Spectroscopic speciation and thermodynamic characterization of the Np(V)-ion complexation

The geochemistry of the actinides is of particular interest for the safety case of a nuclear waste repository. The present work focuses on the complexation of pentavalent actinides An(V) with different organic and inorganic ligands in aqueous solution at elevated temperatures. In the first part of the thesis the complexation of NpO${}_2^{+}$ with different mono- and dicarboxylic ligands (formate, acetate, oxalate, malonate, succinate) and hydroxyl functionalized carboxylates (lactate, malate, tartrate) is studied. These ligands are either naturally occurring substances in the pore waters of clay rocks or serve as model ligands for macro molecular organic compounds like humic substances or polycarboxylate based cement additives. The speciation of the formed complexes and the thermodynamic functions (log ${\beta }_i^0$ (T), ${\Delta }_R$H${}_{m,i}^0$, ${\Delta }_R$S${}_{m,i}^0$) of the corresponding complexation reactions are determined as a function of temperature and ionic strength using Vis/NIR absorption spectroscopy. Additionaly, the structures of the NpO${}_2^{+}$ complexes with oxalate, malonate, succinate and lactate are resolved by EXAFS spectroscopy, ATR-FT infrared spectroscopy and quantum chemical calculations revealing detailed information on the coordination properties of An(V) on a molecular level. In the second part the complexation of NpO${}_2^{+}$ with the inorganic anions fluoride, chloride, nitrate and sulfate is studied at various temperatures and ionic strengths revealing the stoichiometry of the formed complexes and the thermodynamic functions of the respective complexation reactions. These inorganic anions are integral parts of natural groundwaters. Additionaly, fluoride and chloride are available in large quantities in salt rock formations which are in discussion as host rocks of nuclear waste repositories. In the third part of this work the NpO${}_2^{+}$ complexation up to 200 °C is studied for the first time by absorption spectroscopy. Therefor, a high-temperature spectrophotometric cell is developed at the Institute for Nuclear Waste Disposal (INE) at the Karlsruhe Institute of Technology (KIT). The applicability of this setup for speciation studies up to 200 °C is demonstrated by studies on the complexation of NpO${}_2^{+}$ with SO${}_4^{2-}$. The obtained results contribute to an improved understanding of the (geo)chemical behaviour of pentavalent actinides in aqueous systems and provide detailed information on the complexation properties of An(V) on a molecular level. Furthermore, the implementation of a high-temperature spectrophotometric cell for speciation studies on the complexation of NpO${}_2^{+}$ up to 200 °C opens up the possibility for thermodynamic studies at conditions that might occur in the near-field of a nuclear waste repository.