Mineralisation and biomineralisation of radionuclides

Management of contamination from industrial activities and wastes from nuclear power generation and weapons development are arguably amongst the greatest challenges facing humanity currently and into the future. Understanding the mobility of toxic radioactive elements is essential for successful remediation strategies and safe management of our nuclear waste legacy (DEFRA, 2008). Interactions between minerals and radionuclides, such as sorption and precipitation, govern the mobility of the contaminants through the subsurface environment. Microbial metabolic processes (redox cycling or release of metabolites) have the potential to affect drastically these abiotic interactions. Microbially-driven mineralisation processes could provide long-term solid-phase-capture solutions to radionuclide contamination problems and support safety cases for geological disposal of radioactive waste.The recent advancements at the intersection between mineralogy, microbiology and radiochemistry were reviewed with the aid of a cluster analysis (Self-Organising Map). This is a relatively novel method of creating a map of the ? research landscape? which provides a visual summary of the reviewed literature and can help to identify areas of promising and active research as well as less researched interdisciplinary areas. It is the first time this tool has been applied to research literature on this interdisciplinary topic, and it highlighted the need to gain further understanding of ternary systems including bacteria, minerals and radionuclides. The analysis showed that phyllosilicates are of interest, but few studies have explored the properties of the Fe(II)/Fe(III)-containing micas biotite and chlorite.The ability of model Fe(III)-reducing microorganisms to reduce Fe(III) in biotite and chlorite was demonstrated in batch model systems. In chlorite, approximately 20% and in biotite ~40% of the bulk Fe(III) was transformed to Fe(II) by this reduction. To our knowledge, this is the first study to show the availability of Fe(III) in biotite for such reduction and the ability of the model organism Shewanella oneidensis MR-1 to conserve energy for growth using Fe(III) in biotite as the sole electron acceptor. The microbial Fe(III) reduction led to a decrease in the sorption of Cs and Sr by chlorite, but had very little effect on sorption to biotite. The data indicate that remediation strategies based on microbial Fe(III) reduction may exacerbate the movement of Cs and Sr through strata where sorption is dominated by phyllosilicates, particularly chlorite. While microbial Fe(III) reduction had only a slight effect on the sorption properties of biotite and chlorite, it drastically altered their redox properties. Previously bioreduced biotite and chlorite readily removed Cr(VI), Tc(VII) and Np(V) by surface-mediated reduction. The minerals were also able to reduce U(VI), but solution chemistry affected this reaction, reflecting the complexity of the biogeochemistry of this actinide. Overall, this work highlights the importance of decoupling microbial and geochemical processes in developing a holistic understanding of radionuclide behaviour in the environment.This body of work forms the thesis is entitled ? Mineralisation and Biomineralisation of radionuclides? , and was prepared by Diana Roumenova Brookshaw for submission in August 2013 for the degree of Doctor of Philosophy to the University of Manchester.