Optimising Bioremediation End-Points for the Safe and Effective Long-Term Stewardship of UK Radwaste Impacted Land

  • Gianni Vettese

Student thesis: Phd


The development of nuclear energy for both civil and military demands has created a significant legacy of waste and contamination at nuclear facilities worldwide. Understanding the biogeochemical behaviour of radionuclides under environmentally relevant conditions is required to protect the wider environment with respect to contaminated lands and the long-term storage of wastes. This PhD is centralised on the contaminated land scenario and focusses largely on bioremediation and chemical treatment. Bioremediation utilises the microbial interactions to clean a contaminated site; it presents a relatively cheap, selective and in-situ approach that can be used on a large scale. It is a promising alternative to traditional “Dig-and-Dump” or engineered barrier systems which are available but are often expensive, highly invasive and non-selective. Two elements present in radioactive contaminated lands are U and Sr and they are the focus of this project. Under oxidising conditions U is present in its mobile form as U(VI), however by influencing the redox chemistry insoluble U(IV) minerals can precipitate. This reduction can be influenced by microbial metabolisms when stimulated via the addition of an organic electron donor. Conversely, environmental Sr is almost always present as non-redox active Sr(II), the environmental mobility of Sr is thus principally governed by sorption to minerals, where it behaves similarly to other divalent cations such as Mg(II)or Ca(II). Interestingly, both U and Sr can also be incorporated into phosphate biominerals. This thesis focuses on 2 core subjects. The first investigates the mechanism(s) for bioreduction of aqueous U(VI) in model Fe(III)-reducing bacteria Shewanella oneidensis MR1. Whilst the bioreduction of U(VI) to U(IV) is well studied, the mechanism(s) underpinning this transformation are not yet fully understood; previous work has suggested that the bioreduction of U(VI) by model Fe(III)-reducing bacteria proceeds via a pentavalent U(V) intermediate. This work utilises a multi-technique approach which, for the first time, identifies U(V) as an intermediate under environmentally relevant conditions. It boasts the first publication to use state-of-the-art U M4-edge High Resolution X-ray Absorption Near-Edge Structure (HR-XANES) in a microbial system. Furthermore, not only did this work successfully identify a U(V) intermediate, which appears to be more stable than previously thought, but it also discovered long-term stabilisation of U(V) in the post reduction solids. This discovery has led to further research within the Manchester Geomicrobiology Group. The second subject discussed in this thesis compares the long-term effectiveness of various bioremediation and chemical strategies which co-target U and Sr. Here, batch microcosm experiments were set up using a range of treatments which were monitored during incubation and then the remobilisation of the contaminants was studied upon exposure to air or nitrate. Whilst many different treatments have been studied over long timescales, varying experimental conditions means that the results are not directly comparable. The work carried out here provides clear and concise comparisons between seven different bioremediation or chemical systems in the co-treatment of U and Sr. At key time points radionuclide speciation was characterised using X-ray absorption spectroscopy (XAS) and the microbial communities were analysed using 16s rRNA gene analysis. This work identified that systems treated with glycerol phosphate or nanoscale zero-valent iron (nZVI) treatments worked best. Finally, the best 3 treatments were used in flow-through sediment columns to assess U and Sr migration after the intrusion of ground- and seawater; they were compared to a standard electron donor control experiment. This work further demonstrated enhanced stability of solid phase U and Sr contaminants treated using nZVI or glycerolphosphate. It also found that seawater increased t
Date of Award1 Aug 2020
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorSamuel Shaw (Supervisor), Katherine Morris (Supervisor) & Jonathan Lloyd (Supervisor)

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