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.

The polyphenol pathway and Maillard reaction (polycondensation of sugars and amino acids) are regarded as important pathways in natural humification. The significance of linking the Maillard reaction and polyphenol pathways into an integrated humification pathway has been addressed. However, the ability of mineral colloids commonly occurring in tropical and temperate environments to promote the Maillard reaction and integrated polyphenol-Maillard humification pathways remained to be uncovered. Furthermore, the effect of the nature and relative abundance of biomolecules on humification and associated reaction products remained to be studied.<p>The results of this study show that the structure of polyphenols and the relative molar ratio of polyphenol, glucose and glycine, significantly affected humification processes and the associated carbonate formation in the integrated polyphenol-Maillard reaction catalyzed by birnessite. Increasing the molar ratio of ortho-polyphenols (catechol and pyrogallol) to Maillard reagents in the polyphenol-Maillard pathway catalyzed by birnessite enhanced humification while suppressing the formation of rhodochrosite (MnCO3). The opposite trend of MnCO3 formation was observed in the meta-polyphenol (resorcinol)-Maillard reaction system. Increasing the amount of glucose in the integrated catechol-Maillard system under the catalysis of birnessite promoted the formation of Maillard reaction-type humic acid in the supernatant and MnCO3 in the solid phase.<p>The catalytic abilities of commonly occurring mineral colloids from temperate and tropical regions greatly differed in influencing humification processes and products in the Maillard reaction and integrated polyphenol-Maillard pathways. Compared with layer silicate colloids, the poorly ordered Fe and Mn oxides were by far the strongest catalysts of humification reactions in the Maillard and catechol-Maillard pathways. This accounted for the significant difference in reactivity between the sesquioxide-rich Oxisol clay from the high rainfall region of South Africa and the Mollisol clay from the Canadian Prairies. Furthermore, the nature of the mineral colloids also affected the extent of organic C accumulation in the solid phase upon humification, and related mineral surface alteration. The metal oxide- and Oxisol clay-catalyzed Maillard and catechol-Maillard reaction systems had the highest accumulation of organic C in the solid phase, indicating their significance in contributing to C sequestration in the environment.<p>The findings obtained in this study are of fundamental significance in understanding the influence of the atomic bonding, structural configuration and related surface properties of mineral colloids, and the nature and abundance of biomolecules on the abiotic humification pathways and related reaction products in natural environments.

The polyphenol pathway and Maillard reaction (polycondensation of sugars and amino acids) are regarded as important pathways in natural humification. The significance of linking the Maillard reaction and polyphenol pathways into an integrated humification pathway has been addressed. However, the ability of mineral colloids commonly occurring in tropical and temperate environments to promote the Maillard reaction and integrated polyphenol-Maillard humification pathways remained to be uncovered. Furthermore, the effect of the nature and relative abundance of biomolecules on humification and associated reaction products remained to be studied.<p>The results of this study show that the structure of polyphenols and the relative molar ratio of polyphenol, glucose and glycine, significantly affected humification processes and the associated carbonate formation in the integrated polyphenol-Maillard reaction catalyzed by birnessite. Increasing the molar ratio of ortho-polyphenols (catechol and pyrogallol) to Maillard reagents in the polyphenol-Maillard pathway catalyzed by birnessite enhanced humification while suppressing the formation of rhodochrosite (MnCO3). The opposite trend of MnCO3 formation was observed in the meta-polyphenol (resorcinol)-Maillard reaction system. Increasing the amount of glucose in the integrated catechol-Maillard system under the catalysis of birnessite promoted the formation of Maillard reaction-type humic acid in the supernatant and MnCO3 in the solid phase.<p>The catalytic abilities of commonly occurring mineral colloids from temperate and tropical regions greatly differed in influencing humification processes and products in the Maillard reaction and integrated polyphenol-Maillard pathways. Compared with layer silicate colloids, the poorly ordered Fe and Mn oxides were by far the strongest catalysts of humification reactions in the Maillard and catechol-Maillard pathways. This accounted for the significant difference in reactivity between the sesquioxide-rich Oxisol clay from the high rainfall region of South Africa and the Mollisol clay from the Canadian Prairies. Furthermore, the nature of the mineral colloids also affected the extent of organic C accumulation in the solid phase upon humification, and related mineral surface alteration. The metal oxide- and Oxisol clay-catalyzed Maillard and catechol-Maillard reaction systems had the highest accumulation of organic C in the solid phase, indicating their significance in contributing to C sequestration in the environment.<p>The findings obtained in this study are of fundamental significance in understanding the influence of the atomic bonding, structural configuration and related surface properties of mineral colloids, and the nature and abundance of biomolecules on the abiotic humification pathways and related reaction products in natural environments.