Speciation of tetravalent plutonium in contact with humic substances and kaolinite under environmental conditions

Plutonium represents the major contribution to the radiotoxicity of spent nuclear fuel over storage times of up to several hundred thousand years. The speciation of plutonium in aquifer systems is important in order to assess the risks of high-level nuclear waste disposal and to acquire a deep knowledge of the mobilization and immobilization behavior of plutonium. In aqueous solutions, plutonium can coexist in four oxidation states and each one of them has different chemical and physical behavior. Tetravalent plutonium is the most abundant under natural conditions. Therefore, detailed speciation studies of tetravalent plutonium in contact with humic substances (HS) and kaolinite as a model clay mineral have been performed in this work. Plutonium is present in the environment at an ultratrace level. Therefore, speciation of Pu at the ultratrace level is mandatory. Capillary electrophoresis (CE) coupled to resonance ionization mass spectrometry (RIMS) was used as a new speciation method. CE-RIMS enables to improve the detection limit for plutonium species by 2 to 3 orders of magnitude compared to the previously developed CE-ICP-MS. For understanding the behavior of Pu(IV) in aqueous systems, redox reactions, complexation, and sorption behavior of plutonium were studied. The redox behavior of plutonium in contact with humic acid (HA) and fulvic acid (FA) was investigated. A relatively fast reduction of Pu(VI) in contact with HS was observed. It was mainly reduced to Pu(IV) and Pu(III) within a couple of weeks. The time dependence of the Pu(IV) complexation with Aldrich HA was investigated and a complex constant (logßLC) between 6.4 - 8.4 of Pu(IV) was determined by means of ultrafiltration taking into account the loading capacity (LC). The sorption of tetravalent plutonium onto kaolinite was investigated as a function of pH in batch experiments under aerobic and anaerobic conditions. The sorption edge was found at about pH = 1 and a maximum sorption at around pH = 8.5. In the presence of CO2 at pH > 8.5, the sorption of plutonium was decreased probably due to the formation of soluble carbonate complexes. For comparison, the sorption of Th(IV) onto kaolinite was also investigated and consistent results were found. The Pu(IV) sorption onto kaolinite was studied by XANES and EXAFS at pH 1, 4, 9 and the sorbed species on kaolinite surface was Pu(IV). Depending on the pH, only 1 - 10 % of the sorbed plutonium is desorbed from kaolinite and released into a fresh solution at the same pH value. Furthermore, the sorption of HS onto kaolinite was studied as a function of pH at varying concentrations of HS, as a prerequisite to understand the more complex ternary system. The sorption of HA onto kaolinite was found to be higher than that of FA. The investigation of the ternary systems (plutonium-kaolinite-humic substances) is performed as a function of pH, concentration of HS, and the sequences of adding the reactants. The presence of HS strongly influences the sorption of Pu(IV) onto kaolinite over the entire pH range. For comparison, the influence of HS on the sorption of Th(IV) onto kaolinite was also investigated and a good agreement with the results of Pu(IV) was obtained.

The grain-scale distribution and behaviour of melt and fluid in crystalline analogue systems

The production, segregation and migration of melt and aqueous fluids (henceforth called liquid) plays an important role for the transport of mass and energy within the mantle and the crust of the Earth. Many properties of large-scale liquid migration processes such as the permeability of a rock matrix or the initial segregation of newly formed liquid from the host-rock depends on the grain-scale distribution and behaviour of liquid. Although the general mechanisms of liquid distribution at the grain-scale are well understood, the influence of possibly important modifying processes such as static recrystallization, deformation, and chemical disequilibrium on the liquid distribution is not well constrained. For this thesis analogue experiments were used that allowed to investigate the interplay of these different mechanisms in-situ. In high-temperature environments where melts are produced, the grain-scale distribution in “equilibrium” is fully determined by the liquid fraction and the ratio between the solid-solid and the solid-liquid surface energy. The latter is commonly expressed as the dihedral or wetting angle between two grains and the liquid phase (Chapter 2). The interplay of this “equilibrium” liquid distribution with ongoing surface energy driven recrystallization is investigated in Chapter 4 and 5 with experiments using norcamphor plus ethanol liquid. Ethanol in contact with norcamphor forms a wetting angle of about 25°, which is similar to reported angles of rock-forming minerals in contact with silicate melt. The experiments in Chapter 4 show that previously reported disequilibrium features such as trapped liquid lenses, fully-wetted grain boundaries, and large liquid pockets can be explained by the interplay of the liquid with ongoing recrystallization. Closer inspection of dihedral angles in Chapter 5 reveals that the wetting angles are themselves modified by grain coarsening. Ongoing recrystallization constantly moves liquid-filled triple junctions, thereby altering the wetting angles dynamically as a function of the triple junction velocity. A polycrystalline aggregate will therefore always display a range of equilibrium and dynamic wetting angles at raised temperature, rather than a single wetting angle as previously thought. For the deformation experiments partially molten KNO3–LiNO3 experiments were used in addition to norcamphor–ethanol experiments (Chapter 6). Three deformation regimes were observed. At a high bulk liquid fraction >10 vol.% the aggregate deformed by compaction and granular flow. At a “moderate” liquid fraction, the aggregate deformed mainly by grain boundary sliding (GBS) that was localized into conjugate shear zones. At a low liquid fraction, the grains of the aggregate formed a supporting framework that deformed internally by crystal plastic deformation or diffusion creep. Liquid segregation was most efficient during framework deformation, while GBS lead to slow liquid segregation or even liquid dispersion in the deforming areas.

