Effect of O2, H2O2, and HNO2 on the stability of Np(III–VII) in aqueous solutions

Abstract

Published data on reactions of Np ions with О 2 , Н 2 О 2 , HNO 2 , and HNO 3 in solutions of various compositions in a wide pH range are considered. О 2 oxidizes Np(III) in acid solution and Np(IV) and Np(V) in alkaline solutions. Н 2 О 2 exhibits dual behavior. In weakly acidic solutions, it converts Np(III) and (IV) to Np(V), in 0.75-1 M NaHCO 3 it oxidizes Np(V) to Np(VI), whereas in dilute HClO 4 and HNO 3 and in carbonate and alkali solutions it reduces Np(VI), and in alkali solutions it reduces Np(VII). The first step of reduction in most cases is the formation of the Np(VI) peroxide complex, and the next step is the intramolecular charge transfer. In concentrated HNO 3 solutions, H 2 O 2 converts Np(V) to Np(IV) and Np(VI) and then reduces Np(VI). Some radiation-, photo-, and sonochemical reactions occur via formation of excimers, i.e., of dimers arising from excited and unexcited Np ions. The excimer decomposes into two ions with higher and lower oxidation states. In reduction reactions, the excimer eliminates Н 2 О 2 (in addition to the Н 2 О 2 arising as primary product of water radiolysis). In HNO 3 solutions, oxidation of Np ions occurs only in the presence of HNO 2 arising as reaction product or upon radiolysis, photolysis, or sonolysis. The active species are NO 2 -, NO 2 , and NO + present in equilibrium with HNO 2 .Any solutions contacting with air contain О 2 . The Np ions whose oxidation potential is lower than the О 2 /Н 2 О potential in the given medium react with О 2 . Hydrogen peroxide is formed in solutions containing actinides and fission products under the action of α-, β-, and γ-radiation, and HNO 2 is formed in HNO 3 solutions. Solutions of HNO 3 , Н 2 О 2 , and HNO 2 are used in processes for Np recovery and purification, and also in laboratory practice. In this connection, we analyze here the published data on reactions of Np(III-VII) ions with О 2 , Н 2 О 2 , HNO 2 , and HNO 3 . In reviews [1-3] these reactions are not considered.At the same time, in a 0.01 M HClO 4 solution saturated with О 2 , the Np(IV) concentration decreased from 9.81 × 10 -4 to 5.92 × 10 -4 M in 16 h at 25°С. We estimate the bimolecular rate constants of the reactions of О 2 with Np(III) and Np(IV) at 3.5 × 10 2 and 8.8 × 10 -3 l mol -1 s -1 , respectively.

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

“…Although Np­(V) is the most common oxidation state in solution, the rich redox chemistry of Np is likely important in systems containing hydrogen peroxide produced by alpha radiolysis of water near or in contact with radioactive materials. − Specifically, H 2 O 2 oxidizes Np­(IV) to Np­(V/VI) in solutions containing H 2 O 2 and NaHCO 3 , and reduces Np­(VI) to Np­(V) in solutions containing Na 2 CO 3 and H 2 O 2 . This leads to complex chemistry that is dependent on solution conditions and initial oxidation state of Np and warrants more in-depth studies aimed at elucidating Np redox activity as a function of peroxide and hydroxide concentration …”

“…10 This leads to complex chemistry that is dependent on solution conditions and initial oxidation state of Np and warrants more in-depth studies aimed at elucidating Np redox activity as a function of peroxide and hydroxide concentration. 11 In addition to the complex redox chemistry, the complexation of Np with peroxide is not well-understood, and the descriptions of peroxide complexes of Np that have been isolated in the solid state are scarce. Cubic and hexagonal forms of Np(IV) peroxide precipitates were characterized by X-ray diffraction and have similarities to Pu(IV) peroxides.…”