Biogeochemical Cycling of 99Tc in Alkaline Sediments

Abstract

⁹⁹Tc will be present in significant quantities in radioactive wastes including intermediate-level waste (ILW). The internationally favored concept for disposing of higher activity radioactive wastes including ILW is via deep geological disposal in an underground engineered facility located ∼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), ∼35% Tc(VII) removal occurred during bioreduction. Solvent extraction of the residual solution phase indicated that ∼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 >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 oxidized, there was a significant loss of Fe(II) over 1 month (∼50%), yet this was coupled to only modest Tc remobilization (∼25%). Here, XAS analysis suggested that 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.

Keywords: bioreduction; elevated pH; geological disposal; incorporation; iron(II); reoxidation; technetium.