Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.

The migration behavior of actinides and other radioactive contaminants in the environment is controlled by prominent molecular phenomena such as hydrolysis and complexation reactions in aqueous solutions as well as the diffusion and sorption onto minerals present along groundwater flow paths. These reactions significantly influence the mobility and bioavailability of the metal ions in the environment, in particular at liquid-solid interfaces. Hence, for the assessment of migration processes the knowledge of the mechanisms occurring at interfaces is crucial. The required structural information can be obtained using various spectroscopic techniques. In the present study, the speciation of uranium(VI) and neptunium(V) at environmentally relevant mineral – water interfaces of oxides of titania, alumina, silica, zinc, and alumosilicates has been investigated by the application of attenuated total reflection Fouriertransform infrared (ATR FT-IR) spectroscopy. Moreover, the distribution of the hydrolysis products in micromolar aqueous solutions of U(VI) and Np(V/VI) at ambient atmosphere has been characterized for the first time, by a combination of ATR FT-IR spectroscopy, near infrared (NIR) absorption spectroscopy, and speciation modeling applying updated thermodynamic databases. From the infrared spectra, a significant change of the U(VI) speciation is derived upon lowering the U(VI) concentration from the milli- to the micromolar range, strongly suggesting the dominance of monomeric U(VI) hydrolysis products in the micromolar solutions. In contradiction to the predicted speciation, monomeric hydroxo species are already present at pH ≥ 2.5 and become dominant at pH 3. At higher pH levels (> 6), a complex speciation is evidenced including carbonate containing complexes. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3, and they are comparatively discussed with U(VI). For both actinides, the formation of similar species is suggested at pH ≤ 4, whereas at higher pH, the infrared spectra evidence structurally different species. At pH 5, the formation of a carbonate-containing dimeric complex, that is (NpO2)2CO3(OH)3^-, is strongly suggested, whereas carbonate complexation occurs only under more alkaline conditions in the U(VI) system. The results from the experiments of the sorption processes clearly demonstrate the formation of stable U(VI) surface complexes at all investigated mineral phases. This includes several metal oxides, namely TiO2, Al2O3, and SiO2, serving as model systems for the elucidation of more complex mineral systems, and several alumosilicates, such as kaolinite, muscovite and biotite. From a multiplicity of in situ experiments, the impact of sorbent characteristics and variations in the aqueous U(VI) system on the sorption processes was considered. A preferential formation of an inner-sphere complex is derived from the spectra of the TiO2 and SiO2 phases. In addition, since the in situ FT-IR experiments provide an online monitoring of the absorption changes of the sorption processes, the course of the formation of the U(VI) surface complexes can be observed spectroscopically. It is shown that after prolonged sorption time on TiO2, resulting in a highly covered surface, outer-sphere complexation predominates the sorption processes. The prevailing crystallographic modification, namely anatase and rutile, does not significantly contribute to the spectra, whereas surface specific parameters, e.g. surface area or porosity are important. A significant different surface complexation is observed for Al2O3. The formation of innerspheric species is assumed at low U(VI) surface coverage which is fostered at low pH, high ionic strength and short contact times. At proceeded sorption the surface complexation changes. From the spectra, an outer-spheric coordination followed by surface precipitation or polymerization is deduced. Moreover, in contrast to TiO2, the appearance of ternary U(VI) carbonate complexes on the γ-Al2O3 surface is suggested. The first results of the surface reactions on more complex, naturally occurring minerals (kaolinite, muscovite and biotite) show the formation of U(VI) inner-sphere sorption complexes. These findings are supported by the spectral information of the metal oxide surfaces. In this work, first spectroscopic results from sorption of aqueous Np(V) on solid mineral phases are provided. It is shown that stable inner-sphere surface species of NpO2 ^+ are formed on TiO2. Outer-sphere complexation is found to play a minor role due to the pH independence of the sorption species throughout the pH range 4 – 7.6. The comparative spectroscopic experiments of Np(V) sorption onto TiO2, SiO2, and ZnO indicate structurally similar bidentate surface complexes. The multiplicity of IR spectroscopic experiments carried out within this study yields a profound collection of spectroscopic data which will be used as references for future investigations of more complex sorption systems in aqueous solution. Furthermore, from a methodological point of view, this study comprehensively extends the application of ATR FT-IR spectroscopic experiments to a wide range in the field of radioecology. The results obtained in this work contribute to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V/VI), in the environment. Consequently, more reliable predictions of actinides migration which are essential for the safety assessment of nuclear waste repositories can be performed.

