Synthetic influences on neptunium incorporation in naturally occurring copper uranyl phosphates

Neptunium incorporation in the torbernite/meta-torbernite [Cu(UO ₂)₂(PO₄)₂·nH₂O] system has been investigated in this study under both hydrothermal and slow diffusion conditions (at room temperature and 90 C) to examine the role synthetic conditions play on neptunium uptake by uranyl phases and whether hydrothermal incorporation studies feasibly model environmental incorporation. The hydrothermally prepared crystals contained approximately 19 ± 2 and 73 ± 3 times more neptunium on average than crystals grown during slow diffusion at room temperature and elevated temperature, respectively. UV-vis spectroscopy on the mother solution showed that, on average, 51.53(±0.04)% of the original neptunium(V) remains in solution following hydrothermal synthesis versus 37.24(±0.04)% following slow diffusion at room temperature and 73.62(±0.07)% at 90 C. Additionally, solid-state UV-vis-NIR indicated that the incorporated neptunium was predominantly present in the +6 oxidation state in the hydrothermal samples despite the fact that the initial oxidation state of neptunium in solution was +5 and no oxidizing species were present in the reactions. The oxidation state of neptunium in the slow diffusion samples was not able to be determined due to the low incorporation levels. These results suggest that neptunium(VI) may play a more significant role than previously expected in geological repositories.