Investigating Np(VI) Nitrate Speciation Control through Temperature and Optical Spectroscopy

Area of Science:

• Nuclear Chemistry
• Solution Chemistry
• Spectroscopy

Background:

• Accurate f-element speciation is crucial for nuclear processing applications like waste remediation and radioisotope production.
• Fundamental understanding of actinide chemistry, particularly neptunium (Np), remains limited compared to other periodic table elements.
• The speciation of Np-(VI) in nitric acid (HNO3) is not well-established, hindering its effective use in nuclear applications.

Purpose of the Study:

• To investigate the impact of nitric acid concentration and temperature on the spectral properties and speciation of Np-(VI).
• To elucidate the formation of Np-(VI) nitrate complexes and their behavior under varying conditions.
• To provide experimental data for advancing computational models of actinyl ion electronic transitions.

Main Methods:

• Preparation of Np-(VI) samples in 1 to 10 M HNO3, stabilized with (NH4)2[Ce(NO3)6].
• Acquisition of UV-vis-NIR absorbance spectra across a temperature range of 15 to 40 °C.
• Dilution studies monitored using UV-vis-NIR and Raman spectroscopies.

Main Results:

• The 10 M HNO3 spectrum differs significantly, indicating high-symmetry Np-(VI)-nitrate complex formation.
• Temperature variations (15-40 °C) induce systematic changes in spectral features, suggesting temperature-dependent Np-(VI) speciation.
• Dilution studies reveal the presence of multiple Np-(VI) complexes with varying symmetries, including a high-symmetry nitrate complex and lower-symmetry aquo complexes.
• Raman spectroscopy shows the Np-(VI)-O22+ symmetric stretch is sensitive to coordination environment, distinguishing between aquo and nitrato complexes.

Conclusions:

• Np-(VI) speciation in nitric acid is influenced by both nitrate concentration and temperature.
• High-symmetry Np-(VI) nitrate complexes form in concentrated HNO3 and transition to lower-symmetry aquo complexes upon dilution.
• Spectroscopic methods, particularly Raman, can differentiate between Np-(VI) aquo and nitrato complexes, providing valuable data for theoretical modeling.