Etude expérimentale et modélisation, en fonction du pH et de la concentration en NaCl, du système ternaire U(VI)-NaCl-H2O à T = 155°C et pression de vapeur saturante. Une étude expérimentale sur la solubilité de l'U(VI) dans des saumures de NaCl (0.5 – 6 M) en fonction du pH (3-13) a été réalisée à T = 155°C et pression de vapeur saturante. Les résultats obtenus ont montré que la phase solide initiale UO3 se transforme en de multiples phases solides d'uranates de sodium de formule générale Na2xUyO(x+3y) hydratés dont le rapport molaire Na/U croit avec l'accroissement du pH et de la concentration de NaCl. La solubilité de l'U(VI) est directement gouvernée par trois paramètres physico-chimiques : le pH, le ligand Cl- et le cation Na+. Les deux premiers favorisent fortement la solubilité de l'U(VI) sous forme de complexes d'U(VI) avec les ligands OH- et Cl- à pH = 4. A la différence des deux premiers paramètres, le cation Na+ inhibe la solubilité de l'uranium (VI) en le précipitant sous forme de multiples composés d'uranates de sodium très peu solubles. Les résultats de mesures de solubilité de l'U(VI) obtenues servent de base de données de référence pour calculer les variations de la concentration de chaque complexe soluble d'U(VI) en fonction de deux paramètres, pH et [Na], à l'aide d'une équation de type exponentielle d’un paraboloïde elliptique négatif. Un modèle empirique a également été mis au point, à partir des équations de Pitzer, afin de déterminer, de manière continue, les variations des divers paramètres physico-chimiques des ions solubles présents dans une saumure de NaCl en fonction de quatre paramètres : pH, [Na], T et P du milieu réactionnel. Ce modèle est valable jusqu’à [NaCl] = 6M, T = 300°C et P = 100 MPa. / An experimental study on the solubility of U(VI) in concentrated (0.5-6.0 M) NaCl solutions as a function of pH (3-13) has been realized at 155°C and saturation vapor pressure. The obtained results indicate that the initial solid phase UO3 initial solid phase is transformed into multiple solid phases of hydrated sodium uranates with general formula Na2xUyO(x+3y), the Na/U molar ratio of which increases with increasing pH and NaCl concentration. The solubility of U(VI) is directly dependant of three physico-chemical parameters : the pH, the Cl- ligand, and the Na+ cation. The first two parameters strongly favour the U(VI) solubility in the form of aqueous U(VI) complexes with the OH-and Cl- ligands at a pH = 4, while the Na+ cation inhibits the solubility of U(VI) as it precipitates into various low solubility sodium uranates. The results of solubility measurements obtained for U(VI) serve as a reference data base for the calculation of concentration variations of each soluble U(VI) complex as a function of two parameters, pH and [Na], with the use of a negative elliptic paraboloïd exponential. An empirical model has also been developed from the Pitzer equations in order to determine, in a continuous way, the variations of various physico-chemical parameters of the soluble ions present in the concentrated NaCl solutions as a function of four parameters: pH, [Na], T and P of the reaction medium. This model is valid up to [NaCl] = 6M, T = 300°C and P = 100 MPa.

The utilisation of fossil fuels for energy worldwide depletes the natural reserves and at the same time releases billions of tonnes of carbon dioxide and other greenhouse gases into the atmosphere. In order to reverse the negative effects of this accumulation, i.e., global warming and climatic changes, countries around the world are now considering nuclear energy and other cleaner sources of energy as a substitute to the burning of fossil fuels. The deployment of the later technology has progressed slowly due to lack of public support. The general public and environmental lobbyists worry about the discharge of radioactive waste from nuclear power generation and accidents that have occurred in the nuclear power industry in the recent past. One of pollutants of concern is uranium which is discharged from the nuclear generation processes as the highly toxic uranium-6, (U(VI)). U(VI) coming from the reactors is radioactive as well as highly toxic to aquatic life forms. Biological treatment of metal pollutants is viewed as an environmentally friendly alternative to conventional physical/chemical treatment methods, especially in dilute solutions where physical/chemical methods may not be effective. Microbial processes may be applied both as in situ and/or ex situ processes. Microbial consortia, consisting of several species of microorganisms in the form of bioflocs for reducing/removing the pollutants have been used as they preserve the complex interrelationships that exist between species in the source. The results of this study demonstrate the potential of microbial U(VI) reduction as a possible replacement technology for physical/chemical processes currently in use in the nuclear industry. A detailed analysis of the biological reduction of uranium-(VI) was conducted and the following were the main findings of the study: (1) Background uranium concentration in soil from the mine was determined to be 168 mg/kg, a very high value compared to the typical concentration of uranium in natural soils; (2) Among six bacteria species isolated from a uranium mine in Limpopo, South Africa, three anaerobic species – Pantoea sp., Enterobacter sp. and Pseudomonas stutzeri – reduced U(VI) to U(VI) and facilitated the removal of the uranium species from solution. Based on batch studies and cell disruption studies, the laws governing microbial U(VI) reduction were determined and the kinetic parameters for U(VI) reduction were determined. The cultures in this study reduced uranium-U(VI) at a rate better than rates found in literature for other microorganisms. Reduction rates reported in this paper can be used to assess the applicability of bioreduction for uranium removal processes. / Dissertation (MSc)--University of Pretoria, 2011. / Chemical Engineering / unrestricted

