Adsorption Behavior of Neptunium Ions on Pyridine-type Resin in Hydrochloric Acid Solutions

Abstract

The adsorption behavior of neptunium ions on the synthesized pyridine-type resin (Abbreviated as PYAER) in hydrochloric acid solutions whose concentrations are in the 0.10-11.5 M region has been studied at room temperature. Neptunium has many kinds of valence states of ions; mainly tetravalent, pentavalent, and heptavalent ions. We have controlled the neptunium valence states by using redox reactions and confirmed the valence states in hydrochloric acid solutions using an ultraviolet visible nearinfrared absorption photospectrometry at room temperature before the adsorption experiments described above. After adjusting the valence states of neptunium ions, the adsorption mechanisms on the neptunium ions with PYAER was studied by batch-wise adsorption experiments and the estimated mechanisms were compared with those of heptavalent uranium, trivalent lanthanides, and tetravalent plutonium against PYAER. As a result, it was found that the obtained distribution coefficients (Abbreviated as Kd) of neptunium with PYAER increase with increasing of the concentrations of hydrochloric acid solutions and their order was heptavalent > tetravalent ≒ pentavalent. The Kd values of tetravalent plutonium were found to be between the Kd values of neptunium. On the other hand, it was also revealed that trivalent lanthanides show little adsorption ability for PYAER in the examined hydrochloric acid concentration ranges and PYAER was able to adsorb strongly heptavalent uranium ions with increasing concentrations of hydrochloric acid. In brief, the obtained Kd values increased in order of lanthanides, neptunium (≒ plutonium), and uranium. These results support the positive possibility of the mutual separation of uranium, plutonium containing neptunium, minor actinides (Am, Cm), and lanthanides and were therefore found to be valuable data for our proposed reprocessing process.