Laver, Nicholas Robert; (2020) The removal of radionuclides from contaminated water samples using graphene oxide nano-flakes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

One of the main challenges currently facing the nuclear industry is the management, removal and disposal of radioactive material from aqueous environments. Water contaminated with nuclear material has arisen in a number of ways. The majority of nuclear material in aqueous environments is from normal nuclear activities and is currently stored in ponds, silos and tanks. Many of the waste storage facilities in the UK are however nearing the end of their intended lifetime so will shortly need processing. The contamination of water with nuclear material is also not confined directly to activities in a nuclear site, activities such as uranium mining can significantly increase the concentration of radionuclides and other heavy metals in groundwater. There are solutions within the nuclear industry to remediate contaminated water, there is also an ongoing need for more efficient and low-cost solutions. Graphene oxide (GO) has been shown under laboratory conditions to be a highly effective sorbent material for the removal of cations from aqueous environments. The research presented in this thesis is a computational study to better understand the binding interactions between GO and cations to influence its development as a decommissioning solution in the nuclear industry. The research begins in Chapter 3 where a model of GO is developed in density functional theory (DFT) and an insensitivity to the lateral size of the nano-flake for functional group stability on the surface at high level of oxidation is revealed, which had not been reported previously. It was also found from calculations that the formation of epoxy groups is unlikely at low levels of surface oxidation and functional groups tend to aggregate on GO surfaces, which is consistent with experimental observations. The research then continues in Chapter 4 with an investigation into the binding interactions between GO and radionuclide cations. It is found, in agreement with experimental results that tetravalent cations mostly form inner-sphere complexes with GO and divalent cations mostly form outer-sphere complexes. A correlation between formal charge and binding energy is also revealed which is consistent with experimental results. Th(IV) is found to have a low affinity towards the neutral COOH group which exists on GO edges in low pH environments, which is identified as a potential route towards radionuclide selectivity using chemically modified GO. An investigation into the effect of defects in the GO lattice on sorption ability is presented in Chapter 5. It is found that defects have little to no effect on binding to the edge of the GO nano-flake and binding to the surface of defected GO is broadly similar to binding to the surface of defected-free GO. It is found however that the presence of a pore in the GO lattice can promote the hydrolysis of the Th(IV) aquocomplex which significantly increases the stability of the system. Binding to areas of the GO surface containing Stone-Wales defects is also enhanced by a greater density of functional groups. The thesis then concludes in Chapter 6 with a summary of results and a discussion of the potential applications of GO in nuclear decommissioning.

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