Investigation of uranium redox chemistry and complexation across the pH range by cyclic voltammetry
thesisposted on 2013-10-31, 12:52 authored by Mei Chew
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.