The current option for the management of Intermediate-Level Waste (ILW) and High-Level Waste (HLW) in the UK is to store it in stainless steel containers and then placed in a deep underground Geological Disposal Facility (GDF). This may subsequently be backfilled with a cementitious material generating very high pH conditions. The eventual corrosion of the stainless steel canisters containing the waste used for disposal will lead to reducing conditions thereby promoting a low Eh environment. Electrochemical experiments are needed to determine which uranium species is/are present at a particular pH and to model the redox behaviour of aqueous uranium in a potential GDF. The main aim of this project is to use cyclic voltammetry to deduce peak potentials for the various uranium redox couples in aqueous solution across the pH range and in particular the hyperalkaline range, as the surroundings of a GDF will be in high pH conditions. Data in the literature have been obtained only under acidic conditions where they were subsequently extrapolated to obtain data for alkaline conditions in some reports. Is this valid however? Experiments are therefore needed to obtain fundamental data under alkaline conditions to fill in gaps in the literature. In addition to radionuclides, complexing organic ligands present in a cementitious repository could have an important effect on the immobilisation of radionuclides in concrete. This is due to the ability of the ligands to form complexes with cations, thereby enhancing their solubility and mobility in the cement pore water. Four different ligands were investigated in this project that are relevant to nuclear waste disposal which comprised of carbonate, ethylenediaminetetraacetic acid (EDTA), gluconic acid and α-isosaccharinic acid (α-ISA). The peak potentials of each uranium redox reaction in aqueous solution were measured and the potentials were compared in ligand and non-ligand systems. The voltammograms were compared to obtain their similarities and differences in terms of the shape of the cyclic voltammograms, peak potentials, reversibility, current responses and etc. Analysis of the similarities and differences was needed to be able to increase the understanding of the complexation effects of these ligands with uranium under different pH conditions in aqueous solution.

The objective of this research was to investigate lead speciation in the soil/sediment-water environment and to better understand how the species affect lead mobility under different environmental conditions. The research involved both field soil and sediment samples as well as standard lead solutions. Field samples were fully characterized and extracted by aqueous and organic solvents. The results were compared and evaluated with the metal speciation model, MINTEQA2. Hydrolytic polymerization and organic complexation studies were conducted with standard lead solutions under controlled experimental conditions. Results of the field samples showed that pH, dissolved cations, ionic strength, dissolved organic matter, and nature of the soil/sediment matrix play major roles in the distribution and mobility of lead (Pb) from contaminated sites. In the aqueous equilibration experiment, the magnitude of Pb2+ solubilization was in the order of pH4>pH7>pH9. The results were in good agreement with MINTEQA2 predictions. An important finding of the research is the detection of Pb polymerization species under controlled experimental conditions. At pH 5.22, Pb polymeric species were formed at rate of 0.03 per day. The role of Pb complexation with organic matter was evaluated in both field and standard samples. Different methodologies showed three types of organically bound Pb. A very small fraction of Pb, in the ppb range, was extractable from the contaminated soil by polar organic solvents. Sequential extractions show that 16.6±1.4 % of the Pb is organically complexed. Complexation of Pb with fulvic acid provided new information on the extent of Pb association with soluble organic matter. The overall results of this research have provided new and useful information regarding Pb speciation in environmental samples. The results, in several instances, have provided verification of MINTEQA2 model's prediction. They also revealed areas of disagreement between the models prediction and the experimental results. A positive note regarding the experimental work done in the research is the verification of the mass balance in all the repeated experiments.

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