Ces travaux de thèse, effectués dans le cadre des études sur le retraitement des combustibles nucléaires usés par extraction-liquide-liquide, concernent l’étude cinétique d’extraction de l’uranium(VI) et du plutonium(IV) en milieu acide nitrique par un mélange d’extractants de type monoamide. Trois techniques ont été utilisées pour étudier les cinétiques d’extraction et identifier les régimes de transfert : la méthode de la goutte unique, la cellule d’extraction de Nitsch et la cellule à membrane tournante (RMC). Les résultats obtenus avec la goutte unique indiquent que les cinétiques de transfert d’U(VI) et Pu(IV) sont proches. Une étude hydrodynamique des gouttes et la modélisation du transfert de l’U(VI) ont montré que la présence d’une circulation interne à la goutte diminue les résistances diffusionnelles pour des tailles de goutte croissantes. L’influence de la concentration d’U(VI) dans le solvant sur la cinétique d’extraction a montré que le transfert d’U(VI) et Pu(IV) ralentit lorsque la concentration d’U(VI) en phase organique et la viscosité augmentent. L’ensemble de ces résultats couplé à l’étude du transfert de l’U(VI) par la cellule de Nitsch conduisent à supposer une cinétique gouvernée par la diffusion principalement localisée dans la phase organique. Les études réalisées avec la RMC ont permis de déterminer la constante chimique d’extraction de l’U(VI), du même ordre de grandeur que les cinétiques obtenues par la goutte unique et de confirmer la présence d’une réaction chimique interfaciale. Enfin, le transfert d’U(VI) et Pu(IV) s’avère 3 fois plus lent que celui par le TBP mais reste adapté pour un procédé d’extraction à l’échelle industrielle. / This thesis was conducted in the framework of the reprocessing of spent nuclear fuels. The kinetics extraction of uranium(VI) and plutonium(IV) by N,N-dialkylamides or monoamides in an aliphatic diluent were studied from an aqueous nitric solution using 3 different techniques: “single drop technique”, Nitsch cell and Rotating Membrane Cell (RMC). All these experiments were useful to attempt the identification of the transfer process between the phases which can be controlled by kinetic or diffusional regime. The kinetics of extraction of U(VI) by monoamides solvent seems to be similar to that of the Pu(IV). In general, molecular diffusion in organic phase slows down the extraction process and the limiting thickness of organic phase increases with solvent viscosity. The process in “single drop technique” seems to be controlled not only by diffusion but also by the chemical reaction. Then the extraction kinetics of U(VI) has been carried out by Nitsch cell and the RMC. Diffusionnal regime is the limiting step in Nitsch cell and the results confirm that molecular diffusion in organic phase should mainly control the kinetics transfer. Then experiments performed by the RMC indicate that the kinetics is in the same order as transfer coefficient obtained by “single drop” and the chemical reaction occurs at the interface. Finally, these results were compared with data obtained with a TBP solvent (tributyl phosphate) currently used in the PUREX process in order to estimate the interest of such a new solvent. The kinetics of extraction of U(VI) by this monoamide-based solvent is three times lower that of the TBP 30 % but remains fast and suitable for a future industrial process.

Les matériaux cimentaires sont largement utilisés dans la conception et la construction des sites de stockage de déchets radioactifs. Une des manières d’améliorer leur performance est d’introduire des adjuvants organiques dans la structure. La présence de matière organique dans l’eau porale peut affecter la mobilité des radionucléides : les molécules organiques forment des complexes solubles et peuvent être en compétition avec les radionucléides au niveau des sites de sorption. Ce travail avait pour but de comprendre les mécanismes de telles interactions au niveau moléculaire. Le système modèle a trois composantes. D’abord, des phases C-S-H ont été choisies en tant que modèles du ciment.Ensuite, le gluconate est sélectionné en tant que modèle d’additif organique pour sonder les mécanismes d’interaction à l’échelle moléculaire. Un système plus complexe impliquant un superplastifiant (PCE) a été testé. La troisième espèce, U(VI), est représentative d’un radionucléide de la série des actinides. Le développement de la description des effets de postproduction des espèces organiques pour les applications de stockage des déchets radioactifs était l’objectif principal de ce travail. L’étude des systèmes binaires fournit des données de référence pour l’investigation de systèmes ternaires C-S-H/matière organique/U(VI) plus complexes. Des cinétiques et des isothermes de sorption/désorption pour les espèces sur les C-S-H sont mesurés. En parallèle, des modèles atomiques ont été développés pour les interfaces d’intérêt. Les aspects structuraux, énergétiques et dynamiques des processus de sorption sur les surfaces de ciment sont modélisés par la technique de la dynamique moléculaire. / Cementitious materials are extensively used in the design and construction of radioactive waste repositories. One of the ways to enhance their performance is to introduce organic admixtures into the cement structure. However, the presence of organics in the pore water may affect the radionuclide mobility: organic molecules can form water-soluble complexes and compete for sorption sites. This work was designed to get detailed understanding of the mechanisms of such interactions on the molecular level. The model system has three components. First, pure C-S-H phases with different Ca/Si ratios were chosen as a cement model. Secondly, gluconate (a simple well-described molecule) is selected as a good starting organic additive model to probe the interaction mechanisms on the molecular scale. A more complex system involving polycarboxylate superplasticizer (PCE) was also tested. The third, U (VI), is a representative of the actinide radionuclide series. The development of description of the effects of organics for radioactive waste disposal applications was the primary objective of this work. The study of binary systems provides reference data for the investigation of more complex ternary (C-S-H/organic/U(VI)). The interactions are studied by means of both experimental and computational molecular modelling techniques. Data on sorption and desorption kinetics and isotherms for additives and for U (VI) on C-S-H are acquired in this work. In parallel, atomistic models are developed for the interfaces of interest. Structural, energetic, and dynamic aspects of the sorption processes on surface of cement are quantitatively modeled by molecular dynamics technique.

