Biogeochemical Cycling of 99Tc in Alkaline Sediments

99Tc will be present in significant quantities in radioactive wastes including intermediate level wastes (ILW). The internationally favoured concept for disposing of ILW is via deep geological disposal in an underground engineered facility located approximately 200 – 1000 m deep. Typically, in the deep geological disposal environment the subsurface will be saturated, cement will be used extensively as an engineering material, and iron will be ubiquitous. This means that understanding Tc biogeochemistry in high pH, cementitious environments is important to underpin safety case development. Here, alkaline sediment microcosms (pH 10) were incubated under anoxic conditions under no added Fe(III) and with added Fe(III) conditions (added as ferrihydrite) at three Tc concentrations (10-11, 10-6, and 10-4 mol L-1). In the 10-6 mol L-1 Tc experiments with ‘no added Fe(III)’, approximately 35% Tc(VII) removal occurred during bioreduction. Solvent extraction of the residual solution phase indicated that approximately 75% of Tc was present as Tc(IV), potentially as colloids. In both biologically active and sterile control experiments ‘with added Fe(III)’, Fe(II) formed during bioreduction and greater than 90% Tc was removed from the solution, most likely due to abiotic reduction mediated by Fe(II). X-ray absorption spectroscopy (XAS) showed that in bioreduced sediments, Tc was present as hydrous TcO2-like phases, with some evidence for an Fe association. When reduced sediments with added Fe(III) were air oxidised, there was a significant loss of Fe(II) over 1 month (approximately 50%), yet this was coupled to only modest Tc remobilisation (approximately 25%). Here, XAS analysis suggested with air oxidation, partial incorporation of Tc(IV) into newly forming Fe oxyhydr(oxide) minerals may be occurring. These data suggest that in Fe-rich, alkaline environments, biologically mediated processes may limit Tc mobility.