Microbial metabolism can have a controlling influence on the solubility of actinides and fission products in natural and engineered environments, offering non-invasive bioremediation options for the nuclear sector. Most work has focused on uranium removal, with several mechanisms of uranium bioprecipitation explored, including those mediated by the production of surface localised or secreted ligands. Most interest, however, has focused on the microbial redox cycling of uranium, where anoxic reduction of relatively soluble and mobile U(VI) facilitated by specialist metal-reducing bacteria, results in the formation of essentially insoluble U(IV) (e.g. uraninite). There has been a significant body of research addressing such transformations in “far field” aquifer type systems, following “biostimulation” of the extant microbial communities with simple electron donors. These studies have ranged from the mechanisms of bioreduction, including identification of the genes, proteins and other redox active biomolecules involved in electron transfer, to the post-reduction fate of the actinide. These studies have also addressed U(IV) re-oxidation and remobilization, which can be appreciable, under a range of environmental conditions.
Complementary studies have also addressed the fate of other redox active radionuclides (including Np, Pu and Tc) under similar redox cycling scenarios. They show a surprising variation in the behaviour of the elements under reducing and oxidising conditions, and suggest that the impact of biostimulation of anaerobic uranium-reducing bacteria should be accompanied by a deeper understanding of the impact of such processes on a broad-range of toxic and radioactive elements where they are present. There is also an increasing interest in the microbial redox cycling or uranium and other radionuclides in engineered geodisposal environments and wasteforms under biogeochemically more extreme conditions. Recent studies in these areas, and their implications, will also be discussed alongside an overview of complementary approaches to target non-redox active radionuclides.