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Updated: Mar 27, 2026

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films
Published on: November 9, 2015
Evaluating Np(v) disproportionation: effect of Np concentration.
Sara E Gilson1, Megan E Simms1, Frankie D White1
1Radioisotope Science and Technology Division, Oak Ridge National Laboratory 1 Bethel Valley Road Oak Ridge TN 37830 USA gilsonse@ornl.gov.
High neptunium (Np) concentrations complicate Np(v) stability in acidic solutions, with neptunyl self-complexation observed. Np(v) disproportionation was not definitively proven due to competing mechanisms.
Area of Science:
- Radiochemistry
- Solution Chemistry
- Spectroscopy
Background:
- Neptunium (Np) redox chemistry is crucial for nuclear waste management.
- Understanding Np(v) behavior in acidic media is essential for predicting its fate.
- Neptunium disproportionation is a potential reaction pathway influencing its speciation.
Purpose of the Study:
- To investigate the role of Np(v) concentration in driving Np disproportionation.
- To probe Np redox chemistry in nitric acid (HNO3) and hydrochloric acid (HCl) media.
- To understand the stability of Np oxidation states at high concentrations.
Main Methods:
- Visible-near-infrared absorbance spectroscopy was used for in situ analysis.
- Chemically and electrochemically manipulated Np oxidation states were studied.
- The Np(VI/V) ratio was analyzed after electrochemical tuning.
Main Results:
- High Np concentrations made stabilizing pure Np(v) difficult, even in dilute acid.
- A Np(vi) component coexisted with Np(v) in both HNO3 and HCl.
- Neptunyl self-complexation was identified as a key mechanism affecting the Np(VI/V) ratio.
- Np(iv) was stabilized in HNO3, but Np(v) was only briefly pure.
- Np(v) disproportionation was not definitively confirmed by spectral data.
Conclusions:
- High Np concentrations significantly impact Np redox chemistry in acidic solutions.
- Neptunyl self-complexation and radiolysis are potential competing mechanisms to Np(v) disproportionation.
- Further investigation is needed to definitively determine the occurrence of Np(v) disproportionation.
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