Thermodynamics of aqueous biopolymer solutions and fractionation of Dextran

Flory-Huggins interaction parameters and thermal diffusion coefficients were measured for aqueous biopolymer solutions. Dextran (a water soluble polysaccharide) and bovine serum albumin (BSA, a water soluble protein) were used for this study. The former polymer is representative for chain macromolecules and the latter is for globular macromolecules. The interaction parameters for the systems water/dextran and water/BSA were determined as a function of composition by means of vapor pressure measurements, using a combination of headspace sampling and gas chromatography (HS-GC). A new theoretical approach, accounting for chain connectivity and conformational variability, describes the observed dependencies quantitatively for the system water/dextran and qualitatively for the system water/BSA. The phase diagrams of the ternary systems water/methanol/dextran and water/dextran/BSA were determined via cloud point measurements and modeled by means of the direct minimization of the Gibbs energy using the information on the binary subsystems as input parameters. The thermal diffusion of dextran was studied for aqueous solutions in the temperature range 15 < T < 55 oC. The effects of the addition of urea were also studied. In the absence of urea, the Soret coefficient ST changes its sign as T is varied; it is positive for T > 45.0 oC, but negative for T < 45.0 oC. The positive sign of ST means that the dextran molecules migrate towards the cold side of the fluid; this behavior is typical for polymer solutions. While a negative sign indicates the macromolecules move toward the hot side; this behavior has so far not been observed with any other binary aqueous polymer solutions. The addition of urea to the aqueous solution of dextran increases ST and reduces the inversion temperature. For 2 M urea, the change in the sign of ST is observed at T = 29.7 oC. At higher temperature ST is always positive in the studied temperature range. To rationalize these observations it is assumed that the addition of urea opens hydrogen bonds, similar to that induced by an increase in temperature. For a future extension of the thermodynamic studies to the effects of poly-dispersity, dextran was fractionated by means of a recently developed technique called Continuous Spin Fractionation (CSF). The solvent/precipitant/polymer system used for the thermodynamic studies served as the basis for the fractionation of dextran The starting polymer had a weight average molar mass Mw = 11.1 kg/mol and a molecular non-uniformity U= Mw / Mn -1= 1.0. Seventy grams of dextran were fractionated using water as the solvent and methanol as the precipitant. Five fractionation steps yielded four samples with Mw values between 4.36 and 18.2 kg/mol and U values ranging from 0.28 to 0.48.

Structure and properties of nanostructures from polyelectrolytes and porphyrins: Self-Assembly in aqueous solution

In this work self-assembling model systems in aqueous solution were studied. The systems contained charged polymers, polyelectrolytes, that were combined with oppositely charged counterions to build up supramolecular structures. With imaging, scattering and spectroscopic techniques it was investigated how the structure of building units influences the structure of their assemblies. Polyelectrolytes with different chemical structure, molecular weight and morphology were investigated. In addition to linear polyelectrolytes, semi-flexible cylindrical bottle-brush polymers that possess a defined cross-section and a relatively high persistence along the backbone were studied. The polyelectrolytes were combined with structural organic counterions having charge numbers one to four. Especially the self-assembly of polyelectrolytes with different tetravalent water-soluble porphyrins was studied. Porphyrins have a rigid aromatic structure that has a structural effect on their self-assembly behavior and through which porphyrins are capable of self-aggregation via π-π interaction. The main focus of the thesis is the self-assembly of cylindrical bottle-brush polyelectrolytes with tetravalent porphyrins. It was shown that the addition of porphyrins to oppositely charged brush molecules induces a hierarchical formation of stable nanoscale brush-porphyrin networks. The networks can be disconnected by addition of salt and single porphyrin-decoratedrncylindrical brush polymers are obtained. These two new morphologies, brush-porphyrin networks and porphyrin-decorated brush polymers, may have potential as functional materials with interesting mechanical and optical properties.