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.

Considering the worldwide growing discharge of minor actinides and the current need for geological disposal facilities for radioactive waste, this work provides a contribution to the safety case concerning Np transport if it would be released from deep repository sites and moving from alkaline cement conditions (near-field) to more neutral environmental conditions (far-field). The reducing conditions in a nuclear waste repository render neptunium tetravalent, which is assumed to be immobile in aqueous environment due to the low solubility solution of Np(IV). For tetravalent actinide nuclides, the most significant transport should occur via colloidal particles. This work demonstrates the formation of intrinsic neptunium dioxide nanocrystals and amorphous Np(IV) silica colloids under environmentally relevant conditions. The dissociation of the initial soluble Np(IV) complex (i.e. [Np(IV)(CO3)5]6-) induces the intrinsic formation of nanocrystalline NpO2 in the solution phase. The resulting irregularly shaped nanocrystals with an average size of 4 nm exhibit a face-centered cubic (fcc), fluorite-type structure (space group ). The NCs tend to agglomerate under ambient conditions due to the weakly charged hydrodynamic surface at neutral pH (zetapotential ~0 mV). The formation of micron-sized agglomerates, composed of nanocrystals of 2-5 nm in size, and the subsequent precipitation cause immobilization of the major amount of Np(IV) in the Np carbonate system. Agglomeration of NpO2 nanocrystals in dependence on time was indicated by PCS and UV-vis absorption spectroscopy with the changes of baseline characteristics and absorption maximum at 742 nm. Hitherto, unknown polynuclear species as intermediate species of NpO2 nanocrystal formation were isolated from solution and observed by HR-TEM. These polynuclear Np species appear as dimers, trimers and hexanuclear compounds in analogy with those reported for other actinides. Intrinsic formation of NpO2 (fcc) nanocrystals under ambient environmental conditions is prevented by admixing silicic acid: amorphous Np(IV) silica colloids are formed when silicate is present in carbonate solution. Herein, the initial molar ratio of Si to Np in solution lead to the formation of Np(IV) silica particles of different composition and size where Si content determines the structure and stability of resulting colloids. Implications for different electronic structures of Np(IV) in dependence on Si content in the solid phase are given by the shift of the absorption maximum at 742 nm characteristic for Np(IV) colloids, silica excess of 5 times the magnitude of Si to Np reveal a redshift up to 6 nm in the colloidal UV-vis spectrum. Precipitation of Np(IV) particles in the ternary system results in a different coordination sphere of Np(IV) compared to the binary system, and the incorporation of Si into internal structure of Np(IV) silica colloids in coffinite-like structure is confirmed by EXAFS. TEM confirms different kinds of particle morphologies in dependence on the silica content. Silica-poor systems reveal porous particles in the micron-range which consist of irregular cross-linked hydrolyzed Np(IV) silica compartments with pores <15 nm. In contrast, long-term stabilized and silica-enriched systems are characterized by isolated particles with an average particle size of 45 nm. Agglomerates of such isolated Np(IV) silica particles appear as consolidated amorphous solids with a densely closed surface and exhibit no internal fractures. The latter mentioned morphology of Np(IV) silica particles might facilitate the migration behavior of Np(IV) in a stabilized colloidal form under environmental conditions. The silica-enriched particles with densely closed surface are long-term stabilized as colloidal dispersion (>1 year) due to repulsion effects caused by significant surface charge. Particles synthesized from Si/Np = 9/1 carry exclusively negative surface charge in nearly the whole pH range from pH 3 to pH 10 with zetapotential = (-) 5 to (-) 30 mV. The zeta potentials of all particle systems containing silica are significantly shifted to more negative values below pH 7 where the isoelectrical point shifts from pH = 8.0 to 2.6 effecting negative charge under ambient conditions which supports electrostatic stabilization of Np(IV) particles. Particle surface charge at the slipping plane, particle size and shape necessarily depend on the initial magnitude of Si content in solution during particle formation. Particular changes of the morphology and internal structure of different Np(IV) silica colloids by aging are indicated by TEM and XPS. The composition and the crystallinity state of the initially formed amorphous phases partially changed into well-ordered nanocrystalline units characterized with fcc structure. The presence of silicate under conditions expected in a nuclear waste repository significantly influences the solubility of Np(IV) and provoke the stabilization of waterborne Np(IV) up to concentrations of 10-3 M, exceeding Np´s solubility limit by a factor of up 10.000. Neptunium and silicate significantly interact with each other, and thereby changing their individual hydrolysis and polymerization behavior. Silicate prevents the intrinsic formation of NpO2 NCs in fcc-structure, and at the same time, Np(IV) prevents the polymerization of silicate. Both processes result in the formation of Np(IV) silica colloids which possibly influence the migration behavior and fate of Np in the waste repositories and surrounding environments. For tetravalent actinides in general, the most significant transport in the environment would occur by colloidal particles. Therefore, Np(IV) silica colloids could have a significant implication in the migration of Np, the important minor actinide in the waste repositories, via colloidal transport.

