The immobilisation of uranium is an important issue within the nuclear industry due to contaminated land from accidental spillage, weapons testing or mining activities. Within the environment uranium is most commonly found in the +VI oxidation state as the mobile uranyl cation [UO2]2+. Alternatively, the +IV oxidation state can also be found in the environment, forming either an insoluble crystalline uraninite phase, or a more soluble molecular uranium(IV) species. Many endogenous subsurface bacteria can bind and accumulate actinide ions through biosorption and can reduce mobile uranyl(VI) species down to immobile uranium(IV) compounds and mineral phases. This work presents an investigation into the bioreduction process by two anaerobic Gram-negative bacteria, Geobacter sulfurreducens and Shewanella oneidensis MR-1. Luminescence spectroscopy is used to monitor the intensity of uranyl(VI) emission in situ over the course of a 24 hour bioreduction experiment with uranyl(VI) acetate as the electron acceptor and either acetate or lactate as the electron donor. An increase in intensity of the emission around hour three or four during the reduction, followed by an overall decrease, is attributed as the disproportionation of an unstable uranyl(V) intermediate. The role of inner and outer membrane c-typecytochromes as well as flavin secretion is also investigated using three deletion mutants of the S. oneidensis bacteria, which shows that in their absence, the reduction of uranyl(VI) does not occur over the course of 24 hours. The emission of uranium(IV) is also investigated during bioreduction in phosphate media and results show that emission can be observed in aqueous solutions at pH 7 pointing to the presence of a molecular product. One photon confocal and two photon fluorescence microscopy has been utilised for the very first time to directly optically image the bioreduction of uranyl(VI) in combination with luminescence lifetime mapping. The sorption of uranyl(VI) onto the surface of the bacteria with differing lifetimes indicates a direct interaction between uranyl(VI) and surface bound c-type cytochromes, since this variation was not observed in mutant S. oneidensis strains where the cytochromes were not present. Combined, these results have established the applicability of optical spectroscopy and microscopyin tracking the bioreduction of uranium in situ.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:728152 |
| Date | January 2017 |
| Creators | Jones, Debbie |
| Contributors | Natrajan, Louise |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/fluorescence-spectroscopy-and-microscopy-as-tools-for-monitoring-redox-transformations-of-uranium-in-biological-systems(e5420e94-b96e-4ee1-be63-1a3363672014).html |
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