Dans le but d’élucider la chimie de coordination et la structure des complexes formés avec des ligands organiques de la famille des sidérochélates, des études physico-chimiques sur la complexation du fer(III), de certains lanthanides(III) (La3+, Nd3+, Sm3+, Eu3+, Gd3+, Ho3+, Lu3+) et de l’uranium(VI) ont été effectuées. La connaissance des propriétés de complexation sidérophore-actinide est une étape essentielle pour appréhender le comportement à long terme d'un sol contaminé par des radioéléments. Trois acides dihydroxamiques synthétisés au laboratoire ((LCyEt)2–, (LCyPr)2– et (LO)2–) mimant un sidérophore d’origine fongique, l’acide rhodotorulique, ont été évalués pour la chélation du fer(III) par des titrages potentiométriques éventuellement couplés à une détection spectrophotométrique en milieu KNO3 0,1 M. Ces mesures ont permis de confirmer la présence de complexes di- et trileptiques dans les conditions d’excès du ligand. Le modèle chimique comprend au total cinq espèces de formule [Fem(L)lHh](3–2l+h)+ : [Fe(L)]+, [Fe(L)(OH)], [Fe(L)(OH)2]–, [Fe(L)2H] et [Fe2(L)3]. Le traitement numérique des données spectrophotométriques collectées dans le visible nous a conduit à proposer le spectre électronique pour chacune des espèces identifiées. En outre, la spectroscopie de masse par ionisation électrospray (ESI-MS) a confirmé la formation des espèces mono- ([Fe(L)]+) et dileptiques ([Fe(L)2H]). L’étude potentiométrique du ligand dihydroxamique abiotique (LCyPr)2– en présence de sept lanthanides trivalents a permis de proposer un modèle chimique comprenant cinq espèces mono- et dileptiques ([Ln(LCyPr)]+, [Ln(LCyPr)H]2+, [Ln(LCyPr)2]−, [Ln(LCyPr)2H]) et une espèce monohydroxylée [Ln(LCyPr)OH] en milieu KNO3 0,1 M. Par ailleurs, les mesures potentiométriques et spectrophotométriques pour le système UO22+/(LCyPr)2– suggèrent la formation à l’équilibre de cinq complexes d’uranium(VI) entre p[H] 2 et 10. Le modèle comprend les complexes mono- et dileptiques suivants : [UO2(LCyPr)], [UO2(LCyPr)H]+, [UO2(LCyPr)OH]−, [UO2(LCyPr)2]2− et [UO2(LCyPr)2H]−. La combinaison de ces deux techniques nous a permis d’appréhender la spéciation de ce radionucléide en milieu KNO3 0,1 M et de proposer des schémas de coordination pour les différentes espèces mises en évidence / With the aim of elucidating the coordination chemistry and the structure of the complexes formed with organic ligands belonging to the family of siderochelates, physico-chemical studies of the complexation of iron(III), some lanthanides(III) (La3+, Nd3+, Sm3+, Eu3+, Gd3+, Ho3+, Lu3+) and uranium(VI) have been performed. The knowledge of the properties of actinide-siderophore complexes is an essential step to assess long-term behavior of soils contaminated by actinides. Three dihydroxamic acids synthesized in our laboratory ((LCyEt)2–, (LCyPr)2– and (LO)2–) mimicking a fungal siderophore, rhodotorulic acid, have been evaluated with respect to iron(III) chelation by potentiometric and spectrophotometric titrations in 0,1 M KNO3. These measurements revealed the formation of di- and trileptic complexes in the presence of an excess of ligand. The chemical model includes five species of [Fem(L)lHh](3–2l+h)+ general formula : [Fe(L)]+, [Fe(L)(OH)], [Fe(L)(OH)2]–, [Fe(L)2H] and [Fe2(L)3]. The numerical treatment of the spectrophotometric data collected in the visible range, led us to propose the electronic absorption spectrum for each of the identified species. Moreover, electrospray ionization mass spectroscopy (ESI-MS) confirmed the formation of the mono- ([FeL]+) and dileptic ([FeL2H]) complexes. The potentiometric study of the abiotic dihydroxamic ligand (LCyPr)2– in the presence of seven trivalent lanthanides allowed us to propose a chemical model which includs five mono- and dileptic species ([Ln(LCyPr)]+, [Ln(LCyPr)H]2+, [Ln(LCyPr)OH], [Ln(LCyPr)2]–, [Ln(LCyPr)2H]) in 0,1 M KNO3. Finally, potentiometric and spectrophotometric measurements for the UO22+/(LCyPr)2– system suggested the formation at equilibrium to five of uranium(VI) complexes between p[H] = 2 and 10. The model includes the mono- and dileptic [UO2(LCyPr)], [UO2(LCyPr)H]+, [UO2(LCyPr)OH]–, [UO2(LCyPr)2]2–, [UO2(LCyPr)2H]– complexes. The combination of both techniques allowed us to assess the speciation of this radionuclide in KNO3 medium and to propose a coordination scheme for each complex

Le procédé hydrométallurgique le plus utilisé pour extraire l'uranium VI des minerais phosphoriques met en jeu un mélange synergique de deux molécules extractantes : HDEHP et TOPO. Si les combinaisons synergiques sont connues dans le domaine de l'extraction liquide/liquide, les mécanismes qui pilotent la synergie d'extraction sont encore mal décrits. Une approche multi-échelle a été utilisée pour décrire ces mécanismes, combinant deux descriptions qui diffèrent par leur centre d'intérêt, respectivement l'ion pour l'approche moléculaire et les agrégats d'extractants pour l'approche supramoléculaire. Ces deux approches ont parallèlement été rationnalisées par des calculs de dynamique moléculaire. La description obtenue permet de rendre compte de la synergie via la structure des complexes et des agrégats formés pour différents ratios HDEHP/TOPO. De la même façon, des composés bifonctionnels, qui combinent deux fonctions extractantes au sein de la même molécule ont été étudiés et comparés au système HDEHP/TOPO pour identifier l'origine de l'augmentation des performances d'extraction et de sélectivité. / Uranium VI is commonly extracted from phosphoric ores by a well-known process exploiting the synergistic mixture of two extractant molecules : HDEHP and TOPO. In the field of liquid-liquid extraction, synergistic combinations are common but the mechanisms at the origin of the synergy are not well understood. A multi-scale approach has been used to describe these mechanisms, combining two different descriptions : the molecular scale focuses on the ion point of view, while the supramolecular scale focuses on extractants' aggregation. These two approaches have been rationalized by molecular dynamics computations. The results allow describing the synergy through the structure of the complexes and aggregates. With the same approach, some bifunctional compounds, combining the two extracting sites in one molecule, have been studied and compared to the HDEHP/TOPO system in order to identify the origin of their increased capacities in extraction and selectivity.

