Structural and Morphological Influences on Neptunium Incorporation in Uranyl Molybdates

Abstract

The in situ incorporation of pentavalent neptunium has been studied in the structurally related uranyl molybdate frameworks (NH 4 ) 4 [(UO 2 ) 5 (MoO 4 ) 7 ](H 2 O) 5 and (NH 4 ) 2 [(UO 2 ) 6 (MoO 4 ) 7 ](H 2 O) 2 prepared under similar synthetic conditions. The presence of Np(V) was confirmed by UV-vis-NIR spectroscopy in the first compound whereas Np(VI) was identified in the second based on the observation of a unit cell contraction and the lack of a spectral signature for Np(V). The incorporation of neptunium does not affect the overall structure of the host compound based on the crystallographic unit cell parameters. Neptunium appears to preferentially incorporate in the structure of (NH 4 ) 2 [(UO 2 ) 6 (MoO 4 ) 7 ](H 2 O) 2 due to the formation of Np(VI) during synthesis, although higher total uptakes were observed in (NH 4 ) 4 [(UO 2 ) 5 (MoO 4 ) 7 ](H 2 O) 5 due to a higher initial concentration of neptunium in solution despite maintaining the same ratio of U:Np.

Competitive Reaction of Neptunium(V) and Uranium(VI) in Potassium–Sodium Carbonate-Rich Aqueous Media: Speciation Study with a Focus on High-Resolution X-ray Spectroscopy

“…It was also found that Np can be incorporated into the mineral structure by cation exchange or by substitution of a U VI for a Np V or Np VI ion in the mineral framework, with or without a compensating charge-balancing cation substitution . Within the available experimental data, it seems that the structure and/or chemical composition of the U mineral phase is of direct importance to the amount of incorporated Np . In this sense, several laboratory studies have already illustrated that the U alteration products can effectively retain Np by sorption or incorporation processes but then subsequently release it into the aqueous solution …”