + G e ! (aq) + G H + 2G H 2 O 2 .It follows from the data obtained that H 2 O 2 under these conditions is an oxidant with respect to Np(V). It was interesting to examine the behavior of H 2 O 2 not nascent but specially introduced into the solution. EXPERIMENTAL 237Np purified by ion exchange was used. The nitric acid solution of Np(IV) after passing through an ion-exchange column was twice evaporated with perchloric acid up to appearance of white fume. The dry residue of NpO 2 (ClO 4 ) 2 . aq was dissolved in 0.01 M HClO 4 , and NaNO 2 was added to a part of the solution obtained to convert Np(VI) into Np(V), which was then precipitated as hydroxide with aqueous ammonia. After coagulation, the NpO 2 OH precipitate was separated by centrifugation, washed twice with cold double-distilled water, and dissolved in a minimal amount of an HClO 4 solution. The oxidation states of neptunium in solutions prepared were monitored spectrophotometrically on a Shimadzu PC-3100 spectrophotometer (Japan). The neptunium concentration was determined complexometrically after the reduction to Np(IV) [3]. Na 2 CO 3 and NaHCO 3 were of chemically pure and analytically pure grade. Na 2 CO 3 was additionally purified by recrystallization from double-distilled water. Medicinal grade hydrogen peroxide stabilized with sodium dihydrogen phosphate was vacuum-distilled. The working solution...

Neptunyl Peroxide Chemistry: Synthesis and Spectroscopic Characterization of a Neptunyl Triperoxide Compound, Ca2[NpO2(O2)3]·9H2O

“…10,58 A study of Np(V) in solutions containing NaHCO 3 and H 2 O 2 indicated Np(V) was oxidized to Np(VI). 59 As observed for 1, Np(V/VI) 24 clusters formed when hydroxide base and excess H 2 O 2 were added to a Np(V) stock solution. Again, oxidation of Np(V) to Np(VI) occurs rapidly in alkaline, peroxide-rich systems.…”

“…Studies of systems containing Np­(V) and H 2 O 2 are limited. Oxidation of Np­(III) and Np­(IV) by hydrogen peroxide has been observed, , and studies of Np­(VI) interaction with H 2 O 2 mostly indicate that Np­(VI) is reduced to Np­(V), and some Np­(IV) is produced at high H 2 O 2 concentrations. , One study postulated the formation of a Np­(VI) peroxo complex in alkaline conditions on the basis of reaction rates. , A study of Np­(V) in solutions containing NaHCO 3 and H 2 O 2 indicated Np­(V) was oxidized to Np­(VI) . As observed for 1 , Np­(V/VI) 24 clusters formed when hydroxide base and excess H 2 O 2 were added to a Np­(V) stock solution.…”

Oxidative Dissolution of Neptunium(iv) Oxide in Carbonate Solutions

“…In carbonate or bicarbonate solutions, H2O2 not only oxidizes or reduces neptunium but also participates in processes of formation of mixed peroxocarbonate complexes. It was established [18]- [20] that in presence of H2O2 in solutions of NaHCO3-Na2CO3, there occurs both oxidation of Np(IV) to Np(V) and partially to Np(VI), and formation of a mixed peroxocarbonate complex of Np(IV) with structure [Np IV (O2)(CO3)2] 2-. In presence of an excessive quantity of H2O2, Np(V) is oxidized to Np(VI), and then slowly reduces (with decomposition excess of H2O2) to Np(V).…”

“…In presence of an excessive quantity of H2O2, Np(V) is oxidized to Np(VI), and then slowly reduces (with decomposition excess of H2O2) to Np(V). Neptunium(VI) also forms mixed peroxo-carbonate complexes with structure [Np VI O2(O2)(CO3)2] 4or [Np VI O2(O2)2CO3] 4- [18]- [20]. However, the redox equilibria in the aqueous carbonate systems will be complicated by the reactions disproportionation of Np(V), reproportionation of Np(IV), Np(VI), and dissociation of carbonate complexes of Np(V) [21].…”