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Neptunium is a transuranium element, produced in tonne quantities in nuclear reactors. Because it has access to a range of oxidation states, neptunium may undergo redox transformations in the environment and these can have far-reaching effects on its environmental mobility. Here, the reaction of NpO2+ (the soluble and thermodynamically stable neptunium species in oxic systems) with microcrystalline mackinawite is studied. Uptake of neptunium from solution is relatively low (approximately 10% of the total initially present in solution) and independent of initial solution concentration over the range 0.27-2.74 mM and of equilibration time. X-ray absorption spectroscopy (XAS) of the solid sulfide samples indicates nearest neighbor oxygen atoms at distances around 2.25-2.26 A, sulfur atoms at around 2.61-2.64 A, and two more distant shells fitted with iron, at 3.91-3.95 A and 4.15-4.16 A. These observations suggest that on interaction with the sulfide surface reduction of Np(V) to Np(IV) occurs, accompanied by loss of axial oxygen atoms. Neptunium coordinates directly to surface sulfide atoms, in contrast to the behavior previously observed for uranium under similar conditions. These results demonstrate the importance and variability of the speciation of redox sensitive actinides under anoxic conditions.

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