| Summary: | The rapid development of the nuclear industry and the associated production of waste has created a large demand for the design of materials aimed at the removal of metal ions and radionuclides from the environment and for their safe and efficient disposal. Treatment of waste waters from reprocessing or storage of nuclear fuel requires highly selective sequestering materials with characteristics of high capacity and facile separation from water, whilst decommissioning of nuclear installations requires highly sensitive detection methods. The development of magnetic nanoparticles for separation technologies in liquid systems is wide-spread and already in use in medical testing. Generally, the method relies on the nanoparticles exhibition of superparamagnetic behaviour (i.e. external magnetic field-induced magnetisation of the suspension, with redispersion of the nanoparticles at the removal of the field). In this work, the challenge of translating this methodology, via surface functionalisation, for developing nano-sorbent structures to be used in radionuclide remediation from liquid streams, and to use superparamagnetic behaviour for waste form speciation and separation is addressed. Specifically, in this thesis superparamagnetic iron oxide nanoparticles, in the form of magnetite (Fe3O4), were designed as magnetic-adsorbents for uranium in water. Through surface activation with a phosphate-based ligand, magnetite nanoparticles with high uranium adsorption capacity and selectivity were obtained, with characteristics of extremely rapid uptake and sequestration from low concentration solutions. The former feature is crucial if the nanoparticles are to be applied for sequestering and storage purposes, whilst the latter opens the possibility for their sensing application to ascertain presence of uranium in even low amounts. Selectivity of the functionalised nanoparticles towards uranium against other cation species (Mg(II), Ca(II), Sr(II)) is observed. Desorption studies show a strong “irreversible” interaction between the functionalised nanoparticles and uranium, with no release of uranium detected after 14 months of contact with water. The possibility of tuning the nanoparticles’ magnetic properties by the synthesis of different-sized Fe3O4 nanoparticles and/or by developing an inert coating strategy is also shown. This feature would offer the potential of separation of nanoparticles targeting different waste-stream components after a single adsorption step from water. The optimum characteristics commonly exhibited by nanosized materials, such as increased surface area, high number of sorption sites given by a high yield in surface functionalisation, and the nanomagnetic properties, are observed to be crucial for the extremely efficient sequestration of uranium from water. The coating of the same nanoparticles by a silica shell was also exploited for potential applications in infrastructure projects: i.e. sealing of cracks present in deteriorating cementitious barriers for radionuclide containment. Silica coated-Fe3O4 nanoparticles were effectively produced and shown to undergo controlled gelation in NaCl solution.
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