A MULTIMETHOD APPROACH TO ACTINIDE SPECIATION APPLIED TO PENTAVALENT NEPTUNIUM CARBONATE COMPLEXATION

We have adopted a multi-method approach toward understanding aqueous n eptunyl (V) chemistry, bringing together expertise from several chemic al specialties to gain a more complete and accurate understanding of t he complicated and environmentally important solution chemistry of the NpO2+ ion, We provide here an overview of our most recent work in thi s field, studying carbonate complexation. The use of large, bulky cati ons such as (CH3CH2CH2CH2N+ (TEA) in place of the more commonly used N a+ and K+ in the preparation of solutions of NpO2(CO3)(-), NpO2(CO3)(2 )(3-), and NpO2(CO3)(3)(5-) affords a large solubility increase, and a llows for the study of the solution species using a variety of spectro scopic techniques which were previously unusable due to low solution c oncentrations of the ions. The temperature-dependence of carbonate com plexation was studied using C-13 nuclear magnetic resonance (NMR) and near-infrared electronic absorption (NIR) spectroscopies in TBA(NO3) e lectrolyte solutions. These data reveal the solution conditions under which a single species can be prepared, and the molecular structure wa s then elucidated using Raman and EXAFS spectroscopies. EXAFS data for NpO2(CO3)(3)(5-) show unequivocally that the three carbonate ligands bond to the NpO2+ center in a bidentate fashion. The thermodynamic dat a were used to predict the fate and transport of Np(V) in groundwater from to the Yucca Mountain site.