An insightful study on the reduction and extraction of neptunium assisted by molybdenum.

High-level liquid waste (HLLW) generated through nuclear reactions and spent fuel reprocessing contains the critical transuranic element neptunium (Np). Thus, the efficient recovery of Np from HLLW via the TRPO (trialkylphosphine oxide) process is not only beneficial for radioactive contamination remediation but also boosts the recycling of precious nuclear resources. However, this process including Np(V)-Np(IV) conversion is highly dependent on the components coexisting in HLLW, and its underlying mechanism is still not clear, restricting the recovery of Np in multiple scenarios. Herein, the effect of coexisting metal ions on the reduction and extraction of Np(V) was studied, and molybdenum (Mo) was identified to play a vital role in this course. Additionally, in combination with absorption spectral analysis, X-ray photoelectron spectroscopy and solvent extraction, the promotion of the extraction of Np by Mo was elucidated to be due to the catalytic reduction effect both in the aqueous phase and organic phase. This study can help researchers better understand the redox chemistry of Np in the treatment of HLLW.