Many countries intend to achieve the safe management of their radioactive wastes through geological disposal. In addition, radioactively contaminated land is of global concern. To address both of these technical challenges it is imperative to understand the behaviour and subsequent migration of radionuclides in the subsurface. This thesis addresses uncertainties in the behaviour of the long-lived, risk-driving radionuclides U and Np in their most mobile and environmentally relevant oxidation states, U(VI) and Np(V). The formation the U(VI) colloidal nanoparticles is identified under the high pH, low carbonate conditions expected within the near field of a cementitious Geological Disposal Facility (GDF). XAS, SAXS, and TEM have been used to characterise these U(VI) colloids as 60-80 nm clusters of 1-2 nm clarkeite-like (Na uranate) nanoparticles, which are stable in cement leachate for a period of at least 5 years. The reactivity of these U(VI) colloids towards a range of mineral phases was investigated. In the presence of the common rock-forming minerals biotite, orthoclase, and quartz, only limited reactivity was observed with > 80 % of the U(VI) remaining in the filtered fraction after up to 5 years of reaction. In contact with cement, > 97 % of the U(VI) was removed from solution within 1 month. Reversibility studies, luminescence spectroscopy, and XAS suggest that a large portion of the cement associated U(VI) is in a uranophane-like coordination environment, likely incorporated into the C-S-H interlayers or as a stable surface precipitate. Together, this suggests that while U(VI) colloids could form in high pH (> 13) cement leachate, providing an additional pathway for migration, many of them are likely to be removed from suspension by the presence of solid cement, although 2.4 % (1.0 IμM) U(VI) remained in the filtered fraction even after 21 months of reaction. The interaction of aqueous U(VI) and Np(V) with a range of environmentally relevant Mn minerals has also been studied under circumneutral to alkaline conditions. Here, extensive (up to 99 %) uptake of U(VI) and Np(V) was observed in systems containing δ-Mn(IV)O2, triclinic (Na)-birnessite [Na0.5Mn(IV/III)2O4 · 1.5H2O], hausmannite [Mn(III/II)3O4], and rhodochrosite [Mn(II)CO3]. The uptake of U(VI) by δ-MnO2 and hausmannite was found to be partially irreversible, suggesting that these minerals could be particularly important in determining radionuclide migration. XAS indicated that both U(VI) and Np(V) formed edge-sharing bidentate adsorption complexes on the surface of δ-MnO2 and hausmannite, implying that these complexes are responsible for the observed reversibility. These complexes were also identified on triclinic (Na)-birnessite; however, after 1 month of reaction U(VI) was found to have migrated into the triclinic (Na)-birnessite interlayer, replacing Na+. Reaction with all three investigated Mn oxide phases was rapid, with equilibrium being reached within at least 2 weeks. However, whilst U(VI) and Np(V) were both extensively removed from solution in systems containing rhodochrosite, these reactions were much slower, with equilibrium taking up to 4 months to be established. XAS suggested that this was due to the formation of a U(VI) or Np(V) containing precipitate on the rhodochrosite surface.

L’objectif de cette thèse est de quantifier l’évolution de la surface réactive d’un silicate modèle (orthose) lors de son altération hydrothermale et estimer son impact sur la cinétique réactionnelle. L’étude porte sur : (1) l’influence de la présence de couverture de phases secondaires à même la surface de l’orthose, (2) l’impact de l’anisotropie de la structure cristalline de l’orthose et (3) l’effet de la formation de puits de corrosion en surface. Les résultats expérimentaux et numériques mettent en évidence que la vitesse de dissolution de l’orthose et son évolution au cours du temps dépendent essentiellement de sa morphologie.Certaines faces cristallines se dissolvent 10 fois plus rapidement que d’autres, entraînant une augmentation de la proportion de faces rapides au cours du processus et une élévation, jusqu’à un ordre de grandeur, de la vitesse de dissolution globale de l’orthose. Ces résultats ouvrent d’importantes pistes de réflexion sur la méthode adéquate pour rendre compte des cinétiques des interactions fluide/roche sur le terrain ainsi que sur la signification des lois de vitesse et des mécanismes réactionnels déterminés à partir d’expériences sur poudre. / This thesis aims at quantifying the reactive surface area evolution of dissolving K- feldspar, and evaluating the impact on the dissolution kinetics during its alteration in geothermal context. The study focuses on : (1) the influence of secondary coatings on the orthoclase surface, (2) the impact of the anisotropic crystalline structure of orthoclase and (3) the effect of etch pit formation on the mineral surface. Experimental and numerical results highlight that the orthoclase dissolution rate and its evolution over time mainly depends onits morphology. Some orthoclase faces dissolve 10 times faster than others, resulting in an increase of the surface proportion of rapid vs. slow dissolving faces during the process and the increase of up to an order of magnitude of the overall orthoclase dissolution rate. These results question the significance of rate laws and reaction mechanisms determined from powder experiments and the pave to new approaches for investigating mineral reactivity.