The migration behavior of actinides and other radioactive contaminants in the environment is controlled by prominent molecular phenomena such as hydrolysis and complexation reactions in aqueous solutions as well as the diffusion and sorption onto minerals present along groundwater flow paths. These reactions significantly influence the mobility and bioavailability of the metal ions in the environment, in particular at liquid-solid interfaces. Hence, for the assessment of migration processes the knowledge of the mechanisms occurring at interfaces is crucial. The required structural information can be obtained using various spectroscopic techniques. In the present study, the speciation of uranium(VI) and neptunium(V) at environmentally relevant mineral – water interfaces of oxides of titania, alumina, silica, zinc, and alumosilicates has been investigated by the application of attenuated total reflection Fouriertransform infrared (ATR FT-IR) spectroscopy. Moreover, the distribution of the hydrolysis products in micromolar aqueous solutions of U(VI) and Np(V/VI) at ambient atmosphere has been characterized for the first time, by a combination of ATR FT-IR spectroscopy, near infrared (NIR) absorption spectroscopy, and speciation modeling applying updated thermodynamic databases. From the infrared spectra, a significant change of the U(VI) speciation is derived upon lowering the U(VI) concentration from the milli- to the micromolar range, strongly suggesting the dominance of monomeric U(VI) hydrolysis products in the micromolar solutions. In contradiction to the predicted speciation, monomeric hydroxo species are already present at pH ≥ 2.5 and become dominant at pH 3. At higher pH levels (> 6), a complex speciation is evidenced including carbonate containing complexes. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3, and they are comparatively discussed with U(VI). For both actinides, the formation of similar species is suggested at pH ≤ 4, whereas at higher pH, the infrared spectra evidence structurally different species. At pH 5, the formation of a carbonate-containing dimeric complex, that is (NpO2)2CO3(OH)3^-, is strongly suggested, whereas carbonate complexation occurs only under more alkaline conditions in the U(VI) system. The results from the experiments of the sorption processes clearly demonstrate the formation of stable U(VI) surface complexes at all investigated mineral phases. This includes several metal oxides, namely TiO2, Al2O3, and SiO2, serving as model systems for the elucidation of more complex mineral systems, and several alumosilicates, such as kaolinite, muscovite and biotite. From a multiplicity of in situ experiments, the impact of sorbent characteristics and variations in the aqueous U(VI) system on the sorption processes was considered. A preferential formation of an inner-sphere complex is derived from the spectra of the TiO2 and SiO2 phases. In addition, since the in situ FT-IR experiments provide an online monitoring of the absorption changes of the sorption processes, the course of the formation of the U(VI) surface complexes can be observed spectroscopically. It is shown that after prolonged sorption time on TiO2, resulting in a highly covered surface, outer-sphere complexation predominates the sorption processes. The prevailing crystallographic modification, namely anatase and rutile, does not significantly contribute to the spectra, whereas surface specific parameters, e.g. surface area or porosity are important. A significant different surface complexation is observed for Al2O3. The formation of innerspheric species is assumed at low U(VI) surface coverage which is fostered at low pH, high ionic strength and short contact times. At proceeded sorption the surface complexation changes. From the spectra, an outer-spheric coordination followed by surface precipitation or polymerization is deduced. Moreover, in contrast to TiO2, the appearance of ternary U(VI) carbonate complexes on the γ-Al2O3 surface is suggested. The first results of the surface reactions on more complex, naturally occurring minerals (kaolinite, muscovite and biotite) show the formation of U(VI) inner-sphere sorption complexes. These findings are supported by the spectral information of the metal oxide surfaces. In this work, first spectroscopic results from sorption of aqueous Np(V) on solid mineral phases are provided. It is shown that stable inner-sphere surface species of NpO2 ^+ are formed on TiO2. Outer-sphere complexation is found to play a minor role due to the pH independence of the sorption species throughout the pH range 4 – 7.6. The comparative spectroscopic experiments of Np(V) sorption onto TiO2, SiO2, and ZnO indicate structurally similar bidentate surface complexes. The multiplicity of IR spectroscopic experiments carried out within this study yields a profound collection of spectroscopic data which will be used as references for future investigations of more complex sorption systems in aqueous solution. Furthermore, from a methodological point of view, this study comprehensively extends the application of ATR FT-IR spectroscopic experiments to a wide range in the field of radioecology. The results obtained in this work contribute to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V/VI), in the environment. Consequently, more reliable predictions of actinides migration which are essential for the safety assessment of nuclear waste repositories can be performed.