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Pyroprocessing, via electrorefining in a molten salt bath, is a promising treatment route for spent nuclear fuel reprocessing. In order to implement such a technology and ensure its safe operation it is vital to develop on-line techniques to understand and monitor the molten salt and its contents. These tools are technically challenging because of the high temperatures and corrosive environment experienced in molten salt media. Electrochemical, spectroscopic and spectroelectrochemical methods were developed and used to study actinide and fission product behaviour in molten LiCl-KCl eutectic. A spectroscopic furnace was designed and supporting methodology developed in order to allow the acquisition of reproducible quantitative data. The apparatus monitored the precipitation of NdCl3 by the addition of Li2CO3 and PrCl3 by the addition of Li2O in LiCl-KCl eutectic. The precipitates formed were identified as the respective LnOCl. In order to probe actinide behaviour in this hygroscopic medium, dry actinides chlorides were synthesised. The oxidation of uranium metal by BiCl3 in LiCl-KCl eutectic yielded UCl3 while neptunium and plutonium were prepared as Cs2AnCl6 via precipitation in concentrated aqueous HCl by addition of CsCl. The molar extinction coefficients for U(III), U(IV), Np(IV) and Pu(III) were obtained in LiCl-KCl eutectic at 450 áμ’C. The study of the Np(IV)/Np(III) couple via spectroelectrochemical techniques, enabled the determination of the Np(III) molar extinction coefficients. Uranium was studied in LiCl-KCl eutectic using square wave voltammetry, cyclic voltammetry and chronoabsorptometry. The electrochemical techniques benchmarked the results obtained by spectroelectrochemistry. The results from the different techniques were compared to and explained by determining the Gibbs energy and activation energy of U(III) and U(IV). It was concluded that all the mentioned techniques are suitable for the study of high temperature molten chlorides. Because of its capacity to gather numerous data parameters while minimising the number of experiments required and the quantity of material needed, spectroelectrochemical methods were highlighted as the most promising technique for future studies of radionuclides in high temperature melts.

Thesis: M.S., Massachusetts Institute of Technology, Department of Chemical Engineering, 1979 / Includes bibliographical references (leaves 207-212). / by Saeed Tajik. / M.S. / M.S. Massachusetts Institute of Technology, Department of Chemical Engineering

Tens of thousands of metric tons of highly radioactive, nuclear waste have been generated in the United States. Currently, there is no treatment or disposal facility for these wastes. Of the radioactive elements in high-level nuclear waste, neptunium (Np) is of particular concern because it has a long half-life and may potentially be very mobile in groundwaters associated with a proposed underground disposal site at Yucca Mountain, Nevada. Aqueous Np concentrations observed in previous, short-term solubility experiments led to calculated potential doses exceeding proposed long-term regulatory limits. However, thermodynamic data for Np at 25°C showed that these observed aqueous Np concentrations were supersaturated with respect to crystalline NpO 2 . It was hypothesized that NpO 2 is the thermodynamically stable solid phase in aqueous solution, but it is slow to form in an aqueous solution of NpO 2 + on the time scale of previous experiments. The precipitation of NpO 2 would provide significantly lower aqueous Np concentrations leading to calculated doses below proposed regulatory limits. To test this hypothesis, solubility experiments were performed at elevated temperature to accelerate any slow precipitation kinetics. Ionic NpO 2 + (aq) was introduced into very dilute aqueous solutions of NaCl with initial pH values ranging from 6 to 10. The reaction vessels were placed in an oven and allowed to react at 200°C until steady-state aqueous Np concentrations were observed. In all cases, aqueous Np concentrations decreased significantly from the initial value of 10 −4 M. The solids that formed were analyzed by x-ray powder diffraction, x-ray absorption spectroscopy, and scanning electron microscopy. The solids were determined to be high-purity crystals of NpO 2 . This is the first time that crystalline NPO 2 has been observed to precipitate from NpO 2 + (aq) in near-neutral aqueous solutions. The results obtained demonstrate that Np will precipitate as NpO 2 in aqueous solutions thereby leading to significantly lower aqueous Np concentrations.

