Groundwater contaminated with radioactive elements is a pressing environmental issue at current and former nuclear sites. Bioremediation, that is the stimulation of sediment microbial communities to remove radionuclides from solution, is a promising technology that may be used to treat groundwater contaminated with uranium and technetium. The application of two different bioremediation techniques has been investigated via a series of sediment microcosm and pure bacterial culture experiments including: the use of an electron donor to stimulate microbial-reduction of soluble and mobile uranium(VI) and technetium(VII) to insoluble U(IV) and Tc(IV) minerals; and the use of glycerol phosphate to stimulate the precipitation of biogenic uranium-phosphate minerals. Sediment samples were collected from the subsurface underlying the Sellafield nuclear site; to our knowledge this is the first time that such samples have been used in biogeochemical experiments.Microbial U(VI) reduction was stimulated in a variety of different lithology Sellafield sediments via the addition of an acetate/lactate electron donor mix. In the majority of samples U(VI) was successfully removed from solution as U(IV), highlighting the potential for biostimulation to be used to remediate groundwater contaminated with uranium at UK nuclear sites. However, this did not occur in two sediments, and investigations suggested that this may have been due to a paucity of bioavailable Fe(III) and hence low numbers of Fe(III)-reducing bacteria. Questions have been raised concerning whether microbially-reduced U(IV) would be suitable for maintaining low concentrations of uranium in groundwater over long time periods, particular if groundwater conditions become oxidising. Therefore a series of experiments were performed with biogenic U(IV) that had been aged for up to 18 months, to assess whether oxidative remobilisation may occur under strongly oxidising, worst case conditions. Microbially-reduced U(IV) was fully reoxidised via exposure to air, and partially reoxidised by nitrate. Evidence for an increase in the crystallinity of microbially-reduced U(IV) was observed during ageing, but despite this it did not become more recalcitrant to oxidative remobilisation. Microbial Tc(VII) reduction was also stimulated in Sellafield sediment using a range of slow-release proprietary electron donors. Further characterisation work, such as column studies and field trials would be required prior to this stimulated microbial reduction technology being implemented at a UK nuclear site.Biomineralisation of uranium phosphates was stimulated by the addition of glycerol phosphate to a Serratia environmental isolate and to Sellafield sediment for comparative studies. The Serratia species was able to remove U(VI) from solution by multiple metabolic pathways, including via the formation of uranyl(VI) phosphates of the autunite group. Uranium was precipitated in sediment systems as a crystalline U(IV) phosphate mineral similar to ningyoite. This was not susceptible to oxidative remobilisation by nitrate, and was only partially reoxidised via exposure to air under strongly oxidising end member conditions. Therefore this bioremediation technique may be more suitable for achieving long-term removal of uranium from groundwater, although again further characterisation work would be required prior to implementation.
|Date of Award||1 Aug 2015|
- The University of Manchester
|Supervisor||Jonathan Lloyd (Supervisor) & Katherine Morris (Supervisor)|