Knowledge Bank

The speciation of neptunium in water was modeled using density functional theory (DFT) methods. Aqueous neptunium exists in four oxidation states: $+3,+4,+5$, and $+6$. The first two forms exist as the bare ions, but the $+5$ and $+6$ states exist as $(NpO2)+$ and $(NpO2)2+$ species, respectively. The coordination number for the number of water molecules around each of these species is determined and compared to experiment - which uses XANES (X-ray Absorption Near-Edge Spectroscopy) and eXAFS (extended X-ray Absorption Fine Structure) spectroscopy. The DFT calculations include generalized gradient corrections.