<p>Uranium imido compounds have been traditionally studied due to their analogous nature to uranyl compounds. The investigation of uranium imido bonds can open the door to the activation of uranium oxygen bonds, which is important for the recyclization of spent nuclear fuel. This research encompasses further characterization of imido compounds utilizing electrochemical techniques which will better the understanding of these compounds. Utilizing what we already know about uranium imido compounds, further reactivity studies were conducted. Additionally, uranium is a suitable candidate for modeling synthesis of transuranic elements, specifically neptunium. Therefore, additional pathways to synthesize uranium tris(imido) and uranium tertrakis(imido) complexes were explored as a modeling system.  </p>

A database is being developed by the NWMO that will be capable of predicting sorption for key elements of interest onto crystalline and sedimentary rocks in the highly saline groundwaters and porewaters at DGR depths in Ontario, Canada. Pd(II), Np(IV), and Np(V) have all been identified as elements of interest by the NWMO. Sorption experiments of Pd(II) were conducted on illite, bentonite, and shale to investigate the effects that ionic strength (I), and pH have on sorption. Na-Ca-Cl solutions ranging from 0.01 – 6.0 M I with pH values between 3 and 9 were considered under aerobic conditions at 25°C for sorption testing conditions. Sorption data for Pd(II), Np(IV), and Np(V) was used with 2SPNE SC and 2SPNE SC/CE models developed in PHREEQC to model sorption onto illite and montmorillonite at different values of I and pH. These models were also used to test proposed reaction constants from various sources in the existing literature, and to establish and propose optimized surface complexation constants of our own. Optimized reaction constants were compared with the LFER to determine its validity across all test conditions. The LFER was found to not adequately describe sorption of Pd(II), Np(IV), or Np(V) on either illite or montmorillonite. The JAEA TDB, R04, and R07 THEREDA were all tested along with the Davies, Debye-Hückel, SIT, and Pitzer computational methods in order to determine their overall accuracy. The JAEA TDB using the SIT computational method was found to not be capable of modelling Np(IV) onto illite or montmorillonite in 4.0 M I. / Thesis / Master of Applied Science (MASc)

A safe storage of radioactive waste in repositories is an important task to protect humans and the environment from radio- and chemotoxicity. Long-term safety assessments predict the behavior of potential environmental contaminants like the actinides plutonium, uranium, or neptunium, in the near and far field of repositories. For such safety assessments, it is necessary to know the migration behavior of the contaminants in the environment, which is mainly dependent on the aquatic speciation, the solubility product of relevant solid phases, and the retardation due to sorption on surrounding minerals. Thus, an investigation of sorption processes of contaminants onto different minerals as well as the derivation of mineral specific surface complexation model (SCM) parameters is of great importance. Feldspar and mica are widely distributed in nature. They occur as components of granite, which is considered as a potential host rock for a repository in Germany, and in numerous other rocks, and thus also in the far field of nearly all repositories. However, their sorption behavior with actinides has only been scarcely investigated until now. In order to better characterize these systems and subsequently to integrate these minerals into the long-term safety assessments, this work focuses on the investigation of the sorption behavior of U(VI), Np(V), and Nd(III) as analogue for An(III) onto the minerals orthoclase and muscovite, representing feldspars and mica, respectively. All investigations were performed under conditions relevant to the far field of a repository. In addition to the extensive characterization of the minerals, batch sorption experiments, spectroscopic investigations, and surface complexation modeling were performed to elucidate the uptake and speciation of actinides on the mineral surfaces. In addition, the influence of microorganisms naturally occurring on the mineral surfaces and the effect of Ca2+ on U(VI) uptake on the minerals was studied. The obtained sorption curves exhibit a similar characteristic for orthoclase and muscovite. As expected Nd(III) shows the highest amount of sorption followed by U(VI) and finally Np(V). With spectroscopic investigations of the aquatic U(VI) solution in presence of Ca2+, the Ca2UO2(CO3)3 complex could be identified. Furthermore, with spectroscopic methods the U(VI) surface species onto orthoclase could be characterized, of which a novel uranium-carbonate surface species was observed. Based on the results of batch experiments and spectroscopic methods new SCM parameters for the sorption of U(VI), Np(V), and Nd(III) onto orthoclase and for Np(V) and Nd(III) onto muscovite could be derived. SCM parameters for U(VI) sorption onto muscovite confirmed earlier investigations. The obtained SCM parameters increase the amount of data available for sorption processes onto feldspar and mica. With this the relevance of feldspars for the sorption of actinides and lanthanides could be shown. Thus, this work contributes to a better understanding of interactions of actinides and lanthanides, in particular U(VI), Np(V), and Nd(III), with mineral phases ubiquitous in the environment. This in turn adds confidence to long-term safety assessments essential for the protection of humans and the environment from the hazards of radioactive waste.