Identifying the Plutonium Oxidation States

of elements to exhibit four common oxidation states: Pu(III), Pu(IV), Pu(V), and Pu(VI). In addition, plutonium complexes in aqueous solution can change their oxidation state by undergoing disproportionation or reproportionation reactions, or through radiolytic reactions. Because so many plutonium species can form and interact in solution, aqueous plutonium chemistry can be remarkably complex. In environments where different ligands abound, such as the ground waters of Yucca Mountain or the waste materials (residues) produced during the manufacturing of nuclear weapons, the plutonium species need to be understood beyond simple elemental analysis. We need to have information about the possible oxidation states of the numerous species, along with information about their atomic compositions and molecular structures. The plutonium aquo ions are the “baseline” plutonium species in aqueous solution because they only have water molecules attached to a central plutonium ion. Other species form as different ligands replace one or more of the water molecules in the coordination sphere of the plutonium ion.Understanding the aquo ions in detail can therefore provide a starting point for understanding other plutonium complexes. The aquo ions of plutonium in the III and IV oxidation states have the general formula Pu(H2O)n 3+ and Pu(H2O)n 4+, respectively, which are often designated as Pu3+(aq) and Pu4+(aq). But plutonium in the V or VI oxidation state has such a large positive charge that, in aqueous solution, it reacts with water to form the transdioxo (plutonyl) cations PuO2 + or PuO2 2+. The corresponding aquo ions therefore have the general formulas PuO2(H2O)n + and PuO2(H2O)n 2+, which are conveniently designated as PuO2 +(aq) and PuO2 2+(aq). Before we conducted our research, the number of water molecules, n, surrounding the plutonium ions was still open to debate. Several years ago, Los Alamos scientists from three Laboratory divisions began working together to apply x-ray absorption fine-structure (XAFS) spectroscopy to characterize the plutonium aquo ions. (More information about XAFS spectroscopy is found in the article “XAFS” on page 422.) All the measurements were performed at the SPEAR synchrotron x-ray source, which is owned and operated by the Stanford Synchrotron Radiation Laboratory (SSRL). Solutions containing ions in a single oxidation state were prepared by electrochemical synthesis. These samples allowed us to study each aquo ion independently. We verified the oxidation state integrity of the samples before and after x-ray absorption analysis by measuring the electronic absorption spectra. Because the chemical environment surrounding the plutonium ion in each solution was very consistent (the only ligands were water molecules), we could also look for behavioral or structural trends among the four oxidation states. Identifying the Plutonium Oxidation States