Microbially driven transformations of technetium

Technetium-99 (Tc) is a redox active, beta-emitting radionuclide and a mobile contaminant in groundwaters at nuclear facilities in the UK and USA. Additionally, it has been released to the natural environment in authorised and accidental discharges, and is expected to be a radiologically significant component of spent nuclear fuel due to its long half-life (2.13 x 105 y) and potentially high environmental mobility. In terms of its environmental behaviour, the speciation of Tc is the major control on its mobility. Under oxic conditions the highly mobile TcO4 - is expected to dominate whilst, under reducing conditions, poorly soluble reduced species (predominantly Tc(IV)) are expected. Here we apply XAS techniques to determine the speciation of Tc in a range of sediments that have been subjected to biologically mediated redox transformations. In sediments, microbially-mediated reactions control the development of sediment anoxia and, thus control Tc speciation and mobility. Experiments show that microbially mediated reduction of sediments led to reduction of soluble TcO4 - to poorly soluble Tc(IV) during Fe(III)-reduction and the Tc was present as hydrous TcO2 in a range of systems. We also studied the behaviour of Tc on reoxidation of the sediments with air, which is relevant to disturbance of Tc-contaminated sediments, and via microbially mediated “anoxic oxidation” with nitrate. Anoxic oxidation is relevant to proposed bioremediation plans for contaminated sites where reducing conditions are stimulated in the sub-surface by addition of organic matter. In this context, when environmental stewardship of these sites ceases, sub-oxic, nitrate-rich water may flush the anoxic area and mediate reoxidation of the sediments. On air reoxidation 50-80% of the sediment bound Tc was remobilised to solution as TcO4 - whilst on nitrate reoxidation typically < 10% of the sediment bound Tc was remobilised. The speciation of the Tc remaining on the sediments was site- and biogeochemistry-specific with both a mix of hydrous TcO2 and Tc(VII) being the most common form of Tc after reoxidation, but with one air reoxidised sediment containing only hydrous TcO2.