The storage of nuclear waste in deep geological formations is discussed worldwide as the main strategy for nuclear waste management. To ensure the confinement of the nuclear waste, a multiple barrier system which consists of engineered, geo-engineered, and geological barriers will be applied. Thereby, in Germany the definition of the isolating rock zone represents an important safety function indicator. Clay rock is internationally investigated as potential host rock for a repository and represents a part of the geological barrier. In the present work, the natural clay rock Opalinus Clay from the Mont Terri rock laboratory, Switzerland, was studied. In Germany, the direct disposal of the spent nuclear fuel without the reprocessing of the spent fuel is preferred. In case of water ingress, radionuclides can be released from the nuclear waste repository into its surroundings, namely the host rock of the repository. Humic acids, ubiquitous in nature, can be found associated with the inorganic components in natural clay rock (1.5×10–3 wt.% in Opalinus Clay). They can be released under certain conditions. Due to their variety of functional groups, humic acids are very strong complexing agents for metal ions. They have inherent redox abilities and a colloidal conformation in solution. Because of these characteristics, humic acids can affect the mobility of metal ions such as actinides. Furthermore, in the near-field of a repository elevated temperatures have to be considered due to the heat production resulting from the radioactive decay of the various radionuclides in the nuclear waste. This work focuses on the interaction of uranium, as main component of spent nuclear fuel, with Opalinus Clay and studies the influence of humic acid and elevated temperature on this interaction. For investigation of the retention behavior of the clay and the mobility of U(VI) in the system, batch sorption and diffusion experiments were performed. To clarify which U(VI) and humic acid species were present under the applied conditions, aqueous speciation modeling was used. Additionally, the U(VI) speciation in solution and on the clay surface was investigated by spectroscopic methods. Prior to the investigation of the ternary system U(VI) / humic acid / clay, the applied batches of Opalinus Clay were characterized (e.g., specific surface area, carbon content, cation exchange capacity, elemental composition, particle size distribution). Leaching studies with Opalinus Clay in synthetic Opalinus Clay pore water (pH 7.6, It = 0.34 mol/L) and in NaClO4 (pH 3 – 10, I = 0.1 mol/L) were performed to identify the competing ions and their concentrations in the background electrolytes. These data were used to calculate the U(VI) and humic acid speciation in solution. Calcium and carbonate ions are present under pore water conditions as well as in 0.1 mol/L NaClO4 from pH 7 to 8.5, due to dissolution of calcite (mineral fraction in Opalinus Clay). Thus, the U(VI) speciation is dominated by the aquatic Ca2UO2(CO3)3 complex. In the case of pore water, Ca2UO2(CO3)3(aq) is also the dominant U(VI) species in the presence of humic acid, which was corroborated by time-resolved laser-induced fluorescence spectroscopic measurements. A significantly changed speciation was found in 0.1 mol/L NaClO4 in the presence of humic acid. At pH > 7, the negatively charged UO2(CO3)2HA(II)4– complex determines the U(VI) speciation, thus repressing the Ca2UO2(CO3)3(aq) complex. In addition, the speciation of humic acid is influenced from ions leached out from Opalinus Clay. The CaHA(II) complex is the dominating humic acid species in solution. Batch sorption experiments in 0.1 mol/L NaClO4 showed that Opalinus Clay has the strongest retardation effect on U(VI) in the pH range from pH 4.5 to 7. However, under environmentally relevant conditions (pH > 7), the sorption of U(VI) onto Opalinus Clay is very weak. Under pore water conditions, a distribution coefficient (Kd) of 0.0222 ± 0.0004 m3/kg was determined, which was shown to be independent of solid-to-liquid ratios ≥ 60 g/L. In addition, in pore water, the U(VI) sorption onto Opalinus Clay is not influenced by humic acid, which is supported by the speciation results. Extended X ray absorption fine-structure investigations confirmed this batch sorption result. The U(VI) diffusion experiments performed in pore water at 25 °C with Opalinus Clay bore core samples confirmed the Kd value obtained by batch sorption experiments. In the diffusion experiments at 60 °C, a change in the U(VI) speciation occurred. Beside Ca2UO2(CO3)3(aq), a colloidal U(VI) species was formed. Almost equivalent apparent diffusion coefficient (Da) values were determined for the diffusion of the aqueous U(VI) species at 25 and 60 °C through Opalinus Clay. Thus, based on the investigations in the present study the breakthrough of U(VI) through Opalinus Clay is expected to be independent of the temperature and should occur nearly at the same