Spectrophotometry and electrochemical methods were combined (i.e., spectroelectrochemistry) to simultaneously measure neptunium (0.01–1.0 M), nitric acid (0.1–4 M), and temperature (15–45°C). Measuring neptunium (Np) species in aqueous solutions is challenging because Np persists in multiple oxidation states, has high extinction coefficients, is susceptible to hydrolysis, and coordinates with a wide range of ligands. In nitric acid solutions, Np(IV) exists as a hydrated and/or coordinated ion, while Np(V) and Np(VI) persist as the trans-dioxo cations NpO2+ and NpO22+, respectively. In situ thin-layer spectropotentiometry was used to control Np oxidation states (IV/V/VI) while visible/near-infrared absorption spectra were measured, without compromising the optical properties of the sample. Spectrum features including peak intensity, position, and shape were characterized using multivariate data analysis and revealed interionic associations (e.g., between NpO2+ and NpO22+) at high Np concentrations. Key findings from partial least squares regression and multivariate curve resolution analyses will be discussed. This work establishes spectrophotometry to accurately quantify Np process solutions for the Plutonium-238 Supply Program.

Spectroelectrochemistry for characterizing neptunium processing streams to support the plutonium-238 supply program

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ACS Fall 2022

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