Published Online: 15 JUN 2010
Copyright © 2006 John Wiley & Sons, Ltd
Encyclopedia of Inorganic Chemistry
How to Cite
Runde, W. and Goff, G. S. 2010. Neptunium: Radionuclides. Encyclopedia of Inorganic Chemistry. .
- Published Online: 15 JUN 2010
Following the element uranium, neptunium is the fourth element in the actinide series. Although primordial neptunium isotopes formed during the creation of the solar system billions of years ago have long since decayed away, neptunium continues to be formed by the nuclear reactions in naturally occurring uranium deposits. Currently, trace amounts of neptunium isotopes can be isolated from uranium ore minerals. Most of present-day inventory of neptunium originates from human activities, such as nuclear weapons testing, nuclear accidents, or irradiation of uranium fuel for producing nuclear energy. All neptunium isotopes are highly radioactive and pose extreme health hazards to humans when ingested or inhaled. Neptunium exhibits five oxidation states (III–VII) in solution and solid state, but only the IV and V oxidation states are relevant under most environmental conditions. In the +IV state, neptunium is insoluble and forms stable complexes with ligands abundant in natural environments (such as hydroxide, carbonate, or humic substances), and can adsorb onto mobile colloidal matter. In contrast, neptunium(V) forms a soluble dioxo-cation in solution (NpO), which exhibits an increased solubility, weaker complex formation, and higher stability in the aqueous phases of aquatic systems. Thus, neptunium(V) is mostly characterized as the dominant solution species, while Np(IV) is found to be adsorbed on sediments, minerals, and colloidal particles. There is no large-scale use of neptunium in industrial applications.