time. Modeling calculations showed that it would take about 10 years until a detectable amount of 233U(VI) (1×10–9 mol/L) migrates through an 11 mm thick Opalinus Clay sample. Two distinct humic acid size fractions – a large- and a small-sized colloid fraction – diffused through the Opalinus Clay samples. Within three months, the high molecular size humic acid colloids migrated only about 500 µm into the clay, whereas the low molecular size fraction diffused through the entire Opalinus Clay samples and were consequently detected in the receiving reservoirs. These findings demonstrate a filtration effect of the compacted clay. The diffusion experiments revealed that the effect of humic acid on U(VI) diffusion is negligible and, under the studied conditions, independent of temperature. The obtained results contribute to data bases used for modeling of interaction and migration processes in uranium / clay rock systems. Thus, the collected sorption and diffusion data are not only relevant for safety assessment of nuclear waste repositories but also for any clay-containing system present in the environment, where the geochemical interaction with uranium contaminated water plays a role. Concerning the suitability of Opalinus Clay as host rock for a nuclear waste repository, it can be concluded, that Opalinus Clay has a relatively high retardation potential for U(VI). In case of water ingress U(VI) as part of the nuclear waste is released into the clay formation. Under near-neutral pH conditions, it will be complexed by calcium and carbonate ions leached out from Opalinus Clay, whereby Ca2UO2(CO3)3(aq) is formed. This complex is only weakly retarded by sorption onto the clay, which can contribute to an enhanced mobility of U(VI) in the host rock. However, the U(VI) migration through the clay rock is governed by molecular diffusion. This decelerates the migration of Ca2UO2(CO3)3(aq) through Opalinus Clay and thus it represents the decisive retardation process in the investigated system. Additionally, under environmentally relevant conditions, humic acid has no significant influence on U(VI) / Opalinus Clay interaction even at an elevated temperature of 60 °C. This was shown by speciation, sorption, as well as diffusion experiments. / Eine weltweit diskutierte Strategie zum Umgang mit radioaktiven Abfällen ist deren Endlagerung in tiefen geologischen Formationen. Zur Abschirmung der Umwelt vor den schädlichen Einflüssen des radioaktiven Abfalls soll ein Multibarrierensystem bestehend aus technischen, geotechnischen und geologischen Barrieren im Endlager dienen. Dabei ist in Deutschland die Definition des einschlusswirksamen Gebirgsbereichs ein wichtiger sicherheitstechnischer Indikator. Tongestein wird als potentielles Endlagerwirtsgestein und Teil der geologischen Barriere international erforscht. In der vorliegenden Arbeit wurde das natürliche Tongestein Opalinuston aus dem Mont Terri Felslabor, Schweiz, untersucht. In Deutschland wird die direkte Endlagerung des abgebrannten Kernbrennstoffes ohne Wiederaufarbeitung des Brennstoffs favorisiert. Bei Wassereinbruch können Radionuklide aus dem Abfall in die Umgebung des Endlagers freigesetzt werden, d. h. sie können in Kontakt mit dem Wirtsgestein kommen. Ubiquitär in der Natur vorkommende Huminsäuren können mit den anorganischen Komponenten des natürlichen Tongesteins vergesellschaftet sein (1.5×10–3 Gew.-% in Opalinuston). Unter bestimmten Bedingungen können die Huminsäuren freigesetzt werden. Ihre Struktur enthält eine Vielzahl von funktionellen Gruppen, was sie zu starken Komplexbildnern für Metallionen macht. Sie besitzen Redoxeigenschaften und bilden in Lösung eine kolloidale Konformation aus. Aufgrund dieser Charakteristika können sie die Mobilität von Metallionen wie den Actinoiden beeinflussen. Weiterhin sind im Nahfeld eines Endlagers erhöhte Temperaturen zu erwarten, welche aus der Wärmefreisetzung beim radioaktiven Zerfall der verschiedenen Radionuklide im radioaktiven Abfall resultieren. Die vorliegende Studie konzentriert sich auf die Untersuchung der Wechselwirkung von Uran, als Hauptkomponente des endgelagerten abgebrannten Kernbrennstoffs, mit Opalinuston und untersucht dabei den Einfluss von Huminsäure und erhöhter Temperatur. Um das Rückhaltevermögen des Tongesteins gegenüber U(VI) und die U(VI)-Mobilität im System zu ermitteln, wurden Sorptions- und Diffusionsversuche durchgeführt. Zur Klärung, welche U(VI)- und Huminsäurespezies unter den untersuchten Bedingungen vorliegen, wurde die aquatische Speziation berechnet. Zusätzlich wurde die U(VI)-Speziation in Lösung und an der Tonoberfläche mit spektroskopischen Methoden untersucht. Vor der Untersuchung des ternären Systems U(VI) / Huminsäure / Ton wurden die eingesetzten Opalinuston-Chargen charakterisiert (z. B. spezifische Oberfläche, Kohlenstoffgehalt, Kationenaustauschkapazität, elementare Zusammensetzung, Partikelgrößenverteilung). Anschließend wurden Auslaugungsversuche mit Opalinuston in