Made available in DSpace on 2014-12-17T14:07:06Z (GMT). No. of bitstreams: 1 RaimundoJSC_TESE.pdf: 1754370 bytes, checksum: 88d2811dd20d4c281cc4bc5a58cfc6fa (MD5) Previous issue date: 2010-11-16 / The red pottery industry in Piau? state is well developed and stands out at the national context for the technical quality of its products. The floor and wall tile industry, however, is little developed since the state has only one company that produces red clay-based ceramic tiles. This thesis aims at using the predominantly illitic basic mass of the above mentioned industry, with the addition of feldspar and/or kaolin residue in order to obtain products of higher technical quality. Kaolin residue consists basically of kaolinite, muscovite mica and quartz; the feldspar used was potassic. In this experiment, basic mass (MB) was used for experimental control and fifteen formulations codified as follows: F2, F4, F8, F16, F32, FR2, FR4, FR8, FR16, FR32, R2, R4, R8, R16 and R32. All raw materials were dry-milled, classified, formulated and then humidified to 10% water. Thereafter, test samples were produced by unixial pressing process in a rectangular steel matrix (60.0 x 20.0 x 5.0) mm3 at (25 MPa). They were fired at four temperatures: 1080?C, 1120?C, 1160?C, with a heating rate of 10?C/min during up to 10 min in an electric oven, and the last one in an industrial oven with a peak of 1140?C, aim ing to confirm the results found in laboratory and, finally, technological tests were performed: MEA, RL, AA, PA, TRF and PF. The results revealed that the residue under study can be considered a raw material with large potential in the industry of red clay-based ceramic tiles, since the results found both in laboratory and in the industry have shown that the test samples produced from the formulations with up to 4% feldspar and those produced with up to 8% feldspar and residue permitted a reduction in the water absorption rate and an increase in the mechanical resistance while those samples produced with up to 4% residue had an increase in the mechanical resistance when compared to those produced from the basic mass and that the formulation with 2% feldspar and residue presented the best technological properties, lowering the sintering temperature down to 1120?C / A ind?stria de cer?mica vermelha do Piau? ? bem desenvolvida e se destaca no contexto nacional pela qualidade t?cnica de seus produtos. J? a ind?stria de revestimento pouco se desenvolveu, visto que o estado possui uma ?nica empresa que produz revestimento de base argilosa vermelha. Esta tese tem por objetivo utilizar a massa b?sica predominantemente il?tica da ind?stria citada, adicionando ? mesma feldspato e/ou res?duo de caulim com o prop?sito de obter produtos de melhores qualidades t?cnicas. O res?duo caulim ? constitu?do basicamente de caulinita, mica moscovita e quartzo; o feldspato utilizado foi o pot?ssico. Para este experimento, utilizou-se a massa b?sica (MB) para controle experimental e quinze formula??es assim codificadas: F2, F4, F8, F16, F32, FR2, FR4, FR8, FR16, FR32, R2, R4, R8, R16 e R32. Todas as mat?rias-primas foram mo?das a seco, caracterizadas, formuladas, depois umidificadas a 10% com ?gua. Em seguida, foram confeccionados corpos-de-prova por prensagem uniaxial em matriz retangular de a?o (60,0 x 20,0 x 5,0) mm? a (24 MPa), os quais foram queimados em quatro temperaturas: 1080?C, 1120?C, 1160?C, com taxa de aqu ecimento de 10?C/min e patamar de 10 min em forno el?trico, e a ?ltima em forno a rolo industrial e pico de 1140?C com o objetivo de validar os resultados encontra dos em laborat?rio; por ?ltimo, foram realizados ensaios tecnol?gicos: MEA, RL, AA, PA, TRF e PF. Os mesmos mostraram que o res?duo estudado pode ser considerado como mat?riaprima de grande potencial para a ind?stria de pisos e revestimentos cer?micos de base argilosa vermelha, j? que tanto os resultados encontrados em laborat?rio como na ind?stria mostram que os corpos-de-prova produzidos a partir das formula??es com feldspato at? 4% e os produzidos com feldspato e res?duo at? 8% permitiram uma redu??o da absor??o de ?gua e aumento da resist?ncia mec?nica, sendo que os corpos-de-prova produzidos com at? 4% de res?duo tiveram aumento da resist?ncia mec?nica, quando comparados aos corpos-de-prova produzidos a partir da massa b?sica e que a formula??o com 2% de feldspato e res?duo apresentou melhores propriedades tecnol?gicas, reduzindo a temperatura de sinteriza??o para 1120?C