The chemistry of technetium with reference to geological disposal

The understanding of the chemistry of technetium is of vital importance to the development of a geological disposal facility (GDF) for the storage of radioactive waste in the UK. This has led to the research that has been conducted to produce this thesis. As technetium has a highly mobile anionic species, the pertechnetate TcO4-, in general it is desirable to reduce this species to a sparingly soluble TcO2 species. This thesis includes studies conducted using both Tc(VII) and Tc(IV). Solubility studies have been conducted using Tc concentrations of 10-9, 10-10 and 10-11. These were conducted using ligands that are likely to be present such as Ethylenediaminetriacetic acid (EDTA), Nitriloacetic acid (NTA), picolinic acid, Iso-saccarinic acid (ISA), and an ISA analogue gluconic acid. Ligands were chosen that are likely to be present in Low Level Waste (LLW). They would become present in the GDF as they can be used in cleaning and decontamination proceses. These studies were conducted using electrolysis as a reduction technique both before and in the presence of, the anthropogenic complexing agents mentioned previously. It has been established that when the ligands were added to a technetium solution prior to the reduction process taking place there was an apparent inhibition of the full reduction of technetium in solution. Sorption studies of both oxidised and reduced technetium species have been conducted and the results have shown that there is apparent sorption of oxidised technetium to some of the solids that have been tested as well as technetium in a reduced oxidation state of Tc(IV). Solids that have been tested have been chosen to accommodate a broad sprectrum of solids which may be present in, or in close proximity to, a GDF in the UK. These experiments have been conducted after a review of relevant literature to ascertain the level of, if any, work that has been completed using technetium in a reduced form for both sorption and solubility studies. The methods that have been used involved the development of an electrochemical technique which allowed for the rapid and repeatable reduction of a pertechnetate solution electrochemically rather than using a more complicated system involving either tin or iron as a reducing agent.