synthetischem Opalinustonporenwasser (pH 7.6, It = 0.34 mol/L) und in NaClO4 (pH 3 – 10, I = 0.1 mol/L) durchgeführt, um relevante Konkurrenzionen zu identifizieren und deren Konzentration in den Hintergrundelektrolyten zu bestimmen. Die erhaltenen Daten wurden zur Berechnung der U(VI)- und Huminsäurespeziation in Lösung verwendet. Unter Porenwasserbedingungen sowie in 0.1 mol/L NaClO4 von pH 7 bis 8.5 liegen, durch die Auflösung von Calcit (Mineralphase im Opalinuston), Calcium- und Carbonationen in Lösung vor. Dadurch wird die U(VI)-Speziation von dem aquatischen Ca2UO2(CO3)3-Komplex dominiert. Im Falle des Porenwassers ist Ca2UO2(CO3)3(aq) auch in Gegenwart von Huminsäure die dominierende U(VI)-Spezies. Dies wurde durch zeitaufgelöste laserinduzierte fluoreszenzspektroskopische Messungen nachgewiesen. Eine signifikante Änderung der U(VI)-Speziation tritt in 0.1 mol/L NaClO4 in Gegenwart von Huminsäure auf. Bei pH > 7 bestimmt der negativ geladene UO2(CO3)2HA(II)4–-Komplex die U(VI)-Speziation, wobei der Anteil von Ca2UO2(CO3)3(aq) zurückgedrängt wird. Auch die Huminsäurespeziation wird durch die vom Opalinuston ausgelaugten Ionen beeinflusst. So ist der CaHA(II)-Komplex die dominierende Huminsäurespezies in Lösung. Sorptionsversuche in 0.1 mol/L NaClO4 zeigten, dass Opalinuston gegenüber U(VI) den stärksten Retardationseffekt im pH-Bereich 4.5 bis 7 aufweist. Unter umweltrelevanten Bedingungen hingegen (pH > 7) ist die U(VI)-Sorption an Opalinuston sehr schwach. Unter Porenwasserbedingungen wurde ein Verteilungskoeffizient (Kd) von 0.0222 ± 0.0004 m3/kg ermittelt, der von Fest-Flüssig-Verhältnissen ≥ 60 g/L unabhängig ist. Außerdem wird die U(VI)-Sorption an Opalinuston in Porenwasser nicht von Huminsäure beeinflusst. Dies wird durch die Ergebnisse aus den Speziations-rechnungen unterstützt. Röntgenabsorptionsspektroskopische Untersuchungen bestätigten ebenfalls dieses Sorptionsergebnis. Die U(VI)-Diffusionsexperimente in Porenwasser bei 25 °C unter Verwendung von Opalinustonbohrkernstücken bestätigten den Kd-Wert der Sorptionsexperimente. In den Diffusionsexperimenten bei 60 °C trat eine Änderung in der U(VI)-Speziation auf. Neben Ca2UO2(CO3)3(aq) wurde eine kolloidale U(VI)-Spezies gebildet. Für die Diffusion der aquatischen U(VI)-Spezies durch Opalinuston bei 25 und 60 °C wurden annähernd gleiche scheinbare (apparente) Diffusionskoeffizienten (Da) bestimmt. Das bedeutet, der Durchbruch von U(VI) durch Opalinuston ist unabhängig von den hier untersuchten Temperaturen und wird deshalb etwa zum gleichen Zeitpunkt erwartet. Modellierungen zeigten, dass es etwa zehn Jahre dauern würde, bis eine detektierbare Menge an 233U(VI) (1×10–9 mol/L) durch ein 11 mm-dickes Opalinustonbohrkernstück migrieren würde. Zwei verschiedene Huminsäuregrößenfraktionen diffundierten durch die Opalinustonproben – eine große und eine kleine kolloidale Größenfraktion. Innerhalb von drei Monaten migrierten die hochmolekularen Huminsäurekolloide nur 500 µm in den Ton, während die niedermolekularen Huminsäurekolloide durch die gesamten Opalinustonproben diffundierten und dadurch im Auffangreservoir detektiert werden konnten. Diese Resultate demonstrieren den Filtrationseffekt des Tongesteins. Die Diffusionsversuche zeigten, dass der Einfluss von Huminsäure auf die U(VI)-Diffusion, unabhängig von der in dieser Arbeit verwendeten Temperatur, vernachlässigbar ist. Die erhaltenen Ergebnisse tragen zu Datenbanken bei, die für die Modellierung von Wechselwirkungs- und Migrationsprozessen in Uran / Tongestein-Systemen genutzt werden. Das bedeutet, die gesammelten Sorptions- und Diffusionsdaten sind nicht nur für den Langzeitsicherheitsnachweis eines Endlagers für radioaktive Abfälle von Relevanz, sondern auch für jedes tonhaltige System in der Umwelt, bei dem die geochemische Wechselwirkung mit urankontaminierten Wässern eine Rolle spielt. Bezüglich der Eignung von Opalinuston als Wirtsgestein für ein Endlager radioaktiver Abfälle lässt sich schlussfolgern, dass Opalinuston ein relativ hohes Retardationspotential bezüglich U(VI) aufweist. Wenn U(VI) als Bestandteil des radioaktiven Abfalls bei Wassereinbruch im Endlager in die Umgebung freigesetzt wird, wird es unter umweltrelevanten Bedingungen von Calcium- und Carbonationen, welche aus dem Opalinuston herausgelöst werden, komplexiert. Dabei bildet sich Ca2UO2(CO3)3(aq). Dieser Komplex wird nur schwach durch Sorption am Tongestein zurückgehalten, was zu einer erhöhten U(VI)-Mobilität im Wirtsgestein führen kann. Im untersuchten System wird die U(VI)-Migration durch das Tongestein jedoch durch molekulare Diffusion bestimmt. Sie verzögert die Migration von Ca2UO2(CO3)3(aq) durch Opalinuston und stellt somit den maßgeblichen Retardationsprozess im System dar. Huminsäure hat keinen signifikanten Einfluss auf die U(VI) / Opalinuston-Wechselwirkung, sogar bei einer erhöhten Temperatur von 60 °C. Dies wurde mittels Speziationsmodellierungen sowie durch Sorptions- und Diffusionsversuche gezeigt.